Cultural Heritage in Marker-Less Augmented Reality: A Survey

*Hoshang Kolivand, Abdennour El Rhalibi, Mostafa Tajdini, Sarmad Abdulazeez and Pisit Praiwattana*

### **Abstract**

Augmented reality (AR) is considered as one of the most significant technologies in the field of computer graphics and is utilised in many applications. In this chapter, we have presented a brief comprehensive survey of cultural heritage using augmented reality systems. This survey describes the main objectives and characteristics of marker-less augmented reality systems through presenting up-to-date research results in this area. We describe the marker-less technologies in the area of AR, indoor marker-less AR, outdoor marker-less AR, real-time solutions to the tracking problem, real-time registration, cultural heritage in AR, 3D remonstration techniques, as well as presenting the problems in each research.

**Keywords:** virtual heritage, culture heritage, augmented reality, marker-less AR, AR heritage

### **1. Introduction**

Augmented reality is a technology in which user's view of a real scene is augmented with extra virtual information. Augmented reality (AR) registration of virtual objects is required having an accurate tracking or camera pose estimation, but tracking is one of the key technical challenges of AR system [1]. AR has many conceivable applications in a wide range of fields such as education, construction, public health, manufacturing and entertainment. With the increased computational speed and advancement of particular computer technology, augmented reality applications become possible in multidisciplinary fields, for example, education, simulation, entertainment, medical and games. Researches related to augmented reality (AR) and virtual reality (VR) have shown significant growth with the development of interactive computer technology and sophisticated 3D modelling packages [2]. Virtual heritage is considered one of the important fields in the computer-based interactive technologies in virtual reality [3]. It created visual representation of monument, artefacts, building and culture to present openly to global audiences. However, virtual heritage becomes as a platform for promoting the education process, motivating and understanding particular events and historical elements for the use of students and researchers. Augmented reality techniques can be classified into two main categories: vision-based AR and location-based AR [4, 5]. Location-based AR uses the capability of a specific device to determine its position in the world, for example, GPS and then the retrieval relevant information to that location. Then, this information is superimposed into

the output of their device's camera to permit a more natural data presentation compared only using the map alone. Vision-based AR particularly depends on processing the data that is extracted from the images or video frames that have been taken by the device. This kind of AR includes a number of techniques that lend significantly from computer vision to the range, where research progress in AR relies on the progress of the latter [6]. Lately, augmented reality technology has become an accepted technology among scientific community and even public, which is used for merging of real and virtual objects, and mixed it into the real-world environment. However, this technology is used in virtual heritage to improve the visitor experience of a cultural heritage site, as well as, the possibility to present the ancient-ruined building without any damage. In this chapter, we have presented a survey of marker-less AR. This survey is based on the state-of-the-art related to marker-less AR such as indoor marker-less AR, outdoor marker-less AR, real-time solutions to the tracking problem, real-time registration and cultural heritage in AR. Section 3 introduces the marker-based AR and Section 4 is allocated to marker-less AR, while Section 5 presents the researches related to cultural heritage in augmented reality.

Section 6 reveals the issues with virtual heritage in augmented reality and Section 7 presents 3D reconstruction techniques for cultural heritage. Section 8 is all about location in AR. The work is concluded by section 9 which is the conclusion.


**6.1 Plastic recycling strata**

The recycling process for plastics has been subdivided into four components: primary recycling, secondary recycling, tertiary recycling, and quaternary recycling [16–18]. Recovery of production scraps as a pure polymer stream or the extrusion of preconsumer polymer has been identified as the operational component of primary recycling. Secondary recycling involves sorting of waste polymer feedstock, waste polymer size reduction, cleaning the waste polymer flow, drying and extrusion to form desired recycled polymer products [19–24]. Multiple cycles of recycling at primary and secondary stages can be used to recover many polymers while maintaining expected performance characteristics.

Conversion of the polymer feedstock to its constituent monomers can be selected when the first two recycling stages do not offer the desired recycling flexibility, economic opportunity, or product quality [25]. This tertiary stage recycling can complement the earlier, more traditional recycling technology strata (**Figure 4**). A selective approach to converting the feedstock polymer to its monomeric components can retain significant feedstock value with the ability to convert the recycle stream to a spectrum of polymer applications. This depolymerized feedstock can be utilized in a spectrum of pathways leading to chemical production syntheses other than reforming recycled polyethylene terephthalate (rPET) as an upcycling endeavor. Chemical recycling utilizes reactions with the chemical structure of the polymer to convert the polymeric mass to monomers and related chemicals [15, 26–30]. The chemical recycling process permits the removal of material consigned to downcycling recycling technology such as colorants, process additives, and other contaminants. This removal provides a material that can be used as a feedstock for opportunities of upcycling plastic waste to access valueadded chemicals, thereby enhancing economic performance (**Figure 5**).

