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E. W. RUGELEY, T. A. FEILD, JR.,.AND G. H. FREAION Carbide and Carbon Chemicals Corporation, South CharlestonL,W.Vu.
I S Y O S vinyl chlorideGreatly improved solvent resistance, better dye affinity, and acrylonitrile when i t was and higher softening points have been achieved in vinyl discovered t h a t certain of vinyl acetate copolymer fibers, which have been uscd fibers without sacrificing the high strength, nonflammathem are soluble in acetone. bility, exceptional chemical resistance, and immunity to Vinyl chloride copolymers of commercially for almost a decade, are characterized by rot arid mildew that characterize earlier fibers of the molecular lyeight high enough high strength, nonflamvinyl f a n d j - . These improvements are obtained by exto yield attractive fibers are mabilitp, exceptional chemiploiting two unusual properties of the vinyl chloridenot suitable for melt extruea1 resistance, negligible acrylonitrile copolymer resin from which the new fibers sion or similar methods of water absorption, and imare made. In the form of filaments fillma, this resin forming arid they must be niunity to rot and miklewshows a pronounced rise in softening point as a result of soluble, in a practical sense, a combination of properties molecular orientation, and it becomes insoluble and in order to be useful in t h a t distinguishes them from shows a further rise in softening point upon exposure t 3 fiber applicat'ions. T o be all other materials available clcvated temperatures. particularly attractive, they to the textile industry (2, must be soluble in cheap, 1 6 ) . They are also characl o v boiling, chemically stable terized by a relatively 1 o strain-release ~ temperature and solvent's which can be employed on a commercial scale susceptibility to attack by numerous organic solvents, h o w without serious toxicity hazards. As the properties of coever, and primarily for these reasons they have been limited polymers are usually predictable with fair accuracy from the to specialty applicat,ions. properties of homopolymers of the individual components, A new family of vinyl fibers, designated as T'inyon N, was anand as both polyvinyl chloride and polyacrylonitrile having molecular weights within t h e "useful" range are insoluble in nounced in February 1947 ( 1 ) and put into production on a pilot plant scale shortly thereafter. The nen' fibers possess the excelacetone, it was surprising to find that any high molecular weight lent, combination of properties of t'he earlier \-inyon fibers, and in copolymers of the two are acetone-soluble. This discovery inaddition have much higher softening points and much better vited iiiteiisivc study of these copolymers in fiiixrs, cast film, and solvent resistance, and therefore qualify for many applicat,ions coatings applications. from which the vinyls have been barred heretofore because of PROPERTIES OF THE RESIN their limitations in thcse important rcspects. Vinyon S fibers are based upon a copolymer of vinyl chloride and acrylonitrile 77-hich, The copolymer resin used for the manufacture of Vinyon N fibcrs a t the present time contains 56 to 60% of vinyl chloride and in properties and behavior, is surprising in many mays. I t is these unusual featuyes, and the w r i o u s ~ a y in s which they are exhas a specific viscosity, as measured in cyclohexanone (0.2% solution), of 0.25 t o 0.30. Thc rcsin is produced by the polymeriploited, t,hat v d l receive most attention in this paper. The commercial history of vinyl chloride resins dates back to zation unit in the form of a fine powder resembling floiir in appearthe late twenties, and is familiar to a11 who are acquainted XT-ith ance, and like most vinyl chloride polymers and copolyniers i t can be converted into a coherent resinous mass by fluxing on the plastics industrp. Acrylonitrile has been I r n o ~ ~ton chemists heated rolls a t a temperature of about 180 C. The density of the for many years, arid was copolymerized with various vinyl and fluxed resin is about 1.30, and its color ranges from light amber t o acrylic compounds as early as 1931. It first appeared as an important component of a commercial polymer in this country dark brown. depending upon the purity of the sample and the when butadiene-acrylonitrile synthetic rubber was put into proamount of thermal abuse t o vvhich it has been subjected. Vinyon S copolymer resin can be molded by the compression duction in 1940. At, present it is used to the extent of 20 to 40";Ic in nitrile rubber, and in certain relatively new vinylidene chloride technique, but in common with most other resins containing high proportions of acrylonit'rile, its flow characterist'ics are very poor and styrene copolymers. Polymers high in acrylonitrile content, are, as a class, extremely compared wit,h those of vinyl chloride-acetate copolymer resin, for example. These t w o are compared in Some detail in Figure 1, difficult t o fabricate by processes involving resin flow at elevat.ed in which "apparent viscosity,'' as calculated from the flow of a temperatures. They are not amenable to plast,icixation,and their short cylindrical specimen bet'ween parallel plates under a load of troublesome solubility characteristics have discouraged the de2000 pounds per square inch, is plot'ted against temperature. velopment of solution processes. Thc rclatively slow commercial Molded Viiiyon X resin shovc-s a heat-distortion point (A.S.T.M. development of acrylonitrile polymers and copolymers containing D-648) of about 85 C., and for reasons which are discussed in high proportions of acrylonitrile does not, mean, however, that, greater detail below, molded specimens tend t o be fibrous rather research has passed them by. On the contrary, they have been t , h m completely isotropic in nat,ure. Even isotropic specimens studied intensively in many laboratories arid in semicommercial show unusual toughness and dimensional stability under