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16 PIGMENT AND RESIN TECHNOLOGY JULY 1984 A n overview of the principles of the testing and formulation of pressure-sensitive adhesives* *Abstract of paper presented at the Seventh Annual Meeting of The Adhesion Society, Jacksonville, Florida, USA, 13-15 February, 1984. D. Satas Pressure sensitive adhesives are polymeric Pressure-sensitive adhesives are characterised materials of rather specific rheological properties, by three parameters: tack, resistance to peel and exhibitin g both elastic and viscous behaviour. These resistance to creep. This characterisation evolved properties are achieved by either compounding the fro m the use requirements of pressure sensitive pro elastomer with resins and plasticizers, or by tailoring ducts . the polymer properties during polymerisation. Tack indicates the rate of adhesive bond Mos t elastomers can be compounded into formation . It is a fundamental property of pressure- tacky materials, but only some of them are of practical sensitive adhesives which by definition must be tacky. importance for pressure-sensitive adhesives. Natural It is, however, a transient property and once the bond rubber and SBR are the most often used elastomers. is formed, tack is no longer important. There are Butyl rubber, polyisobutylene and silicones are also several tests used to evaluate tack. From the user's used for pressure sensitive products. Block poin t of view a qualitative description of tack, i.e., copolymers , especially styrene-isoprene-styrene, are high , medium, or low, is quite sufficient and a more very important especially for hot melt adhesives. exact quantitative description of this property does Compoundin g of block copolymers is more complica no t contribute much to the choice of the adhesive. ted , because of the presence of two different phases Tack measurements by the adhesive product manufac whic h exhibit different compatibilities. The elasto- turers for quality control purposes are quite useful. meric phase determines the tack and peel properties Resistance to peel is related to the adhesive and it is compounded with tackifying resins, while bond strength and therefore it is expected to relate styrene phase largely controls the cohesive strength. to bond performance. There is an erroneous, but Some polyacrylates and polyvinylethers are widel y spread tendency, to relate a high resistance inherently tacky and can be used as pressure-sensitive to peel to good adhesive performance. adhesives without compounding with tackifying Pressure-sensitive adhesives when peeled are resins. Polyacrylates have become important ad expected to fail adhesively, although under some hesives and their properties are usually adjusted circumstances a cohesive failure also may be ex durin g polymerisation. Presence of various functional perienced . Cohesive failure is more likely to occur groups, molecular weight, degree of crosslinking, at elevated temperatures and at low peel rates. A hydrogen bonding capabilities can be varied during plo t of resistance to peel as a function of peel rate is polymerisatio n and influence the adhesive properties. especially useful to analyse adhesive performance. Polyvinylethers have not achieved the wide use of Resistance to creep is a good measure of the polyacrylates , but vinyl acetate-ethylene copolymers cohesive strength and these tests are important to migh t become important as pressure-sensitive predic t the adhesive behaviour under shear. The adhesives. adhesive should fail cohesively in such tests. Resis Environmental regulations and increased tance to creep-temperature curves are quite informa solvents costs have contributed to the use of aqueous tive in predicting the adhesive behaviour when dispersions and of hot melts. These changes have subjected to a dead load under various conditions. raised additional compounding problems. Aqueous The adhesive properties depend upon the dispersions of acrylic polymers require compounding rheology of the adhesive and it could be expected to improve the coatability. The adhesive properties that tensile modulus, compliance or some other pro of aqueous dispersions are not quite as good as perty expressed as a function of time could be used those of solution polymers and further improvements to describe pressure-sensitive adhesives. Such in the emulsion polymerisation technology are re approaches are interesting and help to understand the quired . The growing use of hot melts require behaviour of pressure-sensitive adhesives, but the improve d thermal stability, since such adhesives correlatio n of such data with the adhesive perform migh t be exposed to elevated temperatures for ance has not been completely successful. prolonge d periods of time.
Pigment & Resin Technology – Emerald Publishing
Published: Jul 1, 1984
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