Scaling properties of contact area of pressure-sensitive adhesives

Two new devices were developed to provide accurate measurements of both the contact area and the tack strength of pressure-sensitive adhesives (PSAs). The first one is the "mechano-optical tack tester" (MOTT), which was designed to apply controlled contact pressure by means of a quartz prism probe, for determined contact times, onto the surfaces of PSA samples. The contact area is measured by the reflection of light at the quartz probe surface, which is in contact with the adhesive. The second device is an "acoustic contact tester" (ACT) that measures the tack strength and the contact area between a silicate glass and an adhesive by the reflection of an acoustic wave. Two ultrasonic sensors of different acoustic wavelengths have been built in order to study the scaling effects of the contact kinetics between an adhesive and the probe. MOTT and ACT experiments on a commercial PSA show that the contact area is the main parameter that governs the tack properties of the PSA. The experiments and the modeling point out that the contact area depends on the compression stress, the roughness, the thickness, and the Young's modulus of the PSA. However, comparison of contact kinetics from MOTT and ACT experiments show that the contact area is a subjective parameter that depends on the wavelength of the reflected beam used for its measurement.     

The mechanics of tack: Viscoelastic contact on a rough surface

This work considers the mechanics of tack in viscoelastic materials. We study a particular tack test in which a flat, rigid probe is brought into contact with the rough surface of a viscoelastic material. The rough surface is modeled as consisting of many spherical asperities of varying heights but all having the same radius. Because of the asperities, the apparent contact area can be much greater than the actual contact area, which is regarded as the key parameter that controls tack. We show how this actual contact area evolves with time under different loading conditions. Our formulation is different from previous theories in that it explicitly accounts for the fact that asperities of different heights are subjected to different loading histories. Explicit solutions are given for the cases of a constant load test, a load relaxation test, and a constant displacement rate test. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1485–1495, 2000     

Interplay between bulk viscoelasticity and surface energy in autohesive tack of rubber‐tackifier blends

The effects of change in surface energy and bulk viscoelastic properties on the autohesive tack strength of brominated isobutylene‐co‐p‐methylstyrene (BIMS) rubber have been investigated by the addition of hydrocarbon resin (HCR) tackifier and maleated hydrocarbon resin (MA‐g‐HCR) tackifier. The addition of compatible HCR tackifier results in a reasonable increment in the tack strength of BIMS rubber by modifying only the bulk viscoelastic properties (compliance, entanglement molecular weight, relaxation time, self‐diffusion, and monomer friction coefficient values) of BIMS rubber to perform better during the course of bonding and debonding steps of the peel test. Incorporation of MA‐g‐HCR tackifier (containing 5–20 wt % of grafted maleic anhydride) steadily increases the tack strength of BIMS rubber further by precisely modifying both the surface energy and bulk viscoelastic properties to perform much better in the bonding and debonding steps. However, beyond 20 wt % of grafted maleic anhydride in the HCR tackifier, the tack strength starts decreasing due to the incompatibility between the blend components, and hence, the bulk viscoelastic properties required for bond formation are severely retarded by the interrelated reinforcing effect and the phase separation effect of the brittle MA‐g‐HCR tackifier in the BIMS rubber. Hence, the polar groups in a tackifier will contribute to significant enhancement of autohesive tack strength only if the bulk viscoelastic property of the rubber‐tackifier blend is favorable for bond formation and bond separation. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 972–982, 2010     

Molecular and nanoscale factors governing pressure‐sensitive adhesion strength of viscoelastic polymers

Pressure‐sensitive adhesives (PSAs) are finding increasing applications in various areas of industry and medicine. PSAs are a special class of viscoelastic polymers that form strong adhesive joints with substrates of varying chemical nature under application of light external bonding pressures (1–10 Pa) over short periods of time (1–5 s). To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, and high cohesive strength and elasticity, which are necessary for resistance to debonding stresses and for dissipation of mechanical energy at the stage of adhesive bond failure under detaching force. For rational design of novel PSAs, molecular insight into mechanisms of their adhesive behavior is necessary. As shown in this review, strength of PSA adhesive joints is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms. At the molecular level, strong adhesion is the result of a narrow balance between two generally conflicting properties: high cohesive strength and large free volume. These conflicting properties are difficult to combine in a single polymer material. Individually, high cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. Evident correlations are observed between the adhesive bond strengths of different PSAs, and their relaxation behaviors are described by longer relaxation times. Innovative PSAs with tailored properties can be produced by physical mixing of nonadhesive long‐ and short‐chain linear parent polymers, with groups at the two ends of the short chains complementary to the functional groups in the recurring units of the long chains. Although chemical composition and molecular structure of such innovative adhesives are unrelated to those of conventional PSAs, their mechanical properties and adhesive behaviors obey the same general laws, such as the Dahlquist's criterion of tack. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

How does tack depend on time of contact and contact pressure?

