The use of biomass to produce bio-based composites and building materials
R.M. Rowell, University of Wisconsin, USA
Abstract:
There are many different types of natural fibers coming from a wide variety of plant sources. These fibers have been used by humans since the beginning of human existence for clothing, heating, shelter, weapons, tools, packaging and writing materials. This chapter will deal with the use of biomass in biocomposites. These will include waferboard, flakeboard, particleboard, fiberboard, geotextiles, filters, and sorbents as well as physical and chemical properties of natural fibers. In most cases, natural resources have been used in composites without any modification for high volume, low cost and low performance products. It is possible to improve property performance through chemical modification so that natural, renewable, recyclable, and sustainable fibers can be used in value added markets.
Key words
biomass; natural fibers; resources; types; isolation; properties; waferboard; flakeboard; particleboard; fiberboard; geotextiles; filters; sorbents; improved performance; value added; emerging trends
25.1 Introduction
25.1.1 History of bio-based composites
We have used wood and other types of biomass for many applications since the beginning of the human race. The earliest humans used biomass to make shelters, cook food, construct tools, and make boats and weapons. There are human marks on a climbing pole that were made over 300,000 years ago. We have found wood in the Egyptian pyramids, Chinese temples and tombs and ancient ships that attest to the use of wood by past societies. The use of natural fibers dates back in history to about 8,000 BC. Linen and hemp fabrics are known to have existed at that time and linen textiles are known to have existed in Europe from about 4,000 BC. Reference is made to the use of textiles as reinforcements of ceramics as early as 6,500 BC. Ramie is known to have been used in mummy cloths in Egypt during the period 5,000 ± 3,300 BC. Grass and straws have been used for many generations as a reinforcing fiber in mud bricks (adobe) and in ancient Egypt 3,000 years ago, pharaoh mummies were wrapped in linen cloth impregnated with salts, resins, and honey to protect and reinforce them. The use of flax for the production of linen dates back over 5,000 years. Pictures on tombs and temple walls at Thebes depict flowering flax plants. Cotton fibers have been found in caves in Mexico that date back over 7,000 years. There are references in early Chinese history to natural fibers for papermaking. Hemp fiber implants that are over 10,000 years old have been found in pottery shards in China and Taiwan. The use of hemp fiber dates back to the Stone Age. Thomas Jefferson drafted the United States Declaration of Independence on hemp paper (Rowell, 2008).
25.1.2 Biomass resources
As we begin the twenty-first century, there is an increased awareness that non-renewable resources are becoming scarce, and dependence on renewable resources is growing. The twenty-first century may be the cellulosic century as we look more and more to renewable plant resources for products. It is easy to say that natural fibers are renewable and sustainable but, in fact, they are neither. Natural fibers come from plants and it is the living plants that are renewable and sustainable, not the fibers themselves. This means that we must put our emphasis on healthy forests and agricultural lands, and to manage and use our ecosystems in ways that do not put them at risk.
25.2 Fibrous plants
In terms of utilization, there are two general classifications of plants producing natural fibers: primary and secondary. Primary plants are those grown for their fiber content, while secondary plants are those where the fibers come as a by-product from some other primary utilization. Jute, hemp, kenaf, sisal, and cotton are examples of primary plants, while pineapple, cereal stalks, agave, oil palm, and coir are examples of secondary plants (Rowell, 2008).
Table 25.1 shows an inventory of some of the major fibers now produced in the world. While wood is the major source of fiber, plant straws and stalks combined are potentially a larger source of fiber than wood. The data for this table were extracted from several sources using estimates and extrapolations for some of the numbers. For this reason, the data should be considered as only a rough relative estimate of world fiber resources. The inventory of many agricultural resources can be found in the FAO database on its website. By using a harvest index, it is possible to determine the quantity of residue associated with a given production of a crop.
