# BIODEGRADABLE MATRICES AND COMPOSITES

EDITED BY : Alessandro Pegoretti, Yu Dong and Miroslav Slouf PUBLISHED IN : Frontiers in Materials

#### Frontiers eBook Copyright Statement

The copyright in the text of individual articles in this eBook is the property of their respective authors or their respective institutions or funders. The copyright in graphics and images within each article may be subject to copyright of other parties. In both cases this is subject to a license granted to Frontiers. The compilation of articles constituting this eBook is the property of Frontiers.

Each article within this eBook, and the eBook itself, are published under the most recent version of the Creative Commons CC-BY licence. The version current at the date of publication of this eBook is CC-BY 4.0. If the CC-BY licence is updated, the licence granted by Frontiers is automatically updated to the new version.

When exercising any right under the CC-BY licence, Frontiers must be attributed as the original publisher of the article or eBook, as applicable.

Authors have the responsibility of ensuring that any graphics or other materials which are the property of others may be included in the CC-BY licence, but this should be checked before relying on the CC-BY licence to reproduce those materials. Any copyright notices relating to those materials must be complied with.

Copyright and source acknowledgement notices may not be removed and must be displayed in any copy, derivative work or partial copy which includes the elements in question.

All copyright, and all rights therein, are protected by national and international copyright laws. The above represents a summary only. For further information please read Frontiers' Conditions for Website Use and Copyright Statement, and the applicable CC-BY licence.

ISSN 1664-8714 ISBN 978-2-88966-051-3 DOI 10.3389/978-2-88966-051-3

#### About Frontiers

Frontiers is more than just an open-access publisher of scholarly articles: it is a pioneering approach to the world of academia, radically improving the way scholarly research is managed. The grand vision of Frontiers is a world where all people have an equal opportunity to seek, share and generate knowledge. Frontiers provides immediate and permanent online open access to all its publications, but this alone is not enough to realize our grand goals.

#### Frontiers Journal Series

The Frontiers Journal Series is a multi-tier and interdisciplinary set of open-access, online journals, promising a paradigm shift from the current review, selection and dissemination processes in academic publishing. All Frontiers journals are driven by researchers for researchers; therefore, they constitute a service to the scholarly community. At the same time, the Frontiers Journal Series operates on a revolutionary invention, the tiered publishing system, initially addressing specific communities of scholars, and gradually climbing up to broader public understanding, thus serving the interests of the lay society, too.

#### Dedication to Quality

Each Frontiers article is a landmark of the highest quality, thanks to genuinely collaborative interactions between authors and review editors, who include some of the world's best academicians. Research must be certified by peers before entering a stream of knowledge that may eventually reach the public - and shape society; therefore, Frontiers only applies the most rigorous and unbiased reviews.

Frontiers revolutionizes research publishing by freely delivering the most outstanding research, evaluated with no bias from both the academic and social point of view. By applying the most advanced information technologies, Frontiers is catapulting scholarly publishing into a new generation.

#### What are Frontiers Research Topics?

Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office<EMAIL_ADDRESS>
# BIODEGRADABLE MATRICES AND COMPOSITES

Topic Editors: Alessandro Pegoretti, University of Trento, Italy Yu Dong, Curtin University, Australia Miroslav Slouf, Institute of Macromolecular Chemistry (ASCR), Czechia

Citation: Pegoretti, A., Dong, Y., Slouf, M., eds. (2020). Biodegradable Matrices and Composites. Lausanne: Frontiers Media SA. doi: 10.3389/978-2-88966-051-3

## EDITORS BIOGRAPHY

## Prof. Alessandro Pegoretti

Full Professor, Head of Polymers and Composites Laboratory

Department of Industrial Engineering, University of Trento, via Sommarive 9, 38123 Trento, Italy

Webpage: http://www.ing.unitn.it/~pegorett/

Email<EMAIL_ADDRESS>
Phone: +39 0461 282452

## Biography

Prof. Alessandro Pegoretti is currently full Professor at the Department of Industrial Engineering, University of Trento. The main aim of his research activity is to investigate the structure-properties relationships in polymer, polymer blends, composites and nanocomposites, with particular attention to their mechanical behavior. Main fields of interests are deformation, yield and fracture mechanics of polymers and composites, processing and characterization of multiphase polymeric materials (micro- and nanocomposites, and blends), durability of polymeric and composite materials, environmentally sustainable polymers and composites (biodegradable, from renewable resources, fully recyclable), polymers and composites with functional properties (electrical conductivity, shape memory, strain and damage monitoring, self-healing, etc.). He is an author or co-author of more than 240 peer-reviewed journal articles, 12 book chapters and 3 patents. He serves as Specialty Chief Editor of Frontiers in Materials - Polymeric and Composite Materials section, associate editor of the Journal of Reinforced Polymers and Composites. He is member of the editorial board member of various Journals such as Express Polymer Letters, Composite Interfaces, Materials, Journal of Composite Science, Applied Sciences, International Journal of Polymer Science, Journal of Applied Biomaterials and Functional Materials, as well as Journal of Nanomaterials.

## Dr. Yu Dong

Senior Lecturer in Mechanical Engineering

School of Civil and Mechanical Engineering, GPO Box U 1987, Curtin University, WA 6845, Australia

Webpage: https://staffportal.curtin.edu.au/staff/profile/view/Y.Dong/

Email<EMAIL_ADDRESS>Phone: +61 8 92669055

Fax: +61 8 92662681

## Biography

Dr. Yu Dong is currently a Senior Lecturer within the School of Civil and Mechanical Engineering at Curtin University, Perth, Australia. His main research interests are polymer nanocomposites, electrospun nanofibers/nanocomposites, green composites, nanomaterial processing and characterization, micromechanical modeling, finite element analysis, statistical design of experiments and engineering education. He has published over 70 peer-reviewed journal articles and 30 fully referred conference papers, written 10 book chapters and edited 3 technical books. He serves as an associate editor of Frontiers in Materials (Polymeric and Composite Materials section) and Applied Nanoscience, Springer Nature with the specialty recognition of nanomaterials and nanocomposites.

## Dr. Miroslav Slouf

Senior Research Fellow, Head of Department of Polymer Morphology

Institute of Macromolecular Chemistry, Czech Academy of Sciences, Heyrovskeho nam. 2, 16206 Prague 6, Czech Republic

Webpage: https://www.imc.cas.cz/en/research/research-departments/polymermaterials-and-technologies/polymer-morphology/staff

Email<EMAIL_ADDRESS>
Phone: +420 296 809 291

### Biography

Dr. Miroslav Slouf is currently a Senior Research Fellow and Head of Department of Polymer Morphology at the Institute of Macromolecular Chemistry, the Czech Academy of Sciences, in Prague, Czech Republic, as well as Associate Professor at the Faculty of Chemistry at the Brno University of Technology, Brno, Czech Republic. He works in the field of morphology, electron microscopy and micromechanical properties of polymer materials. His main research interests comprise biodegradable polymer blends for local release of antibiotics, ultrahigh molecular weight polyethylene for total joint replacements, and relations between structure, macroand micromechanical properties of polymers. He is an author or co-author of more than 250 peer-reviewed journal articles, 4 book chapters and 5 patents. He serves as an associate editor of Frontiers in Materials (Polymeric and Composite Materials section), a member of advisory editorial board in Open Materials Science, and a member of the board of the Czechoslovak Microscopy Society.

# Table of Contents

*09 Editorial: Biodegradable Matrices and Composites* Alessandro Pegoretti, Yu Dong and Miroslav Slouf

## PART A: BIOPOLYMERS, BIOBLENDS AND BIODEGRADABLE MATRICES

*12 Durability of Biodegradable Polymers for the Conservation of Cultural Heritage*

Maria C. Mistretta, Francesco P. La Mantia, Vincenzo Titone, Bartolomeo Megna, Luigi Botta and Marco Morreale

*24 Sustainable Active PET Films by Functionalization With Antimicrobial Bio-Coatings*

Annalisa Apicella, Paola Scarfato, Luciano Di Maio and Loredana Incarnato

Ivan Fortelny, Aleksandra Ujcic, Luca Fambri and Miroslav Slouf

*56 Structure Characterization and Biodegradation Rate of Poly(*e*-caprolactone)/Starch Blends*

Martina Nevoralová, Marek Koutný, Aleksandra Ujčić, Zdeněk Starý, Jana Šerá, Helena Vlková, Miroslav Šlouf, Ivan Fortelný and Zdeněk Kruliš

*70 Tailoring Biodegradability of Poly(Butylene Succinate)/Poly(Lactic Acid) Blends With a Deep Eutectic Solvent*

Emma Delamarche, Agnès Mattlet, Sébastien Livi, Jean-François Gérard, Rémy Bayard and Valérie Massardier

Rasoul Esmaeely Neisiany, Mohammad Saeid Enayati, Amin Kazemi-Beydokhti, Oisik Das and Seeram Ramakrishna

Zain Zaidi, Damia Mawad and Alan Crosky

## PART B: BIOCOMPOSITE MATERIALS

Tassadit Aouat, Mustapha Kaci, José-Marie Lopez-Cuesta and Eric Devaux

Zonglin Li, Christoff Reimer, Maisyn Picard, Amar K. Mohanty and Manjusri Misra

*228 Thermoplastic Starch Composites With Titanium Dioxide and Vancomycin Antibiotic: Preparation, Morphology, Thermomechanical Properties, and Antimicrobial Susceptibility Testing*

Aleksandra Ujcic, Sabina Krejcikova, Martina Nevoralova, Alexander Zhigunov, Jiri Dybal, Zdenek Krulis, Petr Fulin, Otakar Nyc and Miroslav Slouf

*241 Valorization of Food Industries Wastes for the Production of Poly(vinyl) Alcohol (PVA) Biodegradable Composites*

Francesca Ferrari, Raffaella Striani, Carola Esposito Corcione and Antonio Greco

Zainab Waheed Abdullah and Yu Dong

*277 Thermoplastic Starch Composites Filled With Isometric and Elongated TiO2-Based Nanoparticles*

Aleksandra Ujcic, Martina Nevoralova, Jiri Dybal, Alexander Zhigunov, Jana Kredatusova, Sabina Krejcikova, Ivan Fortelny and Miroslav Slouf *290 Cellulose Nanowhisker (CNW)/Graphene Nanoplatelet (GN) Composite Films With Simultaneously Enhanced Thermal, Electrical and Mechanical Properties*

Dongyan Liu, Yu Dong, Yueyue Liu, Na Ma and Guoxin Sui

*301 Insight Into the Current Directions in Functionalized Nanocomposite Hydrogels*

Rasoul Esmaeely Neisiany, Mohammad Saeid Enayati, Paweł Sajkiewicz, Zari Pahlevanneshan and Seeram Ramakrishna

# Editorial: Biodegradable Matrices and Composites

Alessandro Pegoretti 1 \*, Yu Dong2 and Miroslav Slouf 3

*1 Department of Industrial Engineering, University of Trento, Trento, Italy, 2 School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia, 3 Institute of Macromolecular Chemistry, Czech Academy of Sciences, Prague, Czechia*

Keywords: biodegradable polymers, biodegradable composites, biodegradability, material characterization, environmental sustainability, recyclability

**Editorial on the Research Topic**

#### **Biodegradable Matrices and Composites**

Biodegradable and ecofriendly materials attract an increasingly high attention from polymer and composite researchers and manufactures as they can contribute to a more sustainable global development policy, to significantly reduce non-recyclable plastic wastes and limit the impact on the environment. A thorough understanding of both advantages and limitations of biodegradable matrices and composite materials is the key to their reasonable, precisely targeted and effective applications in order to benefit wide communities. The attention in the field of real applications is focused mostly on packaging materials, but automotive, construction, and agriculture are also important fields. As for medical applications, biodegradable matrices and composites are used mostly due to their tunable biodegradability, which is often combined with controlled and localized drug release. The term of biodegradable composites comprises a wide range of at least two-phase hybrid materials in which either fillers or matrix or both must be chosen from biodegradable sources. Hence, those matrices based on biodegradable polymers are also well-recognized as biodegradable matrices. On the other hand, fillers being the minority phase act as a reinforcement in most biodegradable composites. However, with respect to polymer blends, the minority phase often plays a different role as an impact modifier, a plasticizer or a compatibilizer, as opposed to its drug carrier status in medical applications. Particular cautions should always be taken in the case of composites containing synthetic polymers as matrices, whose biodegradability, biocompatibility, and/or environmental impact may be rather questionable. On the other hand, composites containing bio-based biodegradable polymers as matrices can be fine-tuned to exhibit good tensile, impact and creep properties in spite of lower resistance to moisture, leading to poor mechanical performance at high humidity or aqueous conditions.

The Research Topic on Biodegradable Matrices and Composites is focused on recent advances in preparation and characterization of biodegradable polymeric matrices and their composites with the main objective to understand their processing-structure-property relationship at nano-, micro-, and macroscale. Both conventional biodegradable polymers and related biodegradable composites and more recent biodegradable nanocomposites reinforced with nanoparticles and nanofillers are covered to address various aspects, including but not limited to their mechanical, thermal and barrier properties, biodegradability, environmental sustainability, renewability and recyclability, limitations and applications in building constructions, automobiles, medical devices, and material packaging.

