Plastics Construction Materials
University Research on Engineering
Properties of Plastics The manner in which universities,
fulfilling their primary educational functions, can also contribute to a better understanding of the engineering properties of plastics is discussed. Illustrations of such academic-industrial cooperation are cited from the experience of the MCA-MIT Plastics Research Laboratory which has been operative for a number of years. Important products of this research are also described while the need for future effort in certain areas is outlined. Finally, the problems encountered in the study of plastics and their applications by other than engineering groups within the university are mentioned. FREDERICK J. McGARRY Massachusetts Institute of Technology, Cambridge 39, Mass.
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N PERHAPS a broader sense than is implied by the title,
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the purpose of this article is to discuss and illustrate methods by which universities and the chemical industry can cooperatively contribute to the advancement of plastics technology and engineering. The frame of reference of the discussion is the industrially supported MIT Plastics Research Laboratory which was set up in 1946 by the Plastics Materials Manufacturers’ Association, now a group within the Manufacturing Chemists’ Association. Though relatively modest in scale, the experiences associated with the laboratory are believed to be representative, in many respects, of a mutually beneficial academic-industrial relationship, Quite obviously the primary purpose of a university is to train students to think in a manner recognizably valid and rational. At the same time both pedagogical and utilitarian considerations recommend the acquisition of certain facts of a related nature and these, in turn, are used in fulfilling the university’s second important function of contributing fundamental information to our general store of knowledge. It follows, then, that the most important function which university research on plastics can fulfill is the introduction to and the training of its students in the field of plastics technology. While this represents a definite purpose, the methods of executing it are not always clear cut because of the inherent complexity of the subject and the number of formal disciplines upon which it draws. As an example of one approach in the M I T Plastics Research Laboratory which is primarily devoted t o examining physical and mechanical properties of plastics, it has been found advantageous to maintain a small diversified staff with backgrounds including physics, chemistry, mathematics, mechanical engineering, and electronics. Undergraduate students after an introductory or survey course extending over one academic term and providing a broad general knowledge of the subject, are then encouraged to participate in the laboratory activities, in conjunction with the permanent staff. Most often this takes the form of a bachelor’s thesis in which the student executes a rather well defined piece of research, under careful supervision. Usually the primary value associated with such work results from the contact of the student with the laboratory staff; terminology, methods of testing and evaluation, fabricating techniques, mechanical properties, all become somewhat familiar t o him. I n many instances the thesis work may be of inherent value. For example, recent undergraduate thesis work concerned with the directional strength properties of glass fabric-reinforced laminates constitutes the initial phase of an investigation considered essential to this field. July 1955
A second thesis determined the work to produce tensile failure of several types of methacrylates as a function of the strain rate and the temperature of the test. I n conjunction with an industrial fabricator, a third student observed the effect of various fillers on the accelerated weathering behavior of reinforced polyesters. On the graduate level, the work is considerably more fundamental. Such students have more detailed knowledge of a particular field and as a consequence their thesis efforts are of greater scope and intensity. I n this respect, close liaison with advisory people from industry on the technical committee of the laboratory is of great value. STATIC TESTING TECHNIQUES
Perhaps one of the most significant engineering cont>ributions which universities can render is associated with the evaluation of the mechanical and physical properties of polymeric materials. Compared to older, more conventional materials, plastics are unique in that their behavior is highly sensitive to composition, temperature, and time. As a consequence of this sensitivity and in order to accurately assess its nature, a genuine revolution in the philosophy of materials testing has been necessary. The human factor in the operation of testing machinery has been reduced or eliminated through automatic controls; the concept of large deformations has been further developed; positive controls over the temperature and time of test are necessary. Resulting from this has been the development of numerous testing machines and techniques specifically designed for applications