In this post I am writing about two-component silicones, but the principle of cure time also applies to one-component silicones. However, the curing process usually looks a bit different for them.
An RTV silicone can be used for bonding and sealing, moulding, varnishing, to make parts from or to make moulds with. And so on. What all these uses have in common is that during production you want to know when, i.e. how long time after dispensing the silicone, which has solidified sufficiently so that it can be loaded and the parts can be moved or delivered.
As usual with chemicals, there is no clear-cut answer to this question, as it can actually be asked in several different ways.
There is a difference if you mean for example:
- When the silicone has solidified/dried enough to be moved
- When it has solidified enough to be loaded.
- When you can deliver the details to the customer
- When all reactions have been finalised and final strength values etc. have been achieved.
The correct answer to the question WHEN a silicone has cured is: when it has reached its final surface hardness. I.e. a material that should give 50 shore A is fully cured when a hardness measurement gives that result. The time to reach this point varies from silicone to silicone, depending on which curing system it has.
The graph below (Figure 1) shows two examples of 2-component RTV for moulding: TSE3664K and TSE3663. Both silicones cure at room temperature, but TSE3664K has a much higher final hardness than TSE3663.
From the graph we can see that TSE3664K solidifies significantly faster - already after 3 hours from mixing it has reached 45 shore A. TSE3663 does not start to build any surface hardness until after 2-3 hours.
The next graph shows the same curing process, but the scale goes from 0 to 72 hours.
Here you can see some differences. TSE3664K apparently no longer increases in surface hardness after about 36 hours. On the other hand, TSE3663 continues to increase in hardness further into the third day. The conclusion is that TSE3664K has a curing time of 36 hours, i.e. 1.5 days.
TSE3663 would need to be measured a little more to be sure, although the curve flattens out at 72 hours.
Going back to questions 1-4 above, the graphs could be used to answer them as follows for TSE3664K:
1) When a silicone has surface hardness, it can carry a load. Then it can definitely be moved on. This occurs about 1 hour after mixing.
2) Depends on the magnitude and type of load. For a moulding material like TSE3664K, vibration and shaking are the most common loads, sometimes also mechanical pressure. A silicone with a hardness of 30 shore A tolerates pressure reasonably well - and TSE3664K reaches this point after just 1.5 hours.
3) This of course depends on customer requirements, but a benchmark is that 90 % of the final hardness according to the graph is reached after about 8 hours. In most cases, the parts with the silicone can be delivered at that point. The rest cures on the way to the customer.
4) According to the graph in Figure 2, there are no more changes in surface hardness after 36 hours, i.e. the material is fully cured.
