Fiber-Forming Polymers - American Chemical Society

19 Fiber-Forming Polymers MICHAEL J. DREWS, ROBERT H. BARKER, and JOHN D. HATCHER

Downloaded by KTH ROYAL INST OF TECHNOLOGY on August 12, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch019

School of Textiles, Clemson University, Clemson, SC 29631

Chemistry of Man-Made Fiber Formation Polyesters Nylons Spandex Polymers Rayon Acetates Acrylics and Modacrylics Polybenzimidazole and Carbon Fibers Polymerization Process Technology Fiber Formation Postfiber Formation Chemistry and Technology Future Trends

The chemistry and technology of man-made, fiber-forming polymers date back to 1885 when an artificial silk was patented by Chardonnet in France. Since then, these materials have progressed to become the focus of a major global industry with applications ranging from the everyday world of apparel to biomedical and advanced aerospace engineering concepts. For the purpose of discussion of the chemistry and technology of man-made, fiber-forming polymers, the term "synthetic fiber" will be used to denote a l l man-made fibers manufactured from noncellulosic raw materials. The term "cellulosics" will apply to those man-made fibers that are manufactured from cellulosic raw materials. The term "man-made fibers" will apply to a l l fibers except the naturally occurring cellulosic and protein fibers. The manufacture of a l l man-made fibers involves at least three distinct process steps. The first consists of the production of polymers or polymer derivatives suitable for spinning into fibers. In the second step, or spinning, a polymer melt or solution is extruded under pressure through the appropriate spinneret's orifice(s) to form the fiber or fibers. If only a single fiber is produced from a spinneret, it is referred to as monofilament. Multifilament spinnerets produce yarns. The third step is drawing, 0097 6156/85/0285-0441 $07.75/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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APPLIED POLYMER SCIENCE

which i s the stretching, either hot or c o l d , of monofilament or yarn to some m u l t i p l e of i t s as-spun l e n g t h . T h i s step r e s u l t s i n an orientation of the polymer chains and c r y s t a l l i t e s with respect to the f i b e r a x i s , and i t i s c r i t i c a l w i t h r e s p e c t to the u l t i m a t e mechanical properties of the f i b e r . Bundles of yarns from m u l t i p l e s p i n n e r e t s are r e f e r r e d to as tow. I f the tow i s subsequently cut into short specified lengths, i t i s r e f e r r e d to as s t a p l e tow, and the product i s s t a p l e f i b e r . Depending on the man-made f i b e r producer's p r o d u c t i o n equipment, one, two, or a l l t h r e e of the s t e p s j u s t d i s c u s s e d may be executed i n either a batchwise or continuous manner. Table I shows the world-wide production of man-made f i b e r s , the U.S. p r o d u c t i o n of man-made f i b e r s , and the percentage of the world's production of man-made fibers produced i n the U.S. i n 1983. On the b a s i s of these data, which are s i m i l a r to the p r o d u c t i o n f i g u r e s for the past f i v e y e a r s , p o l y e s t e r s (11.20 b i l l i o n l b ) , polyamides (7.00 b i l l i o n l b ) , c e l l u l o s i c s (6.62 b i l l i o n l b ) , and the a c r y l i c s (4.91 b i l l i o n l b ) account for over 95% of the world's manmade fiber production. In T a b l e I I the U.S. p r o d u c t i o n of man-made f i b e r i s f u r t h e r divided into filament and s t a p l e f i b e r , and the U.S. consumption of c o t t o n and wool i s i n c l u d e d . On the b a s i s of these data domestic man-made fibers accounted for over 70% of a l l of the fiber consumed i n the U.S. i n 1982. These numbers, as w e l l as those i n T a b l e I , c l e a r l y i n d i c a t e the importance of the man-made f i b e r p r o d u c t i o n industry with respect to the U.S. and the world's t e x t i l e industry. The remainder of t h i s chapter i s concerned w i t h b r i e f l y summarizing the chemistry and technology of man-made fiber format i o n . An attempt has been made to p l a c e the emphasis on those synthetic and c e l l u l o s i c fibers that represent significance either i n terms of w o r l d p r o d u c t i o n o r , i n the a u t h o r s ' v i e w , i n terms of u n u s u a l or u n i q u e p o l y m e r c h e m i s t r y . More d e t a i l e d and comprehensive g e n e r a l r e v i e w s on v a r i o u s a s p e c t s of these t o p i c s have been published elsewhere (4-10). Chemistry of Man-Made Fiber Formation Of the f i b e r s l i s t e d i n T a b l e I I o n l y the p o l y e s t e r s , polyamides, spandexes, acetates, and rayon are discussed i n t h i s chapter. While the a c r y l i c s and modacrylics are the t h i r d most important c l a s s of commercial f i b e r s ; because t h e i r p o l y m e r i z a t i o n chemistry i s a l s o discussed i n other chapters concerned with v i n y l addition emulsion polymerizations, i t w i l l only be b r i e f l y summarized here. For the same reason p o l y p r o p y l e n e p o l y m e r i z a t i o n chemistry i s a l s o not covered i n t h i s s e c t i o n . However, two a d d i t i o n a l t o p i c s , carbon f i b e r formation and p o l y b e n z i m i d a z o l e s have been i n c l u d e d on the b a s i s of the c u r r e n t i n t e r e s t i n high-performance f i b e r s f o r composite materials. P o l y e s t e r s . Because of the h y d r o l y t i c i n s t a b i l i t y of e s t e r s of a l i p h a t i c acids, v i r t u a l l y a l l commercial polyesters are based on aromatic a c i d s (8). By f a r the most common i s p o l y ( e t h y l e n e terephthalate) (PET):


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U . S . and World-Wide Man-Made Fiber Production i n 1983 (I)

Table I .

