Structure and physicomechanical properties of linear polyethylene containing a ZnO ZnS mixture under UV irradiation

Transcription

1 Plasticheskie Massy, No. 6, 2007, pp. 6 9 Structure and physicomechanical properties of linear polyethylene containing a ZnO ZnS mixture under UV irradiation V. P. Gordienko and V. G. Sal nikov State Southern Scientific Research Test Range, Russian Academy of Sciences Selected from International Polymer Science and Technology, 35, No. 1, 2008, reference PM 07/06/06; transl. serial no Translated by P. Curtis On the basis of the results of earlier research [1, 2] it can be assumed that the introduction into linear polyethylene (PE) of combined photostabilisers (for example, metal oxide/sulphide) should lead to considerable shielding of the structure and physicomechanical properties of thermoplastic materials during UV irradiation. The aim of the present work was to investigate the combined effect of zinc oxide and zinc sulphide on the structure, strength, and deformation properties of linear PE under UV irradiation. The investigation was conducted on high-density linear polyethylene, the viscosity-average molecular weight of which was , the degree of crystallinity 54%, the melting temperature 402 K, and the density 958 kg/m 3. Dispersed zinc oxide and zinc sulphide were used as the additives. The particle size of the additives was smaller than 1 µm. Composites containing vol.% of the given additives were obtained by homogenising filled PE melts at a temperature of 453 K. Such a mixing regime enables composites with a high degree of homogeneity to be produced [3]. A polymer not containing additives was treated by a similar regime. The specimens for investigating the resistance of composites to UV irradiation comprised films of 200 ± 10 µm thickness, obtained by hot pressing. Film specimens were subjected to the unfiltered radiation of a DRT-1000 mercury quartz lamp in an air atmosphere with periodic flipping [4]. The parameters of the supermolecular structure of PE were determined by the X-ray diffraction method on a DRON-3 diffractometer (degree of crystallinity and crystallite fold height) and by optomicroscopic analysis in polarised light on a MIN-8 instrument (average spherulite size). The change in the composition and structure of the PE macromolecules was assessed from the amount of absorbed oxygen, from the content of carbonyl groups, and from the reduction in the molecular weight of the polymer during UV irradiation. The process of oxygen absorption was studied using a laboratory vacuum unit equipped with a differential liquid manometer [5]. The content of carbonyl groups and carboxylates [6] was determined on the basis of the optical densities of the absorption bands at 1720 and 1596 cm -1 respectively (using a UR-20 IR spectrophotometer, differential recording method). The relative viscosity-average molecular weight was determined on a laboratory capillary in a solution of 1-chloronaphthalene at a temperature of 403 K. Additive particles were removed beforehand by filtering of the hot solutions. As the physicomechanical properties of the composites, the tensile stress causing failure σ t and the breaking elongation ε b were assessed; the elongation rate of the specimens was 6.6 x 10-4 m/s. The effect of UV irradiation on the brittleness of the thermoplastic materials was assessed from the time at which the breaking elongation of the irradiated specimens amounted to 10% of the breaking elongation of the initial specimens, τ brittle. Investigation of non-irradiated PE specimens with introduced additives showed that, in contrast to some systems studied earlier [1], the above additives hardly affect the change in the structure of the polymer on the molecular level, and the change in the supermolecular structure is also insignificant. The data obtained (Table 1) indicate only that the additives introduced (up to 6 vol.%) 2008 Smithers Rapra Technology T/27