to plastics technology. Noteworthy among these is the universal plastics testing machine (2) created in the M I T laboratory by both staff and student engineers. This device is capable of tensile, compressive, torsional, and flexural tests-any of these at either a constant rate of crosshead displacement, a constant rate of load increase, or a constant rate of strain increase. I n addition, any program of load time or deformation time can be impressed on the specimen. With it, one can dissociate the effects of rate and type of test, temperature, and composition variations on the various mechanical properties. For example, with the cooperation of several resin producers, a series of methyl methacrylate sheets of varying molecular weights, plasticizer contents, and degrees of cross linkage was prepared after the machine had been constructed. Tensile testing was carried out on specimens from these sheets in a manner such that controlled variations of only a single parameter were allowed in any series of tests. Thus for a certain plasticizer content at a single temperature in a load
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rate type test, the rate of loading would cover, stepwise, a range of 10,000 to 1. By this process it was possible to determine the identity of those parameters having the greatest influence on mechanical characteristics such as tensile strength, modulus of elasticity, and strain a t fracture and to arrange the parameters in the proper order of importance. The value of this development can best be established by the fact that the commercial version of the machine, now manufactured by personnel originally associated with the project, is considered an essential tool in many plastics research laboratories. A by-product of the work with the universal testing machine was the development of a strain-indicating device or extensometer which provides a continuous electrical signal proportional to the logarithmic strain experienced by the specimen (4). It is a clip-on type instrument that records strains as large as several hundred per cent with no damage occurring when fracture of the specimen takes place. The extensometer has been in satisfactory use for several years, and recently it was modified slightly by an industrial research laboratory for their own purposes. ANALYTICAL METHODS
The previously mentioned tensile testing program on methyl methacrylate was a formidable affair embracing, as it did, five molecular weights, five plasticizer contents, five temperatures, two types of test (constant load and constant strain rates) and five speeds for each type. To reduce the quantity of stress-strain information so derived from the graphical representation to a form submissive for engineering use and thereby demonstrate a technique applicable to similar situations with other materials, a simple empirical equation relating stress to strain and involving several constants was employed. The equation was fitted, by machine computation in the M I T Center of Analysis, to the various curves, and then the changes in the equation constants brought about by temperature, composition, and other variations were presented graphically. Several thousand tests were thus portrayed on perhaps fifteen plots, an appreciable condensation, which, however, was of somewhat restricted value since many engineering applications of plastic materials depend on their creep or relaxation properties instead of their stress-strain response under constant rate conditions. To remedy this inadequacy a more fundamental analysis predicated on B modified form of the Boltzman Superposition principle was evolved ( 3 ) . The modification arises in the use of a nonlinear expression to relate stress, strain, and time in contrast to the original Boltzmann concept which was evolved on the validity of a linear differential expression for the same purpose. The analysis utilizes experimental observations from a series of stress relaxation tests a t various stress levels to predict the creep and stress-strain a t constant strain rate behaviors over comparable time intervals. The technique has been successfully applied to materials such as methacrylate, polyethylene, unplasticized vinyl chloride, and Teflon. At present a more accurate and faster apparatus to perform the relaxation tests is being put into operation, and it is hoped that this will permit an extension of the time range of the predictions well beyond that of the relaxation observations. If the latter proves possible then the somewhat involved mathematical manipulations in the process will be simplified and the technique reduced to a form more easily employed in engineering applications. In all this work the contributions from students, both graduate and undergraduate, have been indispensable. More specifically a number of graduate theses have been concerned with the design and operation of the universal machine or with results of experimental investigations employing it as a research tool. As an indication of their response to the laboratory experience, it is worth noting that the proportion of those students taking positions in the plastics industry after completing their academic work is perhaps 60 to 75%, a figure which we find gratifying.
