Title:
Cellulose-based end-grain core material and composites
Kind Code:
A1


Abstract:
A composite structural product is provided which comprises light weight cores made from processed kenaf, balsa or other cellulosic stalks bonded together. These may be conveniently manufactured into core blocks, and later sized and joined as the core member of a composite structure, e.g. laminates, fiberglass walls, or plastic encased products. The other members of the composite could include many different materials, from metals to woods to synthetic polymers such as thermoplastic and thermosetting resins. In one form, the kenaf or balsa stalks are formed into small strips or grindings for use as a light core weight filler in molding processes. If combined with yet other strong materials like bamboo in the core, a light-weight, stronger, and cheaper core can be achieved for, e.g., “plastic wood” applications. Thus, an improved core and composite, and process for making them, is provided that has the ability to overcome the disadvantages of the presently available structural materials.



Inventors:
Ryan, Dale B. (Mitchel, SD, US)
Application Number:
10/246769
Publication Date:
03/13/2003
Filing Date:
09/19/2002
Assignee:
RYAN DALE B.
Primary Class:
International Classes:
B27N3/04; B27N5/00; B29C45/14; B29C47/02; B29C70/48; B32B21/04; D04H1/58; (IPC1-7): D04H1/00
View Patent Images:
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Primary Examiner:
TORRES VELAZQUEZ, NORCA LIZ
Attorney, Agent or Firm:
HOLLAND & KNIGHT LLP (WASHINGTON, DC, US)
Claims:

We claim:



1. A composite comprising a core of plural cellulosic stalks.

2. The composite of claim 1, wherein the core of plural cellulosic stalks comprises bonded stalks of one of the group of kenaf or balsa.

3. The composite of claim 2, wherein the plural cellulosic stalks comprise shaped stalks.

4. The composite of claim 2, wherein the bonded stalks further comprise compressed stalks having a density of less than 20 pounds per cubic foot.

5. The composite of claim 1, wherein the composite further comprises one of the group of a laminate further comprising a first sheet adhered to the core, a fiberglass structure further comprising a shaped structural member and a fiberglass member on opposing sides of the core, and a plastic structural member comprising plastic surrounding the core.

6. The composite of claim 1, wherein the plastic structural member comprises an interior element comprising a plurality of bamboo tape elements and the core and is formed in the shape of one of the group consisting of beams, columns, poles and dimensional lumber.

7. The composite of claim 1, wherein the plural cellulosic stalks comprise compressed stalks bonded with a binder, the binder comprising at least one member selected from the group consisting of maleated polypropylene, maleated polyethylene, maleic anyhdride, hydroxyl methacrylate, silane compounds, N-vinyl pyridine, Nvinyl caprolactam, N-vinyl carbazole, methacrylic acid, ethyl methacrylate, isobutyl methacrylate, sodium styrene sulfonate, bis-vinyl phosphate, divinyl ether-ethylene glycol, vinyl acetate, vinyl toluene, vinylidene chloride, chloroprene, isoprene, dimethylaminoethyl methacrylate, isocetylvinyl ether, acrylonitrile, glycidyl methacrylate, N-vinyl pyrrolidone, acrylic acid, ethyl acrylate, itaconic acid, methyl acrylate, sodium vinyl sulfonate, cetyl vinyl ether, divinyl ether-butanediol, octadecyl vinyl acetate, and a latex glue.

8. A product comprising a core block of plural cellulosic stalks.

9. The product of claim 8, wherein the core comprises stalks of one of the group of kenaf or balsa bonded with a binder, the binder comprising at least one member selected from the group consisting of maleated polypropylene, maleated polyethylene, maleic anyhdride, hydroxyl methacrylate, silane compounds, N-vinyl pyridine, Nvinyl caprolactam, N-vinyl carbazole, methacrylic acid, ethyl methacrylate, isobutyl methacrylate, sodium styrene sulfonate, bis-vinyl phosphate, divinyl ether-ethylene glycol, vinyl acetate, vinyl toluene, vinylidene chloride, chloroprene, isoprene, dimethylaminoethyl methacrylate, isocetylvinyl ether, acrylonitrile, glycidyl methacrylate, N-vinyl pyrrolidone, acrylic acid, ethyl acrylate, itaconic acid, methyl acrylate, sodium vinyl sulfonate, cetyl vinyl ether, divinyl ether-butanediol, and octadecyl vinyl acetate, and a latex glue.

