Polymer Stabilization and Degradation - American Chemical Society

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Stabilization of Polypropylene Multifilaments Utility of Oligomeric Hindered Amine Light Stabilizers ROBERT J . T U C K E R and PETER V. SUSI Polymer Products Division, American Cyanamid Company, Bridgewater, NJ 08807

The light stabilization of polypropylene (PP) fibers places special requirements on the stabilization system due to the high surface to volume ratio, the severe processing conditions, and the post spinning treatments. Earlier systems provided only moderate performance lifetimes thus limiting the markets for PP fibers. The development of oligomeric hindered amine light stabilizers (HALS) has led to PP fibers with greatly increased service lifetimes, thus opening up new applications for this fiber. Some of the factors leading to the enhanced performance obtainable with certain newer oligomeric HALS are described and evaluated. Also, the importance of processing conditions and interactions of HALS with other additives on the light stability of the resulting fiber are discussed. Because of polypropylene s unusual properties, such as light weight and ease of fabrication, it has been used to make a variety of fabrics and is expected to be the growth fiber of of the eighties (1,2). Adding impetus to this expectation was the development of hindered amine light stabilizers (HALS) (3)_ that enable polypropylene (PP) fibers to penetrate new markets. The processes for producing these fibers range from conventional melt spinning for continuous filament and staple through heavy denier monofilaments produced by extrusion into water, with additional large volumes produced from film by slitting or splitting. Even direct production of fabrics from polymer by spun-bonded processes is possible with polypropylene. The deniers of fibers produced by the varied techniques cover a wide range varying from micro deniers produced by melt blowing processes to the heavy deniers used in carpet backings, sacks, bags and rope or cordage (2). Essential to the continued success for this polymer in the fiber market is the ongoing effort to continually enhance the stability of the products produced especially towards oxygen, heat and light. 1

0097-6156/85/0280-0137$06.00/0 © 1985 American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

While the e a r l y HALS gave c o n s i d e r a b l y g r e a t e r l i g h t s t a b i l i t y t o PP f i b e r s than c l a s s i c a l l i g h t s t a b i l i z e r s , they s t i l l f a i l e d to p e r f o r m w e l l i n low d e n i e r f i b e r s . These p l a c e s p e c i a l r e q u i r e m e n t s on the s t a b i l i z e r system due t o t h e h i g h s u r f a c e a r e a , t h e s e v e r e p r o c e s s i n g c o n d i t i o n s u s e d , and t h e use o f v a r i o u s c o l o r a n t s and post spinning treatments such as t e n t e r i n g , l a u n d e r i n g , and d r y cleaning. More r e c e n t l y , o l i g o m e r i c type HALS have been found to provide the b e s t b a l a n c e o f p r o p e r t i e s f o r most a p p l i c a t i o n s . The products a t t a i n their improved structural properties without a substantial reduction i n h i n d e r e d amine c o n t e n t , thus r e t a i n i n g a high s p e c i f i c a c t i v i t y . Data a r e p r e s e n t e d showing the s u p e r i o r performance o f c e r t a i n o l i g o m e r i c HALS i n PP m u l t i f i l a m e n t s . Experimental As used h e r e i n , y a r n d e n i e r i s t h e number o f grams p e r 9000 m e t e r s . Y a r n t e n a c i t y i s the t e n s i l e s t r e s s expressed as force per unit l i n e a r d e n s i t y o f the u n s t r a i n e d s p e c i m e n , i n g r a m s - f o r c e p e r d e n i e r (gf/den.). The m a j o r i t y o f t e s t s were c a r r i e d out on m u l t i f i l a m e n t y a r n s p r e p a r e d from H e r c u l e s PRO-FAX 6401 p o l y p r o p y l e n e powder. To the base polymer was added 0.05% c a l c i u m s t e a r a t e and 0.1% 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate (processing stabilizer). Additives were d r y b l e n d e d i n t o the powder and t h e r e s u l t i n g b l e n d s e x t r u d e d a t 227°C and p e l l e t i z e d . The p e l l e t s were spun i n t o y a r n s , u s i n g a NRM e x t r u d e r a t 280°C w i t h a 30 h o l e d i e , and the y a r n s were t h e n drawn a t a 6:1 r a t i o i n two s t a g e s and g i v e n a 2Z t w i s t . The y a r n s (240/30 d e n i e r ) were woven i n t o t e s t s t r i p s (40 y a r n s p e r i n c h ) and used f o r exposure studies. Accelerated light stability studies were carried out i n an A t l a s Xenon A r c Weather-Ometer (WOM) w i t h a 6500 Watt b u r n e r . Operating conditions were 30% r e l a t i v e humidity, 44°C ambient temperature and a b l a c k p a n e l temperature o f 65 + 3 ° C . F o r the GM-WOM t e s t , an A t l a s twin globe e n c l o s e d c a r b o n a r c u n i t was used w i t h a 3.8 hour l i g h t c y c l e and 1 hour water m i s t c y c l e . The ambient temperature was 72°C with a black panel temperature of 89° + 3°C d u r i n g t h e l i g h t c y c l e . T e n t e r i n g was s i m u l a t e d by h e a t i n g the y a r n s a t 120°C f o r 20 minutes i n a c i r c u l a t i n g a i r oven. In t h e l a u n d e r i n g t e s t , yarns were machine washed with d e t e r g e n t and d r i e d t h r e e t i m e s . F o r the d r y c l e a n i n g t e s t s , y a r n s were c o m m e r c i a l l y d r y c l e a n e d t h r e e t i m e s . In a l l s t u d i e s , the f a i l u r e p o i n t was a 50% l o s s i n o r i g i n a l breaking strength of the y a r n s as measured by I n s t r o n t e n s i l e p r o p e r t y measurements. HALS s t r u c t u r e s a r e shown i n F i g u r e 1 . Results

