Plastics Construction Materials
-
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. T h e 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.
-4CCEPTED
February 24, 1965.
Modified Stvrenes for Structural Applications ROBERT H. STEINER Atlas Mineral Products Co.,Mertztoum, Pa.
I
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
INDUSTRIAL
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. T h e 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
AND ENGINEERING
CHEMISTRY
1307
Because of their inherent physical and chemical properties, styrene copolymers and copolymer blends are being used considerably as plastic materials of construction. A brief survey of the physical properties of the commercially available types is presented to indicate their versatility of application. The economic advantages of low specific gravity and high heat distortion are stressed. Styrene copolymer sheets produced by molding or extrusion lend themselves to solvent cementing and vacuum forming techniques for the production of appliance and equipment. Perhaps the largest use of these plastics at the present time is the extrusion of pipe for petroleum and chemical industry applications. Molded fittings are available for rapid assembly of pipeline installations. Pipe can readily be bent by standard heat-forming methods. A study of the resistance of styrene copolymer blends to acids, alkalies, salts, and organic solvents is presented. Weight and tensile strength changes were studied over a temperature range.
these deficiencies. Structural applications are also discussed but are limited to materials of construction in which resistance t o chemical attack is an important factor. Studies on the resistance of the more important types of styrene plastics for these uses are presented. RING MODIFIED STYRENE POLYMERS
Among the early commercial attempts to produce improved polystyrene resins was the chemical modification of the monomeric styrene molecule. The extensive patent literature attests to the many man-years of research devoted to this approach to the problem (5). The only product of this type produced commercially was poly(dich1orostyrene) (IO). Heat distortion values of 230' to 250" F. were obtained which permitted hot water sterilization of molded objects. Most of the other excellent properties of polystyrene were retained and, in addition, the high chlorine content conferred nonflammability properties. However, the impact resistance was not improved and, as expected, the specific gravity was increased. Because of the high cost of the monomer, dichlorostyrene polymers have not achieved large commercial acceptance except for specialized applications. STYRENE COPOLYMERS
A much more fruitful modification of styrene polymers has been the copolymerization of styrene with minor amounts of other vinyl monomers. Over 1000 patents and academic papers have been published on the subject (4). Although almost every conceivable type of copolymer has been investigated, relatively few styrene copolymers have shown sufficient promise to warrant commercial consideration. Among the first products exploited were a series of styrenefumaro-dinitrile copolymers ( 2 , I O ) . These products had heat distortion points in the range 220' to 250' F. The most highly modified types lacked the excellent molding characteristics of straight polystyrenes, but the lower types were completely satisfactory in this respect. Unfortunately, the high cost of these products has limited their commercial acceptance. Another early attempt a t copolymerization involved the use of small amounts of p-divinylbenzene, a bifunctional monomer ( 2 , 8, I O ) . As little as 1% of this compound produced crosslinked resins with heat distortion temperatures up t o 235" F. These products are, of course, no longer thermoplastic and cannot be molded. They are available only as cast rods and sheets for
1308
