CA2373339C - Hemp hurd composite panels - Google Patents

Abstract

A method of forming a structural member from hemp hurd, substantially free of hemp bast fibres is disclosed. Hemp stalks are decorticated and the hurd material divided into hurd strands which are mixed with a binder and pressed into the structural member. Also disclosed are structural members comprising hurd strands substantially free of hemp bast fibres. Hurd strands may be mixed with wood strands to form wood/hurd composite structural members.

Description

02-07-2001 ~~ MAN 02.29 PM BENNETT JONES FAX N0, 780 421 1y51 r. ue ' CA 02373339 2002-O1-07 I~:EMP HURD CUMPUSITE pANLLS
F IF,L)U OF TItI'E )<NVENTION
The present invention rrlatcs to composite strucniral members such as panels, boards or beams which comprise hemp hurds.

industrial hemp is a bast fibre plant similar to flax. Bast plants produce two types of ~brcs. The outer bast fibres run longitudinally along the perimeter of the stalk. Rast fibres are very strong in tension and are typically used in textile manufacture, cordage and specialty papers.
Hemp bast fibxes have also been used in medium density fibreboard.
There is significant interest in producing non-wood lignocellulosic structural building material, particularly with a view to forest conservation and the utilir~tion oEa waste material.
For example, cereal straw has been successfully incorporated into an oriented strand board ("OSJ3") with strength characteristics which are similar to or better than wood pSB of similar ihiclu~ess. A description of a method and apparatl~s of fabricating a cereal straw panel may be found in co-owned U,S. Patent No. 5,932,038.
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AMENDED SHEET

A significant portion of a hemp plant is the hurd, which is the core fibrous material in the stalk. The Kurd has been dismissed as a potential structural material due to its short fibre length and its low density. It is only marginally suited for paper production. In hemp processing, it is considered a waste product with limited value mainly as an absorptive material. This is unfortunate because the Kurd may make up 70 to 7~°,% of the hemp plant stalk mass.
It is known to use hemp hurd material with a cementitious binder to make a bulk material that may be handled in a similar manner to cement in that it may be poured into a mould or troweled onto a surface. However, there is no disclosure in the prior art that teaches the use of hemp hurd fibre in a structural member such as a board, a panel or a beam.
SUMMARY OF THE INVENTION
The present invention is directed to methods of producing structural members comprising hemp hurd fibres and to such structural members themselves.
Accordingly, in one aspect of the invention, the invention comprises a method of forming a structural member comprising the steps of:
(a) processing a hemp plant to produce hurd which is substantially free of bast fibres;
(b) splitting the Kurd longitudinally to produce hurd strands;
(c) adding binder to the hurd stands; and (d) pressing the hurd strands into a desired shape and allowing the binder to set.

-2-SUBSTITUTE SHEET (RULE 26) The hurd strands may comprise substantially elongate strands having a length greater than about 20 millimetres and aspect and slenderness ratios greater than about 3:1.
Shorter strands having aspect and slenderness ratios as low as about 1:1 may have an application in producing thin-stiff panels of limited bending strength. The structural member may be a board, a panel or a beam.
The method rnay further comprise the step of orienting a majority of the hurd strands such that the Kurd fibres are substantially parallel in the structural member.
In one embodiment which emphasizes structural strength, the Kurd strands may be pressed such that the structural density is greater than about 500 kilograms per cubic meter. In another embodiment which emphasizes insulative and sound absorptive properties, the hurd strands may be pressed such that the structural member density is less than about 350 kilograms per cubic meter.
In one embodiment, the Kurd strands may be mixed with wood strands to produce a composite wood/hurd structural member.
In another aspect of the invention, the invention comprises a structural member comprising oriented hemp Kurd strands and a resin wherein said member is substantially free of hemp bast strands.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of exemplary embodiments with reference to the accompanying drawings in which:
Figure 1 is a graphical representation of a pressing cycle used in one embodiment of the present mventton.

