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LDPE / HDPE /LLDPE/ Metallocene
/ Table 1 (Film Properties) / Table
2 (Additives)
Technical Information
(Excerpt
from “A Guide to Polyolefin Film Extrusion”, Equistar
Chemicals, LP (Formerly Quantum Chemical Corp.)
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.
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.
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
resins’ densities range from 0.900 to 0.939 grams per cubic
centimeter (g/cm3)
- LDPE
resins range from 0.916 to 0.925 g/cm3
- MDPE
(medium density) resins range from 0.926-0.940 g/cm3
- HDPE
resins range from 0.941 to 0.965 g/cm3
- PP
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)
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.)
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.
Comonomers
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 comonomer -- and these combinations are called
copolymers. Many film extrusion grades of LLDPE, LDPE,
HDPE and PP are made
with Comonomers. These side chain groups provide specific property
improvements.
The
Comonomers
used most often with LLDPE and HDPE are collectively
called alpha olefins. They include butene, hexene and others. Other
Comonomers 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
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
- Additives)
OUR BLOWN FILM LINE
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