Reactive Oligomeric Light Stabilizers - ACS Publications

30

Reactive Oligomeric Light Stabilizers

Downloaded by EMORY UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch030

Š. Chmela and P. Hrdlovič Polymer Institute, Slovak Academy of Sciences, 842 36 Bratislava, Slovak Republic

A new class of photoreactive weak link in its structure

oligomeric

light stabilizer

has been synthesized

containing

and tested in

a

polypro-

pylene (PP). The light-sensitive 1-phenyl-2-propenone (VPK) was used as the weak link. The other monomers were as follows: methyl-4-piperidylacrylate benzophenone

(2HAB),

a sterically hindered

(TMA),

and n-octadecylacrylate

(ODA).

amine as the light stabilizing

a UV absorber. ODA improves the compatibility -polar PP. For terpolymers

2,2,6,6-tetra-

2-hydroxy-4-(2-acroyloxyethoxy)-

TMA-ODA-VPK

TMA contains

unit, and

2HAB

is

of oligomers with nonwith similar

molecular

mass, the stabilizing efficiency depends on the VPK content. The higher the VPK content the higher the efficiency. - O D A - V P K this dependence

For terpolymers

low. The reasons for this different behavior are

Τ Γ Η Ε

2HAB-

is not valid and the efficiency is extremely discussed.

Q U E S T F O R I M P R O V E D R E S I S T A N C E o f plastics a n d

fibers

to

degradation

b y o u t d o o r w e a t h e r i n g has r e s u l t e d i n t h e d e v e l o p m e n t o f several classes o f fight

stabilizers ( I ) . T h e m o s t successful a n d r a p i d l y g r o w i n g stabilizer class

at p r e s e n t is t h e h i n d e r e d a m i n e l i g h t s t a b i l i z e r ( H A L S ) f a m i l y . T h e m a j o r i t y of commercial products

o f this class are derivatives o f 4 - s u b s t i t u t e d

2,2,6,6-

tetramethylpiperidine. T h e result o f very extensive research i n several c o u n tries is a reasonable H A L S

understanding

o f the mechanism o f H A L S

a n d their conversion products

protection.

a c t as m u l t i f u n c t i o n a l a d d i t i v e s d u r i n g

stabilization. B e c a u s e o f this c o m p l e x i t y s o m e details o f their action are still controversial

(2).

I n addition to suitable c h e m i c a l structure, a n effective additive has t o have g o o d physical properties. F i r s t o f all, it has to have g o o d processing stability, g o o d t h e r m a l stability, a n d sufficient retention i n the p o l y m e r substrate. O t h e r important aspects are g o o d dispersion i n the p o l y m e r d u r i n g processing a n d sufficient mobility d u r i n g service lifetime. S o m e o f these requirements

0065-2393/96/0249-0473$12.00/0

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

might

POLYMER DURABILITY

474


be achieved easily by increasing the molecular mass of the additive. But the mobility of the additive decreases with increasing molecular mass. Generally, the stabihzing efficiency for a particular additive decreases with increasing molecular mass (3, 4). It is necessary to find the optimal region of molecular masses for each particular oligomeric additive with respect to its application. Alternately, we have tried to solve this problem by preparing oligomeric additives with relatively high molecular mass but with some content of statis tically distributed Hght-sensitive units. The role of these units on the action of light is to cleave the oligomeric chain of the additive and thus produce shorter fractions with higher mobility.

Experimental Details Starting monomers were 2,2,6,6-tetramethyl-4-piperidylacrylate (TMA), 2-hydroxy-4-(2-acroyloxyethoxy)benzophenone (2HAB), n-octadecylacrylate (ODA), and l-phenyl-2-propanone (VPK). Their preparation is described elsewhere (5, 6). Terpolymers ( T M A - O D A - V P K and 2 H A B - O D A - V P K ) and copolymers ( T M A ODA, T M A - V P K , O D A - V P K , and 2 H A B - O D A ) were prepared by radical coïolymerization under nitrogen in benzene by using 2-2 -azobisisobutyronitrile AIBN) as the initiator. The limiting viscosities ( T ^ C ; intrinsic viscosity) were determined in benzene at 30 °C. Polypropylene (PP) powder (Tatren H P F , Slovnaft s.e., Bratislava, Slovak Republic) and the additives (0.2 wt %) were mixed and homogenized in a Brabender Plastograph (Duisburg, Germany) at 190 °C for 5 min. The bulk polymer was then pressed into 0.15-0.20-mm films in an electrically heated laboratory press at 200 °C for 1 min. Films were irradiated in a merry-goround at 30 °C. A medium-pressure mercury lamp was used as a source of irra diation (λ < 310 nm). Absorption at the 1700-1750 c m region was monitored by IR spectroscopy (IR-75, Carl Zeiss, Jena, Germany). UV spectra were measured on a Specord UV-VIS and M-40 (C. Zeiss, Jena, Germany). ,

