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The evaluation of internal cracking in polyurethane adhesives
Author(s) -
Swanson Frank
Publication year - 1977
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760170211
Subject(s) - materials science , prepolymer , curing (chemistry) , polyurethane , adhesive , cracking , composite material , layer (electronics)
In an evaluation of low modulus polyurethanes as adhesives, systems based on 1, 4 butanediol‐extended methylene diisocyanate polymer displayed a tendency to crack during cure. Cracking occurred when these systems were gelled at low temperatures in thin layers where adhesion places constraints on dimensional change. A test technique was developed that evaluates the tendency for polyurethane adhesives to develop internal cracks during cure. The test consists of casting the material between the walls of two eccentric beakers and observing the behavior of the system as cure proceeds. The 1, 4 butanediol‐extended methylene diisocyanate system was studied by systematically evaluating the influence of processing variables on cracking. The variables studied included curing agent to prepolymer stoichiometry, moisture content of the curing agent, effect of pre‐evacuating the prepolymer, the inclusion of additives such as silicone antifoam agent and silane coupling agents, and time/temperature cure conditions. The following factors were identified as being primarily responsible for cracking in these polyurethane systems: (1) the stoichiometry of the curing agent to the prepolymer, which must be held within a narrow range; (2) the water content of the curing agent which must be minimized; and (3) additives, particularly silane coupling agents, which enhance adhesion of the urethane to the container walls and also prematurely terminate chain extension.