Evaluating Plastics for the Chemical Industry The unique properties of plastics can be used to good advantage in the chemical industry. However, plant designers must become familiar with the general properties of plastics as compared to the properties of more conventional materials of construction. A comparison of such general properties is made together with a general property comparison for the generic plastic groups. Environmental conditions such as elevated temperature, high moisture, chemical attack, and outdoor weathering all effect the properties of plastics. Specific information developed over the past 10 years on reactions of plastics to different environments is given. Factors controlling the economics of plastics use are reviewed, and recommendation is made that specific application problems be discussed with the fabricator or material supplier.
C. HOWARD ADAMS AND R . A. MCCARTHY Monsanto Chemical Co., Springfield, iMass.
T
H E widespread acceptance of plastics as materials of construction has undoubtedly been hampered by the difficulty found in correlating physical properties with behavior in actual use. The situation is further complicated by the many kinds of plastics available, the variety of formulations of each, and the methods of fabricating them; these variables account for a confusing multiplicity of end products with different properties. The problem of correlating properties with engineering behavior is one that is receiving top priority attention in the research laboratories of the industry. Here, fundamental physical properties are under study with accumulation of actual performance data, an important aspect of the work. However, the complexity problem appears only to be an educational one. If the industry can direct the thinking of designers along the lines of the characteristic behavior of a given plastics family (generic group), the designer will learn the proper plastics for his plant and will use them more wisely. His ability t o work with new materials has already been demonstrated in other areas. Certainly the many metal alloys in use today do not seem to have been a deterent to the alert designer ($3). Plastics have unique properties which can be used to good advantage. Considered in this light rather than as substitute materials, t h e y are filling a place now vacant or occupied by iess 1294
satisfactory materials. The selection of the proper plastic must be made in accordance with normal engineering procedure: 1. Definition of the performance required of the finished product or structure 2 . Selection of the material which best meets these performance requirements and which can easily be fabricated 3. Analysis of the economics of the finished plastic component or structure ( 2 2 )
If plastics are given a place in the pool from which materials of construction are selected, they will fit into their proper niche. The purpose of this discussion is to acquaint those interested with some of the properties of plastics, particularly those properties which cause the uninformed t o shy away. There are a lot of people like the one who was being shown an experimental accoustical ceiling tile of plastic material. He contemptuously crushed it underfoot saying, “What if somebody tried t o walk on this stuff?” PLASTICS AND CONVENTIONAL MATERIALS
The properties of plastics of greatest importance in chemical industry applications are corrosion resistance and low density. Other properties of importance, specific to certain plastic types, are shock resistance, dimensional stability, ease of fabrication,
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and low cost (24). I n order to show plastics as a class of materials in their proper perspective with the more conventional materials of construction, typical ranges of behavior are compared in Table I. It should be remembered that the properties can vary widely within the class just as they do with the glass, hard rubber, steel, aluminum, copper, and wood materials that have been selected for comparison. The strength/specific gravity relationship is often cited in the trade literature on plastics as favoring the use of these materials over steel. However, since this relationship is subject to many limitations (4), only room temperature tensile strength comparisons are given. The wide range for plastics encompasses the ranges of aluminum, glass, wood, and hard rubber, while the upper range of plastics approaches steel and copper. If equated to the same weight of material, which must be done with caution, plastics compare favorably with other structural material. However, even the direct comparihon of tensile strengths shows that plastics are entitled to an entry into many applications. With regard to rigidity, as shown by the modulus of elasticity in tension (Table I), the plastic range, which covers from the flexible to the rigid, indicates more versatility in these materials than is found in rubber or wood. Plastics are less rigid than glass, steel, aluminum, and copper. I n applications requiring a very rigid material, the use of plastics is limited, but where shock resistance and vibration damping effect are of importance, the scope of application is broadened. Hardness, based on the MOH scale, shows that plastics bracket the range of rubber, wood, and aluminum, and are close to copper in their upper range. They are softer than glass or steel. The hardness characteristic is important not as a direct measure of engineering behavior, but as an indication of creep, abrasion resistance, and machineability. A discussion of the mechanical properties of plastics must, of course, include something concerning creep. Creep or cold flow is the amount of permanent set caused by a continued application of a fixed stress. It can b e a problem with plastics that must bear a load. Some representative plastics were tested at 4000 pounds per square inch load in compression a t 120” F. for 24 hours. The amount of