load, development plants during recent years, and considerable progress however, and several critical, if small scale, applications have has been made in the development of copolymers which are atbeen found for molded resin which justify the relatively complitractive despit'e the difficulties attending the use of acrylonitrile. cated molding operations required. Progress has also been made ia the development of new techThe solubility characteristics of acrylonitrile copolymers are niques for handling t'hese and other intractable synthetic resins. considerably more complex than those of the familiar vinyl, vinylRecent' patents issued to Carbide and Carbon Chemicals Corpoidene, and cellulose resins (13),for example, and fcir general staterat'ion (23: $43, E . I. du Pont de Nemours & Company ( 8 ) ,and ments about solubility behavior are possible. The unexpec.tcd General Elect'ric Company (111, among ot'hers, reflect activit'y acetone solubility of certain vinyl chloride-acrylonit d o n g both lines as early as 1940. mers, mentioned above, is also found in certain vinyl Attention was first attracted to copolymers of vinvl chloride 1724
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INDUSTRIAL AND ENGINEERING CHEMISTRY
IC ride-acrylonitrile copolymers and styrene-acrylonitrile copolymers. I n the case of vinyl chloride copolymers, within the range of molecular weights corresponding t o specific viscosities of 0.10 to 0.50, acetone solubility extends over the range from about 20 t o about 55% of acrylonitrile, t h e practical limits depending t o some extent on molecular weight. Copolymers low in acrylonitrile content show about the same solubility characteristics as polyvinyl chloI I ride ( I @ , but as acrylonitrile content is increased, the resins become progressively less IO6 soluble in higher aliphatic ketones such as methyl isobutyl ketone, and more readily soluble in acetone. Copolymers in the range of composition preferred for fibers show only slight solubility in methyl ethyl ketone, and are virtually insoluble in methyl isobutyl ketone, butyl acetate, and other common solvents. The cyclic ketones cyclohexanone and isophorone dissolve these resins readily, as does dimethyl W e formamide, b u t tolerance for diluents i.s low. a Some coupler action is shown by small amounts P a of certain solvents such as tetrahydrofuran or methyl ethyl ketone when blended with acetone. Thus the list of practical solvents is small, and while this imposes limitations upon vinyl chloride-acrylonitrile copolymers in surface coatings and cast film applications, it does make possible lacquers and films having good resist ance to many solvents. Solubility limitations can be overcome t o some extent by preparing dispersions of finely divided resin in nonsolvents such as the higher aliphatic ketones, following the organosol techniques developed for vinyl chloride-vinyl acetate copolymers (18). 140 The solubility characteristics summarized above are reflected in the plasticizer compatibility of vinyl chloride-acrylonitrile copolymers, and it is found t h a t relatively few of the plasticizers ordinarily employed in connection with vinyl chloride resins are compatible with Vinyon N resin. The difference in plasticizer susceptibility is so great, in fact, t h a t powdered Vinyon X resin can be used as a filler, or Vinyon N fabric can be used as a reinforcing core (E%), for vinyl elastomeric compounds based upon many of the more popular plasticizers. Panticizcr 8 (0- and p-toluene ethyl sulfonamides) and tetrahydrofurfuryl phthalate have been found compatible with Vinyon N resin, however, and elastomeric compounds characterized by high strength and great toughness can be obtained with these plasticizers. Vinyon N resin, like other polymers and copolymers of vinyl chloride, becomes markedly less soluble as a result of prolonged exposure t o elevated temperatures ( 7 ) . This tendency can be enhanced by incorporating certain alkaline materials such as zinc oxide, hexamethylenetetramine or aiethylenetriamine, or rubber accelerators such as 808, BJF, Tuads, or Monex, into the resin. T h e rates of insolubilization of Vinyon N resin and of Vinyon vinyl chloride-acetate' resin, as indicated by weight per cent of cast films extractable with boiling acetone, are shown in Figure 2. Molded specimens also tend t o become acetone-resistant as a result of prolonged heating, and acetone-absorption measurements on molded Vinyon N resin and polyvinyl chloride, respectively, yield curves similar to those of Figure 2. The relatively high rate of insolubilization of Vinyon N resin is a matter of considerable pr$ctical importance, particularly in ' view of its pronounced resistance t o other common solvents. Furthermore, the resin attains a high degree of solvent resistance with little if any decline in mechanical strength. This enhanced solvent resistance is obtained a t the cost of only some darkening
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FLOW C H A R A C T E R I S T I C S F RESINS AT E L E V A T E D
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TEMPERATURE, DEGREES CENTIGRADE
