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Displays in electronic devices visualize digital information, which is essential for humans to ultimately understand and use the Internet of Things (IoT) technology. Along with the evolution of several electrical components for flexible displays, developing flexible pressure-sensitive adhesives (PSA) is also important. In this study, a pattern of crosslinking density was applied to a PSA during the ultraviolet (UV) curing process to secure adhesion, transparency, stress relaxation, and supplement the strain recovery performance. We measured the modulus of each pattern region by atomic force microscopy (AFM) and confirmed whether the UV pattern PSA was prepared through the implementation of differential crosslinking density. Additionally, we measured dynamic mechanical analysis (DMA) strain recovery test, rheometer, peel strength, probe tack, and transmittance to confirm the effect of the given crosslinking density pattern on the overall physical properties of PSA.

Pressure-sensitive adhesives (PSAs) are used in various industries, and have been widely applied in smart devices. In this study, the adhesion properties, viscoelastic properties, and gel fraction of PSAs were investigated after primary and secondary curing to understand their UV-curing behavior. In addition, the impact of concentration of crosslinking agent on the crosslinking density and the degree of crosslinking was investigated. In general, as more crosslinking agent was added, the hardness increased, but this result can be explained by the difference between “crosslinking density” and “degree of crosslinking” at the highest concentration of crosslinking agent actually led to a decrease in “degree of crosslinking”.

Cross-linking is an essential element in enhancing polymers’ mechanical properties in adhesives, elastomers, and hydrogels. However, covalent cross-linking introduces a trade-off relationship betwe...

In this study, we reports the first quantification details of the pencil hardness, eraser abrasion resistance and steel wool abrasion resistance of optically-transparent and biaxially-oriented poly(ethylene terephthalate) (PET) films in various thicknesses before and after hard coating (HC) and/or optically clear adhesive (OCA) coating. Surprisingly, the PET base films were determined to exhibit unrealistically very low hardness and abrasion resistances although their stress-strain behavior followed a ductile and tough plastic nature as a high performance engineering plastic: the pencil hardness was only ≤ 9B even at a low loading force, 0.100 kgf, and the onset loading force to cause scratches was far below 0.013 kgf in both eraser and steel wool abrasions. These collectively confirmed that, without surface protections, PET base films are not suitable for any advanced applications. Such the low hardness and poor abrasion resistances could be improved substantially by only thin HC coating of di(trimethylolpropane) tetraacrylate (HC-a, an ultraviolet curable acrylic resin). In addition, the OCA coating based on an acrylic copolymer adhesive provided adhesion function to the PET films, showing excellent adhesion strength onto chemically-toughened glass. However, the OCA layer causes cushioning effect because of its easy plastic deformation nature, making negative impact on the hardness and abrasion resistance performances of the PET film. Thus, the OCA coating should be thinned until retaining good adhesion to adherends. Overall, PET films coated with HC and/or OCA are suitable for advanced applications in various fields including smart phones, foldable phones, and flexible display devices.

The era of the Internet of Things (IoT) is becoming a reality, and the display equipment is at its center. The display device is the window that connects the IoT and humans, and is evolving into various form factors convenient to human use, such as foldable mobile phone, flexible wearable device, paper-like display, and rollable TV. Even though transparent flexible electrodes and substrates have been developed, their assembly remains a challenge. Therefore, the importance of optically clear acrylic pressure-sensitive adhesives (PSAs) that ensure device integrity by easily stretched and mitigating the stress during repeated stretching and bending process is being emphasized. In this study, elongation and recovery features were identifiable as key flexible properties of PSAs. These properties were estimated by measuring shear strain and strain recovery using a universal testing machine and by dynamic mechanical analytics respectively. Compared to conventional UV metal halide curing, a newly introduced ultraviolet light-emitting diode (UV LED) curing process resulted in different PSA property variables, namely UV exposure intensity and UV exposure time. Fully cured acrylic PSA realized under UV LED exposure level 30 for 8 s demonstrated the highest shear strain with excellent strain recovery of ~23,500% and 94% respectively. Under these conditions, effective material adhesion performance was achieved while retaining optical transparency and thus fulfilling the dual property requirement for non-opaque material adhesiveness necessary for use in flexible displays.

Pressure-sensitive adhesives (PSAs) and UV-curable acrylic systems are widely utilized. In particular, PSAs have currently attracted significant attention because of their utility and applicability for manufacturing re-useable modules for smart devices. In this study, adhesion properties, UV-curing behavior, viscoelastic properties, and gel fraction were investigated. The UV/UV step curing process was utilized to overcome limitations, such as re-work or gap filling. The difference in physical properties between primary curing and secondary curing was confirmed. In addition, by adjusting the content of the crosslinking agent, the experiment based on controlling the structure of the semi-interpenetrating polymer network was conducted. We determined that physical properties varied with the crosslinking agent content. The degree of crosslinking was high, whereas the crosslinking density was low for 0.1–0.3 phr. However, both the degree of crosslinking and the crosslinking density increased for 0.5–1 phr.

UV-polymerizable and UV-crosslinkable pressure-sensitive adhesive acrylic compositions are widely applied for high-quality carrier-free films, one-sided or double-sided self-adhesive tapes. Comprehensive studies of the properties of novel acrylic PSAs for carrier-free films or adhesive tapes were carried out. Solvent-free acrylic pressure-sensitive adhesive layers were UV-crosslinked using various photoreactive crosslinking agents. Polymer content during UV-polymerization, prepolymers viscosity, temperature changes of prepolymer during UV-initiated polymerization process were investigated. The chemical structure of prepolymer and polymerized acrylic film were examined using FTIR spectroscopy. Prepolymer mixtures were transferred on the transparent carriers to form 1 mm thick polymer films, and then UV-crosslinked after the addition of the selected acrylic crosslinker. Final adhesive films were then tested to evaluate their functional properties as the tack, peel adhesion, and shear strength at 20 °C and 70 °C. From the results reveal that both concentrations of acrylic acid in the prepolymer, and cross-linking agent in the final polymer layer, have a significant influence on the functional properties of the produced PSA films.

