Preparation And Properties Of Fast-cure And Heat-resistant, High Strength Phenolic Resin Injection Molding Materials

Phenolic resins and phenolic injection molding materials belong to an important group of thermosetting polymers with very good fire resistance, dimensional stability, and excellent insulation properties. However, some key issues, such as structure and curing of phenolic resins and chemorheology of phenolic injection molding materials, and viscoelastic and postcured characteristics of phenolics still remain to be resolved. In this work, phenolic injection molding materials with fast-curing characteristics, phenolic injection molding materials with high mechanical and heat resistance properties, and the postcured properties of different phenolic resin materials reinforced with wood flour and glass fiber also investigated systematically.Injection properties of phenolic injection molding materials are related to curing and flowability. The key technical point in preparing phenolic injection molding materials with fast-curing characteristics is to solve the antinomy between fast cure and excellent flowability of materials. The work aims at balancing the relationship of fast cure and excellent flowability by adjusting the phenolic resin structure.Firstly, the effects of pH value, catalyst dosage, and the phenol/formaldehyde mole ratio on the stability of synthesis and resin properties are investigated. It is found that the key factor of reaction stability is the control over the methylol concentration. The form and ratio of ortho linkage to para linkage(hereinafter referred to as O/P ratio) of phenolic resins, which can be quantitatively measured by ¹³C-NMR.The activation energy of curing reaction and the curing process of phenolic reisn with varying O/P ratio are studied by a nonisothermal differential scanning calorimetry (DSC). The activation energy decreases with increasing the O/P ratio, while a lower activation energy indicats that the reaction proceeds faster at a given temperature. Many microgels grow without restraints in the initial stage of curing. The growth rate increases with increasing the O/P ratio. As the reaction proceeds, the number of microgels increases, the particles are more densely distributed, the curingprocess becomes diffusion-controlled, and there is little effect of the O/P ratio on the rate. The higher the curing temperature or the higher O/P ratio, the more constituents and the larger quantity of small molecules in the cured resin.In order to adjust the relationship of cure and flowability of phenolic resin fast curing injection molding materials, the effects of rolling conditions and O/P ratio on the material shaping time and flowability are studied. It is showed that the rolling temperature and time have evidently effects on the shaping time and flowability. The extent of flowability mainly depends on the O/P ratio. The higher the O/P ratio, the more obvious the tendency of flowability reduction. The shaping time is shortened with increasing the rolling temperature or time, but the O/P value has little effect on the shaping time. Under the optimized rolling conditions, the key factor on shaping time is the O/P ratio, the shaping time is shortened with increasing the O/P ratio, while the O/P value has little effect on the flowability of phenolic resin injection molding materials.The rheological behavior of phenolic resin injection molding materials is studied with brabender experiments. A rheological model based on the dual-Arrhenius equation is established and used to simulate the rheological behavior of the materials. The model predication is in a good agreement with experimental data. The injection-processing window of the materials can be well determined based on the developed model.The composites of magnesium hydroxide sulfate whisker and phenolic resin are prepared by an in-situ filling technique. Whiskers pre-treated with a coupling agent exhibit good dispersity. With TGA and DSC analysis, it is found that the phenolic resin has a better thermal stability at high temperatures and higher curing rates when a small amount of the pre-treated whisker is introduced by the in-situ filling technique. The flexural and impact strengths of the composite molding materials are also increased significally.A dynamic mechanical analysis (DMA) and a scanning electron microscope (SEM) are used to investigate the degree of resin curing and the state of glass fibers dispersion and wetting of the reinforced materials. The blend has more looselycrosslinked structures and lower cure degrees than the pure resol or novolac. The glass fibers are dispersed and wetted better in the blend. The blend is therefore prepared to obtain the phenolic injection molding materials with improved impact and flexural strengths and heat-resistance.Viscoelastic characteristics of the cured glass fiber-reinforced phenolic materials are investigated through glass transition, postcured and degradation reaction processes at high temperatures up to 350 °C. A typical glass transition of the crosslinked thermoset polymer is followed by irreversible postcured and degradation reactions, which exhibit an increasing storage modulus. A postcured and degradation master curve is constructed using the vertical and horizontal shift factors, both of which comply well with the Arrhenius equation. Using an analogy to the standard linear solid (SLS) model, a viscoelastic modeling methodology is developed to characterize the temperature-dependent complex moduli of the glass fiber-reinforced phenolic materials. The calculated master curve and storage modulus of glass fiber-reinforced phenolic materials agree well with the experimental data.N-Phenyl maleimide-modified novolac resin is synthesized by a condensation polymerization of N-phenyl maleimide and formaldehyde with phenol slowly feed to a reactor in the present of an acid catalyst. The polymerization conditions are optimized to obtain a high reaction conversion of PMI and a proper viscosity of PPMF. It is found that the reaction competition and conversion ratio for PMI are enhanced with an increase in the feeding time of phenol. As the reaction reflux time is increased, the conversion of PMI and the viscosity of PPMF are increased. At higher reaction temperatures or more PMI, the conversion of PMI decreases. The resins are characterized by FTIR, BC-NMR and thermal analyses. It is found that the N-phenyl maleimide groups are combined into the chains of novolac. The N-phenyl maleimide content of PPMF around 33% can be reached. The DSC curves reveal two curing peaks. The one at 150°C is attributed to the condensation reaction of methylol group formed in a minor quantity on the phenyl rings. The other at around 250°C is attributable to the addition curing reaction of the maleimide groups. A TGA analysis shows that the cured PPMF has better thermal properties.The effect of postcure on the properties of different cured phenolics is studied. The properties of cured phenolics after postcure, such as heat-resistance, mechanical properties, weight loss, water absorption, and electrical properties are measured. A DMA is used to investigate the effects of postcuring on phenolics materials. It is found that the storage modulus of the materials can be increased while the damping capacity decrease by the postcure, which yields an increase in crosslinking density. The results show that the improvement of mechanical and heat-resistant properties of moldings is due to an increase of crosslinking density and the interfacial bonds between the fibers and the resin matrix by the postcure. The results also show that the decrease of water absorption of moldings is due to the increase of hydrophobic nature of wood flour included in moldings by postcure. The reasons for the improvement of the electrical properties by postcure are also discussed.