machining into small electrical components. Because of this limitation, styrene-divinylbenzene copolymers have attained only limited commercial usage. Copolymers of styrene with butadiene, in the form of Buna S or GR-S, have accounted for more tonnage production than any other synthetic high polymeric material. This product, which contains approximately 27% styrene, is a true vulcanizable elastomer and is not within the scope of this discussion. Other styrene-butadiene copolymers, in which styrene is the major component, have also found many industrial applications ( 1 , 1 1 , I C ) . These copolymers, which may or may not be vulcaniaable, are tough, horny resins with lower heat distortion points than polystyrene and low impact resistance. They have found wide use as reinforcing resins for natural and synthetic rubbers. Great improvements in hardness, stiffness, and abrasion resistance have been reported. High styrene-butadiene copolymers also serve as vehicles for excellent masonry protective coatings, based either on solvent systems or aqueous dispersions. However, these polymers have not been used for structural purposes. Styrene-isoprene copolymers have also been produced for essentially the same end uses as the butadiene copolymers. Acrylonitrile has been copolymerized with styrene to produce moldable products having increased heat distortion temperatures (190' to 210" F.) (16) and greater strength characteristics than standard polystyrene resins. One of the more important properties of this type of copolymer is the high flexural strength a t elevated temperatures. These resins can be fabricated into sheet up t o inch in thickness by calendering and extrusion processes. These sheets can be laminated into thicker sheets on conventional presses. Vacuum forming of such sheet is possible, but much of the acrylonitrile copolymer produced is used in injection molding applications. Styrene-acrylonitrile copolymers a h o show improved weathering characteristics as compared to straight polystyrene. Recently, a t least four manufacturers have introduced new styrene copolymers with extremely high impact strengths coupled with moderately high heat distortion temperatures and good processing characteristics. All of these products are opaque. These new products have great potential use as the structural plastics but have not been completely evaluated as yet. Their high cost will probably limit their acceptance as molding materials to special uses. Costs will undoubtedly be reduced as production is increased. Unfortunately, the nature of the comonomers used in these products has not been discloeed as yet.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 7
i
c
Plastics Construction Materials STYRENE BLENDS
The most important class of modified styrene plastics for structural applications consists of blends of polystyrene or styrene copolymers with compatible elastomeric materials (6, 7, 9). These materials have been variously referred to as styrene alloys, styrene-rubber blends, and styrene-rubber plastics. These products are all characterized by their improved impact strength which can be attributed t o the increased extensibility as compared to unmodified polystyrene. I n general, the ultimate elongation under tension is approximately ten times higher. There are two basic types of blends available commercially. One type consists of polystyrene combined with a minor amount (presumably 10 to 25%) of a styrene-butadiene rubber. These products, characterized as medium impact materials, have lower tensile strength, higher impact resistance, and approximately equal heat distortion values in comparison with the unmodified polystyrene. Costwise, they rank between polystyrene and the acrylonitrile copolymer blends. The most interesting styrene-based plastics for structural purposes are the styrene-acrylonitrile copolymers blended with elastomeric butadiene-acrylonitrile (Buna N ) copolymers. Most commercial products contain 10 t o 25% of rubber. These products, referred to hereafter as acrylonitrile copolymer blends, have been used commercially for a t least 5 years for the production of pipe, pipe fittings] and duct work. It must be remembered that this nomenclature does not signify