Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Autogenously or by activation of dry coated particles
Reexamination Certificate
2001-01-22
2004-02-17
Lechert, Jr., Stephen J. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Autogenously or by activation of dry coated particles
C264S126000
Reexamination Certificate
active
06692670
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the manufacture of synthetic boards having thermosetting binders and, more specifically, to fiberboards having isocyanate binders.
BACKGROUND ART
For many years, fiberboards have been manufactured from wood or agricultural substrates using thermosetting binders. Formaldehyde-based binders, such as urea formaldehyde and melamine formaldehyde have traditionally dominated the fiberboard industry during that time. Isocyanate-based binders, particularly diphenylmethane diisocyanate (MDI) binders, however, offer some significant advantages over formaldehyde-based binders, including superior physical and moisture-resistance properties and the elimination of formaldehyde emission hazards.
Although isocyanate binder technology for fiberboard manufacture has been available for many years, isocyanates have not gained widespread commercial acceptance, primarily because of cost. MDI-based isocyanates tend to be more expensive per pound than formaldehyde-based binders, but are used at a lower dose rate, partially offsetting the cost disadvantage. For example, MDI-based binders generally comprise about 6% of total finished fiberboard weight compared to as much as 20% of total fiberboard weight for urea formaldehyde. Also, in many instances, MDI-based binders cure more slowly than urea formaldehyde or melamine formaldehyde resins.
Fiberboard is typically manufactured via a multi-step process. Typically, wood chips (or other suitable materials) are fed into a digester in order to soften them via exposure to steam and high pressure. This process also breaks down some of the lignin within the wood chips. The treated chips are then fed to a refiner, where they are separated into their component fibers by intense mechanical forces. The hot, wet fibers exit the refiner, and are rapidly transported via steam through a “blowline”. Typically, binders are added to the fibers via blowline injection.
The use of low dosage isocyanate-based binders via blowline injection poses a different set of challenges than the dispensation of high dosage formaldehyde based binders. Because such low dosages of isocyanate-based binders are used, great care must be taken to distribute the binder evenly throughout the panel. This task is further complicated by the fact that isocyanates react very quickly with water to form polyureas at elevated temperatures, such as the temperatures experienced in the blowline.
The blowline deposits the binder treated fibers into a dryer, and eventually into forming and pressing devices which produce the final panels. Ideally, polymerization of the binder into its final thermoset form takes place in the hot press, not prior to pressing the fiberboard into its final form. Because isocyanates are so reactive with water at high temperatures, and because water and high temperatures are so prevalent in the fiberboard manufacturing process, it is extremely likely that a great deal of isocyanate is converted to polyurea prior to pressing, which can lead to the formation of solids, which foul the blowline or the dryer. Also, pre-polymerization renders a significant portion of the binder inactive, greatly reducing bonding efficiency. It is also likely that some isocyanate is volitalized in the drying process, and thus lost in the process.
In order to protect isocyanate-based binders from the harsh conditions of the typical fiberboard manufacturing process, emulsifiable isocyanates have been developed. These modified isocyanates can be mixed with water using an in-line static mixer, and subsequently injected into the blowline. Emulsification prevents the build up of isocyanate-water reaction of products on the walls of the blowline and helps to prevent premature reaction of the binder. Emulsification also helps to increase the volume of liquid being dispensed, which helps to achieve a more even distribution of binder throughout the fiberboard. Unfortunately, modifying an isocyanate to make it emulsifiable entails additional manufacturing costs, which make these products less cost effective.
Another cost-disadvantage of isocyanate binders is their need for a release agent, in order to avoid sticking to metal press platens. Formaldehyde based binders do not normally require release agents. In industrial practice, water based release agents are typically emulsified in line with an emulsifiable MDI just prior to isocyanate emulsification, and “blowline” injection. The need for release agent further helps to increase the volume of fluid being dispensed, but adds cost to the system.
All of the technology described above is well known. The process for using isocyanate binders for the production of fiberboard via “blowline” injection is described in, for example, U.S. Pat. No. 4,407,7771, issued to the Celotex Corporation in 1983. The use of emulsifiable isocyanates is described in, for example, U.S. Pat. No. 3,996,154 issued to ICI Americas Inc. in 1976. The use of wax release agents in conjunction with isocyanate binders is described in, for example, U.S. Pat. No. 4,388,138 and U.S. Pat. No. 4,396,673, both issued to ICI Americas Inc. in 1983. The use of an in-line mixing apparatus for dispensing isocyanate binders into the “blowline” of an medium density fiberboard (MDF) manufacturing process is described in, for example, U.S. Pat. No. 5,093,058 issued to the Medite Corporation in 1992.
Despite all of this known technology, the use of isocyanates in fiberboard manufacture had remained minor, because of the various cost effectiveness limitations discussed above. The present invention concerns a novel isocyanate-based composition, and particularly an MDI-based composition, for binding fiberboards, which offers significant advantages over the isocyanate technology available previously.
DISCLOSURE OF THE INVENTION
In accordance with the present invention it has been discovered that improved results are achieved if a binder comprising a polymeric MDI having a relatively low diisocyanate content is employed in the fiberboard manufacturing process.
The method of the invention broadly relates to forming fiberboards comprises the steps of:
1) providing fibers to a blowline;
2) providing a relatively low diisocyanate content polymeric MDI to the blowline to treat the fibers with the polymeric MDI;
3) at least partially drying the treated fibers; and
4) pressing the treated fibers to form the fiberboard.
The present invention also relates to fiberboards fabricated with such binders comprising polymeric MDI.
BEST MODE FOR CARRYING OUT THE INVENTION
As indicated above, processes for preparing fiberboards, such as medium density fiberboards, are known to those skilled in this art. Any of the conventionally employed processes can be used in this invention.
Polymeric MDI (polymethylene polyphenylene polyisocyanate) binders have been used in the preparation of fiberboards. These binders contain a mixture of isocyanates generally including a relatively high content of diisocyanate molecules. For example, commercially available polymeric MDI generally consists of about 48 to 50% diisocyanate molecules, about 22 to 26% triisocyanate molecules, with the remainder consisting of larger oligomers. Examples of such commercially available polymeric MDI is Rubinate 1840 isocyanate and Rubinate M isocyanate, available from Huntsman Polyurethanes located in West Deptford, N.J.
Suitable polymeric MDIs for use in the invention also include emulsifiable, polymeric MDIs. These materials also contain a mixture of isocyanates generally including a relatively high content of diisocyanate molecules, as described above, but are altered by known techniques to be made emulsifiable. An example of such a commercially available emulsifiable, polymeric MDI is Rubinate 1780, available from Huntsman Polyurethanes, located in West Deptford, N.J.
In accordance with this invention it has been discovered that improved results are achieved if the diisocyanate content of the polymeric MDI is reduced. Preferred results are achieved by utilizing a polymeric MDI having a diisocyanate content of les
Burns Steven B.
Kolaczyk Tina M.
Moriarty Christopher J.
Graham Nicole
Huntsman International LLC
Lechert Jr. Stephen J.
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