ELSEVIER

Po/ymer Degradamn and Stability 60 ( 1998) 195-204

Q 1998 Elsevier Science Limited

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PII: s0141-3910(97)00141-9 0141-3910!98/$19.00

Thermal and photo-oxidative stabilisation of polyolefin films: mode of action of novel

naphthalene-hindered piperidine derivatives

Ricardo Acosta Ortiz,” Eduardo A. Romero Salas” & Norman S. Allen”* Tentro de Investigacidn en Quimica Aplicada, Departamento de Aditivos para Polimeros, Bolvd. Enrique Reyna Hermosillo

No. 140, Saltillo, Coahuila, C.P. 25 100, Mkxico bChemistry Department, Manchester Metropolitan University, Faculty of Science and Engineering, Chester Street,

Manchester Ml 5GD, UK

(Received 10 June 1997; accepted 17 July 1997)

Four novel naphthyl and naphthoate derivatives of tetramethyl and penta- methylpiperidine (HALS) have been synthesised, characterised and evaluated as thermal and light stabilisers in both high density polyethylene (HDPE) and polypropylene (PP) film using FTIR analysis. The naphthalene moiety is found to induce good dual thermal and light stabilising activity in the polymers. In terms of thermal oxidative stability, the secondary HALS were better than the tertiary HALS systems and vice-versa for photostabilisation. From mass spectrometric evidence and photochemical studies the naphthalene and naphthyl moieties sever rapidly during the early stages of degradation, resulting in effective oxygen scavenging products protecting the HALS moiety from oxidation. 0 1998 Elsevier Science Limited. All rights reserved

1 INTRODUCTION

Naphthalene is very well known as a quencher of excited states of photosensitive polymers.1-7 David et al.‘_5 studied the energy transfer from the tri- plet states of poly(vinylbenzophenone) and poly (phenyl vinyl ketone) to naphthalene. They found that main chain scissions are inhibited by naph- thalene when the polymers are irradiated as films or in solution at room temperature with light at 366 nm. Photo-crosslinking of poly(vinyl- benzophenone) is also prevented when the poly- mer is irradiated in the presence of naphthalene. Thus, the quenching abilities of naphthalene appear to have the potential for preventing photo- deterioration of polymers such as polyethylene and polypropylene. Allen et. d7 have already synthe- sised two novel stabilisers which contain a naph- thy1 ring, and observed that they operated as effective quenchers of excited states of active a,#?- unsaturated carbonyl compounds.

*To whom correspondence should be addressed.

195

In this study we have synthesised four (hindered piperidine) HALSnaphthalene derivatives (Fig. 1) and examined their performance as thermal and UV stabilisers in both HDPE and PP films by FTIR analysis. The potential quenching and oxy- gen scavenging abilities of the naphthyl ring cou- pled with the radical scavenging behaviour of hindered piperidines were linked in one molecule in order to assess the stabilisation capability of the combined moieties in a singular structure. Fluor- escence analysis and mass spectrometry have also been utilised to determine the relative stability of the stabilisers.

2 EXPERIMENTAL

2.1 Materials

The naphthyl acetic acid, naphthoyl chloride, thionyl chloride, cyanuric chloride, triethyl amine, 2,2,6,6, tetramethyl piperidin-4-01 and 1,2,2,6,6_penta- methyl piperidin4-01, calcium hydride, sodium carbonate, sodium hydroxide, silica gel powder

196 R. A. Ortiz et al.

(60 mesh) and deuterated chloroform were obtained from the Aldrich Company (USA). The methylene chloride, toluene, hexane and ethyl acetate were purchased from JT Baker. The polypropylene pow- der (PP) containing no commercial additives was supplied by ICI Chemicals and Polymers Group Ltd (Wilton, UK); high density polyethylene (HDPE) was obtained from BP Chemicals (UK).

2.2 Characterisation of products

Infra-red spectra were recorded with a Nicolet Magna 550 Fourier Transform Infrared Spectro- photometer. ‘H and 13C NMR spectra were obtained on a Varian Gemini FT-NMR, with CDC13 as the solvent and TMS (tetramethylsilane) as a standard. UV-Visible spectra were performed on a HP 8452 spectrophotometer and the mass spectra were recorded using a GC-MS HP 5971 instrument. Elemental analysis was carried out on a Perkin Elmer 2400 series II instrument. Fluores- cence spectra were obtained using a Perkin Elmer LS-SOB instrument.

