What is casting rubber?

Casting rubber refers to a pourable material that displays the properties of rubber after curing. There are various synthetic materials with this property, such as certain polyurethanes and epoxies. However, when we talk about casting rubber, silicone rubber is usually meant.

Silicones as casting rubber are an excellent alternative to natural casting rubber or natural rubber. They retain their rubber-like properties at both low and high temperatures. In addition, silicones last much longer than traditional rubber and are more resistant to chemicals.

Silicones suitable as casting rubber are divided into two categories based on their chemical properties: condensation-curing silicones (also called tin-curing silicones) and addition-curing silicones (also called platinum-curing silicones).

What is the difference between addition and condensation silicones?

Detailed explanation of condensation and addition silicones

The chemical difference between condensation and addition silicones lies in the way polymerisation takes place.

Condensation silicones

• Condensation silicones form long chains during the curing process. Part of the material condenses (separates out), which results in a decrease of the total volume. As a result, condensation silicones have more shrinkage than addition silicones.
• They release tin salts during curing. These salts are harmful to health, which is why moulds made of condensation silicone are not suitable as a food mould.
• After mixing the A and B components, they keep reacting and forming chains continuously. This means they shrink further and become more brittle over the years. The advantage is that they also adhere to old condensation silicone. With two-part moulds it is therefore essential to maintain a good separation between the two layers to prevent unwanted adhesion.
• They need moisture to cure and draw this from themselves and from the environment. This is why curing proceeds faster at high humidity. Seal condensation silicones well after use, otherwise the moisture evaporates.

Addition silicones

• Addition silicones show virtually no shrinkage. They form long chains without releasing substances during curing.
• In the liquid state they can still react with certain substances, which can cause poisoning. Sulphur, nitrogen, amino compounds and metal salts play a role in this.
• To force a complete cure and prevent unreacted material, it is recommended to post-cure addition silicones after curing, that is, to place them in an oven at 80 to 100 degrees Celsius for a considerable time.
• They need heat to cure and cure faster at higher temperatures. At low temperatures, already noticeable below 18 degrees Celsius, many addition silicones can cure poorly or not at all. Rule of thumb: for every 10 degrees higher temperature the processing and curing time halves, and for every 10 degrees lower temperature it doubles.

Resistance to chemicals and contamination

• After curing, condensation silicones are less resistant to chemicals and resins than addition silicones. During the curing process, however, condensation silicones cope better with contamination.
• Addition silicones can have problems curing when contaminated by substances such as tin salts, various adhesives, phosphorus, arsenic, nitrogen, 1-component silicones and sulphur (even sulphur-containing latex gloves). After curing, addition silicones can withstand many products. The biggest problems are usually petroleum derivatives and strong bases or acids.

Mixing ratios

• With condensation silicones the mixing ratios are less critical. Slightly more catalyst speeds up curing and shortens the working time. Too large an excess can reduce strength, too little catalyst gives very slow curing, and too much catalyst makes the end product age and become brittle faster.
• Addition silicones must be mixed very accurately. Even small deviations can cause the silicone to stop curing. In the best case this can still be remedied with a post-curing process.

Summary: condensation versus addition