Table 25.1
| Source | Metric tons |
| Wood | 1,750,000,000 |
| Straw | 1,145,000,000 |
| Stalks | 970,000,000 |
| Bagasse | 75,000,000 |
| Reeds | 30,000,000 |
| Bamboo | 30,000,000 |
| Cotton staple | 15,000,000 |
| Core fiber | 8,000,000 |
| Papyrus | 5,000,000 |
| Bast fiber | 2,900,000 |
| Cotton linters | 1,000,000 |
| Grasses | 700,000 |
| Leaf | 480,000 |
| Total | 4,033,080,000 |
25.3 Fiber types and isolation
25.3.1 Fiber types
There are many ways to classify natural fibers. Some authors classify fibers as to industrial use, i.e. papermaking, textile, composites. Others use systems such as hard and soft fiber, long and short fibers, cellulose content, strength, color, etc. The most common classification for natural fibers is by botanical type. Using this system, there are six basic types of natural fibers:
1. bast fibers such as jute, flax, hemp, ramie, and kenaf;
2. leaf fibers such as banana, sisal, agave, and pineapple;
3. seed fibers such as coir, cotton, and kapok;
4. core fibers such as kenaf, hemp, and jute;
Table 25.2 gives a more complete list of fiber types. Some plants yield more than one type of fiber. For example, jute, flax, hemp, and kenaf have both bast and core fibers. Agave, coconut, and oil palm have both fruit and stem fibers. Cereal grains have both stem and hull fibers (Rowell, 2008).
Table 25.2
Six general types of natural fibers
| Bast | Leaf | Seed | Core | Grass/reeds | Other | ||||
| Fibers | Pod | Husk | Fruit | Hulls | |||||
| Hemp | Pineapple | Cotton | Kenaf | Wheat | Wood | ||||
| Ramie | Sisal | Kapok | Jute | Oat | Roots | ||||
| Flax | Agava | Loofah | Hemp | Barley | Galmpi | ||||
| Kenaf | Henequen | Milk weed | Flax | Rice | |||||
| Jute | Curaua | Coir | Bamboo | ||||||
| Mesta | Banana | Oil palm | Bagasse | ||||||
| Urena | Abaca | Rice | Corn | ||||||
| Roselle | Palm | Oat | Rape | ||||||
| Cabuja | Wheat | Rye | |||||||
| Albardine | Rye | Esparto | |||||||
| Raphia | Sabai | ||||||||
| Curauà | Canary | ||||||||
| grass | |||||||||
25.3.2 Isolation of bioelements
For biocomposites such as chip, flake and strand boards, wood is the major source of these elements. Green wood is debarked and either run through as chipper, flaker or strand-making equipment. The chips, flakes or strands are then dried, an adhesive is added and a mat of the elements is formed. The mat is then run into a press, the press is closed and heated for the assigned time. The panels are cooled, edge cut to size, face sanded to a standard thickness and stacked for shipment.
For fiberboards, many different types of biomass plants can be used. Fiber can be isolated from the plant using many different types of procedures from simple mechanical methods to chemical treatments.
It has been known since ancient times that fibers from plants such as jute, flax and hemp could be isolated by allowing the plant to rot either in the field or in a pond and then beaten on rocks to loosen the fibers from the plant. In the process, the bark is removed by enzymatic action from microorganisms (molds and bacteria) in the water. This process is also used for hemp and kenaf. The entire plant is placed in a pond and the natural decay process removes the bark and separates the long bast fiber from the core or stick. The process mainly removes pectic substances which frees the fiber bundles from the woody core. The process takes 2–3 weeks (shorter times if the water is warm). Once the bark has been removed, the fiber is removed and dried.
Jute, hemp and kenaf fibers can also be isolated using a decorticating machine. The plant is squeezed through slotted rollers that spread the plant apart and then slotted rollers with cutting edges cut and separate the fibers from the other plant tissue. The plant may have to be run through the decortication equipment several times for complete fiber isolation.
Another mechanical separation method carried out on cotton uses a cotton gin. A cotton gin separates the cotton fibers from the seed pods and the seeds. It uses a combination of a wire screen and small wire hooks that pull the cotton through the screen while brushes continuously remove the loose cotton lint to prevent jams.
