the research department of courtaulds limited

The Research Department of Courtaulds LimitedAuthor(s): A. H. WilsonSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 142, No.908 (May 27, 1954), pp. 289-305Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/82806 .

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The Research Department of Courtaulds Limited

By A. H. Wilson, F.R.S.

Director in Charge of Research and Development

(Lecture delivered 12 November 1953?Received 12 November 1953)

[Plates 23 to 26]

1. Inteoduction

Before I describe the Research Department of Courtaulds Ltd, I must first give a brief description of the activities and organization of the Company as a whole.

Broadly speaking, Courtaulds Ltd is engaged in all aspects of the man-made fibre

industry, including primary raw materials at one end and garments at the other,

though the industrial effort is far from uniformly spread between these limits, the

major strength of the Company being concentrated in the production of the

cellulosic-based fibres known as viscose and cellulose acetate rayon. In addition, the Company has large investments in subsidiary and associated rayon producing

companies in Australia, Canada, France, Germany, Italy, Spain and the U.S.A., while in England the manufacture of Cellophane is carried out by the subsidiary

company of British Cellophane Ltd, and that of nylon by the associated company British Nylon Spinners Ltd, so that the parent company is partly a manufacturing

organization and partly a holding company. All the subsidiary and associated

companies have their own research departments of varying sizes and complexities but I shall not attempt to describe these, and I shall confine my remarks to the

research department of the parent company. As I have already mentioned, the major industrial activity of Courtaulds Ltd

in the United Kingdom is the production of cellulosic-based fibres. The rayon factories run 168 hours a week, there being four shifts. The number of hourly-paid

operatives is at present about 15000 and the sales are of the order of ?50000000

per annum. The value to be placed upon the fixed assets is largely a matter of

opinion, but in considering the effect of technical innovations upon the operations of the Company the fixed assets can be assumed to be worth something of the order

of ?80000000. The number of operatives is therefore small; the capital investment

is high and the turnover is relatively low.

The rayon and synthetic fibre industry is essentially a mass -production industry

producing continuous filament yarns and staple fibres for use in the textile industry. Continuous filament yarn, as its name suggests, consists of a number of filaments

forming an unbroken thread. The number of filaments in a thread varies from

about 15 to 500, their diameter is of the order of one-thousandth of an inch and

their length may run up to about 200 miles. Staple fibre, on the other hand, is

similar to cotton, wool or linen in that it has been cut up into portions of a pre? scribed length, ranging from lj to 8 in., and which must, like the naturally

occurring fibres, be spun into a thread before it can be used in a loom or in a

Vol. 142. B. (27 May 1954) [ 289 ] 20



290 A. H. Wilson

knitting machine. Each particular man-made fibre can be produced as continuous

filament or as staple fibre, and so the fibre-producing factories of Courtaulds Ltd

produce only four major products, namely viscose and cellulose acetate continuous

filament yarns and staple fibres (six products if we include the associated company British Nylon Spinners Ltd), the average output in the last few years being of the

order of 45000 tons of continuous filament yarn and 65000 tons of staple fibre.

2. General description or the industrial processes and their

EFFECT UPON RESEARCH TECHNIQUES

The rayon and synthetic fibre industry has very much in common with the

heavy chemical industry, but it has certain distinctive features of its own, and it is

on these I wish to concentrate. In the first place, fibre-forming substances, whether

natural or partly synthetic, are necessarily linear polymers of high molecular

weight. Gas-phase reactions of high polymers are uncommon, and one has there?

fore to deal either with solid-phase reactions or with liquid-phase reactions in

highly viscous liquids. In the second place the polymer must be made into a

continuous thread by being extruded through a die or jet, a process which is always known as spinning. There are three entirely different spinning processes, the wet

spinning process in which a solution of the polymer is precipitated by being extruded

into a non-solvent, the dry or evaporative spinning process in which a solution of

the polymer in a volatile solvent is extruded into hot air, the solvent being removed

by evaporation, and the melt spinning process in which the polymer is melted and

after passage through the holes in the jet is solidified by cooling. In all cases

stretching of the freshly formed fibre is an essential part of the process. The wet

spinning process is used for producing viscose (in which case there is a chemical

reaction as well as a coagulation), the dry spinning process for cellulose acetate

and the melt spinning process for nylon. In the viscose process, the primary raw material is a pure form of cellulose

derived from wood pulp. This is treated with caustic soda and carbon disulphide to produce sodium cellulose xanthate, which is one of the few water-soluble

cellulose compounds. The xanthate is dissolved in dilute caustic soda to form

a thick syrupy liquid, known as viscose, which is extruded into a bath containing about 10 % sulphuric acid, 20 % sodium sulphate and from 1 to 4 % zinc sulphate. The xanthate is decomposed by the sulphuric acid and the original cellulose is

regenerated and precipitated; it is then purified and dried.

In the cellulose acetate process, wood pulp or cotton linters is acetylated by means of acetic anhydride, using sulphuric acid as a catalyst and either acetic acid

or methylene chloride as a solvent, a number of reaction stages being employed in order to control the degree of acetylation and the chain-length of the substituted

cellulose molecules. The cellulose acetate polymer is then precipitated and dried, and is dissolved in acetone to form a highly viscous solution which can be dry spun.