3 SYNTHESIS OF THE STABILISERS

3.1 Synthesis of 2,2,6,6,-tetramethyl piperidin-Co1 3.2 Synthesis of 1,2,2,6,6_pentamethyl piperidin-4- naphthoate 01 naphthoate

In a three neck round-bottomed flask fitted with a condenser, thermometer and magnetic stirring, 75cm3 of dry toluene were added. Then 6.29g (40 mmol) of 2,2,6,6-tetramethyl piperidin4-01 were placed in the flask followed by dropwise addition of 2.81 g (20 mmol) of naphthoyl chloride dissolved in 10 cm3 of dry toluene. Argon gas was delivered into the reaction mixture to prevent hydrolysis of the acyl chloride. The solution was then refluxed for 12 h and monitored by TLC using a mixture of hexane:ethyl acetate (7:3) as eluent. At

R = H ADDITIVE 1 R = H ADDITIVE 3

[2,2,6,6-TETRAMETHYLPlPERlDlN4-YL]- [2,2,6,6-TETRAmETHYLPIPERIDIN-4-YLl- NAPHTHOATE 2-NAPHTHYLACETATE

R = CH3 ADDITIVE 2 R = CH3 ADDITIVE 4

[1,2,2,6,6-PENTAMETHYLPIPERlDlN-%YL]. [1,2,2,6,6-PENTAMETHYLPIPERlDlN-4- NAPHTHOATE YL]-2-NAPHTHYLACETATE

Fig. 1. Structures of naphthalene HALS derivatives.

the end of this time, the reaction mixture was fil- tered, yielding a clear yellow solution. The solid was then dissolved in a KOH saturated solution followed by ether extraction three times. The organic layer was then added together with that obtained during filtration and the mixture flushed with hydrogen chloride to form the hydrochloride of the product. The salt obtained in this way was filtered and the desired product obtained by neu- tralisation with sodium carbonate giving a yield of 69.5%. The melting point of the synthesised com- pound was 59-60°C. Elemental analysis calculated for C2eHZ502N was C=77.13% and H=8.09%, while it was found to be C= 76.03% and H = 8.48%. The signals found by NMR in CDC13 using TMS as standard were as follows: 1.4 ppm [s, CH,, 6H]; 1.6 ppm [s, m, 6H]; 2.4 ppm [m, CH,, 4H]; 5.8 ppm [tt, CH, IH]; 7.8 ppm [m, Ar-H, 3H]; 8.15 ppm [d, Ar-H lH]; 8.25 ppm [d, Ar-H lH]; 8.4 ppm [d, Ax,’ lH]; 9.2 ppm [d, Ax, lH]. The FT-IR spectrum determined in a KBr disc showed bands at 3398 cm-’ for N-H; at 3052 cm ’

for C-Ar; at 1704 cm-’ for C = 0 and 1240 cm ’

for C-O. By mass spectroscopy, signals were seen at 3 11 and 296 m/e corresponding to [M ‘1 and [M-CH3], respectively.

The same apparatus and technique were used as described above. Here, 1,2,2,6,6_pentamethyl piperidin-4-01 (6.85 g, 40 mmol) was dissolved in dry toluene. Then, naphthoyl chloride (3.81 g, 20mmol) was added slowly by means of a funnel. The reaction mixture was then raised to boiling point and maintained for 7 h. After treatment with hydrogen chloride and neutralization of the salt, the desired product was obtained in a yield of 41%. The melting point was 75-77°C. The elemental analysis calculated for C2iHZ702N was C = 77.5% and H = 8.36%; the following values were found: C= 76.32% and H = 8.92%. NMR signals in CDC13 as solvent and TMS as standard are: 1.2 ppm [s, g-s, 6H]; 1.25 ppm [s, g-m, 6H]; 2.1 ppm [m, s, 4H], 2.3 ppm [s, N-CC, 3H], 5.4 ppm [tt, CH, lH]; 7.5 ppm [m, Ar-H, 3H]; 7.9 ppm [d, Ar-H, lH]; 8 ppm [d, Ar-H, lH]; 8.2 ppm [d, Ar-H, lH], 9 ppm [d, Ar-H, lH]. FT-IR showed bands at 3060cm-’ (Ar-H), 1701 cm-’ (C = 0); 1248cm-’ (CO); 1133cm-’ (C--N). Mass spec- trometry gave the following data: [M ‘1 at 325 mje and [M-CH3] at 3 10 m/e.

