Preparation Of Benzylidene Acetals By Solid State Reaction And Their Effect On Heterogeneous Nucleation Process Of Isotactic Polypropylene

Controlling crystallization behaviors and crystallization morphology of isotactic polypropylene (iPP) through adding nucleating agents to reach the objective of improving the properties of iPP is a very effective and practical means. Sorbitol acetals are used as nucleating agents in isotactic polypropylene (iPP) to improve the transparency and rigidity, and reduce circulation time of processing. The traditional preparation processes of benzylidene acetals need a lot of heat and solvent because the gelation of the benzylidene acetals is bad for the mass transfer, which makes a lower yield. Therefore, it is very significant to develop a novel preparation process of benzylidene acetals and study the interaction mechanism between benzylidene acetals and iPP.Firstly, a novel preparation process of benzylidene acetals by solid state reaction was developed to synthesizel,3:2,4-di-p-methylbenzylidene sorbitol (DMBS),1,3:2, 4-di-chlorobenzylidene sorbitol (DCBS) and asymmetric p-methylbenzylidene/ p-chlorobenzylidene sorbitol (SCM). In the optimum reaction condition, the yield of DMBS was 93%. Compared to traditional preparation process, the solid state preparation process does not need solvent. By HPLC and mass spectrum analysis, the product of asymmetric SCM contained 24% DMBS,19% DCBS and 57% SCM.Secondly, the effect of molecular structure of benzylidene acetals on their solubility in iPP was studied, selecting mono-p-methylbenzylidene sorbitol (MMBS, containing 4 hydroxyl and 1 benzylidene groups), DMBS and DCBS(containing 2 hydroxyl and 2 benzylidene groups), tri-p-methylbenzylidene sorbitol(TMBS, containing threes benzylidene groups only) and di-(p-methylbenzylidene) pentaerythritol(DMBP, containing two benzylidene groups only) as objects for studying. The DSC results indicated that the less hydroxyl groups the benzylidene acetal has, the more the benzylidene acetal dissolves into polypropylene is. The saturated concentration of MMBS, DMBS, DCBS, TMBS and DMBP in iPP were 0.15%,0.2%,0.2%,0.8% and 4%, respectively, where the crystallization temperature(Tc) increased to 120.8℃,128.2℃,127.9℃,128.1℃and 128.9℃, respectively, from the Tc the virgin iPP 112.4℃. It is suggested that the number of hydroxyl and benzylidene groups not only affect on the solubility of benzylidene acetals in polypropylene, but also affect on the nucleation of benzylidene acetals in iPP. Especially, from the phase behavior of SCM compounds in iPP, it was found that there is a'salt out effect'between the solubility of different benzylidene acetals, and the solubility of compound is equal to that of any benzylidene acetal in the compound.Thirdly, the effect of the molecular structure of benzylidene acetals on the transparency and mechanical property of iPP were studied selecting MMBS, DMBS, DCBS, TMBS and DMBP as samples. The results showed that the haze of iPP decreased first and then increased, with the number of hydroxyl group of benzylidene acetals increased. The haze of iPP contained MMBS, DMBS, DCBS and TMBS were decreased by 13.0%,40.9%,57.8% and 9.0%, but the addition of DMBP made the haze of iPP increase by 17.8%, because the saturated concentration is too high to improve that transparence of iPP. On the mechanical property, the effect of molecular structure of benzylidene acetals has the same effect. Under the saturated concentration, the addition of these benzylidene acetals made the flexural modulus increase by 19.7%,35.8%,29.3%,28.4% and 18.3%, respectively. When several kinds of benzylidene acetals compounded, the nucleation of compound is among that of components. Based on this conclusion, the nucleation of SCM compound is better than DMBS and DCBS indicated that SCM has higher nucleation efficiency.Fourthly, the non-isothermal crystallization behaviors of iPP were investigated by DSC. The Mo and Caze methods were used to describe the non-isothermal crystallization kinetics of iPP nucleated with different content of DMBS. The Kissinger method was applied to calculate the crystallization energy. The results indicated that the addition of DMBS can increase the crystallization rate and decrease the half crystallization time of iPP remarkably. The nucleation model of iPP with DMBS is three dimensions homogenous nucleation, and would form shish kebab crystallization when the concentration of DMBS exceeds 0.20%. Under non-isothermal condition, the addition of DMBS increases the crystallization activation energy so that the crystallization of iPP is hindered, but total crystallization rate increased because nucleation is accelerated due to the existence of large amount of nuclei of DMBS.Fifthly, the nucleation mechanism of benzylidene acetals in iPP was investigated by polarizing microscope, two dimensions infrared, and rheometer. The results of polarizing microscope showed that the lamellar crystals of iPP are oriented growth and perpendicular to the surface of DMBS. The testing of viscosity and relaxation time spectrum on rheometer indicated that there is an intermolecular interaction between polypropylene molecule and crystal of DMBS at 200℃. Further, the results of two dimensions infrared indicated that the change of the bending vibration of side group methyl of iPP occurs before that of the C-C stretch in the induction stage of crystallization, when the benzylidene acetals are presence, which is opposite to that of virgin iPP, which indicated that the intermolecular interaction occurs between benzylidene groups of acetal and methyl groups of iPP. Finally, the effect of calcium stearate on the stability in iPP was investigated. The results indicated that the acidic catalyst residue in the DMBS will lead to the decomposition of DMBS at the process of iPP, and the calcium stearate can neutralize the residue acid catalyst and improve the nucleation efficiency at the concentration 0.05%.