Synthetic Resin Design ALBERT G . CHENICEK Interchemical Corporation, New York City
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YNTHETIC resins were known long before chemists became interested in making them. Obtained as undesirable end products during chemical reactions carried out for other Purposes, such products were described as tars, pitches, amorphous materials, gums, syrups, and so on. They are still undesirable in most instances because the chemist has learned how to produce superior, more useful materials. However, the means by which these improved products are formed are the same as those which gave products destined for the waste jar. The chemist has now learned how to choose his starting materials and to control their reaction to give a substance the properties he desires. He did that by making innumerable resinous materials, as did his predecessors, but in addition, he applied a new knowledge of chemical reactions to what he found. Then by picking a few combinations here and there he succeeded in making something that could be sold for purposes other than covering roofs or paving roads. The ultimate goal of a resin chemist is to be able to "tailor make" a resin with specific properties by proper choice of raw materials and conditions of reaction. He is on the way, bnt the path is faint and the obstacles
mter the
DESK-TOP CHEMISTRY
idea had been the necessity polyfunctionality for resin formation was easy to understand. If each molecule has only one functional group and i t is made to react with another monofunctional
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I'K:UKK ~.-MOI.HCLII.HS OF I)II'YI;KRSI PUNCTIONALITV CA The procedure of ,+, stir, heat, and hope_came to H E K e ~ n n s n ~ rnv ~ nH ~ m n h . 5( I ~ ~ P I : N C T K N A L )AND PPRK an end when it was recognized .that t h e process of resin CLIPS ~ R I P V X C T I I I S A I . ) formation is one in which molecules react with each other and more molecules react with each successive compound, a larger molecule is formed which has UI, product to form a long chain, the agglomeration of residual functionality and cannot react further to chains making up the resin. A consideration of the produce a chain. To illustrate this, suppose our moleaccumulated types of compounds and reactions which cules are hairpins (get some, if you can, and try this led to resins brought out the concept of functionality, yourself) each loop of which corresponds to a functional a concept fathered to a large extent by the work of group in a real molecule (Figure I ) . We find that twc~ Kienle and Carothers. of the hairpins can be hooked together but the open Kienle, among others, worked on the production of ends prevent the building up of a chain. The reaction resinous materials by the esterification of alcohols with we have carried out corresponds to that of ethyl alcohol acids. Such reactions were old, hut i t was found neces- with acetic add to give ethyl acetate but no resin molesary to have a t least two hydroxyl groups in the alcohol cule. (Because such an esterification reaction involves and two carboxyl groups in the acid before a resin could the loss of a molecule of water, this illustration of a be obtained. In other words, each reactant had to be condensation reaction, and those following, are not bifunctional. This was the basic requirement for resin strictly applicable in terms of mechanism of reaction. formation. The primary purpose of these examples is to emphasize Carothers extended the idea of functionality to the role of functionality.) include many other types of reaction. He was able Let us take an imaginary bifunctional molecule, a to prepare polymers by such common reactions of or- paper clip, and react it with some hairpins. We can ganic chemistry as esterification, amide, acetal, and hook two hairpins to one clip to give a chain of three anhydride formation, and etherification, by always molecules, but can go no further. In this instance bearing in mind that i t was necessary to have molecules we reacted ethyl alcohol with phthalic acid to form containing a t least two groups which were capable of diethyl phthalate, but again, no resin molecule resulted. You know now that you produced imaginary resin reaction. Thus order came to the process of resinification. molecules when you made chains of paper clips at some
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time or other. For the sake of a more accurate analogy, suppose we take two different sizes and make the rule that a big clip can react only with a little one. Here we have two different molecules which can react with each other, but not with themselves, and we get a resin molecule. Such is the reaction between phthalic auhydride and ethylene glycol to give an alkyd resin molecule. If we build enough chains and mix them together we have an imaginary resin. Instead of two different molecules one kind of molecule containing two different groups can be used. Under the proper conditions either combination gives resins. To get back to our paper clips again: in place of the different clips used before, we shall take all one size, but impose the restriction that only unlike ends can be hooked together. We can still build a chain if we turn the clips the right way. Our chain is an example of the reaction product of, let us say, glycollic acid with itself. This acid has both a hydroxyl and a carbl~xylgroup, permitting inter-esterification. There is an additional kind of bifunctional molecule typified by styrene. Bifunctionality is obtained by the opening of a double bond to form two reactive ends. For the purpose of illustration we shall use safety pins, but it should be remembered that this, like other analogies, may fail if carried too far. Each bar of the safety pin represents a bond. With a little pressure corresponding to the required activation energy, one bond can be opened. The free point representing one functional group can then he inserted into the opening in the head (the second functional group) of a second pin and so on to form a chain. This serves to present a rough picture of addition polymerization made possible by the hifunctionality of the double bond. RESIN NETWORKS
