High-Tech Fibrous Materials - American Chemical Society

Chapter 21

Stabilization of Polypropylene Fiber and Tape for Geotextiles Robert L. Gray

Downloaded by UNIV LAVAL on October 28, 2015 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch021

CIBA-GEIGY Corporation, Ardsley, NY 10502

Recent advances in resin and stabilization technologies have expanded applications of polyolefin fibers in the carpet and geotextile industries. The stabilization requirements of applications such as nonwoven geotextiles, outdoor carpet, or artificial turf demand the use of high performance additives. New, more effective Hindered Amine Light Stabilizers (HALS) have been developed which offer significant advantages over traditional light stabilizers. This paper will discuss process, thermal, and light stability of polypropylene fiber and tape as well as stabilizer extraction resistance. Geosynthetics have become increasingly important i n the construction and environmental industries. Included i n the general geosynthetic c l a s s i f i c a t i o n are: geomembranes, geonets, geotextiles, and geogrids. Due to t h e i r excellent transmissivity properties, geotextiles are often used as substitutes for natural materials such as gravel or sand (1). A v a r i e t y of polymers including polyolefins, polyethylene terephthalate, and p o l y v i n y l chloride are currently being used as geosynthetics. Selection of the polymer i s based on the s p e c i f i c physical and chemical requirements for each i n d i v i d u a l a p p l i c a t i o n (2.3). Polyolefins are often the polymer of choice for use i n many of these applications due to their unique physical properties, excellent d u r a b i l i t y and chemical resistance (4.5). This work discusses s t a b i l i z a t i o n of polypropylene fiber for use i n geotextiles. Degradation. As with many polymeric materials, the mechanism for p o l y o l e f i n degradation i s generally considered to proceed v i a a r a d i c a l chain

0097-6156/91/0457-O320$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.


21. GRAY

321 Stabilization of Polypropylene Fiber and Tape for Geotextiles

pathway (6) (Figure 1 ) . I n i t i a t i o n t y p i c a l l y occurs through exposure to heat, shear, and/or UV r a d i a t i o n . A d d i t i o n a l l y , the presence of reactive impurities such as residual catalyst or chromophoric groups can a i d i n r a d i c a l i n i t i a t i o n . Propagation steps are proposed to involve a reaction with oxygen which can r e s u l t i n the formation of hydroperoxides. Subsequent decomposition of these unstable hydroperoxides lead to the further production of reactive r a d i c a l s . The effect of t h i s degradation process on the physical c h a r a c t e r i s t i c s of geosynthetics i s of c r i t i c a l importance. Degradation of polyolefins results i n two opposing effects on molecular weight d i s t r i b u t i o n . Chain s c i s s i o n (decreased molecular weight) and crosslinking (increased molecular weight) may both occur during aging. Polyethylene t y p i c a l l y experiences less chain s c i s s i o n than c r o s s l i n k i n g , thus the melt flow rate i s usually observed to decrease, r e f l e c t i n g an o v e r a l l increase i n molecular weight. Conversely, polypropylene primarily undergoes chain s c i s s i o n which results i n an increased melt flow rate. The p r a c t i c a l consequences of these reactions are: d i s c o l o r a t i o n , surface crazing (formation of surface micro-cracks) embrittlement, and loss of mechanical properties (elongation, impact strength, t e n s i l e strength). Mechanisms of S t a b i l i z a t i o n Thermal S t a b i l i z a t i o n . An effective method of thermal s t a b i l i z a t i o n i s through the use of a r a d i c a l terminating antioxidant. The most common class of antioxidant for r a d i c a l termination i s a hindered phenol (7). The mechanism (Figure 2) by which these compounds function i s by abstraction of H from the hindered phenol by the reactive peroxy-radical. This produces a less reactive, resonance s t a b i l i z e d phenolic r a d i c a l . Peroxycyclohexadienones can then be formed after reaction with an second peroxy-radical (8). This type of s t a b i l i z a t i o n i s effective at temperatures encountered during both melt processing and long term heat aging. Phosphites (9-13) and thioethers (14-17) are effective at decomposing hydroperoxides into stable non-radical products. As previously discussed these peroxy moieties are thermally and p h o t o l y t i c a l l y unstable and t y p i c a l l y decompose producing two r a d i c a l products. Interrupting this process through the use of phosphites or thioesters can s i g n i f i c a n t l y reduce the l e v e l of r a d i c a l i n i t i a t i o n . Phosphites are very effective during melt processing ( t y p i c a l temperature range: 220-315oC), providing color and melt flow rate s t a b i l i t y . Thioethers ( t y p i c a l l y used i n combination with hindered phenols) are used for elevated temperature (>100oC) applications. Light S t a b i l i z a t i o n . UV s t a b i l i z e r s can generally be categorized into three types. The f i r s t class of s t a b i l i z e r s function by screening the polymer through absorption of UV r a d i a t i o n . These s t a b i l i z e r s absorb UV radiation and prevent polymer degradation. By undergoing a reversible tautomeric process (18.19), they dissipate the UV energy as heat. As shown i n Figure 3, the enol form of the UV absorber (UVA) absorbs the r a d i a t i o n and i s


