Enhancing Silicone Rubber: A Guide to Additives for Improved Performance and Sustainability

Did you know that adding the right type of additives to silicone rubber can significantly improve its properties and performance? From plasticizers to reinforcements, colorants to flame retardants, there are a wide range of additives available that can enhance the versatility and functionality of silicone rubber.

In fact, according to industry experts, the use of additives in silicone rubber has become increasingly common in recent years, with the global market for silicone additives expected to grow at a CAGR of 6.2% from 2020 to 2025 (1). This is due in part to the unique properties of silicone rubber, including its resistance to high temperatures, chemicals, and weathering, as well as its flexibility and durability.

But what exactly are these additives, and how do they enhance the properties of silicone rubber? In this article, we’ll explore the various types of additives used in silicone rubber, their respective roles and benefits, and best practices for selecting and compounding them. Whether you’re a manufacturer or engineer working with silicone rubber or simply curious about the science behind this versatile material, we hope you’ll find this article informative and helpful.

Plasticizers

Plasticizers are additives that are commonly used in silicone rubber to improve its flexibility and durability. They work by reducing the glass transition temperature (Tg) of the material, which makes it more pliable and resistant to cracking or tearing.

Plasticizers are typically small, low-molecular-weight compounds that can be incorporated into the silicone rubber matrix through physical or chemical interactions. Commonly used plasticizers in silicone rubber include dimethyl phthalate (DMP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP).

How plasticizers can enhance the properties of silicone

In silicone rubber, plasticizers help to increase the material’s elongation at break and reduce its modulus, which improves its ability to withstand deformation and stress without rupturing. This is achieved by disrupting the packing and alignment of the polymer chains in the material, which allows them to move more freely and flexibly. Plasticizers can also improve the material’s low temperature performance by preventing it from becoming brittle or stiff in cold environments.

Negative effects & Countermeasures

However, it’s important to note that the use of plasticizers in silicone rubber can also have some drawbacks. Plasticizers can migrate out of the material over time, which can lead to issues with leaching or loss of properties. Some plasticizers can also have negative environmental impacts, such as being toxic or difficult to biodegrade.

To address these concerns, manufacturers may opt to use alternative plasticizers or to develop plasticizer-free formulations for silicone rubber. For example, recent studies have explored the use of bio-based plasticizers, such as epoxidized soybean oil (ESO), as a more sustainable and eco-friendly alternative to traditional phthalate-based plasticizers (2).

In summary, plasticizers are effective additives for improving the flexibility and low temperature performance of silicone rubber. However, careful consideration should be given to their selection and use to ensure optimal performance and minimize potential negative impacts.

Flame Retardants

Flame retardants are a type of additive commonly used in silicone rubber products to improve their safety and compliance. Flame retardants work by interfering with the combustion process, either by reducing the flammability of the material or by inhibiting the spread of flames. This is achieved through a variety of mechanisms, including releasing flame-quenching gases, forming a protective char layer, or diluting the combustible gases. Thereby, flame retardants are especially important for products used in high-risk environments such as aerospace, automotive, and electronics.

Safety and compliance of silicone rubber products

One of the key benefits of flame retardants is that they can help silicone rubber products meet stringent safety and compliance standards. For example, UL94 is a widely recognized test standard used to evaluate the flammability of plastic materials. By incorporating flame retardants, silicone rubber products can achieve higher UL94 ratings, indicating greater resistance to ignition and flame spread.

Development history and common types

The use of flame retardants in silicone rubber dates back several decades, with the earliest flame retardants being based on halogenated compounds such as brominated flame retardants (BFRs) and chlorinated paraffins. However, concerns about the potential toxicity and environmental impact of these compounds have led to the development of alternative flame retardants, such as organophosphates, nitrogen-based compounds, and metal hydrates.

Organophosphates, such as triphenylphosphate (TPP), work by releasing phosphorus-containing compounds that interfere with the combustion process at the gas phase. TPP is effective at reducing the flammability of silicone rubber and has excellent thermal stability. However, it can have limited effectiveness in suppressing flames and may release toxic byproducts when heated.

