Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy

In this work aluminium alloy surfaces have been subjected to three different methods of surface pre-treatments such as solvent degreasing, FPL (Forest Products Laboratory) etching and priming using an epoxy based primer. The treated surfaces were evaluated for surface energy, contact angle, surface topography, surface roughness and adhesive strength characteristics. The influence of surface pre-treatments on the variation of polar, dispersive and total surface energy of the surfaces is addressed. A wettability test was performed on the surfaces using an epoxy adhesive in order to assess the influence of the pre-treatment techniques on substrate/adhesive interaction. Theoretical work of adhesion values for the various pre-treated surfaces were calculated using the contact angle data and further tested experimentally by adhesive bond strength evaluation by tensile testing of a single lap aluminium-epoxy-aluminium assembly. The method of surface pre-treatment showed a profound effect on the surface topography and roughness by AFM. This study reveals that a combination of high surface energy and high surface roughness of the substrate along with good wettability of the adhesive contributed to the highest joint strength for the aluminium alloy through the FPL etching pre-treatment.     

On the use of Washburn's equation for contact angle determination

A systematic study on the possibility of Young's contact angle determination from Washburn's equation was performed using the so-called thin-layer wicking technique in which the rate of penetration of a liquid into the porous layer of a solid is measured. Commercial (Merck) SiO₂ deposited on the glass plate for thin-layer chromatography was used as a model solid and n-alkanes (from pentane to hexadecane), diiodomethane, and α-bromonaphthalene were employed as the probe liquids. It was shown that the contact angle calculated from Washburn's equation was not equal to Young's contact angle of a drop of the same liquid, placed on a flat surface of the solid. Consequently, the solid surface free energy components calculated using contact angles from Washburn's equation are not the true values. However, the approach previously suggested by us has been verified again, as it gives consistent values of the surface free energy components determined from all the liquids used.     

Tailoring intrinsic hydrophobicity and surface energy on rough surface via low-T Cassie–Wenzel wetting transition method

Wettability is an important parameter of micro/nanostructured composites. The measurement of apparent contact angle is strongly affected by surface roughness, which induces some challenges to study the intrinsic hydrophobicity correlating to the nature of chemistry. Carbon-Nafion composites exhibited about 30° decrease in apparent contact angle from 30 to 10°C due to the condensation of water vapor into cavities, suggesting a significant Cassie–Wenzel wetting transition phenomenon. The focus of this work has been on the first-time use of a low-T Cassie–Wenzel wetting transition method to evaluate Young's (ideal) contact angle and surface free energy. A maximum Young's contact angle (113°) and minimum total surface energy (12 mJ/m²) were determined at Nafion content of 70 wt%, indicating the orientation effect that sulfonate groups in Nafion preferentially pointed toward polar carbon. This approach provided the reasonable prediction of intrinsic hydrophobicity, especially when a rough solid surface is not easily wetted by liquids.     

Instability of the three-phase contact region and its effect on contact angle relaxation

Contact angle relaxation was measured for captive air bubbles placed on solid surfaces of varying degrees of heterogeneity, roughness, and stability, in water. The experimental results indicate that both advancing and receding contact angles undergo slow relaxation in these water-air-solid systems, due to instabilities of the three-phase contact line region. It is shown that the advancing contact angle decreases and the receding contact angle increases for many systems over a period of a few hours. Also, examples of reverse progressions are reported. Additionally, in extreme cases, the contact angle oscillates down and up, over and over again, preventing the system from stabilization/equilibration. Four different mechanisms are proposed to explain the contact angle relaxation. These include (i) pinning of the three-phase contact line and its slow evolution; (ii) the formation of microdroplets on the solid surface and their coalescence with the base of the gas bubble, which causes dynamic behavior of the three-phase contact line; (iii) deformation of the solid surface and its effect on the apparent contact angle; and (iv) chemical instability of the solid.     

粗糙固体平面上液滴的接触角滞后的简单流体静力学模型

The phenomenon of hysteresis of contact angle is an important topic subject to a long time of argument.A simple hydrostatic model of sessile drops under the gravity in combination with an ideal surface roughness model is used to interpret the process of drop volume increase or decrease of a planar sessile drop and to shed light on the contact angle hysteresis and its relationship with the solid surface roughness. With this model, the advancing and receding contact angles are conceptually explained in terms of equilibrium contact angle and surface roughness only,without invoking the thermodynamic multiplicity. The model is found to be qualitatively consistent to experimental observations on contact angle hysteresis and it suggests a possible way to approach the hysteresis of three-dimensional sessile drops.     