The tack of polymer melts on rigid substrates under conditions of short contact times and low pressures is examined. The substrate is modeled as a random rough surface with a distribution of asperities heights. The true contact area between the adhesive and the substrate is calculated for a given total load and elastic modulus of the substrate. The dependence of tack on contact time is accounted for by introducing the relaxation of the adhesive through a time-dependent elastic modulus. For relatively high pressures the tack is predicted to scale with 1/E so that for short contact times, t_c, the tack is predicted to scale with (t_c/τ_e)^1/2, where τ_e is the entanglement time. For lower pressures this simple scaling law is no longer valid and we predict a complex variation of tack with contact time and molecular parameters. © 1996 John Wiley & Sons, Inc.     

Mobility of a tackifying resin in a pressure-sensitive adhesive

A detailed study of the mobility of a tackifying resin in a pressure-sensitive adhesive (PSA) has been done for the first time. The objective of this work is to relate changes in adhesive performance with tackifier loading to tackifier mobility. Tackifiers are low-molecular weight resins that improve the overall performance of PSAs. They increase the adhesive tack or the ability to form a bond of measurable strength after brief contact under slight applied pressure. In this study the diffusion of n-butyl ester of abietic acid (n-BEAA) in either polyisoprene (PI) (M_w = 195,000 M_w/M_n ∼ 1.05) or poly(ethylene-propylene) (PEP) (M_w = 40,000 M_w/M_n ∼ 2.30) was measured by Pulsed Gradient Spin Echo-Nuclear Magnetic Resonance (PGSE-NMR) as a function of both tackifier concentration and temperature. The concentration dependence of the tackifier's diffusion coefficient was weak for both systems. The weak variation in mobility with composition for the PI/n-BEAA system was consistent with that system's weak variation in tack with composition. On the other hand, blends of PEP/n-BEAA showed only modest variation in mobility, even though these adhesive systems showed appreciable enhancement of tack at intermediate compositions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 373–381, 1998     

Effect of nonlinear viscoelastic properties on tack

The influence of rheological and surface properties on adhesive tackiness are studied. In particular, the importance of the elongational properties is emphasized in a model, which considers only the adhesive contribution while neglecting the importance of cavitation and surface roughness. This simple analysis allows us to recover the different types of curves (i.e., different adhesive materials) obtained in the literature on tack. Elastic, strain‐hardening, and viscous adhesive materials are considered. The question of the importance of surface properties is raised and discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3139–3149, 2003     

Viscoelasticity and tack of poly(vinyl pyrrolidone)–poly(ethylene glycol) blends

The adhesive properties of blends of high molecular weight poly(vinyl pyrrolidone) (PVP) and low molecular weight poly(ethylene glycol) (PEG) were systematically investigated with a probe test and correlated with their viscoelastic properties. The material parameters that were varied were the PEG content (31–41 wt %) and the hydration rate. The 36% PEG showed the best balance of properties for a pressure‐sensitive adhesive. At low debonding rates, the debonding took place through the formation of a fibrillar structure, whereas at high debonding rates, the debonding was brittle. This transition was attributed to the breakage and reformation of hydrogen bonds between PVP units and OH groups on PEG during the large strain of the polymer chains in elongation. This transition was observed, albeit shifted in frequency, for all three compositions, and the characteristic relaxation times of the hydrogen‐bonded network were estimated. A comparison between the tack properties of the adhesives and their linear viscoelastic properties showed a very strong decoupling between the small‐strain and large‐strain properties of the adhesive, which was indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen during tensile tests, was attributed to the peculiar phase behavior of the blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2395–2409, 2002     

