There are several terminologies used to refer to materials used in the manufacture of silicone-based products. As such, it can be quite confusing if you are trying to understand a certain material for decision-making purposes. One of these terms is silicone elastomers. In this article, you will learn everything you need to know about this term.
What is a Silicone Elastomer?
The definition of a silicone elastomer can be built from two parts — silicone and elastomer. First, silicone is technically plastic. However, its formation, molecular structure, and properties are different from regular plastics. Chemically, it can be classified as a synthetic polymer made of silicon-oxygen compounds with repeating methyl groups.
Elastomers on the other hand are rubber-like materials. Chemically, they are polymers but they have poorly cross-linked compounds. This is what gives them an elastic nature. Synthetic rubber is an example of an elastomer.
With this in mind, we can deduce that silicone elastomers are loosely cross-linked synthetic polymers. The main difference between silicone elastomers and plastic is that the curing process creates a permanent bonding that can’t be modified under high heat.
In other words, the terms silicone and silicone elastomer can be used interchangeably to mean the same thing. In material science, the terms Polysiloxanes and Siloxanes are also used to refer to silicone elastomers. Finally, the international community uses VMQ, PMQ, and PVMQ as the standard abbreviations of silicone.
Overview of silicone elastomers
Silicone has several properties and characteristics that set it apart from other types of synthetic polymers. It has unique qualities that make it suitable for use in specific environments. Let us explore these properties.
Chemical Properties of Silicone Elastomers
The chemical property of a substance indicates how its molecular structure is impacted when subjected to certain conditions. Silicone elastomers have the following chemical properties.
The chemical resistance of silicone is its ability to sustain corrosive environments without degrading. It is also known as inertness. Generally, silicone elastomers are compatible with many chemicals. This means its chemical tolerance is high. However, its worth noting that there are certain extents to which the resistance wears off.
A resistant chart is used to indicate resistant levels of silicone to chemicals. This chart outlines the chemicals silicone is resistant to and to which extent. Broadly, chemicals can be found in acids, greases, oils, and organic compounds. You can determine the specific tolerance of silicone based on the environment you intend to use the material.
While elastomers are technically unaffected by heat, there are extents to which a silicone-based product will sustain thermal energy. For instance, silicone elastomers can remain undeformed at temperatures of up to 150o C. Most polymers in the same class of plastics melt at significantly low temperatures.
The excellent thermal or heat resistance of silicone makes it suitable for manufacturing products used in high-heat environments. A good example is kitchenware.
Another term used to refer to UV resistance is weatherability. It dictates the behavior of silicone when exposed to ultraviolet rays primarily from the sun. Resistance to UV is vital provided that most silicone products are made for outdoor applications.
The ability to resist UV light is essential to ensure the molecular structure of silicone remains intact. Long exposure to UV light affects the elasticity of silicone elastomers which are useful in making static sealants and gaskets. Luckily, silicone elastomers are highly resistant to UV degradation.
Oil and Solvent Resistance
Oils and solvents are compatible with silicone elastomers and they have minimal impact on their structure and form. In other words, this material is unaffected by chemical agents found in mineral oils and polar organic compounds. Particularly, silicone has excellent resistance to most oils below 100oC.
However, it is worth noting that silicone swells when exposed no non-polar organic compounds like benzene, toluene, and gasoline. The good news is that the swelling will disappear after it has been withdrawn from the compound.
Electrical Current Resistance
Silicon, an element in silicone, is a poor conductor of electricity. Therefore, it suggests that silicone has a high electrical current resistance. This chemical property makes it an excellent insulating material for electrical equipment. The resistance is rarely affected by external factors like temperature and electrical conductors like water. For this reason, silicone elastomer can be considered a dielectric material.
Mechanical Properties of Silicone Elastomers
These properties demonstrate how silicone elastomers behave when subjected to external stress in the form of force. In most cases, they are measured on cured silicone elastomers to get actual values that products will exhibit.
Elongation at break
If silicone has excellent elongation, it means it can withstand the expected stress induced by the stretching of a silicone product.
Elongation at break is the length the elastomer extends before it breaks. In this case, the longer the distance, the better the resistance to break. Different silicone elastomers have varying percentages of elongation at break, which are usually high. This property is tested together with tear and tensile strengths since they are related.
Hardness is a measure of the resistance to deformation under compression pressure. In other words, how does a silicone elastomer sustain compression force? This is also called a durometer and its assigned different scales based on the resistance.
Silicone elastomers can be soft or hard. This depends on the density of the cross-linking agents and other characteristics. The reason for the different scales of durometers is that both soft and dense materials are useful in specific applications.
