SynopsisBlends of elastomers with the proper concentration of appropriate low molecular weight resins exhibit performance as pressure sensitive adhesives. Viscoelastic properties, which may be related to adhesive performance, were measured on 1:l blends of rubber and resin using a mechanical spectrometer. Significant differences in viscoelastic properties were observed depending upon the resin structure. On plots of G' and tan 6 vs. temperature, the addition of a compatible resin produces a pronounced shift of the tan 6 peak to a higher temperature and reduces the modulus in the rubbery plateau. An incompatible resin results in a minor shift in the tan 6 peak of the elastomer along with the appearance of a second peak at higher temperature, attributed to a second phase which is predominantly resin. Also, the modulus is increased in the rubbery plateau. A polystyrene resin, M,, about 900, is shown to be incompatible with natural rubber but compatible with styrene-butadiene rubber. A cycloaliphatic poly(viny1 cyclohexane) resin, M,,, about 650, prepared by hydrogenating the polystyrene resin, is compatible with natural rubber, but incompatible with styrene-butadiene rubber. An alkylaromatic poly(tert-butylstyrene) resin, Mu about 850, which is intermediate in aromaticity between the aromatic polystyrene resin and the cycloaliphatic poly(viny1 cyclohexane) resin, is compatible with both natural rubber and styrene-butadiene rubber. Therefore, the structure of the resin is very important in adjusting the viscoelastic properties of a rubber-resin blend to achieve pressure sensitive adhesive performance.

SynopsisThe viscoelastic properties of a rubber-resin blend, which influences performance of the blend as a pressure-sensitive adhesive, depend upon the structure of the resin as well as its molecular weight. The effect of the concentration of a compatible resin in the blend was examined using a mechanical spectrometer. Four types of resins were used. These are the rosin esters, polyterpenes, pure monomer resins such as polystyrene and poly(viny1 cyclohexane), and petroleum stream resins. Each was examined in blends with both natural rubber and styrene-butadiene rubber over a range of concentrations. It is shown that the temperature of the tan 6 peak for compatible systems can be predicted by the Fox equation, T;l = WIT$ + WzTal, where W1 and Wz are the weight fractions of the resin and rubber, respectively, and the 2'; s are the tan 6 peak temperatures in K. The plateau modulus GR for a blend can be identified as the G' value in the rubbery plateau at the point where tan 6 is at a minimum. The relationship between GR and GRp, the plateau modulus for the undiluted elastomer, is shown to be proportional to the volume fraction of the elastomer raised to the 2.3-2.4 power for natural rubber with six different compatible resins. The exponent for styrenebutadiene rubber is 2.5-2.6 with four different resins. Using these relationships, both the tan 6 peak temperature and plateau modulus can be predicted for a rubber-resin system from data on the unmodified elastomer and on one typical rubber-resin blend.

SynopsisIn blends of rubber and low molecular weight resins, the compatibility of the system controls the viscoelastic properties and ultimately the performance of the composition as a pressure sensitive adhesive. The effect of the resin molecular weight on compatibility was examined by studying rubber-resin blends prepared from resins which represent a range of molecular weights. Viscoelastic properties were measured using a mechanical spectrometer on 1:l blends of rubber and a series of polystyrene resins and poly(vinylcyc1ohexane) resins. Based on plots of G' and tan 6 vs. temperature, blends of natural rubber and polystyrene resin show incompatibility at resin M,,, of about 600 and above. Blends of natural rubber and poly(viny1 cyclohexane) are incompatible at resin M, of about 1800, but are compatible at M , of about 650. Blends of styrene-butadiene rubber and polystyrene resins are compatible at resin M,,, of about 650 but appear to contain a low volume incompatible phase at M , of about 900. Therefore, the compatibility of a rubber-resin blend depends upon the molecular weight of the resin. Even systems expected to be compatible will show evidence of incompatibility as the molecular weight of the resin is raised above some limiting value.

Peroxides are preferred for heat curing vinylmethylsilicone (VMQ) elastomers because the free-radical-initiated crosslink does not reduce the inherent stability of the polymer. The objectives of this work were (1) to obtain information on the relationship between peroxide concentration and the physical properties of heat-cured silicone elastomers, and (2) to explain the suspected difference in efficiency of two peroxides, α,α′-di(tert-butylperoxy)-m/p-diisopropylbenzene (DBPIB) and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DBPH). VMQ bases and gums were cured with DBPIB and DBPH over a range of peroxide concentrations. Higher tensile modulus and higher delta torque (oscillating disk rheometer data) were observed when curing with DBPIB over the entire concentration range. This indicates that DBPIB produces a higher crosslink density than DBPH at equivalent molar concentrations. A series of calculations were performed, using computational chemistry techniques, to gain insight into the reason for the observed difference in crosslinking efficiency between DBPIB and DBPH. These calculations show that DBPIB is not more efficient than DBPH in abstracting hydrogen from the methyl groups of sihcone elastomers. The predominant cause for the difference in efficiency is related to the stability of the DBPH free radical. This radical is formed from hydrogen abstraction at the central ethylene moiety of DBPH by neighboring peroxy fragments. Therefore, DBPH is more susceptible to hydrogen abstraction, which consumes radicals in nonproductive (noncrosslinking) pathways This difference in peroxide efficiency may apply to other polymers.

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