The nonselective technologies found useful at the last recycling stage are generally thermolytic such as pyrolysis and hydrocracking technologies that convert the polymer feedstock into monomer and pyrolytic oils [31, 32]. At a quaternary stage, incineration is employed to recover the energy content of the waste polymer feedstock [33–35]. The relegation of polymer feedstock to quaternary recycling can be attributed to feedstock complexity, processing economics, waste unsuitable to other forms of recycling, market forces, and waste polymer treatment demands and capabilities [24, 36, 37]. This stage of recycling recovers little value of the feedstock and may contribute to greenhouse gas emissions having other unintended environmental consequences. Selective chemical recycling of polymers has become more

**Figure 5.** *Tertiary plastic recycling strata.*

*Are Reliable and Emerging Technologies Available for Plastic Recycling in a Circular… DOI: http://dx.doi.org/10.5772/intechopen.101350*

attractive to engage significant process funding recently, since chemical depolymerization is designed to form the original monomers of the recycled polymer in successive recycling to approach continuous recyclability [38–40].


*Article* **Selection of a PCM for a Vehicle's Rooftop by Multicriteria Decision Methods and Simulation**

**Juan Francisco Nicolalde 1,\* , Mario Cabrera 1 , Javier Martínez-Gómez 1,2 , Rodger Benjamín Salazar 3 and Evelyn Reyes 1**

	- Instituto de Investigación Geológico y Energético (IIGE), Quito 170518, Ecuador

#### **Featured Application: Energy efficiency and thermal comfort on vehicles.**

**Abstract:** The automotive industry is one of the most contaminant; for this reason, solutions in efficient matter has been proposed over the years. This research contributes to this subject by evaluating the thermal comfort in the internal air of a vehicle by using a 20 mm layer of a phasechange material attached to the rooftop interior of a car. The phase-change material selection is based on a list of other materials proposed in previous research and chosen by multicriteria decision methods. In this sense, the material savENRG PCM-HS22P proved to be the best. Moreover, a simulation using the finite elements method showed how the PCM reduced the temperature of the air by 9 ◦C when heating and by 4 ◦C when the temperature drops. To conclude, the multicriteria selection methods chose the best material to absorb energy during the charging process and released it during the discharging event in this automotive application.

**Keywords:** phase-change material; multicriteria decision; finite element; automotive; energy storage

### **1. Introduction**

The utilization of air conditioning and heating systems to control passengers' comfort in the automotive industry impacts the fuel consumption and economy, which makes it necessary to improve the fuel efficiency [1–3], where the internal temperature of the vehicle's air, as a measurable factor of discomfort, should be controlled between 23 ◦C and 28 ◦C [4]. In order to handle this, the development of energy-efficient solutions have been implemented, one of these being Thermal Energy Storage (TES), which is the most efficient in using the available heat resources [5,6]. In this sense, Latent Heat Storage (LHS) is responsible for the accumulation of energy. In phase-change materials (PCM), this phenomena takes place at a molecular level and triggers the transition between phases [7], producing an endothermic reaction when melting and an exothermic reaction when changing from liquid to solid [8]. In this sense, the performance of the PCM depends on the climate and the amount of PCM [9]. Moreover, when talking about LHS, PCMs are classified in organic PCMs, inorganic PCMs and eutectic mixtures; these last ones are mixes of two or more composites that melt and solidify together [10].

The utilization of PCMs has been widely studied for the properties presented during the phase change; in this sense, naming the research of Bakan et al., it has been stablished the importance of the crystallization during the phase change and the requirement to study the crystal growth in different temperature ranges. In this way, the crystal growth of the PCM Ge2Sb2Te2, in a range of temperatures between 300 K and 870 K, has been studied [11], allowing to develop a PCM with promising nanophotonic applications that has

**Citation:** Nicolalde, J.F.; Cabrera, M.; Martínez-Gómez, J.; Salazar, R.B.; Reyes, E. Selection of a PCM for a Vehicle's Rooftop by Multicriteria Decision Methods and Simulation. *Appl. Sci.* **2021**, *11*, 6359. https:// doi.org/10.3390/app11146359

Academic Editor: Ioannis Kartsonakis

Received: 27 May 2021 Accepted: 24 June 2021 Published: 9 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to be investigated [12]. Furthermore, the thermal applications of PCMs were investigated to reduce the indoor temperatures and reduce the internal air temperature to reach a maximum [13] by storing solar energy [14]. On the other hand, in the automotive industry, PCMs have been used as thermo stabilizers for batteries of electric and hybrid vehicles, by storing the overheat since natural and forced convection are not as efficient [6]. The size of the radiator and the cooling fan also can be reduced by using PCMs and help the cold ignite as well as recover the latent heat [15,16].