10. A method of forming a composite product comprising: providing a mold defining a shape for the product; positioning a cellulosic stalk core into the mold so as to allow a desired thickness of plastic to be injected around the core; injecting heated liquid plastic into the mold to said a desired thickness; allowing the plastic to cool and to solidify; and removing the product from the mold.

11. The method of claim 10, wherein the step of positioning the core further comprises positioning along with the core a bamboo element coated with a binding agent comprising one of the group of maleated polypropylene, maleated polyethylene, maleic anyhdride, hydroxyl methacrylate, N-vinyl pyridine, N-vinyl caprolactam, N-vinyl carbaxole, methacrylic acid, ethyl methacrylate, isobutyl methacrylate, sodium styrene sulfonate, bis-vinyl phosphate, divinyl ether-ethylene glycol, vinyl acetate, vinyl toluene, vinylidene chloride, chloroprene, isoprene, dimethylaminoethyl methacrylate, isocetylvinyl ether, acrylonitrile, glycidyl methoacrylate, N-vinyl pyrrolidone, acrylic acid, ethyl acrylate, itaconic acid, methyl acrylate, sodium vinyl sulfonate, cetyl vinyl ether, divinyl ether-butanediol, octadecyl vinyl acetate, latex glue, acrylic acid, maleic anhydride, or salt or ester derivatives thereof.

12. A method for forming a core for use in composites, comprising: a. coating plural cellulosic stalks with a binder; b. forcing said stalks together allowing the binder to adhere said stalks into a bound core block.

13. The method of claim 12, further comprising: c. cutting said core block into plural shaped core sheets.

14. The method of claim 12, wherein step b further comprises compressing said stalks to form a denser bundle of stalks while allowing the binder to adhere.

15. The method of claim 12, wherein step a is preceded by shaping said stalks into a desired form for use in forming said bound core block.

Description:

RELATED APPLICATIONS

[0001] This application is related (a) as a continuation in part to U.S. application Ser. No. 09/960,204, filed on Mar. 2, 1999, which is a continuation-in-part of application Ser. No. 08/908,589, now U.S. Pat. No. 5,876,649, which is a continuation-in-part of application Ser. No. 08/696,484 filed Aug. 14, 1996; and (b) as a non-provisional application continuing in part from (i) U.S. provisional application no. 60,323,290, filed on Sep. 19, 2001, and (ii) U.S. provisional application no. 60,330,214, filed on Oct. 18, 2001, all of which applications are fully incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

[0002] The invention in general relates to the field of composite materials, and more particularly to load carrying, ornamental and other structural materials with cellulosic cores, and methods to make such cores and composites.

BACKGROUND

[0003] The composites industry is growing as the demand increases for plastic products, light weight materials and wood alternatives. The automotive arena is a prime example as plastic parts continue to replace those formerly made of metal or other heavier materials.

[0004] However, the high cost of core material in the composites industry remains a significant barrier to expansion. Many people within the composites industry believe that the availability of a lower-priced core material would lead to an even bigger expansion of the field. The market for carbon-fiber products expands exponentially whenever the cost of the fiber drops. By the same token, other products that might have been considered for composite construction have been forced out when there is too high a cost for the component raw materials. Those same product could, with a reasonably priced core material, be very cost competitive. Thus, a low cost core material could play a key role in expanding the whole composite plastics industry.

[0005] Balsa wood has emerged as the core of choice in this type of fabrication, since virtually all of the “wood-type” cores are much too heavy to allow their use as core material. The drawback to balsa, though, is its expense. This occurs because balsa has a long and difficult growing cycle, higher transportation costs since it only grows in the tropics, and a limited number of suppliers controlling the market.

[0006] Thus, when from time to time people have considered composite projects designed to further replace metal, concrete, or wood, especially on large projects such as bridge decks, many fail to get off the drawing boards due to the high cost for core materials in those large volumes. Further, it is doubtful that in the near future we will find lower prices for other composite materials such as resins, or innovative low cost reinforcing fibers (to replace glass, Kevlar, or carbon). Thus, the lack of a less expensive alternative to balsa cores is a significant problem for growing the composites industry.

[0007] One of the largest potential markets is still out of reach of composites due to the high price of materials and that is highway bridges. Ten years ago the Federal Department of Transportation proposed an all-plastic bridge structure and cited benefits including minimum (if any), maintenance that would be required once the bridge is installed. Current steel/concrete structures require maintenance within three years of installation and need replacement much sooner than was originally thought. The cost to the Federal Bridge Inventory is enormous and today over 260,000 U.S. bridges are in need of repair or replacement. Several composite bridge demonstration programs have come and gone, victims of the cost analysis, there is currently a 300 to 500 percent price premium over an equal steel and concrete span. An inexpensive structural core could dramatically change this formula and, in sufficient quantity, the core cost reduction could make a composite highway bridge quite competitive. Further, a lightweight load-carrying beam, column or cross-tie that would not be sensitive to seismic or temperature changes (as is the case with concrete) would be a very desirable replacement for concrete.

