Silicone and epoxy are interesting compounds. They exhibit somewhat similar characteristics but they are quite different. Both compounds are classified as thermosetting polymers. Thermosets are compounds whose molecular structure cannot be reversed after they have been cured. This article explores this concept and more to bring out the comparison between silicone and epoxy.
Introduction to Thermosetting Polymers
Generally, there are three types of polymers;
- Elastomers – these are polymers that can deform to a large degree when stretched without breaking. Also, they have a high tolerance to heat. All rubbers are types of elastomers.
- Thermosetting plastics or thermoplastics – These polymers burn but do not melt easily when exposed to high temperatures. They are also brittle and have limited elasticity.
- Thermosetting polymers – These types of polymers are cured by either heat or special vulcanization systems. They are amorphous in nature.
Thermosetting polymers are made by hardening a viscous liquid in the presence of heat and additives to form a rigid but flexible compound. This process of curing is irreversible unless special mechanisms are applied.
The starting material in the curing process of epoxy thermosets is known as a resin, a type of prepolymer compound with certain properties. Epoxy is a great example of a thermoset that is made from a prepolymer resin.
On the other hand, silicone is technically classified as an elastomer. Moreover, some of its properties make it a thermoset as well. This is seen in some forms of the compound as we will discuss later.
Most thermosets are copolymers. This means that they have a base monomer which acts as the reactant and a secondary comonomer which helps the formation of the crosslinks between or among the monomers. These crosslinks are what give the polymer its properties. If you are confused by all this, keep reading and you will understand these compounds better.
Overview of Epoxies and Silicones
Epoxies and silicones are some of the most common thermosetting polymers. Let us define them and explore their synthesis. Doing this will help us create a basis for the comparison of the properties and characteristics of the two compounds.
Epoxy: What is epoxy?
Epoxy is a compound classified as a synthetic resin, produced from two basic petrochemicals namely Epichlorohydrin and Bisphenol-A. Two words that stand out here are synthetic and resin. The former means that it is artificially made and the latter means that it is a mixture or solution of organic compounds.
Resins can also be obtained from plants. For instance, Amber is a common plant resin found in certain plants. This type of resin is used for its fragrance qualities. This should tell you that a resin by itself has limited properties. It is until other additives such as a hardener give it properties such as strength.
The two petrochemicals used to make synthetic resin, Epichlorohydrin, and Bisphenol-A are obtained from crude oil. That said, they could have negative implications on human life. Besides, there have been health and safety concerns about Bisphenol-A.
Epichlorohydrin is the base compound in the epoxy resin. This petrochemical is made from propylene, a common hydrocarbon found in petroleum. Propylene is also used to manufacture some types of synthetic rubbers.
Bisphenol-A or BPA for short, is the secondary hydrocarbon in epoxy resin. This petrochemical is made by reacting phenol and acetone.
When these two hydrocarbons are combined, they form a flowy substance with a low viscosity (flow rate is low). Additionally, the substance lacks several properties at this stage. This is because it has not undergone curing which is facilitated by a hardener. These hardeners initiate the reaction that transforms resin into an adhesive or coating material.
Silicone: What is silicone?
The scientific name of silicone is polysiloxane. This name implies that silicone is a polymer made from siloxane molecules.
Silicone is among the most common synthetic elastomers in the market. Its excellent heat resistance and strength properties have positioned it at the helm of almost every manufacturing line. However, what makes it so special is not its properties but its availability.
You see, silicon, the primary element in the siloxane compound, is almost inexhaustible in the world. This is because this element is found in almost all types of rocks. The only limitation of the availability of silicone is the ability to extract silicon from these rocks.
However, silicone is not only made from silicon. The monomers, siloxanes, are comprised of oxygen and hydrogen atoms. It is worth noting that the absence of carbon atoms in this molecular structure makes silicone an inorganic polymer. The upside to this is that silicone ends up having high thermal stability, flame retardancy, and low-temperature flexibility.
Silicone is used in harsh conditions since it can sustain stressors such as corrosive elements, high heat, and tensile strength. Additionally, there are speciality grades of silicone that can handle more challenging environments.
