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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
211
Several classes of ABS which show the above general
characteristics but with specific attributes are recognized. One supplier for
example classifies ABS materials into the following categories:
• general purpose grades;
• fire retardant grades;
• improved heat resistance grades;
• enhanced chemically resistant grades;
• static dissipation grades;
• extrusion grades;
• fire retardant extrusion grades;
• transparent grades electroplating grades blow moulding grades.
Over the years there has been some difference in the balance of use
between UPVC and ABS in the United States compared with Western
Europe. This was due largely to the earlier development in Western Europe
of UPVC and in the United States of ABS. Thus, for example, whilst ABS
consolidated its use for pipes and fittings in the United States, UPVC was
finding similar uses in Europe. Whilst some of these traditional differences
remain, ABS is now well established in both Europe and the United States.
As well as unplasticized PVC, ABS also finds competition from
polypropylene. In recent years polypropylene has been the cheaper material
on a tonnage basis and even more economic on the more relevant volume
basis. On the other hand the properties listed above, in particular the
extreme toughness and superior heat distortion resistance, lead to ABS
being preferred in many instances. Because ABS, typically, has a higher
flexural modulus than polypropylene, mouldings of the latter will have to a
wall thickness some 15-25% greater in order to show an equal stiffness. It
is also interesting to note that because of its higher specific heat as well as
possessing a latent heat of fusion, polypropylene requires longer cooling
times when processing.
Because of their toughness and good appearance ABS polymers
have become regarded as a de luxe form of polystyrene, their biggest
drawbacks being their limited weathering resistance and relatively high
cost. It is one of the few major polymers where there is different pattern of
use in North America compared with Europe.
In Western Europe the largest user is the vehicle construction
industry where ABS has been used for fascia panels, door covers, door
handles, radiator grilles, ventilation system components, heater housings,
seat belt fastenings, console panels, loudspeaker housings, interior trim and
212
other uses. For some years there was extensive use of electroplated ABS.
CH
2
CH
OH
CH3CHO
CH2CH
OCCCH
3
+ CH3OH
CH
2
CH
OH
+ CH3COOCH
3
Whilst this continues to be used for nameplates, reflectors and other parts
where a bright reflecting surface is a requirement, it has tended to fall out
of favour simply for decoration.
The use of ABS has in recent years met considerable competition
on two fronts, particularly in automotive applications. For lower cost
applications, where demands of finish and heat resistance are not too
severe, blends of polypropylene and ethylene-propylene rubbers have
found application. On the other hand, where enhanced heat resistance and
surface hardness are required in conjunction with excellent impact
properties, polycarbonate-ABS alloys have found many applications. These
materials have also replaced ABS in a number of electrical fittings and
housings for business and domestic applications. Where improved heat
distortion temperature and good electrical insulation properties (including
tracking resistance) are important, then ABS may be replaced by
poly(butylene terephthalate).
In the US the largest single application area is for pipes and fittings
whereas in Western Europe the corresponding market is largely dominated
by unplasticized PVC. This is largely a reflection of the earlier
development of methods of handling unplasticized PVC in Europe than
was generally the case in the USA.
Other important application areas in both regions are household
appliances, consumer electronic equipment, refrigerator sheeting, toys,
telephones, office equipment, recreational equipment, luggage and as a
modifier for PVC.
2.3 Polymeric Alchohols and Their Derivatives
2.3.1 Poly(vinyl alcohol)
Vinyl alcohol does not exist in the free state and all attempts to
prepare it have led instead to the production of its tautomer, acetaldehyde.
Poly(vinyl alcohol) is thus prepared by alcoholysis of a po ly(vinyl
ester) and in practice poly(vinyl acetate) is used:
The term hydrolysis is sometimes incorrectly used to describe this
213
process. In fact water does not react readily to yield poly(vinyl alcohol)
and may actually retard reaction where certain catalysts are used.
