Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Plastics technology. Часть 1. Учебное пособие.pdf
51
Syndiotactic polypropylene first became available in the 1990s
Parameter
Syndiotactic PP
Isotactic PP
Density (g/cm3)
Tm (°C)
0.9
168
0.9
163
(Fina, Mitsui Toastu, Sumitomo) and more recently has been marketed by
Dow. Currently this polymer is more expensive than other polypropylene
both because of catalyst costs and the small scale of production.
Syndiotactic materials are generally softer, tougher and more
transparent than isotactic materials but exhibit similar melting points
(Table 2).
Table 2 – Comparison of some properties of syndiotactic and isotactic
polypropylene
Elastic modulus (MPa)
Impact strength (-23°C), kJ/m
Crystallinity
number of other additives. Of these the most important are:
filler the use of such materials is more common than with polyethylene.
About 3% of polypropylene compounds are filled with talc and these have
found use in both injection moulding and sheet applications. The improved
stiffness and heat deformation resistance has led to the use of such
compounds for the manufacture of heater housings, car mounting
components and several domestic appliances. Talc-filled polypropylene
sheet is also used as an alternative to carton board. More recently there has
been increasing interest in the use of calcium carbonate, which may be used
at levels as high as 50 parts per hundred (pts phr). In comparison to the
talc-filled grades the calcium-carbonate-filled grades are claimed to have
higher impact strength, brighter colour, higher thermal stability, improved
fatigue strength but lower stiffness. Tensile strength is markedly reduced
with both fillers.
2
61
80
30-40
16.5
16
40-60
Additives for isotactic polypropylene
Commercial grades of polypropylene may be blended with a
(1) Fillers.
(2) Rubbers.
(3) Pigments.
(4) Carbon black and ultraviolet absorbers.
(5) Antioxidants.
(6) Nucleating agents.
Whilst most of the polypropylene produced is used without mineral
52
Glass fibres are used to confer enhanced strength and rigidity.
Substantial improvements are, however, only realized after a coupling
reaction takes place between organofunctional silanes on the glass fibre and
reactive groups introduced into the polypropylene molecule. Asbestos
fibres have also been used but concern over health hazards and other
factors have led to declining use with polypropylene.
In the early stages of development of polypropylene rubbers,
particularly butyl rubber, were used to reduce the brittleness of
polypropylene. Their use declined for some years with the development of
the polypropylene copolymers but interest was greatly renewed in the
1970s. This interest has been centered largely around the ethylenepropylene rubbers which are reasonably compatible in all proportions with
polypropylene. At first the main interest was with blends in which the
rubber content exceeded 50% of the blend and such materials have been
designated as thermoplastic polyolefin elastomers. There is also increasing
interest in compounds with less than 50% rubber, often referred to as
elastomer-modified thermoplastics. It is of interest to note that the rubbery
component is capable of being diluted with large amounts of carbon black
and mineral oils to reduce costs substantially.
In general the selection of pigments for polypropylene follows the
same considerations as for polyethylene. Because of the higher processing
temperatures and the lesser resistance to oxidation, selection does,
however, require rather more care.
To improve the resistance to ultraviolet light c arbon bla ck is often
useful as a light screen. Its use in fibres and films is clearly very r estricted
and in these instances ultraviolet absorbers and/or quenching agents are
used. Recent developments include the greater use of hindered amine and
nickel compounds.
Antioxidants are necessary components of all polypropylene
compounds and the selection of such ingredients is an important factor in
determining the success of a given commercial material. For optimum
processing stability a single antioxidant of the phenol alkane type, for
example 1,1,3-tris-(4-hydroxy-2-methyl-5-t-butylphenyl)butane (Topanol
CA), tends to give the best results.
Processing characteristics
Polypropylene may generally be processed by methods very similar
to those used with the polyethylene, particularly high-density polyethylene.
The main differences are the lower specific heat and the greater sensitivity
53
of flow properties to temperature and shear rate. The moulding shrinkage is
lower than with polyethylene but higher than with polystyrene. Most
processing operations involve the use of melt temperatures in the range
210-250°C. Because of the tendency of polypropylene to oxidise, heating
times should be kept down to a minimum.
