Physico-Chemical Criteria for Maximum Adhesion. Part I: Theoretical Concepts and Experimental Evidence

This paper presents an analysis of the literature with the aim of defining basic criteria and developing a general model to describe joint strength. Two particular cases of the relationship: cosθ = f(γ_L) have been identified as prerequisites for further analysis of interfacial phenomena and conditions governing their existence were discussed.

The fact has been pointed out, based on available experimental results, that for the most important case in practice where 0.6 ≤ cosθ ≤ 1.0, the relation cos θ = f(γ_L) can be treated as rectilinear. This finding will be utilized in the comprehensive development of criteria defining joint performance in Part II.

Variability of the interaction factor Φ for various systems has been investigated in relation to cos θ, for the identified particular cases of the relationship cos θ = f(γ_L) A special value of the interaction factor, ₀, was found. The importance of the rectilinear particular case of cos θ = f(γ_L) was shown, which involves constant factor Φ₀ instead of variable Φ.     

Interface/interphase engineering of polymers for adhesion enhancement: Part II. Theoretical and technological aspects of surface-engineered interphase-interface systems for adhesion enhancement

Part I of this paper reviewed the theoretical principles of the macromolecular design of polymer interface/interphase systems for obtaining maximum adhesion and fracture performance of adhesively bonded assemblies. In Part II a novel, relatively simple and industry-feasible technology for surface-grafting connector molecules is demonstrated and discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theory, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Thermodynamic Criteria for the Maximum and Minimum Strength of Fibre-Reinforced Composite Materials

The overall performance and reliability of composite materials are, in most cases, dependent upon the behaviour of the reinforcement-matrix interface, particularly upon its ability to transfer stress.

A theory for predicting thermodynamic conditions for the maximum and zero-adhesion at the reinforcement-matrix interface is tested in this paper, based on experimental data. Proposed is a model of the relationship between mechanical properties of composite materials (tensile strength, flexural strength, Young's modulus and impact resistance) and energetic properties of matrix and reinforcement expressed by the energy ratio a = γ_l/γ₂.     

INTERFACE/INTERPHASE ENGINEERING OF POLYMERS FOR ADHESION ENHANCEMENT: PART II. THEORETICAL AND TECHNOLOGICAL ASPECTS OF SURFACE-ENGINEERED INTERPHASE-INTERFACE SYSTEMS FOR ADHESION ENHANCEMENT

Part I of this paper reviewed the theoretical principles of the macromolecular design of polymer interface/interphase systems for obtaining maximum adhesion and fracture performance of adhesively bonded assemblies. In Part II a novel, relatively simple and industry-feasible technology for surface-grafting connector molecules is demonstrated and discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theory, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Interface/Interphase engineering of polymers for adhesion enhancement: Part I. Review of micromechanical aspects of polymer interface reinforcement through surface grafted molecular brushes

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material.It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples.Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Mode-l Fracture Behaviour of Adhesive Joints. Part II. Stress Analysis and Constraint Parameters

The constraint effect on the fracture behaviour of a rubber-modified epoxy was investigated using compact tension (CT) adhesive joints. An elastic-plastic finite element analysis was conducted to evaluate the stress distribution ahead of the crack tip in the bulk adhesive and adhesive joints of different bond thickness. The models with sharp and finite radius crack tips were evaluated in the analyses. The constraint effect of adherends on the stress triaxiality ahead of the crack tip in the adhesive joints were discussed. The constraint parameters were investigated using the J-Q theory and the J-CTOD relationship. It was found that as the adhesive thickness was increased, the stress triaxiality ahead of the crack tip was relieved by the remarkable deformation of the adhesive material. Similarly, the crack tip constraint was reduced with increasing bond thickness so that the fracture energy increased towards the value of the bulk adhesive. A higher constraint was associated with a lower fracture energy and vice versa. Furthermore, the J-integral did not have a unique relationship with the crack-tip opening displacement (CTOD) for different adhesive bond thickness, as this depends on the constraint around the crack tip. The results of this study will help improve reliability assessment of adhesive joints in engineering applications.     

