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tapraid5/vnl-spevnl/vnl-spevnl/vnl00106/vnl0040d06a heckt S⫽10 1/6/06 11:07 Art: 0306-03 Input-rd

Synergistic Profiles of Chain-Breaking Antioxidants With Phosphites and Hindered Amine Light Stabilizers in Styrene–Ethylene–Butadiene–Styrene (SEBS) Block Copolymer

Cristina Luengo, Norman S. Allen, Arthur Wilkinson, Michele Edge Chemistry and Materials, Faculty of Science and Engineering, The Manchester Metropolitan University, Manchester, UK M. Dolores Parellada, Juan A. Barrio, V. Ruiz Santa Repsol-YPF, Carretera de Extremadura, N-V. Km. 18, 28931 Mostoles, Spain

The photostabilization of poly(styrene-b-ethylene-cobutylene-b-styrene) (SEBS) by phosphite/p-hydroxybenzoate antioxidants and hindered phenol/hindered amine light stabilizers (HALS) was studied by using a variety of spectroscopic methods, including FTIR, UV, and luminescence spectroscopy coupled with crosslinking and hydroperoxide analysis. The results were compared with those obtained for hindered phenols and their combinations with phosphite antioxidants. All the stabilizing packages stabilized the SEBS in terms of the inhibition of discoloration and the formation of hydroperoxides, acetophenone, and oxidation products, as well as chain scission and disaggregation of the styrene units. Although phosphite/p-hydroxybenzoate combinations appeared to reduce the formation of oxidation products, they did not show any remarkable enhancement in longterm stabilization with respect to phenolic/phosphite antioxidants. On the other hand, strong synergistic profiles were found with the HALS. Mobility and diffusion impediments in the polymeric material appeared to play an important role in the stabilizing activity of the HALS. J. VINYL ADDIT. TECHNOL., 12:000 – 000, 2006. © 2006 Society of Plastics Engineers

INTRODUCTION A previous study on the photostabilization of poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) by hindered phenols and their combination with phosphite antioxidants showed that the addition of a hindered phenol photo-stabilizes the SEBS in terms of inhibition of discoloration and

Received ???; accepted ??? Correspondence to: Norman S. Allen; e-mail: [email protected] DOI 10.1002/vnl.0306 – 03 Published online in Wiley InterScience (www.interscience.wiley. com). © 2006 Society of Plastics Engineers

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2006

the formation of hydroperoxides, acetophenone, and oxidation products, as well as chain scission and disaggregation of the styrene units, through its alkyl-radical scavenging action [1]. Furthermore, a strong beneficial effect was found with combinations of a hindered phenol and phosphite antioxidant, especially with an increase in the phosphite concentration due to its inhibition of radical formation by the reduction of hydroperoxides to alcohols. Because of the important role that hydroperoxides had showed to have during the photooxidative degradation of SEBS [1], in this study the suppression of hydroperoxide formation was attempted by any possible synergistic profiles by phosphite/p-hydroxybenzoate combinations. Although alkyl substituted p-hydroxybenzoates can stabilize the polymers by suppressing hydroperoxide formation, their main stabilization mechanism is as chain-breaking hydrogen donors [2]. Their stabilizing mechanism is similar to that of the hindered phenolic antioxidants. However, this type of structure is more highly selective and reacts with alkoxy and hydroxy radicals by hydrogen atom transfer [3]. It is also much more stable than the phenolic antioxidants because of the electron withdrawing effect of the p-carboxyl group [4]. An advantage of these compounds is that they are resistant to photodimerization in the polymer due to the presence of the carboxylic acid group attached to the para position of the phenyl ring, thus, preventing further photoreactions in which yellowing stilbene-quinones are formed [5]. Another advantage of this type of chain-breaking antioxidant is the ability to synergize with other hydroperoxide decomposer antioxidants such as phosphites and hindered amine light stabilizers (HALS). Stabilization by means of more specific light stabilizers, such as HALS, was also studied in this work in synergistic formulations. Sterically hindered amines based on tetram-


Scheme 1.

S1

S2

Stabilization mechanisms by hindered amine light stabilizers (HALS)

