Continuous extrusion process using organic waste materials

A substantially continuous method for forming a structural panel of indefinite length is disclosed. The method includes the steps of preparing a settable mixture, conveying the mixture in a flowable state to a load chamber, progressively forcing the mixture from the load chamber through an inlet toward an outlet of an open ended mold chamber by a compression device at least partially curing the mixture within the mold chamber, resisting movement of the mixture through the mold chamber until a predetermined consolidation pressure or density within the mold chamber is achieved, subsequently allowing movement of the mixture through the mold chamber in response to further mixture being forced through the inlet by the compression device, and allowing the mixture to cure, thereby progressively forming a structural panel of substantially indefinite length.

FIELD OF THE INVENTION
 The present invention relates to an apparatus and method for forming a
 structural panel of indefinite length.
 BACKGROUND OF THE INVENTION
 This invention has been developed primarily for use in the manufacture of
 pallets and building panels, using rice husk or hull. Accordingly, the
 invention will be described primarily in relation to these applications
 and this material. It will be appreciated, however, that the invention is
 not limited to these particular fields of use.
 One application for structural panels is in the construction of pallets
 which, as is well known, are widely used for the storage, transportation
 and handling of bulk materials. The vast majority are formed from timber
 planks and beams nailed together to define a generally planar support
 surface and longitudinal channels adapted for engagement by forklift
 tynes. A major problem with such panels, however, is that the nails tend
 to work loose over time as a result of timber shrinkage, impact damage,
 general wear and tear, and misuse. Once the nail heads begin to protrude,
 bagged products such as grain and other commodities begin to be subjected
 to unacceptable damage. This renders the pallets effectively unusable,
 without costly maintenance and repair. Timber pallets are also prone to
 splintering, which gives rise to similar problems. A further problem with
 wooden pallets is that in wet environments, the wood can rot, or worse
 still, provide a damp environment in which harmful bacteria can breed.
 In an attempt to overcome these difficulties, particularly in the food
 industry, plastic moulded pallets have been used. It has become apparent,
 however, that such pallets will not withstand the heavy duty cycle to
 which they are inevitably subjected. In an attempt to overcome this
 problem, reinforced plastic pallets have been produced with multiple
 strengthening webs and ribs. However, these are subject to a further
 disadvantage in that the numerous additional recesses and crevices defined
 between and around the strengthening webs are difficult to clean
 effectively, and hence allow fungi and bacteria to thrive. Again, this is
 particularly disadvantageous in the food industry. As a further
 alternative, metal pallets have been produced. However, it has been found
 that in order to provide the necessary structural integrity, the weight
 becomes excessive and/or the cost commercially unviable.
 In view of the above, there remains a long felt need for a pallet with a
 substantially smooth, continuous support surface, which is formed without
 nails, is easy to clean, is repairable, and which has a heavy duty service
 capability.
 Another application for structural panels is in the field of housing
 construction. In conventional building techniques, the walls are normally
 constructed by first erecting a timber frame. The frame is normally clad
 internally with a suitable laminate material such as plasterboard whilst
 the external walls are normally formed from weatherboard, brick veneer, or
 other more contemporary cladding materials such as aluminium sheeting.
 These conventional techniques are relatively labour intensive and costly,
 partly because workers from numerous specialised trades are required.
 These include builders, carpenters, brick layers, joiners, plasterers and
 the like, all of which add to the overall cost. A further disadvantage is
 that these construction techniques consume relatively large quantities of
 valuable and diminishing resources, especially timber, which are
 progressively increasing in cost as supplies become less available. A
 further disadvantage is that conventional housing construction techniques
 require the vast majority of the construction work to be conducted on
 site, by skilled labour. There is little scope for initial prefabrication
 or modular construction and minimal scope for modifying or restructuring a
 house in a cost-effective manner, once built. Moreover, in the event that
 a house needs to be demolished or relocated, there is minimal scope for
 recycling or reusing the constituent materials, which in such
 circumstances are largely wasted.