In both processes, the difficulties which arise in the first stages of manufacture

are partly chemical, such as ensuring a regular degree of substitution along the

cellulose molecule and of avoiding excessive depolymerization of the natural

polymer, and partly physical, due to the low heat-transfer coefficients in hetero-



The Research Department of Courtaulds Limited 291

geneous solid and liquid reactions. They are, however, similar to problems which

occur in other industries, though not in such an extreme form. Special difficulties,

however, arise because one has to deal with natural or synthetic polymers whose

characteristics are not well defined. In many cases, results obtained from basic

researches or from pilot plant investigations cannot be directly translated into

industrial practice without considerable modifications, and ultimately large-scale

experimentation is required. The spinning processes are characteristic of the man-made fibre industry and

demand an unusual combination of chemical, physical and mechanical techniques, while the phenomena to be dealt with are exceedingly complex and take place with

great rapidity. Moreover, the product is judged by its physical and textile charac?

teristics rather than by its chemical properties. For these reasons it is not possible to make a complete study of the process by measurements of rates of diffusion,

reaction, etc., and a more direct approach is necessary. The standard research tool

in research on fibres is the single-end spinning machine, which consists of a single extrusion jet followed by a number of devices for stretching and relaxing the

freshly formed fibre and, if necessary, treating it with various chemical reagents to improve or modify its properties. Promising results obtained on single-end

spinning machines have to be tried out on a larger scale involving the use of, say, a five or ten end experimental machine capable of producing a hundred pounds or

so a week of a fibre for more detailed evaluation. Many products will, of course, be rejected at this stage, but if it seems likely that a new or modified fibre has

commercial possibilities it is then necessary to produce it in reasonable quantities for a considerable time in order to investigate the reproducibility of the properties of the fibre, which is of paramount importance, and the behaviour of the fibre in

use as garments. This means that the fibre must be produced and used essentially under factory conditions, and the facilities in the research department have to be

very extensive, ranging from small-scale apparatus for basic research to factory- scale equipment for the production and assessment of considerable weights of

fibres. There is therefore a very high ratio of technicians and operatives to graduate

staff, and the consumption of raw materials is a very large element in the running cost of the research department. Products are not, however, produced on a semi-

commercial scale unless they have some commercial value, and a proportion of the

development costs can be recovered by sales.

Photographs of typical single-end spinning machines and of larger spinning

plants in the Research Department are shown in plates 23 to 26.

3. Organization of the research department

The Research Department is decentralized and consists of six independent

laboratories, three at Coventry and one each at Booking, Essex, at Droylsden, Manchester and at Maidenhead, Berkshire. Large-scale development work is

carried out in the first four of these laboratories, whose activities are therefore

somewhat sharply divided into two, it being desirable to segregate the basic

research and initial development work from the semi-industrial activities. The



292 A. H. Wilson

Coventry laboratories are essentially devoted to research on the production of

man-made fibres, one of them dealing with viscose rayon fibres and a second with

cellulose acetate fibres and synthetic fibres in general. The third organization at

Coventry is the Chemical Engineering Section which deals with processes ancillary to fibre manufacture, including the production of the chemicals required and the

recovery of the solvents, coagulating baths, etc., used in the spinning processes. The Textile Research Laboratory at Bocking is concerned with the weaving,

knitting, dyeing and finishing of fabrics, and in addition provides the facilities for

the evaluation of the new types of fibres discovered in the Coventry laboratories.

The laboratory at Droylsden concentrates upon the dyeing of fibres in bulk before

weaving or knitting, while the aim of the laboratory at Maidenhead is the discovery of synthetic fibres of radically new types.

In 1952 the staff of the Research Department consisted of approximately 200 graduates, 450 technical staff and 400 operatives. The total research expenditure was about ?900000 of which ?750000 was the cost of research and small-scale

development, while semi-commercial projects cost about ?150000. The gross ex?

penditure on semi-commercial development was about ?600 000, but, as I mentioned

above, it is usually possible to recover a substantial proportion of this expenditure

by sales of the products, and in 1952 the credits were of the order of ?450000. The

cost of semi-commercial development is liable to considerable fluctuations since

the incidence of the projects which reach this stage is necessarily erratic, and the

proportion of the expenditure which can be recovered by sales will vary widely from project to project.

4. General post-war problems

During the war the rayon industry was classed as an inessential industry and

many of the Company's factories were closed down, while the Research Department was reduced to a very low level of activity. When it was possible to start to

reconstitute the Research Department, it was found that the problems to be dealt

with differed substantially in character and in magnitude from those of the pre-war

period. In the first place the rayon industry was expanding very rapidly in the

1930's and it was clear that the productive capacity of 1939 would be insufficient

to meet the post-war demand. In the second place, the great increase in the cost

of capital equipment had considerably increased the ratio of capital invested to

turnover, so that pre-war plant designs had become completely uneconomic.