322

HIGH-TECH FIBROUS MATERIALS

Initiation PH *RI: Reactive Impurities (residual catalyst, chromophoric groups or precursors of chromophoric groups) Propagation

P* + 0,2 P0 * + PH POOH 2POOH PO* + PH •OH + PH


Termination

2P0 * P0 * + P# P# + P* 2PO, 2

2

Figure 1.

P0 * POOH + ? • + *OH POO* + PO* + H 0 POH + P* H 0 + ?• 2

2

A/UV

2

2

POH + P=0 + 0 POOP P -P POOP + O,

Auto-oxidation Mechanism of

2

Polyolefins

• Trap POO* radicals by donating H* • Better H» donors than substrate • Sacrificial (not catalytic) stabilizer O

OH

•

Produces

Figure 2.

> Phenoxyl radicals > Cyclohexadienone derivatives

Hindered Phenol S t a b i l i z a t i o n Mechanism



21.

GRAY

323 Stabilization of Polypropylene Fiber and Tape for Geotextiles

converted into i t s keto-tautomer. This energy i s then dissipated as heat i n the reverse reaction. The most commonly used UV absorbers are substituted benzotriazoles or benzophenones. A disadvantage of UVA's i s that their effectiveness i s l i m i t e d by Beer's Law which requires high concentrations and s u f f i c i e n t polymer thickness. In colored samples, the pigment serves as a screening agent; therefore, the benefit of UVAs are often s i g n i f i c a n t l y reduced. Another class of s t a b i l i z e r s act as UV quenchers or energy transfer agents. These compound function by quenching molecules that have become excited and returning them to the ground state before homolytic bond cleavage can occur (20.21). This class of s t a b i l i z e r s are t y p i c a l l y nickel-based coordination complexes. The most recent class of l i g h t s t a b i l i z e r s are the Hindered Amine Light S t a b i l i z e r s (HALS) which function by interrupting the r a d i c a l chain degradation mechanism. The proposed s t a b i l i z a t i o n mechanism (Figure 4) involves the formation of a n i t r o x y l r a d i c a l from the oxidation of the hindered amine (22), (Klemchuk, P. P . ; Gande, M. E . ; Cordola, E . , Polym. Peg, and Stab., i n p r e s s . ) . This n i t r o x y l r a d i c a l can i n turn react with free r a d i c a l s i n the polymer to eventually y i e l d nonradical products. In contrast to the s a c r i f i c i a l mechanism proposed for hindered phenolic antioxidants, i t i s believed the n i t r o x y l r a d i c a l can be repeatably regenerated and thus function i n an efficent manner. This may explain the high l e v e l of UV s t a b i l i z a t i o n achieved at r e l a t i v e l y low concentrations. Fiber S t a b i l i z a t i o n S t a b i l i z a t i o n of polypropylene multifilament fiber requires special s t a b i l i z a t i o n approaches. Fiber processing expose the polymer to high shear and often severe extrusion conditions. These conditions require a high performance s t a b i l i z e r package. The high surface-area-to-volume r a t i o of the finished polypropylene fiber product requires s t a b i l i z e r s r e s i s t a n t to v o l a t i l i z a t i o n and extraction. A d d i t i o n a l l y , many f i b e r applications demand color s t a b i l i t y through processing, aging, and exposure to NOx type gases (gas fade). Results and Discussion Primary Antioxidant. Polyolefin f i b e r i s often s t a b i l i z e d through the use of a hindered phenol as primary antioxidant i n combination with a phosphite. Selection of the hindered phenol (Figure 5) w i l l depend on the p a r t i c u l a r performance requirements needed for each a p p l i c a t i o n . Issues to be considered are color development (aesthetics), thermal s t a b i l i t y requirements, and extraction/chemical resistance. Secondary Antioxidant. During processing a phosphite can be used to s a c r i f i c i a l l y s t a b i l i z e the polymer thus preserving the primary antioxidant for l a t e r use as a long term thermal s t a b i l i z e r . The structure of the phosphite (Figure 6) can have a dramatic effect on both s t a b i l i z a t i o n performance and other physical properties