Until now we have considered only the classes of monomeric compounds which react to form thermoplastic, soluble resins. Insoluble thermosettine resins may be derived from molecule; which possess more than
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two functional groups. Long-chain molecules which constitute a resin can be separated by the intrusion of solvent molecules or by the activating &ect of heat. If, however, these chains become fastened to each other by cross-linking made possible by additional functionality, then a network is formed which prevents such separation of the chains. The resin is then thermoset and usually resistant to the action of solvents and heat. To demonstrate the process of cross-linking we will need paper clips with three or more corners all of which can be connected to other clips (Figure 2). In the complex structure resulting all the molecules are attached to each other through chains of varying length. Thus any piece of well-cured resin may be considered to consist of one large molecule. It will be noted that all the molecules making up the resin need not be trifunctional; a few such molecules will sufficeto produce a network by cross-linking. The fewer the cross-links, and therefore the larger the openings between chains, the more readily will the resin he affected by solvents and heat. We have been able to control the growth of our imaginary molecular chains by putting the molecules together as we wished. In actual chemical reactions between polyfnnctional molecules, however, resins are not always obtained. Even in the presence of activating agents such as catalysts, heat, and light two similar reactions can take place leading to nonresinons products. These reactions both involve cyclization of the reacting molecules. This simply means that the reactive ends of the same molecule, the functional groups involved in the polymerization, become hooked together and functionality is lost. Make a bracelet with paper clips and you have no ends to which to add more clips. It is important to realize that with molecules having a functionality higher than two there are still reactive points even in a cyclized molecule and therefore further reaction can proceed. Ring formation can take place after a number of molecules have reacted together or i t may occur within one molecule if its functional groups are capable of reaction with each other. Such reactions leading to relatively simple molecules are more likely to take place in dilute solution where the chance of ends of the same molecule coming together is greater than that of their contacting other molecules because of greater separation by solvent molecules. Another factor which prevents resin formation is commonly called steric hindrance. In examples where hindrance occurs the structure of the molecules involved is such that the functional groups are shielded by other groups and reaction cannot take place. I t is like trying to reach a dime dropped through a grating --one's hand just will not go through. In addition to these complications, some molecules, which theoretically possess the requisite functionality and seemingly are not subject to steric hindrance, just will not react. These compounds generally have isolated double bonds which appear to need more energy for reaction than is suo~liedunder conditions which do not decompose the molecule itself
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Many of the commercial resins developed in the past were first synthesized and then utilized for whatever purpose they happened to he suited. On the chemists' drawing boards is the molecular architecture of resins of the future. The designs are governed by specifications for certain uses; they are based upon general principles relating physical structure and chemical nature to physical properties. Knowledge of these connections has not advanced far enough to permit accurate prediction of the properties of a finished resin by inspection of a formula on paper; nevertheless, enough is known to limit choice of materials and conditions of preparation to obtain a resin of certain properties and to serve as a guide for further changes. The foundations of this embryo science of resin design rest Rom~ HEXAME AM ETHYLENE DIAMINE A N D ADIPIC ACIDFORM upon an understanding of how size, shape, arrangement, POLYAM~D RESIN(NYLON). and composition of the molecules-the building blocks The second reaction is that of the bifunctional com- -influence the properties of resinous substances. pound, formaldehyde, with the tetrafunctional comFORCES BETWEEN MOLECULES pound, nrea, to form a urea resin. The high functionFor the present purpose a resin may be defined as a ality of the urea permits the formation of cross-links collection of molecules, generally of different sizes, and results finally in the production of a nonthermohaving a relatively high molecular weight. The forces plastic insoluble resin. The mechanism by which this which bind the large resin molecules together to make occurs is a subject of debate but the schematic representhe mass of resin are important factors in determining tation in Figure 4 will serve to illustrate the essential its physical properties. These attractive forces are points.
SUCCESSFUL CHAINS Let us look a t two reactions which have been mastered sufficiently to produce resins on a commercial scale. When the bifunctional amine, hexamethylene diamine, reacts with the bifunctional acid, adipic acid, the result is a linear, thermoplastic superpolyamide known as nylon. (See Figure 3.)
