| Due to the influence of crystallization characteristics such as high crystallinity,high crystallization rate and grain size,polyformaldehyde(POM)is prone to defects such as surface sag,warping and even internal cavity when forming thick walls or large parts,so that POM can only form thin-wall and small-size parts.Forming between thick wall and large size and continuous forming of plate/bar in Germany is still a problem for the industry.This severely restricts the size and application of POM artifacts.If satisfies the crystallization rate of POM decreased when forming of rapid prototyping,but maintain a high degree of crystallinity and formation of fine grains,to ensure that the products just characteristic and products with high strength and high toughness,to realize the POM thick wall,large size products forming,this paper adopts the spherulite growth inhibitors and nucleating agent on the crystallization process of POM collaborative control,to achieve the above purpose.On the one hand,nucleating agent is used to reduce the nuclear energy and increase the density of crystal nuclei,so as to obtain high crystallinity and refine grains.On the other hand,spherulite growth inhibitors are used to interfere with the migration of POM molecular chains to the grain surface and the orderly arrangement process on the side of the chip,forcing the crystallization rate to decline to meet the needs of rapid prototyping.It is necessary to coordinate the contradiction between the decrease of crystallizatio n rate of spherulite growth inhibitor and the increase of crystallization rate of nucleating agent.The equilibrium inhibitor reduces the trade-off between both crystallization rate and crystallinity in order to keep the POM mild and clean at a high crysta llinity.In this paper,several polymers with different degrees of action with POM were selected as spherulite inhibitors,namely TPU,N BR and PF.In the process of processing,these polymers and POM form an aggregate structure that inhibits the growth of spherulites and interferes with the orderly arrangement of chain segments.Due to polyformaldehyde itself has a good capability,some for sale on the market of nucleating agent such as sorbitol,phosphate and most inorganic nucleating agent have does not display the nucleation effect on POM,after a lot of research,the topic selection of a certain particle size MWCNT,MMT and diatomite as the nucleating agent,research under the nucleating agent and the inhibitor synergy regulation,on the crystallization behavior of POM,non-isothermal crystallization kinetics,nucleation time,nucleation density and the influence of spherulite growth rate.The results show that:PF under the regulation for inhibitors and nucleating agent,found that the movement of the POM to the low temperature crystallization peak temperature,PWHH widen,the crystal ization rate decreases,and under the control of PF/MWCNT,crystallinity have further ascension instead,through TEM fo und in PF/POM and three types of nucleating agent blend,diatomite and MMT were scattered in the PF phase,not only cannot exert itself nucleation induction and regulation ability of crystallinity,on the contrary make PF inhibition effect is better,but t he MWCNT were scattered in the POM phase,greatly reduces the POM crystallization nucleation energy barrier,Make its crystallinity higher than that of pure POM;In PF and under the control of nucleating agent,to achieve the same degree of crystallinity o f the crystallization time is significantly lower,and the other in the polarized images,view spherulite size decrease markedly,and the chip number has increased,according to the surface free energy of nucleation parameters and folded chain found that PF/MWCNT,compared with pure POM POM spherulite growth rate increased significantly,the last under the twin-screw extruder to enlarge regulation results of PF/MWCNT,found under its control,POM can realize moderate crystallization rate and maintain a high degree of crystal inity and fine grains.When NBR as inhibitor,respectively with three kinds of nucleating agent found that common regulation under its control,the crystallization peak temperature of POM to move to low temperature,PWHH widen,the crystallization rate reduced.However,the crystal enthalpy of POM was significantly reduced,and the other two-step method could disperse more nucleating agents in the POM phase than the one-step method,so as to better exert the ability of controlling crystallinity.Through relative crystallinity and crystalline/crystalline temperature figure found that under the control of NBR and nucleating agent,arrived at the same time and temperature of the crystallinity were significantly lower,under its control,the crystallization rate is reduced,and inhibits the high temperature of POM into nuclear capability,higher content of NBR,time of arrival in the same degree of crystallinity and temperature change is more obvious,shows that the crystallization rate of POM can be further reduced,and the other according to different Avrami constant n,mixing way can be found to impact how its nucleus and the growth.Under the control of the polarizing image of the hot table,the size of spherulites in the field of view significantly decreased and the number of spherulites significantly increased under the control of 1wt%MWCNT/5wt% NBR.Under the control of 1wt%MWCNT/5wt% NBR,the nucleation parameters and the surface free energy of the folded chain increased,which showed that the growth rate of spherulites slowed do wn.NBR and nucleating agent can effectively control the crystallization rate of POM,but the crystallinity of POM tends to decrease.control of NBR and nucleating agent,arrived at the same time and temperature of the crystallinity were significantly lower,under its control,the crystallization rate is reduced,and inhibits the high temperature of POM into nuclear capability,higher content of NBR,time of arrival in the same degree of crystal inity and temperature change is more obvious,shows that the crystallization rate of POM can be further reduced,and the other according to different Avrami constant n,mixing way can be found to impact how its nucleus and the growth.Under the control of the polarizing image of the hot table,the size of spherulites in the field of view significantly decreased and the number of spherulites significantly increased under the control of 1wt%MWCNT/5wt% NBR.Under the control of 1wt%MWCNT/5wt% NBR,the nucleation parameters and the surface free energy of the folded chain increased,which showed that the growth rate of spherulites slowed down.NBR and nucleating agent can effectively control the crystallization rate of POM,but the crystallinity of POM tends to decreas.When TPU as inhibitors used with three kinds of nucleating agent,through the experimental data of the project group before,found that the higher the Tm of the TPU best inhibiting effect on POM,so choose TPU-4 as the inhibitors of this chapter,under its control,move to the low temperature crystallization peak temperature,half peak width broadening,crystallization rate is reduced,the other 10 wt % in 1.5 wt % MMT/TPU-4,under the control of the time required to reach the same degree of crystallinity by 0.2 min,lower crystallinity of only 1.85%,regulation of POM to achieve effective through hot stage polarized microscopy observation,It was found that under the control of TPU-4 and nucleating agent,the nucleation time of POM increased significantly,and the number of spherulites in the same field a lso increased.At the same time,under the control of tpu-4 and nucleating agent,the nucleation parameters and the surface free energy of folded chain were both higher or consistent with those of pure POM,and the spherulite growth rate was significantly lower than that of pure POM at the same crystallization temperature.In summary,it was found that under the regulation of 1.5wt%MMT/10wt% tpu-4,the spherulite growth rate of POM significantly decreased,and the crystallinity only decreased by 1.85%,achieving effective regulation. |
PDF Full Text Request