Polymer Durability - American Chemical Society

Downloaded by UNIV OF TENNESSEE KNOXVILLE on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch033

33 Influence of Titanium Dioxide Pigments on Thermal and Photochemical Oxidation and Stabilization of Polyolefin Films Norman S. Allen and Hassan Katami Department of Chemistry, Faculty of Science and Engineering, Manchester Polytechnic, Chester Street, Manchester M1 5GD, United Kingdom

The

thermooxidative

low-density taining

nine types

spectroscopy

activation

actions

was confirmed

a hindered film.

the

stabilizer

surface

of

of con

by

IR

temperature,

The role of the sen

on the

rate of

photooxidation.

by hydroperoxide

thick)

were studied

thermooxidative

pigments

degradation

(100-μm

as a function

were calculated.

analysis.

polymer

The behavior breakdown

of

The complex

with a phenolic

were also examined

effects are discussed

particle

pigments

analysis

during

for

dioxide

piperidine

Stabilizer

the pigment

materials

dioxide

energies

as catalysts

of titanium

and photooxidative film

effects of the pigment

was more important

the pigments polymer

of titanium

or stabilizing

oxidation

(LDPE)

and hydroperoxide

and apparent sitizing

(oven aging)

polyethylene

antioxidant in

in terms of additive

versus the photocatalytic

of the

inter and

polypropylene

adsorption

onto

effect of the

pig

ment.

P I G M E N T S ARE WIDELY USED FOR T H E COLORATION of thermoplastics i n

many commercial applications. Although they are used primarily to impart color to the polymer they can nevertheless have a marked influence on the thermal and photochemical stability of the polymer material (I). For example, by absorbing or screening light energy they can exhibit a protective effect, or they may be photoactive and sensitize the photochemical breakdown of the polymer. With regard to these effects there are four principal factors that can influence the photostability of a pigmented-polymer system (2):

0065-2393/96/0249-0537$12.00/0 © 1996 American Chemical Society

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

538

POLYMER DURABILITY

• • •


•

the intrinsic chemical and physical nature of the polymer itself (1) the environment in which the system is used the chemical and physical nature of the pigment and its concentration (3-6) the presence of antioxidants and light stabilizers as well as other ad ditives (7, 8)

The ability of pigments to catalyze the photooxidation of polymer systems has received much attention in terms of the mechanistic behavior of the pig ments. In this regard much of the information originates from work carried out on T i 0 pigments in both polymers and model systems (J, 2). To date there are three mechanisms of the photosensitized oxidation of polymers by T i 0 , and for that matter other white pigments such as ZnO: 2

2

1.

The formation of an oxygen radical anion by electron transfer from photoexcited T i 0 to molecular oxygen (9). A recent modification of this scheme involves a process of ion-annihilation to form singlet oxygen, which then attacks any unsaturation in the polymer (JO). 2

Tio + o 2

> (Tior + o- )(i)

h v

2

2

(I) - » T i 0 + Ό 2

2

(ion-annihilation)

(I) + H 0 -> T i 0 + HO* + H 0 " 2

2

2 H 0 ' -> H 0 2

RCH 2.

2

= CHR' + Ό

2

2

2

+ 0

2

2

-> R C H = C H C H ( O O H ) R '

Formation of reactive hydroxyl radicals by electron transfer from wa ter catalyzed by photoexcited T i 0 (11). The T i ions are reoxidized back to T i ions to start the cycle over again. 3 +

2

4 +

H

H 0 — 2

Ti0

2

[Τι *] + e' -> [TP-] + o 2 -> 4

3.

+ ë (aqu)+OH

+

[TP] [Ti ] 4+

Irradiation of T i 0 creates an exciton (p) that reacts with the surface hydroxyl groups to form a hydroxyl radical (12). Oxygen anions are also produced and are adsorbed on the surface of the pigment par ticle. They produce active perhydroxyl radicals. 2

T

i0

e' + (p)

2

O H - + (p) ->

HO*

T i * + e' ->

Ή*

4

3


33.