[0008] That said, there are still many composite products, particularly those based on sandwich-core construction. Examples include pleasure boats, yachts, commercial ships, aircraft, tubs and showers, industrial doors and rail-car doors. Naval vessels are now being produced utilizing “Stealth” technology, a technique provided by the radar transparency of certain composites. Kit-planes, private aircraft, commercial airplanes, and military fighters/bombers all utilize light-weight sandwich core construction. Over 3 million boats have been cored with balsa, and Boeing has for many years used balsa cores, clad on both sides with aluminum, for airplane passenger and cargo floors. Elevators, railroad cars, bathtubs/shower stalls, yacht and airplane furniture, and large industrial doors are further examples of products with sandwich cores. While there is demand for composites in all these applications, all would significantly benefit from a lower-cost alternative to today's current core technologies.

[0009] Another advantage of a bigger composite industry could come in the reduction in the amount of forests that need to be chopped down to supply man's needs. One particular application that has been traditionally filled by wood products is the construction of cross beams and pallets. A typical wood pallet is approximately 40 inches by 48 inches by 5 inches and comprises a plurality of top slats and bottom slats supported on edge oriented 2×4″ timbers. The market for such pallets is several million each year. While this market is a substantial drain on the timber industry, such wood pallets are not a preferred pallet for the food industry. In the food industry, contamination is a problem and efforts have been made to create a sanitizable pallet for re-use. Various efforts have been made to create a plastic pallet but such efforts have been largely unsuccessful for at least two reasons. A first reason is that plastic, as its name implies, will deform in response to load and therefore creates a failure condition when loaded pallets are mounted on edge racks in warehouse storage. A second problem is that plastic is substantially more expensive than wood raising pallet costs by several multiples. Accordingly, it would also be advantageous to provide a further structural substitute or supplement for wood and plastic in the pallet industry.

[0010] Just such a solution to the problems noted above and more, is made possible by my invention.

SUMMARY

[0011] An illustrative summary of my invention, with particular reference to the detailed embodiment described below, includes an composite structural piece is provided which comprises light weight cores made from processed kenaf, balsa or other cellulosic stalks and branches bonded together. These may be conveniently manufactured into core blocks, and later sized and joined as the core member of a composite structure. The other members of the composite could include many different materials, from metals to woods to synthetic polymers such as thermoplastic and thermosetting resins. In one form, the kenaf or balsa stalks are formed into small strips or grindings for use as a light core weight filler in molding processes. If combined with yet other strong materials like bamboo in the core, a light-weight, stronger, and cheaper core can be achieved in some applications. Thus, an improved core and composite, and process for making them, is provided that has the ability to overcome the disadvantages of the presently available structural materials.

THE DRAWING

[0012] The invention may be more readily appreciated from the following detailed description, when read in conjunction with the accompanying drawings, in which:

[0013] FIGS. 1a and 1b illustrate different perspective views of kenaf core blocks and sized cores according to an embodiment of the invention;

[0014] FIGS. 2a and 2b illustrate cross-sectional views of alternative kenaf cores, with FIG. 2a showing a core made with shaped kenaf stalks and FIG. 2b showing a loosely joined core;

[0015] FIGS. 3a through 4c illustrate examples of applications using a kenaf core, with FIG. 3a showing a perspective view of a laminate structure, FIG. 3b showing is a cross-sectional view of a scored core joined to a curved structure, FIG. 4a showing a perspective view of a beam containing a synthetic polymer outer layer and an interior comprising bamboo fiber and a cellulosic prepared according to the molding method described below; FIG. 4b is an elevational view of a rod or column made according to a preferred embodiment of the invention; and FIG. 4c is a perspective view illustrating a coating die for producing the composite of FIG. 4a.

