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Файл:Plastics technology. Часть 1. Учебное пособие.pdf
31
Properties of LLDPE and VLDPE
Parameter
VLDPE
EVA
Density (g/cm3)
Stress crack time (h)
0.910
600
0.926
240
As with LDPE and HDPE mater ials, there is a wide range of linear
low-density polyethylene (LLDPEs). Primarily competitive with LDPE, the
“linear low” materials have found rapid acceptance because of their high
toughness (at low, normal and high temperatures), tensile strength,
elongation at break and puncture resistance compared to LDPE materials of
similar melt flow index and density. More specifically the improved
resistance to environmental stress cracking has been emphasized by
suppliers as also has the ability to use dishwashers to clean LLDPE kitchen
utensils, a consequence of the higher heat deformation resistance.
The very low density materials (VLDPEs) introduced in the mid1980s are generally considered as alternatives to plasticized PVC and
ethylene-vinyl acetate (EVA) plastics. They have no volatile or extractable
plasticizers as in plasticized PVC nor do they have the odour or moulding
problems associated with EVA. Whilst VLDPE materials can match the
flexibility of EVA they also have better environmental stress cracking
resistance, improved toughness and a higher softening point.
Some comparative data for a VLDPE copolymer based on ethylene
and oct-1-ene and an EVA material (91% ethylene, 9% vinyl acetate) are
given in Table 1.
Table 1 – Comparison of VLDPE and EVA (9% VA)
MFI
Tear strength (N/mm
Elongation at break (%)
Vicat temperature (°C)
Low-temperature brittle point (°C)
Hardness (Shore D)
Properties of metallocene-catalysed polyethylene
Metallocene-catalysed polyethylene exhibits the general
characteristics of polyethylene. Furthermore they are more like low density
polyethylene (LDPE and LLDPE) than HDPE. As with LLDPE they are
usually copolymers containing small quantities of a low molecular weight
α-olefin such as but-1-ene, hex-1-ene and oct-1-ene. The property
differences largely arise from the narrow molecular weight distribution, the
more uniform incorporation of the α-olefin and the low level of
2
)
7
11.4
710
78
-135
42
9
6.1
475
51
-130
32
32
polymerization residues (about one-tenth that of Ziegler-Natta catalyzed
LLDPE).
It is generally claimed that metallocene polyethylene (often
abbreviated to m-PE) exhibit superior mechanical and optical properties as
well as better organoleptic properties (resulting from the lower residue
levels).
Narrow molecular weight distribution polymer s such as m-PE are
less pseudoplastic in their melt flow behaviour than conventional
polyethylene so that given an m-LLDPE and a conventional LLDPE of
similar melt index (measured at low shear rates), the m-LLDPE will have a
much higher melt viscosity at the high shear rates involved in film
processing. The polymers are also more susceptible to melt fracture and
sharkskin.
Metallocene-catalysed ver y l ow density polyethylene (m-VLDPE)
has become available with densities of as low as 0.903. This i s of use for
sealing layers of multi-layer films since sealing can commence at lower
temperatures than with conventional materials such as LLDPE and EVA
with the polymer seal exhibiting both cold strength and hot tack strength.
Additives
Although polyethylene can be, and indeed often is, used without
additives a number may be blended into the polymer for various reasons.
These additives can be classified as follows :
(1) Fillers.
(2) Pigments.
(3) Flame retarders.
(4) Slip agents.
(5) Blowing agents.
(6) Rubbers.
(7) Cross-linking agents.
(8) Antioxidants.
(9) Carbon black.
(10) Antistatic additives.
Fillers, important constituents of many plastics materials, are rarely
used with polyethylene since they interfere with the crystallinity of the
polymer and often give rather brittle products of low ductility. Carbon
black has some reinforcing effect and is of use in cross-linked polymers. It
is also of some use in introducing a measure of conductivity to the polymer.
