**3. Sensory feedback by electrical stimulation**

### **3.1. Purpose**

(a)Naive (b)Neuron transplantation (c)Surgical control

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

**Figure 3.** Controlled reproduction of ankle motion and measurements of dorsiflexed angles. The ankle motions were restored with electrical stimulation of the peroneal nerve in neural transplantation group (A) as compared to surgical

The transplanted rat embryonic motoneurons could survive in the rat peripheral nerve, and transplanted motoneurons formed functional motor units. Transplantation of motoneurons into peripheral nerve provided the reproduction of ankle dorsiflexion. The transplantation of embryonic neurons into peripheral nerve could restore ankle motions in conjunction with electrical stimulation, even though no neural connection between central nervous system and

Althoughallratswere able toflex their ankle,the smallnumberof axonswaspresentinthe tibial nerves of cell transplantation group as compared with naive group. Endoneurial environment in peripheral nerve may not provide suitable condition for motoneurons. Transplanting

**Figure 2.** Myelinated axons in tibial nerve were assessed with toluidine blue stain.

control group (B).

Biomedical Engineering

18

**2.4. Discussion**

muscle were present.

The purpose of this section is to show the possibility of sensory feedback in nueroprosthetic devices. If we have a way to send the signals from the assembled sensors to sensory nervous

system, leading neuroprosthetic devices with two‐way communication will be achievable. In this Section, we show a preliminary study on the possibility of sensory feedback via axial fibers of peripheral sensory neurons. We have tested several patterns of electrical signals and the placements of surface electrodes for carrying sensory information to brain via axial fibers of neurons. In a preliminary experiment with one channel electrical stimulation, the amplitude modulation of the stimulation voltage achieves only 2 bit resolution. Then, we increase the channels and take a method of frequency modulation of the stimulation electrical signal.

The applied voltages were tuned for each subject in similar way for the good identifiabil‐ ity. The electrical stimulation of surface electrodes with about 2000 Hz gives some feeling of vibration around at the electrodes without any pain. The stimulation frequency of the channel (C, D) was set at five frequencies: 2000 Hz, 2001 Hz, 2002 Hz, 2004 Hz and 2010 Hz. The change of stimulation frequency on the channel (C, D) causes different sensa‐ tions to the subjects. More specifically, the subjects sense the frequency difference be‐ tween the two channels in addition to the base frequency 2000 Hz. In other words, the subjects sense the interference potential of the pair electrical potentials. For an example, 1 Hz sensation will be felt by the subject if the frequencies of 2000 Hz and 2001 Hz are selected. We applied electrical stimulations of 2000 Hz to a subject by using the pair of the electrodes (A, B), and set the same and different frequencies for the pair of the electro‐ des (C, D) for the discrimination experiments. The phase shift was zero degree when the both frequency were set at 2000 Hz. After enough trials for each combination of the stimulation frequencies, each subject carried out 40 trials of the discrimination. In the trials, all combinations of frequencies were presented in random order. Five subjects were asked

Neural Interfaces: Bilateral Communication Between Peripheral Nerves and Electrical Control Devices 21

The average rate of the correct answers over 40 tests per subject was 90%. All errors were occurred in the cases of nearest frequencies difference of the two pairs electrodes. This results show that the difference of the two channel stimulation frequencies is key point for the identifiability of stimulations. The bigger difference in stimulation frequencies, the higherrate

The simple amplitude modulation of one channel electrical stimulation achieves only 2 bits resolution for transferring sensory information to brain. On the other hand, frequency modulation with interference potential of the pair electrical potentials has achieved a high resolution which corresponds to the number of combination of given different stimulation frequencies. For an example, we can obtain 3.5 bits (1 digit in decadal system) resolution if we use five different frequencies. We can increase number of the channels; thus, interference potential of the pair electrical signals with more than two pairs of electrodes holds forth the possibility that we can achieve a high resolution for sensory feedback via axial fibers of

Further research on multi‐channel electrodes, specially more than three channels, will be required to clarify the effect of multiple interference of stimulation frequencies on the sensory resolution of transferring the information. The frequency range which is most appropriate for transferring sensory information should be also clarified. It is common that electrical stimulations with a certain frequency band involve a pain of the subject. Appropriate frequency bands should be identified not only to achieve a high resolution,

to identify the combinations of the electrical stimulations.

of the correct answers will be achieved.

**3.3. Results**

**3.4. Discussion**

peripheral sensory neurons.

but to avoid subjectʹs pain.

### **3.2. Methods**

One pair of surface electrodes, one anode and one cathode, is usually used for one channel electrical stimulation. The two pairs of surface electrodes in our experiments are shown in Figure 4a. The placements of the electrodes were well controlled for each experiment to certify the repeatability based on the markers on skin shown in Figure 4b. We set the two pairs of electrodes (A, B) and (C, D), and made the electrical current flow from A to B or from C to D in each pair. The duty ratio of stimulation voltages was 5%, and the frequency was set at 2000 Hz for the channel (A, B). The duty ratio 5% was defined because we found it looks good for all five subjects to identify the stimulation frequency in several trials with one channel stimulation.

**Figure 4.** The electrodes placement

The applied voltages were tuned for each subject in similar way for the good identifiabil‐ ity. The electrical stimulation of surface electrodes with about 2000 Hz gives some feeling of vibration around at the electrodes without any pain. The stimulation frequency of the channel (C, D) was set at five frequencies: 2000 Hz, 2001 Hz, 2002 Hz, 2004 Hz and 2010 Hz. The change of stimulation frequency on the channel (C, D) causes different sensa‐ tions to the subjects. More specifically, the subjects sense the frequency difference be‐ tween the two channels in addition to the base frequency 2000 Hz. In other words, the subjects sense the interference potential of the pair electrical potentials. For an example, 1 Hz sensation will be felt by the subject if the frequencies of 2000 Hz and 2001 Hz are selected. We applied electrical stimulations of 2000 Hz to a subject by using the pair of the electrodes (A, B), and set the same and different frequencies for the pair of the electro‐ des (C, D) for the discrimination experiments. The phase shift was zero degree when the both frequency were set at 2000 Hz. After enough trials for each combination of the stimulation frequencies, each subject carried out 40 trials of the discrimination. In the trials, all combinations of frequencies were presented in random order. Five subjects were asked to identify the combinations of the electrical stimulations.

#### **3.3. Results**

system, leading neuroprosthetic devices with two‐way communication will be achievable. In this Section, we show a preliminary study on the possibility of sensory feedback via axial fibers of peripheral sensory neurons. We have tested several patterns of electrical signals and the placements of surface electrodes for carrying sensory information to brain via axial fibers of neurons. In a preliminary experiment with one channel electrical stimulation, the amplitude modulation of the stimulation voltage achieves only 2 bit resolution. Then, we increase the channels and take a method of frequency modulation of the stimulation electrical signal.

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

One pair of surface electrodes, one anode and one cathode, is usually used for one channel electrical stimulation. The two pairs of surface electrodes in our experiments are shown in Figure 4a. The placements of the electrodes were well controlled for each experiment to certify the repeatability based on the markers on skin shown in Figure 4b. We set the two pairs of electrodes (A, B) and (C, D), and made the electrical current flow from A to B or from C to D in each pair. The duty ratio of stimulation voltages was 5%, and the frequency was set at 2000 Hz for the channel (A, B). The duty ratio 5% was defined because we found it looks good for all five subjects to identify the stimulation frequency in several trials with one channel

**3.2. Methods**

Biomedical Engineering

20

stimulation.

**Figure 4.** The electrodes placement

The average rate of the correct answers over 40 tests per subject was 90%. All errors were occurred in the cases of nearest frequencies difference of the two pairs electrodes. This results show that the difference of the two channel stimulation frequencies is key point for the identifiability of stimulations. The bigger difference in stimulation frequencies, the higherrate of the correct answers will be achieved.

#### **3.4. Discussion**

The simple amplitude modulation of one channel electrical stimulation achieves only 2 bits resolution for transferring sensory information to brain. On the other hand, frequency modulation with interference potential of the pair electrical potentials has achieved a high resolution which corresponds to the number of combination of given different stimulation frequencies. For an example, we can obtain 3.5 bits (1 digit in decadal system) resolution if we use five different frequencies. We can increase number of the channels; thus, interference potential of the pair electrical signals with more than two pairs of electrodes holds forth the possibility that we can achieve a high resolution for sensory feedback via axial fibers of peripheral sensory neurons.

