Weak Boundary Layer Research Articles

Effect of Low Molecular Weight Oxidized Materials and Nitrogen Groups on Adhesive Joints of Polypropylene Treated by a Cold Atmospheric Plasma Jet.

Polypropylene is a typical representative of synthetic polymers that, for many applications including adhesive joints, requires an increase in wettability and chemical surface reactivity. Plasma processing offers efficient methods for such surface modifications. A particular disadvantage of the plasma jets can be the small plasma area. Here, we present a cold atmospheric plasma radio-frequency slit jet developed with a width of 150 mm applied to polypropylene plasma treatment in Ar, Ar/O and Ar/N We identified two main parameters influencing the tensile strength of adhesive joints mediated by epoxy adhesive DP 190, nitrogen content, and the amount of low molecular weight oxidized materials (LMWOMs). Nitrogen functional groups promoted adhesion between epoxy adhesive DP 190 and the PP by taking part in the curing process. LMWOMs formed a weak boundary layer, inhibiting adhesion by inducing a cohesive failure of the joint. A trade off between these two parameters determined the optimized conditions at which the strength of the adhesive joint increased 4.5 times. Higher adhesion strength was previously observed when using a translational plasma gliding arc plasma jet with higher plasma gas temperatures, resulting in better cross linking of polymer chains caused by local PP melting.

Supercloseness of finite element method on a Bakhvalov-type mesh for a singularly perturbed problem with two parameters

In this paper, the linear finite element method on a Bakhvalov-type mesh is applied to a singularly perturbed problem with two parameters. There is a strong exponential boundary layer and a weak exponential boundary layer in the solution of this problem. In the analysis, we discuss three cases and obtain the corresponding uniform convergence and supercloseness results by using different analysis techniques. Numerical tests confirm our theoretical results.

Use of very weak approximate boundary layer solutions to spatially nonsmooth singularly perturbed problems

We consider singularly perturbed Dirichlet problems which are, in the simplest nontrivial case, of the typeε2u″(x)=f(x,u(x)) for x∈[0,1],u(0)=u0,u(1)=u1. For small ε>0 we prove existence and local uniqueness of solutions u=uε, which are close to functions of the typeu¯ε(x)=u¯(x)+φ0(x/ε)+φ1((1−x)/ε) with f(x,u¯(x))=0 for x∈[0,1] and withφ0″(ξ)=f(0,u¯(0)+φ0(ξ)) for ξ∈[0,∞),u¯(0)+φ0(0)=u0,φ0(∞)=0,φ1″(ξ)=f(1,u¯(1)+φ1(ξ)) for ξ∈[0,∞),u¯(1)+φ1(0)=u1,φ1(∞)=0, and we show that ‖uε−u¯ε‖∞→0 for ε→0. We do not suppose monotonicity of the correctors φ0 and φ1. And, mainly, we do not suppose any regularity besides continuity of the functions f(⋅,u) and u¯. Hence, the functions u¯ε approximately satisfy the boundary value problem for ε≈0 in a very weak sense only, and one cannot expect that ‖uε−u¯ε‖∞=O(ε) for ε→0, in general. For the proofs we use an abstract result of implicit function theorem type which was designed for applications to spatially nonsmooth singularly perturbed boundary value problems with nonsmooth approximate solutions.

Degradation Mechanism of Interfacial Adhesion between Screen-Printed Ag/Polyimide in Temperature/Humidity Environment

The effect of temperature/humidity (T/H) treatment at 85 ℃/85% relative humidity (R.H.) on the peel strength between screen-printed Ag/polyimide (PI) substrate was evaluated using a 180° peel test. Initial peel strength was 22.22 ± 1.00 gf/mm, and then decreased to 0.47 ± 0.22 gf/mm after 500 h at 85 ℃/85% R.H. treatment condition. And, the peeled locus was changed from Ag/PI interface to mixed mode of Ag/PI interface and shallow cohesive inside PI near Ag/PI interface. The decrease in peel strength during T/H treatment is related to formation of weak boundary layer inside PI near Ag/PI interface by hydrolytic degradation of PI due to long-term moisture penetration into the Ag/PI interface.

