Pressure-sensitive Adhesive Films Research Articles

Surface enrichment layers in pressure-sensitive adhesive films

Pressure sensitive adhesives (PSAs) are used for many different applications, such as for example adhesive foils or binding materials. A prominent class of PSA films is based on statistical copolymers. Typically two or three different monomers are combined into the statistical copolymer to balance the different requests of the PSA [1-4]. The adhesive properties of the PSA films are mainly determined by surface-near regions. However, detailed information about surface structures of such adhesive films is still missing.

Effect of Amphiphilic Additives on the Behavior of Water-Based Acrylic Pressure-Sensitive Adhesives during Paper Recycling

Pressure-sensitive adhesives (PSAs) in recovered paper reduce efficiency and increase operating costs for paper recycling mills. Increased PSA fragmentation during pulping and the corresponding reduction in screening efficiency are indications that a PSA will likely interfere with paper recycling. Water-based PSAs, which dominate the label market, have complex formulations that include several amphiphilic materials, i.e., emulsifiers, dispersants, and wetting agents. Increasing the amount of these surface-active materials increased adhesive fragmentation during pulping, and thus reduced screening removal efficiencies. Accompanying the reduction in size was a distinct change in morphology of adhesive particles, which assumed a less collapsed structure during repulping. The presence of surface-active materials also appeared to facilitate the removal of fiber from PSA films during repulping, reducing the importance of paper facestock properties in determining fragmentation behavior. The findings presented here combined with results reported previously provide more complete guidelines for the synthesis and formulation of recycling-compatible acrylic water-based PSAs.

Macromol. Mater. Eng. 7/2007

Cover: The picture on the cover shows an in-situ characterisation of a pressure sensitive adhesive film with the mechanical tack test and synchrotron radiation based ultra small angle scattering. This combination allows to tailor the adhesive properties and to understand the de-bonding mechanism. Further details can be found in the article by P. Müller-Buschbaum,* T. Ittner, E. Maurer, V. Körstgens, and W. Petry on page 825.

Control of Refractive Index of Pressure-Sensitive Adhesives for the Optimization of Multilayered Media

We describe the method of controlling the refractive index of pressure sensitive adhesives (PSAs) from 1.461 to 1.531 by changing the ratio of component monomers. The difference in refractive index between recording and PSA layers determines the penetration depth for recording and reading light due to multiple reflections at layer interfaces. We fabricated multilayered media that contained one, five and ten recording layers and measured their transmittances. The readout signal intensities of ten-layered media, which were fabricated using PSA films of various refractive indices, were measured. We found that a smaller refractive-index difference between recording and transparent layers led to a larger transmittance of the medium and that a refractive index difference smaller than 0.01 provided a signal sufficient for distinguishing each layer. We also calculated reflection intensity distribution for the optimization of the structure of multilayered media.

Probing particle structure in waterborne pressure-sensitive adhesives with atomic force microscopy

There is a need to know the nanostructure of pressure-sensitive adhesive (PSA) films obtained from waterborne polymer colloids so that it can be correlated with properties. Intermittent-contact atomic force microscopy (AFM) of an acrylic waterborne PSA film identifies two components, which can be attributed to the polymer and the solids in the serum (mainly surfactant). It is found that when the average AFM tapping force, F av , is relatively low, the polymer particles appear to be concave. But when F av is higher, the particles appear to have a convex shape. This observation is explained by a height artefact caused by differences in the indentation depths into the two components that vary with the tapping amplitude and F av . To achieve the maximum contrast between the polymer and serum components, F av should be set such that the indentation depths are as different as possible. Unlike what is found for the height images, the phase contrast images of the PSA do not show a reversal in contrast over the range of tapping conditions applied. The phase images are thus reliable in distinguishing the two components of the PSA according to their viscoelastic properties. At the surface of films dried at room temperature, the serum component is found in localized regions within permanent depression into the film.

