Development and Characterization of Polypropylene, Cellulose Acetate, Magnesium Hydroxide Materials

Abstract

The urgency to develop and commercialize multi-component materials containing bio-based material is growing. Such materials can reduce the widespread dependence on petroleum and at the same time can reduce pollution while contributing to the economy. The demand for polymeric materials in applications such as automotive components, building materials, and the aerospace industry is increasing; however, one of the main drawbacks to use polymeric materials is their limited fire resistance. The objective of this work was to develop polypropylene plasticized cellulose acetate materials and to explore the use of magnesium hydroxide to enhance the flame retardant properties of these materials. Specifically, the thermal stability, flammability, crystallization, and mechanical properties were investigated. Material fabrication was accomplished using a two-step extrusion process. In the first step, cellulose acetate was plasticized with 30% triethyl citrate. The effects and viability of triethyl citrate as a plasticizer for the cellulose acetate were investigated by using differential scanning calorimetry, X-ray diffraction, and thermogravimetric analysis. Based on differential scanning calorimetry, the glass transition and the melting temperature were lowered by 39 °C and 77 °C, respectively. The effects of triethyl citrate on the degree of crystallinity of cellulose acetate was examined by X-ray diffraction. The degree of crystallinity of plasticized cellulose acetate was lower than that of cellulose acetate. According to thermogravimetric analysis, triethyl citrate lowered the thermal stability of the cellulose acetate by shifting the onset temperature of degradation and the temperature of maximum weight loss to lower temperature. According to the aforementioned analysis, cellulose acetate and cellulose acetate plasticized with triethyl citrate is applicable to be blended with polypropylene. Formulation of the polypropylene materials was investigated according to the thermal analysis by the thermogravimetric analysis. Choosing the level of each component in the fabrication was developed sequentially, polypropylene-grafted-maleic anhydride, plasticized cellulose acetate, and magnesium hydroxide. Different levels of polypropylene-grafted- maleic anhydride (1, 2, 3 wt.%), plasticized cellulose acetate (20, 30, and 40 wt.%), and magnesium hydroxide (10, 20, 30 wt.%) were used in order to choose the optimum level of each component. Based on the thermal analysis and activation energy estimation, the highest level of each component was chosen to fabricate the multi-component material. The results of scanning electron microscopy imaging showed that plasticized cellulose acetate had better compatibility with polypropylene matrix. The addition of plasticized cellulose acetate was affected negatively by the addition of magnesium hydroxide. Elemental mapping was carried out by energy dispersive X-ray spectroscopy attached to the scanning electron microscope. The results show good dispersion of plasticized cellulose acetate and magnesium hydroxide. After choosing the level of each component, which was 3 wt.% , 40 wt.% and 30 wt.% for polypropylene-grafted-maleic-anhydride, plasticized cellulose acetate, and magnesium hydroxide, respectively, the extruded materials were produced and cut into pellets. Injection molding and hot press compression molding were used to prepare the samples for characterization. Various characterization techniques were used to evaluate the inclusion of plasticized cellulose acetate and magnesium hydroxide in the polypropylene matrix. Thermal stability and kinetic studies were investigated by using thermogravimetric analysis (TGA) under non-isothermal conditions. The thermal stability of the polypropylene materials was evaluated through their TGA and DTGA curves at four heating rates (5, 10, 20, and 30 °C/min ). The results revealed that polypropylene materials with presence of plasticized cellulose acetate and magnesium hydroxide had good thermal stability where the thermal decomposition took place over a wide range of temperature and the maximum weight loss temperature was shifted to higher temperature. The Kissinger, Kissinger-Akhira-Sunose (KAS), and numerical integration methods were employed to estimate the activation energy. The activation energy of polypropylene materials with presence of plasticized cellulose acetate and magnesium hydroxide was higher than those of other polypropylene materials. Flammability and combustion behavior were examined through vertical burning, oxygen index, cone calorimeter and adiabatic bomb calorimeter tests. The results show that polypropylene with plasticized cellulose acetate and magnesium hydroxide ranked as V-0 according to the vertical burning test. The limiting oxygen index of polypropylene containing plasticized cellulose acetate and magnesium hydroxide was higher than that of polypropylene by 29%. According to the cone calorimeter test, the peak heat release rate and total heat release when the plasticized cellulose acetate and magnesium hydroxide were present in the polypropylene matrix were lower than that of polypropylene by 80% and 30%, respectively. The carbon monoxide and carbon dioxide yields revealed that polypropylene materials with plasticized cellulose acetate and magnesium hydroxide were significantly lower than that of polypropylene. The effective heat of combustion, estimated from the cone calorimeter and the heat of combustion from adiabatic bomb calorimeter, confirmed that the polypropylene material with plasticized cellulose acetate and magnesium hydroxide was less exothermic due to the reduction in the estimated heat of combustion. According to the stoichiometry of the carbon in the fuel and combustion products, the polypropylene material with higher yield of residue in the form of soot and char showed better flame retardancy than material with lower yield of residue. The results showed that polypropylene containing plasticized cellulose acetate and magnesium hydroxide had a higher yield of residue. Non-isothermal crystallization and nucleation morphology of the polypropylene materials was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. DSC thermograms showed that the crystallization temperature of polypropylene materials shifted to lower temperature with presence of polypropylene-grafted-maleic anhydride, plasticized cellulose acetate, and magnesium hydroxide. Development of the relative crystallinity was determined at four cooling rates. Avrami model was employed to analyze the data obtained from the DSC. Polarized optical microscopy was used to show the nucleation and crystal growth of the pure polypropylene and polypropylene plasticized cellulose acetate materials. Nucleation activity was estimated for the polypropylene materials. The results demonstrated that polypropylene-grafted-maleic anhydride, plasticized cellulose acetate, and magnesium hydroxide modified the nucleation and the crystal growth of the polypropylene materials. The mechanical properties of the polypropylene materials showed a marginal reduction in tensile strength and the elongation at break due to the inclusion of the plasticized cellulose acetate and magnesium hydroxide. The reduction was 13% and 30% due to the addition of plasticized cellulose acetate and magnesium hydroxide, respectively. However, the addition of plasticized cellulose acetate and magnesium hydroxide increased the Young’s modulus of the polypropylene materials. According to the impact test, there was a reduction in the impact strength of the polypropylene materials due to the addition of plasticized cellulose acetate and magnesium hydroxide. The impact strength reduced by 35% and 80% due to the addition of plasticized cellulose acetate and magnesium hydroxide, respectively. Differential scanning calorimetry and X-ray diffraction were used to study the effect of plasticized cellulose acetate and magnesium hydroxide on the degree of crystallinity and the crystal forms of the polypropylene materials. The addition of plasticized cellulose acetate reduced the degree of crystallinity of the polypropylene. On the other hand, plasticized cellulose acetate induced the ß crystal form, positively influencing the thermal and mechanical properties. The addition of cellulose acetate and magnesium hydroxide increased the degree of crystallinity. The most significant finding to emerge from this study is the feasibility of obtaining materials from polypropylene, plasticized cellulose acetate and magnesium hydroxide. The results of this study indicate that the materials have improved thermal stability and flame retardancy with the combination of plasticized cellulose acetate and magnesium hydroxide. Furthermore, the level of magnesium hydroxide could be reduced and reasonable flame retardancy maintained compared to the level reported in the literature. The inclusion of the plasticized cellulose acetate and magnesium hydroxide affected some mechanical properties of the material. Polypropylene material containing plasticized cellulose acetate and magnesium hydroxide would be suitable for interior automotive components and other non-structural building materials such as electrical power insulation.