Pressure sensitive adhesive wetting

Pressure-Sensitive Adhesives. SiHcoae PSAs are used primarily ia specialty tape appHcatioas that require the superior properties of siHcoaes, including resistance to harsh chemical environments and temperature extremes (398,399). SiHcone PSAs are also used ia appHcatioas requiring long service Hfe, electrical iasulatioa, and protection from moisture. Another distinctive advantage of siHcone PSAs is their abiHty to wet low surface energy tape substrates such as PTEE. 

NR adhesives can be divided in two types wet bonding and dry bonding. Wet bonding adhesives are applied on substrates in a fluid state, the bond being formed by drying. The dry bonding NR adhesives are pressure-sensitive adhesives because the bond is created under pressure. 

Film Adhesive. A further type which is of importance commercially is the film adhesive which exists usually as pre-formed shapes on suitable substrate materials (often, siliconized release paper). When being applied, such adhesives are activated by heat or pressure and so converted into a semi-fluid condition, when they can wet the surface to be bonded. The pressure-sensitive adhesives are an example of this type—the application of finger pressure alone being sufficient to form the bond. 

The primary mode of bonding for a pressure-sensitive adhesive is not chemical or mechanical but rather a polar attraction to the substrate. This always requires pressure to achieve sufficient wet-out onto the surface thereby providing adequate adhesion. The four main varieties of pressure-sensitive adhesives are derived from rubber-based, acrylic, modified acrylic and silicone formulations. Release liners are used to carry the sticky label and enable it to be printed. The release liners are normally paper, treated with a very thin silicone coating to allow the label to be peeled away easily without tearing. Some transfer of the silicone into the adhesive is inevitable. 

ADHERE-TEX 1510 is a semi-permanent pressure sensitive adhesive which develops outstanding tack when the cast film is dried. The adhesive properties may be modified by dilution with water, thickening, or blending with other polymers such as TEX-WET 1089, a polyvinyl acetate emulsion. 

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Hou et al. [42] studied the effect of AA in pressure-sensitive adhesives composed of a terpolymer of 2-ethylhexyl acrylate, VAc and AA. Like Gajria et al. they noticed a dependence of latex swelling with polymerization procedure. Hou et al. noted that batch polymerized latexes produced greater viscosity responses with pH adjustments. They demonstrated that addition of sodium chloride effectively reduced the pH-thickening response curve. Hnally, Kries and Sheiman [43] studied the influence of polymerizing various fimctional monomers on the wet adhesion of VAc/B A latexes. A number of monomer types were studied, such as polymoizable monomers containing amines, amides and acetoacetates, alone and in combination with each other. The ureido functional monomras exhibited supoior wet adhesion. 

Uses Epoxy modifier esp. suited for use in wh. and pastel coatings, and in hot-melt, hot-melt pressure-sensitive, and pressure-sensitive adhesives diluent for low-vise, epoxy systems pigment wetting agent plasticizer for epoxy system injections provides impermeable membrane between asphaltic surfs, and concrete bridge decks Properties Gardner 1 resin sol. in alcohols, chlorinated hydrocarbons, esters, ethers, ketones (except acetone), aromatic, naphthenic, and terpene hydrocarbons partially sol. in paraffinic hydrocarbons m.w. 225 vise. 1.75 poises hyd. no. 50 Toxicology TSCA listed Nevoxy EPX-L5 [Neville]... 

Adh6SiV6 Tack. Adhesives that exhibit adhesive tack are often called pressure-sensitive adhesives (PSAs), since joint strengths depend on the pressure applied during bonding. In practice, PSAs are usually carried on a backing tapes and labels are examples. In order to secure rapid wetting on common surfaces, a PSA must have a creep compliance after 1 s greater than about 10" m /N... 

Thus, in brief, a successful pressure-sensitive adhesive not only has low resistance to small strain deformation in order to facilitate wetting, but also it can resist large strains without flowing apart easily. Certain neat elastomers such as acrylate-based PSAs possess these features and are intrinsically tacky without additives. Other PSAs are formulated by diluting high molecular weight rubbers with special resins called tackifiers. 

The surface applications of silicone products, particularly those based on polydimethylsiloxane (PDMS), are many and varied (7). Familiar examples include release liners for pressure-sensitive adhesives (PSAs), antifoaming agents, and water-repellent treatments for a wide variety of substrates. This broad diversity of application is a direct consequence of the low surface energy of PDMS which is lower than most other polymers except for those based on aliphatic fluorocarbon moieties. Despite the commercial importance of this aspect of the properties of polydimethylsiloxane, there is no fully satisfactory contact angle characterization of PDMS yet available. There is no lack of potential candidates part of the difficulty in identifying a definitive study lies in the breadth of these diverse wetting... 

Normally adhesives are applied in a liquid state to the substrate, wet the surface, and solidify to generate a bond, for example by evaporation of a solvent, cooling of a hot melt, or formation of a cross-linked polymer. Pressure-sensitive adhesives do not change their physical state, but they also must wet the surface of the substrate like a liquid and sustain loads like a solid. There have been many discussions and investigations concerning how they fulfill these conflicting demands. 

Pressure-sensitive adhesion is determined by two key factors. First, the surface must be contacted to such a degree that molecular attraction forces come into action (wetting). The second factor is the energy of deformation of the viscoelastic adhesive until rupture or separation from the surface occurs. 

Pressure-sensitive adhesives seem to possess a critical balance between adhesion and cohesion. The phenomenon of pressure-sensitive tack has been described as a complex function of flow and diffusion. The role of resins in pressure-sensitive adhesives has been interpreted recently as improving tack by a two-step process (1) reducing the modulus and viscosity of the adhesive to give faster, more complete wetting of the substrate and (2) raising the Tg of the adhesive. Plasticizers and solvents accomplish the first objective but not the second This interpretation is consistent with the... 

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