Study On Structure And Properties Of Dynamically Cured Polypropylene/Novolac Blends

Developing polypropylene (PP) based materials with good mechanical properties by the means of modification has been an important way to widen application fields of PP. Reinforced PP, toughened PP, antistatic and conductive PP composites are the important areas to modify PP. In this paper, dynamic vulcanization is applied to thermoplastic/thermosetting resin systems, and to prepare new types of PP/Novolac type phenolic resin (Novolac) and elastomer toughened PP/Novolac blends through dynamically curing a Novolac resin in different systems. As Novolac and PP are immiscible, proper compatibilizer and elastomer were chosen to prepare good mechanical properties of dynamically cured blends. And the preparation, structure and properties of dynamical cured PP/Novolac and PP/elastomer/Novolac blends were studied.PP or PP/Novolac resin were chosen as the matrix and filled with CB. The electrical properties, morphology and rheological behavior of the PP/CB and PP/Novolac/CB composites were investigated systematically. Our work was mainly on the CB distribution in the PP/Novolac matrix, and the effects of blend composition, dynamic cure, compatibilizer and processing sequence on the morphology and properties of the composites.Firstly, the feasibility of dynamically curing a Novolac resin during melt-mixing was studied. Maleic anhydride-grafted PP (MAH-g-PP) was used as a compatibilizer for imcompatible PP and Novolac blends. The effects of dynamic curing, compatibilizer content, Novolac content and curing agent content on mechanical properties of PP/Novolac blends were investigated. The mechanical properties, crystallization behavior, rheological behavior, thermal stability and dynamic mechanical thermal analysis of PP/Novolac blends were studied.Experimental results showed that the dynamically cured PP/MAH-g-PP/Novolac blends have better mechanical properties than the uncured PP/Novolac, uncured PP/MAH-g-PP/Novolac and dynamically cured PP/Novolac/HMTA blends. The dynamic curing of Novolac increased the modulus and stiffness of the PP/Novolac blends. The compatibilizing together with dynamic curing could further improve the mechanical properties. With increasing Novolac content, tensile strength, flexural modulus and flexural strength of dynamically cured PP/MAH-g-PP/Novolac blend increased significantly, while the elongation at break dramatically deceased. Those blends with Hexamethylenetetramine (HMTA) as a curing agent had good mechanical properties at HMTA content of 10 phr (relative to Novolac content). The optimum content of MAH-g-PP was 10wt%. FTIR analysis and extraction test indicated the hydrogen-bond interaction between the hydroxyl groups of Novolac and the carbonyl groups of MAH-g-PP. MAH-g-PP can react with HMTA, leading to the formation of graft copolymers between MAH-g-PP and cured Novolac resin.SEM observation showed that the dynamic curing of Novolac could prevent Novolac particles from aggregation in PP matrix and make the dispersed phase small in dynamically cured PP/Novolac blends. MAH-g-PP together with dynamic curing could lead to the fine domains in blends. Thermogravimetric analysis indicated that the incorporation of Novolac into PP could improve the thermal stability of PP. Dynamic curing can further improve the thermal stability of blends, and the dynamically cured PP/MAH-g-PP/Novolac blends had the best thermal stability and char yields among all the PP/Novolac blends. And it is observed that the higher the content of Novolac, the more thermally stable the blend. The rheological analysis showed that PP and PP/Novolac blends presented evident shear thinning behavior. At the same content of Novolac resin, the dynamically cured PP/Novolac blends exhibited higher viscosity in the whole frequency range than the uncured samples. The compatibilizing together with dynamic curing could further increase the complex viscosity, the storage modulus and loss modulus over the entire frequency range. With the increasing Novolac content, the viscosity and modulus of PP/MAH-g-PP/Novolac blends increased gradually. DMTA analysis showed that dynamic curing could evidently increase the storage modulus E' of blends. Due to the high density of cross-linked Novolac resin, the Tg of Novolac resin of blends disappeared.Isothermal and non-isothermal crystallization behaviors were analyzed using DSC. The results showed that the crystallization of PP in PP/Novolac blends was strongly influenced by crystallization temperature, particles size of Novolac, cross-linking and MAH-g-PP. Uncured and cured Novolac particles in the PP/Novolac blends could act as effective nucleating agents, accelerating the crystallization of PP in the blends. The compatibilizer MAH-g-PP together with dynamic curing could dramatically increase the crystallization rate of PP. And the smaller the Novolac particles were, the more effective the nucleating agent for PP crystallization. Avrami equation was used to analyze the isothermal crystallization kinetics of PP and PP/Novolac blends. The crystallization thermodynamics were estimated using the Hoffman theory. The incorporation of Novolac resin to PP could decrease the chain folding energy (σe) of PP. dynamic curing and compatibilizer can further decreasedσe of PP. The