Cure Reaction Mechanism

The important classes of polymerisation mechanisms are also used in thermosetting systems. The cure reaction mechanism will be one determining factor for the evolution of the system. Results for several polymerisation systems will be discussed in this chapter, and some background information about the reaction mechanisms is given in this section. 

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The mercaptan hardener is one of a very few hardeners which can cure the resin at low temperature even below the freezing point in a few minutes using basic catalysts such as amines. There are only very few studies of the curing reaction mechanism and the physical properties of the cured resin available 42 45). 

Starting from the cure reaction mechanism, a proper cure rate law, describing the evolution of the system from initial to final state, can be proposed. In the case of a mechanistic approach, in which the reaction model consists of a set of chemical reaction steps, a set of (stiff) coupled differential equations has to be solved to describe the evolution of the important reacting species of the system. In this case, effects of the composition of the fresh reaction mixture (such as a stoichiometric unbalance of resin and hardener, the concentration of accelerator, initiator or inhibitor) and the influence of additives (such as moisture and fibres in composites) can be studied. Because this set of equations may be rather complex and/or even partly unknown, various simplifications have to be made. 

Wang, M.H., Yu, Y.F., and Li, S.J. (2007) Polymerization-induced phase separation in polyethersulfone modified epoxy resin systems effect of curing reaction mechanism. Sci. China Ser. B. 50 (4), 554-561. 

Keywords Liquid crystalline thermosets, liquid crystalline epoxy resins, curing reactions, mechanical properties, thermal properties... 

Rapid kinetics or in-situ observation of the curing reaction mechanism using time-resolved FTIR spectroscopy [13] can provide valuable information about the volume collapse or hardening route in the complex ceramer network. The residual porosity V in the coating structure can be determined by FTIR analysis using the following equation [14] ... 

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM). 

Nonblack fillers such as the precipitated siHcas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between mbber and filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevents this competition thereby maintaining the desired cure characteristics. Neutral fillers such as calcium carbonate (whiting) and clays have Httie or no effect on the cure properties. 

Kinetics and mechanism of curing reactions using ylides as a new curing agent. 

This reasoning does not account for segments that are not properly attached to the network nor for additional crosslinks and any deviation from the outlined mechanism of the phenol curing reaction. Side reactions are not considered. 

The kinetics of resole cure reactions monitored via FTIR suggest that a diffusion mechanism dominates below 140°C. The cure above 140°C exhibits a homogeneous first-order reaction rate. The activation energy of the cure reaction was -"-49.6 kJ/mole.66... 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

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