Unstable Prepolymer System

There is only one unstable prepolymer system widely used in commercial practice, namely Vulkollan from Bayer, and this will be described as representative of this class of urethane elastomers. 

Formulation (pbw) Multrathane F66 100 1,4-Butane diol 4 8 Diethylene glycol 2 0 Multrathane F66 100 1,4-Butane diol 6-5 Multrathane F66 100 Multrathane XA 14 3 Multrathane F66 100 Diphenylmethane diisocyanate 28 7 1,4-Butane diol 16 

Vibrathane type Processing temperature ( C) 6005 Polyester 100 6008 Polyester 93 6004 Polyester 88 A682 Polyester 71 6006 B602 Polyester Polyether 65 100 B600 Polyether 100 B601 Polyether 71 B605 Polyether 110 

Vulkollan is the trade name applicable only to polyurethanes made from polyesters, NDI and chain extenders supplied by Bayer. Materials with somewhat similar properties can be made by substituting MDI for the NDI, but they can no longer be described as Vulkollan. The Vulkollan basic system became available in the early 1950s and has since remained essentially unchanged. 

Two polyesters in particular are used in the manufacture of Vulkollan, i.e. Desmophen 2000 and Desmophen 2001 (see Table 5.13). Desmophen 2000 is a linear polyethylene adipate with an approximate molecular weight of 2000 and a low acid number, and can be considered to be the normal polyester to be used. Demophen 2001 is again a linear polyester with a similar low acid number but consists of a mixed glycol polyester. The effect of using this mixed polyester is to reduce the crystallinity and make the final product more suitable for use at lower temperatures. At normal temperatures, however, Desmophen 2001 products have slightly inferior properties to products from Desmophen 2000. 

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Fig. 5.5. Manufacturing route for urethane based on unstable prepolymer systems (Vulkollan) (from Wright Gumming, 1969).

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. 

The prepolymer should be used on a first-in, first-out (FIFO) basis, as it may deteriorate slowly on standing. During the normal manufacturing process, the system is stabilized to reduce the aging process. Systems based on the unstable prepolymers (such as NDI-based materials) only have a life of several hours, so this must be taken into account. 

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