Adhesion and friction performance of DLC/rubber: The influence of plasma pretreatment

Diamond-like carbon (DLC) films are deposited on rubber surfaces to protect the rubber components, and surface pretreatment of the rubber substrates prior to the film deposition can improve the adhesion between the DLC films and the rubber. Thus, the principal purpose of this work concentrates on determining the effects of argon (Ar), oxygen (O2), nitrogen (N2), and hydrogen (H2) plasma pretreatments on the adhesion and friction performance of the DLC films deposited on rubber (DLC/rubber). The results indicated that the Ar plasma pretreatment promoted the formation of a compact layer on the rubber surface. By contrast, massive fillers were exposed on the rubber surface after oxygen or nitrogen plasma pretreatments. Moreover, the typical micrometer-scale patches divided by random cracks were observed on the surface of DLC/rubber, except for the sample pretreated with oxygen plasma. The adhesion of DLC/rubber was found to strengthen with the removal of weak boundary layers and the generation of free radicals on the rubber surface after plasma pretreatment. The tribo-tests revealed that DLC/rubber with O2, N2, and H2 plasma pretreatments cannot achieve optimal friction performance. Significantly, DLC/rubber with Ar plasma pretreatment exhibited a low and stable friction coefficient of 0.19 and superior wear resistance, which was correlated to the high adhesion, good load-bearing of the rubber surface, and the approximate sine function of the surface profile of the DLC film.

Investigation of the Effects of Rice Husk and Rice Straw on the Properties of Lightweight Fibrecrete Blocks

Uganda’s progress towards achieving sustainable growth is curtailed by poor and limited infrastructure, over 60% of the urban population live in slums with poor quality housing and hygiene, 44% of the households sleep in one room, the situation is more critical in Kampala where about 70% sleep in one room. About 67% of Ugandans live in dwellings with brick walls while 28% live in dwellings of mud walls reinforced with timber poles. Over 90% of framed structure in Uganda are built from reinforced concrete structures with bricks or block infill walls that are heavy thus influencing the type and size of structural members hence its cost. This research focused on use of Rice Husks (RH) and Rice Straws (RS) in lightweight fibrecrete blocks for benefit of their reduced weight, reducing environmental pollution, low processing energy, and availability at modest cost. In this research, the fibres were alkali-treated using sodium hydroxide solution prepared by dissolving 15g of sodium hydroxide pellets in 1 liter of water heated at 100°C for 60 minutes. This was aimed at increasing surface roughness, expose cellulose to fibre surface, improve fibre/matrix adhesion, eliminate weak boundary layers and remove superficial lignin layer. The alkali treated fibres were used to prepare one hundred twenty one (121) fibrecrete blocks of RH and RS with varying proportions of 0%, 10%, 20%, 30%, 40%, and 50% fibre and tested for their density, compressive strength, water absorption, thermal conductivity, fire resistances and microstructure. The densities of lightweight fibrecrete blocks ranged between 1947-1485kg/m³ with a reduction of 7.9 - 30.5% compared to aggregate blocks of 2138kg/m³ density. Compressive strength of the blocks varied between 1.53 - 5.36 N/mm² and 1.28 - 3.48 N/mm² for RH and RS blocks respectively with control block having compressive strength of 7.2N/mm² at 28 days. Compressive strength loss in the lightweight fibrecrete blocks tested for fire resistance ranged between 10.7- 34.3% and 6.8-73.7% for RH and RS-blocks respectively while the water absorption ranged between 5.8-7.8% for RH and 6.4-11.3% for RS blocks and 3.6% for aggregate blocks. Thermal conductivity was improved by 13.4-64.8%. The researcher concluded that, RH-40 and RS-20 blocks have the most appropriate proportions for lightweight fibrecrete blocks. This would reduce Environmental degradation, utilize agricultural waste, increase the income of the farmers, reduce construction cost due to reduced density of fibrecrete blocks, and increase thermal insulation hence thermal comfort of the occupants.

Surface Modification and Adhesion Mechanism of Isotactic Polypropylene with Low-Energy Electron-Beam Treatments.

