Polyols polyether/polyester

The synthesis of PU can be carried out by the reaction described in Eq. (2.24). If the functionality of the hydroxy-containing compounds or the isocyanate is increased beyond 2, branched and possibly crosslinked polymers are produced. Because the nature of the polyol (polyether, polyester, polybutadiene, etc.) and isocyanate components can vary widely, PU are among the most versatile polymers, producing a wide variety of materials such as elastomers, foams, coatings, adhesives, or fibers. 

Polyester Polyols. Initially polyester polyols were the preferred raw materials for polyurethanes, but in the 1990s the less expensive polyether polyols dominate the polyurethane market. Inexpensive aromatic polyester polyols have been introduced for rigid foam appHcations. These are obtained from residues of terephthaHc acid production or by transesterification of dimethyl terephthalate (DMT) or poly(ethylene terephthalate) (PET) scrap with glycols. 

In addition, polyester polyols are made by the reaction of caprolactone with diols. Poly(caprolactone diols) are used in the manufacture of thermoplastic polyurethane elastomers with improved hydrolytic stabiHty (22). The hydrolytic stabiHty of the poly(caprolactone diol)-derived TPUs is comparable to TPUs based on the more expensive long-chain diol adipates (23). Polyether/polyester polyol hybrids are synthesized from low molecular weight polyester diols, which are extended with propylene oxide. 

The polyols used are of three types polyether, polyester, and polybutadiene. The polyether diols range from 400 to about 10,000 g/mol. The most common polyethers are based on ethylene oxide, propylene oxide, and tetrahydrofuran or their copolymers. The ether link provides low temperature flexibility and low viscosity. Ethylene oxide is the most hydrophilic and thus can increase the rate of ingress of water and consequently the cure rate. However, it will crystallize slowly above about 600 g/mol. Propylene oxide is hydrophobic due to hindered access to the ether link, but still provides high permeability to small molecules like water. Tetrahydrofuran is between these two in hydrophobicity, but somewhat more expensive. Propylene oxide based diols are the most common. 

PUR are a broad class of highly cross-linked plastics prepared by multiple additions of poly-functional hydroxyl or amino compounds. Typical reactants are polyisocyanates [toluene diisocyanate (TDI)] and polyhydroxyl molecules such as polyols, glycols, polyesters, and polyethers. The cyanate group can also combine with water this reaction is the basis for hardening of the one-part foam formulations. 

Polyether-polyester polyol hybrids, 25 472 Polyetherimide-polysiloxane block copolymers, 24 698, 704, 708... 

Foams with enhanced properties have been developed with the liquified wood in the presence of polyether polyols and polyester polyols [28]. An example is shown in Fig. 2. 

Second-generation solventless polyurethane adhesives Two-component second-generation adhesive systems are the most widely used of the 100% solids products. These two-part adhesives comprise an isocyanate terminated prepolymer based on the reaction product normally of MDI and polyols (polyether or polyester) in the presence of excess isocyanate, and a polyol (polyether or polyester based). 

The polyols which function as modifiers include ethylene glycols, 1,4-butanediol, polyether polyols and polyester polyols. In recent years aromatic polyesters prepared from reclaimed PET (polytetraethylene terephthalate) or the distillation residue of DMT (dimethylterephthalate) have appeared as modifiers for urethane-modified isocyanurate foams (73, 78). These aromatic polyesters are produced by the transesterification of reclaimed PET or DMT distillation residue. 

Polyurethanes are produced by reacting a polyisocyanate with a polyol (or with other chemicals such as polyethers, polyesters, and glycols, which have hydroxyl groups). 

Preparation of Cast Elastomers. The cast elastomers were prepared in a two-step procedure. First prepolymers were made from one polyether polyol (poly(oxy-tetramethylene) glycol of 1000 M.W., (POTMG)) and two polyester polyols (adipate polyester of 2000 M.W. (PAG) and polycaprolactone of 1250 M.W. (PCL)) by reaction with the corresponding diisocyanates (MDI, PPDI, CHDI or NDI) at an NCO/OH ratio of 2/1. The temperature was maintained at 80°C and periodic samples were withdrawn to determined the isocyanate content. When the isocyanate content of the mixture reached within 0.3% of the calculated value, the reaction was stopped by cooling. The prepolymer could be kept for a period of six months in the absence of moisture. The isocyanate-terminated prepolymers were then chain-extended with... 

The polyols employed in the manufacture of polyurethanes are divided into two main classes, polyether-polyols and polyester-polyols, the former bong the most important. commercially. 

The resulting polyol, alone or in combination with classic rigid polyols, polyethers or polyesters, has a good potential use in fabricating rigid PU foams, especially in very reactive systems ( spray foams). 

By hydrolysing a polyether PU, a polyether polyol is obtained with a similar structure to those of the initial virgin poly ether polyol. For polyester PU the hydrolysis reaction is more complicated because the esteric groups of polyesters are hydrolysed back to monomers, such as diacids and glycols or polyols (reaction 20.3). 

From high MW triols or low branched oligo-polyols (MW = 3000-6500 daltons) polyethers, polyesters, filled polyols (polymer polyols), are obtained elastic PU with a low degree of crosslinking (flexible and semiflexible foams, coatings etc). 

From low MW oligo-polyols (400-1000 daltons polyethers, polyesters, Mannich polyols, aromatic polyesters, oleochemical polyols, etc.), are obtained rigid, hard polyurethane structures (rigid PU foams, wood substitutes, etc). 

Based on the data in Table 21.1, the relative cohesive energy of oligo-polyols polyether polyols, polyester polyols and polyhydrocarbon polyols (for example polybutadiene or hydrogenated polybutadiene) occur in the following order ... 

Polyurethanes are formed by polymerization of isocyanates with polyether polyols or polyester polyols. The products are quite versatile and can be manufactured as either foamed or elastomerics. In addition, a small amount of isocyanates are used for adhesives and coatings and a minor quantity of monoisocyanates are used as precursors for carbamate pesticides. Phosgene is a key reactant in the commercial processes for all of the isocyanates. 

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