Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications

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.     

Application of selected 2-methylbenzothiazoles AS cationic photoreactive crosslinkers for pressure-sensitive adhesives based on acrylics

Since their introduction half a century ago, acrylic pressure-sensitive adhesives have been successfully applied in many fields. They are used in self-adhesive tapes, label signs, marking films and protective films as well as in medical pharmaceutical applications for plaster, in dermal dosage systems and in a wide range of biomedical electrodes. In the last 15 years or so, the UV technology, especially UV-crosslinking, is well established in the market and allows the production of UV-crosslinkable pressure-sensitive adhesives (PSA) based on acrylics with interesting performance. So much so that the larger manufacturers of pressure-sensitive adhesive materials and their suppliers now use very expensive equipment to study pressure-sensitive adhesive behavior: tack, peel adhesion and shear strength. The balance between adhesive and cohesive strength after the crosslinking process is very important and critical for properties of acrylic PSA in form of self-adhesive films. In this work the cationic UV-crosslinking of acrylic PSA containing epoxy groups in their structure and additionally cationic photoinitiators based on 2-methylbenzothiazoles as photoreactive crosslinkers have been investigated using UV-lamp as ultraviolet sources. The investigated acrylic PSA were synthesized from 80 wt% of butyl acrylate, and 20 wt% of glycidyl methacrylate. The use of selected photoreactive crosslinkers: 1,5-bis[N,N׳-(2-methylbenzothiazolium)]pentane diiodide and 1,10-bis[N,N׳-(2-methylbenzothiazolium)]decane diiodide allows manufacturing of high quality PSA materials with interesting properties, such as high tack, high peel adhesion, and excellent shear strength.     

Effect of crosslinker reaction rate on film properties for thermoset coatings

With thermoset coatings that employ room temperature crosslinkers, there is often a competition between film formation and crosslinking. In order to meet the early use/handling requirements for the coated substrate, formulators sometimes need to use crosslinkers with very fast room temperature reaction rates. However, when the coatings are based on aqueous latex resins, sufficient time must be allowed for latex particle coalescence to occur prior to crosslinking, or the resulting films will be partially or completely precross-linked, which is detrimental to many coatings properties. In this paper the effect of the crosslinker reaction rate on various film properties of coatings based on acrylic and acrylic/styrene latex resins will be investigated using several model carbodiimide-functional crosslinkers which have different reaction rates. Presented at the 76th Annual Meeting of the Fedeation of Societies for Coatings Technology on October 14–16, 1998, in New Orleans, LA. 727 Norristown Rd., Spring House, PA 19477, E-mail: RSCWTB@rohmhaas.com.     

UV-initiated crosslinking of photoreactive acrylic pressure-sensitive adhesives using excimer-laser

UV-initiated crosslinking technology is well established in the market and allows the production of a wide range of ultraviolet (UV)-crosslinkable pressure-sensitive adhesives (PSA) with interesting features. The balance between such properties as adhesive and cohesive strengths within the crosslinked self-adhesive coatings is critical for their performance. The UV crosslinking of acrylic PSA, especially for following properties: tack, peel adhesion, and shear strength of self-adhesive polymer layers, has been investigated using UV excimer-laser and UV lamp as UV sources. It was observed that after UV crosslinking of acrylic PSA using excimer-laser in comparison with typical UV lamp, high-quality PSA products with excellent properties, such as tack, peel adhesion, shear strength, and shrinkage were received.     

Photocrosslinking of solvent-based acrylic pressure-sensitive adhesives (PSA) by the use of selected photoinitiators type I

This study investigated the photocrosslinking of solvent-based acrylic pressure-sensitive adhesives (PSA) containing selected photoinitiators type I, known as α-cleavage photoinitiators. Photocrosslinking of PSA, especially of acrylic PSA, is well established crosslinking process using the UV radiation technology. UV-initiated crosslinking of acrylic PSA allows the synthesis of the wide range of UV-crosslinkable PSA with the interesting features. Especially, the important balances of properties such as adhesive and cohesive strength which are typically critical for the application performance can be achieved by this technology. The selection of suitable photoinitiator plays an important role to obtain the optimum properties of acrylic PSA including tack, peel adhesion, and shear strength. In this study, the investigations on different saturated conventional photoinitiators of type I for solvent-based PSA were carried out. The effects of photoinitiator concentration, UV crosslinking time and UV dose on the tack, peel strength, and shear strength were explored in detail for guiding the choice of photoinitiators to fabricate advanced PSA for industrial usage.     