 All of the above problems are particularly significant in developing
 countries where the availability of skilled labour is limited and the cost
 of conventional building materials is often prohibitive. In an attempt to
 ameliorate some of these problems, various low cost building materials
 have been developed, such as fibre reinforced cement (FRC) sheeting which
 can be applied, relatively easily, to a timber framing structure. However,
 this form of construction gives rise to a "flimsy" subjective impression
 which is often seen as undesirable compared to brick construction, which
 gives the impression of solidity. The insulation properties are also
 minimal, which often necessitates an intermediate layer of insulation in
 the wall cavity, further adding to the overall cost. Moreover, because the
 cladding sheets have minimal structural integrity, the conventional
 framing structure is still required.
 In a further attempt to address these problems, the use of composite panels
 has also been proposed. Panels of this type typically incorporate a series
 of laminates fabricated from different materials designed to achieve the
 desired strength to weight characteristics, as well as to provide thermal
 and acoustic insulation properties. A major problem with known fabrication
 techniques, however, is that there is a practical limit to the maximum
 length of the individual panels. This in turn leads to the requirement for
 smaller panels to be joined end to end to form a combined panel assembly
 of the necessary size. Typically, however, inadequate techniques for
 joining the panels have resulted in such structures being relatively weak.
 The resultant loss of structural integrity has, in turn, resulted in the
 potential strength characteristics not having been realised in larger
 scale applications particularly housing. For this reason, composite panels
 have tended only to be used to form internal partitions and non-load
 bearing walls, where significant structural integrity is not required.
 Accordingly, a separate framing structure is still required and the
 inherent problems associated with conventional building methods have
 remained largely unsolved.
 In view of the above factors, there remains a long felt need for a
 cost-effective alternative to existing housing construction techniques,
 which would provide an alternative to the diminishing supplies of raw
 materials, require less use of skilled labour particularly on site, would
 lend itself to prefabrication, and which would be modular to some extent
 so as to allow the flexibility for structures to be built, altered or
 moved as required at minimal additional cost.
 The present invention relates to the manufacture of structural panels and
 associated products using waste organic material, and in particular rice
 hull. Each year in Australia and around the world, the processing of rice
 for human consumption involves the removal of millions of tonnes of rice
 husks or "hulls". These hulls are particularly difficult to dispose of,
 because, being formed substantially of lignin, they are essentially
 waterproof and resistant to biodegradation. Furthermore, the relatively
 high silicic acid content limits their use as cattle fodder. Even rodents
 and insects will not consume them.
 One means of disposal is to incinerate the hulls at high temperature, which
 substantially reduces their volume. However, this process is relatively
 energy inefficient, expensive, and has undesirable effects on the
 environment, particularly in terms of atmospheric pollution. Due to the
 difficulties of disposal, some attempts have also been made to use waste
 rice hulls and similar organic waste materials constructively. They have,
 for example, been used as insulation in building cavities and other
 applications. However, this use has hitherto been severely limited due to
 the lack of structural integrity of the raw material. In an attempt to
 address this problem, rice hulls have also been mixed with various binding
 agents. For example, it has been known, although not commonly, to employ a
 binding agent curable by RF (radio frequency) radiation in order to
 consolidate the individual rice hulls. However, this has also been found
 to be an expensive process which is not generally cost effective on a
 commercial scale. Moreover, to date, the products from this and other
 techniques have been excessively weak and prone to crumbling. More
 particularly, they have lacked the requisite structural integrity for use
 as a self-supporting structural material, which has substantially limited
 their applicability in many potential fields of use, including the
 building industry.
 In view of the above, it will be appreciated that a process capable of
 forming a structural product using waste rice hull would address the
 significant problem of waste disposal, whilst at the same time providing a
 useful and commercially viable alternative to the diminishing supplies of
 raw building materials, particularly timber. To date, however, no such
 process or product has been found.
