Polypropylene V-0: Halogenated and Halogen-Free Formulation Approaches

Polypropylene combines low density, good mechanical properties, chemical resistance and ease of processing. From a fire-safety perspective, however, PP has a serious drawback: it ignites readily, burns intensely and is prone to melt dripping.

There are two main approaches to addressing this issue: halogenated and halogen-free flame-retardant systems.

What UL 94 V-0 Requires

The UL 94 test assesses more than whether the material self-extinguishes. For a V-0 rating, the afterflame time following each flame application must not exceed 10 seconds. In addition, flaming drips must not ignite the cotton indicator placed below the specimen.

Melt dripping is particularly important for PP, so the formulation must simultaneously control:

• the burning rate;
• the duration of self-sustained burning;
• melt behavior;
• the formation of a protective layer;
• the retention of mechanical properties.

Halogenated Systems: High Efficiency at a Comparatively Low Loading

The conventional approach to flame-retarding polypropylene uses a halogen-containing flame retardant, most often a brominated one.

Such formulations may also contain synergists, such as antimony compounds. The main action takes place in the gas phase, where decomposition products of the flame-retardant system suppress combustion chain reactions.

One advantage of this approach is its high efficiency. The required flame resistance can be achieved with a lower additive loading than in some halogen-free systems.

This helps preserve:

• PP flow properties;
• mechanical properties;
• the compound’s comparatively low density;
• ease of processing.

However, this formulation remains halogen-containing. Its use may be limited by the requirements of a particular customer, industry or specification concerning halogen content and combustion products.

Revaguard BG08P: A Halogen-Antimony Flame-Retardant System on a Polyolefin Matrix

Revaguard BG08P is a polyolefin-based compound containing a combination of a halogenated flame retardant and an antimony compound as a synergist. The product is a ready-to-process masterbatch or compound intended to give polypropylene and other polyolefins a UL 94 V-0 flame rating. The combination of components in BG08P follows the conventional approach to flame-retarding polyolefins, in which the main action takes place in the gas phase.

Halogen-Free Systems: Controlling Combustion Through a Protective Layer

Phosphorus-containing and phosphorus-nitrogen systems, including intumescent flame retardants, are often used for halogen-free PP. When heated, such a system can form an expanded char layer on the material’s surface, limiting heat transfer and the release of combustible decomposition products.

Intumescent systems are one of the main approaches to producing flame-retardant PP. However, achieving the required performance often calls for a substantial flame-retardant loading, which may impair mechanical properties and affect processing.

In addition to phosphorus-nitrogen systems, other options include:

• metal hydroxides;
• red phosphorus;
• other phosphorus-containing flame retardants;
• synergistic combinations of several flame-retardant mechanisms.

There is therefore no single universal formulation for halogen-free PP rated V-0.

Revaguard NP-07: A Halogen-Free Intumescent System for Polyolefins

Revaguard NP-07 is a ready-to-use flame-retardant compound based on a phosphorus-nitrogen intumescent system, intended to impart flame resistance to polypropylene and other polyolefins. Unlike halogen-antimony systems, NP-07 uses a physicochemical protection mechanism based on the formation of an expanded char layer on the material’s surface when exposed to flame. The product combines all three key components required for the intumescent mechanism in one compound, simplifying dosing and helping achieve reproducible results.

Why One Flame Retardant Cannot Simply Be Replaced with Another

Halogenated and halogen-free systems work through different mechanisms and require different compound designs.

When switching to a new flame-retardant system, it is necessary to reassess:

1. the PP grade and melt flow index;
2. filler content;
3. the effect of pigments and colorants;
4. mechanical properties;
5. melt behavior during combustion;
6. product thickness;
7. processing conditions.

Thickness is especially important. A UL 94 rating is assigned to a material based on the tested specimen, so a V-0 rating at one thickness cannot automatically be applied to a thinner part.

Choosing the Right Approach

A halogenated system can be a rational choice when high flame-retardant efficiency, low additive loading and retention of mechanical properties are priorities, and there is no Halogen Free requirement.

A halogen-free system becomes necessary when required by the finished product specification or the customer. In this case, the technologist’s main task is to achieve V-0 without excessively compromising PP’s mechanical properties or processability. In both cases, V-0 is the result of the entire formulation, not a single additive.

Revada Plastic specialists help select a flame-retardant system for polypropylene based on the required UL 94 rating, product thickness, compound composition and processing technology.