Controlling the hydrophilicity of microchannels with bonding adhesives containing surfactants

In this study, a novel protocol to control the hydrophilicity of microchannels as well as to bond the microfluidic devices in all polymer-based biomedical devices is suggested. It is demonstrated that the contact angle of water in the microchannel surface can be tuned using conventional pressure-sensitive adhesives (PSA) containing surfactants. The surfactants are immersed in a hydrophobic PSA film in hydrophobic environments (air). The water–adhesive interface reconstructed quickly and became hydrophilic upon contact with water. Both the contact angle and volumetric flow rate (VFR) of water in microchannels could be systematically controlled by changing the concentration of the surfactants in the PSA film and by using different types of surfactants with different chemical structures. The VFR of water changed in a two-order range (1–102 nl s−1) when the concentration of surfactants was varied from 1% to 10%. We believe that the use of surfactants favors mainly the control of flow rate and the protocol suggested here to control the flow velocity in a broad range should find a wide application. Although the hydrophilic coating formulated with surfactants has proven effective in improving the flow rate of liquid, the surfactants located at the interface could be dissolved into the fluid samples. And the beneficial result of the use of surfactants is restricted to microfluidic devices where the liquid stays briefly in contact with the channel walls. The protocol suggested in the research can be directly applied to the development of low-cost, high-speed and reliable bonding techniques for disposable polymer-based microfluidic devices.

Probe tack of tackified acrylic emulsion PSAs

Water-based acrylic, pressure sensitive adhesives (PSAs) were prepared by addition of aqueous rosin esters dispersions. PSA films were laminated to corona-treated PET film of 50 μm average thickness by transfer-coating. The glass transition temperature ( T g) was measured using differential scanning calorimetry (DSC). The probe tack test was conducted using texture analyzer. The tack performance of tackified emulsion PSAs were affected by the T g of bulk PSAs, as well as by the miscibility and tackifier contents.

Evaluation of temperature-dependent adhesive performance via combinatorial probe tack measurements

We describe the design and application of a temperature gradient probe tack apparatus for investigating the adhesive performance of model pressure-sensitive adhesives (PSAs). In particular, we illustrate a probe tack apparatus for studying the effect of temperature on three critical adhesion identifiers: adhesion energy, elongation at break, and debonding mechanisms. The measurement temperature is varied across the PSA film using a gradient temperature stage constructed from a transparent sapphire plate with a heating and cooling source positioned at opposite ends. The transparent substrate allows visualization of the contact area and debonding mechanisms during the test. The gradient temperature stage is integrated onto a motorized x-y stage, enabling a matrix of probe tack tests to be conducted across the PSA film at different sample temperatures. We use a spherical probe to evaluate the adhesive performance of a 150μm thick model poly(styrene-b-isoprene-b-styrene) PSA film between a temperature range of 10 °C to 100 °C. We demonstrate that this apparatus is a viable combinatorial design for tack measurements and may be extended to more complicated two-dimensional gradient films.

Assessing the effect of latex particle size and distribution on the rheological and adhesive properties of model waterborne acrylic pressure-sensitive adhesives films

The adhesive and rheological properties of model acrylic pressure-sensitive adhesive (PSA) films prepared from high solid emulsions with different particle sizes and distributions have been investigated with a customized probe tack apparatus. For each emulsion, the monomer composition and gel content were kept constant but different average particle sizes and distributions were used. Adhesive films 100 μm thick were then prepared from these emulsions and their rheological properties in the linear regime and adhesive properties were systematically characterized. Surprisingly, both the rheological and adhesive properties were found to be very dependent on the initial latex particle size distribution. A series of experiments were carried out to assess the adhesive properties of films made from blends of small- and large-particle-size latexes. Using the probe tack test, a maximum in adhesion energy of the dry films was found for 60% of small particles in the blend, a composition clearly different from that giving a minimum viscosity of the latex implying that optimizing for properties may not be equivalent to optimizing for processing in these adhesive applications. Finally, the adhesive properties of two multimodal latexes with different particle size distributions were investigated. Both gave significantly higher adhesion energies and clear evidence of a fibrillar detachment process. This important result suggests that the spatial distribution of gel domains in the dry film and the molecular connectivity between those gel domains also play an important role in controlling its adhesive properties.

