BY HERMAN MARK
THE purpose of this article is:
(1) to review briefly the-outstanding properties and production possibilities of a number of new fully synthetic fibers;(2) to compare them with synthetic fibers based on cellulose; and
(3) to attempt an outline of the advances cellulose chemistry and technology will have to achieve if fibers based on this material are to compete successfully with the new products to be considered under (1) below.
During the last twenty years chemical industry has started producing a number of highpolymeric substances which have proved very suitable for processing into fibers and films. These highpolymers are being made from coal, oil, air, and other basic raw materials by means of a two-step process. First, from the basic raw materials are produced low molecular weight intermediates, the so-called basic units or monomers. From them the polymers are then built up by a process of polymerization, which unites the small basic units having molecular weights around one hundred by strong chemical bonds to form large molecules having molecular weights of several hundred thousands and, in most cases, possessing the character of a long flexible chain. The art of polymerization has been greatly developed during the last 10 or 15 years and there can be little doubt that it will further develop in the near future. Many leading chemists in all countries feel themselves attracted by the peculiar nature of the polymerization reactions, and extensive research is being done in this field in many universities and public research institutions. It is significant for the general interest shown that many seminars, symposia, and special conferences are being held every year to discuss problems of polymer production, characterization, and utilization. A special Institute of Polymer Research has been created at the Polytechnic Institute of Brooklyn, and a Journal of Polymer Science has been published in the United States of America since 1946. This public interest is stressed by intensive research work in many industrial organizations, and it is probably a conservative estimate to put the total number of highly qualified research chemists, physicists, and engineers specialized in this field at around 5,000 to 6,000. As a consequence of this interest and of the resulting research activities, it seems safe to predict that within a short period:
(a) The price of presently known basic materials, such as dibasic acids, diamines, glycols, vinyl and acrylic derivatives, will gradually decrease and may go down in particularly favorable cases to between $0.101 and $0.20 per pound (1 lb. = 0.4536 kg.).
1 The currency referred to throughout this report is the U. S. dollar.
(b) The efficiency and economy of polymerization techniques will gradually improve: this is bound to lead to better and cheaper polymers. Again, it seems fair to assume that the cost of the polymerization step may level off at $0.05 to $0.10 per lb. of the final polymer, so that in favorable cases the price of fully synthetic fiber formers may reach levels ranging as low as from $0.15 to $0.30 per lb.
(c) Entirely new monomers and polymerization processes will be discovered, which will either make possible even cheaper fiber-forming polymers than those mentioned under (b) or will enable us to produce fully synthetic fibers of greatly superior properties which could be used for purposes inaccessible to the present artificial fibers based on cellulose, such as viscose rayon, copperammonia rayon, and cellulose ester rayon.
With this probable development pending in the not too distant future, it is interesting to consider the question: How far will cellulose as a successful competitive polymer for fiber formation be endangered by the synthetic polymers and how can its position be maintained in the future or even improved by active and well co-ordinated research in the cellulose field?
Consideration of this question should start by an enumeration of the fully synthetic fibers, which have been developed recently and are beginning to invade the various branches of the textile industry.
The most important of them is nylon. Nylon is a generic term for a large class of polymeric compounds. The essential discoveries were made by Carothers and his associates in the laboratories of E. I. du Pont de Nemours and Company about twenty years ago.2 Nylons are produced by combining dibasic acids with diamines or glycols and constitute high-melting substances (melting points around and above 250° C.) of high resistance to the attack of acids, alkalis, and or genie solvents. These polymers may be spun into fine filaments or east into thin films, either from solutions or directly from the melt. Generally they are drawn and twisted after the spinning process to obtain very strong and tough fibers of a pleasant luster that compare favorably with all natural fibers and with all artificial filaments produced from materials based on cellulose and its derivatives. The table shows some figures to outline this situation. To date many hundred different nylons have been synthesized and it has been found that they cover a whole range of fiber formers, out of which it is possible to spin filaments of widely varying properties. Thus research chemists of the du Pont Company3 and of the Bell Telephone Company4 have found that filaments and fibers of certain nylon types can be produced having a low initial modulus and a high reversible elongation. In their behavior these samples come very close to natural or synthetic rubbers. On the other hand, it has been possible to produce fibers of great stiffness and high tensile strength by using aromatic dicarboxylic acids5 in the formation of the polymer.
2 W. H. Carothers, Collected Papers on Highpolymeric Substances. Edited by II. Mark and G. S. Whitby. New York: Interscience Publishing Company. 1940.3 See lectures given by Dr. Whitbacker at various meetings during 1945 and 1946 of the American Chemical Society and the American Association for Advancement of Science.
