Enhancing exterior performance of clear coatings through photostabilization of wooden surfaces. Part 1: Treatment and characterization

The aim of this work was to produce a less photodegradation-prone substrate for clear coating by partially delignifying the surface cells of Pinus radiata boards to a depth of 2–3 mm while maintaining the integrity of the wood surface tissues in the delignified zone. To achieve this, several surface oxidative delignification treatments were trialled in the method development process and peracetic acid was chosen as the method for refinement and deployment. The treatment method was optimized to yield a significant degree of delignification compatible with the aim of producing a photostabilized yet intact wooden surface. A preweathering technique was also used as a second delignification method for producing photostabilized boards. Microscopic and chemical techniques were used to characterize the effects of chemical and preweathering treatments, which produced delignified surface envelopes 2–3 mm and 100 μm deep, respectively. Acetyl bromide lignin analyses, infrared spectroscopic analyses, and density changes of 30-μm-thick sections of peracetic-acid-treated samples, as a function of depth from the wood surface, suggested that a partial delignification had occurred that diminished with depth. Light and transmission electron microscopy provided evidence of delignification at the cellular level. In the surface layers of peracetic-acid-treated boards, all cell wall regions were delignified, with the middle lamella being the most severely affected. Lignin appeared to be completely removed from the cell corner middle lamella regions, but tracheids were still joined in other parts of the middle lamella. The S₁, S₂, and S₃ walls were also delignified. In subsurface layers, cell walls were only partially delignified and the tissues held their integrity. In contrast, in the preweathered boards, cell walls in the outermost layers were completely separated at the middle lamella from photodegradation. Preferential lignin removal over that of hemicelluloses was achieved via oxidative treatment of solid wood. The outcome of this was the successful fulfillment of our aim to produce partially delignified wooden surfaces that retained sufficient strength and aesthetic appearance and were suitable for application in wooden structures, provided that the necessary protection measures were carried out. Due to the narrow surface zone produced, machining of such surfaces would not be recommended as the most delignified zone would be removed first. In Part 2 of this series, the photostability of treated boards, which had been clear coated and weathered, will be considered.     

Photocurable epoxy/cubic silsesquioxane hybrid materials for polythiourethane: Failure mechanism of adhesion under weathering

A new inorganic–organic hybrid coating containing epoxy‐functionalized cubic silsesquioxane (CSSQ) has been developed, which can be polymerized cationically by UV radiation. This solvent‐free solution can be used as hybrid coating for polythiourethane (PTU) substrate. The surface properties of the coating film were determined by adhesion and scratch resistance. The excellent adhesion of coating films on the substrate was observed at the initial stage before weathering, but deteriorated after exposure to the sunshine. The low viscosity of hybrid coating solution (～ 15 mPa s) leads to fast curing and the formation of hybrid coating film during the photopolymerization reaction. The adhesion failure was evaluated by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), and X‐ray photoelectron spectroscopy (XPS) analyses. AFM images showed that the surface is smooth at the initial stage, but a texture surface was developed after weathering. The shrinkage of the hybrid film due to the increase in crosslinking density by postpolymerization would affect the surface roughness after weathering. XPS analysis indicated that the adhesion failure occurred by photodegradation of the PTU substrate during weathering. The weathering resistance was significantly improved by adding UV absorbers, which protected the polymer substrate from the photodegradation. The advantages of the hybrid coating include fast cure speed, solvent‐free formulation, and improved surface properties of the coating film. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Weathering performance of spruce coated with water based acrylic paint modified with TiO2 and clay nanoparticles

This work examines the effect of wood specimens coated with water based acrylic coatings modified with TiO₂ and clay nanoparticles against weathering strain. The long-term durability of the specimens towards climate strain was studied within a relatively short time frame by accelerated climate ageing. The surface changes that occurred as a result of photodegradation of the specimens subjected to accelerated climate exposures were studied using colour measurements and Fourier transform infrared (FTIR) spectroscopy. The results revealed a significant decrease in the intensity of lignin bands attributed to degradation of the lignin component of the wood. However, the intensity of the lignin photodegradation was lower for coated specimens, with slightly lower degradation for the specimens coated with paints modified with TiO₂ and unmodified montmorillonite clay nanoparticles.     