World-Wide, b i l l i o n lb

Fiber


Fiber-Forming Polymers

Polyester

U.S., b i l l i o n lb

U.S., %

11.20

3.54

31.6

Polyamides

7.00

2.42

34.6

Acrylics

4.91

0.67

13.6

Cellulosics

6.62

0.63

9.5

3

2.29

0.90

39.0

Other

Includes the o l e f i n f i b e r s as w e l l as others.

Table I I .

Summary of U . S . Man-Made Fiber Production Data i n 1982 (2)

Fiber Class

Yarns + Monofilaments, million lb 195.2

Acetate

46.6b

Rayon

Staple + Tow

4.0 355.0

1,246.1

686.0

A c r y l i c + Modacrylic

—

624.1

Olefin

582.2

O l e f i n + Vinyon

—

Nylon + Aramid

Polyester

1,213.6

— 138.2 1,955.2 —

T e x t i l e glass fiber

899.2

Raw Wool

—

109.2

Raw cotton

—

2,491.1

a

1981 Data and does not include c i g a r e t t e tow production (_3). bl981 Data ( 3 ) .


a

444


0

u (CH -CH -0-C


2

2

0

-(o)-c-of

x

The p o l a r i t y of the e s t e r l i n k a g e produces s u f f i c i e n t i n t e r c h a i n f o r c e s to a l l o w these polymers to d e v e l o p good f i b e r - f o r m i n g p r o p e r t i e s w i t h number average m o l e c u l a r weights i n the 15 to 25 thousand range. PET i s prepared commercially by either ester interchange (EI) or d i r e c t e s t e r i f i c a t i o n processes (11, 12). For many y e a r s , the EI process was the o n l y one p r a c t i c a l because of the d i f f i c u l t i e s encountered i n p u r i f i c a t i o n of t e r e p h t h a l i c acid (TA). The dimethyl t e r e p h t h a l a t e (DMT) was much e a s i e r to p u r i f y , and t h e r e f o r e much more r e a d i l y a v a i l a b l e as a raw m a t e r i a l . In the normal EI batch process, ethylene g l y c o l and DMT are mixed i n a r a t i o ranging from 1.9 to 2.5 along with a t r a n s i t i o n metal c a t a l y s t at a l e v e l ranging from 50 to 150 ppm. Commonly, manganese a c e t a t e i s used as the c a t a l y s t , although s a l t s of calcium, magnesium, z i n c , and titanium have a l s o been employed. Temperatures are raised to 180 to 220 °C over a p e r i o d of time, and the e v o l v e d methanol i s removed by d i s t i l l a t i o n . During t h i s phase, bis(hydroxyethyl)terephthalate (BHET) ( S t r u c t u r e I) i s formed a l o n g w i t h l e s s e r amounts of dimer and trimer (Scheme I), When the e s t e r i n t e r c h a n g e step i s completed, a phosphorus compound, u s u a l l y an organic phosphate, phosphite, or polyphosphoric a c i d , i s added to complex the EI c a t a l y s t and thus d e a c t i v a t e i t . Antinomy o x i d e , or l e s s commonly germanium or t i t a n i u m o x i d e s , i s added at a l e v e l of 150 to 450 ppm to s e r v e as a p o l y c o n d e n s a t i o n (PC) c a t a l y s t . The mixture i s heated to 280-295 °C, and a vacuum of 0.1 mmHg or l e s s i s a p p l i e d to a i d i n the v o l a t i l i z a t i o n of the e t h y l e n e g l y c o l formed d u r i n g PC. The r e a c t i o n i s a l l o w e d to proceed u n t i l the increase i n reaction v i s c o s i t y , as determined by power consumption f o r s t i r r i n g , i s at a l e v e l t h a t i n d i c a t e s t h a t the d e s i r e d m o l e c u l a r weight has been o b t a i n e d . As the degree of polymerization increases, the inherent r e a c t i v i t y of the functional groups becomes l e s s of a l i m i t i n g factor than the a b i l i t y to remove e t h y l e n e g l y c o l from the h i g h l y v i s c o u s m e l t . Because of t h i s s i t u a t i o n , the PC reaction i s u s u a l l y c a r r i e d out i n equipment that maximizes the melt surface area by s t i r r i n g and f i l m generation on the reactor w a l l s . When the desired molecular weight has been achieved, the molten polymer i s extruded as a rope, commonly r e f e r r e d to as s p a g h e t t i , which i s quenched i n a water bath, cut to c h i p , and d r i e d . The dried chip i s then melt spun. As p u r i f i e d t e r e p h t h a l i c a c i d became more a v a i l a b l e as a raw m a t e r i a l , processes were developed based on d i r e c t e s t e r i f i c a t i o n of the TA (Scheme I I ) . These processes have the obvious advantages of an e s s e n t i a l l y s i n g l e - s t a g e p r o c e s s , a l t h o u g h they are u s u a l l y c a r r i e d out i n at l e a s t two phases. T h i s method, however, a l l o w s the use of a s i n g l e c a t a l y s t and g r e a t l y reduces the need f o r phosphorus s t a b i l i z e r s , a l t h o u g h s m a l l amounts are almost always added as complexing agents f o r t r a c e metal i m p u r i t i e s . Being a s i n g l e p r o c e s s , the d i r e c t e s t e r i f i c a t i o n route i s a l s o much more amenable t o c o n t i n u o u s p r o d u c t i o n , as w e l l as i n t e g r a t e d polymerization and spinning processes.