蒸汽冷凝型态的表面自由能差判据

提出了蒸汽在固体表面上冷凝方式的表面自由能差判据 ,即冷凝温度下液体表面自由能与固体表面的表面自由能差大于 33.3mJ·m^-2 时蒸汽在该表面上呈现滴状冷凝的必要条件 .当表面自由能差在 0与 33.3mJ·m^-2范围内 ,表面强化冷凝传热的效果将取决于表面自由能差值的大小 ,差值越大 ,强化效果越明显 .这对深化表面涂层强化冷凝传热的机理以及选择传热表面涂层材料具有指导意义 .通过与文献报道实验结果的比较证实 ,该判据排除了静态接触角判据中测量温度的影响 .     

Surface energetics and acid-base character of sized and unsized paper handsheets

The surface energetics and acid-base character of paper handsheets were investigated using dynamic contact angle analysis. The surface energies were calculated using both geometric and harmonic mean methods. The surface acid-base property was characterized by calculating the work of acid-base interaction according to Fowkes' theory. To evaluate the effect of sizing on the paper surface properties, handsheets with various sizing treatments were studied in comparison with unsized handsheets. It was found that the sizing on the paper handsheets tends to reduce the surface energy and cover the acid-base sites. The results also show that the handsheet surface can be characterized directly using contact angle analysis.     

超疏水/超双疏表面自修复方式的研究进展

超疏水/超双疏材料存在耐久性和稳定性的问题,因此很大程度影响了超疏水/超双疏材料的实际应用。受自然界中荷叶、三叶草等超疏水表面在受到破坏后,表面粗糙结构和表面组成可以恢复直到生物死亡的启发,科研学者通过不断探索研究出修复超疏水/超双疏材料的一些方式。本文从低表面能物质和表面微观结构的自修复角度出发,综述了影响超疏水/超双疏表面的自修复方式。当超疏水/超双疏表面受到物理破坏或者化学破坏时,失去超疏水以及超疏油性能,在温度、相对湿度、机械、UV等诱导条件下,低表面能物质迁移至表面完成自修复过程以及表面微观结构的自修复,从而使超疏水以及超疏油性能得以恢复。价格低廉的环保材料和系统性地研究自修复的机理是将来超疏水/超双疏自修复材料的主要研究方向。     

不同涂层材料表面CaCO_3污垢特性的研究

采用对比实验的方法,通过对ECTFE、FEP、PTFE和PFA4 4种涂层材料表面的粗糙度、接触角、表面能测量和计算,考察了这些表面特性参数对CaCO3污垢形成的影响。实验结果表明,材料表面特性对污垢形成有较大影响,材料的表面能越低,污垢沉积量越小,而表面的粗糙度、接触角对CaCO3污垢形成的影响还未发现明显的规律。     

固体材料性质对生物垢形成的影响

从固体材料表面性质入手 ,探讨了材料的表面粗糙度、表面自由能、界面能以及材料与生物垢之间的界面能对生物垢形成的影响。实验结果表明 :在 0 2 9～ 1 82 μm内 ,材料的表面粗糙度越大越有利于生物垢的形成 ;在 11 7～ 38 3mN/m内 ,材料的表面自由能和固 -液界面能与生物垢形成之间没有简单、直接的关系 ;在 1 3～ 18 0mN/m内 ,固体材料与生物垢之间的界面能对生物垢的形成存在一个极大值 ,此时的界面能约为 3 4mN/m。     

Evolution of surface chemistry and physical properties during thin film polymerization of thermotropic liquid crystalline polymers

By applying a novel thin film polymerization technique, X-ray photoelectron spectroscopy (XPS), and the Lifshitz-van der Waals acid-base (LWAB) theory, we have determined the time evolution of surface chemistry and surface free energy during the polymerization of liquid crystalline poly(p-oxybenzoate/2,6-oxynaphthoate) at a molar ratio of 50/50. The surface free energy components of these main-chain liquid crystalline copolyesters were calculated from contact angle measurements using a Ramé-Hart goniometer and a three-liquid procedure (water, glycerol, and diiodomethane). The experimental data suggest that the Lewis base parameter (y-) during thin film polymerization decreases rapidly with the progress of polymerization, while the Lewis acid parameter (γ⁺) and the Lifshitz-van der Waals parameter (γ^LW) are almost invariant. The surface roughness data measured by atomic force microscopy (AFM) suggested that the increase in water contact angle (or the decrease in y-) was not caused by the change in surface roughness, but by the change in surface chemistry, i.e. due to the reaction of acetoxy and carboxy groups to release acetic acid during the polymerization reaction. In addition, the XPS results coincide with our previous Fourier transform infrared spectroscopy results showing that the condensation polymerization is much faster in the beginning than in the later stages. Consequently, the decrease in y- in the early stages of the polymerization is well explained.     

