N-(Substituted phenyl)itaconimide–phenyl salicylate blends as organic stabilizers for plasticized poly(vinyl chloride) against photo-degradation

Polymer Degradation and Stability 92 (2007) 1003e1008 www.elsevier.com/locate/polydegstab

N-(Substituted phenyl)itaconimideephenyl salicylate blends as organic stabilizers for plasticized poly(vinyl chloride) against photo-degradation Nadia Ahmed Mohamed*, Mona Mohamed Fahmi Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Received 10 January 2007; accepted 5 March 2007 Available online 18 March 2007

Abstract The effect of blending some N-(substituted phenyl)itaconimide derivatives, Ne(RPh)II, (R: eH, or eOMe) with phenyl salicylate UV absorber on the stabilizing efficiency in photo-degradation of PVC plasticized with dioctyl phthalate (DOP) has been investigated. Blending was effected in the range of 0e100 wt% of the itaconimide relative to reference stabilizer. The stabilizing efficiency was evaluated by measuring the length of the induction period (Ts), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination on one hand, and the extent of discoloration of the degraded polymer on the other. The efficiencies are also evaluated by determining the amount of gel formation as well as the intrinsic viscosity of the insoluble and the soluble fractions of the degraded polymer. The results show a true synergistic effect from the blending of itaconimide derivative with phenyl salicylate UV absorber. Blending of the stabilizers improves the Ts values, decreases the rate of dehydrochlorination, and lowers the extent of discoloration and the gel content of the polymer. The synergism attains its maximum when both the itaconimide and the reference stabilizers are taken in equimolar ratios. The observed synergism may be attributed to the combination of mechanisms by which the itaconimide and the reference stabilizer work. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Plasticized poly(vinyl chloride); Blended photo-stabilizers; Potentiometric determination: discoloration; Gel content; Intrinsic viscosity; Synergistic effect

1. Introduction A prerequisite for good performance of PVC in many applications is its resistance to sunlight. PVC in general is fabricated into useful products by operations which invariably involve heat stress (temperature up to 200  C). Thus, heat stabilizers should be added. The time of PVC fabrication is relatively short. Measurements of the so-called residual thermal stability of PVC mixture [1e4] and determination of the amount of stabilizer consumed [4,5] after various

* Corresponding author. E-mail address: [email protected] (N.A. Mohamed). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.03.008

processing times indicate that most of the stabilizer remains unreacted. Thus, the final product contains large amounts of thermal stabilizer, whose effect on photo-degradation may differ substantially from its effect on thermo-degradation. In this connection, various investigators have studied the photodegradation of PVC in the presence of the commonly used tin [6,7] or soap [8,9] thermal stabilizers. In spite of the fact that the above-mentioned classes of stabilizers are quite efficient at the industrial level, the metallic residues from some of these organo-metallic compounds (lead, cadmium, zinc or tin) may present serious environmental problems [10,11]. Further, their by-products, mainly metal chlorides, accumulated during the reaction of these stabilizers with polymeric chains are considered as strong catalysts for the subsequent

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dehydrochlorination and are responsible for sudden blackening of certain formulations [12e14]. Meanwhile, it is possible to struggle against these inconveniences by using different types of organic co-stabilizers (oxiranes, polyols, etc.), but the manipulation of these systems remains delicate [15,16]. For this, recently a new trend has been established based on the use of organic stabilizers for the thermal [17e27] and consequently for the photo-stabilization of PVC [28,29]. We have suggested that various N-(substituted phenyl)itaconimide derivatives are efficient thermal and photo-stabilizers for rigid [26,29] and plasticized PVC [27,30]. In addition to the ability of the itaconimides to absorb UV radiation, their stabilizing action is attributed to their ability not only to intervene with radical chain degradation of the polymer through blocking the odd electron sites created on PVC chains, but also to act as powerful dienophiles which disrupt polyene formation along the degraded PVC chains. However, the industrial reference stabilizer phenyl salicylate acts as a UV absorber. In order to find a wider application for the suggested derivatives, it became of interest to investigate the effect of blending some of the itaconimides together with phenyl salicylate on the photo-stabilizing efficiency for plasticized PVC. The industrially known dioctyl phthalate (DOP) plasticizer was used for this investigation.

