Process for making a filler reinforced thermoplastic composites having biaxially oriented components

There is disclosed a process and apparatus for the continuous production of a thermoplastic product having biaxially oriented components. Preferably the thermoplastic compound includes filler. The process may be used to manufacture hollow profiles or sheets. The product is made by a continuous extrusion process which biaxially orients the components by forcing a molten thermoplastic compound through a contoured die with a contoured mandrel therein and then cooling the compound to preserve the imparted orientation. With respect to the sheets a biaxially oriented hollow profile is formed, then cut along the axial direction and then flattened into a sheet.

FIELD OF THE INVENTION 
The present invention relates to processes and apparatus for the 
manufacture of filler reinforced thermoplastic composite materials with 
biaxially oriented polymer components. The products formed by the present 
invention can be of hollow profiles or sheets. 
BACKGROUND OF THE INVENTION 
It is well known that the properties of thermoplastic polymers and 
thermoplastic/filler composite material can be enhanced by orienting the 
thermoplastic molecules and filler (when present). Further, for certain 
applications it is desirable to orient the molecules and filler in two 
directions. Accordingly much research has been directed towards developing 
methods for producing such products. 
One method proposed for producing biaxially oriented products is solid 
state extrusion. For example U.S. Pat. No. 4,801,419 (Ward et al., 1989) 
and Ward et al., Plast. Rub. Comp. Proc. Appl. 19 (1993) describe a 
process to produce uniaxially and biaxially oriented hollow profiles by 
deforming a pre-extruded, unoriented hollow workpiece comprising 
orientable, thermoplastic polymer by passage in the solid phase through a 
die having both an entry and an exit side. The hollow workpiece is 
provided at the entry side of the die, while tension is applied to the 
hollow workpiece from the exit side of the die. Two important elements 
disclosed in these publications are the temperature of the polymer during 
deformation and the drawing or tension applied to cause this deformation. 
The temperature must be well below the melting point of the polymer, thus, 
the deformation of polymer took place in the solid phase. The tension 
applied must be high enough to cause solid state deformation but low 
enough so as not to cause tensile failure. A special grip is needed to 
hold and pull the workpiece from the exit side of the die. As the result, 
this process is a batch process. 
A similar process is described in U.S. Pat. No. 5,096,634 (Tsadares and 
Anastassakis, 1992) which describes a process for producing biaxially 
stretched, unfilled thermoplastic tube (especially polyvinylchloride) by 
steps of extruding an unoriented thermoplastic tube through a die in the 
melt/viscous state, cooling the tube and drawing it at the same speed as 
the extrusion speed, passing the tube next over a conical expanding 
portion of a stretching mandrel (a second die) and drawing the expanded 
tubing at the exit at a speed higher than the entering speed, thus, 
biaxially stretch the tube. The stretching is resulted from the 
differences in the drawing speed of the tubing before and after the 
conical die. 
It is desired to produce hollow profiles or sheet with increased mechanical 
strength properties in two directions, where the strength properties in 
both directions can be designed or engineered according to the desired 
needs. It is also desirable to produce these superior products 
continuously using commercially available equipment at a cost comparable 
to or lower than existing products. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has been found that fibre and 
particulate reinforced thermoplastics of any molecular weight of 
exceptionally high strength and modulus in two directions (biaxially 
oriented), can be made by extrusion processes which biaxially (in two 
directions) orient the thermoplastic polymeric chains and the fibrous 
filler/particulate having an aspect ratio greater than unity during 
extrusion, and which freeze them in that orientation, substantially 
preventing relaxation of polymeric chain. In this process, the extruder is 
responsible for pushing the composite material through the die assembly, 
thus creating biaxial orientation. Application of tension at the exit side 
of the die must be avoided because the composite material has very low 
tensile strength at the processing condition due to high filler content. 
The present invention provides a process for the continuous production of a 
thermoplastic product having biaxially oriented components. The process 
comprises the steps of providing a thermoplastic compound comprising 
thermoplastic polymer. The thermoplastic compound is brought to a 
temperature in a predetermined range just above and including the 
softening point temperature of the thermoplastic compound thereby 
producing a molten thermoplastic compound. The molten thermoplastic 
compound is forces through a die and an interior cavity is formed therein. 
