Understanding SiH Crosslinkers and Chain Extenders in RTV-2 Silicone Systems - News

In the world of RTV-2 (Room Temperature Vulcanizing) silicones, crosslinkers play a decisive role in defining mechanical performance, processing behavior, and cure kinetics. Among the most critical are SiH-functional crosslinkers, which react with vinyl-functional polydimethylsiloxanes (PDMS) under platinum-catalyzed hydrosilylation to form the durable Si–CH₂–CH₂–Si network that gives silicone elastomers their unique balance of flexibility and strength.

BRB offers a comprehensive range of crosslinkers and chain extenders to support diverse formulations needs.

What are SiH Crosslinkers?

SiH crosslinkers are organosilicon compounds that contain multiple silicon-hydrogen (Si–H) groups per molecule. When mixed with vinyl-terminated PDMS, these groups react via hydrosilylation, forming covalent bridges between polymer chains.

The reaction rate follows second-order kinetics:

r = k [Si–H] [CH=CH₂]

where the rate constant k depends on temperature, catalyst concentration, and the type of inhibitor used.

Key parameters influencing the reaction include:

SiH content: number of reactive hydride groups
Viscosity and molecular weight of the crosslinker
Vinyl content of the base polymer
Catalyst and inhibitor balance

The Effect of SiH Content and Crosslinker Viscosity

The SiH content of a crosslinker directly determines crosslink density, while viscosity affects mixing, flow, and network uniformity.

Parameter

High SiH content

Low SiH content

Crosslink density

↑ Higher (stiffer network)

↓ Lower (softer elastomer)

Cure rate

↑ Faster

↓ Slower

Shrinkage

↑ More

↓ Less

Elastic modulus

↑ Increases

↓ Decreases

Similarly:
Low-viscosity crosslinkers → Better flow, uniform cure, longer pot life
High-viscosity crosslinkers → Stiffer networks, ideal for structural silicones

Effect of SiH Content and Crosslinker Viscosity on RTV-2 Silicone Properties

The graph below illustrates the relationship between SiH content and key performance indicators such as modulus and elongation for low- and high-viscosity crosslinkers.

BRB’s Range of Crosslinkers

BRB product name

Type of SiH

Viscosity (cSt)

SiH content (mmol/g)

BRB Crosslinker 434H4

Pendant

4.0

BRB Crosslinker 1595H7

Pendant

7.0

BRB Crosslinker 1738H1.9

Pendant

1.9

BRB Crosslinker 959H2.5

Pendant & end-capped

2.5

The Role of Chain Extenders

While crosslinkers build the 3D network, chain extenders modify the polymer chain length before crosslinking occurs.

Component

Structure

Function

Crosslinker

PDMS backbone with in-chain SiH groups

Create network junctions

Chain Extenders

PDMS backbone end-capped with SiH groups

Extend chains linearly before curing

The combination of both enables formulators to fine-tune elasticity, mechanical strength, cure rate, and pot life. By adjusting the ratio of crosslinkers to chain extenders, materials can range from soft gels to tough elastomers.

BRB’s Range of Chain Extenders

Product name

Type of SiH

Viscosity (cSt)

SiH content (mmol/g)

BRB Modifier 1439

End-capped

2.5

BRB Modifier 1449

End-capped

1.3