of color, as opposed to the blackening of other vinyl chloride resins ( 2 4 ) . Polyacrylonitrile and all commercial copolymers of acrylonitrile known to the authors are Characterized by a yellow to amber color which becomes more intense as heating is prolonged. Although numerous methods have been advocated in the literature for obtaining acrylonitrile resins of superior color (IO), neither a satisfactory explanation of the characteristic color nor a satisfactory method of avoiding i t has been advanced. The color problem is being studied activcly a t the present time, but this study has not progressed to the point of yielding a comprehensive hypothesis t o explain the color and color change of acrylonitrile resins. It does appear, however, t h a t in color and color-stability characteristics Vinyon K resin resembles other acrylonitrile copolymers and polyacrylonitrile itself, more closely than it does polyviny! chloride. Tn certain respects-in its tendency to bleach in strong sunlight, for example-it differs markedly from polyvinyl chloride. The cyanide group has been found to be much more firmly bound to the macromolecular chain than is the chlorine group in vinyl chloride resins, and, as noted above, Vinyon N resin resists blackening much more strongly than do copolymers containing high proportions of vinyl chloride. These and other points of evidence make i t appear doubtful t h a t the conjugated-double-bond hypothesis (6),commonly accepted in explanation of the discoloration of polyvinyl chloride, is adequate to explain the color behavior of Vinyon N resin. Vinyon N films, fibers, and molded articles show a pronounced rise in softening point as a result of prolonged exposure to elevated temperatures-an effect which is of a higher order of importance
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In the latter process, a well filtered acetone solution containing approximately 25v0 of resin and 0 small amounts of heat and light stabilizers is a mct.ercd under pressure through standard stainW less steel spinnerets. The fine st,reams of solution z thus formed emerge a t the top of a vwtical spin0ning tube xi-hich is 8 inches in diameter and 20 feet 0 long, into a countercurrent st,reaIn of h o t air. The E 4 solvent is flushed off, leaving continuous filaments which are gathered under slight tension a t W the bottom of the tube, and mound externally m on spools by means of a cap-spinninx device. Fibel, 2 size may be varied through a considerable range, U a but filament deniers of 10 t o '20 reprcscnt t,he 3 optimum, as judged on the basis of both yarn verW satility and operating ease. Thc optimum rate W 2 of yarn travel through the spinning cell dcscribcd 0 W c here,ia about 100 nietcre per minute. 0 U The processing operations to which the yarn is HEATING PERIOD. HOURS AT 150 DEGREES C E N T I G R A D E subjected after spinning depend upon the kind of application for which it is intended. As spun, Figure 2. Effect of Prolonged Heating on Solubility of t8hemacromolecules that make up the fiber are Vinyl Chloride Resins essentially unoriented, and the yarn a t this stago has a tenacity of about 0.7 to 0.9 gram per denier and ultiniate extensibility of 5 t o 40%,, depending upon the ratt: in fibers than the insolubilization effect discussed above. The of loading. It is chardcterized by moderate stiffness and goocl magnitude of the rise in softening point, like softening point itself, resiliency, and it, is attractive for applications where these propdepends to a very great extent upon the magnitude of the load erties are desired, but where modest tensile strength is adequate. employed in ina.king the measurement, arid upon the rate of temP a r u intended for such applications is usually st,abilized dimcnperature rise. In molded specimens, the heat-distortion point siorially by immersion in boiling water, or by exposure to air at rises from about 85 O to 89-90 C, as a result of 2 hours' heating temperatures of 100" to 150" C. It is readily processable on a t 150" C. The heat-distortion point, may be raised further, t o standard textile equipnicnt. 97-98" C., by incorporating 2% of zinc oxide in the resin before Vinyon N fibers, like many synthetics, attain very high degrees heating. According t o the print-point test employed in connocof molecular orientation as a result of stret'ching. Tenacity intion with surface coatings, 2 hours' heating a t 150' C. raises t'he creases regularly with degree of stretch; ult'imate elongat,ion desoftening point of the film from 93' to 127' C.-these values indicreases and stiffness moddus increases as the macromolecules arc cating the temperature at which the surface shows marring by a forced into parallel alignment. Vinyon N yarn cannot be coldrough second surface under a load of 1 pound per square inch. drawn at practical rates, but the drawing or stretching operation It is shown below that the service-temperature limitation on proceeds smoothly a t elevated temperatures. The stretch imstretched fibers may be raised by as much as 65 ' C. by similar pcked is retained if tension is maintained until the temperature of heat treatment. the yarn declines by a few degrees. In present semicommcrcial operations, the yarn is stret,ched continuously in specially deMANUFACTURING PROCE 5SE S signed steam cells. The actual mechanics of the stretching operation are based upon the thermoplastic nature of the resin, and it is Vinyl chloride and acrylonitrile are copolymerized by a n emulcarried out in either a steam or a hot air atmosphere at temperasion process (24)similar to that used for the copolymerization of tures ranging from 117 to 170 ' C. , depending upon the degree of butadiene and styrene, and also similar to other copolymerization stretch desired and the previous history of the spun fiber. As is reactions involving vinyl chloride. I n sharp contrast t o the true of other thermoplastic yarns, most properties improve up t o a readiness with which vinyl compounds and acrylic compounds polymerize and copolymerize, the vinyl chloride-acrylonitrile copolymerization reaction tends to proceed slowly under ordinary too. conditions, but it can be accelerated by catalyst systems of the redox type (3,d ) , and controlled by means of the usual degraders I so FIGURE 3 and modifiers. The copolymerization ratio of vinyl chloride and acrylonitrile is rather high in favor of acrylonitrile, as indicated by Figure 3, and it is necessary to feed acrylonitrile continuously during the copolymerizat'ion reaction in order to obtain resins of t>hedegree of compositional uniformity desired for fiber applications. Coagulation, washing, and drying of the resin must be carried out with due regard for its heat reactivity, but IIO particular difficulty in this respect is encountered in practice. As metallic contamination has an adverse effect upon bot,h the color and the color stability of resin and yarn, stainless steel or glasslined equipment is used for the polymerization, coaguiation, and drying operations. The conversion of the resin t o fiber or yarn form is accomplished by either the dry- or the wet-spinning process, depending upon the particular qualities desired in the finished product (28). The wet process is particularly well suited to the production of staple fiber and heavy denier continuous-filarnent yarn. The dry spinning process is ordinarily employed for the manufacture of con0 IO 20 30 40 50 60 70 80 90 100 tinuous-filament yarn in fine and medium sizes, and it is the more VINYL CHLORIDE I N M O N O M E R , P E R CENT B Y W E I G H T widely applicable of the two. c z
