Transesterification polyurethane

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. 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

The chemistry of the glycolysis of polyurethanes is complicated by the fact that there are additional groups in the polymer such as ureas, allophanates, and biurets, and die PURs may be crosslinked. In die presence of the appropriate glycols and at about 200°C, PURs undergo transesterification to form polyols. Under the same conditions, ureas undergo glycolysis to form urethanes and amines (Fig. 10.5). 

These materials are segmented copolyether esters formed by the melt transesterification of dimethyl terephthalate, poly(tetramethylene ether) glycol and 1,4-butane diol. As with the thermoplastic polyurethanes, one can describe a hard segment and a soft segment, the hard segments forming crystalline areas which act as pseudocrosslinks . 

One of the more advanced technical offerings from castor oil is a line of polyester diols, triols, and higher functional polyols derived from 100% castor oil as products for the preparation of polyurethane prepolymers and elastomers [68]. The Polycin line of polyols are prepared by transesterification of ricinoleic acid and derivatives. The producers (Vertellus) offer diol and triol products, as well as a recently developed series of diol and triol glyceryl ricinoleate esters that are stated to be prepared from 100% castor oil, making them fully renewable in content. The products are recommended for coatings, sealants, and adhesive applications. 

Dialkyltin dicarboxylates, e g. Bu2Sn(02CCu 1123)2, are useful catalysts for polyurethane foam formation and as cross-linking agents for room-temperature vulcanization. Monobutyltins have been used as catalysts in transesterification and esterification reactions, e.g. in the reaction between phthalic anhydride and 2-ethylhexanol. 

Transesterification performed at 170-200°C in the presence of aliphatic carboxylic acids, such as adipic acid, after the depolymerization of PET in EG and propylene glycol leads to unsaturated polyesters. These materials are used in foam production or for the production of polyurethanes and polyester polyol copolymers [7-9]. 

In parallel with these developments, organotin compounds have found a variety of applications in industry, agriculture, and medicine, though in recent years these have been circumscribed by environmental considerations. In industry they are used for the stabilization of poly(vinyl chloride), the catalysis of the formation of the polyurethanes, and the cold vulcanisation of silicone polymers, and also as transesterification catalysts. 

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-Isooctane, and methanol-water-lsooctane. The resulting decrease In the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As Indicated by the decrease In properties, reaction with methanol Is Initially faster, but the reaction rate with water Increases with time — presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy Indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. 

Extensive degradation of polyester based thermoplastic polyurethane samples occurs due to hydrolysis In the presence of water and transesterification In the presence of methanol. 

Dimethyl terephthalate for the production of polyethylene terephthalate) is produced by the cobalt salt-catalyzed oxidation of p-xylene with oxygen (reaction 1.15).209 In this free radical process, some biphenyl derivatives are formed. In addition, triesters are formed from any trimethylbenzenes in the feed. Thus, the still bottoms contain several compounds, which are all methyl esters. Hercules found that transesterification of this mixture with ethylene glycol led to a mixture of polyols that could be used with isocyanates to form rigid polyurethanes. For the price, the Terate product was hard to beat. 

Thesecompounds, widely employed at the outset of the development of polyurethanes have been displaced from their market by polyether-polyols. For the production of rigid foams however, use is still made of aromatic polyols produced by the transesterification of dimethyl terephthalate by glycoL... 

Scheme 3.2 Synthesis of phosphorus-containing polyurethanes via transesterification of diesters of H-phosphonic acids with urethane diols.

The chemo-enzymatic synthesis of polyurethanes has been reported through the inter-esterification of castor oil and linseed oil at ambient temperature, using lipase as a catalyst and foUowed by treatment of the inter-esterified product with TDI. In the first step, partial esters are prepared by transesterification of soybean and linseed oils with n-butanol in the presence of lipozyme (a lipase) as the catalyst. The partial esters are then reacted with different diisocyanates to obtain a series of polyurethanes. The reaction of polyhydroxy compounds (transesterification reaction between different compositions of castor oil and glycolysed poly(ethylene terephthalate)) with diisocyanates offers a polyurethane network for new insulating coating applications. ... 

Polyurethanes prepared via transesterification of soybean and linseed oils with n-butanol in the presence of lipozyme (a lipase), also possess these improved properties, particularly in the case of MDI-based polyurethanes. Mechanical properties such as tensile strength, percentage elongation, elastic modulus, wear resistance, tear resistance. Shore A hardness, and thermostability of vegetable oil-based polyurethane IPNs showed significant improvement at a critical vinyl or acrylic polymer level. 

Polyurethane dispersions can also be prepared by a nonisocyanate process (35). An aliphatic polyester diol is reacted with a bis(2-hydroxypropyl carbamate), a triol, and a transesterification catalyst, such as dibutyltin oxide. Carboxy functionality is then introduced by reacting some of the remaining hydroxy groups with an anhydride. 

Carbamates can also be modified by transesterification [15, 16], An important example of this last reaction, which is already applied on an industrial scale for the synthesis of plasticizers, is the direct synthesis of polyurethanes from alkyl aryldicarbamates and diols, without the need for producing the diisocyanate at any stage of the process [16],... 

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