Î

- 1

Results and Discussion S y n t h e s i s o f A d d i t i v e s . Two types of terpolymers were prepared (Table I). The first ones were the terpolymers containing the H A L S unit T M A (1). The second type consisted of terpolymers with 2 H A B (2), which is a U V absorber stabihzing unit. V P K (3) was used as a fight-sensitive unit i n both cases. From the structural formulae it is clear that all comonomers are rather polar compounds. To increase compatibility with nonpolar PP, O D A (4) was used as the third comonomer. To verify importance and contribution of the particular comonomers, T M A - O D A , O D A - V P K , T M A - V P K , and 2 H A B - O D A were prepared. Two groups of additives were prepared according to molecular mass by using dif ferent initiator concentrations. The first group (polymers A , B, and D in Table I) had higher molecular masses and T| /c values of 110-180 mL/g; the second group (polymers C and E - K in Table I) had relatively lower molecular masses and T) /c values of 40-80 mL/g. sp

sp


30.

C H M E L A & HRDLOVIC

Reactive Oligomerie Light Stabilizers

475

Table I. Polymerization Characteristics of Copolymers and Terpolymers


Polymer

Structure

VPK 2HAB AIBN (wt %) (wt %) (wt %Y

A

TMA-ODA-VPK

4.8

Β

TMA-ODA-VPK

10.2

C

ODA-VPK

9.6

D

TMA-ODA

0

Ε

TMA-ODA

0

F

TMA-VPK

12.2

G

TMA-ODA-VPK

8.2

H

TMA-ODA-VPK

I

J

Κ

(mL/g)

a

— — — —

0.1

2.98

164

0.1

2.61

111 83

0.1

0

0.1

3.06

180

0.7

2.44

66

0.7

5.78

55

0.7

2.64

62

2.5

— — —

0.7

2.78

64

2HAB-ODA

0

53

0.1

2HAB-ODA-VPK

W 5

43

0.1

40

47

0.1

— — —

2HAB-ODA-VPK

d

Calculated from UV spectra. Êlemental analysis; %N calculated for TMA-ODA = 2.61% (ratio 1:1). intrinsic viscosity for benzene; c = 7 g/L. Value determined from monomer mixture. rf

l


80 52

476

POLYMER DURABILITY

Compositions of the terpolymers and copolymers were roughly the same as the composition of the starting monomers mixture [copolymerization par ameters for T M A and O D A are r = 0.86 and r = 0.89, respectively (7)]. Terpolymers and copolymers were white or slightly yellowish powders or rub bery materials. The presence of V P K units was evident from U V and IR spec tra of carbonyl absorption at 1680 c m " . 2

x


1

Photolysis i n Solution. A Norrish type II reaction leads mainly to main-chain scissions in polymer containing carbonyl groups and 7-hydrogen atoms (8, 9). Terpolymers and copolymers containing V P K units from Table I represent this type of polymer. The changes in molecular mass of prepared terpolymers and copolymers are shown in Figures 1 and 2. The molecular masses of copolymers D , Ε ( T M A - O D A ) and I ( 2 H A B O D A ) do not contain the light-sensitive V P K unit and do not change through out the photolysis in benzene. O n the other hand, all copolymers and ter polymers (with the exception of the terpolymers 2 H A B - O D A - V P K ) containing V P K units exhibited a remarkable decrease in molecular mass, es pecially at the first stage of photolysis. The mechanism of the main-chain scission reaction for terpolymers T M A - O D A - V P Κ is shown in Scheme I. The rate of decrease in molecular mass and the final molecular mass depend on the V P K content. The greater the V P K content, the higher the rate and the

200Γ

01 0

, 10

, 20

, 30

I 40

IRRADIATION TIME (min) Figure 1. Course of molecular mass changes of copolymers and terpolymers during photolysis in benzene solution: •, Α; Δ, B; O, C; V , D; and Ο, i.


30.


1

TMA/ODA ν

-TMA/ODA/VPK

^

> \ \

40

1

(E)

Γ7

v

2.5 wt%

.