The blow moulding of polypropylene is generally considered as
being more difficult than for polyethylene. However, providing sufficient
care is paid to both equipment design and operating conditions good
mouldings may be obtained. The basic difficulty is to reconcile the need for
a homogeneous melt which requires high melt temperatures and extensive
shear mixing with the tendency of these conditions to cause polymer
degradation. In order to work at the lowest practical temperatures very
good temperature control is necessary for all stages of the process and in
addition the machine should be of robust construction. This is because at
the low temperatures high pressures will be developed in the barrel and in
particular the thrust bearings must be well designed. To minimise wear the
extruder barrel should have a continuous hardened steel liner.
Applications
There are many factors which must be taken into account and the
choice of a particular polymer for a given application will depend on a
careful study of the product requirements and the properties of potential
materials. Polypropylene homo-polymers and copolymers have found
applications for mouldings where such properties as good appearance,
environment stress cracking resistance and good heat resistance are of
import ance. One further particularly useful property of polypropylene is the
excellent resistance of thin sections to continued flexing. T his has led to the
introduction of a number of one piece mouldings for boxes, cases and
automobile accelerator pedals in which the hinge is an integral part of the
moulding. The special copolymer grades with their higher impact strength
and lower brittle point have absorbed a large part of this market. Typical
mouldings include hospital sterilizable equipment, luggage, stacking chairs,
washing machine parts, toilet cisterns and various car parts such as dome
lights, kick panels, door frame parts, accelerator pedals and car battery
cases.
There has been considerable interest in physically modified
polypropylene for injection moulding. For example, elastomer-modified
blends are widely used in the car industry for such purposes as bumpers
and radiator grilles, fascia panels and protective strips. Outside of the car
54
industry there has been interest in the material for ski boots. Talc-filled
grades are used where extra rigidity and heat deformation resistance are
required whilst the cheaper calci um-carbonate-filled grades have replaced
polystyrene in such applications as flower pots. Further property
enhancement may be obtained by the use of coupled glass fibre as a filler,
on occasion in conjunction with a blowing agent to give a glass-reinforced
structural foam such as that used for the outer tank of a well-known
domestic washing machine.
One particular growth area for polypropylene mouldings is for thinwall packaging such as margarine tubs. This is largely at the expense of
polystyrene and arises partly from economics and partly from the wish to
have a product free of residual styrene monomer.
Non-oriented polypropylene film, which is glass clear, is used
mainly for textile packaging but also as a confectionery wrap and for
packaging “fast-turnover” food. It also finds some acceptance in laminate
production where a high heat resistance is required for the packaging of
fish and meat products. Oriented film is more important because of its
greater clarity, impact strength and barrier properties. Coated grades are
used for bread and biscuit wrapping, for packaging potato crisps and as a
capacitor dielectric. These applications are mainly at the expense of
regenerated cellulose film but the latter has been more difficult to replace
for cigarette wrapping.
Monoaxially oriented film tapes have been widely used for carpet
backing and for sacks. In the latter case the resistance to rotting is of
import ance and the material has widely replaced jute. Fibrillated oriented
tape has also made large inroads into the markets of another natural fibre,
sisal, for twine and string purposes. Polypropylene straps have also gained
rapid and widespread acceptance for packaging, combining strength,
lightness and scope for attractive patterns on the strap surface.
Amongst hydrocarbon polymers polypropylene has been uniquely
successful as a fibre. Its comparatively low cost and excellent wear together
with resistance to staining have led to a major use in carpets as the tufting
material. It has also found some use for blankets. As softer grades of fibre
become available and techniques are developed for dyeing, polypropylene
may be expected to extend its range of fibre applications.
The polymer has found some small-scale outlets in other directions
such as sheet, pipe and wire coating. Consumption of the polyme r in these
directions is, however, dependent on finding applications for which
polypropylene is the most suitable material.
55
Atactic and syndiotactic polypropylene
Atactic polypropylene may be obtained either as a by-product of
the manufacture of isotactic polypropylene or by specific processes
designed for its direct production.
