Phase inversion rubber content

The reactor operates at a steady state with a polymer solids level above that at which phase inversion occurs and up to 70% polymer solids. Operation at such a polymer solids content ensures that upon addition the rubber immediately forms small particles containing a monomer component, dispersed in the partially polymerized reaction mixture. 

Spontaneous phase inversion (no shear) has been described by Keskkula [12]. This was demonstrated during the quiescent polymerization of styrene-polybutadiene mixtures containing less than 3 wt% polybutadiene. For industrially important systems (higher rubber content), a minimum amount of shear is required [13]. If no adequate agitation is applied, the system will solidify in the emulsion state before the inversion point. The final product will then consist of a continuous phase of a crosslinked polybutadiene network with dispersed SAN particles. Such a material will not have the typical properties of ABS. 

At a higher rubber content, small (several micrometers) and large (several hundred micrometers) DGEBA particles are dispersed in the continuous rubbery triglyceride phase. The large particles probably contain occlusions of much smaller rubbery particles. This is the end of the phase inversion. 

A further increase in the rubber content leads to a complete phase inversion in all four formulations, with the formation of two-phase thermosets... 

In all four formulations, however, there is also a pronounced dependence of the phase-separation process on the nature of the diamine and the epoxi-dized triglyceride rubber used for their preparation. As mentioned earlier, the DGEBA/DDS matrix is more polar than the DGEBA/DDM matrix because DDS is more polar than the DDM diamine. ESR is more polar than VR because the epoxy functionality of the initial ESO is twice as high as the epoxy functionality of VO. Both the particle-size distribution and the rubber content at which phase inversion occurs are determined by the miscibility of the two phases of the four types of formulations discussed. 

Phase Inversion. The rubber content required for phase inversion increases gradually for the formulations with increasing miscibility of their two phases (Table VI). In this regard, the four types of formulations follow exactly the same order as discussed previously for particle-size distribution. This relationship between rubber content, miscibility, and phase inversion is probably due to a different DGEBA partition in the two phases of the formulations. A tentative mechanism for the phase-inversion process is proposed in this chapter. 

Table VI. Percentage of Rubber Content at Which Phase Inversion Occurs in Different Rubber-Modified DGEBA Thermosets...

ESR is more polar than VR. DGEBA partition in soybean particles is therefore lower than in vemonia particles, and the apparent volume fraction of soybean rubbery particles is smaller than the vemonia volume in the corresponding formulations with the same rubber content. As a result, phase inversion of ESR formulations requires a much higher rubber content. Indeed, phase inversion in DGEBA/DDS/ESR formulations starts at a 30% rubber content. 

All formulations with a rubber content below that required for phase inversion form only small rubbery particles. The initial homogeneous formulations reach their saturation point at a later (advanced) curing stage, when most of the DGEBA monomer has been already consumed. The phase-separation process, therefore, proceeds practically without DGEBA partition in the rubbery phase. At a higher rubber content (above phase inversion), the phase-separation process follows a similar mechanism, but only small DGEBA particles are formed. 

Morphology. Phase inversion in polymer mixtures occurs when the volume fraction of the dispersed phase becomes equal to or exceeds 0.5 (14). The driving force is to minimize the interfacial energy of the system. This is not the case here because the volume fraction of the rubber-rich phase at phase inversion is about 0.85. After inversion, the fraction of the continuous rubber-rich phase is only 0.28, and it increases to 0.63 at 12.5% rubber content. Initially, the components are fully soluble and compatible, but as the reactions proceed, the molecular weight of the products increases and phase separation results. The ability to separate and invert is dependent on the viscosity of the medium. The unsaturated polyester forms a gel at conversions as low as 2 to 5%, and both the ability to separate and to invert is impeded. Thus the morphology depends on the two competing effects of phase inversion and... 

The phase-within-a-phase-within-a-phase morphology, depicted in the right-hand portion of Figure 6.6 after phase inversion, looks much like that of high-impact polystyrene prepared by the graft technique (see Figure 2.3). In both, polystyrene forms the matrix, as well as the contents of the cellular structure within the rubber (castor oil) droplets. 

A study by A.K. Sen and G.S. Mukheijee (Sen and Mukheijee, 1993) reported the use of inverse gas chromatography to investigate the thermodynamic compatibility of blends of PVC and nitrile rubber (NBR) as a function of blend composition and acrylonitrile content of NBR. The values of the polymer-polymer thermodynamic interaction parameters and the solubility parameter of the polymers and then-blends were determined with the help of the measured retention data for various polar and nonpolar probes in the pure and mixed stationery phases of these polymers. The two polymers exhibited fair compatibility which increased with increasing content of acrylonitrile in NBR. 

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