Pressure-sensitive Adhesive Films Research Articles

Pressure-Sensitive Adhesives under the Influence of Relative Humidity: Inner Structure and Failure Mechanisms.

Model pressure-sensitive adhesive (PSA) films of the statistical copolymer P(EHA-stat-20MMA), which comprises 80% ethylhexyl acrylate (EHA) and 20% methyl methacrylate (MMA), are studied. The PSA films are stored under different relative humidities from <2% to 96% for 24 h and subsequently investigated concerning the near-surface composition profile by measuring X-ray reflectivity (XRR) and tack performance. For both types of measurements, special custom-made sample environments are used, which ensure constant temperature and relative humidity during the XRR and tack measurements. Different failure mechanisms of the adhesive bond are found by adjusting the relative humidity. XRR measurements evidence enrichment layers in vicinity to and at the surface depending on the provided relative humidity during the postproduction treatment, which also influence the tack performance. This finding is supported by tack measurements using punches with different roughness.

Preparation and adhesion performance of transparent acrylic pressure sensitive adhesives for touch screen panel

Transparent acrylic pressure sensitive adhesives (PSAs) comprised semi-interpenetrated structured polymer networks were prepared with different co-monomer compositions. Emphasis was placed on the effect of functional groups in the co-monomer including morpholine and tetrahydrofurane moieties in the typical acrylic PSA formulation. The synthesized acrylic PSA syrups were characterized and the optical properties of the acrylic PSA film were also examined by ultraviolet–visible spectroscopy, haze meter, and prism coupler. Acrylic PSAs exhibit high transparency in the visible wavelength region. Adhesion performance was measured by the peel strength, cohesion strength, and probe tack tests. With increasing 4-acryloyl morpholine monomer concentration in the acrylic PSAs, the peel strength, cohesion strength, and probe tack increased.

Removable acrylic pressure-sensitive adhesives activated by UV-radiation

Photo-activated removable acrylic pressure-sensitive adhesives (PSAs) were prepared by mixing a preformed polyacrylate resin with a multifunctional crosslinker, such as ethylene glycol dimethylacrylate (EGDMA), bisphenol A ethoxylate diacrylate (BAED), or trimethylolpropane triacrylate (TMPTA), and followed by a UV-curing process after film coating. The UV-curing process extensively increases the cohesive strength of PSA by increasing its molecular chain length, and the PSA can be removed from the substrate cleanly. The release of the PSA film can also be set to a requested time by applying a UV-curing process. It is shown that certain recipes with a combination of crosslinker and UV light showing excellent removability, such as using BAED with Irgacure 907, or EGDMA with Darocur 1173.

Silicone Adhesive Matrix of Verapamil Hydrochloride to Provide pH-Independent Sustained Release

Providing pH-independent oral release of weakly basic drugs with conventional matrix tablets can be challenging because of the pH-dependent solubility characteristics of the drugs and the changing pH environment along the gastrointestinal tract. The aim of the present study was to use a hydrophobic polymer to overcome the issue of pH-dependent release of weakly basic model drug verapamil hydrochloride from matrix tablets without the use of organic buffers in the matrix formulations. Silicone pressure-sensitive adhesive (PSA) polymer was evaluated because of its unique properties of low surface energy, hydrophobicity, low glass transition temperature, high electrical resistance, and barrier to hydrogen ion diffusion. Drug release, hydrogen ion diffusion, tablet contact angle, and internal tablet microenvironment pH with matrix tablets prepared using PSA were compared with those using water-insoluble ethyl cellulose (EC). Silicone PSA films showed higher resistance to hydrogen ion diffusion compared with EC films. Verapamil hydrochloride tablets prepared using silicone PSA showed higher hydrophobicity and lower water uptake than EC tablets. Silicone PSA tablets also showed pH-independent release of verapamil and decreased in dimensions during drug dissolution. By contrast, verapamil hydrochloride tablets prepared using EC did not achieve pH-independent release.

