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Heat insulation and fire resistance using composite materials (June 1999)

   
Greg Thomas

Introduction

Composite materials are used in many industry sectors as advantages of low density, corrosion resistance and mechanical properties are realised. All generic composites - including polymeric, fibre reinforced, ceramic and sandwich panels - have found high volume and niche applications in transportation, aerospace, marine/offshore and construction industries, aided by comparative requirements and transfer of technology ( fig.1).
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Fig.1 Industrial applications of composite materials.

In many applications, materials are required to possess fire resistance and/or low heat transfer for protection of people, equipment, and increasingly, the surrounding environment. Typical applications are described in Table 1. In such applications, structures are required to conform to mandatory fire regulations. For example, marine and offshore industries are controlled by Safety Of Life At Sea (SOLAS) regulations [1] , while ISO834 [2] and ISO1182 [3] describe mandatory requirements in building and construction elements. Such regulations relate to smoke and toxic fume, transmission of heat and combustibility.

This paper concentrates on the use of composites for low cost fire resistant applications (e.g. fire doors and panels) for which conventional materials (polymer foams, ceramic foams, woods etc) possess a number of disadvantages (including density, cost, Health and Safety or poor fire performance). There is a need therefore for a low density core material which can be used for fire resistant panels. 

Table 1 Typical fire resistance applications.

ApplicationIndustry Sector
Fire walls/door Building, Marine, Offshore
Cabin/cargo hold walls Aerospace
Ceilings/walls/partitions (Rail) Transportation
Protection and heat shields
e.g. exhaust manifolds,
reduction of heat signature
Military, Motorsport, General

Nature of Fires

Fires are categorised as either cellulosic or hydrocarbon [4] . Cellulosic fires are typified by burning timber and upholstery, commonly experienced in buildings and offshore accommodation modules (Class A in fig.2). Hydrocarbon fires are more severe, typified by burning oils and fuels, such as in offshore situations. The categorisation of hydrocarbon fires (Class H in fig.2) is developed from the Class A building industry fires. Finally, offshore structures often require 'jet fire' resistance (i.e. explosive fires) which are more severe than furnace type fires due to stresses caused by flames impacting on surfaces.
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Fig.2 Fire class curves

For SOLAS [1] regulations, non-combustible and non-toxic materials must ensure that the mean back face temperature of a panel is no more than 139°C above the initial temperature for the fire test duration (e.g. H120 - 120mins), and that no single point is greater than 180°C above the initial. Due to the different severity of each fire category (A and H), a range of protection materials are employed.

Fire protection/Heat insulation materials

Materials for fire resistance and heat insulation can be either passive or active in nature. Passive techniques, such as protective skins or cores, rely on retarding temperature increases by insulation. Active techniques employ a cooling medium to remove heat, such as flowing water in composite pipes. Panel applications primarily employ passive techniques due to low cost and ease of design.

Low density, low cost mineral wool cores are available which meet A15-H120 fire classes. Adhesion to skin materials and mechanical strength are low, plus increasing concern in Europe about fibre aspect ratios, potentially raises significant Health and Safety concerns. Balsa wood possesses low density, although in a fire situation, it chars, emitting smoke. Furthermore, moisture absorption causes swelling and panel distortion. Hardwood overcomes many of these problems, but high density and cost prove restrictive.

Polymer foams, such as polyurethanes, and polymer skins, such as epoxies and phenolics, have been used in many industries, but low mechanical properties, Health and Safety requirements during processing, (often toxic) fume emission under fire conditions, plus degradation of most polymers above 200°C may limit their use. Brominated or halogenated compounds have been used extensively as fire retardants, but increasing European legislation may phase out these materials on Health and Safety grounds. Inorganic materials, such as foamed glass or ceramics, provide low thermal conductivity and excellent fire retardance, but brittleness, high cost and processing difficulties have limited their use.

There is an industrial need, therefore, for a core material which embodies the advantages, and none of the disadvantages, of the above materials. One such material, known as Barrikade TM, has been developed by TWI.

Fire resistance using Barrikade TM

Barrikade TM (patented) is a low density, fire resistant inorganic material, consisting of vermiculite particles and a sodium silicate blend binder (in principle, any inorganic binder could be used). Although still in its development stage, Barrikade TM has shown no combustion or flame spread in its limited flame and fire trials. The inert nature of its constituent materials ensures that no smoke or toxic fumes are produced in a fire situations.

Barrikade TM can be processed in various ways, including moulding, pressing and potentially spraying, allowing both cores and protective skins to be manufactured. Once cured (typically below 100°C) Barrikade TM can be cut and machined using conventional tools in the same manner as wood. Only Health and Safety precautions when handling dust apply.

The use of vermiculite and a sodium silicate blend, plus pockets of trapped air, provides a low thermal conductivity (typically 0.06 W/m/K) and low heat release rate. Initial trials have shown that a 20mm thick panel exposed to flames of 1000°C resulted in a back face temperature of 170°C after 80 minutes ( fig.3).

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Fig.3 Temperature profile for Barrikade TM panel

Control of processing parameters (such as mix ratio, cure time and temperature) enables Barrikade TM to be produced in densities of 150-350 kg/m 3, and thicknesses of 3-150mm. Thermal and mechanical properties can also be controlled ( fig.4).

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Fig.4 Flexural strength vs density

Typically, 250kg/m 3 panels provides a flexural strength of 0.31MPa (6.0MPa with 1.6mm aluminium skins) and a compressive strength of 0.65Mpa, which compares favourably with competitive materials (e.g. ceramic foam shows 0.31MPa and 0.10MPa respectively). The major advantages which Barrikade TM possesses over competitors is its low density, low Health and Safety concerns and low cost (typically £20/m 2 (25mm thick), compared with £30/m 2 for Balsa wood, £32/m 2 for ceramic fibreboard and £35/m 2 for foamed ceramics).

Typical applications for Barrikade TM

  • Low cost fire doors
  • Fire resistant walls/partitions
  • Offshore accommodation modules
  • Heat shields
  • Low temperature insulation
  • Protection of control modules, computers, electronics, safety deposit boxes

References

1   International SOLAS protocol, IMO, London 1986
2   ISO 834 -Fire resistance tests, building construction, ISO 1975
3   ISO 1182 - Non-combustibility tests, building materials, ISO 1990
4 F Barnes and DS Ness 'The development and testing of the ProTek offshore fire and blast protection system' Polymers in a Marine Environment, IMechE, London, 1991.

 

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