In contrast, nitrogen-based flame retardants, such as melamine cyanurate (MCA), form a protective char layer that insulates the material from the heat and oxygen. MCA has high thermal stability and does not release toxic byproducts, making it a safer and more eco-friendly option. However, it may not be as effective as other flame retardants in reducing flammability.

Metal hydrates, such as aluminum trihydrate (ATH), release water vapor when heated, which cools down the material and suppresses the flame. ATH is an effective and widely used flame retardant, with high thermal stability and low toxicity. However, it may not be as effective in reducing flammability as some other types of flame retardants.

Recent advances in flame retardant technology have focused on developing more eco-friendly and sustainable alternatives, such as bio-based flame retardants derived from renewable sources. These alternatives are designed to offer comparable or even superior performance to traditional flame retardants, while also minimizing their environmental impact.

Case studies

Case studies have demonstrated the effectiveness of flame retardants in silicone rubber products. For example, in a study published in the Journal of Applied Polymer Science, researchers evaluated the flammability of silicone rubber compounds with and without ATH. They found that the addition of ATH significantly improved the flame resistance of the material, reducing the peak heat release rate and total heat release by up to 70% (3). Another study published in Polymer Testing demonstrated the effectiveness of a silicone rubber coating containing a halogenated flame retardant in reducing the flame spread and smoke emission of polyurethane foam (4).

In conclusion, flame retardants are a valuable type of additive in silicone rubber products, improving their safety and compliance while still retaining the unique properties of silicone rubber.

Colorants

Colorants are additives that add color to silicone rubber products. They are commonly used in a variety of industries, including automotive, consumer goods, and medical devices.

the Role and principle of colorants

Colorants play an important role in silicone rubber products, providing both aesthetic and functional benefits. The primary function of colorants is to add color to the silicone rubber, allowing for a wide range of colors and visual effects. Colorants can also provide functional benefits, such as UV resistance or improved product identification.

The addition of colorants to silicone rubber is achieved through a process known as dispersion. During the mixing process, the colorant is added to the silicone rubber raw materials and mixed until it is evenly distributed throughout the compound. This ensures that the color is consistent throughout the final product.

In addition to the visual effects, colorants can also impact the physical and chemical properties of silicone rubber. For example, some colorants may improve the thermal stability or UV resistance of the material, while others may have an impact on the material’s chemical resistance or flexibility. The properties of the colorant itself, such as its particle size and chemical composition, can also impact the properties of the final product.

Types of colorants used in silicone rubber

There are several types of colorants used in silicone rubber, including pigments, dyes, and masterbatches. Pigments are finely ground powders that are dispersed in silicone rubber to create a solid color. Dyes, on the other hand, are liquid or powder additives that dissolve in silicone rubber to create a translucent or transparent color. Masterbatches are pre-dispersed pigments or dyes that are mixed with silicone rubber during compounding to create a specific color.

Methods of incorporating colorants into silicone rubber

Colorants can be incorporated into silicone rubber using a variety of methods. One common method is to mix color paste with silicone raw materials during compounding. The color paste is typically a pre-dispersed mixture of pigments or dyes in a carrier resin or oil. The paste is added to the silicone raw materials during the mixing process, and the resulting compound is then processed into the final product.

Another method of incorporating colorants into silicone rubber is to use masterbatches. Masterbatches are pre-dispersed pigments or dyes that are mixed with silicone rubber during compounding to create a specific color. Masterbatches offer several advantages over color paste, including better color consistency and improved processing characteristics.

The method used to incorporate colorants will depend on the specific application and the desired color effect. For example, injection molding is commonly used for high-volume production of small parts, while compression molding is better suited for larger, more complex parts. The mixing process should be carefully controlled to ensure that the color is evenly distributed throughout the silicone rubber compound. This will help to ensure color consistency between different parts or batches.

It’s important to note that different types of colorants may require different processing conditions. For example, some colorants may require higher processing temperatures or longer mixing times to achieve optimal dispersion. Manufacturers and engineers should work closely with their material suppliers to determine the best processing conditions for their specific colorant and application.

Case studies

One example of the use of colorants in silicone rubber products is in the healthcare industry, where color coding is often used to differentiate between different types of medical devices. For example, silicone rubber tubing used for enteral feeding is often color coded to ensure that the right type of tubing is used for each patient. Colorants can also be used in automotive and consumer goods applications to add decorative or functional color to products.