324

HIGH-TECH FIBROUS

Figure 3. Tautomeric Process by Which Benzotriazoles Other UV Absorbers) Dissipate Energy

R = O +

Figure 4.

MATERIALS

(and

R'OH

HALS S t a b i l i z a t i o n

Mechanism



Octadecyl 3,5 di-tert-butyl-4hydroxyhydrocinnamate

1,3,5-trls(35-di-tert-butyl-4hydroxy-benzyl)-s-triazine 2,4,6(1 H, 3H, 5H)trione

Calcium bis[monoethyl(3,5-di-tertbutyl-4-hydroxybenzyl)phosphonate (50% with polyethylene wax)

AO-2

AO-3

AO-4

Figure 5.

Irganox 1425wl

Irganox® 3114

Irganox 1076

Irganox® 1010

Trade Name

0

CH,

J CH,

2

2

^

2

2

OC H

5

^ - C O C ^

o

2

H

I

CH

OH

C

o

-^

2

CH CH C-0-CH -

HO-^-CH -P-0

H

Structure

Hindered Phenolic Antioxidant Structures

Tetrakis[methylene(3,5-di-tert-butyl-4 hydroxyhydrocJnnamate)]methane

AO-1

t

Chemical Name

Abbrev.



Bis(2,4-di-t-butylphenyl) pentaerythritol diphosphite

Phos-2

Ultranox® 626

Irgafos® 168

Trade Name

Structure

Phosphite Processing S t a b i l i z e r Structures

Tris(2,4-di-tert-butylphenyl) phosphite

Phos-1

Figure 6.

Chemical Name

Abbrev.


21. GRAY

Stabilization of Polypropylene Fiber and Tape for Geotextiles 327


such as h y d r o l y t i c s t a b i l i t y (PHOS-1 being s i g n i f i c a n t l y more r e s i s t a n t to hydrolysis than PHOS-2). Figure 7 shows the s t a b i l i z i n g effect on melt flow of phenol/phosphite additive packages. The large change i n melt flow rate found between the compounded r e s i n and f i n a l fiber indicates a change i n molecular weight and thus the degree of degradation. Light S t a b i l i z e r s . Light s t a b i l i t y i s an important issue for t e x t i l e s and i s p a r t i c u l a r l y c r i t i c a l for geotextiles which may experience extended outdoor exposure. The l i g h t s t a b i l i z e r s evaluated i n this study are shown i n Figure 8. Hindered Amine Light S t a b i l i z e r s (HALS) outperform t r a d i t i o n a l UVAs even at one t h i r d the concentration as demonstrated i n Figure 9 which shows actual outdoor weathering of natural polypropylene tapes. In TiO pigmented f i b e r , the addition of HALS results i n a tenfold increase i n l i g h t s t a b i l i t y (Figure 10). A concentration effect exists where s t a b i l i t y can be increased with increasing HALS concentration. The high performance c h a r a c t e r i s t i c s of these HALS are due not only to the regenerative mechanism but also to excellent permanence i n the polypropylene. HALS-1 and HALS-2 are both oligomeric; consequently, these s t a b i l i z e r s do not r e a d i l y v o l a t i l i z e during processing and cannot be e a s i l y extracted. Table I shows the extraction resistance of the HALS during wash

Table I . Extraction Resistance of HALS-1 & HALS-2 i n Polypropylene Multifilament: Washing Test

Base S t a b i l i z e r :

