1.5
1.o
0.5 As uses increase, volume goes up
Partially Acetylated (PA) Cotton A chance for cotton to capture new markets through
b b
good mildew and rot resistance superior heat and scorch resistance
EARL V. ANDERSON, Assistant Editor In Collaboration with ALBERT S. COOPER, Jr. Southern Utilization Research Laboratory. New Orleans. La.
Over-all view of SRRL’s continuous range for acetylating cotton, with hood removed. Presoak tanks, flrst step in process, are a t near end
608
INDUSTRIAL AND ENGINEERING CHEMISTRY
M O R E AND MORE, specialization is becoming a keyword in American industry. The textile industry is no exception. Most modern synthetic fibers have specifically built-in properties, designed for a particular application. This is one major reason why King Cotton is teetering on its regal perch. Cotton economists recognize this and realize that the general purpose fiber, either natural or synthetic, is a thing of the past. If cotton is to capture new markets and regain some of those it has lost, special properties must be built into it. Fortunately, this can be done chemically. Cotton cellulose has reactive groups u hich make it possible to chemically modify the fiber. The resulting fibers usually have physical and chemical properties vastly different from untreated cotton, but maintain their textile properties and fibrous nature. The Southern Regional Research Laboratory, New Orleans, La., has done considerable work on many chemically modified cotton fibers, but the one which it has investigated most extensively is partially acetylated, or PA. cotton. PA cotton has several improved properties over untreated cotton but the most important are its excellent mildew and rot resistance and its superior heat and scorch resistance. SRRL first began its work with PA cotton during the World War I1 years when there was an acute need by the military for fabrics with
Pros and Cons of PA Cotton CON
PRO Heat Resistance. Physical tests show that PA cotton withstands sustained temperatures up to 320 ' F, with an increased practical service life about four times that of untreated cotton. Scorch Resistance. Visual tests show that PA cotton withstands 30 minutes of direct contact with an electric iron a t 400' F. without appreciable discoloration. M i l d e w Resistance. Pure culture tests show that P.4 cotton is completely resistant to mildew attack. Rot Resistance. I n soil burial tests, PA cotton retained 90% of its original breaking strength after lveeks to untreated
cotton which was completely destroyed after 1 week. D y e i n g . Can be dyed with direct acetate dyes, I n some cases: it is ilill,lllil,1,,111,III
better to vat-dye cotton before acetylating. Moisture Regain. Less than half that of untreated cotton. Acid Resistance. .4bout tL,.ice that of untreated cotton. Resistance to Corrosive Atmospheres. PA Cotton resists a hydrogen chloride-air mixture roughly 10 times better than untreated cotton. In a nitrogen dioxide-air mixture, PA Cotton:s loss of
strength is only half that of normal fabric. Electrical Resistance.
Like simi-
la' such as acetate rayon> paper! and lulose acetate plastics, d.c. current insulation resistance of p.4 cotton is many thousand times that of ordinary cotton, depending upon degree of acetylation, number of washings, and humidity.
R e d u c e d Tenacity. Acetylated fibers, yarns, and fabrics have about the same breaking strength as they have before treatment. However, if the additional weight of the acetyl groups is considered. tenacity is reduced by a percentage approximately equal to the percentage of the added lveight. Flex Abrasion. Reduced 50 to s5% depending upon the and the control exercised during reaction. Tear Strength. Reduced 40 to 60y0 depending upon fabric consrrucrion. Flat Abrasion. About equal to UnaCetYlated cotton. Elongation. Usually reduced from 10 to 15%.