ALLEN & KATAMI

Influence of Titanium Dioxide Pigments Ti * + 0 -» 3

2

[ T i - 0 " ] adsorbed + H 0 ->


4+

2

2

539

[ T i - - 0 i adsorbed 4 +

Ti

4 +

2

+ HO~ + H0 * 2

In this chapter we attempt to address a number of the previously mentioned factors related to the influence of titanium dioxide pigments on the thermal and photooxidative degradation of low-density polyethylene ( L D P E ) . The first factor related to the inherent stability of the polymer is addressed to some degree with regard to the importance of the pigment type on the formation of hydroperoxide and carbonyl groups, whereas the second factor, which re lates to environmental effects, is interrelated in terms of the influence of temperature during thermal and photochemical aging processes. The third factor related to pigment structure is examined on a wide basis through a study of nine different pigment types. The fourth factor on stabilization is examined for polypropylene film in terms of the interactions of the titanium dioxide pigments with a hindered-piperidine light stabilizer and a hindered phenolic antioxidant.

Experimental Materials. Polymer (LDPE) film samples (100-μηι thick) were prepared and supplied by Tioxide (UK) Ltd., Stockton-on-Tees, U.K. Masterbatches were prepared at a pigment loading of 50% by weight in an M F I 20 L D P E , with an addition of 0.5% w/w (on the weight of pigment) zinc stéarate by using a Banbury laboratory internal mixer. The masterbatches were then reduced to a melt-flow index (MFI) 7 grade of L D P E and films were prepared on a film tower. The final concentrations of the titanium dioxide pigments used were eventually reduced to 0.5% w/w by using the film tower. The codes used for the different pigment types studied are shown in Table I together with their various properties. The durability is defined commercially in terms of their overall weatherability. The stabilized polypropylene film samples were prepared by using a Brabender Plasticorder (Duisburg, Germany) and processing tor 10 min at 190 °C by using 0.1% w/w each of a hindered piperidine light stabilizer, bis(2,2,6,6-tetraTable I. Properties of Titanium Dioxide Pigments Crystalline Sample Modification Coated A Β C D Ε F G H I

Anatase Anatase Rutile Rutile Rutile Rutile Rutile Rutile Rutile

No light Light light Light Medium Medium Heavy No

Crystal Size

Photodurability

Fine Fine Fine Fine Medium Fine Medium Medium Fine

Poor Poor Medium Medium Good Very good Very good Excellent Poor

NOTE: All samples underwent organic treament.


POLYMER DURABILITY

540


methyl-4-piperidinyl)sebacate (Tinuvin 770), and a hindered phenolic antioxidant, pentaerythrityltetra-3-(4-hydrox^^ (Irganox 1010), both supplied by Ciba-Geigy (UK) Ltd., Manchester. The anatase pigments used were samples A and B, whereas the rutile pigments were samples I and G for uncoated and coated grades, respectively, at a 1% w/w concentration. Films (200-μτη thick) were then compression molded at 200 °C for 1 min. Thermal and Photooxidation. Polymer films were exposed in a SEPAP S aire m 30:24 unit (designed by the Université Blaise-Pascal Clermont-Ferrand II, France) using four 500-W high-pressure Hg/W fluorescent lamps (wavelengths > 300 nm). The temperature of light exposure was carried out at 50, 60, 70, 80, and 90 °C. The 400-W medium-pressure Hg lamps normally used in this light exposure unit were replaced by the fluorescent lamps. Polymer film samples were also aged in hot air ovens over the same temperature range. The rates of photooxidation of the polymer films were monitored by meas uring the buildup in the concentration of the nonvolatile carbonylic oxidation products absorbing at 1710 c m in the IR region. IR spectra were recorded by using a Mattson Alpha-Centauri Fourier transform IR (FTIR) instrument. Oxi dation rates were determined via the well-established carbonyl index (C.I.) method (I): - 1