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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t o shrink appreciably during the process. In this case, heat modification raises the yield point of the yarn and markedly improves its dimensional stability. When stretched yarn is heat-modified on rigid bobbins, a final product is obtained which is characterized by excellent tenacity, high yield point, a n d relatively low ultimate elongat ion. When the yarn is heat-modified in collapsible packages t o allow a controlled amount of shrinkage, the product has slightly lower tenacity and higher ultimate elongation. The more cevere the conditions of heat modification, the greater is the dimen~ionalstability of the yarn a t elevated temperatures, and the greater is its resistance to solvents. Highly modified yarns, for example, can withstand boiling acetone without damage, and can be coated with Vinyon N resin or other resins in acetone solution without suffering m y loss in strength, Before the heat-modification step, the yarn is subjected to a standard twisting operation onto spools or packages designed t o meet the requirements of the heating operation. I n the present development plant, the various spinning, stretching, and heatmodification steps are, for reasons of flexibility, separate operations, but other experimental work has demonstrated that foi most purposes these steps can be combined into one continuous pi oress with probable improvement in over-all quality and substantial savings in operating costs. EFFECT OF PROCESSING ON SHRINKAGE CHARACTERISTICS OF YARNS
TEMPERATURE, DEGREES CENTIGRADE
certain degree of stretch; in the case of Vinyon N yarn, satisfactory combinations of properties are obtained by stretrhing the spun yarn ten- to thirteen fold. Stretches as high as 2500%, based upon spun yarn, and filament deniers as low as 0.35, have been attained in experimental work, but in practice the filament .denier of stretched yarn usually falls within the range 0 75 to 2.0. Stretched Vinyon N yarn has temperature characteristics different from those of unstretched yarn because the macromolecules, being more or less completely extended, are in a low entropy state and will return t o a state of higher probability if the temperature is raised t o the point where relaxation is possible. Although i t is not subjected to a stretching operation, some strains are imparted to spun yarn during the spinning process, and when this yarn is heated it begins to show some shrinkage at about 60" C. and in boiling water i t shrinks about 20%. Stretched yarn also begins to show some shrinkage a t a temperature of about 65" C., and a t higher temperatures show3 greater shrinkage than unstretched yarn, as the molecular strains inherent in stretched yarn are considerably greater than those in unstretched yarn. The amount of shrinkage suffered at any given temperature by yarn made with a given degree of stretch is highly reproducible, and because there are definite applications for yarn with this characteristic @ I ) , some stretched yarn is marketed without further treatment other than standard twisting and packaging operations. For the more general applications in which better dimensional stability 'or superior resistance t o solvents is required, however, some degree of heat treatment is indicated. The major variables in the heat-modification process are temperature, time, and allowed shrinkage. Unstretched yarn is heat-modified while wound on rigid bobbins, and is not allowed
Spun yarn, as obtained from the spinning cell, shows a maximum of about 25% shrinkage when heated t o temperatures well above its strain-release temperature. It contains the minimum amount of stretch consistent with practical spinning operations, and the degree of molecular orientation which prevails is very low compared with that achieved in yarns stretched tenfold, for example, after spinning. I n Figure 4,shrinkage-temperature curves of the spun yarn in both wet and dry atmospheres are compared with a relative volume-temperature curve of the type ordinarily employed for determining second-order transition temperature. The second-orappears to lie at 91 C.-agreeing fairly der transition point (6,Q) well, as would be expected, with the heat-distortion point of 85 O C. and with the print-point value of 93" C. reported above-and it also corresponds t o the point of maximum slope of the shrinkage-temperature curve of spun yarn when heated in a dry me-
TEMPERATURE, DEGREES CENTIGRADE
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TE M P E R ATU R E, D E G R E E S C E N T I G R AD E
