PLANT STEM BREAKING APPARATUS AND METHOD

Abstract

A stem breaking apparatus includes a roller having unevenly spaced radial projections and is useful in a method of extracting bast fibers from a plant stem. The roller is one of a pair of intermeshed counter-rotating rollers between which the plant stem is passed to break the woody core of the stem for separation from the bast fibers. Pairs of adjacent projections are angularly spaced by a larger amount that the individual projections of each pair. The rollers provide a larger support span than conventional stem-breaking rollers, resulting in gentler breaking of the woody core and preventing or eliminating the kinks and inter-fiber splitting associated with conventional stem breaking. Pre-compression of the stems before breaking offers further advantages.

Claims

1. A stem breaking apparatus comprising a roller having unevenly spaced radial projections.

2. The apparatus of claim 1, further comprising a tooth including a plurality of the radial projections.

3. The apparatus of claim 1, further comprising a plurality of teeth, each radial projection being provided by one of the teeth, wherein an angular spacing between adjacent teeth is greater than an angular spacing between at least one pair of the radial projections.

4. The apparatus of claim 1, further comprising a plurality of teeth extending from a minor diameter to a major diameter of the roller, each radial projection being provided by one of the teeth and extending to the major diameter.

5. The apparatus of claim 1, wherein each radial projection extends from a minor diameter to a major diameter of the roller, and wherein at least one radial projection has a surface that intersects an adjacent radial projection at a diameter between the minor and major diameters.

6. The apparatus of claim 1, wherein the roller is a first roller having a plurality of teeth intermeshed with a plurality of teeth of a second roller, each roller having unevenly spaced radial projections, and wherein each radial projection is provided by one of the teeth of the respective roller.

7. The apparatus of claim 1, wherein the roller is a first roller, the apparatus further comprising a second roller having radial projections intermeshed with the radial projections of the first roller such that a pair of the projections of the first roller lies between a pair of the projections of the second roller where the rollers oppose each other at a roller gap.

8. The apparatus of claim 1, wherein the roller is a first roller and the unevenly spaced radial projections are arranged in an angular pattern, the apparatus further comprising a second roller having unevenly spaced radial projections arranged in the same angular pattern and intermeshed with the radial projections of the first roller.

9. The apparatus of claim 1, wherein the roller is a first roller, the apparatus further comprising a second roller having radial projections intermeshed with the radial projections of the first roller such that a plant stem passed between the rollers is subjected to four-point bending.

10. The apparatus of claim 9, wherein, during the four-point bending, the plant stem spans a distance between a first radial projection and a second radial projection of the first roller and additionally spans a distance between a third radial projection and a fourth radial projection of the second roller.

11. The apparatus of claim 10, wherein the third radial projection and the fourth radial projection extend between the first radial projection and the second radial projection.

12. The apparatus of claim 10, wherein each of the third and fourth radial projections is provided by a tooth of the second roller, and wherein each of the first and second radial projections is provided by a respective tooth of the first roller.

13. A method of extracting bast fibers from a plant stem, the method comprising bending the plant stem by an amount sufficiently high to break a woody core of the plant stem and sufficiently low to prevent formation of kinks in a majority of the bast fibers surrounding the woody core.

14. The method of claim 13, further comprising the step of subjecting the plant stem to four-point bending between a pair of rollers, during which the plant stem spans a distance between adjacent teeth of one of the rollers and spans a distance between adjacent projections of a tooth of the other roller.

15. The method of claim 13, further comprising the step of passing the plant stem between a pair of rollers each having unevenly spaced radial projections.

16. The method of claim 15, further comprising the step of compressing the plant stem before passing the plant stem between the pair of rollers.

17. The method of claim 16, wherein the step of compressing is performed below a threshold compressive force to prevent damage to the bast fibers.

