Polyurethanes pre-polymers

Polyurethane pre-polymer or thermoplastic polyurethanes are prepared by the reaction of polyester or polyether diols with diisocyanates. The reaction can be conducted in bulk, in a solvent, or in an aqueous solution or dispersion. Unless the reaction is carried out at an elevated temperature a catalyst is required. Organotin compounds are the prime catalyst for this reaction. 

Using this technique, a large variety of polyurethanes have been prepared from different vegetable oils. Natural polyols like castor oil (generally trifunctional) are directly reacted with diisocyanates to obtain branched polyurethanes, although it is difficult to control the reactivity. However, bifunctional castor oil can be polymerised with diisocyanates in the presence of suitable chain extenders and catalysts to produce polyurethanes in a more controlled manner (Fig. 6.4). A castor oil polyol-based polyurethane network can also be prepared from epoxy terminated polyurethane pre-polymer with 1,6-hexamethylene diamine. Epoxy terminated pre-polymer is obtained by the reaction of glycidol and isocyanate terminated polyurethane pre-polymer of castor oil polyol, poly(ethylene glycol) (PEG) and 1,6-hexamethylene diisocyanate. ... 

A three-enzyme electrode system, such as needed for creatinine measurement, poses a more difficult enzyme-immobilisation problem, in that different enzymes have different immobilisation requirements and their microenvironmental interrelationships need to be optimised. For one creatine sensor, the requisite creatine amidinohydrolase and sarcosine oxidase were immobihsed in polyurethane pre-polymer and PEG-hnked creatinine amidohydrolase was attached via diisocyanate pre-polymer to create a polyurethane adduct [14]. The likelihood of enzyme inactivation with chemical immobih-sation is high, but provided an enzyme preparation survives this, long-term stability is feasible. In the case of these three particular enzymes, a loss of activity resulted from silver ions diffusing from the reference electrode the material solution was to protect the enzyme layer with a diffusion-resisting cellulose acetate membrane. 

Arquad 2.10-50 Arquad 2C-70 Nitrite Arquad HTL8(W) MS-85 Polyurethane pre polymer... 

Both of the simple polyurethanes (TDI-PU and MDI-PU) were synthesized according to a well known solution polymerization technique (10). The polyurethane elastomer (MDI-PUE) was prepared by a pre-polymer method (11). 

Processing of polyurethanes is different from other elastomers in that most polyurethanes are cast into a mold. The mixing of the pre-polymer and curative can be done with specially designed machines in large shops or by hand in small shops or for small runs. 

Cyclic derivatives of type III include cyclic Mannich bases, such as dihydroben-zoxazines 497, employed as detergents for lubricating oils, - and cyclic urcides 498, precursors of crosslinking agents for fabrics, as well as other cyclic derivatives prepared by conversion of Mannich bases. Macromolecular derivatives of type IV are relatively small in size and have branched (star-shaped) structures they are of considerable importance as, for example, corrosion inhibitors 499, plastics stabilizers 500, - pre-polymers for epoxy-based electrophoretic paints, and polyols in polyurethane synthesis. ... 

In coating applications, urethane chemistry is used to cross-link coatings and also to prepare thermoplastic polyurethanes or isocyanate-terminated pre-polymers. 

Polyurethanes are one of the more commercially useful polymers and are a major commodity item. A series of papers have reported attempts to incorporate phosphazene oligomers into polyurethanes. Cyclic trimer (54) has free hydroxyl groups that was reacted with prepared isocyanate pre-polymer. The loading process increased thermal stability, lowered temperature resistance and hydrophobity however mechanical stability was slightly less. 

As the prepolymer reacts with moisture, this must be rigorously excluded from the can, since it will not form a skin on the paint but will eventually gel it right through. Another difficulty is the moisture found on most pigment surfaces. This must be removed, possibly by reaction with monoisocyanate, before the pre-polymer can be pigmented. If it is not, gelation occurs. The main outlet for this type of finish - which has the typical polyurethane virtues of toughness and abrasion-resistance - is as a floor varnish. 