Synthesis of Self‐Crosslinkable Water‐Borne Pressure Sensitive Adhesives

Self‐crosslinkable acrylic pressure‐sensitive adhesives (PSAs) functionalized with 3‐isopropenyl‐α,α‐dimethylbenzyl isocyanate (TMI) are prepared by starved, seeded semi‐batch emulsion polymerization. n‐Dodecanethiol is used as a chain transfer agent (CTA) and EF‐800 as an anionic surfactant. A three‐level factorial design (32) is performed by varying the TMI and CTA concentrations. Increasing the TMI decreases the rate of polymerization. Nonetheless, high monomer conversions and zero coagulum are observed at the end of all reactions. The latexes are used to prepare PSA films for the evaluation of tack, peel strength, and shear strength. Crosslinking reactions caused by the isocyanate groups of TMI lead to films with very high gel content, even in cases where the CTA concentration is high. As a result, shear strength is greatly improved. The same films preserve sufficient tack but display low peel strength. Empirical models of the tack and peel strength data indicate that TMI has a significant effect on these properties.

Surface Enrichment by Conventional and Polymerizable Sulfated Nonylphenol Ethoxylate Emulsifiers in Water-Based Pressure-Sensitive Adhesive

Comparisons of properties are made for pressure-sensitive adhesives (PSAs) generated via emulsion polymerization using both conventional and reactive emulsifiers. The emulsifiers are ammonium salts of sulfated nonylphenol ethoxylates with similar chemical structures and hydrophilic–lipophilic balances. The polymerizable surfactant possesses a reactive double bond at its phenyl ring allowing for participation in free-radical polymerization reactions. Incorporation of the polymerizable surfactant into the adhesive polymer was confirmed via reverse-phase liquid chromatography. Surfactant distributions in PSA films were characterized using confocal Raman microscopy and contact angle measurements. Increasing conventional surfactant concentrations in emulsion polymerization reactions resulted in proportional increases in concentrations at the surfaces of latex-cast films. No surface enrichment of adhesive films was observed when reactive emulsifiers were employed. Variations in measured depth profiles and conta...

UV-curable pressure sensitive adhesive films: effects of biocompatible plasticizers on mechanical and adhesion properties

We developed a new approach to fabricate pressure sensitive adhesive (PSA) hydrogel films for dermatological applications. These hydrogel films were fabricated using polyvinylpyrrolidone (PVP), poly(ethylene glycol) diacrylate (PEGDA) and polyethylene glycol (PEG) with/without propylene glycol (PG) via photo-polymerization. Hydrogel films with a thickness ranging from 130 to 1190 μm were obtained. The surface morphology and drug distribution within the films were found to be uniform. The effects of different factors (polymeric composition, i.e., PEG/PG presence and film thickness) on the functional properties (i.e., rheological and mechanical properties, adhesion performance and drug distribution) of the films were investigated. The addition of plasticizers, namely PEG and PG, resulted in a simultaneous increase in elasticity and adhesiveness of these hydrogels, via the formation of hydrogen bonds, which has a direct correlation with their adhesion properties. The new approach is potentially useful for industrial applications, due to the simple procedure, precise control over film thickness, minimal usage of solvents and adjustable mechanical, rheological and adhesive properties.

Structure and Macroscopic Tackiness of Ultrathin Pressure Sensitive Adhesive Films

Ultrathin layers of the statistical copolymer P(nBA-stat-MA) with a majority of n-butyl acrylate (nBA) and a minority of methyl acrylate (MA) are characterized with respect to the film morphology and the mechanical response in a probe tack test. The probed copolymer can be regarded as a model system of a pressure sensitive adhesive (PSA). The films are prepared by spin-coating which enables an easy thickness control via the polymer concentration of the solution. The film thickness is determined with x-ray reflectivity (XRR) and white light interferometry (WLI). Grazing incidence small angle x-ray scattering (GISAXS) provides detailed and statistically significant information about the film morphology. Two types of lateral structures are identified and no strong correlation of these structures with the PSA film thickness is observed. In contrast, prominent parameters of the probe tack test, such as the stress maximum and the tack energy, exhibit an exponential dependence on the film thickness.

Reorganization of the Near-Surface Composition in Pressure Sensitive Adhesive Films Stored in Nitrogen Atmosphere

The reorganization of the near-surface composition of a pressure sensitive adhesive (PSA) film upon exposure to nitrogen atmosphere is investigated. PSA films are prepared via solution casting of the statistical copolymer P(EHA-stat-20MMA) which consists of 80% ethylhexylacrylate as the sticky majority component and 20% methylmethacrylate as the glassy minority component. The resulting films are exposed to a nitrogen atmosphere representing an apolar and inert gas. This treatment causes a reorganization of the inner morphology in the PSA film. Using X-ray reflectivity measurements it is shown that the freshly prepared films show a solubility-driven enrichment of the PMMA component at the very surface. Due to the interaction with the apolar nitrogen atmosphere the less polar component of the copolymer, PEHA, enriches on top with time. The timescale of this internal rearrangement lies in the order of days; a complete inversion is found to take 12 days. An investigation of the local homogeneity of the sample surface on a nanoscale with atomic force microscopy proves a smooth and featureless surface right after casting.