These reactions ditrerentiate between two main types of resins. Nylon, polystyrene, Lucite, Vinylite, and Saran, and other thermoplastic commercial resins are produced if the functionality is two. Functionality greater than two results in thermosetting products such as nrea, phenolic, and alkyd resins. Polyfunctionality is mandatory for resin formation, while the degree of functionality determines the type of resin obtained. Thus the complexities of resin chemistry are systematized in large part by these simple principles. Proper functionality, then, is the first consideration in choosing starting materials for preparing a resin. The next problem is to build into this resin the properties which will fit it for specific practical applications.
termed secondary valences or van der Waals' forces, as contrasted to primary valences which form the chemical bonds between atoms such as those which constitute the resin molecules (Figure 5). Secondary valences result from the presence of electrons in the outer electronic shell or orbit of atoms; these electrons are not a part of the primary valence bonds. If we magnetize a number of chains of paper clips to make them cling together, we will have a rough analogy of how secondary valences bind molecules together. In orpanic com~onndsthe most common atoms which " contain such "unshared" electrons are oxygen, nitrogen, sulfur, and chlorine. Atoms of this type are termed polar and their presence in a molecule leads to unbalanced electrical fields within the molecule, i. e., polarity. Table 1 lists common groups containing polar atoms in increasing order of their effect in binding molecules together.
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Completely substituted carbon atoms have all four
up of molecules of low polarity would have little strength, while resins containing many polar groups have none t o contribute to secondary valences. In a would be tough. This generalization is subject to carbon-carbon double bond the two pairs of electrons many exceptions, because other factors in addition to shared between the two atoms, especially if the atoms degree of association determine the strength of resins. are unsymmetrically substituted, cause a state of This is illustrated by answering the question, "How polarity intermediate between that of a saturated car- can hydrocarbon resins be hard, brittle products, such bon atom and an atom like oxygen. as polyindene, tough plastic materials like polystyrene, The presence of polar atoms in molecules will affect as well as elastic, like rubber?" Here is a series of the attractive forces between them. Their con- resinous materials, all of low polarity, varying conceutration per unit volume of resin will determine the siderably in properties. The answer lies in the length degree of association of the molecules. As the num- of the polymer chains and the shape of these chains. ber of polar atoms increases in a molecule there is a It has been well established that resins such as polycorresponding increase in the number of points of at- styrene do not acquire the property of toughness until tachment by means of secondary valences. the average molecular weight of the molecules exceeds hlolecules may, of course, become attached through a certain value, roughly 8000. The very low molecular primary valences, in which case they become part of weight polystyrenes and polyindenes formed by the the same molecule. I n such cases primary valences use of acid catalvsts are exceedindv ', ,brittle. T o underare the result of higher functionality. Resins of this stand this variation, visualize two strings, one comtype constitute the thermosetting class which will be posed of long fibers and the other of short. T o break discussed later. the string with long fibers requires more force, because fibers must be broken, whilethe short ones are easily LENGTH AND S H A P E OF CHAINS pulled apart (Table 2). What happens to our building blocks when force is while they have high molecular weight, the mole. to a resin de~erldsupon the quality of Our cules of rubbery hydrocarbons-elastomersare not electronic "mortar." T o break a resin requires enough polymers, but are, instead, actually force t o separate molecules and possiblr to break kinked and curled polymers: Stretching an elastomer atomic bonds. One would conclude that resins made straightens the molecules into a parallel alignment, TAH1.E 1 I t was not until x-ray methods were applied t o resiCoansrva *:rru.:,an us XorHcuLAn Gnvurr nous cohesion E . ~ ~ ~ ~materials that experimental evidence for strucGOUP (c~I./M~I.I tural regularity of their molecules was obtained. Some Methylene -CH, 990 1,680 resins under certain conditions give x-ray diffraction 4 ~th~ e t h y ~ -CH* 1,7110 patterns similar to those obtained from crystalline Chloride -CI 8.40D ~~i~~ NH:, 3,530 substances (Figure 6). This means that the molecules Ketone =CO 1,270 of the resins, as in crystals, are arranged in a definite, Aldehyde -CHO 4.71IU E E ~ ~C O ~ OCHI a.600 regular manner. Stretched rubber shows this crystalAcid -COOH 8,970 line behavior while unstretched rubber does not 1U.UIIlI lmide =CNH id^ C ~ > W H I 13.200 (Figure 7). While the molecules can be forced into of their outer electrons as part of chemical bonds and
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TABLE 2 CH*N*E
T ~ ~(guttxp r~r c h . ~ Polystyrene Polystyrene Polymethyl metharrylstr Polyamid (nylon)
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Pala"1y 9cr Unil Volunrr Low
W~lerulor Weight High
Resin Cia-pdyisuprene (rubber) ~
nigh Low High High Hish High
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~ LOW LOW LOW Medium Medium High
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iYIolerulnr Arrowemcnl Un3trctehcd-Rendom Stretehed-oriented ~ ~ ~ ~ ~~~d~~ Random Random Random Oriented (eri.talllne) Three dimensional, moss-linked