 It is an object of the present invention to overcome or substantially
 ameliorate at least some of the disadvantages of the prior art.
 SUMMARY OF THE INVENTION
 Accordingly, in a first aspect, the invention provides a substantially
 continuous method for forming a structural panel of indefinite length,
 said method comprising the steps of preparing a settable mixture including
 an organic particulate base material consisting at least predominantly of
 rice hull and a binder, conveying the mixture in a flowable state to a
 load chamber, progressively forcing the mixture from the load chamber
 through an inlet toward an outlet of an open ended mould chamber of
 substantially constant cross-sectional dimensions by compression means, at
 least partially curing the mixture within the mould chamber, resisting
 movement of the mixture through the mould chamber until a predetermined
 consolidation pressure or density within the mould chamber is achieved,
 subsequently allowing movement of the mixture through the mould chamber in
 response to further mixture being first forced through the inlet by the
 compression means, allowing the mixture to cure, and hereby progressively
 forming a structural panel of substantially indefinite length.
 The base material may further include wheat husk, saw dust or other
 suitable organic or waste products.
 In one preferred embodiment the binder is a thermosetting resin binder.
 More preferably, the binder consists substantially of a thermosetting
 phenolic resin. The curing or setting process is preferably initiated by
 heating the mixture within the mould chamber.
 Alternatively, the binding medium may include materials such as
 cementitious binders, to provide the characteristics of masonry. Whilst a
 thermosetting binder is preferred, it will be appreciated that two part
 chemical setting resins could be used. Binders curable by ultraviolet
 radiation or other stimulants are also envisaged.
 In other embodiments, additives such as polyvinylacetate or fortified urea
 formaldehyde may form part of the binding agent for specific purposes such
 as to provide antibacterial properties. Other agents may also be added to
 the binding agent to modify the fire, water, weather or rodent resistance
 of the cured products. For example, it has been found that the use of a
 magnesite binder substantially improves the fire retardant characteristics
 of the resultant product.
 Preferably, the mixture is formed from a combination of shredded rice hull
 mixed with a phenolic resin binder in a ratio of approximately 5 to 1 by
 mass, although different ratios may be used depending upon the desired
 characteristics of the final product. Desirably, the phenolic resin is
 potassium hydroxide based with up to around 60% by weight of solids. In
 another form, a sodium hydroxide based phenolic resin may be used,
 incorporating up to around 40% by weight of solids. Alternatively, a
 powdered phenolic resin consisting of 100% solids may be used.
 The curing temperature is preferably between 50.degree. and around
 200.degree. C. and is ideally around 160.degree. C. The curing pressure
 within the mould cavity is preferably between 2 and around 80 pounds per
 square inch, depending upon the desired characteristics of the finished
 product. More preferably, the curing pressure is between 15 and around 50
 pounds per square inch, and most preferably around 40 pounds per square
 inch.
 In one preferred embodiment, the step of resisting movement of the setting
 mixture is achieved by means of mutually opposed pressure plates engaging
 opposite sides of the panel through appropriately spaced apertures in the
 mould. The pressure plates in one preferred embodiment are actuated by
 respective hydraulic rams in response to a control signal indicative of
 the pressure or density of the mixture within the mould chamber. In other
 embodiments, however, it has been found that the frictional resistance
 between the setting mixture and the surrounding surface of the mould
 chamber inherently provides the required degree of resistance, so that
 this step is performed automatically, without the need for independent
 external pressure application means. This is, however, subject to
 selection of appropriate materials, mould shape, compression force, curing
 rate and other relevant parameters.
 In a particularly preferred embodiment, a self-centring mandrel is disposed
 to extend axially through the mould chamber such that the moulded panel is
 formed with a corresponding longitudinally extending hollow channel. At
 least two such mandrels are preferably disposed in parallel side by side
 relationship. The mandrels are preferably heated to allow relatively
 uniform curing of the panel from within.
