Design and development of self-stratifying systems as sustainable coatings

Thermosetting polyurethane coating formulations which stratify into two distinct layers after application have been prepared. In a thermosetting system the polymer phase separation among other factors is seen to be kinetically controlled and have less dependence on parameters such as surface tension gradient and polymer solubility. Certain coating applications including those used in architectural, automotive, industrial maintenance and aerospace require a combination of primer/topcoat or basecoat/clearcoat layers. These multilayered systems necessitate complex application and curing procedures. Multiple product formulation, application and processing steps not only require significant amounts of processing time, and contributing to environmental waste generation and pollution they also consume excessive amount of energy and manpower until a solid film has been produced. It would be desirable to reduce the number of layers to a minimum while providing the equal or better overall performance. In this paper we report the design and development of thermosetting polyurethane coatings that self-stratify to two distinct phases upon application and cure. It was demonstrated that kinetically controlled reactions aided by incompatible polymers possessing gradient surface free energies can afford self-stratifying coatings. This investigation also revealed that solvent type, evaporation rate, and gravity did not contribute to stratification of thermosetting coatings. Prototype pigmented systems were applied and cured and characterized by FTIR, SEM/EDX and were evaluated using standard coating test methods. The results of this investigation provide a new understanding of stratification phenomenon and establish that preferentially reactive polymers can be a basis for formulating cross-linkable self-stratifying coatings.     

Process limits in two-layer reverse roll transfer

Reverse roll coating in which a thin single layer of liquid is applied onto a substrate has been used in industry for decades and has been extensively analyzed in the literature. Modern coatings, however, are often composed of more than one layer to improve the product performance and to reduce the manufacturing cost. Premetered methods such as slot, slide, and curtain coatings are typically used to produce such multilayer coatings. If the caliper of the substrate to be coated is not constant, then the coating gap and consequently the final film thickness deposited on the web will also be nonuniform. In this study, we focused on the use of reverse roll technique with slot die liquid delivery system to produce a uniform thin two-layer coating. The use of this coating technique to produce such a coating has not been previously explored. The liquid film surface as it is transferred from a rigid steel roll to a deformable urethane-covered roll was visualized to find out how the uniformity of the two-layer coating is affected by the speed ratio between two rolls, layers’ wet thicknesses, and liquid viscosities. The effect of these parameters on the ribbing frequency and amplitude was also investigated. The results show that in the two-layer coating, as in the single layer reverse transfer, there is a critical web speed above which ribbing occurs. The critical speed is determined by the bottom layer viscosity.     

Study of degradation of organic coatings in seawater by using EIS and AFM methods

In this article, electrochemical behaviors and their topography observation for four organic coatings used in seawater, by using both electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM) methods to study environment behaviors of different coatings as well as the effects of their film formation, pigments, and fillers on anticorrosion behaviors, were measured. The results show that polyurethane, epoxy, and chlorinated rubber coatings all present one capacitive loop in their tested EIS which contains phenomenally only one time constant, whereas alkyd coating presents two capacitive semicircle arcs. With two capacitive loops, the capacitive semicircle in the high frequency range represents barrier layer property, but the semicircle in the low frequency range represents corrosion reaction of metals under the film. Polyurethane coating used in seawater has well anticorrosion property in seawater immersion test. The appearance features of different layers are visible different between different layers of tested coatings at their surface topography. The property of polyurethane paint film coated on metal is better than other layers, and film of alkyd coating has many pits at its surface by observing the layer's images. AFM photos imaged have also been used to further detail surface topography for four organic coatings, and to approve effects of topography of these coatings on its electrochemical behaviors, from two views of both height and phase modes. It is beneficial to explain deeply the environment behaviors and degradation mechanism of organic coatings. To further study failure of these organic coatings and dynamic processes of corrosion of metal under the film, two equivalent circuit models, according to these tested EIS, have been suggested to explain the corrosive kinetics of these four coatings. To polyurethane, epoxy, and chlorinated rubber coatings used in seawater which have good protection effects for substrate metal, the diffusion process for water, from their layer's surface to interface of film/metal, is mainly controlled factor for degradation. However, the electrochemical reaction process has may become a control procedure for corrosion of alkyd coated metal. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Layer-by-layer modification of thermoplastic coatings to improve adhesion