Photodegradation of Natural Wood Veneer and Studies on Its Color Stabilization for Automotive Interior Materials

The aim of this study was to investigate the color and chemical changes of natural wood veneers after ultraviolet (UV) exposure for automotive interior materials. To control the photodegradation, chemical treatments with hydrogen peroxide (H₂O₂) and sodium hypochlorite (NaClO) solution were applied to the veneer under different concentration conditions. The veneers treated with H₂O₂ showed strong color changes on the surface after the UV test. On the other hand, the veneers treated with NaClO solution (2–3% concentration) showed excellent color stability compared to the untreated veneer because NaClO partially removed lignin compounds on the veneer surface. NaClO can improve the color stability by destroying the lignin structures. However, 3% NaClO condition was seen to cause surface damage although this treatment condition had the best effect on photoprotection. Therefore, treatment with 2% NaClO was the best condition in this study for against color change and photodegradation caused by UV light.     

Enhancing exterior durability of jack pine by photo-stabilization of acrylic polyurethane coating using bark extract. Part 1: Effect of UV on color change and ATR–FT-IR analysis

Heat-treated wood is a value-added product but its exposure to various environmental factors leads to discoloration of wood surface due to the photochemical reactions. Discoloration has become an important economic problem for wood industries since product specifications are now more demanding. In addition, stricter environmental legislations necessitate the development of environmentally friendly transparent coatings with minimal use of chemicals which balances aesthetic and protection. In this study, the acrylic polyurethane coating was improved with the addition of natural antioxidant (bark extract) and lignin stabilizer alone or in combination to enhance the resistance of this coating to different weathering factors. An accelerated aging test was conducted with the aim of comparing the acrylic polyurethane coatings containing different additives with the commercially available pigmented solvent borne coating used by industry and organic UV absorbers. The modifications in chemical structure of coatings were characterized by ATR–FT-IR analysis. The color change data showed that the coating containing bark extract was the most effective and performed better than the industrial coating. However, the visual assessment showed that the coating containing bark extract and lignin stabilizer had the best performance. FT-IR analyses suggested that the chain scission reaction took place throughout the weathering but its effect was not significant for any of the coatings.     

Natural weathering performance and the effect of light stabilizers in water-based coating formulations on resin-modified and dye-stained beech-wood

This study assesses the performance of untreated and resin-modified beech-wood (Fagus sylvatica L.) during outdoor weathering. Boards modified with thermosetting N-methylol melamine (NMM) and phenol–formaldehyde (PF) resins, which were partly dye stained, were coated solely with a waterborne acrylic binder and formulations containing the same binder with different types and contents of photo-protective additives. Most modifications of the wood substrate changed the original color of wood, except for sole NMM modification. Changes in mass and capillary water uptake during exposure of modified, uncoated and coated boards were less than those of respective controls. Surface defects and cracks were clearly fewer on modified wood than on the controls, but no clear difference was observed among the topcoats containing UV-protective agents (UV-PA). The color stability during outside weathering depended on the treatment and coating formulation. Untreated and NMM-modified boards became grayer, and the NMM-dye-modified boards turned to a lighter gray, while PF-modified boards adopted a darker, blackish color. The weathered coating on the modified boards, particularly with PF resin, showed less blistering, flaking, and cracking than that on the controls. UV-PA stabilized the color and adhesion on all boards compared to the sole binder formulation. We conclude that wood modification with NMM and PF resin improves the natural weathering performance of wood coated with acrylic coatings. Combination of modification with staining enables diversification of the optical appearance.     