C.I. = Log (/ /i )/D X 100 10

0

t

where D is film thickness in μηι, ί is the initial light intensity at 1710 c m , and i is the transmitted light intensity at 1710 c m . From the C.I. rate curves, rate constants were obtained from tangential fits over the C.I. range 0.1-0.2 with the aid of an SE Apple Macintosh system via the use of Cricket Graph software (Computer Associates International Ltd.). -1

0

-1

t

Hydroperoxide Measurement. Hydroperoxide determinations were car ried out by using the iodiometric method of analysis (10). Polymer film samples (1 g) were cut into small pieces and placed into a pear-shaped 100-cm flask containing sodium iodide (0.1 g) ( B D H Ltd., U.K), 9.5 c m of Analar-grade propan-2-ol ( B D H Ltd.), and 0.5 cm of glacial acetic acid ( B D H Ltd.). The combination was then refluxed vigorously for 30 min with a control solution that did not contain polymer. The yellow I " was then measured spectrophotometrically at 380 nm (wavelength maximum at 357 nm) (Perkin-Elmer Lambda 7 spectrometer) to avoid any additive absorbances, and a calibration curve was set up using 70% w/ w cumene hydroperoxide as a standard (Aldrich Chemical Company, U.K). The data were cross-checked using FTIR spectroscopy (II). 3

3

3

3

Results and Discussion T h e r m a l Oxidation. The effects of thermal oxidative aging on the unpigmented and pigmented L D P E film samples show a number of interest ing features. Rates of C.I. versus oven-aging time in hours, particularly at the higher temperatures, show a strong overlap for many of the plots. A typical example is shown in Figure 1 for the 90 °C data. Typical temperature effects on oven aging are also illustrated i n Figure 2 for sample G . F o r an easier


ALLEN & KATAMI

Influence of Titanium Dioxide Pigments

541


33.

Figure 1. C.I. versus oven-aging time (h) for LDPEfilms(100-μηι thick) at 90 °C containing no Ti0 (Z; O) and 0.5% w/w of pigments. For key, see Table 2


POLYMER DURABILITY

542


0.4

0.3 h

OVEN AGING TIME (hrs) Figure 2. C.I. versus oven-aging time (h) for LDPEfilms(100-μη thick) containing pigment G (0.5% w/w) at various temperatures (°C).



33.

ALLEN & KATAMI

543


comparison of the data, Table II compares the times to 0.1 C I . for all the aged L D P E films; this time is often taken as the time to embrittlement for this polymer. The first and most interesting feature is the observation that all the pig ment types sensitize the thermal oxidative aging of the L D P E at all temper atures. The initial decrease i n carbonyl may be due to a catalyzed decomposition of the initial carbonyl groups present in the polymer films. The second point is that the rate of oxidation accelerates with increasing temper ature, as would be expected, and becomes autocatalytic above 70 ° C Thus, at 50 °C the order in thermal stabilization is control film (Z) > > E , F , H , and I » Β and G > > D » C and A. At higher temperatures the order is very similar, although the differences between the pigments are reduced. The or ders of stability are seen to be reasonably similar over the entire temperature range, and the uncoated anatase behaves as the most powerful sensitizer. Apart from pigment G all the durable, coated, rutile forms are less active as sensitizers, and two of the lightly coated, fine crystal rutile grades are the more active types (C and D ) . The relationship between temperature and pigment type is compared in more detail i n Figure 3 as a plot of time to 0.1 C.I. versus oven-aging tem perature. This data shows that as the temperature of aging is increased the pigment type becomes less important i n terms of its catalytic activity on pol ymer oxidation. Hydroperoxide species are known to be important in the initiation and propagation steps of L D P E oxidation reactions. Figure 4 shows a plot of the hydroperoxide concentrations versus oven-aging time in hours at 90 °C for the various unpigmented and pigmented polymer film samples. The actual data values are given in Table III. Again, all the pigments sensitize the formation of hydroperoxide groups during aging of the polymer compared with the con trol sample without pigment. Furthermore, from the initial values before oven aging, all the pigmented polymer samples exhibit higher hydroperoxide con centrations than that of the control film. Thus, during the processing operation Table II. Time (h) to 0.1 C.I. for Pigmented L D P E Films during Oven Aging 90 °C 80 °C 70 °C 60 °C Sample 50 °C Ζ (control) A Β C