Thirdly, many of the raw materials required by the industry were of American

origin, and their purchase was limited by the shortage of dollars, and, finally, con?

siderable changes in the type of product required had taken place during the war,

mainly due to the greatly increased demand for viscose tyre fabric and to the first

successful launching of a synthetic fibre, namely nylon, on a commercial scale in

1940. The general effect of these circumstances was first that a considerable and

immediate expansion of production was required at the minimum capital cost, while existing processes had to be modified to meet the changed pattern of demand;

secondly, that new sources of raw materials had to be found which would render

us less dependent upon imports from dollar countries; and thirdly, as a much




longer term project, that a research programme on new synthetic fibres should be

undertaken.

The major items in the research programme were partly of a relatively short-

term nature which could be solved in some two to five years, while others which

were much more speculative were such that their outcome could not be forecast

and in any case would require many years of work. The short-term programme was

complicated by the fact that the construction of new factories had to proceed before the research programme had got fully under way. It was therefore necessary to anticipate to a large extent the successful completion of researches which, while

promising, were not fully worked out, and to proceed with design, construction

and research simultaneously, in the hope that all the design data assumed would

be substantially confirmed and the projects properly integrated at a late stage in

their construction. A further complication was that our technical headquarters at

Coventry had suffered severe war damage, so that our research and development facilities were insufficient for a research effort on the pre-war scale and were quite

inadequate for the solution of the vastly expanded post-war problems. Owing to

the difficulty in obtaining new buildings in the Coventry area, only a limited

increase in facilities could be provided and this increase was initially confined to

the construction of medium and large pilot plants to implement the short-term

research programme, while the provision of new laboratories has had to be delayed until this year.

The longer term research programme was concerned with seeking for major

changes in the properties of existing fibres and of investigating new synthetic fibres. The great success of nylon after more than ten years of development work

on a very large scale caused an enormous amount of work to be undertaken all over

the world in the hope of repeating that success. It is not very difficult, by expending sufficient time and money, to produce new fibres of one kind or another, but, unless

they have very special technical and economic advantages over existing fibres, it

is not possible to establish them on a sufficiently firm basis or on a sufficiently large scale to ensure their survival. There is no room for a large number of highly priced

speciality fibres. Most proposed new fibres must be rejected at a very early stage, since the cost of the tentative evaluation of a fibre may well run into six figures, and if a project is to be pursued intensively the development costs will run into

millions of pounds. The chances of success in this field are therefore small, but the

prize is so large that a considerable effort is worth while. If it were possible to

dispense with elaborate pilot plants for the production of a new fibre and with the

long and detailed programme required to evaluate its merits, the situation would

be radically changed. But at the moment there seems to be no hope of predicting the properties and usefulness of a fibre from its chemical constitution or from the

physical properties of the bulk polymer. The properties which can be measured in

the laboratory such as the melting point, the strength, the extensibility and the

dyeing characteristics of a fibre serve mainly to indicate whether it is worth

a detailed investigation or not.

It is impossible for me to give briefly a balanced account of the implementation

during the past seven years of the research programme outlined above, and it



294 A. H. Wilson

would also require me to give many more details of the background of the industry if the true significance of the individual items in the programme were to be

intelligible. I shall therefore give instead a description of some typical researches,

choosing them mainly because of their economic importance but also because they can be understood without an intimate knowledge of the technology of the industry.

5. Some typical problems

5-1. The raw materials for the viscose process are wood pulp, caustic soda, carbon

disulphide and sulphuric acid, of which only caustic soda is normally produced from materials indigenous to the United Kingdom. The United Kingdom supplies of pulp are obtained from North America and Scandinavia, and, although on

a global basis the supply and demand for wood pulp are approximately in balance,

there is a deficiency in the supply from non-dollar sources. It was considered that

for national reasons it was desirable to have a source of supply in the sterling area

for some proportion of the wood pulp requirements, and an investigation was

made into possible Commonwealth sources. In general, the high quality wood

pulp required for rayon manufacture, usually known as dissolving pulp, is made

from softwoods such as spruce, whereas the Commonwealth wood supplies are

mainly hardwoods, and the utilization of hardwoods for dissolving pulp is by no

means easy. However, a survey of possible sources of timber showed that Euca?

lyptus saligna, which grows very readily in plantations in parts of Natal and

Zululand, had much to recommend it, and a research programme was instituted

to see whether an acceptable dissolving pulp could be produced from this wood.

The normal method of producing pulp is to separate the cellulose from the lignin

by cooking wood chips in a calcium bisulphite solution in digesters of up to 150 m3

capacity at a temperature of the order of 150? C, and afterwards to purify and

bleach the crude cellulose. Although the pulping process is too complex to be

capable of detailed scientific explanation, the broad chemistry of the process has

become increasingly understood in recent years, the action of the cooking liquor

being to sulphonate the lignin and make it soluble in water. It is thought that the

lignin molecule contains two groups which react differently with sulphur dioxide.