The edited book for this Research Topic comprises a special issue collection of 24 contributions represented by 1 mini review, 3 full review, 1 perspective, and 19 original research articles, which are classified into Part A: Biodegradable Polymers, Blends, and Matrices and Part B: Biodegradable Composite Materials. In Part A, Mistretta et al. investigated mechanical, structural and optical properties of polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), and

Edited and reviewed by: *Patricia Krawczak, IMT Lille Douai, France*

\*Correspondence: *Alessandro Pegoretti<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Polymeric and Composite Materials, a section of the journal Frontiers in Materials*

> Received: *13 June 2020* Accepted: *17 July 2020* Published: *27 August 2020*

#### Citation:

*Pegoretti A, Dong Y and Slouf M (2020) Editorial: Biodegradable Matrices and Composites. Front. Mater. 7:265. doi: 10.3389/fmats.2020.00265*

**9**

PBAT/PLA blends, as well as their surface properties and water vapor permeability before and after the exposure to UV irradiation in order to evaluate their durability and suitability for the conservation of culture heritage. Apicella et al. developed a sustainable food-packaging solution with antimicrobial effectiveness and high functional performance by spreading PLA/ Ethyl-Nα-dodecanoyl-L-arginate (LAE) coating solution on a recyclable polyethylene-terephthalate (PET) substrate. E. coli CECT 434 strain as a pathogenic agent was employed to test antimicrobial activity of such multilayer films in liquid culture media. Ten percentage of LAE was found to be the minimum concentration to warrant total inhibition without significantly altering the functionalities of the developed systems. Iozzino et al. prepared biphasic samples (half amorphous and the other half crystalline) of PLA using micro-injection molding. Hydrolysis tests were carried out to monitor their degradability, which indicated that crystalline regions exhibited slightly better resistance to the hydrolysis as opposed to amorphous phase. Fortelny et al. reported a comprehensive review on morphology, compatibility, and mechanical properties of PLA/ poly(ε-caprolactone) (PCL) blends in order to control blend toughness and crystallinity so that the blending of PLA and PCL can result in desirable materials with well-tailored mechanical properties. Among other things, the review showed how to prepare super-tough PLA/PCL blends without compatibilization. Nevoralová et al. evaluated the effect of blending PCL with thermoplastic starch (TPS) on the final biodegradation rate of such blends in both compost and soil environments. Very fast biodegradation with the initial similar rate to that of pure TPS in both environments was clearly identified for the blends containing 70% TPS with a co-continuous morphology. Whereas, 30% TPS blends demonstrated the particle morphology of starch phase in PCL matrix, indicating a dominant effect of the matrix on the biodegradation course. Delamarche et al. investigated the relationships between physical properties and tailored biodegradability for poly(butylene succinate) (PBS)/PLA blends using a deep eutectic solvent (DES). It was shown that molar masses, crystallinity, yield, morphologies, and surface properties were crucial to control the biodegradability of blend materials. Aliotta et al. studied a multiphase ternary system comprising two different elastomers of PBAT and polyolefin elastomer grafted with glycidyl methacrylate (POE-g-GMA) being added into PLA matrix, as compared with PLA and PLA/PBAT binary blends with maximum 10 wt% PBAT. It was found that the best compromise between impact, tensile properties and biodegradability content could be achieved by using a reactive plasticizer EJ-400. Rigotti et al. evaluated the effect of blending poly(3-hydroxybutyrate) [P(3HB)] to linear low density polyethylene (LLDPE) at the P(3HB) concentrations in the range of 10–50 wt% on hydrolytic degradation, water absorption, thermal and mechanical properties as a function of hydrolysis time. It was demonstrated that the use of biodegradable plastic P(3HB) at a relative amount of 20 wt% to substitute for LLDPE could successfully develop a desirable material in order to meet processability requirements with better tailored mechanical properties even after prolonged hydrolytic degradation treatment. Neisiany, Enayati, Kazemi-Beydokhti et al. provided a mini review on current state-of-art development of multilayered biodegradable electrospun membranes along with new insights into the future of tailored membranes toward practical applications. Mohammadzadehmoghadam and Dong investigated the impact of gelatin content on material properties and structures of silk fibroin (SF) nanofibers crosslinked with glutaraldehyde vapor (GTA) to understand the potential application of such biodegradable material matrices in tissue scaffolding. The detailed characterization was performed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), mechanical testing, degree of crosslinking test, water uptake test, water contact angle measurements, X-ray diffraction (XRD) analysis, as well as biocompatibility assessment using fibroblast cells. Adverse effect of GTA crosslinking was detected to reduce the capacity of supporting the cell activity despite improved mechanical properties of nanofiber mats. GTA optimization became essential to further modulate physicochemical properties of SF/gelatin nanofiber mats in order to achieve stable materials with favorable bioactive properties and more active cellular response in tissue engineering. Zaidi et al. reported the incorporation of epoxidized natural rubber (ENR) as a biodegradable toughening agent, which could enhance toughness properties of polyhydroxybutyrate-covalerate (PHBV) biodegradable composites and also improve their biodegradability in a more substantial manner, when compared with the addition of commonly used toughening agent PBAT.

In Part B, Aini et al. presented the detailed review with respect to the performance of lignin-filled rubber composites using different approaches including mixing methods, surface modification hybrid fillers and so on in order to gain the insights to the advances in the development of green rubber products. Girijappa et al. thoroughly reviewed different sources of natural fibers, their properties as well as the effect of modification and treatments on natural fibers, etc. with major applications targeting the reinforcements for polymer composites. Salim et al. investigated the effect of fiber surface treatments (i.e., alkali treatment, alkali treatment at elevated temperature, and hear treatment) on the durability of kenaf fiber reinforced acrylic based polyester composites under accelerated weathering exposure for potential applications in automotive industry. Moreover, the excellent fiber-matrix adhesion associated with these treatments may also decrease the water absorption rate by such composite materials. Aouat et al. assessed the influence of combined humid atmosphere and temperature on the morphology, chemical structures, and physical properties of neat PLA, PLA/PLA-grafted-maleic anhydride (PLA-g-MA)/microcrystalline cellulose (MCC) and PLA/PLA-g-MA/cellulose nanowhisker (CNW) biodegradable composite fibers. It has been found that the durability of PLA fibers to hygrothermal degradation can be established in the following order: PLA > PLA/PLA-g-MA/MCC > PLA/PLAg-MA/CNW where the MCC and CNW concentrations are 1 wt% and PLA-g-MA content is 7 wt%. Slouf et al. successfully prepared biodegradable microfibrillar PCL/PLA/organophilic montmorillonite composites. It was reported that the macroand micromechanical properties of all intermediate products and final microfibrillar composites were related not only to the composition and morphology, but also to the crystallinity of both components. In particular, the modulus of such composites reached almost twice higher value in comparison with the original PCL matrix. Li et al. provided a physical and chemical analysis of chicken feather (ChF) biocarbon generated at different pyrolysis temperatures, which were further combined with PLA matrix to generate 100% sustainable composites. The authors analyzed their thermal properties, mechanical properties, and composition. The valorized ChF as sustainable natural fillers can be a safe and suitable alternative used for composite applications in order to reduce their waste disposal. Ujcic, Krejcikova et al. demonstrated a two-step preparation of TPS using both solution casting and melt mixing to prepare TPS/TiO2 composites with homogeneous filler dispersion even at high filler concentrations up to 20 wt%. The same approach was also employed to uniformly disperse antibiotic (ATB) vancomycin at 10 wt% in TPS systems. The interactions among TPS, TiO2 particles, and ATB molecules gave rise to the shift of glass transition temperatures to higher values along with stiffening effect on corresponding composites. It has been proven through standard antimicrobial susceptibility tests that neither thermal processing nor the addition of TiO2 influence the ATB, which remains active in both TBS/ATB and TPS/TiO2/ATB composites. Fully biodegradable TPS composites are believed to be very promising materials for biomedical applications with respect to local release of antibiotics. Ferrari et al. developed new green composite material with the incorporation of natural additives such as shredded walnut shells and organic fraction of municipal solid waste (MW) into poly(vinyl) alcohol (PVA) for the improvement of mechanical properties of composites. The addition of walnut shells indicated the increases in glass transition temperature and flexural modulus reached about 50% along with the reduction in flexural strength by ∼30%. Furthermore, when added with a low amount of water, the inclusion of MW resulted in increasing strain at break by 100%. Bugatti et al. evaluated the recovery and upgrade of tomato processing residues to be used for the manufacture of innovative green composites based on tomato peels (TPs) and natural halloysite nanotubes (HNTs) loaded with carvacrol as a natural antibacterial agent. Thermal properties of such biodegradable composites were improved with increasing the filler loading as opposed to worse mechanical properties, which especially took place at higher loading levels associated with the poor interaction between HNTs and matrix. No dramatic increase was detected in the degree of hydrophilicity of biodegradable composites. This phenomenon might be attributed to the role of hydrophobic carvacrol acting as hydrophobic coating for HNTs. Finally prepared biodegradable composites exhibited a long-term release of antimicrobial agents, greatly benefiting their wide applications in food packaging. Abdullah and Dong mentioned the comprehensive material development of PVA/starch (ST)/HNT biodegradable composite films for sustainable packaging. Such films possessed high water resistance, good biodegradability, acceptable transparency, as well as overall migration rates to target sustainable food packaging particularly for lipophilic and acidic foodstuffs. Ujcic, Nevoralova et al. compared wheat and tapioca TPS and their composites with TiO2-based nanoparticles (i.e., isometric TiO2 nanoparticles and high-aspect-ratio titanate nanotubes) where TPS composites were prepared by a combined technique using solution casting and melt mixing. Such a combined technique was proven to be effective to the manufacture of TPS composites with homogeneous filler dispersion regardless of starch sources and types. The type of TPS matrix (i.e., wheat or tapioca) was shown to have significant impact on the properties of resulting composites in which isometric TiO2 nanoparticles yielded very good filler dispersion while TiNT nanotubes tended to form microsized aggregates within both TPS matrices. Liu et al. successfully developed CNW/graphene nanoplatelet (GN) composite films with respect to the GN role as rigid fillers, and further evaluated filler-matrix interactions to concurrently improve their thermal, electrical, and mechanical properties, which were difficult to achieve in conventional nanocomposite materials. Such CNW/GN composite films are anticipated to be potentially effective multifunctional materials in the field of electronic packaging. Finally, Neisiany, Enayati, Sajkiewicz et al. briefly reviewed recently reported work on nanocomposite hydrogels based on chiral functionalized nanomaterials. This work can pave the way for the development of biodegradable hydrogels toward real practical applications.

In summary, this edited book offers a diverse range of biodegradable polymers, their blends and biodegradable composite materials, focusing on their fabrication, characterization and widespread applications. It can be deemed as a good overview for materials engineers and scientists, experienced researchers and postgraduate students, as well as industrial staff working in research & development sectors to rapidly move forward to innovative and ecofriendly materials for significantly reducing conventional petroleum-based plastic wastes at disposal.

## AUTHOR CONTRIBUTIONS

YD prepared the initial editorial draft while AP and MS provided further comments and feedback. YD, AP, and MS jointly finalized the editorial work. All authors contributed to the article and approved the submitted version.

## ACKNOWLEDGMENTS

We would like to thank all the authors, reviewers and handling editors to make every success in this Research Topic. We are also indebted to the editorial team of Frontiers in Materials including Mina Roussenova, Paola Morelli, Alexander Stephens, and Michelle Cook to assist in every step from Research Topic proposal, article review and production to final publications.

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Pegoretti, Dong and Slouf. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Durability of Biodegradable Polymers for the Conservation of Cultural Heritage

Maria C. Mistretta1 , Francesco P. La Mantia1 \*, Vincenzo Titone1 , Bartolomeo Megna1 , Luigi Botta1 and Marco Morreale2

1 Department of Engineering, University of Palermo, Palermo, Italy, 2 Faculty of Engineering and Architecture, Kore University of Enna, Enna, Italy

The use of polymers for conservation of cultural heritage is related to the possibility to slow down or stop natural deterioration which, in many cases, corresponds to stopping the entrance of liquid water and to favor spontaneous water vapor removal. Unfortunately, hydrophobicity is generally favored by surface roughness and thus competitive with transparency. It is therefore important to find an optimal balance hydrophobicity, transparency and durability (especially to photooxidation). However, polymers typically used for applications in this field come from non-renewable resources and are not biodegradable. In this work, the mechanical, structural, and optical properties of PLA, PBAT, and a PBAT/PLA blends, as well as surface properties and water vapor permeability, were investigated before and after exposure to UV irradiation, in order to evaluate their durability and suitability for conservation of cultural heritage.