PCMs have a wide variety and different properties that are needed for different applications [17]; in this sense, multicriteria decision methods (MCDM) have been used in the selection of PCMs over several areas with great results. The methods used in this research are the Analytic Hierarchy Process [18], the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) [19], the VIKOR method [20] and the Complex Proportional Assessment Methods (COPRAS) [21]. Moreover, the selection by MCDM was validated by simulations of the superplastic forming for a vehicle's components [22], as well as the selection by MCDM and simulation to enhance the PCM by nanoparticles [23]. In this sense, finite elements analysis has been useful in automotive design, such as in disk brakes and its thermo-mechanical behavior [24].

The building industry has had a leading role in the research of simulated PCMs selected by MCDM, but the automotive sector does not much apply these tools for this benefit. With this previous knowledge, this research aims to select the best PCM by MCDM means based on a bibliographical research, to be applied in a 20 mm layer of PCM that will store energy in the charging process and release it in the discharging event to control the internal air temperature, and where this phenomenon will be simulated by computer-aided engineering.

#### **2. Materials and Methods**

### *2.1. Materials Determmination*

Rastogi et al. did a research where 35 PCMs passed through a MCDM selection, where the candidates were considered for heating, ventilation and air-conditioning application in buildings; also in this investigation, the author takes in consideration the Figures of Merit (FOMs), for the performance of the heat extraction per unit volume (FOM1) and the response time of the material (FOM2). Thus, these criteria will be used as the base, where 15 with the best specific heat capacity will be considered for the MCDM proposed for automotive rooftop applications. However, in this selection, some materials from the same family have the same specific heat capacity; for these materials, the one with the best heat extracted per unit volume (FOM1) calculated is considered. In this way, the materials PlusICE PCM A22, PlusICE PCM A23, PlusICE PCM S19, PlusICE PCM S21, PlusICE PCM S25, Rubitherm GmbH PCM SP21E2, Rubitherm GmbH PCM SP25E2, Rubitherm GmbH PCM RT21, Rubitherm GmbH PCM RT24, Rubitherm GmbH PCM RT25, Rubitherm GmbH PCM RT27, Rubitherm GmbH PCM RT21HC and Rubitherm GmbH PCM RT22HC are rejected and, in the case of the materials Rubitherm GmbH PCM SP24E and Rubitherm GmbH PCM SP26E that have the same values, the series SP24E stands for its wide range of phase-change temperatures [25]. In this sense, Table 1 displays the materials considered with its thermal properties. Furthermore, it is important to point out that this investigation will use the lowest thermal phase-change temperature and characteristics in the liquid state; also, an M index was added to the materials as a label to be used in the MCDM to be used instead of the full name. On the other hand, Table 2 shows the results of the calculations made for the figures of merit [25].

#### *2.2. Analytic Hierarchy Process (AHP)*

For this automotive application, the following characteristics are searched for:

**Table 1.** List of phase-change materials and their thermo-physical properties.

**Table 2.** FOMs results.

These requirements are fulfilled by the candidates and the AHP method allows to weight them; this process is developed in the research of Odu. G.O. [26]. Moreover, the method requires a label for the criteria, which is displayed as follows:

#### *2.3. Method VIKOR*

Following the method used by Shekhovtsov and Salabun [27], the VIKOR method solves selection problems looking for a ranking where the alternative and criteria that will be used is the solution closest to the ideal [28]. This method requires that the criteria described before have the following considerations:

• T1 Higher = Better;

With these in mind, the VIKOR method will be performed as the named previous research.

#### *2.4. TOPSIS Method*

The TOPSIS method takes and classifies a finite number of alternatives by similarity to the ideal solutions, looking for the alternatives to have the shortest distance to the ideal positive solution and the farthest from the negative [28]. The steps proposed by Shekhovtsov and Salabun [27] are followed and the cost and profit criteria are the same that were determined by the VIKOR method.