and D i s c u s s i o n

Most h i n d e r e d amine light stabilizers have evolved from the discovery (4) that compounds containing a 2,2,6,6t e t r a m e t h y l p i p e r i d i n e moiety can stabilize polymers against photodegradation and t h i s m o i e t y has been i n c o r p o r a t e d i n t o HALS o f v a r i o u s types ( F i g u r e 1 ) . Much has been p u b l i s h e d on the mechanism of a c t i o n o f HALS and t h e l i t e r a t u r e i n t h i s a r e a has r e c e n t l y been c r i t i c a l l y reviewed ( 5 , 6 ) . W h i l e the complete mechanism of action has not been fully elucidated, the h i g h performance o f HALS i s g e n e r a l l y a t t r i b u t e d t o the a b i l i t y o f t h e i r o x i d a t i o n products to

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11. TUCKER AND SUSI

Stabilization of Polypropylene Multifilaments


139


140

act as r a d i c a l scavengers i n a c y c l i c s e l f - p e r p e t u a t i n g shown i n E q u a t i o n s 1 through 3 ( 7 , 8 ) . VH / V o -

oxidation +

R.

^ ^ >

/

V o /

(1)

N-0-R

(2)

N

f a s h i o n as

/

V-O-R + /

ROO-

*

V o - +

ROOR

(3)