2 Table 1. Dependence of parameters of supermolecular structure of PE on content of additives Structural parameters Additives,* vol.% D spher, * m ZnO ZnS ZnO + ZnS (1:1) K,* % ZnO ZnS ZnO + ZnS (1:1) L 002, * nm ZnO ZnS ZnO + ZnS (1:1) * D spher, average size of spherulites; K degree of crystallinity; L 002 size of crystallites; content of additives Table 2. Effect of UV irradiation on molecular weight (x 10-3 ) of PE containing 2 vol.% additives t irr, *h PE PE + additives ZnO ZnS ZnO + ZnS (1:1) * t irr is the time from the start of UV irradiation promote slight amorphisation (3 4%), a certain reduction in spherulite size, and an increase in the defectiveness of the polymer crystallites. Such insignificant changes in crystal structure should not have any significant effect on transformation of macromolecules under UV irradiation. In spite of the actual identity of the structure of nonirradiated PE specimens, the rates of photooxidation and photodegradation of the initial polymer and of the polymer with additives (optimally 2 vol.% [1, 2]) differ appreciably under UV irradiation. Oxygen absorption and the accumulation of carbonyl groups increases with increasing UV irradiation time of the initial PE and of PE containing 2 vol.% additives, but there is a general tendency towards slowing down of the indicated processes in the order PE > ZnS > (ZnS + ZnO) > ZnO (Figure 1). From Table 2 it can also be seen that reduction in the molecular weight of PE specimens occurs in accordance with the intensity of oxygen absorption and carbonyl group accumulation. Most resistant to photodegradation is PE containing zinc oxide, and least resistant are the initial polymer and the polymer containing zinc sulphide. The mixture of zinc oxide and zinc sulphide in a ratio of 1:1 occupies an intermediate value for all investigated parameters of the molecular structure of the UV-irradiated PE specimens (Figure 1; Table 2). It must be pointed out that the oxygen absorption and the accumulation of oxygen-containing groups, and also the reduction in molecular weight, in PE under UV Figure 1. Dependence of amount of absorbed oxygen (a) and optical density of 1720 cm -1 absorption band (b) on UV irradiation time of initial PE and of PE containing 2 vol.% ZnS, 2 vol.% ZnO + ZnS (1:1), and 2 vol.% ZnO T/28 International Polymer Science and Technology, Vol. 35, No. 6, 2008

3 irradiation for specimens containing a ZnO ZS mixture are not proportional to the components, and their values are closer to those of PE with 2 vol.% ZnO. This indicates the predisposition of the given material towards greater resistance to photooxidation and photodegradation than would follow from the additive quantities (Figure 1; Table 2). Also characteristic is the fact that an identical molecular weight is possessed by specimens of PE with 2 vol.% of ZnS after UV irradiation for 20 h, PE with 2 vol.% of a ZnO + ZnS mixture (1:1) after UV irradiation for 60 h, and PE with 2 vol.% of ZnO after UV irradiation for 100 h. The change in the supermolecular structure of the initial PE and of PE containing 2 vol.% of additives under UV irradiation is insignificant but has common trends: increase in spherulite size and in degree of crystallinity, and reduction in crystallite fold height of the polymer (Table 3). The greatest stability of the parameters of the supermolecular structure of PE under UV irradiation of specimens is observed for the polymer containing zinc oxide, and the least stability is observed in the initial PE and in the PE containing zinc sulphide. Such a position is consistent with the change in structure of the polymer on the molecular level (Figure 1; Tables 2 and 3). In the presence of zinc oxide, the parameters of the supermolecular structure of PE changed negligibly during UV irradiation for 100 h, evidently as a consequence of the fact that this additive effectively slows down the process of photodegradation of the macromolecules. It is well known [7] that, with exposure of PE to UV irradiation, ordering of the relatively short and mobile fragments of the amorphous chains formed during photodegradation can occur, with the emergence of a new crystalline phase (crystallites with an imperfect internal structure). Reduction in the intensity of photodegradation may lead to a reduction in the yield of relatively low molecular weight fragments of macromolecules responsible for the formation, during UV irradiation, of additional crystallites of small size, on account of which there is a reduction in the average size of the PE crystallites, an increase in total crystallinity, and growth in polymer spherulites (Table 3). The photooxidative degradation of macromolecules, accompanied with the formation of additional highly defective crystalline regions in the structure of the irradiated polymer and with growth of its spherulites, leads to deterioration in the physicomechanical characteristics of such a polymer [1, 2]. On the other hand, reduction in the intensity of oxidation and degradation processes of PE and also the formation of chemical compounds at the interface of components during UV irradiation of a polymer containing additives [6] should lead to stability of the properties of the materials over a fairly long time. The experimental data obtained (Figure 2) confirm the position set out above. Figure 2. Dependence of tensile stress causing failure (a) and breaking elongation (b) on UV irradiation time of initial PE and of PE containing 2 vol.% ZnS, 2 vol.% ZnO + ZnS (1:1), and 2 vol.% ZnO Table 3. Parameters of supermolecular structure of PE containing 2 vol.% additives before (numerators) and after (denominators) UV irradiation for 100 h Structural parameters PE PE + additives ZnO ZnS ZnO + ZnS (1:1) D spher, * m K,* % L 002, * nm * D spher average size of spherulites; K degree of crystallinity; L 002 size of crystallites 2008 Smithers Rapra Technology T/29