Materials that can switch adhesive properties based on external stimuli are required in several industries for temporary bonding or transfer processes. Previously studied materials achieve this under restricted conditions (hydration, heat, and long switching times), and some materials have limitations related to reuse because of irreversible reactions or residue formation on substrates. Herein, a rapid photoresponsive switchable pressure-sensitive adhesive (PSA) fabricated using an acrylic polymer and an aliphatic monomer containing azobenzene is reported. The adhesion force of the proposed PSA can be switched by photoisomerizing the azobenzene moiety. The process induces the transition of surface energy and modulus of the PSA. Ultraviolet and visible light irradiation can switch the probe tack force from 200 to 4 kPa within 15-30 s. Adhesion switching is possible in a state wherein the PSA remains adhered to a substrate. Mini-LEDs are selectively transferred from the carrier PSA to a polydimethylsiloxane substrate following the process of partial adhesion switching of the PSA. The novel and switchable PSA, which exhibits a selective and repeatable adhesion switching property and high switching ratio when stimulated by light stimuli, may be potentially used to realize the mini-LED or micro-LED transfer processes.

Unlike traditional adhesives with a fixed adhesive force, switchable adhesives, which have an adhesive force that can be adjusted by external stimuli, are specifically designed to be released according to user demand, or to enable the transfer of fine electronic devices. Previously developed switchable adhesives have limitations such as a slow switching rate, narrow adhesion modulation range, or the lack of reusability. Thus, we fabricated switchable pressure-sensitive adhesives (PSAs) that can overcome these limitations. The adhesive force of each switchable PSA, which comprises an azobenzene-containing acrylic polymer and low molecular weight compounds, was designed to be activated/deactivated via ultraviolet (UV) and visible light irradiation. The adhesive force and UV intensity required for the switch were found to be dependent on the aliphatic chain length of the compound. The adhesive force of the SP-C10, i.e., a switchable PSA containing a azobenzene compound with an aliphatic chain of 10 hydrocarbons, increased to 3.5 N from nearly zero in response to only 30 s of low-level (25 mW cm−2) UV irradiation. Additionally, SP-C10 did not lose its adhesive force even after 30 cycles of repeated adhesion switching. The mechanism of adhesion switching influenced by UV intensity and the structure of low molecular weight azobenzene compounds are also reported.

Cross-laminated timber (CLT) is widely used as a building material, and recent interest in this material has increased rapidly due to environmental problems. For instance, the phenol resorcinol formaldehyde (PRF) used as an adhesive in CLT can release toxic gases during a fire, resulting in adverse health effects. These properties cause concern regarding the use of CLT as a building material and restrict its use. In this study, the effects of the particle size and size distribution of clays on the thermal properties and flame retardancy of composites were investigated. Ethylene vinyl acetate (EVA)/clay adhesive sheets were prepared to substitute for the PRF that is used to cohere the timber in CLT. To increase the dispersibility of clay inside the EVA matrix, the clays were organically treated using cetyltrimethylammonium bromide (CTAB), and the interlayer spacing was determined via X-ray diffraction. In addition, the EVA/clay adhesive sheets presented a broad peak due to their good dispersion. As the clay contents increasingly degraded, the thermal properties, such as the degradation temperature and final residue, of the EVA/clay adhesive sheets enhanced. However, the adhesive sheets showed continuous improvement only for a specific particle size and size distribution due to the difference in the degree of organic treatment. The results from cone calorimetry showed a decrease in the heat release rate (HRR) as the clay content increased. With respect to the flame retardancy of EVA/clay adhesive sheets, clay with a large particle size is more effective than clay with a small particle size, while the size distribution does not have a strong influence.

Fixing layers in flexible displays requires acrylic pressure-sensitive adhesives (PSAs) that can withstand deformation and folding stress. In this study, the effects of a styrene–isoprene–styrene (SIS) elastomer on the adhesion performance and recovery of an acrylic PSA were investigated. In simple blends, the peel strength and probe tack of a PSA sheet decreased slightly as the SIS content increased owing to the elastomer’s soft nature. In contrast, the lap shear stress increased slightly owing to entanglement of the SIS polymer chains. In stress relaxation tests, the effect of SIS on PSA recovery was limited by weak bonding. Therefore, the use of thermal crosslinking to improve bonding within PSA/SIS blends was examined. Although increased crosslinking decreased the peel strength and probe tack, the adhesion properties were maintained and the lap shear stress and strain increased. Further, PSA recovery improved in stress relaxation tests, but a specific strain could not be endured above 1 phr crosslinking agent. Thus, crosslinking with an elastomer plays an active role in the adhesion performance and recovery of acrylic PSAs. However, as the PSA sheet becomes opaque in the presence of SIS, the display applications of this system are limited and further improvements are necessary.

A silicone acrylic pressure-sensitive adhesive (SPSA) was synthesized using a silicone macro-azo-initiator as an adhesive for low surface energy substrates. The SPSA had a high peel strength value on an SUS (steel use stainless) substrate due to its low surface energy; however, it had very low peel strength values on low surface energy substrates such as PP and PE. In this study, a silicone urethane dimethacrylate (SiUDMA) oligomer was synthesized as a modifier for the SPSA. The SiUDMA had a much lower viscosity and a slightly higher surface energy than the SPSA. The addition of SiUDMA increased the peel strength of the SPSA regardless of the substrate. In the case of the PP and PE substrates, the increase of peel strength with SiUDMA addition was remarkably large when compared to the SUS substrate because SiUDMA significantly reduced SPSA viscosity and improved its fluidity, consequently facilitating the wetting of the rough surfaces of PP and PE. The rough surfaces of PP and PE could also aid the spreading of SiUDMA-modified SPSA and enlarge the interfacial area. On the other hand, UV curing of SiUDMA and the SiUDMA-modified SPSA resulted in a serious decrease in the peel strength on the PP and PE substrates due to the decrease in the fluidity of the SPSA material, but the SAFT of SPSA increased with UV irradiation.