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445


0

CH3-O-C- ( 0 / " - ° _

C H

3

(DMT) + HO —CH —CH —OH 2

2

Mn(0A )

c 2

EI

180—220°C CH3OH

g

+ . .

0

3-CH -CH -0-i- and c e l l u l o s e . T h i s process forms a s k i n around the f i l a m e n t which acts as a semipermeable membrane. Diffusion of water out of the filament i s favored by the high i o n i c strength of the spin bath and r e s u l t s i n c o n c e n t r a t i o n and c o a g u l a t i o n of the remaining viscose. Concurrent d i f f u s i o n of hydronium i o n i n t o the f i l a m e n t n e u t r a l i z e s the viscose and regenerates the c e l l u l o s e . In the area between the c o a g u l a t i o n p o i n t and the n e u t r a l i z a t i o n p o i n t the polymer e x i s t s as a g e l . I t i s o n l y i n t h i s form t h a t drawing can occur. Because of t h i s r e s t r i c t i o n , dopes f o r the p r o d u c t i o n of high-strength rayon may contain a d d i t i v e s such as amines to slow the +


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n e u t r a l i z a t i o n and extend the opportunity for drawing and alignment of the c h a i n s . I n c r e a s e s i n c h a i n order may a l s o be a c h i e v e d by i n c l u d i n g Zn*^ s a l t s i n the spin baths. These ions coordinate with the xanthate s u l f u r on adjacent c h a i n s and form temporary c r o s s links. Acetates. Two forms of c e l l u l o s e a c e t a t e are of commercial importance for t e x t i l e fibers (19, 20). Primary c e l l u l o s e acetate, or t r i a c e t a t e , i s almost completely e s t e r i f i e d and has 10% or fewer free hydroxyls. Secondary acetate, or diacetate, i s the more common f i b e r m a t e r i a l . Because i t has 25-30% free h y d r o x y l s , i t i s more h y d r o p h i l l i c , s o l u b l e i n more polar s o l v e n t s , and l e s s thermoplastic than t r i a c e t a t e . Both m a t e r i a l s are made from c e l l u l o s e p u l p by p r e s w e l l i n g w i t h a c e t i c a c i d and then e s t e r i f i c a t i o n by u s i n g a s u l f u r i c a c i d - a c e t i c anhydride mixture. The reaction i s c a r r i e d to a point at which e s s e n t i a l l y a l l of the hydroxyls are converted to e i t h e r a c e t a t e or s u l f a t e e s t e r s . Further reaction a l l o w s t r a n s e s t e r i f i c a t i o n of the s u l f a t e s to y i e l d the t r i a c e t a t e . Hydrolysis of the mixed t r i e s t e r causes s e l e c t i v e cleavage of the s u l f a t e s w i t h p a r t i a l removal of the a c e t a t e s to y i e l d the d i a c e t a t e . The process i s c a r r i e d out i n t h i s manner because any a t t e m p t t o go d i r e c t l y to the d i a c e t a t e w o u l d r e s u l t i n a heterogeneous block copolymer system because c e l l u l o s e i n accessible r e g i o n s would r e a c t to the t r i a c e t a t e stage before s i g n i f i c a n t penetration of the c r y s t a l l i n e regions could be accomplished. After p r e c i p i t a t i o n , n e u t r a l i z a t i o n , and drying of the product, i t i s d i s s o l v e d i n a v o l a t i l e o r g a n i c s o l v e n t and dry spun. The d i a c e t a t e i s s o l u b l e i n acetone, but the l e s s p o l a r t r i a c e t a t e requires a solvent such a methylene c h l o r i d e . A c r y l i c s and Modacrylics. The f i b e r - f o r m i n g a c r y l i c polymers are produced i n aqueous media by using f r e e - r a d i c a l - i n i t i a t e d addition polymerizations. To provide d y e s t u f f a c c e s s i b i l i t y and s i t e s f o r dye binding they contain s m a l l amounts ( l e s s than 15% by weight) of one or more comonomers such as 2 - v i n y l p y r i d i n e , N - v i n y l p y r r o l i d o n e , a c r y l i c a c i d , m e t h a l l y l s u l f o n i c a c i d , and v i n y l acetate, or a c r y l i c esters. The m o d a c r y l i c f i b e r s c o n t a i n s i g n i f i c a n t l y h i g h e r c o n c e n t r a t i o n s of comonomers than the a c r y l i c s (up to 65% by weight). The comonomers of choice are v i n y l i d e n e c h l o r i d e , v i n y l c h l o r i d e , and possibly acrylamide. The high halogen content of most modacrylics makes them r e l a t i v e l y flame r e s i s t a n t but a l s o lowers t h e i r melting points and reduces t h e i r heat s t a b i l i t y r e l a t i v e to the a c r y l i c s (21). A c r y l i c and modacrylic fibers are produced by either dry or wet s p i n n i n g . As a r e s u l t of the s t r o n g i n t e r m o l e c u l a r a t t r a c t i o n s present i n the a c r y l i c s , the o n l y s o l v e n t s t h a t are s u i t a b l e are those t h a t are very p o l a r and thus capable of d i s r u p t i n g these secondary v a l e n c e bonds. These i n c l u d e tf,tf-dimethylformamide, d i m e t h y l s u l f o n e , d i m e t h y l s u l f o x i d e and d i m e t h y l acetamide. Modacrylics, however, are s o l u b l e i n more v o l a t i l e , lower p o l a r i t y s o l v e n t s such as acetone. After spinning the r e s i d u a l s o l v e n t i n a c r y l i c s must be removed by washing, and the f i b e r s are drawn either dry ( i n a hot a i r oven or over-heated r o l l s at 80-110 °C) or wet ( i n steam or hot water at 70-100 °C). F i n a l l y the yarns must be d r i e d