[0016] FIG. 5 illustrates a side elevational view of a machine for making core blocks according to a preferred embodiment of the invention;

[0017] FIGS. 6a through 6c illustrates a side elevational view of machines for making extruded or molded composite according to a preferred embodiment of the invention, with FIG. 6a showing an extrusion machine having a circular die, FIG. 6b showing a rectangular die capable of use as a replacement for the circular die in the extrusion machine of FIG. 6a, and FIG. 6c showing a cooling bath and a molding apparatus used according to this invention;

[0018] FIG. 7 illustrates an exploded view of one form of structural pallet using at least some of the teachings of the present invention;

[0019] FIG. 8 illustrates a mold for producing the pallet of FIG. 7.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT OF THE INVENTION

[0020] A solution to the problems described above is provided by a new core and processes associated with this core according to my invention. This invention is illustrated below in connection with several presently preferred embodiments.

[0021] One particularly useful embodiment is found in the application of bound kenaf as the core material for a composite. This has come as something of a surprise, as kenaf is a well-known plant that has been used for literally thousands of years. Kenaf was first used to make ropes and bags and braided materials. More recently, interest has focused around the outer “bast” fiber. Some attempts have been made to use this bast fiber in the paper industry, although with little success against the predominate wood pulp, as well as in side markets for auto components, cordage, and as a replacement for items like burlap. However, very little attention has been paid to the balsa-like inner core, with some of the few exceptions being use for carved toys in Thailand, kitty litter, particle board particulate, and recently as ground up core for filling fabric tubes—used to contain oil spills by soaking up the liquid. In most of these cases the inner core has been destroyed, a typical by-product of many of the processes used to obtain the outer core. This latter reason, coupled with the common view that the inner core is a waste product, may well be why products requiring long undamaged kenaf sticks have not been forthcoming.

[0022] Until now the composites industry has substantially relied upon end-grain balsa wood cores as the material of choice. This is largely due to balsa's low weight and strength, as balsa averages 9 pcf (pounds per cubic foot) density whereas other woods are 30 pcf and up. However, balsa has two big disadvantages that limit a wider-spread use—its high price and limited tropical growth zone. In production it is also wasteful—only the trunk is used in forming the rectangular boards that are later joined into blocks and sawed into balsa cores—and the end product can have an uneven density across the core.

[0023] Kenaf has none of these disadvantages. Like balsa, kenaf averages 7 to 9 pcf density. Unlike balsa, it is a fraction of the cost and can be grown in many different countries—including the U.S. As the inventor has discovered, kenaf can also be processed to create cores of substantially uniform density but across a wider range and grade, from under 7 to over 20 pcf It can be harvested quickly, and its production is not wasteful like that of balsa since smaller stalks are used. In fact, in an alternative embodiment the inventor has also found that the smaller diameter balsa waste can also be salvaged and made into cores using the teachings of the invention. In both cases the resulting core appears to be stronger than prior balsa cores in compression and core-shear, apparently due to the smaller diameter stalks. Core-shear is an important test used for evaluating the dynamic behavior of a composite sandwich cored structure, and the load-carrying ability in compression is used in determining the overall strength of the sandwich-cored part.

[0024] In a preferred embodiment, a group of stalks are pressed and bonded together to form a very strong “end-grained” block. This is illustrated in FIGS. 1a and 1b, in which stalks 12,14 are bonded together and compressed to form a block of stalks 10. When later trimmed and sliced horizontally, an end-grain core 15 is formed. This is particularly useful, as end-grain core materials are a proven form of composite construction due to the substantial load-carrying ability that cellulose fibers exhibit when standing on end—or put another way, when the fiber direction coincides with the load path.

[0025] It may be noted that kenaf inner fiber is the only other cellulosic besides balsa trunks available in the preferred weight range of from seven to nine pounds per cubic foot density. At this core weight a composite part can be over thirty five times stronger and stiffer than non-composites but with only as little as 3% or less addition to overall product weight. If, e.g., an engineer needs to equal this strength and stiffness with solid fiberglass laminate, the cost and weight would soar dramatically.

[0026] In making a kenaf core, if smaller diameter sticks (approximately ⅜″ to 2″ diameters) are used for the compressed core, that will result in varying densities from around 9 to 20 pcf. These can be made in convenient, e.g., one pound, increments. Further, kenaf can be grown with a plant base diameter up to 5″. At these sizes it becomes feasible to machine shape the core, e.g., to a square or hexagonal cross-section (see cores 22 in FIG. 3a). This can be advantageous, in that it permits shaped stalks to be glued together into a block with little or no press tonnage. Since it is the compression of the stalks (which is necessary to remove gaps or inclusions in unshaped stalks) that leads to higher densities, this feature allows the blocks with shaped stalks to achieve premium-grade densities of less than 10 pcf (e.g., at increments between 7 to 9 pcf). In this way one can now achieve a full range of product densities that is easily gradable with repeatable performance and costs.

