SHARP, John, G. (11A Park Avenue, Stirling FK8 2QR, GB)
Claims
1. A method of manufacturing fiberboard from lignocellulose-containing fiber material, including pre- treatment of the material and then disintegrating, gluing, drying and pressing the material to a fiberboard product, c h a r a c t e r i z e d i n that a complimentary substance is supplied to the fiber material after the disintegration, which substance is reacting with formaldehyde liberated from the fiber material thereby in-situ generating a formaldehyde binder for gluing the fiber material.
2. A method according to claim 1, c h a r a c t e r i z e d i n that the formaldehyde liberation is increased by exposing the fiber material during the pre-heating to a temperature of 80 - 200 0 C for a retention time of 1 - 10 minutes.
3. A method according to claim 1 or 2, c h a r a c t e r i z e d i n that the formaldehyde liberation is increased by exposing the fiber material during the pre-heating to treatment of chemicals such as sulfite, hydroxide, bisulfite and/or metabisulfite from sodium, calcium, magnesium and/or potassium at a level of 0.25 - 10% solid component.
4. A method according to any of claims 1 - 3, c h a r a c t e r i z e d i n that as complimentary substance is used a formaldehyde containing resin where the molar ratio formaldehyde : resin is less than unity.
5. A method according to claim 4, c h a r a c t e r i z e d i n that the complimentary substance is any of urea formaldehyde resin and melamine-urea formaldehyde resin with a molar ratio formaldehyde : urea and formaldehyde : urea-melamine, respectively, of less than unity.
6. A method according to claim 4, c h a r a c t e r i z e d i n that the complimentary substance is a phenol formaldehyde resin with a molar ratio formaldehyde : phenol of less than 0.85. |
Manufacture of fiberboard
MDF or medium density fiberboard is a product in which lignocellulose-containing material such as wood or woody material is disintegrated to fiber bundles and fibers. Such fibers are thereafter admixed with a binder (glue) before or after drying and formed into a mat, which is subsequently pressed in, usually, a hot press to form the final fiberboard product .
The binder employed is usually a thermosetting resin based on condensates of formaldehyde such as urea formaldehyde, melamine-urea formaldehyde or phenol formaldehyde. During the hot pressing of such panels the binder is fully cured in order to produce a stable and sufficiently strong panel product
In such co-condensates there is traditionally a molar excess of formaldehyde with respect to the urea, melamine or phenol etc. This is traditionally employed in order to produce sufficient panel strength and other physical properties together with competitive production economics. In the case of a phenol formaldehyde resin, therefore, the type traditionally employed is, thus, a so-called resol type. Further more, resins of urea formaldehyde (UF) or melamine- urea formaldehyde (MUF) so employed have a molar ratio F:U or F: (U+M) of greater than unity.
The use of such excess proportions of formaldehyde has the disadvantage of liberating formaldehyde from the finished panel emanating from residual unreacted formaldehyde and/or as a consequence of the inherent reversibility of many of the reactions of formaldehyde within the product.
Furthermore, particularly when the resins are applied prior to fiber drying it is often found that formaldehyde can be detected in substantial quantities in the drier exhaust gases. Thus, formaldehyde may be liberated from not only the finished product, an issue which is being constantly addressed in the seeking of ever-lower emission levels, but also from the fiber drier as well as other emissions from the manufacturing process and particularly from the hot press.
The present invention is directed to the above mentioned environmental problems . According to the invention the emission of formaldehyde can be substantially reduced and instead the formaldehyde liberated in the process can be utilized within the process. The characterizing features of the invention are defined in the claims.
It has been discovered that emissions of significant proportions of formaldehyde can be found in drier exhaust even in the absence of any applied resin. This demonstrates the ubiquitous nature of formaldehyde as a by-product of even natural processes. Examples of this include the baking of bread, atmospheric breakdown of terpene emissions from pine forests, many combustion processes etc.
What has been furthermore discovered is that the release of formaldehyde from the typical MDF drier emanates from the processing of the various components of the wood which depends on the particular conditions of pre-treatment prior to introduction of the wood in chip form to the disintegrating apparatus.
Thus, typically in the MDF process temperatures of 80- 200 0 C for retention times of from 1 to 10 minutes are employed in order to soften the inter-cellular lignin sufficiently to allow for economical disintegration energy consumption. Such pre-treatment is traditionally carried out in a pre-heater pressure vessel, which is placed directly before the in-feed of a refiner, which is used as the disintegrating apparatus.
Additionally, in common with some pulp practice, certain chemicals such a sodium sulfite in combination with sodium hydroxide, sodium bisulfite or sodium metabisulfite may be employed to accelerate the softening action on the lignin to furthermore reduce disintegration energy consumption. Alternatively, calcium, magnesium or potassium in these chemicals could be substituted for sodium. The amount of chemicals could be at a level of 0.25 - 10% solid component on BDT (bone dry ton) of fiber material.
The outcome of such procedures has been found to exacerbate further the liberation of formaldehyde as increasing components of the wood are exposed to hydrolytic action and to increasing degree with increasing severity of pre-treatment conditions. This would normally be considered a major disadvantage of such procedures and has precluded the employment of such practice.
However, according to the present invention such generated formaldehyde may be utilized, on the contrary, to advantage in that, by using such pretreatment conditions, the energy consumption of the refiner can be substantially reduced. This in itself confers process economic advantage but, importantly, such generated formaldehyde can be gainfully employed to provide an in-situ source of the principal component of the binding resin (binder) . Thus, in place of a binder with a molar excess of formaldehyde, which would result in excessive levels of formaldehyde from the panel product, merely a complementary substance is utilized. This complimentary substance is a substance with the contrary composition of a molar insufficiency of formaldehyde, i.e. a molar ratio of less than unity.
In the case of a phenol formaldehyde (PF) binder, a suitable complimentary substance is a novolac type resin with a ratio F: P of less than 0.85. Similarly, a UF or MUF resin used as a complimentary substance should have a molar ratio of less than unity. The action of such a resin is to effectively scavenge the formaldehyde generated during the pre-treatment prior to disintegration of the fiber material in order to complement the quota sufficient to produce sufficient subsequent strength and other physical properties from the pressed panel and attainment of economic pressing time .
Preferably, the complimentary substance is supplied in the blow line after the refiner
The in-situ generation of formaldehyde, when utilized in such a manner, further complements the process economic
advantages since the binding resin is normally purchased at a cost representing a major proportion of the overall production cost.
The invention, of course, is not restricted to the examples disclosed above, but can be varied within the scope of the invention idea as defined by the claims .