33
Somewhat better results with non-black fillers may be achieved with the
use of silane and titanate coupling agents and compounds with increased
rigidity and tensile strength compared with unfilled polymer may be
obtained. However, unlike polypropylene, mineral-filled polyethylene has
remained unimportant. A number of pigments are available for use in
polyethylene. The principal requirements of a pigment are that it should
have a high covering power/cost ratio and that it should withstand
processing and service conditions. In the case of polyethylene special care
should be taken to ensure that the pigment does not catalyze oxidation, an
effect observed with a number of pigments based on cobalt, cadmium and
manganese. Other adverse effects have also been reported with hydrated
chromic oxide, iron blues, ultramarine and anatase titanium dioxide. For
electrical insulation applications pigments such as cobalt blues, which
cause a rapid rise of power factor on aging, should be avoided.
Polyethylene burns readily and a number of materials have been
used as flame retarders. These include antimony trioxide and a number of
halogenated materials.
Layers of low-density polyethylene film often show high c ohesion,
or “blocking”, a feature which is often a nuisance on both processing and
use. One way of overcoming this defect is to incorporate anti-blocking
agents such as fine silicas. In addition slip agents may be added to reduce
the friction between layers of film. Fatty acid amides such as oleamide and,
more importantly, erucamide, are widely used for this purpose. Polymers
with densities of above 0.935 g/cm3 show good slip properties and slip
agents are not normally required for these products.
Products with very low dielectric constant (about 1.45) can be
obtained by the use of cellular polymers. Blowing agents such as 4,4'-
oxybisbenzenesulphono-hydrazide and azocarbonamide are incorporated
into the polymer. On extrusion the blowing agent decomposes with the
evolution of gas and gives rise to a cellular extrudate. Cellular polyethylene
is a useful dielectric in communication cables.
Although many rubbery materials show varying compatibility with
polyethylene the only elastomeric materials used in commercial compounds
are polyisobutylene (PIB) and butyl rubber. Polyisobutylene was originally
used as a “plasticizer” for polyethylene but was later found also to improve
the environmental stress cracking resistance. Polyethylene is sometimes
blended with ethylene-propylene rubber. In this application it is most
commonly used as an additive to the rubber, which in turn is added to
polypropylene to produce rubber-modified polypropylenes. In addition up
34
to 20% of ethylene-propylene rubber may be used in blown film
applications.
Vulcanized (cross-linked) polyethylene is being used for cable
application where service temperatures up to 90°C are encountered. Typical
cross-linking agents for this purpose are peroxides such as dicumyl
peroxide. The use of such agents is significantly cheaper than irradiation
processes for the cross-linking of the polymer. An alternative process
involves the use of vinyl silanes.
When polyethylene is to be used in long-term applications where a
low power factor is to be maintained and/or where it is desired to provide
thermal protection during processing, antioxidants are incorporated into the
polymers. Although amines have been used widely in the past phenols are
now used almost exclusively.
Antistatic additives are widely used to reduce dust attraction and
also in films to improve handling behaviour on certain types of bag making
and packaging equipment. Whilst at one time it was more common to apply
an antistatic agent to the surface by wiping, dipping or spraying, increasing
use is now being made of antistatic agents which are incorporated into the
polymer mass during normal compounding and which migrate to the
surface with the passage of time. This approach has the advantage that the
extra coating process is avoided and also that any layer of material removed
by normal handling will be replaced by material which will migrate out of
the mass. The selection of such “antistats” is critical and will depend
particularly on the polymers used and on the thermal stability required. For
a given polymer the agent should have a limited compatibility and a high
diffusion rate in order to produce an antistatic layer as soon as possible
after manufacture. Whereas quaternary ammonium compounds are widely
used for polystyrene, polyethylene glycol alkyl esters would appear to be
preferred in polyethylene compositions. The actual chemical composition
of the rather small number of antistatic agents so far found suitable is rarely
disclosed by the suppliers.