Thermal and photo-oxidative stabilisation of polyoleJinJi1m.s 197

3.3 Synthesis of [2,2,6,6-tetramethyl piperidin-4-011 2-naphthyl acetate

Naphthyl acetic acid 0.93 g (5 mmol) and cyanuric chloride 0.46g (2.5mmol) were added to a round- bottomed flask containing calcium chloride dried acetone (50cm3). Argon gas was delivered into the system in order to provide an inert atmosphere. Then triethylamine (0.505 g, 5 mmol) was placed in the flask. The reaction mixture was stirred at room temperature for 3 h. At the end of this time the mixture was filtered off and the acetone partially evaporated under reduced pressure. The residue was redissolved in 70cm3 of dry toluene and then placed in a flask fitted with a condenser, thermo- meter, magnetic stirrer and gas inlet. The residual acetone was distilled off and 0.79g of 2,2,6,6-tet- ramethyl piperidin4-01 (5 mmol) was added to the reaction mixture and the temperature raised to boiling point. The reflux was maintained for 4 h and the advance of the reaction was monitored by TLC. When the reaction was finished, the solids in the reaction mixture were filtered off. The solvent of the filtrate was evaporated under reduced pres- sure. The residue was dissolved in ether, and the resulting solids filtered. Then a stream of hydrogen chloride was passed through the solution and the hydrochloride salt obtained was filtered and neu- tralized with sodium carbonate to pH 7. The com- pound was obtained in a yield of 41%. The melting point was 52°C. The elemental analysis calculated for C21H270zN is C = 77.5% and H = 8.36%; that found was C= 76.52% and H= 8.54%. NMR signals in CDC13 are as follows: 1 .15 ppm [s, g-s, 6H]; 1.25 ppm [s, g-CC, 6H]; 1.9 ppm [m, s, 4H]; 4.1 ppm [s, B-C = 0, 2H]; 5.2 ppm [tt, C& lH]; 7.5 ppm [m, Ar-H, 4H], 7.85 ppm [m, Ar-H, 2H]; 8.05 ppm [d, Ar-H, lH]. The FT-IR spectrum showed the following peaks: 3328 cm-’ (N-H); 3045 cm-’ (Ar-H); 1723 cm-’ (C = 0); 1242cm-’ (C-O). M ass spectrometry showed peaks at 325 and 310 m/e corresponding to [M+] and [M-CH3], respectively.

3.4 Synthesis of [ 1,2,2,6,6-pentamethyl piperidin-C 011 2-naphthyl acetate

In this case the same apparatus and technique were used as described above. Naphthyl acetic acid (5 g, 26.85 mmol) and cyanuric chloride (2.47 g, 13.42 mmol) were placed together in 50 cm3 of dry acetone. Then triethylamine (2.71 g, 26.85 mmol) was added to the flask. After the treatment, 4.59g

(26.85 mmol) of 1,2,2,6,6_pentamethyl piperidin-4- 01 was added to the reaction mixture. The ester obtained was purified by means of a chromato- graphic column filled with silica gel using a mixture of hexane:ethyl acetate (50:50) as eluent. A high boiling point liquid was obtained in a yield of 21.5%. Elemental analysis calculated for C22H2902N gave C = 77.83% and H = 8.61%. The values found were C = 77.49% and H = 8.39%. iH NMR signals gave the following: 1 ppm [s, g-B, 6H]; 1.15 ppm [s, g-B, 6H]; 1.85 ppm [dd, CH2, 4H]; 2.25 ppm [s, N-m, 3H]; 4.05 ppm [s, m- C=O]; 5.1 ppm [tt, CH, lH]; 7.5 ppm [m, Ar-H, 4H]; 7.8 ppm [m, Ar-H 2H]; 8.05 ppm [d, Ar-H lH]. The FT-IR &&&urn showed the following peaks 3052 cm-’ (Ar-H); 1736 cm-’ (C = 0); 1255cm-’ (C-O) and 1163 cm-’ (C-N), respec- tively. Mass spectrometry gave peaks at 339 and 324 m/e corresponding to [M ‘1 and [M-CH3].

