Polyurethane adhesives temperature effects

Inspection of Figure 5 shows a very broad glass-to-rubber transition range which extends from below -100°C to above 0°C for the polyurethane adhesive. The relaxation modulus E(t) - 400 Kg/cm which occurs at the rubbery inflection temperature - 40°C - 313 K describes an effective molecular weight M as defined by kinetic theory of rubber elasticity ... 

Melt glues are amorphous and/or partially crystalline polymers above their glass transition temperatures or melt temperatures. Their viscosities should not be too high so that they can wet surfaces well, and not too low so that they do not flow away from where they are applied. Best results are obtained for viscosities of about 10-1 000 Pa s. Poly(ethylenes), poly(ethylene-co-vinyl acetate), poly(vinyl butyrals), versamides, polyamides, aromatic copolyesters, polyurethanes, bitumens, and asphalts, for example, are used as melt glues. The adhesive effect is produced by solidification of the melt glue. 

Polyurethanes similar to those utilized as millable elastomers (Section 14.6.2) may be used as adhesives provided the molecular weight is low enough to permit solubility in appropriate organic solvents. Isocyanates are generally the preferred cross-linking agents for these adhesives since they are effective at low temperatures. Such adhesives give flexible bonds and are used in the shoe industry. 

Experiments and results. The viscoelastic material chosen was an optically smooth polyurethane, reconunended for dynamic studies in photoelasticity SM4 Vishay, with E = 3.6 MPa). The surfaces were wiped with an alcohol-soaked cloth, dried with warm air and left, sheltered from dust, for 30 min for the equilibrium with room temperature to be reached. Then, two strips of thickness /i=3.175 mm with various lengths L and widths b varying in the range 5-20 cm and S-20 mm, respectively, w e gently superimposed, and they adhered under only molecular attraction forces, without additional adhesive. In order to avoid the dwell time effect (/9), strips were coupled during the same contact duration 30 min, for all experiments. Moreover, temperature (23°C) and humidity (84%) were kept constant. In these conditions, the reproducibility is better than 3%. 

Adhesives are usually formulated with additives to protect or to counteract the effects of the degrading agent to which the polymer is most susceptible. In the case of materials normally used at atmospheric temperatures, if susceptible to oxidation, an antioxidant is added. The degradation of polyurethanes by hydrolysis can be mitigated by the use... 

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