Polypropylene Random Copolymer Knowledge Introduction
Polypropylene Random Copolymer
Polypropylene random copolymer is also a kind of polypropylene, its basic structure of the polymer chain is modified by adding different kinds of monomers. Ethylene is the most common monomer that causes changes in the physical properties of polypropylene. Compared to PP homopolymers, random copolymers have improved optical properties (increased transparency and reduced haze), improved impact resistance, increased flexibility, reduced melting temperatures, and reduced thermal fusion temperatures Simultaneously with the homopolymer in terms of chemical stability, water vapour barrier properties and organoleptic properties (low odor and taste).
PP random copolymers combining improved transparency and impact strength have been developed for use in blow molding, injection molding, film and sheet extrusion processing for food packaging, pharmaceutical packaging materials and consumer products.
Chemistry
PP random copolymers generally contain 1 to 7% by weight of ethylene molecules and 99 to 93% by weight of propylene molecules. On the polymer chain, ethylene molecules are randomly inserted between the propylene molecules. In this random or statistical copolymer, the majority (usually 75%) of the ethylene is incorporated in a single-molecule insertion, called the X3 group (three consecutive ethylenes [CH2] are sequentially arranged in the main On the chain, this can also be seen as an ethylene molecule interposed between two propylene molecules.
Another 25% of the ethylene is incorporated into the main chain in a multi-molecule insertion manner, also called the X5 group, because there are 5 consecutive methylene groups (two ethylene molecules intercalated between two propylene molecules). It is difficult to distinguish X5 and higher groups such as X7, X9, etc. In view of this, the XS and higher group ethylene contents are counted together as >X3%.
The randomness ratio X3/X5 can be determined. When the percentage of groups above X3 is large, the crystallinity of the copolymer will be significantly reduced, which has a great influence on the final properties of the random copolymer. The effect of a very high content of ethylene in the copolymer on the crystallinity of the polymer is similar to that of a high atactic polypropylene content.
Random PP copolymers differ from homopolymers because the random insertion of ethylene molecules in the polymer backbone hinders the crystallographic alignment of the polymer molecules. The decrease in the crystallinity of the copolymers results in changes in the physical properties: the random copolymers have lower stiffness, improved impact resistance and better transparency than PP homopolymers. Ethylene copolymers also have a lower melting temperature, which is an advantage when they are used in certain aspects.
Random copolymers contain more of extractables and atactic PP, as well as much higher ethylene content polymer chains. This higher content of extractables, depending on the polymerization process, is present in varying degrees in all commercial copolymer materials and causes difficulties in meeting the requirements of the Federal Food Administration (FDA) regarding food contact.
Manufacturing method
Ethylene/propylene random copolymers are prepared by the simultaneous polymerization of ethylene molecules and propylene molecules using the same reactors as the PP homopolymers. The ethylene molecule is smaller than the propylene molecule and the reaction is faster than (reaction activity is about ten times) propylene. This reduces the stereospecificity of the catalyst and increases its activity, resulting in an increase in the production of atactic polypropylene. In order to reduce the formation of such a random matter, it is necessary to lower the reaction temperature, thereby reducing the activity of the catalyst and reducing the content of atactic isomers in the final product, resulting in a product with more balanced performance.
Random copolymers with high ethylene content (>3%) are difficult to handle in the production process and it is difficult to carry out the polymerization in hexane diluent because of the secondary by-products of the reaction (atactic polypropylene and ethylene content) Very high copolymers) are soluble in hexane. This also applies to bulk polymerization of liquid propylene, albeit at lower solubility. The large amount of by-products produced by the hexane dilution process must be separated out at the hexane recycle stage, which increases the total production cost, but can result in a cleaner polymer with a smaller amount of soluble components. In the bulk polymerization process, these impurities can remain in the polymer and cause troubles when handling the flake material. Moreover, the final copolymer product contains more soluble impurities. The use of organic solvents for secondary cleaning can remove most of the impurities but increase the overall production cost of the copolymer. Generally, when the content of by-products is high, the flake-like random copolymer may become more viscous, and this problem is more prominent when the ethylene content is more than 3.5% by weight.
Increased processing problems and lower reactor temperatures result in lower production rates of random copolymers. And the production cycle of random copolymers is usually very short. These factors make the total production cost of random copolymers higher than that of homopolymers, especially for random copolymers with a high ethylene content.
The decrease in the melting point of the copolymer is directly related to the ethylene content. It is reported that the melting point of the copolymer is as low as 152°F when the ethylene content is 7%. The effect of X3 content on the melting point of the copolymer was greater than that of children and higher gene content. It also depends on the catalyst itself, and its ability to replace the group X3 with ethylene in place of the X3 group.
performance
Physical properties: In general, random PP copolymers have better flexibility and lower rigidity than PP homopolymers. They also maintain moderate impact strength when the temperature drops to 32°F, and when the temperature drops to -4°F, usefulness is limited. The flexural modulus of the copolymer (cutting modulus at 1% strain) is in the range of 483 to 1034 MPa, while the homopolymer is in the range of 1034 to 1379 MPa. The effect of the molecular weight of the PP copolymer material on rigidity is not as great as that of the PP homopolymer. The notched Izod impact strength is generally in the range of 0.8-1.4 ft-lb/in.
Chemical resistance: random PP copolymer to acid. Alkali, alcohol, low-boiling hydrocarbon solvents and many organic chemicals have a strong resistance. At room temperature, the PP copolymer is essentially insoluble in most organic solvents. And, when exposed to soap, soapy lye. When used in aqueous reagents and alcohols, they do not suffer from environmental stress fracture damage as do many other polymers. When in contact with certain chemicals, especially liquid hydrocarbons. Chloroorganics and strong oxidants.
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