Progress in the Light Stabilization of Polymers - ACS Symposium

3 Progress in the Light Stabilization of Polymers T O S H I M A S A T O D A , T O M O Y U K I K U R U M A D A , and KEISUKE M U R A Y A M A

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Chemical Research Laboratories, Research Institute, Sankyo Co., Ltd., 2-58, Hiromachi 1-chome, Shinagawa-ku, Tokyo 140, Japan

Since their i n i t i a l production by our laboratories about ten years ago, hindered amine light stabilizers (HALS) have become established as excellent light stabilizers of polymers. This review deals with their synthesis, evaluation and development. Hindered Amine Light Stabilizers (HALS) are a new class of highly efficient stabilizers protecting polyolefins and other polymers against light-induced deterioration. They were i n i t i a l l y developed into commercial products in our laboratories. In this review we describe the details of how they were synthesized, evaluated and developed. In order to find a new potential stabilizer, our initial efforts were directed toward synthesizing new stable nitroxyl radicals and evaluating their stabilizing activity. Although these stable radicals were good light stabilizers, they could not be used commercially since they contributed color to the polymers. Finally, intensive studies led to the discovery that hindered amine compounds derived from 2,2,6,6-tetramethyl-4-oxopiperidine may be converted into the corresponding stable nitroxyl radical and are excellent light stabilizers for polymers. Derivatives of 2,2,6,6-tetramethylpiperidine were synthesized and tested, and as a result, esters of 4-hydroxy2,2,6,6-tetramethylpiperidine were selected as practical light stabilizers, particularly for polyolefins. Degradation and Stabilization Oxidation reactions are generally believed to involve a free radical chain reaction as proposed by Bolland and Gee Q). The main steps of this reaction are: (1) R. + 0

2

R00- + RH

>R00> R00H + R-

(2) (3)

0097-6156/85/0280-0037$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

38

POLYMER STABILIZATION AND DEGRADATION ROOH

heat or l i f i h t ^

2 ROOH

>R0-

R0- + RH


+

(A)

# Q H

+ ROO- + H 0 2

>ROH + R.

HO- + RH 2 ROO

R Q
HOH + R. ^

Product ketones, alcohols, e t c ,

(5) (6) (7)

(8)

Radicals formed during i n i t i a t i o n react with oxygen leading to chain reactions. G e n e r a l l y , R e a c t i o n 2 i s f a s t e r than Reaction 3. The decomposition of hydroperoxides by heat or UV l i g h t (Equation 4) causes f o r m a t i o n of alkoxy and hydroxy r a d i c a l s l e a d i n g to c h a i n branching. UV absorbers can absorb t h i s harmful energy, s l o w i n g i n i t i a t i o n reactions, while antioxidants (InH) act to interfere with propagation according to the following reaction: ROO- + InH

> ROOH + In-

(9)

F i n a l l y , peroxide decomposers can decompose hydroperoxides i n a nonr a d i c a l process to interfere with Reaction 4. T y p i c a l examples of the s t a b i l i z e r s g e n e r a l l y used to prevent the above c h a i n r e a c t i o n a r e : (a) UV absorbers — 2 ( 2 - h y d r o x y - 3 tert-buty 1-5-methylpheny 1 ) - 5 - c h i or oben zo t r i a zo le an d 2-h yd ro xy - 4 octoxybenzophenone; (b) Antioxidants — 3 , 5 - d i - t e r t - b u t y l - 4 - h y d r o x y toluene and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate; (c) P e r o x i d e decomposers — d i l a u r y l thiodipropionate. In a d d i t i o n quenchers such as the o r g a n i c n i c k e l complex, N i ( I I ) b i s ( d i i s o p r o p y l d i t h i o c a r b a m a t e ) are used for the deactivation of the e x c i t e d s t a t e s of the chromophoric groups r e s p o n s i b l e for l i g h t initiation. Since p o l y o l e f i n s r e a d i l y undergo o x i d a t i v e d e t e r i o r a t i o n on exposure to UV l i g h t , c o n v e n t i o n a l s t a b i l i z e r s were found to be unsatisfactory for long-term outdoor applications. Therefore, more effective s t a b i l i z e r s for polyolefins were desired. Stable Nitroxyl Radicals Since UV absorbers cannot completely prevent the i n i t i a t i o n reactions above, we f e l t t h a t better a n t i o x i d a n t s were necessary to o b t a i n improved l o n g - t e r m p h o t o - o x i d a t i v e s t a b i l i t y of p o l y o l e f i n s . We reasoned that stable r a d i c a l s which scavenge r a d i c a l s formed i n the o x i d a t i v e process might compensate for the i m p e r f e c t i o n s of the absorbers. On the basis of the assumption that the d i a r y l n i t r o x y l r a d i c a l s , rather than t h e i r parent d i a r y l a m i n e s , are the a c t i v e a n t i f a t i g u e s p e c i e s i n rubber p r o t e c t i o n , we f i r s t attempted to obtain new stable n i t r o x y l r a d i c a l s . The f i r s t radicals we looked at, the alkylaminotropone nitroxy r a d i c a l s (1), were unstable (2). However, s i n c e Neiman e t a l . (_3) had reported that 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl (2) was extremely stable, we turned to n i t r o x y l r a d i c a l s derived from f u l l y hindered amines. In t h i s section we w i l l describe the synthesis of new stable n i t r o x y l r a d i c a l s and their evaluation i n polypropylene.