### Edited by:

Alessandro Pegoretti, University of Trento, Italy

#### Reviewed by:

Ralf Schledjewski, University of Leoben, Austria Veronique Michaud, École Polytechnique Fédérale de Lausanne, Switzerland

\*Correspondence: Francesco P. La Mantia<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Polymeric and Composite Materials, a section of the journal Frontiers in Materials

> Received: 27 March 2019 Accepted: 13 June 2019 Published: 28 June 2019

#### Citation:

Mistretta MC, La Mantia FP, Titone V, Megna B, Botta L and Morreale M (2019) Durability of Biodegradable Polymers for the Conservation of Cultural Heritage. Front. Mater. 6:151. doi: 10.3389/fmats.2019.00151 Keywords: biodegradable polymers, durability, cultural heritage, conservation, mechanical properties, permeability

## INTRODUCTION

The most important characteristics of polymers for conservation of cultural heritage are related to the capability to slow down, or possibly stopping, the ongoing deterioration; however, since one mostly causes of deterioration are related to the presence of water, the most important goal is to stop the entrance of liquid water and to facilitate spontaneous water vapor removal. Therefore, hydrophobicity is particularly important. However, one of the surface properties most affecting the actual hydrophobicity is the roughness: typically, hydrophobicity increases on increasing the surface roughness. Since the latter is competitive with transparency (which typically decreases on increasing the roughness), a further challenge is to tailor the polymer coating thickness in order to find the best compromise in terms of hydrophobicity, transparency, and durability. Furthermore, a good durability requires a suitable resistance to photooxidation and a good water vapor permeability after treatment.

The first category of polymeric materials to be actually investigated for application in this field was represented by acrylic polymers (Feller, 1978, 1994; Favaro et al., 2006). Their actual resistance to water and photooxidation was evaluated (Melo et al., 1999; Borgia et al., 2001; Chiantore and Lazzari, 2001) finding an unsatisfactory stability under UV irradiation, as well as inadequate water barrier properties over time. Thus, fluorinated polymers were taken into account as possible alternatives, showing some encouraging results, but also questionable surface adhesion properties (Frediani et al., 1982; Torrisi, 2008; Licchelli et al., 2013). Therefore, further efforts were focused on copolymers or blends from both acrylic and fluorinated polymers (Mazzola et al., 2003; Malshe and Sangaj, 2009). Furthermore, silanes and siloxanes (Tsakalof et al., 2007; Vacchiano et al., 2008; Ershad-Langroudi et al., 2017) also showed interesting properties in terms of impermeability and protection against color loss, although these were strongly dependent on the specific substrate. Polymers from cellulose have been extensively used and studied for application on cultural heritage, finding that both cellulose ethers (Feller and Wilt, 1990) and nitrate (Selwitz, 1988) have some stability issues. Finally, PEG was recently proposed in order to consolidate collapsible soil, highlighting interesting properties (Zimbardo et al., 2019).

However, all of the above discussed solutions are based on traditional polymers, coming from non-renewable resources and not biodegradable, with the well-known related issues of environmental impacts: the increasing demand for better use of natural resources and reduction of the environmental impacts related to the use of plastics currently require the search for alternative and more environment-friendly solutions (Bastioli, 2005; Song et al., 2009; Lo Re et al., 2013; Morreale et al., 2015). Furthermore, one of the main problems related to the use for conservation of cultural heritage (especially, for stone conservation) is related to reversibility (Andreotti et al., 2018). In fact, any protective coating should be removable in the future if needed, or at least retreatable. Unfortunately, traditional organic coatings have been found to be hardly removable (even using solvents) after some years of exposure to outdoor conditions, and go on aging and progressively losing their properties. This suggests, therefore, the use of biodegradable materials in replacement for non-biodegradable one; in particular, biodegradable polymers should spontaneously disappear from the underlying stone, after their properties (namely, water barrier and repellence properties) are lost (Andreotti et al., 2018).

With regard to the actual suitability of biodegradable polymers for cultural heritage conservation, the Literature does not report a high amount of data in this context.

Ocak et al. (2009) studied the behavior of some biodegradable polymerssuch as zein, chitosan, polyhydroxybutyrate (PHB), and poly-lactic acid (PLA) in terms of protection capability of marble surfaces under SO2. They found that the latter two were less suitable to protect against sulphatation.

Giuntoli et al. (2012) prepared some PLA samples, functionalized through fluorinated alcohols as co-initiators of polymerization, finding that the polymers coming from the L-lactide showed higher water protection in comparison to those coming from a racemic mixture of L- and D-lactide.

Ocak et al. (2015) investigated the behavior of a PLAmontmorillonite nanocomposite in terms of protection of marble surfaces against air pollution. The presence of the nanofiller evidenced an increased hydrophobicity, and therefore higher impermeability to environment waters and, at the same time, reduced water vapor permeability.

Sacchi et al. (2012) performed several investigations on different systems such as a lactic acid omopolymer and two lactic acid copolymers (copolymerized with a commercial low molecular weight perfluoropolyether). Their suitability to protect marble against aging (simulated by thermohygrometric cycles and UV irradiation) was studied, finding an enhancement of the water-repellent behavior in comparison to non-fluorinated PLA; this was kept even after thermohygrometric aging, while UV aging led, in some cases, to a detachment between the polymer layer and the marble substrate, depending on the kind of marble examined.

Andreotti et al. (2018) studied the behavior of PHA for stone protection, finding interesting results in terms of effectiveness and compatibility, while further improvements of durability and applicability still seem necessary.

Xing et al. (2017) prepared PBAT/lignin UV-blocking films. More in details, they grafted bio-based 10-undecenoic and oleic acids on soda lignin via solvent methods and then blended the lignin, as well as the lignin ester derivatives, with PBAT. They found good lignin dispersion, thermal stability and retaining of tensile properties up to 10–20 wt% filler loading, and very good UV-barrier properties even after 50 h of irradiation.

It can be deduced from this brief literature report, that the suitability of a polymer system as protective layer on stone is dependent on its durability, which in its turn is related to the loss of hydrophobicity. The latter can depend on photooxidation (photooxidation, in fact, leads to the increase of polar groups on the surface, and therefore to an increase of hydrophilicity). Furthermore, most of the available information regards PLA, while less information is available about other polymers, such as for instance PBAT.

The photooxidation behaviors of PLA and PBAT have been reported in the literature (Ikada, 1997; Tsuji et al., 2006; Janorkar et al., 2007; Nakayama and Hayashi, 2007; Kijchavengkul et al., 2010; Zaidi et al., 2010; Gardette et al., 2011; Stloukal et al., 2012). With regard to PLA, the mechanisms involved are a photolytic mechanism leading to the breaking of the C-O bond in the macromolecular chain and a photooxidative mechanism leading to the formation of hydroperoxides and, in turn, of carboxylic acids and ketones (Janorkar et al., 2007); moreover, also the Norrish II mechanism can be involved (Tsuji et al., 2006; Nakayama and Hayashi, 2007), although under UV irradiation at wavelengths higher than 300 nm, different chain scission mechanisms were found (Gardette et al., 2011). With concern to the PBAT, degradation can occur via Norrish I and II chain scission, and can also lead to significant crosslinking phenomena due to the free radicals generated during the Norrish I path (Kijchavengkul et al., 2010; Stloukal et al., 2012).

In this work, therefore, we focused on the investigation on the mechanical and optical properties of PLA, PBAT, and a PBAT/PLA blend, with particular concern on the behavior after exposure to UV irradiation. The main objective was to evaluate the suitability and durability of these materials for conservation of cultural heritage.

## EXPERIMENTAL

The materials used in this work were: a PLA sample, commercially known as Ingeo 4032D (NatureWorks, USA), having a MFI equal to 7 g/10 min (at 210◦C and 2.16 kg), a density of 1.24 g/cm3 and a melting point between 155 and 170◦C; a PBAT sample, commercially known as Ecoflex (BASF, Germany) with a MFI between 2.7 and 4.9 g/10 min; a sample of PBAT/PLA blend, commercially known as Ecovio (BASF, Germany) with a MFI between 5 and 11 g/10 min. The neat, non-aged systems were compared also to a reference commercial fluoropolymer sample, Fluoline HY (CTS, Italy), that is a high molecular weight fluorinated elastomer, typically used as protective layer for porous stone substrates.

Sheets of all the samples (previously conditioned in a vacuum oven at 70◦C for 4 h) were prepared by compression molding at T = 170◦C (except for the PLA which was molded at 190◦C) in a Carver (USA) laboratory press.

The obtained specimens (thickness of about 300µm) were exposed to accelerated weathering in a "Q-UV" chamber (Q-Labs Corp., USA) containing eight "UVB-313" lamps up to about 128 h, according to the procedures described elsewhere (Morreale et al., 2013) at 70◦C.

Mechanical (tensile) tests were carried out according to ASTM D638 both on weathered and unweathered specimens (90 × 10 × 0.3–0.5 mm) using an Instron (USA) mod. 3365 universal testing machine. The elastic modulus was measured at 1 mm/min deformation speed. When the deformation reached 10%, the crosshead speed was increased to 100 mm/min until final breaking. The values of the elastic modulus, E, tensile strength, TS, and elongation at break, EB, were calculated as average of 10 tests, with an adequate reproducibility (±7%).

FTIR spectra in ATR mode were obtained by using a Perkin-Elmer (USA) Spectrum One spectrometer, using the embedded SpectrumOne software. The spectra were obtained through 16 scans with a 4 cm−1 resolution. Measurements were obtained from the average of triplicate samples. Particular attention was focused on the carbonyl peak centered at 1,720 cm−1 , which was used to measure the dimensionless carbonyl index (ratio between the carbonyl peak area of the sample at a given photooxidation time and carbonyl peak area of the unirradiated sample) in order monitoring the photooxidation.

UV-Vis spectra were obtained on triplicates using a Specord 252 (Analytik Jena, Germany) spectrometer in the 190–1,100 nm range.

Melt Flow index (MFI) values were measured using a CEAST (Italy) equipment at 170◦C under a 2.16 kg load, as the average of four measurements (data reproducibility: ±5%).

The cross-linked fraction was evaluated by measuring the residual gel fraction obtained after Soxhlet extraction using boiling tetrahydrofuran (THF) on triplicates.

Enthalpy of fusion of the samples was measured by differential scanning calorimetry (DSC), using a Shimadzu (Japan) DSC-60 apparatus, with 5◦C/min heating rate from 40 to 200◦C, as the average of four measurements (data reproducibility: ±6%).

For the blend sample, the enthalpies of fusion of the two phases are reported.

Contact angle measurements (in order evaluating wettability of the investigated materials) were performed using a First Ten Angstrom (USA) FTA1000C system, with demineralized water (on four samples, data reproducibility: ±4%).

Water vapor permeability measurements were performed on samples (diameter 4.5 cm) coming from the above mentioned compression-molded sheets, at 25◦C and 50% relative humidity, using a BYK-Gardner (Germany) permeability cup. Permeability constant was calculated as the average of four measurements by normalizing the measured weight loss of each sample against the sample thickness (data reproducibility: ±7%), according to the procedures described elsewhere (Iizuka et al., 2016; Hendrickx et al., 2017).

region (dotted line), as a function of the irradiation time.

## RESULTS AND DISCUSSION

In **Figure 1**, the dimensionless MFI values are reported, as a function of the irradiation time.

The gel fraction values of the three pristine (unirradiated) samples were obviously zero; after 128 h irradiation they were found to be still zero in the PLA sample, while they increased to 6.8 in the PBAT, and 22.6 in the PLA/PBAT.

The photooxidation of PLA, therefore, gives rise to a decrease of the molecular weight—increase of the MFI value—and no presence of cross-linking was detected. These results indicate that the photooxidation proceeds to breakage of the macromolecules

and oxidation of the carbon atoms. PBAT and the PBAT/PLA blend show similar behaviors, with a decrease of molecular weight but, on the contrary, a formation of cross-linking structures is observed in both of the systems. According to the reported results, it is evident that the cross-linking occurs only in the PBAT phase and that the increase of the crosslinking in the blend, in comparison to neat PBAT, suggests that PLA degradation (which follows a chain scission path) and the related degradation products promote PBAT degradation (which, on the other hand, mainly follows a crosslinking path). The observation of **Figure 1** further indicates that the PBAT undergoes significant crosslinking phenomena (in particular, after 32 h) resulting in a

decrease of the MFI, and this is even more important in the blend, where the crosslinking of the PBAT fraction is able to lead to a decrease of the MFI even though the greater part of the blend is composed by PLA. On the other hand, the role of the chainscission degradation of PLA gains importance on increasing the photooxidation time, leading to a gradual reversal of the trend.

The enthalpies of fusion of the three samples, virgin and photooxidized, are reported in **Table 1**.

It can be observed that, in substantial agreement with the crosslinking (gel fraction) data, the crystallinity generally decreases on increasing the crosslinking rate.

**Figures 2**–**4** report the ATR spectra of virgin and photooxidized PLA, PBAT, and PLA/PBAT blend, respectively.

As regards the PLA, the main peaks to be observed are: 871 cm−1 (-CH- bending), 1,453 (-CH3 bending), 1,745 cm−1 (C=O axial deformation vibration), 1,087, 1,129, and 1,184 cm−1 (stretching of the –CO- bond).

In the case of PBAT, the main peaks are: 729 cm−1 (-CH2 stretching), 1,104, 1,120, and 1,165 cm−1 (-CO- stretching), and 1,730 cm−1 (C=O axial deformation vibration). The blend shows the typical peaks from both of the two components, in particular at 1,745, 1,730, 1,453, 1,087–1,104–1,165, 871, and 729 cm−1 . The reported peaks are in agreement with literature data (Dias Fernandes et al., 2017).

