Halthane adhesives

Hamnon, H.G. Althouse, L.P. Hoffman, D.M. "Development of Halthane Adhesives for Phase 3 Weapons Summary Report" UCRL- 52943, December, 1980. 

Compositions of Halthane adhesives are given in Tables I and II. Information on polymerization procedures is described elsewhere (ji). 

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 1. The dynamic viscosity (G"/w in pascal seconds) of Halthane 73-18 segmented polyurethane adhesive increases with time until the chemorheology of cure is complete. Cure curves for four different temperatures show a positive temperature coefficient which should be proportional to the overall reaction rate.

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. 

Figure 5. The change in dynamic viscosity of Halthane 87-1 segmented poly(urea-urethane) adhesive with time increases dramatically with temperature. Since the initial viscosity decreases with temperature, the plot cross through each other.

Figure 6. Because of the rapid increase in dynamic viscosity of Halthane 88-2 poly(urea-urethane) adhesive caused by a higher concentration of HMDI- aromatic diamine chain extender, the initial viscosities are difficult to determine accurately.

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. 

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 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.

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...

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