Calculated values of the adhesion of phenol-formaldehyde oligomers to crystalline cellulose II

The energies of interaction of phenol, monomethylol and dimethylol phenols, trimethylol phenol, and of three dihydroxydiphenylmethanes with the surfaces of an elementary model of the crystallite of cellulose II were obtained by molecular mechanics techniques. The results indicated that in the case of phenol-formaldehyde (PF) oligomers, methylolation at least in the monomeric phenol species does not enhance adhesion, with the exception of the case of trimethylol phenol. This appeared to be due to strong steric hindrance counteracting the normally highly attractive interaction induced in resins by methylol groups. All the phenolic oligomers, with the exception of one, were still found by computation to present strong attractive interactions to the cellulose substrate, confirming again the strong adhesion of PF resins to lignocellulosic substrates which has been found experimentally many times before. In the complex case of the separation of a homologous series of oligomers of increasing molecular mass, the good correspondence between the computed results and the experimental chromatography R_f values indicated that computationally obtained energies of adhesion are capable of modelling well experimental realities. This result is of interest because the hydroxybenzyl alcohols and dihydroxydiphenylmethanes do not belong to the same homologous series. The model, at least for these compounds, did not require additional modelling of water molecules other than introducing its dielectric constant in the electrostatic forces calculations.     

A molecular mechanics approach to the adhesion of urea-formaldehyde resins to cellulose. Part 2. Amorphous vs. crystalline Cellulose I

The average adhesion of urea-formaldehyde (UF) resins to amorphous cellulose appears to be lower than that to crystalline Cellulose I. Water appears to be able to displace UF resins on many sites of amorphous cellulose. Selected high attraction sites appear to still be able to give adequate adhesion. There are indications that UF resins' wetting of amorphous cellulose is independent of the resin water content. The energies of interaction of UF oligomers with amorphous cellulose and Cellulose I help in distinguishing between adhesion of UF resins of different stages of condensation. It is also shown that the molecular mechanics method outlined in this article can be used to scan UF adhesive formulations as regards resin adhesion to lignocellulosic substrates.     

Molecular dynamics simulation on the adhesion mechanism at polymer-mold interface of microinjection molding

In the demolding process of microinjection molding, the defects of microstructures are often caused by the strong adhesion between polymer and mold. In order to study the adhesion mechanism, the molecular dynamics (MD) method was proposed to simulate the adsorption of cycloolefin copolymer (COC) molecules on mold surfaces. The evolution snapshots of COC molecular chains of three interfacial models were obtained to directly demonstrate the adhesive strength of interfaces. Meanwhile, the work of adhesion, the relative concentration, the potential energy, and the radial distribution function (RDF) were calculated to explain the interaction mechanism of polymer-mold interfaces. The simulation results showed that the COC-Ni interface had the largest work of adhesion and the lowest potential energy, compared with other two interfaces. The van der Waals (VDW) energy, which mainly derived from the interaction between H atoms in COC and the mold material was the only nonbond interaction energy at the COC-Ni and COC-Si interfaces, while the electrostatic energy existed in COC-Al₂O₃ interface. In order to reduce the adhesion between polymer and mold, fluorine (F) element could be doped into the Ni mold.     

A molecular mechanics approach to the adhesion of urea-formaldehyde resins to cellulose. Part 1. Crystalline Cellulose I

The energies of interaction of urea, methylol ureas, and urea-formaldehyde (UF) condensates, methylolated and non-methylolated, linear and branched, up to trimers, with the surfaces of an elementary model of the crystal of Cellulose I were obtained by molecular mechanics techniques. The results indicated, firstly, that methylolation enhances adhesion, especially at low molecular weights, while branching tends to decrease it; secondly, that adhesion of UF resins to the cellulose surface can be enhanced by shifting the resin preparation conditions to increase the proportion of species having higher specific adhesion. The theoretical results obtained are in agreement with published experimental evidence. While urea resins show stronger average affinity for cellulose than the average affinity of water, this trend is less marked than in phenol-formaldehyde (PF) resins. The results obtained also appear to infer that the lack of water resistance of UF resins is mainly due to the instability in water of the internal, covalent, aminoplastic bond rather than to UF adhesion to cellulosic substrates. Resin-substrate H-bonding was shown to be of lesser importance in UF than in PF resins.     