456



and heat s t a b i l i z e d to c o n t r o l f i b e r s h r i n k a g e d u r i n g subsequent processing. P o l y b e n z i m i d a z o l e and Carbon Fibers. Polybenzimidazole (Structure XIV) f i b e r s are s p e c i a l t y f i b e r s with high melting points, very high d e c o m p o s i t i o n t e m p e r a t u r e s (>500 ° C ) , good s t r e n g t h , and e x t e n s i b i l i t y . They are e s s e n t i a l l y nonflammable and have an u n u s u a l l y high moisture r e g a i n for a s y n t h e t i c f i b e r (>10%). Because of the t o x i c i t y of the monomers, they must be manufactured i n c o m p l e t e l y c l o s e d systems w i t h s p e c i a l h e a l t h and s a f e t y precautions. The most c i t e d p r e p a r a t i o n i n v o l v e s the m e l t c o n d e n s a t i o n p o l y m e r i z a t i o n o f 3 , 3 - d i a m i n o b e n z i d i n e or i t s tetrahydrochloride and diphenyl isophthalate (21-23). f

XIV

Carbon f i b e r s have been the s u b j e c t of e x t e n s i v e r e s e a r c h and i n v e s t i g a t i o n (24). They are characterized by t h e i r high strength and m o d u l i as w e l l as high-temperature r e s i s t a n c e . As a r e s u l t , carbon f i b e r s have been the f i b e r of c h o i c e f o r use as the r e i n f o r c i n g component i n l i g h t w e i g h t , high-performance composites. A l l carbon fibers are made by the carbonization of preformed f i b e r s . These p r e c u r s o r s may be a c r y l o n i t r i l e homopolymers, high-tenacity rayon f i l a m e n t y a r n s , or mesophase p i t c h f i b e r s from c o a l or petroleum p i t c h . The three process steps common to the production of carbon f i b e r s from the above p r e c u r s o r s are p r e o x i d a t i o n at temperatures r a n g i n g from 200 to 400 °C to prevent random c h a i n s c i s s i o n decompositions; carbonization at temperatures ranging from 1000 to 1400 °C; and g r a p h i t i z a t i o n at temperatures ranging from 1600 to 3000 °C to produce the f i n a l carbon f i b e r s . Polymerization Process Technology In g e n e r a l terms a man-made f i b e r p o l y m e r i z a t i o n scheme can be c l a s s i f i e d as e i t h e r a batch or a continuous p r o c e s s . In a pure batch process the polymerization step i s c a r r i e d out separately from f i b e r formation i n r e a c t o r s t h a t r e c e i v e d i s c r e t e charges of monomer(s). In a continuous p o l y m e r i z a t i o n (CP), monomer i s fed c o n t i n u a l l y into the reactors, and polymer i s c o n t i n u a l l y removed downstream. For some polyamides and polyesters, f i b e r formation may or may not be an i n t e g r a l p a r t of the CP l i n e . Most modern p o l y m e r i z a t i o n schemes are continuous p r o c e s s e s , and these are s l o w l y replacing much of the older batch technology. In the design of any polymerization reaction scheme, important c o n s i d e r a t i o n must be g i v e n to maximizing the p r o d u c t i o n of h i g h quality chip. At the temperatures t h a t f a v o r h i g h r a t e s of p r o d u c t i o n , r e a c t o r r e s i d e n c e times must be minimized to a v o i d degradation of the polymer. E f f i c i e n t recovery of unreacted monomer as w e l l as low-energy process requirements are a l s o important design criteria. Of the f i b e r s l i s t e d i n T a b l e I I , p o l y e s t e r and polyamide polymers are made by hot m e l t p o l y m e r i z a t i o n s . Because the



