3 December 2013

When is a silicone fully cured?

In this post, I am writing about two-component silicone, but the principle of the curing time also applies to one-component silicone. However, the course of the outbreak usually looks a little different for them.

An RTV silicone can be used for gluing and sealing, instillation, varnishing, to make details from or to make molds with. And so on. Common to all these areas of use is that during production you would like to know when, i.e. how long time after the silicone has been dispensed, as it has hardened sufficiently so that it can be loaded and the parts can be moved on or delivered.

As usual when it comes to chemicals, there is no crystal clear answer to the question, because the question can actually be asked in several different ways.

There is a difference if you mean, for example:

  1. When the silicone has solidified/dried enough to be able to move it
  2. When it has solidified enough to load it.
  3. When you can deliver the details to the customer
  4. When all reactions have been completed and final strength values etc. have been reached.

The correct answer to the question WHEN a silicone has cured is: when it has reached its final surface hardness. That is, a material that should give 50 shore A is fully hardened when a hardness measurement gives exactly that result. The time to get there varies, of course, from silicone to silicone, depending on which curing system it has.

The graph below (Figure 1) shows two examples of 2-component RTV for casting: 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 of 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 heart rate, 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. In contrast, TSE3663 continues to increase in hardness further into the third day. The conclusion is that the TSE3664K has a curing time of 36 hours, i.e. 1.5 days.

TSE3663 would need to be measured a bit more to be sure, although the curve flattens out at 72 hours.

Coming back to questions 1-4 above, using the graphs one could answer them as follows for the 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 how big the load is and what type of load it is. For an injection molding 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 the TSE3664K gets there after just 1.5 hours.

3) This of course depends on the customer requirements, but a benchmark is that 90 % of the final hardness according to the graph is achieved after about 8 hours. In most cases, the details with the silicone can be delivered in that position. The rest hardens on its way to the customer.

4) According to the graph in Picture 2, no changes in surface hardness take place after 36 hours, i.e. then the material is fully hardened.

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