Waterborne Acrylic Pressure Sensitive Adhesives Research Articles

Improving the properties of water-borne pressure sensitive adhesives by using non-migratory surfactants

The adhesive performance of high solids content water-borne acrylic pressure sensitive adhesives synthesized using polymerizable surfactants (Latemul®PD-104 and Sipomer®Pam-200) or a polymerizable stabilizer (Sipomer®Cops-1) has been studied. The use of a high pH during the polymerization process has a deleterious effect on the final adhesive properties because gel polymer is not formed. Interestingly, at low pH it is found that the surfactant concentration used during the polymerization process has also an effect on the polymer microstructure; the higher the surfactant concentration, the higher the final gel content of the latex. When polymerizable surfactants or a stabilizer are used the peel strength of the final films is considerably improved due to the surfactant incorporation into the polymer backbone and hence reduced surfactant migration to the air-film interface. Work of adhesion and shear adhesion failure temperature (SAFT) are not noticeably affected by surfactant migration but their performance is affected by the surfactant employed.

Environment-Friendly Acrylic Pressure Sensitive Adhesives (PSAs) with Improved Antistatic Property

A series of acrylic PSAs were synthesized by pre-emulsion addition method and characterized by FT-IR spectroscopies. Electrical and mechanical properties of antistatic waterborne acrylic PSAs containing electrically conductive fillers were investigated in this paper depending on the component of monomers, reactive emulsifiers, and the type of conductive fillers. PSA synthesized by reactive emulsifier showed higher physical properties and its composite with 1wt% of conductive fillers showed good dispersity with optimal peel strength.

Synthesis of Thermally Stable Waterborne Acrylic Pressure Sensitive Adhesives (PSAs) for Application to LCD Devices

The new thermally stable acrylic pressure sensitive adhesives (PSAs) were synthesized by pre-emulsion addition method, characterized by FT-IR spectra and differential scanning calorimetry (DSC), and evaluated their physical properties based on reactive emulsifiers and tackifiers. Experimental Tg of PSAs with different monomer compositions was similar to its theoretical values. The PSAs with reactive emulsifier showed higher mechanical and thermal properties than those with conventional emulsifier. The novel thermally stable acrylic PSA synthesized from 2-ethylhexyl acrylate (2-EHA) and butyl acrylate (BA) as main monomers, and methylmethacrylate (MMA) and acrylic acid (AAc) a cohesive monomer, and reactive emulsifier and tackifier, showed excellent physical properties such as initial, moisture and heat resistant peel strengths, which are considered to be one of the most important properties to be applied for liquid crystalline display (LCD) devices.

Synthesis and Properties of Thermally Stable Pressure Sensitive Adhesives for LCD Devices

Waterborne acrylic pressure sensitive adhesives (PSAs) were synthesized by pre-emulsion addition method, characterized by FT-IR, 1H NMR spectra and differential scanning calorimetry (DSC), and evaluated their physical properties based on main and cohesive monomers. The waterborne acrylic PSA synthesized from 2-ethylhexyl acrylate (2-EHA) and butyl acrylate (BA) as main monomers, and 10 wt% of methyl methacrylate (MMA) as a cohesive monomer, showed excellent physical properties such as initial, moisture and heat resistant peel strengths, which are considered to be one of the most important properties to be applied for liquid crystalline display (LCD) devices.

Viscoelastic properties and peel strength of water-borne acrylic PSAs for labels

The use of acrylic emulsion pressure-sensitive adhesives (PSAs) in labels continues to increase due to their environmental friendliness, good aging resistance and excellent processability during coating process. This study has investigated the acrylic emulsion/tackifier blend systems by examining their thermal properties as indicated by the glass transition temperature (T g), by conducting optical microscopy to determine their structure, by exploring their viscoelastic properties with the advanced rheometric expansion system (ARES), and by measuring their peel strength with the 180° peel test. The acrylic emulsion/tackifier blends with low softening point and low molecular weight exhibited only a single T g. However, the acrylic emulsion/tackifier blends with high softening point and high molecular weight showed two T g values. Microscopic observation of blends with miscible tackifier did not show any evidence of tackifier traces, whereas the microscopic structures of blends with immiscible tackifier showed dispersed tackifier particles. As the tackifier content increased, the rubbery plateau region decreased for the miscible blends but increased for the immiscible blends. The 180° peel strength of the miscible blends was dependent on the viscoelastic properties and was influenced by incorporation of tackifiers in the PSA systems. However, the peel strength of the immiscible systems did not depend on the viscoelastic properties at low tackifier content, but it decreased as the tackifier content increased in accordance with the increase of the storage modulus G′.

