High-Tech Fibrous Materials - American Chemical Society

Chapter 17

Printing Technology for Aramid Fabrics James D. Hodge and E. Anne Dodgson

Downloaded by COLUMBIA UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch017

Spruance Research Laboratory, E. I. du Pont de Nemours and Company, P.O. Box 27001, Richmond, VA 23261

Fabrics of Du Pont NOMEX aramid fiber can be printed by a conventional rotary screen printing process if the aramid fiber structure is open to allow dyes to migrate in and there is a structural feature to facilitate binding of the dye molecules to the fiber substrate. With Nomex this is achieved by imbibing surfactants into the water swollen fiber structure prior to initial drying. The surfactant provides a much more open structure as well as providing an ionic binding site for the dyes. The special problems of printing military camouflage fabrics where the colors, dye fastness properties, and infrared reflectance properties are critical are discussed.

The primary f u n c t i o n o f thermal p r o t e c t i v e c l o t h i n g i s t o minimize o r even e l i m i n a t e p h y s i c a l harm as a r e s u l t o f a f i r e . However, beyond t h i s obvious requirement o f flame p r o t e c t i o n , i t i s h i g h l y d e s i r a b l e and o f t e n necessary t o p r o v i d e p r o t e c t i v e c l o t h i n g i n v a r i o u s c o l o r s . T h i s d e s i r a b i l i t y f o r a v a r i e t y o f c o l o r s can be from a need f o r s t y l i n g as w i t h uniforms f o r race c a r d r i v e r s or from a need f o r h i g h v i s i b i l i t y as w i t h t h e outer s h e l l o f t h e launch and recovery s u i t s used by t h e space s h u t t l e crews. Quite t h e opposite i s r e q u i r e d f o r flame r e s i s t a n t m i l i t a r y camouflage; t h a t i s , c o l o r a t i o n i s necessary t o impart i n v i s i b i l i t y . Because o f the h i g h l y c r y s t a l l i n e nature and h i g h Tg o f aramid f i b e r s , such as Du Pont's NOMEX, i t i s extremely d i f f i c u l t t o dye aramid f a b r i c s i n c o n v e n t i o n a l dyeing processes. C o l o r e d NOMEX with the best o v e r a l l p r o p e r t i e s ( f a b r i c s t r e n g t h and c o l o r f a s t n e s s ) i s achieved by i n j e c t i n g c o l o r a n t s i n t o t h e s p i n dope p r i o r t o f i b e r spinning. These c o l o r a n t s can be dyes o f a v a r i e t y o f c l a s s e s o r pigments. No m o l e c u l a r b i n d i n g o f t h e c o l o r a n t i s necessary i n t h i s procedure s i n c e t h e c o l o r a n t i s p h y s i c a l l y o r mechanically locked i n t o the f i b e r s t r u c t u r e . Dyeing NOMEX i n f a b r i c form can only be achieved by u s i n g

C^-6156/91/0457-O270$06.00/0 © 1991 American Chemical Society

In High-Tech Fibrous Materials; Vigo, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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c a r r i e r s , u s u a l l y a powerful organic s o l v e n t , t o s w e l l o r d i s r u p t t h e s t r u c t u r e t o permit dyes t o migrate i n t o t h e f i b e r . In t h i s case t h e r e must be some s o r t o f molecular bonding o f the dye t o t h e aramid s t r u c t u r e and o n l y c a t i o n i c (basic) dyes w i l l b i n d t o the NOMEX polymer s t r u c t u r e . F a b r i c s o f NOMEX w i l l dye t o deep shades (with a c a r r i e r ) u s i n g c a t i o n i c dyes but w i l l only l i g h t l y s t a i n u s i n g a c i d dyes. T h i s a f f i n i t y o f c a t i o n i c dyes f o r the meta aramid s t r u c t u r e i s not w e l l understood. The most obvious bonding s i t e i s t h e f r e e c a r b o x y l end group. On the other hand, t h e r e are very few f r e e amine ends a v a i l a b l e as s i t e s f o r bonding t o a c i d dyes. The amide groups may a l s o p l a y a r o l e s i n c e amides a r e h y b r i d s t r u c t u r e s o f two resonance forms r e s u l t i n g i n t h e carbonyl oxygen being somewhat electronegative. Van der Waals f o r c e s between the aromatic r i n g s o f the polymer and dye may a l s o be a f a c t o r ; however t h i s would apply e q u a l l y t o a c i d o r b a s i c dyes (1).

