COMPRESSIONAL BEHAVIOR OF INKED OPEN-CELL FOAMS

The mechanical behavior of various classes of inked and non-inked (dry) open-cell foam rollers has been investigated from stress/strain measurements in compression. Hysteresis, creep, and recovery in dynamic loading were used to differentiate the rollers as to their utility in printing applications. Creep, epsilon(c)(t), and recovery, epsilon(r)(t), were determined in a compression mode from force measurements and strain decay, epsilon(t), i.e., DELTA-epsilon(c)(t) = epsilon(o) - epsilon(t) [epsilon(o) = epsilon(t = O)] or epsilon(c)(t) = 2-epsilon(o) - epsilon(t). The creep function, DELTA-epsilon(c)(t), represents the plastic strain, epsilon(pl)(t = t(h)), and is uniquely defined from the recovery function, DELTA-epsilon(c)(t) = epsilon(r)(t = t(h)), where t(h) is the hold or contact time. The recovery results for a polyurethane ester (PUE) and acrylonitrile/butadiene (AB) rollers (dry/ink), poly(vinylidene fluoride) film (PVF2) (air/vapor) and low-density polyethylene film (LDPE) (air/vapor) were found to fit a master curve of the form F(r)(theta) = EXP[ - K(r)(t(h)theta] = [epsilon(r)(t) - epsilon(infinity)(t(h))]/[epsilon(o)(t = O) -epsilon(infinity)(t(h))] at a reduced time of K(r)(t(h))theta{theta = t/t(h) and K(r)(t(h)) = k'r(t(h))t(h) = C(O)/(t(h))alpha-1 (where C(O) depends on the material's "dry" or "wet" state, alpha is a function of the type of material, and epsilon(infinity) is the permanent set). These empirical results are consistent with the observed decreases in print intensity during transfer to a paper substrate and weight changes of the roller, i.e., creep and recovery are important in the printing characteristics of a given roller material. Other factors of importance in the overall transfer and print quality, but of longer-term considerations, are diffusion processes within the polymer and the nature of the polymer (e.g., porosity, chemical constitution, surface and interfacial tensions).