Further research on multi‐channel electrodes, specially more than three channels, will be required to clarify the effect of multiple interference of stimulation frequencies on the sensory resolution of transferring the information. The frequency range which is most appropriate for transferring sensory information should be also clarified. It is common that electrical stimulations with a certain frequency band involve a pain of the subject. Appropriate frequency bands should be identified not only to achieve a high resolution, but to avoid subjectʹs pain.

segments. It is possible to extend this type of neuroprosthetic system with portable sensors

Neural Interfaces: Bilateral Communication Between Peripheral Nerves and Electrical Control Devices 23

The purpose of this section is to conduct walking simulation which will play an importantrole of our succedent neuroprosthetic system. The basic concept of the neuroprosthetic system is shown in Figure 5. The role is to check whether the designed artificial CPG controller works adequately or not, and whether the sensing system is able to provide necessary information forthe controllerʹs adapting to the environmental changes. Thus, the walking simulation given in this section means the preliminary study of motion planning and control for our succedent

The neuro‐musculo‐skeletal model of rat is composed of a rigid link system, joint torque generators, neural controllers of CPGs with the upper level triggers. The inertial properties of the entire body are represented by a three dimensional 17‐rigid‐link system with 16 joints. All the joints are one degree‐of‐freedom, and the axes of the joints are perpendicular to sagittal plane. The schematic views in sagittal and horizontal planes are given in Figure 6. The dappled

A viscoelastic passive moment induced by soft tissues and a passive moment of nonlinear elasticity induced by tendons and bones assumed to act on each joint. The characteristic is

*k k*

parameters are constants representing the joint characteristics. These parameters were determined from the estimations for human joints [23]. The inertia properties of each body segment, such as the mass and the moment of inertia, were estimated from the size of anato‐

(a)view in sagittal plane (b)view in horizontal plane

x

50mm

(1)

*k c*

*i* is the *i*‐th joint angular velocity variable, and another

x

z

y

1 23 4 5 6 , exp exp *J JJ J J J J i i i ii i i i i i i i i ii passive k k k*

.

mized each body segment with the average density of rat body [24].

y

z

110mm

and other electrical devices to more practical one for disabled persons.

neuroprosthetic system.

given by

60mm

**4.2. Neuro‐musuculo‐skeletal model**

**Figure 6.** Assumed skeletal system of rat.

where *θi* is the *i*‐th joint angle variable, *θ*

circles indicate the center of mass of each segment.


**3. Types of pressure-sensitive adhesives and applications**

Self-adhesive materials are called, in the adhesives trade, .pressure-sensitive adhesives. PSAs are designed to stick on almost any surface by simple contact under light pressure. Among the different classes of adhesives, pressure-sensitive adhesives (PSAs) are perhaps the most common type found in consumer products. Selfadhesive tapes and labels of all kinds are ubiquitous in everyday life. Despite the fact that PSAs are supposed to adhere to substrates, they are different from other adhesives in many ways. First, PSAs are mainly nonstructural adhesives; they cannot be compared with epoxies for structural applications. Moreover, PSAs mainly adhere to a surface upon contact without any chemical reaction [6, 15, 23, 26].

Pressure-sensitive adhesives can be classified by the type of ingredients used to make them (e.g., rubber-based, acrylic and silicone) and by their state (e.g., organic solvent solution, emulsion, solid (hot melt) and aqueous solution) (**Figure 12**).

Emulsions are dispersion of particles containing acrylic polymer in water (latexes). Labels and Packaging Materials (LPM) has manufacturing infrastructures in USA and Europe and wide manufacturing companies to provide latexes around the world. There are also acrylic polymers in petroleum-based solvents—usually of low to moderate solids content. End users prefer the performance of solvent based acrylic polymers although they are more expensive than their water borne counterparts (latexes) [7, 15]. Hot melt PSAs are based on block copolymers. Oils, plasticizers and tackifiers are the necessary components of the formulation needed to obtain appropriate performance. Most hot melts (HM) are dependent on C5/C9 streams from petroleum distilleries. The other PSA acrylic grade UV compositions, which are solventless, produce films with higher thickness, including foam-like adhesives. The two-stage process creating performance advantages in the second stage of curing gives the opportunity of very high molecular weight reactions which can't be cast from solvent. Warm melts (100% solids materials) are either made in solvent and stripped (AC resin) or polymerized to near 100% and coated. They use hot-melt equipment (high coating speeds) and give performance of solvent benefits—high coating speed and solvent-like performance simultaneously.

**Figure 12.** *Different types of PSAs by their state.*

PSA are constructed based on these elements as follows:

Common structure of different tapes and labels are presented in **Figure 13**. For commercially used PSA systems with excellent performance levels, synthetic polymers based on acrylics, silicones, polyurethanes, and polyesters are preferred (**Figure 14**) [27].

From historical point of view, the first PSAs were rubber based. They are still the cheapest to produce and also the simplest to formulate for they are typically compounds of natural rubber and tackifier which is miscible with the rubber in approximately equal proportions. First versions of rubber based PSAs were not cross-linked, but today a cross-linking step is added to avoid flow [1, 6, 22, 23, 26].

Types of rubber based PSAs are as follows:

Acrylic PSAs are the best choice for optimization and formulation using copolymerization of a long side-chain acrylic such as *n*-butyl acrylate (BA) or 2 ethylhexyl acrylate (2-EHA) with a low glass transition temperature *Tg* (soft monomers), a short side-chain acrylic such as methyl methacrylate (MMA) (hard monomer) to

**99**

*Pressure-Sensitive Adhesive Joints*

colloidal stability [28].

*Commercially used PSA systems.*

**Figure 13.**

**Figure 14.**

solvent-free acrylic PSA (**Figure 15**) [7, 15, 27].

tance for securing operating theatre sheets [27, 29–31].

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Common structure of different tapes and labels.*

adjust the *Tg*, and acrylic acid to improve adhesion, optimize elongational properties (their mechanical response to deformation in uniaxial extension) and control

Acrylates and other suitable monomers such as vinyl acetate, acryl amide, and acrylonitrile are copolymerized to synthesize an acrylic copolymer with optimized adhesion properties. Crosslinking agents are also added for cohesion improvement. Acrylics can be synthesized in organic solvents in which no further formulation happens except for the time it is needed to modify their properties. Acrylics can also be synthesized in water (water borne) but here it is needed to add materials called emulsifiers to make the polymer dispersible in water. The third group of acrylics is

Acrylic latex based PSAs are used primarily in the production of different kinds of tapes such as industrial single-sided, double-sided, and transfer adhesive tapes. Another important application of emulsion pressure-sensitive adhesives resides in the manufacture of water-soluble labels. The newest application of them is in the medical products in which neutral electrodes and adhesive tapes are of high impor-

An issue in the field of PSAs is to give them some water solubility depending on the hydrophilic properties of the groups incorporated into the polymer chain. The *Tg* of the water-soluble monomers (**Figure 16**) is the main property for the water solubility and also adhesive properties of the synthesized copolymers [28].

In spite of advantages of water borne acrylic PSAs, the solvent-borne counterparts have been marginally preferred in high-performance applications because of their superior adhesive performance at high temperatures (**Figure 17**). The latter

#### **Figure 13.**

*Adhesives and Adhesive Joints in Industry Applications*

(ethylene-vinyl-acetate).

permanently with a load.

(**Figure 14**) [27].

ening, maintaining a barrier, etc.

Types of rubber based PSAs are as follows:

of styrene domains such as SIS and SBS.

tackifier for enhancement of the adhesion strength.

its vulcanized properties.

PSA are constructed based on these elements as follows:

• **Adhesive:** Typical adhesives are acrylic, natural rubber, or synthetic rubber. Special adhesives with a smaller application base are silicone, butyl, or EVA

• **Backings:** A backing or carrier is a relatively thin, flexible, web based film. Backings can be a paper, film, foil, foam, fabrics, nonwovens, or laminates which are surely chosen based on the application—such as cushioning, damp-

• **Primer:** A primer is used to increase adhesion of the adhesive to the backing and can be a liquid substance coated onto the backing surface, or a physical treatment, such as corona or flame treatment. This stage is of high importance if the tape needs to be removed cleanly, or if the tape is subjected to stress

• **Release Coating:** Release coating is a treatment process which should be applied on the backing of a single-sided tape, and the release liner for a double-sided tape. This process ensures stable unwinding of the tape during the application especially for automatic applications in which the tape can be unwound at high speeds. Different release forces can be used for either easy unwinding or harder unwinding, depending on the customer's needs.

