PP random co-polymers


Introduction

Polypropylene random copolymers are a type of polypropylene in which the basic structure of the polymer chain has been modified by the incorporation of a different monomer molecule. Ethylene is the most common comonomer used. This causes changes to the physical properties of the PP. In comparison with PP homopolymers, random copolymers exhibit improved optical properties (increased clarity and decreased haze), improved impact resistance, increased flexibility, and a decreased melting point, which also results in a lower heat-sealing temperature. At the same time they exhibit essentially the same chemical resistance, water vapour barrier properties, and organoleptic properties (low taste and odour contribution) as PP homopolymer.

Random copolymer PPs were developed to combine improved clarity and impact strength, and are used in blow moulding, injection moulding, and film and sheet extrusion applications. They are used in food packaging, medical packaging, and consumer products.

Chemistry

Random copolymer PP typically contains between 1 and 7 wt.-% of ethylene molecules and 99 to 93 wt.-% propylene molecules. The ethylene molecules are inserted randomly between the propylene molecules in the polymer chain. In such random or statistical copolymers the majority (usually 75%) of the ethylene is incorporated as single molecular insertions termed X3 groups (these have three consecutive ethylene [CH2] molecules in sequence in the polymer chain). These also can be viewed as one ethylene molecule inserted between two propylene molecules.

About 25% of the ethylene is incorporated in multiple molecular insertions. These are called X5 because there are five methylene groups in sequence (two ethylenes inserted together between two propylenes). It is difficult to distinguish between X5 and higher groups (X7, X9, etc.). Because of this, the content of multiple ethylene insertions for X5 and higher groups is usually reported as >X3%.
The randomness ratio (X3/X5) also can be determined. A large percentage of >X3 groups will significantly decrease the crystallinity of the resulting copolymer. This has important effects on the final properties of the random copolymer. Very high levels of ethylene in the copolymer have similar effects on the crystallinity of the polymer, as do high levels of atactic PP.

Random PP differs from homopolymer because the ethylene molecules randomly inserted into the polymer backbone hinder the crystal-type arrangement of the polymer molecules. This reduction in copolymer crystallinity is responsible for the modification of physical properties: random copolymers have reduced stiffness, higher impact resistance, and much better clarity than homopolymer PP. Ethylene copolymers also have lower melting temperatures which give them advantages in some applications.

Random copolymers also have higher levels of extractable materials and atactic PP, and polymer chains with much higher levels of ethylene (to 15wt.-%).These higher extractable levels occur to one degree or another with all commercial copolymer materials (depending on the polymerization process), and can cause problems in meeting FDA food contact regulations for some applications.

Manufacture

Ethylene/propylene random copolymers are produced by the simultaneous polymerisation of propylene and ethylene molecules in the same reactors used to produce homopolymer PP. Ethylene molecules are smaller than propylene molecules, and react faster (they are about ten times more reactive). This makes the catalyst less stereospecific but more active, which results in an increased production of atactic PP. To reduce such atactic production, the temperatures at which the reaction is carried out are reduced. This lowers the activity of the catalyst and so reduces the eventual atactic content, resulting in the production of a copolymer product with a better balance of properties.

Random copolymers with high levels of ethylene (>3%) are more difficult to handle in production. Polymerizing high ethylene copolymers in hexane diluent is more difficult because of the solubility in hexane of the secondary by-products of the reaction (atactic PP and copolymer with very high ethylene content). This also is true in bulk polymerization with liquid propylene, although the solubilities are lower. Thus, the hexane diluent process produces a large amount of by-product that must be separated in a hexane recycle stage, adding to the overall manufacturing cost. It does, however, result in cleaner polymer with lower levels of extractable components.

In a bulk process, these residues tend to stay in the polymer and cause handling problems with flake materials. Also, the final copolymer product tends to have higher residual extractable components. Secondary washing steps that use organic solvents can remove a large part of these residues, but this also serves to increase the overall cost of manufacturing the copolymers. Generally, random copolymer flakes are stickier because of these high level of by-product materials, and this problem becomes critical when ethylene levels are greater than 3.5 wt.-%.

Increased handling problems and lower reactor temperatures lead to lower production rates for random copolymers. Also, random copolymers usually are produced in much shorter runs. These factors contribute to the overall higher production cost for random copolymers compared to homopolymers, especially for the higher ethylene types.

Reduction of the copolymer's melting point is directly related to ethylene content. Thus, melting point values as low as 60oC. have been reported with copolymers with 7 wt.-% ethylene. The X3 content has a greater effect on the copolymer melting point than the content of X5 and higher sequences. It also depends on the catalyst itself and its ability to incorporate ethylene as X3 sequences instead of as X5 sequences.

Properties

Physical properties: Generally, random co-polymers are more flexible and less stiff than homopolymer PP. They have moderately better impact strength at temperatures down to 0oC, and they have limited utility down to -25oC. The molecular weight of the material has a greater effect on the stiffness for homopolymers than it does for copolymers.

Chemical resistance: Random copolymers are highly resistant to attack by such chemicals as acids, alkali's, alcohols, low-boiling hydrocarbon solvents, and many inorganic chemicals. At room temperature, PP copolymers are essentially insoluble in most organic solvents. Also, they are not susceptible to environmental stress cracking failures when exposed to soaps, soap solutions, wetting agents, and alcohols, as are many other polymers. Contact with some chemicals-particularly liquid hydrocarbons, chlorinated organic compounds, and strong oxidizing acids-can cause surface crazing or swelling. Generally, non-polar compounds are absorbed more easily by PP than are polar chemicals.

Barrier properties: Both homopolymer and copolymer PP have very low water vapour permeability. Resistance to permeation by gases is fair to moderate. These can be improved through orientation.

Stretch-blow moulded PP bottles improve the moisture vapour resistance and the O2 permeability.

Electrical properties: Generally, PP has excellent electrical properties-including high dielectric strength, low dielectric constant, and low dissipation factor. However, homopolymers are the usual choice for electrical applications.

Applications

Random copolymer PP is mainly used in film, blow moulding, and injection moulding applications where high clarity is a requirement. Because of their lower sealing initiation temperatures, higher-ethylene copolymers find wide use as special sealing layers in co-extruded film structures.