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has been chilled (I@. Raw rubber in this stretched, "frozen" condition is reported to show an intense fiber diagram on x-ray analysis, and to have mechanical properties approaching t'hosc of a n inelastic fibrous material. This fact, supported by the additional fact t,hat stretched specimens are highly resistant t o benzene while unstretched specimens go into solution readily a t the same temperature, leads to the hypothesis t h a t crystallization of the macromoleeulcs, as a result of &etching, is responsible for the rise in strain-release temperature Vinyon N fibers, like raw rubber, show an increase in degree of crystallinity and in size of crystallites as a result of stretching, but' the degree of crystallinity is so lo^, compared with bhat of stretched rubber, nylon, polyethylene, or saran, that the magnitude of the influence of crystallinity on strain-release temperature is open to question. Unatretched fibers show a broad, hazy ring similar to that, found in diagrams of copolymers in which vinyl chloride predominates, and it is therefore believed that, the degree of crystallinity in unstretched fibers is very low, and that such crystallites as do exist are very small. X-ray diagrams of stretched Vinvon N fibers are considerably sharper than those of unstretched fibers, however, and though the patterns are still weak compared with those of the familiar crystalline materials, it is apparent t h a t both the degree of crystallization and the average size of t h e crystallites become appreciably greater as a result of stretching. This IS in sharp contrast to the behavior of such essentially noncrvstalline materials as vinyl chloride-acetate copolymers, and also of highly crystalline materials such as polyethylene, neither of which shows any appreciable change in degree of crystallinity or crystallite size as a resuli of stretching. Although the actual influenee of crystallinity on the shiinkage-temperature. characteristics of stretched Vinyon S fibers is still a matter of speculation, therefore, it does appear t h a t Vinvou S yarn conforms, at least qualitatively, with the prevailing theories corei ing the behavior of rubber. T h e shrinkage-tempeiature properties of five different grades of Vinyon N yarn in dry heat are shonn in Figure 8. Similar relationships are observed in aqueous media, but as in the case of the spun yarn, shrinkage a t any given temperature is higher by a fern per cent in water or steam than in a dry medium. D a t a on the effect of mater are presented in Table I. Y
dium. Figure 4 also shows t h a t water reduces the apparent softening point of spun yarn by appr2ximately 6" C. It is a n interesting fact that the shrinkage-temperature characteristics of Vinyon AT yarns are better a t very high degrees of stretch than a t intermediate degrees of stretch. This fact is brought out in Figures 5 and 6, which show shrinkage-temperature curves of spun yarn, and of yarns obtained by stretching spun yarn one-, three-, five-, eight-, and approximately thirteen fold. It is apparent from these curves that dimensional stability is poorest in yarns stretched three- to fivefold-a fact t h a t is brought out somewhat more clearly in Figure 7 , in which shrinkage a t several different temperatures is plotted against nominal stretch. The behavior of stretched fibeis, summarized in Figures 5 , 6, and 7, is of course a matter of considerable practical importance, but the data presented are of little theoretical significance in the form shown there because the actual state of strain inherent in each of the various samples is ignored. If the data are reduced t o a more or less common basis, hoLT-ever, by calculating the per cent actual stretch prevailing in each sample-that is, by expressing the length of each in terms of per cent of its fully relaxed lengthit appears that Vinyon N fibers become progressively more stable a s the degree of stretch is increased above about 500'%. Specific d a t a on this point are unfortunately not ready a t the present time because of several inconsistencies which are believed to be attributable to uncertainty as to the true relaxed length of certain of the samples. The authors are confident, however, t h a t more refined data will bear out these preliminaiy indications. T h e phenomenon of increased softening point as a result of molccular orientation is not unique among high polymers, but it is rare among those emp1oyi.d as fibers. Since the fully extended condition is a highly improbable one for a macromolecule to assume, the tendency of the molecules to retract at any given temperature is more pronounced, the higher the degree of molecular orientation. I n the normal case, the influence on shrinkage-temperature characteristics exerted by the degree of molecular orientation prevailing in the fiber is a t most only minor, b u t it is a n adverse rather than a favorable influence. The opposite effecta n increase in strain-release temperature as a result of stretching -is observed in the ease of ran- rubber. Thus it is reported in the literature t h a t the softening point of raw rubber is in the neighborhood of 18' C. but may be raised to 34' C. by stretching a t elevated temperatures and maintaining the stretch until the sample
NOMINAL
STRETCH, PER CENT
Figure 7. Effect of Imposed Stretch on Shrinkage Characteristics of Unmodified Vinyon N Fibers
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September 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
OF WET- AND DRY-SHRINKAGE TABLE I. COUPARISOX
CHARACTERISTICS OF VINYONN YARNS
Shrinkage, Per Cent Heat-&~odifioetion .kt 80' c. A t 100' c. At 120' c. Yarn Type Treatment Dry Wet Dry Wet' Dry Wet SOZZ None 8.5 13.0 15.0 2 1 . 0 26.0 53.0 2.2 37.4 0 0 1.0 NOHU 6 h o u r s a t 110' G a 0 0.5 0 6.0 0 0 0 NORU 3 hoursat 135' 3.0 0 0 0 5.4 NORT 3hoursa.t15O0 C.b 0 Controlled shrinkage allowed during heat-modification process. b No shrinkage allowed during heat-modification process.
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yarn for very short periods a t elevated temperatures and, according to x-ray studies, the crystallite rearrangement t h a t occurs during heat niodification is virtually complete at the end of the first second. The temperature characteristics and solvent resistance of highly modified yarns have not thus far been achieved by continuous annealing, however, and present evidence indicates that some degree of actual cross linking is probably required in order to realize the ultimate in yarn charactcristics.