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On the other hand, those students who choose to do only course work to the exclusion of laboratory activities a t least develop a concept for the advantages and limitations of plastics as engineering materials. This, too, is important since misguided efforts to apply plastics in unsuitable applications can often constitute a serious hazard to large segments of the industry. DYNAMIC TESTIXG TECHNIQUES
Another field in which the laboratory has been active is that of dynamic or high speed tests, of both destructive and nondestructive types. This is important not only because there are numerous applications involving impact or rapid cyclic loading wherein plastics can be advantageously used if their propertiee under such conditions are well known but also because the response of plastics to high frequency-low energy disturbances often reveal information about the internal structure of the material. I n the destructive category, a device was developed for tensile testing of plastic materials in which the interval from no load to fracture is of 2 to 10 milliseconds’ duration, during which the stress-strain behavior of the material is observed ( 5 ) . This information is derived by electromechanical and electronic means and photographically recorded for analysis. With the apparatus it is also possible to study the behavior of synthetic adhesive bonds under rapid loading conditions, and a very interesting research of this nature is now under way by personnel in the chemical engineering department. Another apparatus, of a vibratory, nondestructive nature, has been developed and applied to the study of adhesive bonds ( 1 ) . Operating in the low kilocycle region, it detects the changes in the resonance characteristics of a composite cylindrical rod (metal and adhesive) exposed to a variety of thermal deterioration actions. Such changes, in turn, can be correlated with the tensile strength of the bond a6 determined by a destructive test. A third technique for high speed examination, this of a nondestructive nature, employs bursts of high frequency ultrasound passing through the plastic material (6). Changes in the velocity and the apparent energy absorption are the parameters observed as functions of frequency, temperature, composition, and physical state, thus providing some indication of the elastic and viscous components of the material’s behavior. This work is considerably more fundamental than what would usually be classed as engineering research through applications of the apparatus to various control functions have been successful. As an example, certain subtle variations in the characteristics of glass fiber-reinforced laminates appear to be detected by this means. All of these rapid test techniques, incidentally, have been graduate thesis research projects. Again on a fundamental level, a basic study of metal surfaces has recently been initiated in an attempt to broaden our understanding of the subject of polymer-metal adhesion. This research proposes to first prepare completely uncontaminated solid metallic surfaces, verify their nature both before and after controlled contamination, and then to observe the effect of the known contaminant on the properties and behavior of a subsequently formed adhesive bond, Because of the number of specialized fields contributing to progress on such work, it, perhaps better than any of the projects previously mentioned, well illustrates how the diverse resources of a university can effectively and economically be brought to bear on a problem of theoretical and practical importance. ENGIh-EERING APPLICATIONS
Implicit in all the work mentioned is the eventual application of its results to engineering situations. I n this respect the counsel and direction received from the technical committee, composed of representatives from the industrial sponsors of the laboratory, is extremely valuable. Their recognition of both immediate and future technical problems in the plastics industry
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Plastics Construction Materials
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is perceptive, and certain applications of apparatus or techniques are readily apparent to them though well removed from what may have been the original purpose. As our methods of mechanical property evaluation become more informative and sensitive, the need for rational engineering design criteria continues and in many instances, because of economic pressure, increases. For those materials which appear more immediately suitable for load-bearing applications (the reinforced plastics) certain design procedures may be extrapolated from other fields where the body of experience is more extensive. An illustration of the latter is provided by wood, a nonisotropic material, exhibiting many of the properties associated with reinforced plastics and for which a reasonably effective and fundamentally sound design procedure exists. Prior to any adoption of such an extrapolation product, however, a comprehensive testing program t o prove its validity would be necessary, and this constitutes a formidable task of analysis and interpretation. Loading conditions producing other than simple uniaxial stress situations should be studied since few structural members undergo a simple