The ATR spectra of the PLA show a decrease of the peak located at 1,745 cm−1 , thus indicating a reduction of the esters, but at the same time the overall carbonyl peak area does not change significantly, thus suggesting chain scission degradation, in agreement with the hypotheses derived from the previously discussed data. With regard to PBAT, a somewhat similar decrease and a widening of the carbonyl peak was observed, but in this case also two shoulders at 1,790 and 1,750 cm−1 appear, related to the formation of free carbonyls and low molecular weight esters, respectively. These two findings suggest that Norrish-I type chain scission reactions occurred (Gardette et al., 2011).

Furthermore, a broadened peak around 1,000 cm−1 was observed, indicating the formation of C=C bonds, thus suggesting that also Norrish-II type chain scission reactions occurred (Gardette et al., 2011).

With regard to the blend, the peak modifications substantially mirror those of the individual components. However, such peak modifications are significantly sharper.

In order to deepen the analysis, the dimensionless carbonyl peak area (calculated as the ratio between the peak from 1,600 to 1,860 cm−1 at a given photooxidation time and the same before photooxidation) as a function of irradiation time is reported in **Figure 5**.

As previously discussed, a reduction of the carbonyl peak upon increasing the UV exposure time should happen, mainly attributable to chain scission reactions (especially Norrish I type). However, as the oxidation goes on, it gives rise to different oxidation species such as free carbonyls and low molecular

TABLE 2 | Elastic modulus, tensile strength and elongation at break of the three unirradiated samples.

weight esters, which allow the overall carbonyl peak area to not decrease much, or to keep practically constant; in the case of PBAT, C=C bonds are formed, reasonably due to Norrish II type chain scission reactions, which slightly increase the related peak area (between 900 and 1,100 cm−1 , see dotted line in the same **Figure 5**).

The degradation has a deep effect on the mechanical properties that can strongly change during the photooxidation. In **Figures 6**–**8**, the dimensionless values of the elastic modulus, E, tensile strength, TS, and elongation at break, EB, are drawn as a function of the irradiation time for the three materials (PLA values reported only up to 64 h, since

the samples became excessively brittle after that exposure time). The dimensionless values have been calculated by dividing the values at each irradiation time by those of the same virgin (unirradiated) polymer, which are reported in **Table 2**.

As expected, the blend has intermediate properties in terms of rigidity and deformability, while the tensile strength experiences antagonistic effects.

It can be observed that the photooxidation had significantly detrimental effects on the ductility (EB) of the materials, in

agreement with the previous discussions on the reduction of molecular weight and crosslinking phenomena. With regard to the tensile resistance (TS), all of the three materials undergo (as expectable on the basis of the above consideration) a reduction, but with remarkable differences: while the PLA undergoes limited worsening of the TS, the PBAT undergoes a drop. This may be due to the different embrittlement kinetics of the two materials (see **Figure 8**), which lead to premature breaking of PBAT. On the other hand, the blend shows a different behavior, with significantly lower reduction of the TS in comparison to the PBAT, in apparent contrast with the deductions that could be drawn on the basis of the embrittlement. This is explainable, however, considering that the blend experienced increased crosslinking phenomena, which mitigate the loss of tensile resistance. At the same time, there is some increase or no change of the modulus, due to the above discussed phenomena of crosslinking and chain scission.

Due to its use for covering cultural heritage, the optical properties play a very important role.

**Figure 9** shows the UV spectra of virgin fluoropolymer, PLA, PBAT, and PBAT/PLA blend, while **Figures 10**–**12** report the comparison to the spectra obtained after 128 h photooxidation of the same samples. The transmittance (and thus transparency) in the visible range (380–740 nm) of the reference fluoropolymer is, on average, slightly higher, however the values of the biodegradable polymers appear to be comparable. On increasing the photooxidation time, it slightly decreases in the PBAT, while the reduction is higher in the PLA; however, in the case of the blend, in the 490–780 nm range, the values are almost unchanged. This suggests that the investigated polymer blend should be suitable for cultural heritage applications, in terms of maintaining the main optical properties over time, although the use of suitable antioxidants would obviously improve the behavior.

Contact angle measurements at different photooxidation times were measured, and are shown in **Figure 13**.

It was not possible to measure the values of the PLA samples irradiated more than 64 h since the specimens were significantly damaged due to the photooxidation. It is observed that the contact angle decreases on increasing the photooxidation time, due to the increasing amount of polar groups formed during the degradation (as discussed previously) and the increasing rugosity of the surfaces. This is a further confirmation of the interpretations provide with regard to the results coming from spectral analysis.

Finally, the measured values of specific water vapor permeability were found to be 1.35 ± 0.07 g/mm in the reference fluorinated elastomer; with regard to the biodegradable systems, it was found to change from 5.41 ± 0.32 to 2.95 ± 0.37 g/mm in the case of PBAT, and from 6.79 ± 0.47 to 2.88 ± 0.2 g/mm in the case of the PBAT/PLA blend. Therefore, the biodegradable systems are in the same order of magnitude and thus comparable to the reference fluoropolymer. Also in this case, it was not possible to measure the values of the PLA samples irradiated more than 64 h since the specimens were significantly damaged due to the photooxidation It is thus observed that, after the UV aging treatment, the specific permeability significantly decreases in comparison to the neat unirradiated systems. This is in agreement with the previously discussed results, since it can be explained on the basis of the increased crosslinked fraction and the variations of crystallinity degree.

## CONCLUSIONS

In this work, the mechanical, structural and optical properties of PLA, PBAT, and a PBAT/PLA blends, as well as surface properties and water vapor permeability, were investigated before and after exposure to UV irradiation. The main objective was to evaluate the suitability and durability of these materials for conservation of cultural heritage.

Photooxidation followed a complex path, involving mainly chain scission in the PLA and crosslinking in the PBAT, while synergistic effects were found in the blend.

The mechanical properties of the blend were intermediate between those of the neat PLA and PBAT; however, the UV exposure led to different worsening of the three systems, pointing out that the blend undergoes more limited decay of the main mechanical properties, in comparison to the neat PBAT.

Water vapor permeability was found to decrease on increasing the exposure time; the UV transmittance in the visible range slightly decreased in the PBAT, while the reduction was higher in the PLA; in the case of the blend, the values in the 490–780 nm range were almost unchanged, although the overall transmittance was lower.

The obtained results suggest that the PBAT/PLA blend investigated in this work could be suitable for cultural heritage

## REFERENCES

applications, and that the next advisable steps should consist in experimenting the addition of suitable antioxidants in order to improve the overall performance.

## DATA AVAILABILITY

The datasets generated for this study are available on request to the corresponding author.

## AUTHOR CONTRIBUTIONS

FL and MCM designed the experiments. VT, MCM, and LB performed the experiments. All of the authors analyzed the data. FL supervised the work. FL, MM, and BM wrote the paper (original draft, reviewing, and editing).

## FUNDING

This work was financially supported by Italian Ministry of Education, University and Research, PRIN 2015WBEP3H. Monitoraggio, Consolidamento, Conservazione e Protezione di Beni Culturali.

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Mistretta, La Mantia, Titone, Megna, Botta and Morreale. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Sustainable Active PET Films by Functionalization With Antimicrobial Bio-Coatings

Annalisa Apicella, Paola Scarfato, Luciano Di Maio and Loredana Incarnato\*

*Department of Industrial Engineering, University of Salerno, Fisciano, Italy*

The realization of antimicrobial films through the incorporation of active agents into a polymer matrix is a promising alternative to the direct addition of antimicrobials into the food matrix. To this aim, the goal of this work was to develop a sustainable, food packaging solution with antimicrobial effectiveness and high functional performance, based on Ethyl-Nα-dodecanoyl-L-arginate (LAE). Active biodegradable coatings, easy soluble to be removed, were realized by spreading a Polylactic acid/LAE coating solution, at different antimicrobial concentration (from 0 to 20%), on a recyclable Polyethylene-terephthalate substrate. The antimicrobial activity of the multilayer films was tested *in vitro* against *E.Coli* CECT 434 strain as pathogenic agent in liquid culture media. Moreover, the LAE chemical interaction with the PLA matrix was investigated, as well as its effect on the adhesion, wetting, optical and barrier properties of the films. The results pointed out that that the minimum LAE concentration incorporated already guarantees an antimicrobial activity comparable to commercial antimicrobial packaging solutions, and that, among the systems investigated, 10% LAE is the minimum concentration guaranteeing total inhibition without significantly altering the functionalities of the developed systems.

#### Edited by:

*Alessandro Pegoretti, University of Trento, Italy*

#### Reviewed by:

*Massimo Messori, University of Modena and Reggio Emilia, Italy Fabrizio Sarasini, Sapienza University of Rome, Italy*

\*Correspondence:

*Loredana Incarnato<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Polymeric and Composite Materials, a section of the journal Frontiers in Materials*

> Received: *01 August 2019* Accepted: *18 September 2019* Published: *02 October 2019*

#### Citation:

*Apicella A, Scarfato P, Di Maio L and Incarnato L (2019) Sustainable Active PET Films by Functionalization With Antimicrobial Bio-Coatings. Front. Mater. 6:243. doi: 10.3389/fmats.2019.00243* Keywords: antimicrobial packaging, ethyl-Nα-dodecanoyl-L-arginate (LAE), polylactic acid (PLA), PET, coating, biodegradable film

## INTRODUCTION

Active packaging solutions have gained increasing attention in the last years from the food industry, since microbial contamination after processing is one of the major causes of foodborne diseases, representing both a public concern and an economic issue (Higueras et al., 2013; Coronel-León et al., 2016). At the same time, consumers demand is increasingly addressed toward the consumption of fresh-like, safe food products, with high organoleptic quality, prolonged shelf-life, and with fewer chemicals addition (Apicella et al., 2019).

The realization of antimicrobial packaging films, through the incorporation of active molecules into the bulk matrix, represents an innovative approach capable to avoid the direct addition, by dipping or spraying, of large antimicrobial amounts onto the food surface, where a large portion of spoilage and contamination occurs. As a matter of fact, the rapid diffusion of the antimicrobial into the food matrix results in an immediate reduction of bacterial populations, but does not allow controlling the metabolism growth of surviving biomass after depletion of antimicrobial residues (Chi-Zhang et al., 2004; Landi et al., 2014). The use of polymers as carriers, instead, allows the gradual delivery of the active agent during the storage and distribution of food packaging, with less antimicrobial concentration, tunable release and tailor-made applications (Lagaron, 2011).

The main challenges in this field are related to the production of effective packaging systems, with good functional properties, realized through easy-scalable, conventional technologies in use in food packaging industry and, last but not least, with high eco-sustainability (Scarfato et al., 2015a; Apicella et al., 2018a). In fact, the recent concerns toward waste-management issues and shortage of resources, have increasingly shifted the research focus to the development of eco-compatible and sustainable packaging solutions, by means of biomass-derived biodegradable or completely recyclable polymers, or a combination of both (Scarfato et al., 2015b; Zainab and Dong, 2019).

At the same time, the selection of the proper antimicrobial, as well as the choice of its optimal loading, is an important and not an easy task in the design and application of antimicrobial packaging (Becerril et al., 2013; Wicochea-Rodríguez et al., 2019). Some of these substances can modify the organoleptic profile of the packaged foodstuff, or can be used only for a small variety of foods, or can affect the functional properties (i.e., gas permeability, tensile strength, transparency, thermal stability, etc.) of the packaging material, or can have migration limits (European Commission, 2009; Aznar et al., 2013; Muriel-Galet et al., 2014; Paciello et al., 2015; Bugatti et al., 2019).

One of the most innovative and powerful antimicrobial compounds, not showing much of these drawbacks, is Ethyl-Nαdodecanoyl-L-arginate (LAE). LAE is a derivative of lauric acid, L-arginine and ethanol, which showed an extensive spectrum of antimicrobial activity against Gram positive and Gram-negative bacteria, as well as yeasts and molds (Infante et al., 1997; Bakal and Diaz, 2005; Pezo et al., 2012). It interacts, as a cationic surfactant, on the cytoplasmic membrane of microorganisms increasing the cell permeability without causing lysis (Nerin et al., 2016). Among the major positive features, LAE has been classified as Generally Recognized as Safe (GRAS) and food preservative by the Food and Drug Administration (FDA), is chemically stable, has low cost, and does not provide any taste or odor, in compliance with European Regulation on active food packaging materials (Otero et al., 2014).

Previous published researches already addressed the realization of packaging films incorporated with LAE, realized by solvent casting technique (Muriel-Galet et al., 2012; Rubilar et al., 2016; Moreno et al., 2017; Haghighi et al., 2019). However, to the best of our knowledge, limited information is available concerning the development of sustainable packaging solutions based on PET films, functionalized with LAE-activated bio-coatings, and produced with conventional techniques easy-scalable at industrial level.

On the basis of the aforementioned considerations, the goal of this study was to develop antimicrobial, multifunctional, and sustainable food packaging films, based on LAE, by means of coating technology. The multilayer films were realized by spreading an amorphous PLA coating layer (C), incorporated at different concentrations of LAE, on a Bi-oriented PET substrate (S). PET was selected as web layer thanks to its excellent functional properties, as oxygen and water vapor barrier, high tensile strength, and thanks to its complete recyclability (Di Maio et al., 2017). Amorphous PLA coating already demonstrated to provide sealant layer to polyester films (Barbaro et al., 2015), in addition to its good optical properties, good processability, and environmental benefits (Scarfato et al., 2017). In addition, the biodegradable layer is easy to be removed by non-toxic solvents, ensuring the complete recovery of the recyclable substrate (Bugnicourt et al., 2013), and the coating technology ensures to avoid thermal stresses to heat sensitive active compounds.

