ELSEVIER

The synthesis and evaluation of novel photosensitisers for controlled degradation of

polyethylene -

Ricardo Acosta,‘* Maricela Garcia,” Alfred0 Resales,” Conception Lydia Berlanga,” Graciela Arias” & Norman S. Allen”

Gonzalez,”

“Centro de lnvestigacion en Quimica Aplicada, Boulevard Enriyue Reyna Hermosillo #14/l. Saltillo. C’orrhuila. Me.uico “Chemistry Department, Faculty of Science and Engineering, Manchester Metropolitan University, Chester Street. Manchester

Ml 5GD, UK

(Received 24 July 1995: accepted 16 October 1995)

Six novel additives have been synthesised by substitution of benzophenone in the 4-position with 2-hydroxybenzophenone or 4-amino-2,2,6,6_tetramethyl piperidine groups. The spacing between the chromophores was varied by alkyl groups of n = 4, 6 and 8 CH2 units. The additives were incorporated into polyethylene and the rates of thermal and photo-oxidation monitored using FTIR spectroscopy. The data show a number of interesting features that appear to be dependent not only on the nature of the stabiliser moiety but also the value of n. Their behaviour is compared with mixed compositions based on benzophenone with 2-hydroxy-4-octoxybenzophenone and 2,2,6.6-tetramethyl- 4-piperidinyl sebacate. The possibility of these compounds for the control of thermal and photo-oxidation of polyolefins is discussed. 0 1996 Elsrvicr Science Limited

1 INTRODUCTION

Photodegradable plastics have been proposed as an answer to the problem of environmental pollution by polymeric materials. These kinds of materials are -mainly used in the packaging industry as well as in agriculture, where it is desirable to retain their physical properties for a specific period of time after which they photodegrade rapidly.’ Although the polymers are inherently photo-unstable, faster photodeg- radation can be achieved by using copolymers of ethylene-carbon monoxide (Et-CO)’ or vinyl ketone copolymers.’ Here the ratio of carbonyl groups to monomer controls the photosensitisa- tion of the polymer.

The addition of photosensitisers to the polymer can also result in acceleration of the degradation.‘.’ A combination of a photo- stabiliser with a photosensitiser can be an

improved way to control the lifetime of polymers, especially polyolefins, by varying the ratio of these compounds, as in the case of the dithiocarbamate system designed by Scott and Gilead.6 Another system, studied by Allen,’ uses a photoantioxidant, Tinuvin 770 and a photosen- sitiser, such as anthraquinone. When a mixture of photoantioxidant and photosensitiser was used, an intermediate value for the induction period was obtained. Following from this, it was decided to develop new combined stabiliser-sensitiser systems for the control of polymer photosen- sitisation rates in a similar manner to that of active co-monomers.

The aim of this work was to study the effect of connecting a photostabiliser moiety such as a hindered piperidine or an ortho-hydroxy- benzophenone with a photosensitiser, such as benzophenone, on the photostability of the poly- ethylene films. For comparison thermo-oxidative

* To whom correspondence should be addressed. stability has also been studied. II

12 R. Acosrrr et al.

2 EXPERIMENTAL

2.1 Materials

All reagents were purchased from Aldrich and used as received. HDPE was obtained from BP Chemicals (UK). 2,2,6,6-Tetramethyl-4-piper- idinyl sebacate (Tinuvin 770-‘.“, Ciba-Geigy) was donated by Ciba-Geigy (UK) Ltd, Manchester, and 2-hydroxy-4-octoxybenzophenone (Cyasorb .W”) was purchased from American Cyanamid (Stamford, USA).

2.2 Characterisation of products

IR spectra were recorded with a Nicolet Magna 550 Fourier transform infra-red spectrophoto- meter. ‘H NMR spectra were obtained on a Varian Gemini FT-NMR, with CDCl, as the solvent and TMS as the standard.

2.3 Synthesis of the stahilisers

2.3.1 4-(&hromooctoxy)benzophenone 4-Hydroxybenzophenone (3.96 g, 20 mmol) was added to an aqueous solution of sodium hydroxide (22 mmol in 30 ml of water) in a 100cm3 round bottom flask provided with condenser, thermometer, and magnetic stirring, and the reaction mixture was left for 15 min. The solution turned green indicating the presence of the phenoxide. 60cm” of isopropyl alcohol were added with strong agitation, followed by dibromooctane (16.32 g, 60 mmol). The tempera- ture was raised to boiling point and reflux continued until the green colour of the mixture disappeared.

