Polyolefins, functionalised

In general, there are two ways to functionalise polyolefins via direct copolymerisation of an olefin with a polar monomer and via chemical modification of preformed polymers [508 514]. 

Maleic anhydride-functionalised polyolefins feature in Solvay s Priex range of compatibilising resins, some of which have very low viscosities so that they can be emulsified in aqueous systems. They are used to compatibilise resins with wood fibres in the production of Wood-Stock wood-plastics composites for the automotive industry. (There is also another type of Priex product, consisting of ionomers designed to improve the mechanical properties of polyolefins.)... 

Polyamide (PA)/maleic-anhydride (MA)-functionalised polyolefin (PO) blends provide a compatibility strategy consisting of maleation of the PO phase followed by a reactive processing favouring copolymer formation by imidisation between the anhydride and amine chain ends. The PO is most often ethylene-propylene copolymer rubber, PP or PE, and the PA is commonly PA6 or PA6,6 [49-58]. 

Other recent studies which involve and illustrate the power of the FTIR technique include surface studies of PVC systems with PMMA [192] and poly(e-caprolactone) (PCL) [193, 194] PVC with styrene/acrylonitrile copolymers [195] polyester/nitrocellulose [196] EVA copolymer with PVC and chlorinated polyethylene (CPE) [197] and interactions in blends involving p-sulphonated polystyrene [198, 199]. FTIR techniques have been used to map the phase diagram of an aromatic polyamide-poly(ethylene oxide) blend [200], while microscopy-FTIR has been used to obtain information on intermolecular interactions and conformational changes in specific domains in functionalised polyolefins with PVC or polystyrene [201]. Segmental motions and microstructure studies from combined DSC and FTIR measurements have been used to interpret solid-state transitions in miscible rubber blends [202]. 

The ability to downgauge, decrease part weight, improve barrier properties and reach new levels of product performance are propelling polyolefins into new markets previously dominated by other plastics. The high growth rate in PP production capacity is mainly being driven by the ability of PP to replace other resins on a cost/performance basis. For example, functionalisation of PP by incorporation of acrylic functionality has extended its weatherability performance. Interpolymer competition will have a significant impact on the amount and type of additives used. 

The use of borane-containing monomers clearly presents an effective and general approach in the functionalisation of polyolefins, which has the following advantages stability of the borane moiety to coordination catalysts, solubility of borane compounds in hydrocarbon solvents (such as hexane and toluene) used as the polymerisation medium, and versatility of borane groups, which can be transformed to a remarkable variety of functionalities as well as to free radicals for graft-form polymerisations. The functionalised polymers are very effective interfacial modifiers in improving the adhesion between polyolefin and substrates and the compatibility in polyolefin blends and composites [518],... 

It is worthwhile to note that chemistry plays a major role in the morphology and control of mechanical properties in complex systems like PPE blends with crystalline polymers, such as polyolefins, polyamides (PA) and polyesters (18). The amount of copolymer formed during the reactive extrusion between functionalised PPE and PA has a significant effect on the impact-strength of blends. The latter levels off only above 10% of copolymer. 

Despite the large number of studies on MA grafting and the commercial success of MA grafted polyolefins, the chemical mechanism involved in the functionalisation process is not fully understood. Several studies have shown that the reaction pathways depend on the polyolefin molecular structure. When a peroxide is used as initiator, crosslinking or chain scission may occur simultaneously with the grafting reaction. The dominant side reaction for PE is crosslinking [68-76] and for PP is chain scission [77, 78]. 

Where the matrix polymer undergoes a polymerisation, or crosslinking process, during composite formation then it is fairly simple to choose a suitable polymer reactive group (e.g., a carbon=carbon double bond). However, with polymers such as the polyolefins, this can be difficult. Highly reactive functionalities, such as azides, are sometimes used in this case. It is also possible to pre-functionalise a part of the polymer matrix, by grafting on a species such as an acid, anhydride or alkoxy-silane, in a separate process. 

In-line functionalisation of polyolefins with RS resulted in better dispersion and adhesion when blended with PA. Without modification the PA drops, dispersed in a polypropylene matrix, are large and poorly attached to the continuous phase as shown in Fig. 5a. In the presence of RS considerably smaller PA drops and better contact between the phases can be seen in Fig. 5b. 

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