One group reacts in weakly acid solutions and this position is blocked by the

presence of strong acids, while the second group only reacts in strongly acid

solutions. Initially the reaction rate is controlled by the rate of diffusion of liquid into the wood, and since the sulphonic acids produced are very strong acids it is

necessary to add a base in order to prevent the liquid inside the wood from

becoming too acid before all the groups of the first type are sulphonated. An upper limit to the concentration of base is set by the requirement that the amount of

base should only be sufficient to neutralize the sulphonic acid groups in the first

position, so that ultimately the solution will become sufficiently acid to enable the

groups in the second position to be sulphonated. If eucalyptus is cooked under the same conditions as spruce, the reaction is

incomplete, the semi-digested chips are bright red in colour and the yield of good

pulp is small. The general picture of the pulping process given above suggested that the difficulties in cooking eucalyptus were due to itb compact structure and




to the consequential low diffusion rate of the cooking liquor, and more particularly of the base, into the chips. If this is so, then if cooking conditions suitable for

softwoods are used for eucalyptus, strongly acid sulphonic groups will be formed

in the interior of the wood chips and they will not be locally neutralized by the

base which is present in the bulk cooking liquor. The strongly acid conditions thus

produced will favour an undesirable side reaction of the lignin which will interfere

with its subsequent dissolution. The effects of the slow penetration of the base into

the compact eucalyptus structure can, however, be counteracted by increasing the base concentration and by increasing the liquor/wood ratio. In addition the

temperature must be kept low in the early stages of the reaction since the tem?

perature coefficients of the reaction rates are higher than those of the diffusion

constants.

By investigating a wide range of the variables involved, it was found possible to obtain a set of conditions under which satisfactory pulp could be produced while

not increasing the cooking time unduly, though the degree of control required is

greater than usual on account of the susceptibility of the resulting pulp to excessive

degradation. The building of a plant at Umkomaas in Natal to produce 40000 tons

a year of dissolving pulp is proceeding at present in conjunction with Snia Viscosa

of Italy and the Industrial Development Corporation of South Africa. This is

a large project which has involved many considerations (apart from the design data for the chemical process) which differ substantially from those associated

with pulp plants in northern countries. In most areas where pulp plants are

situated, water, for example, is plentiful. In South Africa, on the other hand, large

quantities of perennial water are difficult and expensive to obtain. The local con?

ditions have therefore necessitated a considerable amount of investigational work

to verify the feasibility, both from the technical and from the economic points of

view, of the engineering design proposed, and have given rise to problems which in

many ways are as difficult as those encountered on the purely chemical side of

the project. 5-2. My second example also concerns the supply of raw materials, in this case

for cellulose acetate, the normal raw materials for which are cotton linters (or a mixture of cotton linters and very pure wood cellulose) and acetic anhydride. The latter is commonly made from ethyl alcohol, which has been in relatively short

supply in the United Kingdom for some time. With the development of oil-

cracking installations in this country, the use of acetone, derived from propylene, as an alternative raw material to ethyl alcohol became a possibility. An increase

in the supply of acetic anhydride was in any case essential, and it seemed desirable

to have more than one route to this essential raw material in the absence of any decisive economic factor singling out one route to the exclusion of all others.

The basis of the process is the thermal cracking of acetone to produce ketene

and methane according to the reaction

(CH3)2CO -> CH2: CO + CH4.

The subsequent absorption of ketene in acetic acid gives acetic anhydride. When

it was decided that this process was worth investigating, it was found that the



296 A. H. Wilson

published data were useless for industrial purposes since they referred to cracking conditions in silica or copper tubes. It was therefore necessary to institute

laboratory and pilot-plant work to determine the course of the reaction in the

types of tubes which could be used in a full-scale plant and with rates of gas flows

in the practical range. A 25/20 chromium-nickel steel was selected as the material

with the best thermal and mechanical properties at the temperatures involved, which he in the range 650 to 700? C, but difficulties were immediately encountered

since this steel catalyzes the dehydrogenation reaction

(CH3)2CO -> 3H2 + CO + 2C.

There are, in addition, a number of other side-reactions which also proved trouble?

some. It was therefore first necessary to inhibit the dehydrogenation reaction and

establish conditions in which the main reaction predominates, and then to investi?

gate the efficiency of conversion of acetone to ketene. At the low temperature end

of the cracking range, the ketene-selectivity (the ratio of the amount of acetone

80

GO 10 20

acetone conversion (%)

30

Figure 1. The selectivity of the thermal cracking of acetone to ketene as a function of the acetone decomposed.

converted to ketene to the total amount of acetone consumed) is high, but the

amount of acetone which reacts is small. If the temperature is increased, the

percentage of acetone decomposed is increased but the ketene-selectivity falls off,

mainly due to the secondary decomposition of the ketene itself, the general trend

of the results being as shown in figure 1. It is therefore necessary in an industrial

plant to arrange for the recycling of the unreacted acetone. Since acetone is an

expensive raw material it is desirable to make the ketene-selectivity as high as

possible, but on the other hand if too much acetone is recycled the plant installa?

tion costs are prohibitive. Accordingly, an acetone conversion of 25 % per pass was used for the design of the full-scale plant, since this gave the lowest overall

running cost. The initial operation of the full-scale plant was not without its

difficulties, due mainly to the secondary decomposition of ketene and the manifold

reactions of its decomposition products. These problems were overcome one by




one, and we have now achieved a desirable diversification of the sources of supply of acetic anhydride in this country.

