Properties of polypropylene

Cast films provide a high clarity, heat sealable film and are primarily used as an overwrap for boxes and other packaging. These films have a lower density than cellophane and provide a longer product shelf life. Properties of polypropylene films are given in Table 12. 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

As shown in the previous section the mechanical and thermal properties of polypropylene are dependent on the isotacticity, the molecular weight and on other structure features. The properties of five commercial materials (all made by the same manufacturer and subjected to the same test methods) which are of approximately the same isotactic content but which differ in molecular weight and in being either homopolymers or block copolymers are compared in Table 11.1. 

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

The standard melt flow index machine is often used for characterising the flow properties of polypropylene and to provide a rough measure of molecular weight. Under the conditions normally employed for polyethylene (2.16 kg load at 190°C) the flow rate is too low for accurate measurement and in practice higher loads, e.g. 10 kg, and/or higher temperatures are used. It has been found that a considerable pressure drop exists in the barrel so that the flow towards the end of a test run is higher than at the beginning. 

The electrical properties of polypropylene are very similar to those of high-density polyethylenes. In particular the power factor is critically dependent on the amount of catalyst residues in the polymer. Some typical properties are given in Table 11.3 but it should be noted that these properties are dependent on the antioxidant system employed as well as on the catalyst residues. 

Table 11.4 Effect of percentage stretch on tensile properties of polypropylene film ...

Two random copolymers of this type are of importance, ethylene-propylene copolymers and ethylene-but-l-ene copolymers. The use and properties of polypropylene containing a small quantity of ethylene in stereoblocks within the molecule has already been discussed. Although referred to commercially as ethylene-propylene copolymers these materials are essentially slightly modified polypropylene. The random ethylene-propylene polymers are rubbery and are discussed further in Section 11.9. 

Metallocene isotactic polypropylenes (MET.PP) are accessible with different melting points under the commercial range of melt flow rate (Fig. 11). The variation of melting points in these polymers is linked with the presence of different lengths of isotactic sequences. Mechanical properties of polypropylene... 

Articles made from polypropylene have good electrical and chemical resistance and low water absorption. Its other useful characteristics are its light weight (lowest thermoplastic polymer density), high abrasion resistance, dimensional stability, high impact strength, and no toxicity. Table 12-3 shows the properties of polypropylene. 

Table 8-12 Effect of molecular orientation on the impact properties of polypropylene films...

Preparation and properties of polypropylene/clay nanocomposites, Journal of Materials Research 8 11-47. 

Shanmugharaj, A.M., Kim, J.K., and Ryu, S.H., UV surface modification of waste tire powder Characterization and its influence on the properties of polypropylene/waste powder composites. Polymer Test., 24, 739, 2005. 

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. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Sometimes the hazy optical properties of polypropylene are a detriment to its use in thin films. To address this problem, as well as to improve physical properties and reduce material costs, we can orient the molecular chains by stretching the polymer film after it has left the extrusion die. There are two methods of orienting films, film blowing, shown in Fig. 11.6, and tentering, shown in Fig, 19.6. 

A polypropylene square slab, 0.3 m on a side, bums in a steady manner. Polypropylene does not char, and it can be considered that its radiation characteristics do not vary, with the flame radiative fraction at 0.38 and the flame incident heat flux to the fuel surface at 25 kW/m2. It burns in a wind as shown in various conditions as specified. Its convective heat transfer coefficient can be taken as 50 W/m2 K. The properties of polypropylene are listed as follows. 

The solubility properties of polypropylene have been similar to that of polyethylene. Although polypropylene is insoluble at room temperature, yet it is soluble in hydrocarbons and chlorinated hydrocarbons at temperature above 80°C. 

Most properties of polypropylene are similar to polyethylene but polypropylene has higher softening point and at 140°C polypropylene still retains its shape. Polypropylene is more susceptible to oxidation by air at higher temperature. Cross-linking, chlorination and other reactions lead to degradation of Polymer chain and are not very useful commercially. 

Danilova-Volkovskaya G.M. The effect of processing parameters and modifiers on the properties of polypropylene and PP-based composite materials. — Doctoral Thesis, (technical sciences). 2005, p. 273. 

Baars et al. recently investigated the host-guest properties of polypropylene imine) dendrimers functionalized with tris-3,4,5-tri(tetraethyleneoxy)benzoyl units (Figure 16.15) [191]. These hosts are highly soluble in a broad range of solvents, from apolar solvents such as toluene to polar aqueous media. 

Mahlberg, R., Niemi, H.E.M., Denes, F. and Rowell, R.M. (1998). Effect of oxygen and hexameth-yldisiloxane plasma on morphology, wettability and adhesion properties of polypropylene and lignocellulosics. International Journal of Adhesion and Adhesives, 18(4), 283-297. 

The typical properties of polypropylene foams for sandwich technology are ... 

Atactic. A characteristic of the spatial configuration of atoms or groups in a polymer chain. Atactic indicates a random distribution- of those atoms or groups, i.e., no symmetry to the spatial configuration. This characteristic is important, for example, in determining the properties of polypropylene. 

More recently, Lin and Masuda [47] measured the viscoelastic properties of polypropylene melts filled with small (0.15 pm) and larger (4.0 pm) calcium carbonate particles. The dynamic modulus and viscosity were found to rise with filler loading especially at low frequencies. With highly filled compositions (at... 

The viscoelastic properties of polypropylene melts containing magnesium hydroxide fire retardant fillers have been studied using parallel plate dynamic rheology [36]. In this work the filler variants differed in particle size, surface area and morphology, ranging from approximately spherical particles formed... 

Watson, G.L. (1987) A study of the fire and mechanical properties of polypropylenes filled with sea water magnesium hydroxide, PhD thesis, Brunei University, UK... 

Effect of silane treatment on the physical properties of polypropylene/E-glass bead composites... 

The superior properties of polypropylene terephthalate) (PPT) polymer and fibers over the chemically analogous poly(ethylene terephthalate) (PET, used for soda bottles) and poly(butylene terephthalate) (PBT) have been well known for several decades PPT fibers are much more elastic and less brittle than PET and offer better recovery from stretching than PBT they are also easier to dye than either PET or PBT. Compared to the intermediate for PET, ethylene glycol, which is available inexpensively from ethylene oxide, and to that for PBT, butanediol, likewise available inexpensively from butene or butadiene, the intermediate for PPT, 1,3 propanediol (1,3-PPD or PDO), was not - and on a large scale is still not - available. Three processes, two chemical ones and one biotechnological, compete to change this situation (Figure 20.10). 

The properties of polypropylenes depend mainly on the regio- and stereospecificity of the inserted propylene units which influence the microstructure. The microstructure of polypropylene in terms of the enchainment of... 

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