Recently, smaller-size electron-beam (EB) accelerators have offered EB irradiation in laboratory systems. Therefore, polymer surface treatments with low-energy EB have been developed in the past years. For high adhesion strength, low-energy EB treatment is also a promising method in comparison to plasma surface treatment. In the plasma treatment, the mechanism of the effect on the adhesion properties has been proved and the excess treatments led to the formation of a weak boundary layer and reduction of adhesion strength. In contrast, the low-energy EB possesses high penetration ability. In this work, we focused on the surface treatments of isotactic polypropylene (it.PP) with low-energy EB irradiation for adhesion. The dependence of adhesion strength on the absorbed dose of electron beam was evaluated, and the mechanism of electron beam on the adhesion properties was investigated from various perspectives of surface properties and morphology. Compared to that of plasma-treated it.PP, the adhesion strength of it.PP with electron-beam irradiation increased drastically. We proved that the radical was generated in the substrates after electron-beam treatments and would form covalent bonds between adhesives and substrates, which achieved higher adhesion than plasma treatments. In addition, the electron beam reached effectively a deep region from the top surface of the substrates and provided larger adhesion strength.

Investigation on the adhesion improvement of organic coatings on the surface of hot-dip galvanized steel

In order to solve the problem of poor adhesion of organic coatings on the surface of hot-dip galvanizing, we conducted a series of experiments to select the suitable pre-treatment method and type of the coatings. We adopted temperature shock & alternating damp heat combined test and waterproof test to simulate the service conditions, and investigated the adhesion of organic coating on the surface of hot-dip galvanizing in the wet environment. With the XPS analysis, we discussed the avoidance of formation of weak boundary layer on the hot-dip galvanizing surface by pre-treatment, and the improvement on adhesion of the organic coatings. We also proposed the pre-treatment method of coupling treatment with silane or combined alkali washing & grinding procedures, by which the organic coating on the hot-dip galvanizing surface performed excellent adhesion and water proof ability against wet conditions.

Adhesion and viscoelastic property of poly(ethylene-co-vinyl acetate) based hot melt adhesives- effects of tackifier and wax

A series of hot-melt adhesives (HMAs) were made from poly(ethylene-co-vinyl acetate) (EVA) and a glycerol ester of partially hydrogenated rosin (Staybelite Ester 10, SE10) in various ratios. A content of 70 parts of SE10 per hundred parts (phr) of EVA showed a maximal peel strength. The effect of decreasing the rubbery plateau G′ value and increasing the glass transition temperature while increasing the SE10 content seems evident in the peel strength study. The trend also showed that the tackifier became less effective when the Tg's of the adhesive mixture were higher than a limit temperature (i.e., 15 °C).With 70 phr of SE10, the compatibility of the adhesive blend was further improved by adding 10 phr of wax, and the recipe displayed an even higher peel strength value. The presence of wax leads to ultrahigh cohesive strength, which is related to the rapid increase in crystallinity of the adhesive mixture. Formulating with a larger amount of wax worsens the peel strength, which may be explained by the formation of a weak boundary layer of the adhesive mixture on the surface of the adhesive blend.

On the Effect of Heat Treatments on the Adhesion, Finishing and Decay Resistance of Japanese cedar (Cryptomeria japonica D. Don) and Formosa acacia (Acacia confuse Merr.(Leguminosae))

In Taiwan, it is important to maintain sustainable development of the forestry industry in order to raise the self-sufficiency of domestic timber. Japanese cedar (Cryptomeria D. Don and Formosa acacia (Acacia confusa Merr.(Leguminosae)) have abundant storage options and are the potential candidates for this purpose. Heat treatment is a new environment-friendly method used to enhance the dimensional stability and durability of wood. On treatment, a surface with new characteristics is produced because of wood component changes. Consequently, an inactivated surface and a weak boundary layer are generated, and the wettability for adhesives and coatings is reduced. Furthermore, it decreases the pH value of the wood surface, and results in delay or acceleration during the curing of adhesives. This phenomenon must be paid attention to for practical applications of heat-treated wood. Ideal heat-treated conditions of C. japonica and A. confusa woods with productive parameters such as temperature, holding time, heating rate, and thicknesses of wood were identified in our previous study. In this research work, we focus on the normal shear strength of heat-treated wood with adhesives such as urea-formaldehyde resin (UF) and polyvinyl acetate (PVAc), and the finishing performances of heat-treated wood with polyurethane (PU) and nitrocellulose lacquer (NC) coatings as well as assessing the decay-resistance of heat-treated wood. The results show that heat-treated wood had a better decay resistance than untreated wood. The mass decrease of heat-treated wood was only 1/3 or even less than the untreated wood. The normal shear strength of heat-treated wood with UF and PVAc decreased from 99% to 72% compared to the untreated wood, but the wood failure of heat-treated wood was higher than that of the untreated one. Furthermore, the adhesion and impact resistance of wood finished by PU and NC coatings showed no difference between the heat-treated wood and untreated wood. The finished heat-treated wood had a superior durability and better gloss retention and lightfastness than that of the untreated wood.