0

0

II

II

+

o I

NHX^-,NH=C

il

(The

J

r

II

Meta Aramid S t r u c t u r e Two Resonance Forms o f the Amide Group A r e Shown)

These techniques o f adding c o l o r a n t s t o t h e s p i n dope and c a r r i e r dyeing f a b r i c s a r e f i n e f o r p r o t e c t i v e c l o t h i n g i n s o l i d shades. But f o r a p r o t e c t i v e apparel where p r i n t e d c o l o r s a r e r e q u i r e d , such as m i l i t a r y camouflage, t h e r e a r e a new s e t o f problems: t h e requirements o f the p r i n t i n g process and l i m i t a t i o n s on t h e d y e s t u f f s t h a t may be used. There are two p r i n c i p a l p r i n t i n g procedures used today i n t e x t i l e p r i n t i n g : pigment p r i n t i n g and dye (or wet) p r i n t i n g . The most widely used procedure i s pigment p r i n t i n g where t h e pigment c o l o r a n t i s bound t o the f i b e r s u b s t r a t e with a thermoset r e s i n . T h i s method i s not acceptable f o r thermal p r o t e c t i v e camouflage because o f f l a m m a b i l i t y problems a s s o c i a t e d w i t h t h e b i n d e r r e s i n . With the pigment c o l o r a n t o n l y on t h e s u r f a c e o f t h e f i b e r , wash and c r o c k f a s t n e s s a r e a l s o problems with pigment p r i n t s , e s p e c i a l l y w i t h m i l i t a r y s p e c i f i c a t i o n s around these parameters. As a consequence, dye p r i n t i n g i s t h e only acceptable method f o r m i l i t a r y camouflage p r i n t i n g at present. There i s a f u r t h e r l i m i t a t i o n on t h e c o l o r a n t s a v a i l a b l e f o r m i l i t a r y camouflage. The c o l o r s , o r shades, a r e chosen t o reduce t h e c o n t r a s t o f b a t t l e dress uniforms w i t h the n a t u r a l background. N e a r l y a l l camouflage p a t t e r n s are composed o f v a r i o u s shades o f green, brown and b l a c k . There i s , however, another important criteria: t h e shades must a l s o minimize t h e c o n t r a s t i n t h e near i n f r a r e d r e g i o n (600-900 nm) t o a v o i d d e t e c t i o n by n i g h t s u r v e i l l a n c e devices. T y p i c a l i n f r a r e d r e f l e c t a n c e o f some common dye c l a s s e s a r e shown i n F i g u r e 1. The dye c l a s s e s t h a t c u r r e n t l y meet t h e IR r e f l e c t a n c e requirements o f t h e m i l i t a r y , both here and abroad, a r e l i m i t e d t o a c i d dyes and vat dyes (2). T h i s p r e s e n t s a unique problem f o r NOMEX s i n c e aramids are n o t


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100

o' 600

'

' 700

•

'

'

800

900

'

1 1000

Wavelength (nm) Figure 1. Effect of Dye Class on IR Reflectance.