•

R e c e n t l y , most e f f o r t s i n the HALS a r e a have centered around obtaining the b e s t s t a b i l i z a t i o n a t the lowest c o s t i n a v a r i e t y o f demanding a p p l i c a t i o n s such as p o l y p r o p y l e n e m u l t i f i l a m e n t s . While the stabilizing activity o f HALS i s c e n t e r e d around t h e h i n d e r e d p i p e r i d i n e n i t r o g e n , the r e s t o f t h e m o l e c u l e s t i l l has an i n f l u e n c e on o v e r a l l p e r f o r m a n c e . It i s f e l t that HALS c o n c e n t r a t e i n the amorphous area o f p o l y o l e f i n s where d e g r a d a t i o n i s more l i k e l y t o o c c u r due t o i n c r e a s e d oxygen d i f f u s i o n and a l a c k of crystalline order. Some c h e m i c a l s t r u c t u r e s may be more s u i t a b l e o r s t e r i c a l l y more f a v o r a b l e than o t h e r s i n a l l o w i n g closer association o f the HALS w i t h potential damage s i t e s . T h i s " c o m p a t i b i l i t y " c a n be a very important HALS a t t r i b u t e and an i m p o r t a n t factor i n the e f f e c t i v e n e s s o f a HALS over the l i f e o f a polymer s u b s t r a t e . The stabilization of polypropylene yarns is a demanding a p p l i c a t i o n because o f the e x t r e m e l y high surface area (9) and intimate exposure of the f i l a m e n t s t o oxygen and l i g h t throughout t h e i r very thin cross section. Table I shows the e f f e c t of thickness on the t h e r m a l s t a b i l i t y o f p o l y p r o p y l e n e c o n t a i n i n g , as the a n t i o x i d a n t s y s t e m , 0.1% tetrakis[methylene(3,5-di-tert-butyl4-hydroxyhydrocinnamate)]methane and 0.3% distearylthiodipropionate. At 1 5 0 ° C , an 8 d e n i e r p e r f i l a m e n t y a r n f a i l s much sooner than a 0.4 m i l t h i c k f i l m which i n t u r n i s l e s s s t a b l e than a 4 . 0 m i l f i l m d e m o n s t r a t i n g the d r a m a t i c e f f e c t o f sample thickness on thermal s t a b i l i t y .

Table I .

PP Thermal S t a b i l i t y v e r s u s

Sample T h i c k n e s s 4.0 m i l F i l m 0.4 m i l F i l m 240/30 D e n i e r Y a r n (8 d p f )

Thickness

Hours t o E m b r i t t l e m e n t at 150°C >500 200 25

In Table II, the e f f e c t of f i l a m e n t d i a m e t e r on t h e l i g h t s t a b i l i t y of polypropylene yarn, containing 0.1% o c t a d e c y l 3,5di-tert-butyl-4-hydroxyhydrocinnamate as the the a n t i o x i d a n t , i s shown. F a i l u r e was the time t o 50% o r i g i n a l b r e a k i n g s t r e n g t h . The lower d e n i e r f i b e r showed a g r e a t l y reduced l i f e t i m e due t o i t s much s m a l l e r c r o s s s e c t i o n , and thus increased exposure to l i g h t and oxygen.


11. TUCKER AND SUSI Table I I . D e n i e r per Filament 146 8

Stabilization of Polypropylene Multifilaments PP L i g h t S t a b i l i t y v e r s u s

F i l a m e n t Diameter 6.0 m i l 1.4 m i l s

Thickness

Hours t o F a i l u r e Xenon WOM 750 200

The s e v e r e processing conditions used t o produce low d e n i e r f i b e r s r e q u i r e a s t a b i l i z e r w i t h v e r y good t h e r m a l s t a b i l i t y and low volatility. V o l a t i l i t y o f the s t a b i l i z e r can a l s o be an important f a c t o r i n the e f f e c t i v e n e s s o f a HALS under e n d - u s e c o n d i t i o n s . The volatility of several commercial HALS, as measured by thermogravimetric a n a l y s i s (TGA), is shown i n Table III. The oligomeric HALS (3-6) have the best thermogravimetric p r o f i l e , showing low p r o d u c t l o s s a t h i g h t e m p e r a t u r e s , and s h o u l d survive polymer processing better than HALS 1 and 2, as w e l l as remain i n the h i g h s u r f a c e a r e a f i b e r s over t i m e . Table I I I .