[0027] An additional advantages of making cores this way is that as the small sticks become coated with the binder/adhesive and pressed into blocks, they produce a core with little chance for water migration from side to side. They do not have the capillary problems that balsa does, which means kenaf does not have to be sealed against wicking—a problem in lamination or fiber glass processes where the resin is soaked away too rapidly. On the other hand, there is a tiny “pith” center hole formed within the stalk that can be advantageous when this core is used with thermoplastic matrices, allowing the plastic to flow through the pith. Further, the small diameter columnar arrangement can carry more compressive loads for a given core density and more shear strength. Kenaf itself has more advantages, in that its annual growing season (plant in May, harvest in September) lends itself to easier manipulation than a tree product. Some might consider it an advantage that this plant can be described as Tree-Free. At a very low cost-to-produce, this core can now be used in many lower-cost building-products such as lightweight plywood, insulating panels, and even thin architectural facings (e.g., ⅛″ stone) could be bonded to it.

[0028] During the past ten to fifteen years a typical kenaf processing technique has emerged that fits well with the current uses for the fiber. The plants are grown to their maximum height (typically 12 to 18 feet), and left to dry on the stalks uncut. What does not work well is the typical approach of next harvesting the dried stalks with a foliage chopper, cutting the stalks into short lengths (<1 inch to 4 inches), and pass these through a machine that removes the outer fibers by pulverizing the inner core. Rather, it is important in most applications for the inner core to remain undamaged. Machines used in other applications such as cutting corn stalks and the sugar cane industry may well be useful, and a preferred machine would be field portable so that this process could take place where the kenaf is grown. Other techniques, such as retting used in Thailand to soak kenaf to allow the outer bast to be loosened, may be useful in removing the bast. In any event, it is preferred to remove the bast (or barks in the case of bamboo sticks) so the inner core is processed before pressing. Of course, if a bigger kenaf stalk is used, the process of shaping/routing the stalk may be sufficient to remove the outer bast.

[0029] The dimensions of a kenaf core block is a matter of design choice, and could be as long as the stalk lengths (12 to 18 feet). In most applications a shorter length is sufficient (preferably 18″ or less, but under 6″ tends to be less practical), whatever is convenient for the binding process, as it is anticipated most end cores 15 will be sawn to shorter thicknesses, down to fractions of an inch. The width will vary by application, only limited by the size of the press (e.g., producing cores of 2′ by 4′ slabs or more). It should also be noted that kenaf tends to evenly taper from bottom to top, so it is preferred to lay kenaf stalks alternating in opposite directions so the tapers of adjacent stalks match, and the finished core is more approximately right-angled/rectangular in shape.

[0030] While kenaf inner-core fiber is a preferred material for forming the above described composite construction, applicant believes that other types of stalks or plant limbs could also be used in this construction. In particular, it is believe that the composite construction could be formed by harvesting younger balsa wood trees at an early stage (when they are still stalk-sized at about 5″ or less). Left-over branches could also be used. All these smaller-dimensioned or stalk-sized cellulosics are collectively referred to herein as “cellulosic stalks” (or separately as, e.g., “balsa stalks”) by which is meant stalks, branches or trunks of about 5″ or less in diameter from cellulosics like balsa and kenaf that have compressed core densities of about 20 pcf or less or uncompressed core densities of about 20 pcf or less (more preferably 13 pcf or less). In processing a cellulosic stalk, one could strip bark by any convenient means, and size the length as convenient for the binding process. One advantage of this construction with balsa is that it allows the use of the balsa wood trees in an early stage, rather than having to wait several years for the balsa wood to mature to diameters sufficient for the typical method of producing (uncompressed) balsa core blocks.

[0031] Turning now to FIGS. 3a through 4b, several types of composite end-products are shown illustrating the range of applications to which the kenaf core can be put. In FIG. 3a, a laminate 30 is shown, such as is found in plywood, furniture, facing, and other constructions. The kenaf core 32 is there sandwiched between two materials 31 (e.g., wood, metal, etc.), and in most applications is preferably adhesively joined to the other sheets. Note that while the kenaf stalks were joined by an adhesive when making a core, this is primarily to keep the core in tact before joinder with or placement in the other composite materials. Once joined, the adhesive is not typically needed for any lateral strength; one skilled in the art will readily appreciate how to determine the amount of adhesive to use based on the end applications.

