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Poly(viny1 acetate) and its Derivatives
Poly(viny1 acetate) 387
- 118 kJ/mole
CH, . COO
OOC . CH,
CH, . COO
In a typical system the reaction vessel is at 75-80°C and the vinyl acetate
formed is swept out into a condenser at 72-74°C by means of circulating excess
acetylene. This prevents distillation of higher boiling components but allows the
vinyl acetate and acetylene through. The former is separated out by cooling and
the acetylene recycled.
Vapour phase synthesis may be carried out by passing a mixture of acetylene
and acetic acid through a reaction tube at 210-215°C. Typical catalysts for this
reaction are cadmium acetate, zinc acetate and zinc silicate. The monomer in
each of the above mentioned processes is purified by distillation.
Purified monomer is usually inhibited before shipment by such materials as
copper resinate, diphenylamine or hydroquinone, which are generally removed
before polymerisation. The monomer is a sweet-smelling liquid partially miscible
with water and with the following properties: boiling point at 760mmHg,
72.5"C; specific gravity at 20"C, 0.934; refractive index nD20, 1.395; vapour
pressure at 20°C 90mmHg.
In 1953 the Celanese Corporation of America introduced a route for the
production of vinyl acetate from light petroleum gases. This involved the
oxidation of butane which yields such products as acetic acid and acetone. Two
derivatives of these products are acetic anhydride and acetaldehyde, which then
react together to give ethylidene diacetate (Figure 14.2.)
OOC . CH,
Exposure of the ethylidene diacetate to an aromatic sulphonic acid in the
presence of five times its weight of acetic anhydride as diluent at 136°C will
yield the following mixture: 40% vinyl acetate; 28% acetic acid; 20% acetic
anhydride; 4% ethylidene diacetate; 8% acetaldehyde.
The latter four products may all be reused after separation.
In recent years vinyl acetate has been prepared in large quantities by oxidation
of ethylene. If ethylene is passed into a solution of palladium chloride in acetic
acid containing sodium acetate, then vinyl acetate, ethylene diacetate and
acetaldehyde are produced, the vinyl acetate being obtained in good yields by the
reaction shown in Figure 14.3
388 Poly(viny1 acetate) and its Derivatives
OOC . CH,
The ethylene oxidation process can be carried out in either a liquid or a vapour
phase but the latter method is often preferred because it avoids corrosion
problems and the use of solvents.
A one-stage process for producing vinyl acetate directly from ethylene has also
been disclosed. In this process ethylene is passed through a substantially
anhydrous suspension or solution of acetic acid containing cupric chloride and
copper or sodium acetate together with a palladium catalyst to yield vinyl
Vinyl acetate may be easily polymerised in bulk, solution, emulsion and
suspension. At conversions above 30%, chain transfer to polymer or monomer
may occur. In the case of both polymer and monomer transfer two mechanisms
are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the
---CH, -CH -
OOC . CH,
OOC . CH,Radical
The radical formed at either the tertiary carbon atom or at the acetate group
will then initiate polymerisation and form branched structures.
Since poly(viny1 acetate) is usually used in an emulsion form, the emulsion
polymerisation process is commonly used. In a typical system, approximately
equal quantities of vinyl acetate and water are stirred together in the presence of
a suitable colloid-emulsifier system, such as poly(viny1 alcohol) and sodium
lauryl sulphate, and a water-soluble initiator such as potassium persulphate.
Polymerisation takes place over a period of about 4 hours at 70°C. The
reaction is exothermic and provision must be made for cooling when the batch
size exceeds a few litres. In order to achieve better control of the process and to
Poly(viny1 alcohol) 389
obtain particles with a smaller particle size, part of the monomer is first
polymerised and the rest, with some of the initiator, is then steadily added over
a period of 3-4 hours. To minimise the hydrolysis of vinyl acetate or possible
comonomers during polymerisation, it is necessary to control the pH throughout
reaction. For this purpose a buffer such as sodium acetate is commonly
14.2.3 Properties and Uses
Poly(viny1 acetate) is too soft and shows excessive 'cold flow' for use in
moulded plastics. This is no doubt associated with the fact that the glass
transition temperature of 28°C is little above the usual ambient temperatures and
in fact in many places at various times the glass temperature may be the lower.
