Sawtooth-shaped stringiness with front frame formation for polyacrylic pressure-sensitive adhesives with two different molecular structures

The formation of sawtooth-shaped stringiness during 90° peeling was investigated using crosslinked poly(n-butyl acrylate–acrylic acid) and poly(2-ethylhexyl acrylate–acrylic acid) random copolymers with an acrylic acid content of 5 wt.% and different crosslinking degrees as pressure-sensitive adhesives (PSAs). The gel fraction was measured by toluene extraction of PSA, and it increased with crosslinker content for both systems. The observed stringiness was sawtooth-shaped, but there were three different types; both the typical sawtooth shape and the frame formed at the front tip with interfacial failure, and the sawtooth shape formed with cohesive failure. The change in the stringiness shape was affected strongly by the gel fraction of PSA. The peel rate under constant peel load was measured and revealed that the peel rate was lowest upon formation of the front frame type. A good relation was found between peel rate and peel strength, with a greater peel strength upon formation of the front frame type. The concentrated stress at the peeling tip is released by progress of peeling and deformation of the adhesive layer (stringiness) for no frame type. On the other hand, the sufficient interfacial adhesion delays the progress of peeling, and the applied larger stress causes cavitation in the PSA layer for front frame type. The formed cavity grows and the front frame type formed as a result. That is, internal deformation occurred preferentially over peeling. In order to improve the peel strength, the front frame type is the most useful stringiness shape.     

Influence of the degree of crosslinking on the stringiness of crosslinked polyacrylic pressure‐sensitive adhesives

The stringiness of crosslinked polyacrylic pressure‐sensitive adhesive (PSA) was observed during 90° peeling under the constant peel load. The random copolymer of butyl acrylate with 5 wt % acrylic acid crosslinked by N,N,N′,N′‐tetraglycidyl‐m‐xylenediamine was used as PSA. All observed stringiness upon peeling was sawtooth‐shaped, but it could be classified into three types dependent on the degree of crosslinking. The typical sawtooth‐shaped stringiness with interfacial failure was observed at the relatively higher crosslinker content ranging from 0.008 to 0.016 chemical equivalents (Eq.), where the PSA has high cohesive strength and low interfacial adhesion. The frame formed at the front end of stringiness at the content ranging from 0.002 to 0.004 Eq. Sufficient interfacial adhesion and deformability generate large internal deformation of the PSA layer. Internal deformation occurred preferentially over peeling as a result of front frame formation. The mode of peeling was changed from cohesive failure to interfacial failure in this range of crosslinker content. The sawtooth‐shaped with cohesive failure was observed at the lower content ranging from 0 to 0.001 Eq. The PSA has high interfacial adhesion and low cohesive strength, and thus exhibited cohesive failure. The PSA after peeling remained in the shape of belts. It was found that the shape of stringiness is strongly dependent on the balance between the interfacial adhesion and the cohesive strength of PSA. When the sawtooth‐shaped stringiness with frame formed, the peeling rate was lowest. This means the peel strength should be the maximum in this shape of stringiness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40336.     

Influence of the interfacial adhesion on the stringiness of crosslinked polyacrylic pressure‐sensitive adhesives

The stringiness of crosslinked polyacrylic pressure‐sensitive adhesives (PSA) was observed during 90° peeling under a constant peel rate with various adherends in order to clarify the influence of interfacial adhesion on the stringiness behavior. The crosslinked random copolymer of butyl acrylate with 5 wt % acrylic acid was used as a representative PSA. Poly(methyl methacrylate) (PMMA), polycarbonate (PC), poly(vinyl chloride) (PVC), fused quartz plates and some surface‐modified poly(ethylene terephthalate) films were used as adherends. The films were pasted on a glass plate using a cyanoacrylate adhesive. The 180° peel strength was higher in the order of PVC >> PMMA ≈ PC > other adherends. All observed stringiness was sawtooth‐shaped, but the stringiness width and length were longer in the same order. The number of sub‐branches formed at the tips of the strings was much more for the PVC, PMMA and PC adherends. Frames formed at the front end of the strings in the case of PVC adherend. Sufficient interfacial adhesion generates large internal deformation of the PSA layer. Internal deformation occurred preferentially over peeling as a result of front frame formation. The string length and the peel load required for the constant peel rate have good correlation with the peel strength. The estimation of generated inner stress in the fibrils of the strings was possible by analysis using the string length for various adherends and the stress–strain curve of pure PSA. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40869.     