Common structure of different tapes and labels are presented in **Figure 13**. For commercially used PSA systems with excellent performance levels, synthetic polymers based on acrylics, silicones, polyurethanes, and polyesters are preferred

From historical point of view, the first PSAs were rubber based. They are still the cheapest to produce and also the simplest to formulate for they are typically compounds of natural rubber and tackifier which is miscible with the rubber in approximately equal proportions. First versions of rubber based PSAs were not cross-linked, but today a cross-linking step is added to avoid flow [1, 6, 22, 23, 26].

• Natural rubber: They must be cross-linked for increasing cohesive strength.

• Reclaimed rubber: This rubber is used to coat the calender surfaces considering

• SBR: This grade must be combined with natural rubber and a small volume of

• Block copolymer: It should be plasticized by heating process via the formation

• Polyisobutylene: This rubber is a blend of high and low molecular weight resins

Acrylic PSAs are the best choice for optimization and formulation using copolymerization of a long side-chain acrylic such as *n*-butyl acrylate (BA) or 2 ethylhexyl acrylate (2-EHA) with a low glass transition temperature *Tg* (soft monomers), a short side-chain acrylic such as methyl methacrylate (MMA) (hard monomer) to

made by partial crosslinking of butyl rubber and addition of fillers.

**98**

*Common structure of different tapes and labels.*

adjust the *Tg*, and acrylic acid to improve adhesion, optimize elongational properties (their mechanical response to deformation in uniaxial extension) and control colloidal stability [28].

Acrylates and other suitable monomers such as vinyl acetate, acryl amide, and acrylonitrile are copolymerized to synthesize an acrylic copolymer with optimized adhesion properties. Crosslinking agents are also added for cohesion improvement. Acrylics can be synthesized in organic solvents in which no further formulation happens except for the time it is needed to modify their properties. Acrylics can also be synthesized in water (water borne) but here it is needed to add materials called emulsifiers to make the polymer dispersible in water. The third group of acrylics is solvent-free acrylic PSA (**Figure 15**) [7, 15, 27].

Acrylic latex based PSAs are used primarily in the production of different kinds of tapes such as industrial single-sided, double-sided, and transfer adhesive tapes. Another important application of emulsion pressure-sensitive adhesives resides in the manufacture of water-soluble labels. The newest application of them is in the medical products in which neutral electrodes and adhesive tapes are of high importance for securing operating theatre sheets [27, 29–31].

An issue in the field of PSAs is to give them some water solubility depending on the hydrophilic properties of the groups incorporated into the polymer chain. The *Tg* of the water-soluble monomers (**Figure 16**) is the main property for the water solubility and also adhesive properties of the synthesized copolymers [28].

In spite of advantages of water borne acrylic PSAs, the solvent-borne counterparts have been marginally preferred in high-performance applications because of their superior adhesive performance at high temperatures (**Figure 17**). The latter

**Figure 15.** *Acrylic PSAs different types.*

**101**

*Pressure-Sensitive Adhesive Joints*

polymer chain (**Figure 19**) [36].

**Figure 17.**

lamps in the production line.

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

Structural bonding tape (Structural PSAs) is an acrylic pressure-sensitive tape impregnated with an epoxy resin being cured under heat. This adhesive is cured in an oven at about 145°C for 20 min. The composition of structural PSA in which we have inherently pressure-sensitive before the curing process is a combination of soft monomers of acrylate with low *Tg* and hard monomers of epoxy with high *Tg* in the

*Applications of solvent based PSAs such as examples of custom applications for sealing, gasketing, shielding and sound insulation, and foam tapes and adhesives are used in automotive or other industrial applications where*

*strength, heat, or moisture resistance is required (close-up application of heated mirror).*

Pressure-sensitive thermosetting adhesive based on acrylic ester/epoxy resin is an initially tacky and conformable thermosetting adhesive produced by blending comprising epoxy resin or a mixture of about 20–60 wt. % epoxy resin and 0, 5–10 wt. %

The other PSA grade which is currently available for industrial application is crosslinkable acrylic hot melt PSAs (HMPSAs) designed to be cross-linked by a UV-lamp or UV-laser light. HMPSAs propose advantages such as relatively low application viscosity at higher temperatures, high processing speeds, aggressive tack, high resistance to plasticizers and solvents, and too high heat resistance after curing process. There are chemically built-in photoreactive functional groups on acrylic chains in UV-acrylic hot melts. UV grades are solvent free and are processed as hot melts. The adhesive film is cross-linked with UV radiation after the acrylic film has been coated with the

It is easy to coat heavy depositions of UV grades of acrylic PSA at a good speed since these are 100% solid adhesives. Using photoreactive diluents viscosity can be modified within certain limits and also, the UV dose can be controlled by adjusting the power of UV lamps and the speed at which the substrate is passed under the

hardener being activated by heating for hardening of the epoxy resins [36].

UV-acrylic hot melt for self-adhesive material production [28, 33].

#### **Figure 16.**

*Structure of emulsion acrylic based PSAs.*

one have much higher shear strength, together with higher peel and tack properties, for the aforementioned applications considering the continuous network morphology made by physical entanglements of acrylic chains in the solvent-borne adhesives when the solvent has evaporated [32]. In comparison, the microgels in the latex particles retain their discrete morphology in the adhesive film [33]. This morphology decreases the shear strength of the latex based PSAs because of the poor interconnections between the microgels and the linear polymers in the structure of the film [34, 35].

UV PSAs have the characteristic of being a light-cured acrylic adhesive (thermoset). UV PSAs are nonflammable and solvent free. UV adhesives offer advantages of lower VOCs and better environmental compliance. In the case of saturated acrylic pressure-sensitive adhesives, the photogeneration of initiator radicals by α-cleavage photoinitiators or H-abstraction photoinitiators is followed by a reaction with the acrylic chain that produces a new radical reacting with a neighboring acrylic chain (**Figure 18**) [27].

Totally, acrylic PSA categories are as follows: Water-soluble In this grade, monomers having carboxyl groups are copolymerized. Non–cross-linked

In this one, monomers having low and high Tg are copolymerized to produce a PSA.

Cross-linked

Here, a low Tg monomer is cross-linked by blending with a crosslinking agent.

**Figure 17.**

*Adhesives and Adhesive Joints in Industry Applications*

one have much higher shear strength, together with higher peel and tack properties, for the aforementioned applications considering the continuous network morphology made by physical entanglements of acrylic chains in the solvent-borne adhesives when the solvent has evaporated [32]. In comparison, the microgels in the latex particles retain their discrete morphology in the adhesive film [33]. This morphology decreases the shear strength of the latex based PSAs because of the poor interconnections between the microgels and the linear polymers in the struc-

UV PSAs have the characteristic of being a light-cured acrylic adhesive (thermoset). UV PSAs are nonflammable and solvent free. UV adhesives offer advantages of lower VOCs and better environmental compliance. In the case of saturated acrylic pressure-sensitive adhesives, the photogeneration of initiator radicals by α-cleavage photoinitiators or H-abstraction photoinitiators is followed by a reaction with the acrylic chain that produces a new radical reacting with a neighboring acrylic chain

In this grade, monomers having carboxyl groups are copolymerized.

In this one, monomers having low and high Tg are copolymerized to produce a

Here, a low Tg monomer is cross-linked by blending with a crosslinking agent.

**100**

PSA.

ture of the film [34, 35].

*Structure of emulsion acrylic based PSAs.*

**Figure 16.**

**Figure 15.**

*Acrylic PSAs different types.*

(**Figure 18**) [27].

Water-soluble

Cross-linked

Non–cross-linked

Totally, acrylic PSA categories are as follows:

*Applications of solvent based PSAs such as examples of custom applications for sealing, gasketing, shielding and sound insulation, and foam tapes and adhesives are used in automotive or other industrial applications where strength, heat, or moisture resistance is required (close-up application of heated mirror).*

Structural bonding tape (Structural PSAs) is an acrylic pressure-sensitive tape impregnated with an epoxy resin being cured under heat. This adhesive is cured in an oven at about 145°C for 20 min. The composition of structural PSA in which we have inherently pressure-sensitive before the curing process is a combination of soft monomers of acrylate with low *Tg* and hard monomers of epoxy with high *Tg* in the polymer chain (**Figure 19**) [36].

