Modifier 2.5 – Si-H Terminated Chain Extender for Addition Cure Silicones

Si-H terminated chain extender for addition cure silicones

Modifier 2.5 is a linear silicone fluid that carries a reactive Si-H group at both ends of the molecule only, with no Si-H along the chain. During addition cure those two end groups react with vinyl groups, so the polymer chain is extended instead of adding further crosslinking points. The result is a lower crosslink density, a lower hardness and more elongation, at an SiH content of approximately 2.5 mmol/g and a viscosity of approximately 5 cSt. Depending on stock we supply this product as BRB Modifier 1439 or as AToro CE-5.

This product is intended for the business market. For larger quantities, a specific variant or custom products you can reach us at info@siliconesandmore.com.

Key benefits

• Lowers hardness without lowering the crosslinker: the two terminal Si-H groups extend the chain and lower the average functionality of the system. You control hardness with a separate raw material rather than with the crosslinker dosage.
• Terminal Si-H only: no reactive groups along the chain, so no unwanted additional crosslinking.
• Low viscosity, approximately 5 cSt: keeps formulations free flowing and easy to degas, also at high filler loading.
• Cures without by-products: hydrosilylation with vinyl functional components produces no cleavage products, so no shrinkage through evaporation.
• Suitable for gels: makes it possible to formulate soft, addition cure gels and encapsulants.
• Low dosage: the high SiH content per gram means little product is needed to see an effect.
• Synergy with crosslinkers: combined with a multifunctional SiH crosslinker you tune hardness, elongation and tear strength independently.

Typical applications

• ✓ Lowering the hardness of addition cure RTV-2 silicones
• ✓ Formulating soft, addition cure gels and encapsulants
• ✓ Chain extension in RTV and LSR formulations
• ✓ Free flowing casting systems where viscosity has to stay low
• ✓ Fine tuning hardness and elongation alongside a multifunctional SiH crosslinker
• ✓ Damping and vibration absorbing gels for electronics

Processing

1. Add Modifier 2.5 to the component that contains the crosslinker, that is the component without platinum catalyst. Si-H and platinum together in one component causes hydrogen evolution and an unstable stock product.
2. Stir the product in until the mixture is homogeneous. The low viscosity makes this easy, also when cold.
3. There is no fixed dosage. Start low and work up in small steps, assessing hardness, elongation and cure speed on a test casting at every step.
4. Keep an eye on the overall SiH:Vinyl ratio. Modifier 2.5 raises the SiH content of your system but adds only two reactive groups per molecule, lowering the average functionality.
5. Degas the mixture under vacuum before casting.
6. Test every change on a small scale before scaling up. Besides hardness, working time and cure time usually change as well.

We deliberately do not give a fixed dosage table. The effect depends on your vinyl polymer, your crosslinker, the filler and the final hardness you want, and can only be established reliably on your own formulation.

Points to watch

• ✓ Flammable liquid category 3, flash point approximately 55 °C for the BRB variant. Keep the product away from heat, hot surfaces, sparks and open flame, do not smoke in the work area and take precautionary measures against static discharge. The product is sensitive to static discharge.
• ✓ Very toxic to aquatic life with long lasting effects. Avoid release to the environment and to the sewer, and dispose of residues and packaging through an approved waste processor.
• ✓ Si-H reacts with water, alcohols, acids and bases and with many metals and metal compounds, releasing hydrogen gas. Store dry, in the original tightly closed packaging, and keep tools clean and dry.
• ✓ Do not mix with oxidising agents.
• ✓ Above 150 °C traces of formaldehyde may be released. Provide sufficient extraction when heating.
• ✓ The BRB variant contains D4, D5 and D6 and according to BRB meets the PBT and vPvB criteria of REACH Annex XIII. Check whether your application falls under the restrictions of REACH Annex XVII before scaling up.
• ✓ Wear nitrile gloves of at least 0.4 mm with a breakthrough time of at least 480 minutes, safety glasses with side shields, protective clothing with long sleeves and antistatic footwear.

Professional support

Not sure how much you need to go from Shore A 30 to Shore A 15, or whether chain extension or a lower crosslinker dosage is the better route in your case? Contact us at info@siliconesandmore.com. Tell us which vinyl polymer and crosslinker you use and the final hardness you are aiming for, and we will help you think through the formulation and which of the two variants fits best.