Interface Bonding Mechanism of Recycled Asphalt Mixture

With more and more asphalt pavement entering into maintenance period, a large number of aged and damaged asphalt pavements are excavated to reclaimed asphalt pavement. Recycling of old asphalt pavement into asphalt mixture is one of the important ways to solve this kind of waste material. However, the water stability of reclaimed asphalt mixture is difficult to meet the requirements of actual pavement performance, resulting in insufficient pavement durability. Water vapor weakens the bond between asphalt and aggregate and asphalt paste. In this paper, the failure mechanism of interfacial bond in asphalt mixture is summarized by using weak boundary layer, electrostatic force, chemical bond, mechanical bonding and surface free energy from the view of microstructure. Further more, the research direction of interfacial bonding mechanism of recycled asphalt mixture is put forward.

Corona Treatment for Nanotransfer Molding Adhesion

Corona discharge treatment offers a facile, robust, and scalable technique for adhesion promotion in nanotransfer molding. In contrast to conventional plasma treatment, which requires vacuum operation, corona treatment at ambient pressure enabled patterning of woodpile structures of PDMS with smaller feature dimensions (70 nm line width) and across a wider range of treatment doses. Similarities and differences between plasma (batch) and corona (continuous) treatment were established using dyne testing to capture the effects of treatment on surface wettability of PDMS. Evaluating spatiotemporal evolution of wettability through dyne tests enabled a common axis for direct comparison of the two process configurations in terms of dose. Both treatment types formed surface films of oxidized PDMS, which were characterized through compressive buckling tests of the PDMS–oxide bilayer. Corona treatment, by forming a thinner oxide film, extended the range of treatment doses and feature sizes exhibiting successful pattern transfer. Reduction in adhesion at high treatment dose was attributed to a weak boundary layer at the bonded interface. Demolding was studied via peel tests and AFM analysis to reveal that both plasma and corona treatment etch the stamp material (PFPE), underscoring the benefits of low treatment dose for process throughput as well as stamp lifetime. While both treatments displayed comparable etch rates as a function of dose, microscopic bumps on corona-etched stamps indicate a higher surface temperature compared to plasma-etched stamps. By optimizing adhesion for nanotransfer molding with a surface treatment well-suited to continuous processing, corona treatment provides a practical and economical approach to layer-by-layer additive manufacturing at the nanoscale.

Evaluation of crack sealant adhesion properties under complex service ambient conditions based on the weak boundary layer (WBL) theory

The use of crack sealant is an effective, simple, and extensive maintenance approach to repairing cracks. However, the extended exposure of sealants to complex ambient conditions have negative impacts on sealant properties, causing gradual degradation of sealant performance and even more seriously, premature failure during their service. According to fracture modes of the failure interface, a weak boundary layer reportedly exists and results in adhesion failure of crack sealants. This work studied three environmental factors, namely, temperature, water, and impurities, to determine the effects of hostile ambient exposure on different crack sealants commonly used in hot and moderate climates. Tensile test was the main evaluation method, and the weak boundary layer (WBL) theory was introduced to illustrate the adhesion failure between sealant and crack wall. Analysis of experimental results showed that the lower temperature, the longer soaking time and the impurities would precipitate the eventual formation of a WBL. Sealants reached a glassy state and were likely to acquire brittle fracture at the interface at −30 °C. When soaked, sealants gradually hardened and water penetrated into the pores on the adhesive surface of the crack wall. Impurities at the interface hindered adhesion and reduced the contact area between sealant and crack wall. Consequently, the adhesion ability between interfaces dropped sharply and thus caused adhesion failure.