Pressure-sensitive thermosetting adhesive based on acrylic ester/epoxy resin is an initially tacky and conformable thermosetting adhesive produced by blending comprising epoxy resin or a mixture of about 20–60 wt. % epoxy resin and 0, 5–10 wt. % hardener being activated by heating for hardening of the epoxy resins [36].

The other PSA grade which is currently available for industrial application is crosslinkable acrylic hot melt PSAs (HMPSAs) designed to be cross-linked by a UV-lamp or UV-laser light. HMPSAs propose advantages such as relatively low application viscosity at higher temperatures, high processing speeds, aggressive tack, high resistance to plasticizers and solvents, and too high heat resistance after curing process. There are chemically built-in photoreactive functional groups on acrylic chains in UV-acrylic hot melts. UV grades are solvent free and are processed as hot melts. The adhesive film is cross-linked with UV radiation after the acrylic film has been coated with the UV-acrylic hot melt for self-adhesive material production [28, 33].

It is easy to coat heavy depositions of UV grades of acrylic PSA at a good speed since these are 100% solid adhesives. Using photoreactive diluents viscosity can be modified within certain limits and also, the UV dose can be controlled by adjusting the power of UV lamps and the speed at which the substrate is passed under the lamps in the production line.

**Figure 18.** *General examples of the chemical structure of unsaturated photoinitiators [27].*

#### **Figure 19.**

*The polymer morphology of a structural acrylic adhesive.*

Pressure-sensitive hot melt adhesives are thermoplastic rubber-based adhesives. They are characterized by the fact that they do not fully set, instead remaining permanently sticky. This allows for an excellent adhesive bond even when the glue is cold. In this case, the required contact pressure is the key to creating a sufficient coat between the contact adhesive and the joined part, which is why this group of adhesives is also referred to as "PSA-pressure-sensitive adhesives" (**Figure 20**).

Styrenic block copolymers based PSAs are the latest type which are introduced into the market. They are generally blends of triblocks of styreneisoprene-styrene (SIS) and styreneisoprene diblocks blended with a low-molecular-weight and high-*Tg* resin based on C5 rings that is miscible with the isoprene phase and not miscible with the styrene phase. It is compulsory to state that a tackifying resin is a necessary component of approximately any class of PSA [26].

Silicone pressure-sensitive adhesives are high-performance adhesives that can be utilized over a wide range of temperatures, from −40 to 300°C (**Figure 21**).

They bond to both low energy and high-energy surfaces. Silicone polymers based on gum contain dimethylsiloxy and diphenylsiloxy groups (**Figure 22**) [37].

The molecular weights of solvent-borne silicones are preferably in the range from 500,000 to 1,500,000 Daltons [38]. The crosslinking of silicone PSAs containing methyl or/and phenyl groups can be achieved thermally between 120 and 150°C using organic peroxide.

With respect to performance, the characteristics of PSA are generally determined by their ingredients. The insert to the right gives a brief summary based on ingredient type [1, 4, 13, 14].

**103**

**Figure 22.**

*Silicone PSA structure.*

*Pressure-Sensitive Adhesive Joints*

labels, films).

properties).

**Figure 20.** *PSA hot melt.*

**Figure 21.**

*Silicone PSA application in electronic circuits.*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

The main applications are as follows:

Temporary bonding (re-bonding not required; easy removal preferred). Bonding "hard to bond adhered (e.g., PP, PE, PTFE, and EPDM).

It's important to select the appropriate type of PSA for your needs.

Temporary bonds (e.g., adhesive processing of nameplates, adhesive coatings on

Primary processing of adhesive components (e.g., double-sided adhesive tape). Repeated peeling and adhesion (requiring enhanced re-adhesion and peeling

#### *Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

The main applications are as follows:

Temporary bonding (re-bonding not required; easy removal preferred).

Bonding "hard to bond adhered (e.g., PP, PE, PTFE, and EPDM).

Temporary bonds (e.g., adhesive processing of nameplates, adhesive coatings on labels, films).

Primary processing of adhesive components (e.g., double-sided adhesive tape). Repeated peeling and adhesion (requiring enhanced re-adhesion and peeling properties).

It's important to select the appropriate type of PSA for your needs.

**Figure 20.** *PSA hot melt.*

*Adhesives and Adhesive Joints in Industry Applications*

*General examples of the chemical structure of unsaturated photoinitiators [27].*

Pressure-sensitive hot melt adhesives are thermoplastic rubber-based adhesives.

Silicone pressure-sensitive adhesives are high-performance adhesives that can be

With respect to performance, the characteristics of PSA are generally determined by their ingredients. The insert to the right gives a brief summary based on

utilized over a wide range of temperatures, from −40 to 300°C (**Figure 21**). They bond to both low energy and high-energy surfaces. Silicone polymers based on gum contain dimethylsiloxy and diphenylsiloxy groups (**Figure 22**) [37]. The molecular weights of solvent-borne silicones are preferably in the range from 500,000 to 1,500,000 Daltons [38]. The crosslinking of silicone PSAs containing methyl or/and phenyl groups can be achieved thermally between 120 and 150°C

They are characterized by the fact that they do not fully set, instead remaining permanently sticky. This allows for an excellent adhesive bond even when the glue is cold. In this case, the required contact pressure is the key to creating a sufficient coat between the contact adhesive and the joined part, which is why this group of adhesives is also referred to as "PSA-pressure-sensitive adhesives" (**Figure 20**). Styrenic block copolymers based PSAs are the latest type which are introduced into the market. They are generally blends of triblocks of styreneisoprene-styrene (SIS) and styreneisoprene diblocks blended with a low-molecular-weight and high-*Tg* resin based on C5 rings that is miscible with the isoprene phase and not miscible with the styrene phase. It is compulsory to state that a tackifying resin is a necessary

component of approximately any class of PSA [26].

*The polymer morphology of a structural acrylic adhesive.*

**102**

**Figure 18.**

**Figure 19.**

using organic peroxide.

ingredient type [1, 4, 13, 14].

**Figure 21.** *Silicone PSA application in electronic circuits.*

**Figure 22.** *Silicone PSA structure.*

**Figure 23.** *Screen printing: (a) Pattern making (b) Printing.*

Screen printing which is one application of PSAs initiates with stretching a piece of screen (mesh) over a frame and securing it into place. After that, a photoreactive emulsion that cures when light is irradiated onto it is applied to the mesh. Then, the desired pattern is placed onto the emulsion-coated mesh (**Figure 23**). In the next stage, the sealant emulsion was used to form a resist when being exposed to light. To remove the unhardened emulsion, the screen is washed, and the pattern emerges as an opening not covered by the resin which can be used as the plate for screen printing. For printing, ink is then placed inside the frame, and the plate is placed over the substrate to be printed. The pattern is transcribed onto the substrate when the ink is pushed out of the opening with the linear pressure from the squeegee. This process can be repeated many times to mass produce substrates with the same pattern [39].

Before the film assembly on LCD, other processes including punching, transporting and inspecting are necessary. Thus, the application of a protective film is compulsory to protect the surface of the optical films from contaminants or scratching. A sketch of a polarizing plate with a protective film is demonstrated in **Figure 24**. The substrates of the protective film are typically fabricated using polyethylene (PE) or polyethylene terephthalate (PET) with pressure-sensitive adhesives (PSAs) coated on one side. PSAs are viscoelastic materials that can adhere to various substrates upon application of light contact pressure in short contact time [40].

There are two different major classes of medical adhesives available [41]:

**105**

**Figure 24.**

*A polarizing plate with a protective film.*

*Pressure-Sensitive Adhesive Joints*

adhesion.

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

ings and urethane implants.

• Transdermal drug delivery.

• Adhesive drapes.

• Adhesive bandage or plasters.

example, a tracheostomy.

• Electro medical procedures.

• Microporous tapes.

drug master files [42].

wound protection. The applications for PSAs are:

• Polyvinyl ether-based adhesive, which are used in permeable skin patches.

• Polyvinyl pyrrolidone-based adhesives, which are used for high moisture absorption applications, such as ostomy bags and mounts, without the loss of

• Urethane-based adhesives, which are used for sustained released wound dress-

The biggest amount of the pressure-sensitive adhesives on the market is for

• Ostomy bag mounts: construction of an opening in the human skin. For

Drug-loaded adhesive patches delivering the therapeutic agent, when applied to the skin, at a controlled rate, through the skin to the systemic circulation and to the target organs are transdermal drug delivery (TDD) systems. For maintaining intimate contact between the patch and the skin surface, adhesives are used in TDD applications. Many types of adhesives might be considered to be used for application of TDD patches although PSAs are preferred. There are other factors to be considered in the selection and use of PSAs for TDD systems in addition to the usual properties of PSAs such as tack, adhesive and cohesive strength including compatibility, biocompatibility, delivery system compatibility, formulation, and acceptable regulatory status being equivalent supporting documentation or the availability of

The adhesive matrix or drug-in-adhesive (DIA) design in its simplest form is directly

applied into the PSA matrix. DIA's design configuration is shown in **Figure 25a**.