D Ε F G H

I

9000 4260 6200 4300 5200 7800 7600 6020 7380 7300

3400 1800 2570 1690 2260 2745 2957 2120 2720 2957

830 402 511 509 649 653 680 611 740 739

383 330 340 332 350 440 460 383 400 380


264 165 193 170 212 250 248 217 244 220

POLYMER DURABILITY

544

1


10000 ι—•—ι— —•—•—ι—'—r

40

50

60

70

80

90

100

O V E N AGING T E M P E R A T U R E (C) Figure 3. Time to 0.1 C.I. versus oven-aging temperature for LDPEfilms(100μπι thick) containing no Ti0 (Z; O) and 0.5% w/w of pigments. For key, see Table I. 2

in the Banbury mixer the titanium dioxide pigments are catalyzing the for mation of hydroperoxide groups, and both the uncoated anatase and lightly coated rutile pigments are the most active types. From the initial data in Table III, pigment activity follows the order I > A > D > B > C > G > E > F > H . Again, the coated, durable, rutile types are the least active in this regard, followed by the fine crystal grades. Pigment


ALLEN & KATAMI


545


5000

0

10

20

30

40

50

60

OVEN AGING TIME (hr)

Figure 4. Hydroperoxide concentration (ppm) versus oven-aging time (h) f LDPEfilms(100-μπι thick) at 90 °C containing no Ti0 (Z; O) and 0.5% w/w of pigments. For key, see Table I. 2


POLYMER DURABILITY

546

Table III. Hydroperoxide Concentrations ^g/g) for Pigmented LDPE Films during Oven Aging at 90 °C 50 h 20 h 10 h 5h Oh Sample Ζ (control) A Β C Downloaded by UNIV OF TENNESSEE KNOXVILLE on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch033

D

Ε F G H I

210 1660 1620 1455 1633 1350 760 1440 607 1676

760 1742 1796 2428 1802 1750 956 1505 780 2953

640 1920 1824 2840 2240 2200 1015 1540 1350 3053

1068 2500 1860 3400 2508 2400 1300 1640 1995 3150

1504 3950 3840 3616 3128 3800 3600 4200 3400 4550

I is essentially classed as an uncoated fine-crystal grade of rutile and is more active than anatase in terms of hydroperoxide formation. The coated anatase pigment is much less active. Thus, the nature of the coating treatment appears to be quite important in terms of actual contact between the surface of the titanium dioxide particles and the polymer matrix. However, there are major differences among the rates of C.I. formation and those related to hydro peroxide formation. Hydroperoxides behave as potential initiators and intermediates during oxidation of the polymer. Depending on the temperature, they will be catalytically decomposed by the titanium dioxide pigments at rates depending on the pigment type. P h o t o o x i d a t i o n . The photooxidation results are different and show a number of interesting and related trends. Figure 5 shows the rates of carbonyl formation versus irradiation time at 50 °C i n the S E P A P unit. The effects of temperature for pigment G are shown in Figure 6. Actual data values are shown in Table IV for all the pigment types. A l l the rates of carbonyl formation are autocatalytic irrespective of the temperature of exposure. Also, pigment activity increases with increasing tem perature of light exposure from 50 °C to 90 °C. In the 90 °C case all the pigments, with the exception of H , operate as photosensitizers of carbonyl formation in L D P E . Obviously, during photothermal aging active hydro peroxide and carbonyl groups are formed in-situ, and they can accentuate the photooxidation rate of the polymer. Figure 7 shows a plot of the time to 0.1 C.I. for the polymer films versus exposure temperature from 50 °C to 90 °C. Differences between pigment types are reduced with increasing temperature, but not to the same degree as with thermal aging. At 50 °C the order i n pigment activity in terms of ability to stabilize the polymer is H > Ε > F > G > D > C > Control, whereas the order for sensitization pigments is Β > I > A . The uncoated pigments are the most active, followed by the fine-crystal


ALLEN & KATAMI


547


33.

