Silicone vs Rubber: Comparing the Common Properties of Natural and Synthetic Elastomers

Some polymeric materials share similar traits. Still, they have unique properties that define them. Silicone and rubber are examples of these polymers. Both exhibit similarities since they are elastomers. However, they are different at the molecular level and have specific applications. This article highlights these differences and similarities.

Silicone and Rubber as Elastomeric Materials

In polymer science, substances are classified based on factors like molecular forces, sources, mode of synthesis, etc. Molecular forces are widely used to classify polymers. This classification gives us elastomers, among other types of polymers

Generally, the term elastomer is used to describe viscoelastic polymers. These are polymers that can stretch and return to their form without deformation. Elastomers have intermolecular cross-linkages that hold the monomer bonds together. These bonds are weak to allow the elastomer to stretch when pulled.

Elastomers are either natural or synthetic. Whereas rubber is considered a perfect example of a natural elastomer, silicone is a great example of a synthetic elastomer. As you will see throughout the article, the two have many similarities and differences, when we consider their properties.

What is Silicone?

The chemical term for silicone is polysiloxane. Meaning it is a polymer made of repeating units of siloxanes. These siloxanes are synthetic compounds made up of silicone and oxygen elements. Therefore, you can say that silicone is a synthetic polymer made up of silicone-oxygen compounds. 

What we described here is the base silicone material. Meaning, there are many variations of silicone depending on synthesis methods and added elements. For instance, there are several grades of silicone manufactured to sustain specific conditions.

How is silicone synthesized?

Silica sand is the main ingredient in the synthesis of silicone. Using various processes, the silicon compound is extracted from this sand. 

Silicon undergoes other series of reactions including the addition of methyl chloride, distillation, hydration, and finally polymerization. There are standard steps in this process, but they mainly depend on the manufacturer. The final step is polymerization which yields polysiloxane — the silicone polymer.

Manufactured silicone will undergo vulcanization processes to improve its elasticity at given temperatures. Also, this last step involves grading silicone and preparing it for the milling of silicone-based products.

What is rubber?

 

For this article, rubber is used to mean natural rubber. It is crucial to specify this to clarify the difference. This is because silicone is considered a type of rubber.

Broadly, rubber is an elastic material that has many desirable properties. It exists naturally in the latex extract from various trees. Historically, it was first discovered in Hevea brasiliensis, an indigenous Brazilian tree. 

The natural rubber polymer is made up of repeating units of isoprene monomers. This is why natural rubber is also known as polyisoprene. 

How latex is converted into rubber

The process of making natural rubber begins after the latex suspension is tapped from the bark of Hevea brasiliensis. The milky white aqueous substance is cleaned and acids are added to it to help coagulate the rubber particles present in the latex. This initial process might vary depending on the production standards followed.

The coagulated rubber undergoes mastication to improve its elasticity. Certain chemicals are also added to further enhance the inherent properties. However, the end product in this step is still in an unstable condition. 

The final step is vulcanization. This step involves treating the calendared rubber with sulfur and heat to harden it. Vulcanization is necessary to ensure the rubber is strong enough for applications that require hard rubber. However, this process does not affect the elasticity of natural rubber.

Silicone vs Rubber: Comparison of Common Properties

The best way to understand the difference between silicone and rubber is to compare the common properties. In other words, there are features that both elastomers share, but each has a degree of the property. This will become clearer when we start comparing the similarities and differences. 

Elasticity

Elastomers are known to be viscoelastic. In other words, they are able to stretch when pulled. Let us see how elastic silicone is compared to rubber.

Silicone

Silicone is quite elastic and stretchy. The helical structure of the silicone molecules contributes to this high elasticity. The cross-linking of the monomers forms a conical or cylindrical shape. Put simply, the elasticity of silicone is rooted in its molecular structure. 

Silicone retains its elasticity in both low and high temperatures. Therefore, this widens the range of high-stress applications suitable for silicone. 

The elasticity of silicone is measured using a test known as the modulus of elasticity or Young’s Modulus (E). The modulus is given in Megapascals (MPa) and the values range depending on the test conditions and the grade of silicone used. 

It is worth pointing out that the elasticity of rubber, like most of its properties, can be enhanced during its synthesis. Therefore, you can get custom elasticity if you intend to use silicone in high-stress environments.