(H)

λ

2HARIODA/VPK

(J) —U~

\^ÎMAÔDA/VPK Downloaded by EMORY UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch030

411


8.2wtyT(G)

-

—D--ZriZ^3^——

—o~

TMA/VPK 12.2 wVL (F)

0

ι

1

40

80

120

IRRADIATION TIME (min) Figure 2. Course of molecular mass changes of copolymers and terpolymers during photolysis in benzene solution: V , E; •, F; O , G; Δ , H; and O, J. smaller the final molecular mass (compare the course of changes for polymers Η and G in Figure 2). The molecular mass of terpolymer J did not change despite the presence of the V P K fight-sensitive unit. The explanation for this might be in the dif ference between the extinction coefficients for 2 H A B and V P K . The 2-hydroxybenzophenone units in 2 H A B that are around the V P K molecule can absorb all the light because their extinction coefficient, ε, is 100 times larger [ε = 8700 L/(mol X cm) and ε^κ 83 L/(mol X cm) at 330 nm]. Another possible mechanism is energy transfer from the excited triplet state of V P K (340 kj/mol) to the 2 H A B structure, which has a lower triplet energy (260 kj/ mol; references 10 and 11). O n the other hand, degradation may occur at a correct ratio of V P K and 2 H A B . The preparation conditions for a terpolymer of this type were not optimized. 2ΗΑΒ

Photostabilizing Efficiency i n PP. We investigated the action of the light-sensitive V P K unit in the copolymer without the light-stabilizing unit (copolymer C). This copolymer acts neither as an inhibitor nor as a sensitizer, and the course of the photooxidation is the same as for unstabilized P P film (Figure 3). This result means that the absorption of light by phenyl ketone groups is not sufficient to act as a U V screener or absorber. O n the other


POLYMER DURABILITY

STATISTICAL T E R P O L Y M E R

-TMA-ODA-VPK-TMA-ODA-TMA-ODA-ODA-TMA-VPK-ODA-

NORRISH T Y P E II

CH

2

CH

CH

hv

6

2

CH

2

CH


CH

|

C

Ο

(CH2)i7

CH

3

Scheme I.



30.

CHMELA

0

&

HRDLOVIC

Reactive Oligomeric Light Stabilizers

100

200

300

IRRADIATION TIME(min)

479

400

Figure 3. Rates of photooxidation of PPfilms(ca. 0.2 mm) containing additives (0.2 wt %): V, no additive; Ο C; •, D; O, A; and Δ, B. hand the photolytic process and its products do not sensitize or inhibit pho tooxidation of P P under these irradiation conditions. Among the terpolymers with higher molecular mass, A , B, and D , the highest efficiency was obtained with terpolymer Β followed by terpolymer A wt% (Figure 3). The minimum efficiency was exhibited by copolymer D . The molecular mass of D did not change during irradiation in benzene solution (Figure 1). Its molecular mass is not expected to change in P P film, either. We assume that such a large molecule is not able to migrate at all, and because it is completely immobile, it remains in the same position at all times. Ac cording to Figures 1 and 3, the stabihzing efficiencies for A, B, and D increase with decreasing molecular mass and, what is more essential, with increasing V P K content. The importance of V P K content is clear from Figure 4, where the efficiencies of copolymers G and H , which have the same molecular mass but different V P K contents, and copolymer Ε are compared. The increasing of V P K content from 0 to 2.5 to 8.2 wt % resulted in an increase in the stabihzing efficiency from 150 to 350 to 700 h, respectively, to reach carbonyl absorption (A = 0.2). These differences suggest a possible role of stabilizer migration. The initial distribution of the stabilizers in P P is supposed to be similar, because molecular masses are almost the same for all three additives. Copolymer Ε without V P K units retains its original molecular mass which is too high for effective action; its stabihzing efficiency is the smallest (4). Both terpolymers react with fight by Norrish type II reactions and the result is main-chain scission. Fragments from terpolymer G , which has a higher V P K content, have smaller molecular mass than fragments of co



480

POLYMER DURABILITY

200

400 IRRADIATION

600 TIME

800

(h)

Figure 4. Bates of photooxidation of PPfilms(ca. 0.2 mm) containing additives (0.2 wt %): V, no additive; O, F; •, Ε; Δ, Η; and Ο, G. terpolymer H , which has a lower V P K content. Consequently, the highest efficiency is exhibited by terpolymer G and is due to the highest migration ability of its smaller fragments. Having the smallest fragment size itself is not a sufficient condition for effective action. The chemical composition of fragments with respect to their physical properties is also very essential. Copolymer F, which contains the stabihzing unit in an even higher content but does not contain the O D A unit with the long alkyl chain, exhibited the highest rate of sphtting reaction in benzene (Figure 2). It also has fragments of the smallest size. Regardless, its stabilizing efficiency is extremely low (Figure 4), and the course of carbonyl product accumulation is almost the same as for unstabilized PP. The absence of O D A units increases polarity and decreases compatibility, which results in the too-polar fragments not being able to migrate in nonpolar P P to the places where photooxidation is occurring. Quite a different situation is found in the case of terpolymers containing 2 H A B ( 2 H A B - O D A - V P K ) instead of T M A ( T M A - O D A - V P K ) . The stabihz ing efficiency of terpolymers J and Κ is extremely low, even a bit lower than the efficiency of copolymer I (Figure 5). No interdependence exists between efficiency and V P K content, as is the case of T M A - O D A - V P K terpolymers. The reason ensues from the different behavior of 2HAB-containing terpoly mers upon photolysis in benzene solution. Contrary to the terpolymers T M A O D A - V P K , the molecular masses of terpolymers 2 H A B - O D A - V P K do not change.


