Polyurethane curatives

Ethacure E300 107 Di-(methylthio)toluene diamine DMDTA 

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There are a number of different polyurethane curatives available based on methylene dianiline (see Figure 2.20) with different groups next to the amine group (NH2). MOCA with the chlorine atoms has been the most successful. 

MCHD from ring reduction of I DA (60,78) has been cited as an epoxy curative (79) and is available from Air Products and Chemicals as a developmental cycloaHphatic diamine. Ring reduction of stericaHy hindered arylenediamines such as diethyltoluenediamine [68479-98-1J -ptovides slower-reacting alkylated 1,3-cyclohexanediamines for polyurethane, polyurea, and epoxy use (80). 

Only a few commercial uses for TDA per se have been found. In epoxy curing appHcations, 2,4- I DA has been used as a component of a eutectic mixture with short chain aUphatic glycidal ether resins (46) as well as by itself (46,47) TDA (46) and single isomers (47) are also used as amine curatives. TDA can be used as a chain extender in polyurethanes (48,49). TDA is cited as a monomer in making aromatic polymers with unique properties, eg, amorphous polyamides (50), powdered polyamides (51), and low melting, whoUy aromatic polyamides (52). 

A study was done measuring the thermal oxidative stability of polyurethanes made from PPG polyols, varying the isocyanate curative. Oxygen absorption was... 

Polyester polyurethanes are usually based on a blend of a quasi-prepolymer (polyester/MDl) and a diol/polyester suitable for spray-up operation. An alternative is to use a solvent-containing system using blocked curatives to give an extended pot-life of 2 to 3 hours enabling them to be brush, roller or spray applied. 

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. 

The two primary hydroxyl groups provide fast reaction rates with diisocyanates, which makes this diol attractive for use as a curative in foams. It provides latitude in improving physical properties of the foam, in particular the load-bearing properties. Generally, the ability to carry a load increases with the amount of 1,4-cydohexanedimethanol used in producing the high resilience foam (95). Other polyurethane derivatives of 1,4-cyclohexanedimethanol indude elastomers useful for synthetic rubber products with a wide range of hardness and elasticity (96). 

The major problem is moisture that is absorbed into the polyurethane system or into the curative and auxiliary materials. Free water will liberate carbon dioxide when the chain extension is carried out. It is important to keep the reactants dry, as any moisture that may have come in contact with the prepolymer will react to give an amine and carbon dioxide. This amine reacts with more isocyanates to form a disubstituted diamine. The reaction is outlined in Figure 2.9. 

However, one of the main points to be considered in the making of a polyurethane elastomer (apart from the reactivities of the isocyanate and curative) is the water absorption. Special precautions must be taken to keep a diol curative (such as butanediol) sufficiently dry for use. Diols have to be kept dry with molecular sieves and dry nitrogen blanketing. 

A polyurethane made from a 90 Shore A PTMEG prepolymer and the two curatives (DuPont) both had excellent physical properties, but the pot lives for the two were ... 

Polyurethane systems can be produced to have near-equal or easy mixing component ratios (by either weight or volume). The prepolymers in this type of system are called quasiprepolymers. Quasiprepolymers are normally taken as having at least a fourfold molar excess of isocyanate in the isocyanate side of the system. The other side contains the remainder of the polyol, curative, and any catalysts. 

In cross-linked polyurethanes, there are actual chemical bonds formed in a three-dimensional manner. The main difference is that with the cast-able polyurethanes the actual chemical structure consists of two major zones, a hard zone and a soft zone. This is not as pronounced in the cross-linked polyurethanes. A typical example is the addition of TMP (trimethy-lol propane) to polyurethane to make it softer. This curative will make the material much softer as it breaks the hard segment zoning up to a certain degree. They do, however, give a material with improved compression set properties. 