that acrylonitrile is the major component. They are characterized by high impact strength, fairly high heat distortion temperatures, and excellent molding and extrusion properties. Their cost is higher than that of the polystyrene-GR-S blends b u t lower than the new high impact styrene copolymers discussed in the previous section. PhysicaI Properties. The most significant physical properties of the most important commercial modified styrene plastics are given in Table I. Data on a typical unmodified polystyrene are included for comparative purposes. It must be emphasized that the values given are representative of groups of products and are not intended to typify any specific commercial product. Information on individual products can be obtained from the resin manufacturers. STRUCTURAL APPLICATIOSS OF ACRYLONITRILE COPOLYMER BLENDS
copolymer sheet can be heat-formed into component parts which can then be assembled by solvent welding or cementing techniques. Extruded angles of the same material are used to form corners and joints with strengths approaching that of the sheet itself. Also, special slotted extrusions with H or L cross sections have been made available for rapid assembly. However, it is necessary t h a t the sheet components be cut with great accuracy to ensure tightly fitted connections. As this method is limited t o duct work having square cross sections, the great design flexibility available with the weldable thermoplastics is lacking. On the other hand, the higher heat distortion temperatures of the acrylonitrile copolymer blends permit their use a t slightly higher temperatures than more widely used plastic materials of construction. Tanks. Although the same jointing techniques may be used for the construction of self-supporting tanks, the lower flexural modulus of the styrene copolymer blends have limited their use in this field. Pipe. Acrylonitrile copolymer blends have found their greatest acceptance in the form of extruded pipe. The availability of suitably compounded resins has permitted commercial extruders t o produce uniform pipe in iron pipe standard sizes (IPS) from to 6 inches. I n general, two wall thicknesses are available: Schedule 40 (standard wall) and Schedule 80 (heavy wall). Solvent welded fittings may be used with either type of pipe, b u t Schedule 80 is usually threaded and joined with corresponding threaded fittings. Injection molded threaded fittings are available in standard sizes up to 4 inches in the form of TIS, 45" elbows, 90' elbows, unions, couplings, reducer couplings, flanges, and caps. The use of threaded pipe with suitable pipe dope permits more rapid installation and does not require specially trained workmen. Furthermore, repairs or relocation of pipelines can be accomplished without cutting out and discarding the fittings as is necessary with solvent welded Schedule 40 pipe. However, permanent leakproof joints are frequently formed by coating the threaded sections with a solvent or a cement before assembly. The determination of maximum service conditions for any type of plastic pipe is a problem of considerable interest t o the chemical engineer. Unfortunately, long term tests are lacking, but both the raw materials manufacturers in their own laboratories and the pipe extruders through a cooperative project a t Battelle Memorial Institute are investigating this problem. Short term bursting pressures for various sizes of acrylonitrile copolymer blend pipe have been determined (7). These results agree with calculated burst strengths based on the application of the Lam4 or Barlow formulas which take into account pipe dimensions and tensile strength. These results are difficult t o interpret because the important factor of creep under tensile stress is ignored in tests of this type. The most common prac-