The thermoplastic compound undergoes circumferential stretching flow 
through the die to impart circumferential orientation to at least some of 
the thermoplastic polymer and the thermoplastic compound undergoes axial 
stretching flow through the die to impart axial orientation to at least 
some of the thermoplastic polymer. Thereby a molten oriented thermoplastic 
compound having a hollow profile is produces. The molten oriented 
thermoplastic compound is cooled after imparting circumferential and axial 
orientation to a temperature below the softening point temperature of the 
thermoplastic compound to preserve the imparted orientation therein and to 
solidify the thermoplastic compound. Optionally the process may include 
the steps of axial cutting the molten oriented thermoplastic compound and 
gradually flattening the cut molten oriented thermoplastic compound into a 
sheet having biaxially oriented compounds and whereby during cooling the 
sheet is solidified. 
In another aspect of the invention there is provided a device for 
manufacturing thermoplastic product having biaxially oriented components. 
The device comprises a reservoir for bringing the thermoplastic compound 
to a temperature in a predetermined range just above and including the 
softening point temperature of the thermoplastic compound. Downstream 
thereof is a die having internal walls, an inlet, an outlet and a passage 
communicating between the inlet and outlet. The circumference of the 
outlet is greater than that of the inlet. A contoured portion of the 
internal walls define a contoured passageway having an inlet and an 
outlet, wherein in use the thermoplastic compound undergoes axial 
stretching flow through the passageway to impart axial orientation to at 
least some of the thermoplastic polymer. A mandrel is positioned in the 
die and extends through the contoured passageway. The mandrel has external 
walls and a contoured portion in registration with the contoured portion 
of the die. The cross sectional area of the inlet of the contoured 
passageway is greater than the outlet and wherein in use the mandrel 
causes a cavity to be formed in the thermoplastic compound to produce a 
hollow profile and the thermoplastic compound undergoes circumferential 
stretching flow through the contoured passageway to impart circumferential 
orientation to at least some of the thermoplastic polymer. There is 
provided support for the mandrel. An extruder forces the thermoplastic 
compound through the die. The device also comprises a cooling means 
downstream of the passageway for cooling the thermoplastic compound.

DETAILED DESCRIPTION OF THE INVENTION 
In the present process a thermoplastic/filler composite material is 
continuously extruded from an extruder through a die assembly comprised of 
an adapter, a reservoir, a mandrel, a mandrel support, a die and a 
calibrator. The adapter joins the extruder to the rest of the die 
assembly. The reservoir is an extended tubular section and in the 
reservoir the composite material is allowed to stabilize. As well, in the 
reservoir the temperature of the bulk composite material is adjusted to 
the desired deformation temperature for biaxial orientation of the polymer 
molecules and fibrous filler particulates. Lubricant is distributed evenly 
at interfaces between the internal surface of the reservoir and the 
composite material thus reducing shear at interfaces and promoting plug 
flow with homogeneous speed. The mandrel serves two purposes: firstly the 
portion of the mandrel in the reservoir serves to convert the flow of the 
molten composite material from the extruder to an annular flow before 
entering the die and secondly the portion of the mandrel in the die 
determines the flow channel in the die in conjunction with the contour of 
the die. Liquid lubricant is also introduced to the interfaces between the 
composite material and the internal surface of the die and between the 
composite material and the external surface of the mandrel by injecting 
lubricant through channels in the mandrel holder or adapter. Both spider 
or spiral type of mandrel are common, but spiral mandrel is preferred 
because it is structurally more sturdy and it can prevent weld lines along 
the pipe/tube which can cause weak points in the radial direction. The 
temperature of the reservoir housing and portion of the mandrel in the 
reservoir are usually set below the temperature of the bulk composite 
material to provide cooling to the bulk composite material. The dimensions 
and contours of the mandrel and the die induce elongational flow (rather 
than shear flow) in both the axial and circumferential directions which 
are essential for orienting polymer molecules and filler particulates in 
both directions. To facilitate the alignment of the polymer molecules and 
the filler particulates, the temperature of the bulk composite must be 
above the softening point of the polymer. To retain the orientation the 
composite must be rapidly frozen/solidified, thus, the temperature of the 
bulk composite must be close to the solidification temperature as it goes 
through the die. The preferred temperature of the bulk is 
1.degree.-10.degree. C. above the melting point or softening point of the 
polymer. Solidification is achieved by strain hardening of the polymer 
during biaxial stretching as well as additional cooling provided in the 
calibrator located next to the die assembly. The final dimension of the 
product is maintained in the calibrator. Vacuum calibrators may be used to 
further improve product quality and are desirable if the tolerance of the 
final dimension of the hollow product is critical. Biaxially oriented 
sheet can be produced by modifying the die and calibrator in which the 
annular flow of a circular pipe near the end of the die is cut in the 
axial direction and gradually converted into a sheet. 