Investigation into the mechanism of tack of rubbers

The tack of cis-polybutadiene was measured by means of a Skewis tackmeter, and the results, which showed considerable variability, could be represented by a double exponential cumulative distribution function of Gumbel. It was concluded that there exists a linear relationship between the applied forces (contact and break) and the time that these forces are applied in the tack test. The mechanism of tack is considered in terms of two distinct processes: (a) the development of a bond and (b) the strength of the bond thus formed. The latter is rationalized by applying Eyring's absolute rate theory. A simple equation is derived which predicts an inverse linear relation between the breaking time and breaking force, which is in agreement with the experiment. The theory also suggests that the area of actual interpenetration of polymer interfaces depends largely on the contact pressure, whereas the depth of penetration appears to depend to a similar degree on both contact pressure and contact time.     

SEED SEMICONTINUOUS EMULSION MULTI-COPOLYMERIZATION OF (METH) ACRYLATES WITH HIGH-SOLID CONTENT: EFFECT OF THE OPERATION CONDITIONS

The seeded semicontinuous emulsion multi-copolymerization of butyl acrylate (BA),2-ethylhexyl acrylate (2EHA), methyl methacrylate (MMA), 2-hydroxyl propyl acrylate(HOPA) and acrylic acid (AA) was used to prepare the acrylic latexes with high-solidcontent. The effects of monomer emulsion feed rates (R_a) and (R/E)_E values, the ratio ofemulsifier amount between the initial charge (R) and the addition monomer emulsion (E),on the polymerization reaction features, the viscosities, surface tensions,particle sizes andparticle sizes distributions of latexes,T_g and the insoluble fractions of films, the 180° peelstrength, tack and holding power of pressure-sensitive adhesive (PSA) tapes, preparedfrom the latexes, were studied. Experimental study shows that the grafting and cross-linking fraction in the PSA tapes must be controlled within a suitable range to keep thebalance of the 180° peel strength, tack and holding power.     

A detailed elastic analysis of the flat punch (tack) test for pressure‐sensitive adhesives

Detailed finite element calculations are carried out in order to study the mechanical response of a compliant layer sandwiched between a rigid cylindrical flat punch and a rigid substrate. Two cases of practical interest are considered: one in which the layer is perfectly bonded to the punch and the substrate and one in which the interface between the punch and the layer is frictionless. The substrate is assumed to be perfectly bonded to the adhesive layer in both cases. Analytic expressions are obtained for the stresses away from the edges, and the effect of lateral constraint is examined. The compliances of the loading systems for both cases are obtained numerically, and accurate analytic expressions are determined based on these numeric results. The nature of the stress fields near the contact edge are explored, and their connections with the energy release rate are determined. The relevance of these calculations to two recent adhesion tests is discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2769–2784, 2000     

The Role of Components in Waterbased Microsphere Acrylic Psa Adhesive Properties

In this paper the batch suspension copolymerization of ethyl acrylate/2 ethyl hexylacrylate (EA/2-EHA) for production of suspension-based microsphere acrylic pressure sensitive adhesives (PSA) is presented. The effects on the adhesion properties of PSA different process (reaction temperature and stirrer speed) as well as chemical parameters (amount of EA, initiator concentration) are discussed. The conversion was monitored in-line using Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy and the results were compared with the standard gravimetrical method. The glass transition temperatures (T_g) of the PSAs were measured using differential scanning calorimetry (DSC) technique, while molecular weight distribution (MWD) was determined by gel permeation chromatography (GPC). The adhesion properties of PSAs were characterized via the measurements of tack, peel adhesion and peel strength. The results of the experiments have shown that the kinetics of the suspension polymerization for production of PSAs is significantly affected by temperature of polymerization and the initiator concentration, but are shown to be relatively independent of the EA amount and the stirrer speed. The tack, peel and shear strength depend on the mean particle size and particle size (PS) distribution (PSD) and T_g. The mean particle size and PSD depend primarily on the stirrer speed during the PSA synthesis process, while the T_g is most affected by amount of EA used for the synthesis. The results have also shown a rather unexpected relationship between MWD of the PSAs and the applicative properties: tack, peel and shear are seen to be increasing to the decreasing values of weight average MWD, which is the exact opposite of the previously published research. The most likely explanation for this relationship is the formation of a gel during the synthesis of PSA.     