A white precipitate appeared on cooling of the mixture, which was purified by column chromat- ography using hexane-ethyl acetate-chloroform (75:8.5:16.5) as eluent. The melting point of the desired product was 58-60°C.

The -‘H NMR signals in CDCl, for the 4-bromooctoxybenzophenone were as follows: 1.4 ppm (m, -CH,-aliph, 8H); 1.85 ppm (m, -CH,-aliph, 4H): 3.4 ppm (t, CH,-Br, 2H): 4.05 ppm (t, -0-CH,-, 2H); 6.95 ppm (d, Ar-H, 2H): 7.5 ppm (m, Ar-H, 3H): 7.8 ppm (m, Ar-H, 4H). FTIR analysis in KBr discs gave peaks at 2924 cm ’ due to C-H aliph stretch: 1644 cm-’

aromatic C=O, 1602 cm ’ aromatic C-C stretch: 1256cm ’ C-O stretch; 689cm ‘. C-Br.

2.3.2 Synthesis of 4-(6-iodohexyloxybenzophe- none) The synthetic procedure for the hexyloxy derivative was the same as in the case of the octoxy derivative only using diiodohexane. A white solid with a melting point of 50-51°C was obtained. The ‘H FT-NMR analysis confirmed peaks at 1.5 ppm, (m, -CH,- 4H), 1.85 ppm (m, -CH,- 4H); 3.45 ppm (t. CHI-Br, 2H); 4.05 ppm (t, -OCH2-, 2H); 6.95 ppm (d, Ar-H, 2H): 7.5 ppm (m, Ar-H, 3H); 7.8 ppm (m, Ar-H, 4H). The FTIR signals obtained using a KBr disc were as follows: 2938 cm ’ due to C-H aliph; 2868cm ‘: 1637 cm ’ aromatic C=O: 1602 cm ‘, aromatic C=C stretch: 1243 cm ’ C-O: 688 cm ’

for C-Br.

2.3.3 Synthesis of 4- (4-iodobutoxybenzophenone) In this case the method was as mentioned above, only using diiodobutane and the melting point of the product was 47-48°C for the 4-carbon derivative. ‘H NMR signals observed were: 2 ppm (m, -CH2-, 4H); 3.45 ppm (t, CH?-Br, 2H); 4.05 ppm (t, -OCH,-, 2H); 6.9ppm (d, Ar-H, 2H); 7.5 ppm (m, Ar-H, 3H); 7.8 ppm (m, Ar-H, 4H). The FTIR signals were as follows: 2932 cm ’

for the C-H aliph; 2876 crn~-‘; 1639 cm ’ for aromatic C=O: 1604 cm-‘: 1250cm ’ C-O; 702 cm -’ for C-Br.

2.3.4 Synthesis of the 2-hydroxy-4-/(4-octoxyben- zophenone)]benzophenone In a three neck round bottom flask fitted with a condenser, thermometer and magnetic stirring, 2,4-dihydroxybenzophenone (0.428 g, 2 mmol) was added to an aqueous solution of potassium hydroxide (2.2 mol of the base) and tetra- butylammonium bromide (0.147 g, 0.4 mmol) as phase transfer catalyst. Afterwards, 60 cm3 of isopropyl alcohol and 4-bromo octoxyben- zophenone (0.778g, 2 mmol) were added to the reaction mixture with vigorous stirring. The solution was then refluxed and monitored by TLC until the alkylated derivative disappeared. The reaction mixture was allowed to cool and then 100 cm’ of benzene were added. The organic phase was separated, dried with magnesium sulphate, and then evaporated. The residue was purified by column chromatography using a mixture of hexane-ethyl acetate-chloroform

(6.5:2.5:1) as eluent. A green solid was obtained with a melting point of lOl-103°C.

The ‘H NMR signals in CDCI, for the eight carbon derivative were as follows: 1.45 ppm

(undef. -CH,-aliph, 8H); 1.95 ppm (undef, -CH,-aliph, 4H): 4.05 ppm (m, -0-CH,-, 4H); 6.4 ppm (dd. Ar-H, IH); 6.5 ppm (d. Ar-H, 1H); 7 ppm (d. Ar-H, 2H); 7.6 ppm (m, Ar-H, 9H); 7.8 ppm (m, Ar-H, 4H): 12.7 ppm (s, o-OH, 1H). The FTIR bands obtained for a KBr disc were:

3424 cm ‘, for OH: 3059cm ’ aromatic C-H: 2933 cm ’ aliphatic: 1649cm ’ C=O aromatic:

163Scm ’ C=O enolic aromatic; 1605 cm ’

aromatic C=C stretch: 1291 cm ’ C-O.