Rain Protnirr Elastic. oft ~ mantie ~ ~ ~ mastic, hard, brittle Plastic, hard, tough Plastic, hard, tough Tough Sber Infusible. inroluble. hlrd. brittle
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alignment by the process of stretching they do not remain in that state when the external force is released. This is because the forces between the hydrocarbon molecules in the stretched position are less than the forces which cause contraction. It should be mentioned that the sulfur cross-links in vulcanized rubber probably assist in pulling the molecules back to their original state. The use of large / amounts of sulfur, resulting in increased cross-linking, gives hard rubber. With molecules of greater polarity, cohesive forces are increased, elastic properties are lost, and the applied forces lead to fairly permanent deformation. With many thermoplastic resins the deformation is not entirely permanent because these exhibit the phenomenon of "elastic memory." Thus they may be deformed by the use of heat and pressure and retain their new shape for some time, but slowly at ordinary temperatures and more rapidly a t elevated temperatures they tend to return to their original form. Long molecules containing strongly polar groups that are not highly concentrated comprise fiber-forming resins, if their shape is such that they can be fitted together. To get maximum strength in a fiber the filaments of resin are stretched. As with rubber this procedure results in a regular alignment of molecules as shown by the crystal-like pattern of the x-ray diagram. Unlike rubber, however, the molecules do not return to their original position. The reasons seem to be that the forces due to secondary valences are strong enough to hold the molecules in their new positions and the molecules fit together more closely. I t is quite likely that molecules of fiber-forming resins are not kinked and cwled so much as those of rubber and therefore the elastic force to be overcome isles;. CLASSIFICATION OF RBSINS
O F RUBBER (CIS-POLYISOPRENE) AT FIGURE 8.-STRUCTURES TOP,AND THE PLASTIC. GUTTAPERCHA (TRANS-POLYISOPRENE), AT BOTTOM. SPATIAL ARRANGRMENTS HELP EXPLAIN DmWRENT PROPERTIES.
it easier for the chains to fit together and probably does not allow for as much curling of the molecules as the cis-arrangement. When stretched, therefore, the rubber molecules can slip past each other more easily than gutta percha molecules, and in addition the force pulling them back to their original position is greater. Both of these conditions result in greater elasticity. For fibers, the polarity per unit volume must be greater than that of elastomers. A balance must be attained, however, because excessive polarity results in brittleness; if too low, the chains do not fit together well and fail to remain in a "crystallized" state after stretching. The molecular size must also be great enough to provide flexibility. The class of plastics then consists of polymers made up of molecules of all degrees of polarity. It is only when special properties are developed by means of a combination of polarity, molecular size, and molecular shape that the material is called an elastomer or a fiber.
Whether or not a resin will fall into the class of fibers, plastics, or elastomers, therefore, will depend upon the polarity per unit volume, the molecular size, OTHER PHYSICAL PROPERTIES and molecular shape. No single one of these variables The interpretation of such properties of plastics as will determine the nature of the product; the properties of a resin depend upon all three. Molecules of hardness, impact strength, adhesion, softening point, low polarity tend to give elastomers if the chains are solubility, and abrasion resistance in terms of moleculong enough and if the molecules are curled so that the lar size, shape, arrangement, and polarity is a more chains do not fit together well. If all these conditions difficult matter. It is perplexing part& because these are not met the result will be a plastic rather than an properties are not sharply defined, but the real difficulty elastomer. The difference between rubber and gutta is lack of knowledge. Measurements, developed mainly percha illustrates this (Figure 8). Here are two sub- from the practical viewpoint, are usuaUy a result of stances of the same chemical composition (polymerized combined properties rather than one alone. The polymers of a series of esters of methacrylic acid isoprene) and both of high molecular weight, hut rubber is highly elastic while gutta percba is a plastic. The with diierent alcohols illustrate these changes in propdifference lies in the shape of the molecules. These erties (Table 3). As the length of the alcohol chain is isoprene units of rubber are linked together in such a increased the softening point decreases and there is an way that the largest groups on the carbon atoms joined increase in flexibility. When the chain is long enough, by the double bonds have a &-arrangement. In gutta the polymer is a liquid. Lengthening the hydrocarbon percha the groups have a trans-arrangement. Cis chain of the alcohol acts to decrease the number of means that these groups are projected from the mole- secondary valences per unit volume. As the number cule in the same plane, while trans indicates that they of points of attachment is thereby reduced, less energy are in dzerent planes. The trans-arrangement makes or work (as heat) is required to separate the molecules
FIGURE9.-VINYL CHLORIDE POLYMERIZATION SHOI\.N WITH Am OF FISHER-HIRSCHFELDER ATOMMODELS
and they become soft a t lower temperatures. This is analogous to the method used to obtain the values in Table 1 which were calculated from the heat required to vaporize liquids. This heat is a measure of the energy needed to separate the molecules far enough for the liquid to become a vapor.