[0032] In FIG. 3b, the core has been adapted for use in a curved end-product. This product could be any shaped structure, e.g., a hull on a boat. In the case of a boat, a curved structural support 36 is provided, and an outer fiberglass layer 38 is added after the core has been positioned. To allow the core to shape to the curve, it is preferably adhered to a scrim 35, and then scored (e.g., cut to ⅓ of the depth of the core 34). The kenaf core 34 will separate along the scores, but the core remains positioned/adhered to the scrim allowing its placement in tact along member 36. A filler resin 37 may be added along the valleys formed by the scoring/separation, followed by the application of the fiber glass outer layer 38. In this manner, the kenaf core provides a light-weight core that is much less expensive than other typical materials like foam and without their disadvantages.

[0033] In FIG. 4a, another embodiment of a composite using a cellulosic stalk core is illustrated. In this embodiment a light-weight plastic-covered beam 40 is made using a multiple-layered core. The central carrier core 43 is preferably the cellulosic stalk core appropriately sized for use in beam 40. This carrier core is adhered to strengthening members 41, which may conveniently be made of bamboo linear fiber in the form of a tape, as more fully described in U.S. application Ser. No. 09/960,204, incorporated by reference above.

[0034] This three member core 41, 42 is then extruded by any convenient means to form the desired shape and thickness of thermoplastic material 42 surrounding the core 41, 42. One preferred approach would include a plastics extruding machine (FIG. 6a) connected to a die 65 that allows the core 43/bamboo tape 41 core assembly is inserted into a mold 68 and positioned so as to allow clearance for the plastic 42 matrix to flow around all exposed surfaces in desired thicknesses. The mold 68 is heated and connected to an extruder 60 or large injection molding machine. Some molds may require a vacuum to be pulled by a vacuum system 67 on the interior of the mold 68 prior to injection. The synthetic polymer 42 is then injected, the mold 68 is chilled, and the resulting composite structure 40 is removed from the mold 68.

[0035] A cross-arm for a utility pole may be similarly constructed. Bamboo linear fiber 41, e.g., in the form of a tape, is treated with at least one bonding material and is bonded to the central carrier core 43. This assembly is forced through die 65 to produce a rectangular beam cross-section, as shown in FIG. 4A. The die 65 operation, the extracting, and cooling are identical to the operation described for producing poles or pilings. A power saw 66 travels beside the piece, sawing it to a desired length without slowing the process. The thus-prepared composite structure 40 is transferred to water-cooled bath 64 where it is cooled to ambient temperatures; and the sawed ends may also be capped.

[0036] Another embodiment for laying forming the composite of FIG. 4A is shown in the coating die 48 FIG. 4C. In this approach, the bamboo tape 41 is fed into a coating die and laid onto the carrier core 43, which is being fed through the aligning tray 49. The plastic in from the extruder is fed onto the bamboo 41 and core 43 via a ribbon film, thus providing the plastic coating 42 for surrounding the core.

[0037] With reference to FIG. 4b, in preparing poles 47 or the like, a similar process may be used if, e.g., a bamboo tape 45 is wound around a kenaf core 44, and molded or extruded to add plastic outer layer 46. Alternatively, one may produce the same without any bamboo tape strengthening layer 45, and even without applying a binder in making the core. In this case the kenaf stalks are positioned in the mold or extruder so as to permit the thermoplastic or resin to flow around the stalk core 44. In yet another alternative, the kenaf may be more finely chopped or pulverized, and added to a first plastic matrix to extrude together as the center core (similarly as can be done with fine bamboo fiber). Unlike bamboo and similar fibers, where they are used for additional strength, this type of product is preferably used for applications where the kenaf is not being relied on for its strength (particularly in the case of the pulverized kenaf), but rather as a light-weight and inexpensive filler.

[0038] It should also be noted that kenaf and similar density cellulose products could be used where the wood core is a lightweight cellulose product that is used as a filler in a thermoplastic material so that the resulting product can be machined or processed to various shapes. Thermoplastics such as polyethelyne are suitable for this type of application. In such applications, the thermoplastic material does not bond well to the core material and simply relies on the core material as a filler to displace plastic and make the construction lighter and less expensive.

