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221
the extent of nitration being determined by the strength of the nitrating
acid.
Because of the insolubility of cellulose it is not possible to carry
out uniform esterification with the lower organic acids (acetic acid,
propionic acid etc.) and in those cases where incompletely substituted
derivatives are required a two-stage reaction is employed. This involves
total esterification in a medium in which the ester dissolves, followed by
the uniform removal of some of the substituent groups (this now being
possible in solution) by hydrolysis.
2.4.1 Cellulose Esters
The cellulose esters are useful polymers for the manufacture of
plastics. Until about 1950 they did in fact form the most important group of
thermoplastics materials.
The most important of the esters is cellulose acetate. This material
has been extensively used in the manufacture of films, moulding and
extrusion compounds, fibres and lacquers. As with all the other cellulose
polymers it has, however, become of small importance to the plastics
industry compared with the polyolefins, PVC and polystyrene. In spite of
their higher cost cellulose acetate-butyrate and cellulose propionate appear
to have retained their smaller market because of their excellent appearance
and toughness.
The early 1990s have also seen much interest in biodegradable
cellulose acetate compound s.
The doyen of the ester polymers is cellulose nitrate. Camphormodified cellulose nitrate has been known for over 120 years and still
retains its use in a few specialized applications.
Cellulose nitrate
Preparation
The reaction between cellulose and nitric acid is one of
esterification. It is possible to achieve varying degrees of esterification
according to the number of hydroxyl groups that have been replaced by the
nitrate group. Complete substitution at all three hydroxyl groups on the
repeating anhydroglucose units will give the explosive cellulose trinitrate,
containing 14.14% nitrogen. This material is not made commercially but
esters with lower degrees of nitrate are of importance.
Industrial cellulose nitrates or “nitrocellulose” (as they are often
erroneously called) have a degree of substitution somewhere between 1.9
222
and 2.7 and that materials with lower degrees of substitution are used for
plastics applications.
The nitration process involves the steeping of cotton linters into a
mixture of nitric and sulphuric acids and subsequent removal, stabilization,
bleaching and washing of the product. Subsequent conversion into plastics
materials involves displacing residual water by alcohol, mixing the
alcohol-wet nitrate with camphor and other ingredients, seasoning the
rolled hides, pressing and finally cutting to shape.
Before nitration the moisture content of the purified linters is
reduced to well below 5% since the presence of water will modify the
progress of the reaction and tends to produce undesirable products. The
drying operation is carried out by breaking open the cotton linters and
passing along a hot air drier.
The nitration bath normally contains sulphuric acid as a
condensing age n t a n d a t ypi cal bath for prod uc i n g a c el lulose ni t r ate with a
nitrogen content of 11% would be
Nitric acid 25%
Sulphuric acid 55%
Water 20%
In a typical process 545 kg of the mixed acids are run into the
reaction vessel and 13.5 kg of the dried cotton linters are added. The
mixture is agitated by a pair of contra-rotating stirrers and nitration is
allowed to proceed at about 35-40°C for 20 minutes. It is interesting to
note that the cellulosic material retains its fibrous form throughout the
nitration process.
On the completion of nitration the batch is dropped from the
reaction vessel into a centrifuge and the acid mixture spun off and
recovered. The nitrated linters, which still contain appreciable quantities of
acid, are then plunged into a drowning tank, where the nitric acid is diluted
with a large volume of water. The resultant ester is then pumped, as a
slurry, into storage vats which may hold the products of several nitrations.
The product at this stage is unstable. It has been shown that some
of the sulphuric acid reacts with the cellulose hydroxyl group to form
sulphates. These tend to split off, re-forming sulphuric acid to initiate an
autocatalytic decomposition which can lead, and has in the past led, to
disastrous explosions. The remedy lies in removing the sulphate groups in
a stabilization process by boiling the cellulose ester with water that
contains a controlled trace of acid for several hours. Side effects of this
process are a reduction in molecular weight and in the nitrogen content.
223
The stabilized nitrate may then be bleached with sodium
CH
3
C
CH
2
CH
2
CH
CH
2
C
C
CH
3
CH
3
O
hypochlorite, centrifuged to remove much of the water in which the
polymer has been slurried and dehydrated by displacement with alcohol
while under pressure in a press. In these processes approximately 160000
litres of water are used for every ton of cellulose nitrate produced. Control
of purity of the water is important; in particular the iron content should be
as low as 0.03 parts per million since iron can adversely affect both the
colour and heat stability of the polymer.
Manufacture of celluloid sheets
Although originally a trade name the term celluloid has come into
general use to describe camphor-plasticized cellulose nitrate compositions.
The rather unexpected plasticizing effect of camphor was first
appreciated by Hyatt over a hundred years ago and, in spite of all that has
been learned about polymers since then, no superior plasticizer has yet
been discovered.