Assessment of a Five-Layer Laminate Technique to Measure the Saturation Solubility of Drug in Pressure-Sensitive Adhesive Film

A five-layer laminate technique is used to determine the saturation solubility of a drug in a thin polymer film, c(p)(s), of pressure-sensitive adhesive (PSA) used to prepare transdermal patches. A drug-loaded donor polymer film is attached via a separating membrane to an initially drug-free acceptor polymer film. Diffusion of drug occurs into the acceptor up to saturation solubility equilibrium. This systematic study of the technique using the drug tamsulosin and the PSA Duro Tak 87-4098 was a kinetic analysis of the diffusion process. It was found that the technique gives an equilibrium value for c(p)(s) in a PSA polymer film in the presence of crystalline phase of the drug in the donor. A highly permeable Perthese-separating membrane caused overshoot in the acceptor, most likely induced by initial supersaturation of the donor. Change to a less-permeable ethylene-vinyl-acetate-separating membrane avoided overshoot, but gave a prolonged time, >300 days, to equilibrium. Preloading the acceptor accelerated the equilibration process to approximately 50 days with the Perthese. Suitable experimental conditions are identified that, if performed correctly, allow the technique to give an equilibrium value for c(p)(s).

Probing the Near-Surface Composition Profile of Pressure Sensitive Adhesive Films with X-Ray Reflectivity

The composition of pressure sensitive adhesive films as a function of distance from the sample surface is probed with x-ray reflectivity. A two-component statistical copolymer with ethylhexylacrylate as the majority component and methylmethacrylate representing the minority component is chosen as a sample system. Different preparation protocols are employed to tailor the near-surface composition profile. In this context, doctor-bladed films exhibit a different near-surface composition as compared with solution cast samples. For the latter case, a small surface roughness can be ensured by either using slowly evaporating solvents or reducing the solvent evaporation speed by providing an appropriate atmosphere surrounding the drying solution. For freshly prepared samples, the use of non-polar solvents in combination with the corresponding solvent vapor atmosphere leads to a surface enrichment of ethylhexylacrylate whereas a humid environment, independent of the choice of solvent, favors enrichment of the more polar methylmethacrylate. We finally show that the installed composition profiles change with time and depend on the storage conditions.

Influence of polymer microstructure on the performance of post‐treated latex‐based pressure sensitive adhesives

AbstractLatex‐based butyl acrylate (BA)/acrylic acid (AA)/2‐hydroxyethyl methacrylate (HEMA) pressure sensitive adhesive (PSA) films with various microstructures were heated to improve their performance. The treated PSA films showed significantly better performance than original latex‐based PSA films with similar polymer microstructures. The effect of the heat treatment depended on the polymer microstructure of the untreated PSA films (or corresponding latices). Decreasing the amount of very small sol polymers (i.e., Mx &lt; 2Me) in gel‐free untreated PSA films, or both very small (i.e., Mx &lt; 2Me) and very large sol polymers (i.e., Mx &gt; 20Me) in gel‐containing untreated PSA films led to treated PSA films with significantly better performance. (Note: Me is the molecular weight between two adjacent entanglement points in a polymer material.) In addition, simultaneously increasing the sol polymer molecular weight (Mw) as well as the size of the chain segments between two adjacent cross‐linking points (Mc) of the gel polymer in the original PSAs resulted in treated PSA films with better performance. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Tailoring the near-Surface Composition Profiles of Pressure-Sensitive Adhesive Films and the Resulting Mechanical Properties

We present a possibility of tailoring the near-surface composition profiles of pressure sensitive adhesive (PSA) films by an exposure to atmospheres of different relative humidities (RHs). The statistical copolymer P(EHA-stat-20MMA) with a majority of ethylhexylacrylate (EHA) and a minority of methylmethacrylate (MMA), being cast from a toluene based solution, is chosen as a model system. The near-surface composition profile is probed with X-ray reflectivity. All probed samples show an enrichment of PMMA at the sample surface; however, the near-surface PMMA content strongly increases with increasing RH. The influence of the RH on the composition profile is present down to a depth of 50 nm. Therefore the surface tensions being derived from contact angle measurements do not show any measurable humidity dependence. In contrast, in a mechanical tack test with a smooth punch surface, a strong influence is probed. This observation can be explained by considering the integrated PMMA content over an appropriate near-surface region and the resulting impact on the cavitation process.