Materials

Part by weight

PVC Plasticizer Photo-stabilizer(s) Lubricant (IrgawaxÔ)

100 g 30 g 10 mmol 2g

The resulting solution was then poured into a Petri dish of known diameter. The films formed on complete evaporation of the solvent at room temperature were then heated in a vacuum oven at 70  C to remove any residual THF. The films were 0.1  0.02 mm thick in all cases. Blending of the stabilizers was effected in the range of 0e100% of the itaconimide stabilizer relative to phenyl salicylate reference stabilizer. The overall mixed stabilizer concentration was kept constant at 10 mmol/100 g PVC. 2.4. Photo-degradation Photo-degradation was effected in air by a fluorescent lamp (Philips TKL 05, 40 W) emitting a Gauss-shaped spectrum between 300 and 500 nm with lmax ¼ 365 nm. The light intensity at the sample surface level (7.2  1018 photons cm2 h1) was determined by actinometry [33] using a polycarbonate film calibrated with o-nitrobenzaldehyde. The exposure temperature was 40  C.

2. Experimental

2.5. Method of evaluation of the stabilizing efficiency

2.1. Materials

A continuous potentiometric method for measuring the dehydrochlorination rate has already been described [34]. The extent of discoloration of the polymer samples (irradiated for 960 min) was measured colorimetrically at l ¼ 500 nm using a PerkineElmer UV/visible Spectrophotometer, Lambda 3. The gel content of the degraded polymer samples was determined by dissolving constant weights (0.5 g) of each sample photolysed for various time intervals, in 20 ml THF at 35  C overnight. The insoluble fractions were then separated by centrifugation, washed with the solvent and dried to constant weight. The % gel content was calculated as follows:

PVC (suspension) from the National Plastic Company (Sabic, Kingdom of Saudi Arabia) with K-value of 67; phenyl salicylate from El-Nasr Company for Medicinal Chemicals (Egypt); dioctyl phthalate (DOP) from S.D. Fine-Chem. Ltd. (India); and Irga wax-280 from Ciba Specialty Chemicals were used for this investigation. Tetrahydrofuran (THF) was obtained from Prolabo Company, analytical grade (France). 2.2. Preparation of the stabilizers Two N-(substituted phenyl)itaconimide derivatives, namely N-(phenyl)itaconimide (NePhII), mp 112e113  C and N-(4-methoxyphenyl)itaconimide [Ne(4-MeOPh)II], mp 105e106  C, were prepared by the two-step procedure originally developed by Akashi [31] and Mehta et al. [32]. The elemental analyses and characteristic IR peaks of the various Ne(RPh)II derivatives agreed well with the theoretical values.

% Gel ¼ ðW2 =W1 Þ  100 where: W1 ¼ weight of the original sample and W2 ¼ weight of the insoluble fraction. Intrinsic viscosity measurements were carried out on 0.5% solution of the polymer samples, in THF, at 30  C using an Ubbelohde suspended-level dilution viscometer with negligible kinetic energy correction.

2.3. Preparation of PVC samples 3. Results and discussion Films of plasticized PVC containing either the reference stabilizer, the material under investigation, or blends of various molar ratios of them were prepared by mixing the THF solutions of the polymeric material, plasticizer, the stabilizer and Irga wax and stirring the resulting solution well to attain maximum homogeneity. The plasticized PVC films were formulated as follows:

3.1. Effect of blending itaconimides with phenyl salicylate on the photo-stabilization of plasticized PVC It was suggested that the stabilizing action of the itaconimide stabilizers involves a radical mechanism which disrupts the radical photo-degradation process of PVC through