3.5 Polymer sample preparation

The stabilisers at concentrations of 0.05, 0.1 and 0.15% w/w were solvent-blended into the polymers using dichloromethane as solvent and then com- pression-moulded into films (15&200 pm thick) at 150°C for HDPE and 170°C for PP. Table 1 gives the relationship of the codes used for the different formulations.

3.6 Thermal and photo-oxidation

The films were heated in an air draught oven set at 110°C for HDPE and 130°C for PP. The samples were irradiated in a QUV test chamber with alter- nated cycles of irradiationcondensation according

Table 1. Codes for the different additive formulations used in this study

Additive 1 (Al)

Additive 2 (A2)

Additive 3 (A3)

Additive 4 (A4)

Formulation 5 (F5)

Formulation 6 (F6)

Formulation 7 (F7)

Formulation 8 (F8) Formulation 9 (F9)

[2,2,6,6-Tetramethylpiperidin-4-011 napthoate

[ 1,2,2,6,6 Pentamethylpiperidin-4-011 napthoate

[2,2,6,6 Tetramethylpiperidin-4-011 2-naphthyl acetate

[ 1,2,2,6,6-Pentamethylpiperidin-4-011 2-naphthyl acetate

Naphthalene + 2,2,6,6_tetramethyl- piperidin-Co1

Naphthalene + 1,2,2,6,6_tetramethyl- piperidin-Co1

2,2,6,6-Tetramethylpiperidin-4-01

1,2,2,6,6-Pentamethylpiperidin-4-01 naphthalene

198 R. A. Ortiz et al.

to the G 5388 ASTM norm. This norm established temperatures of 50 and 60°C for the irradiation and the condensation periods, respectively.

The rates of polymer oxidation were measured by monitoring the rate of formation of the non- volatile carbonyl oxidation products in the region 1700-l 800 cm-’ using a Nicolet Magna spectro- meter. The carbonyl index was determined using the equation:

carbonyl index = [{log,,(Zo/l,)/d} x 1001

where d = film thickness in pm; Ia = incident light intensity; It = transmitted light intensity.

The systems used for comparison were a mixture of naphthalene and 2,2,6,6-tetramethyl piperidin-4- 01 and a mixture of naphthalene and 1,2,2,6,6- pentamethyl piperidin-4-01 in such a ratio that the molar equivalents of the active groups of the com- mercial additives would be the same as in the syn- thesised compounds.

4 RESULTS AND DISCUSSION

4.1 Ageing studies

The influence of the naphthalene esters on the thermal and light stability of polyethylene (HDPE) and polypropylene (PP) films has been examined and compared to the corresponding activities of simple tetra and pentamethylpiperidinol com- pounds. Formulations were also prepared to deter- mine the activity of naphthalene itself on a molar percentage basis for equivalence. Oxidation rates have been determined by measuring the rate of car- bony1 growth in the films through FTTR analysis and the respective absorption peak maxima. Embrittle- ment values for the films are normally taken at 0.06 carbonyl units as a comparison of stabilities.

4.2 HDPE thermal ageing

Embrittlement data for the oven-aged HDPE films are compared in Table 2 for all the formulations at three concentrations: 0.05, 0.1 and 0.15%. Figure 2 compares the actual carbonyl oxidation curves at 0.1% concentration. The most interesting feature of the ageing data is that both the secondary amine derivatives (additives 1 and 3) exhibit higher thermal stabilising performance than that of both the tertiary analogues (additives 2 and 4). Fur- thermore, strong autosynergism is observed for

Table 2. Embrittlement time for HDPE films during oven age- ing at 110°C

Sample Embrittlement time in hours (C.I. =0.06)

0.05%b O.l%b O.l5%b

Control 199 199 199 Additive 1 (Al) 507 (303) 416 (402) 474 (451) Additive 2 (A2) 329 (490) 270 (496) 434 (446) Additive 3 (A3) 503 (303) 603 (402) 591 (451) Additive 4 (A4) 533 (490) 306 (496) 353 (446) Formulation 5 (F5) 275 232 212 Formulation 6 (F6) 212 243 215 Formulation 7 (F7) 82 170 234 Formulation 8 (F8) 269 264 229 Naphthalene (F9) 221 232 217

