Note: Descriptions are shown in the official language in which they were submitted.
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0 Title: WOOD FIBER POLYMER COMPOSITE EXTRUSION AND
METHOD
TECHNICAL FIELD
The invention relates to a composite polymer/wood
fiber extrusion and a method for making the same. More
specifically, the invention relates to a foamed
cellulosic/polymer extrusion and a method for making the
same.
BACKGROUND OF THE INVENTION
Composite wood fiber/polymer extrusions have been
available for a number of years. The art with respect to
the manufacture of such extrusions, particularly combining
wood fibers having a mesh size between approximately 40
mesh and 80 mesh, and thermoplastic polymers, primarily
polyolefins is well developed. An early application for
such a composite related to the extrusion of a mixture
comprising 50% by weight wood fiber and 50% by weight
polypropylene for use in car door panels and other
interior automotive parts. This process had significant
economic advantages, particularly in the early.70's when
wood fiber was essentially a low or no cost waste product
from wood processing facilities and the price of petroleum
was relatively unstable. Extruders could vary the
percentage of waste wood, cellulosic material in the
extrusion depending on the price of polypropylene feed
stock which was, of course, dependent upon the price of
oil. Other extruders recognized not only the economic
merit of such a product but also recognized that a variety
of wood only products, such as decking, pallets, and
containers could be replaced with wood/thermoplastic
extrusions because the price of virgin wood was climbing
rapidly. Extruders eventually acquired the ability to co-
extrude waste wood products with polyvinyl chloride
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0 thermoplastics as well as polypropylenes and polyethylenes.
Problems relating to co-extrusion'of wood fibers and
a thermoplastic polymer component are well explained in
United States Patent No. 5,851,469 to Muller et al. issued
December 22, 1998,
Muller et al. described the typical
prior art steps tor co-extruding a thermoplastic polymer
with wood fiber. In a first step, the wood fiber i$ dried
using conventional techniques to a moisture content of
less than 8% by weight. In a second step the wood fiber
and plastic material are preheated to a temperature of
approximately 176 F. to 320 = F. In a third step, the
materials are mixed or kneaded at a temperature of 248 F.
to 482 F. to form a paste. In a fourth and final step,
the paste is either injection molded or extruded into a
final form. If the paste is extruded, the extrudate must
be calibrated and cooled. The Muller et al. reference
specifically addresses the problem of controlling the
temperature of the extrudate through various stages of the
extrusion process to prevent undesirable sheer stresses
from arising during the extrusion process. Muller et al.
also teach that a particular problem involved with wood
fiber/thermoplastic composite extrudates involves
volatiles in the wood component boiling off at extrusion
temperatures causing an undesirable foaming of the
extrudate.
U.S. Patent No. 5,746,958 to Gustafson et al. further
explains that particularly when using post-consumer
polymers (usually polyethylenes) the vagaries of the
characteristics of this component, when combined with the
problem of wood volatile boil off creates difficulties in
producing a uniform composite extrudate. Specifically,
Gustafson et al. teach that a high volume extruder must be
fed a minimum volume of a continuous product (e.g. feed
stock) stream. To satisfy this demand within the
parameters of the problem discussed above, Gustafson et
al. teach a method of pelletizing the thermoplastic
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0 component so as to produce a uniform feed stock having
known characteristics. Two or more different
thermoplastic, pelletized feed stocks are provided and
then blended with wood fibers to produce an extrudate
having consistent quality characteristics.
U.S. Patent No. 5,435,954 to Wold describes methods
for molding wood fiber/thermo-setting resins to produce
oriented strand board type products and is thus
illustrative of the differences between continuously
extruding thermoplastic wood fiber/thermoplastic
extrusions and hot press molding of wood fiber/thermo-
setting composite products. U.S. Patent No. 5,759,680 to
Brooks is believed to disclose the current state of the
art for preparing a wood fiber/thermoplastic extrusion
suitable for use in the building trades.
U.S. Patent No. 5,486,553 to Deaner et al. discloses
a polymer/wood thermoplastic composite structural member,
suitable for use as a replacement for a wood structural
member, such as for window components. The preferred
thermoplastic component is polyvinyl chloride (PVC) and
sawdust. In a preferred embodiment of the invention, a
double hung window unit is disclosed having cell, jamb and
header portions comprising hollow, multi-compartment
lineal extrusions which can be made from the disclosed
thermoplastic polymer/wood fiber composite. The resulting
extrusion has mechanical properties which are similar to
wood, but have superior dimensional stability, and
resistance to rot and insect damage as compared to
conventional wood products.