 According to a second aspect, the invention provides an apparatus for
 forming a substantially continuous structural panel of indefinite length,
 said apparatus comprising mixing means to form a settable mixture,
 transfer means to convey the mixture in a flowable state to a load
 chamber, compression means adapted progressively to force the mixture from
 the load chamber through an inlet toward an outlet of an open ended mould
 chamber of substantially constant cross-sectional dimensions, resisting
 means adapted initially to resist movement of the mixture through the
 mould chamber until a predetermined consolidation pressure or density
 within the mould chamber is achieved and adapted subsequently to allow
 movement of the mixture through the mould chamber in response to further
 mixture being forced through the inlet by the compression means, thereby
 progressively to form a structural panel of substantially indefinite
 length.
 Preferably, the apparatus further includes heating means to initiate curing
 of the mixture within the mould chamber. Preferably, the compression means
 take the form of at least one hydraulic ram connected to a piston adapted
 upon reciprocation to force the mixture from the load chamber to the inlet
 end of the mould. Preferably, the resisting means include mutually opposed
 pressure plates engaging opposite sides of the panel. The pressure plates
 are preferably actuated by respective hydraulic rams.
 In the preferred embodiment, the apparatus further includes sensor means
 adapted to generate a control signal indicative of pressure or density
 within the mould chamber and control means adapted to regulate the
 pressure plates in response to the control signal.
 The apparatus preferably also includes cutting means in the form of flying,
 shears adapted to cut the panel into discrete sections of preselected
 length.

PREFERRED EMBODIMENT OF THE INVENTION
 Referring firstly to FIG. 1, the invention provides an apparatus 1 for
 forming a substantially continuous structural panel 2 of indefinite
 length. The apparatus includes mixing means in the form of a mixing drum 3
 supported for rotation about a generally horizontal axis 4 by an external
 drive mechanism (not shown). Mixture from the drum, 3 is fed via a
 transfer chute 5 to a load chamber 6. In the mixing drum, rice hulls and
 other suitable materials are mixed with a thermosetting resin binder to
 form a flowable mixture, as described more fully below.
 The apparatus further includes compression means in the form of an
 hydraulic ram assembly 10 including a piston 11 adapted to reciprocate
 through the load chamber 6. In doing so, the ram 10 forces mixture
 progressively from the load chamber through an inlet 15 toward an outlet
 16 of an open ended mould chamber 17.
 The outer wall 18 of the mould chamber 17 defines a generally rectangular
 mould cavity of substantially constant cross sectional profile, which
 corresponds to the external profile of the moulded panel 2 (see FIG. 2).
 The mould further includes a pair of self-centring mandrels 20 which
 extend axially at least part way along the mould cavity. The mandrels thus
 define respective axially extending channels 21 which extend through the
 panel, as best seen in FIG. 2. The outer walls 18 and the mandrels 20
 defining the mould chamber are preferably heated to a temperature of
 around 160.degree. C., so as to initiate relatively uniform curing of the
 thermosetting resin, within the mould.
 The upper and lower outer walls of the mould are formed with respective
 apertures 25 to accommodate pressure plates 26. The pressure plates are
 connected respectively to a pair of mutually opposed hydraulic actuators
 27. These actuators act in unison such that extension of the actuators
 urges the pressure plates toward one another, thereby progressively
 resisting movement of the panel through the mould chamber. Conversely,
 retraction of the actuators progressively moves the pressure plates apart
 and thereby reduces the resistance to movement of the panel through the
 mould. As an alternative to apertures 25, the entire mould cavity may be
 divided into two longitudinally spaced halves, with the pressure plates
 positioned in between. In a further alternative, the pressure plates may
 be disposed beyond the outlet end 16 of the mould cavity. However, the
 intermediate location is preferred since in that position, the remote ends
 of the floating mandrels 20, which extend only part way along the mould
 cavity, are sandwiched between the pressure plates to provide internal
 support for the panel, which may be only partially cured at that point.
