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231
compatibility with the polymer. This may be expected from solubility
parameter data. It is often used in conjunction with dimethyl phthalate and
has the added virtues of imparting flame resistance and improved water
resistance. It is more permanent than DMP. Triacetin is less important,
although it is compatible, it is also highly volatile and lowers the water
resistance of the compound. Today it is essential to prepare low-cost
compounds to allow cellulose acetate to compete with the synthetic
polymers, and plasticizers such as ethylphthalyl ethyl glycollate, which are
superior in some respects, are now rarely used.
Small amounts of stabilizer (1-5%) are normally added to improve
weather resistance. These materials are the usual ultraviolet light absorbers
such as phenyl salicylate and various benzoates. Triphenyl phosphate also
has a beneficial influence.
Other ingredients may include cheapening extenders such as castor
oil, colouring agents, lubricants and, rarely, fillers.
Compounding may be carried out by either a wet or a dry process.
In the wet process, now obsolescent, the ingredients are mixed as a viscous
solution in acetone in a dough mixer. The resulting dough is then rolled on
a hot two-roll mill to evaporate the bulk of the solvent. It is then necessary
to “season” the resulting hides until the solvent content is reduced to a
tolerably low level.
Dry processes which obviate solvent difficulties are now preferred
and are similar to those employed with the major thermoplastics. They
include the use of two-roll mills, internal mixers, extruders and extrusion
compounders.
Properties of cellulose acetate plastics
Cellulose acetate plastics have no really outstanding properties.
Their continued use for mouldings and extrusions depends on their
toughness and good appearance at a reasonable cost, although somewhat
above the prices ruling for the major vinyl plastics, PVC, polyethylene and
polystyrene. In common with most other plastic materials they are capable
of unlimited colour variations, including water-white transparency.
Processing is quite straightforward provided the granules are dry.
Compared with the major vinyls cellulose acetate plastics have a
high water absorption, poor electrical insulation characteristics, limited
aging resistance and limited heat resistance and are attacked or dissolved
by a wide variety of reagents.
A wide range of cellulose acetate compounds are commercially
232
available. The properties of these compounds depend on three major
Parts DMP per 100 parts polymer
37.8
30.0
22.6
Flow temperature, °C
130
140
152
factors:
(1) The chain length of the cellulose molecule.
(2) The degree of acetylation.
(3) The type and amount of plasticizer(s).
During production of cellulose acetate from cellulose a certain
amount of chain degradation takes place. As a result the degree of
polymerization of commercial acetate esters is usually within the range
175-360. It is convenient to assess the chain length by solution viscosity
methods. Products differing in viscosity will be produced by varying the
source of the original cellulose and by modifying reaction conditions. Since
marked batch-to-batch variations in the viscosity of the finished product
occur in practice, products of specified viscosity are obtained by blending.
The greater the molecular weight the higher is the flow temperature
and the heat distortion temperature. Variations in molecular weight, in the
normal range, however, have less effect than do variations in the degree of
acetylation and in the plasticizer used.
Increasing the degree of acetylation from that corresponding to a
diacetate will obviously reduce the hydroxyl content and this will increase
the water resistance. The polymer also becomes less polar and the solvent
properties correspondingly alter. Increase in the degree of acetylation
reduces the hardness, impact strength and water absorption but increases
the “flow temperature”.
Cellulose acetate plastics are generally produced using polymers
from a fairly narrow range of molecular weights and degrees of acetylation.
In practice the greatest variation in properties is achieved by modifying the
type and amount of plasticizer. Table 19 shows the influence of varying the
amount of plasticizer on several important properties of cellulose acetate.
Table 1 9 – Influence of amount of plasticizer (dimethyl phthalate) on some
physical properties of cellulose acetate compositions
Property
Elongation, %
Tensile strength, MPa
Flexural strength, MPa
Water absorption, %
Loss in weight (1 week 150°C), %
8.0
45
77
2.13
2.40
6.5
56
98
2.47
0.86
5.0
68
120
3.24
0.53
233
The so-called flow temperature cannot be considered to be either
the processing temperature or the maximum service temperature. It is the
temperature at which the compound is forced down a capillary of fixed
dimensions by a fixed load at a specified rate. It is thus of use only for
comparison and for quality control purposes.