Apparatus and Method for Producing Hollow Products 
Referring first to FIG. 1 there is illustrated an apparatus 10 for 
producing biaxially oriented hollow products made of filler reinforced 
thermoplastic composites using the process of the present invention. The 
apparatus consists of, in sequence moving downstream, an extruder 12, a 
die assembly 14, a cooler or vacuum tank 16, a puller 18, a cut-off saw 20 
and a lubricant pump 22. 
Extruder 12, which may be one of many different types including single or 
twin screw extruders known to those skilled in the art, is used to melt 
and convey the plastic composite material through a passageway in the die 
assembly 14 shown in detail in FIG. 2. The processing conditions in 
extruder 12 are chosen to ensure that the composite material is completely 
melted and mixed without causing excessive torque to the extruder drive. 
FIG. 2 shows a cross-section of die assembly 14 having a spiral type 
mandrel support. By way of example, die assembly 14 is configured for 
producing a circular pipe product using the process of the present 
invention, however it will be appreciated by those skilled in the art that 
this process applies to a variety of configurations. Die assembly 14 
comprises an adapter section 24 attached to the end of the extruder barrel 
(not shown), a spiral type mandrel support 26, a mandrel 28, a reservoir 
30, a die 32 and a calibrator 34. The conveying action of the extruder 
screw located in the extruder barrel forces the molten 
filler/thermoplastic composite through a breaker plate (not shown) into 
adapter section 24. 
The shape of adapter section 24 is designed so that it can be attached at 
its upstream end to the extruder barrel by any means known in the art so 
that there is a gradual transition from the upstream section to the 
downstream section of adapter 24. The downstream section of adapter 24 is 
attached to mandrel support 26 which in turn supports mandrel 28. 
Mandrel 28 determines the internal shape of the hollow profile along the 
die assembly 14. Mandrel support 26 is a typical spiral mandrel support. 
Spiral mandrel support 26 creates more uniform distribution of material 
than other conventional mandrel supports without any weld lines. Lubricant 
is injected through channel 36 to the interface between the external 
surface 37 of the mandrel 28 and the composite material. Lubricant is 
injected through channel 38 to the interface between the internal surface 
39 of reservoir 30 and the composite material. Channels 36, 38 are 
preferably shaped and dimensioned to ensure that the lubricant is 
distributed evenly around both interfaces, thereby coating both surfaces 
of the composite material. A lubricant may also be mixed in the 
thermoplastic/filler composite. Typically, the temperature of the 
reservoir 30 and the mandrel 28 in the reservoir section are set close to 
the softening point temperature of the bulk composite material. 
As illustrated in FIG. 1, the lubricant may be injected by a device 22 such 
as a metering pump, syringe pump, gear pump or other apparatus known in 
the art which can consistently deliver the necessary amount of lubricant 
at a sufficiently high pressure to overcome the pressure inside reservoir. 
The lubricant is used to ensure that the annular flow through the die 
assembly 14 is mainly plug or elongational flow (minimal shear flow). 
Suitable lubricants include silicone oils (Dow Corning Inc.), liquid 
paraffins, glycerol, fatty amides (Kenrich Chemical Co.) and titanates. 
The length of the reservoir or straight section 30 is chosen so that there 
is sufficient time for the lubricant to spread evenly around the 
interfaces between the inside of reservoir 30 and the composite material 
and the outside of mandrel 28 and the composite material. In addition, 
reservoir 30 must be of sufficient length to ensure that the composite 
material enters die 32 at the desired temperature and that the temperature 
of the thermoplastic/filler composite is as uniform as possible throughout 
its cross-section. 
