New photostabilizers for polystyrene based on 2,3-dihydro-(5-mercapto-1,3,4-oxadiazol-2-yl)-phenyl-2-(substituted)-1,3,4-oxazepine- 4,7-dione compounds
© Yousif et al.; licensee Springer. 2013
Received: 15 December 2012
Accepted: 27 February 2013
Published: 12 March 2013
The photostabilization of polystyrene (PS) films by 2,3-dihydro-(5-mercapto-1,3,4-oxadiazol-2-yl)-phenyl-2-(substituted)-1,3,4-oxazepine-4,7-dione compounds was investigated. PS films containing concentration of complexes 0.5% by weight were produced by the casting method from chloroform as a solvent. The photostabilization activities of these compounds were determined by monitoring the carbonyl and hydroxyl indices with irradiation time. The changes in viscosity average molecular weight of PS with irradiation time were also tracked (using benzene as a solvent). The quantum yield of the chain scission (Φcs) of these complexes in PS films was evaluated and found to range between 3.31 × 10-6 and 7.89 × 10-6. Results obtained showed that the rate of photostabilization of PS in the presence of the additive follows the trend (I > II > III > IV). According to the experimental results obtained, several mechanisms were suggested depending on the structure of the additive like UV absorption, peroxide decomposer and radical scavenger.
Polystyrene is one of the important commercial polymers, widely used in various industrial fields. One of the important uses of PS is in the manufacture of cover signals lamp of some automobiles. PS is subjected to the irradiation of sunlight on outdoor exposure (Safy & El-Laithy, 1994).
Many polymers undergo thermal oxidative degradation during processing. Over longer periods at ambient temperature polymers also deteriorate in the solid state through autooxidation and photooxidation. In outdoor applications where the materials are exposed to UV solar radiation, the energy of this radiation is sufficient to initiate photochemical reaction leading to degradation. Plastics are commonly protected against such deterioration by the addition of antioxidants, light and heat stabilizers (Yousif et al. 2010).
There is a great interest at present in the photo-oxidative degradation of polymeric materials because macromolecules have increasingly widespread commercial applications. Synthetic, semisynthetic and natural polymers undergo degradation when exposed to the natural (Grassie & Scott, 1985).
All commercial organic polymers degrade in air when exposed to sunlight as the energy of sunlight is sufficient to cause the breakdown of polymeric C-C bonds as a consequence of degradation. The resulting smaller fragments do not contribute effectively to the mechanical properties and the polymeric article becomes brittle. Thus the life of thermoplastics for outdoor applications becomes limited due to weathering (Andrady et al. 1998).
Almost all synthetic polymers require stabilization against adverse environmental effects. It is necessary to find a means to reduce or prevent damage induced by environmental components such as heat, light or oxygen. This can be achieved through addition of special chemicals, light or UV stabilizers, that are selected to be compatible with the resin and the specific application considered. The photostabilization of polymers may be achieved in many ways. The following stabilizing systems have been developed, which depend on the action of stabilizer. a) Light screeners. b) U.V. absorbers, c) Excited state quenchers, d) Peroxide decomposers and e) Free radical scavengers, of these it is generally believed that types c), d) and e) are the most effective.
There has been no attempt to investigate the photostabilization of PS films using 1,3-oxazepine compounds containing 1,3,4-oxadiazole units. The design of 1,3-oxazepine compounds and their use as photostabilizing agents for polystyrene are reported herein.
Results and discussion
2,3-dihydro-(5-mercapto-1,3,4-oxadiazol-2-yl)-phenyl-2-(substituted)-1,3,4-oxazepine- 4,7-dione compounds have been used as additives for the photo stabilization of PS films. To assess the effectiveness of these additives for the photostabilization of PS films changes in the infrared spectra of these materialswere monitored as a function of irradiation time at 313 nm. This irradiation altered the structure of the polymer as noted by distinct changes in the spectra. Most notable was the appearance of absorption bands characteristic of carbonyl (1720 cm-1) and hydroxyl groups (3450 cm-1). (Andrady & Searle, 1989).
I) Variation of Ps molecular weight during photolysis in the presence of by 1,3-oxazepine compounds
Where m is the initial molecular weight.
The values of α of the irradiated samples are higher when additives are absent and lower in the presence of additives compared to the corresponding values of the additive free PS. In the initial stages of photodegradation of PS, the value of α increases with time, these indicators indicates a random breaking of bonds in the polymer chain.
Quantum yield (Φcs) for the chain scission for PS films (40 μm) thickness with and without 0.5 (wt/wt) additive after 250 hrs irradiaton time
Quantum yield of main chain scission (Φcs)
PS + I
PS + II
PS + III
PS + IV
The explanation for low values of Φcs is that in macromolecule of PS, the energy is absorbed at one site, and then the electronic excitation is distributed over many bonds so that the probability of a single bond breaking is small, or the absorbed energy is dissipated by non reactive processes.
II) Suggested mechanisms of photostabilization of Ps by 1,3-oxazepine compounds
I) Films preparation
Commercial polystyrene supplied by Petkim Company (Turkey) was re-precipitated from chloroform solution by alcohol several times and finally dried under vacuum at room temperature for 24 hours. Fixed concentrations of polystyrene solution (5 g/100 ml) in chloroform were used to prepare polymer films with 40 μm thickness (measured by a micrometer type 2610 A, Germany). The films were prepared by evaporation technique at room temperature for 24 hours (Sastre et al. 1990). To remove the possible residual chloroform solvent, film samples were further dried at room temperature for three hours under reduced pressure. The films were fixed on stands specially used for irradiation. The stand is provided with an aluminum plate (0.6 mm in thickness) supplied by Q-panel company.
II) Irradiation experiments
Accelerated testing technique Accelerated weatherometer Q.U.V. tester (Q. panel, company, USA), was used for irradiation of PS films. The accelerated weathering tester contains stainless steel plate, which has two holes in the front side and a third one behind. Each side contains a lamp (type Fluorescent Ultraviolet Lights) 40 Watt each. These lamps are of the type UV-B 313 giving spectrum range between 290–360 nm with a maximum wavelength 313 nm. The polymer film samples were vertically fixed parallel to the lamps to make sure that the UV incident radiation is perpendicular to the samples. The irradiated samples were rotated from time to time to ensure that the intensity of light incident on all samples is the same.
III) Photodegradation measuring methods
As = Absorbance of peak under study
Ar = Absorbance of reference peak
Is = Index of group under study
Determination of average molecular weight using viscometry method.
[η] = the intrinsic viscosity
K, α are constants depending upon the polymer-solvent system at a particular temperature.
C = Concentration of polymer solution (g/100 ml).
Where: C = concentration; A = Avogadro’s number; = the initial viscosity–average molecular weight; [ηo] = Intrinsic viscosity of PS before irradiation; Io = Incident intensity and t = Irradiation time in second.
In the work described in this paper, the photostabilization of PS films using 2,3-dihydro-(5-mercapto-1,3,4-oxadiazol-2-yl)-phenyl-2-(substituted)-1,3,4-oxazepine4,7-dione compounds were studied. These additives behave successfully as photostabilizer for PS films. These additives stabilize the PS films through UV absorption or screening, peroxide decomposer and radical scavenger mechanisms. The compound I was found to be the most efficient in photostabilization process according to the photostability and mechanisms mentioned above. These mechanisms support the idea of using 1,3-oxazepine compounds
The authors would like to thank University of Al-Nahrain, and Universiti Kebangsaan Malaysia for research grant UKM-GUP-NBT-08-27-113 and UKM-OUP-2012-139 and for financial support and technical assistance on this work.
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