creep that occurred expressed in per cent of the original length ranged from 0.35 to 64%. Hard rubber subjected to the same test was in the same range with values of from 0.5 to 80% ($0). Compare this with the 1% creep of brass and aluminum at 400’ F. and 18,000 and 15,000 pounds per square inch, respectively, for 10 years (28). Long time strength behavior, though infrequently a design criterion, is much in the same vein. Experimental evidence shows that one unreinforced thermosetting material will support a load for 5 years that is 25% of its original load bearing strength. Similarly, a typical styrene plastic will support a fixed load for 1 year that is 10% of its original strength (32). Certainly these properties must be considered when using plastics. There is a common belief that the thermal properties of plastics relegate them to rather low temperature applications, but as shown by the maximum use temperature (Table I), plastics are comparable with rubber and wood and, in certain instances, the less heat-resistant alloys of aluminum and copper. Glass and steel have superior heat-resistant properties. Other thermal properties which must be considered, in applications where heat and temperature are important, are thermal conductivity and thermal coefficient of expansion. The low value of thermal conductivity for plastics is an advantage in certain applications, but if heat is to be conducted away rapidly, metals have the advantage. I n many industrial applications, this is not a problem since only permanence in a heated environment is required. Likewise, the high thermal coefficient of expansion of plastics can be handled by careful design. If plastics are t o be used in combination with metals, this characteristic behavior should be recognized and tough, ductile plastics used. Such
July 1955
materials are required t o satisfactorily withstand the stresses developed at the juncture of the metal and plastic. Plastics have come into their own in the chemical industry in the field of corrosion resistance. Resistance to attack by ionic, oxidizing, and organic solvents (Table I) shows that, on an overall basis, only glass is superior to plastics. Since corrosion resistance is the major problem in the chemical industry, designers often select the materials that will stand u p and then design “around” some of the less favorable properties of the plastic. This seems to be a good approach since it recognizes that the perfect material has yet to be found. As a final comparison with other materials, costs are considered. Unless the specific application is known, little can be gained b y a pound-by-pound or cubic foot-by-cubic foot comparison of cost of plastics and other materials of construction. For tanks, plastics are comparable in cost with aluminum and cost 20% more than galvanized steel (16). This does not take into account the lower cost of erection and maintenance for the plastic. Phenolic-glass and polyester-impregnated glass cost about four times as much as aluminum or stainless steel, on a pound basis, but fabricating into sheeting makes the cost comparable ( 8 7 ) . For piping of small sizes, the rank in order of increasing cost is iron, plastics, aluminum, copper, and stainless steel. For larger sizes, the order is iron, aluminum, plastics, copper, and stainless steel (7). Depending on the application, the fabrication costs are often the controlling factor. It often costs approximately ten times as much to tool up for steel materials as it does for plastics (SO). This would mean that, for short rune, plastics costing u p to three times as much as steel would be competitive ( 1 7 ) . Often the ease of processing can be controlled on longer runs since plastics db not require extensive finishing operations. This is a factor in the production of radio cabinets where the cost ratio between plastics, metal, and wood was found to be 1:1.1 : 1.2 (8). I n general, i t can be said that cost savings up to 20% can be expected by the selection of plastics where short runs of complicated shapes are made or on long runs where ease of fabrication is an advantage, initial costs of materials not being far different. The former is probably most applicable to chemical industry applications. Summing up, it can be said that we are dealing with a material that is fairly strong, hard, and flexible, that has good chemical resistance, and that may be more economical to use. On the other hand, it must be used a t relatively low temperatures and has severe creep properties which must be considered. GENERIC GROUPS O F PLASTICS
Once the general performance perspective for plastics in relation to conventional materials is known, the characteristics of the major generic groups of plastics should be considered. However, before discussing the generic groups, i t is necessary to point out a basic classification of plastic types-e.g., thermosetting and thermoplastic. I n general, the thermoset materials are more heat resistant and stronger than the thermoplastics. The ranges shown for each group may be due to plasticization, copolymerization, reinforcement, or blending with other polymers. The room temperature tensile strength (Table I) shows that the thermosetting materials are uniformly high, with the reinforced plastic exhibiting the highest strength because of its reinforcement. The remainder are approximately the same and are restricted t o lower working Rtresses. Ethylene and plasticized vinyl plastics are the exceptions, being characterized by very low strength. (The halocarbon group shown in the table refers to various fluorocarbon compounds.) Rigidity as shown b y the elastic modulus in tension (Table I) varies over a 1000-fold range. The reinforced thermosetting plastics are the stiffest, with the reinforcing agent again being the controlling factor. Most of the other materials are cornparable, with the exception of the more flexible ethylene, halocarbons, and plasticized vinyl plastics.