Description

DESCRIPTION OF EMBODIMENTS

[0020] Described below are an apparatus and method for extracting natural fibers from plants stems. These innovations are particularly useful with plants containing bast fibers, such as flax, hemp, ramie, jute, kenaf, and okra, to name a few examples. The bast portion of a plant stem, which contains the bast fibers, surrounds and is bound to a woody core of the stem. Extracting the fibers generally includes the steps of retting, breaking, cleaning, and refining.

[0021] Retting involves decomposition of the substances (e.g., pectin) binding the bast to the core of the stem and binding bast fiber bundles to one another. Retting can be accomplished via microbial decomposition (e.g., dew retting), enzymatic decomposition, and/or chemical decomposition. Breaking involves subjecting retted and dried stems to mechanical stresses (e.g., bending stresses) to break the woody core into small pieces referred to as shives that are allowed to fall away from the bast material after breakage. In one example, retted stems are fed through gear-like machinery to perform the breaking task. After the bulk of the woody core is separated from the surrounding bast fiber bundles during breaking, the fiber bundles may be cleaned in shaking and scutching processes, in which any residual woody material and short fibers are removed. Refining may include a hackling process, in which the cleaned bast fiber strips or bundles proceed through fine pins to be straightened and reduced to finer technical fibers.

[0022] Some of these processes can introduce defects into the fibers. For example, conventional breaking processes often result in kinks along the lengths of the fibers, which introduce weak spots along the fibers and reduce their strength accordingly. Some fibers are weakened more than others, with the result being overall weaker fibers with a high degree of variation in mechanical properties, which, while less important in the textile industry, represents a major problem with natural fibers for use as the reinforcing medium in polymer composites.

[0023] FIG. 1 is a schematic side or cross-sectional view of a portion of an embodiment of a stem breaking apparatus 10. The apparatus 10 includes a pair of counter-rotating rollers, including a first roller 12 and a second roller 14. Each illustrated roller 12, 14 includes a plurality of angularly spaced teeth 16, and each illustrated tooth 16 includes a plurality of radial projections 18. Each tooth 16 extends from a base 20 at a minor diameter Dm to a distal end 22 at a major diameter DM of the respective roller 12, 14. Each radial projection 18 also extends from the minor diameter Dm to the major diameter DM of the respective roller 12, 14. The teeth 16 of the first roller 12 are intermeshed with the teeth of the second roller 14. But, unlike gear teeth, the respective teeth 16 of each roller 12, 14 are not in contact with each other. To be considered intermeshed, the respective major diameter DM of each roller 12, 14 must intersect the other, and, when a tooth 16 of one roller is located along a line L extending between the centers of the rollers, that same tooth is positioned between adjacent teeth 16 of the other roller.

[0024] While conventional stem breaking rollers are defined by a series of evenly spaced radial projections (e.g., flutes or lobes), the illustrated rollers 12, 14 have unevenly spaced radial projections 18. That is, the angular or circumferential distance between at least one pair of adjacent radial projections 18 is different from the angular or circumferential distance between at least one other pair of adjacent radial projections 18. As used here, adjacent refers to the next in sequence along the outer perimeter of the roller. By way of example in FIG. 1, the angular distance between adjacent radial projections 18A and 18B of the second roller 14 is approximately 8?, which is different from the angular or circumferential distance between adjacent radial projections 18B and 18C, which is approximately 28?, as measured between the angular center of each projection 18. The angular spacing between adjacent teeth 16 is also greater than the angular spacing between at least one pair of the radial projections 18. By way of example, the angular spacing between adjacent teeth 16 of the rollers 12, 14 of FIG. 1 is 36? degrees, which is greater than the angular spacing between both pairs of radial projections 18A-18B and 18B-18C.

[0025] The illustrated rollers 12, 14 have ten evenly spaced teeth 16 and twenty unevenly spaced radial projections 18 along their respective perimeters. The number of teeth 16 per roller may be a function of the diameter of the roller, among other variables. Although the radial projections 18 are unevenly spaced, they are arranged in a repeating angular pattern with two evenly spaced projections 18 provided by each of the evenly spaced teeth 16. The angular pattern with uneven angular spacing among the radial projections may be formed by a first set of evenly spaced projections including projections 18A and 18C, and a second set of evenly spaced projections, including projection 18B, for example.