• Intravenous needle attachments, like morphine perfusion.

#### *Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

Screen printing which is one application of PSAs initiates with stretching a piece of screen (mesh) over a frame and securing it into place. After that, a photoreactive emulsion that cures when light is irradiated onto it is applied to the mesh. Then, the desired pattern is placed onto the emulsion-coated mesh (**Figure 23**). In the next stage, the sealant emulsion was used to form a resist when being exposed to light. To remove the unhardened emulsion, the screen is washed, and the pattern emerges as an opening not covered by the resin which can be used as the plate for screen printing. For printing, ink is then placed inside the frame, and the plate is placed over the substrate to be printed. The pattern is transcribed onto the substrate when the ink is pushed out of the opening with the linear pressure from the squeegee. This process can be repeated many times to mass produce substrates with the same pattern [39]. Before the film assembly on LCD, other processes including punching, transporting and inspecting are necessary. Thus, the application of a protective film is compulsory to protect the surface of the optical films from contaminants or scratching. A sketch of a polarizing plate with a protective film is demonstrated in **Figure 24**. The substrates of the protective film are typically fabricated using polyethylene (PE) or polyethylene terephthalate (PET) with pressure-sensitive adhesives (PSAs) coated on one side. PSAs are viscoelastic materials that can adhere to various substrates

upon application of light contact pressure in short contact time [40].

• Removable adhesives (pressure-sensitive adhesives).

• Permanent or tissue adhesives (glue).

There are two different major classes of medical adhesives available [41]:

• Other than acrylics, there are other types of PSAs for medical applications:

• Silicone-based adhesives, which are used in transdermal drug delivery systems (drug delivery). These devices are in need of an inert adhesive and biocompat-

**104**

**Figure 23.**

*Screen printing: (a) Pattern making (b) Printing.*

ible system.

The biggest amount of the pressure-sensitive adhesives on the market is for wound protection. The applications for PSAs are:

Drug-loaded adhesive patches delivering the therapeutic agent, when applied to the skin, at a controlled rate, through the skin to the systemic circulation and to the target organs are transdermal drug delivery (TDD) systems. For maintaining intimate contact between the patch and the skin surface, adhesives are used in TDD applications. Many types of adhesives might be considered to be used for application of TDD patches although PSAs are preferred. There are other factors to be considered in the selection and use of PSAs for TDD systems in addition to the usual properties of PSAs such as tack, adhesive and cohesive strength including compatibility, biocompatibility, delivery system compatibility, formulation, and acceptable regulatory status being equivalent supporting documentation or the availability of drug master files [42].

The adhesive matrix or drug-in-adhesive (DIA) design in its simplest form is directly applied into the PSA matrix. DIA's design configuration is shown in **Figure 25a**.

**Figure 24.** *A polarizing plate with a protective film.*

#### *Adhesives and Adhesive Joints in Industry Applications*

**Figure 25.** *Typical designs of TDD systems.*

**Figure 26.** *Structure of polyurethane based PSA.*

The adhesive matrix transfers several properties, including peel adhesion, control over drug delivery rate and storage of the drug, and it also governs their partitioning into the stratum corneum. An impermeable backing film supports DIA matrix on the topside and on the side facing the skin there is a lamination with a removable release liner [1, 42].

There is also a contribution of the extraordinarily diverse chemistry of polyurethanes to the development of efficient adhesives applying only to a limited degree to PSAs for which the number of patent publications is surprisingly small. Solvent free 2-K PSAs on the basis of polyether polyols and isocyanates are available on the market (**Figure 26**) [27].

Previous studies have shown [43] that aliphatic polyesters with molecular weights ranging from 10,000 to 50,000 Dalton and *Tg* between −25 and − 47°C can give us excellent adhesion. An example of this method is the acrylation of polyester PSAs containing end groups of hydroxyl. Introduction of an unsaturated architecture can also be practically achieved via the reaction of the OH-groups of a polyester chain with the corresponding acrylic or unsaturated isocyanate compounds [44].

### **4. Manufacturing of PSAs**

The manufacture of pressure-sensitive raw materials and adhesives can be carried out off -line or in-line. As illustrated in **Figure 27**, both technological

**107**

**Figure 28.**

*Manufacturing steps for PSPs [24].*

**Figure 27.**

*Pressure-Sensitive Adhesive Joints*

in PSP.

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

modalities include various chemical technologies. Generally, off -line synthesis yields pressure-sensitive raw materials or PSAs; in-line technology leads to simulta-

As illustrated in **Figure 28**, off-line synthesis (1) allows polymer-based PSA formulation to be coated (2) and (if necessary) transforms (mainly by cross-linking)

Pressure-sensitive products (PSPs) are multilayer constructions based on solid

state materials and coated component. Therefore, the production of the PSPs includes the manufacture and assembly of the individual components. The manufacture of the components includes the manufacture of both the solid-state carrier materials (face stock and release liner) and the coating components (adhesive, adhesive, primer, etc.) Generally, the buildup of the PSP from its components is carried out by coating and laminating and leads to a web-like product that must be confectioned. **Table 1** presents the manufacturing technology of main PSPs and demonstrates that the manufacture of PSPs by coating of a solid-state carrier with a liquid pressure-sensitive adhesive (PSA) is the main procedure to produce various

*Principles of manufacture technology for pressure-sensitive raw materials and adhesives [24].*

neous PSA and PSP (pressure-sensitive products) manufacture [24].

#### *Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

The adhesive matrix transfers several properties, including peel adhesion, control over drug delivery rate and storage of the drug, and it also governs their partitioning into the stratum corneum. An impermeable backing film supports DIA matrix on the topside and on the side facing the skin there is a lamination with a removable release liner [1, 42]. There is also a contribution of the extraordinarily diverse chemistry of polyurethanes to the development of efficient adhesives applying only to a limited degree to PSAs for which the number of patent publications is surprisingly small. Solvent free 2-K PSAs on the basis of polyether polyols and isocyanates are available on the

Previous studies have shown [43] that aliphatic polyesters with molecular weights ranging from 10,000 to 50,000 Dalton and *Tg* between −25 and − 47°C can give us excellent adhesion. An example of this method is the acrylation of polyester PSAs containing end groups of hydroxyl. Introduction of an unsaturated architecture can also be practically achieved via the reaction of the OH-groups of a polyester chain with the corresponding acrylic or unsaturated isocyanate compounds [44].

The manufacture of pressure-sensitive raw materials and adhesives can be carried out off -line or in-line. As illustrated in **Figure 27**, both technological

**106**

market (**Figure 26**) [27].

*Structure of polyurethane based PSA.*

*Typical designs of TDD systems.*

**Figure 26.**

**Figure 25.**

**4. Manufacturing of PSAs**

modalities include various chemical technologies. Generally, off -line synthesis yields pressure-sensitive raw materials or PSAs; in-line technology leads to simultaneous PSA and PSP (pressure-sensitive products) manufacture [24].

As illustrated in **Figure 28**, off-line synthesis (1) allows polymer-based PSA formulation to be coated (2) and (if necessary) transforms (mainly by cross-linking) in PSP.

Pressure-sensitive products (PSPs) are multilayer constructions based on solid state materials and coated component. Therefore, the production of the PSPs includes the manufacture and assembly of the individual components. The manufacture of the components includes the manufacture of both the solid-state carrier materials (face stock and release liner) and the coating components (adhesive, adhesive, primer, etc.) Generally, the buildup of the PSP from its components is carried out by coating and laminating and leads to a web-like product that must be confectioned. **Table 1** presents the manufacturing technology of main PSPs and demonstrates that the manufacture of PSPs by coating of a solid-state carrier with a liquid pressure-sensitive adhesive (PSA) is the main procedure to produce various

#### **Figure 27.**

*Principles of manufacture technology for pressure-sensitive raw materials and adhesives [24].*

**Figure 28.** *Manufacturing steps for PSPs [24].*

#### **Table 1.**

*Manufacturing technology of main PSPs [24].*

PSPs. The principal coating component is the PSA. Other coating components are used to allow the anchorage of the adhesive on the carrier (e.g., primer) and its debonding from the laminate (e.g., release agent) and to protect or finish the adhesive or the carrier (e.g., printing inks, lacquers) [1, 6, 17, 24].

