What is Liquid Silicone Rubber?
Liquid Silicone Rubber (LSR) is a non-toxic, heat-resistant, and highly resilient flexible thermoset material. Its rheological behavior is characterized by low viscosity, rapid curing, shear thinning, and a relatively high coefficient of thermal expansion. LSR is a two-part liquid system consisting of Part A containing a platinum catalyst and Part B containing a silicone hybrid crosslinker and an inhibitor to provide a reasonable pot life.
Properties of Liquid Silicone Rubber
Rheological Properties
LSR exhibits unique rheological properties that make it suitable for various applications. It has a low viscosity, typically ranging from 1 to 1,000 Pa·s at 25°C, allowing for easy processing and injection molding. Additionally, LSR exhibits shear thinning behavior, which facilitates flow during molding and filling of intricate cavities.
Mechanical Properties
LSR possesses excellent mechanical properties, including high tensile strength, tear strength, and elongation at break. It can achieve a hardness of at least 75 on the Type-A durometer scale and an elongation at break of at least 200%. These properties make LSR suitable for applications requiring flexibility, durability, and resistance to wear and tear.
Thermal and Chemical Resistance
LSR exhibits outstanding thermal stability and resistance to high temperatures, making it suitable for applications involving exposure to elevated temperatures. It also demonstrates excellent chemical resistance, withstanding exposure to various solvents, oils, and chemicals.
Electrical Properties
LSR is an excellent electrical insulator, with a breakdown strength of up to 42 kV/mm. This property, combined with its thermal stability, makes LSR an ideal material for electrical and electronic applications, such as insulation for high-voltage components and encapsulation of electronic devices.
Biocompatibility and Purity
LSR is a non-toxic and biocompatible material, making it suitable for medical and food-grade applications. It is free from contaminants and potential biological pollutants, ensuring safety and hygiene in sensitive applications.
Thermal Conductivity
While pure LSR has relatively low thermal conductivity, the addition of fillers like hexagonal boron nitride particles can significantly enhance its thermal conductivity, reaching up to 0.96 W·m⁻¹·K⁻¹, an increase of 557% compared to pure LSR. This property makes LSR suitable for applications requiring efficient heat dissipation, such as LED packaging and electronic encapsulation.
Adhesion and Bonding
LSR exhibits excellent adhesion to various substrates, including thermoplastics like polyamide (PA), polybutylene terephthalate (PBT), polyphenylene sulfide (PPS), and polyphthalamide (PPA). Self-bonding LSRs and adhesion additives can further enhance the bonding capabilities, eliminating the need for primers and simplifying the manufacturing process.
The unique combination of properties exhibited by LSR, including its rheological behavior, mechanical strength, thermal and chemical resistance, electrical insulation, biocompatibility, and potential for enhanced thermal conductivity and adhesion, make it a versatile material suitable for a wide range of applications across various industries.
Production of Liquid Silicone Rubber
LSR Compounding and Base Preparation
The key components for LSR base preparation include:
- Vinyl-containing organopolysiloxane
- Reinforcing silica filler with high surface area (≥50 m2/g)
- Silane surface treatment agent
- Water or ammonium solution
The typical process involves:
- Kneading (A), (B), (C), and (D)
- Heat treatment of the kneaded mixture
- Further kneading with additional (C)
- Final heat treatment
This produces a low viscosity, stable LSR base compound.
LSR Composition Formulation
The LSR base is combined with other components to form the final curable composition:
- Vinyl-containing polysiloxanes of different viscosities
- Non-functional dimethylpolysiloxane
- Platinum catalyst
- Crosslinking agent (e.g. hydride functional polysiloxane)
- Inhibitors
The components are precisely dosed, mixed, and deaerated.
LSR Processing and Molding
LSR is suitable for injection molding due to its low viscosity:
- Transferred from supply pump to mold via heated barrel and runner
- Injected into the heated mold cavity
- Rapidly cured by addition reaction at elevated temperatures
Advantages include rapid curing, high production efficiency, and over-molding capability onto heat-sensitive substrates.
Applications of Liquid Silicone Rubber
Medical and Healthcare Applications
LSR is widely used in the medical industry due to its excellent physical properties, chemical resistance, and heat resistance. Medical-grade LSR offers advantages like biocompatibility, ease of sterilization, and transparency. Its soft feel against the skin makes it an ideal choice for medical devices in contact with the body, and it meets stringent FDA standards. Key applications include:
- Flow control valves, syringe stoppers, and other components
- Balloon catheters, cardiovascular catheters, tubing for feeding, drainage, and peristaltic pumps
- Compression bars, multi-port urological catheters, infusion sleeves, and test chambers
- Wire/fluid-path co-extrusion, ear plugs, hearing aids, shunts, septa, seals, valves, stoppers, and clips
Automotive Applications
LSR’s durability, tensile strength, high heat stability, fatigue resistance, and tear resistance make it an excellent choice for components used in automotive engine compartments. Many OEMs and tier 1/2 automotive suppliers use LSR to manufacture various parts, including:
- O-rings, buttons, components, gaskets, and wire harness assemblies
- LSR can also be directly over-molded onto plastic or metal parts
Consumer Products
LSR’s diverse properties make it suitable for manufacturing various consumer goods, ranging from kitchen bakeware and plastic shower heads to electronic products and bottle dispensers.