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TabIe I. Properties of Construction Materials PLASTICS AND CONVENTIONAL MATERIALS Plastic
Glass
Hard Rubber
Tensile strength, lb./sq. inch x 102 0.5-55 (6,12, 16) 1-10 (4) Modulus of elasticity. lb./sq. inch X 106 8-10 (4) 0.001-3 1, 6,16) Hardness MOH scale 7.1-7.3 (15) 0.2-3.5 13) 100-500 16) Av. max. use temp., E”. 360-1650 (7) Thermal conductivity, sq. cal./ sec./sq. om./’ C./cm. X 10 -4 1-17 (16) 18-40 (7) coefficient of expansion C. X 10-4 3-360 (16) 6-102 (7) Chem;cal resistance (relative rating, 10 best) Ionic Oxidizing Organic
2-8.3 (23)
i
Tensile strength0. lb./sq. inch x loa (16). . . - .. . Moddus ot slasticity*, Ib./sq. inch X 1 0 @ i n t e n s i o n ( l 6 ) Rockwell hardnessa ( 2 6 ) Toughness o-3y,n h;rr+aa+\ “ “ ‘ 6 ,ARelative &+:=Modulus of elasticity (16) Izod impact ( 1 6 ) . . Tensile strength and elongation (16) Av. max. use temp ’ F. Chemical resistanze (relative
Amino
5-9
5-10
.
0.35-5.0 1.0-1.13 M50-120 M115-125 8 7.5
Aluminum
3.5-7.0 (8) 0.01-80 (20)
376-1840 (23)
6.0-55
0.5-12
‘(I”IY6,
A-
2.5-14 (16)
2700-5120 (23)
1200-5200 (IS)
1.0-9.0(16)
14-28 (23)
20-40 (13)
2.0 (16)
5-12
0 001-0.5 0.05-0.38 0.18-0.6 M5-80 R50-120 M20-90 14 15 10
Ye’LrU”,
32-70 (8, 15)
0 9-2.0 18) 018-2 215-380 Ol.28) (16)
11-20 (IS)
1.9-8
Wood
14-19 13) 400-550 IZS) 3.3-4.0 (9)
8-17 (1, 6) 2.7-3.5 ( 1 3 ) 300-500 (2)
GENERIC GROUPSO F PLASTICS Reinforced Plastics Vinyl Cellulosic Styrene
0.5-3 0 M90-120 26
Copper
11-40 (6,13)
40-330 (6,2 3 )
27-29 (1, 6,IS) 0.1-0.2 ( 1 5 ) 0.3-0.4 (19) 4.3-8.2 (13) 112-300 (19) 750-1000 ( 2 5 )
Therms:
Phenolic
Steel
Aorylh
Ethylene
Nylon
Halocarbon
6-10
1.4-2.4
6.5-11.0
1.8-5.7
0.38-0 50 0.02-0.03 0.07-0.3 0.05-0.020 M80-105 R11 R45-118 R20 8 17 16 15
6.5 0.5
10 10
3 3
3 6
3 4
4
1
2
1 10
3 5
3.5 4
1 212-450
0.5 170-300
6 163-400
8 100-200
6 115-170
3 140-225
2 140-200
6 100-185
8 140-250
7.5 390-500
6
-”mu,
Ionic (3 24, 16) 6 7 5 8 7 4 10 7 8 4 Oxidizdg (3, 14, 16) 3 3 3 6 4 3 2 7 Organic (8,24, !6) 9 7 7 5 2 3 2 5 8 $0.18-0.27$0.18-0.67 $0.20-1.03 $0.35-0.84 $0.40-0.90$0.33-0.83 $0.70-1.00 $0.43-0.53 $1.60-2.26 :oat of rawmaterial/lb. (11) a Properties at 73.4” F. and 50% rel. humidity.