Investigation of shear failure mechanisms of pressure‐sensitive adhesives

We report new experiments investigating the failure mechanisms in shear, of thin layers of acrylic pressure‐sensitive adhesives (PSA). We have developed a novel experimental device able to shear a soft adhesive layer confined between a rigid hemispherical lens and a rigid glass substrate. Using the resources of in situ contact visualization, the nonhomogeneous deformation of the layer and the shear failure processes were observed optically. Depending on the rheological properties of the adhesive, ratios of the contact radius over the layer thickness of 10–30 were achieved, mimicking well the contact conditions encountered in a thin adhesive layer within a joint. When the adhesive was weakly crosslinked, we observed a fluid‐like behavior and could measure a reasonable value for the viscosity of the PSA, implying that flow can occur in the layer and failure will occur by creep. On the other hand, for a more crosslinked adhesive, closer to what is used in applications, a stick‐slip peeling behavior was observed, which involves a coupling between peeling mechanisms at the leading edge of the contact and interfacial slippage. Such a process suggests a failure by fracture rather than by creep. Interestingly, the peeling mechanisms and the associated stress levels change significantly when the layer becomes as thin as 20 μm, implying a fracture process that is controlled by a critical energy release rate in shear G_IIc rather than by a critical shear stress causing failure of the interfacial bonds. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3316–3330, 2005     

Electron microscopy of high‐pressure crystallized poly(ethylene terephthalate)

The high‐pressure crystallized poly(ethylene terephthalate) samples were investigated with scanning electron microscopy. The striation appearance, which is the most common feature of polymer extended‐chain crystals, was clearly observed. Poly(ethylene terephthalate) extended‐chain crystals with thickness up to 17 m were obtained at high pressure. Fibrous crystals were also formed at high pressure. The fracture behaviors, which affected the exposure of the striations, were also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1612–1616, 2000     

Influence of thermal reactive crosslinking agents on the tack,peel adhesion,and shear strength of acrylic pressure-sensitive adhesives

In recent decades, the basic technology of pressure-sensitive adhesive (PSA) acrylics has developed into a sophisticated science. The main properties of acrylic PSAs such as tack, peel adhesion, and shear strength are determined to a large extent by the kind and quantity of crosslinking agents added to the synthesized PSAs. In order to improve their adhesive (tack, peel adhesion) and cohesive (shear strength) properties, a wide range of amino resin thermal crosslinkers are tested. An acrylic PSA based on 2-ethylhexylacrylate, n-butyl acrylate, and acrylic acid was synthesized by performing a radical polymerization in ethyl acetate. After the addition of amino resins to the acrylic PSA and carrying out thermally initiated crosslinking processes to prepare one-sided self-adhesive tapes, their properties were assessed.     

UV-curing behavior and adhesion performance of polymeric photoinitiators blended with hydrogenated rosin epoxy methacrylate for UV-crosslinkable acrylic pressure sensitive adhesives

UV-crosslinkable polyacrylates were synthesized for use as pressure sensitive adhesives (PSAs). These polyacrylates acted as polymeric photoinitiators due to the benzophenone incorporated into their backbones. Hydrogenated rosin epoxy methacrylate (HREM; based on hydrogenated rosin and glycidyl methacrylate) was also synthesized as a tackifier, and blended at different levels with the synthesized, UV-crosslinkable polyacrylates for use as PSAs. The effect of the new tackifier, HREM, on the properties of the UV-crosslinkable PSAs was examined in comparison with the properties exhibited by PSA/hydrogenated rosin blends. The characteristics of these PSA/tackifier blends were examined by Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and an advanced rheometric expansion system (ARES). In addition, the adhesion performance of the PSA blends was investigated using probe tack tests. DSC and ARES revealed all the PSA blends with HREM or hydrogenated rosin to be miscible at the molecular level. The glass transition temperature (T_g) of HREM was −25.6 °C, which is lower than that of other commercially available rosin tackifiers. FTIR revealed changes in the relative concentration of benzophenone groups in the PSAs at 1580 cm⁻¹, which demonstrated that the crosslinking efficiency is proportional to the benzophenone content and UV dose, but decreases with increasing hydrogenated rosin content. However, the reduced crosslinking reaction efficiency was improved in the PSA/HREM blends due to the low T_g of HREM which only slightly increased the T_g of the PSA blends. Moreover, the relative initial decrease in the probe tack of the PSA/HREM blends was lower than that of the PSA/hydrogenated rosin blends after UV irradiation.     