One of the causes leading to low bond strength between a coating and a substrate (adhesion strength) - if coatings are formed at elevated temperatures in air - is assumed to be a weak boundary layer generated in the region of adhesional contact: the boundary layer consisting mostly of low-molecular-weight products resulting from thermal oxidative degradation of the polymer. It has been verified experimentally that products of oxidation diffuse from the coating surface layer to the contact area. The oxidation process is supposed to be localized within that surface layer. A method has been devised to determine the thickness of the layer, and model experiments have been conducted to show that low-molecular-weight products of oxidation deteriorate the adhesion strength. Ways have been found to increase the adhesion strength of coatings by means of modification of the coating applied in a layer-by-layer manner. The idea is to introduce separately such modifiers as antioxidants, inorganic fillers possessing high adsorption capacities, and crosslinking agents into the coating surface layer. This method of coating modification allows one to eliminate the negative effects of the low-molecular-weight products generated in the surface layers during the formation.     

Influence of plasma nitriding treatment on the adhesion of DLC films deposited on AISI 4140 steel by PVD magnetron sputtering

Duplex surface treatments composed of diamond like carbon (DLC) coating followed by plasma nitriding have drawn attention for a while. In this study, AISI 4140 steel substrates were plasma nitrided at different treatment temperatures and times. Then, DLC films were deposited on both untreated and plasma nitrided samples using PVD magnetron sputtering. The effect of different plasma nitriding temperatures and times on the structural, mechanical and adhesion properties of DLC coatings was investigated by XRD, SEM, microhardness tester and scratch tester, respectively. It was found that surface hardness, intrinsic stresses, layer thickness values and phase distribution in modified layers and DLC coating were the main factors on adhesion properties of duplex coating system. The surface hardness and residual stress values of AISI 4140 steel substrates significantly increased with both DLC coating and duplex surface treatment (plasma nitriding + DLC coating). Increasing plasma nitriding temperature and time also increased the diffusion depth and the thickness of modified layers. Hard surface layers led to a significant improvement on load bearing capacity of the substrate material. However, it was also determined that the process parameters, which provided lower intrinsic stresses, improved the adhesion properties of the duplex coating system.     

Self-stratifying coatings based on Schiff base epoxy resins

Schiff base epoxy resin was used as a component of self-stratifying compositions, in which different acrylic resins were used. The degree of stratification was determined using the ATR FTIR spectroscopy and microscopy technique. Results were compared with those for commercial epoxy resins with a comparable epoxy number. A determination was made of the mechanical properties of the coatings obtained. Based on the previous studies, it was concluded that Schiff base epoxy resin in a film-forming composition behaves in a similar manner to BPA-based epoxy resins. The best mechanical properties and a high degree of stratification were achieved using a Schiff base epoxy resin/poly(isobutyl methacrylate) composition. Use of a self-stratifying composition not only reduces the time and cost of preparing a multilayer coating system, thanks to the formation of two layers in one application, but also causes the Schiff based epoxy coatings to become more flexible.     

Polymer structure and properties of heterophase and self-stratifying coatings

Phase separations in bulk and during the film forming process on a substrate, accompanied with the evaporation of solvents and chemical reactions of curing, have been studied in incompatible polymer blends (combinations of solid epoxy resin with selected thermoplastic resins, priorly dissolved in organic solvents) in order to be able to control the heterophase polymer structure of coatings. The resultant polymer/ polymer heterogeneous and sometimes non-homogeneous-in-layer (ultimately double-layer) polymer structures of coatings were evaluated by scanning electron microscopy (SEM). A few driving forces for self-stratification and the formation of double-layer coating structures, where each layer is enriched with a certain polymer component, are discussed as possibly associated with selective wetting of a substrate, phase contraction, interfacial tension gradients, etc. Examples are given to illustrate non-additive changes of structure-dependent coating properties with the composition of the epoxy/thermoplastic resin blends, which is characteristic for polymer/polymer heterophase materials. Depending on the phase state of the binder in the paint (formulated with the use of a selected incompatible polymer blend, combination of solvents and sometimes with an additive, controlling the phase separation process) and conditions of application to a substrate and film formation, various polymer/polymer heterophase or self-stratifying one-coat coatings can be obtained. Some of them, particularly those that combine partial stratification with polymer/polymer heterogeneity, are capable of meeting the demands for advanced performance due to improved combinations of bulk and surface properties.     

The influence of substrate absorbency on coating surface chemistry

The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS).

Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates.     

Environmentally friendly wooden-based coatings for thermal insulation: Design,manufacturing and performances

The design of an innovative protective and thermal insulating coating is investigated. The coating is composed of two superimposed layers; the innermost layer consists of dried beech dust dispersed in a diluted polyurethane binder, while the outermost layer is a conventional decorative hybrid epoxy-polyester powder coating. Each layer was sprayed on a metal substrate and baked at moderate temperature to consolidate the coatings and establish their full properties. The morphological features, mechanical response, protective and thermal insulating performance of the coatings were experimentally analysed by varying their structure and dried beech dust concentration in the binder. These coatings exhibit high potential in terms of thermal insulation and also show remarkable behaviour in terms of visual appearance, adhesion to the substrate and long lasting.     

Improving the adhesion between epoxy coatings and aluminium substrates

Two different methods were followed to improve the adhesion and durability of the adhesion of a commonly used epoxy coating on an aluminium substrate. The first method was by application of a thin polymeric layer, having a thickness of around 10 nm, on the aluminium substrate prior to application of the epoxy coating. The functional groups in the polymers were chosen so as to be capable of chemisorption to the oxide surface and should also to be capable of being involved in the curing reaction of the epoxy resin. These polymers were poly(acrylic acid) (PAA), poly(ethylene-alt-maleic anhydride) (PEMah) and poly(vinyl phosphonic acid) (PvPA). An investigation of the interphasial region between the epoxy coating and the aluminium substrate in the final cured system showed that the polymeric layers were indeed involved in the curing reaction with the epoxy.

For the poly(ethylene-alt-maleic anhydride)-based system, this resulted in the formation of a cured, mixed poly(ethylene-alt-maleic anhydride)/epoxy interphasial region between coating and substrate while for the two other polymers, a weakly cured interphasial region was formed. The second method of adhesion and durability improvement was by hydration of the aluminium substrates, performed by immersion in boiling water. This procedure results in the formation of a porous pseudoboehmite oxyhydroxide layer. The epoxy coating was found to be capable of fully penetrating into the layer. The adhesion of the epoxy coatings was tested initially and after exposure to 40 °C water and 40 °C 5% acetic acid. The poly(ethylene-alt-maleic anhydride)-based system resulted in a very good initial adhesion and durability in presence of water for the epoxy coating, while the systems based on the other two polymers did not. The pseudoboehmite-based system also resulted in very good initial adhesion and durability in the presence of water. None of the improved systems were, however, found to be able to withstand 40 °C 5% acetic acid and showed severe corrosion underneath the epoxy coating.     

Gradient MoSi2-borosilicate glass high emissivity coating with enhanced contact and impact damage resistance

High emissivity coatings on fibrous insulation tiles played an important role in thermal protection systems and thereby intrigued many researchers; however, there was little emphasis on the mechanical properties of the coatings. In this study, a gradient MoSi₂-borosilicate glass coating with a dense surface layer and a porous interlayer was designed for mullite fibrous ceramics. Mechanical properties and structure parameters of the coating layers and the substrate were studied. The gradient coating was compared with a monolayer dense coating of the same composition and same surface density in contact damage resistance, impact resistance and emissivity. Compared with monolayer dense coating coated substrates, the gradient coating coated ones exhibited two times higher load bearing capacity in Hertzian indentation test at the same displacement of 1?mm; they appeared to be stiffer and harder at constant load of 20?N, and showed better impact resistance at impact energy range of 0.25–0.75?J in the falling weight test; besides, fatal radial cracks were not observed in gradient coatings after the tests. In addition, the gradient coating had higher emissivity (0.838) than the monolayer dense coating (0.816) because of the significant absorptivity increase and reflectivity decrease by small gradual slopes in the rough surface.     

