Polyurethane elastomers, dynamic mechanical

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

Table III. Formulations and Dynamic Mechanical Spectroscopy Results for Polyurethane Elastomers...

Our own work was motivated by the hope that more detailed information on the postcure process of polyurethane elastomers could be obtained by combining the quantitative measurements like the enhancement in the mechanical properties in time with various dynamic processes which take place concomitantly, similarly as performed by Toffey et al in the case of cellulose crosslinking by urethane groups when the dynamic mechanical thermal analysis (DMTA) was used [341],... 

The present book is organized into 6 chapters. Chapter 1 describes general aspects on the chemistry of polyurethane elastomers their origins and development, the principles and synthesis mechanisms, as well as general considerations on the main chemical parameters that define such materials, i. e. diisocyanate, macrodiol and chain extender. Selected considerations regarding the reactivity of diisocyanates, the hydrogen bonding and its dynamic and quantum aspects are also discussed in this chapter. 

The way in which the chemical structure of the various chain extenders of Table 3.16 influences the thermal stability of polyurethane elastomers based on the molecule Capa 225/CHDI/chain extender in the molar ratio 1 2 1, respectively, is given in the following figures and tables. For example, Fig. 3.7 shows the dynamic mechanical thermal properties of a series of the polyurethane elastomers in which the variable is the chain extender. The temperature at which the value of the storage modulus (log E ) changes significantly is considered to indicate the limit of thermal stability of the polyurethane elastomers. 

Fig. 5.11. Dynamic mechanical thermal analysis (DMTA) properties of PPDI and CHDI polyurethane elastomers.

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. 

Rosthauser J W, Haider K W, Steinlein C and Eisenbach C D (1997) Mechanical and Dynamic Mechanical Properties of Polyurethane and Polyurethane/Polyurea Elastomers Based on 4,4 -Diisocyanatodicyclohexyl Methane, J Appl Polym Sci 64 957-970. 

So one can maximise the benefit from ultra-low monol PPG polyols in urethane systems such as MDI/BDO cured elastomers by understanding their fundamental differences in comparison to other high-performance polyols like PTMEG. A detailed look at the effect of the polyol MWD on the mechanical and dynamic properties of polyurethanes follows. 

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