Лако-красочные материалы — производство

8.4.1

Introduction

For various reasons, wood belongs to one of the most ancient and popular materials of mankind. With the exception of deserts, it is available everywhere and in abundance. Unlike metal or stone, wood is easily obtained and processed by means of machining technology. The life cycle analysis of wood and wood-based materials is excellent, since it is a regrowing raw product. Another important aspect of wood is that it is associated with positive feelings; people have a sense ofwell-being iftheir living and working environment is entirely or partly made from wood. The reason for this is that wood feels ‘warm’ owing to its excellent insulating characteristics. By absorbing and emitting moisture, it regulates the interior climate and is therefore a precious material in terms of building biology. Its acoustic performance — that is, the propagation and shielding of sound waves — also differs from that of other materials. This property is not only taken advantage of when building musical instruments, but also very often when furnishing concert halls, since wood creates excellent sound effects.

From the technological point of view, wood is a complex material. Due to its pronounced fibrous structure, most of the physical characteristics — particularly the mechanical ones — are highly anisotropic. For example, depending on the species, the tensile strength perpendicular to the direction of the fibers (perpendicular tensile strength) amounts to only 10-15% ofthe parallel tensile strength. The latter, however, is quite considerable. Beech, for example, has a parallel tensile strength of 120 N mm-2 at a green density (12% moisture content) of 0.7 kg dm-3. By comparison, mild steel has values ranging between 310 and 630 Nmm-2 at a density of 7.8kgdm~3 [19]. Consequently, all things being equal, lightweight constructions are possible when using wood instead, provided that the designer arranges for the majority of the important forces to produce only tensile or flexural stresses.

But wood is not just wood; it rather reflects the natural variety ofthe species from which it is obtained. Depending not only on the species but also on the site of growth and its climatic history, there is a wide array of wood densities, strengths, colors and grain patterns, and this is why construction engineers and designers are so much attracted by this material. At a glance, wood appears to be a ‘good-natured’ substrate for bonding purposes. In the drying process, the capillary water evaporates, creating a porous structure and enabling the adhesive film to build up mechanical interlocking in the wood structure. The chemical structures of the main components of the wood matrix (i. e. cellulose and lignin), with their hydroxyl groups and other polar functional groups, offer various possibilities for the formation of hydrogen bridge bonds and even covalent chemical bonds in the adhesive layer. This is the reason why the most ancient wooden objects and artifacts ofthe history of mankind ever found by archaeologists already also bear witness to the bonding of wood. These assemblies were nonstructural — that is, they were nonloaded assemblies — and the glues used to

create them originated from Nature, being obtained from plants and animal organs. The production of glutine glues (i. e. glues made from skin or bones) required an advanced craftsmanship (see Section 5.9), and it is no wonder that works of art and articles for daily use produced with these glues were considered extremely precious. As a consequence, they were found, for example, among the grave goods of the pharaohs of Ancient Egypt. Moreover, even the glue itself was considered precious; among the treasures of the tomb of Tutankhamun was found a glue tablet that had accompanied the pharaoh on his last journey, together with gold and gems. Pliny the Elder (23-79 ad) was the first to scientifically and systematically describe gluing in his 37-book work Naturalis Historia (see Chapter 2). He noted that specific species of hardwood were more difficult to bond that softer types of wood — a fact that still applies to modern adhesives. During the first half of the 20th century, wood and wood adhesives were the key materials for the cutting-edge technology of the time. Today, designations such as ‘Propellerleim’ (a glue once developed for the assembly of wood propellers), are reminiscent of these applications. Even jet aircraft such as the British De Havilland ‘Vampire’ (Figure 8.17) were made from wood. Immediately after the Second World War there was a flood of new testing methods and standards aimed at increasing the safety and reliability of wood constructions. Yet, it is in the nature of standards to be conservative and to hinder the market entry of new systems. This applies in particular to standards that prescribe the use of specific materials, and in this case the new materials had to fight for acceptance via lengthy special approvals. Unfortunately, all of this proved detrimental to the advancement of technology.

In the modern context of wood bonding, it is useful to consider two dimensions. On the one hand, there is a difference between ‘structural’ and ‘nonstructural’ bonding, and on the other hand between ‘industrial’ and ‘handicraft’ bonding. Handicraft production is characterized by small series sizes, most often even one part. Generally, when fabricating unique pieces or small batches, process optimization in terms oftype approval or load calculations is too labor-intensive or uneconomic, and therefore, structural (i. e. load-bearing) bonding is rather an exception here. For loaded joints,

such as shelves in furniture, mechanical fastening techniques (e. g. dowels, screws or nails) are used, the advantage being that special fixing or positioning tools are not needed to hold such joints in place. The assembly of structural elements at the construction site is also a handicraft situation, since on most occasions on the construction site there is no possibility of applying pressure to a bonded joint. Therefore, frictional fastening techniques are preferred for fitting or roofing purposes.

In industrial production, however, there are no such limitations, as large numbers of pieces justify investment in complex facilities, and the development of refined processes, predominantly motivated by aspects of economy and sustained quality. Economy is an important factor because high investment costs need to amortize, while sustained quality is required to prevent the multiplication of flaws in series production that may have disastrous consequences in terms of liability and damages [20-22].

8.4.2