Epoxy vs Silicone: Material Synthesis
The synthesis of silicone and epoxy differs quite a bit. This is because the end product has distinct properties and its uses are different as well. All the same, both undergo polymerization of a base monomer with the addition of certain agents to promote curing and cross-linking. Let us explore more about how these thermosets are made.
How Epoxy Is Made
Epoxy is made either industrially or in small portions through DIY methods. For DIY (do it yourself), the maker buys the resin and hardener separately, mixes them together, and leaves the solution to cure and form a hard thermoset. In this approach, they can also add dye to change the color from transparent to their color of choice.
The industrial approach is basically bent on creating these resins from the base chemicals. Since epoxy cures fast and the reaction is irreversible, a lot of caution is exercised. Particularly, they ensure that the resin and hardener are not in contact before the curing process.
Compounds That Make Epoxy
When defining epoxy, we stated that it is made from resin monomers, hardeners, and secondary agents that initiate and accelerate the curing/hardening process. In fact, the term epoxy is used to describe the hardened compound.
- Resin monomer. Technically, Epichlorohydrin and BPA are not monomers in the actual sense. However, in the context of epoxy resin synthesis, which is a polymerization reaction, the two can be termed monomers.
- Hardener. The curing process of epoxy resin is facilitated by several types of hardeners, with the most common ones being polyamides, amines, and anhydrides. The choice of the hardener to use will depend on several factors such as the cure period, surrounding conditions, and the nature of the project.
- Accelerator. An accelerator does exactly what it implies — it speeds up the curing process of the epoxy resin.
The compounds used to make epoxy can vary depending on several factors. In most cases, they are dictated by the manufacturer, who considers other factors such as cost, intended use, and other important considerations.
How Silicone is Made
Silicone is mainly produced by the polymerization of siloxane monomers. These monomers are obtained from the hydrolysis of silicon chlorides. Hydrosis introduces the hydrogen and oxygen atoms to the compound.
Polymerization of Siloxane Monomers
The specific polymerization technique used for siloxane is called ring-opening polymerization (ROP). This is a special type of polymerization that results in polymers with high molecular weight. This is essential and it is what gives silicone excellent properties as an elastomer.
During this reaction, additives such as modifiers, crosslinkers, and fillers are added. Each additive has a specific function in the whole reaction. For instance, fillers are added to reinforce the properties of silicone. This means that in latex form, most of the properties of silicone are not advanced enough. Fillers such as silica are used to reinforce these properties.
Epoxy vs Silicone: Properties
Another way of comparing epoxy and silicone is by looking at their individual properties. Comparing them directly would not yield the best comparison since the materials are made for different purposes. However, some of these properties overlap.
Properties of Silicone
Regular silicone has several properties. By regular, we mean the general grade or variation of the material. This is because there are speciality grades and those have quite specific properties. All in all, the general properties will be found in these specialty grades, with better or lesser values.
Elastic and Flexible
This is arguably the highlight of every elastomer. If anything, the definition of an elastomer is a material that has elastic properties. Tests have revealed that its elasticity region can go as high as 1000%.
The elasticity of silicone is quite high. What this means is that the compound can be stretched to almost thrice its original size without showing any signs of deformation. When something is said to be elastic, it means that it can stretch and return to its original shape or size without losing any physical properties.
One of the reasons why silicone has a high elasticity and flexibility is because its molecular structure is helical. This means that the molecular structure of the monomers forms a spiral or helix shape. This contributes to a high degree of extension when pull force is applied.
The other reason for silicone’s high elasticity is because of the weak intermolecular forces of the molecular bonds. While they are weak, this does not mean that the material has a low tension or tear strength.
High Resistance to Mineral Oils
Here is another reason why silicone is highly praised. Generally, the compound is insoluble in virtually all oily substances. One reason for this occurrence is that the carbon atoms in the silicone compound hinder the dielectric properties of mineral oil. This means that carbon worsens the innate ability of oil to resist electrical charge.
Resistance to mineral oils is essential. It makes silicone a reliable material for manufacturing components used in places where these oils are in plenty. Also, it means that silicone can have a longer lifespan hence providing more value for you.