<lThese values were obtained by interpolation in the degrada- tion curves. %Zoncentration of the additives (w/w); in the case of the mix- tures of naphthalene+piperidinols the molar % of the stabi- lising moieties of the synthesised additives was considered to keep the same concentration. Numbers in parentheses represent additivity effect of the mix- tures of naphthalene + piperidinols.

additive 3 at all concentrations and a less pro- nounced effect is observed for additive 1. It is noted that all four additives display high activity at low concentrations, with naphthalene itself dis- playing some thermal stabilisation. From the car- bony1 index curves additives 1 and 3 exhibit strong autoretarding effects on polymer oxidation. High molecular weight HALS systems are known to exhibit properties of thermal stabilisers and that this is consistent with their ability to react with oxygen and alkyl radicals as well as their stoichio- metric reaction with hydroperoxides. In this case, therefore, the stabilisation activity is associated in the main with the HALS moiety with the naphthalene providing the high molecular weight

Fig. 2. Carbonyl index versus oven ageing time (h) of HDPE films (200 Mm thick) containing additive formulations at 0.1% w/w equivalent. (X) Control, (X) Al, (0) AZ, (0) A3, (W)

A4, (0) F5, (A) F6, (A) F7, (+) F8 and (0) F9 at 110°C.

Thermal and photo-oxidative stabilisation of polyolefinjilms 199

link reducing volatility. The higher performance of the secondary over the tertiary amine HALS is possibly due to the enhanced ability of the former to scavenge oxygen more effectively via oxidation of the amino group (N-H) rather than that of the alkylamino group (N-CH,).

4.3 HDPE photo-oxidation

Embrittlement data for the HDPE films during UV exposure are compared in Table 3. The carbonyl index plots for the UV exposure are shown in Fig. 3 for formulations at a concenration of 0.1%. High UV stabilisation performance was observed for all four additives over the complete concentration range when compared with the effects of the mix- tures with naphthalene as well as the single tetra and pentamethylpiperidinol compounds. In this case the nature of the HALS functionality appears to be reversed from that observed in thermal sta- bilisation, with the two tertiary-based systems exhibiting the higher stabilisation activity. Alone the naphthalene is a photosensitiser whereas when coupled to the HALS group it exhibits high stabi- lity. The strong autoretarding effects of the two tertiary HALS 2 and 4 are shown in Fig. 3 while the other formulations are autocatalytic in carbo- nyl growth. The high stabilisation of the naphthyl derivatives may be associated with the inherent stability and synergy within the structures them- selves. Taking the naphthoate derivatives first, these can break down as shown in Scheme 1 to

Table 3. Embrittlement times for HDPE films during UV ageing in a QUV unit

Sample Embrittlement time in hours (C.I. = 0.06)

O.O5%b O.l%b O.l5%b

Control 217 217 217 Additive (Al) 462 (342) 552.5 (359) 683 (351.5) Additive 2 (A2) 618 (322) 651 (337.5) 743 (353.5) Additive 3 (A3) 453 (342) 500 (359) 635 (351.5) Additive 4 (A4) 468.5 (322) 645.5 (337.5) 656.5 (353.5) Formulation 5 (FS) 230 204.5 202.5 Formulation 6 (F6) 175 181.5 176.5 Formulation 7 (F7) 172 207 202.5 Formulation 8 (F8) 152 185.5 204.5 Naphthalene (F9) 170 152 149

These values were obtained by interpolation in the degrada- tion curves. bConcentration of the additives (w/w); in the case of the mix- tures of naphthalene+piperidinols the molar % of the stabi- lising moieties of the synthesised additives was considered to keep the same concentration. Numbers in parentheses represent additivity effect of the mix- tures of naphthalene + piperidinols.

Fig. 3. Carbonyl index versus QUV irradiation time (h) of HDPE films (200pm thick) containing additive formulations at 0.1% w/w equivalent. (X) Control, (X) Al, (0) A2, (0)

A3, (H) A4, (0) F5, (A) F6, (A) F7, (+) Fg and (0) F9.

produce naphthalene radicals. Homolysis may also occur at the piperidine ring resulting in the forma- tion of an aromatic carboxyl radical. Both mechanisms have been confirmed in earlier photo- chemical and flash photolysis studies on aromatic carboxylates. 8,9 The aromatic carboxyl radical is known to split off carbon dioxide rapidly to pro- duce naphthalene radicals.‘O In the presence of oxygen this radical species can scavenge oxygen to form aromatic peroxy radicals as shown in Scheme 2. Both naphthyl acetate derivatives undergo simi- lar processes as shown in Scheme 3. Here scission is

_ \ /

8 \ / -co2 hdAH ---I hf = 127

o,.+.

