Polypropylene (PP) Technical University of Gabrovo Milena Koleva General General Polypropylene or polypropene (PP) is a thermoplastic polymer, made from the monomer propylene (propene): Its molecular mass varies within the 80 000 200 000 range. Polypropylene is produced using the stereo specific polymerization method, at low pressure, in the presence of Ziegler Natta catalysts. The proper choice of catalyst can produce isotactic, syndiotactic or atactic polypropylene, or a combination of these. Most commercially available polypropylene is made with titanium chloride catalysts, which produce mostly isotactic polypropylene. With the methyl group consistently on one side, such molecules tend to coil into a helical shape; these helices then line up next to one another to form the crystals that give commercial polypropylene its strength. More precisely metallocene catalysts offer a much greater level of control. These catalysts use organic groups to control the monomers being added. In addition to this qualitative control, they allow better quantitative control, with a much greater ratio of the desired tacticity than Ziegler Natta techniques. They also produce higher molecular weights than traditional catalysts, which can further improve properties. Production methods A rubbery PP can also be made by a specialized synthesis process. To produce it, a catalyst can be made, which yields isotactic polypropylene, but with the organic groups that influence tacticity held in place by a relatively weak bond. After the catalyst has produced a short length of polymer which is capable of crystallization, light of the proper frequency is used to break this weak bond, and remove the selectivity of the catalyst, so that the remaining length of the chain is atactic. The result is a mostly amorphous material with small crystals embedded in it. Since each chain has one end in a crystal but most of its length in the soft, amorphous bulk, the crystalline regions serve the same purpose as vulcanization. Unlike traditional rubber, rubbery PP can be melted and recycled, making it a thermoplastic elastomer. Polypropylene can have an isotactic structure (its methyl groups are located on one side of the chains), or a syndiotactic structure (the methyl groups are located alternatively on either side of the polymer chain) (Fig. 1). Atactic polymer has disorderly located methyl groups. Stereoblock polymer has both isotactic and tactic segments. The commercial polymer is most often a mixture of polymers having Structure Polypropylene 1
different structures, the ratio of which depends on the conditions of the polymerization process. Isotactic polypropylene with a minimum content of atactic fraction (5 20%) is the most valuable for practical purposes. Fig.1. Segments of polypropylene with isotactic (above) and syndiotactic (below) tacticity. During processing (75 80 C) isotactic polypropylene crystallizes, which adds a number of valuable properties to the product. Properties Polypropylene has an intermediate level of crystallinity between that of low density polyethylene (LDPE) and high density polyethylene (HDPE). Although it is less tough than HDPE and less flexible than LDPE, it is much more brittle than HDPE. Its tensile behaviour is more dependent on load speed and temperature than that of polyethylene the lower the tensile speed, the higher the values of its mechanical characteristics. Its Young s modulus is intermediate between that of low density polyethylene and high density polyethylene. Physical and mechanical properties Polypropylene is rugged, often somewhat stiffer than some other plastics. It has very good resistance to fatigue. Table 1. Physical properties of polypropylene. Physical Properties Density 0.9 1.44 g/cm 3 Water Absorption 0.01 0.1 % Moisture Absorption at Equilibrium 0.1 % Environmental Stress Crack Resistance Melt Flow Linear Mould Shrinkage 1000 hours 0.5 136 g/10 min >0.0025 cm/cm Polypropylene has considerably better physical and mechanical properties than polyethylene. Its tensile behavior depends to a great extent on the load speed and temperature. Its typical mechanical properties are given in Table 2. Polypropylene 2
Table 2. Typical mechanical properties of polypropylene Mechanical Properties Hardness, Rockwell M 70 113 Tensile Strength, Ultimate 17.9 80 MPa Elongation at Break 2.5 900 % Modulus of Elasticity 0.008 8.25 GPa Izod Impact, Notched 2.2 21 kj/m 2 Charpy Impact Unnotched 8 28 kj/m 2 Tensile Creep Modulus, 1 hour Tensile Creep Modulus, 1000 hours 550 700 MPa 220 440 MPa Polypropylene has higher melting and destruction points than polyethylene. It is also more heat resistant, which determines the wider temperature range in which it can be used up to 110 130 C. However, it is less cold resistant than polyethylene. Products made of polypropylene withstand heat treatment above the boiling point; therefore they can be sterilized without any effect on their shape or mechanical properties. Thermal characteristics The brittleness temperature of polypropylene (and its cold resistance, respectively) is 5 15 C. The main thermal properties of polypropylene are presented in Table 3. Table 3. Thermal properties of polypropylene Thermal Properties CTE, linear 20 C Specific Heat Capacity Thermal Conductivity 25 185 μm/m. C 2 J/g. C 0.1 0.13 W/m.K Maximum Service Temperature, Air 44 148 C Melting Point 130 168 C Vicat Softening Point 35 148 C Glass Temperature 100 105 C Oxygen Index 18 % The electrical characteristics of polypropylene are presented in Table 4. Electrical Properties Polypropylene 3