Camphor was originally obtained from the camphor tree Laurus
camphora in which it appeared in the optically active dextro-rotary form.
Since about 1920 the racemic (±) mixture derived from oil of turpentine
has been more generally used. By fractional distillation of oil of turpentine
the product pinene is obtained. By treating this with hydrochloric acid,
pinene hydrochloride (also known as bornyl chloride) may be produced.
This is then boiled with acetic acid to hydrolyse the material to the racemic
borneol, which on oxidation yields camphor. Camphor is a white
crystalline solid (m.p. 175°C) with the structure shown in Figure 50.
Figure 50
It has a low solubility parameter (15.3MPa
considerably from that of the cellulose dinitrate (21.8 MPa
indicates that compatibility is not simply due to similarities of cohesive
1/2
) which differs
1/2
). This
forces but also to some form of interaction probably involving the carbonyl
group.
224
Mixing of the ingredients is carried out in steam-heated dough
mixers fitted with solvent extraction hoods. A typical charge would consist
of 10.9 kg of cellulose nitrate and 3.6 kg of camphor. The residual alcohol
in the nitrate develops a powerful solvent action and the cellulose nitrate
loses its fibrous form and the whole mix becomes a gelatinous mass.
Typically mixing temperatures are of the order of 40°C and mixing times
are approximately 1 hour.
Pigments or dyes may be added at this stage and where clear waterwhite sheet is required a small amount of a soluble violet dye is added to
offset the faintly yellow colour of the natural mix. Stabilizers such as zinc
oxide, zinc acetate or urea may be added to prevent the composition from
developing acidity.
The celluloid dough is then filtered by forcing through a pad of
calico and brass gauze backed by a heavy brass plate at a press of about 1.5
tons per square inch. Any undesirable foreign matter is thus separated from
the dough.
The filtered dough is then returned to a mixer and the alcohol
content reduced to 25% by kneading under vacuum. Further reduction in
the alcohol content is brought about by rolling the compound on a hooded
two-roll mill. The milled product is then consolidated on a two-bowl
calender and sheeted off in hides about 1/2 in thick. At this stage the
solvent content is between 12 and 16%.
A number of hides are then laid up in a box mould with a grooved
base. The mould is then loaded into a press and heated with hot water to
consolidate the mass. Great care must be taken to avoid overheating since
this can cause disastrous explosions. It is not unknown for the head of a
hydraulic press to have been blown through the roof of the press shop as a
result of overheatin g the celluloid. Press temperatures are typically about
75°C and pressure about 3.5 MPa.
After pressing, the moulded slab is allowed to cool and then sliced
using a horizontally reciprocating knife. The thickness of the sliced sheet
may range from 0.012 to 2.5 cm. Attractive mottled effects may be
produced by plying sliced sheets, repressing and then subsequently
reslicing on the bias. Bias-cut sheets of different patterns may then again
be pressed and the process repeated indefinitely. In this way complex but
reproducible patterns may be built up.
The sliced sheet will still contain large quantities of alcohol and it
is necessary to “season” the sheet at elevated temperatures. This may only
225
take three days at 49°C for 0.025 cm thick sheet but will take about 56
days for 2.5 cm thick blocks. The removal of alcohol, as might be
expected, is accompanied by considerable shrinkage. Fully seasoned sheet
has a volatile content of 2%, the bulk of which is water but there is some
residual alcohol. The sheet may be fully polished by heating in a press
between glazed plates under pressure for a few minutes. Because the
material is thermoplastic it is necessary to cool it before removal from the
press.
It is also possible to extrude alcohol-containing celluloid
compositions through either ram or screw extruders under carefully
controlled conditions. The process is now believed to be universally
obsolete.
Sheet and block may be machined with little difficulty providing
care is taken to avoid overheating and to collect the inflammable swarf.
Structure and properties of celluloid
Nitration of cellulose followed by plasticization of the product
with camphor has the effect of reducing the orderly close packing of the
cellulose molecules. Hence whereas cellulose is insoluble in solvents,
except in certain cases where there is chemical reaction, celluloid is soluble
in solvents such as acetone and amyl acetate. In addition the camphor
present may be dissolved out by chloroform and similar solvents which do
not dissolve the cellulose nitrate.
The solvation by plasticizer also gives celluloid thermoplastic
properties owing to the reduction in interchain forces. On the other hand
since the cellulose molecule is somewhat rigid the product itself is stiff and
does not show rubbery properties at room temperature, cf. plasticized PVC.
As may be expected from such a polar material it is not a
particularly good electrical insulator, particularly at high frequencies. The
high dielectric constant is particularly noteworthy.
The chemical resistance of celluloid is not particularly good. It is
affected by acids and alkalis, discolours on exposure to sunlight and tends
to harden on aging. More seriously it is extremely inflammable, this being
by far the greatest limitation of the material.