Q
GENERAL PROPERTIES OF VINYON N YARNS
The stress-strain properties of several grades of Vinyon iK yarn, measured by means of a Scott Serigraph, Model IP-2, are The curve labeled NOZZ in Figure 8 is the same as that correshown in Figure 9. The data summarized there were obtained sponding to 1289% nominal stretch in Figure 6. _. It is apparent on specimens which were saturated with water a t room temperafrom the other curves that heat modification brings about decisive ture, but because the water absorption of Vinyon N is very low improvement in the temperature characteristics of the yarn. (17) the curves of Figure 9 are essentially the same as those obFor example, Grade NORT yarn, which is heat-modified under tained on dry samples. The increase in tenacity brought about by stretching is clearly apparent, and i t will be noted also t h a t conditions t h a t allow no shrinking during the process, can withstand a service temperature approximately 60' C. higher than heat modification at 110" and 135" C., respectively, causes the Grade NOZZ (stretched, unmodified) yarn in applications where yield point of the fiber t o become more pronounced than it is in 2.5 % shrinkage is considered the maximum allowable, and some stretched, unmodified yarn, and increases its ultimate extensibil65" C. higher in applications where 574 shrinkage can be tolerity. These latter changes are primarily the result of the conated. The insolubilization phenomenon discussed above is manitrolled shrinkage allowed during heat modification at these temfested in fibers to a somewhat higher degree than in films or peratures. Yarn of Grade NORT, which is heat-modified a t molded specimens. Appreciable resistance to acetone is achieved 150 O C. under tension, has stress-strain characteristics similar to in Grade XOHU yarn, heat-modified for 6 hours at 110"C., and those of stretched, unmodified yarn. resistance becomes progressively more pronounced until it is esColor and color stability of the yarn in sunlight appear t o be sentially complete in Grade NORT yarn, which is heat-treated for closely related, particularly when metallic contamination is a n 3 hours at 150" C. appreciable factor. As in the case of other vinyl chloride resins, The heat-modification phenomenon has been the subject of exsmall amounts of stabilizer can be used to good advantage, and tensive study for several years, not only because of its great prac: in practice one or more of these is added to the spinning solution. tical importance in Vinyon N fibers, but also because of its posThe most effective heat stabilizers for Vinyon N yarn found thus far belong to the organo-tin group (20, 69), and these same stasible implications in connection with other vinyl fibers. The heatmodification effect,like the orientation effect discussed above, has bilizcrs are remarkably effective in preventing discoloration of the not yet been completely explained, but certain facts have been yarn as a result of exposure to sunlight. I n Figure 10, color, as established which are of considerable interest. measured by blue-light reflectance, is plotted against hours of exX-ray analysis shows that prolonged heating of Vinyon N resin posure to Florida sunlight (68). Stretched unmodified yarn and films, molded specimens, and unoriented fibers brings about an two grades of stretched heat-modified yarn, all containing 2% of increase in degree of crystallinity, just as does stretching, and tin stabilizer, are included in the chart; cotton and viscose rayon t h a t the magnitude of the change depends upon the severity of are also included for purposes of reference. All grades of Vinyon the heat treatment. It shows also t h a t when stretched fibers are N have about the same color after 200 Florida sun-hours as they heat-modified under no tension, a t temperatures below ~ o s e had originally, and the over-all effect of sunlight on color is slight which bring about complete relaxation of the fibers, some loss of bleaching during the early stages of exposure, followed by a slow crystallite orientation occurs, as would be expected, but crystalreturn to the original shade. lite size increases. Heat modification under tension great enough Prolonged exposure to strong sunlight reduces the mechanical t o prevent shrinkage causes the same change in crystallinity, but strength of Vinyon N yarn to a n extent that depends in some the amount of disorientation of crystallites is reduced. Following the same reasoning as t h a t outlined above in connection with stretching, therefore, it would seem that the superior temperature characteristics of heaj-modified specimens, whether films, moldings, or fibers,isattributable, at least in part, t o enhanced crystallinity. Research along other lines has yielded evidence of actual cross linking of the macromolecules as a result of prolonged heating. It has been found, for example, that the decline in acetone solubility of cast Vinyon N films can be correlated directly with the concentration of double bonds formed as a result of heating, just as i t can in the case of vinyl chloride-acetate copolymer films. The intrinsic viscosity of heat-modified (but still completely soluble) resin has been found to increase directly with the severity of the heat treatment. Yarn having temperature characteristics simiTEMPERATURE, DEGREES CENTIGRhDE lar to those of yarn heat-modified to intermediate degrees can be obtained by annealing stretched Figure 8. Dry-Heat Shrinkage of Vinyon N Yarns
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z n W K a W v)
I K 4 l9
a d
s
. Figure 9.
ELONGATION, PER CENT
Stress-Strain Properties of Vinyon
N Yarns
Vol. 40, No. 9
stability in cellulose acetate. The temperatures Employed in the use of acetate dyes are materially higher than those normally used for dyeing acetate rayon, but the higher temperatures do not semi t o pose any serious problems hledium to deep shades can be obtained a t or slightly below 100" C , or, Kith the aid of dyc assistants such as p-hydroxydiphenyl, a t temperatures around 85 ' C. Depth of color and degree of penetration depend to some extent upon concentration of dyestuff in the bath, and for a givm ratio of dyestuff to fabric, best results a l e obtained by keeping the bath iatio a t a minimum. Soap and other detergents in veiy small amounts serve as leveling agents, but in higher concentrationsthey seriously retard the exhaustion of dyestuff onto the fiber. The affinitv of Vinyon P;yaIn for direct and acid dyes is lower than its affinity for acetate dyes, but the use of dye assistants or of certain treatments a t ternperatures above 100' C. has yielded generally encouraging results in laboratory experiments. Considerable progress has also been made in the laboratory in the use of v a t dyes, and techniques have been developed for obtaining medium shades with very good resistance t o sunlight and t o scouring. Satisfactory leveling and acceptable resistance to crocking have, however, not yet been attained with v a t dyes. The equilibrium moisture content of Vinyon N fibers is low compared with t h a t of most other textile materials. I t is higher than that of Vinyonvinyl chloride-acetate fiber, however, and presumably for this reason static electricity does not pose the same problems encountered in connection with the older vinyl yarns. Treatment of fibers with hydrophilic materials, for the purpose of reducing static electricity, is a familiar procedure in textile operations, and one t h a t can be followed to good advantage in connection with Vinyon N yarns and fabrics. As would be expccted, the degree of success enjoyed by a n antistatic treatment of thir: kind depends t o a verv great extent upon the degree of permanence of the hydrophilic cfJating,and in thir respect, polyethyleneimine stearamide (26. a7j yield.: promising results when applied to Vinyon N yarn or fabric. Applied in the form of ail aqueous solution a t temperatures near the boiling point, this compound appears t o penetrate very deeply into the fiber and wccessfully withstands repeated laundering. Polyethyleneimin(~stearaniide has been tested extensively for toxicity hazards and allergy reactions, and appears to be satisfactory froni this point of view.
