Excess of diisocyanate

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

The high exotherm and the number of feeds have led to the popularity of the quasiprepolymer variation of the one-shot technique. Quasiprepolymers are a version of the standard prepolymer method, where some of the polyol is reacted with an excess of diisocyanate. This is later reacted with the remainder of the polyols, including the chain extender and some catalysts. The advantage of this scheme is that the exotherm is spread between the preparation of the quasiprepolymer and the final cured part. In MD1 systems, BDO is often used as the curative. As the molecular weight of BDOs is low, the amount added is low. By dissolving it in the polyol, a more even mix ratio can be obtained. 

The acrylic groups were added in two steps first by addition of an excess of diisocyanate and then reaction with hydroxy-2 ethylacrylate. 

Polyurethanes with a few chromophoric hard segment units which do not show phase separation can be obtained by a polyaddition reaction of only a macrodiol and a diisocyanate in nearly stochiometric amount (slight excess of diisocyanate) together with a small amount of a difunctional chromophore (DAAB), counterbalancing the diisocyanate excess. The structure of such polyurethanes investigated here are given in Fig. 13. 

Most urethane adhesives are based on urethane prepolymers. A prepolymer is made by reacting an excess of diisocyanate with a polyol to yield an isocyanate-terminated urethane as shown in Fig. 11. Prepolymers may have exeess isocyanate present ( quasiprepolymers ) or they may be made in a 2 1 stoichiometric ratio to minimize the amount of free isocyanate monomer present. Most moisture-cured prepolymers are based on 2 1 stoichiometric ratios. Two-component adhesives generally are based on quasi-prepolymers, which use the excess isocyanate to react with either chain extenders present in the other component or with the substrate surface. 

In contrast in the two-step polymerization technique (Fig. 1.6), in the first step of the reaction a prepolymer is produced through the reaction of a SS oligomer with an excess of diisocyanate, followed by chain extension with a short diol(urethane or ester) or a diamine (urea, urethaneurea, amide, or ester-amide) to form the HS and also to increase the overall molecular weight of the polymer. 

This formulation contains 2-55 mol of difunctional hydroxy compound compared with 2-86 mol of diisocyanate, an excess of 0-31 mol of diisocyanate. The presence of 0 5% water in the Desmophen 2000 would remove the excess diisocyanate and result in the equivalent of a deficiency of 0-24 mol of diisocyanate. Since an excess of diisocyanate is required the water content of the Desmophen 2000 must be reduced, and to obtain consistent results it is recommended that it be reduced to less than 0 05%. Apart from altering the stoichiometry, the water can cause premature chain extension and gas evolution, both of which are detrimental to the process. It is obvious that polyesters must be very dry before being synthesized into the prepolymer. 

Two-stage crosslinking, in which in the first stage is the synthesis of a prepolymer containing two isocyanato endgroups in the classical way of reaction (a diol either of low or of high molecular weight with an excess of diisocyanate) and the second step to form the network, can be accomplished by... 

This limitation is overcome when the elastomer is cast from a mixture consisting of a linear polyester or poly ether, diisocyanate and glycol or diamine. In this process, the hydroxy-terminated polymer (e.g., poly(ethylene adipate) or poly(oxytetramethylene) glycol) is treated with an excess of diisocyanate (e.g., naphthylene 1,5-diisocyanate, tolylene diisocyanate or diphenylmethane 4,4 -diisocyanate). The product is a pre-polymer which is, in effect, a mixture of isocyanate-terminated polymer and unreacted diisocyanate. The pre-polymer is then mixed with either a glycol (e.g., 1,4-butanediol or 1,6-hexanediol) or diamine (which is usually a deactivated amine such as 3,3 -dichloro4,4 -diamino-diphenylmethane (MOCA)). The mixture is poured into a heated mould where it quickly sets. After about 30 mins the casting is removed from the mould and cured at about 110°C for 24 hours. 

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 ... 

Moisture-curing systems are based on isocyanate-terminated branched prepolymers which cure by interaction with atmospheric moisture. The prepolymers are prepared by reaction of an excess of diisocyanate (usually tolylene diisocyanate) with a polyol (polyester, polyether or castor oil). It is important that the residual free tolylene diisocyanate content should be low to reduce toxic vapour hazards. When the pre-polymer is exposed to moisture, the following reactions occur and cure is effected ... 

Unstable prepolymer systems are represented by the Vulkollan process. A prepolymer is first prepared by reacting polyol with an excess of diisocyanate typically at about HO C. In the specific case of Vulkollan the polyol is an aliphatic polyester of molecular weight c. 2000, and the isocyanate is 1,5-naphthalene diisocyanate. Because this isocyanate tends to become involved in side reactions leading to cross-linking the prepolymer is unstable. It has thus to be used within a few minutes of preparation by mixing with a chain extender, typically butane diol, and cast into a mould. A subsequent hot air cure for 24 hours at 100°C is desirable to obtain optimum properties. 

In a typical manufacturing process a prepolymer is first produced by reacting a linear terminal dihydroxy polyester or polyether of molecular weight 800-2500 with an excess of diisocyanate (usually 4,4 -diphenylmethane diisocyanate) at about 80-120 C. The product at this stage is an isocyanate-terminated prepolymer plus free (unreacted) isocyanate. This blend is then reacted with a chain extender, usually a glycol such as 1,4-butane glycol, to give a chain with a hard... 

When an excess of diisocyanate is used, crosslinking occurs through formation of an allophanate, which is the reaction product of an already formed urethane group and an isocyanate... 

Prepolymers can be cured by an excess of diisocyanate or by a combination of triols and polyols with prepolymer. The formation of urethane linkages by reaction of hydroxyl groups with isocyanate groups is catalyzed by a number of organometallic compounds. Most commonly, tin compounds such as stannous octoate or dibutyltindilaurate are used. Many other metal compounds have been used, including those of cobalt, lead, and copper (23). 

The preparation of foams implies the presence of porogens. Two methods are used for this purpose. The first one involves the in situ generation of carbon dioxide by a controlled reaction of water with an excess of diisocyanate in an initial phase. It is a useful solution but expensive due to the cost of isocyanates it is, however, the method chosen for the production of the flexible foams. It is sometimes also used for rigid foams... 

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