Influence of selected photoinitiators on important properties of photoreactive acrylic pressure‐sensitive adhesives

Photoreactive solvent‐borne pressure‐sensitive adhesives are not commercially available in the market. The reason for it is that the UV‐initiated crosslinking has sense only in the case of solvent‐free self‐adhesive systems. Investigations conducted in Institute of Chemical Organic Technology have shown that the photoreactive solvent‐borne acrylic PSA are conventional crosslinked solvent‐borne acrylic PSA used as crosslinking agents typical metal chelates as titanium acetylacetonate (TiACA), aluminum acetylacetonate (AlACA) or thermal reactive crosslinker melamine‐formaldehyde resin Cymel 303 clear considered. The main purpose of the investigation was to study the influence of diverse photoinitiators on main properties, such as shrinkage, tack, peel adhesion, and shear strength of solvent‐based acrylic pressure‐sensitive adhesives. The interesting alternative to conventional photoinitiators is unsaturated photoinitiators described in this article. Following unsaturated photoinitiators were used: 4‐acryloyloxy benzophenone, 4‐acryloyloxyethoxy benzophenone, and 4‐acryloyloxybutoxy benzophenone. The influence of the crosslinking agents or crosslinking methods was determined in relation to the reaction time and to the concentration versus adhesion properties. The increase of photoinitiator concentration causes in the decrease of the shrinkage. Increasing the UV dose during the crosslinking of acrylic PSA film leads clearly to better shrinkage resistance. The best results of the lowest shrinkage value of 0.35% were given by using 4‐acryloyloxy benzophenone. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

水性聚氨酯与丙烯酸酯乳液交联反应的研究

将由双丙酮丙烯酰胺（DAAM）参与共聚的丙烯酸酯乳液与含有肼基的聚 氨酯水分散体混合后，得到了交联型聚氨酯／丙烯酸酯复合乳液，利用红外光谱和透射电镜技术证实了酮羰基与肼基之间的交联反应的发生。对乳液膜性能的研究结果表明，交联反应极大地提高了乳液膜的耐水性，耐溶性，断裂强度，断裂伸长率。     

Influence of iron carbide filler in carbon matrix on the adhesive properties of acrylic pressure-sensitive adhesives

Inorganic fillers can improve coating properties, such as scratch resistance and UV stability and can significantly enhance the fillers usability in coatings and realize new market opportunities. In the pressure-sensitive adhesive (PSA) technology the inorganic fillers are used to change the very important properties of pressure-sensitive adhesives, such as tack, peel adhesion and shear strength. In the current study, the above mentioned properties of synthesized acrylic PSA using iron carbide filler in carbon matrix were investigated. The acrylic PSA containing iron carbide filler (Fe₃C,C) was examined with SEM/EDX technique and the PSA adhesive and cohesive properties were examined by using a special strength machine according to AFERA tests. The conclusion is that the application of this kind of fillers allows the manufacturing of self-adhesive materials with moderate adhesiveness and very good removability.     

Improvement in wettability of pressure‐sensitive adhesive on silicon wafer using crosslinking agent with siloxane groups

Acrylic copolymers are prepared by radical polymerization of 2‐ethylhexyl acrylate, ethyl acrylate, and acrylic acid followed by crosslinking to manufacture the pressure‐sensitive adhesives (PSAs) for silicon wafer protection. Both higher reliability and wettability are required for the protective acrylic PSAs in the semiconductor processing applications. The siloxane linkages are introduced in the acrylic PSAs via crosslinking with siloxane‐containing crosslinking agent to modify the thermal and wetting properties of PSAs efficiently. The more efficient formation of crosslinked network structure was achieved with higher content of tetra‐functional crosslinking agent, and the surface energy of PSAs decreased significantly with increasing the content of siloxane linkage resulting in the improved areal wetting rate. The thermal stability of PSAs was also improved significantly by incorporation of siloxane linkages. The adhesion properties such as peel strength and probe tack of acrylic PSAs decreased significantly by increasing the content of either crosslinking agent or siloxane linkage. The acrylic PSA with siloxane group showed both satisfactory wetting and clean debonding properties for the optimal protection of thin silicon wafers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