2J.5 2-H?inro.ul,-4-[(4-he.~~l~~.~~benzophenone)]- henzophenone The melting point for the six carbon derivative is 109-l 10°C and its NMR signals are: 1.55 ppm

(undef. -CH,-aliph, 4H); 1.85 ppm (undef, -CH,-aliph. 4H): 4.05 ppm (m, -0-CH,-, 4H); 6.4 ppm (dd. Ar-H, 1 H); 6.5 ppm (d. Ar-H, 1H); 7 ppm (d. Ar-H, 2H): 7.6ppm (m, Ar-H, 9H):

7.X ppm (m, Ar-H, 4H); 12.7 ppm (s, o-OH, 1H). The FTIR bands for a KBr disc are: 3431 cm -’ for the O-H: 3052 cm ‘, for aromatic C-H: 1652cm ‘: for aromatic C=O; 1635 cm ‘, for enolic C=O: 1263 cm ’ for C-O.

2.3.6 .?- Hyrlroxy-4-/(4-hutoxyhenzophenone)/- benzophrtlone The melting point for this compound was 13 1-132°C. The ‘H NMR signals for the four carbon derivative are the following: 2 ppm (s. -CH,-aliph. 4H); 4.05 ppm (s, 0-CH2-, 4H): 6.4 ppm (dd, Ar-H, 1H): 6.5 ppm (d, Ar-H, 1H): 7 ppm (d, Ar-H, 2H): 7.6ppm (m, Ar-H, 4H):

7.X ppm (m. Ar-H, 9H): 12.7 ppm (s, -OH, IH). FTIR bands in KBr are: 3445 cm-‘, for O-H: 3059 cm ’ for C-H aromatic: 2947 cm ‘, for C-H aliph: 1650 cm ‘, for aromatic C=O, 1634 cm I_

for enolic C=O. 1291 cm ‘, for C-O.

2.3.7 Syd~csi.s of the 4-[(4-amino-2,2,6,6-tetra- methyl~,i~~c~ririinl’l)octoxv/hmzophenone To a 100 ml three neck round bottom flask fitted with condenser, thermometer and addition funnel, were added 4-amino-2,2,6,6_tetramethyl piperidine (3.46 g, 22.16 mmol) and 20 cm’ of water. This was followed by sodium bicarbonate (0.5817 g, 6.925 mmol) and tetrabutylammonium bromide (0.4 g, 1 .l mmol) and the temperature raised to boiling point. The 4-(8-

bromooctoxy)benzophenone (2 g, 5.54 mmol) was then added dissolved in 25 cm’ of benzene. The reaction mixture was left refluxing for 54 h and monitored by TLC until the spot corres-

ponding to the alkylated benzophenone

disappeared. When the reaction was complete, the two

phases were separated, and the organic layer washed with water to remove any unreacted

4-amino-2.2,6,6-tetramethyl piperidine. The or- ganic layer was then purified by extraction with 10% hydrochloric acid. The extract was neutral- ised with 10% NaOH and extracted with

chloroform followed by evaporation of the

solvent leaving a brownish solid with a melting point of S8-60°C.

The aqueous layer was extracted with chloro- form to recover the 4-amino-2,2,6,6_tetramethyl piperidine and the extracted amine was distilled to purify it. A white solid with a melting point of 63-65°C was obtained. ‘H NMR signals for the amine with 8 carbons are: 1.2 ppm (d, gem-CH,, 12H); 1.4 ppm (m, CH,-aliph, 12 H): 1.85 ppm (m, piperidine-CH1, 4H): 2.65 ppm (t, NH-CH,. 2H): 2.9 ppm (tt, piperidine-CH-, 1 H); 4.05 ppm (t. 0-CH,, 2H): 6.95 ppm (d. Ar-H, 2H): 7.5 ppm (m, Ar-H, 3H): 7.8 ppm (m. Ar-H, 4H). FTIR bands (KBr disc) were 3424cm ‘. for the N-H: 2933cm ‘, for the aliph C-H: 2856 cm ‘: 1656cm ‘. for aromatic C=O: 1600 cm ‘, for aromatic C=C stretch: 1473 cm ’ for the gem- dimethyls: 1256 cm ‘. for C-O: 70X cm ‘.