For flexible displays, recovery and relaxation of acrylic pressure-sensitive adhesives (PSA) must be enhanced; however, only a few studies have focused on their optimization. High cross-linking density of the PSA leads to improved recovery but deteriorates the stress relaxation; thus, it is difficult to perform optimization by simply controlling the cross-linking density. Herein, it was determined that a UV-patterned PSA with both high and low cross-linking densities in a single layer enables the optimization of both recovery and relaxation. By introducing the UV-patterned PSA, the elasticity and recovery largely deteriorated but the stress relaxation was significantly improved, compared to that of the nonpatterned PSA, and this effect was enhanced with increases in the applied strain. Thereby, the recovery and relaxation were well-optimized with both values above 71% only at 300% strain. The recovery and relaxation of PSA had, respectively, a positive and negative correlation with the storage modulus.

The challenge of developing epoxy-foam adhesives for automotive applications lies in the difficulties associated with finding the optimal composition ensuring both high expansion ratio and mechanical strength. Herein, epoxy-foam adhesives based on epoxy composites were prepared and evaluated for automotive applications. The curing behavior was studied by differential scanning calorimetry, and the morphology was characterized non-destructively by micro X-ray computed tomography. Increasing the content of rubber-modified epoxy (RME) retarded the curing reaction by increasing the expansion ratio and affected the lap shear strength. A high expansion ratio and suitable mechanical strength was observed for epoxy-foam adhesives containing 10 or 15 wt% RME.

Automotive structural adhesives must show a steady toughness performance in the temperature range of −40 °C to 80 °C, considering their actual usage environments. Core-shell rubber (CSR) nanoparticles are known to enhance the toughness of epoxy systems. In this study, a CSR, pre-dispersed, diglycidyl epoxy of bisphenol A (DGEBA) mixture at 35 wt % (KDAD-7101, Kukdo Chemical, Seoul, Korea) was used as a toughener for an automotive structural epoxy adhesive system. A simple, single-component, epoxy system of DGEBA/dicyandiamide with a latent accelerator was adopted, where the CSR content of the system was controlled from 0 to 50 phr by the CSR mixture. To determine the curing conditions, we studied the curing behavior of the system by differential scanning calorimetry (DSC). Modulus variations of the cured bulk epoxies were studied using a dynamic mechanical analyzer (DMA) in the dual cantilever mode. The flexural modulus of the cured epoxies at various temperatures (−40, −10, 20, 50, and 80 °C) showed the same tendency as the DMA results, and as the flexural strength, except at 0 phr. On the other hand, the strain at break exhibited the opposite tendency to the flexural modulus. To study the adhesion behavior, we performed single-lap joint (SLJ) and impact wedge-peel (IWP) tests. As the CSR content increased, the strength of the SLJ and dynamic resistance to the cleavage of the IWP improved. In particular, the SLJ showed excellent strength at low temperatures (32.74 MPa at 50 phr @ −40 °C (i.e., an 190% improvement compared to 17.2 MPa at 0 phr @ −40 °C)), and the IWP showed excellent energy absorption at high temperatures (21.73 J at 50 phr @ 80 °C (i.e., a 976% improvement compared to 2.07 J at 0 phr @ 80 °C)). The results were discussed in relation to the changes in the properties of the bulk epoxy depending on the temperature and CSR content. The morphology of the fracture surface was also provided, which offered useful information for composition studies using the CSR/epoxy system.

Owing to their excellent properties, such as transparency, resistance to oxidation, and high adhesivity, acrylic pressure-sensitive adhesives (PSAs) are widely used. Recently, solvent-free acrylic PSAs, which are typically prepared via photopolymerization, have attracted increasing attention because of the current strict environmental regulations. UV light is commonly used as an excitation source for photopolymerization, whereas visible light, which is safer for humans, is rarely utilized. In this study, we prepared solvent-free acrylic PSAs via visible light-driven photoredox-mediated radical polymerization. Three &alpha, haloesters were used as additives to overcome critical shortcomings, such as the previously reported low film curing rate and poor transparency observed during additive-free photocatalytic polymerization. The film curing rate was greatly increased in the presence of &alpha, haloesters, which lowered the photocatalyst loadings and, hence, improved the film transparency. These results confirmed that our method could be widely used to prepare general-purpose solvent-free PSAs&mdash, in particular, optically clear adhesives for electronics.

Acrylic pressure-sensitive adhesives (PSAs) are used as fixatives between layers of a display. PSAs&rsquo, function is an important factor that determines the performance of the display. Of the various display types available, the touch screen panel (TSP) of smart devices is firmly related to the relative permittivity of the elementals. Therefore, adjusting the relative permittivity of the PSA is indispensable for driving the TSP. Accordingly, selected acrylic pre-polymers were polymerized and the pre-polymer was blended and cross-linked with monomers with different chemical structure to adjust the relative permittivity. The monomers were hexametyldisiloxane (HMDS), N-vinylcaprolactam (NVC), tert-butyl acrylate (TBA), and isooctadecyl acrylate (ISTA). The gel fraction and transmittance as a function of the monomers show a similar result to the pure acrylic PSA. However, the gel fraction value decreased to about 90% and the transmittance decreased to about 85%, due to the immiscibility between nonpolar HMDS and acrylic PSA. On the other hand, the adhesion properties were improved when NVC was added because of the polarity of the nitrogen group. In addition, the relative permittivity of the PSA decreased regardless of the monomer chosen. There was, however, a difference in the optimal content of each monomer, and NVC decreased from 4 phr content to about 3.4 in reducing relative permittivity. Through the above results, it was confirmed that NVC having a nitrogen group is most advantageous in lowering adhesion properties and relative permittivity, and necessitates further research based on the findings.

