Polyolefins modifications, polymers

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Machado, A.V., van Duin, M., Covas, J.A. Monitoring Polyolefin Modification Along the Axis of a Twin-screw Extruder. II. Maleic Anhydride Grafting, Journal of Polymer Science, Part A Polymer Chemistry, 38,3919-3932 (2000). 

The synthesis of new polymeric materials having complex properties has recently become of great practical importance to polymer chemistry and technology. The synthesis of new materials can be prepared by either their monomers or modification of used polymers in industry. Today, polystyrene (PS), which is widely used in industrial applications as polyolefins and polyvinylchlorides, is also used for the production of plastic materials, which are used instead of metals in technology. For this reason, it is important to synthesize different PS plastic materials. Among the modification of PS, two methods can be considered, viz. physical and chemical modifications. These methods are extensively used to increase physico-mechanical properties, such as resistance to strike, air, or temperature for the synthesizing of new PS plastic materials. 

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

Alkanesulfonates are also suitable for antistatic modification of polyolefins, particularly polethylene. In addition, they improve the flow of polymer melts during processing. Typically added amounts are 1-2 phr. 

Roy Choudhury N. and Bhowmick A.K., Compatibilization of natural rubber-polyolefin thermoplastic elastomeric blends by phase modification, J. Appl. Polym. Sci., 30, 1091, 1989. 

A good example of a reactive modifier which has been used (14) to enhance properties of polyolefins is maleic anhydride (MA). The formation of maleic adduct in polypropylene (PP), for example, can be used to effect several modifications e.g. to improving hydrophilicity, adhesion and dyeabflity. Moreover, the polymer-maleic adduct has an availabla additional functionality to effect other chemical modifications for achieving the desired material design objectives. Reactions of MA with polymers in solution are described in the patent literature (15). 

Favorable rheological properties are an essential requirement for the commercialization of polyolefins like polyethylene. The ease of processability of the polymer melt, obtained through modifications in the microstructural features, is as important as the end use mechanical properties of these polymers. Presence of long-chain as well as short-chain branching, LCB and SCB, respectively, more or less dictates the rheological behavior of most commercial... 

Chlorination is one of the most interesting processes for polymer modification, and is usually carried out by means of catalysts or by UV irradiation. Since 1960, radiation-induced chlorination of polyethylene and polypropylene has been studied, especially by Soviet workers (1-3). As the polymers used in that work are insoluble in the usual solvents at normal temperature, chlorination was done in the heterogeneous phase— for example, by leading continuously a stream of chlorine over the finely ground polymer or through an aqueous suspension of the polyolefin. It is, therefore, difficult to compare the results obtained under the different conditions used. 

Abstract Polyolefins such as polyethylene, polypropylene and their copolymers have excellent bulk physical/chemical properties, are inexpensive and easy to process. Yet they have not gained considerable importance as speciality materials due to their inert surface. Polyethylene in particular holds a unique status due to its excellent manufacturer- and user-friendly properties. Thus, special surface properties, which polyethylene does not possess, such as printability, hydrophilicity, roughness, lubricity, selective permeability and adhesion of micro-organisms, underscore the need for tailoring the surface of this valuable commodity polymer. The present article reviews some of the existing and emerging techniques of surface modification and characterisation of polyethylene. 

Polyolefins are amongst the most widely used polymers. Polyethylene in particular holds a unique status due to its excellent user-friendly properties. Tailoring of its surface properties would open an avenue to the most lucrative markets. The present article reviews some of the existing as well as emerging techniques of surface modification and characterisation of polyethylene. 

---