10 10
10 $5.50-8.00
SOURCES OF DATA
(1) Boonton Molding Co., Boonton, N. J., “Ready Reference for Plastics,” p 54, 1944. (2) Chemical Engineers’ Handbook (J. H. Perry, editor), p. 424. McGraw-Hill, New York, 1950. (3) Corrosion Handbook (H. H. Uhlig, editor), p. 363, Wiley, New York, 1948. (4) Encyclopedia of Chemical Technology (Kirkland and Othmer, editors), Interscience, New York, 1951. (5) Francis, J. J., Product Eng., 22, No. 2, 85-108 (1951). (6) Handbook of Chemistry and Physics, 35th ed., pp. 1962-73 Chemical Rubber Co., Cleveland, Ohio, 1953. (7) Materials & Methods, 37, No. 5, 130 (1953). (8) Ibid., 38, No. 1, 94 (1953).
(9) Mechanical Engineers’ Handbook (L. S. ,Marks, editor), 5th ed., p. 606, NIcGraw-Hill, New York, 1951. (10) Nason, H. K., Carswell, T. S., and Adanis, C. H., Am. SOC. Testing Materials, Spec. Tech. Publ. 78, 1949. (11) Plastics W o r l d , 12, No. 6, 7 (19541. (12) PToduct Eng., 14, No. 9, 583 (1943). (13) Reinhold Publishing Corp., New York, N. Y . , “Portfolio of Material Engineering File Facts,” 3rd ed., 1951. (14) Seymour, R. B., SPE Journal, 10, No. 1, 15 (1954). (15) Technical Data on Plastics, Manufacturing Chemists’ Assoc., Washington, D. C., October 1952. (16) Wangaard, R. E., “Mechanical Properties of Wood,” Wiley, Kew York, 1950.
T h e Rockwell hardness data show the rigid materials to b e about the same throughout, with the flexible vinyl, ethylene, and halocarbon plastics appreciably lower. T h e engineering significance of the hardness measurement has already been discussed. For plastics, particularly laminated thermosets, i t has been found useful in following t h e cure of “punch stock.” Hardness data are most often of value for analytical and quality control purposes. T h e question might now arise as to how these groups compare in “toughness” or energy-absorbing ability. Sharp differences in rigidity, some similarity in hardness, and appreciable variations in tensile strength have been observed, but which groups are tough? The definition of “toughness” is a difficult one, despite the fact that the notched impact test is often looked upon as an accepted measure. It is obvious t h a t this test gives data for only one point on the energy absorption versus speed curve, and measures a composite of properties such as flexibility, fatigue (resistance to flexing above the yield point), ductility (elongation a t failure), and damping factor (energy converted to heat) ( I ) . If i t is assumed that toughness is a composite of these properties, and it seems reasonable to do so, and if toughness is defined as the ability of a material to adsorb a rapid applied energy load without breaking, it is apparent that several properties must b e weighed in its estimation. Reinforced plastics show their ability t o take mechanical abuse, due in large measure to their
high strength. Resilience of ethylene and vinyl plastics is responsible for their high order of toughness. The rest of t h e materials are roughly comparable with the acrylics and, unless modified, are low in toughness. CoId flow or creep also varies with the generic groups. T h e cellulosics, nonrigid vinyl, ethylene, and halocarbon plastics exhibit high orders of creep, with deformation under 4000 pounds per square inch running from 2 to 60% for the cellulosics and 10 to 45% for the nonrigid vinyls. The other materials, ethylene and halocarbon plastics excluded, are in the range 0.2 t o 5% depending on their composition (33). Thermosets are, in general, superior t o thermoplastics in resistance to continuous heat. T h e majority of the thermoplastics have a top use limit a t about t h e boiling point of water. Sylon has somewhat better resistance under the right loading conditions, being capable of resisting temperatures u p to 300’ F The most heat-resistant plastics are the halocarbons, with certain ones having upper limits of approximately 500° F. This is beyond t h e range of phenolics and well above the charring point of wood. Chemical resistance varies widely among the various plastics and is completely dependent on the chemical composition. Resistance t o organic solvents is generally only fair for the thermoplastics. However, nylon and poly(viny1idene chloride) (saran) can withstand attack b y the more common solvents, with the
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Plastics Construction Materials exception of hot benzene. Halocarbons are almost completely resistant t o attack b y chemicals. Table I shows the comparative resistance of the various plastics to ionic, oxidizing, and organic solvents. The halocarbons are uniformly excellent, vinyl and ethylene plastics show good resistance, and cellulosics and acrylics are, in general, fair. I n testing for this property, the impurities normally found in the chemical to which plastics are to be subjected should not be removed, for they often play as important a part in chemical attack as does the major constituent (6). Cost is a factor in deciding between plastic groups. Based on their chemical resistance performance, the halocarbons look inviting and are for many industrial uses. However, their high price restricts their area of application to those critical uses where a cheaper material cannot do the job. The phenolics, polyesters, and ethylene plastics are in the low raw material cost area, as contrasted to nylon and t h e halocarbons which are expensive. Acrylic compounds are slightly higher than the intermediate materials. Desirable properties must be paid for in raw material costs, but this cost may be regained later in terma of lower cost of fabrication or better service performance. PLASTICS AND THEIR ENVIRONMENT