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


3.

TODA ET AL.

39

Light Stabilization of Polymers

2,2,6,6-Te t r amet hy 1 - 4 - s u b s t i t u t e d p i p e r i d i n e - 1 - o x y I s . Neiman's stable n i t r o x y l r a d i c a l (2) was prepared i n our laboratory and tested f o r i t s s t a b i l i z i n g a c t i v i t y on polymers. Its light s t a b i l i z i n g effect was superior to that of conventional UV absorbers; however, i t was unstable a t e l e v a t e d temperatures. Heating c r y s t a l s o f (2) a t 105-110°C for 3 hours under nitrogen gave l-hydroxy-2,2,6,6-tetramethyl-4-oxopiperidine (3) and a trace of 2,6-dimethyl-2,5-heptadien-4one (4). On the other hand, when c r y s t a l s of (2) were a l l o w e d to stand at room temperature for s i x months, l-hydroxy-2,2,6,6-tetrameth y l -3 - ( 2 , 2 , 6 , 6 - te t r a me t hy 1- 4- ox op i p er i d inoxy )-4-oxopiper i dine ( 5 ) was o b t a i n e d as a p r e c i p i t a t e from the l i q u e f i e d substance. We believe the decomposition of (2) proceeds as shown i n Chart 1 (4). The heat s t a b i l i t y of a d d i t i v e s i s an important f a c t o r s i n c e polymer processing i s done at high temperatures (150-280°C). Therefore we considered ways t o o b t a i n more t h e r m a l l y s t a b l e n i t r o x y l radicals. If the hydrogen alpha to the keto group of (2) i s involved i n i t s decomposition as shown i n Chart 1, analogous compounds having no keto group should be more stable than (2). Indeed, after heating under the conditions described above, both 2,2,6,6-tetramethylpiperid i n e - l - o x y l (6) and 4 - h y d r o x y - 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e - l - o x y l (7) were almost completely recovered. OH

Additional examples of stable r a d i c a l s derived i n our laboratory from 4 - k e t o p i p e r i d i n e - l - o x y l (2) are shown i n Chart 2. Substituted 4-oxoimidazolidine-l-oxyls. We found that the hydrogen peroxide o x i d a t i o n of 2,5-bis(spiro-l-cyclohexyl)-4-oxoimidazolidine (17) (9) prepared by self-condensation of 1-amino-l-cyanocyclohexane (16) yielded the corresponding n i t r o x y l r a d i c a l (27) (10).

(16)

(O)

(27)



