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.
Gut immunity in Lepidopteran insects.
Lepidopteran insects constitute one of the largest fractions of animals on earth, but are considered pests in their relationship with man. Key to the success of this order of insects is its ability to digest food and absorb nutrition, which takes place in the midgut. Because environmental microorganisms can easily enter Lepidopteran guts during feeding, the innate immune response guards against pathogenic bacteria, virus and microsporidia that can be devoured with food. Gut immune responses are complicated by both resident gut microbiota and the surrounding peritrophic membrane and are distinct from immune responses in the body cavity, which depend on the function of the fat body and hemocytes. Due to their relevance to agricultural production, studies of Lepidopteran insect midgut and immunity are receiving more attention, and here we summarize gut structures and functions, and discuss how these confer immunity against different microorganisms. It is expected that increased knowledge of Lepidopteran gut immunity may be utilized for pest biological control in the future.