The prepolymer as received from the manufacturer has a simple chain that has been terminated with an isocyanate. The isocyanate ends with this magical NCO group. The NCO is the reactive part of it. The higher the percentage of NCO in the prepolymer, the harder the material will be. An 80 Shore A will have an NCO of approximately 3.1 to 3.2%, whereas a 75 Shore D will have an NCO content of about 11.2%. To obtain the chain extension, one must add an appropriate amount of an amine or diol curative. For every curative, there is a different amount that must be added. The manufacturers of the prepolymers and curatives will give the appropriate factors for mixing the polyurethane. The prepolymer must be heated before use. This is to reduce the viscosity of the material as well as to obtain the correct cure rate and complete cure time. 

The final properties of the polyurethane can be enhanced by using slightly more or less than the theoretical amount of curative needed to react with all the available isocyanate groups. Table 4.1 details the changes in properties with respect to the level of curative. 

Plasticizers can perform several rolls in a polyurethane compound. There are two major groups of plasticizers used. The most common is the nonreac-tive plasticizers such as the ester group, and the second is the long-chained diols that act as a combined curative and plasticizer. In the case of the reactive material, this must be taken into account when calculating the amount of curative to be used (see Appendix 2). 

There are two main times during the processing of polyurethane where vacuum degassing may be required. The first is the prepolymer as supplied by the manufacturer. The second is after mixing the prepolymer with the rest of the system (curative, pigment, extenders, and fillers). 

For an epoxy resin with epoxide value of 183 if hardened with Ethacure 100, one would need (44.6 x 100)/183 = 24.5 ppw of E100 for 100 pbw of resin. The required amounts of curative for the polyurethane and the epoxy are calculated separately. 

TDI-based polyurethanes produce the best properties when further chain-extended with amine-based curatives. The overall properties can be increased if only the 100% 2,4 isomer is used. These materials are generally not suited for use with food. MDI-based polyurethanes have good overall properties and as they are predominately cured with a diol, they can obtain FDA approval more readily. 

The normal operating temperature of polyurethane is between ambient and 120°C (dry conditions). At these temperatures, the properties are normally at their best. The envelope can be increased by the use of the most appropriate curative and backbone. For moist conditions, the normal maximum temperature is 80°C using a PTMEG polyether, whereas it drops to 50°C for conventional polyesters. The newer esters have more hydrolysis resistance and can be used at slightly higher temperatures. The dry and moist temperatures can be extended by using newer isocyanates such as PPDI. 

In processing polyurethanes, the hardness is not greatly affected by the mixing ratios. A material mixed at 80% of the theoretical curative level has greatly different overall properties to one mixed at 100%. The hardness is still at the same level. Figure 7.6 shows the hardness variations with different curative variations. 

This moisture has several effects. Initially, it will react with any remnants of isocyanates and curatives. Moisture acts like a plasticizer, helping to give the final properties of the polyurethane. Under the influence of heat, it will cause some breakdown of the material, especially in the polyesters. 

Carbodiimides have been found to stabilize the polyester polyurethanes by blocking the carboxyl group formed when the chain has been broken. This helps prevent further autocatalytic degradation. The action is most pronounced when amines are used as the curative in caprolactone polyurethanes. 

The hardness of polyurethanes can be obtained by either the density of the hard segments, the choice of curative, or the addition of plasticizer. The importance of hardness is to control the rigidity of the part (if not reinforced) or its softness to conform to the shape of some other object it passes over. For very low hardness (< 60 A) applications such as rolls, TDI esters are the most suited, whereas ether-based materials are least suited. Hard materials do have better abrasive resistance than soft materials. 

Polyurethanes based on MDI have the greatest flexibility in the choice of curatives and blends of curatives that can be used to obtained the desired properties. In cast elastomers, the hygroscopic nature of the curatives must be taken into account and proper handling and storage used otherwise, there may be a very high reject rate. Catalysts are also more readily used with the MDI materials. Organic acids such as adipic or oleic acids work better with TDI materials than MDI systems. 

