Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2002-02-08
2003-03-11
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S398100, C427S421100, C427S551000, C427S553000, C427S557000
Reexamination Certificate
active
06531190
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for adjusting the quality factors of whole wood surfaces.
BACKGROUND OF THE INVENTION.
Whole wood is used in building and furniture industry as plywood, board, solid wood, construction wood and plates. Usually whole wood contains colour defects that must be removed in order to enable industry to utilise said wood. Among others, there may be uneven colouring, non-desired colouring as well as discolorations created during the drying process. As regards thermally treated wood, for instance, it is usually necessary to even out the shade differences of the wood surface. At present whole wood colour defects are usually removed mechanically, because the field is lacking an effective and sufficiently rapid method for adjusting whole wood colour defects which would also maintain the strength properties of the wood in question. In addition to removing colour defects, the industry also needs the adjusting of other factors that affect the colour shades of whole wood.
In non-industrial scale, whole wood colour defects have been removed by using various bleaching methods, where the process conditions depend on the type of wood in question, its hardness, purpose of use etc. Particularly as regards North-European wood species, however, there are very few economical and for mass production sufficiently rapid and effective colour defect removing methods available for the carpentry and building industry. Another remarkable drawback with known methods for adjusting whole wood colour defects is that they cannot effectively adjust other quality factors of whole wood surfaces, i.e. surface hardness and other factors affecting the colour shades of the surface, but they are exclusively concentrated on adjusting individual quality parameters, such as lightness. In part the latter is due to the fact the current whole wood bleaching methods are too slow for achieving an adequately controllable adjusting method for whole wood surface shades.
Whole wood colour shades are adjusted by bleaching the wood surface to a desired lightness, uniformity of colour and reflectivity. The most important bleaching chemicals used in the bleaching of whole wood have been ozone, Na chlorite, Na hypochlorite and hydrogen peroxide. A good, industrially usable whole wood bleaching chemical should be economic, as ecological as possible and safe in use. At present the general tendency is to give up chlorine-based bleaching agents owing to their environmental hazards, and as regards ozone, it is expensive an technically difficult to use. Ammonia has also been used to some extent, but its use has been limited owing to health hazards. Among the above mentioned bleaching chemicals, only hydrogen peroxide fulfils all of the mentioned criteria: as an industrial chemical agent, it is generally available, fairly economic, ecological and safe in use.
In the prior art, hydrogen peroxide has usually been used for bleaching whole wood in alkaline conditions. The bleaching effect of hydrogen peroxide in alkaline conditions is mainly based on the effect of the perhydroxyle HOO ion that reduces the quinoid structures of cellulose. The efficiency of hydrogen peroxide bleaching in alkaline conditions depends on the temperature according to the Arrhenius' rule k=Ae
−RT
, in which case a rise in the temperature increases reaction speed. However, raising the temperature also softens the wood material; this is due to the breaking-up reactions of carbohydrates, caused by the alkaline conditions. With respect to industrial bleaching of whole wood, it is extremely difficult with hydrogen peroxide to obtain both a bleaching time that is sufficiently short and simultaneously an adequate bleaching efficiency, in order to achieve a desired bleaching capacity and result. If the bleaching temperature is raised in order to increase the processing capacity and to ensure bleaching efficiency, we are fairly soon faced with problems relating to the physical properties of the wood material, for instance surface hardness.
The company Finnish Peroxides Oy recommends that whole wood hydrogen peroxide bleaching should be carried out at the pH value 11, in the presence of a sodium silicate stabiliser. The bleaching temperature is 60° C., and the bleaching takes place by immersing the wood in the solution for the duration of 60 minutes. This type of bleaching method is well suited for the bleaching of individual wood products, but as regards such furniture and carpentry industry that operates on a mass-production scale, the processing time is far too long. From the U.S Pat. No. 3,690,922 there is known a bleaching method for whole wood or plywood, in which method chromophoric groups are first attempted to be eliminated by oxidising them in the pre-treatment step with a persulphate solution that also contains surface active and buffering agents. After eliminating the chromophoric groups, the wood is treated with another bleaching agent, such as hypochlorite or hydrogen peroxide solution. After the described steps, there are carried out wood tempering, pH adjustment and wood drying steps. In an industrial scale, the drawback with this kind of process is its slowness, its multiple steps as well as the use of several different chemicals, which increases the operating expenses.
Owing to the length of the required bleaching period, it has been attempted to intensify the bleaching effect of hydrogen peroxide in several different methods by means of several different hydrogen peroxide activators. In the DE patent publication 1,792,211, the employed activator is ammonia and sodium carbonate, in the SE publication 325,124 the employed hydrogen peroxide activator in the overpressure bleaching of veneer doors is ammonia, and in the patent publication U.S. Pat. No. 3,708,889, ammonia is used both as a bleaching activator and a bleaching agent. Although the use of a bleaching activator solves many problems in hydrogen peroxide bleaching, an increase in the use of bleaching chemicals and a resulting increase in the operating expenses may still easily become a problem.
In several cases, it has been attempted to perform hydrogen peroxide treatment in acidic conditions, at a raised temperature without any activators that increase the chemical costs. One of these methods is the bleaching method known from a FR patent publication, wherein wood that has become greyish is immersed in a hydrogen peroxide solution in acidic conditions for a relatively short period. However, in order to ensure the uniformity of the bleaching result, the wood must in this process be pressure treated twice, which makes the method remarkably more complicated and increases the processing expenses.
Among the above described known methods there are not set forth any such overall adjusting methods for whole wood surface quality factors that would be industrially suitable, simple and sufficiently rapid. Thus the main object of the present invention is to eliminate the drawbacks of the prior art as described above and to realize a particularly industrially suitable control method for wood surface quality factors, in this case colour shades and surface hardness.
SUMMARY OF THE INVENTION
The first object of the invention is to realize an adjustment method for wood surface colour shades, which method is expressly suitable for industrial-scale serial production, i.e. production that is carried out on a conveyor belt, in which method the process control factors that affect the surface colour shades (further on process control factors or, for the sake of simplicity, control factors) can be managed so, that the desired wood surface quality factors are achieved, said factors including the desired wood surface colour shades, i.e. lightness, uniformity of the shades and reflectivity of the surface, as well as a sufficient wood surface hardness, i.e. the physical strength properties of the wood. In industrial-scale mass production, the management of the control factors also is important because the quality factor requirements set for the wood may vary betw
Altonic Oy
Pianalto Bernard
Young & Thompson
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