The design of die 32 is important to the success of the process. First, the 
exit section 40 of die 32 must correspond to the section of the desired 
profile. Second, the dimensions or contour of die 32 and the contour of 
mandrel 28 must be carefully designed in order to create biaxial 
orientation, while maximizing the throughput and optimizing the surface 
appearance of the skin. The internal shape of die 32 at the exit 40 will 
define the external shape of the hollow profile and the external shape of 
the mandrel 28 at the exit 40 will define the internal shape of the hollow 
profile. The following paragraphs illustrate the important aspects in 
designing die 32. A circular hollow profile is used as an example to 
simplify the illustration, but it will be appreciated by those skilled in 
the art that these principles apply to any other shapes (rectangular, 
triangle, etc.). 
FIG. 3(a) shows a diagram of a pipe 42 with internal and external radii of 
R and R.sub.o respectively and wall thickness d=R.sub.o -R.sub.i. Pipe 42 
is elongated in the x direction. The term biaxial orientation refers to 
orientation in x direction and in the circumferential direction. To 
manufacture pipe with biaxially oriented components, the 
thermoplastic/filler composite material must experience elongational or 
stretching flow (not shear flow) in two directions. The stretching flow in 
the x (axial) direction is caused by diminishing bulk cross-sectional area 
of the tube/pipe along x direction. Bulk cross-sectional area is defined 
as the area of the bulk profile normal to the axial or x direction. For a 
pipe, it can be calculated as .pi.(R.sub.0.sup.22 -R.sub.i.sup.2). The 
stretch (draw) ratio in x or axial direction is defined as the ratio of 
the bulk cross-sectional areas of the entrance and the exit of the die. 
The stretching flow in the circumferential direction is caused by 
increasing the circumference of the die from the entrance to the exit of 
the die. The circumferential stretch (draw) ratio is defined as the ratio 
of the average circumferences (or average radii) at the exit and the 
entrance of the die. The circumference can be calculated from 2 .pi.R, 
where R is radius. The term average refers to intermediate between 
internal and external values, R=(R.sub.o +R.sub.i)/2. Therefore, the 
criteria for a die of the present invention, which causes the polymer 
components to be biaxially oriented during the process, is that the radius 
or circumference of the die opening is greater at the exit 40 (FIG. 2) 
than at the entrance 41 and the bulk cross-sectional area is greater at 
the entrance 41 than at the exit 40. Axial stretch ratios are typically 
1.25:1 to 4:1. Similarly, circumferential stretch ratios are typically 
1.25:1 to 4:1. 
Another important criteria for a successful biaxial orientation process is 
the stretch rates (the speed of stretching) in both orientation 
directions. Along with the material properties, in particular the 
rheological properties, the stretch rates and the stretch ratios in both 
directions determine the contour of the die housing and the mandrel. The 
best results are usually obtained for a design which impart constant or 
decreasing stretch rates. To achieve stretching, i.e., through 
elongational flow, lubricant at the interfaces between the composite 
material and the metal surfaces of die 32 and mandrel 28 is required. 
Otherwise, shear flow, which is not effective for stretching, will 
dominate. 
The final element of die assembly 14 is calibrator 34, which has the same 
cross-section as the adjacent exit 40 of the die 32 and consequently the 
same section as the desired profile of the product. The portion of mandrel 
28 with a constant cross-section extends into the calibrator 34 partially 
or all the way. The main function of calibrator 34 is to maintain the 
dimensional stability of the hollow profile. Vacuum calibrator or vacuum 
tank 16 (FIG. 1) may be used at the end of the die assembly if the 
tolerances of the dimension of the final product are critical. 
The temperature of die 32 and preferably, also the temperature of the 
composite material entering the die 32 is generally a few degrees above 
the softening point of the resin mixture, that is 1.degree.-10.degree. C. 
preferably 2.degree.-5.degree. C. above the softening point. The composite 
material is partially solidified as it exits the die section due to strain 
hardening resulting from stretching the material in the die. Further 
solidification takes place in calibrator 34. Rapid cooling is necessary to 
preserve the imparted orientation of the polymer molecules and the filler 
particulates. If desired a cooler or vacuum tank 16 may be used, see FIG. 