1,
good mildew and rot resistant characteristics. But the history of PA cotton does not start there for in 1901. two British scientists, Cross and Bevan (5, 7), reported the earliest preparation of fibrous partially acetylated cotton yarn. An acid catalyzed process for producing it was patented in 1933 (7, 76). Another British product called Cotopa (or Crestol, if applied to mercerized cotton). which is produced by a zinc chloride catalyzed process, has been made commercially since 1927, primarilv as a specialty item ( 7 3 . 74). Reportedly, Hungary also produced a PA cotton called Izopa (75, 77, 78). After \Vorld War 11, interest in the mildew and rot resistant properties of PA cotton waned and SRRL concentrated on evaluating the material's resistance to heat and scorching. I t also developed both a batch and a continuous process for making PA cotton. Yarn or fabric can be treated by either process on standard or slightly modified textile equipment. I n addition, SRRL is also developing a batch process to treat cotton raw stock. I n 1952-53. two U . S . companies became interested in commercializing these processes. Proctor Electric, Philadelphia. Pa., chose the batch process. After considerable development work, it successfully transferred the process from SRRL's semi-pilot scale to commercial textile equipment. William E. Hooper
Last two sections of SRRL's continuous acetylating range. Hot-air dryer, with doors open, is at right. Batcher, left, is shown with yard beam on take-up shaft
VOL. 51, NO. 5
MAY 1959
609
PRESOAK EVAPORATIVE COOLING
I/EC
1
RETURN TO TANKS 1
PLANT PROCESS SERIES
BRINE RETURN
d REACTION
15 PSL DlR
I
SINGLE LAYER HOOD
I
COLD
p--
'
R
I 1
I
1
PA COTTON TO 6ATCHER+--d
NOTE DOTTED LINES REPRESENT COTTON FA6RIC.
\
PIPE CONNECTION DRYING
TO DRAIN
WASHING SRRL CONTINUOUS RANGE
Flowsheet for the continuous acetylation process of cotton, Southern Utilization Research Laboratory, New Orleans, La.
61 0
INDUSTRIAL AND ENGINEERING CHEMISTRY
P A COTTON & Sons Co., Baltimore, Md., meanwhile, obtained a research and development contract from Army Ordnance, which was interested in PA cotton for powder bags. Hooper began purchasing equipment for the continuous process in July 1952, and in March 1953, made its first runs. The company had hoped to obtain another contract for full scale Ordnance performance tests, which would require 300,000 yards of material. However, the army decided upon other fabrics and never issued a contract. Since then, Hooper has intermittently acetylated many samples for potential users, but in September 1957 ceased operations completely. Eastman Chemical Products Go. also acetylates sample amounts with the batch process today, but Proctor remains the only company in the U. S. which produces PA cotton commercially. Chemical Nature of PA Cotton
Cotton is essentially made up of long chains of cellulose molecules. These, in turn, are composed of cellobiose units having the following molecular structure :
structure of crystalline cellulose with interspersed cellulose triacetate, averaging nearly one acetyl per glucose unit (21% acetyl by weight) throughout the entire mass. Physically, PA cotton is odorless, nontoxic, and unchanged in color. Its appearance and texture differ only slightly from those of untreated cotton. Higher average degrees of acetylation (above DS = l), however, produce some stiflness in the fibers ( 3 ) . Highly acetylated material (above 25% acetyl content) also becomes thermoplastic. PA cotton is much better than untreated cotton in many respects, but by far the most important of its properties are its improved heat, scorch, mildew, and rot resistance-important because it is based on these properties that PA cotton hopes to gain large neiv or expanded markets. Acetylation also adversely affects some desirable characteristics of cotton. I t reduces the material’s tenacity, flex abrasion, and tear strength, but not enough to hinder its usefulness.