0.075% A0-4 0.075% Phos-1 0.10% calcium stearate

Pigmentation 0. 25. T i 0 ( r u t i l e ) 2

Denier Wash Procedure

Light S t a b i l i z e r HALS-1 HALS-1 HALS-2 HALS-2

130/37 1. Washed i n 7% soap solution for 30 min. @ 60oC 2. Dried for 30 min. @ 60oC 3. 5 Cycles

Untreated 0.25% 0.49% 0.38% 0.64%

% Concentration Treated 0.27% 0.50% 0.40% 0.67%



328


Unstabilized 0.05% AO-3/.1% Phos-1 0.05% AO-3/.1% Phos-2

0

10 20 30 40 Melt Flow Rate (230°C/2.16 kg)

Base: PP homopolymer, 0.1% CaSt & 0.25% Ti0 Extruder Temp: Compounding 450°F, Fiber 500°F

50

2

Figure 7 . Processing S t a b i l i t y - Melt Flow Change From P e l l e t s to Fiber



,

i

(

r

f

Figure 8.

2

2

17

2

2 6

OJ

-N-(CH ) -N-

O

CH, NH—C—CH,—C—CH, CH, CH,

N

3

r *i— J CH,

H C CH 3

2

8

OC H

(CH ) -OC-(CH ) -C40-CH,

H3CCH3

Structure

Light S t a b i l i z e r Structures

H

N-N-bis(22,66 tetramethyl-4Chimassorb™ 944FL piperidinyl)-1,6-hexanediamine, polymer with 2,4,6-trichloro-1,3,5triazineand 2,44-trimethyM,2pentanamine

l

HALS-2

f

Dimethyl succinate polymer with Tinuvin® 622LD 4-hydroxy-22,6,6-tetramethyl-1 piperidine ethanol

HALS-1

Cyasorb® UV-531

2-Hydroxy-4-n-octyloxy benzophenone

UVA-1

Trade Name

Chemical Name

Abbrev.


330

HIGn-TECH FIBROUS MATERIALS

Kilolangleys to Failure in Florida

(«3 months)

None I

(«6 months)


0.3% UVA-1 0.1% HALS-1

(~8 months)

0.1% HALS-2 0

(«10 months) 25

50

75

100

125 150

Base: PP homopolymer, 0.1% CaSt, 0.05% AO-1, 0.05% Phos-1 Figure 9.

HALS i n 2 M i l Polypropylene Tape

Days to 5 0 % Retention of Tensile Strength

• •

0.3% H A L S 0.6% H A L S

No H A L S

HALS-2

HALS-1 0

50

100

150

Base: PP homopolymer, 0.1% CaSt2, 0.25% Ti0

2

200

250

& 0.1% AO-4

Figure 10. HALS i n 130/37 Denier Multifilament PP Fiber Xenon Arc Weatherometer Exposure


21. GRAY


cycles at 60°C. After 5 cycles, no loss of HALS was detected by t o t a l nitrogen analysis. An analogous study was c a r r i e d out simulating dry cleaning (Table I I ) . HALS-1 experienced a 31% loss

Table I I . Extraction Resistance of HALS-1 & HALS-2 i n Polypropylene Multifilament: Dry Cleaning

Base S t a b i l i z e r :

0.05% AO-4 0.10% calcium stearate

Pigmentation

0. 25. T i 0 ( r u t i l e )


2

Denier 130/37

Wash Procedure

HALS-1 Light S t a b i l i z e r HALS-1 HALS-2 HALS-2

1. 2. 3.

30 min. @ 30C i n perchloroethylene Dried for 30 min. @ 60C 5 Cycles % Concentration Untreated Treated 0.29% 0.51% 0.26% 0.46%

