Film Extrusion Guide.pmd - LyondellBasell

H H
C = C
H H
Figure 1. Ethylene monomer
molecular structure
H H H H H H H H H H
C C C C C C C C C C
H H H H H H H H H H
Figure 2. Polyethylene molecular
chain.
Figure 3. Polyethylene chain with
side branches.
Figure 4. Polyethylene chain with
side branches.
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.
Polyolefin resins are classified
as thermoplastics, which means
that they can be melted, solidified
and melted again. This contrasts
with thermoset resins which, once
molded, cannot be reprocessed.
Most polyolefin resins for film
extrusion generally are used in
pellet form. The pellets are about
1/8 inch thick and 3/16 inch in
diameter, usually somewhat
translucent and white in color.
Polyolefin resins sometimes will
contain additives, such as thermal
stabilizers. They also can be
compounded with colorants,
antistatic agents, slip and antiblock
additives, UV stabilizers, etc.
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. The basic building
6
blocks for the gases from which
polyolefins are derived are hydrogen
and carbon atoms. For polyethylenes,
these atoms are combined
to form the ethylene monomer,
C 2 H 4 , i.e., two carbon atoms and
four hydrogen atoms (see Figure
1). In the polymerization process,
the double bond connecting the
carbon atoms is broken. Under the
right conditions, these bonds
reform with other ethylene molecules
to form long molecular
chains (Figure 2). The resulting
product is polyethylene.
For polypropylene, the
hydrogen and carbon atoms are
combined to form the propylene
monomer, CH 3 CH=CH 2 , which has
three carbon atoms and six
hydrogen atoms (Figure 3). The
third carbon atom remains pendant
and protrudes from the spiraling
backbone chain.
Ethylene copolymers, such as
EVA and EMA, are made by the
polymerization of ethylene units
with randomly distributed
comonomer groups, such as vinyl
acetate (VA) and methyl acrylate
(MA).
The polymerization of
monomers creates a mixture of
molecular chains of varying
lengths. Some are short, while
others are enormously long,
containing several hundred
thousand monomer units. For
polyethylene, the ethylene chains
have numerous side branches. For
every 100 ethylene units in the
molecular chain, there are about
one to 10 short or long branches.
The branches radiate in three
dimensions (Figure 4).
Chain branching affects many
polymer properties including
density, hardness, flexibility and
transparency, to name a few. Chain
branches also become points in the
molecular network where oxidation
may occur. In some processing
techniques where high temperatures
are reached, oxidation can
adversely affect the polymer’s
properties.