4 See, for example, W. O. Baker and C. S. Fuller, "Macromolecular disorder in linear polyamides. Relations of structure to physical properties of copolyamides," Journal of American Chemical Society, 64 (1942) 2399.
5 See, for example W. T. Astbury and C. J. Brown. "Structure of Terylene," Nature, 158 (1946) 871.
Other interesting fully synthetic fibers have been produced recently by spinning solutions or melts of polyvinyl derivatives and polyacrylic derivatives, such particularly as polyvinylchloride, polyvinylidene chloride, polyacrylonitrile, and copolymers from them. Fibers and films of these materials have already been produced in commercial quantities and have appeared on the market under the names of Vinyon, Saran, Fiber A, P-C fiber, etc. They also cover a wide range of properties, from soft, pliable, and highly elastic films to rigid, strong, and tough fibers. Most materials are highly resistant against attack by any chemical and some of them exhibit excellent stability against oxidation and sunlight.
The table also contains a few figures comparing representative values of cellulose fibers with those fibers based on polyvinyl and polyacrylic derivatives.
The figures presented in this table and the remarks contained in the preceding paragraphs concerning the general properties of fully synthetic fibers raise the question: What are the fundamental molecular properties of these synthetic fiber-forming polymers responsible for their outstanding mechanical, thermal, and chemical behavior? And, in what respect does cellulose differ from them, if at all?
There are a number of factors characterizing a polymeric material and it may be best to start by listing the most important of them.
First, there is the molecular weight of the polymer. Experience has shown that really good mechanical properties can be expected only from polymers of high molecular weight and that, in general, the properties improve, roughly, in proportion to the molecular weight. The molecular weight is also proportional to the length of the chain which each polymer molecule represents; the greater this length, the stronger, generally, the mutual cohesion between adjacent polymer molecules in the fiber and the higher the latter's tensile strength. Our present knowledge of polymerization techniques makes it possible to produce polymers of greatly different molecular weights and, in particular, enables us to synthesize materials having a high average molecular weight.
In the case of cellulose, we use a polymer that occurs in nature with a very high molecular weight and degrade it during the various steps of manufacturing into a mixture of larger and smaller molecules, which is then processed into fibers or films. In order to get the best fiber out of a given polymer, it is essential not only that the average molecular weight be of a certain sufficiently high value, but also that the shape of the molecular weight distribution curve be such that the best possible mechanical properties should result. Preponderant in this respect appears to be the absence of any kind of material of very low molecular weight. There are indications that the molecular weight distribution curves of fully synthetic polymers are more favorable for the development of optimum textile properties than those of the cellulose polymers being used at present for the production of the various types of rayon.
Another important factor for the final properties of fibers or films made from any given polymer is the chemical character of the polymer. The long chain molecules or cellulose are linked together by oxygen bridges formed during the growth of the molecule by the elimination of water. These so-called glucosidic bonds are very stable towards all organic solvents, are stable to alkali in the absence of oxygen, but are rather easily attacked by acids. As a consequence, fibers made from cellulose deteriorate rapidly under the influence of weaker acids even, whereas some of the fully synthetic fibers are nearly completely acid-resistant. This property is closely connected with the intrinsic chemical structure of cellulose as a polysaccharide and it seems, therefore, that there is no way to escape the unpleasant consequences of this fact. Recently, however, it has been found that it is possible to stabilize cellulose fibers against acids by impregnating them with small amounts of certain synthetic resins, which penetrate most of the fine cracks and capillaries of the fiber, cover the internal surface with a very thin acid-resistant film, and thereby protect the cellulosic material from hydrolytic deterioration.
Then there is the internal flexibility of the long chain molecules of the polymer. Experience has shown that flexible chain molecules can be more easily spun and drawn into fibers and east into films than the intrinsically stiff molecules of cellulose. Conversely, however, stiff chain molecules produce fibers of a higher initial modulus and a higher ultimate tenacity. The molecules of nylons and polyvinyl derivatives are essentially flexible and this property explains the fact that fibers and films of very high, reversible elasticity are relatively easy to produce from such polymers. On the other hand, the molecules of cellulose, consisting of a chain of consecutive glucose rings, are intrinsically stiff and have a strong tendency to aggregate into lightly packed bundles, the so-called cellulose micells. It is not possible to produce from cellulose, therefore, fibers possessing the rubber character of certain nylons or the high resilience characteristic of wool or of certain fully synthetic fibers made from polyvinyl and polyacrylic derivatives.