Comparative determination of TiO2 surface free energies for adhesive bonding application

The development of durable bonds using titanium adherens has been investigated from the point of view of surface energy theoretical models measurements. The traditional Chromium Acid Anodization, which provides excellent durability, has to be phased out due to the use of hazardous Cr (VI) in the bath and as a result, special attention is paid to the sodium hydroxide anodizing and other alkaline chemical etchers. There are hardly any references on the surface free energy of adhesive titanium oxide coatings and therefore the objective of this work was to evaluate the surface and interface energy parameters of the various types of alkaline chromate free surface treatments using Neumann, Fowkes and van Oss–Chaudhury–Good methods in order to determine which method provides greatest differentiation between the coatings. Results show that Fowkes method produced the greatest variance in surface energies of the compared surface treatments and hence can be considered as better suited for more accurate discrimination between the oxide surface treatments on Ti–6Al–4V alloy. Although, in the case of model liquids, i.e. water and diiodomethane, the trends obtained for contact angles, surface energies, works of adhesion and solid/liquid interface energies all correlated between each other, a disagreement between the trends of solid/liquid interface energies calculated using Fowkes and van Oss–Chaudhury–Good methods for surface treatment/adhesive resin was obtained. In case of real adhesive systems, the use of work of adhesion appears more adequate in order to discriminate the surface treatments. Based on these findings the anodization in the tested alkaline bath after a previous alkali etching in the same bath is recommended, although adhesion test has to be still performed.     

A new technique for evaluating ink–cellulose interactions: initial studies of the influence of surface energy and surface roughness

Ink–cellulose interactions were evaluated using a new technique in which the adhesion properties between ink and cellulose were directly measured using a Micro-Adhesion Measurement Apparatus (MAMA). The adhesion properties determined with MAMA were used to estimate the total energy release upon separating ink from cellulose in water. The total energy release was calculated from interfacial energies determined via contact angle measurements and the Lifshitz–van der Waals/acid–base approach. Both methods indicated spontaneous ink release from model cellulose surfaces, although the absolute values differed because of differences in measuring techniques and different ways of evaluation. MAMA measured the dry adhesion between ink and cellulose, whereas the interfacial energies were determined for wet surfaces. The total energy release was linked to ink detachment from model cellulose surfaces, determined using the impinging jet cell. The influences of surface energy and surface roughness were also investigated. Increasing the surface roughness or decreasing the surface energy decreased the ink detachment due to differences in the molecular contact area and differences in the adhesiom properties.     

Plasma modification of cellulose fibers for composite materials

Composites of natural fibers and thermoplastics can be combined to form new enhanced materials. One of the problems involved in this type of composites is the formation of chemical bonds between the fibers and the polymers at the interface. This work presents a study where low energy glow discharge plasmas are used to functionalize cellulose fibers implanting polystyrene between the fibers and the matrix that improve the adhesion of both components. The interface of polystyrene was synthesized by continuous and periodic glow discharges on the surface of the cellulose fibers. The results show that the adhesion in the fiber–matrix interface increases with time in the first 4 min of treatment. However, at longer plasma exposures, the fiber may be degraded reducing the adhesion with the matrix. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3821–3828, 2006     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

A molecular simulation analysis of influence of lignosulphonate addition on properties of modified 2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid based pressure sensitive adhesive

A novel PSA formulation that incorporates a bio-sourced lignosulphonate has been proposed. A molecular dynamics simulation study is attempted to explore the effect of lignosulphonate addition on the adhesion, thermal and mechanical properties of a conventionally used acrylic PSA composed of 2-EHA, MMA and AA. A good agreement of conventional PSA density and glass transition temperature, T_g, with literature estimates confirmed the accuracy of the molecular simulations. It is observed that there is an increase in PSA-substrate interaction energy with an increase in lignosulphonate content despite an increase in T_g due to its addition. This is proposed to be primarily due to an increase in polar groups contributed by lignosulphonate. The availability of more polar groups in bulk and increased density of these groups due to significant migration to interface results in an increase in interfacial energy, and hence, improved PSA adhesion. The shear modulus is observed to increase with increase in lignosulphonate content indicating its effectiveness to resist PSA shear deformation. Simulations suggest that in order to form an industrially useful adhesive, that may work well under RT conditions possessing an optimum cohesive strength and surface adhesion, a PSA formulation with ~15 wt.% lignosulphonate may be used.     