Crosslinked acrylic pressure sensitive adhesives. 3. Effect of adherend on film formation

The reaction of isocyanate in pressure sensitive adhesive (PSA) films adhered on to various adherends having different surface tensions was monitored by depth profiling using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. In the latter stages of crosslinking reaction, unreacted isocyanate and its derivatives exist more in the bulk of the PSA than in the interfaces between PSA and adherends which are Teflon sheet and PE film having relatively lower surface tensions. In the case of using stainless steel having relatively higher surface tension as adherend, opposite segregation was observed compared to Teflon and PE. From X-ray photoelectron spectroscopy (XPS), it was revealed that N atoms exist more in surface than in bulk when stainless steel is used as adherend. We conclude that polyisocyanates migrate in the PSA film in order to minimize the magnitude of interfacial free energy between the PSA and the adherend, which leads to the change of surface tension of PSA film.

Role of Paper Face Stock Properties in Determining Removal Efficiencies of Pressure-Sensitive Adhesive Films from Recycling Operations

Results are presented from a study on the influence of paper face stock properties on the screening removal of two commercial thermoplastic or hot-melt pressure-sensitive adhesives (PSAs). Significantly lower removal efficiencies were found when the adhesives were attached to commercial face stocks as opposed to untreated paper. These differences were only apparent for temperatures within the plateau region of the thermoplastic formulations and were maximized near their shear adhesion failure temperatures (SAFTs). It is proposed that the collapse of PSA films is inhibited by the presence of residual fiber from the laminate face stock, resulting in higher film stress and increased fragmentation. Detachment of the face stock is likely controlled by its strength and adhesion to PSA surfaces during repulping. Removal efficiencies of laminates involving commercial PSAs and fiberizable face stock treated with a poly(amide epichlorohydrin) wet-strength resin were found to correlate with the paper's loss in tensile strength after water saturation (i.e., tensile loss). Removal efficiencies decreased with increasing face stock size (alkenyl ketene dimer) concentration, which should increase PSA−face stock adhesion in aqueous environments. The sigmoidal function previously presented for modeling removal efficiency−temperature data of thermoplastic formulations was found to provide an accurate fit of data for laminates involving various wet-strength and sized face stocks. However, the T50 or inflection points of these curves were shifted to values lower than the SAFTs for the attached PSAs. T50 values were found to be proportional to both face stock tensile loss and surface energy.

Control of structure and tack properties of acrylic pressure‐sensitive adhesives designed by a polymerization process

AbstractFree radical emulsion polymerization of methyl methacrylate (MMA) and 2‐ethylhexyl acrylate (EHA) results in the synthesis of pressure‐sensitive adhesives (PSAs) with good tack properties. Management of both the copolymer composition and the polymerization process allows one to control the behavior of the PSA. Semicontinuous (SC) processes create polymer particles whose instantaneous composition is close to that of the feed particle The SC Mixture process (continuous feeding with comonomer blends) affords nearly homogeneous latex particles and PSA films. The SC Gradient process (separate feedings at inversely varying rates) affords heterogeneous particles and films. The Batch process leads to somewhat heterogeneous films, but the hard (MMA‐rich) microdomains are made compatible with their soft (EHA‐rich) matrix because of the assumed formation of tapered‐type copolymers. Tack measurements indicate the importance of the particle and film structures. Too much hardness or softness leads to unacceptable lacks of adhesion and cohesion, respectively. Homogeneous structures prove adequate, but their tack properties collapse with rising temperature. Heterogeneous structures, with extensive phase segregation, prove unsatisfactory because they lack adhesion and cohesion. Finally, the association of well‐balanced composition and compatible heterogeneity is the criterion for suitable PSA behavior. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2749–2756, 2003

Various ways to control the bulk properties of pressure sensitive adhesives

The widespread industrial use of pressure sensitive adhesives (PSAs) is based on the advantage that PSAs can maintain sufficient bond strength, as well as, if needed, they can also be cleanly removed. These two essential requirements must be balanced to be a good PSA, and are not obtained without an accurate control of rheological properties. In this study, a new type of creep test is devised to measure the rheological behavior of thin film PSAs with high precision. Based on this technique, we studied four different methods to control the viscoelastic properties of PSAs. These are (1) control of the amount of crosslinking agent, (2) use of plasticizers, (3) adjustment of PSA film thickness, and, (4) construction of PSA double-layers. It was found that the levels of creep could be controlled over a wide range by adjusting the amount of crosslinking agent. Samples containing plasticizers behaved similarly to the PSAs with less amount of crosslinker (thus more mobile). The increase of film thickness also led to an increase of creep in a similar fashion as noted above. The creep behavior of the double-layer PSAs, composed of two PSA layers with different levels of crosslinking and thickness, followed a reasonable prediction: the total deformation, δt, was always less than the amount predicted by the two-phase model in which the limits are determined by two single layers of soft and hard PSA; however, δt was greater than the level of creep based on the one-phase model in which a complete mixing of crosslinker within the two PSA layers is assumed. In the last part, some characteristic creep behaviors of selected PSAs are discussed based on a linear viscoelastic model.