Morphology and failure in nanocomposites. Part II: surface investigation

Polymer composites filled with calcium carbonate (CaCO₃) nanofillers (<100 nm), and kaolin filler of layered structure, both well suited to create nanocomposites, were analysed. The aim of this study was to investigate the influence of surface properties of the filler and matrix on the adhesion parameters at the interface in composites. The inverse gas chromatography, contact angle and capillary measurements were used for the surface characterization of filler and matrix. Although these methods are based on different assumptions, we found the same trends in the effects of filler surface treatment and/or matrix chemical structure on the changes in the dispersive and polar components of the surface energy. The energetics of the filler and matrix was varied in order to investigate the work of adhesion, interfacial energy and coefficient of spreading, and their influence on the composite properties. We found that the surface treatment of calcium carbonate filler lowered the filler surface energy and the work of adhesion in the composite with poly (vinyl acetate) matrix. The mechanical, thermal and morphological properties of the composite with treated CaCO₃, measured in the first part of this paper, indicated a weak and thin interphase. In the composite with kaolin filler the higher interaction with the polyacrylate copolymer matrix based on styrene as compared to the one based on methyl methacrylate, was confirmed by the higher work of adhesion in the interphase, resulting in a stronger reinforcing of the composite.     

Adhesion Science and Surface Analysis. Typical Examples

A number of surface modification methods using mechanical, chemical, electrochemical or physical processes have commonly been employed to treat adherend surfaces and improve adhesion in various bi- or multi-layered systems which play a key role in many advanced technologies. Results reported in this review paper deal more particularly with components of polymer-metal systems and provide typical examples of surface analysis obtained by x-ray photoelectron spectrometry (XPS), ion scattering spectrometry (ISS), low-energy electron induced x-ray spectrometry (LEEIXS), optically stimulated electron emission (OSEE) and Fourier transform infra-red spectrometry (FTIR). These examples concern, on the one hand, metallic adherends (aluminum, stainless steel, zinc-coated steel, gold) the surfaces of which were modified by cleaning, etching, oxidation, conversion or chemical grafting and, on the other hand, bonded joints (polyester-steel and epoxy-aluminum systems) which were delaminated using a peel test and a three-point flexure test, respectively.     

Optimum Polymer - Solid Interface Design for Adhesion Strength: Carboxylation of Polybutadiene and Mixed Silanes Surface Modification of Aluminum Oxide

To understand the optimum design of polymer-solid interfaces for adhesion strength, model polymer-solid interfaces of carboxylated polybutadiene(cPBD) adhered to mixed silane modified Al₂O₃ surfaces were examined. The cPBD, having various ?COOH sticker group concentration φ(X) (0 ～ 10 mol%), was synthesized through high-pressure carboxylation of PBD, while Al₂O₃ surfaces were modified to have various -NH₂ density, φ(Y) (0 ～ 100 mol%), using self-assembly of mixed amine- and methyl-terminated silanes. The coadsorption kinetic model of the two silanes was analyzed through X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and dynamic contact angle (DCA), which gave the capability of controlling the receptor concentration of aluminum oxide surfaces. The polymer surface chain responses after exposure to various media were understood by measuring contact angle changes of various probe liquids. T-peel tests of the model polymer–solid interfaces, as a function of time and sticker and receptor group concentrations showed much longer time dependence than the characteristic time of a bulk polymer chain. Additionally, the classical equation of interface failure was re-examined to see the effects of deformation rate, annealing temperature, and annealing time. A simple scaling analysis of free energy of an adsorbed polymer on a solid surface was extended to predict the adhesion potential of the model polymer–solid interfaces. From the experiments and theory of adhesive vs. cohesive failure, it was found that there existed an optimum product value r* = φ(X)φ(Y)χ of sticker concentration φ(X), receptor concentration φ(Y), and their interaction strength χ, which was approximately 150 cal/mol for this polymer–solid interface. Below or above this optimum product value r*, the fracture energy of polymer-solid interfaces, G _IC, was less than its optimal value, G _lc*.     

Characterization Of Water Quality Criteria for Ozonation Processes. Part II: Lifetime of Added Ozone

The lifetime value of ozone in a water is one of the key parameters for describing the characteristics of a water considering an ozonation process and for planning, optimizing, and steadily adapting the ozonation process. This lifetime is highly sensitive with respect to the chemical composition of the water. Interferences of different radical-type chain reactions make it difficultto predict it solely from analytical characterizations. The waters with respect to the lifetime of ozone, therefore, have to be characterized by direct laboratory experiments. However, because the half-life times depend on the type of operation used for the measurement, it is appropriate to formulate corresponding standardized guidelines that allow comparison whenever the responses of different waters have to be compared, and whenever characterizations have to be followed during the decades during which a water utility applies a process.     