5-3. I mentioned earlier that one of our main preoccupations has been the

extremely high capital cost of post-war plants, which bears very heavily on an

industry with a low turnover. It was therefore necessary to institute a series of

studies of the reaction rates of the various chemical processes concerned to see

whether they could be speeded up, not merely marginally but by large factors.

This comprehensive programme involved both fundamental studies of reaction

kinetics and a detailed investigation of plant capacities. One isolated example of

work of this type is the following. The cellulose acetate used in rayon manufacture is an incompletely substituted

ester, only about 2-4 hydroxyl groups per glucose residue being acetylated. (The number of hydroxyl groups substituted is usually designated by the acetyl value

of the ester. The ' acetyl' value is defined as the percentage of CH3. CO groups

combined in the molecule, expressed in terms of acetic acid. The acetyl values of

62-5 and 48-9 correspond to triacetate and diacetate respectively.) The required

acetyl value of 54 cannot, however, be obtained conveniently in one process and

the reaction is normally carried out in two stages. In the first stage a highly reactive cellulose is acetylated by means of acetic anhydride with sulphuric acid

as a catalyst, the reaction being allowed to proceed until as many of the hydroxyl

groups as possible are substituted. Ideally the product would be cellulose tri?

acetate, but in practice the highest acetyl value reached is about 61.

It is normally desirable to dissolve the cellulose ester as it is formed, and acetic

acid either alone or mixed with methylene chloride is added to the reaction mixture.

When acetylation is complete, it is necessary to carry out a partial hydrolysis of

about one in six of the acetyl groups in the cellulose acetate in order to obtain an

ester with the most desirable properties. Subsequently, the cellulose acetate is

precipitated, washed and dried, while the various solvents are recovered and re?

used. The plant required is expensive and, in the process using methylene chloride

as the solvent, the cost of a single unit of process equipment (excluding services

and buildings) is of the order of ?500000.

In view of the high capital cost of the plant, it was desirable to increase the

reaction rates as far as possible. A detailed analysis of the various reactions

revealed that, by changes in technique, substantial increases in output could be

obtained over those considered normal, the simplest to describe being that

associated with the second stage of the process, namely the hydrolysis. When the acetylation is complete, the reaction mixture contains cellulose acetate,

methylene chloride, acetic acid, acetic anhydride and sulphuric acid. The hydro?

lysis is carried out by adding water and sulphuric acid to the reaction mixture, but in order to avoid precipitation of the cellulose acetate only part of the water

is added at the beginning. Since depolymerization of the cellulose chain occurs

simultaneously with the hydrolysis, the reaction cannot be speeded up indefinitely

merely by raising the temperature, and other methods had to be considered.

An analysis of the hydrolysis against time curve revealed an unexpected feature, since we should expect the rate of hydrolysis to decrease as the reaction proceeds,



298 A. H. Wilson

whereas (curve I, figure 2) the reaction in fact accelerates. A possible explanation of this anomaly is that the reaction instead of being homogeneous is heterogeneous, and this is supported by the configuration of the phase diagram of methylene chloride?acetic acid?water mixtures containing sulphuric acid (figure 3). (The

Figure 2. The rate of hydrolysis of cellulose acetate solutions. Curve I, normal conditions. Curve II, with approximately half the methylene chloride removed.

acetic acid

methylene chloride water 90, sulphuric acid 10

Figure 3. The approximate phase diagram for the system methylene chloride?acetic acid? dilute sulphuric acid. The points I and II refer to the concentrations associated with the curves I and II respectively of figure 2.

ratio of the sulphuric acid to water in the hydrolysis liquors is about 1 to 9.) If we assume that the phase boundary in the actual five-component system containing cellulose acetate is similar to that shown in figure 3, then the liquid in the hydrolysis vessel is an emulsion consisting predominantly of an external phase composed of cellulose acetate, methylene chloride, acetic acid and some water and sulphuric acid together with an internal phase containing the remainder of the water and the

sulphuric acid and some acetic acid. During the hydrolysis, acetic acid is split off from the cellulose acetate and increases the acetic acid/methylene chloride ratio




in the external phase, thereby enabling it to absorb more water and sulphuric acid from the internal phase. In addition, as more hydroxyl groups appear in the

cellulose acetate, the external phase becomes more hydrophilic. As a result, the

internal phase acts as a reservoir of water and sulphuric acid for the external

phase, and as the reaction proceeds there is an increasing concentration of the

hydrolyzing agents in the phase containing the cellulose acetate, and the hydro?

lysis proceeds at an increasing rate.