ethyl piperidine derivatives are effective light stabilizers through various mechanisms: free radical scavenging, hydroperoxide decomposition and excited state quenching [6]. These amines are converted to nitroxyls by oxidation in the presence of peroxy radicals and to hydroxylamines by oxidation with peracids (Scheme 1) [7,8]. In general, the operating mechanism of hindered amine stabilizers is explained by the reaction of alkyl radicals with the nitroxyl radical. Their effectiveness lies in the fact that the reaction rate of nitroxyl radicals with alkyl radicals appears to be only slightly lower than that of macroradicals with oxygen. Therefore, only amines capable of forming stable nitroxyl radicals are effective UV stabilizers [9]. The scavenging of alkyl radicals by nitroxyls leads to retarded oxidation. This process proceeds cyclically and the nitroxyl radical regenerates until it is destroyed via side reactions (Scheme 2). It was found that hindered amines effectively quench the molecular decomposition of polymeric hydroperoxides, which is known to produce vinyl groups in the polymer [10]. There are several possible ways by which the reaction can be quenched. The stabilizer may compete with the polymer for the energy transfer by a photoactive impurity (such as traces of metals) contributing to the excitation of the triplet state. Another possibility is that the stabilizer competes with the reaction with oxygen needed to form an exciplex. Finally, the last possible mechanism is the deactivation of the polymer– oxygen exciplex before it reacts. Unfortunately, antagonist effects occur when hindered amines are used in combination with hindered phenols, sulfides, and phosphites. These compounds decrease, or at best they do not change, the effectiveness of hindered amines [11]. In mixtures with phenolic antioxidants, yel-

lowing has been observed. This yellowing may be due to a weak absorption of visible light by the nitroxyl radicals, or by the formation of colored reaction products, chromophores, in the polymer [12]. EXPERIMENTAL PROCEDURES

Materials The SEBS, Structure 1, was supplied by Repsol-YPF (Madrid, Spain) in the form of pellets and/or sheets and all the samples were of experimental grade. The sample grades together with their molecular weights and antioxidant concentrations are shown in Table 1. Data on molecular weights and Ti contents, measured by Repsol (Madrid), were given in a previous article [13]. Each polymer sample is prepared separately so that the Ti content will differ in each case. Stabilizers A, B, D, and E were supplied by Novaris (Basel, Switzerland) and stabilizer C from Cytec Industries (Stamford, USA). All the samples were specially prepared for this study in order that the control and the stabilized samples had a similar history. The chemical structures of the antioxidants used were shown in Structures 2 (A and B) and 3 (C, D, and E). Ratios of polystyrene to olefin are proprietary. The SEBS samples were compression molded at 205°C, as described previously [13]. All the solvents and chemical products used in this work were of ‘Analar’ purity and obtained from Aldrich Chemical (Gillingham, UK).

Colorimetry Color formation was obtained by the yellowness index (YI), using a Gretag–Magbeth spectrometer, according to the ASTM D1925 procedure.

Scheme 2.

2

Destruction of nitroxyl radicals via side reactions.


Structure 1.

General structure of SEBS triblock copolymer.

DOI 10.1002/vnl

r1

T1

r2-3

tapraid5/vnl-spevnl/vnl-spevnl/vnl00106/vnl0040d06a heckt S⫽10 1/6/06 11:07 Art: 0306-03 Input-rd TABLE 1. SEBS materials for phenolic/phosphite stabilising package.

the

study

of

hindered

Sample code

Molecular weight

Stabilizers (%, w/w)

Ti content (ppm)

1 3 6 10 11 12

High High High High High High

None 0.1% A ⫹ 0.1% B 0.1% A ⫹ 0.2% B 0.05% C ⫹ 0.1% B 0.05% A ⫹ 0.05% D 0.05% A ⫹ 0.05% E

140 140 49 Unknown Unknown Unknown

Sol/Gel Analysis Samples of photoaged SEBS were extracted with toluene in glass tubes maintained in a water bath at 60°C for 24 h, followed by separation of the gel, drying at 60°C for 2 h, and then weighing the residue. FTIR Spectroscopy The samples were placed in a transmission cell fitted to a Nicolet Nexus Fourier-transform infrared (FTIR) Spectrometer (DTGS detector). Spectra of 32 scans were recorded with a resolution of 2 cm⫺1. Hydroperoxide Analysis Hydroperoxide concentration of photooxidized SEBS was quantitatively determined via the reflux iodometric method, using a PerkinElmer Lambda 7 UV/Vis spectrophotometer. The values were measured at wavelength of 420 nm. Luminescence Spectroscopy Luminescence analysis was performed using a PerkinElmer Luminescence Spectrometer Model 50-B. Fluorescence spectra were recorded at ambient temperature and the excitation wavelength of 270 nm on SEBS sheets using a front face accessory, while phosphorescence emission spectra were obtained at 77 K and the excitation wavelength of 290 nm on thin strips of film placed in quart tube cells. Photooxidation Irradiation studies were performed in a Suntest CPS⫹ (Atlas) weathering unit, using a 1.5 kW optical filtered

Structure 2. General structure of the hindered phenol (A) and the phosphite (B) antioxidants.