1. 
Referring to FIG. 1, after solidification to the desired dimension, the 
biaxially oriented hollow extrudate 43 is passed through a puller 18 which 
guides it to the cut-off saw 20. The puller 18 only acts as a guide to 
ensure a straight product and does not provide tension nor does it assist 
the composite material in going through die assembly 14. Tension will 
cause tensile failure in the extrudate due to the low melt strength of 
highly filled composite material. 
An alternate embodiment of the die assembly is shown in FIG. 4 at 44. FIG. 
4 shows a longitudinal cross-section of die assembly 44 having a spider 
type mandrel support. Die assembly 44 is generally the same as that 
described above and only those portions which are different will be 
described and assigned different designation numbers. 
Mandrel support 46 is a typical spider type support. Spider mandrel support 
46 is much simpler in design, but the spider legs may cause weld lines in 
the product and thus weak spots. Lubricant is injected through channel 48 
to the interface between the external surface of the mandrel 28 and the 
composite material. Lubricant is injected through channel 50 to the 
interface between the internal surface of reservoir 30 and the composite 
material. Channels 48, 50 are preferably shaped and dimensioned to ensure 
that the lubricant is distributed evenly around both interfaces, thereby 
coating both surfaces of the composite material. 
Apparatus and Method for Producing Biaxially Oriented Sheets 
Referring to FIG. 3a) and b), cutting biaxially oriented pipe 42 along the 
axial direction and flattening it into a sheet 52 results in a biaxially 
oriented sheet of the same thickness as the wall of the pipe. The 
orientation in the axial direction remains the orientation in the x 
direction, while the orientation in the circumferential direction of the 
pipe becomes the orientation in the y direction, where y is perpendicular 
to x. Therefore, if a pipe is cut along the x direction and opened-up into 
a flat sheet, the biaxial orientation becomes apparent in x and y 
coordinates. 
FIG. 5 shows an apparatus 54 for producing biaxially oriented sheets. 
Generally, biaxially oriented sheets can be manufactured using similar 
equipment as that used to manufacture biaxially oriented pipe described 
above, with some modifications to accommodate the principle of converting 
a pipe into a sheet. Only those differences will now be discussed. 
Extruder 12, cooler or vacuum tank 16, puller 18, cut off saw 20 and 
lubricant pump 22 are the same as for the biaxially oriented pipe 
described above and shown in FIG. 1. Die assembly 56 includes those parts 
of die assembly 10 described above and shown in FIG. 2 in sequence from 
the adapter 24 to the die exit 40. However, instead of a calibrator 34 a 
transformation section 58 (FIG. 6) is attached to the end of die 32 not 
shown in FIG. 5. This transformation section 58 converts the flow of the 
composite material from a pipe 60 into increasingly open pipes, and 
finally into a flat sheet 62. 
Several pairs of rollers 64 (shown in FIG. 5) are located after the 
transformation section 58 to flatten the partially open pipe into sheet 
and/or to maintain the sheet flat before entering cooling bath 16, puller 
18 and cut-off saw 20. FIG. 6 illustrates a schematic diagram of 
transformation section 58 and shows a pipe 60 entering transformation 
section 58 and a planar sheet 62 exiting transformation section 58. 
As discussed above, the principle is to gradually convert the pipe into a 
sheet having a constant bulk cross-sectional area. Since the wall 
thickness of the pipe or the thickness of the resulting sheet is small 
compared to the diameter of the pipe, this transformation does not result 
in failure on the surface of the sheet. FIG. 7 shows stages in the gradual 
transformation of the profile from a circular pipe 60 to a planar sheet 62 
at several locations along the machine direction. During this 
transformation, the composite material is cooled down to freeze the 
orientations created in the die and to condition the material for the 
rolling step. 