Excellent Weather Ability
When a substance is said to have a high weatherability, it means that it can sustain UV and ozone without becoming obsolete. It is a property that is present in most rubber compounds. However, the level of resistance varies from one compound to another.
It is believed that silicone displays exceptional weather ability since it is inorganic. In other words, organic compounds have the best weatherability properties.
Weatherability is one of the properties that affect the lifespan of a compound. In this case, you can expect the lifespan of silicone to be high judging from its excellent weatherability. In fact, silicone can last up to 20 years without showing any signs of physical damage.
High Thermal Stability
Thermal stability is defined as the ability of a substance to remain unchanged when exposed to high temperatures. Also, the material has to do this while maintaining all its mechanical properties like strength, compression set, and elasticity.
Silicone has an operating temperature of 50°C up to +250°C. This means that silicone can remain unchanged in extremely low and high temperatures. This is an important property since it makes silicone quite reliable in those low or high-heat conditions of the environment.
Lower Adhesive Strength Than Epoxy
For the sake of comparison, we can compare the adhesive properties of silicone to epoxy. Adhesion is the binding force a certain material provides between two surfaces. This is a layman’s description of the term.
In its liquid state, silicone can be used as an adhesive. This type of silicone, otherwise known as liquid silicone rubber (LSR) can be used for sealing, coating, and other adhesive functions. It tends to cure like epoxy but with a weaker bond. As such, it is considered a weaker adhesive than epoxy resin.
Properties of Epoxy
If you have interacted with epoxy, you must have noticed that it is an excellent adhesive. If anything, adhesion is the primary use of this compound. For this reason, it has quite specific properties, but also some that are related to silicone.
Strong Adhesive Properties
In manufacturing processes such as rubber-to-metal bonding, one of the common adhesives used to bind the substrates is epoxy. Provided that top-level adhesive properties are required for this process, the adoption of epoxy tells you how strong it is.
Epoxy is also used to form coats for covering cracks and unwanted openings. In such applications, adhesive properties are also required. Moreover, it does not matter the material that the epoxy is bonded with. This is because epoxy can bind with several types of material, regardless of its strength, surface properties, or anything.
The amount of size that epoxy loses after curing is about 1 – 5 % of the original size. This is a negligible amount of loss. In fact, this is the lowest volume loss that any polymeric adhesives in the industry. As such, it means that epoxy is generally better than silicone.
Shrinkage rates of epoxy resins vary depending on the specific formulation. However, generally, most epoxies have low shrinkage. What differentiates them is how low it is.
Like silicone, epoxy has a great deal of resistance to chemicals as well. This means that it can withstand corrosive chemical compounds such as concentrated acids and bases. Similarly, this property is good because it increases the scope of applications of epoxy. In other words, just like silicone, epoxy can be used in harsh conditions.
Although we are comparing this property between two materials with dissimilar molecular compositions, you can attest that both have what it takes. Chemical resistance is really important for compounds that manufacture outdoor components.
High Tensile Strength
In its cured form, epoxy is as strong as top-grade plastics. It goes as far as to resist tension forces. Actually, if it didn’t, it would not be considered a good adhesive. This force is strong enough to sustain mechanical stress.
Epoxy as an adhesive can last a minimum of 20 years without losing its adhesive properties. A duration that long is enough to have had proper service from the bonded substrates. Also, before this period lapses, you will have performed several maintenance practices to keep the adhesive bond tighter.
For an industrial adhesive such as epoxy, a long lifespan is non-negotiable. It makes the material more reliable in several conditions. Also, a long lifespan raises the value of the material, making it more desirable for commercial applications.
Epoxy vs Silicone: Comparing Similarities and Differences
Silicone and epoxy share some similarities. Whether it’s the functionality, the properties, or something else, these compounds have something in common. Similarly, there are differences that show the fine line between these materials. It is what puts them in a separate category of compounds. Let us explore some of these similarities and differences.
Similarities Between epoxy and silicone
Some of the common similarities between general grades of epoxy and silicone include;
- Both can be used as adhesives. Silicone in its liquid form is used as a sealant adhesive. In this specification, silicone is also known as silicone glue. Epoxy is largely used in the same manner. This shared use case is due to the adhesive properties of both compounds.