Either methanol or ethanol may be used to effect alcoholysis but
the former is often preferred because of its miscibility with poly(vinyl
acetate) at room temperature and its ability to give products of better
colour. Where methanol is employed, methyl acetate may be incorporated
as a second solvent. It is also formed during reaction. The concentration of
poly(vinyl acetate) in the alcohol is usually between 10 and 20%.
Either acid or base catalysis may be employed. Alkaline catalysts
such as caustic soda or sodium methoxide give more rapid alcoholysis.
With alkaline catalysts, increasing catalyst concentration, usually less than
1% in the case of sodium methoxide, will result in decreasing residual
acetate content and this phenomenon is used as a method of controlling the
degree of alcoholysis. Variations in reaction time provide only a secondary
means of controlling the reaction. At 60°C the reaction may takes less than
an hour but at 20°C complete “hydrolysis” may take up to 8 hours.
The use of acid catalysts such as dry hydrochloric acid has been
described in the literature but are less suitable when incompletely
“hydrolysed” products are desired as it is difficult to obtain reproducible
results.
Commercial poly(vinyl alcohol) (e.g. Gelvatol, Elvanol, Mowiol
and Rhodoviol) is available in a number of grades which differ in
molecular weight and in the residual acetate content. Because alcoholysis
will cause scission of branched polymers at the points where branching has
proceeded via the acetate group, poly(vinyl alcohol) polymer will have a
lower molecular weight than the poly(vinyl acetate) from which it is made.
Structure and properties
Poly(vinyl acetate) is an atactic material and is amorphous. Whilst
the structure of poly(vinyl alcohol) is also atactic the polymer exhibits
crystallinity and has essentially the same crystal lattice as polyethylene.
This is because the hydroxyl groups are small enough to fit into the lattice
without disrupting it.
The presence of hydroxyl groups attached to the main chain has a
number of significant effects. The first effect is that the polymer is
hydrophilic and will dissolve in water to a greater or lesser extent
according to the degree of “hydrolysis” and the temperature. Polymers with
a degree of “hydrolysis” in the ra nge of 87-89% are readily soluble in cold
water. An increase in the degree of “hydrolysis” will result in a reduction in
214
the ease of solubility and fully “hydrolysed” polymer s are only di ssolved
by heating to temperatures above 85°C.
This anomalous effect is due to the greater extent of hydrogen
bonding in the completely “hydrolysed” polymers. Hydrogen bonding also
leads to a number of other effects, for example, unplasticized poly(vinyl
alcoho l) decomposes below its flow temperature. The polymer also has a
very high tensile strength and is very tough. Films cast from high
molecular weight grades, conditioned to 35% humidity, are claimed to have
tensile strengths as high as 125MPa.
The properties will be greatly dependent on humidity; the higher
the humidity, the more the water absorbed. Since water acts as a plasticizer
there will be a reduction in tensile strength but an increase in elongation
and tear strength.
Because of its high polarity, poly(vinyl alcohol) is very resistant to
hydrocarbons such as petrol. Although the polymer will dissolve in lower
alcohol-water mixtures, it does not dissolve in pure alcohols. As it is
crystalline as well as highly polar only a few organic solvents, such as
diethylenetriamine and triethylenetetramine, are effective at room
temperature. As might be expected, the hydroxyl group is very reactive and
many derivatives have been prepared.
The polymer may be plasticized by polar liquids capable of
forming hydrogen bonds with the hydroxyl groups. Glycerin has been used
for this purpose.
Applications
Poly(vinyl alcohol) is employed for a variety of purposes. Film cast
from aqueous alcohol solution is an important release agent in the
manufacture of reinforced plastics. Incompletely “hydrolysed” grades have
been developed for water-soluble packages for bath salts, bleaches,
insecticides and disinfectants. Techniques for making tubular blown film,
similar to that used with polyethylene, have been developed for this
purpose. Moulded and extruded products which combine oil resistance with
toughness and flexibility are produced in the United States but have never
become popular in Europe.
