Adhesive contact on power-law graded elastic solids: The JKR–DMT transition using a double-Hertz model

A cohesive zone model of axisymmetric adhesive contact between a rigid sphere and a power-law graded elastic half-space is established by extending the double-Hertz model of Greenwood and Johnson (1998). Closed-form solutions are obtained analytically for the surface stress, deformation fields and equilibrium relations among applied load, indentation depth, inner and outer radii of the cohesive zone, which include the corresponding solutions for homogeneous isotropic materials and the Gibson solid as special cases. These solutions provide a continuous transition between JKR and DMT type contact models through a generalized Tabor parameter μ$\mu$. Our analysis reveals that the magnitude of the pull-off force ranges from (3+k)πRΔγ/2$(3+k)\pi R\mathrm{\Delta }\gamma /2$ to 2πRΔγ$2\pi R\mathrm{\Delta }\gamma$, where k$k$, R$R$ and Δγ$\mathrm{\Delta }\gamma$ denote the gradient exponent of the elastic modulus for the half-space, the radius of the sphere and the work of adhesion, respectively. Interestingly, the pull-off force for the Gibson solid is found to be identically equal to 2πRΔγ,$2\pi R\mathrm{\Delta }\gamma ,$ independent of the corresponding Tabor parameter. The obtained analytical solutions are validated with finite element simulations.     

可粘贴超疏水薄膜的制备

通过聚二甲基硅氧烷(PDMS)与碳纤维织物复合, 采用模板法在PDMS聚合物表面构筑微阵列结构, 制备了一种具有可重复粘贴性的超疏水薄膜. 研究结果表明, 该薄膜微结构表面的接触角为154°, 滚动角为14°, 具有低黏附的超疏水特性. 而PDMS与碳纤维织物的紧密结合, 赋予了超疏水薄膜较高的黏接力和力学性能, 断裂强度达到116.96 MPa. 所制备的超疏水薄膜可粘贴于多种材料表面, 同时经过30 d的长时间粘贴以及50次的循环粘贴后, 该薄膜依然保持着较高的黏附性能及超疏水特征, 表明超疏水薄膜具有良好的力学稳定性及耐久性, 满足长时间可重复使用的要求, 可应用于对破损超疏水涂层的快速、 大面积粘贴修复.     

HL-2M 装置环向场线圈匝间绝缘工艺研究

HL-2M tokamak adopts the demountable toroidal field coils (TFCs) structure in order to assemble the vacuum vessel chamber and poloidal field coils (PFCs) integrally. TFCs are subjected to high shear force, which require high electrical performance and adhesive strength between the inter turn insulation and the copper plate. Baking and curing impregnated glass cloth and epoxy glass cloth under different pressure, then adhesive properties have been researched by shear tests. The results show that there are some differences in the adhesive properties between the turn to turn insulation and the copper plate under different structural combinations and different pressures. Appropriate structural combination and pressure of hot baking and curing were selected for TFCs.     

透明硅树脂的合成及应用

本文概述了透明硅树脂的合成、结构、性能、特点以及在各行各业中的应用。     

Adhesion and tack of polymers: Influence of mechanical properties and surface tensions

The adhesion of various polymers used as model adhesives, polyisobutylene, polyacrylates etc. has been investigated by means of an apparatus measuring the adhesive failure energyw in dependence on contact time, contact pressure, rate of separation, and temperature. The adhesive failure energy of adhesive joints formed with low contact pressure during a short contact time is called tack. After a sufficiently long contact time and with a high bonding pressure an adhesive joint exhibits its maximum energy of separationw _m .The viscoelastic properties of the model adhesives were characterized by creep experiments in dependence on time and temperature. The surface tension of the polymer adhesives and adherents could be determined by contact angle measurements. Adhesion measurements of polyisobutylene on a number of adherents were carried out in air and in various liquids in order to obtain information about the influence of surface tension on tack and maximum adhesive failure energy. w _m can be written as the product of two terms: the thermodynamic work of adhesionW _A which is related to the surface and interfacial tensions of adhesive and adherent and a dimensionless function dependent on temperature and rate of separation which describes the viscoeleastic properties of the adhesive and which obeys the rate-temperature superposition principle known from linear viscoelasticity. The tack is related to incomplete bond formation and cannot be described in the same manner. It is, however, strongly dependent on the viscoelastic properties of the adhesive showing a maximum at about 50 to 70 °C above the glass transition temperature. It is, moreover, influenced by the compliance in the plateau range above the glass transition which is determined by the entanglement network of the polymer. Wetting of the adherent by the adhesive is a further important condition for high tack values which is fulfilled if the adherent has a higher surface tension than the adhesive.     