[0026] The sizes of and the spacing between the angular centers of adjacent projections 18 of the same tooth 16 is such that surfaces of the individual projections 18 of each tooth intersect at a minor projection diameter D, which is greater than the minor diameter Dm of the rollers 12, 14 and less than the major diameter DM of the rollers. Projections 18A, 18B on roller 14 of FIG. 1 have respective outer side surfaces 24, 24 facing away from each other and extending between the minor diameter Dm and the major diameter DM. Projections 18A, 18B on roller 14 of FIG. 1 also have respective inner side surfaces 26, 26 facing toward from each other and extending between the major diameter DM and the minor projection diameter D. Stated differently, and following a profile surface of one of the teeth 16 from one side of its base 20 to the to the other, the profile surface extends radially outward from the minor diameter Dm to the major diameter DM, then radially inward from the major diameter to the minor projection diameter D, then radially outward again to the major diameter DM, and finally radially inward again to the minor diameter Dm. The profile surface is smooth and continuous along its full extent, gradually changing in slope along the entire periphery of each roller 12, 14, except at an inflection point at the angular center of each tooth 16, which is at the minor projection diameter D. This shape defines a recess 28 at the distal end 22 of each tooth that partly defines adjacent projections 18 of each tooth 16. In other embodiments, each tooth 16 includes one or more recesses 28 at its distal end 22 and the entire profile surface is smooth and continuous and/or the recess or inflection point is not angularly centered along the respective tooth.

[0027] One result of the uneven spacing among the radial projections 18 is four-point bending of a plant stem P when passed between the rollers 12, 14, as illustrated in FIG. 1. Conventional stem breaking rollers with uniformly spaced radial projections are typically limited to three-point bending. The plant stem P of FIG. 1 is loaded at four distinct points (a-d) along its length as it passes through the gap between the rollers 12, 14. The plant stem P is in contact with the first roller 12 at points b and c and in contact with the second roller 14 at points a and d. Points b and c are located between points a and b where the teeth 16 of the rollers 12, 14 intermesh. The plant stem P spans a distance (a to b) between adjacent teeth 16 of one roller 14 and also spans a distance (c to d) between adjacent projections 18 of a tooth 18 of the other roller 12 while passing across line L.

[0028] FIG. 2 is a schematic side or cross-sectional view of a roller 12 similar to those of FIG. 1, and FIG. 3 is a schematic side or cross-sectional view of a conventional stem breaking roller 12. As illustrated in FIGS. 2 and 3, providing the radial projections 18 with uneven spacing permits a significantly larger support span S than the support span S of a similar number of evenly spaced radial projections 18. The larger support span S results in lower bending stresses in the plant stem for the same amount of stem deflection. This offers a larger processing window in that the change in bending stresses for a given change in the degree of intermeshing ID of the teeth 16 is smaller with a larger support span S. FIG. 4 illustrates a degree of intermeshing ID associated with the rollers 12, 14 of FIG. 1. Where the major diameters DM of the rollers 12, 14 are the same, the degree of intermeshing ID is equal to the difference of the major diameter DM and a distance between the rotational axes of the rollers. The larger processing window makes possible a method of extracting bast fibers from a plant stem that includes bending the plant stem by an amount sufficiently high to break the woody core of the plant stem and sufficiently low to prevent formation of kinks in a majority of the bast fibers surrounding the woody core.

[0029] With reference again to FIG. 2, certain roller characteristics or dimensions that can affect the process are identified. The illustrated roller 12 has a major diameter DM, a minor diameter Dm, a number of teeth N, and a number of radial projections n. Each tooth 16 has a height H and a width W. Each projection 18 has a height H measured from the base of the corresponding tooth 16, a height h measured from the minor projection diameter D, a width w, a load span LS, and a surface radius R. The roller 12 is characterized by a support span S, an inner span width Si, a span ratio S/LS, and an intermeshing clearance IC with respect to an intermeshed roller 14 (see FIG. 4).

