Pressure-sensitive adhesives -adhesion properties

A more complete account of the many varieties of pressure-sensitive products commercially available may be found in the first reference (see also Pressure-sensitive adhesives - adhesion properties). 

Viscoelastic characteristics of polymers may be measured by either static or dynamic mechanical tests. The most common static methods are by measurement of creep, the time-dependent deformation of a polymer sample under constant load, or stress relaxation, the time-dependent load required to maintain a polymer sample at a constant extent of deformation. The results of such tests are expressed as the time-dependent parameters, creep compliance J t) (instantaneous strain/stress) and stress relaxation modulus Git) (instantaneous stress/strain) respectively. The more important of these, from the point of view of adhesive joints, is creep compliance (see also Pressure-sensitive adhesives - adhesion properties). Typical curves of creep and creep recovery for an uncross-Unked rubber (approximated by a three-parameter model) and a cross-linked rubber (approximated by a Voigt element) are shown in Fig. 2. 

The viscoelastic contribution to the fractnre energy in an adhesion test is, in principle, rate and temperatnre-dependent. Where this contribution is significant, it has been possible to nationalise adhesion measurements using the WLF transform-see Adhesion - fundamental and practical. Pressure-sensitive adhesives - adhesion properties and Tensile... 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Ko, C. U., S. L. Wilking, and J. Birdsall. 1995. Pressure sensitive adhesive property optimizations for the transdermal drug delivery systems. Pharm. Res. 12 S-143. 

Figure 18.10 Pressure-sensitive adhesive properties of polyaciylates as a function of temperature (a) tack, and (b) elastic modulus. Data fix>m Druschke as reproduced in Satas [27], (Reproduced with permission.)...

Figure 18.11 Pressure-sensitive adhesive properties as a function of molar mass. (Adapted from Satas [123].)...

Uses Monomer for lube oil additives, pour pt. depressants, paper coatings, textile finishes, paints, varnishes, pressure-sensitive adhesives Properties APHA 200 color m.w. 338.58 sp.gr. 0.864 (20/20 C) f.p. 18... 

The preparation of silicone psas was first disclosed in the mid 1950s when it was observed that blends of silicone resins and polydimethylsiloxane gums (PDMS) produced a material with pressure sensitive adhesive properties (i.e., tack and adhesion under low application pressure). By 1956 a versatile process for the preparation of silicone resins suitable for use in silicone psas was described (6). Rapid progress in the preparation and application of silicone psas was observed from this point to the present. 

A large number of compositions containing poly(vinyl alkyl ethers), multifunctional acrylates and benzophenone were irradiated and examined for their pressure-sensitive adhesive properties. Among the components which were found to confer useful properties on the films were the methyl, ethyl, and isobutyl poly(vinyl alkyl ethers) and the multifunctional acrylate derivatives of neopentyl glycol, pentaerythritol, certain epoxidized oils and an aliphatic polyurethane (9). For our present discussion, however, we will focus on the behavior of the poly(vinyl ethyl ether) (PVEE/neopentyl glycol diacrylate (NPGDA) system. 

Pig. 4. Effect of PVEE/MCEA Ratio on Pressure-Sensitive Adhesive Properties (6 Sec. Cure, 2 % Benzophenone)... 

When isobornyl acrylate was used in place of MCEA, cured films with good pressure-sensitive adhesive properties were again obtained although somewhat longer irradiation times were required. For example, blends of poly (vinyl ethyl ether ) in IBA displayed properties ranging from 2 pli with adhesive failure and over 90 hours of shear to nearly 9 pli (cohesive failure) and 1 hour of shear (Table 7). The latter product was examined closely for complete cure and found to have 1.6 percent unreacted monomer at 6 sec. radiation and 0.23 percent at 9 sec. Again, a large gel fraction was present in the cured adhesive as noted earlier. 

Although the gel content indicated the presence of substantial cross-linking, the pressure-sensitive adhesive properties could be improved further by incorporating a small amount (1 percent) of pentaerythritol triacrylate (PETA). This system developed a peel value of 5.6 pli with adhesive failure and 22 hours of shear with only 6 seconds of cure time in air (Table 7). 

A review of the traditional use of resins as tackif iers for elastomers in solvent-applied adhesives is presented. Following is a discussion of how this technology is extended to water-based systems. Data is discussed on the use of various backbone polymer latexes, modified with the proper resin emulsions to achieve pressure—sensitive adhesive properties. Recommendations are also given for the proper choice of resin type to achieve optimum properties with each specific class of elastomer. 

Emulsion blends of a Cg-Cu alkyl acrylate copolymer and a styrene based polymer (5-30 wt% in the blend) were noted to have an improved balance of pressure sensitive adhesive properties [141]. The blend was prepared by polymerization of the alkyl acrylate copolymer in the presence of the prepolymerized styrene polymer. 