Skin Development during the Film Formation of Waterborne Acrylic Pressure-Sensitive Adhesives Containing Tackifying Resin

Tackifying resins (TR) are often used to improve the adhesive properties of waterborne pressure-sensitive adhesives (PSAs) derived from latex dispersions. There is a large gap in the understanding of how, and to what extent, the film formation mechanism of PSAs is altered by the addition of TR. Herein, magnetic resonance profiling experiments show that the addition of TR to an acrylic latex creates a coalesced surface layer or “skin” that traps water beneath it. Atomic force microscopy of the PSA surfaces supports this conclusion. In the absence of the TR, particles at the surface do not coalesce but are separated by a second phase composed of surfactant and other species with low molecular weight. The function of the TR is complex. According to dynamic mechanical analysis, the TR increases the glass transition temperature of the polymer and decreases its molecular mobility at high frequencies. On the other hand, the TR increases the molecular mobility at lower frequencies and thereby promotes the interdiffusion of latex particles to create a skin layer. In turn, the skin layer is a barrier that prevents the exudation of surfactant to the surface. The TR probably enhances the coalescence of the latex particles by increasing the compatibility between the acrylic copolymer and the solids in the serum phase.

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.

Morphology and elasticity of waterborne acrylic pressure-sensitive adhesives investigated with atomic force microscopy

The morphology of pressure sensitive adhesives (PSAs), especially at the surface in contact with a release liner, is expected to have a dominant influence on the tack strength and energy in an application. We have used tapping-mode atomic force microscopy to determine the morphology at the surfaces of freshly-cast waterborne acrylic PSAs over lateral length scales of a few μm. We demonstrate that topographical features on silicone release liners can be used to pattern the PSA surface in contact with it. Control of the texture of a PSA surface can potentially be exploited to tailor its properties. Latex particle boundaries are much better defined at the air surface of the PSA in comparison to its back face. A series of experiments suggests that this difference results from the distribution of water-soluble species within the dry film. The pressures and processes involved in the transfer lamination process do not alter the PSA morphology. The first reported AFM images of the response of these materials to pressure and shear provide insight into the deformation mechanisms. Amplitude-distance curves on PSA surfaces show that there is a small decrease in tack and an increase in stiffness after ageing for 13 months.

Origins and Effects of a Surfactant Excess near the Surface of Waterborne Acrylic Pressure-Sensitive Adhesives

An excess of anionic surfactant at the surface of waterborne acrylic pressure-sensitive adhesives (PSAs) is identified through the complementary use of atomic force microscopy and elemental depth profiling with Rutherford backscattering spectroscopy. This surfactant is distributed around the particles at the film surface, where it presumably contributes to the stabilization of the particles against coalescence. This result is not consistent with the prediction of a process model of film formation that a coalesced skin layer should form, owing to the polymer's low zero-shear-rate viscosity (5 × 104 Pa s). To investigate the reason for the surface excess, the spatial distribution of water during film formation of the latex PSAs has been determined using magnetic resonance profiling. In the later stages of drying, the water concentration is very low near the surface, and it increases linearly with the depth into the film. The water profiles indicate that the deformation of particles is not accompanied by coalescence but that water-filled capillaries exist at particle boundaries. It is suggested that the transport of surfactant (and water-soluble polymer, ions, and other species) to the surface is driven by the capillary pressure.

Obtaining and Interpreting Images of Waterborne Acrylic Pressure-Sensitive Adhesives by Tapping-Mode Atomic Force Microscopy

The first atomic force microscopy (AFM) images of waterborne acrylic pressure-sensitive adhesives (PSAs) are presented along with details of their optimum scanning conditions. Driving this work is a huge practical need for information about the surface morphology of waterborne PSAs, which are deposited from colloidal dispersions to yield highly tacky, soft surfaces. These surfaces present contradictory requirements for tapping-mode AFM. Whereas soft surfaces require light tapping to avoid surface damage, tacky surfaces require energetic tapping to enable the tip to lift off of the surface. We have made a systematic study of the effects of several key parameters: the cantilever spring constant; the free amplitude of oscillation (Ao); the setpoint value (dsp); and the setpoint ratio (rsp = dsp/Ao), which we have re-defined for a soft surface to account for the indentation depth. Amplitude−distance curves were obtained from the PSA surfaces to evaluate the tip's indentation depth. Reliable images are obtained when these parameters are known and optimized. While the “true” surface of the film is actually rather smooth, images of the sub-surface particle morphology are best obtained with a stiff cantilever (spring constant of 48 N/m) and a large Ao (about 135 nm). Setting rsp close to unity minimizes the indentation of the tip and the resultant surface deformation.