PSAs manufacturing includes the manufacture of the raw materials and also their formulation. PSAs can be found in different forms such as solutions, dispersions, or hot-melt adhesives. Water-borne PSAs can be used in certain cases without any additional formulation. Solvent-based acrylic PSAs may also be used without further formulation. PSA manufacture is originally based on the blending and dissolving of different raw materials, none of which exhibits a PSA character lonely.

For preparation recipes for PSAs, formulating must be done via changing the adhesive composition, by addition of different macro- or micromolecular compounds with elastomeric or viscous, surface active or chemically stabilizing properties; antimicrobial agents must also be added to the adhesive. PSAs should have a Tg of about \_15° to \_5°C. Latexes' sensitivity to bacteria resides in the moderate pH and presence of polyvinyl alcohol. Blending a high Tg, hard resin with a lower Tg elastomer provides a desirable formulation. Considering labels, PSA coating and converting, and end-use require other special performance requirements which can be obtained only through formulating. For the formulation of PSAs, it is needed to produce tailored adhesives for each one of the intended applications. Parameters including the converting technology of the adhesive, the coating technology, the end-use properties, and economical criteria influence the formulation of PSAs. The first reason for formulating is the necessity to improve the adhesive properties. Rubber based PSAs do not possess appropriate intrinsic tack and peel adhesion. Most viscoelastic materials with intrinsic tack being obtained via the synthesis do not show enough peel adhesion to nonpolar surfaces, and soft, tacky PSA raw materials exhibit too low shear adhesion. In cases of water borne or solvent borne acrylic PSAs, the removability of the PSA must be improved. Therefore, a proper balance between high tack, peel adhesion, and high cohesion has to be achieved using formulation. Formulating for high tack and peel (tackification), using viscous formulating agents such as tackifiers and plasticizers decreases the shear adhesion. A high cohesion level may be achieved by the rubber-like component of the adhesive or by partial crosslinking [6, 15, 22, 24].

**109**

is also necessary [1, 6, 7, 15, 51, 52].

PSAs [48–50].

*Pressure-Sensitive Adhesive Joints*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

solubility at 10–30% solids in solvent [6].

There are rubber-like (elastic) and liquid like (viscous) PSA components. Natural raw materials with rubber-like properties and natural resins or plasticiz-

ers may be used in order to formulate an adequate recipe. Physical modification

Natural rubber is used as a solution, and manufacture of the PSAs implies dissolution of the rubber. In fact a slight mechanochemical destruction of the rubber occurs in parallel with the dissolution process. At least theoretically a precalendering (i.e., a controlled depolymerization of the natural rubber) is possible, but not economically viable. There are only a few special uses (like protective films) where mastication of the rubber must be carried out. Masticated rubber was used for tapes. Pressure-sensitive adhesives exhibiting low tack, for bonding onto themselves, also are formulated on the basis of masticated natural rubber [4–4]. The high molecular weight portion of natural rubber is insoluble in solvents and therefore natural rubbers must be milled to a Mooney viscosity of 53–75 or below, to obtain

Solvent-based PSAs are tackified elastomers, tackified or untackified viscoelastomers, or tackified plastomers. Both (the tackified and the untackified recipe) need adhesive solutions as a blending medium; that is, mixing of the components occurs in solvents. Classic, solvent-based adhesives were formulated with natural rubber (NR) and tackifier resins. Later, synthetic rubber-like products were introduced. Acrylics, vinyl acetate (VAc) copolymers, silicones, polyurethanes (PURs), and other viscoelastomers enlarged the choice of raw materials for solvent-based PSAs and the diversity of the required equipment. Solvent-based acrylics need a special design, but less formulation. It is well known that various materials can be used as adhesives [45–47], among which acrylic polymers are the most popular ones for PSA applications because of the inherent unique properties. Acrylic PSAs, generally borne in an organic solvent, must possess correspondingly suitable 180° peel strength, high shear strength and appropriate tack for optical film applications. The adhesive properties of acrylic PSAs are critically determined by the monomer compositions, the initiators, the crosslinking agents and the tackifiers of acrylic

The main differences between solvent-based and water-based systems are that solvent based systems are homogeneous and water-based ones are heterogeneous; solvent-based systems have low molecular weight (limited by the solubility/viscosity of macromolecular compounds), whereas water-based ones possess higher molecular weight; solvent based systems may be formulated without non adhesive components, but water-based systems must include technological additive. Started with rubber–resin PSA solutions, label coating now uses mostly acrylic, waterbased PSAs. Therefore, the formulation of PSAs for labels is mainly a formulation of water-based compounds. In practice, it consists of the blending of liquid-state components, that is, the viscoelastomer dispersion and the tackifier dispersion. It should be taken into account that in a different manner from true solutions, waterbased dispersions have a limited (4-year) shelf life, narrow storage-temperature intervallum (−30 to +70°C), and low shear stability. First step of the manufacturing process is the emulsion polymerization process of acrylic monomers in a polymerization reactor plant. After that, this water borne acrylic polymer solution called latex has to be mixed with other additives to be formulated as a PSA applicable for PSP applications. Blending acrylic dispersions requires a test of compatibility, adjustment of pH, and, after the addition of solvent, plasticizers, or fillers, storage time. Peel improvement after longer storage times for a tackified acrylic dispersion

Hot melts are based on special designed polymers that need complex formulation. Generally, hot-melt PSAs (HMPSAs) are formulated from thermoplastic

allows the use of these materials in a suitable state [22, 27].

#### *Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

Free choice of product components

**Carrier Adhesive Very**

**Manufacturing procedure**

Extrusion coating

**Table 1.**

*Note:* ●*, preferred;* ●○*, possible.*

*Manufacturing technology of main PSPs [24].*

PSPs. The principal coating component is the PSA. Other coating components are used to allow the anchorage of the adhesive on the carrier (e.g., primer) and its debonding from the laminate (e.g., release agent) and to protect or finish the adhesive

**Parameter**

**good**

Coating ● ● ● ●○ Label,

Coextrusion ˗˗ ˗˗ ● ●○ Tape,

Running speed On-line

**Good Fair**

˗˗ ˗˗ ● ●○ Tape,

manufacture

PSP

tape, protection film

protection film

protection film

PSAs manufacturing includes the manufacture of the raw materials and also their formulation. PSAs can be found in different forms such as solutions, dispersions, or hot-melt adhesives. Water-borne PSAs can be used in certain cases without any additional formulation. Solvent-based acrylic PSAs may also be used without further formulation. PSA manufacture is originally based on the blending and dissolving of different raw materials, none of which exhibits a PSA character lonely. For preparation recipes for PSAs, formulating must be done via changing the adhesive composition, by addition of different macro- or micromolecular compounds with elastomeric or viscous, surface active or chemically stabilizing properties; antimicrobial agents must also be added to the adhesive. PSAs should have a Tg of about \_15° to \_5°C. Latexes' sensitivity to bacteria resides in the moderate pH and presence of polyvinyl alcohol. Blending a high Tg, hard resin with a lower Tg elastomer provides a desirable formulation. Considering labels, PSA coating and converting, and end-use require other special performance requirements which can be obtained only through formulating. For the formulation of PSAs, it is needed to produce tailored adhesives for each one of the intended applications. Parameters including the converting technology of the adhesive, the coating technology, the end-use properties, and economical criteria influence the formulation of PSAs. The first reason for formulating is the necessity to improve the adhesive properties. Rubber based PSAs do not possess appropriate intrinsic tack and peel adhesion. Most viscoelastic materials with intrinsic tack being obtained via the synthesis do not show enough peel adhesion to nonpolar surfaces, and soft, tacky PSA raw materials exhibit too low shear adhesion. In cases of water borne or solvent borne acrylic PSAs, the removability of the PSA must be improved. Therefore, a proper balance between high tack, peel adhesion, and high cohesion has to be achieved using formulation. Formulating for high tack and peel (tackification), using viscous formulating agents such as tackifiers and plasticizers decreases the shear adhesion. A high cohesion level may be achieved by the rubber-like component of the adhesive or by partial cross-

or the carrier (e.g., printing inks, lacquers) [1, 6, 17, 24].

**108**

linking [6, 15, 22, 24].

There are rubber-like (elastic) and liquid like (viscous) PSA components.