Applications
Cellulose acetate is used because of its reasonable toughness,
transparency and wide colour range. It is not suitable when good electrical
insulation properties, heat resistance, weathering resistance, chemical
resistance and dimensional stability are important.
The main outlets are for films and sheeting. Because of its clarity
the film is used extensively for photographic purposes and for packaging.
Sheeting is used for a variety of purposes. Thin sheet is useful for highquality display boxes whilst thicker sheet is used for spectacle frames.
Triacetate film is used in the graphic arts, for greetings cards, and
for specialised electrical appli cat ions such as non-conducting separators.
The use of cellulose acetate for moulding and extrusion is now
becoming small owing largely to the competition of the styrene polymers
and polyolefins. The major outlets at the present time are in the fancy
goods trade as toothbrushes, combs, hair slides etc. Processing provides no
major problem provided care is taken to avoid overheating and the
granules are dry. The temperatures and pressure used vary according to
grade. The best injection mouldings are obtained using a warm mould.
Biodegradable cellulose acetate compounds
As a result of development work between the Battelle Institute in
Frankfurt and a German candle-making company, Aeterna, biodegradable
cellulose acetate compounds have been available since 1991 from the
Rhône-Poulenc subsidiary Tubize Plastics. They are marketed under the
trade names Bioceta and Biocellat. The system is centred round the use of
an additive which acts both as a plasticizer and a biodegrading agent,
causing the cellulose ester to decompose within 6-24 months.
The initial use was as a blow moulded vessel for vegetable oil
candles. However, because of its biodegradability it is of interest for
applications where paper and plastics materials are used together and
which can, after use, be sent into a standard paper recycling process.
Instances include blister packaging (the compound is transparent up to 3
mm in thickness), envelopes with transparent windows and clothes pointof-sale packaging.
234
Compared with more common plastics used as packaging
materials, the compound does have some disadvantages, such as a high
water vapour permeability and limited heat resistance, losing dimensional
stability at about 70°C. It is also substantially more expensive than the
high-tonnage polyolefins. Last but not least its biodegradability means that
it must be used in applications that will have completed their function
within a few months of the manufacture of the polymer compound.
Other cellulose esters
Homologues of acetic acid have been employed to make other
cellulose esters and of these cellulose propionate, cellulose acetatepropionate and cellulose acetate-butyrate are produced on a commercial
scale. These materials have larger side chains than cellulose acetate and
with equal degrees of esterification, molecular weights and incorporated
plasticizer, they are slightly softer, of lower density, have slightly lower
heat distortion temperatures and flow a little more easily. The somewhat
greater hydrocarbon nature of the polymer results in slightly lower waterabsorption values (see Table 19).
It should, however, be realised that some grades of cellulose
acetate may be softer, be easier to process and have lower softening points
than some grades of cellulose acetate-butyrate, cellulose acetate-propionate
and cellulose propionate since the properties of all four materials may be
considerably modified by chain length, degree of substitution and in
particular the type and amount of plasticizer.
Cellulose acetate-butyrate (CAB) has been manufactured for a
number of years in the United States (Tenite Butyrate-Kodak) and in
Germany (Cellidor B-Bayer).
In a typical process for manufacture on a commercial scale
bleached wood pulp or cotton linters are pretreated for 12 hours with 4050% sulphuric acid and then, after drying, with acetic acid. Esterification
of the treated cellulose is then carried out using a mixture of butyric acid
and acetic anhydride, with a trace of sulphuric acid as catalyst. Commercial
products vary extensively in the acetate/butyrate ratios employed.