[0039] With respect to both the kenaf and the bamboo, the following binding agents may be advantageously used (and in the case of bamboo, give surprisingly good bonding between the bamboo and the polymer matrix): maleated polypropylene, maleated polyethylene, maleic anhydride, hydroxyl methacrylate, silane compounds, N-vinyl pyridine, N-vinyl caprolactam, N-vinyl carbazole, methacrylic acid, ethyl methacrylate, isobutyl methacrylate, sodium styrene sulfonate, bis-vinyl phosphate, divinyl etherethylene glycol, vinyl acetate, vinyl toluene, vinylidene chloride, chloroprene, isoprene, dimethylaminoethyl methacrylate, isocetylvinyl ether, acrylonitrile, glycidyl methacrylate, N-vinyl pyrrolidone, acrylic acid, ethyl acrylate, itaconic acid, methyl acrylate, sodium vinyl sulfonate, cetyl vinyl ether, divinyl ether-butanediol, MDA and octadecyl vinyl acetate. In the case of kenaf, the lightest and cheapest binder is generally preferred, since the objective is typically for purposes of holding the core shape long enough to cut it, transport it and position it in the composite, and not to provide continuing strength after the composite is formed. Examples of binders that may achieve these purposes are rice latex glue and other latex and petroleum based glues.

[0040] Turning now to FIG. 5, one embodiment of a core block maker 50 having a hydraulic press with feeder and box mold attachment is illustrated. This press has the capability of producing varying sizes of end-grain core blocks, e.g., 2′ by 2′ by 4′ thick. Prior to entering the press, the stalks have been automatically fed through a glue-coater machine that sparingly mists the sticks with the proper adhesive. Feeder is preferably tapered so as the stalks 52 are fed in they are forced together. From there they are automatically transported to the press/box mold 53. There, a hydraulic press 54 compresses the stalks to the desired density, typically using pressures from 700 to 1000 psi. As the stalks travel the tapered zone 51 and a glue drying zone 56 to allow enough time for the binder to set. The bound stalks are advanced past a cutting device 57 and cut to the desired height, producing end-grain core block 58. The core may then be placed in stock within the same facility after quality control checks, optimum moisture content checks, and placed within sealed plastic film to keep the moisture content correct during storage. As customer orders for core sheets are received then blocks are taken from storage, placed on an automatic bandsaw, cut to the proper thickness as ordered, repackaged in sealed plastic bags, inserted into the appropriately sized cardboard boxes, and shipped to the customer. Alternatively, the blocks may be shipped directly to other facilities for storage or subsequent sizing.

[0041] Of course, if a lighter-weight, premium kenaf is desired, much less force (and potentially no more than created by the force of gravity) is needed, since in these applications the kenaf sticks are typically enhanced (thicker) and shaped before consolidating the stalks into block form. These two production methods would be preferably be run simultaneously, as they are able to produce two different grades of core material, each with their own special attributes. The smaller sticks that are forced with high pressure into consolidation would be faster and less expensive to produce, however, the density increase would be substantial, between fifty and seventy-five percent. The un-pressed stick is about seven pounds per cubic foot density (on average), whereas the 1000 psi. formed block, including adhesive, produces core that varies from 10 to 13 (or more) pcf density. Also, because of this density increase, this core is stronger and stiffer in addition to being lower in cost and there are certainly many applications for end-grain core with this weight and performance.

[0042] The second production method minimizes the clamping pressure to form a block, thus producing a core close to the sticks original density, adding only the weight of the glue binder. This would, again, become a premium offering that would be used in applications that were weight critical in the final part or composite. The aviation industry, for example, is willing to pay a premium to save a pound of weight and for obvious reasons its even more critical for aerospace.

[0043] Turning now to FIGS. 7 and 8, another embodiment of a composite and process for making it is illustrated. While various structural devices can be made using the inventive cellulosic stalk/plastic composite of the present invention, one application that is adaptable to such composite is the structural pallet. Pallets are used worldwide for supporting various products for shipment or storage. The use of pallets is so pervasive that standards have been established to define sizes of pallets. In the United States, one standard is the Grocery Manufacturers Association or GMA standard defining a pallet of 40 inches by 48 inches with a bottom structure having a cross-shaped form, i.e., having 4 large openings and a perimeter base. Europe has a different standard in which the bottom structure for the Euro-pallet uses three lengthwise extending boards or braces and the overall dimension is 1000×1200 cm. In general, the standards require that the pallet be able of edge mounting in warehouse racks with a 2000 lb. load and exhibit less than one inch deflection. These standards have created a problem for plastic pallets since plastic flows under load, is substantially weaker than an equivalent volume of wood and costs about 3 times the price of wood. However, plastic does have the advantage of being cleanable or sterilizable.