Thermally Stable, Transparent, Pressure-Sensitive Adhesives from Epoxidized and Dihydroxyl Soybean Oil

Thermal stability and optical transparency are important factors for flexible electronics and heat-related applications of pressure-sensitive adhesives (PSAs). However, current acryl- and rubber-based PSAs cannot attain the required thermal stability, and silicon-based PSAs are much more expensive than the alternatives. Oleo-chemicals including functionalized plant oils have great potential to replace petrochemicals. In this study, novel biobased PSAs from soybean oils were developed with excellent thermal stability and transparency as well as peel strength comparable to current PSAs. In addition, the fast curing (drying) property of newly developed biobased PSAs is essential for industrial applications. The results show that soybean oil-based PSA films and tapes have great potential to replace petro-based PSAs for a broad range of applications including flexible electronics and medical devices because of their thermal stability, transparency, chemical resistance, and potential biodegradability from triglycerides.

The influence of butyl acrylate/methyl methacrylate/2-hydroxy ethyl methacrylate/acrylic acid latex properties on pressure sensitive adhesive performance

Pressure sensitive adhesives (PSAs) were produced using latexes synthesized via a starved seeded semi-batch emulsion polymerization process with butyl acrylate (BA), methyl methacrylate (MMA) and sometimes additional monomers, 2-hydroxy ethyl methacrylate (HEMA) and/or acrylic acid (AA). For the BA/MMA comonomer latexes, the amount of cross-linker (allyl methacrylate), monomer emulsion and initiator solution feeding times, and the BA/MMA weight ratio were manipulated to vary the polymer properties. The performance of PSA films cast from these latexes was evaluated by tack, peel strength and shear strength. The effect of polymer properties on PSA performance was related to the viscoelastic properties of the PSAs. For BA/MMA latexes, it was not possible to greatly improve the peel strength even at great sacrifice of shear strength for PSAs with gel contents of ∼60 wt% or higher. This was because the ratio of the loss modulus at debonding frequency to the storage modulus at bonding frequency did not vary significantly for the conditions studied. The addition of HEMA provided a significant influence on latex polymer properties as well as PSA performance for both high (>60 wt%) and low (∼20 wt%) gel contents. For some cases, tack, peel strength and shear strength were simultaneously and greatly improved by the addition of HEMA. Adding both AA and HEMA while decreasing the amount of emulsifier, also resulted in a PSA with much better performance (i.e., higher tack, peel strength and shear strength) than a BA/MMA PSA with similar gel content (∼60 wt%).

Near-Surface Composition Profiles and the Adhesive Properties of Statistical Copolymer Films Being Model Systems of Pressure Sensitive Adhesive Films

Statistical copolymer films that consist of two types of monomers, a sticky and a glassy monomer, are investigated. These model systems of pressure-sensitive adhesive films are probed with X-ray reflectivity and mechanical tack measurements. For the first time, composition profiles along the surface normal in the near-surface region are monitored. The influence of the type of monomers being copolymerized, the monomer ratio, and the sample age on the near-surface composition as well as the resulting adhesive performance of the films are analyzed. The copolymers contain ethylhexyl acrylate as the majority component and styrene, maleic acid anhydride, or methylmethacrylate as a minority component. Regardless of the composition, we find a surface enrichment of one component in all samples. In the case of freshly prepared samples, this enrichment is driven by solubility due to the preparation based on solution casting. The minimization of the surface free energy results in an internal reorganization and the component of the statistical copolymer with the lower surface tension enriches at the free surface. The mechanical behavior is not dominated by the surface but by the surface-near part of the composition profile.