stress experience in service. Economical and realistic factors of safety under various conditions of use should be determined and substantiated. Techniques of connection and support and their stress characteristics must be investigated. Reactions of reinforced plastic structures to point impact loads are important. These and many other aspects of structural plastics must be explored before their full potentialities can be realized. The use of the more time-sensitive thermoplastic materials in a similar fashion presents challenging opportunities. As a matter of fact, numerous investigators have already developed very useful concepts regarding creep behavior and time dependent moduli which are successfully applied to engineering cases, and recently preliminary work has been done demonstrating how time dependent, loaded bodies can be mathematically transposed to different loading systems on the same geometry, now considered perfectly elastic, thus making possible the use of the extensive literature of elastic stress analysis ( 7 ) . It is a bit of an understatement to say that a concept as powerful as this warrants experimental substanti ation. Another function which the Plastics Research Laboratory fills is that of advising various groups within the institute who are interested in applying plastics in their own fields, Perhaps the most active of such groups are the architects, perceiving, as they
do, the potential advantages in form, coloring, weight and portability, lighting, fabrication, and maintenance which many plastics or plastic-based materials can offer. While the architect quickly appreciates these materials as being inherently valuable rather than regarding them primarily as substitutes, it is often difficult to establish an awareness, in his attitude, of their structural limitations and thereby avoid misapplications of them. Considerable imagination and judgment appear to be necessary to successfully integrate plastics into various forms of architecture without becoming unrealistic or repetitious, and the plastics industry, too, has a challenging responsibility in this respect. It is reassuring to observe the various studies and programs being initiated by industry to meet this challenge cooperatively. CONCLUSION
T o conclude, the instructing and research functions of universities can be applied beneficially to problems of plastics technology. Students are trained in the subject and then, with their teachers, can often make significant and fundamental contributions t o the field. A few examples of the latter, as results of an industry-university effort, have been described. Areas in which further engineering research are needed have been mentioned as well as more general problems associated with the broadening use of plastic materials. LITERATURE CITED
Bockstruck, H. N., Dietz, A. G. H., and Epstein, G., Am. Soc. Testing Materials, Tech. Publ. 138, 1952. Burr, G. S., Dietz, A. G. H., Gailus, W. J., Silvey, J. O., Yurenka, S., A S T M Bull., No. 149 (December 1947). Dietz, A. G. H., and Know-les, J. K., Trans. Am. SOC.Mech. Engrs., 77, 177-86 (February 1955). Diets, A. G. H., and McGarry, F. J., Rev. Sci. Instruments, 2 5 , 740-5 (August 1954). Dietz, A. G. H., Dorses, J.. and McGarry. F. J.. “Methods of Testing Mechanical Properties of Propellants,” interim tech. rept. submitted to Picatinny Arsenal, Dover, N. J., under Contract DA-19-020-ORD-6, January 1953. Dietz, A. G. H., Hauser, E. A., and Sofer, G. A., IND.ENG. CHEM.,45, 2743 (1953). Lee, E. H., “Stress Analysis in Viscoelastic Bodies,” tech. rept. 8 submitted to Picatinny Arsenal, Dover, N. J., under contract NORD 11496, June 1954. RECEIVED for review September 17, 1954.
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February 24, 1965.
Modified Stvrenes for Structural Applications ROBERT H. STEINER Atlas Mineral Products Co.,Mertztoum, Pa.
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N 1953, production of polystyrene molding resins in this country approached 300,000,000 pounds, placing the annual production of this type of plastic behind only the vinyls and phenolics. The reasons for this great popularity are many. Low density, low cost, hardness, excellent injection molding characteristics, and unlimited color possibilities have made polystyrene a favorite material for the fabrication of countless items for the houseware, toy, and novelty industries. The excellent electrical properties and relative insensitivity to moisture have made possible its extensive use in the manufacture of electrical equipment. July 1955
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However, in spite of the tremendous improvements brought about by the resin manufacturers in the past 15 years, straight polystyrene resins still have two fundamental drawbacks which limit their use as structural material. I n the first place, these materials are inherently brittle. The tendency for certain sections of the plastics industry to overlook this fundamental fact has led to many misapplications with the resulting disappointment of the consuming public. The second disadvantage lies in the relatively low heat distortion point which prevents use of polystyrene a t temperatures above 170’ to 180’ F. A brief discussion is presented of the attempts t o overcome
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