Yoshiaki Urahama

Relation between phase structure and peel adhesion of poly(styrene-isoprene-styrene) triblock copolymer/tackifier blend system

The effect of tackifier on the adhesive properties of a model pressure-sensitive adhesive tape was investigated. For this purpose, a model system consisting of poly(styrene-isoprene-styrene) triblock copolymer as the base polymer and a typical aliphatic petroleum resin as the tackifier was prepared. The tackifier content ranged from 10 to 60 wt%. The tackifier used has a good compatibility with polyisoprene, whereas it has a poor compatibility with polystyrene. The 180° peel adhesion was measured. The peel adhesion increased with the tackifier content, while the degree of increase became more significant above 40 wt%. The pressure sensitivity appeared obviously and the maximum peel adhesion was obtained without heating above 40 wt%. The phase structure was determined using pulse 1H-NMR, transmission electron microscopy and dynamic mechanical analysis. A phase structure in which spherical polystyrene domains with a mean size of about 20 nm were dispersed in the polyisoprene continuous phase was observed. It was found that the tackifier-rich phase of the order of nanometers in size was formed in the polyisoprene matrix and the concentration increased with the tackifier content. The tackifier-rich phase seemed to develop the cohesive strength and, thus, it increased the peel adhesion.

Effects of Compatibility of Acrylic Block Copolymer and Tackifier on Phase Structure and Peel Adhesion of Their Blend

The effect of compatibility of polymer and tackifier on the adhesion properties of pressure-sensitive adhesive tape was investigated. For this purpose, a model pressure-sensitive adhesive tape was prepared. A mixture of poly(methyl methacrylate)-block-poly(butyl acrylate)-block-poly(methyl methacrylate) triblock copolymer and a similar diblock copolymer was used as the base polymer. And three kinds of tackifiers, namely a rosin phenolic resin (A), a special rosin ester resin (B) and a hydrogenated cycloaliphatic resin (C) were used. The compatibility with the base polymer was in the order of tackifier A > B > C. Transmission electron microscopy studies confirmed that the tackifier A had good compatibility, where agglomerates of the tackifier with a size of about 10 nm only were observed. Larger and distinct agglomerates of tackifier were observed for tackifier B and the size increased with the tackifier content. The tackifier C formed domains with a mean size of several tens μm. Dynamic mechanical analysis indicated that both the glass transition temperature and modulus increased at low temperature and the modulus at high temperature decreased by the addition of tackifier for tackifiers A and B. This tendency was more remarkable for tackifier A as compared to tackifier B. An increase of the glass transition temperature was never observed for tackifier C. The peel adhesion was in the order of tackifier A > B > C. It was found that the compatibility of tackifier and base polymer had strong effect on adhesion properties.

Influence of crosslinking and peeling rate on tack properties of polyacrylic pressure-sensitive adhesives

Tack properties and peeling behavior of crosslinked polyacrylic pressure-sensitive adhesives were investigated. The model adhesive was a crosslinked poly(n-butyl acrylate-acrylic acid) random copolymer with an acrylic acid content of 5 mol% with various crosslinking degrees. Tack was measured using a probe tack test with probe rates of 1 and 10 mm/s and various contact time. The tack increased with contact time. The degree of tack rising with contact time decreased with an increase in crosslinking degree for 10 mm/s, while the tendency was opposite for 1 mm/s. The temperature dependency of tack was measured with a contact time of 30 s. The tack peak shifted to higher temperatures with an increase in crosslinking degree and probe rate. Peeling behavior was observed using high-speed microscopy. The peeling behavior changed from A to C with the decrease of peeling rate and crosslinking degree. A: Cavitation and peeling progressed simultaneously at maximum stress at 10 mm/s independent on the crosslinking degree. B: Cavitation occurred at the edge of the probe at low stress and spread to the center of the probe at maximum stress at 1 mm/s and high crosslinking degree, then peeled out. C: After B, fibrillation occurred at 1 mm/s with low crosslinking degree. The change of peeling behavior was caused by the following: the interfacial adhesion increased, while the cohesive strength decreased as crosslinking degree and probe rate decreased.