#### *2.5. COPRAS-G Method*

The ranking of the alternatives in the COPRAS method considers the utility degree of the different options by using grey numbers that comes from the grey theory for insufficient information [28]. This method is followed as in the research of Mousavi-nasab [29] and Sotoudeh-anvai [30], where the beneficial and non-beneficial criteria are defined as follows:

#### *2.6. Spearman's Correlation Coefficient*

The different methods obtains different results; to measure the relation between these non-linear results, Spearman's correlation was used. In this sense, this technique quantifies the strength between the variables; if there are no duplicated data, the perfect correlation is +1 or −1. This was calculated following the research of Beltrán and Martínez-Gómez [28].

## *2.7. Simulation*

The simulation of the system takes into consideration the three solids that make up the rooftop of the vehicle, which is made by steel bake hardening, YS260, and cold rolling. This material is described as used for automotive applications, such as roofs, in the software CES-Granta Edupack [31]. The next body represents a layer of material that corresponds to the 20 mm PCM made of the selected material by the MCDM and finally air fills the space; the proposed geometry is displayed in Figure 1. On the other hand, the simulation took two events, the first when the roof is heating up and the PCM is storing energy and the second where the roof is cooling down and the PCM releases the stored energy.

**Figure 1.** The CAD model.

1

#### *2.8. Boundary Conditions*

The simulation of the charging events comes with the parameters investigated by Dadour et al., who found that the temperature of a parked vehicle's black rooftop can start at 12 ◦C and rise to 45 ◦C, provoking a charge event [32]. On the other hand, after reaching this heating peak, a cooling process begins and the temperature reduces to 12 ◦C in 15 h. Where a discharging event takes place, these events are the ones that will be simulated [32]. Moreover, a convection process with a coefficient for free gases is found in the range of 2 *W m*2*K* and 25 *W m*2*K* [33]; for this reason, a middle value of 12 *W m*2*K* will be used to simulate this phenomena. It is important to point out that the initial temperature of the air is the one reached in the charging event; the boundary conditions of these simulations are displayed in Tables 3 and 4 with the mesh data of the simulation.

**Table 3.** Boundary conditions for charge.

**Table 4.** Boundary conditions for discharge.

#### **3. Results and Discussion**

*3.1. AHP Results*

The different processes are shown in the following tables, where Table 5 presents the pair-wise assessment, Table 6 displays the normalization of these and Table 7 exhibits the weighted results of the AHP method, where specific heat capacity has a more significant importance in this application since this property absorbs heat and releases it.

**Table 5.** Comparison matrix of the criteria.

#### **Table 6.** Normalized matrix of the criteria.

```
Table 7. Weighted criteria.
```

Furthermore, the proof that the method was performed correctly is displayed in Table 8, in which the multiplication of the normalized matrix with the weighted matrix is calculated, allowing to subsequently calculate the consistency index (CI), the random index (RI) and consistency relationship (CR), displayed in Table 9. In this case, the consistency is less than 10%, proving that the weights were assessed correctly [34].

**Table 8.** Matrix multiplication.

**Table 9.** Calculation of consistency.

#### *3.2. VIKOR Results*

In previous research, the VIKOR method was found to be the best methodology for selection of materials in automotive applications, since this delivers a compromise set of solutions as the result [35]. In this sense, Table 10 shows the results of the calculations for the VIKOR method where the normalization takes place, taking in consideration the previous weights and the best and worst parameters.

#### **Table 10.** VIKOR calculations.

On the other hand, the ranking is displayed in Table 11, where savENRG PCM-HS22P stands as the best, being an organic PCM that is used in the storage of great energy and which has a low cost.

**Table 11.** VIKOR ranking.

Furthermore, the verification of the method says that VIKOR performed well since Q2−Q1 = 0.285 is bigger than DQ = 0.17, showing an acceptable advantage. Furthermore, the results of Si and Ri demonstrate that the best belongs to the winner M7, meaning that there is an acceptable stability, fulfilling the two conditions that conclude that the method has an acceptable compromised solution.

#### *3.3. TOPSIS Results*

The TOPSIS method, along with AHP, plays an important part to reduce a possible selection of a wrong PCM, which was also studied in the thermal management of electronics [36]. Moreover, the development of the normalized matrix, which takes the original criteria and divides them by the square root of the quadratic summation of all the materials, is shown in Table 12. In turn, the weighted matrix that multiplies the previous criteria by the AHP weight is displayed in Table 13, followed by the beneficial and non-beneficial solutions in Table 14. Lastly, the negative and positive ideal solutions with the closeness index for the ranking result is calculated in Table 15. In these results, again savENRG PCM-HS22P is the optimum material.