-3-SUBSTITUTE SHEET (RULE 26) Figure 2 is a graphical representation of a pressing cycle used in one embodiment of the present invention.
Figure 3 shows the sound absorptive properties of structural members according to one embodiment of the invention.
S
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides for a structural member comprising hemp Kurd fibres bound together by a resin as well as a method of producing such structural members. When describing the present invention, the following terms have the following meanings, unless indicated otherwise. All terms not defined herein have their common art-reco'nized meanings.
A. Definitions The term "hurd" refers to the inner fibrous core of the hemp plant which may be separated from the cortical bast fibres of the hemp plant.
The term "bast" refers to fibres of the cortical bark of the hemp plant.
The term "oriented" refers to positioning of the strands which form the structural members of the present invention such that the majority of the fibres are substantially parallel to each other.
The term "strand" or "strands" refers to a bundle of elementary fibres wherein the length to width ratio (aspect ratio) and the length to thickness ratio (slenderness ratio) are both greater than about 1: l and preferably greater than about 3:1.

-4-SUBSTITUTE SHEET (RULE 26) The term "structural member" refers to a member formed by the methods of the present invention which maintains its structural shape and integrity. It is not intended to restrict the definition of structural member as used herein to members which meet or exceed certain standards to be used in structural or building applications.
Therefore, "structural S member" may include light-weight, low-density members which, by themselves, are unsuitable for load bearing applications.
The term "substantially free of bast fibres" refers to Kurd material which contains less than about 15% bast fibres by weight, preferably less than about 5% bast fibres by weight and more preferably less than about 2% bast fibres by weight.
B. Description In one aspect of the invention, a method for producing a structural member such as a panel, board or beam is provided. Hemp stalks are processed to separate the Kurd from the bast fibres and to produce Kurd pieces in a process referred to herein as decortication. As used herein, decortication shall refer to a process of separating the outer fibrous bark or bast of a hemp plant from its hard. This process may be done manually by stripping or peeling the bast fibres away from the hard. Preferably, decortication is automated using machines and methods designed to decorticate a hemp plant or adapted from machines and methods used to decorticate sugarcane or flax.
In one embodiment, coarse Kurd may be separated from bast fibre by hammer milling hemp stalks through a coarse screen. The size of the screen may be varied to vary the size of the hard obtained. In one preferred embodiment, the screen may be a 38 mm screen.
The hurd/bast fibre mixture is then screened to separate the two constituents. Coarse Kurd produced in this manner may have an average length of about 20 to about 2~ mm. Coarse hard material may be further screened and/or hammer milled to produce finer Kurd material if desired.

-5-SUBSTITUTE SHEET (RULE 26) Longer Kurd strands may be produced by stripping the Kurd from whole hemp stalks which have been pressed and split. Hurd strands greater than about 25 mm in length may be produced in this manner. In this instance, the hurd should be split longitudinally to produce long, slender and narrow strands.
One example of a decortication machine is disclosed in PCT application WO

by Australian Hemp Company Limited. The apparatus includes bond rupture rollers to facilitate the rupture of the bonds between the fibre and the hurd, counter-rotating pressing rollers which flatten the stalk. The stalk is then split longitudinally and processed over decorticating rollers which include a vacuum roller and a toothed roller which cooperate to strip the Kurd from the fibre. Sugarcane separators such as those disclosed in U.S. Patent No.'s 3,567,510, 3,976,498 and 4,312,677 may also be used with minor modifications to handle hemp stalks.
Numerous other alternatives are known to those skilled in the art.
An alternative decorticating technique may be used to produce hurd strands having a length less than about 30 mm. Hemp stalks may be fed through interlocking rollers having flat-topped teeth. The root and land of each roller tooth may be approximately 1 cm each. The result is a corrugated bark material where the hurds have been separated from the bark. The hurd material ranges in size from 30 mm long strands to a fine particulate consistency. Larger hurd material may result if lamer roller teeth are provided on the interlocking rollers.
In one embodiment, the hurd material is cut or produced into strands having a nominal length of 20 mm or longer, preferably in the range of greater than about 50 mm and more preferably in the range of about 100 mm. In another embodiment, the Kurd pieces are cut or produced into strands having a nominal length of less than 20 mm. The longer strands are more suitable for end product members that are intended to be high density structural members although their use is not restricted to such high density products. The shorter strands are more suitable for low density panels or boards having thermal resistivity and/or acoustical absorption