[0044] Referring now to FIG. 7, there is shown an exploded view of a pallet 70 having a solid top panel 72, a bottom panel 74 conforming to GMA standards and a plurality of support blocks or spacers 76 which support and position top panel 72 with respect to bottom panel 74. The top panel 72 may be formed of a plurality of bamboo strips together with a kenaf core (as in, e.g., FIG. 4A) if the strength of a bamboo member is desired; but it may also be solely of the kenaf core, in a preferred form. For weight reduction the top panel 72 and bottom panel 74 may both be cored. The spacers 76 may be injection molded blocks using bamboo/plastic pellets or may be sections of an extruded elongate composite such as shown in FIG. 4A. Bottom panel 74 is formed by multiple overlapping layers of bonded bamboo fibers/tape and kenaf core. All the exposed surfaces of the top and bottom panels and spacers are protected and covered by an outer plastic shell.

[0045] FIG. 8 illustrates a simplified form or mold for producing the GMA pallet of FIG. 7. The mold includes a base 81 having a bottom member 85 to which are attached outer periphery defining side members. The pallet 70 is actually formed in an inverted orientation and the inner surface 85 of base member may desirably be embossed with selected patterns so as to form mating patterns on an upper surface of the pallet to minimize sliding of a load on the pallet's plastic surface. The embossing on the surface 85 is preferably formed as continuous connected grooves such as in a spider web configuration (radial lines intersecting concentric circles) so as to create flow paths for injection of plastic into the mold for covering the bamboo or core. In the use of the illustrated mold, the core (and if bamboo fibers, preferably in the form of woven mats), are laid into the mold base 81. It may be desirable to create a pre-form of core/bamboo fibers, so that the pre-formed base of bamboo can be easily positioned in mold base 81. As will be apparent, plastic molding generally requires that the mold be pre-heated and, while heating apparatus is not shown, those skilled in the molding art will understand various methods and apparatus for pre-heating the mold components to temperatures suitable for molding, e.g., about 400 F. Plastic is then injected into the mold at relatively low pressure so as not to disturb the bamboo fiber mats. Once the mold cavities have been filled with the molten plastic, plastic injection is shut-off and the die is closed to 100% volume to consolidate the pallet.

[0046] The mold assembly of FIG. 8 is provided only by way of example of a method for manufacturing a bamboo/plastic composite pallet. For production of large volumes of pallets, it is anticipated that the mold assembly will be substantially modified and will have other moving elements to speed-up and simplify the molding process. Accordingly, it is not intended that the invention be limited by the illustrated mold assembly or process. For example, it may be desirable to form the pallet in multiple steps such as by molding top panel 72 in one operation, molding bottom panel 74 in another operation and then attaching the top and bottom panels together by adhesively bonding spacers 76 to facing surfaces using a plastic solvent type adhesive or heat to raise the plastic temperature to a bonding state.

[0047] As will be apparent from the discussion above, there are many benefits and possible alternatives for use of cellulosic stalk cores. One major benefit of this type of core to the composites industry is a much lower priced product that does not sacrifice performance, in fact the performance is superior in many ways as compared to the balsa core in use today. There are many potential applications for composite construction products that are not cost justifiable at this time due to the expensive raw materials. A low priced, strong, end-grained core material would open up substantial new markets for composite products.

[0048] Secondary benefits would include such things as sustainable production of kenaf in the United States, making use of the bast (as by products) more feasible given the profitable use of the core. The U.S. might benefit directly as there are many composite applications that utilize sandwich-core construction. For example, the military is a key user of composites due to the increasing need for rapid deployment. The light and fast stealth planes and ships are, due to composite materials, transparent to enemy radar. Ships like minesweepers must not be made of metal as the majority of mines are magnetically detonated, therefore, composite construction is the only viable way to protect the ship and its personnel. Thousands of dollars in savings could be realized utilizing this new core, with no sacrifice in core performance. Military personnel shelters use balsa core to make them light enough to be portable, and there are literally thousands of other applications that are similar. There are also less “exotic” uses for this new core, such as shipping containers, mobile bridges, and troop housing. A much lower priced core material has the potential to save large amounts of money in these areas, and allow new products to emerge in a cost-effective way. Also, a substantial amount of core material can be exported to the foreign countries where composite fabrication facilities exist, and, core production can begin soon, no lengthy wait for commercialization.

[0049] While the invention has been primarily described in connection with kenaf, as noted above it is applicable to other cellulosic stalk plants, and the embodiments discussed above in connection with kenaf should be understood as potentially applying to balsa stalks and others. While the foregoing constitute certain preferred and alternative embodiments of the present invention, it is to be understood that the invention is not limited thereto and that in light of the present disclosure, various other embodiments will be apparent to persons skilled in the art. Accordingly, it is intended that the invention not be limited to the specific illustrative embodiment but be interpreted within the full spirit and scope of the appended claims.