**Table 12.** TOPSIS normalized matrix.

**Table 13.** TOPSIS weighted matrix.

**Table 14.** Max and min TOPSIS criteria value.

## *3.4. COPRAS Method*

The engineering fields have been benefiting from this method by enabling the decision maker to determine the overall efficiency of the different options [37]. In this way, the process of development is displayed in Table 16 with the normalized matrix that divides each position by the summation of all of them for every criterion. Table 17 presents the weighted matrix that is the result of the previous table by every weight of AHP, followed by Table 18 that shows the summation of the beneficial and non-beneficial weights. Lastly, the priority of positions with the level of performance and the COPRAS rank is presented in Table 19, where the results show that the material M7 savENRG PCM-HS22P again is established as the better material among the 15 candidates.

**Table 16.** COPRAS normalized table.

**Table 17.** COPRAS weighted matrix.

**Table 18.** COPRAS solutions.

**Table 19.** COPRAS ranking.

#### *3.5. Spearman's Correlation Results*

Table 20 shows the results of the Spearman's correlations where only the correlation COPRAS-TOPSIS has a very good relation; thus, the rest of them does not have a good relation although they indicate a positive correlation. However, it can be seen that the three methods show consistency in their results regarding the best material, as studied before [27].

**Table 20.** Spearman's correlation.

Moreover, even though the correlation shows that it is not perfect, it agrees with the fact that the material savENRG PCM-HS22P was chosen as the best, and since it is the one that will be simulated, Table 21 displays its properties.

Comparing the selection results with what Rastogi et al. had done, there is a difference where they prioritized the Figures of Merit since air conditioning systems require that the extraction of heat be primordial [25]. On the other hand, this application demands heat storage and release. Moreover, in the research of Socaciu et al., the best materials come from the same family as our best (SavEnrg PCM-Hs22P), also in thermal comfort applications in the automotive industry [38], corroborating that our selection is optimum.

#### *3.6. Simulation Results*

By reducing the difference in temperature between the internal air and body heat, the thermal comfort of the user can be controlled [39]. In this sense, the simulation with a raising temperature in the roof showed that without PCMs there is not much difference in the internal air, resulting in a high temperature in the cabin, as shown in Figure 2a. However, by using a 20 mm layer of the selected PCM, the temperature did not rise as much, and presented a decrease of near 9 ◦C, which is a congruent result compared with previous research that managed almost the same temperature degree with other PCMs [40–42]. This difference is showed in Figure 2b.

The discharging event, on the other hand, showed that while the temperature of the rooftop dropped to 12 ◦C, the internal air stood 4 ◦C warmer, meaning that the PCM maintain the internal comfort by releasing its stored energy into the cabin; this result is displayed in Figure 3.

Other researchers proved that the utilization of PCMs delayed the heat flux, making them more effective for isolation since the wall loses less energy, which helps to maintain a better temperature [9,43].

**Figure 2.** Charging the simulation comparison. (**a**) PCM 0 mm (**b**) PCM 20 mm **Figure 2.** Charging the simulation comparison. (**a**) PCM 0 mm (**b**) PCM 20 mm.

**Figure 3.** Discharge.

## **4. Conclusions**

**Figure 3.** Discharge. Starting with a list of 15 PCMs, savENRG PCM-HS22P was selected as the best by the utilization of the AHP, VIKOR, COPRAS and TOPSIS multicriteria methods. Furthermore, a Spearman's correlation showed that the methods are consistent, and the selection is optimum.

3

The simulation showed that the material savENRG PCM-HS22P improves the thermal comfort in the cabin by reducing the internal temperature of the air by 9 ◦C in the charging event, and in the discharging simulation it was also showed that the PCM allows to maintain a better temperature inside the cabin by making a differentiation of 4 ◦C warmer, both with a 20 mm layer of the selected PCM.

It is demonstrated that the use of a PCM layer of 20 mm can improve the thermal comfort in the vehicle, reducing the need to use the heater or the air conditioner.

**Author Contributions:** Conceptualization: J.M.-G. and J.F.N.; methodology: M.C., J.M.-G. and J.F.N.; software: M.C., J.M.-G. and J.F.N.; validation: M.C., J.M.-G., J.F.N., R.B.S. and E.R.; formal analysis: E.R., J.F.N. and J.M.-G.; investigation: E.R., J.F.N. and J.M.-G.; resources: E.R.; writing—original draft preparation: J.F.N.; writing—review and editing: E.R., R.B.S. and J.M.-G.; visualization: J.F.N.; supervision: J.M.-G.; project administration: J.M.-G.; funding acquisition: J.M.-G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research takes part of the project 'Selection, characterization and simulation of phase change materials for thermal comfort, cooling and energy storage'. This project is part of the INEDITA call for R&D research projects in the field of energy and materials. This research is part of the project P121819, Parque de Energias Renovables, founded by Universidad International SEK.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data reported in this research can be found at https://drive.google. com/file/d/19rI2-2r6nYN0C3Ymjiegf55bdq4jrB8Z/view?usp=sharing (accessed on 15 January 2021).

