Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-05-03
2001-07-03
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S015000
Reexamination Certificate
active
06255368
ABSTRACT:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER
FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
BACKGROUND OF THE INVENTION
The present invention relates to wood filled plastic and methods for manufacturing wood filled plastic.
Plastic can be a cost-effective material for many applications. For some applications, however, the lower cost plastics such as polyethylene, polypropylene, and polystyrene lack sufficient stiffness. Wood, on the other hand, has high stiffness. But wood of high quality, which is free of knots and as a uniform grain such that it is suitable for use in mass production products, is expensive and, in recent times, a reliable source of such wood has been difficult to find. Plastic has a lower modulus of elasticity than wood, and hence a stiffness which is less than that required for many applications. This is particularly the case for relatively low cost plastics such as polyethylene, polypropylene, and polystyrene. Wood, while having a substantially higher modulus of elasticity than common plastics, is subject to attack by insects, fungus, and mold. Furthermore, wood, because of its tendency to absorb and lose moisture and the resultant dimensional changes, is dimensionally unstable under certain conditions.
Wood fibers and more generally cellulosic materials may be obtained at low cost and are often available as a waste product or readily manufactured from a waste stream.
By combining plastic with cellulosic materials, composite materials have been developed which combine many of the advantages of wood and of plastic while avoiding the disadvantages of either material. The plastic-wood fiber compounds which are sold as a substitute for plastic have stiffness levels approaching or surpassing those of wood while being available at a cost which is comparable or less than that of plastic with similar structural properties. The composite materials are manufactured by mixing finely divided cellulosic material into molten plastic. The mixed plastic is extruded through a die to form pellets similar in shape to the plastic pellets which are sold as the basic material for manufacturing plastic articles. Although a part manufactured of a cellulosic-plastic composite will typically be designed with that material in mind, standard extrusion and injection molding techniques can be used to fabricate parts with cellulosic-plastic composites.
The addition of cellulose to plastic presents a problem if the hot material during manufacture is exposed to moisture and thus caused to absorb undesirable moisture. It is thus critical that the manufacturing process control and eliminate moisture in the final product.
What is needed is a method of manufacturing a plastic cellulosic composite which removes moisture and prevents moisture from being reabsorbed into the finished product.
SUMMARY OF THE INVENTION
The process of this invention produces a wood-plastic composite material. The process employs a twin screw extruder with parallel segmented screws. The screw segments are positioned on two spline shafts and allow the various required functions of mixing, deaerating and removal of moisture followed by extrusion. Plastic, preferably polypropylene, polyethylene, or polystyrene, is fed to the main throat of the extruder where it is conveyed along a barrel formed by the case of the extruder. The plastic is heated while passing through a high shear area and becomes molten. Cellulosic fiber is positively fed into the extruder approximately one-quarter of the way down the extruder barrel by a twin-screw side stuffer. A vent is provided for entrained air in the cellulosic material to escape. The fiber and the plastic polymer then pass through a low shear mixing area in the extruder. The temperature is controlled by limiting shear rates and by providing a water-cooled jacket to prevent degrading of the cellulosic material. The maximum temperature is limited to about 400 degrees Fahrenheit and is preferably below 390 degrees Fahrenheit.
The molten mixture of polymer and cellulosic material is then conveyed to a devolatilization area in the extruder which incorporates an atmospheric vent. The mixture is moved through the atmospheric vent by drag-flow. Use of conveyor elements which produce drag-flow keeps the mixture from migrating up the vent and allows the release of some of the moisture entrapped in the fiber. To separate the devolatilization zone from the following zone, a melt seal is formed using a small low-shear mixing element on the twin spline shafts. A second devolatilization zone is formed where additional moisture is removed. The second zone employs a vacuum of 0.6 to 0.95 bar or about 18 to 29.5 inches of mercury vacuum.
Following the second devolatilization zone is a pumping section which raises the pressure of the mixture of polymer and cellulosic material to between 1,000 and 6,000 psi. The high pressure mixture is extruded through a multiplicity of small holes and separated into individual pellets by a four-bladed rotating knife. Calcium carbonate, talc or other mineral is blown onto the pellets as they are formed to prevent them from sticking to each other. The pellets are cooled to room temperature by fluidized air beds which rapidly cool the pellets without immersing them in water.
It is an object of the present invention to provide a method of manufacturing a plastic cellulosic mixture with a very low moisture content.
It is a further object of the present invention to provide a method of combining cellulosic material in plastic which does not degrade the combined cellulosic material.
It is another object of the present invention to provide a process for the devolatilization of a molten plastic and cellulosic mixture.
It is a still further object of the present invention to provide a method of cooling plastic pellets containing wood fiber without subjecting the pellets to contact with water.
It is a yet further object of the present invention to provide a plastic-cellulosic material mixture in pellet form with a very low moisture content and a cellulosic material content of 20 to 80 percent.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.
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English Brent W.
Gohr Kevin P.
Cain Edward J.
Gohr Kevin P.
Lathrop & Clark LLP
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