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FIELD OF INVENTION
This invention relates generally to the field of the composition and
manufacture of particle board. More particularly, this invention relates
to the field of manufacturing particle board from lignocellulosic
materials including scrap lignocellulosic materials, and to the
manufacture of particle board including as one of the components thereof
plastics and scrap plastic materials, and to the manufacture of particle
board by means of extrusion.
BACKGROUND OF INVENTION
An important problem in modern society is to make the maximum use of all
available materials, including the reuse of scrap materials back into
useful products. One component of the modern emphasis on recycling
materials is the use of scrap materials in the production of particle
board.
The customary method for the manufacture of particle board is to use wood
in the form of sawdust, wood particles and fibers which have been
defibrated by a mechanical or chemical defibrating process. These
materials are typically mixed with a form of adhesive or binder. There are
three general catagories of binders commonly in use. Formaldehyde-based
binders are one category of binder commonly used, although this material
is under increasing scrutiny due to the toxicity of the underlying
formaldehyde compounds. A second customary form of binder material in
common use are polyurethane-type binders, including under this general
term specific compounds such as polymeric diphenylmethane diisocyanate and
related compounds generally referred to as "MDI". The third general
category of binder involves phenolic-based binders. The phenolic-based
binders tend to be more costly, but offer quality advantages in particle
boards of higher value.
The customary process of manufacturing particle board involves the blending
of the binder with the wood or wood-related particles into a substantially
dry blend in which typically about 6% by weight would be binder. This
blend is deposited onto a moving mat or belt (hereinafter "belt" for
simplicity) either by an "air laying process" or laid onto the belt
following wetting; both processes well known in the field. The material
following deposition is typically held in place on the belt by means of a
vacuum suction applied from below the moving belt, which is typically made
to be porous for precisely this purpose. Typically, to manufacture
particle boards of three-fourths inch, a deposition on the belt of up to
15 inches may be required. The final particle board is manufactured by
pressing the material deposited on the belt into the final particle board
with the required density. Commonly, the material will be pressed in a
two-step process. A prepressing applied to the material on the mat may be
used to reduce the size (that is, thickness) of the material prior to
pressing to final density. Pressing to final density then follows, often
accompanied by simultaneous heating of the material. Both continuous
pressing as well as pressing in a batch process are used in the industry.
The density of the final particle board produced by this process is
predetermined when material is deposited on the moving belt by adjusting
the amount of material (per square foot) deposited on the moving belt
prior to pressing.
An alternative process for the manufacture of particle board is to use an
extrusion process. In the manufacture of extruded particle board, the
starting material is likewise wood, wood-related or other lignocellulosic
products blended together with a binder or other substances which impart
desirable properties to the final product. For extruded particle board,
however, it is not necessary to include in the blend of starting materials
a refined wood or lignocellulosic fiber. Instead, it is common in the
production of extruded particle board to include plastic (typically scrap
plastic) in the blend of starting material. The amount of plastic may vary
somewhat from process to process, but typically an equal 50%-50% blend of
plastic and wood products (by weight) are included in the starting
material for extrusion. The inclusion of plastic imparts a viscosity to
the starting blend facilitating extrusion through a die. This process is
most useful for the production of long lengths of material with typical
cross-sectional dimensions much less than their typical length dimensions.
Such material may include floor boards, planking, moldings, window and
door frames, etc. Essentially any plastic material which may itself be
formed by an extrusion process can be employed along with wood in the
manufacture of extruded particle board.
The production of extruded particle board typically does not involve the
addition of binders to the starting blend (the plastic acting as the
binding agent in this sense). However, "coupling agents" for binding the
wooden material to the plastic are required to be included in the blend
for extrusion. Coupling agents are essentially materials for the binding
the wood and wood fiber to the plastic as the typical wood tends not to
bind directly to the plastic without the assistance of special materials
for facilitating the bonding. A common coupling agent is commercially
available from E. I. du Pont under the tradename "Epolene".
This customary approach to production of particle board by means of
extrusion described above successfully gives a material with a good
surface appearance. However, the product produced thereby suffers from an
important drawback in that the physical properties more closely resemble
those of a filled plastic than those of particle board. Due to the
relatively large proportion of plastic contained in the material
(typically approximately 50%), the properties of the final material are
rather like those of a plastic filled with an additive rather than those
of a particle board. Thus, it is perhaps misleading to refer to such
products as "extruded particle board." Rather a more correctly descriptive
name would be "extruded plastic with wood filler".