**Conflicts 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.

#### **References**


**2. Characterization of Kapton HN films**

electrical properties as well as exceptional thermal and chemical stability. However, such films feature an inert and highly hydrophobic surface. Hydrophilic (e.g. water-based) fluids (solutions, suspensions, inks, *etc*.) will ball up on such surfaces (due to "lotus effect") [1, 2]. However, for fabrication of an entire electronic device, both organic solvent- and water-based fluids are usually needed to deposit functional materials on the same substrate surface. As a result, surface modifying polyimide substrates to reduce their inherent surface hydrophobicity and/or inertness is usually needed to allow for the continuous and uniform deposition

Traditionally, polyimide substrates are surface modified with a number of methods including plasma [3, 4] and ion-beam [5, 6] etching, UV/ozone exposure [3, 7], acid [3, 8] and/or base [9, 10] treatments, and laser ablation [11, 12]. These methods, however, usually compromise the structural integrity and the properties (such as the cohesive strength, and the thermal and chemical stability) of the polyimide substrate, since they utilize relatively harsh conditions to oxidize and/or tear out part of the surface polyimide. Additionally, the wastes and by-products (such as acrolein which is extremely irritating, strong bases and acids which are corrosive, and benzene which is carcinogenic) generated from these harsh treatments can raise serious environmental and safety issues (especially when the treatments are performed indoors and/ or in large scales). For example, incubation with a sodium hydroxide solution has been one of the most common traditional methods to tune the surface properties of Kapton polyimide substrates [9, 10, 13], but such a treatment not only generates highly corrosive strong base waste but also tears out some surface polyimide resulting in pits in the Kapton surface [14]. The defects on structurally damaged Kapton films would result in not only poor deposition quality of the device components but also weakened mechanic strength of the resulting devices. The increasingly growing of flexible electronic devices (such as flexible displays [15], electronic paper [16, 17], photovoltaic cells [18, 19], sensors [1, 2, 20–23], LEDs [24], electronic textiles [25], RF tags [26], and electrochemical devices [27], *etc*.) is calling for mild and environmentally friendly surface modification approaches which can minimize the compromise to the structural integrity and the properties of Kapton polyimide substrates while efficiently tuning the surface properties of the substrates. To take full advantage of the properties of Kapton HN films, any surface modification to such films should avoid as much as possible compromising their structural integrity and properties. For extremely thin Kapton HN films, such as Kapton 30HN (thickness 7.5 μm), 50HN (thickness 12.7 μm), and 75HN (thickness 19.1 μm), it is criti-

cal to make sure that their surface modification is non- or minimally destructive.

charges on the slip additive particles.

Kapton HN films have a slip additive incorporated in the polyimide matrix to enhance their mechanical properties [13]. The nature of the additive, however, has been very scarcely reported in the literature. Williams *et al.* has mentioned, but without providing supporting data, that the additive in Kapton HN films was calcium phosphate dibasic (CaHPO4

This chapter first describes the characterization of Kapton 500HN films particularly of their slip additive, then introduces two recently-developed mild and environmentally friendly wet chemical approaches for surface modification of Kapton HN films to allow for not only great printability of both water- and organic solvent-based inks but also strong adhesion between the inkjet-printed traces and the surface modified substrates. Unlike the aforementioned traditional Kapton surface modification methods which target, and oxidize and/or tear out part of, the surface polyimide matrix, the approaches described in this chapter target the electric

) [14].

with both organic solvent- and water-based fluids.

