Dicumyl Peroxide (DCP) as a Flame Retardant Synergist in EPS

Expanded polystyrene (EPS) is widely used in thermal insulation, packaging and construction applications. Because polystyrene is inherently combustible, flame-retardant systems are commonly incorporated into EPS formulations where improved fire performance is required.

In addition to the main brominated flame retardant, some EPS formulations use Dicumyl Peroxide (DCP) as a flame-retardant synergist. DCP does not replace the primary flame retardant. Instead, it can be used together with brominated flame retardants such as FR-130 or Brominated SBS to improve the overall efficiency of the flame-retardant system.

What Is Dicumyl Peroxide?

Dicumyl Peroxide, commonly abbreviated as DCP, is an organic peroxide with CAS No. 80-43-3.

DCP is widely known as a crosslinking and radical-generating agent in polymer processing. In EPS applications, it may also be used as a flame-retardant synergist together with brominated flame retardants.

Its role is different from that of the primary flame retardant. The brominated flame retardant provides the main flame-retardant effect, while DCP can support the system through radical-generation mechanisms during combustion.

For product specifications, see our Dicumyl Peroxide (DCP) – ST-DCP product page.

Why Is DCP Used in EPS Flame-Retardant Systems?

A flame-retardant synergist is an additive that helps improve the effectiveness of the main flame retardant.

In an EPS formulation, DCP may be used together with brominated flame retardants to enhance flame-retardant performance without acting as the main bromine source.

The main reasons DCP may be included are:

• To improve the efficiency of a brominated flame-retardant system
• To support radical reactions during combustion
• To help optimize the overall additive package
• To work together with flame retardants such as FR-130 or Brominated SBS

The exact effect depends on the formulation, resin system, additive dosage and required fire-performance standard.

DCP with FR-130

FR-130 is a brominated flame retardant commonly used in EPS and XPS applications.

In some EPS formulations, DCP can be used together with FR-130 as part of a combined flame-retardant system.

FR-130 provides the primary brominated flame-retardant function, while DCP acts as a synergist. This combination can help manufacturers optimize flame-retardant efficiency depending on their production process and target performance.

For detailed product information, see our FR-130 Flame Retardant – ST-130 page.

DCP with Brominated SBS

Brominated SBS is a polymeric brominated flame retardant used in EPS and XPS.

Compared with smaller-molecule brominated additives, Brominated SBS offers a high-molecular-weight polymeric structure, good thermal stability and lower migration tendency.

DCP may also be used as a synergist in formulations containing Brominated SBS, depending on the required flame-retardant performance and processing conditions.

For technical specifications, see our Brominated SBS – ST-970 page.

DCP, FR-130 and Brominated SBS: Different Roles

The key point is that DCP is not an alternative to FR-130 or Brominated SBS.

Instead:

• FR-130 or Brominated SBS provides the principal flame-retardant effect.
• DCP may be added to improve or support the overall flame-retardant system.

This difference is important when designing an EPS formulation.

How Does DCP Work as a Synergist?

Organic peroxides such as DCP can decompose to generate free radicals.

In a flame-retardant system, these radical reactions may interact with the decomposition behavior of brominated flame retardants and the polymer matrix.

The exact mechanism can depend on formulation and combustion conditions, but the practical objective is to improve the overall effectiveness of the flame-retardant package.

Because DCP is a reactive peroxide, its use should be evaluated carefully with respect to:

• Processing temperature
• Dosage
• Storage conditions
• Compatibility with other additives
• EPS production process
• Required fire-performance level

DCP Is Also a Crosslinking Agent

Outside EPS flame-retardant applications, DCP is widely used as a crosslinking agent in polymers.

Typical applications include:

• Polyethylene
• EVA
• Rubber compounds
• Cable insulation
• Elastomers
• Other peroxide-cured polymer systems

This dual functionality is one reason DCP is widely used across the polymer industry.

However, its function in EPS flame-retardant formulations should be evaluated separately from its conventional crosslinking applications.

Factors Affecting DCP Dosage in EPS

There is no universal DCP dosage suitable for every EPS formulation.

The optimum level depends on several factors, including:

• Type of brominated flame retardant
• Bromine content
• EPS density
• Bead formulation
• Production temperature
• Other additives in the system
• Required flame-retardant standard
• Processing stability

For this reason, DCP dosage should be established through formulation testing and production trials.

DCP vs Primary Flame Retardants

One common misunderstanding is to treat DCP as the main flame retardant in EPS.

This is not its primary role.

DCP should be regarded as a synergistic additive, while products such as FR-130 and Brominated SBS remain the principal brominated flame retardants.

A practical EPS flame-retardant formulation may therefore contain:

• A primary brominated flame retardant
• DCP as a synergist
• Other processing or coating additives depending on the formulation

Related EPS Additives

In addition to DCP and brominated flame retardants, EPS manufacturers may also use:

• Polymerization initiators such as BPO and TBPB
• Coating agents
• Lubricants
• Antistatic additives
• Processing aids

These additives perform different functions and should be selected as part of the complete EPS production system rather than in isolation.

Conclusion

Dicumyl Peroxide (DCP) can be used as a flame-retardant synergist in EPS formulations, particularly in systems containing brominated flame retardants such as FR-130 or Brominated SBS.

DCP does not replace the primary flame retardant. Instead, it can support the overall flame-retardant system through its radical-generating behavior.

For EPS manufacturers, the final formulation should be based on production trials, flame-retardant requirements, processing conditions and the complete additive package.

Senthos supplies ST-DCP for polymer applications and EPS flame-retardant systems. Contact us for technical specifications, samples or product information.