Precision coating on an industrial scale: requirements, technologies, equipment
Reasonable interest of woodworkers, furniture makers and builders to improve performance and technological properties of wood led to the development of laminated wood materials and structures, plate materials, such as chipboard, LVL, OSB, fibreboard, MDF and products from modified wood (TMW, Destam, Lignamony, Kebony, Accoya). In the production of such composite wood materials are used various kinds of binders, adhesives and chemical properties of natural activators of wood substance. Different kinds of binders, adhesives and chemical activators of properties of natural wood are used in the production of such composite wood materials. Thus, the increasing requirements to environmental, economic and operational efficiency of new wood-based materials and production technologies of the XXI century lead to an understanding that the natural wood itself is a complex composite, consumer properties of which can be targeted modified with use of the latest achievements of science and technology.
The constantly renewable natural wood substance by its microstructure is an excellent composite material similar to modern concrete panels: the reinforcement durable fibres consisting of cellulose microfibrils, which are twisted in a certain way with the size ranging from 2 to 50 nanometers, are packed and oriented in the ligno-carbohydrate solution of lignin and hemicellulose with the size from 20 to 80 nanometers over the cross section. This hemicellulose can be considered an intermediate binder interlayer between the cellulose fibres s and basic lignin solution. For various wood species the parent ligno-carbohydrate matrix of wood material has a well-defined crystal structure at the macrostructural level.
It should be noted that the fragmentary chemical composition of the natural lignin and hemicellulose is close (although more complex) to the chemical composition of binders used in the production of wood-based materials, for that reason, some components of modern resins are obtained from natural wood by using expensive chemical technologies. Companies are offered lignin at the price of lumber kind of "as a load", as it often negatively affects any further manufacturing products. Therefore, of a great practical interest is the use of natural components of wood material by intentionally activating the desired structural components of the biocomposite matrix and supplying at the micro level the needed reactive additives and activators to the wood substance. This allows obtaining a new modified wood composite with pre-determined properties without changing the textural integrity of the wood, its positive properties but giving it new operational or design properties expected by consumers. Despite the seeming complexity of this approach, the real production processes used on the specially designed equipment with an automated control system will be understood by trained professionals, and they enable a wood-processing company to get to a new level of profitability, apply fundamentally new wood processing technologies and creating new market sectors.
Structurally-modified wood substance
In order for the modern timber processing enterprises to turn a lignin-carbohydrate matrix from a "load" into a "bonus", experts of the Stroylab company established by the Ulyanovsk nanocentre ULNANOTECH in 2012, develop processes, new equipment and materials that meet the criteria of the XXI century technologies. Based on the current knowledge of the microstructure structure of natural wood, laboratory assistants are developing technologies and equipment for the manufacture of structurally-modified wood substance (SMWS) obtained in an array (sawn wood) and in the form of production waste (sawdust, chips, bark, cellulose-containing waste of industrial enterprises). The main area of activity is the adaptation and implementation of unique technology and materials in industrial production to meet modern product requirements.
The proposed SMWS product manufacturing technology provides for impregnation of the feedstock with the modifier solution to improve the properties of the biopolymer matrix of wood at the micro level, and the piezothermal treatment of semi-finished products to obtain a product with the right properties for consumers. The activated aqueous urea solution, which creates an energetically active clathrate channel in the process, is used as a base modifier of the wood substance. The diameter of the clathrate channel in the narrow part is 0.49 nm, in the wide part about 0.6 nm, however complex with urea may form any substances whose molecules have a cross sectional diameter smaller than the diameter of the clathrate. This allows the process man to pick up the necessary target additives to modify the wood, i.e. substances give the wood the desired properties by interacting with the structural components of a biopolymer matrix. Since urea is a synergist or a compound multiplying the beneficial effects of additives, their bulk quantity is insignificant.
A feature of the SMWS technology is that base solution of the modifier is fully compatible with the natural biopolymer matrix of the wood substance, and its ability to deliver specially selected additives at the micro allows to synthesize nanoscale components of the directed modification of the wood substance without disrupting its natural macrotexture. The ecological compatibility of SMWS and process safety are guaranteed by the fact that urea is not only a natural fertilizer for plants but also a food additive (E927b according to the international classification). The targeted additives used in minimum quantities are seamlessly included in wood substances and allow adapting the process to any wood species in any of its condition although freshly cut wood is preferable.