measure on its degree of heat modification. Because of the long periods of time required to evaluate light stability reliablv, comprehensive data are not yet available on Vinyori N yarns containing t h e newer stabilizers. I n tests which have been completed, however, Grade NOZZ yarn has shown less than 3070 decline in tenacity after 200 Florida sun-hours (approximately 40 days) of exposure. I n parallel tests, NORU yarn showed a decline of 4970, while unmercerized cotton, acetate rayon, viscose rayon, polyamide, and Vinyon chloride-acetate fiber showed declines of 43, 51,73,36, and l % ,respectively. Stretched, unmodified (Grade KOZZ) Vinyon K yarn shows no change in modulus or yield point as a result of aging in sunlight, and the effect of TECHiYQLOGICAL SIGUIFICANCE exposure is t o shorten the stress-strain curve. Similar effects Vinyon N Trams make the unique properties of vinyl resins mere noted in connection with cotton and viscose rayon. Grade available for the first time t o important branches of the textile inKORU yarn responds t o exposure in almost the same manner, but dustry which could not use the earlier vinyl yarns because of their shows a tendency t o reach slightly higher elongation a t break than temperature- and solvent-resistancr lirnitations. Extcnsi\ e diswould be expected from the original stress-strain curve. Energv required to break the fiber is therefore slightly greater than would be inferrpd from the ratio of tenacities before and after exposure. 100 The decline in mechanical strength o f Vinyon S 90 yarn upon outdoor aging appears t o occur through c the mechanism of chain scirsion, as it does in poly70 0 vinyl chloride, cellulose acetate (15), and other K 6o synthetic yarns. The decline in tenacity is accomW p 50 panied by a decline in specific viscosity, but the W' chain-scission hypothesis does not appear to explain 40 loss in strength completely. Thus, using a given 2 0 light stabilizer, specific viscosity can be correlated 30 Lsl with tenacity fairly well, but the relationship bea tween the two is not the same when a different I + stabilizer is employed. E 20 -I Primarily because t h e temperature limitations W a on Vinyon N yarns are much less severe than those m on the earlier Vinyon yarns, the new family is greatly superior t o the old in dyeing characteristics. 10 Dispersed acetate dyes yield colors which are fast 0 50 100 150 200 t o laundering, have excellent crock resistance, and FLORIDA S U N H O U R S are not subject to gas fading. T h e sunlight stability Figure 10. Effect of Sunlight Exposure on Color df Knitted Fabric of these dyes in Vinyon h- yarn does not equal their
2
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1948
cussion of the potentialities of Vinyon N yarns would be out of place in this article, but it is pertinent to point to a few broad examples by way of illustration, Thus the industrial applicatibns of Vinyon yarns have been greatly expanded, and their inherent nonflammability can now be exploited in upholstery and drapery fabrics and in (26). Their shrinkage characteristic make possible new inexpensive processes for the manufacture of familiar seersuckers and crepes, and also novel fabrics which could be made from the older fibek only by more complicated processes. Similarly, the controllable thermoplasticity of certain grades of fiber makes possible interesting open netting which can be readily shaped into items of wearing apparel such as light-weight hats and shields. The whole program of research on vinyl fibers is based upon the fact that vinyl fibers, as a class, offer a combination of properties which is outstanding in many respects, and which cannot be approximated by any other fiber known today. A great number of chlorine-bearing resins have been studied intensively over a period of years, in the search for an alternative t o vinyl chlorideacetate copolymer which would have a substantially higher softening point than the older copolymer and yet be similar to it in all other essential properties. T h e Vinyon N family represents a substantial step forward along this line. LITERATURE CITED
(1) Anon., Rayon Textile Monthly, 28, 178-9 (1947); Chem. Inds., 61,589 (1947). (2) Anon., Silk & Rayon, 18, 36-40, 372-4, 438 (1944); Textile World, 97,No. 9,113-28 (September 1947). (3) Bacon, R. G. R., Trans. Paradau Soc.. 42.140 (1946). (4) Baxendale, J . H., Evans, M . G., and Park, G. S., Ibid., 42, 155 (1946). ( 5 ) Boyer, R. F., J . Phys. Colloid Chem., 51,80(1947). (6) Boyer, R.F.,and Spencer, R. S.,J . Applied Phys., 15,398(1944) (7) Chaney, N. K.,and Dexter, W. B. (to Carbide and Carbon Chemicals Corp.), U. S.Patent 2,060,035(Nov. 10,1936). (8) Charch, W. H., Finzel, T. G., Hansley, V. L., Houtz, R. C., Latham, G. H., Merner, R . R., and Rogers, A. 0. (to E. I.
