Polyurethanes in Fire Protection

Polyurethanes (PU) are plastics, usually manufactured from petroleum-derived raw materials, and therefore combustible materials. Nevertheless, their use in fire protection applications is on the rise. Combustible fire protection: Can this work, and how can it be achieved? A short overview of fundamentals, requirements and applications.

Polyurethanes in Fire Protection
Fig.1: Functioning principle of intumescence a) Penetration seal after exposure to flames, b to e) Functioning principle of collars. (Source: ZAPP-ZIMMERMANN)

By Philipp Toth. The goal of fire protection in buildings and other industrial facilities such as rail vehicles or ships is to prevent personal injury and property damage. In many areas national and international laws, standards and guidelines strictly regulate fire protection measures, based on the two concepts of preventive (prevention and limitation of fires) and defensive (fire fighting) fire protection. Preventive fire protection is itself divided into three areas: structural, engineering and organisational fire protection. Solutions for preventive engineering fire protection include smoke and fire detection systems, sprinklers and inert gas extinguishing systems, as well as systems for oxygen reduction. Examples of preventive structural fire protection are firewalls and fire doors, fire-resistant glass, fire penetration seals and the use of fire-retardant materials.

Polyurethanes in Fire Protection
Fig.2: Two-component PU fire protection systems: a) Gas-tight for sealing of gas lines, b) Conventional for general application, c) Water-tight for rail vehicles (Source: ZAPP-ZIMMERMANN)

Use of plastics

To reduce the fire load it is recommended – where possible – to use non-combustible materials, which include most mineral-based and metal materials. In many applications, however, modern plastics are superior to mineral-based materials with respect to material properties, design options and production processes. Most plastics, however, are based on petroleum or increasingly on renewable raw materials, making them generally combustible. To reduce the potential hazard in the event of a fire, they are therefore treated with special additives, positively effecting flammability, flame spreading, drip behaviour in fire, and smoke composition.

Flame retardants and functioning principles

Among these additives, various modes of action exist. Substances such as aluminium trihydroxide (ATH) withdraw energy from the flames (cooling effect) combined with the formation of water as a non-combustible gas (dilution effect). Halogenated flame retardants directly inhibit the chemical combustion process due to the released radicals (flame poisoning). Although the latter additives are highly effective, their toxic combustion products as well as their persistence combined with potential hormone-like effects have led to stricter regulations and even bannings of some of these substances.

Another important principle is known as intumescence, the formation of an insulating carbonaceous layer with a simultaneous increase in volume. In this process an expanding foam body forms on the component, protecting the remaining material from oxygen, heat and flames. Various organic and inorganic materials as well as their combinations can cause this effect. In general, materials that form a stable matrix in case of fire can be expanded by additional gas-forming substances. Typical examples of this group are (poly) phosphates. Intumescent fire protection products are mainly applied as coatings or penetration seals.

Intumescence

In penetration seals, the increase in volume is a distinct advantage of intumescent systems. An intumescence factor > 1 corresponds to a larger volume of the components in case of fire than in the original state when installed. This increase in volume can fill damaged areas of penetration seals such as gaps or holes that occur for example during combustion of flammable penetrations. By choosing suitable additives it is possible to adjust both the extent and the force of the intumescent process (expansion force) and the solidity of the resulting insulating coating. The inserts of collars, for example, have a high intumescence factor together with a high expansion force, so that they can compress plastic pipes and seal the remaining opening after they have burned away.

Polyurethanes in Fire Protection
Fig.3: Examples of different moulded components: a) Blocks, b) Plugs, c) strips, d) Complex moulded components for special applications, e) Ventilation grilles. (Source: ZAPP-ZIMMERMANN)

PU fire protection systems in detail – Processing and use

Development

The first intumescent fire protection products made of polyurethane (PU) for penetration seals in building construction appeared on the market more than 25 years ago. Continuous development and improvement of the product properties led to the expansion of these systems to various international markets, as well as other areas of use such as civil engineering, rail vehicle construction and ship building.

Polyurethanes in Fire Protection
Fig.4: Combination of moulded components with two-component in-situ foam (Source: ZAPP-ZIMMERMANN)

Customization of properties

The material properties of PU can be adjusted in a wide range, making It one of the favourite materials for industrial use. Typically composed of polyols and isocyanates, the exact choice of these two base materials and their relative ratios influences properties such as density, hardness or elasticity, without the need for plasticizers or other additives. PU-foams can in most cases be achieved by adding water, which in reaction with the existing isocyanates forms gaseous carbon dioxide (CO2), which serves as an expanding agent.

Other additives are used to control, for example, the speed of the cross-linking reaction and the viscosity of the raw materials. These two properties are especially relevant for two-component in-situ foams and casting compounds, which deliver virtually constant end results regardless of the ambient conditions, as long as the material is at the correct temperature.

Shaping

Polyurethane is an outstanding material for moulded components. With the right moulds it is possible to produce complex three-dimensional moulded components that are comparable to such produced by injection moulding processes. Especially in controlled applications such as rail vehicle construction, this enables custom solutions that save time during installation.