The above considerations suggested that the hydrolysis could be speeded up, without increasing the rate of depolymerization, by changing the composition of

the liquid during hydrolysis so as to make it homogeneous. This could be effected

by distilling off some of the methylene chloride at the beginning of the hydrolysis instead of at the end. However, a limiting factor is the necessity of avoiding

gelation, and it was found that only about one-half of the methylene chloride could

be safely removed at this stage. This is insufficient to bring the reactants into the

homogeneous region, as is shown by the hydrolysis curve still being concave down?

wards (curve II, figure 2), but the reaction rate is considerably increased.

The application of the principles outlined above in the full scale plant required considerable modifications to the details of the process, but, when all these were

worked out and put into practice, it was found that the shortening of the time-

cycle resulted in an increase in output of the order of 50 %. 5-4. I now turn to an older problem, but one which has dominated the develop?

ment of the viscose yarn industry in the past fifteen years. In the first thirty years of the viscose process, the underlying scientific principles were imperfectly under?

stood. It was, for example, not until 1932 that cellulose was finally accepted to

be a linear polymer. Considerable progress was, however, made by empirical

experimentation, and by the early 1930's the value of a coagulant containing

sulphuric acid, a high concentration of sodium sulphate and a small amount of

zinc sulphate (about 1 %) was recognized. It was also realized that increased

strength could be obtained at the expense of a reduced extensibility by stretching the freshly-formed filament, the work of the rupture remaining more or less

constant while the orientation of the cellulose molecules along the fibre axis was

increased.

When the alkaline solution of cellulose xanthate is extruded into the coagulant,

coagulation begins at the filament surface by a variety of processes which occur

with great rapidity, and a superficial skin is formed. Water then passes from the

interior of the filament under the influence of osmotic forces so that the skin

collapses and the cross-section of the filament may be highly crenellated. Simul?

taneously the constituents of the coagulant diffuse into the filament, and, under

the acid conditions which then prevail, the cellulose xanthate decomposes rapidly and the final product consists entirely of regenerated cellulose. It is important that the orientation of the fibre structure by stretching should take place before

the regeneration to cellulose is completed. The major variables in the production of viscose rayon are as follows:

(a) The composition of the viscose, in which are included the concentrations

of cellulose and alkali, the degree of xanthation and the viscosity.



300 A. H. Wilson

(6) The composition and temperature of the coagulant.

(c) The time of immersion of the filaments in the coagulant as determined by the length of immersion and the speed of take up.

(d) The denier and the cross-section of the individual filaments. (The denier

is the weight in grams of 9000 metres of a filament.)

(e) The degree of stretch applied to the filaments immediately after their

withdrawal from the coagulant.

(/) The conditions under which stretching occurs.

The above factors are interrelated, but they are not all of comparable importance, and the intrinsic properties of the fibre are largely decided by the conditions under

which the initial coagulation takes place, i.e., within the first tenth of a second

of the existence of the fibre. Thereafter, the properties of the fibre can be changed,

but, if one characteristic is improved, another will suffer a consequential change. One of the most important factors is the concentration of zinc sulphate in the

coagulant, since this strongly influences the work of rupture, and an increase in

the concentration of zinc sulphate gives an increased extensibility when all the

other conditions are kept constant (see table 1).

Table 1. Effect of zinc sulphate in the coagulant on the extensibility

of viscose rayon (all the other factors being kept constant)

tenacity % % ZnS04 (g/denier) extensibility

0 1-9 15 1 20 19 2 2-1 23 4 2-1 28

In seeking methods for producing yarns with high works of rupture but with

high strengths instead of with high extensibilities, it was found that a large degree of stretch could be applied to the filament without its breaking by hot drawing the

filament in a bath of dilute sulphuric acid near the boiling point. The effect of the

hot stretch bath appears to be due partially to normal thermal plasticization and

partially to the transient swelling which takes place as the thread enters the hot

liquid. Figure 4 shows the effect of conditions in the stretch bath on the tension

produced by a given degree of stretch. Further, by changing the amount of applied stretch it is possible, as mentioned above, to exchange extensibility for tenacity over a wide range (figure 5).

If the work of the rupture remained strictly constant, the relation between

tenacity and extensibility would be roughly hyperbolic, and to different types of

yarns having different works of rupture would correspond a family of hyperbolas. The experimental data are shown in figure 6 and it is clear that normal viscose

yarn and the high-strength yarns belong to different series. (High tenacity viscose

yarns can be obtained by the ' hot stretch process' outlined above, while the very

high tenacity yarns can be obtained either by stretching and saponifying cellulose

acetate yarns or by decomposing cellulose xanthate in concentrated sulphuric acid.) There is presumably a limiting hyperbola which represents the properties of ideal




cellulose fibres which are free from fine-structure defects. Much of the research

in single-end spinning is concerned with trying to determine this limiting hyperbola and of finding methods for approaching it. The Courtaulds 'hot stretch' process is

one of the successful attempts in this direction, and it forms the basis for the most

I

1

20 40

% applied stretch

Figure 4. The effect of hot dilute sulphuric acid in re?

ducing the tension in a freshly formed viscose filament. 1, normal air stretch; 2, stretch through hot dilute acid.