The antimicrobial effectiveness of the active films was assessed by means of microbiological tests, using E. Coli CECT 434 strain as pathogenic agent. Moreover, the produced systems were characterized in order to ascertain the effect of the active phase on the chemical, morphological, and functional properties of the polymeric films.

## MATERIALS AND METHODS

## Materials

Commercial biaxially oriented poly(ethylene terephthalate) (BOPET) film (Nuroll S.p.a, Italy), with 23µm thickness and corona treated surface, was used as substrate (S). PLA4060 (Natureworks, Minnetonka, USA), characterized by a D-lactide content of 12 wt%, was used for the coating layer (C). Ethyl-Nαdodecanoyl-L-arginate (LAE) was provided by Vedeqsa Grupo LAMIRSA (Terrassa, Barcelona, Spain). All the solvents used were analytical grade.

## Preparation of the Active Systems

Antimicrobial coated films were realized according to the method explained by Barbaro et al. (2015), with some modifications. The PLA coating solution was prepared by dissolving the polymer in acetone (mass ratio 20:80) and subsequently adding LAE at different percentages (0, 5, 10, and 20% w/wt). The casting mixture was spread on the BOPET substrate by means of a K Hand Coater (RK, Printocoat Instruments Ltd., Litlington, UK), equipped with stainless steel closed wound rod, with wire diameter equal to 0.64 mm, yielding final coatings with an average thickness of the coating layer comprised between 7 and 10 microns.

**Table 1** resumes the list of the prepared films (named SC, SC5, SC10, and SC20). The PET substrate (S) was also used as comparison.

## Characterization Methods

## ATR-FTIR Analyses

Fourier Transform Infrared spectra of the films were collected by a Thermo Scientific Nicolet 600 FT-IR, equipped with a

Smart Performer accessory for attenuated total-reflection (ATR) measurement using a ZnSe crystal. The operating spectral range was set at 650–4,000 cm−1 , with a resolution of 4 cm−1 and 64 scans per sample. Normalization and peak integration was performed using Omnic software.

### Field Emission Scanning Electron Microscopy (FESEM) Analyses

Field Emission Scanning Electron Microscopy (SEM) analyses were conducted on the film sections, which were cut cryofracturing them in liquid nitrogen normally to the extrusion direction, sputter coated with gold (Agar Auto Sputter Coater mod. 108A, Stansted, UK) at 30 mA for 160 s, and analyzed using a field emission scanning electron microscope (mod. LEO 1525, Carl Zeiss SMT AG, Oberkochen, Germany).

#### Delamination Tests

In order to evaluate the adhesion strength of the PLA coatings, delamination tests were carried out by SANS dynamometer (mod. CMT 4000 by MTS, China), equipped with a 10 N load cell, according with the standards ASTM F88-00 and ASTM 2029. In particular, the coated films were cut in strips of 200 × 25 mm2 area, sealed with a Brugger HSG-C (Germany) heat sealing machine at 120◦C for 1s, by applying a force of 690 N, and stored for 48 h in environmental condition prior to analysis. Then, the bonding strength was evaluated in tensile mode at 250 mm/min until delamination failure of the seal. The maximum load reported is considered as the maximum bond strength of the coating on a specific substrate (expressed as N/25 mm). For each sample type, at least 10 measurements were performed to assess the reproducibility of the results.

### Static Water Contact Angle and Surface Tension Characterization

Static contact angle measurements were performed with a First Ten Angstrom Analyzer System 32.0 mod. FTA 1000 (First Ten Angstroms, Inc., Portsmouth, VA, USA), according to the standard test method ASTM D5946. The drop volume was taken within the range where the contact angle did not change with the variation of the volume (2 ± 0.5 µL). Each reported value of the θ angle is the average of at least 10 replicate measurements. The dispersion (γ d s ) and polar (γ p s ) components of the surface energy (SE) for all the samples were calculated according to the Owens-Wendt geometric mean equation (Owens and Wendt, 1969), using distilled water and ethylene glycol as testing liquids. The SE components (mN/m) for water are: γ = 72.1, γ d = 19.9, γ p = 52.2, and for ethylene glycol are: γ = 48, γ d = 29, γ p = 19 (here γ is the total SE, and γ d and γ p are the dispersion and polar components, respectively) (Zonder et al., 2014).

### Antimicrobial Activity Assays

The antimicrobial effectiveness of the films was tested against the Gram-negative bacteria E. Coli CECT 434 (ATCC 25922), selected because of its relevance in food industry. Stock culture was stored at −18◦C in Nutrient Broth (Sigma-Aldrich, Missouri, USA) with 40% Glycerol. Subcultures were grown overnight in Tryptic Soy Broth (TSB, Sigma-Aldrich) plus 0.6% Yeast Extract (YE, Sigma-Aldrich) at 37◦C prior to each experiment.

The minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of LAE against the selected strain was determined in TSBYE. 1.2 g of LAE was diluted in 100 mL of Milli-Q water to obtain a concentration of 12,000 ppm, and then serial dilutions between 80 and 4µg/mL were made up in sterile TSBYE to study MIC and MBC. Hundred microliter of ∼104 CFU/mL of microorganism in exponential phase was inoculated in each test tube. The tubes were incubated at 37◦C overnight, then, 100 µL from each tube was plated and incubated at 37◦C overnight (M100-S18, 2008). MIC was reported as the lowest antimicrobial concentration that inhibited the growth of the pathogen microorganism, while the MBC was defined as the lowest concentration at which no colonies growth was observed in the medium (TSBYE), and they were not culturable after plating onto Tryptic Soy Agar (TSA, Sigma-Aldrich) (Muriel-Galet et al., 2012; Higueras et al., 2013).

In vitro microbial tests on the films were performed by cutting 2.5 × 2.5 cm2 of multilayer film samples at different LAE concentration (0, 5, 10, and 20%), sterilizing them by UV lamp on both sides, and placing them in a sterile tube containing 5 ml of TSBYE medium. Aliquots containing 100 µL of ∼104 CFU/mL of microorganism in exponential phase were inoculated in each test tube, and incubated at 37◦C and 300 rpm overnight. Depending on the turbidity of the tubes, serial dilutions with physiological saline were made and plated in Petri dishes with 20 mL TSA culture medium. Colonies visible to naked eye were counted after incubation at 37◦C overnight (Paciello et al., 2013). The negative controls (i.e., films in liquid medium without E. Coli) were also prepared. Counts were performed in triplicate.

### Water Vapor Permeability Tests

Water vapor permeability was measured by M7002 Water Vapor Permeation Analyzer (Systech Instruments Ltd, Oxfordshire, UK) according to the standard ASTM F 1249. Films were tested at 23◦C and 50% R.H., and the results, performed in triplicate, were expressed as PWV (g m/(m2 Pa s)), calculated as the following equation (Li et al., 2015):

$$P\_{WV} = \frac{WVTR \times L}{\Delta P} \tag{1}$$

where WVTR is the water vapor transmission rate (g/m2 s) measured through the film, L is the average film thickness (m), and 1P is the partial water vapor pressure difference (Pa) across the two sides of the film.

#### Optical Analyses

The optical properties of the films were evaluated by measuring the UV-Visible transmittance of the films from 200 to 800 nm with Perkin Elmer UV-Visible Spectrophotometer Lambda 800. The transparency of the films was evaluated by measuring the Transmittance % of visible light at 550 nm, according to the ASTM D1746-03.

## RESULTS AND DISCUSSION

## ATR-FTIR Analyses

Possible changes in intra- and intermolecular interactions due LAE addition into the PLA matrix were explored by Fourier transform infrared spectroscopy (FT-IR), which is sensitive to the structural conformation and local molecular motions. Due to the limited penetration depth of infrared radiation into the sample in the ATR-FT-IR measurement geometry, the spectra were collected on the thin PLA coating layers (namely C, C5, C10, and C20), at different percentages of antimicrobial, and compared to the spectrum of LAE, which is shown in **Figure 1A**. The absorption band at 3,320 cm−1 corresponds mainly to the ν(N-H) stretching vibration of hydrogen bonded N-H functionalities. The double peaks at 2,927 and 2,850 cm−1 can be assigned to antisymmetric and symmetric stretching vibrations of CH3 and CH2 functionalities, νas(CH3/CH2) and νs(CH3/CH2), respectively. The small peak around 1,740 cm−1 suggest the presence of carbonyl groups, the one at 1,560 cm−1 corresponds to the δ(N-H) bending vibrations (amide-II) combined with ν(C–N) stretching, the peak at 1,655 cm−1 is due to the ν(C=O) stretching vibration (amide-I), while the band situated around 1,027 cm−1 can be assigned to a ν(C–O) stretching vibration. Similar outcomes shown by other authors (Haghighi et al., 2019).

As it is possible to observe from **Figures 1B–D**, the PLA characteristic absorption peaks remains essentially unaltered, and the LAE characteristic absorption peaks are observable only by increasing the antimicrobial content in the coating layers. In particular, for the samples at 10 and 20% LAE (C10 and C20, respectively), the absorption bands at 2,927, 2,850, 1,655, and 1,027 cm−1 are detectable. The absence of changes in the intensities and positions of the major PLA and LAE bands suggests the realization of a physical mixture, with no chemical interaction between the two phases. Similar results were also reported by other authors (Rubilar et al., 2016; Gaikwad et al., 2017).

## FESEM Analyses

As it is known, the microstructural properties of composite films depend on the compatibility between all the film components, affecting the final physical, mechanical, barrier, and optical properties (Attaran et al., 2015). To this aim, the distribution of the active phase inside the PLA matrix and the quality of the interlayer adhesion were investigated through SEM analyses. **Figures 2A,B** show the cross-section micrographs of SC and SC10 films, respectively, taken as an example. Images analyses display the PLA coating layer, in which few small voids are recognizable due to solvent evaporation, over the BOPET substrate, characterized by an oriented morphology. The absence of visible LAE powder domains inside the coating layer highlights the good dispersion and homogeneous distribution of the antimicrobial into the polymer matrix. Moreover, the absence of voids in the cross sectional area of the investigated films pointed out the good adhesion of the coating layers on the substrate.

## Evaluation of Adhesion Strength and Surface Wettability

To determine the adhesion strength, delamination tests were performed as described in the methods section. The delamination occurred by separating the coating, thermally sealed at the defined temperature, by means of a dynamometer. The bonding strength, reported in **Table 2**, represents the force required to delaminate the PLA coating from the web.

The results show that the adhesion strength of the PLA coating layers was not significantly affected by the antimicrobial addition up to LAE concentrations equal to 10% (SC10 sample), whereas a decrease in the bonding strength for the SC20 film, equal to 2.56 ± 0.35 N/25 mm, was observed.

The adhesion strength strictly depends on the amount of polar and dispersion bonds between the substrate film and the coating (Lindner et al., 2017).

To this aim, the polar (γ p s ) and dispersion (γ d s ) components of the surface energy were calculated from water and ethylene glycol static contact angle measurements, as described in Par. 2.3.4, and displayed in **Table 2**.

The reported surface energy values show that the adhesion force is optimal when the γ p s and γ d s values are of the same order of magnitude, and maximum when their ratio is close to unity. On the other hand, when the difference between the γ p s and γ d s values is more pronounced, as in the case of SC20 sample, where it is of one order of magnitude, a significant drop in bonding strength is observable, which however remains acceptable for the film application in food packaging.

## Antimicrobial Activity of the Active Bio-Coated Films With LAE

The antimicrobial activity of the multilayer films against E. Coli growth was then determined by the liquid medium method described in the experimental section.

A preliminary study, previously carried out on only the PLA coating layers, showed the antimicrobial effectiveness of the coatings in releasing the active agent and inhibiting the proliferation of microorganisms, with a total inhibition at the LAE concentration of 20% (Apicella et al., 2018b).

However, in the real packaging applications, the release does not occur symmetrically on both sides of the coating, and the different layout of systems can determine differences in the mass transport of the active agent. Therefore, it is necessary to study in depth the antimicrobial efficacy of the active multilayer films in real geometry and application conditions.

In first analysis, the MIC and the MBC for LAE were evaluated in the test conditions described in the methods section. The corresponding values are reported in **Table 3**, and are equal to 51 and 63 ppm, respectively. These values, different from previous results reported in the literature, highlight the influence of medium composition, of the method used (in terms of incubation time, inoculum concentration, and temperature), as well as of the strain used, in the quantification of the activity of the antimicrobial agent (Muriel-Galet et al., 2012).

Afterwards, the microbiological tests were conducted on the SC samples, at different LAE concentrations (0, 5, 10, and 20%)

and with exposed surface area equal to 6.25 cm2 , according to the conditions described in the experimental section.

The results are reported in **Table 3**, and are expressed as logarithm of colony forming units (Log(CFU)) and log reduction value (LRV). Data regarding the control sample are also reported as reference. For further comprehension, **Figure 3** shows the pictures of the tested tubes after incubation at 37◦C overnight, in comparison with the control sample.

TABLE 2 | Bonding strenght (N/25 mm), static water (CAw), and ethylene glycol (CAEG) contact angles, and dispersion (γ d s ) and polar (γ p s ) components of the surface energy for the neat PET substrate film (S) and for the SC coated films at 0, 5, 10, and 20% LAE concentration.