The effects of substrate surface properties on tack performance of acrylic Pressure-Sensitive Adhesives (PSAs)

The effects of substrate surface free energy (SFE) and substrate roughness on tack performance of adhesive tapes containing synthesized model acrylic pressure-sensitive adhesive (PSA) have been investigated. In order to study the influence of substrate SFE on tack the adherents with the same surface roughness (expressed by selected amplitude parameters) were used: PTFE, PP, PE, ABS, PC, PMMA, stainless steel and glass. The relationship between substrate roughness and tack was investigated using two polypropylene plates (PP and PP_rough) characterized as having the same wettability (SFE). For tack determination the most common method in the PSA tapes industry was employed (loop tack test). The conducted experiments showed that substrate SFE is a crucial factor governing tack properties of acrylic PSAs. In general, a larger difference between the SFE values of the substrate and adhesive were correlated with greater tack values. The dependence of tack and SFE was significantly influenced by crosslinking degree and layer coat weight of model acrylic PSA. The experiments carried out in the second part of the study revealed that the adhesive׳s viscoelastic properties control the tack properties on rough substrates, however, the final tack performance was found to be strongly affected by the level of substrate roughness and PSA thickness.     

Crosslinking of pressure sensitive adhesive based on water‐borne acrylate

This publication shows how the kind of crosslinking agents and their contents influence important properties of acrylic based pressure‐sensitive adhesive (PSA) dispersions such as tack, adhesion and cohesion. Synthesized PSAs based on acrylic polymers, containing 2‐ethylhexyl acrylate, butyl acrylate, vinyl acetate, styrene and acrylic acid are used in the preparation of self‐adhesive dispersions used as coating of polyethylene foams and poly(vinyl chloride) and polyester foils. © 2003 Society of Chemical Industry     

Post-crosslinking of water-borne urethanes

Water-borne polyurethanes are used as coatings because of their high performance. Polyurethanes deliver excellent abrasion resistance, tensile strength, impact strength and good low temperature flexibility. Due to process considerations, most water-borne urethane dispersions are linear thermoplastic polymers which can only contain low levels of pre-crosslinking. To improve chemical resistance, urethane dispersions can be post-crosslinked during or after film formation by a variety of crosslinkers. This paper will review the effects on coating properties after crosslinking with melamine, aziridine, epoxy silane, carbodiimide or water-dispersed isocyanate.     

Urethane/acrylic composite polymer emulsions

Blends of waterborne urethane and acrylic polymer systems were studied to obtain a composite emulsion that would have all of the advantages of the two polymers without their associated disadvantages. An approach to achieve extensive polymer-polymer interactions through crosslinking reactions was studied to optimize the positive aspects of each polymer. The crosslink system used an acrylic polymer emulsion containing keto or aldo groups and a polyurethane dispersion incorporating a hydrazine group. The degree of crosslinking was determined by FT-IR Single package, ambient temperature crosslinking emulsions were obtained by using this system. In addition to the excellent properties these two polymers normally possess, the crosslinked blends exhibit synergistic effects in film properties, such as good solvent resistance and low heat sensitivity over a wide range. Composite polymers of this type could be useful in applications where high durability is required: tennis court coatings, floor coatings, laminating adhesives. and paper and textile finishes.     

Effect of crosslinkers on adhesion properties of electron beam curable polyurethane pressure sensitive adhesive

Polyurethane (PU) based pressure-sensitive adhesive (PSA) using monofunctional urethane acrylate (MFUA) and difunctional urethane acrylate (DFUA) has been cured using electron beam (e-beam) irradiation. The effect of varying e-beam doses on the adhesion properties viz. peel adhesion, shear adhesion and initial tack of PSA has been studied. Effect of various crosslinkers viz. methylene diphenyl di-isocynate (MDI), isophorone di-isocynate (IPDI) and triallyl isocynurate (TAIC) on adhesion properties as well as on moisture vapor transmission rate (MVTR) and gel content of the PSA has also been studied. It was observed that with increasing dose and concentration of crosslinkers all the adhesion properties viz. peel adhesion, shear adhesion strength and initial tack were increased and reached a plateau, and then a decline was observed with further increasing of the dose and crosslinkers concentration. Similarly, MVTR decreased and the gel content increased with increasing dose of electron beam.     