Tear strength is a crucial measure of how silicone performs under mechanical stress. It is a measure of the ease of tear of silicone that has been chipped. Since silicone requires less strength to tear, it implies that it has poor tear resistance.
This property simulates the wear and tear conditions that are expected when a silicone product is in use. The better the resistance, the more the product can sustain high-tension environments. Tear strength is measured in pounds per inch (ppi).
The force required to break silicone when it’s under tension is called tensile strength. When a material has a high tensile strength, it means it requires a lot of force to break it. This is the case with silicone. While the general strength is usually high, the specific force will depend on the specific type of silicone elastomer in question.
While some types of elastomers have excellent abrasion resistance, silicone is unable to sustain abrasive environments. Abrasion is the wear caused by repeated aggression of the outer surface of a material.
Silicone elastomers have poor abrasion resistance due to the low tear resistance. This is why silicone is recommended for products that are mostly static rather than dynamic. If the movement of the product is limited, the silicone can function optimally without having to wear off quickly.
The chemistry of a Silicone Elastomer
Understanding silicone at a molecular level is beneficial in many ways. For one, you will appreciate how the material is synthesized. Secondly, you will easily differentiate between the various forms of silicones.
In the definition of silicone elastomers, we already established that silicones are polymers with silicon-oxygen compounds and methyl groups. Silicon exists naturally in various rocks and it is extracted using special processes.
Silicon combines with oxygen to form silicon dioxide (SiO2). In the lab, this compound undergoes polymerization, which involves the addition of methyl groups, to form silicone or silicone polymer. That is the basic explanation of how silicone is synthesized. Obviously, it undergoes other purification and refining procedures to make it usable.
Thermoplastic Elastomers (TPEs) vs thermosets
Regulatory bodies overseeing the usage of synthetic polymers devised a classification to distinguish the various forms of elastomers. In this context, we have two closely related but different classes. These are thermoplastic elastomers and thermosets. Both have rubber properties such as stretching.
The main difference between these two classes of elastomers is that thermoplastic elastomers can be deformed by heat while thermosets cannot. In other words, thermoplastics can melt at certain temperatures.
The cross-linking technique used to bind the thermoset elastomers creates a permanent covalent bond during curing. This bond is loose to make it flexible but strong enough to prevent deformation under high temperatures.
Silicone elastomers as thermosets
The characteristics of thermosets we have discussed above define silicone elastomers. If anything, silicone is one of the many examples of thermosets. Silicone elastomers are a special type of plastic known as thermosetting plastic. What sets them apart from regular plastics is the curing process.
During the curing process of silicone, the curing agents alter the molecular structure and bonding on the various groups. The covalent bond created is permanent meaning that it is hard to change unlike for thermoplastics. Fundamentally, this is why silicone is almost impossible to melt or has a high tolerance to temperature.
Silicone Elastomer Testing
The presence of properties of silicone need to be verified before milling or manufacturing processes begin. Moreover, there are other tests carried out during the manufacturing processes.
ASTM Testing standards for Silicone Elastomers
ASTM is an international body that regulates standards for dealing with various materials including silicone. This body has guidelines for silicone elastomer tests that indicate the approved values, and other regulatory measures.
Silicone elastomer testing standards are found under rubber standards. It provides general guidelines for tests. The specific tests will be determined based on the particular type of silicone elastomer being used.
Major Silicone Elastomer Properties Tests
The application, fabrication process, and type of silicone are primarily what determine the appropriate tests. This is because these tests will confirm the needed properties and dictate the manufacturing method.
Another reason why tests are narrowed down is that there are as many tests available as there are silicone properties. To expound, virtually all properties can be tested using multiple tests. For instance, UV resistance alone can be tested in multiple chemical and physical tests. Click here to explore the UV resistance test standards for silicone.
General tests that are conducted on all silicone elastomers include durometer, modulus of elasticity, and modulus of elongation. These tests are accurate at simulating how the silicone product will handle the mechanical stress.
Factors Affecting the Performance of Silicone Elastomers
While silicone is nearly immune to many stressors, there are circumstances that greatly challenge its performance. There are factors that dictate how good or bad the performance of a silicone product would be. Here are some of these factors.
Aging and Degradation Accelerators
Over time, the quality of a silicone product deteriorates. This is regardless of whether it is in use or storage. The loss of quality can be accelerated or slowed by the environment, storage conditions, and quality.
Unfavorable conditions can prematurely age a silicone product. To clarify, if the silicone is used in an environment where it does not tolerate, it would age and degrade faster than expected. Similarly, if it is stored in an environment with certain stressors like ozone, it tends to diminish at a much higher rate. In other words, its properties decline.