It should be noted that the orientation of the nanoscale modification grid is determined by the microstructure of the wood biopolymer matrix, so the modes of piezothermal treatment should be adjusted taking into account the special additives used as well as the size and type of the raw materials processed.
One of the development activities is the creation of industrial equipment which can not only vary the process parameters of modification depending on the raw materials used but also actively diagnose and adjust processes with the automated control systems and digital technologies. Allowance is also made for the fact that in the context of the present shortage of lumber, the high cost of transportation as well as reduced cost of finished products, the SMWS equipment should be not only a large-scale, such as the chipboard production, but also mobile and affordable for an average-level entrepreneur.
Manufacturing molded products
In cooperation with the leading Russian manufacturers of the wood modification and impregnation machines as well as with chipboard manufacturing facilities, the laboratory assistants are involved in several innovative projects for the production of SMWS products. For example, the Dream Wood company (Ulyanovsk) is expected to manufacture on a monthly basis up to 250 cubic metres of solid flooring and decking, building boards and construction timber made of SMWS. It is planned that the operational characteristics and source species subject to modification will be agreed with the customer. Table 1 shows the design performance indicators for a set of Dream Wood terraced covering, Table 2 shows the comparative properties SMWS for structural molded used for construction purposes.
Another innovative project in progress is the production of environmentally friendly woodchip material (EFWCM) that is SMWS-based. The proposed technology concerning the use of natural components of wood substances eliminates from the traditional chip board production process toxic resins and waste, which may only be disposed of at special landfills, reduces power consumption, recycle the so-called "old" waste that have laid for several years in dumps of wood processing facilities. The commercial introduction of the EFWCM technology will cut a number of units without influencing the production process concept but only upgrading the standard equipment.
The project initiators also intend to develop and establish test output mobile lines for the production of EFWCM. To reduce the cost of production, the modern chip board production technologies require larger capacities. However, this leads to problems associated with the supply of raw materials and increased costs due to increased logistics costs for the transportation of balances, often it concerns the commercial timber. It is unprofitable to transport chips at all. A mobile line for the production of EFWCM can be placed in close proximity to large woodworking plants, and as soon as the accumulated stocks of raw materials are depleted, it can be taken to another customer to remove heaps of sawdust and wood chips. Thus, the raw materials logistics will be limited to the supply of a relatively small amount of modifier in standard packaging from the manufacturer
Promising areas of development are related to the construction of high-quality low-cost housing with the use of the versatile modified wood module (VMWM) which is a combination of glue-free connection with the help of discrete embedded inserts (cords) of two profiled SMWS bars (walls). The module dimensions are standardised and allow you to assemble both the wall elements of houses and floor elements, roofing elements and load-bearing structures, stairs, roof etc. In the internal cavities utilities (power lines, water and gas pipelines) can be placed followed by the filling of the porous insulation boards made of SMWS.
Building elements made of the modified low-value wood species will reduce the value of a house, greatly enhance its strength properties and functional quality, fire resistance, bio-proofness and moisture resistance without the need for additional finishing and with a very long period of operation.
Processing the sinkers
An important trend in resource-saving technologies is the creation of a flooded timber processing technology. The amount of sunken wood in the Russian rivers, which can be used as commercial timber in the construction and furniture industries, is more than 30 million cubic metres. It is economically more reasonable to transform more than 50 million cubic metres into EFWCM in the finely divided state. These amounts are not demanded by pulp and paper mills because their physical and chemical properties are not suited for existing technologies. Large amounts of sunken wood and waterlogged wood cause serious problems from the points of view of ecology and operation of hydraulic facilities. At the moment, the recycling combustion, pyrolysis and pellet production are used in manufacturing.
According to the Central Research Institute of Wood Rafting, the cost of industrial lifting of one cubic metre of the sunken wood to the coast averages 3,500 roubles, and the cost of a cubic metre of the sunken wood dried in the log for the manufacture of furniture reaches $ 4,000. The problem is that it is quite expensive to dry the sunken wood logs with known methods, and when stored in stacks at the place of lifting they begin to crack already after two days. When grinding the sunken wood chipboard manufacturers are not satisfied with the quality of sawdust, so these raw materials are usually burned.
With the SMWS technology these volumes (over 85 million cubic metres) can be used in the form of an array or sawdust. To solve the sunken wood storage problem on the coast, it is advisable to create some special sheeting pre-impregnated with an active modifier for logs to be wrapped up shortly after lifting that eliminates wood cracking.