Prepolymer unstable

Initially, the water slowly reacts with the isocyanate. However, the reaction can be catalyzed with an appropriate catalyst, such as dibutyltin dilaurate or a morpholine tertiary amine catalyst. The isocyanate will react with water to form a carbamic acid, which is unstable and splits off carbon dioxide, to produce a terminal amine end group (see p. 76 in [6]). This amine then reacts with more isocyanate-terminated prepolymer, as shown above, to form a polyurea. This process repeats itself, building up molecular weight and curing to become a polyurea-polyurethane adhesive. 

MAPO ( see Table IV) has also been used effectively as a curing agent for prepolymers containing carboxylic acid, and like BITA it undergoes homopolymerization and oxazoline formation, particularly in the presence of ammonium perchlorate. The polymer network formed, however, is unstable and softens rapidly when exposed to higher temperatures. This phenomenon is caused by the presence of three phosphorus—nitro-... 

The unstable prepolymers are typified by the Bayer Vulkalon polyurethane. Vulkalon is generally made by reacting naphthalene diisocyanate (NDI) and hydroxyl terminated polyesters to form a prepolymer. The reactivity of the... 

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. 

As D Alelio s patent embodies conditions of low omdensation temperatures and so, just like some of the aforementioned solution polymerizations , teaches a>nditions beneficial to reactant and nx>duct integrity, it permits the preparation of polybenzimidazoles from monomers too unstable thermally or chemically for use in the melt or poly(phosphoric acid) solution processes, examples of sudi monomers being te-trahaloterephthalaldehydes or the crosslinkable 3-vin)fisophthalaldehyde. It does not, however, provide for the isolaticm and application of prepolymers and, thus, for utilization of die two-stage approadi so beneficial from a processing and application standpoint. 

The reactive monomer responsible for the cross-linking reaction is normally styrene, vinyl toluene, methyl methacrylate, or diallyl phthalate. The unsaturated polyester prepolymers are, by themselves, relatively stable at ambient temperatures. When they are added to the cross-linking monomers, however, the blend is extremely unstable. To prevent premature gelation and to control the cross-linking process, inhibitors like hydroquinone are added to the blend. The cross-linking process is usually initiated by an organic peroxide added by the end user. The peroxide may be activated by heat. For room-temperature cure, accelerators and activators such as cobalt salts are also added. The cross-linking reaction can be represented as follows (Equation 5.2) ... 

Vulkollan-type rubbers suffer from the disadvantage that the prepolymers are unstable and must be used within a day or two of their production. Moreover, these rubbers cannot be processed with conventional rubber machiners, so the products are usually made by a casting process. Attempts were then made to develop other polyurethane rubbers which could be processed by conventional techniques. 

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

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. 

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. 

Polyanhydrides are among the most hydrolytically unstable polymers. These polymers were prepared first in 1909, subsequently investigated as potential textile fibers but found unstable melt-polycondensation. Here, starting from a dicarboxylic acid monomer, a prepolymer consisting of a mixed anhydride of the diacid with acetic acid is formed (Scheme 3). The polymer is obtained by subsequently heating the prepolymer under vacuum to eliminate the acetic anhydride by-product. Several aliphatic dicarboxylic acid compounds (sebacic acid (SA), fumaric acid (FA), etc.) and aromatic... 

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