Poly(vinyl alcohol) will function as a non-ionic surface active
agent and is used in suspension polymerization as a protective colloid. In
many applications it serves as a binder and thickener is addition to an
emulsifying agent. The polymer is also employed in adhesives, binders,
paper sizing, paper coatings, textile sizing, ceramics, cosmetics and as a
steel quenchant.
215
Japanese workers have developed fibres from poly(vinyl alcohol).
The polymer is wet spun from warm water into a concentrated aqueous
solution of sodium sulphate containing sulphuric acid and formaldehyde,
the latter inso lubi li s ing the alcohol by formation of formal groups.
2.3.2 Poly(vinyl acetals)
Treatment of poly(vinyl alcohol) with aldehydes and ketones leads
to the formation of poly(vinyl acetals) and poly(vinyl ketals), of which only
the former products are of any commercial significance:
CH
2
CH CH
OH
CH
2
OH
CHCH
2
OH
CHCH
CHCH
2
2
OH
OH
CHCH
2
OH
O
C
H
R
CH2CH
O
R H
O
C
CH
R
2
O
C
H
CH
CH
O
R H
CH
2
C
CH2CHOHCH2CHCH2CH
O
The products are amorphous resins whose rigidity and softening
point depend on the aldehyde used. Poly(vinyl butyral), with the larger side
chain, is softer than poly(vinyl formal). Since the reaction between the
aldehyde and the hydroxyl groups occurs at random, some hydroxyl groups
become isolated and are incapable of reaction. A poly(vinyl acetal)
molecule will thus contain:
(1) Acetal groups.
(2) Residual hydroxyl groups.
(3) Residual acetate groups, due to incomplete “hydrolysis” of
poly(vinyl acetate) to poly(vinyl alcohol).
Poly(vinyl formal)
The poly(vinyl acetals) may be made either from poly(vinyl
alcohol) or directly from poly(vinyl acetate) without separating the alcohol.
In the case of poly(vinyl formal) the direct process is normally used.
In a typical process, 100 parts of poly(vinyl acetate) are added to a
216
mixture of 200 parts acetic acid and 70 parts water, which has been
warmed to about 70°C, and stirred to complete solution. Sixty parts of 40%
formalin and 4 parts sulphuric acid (catalyst) are added and reaction is
carried out for 24 hours at 70°C. Water is added to the mixture with rapid
agitation to precipitate the granules, which are then washed free from acid
and dried.
A number of grades of poly(vinyl formal) are commercially
available (Formvar, Mowital) which vary in degree of polymerization,
hydroxyl content and residual acetate content.
It will be observed that molecular weight has little effect on
mechanical properties but does influence the flow temperature.
The hydroxyl content of commercial material is kept low but it is to
be observed that this has an effect on the water absorption. Variation in the
residual acetate content has a significant effect on heat distortion
temperature, impact strength and water absorption. The incorporation of
plasticizers has the usual influence on mechanical and thermal properties.
The polymer, being amorphous, is soluble in solvents of similar
solubility parameter, grades with low residual acetate being dissolved in
solvents of solubility parameter between 19.8 and 22 MPa
1/2
.
The main application of poly(vinyl formal) is as a wire enamel in
conjunction with a phenolic resin. For this purpose, polymers with low
hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are
used in structural adhesive (e.g. Redux) which are also used in conjunction
with phenolic resin. Poly(vinyl formal) finds some use as a can coating and
with wash primers. Injection mouldings have no commercial significance
since they have no features justifying their use at current commercial
prices.
Poly(vinyl acetal)
Poly(vinyl acetal) itself is now of little commercial importance.
The material may be injection moulded but has no particular properties
which merit its use. It is occasionally used in conjunction with
nitrocellulose in lacquers, as a vehicle for wash primers and as a stiffener
for fabrics.
Poly(vinyl butyral)
As a safety glass interleaver, poly(vinyl butyral) (Butacite, Saflex)
is extensively used because of its high adhesion to glass, toughness, light
stability, clarity and moisture insensitivity.
