Comparison of organic coating accelerated tests and natural weathering considering meteorological data

The protective properties of organic coatings for outdoor applications are generally evaluated by means of accelerated laboratory tests, including electrochemical techniques. The coatings are stressed by different mechanical, chemical, thermal loads and the effects on the protective properties can be measured by using well established electrochemical techniques, like electrochemical impedance spectroscopy, electrochemical noise, etc. An open question is how these accelerated tests can be correlated with natural exposure in different environments.     

Scratch behavior of nano-alumina/polyurethane coatings

Incorporating metal-oxide nanoparticles such as nano-alumina and nano-silica into polymer coatings to enhance mechanical durability is widely utilized in the current antiscratch and mar technologies. In this article, a quantitative study of the effect of a nano-alumina additive on the surface mechanical properties and scratch behavior of a two-part polyurethane coating is reported. An instrumented indenter with a conical diamond tip is used to measure surface mechanical properties (modulus and hardness) and to perform scratch tests, over a wide range of scratch loads. The scratch behavior in terms of the onset of elastic–plastic transition and scratch morphology were characterized by laser scanning confocal microscopy. The scratch results were correlated to the surface mechanical properties and relevant bulk material properties to understand the overall scratch behavior of the coatings. The results show that the scratch behavior of the coatings depends strongly on the concentration of nano-alumina.

Investigation of accelerated aging behaviour of high performance industrial coatings by dynamic mechanical analysis

Dynamic mechanical analysis (DMA) represents one of the most important methods for understanding mechanical behaviour of surface coatings providing a valuable link between chemistry, morphology, and performance properties. In this work, dynamic mechanical properties of several high performance industrial coatings were studied extensively. Four commercially available topcoats namely alkyd modified polyurethane (PU), economy aliphatic PU, high performance aliphatic PU and epoxy modified polysiloxane were selected based on their cure chemistries, volume solids, and overall performance. DMA was used to determine elastic modulus, glass transition temperature (T_g), crosslink density and creep behaviour of these coatings. DMA data were substantiated with mechanical and performance properties. Among the coatings, epoxy modified polysiloxane showed the highest T_g of 65.6 °C as well as crosslink density value of 2.24 × 10⁻³ mol/cc which was attributed to its superior mechanical and performance properties. In addition, topcoats were also subjected to artificial aging process in accelerated cyclic corrosion cabinet and QUV-weatherometer, respectively. The consequent changes in their physico-mechanical properties post exposure were also evaluated using DMA and correlated with other performance properties. After aging, the T_g increased substantially for all the coatings irrespective of their exposure type. For example, T_g of economy aliphatic PU increases from 38.4 °C to 52.9 °C and 51 °C after cyclic corrosion and UV-B weathering, respectively. However, crosslink densities either increased or decreased depending on the type of exposure and cure chemistries. These changes were corroborated using the Fourier transform infrared spectroscopy findings. The outcome of this study is expected to generate new insights into the behaviour of these coatings under dynamic mechanical stress and its relation with long term performance properties.     

Nanomechanical characterization of RB-SiC ceramics based on nanoindentation and modelling of the ground surface roughness

Reaction bonded silicon carbide (RB-SiC) ceramics are the primary structure and mold materials for the optical industry and mostly are machined by means of ultra-precision grinding to achieve a satisfactory surface quality. However, it is not easy to attain the theoretical prediction of surface quality, particularly surface roughness, because of different mechanical characterization of Si/SiC phases inside the RB-SiC ceramics. In this work, the nanoindentation tests were performed to investigate the nanomechanical characterization of individual phase inside the RB-SiC ceramics. On the basis of the nanoindentation results of RB-SiC, a theoretical model was established to predict surface roughness in the ultra-precision grinding process, which considered the different removal mechanisms of Si matrix and SiC particles. The comparison of the prediction results of existing and novel models and single-factor experimental results shows that the novel model was well consistent with the experiment.     

The influence of pre-bond surface treatment over the reliability of steel epoxy/glass composites bonded joints

A main objective of present research is to consider adhesive bonding as a novel maintenance and repairing damaged section trend for fluid transporting tubes. Nowadays, applying glass fiber reinforced epoxy composite patches (GFRECPs) is considered as an alternative rapid and affordable repair system instead of traditional techniques such as removing strained sections. The main problem with repairing metal pipes using GFRECPs is low strength of adhesion between GFRECPs and a steel substrate. To make adhesion strong enough, it is necessary to excite the intrinsic adhesion forces such as dipoles across the interface which consequently increases a bonding strength due to Van der Waals forces; but secondary forces activation depends on surface regulation levels. In fact, providing a surface with a suitable roughness and increased pureness without any polluters is a key parameter achieving a highly resistant GFRECPs-steel adhesion. To do so, samples were prepared using the SiC paper up to 100, 220, 500 and polished to investigate the effect of different roughness levels in the range of 90.77±1.81 to 2.97±0.05 nm. The surfaces, interface features and bonding strength were characterized applying the atomic force microscope (AFM), water contact angle measurements, FE-SEM, single lap shear (SLS) and T-peel (90°) tests. The results revealed that the highest adhesion strength could be achieved at the polished substrate.     