30.



481

02 1

no additive

H—2HAB/0DA/VPK(J)

(I—2HAB/ODA

/

(l)

I

ο

I Id

OQ

/ Downloaded by EMORY UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch030

/

2HAB Monomer

Ο

OQ

W

I

100

300

200 IRRADIATION

TIME

(h)

Figure 5. Rates of photooxidation of PPfilms(ca. 0.2 mm) containing the following additives: Δ no additive; O , J (0.2 wt %); Ο, I (0.2 wt %); and 2HAB monomer (0.1 wt %). Monomer 2 H A B exhibited a much higher stabilizing efficiency at the same concentration of absorbing structural units. The molecular mass of ter polymer J ( M = 60,700) determined by gel permeation chromatography using tetrahydrofuran as the mobile phase is very high in comparison with that of monomer 2 H A B ( M = 312). The greater dispersion of monomelic 2 H A B units in P P acts as a much better U V absorber in comparison with the localized concentration in terpolymer J. Similarly, in the terpolymer case, many ab sorbing units are concentrated in small areas, whereas the rest of P P volume is unprotected. Several authors (12, 13) concluded that the efficiency of 2-hydroxybenzophenone-type stabilizers is based not only on screening effect but also on quenching of the excited states and radical scavenging processes. Because the terpolymers 2 H A B - O D A - V P K (J and K) and copolymer 2 H A D - O D A (I) have rather high molecular masses, they could not be the effective quenchers. The same reasoning is valid for explaining the low effectiveness of radical scavenging by these types of additives. The mobility of the monomer 2 H A B ensures the inhibition of photooxidation during the induction period up to the consumption of about 80% of the monomer (decrease of absorption at 330 nm measured by U V spectroscopy). Then, the oxygen-containing products begin to be formed. For terpolymers J and Κ and for copolymer I, photooxn

w


482

POLYMER DURABILITY

idation starts at about a 10% decrease in absorption at 330 nm. These facts indicate that the polymeric additives act in the same manner as the lowmolecular weight ones, but their low mobility hinders sufficient PP protection. As a result, the induction period is very short, although the polymeric stabi lizers still contain high concentrations of the original, virgin, active, absorbing units.


References 1. Allen, N . S. Chem. Soc. Rev. 1986, 15, 373. 2. Chirinos Padron, A. J. J. Macromol. Sci. Rev. Macromol. Chem. Phys. 1990, C30, 107. 3. Gugumus, F. Res. Disci. 1981, 209, 357. 4. Hrdlovič, P.; Chmela, Š. J. Polym. Mater. 1990, 13, 249. 5. Chmela, Š.; Hrdlovič, P.; Maňásek, Z. Polym. Degrad. Stab. 1985, 11, 233. 6. Karvas, M . ; Jexova, E.; Holcik, J.; Balogh, A. Chem. Prum. 1968, 18, 427. 7. Watrt, J. Diploma Thesis; Polymer Institute: Bratislava, Slovak Republic, 1983. 8. Guillet, J. E. Polymer Photophysics and Photochemistry; Cambridge University Press: 1985; p 261. 9. Hrdlovič, P.; Lukáč, I. In Developments in Polymer Degradation; Grassie, N., Ed.; Applied Science Publications: London, 1982; Vol. 4, p 101. 10. Klopffer, J. J. Polym. Sci., Polym. Symp. 1976, 57, 205. 11. Lukáč, I.; Hrdlovič, P. Eur. Polym. J. 1979, 15, 533. 12. Carlsson, D. J.; Suprunchuk, T.; Wiles, D. M . J. Appl. Polym. Sci. 1972, 16, 615. 13. Wink, P.; Van Ween, T. H . J. Eur. Polym. J. 1978, 14, 533. RECEIVED

for review May 14, 1994. ACCEPTED revised manuscript May 5, 1995.


Reactive Oligomeric Light Stabilizers - ACS Publications

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