Carbon fiber reinforced plastic (CFRP) is currently used as a lightweight material in various parts of automobiles. However, fiber reinforced plastic (FRP) material may be damaged at the time of joining via mechanical bonding, therefore, adhesion is important. When bonding is conducted without surface CFRP treatment, interfacial destruction occurs during which the adhesive falls off along with the CFRP. Mechanical strength and fracture shape were investigated depending on the surface treatment (pristine, plasma treatment times, and plasma treatment times plus epoxy modified primer coating). The plasma treatment effect was verified using the contact angle and X-ray photoelectron spectroscopy. The wettability of the epoxy modified primer (EMP) coating was confirmed through surface morphology analysis, followed by observation of mechanical properties and fracture shape. Based on test data collected from 10 instances of plasma treatment, the EMP coating showed 115% higher strength than that of pristine CFRP. The adhesive failure shape also changed from interfacial failure to mixed-mode failure. Thus, applying an EMP coating during the automotive parts stage enhances the effect of CFRP surface treatment.

Research on solvent-free acrylic pressure-sensitive adhesives (PSAs) has tremendously grown over the last few decades due to the stringent regulations to control volatile organic compound emissions. They are mostly prepared in the presence of photoinitiators using high-energy UV light that causes several issues such as those associated with radiation safety. Herein, for the first time, solvent-free acrylic PSAs were prepared through visible-light-driven photocatalytic free radical polymerization. Importantly, we found that the use of N-vinyl-based monomers noticeably enhances the rate and conversion of polymerizations, thereby eliminating the need for additives (e.g. α-haloester and sacrificial electron donor) that are usually required for photoredox-mediated free radical polymerization, but concurrently needs an additional purification process to remove residues. Combined experiments and quantum chemical calculations suggest that N-vinyl-based monomers facilitate electron transfer between monomers and photocatalysts, which is responsible for the enhanced rate and converison of polymerization. Viscoelasticity, mechanical strength and adhesion performance of acrylic PSAs were well adjusted in a broad range by controlling the monomer composition, suggesting that our new method would replace the existing photoinitiated free radical polymerization.

Acrylic pressure-sensitive adhesives (PSAs) are extensively used to fasten display layers. Although PSAs must show sufficient adhesion for application to existing displays, PSAs must utilize stretching and recovery to mitigate the effects of deformation for application to flexible displays. Therefore, in this study, an ultraviolet (UV)-patterned acrylic PSA was prepared by incorporating a region showing variable degrees of crosslinking achieved using a contrasting UV-patterned film. The gel fraction was measured to confirm the degree of crosslinking, and the adhesion was measured by peel strength, pull-off, and lap shear tests. Dynamic mechanical analysis (DMA) stress relaxation was used to measure the recovery. The highest gel fraction was obtained at a crosslinker content of 1 part per hundred resin (phr) and a UV dose of 1600 mJ/cm2. The low-crosslink-density area was optimized using the 50% gray patterned film. The acrylic PSA prepared with 0.001 phr of 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBT), a light-emitting compound used to visualize the pattern formation, showed the clearest pattern. Although the adhesion of the UV-patterned acrylic PSA initially decreased, it gradually increased for the 2 mm pattern. However, the shear adhesion was maintained without appreciably changing. Although the acrylic PSA could not withstand certain deformation conditions during UV patterning, it was stretchable at a pattern size of 4 mm, and the recovery increased to approximately 60% at a pattern size of 2 mm. The results confirmed that the UV-patterned PSA is suitable for application to flexible displays.

Epoxy, which exhibits tunable mechanical properties, has been investigated to overcome its inherent brittleness and extend its application range. Herein, various epoxy composites, including a toughened polymer, elastomer, and pressure-sensitive adhesive (PSA), with tunable mechanical properties were prepared by adjusting the ratio of a rigid aromatic epoxy and flexible aliphatic epoxy. A flexible aliphatic epoxy that incorporated urethane linkages was used to extend the tuning range; it was synthesized via an eco-friendly route utilizing CO2 without isocyanate. The prepared toughened polymer-like and elastomeric epoxy composites can be used in structural epoxy adhesive (lap shear strength = 27.8 MPa) and flexible electronics (plastic strain PSA . This work proposes a new application of eco-friendly epoxies and a strategy to facilitate the wide tuning range of epoxy materials, including PSAs.

The use of acrylic pressure sensitive adhesives (PSAs) in flexible displays involves their attachment to each layered device. Due to the high industrial demand of flexible displays, acrylic PSAs must necessarily exhibit high flexibility. In this study, the effect of the crosslinking density of acrylic PSAs on their adhesion and flexibility properties has been investigated by incorporating a diisocyanate crosslinking agent into the PSA structure. As the content of the crosslinking agent increased, the measured peel strength and tack of the synthesized PSAs decreased, while the maximum value of the lap shear stress increased. In addition, the maximum stress and shear strain determined for the crosslinked PSA specimens decreased with an increase in the crosslinking agent concentration (although, the stress measured at low strain values was initially increasing until the crosslinking agent content reached 1 phr). The results of stress relaxation testing showed a stress increase at specified strain levels with an increase in the crosslinking agent content up to 0.5 phr, while the results of creep testing revealed that the measured strain was inversely proportional to the degree of elastic recovery. The obtained data indicate that crosslinking generally improves the PSA properties related to their use in flexible display applications; however, very high crosslinking densities produce mostly a negative effect on the PSA flexibility and adhesion characteristics.

