Vulkollan properties

Some typical properties of a Vulkollan-type polyurethane cast rubber and a black-reinforced polyurethane rubber processed by conventional techniques are compared with black-reinforced natural and nitrile rubbers in Table 27.2 ... 

RUBBER (Synthetic). Any of a group of manufactured elastomers that approximate one or more of the properties of natural rubber. Some of these aie sodium polysulfide ( Thiokol ). polychloiopiene (neoprene), butadiene-styrene copolymers (SBR), acrylonitrilebutadiene copolymers (nitril rubber), ethvlenepropylene-diene (EPDM) rubbers, synthetic poly-isoprene ( Coral, Natsyn ), butyl rubber (copolymer of isobutylene and isoprene), polyacrylonitrile ( Hycar ). silicone (polysiloranei. epichlorohy-drin, polyurethane ( Vulkollan ). 

Polyurethane rubbers, in general, and the Vulkollan-type rubbers, in particular, possess certain outstanding properties. They usually have higher tensile strengths than other rubbers and possess excellent tear and abrasion resistance. The urethane rubbers show excellent resistance to ozone and oxygen (in contrast to diene rubbers) and to aliphatic hydrocarbons. However, they swell in aromatic hydrocarbons and undergo hydrolytic decomposition with acids, alkalis, and prolonged action of water and steam. 

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. 

PHYSICAL PROPERTIES OF CELLULAR POLYESTER URETHANES (VULKOLLAN) (FROM WRIGHT CUMMING, 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. 

In spite of the processing limitations imposed by the use of unstable prepolymers this group of casting processes, particularly that used for making Vulkollan, remains important because of the good mechanical properties exhibited by the cross-linked rubber. 

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