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c e l l u l o s i c s are based on a natural polymer, they are not polymerized i n p r o d u c t i o n . The a c r y l i c s are e m u l s i o n p o l y m e r i z e d , and the p o l y o l e f i n s may be polymerized either as a s l u r r y i n a hydrocarbon s o l v e n t , i n b u l k , or i n gas-phase r e a c t o r s . Although every manufacturer adapts h i s equipment to h i s p h y s i c a l p l a n t and p r o d u c t i o n needs, t h e r e are process steps c h a r a c t e r i s t i c of each type of man-made f i b e r p o l y m e r i z a t i o n t e c h n o l o g y . T h i s f a c t i s i l l u s t r a t e d by two examples of hot m e l t p o l y m e r i z a t i o n processes from the recent patent l i t e r a t u r e ; these are b r i e f l y d i s c u s s e d i n the f o l l o w i n g and are presented schematically i n Figures 2 and 3. In F i g u r e 2 a schematic r e p r e s e n t a t i o n f o r the c o n t i n u o u s p o l y m e r i z a t i o n of PET from e t h y l e n e g l y c o l and TA i s shown (25). The primary e s t e r i f i e r i s a multicompartment r e a c t i o n v e s s e l operated above the p a r t i a l pressure of the g l y c o l at the reaction temperature employed, which i s t y p i c a l l y around 180 °C. The secondary e s t e r i f i e r i s of s i m i l a r design to the primary except that i t i s operated at a reduced p r e s s u r e to remove water and g l y c o l . The p r o d u c t o f the s e c o n d a r y e s t e r i f i e r i s f e d t o t h e l o w polymerizer where the polycondensation phase of the reaction begins. The low p o l y m e r i z e r i s a l s o a m u l t i z o n e r e a c t o r where t h e temperature and p r e s s u r e are p r o g r e s s i v e l y increased and reduced, r e s p e c t i v e l y . The product from the low polymerizer i s fed i n t o the high polymerizer where the temperature i s brought to i t s f i n a l value of 280-295 °C, and the p r e s s u r e f u r t h e r reduced. To maximize the molecular weight of the polymer w h i l e reducing degradation at these temperatures, the m e l t s u r f a c e area i s kept l a r g e i n the h i g h polymerizer by f i l l i n g the reactor to only one-tenth to one f i f t h of i t s volume capacity. This condition a l s o minimizes the d w e l l time of the melt i n the reactor at these elevated temperatures. A continuous process for polymerization of nylon 6,6 i n which a f l u i d i z e d bed s o l i d state polymerization reactor i s used as the high polymerizer i s represented schematically i n Figure 3 (26). In t h i s process the low m o l e c u l a r weight polymer i s produced i n a f i l l e d pipe reactor located just upstream of the spray d r i e r . The l i q u i d product of t h i s step i s then sprayed i n t o a hot i n e r t gas atmosphere where the water i s flashed off and a fine powder i s produced. This powder i s fed i n t o an opposed-flow, f l u i d i z e d bed reactor at 200 °C where the h i g h m o l e c u l a r weight polymer powder i s generated at temperatures w e l l below the 255 °C melting point of nylon 6,6. The powder i s then melted i n the e x t r u d e r and c o n v e r t e d i n t o f i b e r or chip. A d d i t i o n a l examples of polymerization processes can be found i n a recently published review of fiber-forming polymerization patents by Robinson (27). A d e t a i l e d comparison of batch and continuous polymerization for nylon 6,6 can be found i n a review by Jacobs and Zimmerman (15). In another review Short has summarized the current s t a t e of p o l y p r o p y l e n e p o l y m e r i z a t i o n t e c h n o l o g y and c a t a l y s t development (28). Fiber Formation The p h y s i c a l properties of the polymer often d i c t a t e the spinning method t h a t must be used f o r f i b e r f o r m a t i o n . For example, i f the melt temperature i s above the thermal degradation temperature, the polymers cannot be melt spun. Such polymers must be l i q u i f i e d with




458

Spinneret

Figure 1.

MONOMER

Regeneration of viscose rayon.

MIXER

I

Mr

GLYCOL

4

J[J!

• PRIMARY ESTERIFIER

L

. iVAPOR

J

G

ft OARY ESTERIFIER

LOW

—V.

/*

POLYMERIZER

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Figure 2.

j j ^EXTRUD

I I I I I Schematic of d i r e c t e s t e r i f i c a t i o n , continuous polyester p o l y m e r i z e r (25).



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s o l v e n t s for spinning i n either a wet or dry solvent removal system. These systems are s i m i l a r to the melt spinning process i n that flow of the polymer and solvent s o l u t i o n s must be established within a spinneret channel (extrusion). However, u n l i k e melt spinning, wet and dry spinning require the p r e c i p i t a t i o n of the polymer either i n a s o l u t i o n bath f o r wet s p i n n i n g or by e v a p o r a t i o n of the s o l v e n t from the f i b e r f i l a m e n t i n dry s p i n n i n g . To be s p i n n a b l e the polymer melt or s o l u t i o n must be c a p a b l e of forming c o n t i n u o u s , f l u i d threads when extruded. These f l u i d threads must be e a s i l y transformed into s o l i d fibers that, after spin f i n i s h a p p l i c a t i o n , possess the physical properties required for subsequent processing (29). Polymers from Table I I that are t y p i c a l l y wet or dry spun are aramids, a c r y l i c s , modacrylics, and c e l l u l o s i c s . The polyesters, polyamides, and p o l y o l e f i n s are melt spun f i b e r s . During the past decade, s i g n i f i c a n t advances have been made i n the m e l t p r o c e s s s p i n n i n g e q u i p m e n t , w h i c h i s r e p r e s e n t e d s c h e m a t i c a l l y i n F i g u r e 4. Improved extruder (melter) design has been achieved by a better understanding of the a n a l y t i c a l treatment of non-Newtonian f l u i d - f l o w r h e o l o g y (30, 31). Increases i n p r o d u c t i o n r a t e s have been accomplished by s h o r t e n i n g r e s i d e n c e times and improving i n t e r n a l mixing. These changes have resulted i n p r o d u c t i o n r a t e s t h a t are more than double those t y p i c a l f o r e x t r u d e r s d u r i n g the 1970s. In t h i s same p e r i o d , there have been s i g n i f i c a n t improvements i n the f i l t e r s t h a t are necessary to p r e v e n t the s p i n n e r e t ' s c a p i l l a r i e s from becoming b l o c k e d . O r i g i n a l l y these f i l t e r s were sand and w i r e mesh s c r e e n s , but developments i n powdered metal and nonwoven s t a i n l e s s wire f i l t e r s have improved f i l t e r l i f e and e f f i c i e n c y . The development of large i n l i n e changeable f i l t e r s upstream of the metering pumps i n m e l t spinning has further extended by t e n - f o l d the time required before shutdown for spinning f i l t e r changes. Spinning speeds have a l s o been increased d r a m a t i c a l l y during the past decade. In addition to increasing production, higher spinning speeds can be used to f i n i s h and orient the yarn while s t i l l on the extrusion/spinning l i n e (spin drawing). In general, as the spinning speed i n c r e a s e s , the a i r drag on the f a l l i n g f i l a m e n t s begins to become s i g n i f i c a n t . For example, at s p i n n i n g speeds exceeding 3000 m/min, PET filaments are stretched by the a i r drag to induce a preorientation of the molecular chains i n the d i r e c t i o n of the f i b e r axis without s i g n i f i c a n t c r y s t a l l i z a t i o n . This p a r t i a l l y oriented yarn (POY) i s e s p e c i a l l y s u i t a b l e f o r draw t e x t u r i z i n g , and i t s commercialization has been c a l l e d one of the more s i g n i f i c a n t new developments i n t e x t i l e yarns (11). On a l i m i t e d s c a l e t h i s process has been s u c c e s s f u l l y extended to 10,000 m/min. To achieve higher speeds, which should r e s u l t i n spun yarns with even higher as-spun strength, i t w i l l probably be necessary for mechanical take-up to be replaced with more exotic winding devices. Conventional, low-speed spun fibers must be further finished by drawing, a l t h o u g h POY yarns are t y p i c a l l y draw t e x t u r i z e d . The drawing process adds s t r e n g t h by o r i e n t a t i o n of the m o l e c u l a r structures. Normal e x t e n s i b i l i t y and proper tenacity are added by drawing the spun f i b e r s to s e v e r a l times t h e i r as-spun length. The t e x t u r i n g process produces permanent crimp, l o o p s , c o i l s , or c r i n k l e s i n the yarn t h a t r e s u l t i n p r o p e r t i e s of s t r e t c h , b u l k , absorbency, and improved hand (32). There are three basic types of