Application of profilometry and fractal analysis to the characterization of coatings surface roughness

Quantitation of the analysis of coatings surface roughness by a contact profilometer was undertaken to obtain improved repeatability and meaningfulness of data. This effort involved interfacing the profilometer to a personal computer for data acquisition and analysis, optimizing operational variables involved in the measurement process and improving methods for quantitative data analysis of the roughness profiles. The quantitative methods of data analysis explored included the use of various surface profile roughness averages, power spectrum analysis through zone integrals, auto-correlation function analysis and fractal analysis of the surface roughness profile. Quantitative parameters obtained from the above methods were correlated with visual and optical methods for evaluating appearance for a variety of coatings. The superiority of correlation with evaluations of appearance and repeatability of multiple runs will be demonstrated for fractal numbers obtained from a fractal analysis of surface roughness profiles.     

Photo-induced deformation behavior depending on the glass transition temperature on the surface of urethane copolymers containing a push-pull type azobenzene moiety

Urethane copolymers containing a push-pull type azobenzene moiety with the same dye content were synthesized to investigate the relationship between photo-induced deformation and molecular mobility. The copolymers exhibit different glass transition temperatures (T_g), from 46 to 143 °C, due to their different main chain structures. An indented nanostructure induced by the optical near field around the polystyrene microspheres and a surface relief grating (SRG) induced by exposure to a two-beam interference pattern were examined using films of copolymers. We found the dependency of the deformation efficiency on T_g was inverted depending on the irradiation power. The deformation depth increased with T_g under high power irradiation in both the indented nanostructure and the SRG forming experiments. In contrast, the deformation depth of the SRG decreased with increasing T_g under low power irradiation. The discovery of this inverted tendency suggests that, in addition to the molecular mobility, we should consider other factors in the deformation mechanism, such as the recovery of deformation, the degree of plasticization, and the thermal effects.     

Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positron annihilation lifetime measurements

The glass transitions of acrylonitrile-butadiene rubber (NBR)/organoclay nanocomposites with various silicate contents were investigated using positron annihilation lifetime spectroscopy (PALS). The nanocomposites were prepared through melt intercalation of NBR with various concentrations of organoclay (OC30B) modified with the organic modifier, methyl tallow bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH), i.e., Cloisite^® 30B. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements of the NBR/OC30B nanocomposites showed that the NBR chains were intercalated between the silicate layers, thereby increasing the gallery heights of the organosilicates. The glass transition temperature of NBR was determined using differential scanning calorimetry (DSC). However, it seemed to be very difficult to clearly resolve the very small differences in T_gs caused from various loading of nanosized silicate in NBR/OC30B nanocomposites. Hence, we performed positron annihilation lifetime spectroscopy (PALS) on NBR/OC30B nanocomposites containing various amounts of OC30B (1-10 wt%). Significant changes in the temperature dependencies of free volume parameters (i.e., lifetimes and intensities) were observed at the transition temperature, T_g,PALS, and the T_g,PALS values were found to increase with increasing organoclay content in the samples. These observations are consistent with PALS having a higher sensitivity in the detection of very small changes in free volume properties. The present findings thus highlight the usefulness of PALS for studying phase transition phenomena in polymeric materials with nanoscale structural variations.     

Investigation of the glass transition temperature and damping of an acrylic/epoxy bonding tape using the peak damping method

The dynamic properties of a structural tape used for adhesive bonding applications have been measured at different temperatures to determine its glass transition temperature and damping properties. For this purpose, free layer beams consisting of a base layer steel and the tape layer were vibrated using a resonant beam technique with free-free end conditions. To measure the dynamic values (elastic modulus and loss factor) of the tape, the necessary equations were derived and the frequency dependence of the beams was investigated from –55°C to +60°C. Three beams with different layers were tested. Results have shown that as the temperature increases, the elastic modulus of the tape decreases, while the loss factor of the tape increases up to 20°C and then decreases to a constant level. The results from the three beams are in agreement, showing that the glass transition temperature of the tape is about 20°C, which implies viscoelastic properties at room temperature.     

