| In the polymer production process,the unreacted monomers,solvents and other small molecule volatiles need to be removed from the polymer to reduce the negative effects on product quality,post-system safety and the environment caused by high product volatile content.The extruder is often applied to deeply devolatilize the product in the traditional devolatilization process.Although the process is simple and the technology is relatively mature,the devolatilization efficiency of traditional devolatilization process cannot meet the increasingly stringent quality requirements and environmental protection standards.Therefore,the development of high-efficiency polymer devolatilization technology has been brought to the fore at home and abroad.Compared with other devolatilization enhanced technologies,adding devolatilization additives in the extruder supplemented with vacuuming,can not only achieve high-efficiency devolatilization,but also make little changes to the existing process and reduce transformation costs,which has broad industry application prospects.The polymer successively undergoes foaming devolatilization and difusion devolatilization stages in the extruder and the process is extremely complicated.Bubbles exist in the molten polymer even during the diffusion devolatilization where the volatile content is very low.The main factors that determine the degree and rate of extruder devolatilization are the thermodynamic properties of the polymer-volatile system,the diffusion properties of volatile in the polymer,and the bubble nucleation,growth,and breakup rate in the polymer melt.The bubble evolution in the polymer melt involves a series of processes such as bubble nucleation,growth,coalescence,breakup and so on.The study of the growth and evolution of bubbles in the polymer melt and the enhancement mechanism of additives on foaming and polymer devolatilization can provide theoretical guidance for the design optimization and process strengthening of extruder,which help it to realize the cost reduction and efficiency increase of polymer devolatilization process.In this study,high-speed camera and image recognition technology of bubble parameters were used to study the nucleation,growth,breakup,and collapse of n-hexane bubbles in high-density polyethylene(HDPE)melts in the static melting visualization setup and the sliding plate shearing melting visualization setup.Then established a visual evaluation method of bubble behavior in HDPE melt.And studied the effect of additives on bubble behavior in static polymer melt to obtain the optimal additive formula and concentration.Finally,the effects of additives and operating parameters on the extrusion devolatilization and product properties of HDPE were investigated on a laboratory extruder.The main research contents and achievements of this paper include.The main research contents and results of this study include:(1)The static melting visualization setup was established,and the high-speed camera was applied to study the bubble evolution behavior of n-hexane in the HDPE melt.It was found that n-hexane bubbles would undergo nucleation,growth,coalescence and collapse.During the process,there are two bubble collapse modes:small bubbles(d<0.10 mm)will directly collapse after nucleation,and large bubbles(d>0.10 mm)will collapse after grow until disappear.The influence of temperature on the bubble formation process was also investigated,it was found that as the temperature increased,the mean bubble diameter became larger,the initial bubble number increased and the initial bubble diameter distribution changed from a unimodal distribution(0.06-0.08 mm)to a bimodal distribution,and a new distribution peak(0.10-0.12 mm)was generated.The influence of temperature on the bubble collapse rate was investigated,and a single parameter(collapse factor n,which characterized the bubble collapse rate)collapse model was established.The deviation between the predicted value of the model and the experimental value was less than 1%.As the temperature increased,the collapse factor n increased,and the bubble collapse rate became faster.(2)The sliding plate shearing melting visualization setup was established and the continuous shearing experiment was used to simulate the shearing area of the extruder,and the non-shearing area in the extruder was simulated by abruptly stopping the shearing after continuous shearing for 1 min.Then,the high-speed camera was applied to study the bubble evolution behavior of n-hexane in the HDPE melt under shearing.It was found that as the shear rate increased,the nucleation rate of bubbles in the HDPE melt increased.Moreover,the mean bubble diameter in the high pressure area decreased monotonously with time until bubbles disappeared and the mean bubble diameter in the low pressure area increased linearly with time.A modified prediction model of bubble coalescence time in the high-pressure area was established,and the relative deviation between the predicted value of the model and the experimental value was less than 1%.It was also found that there were three modes of bubble breakup under shearing:tensile breakup,erosive breakup in the melt and bubble breakup at the gas-liquid interface.With the increase of the shear rate,the dominant factor of bubble tensile breakup will change from melt viscosity to bubble size.And the probability of tensile breakup will first increased and then decreased with the increasing of the capillary number.The shearing effect can shorten the bubble removal time.According to the research results,the schematic of the foam devolatilization in the shear field of the extruder was established.(3)The influence of water and surfactant on the bubble behavior in the HDPE melt was investigated in the static melting visualization setup.It was found that the water vaporized in the melt to promote bubble nucleation,and make the mean bubble diameter decreased and the collapse rate increased;as the water concentration increased,the relative bubble collapse factor n/no first decreased and then increased.Surfactant could reduce the surface tension of the melt and promote the growth of bubbles to enhance devolatilization,but too much surfactant will make the bubbles collapse rate decrease.Compared with water,water and surfactant mixed additive significantly strengthen the bubble nucleation process in HDPE melt.Furthermore,through the HDPE extrusion devolatilization experiment in the small extruder under the action of additive,the optimal additive formula(msurf:mwater=1:7)and concentration(1000 ppm)were obtained,and it was found that the additive have little effect on the mechanical properties of HDPE products.The influence of extruder temperature,vacuum degree,and screw speed on the volatile content in the HDPE product was also investigated,and the optimal operating temperature(220?)and rotation speed(250 rpm)of the extruder were obtained.The volatile content in the product decreased as the degree of vacuum increased.Under the optimal operating conditions,the volatile content in HDPE after adding the additive was reduced by 34% compared with the condition without additive. |
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