Polymer properties polypropylene

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Polymers of this sort possess an interesting combination of properties. They are clear and tough (although notch sensitive) and exhibit a level of flexibility somewhat higher than that of polypropylene. Typical properties are given in Table 16.6. 

The most common backbone structure found in commercial polymers is the saturated carbon-carbon structure. Polymers with saturated carbon-carbon backbones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates, are produced using chain-growth polymerizations. The saturated carbon-carbon backbone of polyethylene with no side groups is a relatively flexible polymer chain. The glass transition temperature is low at -20°C for high-density polyethylene. Side groups on the carbon-carbon backbone influence thermal transitions, solubility, and other polymer properties. 

Phadke, A.A. and De, S.K., Effect of cryo-ground rubber on melt flow and mechanical properties of polypropylene, Polym. Eng. Sci., 26, 1079, 1986. 

Choice of sensitizer is related to the base polymer properties. Besides triplet sensitizers, dyes(4, 5), metal salts(11, 12), and radical initiators(7, 11, 12) have been used in photografting. These sensitizers other than triplet sensitizers are, however, not capable of initiating surface photografting onto polyolefins. Although benzoin isopropyl ether has been used for photografting of polypropylene, the reaction conditions seem to be in favor of deep grafting(12). 

Table 3.5 shows some examples of the property effect ratios for mineral filler-reinforced polypropylene. The effect ratio is the performance of the reinforced polymer divided by the performance of the neat polymer. Properties of low-level glass fibre reinforced polypropylene are given for comparison. 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene-propylene rubbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance. 

Propylene (C3H6) is a gas with chemical, physical, and toxicological properties very similar to those of ethylene. It, too, is a simple asphyxiant. Its major use is in the manufacture of polypropylene polymer, a hard, strong plastic from which are made injection-molded bottles, as well as pipes, valves, battery cases, automobile body parts, and rot-resistant indoor-outdoor carpet. 

It is worth noting that the absolute configuration of the tertiary carbon atoms in isotactic or syndiotactic polypropylene molecules has, in principle, no practical significance for the polymer properties, which might result from the presence of these carbon atoms in the main chain of the macromolecule. 

Thus far, we have considered addition polymerization routes - either catalyzed or uncatalyzed. Although this is sufficient to describe the synthesis of common packaging materials such as polyethylene, polypropylene, polystyrene, etc., other classes of polymers such as nylon, PETE, and polyacrylamide are generated through step-growth mechanisms. Although the synthetic pathway for these polymers is more straightforward than addition polymerization, there are many intricate considerations that affect overall polymer properties. 

Figure 5.34. Tacticity vs. bulk properties for polypropylene polymers. The homogeneous" and heterogeneous notations refer to whether the entire polymer chains, or only regions, are of a certain tacticity.

Proprietary catalyst and donor systems The Dow Chemical Co. manufactures and provides a family of polypropylene catalysts (SHAC catalyst) and a family of external electron donors (SHAC ADT) for use in the UNIPOL PP process. SHAC catalyst offers high catalyst activity (up to 35 kg/g-hr using conventional external donor and up to 60 kg/g-hr using a SHAC ADT) and competitive polymer properties. SHAC ADT performs three functions ... 

The combination of SHAC catalyst and SHAC ADT enables Dow s catalyst systems to achieve low cost and easy production of polypropylene polymer with superior properties. The exceptional performance of the SHAC catalyst systems also finds applications in other PP processes. 

Chemical modification of polymers affords an opportunity to modify undesired properties that would limit or even invalidate the potential usefulness of these materials. Polypropylene polymers are a good example, because their applications depend on their stereoregularity. Atactic homopolymer, a by-product from industrial polymerization reactors, lacks good material properties. However, its chemical modification by grafting with polar groups, one can convert it into a useful new material, as suggested by Natta (37). 

Figure 3.32 Variation of amorphous and crystalline contributions to the birefringence of polypropylene film with elongation (from Samuels RJ, Structured Polymer Properties, Wiley, 1974).

Figure 8.1 5 The crystal orientation of polypropylene fibres and films as a function of the true strain in the deformation process draw temperatures ( ) 135 °C, (a) I 10 °C. The prediction of the pseudo-affine model is shown as a dashed line (from Samuels RJ. Structured Polymer Properties, Wiley, 1974).

S. Bualek-Limcharoen, 1. Samran, T. Amomsakchai, and W. Meesiri. Effect of compatibUizers on mechanical properties and morphology of in-situ composite film of thermotropic liquid crystalline polymer/polypropylene. Polym. Eng. ScL, 39 312-320, 1999. 

---