Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Forming articles by uniting randomly associated particles
Reexamination Certificate
2000-05-17
2003-06-24
Lechert, Jr., Stephen J. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Forming articles by uniting randomly associated particles
C264S405000, C264S460000, C264S463000, C264S485000, C264S488000, C264S109000
Reexamination Certificate
active
06582648
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the manufacture of moulded bodies from comminuted, especially cellulosic material, in particular the manufacture of chipboard, fibreboard or OSB (oriented strand board), in which the prepared material is mixed with a hardenable binder, the mixture being brought to a moulding station, and compressed by way of pressure into a moulded body, whereupon the binder is hardened.
Such a process is generally known, and is utilized for the manufacture of chipboard or fibreboard, generally speaking. Therein utilized are thermally hardenable binders such as urea-formaldehyde resin, melamine-formaldehyde resin, isocyanate, phenol-formaldehyde resin, among others. From the chemical point of view, the hardening corresponds to a thermally accelerated polymerization or polycondensation reaction. In the manufacture of chipboard, the dried and binder-coated chips are led to large format staged presses or cycling presses (discontinuous manufacture), or undergo a continuous process (continuous manufacture), for example, according to the Conti-Roll-process, wherein an endless bed of chips passes along a pressure pathway between gradually converging conveyor belt reaches and/or a roller nip, by which the compression is attained.
Production quantities of such installations are decidedly limited by the comparatively slow hardening procedure. The limiting factor is in particular the passage of the heat from outside to the middle of the panel. To achieve acceleration, the so called “steam impact effect” is used. According to the latter, steam passes by condensation from the hot surface of the panel to the panel centre, and accelerates the transfer of heat. This acceleration however has physical limits, since in the interior of the panel, the steam pressure will adjust itself depending upon the pressure at the exterior, and on the temperature. When, at the end of the compression process, the pressure from the exterior diminishes, the steam pressure within the panel can be too high, resulting in a bursting of the panel, specifically an explosion of the panel at its interior.
An important capacity indicator for a chip panel or a fibre panel production installation is the press factor, which refers to the time required for the panel to harden in the dimension perpendicular to the panel surface. The panel thickness is a factor on the basis of which one can calculate the maximum possible forward feed (in the case of continuous manufacture) or the maximum possible cycles per unit time in the case of a cyclical press, which allows the capacity of the installation to be determined. Typical press factors are in the region of 3 to 6 s/mm for Conti-Roll-installations, and between 5 and 9 s/mm for cyclical installations. As an example, the hardening of a 19 mm panel with a press factor of 5 s/mm results in a manufacturing time of 95 seconds.
The steam impact effect, which is advantageous for the acceleration of the hardening, has the further disadvantage that the product moisture at the surface of the panel is practically nil, and significantly climbs toward the interior, producing an inhomogeneous moisture profile. From the point of view of a stable product, however, a homogeneous moisture profile is important to strive for, since this is reached, in practice, only after storage lasting several weeks. The working and particularly the cashiering of panels with significantly inhomogeneous moisture profiles leads to problems of quality. Further, continuously rising installation outputs have led to a lower product moisture content, which is now below the moisture content of the product in its typical use (uniform moisture). The product thus endeavours to remove moisture from its surroundings.
The use of high-energy electron radiation (gamma radiation, roentgen radiation, ionizing radiation) for the hardening of organic synthetic resin is already known. Described in AT 338499 is the impregnation of chipboard and fibre board with radiation-hardenable components in order to attain specified technological characteristics. In this reference, panel material is manufactured in a hot press procedure according to the standard process. Following this, in the alternating pressure process, is an impregnation with the irradiation-hardenable components, and their hardening utilizing electron radiation energy. With this subsequent treatment, the expectation is that the mechanical characteristics of the panel and its dimensional stability in the presence of water will be improved, so that the actual manufacture of the panel can be carried out with a significantly reduced amount of thermally hardenable binder. One radiation-hardenable binder is a mixture of unsaturated oligomers (at least 30% by weight), acrylonitrile (1-30% by weight), unpolymerized additional materials (maximum 30% by weight) and the rest up to 100% by weight of vinylic unsaturated monomers. Examples of unsaturated monomers are: polyester resins, acrylic resins, diallylphthalate-prepolymerisate, or an acryl-modified alkyd resin, epoxy resin or urethane resin. Additional material may include polymerisation-accelerators.
In this case it is not a matter of manufacturing a panel by way of electron bombardment, but rather a subsequent refinement in a downstream process, utilizing electron bombardment for the improvement of the panel characteristics. The actual manufacture of the panel takes place, here as well, through the use of a thermally hardenable binder as well as the application of heat in the press region, with a complete hardening of the binder contained in the compressed panel. As such, the previously mentioned disadvantages—a limitation in capacity due to the time necessary for heat transfer, an inhomogeneous dampness profile and the danger of the panel exploding—are basically not avoided.
From U.S Pat. No. 3,549,509 it is known to manufacture a moulded body by the use of radiation-hardenable binder material. In this process, wood dust or sawdust is mixed with a radiation-hardenable liquid monomer, placed into a mold, therein compressed, and hardened by the operation of radiation energy. The source of the radiation can be radioactive electron emitters (for example cobalt-60) or ionizing radiation sources (for example x-rays). In accordance with these examples, the hardening takes place in a cobalt-60 radiation chamber. Methylacrylate; methylmethacrylate and propylacrylate are suggested as radiation-hardenable monomers.
The hardening in a closed mold (radiation chamber) and the comparatively slow hardening of monomers subjected to gamma radiation both interfere with attaining a high manufacturing capacity. Accordingly, this known process is also not to be recommended for the manufacture of comparatively thick plates or bodies moulded from chips or fibres, but rather only for thin coatings on products already of stable shape which are first deposited in the mold.
SUMMARY OF THE INVENTION
It is an object of the invention to carry out the manufacturing process so that an increased production capacity is possible, without having to deal with a problematical moisture content, or an inhomogeneous moisture distribution.
This object is attained by way of the originally described process which, in accordance with the invention, is characterized in that the material is mixed with a binder which is hardenable by electron beam energy, and in that after compression, the binder is hardened by electron radiation.
The invention is based on the fact that the activation and hardening of the binder material, contrary to thermally hardended binders, are caused by high energy radiation from an electron beam accelerator. The capacity thereof is essentially determined by two values: the accelerator voltage in MeV, which is responsible for the wide field of the energy within the irradiated body, and the energy quantity (beam capacity, dose) directed from the irradiator to the irradiated body, which is the product of the accelerator voltage times the accelerator current. The beam capacity determines the quantity of energ
Fritz Egger GmbH & Co.
Lechert Jr. Stephen J.
Striker Michael J.
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