无机型人造石表面紫外光固化涂膜研究

以KH-550为打底液对无机型人造石表面进行预处理,然后以紫外光固化涂料罩光.讨论了低聚物和活性稀释单体的组成、消泡剂种类和KH-550的用量对人造石表面紫外光固化涂膜性能的影响.结果表明,以质量分数为3%的KH-550对人造石表面进行预处理后,按低聚物55%,活性稀释单体40%,光引发剂和助剂分别为4%和1%的质量分...     

Weathering performance of waterborne acrylic coating systems on flat-pressed wood–plastic composites

This study applied coating systems containing two different waterborne acrylic resins with an ultraviolet (UV) absorber on the surfaces of polyvinyl chloride-based flat-pressed wood–plastic composites (WPCs) evaluated and the effects of waterborne acrylic coating on the performance of WPCs in outdoor conditions. The results showed that waterborne acrylic coating systems enhanced the surface quality of WPC. The decrease in flexural strength was found to reach up to 22%, while it was about 25% for modulus. The color changes on the surface decreased by 55% as a result of the UV absorbability of the coating systems. Moreover, the chemical changes in the composites were found to be almost nonexistent in the attenuated total reflection–Fourier transform infrared spectroscopy for WPCs coated with both formulations. The light microscopy images revealed that the coating systems minimized deformation on the surface. In comparison to the control samples, it was seen that the surface roughness of the WPC was also improved by using waterborne acrylic coating systems. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48518.     

AMMONIA CATALYZED ORGANOSOLV DELIGNIFICATION OF POPLAR

A parametric investigation of NH₄OH catalyzed solvent delignification of poplar was conducted to define pretreatment conditions which would yield an optimal separation of the biomass components and an enzymatic susceptible solid carbohydrate phase. Delignification parameters of interest included concentration of NH₄OH, time and temperature of the reaction, and type of solvent. The addition of 0.82 M NH₄OH to the delignification liquor increased lignin removal and decreased carbohydrate degradation, but increasing NH₄OH concentration had no additional effect. At lower reaction temperatures, the extent of delignification increased with reaction time; at higher temperatures, a “relignification” of the pretreated wood was observed. The delignification and hemicellulose solubilization were modelled and rate constants reported. No major difference between three potential pulping solvents—ethanol, butanol, phenol—was observed. The enzymatic susceptibility of pretreated wood samples was approximately 6-fold greater than that of the untreated poplar. UV absorbance was used to qualitatively characterize the soiubilized lignins.     

Alteration of the Surface Energy of Wood Using Lignin-(1-Phenylethene) Graft Copolymers

Graft copolymers of lignin, made by free radical graft copolymerization of 1-phenylethene on lignin, increased the contact angle of water on birch wood (Betula papyrifera) and decreased the critical surface tension of the wood when coated onto the wood surface from an N, N-dimethylformamide solution. The coating of copolymer changed the wood from hydrophilic (～50°) to hydrophobic (110°).

The most pronounced change in contact angle was produced by benzene-soluble extracts of the reaction product. These extracts contained lignin with long poly(1-phenylethylene) sidechains and pure poly(1-phenylethylene). They produced surfaces with the numerically highest contact angles with water and changed the wetting behavior of the surface more than physical mixtures of lignin and poly(1-phenylethylene) or either of the pure polymers.

Despite coating weights of less than 100 mg/cm, the critical surface tension of the birch wood coated with the various lignin-(1-phenylethylene) graft copolymers ranged from 26.9 to 44.9 dynes/cm while the uncoated birch had a critical surface tension of 49.6 dynes/cm.     