Processing
Although plastics materials may in principle be processed in a
variety of physical states (in solution, in emulsion, as a paste or as a melt),
melt processing is used almost exclusively with polyethylene. The main
features to be borne in mind when processing the polymers are:
(1) The low water absorption of the polymer avoids the necessity
of predrying before processing except where hygroscopic additives are
present.
35
(2) The tendency of the material to oxidize in air particularly at
melt temperatures means that the melt should come in contact with air as
little as possible.
(3) Although processing temperatures are low compared with many
plastics the specific heat, which varies with temperature, is high. As a
consequence more heat needs to be put in and taken out of polyethylene
during processing than for other major thermoplastics. This means higher
energy costs to raise the temperature and longer cooling times after
shaping.
(4) The me lt vi sc os it y is hi ghly n on -Newtonian in that the apparent
viscosity drops considerably with increasing shear rate. Melt viscosities are
about the average encountered with plastics materials but there is a
considerable variation between grades.
(5) The high degree of crystallization, which leads, among other
things, to a high shrinkage on cooling.
(6) The short polymer relaxation times.
(7) A rather sharp melting point.
Polyethylene is processed by a wide variety of techniques. There is
insufficient space here to deal adequately with the principles and practice of
these processes or even with the particular characteristics of polyethylene
being fabricated by these processes. Compression moulding is used only
occasionally with polyethylene. A very large number of products are
produced by injection moulding. Many articles, bottles and containers in
particular, are made by blow moulding techniques of which there are many
variations. Another moulding process based on the extruder is “extrusion
moulding”. Approximately three-quarters of the polyethylene produced is
formed into products by means of extrusion processes.
Calendering processes, of great importance in the production of
sheet materials from PVC compounds, are little used with polyethylene
because of the difficulty in obtaining a smooth sheet. Commer cial products
have, however, been made by calendering low-density polymer containing
a small amount of a peroxide such as benzoyl peroxide to give a stiff but
crinkly sheet (Crinothene) which was suitable for lampshades and other
decorative applications.
The oldest-established of the powder processes is fluidized-bed
coating. In the process a metal object which is to be coated with
polyethylene is heated to about 160-250°C and then suspended in a
fluidized bed of powdered polymer.
36
Whereas dipping and spraying techniques require the use of fine
powder, the coarser ground powders may be tolerated in powder moulding
techniques. Once again there are a number of variants of the techniques
such as the Engel process but in principle they involve pouring powder into
a heated low-cost mould. Some of the powder close to the mould fuses and
sinters on to the mould, while the excess is poured out. The mould and the
adhering powder are then heated until the powder forms a smooth
continuous layer, the assembly is cooled and the resultant moulding
removed from the mould. This process is very suitable for large objects but
because of the long process time is less suitable for small articles. One
important variation of the process is rotational moulding. In this case the
required amount of powder is added to the mould, which is completely
closed and then rotated in an oven about two axes. The powder melts and is
distributed over the walls of the mould. The mould is then cooled whilst the
moulds are rotating.
Powdered polyethylene is also used to bond cloth interliners to
garments. Automobile carpets are often made with a polyethylene rather
than a latex backing because the polyethylene-backed carpet can be heatformed so that the carpet will fit the shape of the car floor and cutting and
sewing operations are consequently eliminated.
It is common practice to print upon certain products such as films
and bottles. Because polyethylene is chemically inert it is not possible to
obtain a good ink adhesion directly on to the polymer surface. To overcome
this problem the surface of the polymer is usually modified in order to
provide an oxidized layer that will enable the ink to adhere. Modification
may be carried out either by treatment with a naked flame or, more
commonly, by subjecting the surface to a high-voltage discharge. The
presence of slip agents and antistats may complicate the process and in the
case of polyethylene film the modification is usually carried out
immmediately after extrusion before the wind-up stage and before an
appreciable concentration of slip agent or antistatic additive has migrated to
the surface. Once the surface has been modified it is possible to print on to
the film without undue difficulty.