Synoptic and local circulations associated with events of high particulate pollution in Valparaiso, Chile

This study discusses the synoptic situations associated with cases of high pollution in terms of black carbon (BC) concentrations observed at the research site during the four months of the VAMPIRE campaign that is described fully in the companion study by Marín et al. (2017). Cases in which the concentration of BC exceeded the 85th percentile were selected to evaluate composite synoptic situations, resulting in a total of fifteen cases analyzed. These fifteen cases occurred during one of the three prevalent synoptic situations identified and labeled as: i) Coastal low, characterized by weak easterly winds and low planetary boundary layer (PBL) height due to an enhancement of the temperature inversion before the arrival of the coastal low; ii) Pre-frontal, also characterized by low PBL height, with a cold front approaching from the South and the coastal low retreating northwards, also associated with pollution episodes in Santiago (Rutllant and Garreaud, 1995); and iii) SEP anticyclone characterized by large stability, very low gradient in the mean sea level pressure and weak westerly-southwesterly wind due to the dominance of the southeast Pacific anticyclone. Coastal low cases were observed not only during winter but also in all four months of the VAMPIRE field campaign, while Pre-frontal cases were observed only at the end of July and August 2014. All three synoptic situations are consistent with a reduced synoptic forcing, in which the mesoscale sea-land breeze predominates and advects large pollutant concentrations from local/regional emission sources to the research site. Detailed analysis is presented here for two case studies: one Coastal low and one Pre-frontal, combining the observations of pollutants as well as mesoscale modeling with the WRF model to estimate back-trajectories and evaluate the relative role of the synoptic and mesoscale forcing on the pollution episodes in Valparaiso. These synoptic conditions and their interplay with meso-scale circulations identified during VAMPIRE provide evidence and insight that will aid in future air quality forecasts for Valparaiso.

Comparison between adhesion properties of adhesive bonding and adhesive-free adhesion for heat-assisted plasma-treated polytetrafluoroethylene (PTFE)

ABSTRACTHeating during plasma treatment, known as heat-assisted plasma treatment, has recently reported to positively affect the adhesion properties of polytetrafluoroethylene (PTFE). In the present study, the adhesion properties of adhesive bonding and adhesive-free adhesion were compared for plasma-treated PTFE with different plasma treatment times and with or without heating during the plasma treatment. The relations among adhesion strength, plasma treatment time, radical density ratio, surface morphology, and surface hardness were investigated. No correlation was found between the adhesion strength and the radical density ratio or between the adhesion strength and the oxygen-containing-functional-group ratio. In contrast, correlation was observed between the adhesion strength and the surface hardness. In addition, the heat-assisted plasma treatment time affected the recovery of the weak boundary layer on the PTFE surface. Adhesive-free adhesion was found to require a longer heat-assisted plasma treatment time than adhesive bonding in order to achieve a high adhesion strength such as 1 N/mm.

Reaction of ozone with polybenzimidazole (PBI)

ABSTRACTOzone reacted with the polybenzimidazole (PBI) film surface and X-ray photoelectron spectroscopy (XPS) detected a rapid increase in O atom concentration up to a saturation level of 27 ± 1 atomic %. Atomic force microscopy measurements showed little change in surface roughness. The water contact angle of the treated film decreased by ca. 60% compared to untreated PBI film indicating an increase in hydrophilicity and hydrogen bonding due the formation of the polar oxygenated functional groups. With treatment, the curve fitting of the XPS chemical states showed a decrease in the C–C sp2, C–C sp3, and C=N functional groups, and an increase in C–N, C=O, O–C=O, O–(C=O)–O, and N–C=O moieties which were explained using mechanisms associated with the dissociation of the primary ozonide formed from the addition of ozone to the C=C and C=N bonds and reaction with the amine groups in PBI. Washing the treated surface with water partially washed away the O concentration which indicated the formation of a weak boundary layer due to the breakage of bonds in the decomposition of the primary ozonide.

Surface Modification of Polystyrene with O Atoms Produced Downstream from an Ar/O2 Microwave Plasma

Because discarded polystyrene (PS) is little affected by degrading agents, PS was treated with a remote microwave (MW) plasma discharge of an Ar/O2 mixture in the absence of radiation to increase wettability and introduce functional groups which make the waste more liable to degradation and useful for technological applications. X-ray photoelectron spectroscopy (XPS) detected decreases in the aromatic sp2 and aliphatic sp3 carbons with treatment and, initially, increases in C–O and carbonyl groups, present in the formation of ethers, epoxides, alcohols, ketones and aldehydes. At longer treatment times, ester, O–C=O; carbonate-like, O–(C=O)–O; and anhydride, O=C–O–C=O; moieties are observed with an overall oxygen saturation level of 23.6 ± 0.9 at% O. Atomic Force Microscopy (AFM) measurements detected little change in surface roughness with treatment time. Advancing water contact angle decreased by ca. 50% compared to pristine PS indicating an increase in hydrophilicity because of oxidation. Washing the treated samples in deionized water decreased the oxygen concentrations at the saturation treatment times down to 18.6 ± 1 at% O due to the washing away of a weak boundary layer.