Tuning the mechanical properties of UV coatings towards hard and flexible systems

The standard resins of radiation-curable coatings provide either hard or flexible coatings dependent on the type of chemistry used. Whereas aromatic epoxide acrylates usually give hard and brittle coatings, urethane acrylates are known for their flexibility. Since the radiation curable systems should not contain solvents, the desired low viscosity for the specific application is adjusted with reactive monomers. This normally prevents the use of flexible high-molecular-weight polymers. On the other hand, the viscosity of dispersions is determined by the solid content only and not by the molecular weight of the polymers used. Thus, waterbased UV-curable coatings are one strategy out of this dilemma in order to combine the flexibility of higher-molecular-weight polymers with the hardness of highly crosslinked acrylates. The mechanical data of conventional and waterbased UV coatings are discussed in dependency on glass transition temperature and elastically effective chain length between crosslinks.     

Effect of physical aging on thermal stress development in powder coatings

Thermal stress and physical aging are inherent to thennosetting systems (such as powder coatings) and may affect the coating durability leading to damage such as detachment and cracking. Both phenomena occur principally below the glass transition temperature (T_g) of the coating and affect each other. It is shown that the measurement of stress, as a function of temperature of coatings aged at different temperatures and during various times, represents a simple and interesting way to study these phenomena. The results obtained which show changes in the stress magnitude with aging are explained in terms of stress relaxation and structural recovery. The latter process is especially evident in the T_g region and can prevent the correct determination of the T_g by means of thermal stress measurements. The thermal expansion coefficient and the elastic modulus, two properties directly affecting the thermal stress magnitude were determined separately, and agree well with the proposed interpretation of experimental data. The linear dependence of thermal (compressive) stress on the logarithm of time indicates the possibility of predicting the effect of physical aging.     

Thermal characterization on the relaxation transition of the phenylphosphonate-modified epoxy copolymers by thermally stimulated current technique and relaxation mapping analysis

TSC/RMA study on the depolarization relaxation transitions of the phenylphosphonate-modified epoxy copolymers with thiodiphenol (P2-ETP) or bis-phenol A (P2-EBPA) segments were conducted. P2-ETP resin shows a lower T_g than that of P2-EBPA, indicating that divalent thio-linkage (-S-) is flexible than isopropyl (-C(CH₃)₂-) group. Thermal windowing technique of RMA was used to obtain thermokinetic data in which ΔS of P2-ETP is higher than P2-EBPA, thus confirms the easier motion in P2-ETP which then leads to the lowering of the glass transition (T_g). Methods used for the confirmation on the T_g temperature were illustrated from thermokinetic data obtained by RMA measurement.     

The effects of grit-blasting on surface properties for adhesion

A detailed study of the effects of grit blasting with different alumina grits on the surface characteristics of mild steel and aluminium alloy substractes is reported. Non-contacting 3D-laser profilometry was used to characterise surface texture, and surface energy was measured by static contact angle techniques. The chemical composition of the surface was determined by XPS analysis. Adhesion characteristics were investigated by the measurement of strength of lap shear and tensile butt joints using a two-part room temperature curing epoxy adhesive. As initial joint strengths were relatively insensitive to the changes in grit-blasting parameters, further studies were based on joint response to accelerated ageing conditions. The results indicate that the changes in joint properties associated with roughened surfaces cannot be explained simply by the increased roughness characteristics, such as mechanical keying and increased effective bond area. It is evident that changes in physical and chemical properties of the surfaces, arising from the grit-blasting process contributed significantly to the joint behaviour.     

Influence of alternating sequential fraction on the melting and glass transition temperatures of ethylene-tetrafluoroethylene copolymer

The melting (T_m) and glass transition (T_g) temperatures of a series of ethylene (E)-tetrafluoroethylene (TFE) copolymer (ETFE) have been found to show unique dependence on the TFE content with the minimal and maximal points. These behaviors have been interpreted successfully on the basis of the degree of alternation of E and TFE monomeric units along the skeletal chain. The melting point of a perfectly alternating copolymer is estimated to be 295 °C on the basis of the dependence of T_m using a modified Flory’s equation. The corresponding T_g was estimated as 145 °C by applying a modified Gibbs-Damnation’s equation.     