Applications
Polyethylene was introduced initially as a special purpose dielectric
material of particular value for high-frequency insulation. With increasing
availability the polymer subsequently began to be used for chemical plant
and, to a small extent, for water piping. Since World War II there has been
37
a considerable and continuing expansion in polyethylene production and
this, together with increasing competition between manufacturers, has
resulted in the material becoming available in a wide range of grades, most
of which are sold in the lowest price bracket for plastics materials. The
present position of polyethylene as a general purpose thermoplastic
material is due in no small measure to the low cost and easy processability
of the polymer.
The characteristics of polyethylene which lead to its widespread
use may be summarized as follows:
(1) Low cost.
(2) Easy processability.
(3) Excellent electrical insulation properties.
(4) Excellent chemical resistance.
(5) Toughness and flexibility even at low temperatures.
(6) Reasonable clarity of thin films.
(7) Freedom from odour and toxicity.
(8) A sufficiently low water vapour permeability for many
packaging, building and agricultural applications.
To these could also be added the fact that a great quantity of
information is available concerning the processing and properties of this
material and that its properties are reasonably well known and understood
by the public at larg e .
The limitations of the polymer are:
(1) The low softening point.
(2) The susceptibility of low molecular weight grades to
environmental stress cracking.
(3) The susceptibility to oxidation (however, polyethylene is better
in this respect than many other polymers).
(4) The opacity of the material in bulk.
(5) The wax-like appearance.
(6) The poor scratch resistance.
(7) The lack of rigidity (a limitation in some applications but a
virtue in others) .
(8) The low tensile strength.
(9) The high gas permeability.
For many purposes these limitations are not serious whilst in other
cases the correct choice of polymer, additives, processing conditions and
after-treatment can help considerably.
In the early 1990s it was estimated that the geographical
38
breakdown for production capacity was Western Europe 26%, North
America 33%, Japan 8%, Eastern Europe 8% and Rest of the World 25%.
Major applications for LDPE film include heavy duty sacks, refuse
sacks, carrier bags and for general packaging. Many of these uses may now
be considered to be mature and future growth will become more closely
tied to national economic situations. Similar comments may also be made
about the extensive use of polyethylene film in the building industry.
The LDPE film market has now been partly eroded by LLDPE.
Polymerization plants for LLDPE are cheaper to build, easier to operate
and maintain, have lower energy requirements and are more versatile than
the high-pressure plants. For these reasons manufacturing costs are
reduced. There are also some t echnical advantages to the user since films
from LLDPE have a higher impact strength, tensile strength and
extensibility. Such properties allow the possibility of making film of lower
gauge but with the same mechanical performance. LLDPE materials show
lower gloss, greater haze, are less suitable for shrink film and have a
narrower heat sealing range.
There are two main classes of HDPE now on the market:
(1) High molecular weight, broad MWD polymer – used primarily
for blow moulding and pipe.
(2) Low molecular weight, narrow MWD polymer – used widely
for injection moulding and rotational moulding.
Each type is available in varying densities. Bimodal MWD
polymers have become established in some markets. The high molecular
weight component confers impact strength, toughness, stiffness and good
environmental stress cracki ng resistance, whilst the lower molecular weight
peak improves flow behaviour.
High-density polyethylene film is being increasingly used for
carrier bags. It is also finding rapid acceptance as a wrapping material
instead of paper for food products, probably largely a consequence of its
crisp feel and greaseproof nature. HDPE film is also of some interest in
some pseudo-fibre applications.
Polyethylene is an important injection moulding material. Partly
because LLDPE is now cheaper than LDPE, the former is now becoming
the preferred mate ria l for low-stiffness injection mouldings.
Blow moulding is widely used for HDPE. One large application
area is that of bottles for milk and other foodstuffs, household chemicals,
personal toiletries and drug packaging. These take about 70% of the blow
moulding market but other important areas include drums, pails and toys.
39
Another substantial use area for polyethylene is for pipe. Uses
w
M
w
M
include domestic water and gas piping, agricultural piping and, on a smaller
scale ink tubes for ball point pens.