[0030] In the illustrated example, the width w of each projection 18 is approximately twice its surface radius R (w?2R), the width W of each tooth 16 is approximately twice the projection width w (W?2w), the height H of each tooth and projection is half the difference between the major and minor diameters (H=(DM?Dm)/2), the support span S is approximately twice the tooth width W (S?2W), the load span LS of each tooth is approximately twice the radius R of the radial projections (LS?2R), and the inner span width Si is approximately the support span S minus twice the radius R of the radial projections (Si?S?2R). The span ratio is the ratio S/LS of the support span S to the load span LS. The intermeshing clearance IC is half the difference of the inner span width Si and the tooth width W, or (Si?W)/2 and generally represents the circumferential gap between the teeth 16 of each roller where they are intermeshed.

[0031] Corresponding characteristics are labeled on the conventional stem breaking roller 12 of FIG. 3, where the projection or tooth width is approximately twice its surface radius R and the support span S is approximately twice the projection or tooth width (S?4R). The teeth or projections of the roller 12 of FIG. 3 has no load span, with the bending load being concentrated at a single point.

[0032] As noted above, the uneven spacing among projections 18 of the roller 12 of FIG. 2 permits a larger support span S than the support span S of the conventional breaking roller 12, given a similar number of projections 18 and roller diameter. The support span S of the roller of FIG. 2 is approximately 8R, while the support span S of the conventional roller 12 is approximately 4R. An even greater difference between the two rollers 12, 12 is the intermeshing clearance IC, which is an order of magnitude larger for the roller 12 of FIG. 3 than for the roller 12 of FIG. 3. This is due to the much larger ratio Si/W of the inner span width Si to tooth width W provided by the roller 12 of FIG. 2. In the illustrated example, the Si/W ratio is approximately 1.5 for the roller of FIG. 2, and the Si/W ratio is less than 1.1 for the roller of FIG. 3. The corresponding intermeshing clearances are IC?R for the roller 12 of FIG. 2 and IC<0.1R for the conventional roller 12 of FIG. 3. The conventional roller 12 thus causes a much sharper bend and pinching effect on the processed plant stem than does the roller 12 of FIG. 2. The roller 12 having unevenly spaced radial projections 18 thus eliminates a majority of or substantially all of the kinks associated with the conventional roller profile 12.

[0033] Experiments confirm this desirable result. Bast fibers were extracted from flax stems having a 1.5 mm diameter and a length of 25 cm via enzymatic retting and mechanical breaking using different pairs of rollers. After retting and before breaking, the stems were water-rinsed and then dried at 100? C. for 24 hours. Two stem samples, each 5 cm in length, were taken from the middle section of single retted stems. One stem sample was passed three times through one pair of rollers having unevenly spaced projections consistent with FIG. 2 and the above description, and the other stem sample was passed three times through a different pair of conventional rollers consistent with FIG. 3. Remaining woody core material was removed from each stem sample via manual shaking after breaking. The resulting bast fibers were then hackled into technical fibers using a pin frog having 9 pins/cm2. The roller parameters are given below in TABLE I for each pair of rollers used in the experiment.

TABLE-US-00001 TABLE I Comparative Roller w/Unevenly Parameter Roller Spaced Projections Major Diameter, D.sub.M (mm) 69.6 69.5 Minor Diameter, D.sub.m (mm) 60.4 58.1 Number of Projections 22 10 Projection Radius, R (mm) 2.3 2.0 Tooth Width, W (mm) 4.5 8.0 Projection Height, H (mm) 4.6 5.7 Support span, S (mm) 9.4 16 Load Span, LS (mm) 4.0 Inner Span Width, S.sub.i (mm) 4.8 12 Span Ratio (S/LS) 4:1 Intermeshing Clearance (mm) 0.15 2.0

[0034] FIGS. 5-7 are scanning electron microscope (SEM) images of technical flax fibers obtained from the flax stems after being passed through each different pair of rollers and after hackling. FIG. 5 shows a representative portion of a technical fiber FT extracted from a retted stem that was passed between the conventional rollers having evenly spaced projections. The illustrated fiber FT is bent and exhibits inter-fiber separation among the individual or elementary fibers FE.