The lower water absorption, better flow properties and lower
density of CAB compared with cellulose acetate are not in themselves clear
justification for their continued use. There are other completely synthetic
thermoplastics which have an even greater superiority at a lower price and
do not emit the slight odour of butyric acid as does CAB. Its principal
235
virtues which enable it to compete with other materials are its toughness,
excellent appearance and comparative ease of mouldability (providing the
granules are dry). The material also lends itself to use in fluidised bed dipcoating techniques, giving a coating with a hard glossy finish which can be
matched only with more expensive alternatives. CAB is easy to vacuum
form.
A number of injection mouldings have been prepared from CAB
with about 19% combined acetic acid and 44% combined butyric acid.
Their principal end products have been for tabulator keys, automobile
parts, toys and tool handles. In the United States CAB has been used for
telephone housings. Extruded CAB piping has been extensively used in
America for conveying water, oil and natural gas, while CAB sheet has
been able to offer some competition to acrylic sheet for outdoor display
signs.
In the mid-1950s cellulose propionate became commercially
available (Forticel-Celanese). This material is very similar in both cost and
properties to CAB. Like CAB it may take on an excellent finish, provided
a suitable mould is used, it is less hygroscopic than cellulose acetate, and is
easily moulded.
As with the other esters a number of grades are available differing
in the degree of esterification and in type and amount of plasticizer. Thus
the differences in properties between the grades are generally greater than
any differences between “medium” grades of cellulose propionate and
CAB. Whereas a soft grade of the propionate may have a tensile strength
of 14MPa and a heat distortion temperature of 51°C, a hard grade may
have tensile strength as high as 42 MPa and a heat distortion temperature
of 70°C.
Cellulose acetate-propionate (Tenite Propionate-Kodak) is similar
to cellulose propionate. With the shorter side chains, cellulose propionate
and cellulose acetate propionate tend to be harder, stiffer and of higher
tensile strength than CAB. Like CAB they are easy to vacuum form and
also tend to be used for similar applications such as steering wheels, tool
handles, safety goggles and blister packs.
Many other cellulose esters have been prepared in the laboratory
and some have reached pilot plant status. Of these the only one believed to
be of current importance is cellulose caprate (decoate). According to the
literature, degraded wood pulp is activated by treating with chloroacetic
acid and the product is esterified by treating with capric anhydride, capric
acid and perchloric acid. The material is said to be useful as optical cement.
236
2.4.2 Cellulose Ethers
C
CC
C
O C
H
CH2OC2H
5
H
O
OC2H
5
OC2H
5
H
O
H
H
Of cellulose ethers ethyl cellulose has found a small limited
application as a moulding material and somewhat greater use for surface
coatings. The now obsolete benzyl cellulose was used prior to World War
II as a moulding material whilst methyl cellulose, hyroxyethyl cellulose
and sodium carboxymethyl cellulose are useful water-soluble polymers.
With each of these materials the first step is the manufacture of
alkali cellulose (soda cellulose). This is made by treating cellulose (either
bleached wood pulp or cotton linters) with concentrated aqueous sodium
hydroxide in a nickel vessel at elevated temperature. After reaction excess
alkali is pressed out, and the resultant “cake” is then broken up and vacuum
dried until the moisture content is in the range 10-25%. The moisture and
combined alkali contents must be carefully controlled as variations in them
will lead to variations in the properties of the resultant ethers.
Ethyl cellulose
Ethyl cellulose is prepared by agitating the alkali cellulose with
ethyl chloride in the presence of alkali at about 60°C for several hours.
Towards the end of the reaction the temperature is raised to about 130140°C. The total reaction time is approximately 12 hours. The reaction is
carried out under pressure.
If the etherification were taken to completion the product would be
the compound shown in Figure 51.
Figure 51
It is essential that there be sufficient alkali present, either combined
with the cellulose, or free, to neutralize the acid formed by both the main
reaction and in a side reaction which involves the hydrolysis of ethyl
chloride.
removed by distillation and the ethyl cellulose is precipitated by hot water.
