Post curing literally means “after cure”. By definition, this means that there has been an “initial” cure. During preliminary processing, to fix the rubber into its final shape and form, the uncured compound is subjected to high temperature, this causes the chemistry within to react creating the solid rubber as Silicone Engineering recognise it – the reaction is irreversible. Although the compound has now been transformed into a solid piece of rubber, it is not yet in a stable finished state. Silicone Engineering need to finish the process by post curing the rubber.
Why give silicone a post cure?
After the initial cure of the silicone, by-products (volatiles) are left within as a product of the chemical reactions that have taken place. In order to drive off these by-products, Silicone Engineering need to place the silicone in a static oven and post cure at high temperature for a number of hours. The time and temperature can vary but normal is 4 hours at 200°C.
What benefits does post curing have on the silicone?
Several – all arising from the fact that the post cure ensures that anything in the silicone that could react as a result of being at a high temperature, has already reacted.
– The product is rendered stable in terms of reducing changes that take place in the elastomers physical properties due to natural ageing. From the moment of chemical cross linking (sometimes referred to as vulcanisation) the act of post curing does in effect prematurely age the material! At first this sounds bad but by doing this the “rate” of physical changes that take place during service, will be reduced as a result of post curing.
Fact! – The physical properties of any rubber will drop off as the rubber ages either by natural causes or, induced by any number or combination of influences, chemical, heat, pressure, environment… Studies show that the greatest drop off in properties takes place in the early stages thereafter; the reduction begins to level out. By post curing, Silicone Engineering take the silicone past the initial drop off stage so rendering the rubber stable. A good example is in the elongation to break characteristic. After initial curing, the elongation of a standard GP grade would be ~ 410%. After post curing for 4 hours @ 200°C the elongation has reduced to ~ 370% a big drop however, this element will not now reduce so rapidly. This is very important in the aerospace industry where apart from the silicones mechanical abilities, the designer is concerned with changes that will take place over time, by post curing, Silicone Engineering can ensure that the “rate” of property change drop off over time will be reduced. (See graph at end of this section). The biggest fall off in mechanical properties of the silicone always happens in the first couple of hours at a service temperature. The post cure takes the silicone to the point where the rate of fall off has reduced to the minimum practical – enabling long life with reasonably constant performance.
– By post curing, Silicone Engineering drive off the remaining “volatiles” this ensures that the silicone is fit for use in contact with foodstuffs.
– By ensuring that anything that could become volatile at service temperature has been removed from the silicone during the post cure, there will be nothing in the silicone in the service that could escape into the operational environment, preventing potential problems resulting from silicone cross-contamination, fogging, etc.
– Post curing also makes the product dimensionally stable.
How long does it take to cure silicone?
The time to cure silicone depends on a number of factors however, it mainly depends on:
– The concentration of chemical cross linking agent
– The curing temperature
– Curing method used
The curing process is essentially like any other chemical reaction in that an increase in chemical concentration or, an increase in temperature will increase the reaction time. Standards have been set over many years and a rehometer trace (below) will show that after around 60 seconds between the rheometer platens, the silicone has reached ~ 90% full cure.