2.3.8 ((4-nnrino-2,2,6,6-tetr~~~?~c~th~l~~p~~ri~~ir~~l~- hex~lo.‘yjben,-ophenon~ The melting point for this product was lOS-107°C. ‘H NMR signals for the amine with 6 carbons are: 1.2ppm (d, gem-CH,, 12H): 1.6 ppm (undef, -CH,-aliph. 12H); I .9 ppm (dd, piperidine-CH,, 4H); 2.7 ppm (1, NH-CH>, 2H): 2.95 ppm (tt, -piperidine-CH-. 1 H): 4.05 ppm (t. 0-CH?, 2H): 7ppm (d. Ar-H. 2H): 7.55 ppm (m. Ar-H. 3H): 7.Xppm (m, Ar-H. 4H). FTIR bands (KBr disc): 3431 cm ‘. for N-H: 3073 cm ‘, for aromatic C.-H: 2947 cm ‘. for aliph C-H: 1649cm ‘, for aromatic C=O: 1602cm ‘. for aromatic C-C stretch; 1487 cm ’

for the gem-dimethyls, 1256 cm ’ for (‘-0.

2.3.9 4-/14-umino-~ _.2.6.6-ictrrrnlrth~f~?il~~ririi~~~~- blltosvlbenzophenone The melting point for this derivative was 178%1XO”C. ‘H NMR signals for the 4-[(4-amino-

14 R. Acostrr ct al.

Table 1. Photosensitive systems used for comparison with the synthesized additives at the specified concentrations (mg)

II =J II -6 I, -s

System Additive 0.05% 0.1 %I 0.25% 0.05% 0.1 % 0.25% 0.05% 0. I ‘%I 0.25%

Z, Benzophenone 20 3’) 07 1x 37 Y2 17 35 87

Cyasorb 5.3 1 35 70 174 33 6h lhS 31 62 1%

Z, Benzophenone 22 4s 112 21 42 104 20 40 YX

Tinuvin 770 30 59 147 28 55 13X 26 S2 I30 .~

2,2,6,6-tetramethyl piperidinyl)butoxy]benzophe- none are: 1.4 ppm (d, gem-CH,, 12 H): 1.85 ppm

(undef, -CH1-aliph, 4H); 2 ppm (dd, piperidine- CH?, 4H); 2.8 ppm (t, NH-CHI, 2H): 3.1 ppm (tt, piperidine-CH-, 1H); 4.05 ppm (t, 0-CH?, 2H): 6.9 ppm (d, Ar-H, 2H); 7.5 ppm (m, Ar-H, 3H); 7.75 ppm (m, Ar-H, 4H). The FTIR bands (KBr disc) were 3427 cm~ ‘: 2960 cm ‘, due to C-H aliph; 1643cm ’ for the aromatic C=O:

1602cm ‘, for the aromatic C=C stretch:

1262 cm ’ for the C-O bond, 702 cm ‘.

2.3.10 Polymer suntple prcppamtion The stabilisers were solvent blended into the polymer powder using dichloromethane, followed by evaporation and then compression moulding into films (180-200 mm thick) at 170°C.

2.3.1 I Thermal and photo-oxidation The HDPE films were heated in an air draught oven set at 110°C. The samples were irradiated in an IPIC unit in which the samples are fixed to the walls of the chamber and the lamps are rotated

centrally in the unit. The rates of polymer oxidation were measured

by monitoring the rate of formation of the non-volatile carbonyl oxidation products in the region 1600-1800 cm- ’ using a Magna-Nicolet spectrometer. The carbonyl index was deter- mined using the equation:

Carbonyl index =

The systems used for comparison were a mixture of benzophenone and Cyasorb 531 and a mixture of benzophenone and Tinuvin 770, in such a ratio that the molar equivalents of the active groups of the commercial additives would be the same as in the synthesised compounds. Table 1 shows the amounts of the additives used to be equivalent to 0.05, 0.1 and 0.25% by weight of the synthesised additives.

3 RESULTS AND DISCUSSION

The structures of the additives synthesised are shown below. They form two series, 2, in which the stabiliser component is the substituted benzophenone, and Z, in which it is the hindered piperidine.