Thus, the common goal in the production of extruded particle board is to
reduce the amount of plastic which is included in the particle board as
much as possible. The use of less plastic filler along with an increased
proportion of wood material tends to produce a more economical product
with more desirable physical properties, more nearly resembling those of
particle board. The problem heretofore has been in reducing the mount of
plastic material included in the blend while maintaining the extrudability
of the material. A reduction of the amount of plastic tends to reduce the
extrudability of the product, especially in the surface finish. It has
proven to be a serious challenge to reduce the proportion of plastic
materials used in the blend while still extruding a product with a good
surface finish. The typical problem has been that the extrusion with less
plastic results in a rough surface textures in the product.
The present invention demonstrates a blend of wood product and plastic such
that the resulting product is capable of extrusion with good surface
finish. As described below, the present invention demonstrates an extruded
particle board having much less plastic material than in previous products
(typically down to 20% plastic by weight), along with appropriate binders
and coupling agents, while maintaining extrudability and good surface
finish.
SUMMARY OF THE INVENTION
The present invention relates to an improved material and manufacturing
method for extrusion of particle board. It is shown herein that the use of
fine particles of wood (or other lignocellulosic materials in the form of
fine particles, referred to collectively as "wood" herein), in combination
with an extrudable plastic, allows an increased proportion of wood to be
used (typically greater than 50% by weight, wood to plastic) and still
obtain an extrudable particle board with good surface finish. The use of
wood particles finer than approximately 40 mesh is used in the production
of extruded particle board in the present invention. A coupling agent
consisting of approximately 3% by weight of a polyurethane, MDI coupling
agent is also used. This combination of fine wood particles and the above
coupling agent yields good quality extruded particle board having a
majority of the composition thereof consisting of wood, wood-related
products or other lignocellulosic materials.
OBJECTS OF THE INVENTION
A primary object of the present invention is to produce extruded particle
board in which less than approximately 50% by weight of the extruded board
is plastic.
Another object of the present invention is to produce extruded particle
board using waste plastic as one component thereof.
Another object of the present invention is to produce extruded particle
board using waste plastics polypropylene, polyethylene or a combination
thereof as a component.
Yet another object of the present invention is to produce an extruded
particle board using a coupling agent consisting of a polyurethane-type
coupling agent.
Yet another object of the present invention is to produce an extruded
particle board at lower than customary extrusion temperatures.
Yet another object of the present invention is to produce a particle board
resistant to water-induced rot.
Another object of the present invention is to produce a particle board with
improved strength when compared to previous extruded particle boards.
Yet another object of the present invention is to produce a particle board
with improved water resistance and water repellency when compared to
previous extruded particle boards.
Another object of the present invention is to produce a particle board with
improved dimensional stability, when exposed to heat or moisture, when
compared to previous extruded particle boards.
Yet another object of the present invention is to produce a particle board
with improved resistance to chemical attack when compared to previous
extruded particle boards.
Yet another object of the present invention is to produce a particle board
with improved capabilities for reshaping when compared to previous
extruded particle boards.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates to the production of extruded particle board
consisting of a mixture of lignocellulosic material, plastic, and a
suitable coupling agent. Lignocellulosic material includes wood, wood
fibers, wood-related materials, and also materials derived from knaf,
grasses, rice hulls, and other vegetation, all encompassed under the
generic term of "lignocellulosic material". Current technology for
extruding particle board uses wood materials most commonly. Therefore, for
economy of language we will employ the term "wood" to mean all such
lignocellulosic materials, with understanding that various wood-related
products are typically the most common classes of materials expected to be
used in the extrusion of particle board. Typical plastics used in this
process would be polyethylene, high-density polyethylene, polypropylene,
acrylics, polystyrene, polycarbonate, phenolics, polyesters, or other
extrudable plastics. The plastics used in this process may be either scrap
or waste plastics or new materials provided that the waste plastic is
extrudable (which is not the case for typical thermosetting plastics
following the thermal setting step, although the same plastic prior to
thermosetting may be extrudable). Economically, it is believed that waste
plastics would be the preferred component for use in extrusion (along with
waste or virgin wood), perhaps following certain preprocessing such as
grinding, treating etc.
The function of the coupling agent is to bind the plastic and the wood
particles together with the plastic into a form suitable for extrusion.
Without such coupling agent, typical plastics do not bind to wood and,
therefore, do not lead to a mixture suitable for extrusion into particle
board. Various coupling agents as presently in use in the industry have
been described above.
