Study On The Modification, Processing And Application Of Polyoxymethylene

Polyoxymethylene (POM) is one of the five common engineering plastics.Although the domestic capacity of POM has excessed, but the key technologies of synthesis and modification of POM has not been realized localization, which caused the operating rate of production equipment low, the product brand single, the product grade general, the homogenization competition serious. As a result, most of the production of POM business is at a loss, and a part of the production projects have been discontinued, restricting the development of China’s POM industry. This paper firstly studied the synthesis of POM, designed the first device for the synthesis of POM domestically, and synthesized fiber grade POM. Secondly, researches on the reinforcement, the toughening, the flame retardant, the wear resistance and weather resistance, and the low formaldehyde emission of POM were carried out to improve product performances and to rich product brands. Thirdly, the key technologies for the preparation of bars, films and fibers using POM were achieved, and POM fibers were applied to reinforce the concrete and mortar.The achievements expand the applications of POM. The main works are as follows:1. Through systematically analyzed the properties of melt flow, thermal stability, molecular weight and its distribution, crystallinity and molecular chain structure of the commercial POM resins, the quality evaluation method for POM resins was established, and the micro structure information of different POM resins was obtained. On the basis of a large number of laboratory results about the synthesis of POM in autoclave, the first set of two grade continuous extrusion device for POM polymerization was designed and manufactured in China. Using this device, 9 different kinds of POM products were synthesized, and the No. 5 and No. 6 products were demonstrated to be suitable for melt spinning.2. In order to improve the performance of POM, such as strengthening, flame retardant, weather resistance, wear resistance, toughness and low formaldehyde emission, the studies on the modification of POM were performed.(1) In aspects of reinforced POM, the technologies of screw element rearrangement and crystallization interference were applied to prepare fiber reinforced POM composites by common double screw extruder, and the fiber content of the as-prepared POM composites is as high as 40%. At the same time, through combining the special designed melt impregnation die and common twin-screw extruder, the preparation process of continuous fiber reinforced POM composites was developed, and the method could control the fiber content and the ratio of length to diameter of accurately. When the basalt fiber content is 40 wt%, the tensile strength of as-prepared POM composites reached more than 3 times of pure POM.(2) In aspects of flame retardant POM, by compounding nitrogen flame retardant system, phosphorus flame retardant system and nitrogen- phosphorus flame retardant system, high efficient carbon forming agent assisted POM halogen-free flame retardant system and flame retardant masterbatch technology were developed. The vertical burning level of as-prepared POM composites is UL94-VO, owning tensile strength of 50.2 MPa, bending strength of 63.8 MPa and izod notched impact strength of 5.8 kj/m2 at the same time.(3) In aspects of weather resistant POM, using traditional UV absorbers,light stabilizers, antioxidants and weather resistant agents, together with free radical quenching agents, formaldehyde absorbents and UV shielding agents,weather resistant POM composites were prepared after increasing the molecular weight of small molecules through polycondensation and grafting polymerization After 1600 h UV aging, the tensile strength, flexural strength,notched impact strength and elongation at break of the POM composites still keep 110.2%, 106.3%, 87.2% and 90.8%, respectively. And after 1600 h hot oxygen and hot water aging, the mechanical properties of the POM composites change little.(4) In aspects of toughening POM, we have found out a kind of polyurethane elastomer, which is compatible with POM and has the same melting point and viscosity with POM. By using the technologies of crystallization interference and screw assembly, super toughing POM composites were prepared.(5) In aspects of wear resistance and toughening POM, by mixing novolac resin and polyoxyethylene, which both play a role of interference crystallization, PTFE fiber, carbon fiber, basalt fiber and other organic/inorganic fibers, POM composites have the advantages of abrasion resistance and toughness were prepared, and the toughening and wear mechanism were analyzed.(6) In aspects of low formaldehyde emission of POM, by blending devolatilization agents, antioxidants, light stabilizers, formaldehyde scavengers and formic acid absorbents together with POM, and by strengthening the vacuum devolatilization process at the same time, low formaldehyde emission POM was prepared.3. This paper has studied the preparation technologies of POM rods, POM films and POM fibers, especially the preparation technology of POM fiber and the application in reinforcing concrete and mortar.(1) The effects of melt temperature, melt pressure and cooling time on the roundness and core structure of POM rod were researched. It is demonstrated that melt temperature has little influence on the roundness of POM bar. With the melt pressure and cooling time increases, the diameter of white core of POM bar decreases. The preferable preparation process of POM bar is the melt temperature of 180℃, the melt pressure of 7 MPa, and the cooling time of 30 s.(2) Due to the characteristics of high crystallinity and high crystallization speed of POM, we studied the flowability of POM melt under different temperatures. Through improving the traditional blowing process, POM films were prepared.(3) According to the results of DSC and TG analysis and the actual spinning of POM, we determined the optimum temperature of the melt electrospinning of POM is 190℃, and prepared POM micro-fibers. The effects of receiving distance, spinning temperature and POM content on the diameter of POM micro-fibers were revealed.(4) We have designed the melt spinning process of POM fibers,manufactured a set of experimental equipment, and studied the spinning process and the processing conditions of the POM nascent fibers. The influences of the spinneret draft ratio, spinning temperature and drawing speed on the tensile strength and modulus of the obtained POM fibers were also researched. Finally, we have successfully prepared POM fibers with excellent performances.(5) The effects of POM fibers on the flexural and compressive resistance of concrete were studied. When the length is 6 mm, the diameter is 22 μm and the filling amount is 1%, POM fibers reinforced concrete shows the highest performance. Compared to the results of polyvinyl alcohol fibers and polypropylene fibers reinforced mortar, the flow of POM fibers reinforced mortar changes minimum in the same dosage, and POM fibers could obviously improve the frost resistance of mortar.