non-isothermal crystallization kinetics of dynamically cured PP/Novolac blends compatibilized with MAH-g-PP was studied. It was found that the combination of Avrami and Ozawa equations exhibited great advantages in treating the non-isothermal crystallization kinetics in dynamically cured PP/MAH-g-PP/Novolac blends. The activation energies (ΔE) were determined by the Kissinger and Takhor methods for non-isothermal crystallization. The activation energy of PP was greater than those of dynamically cured PP/MAH-g-PP/Novolac blends. And the values ofΔE decreased with increasing the Novolac content. The POM results showed that the spherulite size of PP in PP/Novolac blends were smaller than the pure PP. XRD analysis suggested that the Novolac resin, compatibilizer and dynamic curing could affect the crystal structure of PP. The PP and dynamically cured PP/MAH-g-PP/Novolac blends showed only theα-form. However, PP/Novolac,PP/MAH-g-PP/Novolac and dynamically PP/Novolac blends had two crystal structures,α-form andβ-form.The incorporation of nitrile rubber (NBR) to dynamically cured PP/Novolac blends increased the toughness and stiffness of the blends, when MAH-g-PP and amine-terminated butadiene-acrylonitrile copolymer (ATBN) were used as compatibilizers for blends. Compatibilizing together with dynamic curing could improve the mechanical properties of PP/NBR/Novolac blend. SEM analysis showed that the addition of MAH-g-PP or ATBN to dynamically cured PP/NBR/Novolac blends could decrease the domain size of NBR. MAH-g-PP together with ATBN was used in the blends, which further resulted in an evident reduction in domain size of NBR. TGA analysis showed that the thermal stability of dynamically cured PP/NBR/Novolac blends was better than the uncured blends. MAH-g-PP combines with ATBN could evidently improve the thermal stability of dynamically cured blends as compared to non-compatibilized blends and the addition of MPP or ATBN alone. XRD analysis showed that NBR, compatibilizer, Novolac and dynamic curing could affect the crystal structure of PP in blends.The percolation threshold of Novolac/CB composite (about 12 phr) is higher than that of the PP/CB composite (10 phr). SEM, optical microscopy and extraction experiment showed that in the PP/Novolac blends, CB preferentially localized in the Novolac phase. The addition of CB changed the morphology of the Novolac phase significantly. for the PP/Novolac (70/30) blend, with increasing CB content, the morphology of dispersed Novolac phase was changed from spherical particles to elongated strucutre, and finally formed cocontinuous structure with PP, which formed double percolation and decreased the percolation threshold (8 phr). Processing sequence had an important effect on the volume resistivity and morphology of the CB filled PP/Novolac blends. The distribution of CB, morphology, and volume resistivity of the PP/Novolac/CB composites prepared through three different processing sequences were investigated. Through the change of the composition and processing sequence, the morphology, structure and volume resistivity of composites could be controlled. The addition to the PP can slightly affect the mechanicals of PP/CB composite. However, for the PP/Novolac/CB composites, the higher content of Novolac resin, the worse of mechanical properties in blends. The addition of CB to PP/Novolac blend evidently increased the storage modulus G ', loss modulus G ''and complex viscosityη?. And the viscosity and modulus of PP/Novolac/CB blends increased with increasing CB content.The innovations of this dissertation are listed as follows:(1) The dynamical cure process has been first applied to PP/Novolac blend. When the suitable curing agent and compatibilizer were chosen, the dynamically cured blends had good mechanical properties.(2) Crystallization behavior of PP/Novolac blends was firstly studied. The results showed that Novolac, compatibilizer and dynamic curing affected the crystal structure of PP and resulted in the formation ofβ-form. Uncured and cured Novolac particles in the PP/Novolac blends could act as effective nucleating agents, accelerating the crystallization of PP in the blends. The compatibilizer MAH-g-PP together with dynamic curing could dramatically increased the crystallization rate of PP. And the smaller the Novolac particles were, the more effective the nucleating agent for PP crystallization.(3) The novel methods concerning reinforcing and toughening of PP were reported. The dynamical cure process was applied to PP/NBR/Novolac blends compatibilized with MAH-g-PP and amine-terminated butadiene-acrylonitrile copolymer (ATBN), the prepared blends maintain the stiffness-to-toughness balance.(4) The conducting behavior of the PP/Novolac/CB composites was studied for the first time, and the relation among blend composition, morphology and properties were investigated systematically. It was found that nearly all the CB particles distribute in Novolac resin and the composites could achieve conductivity via double percolation. By adjusting morphology and the distribution of CB in the composites, conductive or antistatic PP composites with stable volume resistivity could be obtained. The PP/Novolac/CB composite could get lower percolation threshold than that of PP/CB and Novolac/CB composites if different processing sequence was applied.