IRRADIATION TIME (hr)

Figure 5. C.I. versus irradiation time (h) in SEPAP for LDPEfilms(100-μπι thick) at 50 °C containing no Ti0 (Z; O) and 0.5% w/w of pigments. For ke see Table I. 2


548

POLYMER DURABILITY


0.4

IRRADIATION TIME

(hrs)

Figure 6. C.I. versus irradiation time (h) in SEPAPfor LDPEfilm(100-μιη thick) containing pigment G (0.5% w/w) at various temperatures. grade, rutile types and then the more heavily coated durable grades. At this temperature the anatase is seen to be more active than the uncoated rutile grade I. At 90 °C the order in pigment activity in terms of stability is H > Control > F > E > G > D > C and Β > I > A. Again the order in activity is similar; the super-durable rutile grade is the more effective stabilizer, and the un-


33.

A L L E N & KATAMI


549

Table IV. Time (h) to 0.1 C.I, for Pigmented L D P E Films during Irradiation Sample Ζ (control) A

Β C


D

Ε F G H

I

50 °C

60 °C

70 °C

80 °C

90 °C

190 63 126 145 193 254 249 226 296 116

177 46 100 127 156 233 156 175 223 82

112 24 65 80 109 140 150 123 128 63

90 12 35 30 48 67 105 59 119 16

86 7 36 36 40 60 75 48 90 12

coated and coated anatase and lightly coated fine-crystal, rutile types are the most active pigments. From this data and that shown in Table III and Figure 4, the photooxidative behavior of titanium dioxide pigments could be related to hydroperoxide formation during processing and thermal oxidation. The light stabilizing effects are evidently due to the pigments operating as U V screeners as well as absorbers. However, there will be a competitive effect with regard to the ability of the pigment to sensitize the thermal oxidation of the polymer.

Activation Energies. From the C.I. rate curves, rate constants were obtained. Arrhenius plots were obtained from the data, from which apparent activation energies (kj/mole) were determined for carbonyl formation. These C. I. values are shown in Table V, and the activation energies are plotted in Figure 8 against the film sample letter. These values will be subject to some degree of error because the oxidation reaction rates are assumed to be first order and therefore are not absolute and are only relative. We also assume that the pigment is well dispersed and that for a 100-μπι film the oxidation is considered homogeneous. From Figure 8, the activation energies for thermal oxidation are significantly higher than those for photooxidation. Overall, during thermal oxidation the activation energies for the pig mented films are lower than that of the unpigmented control (191.86 kj/mole), and this finding is consistent with the data showing that all the pigments operate as thermal sensitizers. For photooxidation, the activation energies are very similar to that of the control (31.02 kj/mole). The only exception is sample D . The slightly higher values for the pigments are indicative of the fact that on photooxidation they operate more as stabilizers and that temperature is more important in controlling the overall rate of oxidation of the polymer. Stabilization of Polypropylene F i l m . The data in Tables V and VI compare the embrittlement times (0.06 C.I.) for the stabilized polypro pylene films with anatase and rutile pigments, respectively. Notably, in poly-


POLYMER DURABILITY


550

40

50

60

EXPOSURE

70

80

90

TEMPERATURE

100

(C)

Figure 7. Time to 0.1 C.I. versus irradiation exposure temperature for LD films (ΙΟΟ-μτη thick) containing no TiO (Z; O) and 0.59c w/w of pigments. F key, see Table I. z


33.