Table 4. Elecrtical characteristics of polypropylene Electrical Properties Electrical Resistivity Surface Resistance 10 14 10 17 Ω.cm 10 10 10 14 Ω Dielectric Constant 2.2 2.3 Dielectric Constant, Low Frequency 2.3 Dielectric Strength 22 140 kv/mm Dissipation Factor 0.0002 0.002 Dissipation Factor, Low Frequency 0.0007 0.00081 Arc Resistance 136 s Comparative Tracking Index 600 V Polypropylene is a chemically resistant material. It is only strong oxidants, such as chlorosulphuric acid, concentrated nitric and sulphuric acids, which have a significant effect on its properties. Continued contact with these at a temperature above 60 C results in destruction of polypropylene. Chemical Resistance At room temperature polypropylene swells slightly in organic solvents. At temperatures above 100 C it dissolves in aromatic hydrocarbons (benzene, toluene). It has good water resistance even after a continuous contact with water for a period of 6 months at room temperature its water absorbing capacity is only 0.5%, and at а temperature of 60 C not more than 2%. Due to the presence of tertiary carbon atoms in its chain, polypropylene is sensitive to the action of oxygen, especially at higher temperatures. As a result it has a far more marked tendency to ageing than polyethylene the process develops rapidly, with sharp deterioration of its mechanical properties. Therefore stabilization of polypropylene is necessary before it is processed into finished products. Compared to polyethylene, polypropylene is less susceptible to cracking under the combined action of external loads and corrosive environment. Polypropylene offers ease of processing with excellent chemical resistance and good mechanical properties. It is commonly processed using conventional methods, such as injection moulding, extrusion, blow moulding, thermoforming, vacuum moulding, and rotational moulding. Polypropylene can be subjected to drilling, turning and welding. It can also be used in the form of filled or reinforced compositions. Glass fiber reinforced polypropylene has improved dimensional stability, resistance to warpage, rigidity and strength. Heat deflection temperature at 264 psi is increased up to 300 F (149 C) for 40% glass fiber reinforced polypropylene. Polypropyleneʹs coefficient of thermal expansion is cut in half with 40% glass reinforcement. Glass fiber reinforced polypropylene, when utilizing a chemical coupling agent, has significantly improved tensile and flexural strengths over regular glass reinforced polypropylene. Polypropylene with 30% chemicallycoupled glass reinforcement has a 180% improvement in tensile strength over the non reinforced polypropylene and a 50% improvement over conventional glassreinforced grades. Processing properties Polypropylene 4
Overall, chemically bonded polypropylene has improved strength characteristics without altering the moduli, heat resistance, electrical properties, or hardness. Talc filled polypropylenes have improved rigidity, hardness, and heat resistance compared to the base resin. The major processing properties are presented in Table 4. Table 4. Processing characteristics of polypropylene Processing Properties Processing Temperature 202 252 C Rear Barrel Temperature 220 C Middle Barrel Temperature 220 C Front Barrel Temperature 220 C Nozzle Temperature 220 C Mould Temperature 35 49 C Drying Temperature 82 C Typical injection moulding conditions for glass fibers filled PP compounds are listed in Table 5. Table 5. Typical injection moulding conditions for polypropylene. Typical Injection Moulding Conditions Temperatures Rear zone 193 216 C Centre zone 199 221 C Front zone 204 227 C Melt 191 232 C Mould Temperature 32 66 C Pressures Injection Hold Back Fill Screw 69 103 MPa 34 69 MPa 0.34 0.69 MPa 25 51 mm/s 60 90 rpm Polypropylene 5
Application The recommended minimum draft angle is 0.7. If smaller draft angles are used, it is necessary to use wide ejection surfaces. The gas removing channels should be low, depending on polypropylene type from 0.01 to 0.02 mm. The low density, good strength characteristics, lack of physiological risks and possibility of sterilization make polypropylene suitable for production of medical items, for packages of food, cosmetic and pharmaceutical products, household articles, toys, etc. Many plastic items for medical or laboratory use can be made from polypropylene which is autoclavable. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminum foil (both heat resistant materials). Plastic pails, car batteries, wastebaskets, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature. Polypropylene is also used for production of tubes, fittings, parts for the chemical and electrical engineering sectors, fibres. It can be metallized. * The ranges given in the tables indicate the minimum and the maximum value of the respective property, found experimentally and published for the different brands and types of the polymer. Trade names Hostalen PP (Basell, NL) Marlex ( Chevron Phillips Chemicals, USA) Moplen (Basell, NL) Ultralen ( Lonza Werke, DE) References 1. Harper, Charles A., Edward M. Petrie. Plastics Materials and Processes, John Wiley & Sons, 2003. 2. Stevens, M.P. Polymer Chemistry: An Introduction. Oxford University Press, 1998 3. White, J. L., D. D. Ghoi. Polyolefins: Processing, Structure Development and Properties, Hanser Gardner Publications, 2004. 4. Chabot, J. F., The Development of Plastics Processing Machinery and Methods, John Wiley & Sons, 1992. 5. Järvelä P. et al., Ruiskuvalu, Plastdata 2000. Polypropylene 6