Adhesion properties of polyurethane pressure-sensitive adhesive

In this study, the adhesion properties of polyurethane (PUR) pressure-sensitive adhesive (PSA) were investigated. The PUR-PSA was prepared by the cross-linking reaction of a urethane polymer consisting of toluene-2,4-diisocyanate and poly(propylene glycol) components using polyisocyanate as a cross-linking agent. The peel strength increased with the cross-linking agent content and exhibited cohesive failure until the maximum value, after which it decreased with interfacial failure. The PUR-PSA exhibited frequency dependence of the storage modulus obtained from dynamic viscoelastic measurements, but did not show dependence of the tack on the rolling rate measured using a rolling cylinder tack test under the experimental conditions used, which is quite different from the acrylic block copolymer/tackifier system. The PUR-PSA showed strong contact time dependence of tack measured by a probe tack test. The tendency was significantly larger than for the acrylic block copolymer/tackifier system. Therefore, the storage modulus increased, whereas the interfacial adhesion seems to be decreased with increase in the rolling rate for this PUR-PSA system. It was estimated that the influence of rolling rate on the interfacial adhesion and the storage modulus was offset, and, as a result, the rolling cylinder tack did not exhibit rate dependency.

Effects of Polystyrene Block Content on Morphology and Adhesion Property of Polystyrene Block Copolymer

Effects of polystyrene block content on adhesion property and phase structure of polystyrene block copolymers were investigated. Polystyrene-block-polyisoprene-block-polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers with different polystyrene block contents in the range from 13 to 35 wt% were used. In the case of the low polystyrene block content (below 16 wt%), a sea-island structure was observed: near-spherical polystyrene domains having a mean diameter of about 20 nm were dispersed in polyisoprene matrix. The phase structure changed from a sea-island structure to a cylindrical structure with an increase of polystyrene block content (over 18 wt%). Peel strength decreased with an increase of polystyrene block content and the pure triblock copolymers had lower peel strength than their blends with the diblock copolymers. Pulse nuclear magnetic resonance studies indicated that molecular mobility of polyisoprene phase decreased with an increase of polystyrene block content, and the molecular mobility was lower in the pure triblock than in the blend. Thus, the peel strength was found to be related to molecular mobility. The adhesion strength of the block copolymer depended on the molecular mobility: high molecular mobility can promote interfacial adhesion.

Temperature Dependence of Tack for Polyacrylic Block Copolymer/Tackifier Blend

The effect of polymer and tackifier compatibility on tack of pressure-sensitive adhesive (PSA) tape was investigated. Poly(methyl methacrylate)-block-poly(butyl acrylate)-block-poly(methyl methacrylate) triblock copolymer was used as the base polymer and three types of tackifiers, an aliphatic petroleum resin (A), a special rosin ester resin (B), and a rosin phenolic resin (C), were mixed at 10–30 wt.%. Compatibility with the base polymer was greater in the order of C > B > A. Tackifier agglomerates of approximately 10 nm were observed by transmission electron microscopy for the PSA with tackifier C. Similar agglomerates were observed for tackifier B, but the size increased with content. The size for tackifier A was several tens of micrometers. Dynamic mechanical analysis indicated that the glass transition temperature of the poly(butyl acrylate) phase increased upon addition of tackifiers B and C. This trend was more significant for tackifier C. It was not observed for tackifier A. The temperature dependence of tack was measured using a probe tack test. Tack was greater in the order of C = B > A. Tack at higher temperatures was superior for greater tackifier C contents. Tackifier and base polymer compatibility has a strong effect on tack and its temperature dependence.

