Effect Of Spherical Growth Inhibitors On Crystallization Of POM

Because of rapid crystallization and high shrinkage(3-3.5%),POM could only directly form thin-walled small-size parts.Thick-walled and large-size parts could not be directly formed because of surface depression,warping,or even internal voids,which restricts the size and application of products.The root cause was the formation of high crystallinity and large grains at fast crystallization rate.Cooperative control of crystallization behavior during the forming process to reduce the crystallization rate to meet rapid prototyping,while maintaining high crystallinity and forming fine grains to ensure the high strength and high rigidity of products was the solution.At the same time,spherulite growth inhibitor and nucleating agent were used to co-regulate the crystallization behavior of POM to satisfy the forming of thick-walled and large-sized parts,that is to say,the inhibitor was used to control or interfere with the ordered arrangement of POM molecular chains.The crystallization rate is forced to decrease to meet the rapid prototyping of large parts.At the same time,nucleating agent is used to reduce the nuclear energy in the environment of restricted growth,increase the nuclear density,ensure the high crystallinity and refine the grain.As part of the work of the research group,a polymer with different degree of interaction with POM was used as the growth inhibitor of spherulite:(1)hydrogen bond with POM:thermoplastic polyurethane(TPU)and thermoplastic phenolic(PF);(2)compatibility with POM:nitrile rubber(NBR).In the processing,the polymer and the POM are formed to form an aggregation state structure for inhibiting the growth of the spherulites,and the orderly arrangement process of the segments was held down or interfered,so that the growth rate of the spherulites was forced to be multi-layered,and the crystallization rate was reduced.The effects of the inhibitor on the crystallization behavior,the non-isothermal crystallization kinetics,the nucleation time,the nucleation density,the nucleation density and the isothermal crystallization kinetics,the crystalline morphology and the crystalline structure of the inhibitor on the POM crystallization behavior,the non-isothermal crystallization kinetics,the nucleation time,the nucleation density and the isothermal conditions were studied.The results show that:When TPU was used as inhibitor,the initial crystallization temperature(T_i)and peak crystallization temperature(T_c)of POM were almost unaffected,and the crystalline enthalpy of POM decreased obviously,indicating that the crystallization was inhibited,and the higher the T_m of TPU,the stronger the interference ability to the crystallization of POM.The crystallization inhibition ability of polyester type TPU is better than that of polyether type.In polyester-type TPU,the inhibitory effect of TPU-4on the crystallization of POM was the most obvious.However,when TPU-4 is5-10wt%,the moderate increase of its dispersion(rotation speed)in the matrix is not conducive to its inhibiting effect on POM crystallization,but it makes the crystallization multi-layer increase a little.The relative crystallinity of POM decreased with the addition of TPU-4,but the nucleation mode and spherulite growth mode of POM did not change much.Although the nucleation time of POM was prolonged by TPU-4,it was not possible to decrease the nucleation rate of POM.The growth rate of POM spherulites decreased obviously with the increase of crystallization temperature,and the growth rate decreased more obviously with the increase of crystallization temperature.However,when the content was 5-15wt%,the change of content had no obvious effect on the growth rate.At the same time,the addition of TPU-4 increases the nucleation parameter K_g and the surface free energy?_e of the folding chain during the crystallization process of POM,indicating that the nucleation difficulty increases and the energy needed to overcome the spherulite growth increases.The grain size of POM was refined to some extent by several kinds of TPU,and the TPU was encapsulated between or excluded from the microfibers of POM,which had no effect on spherulite and no effect on the crystalline structure of POM.When NBR was used as inhibitor,the crystallization of POM was obviously inhibited,which showed that T_i and T_c gradually moved to low temperature,(35)H_c decreased significantly,and FWHM increased.This inhibition was related to the content of NBR and the distribution of its phase morphology:the increase of the content of NBR increases the inhibition of the crystallization of POM.When the content of POM was up to 10wt%,the crystallization of POM was obviously interfered,resulting in a typical fractional crystallization phenomenon.The dispersion of NBR in POM was increased,and the inhibition of POM crystallization was strengthened.The crystallinity of POM decreased with the addition of NBR,and decreased with the increase of NBR content.With the increase of the content of NBR,the Avrami exponent n decreased slightly,and the corresponding crystallization rate constant Z_c decreased.When the crystallization temperature was higher than 156~oC,the nucleation time of POM increases when the content of NBR was 10wt%,but the nucleation density did not decrease but increases to a certain extent.When the content of NBR was less than 10 wt%,the crystallization rate of POM decreases obviously,and the K_g and?_e decrease,and the energy needed to overcome spherulite nucleation decreases,indicating that the nucleation difficulty decreases.Under the inhibition of POM,the spherulite perfection becomes worse and the spherulite microfiber ordering was very poor.NBR was encapsulated between POM microfibers or excluded from the microfibers,so that the POM grains were refined,but the negative photosensitivity of spherulites was not affected.Moreover,NBR had no effect on the crystalline structure of POM.When PF was used as inhibitor,the crystallization of POM was obviously inhibited:the FWHM of DSC increased,which indicated that the crystallization rate was enhanced at multiple levels.With the increase of the content of PF,the T_i and T_c of POM decreased slightly,and the degree of dispersion of PF in the matrix increased,and the crystalline inhibition of PF increased.Under PF inhibition,the grain refinement and spherulite perfection of POM became worse.Especially when the content of PF was up to 7wt%,the grain was broken.The increase of nucleation parameters and the surface free energy of folding chain indicated that the nucleation difficulty increases and the energy needed to be overcome for spherulite growth was increased.The inhibition of PF on the growth rate of POM spherulites depends on its content,and 3wt%PF can significantly decrease the growth rate of POM spherulites.At the same time,it can be concluded that the addition of PF can inhibit the spherulite growth of POM to a certain extent,and the inhibition effect of PF on the crystallization of POM was the best at the content of 7 wt%.In addition,PF had no effect on the crystal structure and negative photosensitivity of the spherulites of POM.