It has a density of 1.19g/cm3 and a refractive index of 1.47. Commercial
polymers are atactic and, since they do not crystallise, transparent (if free from
emulsifier). They are successfully used in emulsion paints, as adhesives for
textiles, paper and wood, as a sizing material and as a 'permanent starch'. A
number of grades are supplied by manufacturers which differ in molecular
weight and in the nature of comonomers (e.g. vinyl maleate) which are
commonly used (see Section 14.4.4)
The polymers are usually supplied as emulsions which also differ in the
particle size, the sign of the charge on the particle, the pH of the aqueous phase
and in other details.
Being an amorphous polymer with a solubility parameter of 19.4 MPa'12, it
dissolves in solvents with similar solubility parameters (e.g. benzene 6 =
18.8MPa1", chloroform 6 = 19.0MPa'/2, and acetone 6 = 20.4MPa'12.
14.3 POLY(V1NYL 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(viny1 alcohol) is thus prepared by alcoholysis of a poly(viny1 ester) and in
practice poly(viny1 acetate) is used (Figure 14.5).
The term hydrolysis is sometimes incorrectly used to describe this process. In
fact water does not react readily to yield poly(viny1 alcoho1)s 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(viny1 acetate) at room
390 Poly(viny1 acetate) and its Derivutives
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(viny1 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(viny1 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(viny1 alcohol) polymer will have a lower molecular weight than the
poly(viny1 acetate) from which it is made.
14.3.1 Structure and Properties
Poly(viny1 acetate) is an atactic material and is amorphous. Whilst the structure
of poly(viny1 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
range of 8 7 4 9 % are readily soluble in cold water. An increase in the degree of
‘hydrolysis’ will result in a reduction in the ease of solubility and fully
‘hydrolysed’ polymers are only dissolved by heating to temperatures above
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, unplasticised poly(viny1 alcohol) 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 claimed2 to have tensile strengths as high as 180001bf/in2
( I 25 MPa).
The properties will be greatly dependent on humidity; the higher the humidity,
the more the water absorbed. Since water acts as a plasticiser there will be a
reduction in tensile strength but an increase in elongation and tear strength.
Figure 14.6 shows the relationship between tensile strength, percentage
‘hydrolysis’ and humidity.
Because of its high polarity, poly(viny1 alcohol) is very resistant to
hydrocarbons such as petrol. Although the polymer will dissolve in lower alcoholwater mixtures, it does not dissolve in pure alcohols. As it is crystalline as well as
The Poly(viny1 acetals) 39 1
DEGREE OF HYDROLYSIS IN
Figure 14.6. Relation between tensile strength and degree of ‘hydrolysis’ for unplasticised poly(viny1
alcohol) film. (After Davidson and Sittig’)
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 plasticised by polar liquids capable of forming hydrogen
bonds with the hydroxyl groups. Glycerin has been used for this purpose.
Poly(viny1 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(viny1 alcohol) will function as a non-ionic surface active agent and is
used in suspension polymerisation 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.
Japanese workers have developed fibres from poly(viny1 alcohol). The
polymer is wet spun from warm water into a concentrated aqueous solution of
sodium sulphate containing sulphuric acid and formaldehyde, the latter
insolubilising the alcohol by formation of formal groups.
14.4 THE POLY(V1NYL ACETALS)
Treatment of poly(viny1 alcohol) with aldehydes and ketones leads to the
formation of poly(viny1 acetals) and poly(viny1 ketals), of which only the former
products are of any commercial significance (Figure 14.7).
392 Poly(viny1 acetate) and its Derivatives
A Poly(Viny1 Acetal)
The products are amorphous resins whose rigidity and softening point depend
on the aldehyde used. Poly(viny1 butyral), with the larger side chain, is softer
than poly(viny1 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(viny1 acetal) molecule will thus contain:
(1) Acetal groups.
(2) Residual hydroxyl groups.
(3) Residual acetate groups, due to incomplete 'hydrolysis' of poly(viny1
acetate) to poly(viny1 alcohol).
14.4.1 Poly(viny1 formal)
The poly(viny1 acetals) may be made either from poly(viny1 alcohol) or directly
from poly(viny1 acetate) without separating the alcohol. In the case of poly(viny1
formal) the direct process is normally used.
In a typical process, 100 parts of poly(viny1 acetate) are added to a 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(viny1 formal) are commercially available
(Formvar, Mowital) which vary in degree of polymerisation, hydroxyl content
and residual acetate content.
Table 14.13 shows the influence of these variables on some properties. The
residual hydroxyl content is expressed in terms of poly(viny1 alcohol) content
and residual acetate in terms of poly(viny1 acetate) content.
The Poly(viny1 acetals) 393
Table 14.1 Influence of structure variables on the properties of poly(viny1 formal)
Various grades of poly(vinyl formal)
Av. D. of P.