Acrylic pressure-sensitive adhesive (PSA) is used to fix each layer of a flexible display. Acrylic PSA needs to satisfy specific elongation and recovery requirements so that reliability of the flexible display can be achieved. For this reason, we aimed to design an acrylic PSA/acrylic elastomer (AE) blend and to study how some viscoelastic and adhesion properties are influenced by the AE content into the mixed, blended system. Samples were characterized by UV&ndash, Vis spectrophotometry for transmittance, texture analysis for adhesion performances, and dynamic mechanical analysis (DMA) for recovery and viscoelasticity. When acrylic PSA/AE was simply blended, the adhesion performance changed due to the influence of the long molecular chains of AE. Based on this result, the AE content was fixed at 10 wt %, and acrylic PSA prepolymer was crosslinked at different concentrations of crosslinking agent. Peel strength and probe tack decreased as the concentration of crosslinking agent increased, as reported in previous studies. On the other hand, as the content of the crosslinking agent increased, recovery characteristics were improved. Additionally, as the content of the crosslinking agent increased, the storage modulus also increased, although the glass-transition temperature was not affected. According to these findings, we successfully proved the possibility of using AE to adjust adhesion performance and recovery of acrylic PSA for designing flexible displays.

Lignin is the second-most abundant component of plant material and has been proven to be a promising natural resource for adhesive and coating applications. Adhesive and coating materials are conventionally obtained from nonrenewable sources such as petroleum-based sources. Growing demand and dwindling resources have resulted in the chemical industry having to increasingly rely on renewable resources. Over the past decade, numerous research studies have been conducted using various methods and techniques to develop lignin as adhesives and coatings because of its renewability. This chapter details the process of lignin recovery, methods developed to synthesize more useful lignin-based chemical products, lignin as adhesive raw material of the future or research energy, lignin-based polymer composites, and developing lignin-based coatings. Processes and methods developed to isolate useful lignin-based chemical products are discussed. Major topics that are discussed in detail include the structure and processing of lignocellulosic biomass to lignin, polymers utilizing lignin as a macro monomer, the synthesis of monomers and polymers from monolignol, polymers from lignin-derived chemicals such as vanillin, and the use of lignin-based coatings in different products. The chapter is ideal for polymer scientists, engineers, and students interested in the development of lignin as adhesives and coatings.

We have previously performed stress relaxation tests to confirm the recovery of acrylic pressure-sensitive adhesives (PSAs) for flexible display applications. In this study, we investigate the influence of the crosslinking density of solvent-based silicone PSAs on their adhesion and recovery using a platinum catalyst (Pt) as a crosslinking agent. The peel strength and probe tack of the silicone PSAs increased in direct relation to the Pt content, until the Pt content reached 0.5–1.0 phr. Although these results prompted us to investigate the recovery of crosslinked silicone PSAs, there is a discrepancy between the low release force of silicone PSAs (0–0.5 phr of Pt) and that of the fluorine release film. For this reason, it was very hard to obtain reliable, reproducible results for silicone PSAs. Instead, we chose to investigate the recovery of a silicone PSA crosslinked with 1.0 phr of a crosslinking agent with a high release force. This combination exhibited the best adhesion performance and recovery across various temperature conditions.

UV–thermal dual curable adhesives were prepared using an acrylic resin with 2,2′-azobis(4-methoxy-2.4-dimethyl valeronitrile), which exhibits ten hour half-life decomposition temperature of 30 °C, in order to address issues related to monomers that cannot be cured in a shaded area during the display bonding process. We investigated the dual curing behavior of the resin with variations in the UV dose and in the thermal radical initiator content, and we also assessed the influence of the dual curing on the mechanical properties. The curing behavior was assessed through the use of photo-DSC and FT-IR conversion. In addition, the degree of crosslinking was investigated via gel fraction, and the thermal properties were measured via DSC and TGA. Peel strength, probe tack and pull-off tests were performed to evaluate the bonding performance of the dual curable adhesives. The adequate level of thermal radical initiator (TRI) content to obtain a high conversion and degree of crosslinking was approximately 0.1 phr. These were the optimum values to adjust the balance between the unstable state and the insufficient contents, and the thermal curing behavior was promoted with an increase in the UV dose and in the thermal initiator content. The gel fraction and the FT-IR ATR results clearly indicate that an enhanced network structure was formed under low UV and high thermal radical initiator content conditions. Therefore, in a low UV dose condition, the mechanical properties improved, even with a low molecular weight due to a high network formation.

Although pressure-sensitive adhesives can be used in a wide variety of applications, their durable temperature range is generally limited due to their weak cohesive power. In this study, melamine-based compounds bearing UV-curable N -methyl acrylate groups and thermal-curable N -methoxylmethyl groups were evaluated as dual-curable materials for PSAs. The photo-curing abilities of MAOs are slightly inferior to those of TMP(EO)TA due to their rigid melamine core, but they could be improved by substituting acrylate groups onto the melamine core. In photo shrinkage, MAOs are less shrunk than TMP(EO)TA because of their greater size. Following the dual curing process, the storage modulus of MAO-containing PSA was maintained at approximately 3E6 dyne/cm 2 , even at high temperature. In particular, MAOs are very tolerant to heat exposure and thus could afford highly cohesive cured materials with a sufficient thermal curing process.

Acrylic pressure sensitive adhesives (PSAs) are used in the production of flexible displays. Acrylic PSAs must have high flexibility and recovery to meet the industry demands for flexible displays. For these reasons, acrylic PSAs were designed to determine the effects of molecular weight and cross-linking on acrylic PSAs. Stress relaxation and creep tests were measured with dynamic mechanical analysis to identify the flexibility and recovery of the acrylic PSAs. The molecular weight and glass transition temperature of the PSAs were measured by gel permeation chromatography and differential scanning calorimetry. A texture analyzer was used to measure the adhesion performance of the acrylic PSAs. With increasing molecular weight, the adhesion performance increased, especially from 86 to 108 K molecular weight PSAs. This is due to the entanglement of the polymer chains. The stress and recovery of cross-linked acrylic PSAs was much higher than that of neat acrylic PSAs. This result is attributed to in...

