Structure and properties

Polypropylene is a thermoplastic synthetic nonpolar polymer, which belongs to the class of polyolefins. Polypropylene (PP) [-CH2-CH (CH3) -] n is the product of the polymerization of propylene C3H6. Its molecular structure was determined by the Italian chemist J. Natta in 1954, which thus discovered the most important class of stereoregular polymers. In this case, the methyl side groups of CH3 in polypropylene chains can be located either regularly or arbitrarily. It is the spatial arrangement of the side groups (CH3-) with respect to the main chain in polypropylene molecules which is of decisive importance for the properties of this polymer, causing the uniqueness of its chemical-physical properties.

On an industrial scale, polypropylene is produced by the polymerization of propylene C3H6 using metallocene catalysts or Ziegler-Natta catalysts. Necessary conditions for the polymerization is the presence of a pressure of at least 10 atm. And temperatures up to 80 ° C. The method of production of polypropylene with the use of the Ziegler-Natta catalyst was developed in 1957, which made possible the industrial production of polypropylene, consisting mainly of macromolecules of the isotactic structure. In addition to isotactic, there are atactic and syndiotactic polypropylenes. However, the main and most important variety is polypropylene having an isotactic molecular structure that is characterized by high hardness, strength, heat resistance and a significant degree of crystallinity.

Polypropylene, having increased resistance to acids, alkalis, salt solutions and other inorganic corrosive media, does not dissolve in organic liquids at room temperature. At elevated temperatures, it swells and dissolves in benzene, carbon tetrachloride, ether and some other solvents. Featuring a low degree of moisture absorption, polypropylene has good electrical insulation properties in a fairly wide temperature range.

Polypropylene is a light crystallizing material that can be produced in the form of granules, both colored and unpainted. Staining is carried out using organic dyes or pigments. There are such basic kinds of polypropylene as homopolymer, or actually isotactic polypropylene, cross-linked polypropylene (PP-X, PP-XMOD), metallocene polypropylene (mPP), block copolymer with ethylene, or copolymer, and random copolymer.

A very important advantage of isotactic polypropylene is the presence of high mechanical properties. Homopolymer, which can be transparent, is characterized by increased rigidity, but at low temperatures is very fragile. Therefore, in conditions of low temperatures, it is preferable to use a block copolymer having a significantly higher impact resistance. Transparency of the material is achieved by combining the use of special technological techniques (reduced form temperature, etc.), as well as the introduction of a nucleating agent. In addition to the above-mentioned useful properties, polypropylene has excellent wear resistance and is easily recyclable.

The main raw material for the production of many types of products in demand on the market, in particular pipes, packaging, swimming pools, etc., is “Polyvuplen” – polypropylene sheet produced by extrusion technology or extrusion, the raw material for which is homogeneous polypropylene (PPH) or granulate of a block copolymer polypropylene-ethylene (PPC). Polypropylene sheets are produced mainly in welding class 003 or 006 (material of welding class 003 is used most often for the production of plastic piping systems). The sheets, in turn, are divided into 2 operational classes, depending on the smoothness, color, smoothness of the surface and a number of other parameters.

Environmental Safety

The most important advantage of “polyvuplen” sheets is their safety for health, since they are environmentally safe both the starting polymers used for their manufacture and the auxiliary additives. The official conclusion on the safety for the health of polypropylene sheets, signed on October 7, 1998 by the Chief Sanitary Doctor of the Czech Republic, clearly demonstrates this. At the same time, polypropylene sheets fully meet all the requirements of the state environmental standards of the Russian Federation.

Practical use

Polypropylene sheets “Polyvuplen” are used, in particular, for the production of tanks, swimming pools, storage tanks, storage tanks and other sealed containers. At the same time, when carrying out installation work using polypropylene sheets, it is necessary to take into account a number of special properties that distinguish them from traditional structural materials.

Polypropylene sheets are easily subjected to such types of machining as cutting, planing, milling, or processing on the same or similar machines that are used for wood processing.

Connecting polypropylene sheets to each other can be done using several basic methods.

A) Mechanical connection using bolts or rivets. This method is used widely enough, however, since polypropylene is a material prone to linear expansion, such a connection will not provide full water resistance and will not be very strong. The main advantage of this method is that the connection is detachable, which in some cases is absolutely necessary.

B) Gluing. This method is also used quite often. Nevertheless, although polypropylene has a high chemical resistance, being able to interact with many of the soluble adhesives, the bonded joints are also very strong. Use in the process of working with polypropylene method of gluing can only be previously consulted with experts in this field.

C) Welding. This method of joining elements of structures from polypropylene is the most reliable and economically advantageous. In turn, in practice, the most common are three main methods of welding.

The highest effectiveness is provided by polyfusion welding, when the places of future joints of the joined elements are first preheated to a certain temperature for a certain period of time, after which they are pressed against each other again with a strictly defined force. The technological process of polyfusion welding is rather complicated and is used mainly in industrial production, but the strength of the joint, reaching 80-90% of the strength of the material itself, is much higher than in the case of welding by other methods. By polyfusion welding, polypropylene sheets of any thickness can be joined.

Somewhat less robust, but also sufficiently reliable seam, obtained by extrusion welding using a hand extruder. The essence of extrusion welding is the application in the process of welding to the seam of additional material in the form of a filler polypropylene wire, which is pre-melted in the screw rotor of a hand extruder. The quality of the seam itself, and therefore the strength of the joint, often suffers because the extruder is a hand-held device, and therefore it is impossible to strictly observe such technological subtleties as welding at a certain rate under a certain pressure. Nevertheless, the method of extrusion welding is used when joining sheets having a significant thickness.

The welded seam, which is formed during the joining of sheets by means of a hot-air gun with a gun, has the least strength. With this welding method, both the additional material and the joints of the parts themselves are heated. The designs of modern hair dryers are not yet perfect enough, so maintaining a given temperature of heated air is extremely difficult. At the same time, the rate of welding is affected by the temperature change: negative consequences can not be avoided either in the case of too slow welding (the material overheats and degrades), or at too high a rate (the heating temperature is insufficient, which affects the strength of the seam). This welding method is applicable only for joining sheets, the thickness of which does not exceed 0.6 cm.

Coefficients of strength of the obtained joints:

Method of polyfusion welding: fast seam – 0,9; Slow seam – 0,8;

Method of extrusion welding: fast seam – 0,8; Slow suture – 0,6;

Method of welding with a hair dryer: fast seam – 0,8; Slow suture – 0,4.

Transportation and storage

Sheet polypropylene is transported and stored in special pallets-pallets. For transportation it is better to use a lorry with a covered body or containers. In this case, the pallets with the transported sheets laid in them must be carefully fixed. To avoid damage to the sheets, other ways of transporting them are not recommended. To store polypropylene sheets it is necessary on equal surfaces, it is desirable in pallets, necessarily laying each sheet by a layer of a packing material. In this case, sheets that are not stabilized from UV radiation should be stored in rooms that are protected from sunlight.

The most important physical and mechanical characteristics

– Density (average) – 0.92 g / cm3
– Resistance to bending – min. 25 MPa
– Elastic modulus at tension – min. 900 MPa
– Modulus of elasticity at bending – min. 800 MPa
– Yield strength at stretching – min. 21 MPa
– Specific toughness: at 23 ° C – min. 40 kJ / m2; At -30 ° C – min. 5 kJ / m2