Polydopamine and Polydopamine-Silane Hybrid Surface Treatments in Structural Adhesive Applications.

Numerous studies have focused on the remarkable adhesive properties of polydopamine, which can bind to substrates with a wide range of surface energies, even under aqueous conditions. This behavior suggests that polydopamine may be an attractive option as a surface treatment in structural bonding applications, where good bond durability is required. Here, we assessed polydopamine as a surface treatment for bonding aluminum plates with an epoxy resin. A model epoxy adhesive consisting of diglycidyl ether of bisphenol A (DGEBA) and Jeffamine D230 polyetheramine was employed, and lap shear measurements (ASTM D1002 10) were made (i) under dry conditions to examine initial bond strength and (ii) after exposure to hot/wet (63 °C in water for 14 days) conditions to assess bond durability. Surprisingly, our results showed that polydopamine alone as a surface treatment provided no benefit beyond that obtained by exposing the substrates to an alkaline solution of tris buffer used for the deposition of polydopamine. This implies that polydopamine has a potential Achilles' heel, namely, the formation of a weak boundary layer that was identified using X-ray photoelectron spectroscopy (XPS) of the fractured surfaces. In fact, for longer deposition times (2.5 and 18 h), the tris buffer-treated surface outperformed the polydopamine surface treatments, suggesting that tris buffer plays a unique role in improving adhesive performance even in the absence of polydopamine. We further showed that the use of polydopamine-3-aminopropyltriethoxysilane (APTES) hybrid surface treatments provided significant improvements in bond durability at extended deposition times relative to both polydopamine and an untreated control.

Adhesives and Adhesion Technologies: A Critical Review

The article critically reviews about adhesives and their various adhesion phenomenons along with their categorization and functions. Benefit and drawbacks of adhesives joining was explained. Requirements for excellent bonding which includes, appropriate selection of adhesive, superior design for joining, surface cleansing, wetting were defined. The review clearly indicates the various theories involved in adhesion are namely mechanical interlocking, electrostatic, diffusion, wetting, chemical bonding, and weak boundary layer. Bond failure modes and their mechanism were elaborated briefly.

Particle aggregates formed during furfuryl methacrylate plasma polymerization affect human mesenchymal stem cell behaviour

Human Mesenchymal Stem cells (hMSCs) are becoming a major focus in biomedical fields. Application of in vitro expanded hMSCs to treat numerous ailments has led to a commercial emphasis on improving hMSC growth ex vivo. Production of substrate independent, novel thin films is one potential tool for production of commercial viable hMSC expansion. Plasma polymerization allow controlled chemical optimisation of large scale surface areas in a substrate independent manner. Previous study shown that plasma polymerized Furfuryl Methacrylate (ppFMA) surfaces allowed primary fibroblast cells adhesion and proliferation. However, under some deposition conditions, particle aggregates formation was observed. These aggregates had the effect of disrupting cell attachment, despite being chemically indistinguishable from the underlying surface. Herein, hMSCs were cultured on ppFMA surfaces to determine their suitability for stem cell culture and observe the effect of particle aggregates on hMSC attachment and growth. Both metabolic and DNA quantification assays showed that surfaces with particle aggregates had lower numbers of attached cells and slower growth. Uniform surfaces without aggregates showed higher cell attachment and growth levels, which were comparable to Thermanox. Phenotypic analysis showed that there was no change to hMSCs phenotype after 7 & 14days of culture on uniform ppFMA surface. Further investigation using time-lapse image analysis indicated that particle aggregates reduced cell attachment by presenting a physically weak boundary layer, which was damaged by intracellular tension during cell spreading. ppFMA surface can provide a stable substrate independent hMSCs expansion interface that could be applied to larger scale bioreactors, beads or scaffolds.