Whilst completely atactic material would be amorphous,
commercial materials have a small measure of crystallinity. This is often
assessed in terms of insolubility in n-hept ane which is usually of the order
of 5-10%. Viscosity average molecular weights are in the range 2000080000 and specific gravities are about 0.86 g/cm3.
In appearance and on handling the material is somewhat
intermediate between a wax and a rubber. It is also semi-tacky. Like
isotactic polypropylene it is attacked by oxygen but unlike the isotactic
material it swells extensively in aliphatic and aromatic hydrocarbons at
room temperature. It is also compatible with mineral fillers, bitumens and
many resins.
For many years atactic polypropylene was an unwanted by-product
but today it finds use in a number of markets and is specially made for
these purposes rather than being a by-product. In Europe the main use has
been in conjuction with bitumen as coating compounds for roofing
materials, for sealing strips where it confers improved aging properties and
in road construction where it improves the stability of asphalt surfaces.
Less important in Europe but more important in USA is its use for paper
laminating for which low-viscosity polymers are used, often in conjunction
with other resins. Limestone/atactic polypropylene blends in ratio 70/30 are
used as back coatings for self-laying carpet tiles. Here the requirements are
non-slip characteristics, good dimensional stability and resistance to lateral
compressive loads as well as low cost. Other uses are as sealing
compounds, for adhesives and, in combination with felt or open-pore
expanded plastics, for automobile vibration damping.
High molecular weight atactic polyropylene is now available. This
is miscible with isotactic polypropylene in any proportion to give
transparent blends of interest in packaging applications.
In the early 1990s syndiotactic polypropylene became available
from a number of sources (Fina, Mitsui Toastu, Sumitomo) and were joined
in the late 1990s by Dow using metallocene catalyst systems. Interest in
these materials is a consequence of their possessing greater toughness,
clarity and heat resistance (softening point) than corresponding isotactic
polypropylene.
56
CH
2
C
CH
3
CH
3
n
Chlorinated polypropylene
The chlorination of polypropylene has been the subject of several
fundamental studies and a variety of products is obtainable according to the
tacticity of the original polymer and to the extent of chlorination.
The polymers have been offered by Sanyo Pulp of Tokyo as filmforming resins of good chemical resistance, and heat and light stability.
Suggested uses include paint vehicles, printing ink binders, overprint
varnishes, adhesives, additives to sealing compounds and waterproofing
agents.
2.1.3 Polyisobutylene
In chronological terms polyisobutylene (PIB) was the first of the
polyolefins. Low polymers were prepared as early as 1873 by Butlerov and
Gorianov and higher molecular weight waxes in 1930 by Staudinger and
Brunner. High molecular weight polymers were produced by IG Farben in
the early 1930s using cationic polymerization methods and polymers based
on these methods are currently available from BASF (Oppanol) and Esso
(Vistanex). The formula for polyisobutylene is:
The pair of opposing methyl groups leads to a low Tg of about 73°C (c.f. -20°C for polybut-1-ene) and the lack of preference for any
particular steric configuration inhibits crystallization in the normal way
although this can be induced on stretching. The methyl groups do, however,
hinder rotation about the main chain bonds so the resulting material is, at
sufficiently high molecular weights, a rather sluggish rubber. It has little
use as a rubber in itself because of its high cold flow but copolymers
containing a bo ut 2% of isop re ne t o intr odu ce un sat urat ion f or cr oss-linking
are widely used.
The homopolymer finds a variety of uses, as an adhesive
component, as a base for chewing gum, in caulking compounds, as a
tackifier for greases, in tank linings, as a motor oil additive to provide
suitable viscosity characteristics and to improve the environmental stresscracking resistance of polyethylene. It has been incorporated in quantities
of up to 30% in high-density polyethylene to improve the impact strength
of heavy duty sacks.
57
2.1.4 Copolymers Containing Ethylene
Many monomers have been copolymerized with ethylene using a
variety of polymerization systems, in some cases leading to commercial
products. Copolymerization of ethylene with other olefins leads to
hydrocarbon polymers with reduced regularity and hence lower density,
inferior mechanical properties, lower softening point and lower brittle
point.