The last factor to be discussed is the specific molecular attraction between the polymer molecules. This attraction depends upon the nature of functional groups (OH, COOH, Cl, CH3, CONH) distributed along the chain molecules. It is of great importance for the ultimate mechanical properties of the fibers or films produced from a given polymer. On the one hand, strong lateral attraction between the individual chain molecules is necessary to provide high tensile strength; on the other hand, too much crossbonding is often responsible for brittleness and breakage at low elongation. The chain molecules of cellulose with their large number of closely spaced hydroxyl groups exhibit a rather high degree of lateral cohesion which, together with the intrinsic stiffness of the chains, causes filaments and fibers of cellulose to be relatively stiff and unextensible, though having very desirable properties as far as initial modulus and ultimate tensile strength are concerned.
Considering the various points discussed above we may summarize the position of cellulose as follows:
Average molecular weight: initially very large; has to be properly reduced during the various pulping and purification processes without producing too many low molecular weight fractions.
Chemical nature of the bonds keeping the backbone chains together: The glucosidic bond is intrinsically sensitive to acids and alkali in the presence of oxygen. This sensitivity can be reduced by the use of protective resins and by a very tight packing of the chains in the fiber (high degree of orientation of the chain molecules).
Internal flexibility of the chain molecules: The chains of cellulose are less flexible than those of other polymers such as nylon or vinyon. This leads to an intrinsic stiffness of fibers and films made out of cellulose which, to a certain extent, can be counterbalanced by the use of plasticizers.
Chain-to-chain attraction: Cellulose molecules exhibit a very strong chain-to-chain attraction, which leads to a high initial modulus and a high ultimate tensile strength but is responsible at the same time for stiffness and brittleness. Substitution of the hydroxyl groups and the use of plasticizers are means of avoiding brittleness in highly oriented cellulose fibers.
The above outline makes it apparent that cellulose has certain intrinsic advantages as fiber former, such as high molecular weight and strong lateral cohesion, but also that it has certain disadvantages as compared with fully synthetic polymers, namely the weakness of the glucosidic bond and the intrinsic stiffness of the chains.
As far as economic considerations are concerned, cellulose should be able to compete successfully even with the cheaper fully synthetic fiber formers, if one considers that in 1939 a very high-grade wood pulp for rayon production was to be had for $0.04 to $0.05 per lb. and that even now high-grade wood cellulose is marketed at not more than $0.08 to $0.10 per lb.
These comparatively low prices would assure cellulose a strong position as a fiber former if they could be in the future combined with:
(a) A molecular weight distribution curve that would exclude the presence of highly degraded constituents.(b) The absence of any groups that would weaken the glucosidic bond beyond its normal sensitivity. Such groups are particularly carboxyl and carbonyl groups attached to the main chains.
(c) A pretreatment of the chains that would replace a fraction of the hydroxyl groups by substituents less prone to lateral attraction, acting therefore as an internal plasticizer.
A considerable amount of fundamental and practical research is being carried out in the cellulose-producing industry along the lines mentioned above, aiming at an improved raw material for the rayon and cellophane producers. It seems probable that gradual advance will be made towards a highly purified, pretreated wood pulp with a modified distribution curve. Such a material should be capable of maintaining, at least for the immediate future, the position of cellulose as one of the most important fiber formers even in the face of mounting competition from fully synthetic polymers.
Finally, it should be pointed out that in many cases the problem of best quality at cheapest price will not be solved by contrasting the use of cellulose with that of a full synthetic polymer, but by using cellulose together with full synthetic polymers such as resins, plasticizers, etc. This combination of the properties inherent in cellulose, a cheap raw material, with those contributed by the somewhat more expensive but more adjustable synthetic polymers has already led to very noticeable results and will, probably, in future dominate more and more the thinking and planning of fiber chemists and technologists.
SOME PROPERTIES OF FULLY SYNTHETIC FIBERS OF THE NYLON AND VINYL TYPE AS COMPARED WITH VISCOSE RAYON FIBERS MADE OUT 0P CELLULOSE.
|
Property |
Viscose Rayon Type Fiber |
Nylon Type Fiber |
Vinyl or Acrylic Type Fiber |
|
Tensile strength dry |
from 1.5 to 5.0 grams per denier |
from 1.5 to 8.0 grams per denier |
from 1.5 to 5.0 grams per denier |
|
Tensile strength wet |
between 30 and 75% of dry strength |
almost the same as dry strength |
almost the same as dry strength |
|
Elongation to break |
from 5 to 25% |
from 10 to 200% |
from 10 to 200% |
|
Range of elastic recovery |
up to 2 or 3% |
up to several 100% |
up to a few 100% |
|
Resistance against - |
|||
|
acids |
poor |
good |
excellent |
|
alkalis |
moderate |
good |
excellent |
|
organic solvents |
excellent |
excellent |
depending upon solvent |
|
heat |
good |
excellent |
moderate |