Typical physical properties of celluloid are compared with other
cellulose plastics in Table 17.
226
Table 17 – Typical physical properties of cellulosic plastics
Property
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Ethyl
Specific
1.35-
1.27-1.32
1.15-1.22
1.19-1.23
1.18-1.24
1.12-
Refractive
1.47-1.48
Tensile
Elongation
at break, %
10-40
5-55
8-80
30-100
45-65
10-40
Flow
145-
115-
Heat
-
Dielectric
3.0-3.5
nitrate
gravity
index (25°C) 1.5 1.47-1.5
strength,
MPa
temperature,
°C
distortion
temperature,
°C
constant 60
Hz
1.40
35-70* 24-76
152
6.7-7.3 3.5-7.5 3.7-4.5
acetate
165
50-100
acetate-
butyrate
17-52
115-165
56-94
acetate-
propionate
1.46-1.49
24-50
150-180 145-180
45-110
3.7-4.0
propionate
1.46-1.49
14-42
51-70
(106 Hz)
cellulose
1.15
1.47
42-62
100-150
50-66
2.7
* - It is necessary to quote a range of figures in most instances
since the value of a particular property is very dependent on formulation.
The high inflammability and relatively poor chemical properties of
celluloid severely restrict its use in industrial applications. Consequently,
the material is used because of the following desirable characteristics.
(1) Water-white transparency of basic composition but capable of
forming highly attractive multi-coloured sheeting.
(2) Rigidity.
(3) Reasonable toughness.
(4) Capable of after-shrinkage around inserts.
Applications
The annual production of celluloid is now negligible compared
with the total world production of plastics.
The one-time important applications in photographic film, in
227
bicycle parts (pump covers and mudguards) and in toys manufactured by a
blowing process from flat sheet are no longer of importance.
Today the principal outlets are knife handles, table-tennis balls and
spectacle frames. The continued use in knife handles is due to the pleasant
appearance and the ability of the material to after-shrink around the
extension of the blade. Table-tennis balls continue to be made from
celluloid since it has been difficult to match the “bounce” and handle of the
celluloid ball, the type originally used, with balls fabricated from newer
polymers. Even here celluloid is now meeting the challenge of synthetic
polymers. Spectacle frames are still of interest because of the attractive
colour. There are, however, restrictions to their use for this application in
certain countries and cellulose acetate is often preferred.
Cellulose acetate
Preparation
The earliest preparation of cellulose acetate is credited to
Schützenberger in 1865. The method used was to heat the cotton with
acetic anhydride in sealed tubes at 130-140°C. The severe reaction
conditions led to a white amorphous polymer but the product would have
been severely degraded and the process difficult to control. Subsequent
studies made by Liebermann, Francimont, Miles, the Bayer Company and
by other workers led to techniques for controlled acetylation under less
severe conditions.
The methods available today may be considered under two
headings, homogeneous acetylation, in which the acetylated cellulose
dissolves into a solvent as it is formed, and the heterogeneous technique, in
which the fibre structure is retained.
The probability of acetylation of any one cellulosic group is
strongly dependent on its position in the fibre. Since they cannot be
dissolved before acetylation it will be realised that some molecules will be
completely acetylated whilst others may be untouched. It is thus necessary
first to acetylate completely the cellulose and the resultant triacetate
material, which is soluble in certain solvents, may then be back-hydrolysed
in solution. Under these conditions the probabilities of hydrolysis of any
acetyl groups in one molecule will be similar to the reaction probabilities
of these groups in another molecule and products with a reasonably even
degree of substitution less than three may be obtained.
The preparation of the acetate by homogeneous acetylation may be
considered in three stages:
228
(1) Pretreatment of the cellulose.
(2) Acetylation.
(3) Hydrolysis.
The aim of pretreatment is to open up the cellulosic matter in order
to achieve more even substitution and to accelerate the main acetylation
reaction. A large number of pretreatments have been described in the
patent literature but in practice exposure to glacial acetic acid is that most
commonly employed.
The acetylation is usually carried out in bronze stirred mixers. The
acetylating mixture normally contains three components, an acetylating
agent, a catalyst and a diluent.
Although acetyl chloride and ketene (CH2=С=О) have been
described in the literature, acetic anhydride has been the commonly
employed acetylating agent.
The reaction between one of the hydroxyl groups of the cellulose
molecule (XOH) and the anhydride is:
XOH + CH3COOCOCH3 → XOCOCH3 + CH3COOH
Similarly, a number of catalysts have been suggested but
concentrated sulphuric acid, first suggested by Francimont in 1879, is
almost universally employed today.
