Crosslinking of rigid PVC by ionizing radiation to improve its thermal properties C. Garc´ ıa-Castan ˜eda a , R. Benavides a,Ã,1 , M.E. Mart´ ınez-Pardo b , R.M. Uribe c , H. Carrasco-  Abrego b , G. Mart´ ınez a a Centro de Investigaci´ on en Qu´ ımica Aplicada, Blvd. Enrique Reyna 140, Saltillo Coahuila, Mexico b Instituto Nacional de Investigaciones Nucleares, Apartado postal 18-1027, Col. Escand´ on, 11801 M exico, D.F., Mexico c Kent State University, College of Technology and Program on Electron Beam Technology, P.O. Box 5190, Kent, OH 44242-0001, USA article info Keywords: PVC Crosslinking Ionizing radiation abstract Rigid PVC formulations containing two different stabilizer systems (tin and Ca/Zn) and TMPTMA as a crosslinking agent were treated with ionizing radiation (gamma and electron beam) at different doses and irradiation atmospheres. The objective was to increase thermal and mechanical properties of this material. Polyene formation was followed through the yellowing index (YI), the extent of crosslinking by gel percentage, thermal resistance by Vicat temperature and the mechanical properties by DMA. Both formulations became colored with irradiation, especially with gamma as a result of a longer treatment time; the gel formation and the Vicat temperature were also higher for gamma treated samples, suggesting that values were enhanced by oxidation. However, DMA elastic modulus traces were almost similar for both treatments. The main difference observed for Ca/Zn samples compared with traditional tin samples was the lower ability of the former system in protecting the material against processing conditions. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction PVC is a thermoplastic polymer of major consumption due to its low production and processing costs. It is used intensively in rigid form especially for hydraulic pipes; however, in case of hot water applications its use is restricted since PVC is highly susceptible to thermal degradation during which it produces sequences of double bonds (polyenes) and releases HCl (Endo, 2002). A possible solution to enhance thermal resistance as well as other physical properties consists in crosslinking the polymer by means of physical or chemical methods. Radiation crosslinking of polymers is a method commonly used in industry because of its capability to induce reactions in the solid state at low tempera- tures and at high rates of processing (Castan ˜eda Facio et al., 2004, 2007 with references therein). At present the use of radiation (high energy) in the polymer area is employed to induce several chemical processes such as polymerization and graft reactions. Nevertheless, from a practical point of view the most important reaction that is obtained when polymers are irradiated is the crosslinking of the chain molecules (Bhattacharya, 2000; Singh, 2001). Considering that PVC has a low yield of chemical crosslinking during irradiation, one must use polyfunctional monomers capable of promoting the crosslinking of the polymer chains. The most effective compounds for such a purpose are the polyfunctional acrylates and metacrylates, especially trifunctional monomers such as trimethylolpropane trimetacrylate (TMPTMA), an usual crosslinking agent, which produces a high concentration of radicals during irradiation (Castan ˜eda Facio et al., 2004; Nethsinghe and Gilbert, 1988). 2. Experimental 2.1. Materials An industrial PVC resin of K value 65–67 was used to prepare formulations. The polyfunctional monomer used was TMPTMA obtained from Sartomer. The main stabilizers were butyl tin mercaptide for formulation 1 and a mixture of Ca/Zn stearates (2/1%w) for formulation 2, both of industrial grade. Tetrahydro- furan (THF) was used as a solvent in the extraction, to evaluate gel percentage. Other components as the filler (CaCO 3 ) and pigment (TiO 2 ) were industrial grade. Description of the components can be seen in Table 1 . ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.08.004 Ã Corresponding author. Fax: +52 844 438 9839. E-mail address: robertob@ciqa.mx (R. Benavides). 1 Sabbatical leave at UNESC, Criciuma (2008–2009), Brasil. Radiation Physics and Chemistry 79 (2010) 335–338