Rubber

The primary reason why elastomers like natural rubber are elastic is because their polymeric structure is made up of long chains of monomers. Also, the crosslinks made by the vulcanizing agents also contribute to the elasticity.

Rubber is highly elastic and does not deform in shape both physically and at the molecular level when stretched. This mechanical property of rubber makes it quite a desirable material for use in situations where movement is inevitable.

The elasticity of rubber is observed when the material is subjected to an external force. Such instances force the material to stretch to a considerable length and deform or increase in length. When the force is released, the material retreats to its initial state without any damage.

Rubber has a stretch limit which is measured by elongation at break. This represents the percentage increase in length that rubber can sustain before it snaps. Elongation at break is dependent on factors like tensile and tear strength. Particularly, the elongation at the break of rubber is 700%.

Differences

The major difference between the elasticity of silicone and rubber is that rubber is more elastic and stretchy than silicone, in their standard configurations. If the molecular composition of silicone is altered chemically, the elongation at break can increase dramatically, making silicone more elastic.

Other differences between these two elastomers in relation to elasticity include;

  • Silicones are stiffer when stretched compared to natural rubber.
  • The elasticity of silicone can be influenced by temperature changes while rubber maintains its stretchability in all conditions.
  • Natural rubber regains its original physical form better and faster after a stretch when compared to silicone.
  • The elasticity of silicone cannot be affected by chemicals natural rubber can be affected significantly depending on the chemicals.

Similarities

Silicone and rubber have sufficient elasticity to sustain tension. Additionally, their elasticity is measured using the same standards since both are considered viscoelastic. This is why they have varying Young’s modulus values. There are not many similarities because of the differences in molecular composition and method of synthesis.

Durability

Durability is the measure of the time it takes for either rubber or silicone to lose its major properties. These elastomers exhibit differences and similarities in durability. 

Silicone 

One of the defining features of silicone is its ability to endure harsh conditions without losing its resistance, for extended periods. Especially, a study revealed that silicone can withstand temperatures up to 200°C for over ten thousand hours without melting. This explains why silicone is used in the automotive and aerospace industries.

The durability of silicone is also experienced in how silicone products require minimal maintenance. This property results from innovative engineering that is applied during the synthesis of this elastomer.

Researchers spend resources to fine-tune the chemical and molecular nature of silicone and make it endure harsh conditions. Moreover, the base ingredient of silicone, silica, is naturally durable, which makes the end product equally durable.

Rubber

Natural rubber has limited durability in certain conditions. High temperature, for instance, melts the rubber after some time. The highest temperatures natural rubber can withstand without deformation is around 80°C, for extended use. 

This elastomer also will undergo degradation quickly when exposed to conditions with corrosive chemicals for extended periods. Other degrading factors include high tension, which accelerates the wear and tear of the rubber.

Mainly, natural rubber cannot withstand these conditions since it is biodegradable and organic. In other words, its chemicals that are non-synthetic or inorganic. Also, their attributes cannot be altered artificially. This means rubber is stuck with the natural characteristics of its components.

Differences

Evidently, silicone is highly durable compared to natural rubber. The silicon elements in silicon give this substance superior qualities. When comparing them in terms of tensile strength, rubber scores higher than silicone. The low hardness and high elasticity of rubber contribute greatly to its high tensile strength.

Similarities

Fundamentally, both elastomers are considered durable. They are both durable to a certain extent. Silicone can sustain most conditions, while rubber is limited to specific environmental conditions.

Temperature resistance

Temperature impacts elastomers differently based on their molecular composition. In this regard, both silicone and rubber experience temperature in different ways. 

Silicone 

Temperature resistance is exactly what it sounds like — the ability to retain form and nature under temperature extremes. 

Silicone resists temperature at all degrees — both low and high. When looking at the resistance to the temperature of silicone, we need to consider two factors. One, there is the impact of heat on the material. Secondly, we should consider the thermal conductivity or heat transfer coefficient.

This material retains its form — both physically and chemically — under room, low, and high temperatures. 

Silicone is also an excellent heat insulator, meaning it does not transfer heat between objects. This property makes it the ideal material for the manufacture of electronic products.

Rubber

The temperature resistance of natural rubber is informed by what’s called glass transition resistance (Tg). Tg is the point at which natural rubber transitions from hard to brittle when exposed to heat. 