1731
du Pont de Nemours & Co.), U. S. Patents 2,404,714-28 (July 23,1946). (9) Clash, R. F., Jr,, and Rynkiewicz, L, M., IND. ENG. CHEM,, 36, 279 (1944). (10) Crawford, J. W. C. (to Imperial Chemical Industries, Ltd.), U. S.Patent 2,054,740(Sept. 15, 1936). (11) D’AleliO, G. F. ($0General Electric CO.) I u. Patents 2,366,495 (Jan. 2,1945),2,412,034(Dec. 3,1946). (12) ~ ~c, c.,~and Blake, i ~ J , T,, “Chemistry , and Technology of Rubber,” pp. 71-2, New York, Reinhold Publishing- Cors., 1937. (13) Doolittle, A. K., IND. ENG.CREM.,30, 189 (1938). (14) Doolittle, A. K., IND. EKG.CHEM.,NEWSED.,18, 303 (1940). (15) Lawton, T. S., and Nason, N. K., Modern Plastics, 22,No. 2,145 (February 1944); IND.ENG:CHEM.,36,1128 (1944). (16) Mauersberger, H. R., “Matthews’ Textile Fibers,” 5th ed., p. 875,New York, John Wiley & Sons, 1947. (17) Morehead, F. F., Testile Research J., 17,96 (1947). (18) Powell, G.N., and Quarles, R. W., Oficial Digest, Federation Paint & Varnish Production Clubs, No. 263,696 (1946). (19) Quarles, R.W., IND. ENG.CHEM., 35,1033 (1943). (20) Quattlebaum, W. M., and Noffsinger, C. A. (to Carbide and Carbon ChemicalsCorp.) ,U.S.Patent2,307,157 (Jan. 5,1943). (21) Rugeley, E.W. (to Carbide and Carbon Chemicals Corp.), U. S. Patent 2,277,782(March 31,1942). (22) Rugeley, E. W., and Feild, T. A., Jr. (to Carbide and Carbon Chemicals Corp.), U. S.Patent 2,418,904(April 15,1947). (23) Rugeley, E.W., Feild, T. A., Jr., and Petrokubi, J. L. (to Carbide and Carbon Chemicals Corp.), U. s. Patent 2,420,565 (May 13,1947). (24) Shriver, L. C., and Fremon, G. H. (to Carbide and Carbon Chemicals Corp.), U. S. Patent 2,420,330 (May 13,1947). (25) Stoops, W. N., and Wilson, A. L. (to Carbide and Carbon Chemicals Corp.), U. S. Patent 2,403,960(May 16,1946). (26) Stowell, E., Papers Am. Assoc. Textile Technol., 3,30 (1947). (27) Wilkes, B. G.,and Denison, W. A. (to Carbide and Carbon Chemicals Corp.), U. S.Patent 2,381,020(Aug. 7, 1945). (28) Wirshing, R.J., IND. ENG.CHEW,33,234 (1941). (29) Yngve, V. (to Carbide and Carbon Chemicals Corp.), U. S.Patents 2,267,777, 2,267,779(Dec. 30, 1941),2,307,092(Jan. 5, 1943), RECEIVEDMarch 4, 1948. of
The word “Vinyon” is a registered trade-mark
Carbide and Carbon Chemicals Corporation.
Friction in the Flow of Sus ensions GRANULAR SOLIDS IN GASES THROUGH PIPE E. G. VOGT‘ AND R . R. WHITE, University of Michigan, A n n Arbor, M i c h . T h e pressure ditrerentials required to produce steady flow of suspensions of sand, steel shot, clQvcr seed, and wheat in air through 0.5-inch commercial iron pipe are presented. These values and data from the literature on the pneumatic conveying of wheat in pipe sizes ranging from 2 to 16 inches in diameter are correlated by the following type of equation for both horizontal and vertical flow:
(Zy (: x 41/3(w -
OL-l=A
.&)k
where A and k are given as empirical functions of the dimensions group
p)pgd’, and
OL
is the ratio of
pressure drop to pressure drop obtained for the flow of pure fluid at the same velocity. The development and limitations of the correlation are discussed.
* Present address, State College of Washington,
Pullman, Wash.
NEUMATIC conveyers have been used commercially in transporting granular solids for many years. Recently the development of fluid catalyst processes has brought further attention t o the need of information on the pressure differentials required to produce flow of suspensions of solids in gases through conduits. For design purposes i t is necessary to know the effect of such variables as pipe size, the amount of fluid and solid flowing, and the properties of the fluid and solid, on friction losses. Although i t is easy t o calculate friction losses in the flow of ordinary fluids through pipe, the relations ordinarily used for this purpose are not applicable generally t o the flow of suspensions of solids in gases because the meaning of such terms as the density, viscosity, and perhaps velocity of suspensions is rather obscure. PREVIOUS INVESTIGATIONS
Although the literature contains a large number of references on pneumatic conveying and fluid catalyst processes, most of the information is qualitative in nature. Cramp and Priestly (3, 4)
