Search

This paper shows the microscopic evidence (obtained by scanning electron microscopy - SEM) of the structural degradation of wood caused by weathering. Typical damage on radial and tangential surfaces of softwoods due to natural or artificial exposure is presented. The order of appearance of particular structural changes enables the distinguishing between various levels of degradation according to the occurrance and intensity of the changes on certain anatomical features. Subsequently the conditions of the coating, of the substrate and of their link were assessed after exposure of coated samples. Two types of European softwood species were used as a substrate, namely pine and spruce, and they were used either as coated panels or as microtomed wood sections ('thin strips'), which were exposed behind a detached film of a coating. Solvent borne (sb) coatings were compared with modern water-borne (wb) coatings. Semitransparent stains were compared with opaque white paints. The paints did not penetrate the cell wall but firmly adhered to the S3 layer of the lumina. The solvent-borne coatings (both paint and stain) penetrated deeper into the wood surface than the water-borne coatings.The opaque paints fully protect the wood from the effect of light through a 14-month period of natural exposure and maintain the coherent protective coating. The water-borne paint exhibits more brittle characteristics on the fractured transverse surface than the solvent-borne paint. The stains vary in their protective effectiveness. The thick water-borne stain shows a tough, sound failure mode, good adhesion but very poor penetration into wood surface. The thin coating of the solvent-borne stain is degraded by light, and the interface with the wood is affected, resulting in the development of brittleness of both wood and coating and leading to adhesion failures preceding the detachment of the film.

An ESEM (Environmental Scanning Electron Microscopy) technique has been applied to wood objects. ESEM investigations can be performed through several operational modes that offer various sets of environmental and imaging conditions. The comparison of ESEM micrographs with conventional SEM images revealed specific advantages and shortcomings of ESEM technique in studies on genuine and painted wood objects. Merits of the application of ESEM technique for wood are related to the absence of preparation artifacts, such as sputtering irregularities or defects due to shrinkage in vacuum drying. Great ESEM advantages over high-vacuum SEM, such as conduction of dynamic experiments within the chamber, are represented with a sequence of condensation, freezing and drying on a wood specimen. The wood imaging using ESEM proved inferior to that of SEM in terms of lower magnification, sharpness and contrast. However, the article offers guidance for assessment of influential operating parameters and their selection for the optimization of the ESEM work with wood. It may result in micrographs of sufficient resolution, definition and optical quality for study of wood structure on cellular and even intra-cellular level.

A study using the ESEM (Environmental Scanning Electron Microscopy) technique has been performed on wood objects in order that the particular advantages, possibilities and limitations of this microscopic tool are being assessed. In contrast to conventional high vacuum SEM, in ESEM specimens can be invastigated in a gaseous atmosphere, usually of water vapor. This enables the observation of non-conductive, polymeric, composite and porous materials (such as wood) in their natural state, without drying, evacuating or sputtering them with a layer of carbon or metal. Further advantages include observations in a wide range of temperatures, conduction of dynamic processes such as condensation, freezing and thawing of the specimen during observation, or mechanical testing. The imaging quality of ESEM for natural samples, however, is inferior to that of conventional SEM, and the specimens are liable to beam damage. The process of acquiring an image in ESEM is more complex than in SEM, demanding the optimisation of a number of interacting parameters. These include the physical conditions of the specimen, conditions of the chamber environment and electronic parameteers of the formation and optimisation of the image. The work on the ESEM can be performed through several operational modes that offer various sets of environmental and imaging conditions. This article presents a guidance for assessment of influential and imaging conditions. This article presents a guidance for assessment of influential operating parameters and their selection for the optimization of the ESEM work with wood.

An extensive study using the ESEM (Environmental Scanning Electron Microscopy) technique has been performed on wood objects in order that the particular advantages, possibilities and limitations of this microscopic tool are being assessed. The aim of this report is to present the valuable experience gained in optimizing operational and imaging conditions in ESEM during studies on wood and painted wood.

