Over the past decades, allowable harvests of old growth in the Pacific Northwest have been reduced. In light of this restriction, it has become increasingly difficult to provide end users with the quality lumber they once enjoyed. Furthermore, diminished supply of larger dimension timbers has created high pricing. Because of these changes, new products and producers have emerged to fill this need with composite and engineered wood products.
A number of existing manufacturing plants are currently experiencing shortages in their supply of fiber. When such plants can only operate five days out of the week instead of seven days a week, managers are painfully aware of the additional start-up costs and lost potential profits from a process designed to be continuous. Their current fiber supply restricts them from taking advantage of these improvements. The incorporation of hemp could readily supplement their current fiber supply. A transitional period of accommodation would be necessary, as a manufacturing plant would need to make the technical adjustments necessary to utilize this new material. A sufficient foundation, however, has already been laid by research universities and other organizations to allow for the speedy acclimation of alternative fiber crops.
Bast Plants as a Fiber Resource
Bast fibers have been grown for centuries throughout the world. Bast plants are characterized by long, strong fiber bundles that comprise the outer portion of the stalk. Bast plants include flax, hemp, kenaf, sunn-hemp, ramie, and jute. The focus of our research has been on the species that can grow in temperate regions of the world, namely flax, hemp, and kenaf. These fibrous plants have long been noted for their exceptional strength in cordage and paper.
The word “bast” refers to the outer portion of the stem of these plants. This stringy, vascular portion comprises 10 – 40% of the mass of the stem depending upon the species of bast plant, as well as the particular variety, or cultivar, within a bast plant.
The remainder of the stem inside this bast layer is a different type of fibrous material, which has different names depending upon the species. This inner material is known as shives when referring to flax and sometimes hemp, as hurd in the context of hemp, and as core when from kenaf. For the purpose of simplicity and consistency, we will use the word “core” when discussing this portion of the bast plant.
Overall Advantages of Bast Plants
In general, bast plants possess the following benefits:
- High tensile strength in bast portions, especially in fiber varieties.
- Bast plants have a relatively low specific gravity of 0.28 – 0.62, yielding an especially high specific strength, i.e. strength-to-weight ratio, (Kozlowski, Mieleniak, Przepiera, 1994).
- Generally high fiber productivity rates, rivaling and even surpassing that of the most commercial tree species.
- Potential for even greater productivity, bast portions, and mechanical properties through focused genetic breeding.
Overall Limitations of Bast Plants
In general, bast plants also have the following limitations:
- Rotations vary based on seasonal differences.
- Limited research for composite applications in North America.
- Lack of related agricultural infrastructure in North America.
- Relatively high absorption of moisture in core portion.
- Diminished board properties when using core for particleboard.
- Difficulty in handling long fiber bundle lengths for processing.
- Difficulty in applying binder to long fiber bundle lengths.
A notable bast fiber crop is hemp. This plant was probably first grown in Central Asia from where it spread to China where it is thought to have been grown for 4,500 years. It was originally grown for its fiber, then around 900 BC also became known for its narcotic qualities. Hemp is a strong, durable, though harsh bast or phloem fiber, having a core which is characteristic of hardwood fiber. The bast portion is typically 14%. Hemp is an annual plant which at maturity develops a rigid, woody stem ranging in height from 1.2 – 5 m, (3.9 – 16.4 ft), and having a diameter from 4 to 20 mm, (0.16 – 0.79 in), (Hayward, 1948; Berger, 1969).
Hemp varieties planted in the temperate zones fall into two groups, namely the northern and the southern varieties. The latter require high temperatures and a long vegetative period, and consequently grow taller and yield more fibre. Overall, hemp is a tough plant that grows quickly and produces abundant seed and readily adapts to different niches or areas.
According to European statistics, Hungary has some of the greatest yields of hemp stalk. The yield of hemp stalk from Hungary can readily produce 9 metric tons/hectare, (4 tons/acre). Highest yields in Hungary reach 11 metric tons/hectare, (4.9 tons/acre). Hemp varieties have a growing season of approximately 143 days, (Berger, 1969; Helm, 1995; de Meijer, 1993).