Poly(viny1 alcohol) (%)
Poly(viny1 acetate) (%)
Flow temperature (“C)
Tensile strength (lo-’
Impact strength (hod f in
X f in) (ft lbf in -’)
Water absorption (%)
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 plasticisers 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’”.
The main application of poly(viny1 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(viny1 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.
14.4.2 Poly(viny1 acetal)
Poly(viny1acetal) 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.
14.4.3 Poly(viny1 butyral)
As a safety glass interleaver, poly(viny1 butyral) (Butacite, Saflex) is extensively
used because of its high adhesion to glass, toughness, light stability, clarity and
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
394 Poly(viny1 acetate) and its Derivatives
poly(viny1 alcohol) rather than by the direct process from poly(viny1
In a typical process 140 parts of fully ‘hydrolysed’ poly(viny1 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
The solution of poly(viny1 butyral) is diluted with methanol and the polymer
precipitated by the addition of water during vigorous agitation. The polymer
is then stabilised, washed and dried.
Highly ‘hydrolysed’ poly(viny1 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 plasticised with an ester such as
dibutyl sebacate or triethylene glycol di-2-ethyl butyrate, about 30 parts of
plasticiser 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(viny1 butyral) film.
Laminated safety glass has now become standard for automobile windscreens and is used for aircraft glazing.
14.5 ETHYLENE-VINYL ALCOHOL COPOLYMERS
If ethylene is copolymerised with vinyl acetate, and the vinyl acetate
component ‘hydrolysed’ to vinyl alcohol, a material is produced which is in
effect a copolymer of ethylene and vinyl alcohol.
The material is produced by Kurardy and Nippon Gohsei in Japan and was
also produced up until 1993 by Du Pont. Global nameplate capacity has
increased from about 30 000 t.p.a. early in the 1990s to 60 000 t.p.a. at the end
of the millenium. The material is commonly referred to in the abbreviated form
EVOH but occasionally also as EVAL and EVOL.
Certain copolymers of this type have been found to have excellent gas
barrier properties, with the dry polymer having an oxygen permeability only
about 1/ I 0th that of polyvinylidene chloride. Unsurprisingly, the copolymer has
a high moisture absorption and a high moisture vapour transmission rate.
Where the material is swollen by water, gas permeability is also higher.
For reasons explained below, the effect of increasing the ‘vinyl alcohol’
content in EVOH is quite different to that of increasing the vinyl acetate
content in EVA. In the case of ethylene-vinyl acetate (EVA) copolymers,
increasing the vinyl acetate content up to about 50% makes the materials less
crystalline and progressively more flexible and then rubbery. In the range
40-70% vinyl acetate content the materials are amorphous and rubbery, whilst
above 70% the copolymers become increasingly rigid and brittle.
Commerical grades of EVOH typically have ‘vinyl alcohol’ contents in the
range 56-71%, but in contrast to the corresponding EVA materials these
copolymers are crystalline. Furthermore, an increase in the ‘vinyl alcohol’
content results in an increase in such properties as crystalline melting point,
tensile strength and tensile modulus together with a decrease in oxygen
permeability. This is a reflection of the fact that the ethylene and vinyl alcohol
units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1).
Poly(viny1 cinnamate) 395
Table 14.2 Typical properties of EVOH copolymers (For purposes of comparison the grades
selected all have a MFI (2.16kg, 190OC) of 1.7-1.8. Grades with other MFI values are also
r, (by D W ("C)
Tg (by DSC) ("C)
Tensile strength (MPa)
Elongation at break (%)
Tensile modulus (MPa)
Oxygen permeability cc.20 p/m2 24 h atm.
0% RH, 20°C
25% RH, 25°C
Ethylene content (mole %)
Some typical properties of some commercial EVOH polymers (SoamolNippon Gohsei) are given in Table 14.2.
As is to be expected, the table shows that as the humidity is increased,
causing swelling and an increase in the interchain separation, so the oxygen
permeability increases. Also, as expected, the percentage increase is greater the
higher the vinyl alcohol content.
Because of the excellent gas barrier properties, EVOH is of interest as a
packaging material. However, because of its high water absorption it is usually
used as an internal layer in a co-extruded film, sheet, bottle or tube. For
example, the system HDPE-EVOH-EVA may be used as a barrier film for
packaging cereals, and the system polystyrene-EVOH-polystyrene for packaging coffee and cream, whilst the system polystyrene-EVOH-polyethylene has
the additional advantage of heat sealability.
