Study of Effect of NCO/OH Molar Ratio and Molecular Weight of Polyol on the Physico-Mechanical Properties of Polyurethane Plaster Cast
2 Abstract: Polyurethane plaster cast was an open weave fabric bandage, prepared by coating polyurethane prepolymer on raschel knitted fiberglass and polyester fabric. Curing takes place by coming in contact with water to form hard rigid cast. Polyurethane prepolymers were synthesized by reacting molar excess of diphenyl methane diisocyanate (MDI) with polypropylene glycol (PPG) at 60°C for 2 hours. This effect of different NCO/OH molar ratio and molecular weight of polyol on the tack free time, setting time, diametral compression strength, percent elongation and tensile strength of polyurethane plaster cast were investigated.. It was observed that with increase in NCO/OH molar ratio, tack free time and setting time of the cast decreases, whereas diametral compression strength increases. Tensile strength was determined in universal testing machine and was found to increase with increasing NCO/OH ratio while opposite trend was observed in case of percent elongation. With increase in polyol molecular weight, tack free time and setting time of the cast increases, whereas diametral compression strength decreases. Blended polyols i.e. PPG 1000 with PPG 400 in 50/50 ratio gives tensile strength and diametral compression strength in between their unblended counterparts. In comparison to fiberglass cast, polyester cast has higher percent elongation and diametral compression strength.
STRUCTURE-TENSILE PROPERTIES OF POLYURETHANES
Abstract A series of hydroxyl terminated polybutadiene-(HTPB) based polyurethanes (PUs) were synthesized by solution polymerization with different types of diisocyanates, hard segment contents, NCO OH ratio and polymerization methods. Spectroscopic and stress-strain studies were carried out to elucidate the structure-property relationships existing in these polymers. It was found that the binding of the hard segment and the flexibility of soft segments had subtle effects on the properties of stress-strain at break. The segregation between hard and soft segments was manifested by incompatibility between hard and soft segments, steric hindrance of urethane groups and the extent of interchain hydrogen bonding. The hydrogen bonding index (HBI) measured by FTIR was employed to show the extent of interchain hydrogen bonding, which affects the segregation and hence the stress-strain property of the prepared films. Films with higher hard segment content and NCO OH ratio, 4,4′-diphenylmethane diisocyanate (MDI) and the one-stage polymerization method exhibit higher stress at break and lower strain at break. The measurement of FTIR was used to identify the degree of phase segregation and to correlate the stress-strain properties of PUs with different compositions.
Thermal and mechanical properties of solution polymerized segmented polyurethanes with butadiene soft segments
Abstract A series of HTPBD containing polyurethanes of high molecular weight have been synthesized in solution. The value of the soft segment Tg is very close to that of the free HTPBD and independent of hard segment content indicating complete or very nearly complete phase segregation. Since the hard segments of TDI/BDO are amorphous, the driving force for phase segregation must arise from the large degree of incompatibility between the polar hard segment and nonpolar soft segment. Furthermore, in these samples there is also no opportunity for hydrogen bonding between hard and soft segments to enhance compatibility. The values of the hard segment glass transition increase with the average hard segment length following a Fox-Flory type relationship. In contrast to the segment Tg observed in bulk polymerized samples, only a single hard segment Tg occurred in the present study. This indicates that the double Tg behaviour is a result of the heterogeneous nature of the bulk polymerization. With increasing hard segment content, the properties vary from soft to rigid elastomers, and rubber roughened plastics. This variation in properties is caused by changes in the sample morphology which depends upon the relative fractions of hard and soft segments. Mechanical properties show marked improvement over the corresponding bulk polymerized samples. Unlike polyester and polyether urethanes, these materials evidence no change in the soft segment Tg following thermal treatment and no effect of thermal history on the mechanical properties.