20

% extension

Figure 5. The effect of spinning stretch on the load?elongation curves of air-dry high tenacity viscose yarns. Arrow shows increasing stretch and orientation.

\x

0 20 40

% extensibility

Figure 6. The tenacities and extensibilities of different types of regenerated cellulose yarns. ?, flax; O, cotton; a, very high tenacity yarns; b, high tenacity yarns; c, textile viscose

yarns.



302 A. H. Wilson

widely used method of producing viscose high-tenacity yarn for tyre fabric, over

75 % of the world's output being produced by this process. It is still not possible to explain precisely the mechanism of the specific action

of zinc in the coagulant as an aid to the development of good mechanical pro?

perties. The observable phenomena are (1) that zinc ions are highly effective

coagulants, (2) that they appear to slow down the decomposition of cellulose

xanthate, (3) that the freshly formed thread is highly shrunk, and has a high cellulose

concentration. These effects may be interdependent and may result from the ability of zinc as a divalent ion temporarily to cross-link adjacent cellulose chains by the

formation of zinc cellulose xanthate. The net effect may well be to avoid too early a consolidation of the fibre-structure by random crystallization and thereby render

it possible for a more ideal structure to be obtained as a result of the stretching

process. There is evidence to suggest that both the crude internal structure of the

filament and the texture of its fine structure, namely, the pattern of the amorphous and crystalline regions, are profoundly affected by the zinc concentration in the

coagulant. 5-5. My last example does not relate to processes already in commercial pro?

duction but to work of longer-term interest arising out of the search for radically new types of synthetic fibres. The most prized of the natural fibres, namely silk

and wool, are proteins, but so far the course of development of man-made fibres

has been to try to obtain desirable textile properties by using natural or synthetic

polymers of quite different chemical compositions from the proteins. The older

man-made fibres are based upon cellulose while the newer synthetic fibres either

in or nearing commercial production are drawn from the poly amide, the polyester and the polyacrylic fields. The synthesis of a true protein fibre is a formidable task,

and the question whether it would ever be economically feasible or not remains to

be answered in the distant future, but a start has been made which has produced some results of scientific interest and which has brought to light the nature of the

problems to be solved.

Under suitable conditions it is possible to stretch wool fibres reversibly by as

much as 100 %, and this has long been interpreted as showing that wool molecules

normally exist in a folded configuration, usually designated an a-configuration, which by stretching can be continuously deformed into an extended ^-configura? tion. Silk, on the other hand, normally only exists in the ^-configuration. These

conclusions are supported by the X-ray diagrams of the fibres, but the molecules

are so complex and the X-ray diagrams so diffuse that it is difficult to obtain as

exact information as one would like.

The study of the simple amino-acids and their dimers, trimers, etc., which are

the constituents of the natural proteins, has provided important information about

bond lengths and angles, but the polymers built up step by step have no fibrous

characteristics, which only become apparent when the degree of polymerization is at least 100. The recent discovery that polypeptides with degrees of polymeriza? tion of the order of 100 to 500 can be obtained by the polymerization or copoly- merization of the carbonic anhydrides of amino-acids has, however, made it possible for considerable further progress to be made. A variety of synthetic polypeptides




of known composition can now be prepared, and the study of their configurations is very much simpler and more unambiguous than it is for naturally occurring

proteins. The main tools available for this study are X-rays and polarized infra-red

radiation. The use of polarized infra-red radiation makes it possible to determine

the direction of the vibrations of the CO andNH groups, while the high crystallinity of some of the polymers enables the crystal structure to be determined from the

X-ray diffraction data with more certainty than is usual in fibres. Since most of

the polypeptides are soluble in dichloracetic acid, fibres or films of polypeptides can readily be spun or cast from solution, and the long molecules can be oriented

by stretching, so that very favourable experimental conditions can be obtained.

When a polypeptide is precipitated from solution in dichloracetic acid it is

normally found to be in a folded form. Immersion in formic acid, however, or

mechanical stretching, generally transforms it into an extended /?-form. The

original a-form can be regained by immersing the ^-modification in suitable

liquids, e.g., m-cresol or dichloracetic acid, or, in many cases, by heating. It is,

therefore, possible to obtain the same polymer in either form and to study the

transitions between them.

The fully extended /?-configuration of a single chain is shown schematically in

figure 7. The various chains are held together by van der Waals forces and by

hydrogen bonds between the NH and CO groups in adjacent chains. The a-con-

CO ORB NH CO CB.E

Figure 7. The extended /^-configuration of a polypeptide chain.

figuration is a spiral in which the hydrogen bonds are formed between distant NH

and CO groups in the same chain, but at present all the detailed evidence available

cannot be fitted completely into any of the possible models that have been

proposed. There is also some doubt as to the exact configuration of the /?-poly-

peptides; the chains are probably not quite fully extended as in figure 6, but may be somewhat collapsed. In view of these uncertainties, in spite of the exceptionally clear nature of the X-ray diffraction data available for synthetic polypeptides

