Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – With liberating or forming of particles
Reexamination Certificate
1994-03-07
2001-03-06
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
With liberating or forming of particles
C264S121000
Reexamination Certificate
active
06197236
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the production of fibreboards from coarse wood particles and isocyanate as binder, in which the wood particles are heated under excess water vapour pressure and simultaneously comminuted to fibres which are then delivered with expansion to a drier by means of a blowing pipe, and dried in the drier. Downstream of the drier the fibres are then formed into mats in the customary manner and treated further to form the finished fibreboards. The fibreboards according to the present invention are medium density fibreboards in which wood chips or inexpensive types of wood can be used as wood particles so as to make better use of the raw material wood. As far as their mechanical and physical properties are concerned fibreboards can be compared to wood chipboards, although they display uniform density and fineness of structure throughout their thickness. In addition they have a smooth, coherent surface which allows them to be readily, processed and subjected to a large number of finishing techniques.
EP Patent No. 0 092 699 discloses a process of the kind mentioned in the introduction. The wood particles are digested in a digester under excess water vapour pressure and delivered to a refiner in which the wood particles are comminuted to hot, wet fibres. The hot, wet fibres are delivered from the refiner to a separator via a blowing device comprising a blowing pipe. The digester, the refiner and the blowing pipe are operated under pressure, of for example 8 bars. The temperatures of the hot fibres are in the order of 150 to 160° C. At the end of the blowing pipe leading to the separator reduction in pressure, i.e. expansion, takes place since the separator is operated at normal pressure. In the known process the isocyanate is added as binder In the region of the blowing pipe, i.e. in a region in which excess pressure prevails. The isocyanate is applied to the hot, wet fibres prior to expansion. In the blowing pipe the fibres are preferably vortexed and the isocyanate is homogeneously distributed on the hot, wet fibres, so that lump formation is avoided. Size specks in the finished fibreboard are also thereby advantageously avoided. The addition of the isocyanate binder to the hot, wet fibres in the blowing pipe does however cause pre-curing of the isocyanate, which means that encrustations may form on the inner periphery of the blowing pipe, which thus gradually becomes blocked. Continuous operation is thus considerably disrupted and stoppages are necessary to allow the blowing pipe to be cleaned.
From “Tendenzen der MDP-Plattenerzeugung” (Trends in the manufacture of median density fibreboards) on pages 379 to 382 of the journal 'Holz als Roh- und Werkstoff' (Wood as a raw and processing material) 36 (1978) it is known to add the binder at the exit to the drier. The fibres are thus also hot as a result of the drying process when the binder is added. It is applied to the thin, hot fibres with the aid of sizing blenders commonly employed in the chipboard industry. Although the binder is applied to the hot fibres under normal pressure, problems with pre-curing also occur in this process which are counteracted by the use of large volume intermediate bunkers. One of the main problems encountered in this conventional process of sizing fibres with precondensed urea resins is the formation of size specks, since it is apparently not possible, despite the intense fixing process, to apply the binder to the fibres uniformly and in the required finely distributed form. Also, intense mixing and even the use of large volume intermediate bunkers requires extensive periods of time, which are not conducive to the prevention of pre-curing of the binder.
The invention is based on the problem of providing a process for the production of fibreboards sized with isocyanate, of the kind mentioned in the introduction, in which there is neither the danger of size speck formation nor of any significant pre-curing of the isocyanate.
DESCRIPTION OF THE INVENTION
According to the invention this is achieved by spraying the isocyanate on to the fibres after they leave the blowing pipe and before they are dried. Surprisingly lump formation in the fibre material is thereby avoided as well as the formation of size specks on the finished fibreboard, despite the fact that the isocyanate is added at an even earlier stage of the production process than at the known point of addition after the drying step. Thus the expansion which occurs at the exit of the blowing pipe and the corresponding reduction in pressure and temperature of the fibres is utilised in a skilful manner, with the result that the isocyanate is no longer applied to the hot and wet fibres in the blowing pipe but to the comparatively colder and drier fibres. The temperature of the fibres is reduced as a result of the evaporation of water during expansion. When the isocyanate is applied to the fibres they are in a state of intense motion and considerable velocity and the isocyanate can thus be applied in a finely distributed form, lump formation thus being counteracted from the outset. Also, the subsequent movement of the fibres sized with the binder in the intermediate transporting apparatus, and in particular in the drier, is utilised for the purpose of completing the uniform, fine distribution of the binder on the fibres. Pre-curing of the isocyanate is advantageously avoided, not only because the isocyanate is applied to comparatively colder and drier fibres, but also—as experiments have shown—because the increase in temperature in the drier and the very short residence time of the fibres therein does not produce any significant pre-curing. It is an important factor that large volume intermediate bunkers and comparatively long residence times in forced circulation mixers arranged downstream of the drier are avoided, with the result that after the sized fibres have been dried, they can be immediately processed further into fibreboards. It is therefore possible not only to produce fibreboards free of size specks but also to reduce the throughput time and above all to reduce the main contact time between the isocyanate and the fibres compared to the two processes known from the prior art.
The isocyanate is sprayed on to the fibres preferably when they are at normal pressure, i.e. after expansion has taken place. At this point in time the reduction in the temperature of the fibres resulting from the evaporation of water has come into full effect and the isocyanate sprayed onto the fibres is thus not heated to temperature ranges of the kind to which it would be subjected if applied in the blowing pipe.
The isocyanate can be sprayed on to the fibres immediately downstream of the exit to the blowing pipe and during the expansion phase. Thus the point chosen for the spraying or injection of the isocyanate on of the fibres is one at which the fibres move at particularly high speeds. The static pressure in the digester, the refiner and in the blowing pipe also no longer exists, but has been transformed into dynamic pressure. As a result only very short contact times advantageously result between the isocyanate ejected from the nozzles and the fibres flying past at high speeds. It is also possible for the isocyanate to be sprayed on to the fibres at the point of their highest rate of flow.
The isocyanate is not sprayed on to the fibres either in the blowing pipe or after the drying process, as described in the prior art, but at the most appropriate point between these two stages. The first preferred point is directly downstream or the exit to the blowing pipe in processes where a separator is provided between the exit to the blowing pipe and the start of the drying process of relieve the drier of a portion of the water vapour of be removed. Another possibility is that of arranging the blowing pipe immediately upstream of the drier and applying the isocyanate to the fibres in the drier, preferably at the beginning of the drying process, so that the motion of the fibres in the drier can be utili
Oldemeyer Wilhelm
Sanders Werner
Teuber Gert
Bayer Aktiengesellschaft
Brown N. Denise
Gil Joseph C.
Jones Kenneth M.
Silbaugh Jan H.
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