2 Flexible Electronics

In this section, a number of characterizations were performed on as-received Kapton 500HN films (a gift from Dupont, Wilmington, DE, USA) particularly on their slip additive. The optical microscopy analysis of the films showed particles of varying sizes which were imbedded in Kapton HN polyimide matrix (**Figure 1a**). These particles have been shown to be the slip additive to the polyimide matrix of Kapton HN [14, 28]. As shown in **Figure 1a**, the majority of the slip additive particles exuded to the substrate surface, which is consistent with a previous observation [29]. The large hump (with 2θ ranging from ˜10° to ˜35°) and the sharp narrow peaks in the X-ray diffraction (XRD) pattern of the Kapton HN films (**Figure 1b**) indicated the presence of amorphous and crystalline components, respectively. Apparently the amorphous moiety was the polyimide polymer and crystalline moiety the slip additive. While calcium phosphate dibasic (CaHPO4 ) might be present in the additive as previously reported [14], the crystalline peaks in the XRD pattern of the Kapton HN films matched very well with those of calcium carbonate (CaCO3 ) (ICDD reference code 04–001-7249) (**Figure 1b**) but did not match any of the CaHPO4 peaks. Significant carbon and oxygen peaks and a small calcium peak showed up in the energy dispersive X-ray spectroscopy (EDX) pattern of the Kapton substrate (**Figure 1c**).

To better characterize the slip additive in Kapton HN films, efforts were made to minimize the interference from the polyimide polymer matrix. Kapton HN films were fired at 800°C for 2 hours in air (this firing treatment has been shown to be efficient to pyrolyze the entire polyimide polymer moiety in Kapton HN [30]) to remove the polyimide polymer. The remaining inorganic components were characterized with scanning electron microscopy (SEM), EDX, and XRD analyses.

**Figure 1.** Kapton HN film characterization. (a) Optical microscopy analysis of a blank Kapton HN film. (b) XRD analysis of the specimen shown in (a) (pattern ①) and reference CaCO3 (pattern ②, ICDD reference code 04–001-7249) (inset: Locally enlarged XRD pattern to show the area with a 2θ of from 50° to 100°). (c) EDX analysis of the specimen shown in (a) (inset: Locally enlarged EDX pattern to show the calcium peak) [2] (licensed under creative commons attribution 4.0 international license).

As shown in **Figure 2a** and **b**, the size of the ash particles varied significantly, from less than 100 nm to several microns, with the large particles probably resulting from the sintering and agglomeration of the fine ones [14]. The EDX analysis of the ash showed the presence of the elements of oxygen, calcium, and phosphorus (**Figure 2c**). The XRD analysis of the ash showed the presence of only calcium pyrophosphate Ca2 P2 O7 (ICDD Reference code 04–009-6231) (**Figure 2d**). Compared with the XRD pattern of the as-received Kapton HN films (**Figure 1b**), the XRD pattern of the pyrolyzed films (**Figure 2d**) indicated the disappearance of CaCO3 and the presence of Ca2 P2 O7 . The composition change was due to the multiple chemical reactions (such as the decomposition of CaCO3 into CaO and CO2 [31]) taken place during the pyrolyzing process. Combining **Figure 1** (characterization of Kapton HN) and **Figure 2** (characterization of the Kapton HN ash resulted from pyrolysis), we can conclude that the additive in Kapton HN was composed of CaCO3 (crystalline) and one or more phosphorus-containing compounds (crystalline or amorphous). Any calcium phosphate compounds, if present as previously reported [14] in the additive, must be either crystalline but in a small amount (i.e. beyond the detection limit of the diffractometer used for the XRD analyses) or amorphous, or both. While the exact nature of the additive in Kapton HN is probably proprietary

and unknown to the public, XRD analyses showed that crystalline CaCO3

**3. A bio-enabled maximally mild layer-by-layer Kapton surface**

CaCO3

HN films.

chloride.

**modification approach**

positively charged additive particles (e.g. CaCO3

charged protamine and heparin molecules.

**3.1. Surface modification of Kapton HN films**

was rinsed with DI water and dried in air at 60°C for 2 hours.

additive in a significant amount (**Figure 1b**). With an isoelectric point of 8.2 [32], crystalline

Protamine has been clinically used to reverse the anticoagulant effects of heparin by binding to it [33, 34]. The development of the present bio-enabled surface modification approach was inspired by the *in vivo* antagonizing interaction of these two clinically used biological molecules. In this surface modification process, negatively charged heparin and positively charged protamine were used to uniformly deposit a thin film of protamine-heparin complex on Kapton HN substrates in a layer-by-layer fashion. The surface modification process was conducted under maximally mild conditions (in aqueous solutions of clinical biomolecules, and at a neutral pH, room temperature and atmospheric pressure). During the process the

binding of the initial heparin (negatively charged) layer via electrostatic interaction. After the initial binding of heparin, the layer-by-layer uniform deposition of the protamine-heparin complex on Kapton HN was realized by the electrostatic interaction between the oppositely

As far as we know, the present bio-inspired method was the first to use environmentally friendly clinical biomolecules for substrate surface modification. It is also the first surface modification approach performed under maximally mild and minimally destructive conditions.