H 0
H I
H
I
0
H-C-H
I
C
C
lI \ H\ I H C-0
/
/I
I
L-0-
I
no
C-C
H/l
k hH
H-C-H
1
H
0 H
N
One half ’ the cellobiose unit is known as an anhydroglucose unit. Cotton is acetylated by esterifying the hydroxyls of cellulose. The hydroxyls react with acetic anhydride in the presence of a catalyst (perchloric acid) to form cellulose acetate (PA cotton) and acetic acid:
Cel-0-COCHs
+ CHICOOH
Thus, the hydrogens of the hydroxyls on the anhydroglucose units are replaced by acetyl groups. In practice, it is not necessary to completely acetylate cotton-that is, substitute with three acetyls on each anhydroglucose unit [Degree of Substitution (DS) = 31 throughout the entire cotton fiber (2). Substitution at about one third (DS = l ) , or even less, of all the sites is adequate. The important thing is to acetylate the more readily accessibIe hydroxyls in the amorphous regions and crystallite surfaces. The product is believed to be a
Continuous Acetylation
The acetylation process, whether it is batch or continuous, consists of six basic steps : presoaking, cooling, catalyst addition, reaction, washing, and drying. However, before it enters the presoak phase, the cotton should be cleansed as thoroughly as possible without degrading it. If the cotton contains sizing, it should be desized. Waxes, pectins, and other noncellulosics which are alkali soluble should also be removed. A general rule at SRRL is to boil the material in a 270 sodium hydroxide solution for 2 hours, rinse in hot water, and cool. However, lighter material requires less time and concentration. SRRL then neutralizes the alkali with an acetic acid sour, water washes again, and dries the cotton. Proper preparation of the cotton yarn or fabric is very important. The more thoroughly pure and uniform cotton produces a more uniform acetylation, For a given degree of heat, scorch, or rot
resistance, less reaction time is required if the starting material is clean. For instance, to activate pure cotton in acetic acid presoak solution at room temperature requires about 45 minutes. Under the same conditions, r‘grey’’ or impure cotton requires about 4 hours. Following the sodium hydroxide bath, cotton is washed in hot Mater. If cold water was used, the alkali-soluble noncellulosics would redeposit on the material. These noncellulosics interfere with the reaction if they remain on cotton. .4n acetic acid sour treatment. followed by water wash, prevents any chance of free alkali or sodium acetate from getting into the reaction mixture where they may react with part of the catalyst and slow the acetylation. Cotton’s water content before it enters the actual process steps is critical, for the amount of water determines to a large extent the amount of chemicals required. \$‘hen cotton completes the preparation treatment, it is dried. It can be processed from this state or it may be allowed to remain in a constant humidity room to regain its normal moisture content (about 6 to 7%). From an economic standpoint, it is probably best to start with dry cotton as less chemicals are required. For instance, P.4 cotton acetylated to 1Sc% acetyl from a dry state has about the same heat resistance as a 21% acetyl product acetylated from normal regain. SRRL uses normal-regain material as it has no facilities for storing dry cotton. The ‘‘activating,’’ or presoak, solution is glacial acetic acid (O.Syowater maximum) with enough acetic anhydride added to conivert the water in the material to acetic acid. The amount of anhydride is as close as possible to the stoichiometric amount required to convert cotton’s normal regain content. SRRL’s presoak feed is approximately 60y0 acetic acid and 4070 acetic anhydride. The cotton remains in the solution for 2 minutes at 180” F., after which it is given as heavy a squeeze as possible to reduce carrv-out (the amount of solution remaining in the material after squeezing) to about 90% of the material’s weight. This carry-out is essentially glacial acetic acid. I t is important to maintain water content below o.5yO in the carry-out because water may cause trouble in the catalyst bath. However, reaction rate and product uniformity are improved by having the carry-out on the water, rather than the anhydride, side. There is also the possibility that reaction will occur in the catalyst addition tank if excess anhydride is carried into this bath. If this happens. heat given off by the acetylation reaction would permit the VOL. 51. NO. 5
M A Y 1959
61 1
perchloric acid catalyst to excessively degrade the cotton cellulose. Cotton, with its acetic acid carry-out, must be cooled before it enters the catalyst addition tank. This can be accomplished by normal evaporation or by cooling cylinders. SRRL's continuous range is equipped to do both. The catalyst for the acetylation process is perchloric acid (3% perchloric in glacial acetic acid). As small a volume as possible is maintained in the addition tank to obtain maximum catalyst efficiency. Only enough solution to cover the material is used. Uniform distribution is also important because the perchloric must uniformly penetrate into all fibers in a concentration high enough to give the desired degree of substitution. The material remains in the catalyst solution for only 30 seconds at room temperature. It receives two dips and two squeezes, with the final squeeze adjusted to give 100 to 120Y0 carry-out. Actually, this carry-out is based on the carry-out from the presoak unit and should be about 20 to 257G greater. Carry-out from the catalyst addition phase is predominantly acetic acid, with some perchloric acid and a little water. Next, the material enters the reaction solution which is about 60y0 acetic acid and 4070 acetic anhydride maintained at 68" F. Feed for the reaction tanks is technical grade anhydride, which contains about 2.5Y0 acetic acid. The rest of the acid in the reaction solution comes from the presoak and catalyst carry-outs and reaction by-product. Time in the reaction tanks determines the amount of acetylation, or acetyl content, of the material. At SRRL, cotton remains in the solution for 3 minutes to gain an 18 to 2070 acetyl
Laundries Offer a Large Potential Market for PA Cotton
...