Ethyl ether and ethyl alcohol which are formed as by-products are
237
The polymer is then carefully washed to remove sodium hydroxide and
sodium chloride and dried.
The properties of the ethyl cellulose will depend on:
(1) The molecular weight.
(2) The degree of substitution.
(3) Molecular uniformity.
The molecular weight may be regulated by controlled degradation
of the alkali cellulose in the presence of air. This can be done either before
or during etherification. The molecular weight of commercial grades is
usually expressed indirectly as viscosity of a 5% solution in an 80:20
toluene-ethanol mixture.
The completely etherified material with a degree of substitution of
3 has an ethoxyl content of 54.88%. This material has little strength and
flexibility, is not thermoplastic, has limited compatibility and solubility and
is of no commercial value. A range of commercial products are, however,
available with a degree of substitution between 2.15 and 2.60,
corresponding to a range of ethoxyl contents from 43 to 50%.
The ethoxyl content is controlled by the ratio of reactants and to a
lesser degree by the reaction temperature.
Whereas mechanical properties are largely determined by chain
length, the softening point, hardness, water absorption and solubility are
rather more determined by the degree of substitution.
Typical physical properties of ethyl cellulose are compared with
those of the cellulose ethers in Table 18.
The solubility of ethyl cellulose depends on the degree of
substitution. At low degrees of substitution (0.8-1.3) the replacement of
some of the hydroxyl groups by ethoxyl groups reduces the hydrogen
bonding across the cellulosic chains to such an extent that the material is
soluble in water. Further replacement of hydroxyl groups by the less polar
and more hydrocarbon ethoxyl groups increases the water resistance. Fully
etherified ethyl cellulose is soluble only in non-polar solvents.
The relationship between degree of substitution and solubility
characteristics is predictable from theory and is summarised in Table 20.
Ethyl c el l u l ose is subj e c t to oxida tive degr a da tion when e x p os e d to
sunlight and elevated temperatures. It is therefore necessary to stabilize the
material against degrading influences during processing or service. In
practice three types of stabilizer are incorporated, an antioxidant such as
the phenolic compound 2,2'-methylenebis-(4-methyl-6-tert-butylphenol),
an acid acceptor such as an epoxy resin for use where plasticizers may give
238
rise to acidic degradation products and an ultraviolet absorber such as 2,4-
Average number of
glucose unit
Solubility
~0.5
2.5-2.8
soluble in 4-8% sodium hydroxide
soluble only in non-polar solvents
dihydroxybenzophenone for outdoor use. Plasticizers such as tritolyl
phosphate and diamylphenol have a beneficial stabilizing effect.
Table 20 – Solubility of ethyl cellulose
ethoxyl groups per
0.8-1.3
1.4-1.8
1.8-2.2
2.2-2.4
2.4-2.5
Ethyl cellulose has never become well known in Europe and apart
from one or two specific applications has not been able to capture any
signifi cant pr oporti on of the mar ket held by the cellulose esters. Although
it has the greatest water resistance and the best electrical insulating
properties amongst the cellulosics this is of little significance since when
these properties are important there are many superior non-cellulosic
alternatives. The principal uses for ethyl is cellulose injection mouldings
are in those applications where good impact strength at low temperatures is
required, such as refrigerator bases and flip lids and ice-crusher parts.
Ethyl cellulose is often employed in the form of a “hot melt” for
strippable coatings. Such strippable coatings first became prominent during
World War II for packaging military equipment. Since then they have been
extensively used for protecting metal parts against corrosion and marring
during shipment and storage. A typical composition consists of 25% ethyl
cellulose, 60% mineral oil, 10% resins and the rest stabilizers and waxes.
Coating is performed by dipping the cleaned metal part into the molten
compound. The metal part is withdrawn and an adhering layer of the
composition is allowed to harden by cooling. Hot melts have also been
used for casting and paper coating.
