How moisture curing silicones work
A 1-component RTV silicone is cured with moisture unlike a 2-component silicone adhesive which cures upon mixing. The moisture curing silicone will cure from the outside in which means that first, a skin will be formed on the surface. This is a relatively fast process, perhaps 10 minutes at normal temperature and humidity. What it means in reality is that this is the time you have after dispensing until your pieces should be bonded (otherwise known as the ”open time”, ”tack free time” or ”skin formation time”). After the skin has formed, the adhesive will continue to cure inwards which takes longer. A normal cure speed is approximately 2 mm/24 hours, counted from the surface of the adhesive that is in contact with air.
The great advantage of choosing a 1-component silicone adhesiva as compared to a 2-component one is not the cure speed, but the ease of dispensing.
Chemically, what happens during curing is that tiny water molecules creep into the silicone. There they chemically react with the components of the silicone adhesive and create a by-product. Depending on the type of adhesive, this by-product can be either an alcohol (alkoxy), acetic (acetoxy), an oxime or more uncommonly, an amine.
The curing speed of 1-component moisture curing silicones is directly dependent on the:
- Relative humidity.
- Design and dimensions of the bond.
The influence of relative humidity should be clear by now – it is the water molecules in the air that are needed to kick off the chemical curing reaction at all. A higher temperature make the molecules in the adhesive more mobile which allows them to react more quickly. However, be careful not to increase the temperature too much (above approximately 60 ˚C). A too high temperature can degrade the components and either prevent cure altogether or impair the end physical properties.
Advantages of moisture curing silicone adhesives
- Bonding, sealing and fixating.
- Adhere to most surfaces.
- Easy handling with no mixing step.
- Soft and elastic, even at low temperatures down to -60 or in some cases even below -100 ˚C.
- High temperature resistance, often between 200 ˚C till 300 ˚C continuosly.
- Contain no solvents and are usually work environmentally friendly.
- High chemical resistance.
- Water resistant.
- Bacteria and fungi do not thrive on silicone which make them suitable for use in humid environments.
- Can withstand harsh outdoor conditions.
- Are available in many colors (even transparent) and consistencies.
- Good electrical properties.
Applications for moisture curing silicone adhesives
1-component moisture curing silicone adhesives have many areas of use in a number of industries: anything from consumer products such as the doors of ovens to applications within aerospace, aviation, denfense, electronics, medical and everything in between.
Tips & tricks
- Do not use a 1-component moisture curing silicone adhesive if you plan on using it in deep sections. The depth is measured from the side of the silicone that is in contact with surrounding air. This means that moisture curing silicones are not suitable for closed-in areas either. In these cases, you will be better off with a 2-component silicone.
- Ig the original packaging of the adhesive is ungainly, the adhesive can easily be re-packaged into smaller syringes.
- Oximes can cause cancer. Please avoid this type of moisture curing silicone if possible and go for an alkoxy or acetoxy curing one instead.
- Do you need some bond strength instantly? Look for a silicone adhesive with a high green strength (wet strength). Silicone adhesives with high green strength have a type of stickiness that ensures that the bond gets some strength even before curing has begun.
- Keep in mind that many different qualities of moisture curing silciones are available. In a more sensitive application, a higher purity grade silicone, than the ones used in bathrooms for example, will probably be necessary. There are even low outgassing options for very sensitive applications such as in space and electronics as well as implant grade ones.
Contact us if you are interested in what moisture curing silicones we can offer for your application.