In addition to traditional colorants, there are also new developments in the field of silicone rubber coloration. For example, the use of nanoscale pigments and additives is becoming more popular, as they can provide more intense and vibrant colors while using less material.

Reinforcements

Reinforcements are a type of additive used in silicone rubber to enhance its mechanical properties. These additives are typically in the form of fibers or particles that are embedded in the silicone rubber matrix. When the silicone rubber is subjected to stress or strain, these reinforcements act as a framework to distribute the load and prevent deformation or failure.

Increased Strength and Stiffness

One of the main benefits of using reinforcements in silicone rubber is increased strength and stiffness. Research has shown that the addition of reinforcements can increase the tensile strength of silicone rubber by up to 400%, and increase its modulus of elasticity by up to 500% (5). Commonly used reinforcements include glass fibers, carbon fibers, and aramid fibers. Glass fibers are particularly popular due to their low cost and high strength-to-weight ratio. Carbon fibers are also popular, especially in applications where weight reduction is a key consideration. Aramid fibers, on the other hand, are known for their high tensile strength and resistance to abrasion.

Improved Mechanical Properties

In addition to strength and stiffness, reinforcements can also improve other mechanical properties of silicone rubber, such as fatigue resistance and impact resistance. For example, adding glass fibers to silicone rubber can increase its resistance to fatigue failure under cyclic loading, making it ideal for applications that involve repeated stress. In fact, the addition of glass fibers can increase the fatigue strength of silicone rubber by up to 50 times (6).

Precautions

It’s worth noting that the selection and compounding of reinforcements can have a significant impact on the properties of the final product. Factors to consider include the type and amount of reinforcement, as well as the processing conditions used to incorporate the reinforcement into the silicone rubber. For example, the orientation of the fibers can have a significant impact on the mechanical properties of the silicone rubber, so it’s important to carefully control the compounding process to ensure optimal orientation.

The use of reinforcements in silicone rubber can significantly enhance its mechanical properties and make it suitable for a wider range of applications. Whether you’re designing automotive seals, medical devices, or consumer products, the addition of reinforcements can help you achieve the strength and stiffness you need to succeed.

Fillers

Fillers are additives that are used in silicone rubber to enhance its mechanical, thermal, and electrical properties. They are typically solid materials that are mixed into the silicone rubber during compounding, and can provide benefits such as increased tensile strength, improved tear resistance, and enhanced compression set.

Fillers can be defined as solid materials that are added to silicone rubber to improve its properties. They are typically inert, meaning they do not react chemically with the silicone rubber matrix, and can be either organic or inorganic in nature.

How Fillers Can Enhance Properties of Silicone Rubber

The addition of fillers to silicone rubber can enhance its mechanical properties, such as increasing tensile strength and tear resistance, as well as improving its thermal and electrical conductivity. This is due to the fact that fillers can reinforce the silicone rubber matrix, creating a stronger and more durable material.

Commonly Used Fillers in Silicone Rubber

There are many types of fillers that can be used in silicone rubber, each with their own unique properties and benefits. Some of the most commonly used fillers include:

Silica – often used as a reinforcing filler to enhance mechanical properties

Fumed Silica – provides similar benefits to silica, but with smaller particle sizes for improved dispersion

Carbon Black – a conductive filler that can improve electrical properties

Glass Fibers – used as a reinforcing filler to improve mechanical properties

Alumina Trihydrate – a flame retardant filler that can improve safety in certain applications

Practical advice

To achieve optimal performance with fillers in silicone rubber, it is important to carefully select and compound the appropriate fillers for the specific application. The following are some advices to consider when working with fillers in silicone rubber:

Consider the desired properties: The type and amount of filler used will depend on the desired properties of the final product. For example, if the goal is to increase tear strength, a reinforcing filler such as silica or glass fibers may be used. If electrical conductivity is important, a conductive filler such as carbon black may be used.

Evaluate processing requirements: Different fillers can have different effects on processing, such as increasing viscosity or affecting curing time. It is important to select fillers that will not negatively impact processing, and to adjust processing parameters as necessary to accommodate the chosen filler.