-6-SUBSTITUTE SHEET (RULE 26)

7 PCT/CA00/00574 properties while retaining some structural strength although their use is not restricted to such low density products.
Hard material or strands may be split longitudinally to achieve desired slenderness and aspect ratios. Preferably, both slenderness and aspect ratios should exceed 3:1, particularly where bending strength is a desired property of the structural member.
The hard strands should have a moisture content in the range of about 5% to about 25%.
If the moisture content is greater, the hard strands should be dried prior to blending with the resin. If the moisture content is too low, it may be preferable to introduce some moisture to the strands before blending with the resin.
The Kurd strands are then blended with a resin binder which will bind the strands together in the end product. Blending machines for wood OSB are well known in the art and are used 1 S with the present invention with no or little modification. Rotary blenders with a spinning disc atomiser are representative of a suitable apparatus. What is necessary is that the resin be evenly distributed with the Kurd strands to maximize internal bonding when the strands are later pressed and the resin cured. Preferred resins include isocvanate resins such as diphenylmethyldiisocyanate ("MDI"). Also suitable, but less preferred. are phenolic resins typically used for wood OSB products or urea formaldehyde resins used in particleboard. The hard strands are compatible with any resin typically used in lignocellulosic composite panels.
As well, a suitable wax may be added with the resin to improve the efficiency of the resin and enhance resistance to moisture and water absorption. Suitable waxes may include slack wax, emulsified wax or powdered wax, as is well known in the art. The wax may be added in the range of about 0.5°,'° to about 1.5% or more. Increasing the wax component beyond this range is possible but does not improve performance nor does it have any substantially deleterious effect.
SUBSTITUTE SHEET (RULE 26) In one embodiment, the Kurd strands may then be oriented such that the strands are substantially parallel. Strand orientation may be achieved with minor modification to machines and methods used for orienting wood strands for OSB or cereal straw, which are well known in the art. In one example, strand orientation is accomplished using a mat orienter which includes a plurality of spinning discs. In one example, the pieces may be vibrated on a corrugated panel before being pressed. The corrugations will cause the hurd strands to align.
In another example, the hurd strands may be dropped on parallel-aligned vertical bars placed in the form of a spaced grid with a width that is less than the strand length. Vibrating or shaking the strands on the grid will cause the strands to fall through and be substantially oriented in one direction. Layers having cross-orientation may be produced where the strand orientation in one layer is oriented perpendicular to the strand orientation in other layers in order to facilitate stiffness and strength both parallel and perpendicular to machine manufacturing direction. For example, a structural panel may have a core and two face layers where the orientation of the strands in the core layer is perpendicular to the orientation of the strands in the face layers.
The mats formed by the oriented Kurd strands are then pressed to compact and consolidate the mat and raise its internal temperature high enough to cure the resin in a reasonable period of time. The time and temperature parameters may vary according to the resin used. In one embodiment, the temperature may reach between about 175° C
to about 235° C and the resin will cure within about 3 to about ~ minutes. Pressing may be monitored and controlled using well -known or commercially available systems such as the PressMANT"' press monitoring system manufactured by Alberta Research Council, Edmonton, Alberta, Canada.
In one embodiment, Kurd strands may be mixed with wood strands used in conventional wood OSB manufacturing to produce wood/hurd composite OSB structural members.
The methods involved in producing such composite members may be as described herein or may be essentially those used in conventional wood OSB panel production which are w-ell known in the art. Alternatively, the Kurd strands may be mixed with other lignocellulosic strands such as straw _g_ SUBSTITUTE SHEET (RULE 26) or flax to produce composite structural members using the methods described herein or similar methods. Methods of producing oriented strand structural members using lignocellulosic material such as wood or straw are well known in the art. A person skilled in the art may have reference to various prior art references such as U.S. Patent No. 5,932,038 entitled "Method of Fabricating a Straw Panel, Board or Beam" or Maloney, T.M., 1993. Modern Particleboard &
Dry-Process Manufacturing, Updated Edition, Miller Freeman Publications, San Francisco, California.
In a preferred embodiment, the structural member is a panel which is formed from three layers; two outer or face layers and core layer. The core layer may be comprised entirely of oriented hurd strands or a mixture of oriented wood and hurd strands while the outer layers are comprised entirely of wood strands.
EXAMPLES
The following examples are provided to illustrate the present invention not to limit the invention claimed herein. Hurd strand panels and wood/hurd composite panels were manufactured as follows:

Panel Group Target Resin Content Mat Values Type LD. Thickness Face Core Construction Density (mm) (k /m3) Hurd (HighHS 1 11.1 640 4% MDI 4% MDI Panels oriented Density HS 2 11.1 608 4% MDI 4% MDI 60%, 40% core -Long HS 3 11.1 586 4% MDI 4% MDI

Strand) Hurd (Low HI 1,2 25.4 320 4% MDI 4% MDI Random Density HI 3,4 25.4 240 4% MDI 4% MDI homogenous -Short HI 5,6 25.4 160 4%MDI 4% MDI panel Strand) Wood WH 0 11.1 640 4% MDI 4% MDI Oriented-no hurds Hurd WH 1 11.1 640 4% MDI 4% MDI 15% hurds in core Mixtures WH 2 11.1 640 4% MDI 4% MDI 25% hurds in core WH 3 11.1 640 4% MDI 4% MDI 35% hurds in core Table 1 Description of Manufacturing Parameters for each Panel Type The panels designated HS 1, 2 and 3 were produced from Kurd strands having nominal dimensions of about 10 mm wide, 100 mm long and ~ mm or less thick. The panels designated HI 1 to 6 and the u~ood/hurd mixture panels designated WH 0 to 3 were produced from hard strands having a nominal length of about 15 to about 20 mm. In all cases, slack wax in the amount of 1 % was added along with the resin. The panels were pressed with pressing cycles adapted from typical wood OSB pressing cycles.
Example 1 The high density, long strand panels were tested for modulus of rupture (MOR), modulus of elasticity (MOE), internal bonding (IB) and thickness swell. The results are compared with CSA standard 0437 "02" in the following table:
Table 2 Physical Properties of HS panels.
Property CSA 043? Hurd Panels "02" HS 1 HS 2 HS 3 MOR M a 29.0 36.3 32.6 29.9 MOE M a 5500 5700 6550 5550 IB M a 0.345 0.733 0.524 0.524 Thickness Swell (MPa) 15.0 17.5 22.9 I, 22.0 As may be seen, the hard panels meet or exceed each of the CSA standards for each of the properties measured except for thickness swell.
Example 2 Low density hard panels were formed as above using a pressing cycle as shown in Figure 1. The low density Kurd panels HI 1-6 did not have the physical strength to meet any structural panel requirements. However, these panels were lighriveight and maintained their integrity, leading to the conclusion that they were useful for insulating and sound absorption properties.