0.20% 0.35% 0.26% 0.49%

while HALS-2 showed no s i g n i f i c a n t loss after exposure. These tests indicate that the HALS evaluated are f a i r l y resistant to extraction. These experiments are, however, somewhat oversimplified and actual extraction resistance may be dependent upon the exposure conditions. HALS as Thermal S t a b i l i z e r s . In addition to l i g h t s t a b i l i t y , thermal s t a b i l i t y i s an important factor i n polypropylene f i b e r stabilization. Based on the s t a b i l i z a t i o n mechanism proposed for HALS, i t might be expected that HALS could function as thermal s t a b i l i z e r s as well as l i g h t s t a b i l i z e r s . Indeed, when used i n combination with AO-2 and PH0S-1, HALS-2 provides three times the thermal s t a b i l i t y of a t r a d i t i o n a l high performance antioxidant AO-1 i n natural polypropylene tape (Figure 11). Figure 12 demonstates that HALS are effective heat s t a b i l i z e r s i n TiOpigmented polypropylene fiber at temperatures below 150oC. The addition of HALS-2 (at 0.30%) to AO-4 increases the thermal s t a b i l i t y by over e i g h t - f o l d . HALS-1 also provides a s i g n i f i c a n t benefit as a thermal s t a b i l i z e r . Thiosvnergists. The thioether, DSTDP, has been used i n combination with primary antioxidants to provide long term thermal stability. As this compound is not p a r t i c u l a r l y effective at low


332

HIGTI-TECH FIBROUS MATERIALS

Base


0.1% AO-1 0.025% HALS-2 + 0.01% AO-2 0.04% Phos-1 0.075% HALS-2 + 0.01% AO-2 0.04% Phos-1 250

500

750

1000

1500

1250

Days to Embrittlement at 90°C Figure 11.

HALS as Heat S t a b i l i z e r s i n PP Tape

Days to Embrittlement at 110°C

• •

0.15% HALS 0.30% HALS

No HALS

HALS-2

HALS-1 100

125

150

175

Base: PP homopolymer, 0.1% CaSt2, 0.25% Ti0 & 0.1% AO-4 130/37 Denier Multifilament 2

Figure 12. HALS as Heat S t a b i l i z e r s i n Polypropylene Fiber


21. GRAY


temperatures (below 50°C) , i t would not be expected to provide a s i g n i f i c a n t increase i n the longvevity of a geosynthetic. At lower test temperatures, HALS provide s i g n i f i c a n t l y higher s t a b i l i t y than systems containing thiosynergists (Table I I I ) . In

Table I I I .

125oC LTHA of Carbon Black F i l l e d PP Plaques

Weight %


Control

PP Homopolymer Calcium Stearate UV Grade Carbon Black AO

Light S t a b i l i z e r

100.0 0.1 2.5 As indicated

Analysis (By Nitrogen Content) 0.1% AO-1 0.1% AO-1 + 0.3% DSTDP 0.1% Phos-1

Control

600

1,900

0.10% HALS-2 0.50% HALS-2

2,200 5,000

1,100 4,700

0.10% HALS-1 0.50% HALS-1

2,000 5,000

2,000 4,000

f a c t , when HALS are combined with thioesters the thermal s t a b i l i t y reduced to less than for systems containing a HALS alone. Carbon Black F i l l e d Systems. Many geotextile applications require grey or black pigmentation. Although carbon black i s often included to increase l i g h t s t a b i l i t y , i t t y p i c a l l y has a negative effect on thermal s t a b i l i t y . As carbon black levels increase, the thermal s t a b i l i t y provided by the base antioxidant system decreases (Figure 13). It i s believed that the tremendous surface area of the carbon black is capable of adsorbing antioxidants, rendering them i n e f f e c t i v e . Apparently, polymeric HALS such as HALS-2 are not as adversely effected by carbon black and can provide a dramatic increase i n heat s t a b i l i t y . T r a d i t i o n a l l y , carbon black alone was believed to provide sufficient light stability. Xenon weatherometer testing on polypropylene tapes has shown that i f a HALS i s added to carbon black f i l l e d polypropylene, carbon black concentrations can be lowered to one quarter t h e i r o r i g i n a l l e v e l and better l i g h t s t a b i l i t y w i l l s t i l l r e s u l t (Figure 14). As discussed above, reducing the carbon black l e v e l has the additional benefit of enhancing other desirable properties. HALS-1 shows better l i g h t s t a b i l i t y performance than HALS-2. This might be attributed to



334


Base: PP homopolymer, 0.1% CaSt2, 0.1% AO-1, 0.1% Phos-1 Vulcan 9 Carbon Black Used Figure 13. Tape

HALS as Heat S t a b i l i z e r s - Carbon Black F i l l e d PP

Days to 50% Retention of Tensile Strength

1000

•

750

• •

2.5% Carbon Black 0.625% Carbon Black 0.125% Carbon Black No Carbon Black

500 250

No HALS

0.3% HALS-2

0.3% HALS-1

Base: PP homopolymer, 0.1% CaSt2, 0.03% AO-1 and 0.07% Phos-1 Figure 14. HALS i n 2 M i l Black Polypropylene Tape - Xenon Arc Weatherometer Exposure


21. GRAY

Stabilization of Polypropylene Fiber and Tape for Geotextiles

greater steric hinderance of the tertiary amine (HALS-1) which may reduce its interaction with the carbon black surface.