40

POLYMER STABILIZATION AND DEGRADATION

When the new stable r a d i c a l (27) showed excellent l i g h t s t a b i l i zing a c t i v i t y f o r p o l y p r o p y l e n e , syntheses of analogous compounds were planned. From a self-condensation reaction only two different substituents may be introduced on the imidazolidine r i n g . Mechanist i c s t u d i e s of t h i s r e a c t i o n u s i n g the N - l a b e l e d compound (16a) showed that the nitrogen of the n i t r i l e group appears only i n the 3p o s i t i o n of the i m i d a z o l i d i n e r i n g (11). The s e l f - c o n d e n s a t i o n of (16a) yielded the corresponding 4-oxoimidazolidine(3N) (17a), as confirmed by i t s mass spectrum (molecular ion at m/e 223.171 c o r r e sponding to C13H92O NN) and the presence i n esr of the corresponding n i t r o x y l r a d i c a l (27a) t r i p l e t . The r e a c t i o n of (16a) with c y c l o hexanone also yielded (17a). The reaction i s considered co proceed as shown i n Chart 3 (11). T h e r e f o r e , i t i s p o s s i b l e to obtain compounds w i t h four d i f ferent substituents at the 2- and 5-positions of the 4-oxoimidazolidine ring by reacting the appropriate carbonyl compound with an alpha aminonitrile formed i n a Strecker r e a c t i o n from a s e l e c t e d ketone. The o x i d a t i o n of these i m i d a z o l i d i n e s by hydrogen peroxide i n the presence of c a t a l y t i c amounts of sodium tungstate i n acetic acid gave new stable n i t r o x y l radicals (12) shown i n Chart 4. Light S t a b i l i z i n g A c t i v i t y of Nitroxyl Radicals. The polymer s t a b i l i z i n g a c t i v i t y of the stable radicals was evaluated as follows: (i) The s t a b i l i z e r (0.25 weight %) was i n c o r p o r a t e d i n t o polypropylene powder by wet b l e n d i n g with e t h a n o l . The r e s u l t i n g mixture was preheated to 215°C for 2 minutes, and then compression-molded, i . e . , at 215°C for 0.5 minutes to y i e l d 0.5 mm s h e e t s . As a c o n t r o l , unstabilized polypropylene sheet was prepared i n a s i m i l a r manner, ( i i ) For testing of l i g h t s t a b i l i t y each test specimen was exposed to l i g h t i n a Fade Meter (carbon arc lamp) at a black panel temperature of 63±3°C and examined p e r i o d i c a l l y by bending test to determine the time to embrittlement. ( i i i ) For testing of thermal s t a b i l i t y each t e s t specimen was p l a c e d i n a f o r c e d a i r c i r c u l a t i o n oven a t 150°C and the embrittlement time was measured by bending t e s t s . Results are summarized i n Table I. As shown i n Table I, these s t a b l e r a d i c a l s showed s t r i k i n g l y higher l i g h t s t a b i l i z a t i o n a c t i v i t y i n polypropylene than that of the UV absorber tested. We f e l t that their a c t i v i t y was related to their r a d i c a l scavenging a b i l i t y . T h i s hypothesis i s supported by the observation that the coupled products (32) and (33) were obtained by the r e a c t i o n of the n i t r o x y l r a d i c a l s (2) and (27), r e s p e c t i v e l y , with a C - r a d i c a l d e r i v e d from AIBN (10). The r a d i c a l scavenging a b i l i t y of the stable n i t r o x y l radicals i s now w e l l known to play a major role i n the mechanism of l i g h t s t a b i l i z a t i o n by hindered amine compounds (13). Unfortunately, the stable n i t r o x y l r a d i c a l s caused yellowing i n polymers. The yellowing may be due to the weak absorption of v i s i b l e l i g h t by the n i t r o x y l radicals themselves or to formation of colored r e a c t i o n products i n the polymers. We observed t h a t y e l l o w i n g occured when n i t r o x y l r a d i c a l s were used i n combination with a phen o l i c a n t i o x i d a n t such as 3 , 5 - d i - t e r t - b u t y l - 4 - h y d r o x y t o l u e n e (34). T h i s may be due to the r e a c t i o n of (2) and (34) which gives a h i g h l y colored product (35) as shown i n Chart 5.


3.

TODA ET AL.

o

u

• > •N 0c 6«

41


>

> N0' c OH

(2)

+

6

(3)

2)

o

+ Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch003

OH

[

( ) 1.

Chart

2.

D e c o m p o s i t i o n of

O. _l

(2) NH NH 2

2

NaCN |(NH4) C0

R2CH2CN

R1NH2

NO*]

5

^R

2

3

3

• R3 k

NC-C-R2

8 -n-Butyl K I

V

J
T x I H

R

2

R4

R

Ri 27 28 29 30 31

Cyclohexyl Me-, MeMe-, Et MeMe2-Me-Cyclohexyl

Chart 4. New s t a b l e n i t r o x y l r a d i c a l s i m i d a z o l i d i n e s by hydrogen p e r o x i d e .