R e l a t i v e V o l a t i l i t y o f HALS by

TGA

a

Temperature ( ° C ) a t X% Weight L o s s T TlO T20 267 236 251 305 291 275 339 318 329 380 331 351 325 301 277 371 401 344 5

HALS HALS HALS HALS HALS HALS a

1 2 3 4 5 6

Heating

Rate 10°C/minute i n A i r

Another i m p o r t a n t a t t r i b u t e o f a HALS is its effect on the thermo-oxidative stability of polypropylene m u l t i f i l a m e n t s . This p r o p e r t y i s i m p o r t a n t i n c e r t a i n e n d - u s e a p p l i c a t i o n s where e l e v a t e d temperature o v e r a p e r i o d o f time is experienced, such as in automobile rear shelf fabrics. The e x c e l l e n t performance o f the o l i g o m e r i c HALS 4 and 6, as determined by 120°C oven a g i n g , i s shown i n F i g u r e 2. The s u p e r i o r a c t i v i t y o f t h e s e p r o d u c t s may be due not o n l y t o t h e i r low v o l a t i l i t y but a l s o t o the p r e s e n c e o f a triazine moiety i n the s t r u c t u r e , which appears t o have a p o s i t i v e e f f e c t on the t h e r m o - o x i d a t i v e s t a b i l i t y o f p o l y p r o p y l e n e . The GM-WOM i s a high temperature (72 ° C ) , high humidity a c c e l e r a t e d w e a t h e r i n g t e s t , s p e c i f i e d by G e n e r a l M o t o r s , f o r f i b e r s and p l a s t i c s for automotive i n t e r i o r a p p l i c a t i o n s . In t h i s u n i t , the o l i g o m e r i c HALS 4 and 6 gave the b e s t p e r f o r m a n c e , a l t h o u g h HALS 2 was a l s o v e r y e f f e c t i v e ( F i g u r e 3 ) . O l i g o m e r i c HALS g e n e r a l l y o u t p e r f o r m o t h e r t y p e s when a t h e r m a l t r e a t m e n t , such as a t e n t e r i n g o r a l a t e x i n g o p e r a t i o n , i s performed on the y a r n s . As shown i n F i g u r e 4, in simulated tentered yarns (heated at 120° C for 20 m i n u t e s ) , the o l i g o m e r i c HALS 3,4 and 6 showed the b e s t performance i n the Xenon WOM. Another i m p o r t a n t c o n s i d e r a t i o n i n s e l e c t i n g a l i g h t s t a b i l i z e r system f o r p o l y p r o p y l e n e y a r n s i s the r e s i s t a n c e to activity loss after l a u n d e r i n g or dry c l e a n i n g . In F i g u r e 5 d a t a a r e p r e s e n t e d


141


80

r

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

HALS-6

0.3% HALS, 0.1% AO, 0.05% CaSt F i g u r e 2. Thermal s t a b i l i z i n g a c t i v i t y i n 240/30 p o l y p r o p y l e n e y a r n ( y a r n t e n a c i t i e s 5.2 ± 4%).

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

denier

HALS-6

0.25% HALS, 0.1% AO, 0.05% CaSt F i g u r e 3. L i g h t s t a b i l i z i n g a c t i v i t y i n 240/30 p o l y p r o p y l e n e y a r n ( y a r n t e n a c i t i e s 4.7 ± 4%).

denier


11.


TUCKER AND SUSI

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

HALS-6

0.25% HALS, 0.1% AO, 0.05% CaSt

Figure 4. Light s t a b i l i z i n g a c t i v i t y i n 240/30 denier polypropylene yarn (simulated tentering).

2000

r

2

3

4

Laundered

6

2

3

4

6

Dry Cleaned

0.25% HALS, 0.1% AO, 0.05% CaSt

Figure 5. Laundering and dry cleaning effects on 240/30 denier polypropylene yarns.