The applications of modified styrene plastics are literally unlimited; however, this discussion is limited to those structural applications where the strength and chemical resistance characteristics are of prime importance. The specific uses considered are fume ducts, pipe, and pipe fittings. At the present time, the only type of styrene plastic found acceptable in these exacting fields are the acrylonitrile copolymer blends. Fume Duct Systems. The use of all-plastic fume duct systems for the handling of Table I. Physical Properties of Styrene Plastics corrosive vapors has provided StyreneNew Polystyrenea solution to one of the maASTM Test Acrylonitrile Styrene GR-S jor corrosion problems in many Method Polystyrene Copolymer Copolymers Blends Specific gravity m e t a 1-w o r k i n g plants. AlD792 1.05 1.08 1.01-1.05 1.05 Tensile strength, lb./sq. though unplasticizedpoly( vinyl inch D638 7000 9000 3000-4000 4000-6000 Elongation, yo D638 2 2 30-100 20 chloride) has been used to a Flexural strength, lb./sq. inch D790 greater extent, acrylonitrile co10,000 17,000 7000-9000 6000-8000 Flexural modulus, lb./sq. polymers have also been used inch D790 450,000 450,000200,000250,000500,000 250,000 450,000 for specific applications (8). H e a t distortion (264 lb./ sq. inch). F. D648 I n contrast to the vinyls, 180 195-208 180-190 160-190 Izod impact strength polyethylene, methacrylates, (77' F.),f t . lb./inch notch D256 0.35 0.5 5-9 1-5 Hardness (Rockwell) D785 R-120 R-125 R65-80 M60-70 and polystyrene, this type Cost, cents/lb. ... 35 65-70 65-70 40-65 Weathering resistance plastic cannot be joined suc(unpigmented) ... Poor Excellent Good Poor cessfully with hot gas welding methods. However, this July 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acrylonitrile Copolymer Blends 1.06 5000 20
7000-8000 250,000
180-190 5-9 R-95 58-65 Fair
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Table 11. Calculated Burst Strengths and Recommended Worlring Pressures of StyreneAcrylonitrile C o p o b ' m e ~ Buna hTBlend Pipe (Schedule 80) Nominal Size, Inches 1
2
Temp., O F. 75 125 150 75 125 150 75 125 150
Max. Recommended Working Pressure, Lb./Sq. Inoh Solvent welded Threaded 284 142 174 87 120 60 189 95 115 58 79 40 154 77 94 47 64 32
Calcd. Burst Strength, Lb./Sq. Inch 1420 870 GOO 945 575 395 770 470 320
tice assumes the safe continuous working pressure to be approximately 20% of the calculated burst strength. This safety factor of 5 has been developed on the basis of fairly wide field experience by a great number of investigators. Most failures which have occurred in field installations have resulted from a disregard of this rule. Table I1 gives the calculated burst strengths for Schedule 80 pipe of various sizes and a t a series of temperatures. These data are based on an assumed tensile strength of 4500 pounds per square inch a t 75' F. These results were derived from the application of the Lam6 formula: (o.d.)2 Burst strength = tensile strength (o.d.)2 (i.d.Y
+
Table 111. Chemical Resistance Tests on StyreneAcrylonitrile Copolymer-Buna N Blends 150' F. 75' F. Resistance, % Weight Tensile change change
Acids Sulfuric 1 0 7 Sulfuric' 5 0 9 Sulfuric: 7 0 8 Hydrochloric, 10% Hydrochloric, 38% Nitric 109' Nitric: 25d Nitric, 50y0 Phosphoric, 85%
Rating
-3.5 -7.5
E E G
0.8 0.6 3.6
0.6
-1.0
E
0.4
1.7
f3.7 -3.0 -32.0
G
-2.3
N E
3.0 2.5 4.4 D 2.7
-2.7 -7.8
E E
0.6
0.2 1.7
1.0
2.3 D 0.2
+1.0
G P
...
Alkalies Sodium hydroxide, 10%
Sodiuni hydroxide, 50% Ammonium hydroxide, 28% Sodium carbonate, 25% Salts Sodium chloride, 10%
Alum, 10% Bleach Solutions Sodium hypochlorite, 5% Chlorine dioxide, 0.6%
Organic Compounds Acetic acid, 10% Acetic acid, 100% Benzene Crude oil Ethanol Formaldehyde, 37% -Heptane D = Disintegrated
E
=
0.5 -0.6
+1,9
-5.0
-10.0
-14.0 -4.2 +13.0 -12.5 -29.0
Rating
E
E N E
F
-3.9
P N F
1.0 0.6
- 6.1 -9.8
F F
1,9
-12,0
...
N
0.6
+0.8
E
0.8
- 11.0
G
0.8 0.7
+1.4
E E
0.3 0.5
-9.0
+1.0
-6.0
E E
1.2 1.4
-10.7 -9.0
F
F
2.5 7.5
... ...
P P
0.7 40 30
-2.0
E
0.9
-6.8
E
1.0
+'1'.4 -7.0 -2.0
N E G R E
...
.... ..
1.8 0.6 -0.2
N
t . .
...
Excellent: highly recommended
G = Good; recommended
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Resistance& Weight, Tensile, change change
F
=
E' N
= =
...
1.2 3.7 0.2
.. .
-6.8 -20.0 -6.1
...
' '
G N E
..