Boreal forest conifer extracts: potential natural additives for acrylic polyurethane coatings for the protection of heat-treated jack pine

Among consumers, heat-treated wood recently gained more importance due its attractive darker color along with several other advantages such as improved biological resistance, dimensional stability, and thermal insulating properties. However, exposure to the exterior environment drastically changes the attractive darker color of this product. From dark brown, it becomes almost white. For this reason, development of a nontoxic transparent coating which can protect the heat-treated wood from photodegradation is very important. Addition of UV stabilizers (UV absorbers, HALS, antioxidant) is necessary for transparent coatings in order to filter the harmful UV/VIS radiation. In this present study, the effectiveness of natural antioxidants, extracted from conifer barks or needles from trees in protecting the color of wood, as additives to acrylic polyurethane coating is examined. Also, their UV/VIS absorption capacities and semi-quantitative component analyses have been presented. The toxicity of these new products was also evaluated on human normal skin cells due to ever increasing environmental legislation. The chemical modification of acrylic polyurethane coating containing conifer bark and needle extract were examined by ATR–FTIR analysis. Promising results were obtained for these natural antioxidants which showed very high UV absorption characteristics and antioxidant activity.     

Coating performance of finishes on wood modified with an N-methylol compound

This study describes the effects of wood modification with modified 1,3-dimethylol-4,5-dihydroxyethyleneurea (mDMDHEU) on the coating performance of solvent- and water-borne finishes. Although the basic colour of wood was slightly changed due to the modification, modified and unmodified samples did not display any colour differences after coating with translucent stains. Blocking of coated wooden substrates (degree of adhesiveness) was dependant on the type of finish system and, in a few cases, on the wood substrate. For all wood substrates, both treated and untreated, solvent-borne alkyd finish penetrated into the surface layers of the wood cells, while water-borne acrylic finishes spread only on the wood surfaces. The solvent-borne alkyd finish dried much more slowly on wood substrates than did the water-borne acrylics. Modification with mDMDHEU did not affect the drying rates of the finishes tested. The wet adhesion depended on the finish systems tested and on the pre-treatment of the wooden substrate. With regard to the pre-treatment, wet adhesion of one water-borne and the solvent-borne finish systems was significantly enhanced through precedent modification with mDMDHEU. The other two water-borne finish systems showed a higher degree of wet adhesion. This study demonstrates that modification of the wood substrate with mDMDHEU is compatible with both water-borne and solvent-borne finish systems and improves their wet adhesion on the wood surface.     

Surface degradation and nanoparticle release of a commercial nanosilica/polyurethane coating under UV exposure

Many coating properties such as mechanical, electrical, and ultraviolet (UV) resistance are greatly enhanced by the addition of nanoparticles, which can potentially increase the use of nanocoatings for many outdoor applications. However, because polymers used in all coatings are susceptible to degradation by weathering, nanoparticles in a coating may be brought to the surface and released into the environment during the life cycle of a nanocoating. Therefore, the goal of this study is to investigate the process and mechanism of surface degradation and potential particle release from a commercial nanosilica/polyurethane coating under accelerated UV exposure. Recent research at the National Institute of Standards and Technology (NIST) has shown that the matrix in an epoxy nanocomposite undergoes photodegradation during exposure to UV radiation, resulting in surface accumulation of nanoparticles and subsequent release from the composite. In this study, specimens of a commercial polyurethane (PU) coating, to which a 5 mass% surface-treated silica nanoparticle solution was added, were exposed to well-controlled, accelerated UV environments. The nanocoating surface morphological changes and surface accumulation of nanoparticles as a function of UV exposure were measured, along with chemical change and mass loss using a variety of techniques. Particles from the surface of the coating were collected using a simulated rain process developed at NIST, and the collected runoff specimens were measured using inductively coupled plasma optical emission spectroscopy to determine the amount of silicon released from the nanocoatings. The results demonstrated that the added silica nanoparticle solution decreased the photodegradation rate (i.e., stabilization) of the commercial PU nanocoating. Although the degradation was slower than the previous nanosilica epoxy model system, the degradation of the PU matrix resulted in accumulation of silica nanoparticles on the nanocoating surface and release to the environment by simulated rain. These experimental data are valuable for developing models to predict the long-term release of nanosilica from commercial PU nanocoatings used outdoors and, therefore, are essential for assessing the health and environmental risks during the service life of exterior PU nanocoatings.     