Two random copolymers of this type are of importance, ethylenepropylene copolymers and ethylene-but-1-ene copolymers. The use and
properties of polypropylene containing a small quantity of ethylene in
stereoblocks within the molecule has already been discussed. Although
referred to commercially as ethylene-propylene copolymers these materials
are essentially slightly modified polypropylene.
The Phillips process for the manufacture of high-density
polyethylene may be adapted to produce copolymers of ethylene with small
amounts of propylene or but-1-ene and copolymers of this type have been
available since 1958. These soon found application in blown containers and
for injection moulding.
The linear low-density polyethylene might be considered as
variations of this type of polymer.
Ethylene has also been copolymerized with a number of nonolefinic monomers and of the copolymers produced those with vinyl acetate
have so far proved the most significant commercially. The presence of
vinyl acetate residues in the chain reduces the polymer regularity and hence
by the vinyl acetate content the amount of crystallinity may be controlled.
Copolymers based on 45% vinyl acetate are rubbery and may be vulcanized
with peroxides. They are commercially available (Levapren). Copolymers
with about 30% vinyl acetate residues (Elvax-Du Pont) are flexible resins
soluble in toluene and benzene at room temperature and with a tensile
strength of about 6.9 MPa and a density of about 0.95 g/cm3. Their main
uses are as wax additives and as adhesive ingredients.
Ethylene-vinyl acetate (EVA) polymers with a vinyl acetate content
of 10-15 mole % are similar in flexibility to plasticized PVC and are
compatible with inert fillers. Both filled and unfilled copolymers have good
low-temperature flexibility and toughness and the absence of leachable
plasticizer provides a clear advantage over plasticized PVC in some
applications. Although slightly stiffer than normal rubber compounds they
have the advantage of simpler processing, particularly as vulcanization is
unnecessary. The EVA polymers with about 11 mole % of vinyl acetate
58
may also be used as wax additives for hot melt coatings and adhesives.
A further class of ethylene-vinyl acetate copolymer exists where
the vinyl acetate content is of the order of 3 mole %. These materials are
best considered as a modification of low-density polyethylene, where the
low-cost comonomer introduces additional irregularity into the structure,
reducing crystallinity and increasing flexibility, softness and, in the case of
film, surface gloss. They have extensive clearance as non-toxic materials.
A substantial part of the market for the ethylene-vinyl acetate
copolymer is for hot melt adhesives. In injection moulding the material has
largely been used in place of plasticized PVC or vulcanized rubber.
Amongst applications are turntable mats, base pads for small items of
office equipment and power tools, buttons, car door protector strips and for
other parts where a soft product of good appearance is required. Cellular
cross-linked EVA is used in shoe parts.
EVA polymers have been important for film manufacture. They are
not competitive with normal film because of the high surface tack and
friction which make them difficult to handle on conventional processing
machinery. However, because of their somewhat rubbery nature, gloss,
permeability, and good impact strength they are of interest as a stretch film
for meat packaging and for cling-wrap purposes. Some EVA is used in
coextrusion processes for the manufacture of laminated film.
Ethylene-ethyl acrylate copolymers are very similar to the
ethylene-vinyl acetate copolymers. The former materials are considered to
have higher abrasion resistance and heat resistance whilst the EVA have
been considered to be tougher and of greater clarity.
For many years use of this material was largely confined to
America and it was seldom met in Europe because of the cheaper EVA
materials available. In 1980, however, BP initiated production of such
materials, whilst in the United States the material is produced by Union
Carbide. The Dow company, whose product Zetafin was the most wellknown grade, no longer supply the copolymer.
Ethylene-acrylic acid copolymers have been known since the 1950s
but for many years found little application. About 1974 Dow introduced
new grades characterized by outstanding adhesion to a variety of metallic
and non-metallic substrates, outstanding toughness and with good rigidity
and tensile strength. Many of the key features are a consequence of
hydrogen bonding via the carboxyl groups causing an effect referred to by
Dow as pseudo-crystallinity.