Waterborne polyurethane resins are produced from maleinised monoglyceride (MMG) of sunflower oil, hydroxy-terminated polybutadiene, toluene diisocyanate and ethylene diamine. The carboxylic acid groups of MMG are neutralised by triethyl amine, making the resin water dispersible. The monoalkylated castor oil (MCO) or dehydrated castor oil (DCO) is treated with a polyether glycol at 120°C, followed by the addition of IPDI and DBTDL. To obtain a series of aqueous polyurethanes, butane diol and dimethylol propionic acids (DMPA) are added and the mixture heated to 70°C for two hours to produce a NCO-terminated pre-polymer which forms salt with triethylamine, giving a water-soluble polymer. The reaction mixture is dispersed in water and a chain extender, ethylene diamine, is added. Two aqueous polyurethanes, MCPU and DCPU, are finally obtained from MCO and DCO, respectively. 

Some hyperbranched polyurethanes with different compositions based on Mesua ferrea L. seed oil, sunflower oil, and so on, have been prepared by using monoglyceride and glycerol or monoglyceride with hyperbranched polyol. Hyperbranched polyurethanes have been prepared from soybean oil-modified hyperbranched polyol obtained via epoxidation and hydrofor-mylation. Castor oil-based hyperbranched polyurethanes have been synthesised using castor oil as the B3 monomer in an A2 -1- B3 approach. The A2 monomer, -NCO terminated pre-polymer was obtained by reacting MDI with PCL. The urethane reaction was carried out at ca. 110°C in the... 

Polytetrahydrofuran (polytetramethylene ether, PTHF) n, A type of polycol made from tetrahydrofuran by ring opening, have the mer [-CH2(CH2)3-0-] and -OH end groups, with low to moderate molecular weights. These polymers have long been used as pre-polymers for polyurethane elastomers. 

Polyurethane fibres of a kind different to those described above have become important within the last decade these are elastomeric fibres, which are commonly called spandex fibres. These products are made either by solution spinning or by reaction spinning. In the first process, a hydroxy-terminated polyester (e.g., an adipate) or polyether (e.g., poly(oxytetramethylene) glycol) is treated with an excess of diisocyanate (e.g., tolylene diisocyanate) to give an isocyanate-terminated pre-polymer similar to those used for cast elastomers (Section 14.6.1). The pre-polymer is dissolved in a strongly polar solvent (e.g., dimethylformamide) and treated with an aliphatic diamine or hydrazine to effect chain extension with hydrazine the following reaction occurs ... 

The solution is then either spun into a coagulating bath or into a hot atmosphere (where the solvent evaporates) to produce a fibre. In reaction spinning, a liquid isocyanate-terminated pre-polymer of the type described above is spun into an aqueous solution of an aliphatic diamine (e.g., ethylene diamine). Rapid reaction occurs on the surface of the filament to give high molecular weight polyurethane. The filament is then treated with water at 70°C to complete reaction in the interior. 

FIGURE 2.2 Monomers can form different shapes of polymer, (a) Randomly coiled linear thermoplastics, e.g. PMMA. (b) Shghdy branched thermoplastics, e.g. PVAC. (c) Highly branched thermoplastics, e.g. polyurethane foam pre-polymer, (d) Cross-hnked polymers with trifunctional junctions, perhaps formed by reaction of (c), e.g. epoxy resin, (e) Cross-linked polymer with tetra-functional junctions, e.g. polyester casting resin. Source Brydson (1982). 

Polymers that set by chemical reaction may react not only within the components of pre-polymer mixtures, but also with the substrate. This occurs principally with pre-polymers that are sensitive to water, such as isocyanates in polyurethanes and silanes. The chemical links can be deliberately created by using a reactive primer, e.g. a silane coupling agent. Many water-soluble polymers, e.g. poly(vinyl alcohol) and cellulose derivatives, may be expected to adhere more strongly and irreversibly because of the presence of reactive groups on degraded objects. 

One-part formulations are composed of an isocyanate pre-polymer in an anhy-drons solvent. As the solvent evaporates, cross-links are produced to form the polymer. Isocyanate and polyol in the two-part formulation react when they are mixed. Except when appUed to a smooth unreactive surface, a polyurethane treatment is irreversible. 

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