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The study of general properties for comparison and selection of plastics is necessary, but one of the designer's prime interests is the properties of the material as a function of the environment in which i t is to be used. Environmental temperatures of use may vary from -60" to +160" F., the air temperature extremes encountered in various parts of the world, without considering process-caused variations. Organic plastics are affected by such changes just as are other materials but to a much greater degree. Plastics that are hard and brittle at room temperature may become soft and ductile at less than 100" F. above this point. Stiffness or elastic modulus decreases with increasing temperature for all plastics. Thermoplastics show a much more substantial change in this characteristic than do the thermosets. Likewise, strength, whether i t be tensile, compressive, or shear, decreases with increasing temperature. Again the thermoplastics are more sensitive t o this temperature change than the thermosets. Elongation, or, in more general terms, deformation, for a given load, increases with temperature. Humidity can be an important factor in controlling the properties of certain plastics. Moisture acts as a plasticizer in many cases and leads to a reduction in strength and stiffness and an increase in ductility and toughness. Plastics with hydrophilic groups such as cellulosic, casein, and the nylon or polyamide materials (compounds with OH, COOH, or PITH groups) are particularly sensitive to the presence or absence of moisture. Conversely, plastics lacking these hydrophilic groups but rich in carbon, hydrogen, and halogens are very resistant to moisture (9). Chemicals, likewise, can lead to deterioration of plastics through the dissolution of cross linkages or plasticizer extraction or absorption. Change due to moisture content and chemical attack is catalyzed b y higher temperatures. Weathering is a factor, which, because of its complexity, defies any broad generalization relative to its effect on the behavior of plastic materials. Those plastics affected b y ultraviolet lightLe., those containing double bond groups such as the carbonylsdegrade with some degree of rapidity in the presence of sunlight. This degradation is evidenced by the significant reduction of atrength, ductility, and toughness ( I O ) . Resistance to sunlight may be substantially improved b y the incorporation of pigments and fillers which absorb or reflect the ultraviolet light at the surface thereby protecting the interior of the plastic. As is the case with moisture and chemical environments, ultraviolet attack is accelerated with increase in temperature, July 1955
The foregoing facts of life for plastics have sent many scurrying back to the conventional materials with equal but more familiar disadvantages. Others want to know just what are the magnitudes of these changes related to environment, Are the changes serious? Can they be coped with? Actually, the importance of the changes depends on the application. I n applications involving low load bearing structures that are required only to maintain their shape at elevated temperatures, the maximum use temperature would be a useful guide t o choosing the right plastic. Heat aging will cause a change in such properties as tensile strength and elongation, but u p to a 25% change after 60 days' exposure to 60" to 80" C. is not held to be excessive for vinyl materials, according to Underwriter'a Laboratories (33). I n other words, if the magnitude of the change is known, i t can be compensated for in the design. If the structure is to carry a permanent load a t an elevated temperature, the problem of deciding which material will stand up best to environmental changes is somewhat more involved. First, as it has already been pointed out, there is the temperature effect on the properties of most of the plastics. An increasing amount of information on the behavior of plastics over the range of possible temperatures (-60' to +160° F.) is becoming available. The reinforced phenolics and polyesters, which are among the best heat-resistant structural materials, increase in impact strength up t o 80% over the range indicated, decrease in flexural strength from 25 to 3070~and decrease in tensile strength from 12 to 25% ( S I ) . Nylon, a medium heat-resistant material, in a given formulation increases in impact strength u p to 450% over the range given, while decreasing in tensile strength from 50 to 60% (31). The lower heat-resistant plastics, of which the cellulosic, ethylene, styrene, and vinyl