We have investigated the curing behavior of dual-curable acrylic resin to solve problems associated with curing of adhesives in shaded areas during display manufacture. A low-temperature curing-type thermal initiator, 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), with a 30°C half-life decomposition temperature was used in the investigation. Dual-curable adhesives were prepared according to the thermal initiator content and ultraviolet (UV) radiation dose. The effects of thermal initiator and UV irradiation on the curing behavior and viscoelasticity were investigated. Using Fourier-transform infrared spectroscopy and gel-fraction analysis, an evaluation was carried out to determine the degree of curing after dual UV/thermal curing. In addition, the real-time curing behavior was evaluated using thermogravimetric analysis, differential scanning calorimetry, and a UV/advanced rheometric expansion system. A lift-off test was carried out to verify the effects of dual curing on adhesion performance. Application of UV irradiation before thermal curing suppressed the thermal curing efficiency. Also, the network structure formed after dual curing with low UV dose showed higher crosslinking density. Therefore, the thermal initiator radical effectively influenced uncured areas with low curing temperature and initiator content without causing problems in UV-curable zones.

Optically clear adhesive (OCA) is a key material for assembling a touch screen panels (TSP). TSPs are now applied to many devices; therefore, their preparation and adhesion performance examination including durability can be beneficial work. Cloud point resistance means that a material does not have a hazy appearance under any environment, especially under sudden temperature and humidity changes. It is a critical property for an OCA because it is directly related to image display on a TSP. A hydrophilic component needs to be incorporated to the OCA, and 2-hydroxyethyl acrylate (HEA) was selected as the monomeric hydrophilic component. The change in adhesion performance under hygrothermal aging conditions was investigated. The aging behavior was also monitored using infrared (IR) and Raman spectra and chemiluminescence (CL) analysis. The HEA-containing pressure-sensitive adhesive (PSA) exhibited a good cloud point resistance and adhesion performance along with stable adhesion properties during aging. This consequence is considered suitable behavior for the prepared PSA as an OCA.

Optically clear adhesives (OCAs) are key components of touch screen panels (TSPs). It is important that OCAs do not affect the transparent electrodes in TSPs because OCAs are contacted to the transparent electrodes. Therefore, N-vinyl caprolactam (NVC) was incorporated in the composition of an acrylic pressure sensitive adhesive (PSA) with excluding an acidic component to maintain the cohesion for OCA preparation. With increasing amounts of NVC, the tack and peel strength of UV-cured PSA increased, but high amounts of NVC led to decreased peel strength. The UV-cured PSA films were placed in a high temperature and humidity chamber for 8 weeks to investigate the durability and corrosion property under hygrothermal conditions. In this study, the corrosion test method using copper foil was suggested as a simple and economical method and was used to evaluate the effect of NVC on the corrosion property of PSA. This method helped identify suitable OCAs that do not have corrosive property. PSA films containing more than 20 wt% of NVC promoted the corrosion of copper foil under hygrothermal aging conditions. The caprolactam ring was opened by moisture, and the PSA structure morphed into a polar structure during the aging process. This change caused a glass transition shift, an increase in the storage modulus at the rubbery plateau, and an increase in peel strength. The surface free energy of the PSA films also increased due to the increase in the polar property. However, high amounts of NVC caused a decrease in the peel strength after 8 weeks of aging because of increased molecular interactions.

Structural epoxy adhesives are commonly used in many industries but are limited by their flammability and low impact resistance. In order to improve the flame retardancy of epoxy adhesives, phosphorus-containing species are mainly used as reactive flame retardants, but these generally cause a decrease in mechanical strength. In this study, an oligomeric phosphorus-containing polyol (P-polyol) was synthesized by step polymerization between phenylphosphonic dichloride and ethylene glycol. The curing behavior, viscoelastic properties, adhesion performance, impact resistance, and flame retardancy of an epoxy resin containing P-polyol were characterized. Especially, impact resistance was evaluated by the impact wedge-peel test, an industry-standard testing method. The flame retardancy of the structural epoxy adhesive improved steadily with the addition of p-polyol. Impact resistance also improved at moderate P-polyol contents (8–10 parts per 100 parts of resin (8–10 phr)) but drastically decreased at a high content (15 phr). Based on this study, by incorporating P-polyol, it is possible to simultaneously improve the impact resistance and flame retardancy of structural epoxy adhesives. We obtained both a high impact resistance (dynamic resistance to cleavage: 16.9 N/mm) and flame retardancy (heat release capacity: 255 J/(g∙°C)) at an optimum P-polyol content (8 phr).

With the development of mobile display shapes from flat to flexible, the conventional glass cover window must be replaced by a film-type window. However, film-type cover windows have low impact resistance owing to their poor shock absorption. Therefore, to realize their application to flexible displays, it is essential to characterize and improve the shock-absorbing characteristics of the display film. In this study, a semi-interpenetrating network (semi-IPN) acrylic pressure-sensitive adhesive (PSA) consisting of a crosslinked network structure with a linear acrylic polymer chain was developed. The influence of the multi-functional monomer content and UV dose on the properties of the semi-IPN PSA was studied. The peel strength of the semi-IPN PSA was higher than 10 N/25 mm, which is generally considered as the PSA standard for mobile displays. With an increase in crosslink density, the storage modulus increased, but the damping factor decreased. To evaluate shock absorption, a falling ball impact tester was used to estimate the shock absorption ratio (ΔF) of the film. The semi-IPN PSA incorporating 30 phr of the multi-functional monomer exhibited higher ΔF (31.2%) than that of a conventional UV-curable PSA film (24.6%). This shows that even with the relatively low damping factor of the semi-IPN PSA, its structure could improve shock absorption. Moreover, we suggested two mechanisms for shock absorption of the semi-IPN PSA, energy dissipation into the PSA layer and extended contact time.