4 In the given case it can be seen that all of the additives indicated lead to stabilisation of the strength and deformation properties of PE under uniaxial elongation in the process of UV irradiation. This is particularly marked after UV irradiation for h. In this case, embrittlement of the initial polymer and a sharp fall in its strength occur. It is also notable that a ZnO ZnS mixture in a 1:1 ratio is closer to ZnO than to ZnS in terms of its protective action on PE against UV irradiation, which is in qualitative agreement with changes in the molecular structure of these polymer specimens (Figure 1; Table 2). In this context it was of interest to clarify the influence of different ZnO and ZnS ratios on the properties of PE before and after UV irradiation (Table 4). An analysis of the results given above indicates a general tendency towards a fall in strength and deformation of PE with increase in additives to 6 vol.%. Here, this tendency is maintained both for individual additions of ZnO and ZnS and for mixtures in any combination. During the UV irradiation of specimens of linear PE for up to 100 h, stabilisation of the strength and deformation properties under uniaxial elongation of PE is observed, and there is an increase in the irradiation time before the onset of brittleness of the filled polymer. The most stable properties are possessed by PE composites containing 3 5 vol.% of zinc compounds. A PE composite containing a mixture of 2% ZnO and 2% ZnS, which retains its deformability under UV irradiation for over 480 h, is noted for particular stability of its properties. Furthermore, after UV irradiation for 100 h, this composite is superior to all the others in terms of the values of the tensile stress causing failure and breaking elongation (Table 4). The results obtained can be explained on the basis of concepts of the formation of zinc carboxylates [6] and the photochemical mechanism of protection of polymers against UV irradiation [8]. From these studies it follows that the formation of chemical compounds at the interface of components in the filled polymer should lead to strengthening of the composite materials and their stability in time, especially under conditions of the action of different aggressive factors [9]. From the data given in Table 5 it can be seen that the optical density of PE absorption at 1596 cm -1, corresponding to the content of zinc carboxylates formed at the interface of components [6], after UV irradiation for 80 h is most significant for a polymer containing 2% ZnO and 2% ZnS. Detailed comparative analysis of the results of the investigation, given in Tables 4 and 5, indicates that the strength and deformation properties under uniaxial elongation of UV-irradiated composites of linear PE are dependent on the content of chemical compounds (in the present case, zinc carboxylates) formed at the interface of the components. Here, the addition to the polymer of a mixture of 2% ZnO and 2% ZnS leads, during UV irradiation of the composite, to synergism in relation to Table 4. Influence of content of additives on tensile stress causing failure and breaking elongation before irradiation and after irradiation for 100 h, and brittleness of PE Content of additives σ t, MPA ε b, % 100 σ t, MPa 100 ε b,% τ brittle, h ZnO ZnS T/30 International Polymer Science and Technology, Vol. 35, No. 6, 2008

5 Table 5. Influence of content of additives on optical density of 1596 cm -1 absorption band of PE after UV irradiation for 80 h Content of additives, % D 1596 ZnO ZnS the stability of these properties of the materials. Thus, it has been established that a promising direction in the shielding of thermoplastic materials from UV irradiation may be the introduction into these materials of combined additives (of the metal oxide/sulphide type). Here, in a number of cases, synergism of the additives introduced, in relation to the stability of the structure and physicomechanical characteristics of thermoplastic materials under UV irradiation, is realised. REFERENCES 1. V. P. Gordienko and Yu. A. Dmitriev, The degradation and stability of polyethylene with small additions of metal oxides under UV irradiation. Polymer Degradation and Stability, 53, No. 1, 1996, pp V. P. Gordienko and V. G. Sal nikov, Effect of sulphides of certain metals on the structure and properties of linear polyethylene under UV irradiation. Plast. Massy, No. 11, 2005, pp V. P. Gordienko, Radiation modification of composite materials based on polyolefins. Naukova Dumka, Kiev, 1985, 176 pp. 4. P. V. Zamotaev et al., Features of the crosslinking of copolymers of ethylene and vinyl acetate. Khimiya Vysokikh Energii, No. 5, 1989, pp N. Grassie and N. A. Weir, The photooxidation of polymers. 1. Experimental methods. J. Appl. Polym. Sci., 9, 1965, pp V. P. Gordienko and Yu. A. Dmitriev, Formation of zinc carboxylates in a polyethylene zinc oxide system under UV irradiation. Vys. Soed., B37, No. 5, 1995, pp V. M. Aslanyan et al., Plast. Massy, No. 10, 1989, pp B. Ranby and J. F. Rabek, Photodegradation, photooxidation, and photostabilisation of polymers. Mir, Moscow, 1978, 676 pp. 9. E. M. Pludeman, Interfaces in polymer composites. Mir, Moscow, 1978, 296 pp Smithers Rapra Technology T/31