Specialty Applications
Different grades of LSR enable its use in various other applications, such as:
- Valves and instruments, membranes and gaskets, irrigation filter parts, pressure outlets
- Check valves and one-way valves, luminous panel buttons
- Self-lubricating LSR for minimizing friction at component interfaces, enhancing device performance [Supplementary Information]
Application Cases
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Liquid Silicone Rubber (LSR) Medical Devices | LSR offers biocompatibility, ease of sterilisation, and transparency. Its soft feel against the skin makes it ideal for medical devices in contact with the body, meeting stringent FDA standards. | Medical devices such as flow control valves, syringe stoppers, balloon catheters, cardiovascular catheters, tubing for feeding, drainage, and peristaltic pumps, compression bars, multi-port urological catheters, infusion sleeves, test chambers, wire/fluid-path co-extrusion, ear plugs, hearing aids, shunts, septa, seals, valves, stoppers, and clips. |
| LSR Automotive Components | LSR’s durability, tensile strength, high heat stability, fatigue resistance, and tear resistance make it suitable for components used in automotive engine compartments. | Automotive components such as O-rings, buttons, gaskets, and wire harness assemblies. |
| LSR Baby Care Products | LSR’s non-toxic, odourless, and hypoallergenic properties make it safe for use in baby care products, providing a soft and comfortable feel against delicate skin. | Baby care products such as nipples, pacifiers, teething rings, and bottle components. |
| LSR Kitchen and Household Items | LSR’s heat resistance, chemical resistance, and non-stick properties make it suitable for kitchen and household items, ensuring durability and ease of cleaning. | Kitchen and household items such as bakeware, utensils, spatulas, and oven mitts. |
| LSR Industrial and Electrical Components | LSR’s electrical insulation properties, heat resistance, and chemical resistance make it suitable for industrial and electrical components, ensuring reliable performance in harsh environments. | Industrial and electrical components such as cable jacketing, connectors, gaskets, and seals. |
Latest innovations in Liquid Silicone Rubber
Composition and Processing Advancements
- Novel LSR formulations with enhanced thermal conductivity by incorporating metal nitride fillers like aluminum nitride and boron nitride, enabling applications in electronics and LED lighting.
- Improved high-temperature resistance through molecular structure modifications, such as incorporating phenyl or trifluoropropyl groups into the siloxane backbone.
- Innovative processing techniques like in-situ aromatization of poly(cyclohexadiene) brushes to obtain ultra-smooth LSR interfaces with tunable thicknesses from 4 to 108 nm.
- Development of self-lubricating LSRs containing built-in lubricants that elute to the surface, reducing friction and improving performance in dynamic seals and medical devices [supplementary information].
Functional and Smart LSR Materials
- Electrically conductive LSRs achieved by incorporating conductive fillers like carbon black, enabling applications in electromagnetic shielding and antistatic coatings.
- Flame-retardant LSRs through the addition of flame retardants, suitable for construction and transportation industries.
- Adhesion-promoting LSRs by blending adhesion promoters or using self-bonding LSRs, enabling primerless bonding to thermoplastics like polyamides and polyphthalamides [supplementary information].
Emerging Applications and Manufacturing Techniques
- 3D personalized printing of LSR for customized medical devices and prosthetics, leveraging the biocompatibility and flexibility of LSRs.
- Encapsulation of high-refractive-index materials with LSRs for optical applications like lenses and waveguides, taking advantage of LSR’s transparency.
- Advancements in liquid injection molding techniques, such as multi-component molding and low-density LSR molding, enabling complex geometries and lightweight parts.
The innovations highlighted above demonstrate the versatility and continuous improvement of LSRs, addressing challenges in thermal management, conductivity, adhesion, and processing. These advancements open up new possibilities for LSRs in diverse fields, from electronics and optics to healthcare and transportation.
Technical challenges
| Improving Thermal Conductivity | Enhancing the thermal conductivity of liquid silicone rubber (LSR) by incorporating thermally conductive fillers like metal nitrides (aluminum nitride, boron nitride) or modifying the molecular structure. |
| High-Temperature Resistance | Improving the high-temperature resistance of LSR through molecular structure modifications, such as incorporating phenyl or trifluoropropyl groups into the siloxane backbone. |
| Surface Modification | Developing innovative processing techniques like in-situ aromatization of poly(cyclohexadiene) brushes to obtain ultra-smooth LSR interfaces with tunable thickness. |
| Self-Lubricating LSR | Developing self-lubricating LSRs containing built-in lubricants that elute to the surface, reducing friction and improving performance in dynamic seals and medical devices. |
| Conductive LSR | Achieving electrically conductive LSRs by incorporating conductive fillers like carbon black, enabling applications in electromagnetic shielding and antistatic coatings. |
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