Studies on the water resistance of acrylic emulsion pressure-sensitive adhesives (PSAs)

Four different types of acrylic emulsion pressure-sensitive adhesives (PSAs) with the same composition of their constituent co-polymers but stabilized by four different anionic surfactants, two conventional low-molecular-weight surfactants (a sodium salt and an ammonium salt) and two anionic monomers (a sodium salt and an ammonium salt) were prepared. The adhesion properties of the four types of PSA tapes coated onto PET (poly(ethylene terephthalate)) sheets were determined with the national standard methods of China. Water absorption and water solubility of PSA films were determined by the gravimetric method. The peel-strength retention of PSA tapes after immersion in water was compared. The results showed that both the adhesion properties and the water resistance of the acrylic PSAs stabilized by anionic monomers were better than that of the acrylic PSAs stabilized by low-molecular-weight surfactants, and the ammonium surfactants were better than the sodium surfactants. These differences were mainly caused by the different migration ability of the four surfactants in the PSA layers and their different hydrophilic nature, as explained in terms of surfactant content at the surfaces of PSA layers with X-ray photoelectron spectroscopy (XPS).

Crosslinked acrylic pressure‐sensitive adhesives. I. Effect of the crosslinking reaction on the peel strength

AbstractFor pressure‐sensitive adhesives (PSAs) composed of poly(co‐ethyl acrylate‐2‐ethylhexyl acrylate‐2‐hydroxyethyl methacrylate) as a base resin and polyisocyanate as a crosslinker, the relationship between the crosslinking reaction and peel strength was investigated. A 90° peel test of cured PSA films under various storage conditions was carried out. At the same time, the isocyanate (NCO) consumption in these PSA films was monitored by attenuated total reflectance/Fourier transform infrared spectroscopy. The peel strength of the PSA compounded with the crosslinker decreased as the NCO groups were consumed. The elevation of the aging temperature promoted the crosslinking reaction and increased the decrement in the peel strength. The peel strength of noncrosslinked and crosslinked PSA films increased with the contact time. A high storage temperature made the increment in the peel strength increase. The addition of the crosslinker to the PSA films reduced the increment in the peel strength. Furthermore, PSA films with residual NCO groups possessed stronger peel strengths than fully cured films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1493–1499, 2003

Development of Environmentally Benign Pressure-Sensitive Adhesive Systems via Modification of Substrate Properties

Results are presented from a study on a novel approach for reducing the negative impact of pressure-sensitive adhesive (PSA) contaminants on paper recycling. Paper substrates are developed that can be easily removed from the recycling process via screening operations and retain their attached PSAs. A model series of hot-melt PSAs consisting of a styrene−isoprene−styrene/styrene−isoprene (SIS/SI) linear triblock, diblock copolymer blend, a pentaerythritol ester of rosin tackifying resin, and a naphthenic process oil were blended in combinations to produce a range of properties. These PSAs were attached to paper substrates with bulk and surface properties that were manipulated using commercial papermaking chemicals and dosages. A poly(amide epichlorohydrin) wet-strength resin was used to inhibit the fiberization of substrates. Removal efficiencies for PSAs coated on this paper were increased, but poor adhesion between the PSA films and paper substrates during repulping limited the improvement. It was found that the retention of the PSA increased linearly with its mechanical loss tangent indicating that properties of the PSA film could be manipulated to optimize removal efficiencies. It was also found that PSA−paper adhesion could be enhanced without restricting PSA properties by introducing an alkenyl ketene dimer size to the paper. The increased adhesion between PSA films and paper in an aqueous environment was achieved by raising the cellulose−water and lowering the cellulose−PSA interfacial energies. Removal efficiencies for PSAs attached to the sized, wet-strength paper substrates were >90% for all of the experimental hot-melt PSAs. Results of this study indicate that minor modifications made with common papermaking additives to paper used in PSA products can significantly reduce their impact on paper recycling.