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s u b s t a n t i v e t o e i t h e r o f these c l a s s e s o f dyes. A d d i t i o n a l c r i t e r i a important i n the s e l e c t i o n o f dyes f o r m i l i t a r y camouflage are good c o l o r f a s t n e s s t o l i g h t , h i g h temperature washing, and c r o c k i n g . U.S. M i l i t a r y camouflage i s a f o u r c o l o r p a t t e r n : l i g h t green, dark green, brown and b l a c k with i n f r a r e d r e f l e c t a n c e s p e c i f i c a t i o n s shown i n the shaded p o r t i o n o f F i g u r e 2. The p r i n t i n g process can be s i m p l i f i e d somewhat i f the f i b e r i s produced i n the l i g h t green base shade. T h i s was e a s i l y accomplished by a d d i t i o n o f the a c i d dyes f o r t h i s c o l o r as p a r t o f the f i b e r p r o d u c t i o n p r o c e s s . The r e a l problem was producing a f i b e r t h a t would accept a c i d dyes i n p r i n t i n g the remaining t h r e e c o l o r s . The mechanism f o r p r i n t i n g with dyes i s s i m i l a r t o aqueous dyeing o f f a b r i c s — t h e dyes must migrate from the aqueous phase i n t o the f i b e r and then b i n d i n some manner t o the f i b e r s t r u c t u r e . The t e x t i l e wet p r i n t i n g process c o n s i s t s o f o v e r l a y i n g the d r y f a b r i c w i t h a h i g h l y v i s c o u s p r i n t paste c o n t a i n i n g the dyes i n the p a t t e r n d e s i r e d , oven d r y i n g the p r i n t e d f a b r i c , steaming, and washing. Dye " f i x a t i o n " (molecular b i n d i n g o f the dye t o the f i b e r s u b s t r a t e ) i s achieved i n the steaming step where a m i n i a t u r e aqueous phase i s c r e a t e d on the f a b r i c s u r f a c e a l l o w i n g the dyes t o migrate i n t o the fiber. Without the use o f a c a r r i e r t o s w e l l the meta aramid s t r u c t u r e , the NOMEX f i b e r must be i n h e r e n t l y s u f f i c i e n t l y open f o r t h i s dye m i g r a t i o n t o take p l a c e . T h i s was the f i r s t problem t h a t had t o be s o l v e d . A c h i e v i n g dye m i g r a t i o n i n t o the s u b s t r a t e i s o n l y h a l f o f the b a t t l e ; the other h a l f i s accomplishing the molecular b i n d i n g t o the s u b s t r a t e . The p r e f e r r e d mechanism o f b i n d i n g a c i d o r b a s i c dyes t o a s u b s t r a t e i s the formation o f an i o n i c bond. F o r example, a t y p i c a l a c i d dye has one o r more s u l f o n a t e groups, o r p o s s i b l y a c a r b o x y l a t e group, i n the s t r u c t u r e as shown i n these examples o f t y p i c a l a c i d dyes:

o

C.I. A c i d Yellow 23

+

C.I. A c i d Blue 25

The s u l f o n a t e (or c a r b o x y l a t e ) anion w i l l be a t t r a c t e d by a c a t i o n t h a t i s p a r t o f (or c o n t a i n e d i n ) the f i b e r s u b s t r a t e . The f i n a l step i n the mechanism i s formation o f an i o n i c bond between the a n i o n i c dye and the s u b s t r a t e with the a s s o c i a t e d counter ions forming a s a l t which w i l l wash out o f the s t r u c t u r e . F a i l u r e t o achieve good chemical b i n d i n g , o r f i x a t i o n , w i l l r e s u l t i n washfastness being poor o r n o n e x i s t e n t . In f a c t , i f l i t t l e o r no dye f i x a t i o n has been accomplished, most o f the dye w i l l wash out d u r i n g the f a b r i c s c o u r i n g s t e p i n the p r i n t i n g p r o c e s s .

Experimental The two b a s i c p r o p e r t i e s t h a t had t o be i n c o r p o r a t e d i n t o an aramid f i b e r s u b s t r a t e t o permit p r i n t i n g w i t h a c i d dyes i n a c o n v e n t i o n a l screen p r i n t i n g process were: (a) an open s t r u c t u r e t o permit dye