217
It also finds miscellaneous applications in textile and metal
coatings and in adhesive formulations. Where it is to be used as a safety
glass interleaver, a very pure product is required and this is most
conveniently prepared from poly(vinyl alcohol) rather than by the direct
process from poly(vinyl acetate).
In a typical process 140 parts of fully “hydrolysed” poly(vinyl
alcohol) are suspended in 800 parts of ethanol; 80 parts of butyraldehyde
and 8 parts of sulphuric acid are added and the reaction is carried out at
about 80°C for 5-6 hours.
The solution of poly(vinyl butyral) is diluted with methanol and the
polymer precipitated by the addition of water during vigorous agitation.
The polymer is then stabilized, washed and dried.
Highly “hydrolysed” poly(vinyl alcohol) is normally used as a
starting point. For safety glass applications about 25% of the hydroxyl
groups are left unreacted. In this application the polymer is plasticized with
an ester such as dibutyl sebacate or triethylene glycol di-2-ethylbutyrate,
about 30 parts of plasticizer being used per 100 parts of polymer. The
compound is then calendered to a thickness of 0.015 in and coated with a
layer of sodium bicarbonate to prevent blocking. To produce safety glass
the film is washed and dried and then placed between two pieces of glass
which are then subjected to mild heat and pressure. Bulletproof glass is
made by laminating together several layers of glass and poly(vinyl butyral)
film.
Laminated safety glass has now become standard for automobile
windscreens and is used for aircraft glazing.
2.4 Cellulose Plastics
Nature and occurrence of cellulo se
Cellulose is the most abundant of naturally occurring organic
compounds for, as the chief constituent of the cell walls of higher plants, it
comprises at least one-third of the vegetable matter of the world. The
cellulose content of such vegetable matter varies from plant to plant. For
example, oven-dried cotton contains about 90% cellulose, while an average
wood has about 50%. The balance is composed of lignin, polysaccharides
other than cellulose and minor amounts of resins, proteins and mineral
matter. In spite of its wide distribution in nature, cellulose for chemical
purposes is derived commerically from only two sources, cotton linters and
wood pulp.
218
Cotton linters are the short fibres removed from cotton seeds after
the long fibres for use in textiles have been taken off by the process of
ginning. Digestion under pressure at temperatures in the range 130-180°C
with a 2-5% aqueous solution of sodium hydroxide will remove the bulk of
the impurities and after a bleaching operation to remove coloured bodies
the residual cotton contains about 99% alpha-cellulose, the term given to
pure cellulose of high molecular weight. The viscosity average molecular
weight of native cellulose is in excess of 500000 but the purification stage
is accompanied by some degradation so that the resultant material usually
has a molecular weight in the range 100000-500000 (600-3000 repeating
glucose units).
Alternatively cellulose is produced from wood via wood pulp. A
number of processes are used in which the overall effect is the removal of
the bulk of the non-cellulosic matter. The most widely used are the sulphite
process, which uses a solution of calcium bisulphite and sulphur dioxide,
the soda process using sodium hydroxide and the sulphate process using a
solution of sodium hydroxide and sodium sulphide. (The term sulphate
process is used since sodium sulphate is the source of the sulphide.) For
chemical purposes the sulphite process is most commonly used. As
normally prepared these pulps contain about 88-90% alpha-cellulose but
this may be increased by alkaline purification and bleaching.
Analysis of pure cellulose indicates an empirical formula C6H10O5
corresponding to a glucose anhydride. There is ample evidence to indicate
that in fact cellulose is a high molecular weight polyanhydroglucose. In
particular it may be mentioned that controlled hydrolysis of cellulose yields
cellobiose, cellotriaose and cellotetraose which contain respectively two,
three and four anhydroglucose units. Complete hydrolysis will give yields
of glucose as high as 95-96%.