Annual Consumption in 500-Lb. Cotton Bale Equivalents
Total cotton consumed in laundry, dry cleaning equipment, and supplies, 1956 Market held by competing materials, 1956 Market potential for PA cotton if it displaced ( I ) : All competing materials 50% of all competing materials 55'% of all untreated cotton All competing materials 50% of untreated cotton 50% of both competing materials and untreated cotton
+
72,630" 85,451a
19,672 9,836 7,409 27,080 17,245
National Cotton Council of America figures.
content. When it leaves, it contains about 25070 carry-out, including acetic acid, anhydride, and perchloric acid. This is squeezed down to give a final carry-out approaching 1 0 0 ~ owhich , is ideal. A more practical limit is about 12070, however. These percentages are based on weight of PA cotton (includes the weight of the added acetyl groups). The acetylation reaction is highly exothermic and as a result, the reacting solution could get quite hot. Consequently, SRRL circulates its solution through a heat exchanger where it is cooled with brine. The solution then returns to the tanks. Reaction squeeze-out must not be
allowed to drain back into the fresh reacting solution; hence, SRRL collects it with a false bottom in its tank. The volume recovered is roughly equal to the amount needed to feed the presoak tanks. Fortunately, this solution contains about the amount of anhydride needed to convert stoichiometrically the normal regain moisture in cotton to acetic acid. But the solution does not feed directly back to the presoak tanks. First, perchloric acid must be neutralized. SRRL does not do this on a continuous basis; instead, it collects the squeeze-out and neutralizes the perchloric acid with potassium acetate ( 7 7). The reaction forms potassium perchlorate, which is insoluble in glacial acetic acid (70). Cotton running through the range is now P.4 cotton and it goes to the wash units. Washing requires about 50 gallons of water per pound of material. The first water which PA cotton encounters is a cold water spray. It is important that this first wash should be with cold water because, if hot water were used, perchloric acid present in the material might hydrolyze the acetyl groups ( 4 ) . Another function of the first cool-water spray is to stop the chemical reactions as material leaves the reaction tank. If the cotton were not cooled, the reaction would continue, again causing product degradation. Wash water can then get progressively hotter; final wash is with 150" F. water. The wash step, like the purification process, is extremely important to product quality. The better the wash after acetylation, the higher the heat resistance of the product for a given acetyl content. Before batching, PA cotton yarn or fabric is dried for 5 minutes at 300" F.
4 SRRL's continuous range for acetylating cotton with hood in place. Here, cotton yarn, moving from right to left, enters presoak tanks The large cooling can in center cools material leaving presoak tanks before it enters catalyst addition section
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INDUSTRIAL AND ENGINEERING CHEMISTRY
P A COTTON SRRL’r Continuous Range
Besides the need for carefully controlling the chemistry involved, the process also requires sound textile practices. SRRL’s pilot range consists of a series of textile units, but which are smaller than commercial scale. As the range is a research tool for several chemical modification treatments of cotton, some changes would undoubtedly be made if it were adopted for partial acetylation alone on an industrial scale. The basic piece of textile equipment in the continuous range is the padderwasher unit.
tains a false bottom to catch the reaction squeeze-out and the padder tanks in the washer section have been enlarged. SRRL built its own hot air dryer with angle iron framing and plywood, lined on the inside with fibrous insulating material and galvanized sheet metal. I t holds approximately 15 yards of material. T h e heating unit, a common unit space heater, is supplied with high pressure steam. A sheet metal baffle divides drying the dryer into two sections-a area and a return passage for recirculating air through the heater. T h e entire range, except the dryer and batcher, is enclosed in a plywood hood coated with acid-resisting paint.
