Polyolefin polymers properties

The growth of polyolefin fibers continues. Advances in olefin polymerization provide a wide range of polymer properties to the fiber producer. Inroads into new markets are being made through improvements in stabilization, and new and improved methods of extmsion and production, including multicomponent extmsion and spunbonded and meltblown nonwovens. 

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). 

Van Krevelen DW, "Group Contribution Techniques for Correlating Polymer Properties and Chemical Structure", in Bicerano J (Ed) "Computational Modelling of Polymers", Marcel Dekker, New York, 1992, Chap. 1. Vasile C, "Degradation and Decomposition", in Vasile C (Ed), "Handbook of Polyolefins", Marcel Dekker, New York, 2nd Ed, 2000, Chap. 17. 

Mechanisms of degradation in condensation polymers, and the stabilisation of these polymers and non-polyolefin polymers such as poly(vinyl chloride) using organophosphites is discussed in terms of the stability of colour, thermal properties and molecular weight. Stabilisation of poly(ethylene terephthalate) and polycarbonate by organophosphites was studied experimentally. 5 refs. 

Titanium-based solid-state catalysts for the industrial production of polyolefin materials were discovered in the early 1950 s and have been continually improved since then (see Section 7.3). Due to the high degree to which they have been perfected for the production of large-volume polyolefin commodities, they continue to dominate the processes presently used for polyolefin production. Despite (or because of) this product-oriented perfection, only limited degrees of variability with regard to some relevant polymer properties appear to be inherent in these solid-state catalysts. 

Molecular weights are not often measured directly for control of production of polymers because other product properties are more convenient experimentally or are thought to be more directly related to various end uses. Solution and melt viscosities are examples of the latter properties. Poly(vinyl chloride) (PVC) production is controlled aceording to the viscosity of a solution of arbitrary concentration relative to that of the pure solvent. Polyolefin polymers are made to specific values of a melt flow parameter called melt index, whereas rubber is characterized by its Mooney viscosity, which is a different measure related more or less to melt viscosity. These parameters are obviously of some practical utility, or they would not be used so extensively. They are unfortunately specific to particular polymers and are of little or no use in bringing experience with one polymer to bear on problems associated with another. 

Although the catalyst - homogeneous or heterogeneous - mainly determines the oligomerization or polymerization process and hence the polymer properties, the technical process also plays an important role. The oligomers or polymers may or may not precipitate during the synthesis, depending on several factors. Hydrocarbons with six or more carbon atoms (n-hexane, n-heptane, decalin, toluene, xylene, and others) are solvents for polyolefins, but semicrystalline polyolefins are not soluble at room temperature. Only amorphous polyolefins can be dissolved. 

Modern polymerization catalysis, as we know it, was triggered by the development of metallocenes and the concomitant understanding of relationships between ligand structure and polymer properties. The manipulation of these useful relationships has led to a renaissance in tlie synthesis of polyolefin materials having new stereoregularities and, therefore, precise control of polymer rheology. 

The simple homoleptic complex Sn(Oct)2 catalyzes lactide polymerization both in solution and in the melt at temperatures >130°C and is the most widely used catalyst for lactide polymerization industrially. As previously mentioned, commercially available lactide with >90% L-LA polymerized with Sn(Oct)2 will generate -90% isotactic PLA. While this material has commercial applications, its thermal and mechanical properties are not suitable for many applications where polyolefins are typically used. One way to improve the polymer properties is by increasing the isotacticity or forming isotactic stereoblock PLA through stereoselective polymerization of L- and D-LA mixtures. However, this cannot be achieved with Sn(Oct)2 and other simple initiators. ... 

Heterogeneous composite materials made from waste or recycled components such as polyolefin polymers and wood fibers were shown to be made compatible by T. Czvikovszky through the use of small amounts of monomers that graft onto the two normally incompatible phases of said mixtures. These compositions demonstrated sufficient material properties to be used in the construction of truck and automotive components, such as door and side panels. To exploit the use of such recycled resources, process techniques in mixing and compounding had to be developed. 

S. P. Chum, C. I. Kao, and G. W. Knight, Structure, properties and preparation of polyolefins produced by single-site catalyst technology, in Metallocene-Based Polyolefins Preparation, Properties and Technology, Wiley Series in Polymer Science, Vol. 1, J. Schiers and W. Kaminsky (eds.), Wiley, New York, 2000, p. 261. 

Polyolefin foams can be produced with closely controlled density and cell structure. Generally the mechanical properties of polyolefins lies between those of a rigid and a flexible foam. Polyolefin foams have a very good chemical and abrasion resistance as well as good thermal insulation properties. Cross-linking improves foam stability and polymer properties. 

However, there are still a number of important polymer properties that can only be measured by laborious and time-consuming off-line analyses. In this category one can include the MWD (especially in dispersed systems and/or for polyolefins), branching and crosslinking density, the gel content, the PSD, among other properties. (Despite the several examples reported in the scientific literature, at present no commercial equipment can ensure the fast and robust on-hne measurement of the entire size distribution of polymer particles in industrial reactors.)... 

A viable process for manufacturing polyolefin-clay nanocomposifes by in situ polymerization requires adequate catalytic activity, desirable polymer microstructure, and physical properties including processibility, a high level of clay exfoliation fhaf remains stable under processing conditions and, preferably, inexpensive catalysf components. The work described in the previous two sections focused on achieving in situ polymerization with clay-supported transition metal complexes, and there was less emphasis on optimization of polymer properties and/or clay dispersion. Since 2000, many more comprehensive studies have been undertaken that attempt to characterize and optimize the entire system, from the supported catalyst to the nanocomposite material. The remainder of this chapter covers work published in the past decade on clay-polyolefin nanocomposites of ethylene and propylene homopolymers, as well as their copolymers, made by in situ polymerization. The emphasis is on the catalyst compositions and catalyst-clay interactions that determine the success of one-step methods to synthesize polyolefins with enhanced physical properties. 

Two types of catalysts used in polymer manufacture are metallic compounds such as aluminium alkyls and titanium halides used in low-pressure polyolefin manufacture. As the presence of residual catalysts can have important effects on polymer properties it is important to he able to determine trace elements which reflect the presence of these... 

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