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phenyl salicylate for 10 h at 40  C, followed by fractional crystallization. The itaconimide derivative was recovered almost quantitatively and no change in its melting point was observed. This implies that no chemical interaction had occurred between the itaconimide and phenyl salicylate. The second involves comparing the UV spectra of the blended stabilizers taken in equivalent molar ratios with those of the pure components before and after photolysis. There is no any interaction or complexation between the components of the mixtures, as no shift in the UV band at 304 nm characteristic for the CH2]CeC]O group has been observed. This provides evidence that the synergistic action obtained is the results of a combination of the different mechanisms by which the itaconimide and phenyl salicylate work, and is the results of the additive stabilizing effect of the components of the blend. Owing to the fact that both the itaconimide and the phenyl salicylate stabilizers act individually in the stabilization process, on their combination an interesting question normally arises which of the two components starts its stabilizing action first? A possible answer can be obtained from the results of the later stages of degradation, as shown in Figs. 1 and 2, the rates of dehydrochlorination in these cases are comparable with that characteristic for NePhII and Ne(4-MeOPh)II rather than for the phenyl salicylate. This behavior is detectable at various molar ratios of the components of the blended stabilizers. The different mechanism by which the investigated and the reference stabilizers work is beyond the observed synergistic effect.

trapping the radical species in the degradation process, by blocking the newly formed radical sites created on the polymer chains, and by absorption of the liberated hydrogen chloride gas. Further, the Ne(RPh)II derivatives act as UV absorbers since they absorb the UV radiation in the range of 280e400 nm with a maximum wavelength at 304 nm (within the emission of the used lamp) as indicated from their UV spectra [30]. However, phenyl salicylate is well known to act as a UV absorber. Thus, blending of these two classes of stabilizers might lead to a complementary action. Thus an attempt was made to investigate the effect of blending the itaconimide stabilizers with phenyl salicylate on the efficiency of stabilization in general. For this purpose, NePhII and Ne (4-MeOPh)II were selected for the blended stabilizer investigations. Blending was effected in the range of 0e100% of the itaconimide derivative relative to the reference stabilizer. The overall blended stabilizer concentration was kept constant at 10 mmol/100 g PVC, and the results represent the average of three comparable experiments for each stabilizer blend. The results of the dehydrochlorination rates of PVC plasticized with DOP, and photo-degraded in the presence of the blended NePhII or Ne(4-MeOPh)II with phenyl salicylate are represented in Figs. 1 and 2, respectively. The results reveal the greater stabilizing efficiency of these blended stabilizers and the existence of a true synergistic effect resulting from the combination of the itaconimide stabilizer with the reference one. The maximum synergism was achieved when the itaconimide (NePhII or Ne(4-MeOPh)II) and phenyl salicylate were blended in equimolar ratios (Figs. 1 and 2). This result would seem to support our observation made earlier [29]. This synergistic effect may be attributed to either to the fact that on blending, a chemical reaction occurs between the itaconimide and the reference stabilizer on photolysis, so that a new product of greater stabilizing efficiency is produced, or it may be due to the sum of the stabilizing effect of the components of the mixture. To investigate this, two experiments were performed; the first involves photolysis a mixture of 1:1 itaconimide either NePhII or Ne(4-MeOPh)II with

3.2. Effect of blended stabilizers on the extent of discoloration for photo-degraded plasticized PVC Fig. 3 shows the effect of blended stabilizers NePhII or Ne (4-MeOPh)II with phenyl salicylate, in various molar ratios, on the extent of discoloration of photo-degraded PVC in the presence of DOP plasticizer. All the samples were irradiated for a fixed time of 16 h. The results reveal that all the blended stabilizers show a lower extent of discoloration than the reference stabilizer, rather than the investigated stabilizers, when

Amount of HCl liberated (10-6 mol HCl / g PVC)

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Fig. 1. Effect of blended stabilizers (NePhII and phenyl salicylate) on the rates of dehydrochlorination of photo-degraded PVC in the presence of DOP plasticizer. The overall blended stabilizers’ concentration was kept constant at 10 mmol/100 g PVC.

