73-series Halthanes

Soft Segment Behavior. The soft segments of all the 73-series Halthanes consist mainly of a low molecular weight... 

MDI. Three low temperature transition maxima are found in the loss modulus (G") of the 73-series Halthanes (see Fig. 1). Two low temperature secondary relaxations below the glass transition of the soft segment are arbitrarily labeled Tp (-100°C) and Ty (-155°C). These relaxations are probably associated with molecular motions in the urethane (9) and polyether (10) components of the soft segment, respectively. The glass transition of the soft segment occurs at about -50°C and is responsible for the drop in the storage modulus G1 by two orders of magnitude. 

Figure 3. Differential scanning calorimeter traces of 73-series Halthanes show the soft and hard segment glass transitions and a melting endotherm. The temperatures of these transitions are consistent with the dynamic mechanical measurements.

Figure 5. Thermogravimetric analysis cruves of 73-series Halthanes show bimodal pyrolysis behavior starting at about 250°C. The rate of change in weight of the adhesive with time, dW/dt is plotted against temperature for a programmed heating rate...

Figure 6. The low-temperature storage moduli of 87- and 88-series Halthanes show a smaller relaxation strength than 73-series Halthanes because of the aromatic...

Figure 7. Differential scanning calorimeter traces of 87- and 88-series Halthanes show the soft segment glass transition and a curing endotherm at about —70° and 100°C, respectively.

This section is subdivided into two parts based on the two types of LLNL Halthane adhesives. The basic distinction between these adhesives is the modulus of the rubbery plateau. Halthane 73-series adhesives are tough, rubbery polyurethanes with a modulus of about 10 Pa at room temperature. On the other hand, Halthanes 87-1, 87-2, and 88-2 are stiff, almost glassy adhesives with a modulus of about 10 Pa at room temperature. 

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.

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