Design, Preparation, Application And Stabilizing Mechanism Of Urea Derivatives As Novel Thermal Stabilizers For Poly(Vinyl Chloride)

It is well known that poly(vinyl chloride)(PVC) is an important thermoplastic material on account of its versatility and low cost. However, PVC always undergoes severe thermal degradation at a relatively low temperature, resulting in the deterioration of the polymer properties. Therefore, thermal stabilizers are necessarily added to protect it from thermal degradation during its processing and applications. At present, lead salts, metal soaps, and organotin compounds are the main kinds of PVC thermal stabilizers. Although these conventional stabilizers have great stabilization, they should be restricted when applied in PVC on account of their toxicity. As a consequence, it has been highly desirable to explore an environmentally friendly stabilizer with non-toxicity and high efficiencyRecently, the attention of many investigators has been focused on organic stabilizers because they are less toxic and safer for using. In this paper, a class of uracil derivatives were synthesized successfully via a precipitation method, and investigated as thermal stabilizers for PVC by Congo red test and discoloration test. The results showed that, these novel uracil derivatives exhibited greater stabilizing efficiency compared with Ca/Zn stabilizers and diphenylthiourea with the same concentration in PVC mixtures. This is attributed to the ability of these compounds to replace the labile chlorine atoms in PVC chains, and absorb of HCl released by the degradation of PVC, which was proved by the FTIR and 1H NMR spectral analysis. Furthermore, an excellent synergistic effect could be obtained when these compounds combined with zinc stearate(ZnSt2) in different mass ratios, and the initial color and long-term stability of PVC products were remarkably improved.Uracil derivatives had been proved to be effective thermal stabilizers for PVC in the previous study and the nitrogen atoms in these compounds played very important roles in the stabilizing reaction. Thus, allantoin, which is structural similar to uracil but contains more nitrogen atoms in its molecule, was investigated as the thermal stabilizers for PVC. The results indicated that allantoin behaved as a good long-term thermal stabilizer for PVC, which was attributed to its ability to absorb hydrogen chloride released from the degradation of PVC. Moreover, allantoin was able to extend the stabilization time of PVC containing ZnSt2/calcium stearate(CaSt2) and postpone “zinc burning”. Fourier transform infrared spectra confirmed that allantoin can react with ZnCl2 to produce an inert complex, which is responsible for postponing ‘‘zinc burning.’’Basic zinc cyanurate(Zn3(C3N3O3)2·ZnO, represented as Zn3Cy2) was synthesized via a precipitation method, and investigated as a thermal stabilizer for PVC by thermogravimetric analysis, Congo red test and discoloration test. The thermal stability of PVC was significantly enhanced with the addition of Zn3Cy2. Compared with ZnSt2, it is observed a significant improvement that Zn3Cy2 could delay the “zinc burning” of PVC. This is attributed to the strong ability of the cyanurate anions in Zn3Cy2 to absorb the hydrogen chloride released by the degradation of PVC. Moreover, mixing Zn3Cy2 with CaSt2 in different mass ratios greatly promoted the thermal stability of PVC. Excellent synergistic effects could be observed when CaSt2/Zn3Cy2 combined with some commercial auxiliary stabilizers. Dibenzoylmethane could increase the initial color of PVC containing CaSt2/Zn3Cy2, pentaerythritol could improve the long-term stabilizing efficiency of CaSt2/Zn3Cy2, and epoxidized soybean oil could improve both initial color and long-term stability.A series of novel ureido organic stabilizers for PVC with different length of alkyl chains were designed and synthesized, which have greater stabilizing efficiency compared with Ca/Zn stabilizers and phenylurea at the same concentration in PVC mixtures. The results of Congo red test, discoloration test, thermogravimetric analysis and Fourier transform infrared spectra showed that the ureido moieties of these urea derivatives have stronger ability to replace the labile chlorine atoms in PVC chains, but weaker ability to absorb hydrogen chloride(HCl) than those in phenylurea. On the other hand, longer alkyl chains in the synthesized organic stabilizers had positive effect in stabilizing efficiency for PVC. Furthermore, mixing these urea derivatives with ZnSt2 in different mass ratios led to a true synergistic effect, the “zinc burning” of PVC products was remarkably postponed.A novel kind of antibacterial thermal stabilizers for transparent poly(vinyl chloride), bis-uracil compounds with different alkyl chain length, was designed and synthesized by the condensation of 6-amino-1,3-dimethyluracil with aliphatic aldehydes. Not only high stabilization efficiency which increased with the increasing of alkyl chain length was obtained to prevent PVC from degradation, special antibacterial properties could also be acquired in the mean time of keeping PVC highly transparent. The results of discoloration test, thermogravimetric analysis, Fourier transform infrared spectra, visible spectroscopy and scanning electron microscopy revealed that the bis-uracil compounds could absorb HCl through acid-base reaction and chemical bonding, and the amine group played a key role in reacting with the inherent structural defects in PVC chains, meanwhile the long alkyl chain of the bis-uracil compounds could reduce their melting points and improve the compatibility with PVC. Comparing with the referential films stabilized by 6-amino-1,3-dimethyluracil and Ca/Zn stabilizers, PVC films stabilized with bis-uracil compounds had excellent transparency. In addition, the measurement of antimicrobial activity suggested that the bis-uracil compounds had the biological activity to inhibit growth of bacteria. We believe that PVC stabilized by such multifunctional stabilizers may widen its applications in biomedical field.