By Mark Harris.
Commercially available sandwich panels use a number of alternative insulating core materials which most commonly include expanded polystyrene, mineral fibre and rigid urethane (including polyurethanes and polyisocyanurates). I will explain here how they are technically different from each other and why this has such a significant impact on their fire performance.
Polystyrene is a thermoplastic polymer which means that the molecules are linear chains without cross-linking. The lack of cross-linking means they have a low melting temperature. Rigid urethanes are not thermoplastics and have varying degrees of cross- linking between molecular chains which results in a thermoset behaviour. This means that much higher temperatures can be sustained before the material degrades resulting in better fire performance.
Mineral fibre insulation comprises non-combustible inorganic fibres held together by an organic resin and bonded to the skins of the sandwich panels using polyurethane adhesive.
When are PIR core sandwich panels used?
PIR core sandwich panels are specified for internal walls that need to provide fire compartmentation, and external walls that need to be fire-resistant to protect adjacent properties. PIR core sandwich panel systems can deliver up to 60 minutes insulation and integrity performance to the standard fire test conditions specified by EN1364-1 Walls and EN1364-2 Ceilings. To satisfy the requirements of the fire test, the panel must be resistant to flame penetration and limit the temperature rise to a maximum of 180°C above ambient temperature on the unexposed surface.
How does PIR provide enhanced fire performance?
PIR insulation, also referred to as polyiso or ISO is a part of the rigid urethane polymer
insulation family. This polymer family consists of a range of materials that are generally described as either polyurethanes (PUR) or polyisocyanurates (PIR). Importantly, there is no clear cut-off point that defines where a polyurethane finishes and a PIR starts – all variants contain a mixture of urethane and isocyanurate chemical bonds.
The chemical constituents are a polyol, methylene diphenyl diisocyanurate (MDI), catalysts, surfactant, water and blowing agent along with additives such as fire retardants in specific formulations. The blowing agent is a gas that is formed from a liquid additive during the exothermic reaction to create the closed cell structure of the insulation. A large proportion of this gas therefore remains contained within the structure of the finished insulation product.
PIR formulations meeting insurers’ fire performance standards are proprietary technology, but in general terms the improvement in fire performance is a result of higher cross-link density which is achieved by polyester polyols, a high MDI content, specialised catalyst systems and high temperature processing.
Fig.13 shows key differences in thermal response influencing the fire performance.
Fig. 13 Graph showing key thermal properties
Of particular importance, and as observed in standard tests and investigation of real fires, the PIR foam formulations meeting insurers’ test requirements do not exhibit the same degradation as other polyurethane foams. In their research into fire behaviour of flexible polyurethane foams, Shields and Ohlemiller reported that this degradation starts to occur at approximately 200ºC and when temperatures in excess of the order of 270ºC are reached. At that time, the material decomposes into the constituent liquid polyol material. This degradation of the thermoset polyurethane is not a true “melting” phenomenon since the degraded material does not re-solidify when the temperature reduces.
Isocyanurate chemical bonds or linkages are stronger than urethane bonds so in insurance industry certified systems this feature, combined with their ring structure and higher cross-link density, results in a more chemically and thermally stable product. By comparison, polyurethane products with predominately urethane bonds have a relatively low cross link density. The greater bond strength and higher cross link density of insurance industry approved PIR core systems mean that the polymer does not exhibit degradation into the polyol component and instead reduces into a carbon char when exposed to fire.
Fig. 15 shows a sample of PIR foam removed from the Suffolk Food Hall fire; it indicates the way in which the material reacts to fire exposure. As evidenced by the formation of pockets within the material when the foam reaches a certain temperature, the blowing gases trapped in the closed cell matrix of the insulation expand and are emitted from the insulation as it becomes permeable by the formation of the larger voids. As the material gets hotter it reduces to leave a stable graphite-like carbon char that continues to provide insulation to the material beneath.
Fig 15. PIR insulation sample removed from Suffolk Food Hall fire
So insulated panels do not all perform in the same way in fire scenarios. The key difference in this thermal response is the reduction of the material into an insulating carbon char rather than its decomposition into the liquid polyol. This feature enables the insulation to retain sufficient physical integrity under fire exposure to achieve both fire resistance and prevention of fire propagation within the material. An on-going programme of real fire investigations is verifying these features and is added to a database that supports the use of the material in performance based fire engineering designs.
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Mark Harris, Building Technology Director