 After a brief rinse with a phosphate buffer (0.2 M, pH 7.0), the Kapton piece was incubated for 10 min with a heparin sodium solution (10 mg/ml, pH 7.0) in the phosphate buffer followed by rinsing three times with the phosphate buffer. The Kapton piece was then incubated for 10 min with a protamine sulfate solution (10 mg/ml, pH 7.0) in the phosphate buffer followed by rinsing three times with the phosphate buffer. This process (heparin/rinse/protamine/rinse) was performed for a total of 5 times. Finally, the Kapton piece

A control surface modification process was conducted to validate the hypothesis that the surface modification process was facilitated by the positive electric charges on the Kapton HN surface. The control process was similar to the standard process described earlier, except that the heparin solution used in each deposition cycle was supplemented with 1 M sodium

 bears positive charges at a neutral or acidic pH. The two recently developed mild and environmentally friendly wet chemical approaches described below both target the surface electric charges borne by the additive particles imbedded in the polyimide matrix of Kapton

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices

was present in the

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http://dx.doi.org/10.5772/intechopen.76450

particles) on the Kapton HN surface enabled

**Figure 2.** Characterization of the Kapton HN ash resulted from the pyrolysis of Kapton HN films at 800°C for 2 hours in air. (a) and (b) SEM images of the ash with low (a) and high (b) magnifications. (c) EDX pattern of the ash. (d) XRD patterns of both the ash (pattern ①) and reference calcium pyrophosphate Ca2 P2 O7 (pattern ②. ICDD reference code 04–009-6231) [2] (licensed under creative commons attribution 4.0 international license).

and unknown to the public, XRD analyses showed that crystalline CaCO3 was present in the additive in a significant amount (**Figure 1b**). With an isoelectric point of 8.2 [32], crystalline CaCO3 bears positive charges at a neutral or acidic pH. The two recently developed mild and environmentally friendly wet chemical approaches described below both target the surface electric charges borne by the additive particles imbedded in the polyimide matrix of Kapton HN films.


*4.2. Practical Application and Analysis of the Implementation of the Second Algorithm for Checking the Stability*

Using the second algorithm for checking the stability, the elements of the pairwise comparison matrix are changed by one. The stability of the method is also set by the different numbers of iterations: 100, 10,000, and 100,000. The value of the relative errors *δ ξ* of the values of the criteria weights is fixed. In each case, ten attempts are made to fix the range of the relative errors. In the first case, the matrix data with a good consistency index are used (Table 1). Table 9 shows the results of ten attempts to check the stability of the method, and the largest relative errors *δ ξ* , with the number of iterations equal to 100, using the first matrix data (Table 1). The results show small *δ ξ* values of the largest relative errors of the weights of the criteria. Comparing the results for the relative errors of the weights of the criteria (Table 10) with the results of the first algorithm (a deviation of 10%) (Table 3), the results have increased on average by 23%.

**Table 9.** The largest relative errors of the criteria weights *δ ξ* , when checking the stability by the second algorithm, 100 iterations, 10 repetitions, 1st matrix.

**Table 10.** The interval of the largest relative errors of the criteria weights *δ ξ* , when checking the stability by the second algorithm, 10 repetitions, 1st matrix.

Checking the consistency of the first matrix, the results of checking the second algorithm for the largest relative errors *δ ξ* (Table 10) are less than with a 10% data deviation, but more than with a 5% deviation. With the number of iterations equal to 100,000, the intervals of the criteria are practically narrowed to one value.

When checking the consistency of the matrices using the first matrix, all the other generated matrices are consistent, in contrast to the second matrix, in which 55–70% of the matrices are inconsistent and discarded (Table 11).

**Table 11.** The percentage of consistent matrices when checking the stability by the second algorithm using the data of the first and second matrices.

The largest relative error *δ ξ* of the results for the second matrix is small (Table 12). The results turn out to be better than when using the first matrix and the same algorithm. This can be explained by the smaller number of consistent matrices used (30–45%).

**Table 12.** The interval of the largest relative errors of the criteria weights *δ ξ* , when checking the stability by the second algorithm, 10 repetitions, 2nd matrix.

In both verification cases, the results show the stability of the method: *δ* = 0.5121 (using the first matrix) and *δ* = 0.7739 (using the second matrix).


**Rodrigo Nogueira de Sousa**

**1**

Section 1

The Role of Mycorrhizae

Fungi in Plant Growth

Department of Soil Science and Plant Nutrition, University of São Paulo, Luiz de Queiroz College, Piracicaba, Brazil Section 1