460

APPLIED POLYMER SCIENCE AQUEOUS SALT SOLUTION 40%-50%

STORAGE


PREHEATER

FLUID BED POLYMERIZER

REACTOR

EXTRUDER HIGH PRESSURE PUMP

Figure 3.

I

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CAST AND QUENCH

Schematic of continuous s o l i d s t a t e p o l y m e r i z e r f o r nylon 6,6 (26).

Figure 4 .

Melt spinning process.



19. DREWS ET AL.


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yarn: stretch, modified s t r e t c h , and bulk yarns. Stretch yarns are c h a r a c t e r i z e d by f a l s e t w i s t or c r i m p i n g . Modified stretch yarns are produced as s t r e t c h yarns but have had the amount of s t r e t c h reduced and the b u l k i n c r e a s e d by heat s e t t i n g a f t e r the f a l s e t w i s t i n g or crimping. Bulk yarns have l i t t l e stretch with increased bulk. Bulk yarns are produced by the stuffer box or edge-crimping methods of t e x t u r i z i n g . There have been s e v e r a l attempts to produce f i b e r s without the use of the c o n v e n t i o n a l e x t r u d i n g and s p i n n e r e t d e v i c e s . In e l e c t r o s t a t i c spinning, for example, an e l e c t r o s t a t i c f i e l d i s used to form the polymer i n t o f i b e r s t r a n d s (33). T h i s method h o l d s promise, but w i l l require s i g n i f i c a n t development before commercial a p p l i c a t i o n can be achieved. The p r o d u c t i o n of bicomponent f i b e r s i n c r e a s e d c o n s i d e r a b l y during the 1970s. Bicomponent f i b e r s are filaments composed of two p h y s i c a l l y d i s t i n c t phases c o n s i s t i n g of different polymers. There are two s i g n i f i c a n t types of bicomponent f i b e r s : side-by-side and sheath-core (5). The components are u s u a l l y polymers of different chemical structure. The side-by-side f i b e r s use polymers that have v a s t l y different p h y s i c a l c h a r a c t e r i s t i c s ; t h i s produces different s h r i n k a g e i n the two s i d e - b y - s i d e polymers and r e s u l t s i n a w o o l l i k e bulk and crimp i n the f i b e r s . These f i b e r s are made by feeding the different polymers i n t o the spinneret at or near the c a p i l l a r y orifice. The sheath-core f i b e r i s u s u a l l y produced to a c h i e v e o v e r a l l strength with the core and wear resistance with the polymer of the sheath. The sheath-core f i b e r s are produced by spinning the two polymers through concentric c a p i l l a r y spinnerets (13). An i n t e r e s t i n g v a r i a t i o n of the bicomponent f i b e r i d e a i s the use of a i r or micropores as the second phase. These f i b e r s have found use i n biomedical a p p l i c a t i o n s as f i l t e r media i n a r t i f i c i a l kidneys. Robinson has reviewed some recent patents i n t h i s area, as w e l l as i n the other segments of polymer e x t r u s i o n , s p i n n i n g , and processing (34). Postfiber Formation Chemistry and Technology The purpose of any p o s t - f i b e r f o r m a t i o n treatment i s to modify an appearance or performance c h a r a c t e r i s t i c of a f i b e r or f a b r i c for a s p e c i f i c a p p l i c a t i o n without l i m i t i n g i t s general use. For example, most man-made f i b e r s could be permanently colored (dope dyed) at the fiber extrusion stage. However, i n p r a c t i c e , except f o r p o l y o l e f i n s , almost a l l c o l o r i s a p p l i e d by u s i n g d y e s t u f f s or pigments i n a l a t e r p r o c e s s i n g s t e p ; t h e r e f o r e , the f i b e r manufacturer or consumer i s not l i m i t e d to a l a r g e q u a n t i t y of permanently colored stock. S i m i l a r considerations apply to the wide v a r i e t y of f i n i s h i n g treatments that may be used to impart s p e c i f i c properties to yarns and f a b r i c s . A l t h o u g h dyeing and r e s i n f i n i s h i n g are by f a r the most important of the post-fiber formation manufacturing steps, the f i r s t f i n i s h to be applied to any man-made f i b e r i s c a l l e d a spin f i n i s h . I t i s a p p l i e d as part of the f i b e r s p i n n i n g process and u s u a l l y consists of at l e a s t three components. These are a wax or heavy o i l t h a t a c t s as a l u b r i c a n t to reduce f i b e r - t o - f i b e r and f i b e r - t o machinery f r i c t i o n ; an a n t i s t a t i c agent to reduce s t a t i c charge b u i l d - u p d u r i n g the p r o c e s s i n g of hydrophobic polymers; and an