Data suggests that hemp, a vital part of the cordage industry throughout the world over a century ago, once had higher yields than those common today. The United States and Canada in recent decades are well noted for their ability to develop high levels of production of agricultural crops relative to the rest of the world. Limited energies by genetic agricultural research organizations in North America have thus far been applied to enhancing the productivity of hemp. With a concerted effort from the North American agricultural research community, it is reasonable to conclude yields substantially greater than those present in Hungary can be achieved in 4 – 10 years of genetic development.
|Stem Yields (dry tons/acre)||Location|
Source: Nelson 1996 and Personal Communication.
Note: Stem yield is only the yield from the bast fiber itself. The amount of raw stalk is 10x as much as the stem yield. So for 4.0 tons per acre of bast fiber, that is 40 tons per acre of stalk.
Advantages of Hemp
Hemp shows the following strengths:
- Hemp’s fiber-bundles are stronger and tougher than those of kenaf, generally comparable to varieties of flax, and most other known fiber species.
- Hemp is generally pest resistant, drought resistant, and light frost resistant.
- With proper leaf removal, hemp has low net nutrient requirements and requires minimal cultivation.
- Hemp provides greater fiber yields in areas generally north of the 40th latitude than most other fiber crops, generally surpassing flax by 10%.
Disadvantages of Hemp
Hemp also has the following weaknesses:
- Restrictions of its growth and cultivation in North America, especially in the United States.
- Lower fiber yields than kenaf and other tropical species in the warmer portions of the United States and more southerly regions.
- Lower bast fiber portions relative to kenaf and flax.
In our initial testing we became aware of the strength of the fiber bundles of the bast fibers. Table 2 below illustrates the fiber bundle tensile strength properties of hemp when compared to those of wood species. (Douglas fir, Southern Pine, Aspen vs. Hemp, Kenaf, Flax). In light of this issue, higher structural applications appear the most promising. This value is an excellent measure of the structural performance we can expect in a particular size and configuration of a product.
Moreover, Table 2 also indicates desirable Length/Diameter Fiber Ratios. These may also contribute to favorable structural characteristics.
|Fibrous Material||Fiber Density (g/cm3)||Average Length (mm)||Average Diameter (um)||L/D Ratio||Tensile Strength (PSI)|
Sources: Wood Handbook; Danforth International; W.S.U., WMEL; Columbus, 1996, Institute of Natural Fibers, U.S.D.A., A.R.S.; The BioComposite Center.
Editor’s Note: According to David, research since this paper was published has shown that high THC variants of cannabis which are not allowed to flower exhibit even higher tensile strengths. While the exact cause is unknown, vegetative state, high-THC variants are even more useful for fiber.
Virtually all of Western Europe, including The United Kingdom, France, The Netherlands, and Germany, as well as Australia, have legalized low-THC varieties of hemp to be grown for industrial purposes. Beginning in 1998, Canada has now legalized the growth of hemp for commercial production and processing. The legalization of the production of industrial hemp is proceeding in several state legislatures in the United States at this time. Industrial hemp that contains only non-leafy material is currently allowed in all of the states for industrial processing.
Editor’s Note: Since this paper was written, there have been exciting advances in the US for hemp and cannabis growers. Keep an eye on current legislation!
A number of viable alternative materials for composite material applications exist. These alternative materials do pose certain limitations and require a certain degree of adjustment in the equipment to accommodate their use in industrial processes.
These materials, when properly selected, however, do hold meaningful promise as either primary or secondary raw materials for a composite facility. The physical properties of agricultural products are sufficiently broad that these materials may be used as possible binding agents, and as low-cost materials for MDF or particleboard.
Bast plants can provide competitively low-cost material suitable for particleboard and MDF applications. Furthermore, the desirable combination of physical, mechanical, and chemical aspects lend the bast fiber plants quite well to high-end structural composite applications. Exciting and profitable opportunities await those who pursue these applications further.
Please see the original paper for a full list of references.
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