Durability enhancement of icephobic fluoropolymer film

An icephobic and superhydrophobic surface was made by the sputtering of fluoropolymer material (PTFE or Teflon^®) on anodized aluminum alloys. The study of this superhydrophobic coating under atmospheric icing conditions showed a 3.5 times reduction of its ice adhesion strength. To evaluate the longevity of such coated surfaces and to assess their potential outdoor applications, their durability was studied after several icing/de-icing cycles. However, these coatings showed weak stability after several icing/de-icing cycles. Plasma argon pretreatment of the anodized aluminum surface was used before sputtering to increase adhesion strength between the anodized aluminum surface and Teflon-like coating. Ice adhesion and contact angle measurements of the pretreated Teflon-like coating indicated clearly that the instability was associated with the low cohesion strength of the Teflon-like film. In order to improve the cohesive strength of the coating, the input power of the discharge was increased during the sputtering process. XPS, SEM, and contact angle analyses showed that an increase in input power renders the Teflon-like coating more stable. The results of ice adhesion measurement showed low variation in ice adhesion strength on such surfaces after 15 icing/de-icing cycles. This coating also showed an excellent stability under UV irradiation and condensation.     

过氧酸预处理时过氧酸与木素的反应机理

综述了过氧酸溶液（过氧乙酸、过氧一硫酸、混合过氧酸和二甲基二环氧乙烷）在预处理浆料时过氧酸与本素之间反应的有关机理,阐述了过氧酸预处理提高后续氧脱木凳的选择性,改善氧脱木素效果的原因。过氧酸作为一类具有很强氧化性的无氯漂剂,随着有关技术的日益发展和完善,完全可以代替有氯漂剂从而实现氯漂白。     

Peracetic acid pretreatment of sugarcane bagasse for enzymatic hydrolysis: a continued work

Previous work has shown that the enzymatic hydrolysis of sugarcane bagasse could be greatly enhanced by peracetic acid (PAA) pretreatment. There are several factors affecting the enzymatic digestibility of the biomass, including lignin and hemicelluloses content, cellulose crystallinity, acetyl group content, accessible surface area and so on. The objective of this work is to analyze the mechanism of the enhancement of enzymatic digestibility caused by PAA pretreatment. Delignification resulted in an increase of the surface area and reduction of the irreversible absorption of cellulase, which helped to increase the enzymatic digestibility. The Fourier transform infrared (FTIR) spectrum showed that the absorption peaks of aromatic skeletal vibrations were weakened or disappeared after PAA pretreatment. However, the infrared crystallization index (N.O'KI) was increased. X‐ray diffraction (XRD) analysis indicated that the crystallinity of PAA‐treated samples was increased owing to the partial removal of amorphous lignin and hemicelluloses and probable physical change of cellulose. The effect of acetyl group content on enzymatic digestibility is negligible compared with the degree of delignification and crystallinity. The results indicate that enhancement of enzymatic digestibility of sugarcane bagasse by PAA pretreatment is achieved mainly by delignification and an increase in the surface area and exposure of cellulose fibers. Copyright © 2008 Society of Chemical Industry     

Effect of polydopamine coating on improving photostability of poly(1,3,4-oxadiazole)s fiber

Effect of polydopamine coating on improving photostability of poly(1,3,4-oxadiazole)s (POD) fiber was studied in detail. Due to strong adhesion ability of polydopamine, the POD fiber could be coated easily by dopamine in Tris–HCl (pH?=?8.5) solution at room temperature with exist of oxygen. The polydopamine coating was characterized by SEM, XPS and ATR-FTIR. It was proved by UV-spectra analysis that the polydopamine coating had a strong absorbance in ultraviolet wavelength and was very stable to ultraviolet. Additionally, it was found by pyrogallol autoxidation method that the polydopamine could efficiently capture superoxide anion radical, which was an important factor that resulted in photodegradation of POD fiber, due to its catechol structure. These two findings implied that polydopamine coating was a potential photoprotection agent. Finally, compared to POD fiber, the tensile test of POD fiber coated by polydopamine further proved that the polydopamine did improve the photostability of POD fiber. And the FTIR analysis of POD coated by polydopamine also confirmed that there was almost no macromolecular change after UV exposure.     