The ether is also used in paint, varnish and lacquer formulations.
soluble in water
swelling in polar – non-polar solvent mixtures
increasing solubility in above mixtures
increasing solubility in alcohol and less polar
solvents
widest range of solubilities
239
Miscellaneous ethers
Only one other cellulose ether has been marketed for moulding and
extrusion applications, benzyl cellulose. This material provides a rare
example of a polymer which although available in the past is no longer
commercially marketed. The material had a low softening point and was
unstable to both heat and light and has thus been unable to compete with
the many alternative materials now available.
Water-soluble cellulose ether – methyl cellulose is prepared by a
method similar to that used for ethyl cellulose. A degree of substitution of
1.6-1.8 is usual since the resultant ether is soluble in cold water but not in
hot. It is used as a thickening agent and emulsifier in cosmetics, as a paper
size, in pharmaceuticals, in ceram ics and in leather tan ning oper ations.
Hydroxyethyl cellulose, produced by reacting alkali cellulose with
ethylene oxide, is employed for similar purposes.
Hydroxypropyl cellulose, like methyl cellulose, is soluble in cold
water but not in hot, precipitating above 38°C. It was introduced by
Hercules in 1968 (Klucel) for such uses as adhesive thickeners, binders,
cosmetics and as protective colloids for suspension polymerization. The
Dow company market the related hydroxypropylmethyl cellulose
(Methocel) and also produce in small quantities a hydroxyethylmethyl
cellulose.
Reaction of alkali cellulose with the sodium salt of chloracetic acid
yields sodium carboxmethyl cellulose, (SCMC). Commercial grades
usually have a degree of substitution between 0.50 and 0.85. The material,
which appears to be physiologically inert, is very widely used. Its principal
application is as a soil-suspending agent in synthetic detergents. It is also
the basis of a well-known proprietary wallpaper adhesive. Miscellaneous
uses include fabric sizing and as a surface active agent and viscosity
modifier in emulsions and suspensions. Purified grades of SCMC are
employed in ice cream to provide a smooth texture and in a number of
pharmaceutical and cosmetic products.
Schematic equations for the production of fully substituted
varieties of the above three ethers are given below (R represents the
cellulose skeleton).
240
R(ONa)
R(ONa)
+ CH3Cl
3n
Methyl Cellulose
3n
+
CH
O
CH
2
2
R(ONH3)
3n
R(OCH3CH2OH)
3n
R(ONa)
+ ClCH2COONa R(OCH2COONa)
3n
3n
+ NaCl
2.4.3 Regenerated Cellulose
Because of high interchain bonding, cellulose is insoluble in
solvents and is incapable of flow on heating, the degradation temperature
being reached before the material starts to flow. It is thus somewhat
intractable in its native form. Cellulose, however, may be chemically
treated so that the modified products may be dissolved and the solution
may then either be cast into film or spun into fibre. By treatment of the
film or fibre the cellulose derivative may be converted back (regenerated)
into cellulose although the processing involves reduction in molecular
weight.
In the case of fibres three techniques have been employed:
(1) Dissolution of the cellulose in cuprammonium solution
followed by acid coagulation of extruded fibre (“cuprammonium rayon” –
no longer of commercial importance). In this case the acid converts the
cuprammonium complex back into cellulose.
(2) Formation of cellulose acetate, spinning into fibre and
subsequent hydrolysis into cellulose.
(3) Reaction of alkali cellulose with carbon disulphide to produce a
cellulose xanthate which forms a lyophilic sol with caustic soda. This may
be extruded into a coagulating bath containing sulphate ions which
hydrolyses the xanthate back to cellulose. This process is known as the
viscose process and is that used in the manufactur e of ray on.
By modification of the viscose process a regenerated cellulose foil
may be produced which is known under the familiar trade name
Cellophane.
The first step in the manufacture of the foil involves the production
of alkali cellulose. This is then shredded and allowed to age in order that
oxidation will degrade the polymer to the desired extent. The alkali
cellulose is then treated with carbon disulphide in xanthating churns at 2028°C for about three hours.
The xanthated cellulose contains about one xanthate group per two