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

A simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41137.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Cracks at adhesive interfaces

The application of Griffith's energy balance argument to cracks at adhesive interfaces is studied. Adhesive interfaces are generally brittle, representing the simplest form of fracture mechanics geometry because cracks are constrained to travel along the interface, giving a defined crack path which eases analysis. Experimentally, such cracks may be propagated along the interface between optically smooth rubber pieces, and measured through the transparent material. The development of adhesive fracture test-pieces since Griffith's time reveals difficulties in his reasoning, and allows improved understanding of the energy balance method. The most important conclusion is that stress does not normally enter the cracking criterion. It is demonstrated experimentally that stress may remain constant while the crack criterion changes. The strength of an adhesive interface is shown to be a meaningless parameter; instead, the work of adhesion, or adhesive energy, which is the work of adhesion together with energy losses, should be used to define the behaviour of cracks at interfaces.     

A Review of Adhesion Mechanisms Using the Peel Test in Air and Liquid Media

This paper deals with the analysis of peel energy of assemblies measured in different environments, i.e. in air and in the presence of liquids, and constitutes a brief review of the work of Professor Schultz' team in this domain. It is shown how such measurements can lead to a better knowledge of the nature as well as of the magnitude of fundamental interactions established at the interface between two solids. Earlier experiments have shown that peel energy can be expressed as a product of three terms corresponding, respectively, to the reversible energy of interfacial adhesion, the hysteretic losses of the bulk materials and the molecular dissipation near the crack front during peeling. This approach is well-verified when only physical interactions (van der Waals) are involved at the interface. However, more complex cases correspond to systems where specific interactions are also established between both materials, in particular acid-base interactions and creation of chemical bonds. In both cases, peel measurements in liquid media can lead to the determination of fundamental parameters, such as the interfacial density of specific interactions at the interface and the acid-base or chemical components of the work of adhesion. Finally, the effect of interdiffusion phenomena on peel energies can also be investigated in the case of elastomer/elastomer assemblies.     

Characterization of the surface energies of functionalized multi-walled carbon nanotubes and their interfacial adhesion energies with various polymers

The surface energies of pristine multi-walled carbon nanotubes (MWCNTs) and MWCNTs functionalized with carboxylic acid (MWCNT-COOH), acyl chloride and ethyl amine were characterized, and the effects of the changes in MWCNT surface energies on the interfacial adhesion and reinforcement of the composites were explored. When the surface energy of pristine MWCNTs was compared to that of functionalized MWCNTs, a decrease in the dispersive surface energy and an increase in the polar surface energy were observed. Interfacial adhesion energies between MWCNTs and various polymers were estimated from surface energy values of MWCNTs and various polymers. Among the MWCNTs, polyethylene, polystyrene and bisphenol-A polycarbonate (PC) had the highest interfacial energy with pristine MWCNTs, while nylon 6,6 and polyacrylamine exhibited the highest interfacial energy with MWCNT-COOH. When tensile properties and adhesion at the interface of PC and nylon 6,6 composites containing MWCNTs were examined, composites having high interfacial adhesion energy exhibited greater adhesion at the interface and reinforcement.     

Effect of spin finish on fiber/binder adhesion in chemically bonded nonwovens

The purpose of this study is to better understand the mechanisms governing the phenomena of fiber/matrix adhesion by controlling the fiber surface properties. This adhesion is evaluated by studying the micromechanical and thermodynamical behavior of the fiber/matrix interface. The complexity of the interactions at the interface requires a global approach that takes into account the chemistry, morphology, and mechanics. The thermodynamical affinity between the binder and fibers is evaluated by the wetting behavior, whereas the mechanical resistance of the fiber/matrix interface is characterized with the pull‐out test. Three distinct approaches are used to classify the different systems according to the nature of the binder and the fiber surface. It is found that there is better adhesion when the spin finish is removed from the fibers, revealing the surface roughness on which the latex can mechanically anchor. © 2006 Wiley Periodicals Inc. J Appl Polym Sci 102: 4092–4100, 2006     

Atomistic modeling: interfacial diffusion and adhesion of polycarbonate and silanes

The performance and strength of many composites, hybrid and thin multi-layered material systems are very much dependent upon the mechanical properties of interfaces. However, continuum mechanics approach to the characterization of interfacial properties has had limited success because it is often unable to incorporate the effects of molecular and chemical interactions into the model. There is therefore a need to understand and study the influence of these factors on mechanical properties such as adhesion strength at a more fundamental level. In the present work, the interfaces of two common coupling agents and matrix polymers in composites are studied with atomistic modeling and simulation. The polymer matrix is polycarbonate (PC) and the coupling agents studied are gamma amino-propyl-triethoxysilane (AMPTES) and stearic-propyl-triethoxysilane (SPTES). Two interface models, SPTES‐PC and AMPTES‐PC were built and the work of adhesion was calculated from molecular dynamics (MD) simulation. The separation of the coupling-agents‐matrix interfaces was simulated using MD calculations and the mechanical properties were obtained. It is shown that the higher work of adhesion of the interface is not equal to higher interfacial toughness.     