Cyclic welding behavior of covalent adaptable network polymers

Surface welding effect of covalent adaptable network (CAN) polymers enables self‐healing, reprocessing and recycling of thermosets, but little is known about their welding behaviors during repeated welding‐peeling cycles. In this article, we study the cyclic welding effect of an epoxy based thermal‐sensitive CAN. Surface roughness is generated by rubbing the sample on sandpapers with different grid sizes. The welding‐peeling cycles are repeated on the same pair of samples for five times, with roughness amplitude and interfacial fracture energy measured in each cycle. It is shown that the roughness gradually decreases during the repeated welding cycles, especially when a long welding time or high welding pressure is applied. Even though lower roughness amplitude promotes the contact area, the interfacial fracture energy reduces due to the increased BER activation energy after long‐time heating. A multiscale constitutive model is adopted, where we incorporate an explicit expression of interfacial contact area as a function of root‐mean‐square roughness parameter. The model is able to capture the evolving interfacial fracture energy during repeated welding cycles by using the measured roughness parameter, network modulus and BER activation energy. The study provides theoretical basis for the design and applications of CANs involving cyclic welding‐peeling operations. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 402–413.     

Diffusion measurements using ATR‐FTIR spectroscopy: Acetone diffusion in polypropylene—use of penetrant fluid pressure to improve sample/IRE contact

In this article, we present a simple method for improving the contact between a film sample and the internal reflection element (or crystal) when diffusion into thin polymer films is measured with attenuated total reflectance (ATR)‐Fourier transform infrared spectroscopy. Intimate film/crystal contact is particularly important for making measurements on premade films and materials that cannot be solution‐coated onto the crystal. This method is based on controlling the penetrant fluid pressure above a threshold value (>230 kPa) in the ATR flow cell. Measurements of acetone diffusion into a commercial polypropylene film at 300 K and varied pressures indicated that the diffusion time constant was constant at pressures above this threshold. We also monitored the absorbance of a polymer band that had no overlap with the acetone spectrum to examine whether adequate sample‐film/crystal contact was reached and maintained. From these observations, we concluded that an apparently good match between the experimental data and a model calculation does not alone justify confidence in the accuracy of the calculated diffusion time constant. Additionally, the practice of using a reference band to correct the uncertainty in absorbance for bands of interest (due to imperfect sample/crystal contact) yielded inconsistent results. We also report further measurements of acetone diffusivity in a polypropylene film at temperatures ranging from 278 to 308 K that yielded an estimated activation energy for diffusion of 98 kJ/mol. © 2000 John Wiley & Sons, Inc.* J Polym Sci B: Polym Phys 38: 1773–1787, 2000     

Influence of Particle Nucleation in Pressure Sensitive Adhesive Properties: Miniemulsion versus Emulsion Polymerization

Miniemulsion polymerization is a promising approach to produce and tailor pressure sensitive adhesives (PSAs). In this paper, a systematic comparison of the adhesive properties of latexes produced by miniemulsion and conventional emulsion polymerization is presented. Specifically, the influence of the total surfactant concentration, chain transfer agent concentration and chemical composition on the final adhesive properties of the polymer 2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid was discerned using a 2³ factorial design for each polymerization method. In addition to the adhesive properties (i.e., loop tack, peel strength and shear strength), molecular weight distribution, particle size distribution (PSD) and glass transition temperature were analyzed. The results show that under the conditions used in this work, it is possible to produce PSAs using miniemulsion polymerization, a process wherein monomer droplet nucleation is the dominant particle nucleation mechanism. The use of a miniemulsion polymerization process, as opposed to the conventional emulsion technique, produced several differences such as larger particles sizes and narrower molecular weight distributions. Focusing on the PSA films that exhibited adhesive rather than cohesive failure, the PSA films generated via miniemulsion polymerization displayed higher values of loop tack and peel strength compared to those produced via conventional emulsion polymerization. Shear strength results were strongly dependent on the amount of gel content and sol molecular weight for both cases.