143

144


showing t h e e f f e c t o f t h e s e o p e r a t i o n s on y a r n s containing various HALS as measured by Xenon-WOM e x p o s u r e . While HALS 2 showed good a c t i v i t y a f t e r l a u n d e r i n g , i t l o s t most o f i t s a c t i v i t y after the dry cleaning treatment. The o l i g o m e r i c HALS 6 outperformed a l l the o t h e r HALS e v a l u a t e d , d e m o n s t r a t i n g good resistance to extraction from the yarns by a h o t aqueous d e t e r g e n t s o l u t i o n o r by o r g a n i c s o l v e n t s used i n d r y c l e a n i n g . The r e s u l t s of a concentration versus light stabilizing activity study with two o l i g o m e r i c HALS (4 and 6) a r e shown i n F i g u r e 6. In t h e GM-WOM u n i t , HALS 6 shows a b e t t e r activity r e s p o n s e t o i n c r e a s i n g c o n c e n t r a t i o n t h a n does HALS 4. A t t h e lower c o n c e n t r a t i o n l e v e l s o f 0.15% t o 0.3%, HALS 6 shows an almost l i n e a r a c t i v i t y increase. The d i f f e r e n c e s o b s e r v e d a r e p r o b a b l y r e l a t e d t o subtle structural differences resulting in altered polymer compatibility. T h i s e f f e c t w i t h HALS 4 has a l s o been seen by o t h e r workers ( 1 0 ) . Another area o f concern i n s t a b i l i z i n g polypropylene f i b e r s i s the development o f c o l o r due t o the s t a b i l i z e r s y s t e m . While most HALS a r e c o l o r l e s s and impart l i t t l e o r no c o l o r on p r o d u c t i o n , " g a s y e l l o w i n g " o f f i b e r s i n use c a n be a c o n c e r n . In T a b l e I V , d a t a a r e shown on the gas y e l l o w i n g resistance of s e v e r a l HALS a t 0.5% concentration i n n a t u r a l polypropylene multifilament. The y a r n s were exposed using a m o d i f i e d AATCC 23-1972 t e s t f o r 1 c y c l e and then e v a l u a t e d u s i n g a gray scale comparator with a 5.0 rating i n d i c a t i n g no change, and a 1.0 r a t i n g d e n o t i n g s e v e r e c o l o r change. With 0.1% p h e n o l i c a n t i o x i d a n t p r e s e n t , HALS 2 and HALS 6 showed a b a r e l y p e r c e p t i b l e c o l o r development, w h i l e HALS 4 d i s c o l o r e d to a greater extent. I n the absence o f t h e p h e n o l i c a n t i o x i d a n t , HALS 4 s t i l l showed a n o t i c e a b l e d i s c o l o r a t i o n , w h i l e the y a r n s containing HALS 6 showed no c o l o r development, i n d i c a t i n g t h a t d i s c o l o r a t i o n i n the presence o f t h e p h e n o l i c a n t i o x i d a n t was due t o the l a t t e r and not t o HALS 6 i t s e l f . Table IV.

HALS HALS HALS HALS HALS

2 4 6 4 6

Gas Y e l l o w i n g R e s i s t a n c e

% Phenolic A . O . 0.1 0.1 0.1 0 0

f O . 5 % HALS M o d i f i e d AATCC 23-1972 t e s t - 1 c y c l e ; change; 1.0 = s e v e r e c o l o r change b

i n 240/30 D e n i e r PP Y a r n s Rating 4.5 4.0 4.5 4.0 5.0

a

1

5.0 - no c o l o r .

P o l y p r o p y l e n e f i b e r s a r e o f t e n pigmented and t h e pigments used can i n f l u e n c e t h e l i g h t s t a b i l i t y o f t h e system ( 1 1 ) . Some improve s t a b i l i t y , some a r e n e u t r a l , w h i l e o t h e r s are deterimenal due t o their prodegradative tendencies or to pigment-stabilizer interactions (12). A comparison o f two o l i g o m e r i c HALS (4 and 6) i n b l u e and r e d pigmented y a r n s i s shown i n T a b l e V . As can be seen, even structurally similar products show d i f f e r e n t relative s t a b i l i z a t i o n e f f e c t i v e n e s s with d i f f e r e n t pigments.


TUCKER AND SUSI




146

T a b l e V.