DOI 10.1002/vnl

xenon source and black body temperature of 50°C (Chamber 40°C; obtained from Atlas technology, Oxford, UK). RESULTS AND DISCUSSION To overcome the degradation and yellowing problem in SEBS, a detailed assessment of photostabilization methodologies by hindered phenol and phenolic/phosphite antioxidant combinations had been undertaken [1]. Because of the important role that hydroperoxides had been shown to have during the photooxidative degradation of SEBS, an attempt was made to minimize the deterioration of the polymer by suppressing hydroperoxide formation through studying possible synergistic profiles of phosphite/p-hydroxybenzoate, the latter being highly selective and only reacts with alkoxy and hydroxy radicals by a chain-breaking donor mechanism [2]. Stabilization by means of more specific light stabilizers, such as HALS, was also studied. HALS not only acts as free radical scavenger and hydroperoxide decomposer, but also as excited state quenchers [6]. The effects of two different HALS in combination with a hindered phenol were assessed. While the HALS utilized in Sample 11 was a difunctional hindered amine type, the one in Sample 12 was of a polymeric nature. The three stabilizing packages produced a blackish discoloration in the unaged material, which had been reported to be observed in mixtures of HALS with phenolic antioxidants [12], possibly due to the formation of colored reaction products in the polymer. Colorimetry results for the three stabilizing packages showed a rapid bleaching out of the initial color to yield a steady state in chromophore formation (Fig. 1). The inhibition of color by the p-hydroxybenzoate containing sample does not appear to show any major enhancement with respect to the highly synergistic phenolic/phosphite stabilizing package (sample 6). Both HALS, especially the difunctional hindered amine, however, proved to be better inhibitors of color formation. Sol/gel analysis showed no formation of gel as previously assessed for high molecular weight SEBS. It was also noticed that the samples did not warp or exhibit surface stickiness until much longer irradiation times compared with the unstabilized material or even the phenolic/ phosphite stabilizing packages. These facts might indicate that the p-hydroxybenzoate and HALS stabilizers reduce the degree of chain scission in the polymer and possibly, as observed in previous experiments for the phenol/phosphite antioxidants in the low molecular weight samples, the degree of crosslinking [1]. FTIR spectra of the stabilized samples showed the same features and degradation pattern as did the unstabilized SEBS [1]. However, the growth of the main peak at 1712 cm⫺1, which had been attributed to the formation of carboxylic acids, was delayed by the addition of the stabilizers. The late appearance of the maxima at 1712cm⫺1 indicates that the initial growth of the broad band is due to other species, such as ketones (1725 cm⫺1) and ␣,␤-unsaturated carbonyls (1700 cm⫺1), and to a lesser degree to the for-


3

F1


Structure 3. E).

F2

General structure of the p-hydroxybenzoate (C) and the difunctional and oligomeric HALS (D and

mation of carboxylic acids. The latter would be augmented at further photooxidation stages and higher irradiation times. The photooxidation rate (Fig. 2) appeared to be significantly reduced by the p-hydroxybenzoate and the difunctional hindered amine, whereas the oligomeric hindered amine did not appear to give any significant enhancement in respect to the phenolic/phosphite antioxidant packages. The oligomeric stabilizer having higher mobility impediment will not diffuse as fast as a smaller particle in the polymer, thus resulting on the reduction of its stabilizing ability. On the other hand, the p-hydroxybenzoate stabilizer, due to its activity in scavenging alkoxy and hydroxyl radicals, will not only delay hydroxyl group formation but also other important oxidation products such as carboxylic acids, which can be formed by either hydrogen atom abstraction by an alkoxy radical or the reaction of acyl and hydroxyl radicals. Hydroperoxides have been shown to play a key role in the initiation of the oxidation of SEBS [1] and hence the high activity of the p-hydroxybenzoate stabilizer is verified through its specificity.

FIG. 1. Yellowness index (ASTM D1925) vs. aging time (h) for SEBS samples.

4


Hydroperoxide analysis revealed that the initial amount of hydroperoxides present as impurities in the unaged material was minimized by the addition of the stabilizers. Addition of the p-hydroxybenzoate antioxidant to the SEBS resulted in a lower rate of hydroperoxide formation in comparison with the unstabilized material, and gradually increased to reach a maximum after about 150 h (Fig. 3). This poor performance by such a selective antioxidant indicates that the formation of hydroperoxides is not the only important reaction in the photooxidation of SEBS. In comparison, the more versatile HALS showed a much smaller growth in hydroperoxides. Unlike the phenolic/phosphite antioxidants, the hydroperoxide formation started from the early stages of irradiation. However, after the initial increase in hydroperoxides, both HALS yielded a steady state at longer irradiation times. Although the oligomeric hindered amine appeared to reach similar hydroperoxide concentrations as did the phenolic/phosphite stabilizing package, the difunctional hindered amine, remarkably, reduced the formation of these species. Fluorescence spectra of the stabilized samples did not show different features to those observed for the unstabi-

FIG. 2.

Carbonyl index vs aging time (h) for SEBS samples.