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Printing Technology for Aramid Fabrics 275

m i g r a t i o n i n t o t h e f i b e r without a c a r r i e r , and (b) a s t r u c t u r a l f e a t u r e t h a t would f a c i l i t a t e b i n d i n g o f an a c i d dye t o t h e substrate. Over the past s e v e r a l years Du Pont has developed p r o p r i e t a r y technology f o r imbibing s u r f a c t a n t s i n t o water swollen NOMEX b e f o r e i t i s d r i e d (3 and 4 ) . I t i s p o s s i b l e t o imbibe as much as 15% o f a a n i o n i c , c a t i o n i c o r even a n e u t r a l s u r f a c t a n t i n t o t h e aramid f i b e r . A f t e r i m b i b i t i o n , t h e subsequent d r y i n g o f the swollen f i b e r removes the water and the f i b e r s t r u c t u r e c o l l a p s e s t o a normal f i b e r . However, t h i s f i b e r now has a more open s t r u c t u r e due t o t h e s u r f a c t a n t and can be e a s i l y dyed without a c a r r i e r . The second b e n e f i t o f t h i s technology i s that i f an i o n i c s u r f a c t a n t i s used, the s u r f a c t a n t w i l l i o n i c a l l y bond t o a dye o f the o p p o s i t e charge. The i n i t i a l work i n v o l v e d imbibing an a n i o n i c s u r f a c t a n t , such as isopropylammonium dodecylbenzene s u l f o n a t e , which produced a s u b s t r a t e t h a t was deeply dyeable with c a t i o n i c dyes. Fabrics p r e p a r e d from f i b e r c o n t a i n i n g the a n i o n i c s u r f a c t a n t c o u l d a l s o be p r i n t e d with c a t i o n i c dyes. Since NOMEX normally has an a f f i n i t y f o r c a t i o n i c dyes, the primary f u n c t i o n o f the a n i o n i c s u r f a c t a n t was t o p r o v i d e the open s t r u c t u r e . To achieve p r i n t i n g with a c i d dyes, the s e l e c t i o n o f s u r f a c t a n t i s more c r i t i c a l s i n c e i t must not o n l y p r o v i d e t h e open s t r u c t u r e , but must b i n d very w e l l t o t h e a c i d dye and form a s t r u c t u r e t h a t w i l l not wash out. In a d d i t i o n , t o improve t h e l i g h t f a s t n e s s a UV screener can be imbibed along with the s u r f a c t a n t . Numerous c a t i o n i c s u r f a c t a n t s were t r i e d and although a l l were quaternary ammonium s a l t s o r ammonium h y d r o c h l o r i d e s a l t s o f one form o r another t h e r e were s i g n i f i c a n t d i f f e r e n c e s i n c o l o r y i e l d i n p r i n t i n g t r i a l s . An i m i d a z o l i n e s a l t was s e l e c t e d as the best s u r f a c t a n t y i e l d i n g t h e best depth o f shade i n a c i d dye p r i n t i n g . The amount o f s u r f a c t a n t i n t h e f i b e r ( s t a p l e through f i n i s h e d f a b r i c ) were determined by s o l v e n t e x t r a c t i o n o f the sample f o l l o w e d by HPLC a n a l y s i s o f the e x t r a c t . Proton magnetic resonance o f the sample d i s s o l v e d i n a s u i t a b l e s o l v e n t can a l s o be used i f the s p e c t r a f o r t h e s u r f a c t a n t and t h e m-phenylenediamine i s o p h t h a l a t e s t r u c t u r e do not i n t e r f e r e w i t h each o t h e r . An approximation o f t h e amount o f t o t a l imbibed m a t e r i a l s can a l s o be done by simple weight l o s s w i t h s o l v e n t extraction. A c c e p t a b l e c o l o r y i e l d on p r i n t i n g was achieved w i t h a c i d dyes f o r t h e dark green and brown shades, but an a c c e p t a b l e b l a c k shade c o u l d not be a t t a i n e d w i t h a c i d dyes. Since the b l a c k c o l o r i s only 16% o f the t o t a l p a t t e r n , i t was p o s s i b l e t o use a carbon b l a c k based pigment p r i n t w i t h a flame r e t a r d a n t b i n d e r t o achieve t h i s c o l o r . An added b e n e f i t o f u s i n g the b l a c k pigment was t h a t t h e IR r e f l e c t a n c e met t h e s p e c i f i c a t i o n o f

High-Tech Fibrous Materials - American Chemical Society

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