TABLE 3 | Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of LAE against *E. Coli*, and antimicrobial activity for the SC coated films at 0, 5, 10, and 20% LAE concentration., expressed as logarithm of colony forming units (Log(CFU)) and log reduction value (LRV).

As expected, the unloaded SC sample was not effective in inhibiting the microbial growth, with an LRV value equal to 0 and a consistent turbidity in all the tubes containing the film samples (**Figure 3A**). On the other hand, the LAE addition caused a consistent (>5 log) reduction of the microorganism viable counts even at the lowest percentage analyzed. In fact, the multilayer film loaded at 5% LAE (SC5) caused a growth reduction of 5.17 log against E. Coli, while LAE concentration equal to 10%, or higher, produced total inhibition against the microorganism tested. What is more, the pictures comparison in **Figures 3B–D** showed a progressive increase in the optical clarity when compared to the control sample, and the disappearance of the bottom cellular deposit at LAE 10 and 20% (SC10 and SC20, respectively). It is worth to point out that the LRV value obtained even at the lowest concentration of LAE, indicates at least a partial deliver of the agent sufficient to achieve an antimicrobial effect comparable with or higher than currently commercially available antimicrobial films and coatings for foods and pharmaceuticals (Molling et al., 2014), while the 10% LAE concentration is the minimum guaranteeing total inhibition of microbial growth among the investigated systems.

These promising results highlight that the developed packaging solutions are efficient even at the minimum concentration of active agent investigated, and pave the way to more in depth studies on the release kinetics, aimed at optimizing the films in terms of antimicrobial concentration and coating thickness.

## Water Vapor Permeability (WVP) Measurements

In order to better investigate the effects of the systems composition on the barrier performance of the samples, water vapor permeability tests were carried out, and outcomes are reported in **Table 4**. All the coated films (SC), with respect to the PET substrate (S), show an increase in the water vapor permeability values. However, this is mainly due to the normalization over the total thickness of the films, in which the amorphous PLA layer does not offer any resistance to the transport of water molecules (Robertson, 2013; Halász et al., 2015). A slight further increase in the water vapor permeability is observable at the highest LAE content, i.e., in the SC20 sample.

## Optical Analyses

In packaging applications, the transmission of visible and ultraviolet light are important parameters to preserve and protect

TABLE 4 | Water vapor permeability (PWV) and UV-Vis transmittance at 550 nm (T550%) for the neat PET substrate film (S) and for the SC coated films at 0, 5, 10, and 20% LAE concentration.

food products until they reach the consumer, as well as to get an attractive transparent package. To evaluate the transparency of samples, UV-Vis measurements were carried out. PET and PLA are known to have excellent optical properties, in this sense the LAE effect on transparency was investigated. In UV-Vis spectra reported in **Figure 4**, the transmittance percentage T% is reported as a function of wavelength for the neat substrate (S) the coated SC films at different content of the active phase. The transparency of the multilayer films, defined as the transparency of visible light in short range of 540–560 nm, was therefore evaluated measuring the transmission at 550 nm (T550%), and the values for the investigated samples are reported in **Table 4**.

As observable from the comparison among the S and SC samples, the PLA coating did not affect the transparency of the

PET substrate, which remains the highest, with a T550% value equal to 84.3% in both films. The further LAE addition to the polymer matrix determines a slight decrease in the transmittance at 550 nm, which becomes more significant at LAE concentration equal to 20% (T550% of SC20 equal to 60.1%). In this latter case,

the increase in film cloudiness, with respect to the SC, SC5 and SC10 samples, is also visible in **Figure 5**.

## CONCLUSIONS

In this work, innovative multifunctional and eco-sustainable antimicrobial packaging solutions have been successfully developed, through a conventional technique commonly applied in packaging industry. The produced films combine the structural and barrier performance of PET with the PLA sealing capacity and the LAE antimicrobial activity.

The chemical, physical and functional analyses carried out on the active multilayer films showed the good dispersion and homogeneous distribution of LAE into the polymer matrix, as well as the good adhesion of the coating layer on the substrate. Moreover, LAE addition did not modify the macromolecular structure of PLA.

Microbiological tests pointed out the effectiveness of the produced systems in releasing the antimicrobial agent and inhibiting microbial growth. In particular, the inhibition of microbial strain was found proportional to the LAE concentration into the PLA matrix, with 5.17 log decrease of viable counts at 5% LAE and total inhibition measured for coating formulations at higher LAE content.

In particular, it was pointed out that, by increasing the LAE concentration up to 10%, the functional performance of the films, in terms of adhesion, water vapor barrier and optical properties, were not significantly affected. On the other hand, increasing LAE content at 20%, a worsening of the PET film functionalities was observed. On the basis of these results, it comes out that the 10% LAE configuration allowed to obtain the best performance in terms of total microbial inhibition and functional properties.

## DATA AVAILABILITY STATEMENT

All datasets generated for this study are included in the manuscript/supplementary files.

## AUTHOR CONTRIBUTIONS

AA and LI: conceived and designed the experiments. AA: performed the experiments and the original draft preparation. AA, LD, and PS: conceptualization. LD, PS, and LI: review and editing of the manuscript. All authors discussed and revised the manuscript.

## ACKNOWLEDGMENTS

The authors would like to gratefully thank Prof. Zvi Hayouka (The Hebrew University of Jerusalem) for valuable discussion, and Prof. Sacha Lucchini (University of Salerno) for his support with microbiological analyses.

## REFERENCES

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Apicella, Scarfato, Di Maio and Incarnato. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Micromolded Polylactid Acid With Selective Degradation Rate

#### Valentina Iozzino, Annarita De Meo\* and Roberto Pantani

*Department of Industrial Engineering, University of Salerno, Fisciano, Italy*

Poly(Lactic Acid), PLA represents a very interesting polymer for industrial applications because of its good processability, the possibility of being obtained from renewable sources, good physical properties, biocompatibility, and biodegradability. The major depolymerization mechanism and the step that controls the rate of PLA biodegradation in compost is represented by the hydrolysis. The characteristic of being degradable is not per se an advantage: the inclination to degrade in the presence of water represents a limit for specific industrial applications, especially for durable components that are designed for long-period utilization such as in the automotive, electronic, and agricultural sectors, as well as in medical applications. Being able to control the degradation rate would be a real advantage: a product should preserve its characteristics during processing and for a time comparable to its application but should be nevertheless fully biodegradable at longer times. Furthermore, a gradient of properties could allow producing samples in which some portions degrade at a faster rate and some others at a slower one. Different methods can be used to influence the degradation rate of PLA, some examples are blending, copolymerization and surface modification. However, these change the physical properties of the material. Any factor influencing the rate of hydrolysis can affect the biodegradation process. The objective of this work is verifying the possibility to modulate the rate of degradation in the same part, in time and at different rates. The method is represented by a technique that influences locally the morphology of the samples. Biphasic samples (half amorphous and the other half crystalline) were obtained by micro-injection molding and the degradation process was monitored by means of hydrolysis tests. The analysis confirmed the crystalline regions show a slightly better resistance to the hydrolysis compared to the amorphous.

#### Edited by:

*Alessandro Pegoretti, University of Trento, Italy*

#### Reviewed by:

*Debora Puglia, University of Perugia, Italy Francesca Lionetto, University of Salento, Italy*

> \*Correspondence: *Annarita De Meo<EMAIL_ADDRESS>
#### Specialty section:

*This article was submitted to Polymeric and Composite Materials, a section of the journal Frontiers in Materials*

> Received: *03 October 2019* Accepted: *14 November 2019* Published: *28 November 2019*

#### Citation:

*Iozzino V, De Meo A and Pantani R (2019) Micromolded Polylactid Acid With Selective Degradation Rate. Front. Mater. 6:305. doi: 10.3389/fmats.2019.00305* Keywords: PLA, micromolding, crystallinity, hydrolysis, mechanical properties

## INTRODUCTION

In the last decades, environmental and economic challenges led scientists and producers to replace, in part, oil-based polymers with biodegradable. Poly(l,l-lactide) acid, hereafter called PLA, represents a good candidate. PLA is a thermoplastic characterized by a high value of strength and a high modulus. The resources that are used to produce PLA, like rice, corn or wheat, are annually renewable. Moreover, it is compostable and can be recycled (Drumright et al., 2000; Sawyer, 2003), producing this polymer consumes carbon dioxide (Dorgan et al., 2001). Another advantage of the PLA is its biocompatibility, especially for biomedical applications, in fact, it is not toxic for local tissues (Farah et al., 2016). Furthermore, the degradation products do not affect tissue healing. The

**34**

use of PLA, that was limited for medical and dental applications in the past, is turning into common applications like packaging and single-use products.

Being interested in the use of PLA in engineering areas and in applications for higher added value, currently, many scientists focused their attention in the production of new biodegradable polymers that have better processability, mechanical properties, and thermal resistance, as well as long durability and stability (Murariu and Dubois, 2016).  The concern for the use of this polymer to satisfy the market needs of bioplastics with a long duration, in sectors such as electronics and automotive components and obtain a processability similar to that of the existing polymers, is causing a rapid growth in PLA production.

The PLA products can be obtained by using the common processing techniques like extrusion, injection molding and compression molding (Garlotta, 2001; McKeen, 2014). Ghosh et al. (2008) verified that the technique for shear controlled orientation in injection molding, called SCORIM, can have an effect on the final energy at break as well as the maximum stress. However, due to its high viscosity, and a relatively narrower processing windows with respect to other resins, only under particular conditions, PLA can be processed by microinjection molding and most of the literature research focused on the microinjection molding of PLA composites or blends (Zhao et al., 2018; Zhou et al., 2018).

To enhance PLA properties, many efforts are undertaken via different techniques and modifications, adding additives (Iozzino et al., 2018) or by realizing physical treatments (Harris and Lee, 2008; De Santis et al., 2017). Some workers proved that by increasing the crystallinity of PLA products the chemical and thermal resistances improve (Gamez-Perez et al., 2011). For this reason, the study of the crystallization kinetics of PLA was of relevant importance (Pantani et al., 2010).

The degradation of biopolymers is the result of two mechanisms: the chemical hydrolysis and the water and oligomer diffusion. In order to estimate the effects of one of the two mechanisms, the hydrolytic degradation kinetics in the chemical regime, the study can be performed in solution (Zhang et al., 1994; De Jong et al., 2001). Different authors suggest an Arrhenius-dependent kinetics (Tsuji, 2003; Weir et al., 2004; Speranza et al., 2014). Many papers concerned the hydrolysis of PLA, studying the elements that can affect the mechanism of chain scission like the molecular weight, the temperature and the chain stereo-configuration (Lostocco and Huang, 1998; Drumright et al., 2000; Gorrasi and Pantani, 2017). Some workers verified that the carboxyl end groups accelerate the hydrolysis of PLA (Li et al., 1990; Hocking et al., 1995), and the kinetics solution is determined by the pH of the degrading medium (De Jong et al., 2001). The mechanism of hydrolysis is still under investigation, in fact, some authors verified that the scission of the chains occurs randomly also in acid (Shih, 1995) or in basic conditions (Belbella et al., 1996). The process of chain scission compared to the end scission determines a substantial reduction of the physical properties (Gleadall et al., 2014). Although, the influence of the stereochemical composition is still not clear (Gorrasi and Pantani, 2013).

The control of the biodegradation rate is interesting for the application in many sectors (Ha and Xanthos, 2010). There are several techniques that can be used to modify the rate of biodegradation. Examples are developments of blends, copolymers and use of particular fillers (Arias et al., 2014; Stloukal et al., 2015; Iozzino et al., 2018).

Literature research reveals that the morphology itself of the PLA can affect its degradation. Since the possibility for the water molecules to enter inside the rigid crystalline regions is extremely limited, the crystalline portion of PLA parts show higher resistance to the hydrolysis respect to amorphous regions. When different crystallinity levels are present in the part, the hydrolysis proceeds breaking the chains in the amorphous regions, therefore, removing the oligomers and monomers that are soluble in water, only intact crystalline regions remain (Tsuji, 2010). Others (Pantani and Sorrentino, 2013) demonstrated, also, that the initial morphology of PLA parts has a high influence on the degradation, in particular on the swelling and flake off processes. Completely amorphous PLA showed a higher degradation than semicrystalline PLA in presence of the same conditions of hydrolytic process (Fukushima et al., 2013). The amount of hydrolysis found for amorphous PLA parts was considerably high. In crystalline polymers the hydrolytic attack of ester bonds can be favorite.

The aim of this work is the production of parts with different morphologies, investigate the effect of the hydrolysis on the degradation of the poly-lactid acid parts and therefore observe the establishment of a profile of the properties inside each sample. In particular, thin PLA bars characterized by different crystallinity along the length were produced by microinjection molding adopting a novel technique that permits the local control of the temperature during and after the process thus affecting the crystallization in a selected region of the sample (De Santis and Pantani, 2016). The crystallinity, the evolution of the molecular weight and the mechanical properties of biphasic samples with different times of hydrolysis were investigated.

## MATERIALS AND METHODS

## Material

The material used in this work is a commercial PLA, the 4032D, supplied by Natureworks. Its D-enantiomer content is approximately equal to 2%. The molecular weight distribution detected by chromatography: Mn 119,000 g/mol and Mw 207,000 g/mol. In order to prevent viscosity degradation, each test was preceded by a drying procedure for the material that was kept for 8 h at 60◦C under vacuum.

