基于大制件快速成型的聚甲醛加工中结晶速率及结晶度协同控制研究

Because of rapid crystallization, high crystallinity and large crystalline grains, the thick walled and large size parts of polyformaldehyde can not be rapid processed. The fundamental way is to synergetic control the crystallization behavior, which make the crystallization rate decrease to meet the rapid prototyping of POM large size parts, and simultaneously maintain high crystallinity and fine crystalline grains to ensure the products with high strength and high rigidity characteristic. However, it is very difficult to decrease the crystallization rate and maintain high crystallinity simultaneously in crystallography. The project plans to use spherulite growth inhibitors to restrict or interfere with the molecular chain ordering process, which forces POM with a decreased crystallization rate to meet its rapid prototyping. At the same time, the nucleation agents are used to reduce the nucleation energy of the confined POM chains, and to increase the nucleation density，ensuring high crystallinity and making the crystalline grain smaller. Ascertain both the restraining mechanism and the influencing patterns of inhibitors on the spherulite growth as well as the crystallization kinetics. Clarify the nucleation mechanism and crystallization acceleration of nucleation agents to POM crystallization with confined growth. Coordinate the contradiction between the crystallization rate decrease by inhibitors and the increase by nucleation agents. Balance the acceptance or reject of the inhibitors decrease both crystallization rate and crystallinity. Obtain the regular patterns between the controlling of crystallization rate and the maintaining of high crystallinity. Achieve a moderate crystallization rate with high crystallinity and fine crystalline grains. The implementation of the project will effectively improve the processing limitations of polyformaldehyde products, and laying a solid theoretical foundation to achieve the rapid processing of thick walled or large size parts of POM.

聚甲醛因快速结晶形成高结晶度和大晶粒使厚壁大尺寸制件无法快速成型，协同调控结晶行为使结晶速率下降满足快速成型、保持高结晶度并形成细化晶粒确保制品高强高刚特性是解决途径，但同时降低结晶速率与保持高结晶度在结晶工程学上难以兼得。项目拟采用结晶生长抑制剂牵制或干扰POM分子链有序排列过程、迫使结晶速率下降以满足快速成型，同时运用成核剂降低生长受限环境中的成核能，提高晶核密度，确保高结晶度并细化晶粒。探明抑制剂对球晶生长和结晶动力学的抑制机理及影响规律；揭示成核剂对生长受限POM晶的成核机理及结晶促进规律。协同控制抑制剂降低结晶速率和成核剂提高结晶速率之矛盾、平衡抑制剂既降低结晶速率又降低结晶度之取舍、建立结晶速率控制和高结晶度保持之间的调控规律、达到“温和结晶并形成高结晶度和细化晶粒”之目的。项目实施将有效改善POM制品的加工局限性，为实现厚壁大尺寸制件的快速成型奠定坚实的理论基础。

针对聚甲醛（POM）快速结晶并形成高结晶度和大晶粒的结晶特性以及由此导致的只能直接成型薄壁小尺寸制件、无法连续快速成型厚壁及大尺寸制件的弊端，项目采用抑制剂干扰链段迁移、增大片晶生长的次级成核能和表面自由能，迫使生长速率下降，同时采用成核剂提高初级晶核密度以补偿因抑制剂导致的结晶度下降，并形成细化晶粒。结果表明：.热塑性酚醛树脂（PF）是POM结晶的高效抑制剂，能显著降低其结晶速率和结晶度（尤其在高分散状态）。3wt%PF可使结晶峰半高宽（FWHM）增大34%，起始结晶温度（Ti）和峰值结晶温度（Tc）分别向低温移动1.2℃和2.0℃；5wt%PF使Ti和Tc分别向低温移动1.6℃和2.5℃，FWHM增大~42%，这是迄今发现的使POM的Tc降低最为显著的最低异质组分用量。非核壳型长链丙烯酸酯聚合物R-1兼具结晶速率抑制剂和成核剂功能，既降低结晶速率又提高结晶度。热塑性聚氨酯（TPU）更倾向降低结晶度，对结晶速率的抑制逊色于PF。NBR的抑制作用强烈依赖其用量和分散。.筛选出三种高效成核剂（MMT、MWCNTs、Diatomite）和一种复配磷酸酯类有效成核剂N，其对POM结晶的诱导成核能力为：MWCNTs>>MMT>Diatomite>N。MWCNTs诱导成核能力尤为突出，1.0wt%MWCNTs使纯POM结晶焓提高21.2J/g。MWCNTs与PF协同可实现使结晶速率下降、却保持高结晶度并形成细化晶粒的调控目标，譬如3wt%PF和0.7wt%MWCNTs使结晶焓比纯POM提高5.3J/g，FWHM增宽40%。成核剂N分别与TPU和R-1协同均可实现调控目标：0.3wt%N协同5wt%R-1使Tc比纯POM低1.5℃，FWHM拓宽~34%；0.3wt%N使TPU抑制下POM的Tc向低温移动1.7℃，FWHM拓宽~36%。三种高效成核剂因选择性分布在抑制剂中而使部分成核作用 “失效”，典型案例为NBR和TPU为抑制剂时，三种成核剂均不能将因抑制剂降低的结晶度提上去。.运用双螺杆挤出机实现了工程化，发现高剪切高分散下调控效果更为显著。1wt%MWCNTs使97/3POM/PF的Tc比纯POM低3.3℃，结晶焓增大10.0J/g，FWHM拓宽48.5%。项目实施为实现POM厚壁及大尺寸制件的连续快速成型奠定了坚实的理论与实践基础。