A L L E N & KATAMI


551

Table V. Interactions of Anatase with Stabilizers in Photooxidation of Polypropylene Film


Additives None Irganox 1010 Tinuvin 770 Irganox 1010 + Tinuvin 770 Uncoated (UN) Anatase Coated (CO) Anatase (UN)Anatase + Irganox 1010 (CO)Anatase + Irganox 1010 (UN)Anatase + Tinuvin 770 (CO)Anatase + Tinuvin 770 (UN)Anatase + Irganox 1010 + Tinuvin 770 (CO)Anatase + Irganox 1010 + Tinuvin 770

Time (h) to 0.06 CI. 35 55 500 455 10 15 45 50 70 110 40 95

NOTE: Film was 100-μπι thick and was processed in a Brabender Plasticorder at 190 °C for 10 min. Pigments were 1% by weight; Irganox 101 and Tinuvin 770 were 0.1% by weight.

propylene, hindered phenolic antioxidants antagonize the light stabilizing effect of hindered-piperidine light stabilizers, and the effect is dependent on the structure of the two stabilizer types (J, 12). This effect is why polypro pylene was chosen for study instead of polyethylene, because polyethylene gives variable effects (13). In the case of die anatase pigment, stabilization of the polymer is not effective when compared with the control films. Thus, even in the presence of a hindered-piperidine light stabilizer, the anatase behaves as a powerful photosensitizer The coated type has only a slightly greater protective effect than the uncoated type. Also, both anatase pigments enhance the antagonistic effect between the hindered-phenolic antioxidant and the hindered-piperidine sta bilizer. O n the other hand, the rutile pigments synergize effectively with both stabilizers, and the coated grade is more effective in each case (Table VI). However, the antagonism between the two stabilizers is enhanced in the pres ence of rutile pigments, and the effect is more pronounced in the presence of the uncoated pigment. The enhanced antagonism may be associated with the ability of the pigments to adsorb the stabilizers onto the pigment particle surfaces and thereby enhance their interaction through the photocatalytic ox idation mechanism.

Conclusions Our results indicate that for thermal oxidative degradation over the temper ature range 50-90 °C, all the titanium dioxide pigments behave as thermal (catalytic) sensitizers. The nature of the pigment appears to control the rate of oxidation at lower temperatures, and uncoated and coated anatase and un-


552

POLYMER DURABILITY


200

I

Z

ι

ί

A

ι

I

B

ι

C

I

ι

D

I

ι

E

I

ι

F

1

ι

G

I

• f

H

• I

I

SAMPLE

Figure 8. Activation energy (kj/mole) versus polymer sample during (O) ov aging and (Φ) irradiation in the SEPAP.


33.

ALLEN & KATAMI


553

Table VI. Interactions of Rutile with Stabilizers in Photooxidation of Polypropylene Film


Additives None Irganox 1010 Tinuvin 770 Irganox 1010 + Tinuvin 770 Uncoated (UN) Anatase Coated (CO) Anatase (UN)Anatase + Irganox 1010 (CO)Anatase + Irganox 1010 (UN)Anatase + Tinuvin 770 (CO)Anatase + Tinuvin 770 (UN)Anatase + Irganox 1010 + Tinuvin 770 (CO)Anatase + Irganox 1010 + Tinuvin 770

Time (h) to 0.06 CI. 35 55 500 455 15 20 180 265 580 850 420 740

NOTE: Film was 100-μπι thick and was processed in a Brabender Plasticorder at 190 ° C for 10 min. Pigments were 1% by weight; Irganox 101 and Tinuvin 770 were 0.1% by weight.

