Curing, phase separation during

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittie. Thus, the utility of unmodified common resins as stmctural adhesives would be very limited. Eor highly cross-linked resin systems to be usehil stmctural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin dexibiUty, and thus increases the fracture resistance of the cured adhesive. Recendy, stmctural adhesives have been modified by elastomers which are soluble within the uncured stmctural adhesive, but then phase separate during the cure to form a two-phase system. The matrix properties are mosdy retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved. 

Figure 3.18 Schematic representation of free energy of mixing for the phase separation during cure (a) via nucleation and growth and (b) via spinodal decomposition...

Figure 3.19 Reaction paths undergoing phase separation during isothermal cure...

Modifier miscibility plays an important role in this preparation. On the one hand, modifiers must be miscible with the reactive system on the other hand, they must phase-separate during cure. Final morphologies are influenced by the phase-separation conditions. 

An interesting method of eliminating the undercure caused by vitrification when using microwave radiation is to modify the formulation, including the use of polar thermoplastic that phase-separates during cure (Chapter 8). The thermoplastic material can convert microwave energy into heat, which enables the thermosetting polymer to devitrify and reach full cure. 

The nature of the functional groups of the HBP is a significant factor for the control of viscosity, miscibility with the thermoset precursors, phase separation during cure, and particle-matrix adhesion. 

Liquid-crystalline polymers (LCP) can be used in thermoset systems as initially miscible modifiers that phase-separate during cure. 

Carboxy-terminated curative, such as CTBN, provides excellent toughening in part due to its miscibility in many epoxy resins. Phase separation during cure is required to obtain toughening, and generally the phase separation requires an elevated-temperature cure. However, by prereacting the CTBN with a portion of the epoxy to obtain an adduct, a room temperature curing toughened epoxy is possible. Adduction reduces the likelihood of early phase separation and maintains the solubility of the elastomer in the uncured resin system. 

In many systems, such as epoxy resins, the rubber toughener may be soluble in the other phase, such as epoxy resin, so the phase separation must be achieved during cure. This is an example of phase separation during reactive processing. 

The process of phase separation during cure arises from the change in the phase diagram as the cure reaction of the epoxy resin progresses. This is shown schematically in Figure 1.34 (Pascault et al, 2002) for a system with an upper critical solution temperature (UCST) in which the lower curve represents the system miscible at room temperature, with the fraction cpRo of elastomer corresponding to the initial composition of the rubber-epoxyresin system before any cure reaction has taken place. 

Thermosetting polymers, like phenolics, epoxies, unsaturated polyesters, etc, are frequently used in formulations containing a low or high-molar-mass rubber, a thermoplastic polymer, an oil, etc, in an amount of the order of 2-50 wt% with respect to the thermoset. This extra component, called the modifier, may initially be immiscible or may phase-separate during cure (reaction-induced phase separation). 

Levita et al. [53] incorporated 10 wt% of a rubber composed of core-shell particles (D = 0.2 pm) and an amine-terminated butadiene acrylonitrile copolymer (ATBN) in different proportions in a DGEBA-based epoxy resin cured with piperidine at 120°C for 12h. The core-shell particles were always immiscible in the epoxy resin while the ATBN phase separated during poly-... 

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