The effect of varied monomer composition on adhesive performance and peeling master curves for acrylic pressure‐sensitive adhesives

A group of pressure‐sensitive adhesives were prepared with constant glass transition temperature, using emulsion polymerization. The monomers chosen were butyl acrylate, 2‐ethylhexyl acrylate, and methyl methacrylate, along with a small amount of acrylic acid. The proportion of acrylic acid monomer was held constant for each polymer preparation but acrylic ester monomer levels were varied. The glass transition temperatures of the acrylate copolymers were measured by using differential scanning calorimetry. Drying and weighing the tetrahydrofuran‐insoluble polymer fractions were used to determine the polymer gel fractions. Films of constant coating thickness were applied to poly(ethylene terephthalate) film and adhesive properties (tack and shear) were examined. Peel was examined through the construction of master curves derived from peel tests conducted over a range of temperatures and peel rates. As the 2‐ethylhexyl acrylate content increased, the latex gel fractions were found to increase. With increasing EHA and gel fraction, peel shear was found to increase. When peel force master curves were compared, divergence in peel master curves occurred as peel rates increased where polymers with higher butyl acrylate contents reached greater peel stress values. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2909–2917, 2004     

Effect of adhesive thickness on the wettability and deformability of polyacrylic pressure‐sensitive adhesives during probe tack test

Effect of adhesive thickness on the wetting and deformation behaviors during probe tack test of pressure‐sensitive adhesive (PSA) was investigated. For this purpose, cross‐linked poly(n‐butyl acrylate‐acrylic acid) [P(BA‐AA)] and poly(2‐ethylhexyl acrylate‐acrylic acid) [P(2EHA‐AA)] random copolymers with an acrylic acid content of 5 wt % and thicknesses in the range of ～15–60 μm were used. Tack was measured using the probe tack test and the fracture energy was calculated from the areas under force–displacement curve recorded during debonding process. From contact time dependence of fracture energy, the rising rate of fracture energy with contact time increased with increasing of adhesive thickness and was P(2EHA‐AA) > P(BA‐AA). The fracture energy was P(BA‐AA) > P(2EHA‐AA) at shorter contact time, whereas it reversed at longer contact time. This was caused by two different interfacial adhesions: the physical wetting of PSA molecules to the adherend surface with contact time and the chemical interaction between the acrylic acid units and the adherend surface. From the force–displacement curve measured under the condition of sufficient interfacial adhesion, both maximum force and displacement—namely, the deformability of PSA during debonding process—increased with adhesive thickness. The degree of increase of deformability was P(2EHA‐AA) > P(BA‐AA). The fracture energy was found to depend on the development of interfacial adhesion during contacting process and the deformability of PSA during debonding process. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43639.     

Stringiness morphology for crosslinked polyacrylic pressure-sensitive adhesives measured by peel testing with constant rate

To clarify the formation mechanism of front frame-type morphology, the stringiness of crosslinked random copolymers of poly(n-butyl acrylate-acrylic acid) during a 180° peel test with a constant tensile rate was examined for various crosslinker contents and rates using a quartz adherend. Cohesive failure occurred for lower crosslinker content and rate, whereas interfacial failure with sawtooth-type stringiness without a frame was observed for higher crosslinker content and rate. Front frame-type stringiness was formed at the boundary of cohesive and interfacial failures. To clarify the formation mechanism, observation was conducted from the start of peel test until the equilibrium state. The sawtooth-type stringiness with branches first formed at the tip. The adjoining branches were connected and the 2D frame was formed only on the adherend surface. The formed 2D frame developed toward the 3D walls and the front frame-type was then completed. This is caused by the surface tension that acts to restrain the increase in the surface area. However, the surface area of the front frame-type morphology was larger than the no frame-type. The larger absorption of peeling stress by the formation of this morphology is expected to contribute to peel strength improvement.     

Effect of Peel Load on Stringiness Phenomena and Peel Speed of Pressure-Sensitive Adhesive Tape

The factors governing interfacial separation in lightly cross-linked polymer adhesives at low pulling rates as demonstrated by their stringiness phenomenon are investigated.

Cohesive failure and adhesive/substrate interfacial separation of uncross-linked polymer adhesives have been adequately explained. However, in lightly cross-linked polymer adhesives, where cohesive failure cannot occur because there is no viscous flow, there are two regions of interfacial separation at low rate and this phemonenon cannot be readily explained by present viscoelastic theories.