Synthesis, polymerisation and degradation of poly(lactide-co-propylene glycol) dimethacrylate adhesives

Fluid ABA triblock poly(lactide-co-propylene glycol-co-lactide)s with 7, 17 or 34 propylene glycol and 2, 4, or 8 lactic acid units in each B and A block, respectively, end capped with methacrylate groups were prepared and polymerisation rates upon blue light exposure for 60, 120 or 240 s using 0.5, 1 or 2 wt.% initiators determined. The shortest monomer formulation, with 0.5 wt.% initiators, was most promising as an injectable degradable adhesive for biomedical applications, achieving 96% polymerisation by 120 s after the start of 60 s illumination, forming then a semi-rigid polymer that in water released linearly with time poly(methacrylic-co-lactic) acid degradation products. Over 14 weeks, 2 mm thick specimens largely maintained their dimensions as water sorption balanced the 19 wt.% material loss. Raising initiator concentration or monomer length reduced polymerisation rate on the lower surface of samples. Increasing the number of lactic acid units in each A block from 2 to 8 also enhanced photocrosslinked polymer water sorption and increased average total mass loss to 60 wt.% over 14 weeks but acid product release rate decreased with time. Monomers produced with longer polypropylene glycol B blocks required longer periods of light exposure for full cure and the final polymers exhibited slower non-linear hydrolytic degradation.     

Modeling via the internal energy balance and analysis of adhesive contact with friction in thermoviscoelasticity

In this paper we introduce and investigate a model for adhesive contact with friction between a thermoviscoelastic body and a rigid support.A PDE system, consisting of the evolution equations for the temperatures in the bulk domain and on the contact surface, of the momentum balance, and of the equation for the internal variable describing the state of the adhesion, is derived on the basis of a surface damage theory by M. Frémond.The existence of global-in-time solutions to the associated initial–boundary value problem is proved by passing to the limit in a carefully tailored time-discretization scheme.     

Inhibation of crystal growth during drying in gels derived from a cheap,mixed metal oxide precursor

The influence of addition of small amounts of either citric acid or lactic acid on the formation of crystalline matter in dried gels derived from a multi-component industrial sol–gel silica precursor has been studied. The sols were water-based and had formic acid as the main acid constituent. A pronounced decrease in the extent of crystallization was observed for both acids, with citric acid being more effective than lactic acid. The results are discussed based on the complexation behavior of the corresponding acids under the studied conditions, and the complexation behavior in solution can be directly linked to the extent of crystallization in the dried gels. However, the sol–gel kinetics followed that expected for a purely silica-based sol, which suggests that the kinetics is mainly controlled by the silica portion of the sol. The results are suggested to be of importance for the industrial use of these sols as binders, as pronounced crystallization in the gels upon drying may lead to mechanical stresses, and thus to a decreased binder performance.     

木质素活化改性制备酚醛树脂胶黏剂研究进展

木质素由于化学结构与苯酚相似,通过活化改性可部分替代苯酚制备木质素改性酚醛树脂胶黏剂。既可降低成本、达到生物质资源高效利用的目的,并且制备的木质素改性酚醛树脂胶黏剂有毒残余较低,具有环保意义,是合成制备生物质高分子材料的重要途径。本文综述了国内外研究人员在木质素活化改性制备酚醛树脂胶黏剂研究领域的最新进展,重点介绍了化学改性、物理改性、生物改性等木质素活化改性方法,比较了不同改性产物制备酚醛树脂胶黏剂的性能,并对影响木质素活化改性制备酚醛树脂胶黏剂实现工业化应用的主要因素进行了分析。     

基于Norland紫外固化胶的热光开关器件

采用新型聚合物材料Norland紫外固化胶（NOA）制备了聚合物M-Z型热光开关器件。对NOA薄膜材料的光学性质进行了表征,采用感应耦合等离子体（ICP）方法制备出形貌良好的波导器件。测得在1 550 nm波长下,长2.2 cm的直波导插入损耗为8.3 dB。在电极上施加直流信号,测得热光开关的消光比为11 dB,驱动功率为85 mW。引入直流偏置网络,获得了器件的开关特性曲线,测得开关器件的上升时间为1.085 ms,下降时间为489.5μs。实验结果表明：NOA材料在热光开关及其它聚合物光波导集成器件的制备中具有很大的应用潜力。