Polyurethanes in Fire Protection
Fig.5: Silicone-coated fire protection block (Source: ZAPP-ZIMMERMANN)

Processability

Especially the soft foams used today for penetration seals feature outstanding processing properties. Moulded components can be adapted with standard insulation knives, if necessary, but can also be made to fit into slightly smaller openings, due to their compressibility. Another advantage of these systems is the elimination of mixing of materials and time-consuming formwork. Reactive systems such as two-component in-situ foams are ideal for sealing openings with numerous or irregular penetrations. During expansion the PU foam body automatically adapts to the closure, eliminating the need for time-consuming custom cutting.

Since two-component foams can be combined with moulded components, large openings with irregular penetrations can also be sealed very quickly by filling empty areas with moulded components and areas with penetrations with in-situ foams.

PU soft foam systems also offer fast retrofitting by removing single moulded components or simple cutting of suitable openings without time-consuming drilling or chiselling.

PU fire protection system in detail – Technical properties

Fire protection

Regarding fire resistance, one must differentiate between the fire properties of the building material and the application. Most PU fire protection materials are combustible and fall under Class E according to EN 13501-1. Proof of the fire protection effectiveness in the application is usually determined based on test procedures for the specific situation. For penetration seals in building construction these are, for example, regulated by EN 1366-3 in Europe and by ASTM E814, also incorporated in the certification standards UL 1479 and FM 4990, in the USA. Fire tests for proving fire resistance are conducted in installations with wall and ceiling elements featuring standard penetrations, such as cables and pipes. In Europe, the control of the temperature profile in the test chamber uses the temperature curve in accordance with ISO 834-1, which is designed to simulate the profile of a solid-substance fire. Other test curves exist for special applications such as tunnel or liquid-substance fires. A basic requirement in all cases is maintaining the room integrity, the prevention of direct flame spread through the penetration seal. In the USA, this F-rating contains as an additional requirement for the mechanical stability of the burned out penetration the so-called hose stream test. In this test, the fire penetration seal is sprayed with a water jet immediately after passing the fire test. The surface of the seal is sprayed several times with water at a pressure of 2.1 bar; any resulting gaps result in failure of the entire test.

Another requirement in many cases is limiting the heat transfer. For the duration of the test, the temperature rise (ΔT) on the unexposed side must not exceed 180 K on the seal surface and the penetrations. In Europe, this criterion is generally mandatory, while in the USA heat transfer is of secondary importance, since the wide use of sprinkler systems can usually suppress the spreading of a fire by means of heat transfer. As a result of these very different requirements, PU fire protection products are designed differently for the respective markets.

Health aspects

The reactive isocyanates in polyurethanes are generally hazardous substances, which can cause allergies and are suspected of being carcinogenic. Due to this, especially the highly volatile specimens of this substance group are increasingly becoming the subject of regulations and restrictions. Since isocyanates are consumed when they react to polyurethane, one must differentiate between the processing of cured and non-cured PU materials when considering the health aspects.

Processing of non-cured PU materials: This category includes typical one- and two-component PU foam in cans and cartridges, which generally contain reactive isocyanates. The safety measures as stated in the according safety data sheet must be followed when processing these materials, for example:

  • Sufficient ventilation of the workplace
  • Use of protective clothing, gloves and goggles
  • Avoidance of skin contact

To assess the actual impact of isocyanates on the workplace, workplace measurements of processing of a two-component fire protection foam with the isocyanate pMDI in a coaxial cartridge with a static mixer attachment in a closed room were conducted. The measured values for both long-term and short-time exposure were below the detection limits. Assuming compliance with the aforementioned safety measures, no dangers arise from this application.

Processing of cured PU materials: This includes PU moulded components and the cured in-situ foams and casting compounds. During curing the potentially hazardous isocyanates react to urethane and urea compounds and cured PU materials are therefore no longer rated as hazardous. To assess their effects on health, the emission behaviour was examined in tests in a test chamber according to EN 16516. The measured values were below the permissible limits. In accordance with various national requirements of European countries, the products are therefore rated as safe.

Durability

European Technical Assessments (ETA) generally assume a life of ten years for PU fire protection products. This value is a general assumption and is stated for all products that can easily be replaced or repaired. For a more realistic assessment of the actual life, in-house tests were conducted for the simulated ageing of different PU soft foams based on the DAfStb Guideline. The results point toward a substantially longer life, which corresponds to the empirical findings since the market launch.

Polyurethanes in Fire Protection (6)
Fig.6: Test set-up for penetration seals (Source: ZAPP-ZIMMERMANN)

Summary

Fire protection measures serve to protect human life and property. Fire protection products on the basis of polyurethane are used in preventive fire protection in buildings and are characterised by diverse adaptable mechanical and technical properties for optimal fire protection.

In extensive tests conducted by certified material testing institutes, these systems consisting of combustible materials successfully demonstrated their fire resistance and have received numerous certifications and ratings with a high application range.

Their increasing worldwide use across industries is due to their outstanding technical properties and economical advantages. PU-based fire protection systems represent equivalent or superior alternatives to the systems otherwise in use.

Author

Dr. rer. nat. Dipl.-Chem. Philipp Toth: Product Manager at ZAPP-ZIMMERMANN GmbH, philipp.toth@z-z.de

The article was published in FeuerTrutz International, issue 1.2019 (January 2019).
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