What is liquid silicone used for

Liquid silicone has become an increasingly popular material for products across many industries due to its unique properties and ease of processing. In this article, we'll examine the composition, benefits, applications, and reasons for the cost of liquid silicone formulations. We'll also look at how liquid silicones differ from solid rubbers and gels.

What is liquid silicone rubber

Liquid silicone rubber (LSR) refers to a platinum-cured silicone polymer with vinyl groups that is processed into a pourable liquid form. It can be injection molded at room temperature into detailed rubber parts.

The chemistry of silicone consists of a silicone polymer backbone chain with organic methyl side groups attached to silicon atoms. This composition gives silicone oils and rubbers their distinctive properties like:

Thermal stability from -100°F to over 400°F
Extreme flexibility and elasticity
Excellent resistance to chemicals and weathering
Low surface tension for easy flow in molds
Electrical insulation

Liquid silicone can be customized into different viscosities and grades for applications requiring specific properties. Adding fillers like silica provides reinforcement, while vinyl groups enable cross-linking into molded rubber parts.

Liquid silicone chemical formula

The basic structure of liquid silicone polymers is:

R2SiO2/2

Where R represents organic methyl (CH3) groups bonded to silicon atoms.

The overall composition is -(Si(CH3)2O)n-

With n representing the number of repeating siloxane monomer units in the polymer chain. Commercial grades have between 300 to 5000 monomers.

Liquid silicones may also contain vinyl (CH=CH2) or other functional groups. But the fundamental non-polar and flexible siloxane backbone gives liquid silicone its unmatched thermal/chemical stability.

What are the benefits of using liquid silicone rubber?

Liquid silicone rubber offers many advantages that make it a versatile material across industries:

Precise molding

Easily fills minute details and complex geometries

Rapid processing

Cures quickly at room temperature

Mechanical properties

High tear strength, low compression set

Thermal stability

Withstands temperatures from -115°F to over 500°F

Chemical resistance

Inert to oils, solvents, water; damaged only by alkalis

Biocompatibility

Non-reactive and hypoallergenic; ideal for medical uses

Low flammability

Produces very little smoke when burned

Electrical insulation

High dielectric strength for electrical embedding

Liquid silicone molding matches the material advantages with automated, high-volume production of detailed rubber components. This enables silicone to replace other rubbers in products across healthcare, automotive, electronics, consumer goods, and other sectors.

What does liquid silicone not stick to?

Thanks to its low surface tension and energy, liquid silicone does not naturally adhere well to most materials. Liquid silicone acts as a release agent and easily peels away from:

Plastics - PE, PP, PVC, acrylic, ABS, nylon
Metals - aluminum, steel, copper, zinc
Wood - veneers, particle board, MDF
Glass - frosted or clear glass
Paper/cardboard
Ceramics - glazed surfaces
Rubber - natural rubber, butyl, neoprene

The slick and frictionless texture of silicone prevents mechanical bonding. Adhesion can be improved by priming surfaces or using silicone specially formulated with adhesion promoters.

Liquid silicone will temporarily stick to bare hands during processing due to tackiness. But it does not permanently bond to skin or surfaces with low surface energy. This allows liquid silicone to be easily molded and removed from non-stick surfaces.

What is silicone commonly used for?

Thanks to a unique combination of properties, silicone is used across a diverse range of applications:

Seals and gaskets - High temperature resistance and flexibility
Medical devices - Biocompatibility and heat resistance for catheters, implants
Cookware handles - Provides grip and insulation from heat
Electrical insulation - High dielectric strength; used in wires and potting electricals
Automotive parts - Resists oils and retains flexibility at high and low temperatures
Molds - Easily releases from mold surfaces; ideal for casting resins and rubbers
Adhesives - Silicone sealants bond glass, metal, plastic, porous materials
Skin care - Silicone oils improve skin feel in lotions and serums
Baking ware - Inherent non-stick properties; withstands oven temperatures

The unique flexibility over a wide temperature range coupled with chemical and thermal stability enables silicone to perform in applications where other materials would fail. Liquid silicone specifically facilitates processing into detailed molded parts.

What is the difference between liquid silicone and soft silicone?

Liquid silicone generally refers to low viscosity silicone fluids and solutions. Soft silicone typically means silicone rubber or elastomers with hardness between 20 - 40 Shore A durometer.

Key differences:

State - Liquid silicone pours and flows; soft silicone retains solid rubber shape
Viscosity - Liquid silicones range from easily pourable to thick fluids; soft silicone is non-flowing solid.
Processing - Liquid can be injected or cast; soft silicone usually molded or extruded
Applications - Liquid used where flow is needed; soft used where rubbery properties are required
Curing - Liquid may cure into elastomers; soft silicone is already cured
Additives - Liquids are primarily polymer; soft contains fillers for reinforcement

So in summary:

Liquid silicones flow easily for casting, coating, molding
Soft silicones are solid, flexible rubber materials

But liquid silicone can become soft silicone rubber through controlled curing in a mold. The process transforms a free-flowing liquid into a rubbery solid.

Is liquid silicone hard or soft?

In its pure liquid polymeric form, liquid silicone is neither hard nor soft - it readily pours and flows with low viscosity.

However, liquid silicone can be converted into either soft flexible silicone rubber or hard rigid silicone through:

Curing Method

Addition cure results in soft, flexible silicone rubber
Condensation cure results in hard, glassy silicone resin

Fillers

Silica filler increases hardness and rigidity
No fillers maintains maximum flexibility

Curing Extent

Partial cure leads to soft gels and rubbers
Full cure maximizes hardness
So the initial liquid silicone can produce a range of end material properties depending on curing formulation and processing.

Some examples:

Liquid silicone rubber (LSR) - Soft, flexible silicone elastomer from addition cure
Room temperature vulcanized (RTV) - Soft silicone rubber from condensation reaction
Silicone resins - Hard, durable thermoset resins from full condensation cure

The versatility of liquid silicone chemistry allows tailoring the final rubber hardness across a wide spectrum from very soft gels to glassy hard resins.

Why is liquid silicone so expensive?

There are several reasons contributing to the relatively high cost of liquid silicone formulations:

Silicone Production - Synthesizing the base silicone polymer requires high temperatures and energy input.

Processing - Multiple steps of polymerization, blending, fillers compounding are needed to produce specific silicone grades.

Precise Chemistry - Tight control and precision is required in synthesis to obtain consistent properties.

Specialized Equipment - Liquid silicone molding needs expensive meter-mix injection molding machinery.

Scale Limitations - Silicone production facilities require major capital investment, limiting competitive suppliers.

Material Purity - Grades like medical or food contact must meet biocompatibility standards.

Custom Formulating - Unique silicone formulations drive up development costs.

Essentially, liquid silicone's specialty attributes of temperature stability, biocompatibility, fluidity, and processing versatility necessitate more tightly controlled chemistry and processing. This results in higher production costs that translate into higher liquid silicone rubber prices compared to conventional polymers.

Conclusion

Liquid silicone delivers an exceptional combination of heat resistance, chemical inertness, electrical insulation, biocompatibility, and processing ease that lend it to a vast range of product applications. Converting liquid silicone polymers into solid rubber parts enables manufacturing of silicone components at high volumes across industrial and consumer sectors. The advanced chemistry and processing capabilities required to produce consistent liquid silicone grades results in higher costs compared to conventional polymers. But for critical applications where silicone's unique performance is indispensable, the material cost is frequently justified.