Microstructure and properties of Ni-WC gradient composite coating prepared by laser cladding

In this study, an Ni-based gradient composite coating reinforced with WC was prepared on a Q345R steel substrate by laser cladding. The Ni-WC composite coating was designed as a multilayer structure with gradient composition. The coating started with a layer of C276 alloy with 10 wt% WC on the substrate, and the subsequent layers were composed of Ni60 alloy with different WC contents (10, 30, and 50 wt% WC). The overall morphology, phase composition, and microstructure of the coatings were investigated. The microhardness and the wear properties of each layer of the coatings were also evaluated. The results showed that the gradient composition design was beneficial for reducing the cracking tendency. The coating was composed of an Ni-based matrix, WC, and multiple carbides and borides hard phases. With increasing WC content in the layers, the hard phases exhibited regional distribution characteristics. The WC reinforcement particles underwent different types of dissolution during the cladding process. From the surface to the substrate, the average microhardness of the coating was 1053.5 HV_0.2, 963.4 HV_0.2, 859.0 HV_0.2, 441.7 HV_0.2, and 260.5 HV_0.2. The wear tests revealed that the coefficient of friction and the wear loss values of the four layers were all lower than those of the substrate, demonstrating enhanced wear resistance.     

自分层涂料的研究进展

在简述自分层涂料的基本概念和发展历程的基础上,介绍了预测理论、设计原则以及表征手段,综述了国内自分层涂料的研究与应用现状,并展望了自分层功能梯度涂料今后的发展方向。     

Contact Damage and Strength Degradation in Brittle/Quasi-Plastic Silicon Nitride Bilayers

A study is made of the damage resistance of silicon nitride bilayers consisting of a hard overlayer (coating) on a soft underlayer (substrate). The two layers are fabricated with different starting powders, to provide distinctive elongate-grain microstructures, and are cosintered, to provide strong interfacial bonding and thus to minimize subsequent delamination. Contact testing with spherical indenters is used to characterize the damage response. The elastic-plastic mismatch between the layers is sufficiently high as to produce distinctive damage modes in the two layers: predominantly cone cracking in the coating, and quasi-plasticity in the substrate. However, the mismatch is also sufficiently low as to preclude secondary transverse cracks of the kind observed in other bilayer systems to initiate immediately beneath the contact at the coating/substrate interface and propagate upward within the coating. The dominant damage mode shifts from coating fracture to substrate quasi-plasticity with increasing contact load and decreasing coating thickness. Significantly, the presence of the soft underlayer inhibits growth of the coating cone cracks as the latter approach and intersect the interface. The underlayer also substantially diminishes strength losses from the contact-induced damage, especially in bilayers with thinner coatings. The implication is that bilayer structures with thin, hard coatings can preserve benefits from the inherent toughness of soft substrate materials, and at the same time afford surface protection (high wear resistance) to the underlayer.     

Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys

Lithium salts are being investigated as leachable corrosion inhibitor and potential replacement for hexavalent chromium in organic coatings. Model coatings loaded with lithium carbonate or lithium oxalate demonstrated active corrosion inhibition and the formation of a protective layer in a damaged area during neutral salt spray exposure. The present paper provides an abridged overview of the initial studies into this novel inhibitor technology for the active corrosion protection of aluminum alloys. Coating defects were investigated by microscopic techniques before and after exposure to corrosive conditions. Scanning electron microscopy analysis of cross-sections of the coating defect area demonstrated that the protective layer comprises a typical three-layered structure, which included a dense layer near the alloy surface, a porous middle layer, and a flake-shaped out layer. Potentiodynamic polarization measurements obtained with a microcapillary cell positioned in the coating defect area and electrochemical impedance spectroscopy confirmed the corrosion protective properties of these protective layers. The long-term corrosion inhibition of the lithium-based coating technology was tested in industrial coating systems.     

Composite multilayered coatings on mild steel

A composite multilayered coating which consisted of an electrodeposited Zn–Fe alloy layer, a zinc phosphate conversion layer, and one, two, or three organic layers was deposited on a mild steel substrate. The adhesion between these multilayered coating and the mild steel substrate was studied with the aid of a scratch testing technique. Observation of the worn surface of different multilayered coatings was performed with the aid of metallurgical microscopy. The same multilayered coatings were examined with FTIR spectroscopy and X-ray diffraction techniques. Finally, the corrosion behavior of bare and multilayered coated mild steel in 0.1 M NaCl solution (pH = 5.5, T = 25°C) was studied.     

