Cure mechanism rates

Cure Rate as Measured by Strength Development of Prototype Joints. Cure rate is an important factor when the expense of jigs and fixturing equipment is high or fast production rates are critical. It is also used as a quality control test to determine if the curing mechanism within the adhesive has changed from lot to lot or if it may have been spoiled by storage, moisture contamination, etc. 

Curing mechanism Chemical reaction with curing agent, leading to uniform cure rate is influenced by temperature and humidity Chemical reaction with moisture in air forms a skin then cures slowly inward... 

Cure Rate Studies. The literature on curing of epoxies indicated two ways to quantitatively assess the difference in cure rate of various curing agents. Fava has reported on cure mechanism studies using differential scanning calorimetry and Dannenberg of Shell has reported on epoxy-cure mechanism studies by near infrared spectroscopy3 (NIR). 

Assuming co-reaction, the cure reaction of a mixture of bis(4-maleimido phenyl) methane and BACY was followed by FTIR [221]. The reaction kinetics, studied by DSC, suggested dependency of cure mechanism on blend composition. The apparent activation energy computed by the Prime method increased with BMI content. The rate maximum at a fractional conversion range of 0.32-0.33 indicated an autocatalytic nature of the reaction. The different pattern of activation energy with fractional conversion for two different blend compositions indicated non-identical cure mechanisms for the two compositions. The cyclot-rimerization of BACY occurred during the cure of a 1 2 molar ratio of BMI and BACY. Since activation parameters derived from DSC method are generally not consistent, and since the cyanate cure can be catalyzed by impurities present in BMI, which was not taken into consideration, the authors conclusions on the cure mechanism based on DSC kinetics can be erroneous. 

In general, the presence of rubber (VTBN) in these acrylate mixtures affects the cure rate due to increases in viscosity, however, no change in curing mechanism is expected. 

Possible trajectories in the metastable region of the conversion vs composition phase diagram a and b represent the compositions of the dispersed phase corresponding to trajectories a and b, respectively (K = phase s ration rate/cure reaction rate. Trajectories a and b are associated to an NG mechanism while trajectory c leads to SD)... 

The cure rates by the triarylsulfonium salts are also influenced by the size of the anions. Larger anions give faster cures. The rates are also influenced by the temperatures. However, they reach an optimum. The mechanisms of photoinitiations by triaiylselenonium salts are believed to be generally similar to those by triarylsulfonium and diaryliodonium salts. 

A highly crosslinked epoxy resin was modified by reactive blending with bisphenol Apolycarbonate. The bisphenol A polycarbonate was dissolved at high temperature in the uncured epoxy resin before the curing process. The physical and chemical interactions between the two components were studied by IR. Isothermal measurements showed that the presence of polycarbonate did not affect the overall curing mechanism but decreased both the initial reaction rate and the final conversion of reactants (278). 

Aerobic acrylics will not cure by themselves due to the absence of air. They require the use of pre-applied activators to initiate the cure mechanism. This property is distinct from anaerobic adhesives which are intrinsically single-component products. Even when accelerators or primers are used to increase their cure rates to meet the demands of rapid assembly techniques, anaerobic adhesives require the absence of air as a necessary condition of cure (1). ... 

The wide choice of chemical functional groups that can be attached to silicone polymers has opened up routes to several cross-linking mechanisms. As a consequence, networks can be varied in a wide range of architecmres. Cure conversion rates can proceed from a fraction of a second to hours, and in a wide range of temperatures. Silicone products respond to very demanding requirements that are dictated by apphcations that organic adhesives cannot respond to. 

The influence of a commercially available additive on BMI resins was investigated and a reduction of the cure shrinkage rate was observed without changes in the cure mechanism. The quality of a laminate of BMI with carbon fiber was improved with the additive (Zhang et al. 2007). 

IR and Raman spectroscopies are both tools that may be useful for monitoring polymerisation rates and cure mechanisms, particularly sequential time-lapse measurements of functional group concentrations at predefined intervals when utilising multichannel Raman or rapid-scan FTIR or FT-Raman detection. 

Melamine resins are not normally used with cathodic systems because the curing mechanism is acid catalysed and under the alkaline conditions prevailing within the film during cure (because of the presence of the - now un-neutralised - amine groups) the curing rate is severely retarded. It is, in theory, still possible to cure cathodic systems with melamine resins but this involves raising the cure temperature by about 30°C, which is usually unacceptable to the customer. 

Using an addition cure mechanism, LSRs exhibit rapid cure once the mass of rubber reaches 110°C. In comparison with the fast peroxide cures (dichlorobenzoyl peroxide), completion of cure occurs in much the same time, but with a longer induction period and faster rate. This non-peroxide system allows LSR to be injection moulded at high temperatures (200-250 °C) with cure times of a few seconds for small parts. Scorch rarely presents a problem as the mould is quickly filled by the flowable liquid, even with very low injection pressures. Typical cure times for LSR as measured on a 12 g part in a Monsanto rheometer are shown in Table 3. 

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