462


e m u l s i f i e r to prevent the separation of the hydrophobic l u b r i c a n t s and h y d r o p h i l i c a n t i s t a t s i n the spin f i n i s h . Spin f i n i s h e s u s u a l l y are temporary f i n i s h e s t h a t are removed by s c o u r i n g p r i o r to any subsequent dyeing or f i n i s h i n g steps. Dyeing i s one of the most u n i v e r s a l of t e x t i l e c h e m i c a l processing steps. Dyes may be a p p l i e d to s t a p l e f i b e r (stock d y e i n g ) , yarns (yarn d y e i n g ) , f a b r i c s , or i n d i v i d u a l t e x t i l e a r t i c l e s . Almost a l l dyestuffs are applied from aqueous s o l u t i o n s or d i s p e r s i o n s , a l t h o u g h t h e r e c o n t i n u e s to be a low l e v e l of i n t e r e s t i n solvent dyeing processing (35). The dyeing process may be e i t h e r batch, i n a v a r i e t y of c l o s e d dyeing systems, or continuous on a continuous dye range. Dyeing c o n d i t i o n s i n batch processes may range from temperatures of 80 °C at atmospheric pressure for rayon to 130 °C under pressure for polyesters. In the continuous dyeing of polyester/cotton f a b r i c s on a thermosol range, temperatures as high as 215 °C are not unusual. Table I I I l i s t s the p r i n c i p a l dyestuff a p p l i c a t i o n c l a s s e s for the f i b e r s i n Table I I , as w e l l as some of the more important dyeing a u x i l i a r y t e x t i l e chemicals. The o b j e c t i v e of postdyeing t e x t i l e f i n i s h i n g i s to impart improved performance c h a r a c t e r i s t i c s i n comparison to those inherent i n a p a r t i c u l a r fiber or f a b r i c construction. Some common examples are i m p a r t i n g flame r e s i s t a n c e f o r p r o t e c t i v e c l o t h i n g , water repellency for outerwear garments, and reduced s t a t i c charge b u i l d up f o r o p e r a t i n g room gowns. In T a b l e IV a b r i e f summary of some f i n i s h i n g o b j e c t i v e s and the broad c h e m i c a l c l a s s e s of t e x t i l e chemicals used to achieve them are presented. Except for c e r t a i n dyeing procedures, t h e r e are three b a s i c steps i n v o l v e d i n most dyeing and f i n i s h i n g processes: a p p l i c a t i o n of the c o l o r or f i n i s h , p r e d r y i n g to remove the excess water, and c u r i n g to f i x or c h e m i c a l l y c r o s s - l i n k the d y e s t u f f or f i n i s h , r e s p e c t i v e l y . This procedure i s commonly referred to as a pad-drycure operation. These are i l l u s t r a t e d by the schematic of a dye range f o r the continuous dyeing of c o t t o n / p o l y e s t e r b l e n d f a b r i c s as shown i n Figure 5. In t h i s example two a p p l i c a t i o n c l a s s e s of dyestuff (one from T a b l e I I I f o r the c o t t o n f i b e r and a d i s p e r s e dye f o r the p o l y e s t e r ) are a p p l i e d s i m u l t a n e o u s l y from the dyebath to the f a b r i c . The f a b r i c i s p r e d r i e d w i t h banks of IR h e a t e r s ( e i t h e r e l e c t r i c or refractory) and then the disperse dye, which i s nonionic and can s u b l i m e , i s t h e r m a l l y f i x e d or thermosoled i n t o the polyester. The c h e m i c a l pad and steamer p a r t s of the range are necessary to provide the chemical components and the reactor for the f i x a t i o n of the c e l l u l o s i c dyes. For vat and s u l f u r dyes on the cotton, a t y p i c a l sequence i n the wash boxes might be a c o l d wash i n box 1, warm washing i n boxes 2 and 3, f o l l o w e d by an o x i d a t i o n i n the next two boxes, f o l l o w e d by hot washes and r i n s i n g i n the remaining boxes. Most commercial ranges have 8-10 boxes, r a t h e r than the four i l l u s t r a t e d i n Figure 5 (36). A t y p i c a l range of the type shown i n F i g u r e 5 would dye 80-120 yd/min. On a f i n i s h i n g range, the drying cans would be replaced by a tenter frame, which i s a h o r i z o n t a l forced a i r oven with the f a b r i c passing through while tensioned at the edges. Modifications that might be found on the range i n Figure 5 or a s i m i l a r range would be the use of a foam a p p l i c a t o r head rather than



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Table I I I .