Mechanistic studies in tackified acrylic emulsion pressure sensitive adhesives

Twenty-three wt % aqueous tackifier dispersion based on glycerol ester abietic acid (Tg = 64°C, Mw = 940) was added to emulsion polymer 50/32/15/3 poly(2-ethyl hexyl acrylate-co-vinyl acetate-co-dioctyl maleate-co-acrylic acid) pressure sensitive adhesive (PSA). From these latices, 25 μm thick films were cast. The films were dried at 25°C for 24 h or at 121°C for 5 min. Dynamic mechanical analysis (DMA) of the films included measuring elastic modulus (G′) and damping factor (tan δ). Under the above drying conditions, the films did not produce significant differences in their DMA and PSA properties as measured by loop tack, peel, and shear holding power. DMA of the tackified acrylic film showed thermodynamic miscibility between the tackifier and polymer regardless of the drying conditions. Microgels formed during emulsion polymerization of the acrylic PSA brought inherent weakness to the tackified film properties. In the neat acrylic PSA film, these discrete networks entangled with the uncrosslinked chains while in the tackified film, these networks could not form entanglements due to the increased molecular weight between entanglements for the uncrosslinked chains. This lack of network entanglements caused shear holding power of the tackified acrylic PSA film to be 4× lower than that of the neat acrylic PSA film. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1965–1976, 2000

A study of the adhesive–skin interface: correlation between adhesion and passage of a drug

The phenomena taking place at the patch–skin interface, in particular the adhesion of a pressure sensitive adhesive (PSA) film loaded with drug and the partition of the drug between the patch and the skin were correlated. The kinetics of adhesion as well as those of drug passage were studied in detail. Adhesion data were collected from peel test either on skin or on a polymer model. Passage of the drug was studied in a simple system composed of PSA film stuck on skin. In some experiments the film was left on the skin throughout the experiment; in others, it was periodically removed and stuck on again to keep the adhesion force constant during the whole of the experiment. We observed a rapid increase of the drug content in the skin until a plateau was reached. One adhesion for the whole experiment or several adhesions gave a similar curve. The main difference was the rate of increase of skin drug content and the value of the plateau. Different hypotheses concerning the relationship between the adhesion of this PSA and changes in the flux of drug have been put forward. However, it is difficult to extrapolate from this model to the in vivo situation because of variation both between individuals and with time.

Tack properties of pressure‐sensitive adhesives

Relevant experiments are essential to clearly understand the role of various molecular (chemical structure, surface energy, composition), experimental (contact time, contact pressure, temperature) or topological (sample roughness and thickness) parameters, on the tack properties of pressure sensitive adhesives (PSA). The “mechano-optical tack tester” (MOTT) is a novel device that we have developed to provide accurate measurements of both the contact area and the tack strength. The MOTT is designed to apply controlled contact pressures by mean of a quartz prism probe, for determined contact times, onto the surface of PSA samples. The probe is then pulled up at controlled rates while the tearing force (tack strength) and the contact area are plotted versus time. The tack energy is then calculated. Using the MOTT, the influence of various parameters (contact pressure, contact area, sample thickness, …) on the tack properties of PSA samples has been studied. The main result lies in the strong dependence of the tack energy on the sample thickness. This points out that the release energy is close to the interface rather than in the bulk of the PSA films, and is a function of the contact area. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1201–1208, 2000

Aging of Pressure Sensitive Adhesives. II: Use of Multidetector Sec

Abstract The use of multiple detector size exclusion chromatography is described for the study of aging of pressure sensitive adhesive films based on styrene-isoprene-styrene block copolymers. In this preliminary investigation of thin films, room temperature oxidation resulted in the formation of an easily detected carbonyl chromophore. While the chromophore concentration was monitored with a UV detector on the chromatograph, the molecular weight distribution was measured with a differential refractometer. Several important general implications of this combination of detectors in SEC are described.