## Samples Preparation

PLA bars were produced by injection molding with a Haake MiniJet Machine and a mold with a cavity geometry of 10 × 4 mm with a thickness of 0.25 mm. Micromolding was carried out at a melt temperature of 220◦C, injection pressure of 150 bar,

post injection pressure of 140 bar. The whole process was carried out under a nitrogen atmosphere. The cavity was inserted in a special mold equipped with a control system for the temperature of the cavity surfaces. This system adopted resistances to heat the surfaces of the cavity by Joule effect and thermocouples that allowed controlling the temperature by switching the current to the heaters. As showed in **Figure 1**, the process involved different steps: after the cavity surfaces were heated up to 160◦C the molten PLA was injected into the thin the cavity (injection step), the material was cooled by switching off the resistances from about 160◦C to about 50◦C in 15 s (cooling step). Then only one zone of the solid injected sample was kept at 105◦C for 500 s (annealing step). Since the PLA has very slow crystallization kinetics, the region of the injection-molded parts that did not experience the annealing step resulted to be fully amorphous. The region that was kept at 105◦C (De Meo et al., 2018) resulted crystalline. Two protocols were used to produce the samples for the degradation study. In protocol "a," after the filling and cooling steps, the region under annealing is the one after the gate. In the protocol "b," the region under annealing is the tip of the sample (**Figure 1**).

## Hydrolysis

The degradation of the PLA parts was investigated by means of hydrolysis tests. Each sample was immersed into distilled water. In line with the ASTM and ISO standards, the biodegradation experiments were carried out at 58◦C. The samples have been put individually in containers with distillated water where the quantity of water [ml] was 800 times the quantity of the dried sample [g]. A thermostatic bath equipped with a lid was adopted to mantain the temperature constant and equal to 58◦C during the hydrolysis experiments. After every 24 h of hydrolysis, the water of each sample was analyzed by using a Crison pHmeter at 25◦C. The water used in this work has a pH value of about 6.5. The water of each sample was replaced by the same amount of fresh distilled water to assure that the pH value of the hydrolysis medium remains constant. This operation was carried out by using a syringe which needle diameter is 0.2 mm. In order to carry out the necessary analysis, the samples were collected and dried in vacuum conditions at

FIGURE 4 | Not hydrolyzed biphasic samples, on the top the one produced by Protocol "a" and bottom by Protocol "b".

60◦C for about 3 h and then weighed. The samples, for the whole hydrolysis test, remain in their vessels, to avoid losses of the hydrolyzed material. All tests were carried out on at least three samples. The results reported refer to the average upon all the tests. The hydrolysis of all samples was carried out for about 60 days. After 60 days, since the samples became fragments with a very small mass an accurate analysis was not possible anymore.

## Calorimetry

Calorimetric analysis were carried out to monitor the crystallinity with the time of hydrolysis. A Mettler DSC822 in flowing nitrogen atmosphere was used for this purpose. To obtain reliable measurements, the indium, which is a standard material, was used to calibrate the heat flow and the temperature Thermograms were obtained by considering a mass of about 5 mg for each sample and the following thermal program:

## Gel Permeation Chromatography (GPC)

As hydrolysis proceeds, the evolution of molecular weight of the microparts was monitored by performing GPC analyses. A HPLC Waters (Milford, MA, USA) fitted out with an autosampler. After dissolving the parts in thetrahydrofuran (THF), considering a ratio between the sample mass and the quantity of solvent at 50◦C equal to 1 [g/mol], a 0.45µm filter Chromafil PTFE was used for the investigation.

## Mechanical Tests

The breaking force was evaluated by means of penetration mechanical tests. The dynamic mechanical measurements were

performed by using the Perkin Elmer DMA 8000 (Waltham, MA, USA). The machine is equipped with a tip that has a diameter of 150µm. For each experiments, the maximum value of force was equal to 7 N and the load rate was 0.2 N/min. A schematic of the device used is shown in **Figure 2**. The sample was placed under the tip with an increasing force up to the maximum allowed value of 7 N (the instrumental limit). DMA returned the values of the force and the displacement of the tip.

## RESULTS AND DISCUSSION

## Micromolding

PLA bars were obtained by microinjection molding. To permit the filling in the microcavity and prevent a premature solidification, a value of temperature between the glass transition

temperature and the melting temperature was applied at the surface of the cavity without changing the temperature of the rest of mold. The special system, as showed in **Figure 3**, is able to increase the temperature of the cavity surfaces from 60◦C, temperature of the mold, to 150◦C. That value was set just before the filling, manually activated at the machine, and was kept until the injection step ended. After that, by switching off the heaters the rapid cooling led the surfaces in less than 10 s to 60◦C. Without extracting the sample from the mold the preparation of each sample included a second step in which an isothermal step (crystallization step) involved only an area of the injected samples. A temperature of 105◦C was applied for 500 s. In these conditions, a low crystallization time is expected (De Meo et al., 2018).

## Hydrolysis

The hydrolysis mechanism represents the main cause is the main of depolymerization in fact, it controls the biodegradation in compost. The hydrolysis rate, therefore, indicates the rate of degradation during composting. The samples were put in distilled water and at the set time were taken out and dried for the needed investigations: optical, calorimetric, GPC and mechanical analysis.

### Optical Observations

All the samples were observed and photographed under polarized light. As showed in the **Figure 4** there was a clear difference between the two regions of the sample: the amorphous region was transparent whereas the crystalline one appeared opaque. Analyzing the images of the samples under hydrolysis it is evident that the crystalline zone of the sample remained unchanged for a time of 12 days (**Figure 5**). The appearance of the amorphous zone changed after the second day turning from transparent to opaque. The same phenomenon was already observed in the PLA (Pantani and Sorrentino, 2013). This could be due to the micro-fractures due to the water moving away from the sample during the drying phase. Obviously, the crystalline zone had a lower water permeability at least for the first 25 days of hydrolysis. From 30 days there were no differences between the two different morphologies. The degradation tests confirmed that crystalline regions had a slightly better resistance to hydrolysis.

## Calorimetric Analysis

From the results of the DSC analysis, before the hydrolysis (samples 0 days) reported in **Figures 6**, **7**, it is evident that there was a difference between the phases: there were crystallization

and melting peaks for the amorphous phase and there was only a melting peak for the crystalline phase. The Glass Transition Temperature and the Melting Temperature exhibited the same values for both phases. Tg and Tm, also, are very similar to those observed in the pellet indicating no degradation had occurred in the manufacturing process, neither in the crystallization step. The degree of crystallization calculated from the analysis of the thermograms is negligible for the amorphous region in all the samples, whereas for the crystalline phase, in the case of both protocols adopted, "a" and "b," the values were between 30 and 35% (**Table 1**). In all the biphasic samples, the crystallization degree reached in the annealing step was then close to the maximum achievable for this grade of PLA when crystallized from the molten state.

During hydrolysis a small piece from each region of the sample was analyzed by calorimetry with the same protocol used for the sample 0 days. **Figures 8**, **9** show the DSC curves obtained from the amorphous zone and the crystalline one respectively. In both cases, the Tg is clear only in the case of the sample 0 days whereas it is not detectable from hydrolyzed samples. Before the hydrolysis, the melting temperature is 169.3◦C for the crystalline zone and 167.3◦C for the amorphous zone. After 40 days of hydrolysis in both regions it is 156.8◦C.

The evolution of the crystallinity degree with the time of hydrolysis is reported in **Figure 10**. As expected, an increase in the crystallinity degree was observed. This result could be due to the erosion of the amorphous regions and the crystallization of the amorphous ones. As the hydrolysis proceeds the crystallization is more fast in the amorphous region than in the crystalline one. The difference in crystallization rate probably indicates a slower degradation of the crystalline zone with respect

FIGURE 8 | DSC curves of amorphous zone of biphasic samples at different time of degradation.

to the amorphous one. After 25 days of hydrolysis, however, no significant difference can be observed between the values of the Xc of the two morphologies.

#### Molecular Weight

observed.

**Figure 11** shows the evolution of the number average molecular weight (Mn) as the hydrolysis of micro-injected biphasic PLA samples proceeds. No differences in the evolution of Mn as the hydrolysis proceeds: after 35 days we can see that Mn for the crystalline zone is a slightly higher than for the amorphous zone. The analysis of the evolution in time of Mn, and the molecular weight of the repeating unit permitted evaluating the kinetic constant of the hydrolysis process, k'= k×ρ/M, where ρ is the density of the polymeric part (about 1.21 Kg/m3 ), M is the molecular weight of the repeating unit (72 g/mol for the PLA 4032D) (Gorrasi and Pantani, 2013). The value k' represents the kinetic constant of the process of hydrolysis:

$$k' = -\frac{d\ln\left(M\_{n-}M\right)}{dt}$$

The kinetic constants calculated by the evolution of the average molecular weight in each region gave a value of 0.086 days−1 for the amorphous and a value of 0.081 days−1 for the crystalline zone showing a small difference between the two regions. This could be ascribed to the process used to produce the samples: the high shear imposed by the micro-injection molding could have generated precursors of crystallites that easily grew once the hydrolysis began. Dimensions and arrangement of the crystalline structure as well as a different orientation from the gate to the tip of the sample probably have an effect on the process highlighting the need for future investigations on the phenomenon.

#### Mechanical Tests

The resistance to the penetration was evaluated with the time of the hydrolysis. In each region of the sample, the mechanical test gave the force applied and the corresponding displacements (**Figure 12**). The maximum force applied was 7 N. Observing the **Figure 13**, it is evident that up to 12 days, no fracture was detected in either phase. After 19 days, the sample did not withstand the force applied and broke. As the hydrolysis proceeds a decreasing mechanical stresses was needed to break the sample. For the crystalline phase the force at break is slightly higher than the one noticed in the amorphous phase.

## CONCLUSIONS

In this work, biphasic samples of PLA were obtained by microinjection molding. For this purpose a new dynamic mold temperature control system for rapid heating and cooling of the mold was used. The system allowed filling a microcavity and produce bars with a thickness of 0.25 mm. Half bar, after the injection, was kept at 105◦C for 500 s to permit a rapid crystallization. The biphasic samples were then hydrolyzed. The hydrolysis process was monitored by considering: the evolution of the degree of crystallinity of the samples by means of the differential scanning calorimetry technique (DSC), the evolution of the molecular weights by means of gel permeation chromatography (GPC), changes in mechanical behavior. For all the sample, in both morphologies, Xc increases as the hydrolysis proceeds. The value of Xc of the crystalline phase, increases more slowly than the Xc of the amorphous phase as the hydrolysis proceeds: this probably indicates a slower degradation of the crystalline zone than of the amorphous zone. The molecular weights decrease for both zones of the two samples. We could

## REFERENCES

observe a slightly slower reduction of Mn with time for the crystalline regions with respect to the amorphous ones as the hydrolysis proceeds. On analyzing the mechanical tests the amorphous zone showed higher frangibility than the crystalline revealing that the crystalline regions present a slightly better resistance to the hydrolysis.

## DATA AVAILABILITY STATEMENT

All datasets generated for this study are included in the article/supplementary material.

## AUTHOR CONTRIBUTIONS

RP designed theexperiments and supervised the work. AD was responsible for the preparation of the samples and wrote the draft of the paper. VI performed the tests of hydrolysis and the characterization of the samples. All the authors reviewed and accepted.

and biodegradation of PLA and its nanocomposites. Poly. Test. 41, 124–132. doi: 10.1016/j.polymertesting.2014.10.015

**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2019 Iozzino, De Meo and Pantani. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

# Phase Structure, Compatibility, and Toughness of PLA/PCL Blends: A Review

#### Ivan Fortelny 1 , Aleksandra Ujcic1 , Luca Fambri 2 and Miroslav Slouf 1 \*

1 Institute of Macromolecular Chemistry, Czech Academy of Sciences, Prague, Czechia, 2 Department of Industrial Engineering, University of Trento, Trento, Italy

Results of the studies dealing with the toughness of polylactic acid/polycaprolactone (PLA/PCL) blends are analyzed with respect to the PCL particle size, PLA matrix crystallinity, and presence of a compatibilizer. It is shown that a high toughness or even "super-toughness" of PLA/PCL blends without a compatibilizer can be achieved for blends with the proper size of PCL particles. Nevertheless, the window for obtaining the super-tough PLA/PCL blends is quite narrow, as the final impact strength is very sensitive to multiple parameters: namely the blend composition, PLA matrix crystallinity, and PCL particle size. Available literature data suggest that the optimal composition for PLA/PCL blends is around 80/20 (w/w). The PLA/PCL(80/20) blends keep high stiffness of PLA matrix and the concentration of PCL particles is sufficient to achieve high toughness. The PLA/PCL(80/20) blends with low-crystallinity PLA matrix (below ca 10%) exhibit the highest toughness for bigger PCL particles (weight average diameter above 1µm), while the blends with high-crystallinity PLA matrix (above ca 30%) exhibit the highest toughness for smaller PCL particles (weight average diameter below 0.5µm). The addition of a compatibilizer may improve the toughness only on condition that it helps to achieve a suitable particle size. The toughness of both non-compatibilized and compatibilized PLA/PCL blends with optimized morphology can be more than 15 times higher in comparison with neat PLA.