Dyestuffs and Chemical A u x i l i a r i e s Used to Dye Man-Made Fibers

Dyestuff class

Class Fiber

Disperse

Polyesters Acetates Polyamides Acrylics

Dispersing agents (sulfonated l i g n i n s ) Carriers/Accelerants (benzene and naphthalene derivatives) Surfactants

Acrylics

Retarders (quarternary ammonium compounds, nonionic surfactants) Surfactants

Basic

Chemical A u x i l i a r y (by c l a s s )

3

0

0

Polyamides

Acid

Leveling agents (nonionic surfactants) Surfactants 0

Direct Vat Sulfur Reactive Azoic Pigments

E l e c t r o l y t e s (NaCl,Na S04) 2

Rayon

Surfactants

0

E l e c t r o l y t e s (NaCl) E l e c t r o l y t e s (NaCl)

Polyolefins (dope dyeing)

a

O n l y the most generally applied dyebath chemical a u x i l i a r i e s have been l i s t e d for i l l u s t r a t i v e purposes. °May be a n i o n i c , c a t i o n i c , n o n i o n i c , or amphoteric depending on applications.


464



Table I V .

Summary of Some Wet Chemical F i n i s h i n g Objectives and the T e x t i l e Chemicals Employed to Obtain Them

Fabric and/or Yarn Property Modified

Chemical F i n i s h i n g Agent (by c l a s s )

Easy care/crease resistance/ wash-wear

JV-methylol derivatives of ureas, triazones, carbamates, melamine

Handle

alkanol amides, polyethylene emulsions, polyacrylates, v i n y l acetates, polyethers

S o i l release

Fluorocarbons

Flame resistance

Phosphonium s a l t s , organohalogens, w/wo Sb20^, halogenated phosphates, v i n y l phosphonates, sulfamates

Static dissipation

Poly(ethylene oxide) g l y c o l s , quaternary ammonium compounds

Water repellancy

Chlorinated paraffin waxes, fluorocarbons, s i l i c o n e emulsions

Abrasion resistance

Reactive s i l i c o n e s , waxes

Pilling

Reactive s i l i c o n e s

Optical brightness

Stilbene d e r i v a t i v e s , oxazoles, triazoles, triazines, phthalimides

Let-Off and Scray

Wash and Treating Dye Chemical Steamer boxes

Figure 5.

Schematic of continuous dye range.


Cans

19.

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a dye pad, or vacuum s l o t extractors after the pads. These would be used to reduce the water t h a t subsequently must be removed i n the d r y i n g s t e p s and thus would s i g n i f i c a n t l y reduce the energy requirements of the process. Radio frequency drying and the use of h i g h - v e l o c i t y a i r have recently been investigated as a l t e r n a t i v e s to conventional drying procedures (37, 38). These and other aspects of the chemistry and t e c h n o l o g y of dyeing and f i n i s h i n g have been e x t e n s i v e l y discussed and reviewed i n the l i t e r a t u r e (39-47). Future Trends The chemistry and t e c h n o l o g y of man-made f i b e r s has changed d r a m a t i c a l l y d u r i n g the past 10 years as evidenced by the i n t r o d u c t i o n of the TA process for p o l y e s t e r , the c o m m e r c i a l i z a t i o n of the aramids, and the increasing use of s o l i d state polymerization. As the man-made f i b e r industry matures, i t becomes more d i f f i c u l t to p r e d i c t whether t h i s trend w i l l c o n t i n u e . In a d d i t i o n , the development of new polymerization chemistry or the introduction of new f i b e r s occurs only r a r e l y . Nevertheless, some developments i n new s p e c i a l t y f i b e r s can almost c e r t a i n l y be expected. In p a r t i c u l a r , an e l a s t o m e r i c f i b e r based on p o l y e s t e r r a t h e r than urethane chemistry and an a n i s o t r o p i c a l l y spun polyester analogous to the aramids would seem to be n a t u r a l t a r g e t s f o r r e s e a r c h and development. Some progress toward these g o a l s has a l r e a d y been made. Any s i g n i f i c a n t advances i n p o l y m e r i z a t i o n t e c h n o l o g y w i l l probably r e s u l t more from the i n c r e a s e d s o p h i s t i c a t i o n of process c o n t r o l and m o n i t o r i n g d e v i c e s r a t h e r than more major equipment d e s i g n changes. By u s i n g t i g h t process c o n t r o l to manipulate m o l e c u l a r weight d i s t r i b u t i o n s , reduce i m p u r i t i e s and s i d e r e a c tions, and increase the a c c e s s i b l e molecular weight range, i t should be p o s s i b l e to more r e a d i l y " t a i l o r make" f i b e r s f o r s p e c i f i c applications. Fiber spinning i s the one area of man-made f i b e r production i n which a major change i n t e c h n o l o g y i s most l i k e l y to occur i n the near future. U l t r a high-speed spinning with nonmechanical devices could d r a m a t i c a l l y affect a l l phases of pre- and postfiber formation chemistry and technology. A l t h o u g h e n v i r o n m e n t a l and energy c o n s i d e r a t i o n s have had considerable impact on the t e x t i l e dyeing and f i n i s h i n g i n d u s t r y , the emphasis i n the recent past has been on equipment modification rather than new chemistry. This trend can be expected to continue as more and more dyeing and f i n i s h i n g processes are automated and process c o n t r o l i s s u b s t a n t i a l l y improved. In summary, i t appears t h a t the man-made f i b e r i n d u s t r y i s l i k e l y to experience more changes i n technology than i n chemistry i n the near future. However, the new technology may very w e l l demand new chemistry and thus p r o v i d e the chemist and engineer w i t h new challenges. Acknowledgment The authors wish to thank the f o l l o w i n g c o l l e a g u e s f o r t h e i r encouragement, h e l p , and s u g g e s t i o n s i n the p r e p a r a t i o n of t h i s manuscript: Dr. R. M e r r i l l , Dr. K. K. L i k h y a n i , Dr. C. C h a i s s o n ,


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Fiber-Forming Polymers - American Chemical Society

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