Synthesis And Characterization Of Bio-based Rigid Polyurethane Foams With Fire Retardancy

Corn is a major food crop in rural northern China. Corn cob is an important agricultural processing residue as well as an important biomass feedstock. Corncob has seldom been used to obtain high-valued products except for the production of furfural as well as active carbon,The purpose of this study is to transfer corn cob into a polyether polyol by liquefaction based on the investigation on its liquefaction behavior, and then to prepare a polyurethane foam with excellent mechanical properties and flame retardant properties by reacting the polyol with isocyanates.In this study, several kinds of polyether polyols were used to replace the conventional polyhydric alcohol liquefied reagents to investigate the liquefaction behavior of corn cob. The chemical composition and the structural characteristics of the major components of corn cob were characterized and the reaction of main components of corncob in the liquefaction process was studied. It is found that the conversion yield of hemicellulose was liquefied most, followed by cellulose and lignin.The influences of the acid type, the acid concentration, the polyol functionality, the reaction time and the temperature on the effectiveness of liquefaction and properties of the liquefaction products were investigated. The conversion yield and the residue yield were used to evaluate the performances o f the catalysts. It was found that the sulfuric acid was the best catalyst in the liquefaction of corn cob while acetic acid gave the weakest performances when used as the catalyst. Among the three polyols, the polyol 303, which has a relatively low molecular weight, gave the highest conversion yield of the feedstock. The residue yield was found to reduce when prolonging the reaction time, but then tended to increase after a certain time point. A reasonable explanation to this result is the re-condensation of the degradation intermediate. Enhancing the temperature favored the degradation of feedstock, but too high temperature usually led to the re-condensation of the degradation intermediate and the increase in residue yield.The weight loss during the lique faction of corn cob in polyol was studied. The weight loss can be assigned to the losses due to thermal decomposition of the plant feedstock and the loss due to the thermal decomposition of the polyol. The decomposition products of both the feedstock and polyol were characterized by GC-MS method. A method of reduce the weight loss was proposed. The plant feedstock could be immersed in the liquefying agent at ambient temperature before the liquefaction. Then the mixture and the catalyst could be batch added into the liquefaction reactor in accompany with continuous stirring in which the rest part of the liquefying agent was preheated to the desired temperature.One phosphate ester of the liquefaction product was prepared by the reaction with polyphosphoripc acid. The modified liquefaction product gave improved compatibility with the polymer than the polyols with the addition of flame retardants. The novel polyol could maintain the performances of the polyurethane, and the flame retardant performance is time lasting.It was also found that, during the modification of liquefaction products by polyphosphoric acid, the increase in the amount of polyphosphoric acid led to the increase in the content of phosphate in the modification product and proportion of phosphate in all phosphorus-containing compounds. Anyway, the proportion tended to decrease when the content of polyphosphoric acid exceeded a certain limit. Optimal conditions to achieve highest phosphate content in the modification product were: polyphosphoric acid / liquefaction products 1: 1 and the reaction time 1h.The modification products of the liquefied corn cob were used to produce rigid polyurethane foam. In this study, the flame retarding properties of the polyurethane foam that prepared from the modified liquefaction products to replace conventional polyols to different proportion were investigated. The increase in the content of polyol phosphate favored the improvement of the flame retardancy. The increase in the content of polyphosphoric acid could also increase the limiting oxygen index(LOI) data. The LOI data of the polyurethane foam could achieve 26.6 when polyol phosphate was used in the preparation of ppolyurethane that is 6.8 higher than that of the control sample.The thermal stability of the rigid polyurethane foam that was prepared from the modified liquefaction product was also investigated. With the increase in the content of polyphosphoric acid, the most rapid decomposing temperature of the foam decreased, and the residual carbon ratio tended to increase, indicating that the addition of polyphosphoric acid described in favor of the flame retardant properties of the polymer.The formation procedure of gaseous products during the polyurethane combustion was studied by TGA- FTIR technique. The combustion process of the bio-based polyurethane foam can be divided into three phases. Carbon dioxide as well as ammonium ion were formed in all the three stages of combustion. In the first phase a number of other gaseous substances, including carboxylic acids, alcohols and anhydrides, were volatilized. In the second phase, the gas also contained alkynes. In the third phase, only CO2 and ammonium ions were formed.