Blueridge Films is a manufacturer plastic film with 29 years of industry experience. Our film products include polypropylene (PP), Linear Low Density Polyethylene (LLDPE), High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE), EVA, Metallocene, and High Molecular Weight High Density Polyethylene (HMW-HDPE). We offer technical assistance with custom formulations.
Polyolefins Are Thermoplastics Derived From Petrochemicals:
Polyolefins are plastic resins polymerized from petroleum-based gases. The two principal gases are ethylene and propylene. Ethylene is the principal raw material for making polyethylene (PE) and ethylene copolymer resins; and propylene is the main ingredient for making polypropylene (PP) and propylene copolymer resins.
Plastic Film Molecular Structure and Composition Affect Properties and Processability
Three basic molecular properties affect most of the properties essential to high quality film extrusion:
- average molecular weight.
- molecular weight distribution.
- crystallinity or density.
These molecular properties are determined by the materials used to produce the Polyolefins and the conditions under which they are manufactured.
Plastic Film Density
Polyolefin resins have a mixture of crystalline and amorphous areas. Molecular chains in crystalline areas are arranged somewhat parallel to each other. In amorphous areas, they are randomly arranged. This mixture of crystalline and amorphous regions is essential to the formation of good film products. A totally amorphous polyolefin would be rubber-like and have poor physical properties; a totally crystalline polymer would be very hard and brittle.
For homopolymer polyethylenes, the higher the resin density, the higher the degree of crystallinity. High density PE resins (HDPE) have molecular chains with comparatively few side chain branches. This allows the chains to pack more closely together. The result is crystallinity up to 85%. Low density PE resins (LDPE) generally have crystallinity from 35 to 55%. Linear low density PE (LLDPE) resins have crystallinity from 35 to 60%. Polypropylene resins are highly crystalline, but they are not very dense.
- LLDPE film resins' densities range from 0.900 to 0.939 grams per cubic centimeter (g/cm3)
- LDPE film resins range from 0.916 to 0.925 g/cm3
- MDPE (medium density) resins range from 0.926-0.940 g/cm3
- HDPE film resins range from 0.941 to 0.965 g/cm3
- PP film resins range from 0.890 to 0.905 g/cm3
- EVA copolymers' densities are functions of the proportion of comonomer incorporated into the resin; as comonomer increases, density increases, but crystallinity decreases.
Higher density, in turn, influences numerous properties. With increasing density, some properties increase. However, increased density also results in a reduction of some properties, e.g., stress cracking resistance and low temperature toughness. (Table1 Film Properties)
Plastic Film Molecular Weight
Atoms of different elements, such as carbon, hydrogen, etc., have different atomic weights. For carbon, the atomic weight is 12, and for hydrogen it is 1. Thus, the molecular weight of the ethylene unit, is the sum of the weight of its six atoms (2 carbon + 4 hydrogen) or 28.
Every polyolefin resin consists of a mixture of large and small chains, i.e., chains of high and low molecular weights. The molecular weight of the polymer chain generally is in the thousands. The average of these is called, quite appropriately, the average molecular weight.
As average molecular weight increases, resin toughness increases. The same holds true for tensile strength and environmental stress cracking resistance (cracking brought on when film is subjected to stresses in the presence of liquids such as solvents, oils, detergents, etc.)
Plastic Film Melt Viscosity
Melt viscosity generally is expressed for polyethylene resins by their melt indices (tested under standard conditions of temperature and pressure). Melt index (MI) is inversely related to the resin's average molecular weight; as average molecular weight increases, MI decreases. Generally, a polyolefin resin with high molecular weight has a low MI, and vice versa.
Molecular Weight Distribution
The relative distribution of large, medium and small molecular chains in a polyolefin resin is important to its properties. When the distribution consists of chains close to the average length, the resin is said to have a "narrow molecular weight distribution". "Broad molecular weight distribution" Polyolefins are those resins with a wider variety of chain lengths. In general, resins with narrow molecular weight distributions have greater stress cracking resistance and better optical properties. Resins with broad molecular weight distributions generally have greater impact strength and greater ease of processing.
Plastic Film Co-monomers
Polyolefins made with one basic type of monomer are called homopolymers. There are, however, many Polyolefins which consist of two or more monomers -- each called a co-monomer -- and these combinations are called copolymers. Many film extrusion grades of LLDPE, LDPE, HDPE and PP are made with Co-monomers. These side chain groups provide specific property improvements.
The Co-monomers used most often with LLDPE and HDPE are collectively called alpha olefins. They include butene, hexene and others. Other Co-monomers used with ethylene to make EMA copolymers, and VA to produce EVA copolymers.
The addition of small amounts of VA to polyethylene results in a resin which extrudes similarly to a polyethylene homopolymer but has the additional properties of increased toughness, lower stiffness and potentially higher clarity. A wide range of properties is possible, depending upon the proportion of VA incorporated and the synthesis conditions used to make the modified resins.
Modifiers, Additives and Tie-Layers for Plastic Films
Numerous chemical modifiers and additives are compounded with polyolefin film extrusion resins. In some grades, the chemical modifiers are added during resin manufacture. These include thermal stabilizers, anti-static agents and slip/anti-block agents. (Table2 - Plastic Additives)
(Excerpt from "A Guide to Polyolefin Film Extrusion", Equistar Chemicals, LP, Formerly Quantum Chemical Corp.)