Investigation of the stringiness phenomenon of peeling pressure-sensitive adhesive tapes at constant loads shows that two peeling speeds exist for any peeling load up to the vicinity of 200 g/25 mm. Also it is clear that stringiness structure differs greatly at each peeling speed. The stringiness phenomenon of each of these two regions is analyzed using Miyagi's observation apparatus. These two measurements are then reversed and a comparison shows that the two peeling speeds correspond to each steady peeling region.

This field of investigation, when added to the present viscoelastic property studies, should lead to a new peeling adhesive theory which, in turn, may lead to the development of new high peel force pressure-sensitive adhesives.     

Resultant synergism in the shear resistance of acrylic pressure‐sensitive adhesives prepared by emulsion polymerization of n‐butyl acrylate/2‐ethyl hexyl acrylate/acrylic acid

Acrylic emulsion pressure‐sensitive adhesives (PSAs) were synthesized by the copolymerization of n‐butyl acrylate with various levels of 2‐ethyl hexyl acrylate (2EHA) and a small constant amount of acrylic acid. The effect of varying the n‐butyl acrylate/2EHA monomer composition on the kinetic behavior of the polymerization and the characteristics of the copolymers prepared in a batch process were investigated. The results showed that increasing the amount of 2EHA in the monomer caused the polymerization rate and the glass‐transition temperature of the acrylic copolymers to decrease. Increasing the amount of 2EHA caused the gel content of the copolymers to decrease, reaching a minimum at 50 wt %; thereafter, the gel content increased at higher 2EHA levels. For the acrylic emulsion, the peel‐fracture energy of the PSAs decreased as the amount of 2EHA in the monomer was increased up to 50 wt %. At higher 2EHA levels, the peel‐fracture energy was relatively constant. Interestingly, a synergistic effect of increased shear resistance at 25 wt % 2EHA was observed without a significant trade‐off in terms of the peel and tack properties. This behavior was attributed to a good interconnection between the microgels and the free polymer chains inside the contacting particles in the adhesive film. Cooperation between various levels of 2EHA in the copolymer structure simultaneously changed the crosslink molecular weight (M_c) of the microgels and the entanglement molecular weight (M_e) of the free chains in the adhesive network morphology. The adhesive performance of the PSAs was found to be correlated with their M_c/M_e values as the 2EHA proportion was varied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

丙烯酸酯乳液压敏胶合成及其常规性能研究

采用半连续种子乳液聚合法制得纯丙乳液压敏胶。研究了胶粘剂厚度、基材厚度、基材类型对压敏胶常规力学性能影响。结果发现,各因素对压敏胶的初粘性和剥离强度影响较大;基材厚度及类型对持粘性影响较小;胶层厚度为15μm左右的压敏胶具有较高性价比。     

Theory and Analysis of Peel Adhesion: Adhesive Thickness Effects

The existing assumptions concerning boundary stress concentrations in peel adhesion are extended to treat the effects of adhesive thickness. In adhesive bonds involving the all-angle peeling of a flexible elastic adherend from a rigid substrate the varying of adhesive thickness is shown theoretically to predict a proportional increase of peel force (P) with adhesive interlayer thickness (a) when the product (βCa) of the cleavage stress concentration β, cavitation scale factor C, and adhesive thickness a is less than unity. When the product (βCa) becomes greater than unity the new theory predicts that cleavage stresses concentrate within a fractional layer of the total adhesive thickness f(a) and the peel force P tends to achieve a constant value P_max. This new theory is verified by experimental studies and the experimental analysis suggests new optimizations in the design and measurement of the peel adhesive bond.     

Low corrosion optically clear adhesives for conducting glass: I. Effects of N,N‐diethylacrylamide and acrylic acid mixtures on optically clear adhesives

This study designs less corrosive optically clear adhesives (OCAs) with good rework properties, where copolymer and glycidyl methacrylate (GMA) are cured by ultraviolet to form OCAs. The copolymer are synthesized by using N,N‐diethylacrylamide (DMA), acrylic acid (AA), and 2‐ethylhexyl acrylate. The DMA and AA could form acid–base interaction and therefore lower the corrosion on metallic substrate caused by AA. Copolymer is applied in OCAs, as different adhesive properties are presented. In terms of the adhesive property of OCAs, the peel strength, shear strength, and transmittance property are decreased when the GMA concentration is increased. The tack and haze are enhanced accordingly. After 7 days' standing at 60 °C and 90% RH, OCAs have no obvious corrosion on the conductive glass circuits, and there is no residue after peeling off. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46277.     

