Preparation And Synergism Effect Of ZnO-mannitol Complexes As Environmentally-friendly PVC Stabilizers

Poly(vinyl chloride) (PVC) is widely used in construction, chemical engineering, electronics and packing material applications, because PVC has many excellent performance, such as superior mechanical, insulating properties, high chemical resistance, flame resistance and low-permeability to gas and moisture. However, owing to its poor thermal stability, thermal stabilizers must be added to PVC during processing. Currently, the commercial thermal stabilizers mainly include lead salts, organotin and metal soaps. Due to the strong toxicity of lead salts and high price of organotin, Ca/Zn stabilizers with low toxicity and low price has been well developed in recent years. However, because of their marked "zinc burning" effect Ca/Zn stabilizers have some defects in long-term thermal stability. With the increasing awareness of environmental protection, it has been highly desirable to explore environmentally-friendly stabilizers with high thermal stability and non-toxicity.In this study, a series of zinc oxide-mannitol complexes (Zn-Ma complex) with different zinc contents were prepared from zinc oxide and mannitol. The element content, chemical composition, crystal structure, thermal performance and morphology of Zn-Ma complexes were characterized by elemental analyzer, atomic absorption spectrometer, Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometer, thermogravimetric analyzer (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM).The components in Zn-Ma complexes were separated by extraction method, and then characterized by FT-IR and atomic absorption spectrometer. The results showed that Zn-Ma complexes consisted of zinc-mannitol alkoxide,zinc oxide and unreacted mannitol, and the content of these components were determined as well.The thermal stability of PVC compounds stabilized by Zn-Ma complexes were systematically investigated by Congo red method, dehydrochlorination method, discoloration tests and torque rheological tests. It was found that the obtained Zn-Ma complexes showed high thermal stability and good lubricity. Amongst the Zn-Ma complexes, Zn-Ma5 and Zn-Ma7 complexes with proper zinc content showed good thermal stability and their thermal stability is better than that of most commercial thermal stabilizers. Based on the results of thermal stability, thermal stabilization mechanism of Zn-Ma complexes was proposed in three aspects:a) replacement of the liable chloride, b) absorption of hydrogen chloride,c) inhibition of zinc burning by chelating with zinc chloride.In order to improve the comprehensive performance of Zn-Ma complexes, calcium stearate (CaSt2), dibenzoyl methane (DBM), epoxidized soybean oil (ESO) and phenyl diisodecyl phosphite (PDDP) were used as auxiliary thermal stabilizers and compounded with Zn-Ma complexes. When CaSt2, DBM, ESO or PDDP was added, the Zn-Ma complexes showed satisfactory performance on inhibiting the initial coloration, long-term stability and static thermal stability due to the synergism effects between Zn-Ma complexes and the auxiliary thermal stabilizers. Besides, the internal lubricity of PVC was improved by the auxiliary thermal stabilizers.