High molecular weight HDPE (HMW-HDPE) is used for blown
film and for demanding moulding and structural uses. Examples are blow
moulded drams for packing dangerous chemicals and pressure piping.
Very hi gh (ultra-high) molecular weight polyethylene (VHMWPE
or UHMWPE) of molecular weights in the range 3-6∙106, with their
outstanding abrasion resistance, low-temperature toughness and low
coefficient of friction, are used in severe operating conditions such as ore
handling and food processing. Such materials are sometimes cross-linked
with peroxides to further improve the abrasion resistance.
VLDPEs, with their very good low-temperat ure impact properties,
are of interest in frozen food packaging.
The excellent electrical insulation properties of polyethylene have
led to extensive use in cable and other wire-covering applications.
Spectacular early uses included undersea cables and airborne radar and the
materials continue to be used in substantial quantities. One particular trend
is the increasing use of cross-linked polyethylene for this area of use. Such
materials have improved heat resistance and in addition have given
generally better resistance to stress cracking. Cellular polyethylene is used
as the insulator for television downlead aerials.
Polyethylene of low and high molecular weight
The most common commercial grades of polyethylene have
number average molecular weights of the order of 10000-40000 (with
corresponding
values in the range 50000-300000). There are,
however, a number of special purpose grades on either side of this range.
Most low molecular weight materials have molecular weights in
the range 1000-10 000 with viscosities of about 1-2 poise at 150°C (c.f.
104-106 poise for conventional polymers) and melting points in the range
80-110°C. Such materials do not exhibit the tenacity and toughness of high
polymers and are essentially wax-like. They are, however, tougher than
conventional waxes and also show very good water and chemical
resistance. Because of these characteristics such low molecular weight
polyethylene are used in wax formulations, wax-paper coatings and as
lubricants in, for example, PVC.
Very high molecular weight polyethylene (
in the range 1-
40
6∙106) prepared by the Zi egl er process have also become available. These
polymers cannot be processed easily in the molten state without
decomposition and it is therefore often necessary to process in the rubbery
phase.
The polymers are linear in nature but, because of the difficulties
large molecules have in crystallizing, the densities are only of the order of
0.94 g/cm3 and properties that are primarily dependent on percentage
crystallinity (density), for example yield strength and stiffness, are slightly
worse than those obtained with linear polyethylene of conventional
molecular weight. The main advantages claimed for the very high
molecular weight polymers are high abrasion resistance and impact
strength, very good stress-cracking resistance and low creep.
2.1.2 Polypropylene
Until the mid-1950s the only polyolefins (polyalkenes) of
commercial importance were polyethylene, polyisobutylene and
isobutylene-isoprene copolymers (butyl rubber). Attempts to produce
polymers from other olefins had, at best, resulted only in the preparation of
low molecular weight material of no apparent commercial value.
In 1954 G.Natta of Milan following on the work of K.Ziegler in
Germany found that certain “Ziegler-type” catalysts were capable of
producing high molecular weight polymers from propyl ene and many other
olefins. By variations on the form of the catalysts used Natta was able to
produce a number of different types of high molecular weight
polypropylenes which differed extensively in their properties. One form,
now known as isotactic polypropylene, was in many ways similar to highdensity polyethylene but with a higher softening point, rigidity and
hardness, whilst another form, the atactic polymer, was amorphous and had
little strength.
Commercial exploitation was very rapid, with isotactic
polypropylene being marketed by Montecatini as Moplen as early as 1957.
From that time on consumption growth rates have been high, with the
material becoming widely used for fibres, films and injection mouldings.
The expiry of the basic patents led to a surge in production capacity in the
1970s which so exceeded demand that the material became available at
abnormally low prices, which in turn stimulated more growth in usage.
Since that time production growth rates for polypropylene have generally
been higher than for the other major tonnage plastics, and in the mid-1980s
polypropylene moved into third place after polyethylene and PVC in the