Synthesis and characterization of ZnO NRs with spray coated GO for enhanced photocatalytic activity

Zinc oxide nanorods, ZnO NRs, were synthesized on a clean glass and coated with graphene oxide (GO) using spray coating method to enhance the photocatalytic activity in wastewater treatment. The ZnO NRs were synthesized using the solution process synthesis that was optimized using Taguchi method. Several synthesis parameters have been optimized and studied to determine the best synthesis parameter to grow ZnO NRs for the photodegradation of organic contaminants. Field emission scanning electron microscopy (FESEM) with EDX, X-ray diffraction (XRD), Raman, ultraviolet visible near-infrared (UV-VIS-NIR), and photoluminescence (PL) spectroscopies were used to investigate the structural and optical properties of the produced nanorods. FESEM images revealed the vertical growth of ZnO NRs as well as layers of GO covering the ZnO NRs' top surface. The Raman study demonstrates the combination peak of GO and ZnO, hence proving the GO layer's successful coating. After the GO coating, decrease in the bandgap of the synthesized photocatalyst was detected by PL and UV–Vis absorption measurements. Under UVC exposure with treatment time of 6 h, the degradation of MB with ZnO NRs/GO photocatalyst reached a degradation percentage of 97.86%, which is greater than the degradation percentage achieved using pristine ZnO NRs, which is 93.28%. The results validated that the coating of GO enhances the photocatalytic activity of the host material, ZnO NRs.     

Duration of wood preweathering: Effect on the service life of subsequently applied paint

Previous studies of the effect of preweathering of wood (weathering of wood prior to painting) on subsequent paint performance have not linked short periods of preweathering (weeks) to paint service life. To examine the link between preweathering and paint service life, we analyzed paint performance (cracking and flaking) after 14 years outside on boards that were preweathered for various amounts of time. We then compared our results with previous results from paint adhesion tests of similar boards that were also preweathered for the same amount of time. There was a direct correlation between the amount of time the siding was preweathered and the long-term performance of paint. Paint on wood preweathered for 16 weeks began to fail after just three years. Paint on wood preweathered for shorter periods lasted longer, but even those boards that were preweathered for as little as one week showed paint failure earlier than boards that were not preweathered. There was also good correlation between paint adhesive strength results from the previous study and outdoor paint performance results from this study, showing that loss of paint adhesion may be linked to paint performance in outdoor field tests. Forest Products Laboratory, One Gifford Pinchot Dr., Madison, WI 53705-2398.     

Kinetics of photodegradation and nanoparticle surface accumulation of a nanosilica/epoxy coating exposed to UV light

Temperature effect on the kinetics of photodegradation and surface accumulation of nanoparticles in an epoxy nanocoating exposed to ultraviolet light (UV) was investigated. A model epoxy coating containing 5% untreated nanosilica was selected. Exposed film specimens were removed at specified UV dose intervals for measurements of chemical degradation of the epoxy component, and nanosilica accumulation on specimen surface release as a function of UV dose for four temperatures. The chemical degradation was measured using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV–visible spectroscopy. Atomic force microscopy was employed to determine the kinetics of nanosilica accumulation on the nanocoating surface during UV exposure. The temperature dependence behaviors of kinetic parameters obtained by various measurement techniques will be used to better understand the degradation mechanism and surface accumulation of nanoparticles in exterior nanocoatings.