Uses Film-former for cosmetics, pharmaceuticals, hairsprays, gels, mousses, lotions, hair thickeners, tints, dyes, antiseptic/anesthetic spray bandages, antibiotic aerosol bandages, spray or rub-on gloves and protective masks solubilizer vise, modifier/stabilizer granulation binder hot melt adhesive for diaper wetness indicator system in repulpable, pressure sensitive adhesives Properties Liq. sol. in aq. and most common org. soivs. 50% act. in ethanol Tbxrco/ogy Very low chronic oral toxicity safe in contact with skin PVP/VA E-635 [ISP]... 

G-5—G-9 Aromatic Modified Aliphatic Petroleum Resins. Compatibihty with base polymers is an essential aspect of hydrocarbon resins in whatever appHcation they are used. As an example, piperylene—2-methyl-2-butene based resins are substantially inadequate in enhancing the tack of 1,3-butadiene—styrene based random and block copolymers in pressure sensitive adhesive appHcations. The copolymerization of a-methylstyrene with piperylenes effectively enhances the tack properties of styrene—butadiene copolymers and styrene—isoprene copolymers in adhesive appHcations (40,41). Introduction of aromaticity into hydrocarbon resins serves to increase the solubiHty parameter of resins, resulting in improved compatibiHty with base polymers. However, the nature of the aromatic monomer also serves as a handle for molecular weight and softening point control. 

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

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. 

Among the different pressure sensitive adhesives, acrylates are unique because they are one of the few materials that can be synthesized to be inherently tacky. Indeed, polyvinylethers, some amorphous polyolefins, and some ethylene-vinyl acetate copolymers are the only other polymers that share this unique property. Because of the access to a wide range of commercial monomers, their relatively low cost, and their ease of polymerization, acrylates have become the dominant single component pressure sensitive adhesive materials used in the industry. Other PSAs, such as those based on natural rubber or synthetic block copolymers with rubbery midblock require compounding of the elastomer with low molecular weight additives such as tackifiers, oils, and/or plasticizers. The absence of these low molecular weight additives can have some desirable advantages, such as ... 

The film that is selected for the application mainly controls the properties these products deliver to the customer. However, in the case of optical applications where transmission of the light is needed, the choice of pressure sensitive adhesive and the quality of the application to the substrate become very critical. Even for the non-optical uses, the PSAs have to perform reliably under sometimes very demanding circumstances. 

Chu, S.G., Viscoelastic properties of pressure sensitive adhesives. In Satas, D. (Ed.), Handbook of Pressure Sensitive Adhesive Technology. Van Nostrand Reinhold, New York, 1989, p. 191. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

The biggest challenge to produce an ultra-thin (about 1 nm) overcoat is to make the coating free of pin-holes while maintaining the durability and tribological properties. In an HDD system, pin-holes can cause much more contaminants from all sources, such as outgas compounds from polymeric foam components, pressure sensitive adhesives, ionic residues from improperly cleaned components and ambient pollutants, which can be detrimental to the tribology and durability of the HDD. Therefore, efforts have been made mainly on the improvement of carbon film [4-7]. 

A different approach, although stdl working with essentially non-fiinctional polymers has been exemplified [114,115], in which, a 100% solid (solvent free) hot melt has been irradiated to produce pressure-sensitive adhesives with substantially improved adhesive properties. Acrylic polymers, vinyl acetate copolymers with small amounts of A,A -dimethylaminoethyl methacrylate, diacetone acrylamide, A-vinyl pyrrohdone (NVP) or A A have been used in this study. Polyfunctional acrylates, such as trimethylolpropane trimethacrylate (TMPTMA) and thermal stabilizers can also be used. 

NR, styrene-butadiene mbber (SBR), polybutadiene rubber, nitrile mbber, acrylic copolymer, ethylene-vinyl acetate (EVA) copolymer, and A-B-A type block copolymer with conjugated dienes have been used to prepare pressure-sensitive adhesives by EB radiation [116-126]. It is not necessary to heat up the sample to join the elastomeric joints. This has only been possible due to cross-linking procedure by EB irradiation [127]. Polyfunctional acrylates, tackifier resin, and other additives have also been used to improve adhesive properties. Sasaki et al. [128] have studied the EB radiation-curable pressure-sensitive adhesives from dimer acid-based polyester urethane diacrylate with various methacrylate monomers. Acrylamide has been polymerized in the intercalation space of montmorillonite using an EB. The polymerization condition has been studied using a statistical method. The product shows a good water adsorption and retention capacity [129]. 

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