I - \ / 8 + co* \ 1’ Scheme I

Scheme 2

200 R. A. Ortiz et al.

3 “’ I

&H2Cd, .g’

i

3 “3 R-H

\ /

8 \ / CH2CO2H + R

Scheme 3

known to occur at the methylene group to produce naphthyl or naphthylcarboxyl radicals. The latter can abstract a hydrogen atom from the polymer generating a carboxylic acid. In the absence of

oxygen, dimerisation of the naphthalene and naphthyl radicals predominates (Schemes 2 and 4). This ability to scavenge oxygen can protect the HALS moiety and thus impart high stability to the polymer. Naphthalene compounds are known to undergo photo-oxidation to produce endoper- oxides (via Scheme 2) which will result in naph- thaquinone formation. Alone this can be detrimental to the polymer during light ageing and impart stability during thermal ageing due to the radical trapping ability of the quinone. Indeed, this is the observed case for both polymers when naphthalene is used alone.

4.4 Polypropylene thermal ageing

Embrittlement data for the oven-aged samples of polypropylene are shown in Table 4 and carbonyl index plots for the 0.1% concentration are shown in Fig. 4. All four additives are seen to be effec- tive thermal stabilisers with both the secondary HALS exhibiting the better performance. This correlates well with the ageing data for HDPE. Again the mixed formulations were not as effective, with naphthalene itself displaying some degree of

Scheme 4

Table 4. Embrittlement time for PP films during oven ageing at 130°C

Sample Embrittlement time in hours (C.I. = 0.06)”

0.05%h o.l%h

Control 9 9 Additive 1 (Al) 28 (26) 30 (38.5) Additive 2 (A2) 26 (25.5) 17 (26.5) Additive 3 (A3) 91.5 (26) 105 (38.5) Additive 4 (A4) 29 (25.5) 17.5 (26.5) Formulation 5 (F5) 13.5 14 Formulation 6 (F6) 15 14 Formulation 7 (F7) 15 27 Formulation 8 (F8) 14.5 15 Naphthalene (F9) 11 11.5

“These values were obtained by interpolation in tion curves.

O.l5%h

9 31 (24)

18 (22.5) 100.5 (24) 30 (22.5)

15 15

15.5 14

8.5

the degrada-

‘Concentration of the additives (w/w); in the case of the mix- tures of naphthalene+piperidinols the molar % of the stabi- lising moieties of the synthesised additives was considered to keep the same concentration. Numbers in parentheses represent additivity effect of the mix- tures of naphthalene + piperidinols.

stabilisation. The carbonyl index growths were all autocatalytic (Fig. 4) due to the high degree of intra-chain hydroperoxidation of the polymer backbone.

4.5 Polypropylene photo-oxidation

Embrittlement data for the UV exposed poly- propylene films are shown in Table 5 and the carbonyl index plots at 0.1% concentration in Fig. 5. In polypropylene the effects are opposite to those seen in HDPE in that both the secondary HALS additives are more effective than the tertiary structures. Also, high autosynergism is observed

Fig. 4. Carbonyl index versus oven ageing time (h) of PP films (200 pm thick) containing additive formulations at 0.1% w/w equivalent. (X) Control, (X) Al, (0) A2, (0) A3. (M) A4.

(0) F5. (A) F6. (A) F7. (+) F8 and (0) F9 at 130°C.