Natural raw materials with rubber-like properties and natural resins or plasticizers may be used in order to formulate an adequate recipe. Physical modification allows the use of these materials in a suitable state [22, 27].

Natural rubber is used as a solution, and manufacture of the PSAs implies dissolution of the rubber. In fact a slight mechanochemical destruction of the rubber occurs in parallel with the dissolution process. At least theoretically a precalendering (i.e., a controlled depolymerization of the natural rubber) is possible, but not economically viable. There are only a few special uses (like protective films) where mastication of the rubber must be carried out. Masticated rubber was used for tapes. Pressure-sensitive adhesives exhibiting low tack, for bonding onto themselves, also are formulated on the basis of masticated natural rubber [4–4]. The high molecular weight portion of natural rubber is insoluble in solvents and therefore natural rubbers must be milled to a Mooney viscosity of 53–75 or below, to obtain solubility at 10–30% solids in solvent [6].

Solvent-based PSAs are tackified elastomers, tackified or untackified viscoelastomers, or tackified plastomers. Both (the tackified and the untackified recipe) need adhesive solutions as a blending medium; that is, mixing of the components occurs in solvents. Classic, solvent-based adhesives were formulated with natural rubber (NR) and tackifier resins. Later, synthetic rubber-like products were introduced. Acrylics, vinyl acetate (VAc) copolymers, silicones, polyurethanes (PURs), and other viscoelastomers enlarged the choice of raw materials for solvent-based PSAs and the diversity of the required equipment. Solvent-based acrylics need a special design, but less formulation. It is well known that various materials can be used as adhesives [45–47], among which acrylic polymers are the most popular ones for PSA applications because of the inherent unique properties. Acrylic PSAs, generally borne in an organic solvent, must possess correspondingly suitable 180° peel strength, high shear strength and appropriate tack for optical film applications. The adhesive properties of acrylic PSAs are critically determined by the monomer compositions, the initiators, the crosslinking agents and the tackifiers of acrylic PSAs [48–50].

The main differences between solvent-based and water-based systems are that solvent based systems are homogeneous and water-based ones are heterogeneous; solvent-based systems have low molecular weight (limited by the solubility/viscosity of macromolecular compounds), whereas water-based ones possess higher molecular weight; solvent based systems may be formulated without non adhesive components, but water-based systems must include technological additive. Started with rubber–resin PSA solutions, label coating now uses mostly acrylic, waterbased PSAs. Therefore, the formulation of PSAs for labels is mainly a formulation of water-based compounds. In practice, it consists of the blending of liquid-state components, that is, the viscoelastomer dispersion and the tackifier dispersion. It should be taken into account that in a different manner from true solutions, waterbased dispersions have a limited (4-year) shelf life, narrow storage-temperature intervallum (−30 to +70°C), and low shear stability. First step of the manufacturing process is the emulsion polymerization process of acrylic monomers in a polymerization reactor plant. After that, this water borne acrylic polymer solution called latex has to be mixed with other additives to be formulated as a PSA applicable for PSP applications. Blending acrylic dispersions requires a test of compatibility, adjustment of pH, and, after the addition of solvent, plasticizers, or fillers, storage time. Peel improvement after longer storage times for a tackified acrylic dispersion is also necessary [1, 6, 7, 15, 51, 52].

Hot melts are based on special designed polymers that need complex formulation. Generally, hot-melt PSAs (HMPSAs) are formulated from thermoplastic

elastomers (TPEs) and viscous (liquid) components. The manufacture of HMPSAs requires mechanical size reduction, micronization of certain elastomers (TPEs are generally pelletized), and molten state mixing of them with other components of the recipe (tackifiers, oils, antioxidants, etc.). Such mixing is carried out discontinuously or continuously [6] with a special apparatus that produces a heat history of the material. For continuous mixers the heat history is very low. Principally, melt mixing can be carried out using a variety of heated mixing equipment. In order of increasing effectiveness, these include vessels stirred with propellers, vessels with high shear (Cowles type) dispersers, planetary mixers, internal mixers (Sigma blade or Banbury type), and modifications of these. In low shear mixers, resins, plasticizers, and stabilizers should be melted first and thermoplastic rubber crumb added incrementally. Where production rates are high enough, continuous mixers are desirable because they reduce degradation of the rubbery mid-block. For hot-melt manufacture, compounding is mixing of the adhesive components at 120 and 200°C, where the shear forces cause a drop in the viscosity. The construction and hydrodynamic characteristics of the mixing equipment may influence the processing performances of HMPSAs; the raw materials used affect it also. For instance, when polyacrylate rubber-based HMPSAs are prepared, high shear mixing is required. As a rule of thumb, increasing mixer shear and temperature lead to better mixing of the adhesive components, a breakdown in molecular weight, and a subsequent increase in tack and reduction in shear performance. In some cases, these effects may be signify cant enough to overwhelm any effects caused by changing the adhesive formulation; for that reason, some researchers choose to solvent blend and solvent coat samples rather than use a mechanical mixer or extruder. Furthermore, adhesive melt viscosity and shear strength are extremely sensitive to melt processing and subsequent breakdown in molecular weight, whereas tack and peel resistance are relatively robust and will remain relatively unchanged [22, 24].

Generally, the manufacture of PSPs includes the manufacture of solid-state components (e.g., carrier materials for face stock and for release liner), the manufacture of liquid adhesive and nonadhesive components (e.g., PSA, release and primer), and their assembly in a continuous web-like material. The PSA can be coated on or built into the bulk carrier material. The classic way to manufacture a PSP is by coating a carrier material with a PSA. Manufacture of the finished product by carrier coating employs a coating technology completed by other converting steps that are common for web-like products. Web finishing is the coating of liquid product components on solid-state web components or their build into solid-state web components. From the technological point of view, PSPs can be manufactured using coating, extrusion coating, or coextrusion. Generally, coated adhesives are components that are deposited on the surface of a carrier material. The manufacture of PSPs by coating is a common procedure for classic products. Coating technology uses various coating methods; their choice depends on the physical state of the adhesive (e.g., solvent-based, water-based, or HMPSA) and PSP grade. For labels, roll-coating and screen printing offer the best results, but blade, cast, and slot-die coating are used also. For tapes, roll-coating, slot-die coating, extrusion, and calendering are mostly used, but blade, cast, and spray coating were suggested as well. Protection films are manufactured by roll and blade coating. Different protective are illustrated in **Figure 29** [22, 24, 28].

Coating methods can be classified as impact (i.e., contact) coating (e.g., roll-coating, screen printing) and nonimpact coating (e.g., spray coating, curtain coating) and according to the dosage of the coated material, that is, whether it is in excess or metered (**Table 2**).

There is also manufacturing of PSPs via extrusion process. Actually, pressuresensitivity of PSPs resides in their coated or built-in pressure-sensitive layer. Buildup

**111**

*Pressure-Sensitive Adhesive Joints*

**Figure 29.**

*a*

*b*

*c*

*d*

*e*

*f Lacquering.*

**Table 2.**

*Different PSA tapes based on protective films.*

Main use in other technologiesc

*Plastic film manufacture (cast, chill-roll film).*

*Plastic film manufacture (plasticized cast film).*

*Working principle of different coating devices [24].*

*Printing (gravure, flexo).*

*Rubber processing.*

*Printing (ink jet).*

**With adhesive excess With adhesive**

Screen coatinga – –

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

of a PSP from its components means the classic manufacture of the finished product which are the carrier material and the PSA, respectively. Practically, other product components such as the release liner and cover film can be built in through one or more layers resulting in a laminate with a complex structure. The PSP can also be manufactured in one step, offering primarily economic advantages if a carrier material with built-in pressure sensitivity is used. As a result, economic considerations forced the development of extrusion-made products. **Table 3** concludes the main properties of the manufacture of adhesiveless and adhesive coated PSPs [22]. When manufacturing PSPs, the final properties of the product and trends in production technology must also be considered. The selection of the PSP components in the manufacturing process of it depends on the product type. There are some limitation in each product class regarding the choice of manufacture equipment and

**Impact coating Nonimpact coating**

**dosage**

Calendaringb,c – – –

Knife coating Slot-diec Air brushf Spray coatingd,f Roll coatinga – – Curtain coatinge