SUBSTITUTE SHEET (RULE 26) The thermal resistivity of these panels was measured and compared with fibreglass batts and loosefill Kurd fibres (CanbioteT'~) and the results shown in the following table:
Table 3 Thermal Resistivity of Hurd Insulation Panels Material * Density R Value /m3 er 25mm Insulation hi h densi HI 1, 2 320 2.1 Insulation medium densit HI 3, 4 240 2.3 Insulation low density HI 5, 6 160 2.7 Fiber lass 160 3.2 Loose-fill Hurd 150 1.8 * R Values for all Kurd panel types at each density were averaged.
As well. the acoustical performance of these Kurd panels was measure and compared with fibreglass bans. The results are shown graphically in Figure 3. The Kurd panels show reasonable sound absorption but not as good as fibreglass.
Example 3 Wood/hurd panels WH 0, 1, 2 and 3 were formed using the pressing cycle shown in Figure 2 and tested for the same physical properties as in Example 1 and further including the 2 1 S hour boil test and linear expansion test (oven dried vacuum pressure soak). The results were compared again with CSA standard 0437 "02" and were as follows:
Table 4 Physical Properties of Wood-Hurd Panel Mixtures Property CSA 0437 "02" WH 0 WH 1 WH 2 WH 3 " a " onl no Kurds 15% Kurds 25% Kurds 35% Kurds _ MOR 29.0 64.0 52.3 48.3 54.2 MPa MPa IB 0.345 1.096 0.799 ~ 0.728 0.655 MPa T.S. 15.0 8.9 9.5 9.9 9.6 2 hr Boil 14.5 I 28.9 28.9 24.0 ~ 27.6 M a LE (ODVPS) 0.35 0.16 0.19 0.22 0.22 SUBSTITUTE SHEET (RULE 26) 02-07-200 ~ 1 MON 02. 29 PM HENNETT JONES FAX N0. 780 421 7951 " ' "

As wcll, these panels were produced with shorter hard strands wish very little aspect or slenderness ratios close to unify. It is reasonable to conclvdc chat woodlhurd panels using longer hurd strands such txs those used in panels I-tS 1-3 would result in stronger composite panels, closer to that of an all-wood panel.
As will be apparent to those skilled in the art, various modifications, adaptations and variations of the foregoing specific disclosure can be made without departing from the scope of the invention as defined by the appended claims.

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AMENDED SHEET

Claims (24)

WHAT IS CLAIMED IS:

1. A method of forming n structural member comprising the steps of:
(a) processing a hemp plant to produce herds which arc substantially free of bast fibres;
(b) splitting the herds longitudinally to produce hard strands;
(c) adding binder to the hurds;
(d) orienting a majority of the hard strands such that the hurd fibres are substantially parallel in the structural member; and (d) pressing the herds into a desired shape and allowing the binder to set.

2. The method of claim 1 wherein the herds arc substantially elongate strands having a length greater than about 20 millimetres and slenderness and aspect ratios greater than 3:1.

3. The method of claim 1 wherein the structural member comprises two or more layers wherein the hard strands are oriented in each layer and the direction of orientation in one layer is perpendicular to the direction of orientation in the other layer.

4. The method of claim 1 or 2 wherein the structural member is a board.

5, The method of claim 1 or 2 wherein the structural member is a panel.

6. The method of claim 1 or 2 wherein the structural member is a beam.

7. The method of claim 1 or 2 whereat the structural member density is greater than about 500 kilograms per cubic meter.

8. The method of claim 1 wherein the hard strands have a length of less than about 20 millimetres.

9. The method of claim 9 wherein the structural member density is loss than about 350 kilograms per cubic meter.

10. The method of claim 1 wherein the binder is an isocyanate resin.

11. The method of claim 11 wherein the isocyanate resin is MD1.

12. The method of claim 1 wherein the structural member also comprises wood strands.

13, A structural member comprising oriented hemp hard strands and a binder wherein said member is substantially fire of hemp bast strands.

14. The structural member of claim 14 which is a board.

15. The structural member of claim 14 which is a panel.

16. The structural member of claim 14 which is a beam.

17. The structural member of claim 14, 15, 16 or 17 wherein the binder is as isocyanate resin.

18. The structural member of claim 18 wherein the isocyanate resin is MD1.

l9. The structural member of claim 14, 15, 16, l7 or 19 further comprising wood StralldS.

20. The structural member of claim 14 wherein the structural member comprises three layers of oriented strands wherein the outer layers comprise wood strands substantially free of hard strands and the core layer comprises hurd strands.

21. The structural member of claim 21 wherein the core layer comprises hard strands up to about 50% by weight and wood strands making up the remainder.

22. The structural member of claim 22 wherein the strand orientation in die core layer is perpendicular to the strand orientation in the outer layers.

23. The structural member of claim 14 wherein the hard strands have a length greater that about 20mm and slenderness and aspect ratios greater than about 3:1.

24. The structural member of claim 14 wherein the hard strands have a length less than about 20mm.