When it comes to picking the right rubber for an application, you must take into consideration a number of material requirements in order to come to the correct decision on the material to use.

There are a vast range of rubbers in today’s market available to engineers, designers and buyers. So much so, that when looking through the list, it is quite easy to get lost in a maze of grades, forms, specifications, thicknesses and hardness’s.

At Silicone Engineering, we understand that with over 2000 formulations to our name, that maze can quickly start to become a headache for customers not familiar with silicone. It is our aim to work closely with you, the customer, to identify the application you intend on using our silicone products for and by using our expertise and knowledge we can then guide you to the correct material and grade.

By understanding your material requirements we can advise on certain grades and/or specifications even if silicone isn’t the right material to use, which in some environments it isn’t.

With this in mind we’ve listed a simple set of questions to think about regarding the environment and performance that the material will be exposed to, it allows us to give the best advice on material choice.

Environment – UV / Ozone / General Weathering

 Will the rubber be exposed to outdoor environments where weathering could affect the rubber’s performance?

If so, you need a rubber that performs well when exposed to these conditions. Cracking, fading and brittleness can all occur if the rubber is not suited for outdoor conditions.

Silicone rubber is one of the best rubbers to use outdoors due to its excellent resistance to environmental conditions.

 

Temperature range

Will the rubber be exposed to extreme temperatures?

What temperatures (high and low) and for how long?

An environment where temperatures fluctuate will need a rubber that can cope with altering heat.

 

 

Silicone Engineering’s materials have extreme temperature resistance from -60°C to 230°C and up to 250°C intermittently.

Rubber Application

• What is the material being used for, where will it end up?

• Is the silicone being used for sealing/gasketing, dampening vibration, shock absorption…?

It is important for us to gain as much detail in this question as possible in order for us as silicone experts to advise the most suitable silicone grade or indeed another rubber to use if we feel silicone is not suitable.

Chemical Resistance

• Will the rubber come into contact with any chemicals or fluids that could cause corrosion or damage?

• If so, how long will it be in contact with the chemical for?

Knowing which chemicals affect particular rubbers is vital and at Silicone Engineering we know that silicone performs well with the majority of chemicals however there are some that silicone should not come into contact with.

 

 

Flame Resistance

• Does the rubber need industry specific flame ratings such as UL-94V0, UL-HB, FAR etc

When discussing specifications and approvals it is important to know if the rubber needs to have full approval or will internal testing be suffice to prove the material can meet these ratings?

Outgassing

• Will the material need to be free from outgassing?

This is very important when looking into hot applications as outgassing can cause fogging of lenses and damage other critical components. This is critical in applications within the Rail, Aerospace and Automotive sectors due to the safety of passengers.

Colour

• Does the material need to be a specific colour or will industry standard colours be OK?

We have the capability to exact colour match if this is required?

 

Cost vs Material Lifespan

• Why should I consider silicone when there are cheaper rubbers on the market?

Some rubbers cost a lot less than silicone, however when assessing performance and lifespan there is a clear difference. Most rubbers need replacing a lot sooner than silicone mainly due to their poorer lifespan therefore making silicone the cheaper rubber over a longer period. When comparing the cost of different rubbers make sure it is comparable in terms of performance and not just cost – silicone becomes the cheaper material over a longer period as it out lasts many other rubbers in application. cost vs lifespan!

Check out our products to find the right silicone product for your business.

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.