End-capped fluoroalkyl-functional silanes. Part II: Modification of polymers and possibility of multifunctional silanes

This work reports the fluoroalkylation of polymer surfaces using novel oligomeric silanes having end-capped fluoroalkyl groups. Polymer surfaces such as cellulose, poly(ethylene terephthalate) (PET), polyethylene, and poly(methyl methacrylate) (PMMA) etc. were effectively modified by these oligomeric silanes as well as the glass surface. From the contact angle measurements, the dispersive and polar components of surface free energies were reduced to 15–20 and 1–3 mJ/m², respectively, and the surfaces were shown to be both highly water- and oil-repellent. Modified cellulose and PET surfaces were analyzed using XPS measurements. In the case of cellulose, a linear correlation was observed between the dispersive component of surface free energy γ_S ^d and the area ratio of the F1s peak to the Si2p peak. In the case of PET, the hickness of siloxane layer on the surface was shown to be less than 8 nm. The modified PET surface showed a high solvent durability against common organic and inorganic solvents except fluorochemicals and alkalis. The structure of the siloxane layer on the modified surface is discussed in terms of a network interphase model. It was also shown to be quite easy to add another function such as hydrophilicity (flip-flop character) and/or antibacterial property in addition to the water- and oil-repellency imparted by fluoroalkyl groups.     

A Combined-Type Fluid-Bed Dryer Suitable for Integration Within a Lignite-Fired Power Plant: System Design and Thermodynamic Analysis

Lignite is becoming a competitive fuel for power plants, offering very high security of supply and cost-effectiveness. However, power plants firing lignite face some thorny issues, such as high carbon dioxide emissions, high investment in construction, etc. Lignite pre-drying is considered an attractive way to tackle these issues, but it consumes a lot of energy and has a high risk of ignition. Thus, a combined-type fluid-bed dryer, with which lignite could be dried safely using some waste from a power-generation process, is proposed in this paper. Both boiler exhaust flue gas and steam extraction of steam turbines are used as heat sources for this kind of dryer. To analyze its thermodynamics, a theoretical model was developed with which a reference case of a 1000 MW air-condensing power plant was performed. The results show that a dryer integrated within the power plant can evidently increase the plant efficiency by approximately 2.55%. The main factors, including the degree of pre-drying, dryer thermal efficiency, and the temperature of dryer exhaust, were analyzed. The results show that the degree of pre-drying has the most obvious influence. A 0.1 increase in pre-drying degree improves the plant thermal efficiency by about 0.62%, while a 10°C decrease of dryer exhaust temperature and a 10% increase in dryer thermal efficiency could improve the plant thermal efficiency by about 0.10% and 0.22%, respectively.     

Base case process development for energy efficiency improvement, application to a Kraft pulping mill. Part II: Benchmarking analysis

A new procedure for benchmarking analysis has been developed to evaluate the energy efficiency of a chemical process. Benchmarking is performed to identify process inefficiencies before developing energy enhancement measures. The new procedure combines typical techniques, such as the comparison with current practice, with utilization of new performance indicators based on exergy and energy content and the targeting by Pinch Analysis and Water Pinch. All process sections and the steam and water utility systems are evaluated. The procedure consists of five phases. In the first phase the data required is compiled. The second phase consists of comparing the energy and water efficiency of the base case to the current practice of the industry. In the third phase, the new energy and exergy content indicators are used to analyze the efficiency of utilities systems and to quantify the heat rejected by the process. In the fourth phase the minimum energy and water requirements are determined. The last phase is a synthesis by which the inefficiencies are identified and guidelines established for process improvement. Interactions between the utilities systems and the process are developed. The procedure has been applied to an operating Kraft pulping mill in Eastern Canada.     

A novel network-based continuous-time representation for process scheduling: Part II. General framework

In the first part of this series of papers we presented a new network-based continuous-time representation for the short-term scheduling of batch processes, which overcomes numerous shortcomings of existing approaches. In this second part, we discuss how this representation can be extended to address aspects such as: (i) preventive maintenance activities on unary resources (e.g., processing and storage units) that were planned ahead of time; (ii) resource-constrained changeover activities on processing and shared storage units; (iii) non-instantaneous resource-constrained material transfer activities; (iv) intermediate deliveries of raw materials and shipments of finished products at predefined times; and (v) scenarios where part of the schedule is fixed because it has been programmed in the previous scheduling horizon. The proposed integrated framework can be used to address a wide variety of process scheduling problems, many of which are intractable with existing tools.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part II: preparation and characterization of MEAs