plastics are examples, increase in impact strength from 15 to 140070 over the range -60" to +160" F. ( S I ) . ( I t would perhaps be more correct to say that the impact strength becomes indeterminate since once a material becomes soft, the specimen will simply bend out of the way-Le., it will not fracture. These data apply only to the materials in rigid or semirigid form.) The flexural strength of this low heat-resistant group decreases b y 35 to BO%, and tensile strength drops off from 35 to 80% over the -60" to +160" F. range. Secondly, there is the temperature-time effect on properties. Instantaneous observations of the effect of elevated temperature can be quite different from those made after exposure to the elevated temperature for substantial periods of time (14). Thirdly, there is the load-time effect on properties of most plastics. A t room temperature, the stress to rupture after 5000 hours of continuous load application is reduced to 8501, of the original value in the case of phenolic laminates, to 67% for poly(methy1 methacrylate) or acrylic, and to 60% for cellulose acetate (one of the cellulosics group) sheet (6). An increase in temperature will lower these values. Thus, data on the temperature-time-strength relationship must be known or obtained for design of load bearing plastics at elevated temperature. Unfortunately, the bulk of strength and deformation data for plastics is based on short term tests. This is true for many other materials of construction. I n general, such data should not be used directly in design calculations unless information on creep and long time strength behavior is available so that the proper safety factor may be applied. However, progress is being made in mathematically defining the temperature-time-strength relationship so that a designer having data available under one set of conditions can calculate how a given material will perform under the conditions of use anticipated in a given application. I n the field of fiber glass-reinforced polyester panels, equations have been developed or adapted relating temperature-time-stress similar to those used for steel. I n other investigations, equations have been developed for acrylic and laminated thermosetting materials ( 2 , 28, 19). The mechanism b y which moisture affects the properties of plastics seems to be primarily one of water absorption. Water
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diffuses through the material by capillary action, saturating the plastic in 11/2 to 2 years on the average ( I S ) . Water vapor on the other hand, requires up to one and one-half times this period to accomplish saturation. Styrene, ethylene, and vinyl plastics have low water absorption, and their properties are but little affected. The acrylics, representative of medium water absorption materials, suffer up t o 30% decrease in original tensile strength as the relative humidity is raised from 0 to lOOyo a t room temperature. The high moisture absorption plastics such as the cellulosics lose up to 'toy0under the same conditions (11). Chemicals, like water, can change the properties of plastics materials, either in a mechanical sense (plasticization) or through chemical attack of the basic polymer structure. Resistance is dependent on the plastic material and the filling and reinforcing materials. Changes are normally greater than for moisture attack. It must be remembered in making use of data showing chemical attack on plastics that higher or lower concentrations of the reagent may make a given chemical more or less reactive and that reagents contaminated or mixed with other reagents may react differently than the pure reagent. Temperature and pressure can advance or retard attack. The degree of chemical attack is indicated by reduced mechanical strength and in certain instances in appearance and electrical property changes. Therefore, because of the number of factors involved, selection of plastic materials should be made after trials of the material in the environment to which it is t o be subjected in use. Short term tests for this property should be indicative. I n such tests made by immersion for 168 hours a t 30" C. ( 1 1 ), an admittedly short time test, the following changes were noted: Styrene plastic, a relatively chemical-resistant material, showed negligible effect t o most acids, both strong and weak, lost up t0.20% in strength when immersed in weak and strong base solutions, and suffered 60 t o lOOyo loss in strength due t o immersion in organic solvents. The phenolics, medium chemical-resistant plastics, showed 20 to 60% loss in strength in strong and weak acids and weak bases, but 60 to 10070 loss in strength due t o strong bases and solvents. Reinforcing materials improved the resistance of the thermosets because they are more resistant in themselves and restrict the chemical attack t o a surface effect. Materials with fair chemical resistance such as the cellulosics showed a 20 to 60% decrease in strength due to contact with strong acids, weak acids, and strong bases and a 60 to. 100% decrease when in contact with weak caustics and organic solvents (19).
