Polyurethane Selection Criteria

Selecting a polyurethane for any application is composed of two stages. Properties Required for Application 

Every application needs to be evaluated on its merits for the range of properties required. A database of previously successful applications is a distinct advantage. An analysis of the main attributes of an application is needed, e.g., compression, shear, static, dynamic, or wear. Many of these factors are not on the supplier s data sheets and the information needs to be obtained from the supplier or by experimentation. 

The overall environmental conditions also must be evaluated. Some of the information may be hard to obtain from the customer due to commercial secrecy. Knowledge of the general type of application needs to be known for example, a simple defoaming agent may be let down in diesel, which has a moderate effect on polyurethanes. The effect of the overall concentration then needs to be evaluated. 

Changes in temperature are an important limitation. The temperature may be from the process stream or it may be generated internally by dynamic work. 

Several factors must be taken into account when the exact grade of material is chosen. The first is the pot life of the system. The pot life controls the time it takes to fill the mold, allowing any entrapped bubbles to rise and to gel off so that it can be placed in the curing oven. This represents a large portion of the physical cycle time. 

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The mechanism of cure has an important influence on the filler choice. In reactive systems such as polyurethanes, water present in formulation adversely affects sealant stability (shelf-life) which imposes the additional selection criterion of water content. If water is present in the filler it must be removed by additional operations either by an expensive drying process or through the use of chemical moisture scavengers which is also an expensive approach. 

Biocompatibility and Mechanical Properties. Currently, their are no suitable artificial materials for the prosthetic replacement of articular cartilage. The biocompatibility is considered the primary criterion in the selection of such a material. In a recent study, Furst and co-workers(10) compared the biocompatibility of the polyurethane to the well known medical grade silicone polymer. The tissue reactions to small polymer discs, inserted in an articulating space—the suprapatellar bursa of rabbits, was examined. The foreign body reaction of the tissue at the implantation site was evaluated at intervals of 7 days,... 

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