Adhesion Reliability of the Epoxy-Cu Interface by Molecular Simulations

Interfacial delamination is one of the most common failure modes in electronic packages. To obtain good reliability of electronic packages, it is important to study the interface properties. In this study, the adhesion reliabilities of the epoxy-Cu interface, which is widely used in electronic packagings, are investigated using molecular dynamics simulation with the effects of temperature, moisture, crosslink conversion, and oxidation degree considered. The results show that adhesion reliability of Cu-epoxy interfaces is almost independent of temperature, strain rate, and crosslink conversion of epoxy resins, while is weakened by increasing moisture contents. The variation of interfacial interaction energy with the increasing tensile deformation is revealed at molecular scale, which is useful to understand the interfacial interaction and interfacial delamination.     

Interfacial phenomena in Al2O3—liquid metal and Al2O3—liquid alloy systems

The sessile drop technique has been used to measure the contact angle of liquid metals and liquid alloys in contact with polycrystalline alumina. The experiments were carried out in argon atmosphere at various temperatures. The measured contact angles exhibit no wettability (θ > 90°). The linear temperature functions of the work of adhesion as well as of the interfacial energy were also determined in the investigated systems. The partial high values of the work of adhesion in several Al₂O₃-liquid metal systems can be attributed to a chemical bond establishment at the interface. The values of the interfacial energies at the melting point of the metals, for non-reactive Al₂O₃-liquid metal systems, vary in a restricted region (2·35–2·75) J m⁻²). An empirical relation is proposed for evaluation of the interfacial energy of the metals at their melting point. The agreement between experimental and calculated values is satisfactory.     

Surface energy, surface topography and adhesion

In this paper are discussed some of the fundamental principles which are relevant to an understanding of the influence that interfacial roughness may have on adhesion. The surface energies of the adhesive, substrate and of the interface between them determine the extent of wetting or spreading at equilibrium. Numerical values for surface energies may be obtained either from contact angle measurements or from analysing force–displacement curves obtained from the surface forces apparatus. The extent to which the relationships, appropriate for plane surfaces, may be modified to take into account interfacial roughness are discussed. For modest extents of roughness, the application of a simple roughness factor may be satisfactory, but this is unrealistic for many of the practical surfaces of relevance to adhesive technology which are very rough, and is ultimately meaningless, if the surface is fractal in nature. Some examples are discussed of published work involving polymer–metal and polymer–polymer adhesion, where the roughness of the interface exerts a significant influence on the adhesion obtained. Roughness over a range of scales from microns to nanometres may strengthen an interface, increasing fracture energy by allowing bulk energy dissipating processes to be activated when the bond is stressed.     

N-1-alkylitaconamic acids-co-styrene copolymers. Surface characterization

A serie of six N-1-alkylitaconamic acids-co-styrene copolymers with alkyl groups varying in side chain length from 3 to 12 was used in this study. The surface behaviour of the copolymers has been studied as function of the side chain length. Contact angle data for two of these copolymer surfaces were obtained in water and several liquids. From this information the surface energy was determined. Differences in the wettability of N-1-alkylitaconamic acid-co-styrene are found. The results are discussed in terms of hydrophobic and polar effect of the copolymers. Results on spread monolayers characteristics of these copolymers on water surfaces are also reported. Surface pressure-area (π-A) isotherms on a pure water subphase exhibit a transition region depending on the length of the alkyl side chain of the itaconamic acid moiety. It was also analyzed the phase transition in the monolayer at the air/water interface by brewster angle microscopy (BAM). Molecular mechanics approach was used to obtain predictions about the local interaction energies between segments. It was possible to conclude that the local interaction energies of propyl and decyl derivatives are quite similar while the hexyl derivative has higher interaction energy. The analysis of the coulombic energies together with the dispersion van der Waals energies (VDW) would be indicative, in first approximation, that carbonyl groups are more exposed in the case of propyl than in the other copolymers studied.