In addition to the above prior art, it is known that
foamed PVC/wood fiber composite extrusions have been
prepared. A foamed extrusion substantially reduces the
amount of polymer necessary per unit volume of extrusion
because the foaming process produces a plurality of
interstitial voids within an otherwise solid extrudate in
cross-section. One disadvantage of this type of extrusion
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0 is that the flexural modulus for this type of a foamed PVC
product is relatively low (e.g. 170,000) whereas the
flexural modulus for ponderosa pine is typically 900,000.
Hollow, extruded profiles can be manufactured with
webs and other internal structural members to produce
virtually any desired macroscopic mechanical property.
However, in extrusions having an extremely high aspect
ratio in cross-section (e.g. slats for Venetian style
blinds) it is mechanically impossible to provide the
extrudate with a wall thickness sufficient to provide the
desired macroscopic mechanical characteristics,
particularly bending moment. In this area of product
application, a product having a solid cross-section from
a foamed material is preferred. Unfortunately, prior
attempts to introduce wood fiber into a foamed polymer
extrudate demonstrates that the wood fiber tends to
counteract the effect of the foaming agent. As a result,
such prior art foamed PVC/wood fiber extrusions have
limited the wood fiber content to 5% by weight or less.
Such a small wood fiber component does little to reduce
the petroleum product content or to improve the mechanical
properties of the extrudate.
Nevertheless, a need exists for a composite extrusion
having a thermoplastic component and a wood fiber
component which uses substantially less thermoplastic
component per unit weight of finished extrusion as
compared to the products made by the processes described
above in the prior art. In addition, a need exists for a
thermoplastic polymer/wood fiber composite extrusion which
is sufficiently rigid to supplant standard solid wood
components in a variety of installations such as Venetian
style window shades and blinds.
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0 SUMMARY OF THE INVENTION
It is therefore an object of the present,invention to
provide a foamed, continuous thermoplastic/cellulose fiber
composite lineal extrusion employing a styrene
5 acrylonitrile (hereinafter "SAN") component, a cellulosic
material component and acrylonitrile butadiene styrene
(hereinafter ABS) resin and a foaming agent.
In a preferred embodiment of the invention, the
extrusion is prepared from a feed stock material
comprising approximately 70% to 90% by weight SAN,
approximately 5% to 25% by weight cellulosic material,
approximately 2% to 27% by weight ABS resin and a trace
amount of lubricant and foaming agent. The SAN feed stock
component is preferably pelletized with the cellulosic
material and is introduced into a conventional multi-screw
extruder and various ratios of medium molecular weight,
high molecular weight, and ultra-high molecular weight SAN
with the'ABS resin. The foaming agent is preferably
injected down stream from a mixing a unit for the above
components and upstream of a forming die connected to the
extruder. The extrusion is then preferably calibrated to
the desired size and shape.
An extrusion prepared by the inventive method
preferably has a heat deflection temperature rating of not
less than 170 F., a flexural modulus of at least 307,000
psi, a coefficient of thermal expansion of not more than
approximately 3.33 x 10-5 inches per inch per degree
Fahrenheit, and a thermal conductivity rating of not more
than approximately 0.6 BTU inch per hour ft2 square degree
Fahrenheit. The preferred cellulosic material is wood
fiber having a mesh size in the range of 40 mesh to 200
mesh, and in the preferred embodiment having a size of
approximately 60 mesh.
The invention has particular utility with respect to
extrusion profiles having a relatively high aspect ratio
in cross-section, such as slats for Venetian style blinds.
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0 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an environmental, isometric view of a
high speed polymer extrusion apparatus for use with the
method of the present invention.
Figure 2 is a schematic representation in block
diagram form of the process of the present invention.
Figure 3 is an enlarged, cross-sectional view of an
extrusion manufactured by the method of the present
invention.
Figure 4 is an alternate embodiment of the extrusion=
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTB
A conventional, twin screw extruder for use with the
method of the present invention is generally indicated at
reference numeral 10 in Figure" 1. The extruder 10
includes a hopper or mixer 12, for accepting a feed stock
consisting of a thermoplastic polymer/ wood composite
pelletized material, a conduit 14 for connecting the
hopper with a preheater 16 for controlling the temperature
of an admixture of the feed stock in the hopper 12, and an
inlet 18 for introducing a foaming agent. The preheater
16 is connected to a multi-screw chamber 20 for admixing
the feed stock with the foaming agent and other
conditioners to be described herein below under controlled
conditions of temperature and pressure. Chamber 20 is
connected to an extrusion die 22 which produces an
extrusion 24. The extrusion is preferably calibrated in
a conventional calibrator 26 to result in a final product
shown in Figures 3 and 4. An appropriate extruding
machine 10 is available from Cincinnati Millacron
corporation, Cincinnati, Ohio, USA.