The type of Silicone Elastomer
There are several types of silicone elastomers that exist. Common ones include high-consistency rubber (HCR), Room Temperature Vulcanization (RTV), and liquid silicone rubber (LSR). Each of these types has its strengths based on its molecular composition and unique properties.
The type of elastomer affects its performance in many ways. For instance, different types have different curing methods during manufacturing. As we saw earlier, curing influences the properties of rubber. Also, the type dictates the product that can be made which in turn controls the setting where it will be used. Performance is influenced by surroundings.
Curing and Manufacturing Process
Silicone elastomers are cured in different ways. Common cure systems include; peroxide systems, platinum-catalyzed, condensation, and oxime systems. These curing systems are augmented by adding heat or pressure.
Since curing is aimed at hardening the elastomer, it can have adverse or positive impacts on its performance. The type of curing system used has an impact on the stability of silicone at the molecular level. This reflects on the properties of the final product. Luckily, stability can be enhanced by performing post-curing on a silicone product.
Silicone products are made for various applications. This means the material can be subjected to either harsh or favorable situations during its lifetime. Silicone sustains its performance depending on the severity of external conditions.
Stressors like temperature, UV light, and tension, reduce the mechanical properties of silicone. However, if the silicone product can be serviced, follow the recommended practices for best results.
Compatibility with Other Materials
The ability to be compatible with other materials is an important aspect that impacts how silicone elastomers perform. Compatibility in this case means the ability to coexist or interact with other materials without losing its properties. Generally, silicone is compatible with most materials such as diluted inorganic acids and bases, and mineral oils.
As much as it’s compatible, the extent to which it can sustain or tolerate certain materials is limited. This limitation can negatively impact its performance. Therefore, it is good practice to use silicone that’s compatible with the surroundings for extended performance.
Advantages of Silicone Elastomers
Silicone is the go-to material for many reasons. Specifically, it has several benefits that make it superior to other materials in its category. Most of the benefits are observed in its supreme properties. Let us summarize them into the following;
- High-quality material: Silicone has many properties that are desirable for various applications. Its quality is suitable for high-performance and hostile environment use since it is quite resistant.
- Durable: This implies that silicone can function in demanding regions for extended periods of time without losing its properties. Durable materials are cost-effective and environmentally friendly.
- Versatile: In terms of application, silicone elastomers are versatile in that they can be used in multiple use cases. Versatility is critical since it means the properties of silicone can be spread to many areas.
Note that most of these benefits will depend on the type of silicone elastomer used. That means the benefits will vary from one type to another.
Disadvantages of Silicone Elastomers
While this material is quite beneficial, it has its drawbacks. There are a number of disadvantages that limits its use, accessibility, and suitability. Some of these demerits include the following;
- Relatively costly: Thanks to its premium properties, silicone comes with a high price tag. Particularly, the initial cost incurred with silicone is high. However, on the bright side, this cost is compensated by its optimal performance with low maintenance needs.
- Lack of compatibility: Although silicone is compatible with some materials, it can lose its properties over extended periods. Moreover, some materials completely deform silicone’s nature almost instantly after contact.
Most of the disadvantages of silicone can be improved or prevented in the manufacturing process. For instance, some properties like tear strength can be enhanced by using certain curing systems.
How to Choose the Right Silicone Elastomer
With many options to choose from and many factors to consider, it can get overwhelming when choosing the suitable type of silicone for your products. If that’s the case, you can base your decision on the following factors;
- Hardness: Silicone has two scales of hardness — soft and rigid. The softness or rigidity of your product will be determined by these scales. Therefore, it makes sense to select the proper hardness.
- Chemical resistance: We discussed that silicone is selectively resistant to certain chemicals. The reason why this factor is vital is that most environments have a combination of natural or synthetic chemicals that can impact silicone.
- Curing method: Your silicone product manufacturer decides on the curing method based on their knowledge. However, you can impact this decision by asking for a specific system as long as it’s applicable.
- Guidelines and regulations: Several bodies such as the ASTM, FDA, and others set regulatory measures that control the use of silicone. For instance, the FDA approves specific grades of silicone for the manufacture of silicone food products. You need to follow these guidelines for compliance purposes.
The above list of considerations is barely exhaustive. In any case, we have not talked about budgets and availability of materials all of which are equally crucial. All in all, basing your decisions on these factors is guaranteed to yield positive outcomes.
This comprehensive article has provided you with an in-depth look at the definitions, properties, benefits, drawbacks, and conditions that affect the performance of silicone elastomers. You now have a better understanding of this material and everything that it pertains. Contact us for more information regarding silicone elastomers.