Toughened plastics phase

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]. 

Assessment of Particle Size Distribution and Spatial Dispersion of Rubbery Phase in a Toughened Plastic... 

A statistical framework has been established for describing the spatial dispersion of second phase particles in a continuous matrix. Based on this scheme a computerized image analysis method has been developed for characterizing the morphology of toughened plastics. 

Bimodal-Distribution Models of the Discrete Phase in Toughened Plastics... 

The program can be used in several ways to model the geometry of real materials. In the modeling of a blend of two rubber-toughened plastic components, the discrete-phase volume and rubber particle-size distribution of each blend component would have to be known. Files containing the actual diameters of discrete-phase spheres would also be required. Finally, the rubber-phase volume in each blend component would have to be multiplied by the components weight fraction in the blend. This assumes that both components have the same continuous phase and, therefore, no volume change when blended. 

In summary, a method has been developed for the placement of bimodal sphere distributions within three-dimensional boundaries. The bimodal distribution is created from the combination of two sphere populations, where each population represents a distinct distribution. The efficient packing of a bimodal distribution of spheres can produce a high volume of the discrete phase in a toughened plastic and a corresponding small interparticle distance. However, combining two materials containing equal discrete-phase volumes of monosized spheres to make a bimodal system does not decrease interparticle... 

It is well known that the main mechanisms of inelastic deformation are shear yielding and multiple crazing in the rigid matrix phase, as well as cavitation in the soft dispersed phase in rubber-toughened plastics and multiphase polymers [42]. Eor many years, these mechanisms have been studied using microscopy techniques. 

In the last two decades, numerous experimental and theoretical studies dealing with reaction-induced phase separation in multiphase polymer systems (mostly porous matrices, toughened plastics, melt processable thermoplastics [143], molecular composites, polymer dispersed liquid crystals, etc.) have been reported. A newcomer in this field should get acquainted with hundreds (possibly thousands) of papers and patents. The intention of this review was to provide a qualitative basis (quantitative occasionally) to rationalize the various factors that must be taken into account to obtain desired morphologies. 

Both the modulus-temperature relationships presented in the preceding sections and the tensile data presented above are strikingly similar to those demonstrated for other rubber-plastic combinations, such as the thermoplastic elastomers (see Chapter 4 and the model system presented in Section 10.13) and the impact-resistant plastics (Chapter 3). The IPN s constitute another example of the simple requirement of needing only a hard or plastic phase sufficiently finely dispersed in an elastomer to yield significant reinforcement. Direct covalent chemical bonds between the phases are few in number in both the model system (Section 10.13) and present IPN materials. Also, as indicated in Chapter 10, finely divided carbon black and silicas greatly toughen elastomers, sometimes without the development of many covalent bonds between the polymer and the filler. 

Bucknall showed that phase domain sizes of the order of 0.5 /lc yield a maximum in impact strength in rubber toughened plastics.It is interesting to speculate whether the phase domain size and organization criteria required for toughening will be as important for SINs as for graft copolymers. 

By blending these polymers, taking care to match melt viscosities and volnme fractions (see eq. 2 below), dual-phase continuity can be achieved (see Fig. 16) (59). In Figure 16a, droplets of material are dispersed in a continuous matrix, a morphology observed in rubber-toughened plastics. Figure 16b illustrates dispersed fibers, observed in liquid crystal polymers and some thermoplastic elastomers. If the fibers or cylinders are infinitely long, they exhibit one-dimensional phase continuity. 

A number of commercial IPN materials and their applications are summarized in Table 3. The value of IPNs to society, of course, lies in their usefulness. IPNs can and are being made into rabber-toughened plastics and a special value seems to lie in their capability of forming tough but flexible materials. IPNs often have another unusual property that of forming co-continuous, very finely divided phases. These tough but flexible materials have suggested biomedical applications. 

HaU R A and Burnstein I (1996) Bimodel-distribution models of the discrete phase in toughened plastics, in Toughened Plastics II Novel approaches in science and engineering, Advances in Chemistry Series 252 (Eds. Riew C K and Kinloch A J) American Chemical Society, Washington, DC, pp. 27-32. 

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