Effect of different skin permeation enhancers on peel strength of an acrylic PSA

Different amounts of two skin permeation enhancers, Oleic acid (OA) and Propylene glycol (PG), were mixed thoroughly with solution of a commercial acrylic pressure sensitive adhesive (Duro‐Tak). Films with different adhesive layer thickness (30 and 60 μm) were prepared by casting of formulations with a film applicator on a PET 80‐μm film and drying of solvents. Peel test was done on different formulations according to ASTM D3330. Surface study and thermal analysis were used for explaining the results. It was shown that the effect of interfacial work of adhesion on peel strength was too low to be considered. PG had no significant effect on peel strength, which was related to effect of hydrogen bonds between PG and copolymer chains acting as crosslinks. OA decreased peel strength significantly, which is due to important changes in copolymer structure. These changes can be found by relatively sharp drop in T_g values. Adhesive–cohesive transition occurred in OA formulations as a result of OA crystals formation. OA migration to surface in concentrations of more than 10 (w/w %) was confirmed by results of DSC and surface study. In contrast with PG, doubling of thickness had no effect on peel strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2987–2991, 2003     

Acrylic pressure‐sensitive adhesive for transdermal drug delivery systems

This article describes the development in the area of resin‐free acrylic pressure‐sensitive adhesive (PSA) based on 2‐ethylhexyl acrylate, methyl acrylate, acrylic acid, N‐vinyl caprolactam, and pregnancy transdermal drug delivery systems, and shows the variety of polymer composition, residue monomers content, quality control of peel adhesion level and repeating during the time, biocompatibility of the acrylic PSA layer, and efficacy in clinical medicine. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Plasticity in peeling

Contrary to classical theory, a high proportion of bond failures by peeling involve progressive plastic adherend flexural yield. Such yield occurs with adherend thicknesses below a critical value, T_c, which is shown calculable by combining elastic peel mechanics with plastic bending criteria. The geometry of such “peel with yield,” and thence the moment-controlled peel forces, can be accounted for only if the adhesive is also recognized as behaving essentially plastically. Subsequent plastic adherend unbending is important with highly extensible adhesives. The geometry of “legging” peel in such cases is best described by fully plastic mechanics. These are derived and shown to account for literature data on dependencies of peel force upon peel rate and adhesive thickness. “Stick-slip” peel phenomena are indicated to be controlled by recurring interacting plastic–elastic transitions, in both adhesive and adherend: adhesive strain rate is critical in such phenomena. Four regimes of peel behavior can therefore apply as adherend thickness (T) increases, with peel forces proportional respectively to T⁰, T^2/3, T^3/2 (above T_c) and finally controlled by moment limitations due to joint configurational constraints (“cleavage”).     

Synthesis of high‐solid‐content,acrylic pressure‐sensitive adhesives by solvent polymerization

In this study, we prepared high solid content (SC), solvent‐based, acrylic pressure‐sensitive adhesives (PSAs) with n‐dodecyl mercaptan as a chain‐transfer agent (CTA) and studied the crosslinking reactions between the crosslinker and the acrylic PSAs. Acrylic PSAs were prepared from 2‐ethyl hexyl acrylate, acrylic acid (AA), and 2‐azobisisobutyronitrile with a solution polymerization process. The results show AA resulted in an effective molecular weight in the acrylic PSAs, as it improved the hydrophilicity with increasing peel strength of the acrylic PSAs. As for the high SC, the molecular weight and system viscosity decreased through the addition of CTA. At a constant AA amount, the addition of CTA decreased the molecular weight and increased the hydrophobicity of the acrylic PSAs; this decreased the peel strength of the acrylic PSAs on the glass. Furthermore, the addition of CTA decreased the molecular weight and improved the acrylic PSAs' surface morphologies and optical properties. The acrylic PSAs produced in this study could meet production needs. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46257.     

Peel mechanics for an elastic-plastic adherend

The force required to propagate a 180° bend in an elastic-plastic strip has been calculated from elementary bending theory. Measured forces for Mylar strips of various thicknesses, bent to various degrees, were in good agreement with these calculated values. The corresponding additional stripping force in a peeling experiment will depend upon the thickness of the elastic-plastic adherend, becoming zero both for infinitesimally thin adherends and for those exceeding a critical thickness t_c and passing through a maximum value at intermediate thicknesses. Published data are in good agreement with these conclusions. For a strongly adhering strip, higher peel strengths are found for a peel angle of 180°, compared to 90°, and the effect is greater than can be accounted for solely by plastic yielding of the adherend. It is attributed in part to greater energy dissipation within the adhesive layer.     