The extruder 10 and calibrator 26 are conventional
apparatus, the operation of which is well understood by
those of ordinary skill in the thermoplastic polymer
extrusion art. The extrusion 24 shown in Figure 3 is a
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0 foamed, continuous thermoplastic/cellulose fiber composite
lineal extrusion adapted for use as a slat or louver in a
window blind construction, commonly referred to as a
Venetian blind. The extrusion has excellent strength to
weight characteristics, and has a workability and surface
finish similar to a milled wood product from a coniferous
tree, such as ponderosa pine. The extrusion has a heat
deflection temperature rating of not less than 170 F., a
flexural modulus of approximately 307,000 psi, a
coefficient of thermal expansion of not more than 3.33 x
10"5 inch per inch per degree F., and a thermal
conductivity rating of not more than approximately 0.6
BTU's per hour per F2. The extrusion preferably also has
a density of not more than approximately 0.6 grams per cm'.
The extrusion produced by the method of the invention
has particular utility with respect to an extrusion, such
as that shown in Figure 3, having a high aspect ratio in
cross-section. Such high aspect ratio extrusions are
often difficult to form as a conventional hollow extrusion
having the desired macroscopic physical properties of
bending moment, workability, screw retention, etc., in a
cost effective manner. Stated another way, it is
difficult to produce a very narrow, hollow extrusion
having a high bending moment, and good screw retention
without employing a complex web structure within the
extrusion and pre-drilled screw holes. While such
structures are technically possible to incorporate in a
hollow extrusion, these features increase the raw material
cost, wall thickness, and engineering complexity of the
die used to produce the extrusion. A foamed extrusion can
be produced which uses significantly less polymer
component per unit length of extrusion than a high aspect
ratio engineered hollow extrusion having similar
macroscopic physical characteristics.
The assignee of the present invention has discovered
that it is possible to produce a foamed thermoplastic
extrusion having wood fiber as a significant component
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0 thereof. Prior attempts to produce a foamed extrusion
having wood fiber as a significant component have been
unsuccessful, as the wood fiber tends to- degrade the
effectiveness of conventional foaming agents. In
particular, polyolefins such as polyethylene and
polypropylene do not adhere well to wood and significant
modifiers are needed (usually a thermo-setting resin, 2%
to 3% by weight). Polyvinyl chloride (PVC) bonds well to
wood fibers because like wood fibers it is a polar
molecule. Unfortunately, prior attempts to foam a PVC/
wood fiber composite extrusion have only been successful
wherein the wood fiber component is 5% by weight or less.
In such low ratios, the wood fiber has little structural
effect on the resulting extrusion and does not achieve any
of the significant advantages of a wood fiber/thermo-
setting polymer extrusion, including rot resistance,
paintability, stainability and workability characteristics
similar to a milled wood product such as pine. It is an
aspect of the present invention that, contrary to
conventional wisdom, a foamed thermo-plastic polymer/wood
composite extrusion can be produced having a high
proportion of cellulosic material content in the form of
wood fiber in the range of 5% to 25% by weight wherein the
principal thermoplastic polymer ingredient is styrene
acrylonitrile (SAN) in the range of 70% to 90% by weight.
Table I illustrates one preferred formulation used for the
production of a foamed, thermoplastic/ cellulosic material
composite extrusion suitable for use as a slat in a window
blind, of the type shown in Figure 3.
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0 TABLE I
INGREDIENT PERCENT RANGE
(by weight)
SAN 70-90
Wood Fiber =- 5-25
ABS 2-8
Lubricant 0.1-0.5
Foaming Agent 0.5-3
An appropriate SAN product is available from General
Electric Specialty Chemicals, Morgantown, West Virginia,
as well as from Kumho, South Korea.' Specifically, the
General Electric products Blendex 570, 576, and 869, as
well as Kumho SAN 350*have proven satisfactory for this
purpose. A suitable ABS component used as a modifier is
General Electric's Blendex 360* product. A suitable
foaming agent is available from Color Matrix of Cleveland,
Ohio, under the designation 80-428-1. Magnesium stearate
has been found to be a suitable lubricant. It is believed
that ethylene-bis-stearimide and calcium stearate in the
same proportions as given above are also suitable
lubricants.
Substantial success has also been achieved by
alloying different molecular weight SAN products. Another
alternate formation is shown in Table II below.
* trade-mark
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0 TABLE II
INGREDIENT PERCENT RANGE
(by weight)
High Molecular Weight SAN 0-85
Medium Molecular Weight SAN 5-90
Ultra-High Molecular Weight SAN 1-5
5 Wood Fiber 5-25
ABS 2-8
Lubricant 0.1-0.5
Foaming Agent 0.5-3
10 It is preferred that the SAN/ wood fiber component be
prepared as a pelletized feed stock for admixture with the
ABS modifier, lubricant and foaming agent. An appropriate
pelletized product is available from Northwoods Company,
Sheboygan, Wisconsin. A typical wood fiber mesh size for
this pelletized product is 60, but an acceptable range may
be from 40 mesh to 200 mesh. The pelletized compound
consists of 20% to 80% by weight medium molecular weight
(MMW) SAN, 20% to 80% wood fiber, and 0.4% to 2.0%
lubricant. A resulting general formula for extrusion is
shown in Table III below.