Isosorbide is newly used as a cross-linking agent to replace petroleum-based cross-linking agents for optically clear pressure-sensitive adhesives (PSAs). To control and determine the optimal concentration of isosorbide, the relationship between the glass transition temperature (Tg), referring to the viscoelastic characteristics of the organic polymeric materials as a key factor not only to synthesize the binders and but also to utilize the final products, and the concentration of isosorbide were investigated by measuring the optical properties, the adhesion performance via the peel strength, the probe tack, and the shear adhesion failure temperature. Consequently, it was found that the relationship provided by the linear plots between Tg and the other properties was strongly affected by the concentration of isosorbide due to the cross-linking density.

Pressure-sensitive adhesives (PSAs) and UV-curable acrylic systems are widely utilized in different industries, with particular attention currently directed at their use in the production of re-useable modules for smart devices. Herein, we developed a two-step UV/UV curing technique to overcome the limitations of dual curing methods conventionally used for PSA preparation and characterized the thus obtained acrylic adhesives by a range of techniques to determine the uncured marginal portion of the pre-polymer, viscoelastic properties, and adhesive properties (peel and tack). Insufficient primary curing resulted in marked physical property deterioration, while the expression of secondary curing was weak when excessive irradiation energy was supplied. Thus, it was concluded that the physical properties of acrylic PSAs could be controlled by adjustment of primary curing energy.

Ultraviolet (UV)-curable urethane acrylic oligomers were synthesized and then modified by the incorporation of silicone diols with different molecular weights to improve thermal stability for temporary bonding and debonding adhesives in a three-dimensional multi-chip package process. The UV-curing behaviors were investigated using photo-differential scanning calorimetry, Fourier transform infrared spectroscopy-attenuated total reflectance, and gel fraction, while the UV-curing kinetics was also studied. In addition, the thermal stability of the samples was checked using thermogravimetric analysis. UV-curing and thermal stability were more affected by the molecular weight of the silicone diols than by UV dose due to both the flexibility and the steric hindrance of the synthesized oligomer structures.

Thin silicon wafers are usually used for many devices and electronic fabrication development. But handling thin wafers are not easy since thin wafers may lose the supporting strength and crack. Using adhesives is the one of the possible solutions for thin wafer handling, and how to synthesize adhesives materials and investigate debonding behaviors for temporary bonding and debonding are an important research. In this work, laser irradiation is considered for debonding temporary adhesives in a 3D multi-chip package process because of its very fast debonding time about few seconds and irradiation stability than thermal or chemical debonding. The thermal curable adhesives were fast cured on high temperature by the laser irradiation. The emphasis is the choice of the specific laser process parameters such as the out-focusing length, the line spacing, and the scan speed. The surface morphology with various sets of these parameters is examined by optical microscopy. Also, peel strength before/after the laser irradiation is investigated. Based on this study, suitable process parameters and conditions are proposed for clean surface of silicon wafers and lower peel strength for easy debonding.

UV curable adhesives with different acrylic functionalities were synthesized for temporary bonding and debonding process in 3D multi-chip packaging process. The aim is to study various factors which have an influence on UV curing. The properties and curing behaviors were investigated by gel fraction, peel strength, probe tack, and shear adhesion failure temperature. The results show that the properties and curing behaviors are dependent on not only acrylic functionalities of binders but also UV doses and coating thickness.

Temporary bonding adhesives for a three-dimensional (3D) multichip package process have been synthesized. To enhance the thermal stability, the adhesives used a fluorinated silicon urethane acrylic binder and ultraviolet (UV) curing for crosslinked network structures. Focusing on different photoinitiator contents and UV doses, the UV-curing behaviors and thermal stability were studied using Fourier-transform infrared spectroscopy attenuated total reflectance, gel fraction, swelling ratio, shrinkage, and thermogravimetric analyses.

Epoxy foam adhesives are widely used for weight reduction, watertight property, and mechanical reinforcement effects. However, epoxy foam adhesives have poor impact resistance at higher expansion ratios. Hence, we prepared an epoxy composite foam adhesive with core&ndash, shell rubber (CSR) particles to improve the impact resistance and applied it to automotive structural adhesives. The curing behavior and pore structure were characterized by differential scanning calorimetry (DSC) and X-ray computed tomography (CT), respectively, and impact wedge&ndash, peel tests were conducted to quantitatively evaluate the resistance to cleavage of the CSR/epoxy composite foam adhesives under impact. At 5 and 10 phr CSR contents, the pore size and expansion ratio increased sufficiently due to the decrease in curing rate. However, at 20 phr CSR content, the pore size decreased, which might be due to the steric hindrance effect of the CSR particles. Notably, at 0 and 0.1 phr foaming agent contents, the resistance to cleavage of the adhesives under the impact wedge&ndash, peel condition significantly improved with increasing CSR content. Thus, the CSR/epoxy composite foam adhesive containing 0.1 phr foaming agent and 20 phr CSR particles showed high impact resistance (EC = 34,000 mJ/cm2) and sufficient expansion ratio (~148%).