Peel Morphology of Acrylate Adhesive Polymers

The morphology of various acrylate adhesive polymers peeled from Pyrex® glass was investigated. The composition ratios of acrylate polymers [poly(2-ethylhexyl acrylate-co-acrylic acid-co-vinyl acetate): P(2EHA-AA-VAc)] are 2EHA/AA/VAc= 100/0/0 (PEHA0000), 95/5/0 (PEHA0500), 92/8/0 (PEHA 0800) and 85/5/10 (PEHA0510) (mol%). The peel pattern of PEHA0000 exhibited a greatly elongated saw tooth, while PEHA0500 and PEHA0800 revealed the characteristic branched stringiness at the edge of the saw tooth. The peel morphology of PEHA0510 exhibited the regular saw tooth. The peel morphological pattern differences between the acrylate adhesive polymers were examined using dynamic mechanical thermal analysis, surface tension and the fluorescence probe technique.     

Polyacrylate emulsion containing IBOMA for removable pressure sensitive adhesives

In the area of pressure sensitive adhesives (PSAs), corona treatment or other preprocessing on the carrier was commonly utilized for the enhancement of the interaction between the adhesive layer and the carrier, with complicated procedures and operations. To make it easy, one‐pot emulsion polymerization using n‐butyl acrylate, isobornyl methacrylate (IBOMA), 2‐hydroxyethyl acrylate and acrylic acid was conducted in the presence of a reactive emulsifier SR‐20 to obtain a reinforced adhesive tape. First, the emulsion had good stabilities, especially freeze‐thaw stability, which was helpful to the fabrication of PSAs. Second, IBOMA increased the binding strength between adhesive layer and adherend. The properties of the final tapes well fulfilled the requirements of this areas, such as that the 180° peel strength of 125.9 N/m when coated on non‐corona surfaces, the tack of 19 #, and the shear holding time of more than 100 h. Third, the tapes produced using the above emulsion could be removed many times, with no PSAs residue on the adherend even when coated on untreated PP, which showed the potential of recycling and environmental protection. Thus, these properties make the polyacrylate emulsion have great potential for practical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42886.     

氢化丙烯酸松香/丙烯酸酯复合乳液的制备及其压敏胶的性能

以氢化丙烯酸松香为增黏树脂,将其溶解在丙烯酸酯单体中,预乳化后采用半连续乳液聚合工艺制备了氢化丙烯酸松香/丙烯酸酯复合乳液。研究了氢化丙烯酸松香的用量对单体转化率、凝胶率、溶胶相对分子质量及其分布、乳胶膜的玻璃化温度(Tg)和压敏胶粘接性能等的影响。结果表明,氢化丙烯酸松香对单体聚合具有一定的阻聚作用,随着氢化丙烯酸松香用量的增加,单体转化率、凝胶率及相对分子质量下降,相对分子质量分布变窄。氢化丙烯酸松香的合适用量为8.0%wt,此时压敏胶的初黏力为15#,180°剥离强度为10.6N.25mm-1,持黏力大于120h。     

Dismantlable adhesion properties of reactive acrylic copolymers resulting from cross-linking and gas evolution

ABSTRACT

The acrylic copolymers involving 2-hydroxyethyl acrylate (HEA) and tert-butyl acrylate (tBA) units as reactive units behave as pressure-sensitive adhesive type dismantlable adhesive materials. In order to clarify the individual role of HEA and tBA units on dismantlability, the 180° peel behavior after the dismantling treatment, i.e., heating in the presence of given amount of acid catalysts, was systematically investigated using the acrylic copolymers involving different amounts of the reactive units. It was revealed that transesterification of HEA units resulted in an increase in the cohesive force and modulus due to an increase in the molecular weight and cross-linking. Deprotection of tBA units, i.e., transformation of tBA to acrylic acid (AA) unit with isobutene evolution, promoted cross-linking by the esterification of AA units and tended to reduce a cohesive force by forming voids in the adhesive layer due to the evolution of isobutene gas. Interfacial failure in the peel tests corresponded with a high degree of cross-linking and increased modulus of the adhesive. Conversely, cohesive failure was associated with reduced cohesive strength of the adhesive layer and a low peel strength.