The Thermal-Oxidative Stability Of Nylon 6 And Its Nanocomposites

Nylon 6 (PA6) is an important engineering plastic, with excellent mechanical properties, remarkable oil and abrasion resistance, easy processability and other properties. However, it is affected by heat, oxygen, light and shear force in processing period, and in practical use. All these factors can cause the oxidation degradation of PA6, leading to a decrease in molecular weight and mechanical properties, so that the service life of material is greatly shortened, and a lot of environmental waste produces. Therefore, the stabilization of polyamide has a significant meaning to the application and development.This thesis is based on the thermal-oxidative stability of PA6 and its nanocomposites. A novel stabilizer was prepared to improve the thermal-oxidative stability of PA6. In addition, inorganic nano-materials with various structure were selected, and the influence of these nano-materials and their surface modification to the thermal-oxidative degradation and thermal-oxidative accelerated aging was studied. The main work and results are as follows.A novel stabilizer containing hindered phenol and hindered amine light stabilizer (HALS) group in one molecule was prepared through the acyl chloride reaction followed by the amidation reaction. Its structure was confirmed by electrospray ionization mass spectrometry (ESI-MS) and proton nuclear magnetic resonance (’H-NMR), Fourier transform infrared spectroscopy (FT-IR). The thermogravimetric analysis (TGA) results under nitrogen and air atmosphere showed beneficial effect of AG-HDM to the thermal and thermal-oxidative stability of PA6. The thermal-oxidative stabilization effect of the novel stabilizer on PA6 was further evaluated in terms of reduced viscosity, tensile properties and chemical structure after accelerated thermal aging in an air circulating oven at 150 ℃. The results indicated that the novel stabilizer could improve the thermal-oxidative stability of PA6 effectively, resulting in the high retention of reduced viscosity and tensile strength, and the low ratio of terminal carboxyl group to amino group. The less condensation water formed, the high persistence and good compatibility of AG-HDM with PA6 matrix were supposed to reasons for its beneficial stabilization effect.Nano-silica was selected as the representative of zero-dimensional inorganic nano-materials. Nano-silica was firstly modified with an aminosilane coupling agent (APTES) and then reacted with the reactive hindered phenol antioxidant, 3-(3,5-di-tert-butyl-4-hydroxyphenyl)- propionic acyl chloride (AG-Cl), to form an antioxidant-immobilized nano-silica (SiO2-APTES-AG). Based on the result of TGA, it was calculated that the loading of antioxidant on the SiO2-APTES-AG was 12.34 wt%. After surface modification, it was found that SiO2-APTES-AG had good suspension stability in formic acid, and also had a uniform dispersion in the PA6 matrix. According to the results of TGA and accelerated aging, it was found that nano-silica which was not modified presented almost no influence to the thermal-oxidative degradation and aging behavior of PA6, while SiO2-APTES-AG can improve the thermal-oxidative stability of PA6 significantly. Compared with the properties of PA6/SiO2-APTES-AG and PA6/AG after aging, the physical loss of antioxidant with a small molecular was decreased after immobilized on the surface of nano-silica, leading to more efficient in preventing PA6 from thermal-oxidative degradation.Attapulgite (ATP) was selected as the representative of one-dimensional inorganic nano-materials. ATP grafted with dendrimer-like polyamidoamine (PAMAM) was prepared by treating original ATP with heat and acid followed by grafting with PAMAM, which was confirmed by FT-IR, TGA and dispersion state in formic acid. The X-ray diffraction (XRD) analysis result indicated that the grafting modification was occurred on the surface of fibrous crystals and did not shift the crystal structure of ATP. On the other hand, it was found that the inclusion of ATP-PAMAM caused the transformation of some a crystalline form to y crystalline form. According to the results of TGA and accelerated aging, it was found that there was no obvious difference of the thermal-oxidative degradation and aging behavior between PA6/2ATP-untreated and pure PA6. However, there was a strong interfacial adhesion between ATP-PAMAM and the PA6 chain, with a uniform dispersion of ATP-PAMAM in the PA6 matrix, which was beneficial to the stabilization of PA6. Therefore, coupled with the favorable effect of abundant amino groups on the surface of ATP-PAMAM, ATP-PAMAM displayed thermal-oxidative stabilization effect to PA6.Montmorillonite (MMT) was selected as the representative of two-dimensional inorganic nano-materials. Epoxy resin-modified montmorillonite (EP-MMT) prepared through solid-state reaction was introduced to the PA6 matrix by melt processing using a typical twin-screw extruder. XRD combined with transmission electron microscopy (TEM) was applied to elucidate the structure and morphology of PA6/EP-MMT nanocomposites, suggesting an exfoliated structure in PA6/2EP-MMT and a partial exfoliated-partial intercalated structure in PA6/4EP-MMT. TGA under air atmosphere and thermal-oxidative accelerated aging results indicated that the incorporation of EP-MMT effectively enhanced the thermal-oxidative stability of PA6. The stabilizing effect was dominant in spite of some decomposition of surfactant. Furthermore, PA6/2EP-MMT with exfoliated structure had less surfactant decomposed, showing comparable stabilization effect of incorporation of 4 wt% EP-MMT to PA6.