Abstract
The thermal stabilities, flame retardancies, and physico-mechanical properties of rice husk-reinforced polyurethane (PU–RH) foams with and without flame retardants (FRs) were evaluated. Their flammability performances were studied by UL94, LOI, and cone calorimetry tests. The obtained results combined with FTIR, TGA, SEM, and XPS characterizations were used to evaluate the fire behaviors of the PU–RH samples. The PU–RH samples with a quite low loading (7 wt%) of aluminum diethylphosphinate (OP) and 32 wt% loading of aluminum hydroxide (ATH) had high thermal stabilities, excellent flame retardancies, UL94 V-0 ratings, and LOIs of 22%–23%. PU–RH did not pass the UL94 HB standard test and completely burned to the holder clamp with a low LOI (19%). The cone calorimetry results indicated that the fireproof characteristics of the PU foam composites were considerably improved by the addition of the FRs. The proposed flame retardancy mechanism and cone calorimetry results are consistent. The comprehensive FTIR spectroscopy, TG, SEM, and XPS analyses revealed that the addition of ATH generated white solid particles, which dispersed and covered the residue surface. The pyrolysis products of OP would self-condense or react with other volatiles generated by the decomposition of PU–RH to form stable, continuous, and thick phosphorus/aluminum-rich residual chars inhibiting the transfer of heat and oxygen. The PU–RH samples with and without the FRs exhibited the normal isothermal sorption hysteresis effect at relative humidities higher than 20%. At lower values, during the desorption, this effect was not observed, probably because of the biodegradation of organic components in the RH. The findings of this study not only contribute to the improvement in combustibility of PU–RH composites and reduce the smoke or toxic fume generation, but also solve the problem of RHs, which are abundant waste resources of agriculture materials leading to the waste disposal management problems.
Highlights
Rigid polyurethane foam (PUf) is a lightweight material, which can be used in various applications.It has been mostly used in building as a thermal insulation material because of its low thermal conductivity, low density, high compressive strength, and high energy absorption capacity [1,2,3,4,5,6,7,8].To increase the mechanical strength, acoustic attenuation, and load bearing, inorganic fillers are often added to PUf formulations [9]
We report the application of rice husks (RHs) as a filler for PUf, with a particular focus on the improvement in flame retardancy of PU–RH
The char residue contents of PU–RH/ATH and PU–RH/OP were 22.2 and 17.0 wt%, respectively. These results confirm that the flame retardants (FRs) increased the thermal stability of the composite by the lower weight loss, shifting it to the higher decomposition temperature and notably higher residual char content than those of PU–RH
Summary
Rigid polyurethane foam (PUf) is a lightweight material, which can be used in various applications. Ye et al [23] studied the fire retardation of rigid PUfs using fine expandable graphite–poly(methyl methacrylate) (pEG–PMMA) composite particles as a fire retardant They reported that the compressive strength, modulus, and flame retardancy increased upon the 10 wt% loading of pEG-PMMA. FRs commercially available on the market, aluminum diethylphosphinate (Exolit OP), having a high char-forming ability, is the most effective nonhalogen FR It has a high phosphorous (P) content, effective fire retardancy, and thermal stability at a high temperature. Based on the previous studies summarized above, the addition of a coadditive/char-forming material along with the phosphorus-based FR could provide an effective flame retardancy. A single phosphorus-based FR compound (OP), having a high phosphorus content and single most used inorganic filler (ATH), were employed to suppress the PU–RH composite flammability. The densities, compressions, and moisture sorption isotherms were measured and analyzed
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