(e.g. poly-L-alanine, R = CH3, and poly-y-methyl-L-glutamate, R = C2H4COOCH3) as compared with natural proteins, it is not surprising that so little progress has

been made in elucidating the details of the structure of fibrous proteins. The unique characteristic of the a-form is a regular folding of the polypeptide

chain involving intra-chain hydrogen bonds. The transformation a -> J5 by formic

acid can be attributed to the breaking of the internal hydrogen bonds by the acid

and their reforming between adjacent chains and perhaps to a smaller extent

between random segments of the same chain. Further, bulky side chains which

prevent the close approach of the main chains will favour the a-configuration, whereas small side chains will favour the /^-configuration. It is noteworthy that

polyglycine, the simplest polypeptide, with R = H, is normally in the /?-form, and

that polyglycine is a large constituent of silk. The difference between the nature



304 A. H. Wilson

of the hydrogen bonds in the two forms leads to a remarkable difference in physical

properties, notably solubility. The <x-forms are in general much more soluble than

the /?, which tend to behave as cross-linked polymers. It is possible that this

difference may have important biological implications. I cannot enter into a discussion of all the interesting points which have arisen

in connexion with polypeptides, and I shall therefore conclude with some general remarks concerning the light which their behaviour may throw upon the dyeing characteristics of fibres.

The natural proteins wool and silk are exceptionally easy to dye, but in general

synthetic fibres do not have good dyeing properties. Since the appearance of

a fabric is so dominated by its colour, difficulty in obtaining the variety, brightness and uniformity of shade required is a serious defect in a fibre, and new dyes and

new methods of dyeing have usually to be investigated when a new fibre is being

developed. In general, dye molecules are large, and unless there are ' holes' in the

fibre structure it may not be possible to introduce the dye molecules into the fibre

at all. A highly crystalline polymer may therefore have too compact a structure

to admit the entry of dye molecules, but if the fibre contains amorphous regions which swell in water, dye molecules can be introduced into the interior of the fibre.

Unless, however, the dye molecules have an affinity for the fibre or are rendered

insoluble, as are vat dyes, they will be too easily removed by washing to be of

any value.

The dyeing of cellulosic fibres is reasonably well understood. The fibres have

considerable amorphous regions and are swollen to a greater or less extent by

water, while the hydroxyl groups can form hydrogen bonds with dyes of suitable

composition. Our knowledge of the dyeing of synthetic fibres is in a more rudi?

mentary state, but the study of the behaviour of polypeptides promises to elucidate

some of the factors involved. In synthetic polypeptides there are three types of

possible dye sites, namely the main polypeptide chain (the backbone), the amino

groups at the ends of the chains, and the side chains. These latter and the properties of the amino end-groups can be varied, as can the accessibility of the different

sites, so that the phenomena are of considerable complexity, and only preliminary work has so far been carried out. The main point which has emerged is that,

although the polypeptide backbone is hydrophilic and has a strong affinity for

most dyes, the presence of sufficient hydrophobic side chains can reduce the

swelling of the fibre to a negligible amount and can screen the backbone entirely. The accessibility of the sites is therefore likely to be the dominant factor in

determining the dyeing behaviour of synthetic protein fibres. In any fibre it is

necessary to have a sufficient number of groups which have an affinity for a dye and it is also necessary for the structure of the fibre to be such that these groups are accessible.

6. Conclusion

Radical improvements to existing fibres or the production of a new fibre will

involve problems similar to those outlined above, and the technological problems which I have barely touched upon will be at least as formidable. If the fibre is



Wilson Proc. Roy. Soc. B9 volume 142, plate 23

Figure 8. Single-end viscose spinning machines: upper, spinning and after-treatments; lower, spinning, stretching and collecting only.

(Facing p. 304)



Wilson Proc. Roy. Soc. B, volume 142, plate 24

Figure 9. Multi-end viscose spinning machine showing upper the regeneration bath, and lower the after-treatment stages.




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based upon a natural polymer the production of the polymer in a suitable form

on a large scale has to be faced, whereas, if the fibre is to be a purely synthetic one, methods of preparation of the monomer and of the subsequent polymerization must be worked out. On account of the large scale of production required, the

chemical engineering aspects of the processes to be investigated will be as im?

portant as the purely chemical ones.

The spinning process may be comparatively simple, as for melt-spun fibres

where the controllable factors are relatively few, or they may be exceedingly

complex, as for wet-spun fibres where chemical reactions take place. They are,

however, intimately bound up with the properties of the polymer, and it is not

possible, in the present state of knowledge, to specify the exact characteristics

that the polymer must have, so that the spinning techniques and methods of

polymer production react upon one another to a very considerable extent. Finally, the fibre must be capable of being woven or knitted into a fabric by more or less

conventional methods and of being dyed in a wide variety of colours with an

acceptable degree of fastness.

It will be seen from the examples given that the study of man-made fibres

covers a very wide range of scientific knowledge. Further, the research programme must be on a large scale if significant results are to be achieved, and since the sum

of money involved in carrying out the work and more particularly in utilizing the

results obtained are considerable, the formulation of a research programme depends as much upon economic considerations as upon scientific possibilities.

Vol. 142. B.



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