UV-crosslinkable acrylic pressure-sensitive adhesives for industrial application

The focus of the development of UV-crosslinkable self-adhesive medical products is on one hand directed toward customer-oriented requirements such as tack, adhesion, cohesion, biocompatibility, and permeability for water vapor or air. The customer wants highly tolerable, breathable products, which are also characterized by very good skin and optimal release. On the other hand, the economic targets of medical products manufactures must be considered. Development in the area of UV-crosslinkable acrylic pressure-sensitive adhesives (PSA) for medical application describes the variety of acrylic composition, residue monomers content, quality control of peel adhesion level and repeating during the time, biocompatibility of the acrylic self-adhesive layers and their practical medical application. The new class of unsaturable copolymerizable photoinitiator, such as 4-acryloyloxy benzophenone was used for the synthesis of photoreactive UV-crosslinkable solvent-borne acrylic PSA. The properties of acrylic PSA were determined as well.     

反光膜用聚丙烯酸酯压敏胶的合成

以醋酸乙烯酯(VAc)为硬单体、丙烯酸异辛酯(EHA)和丙烯酸丁酯(BA)为软单体、丙烯酸(AA)和丙烯酸羟丙酯(HPA)为交联单体、过氧化苯甲酰(BPO)为引发剂和乙酸乙酯为溶剂,制备了反光膜用溶剂型聚丙烯酸酯PSA(压敏胶)。考察了各单体和引发剂含量对PSA性能的影响。结果表明:当w(VAc)=15%、m(EHA)∶m(BA)=3∶1、w(AA)=4%、w(HPA)=6%和w(BPO)=0.8%时,PSA的综合性能优异,并且完全满足反光膜的使用要求。     

Molecular parameters and their relation to the adhesive performance of emulsion acrylic pressure‐sensitive adhesives. II. Effect of crosslinking

Acrylic emulsion pressure‐sensitive adhesive (PSA) films generally have much lower shear holding power than that of their solvent‐borne counterparts for the same peel and tack. This is due to their discrete microgel morphology in the film. In contrast, film cast from solution‐polymerized acrylic PSA forms a continuous network as a result of crosslinking acrylic acid and aluminum acetyl acetonate (AAA) in the film following the solvent evaporation. Novel acrylic emulsion PSA was made by copolymerizing ≤1 wt % isobutoxy methyl acrylamide (IBMA) in the polymer backbone. The IBMA grafted the linear portion of the acrylic polymer with the microgels upon heating the film, which resulted in a significant increase in the shear holding power. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2558–2564, 2001     

Optical properties and adhesion performance of optically clear acrylic pressure sensitive adhesives using chelate metal acetylacetonate

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.     

Effect of propyleneimine external cross-linker on the properties of acrylate latex pressure sensitive adhesives

Acrylate pressure sensitive adhesive (PSA) latexes were synthesized via a starved monomer seeded semi-batch emulsion polymerization process with butyl acrylate (BA), methyl methacrylate (MMA), acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA). These PSA polymers were then cross-linked with trifunctional propyleneimine external cross-linker (SAC-100) to study the cross-linking reaction between carboxylic group of the polymer chain and cross-linking agent. It was found that cross-linking provided a significant influence on the film formation process based on the result of SEM analysis. In addition, with the increase of SAC-100 content, the gel content of the polymer increased significantly, while molecular weight between cross-link points (M_c) and the sol molecular weight (M_w, M_n) of the polymer decreased remarkably. The TGA result showed that the addition of the external cross-linker can enhance the thermal stability of the latex film. Moreover, for the cross-linked adhesive film, the shear strength was improved greatly while at the sacrifice of loop tack and peel strength, when compared with the uncross-linked counterparts. Besides, dynamic mechanical analysis (DMA) was also used to evaluate the viscoelastic properties of the acrylate emulsion PSA film.