PET型光学薄膜用涂层及相关技术研究现状

PET膜以其优异的性能在LCD领域得到了广泛应用,而功能性PET光学膜主要是通过在PET聚酯薄膜表面涂覆各种功能性涂层所得,因此PET型光学薄膜的涂层技术是光学膜的关键技术之一。从光学PET膜用涂层配方、涂层固化方式、涂布工艺和改善膜基材／涂层界面粘接强度的方法等方面,综述了PET型光学膜用涂层技术的国内外研究进展,并提出了未来的发展方向。     

含氟嵌段丙烯酸酯聚合物/环氧树脂自分层涂料的制备与性能

利用引发剂连续再生催化剂原子转移自由基聚合(ICAR-ATRP)合成了一系列结构可控的含有含氟丙烯酸酯和甲基丙烯酸缩水甘油酯(GMA)的两嵌段丙烯酸酯聚合物,即聚(甲基丙烯酸丁酯-co-甲基丙烯酸缩水甘油酯)-b-聚甲基丙烯酸十二氟庚酯[P(BMA-co-GMA)-b-PDFHMA,BGF]。将嵌段聚合物与环氧树脂混合制备自分层涂料。傅里叶变换红外光谱(FT-IR)和扫描电子显微镜-能谱分析仪(SEM-EDS)测试表明,当两嵌段聚合物中P(BMA)-co-GMA与PDFHMA的相对分子质量分别为5 300和2 300、GMA结构单元相对分子质量占总相对分子质量的20%,且BGF用量为10%时,漆膜固化过程中含氟嵌段聚合物可以部分迁移到漆膜表面,共混漆膜氟元素自迁移效果较好,表层氟含量达到20%以上,且增加含氟树脂中GMA含量时漆膜氟元素迁移效果下降;耐紫外老化性测试结果表明,含氟树脂的加入使得漆膜的光泽保持率更好,耐黄变性变化不大;水接触角和耐盐雾性测试结果表明,相对纯环氧树脂,加入10%含氟嵌段聚合物使漆膜的水接触角均提高到了100°以上,自分层漆膜的耐盐雾性都有一定提高,可以有效防止漆膜的起泡。嵌段聚合物的加入量低于4%时,对漆膜的基本力学性能没有影响,但光泽有一定下降。     

Correlation between electrophoretic clearcoats properties and electrochemical characteristics of noble substrates

The use of transparent organic coatings as protection for noble metal layers has a wide application especially in the costume jewellery and glass frames industry. The role of a clearcoat is to protect the noble substrate from scratches, tarnishing, and changes in gloss due to the interaction with the environment and human body. In this study electrodeposited organic coatings have been considered for this purpose. The application of clearcoats by electrophoresis usually assures a good adhesion to the substrate and the formation of a compact and protective layer. Electrophoresis has been developed especially under cathodic deposition and was mainly optimized for corrosion protection of active metals, but in this work both anodic and cathodic e-coats have been applied and analyzed. Since the substrates were noble metals, the surface could show a different reactivity during the application of the deposition voltage. The relationship between electrochemical behaviour of the substrate and the final properties of the clear coatings was investigated with electrochemical tests such as polarization curves, and electrochemical impedance spectroscopy. The correlation between deposition parameters and coatings properties as thickness and presence of defects was discussed according to the electrochemical results.     

Recent progress in corrosion protection of magnesium alloys by organic coatings

The excellent properties of magnesium alloys, especially the high strength/weight ratio, make them desirable materials in the automotive industry. However, their high corrosion susceptibility has greatly limited or even prevented their larger scale use for various applications. Organic coating is one of the most effective ways to prevent magnesium alloys from corrosion. In this report, the recent progress of organic coatings on magnesium alloys and techniques for evaluating the performance of organic coatings are reviewed.As a critical layer in a normal coating system, organic coating has great potential to prevent magnesium alloys from corrosion attack. However, some unsolved problems currently limit the application of organic coatings. Firstly, organic coatings usually have poor adhesion if they are applied without an appropriate pre-treatment. Sol–gel coating or plasma polymerization requires the least pre-treatment prior to deposition. However, the corrosion and wear resistance of these coatings have not been documented. Secondly, it is difficult to prepare a uniform, pore-free organic layer. So, it is usually necessary to apply multiple layers of these coatings to provide sufficient/optimum corrosion and wear resistance. Finally, a number of organic coating techniques are still solvent based, which poses an environmental concern. New water-borne and powder coating technologies should be developed.In order to evaluate the performance of organic coatings on magnesium, both electrochemical and non-electrochemical techniques have been developed. Information from different techniques gives insight into the organic coating/magnesium alloy interface in different aspects. Comprehensive knowledge about the interface is indispensible for understanding the degradation of the organic coating and developing new coating strategies.