ECONOMICS
The use of plastics as materials of construction outdoors has necessitated developing information on the durability of these materials when exposed to the weather. The contributing effects of cyclic wetting and drying, cyclic freezing and thawing, removal of volatile or soluble constituents, and ultraviolet radiation are thought to be the most important factors in the outdoor aging of plastics, Phenolics, in general, weather quite well but warp and crack under conditions of moisture absorption and heat exposure. Heavy sections of urea-formaldehyde are markedly affected by high moisture exposure. Highly plasticized materials such as the cellulosics and nonrigid vinyls lose plasticizer and become brittle because of EL leaching process that takes place in contact with water or high humidity atmospheres (21). Acrylics are not seriously affected by the weather. Vinyl and its copolymers have rather variable resistance t o outdoor exposure which is a function of the manner in which they are formulated and stabilized styrene malleate (and phthalate) polyesterglass-reinforced plastic shows a general decrease in mechanical properties after 3 years' weathering under various climatological conditions (54). There is general agreement that subtropical climates have the worst effect on plastics, but there are excep tions to this. Metropolitan industrial atmospheres have been shown to attack certain plastics quite severely in a short period of time. Recently a weathering program involving styrene plastic
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(crystal and pigmented) and phenolic (black pigmented wood flour filled) where the specimens were exposed in three different geographical locations was completed. The results of this investigation showed that the mechanical properties of unpigmented styrene were seriously degraded by as little as 3 months' exposure, It is believed that the controlling factor was the amount of sunlight reaching the sample. Addition of pigment significantly improved the weather resistance of this materiale.g., the tensile strength decreased only 5 to 20y0 in a 4-year period for all locations. The surfaces of both styrenes were roughened during the exposure period. The subtropical climate (high humidity, high temperature, and moderate rainfall) caused the most severe degradation of the mechanical properties of the phenolic. Tensile strength dropped by 30% compared to 25y0 for a cool temperate climate and 20% for a semidesert region after 4 years of exposure. The humidity factor in the environment 'seems to have the greatest effect on the properties of the phenolic. The specimens exposed in hot, dry climate showed an increase and then a gradual decrease in strength with time. This was probably due t o removal, a t first, of some of the entrapped water left in the specimen by the condensation reaction during molding and curing. Removal of this plasticizing moisture increased the strength which was later reduced b y either degradation of the resin or filler component ( 3 ) . I n general, it can be said that the more opaque materials weather best. As the science of stabilization is advanced, more weather-resistant transparent materials can be expected t o be developed. There is some indication that plastics placed under load and then weathered will age faster than plastics exposed in a nonstress condition. Phenolic and amino (urea-formaldehyde) resins with various fillers were put under a bending stress for 26 months of weathering and lost slightly over 5Oy0 of their original strength ( $ 1 ) . It should be remembered, that longtime applications of stress can lower the strength of the plastic not exposed outdoors. Here, also, it is desirable to have actual environmental data t o predict performance, but weathering is time consuming and accelerated aging dpta correlates only to a limited extent with actual outdoor exposure. This is true partially because it is extremely difficult t o build into one test the many environmental conditions encountered outdoors and partially because the pattern of weather even in one geographic location does not duplicate itself from one year to the next (21).
In the industrial application of any material of construction, its properties and economics must balance. This is tantamount to saying that it must be the cheapest material available for the performance. Low cost can be achieved either through low initial cost of the material or by low maintenance cost. It is obviously not possible t o make any blanket statement either regarding the suitability of the properties of plastics or their economics in chemi,cal industry applications. There are many places where plastic materials are the only materials that can do a given job, and there are many places where they do not warrant the slightest consideration. Two examples of the uses of plastics in the chemical industry can be presented to show how properties and costs work together to control the choice of material of a given application. The first example involves the installation of cellulose acetate butyrate pipe for handling corrosive well water (16). The pipe cost was 5001, more than the material normally used but took half as long to install. The cellulosic pipe lasted four times as long as the conventional material, thereby saving three installations and three purchases of pipe. Finally, pumping costs were reduced since the flow through the pipe was 40% greater when it was made of plastic rather than the conventional material since the smoother interior surface causes less pressure drop. The second example is that of a 500-barrel tank installation. The initial price was 20% greater than for galvanized steel.
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-Plastics However, it weighed only 1600 pounds as compared to the 7940 pounds for steel. Shipping costs and erection costs were much lower for the plastic tank. Likewise, the plastic tank required no maintenance for its surface because of its good chemical resistance. The low thermal conductivity of the plastic tank led to a further saving by cutting down loss of gas ( 1 5 ) . SUMMARY
1
Plastics are relatively new, untried materials on the chemical industry construction scene. I n recognition of this, the intent of this presentation has been to provide information which will help the designer orient his thinking about plastics and what they can do so that he may make a sound decision as to whether he should consider plastic material or one of the conventional materials in a given application. If the decision is to use plastics then he should consider which of the various generic groups will best perform the required function. The final step of choosing a particular formulation or composition and developing the economics is one that is necessarily worked out with the plastics material supplier and the fabricator. REFERENCES (1) Adams, C. H., Am. Soc. Memo to Sec. F, Sub. 1953.