- Both have high thermal stability. A good adhesive or sealant needs to have high thermal stability. This is because of the usage conditions that the material can be used in.
- Both are non-toxic in cured form. While the derivatives of epoxy come from crude oil and natural gas, the final product is usually non-toxic. This is the same case for silicone. Non-toxic materials are preferred since they are safe for the environment and they appeal more to the masses.
- Both share some properties. As you saw when we were discussing the properties of both silicone and epoxy, the two share some properties. For instance, they both have a high temperature tolerance. The thing to note is that the values and extent of these properties are quite different due to the chemical differences of the materials.
Differences between epoxy and silicone
When it comes to differences, you can expect a lot of them between silicone and epoxy. For one, the two compounds are miles apart in terms of chemical makeup. This alone makes them have quite a number of differences.
Some of the major differences between the two are;
- Processing. The synthesis or processing of the two materials is quite different. For one, epoxy can be made using DIY methods using a few ingredients. This is not the same case for silicone which has to be manufactured by a rubber expert to achieve the desired properties.
- Yellowing resistance. Most thermosets experience a phenomenon called yellowing, which is generally caused by degradation. The ability to resist this occurrence is what is known as yellowing resistance, and silicone does a better job at it than epoxy.
- Curing methods. When it comes to curing, silicone and epoxy undergo different techniques to get into a cured state. Specifically, epoxy cures through a chemical reaction while silicone cures through heat, RTV, and other applicable methods.
Epoxy vs silicone: Applications
The last aspect that we can use to compare epoxy vs silicone is the use cases of both materials. You will find that there are similarities in this aspect as well as differences.
Applications of Epoxy
Epoxy is used in a wide range of ways, including;
The term caulking is used to describe methods and techniques used to waterproof something. As such, a caulking compound is a substance used to cover holes, cracks, and openings to make a roof or surface waterproof.
Acrylic Caulk is the most common type of epoxy caulking compound used for this application of the material. This compound forms a strong bond with the substrate and covers any openings that might facilitate water leaks. This epoxy can stick to metal, concrete, plastic, wood, or any other type of building material.
Laminating Composite Materials
Composite materials are substances made by combining two different compounds. A metal-rubber component is an example of a composite. Epoxy is usually used to make composite materials due to its surface properties. It can be applied to metals or other materials and left to cure and form a composite.
Creating Decorative Objects
You have probably stumbled upon DIY epoxy projects on the internet. If not, there is a category of people who mix epoxy resin with dyes to make furniture and all kinds of decorative objects. These projects turn out great especially when epoxy is used to create a tabletop.
Major Applications of Silicones
Silicone has very specific use cases. However, most of its applications resemble those of most rubber compounds. The main difference is that silicone is used in harsher conditions since its properties can withstand these conditions. Some of these applications include;
Silicone, just like epoxy, can be used as a sealant. However, as we have repeatedly said, there is a special type of silicone, i.e. LSR, that is used for this application. LSR is not as effective as epoxy, but it serves the purpose just fine. Therefore, it makes the case for alternative and effective sealants.
There is a high chance that you have handled kitchenware made from silicone. These could include oven mitts, which are quite common. Silicone is used to make kitchenware for its high-temperature tolerance. Silicone products like oven mitts and bakeware can withstand elevated temperatures for extended periods without any damage.
Medical and Food-grade Equipment
Silicone can also be formulated in accordance with FDA guidelines so that it can be used to make food and medical equipment. These guidelines are mostly bent on ensuring that the compound is free of toxins. This is an important and mandatory requirement for any material used to make products that are meant to be used by human beings.
Manufacturing seals, o-rings, and gaskets
In the automotive and aerospace industries, there are many seals, gaskets, and o-rings required in the vehicle body. These components are required to possess strength, resistance to compression set, chemicals, and overall durability. Silicone is a material of choice for these components since it has consistently demonstrated excellent properties.
Epoxy and silicone are common compounds with reliable properties. These characteristics make them useful in various applications. We have seen that, as thermosetting polymers, the two compounds have similarities and differences. Therefore, if you are considering either of the two materials, this information will help you form an informed decision.