Fire Retardant Resins/Additives - A345

Fire Retardant Resins/Additives
Document Type	Article
Document Identifier	345
Relevant Class	Material
Tags	Matrix Material Object
Prerequisites	Material structure Polymer (matrix) structure Thermoplastic polymers Thermoset polymers Material

Overview

This document offers insight into the various types of fire retardant resins and the mechanisms for preventing or slowing down combustion, flame propagation, temperature rise, smoke generation, and emission of toxic fumes. Furthermore, advantages and drawbacks are discussed, accompanied by real-world examples of products that require various levels of fire retardant properties.

Background

As composite structures become more prevalent in the automotive, ground transportation, aerospace, and other commercial industries, it is imperative that suitable fire protection methodologies are developed for these materials. Fire retardants are one of the primary methods used to reduce the fire threat from composite materials. They can be pre-mixed with the resin or gel coat, or applied as a coating during post-production. Generally, the test standards used to evaluate fire retardant materials measure critical parameters such as burn time, temperature rise, and smoke density and toxicity. Various fire retardants have their own advantages and disadvantages as it relates to these fire-testing parameters. Their effect on the final mechanical properties of the part and the manufacturing process must also be considered. In all applications, it is important to understand the regulatory requirements and the goals of the fire protection system in order to select the appropriate fire retardants.

It should also be noted that the field of flame retardancy is ever changing in terms of available/permissible products, regulations for flame retardant use, what they should protect against, and how they should be dealt with at the end of their lifetime.

Application

In this section, we explore mechanisms for enhancing fire safety in composite materials. It begins with an examination of resin additives, outlining three distinct modes of flame retardant action in polymers: gas phase, endothermic, and mineral filler flame-retardants. Then, we discuss the application of phenolic resins, which, while less common in the ground transportation industry, offer remarkable fire-resistant properties. Finally, the section explores the emerging field of polymer nanocomposites.

Resin additives

Three general modes of flame retardant action are used in polymers. The first is gas phase flame-retardants, which reduce the heat released in the gas phase by scavenging reactive free radicals that result in incomplete combustion. This type of flame retardant can include halogenated organics and organophosphorus materials. The second is endothermic flame-retardants, which function in the gas and condensed phases by releasing non-flammable gases, such as water and carbon dioxide, which dilute the fuel and cool the polymer. Examples of this class of flame retardant include aluminum hydroxide, magnesium hydroxide, and some mineral carbonates. Halogenated and phosphorous flame-retardants have a long history of effective use in industry. However, these types of flame-retardants generate additional smoke and carbon monoxide during combustion. Their use is also being reviewed due to potential negative environmental effects.

Mineral filler flame-retardants, such as aluminum hydroxide, magnesium hydroxide, and calcium carbonate are effective at lowering heat release rate and smoke release. However, they are not as effective as other fire retardants on a per weight basis. The need to use large amounts of these types of fire retardants can reduce the mechanical properties of the final part. Additionally, high amounts of fillers can increase the viscosity of the resin. This can lead to processing problems for liquid composite moulding processes like RTM and vacuum infusion.

Phenolic

Phenolic resin is the least common resin system currently used in the ground transportation industry. Difficulty in processing leading to lower mechanical properties is a factor in limiting their widespread use. The curing reaction of phenolic resins releases water vapor and leads to an increased void content. However, high heat resistance and low smoke and toxicity properties during combustion have made them valuable for applications where high temperatures and fire resistance are a concern . Unlike other resin systems, which can produce large amounts of smoke and toxic gas during combustion, phenolics produce up to 50% char by volume and very little smoke or toxic gases. This is of particular importance in underground mass transit applications, where fire, smoke, and toxicity requirements are critical . Examples of phenolics use in the ground transportation industry are in engine firewalls and the manufacture of fiberglass-phenolic flooring systems for buses and trains. Phenolics are also used commonly in aircraft interior applications due to their FST (fire, smoke, toxicity) performance but since they are more difficult to process and tend to have poor mechanical properties, there is a push to use newer flame retardant epoxy systems where possible.

