Nanocomposites of irradiated polypropylene with clay are degradable?
Introduction
Use of radiation to modify the molecular structure and enhancing strain hardening of PP melt, allows to use in areas such as foaming, thermoforming, extrusion coating and blow molding (Gotsis et al., 2004). The high melt strength of a polymer is either due to long chain branching or high molecular weight (Graebling, 2002).
The chemical modification initiated by radicals and the grafting reactions of different monomers on to polypropylene (iPP) in the solid state with the use of grafting at low reaction temperatures, γ, or electron-ray scattering or special peroxides were studied by Rätzsch et al. (2002). Polypropylene irradiated with electrons or gamma-rays undergoes chain scission, and the macroradicals generated form branched macromolecules. Information on the irradiation modification of the molecular structure of PP is still limited in the literature, especially with respect to the controlled generation of long-chain-branches (LCB) (Auhl et al., 2012).
The research group of Prof. Busfield and Appleby (1986), reported the effects of crosslink enhancement by the presence of acetylene during gamma irradiation, on the physical and mechanical properties of polypropylene.
Jones and Ward (1996), in experimental analysis of the radiation-induced crosslinking of a linear low density polyethylene (LLDPE) film, measured gel fractions as a function of dose, both in vacuum and in presence of acetylene gas and in fact crosslinking was accelerated by the use of acetylene.
Grafting of long-chain-branches on PP backbone using acetylene as a crosslink promoter was developed for production of branched PP under gamma radiation process and was reported (Lugao et al., 2007, Oliani et al., 2012).
Concerning polymer nanocomposites (PNCs) area, it has been attracted substantial scientific interest and developments over the last two decades with a huge market opportunity especially for the automotives and packageing industries (Okamoto, 2006). Composites of HMSPP with montmorillonite have few references in the literature (Bhattacharya et al., 2009), mainly considering the aspect of stability under weather conditions or thermal exposition. Polymers exposed to outdoors can degrade through the action of several agents, including solar ultraviolet (UV) radiation; water; pollutants (in gaseous form or, more potently, as acid-rain); elevated temperature. In a majority of cases, the main cause of property deterioration is photo-oxidation, which is initiated by UV irradiation and, as a consequence, much laboratory photo-ageing testing was conducted to determine the weatherability of polymers and to test the effectiveness of stabilizers to improve their weather resistance (White, 2006, Singh and Sharma, 2008, Rivaton et al., 2005, Attwood et al., 2006, Oliani et al., 2010).
Polypropylene undergoes predominantly chain scission under all processing conditions giving rise to pronounced reduction in the molar mass and melt viscosity of the polymer (Al-Malaika, 2003) other than melting temperature and normally increasing of cristallinity.
Abiotic peroxidation of the polyolefins, gives rise to some vicinal hydroperoxides and this process is particularly favored in the poly-α-olefins, such as polypropylene due to the susceptibility of the tertiary carbon atom to hydrogen abstraction via a hydrogen-bonded intermediate. A major proportion of the peroxide product are hydrogen-bonded vicinal hydroperoxides that break down to small biodegradable molecules such as carboxylic acids, alcohols and ketones (Wiles and Scott, 2006, Morlat et al., 2004).
One of the main aspects in the development of polymer nanocomposites is the matrix degradation. In a work of Ramos Filho et al., (2005) a polycationic bentonite clay was added to isotactic polypropylene (iPP). The compounds were prepared by melt intercalation using a twin extruder, similar to work of Komatsu et al., 2014a, Komatsu et al., 2014b. Under 110 °C for up to 165 h, the degradation of the composites was more intense than the unfilled polymer and this may be due to the presence of acidic sites on the clay surface that act as a catalyst to the polymer oxidation, and/or due to salt decomposition, initiating the free radical degradation of iPP.
The aim of present work was to understand the degradation effects of HMSPP/Clay nanocomposites, under thermal and environmental conditions, considering the new morphology created by the irradiation in acetylene, and the presence of clay.
Section snippets
1 Materials and methods
The iPP pellets were manufactured by Braskem and compatibilizer agent, propylene maleic anhydride graft copolymer (PP-g-MA) was supplied by Addivant (Polybond 3200). The clay filler was Cloisite 20A by Southern Clay Products montmorillonite clay quaternary ammonium salt-modified (95 Meq/100 g). The iPP was placed in plastic bags with acetylene that were irradiated in 60Co gamma source at dose of 12.5 kGy in order to obtain the HMSPP. The iPP utilized in this work has no nucleating agent or
Results and discussion
To understand the effects of environmental and thermal ageing in the polypropylene nancomposites, the X-ray fluorescence technique was utilized to evaluate the clay composition, as represented in Table 2.
In Table 2 is observed presence of pro-oxidant elements as: Al, Fe and Mg. In the literature is reported that low quantities of these elements acts as oxidants of polymers by redox reactions with hydroperoxides (Morlat-Therias et al., 2005).
Results of FT-IR are presented in Fig. 2 while
Conclusions
The nanocomposites were obtained with intercalation of the polymer in the lamellar structure of the clay.
The nanocomposites were aged in two different conditions: thermal and environmental. Compared to the H1, iPP modified by irradiation process, the nanocomposites environmental exposition, have the metallic ions that promove the surface oxidation in the early periods of exposition (up to 4 month). Because the competitive mechanism of the “screening effect”, where the clay lamellae acts as a
Acknowledgments
The authors thank CAPES, Brazil; Fapesp, Brazil (2014/26393-1) and CNPQ, Brazil for grants in SWB program, Centre of Science and Technology of Materials – CCTM/IPEN, for microscopy analysis (SEM), the technicians Mr. Eleosmar Gasparin and Nelson R. Bueno, for technical support, Southern Clays, Addivant and Companhia Brasileira de Esterilização (CBE) for irradiating the samples.
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