[0035] FIG. 6 is an enlarged view of a portion of FIG. 5 illustrating kinks along the length of some of the elementary fibers FE and highlighting a splitting location where adjacent elementary fibers FE have begun to split apart from each other in the region near the kinks.

[0036] FIG. 7 shows a representative portion of a technical fiber FT extracted from a retted stem that was passed between the rollers with unevenly spaced radial projections. The illustrated fiber FT is unbent and exhibits no kinks or inter-fiber separation among the individual or elementary fibers FE. Indeed, the individual elementary fibers FE are difficult to discern in FIG. 7 because they remain tightly bundled.

[0037] TABLE II lists mechanical properties of the differently processed fibers. The elastic modulus and tensile strength of the technical fibers from the flax stem sample subjected to the rollers having unevenly spaced projections are significantly higher on average than those of the technical fibers from the flax stem subjected to the conventional rollers. The effect on the strength, stiffness, and consistency of polymer composites using bast fibers processed as disclosed herein is believed to be equally as significant.

TABLE-US-00002 TABLE II Comparative Roller w/Unevenly Parameter Roller Spaced Projections Elastic Modulus (GPa) 37.6 52.5 Tensile Strength (MPa) 350 1032

[0038] In various embodiments, the surface radius R at the distal end 22 of each of the unevenly spaced projections 18 is greater than or equal to the diameter or width of the stem. The width W of each tooth 16 may be in a range from 2R to 4R. The distal end 22 of each projection 18 is preferably a smooth and continuous surface. The span ratio may be in a range from 2.5 to 4.0, although this range may vary based on stem size and other variables. If the span ratio is too low, then stem bending will be sharper and may result in kinks near the load span location. If the span ratio is too high, then the bending will be more similar to a three-point bend causing a sharper bend at the middle of the load span.

[0039] The method may optionally include a compression step. The compression step may include passing the plant stem(s) between rollers having smooth outer surfaces or otherwise applying opposing radial forces to the plant stem. The compression step may be performed after retting and before passing the stems through the toothed rollers. The compression step may be considered a pre-breaking step or may be considered an earlier portion of the breaking step. FIG. 8 includes sequential (1-4) photographic images of the end of a retted flax stem 1.25 cm in length during compression between parallel plates. The magnitude of compression is indicated for each image. The illustrated compression is uniaxial along a direction perpendicular with the longitudinal axis or lengthwise direction of the stem. Other types of compression with a radially inward component are contemplated. Here, the light-colored hollow woody core WC of the plant stem P surrounded by the darker bast can be observed. FIG. 9 is a plot of compressive force vs. degree of compression labeled to correspond to the sequential images.

[0040] In image 1 of FIG. 8, the plant stem P is uncompressed. Between images 1 and 2, the plant stem P exhibits a period of linear elasticity, followed by a period of non-linear behavior, as indicated in FIG. 9. The force-compression curve reaches a local maximum at image 2, where the stem P has begun to buckle at 17% compression and a longitudinal split is observed in the bast layer. The stem exhibits increased compression with decreased applied force between images 2 and 3 up to 27% compression. In image 3, the stem has buckled further, and longitudinal splits are observed in the woody core. Between images 3 and 4, the force-compression curve first plateaus and then begins to rise exponentially as the stem fully collapses.

