Adhesives polyurethane, dynamic, mechanical

Dynamic Mechanical and Thermal Properties of Seven Polyurethane Adhesives... 

Seven polyurethane adhesives have been developed at Lawrence Livermore National Laboratory (LLNL). These adhesives, designated Halthanes were synthesized because of OSHA restrictions on the use of the curing agent methylene bis(2-chloroani1ine). Four of the Halthanes were made fromLLNL-developed 4,4 -methylene bis(phenylisocyanate) terminated prepolymers cured with a blend of polyols three were made from an LLNL-developed prepolymer terminated with Hylene W and cured with aromatic diamines. In this paper we report the dynamic mechanical and thermal behavior of these seven segmented polyurethanes. 

The dynamic mechanical relaxations in the high temperature region are very weak and the glass transition was indistinguishable from the melting point (Fig. 4). However, the mechanical properties of polyurethanes with chemically crosslinked hard segments were quite different from uncrosslinked polyurethanes. In the linear adhesives (73-14 and 73-15), the rubbery plateau ends at the melting point of the... 

Figure L The low-temperature dynamic mechanical spectrum of Halthane 73-14 is typical of the 73-series polyurethane adhesives. Two secondary relaxations, Tp and Ty, are shown as peaks in the loss modulus at —100° and —150°C. The soft segment glass transition, Tg(SS), occurs at about —50°C. The frequency of oscillation was held constant during the measurement at 0.1 Hz.

Figure 12. Viscoelastic properties determined by Dynamic Mechanical Analysis at different temperatures for two-component epoxy (black lines B, C, D, F, H, L, M, O and P) and polyurethane (gray lines R and S) adhesives.

It is well known that adhesives need to be low-modulus elastomers, so that the resin formulation has to be designed to give, after UV-curing, a soft material with a low glass transition temperature (Fg). Fig. 3 shows some typical elastic modulus E) and tan S profiles recorded by dynamic mechanical analysis for a UV-cured polyurethane-acrylate which is suitable for adhesives applications, based on its E and Tg values. It is quite soft, as shown by its Persoz hardness value of 50 s, on a scale that goes up to 400 s for glassy materials. 

Fig. 9. Dynamic mechanical spectra of polyurethane adhesives (logarithmic decrement).

The static moduli failure stresses and dynamic moduli of both filled and unfilled polyurethanes are measured over a range of equihbrium water contents and these results are compared with those obtained from dry controls. The degradation of mechanical properties in the presence of water, and the hydrolytic disruption of filler/matrix adhesion is assessed. A quantitative relationship between the water content and mechanical properties is established and the mechanics of the water/polymer/filler interaction are considered. 4 refs. 

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