DOI 10.1002/vnl

F3


FIG. 3. Growth of hydroperoxide (ppm) with exposure time (h) for SEBS samples.

FIG. 5. Relative acetophenone emission intensity with irradiation time (h) for SEBS samples at 77 K.

CONCLUSIONS

F4

F5

lized SEBS. However, the addition of the stabilizers/antioxidants appeared to impair the reduction in the excimer sites by destroying free radicals and scavenging oxidation products. The addition of the p-hydroxybenzoate induced lower rate in the decrease of excimer emission, in accordance with the gradual formation of hydroperoxides observed, which confirms the close relationship between these species and disaggregation of the styrenic phase (Fig. 4). Addition of HALS showed similar behavior to the mixture of phenol/phosphite, i.e. an initial decrease followed by a steady state. As expected, the difunctional hindered amine impaired, to a greater extent, the reduction in the excimer sites. Phosphorescence analysis showed that the addition of the stabilizers produced a reduction in acetophenone content upon exposure to light (Fig. 5). The induction period for the reduction in acetophenone content was smaller for the phydroxybenzoate and HALS stabilizers compared with that of the phenolic/phosphite antioxidants. At longer irradiation periods an increase in the acetophenone content was observed, which in the sample stabilized with the oligomeric hindered amine suffered a further increase. This increase was also observed at longer exposure times for the difunctional hindered amine stabilizer.

The data show that phosphite/p-hydroxybenzoate antioxidant combinations do stabilize the SEBS material against photooxidation. However, compared to the phenolic/phosphite antioxidant combinations, it appears to exhibit an enhanced performance only by an observed decrease in the main photooxidation product, carboxylic acids, due to its ability to scavenge alkoxy and hydroxyl radicals. On the other hand, strong synergistic profiles were found with the HALS. In particular, they were observed for the difunctional structure, which showed a remarkably enhanced performance compared to the phenol/phosphite antioxidants, in terms of discoloration and the formation of hydroperoxides, acetophenone, and oxidation products as well as chain scission and disaggregation of the styrene units. The poorer performance by the oligomeric HALS was attributed to mobility and diffusion impediments in the polymeric material. ACKNOWLEDGMENTS The authors thank Repsol-YPF, Madrid, for partial financial support of this program of work and for supply of materials.

REFERENCES

FIG. 4. Relative excimer emission intensity with irradiation time (h) for SEBS samples.

DOI 10.1002/vnl

1. N.S. Allen, C. Luengo, M. Edge, A. Wilkinson, M. DoloresParellada, J.A. Barrio, and V. Ruiz Santa Quiteria, J. Photochem. Photobiol. A, 162, 41 (2004). 2. N.S. Allen and M. Edge, Fundamentals of Polymer Degradation and Stabilisation, Elsevier, Oxford, UK (1992). 3. N.S. Allen, A. Parkinson, F.F. Loffelman, and P.V. Susi, Polym. Degrad. Stab., 5, 241 (1983). 4. N.S. Allen, A. Parkinson, F.F. Loffelman, P. McDonald, M.M. Rauhut, and P.V. Susi, Polym. Degrad. Stab., 12, 363 (1985). 5. J.F. Rabek, Photostabilization of Polymers. Principles and Applications, Elsevier, London (1990).


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tapraid5/vnl-spevnl/vnl-spevnl/vnl00106/vnl0040d06a heckt S⫽10 1/6/06 11:07 Art: 0306-03 Input-rd 6. F. Gugumus, “New Trends in Polymer Photostabilisation,” in Current Trends in Polymer Photochemistry, N.S. Allen, editor, Prentice-Hall, London (1995).

10.

7. S. Al-Malaika, “Effects of Antioxidants and Stabilisers,” in Comprehensive Polymer Science, N.S. Allen and J.C. Bevington, editors, Pergamon Press, Oxford, UK (1989).

11. 12.

8. E.G. Rozantsev, Free Nitroxyl Radicals, Plenum Press, New York (1970).

13.

9. N.S. Allen, “Photostabilising Action of Hindered Piperidine Compounds in Commercial Polymers,” in Developments in

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Polymer Photochemistry, Vol. 2, N.S. Allen, editor, Applied Science, London (1981). F. Gugumus, Macromol. Chem. Macromol. Symp., 27, 25 (1989). J. Pospisil, Adv. Chem. Set., 249, 271 (1996). K. Scheim, J. Pospisil, and W. Habicher, Proceedings of the 34th IUPAC Macro, Prague, 8 –16 (1992). N.S. Allen, A. Barcelona, M. Edge, A. Wilkinson, C. Galan Merchan, and V. Ruiz Santa Quiteria, Polym. Degrad. Stabil., in press.

DOI 10.1002/vnl

AQ: 1


AQ1: Kindly update this reference.

DOI 10.1002/vnl


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