Temperature dependence of tack and pulse NMR analysis of polystyrene block copolymer/tackifier system

The influence of tackifier structure on the temperature dependence of tack for a polystyrene block copolymer/tackifier system was investigated. A blend of polystyrene-block-polyisoprene-block- polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers was used as the base polymer. Four different tackifiers were used: special rosin ester resin (RE), rosin phenolic resin (RP), hydrogenated cyclo-aliphatic resin (HC), and aliphatic petroleum resin (C5). Tack at 20 °C increased with the tackifier content for both RE and HC tackifier systems. Tack is affected by two factors: the work of adhesion at the adherend interface and the viscoelastic properties of the adhesive. The good balance of these two factors brought high tack. The adhesive with 10 wt.% tackifier exhibited the highest tack at 20 °C, whereas those with 30 and 50 wt.% tackifier were lower than those systems with 10 wt.% of the RP or C5 tackifiers. The adhesive with overly high hardness lowered the work of adhesion and the tack was not improved with more than 30 wt.%. A compatibility test in toluene solution and in solid state showed that tackifier RE has good compatibility with both polyisoprene and polystyrene, whereas tackifier RP has lower compatibility. Tackifiers HC and C5 had good compatibility with polyisoprene, but poor compatibility with polystyrene, and that of C5 was poorer. Pulse nuclear magnetic resonance (NMR) analyses indicated that tackifiers RE and HC effectively restrict the molecular mobility of polyisoprene phase.

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.

Influences of debonding rate and temperature on tack properties and peel behavior of polyacrylic block copolymer/tackifier system

The influences of debonding rate and temperature on the peel behavior of polyacrylic block copolymer/tackifier system were investigated. Poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) triblock copolymer (MAM) with hard block contents of 23 (MAM-23) and 16 wt.% (MAM-16) and a 1/1 blend with a diblock copolymer (MA) consisting of the same components (MAM-23/MA, total hard block content of 15 wt.%) were used as the base polymer. A special rosin ester was used as a tackifier at various contents in the block copolymer/tackifier system. The peeling process at the probe/adhesive interface during probe tack testing was observed using a high-speed microscope at 23 °C with debonding rate of 10 mm/s. Three different peeling mechanisms were observed. Type A, where peeling progressed linearly from the edge to the center of the probe without cavitation (MAM-23). Type B, where peeling progressed linearly from the edge to the center of the probe with cavitation (MAM-16). Type C, where cavitation occurred over the entire adhesive layer, and peeling initiation was delayed (MAM-23/MA). The peel behavior of MAM-23 changed from Type A to Type B with a decrease of the debonding rate (1 mm/s) or increase of the temperature (40 °C). In contrast, there was no change for MAM-16 and MAM-23/MA. Cavity formation in an adhesive layer restrains peeling; therefore, it is desirable for improvement of the adhesion strength. The tack properties increased with the tackifier content, and the formation of cavitation was less than that for the systems without the tackifier.

Evaluation of the plasticization of ion-selective electrode membranes by pulsed NMR analyses

The potentiometric polymeric membranes for ion-selective electrodes were evaluated by analyses of the proton spin-spin relaxation times T2 with pulsed NMR. The T2 measurements were performed using the Hahn-Echo, Solid-Echo and CPMG pulse sequences. The T2 values and fractions F of each component were obtained by analyses of the FID signals measured with the Hahn-Echo pulse sequence. The softer potentiometric polymeric membrane possessed the main fraction F(L), providing a relatively longer T(2L) value. A linear relationship existed between the weight ratio of the membrane solvent and ln T(2L) (or ln total T2×F). This analysis method could quantify the degree of hardness or softness of the potentiometric polymeric membranes with the differences in the membrane solvent weight. The normalized derivative spectra were acquired from the transverse magnetization M(t) data measured by using the Solid-Echo and CPMG pulse sequences. In the normalized derivative spectra of the potentiometric polymeric membranes, most PVC peaks in the short time region shifted to a larger area of long time regions by plasticization, and the softer potentiometric polymeric membrane incorporating more membrane solvent exhibited a relaxation peak in the relatively longer time region. Thus, the normalized derivative spectra were effective in elucidating the compatibility of the PVC with the membrane solvent.