[0041] Where the method includes a compression step, compression may be applied to the plant stems in an amount that does not exceed a threshold level CT. This threshold level can depend on several factors, including the plant species from which the stems are taken, the average diameter of the stems, etc. The not-to-exceed threshold CT can be determined via a force-compression test as in FIG. 9 or any other suitable method. In the illustrative force-compression curve of FIG. 9, the threshold CT may be selected after the compression force plateaus and/or before the compression force begins to increase exponentially after a period of decreasing compressive force. In the example of FIG. 9, a suitable threshold CT is between 28% and 40% compression. Manually extracted flax fibers from retted stems subjected to 35% compression exhibited an elastic modulus and tensile strength comparable to manually extracted flax fibers from retted stems subjected to no compression.

[0042] Additional experiments indicate that the presence of the recesses 28 (FIG. 1) at the distal ends 22 of the teeth 16 and between the radial projections 18 of the roller achieves an unexpected result. Dynamic mechanical analysis (DMA) was performed with non-rotating static sets of projections simulating the uneven projection spacing discussed above in comparison with the same bending point spacing without the recess at the end of the tooth. In the test condition of FIG. 10, the simulated projection 18 does not have a recess at its end, and in the test condition of FIG. 11, the simulated projections 18 are spaced apart simulating a 28 therebetween. The width W, support span S, and load span LS of the recess-free configuration of FIG. 10 were set the same as the width W, support span S, and load span LS of the configuration of FIG. 11. Specifically, the width W was 20 mm, each support span S was 45 mm, and the load spans LS were 15 mm.

[0043] While FEA analysis indicates substantially similar compressive strain along the top side of a computer-modeled stem for both cases, the experimental data of FIG. 12 tells a different story. Here, retted flax stems subjected to bending by the solid tooth of FIG. 10 exhibit markedly different behavior than retted flax stems subjected to bending by the spaced-projection tooth of FIG. 11. The initial linear elastic region of each force-deflection curve is essentially the same, and the major fracturing event (i.e., peak force) is near 1.5 mm deflection for each. But the configuration of FIG. 11 simulating a recess 28 between projections 18 of the same tooth 16 begins to exhibit non-linear behavior before the peak load and a more gradual breaking of the woody core after the peak load. This smoother, more gradual breaking is in contrast to the multiple sharp breaks associated with the loading condition of FIG. 10.

[0044] The effect of the above-described compression step was also evaluated using DMA and the simulated roller profile of FIG. 11. The bending behavior of an as-retted flax stem is compared with a flax stem that was subjected to 35% compression after retting. As shown in FIG. 13, the slope of the initial linear region of the force-deflection curve for the compressed stem is much lower than that of the as-retted stem, indicating a significant reduction in bending stiffness due to the compression process. This reduction in bending stiffness due to the compression process is greater than can be accounted for based solely on the compression-induced stem thickness reduction. Additionally, the major fracture event occurs at a much higher force in the as-retted stem, and there is a much larger force reduction after fracture in the as-retted stem, indicating a more rapid failure than with the compressed stem, which appears to break more gradually. Both curves plateau at about 3 mm deflection, which is an indicator of an optimum process parameter.

[0045] The plant stem processing method may thus include configuring the stem breaking rollers to subject the plant stem being processed to a threshold amount of deflection between the projections defining a support span. This threshold level can depend on several factors, including the plant species from which the stems are taken, the average diameter of the stems, the support span distance, etc. The threshold can be determined via a force-deflection test as in FIG. 13 or any other suitable method. In the illustrative curves of FIG. 13, the threshold may be selected at or before the deflection at which the bending force plateaus. In the example of FIG. 9, a suitable threshold is between 2.5 mm and 3.5 mm. Proper selection of this threshold can help prevent overbending (i.e., kinks) of the plants stems while ensuring suitable breakage of the woody core.

[0046] It is to be understood that the foregoing is a description of one or more embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.

[0047] As used in this specification and claims, the terms e.g., for example, for instance, such as, and like, and the verbs comprising, having, including, and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.