接触角法在测定固体表面洁净度方面的应用

介绍了接触角法测定固体表面洁净度的基本原理,接触角的测定方法,包括量角法,测高法(小液滴法和平衡液滴法)。影响接触角法测定准确的主要因素为表面粗糙度,此外温度变化及平衡时间对接触角也略有影响。接触角法在生产中的具体应用有水滴法、水膜法和喷雾法,讨论了它们在测定物体洁净度和判断清洗方法优劣方面的应用。     

Plasma Surface Treatment of Aerospace Materials for Enhanced Adhesive Bonding

The increased use of polyphenylene sulphide (PPS) and polyetheretherketone based composites for aircraft structures has highlighted the need for reliable methods of bonding these materials to metallic components such as titanium. Both composite and titanium adhesive bonds exhibit poor long-term durability when exposed to hot/wet conditions, aerospace fluids and solvents. As a result, surface treatments are employed to enhance surface energy, surface roughness and alter surface chemistry to provide better long-term durability. In this initial study the adhesive bonding of glass fibre reinforced GFR-PPS and commercially pure titanium was investigated. Prior to bonding, both materials were plasma treated using argon and oxygen gases in a RF discharge. Surface characterisation was carried out to optimise these treatments. Surface energy and wettability were examined using contact angle analysis, surface roughness was examined using scanning electron microscopy and atomic force microscopy, while X-ray photo-electron spectroscopy (XPS) was employed to study the surface chemistry. Bond strengths were determined using lap shear tests. Initial results reveal that these optimum plasma treatments produce a significant increase in bond strength.     

Optical Emission Spectroscopy Study and 3-D Surface Characterization of Silicone Rubber Exposed to Different Argon RF Plasma Powers

Abstract

The present study was undertaken to investigate the effect of argon plasma treatment on the extent of Foley catheter surface modification. Foley catheter (flexible tube made of silicone rubber, used for urinary catheterization) surface was treated at different argon (Ar) RF plasma powers. The surface modification was assessed by measurement of contact angle. Contact angle study shows decrease in contact angle and increase in surface free energy shows more polar group incorporation at low power plasma treatment. Atomic force microscopy (AFM) revealed an increased average surface roughness proportional to plasma power. Optical emission spectroscopy (OES) shows various states of argon causing surface modification.     

Plasma Surface Treatment of Aerospace Materials for Enhanced Adhesive Bonding

The increased use of polyphenylene sulphide (PPS) and polyetheretherketone based composites for aircraft structures has highlighted the need for reliable methods of bonding these materials to metallic components such as titanium. Both composite and titanium adhesive bonds exhibit poor long-term durability when exposed to hot/wet conditions, aerospace fluids and solvents. As a result, surface treatments are employed to enhance surface energy, surface roughness and alter surface chemistry to provide better long-term durability. In this initial study the adhesive bonding of glass fibre reinforced GFR-PPS and commercially pure titanium was investigated. Prior to bonding, both materials were plasma treated using argon and oxygen gases in a RF discharge. Surface characterisation was carried out to optimise these treatments. Surface energy and wettability were examined using contact angle analysis, surface roughness was examined using scanning electron microscopy and atomic force microscopy, while X-ray photo-electron spectroscopy (XPS) was employed to study the surface chemistry. Bond strengths were determined using lap shear tests. Initial results reveal that these optimum plasma treatments produce a significant increase in bond strength.     

Thermal stability and surface properties of acrylic PSAs modified by hexafluorobutyl acrylate

The article attempted to prepare special acrylic adhesives with preferable adhesion property and better thermal stability by introducing a fluorinated monomer. The FT-IR result showed that fluorinated monomers and acrylic monomers participated in copolymerization successfully. Furthermore, fluorinated groups performed good compatibility with acrylic resins, based on differential scanning calorimetry curve. According to the TG test under different heating rates, the activation energy of PSAs containing different content of fluorinated monomers was calculated to evaluate the effect of hexafluorobutyl acrylate on heat resistance of PSAs. Then, the findings of contact angle test revealed that the fluorinated PSAs also had rather lower surface energy than ordinary PSAs. Finally, the results of peel strength measurements indicated that the fluorinated PSAs demonstrated excellent adhesion property on various materials, especially low surface energy substrates.     