Z,=-NH

3.1 Photo-oxidation where

d = film thickness in mm

I,, = incident light intensity

I, = transmitted light intensity

The photo-oxidation rates of the polyethylene films were measured by monitoring the growth in the non-volatile carbonyl oxidation products using FTIR spectroscopy. Embrittlement values for the polymer were taken arbitrarily at a value

Table 2. Embrittlement times for PE films with the photo- sensitive systems during UV ageing in an IPIC unit

LI \

z,,4 Z,C Ben7ophenone-~l‘inuvin 770

Ben~ophenon~-‘Tinuvin 770

Benzophenone-‘finu\in 770

‘l‘inuvin 770

G, 2 ‘IS

‘,,“:

value

3

h

1

3 6

-1

6

Emhrittlcment time in

hours for irradiated lilms

Without xlditivcs

!. Numher of rythylencs in the aliphatic chain.

of 0.06 carbonyl units. These data are sum- marised in Table 2 for all the photosensitive compositions used. The mixtures used are weight corrected for the value of IZ in the sensitiser structures.

The results show a number of interesting features that appear to be dependent not only on the nature of the stabiliser moiety but also the value of ~1. For the benzophenone-UV 531 mixtures it is noted that the embrittlement times increase with increasing concentration at all equivalent values of II. Thus the stabilizer is evidently more effective at higher concentration and dominates the photosensitising effect of the benzophenone. This is noted from the separate embrittlement times for the benzophenone and UV 5.11 when used alone. Compared with the additive-free film, the photosensitising effect 01 the benzophenone levels after 0.1% w/u. concentration under the irradiation conditions used here.

In contrast. the coupled chromophore system Z, shows a very different pattern of behaviour. In this case with increasing concentration of the additives the photosensitising effect of the benzophenone moiety dominates. This may be associated with the effect of substitution on the 2-hydroxybenzophenone group, influencing the excited state properties of the chromophore and hence its proton transfer rate. This is supported by the observation that the embrittlement times increase to some extent with an increasing value

of II. This effect would further insulate the two chromophores as well as increasing the com-

patibility of the system. It is also noted that the photosensitising effect of the Z, compounds is greater than that of benzophenone itself.

The benzophenone-Tinuvin 770 mixtures show a similar pattern of behaviour to those based on benzophenone-UV 531. Embrittlement times increase with increasing concentration, again demostrating that the stabilising effect of the Tinuvin 770 dominates the photo-oxidation. Corrections for II appear to he significant only at the lower concentration value of 0.05% w/w. The coupled-chromophore Z2 system shows quite a different pattern of behaviour. For II = 4 there is a small increase in stabilisation with increasing concentration of the additive, suggesting that sensitisation and stabilisation are effectively competing with each other. In the case of II = 6, photosensitisation dominates the overall reaction rate, the effect increasing with increasing concentration. When II = 8, photostabilisation is more pronounced at higher concentrations and this may be associated with more effective compatibilisation with the polymer. In fact. the Z2 system with 17 = 8 is an effective photosen- sitiser when used at low concentrations.

Carbonyl index plots for two of the composi-

m 0.05 % A 0. I %

l Control

+ 0.25 5%

0 100 200 300 400 500 6OO

UV Ageing (hrs)

Fig. 1. Carhonyl index versus irradiation time in hours in the EPIC unit for polyethylene films containing the mixtures ol’ Tinuvin 770 and bauophenone at the following concentrations: (A) 0.05%: (A) 0. I “L: ( . ) 0.15’%~: (*)

control polymer.

R. Acostrr et al.

l Control

). 0.25 %

UV Ageing (hrs)

Fig. 2. Index versus irradiation time in hours in the EPIC

unit for polyethylene films containing the synthesised compound Z2(. at different concentrations: ( X ) 0.05%: (W)

0.1%: (A) 0.25%: (+) control polymer.

tions are shown in Figs 1 and 2. Figure 1 illustrates the behaviour of the benzophenone- Tinuvin 770 mixtures. Here the photosensitising effect of the benzophenone at low concentrations

Table 3. Embrittlement times for PE films with the photo- sensitive systems during oven ageing at 110°C at different

concentrations levels

Additives system ‘II’ Embrittlement time in

value hours l’or aged films at

0.05 %

Benzophenone-Cyasorh 531

Benzophenone-Cyasorb 531

Benzophcnone-Cyasorh S31

Cyasorh 53 I Benzophenone

Z, A Z I I3 Z,,

Benzophenonc-Tinuvin 770

Benzophcnone-Tinuvin 770

Bcnzophenone-Tinuvin 770

Tinuvin 770

Z z/z Z 213 Z,, Without additives

_~~ ~_~

4 164

6 80

X 355

240

- I20

4 so

6 64

x 44

4 800

6 73s

8 455

- 550

4 940

6 920

8 1520 -

” Numb ol’ methylencs in the aliphatic chain.