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Amount of HCl liberated (10-6 mol HCl / g PVC)

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Irradiation time (min) Blank PVC N-(4-MeOPh)II (100%)

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Fig. 2. Effect of blended stabilizers [Ne(4-MeOPh)II and phenyl salicylate] on the rates of dehydrochlorination of photo-degraded PVC in the presence of DOP plasticizer. The overall blended stabilizers’ concentration was kept constant at 10 mmol/100 g PVC.

they are used separately. In all cases, the lowest degree of discoloration and consequently the best color stability has been achieved at equimolar ratio between the itaconimide and phenyl salicylate. This implies an additional proof for a synergistic effect of the blended stabilizers. 3.3. Effect of blended stabilizers on the extent of gelation of photo-degraded plasticized PVC The results of the extent of gelation of PVC plasticized with DOP and photo-degraded in the presence of blended NePhII or

Ne(4-MeOPh)II with phenyl salicylate are shown in Figs. 4 and 5, respectively. These results show the lower extent of gel formation in the presence of blended stabilizers as compared with both the investigated and the reference stabilizers, when they are used separately. This provides another evidence for a synergistic effect of the blended stabilizers. Again, the maximum synergism was achieved when the itaconimide and phenyl salicylate were blended in equimolar ratio. On the other hand, the lower viscosities of the soluble fractions of the irradiated polymer stabilized with blended stabilizers imply an additional proof for their synergistic action (Figs. 6 and 7).

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Fig. 3. Effect of blended stabilizers (itaconimide and phenyl salicylate) on the extent of discoloration of photo-degraded PVC for 960 min, in the presence of DOP plasticizer.

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Fig. 4. Effect of blended stabilizers (NePhII and phenyl salicylate) on the extent of gelation of photo-degraded PVC, in the presence of DOP plasticizer.

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3.4. Role of plasticizer in the photo-degradation of PVC in the presence of the blended stabilizers It is well known that different plasticizers play an important role in the photo-aging process for many polymeric systems.

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Fig. 5. Effect of blended stabilizers [Ne(4-MeOPh)II and phenyl salicylate] on the extent of gelation of photo-degraded PVC, in the presence of DOP plasticizer.

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Fig. 7. Effect of blended stabilizers [Ne(4-MeOPh)II and phenyl salicylate] on the intrinsic viscosity of the soluble fractions of photo-degraded PVC, in the presence of DOP plasticizer.

Thus, according to Williams [35] irradiating PVC plasticized with phthalate esters, with both far UV (l mainly 254 nm) and near UV (l > 280 nm) light, initiates a complex series of changes resulting in the formation of insoluble gel fraction which consisted of PVC associated with unextracted phthalate. The PVC gel was strongly coloured and was found to be dehydrochlorinated to a significant extent. Further, it is established that phthalate esters have low light stability and their degradation occurs by a radical decomposition [35]. In accordance with the aforementioned results, either in the absence or in the presence of the investigated blended stabilizers, we find an appreciable difference between the Ts values, the rate of dehydrochlorination, the extent of discoloration and the gel content of plasticized PVC and those obtained when PVC was used in its rigid form [29]. The results indicate the role played by DOP plasticizer in lowering the efficiency of the stabilization. Thus, with no doubt, that a part of the stabilizer is consumed with the radicals resulted from the photo-degradation of DOP itself which results in lowering the efficiency of PVC stabilization in general. 4. Conclusions

Irradiation time (min) Blank PVC N-PhII (100%) N-PhII + Phenyl salicylate (70%: 30%)

Phenyl salicylate (100%) NPhII + Phenyl salicylate (30%: 70%) N-PhII + Phenyl salicylate (50%: 50%)

Fig. 6. Effect of blended stabilizers (NePhII and phenyl salicylate) on the intrinsic viscosity of the soluble fractions of photo-degraded PVC, in the presence of DOP plasticizer.

Blending of the itaconimide stabilizers with phenyl salicylate reference stabilizer showed a true synergistic effect which is illustrated by a remarkable improvement in the Ts values, the rates of dehydrochlorination, the degree of discoloration, as well as the extent of gelation of the photo-degraded polymer and this improvement attains its maximum when both the investigated and the reference stabilizers are taken in

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equimolar ratios. The observed synergistic effect is most probably attributed to the combination of the mechanisms by which the itaconimides and the reference stabilizer work. Thus, while the investigated stabilizers work through radical trapping and blocking the radical sites on the polymer, the reference one absorbs the UV radiation. From the above, it is possible to recommend the use of itaconimides as co-stabilizers with phenyl salicylate UV absorber for plasticized PVC against photo-degradation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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