 A pressure sensor 30 is positioned to provide a control signal indicative
 of pressure or density of the mixture within the cavity. This control
 signal is used to regulate the actuators 27 and hence the pressure plates
 26 in unison such that movement of the partially cured mixture through the
 mould, is only permitted when a predetermined consolidation pressure or
 density is achieved. Thus, by repeated compression strokes of the ram 10,
 a predetermined density of the mixture is achieved at the sensor 30
 whereupon the process controller activates the actuators 27 to cause a
 momentary reduction of pressure on the plates 26. This allows the
 compressed mixture to advance a limited distance through the mould
 chamber. When a reduction in density or pressure below a predetermined
 threshold level is registered at the sensor 30, the process controller
 reapplies pressure to the plates 26 via the actuators 27 to brake or
 retard further progress of the partially cured panel through the mould. In
 this way, the downstream portion of the panel itself forms a choke or end
 stop for the next compression stroke of the ram 10, as it forces fresh
 mixture into the inlet end of the mould chamber. This mechanism ensures
 that the finished product has the required characteristics in terms of
 strength, density and structural integrity.
 It has been found that in some particular configurations of the invention,
 the frictional resistance between the setting mixture and the surrounding
 surface of the mould chamber inherently provides the required level of
 restraining force, so that the braking function is effectively performed
 automatically, without the need for an independent pressure application
 mechanism. This is, however, subject to the particular materials in the
 mixture, the mould shape, the degree of compression, the curing rate of
 the binder and other relevant parameters.
 The outer walls of the mould and the floating mandrels are heated to a
 temperature of approximately 160.degree. C. to initiate the curing of the
 thermosetting resins in the mixture within the mould. The temperature, the
 compression rate and other parameters are selected such that the curing
 cycle is substantially completed by the time the panel reaches the outlet
 end of the mould.
 Beyond the outlet 16 of the mould, a set of travelling or "flying" shears
 40 are disposed to cut the panel into sections of predetermined length.
 The shears are mounted on a carrier with an optical or other suitable
 tracking mechanism which enables the shears to form a straight cut
 perpendicular to the longitudinal axis of the panel, notwithstanding the
 progressive and possibly intermittent movement of the panel through the
 mould cavity. Flying shears of this nature are well known to those skilled
 in the art, and so need not be described in further detail.
 The mixture itself is preferably formed predominantly from shredded or
 ground rice hull and a phenolic resin, in a ratio of approximately 5 to 1
 by mass, although different ratios may be used depending upon the desired
 characteristics of the final product. The preferred phenolic resin is
 potassium hydroxide based with up to around 60% by weight of solids.
 Alternatively, a sodium hydroxide based phenolic resin may be used,
 incorporating up to about 40% by weight of solids. As a further
 alternative, a powdered phenolic resin consisting of 100% solids may be
 used. Advantageously, such phenolic resin binders contribute antibacterial
 properties to the finished product, which is of particular benefit in the
 food industry. It should be appreciated, however, that other organic waste
 materials such as sawdust, wheat husk, and the like may be used either as
 an alternative to or in combination with rice hull. Furthermore, fillers
 and extenders such as rubber crumb, sisal, jute fibres, veld grass,
 coconut husk and the like may also be incorporated to provide the desired
 strength, weight, impact resistance and other properties. It is also noted
 that other binders may be used. For example, a magnesite binder can be
 employed to provide additional fire retardant characteristics, whilst
 cementitious binders may be used to provide masonry-like qualities.
 Further additives such as polyvinyl acetate or fortified urea formaldehyde
 may also form part of the binding agent for specific purposes, for example
 to modify the fire, water, weather or rodent resistance of the cured
 products.