Current usage is almost entirely associated with the good adhesion
59
to aluminium. Specific applications include the bonding of aluminium foil
to plastics films, as the adhesive layer between aluminium foil and
polyethylene in multilayer extrusion-laminated non-lead toothpaste tubes
and in coated aluminium foil pouches. Grades have more recently become
available for manufacture by blown film processes designed for use in skin
packaging applications. Such materials are said to comply with FDA
regulations.
A terpolymer rubber was introduced by Du Pont in 1975 (Vamac).
This is based on ethylene, methyl acrylate and a third, undisclosed,
monomer containing carboxylic acid groups to act as the cure site.
In September 1964 the Du Pont company announced materials that
had characteristics of both thermoplastics and thermosetting materials.
These materials, known as ionomers, are prepared by copolymerizing
ethylene with a small amount (1-10 % in the basic patent) of an unsaturated
carboxylic acid such as acrylic acid using the hi gh-pressure process. Such
copolymers are then treated with the derivative of a metal such as sodium
methoxide or magnesium acetate with the result that the carboxylic group
appears to ionise. It would seem that this leads to some form of ionic crosslink which is stable at normal ambient temperatures but which reversibly
breaks down on heating. In this way it is possible to obtain materials which
possess the advantages of cross-linking at ambient temperatures, for
example enhanced toughness and stiffness, but which behave as linear
polymers at elevated temperatures and may be processed and even
reprocessed without undue difficulty. In the case of the commercial
materials already available (e.g. Surlyn-Du Pont) copolymerization has had
the not unexpected effect of depressing crystallinity although not
completely eliminating it, so that the materials are also transparent. Other
properties claimed for the ionomers are excellent oil and grease resistance,
excellent resistance to stress cracking and a higher moisture vapour
permeability (due to the lower crystallinity) than polyethylene.
The commercial grades available in the 1970s used either zinc or
sodium as the cross-linki ng ion and ranged in melt flow index from 0.4 to
14. The main application of the ionomer resins has been for packaging film.
The polymer is particularly useful in composite structures to provide an
outer layer with good heat sealability. The puncture resistance of film based
on ionomer film has the puncture resistance of a LDPE film of twice the
gauge.
Ionomer resins today have a large portion of the golf ball cover
market. They are considered superior to synthetic trans-polyisoprene in
60
being virtually cut-proof in normal use and they also retain a greater
resiliency over a wider temperature range.
Other uses of ionomer resins are in footwear. Low-cost grades have
been used for parts of shoe heels whilst grades of increased flexibility are
among a wide range of polymers contesting the market for ski boots.
It is to be noted that polymers with ionic groups attached along the
chain and showing the properties of both polymers and electrolytes have
been known for some time. Known as polyelectrolytes, these materials
show ionic dissociation in water and find use for a variety of purposes such
as thickening agents. Examples are sodium polyacrylate, ammonium
polymethacrylate (both anionic polyelectrolytes) and poly-(N-butyl-4vinyl-pyridinium bromide), a cationic poly-electrolyte. Also somewhat
related are the ion-exchange resins, cross-linked polymers containing ionic
groups which may be reversibly exchanged and which are used in water
softening, in chromatography and for various industrial purposes. In
general, however, the polyelectrolytes and ion-exchange resins are
intractable materials and not processable on conventional plastics
machinery. The value of the ionomer is that the amount of ionic bonding
has been limited and so yields useful and tractable plastics materials. It is
also now possible to envisage a range of rubbers which vulcanize by ionic
cross-linking simply as they cool on emergence from an extruder or in the
mould of an injection moulding machine.
2.2 Polymers of Unsaturated Aromatic Hydrocarbons
2.2.1 Polystyrene
It may well be argued that the history of polystyrene is more
closely bound up with the history of the 20th century than is the case with
any other plastics material.
In 1930 BASF, then part of IG Farben, installed a plant for
producing 100 tonnes of polystyrene per annum and in 1933 the first
injection moulded articles were produced. In the US semi-plant-scale work
at the Dow Chemical Company showed promise of commercial success in
1934. As a consequence there became available shortly befor e World War
II a material of particular interest because of its good electrical insulation
characteristics but otherwise considerably inferior to the polystyrene
available today. Because of these excellent electrical characteristics prices
were paid of the order of several dollars per pound for these polymers.