Modifications of Surfactant Distributions and Surface Morphologies in Latex Films Due to Moisture Exposure

Migration of surfactants in water-based, pressure-sensitive adhesive (PSA) films exposed to static and cyclic relative humidity conditions was investigated using confocal Raman microscopy (CRM) and atomic force microscopy (AFM). Studied PSA films contain monomers n-butyl acrylate, vinyl acetate, and methacrylic acid and an equal mass mixture of anionic and nonionic nonylphenol ethoxylate emulsifiers. A leveling of surfactant concentration distributions is observed via CRM after films stored at 50% relative humidity (RH) are exposed to a 100% RH for an extended time period, while relatively small increases in surface enrichment occur when films are stored at 0% RH. Use of CRM for binary mixtures containing anionic or nonionic surfactant and latex that has undergone dialysis to remove nonpolymeric components indicates that surfactant-polymer compatibility governs to a great extent surface enrichment, but not changes observed with humidity variations. AFM images show that upon drying latex coatings, surfactant and other additives collect in large aggregation regions, which protrude from film surfaces. These structures are absent at high humidity, which appears to result from lateral spreading across the polymer surface. When humidity is reduced, aggregation regions reform but appear to be smaller and more evenly dispersed, and by cycling humidity between 0 and 100% RH, interfacial enrichment can be seen to diminish. Presented results provide greater insights into the distribution behavior of surfactants in latex films and potential mechanisms for observed issues arising for these systems.

A Comparison of Tackified, Miniemulsion Core−Shell Acrylic Latex Films with Corresponding Particle-Blend Films: Structure−Property Relationships

Tackifying resins (TRs) are often added to pressure-sensitive adhesive films to increase their peel strength and adhesion energy. In waterborne adhesives, the TR is dispersed in water using surfactants and then blended with colloidal polymers in water (i.e., latex). In such waterborne systems, there are problems with the colloidal stability and difficulty in applying coatings of the particle blends; the films are often hydrophilic and subject to water uptake. Here, an alternative method of making waterborne, tackified adhesives is demonstrated. The TR is incorporated within the core of colloidal polymer particles via miniemulsion polymerization. Atomic force microscopy (AFM) combined with force spectroscopy analysis reveals there is heterogeneity in the distribution of the TR in films made from particle blends and also in films made from miniemulsion polymers. Two populations, corresponding to TR-rich and acrylic-rich components, were identified through analysis of the AFM force-displacement curves. The nanoscale maximum adhesion force and adhesion energy were found to be higher in a miniemulsion film containing 12 wt % tackifying resin in comparison to an equivalent blended film. The macroscale tack and viscoelasticity are interpreted by consideration of the nanoscale structure and properties. The incorporation of tackifying resin through a miniemulsion polymerization process not only offers clear benefits in the processing of the adhesive, but it also leads to enhanced adhesion properties.

Characterizing the Distribution of Nonylphenol Ethoxylate Surfactants in Water-Based Pressure-Sensitive Adhesive Films using Atomic-Force and Confocal Raman Microscopy

Surfactant distributions in model pressure-sensitive adhesive (PSA) films were investigated using atomic force microscopy (AFM) and confocal Raman microscopy (CRM). The PSAs are water-based acrylics synthesized with n-butyl acrylate, vinyl acetate, and methacrylic acid and two commercially available surfactants, disodium (nonylphenoxypolyethoxy)ethyl sulfosuccinate (anionic) and nonylphenoxypoly(ethyleneoxy) ethanol (nonionic). The ratio of these surfactants was varied, while the total surfactant content was held constant. AFM images demonstrate the tendency of anionic surfactant to accumulate at the film surfaces and retard latex particle coalescence. CRM, which was introduced here as a means of providing quantitative depth profiling of surfactant concentration in latex adhesive films, confirms that the anionic surfactant tends to migrate to the film interfaces. This is consistent with its greater water solubility, which causes it to be transported by convective flow during the film coalescence process. The behavior of the nonionic surfactant is consistent with its greater compatibility with the polymer, showing little enrichment at film interfaces and little lateral variability in concentration measurements made via CRM. Surfactant distributions near film interfaces determined via CRM are well fit by an exponential decay model, in which concentrations drop from their highs at interfaces to plateau values in the film bulk. It was observed that decay constants are larger at the film-air interface compared with those obtained at the film-substrate side indicating differences in the mechanism involved. In general, it is shown here that CRM acts as a powerful compliment to AFM in characterizing the distribution of surfactant species in PSA film formation.

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

AbstractMiniemulsion 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.