The synthesis and characterization of acrylic polymer/Na-montmorillonite (Na-MMT) clay nano-composites pressure sensitive adhesives (PSA) are researched. The PSA/clay nano-composites were synthesized by in-situ emulsion polymerization and mechanical blending. And then, different amounts of nanoclay were dispersed in 2-ethylhexyl acrylate (2-EHA)/n-butyl acrylate (BA)/methyl methacylate (MMA)/acrylic acid (AA) monomer mixture, which was synthesized using in-situ emulsion polymerization technique. Morphological observation was carried out using X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). Viscoelastic properties of PSA/clay nano-composites were analyzed using advanced rheometric expansion system (ARES). The adhesion performances of synthesized PSA/clay nano-composites were determined by measurements of peel strength, probe tack and shear adhesion failure temperature.

Optically clear acrylic pressure-sensitive adhesives (PSAs) with different co-monomers were synthesized. This study employed metal chelate aluminum acetylacetonate and zirconium acetylacetonate as curing agents. The optical properties of the acrylic PSAs were examined by UV–visible spectroscopy and a prism coupler. In addition, the adhesion performance was obtained by assessing the peel strength, the tack, and the shear adhesion failure temperature. The decrease in the adhesion performance may be related to a higher crosslinking density, which also resulted in a higher gel content.

Acrylic pressure–sensitive adhesives (PSAs) were synthesized by solution polymerization using zirconium carboxyethyl acrylate (ZrCEA) with methyl aziridine derivatives (MAZ) as a curing agent. The acrylic PSAs were characterized by Fourier transform-infrared spectroscopy and gel contents. The viscoelastic properties of the acrylic PSAs were determined using an advanced rheometric expansion system. The adhesion performance of the acrylic PSAs was determined by measuring the probe tack, peel strength, shear adhesion failure temperature, and holding power. The optical properties of the acrylic PSAs were evaluated by the transmittance and refractive index. The results show that the adhesion performance and optical properties of the acrylic PSAs are influenced by the ZrCEA and MAZ content.

Acrylic pressure–sensitive adhesives (PSAs) with 2-phenoxy ethyl acrylate (PEA) were polymerized using UV-curing technology. This study examined the effects of PEA content and UV dose. The photo-polymerization behavior of the pre-polymer was examined by viscosity measurements, real-time Fourier transform infrared spectroscopy, and photo-differential scanning calorimetry. The curing behaviors of the acrylic PSAs were investigated by shrinkage test, a modular advanced rheometer system, and gel content. differential scanning calorimetry and Advanced Rheometric Expansion System were used to characterize the acrylic PSAs. Adhesion performances were measured by probe tack, peel strength, and shear adhesion failure temperature. The optical properties of acrylic PSAs were examined by UV–visible spectroscopy and prism coupler. The PEA content had a larger effect on improving the optical properties, than did the UV dose. The transmittances of the acrylic PSAs with 95%. The refractive indices of the a...

To improve the thermal stability of general acrylic pressure-sensitive adhesives (PSAs), polydimethylsiloxane (PDMS) was used and UV curing was employed. Silicone urethane methacrylate (SiUMA) was synthesized and introduced into acrylic PSAs for a semi-interpenetrating polymer network structure. The structure of the SiUMA was investigated through C NMR, H NMR, and FT-IR. The kinetics and behaviors of SiUMA (S1) were found by adding photoinitiators (PI) of 0.5, 1.0, 5.0, and 10 phr in a binder, which were examined using the photo-DSC (pDSC). After setting PI as 5.0 phr in a binder and UV intensity as 1000 mJ/cm2, the SiUMA, which was prepared by a radical polymerization, was added to acrylic PSA to 20, 40, 60, and 80% composition, and its kinetics and behaviors were analyzed by pDSC. Finally, the peel strength was checked to evaluate adhesion performance of the acrylic PSAs. The reaction rate was increased with increasing amounts of S1 and PI. Peel strength was dropped sharply with increasing crosslinking ...

Multi-chip packages (MCP) refers to a packaging configuration, connected via wirebonds to a multilayer circuit board, and protected by either a molded encapsulant or a low-cost ceramic package. As it requires high processing temperature, the adhesives for MCP need to show proper adhesion and thermal stability at high temperature. This study employed semi-interpenetrated (semi-IPN) structured polymer networks using UV-curing with acrylate terminated dual-curable urethane epoxy adhesive, a dipentaerythritol hexacrylate (DPHA), using hydroxydimethyl acetophenone as photo-initiator. UV-curing and thermal stability focused on different photo-initiator contents were investigated using photo-DSC, FTIR-ATR spectroscopy, gel content and TGA. The results show that UV-curable acrylic formulations with different content of photo-initiator affects UV-curing and thermal stability.

Optically clear PSAs-TiO2 nano-composites were investigated for the purpose of display. TiO2 nano-particles can be directly incorporated into the polymer matrix to form high refractive index PSAs-nanocomposites films. Moreover, this study also employed semi-interpenetrated structured polymer network through the process of UV-curing with high refractive trifunctional acrylic monomer. The optical properties of PSAs-TiO2 nano-composites were examined by using UV-visible spectroscopy and a prism coupler. Viscoelastic properties were obtained by ARES and adhesion performance was measured by the peel strength, probe tack and shear adhesion failure temperature. Furthermore, curing behaviors of the PSAs-TiO2 nano-composites were investigated by using FTIR-ATR and gel content.

Acrylic pressure-sensitive adhesives (PSAs) need to show proper adhesion and improved wettability on the silicon wafer as the wafer becomes thinner. The acrylic copolymers were synthesized by solution radical polymerization of 2-ethylhexyl acrylate, ethyl acrylate, and acrylic acid with AIBN as an initiator. Adhesion performance and wettability of acrylic PSAs were studied depending on the content of lauryl side chains and the degree of crosslinking. The introduction of lauryl side chain was characterized by Fourier transform infrared spectroscopy. The adhesion performance of acrylic PSAs having lauryl side chain was investigated using contact angle, wettability, probe tack, peel strength, and cohesiveness tests. The wettability of acrylic PSAs was improved significantly with increasing the content of lauryl side chain.