Conclusions Effective stabilization of polypropylene fiber for geotextile applications requires an additive system capable of preventing polymer degradation during both processing and aging. This is best achieved through a combination of primary antioxidants, phosphites, and HALS. Polymeric HALS provide exceptional extraction resistance and stabilization (both thermal and light) in polypropylene fiber and tape. In applications requiring the presence of carbon black, high molecular weight HALS show superior performance presumably due to their ability to avoid interaction with the carbon black surface. Long term thermal stability of these products can be dramatically improved with the addition of the proper HALS. The addition of tertiary hindered amines to carbon black containing systems appears to provide excellent UV stability at significantly lower carbon black levels. Acknowledgments The contributions and research efforts of the Polyolefin Additives Laboratories in both Ardsley, NY and Basel, Switzerland are gratefully acknowledged. Special appreciation is also extended to Ciba-Geigy Corporation for permission to use the data presented. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Charron, R. M. Geosythetics '89 Conference Proceedings, San Diego, CA, 1989. White, D.F.; Verschoor, K.L. Geotechnical Fabricsreport 1989, 7(3), 16. Ford, J. E . , Proc. Fourth International Conference on Polypropylene Fibres and Textiles, Nottingham, UK, September 1987. Tisinger, L.G. Geotechnical Fabricsreport 1989, 7(3), 22. Ramaswamy, S. D.; Rathor, M. N., Proc. Fourth International Conference on Polypropylene Fibres and Textiles, Nottingham, UK, September 1987. Bolland, J . L . ; Gee, G., Trans. Faraday Soc., 1946, 42, 236, 244. Das, P. K.; Encinas, M. V.; Scaiano, J . C.; Steenken, S., J. Am. Chem. Soc., 1981, 103, 4162. Pospisil, J., Adv. in Polym. Sci., 1980, 36, 69. Denney, D. B.; Goodyear, W. F.; Goldstein, B., J . Am. Chem. Soc., 1960, 82, 1393. Zuech, E. A., Chem. Comm., 1968, 1182. Chang, B. C.; Denney, D. B.; Denney, D. Z.; Marsi, K. L . , J. Am. Chem. Soc., 1969, 91, 5243. Denney, D. B.; Jones, D. H., J . Am. Chem. Soc., 1969, 91, 5821.


335

336



13.

Walling, C.; Rabinowitz, R., J. Am. Chem. Soc., 1959, 81, 1243. 14. Oae, S. "Organic Chemistry of Sulfur", Plenum: New York, 1977, 529-532. 15. Block, E., J. Am. Chem. Soc., 1972, 94, 642. 16. Shelton, J. R.; Davis, K. E., J. Am. Chem. Soc., 1967, 89, 718. 17. Davis, F. A.; Awad, S. B.; Jenkins, R. H., Jr.; Billmers, R. L.; Jenkins, L. A., J. Org. Chem., 1983, 48, 3671. 18. Calvert, J. G.; Pitts, J. N., Jr.; "Photochemistry", John Wiley & Sons, Inc., New York, 1967, 419. 19. Goeller, G.; Rieker, J . ; Maier, A.;Stezowski, J. J . ; Daltrozzo, E.; Neureiter, M.; Port, H.; Wiechmann, M.; Kramer, H. E. A., J. Phys. Chem., 1988, 92, 1452. 20. Harper, D. J . ; McKellar, J. F.; Turner, P. H., J. Appl. Polym. Sci., 1974, 18, 2805. 21. Allen, N. S.; Homer, J . ; McKellar, J. F., Macromol. Chem., 1978, 179, 1575. 22. Klemchuk, P. P.; Gande, M. E., Makromol. Chem., Macromol. Symp., 1989, 28, 117. RECEIVED August 2, 1990


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