Transparent polyurethanes

Vapor permeable adhesive films are of use in those wounds in which granulation tissue is established and wound exudate is declining. These products were developed as materials that would, in part, mimic the performance of skin. The resultant products were transparent, synthetic adhesive films generically described as vapor permeable adhesive membranes, and comprised of transparent polyurethane or other synthetic films of low reflectance, evenly coated on one side with a synthetic adhesive mass. The adhesive is cohesive and inactivated by contact with moisture and will not therefore stick to moist skin or the wound bed. The films are permeable to water vapor, oxygen, and carbon dioxide but occlusive to water and bacteria and have highly elastomeric and extensible properties. They are conformable, resistant to shear and tear, sterile and particle free. 

A problem in producing transparent polyurethanes arises from the tendency of some polyols to crystallize (cold-harden) on ageing. This is a particular problem with soft and flexible urethanes as these require the use of high molecular weight polyols and there is an increase in polyol crystallization tendency with molecular weight increase. 

Reaction temperature is observed to have a profound influence on the transparency and tensile properties of these PUs. Figure 12.2 illustrates this for the synthesis of the typical transparent polyurethane (T1 in Table 12.4) which was made at 85, 90, 100, 120 and 130°C. The reaction rate was monitored using an infrared technique which measured the disappearance of the isocyanate peak at 4-4 microns. 

The type of chain extender used was found to play a dominant role in the preparation of flexible, transparent polyurethanes, as mentioned previously. Use of low block ratios (i.e. low hard-segment content) was found necessary to obtain soft materials. However, soft-segment crystallization can result in opacity, so care is necessary. Samples T20, T21 and T22 of Table 12.4 are examples where soft-segment crystallization has occurred, and the resulting materials show opacity and high hardness. 

Butt joint specimens were constructed using clear PMMA as the adherends and transparent polyurethane as the adhesive with a wide variety of thickness to diameter t/D) ratios [12-14]. An experimental setup was constructed to facilitate observation of the bond line while the specimens were loaded in tension. Experimental observations were consistent with numerical predictions. Failure initiated at either the center or outside edge of the bonded surface, depending on the particular geometry tested. Further, the load required to sustain failure was... 

Menezes, M. and D. F. Watt. 1992. Modeling of Infrared Heating of Transparent Polyurethane. Society of Plastics Engineers Technical Papers, 38, 109-113. 

High molai mass polyuiethanes weie obtained from condensation of 4,4 -(hexa luoioisopiopylidene)bis(phenylchloiofomiate) with various diamines (125). These polymers could be cast into transparent, flexible, colodess films or spun into fibers which showed promise as crease-resistant fabrics. Other polyurethanes discovered are good candidates for naval and aerospace apphcations (126). 

This process involves the suspension of the biocatalyst in a monomer solution which is polymerized, and the enzymes are entrapped within the polymer lattice during the crosslinking process. This method differs from the covalent binding that the enzyme itself does not bind to the gel matrix. Due to the size of the biomolecule it will not diffuse out of the polymer network but small substrate or product molecules can transfer across or within it to ensure the continuous transformation. For sensing purposes, the polymer matrix can be formed directly on the surface of the fiber, or polymerized onto a transparent support (for instance, glass) that is then coupled to the fiber. The most popular matrices include polyacrylamide (Figure 5), silicone rubber, poly(vinyl alcohol), starch and polyurethane. 

P.R.248 exhibits excellent bleed resistance in plasticized PVC. Transparent specimens equal step 8 on the Blue Scale for lightfastness, while 1/3 SD samples match step 7. The pigment does not perform as well in terms of weatherfastness, for instance in PVC plastisols for coil coating. P.R.248 was also recommended for use in elastomers, polyurethane, and unsaturated polyester. 

The isosorbide polyurethane based on the aliphatic diisocyanate P(I-HMDI) is flexible. It is a thermoplastic with a glass transition temperature of 110°C and softening temperature of 190°C. Both transitions are well below its degradation onset which occurs at approximately 260°C. It forms good films by evaporation from its solutions and colorless transparent compression moldings. 

As we will show, biocompatibility of a device can be approached from two perspectives. This is most clearly established in a discussion of how blood or plasma interacts with a foreign body. We can approach a project with the goal of allowing the body to react to the device and encapsulate it by a natural process, or alternatively, adopt a strategy to make the material as transparent as possible. Both techniques have been used with some degree of success. It is testimony to the versatility of polyurethanes that either approach can be accomplished with a simple change in the polyol. 

Rubbers rely on fillers (both reinforcing and nonreinforcing) to obtain their properties. The curing system also produces a dirty-colored material. To color a rubber is difficult, and only a few basic colors are used. To obtain a transparent rubber, special latex or synthetic cis-polyisoprene must be used, and the use of a peroxide cure is normal. Polyurethanes can be colored any color, but the yellowing of aromatic systems must be taken into account. Aliphatic systems can give transparent nonyellowing systems. 

Numerous polymers require such a precursor for instance, by polycondensation with hexamethylene diisocyanate, Keller [113] developed interesting polyurethanes which are amorphous, insoluble and transparent when they are prepared at temperatures higher than 75 °C, whereas they are brittle when the reaction is performed at a temperature lower than 75 °C. 

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