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71.
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Low pressure sheet molding compound (LPMC,1.0-3.0 MPa,95-103 ℃) is a new kind of thermosetting material with crystalline polyester as a physical thickenner.LPMC is different from conventional SMC using an earth oxide thickening agent (e.g.MgO) as chemical thickenner,it relies on the physical thickening of crystalline polyester.Crystalline polyester resin is the key material to mold LPMC parts.Currently there was no report about the thickening mechanism of crystalline polyester in LPMC.In this article,crystalline polyester resins,whose melting points were between 45 ℃ and 89 ℃,were synthesized by a two-step esterification.The melt points of crystalline polyesters are controlled by regulating the mol ratio of the two glycols and the two acids.And by means of varying the content of crystalline polyester resin,the thickening effect on resin paste is investigated.In addition,the thickening mechanism of crystalline polyester in LPMC was investigated by FTIR and DSC analysis.The effects of the diameters and viscosity of crystalline polyester on the rheological property and fiber distribution of LPMC sheets were studied,too.Results show that the thickening effect is excellent when the weight content of crystalline polyester resin is 3%.And there exists three kinds of functions acting in the process of thickening:swelling,hydrogen bonds and induction crystallization.During the preparing process of resin paste in LPMC,the temperature of resin paste must be kept at 90 ℃.In addition,crystalline polyester make LPMC have a perfect fluid property.When the viscosity of LPMC sheet is beyond 1 kPa s,the fiber orientation is not obvious.But when the viscosity of LPMC sheet is about 500 Pa s,the fiber shows a certain degree of orientation.Moreover the study of physical and chemical thickening mechanism of crystalline polyester and the rheological discipline of LPMC sheets in the hot mould will provide the researchers and enterprises with theory guidance. 相似文献
74.
通过增加室温混合阶段并优化关键工艺参数的方式改进了聚氯乙烯(PVC)搪塑粉料的传统混料工艺,针对粒径60~80nm的活性CaCO_3和粒径100~120nm的金红石型钛白粉等纳米颗粒,对比研究了新混料工艺和传统混料工艺对无机纳米颗粒均匀分散的影响,并定量分析了在室温混合阶段的不同工艺参数下PVC搪塑粉料的流动性和搪塑制品的力学性能及热稳定性。结果表明:优化后的干混工艺不仅有效提高了搪塑表皮中纳米颗粒分散均匀性和搪塑粉料的流动性,而且明显改善了搪塑表皮的断裂伸长率、热稳定性和耐热挥发损失性。 相似文献
75.
目前在土木工程,建筑,汽车等领域里使用的复合材料中,玻璃纤维是用量最大的增强材料。由于保护地球环境的呼声日趋高涨,天然纤维被期待着代替源于石油而且再利用困难的玻璃纤维,成为绿色复合材料的必要材料之一。本文通过注射成型工艺制作了玻璃短纤维,大麻短纤维以及混杂型纤维增强复合材料,并在拉伸实验中应用两种不同频率的声发射技术检测了拉伸断裂特性。实验发现,随着大麻纤维的加入和混杂复合材料绿色度的增加,复合材料的拉伸弹性模量随之线性增大,而拉伸强度基本保持不变。当大麻纤维的含量超过20wt%的时候,拉伸强度开始降低。在不同频率的声发射实验中,混杂型复合材料的声发射事件的产生都比单一纤维增强型复合材料要来的晚,也就是说,纤维的混杂有助于推迟微裂纹的产生。 相似文献
76.
《材料与设计》2015
This work evaluated the processibility of a low-isotacticity polypropylene (PP) powder by selective laser sintering (SLS), and systematically analyzed and compared the melting and crystallization characteristics, crystalline structure, tensile properties and thermo-mechanical properties of the PP specimens fabricated by SLS and injection molding (IM). The results show that the PP powder has a nearly spherical shape, smooth surfaces, appropriate particle sizes, a wide sintering window and a low degree of crystallinity, consequently indicating good SLS processibility. In SLS, the molten PP continues to maintain at a high part bed temperature until the whole manufacturing process finished, thus demonstrating a low cooling rate. This gives rise to a high degree of crystallinity, formation of γ phase and coarse microstructure. On the contrary, in IM, the fully molten PP is rapidly cooled down to room temperature after injection, and thus show a higher cooling rate and rapid crystallization, leading to a lower degree of crystallinity, absence of γ phase and finer microstructure. Owing to these differences in crystallization characteristics and crystalline structure mentioned above, the SLS PP parts exhibit higher tensile strengths, tensile moduli and storage moduli, but lower elongation at break, toughness and glass transition temperatures, compared with the IM counterparts. 相似文献
77.
以双酚S为原料,采用溶剂法合成了一种含砜基的苯并恶嗪(BOZ–S),并用不同长度的高性能维纶短切纤维(PVAF)为增强材料,通过预浸料模压成型制备出BOZ–S改性酚醛树脂(PF)模塑料。利用傅里叶变换红外光谱仪、核磁共振仪、差示扫描量热仪以及扫描电子显微镜等分别对BOZ–S的单体结构、PF/BOZ–S改性体系的固化特性、改性PF模塑料的力学性能、热性能以及冲击断口进行了分析研究。结果表明,成功合成出了预期结构的BOZ–S,且固化时存在两种放热反应;改性树脂体系的各项性能与纯PF相比均有明显提升,且纤维的长度越长,模塑料的力学性能提升越大,其中PF/BOZ–S质量比为80/20、纤维长度为36 mm时,体系的综合性能最佳,冲击强度和弯曲强度分别为88.4 k J/m~2和175.9 MPa,弯曲弹性模量达到8.8 GPa,马丁耐热温度为164.3℃。 相似文献
78.
为解决黏结性煤在干馏过程中的黏结问题,采取配煤成型方法,在实验室条件下,以某黏结性原煤为基本原料,通过配入一定比例的不黏结煤和加入适当比例的黏结剂,进行配煤成型破黏研究,重点考察了成型过程中2种原料煤的粒度和配比、黏结剂用量和成型水分等因素对配煤成型破黏效果的影响,最终确定试验煤样配煤成型破黏的最优工况。结果表明:在黏结性煤(粒径≤0.5 mm)、不黏煤(粒径≤1 mm)配比为50∶50,黏结剂用量为1.0%,成型水分为15%的条件下进行配煤成型,得到的型煤干燥后冷压强度可达363 N/个,型焦冷压强度达597 N/个,且低温干馏试验中没有黏结现象,满足了国富(GF)干馏炉的入炉要求。 相似文献
79.
For the purpose of obtaining fully dense B4C with micro hole array of high quality and precision, synthesis of B4C powders were carried out by micro powder injection molding. Five kinds of additive systems were used and their influences on mechanical properties were investigated. The relative density, Vickers hardness, bending strength, and fracture toughness of the B4C ceramics with BS10AY additive sintered at 2000 °C for 2 h could reach 97.56%, 3580.4 HV, 355.3 MPa and 5.87 MPa m1/2, respectively. The temperature was generally 100–200 °C lower than reported temperatures which was attributed to the additives. The improvement in mechanical properties was attributed to grain refinement. A mixture of intragranular and transgranular fractures occurred due to the fine microstructures and the additive systems in the B4C ceramics after sintering. Micro-hole array with the diameter of 450 μm and the length-diameter ratio of more than 8 were obtained. The inhomogeneous filling of feedstock from the substrate to the thin wall between two adjacent micro holes caused the inhomogeneous shrinkage of the substrate and micro holes. 相似文献
80.