The fact that, whereas glucose is a strongly reducing sugar,
cellulose is almost non-reducing indicates that the linkage between the
anhydroglucose units occurs at the reducing carbon atom. As cellobiose,
known to consist of two glucose units joined by β-linkage, rather than
maltose with the α-linkage, is one of the stepwise degradation products the
evidence is that cellulose molecules are made up of many anhydroglucose
units joined together by beta-glucosidic linkages:
219
C
C C
C
OC
OH
C
C O
C
CC
H
CH
2
OH
H
H
HO
H
H
OH
O
H
H
H
OH
CH
2
OH
OH
H
H
OH
C
C C
C
OC
OH
CH2OH
H
H
HO
H
H
OH
OH
H
H
OH
CH2OH
OH
H
H
O
C
C O
C
CC
H
H
Maltose
O
OH
OH
O
O
CH2OH
CH2OH
O
OH
OH
O
CH
2
OH
O
OH
OH
O
CH2OH
O
OH
OH
O
Study of the structure of cellulose (Figure 49) leads one to expect
that the molecules would be essentially extended and linear and capable of
existing in the crystalline state. This is confirmed by X-ray data which
indicate that the cell repeating unit (10.25 Å) corresponds to the cellobiose
repeating unit of the molecule.
Figure 49
Although it might be anticipated that, because of the abundance of
hydroxyl groups, cellulose would be water soluble this is not the case. This
is because the regular spacing of hydroxyl groups, particularly in the
crystalline zones, facilitates extensive hydrogen bonding. Thus although
cellulose is somewhat hygroscopic, intermolecular bonds are too great for
solution to occur.
Cellulose may be degraded by a number of environments. For
example, acid-catalysed hydrolytic degradation will eventually lead to
glucose by rupture of the 1,4-β-glucosidic linkages. Intermediate products
may also be obtained for which the general term hydrocellulose has been
given.
220
A wide variety of oxidation products, oxycelluloses, may also be
produced. Oxidation may occur at a number of points but does not
necessarily lead to chain scission.
Of somewhat greater technical interest are the addition compounds
and the cellulose esters and ethers. Of the apparent addition compounds the
most important is alkali cellulose produced by steeping cellulose in caustic
soda and considered to be of general form (С
6Н10O5)x
(NaOH)y rather than
a sodium alcoholate compound. Alkali cellulose is a particularly important
starting point in the manufacture of cellulose ethers. The ability of aqueous
cuprammonium hydroxide solutions to dissolve cellulose appears to be
dependent on addition compound formation.
Many cellulose derivatives have been prepared of which the esters
and ethers are important. In these materials the hydroxyl groups are
replaced by other substituent groups. The degree of substitution is the term
given to the average number of hydroxyl groups per anhydroglucose unit
that have been replaced.
Therefore a fully substituted derivative would have a degree of
substitution of 3.0 whilst a cellulosic material in which on average 1.8
hydroxyl groups per glucose unit had been replaced would have a degree
of substitution of 1.8. Commercial derivatives usually have a degree of
substitution of less than 3.0, the actual value chosen being determined by
the end-use.
The likelihood of any given hydroxyl group reacting will be
determined largely by its position in the molecule and the position of the
molecule in the fibrous structure. The reaction rate is largely determined
by the rate of diffusion of the reagent and this is much greater in
amorphous regions than in the crystalline areas. It is desirable in the
preparation of derivatives that uniform substitution should occur, or at least
that the hydroxyl groups in one molecule should have the same chance of
reaction as those in another molecule. If this is not the case molecules on
the surface of a cellulose fibre may well be fully substituted while
molecules disposed in the centre of the fibre will be completely unreacted.
When reaction is carried out homogeneously in solution this state
of affairs more or less exists and it is possible to achieve a statistically
random degree of substitution. (It is to be noted that the primary hydroxyl
groups will be more reactive than the secondary hydroxyl groups.)
The nitration of cellulose is unusual in that uniform reaction takes
place even though the fibrous structure is retained. This is explained by the
fact that nitration is an equilibrium reaction unaffected by fibre structure,