 FIG. 2 shows a panel section 45 formed using the apparatus of FIG. 1 with
 dimensions suitable for use as an extruded cavity wall panel, as shown in
 FIG. 3. Such panels are waterproof due to the inherent water resistant
 characteristics of the rice hull, and are also resistant to rodents and
 other pests. The product has sufficient structural integrity to obviate
 the need for an internal framing structure. Moreover, the thermal and
 acoustic insulation characteristics are excellent, due in part to the
 cavities running longitudinally through each panel, thereby obviating the
 need for any additional insulation materials. Thus, the invention provides
 a modular structural building panel which requires no internal or external
 framing other than horizontal and vertical tie rods 46 and 47 (as shown in
 FIG. 3) which simply connect the panels together. The roof may be formed
 by further panels and tiled if necessary. The panels may be cut to the
 required height on site, or alternatively, may be supplied pre-cut and
 simply assembled using non-skilled labour. In this way, the invention
 provides the basis for an entirely new construction technique for low cost
 housing which is particularly applicable in developing countries where
 material resources and skilled labour are both scarce. An example of a low
 cost housing project in accordance with the invention is shown in FIG. 4
 where three dwellings 50 are connected in a terraced configuration. It
 will be appreciated, however, that any number of dwellings may be provided
 in any desired configuration depending upon the availability of space,
 cost constraints and other factors. Doorways may be formed simply by the
 omission of a panel in the desired location, whereas window openings may
 be formed by cutting openings where required.
 Referring to FIG. 5, in order to further enhance the tensile strength of
 the panel post tensioning wires 48 may be preheated to 180.degree. C. and
 then fed into the load chamber and positioned in the mix at locations
 corresponding to points A, B, C and D in the formed panel. Preferably, the
 post tensioning wires are deformed in profile so as to allow positive
 lodgement points for the mixture to adhere to.
 As the preheated post tensioning wires are entrapped in the high density
 cured mix, when the mixture cures and cools the wires (having a
 significantly greater co-efficient of expansion) will contract
 considerably more than the relatively inert mixture, thereby exerting a
 post tensioning action on the cured panel. This in turn greatly increases
 the structural integrity of the panel, enabling it to be used as a
 load-bearing flooring section, particularly in high rise and multi-storey
 buildings.
 The panel of FIG. 2 may also be formed in an appropriate size, ideally 1170
 mm.times.1170 mm, to form a pallet. Advantageously, pallets formed in this
 way have a smooth upper surface so as not to damage the products being
 transported. Moreover, since no nails are required for assembly, there is
 no possibility of these working lose over time so as to damage bagged
 products or other commodities. The pallet design incorporates a minimum
 number of crevices and other inexcessible spaces where bacteria might bred
 and is relatively easy to clean. Because of the inherent properties of the
 rice hull, the pallet is also water resistant and the resin binders
 provide additional antibacterial properties. It has further been found
 that the pallet can tolerate a heavy duty service environment with minimal
 damage.
 Perhaps most significantly, however, both the building panel and the pallet
 are formed substantially from materials which would otherwise simply be
 disposed of as waste at considerably less cost. For this reason, the panel
 is particularly inexpensive to produce.
 It should also be appreciated that the invention lends itself to numerous
 other applications. For example, with appropriate mould shapes, the method
 and apparatus of the invention are readily adapted to form water or
 sewerage pipes, of indefinite length, thereby minimising the cost of
 installation. Other products include mine props, road side marker posts,
 structural building blocks, and virtually any other product having a
 substantially constant cross sectional profile in at least one direction.
 In fact, even this limitation is not fundamental, since the panel or
 product can be milled, machined or otherwise re-shaped after the initial
 moulding process to produce a structural element of virtually any desired
 shape, primarily from a waste material. In all these respects, the
 invention confers numerous practical and commercially significant
 advantages over the prior art
 Although the invention has been described with reference to specific
 examples, it will be appreciated by those skilled in the art that the
 invention may be embodied in many other forms.