A l o o k at the Market
I1 Pad d er-washer
unit
This basic unit contains a three-roll tdder in the center and a washer section on each end. The complete range contains five combination padderwasher units, a hot air dryer, and a batcher. Some of the padder-washer units were modified for better use in the chemical processes inkolved (72) ; e.g., the padder in the reaction section con-
The market outlook for PA cotton is deceiving. All of the many outlets which call for the use-qualitites of PA cotton add up to a market of several hundred thousand bales annually. But this is very unrealistic. Treating costs alone eliminate many of these outlets and lack of other needed qualities of PA cotton eliminates still more. -4ccording to SRRL’s market research study, only those markets which require heat and scorch resistance as primary usequalities are considered “practical potential markets” for PA cottton at this time. Products calling for mildew and rot resistance can be made more economically with other materials. Because PA cotton must base its selling principally on its superior heat and scorch resistance, it is limited almost exclusively to the industrial market, and the laundry industry seems to be the most promising target. These proper-
This shows how speed of rollers is controlled. As cloth changes speed, it raises or lowers the dance roll, which adjusts variable speed transmission through a chain and sprocket arrangement
ties are not important in the apparel field or for household uses, with the obvious exception of home ironing board covers, Proctor’s principal outlet. Of course, other markets may spring up after PA cotton is in full-scale commercial production and its price decreases. For several years, cotton consumption has been steadily declining in the laundry industry. It has dropped from slightly more than 134,000 bales of 500 pounds in 1947 to less than 73,000 bales in 1356. These losses are about equally distributed between flatwork ironer materials and press materials. Chiefly responsible for these losses are the inroads made in the laundry market by nylon, dacron, asbestos, steel wool, and cottonnylon blends. Pricewise, untreated cotton fabrics range from 2 to 10 times less than competition. But, because of several quality deficiencies, it does not match competing fabrics in terms of cost-in-use. The future of cotton in commercial laundries depends upon improvements in quality, such as PA cotton offers, which uill lengthen its useful life-important because of labor costs and lost time in changing materials. Even then, laundry suppliers must furnish a lot of sales push on cotton’s behalf and this push won’t come unless PA cotton’s profit margins are equal to or greater than those of competing materials.
I
For detailed cost analysis, see “The Partial Acetylation of Cotton: Cost Analysis Application” b y K. M. Decossas et-al., page
615.
I
b
Close-up of one padder-washer unit o f SRRL’s continuous acetylating range, threaded with cotton fabric. Large rolls are squeeze roils supported b y phenolic laminate blocks. Air cylinder applies pressure to squeeze rolls
VOL. 51,
NO.
5
MAY 1959
613
PA cotton hot head press cover used 16 days is compared with an untreated cotton cover, on the press, used only 4 days. These same covers are shown after removal from press on right The American Institute of Laundering, in cooperation with the National Cotton Council and SRRL, has conducted service tests with PA cotton for laundry use. Although these tests are still incomplete, results indicate that : 0 Service life of PA cotton is about four times that of untreated cotton. 0 Service life of PA cotton ranges from 7574 to 1 0 0 ~ of o that for nylon. 0 PA cotton double-faced flannel has service life about equal to that of nylon. 0 PA cotton fabric is unsuitable for automatic shirt presses because starch from the shirts adheres to it and the buttons punch holes into it. I n its study, SRRL found that PA cotton definitely has a cost-performance advantage over principal competing materials. Considering estimated cost of producing PA cotton (page 615) and service life, SRRL figures that the costperformance ratio of PA cotton to its competition ranges from 1 : 2.27 to 1 : 1.30,
depending upon the competing material and end use (7). Armed with these statistics and a few assumptions, SRRL estimated market potential of PA cotton for commercial laundries at several penetration levels. Its estimates run from 7,500 to over 17,000 cotton bale (500 lb.) equivalents annually, depending on the assumed market penetration. I n addition, the home-ironing-boardcover market is estimated to be from 35,000,000 to 50,000,000 covers annually. However, of this total, only 20%, or 7,000,000 to 10,000,000 covers are in the “over $1.50’’ price range, in which PA cotton must compete. If PA cotton’s longer service life lowered repeat purchases, as it probably would, its practical potential market would be no more than 2225 bales annually, calculated on a cotton equivalent basis. Price and eye appeal are the big factors in selling ironing board covers.
