Prepolymer systems

Both TDI/MOCA and MDI/diol prepolymer systems are of importance for harder grades of printer s rollers and the wear-resisting applications such as pipes, pumps and impellers used in the mining and quarrying industries. 

In addition to one-shot processes, quasi-prepolymer systems are used commercially with rigid polyether foams. The quasi-prepolymer is commonly produced using excess TDI rather than diphenylmethane di-isocyanate. Sinee the former isocyanate is light in colour and the latter dark, quasi-prepolymer foams... 

Figure 5 Urethane polymerization for three prepolymer systems.

There is another method, called the quasi-prepolymer technique, which is similar to the full-prepolymer process but utilizes prereacted isocyanates in the 10-20% NCO range. This eases processing compared to full-prepolymer systems, requiring lower temperatures and volume ratios typically from 4 1 to nearly 1 1 (polyol to isocyanate). 

In the prepolymer system, the diisocyanate and polyol (either a polyether or a polyester) are reacted to give a prepolymer which may be either a liquid or a waxy solid. The reactant ratios used ensure the prepolymer contains isocyanate groups at the chain ends. 

PUs are also widely used as coating materials sold as finished polymers, two-part systems, and prepolymer systems. Water-based PU systems are now available allowing easy home use. Aromatic diisocyanate-derived coatings generally offer poor external light stability while aliphatic-derived systems offer good light stability. 

Urea—formaldehyde and melamine—formaldehyde reagents are resin formers, which not only cross-link cotton but also copolymerize with themselves. These have been used both as simple cross-linkers or prepolymer systems. If too much of the polymerization is concentrated on the fiber surface, the fabric may be sufficiendy stiffer that it takes on a boardy character. As such, the finisher must control the action of agent to give the desired crisp hand but prevent the development of boardiness. Melamines have been recommended for applications when complete shrink resistance is required. However, both finishes were rejected for the white-shirt market because of loss of strength when hypochlorite bleach is used on account of vulnerable NH groups and the ensuing discoloration (37). 

The ultimate tensile strength and the modulus at various strains are always quoted in technical literature. These are very good quality control values and give an indication to the type (ether, ester, MDI, IDI) and nature of the prepolymer system used. These tests are also valuable in evaluating the chemical and aging effects on polyurethanes. Where polyurethanes are used in tension, the amount they are stretched is normally no more than 30%. The highest extension is on the order of 100%. 

Saturated polyesters can be also added to the epoxide/cyanate prepolymer systems [81,88],... 

Polymers bearing side-chain anthraquinone moieties have also been reported. In particular, poly(2-acrylamidoanthraquinone) [poly(AAQ)] is found [51] to exhibit, in combination with a tertiary amine such as p-diethylaminobenzoate, higher solubility and the same photoactivity as the monomer in a mixed vinyl-urethane/triethyleneglycol dimethacrylate prepolymer system, when irradiated by visible light. 

Epoxy Prepolymers. These prepolymers are commonly formed from Bisphenol A and epichlorhydrin (Figure 4). A variety of other materials are also employed, but to a lesser extent. The lowest member of the series might well be considered structoterminal, while all the others must be considered as structopendant prepolymers. Molecular weights range from a few hundred to about 4000 for commonly used industrial epoxy prepolymers. Most common cross-linking agents are amines and anhydrides however, epoxies may also be combined with a variety of other prepolymer systems (25). 

The semi-prepolymer systems have the same advantages as the one-shot, two-component premix systems. 

Uniroyal Chemical, MDI-Prepolymer Systems - Viable Alternatives to MOCA Cured Urethanes. April 18, 1978. 

A wide variety of monomers and prepolymers are used to produce PC. The polymers most frequently used are based on five types of monomers or prepolymer systems ... 

More recently (171) a two-component 100% solids prepolymer system, based on TDI and poly(oxypropylene) glycols, has been disclosed by Alva-Tech, Inc. This system can be run on a conventional urethane or vinyl plastisol casting line and has been suggested as a replacement for vinyl coatings for fabrics. 

Quasi-prepolymer systems based on polyester polyols and polyether polyols are becoming important in shoe soling, the former being most wear resistant and the latter the easiest to process. 

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