Minimum cure temperature measurements

A network structure model has been developed from which a parameter that correlates well with physical measures of paint cure can be calculated. This model together with a kinetic model of crosslinking as a function of time and temperature has been used to evaluate the cure response of enamels in automotive assembly bake ovens. It is found that cure quality (as measured by the number and severity of under and overbakes) is good for a conventional low solids enamel. These results are in agreement with physical test results. Use of paints with narrower cure windows is predicted to result in numerous, severe under and over bakes. Optimization studies using SIMPLEX revealed that narrow cure window paints can be acceptably cured only if the bake time is increased or if the minimum heating rate on the car body is increased. 

The most common method of measurement is to run the compound in the mooney viscometer until the viscosity shows a 5 point rise above the minimum. The viscosity of the compounded rubber at the processing temperatures can also be obtained from the minimum of the curve. The values normally taken from the cure curve are ... 

To determine the wet bond strength coated panels were immersed in distilled water for 1500 h, removed and discs 25.4 mm in diameter stamped from them. The surfaces were wiped with a dry tissue and bonded between two cylindrical test pieces using a polyamide cured epoxide adhesive and immediately placed in a sealed container at 100% RH for the adhesive to cure. After 16 h the specimens were broken on an Instron Universal Test Machine with minimum delay. Recovered values were measured after the panels had dried out at room temperature and humidity for 7 days. Clearly, it is unlikely that the values reported represent the minimum bond strengths, as some drying out is almost inevitable, but the values are directly comparable. 

The literature is rich in examples of this upside-down minimum and the secondary maxima arising from electrode polarization40 43 . Figure 19 shows Lawless superposition of a temperature ramp, the viscosity, and the measured tan 8X for the cure of an Avco 5505 epoxy resin with parallel plates in the presence... 

The determination of set in tensile strain is much less commonly specified than that in compression, although in principle it is a particularly straightforward procedure a strip, dumbbell or ring lest piece of known reference length is. stretched to a given extension, exposed in this condition to a combination of temperature and time, and then released for a specified period before measurement of the reference length. One clear advantage for the test, as say a measure of the state of cure, is that test pieces can be cut from even the thinnest of latex films, whereas the minimum ply thickness of a laminated compression set test piece is 2 mm. 

Model-free kinetics software employs numerical integration methods to measure activation energy versus conversion from cure exotherms at three or more heating rates, or from isothermal data at three or more temperatures. In both cases a minimum of four runs is recommended. Predictions like conversion-time plots and calculated DSC curves are made using Eq. (3.31). An advanced version of MFK software allows analysis of data from arbitrary heating programs, such as combined ramp and isothermal. A drawback of the commercial software is that a discrete mathematical relationship is not produced that can be exported and incorporated into cure models. 

Current industrial practice in the rubber industry is for these instruments to measure a property, usually stiffness, changing progressively with time at a set temperature. The temperature may be set so as to equate to typical cure conditions, or may be of an arbitrary nature if used for quality-control testing. The sample of unvulcanised rubber used will have a hot stiffness or viscosity before any crosslinking occurs. This will increase from a minimum value to a... 

The adhesive film, unsupported, is cured between shimmed Teflon sheets in the normal cure cycle to yield films. 015 to. 025 inches thick. Dogbone tensile specimens are cut from the film, measured, and conditioned at the desired temperature-humidity for a minimum of 72 hours. This duration was found to be adequate for the films to attain equilibrium water content, and samples showed no change upon further exposure. 

The SWNT ropes used were the same as those used for bundle strength measurements discussed earlier. The matrix material used was Epicote 1006 epoxy resin, a room temperature curing system. 32 dog-bone SWNT/epoxy specimens with dimensions of 40 mm x 3.5 mm x 0.4-0.6mm were obtained. The gauge length of die specimens was about 15 mm, and the length of SWNT ropes embedded in the epoxy was about 20 mm. Details of the fabrication method can be foimd elsewhere. The volume fraction of SWNT ropes in the composite was controlled widiin die range of 0.1-0.9%. Specimens were cyclically tested by an Instron 8800 Microforce Tester under tension-tension at 5 Hz, using a sinusoidal wave function at R ratio (ratio of minimum to maximum cyclic stress) of 0.1. 

T5 The scorch time at lower temperature is of importance too. This can be obtained by using a Mooney Viscometer at lower temperature. A Mooney Viscometer is also used to measure the viscosity of the compounds (important for dictating injection-moulding behaviour). The viscometer is also used to assess the tendency to scorch, and sometimes the rate of cure of a compound. A useful estimate of scorch behaviour is represented by T5, the time taken from the beginning of the warm-up period to that at which the Mooney value rises five units above the minimum value. 

---