Ensure proper dispersion: To achieve optimal properties, it is important to ensure that fillers are evenly dispersed throughout the silicone rubber matrix. This can be achieved through careful mixing and compounding, using appropriate equipment and techniques such as high-shear mixing or roll milling.

Control filler loading: Overloading with filler can lead to negative effects on processing and final properties, such as reduced elongation and increased brittleness. It is important to control the amount of filler used and to adjust as necessary to achieve the desired properties.

Consider regulatory and safety requirements: Some fillers may have regulatory or safety considerations, such as flame retardants or heavy metals. It is important to understand and comply with any relevant regulations and to choose fillers that meet necessary safety requirements.

Additive Selection and Compounding

Choosing the right additives for silicone rubber can be a complex process, as it involves a careful consideration of factors such as desired properties, manufacturing process, and regulatory compliance. Here are some best practices to keep in mind when selecting and compounding additives for silicone rubber:

Start with a clear understanding of the desired properties

Before selecting any additives, it’s important to have a clear understanding of the specific properties you want to enhance in the silicone rubber product. For example, if you want to improve its mechanical strength, you might consider using reinforcing fillers or fibers. If you want to make it more flexible, you might consider using plasticizers.

Consider the manufacturing process

Different additives may have different effects on the manufacturing process of silicone rubber products. For example, some fillers may require longer mixing times, or may affect the cure time of the material. It’s important to consider the manufacturing process when selecting and compounding additives, to ensure that the process remains efficient and cost-effective.

Ensure regulatory compliance

Many additives used in silicone rubber products are subject to regulatory requirements, such as restrictions on hazardous substances or flammability. It’s important to ensure that any additives selected for use in silicone rubber products comply with relevant regulations, to avoid any legal or safety issues down the line.

Work with a trusted supplier

When sourcing additives for silicone rubber, it’s important to work with a trusted supplier who can provide high-quality materials that meet your specific needs. Look for a supplier with a strong track record of providing reliable materials and technical support.

Conclusion

In conclusion, the use of additives in silicone rubber has proven to be an effective way to enhance the material’s properties, opening up new possibilities for its use across a wide range of industries. From plasticizers and flame retardants to colorants, reinforcements, and fillers, there are numerous additives available to manufacturers and engineers seeking to improve the performance and safety of their silicone rubber products.

As we look to the future, we can expect continued innovation and development in the field of silicone rubber and its additives. New technologies and materials are being developed to further enhance the properties of silicone rubber, and we can anticipate the material being used in increasingly diverse applications.

To capitalize on the benefits of additives in silicone rubber, we call on manufacturers and engineers to consider the use of these materials in their own products. By taking a strategic and thoughtful approach to additive selection and compounding, we can improve the performance and safety of silicone rubber products while minimizing their environmental impact.

Ultimately, by working together to develop and refine the use of additives in silicone rubber, we can create a more sustainable and innovative future for this versatile and valuable material.

References

• “Silicone Additives Market – Growth, Trends, COVID-19 Impact, and Forecasts (2020 – 2025)” – ResearchAndMarkets.com. Business Wire. December 22, 2020.
• L. Jiang et al., “Soybean oil-based polyol ester as a novel bio-based plasticizer for poly(vinyl chloride),” Journal of Applied Polymer Science, vol. 136, no. 16, 2019.
• Pavan, A. K., et al. “Effect of Aluminum Trihydrate on Fire Retardancy and Physical Properties of Liquid Silicone Rubber.” Journal of Applied Polymer Science, vol. 131, no. 15, 2014.
• Hu, Y., et al. “Synergistic effect of intumescent flame retardant and silicone rubber coating on fire performance of polyurethane foam.” Polymer Testing, vol. 87, 2020.
• Jancovicova, V., and Simonikova, Z. (2019). The influence of reinforcement on mechanical properties of silicone rubber. Materials Science Forum, 948, 260-265.
• Sengupta, S., and Bandyopadhyay, S. (2015). Effect of Glass Fiber Reinforcement on Mechanical Properties of Silicone Rubber. Materials Science Forum, 830, 13-20.