Cathode catalyst layers were prepared and characterized as part of membrane electrode assemblies (MEA) and catalyst coated membranes (CCM) on the basis of carbon supported methanol tolerant RuSe _x catalysts. Preparation parameters varied were: catalyst loading (0.5–2 mg RuSe _x cm⁻²), PTFE content (0, 6, 18 wt.%), carbon support (Vulcan XC 72 or BP2000), and fraction of RuSe _x in the carbon supported catalysts (20, 44, 47 wt.%). The MEAs and cathode catalyst layers were electrochemically characterized under Direct Methanol Fuel Cell (DMFC) operating conditions by recording polarization curves, galvanostatic measurements, and impedance spectra. The morphology of the catalyst layers was investigated by means of confocal laser scan microscopy (CLSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. MEAs with Ru(44.0 wt.%)Se(2.8 wt.%)/VulcanXC72 cathode catalyst achieved the highest performance of all RuSe _x catalysts investigated, i.e. ∼40 mW cm⁻² at 80 °C under ambient pressure and λ_MeOH = λ_air = 4. This is 40% of the value obtained with commercial platinum cathode catalyst under the same operating conditions. The RuSe _x catalysts investigated are stable over a period of more than 1,000 h. This was confirmed by TEM and XRD measurements, where no increase in mean RuSe _x particle size (∼5 nm) after fuel cell operation was found. Enhancement of specific catalyst activity, mass transport, and active surface offer potential for a further improvement of RuSe _x catalyst layers.     

A Model for the Diffusion of Moisture in Adhesive Joints. Part II: Experimental

The concentration dependence and temperature dependence of the diffusion coefficient, as discussed in Part I, are validated with literature data on poly(styrene) and on poly(vinylacetate). The effect on diffusivity, of a uniaxial tensile stress state and of a biaxial tensile stress state, is measured with permeation tests on stretched poly(ethyleneterephthalate) (PET) films. The influence of semi-crystallinity is briefly discussed. Further, diffusivity measurements under a tensile stress state, under a compressive stress state, and under a pure shear stress state are performed on Ultem^® polyimide films, using a modified sorption technique. Good agreement between theoretical predictions and experiment is found. Finally, predictions by the solubility model discussed in Part I are compared with data on low density polyethylene and on Ultem polyimide.     

A Stress Singularity Approach for the Prediction of Fatigue Crack Initiation in Adhesive Bonds. Part 1: Theory

Since crack initiation in adhesive bonds tends to occur near the interface corners where the stress fields are singular, we define a fatigue initiation criterion using stress singularity parameter, Q (a generalized stress intensity factor) and the singular eigenvalue, λ.

Hattori et al., successfully used a generalized stress intensity factor to characterize the static strength of bimaterial interfaces. We show that this criterion is only appropriate for situations in which the adhesive contact angle is no larger than 90° and the modulus ratio (adhesive to adherend) is smaller than 0.1. Fortunately, these conditions are often met in real joints, permitting the use of a single eigenvalue approach. We then extend this criterion to the case of fatigue arising from mechanical, thermal, or hygroscopic cycling.

In preparation for Part 2 (experimental), the special case of an epoxy wedge on a flat aluminum substrate is considered. The singularity is analyzed both analytically and numerically. The scale of the region dominated by the singularity is found to be of the order of 100 μm. The size of the plastically yielded zone near the apex is found to decrease extremely rapidly as the stress intensity factor goes down, thereby increasing the applicability of the method at the low stress levels often encountered in fatigue.     

A study on UV-curable adhesives for optical pick-up: II. Silane coupling agent effect

Ultraviolet curable optical pick-up adhesive composites composed of acrylic and methacrylic mono-, bi- and trifunctional monomers and two kinds of urethaneacrylate oligomers have been easily prepared. In this paper, we studied the influence of various silane coupling agents to enhance the adhesion property in thermal and humid conditions by acting as both inorganic surface and organic polymer modifiers. Among them, vinyltriethoysilane (VTES) provides good adhesion effects on acrylate binder as well as silica surface. VTES added adhesives were studied by the real time FT-IR. The reliability of adhesives was explained by thermal-mechanical analysis (TMA) and then durability was tested by water boiling test. These UV-adhesives were successfully applied and settled in CD and DVD optical pick-up bodies in commercial application.     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

A review of methods for determining structural fire severity—Part II: Analysis and review

There is a risk of a building suffering unsustainable structural damage in the event of a large fire. Therefore, it is necessary to design buildings to withstand expected fires. A widely used simplified calculation method is the so-called “time-equivalence” method. There are significant concerns about the suitability of this method. This paper is part II of a twofold study examining the state of the art of time-equivalence methods. The purpose of this paper is to identify methods and/or analysis concepts which show the potential for use in modern design. A SWOT (Strengths, Weaknesses, Opportunities, and Threats) analysis is used for this purpose. However, as there is a large number of time-equivalence methods to assess, a numerical case study is first undertaken to identify methods which have sufficient accuracy to warrant further study. These analyses found that, while none of the time-equivalence methods studied have sufficient accuracy for use in their present form, the methods derived using the equal energy concept provide a good basis to model the effects of fire on a structure. This study recommends that a new time-equivalence method be developed using the equal energy approach.