Synthesis and characterization of flexible polyurethane foam from liquid natural rubber-based polyol / Siti Zaleha Isa

Siti Zaleha , Isa (2011) Synthesis and characterization of flexible polyurethane foam from liquid natural rubber-based polyol / Siti Zaleha Isa. Masters thesis, Universiti Malaya.

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Abstract

Flexible polyurethane (PU) foams were prepared using a liquid natural rubber (LNR) -based polyol. At the first stage, the polyol was synthesized by in –situ reaction of latex and hydrogen peroxide using sodium nitrite as a degradation agent. At second stage, polyurethane foams were prepared from reaction of the LNR-based polyol with methylene diphenyl 4,4-diisocyanate (MDI) in the presence of catalysts, silicone surfactant, chain extender, and water as the blowing agent. In this work, thermal degradation of NR was carried out at different temperature to obtain LNR-based polyol by varying the reaction times and concentration of H2O2/isoprene ratio. The effect of H2O2/isoprene ratio, the molecular weight and polydispersity of the LNR-based polyol decrease with increasing of H2O2/isoprene ratio, whereby the average molecular weight of the LNR-based polyol decrease almost half from 31000 to 16000 when H2O2/isoprene ratio increase twice from 0.6 to 1.2 mol. These phenomena were due to the chain-scission that may have taken place simultaneously with epoxidation and hydrolysis, resulting in reduced molecular weight of LNR, having the epoxide and hydroxyl groups. Meanwhile, the increase of the H2O2/isoprene from 0.6 to 1.0 mol had led to an increase of the hydroxyl value. However, when the ratio H2O2/isoprene was increased to 1.2 mol, the hydroxyl value decreased. This phenomenon may be due to the formation of hydrofuran. Besides, the LNR-based polyol synthesized using temperature at 90oC has high the hydroxyl value. The average molecular weights (Mw) of the LNR-based polyols decreased with increasing reaction time while polydispersity (PDI) indices increased. The average molecular weight, Mw in the final product is 15368. The PDI increases with increasing of the reaction time from 6 to 24 h. However, when the reaction time was increased to 36 h, the PDI decreased rapidly. It can be concluded that the longer reaction time, the lower will be the molecular weight of the products formed. The LNR-based polyol was used in the production of flexible PU foam. Generally, flexible PU foam is formed by simultaneous reaction between an isocyanate with polyether polyol and water. The increasing water content in the polyurethane formulation produces foams with lower density. Meanwhile, the compression stress of the foams is increases with the increasing of water content in the formulation. These phenomena were due to the formation of urea hard segment when the amount of water content increases. The tensile strength and tear strength of the foams increased with increasing water content. The tensile strength (10.9 kg cm-2 for H and 10.7 kg cm-2 for K) and tear strength property (0.28 N mm- 1 for H and 0.27 N mm-1 for K) of the foams were at a maximum with 5 g of water content. Meanwhile, the water content of 4 g was chosen as the optimal water content as it produced foam with highest elongation at break (75 % for H and 88 % for K). However, at low water content (2 g), cell structures are smaller and also more uniform. The foams produced using higher water content is thermally more stable. Irrespective of the starting polyol, increasing of isocyanate index from 90 to 100 causes an increase of foam density but when the isocyanate index increases from 100 to 110, a reduction of the foam density occurred. Foams made using isocyanate index of 100 possessed the highest density (46 ± 1 kg m-3), in comparison to foams having isocyanate index of 90 and 110, which gave lower densities of 45 ± 1 kg m-3 and 44 ± 1 kg m-3 respectively. Meanwhile, the compression stresses for both MI and NI samples of the PU foams increased with increasing isocyanate index from 90 to 110. The tensile strength and tear strength of the foams increased with increasing of isocyanate index from 90 - 100 but decreased for 110. The isocyanate index of 100 gave the maximum tensile strength for both MI and NI (9.9 kg cm-2) and tear strength (0.27 N mm-1 for MI and 0.16 N mm-1 for NI) respectively. Meanwhile, the elongation at break for foams of MI and NI are decreased when isocyanate index increased. The foams produced from higher isocyanate index are more stable thermally.

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