In the case of EVOH being used as an interlayer with polyethylene or
polystyrene, it is necessary to use additional adhesive layers such as
an ethylene-vinyl acetate-maleic anhydride terpolymer (e.g. OrevacAtochem).
While EVOH is of interest primarily for food packaging applications attention
is now being turned to non-food outlets such as automotive fuel tanks, floor
heating pipes and toothpaste tubes.
14.6 POLY(V1NYL CINNAMATE)
Poly(viny1 cinnamate) is not used in the traditional areas of plastics technology
but its ability to cross-link on exposure to light has led to important applications
in photography, lithography and related fields as a photoresist.
The concept of a Photoresist is of great antiquity and has a number of features
of interest relating to plastics. In Ancient Egypt mummies were wrappted in linen
cloths dipped in a solution of oil of lavender containing high molecular mass
bituminous material (Chapter 30) which was known variously as Syrian Asphalt
or Bitumen of Judea. On exposure to light the product hardened and became
insoluble. The evidence is that some form of cross-linking occurred.
396 Poly(viny1 acetate) and its Derivatives
At the beginning of the nineteenth century, an amateur Egyptologist, J.
Nictphore Niepce, became interested in the process and in 1822 he adapted it to
produce the first permanent photograph. It also played an important role in the
development of lithography. In essence surfaces exposed to light become
insoluble and cannot be removed by solvents whilst unexposed surfaces remain
soluble and can be so removed. This is the concept of a negative photoresist.
(There also exist positive photoresists, including some phenolic resins, which
become more soluble on exposure to light). Today photoresists are used in the
fabrication of solid-state electronic components and integrated circuits and
poly(vin1y cinnamate) is one of the longest established materials of this type.
As with poly(viny1 alcohol), poly(viny1 cinnamate) is prepared by chemical
modification of another polymer rather than from ‘monomer’. One process is to
treat poly(viny1 alcohol) with cinnamoyl chloride and pyridine but this is rather
slow. Use of the Schotten Baumann reaction will, however, allow esterification
to proceed at a reasonable rate. In one example4 poly(viny1 alcohol) of degree of
polymerisation 1400 and degree of saponification of 95% was dissolved in water.
To this was added a concentrated potassium hydroxide solution and then
cinnamoyl chloride in methyl ethyl ketone. The product was, in effect a vinyl
alcohol-vinyl cinnamate copolymer Figure 14.8)
To make a photoresist poly(viny1 cinnamate), or a high vinyl cinnamate
copolymer, is dissolved in a solvent such as methylene dichloride and the
solution is coated uniformly over the substrate by a process such as spin casting.
After evaporation of the solvent a masking material (which in the case of a
simple demonstration could be a paper clip) is placed on the resist and the
assembly is exposed to ultraviolet light. The exposed surfaces are then
insolubilised. After exposure the mask is removed and soluble matter dissolved
in a solvent such as cellosolve acetate and this exposes the substrate in the shape
of the mask. This may then be etched or otherwise treated as required. By the use
of appropriate sensitisers such as, 1,2-benzanthraquinone or Michler ’s ketone the
cross-linking may be brought about by visible light. The cross-linking is believed
to involve the production of a four-membered cyclobutane ring (Figure 14.9).
14.7 OTHER ORGANIC VINYL ESTER POLYMERS
Polymers from many other vinyl esters, such as vinyl propionate, vinyl caproate,
vinyl benzoate, vinyl stearate and vinyl laurate, have been prepared on a
commercial scale. As is to be expected, increasing the length of the side chain
reduces the softening point of the polymer so that polymers similar in many ways
to the higher acrylates and methacrylates may be obtained. It is also of interest
to note that, as with acrylates and methacrylates, the glass transitions of the
polymers go through a minimum with about twelve carbon atoms in the side
chain, side-chain crystallisation becoming important with higher homologues.
Of the higher vinyl ester homopolymers only poly(viny1 propionate) is
currently believed to be of commercial value, being marketed as Propiofan
(BASF) for surface coating application where greater alkali resistance is possible
than with the normal vinyl acetate based copolymers.
Whilst vinyl acetate is reluctant to copolymerise it is in fact usually used today
in copolymers. Two of particular interest to the plastics industry are ethylenevinyl acetate (Chapter 11) and vinyl chloride-vinyl acetate copolymers (Chapter
12). In surface coatings internal plasticisation to bring the Tg to below ambient
temperatures and thus facilitate film forming is achieved by the use of ethyl
acrylate, butyl acrylate, 2-ethylhexyl acrylate and dialkyl maleates and
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