Since the temperature resistance of natural rubber is low, its Tg is usually quite low as well, with estimates of -70°C. The inability of rubber to resist temperature is a limiting factor to its range of applications. In other words, rubber is not suitable for applications where extreme temperatures are anticipated.

Differences

Clearly, silicone and rubber have varying degrees of temperature resistance. Silicone has the molecular composition required to withstand heat while rubber has limitations. Also, silicone has better thermal stability compared to rubber. Basically, silicone outperforms rubber in terms of temperature resistance.

Similarities 

Both elastomers share the privilege to have wide ranges of temperature. This means that they have a range of temperature values that they can withstand. Temperature resistance is an essential and desirable property for elastomers.

Chemical resistance

Chemical resistance is the opposite of chemical reactivity. It represents the ability of materials to resist corrosion and abrasion of chemicals. Elastomers have varying degrees of resistance. It is determined by factors.

Silicone 

The stability of silicone under exposure to chemicals depends on factors like period, type of chemical, and its reactive level. Chemicals exist in various compounds in nature. Acids, for instance, are corrosive chemical compounds that silicones are exposed to.

Generally, silicone goes unaffected by most acids. This property is also known as chemical inertness. It is resistant to oxidation and dissolution caused by chemicals found in acids and other sources.

However, silicone is not fully inert. In other words, it can be affected by certain chemicals over extended periods of exposure. Particularly, synthetic chemicals or highly reactive acids such as sulphuric acids can corrode the outer surface of silicones. Luckily, silicone hybrids like fluorosilicone have improved chemical resistance.

Rubber

There is a huge number of known chemicals. As such, determining the resistance of rubber to these chemicals is challenging. Some resources like this resistance chart show you the level of resistance of natural rubber.

Natural rubber dissolves when it comes into contact with naphtha and turpentine solutions. Benzene can also damage rubber to a great extent if exposed for extended periods. This chemical leads to swelling, discoloration, and cracking of rubber, all of which affect its mechanical properties.

Differences

The level of resistance to chemicals varies between the two elastomers. As you would expect, synthetic rubbers are reinforced chemically to make them more resistant. This ensures that silicone products can perform optimally in environments with high levels of corrosive and reactive chemical compounds.

Similarities 

Both elastomers resist chemical disturbance. They can withstand chemicals without complete loss of rigidity.

The chemical resistance of silicone and rubber is influenced by their molecular composition and structure. Also, the types of intermolecular bonds holding the monomer units account for the resistance. 

Tensile strength

In the context of elastomers, tensile strength is defined as the amount of stress required to break the material when stretched. This property is important since it tells you how much force silicone or rubber can sustain.

Silicone 

There are many types of silicone and each has its own tensile strength. This force is given in Megapascals (MPa) and it influences other properties such as tear strength and elongation at break. These properties have a minimum and maximum value depending on the type of silicone tested.

HTV and LSR are commonly used silicones. Generally, LSR has better tensile strength than HTV. This is primarily because of the low compression set — a parameter used to inform how fast silicone recovers after its compression force is exerted on it. Silicone also has a high durometer (hardness) compared to other silicones.

Rubber

Like most elastomers, the tensile strength is measured after it has been vulcanized, or cured. Rubber has relatively good tensile strength and it can withstand tension to a certain extent. 

Here are some specific values related to the tensile strength of natural rubber.

 

Property Values
Durometer hardness range 30 – 90 Shore A 
Range of tensile strength  500 – 3600 PSI
Elongation at break (%) 300 – 900%

 

If the rubber is meant to be used in low-tension conditions, these values are reliable. This means the material can sustain compression and tension forces without snapping or breaking.

Differences

Rubber has a better tensile strength than silicone. This is because its molecular structure is denser compared to that of silicone. The cross-links of rubber are made of strong intermolecular bonds that reinforce its durometer. Silicone resists breakage when stretched, but not as compared to rubber.

Similarities 

There are not many similarities between silicone and rubber in regard to tensile strength and resistance. Nonetheless, both elastomers are quite stretchy and flexible. This property makes them endure tension.

Rebound resilience

silicone

Resilience informs the speed of recovery that elastomers exhibit after force has been applied to them. Technically, this property is expressed as a ratio of energy exerted to energy dissipated after a deformation has occurred due to force.