(8) Ibid.. KO.6, p. 91. (9) Carswell, T. S., and
Nason, H. K., Am. Soc. Testing Materials, Philadelphia, Pa., Special Tech. Publ. 59, p. 31, 1944.
(10) ( 1 1) (12) (13) (14)
Ibid., Ibid., Ibid., Ib(d.,
p. 35. p. 335. pp. 336-7. p. 341.
Carswell, T. S., Telfair, D., and Haslanger. R. U., M o d e r n Plastics, 20, NO. 6,79-82 (1943). (15) Chrm. Eng.. 59, No.12,218 (1952).
(16) Ibid., p. 219. (17) Chem. E n g . News,32, No. 22, 2180 (1054). (18) Ibid., No. 27, p. 2696. (19) Dietz, A. G. H., presented at annual -4S;CIE meeting, December 1953. (20) Glick, S. E., I n d i a Rubber WorZd, 125, 192-4 (1951). (21) Grimm, G. O., Brit. Plastics, 19, No. 212, 22-30 (1947). (22) Larson, W. S., Product Eng., 15, 845-8 (1944). (23) Mason, J. P., and Manning, J. F., “Technology of Plastics and Resins,” p. 53, Van Nostrand, New York, 1945. (24) Ibid., p. 54. (25) Ibid., p. 115. (26) Manufacturing Chemists’ A4ssociation, Washington, D. C . , “Technical Data Book on Plastics,” October 1952. (27) M o d e r n Plastics, 31, No. 77, 73 (1954). (28) hIoore, H. E., and Moore, M. B., “Textbook of Materials of Engineering,” McGraw-Hill, New York, 1941 (29) Reinhart, F. W., SPE N e w , 4, 3 (1948). (30) Rose, K., Materials & Methods, 37, No. 1, 87 (1953). (311 Sasso, J., “Plastics Handbook for Product Engineers,” pp. 10252, McGraw-Hill, New York, 1946. I
Testing Materials, Philadelphia, Pa., VI, ASTM Committee D-20, August
(2) Ibi$.;Special Tech. Publ. 132, 1952. (3) Adams, C. H., and Taylor, J. R., I b i d . , Preprint 54-SA-68,1954. (4) Adams, C . H.. Findley, W. N., and Stockton, F. D., Dresented (5)
Construction 1Materials
at annual ASME meeting, December 1954. Bruner, W. M., and Waynes, P. J., Chem. Eng., 61, No. 7, 38,
f1953). ‘6) Ibid., p.’ 194. 7 Campbell, J. B., Materials & Methods, 38, No. 5, 89 (1953).
(32) Staff, C . E., Quackenbos, H. M., end Hill, J. M., M o d e r n Plasfics, 27, NO.6, 93-6 (1950). (33) Underwriter’s Laboratories, New York, N. Y., “Standards for (34)
Thermoplastic Insulated Wires,” 1954. Yustein, 5. E., Winans, R. R., and Stark, H. J., A S T M Bull.,
196, 29, 1954. RECEITEDfor review September 17, 1954.
ACCEPTEDM u o h 21, 1Q5&
Engineering Plastics into Chemical Plants Maximum economic utilization of plastics as engineering materials cannot be effected until the design engineer, the construction engineer, and the maintenance and safety engineers have more complete and specific information. Engineers require measured data that will permit design and construction of equipment with assurance that such equipment will perform adequately under process conditions. Data such as chemical resistivity, stress to rupture, creep strength, and fatigue strength should be available for the entire range of allowable working temperatures. The materials employed should be of standard quality and dimensions. Standard methods of fabrication, erection, and maintenance must be developed. “Areas of ignorance” are emphasized in order to encourage development of fundamental data to keep pace with the growing application of plastics.
H. E. ATKINSON E. I . d u Pont de Nemours 6% Co., Wilntington, Del.
P
LASTICS occupy the headlines today in prominence and
profusion The uninformed reader may easily conclude that a line of new engineering materials is fully developed to solve the corrosion problems of the chemical industry or t o effect cost reduction. However, before maximum economic utilization of plastics can be effected, engineers must have more complete and specific information. The purpose of this article is to show the
July 1955
needs of design engineers, construction engineers, and maintenance and safety engineers. A review of the sequence of events in engineering plastics into chemical plants will also point out the important role of research chemists, resin manufacturers, molders or extruders, fabricators, and the engineer himself in satisfying the needs. It is unreasonable to expect that all data required on plastics
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