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Reduction of environmental and health impact of soft PVC

Involved People: Alfonso Maffezzoli, Antonio Greco, Giuseppe Mele
Soft PVC is employed for the manufacturing of a wide range of products with different properties and a relatively low cost. The utilization of soft PVC is restricted by the poor thermal, chemical and mechanical resistance properties. Phthalates, adipates, and phosphates are the main compounds used as plasticizers. Some of these compounds, such as bis-(2-ethylhexyl) phthalate (DEHP), can be toxic when extracted from the polymer by the action of some solvent. At the same time, plasticizer migration can modify the properties or can make useless the materials for some applications because of a general loss of properties. As a consequence of this, in recent years phthalates have been subject of severe revisions, and some limitations in their use. In Europe three phthalates, among them DOP, have been banned for the production of toys which can come in contact with the mouth of infants. Nevertheless, most soft PVC still are produced with phthalates, due to their high versatility and low costs. The aim of the research activity is the reduction of the health impact of soft PVC, which is attained by reducing the migration of plasticizer by chemical modification of the PVC or by use of non toxic plasticizer. The main research activities are reported:
  • Chemical modification of PVC by reactive processing. PVC polymer can be efficiently crosslinked by different chemical species or high energy irradiation. Crosslinking of the PVC introduces covalent bonds between polymeric chains, thus reducing the mobility of diffusing species inside the polymer matrix. Crosslinking has been performed by means of di-functional amines. During thermal treatment of the PVC plastisol, a sharp viscosity increase can be observed when the plasticizer is absorbed in the polymer, as reported in Figure 1. The presence of the crosslinking agent, which introduced covalent bonds between polymer chains, is responsible of a further viscosity increase when crosslinking takes place, as also reported in Figure 1. The incorporation of 2 phr crosslinking agent involves a gel content of 70%. The incorporation of the crosslinking agent also involve a significant weight loss during thermal treatment, which is due to degradation of the polymer during crosslinking. This is due to the fact that HCl, which catalyzes degradation of PVC, is formed during crosslinking. The use of peroxides as crosslinking agents involves a reduced thermal stability with respect to amine crosslinked PVC, and a significant degradation of the polymer during reactive processing. Results obtained in the present activity show that crosslinking usually involves an unacceptable viscosity increase of the polymer, which is very often accompanied by some degradation of the polymer matrix.
  • Derivation of PVC plasticizer from renewable resources. Plasticizers for PVC can be obtained through the chemical modification of cardanol, a prepolymer deriving from the Cashew Nut Shell Liquid (CNSL) a natural resources with low cost and low toxicological impact. In particular CNSL, a by product of the production of the cashew nut (anacardium occidentalis), is characterized by costs compatible with those of the commercial full synthetic phthalate plasticizers. The high polarity of cardanol molecule, due to the presence of the -OH group bond to the benzene ring, makes it unsuitable as primary plasticizer. Cardanol could be used as secondary plasticizer for PVC, in combination with other plasticizers, to reduce the costs. Chemical modifications of cardanol can increase its compatibility with PVC: esterification and epoxiydation have been successfully attempted. Tg and viscosity evolution of PVC plasticized with the new plasticizers and with DOP are very close, as shown in Figure 2 and Figure 3. Mechanical properties of soft PVC obtained with new plasticizer are comparable with those obtained on DOP plasticized PVC, as reported in Figure 4. Thermal stability is improved by the use of the new plasticizers (Figure 5).

Figure 1: evolution of viscosity during thermal treatment of soft PVC using DOP and crosslinking agent (ref 1)
Figure 2: evolution of glass transition temperature as a function of plasticizer content for new and phthalate plasticizer (ref 6)

Figure 3: evolution of viscosity during thermal treatment for soft PVC obtained with new and phthalate plasticizer (ref 6)

Figure 4: stress-strain curves for soft PVC obtained with new and phthalate plasticizer (ref 6)

Figure 5: TGA curves for soft PVC obtained with new and phthalate plasticizer (ref 6)


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University of Salento   FacoltÓ di Ingengeria    Department of Engineering for Innovation