coated fine-crystal rutile types are the most active. The coated fine-crystal rutile grades are less active i n promoting oxidation of the polymer and are followed by the least active, coated, durable, rutile grades. The rates of car bonyl formation are also autocatalytic above 70 °C and are less dependent on the pigment type as the temperature is increased. A l l the pigments operate as thermal aging sensitizers for hydroperoxide formation during processing. The trends in activity on photooxidation are sim ilar to those for thermal oxidative aging, except that the rates of carbonyl formation are autocatalytic over the whole temperature range studied, and some of the pigments behave as stabilizers. The stabilizing effect of many of the pigments, however, is converted into a sensitizing effect as the tempera ture is increased during irradiation. The role of the sensitizing or stabilizing effects of the pigment on the rate of polymer oxidation is more important during photooxidation. From the rates of carbonyl formation, activation en ergies were determined and were significantly higher for thermal aging (128.40-191.86 kj/mole) than for photooxidation (30.79-43.08 kj/mole). F o r thermal aging, all the pigments lowered the activation energy for carbonyl formation, whereas for photooxidation many of the pigments had little overall effect apart from a small increase. However, these values were taken i n a carbonyl region where only minor differences existed in rates of polymer ox idation. The actual times taken for the polymer samples to achieve a given C.I. value will be different and therefore not related to the activation energies obtained here. The rates of thermal and photooxidative aging of L D P E are interrelated with the catalytic formation of hydroperoxides by the titanium dioxide pig ments during processing, and the pigments play an important role i n the pho-



554

POLYMER DURABILITY

tothermal oxidation of the polymer. This effect is, in turn, dependent on the crystal size and structure of the titanium dioxide pigment used and the nature of its surface treatment. In terms of the light stabilization of polypropylene, the use of a hinderedpiperidine light stabilizer, Tinuvin 770, is ineffective against the photosensi tizing effect of anatase. In the case of rutile pigments, strong synergism is observed with the antioxidant and light stabilizer. The antagonism between the antioxidant and light stabilizer is dependent on the nature of the surface treatment on the rutile particle surface.

Acknowledgments We would like to thank Tioxide (U.K) Ltd., Stockton-on-Tees, United King dom, for partial financial support for Hassan Katami. We also thank R. E . Day, C . Watson, and J. Lawson of the Research Laboratories for helpful dis cussions throughout.

References 1. Allen, N. S. In Degradation and Stabilisation of Polyolefins; Allen, N . S., Ed.; Elsevier Science: London, England, 1983; Chapter 8, ρ 337. 2. Allen, N. S.; McKellar, J. F. Photochemistry of Dyed and Pigmented Polymers; Applied Science: London, England, 1980; p 247. 3. Kaempf, G.; Papenroth, W.; Holm, R. J. PaintTechnol.1974, 46, 56. 4. Allen, N . S.; McKellar, J. F. Wilson, D. J. Photochem. 1977, 7, 319. 5. Allen, N. S. Bullen, D. J. McKellar, J. F. J. Mater. Sci. 1979,14, 1941. 6. Day, R. E. Polym. Degrad. Stab. 1990, 29, 73. 7. Allen, N . S. McKellar, J. F.; Wood, D. G. M. J. Polym. Sci. Polym. Chem. Ed. 1975, 13, 2319. 8. Allen, N . S.; Gardette, J. L.; Lemaire, J. Dyes Pigments 1982, 3, 295. 9. Carlsson, D. J.; Garton, Α.; Wiles, D. M. Macromolecules 1976, 9, 695. 10. Carlsson, D. J. Wiles, D. M. Macromolecules 1969, 2, 597. 11. Carlsson, D. J.; Lacoste, J. Polym. Degrad. Stab. 1991, 32, 377. 12. Allen, N . S.; McKellar, J. F. Plast. Rubber Mater. Appl. 1979, 4, 170. 13. Allen, N . S. Hamidi, Α.; Williams, D. A. R.; Loffeleman, F. F. MacDonald, P.; Susui, P. Plast. RubberProc.Appl. 1986, 6, 109. ;

;

;

;

;

;

RECEIVED

1995.

for review January 26, 1994.

;

ACCEPTED

revised manuscript January 10,


Polymer Durability - American Chemical Society

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