H

3

2

Cyclohexyl H, n-Propyl MeMeMe-, hButyl, MeEtCyclohexyl H, n-Undecyl H, PhCyclohexyl 2-Me-Cyclohexyl

17 Cyclohexyl 18 Cyclohexyl 19 Me-, Me20 Me-, Me21 Me-, Me2 2 MeMe23 Cyclohexyl 24 Cyclohexyl 25 MePh2 6 2-Me-Cyclohexyl

R R / ^ N ^ R4 I 0»

2

4

R3

2

N H

RnV

Rs H Q

2

° \ 3 "

C

H

2

~*

#

H

O

"

-

4

Cyclohexyl Me-, MeMeEtCyclohexyl 2-Me-Cyclohexyl

from t h e o x i d a t i o n o f

^ ^ "

C

H

2

"

C

H

2

^ ^ J

^ > 2

CH-CHP(

>=0

+( ) 3

(35) Chart 5 .

R e a c t i o n s of 2 and 34.

Light

2 " x CI

(38)

(36)

(28)

(42)

>0^< +

>0< CI

(37)

Light

# c |

^ *0N" < I O. ~~

(39)

(PhCH ) Hg 2

2

^ jC^X:

C

I CH Ph 2

L i g h t

( P h C H 2 ) 2 H

(40)

C h a r t 6. R a d i c a l s g e n e r a t e d by p h o t o l y s i s c h l o r a m i n e s 36 and 3 7 .

(6)

9 * N' H (41)

of s o l u t i o n s o f N-



3.

TODA ET AL.


45

Synthesis and S t a b i l i z i n g _ A c t i v i t y of Hindered Amines. As mentioned previously, the hindered piperidine compounds showed excellent 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 polypropylene. In order to find more e f f i cient compounds, various derivatives of 2,2,6,6-tetramethyl-4-oxopip e r i d i n e (42) were s y n t h e s i z e d and t e s t e d . In t h i s s e c t i o n we describe some t y p i c a l examples from the great number of derivatives prepared i n our laboratory. In analogy to the Bucherer reaction, we carried out the reaction of 4-amino-4-cyano-2,2,6,6-tetramethylpiper i d i n e (46) with phenyl isocyanate. Heating the r e s u l t i n g ureido compound i n benzene gave the imino-hydantoin (47), while heating i n aqueous hydrochloric acid gave 3-phenylhydantoin (48). S i m i l a r l y , r e a c t i o n of 4 - c y a n o - 4 hydroxy-2,2,6,6-tetramethylpiperidine (49) with p - t o l y l i s o c y a n a t e gave the imino-oxazolidone (50) and the oxazolidinedione (51). Compounds having various substituents at the 3 - p o s i t i o n were prepared (Chart 8). The l i g h t - s t a b i l i z i n g a c t i v i t y of these compounds and the 4-oxoimidazolidines described e a r l i e r are l i s t e d i n Table II (22,23). Table I I .

L i g h t - s t a b i l i z i n g A c t i v i t y of Hindered Amines

Compound m.p. ( b . p . ) Number °C 42 (105/18mmHg) 47 177 52 196 53 157 54 115 48 143 55 187 56 166 57 96 50 152 58 95 51 156 174 59 60 107 17 220 170 19 23 (170/0.0005mmHg) 25 131 UV absorber None

Embrittlement (hours) 320 220 300 400 800 860 660 660 800 60 80 60 200 40 180 120 380 600 80 40

a) Tested by the method described for Table I . Syntheses of hydantoin compounds from 1 , 3 , 5 - t r i a z a - 7 , 7 , 9 , 9 tetramethylspiro[4.5]decane-2,4-dione (61) were also planned, since derivatives of t h i s type showed high a c t i v i t y compared to o x a z o l i dones and 4 - o x o i m i d a z o l i d i n e s . F u r t h e r , three d i f f e r e n t types of


46


H2O I O II CH3CCH3+NH3

, f

CaCl2

H

(43)

CH3

H

(42)

O/f CH3CCH3

\ ^

Chart 7 . ammonia.

i O

^ N j
H


CH3COCH3

CaCl2 or NH4CI

1H

2

O

*

(44)

42

(45)

< >

Compounds o b t a i n e d from the r e a c t i o n of acetone and

N-R RNHCONH CN

H N^CN 2

RNCO *N' H (46)

NH

47 -Ph 52 -Cyclohexyl 53 -Et 5 4 -Octadecyl

3

HO^CN

48 55 56 57

R: -Ph -Cyclohexyl "Et -Octadecyl

51 59 60

-p-Tolyl -Cyclohexyl -Octadecyl

RNHCOO^ CH RNCO

A

H (49) 50 58

Chart 8.

