What is liquid silicone?
Liquid silicone has become an increasingly popular material in recent years due to its unique properties and wide range of applications.
In this article, I will provide an in-depth look at what exactly liquid silicone is, what it is used for, what it is made of, how it compares to regular silicone, the differences between it and medical-grade silicone, why it is more expensive than other silicones, the different types available, and whether liquid silicone can be considered medical-grade.
Liquid silicone is a silicone of high purity, cured with platinum, and characterized by low viscosity and fluidity. Formulated from siloxane polymers, it attains its fluid state through meticulous control of polymerization levels during manufacturing. This yields silicone chains capable of smooth interflow.
Unlike regular solid silicone which maintains its shape, liquid silicone has a flowing, almost water-like consistency. However, it is chemically inert and does not actually contain any water. The viscosity of liquid silicone can vary, from thin and runny to thick and gel-like. But in general, anything labeled as liquid silicone will have a low viscosity compared to conventional rubber or silicone elastomer.
What is Liquid Silicone Used For?
The fluidity and unique properties of liquid silicone make it suitable for a wide array of applications
Moldmaking
The low viscosity allows liquid silicone to flow easily into intricate molds and pick up fine details. Heat-cured liquid silicone rubbers are especially popular for moldmaking due to their convenience and accuracy.
Prototyping
Liquid silicone can quickly produce detailed prototypes and models when poured into 3D printed or CNC machined molds. Rapid prototyping with liquid silicone enables faster design iteration.
Soft robotics
Flexible and stretchable liquid silicone elastomers allow for inflatable bladders and deformable components in soft robotics research. The material is also biocompatible.
Medical devices
Liquid silicone has been used to produce custom medical-grade parts like catheter tubing, seals, gaskets, and various implants.
Consumer goods
The ability to cast detailed and flexible parts makes liquid silicone suitable for consumer products like phone cases, bakeware, ice cube trays, ear plugs, and more.
Textiles
Applying liquid silicone to textiles can make them waterproof and increase durability. This is done by dipping or infusing the fabric.
What is Liquid Silicone Made Of?
The base components of liquid silicone are siloxane polymers, similar to regular solid silicone
Siloxane backbone
This is composed of alternating silicon and oxygen atoms that form the core structure of the polymer chains. The siloxane backbone provides silicone its distinctive properties.
Organic side chains
Methyl or other hydrocarbon groups bonded to the silicon atoms make up the side chains. Different side chains produce silicones with varying properties.
Cross-linking agents
Networks of interconnected polymer chains are formed using cross-linking chemicals like hydrosilane or vinyl. More cross-links result in less fluidity.
Additional ingredients include:
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Platinum catalyst - Platinum complexes enable the cross-linking reaction between vinyl and hydrosilane groups on siloxane chains.
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Inhibitors - Additives that control the polymerization rate and pot life by temporarily deactivating the platinum catalyst.
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Fillers - Reinforcing fillers like silica provide thixotropic properties and increase hardness.
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Pigments - Pigments are dispersed throughout colored liquid silicone rubbers.
Liquid Silicone vs Silicone
Liquid silicones have key differences from conventional solid silicone:
Viscosity - Liquid silicones have a much lower viscosity, allowing them to readily flow before curing. Solid silicone remains rubbery.
Polymerization - Liquid silicones undergo less polymerization, resulting in shorter polymer chains and lower molecular weight.
Additives - The degree of cross-linking is reduced in liquid silicone. Plasticizers and viscosity modifiers are added.
Curing process - Heat and moisture-cured liquid silicones only partially crosslink upon curing. Additional post-curing is optional.
Mechanical properties - While solid silicone is rubbers with high strength and durability, liquid silicones have lower tensile and tear strength.
Liquid Silicone vs Medical Silicone
While liquid silicone has some medical uses, there are important differences from medical-grade silicone
Biocompatibility testing
Medical silicone undergoes biocompatibility testing to evaluate biological risk. Liquid silicone does not necessarily require this.
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Purity standards
Medical silicone adheres to biological risk material purity standards set by pharmacopeia and the FDA. Most liquid silicones do not.
02
Production environment
Medical silicone is produced in strictly regulated cleanroom environments to prevent contamination. Regular liquid silicone production does not need this level of control.
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CYP3A4 testing
Medical silicone intended for implantation requires testing for lack of CYP3A4 induction. Other liquid silicones do not need testing.
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Traceability
Documentation for material traceability is mandated for medical silicone. Traceability requirements are less stringent for industrial liquid silicones.
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Why is Liquid Silicone so Expensive?
There are several reasons contributing to the generally high prices of liquid silicone products:
Manufacturing complexity - Producing consistent, high purity liquids that meet specifications requires a great degree of processing control and expertise. This results in higher costs.
Raw material costs - Medical-grade liquid silicones utilize the highest purity polysiloxane polymers as precursors. These specialized polymers are expensive.
Handling precautions - Careful handling procedures are required during manufacturing and transport to avoid contamination of the liquids. This adds cost.
Custom formulations - Formulating liquid silicones for specific physical properties and cure profiles requires extra steps. Unique formulations are more expensive.
Small batch sizes - Liquid silicone procurement often involves smaller custom batch sizes rather than large bulk orders, which increases cost per volume.
Limited supply - The niche applications of liquid silicones mean fewer overall producers. Lack of suppliers drives up prices.
Certifications - Meeting various biocompatibility, medical, and regulatory standards certification adds to the expense.
What are the 4 Types of Silicone?
Elastomers - Highly flexible and durable rubbers made of interlinked polymers. Used for seals, gaskets, coatings.
Gels - Soft, thick, sticky gels with high elongation. Applications include lubricants, heat transfer.
Fluids - Low viscous, pourable silicone fluids. Includes the liquid silicones discussed in this article.
Resins - Brittle solid resins with high thermal stability. Used as binders, encapsulants, and adhesives.
Liquid silicones specifically fall under the fluid type, while medical grade silicone commonly comes in the elastomer form.
Is Liquid Silicone Medical Grade?
Liquid silicone on its own does not automatically equate to medical-grade silicone. However, liquid silicone rubbers and gels can potentially achieve medical-grade certification through:
Stringent quality control of chemical composition, viscosity, and physical properties
Testing for biologically inert properties and passing biocompatibility requirements
Adhering to medical device regulatory standards and compliance
Validation of manufacturing environment sterility and contamination prevention
So in summary, while not all liquid silicones are medical-grade, some high purity formulations, when properly tested and processed, can potentially meet the requirements for medical device applications and come into some level of contact with the human body.
Conclusion
Liquid silicone is a unique silicone-based polymer fluid capable of flowing and molding before curing into flexible rubber parts. It is produced from siloxane backbones with organic side chains, cross-linking groups, and additives to achieve specific viscosities and properties.
The fluidity, chemical resistance, and temperature stability of liquid silicone make it suitable for producing detailed molds, prototypes, consumer goods, and medical devices. Liquid silicone differs from conventional solid rubbers in viscosity, curing process, additives, and mechanical properties.
While more expensive than other silicones, the manufacturing complexity, purity, quality control, and niche applications of liquid silicone impact its higher costs. With proper formulation and rigorous testing, some liquid silicones can meet the stringent standards required to be considered medical-grade for use in medical devices and human contact.
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In particular, it has outstanding advantages in innovative organic liquid foam rolls, sheets, foamed silicone strips, foamed silicone rings, thermal conductivity, potting, sealing, room temperature vulcanization rubber, research and production technology, etc.