Selection and Pre-treatment of Starting Materials 
The resin component may comprise virgin or recycled (waste) thermoplastics 
derived from the polyolefin family (polyethylenes, polypropylenes and 
copolymers thereof), vinyls (chiefly copolymers of vinyl chloride), 
styrenics (including ABS and maleic anhydride copolymers), polyesters 
(including polyethyleneterephthalate and polybutyleneterephthalate), 
polycarbonates and polyamides (nylons). The process can be carried out 
with recycled or waste resins including commingled resins. For economic 
reasons, the granulated (chipped) resins recovered from plastic bottles or 
film (prior to pelletizing) are preferred since pelletizing greatly 
increases the material cost. This process excludes thermosetting resins 
such as phenolics, formaldehides, polyesters and epoxy resins. For best 
results it is desirable to employ resins having a large melt strength so 
that the material can survive high stretching ratio without tensile 
failure. Since melt strength (and strain hardening) increases as the 
temperature is decreased, it is desirable to extrude near its softening 
point, where the viscosity is high. There is a lower limit to the 
temperature as the extrudate becomes too viscous to extrude and the 
pressures become excessive. The high viscosity near the softening point is 
also retained for a sufficiently long period of time (at least several 
minutes) so that the molten extrudate has sufficient time to solidify and 
freeze the polymer orientation imparted by the die. In the case of melt 
extrusion as normally practised in industry the relaxation times of the 
extrudates are often measured in seconds so that the majority of the flow 
orientation is lost before the extrudate has solidified. Long relaxation 
times and pure elastic deformation (as opposed to viscous deformation) are 
also favoured by the use of high molecular weight resins. The choice of 
molecular weight of the polymer may be limited depending on the ability of 
the latter to be mixed with the filler so that a compromise is usually 
necessary between the ease of mixing and retention time of orientation in 
the extrudate. Therefore the highest molecular weight consistent with the 
ease of mixing is normally preferred. 
The filler content may be inorganic or cellulosic that are in the form of 
fine particulates or short fibres before mixing with the thermoplastics. 
Inorganic reinforcing fillers, i.e., filler particulates/fibres with 
aspect ratios (ratio of length to diameter) greater than four, are 
preferred. The inorganic filler may include mica and talc flakes, short 
glass and carbon fibres, calcium carbonate and clay. The cellulosic filler 
material may be derived from wood/forest by-products and wastes, such as 
saw dust, wood flour or ground wood and ground paper (newspaper and 
cardboard) and agricultural by-products such as ground rice hulls, straws, 
corn husks etc. 
High intensity thermokinetic mixing is used in the process of the present 
invention, discussed further below, in order to disperse and further 
disintegrate filler particulates into tiny fragments. As a result, the 
quality of the fibre composite is not particularly critical and even very 
short fibres may be usefully employed that may be otherwise be considered 
of no commercial value, for example waste sludge from paper recycling and 
fines from pulp mills. Techniques to produce fine, but free flowing, 
reinforcing filler are already well known. A weighed quantity of filler is 
admixed with an appropriate resin and subjected to intensive mixing in a 
thermokinetic mixer, such as a Gelimat (Draiswerke) and K-Mixer 
(Synergistics), or twin screw kneader or extruder. This intensive mixing 
not only separates the loosely bonded fibres/particulates from each other 
but further disintegrates the individual fibre/particulates into a smaller 
size. It is sometimes necessary to employ dispersing/coupling agents in 
order to disperse and compatibilize the non-polar resin with highly polar 
filler components. These surfactants preferentially wet the surface of the 
fibres/particulates (thereby, increasing the degree of dispersion) and 
provide increased adhesion (coupling) between the surface of the 
fibres/particulates and the polymer matrix. It has been found useful to 
employ carboxylated or maleated polyolefins as dispersing agents and/or 
coupling agents in polyolefin polymers. The quantity of 
dispersing/coupling agent required depends upon the surface area of the 
filler component, and is usually 0-5 parts per hundred by weight of the 
filler constituent. The filler concentration may vary from about 0 to 80% 
by weight but the mixing becomes difficult at filler concentration greater 
than 50%. 
It will be appreciated by those skilled in the art that the processes of 
the present invention has been described for a thermoplastic/filler 
composite material would also apply to a thermoplastic polymer alone. 
However, preferably the process will be used with a thermoplastic/filler 
composite material since it can utilize filler.