R: -p-Tolyl -Octadecyl

Compounds having v a r i o u s s u b s t i t u e n t s a t the 3 - p o s i t i o n .


3.

TODA ET AL.

p i p e r i d i n e a c e t a l s were s y n t h e s i z e d (Chart 9), and t h e i r s t a b i l i z i n g a c t i v i t i e s are shown i n Table III (24).


Table III.

47


L i g h t - s t a b i l i z i n g A c t i v i t y of

Compound m.p. ( b . p . ) Number °C 63 125 64 53 65 139 66 (128/2mmHg) 67 (194/3mmHg) 68 137 69 203 70 212 UV absorber None

light-

Piperidine-spiroacetals

Embrittlement Time" (hours) 160 1540 800 460 600 220 300 1340 80 40

a) Tested by the method described for Table I .

Catalytic hydrogenation of (42) led to 4-hydroxy-2,2,6,6-tetramethylpiperidine (71) which was derivatized by known methods as shown i n Chart 10. Since i t was expected that bifunctional compounds would exhibit more s t a b i l i z i n g a c t i v i t y than monof u n c t i o n a l ones, p o l year boxy l i e acid p i p e r i d i n o l esters were synthesized. Their s t a b i l i z i n g a c t i v i t i e s are summarized i n Table IV. The l i g h t - s t a b i l i z a t i o n a c t i v i t y of several esters was tested by an improved t e s t method (25). T h i s method was c a r r i e d out as f o l lows: (i) Preparation of the test specimen. A mixture of 100 parts of polypropylene powder, 0.2 parts of the antioxidant, octadecyl 3(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate, and 0.25 parts of the s t a b i l i z e r was needed for 10 minutes at 200°C i n a Brabender p l a s t i Corder to give a homogeneous m a t e r i a l . T h i s m a t e r i a l was then pressed to a thickness of 2-3 mm i n a laboratory press. A portion of t h i s sheet was pressed for 6 minutes at 260°C i n a h y d r a u l i c press and then immediately placed i n cold water, yielding a 0.5 mm sheet. Following the same procedure, a 0.1 mm f i l m was obtained from the 0.5 mm s h e e t . T h i s f i l m was cut i n t o t e s t pieces 50x120 mm. P o l y p r o pylene films without any s t a b i l i z e r s were prepared as above and used as c o n t r o l s , ( i i ) T e s t i n g of l i g h t - s t a b i l i t y . The t e s t specimens were aged as d e s c r i b e d for Table I above. The exposed f i l m s were subjected to tension tests at regular i n t e r v a l s , and the times r e c o r ded when the test pieces contracted to 50% of their o r i g i n a l extension (Half L i f e Time, HLT).

American Chemical Society Library 1155 16th St., N.W. In Polymer Stabilization and 0.C. Degradation; Klemchuk, P.; Washington, 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

48


NaCN/(NH4) C0 2

3

>

H

xT

'

V,

HN

N

o

~ T

V

0

H X ~ \ ,N-pO > r - A -

NH

o

61


M

e

C

(

C

H

2

°

H

)

^

62

HN^YY" \—

/

^ C O O ^

O-/V_oH

2

D W

...»

HOCH(CH OH) * 2

N

-

R

R-Octyl

HN^YY* \ — ' 0-^\—OCOR

63

)H/°n

H (42)

/

* "OCT

v

RX D

««

64 R:-Me

0

y-v p-i »

, «

— OCOR

65

6 6 R: -Me 67 : - P h

68

69 R: -Me 70 :-CH2Ph

C(CH20H)4

Chart 9. S y n t h e s i s of three d i f f e r e n t types of p i p e r i d i n e and h y d a n t o i n compounds.

acetals

Tun

^ N ^ H

I

RCOOMe

RNCO 0CONHR

r^S J

L H

OCOR

7 2 -Et 73 -Cyclohexyl

J L ^ N ^ H

MeOCORCOOMe

OCO-R-COO R: R: 74 -Me -Me r^S /\ 7 7 --CH2( C -Heptadecyl.J 75 -Heptadecyl L J L 7 8-I H ) -(CH2) -Ph 76 -Ph >^N^ 7980 0P Phenylene H H 8 2

4

Progress in the Light Stabilization of Polymers - ACS Symposium

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