Thermal and photo-oxidative stabilisation of polyolefin$lms 201

Table 5. Embrittlement time for PP films during UV ageing in QUV unit

Sample Embrittlement time in hours (CL = 0.06)

O.O5%h O.l%b O.l5%b

Control Additive 1 (Al) Additive 2 (A2) Additive 3 (A3) Additive 4 (A4) Formulation 5 (F5) Formulation 6 (F6) Formulation 7 (F7) Formulation 8 (F8) Naphthalene (F9)

42 70 (143.5) 88 (108)

76 (143.5) 83 (108)

70 104 106 72 37

114.:;83.5) 90.5 (113.5) 136.5 (83.5) 72 (I 13.5)

83.5 82

46.5 76.5 37

42 153.5 (121) II6 (132) 154 (121)

60.5 (132) 73 71 73 84 48

uThese values were obtained by interpolation in the degrada- tion curves. bConcentration of the additives (w/w); in the case of the mix- tures of naphthalene+piperidinols the molar % of the stabi- lising moieties of the synthesised additives was considered to keep the same concentration. Numbers in parentheses represent additivity effect of the mix- tures of naphthalene + piperidinols.

for these secondary HALS derivatives only at con- centrations of 0.1 and 0.15%. At the lower con- centration of 0.05% pro-oxidation is observed. Physical mixtures of the model HALS and naph- thalene were also antagonistic except at the 0.05% concentration. It is interesting to note that the naphthalene itself changes from a sensitiser at 0.1% to a stabiliser at 0.15% concentration. Thus, in polypropylene the high rate of hydroperoxida- tion of the polymer is a crucial factor in control- ling the rate of oxidation and this, in turn, is linked to the additive concentration. Higher con- centrations of the secondary amine and the naph- thalene are required to scavenge the oxygen

Fig. 5. Carbonyl index versus QLJV irradiation time (h) of PP films (200 pm thick) containing additive formulations at 0.1% w/w equivalent. (X) Control, (X) Al, (0) A2, (0) A3, (H)

A4, (0) F5, (A) F6, (A) F7, (+I F8 and (0) F9.

effectively and compete with the tertiary macro- alkyl radicals.

4.6 Mass spectral analysis

In the mass spectral analysis of the stabilisers fragmentation patterns were observed confirming the mechanistic features outlined in Schemes 1 and 3 for the respective naphthoate and naphthyl acet- ate derivatives. Naphthalene radical fragments were observed with a peak at 127 m/e as shown in Scheme 1 for stabilisers 1 and 2. For stabilisers 3 and 4 the fragmentation pattern gave a peak at 141 m/e due to the naphthyl radical fragment. A typical fragmentation spectrum is shown in Fig. 6 for stabiliser 3 showing the peak at 141 m/e.

4.7 Spectroscopic properties

Absorption spectra of the stabilisers have been recorded in 2-propanol and cyclohexane to deter- mine the extent of absorption in the near UV region. Wavelength maxima and extinction coeffi- cients are compared in Table 6. Stabilisers 1 and 2 exhibit extended absorption maxima into the near UV region while stabilisers 3 and 4 exhibit only a weak tail extending from their longest wavelength bands at 280 nm. Thus, the absorption capacities of these stabilisers are generally very low. However, they will be susceptible to photolytic processes as described in Schemes 1 and 3.

The naphthalene moieties are also expected to be fluorescent and this is shown by the emission spectra of the four stabilisers in Fig. 7. The spectra of stabi- lisers 3 and 4 with a wavelength maximum at 340 nm are close to that of naphthalene itself and repre- sentative of emission from a lowest excited singlet nrc* state. The emissions from stabilisers 1 and 2 are less well structured with a broad maximum at 342nm. The electron-withdrawing effect of the attached carboxyl group would reduce the n electron density in the aromatic rings and hence obscure the vibrational structure. Solution spectra of the stabi- lisers are also compared in Fig. 8 at 4x 10e3 M con- centration. While the spectra of stabilisers 3 and 4 are better resolved, those for stabilisers 1 and 2 are very broad and markedly red shifted to 370 nm. At this concentration, coupled with the magnitude of the red shift, it would appear that stabilisers 1 and 2 are forming excimers due to increased molecular interactions. This would place the naphthalene ring fragments on photolysis in a favourable position for recombination processes as shown in Scheme 2.

202 R. A. Ortiz et al.

MASS SPECTRAL ANALYSIS OF DECOMPOSITION OF [2,2,6,6-TETRAMETHYLPIPERIDIN-4-YL]-2- NAPHTHYLACETATE

bundance

1400000 i

Scan 934 (13.897 min): w34.D 1 -7’

1200000 -

1000000 -

800000 -

600000 -

400000 i

58

141 42

115 82 ‘..J jj, 168 186 207

310 ! 326 /,

/z--> 50 lb0 150 200 ,

250 300

Fig. 6. Mass spectrum of stabiliser 3.