I 10°C

0. I D/o 0.25%

20s IO0

122 I OS

122 72

124 1.55

I.12 12s

1x2 I X0

200 41s

IS0 SOS

74s 510

560 470

so0 X20

x75 1180 x30 X80

1050 930

1200 1500

I65

is gradually superseded by the light stabilising effect of the Tinuvin 770. A similar effect is shown for the Zz system where II = X in Fig. 2. At low concentration of the additive, photosensitisa- tion is dominant with stabilisation occuring at higher concentrations.

3.2 Thermal oxidation

The data on thermal oven ageing oxidation show a markedly different pattern of behaviour (Table 3). Firstly, compared with the additive-free control film both benzophenone and Cyasorb 531 are prooxidants, with the former being the more effective. The only exception is the latter at 0.05% w/w concentrations. Mixtures of ben- zophenone with Cyasorb 531 exhibit variable, mainly negative effects on the polymer stability. Secondly, except when equated to n = 6, the thermal sensitising effect of the benzophenone dominates with increasing concentration. This is in complete contrast to the effects observed on irradiation. Thirdly, the 2, system at all values of rz reverts from being a powerful thermal sensitiser at low concentration to a thermal stabiliser: at concentration values higher than 0.25% w/w, thermal stabilisation dominates.

In the case of Tinuvin 770 thermal stabilisation dominates the reaction, the effect increasing with increasing concentration. When admixed with benzophenone the thermal stabilisation effect is reduced and further decreases with increasing concentration except when equated to y1 = 8. For the Z, system thermal stabilisation dominates and increases with increasing n and concentra- tion. In fact when n = 8 the additive behaves as an effective thermal stabiliser. Such a system would have important implications therefore in preserving the physical characteristics of the polymer yet behaving as a photosensitiser- stabiliser during subsequent exposure to outdoor weathering.

4 CONCLUSIONS

The data show a number of interesting features that appear to be dependent not only on the nature of the stabiliser group but also on the value of ~1. For the single chromophore series Z, the photosensitising effect of the benzophenone moiety dominates over the stabilising effect of the 2-hydroxybenzophenone chromophore. This

may be associated with the effect of substitution on the 2-hydroxybenzophenone moiety influenc- ing the excited state properties of the chro- mophore and hence its proton transfer rate. It is also evident that the Z, system is more effective as a photosensitiser than is benzophenone.

The Z, system shows a quite different pattern of behaviour. When II = 8 for example, photo- stabilisation is more pronounced at higher concentrations and this may be associated with more effective compatibilisation with the poly- mer. On the other hand, at low concentrations the Z, system with IT = 8 is quite an effective photosensitiser.

On thermal ageing the Z, systems at all values of YI change from being powerful thermal sensitisers at low concentrations to thermal stabilisers at higher concentrations. For the single Z, system thermal stabilisation dominates and increases with increasing 11 and concentration. These additives behave as thermal stabilisers. These systems therefore appear to exhibit ideal properties for the control of thermal and photo-oxidation of polyolefins. They would effectively protect the polymer during the melt processing, thus retaining the physical charac- teristics of the polymer, and yet behave as a

photosensitiser-stabiliser during subsequent ex- posure to outdoor weathering. the effect depending upon the value of ‘n‘ as well as the additive concentration.

ACKNOWLEDGEMENT

The authors thank the Mexican Council of Science and Technology (CONACyT) for their financial support for this project.

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I. 2.

-3 _ 1.

5.

6.

7.

Klemchuk. P.. Po/ym. Degrd. Strth.. 27 (1990) 1X3. Johnson. R., In Ptw. Symp. on De,gruduhle Pltr.stic\. Vol. IO. Society of the Plastics Industry. Washiqyon. DC‘, 19x7. p. 22. Guillct. J. E.. I;S Patent 3 753 962, 3 XI I WI: 3 X53 814. Omichi. H.. De,gvdtrtior~ trrd .Stuhili:~rtiorz of Po/w/rfirt~ . cd. Norman S. Allen. Applied Science Publishers. 10x3, p. 1x7. Mellor. D.C.. Moir, A.B. & Scott. (i.. I%,-. /‘o/w. ./.. 9 (1073) 219.

Scott. G. & Gilead. D.. I)c,uc,/r,prllc,,r/.~ iti Po/vr~zcv Strrhi/r,ctrio,l--.F~~~-~, ed. G. Scott. Applied Scicncc Puh- lishers. London. lYX2. p. 71. Allen. h‘.S.. Mtrkrornol. (‘/rcwr.. 181 (IYXO) 2313.