S t a b i l i z a t i o n o f Pigmented PP M u l t i f l a m e n t

Stabilizer HALS 4

Hours t o F a i l u r e GM WOM Blue Red 390 620 --370 775 565 855 — 375 1230

% Concentration 0.2 0.4 0.5 0.8 0.2 0.4 0.5 0.8

HALS 6

a

%00/34 D e n i e r b l u e y a r n ; 300/70 d e n i e r r e d y a r n

^_

F i n a l l y , t h e importance o f u s i n g optimum p r o c e s s i n g c o n d i t i o n s i s i l l u s t r a t e d i n T a b l e V I . Here s i m i l a r f o r m u l a t i o n s were examined but t h e s p i n pack temperature used i n t h e p r e p a r a t i o n o f t h e y a r n s was i n c r e s e d . Both t h e o l i g o m e r i c HALS 4 and 6 gave y a r n s with improved l i g h t s t a b i l i t y when p r o c e s s e d a t the h i g h e r temperature. Table VI.

E f f e c t o f P r o c e s s i n g C o n d i t i o n s on Yarn

Stabilizer HALS 4 HALS 6

240/30 D e n i e r y a r n s ST = s i m u l a t e d

Processing Temperature 265~*C 280 °C 265 °C 280 °C (0.25% HALS); NT

Stability

3

Hours t o F a i l u r e Xenon WOM ST GM WOM NT 390 785 675 980 1878 1772 475 840 710 1067 2440 1950 non-tentered;

Summary In polypropylene m u l t i f i l a m e n t s , o l i g o m e r i c h i n d e r e d amine l i g h t s t a b i l i z e r s have been found to offer superior light stabilizing activity. Properties such as good thermal stability, low v o l a t i l i t y , c o m p a t i b i l i t y , and e x t r a c t i o n r e s i s t a n c e have been shown t o be important f a c t o r s f a v o r i n g t h e use o f o l i g o m e r i c HALS i n polypropylene fiber applications. Differences i n molecular s t r u c t u r e among t h e o l i g o m e r i c HALS a r e p r o b a b l y r e s p o n s i b l e f o r t h e b e s t o v e r a l l s t a b i l i z i n g a c t i v i t y o b s e r v e d w i t h HALS 4 and 6. Acknowledgements The a u t h o r s w i s h t o acknowledge the c o n t r i b u t i o n made t o t h e s e studies by co-workers and thank the management o f American Cyanamid Company f o r p e r m i s s i o n t o p u b l i s h t h e work and M i s s J . C. W i l l i a m s f o r t y p i n g the manuscript.


11.

TUCKER AND SUSI


Literature Cited 1. Blore, J. H. Knitting Times November 6 1978, p. 24. 2. Polypropylene Fiber Symposium, New York, N.Y., 1981 3. Patel, A. R.; Usilton, J. J. In "Stabilization and Degradation of Polymers"; Allara, D. L.; Hawkins, W. L., Eds.; ADVANCES IN CHEMISTRY SERIES No. 169, American Chemical Society: Washington, D. C., 1978; p. 116. 4. Sankyo Co. Ltd. British Patent 1 196 224, 1970. 5. Sedlar, J.; Marchal, J.; Petruj, J. Polymer Photochemistry 1982, 2, 175. 6. Dagonneau, M. et al. Rev. Macromol. Chem. Phys. 1982, C22 (2), 169. 7. Carlsson, D. J.; Grattan, D. W.; Wiles, D. M. Organic Coatings and Plastics Chemistry 1980, 39, 628. 8. Durmis, J. et al. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 549. 9. Carlsson, D. J.; Wiles, D. M. J. Macromol. Sci., Rev. Macromol. Chem. 1976, C14, 65. 10. Gugumus, F.; Linhart, H. Chemicke Vlakna 1982, 32, 94. 11. Klemchuk, P. P. Polymer Photochemistry 1963, 3, 1. 12. Uzlmeier, C. SPE Journal 1970, 26, 69. RECEIVED October 26, 1984


Polymer Stabilization and Degradation - American Chemical Society

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