The change from thermoplastic to thermosetting resins is one of linking the polymer molecules through primary bonds. This cross-linking was illustrated by the three-comered paper clips (Figure 2). Experimental facts uphold the prediction that the resin becomes harder and more brittle with a much higher softening point. Some thermosetting resins soften a t elevated temperatures but do not flow. At these temperatures the attractive forces due to secondary valences have been overcome and the molecules can separate slightly. The chemical bonds between the original molecules, however, remain unbroken and thus prevent flow. Solubility is a property analogous to softening point. The weaker the forces holding the molecules together the more easily are they separated by solvent molecules. Thermosetting resins a t first are soluble. When some chemical cross-links are formed the resin swells, hecause complete separation of the molecules is prevented. The resin finally reaches a stage where i t is practically nudected by solvents. On the basis of this reasoning, hardnessiu addition to softening point and solubilityshould he a measure of intermolecular forces. Whether molecules are held together by van der Wads' forces, chemical bonds, or mechanical entanglement, they are separated mechanically by whatever instrument is used to measure hardness, rather than by heat or solvent molecules. Hardness measurements are generally of two types: indentation and scratch hardness. Both of these actually are often a measure of plastic flow under pressure rather than rupture of the material. With resinous materials in particular, the methods are subject to the effect of a time factor which makes accurate measurement difficult. There is a need for a standard definition of hardness as a property of resins and for a method of measurement. SUMMARY
Generalizations have been presented by means of which resins may be classified as elastomers, plastics. Soflening Paint. or fibers. It should not he thought that by this means Eslcr 'C. Mechanical PrabnlCr each resin can be fitted neatly into a class on the basis Methyl 125 Hard. *mng of its molecular architecture. A beginning has been 65 Ethyl Tough Tough, flexible N-propyl 38 made in this direction and that is important. We do Clear, strong 1ro-pmpyl 95 N-butyl 33 Flexible, strong not need more architects to design new molecules as Iso-hutyl 70 Slightly brittle much as we need engineers to analyze resin structure in Sec-hutyl 62 Slightly brittle Tnl-smgl Brittle 76 terms of the properties of the building blocks. From OetYl Gel Liquid their results the architect can design his molecules and Lauryl vi.eous liquid Liquid know what the properties of the resins will be before When the alcohol chain becomes branched as in the they are built. At present the entire structure must be iso-, secondary, and tertiary alcohols the softening built and tested. If the calculations are faulty, the point goes up. The best explanation of this result may resin fails and a new one must he built. The trialbe that branched chains become entangled with each and-error method characterized the beginning of resin other to form mechanical points of attachment between chemistry. We are now a t the stage of resin design. polymer chains. The concept of functionality brought order to the Such a picture could he used to explain why poly- first field; we are looking for the key to the second. styrene has a fairly high softening point for a substance ACKNOWLEDGMENT of low polarity. The volume of the phenyl group may be sufficient to hinder separation of polymer chains. The writer wishes to express his appreciation to Dr.
Earl K. Fischer f o r his valuable assistance in the preparation of t h i s article. He is also indebted to Mr. Daniel S m i t h for the x-ray diffraction diagrams. RBPBRBNCES (1) BENTON, J. CHEM.Bnuc., 21, 144-7 (1944). (2) CAROTHERS, "Collected Papers;' Vol. I, High Polymer Series, Interscience Publishers. Inc.. New York. 1940. (3) ELLIS, "The Chemistry of Synthetic Resins." Vols. I , 11, Reinhold Publishing Corp.. New York. 1935.
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(5) Haussn. J. C ~ MEonc.. . 21, 15-17 (1944). (6) Houwrm, "Elastiaty, Plasticity, and Structure of Matter." Cambridge University Press, 1937. (7) MARK,''Physical Chemistry of High Polymer Systems," Vol. 11.High Polymer Series, Interscience Publishers, Iuc. N e w York, 1940.
(8) R o ~ o sSTAUOINGER, , AND VIEWEG, "Fortschritte der Chemie. Physik und Technik der makromolekulareu Stoffe." J. F. Lehmann Verlag, Berlin, 1939. (9) SHOR. J. CHEM.EDUC., 21, 88-92 (1944).
