The chemical modification of nitrile rubbers
Nitrile rubbers (copolymers of butadiene and acrylonitrile) are widely used throughout the aerospace, automobile, construction and footwear industries. The useful properties of nitrile rubbers include excellent resistance to the action of oils, petroleum fuels and solvents, combined with good heat and abrasion resistance. These properties derive from the polar nature of the cyanide group on the polymer chain and rubbers with high oil resistance contain 40 - 50 mole % acrylonitrile. Unfortunately, at low temperatures the elastomeric properties of such rubbers are poor. One of the objectives of this work was to investigate whether the working temperature range of nitrile rubbers could be extended by chemicalmodification, in particular by grafting poly (tetrahydrofuran) (PTHF) onto the nitrile rubber. PTHF is a low melting, crystalline polymer with a glass transition temperature, Tg, of -84°C, above which it is rubbery. The polymerisation of THF can be initiated by the generation of suitable carbonium ions which attack the ethereal oxygen of the THF molecule forming an oxonium ion. Propagation occurs via this oxonium ion intermediate by a cationic, ring-opening mechanism. This 'living' polymerisation can be terminated by many nucleophilic reagents such as water, methanol, etc. In view of the polymerisation characteristics of THF, it can be grafted to a nitrile rubber backbone by two distinct methods. The first is a 'grafting from' technique, which involves initiation of the THF polymerisation from carbonium ion sites generated on the polymer backbone. In these experiments, described in Chapter 2, the parent rubbers were first functionalised by dihalocarbenation, bromination and allyl bromination, and on reaction with a suitable silver salt cationic centres were generated on the backbone. This method proved to be unsuitable for preparing well-defined graft copolymers of PTHF. The second process, a 'grafting onto' technique, involved a 'coupling' reaction between 'living' PTHF and functional groups previously attached to the nitrile rubber backbone. This method, which was finally chosen to prepare a range of graft copolymers of PTHF, is described in Chapters 3 and 4. One to 3 mole % of the diene residues in the nitrile rubber were first epoxidised then treated with HC1 to open the epoxide rings to yield chlorohydrin groups. The hydroxyl substituents functioned as terminators in a 'coupling' reaction with the 'living' PTHF to form a graft copolymer. Since polymerisation of THF at room temperature is fairly slow the lengths of the grafted chains could be controlled by monitoring the time from initiation to termination. The graft copolymers and precursors were examined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and for solvent resistance. The DSC results show that grafting with PTHF reduces the Tg of low nitrile rubbers but has little effect on the Tg of high nitrile rubbers. This is believed to be due to a greater degree of phase separation of the PTHF component in the high nitrile rubbers. Consequently, the physical properties of these PTHF-high nitrile rubber grafts are not those of a homogeneous amorphous blend of the two components, TGA of the PTHF graft copolymers suggest that the PTHF graft destabilises the nitrile rubber to heat. However, PTHF is well known to be susceptible to hydroperoxidation and it is likely that thisc effect is attributable to peroxides rather than the PTHF chains per se. An antioxidant should be capable of inhibiting hydroperoxidation. Finally, the solvent resistance of the PTHF graft copolymers, especially from high nitrile rubbers, is lower than that of the original nitrile rubber.