Adhesion and Wettability Characteristics of Chemically Modified Banana Fibre for Composite Manufacturing

In this work banana fibre was chemically modified using various chemical agents. The surface energy of the fibre is an important parameter and one which governs the interaction of fibre with polymeric matrices. This paper describes the influence of various chemical treatments on the surface energy of the banana fibre investigated by contact angle measurements, spectroscopic analysis and surface morphology studies. The surface energy, work of adhesion, polarity, spreading coefficient, interfacial energy and interaction parameter were determined in the case of raw and chemically modified fibres. Chemical modification has been found to have a profound effect on the surface energy. The polar and dispersive components of the surface energy were also found to be dependent on the chemical treatment involved. The chemical modifications done in this work were: alkali treatment, silanation, benzoylation, formylation, potassium permanganate treatment and acetylation. Of all the modifications, the relative surface energy was found to be a maximum for alkali treated fibre and minimum for silanated fibre. Contact angle measurements were found to be an effective tool in predicting the possible interaction of the fibres with phenol formaldehyde matrix resin. Atomic force microscopy roughness analysis revealed a significant decrease in surface roughness for the chemically modified fibre. An increase both in fibre/matrix adhesion and interfacial shear strength has been observed for all surface modified fibres except for those modified by benzoylation and acetylation.     

The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces

Examples of experimental contact angle data for varying drop and bubble volumes on different solids whose surfaces are smooth and homogeneous, rough and homogeneous, smooth and heterogeneous, and covered with unstable organic films are presented. The ideas and theoretical models as proposed in the literature for the interpretation of contact angle/drop (bubble) size relationships are critically reviewed. It is shown that major factors affecting the contact angle variation with drop (bubble) size such as surface heterogeneity, roughness, and stability, have been identified in the literature. However, there is still a need for experimental work with well-defined and well-characterized solid surfaces. Theoretical models that have been proposed in the literature are still inadequate. Advanced modeling of liquid behavior at heterogeneous and rough surfaces is required to understand further, and to predict, the contact angle/drop (bubble) size relationships at imperfect surfaces.     

The Effect of Drop (Bubble) Size on Contact Angle at Solid Surfaces

Examples of experimental contact angle data for varying drop and bubble volumes on different solids whose surfaces are smooth and homogeneous, rough and homogeneous, smooth and heterogeneous, and covered with unstable organic films are presented. The ideas and theoretical models as proposed in the literature for the interpretation of contact angle/drop (bubble) size relationships are critically reviewed. It is shown that major factors affecting the contact angle variation with drop (bubble) size such as surface heterogeneity, roughness, and stability, have been identified in the literature. However, there is still a need for experimental work with well-defined and well-characterized solid surfaces. Theoretical models that have been proposed in the literature are still inadequate. Advanced modeling of liquid behavior at heterogeneous and rough surfaces is required to understand further, and to predict, the contact angle/drop (bubble) size relationships at imperfect surfaces.     

Surface structure,topology, and liquid wetting behavior in oriented polymers

The structure and the properties of oriented polymer surfaces were studied for three series of uniaxially oriented films of polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET). The surface structure was characterized in terms of relative crystallinity and molecular orientation along with topology and roughness by using FT-IR-ATR dichroism technique, optical microscopy and surface profilometer. In all three polymers, the surface orientation function increases with draw ratio. The relative surface crystallinity and the trans con-former also increases for PP and PET, respectively. In uniaxially drawn PP, the surface becomes rough with increasing draw ratio and the roughness is anisotropic with peaks and valleys elongated along the draw direction. For drawn PP, the equilibrium contact angles for four different liquids all exhibit anisotropy with higher values in perpendicular direction than that in parallel to the draw direction. In contrast, both drawn PET and PS films show smooth surfaces, and the equilibrium contact angles were all isotropic. When roughness is removed from the drawn PP by polishing without altering the molecular orientation, the anisotropy becomes negligible and the contact angles approach the value for undrawn PP. When surface roughness was created deliberately on undrawn PET and PS films, the contact angle anisotropy was clearly observed. Therefore, the anisotropy in surface topology rather than the molecular orientation seems to play a dominant role in developing anisotropic wetting behavior. The equilibrium contact angles for smooth surfaces have been calculated using the experimentally obtained roughness and anisotropic contact angle data from the rough surface. These values are in reasonable agreement with the measured contact angles for smooth surfaces, suggesting that the observed contact angle anisotropy can be attributed entirely to the roughness anisotropy rather than to the molecular orientation.