Keywords: biopolymer blends, poly(lactic acid), polycaprolactone, impact strength, crystallinity, particle size distribution

## INTRODUCTION

Poly(lactic acid) (PLA) is frequently reported as one of the most promising biodegradable polymers synthesized from natural resources as it can be used in many technical applications, especially in packaging (Lunt, 1998; Garlotta, 2002; Auras et al., 2004; Averous and Pollet, 2012). At the same time medical grade PLA continues to be a favorable material for many medical applications such as the tissue engineering (Patrício et al., 2013) and bone fixation devices (Todo et al., 2007) due to its well documented biocompatibility, full biodegradability, and high stiffness resulting from the relatively high glass transition temperature (Tg) (Fambri and Migliaresi, 2010). Preparation from natural resources and biodegradability suggest PLA as substitution of some plastics made from fossil fuels in a broad range of applications. However, brittleness is a strong drawback for practical applications of neat PLA. Various methods of PLA toughening were summarized by Krishnan et al. (2016). Generally, most efficient method for an improvement of toughness of a brittle polymer is

Edited by:

Alfonso Maffezzoli, University of Salento, Italy

#### Reviewed by:

Veronique Michaud, École Polytechnique Fédérale de Lausanne, Switzerland Johnny De Nardi Martins, Federal University of Santa Catarina, Brazil

> \*Correspondence: Miroslav Slouf

<EMAIL_ADDRESS>
#### Specialty section:

This article was submitted to Polymeric and Composite Materials, a section of the journal Frontiers in Materials

> Received: 26 June 2019 Accepted: 09 August 2019 Published: 27 August 2019

#### Citation:

Fortelny I, Ujcic A, Fambri L and Slouf M (2019) Phase Structure, Compatibility, and Toughness of PLA/PCL Blends: A Review. Front. Mater. 6:206. doi: 10.3389/fmats.2019.00206 its blending with soft, ductile polymers (Bucknall, 2000; Horak et al., 2005). Poly(ε-caprolactone) (PCL) is soft, biocompatible and biodegradable semicrystalline polyester (Pitt, 1990; Krishnan et al., 2016) with a rubbery amorphous phase at room temperature as its Tg is around −60◦C and its melting temperature is in the range of 55–70◦C. Therefore, PLA/PCL blends should keep favorable biocompatibility and biodegradability and should show enhanced impact strength with respect to neat PLA. The main objective of the preparation of PLA/PCL blends, where PLA is a major component, is the substantial enhancement of PLA toughness with minimum reduction of its stiffness.

Further problem for many technical applications of PLA and PLA/PCL blends is their mechanical performance at elevated temperature. Commercial grades of PLA are mixtures of Lisomer and a small amount of D-isomer (Standau et al., 2019). Therefore, commercial PLA's are semicrystalline with melting temperature, Tm, between 140 and 180◦C in dependence on the enantiomeric purity (Fambri and Migliaresi, 2010) and glass transition temperature, Tg, which usually ranges from 55 to 65◦C (Auras et al., 2004; Krishnan et al., 2016). However, the rate of the PLA crystallization is slow (Auras et al., 2004; Liu et al., 2014; Murariu et al., 2015; Zhang et al., 2018), frequently slower than the cooling rate used in common polymer processing devices (Zhang et al., 2018). Consequently, the crystallinity of PLA in its blends processed by common procedures tends to be very low, typically lower than 10% (Bai et al., 2012, 2013; Ostafinska et al., 2015, 2017). Therefore, the softening temperature of products prepared from neat PLA and PLA/PCL blends at common processing conditions is controlled by the Tg of PLA. At room temperature, the differences between the moduli of PLA with a low (below 10%) and high crystallinity (above 30%) are not critical. The same is true for PLA/PCL blends. It follows from the literature data (Perego et al., 1996; Kelnar et al., 2016) that the ratio of moduli of materials containing PLA with a high and with a low crystallinity does not exceed 1.25. At elevated temperatures, however, the moduli of lowand high-crystallinity PLA may differ significantly: Bai et al. (2013) found that the modulus of PLA/PCL (80/20) blends with a high-crystallinity PLA matrix was 20 times higher than the modulus of an analogous blend with a low-crystallinity of PLA matrix at the temperature of 80◦C. Our measurements at the same temperature led to even higher moduli ratio of 100 for neat PLA and the ratio of 75 for PLA/PCL (80/20) blend. These results discriminate PLA and PLA/PCL blends with a low crystallinity of PLA for many technical applications where good mechanical properties of the material at elevated temperatures are required. Crystallinity of PLA can be enhanced by the annealing during processing and/or by the addition of a convenient nucleation agent.

This review is focused mostly on the toughening of the most common, commercially available PLA polymers, which consist of L-isomer with a small amount of D-isomer, as described above. Other PLA polymers include poly(L-lactic acid) with 100% enantiomeric purity (PLLA), which is polymerized from the pure L-isomer, poly(D-lactic acid) (PDLA), prepared from pure D-isomer, and poly(D,L-lactide) (PDLLA), which derives from a mixture of the L- and D-isomers. Pure PLLA and PDLA can achieve a higher crystallinity than common commercial PLA (Nakajima et al., 2017) but their rate of crystallization is still quite low. PDLLA cannot crystalize. PLLA and PDLLA are commonly used in biomedical applications. We use the above abbreviations in the relation to samples described in the literature. However, it should be mentioned that some authors describe PLA containing small fracture of D-isomer as PLLA.

The impact strength of polymer blends depends on the size distribution of the dispersed particles of the soft polymer in the matrix of the brittle one (Bucknall and Paul, 2009, 2013). Generally, super-tough polymer blends can be obtained only for a certain range, often quite narrow, of sizes of soft polymer particles. The theory of impact behavior of polymer blends is quite complex (Bucknall and Paul, 2013). The lower bound of the suitable particle sizes is given by the minimal size of soft particles needed for their efficient cavitation. The upper bound for a certain blend composition is given by inter-droplet distance which should be shorter than a critical distance for development of cracks or shear bands in the polymer matrix. The optimum particle size depends on dominating mechanism of energy dissipation in the matrix (multiple crazing or shear yielding). Small elastomer particles (weight-average diameter, dw, in the range 0.2–0.4µm) are efficient when shear yielding dominates the toughening mechanism, e.g., for ductile matrices of polypropylene or polyamide, whereas larger particles (dw between 2 and 3µm) are more effective when multiple crazing dominates, e.g., for brittle matrices of polystyrene or poly(methyl methacrylate) (Bucknall and Paul, 2009). Thus, the optimum particle size distribution depends on the structure of the matrix, especially on its crystallinity.

In order to achieve optimal particle size distribution for toughening, it is necessary to understand the relation among the processing conditions of PLA/PCL blends, the rheological properties of the components and the resulting morphology of the blends. Polymer blends are mostly prepared by melt mixing in extruders or batch mixers. They are further processed by injection molding, extrusion, blow molding or compression molding. The phase structure of polymer blends is formed and developed during their compounding and processing. Evolution of the phase structure in flowing molten polymer blends has been studied intensively experimentally (mostly for blends of synthetic polymers) and theoretically during last 40 years (Horak et al., 2005; Huang, 2011; Fortelny and Juza, 2019). Evolution of the droplets-in-matrix morphology (which is typical of PLA/PCL blends discussed in this contribution) during simple shear or elongational flow is qualitatively well understood. However, quantitative prediction of particle size distribution in real systems is extremely challenging due to complexity of the flow fields in mixing and processing devices and complex rheological behavior of molten polymers. The results of previous studies can be briefly summarized as follows: (i) The particle sizes in the blends containing the same components and prepared at the same conditions increase with the concentration of the minor component. (ii) The smallest particle size is usually obtained when viscosities of the dispersed phase and matrix are comparable. (iii) The particle size increases with interfacial tension between the blend components. (iv) The particle size in molten blends increases in quiescent state and in slow flow, which means that an increase in the particle size appears during compression molding. (v) Addition of a compatibilizer decreases the size of the dispersed particles during mixing and stabilizes the blend morphology at processing; this effect is stronger for blends with a higher content of the dispersed phase.

This review aims at the evaluation of the results of studies of the morphology, compatibility and mechanical properties of PLA/PCL blends with respect to the control of the blend toughness and crystallinity. The conclusions of this contribution should answer the question if and how the blending of PLA with PCL can lead to the material with required mechanical properties.

## TOUGHNESS OF PLA/PCL BLENDS WITHOUT COMPATIBILIZERS

Many papers have been focused on the morphology and mechanical properties of PLA/PCL blends. As for the stiffness, all studies are in agreement that the modulus and yield strength of PLA/PCL blends decreases with the PCL content. This behavior is typical of the blends combining stiff and soft polymers (Horak et al., 2005) and thus the stiffness is not discussed in the following text. However, the results related to the toughening of PLA by means of blending with PCL seem to be contradictory at first sight. These results are discussed below. The ratios of toughness, aBl/aPLA, and elongation at break, εBl/ /εPLA, of PLA/PCL blends to neat PLA together with crystallinity of PLA, w PLA c , and average diameter of PCL particles, d, are summarized in **Table 1**.

## Studies Reporting Low Toughness of PLA/PCL Blends

None or weak enhancement in the toughness of PLA by its blending with PCL has been found in a major part of previous studies. López-Rodríguez et al. (2006) found very small increase in strain at break (to 1.3%) together with strong decrease in strength at yield and strength at break for PLLA/PCL (80/20) blends in comparison with neat PLLA. Carmona et al. (2015) found only a small increase in elongation at break for an extrusion prepared PLA/PCL (50/50) blend with respect to PLA but strong decrease with respect to PCL. Gardella et al. (2014) and Monticelli et al. (2014) obtained the same elongation at break for PLA/PCL (70/30) blend prepared in a batch mixer as for neat PLA. The blend contained remarkable amount of large PCL particles.

Tsuji and Ikada (1996) studied the morphology and the properties of the blends of poly(D,L-lactide) containing 50% of D-isomer (PDLLA) with PCL, which were prepared by solution casting. They found that the dependence of the elongation at break on the PDLLA/PCL composition is non-monotonic with a minimum around 50/50 composition. Similar dependence of the elongation at break on the composition of the blends prepared from 100% L-isomer of PLA (PLLA) with PCL was obtained by Tsuji et al. (2003). Simoes et al. (2009) found negligible increase in elongation at break and about twice TABLE 1 | Ratio of toughness, aBl/aPLA, and ratio of elongation at break, εBl/εPLA, of PLA/PCL blends and neat PLA as a function of diameter of PCL particles, d, and crystallinity of PLA matrix, wPLA c .

aPLLA matrix; bMaximum of graphical diameter distribution; cEstimated from the figure; dUnnotched Charpy impact strength; eFracture energy from tensile testing; f Number average of d; g Instrumented impact testing; hWeight average of d; iNotch Izod impact strength; jVolume average of d; kNotched Charpy impact strength; lOur unpublished result, blend containing 1% of talc.

larger Charpy impact strength for PLA/PCL (75/25) blends in comparison with neat PLA. Mittal et al. (2015) found only a small increase in the value of elongation at break of the PLA/PCL (50/50) blends with respect to neat PLA, deeply below the additive value.

Vilay et al. (2009) studied the morphology, thermal behavior, and mechanical properties of PLLA/PCL blends. They found only weak increase in the elongation at break with the increasing content of PCL in the PLLA/PCL blends. Similar result for elongation at break was obtained also by Finotti et al. (2016) for PLA/PCL blends with commercial PLA. Only very small increase in the impact strength was found for PLLA/PCL (85/15) blends having crystallinity of PLLA matrix about 25% by Todo et al. (2007). Authors of the above studies and authors of a review considering these results (Imre and Pukánszky, 2013) concluded that PLA and PCL are incompatible polymers, i.e., that their large interfacial tension results in coarse morphologies and inferior mechanical performance. Therefore, the compatibilization was regarded as necessary for the preparation of PLA/PCL blends with high toughness.

Recently, Kassos et al. (2019) studied the effect of concentration of PCL on mechanical properties of samples of PLA/PCL blends prepared by the mixing in a twin screw extruder, followed by an injection molding. They found that the notched Izod impact strength increased with increasing content of PCL. However, this increase was quite weak: The impact strength of PLA/PCL (70/30) was about twice of that for neat PLA. Moreover, they observed strange dependence of elongation at break on PCL concentration: After strong increase for PLA/PCL (95/5) blend, the elongation at break decreased for higher amounts of PCL (compositions 80/20 and 70/30) to the values comparable with neat PLA.

## Studies Reporting High Toughness of PLA/PCL Blends

Surprisingly enough, the results of a few other studies are in a partial or full contradiction with the conclusions in the previous section. Takayama et al. (2011) studied the character of the fracture in PLA/PCL blends without and with a compatibilizer. They found ductile character of the fracture in the quenched PLA/PCL blends. On the other hand, brittle fracture was detected for PLA/PCL blends annealed with the aim to enhance crystallinity of PLA. Yeh et al. (2009) found a steep increase in the strain at break with the content of PCL above 10% for PLA/PCL blends. Similar dependence of the strain at break on the content of PCL in PLA/PCL blends was found also Zhao and Zhao (2016) for injection molded samples.