Fair: usable for short terms only Poor; not recommended Notresistant
Another complicating factor in the determination of safe working pressure is the burst strength of the joints and pipe fittings. These joints are almost always weaker than the Pipe itself, especially in threaded pipe where the effective strength of the pipe may be reduced by as much as 50%. For this reason, a Safety factor of 10 is usually recommended for piping Systems of this type. The support of horizontal runs of acrylonitrile copolymer blend pipe also requires special consideration. Continuous support in a channel iron is recommended for high temperature lines to minimize sagging, b u t standard pipe supports placed at 3- or 4-ioot intervals are satisfactory for normal temperature conditions. The supports should be free to permit expansion and contraction. CHEMICAL RESISTANCE OF ACRYLONITRILE RESINS
COPOLYMER
Perhaps the most important property of plastics which makes their use in the industrial field so attractive is their resistance t o deterioration under conditions where most metals are subject to rapid attack. Polystyrene exhibits excellent resistance to all nonoxidizing inorganic acids, alkalies, salts, and polar organic solvents such as the lower alcohols and glycols. It is attacked b y strong oxidizing acids such as sulfuric acid above 80% and nitric acid above 25%, as well as most nonpolar solvents and aliphatic hydrocarbons. All the commercial types of modified styrene plastics are somewhat poorer in resistance, especially with regard to oxidizing acids. Considerable work has been done on methods of determining chemical resistance of plastics ( l a , IS). The simplest type of test is the determination of weight change after immersion in various solutions. Long term studies have shown that for most nonreinforced plastics, these weight change studies correlate well with changes in physical properties. Weight change studies of a commercial acrylonitrile copolymer blend at 75" and 150' F. are presented in Table 111. These tests were run in duplicate on 1 X 4 X inch samples for 30 days. At the conclusion of the tests, the samples were broken under tension and the percentage change in tensile strength from the unexposed material was calculated. The rating figures given are based on practical experience with a wide range of similar materials under actual use conditions. I n addition t o these total immersion tests, samples were exposed to various corrosive gases for 30-day periods a t 75" and 150' F. No significant weight changes were found, but ratings were assigned on a basis of visual observation. The results are given in Table IV. Weathering Resistance. I n contrast t o its chemical resistance, unpigmented polystyrene is badly affected by outdoor weathering as indicated by crazing, yellowing, and marked decrease in physical properties. Styrene-acrylonitrile copolymers, on the other hand, show relatively little effect. The polystyrene-Buna S and acrylonitrile copolymer blends are also poor in weather resistance in spite of the fact that they are by nature opaque. I n actual use, practically all industrial products fabricated from styrene copolymer blends are heavily pigmented. The ultraviolet screening effect produced b y pigmentation improves
Table IV.
Resistance of Styrene-Acrylonitrile CopolymerBuna N Blends to Corrosive Gases Gas
Chlorine, dry Chlorine, moist Hydrogen sulfide, dry Sulfur dioxide dry Sulfur dioxide: moist
INDUSTRIAL AND ENGINEERING CHEMISTRY
Resistance Rating a t 75O F. 150' F. E G G F E E
E
-
E E G
Vol. 47, No. 7
Plastics Constructioii Materials the sunlight resistance to the point v-here black and gray extruded pipe has withstood exposure in Florida for 6 months with no significant effect on Izod impact strength, flexural strength, or hardness. Temperature Limitation. As discussed previously, the ASTM heat distortion points for the acrylonitrile copolymer blends are in the range 180" to 190" F. It must be kept in mind that this is a n arbitrary test method and cannot be interpreted literdly as the maximum usable temperature for any size pipe. In general, a safe rule-of-thumb based on practical experience for most thermoplastic structural materials is to limit their maximum temperature range to 20" F. below the ASTM heat distortion point. Thus, with acrylonitrile copolymer blends, 160' to 170" F. would be the recommended maximum. Slightly higher temperatures can be used intermittently if the pipeline is continuously supported in the earth or in angle irons and very low pressures are used. With any high temperature service, satisfactory expansion loops must be designed to accommodate the linear expansion of the pipe. SPECIFIC APPLICATIONS OF ACRYLONITRILE COPOLYMER BLEND PIPE