The diluent, which is usually a solvent for the acetate, facilitates
the reaction, particularly in respect of temperature control. Acetic acid is
generally employed either alone or in conjunction with other materials for
this purpose. It may be added initially but is also formed during the
acetylation of the cellulos. A mild acetylating agent in itself it is not merely
a diluent but influences the course of the reaction. The low boiling solvent
methylene dichloride (b.p. 40°C) is now commonly used in conjuction with
acetic acid. An advantage in using methylene dichloride is that excessive
exothermic heat may be removed as latent heat of evaporation as the
methylene chloride boils. Bubbles formed during boiling or “simmering”
may also assist in mixing of the reaction blend.
In the so-called Dormagen process developed by IG Farben the
cotton was first preheated with 30-40% of its own weight with glacial
acetic acid for 1-2 hours. The pretreated material was then fed to the
acetylizers, which consisted of horizontal bronze cylinders. For every 100
parts of pretreated cellulose there was added the following acetylating
mixture, previously cooled to 15-20°C:
300 parts acetic anhydride
400 parts methylene dichloride
229
1 part sulphuric acid
Degree of
Acetyl
3
Acetic acid
Solubility
Uses
2.2-2.3
36.5-38.0
52.1-54.3
soluble in
chloroform
injection
Cooling water was passed through jackets surrounding the reactor
in order to prevent the temperature from exceeding 50°C.
Esterification is complete in 5-6 hours and the product at this stage
is known as primary cellulose acetate.
Current acetylation techniques may be based on continuous
production similar to that employed in the Dormagen process or batchwise
in dough-type mixers.
Whatever acetylating technique is used, this is then followed by the
ripening operation. The “ripening” is carried out without isolating the
triacetate. Dilute acetic acid or water is added to the acetylizing mixture,
about 20-25% based on the weight of the cellulose. Hydrolysis is allowed
to proceed for a number of days, typically for about 72 hours. The progress
of hydrolysis is followed by checking the solubility of the acetate in
alcohol-benzene and acetone solutions. When the required degree of
substitution has been reached the cellulose acetate is precipitated by
judicious addition of water to the stirred mixture. It is then washed
thoroughly and dried in an electric or vacuum oven. Care must be taken in
the operation of the precipitation stage since otherwise lumps are likely to
be formed. For certain applications, such as for photographic film, further
purification operations may be carried out.
The product at this stage is referred to as secondary cellulose
acetate. Different degrees of acetylation are required for different endproducts and these are indicated in Table 18.
Table 1 8 – Influence of degree of substitution on the properties and uses of
cellulose acetate
substitution
2.3-2.4
2.4-2.6
2.8-3.0
content (%
–COCH
)
38.0-39.5
39.5-41.5
42.5-44
yield (%)
54.3-56.4
56.4-59.3
60.7-62.8
acetone
insoluble in
acetone,
soluble in
moulding
film
lacquers
triacetate
film and
fibre
230
Compounding of cellulose acetate
The cellulose molecule is rigid and forms strong hydrogen bonds
with adjacent molecules. It is thus insoluble and decomposes before
softening on heating. Partial replacement of hydroxyl groups by acetyl
groups has a number of effects:
(1) It reduces interchain hydrogen bonding.
(2) It increases interchain separation.
(3) It makes the polymer less polar – the polarity depending on the
degree of acetylation.
Because of these influences cellulose acetate can be dissolved in a
variety of media, although a liquid suitable as a solvent for cellulose
acetate with a degree of substitution of two would be unlikely to be a
solvent for acetates with degrees of substitution of either one or three.
Although acetylation thus renders the cellulosic structure soluble,
cellulose acetate will still decompose below its softening point. It is thus
necessary to compound cellulose acetate with plasticizers in order t o o b tain
plastics materials of suitable flow properties. Other ingredients are also
added at the same time.
Although the prime function of plasticizers in cellulose acetate is
to bring the processing temperature of the compound below the polymer
decomposition temperature, it has additional values. An increase in the
plasticizer content will reduce the melt viscosity at a given temperature and
simplify processing. The physical properties of the finished product will be
modified, increasing toughness but reducing the heat distortion
temperature, the latter not being an important property in most cellulose
acetate applications.
Although many plasticizers have been suggested for cellulose
acetate very few have been used in practice. The most important of these
are dimethyl phthalate (δ = 21.4 MPa
triphenyl phosphate (δ = 20.0 MPa
parameter within one unit of that of cellulose diacetate (~22.0 MPa
1/2
), triacetin (δ = 20.3 MPa
1/2
), each of which have a solubility
1/2
1/2
) and
).
Of these dimethyl phthalate (DMP) is used in most compositions.
It is cheap, has a high compatibility with secondary cellulose acetate and is
efficient in increasing flexibility, toughness and the ease of flow at a given
temperature. Its principal disadvantages are its high volatility and the fact
that it increases the flammability of the compound. Similar in compatibility
but rather less volatile is diethyl phthalate. This material has less of an
influence on flexibility and flow properties than the methyl ester.
Triphenyl phosphate is a crystalline solid which has less