Corrosion Problems Galore Acetic acid, anhydride, and perchloric acid are corrosion and handling headaches of the first order. I n selecting materials for its equipment, SRRL has taken this into account. T h e padder-washer units are Type 304 stainless steel. For SRRL’s intermittent use, this material is satisfactory. However, for day-in, day-out acetylation, a phenolic coating inside the tanks would be required. The immersion rolls are also stainless steel and all of the rolls are supported in phenolic laminate blocks. I t is very difficult to control acetic acid leaks from pipe joints and through pump packing. A latex cement dissolved in methyl ethyl ketone has given best results so far as a pipe dope on threaded stainless steel joints. For flanges, Teflon has proved unsatisfactory. S R R L now uses neoprene but believes that a Teflon “envelope” over neoprene would be even better. SRRL has had difficulty with Teflon as a pump packing. If it is tightened down to stop leaks completely, the material burns out. However, if the acid is allowed to leak just a little, it lubricates the packing and satisfactory performance results. SRRL has also used blue asbestos as a pump packing but is now considering using it impregnated with Teflon. Alone it requires too frequent repacking.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
PA cotton’s progress in this market has been slow because the price of treated covers is high and their appearance is no better, and in some cases, not as good as lower-priced covers-for instance, aluminum-covered silicone-treated material. With effective promotion, and if its price goes down as production increases, PA cotton could reach its estimated market potential in this field and possibly surpass it. References (1) Barlow, F. D., Jr., Cooper, A. S., Jr., Vix, H. E. L., “Preliminary Evaluation of Potential Markets for Partially Acetylated Cotton,” unpublished. (2) Buras, E. M., Jr., others, Am. Dyestuj R e p . 43, 203 (1954). (3) Buras, E. M., Jr., Persell, R., U. S. Dept. Agriculture ARS-72-4, September 1956. (4) Cooper, A. S., U. S. Dept. Agr., Textile Inds. 116, 97 (January 1952). (5) Cross, C. F., Bevan, E. J., “Researches on Cellulose 1895-1900,” p. 40, Longmans, London, 1901. (6) Fisher, C. H., Perkerson, F. S., “Chemical Modification of Cotton: Progress and Current Status,” 28th Annual Meeting, Textile Research Institute, New York, March 13-14, 1958, unpublished. (7) . , Goldthwait. C. F.. Buras. E. M.. Jr.. Cooper, A. S . , T e i t i l e Reiearch J : 21; 831 (November 1951). (8) Goldthwait, C. F., McLaren, J., Voorhies, S. T., Jr., Textile World 96, 115 (February 1946). (9) Heuser, E., “The Chemistry of Cellulose,” 660 pp., Wiley, New York, 1944. (10) Keating, E. J., Cooper, A. S., U. S. Patent 2.816.003 iDec. IO, 1957). (11) Keating, E . J:, others, Am.’ Dyest$ Rebtr. 44. 65 (Jan. 31. 1955). (12) ‘Keatihg, i. F., Kdating; E. J., U. S. Patent 2,596,154 (May 13, 1952). (13) Rheiner, A., Angew. Chcm. 46, 675-81
I1 \ -911) __-,. (14) Rheiner, A., U. S. Patents 1,861,320 (May 31, 1932); 1,926,498 (Sept. 12> 1933); 1,958,315 (May 8, 1934); Ger. Patents 525,084 (Sandoz) (Nov. 14, 1926); 530,395 (Sandoz) (Feb.320, 1929). (15) Skau, E. L., private communication. (16) Thaysen, A. C., Brit. Patents 399,952 (Oct. 18, 1933); 411,930 (June 21, 1934); 486,901 (June 13, 1938). (17) Zilahi, M., Mbczlr, E., Faserforsch. u. Textiltcch. 8 (5), 192 (1957). (18) Zilahi, M., Oswald, L., Magyar Texttltech. 6, 204 (1956).