An Interactive Tool for the Optimization of Freeze-Drying Cycles Based on Quality Criteria

Among existing dehydration methods, freeze-drying has unique benefits for the stabilization and preservation of biological activity of pharmaceutical products but remains an expensive and time-consuming process. A user-friendly software tool was developed, allowing for interactive selection of process operating condition profiles in order to maximize process productivity while insuring product quality preservation. The software is based on a dynamic, one-dimensional heat and mass transfer model, which can accurately represent both the primary and secondary drying stages and the gradual transition between them. The model was validated in a wide range of operating conditions: ? 25 to + 25°C shelf temperature and 10 to 34 Pa total pressure. By comparing a reference sucrose solution with a formulated pharmaceutical product containing polyvinylpyrrolidone (PVP), it is shown that controlling product properties such as glass transition temperature and sorption isotherm can reduce the minimum achievable cycle duration by 12 h (33%).     

An Interactive Tool for the Optimization of Freeze-Drying Cycles Based on Quality Criteria

Among existing dehydration methods, freeze-drying has unique benefits for the stabilization and preservation of biological activity of pharmaceutical products but remains an expensive and time-consuming process. A user-friendly software tool was developed, allowing for interactive selection of process operating condition profiles in order to maximize process productivity while insuring product quality preservation. The software is based on a dynamic, one-dimensional heat and mass transfer model, which can accurately represent both the primary and secondary drying stages and the gradual transition between them. The model was validated in a wide range of operating conditions: - 25 to + 25°C shelf temperature and 10 to 34 Pa total pressure. By comparing a reference sucrose solution with a formulated pharmaceutical product containing polyvinylpyrrolidone (PVP), it is shown that controlling product properties such as glass transition temperature and sorption isotherm can reduce the minimum achievable cycle duration by 12 h (33%).     

Crystallographic orientation effect on the polishing behavior of LiTaO3 single crystal and its correlation with strain rate sensitivity

Lithium tantalite (LiTaO₃ or LT) single crystal has been extensively applied in the fields of electro-optical and piezoelectric devices. As a typical anisotropic material, the crystallographic orientation effect on its machining responses, i.e., surface roughness and material removal rate (MRR), is not yet well understood. In the present work, we investigated the polishing responses of the three typical crystallographic orientations for LT single crystal under a series of rotation speeds. The results showed that both the rotation speed and crystalline orientation had little effect on the quality of polished surface. While for the MRR, it was almost linearly increased with increasing rotation speed for all of the three planes, among which the enhancements of MRR on Y-42° and Y-36° planes were more pronounced than that on X-112° plane. The scratch features and friction coefficients were investigated using a nanoindentation system under various velocities. The nano-hardness values were obtained under various strain rates and, hence, the strain rate sensitivities (SRS) were determined as 0.0172, 0.0455, and 0.043 for X-112°, Y-42°, and Y-36° planes, respectively. The better mechanical properties and “plastic” removal mechanism of X-112° plane resulted in the lower value of MRR. Also, the much lower SRS corresponded well with the relatively less sensitivity of MRR with rotation speed on X-112° plane. Results of this study suggested that the plastic parameter of SRS could provide as an excellent indicator to bridge the machining response and intrinsic deformation mechanism for brittle single crystal ceramics.     

Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives

The effects of post-curing and cure temperature on the glass transition temperature, T _g, and the mechanical properties of epoxy adhesives were studied. T _g was measured by a dynamic mechanical analysis apparatus developed in-house and the mechanical properties of the adhesives (yield strength, Young’s modulus and failure strain) were measured by a tensile machine. The relationships between T _g and mechanical performance under various post-cure conditions were investigated. The curing process was the same for all tests, consisting of an initial stage performed at different temperatures followed by cooling at room temperature. Three sets of specimens were considered, sharing the same initial cure process, but with a different post-curing procedure. In the first set, the specimens were only subjected to a curing process; in the second set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature below the T _g of the fully cured network, T _g∞; and in the third set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature above the T _g∞. When post-cured at a temperature above T _g∞, the mechanical and physical properties tend to have a constant value for any cure temperature.     

Effect of a moving flame on the temperature of polymer coatings and substrates

A computational model is developed to predict the temperature profile over an organic coating on a metal surface as a result of the action of a moving flame. The deflection of the flame as it impinges on the surface is simulated and its consequent heat transfer to the polymer is determined. The scanning action of the flame across the substrate is quantified and the temperature profiles within the polymer are calculated. The results show a substantial build up of temperature at the surface and large temperature gradients throughout the thickness, which are due to the low thermal conductivity of polymers. This can be particularly detrimental for polymers owing to their low softening and decomposition temperatures. The model can be applied to flame impingement on a bulk polymer or on an organic coating on a metal substrate. The research shows the risks of a moving flame overheating a polymer surface and indicates remedial measures.