Polymer nanocomposites

This is a relatively recent addition to fire retardants for resins, and consists of polymers filled with finely dispersed nanoscale particles. This produces a condensed-phase flame retardant that slows the mass loss rate of the resin through developing a nano-particle rich fire protection barrier. This lowers the peak heat release rate and melting/dripping during fire. However, it does not lower the total heat release of the fuel. Employing polymer nanocomposites is not enough for passing regulatory tests, though they do help retard flame growth. Nanoclays are widely employed as flame inhibitors in commercial applications, yet their utility is constrained by their compatibility with specific polymers and the challenge of achieving complete mixing or exfoliation within the polymer matrix. Carbon nanotube are not limited to what resins they can be used in, but are more expensive and potentially hazardous to human/environment health. It should be noted that options that involve additives may influence the processing of materials, and there can be concerns about fiber reinforcements filtering these additives out of the resin, especially during liquid composite moulding processes.

Practice (Case Studies/Examples)

Examples of consumer products that require high fire resistance are cookware, ashtrays, and electronic components. Other areas where fire retardant resins are typically used are transportation, cladding on buildings, interior building panels, roofing and equipment exposed to high temperatures.

Fire Testing for Mass Transit

Evaluating the flammability of composites plays a vital role in material approval, especially in the context of mass transit applications. In order to gain approval from transit authorities, bus components are required to pass the specifications laid out in Federal Transit Administration Docket 90-A – Recommended Fire Safety practices for Transit Bus and Van Materials Selection. Docket 90-A specifies that the panels must achieve a 15 minute rating according to ASTM E119 – Standard Test Methods for Fire Tests of Building Construction. ASTM E119 consists of a test specimen exposed to constant specified temperature for a specified period of time. It allows for comparison of different materials under the same fire conditions, measuring the transmission of heat and hot gases through the test specimen. There are also provisions for measuring the load carrying capacity during heat exposure. The test does not evaluate the fire hazard due to smoke, toxic gases or other combustion products, or the flame spread over the surface.

Due to the relatively high cost of ASTM E119, it is possible to screen components using lower cost test methods before proceeding to the final ASTM E119 test. ASTM E162 provides a method for comparing the surface flammability of specimens when exposed to a constant level of radiant heat energy. This test is lower cost partly because it can be done on a laboratory scale, using small specimens. This provides a measurement of the rate of flame spread for a representative material/assembly. Like ASTM E119 there are no conclusions that can be draw regarding smoke, toxicity and other fire hazards.

Fire Testing for Aircraft

For aircraft, Title 14 of the Code of Federal Regulations (14 CFR) Part 23, Section 853 and Part 25, Section 853 addresses various aspects concerning fire protection. These regulations pertain to the flammability characteristics of materials used in the airplane's passenger and crew compartments of normal and transit airplanes, respectively. Canadian Aviation Regulations [CAR 525.853] outlines similar standards and requirements for materials used in aircraft interiors to minimize the risk of fire and ensure the safety of passengers and crew. They set standards for materials used in interior components such as seat cushions, upholstery, carpets, luggage compartments, and other components to ensure they meet specific flammability tests, thus minimizing their contribution to fire spread or intensity. The standards outline testing procedures and criteria for evaluating the flammability of materials. Materials used in airplane interiors must be approved by the Federal Aviation Administration (FAA) and approval typically involves submitting test reports demonstrating compliance with the flammability requirements.

Advisory Circulars (AC) issued by the Federal Aviation Administration (FAA), and similar guidelines issued by Transport Canada or other relevant regulatory authority, provide guidance and information on various aspects of aviation regulations, procedures, and practices. AC23-2A provides guidance on flammability testing procedures and requirements for materials used in aircraft construction, particularly focusing on interior components. The AC details the procedures and test methods for evaluating the flammability characteristics of materials used in aircraft interiors. This may include tests such as the vertical flame test, smoke emission test, and heat release test. It also provides guidance on how materials are classified based on their flammability characteristics and how the material classification impacts its use in different areas of aircraft interior. The AC specifies documentation and data required to demonstrate compliance with flammability testing requirements. This includes test reports, material specifications, and other relevant documentation.

The FAA Fire Test Handbook is another resource for standardized procedures for conducting fire tests on materials used in aircraft construction. Aspects of tests such as test equipment and instrumentation, sample preparation, data analysis and documentation requirements are discussed in this handbook.

Conclusion and Further Information

A variety of options exists for fire retardant polymers, each carrying its own set of advantages and drawbacks. With the increasing use of polymers and composites, and as environmental protection and human health concerns become more important, careful selection of fire retardants is necessary to balance the regulatory needs, costs, and the long term performance of the product in the marketplace. Several test methods are available to determine how fire retardant a resin is, and the choice of test method should be based on regulatory needs, final application, and testing cost.

References

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Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.