in diameter. Failures after various lengths of service were reported in five cases. Of these, three failures were caused by excessive temperature, pressure, or thermal expansion while two were attributed to material failure. I n these two cases, the failures occurred a t the joints which were of the solvent welded type. Although this sampling is too small to permit statistical analysis, it indicates that acrylonitrile copolymer blend pipe has a definite place in the handling of corrosive materials. It further points out the necessity for an intelligent understanding of the limitations of the material by the personnel concerned with its installation and maintenance. Surveys of this type are urgently needed to present unbiased and straightforward information that will serve to limit future misapplication and unjustified condemnation of a basically sound material of construction. ACKNOWLEDGMENT
,
The author wishes to express his gratitude to C. F. DeLong and C. S. Reinert for their assistance in the preparation of this paper. LITERATURE CITED
Acrylonitrile copolymer blend pipe has been used in rapidly increasing quantities for 5 years in the chemical process, petroleum, and gas industries. I n the chemical industries, pipe of this nature has been used successfully to carry dilute sulfuric acid, bleach solutions, salt solutions including ferric chloride, ammonium sulfate, and sodium bisulfite, vegetable oils, higher fatty acids, and both dry and moist chlorine gas. It has been extremely satisfactory in the manufacture of water-conditioning equipment where brine, weak alkali, and weak acid solutions are handled. I n the petroleum field, acrylonitrile copolymer blend pipe has been used in well flow lines, salt water and oil gathering lines, and gas lines. A4recent study (6) of oil field installations conducted by the XACE Unit Committee T-1J of Group Committee T-1 has provided valuable information on performance under the most strenuous conditions. Data were obtained on 20 installations totaling almost 22,000 feet of pipe varying from 11/2 to 5 inches
(1) Aiken, W. H., Modern Plastics, 26, No. 2, 99 (1948). (2) Boundy, R. H., and Boyer, R. F., "Styrene," p , 307, Reinhold, New York. 1952. (3) Ibid., pp. 736-810. (4) Ibid., pp. 960-1057. ( 5 ) Bradley, B. W., and Thornton, W. M., Katl. Assoc. Corrosion Engrs., Reat. No. 6 to Committee T-lJ, Feb. 15, 1954. (6) Eliot,-P. M.,-ij'PE Journal, 8, N o . 2, 15 (1952). (7) Zbid., 10, No. 5 , 26 (1954). (8) Haldeman, W. A,, and Wesp, E. F., ISD. ENG.CHEM.,47, 1359 (1955). (9) Modern Plastics, 30,No. 3, 91 (1952). (10) Modern Plastics Encyclopedia, 1947, p. 155. (11) Ibid., 1949, p. 281. (12) Seymour, R. B. and Steiner, R. H., Chem. Eng., 60, N O . 1,254 (1953). (13) Seymour, R. B., and Steiner, R. H., C h ~ mEng. . Progr., 48, S o . 11,586 (1952). (14) Snyder, C. J., SPE Journal, 10, No. 5 , 38 (1954). (15) Whitaker, J. S., Ibid., 10, No. 5 , 30 (1954).
. ,
RECEIVED for review September 17, 1954.
-4CCEPTlnD
April 22, 1955.
Time- and Temperature-Dependent Modulus Concept for Plastics C. H. WEBER, E. N. ROBERTSON, AND W. F. BARTOE Rohm h Haas Co., Bristol, P a .
c
LASSICAL engineering formulas have been used with only partial success in calculating the deformation of plastics in various structural applications. The mechanism of deformation of linear amorphous polymers is considered to be made up of at least three deformation systems ( 5 ) . The first deformation is instantaneous and is considered to be truly elastic and instantly recoverable. This deformation is considered to be due to change of bond angles. The second and third deformations are time dependent and may lead to irrecoverable deformation or permanent set. These are considered to be due to uncoiling and slippage of the polymer chains past one another. I n this paper, no differentiation is made between the latter two types of deformation. Elastic deformation occurs almost instantaneously with loading July 1955
and is completely recovered on unloading a material. On the other hand, additional deformation which does not follow a straight-line stress versus strain relationship and is not completely recoverable will occur if a constant load remains on a plastic specimen. This is true with plastic specimens even when the stress is considerably below the yield or breaking point of the material. Design engineers are usually concerned with the total deformation of plastic structural parts, whether or not such deformation is of the instantaneous or of the plastic type. I n this study of the deformation phenomena of some unoriented polymers, we have devised a method of characterizing the polymer by utilizing a beam continuous over two supports with two external sym-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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