The heat in the sandwich - homegrown fire resistant composite out performs competitors

Paul Burling obtained his Higher National Certificate in Production Engineering from Cambridge College of Art and Technology. He has extensive experience in the project management of large commercial projects worldwide. These include in depth knowledge of composite material for military and commercial applications.

Greg Thomas graduated from Birmingham University in Materials Engineering in 1995 and joined TWI later that year. Since then he has been widely involved in adhesive technology and composite materials including the generation of Design Guidelines for the Offshore Industry. He is currently working in the Advanced Materials, Processes and Microtechnology group at TWI.

Composite materials are used where their best features such as low density, corrosion resistance and mechanical strength make them unique for the job in hand. Here Paul Burling and Greg Thomas describe the development of a TWI-invented low cost lightweight sandwich structure that can be used for fire resistant panelling.

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).

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.

Table 1: Typical fire resistance applications and industry sector

Cabin/cargo hold walls
    aerospace

Ceilings/walls/partitions
    (rail) transportation

Protection and heat shields eg exhaust manifolds, reduction of heat signature
    military, motorsport, general

Such regulations relate to smoke and toxic fume, transmission of heat and combustibility. These regulations are becoming increasingly strict with respect to maximum organic content, maximum allowable temperature increases and spread of flame. Existing fire resistant materials may experience difficulties in achieving the requirements defined by these standards, which may have significant results throughout many industry sectors. It is therefore essential that the correct materials, which can meet these tough regulations at acceptable cost and mechanical performance, are selected for fire resistance applications.

This article concentrates on the use of core materials in sandwich structures for low cost fire resistant applications, eg 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.

Nature of fires

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

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 ( eg H120 - 120min), and that no single point is greater than 180°C above the initial. Due to the different severity of each fire category (A and B), a range of protection materials is used.

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 use a cooling medium to remove heat, such as flowing water in composite pipes. Panel applications primarily use 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, and increasing concern in Europe about fibre aspect ratios, potentially raises significant Health and Safety concerns. Furthermore, their organic content may raise concerns with the new non-combustion regulations. 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 which can meet these requirements has been invented and developed at TWI, and is known as Barrikade®.

Fire resistance using Barrikade®

Barrikade® can be processed in various ways using conventional tools and ovens, including moulding, pressing and potentially spraying, allowing cores, mouldings and protective skins to be manufactured. Once cured (typically below 100°C) Barrikade® 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 0.06-0.15 W/m/K, dependent on density of material used) 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.4). Tests have shown that Barrikade® (at a core density of 250kg/m ³ ) will withstand a butane fire for well over one hour without degradation.

Control of processing parameters (such as mix ratio, cure time and temperature) enables Barrikade® 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.5).

Typically, 250 kg/m ³ panel provides a flexural strength of 0.40MPa (6.0MPa with 1.6mm aluminium skins) and a compressive strength of 0.70MPa, which compares favourably with competitive materials ( eg ceramic foam shows 0.31 and 0.10MPa respectively). Adhesion to skins (such as wood, aluminium, steel, composites) can be achieved using the silicate-based binder, although other adhesive materials can be used to match application requirements.

The major advantages which Barrikade® possesses over competitors are 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). A typical comparison of Barrikade® with common core materials is illustrated in Table 2.

Table 2: Comparison of typical core materials

Typical applications for Barrikade®

• 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

Health and Safety

The limited data published on the properties of vermiculite indicate its Health and Safety risks are low, with the exception of normal considerations when high dust levels are generated. In the past, there was concern that vermiculite contained asbestos. Currently, vermiculite is screened for asbestos and is therefore free of this contaminant.

Alkali silicates may cause caustic burns to the eyes and skin, and are harmful by ingestion. If appropriate precautions are followed, Health and Safety risks are minimal.

Due to the inert nature of vermiculite and the cured binder, Health and Safety risks during fire exposure are minimal.

Current situation

TWI has developed the Barrikade® material to a position where confidence in its flame resistance, heat insulation, mechanical properties and processing characteristics has been gained. A patent for this material has been applied for by TWI. Companies who have a potential application can apply for a licence through Iain Smith, TWI Commercial Manager.

All work carried out as single client work is strictly confidential to the client. TWI is willing to enter into Confidentiality Agreements to help develop products. Therefore, if you have an application or product that is sensitive and you wish to discuss the possibilities for Barrikade® in more detail, please contact either Paul Burling or Felicity Chipperfield at TWI.

References