An extensive study using the ESEM (Environmental Scanning Electron Microscopy) technique has been performed on wood objects in order that the particular advantages, possibilities and limitations of this microscopic tool are being assessed. The aim of this report is to present the valuable experience gained in optimizing operational and imaging conditions in ESEM during studies on wood and painted wood. This set of guidelines and instructions, along with the thorough and critical literature review, should help other researchers to utilize ESEM in more effective way for analysis of wood and other non-conductive polymeric materials, particularly of biological origin. In contrast to high-vacuum microscopy, ESEM is a relatively new development stage in electron microscopy, featuring a physical system in which electrons need not travel through vacuum in the chamber, but through the gaseous atmosphere which usually consists of water vapour. This enables the observation of non-conductive, polymeric, composite and porous materials (such as wood or painted wood) in their natural state, without drying, evacuating or sputtering them with a layer of carbon or metal (Au, Ag, Pt). Further advantages include observations of the object in a wide range of temperatures (up to 1000 °C), dynamic processes such as condensation, freezing and thawing of the specimen during observation (shown in this presentation), mechanical testing and other. All these options open extraordinary possibilities in microscopy that offers three-dimensional visual impression during micro-structural analysis of wood. The imaging quality of ESEM for natural samples, however, is inferior to that of classical SEM (C-SEM), especially at magnifications greater than 2000 times. Furthermore, the process of acquiring an image is much more complex in ESEM than in CSEM, demanding the optimisation of a number of interacting parameters. These include the physical conditions of the specimen, conditions of the chamber environment and electronic parameters of the formation and optimisation of the image. This poses significant demands on the knowledge and skills of the microscope operator, both about the material characteristics and microscopic routines. The analysis often requires time-consuming procedures and benefits from co-operation between the microscope operator and material scientist who supplies the object, especially in conducting dynamic, in-situ experiments within the chamber. The work on the ESEM can be performed through several operational modes, one of which is a classical (or high vacuum) mode. The environmental modes embrace a low vacuum (LV) mode, low vacuum – short distance (LVSD) mode and wet or ESEM mode. These operational systems offer various sets of environmental and imaging conditions and should be differentiated in order that optimum imaging results are obtained for a given wood object. Wood is a very difficult material for microscopic observation because of its variable chemical composition, great structural diversity, high porosity and irregular conductivity. ESEM analysis of wood requires that the essential properties of the object are known and the operational mode chosen accordingly. The report presents the analysis of interactions between influential operational and imaging parameters for the work with wood, painted wood and similar materials. The presentation also brings the observation of various painted wood specimens, which are to serve as guidelines for other ESEM operators and material scientists. A list of possibilities for further work indicates that the range of objects and possible applications of ESEM technique exceeds by far the present scale of implementation and potentials of this versatile and powerful tool in wood research.

For the purpose of evaluating the performance and durability of modern wood coatings with low organic solvent content, natural and artificial exposure trials of various specimens were performed and subsequently the conditions of the coating, of the substrate and of their link was assessed by scanning electron microscopy (SEM). Two types of European softwood species were used as a substrate, namely pine and spruce, and they were used either as coated panels or as microtomed wood sections ('thin strips'), which were exposed behind a detached film of a coating. Solvent borne (sb) coatings were compared with modern water-borne (wb) coatings. Semitransparent stains were compared with opaque white paints. Pine and spruce did not show significant differences in their failure modes, film-holding properties or UV light resistance under semi-transparent coatings. However, spruce was shown to undergo somewhat smaller structural changes with weathering. The paints did not penetrate the cell wall but firmly adhered to the S3 layer of the lumina. The solvent-borne coatings (both paint and stain) penetrated deeper into the wood surface than the water-borne coatings. The opaque paints fully protect the wood from the effect of light through a 14-month period of natural exposure and maintain the coherent protective coating. The water-borne paint exhibits more brittle characteristics on the fractured transverse surface than the solvent-borne paint. The stains vary in their protective effectiveness. The thick water-borne stain shows a tough, sound failure mode, good adhesion but very poor penetration into wood surface. The thin coating of the solvent-borne stain is degraded by light, and the interface with the wood is affected, resulting in the development of brittleness of both wood and coating and leading to adhesion failures preceding the detachment of the film. Thin strips with detached films of coatings realistically represent the surface of coated wood specimens, and are easy to handle, to detach and subsequently to investigate both mechanically and microscopically. The combination of strength measurements and the microscopic evidence proved particularly effective for the interpretation of the changes occurring at the very surface of wood components i.e. at the wood-coating interface. The microscopic observations confirmed that the progress of wood photodegradation can be successfully detected by the combination of the state of deterioration of anatomical elements on radial surfaces and by fractographic evidence on cross sections.