There are two major chemical areas of health concern in the polyurethane industry. The first is isocyanates, in their free form as a liquid or in the vapor form. The second is amines. These can be either curatives or catalysts. One curative, MOCA (CAS number 101-14-4), is classed as a suspect carcinogen in most parts of the world and is subject to many rules and regulations regarding its use. MOCA (MBOCA) is known by many different names, which are listed on various websites such as www.chemindustry.com/chemicals. 

The two main groups of curatives used in the castable polyurethane industry are ... 

Amine or cyclic fragments. A class of compounds used as catalysts or curatives in polyurethane elastomer and foam reactions. Amines are characterized by having N, NH, or NH2 groups in the molecule. 

Gel time For polyurethanes, the interval of time between mixing together the polyol and diisocyanate or prepolymer and curative and the formation of a nonflowing, semisolid, jelly-like system. 

Polyurethanes are made by extending chains of a prepolymer made from a macro diol and a diisocyanate. The prepolymer is further extended with a diol or an amine curative. The long chains form a solid which is relatively weak. When the part is given a longer heat treatment, the molecules align themselves and intermolecular bonds (hydrogen bonding) are formed. At this point the full mechanical properties are established and the material, if suitably formulated, has excellent mechanical and chemical properties. 

When the prepolymer chain is extended with a diol, the polymer formed has only urethane linkages. The polymer formed with the diamine chain extender is strictly a polyurethane polyurea. The first urethane component is from the initial chain extension when the prepolymer is prepared. A diamine curative will form urea linkages (Figure 2.3) between chains. 

NSC 406696 Suool B Tetramethylene 1,4-diol Tetramethylene glycol 1,4-Tetramethylene glycol. Curative, chain extender provides reactive H-source in prepolymer production used to provide hard segments in PU. Used as an intermediate and in polyurethane formulation in the hard segment as a curative. Crystals mp = 20.1° bp = 235° = 1.0171 Xm = 291 nm (e = 200, H2SO4) freely... 

Uses Functional polyol for polyurethane industry for industrial/consumer RIM and structural polymers, dynamic elastomers, adhesives, binders, coatings, and sealants reactive diluent, dispersant for solvent coatings, etc. flexibilizerfor PU PU curative and crosslinker ManufJDistrib. Allchem Ind. BASF Bayer Huntsman Polyurethanes Witco... 

Polyol, rigid Polyoxypropyleneamine Polyoxypropylene triol Uses Functional polyol for polyurethane industry for industrial/consumer RIM and structural polymers, dynamic elastomers, adhesives, binders, coatings, and sealants reactive diluent, dispersant for solvent coatings, etc. flexibilizer for PU PU curative and crossiinker Manuf./Distrib. Aiichem Ind. http //www.aiichem.com] BASF http //www.basf.com] Bayer http //www.bayerus.com] Huntsman Poiyurethanes http //www.icipu. com] http //www. rubinate. com Trade Name Synonyms Adeka Polyether PR-3007 [Asahi Denka Kogyo http //www.adk.co.jp]] Baycoll ND 1110 [Bayer/Fiber, Addits., Rubber http //www.bayerus.com]] Baycoll ND 2060 [Bayer/Fiber, Addits., Rubber http //www.bayerus.com]] Baycoll NT 1380 [Bayer/Fiber, Addits., Rubber http //www.bayerus.com]] Baycoll NT 5028 [Bayer/Fiber, Addits., Rubber http //WWW. bayerus. com]... 

Sealants are divided into groups according to the generic names of polymer base. The main groups include polyurethanes, silicones, acrylics, polysulfide and others (PVC, polybutylene, styrene-butadiene-styrene copolymers, polychloroprene, and several others). The amount of solvent used in sealants is controlled by the standards which previously divided sealants into two groups these below 10% VOC and those above. Recently, a provision was made to include water-based acrylics and the limit of VOC for class A sealants was increased to 20%. Polyurethane sealants and structural adhesives can be made without solvent (the first solvent-free polyurethane sealant was made in 1994). Solvents are added to reduce sealant viscosity and to aid in the manufacture of polymer. Typical solvents used are mineral spirits, toluene, and xylene. A small amount of solvent is emitted from curatives... 

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