**With adhesive excess**

**With adhesive dosage**

#### **Figure 29.**

*Adhesives and Adhesive Joints in Industry Applications*

elastomers (TPEs) and viscous (liquid) components. The manufacture of HMPSAs requires mechanical size reduction, micronization of certain elastomers (TPEs are generally pelletized), and molten state mixing of them with other components of the recipe (tackifiers, oils, antioxidants, etc.). Such mixing is carried out discontinuously or continuously [6] with a special apparatus that produces a heat history of the material. For continuous mixers the heat history is very low. Principally, melt mixing can be carried out using a variety of heated mixing equipment. In order of increasing effectiveness, these include vessels stirred with propellers, vessels with high shear (Cowles type) dispersers, planetary mixers, internal mixers (Sigma blade or Banbury type), and modifications of these. In low shear mixers, resins, plasticizers, and stabilizers should be melted first and thermoplastic rubber crumb added incrementally. Where production rates are high enough, continuous mixers are desirable because they reduce degradation of the rubbery mid-block. For hot-melt manufacture, compounding is mixing of the adhesive components at 120 and 200°C, where the shear forces cause a drop in the viscosity. The construction and hydrodynamic characteristics of the mixing equipment may influence the processing performances of HMPSAs; the raw materials used affect it also. For instance, when polyacrylate rubber-based HMPSAs are prepared, high shear mixing is required. As a rule of thumb, increasing mixer shear and temperature lead to better mixing of the adhesive components, a breakdown in molecular weight, and a subsequent increase in tack and reduction in shear performance. In some cases, these effects may be signify cant enough to overwhelm any effects caused by changing the adhesive formulation; for that reason, some researchers choose to solvent blend and solvent coat samples rather than use a mechanical mixer or extruder. Furthermore, adhesive melt viscosity and shear strength are extremely sensitive to melt processing and subsequent breakdown in molecular weight, whereas tack and peel resistance are relatively robust and will remain relatively unchanged [22, 24]. Generally, the manufacture of PSPs includes the manufacture of solid-state components (e.g., carrier materials for face stock and for release liner), the manufacture of liquid adhesive and nonadhesive components (e.g., PSA, release and primer), and their assembly in a continuous web-like material. The PSA can be coated on or built into the bulk carrier material. The classic way to manufacture a PSP is by coating a carrier material with a PSA. Manufacture of the finished product by carrier coating employs a coating technology completed by other converting steps that are common for web-like products. Web finishing is the coating of liquid product components on solid-state web components or their build into solid-state web components. From the technological point of view, PSPs can be manufactured using coating, extrusion coating, or coextrusion. Generally, coated adhesives are components that are deposited on the surface of a carrier material. The manufacture of PSPs by coating is a common procedure for classic products. Coating technology uses various coating methods; their choice depends on the physical state of the adhesive (e.g., solvent-based, water-based, or HMPSA) and PSP grade. For labels, roll-coating and screen printing offer the best results, but blade, cast, and slot-die coating are used also. For tapes, roll-coating, slot-die coating, extrusion, and calendering are mostly used, but blade, cast, and spray coating were suggested as well. Protection films are manufactured by roll and blade coating. Different

**110**

metered (**Table 2**).

protective are illustrated in **Figure 29** [22, 24, 28].

Coating methods can be classified as impact (i.e., contact) coating (e.g., roll-coating, screen printing) and nonimpact coating (e.g., spray coating, curtain coating) and according to the dosage of the coated material, that is, whether it is in excess or

There is also manufacturing of PSPs via extrusion process. Actually, pressuresensitivity of PSPs resides in their coated or built-in pressure-sensitive layer. Buildup *Different PSA tapes based on protective films.*

#### **Table 2.**

*Working principle of different coating devices [24].*

of a PSP from its components means the classic manufacture of the finished product which are the carrier material and the PSA, respectively. Practically, other product components such as the release liner and cover film can be built in through one or more layers resulting in a laminate with a complex structure. The PSP can also be manufactured in one step, offering primarily economic advantages if a carrier material with built-in pressure sensitivity is used. As a result, economic considerations forced the development of extrusion-made products. **Table 3** concludes the main properties of the manufacture of adhesiveless and adhesive coated PSPs [22].

When manufacturing PSPs, the final properties of the product and trends in production technology must also be considered. The selection of the PSP components in the manufacturing process of it depends on the product type. There are some limitation in each product class regarding the choice of manufacture equipment and

#### *Adhesives and Adhesive Joints in Industry Applications*

**Table 3.**

*Major raw materials and manufacturing processes for PSPs [22].*

laminate components namely printing technology consideration in label manufacture which determines the choice of the label components and manufacturing method. The manufacture of labels should begin with the design process and also include digital artwork and reproduction, origination and pre-press electronic design and artwork plate types, and plate making,. Changes in technology go back to the material components or new technologies used in production. The printing industry has undergone changes to meet these standards. Water-based, solvent-free, and 100% solids adhesives are being used more and more to meet the law of the toxic products emission standards. Food laws get tougher in controlling the effect of migration of various components of the package and also their interaction with the packed products. These changes motivate the use of solvent-free and adhesive-free products.

### **5. Conclusions**

As wholly discussed in this chapter, PSAs as viscoelastic, non-Newtonian flow behavior which is based on the macromolecular nature of the adhesive are increasingly replacing more traditional adhesives because of their ease of use and safe manipulation. Environmental limiting laws drive research toward other production methods to coating from solution in spite of the developments of energy-efficient solvent-recovery methods. Acrylic latexes are so popular for general-use PSAs. However, there is a limitation for their access to the most demanding applications due to the presence of surfactants in their microstructure, which are not easy to eliminate and the lesser control over the molecular structure. The manufacture of pressure-sensitive raw materials and adhesives can be carried out off -line or in-line. An increasingly popular production method is hot-melt, or solventless, technologies. In this process, the viscosity should be lowered enough for an easy coating without the use of any volatile compound. Regarding end-use properties, the future trend for PSAs will be in the direction of heterogeneous polymer structures and also on the incorporation of additional functionalities such as thermal or electrical conductivity or controlled drug release.

**113**

**Author details**

Jaber Khanjani

Department of Chemical, Materials and Polymer Engineering, Buein Zahra Technical University, Buein Zahra, Qazvin, Iran

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to<EMAIL_ADDRESS>
provided the original work is properly cited.

*Pressure-Sensitive Adhesive Joints*

*DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Pressure-Sensitive Adhesive Joints DOI: http://dx.doi.org/10.5772/intechopen.85667*

*Adhesives and Adhesive Joints in Industry Applications*

**Raw materials Manufacturing**

*Major raw materials and manufacturing processes for PSPs [22].*

**procedure**

Ethylene-vinyl acetate copolymers Extrusion, coextrusion Protective film, business forms Ethylene-olefin copolymers Extrusion Protective film, decoration film

laminate components namely printing technology consideration in label manufacture which determines the choice of the label components and manufacturing method. The manufacture of labels should begin with the design process and also include digital artwork and reproduction, origination and pre-press electronic design and artwork plate types, and plate making,. Changes in technology go back to the material components or new technologies used in production. The printing industry has undergone changes to meet these standards. Water-based, solvent-free, and 100% solids adhesives are being used more and more to meet the law of the toxic products emission standards. Food laws get tougher in controlling the effect of migration of various components of the package and also their interaction with the packed products. These changes motivate the use of solvent-free and adhesive-free products.

As wholly discussed in this chapter, PSAs as viscoelastic, non-Newtonian flow behavior which is based on the macromolecular nature of the adhesive are increasingly replacing more traditional adhesives because of their ease of use and safe manipulation. Environmental limiting laws drive research toward other production methods to coating from solution in spite of the developments of energy-efficient solvent-recovery methods. Acrylic latexes are so popular for general-use PSAs. However, there is a limitation for their access to the most demanding applications due to the presence of surfactants in their microstructure, which are not easy to eliminate and the lesser control over the molecular structure. The manufacture of pressure-sensitive raw materials and adhesives can be carried out off -line or in-line. An increasingly popular production method is hot-melt, or solventless, technologies. In this process, the viscosity should be lowered enough for an easy coating without the use of any volatile compound. Regarding end-use properties, the future trend for PSAs will be in the direction of heterogeneous polymer structures and also on the incorporation of additional functionalities such as thermal or electrical

Polyethylene Extrusion Protective film

Ethylene-acrylate copolymers Coextrusion Protective film, tape Butene-isobutene copolymers Coextrusion Protective film, tape Vinyl acetate copolymers Extrusion Business forms

Vinyl acetate copolymers Extrusion, casting Tape

**PSP**

**112**

**Table 3.**

**5. Conclusions**

conductivity or controlled drug release.