Table 6. Wavelength maxima and extinction coefficients of the additives l-4 in different solvents

Sample 2-Propanol Cyclohexane

Wavelength (nm) E in I molP’ cm ’ Wavelength (nm) e in I mol -’ cm-’

Additive I 242, 284, 326 741, 757, 742 244, 282, 320 730, 758, 732 Additive 2 246. 286. 320 747. 765, 750 244, 282, 324 720, 754, 724 Additive 3 232, 272 630, 708 244, 282 747, 763 Additive 4 244, 282 758, 765 244, 280 731,153

I Fluorescence in polypropylene

Emission h (nm) Emission h (nm)

Fig. 7. Fluorescence emission spectra of ( -----) stabiliser 1, Fig. 8. Fluorescence emission spectra of ( ----) stabiliser I, (-. -. -) stabiliser 2, (- - -) stabiliser 3 and (. .) stabiliser 4 in (- -. -) stabiliser 2, (- - -) stabiliser 3 and (, .) stabiliser 4 in

polypropylene film (excitation at 3 15 nm). 2-propanol at 4x lO--3 M (excitation at 3 I5 nm).

Fluorescence

Thermal and photo-oxidative stabilisation of polyolejinjilms 203

600 c

Additive 4 in PP irradiation time

Emission h (nm)

Fig. 9. Influence of irradiation in a QUV exposure unit on stabiliser 4 in PP film measured by fluorescence emission

(excitation at 3 15 nm).

During irradiation all the stabilisers photolyse rapidly. In the case of stabilisers 3 and 4 the fluorescence spectra show a marked decrease in intensity over a short irradiation period of 38 h with no change in emission wavelength maxima. An example is shown in Fig. 9 for stabiliser 4 in PP. Here the naphthyl radicals formed via Scheme 4 will photo-oxidise rapidly producing no new fluorescent products. In the case of stabilisers 1 and 2, whilst there is also a rapid photolysis, the emis- sion spectrum shifts gradually to the red indicating the formation of a more highly conjugated fluor- escent product. An example is shown in Fig. 10 for stabiliser 2 in the HDPE film. Scheme 2 depicts the coupling of naphthyl radicals to form

0 hr

60

ti‘

Additive 2 in HDPE irradiation time

Emission )c (nm)

Fig. 10. Influence of irradiation in a QUV exposure unit on stabiliser 2 in HDPE film measured by fluorescence emission

(excitation at 3 15 nm).

a bis-naphthyl product. The formation of such products via an excimer conformation would therefore appear to be highly likely in the first instance.

5 CONCLUSIONS

The ageing data on HDPE films indicate that both the secondary amine derivatives (additives 1 and 3) exhibit higher thermal stabilising performance than that of either of the tertiary analogues (additives 2 and 4). Furthermore, strong autosynergism is observed for additive 3 at all concentrations and a less pronounced effect is observed for additive 1. It is found that all four additives display a high activity at low concentrations, with naphthalene itself displaying some degree of thermal stabilisa- tion. High UV stabilisation performance was also observed for all four stabilisers in HDPE over the complete concentration range when compared with the effects of the mixtures with naphthalene as well as the single tetra and pentamethylpiperidinol compounds. In this case, the nature of the HALS functionality appears to be reversed from that observed in thermal stabilisation, with the two ter- tiary-based systems exhibiting the higher stabilisa- tion activity. All four additives are seen to be effective thermal stabilisers in PP with both the secondary HALS exhibiting the better perfor- mance. This correlated well with the ageing data for HDPE. In polypropylene the effects are oppo- site to those seen in HDPE during irradiation in that both the secondary HALS additives are more effective than the tertiary structures. Also, high autosynergism is observed only for these secondary HALS derivatives at concentrations of 0.1 and 0.15%. At the lower concentration of 0.05%, pro- oxidation is observed.

The high thermal and photostabilisation of the naphthyl derivatives may be associated with the inherent stability and synergy within the structures themselves. Naphthalene and naphtyl radicals are formed during thermal and photo-oxidation which can effectively scavenge oxygen thus minimising polymer hydroperoxidation.

ACKNOWLEDGEMENT

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

204 R. A. Ortiz et al.

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