synopsisStudies on t,he peel behavior of pressuresensitive tape comprising a polyester backing and polyacrylate adhesive have shown that, in peeling from a plane glass surface, three fundamentally different modes of peeling may be distinguished, depending upon the rate of pulling. At low rates, deformation by flow of the adhesive appears to determine the peel behavior and the peel force is strongly rate dependent. At high rates, little or no viscous deformation of the adhesive occurs and the peel force is independent of rate. At intermediate pulling rates, cyclical instability of mode of failure involving alternate storage and dissipation of elastic energy in the backing, results in the phenomenon of "slipstick" peeling, in which failure is jerky and regular. Results have been obtained which show how the pulling rates a t which transitions from one mode of peel to another occur, and the peel force values for a given type of failure, depend upon such factors as molecular weight of adhesive, thickness of adhesive, thickness of backing film, and angle of peeling.
SynopsisMixtures in various proportions of natural rubber (NR) and each of two tackifier resins, a poly-/3-pinene and a modified pentaerythritol rosin ester, were used as the adhesive layer in joining a flexible polyester strip to a plane glass substrate. Measurements of the force required to peel the strip from the glass at a 90° angle were made over a range of pulling rates at several temperatures. Application of time-temperature superposition enabled a master curve of (reduced) peel force versus (log) pulling rate at a standard temperature (296 K) to be obtained for each adhesive composition.The master curves showed, in increasing order of pulling rate, some or all of four different modes of peeling: (i) peeling with viscous adhesive response, (ii) peeling with rubbery response, (iii) oscillatory or slip-stick peeling, and (iv) peeling with glassy adhesive response. In general, transitions between the different peeling modes were quite abrupt. Increase in concentration of tackifier resin caused displacement of the master curve toward lower pulling rates [an effect interpreted in terms of an increasing adhesive glass temperature (T,)], and a superimposed displacement of the transition between peeling modes (i) and (ii) toward higher pulling rates-an effect attributed to reduction in adhesive average molecular weight. The influence of the tackifier resin in modifying the viscoelastic characteristics of the adhesive was further demonstrated in a comparison of the peel force master curves with corresponding master curves of dynamic storage modulus.
The dynamic viscoelastic response of blends in various proportions of natural rubber with each of two tackifier resins, a poly(β‐pinene) and a pentaerythritol ester of hydrogenated rosin, has been investigated. Results are presented in the form of master curves of the modulus G′r and the viscosity η′r against frequency. The two resins show remarkably similar behavior in modifying the viscoelastic behavior of the rubber; the most obvious effects of increasing resin concentration are (a) a displacement of the transition zone toward lower frequencies, (b) a reduction in width and eventual elimination of the “rubbery plateau,” and (c) a displacement of the terminal zone in the direction of higher frequencies. The effect (a) is interpreted in molecular terms as a restriction of segmental motion and may be quantitatively evaluated in terms of reduced fractional free volume and increased monomeric friction coefficient. Effects (b) and (c) are explicable in terms of reduced average molecular weight, with consequent reduction in entanglement coupling and resistance to viscous flow. Quantitative analysis of the results, using relaxation time spectra, lends support to the iso‐free‐volume theory of the glassy state and shows a correlation between fractional free volume and monomeric friction coefficient.
A good tackifler resin appears to require a combination of high T9 at low molecular weight, and for this a highly condensed alicyclic structure appears to be desirable. Except for the rosin derivatives, the chemical structures of common tackifier resins are not fully resolved and some more definitive studies are needed. In addition, a high degree of miscibility of resin with rubber is important. Although some common tackifier resins cause phase separation over certain concentration ranges when blended with rubbers, this is probably exceptional, and it seems likely that most adhesive compositions are single phase in this respect. Where phase separation does occur, there is a need for fuller elucidation of the phase morphology and composition of the phases. The widely-held view that a two-phase morphology is necessary for the development of high tack appears to be incorrect. The few studies which have been carried out so far indicate that the surface energy of a rubber is modified only slightly by the incorporation of a tackifier resin. It is unlikely that this surface energy change can seriously affect the tack value as measured by the common form of probe test. The viscoelastic behavior of rubber-resin blends provides an adequate explanation for the phenomenon of tack. Over long response times, the low modulus (high compliance) of the blend compared with that of the rubber alone allows a high degree of intermolecular contact to be achieved during the bonding stage of a tack test. Over short response times, corresponding to the de-bonding stage of the test, the blend shows transition zone response with high energy dissipation and consequent high separation force. For further progress in our understanding, however, there is a need to distinguish clearly between the bonding and debonding effects of the resin and also for a more definitive study of the factors controlling the bonding stage.
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