Hard segments adhesive

Table II. Prepolymer and curing agent forumuations for polyurea hard segment adhesives.

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

Figure 4. The large broad "maximum" in the loss tangent isotherm of 73-19 polyurethane adhesive at 40 C is attributed to hard segment development. This peak is not observed above 50 C because the hard segment glass transition temperature has been exceeded.

One further phenomena observed In all 73 systems Is the decrease In opacity on curing at elevated temperatures. Above about 60 C the poly(tetramethylene glycol) and excess Isocyanate become miscible. This miscibility may be assisted by the fact that the MDI-BDO hard segments are above their glass transition temperature. To an extent which has not been quantified as yet, llquld-llquld phase separation of MDI and MDI terminated polyol In the prepolymer at low temperatures preslsts Into the final adhesive. The dynamic mechanical behavior of transparent or opaque adhesive, l.e., cured at 60-100 C compared to room temperature are virtually Identical. Similar Immlsclblllty has been observed In other prepolymers (20). This does not appear to adversely affect the adhesive properties of these Halthanes. 

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 4. The high-temperature shear storage and loss moduli of Halthane 73-14 and 73-19 adhesives are controlled by the presence or absence of the cross-linking agent quadrol in the hard segments. In the linear urethane (73-14), viscous flow follows the melting of the hard segments, whereas in the cross-linked urethane (73-19), the modulus drops only when the polymer begins to degrade.

As in the case of 73-series adhesives, degradation of 87 and 88-series polyurethanes was bimodal (Fig. 9). These adhesives were somewhat more stable than the 73-series (compare Fig. 6 with Fig. 9). Volatilization began at about 250°C. Assuming that the hard segments degrade first, this implies that the polyurea hard segments of 87- and 88-series polymers have better thermal stability than the polyurethane hard segments of the 73-series adhesives. 

There are several ways of changing the properties of segmented polyurethane copolymers. In this study, the chemical structure of the hard segments was changed to adjust the modulus of the adhesive. Further fine tuning is possible by varying the concentration of the different blocks. For example, the 87-series adhesives have a lower modulus than the 88-series adhesives because their hard segment concentration is lower. Studies of concentration effects (16) have shown that much wider variation of modulus is possible tTian was achieved here. 

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