Silicone

Considering the rebound resilience of silicone is a smart move. It will help you decide if the material is suitable for the intended applications. 

This property is influenced by factors like polymer specifications including the weight and the ratio of monomers used.

Silicone has a low rebound resilience. This is because silicone crosslinks affect the free movement of molecules in the structure. This limitation causes a slower rate of recovery from the impact caused by force. However, this property can be improved by adjusting the molecular composition of silicone.

Rubber

The polymeric structure of natural rubber is mostly linear. Therefore, this gives the molecules enough freedom to move around. Resultantly, this movement contributes to a faster rate of recovery. In other words, rubber has excellent rebound resilience.

Differences

The main difference is that natural rubber has more rebound resilience than silicone. It recovers faster due to its linear molecular structure. Silicone’s non-linear molecular structure limits the movement of molecules, which hinders the speed of recovery after deformation.

Similarities 

Silicone and rubber have the ability to regain form after deformation. In other words, they are both resilient. Also, this property is measured in a similar manner. The test is physical with specialized tools. Internationally, the tests are described by ISO 4662:2009.

Aging and weather ability

Scientists have conducted various tests such as irradiation to test the aging of synthetic and natural elastomers. The tests have revealed interesting results that can tell us how silicone and rubber ages.

Silicone 

This study showed that silicone can sustain radiation for 1000 hours and its surface would remain unchanged. However, the surface became considerably porous, but not much to affect its function.

As a synthetic elastomer, silicone has near-perfect molecular composition to prevent weathering under radiation and other external stressors. Temperature is another aging accelerator of elastomers. Provided that silicone can sustain high and low-temperature conditions, it is mildly aged by temperature.

Rubber

Rubber ages quicker than silicone. It also weathers at a faster rate. Aging can be defined in many ways. For rubber, hardening after long exposure to sunlight counts as aging. 

Some studies revealed that rubber hardens when exposed to sun rays for extended periods. This hardening leads to a decrease in elongation at break, thereby causing the material to become brittle and less stretchy.

The discoloration is another aging feature that is observed in rubber. Essentially, most environmental factors affect the structural integrity of natural rubber.

Differences

Silicone ages less than natural rubber. The effect of radiation, for instance, is much more significant on rubber than silicone. This is because, unlike rubber, silicone’s inorganic elements like silicon are resistant to the effects of radiation.

Similarities 

The main similarity between silicone and rubber is that both have a lifespan. It happens that silicone has a longer lifespan than rubber due to its superior properties. In their standard formulations, they are affected by too much exposure to infrared radiation.

Silicone vs Rubber: Advantages and Disadvantages

In what ways is rubber or silicone beneficial or disadvantageous? This is a question you need to ask yourself if you are considering these materials for your business. Let us tabulate these pros and cons based on the properties of the elastomers.

Comparing the advantages and disadvantages of silicone vs rubber 

Advantages Disadvantages
Silicone – High temperature resistance

– High chemical resistance 

– High water resistance

– Able to retain shape after deformity

– Long lifespan 

– Requires minimal maintenance 

– High thermal stability

– High electrical resistance 

– Relatively costly

– Low tensile, tear, and elongation

Rubber – High elongation at break at 700%

– High rebound resistance

– Excellent tensile strength 

– Biodegradable 

– Cost-effective

– Limited availability

– Ages fast 

– Low chemical inertness 

– Low resistance to high heat

– Prone to ozone degradation

Which material is right for you?

Choosing between silicone or rubber could prove challenging. This is because both have their strengths and weaknesses. A more straightforward approach could be evaluating their pros and cons based on the intended application.

For instance, if the environment of use requires high heat tolerance, then silicone would be best suited for this scenario. In most cases, you will have to consider the properties as the base factors. 

Other equally important factors are the use cases. Some industries benefit better from less-toxic materials. For example, if you are making kitchenware, you should consider silicone products since they are less toxic. Additionally, silicone’s thermal stability is useful in this application.

Conclusion

This comprehensive article has outlined how silicone and natural rubber compare against each other in terms of their properties. It revealed that both elastomers share differences and similarities. Above all, each has its strengths and weaknesses. If anything, the specific elastomer is defined by its unique properties.

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