In addition to the above materials, certain substantially inert filler
materials may also be added to the mixture for extrusion. Typically about
10% by weight filler might be included in an extrusion mixture. For
example, certain non-extrudable waste plastics can be added to the mixture
and extruded into particle board as an environmentally-friendly means for
the recycling and reuse of such scrap. Typical waste plastics which might
be used as fillers include, melamine, thermosetting plastics following the
thermoset, polyvinylchloride, polyesters, etc. Various other materials may
be used as inert fillers, such as calcium carbonate, glass (typically as
fibers or beads), ceramics, or other filler materials as would be obvious
to those skilled in the art. The use of inert fillers would typically be
to reduce costs by replacing a higher cost component with a lower cost
filler material. Also, another typical motivation for the use of filler
materials would be as a means of recycling and reuse waste materials which
otherwise present a challenging disposal problem (such as scrap melamine).
Combinations of filler materials can also be used with no essential
change.
Certain additives may also be added to the extrusion mixture in order to
impart desirable properties to the final extruded board. For example,
water or fire resistance can be imparted to the extruded board by the
addition of certain chemicals. Rot or insect resistance can also be
imparted. The specific chemicals leading to such improved properties in
the final extruded product are not the subject of the present invention
but will be apparent to those skilled in such arts.
Previous efforts to manufacture extruded particle board from wood and
plastic have had to deal with at least one major challenge. In order to
extrude a particle board with a good surface finish, it was necessary to
use a relatively large proportion of plastic in the extruded mixture
(typically 50% or greater by weight of plastic). This tends to increase
costs, and produce a particle board with properties more like those of a
filled plastic than wood. Increasing the proportion of wood in the
extruded mixture typically leads to a board with a relatively poor surface
finish which must be machined (or sawed in the fashion of wood) to produce
a commercially acceptable surface finish on the product. Machining or
sawing increases the cost of the process and, perhaps more important,
produces waste in the form of sawdust or scraps of extruded particle board
which must itself be disposed of in an environmentally acceptable manner.
Thus, the challenge to which the present invention is addressed is to
increase the proportion of wood in extruded particle board while
maintaining an acceptable surface finish.
An important feature of the present invention is the use of fine particles
of wood in the mixture to be extruded. It is found in the present
invention that wood particles finer than about 40 mesh (that is, particles
passing through a screen having 40.times.40 openings per square inch) will
permit extrusion into a particle board with acceptable surface finish and
simultaneously permit the use of less plastic in the extrusion mixture.
The present invention allows greater than 50% wood to be used in the
mixture, and typically up to about 70-80 % wood can be used in the form of
particles finer than approximately 40 mesh.
The present invention does not appear to be limited in terms of how fine
the wood particles may be and still lead to acceptable extruded particle
board. Wood particles as fine as 200 mesh may be used in the practice of
the present invention.
In combination with the use of fine particles of wood, the present
invention uses a coupling agent consisting of a polyurethane coupling
agent (typically a polymeric diphenylmethane diisocyanate-MDI). This
coupling agent may be combined with a suitable diluent in order to save
costs. It is found in the present invention that dilution of the above
coupling agent with approximately 10% to 15% by weight of furfural is
preferable.
The coupling agent used in the present invention is typically added to the
mixture for extrusion in an amount equal to about 3% by weight. This
coupling agent tends to add water resistance to the extruded product while
avoiding the necessity of using a relatively costly commercial brand name
product such as "Epolene" as the wood-plastic coupling agent.
Thus, the use of fine particles of wood and a suitable coupling agent leads
to the manufacture of extruded particle board having a much higher
fraction of wood, typically exceeding 50% and up to 70-80% by weight. With
such a high fraction of wood, it is not necessary to use extrusion
temperatures as high as required in process having less wood in the
extrusion mixture. When a larger fraction of plastic is present in the
extruded mixture, typical processing temperatures of around 360 deg. F.
are used. However, by utilizing the present invention in which a higher
fraction of wood is employed, lower processing temperatures may be
successful used. The present invention uses processing temperatures of
about 260 deg. F., reducing the energy required in the present process
and, hence, reducing processing costs.
The use of fine particles of wood along with the coupling agent of the
present invention allows different types of wood to be mixed into a single
extruded particle board. The finer particles of wood seem to facilitate
joining different types of wood and other materials into a single extruded
particle board. However, the detailed mechanism by which this occurs has
not been scientifically determined.
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