TABLE III
INGREDIENT PERCENT RANGE
(by weiqht)
Northwoods Pellets 6-90
MMW SAN 0-85
I-IMW SAN 0-85
UHMW SAN 1-5
ABS 2-8
Foaming Agent 0.5-3
A particular preferred embodiment of the invention is
shown in Table IV below.
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0 TABLE IV
INGREDIENT PERCENT
Northwoods Pellet 26
Kumho SAN 350 65
GE B-869 UHMW SAN (Stiffener) 2
GE B-360 ABS (Modifier) 5.2
Color Matrix Foaming Agent 80-428-1 0.8
In Figure 2, the SAN/wood fiber pelletized feed stock is
added into the hopper or mixing unit 12, along with the
additional Ultra High Molecular Weight (UHMW) SAN
stiffening agent, ABS resin modifier and either Medium
Molecular Weight (MMW) SAN or High Molecular Weight (HMW)
SAN. The ratios of UHMW to MMW or HMW SAN can be varied
in accordance with the skill level of the artisan to
provide an extrusion having varying macroscopic physical
properties. Once mixed, the resulting compound is gravity
fed through the conduit 14 to the extruding chamber 20.
The foaming agent is added on line by way of inlet 18
through a peristaltic pump Model CM100 manufactured by
Color Matrix of Cleveland, Ohio. The pump speed can range
from 7 rpm to 12 rpm according to the feed rate of the
feed stock and speed of the mixer. The extrusion 24
appears at the exit of the extrusion die 22 in the desired
form. An appropriate extruder 10 is a Model CM 55
manufactured by Cincinnati Millacron, Batavia, Ohio.
The extrusion 24 shown in Figure 3 can be used as
wood product replacement in a wide variety of
applications. One application utilized by the assignee of
the present invention is as a,slat for a window blind.
Those of ordinary skill in the art will appreciate other
applications suitable for the extrudate of the present
invention when extruded in a variety of cross-sectional
shapes. The extrusion has physical characteristics
remarkably similar to ponderosa pine and superior to rigid
PVC and foamed PVC products. Table V illustrates results
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0 of tests conducted by the assignee of the present
invention comparing various physical properties of the
inventive extrusion manufactured by the method of the
present invention compared to- rigid PVC and two
competitive foamed PVC products.
TABLE V.
RIGID 1ST 2ND INVENTIVE INVENTIVE PONDEROSA
PVC FOAMED FOAMED EXTRUDATE EXTRUDATE PINE
PVC PVC w/PVC Cap
PRODUCT PRODUCT
Heat 145 F 151 F. 153 F. 175 F. 165" F. N/A
Deflection (165 F.)
Temperature
ASTM D648
Vicat 190 F. 173 F. 179 F. 217 F. 219 F. N/A
Softening
Point
ASTM D1525
Flexural 390,000 128,000 257,000 307,000 220,000 1,290,000
Modulus psi psi psi psi psi psi
ASTM D7 90
Direct 456 lbf 242 lbf 291 lbf 527 lbf 319 lbf 163 lbf
Screw (ASTM
Withdrawal D1761)
ASTM D1037
Hardness, 82 83 62 79 56
Type
Durometer
Coefficient 3.59 x (1.8 x 3.33 x 3.19 x 2.5 x
of Thermal 10"5 10"6 10-5 10'' 10-6
Expansion in/in/ F in/in/ F in/in/ F in/in/'F
in/in/F
0
Thermal 0.69 0.46 0.45 1.6-2.9
Conduct- btu-inch btu-inch btu-inch
ivity ft2-hr-'F ft2-hr- F ftz-hr- F
ASTM D177
Water 0.09% 0.456 0.561 5.16% 17.2%
Absorption
ASTM D1037
Density 90.5235 43.077 39.3309 31.8393
1b/ft3 lb/ft lb/ft lb/fl
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0
Figure 4 illustrates an alternate embodiment of the
invention in which the extrusion 24 is co-extruded with a
polyvinyl chloride cap stock 50. The cap stock is co-
extruded in a manner well known to those of ordinary skill
in the thermoplastic extrusion art.
Those of ordinary skill in the art will, upon
reviewing the above disclosure conceive of other
embodiments and variations of the invention. Therefore,
the invention is not to be limited by the above
description, but is to be determined in scope by the
claims which follow.