Plants can be broken down into fiber bundles and single fibers by grinding or refining. In the grinding process, the wood is mechanically broken down into fibers. In the refining process, wood chips or plant parts can be broken down into fibers between one or two rotating plates in a wet environment to fibers. If the refining is done at high temperatures, the fibers tend to slip apart due to the softening of the lignin matrix between the fibers, and the fibers have a lignin rich surface. If the refining is performed at lower temperatures, the fibers tend to break apart and the surface is rich in carbohydrate polymers (Rowell, 2008).
Chemical separation can also be carried out by chemical pulping. Dilute alkali or dilute acids can be used to separate fiber bundles. Pulping is usually done to reduce the fiber bundles to separate fibers for paper making. Alkali, acid, and organo-solv pulping methods have been used to isolate fibers from plants.
25.4 Fiber properties
The chemical properties of some natural fibers are given in Table 25.3. The mechanical properties of some natural fibers are shown in Table 25.4. The tensile and flexural properties are shown in Table 25.5.
Table 25.3
Chemical composition of some natural fibers
| Type of fiber | Cellulose | Lignin | Pentosan | Ash | Silica |
| Stalk fiber: Straw | |||||
| Rice | 28–48 | 12–16 | 23–28 | 15–20 | 9–14 |
| Wheat | 29–51 | 16–21 | 26–32 | 4.5–9 | 3–7 |
| Barley | 31–45 | 14–15 | 24–29 | 5–7 | 3–6 |
| Oat | 31–48 | 16–19 | 27–38 | 6–8 | 4–6.5 |
| Rye | 33–50 | 16–19 | 27–30 | 2–5 | 0.5–4 |
| Cane fiber | |||||
| Sugar | 32–48 | 19–24 | 27–32 | 1.5–5 | 0.7–3.5 |
| Bamboo | 26–43 | 21–31 | 15–26 | 1.7–5 | 0.7 |
| Grass fiber | |||||
| Esparto | 33–38 | 17–19 | 27–32 | 6–8 | – |
| Sabai | – | 22 | 24 | 6 | – |
| Reed fiber | |||||
| Phragmites Communis | 44–46 | 22–24 | 20 | 3 | 2 |
| Bast fiber | |||||
| Seed flax | 43–47 | 21–23 | 24–26 | 5 | – |
| Kenaf | 44–57 | 15–19 | 22–23 | 2–5 | – |
| Jute | 45–63 | 21–26 | 18–21 | 0.5–2 | – |
| Hemp | 57–77 | 9–13 | 14–17 | 0.8 | – |
| Ramie | 87–91 | – | 5–8 | – | – |
| Core fiber | |||||
| Kenaf | 37–49 | 15–21 | 18–24 | 2–4 | – |
| Jute | 41–48 | 21–24 | 18–22 | 0.8 | – |
| Leaf fiber | |||||
| Abaca | 56–63 | 7–9 | 15–17 | 3 | – |
| Sisal | 47–62 | 7–9 | 21–24 | 0.6–1 | – |
| Seed hull fiber | |||||
| Cotton linter | 99–95 | 0.7–1.6 | 1–3 | 0.8–2 | – |
| Wood fiber | |||||
| Coniferous | 40–45 | 26–34 | 7–14 | < 1 | – |
| Deciduous | 38–49 | 23–30 | 19–26 | < 1 | – |
Table 25.4
Mechanical properties of some natural fibers
| Fiber | Density (g/m3) | Length (mm) | Diameter (μm) | Elongation at break (MPa) |
| Cotton | 1.21 | 15–56 | 12–35 | 2–10 |
| Coir | 0.3–3.0 | 7–30 | 15–25 | |
| Flax | 1.38 | 10–65 | 5–38 | 1.2–3 |
| Jute | 1.23 | 0.8–6 | 5–25 | 1.5–3.1 |
| Sisal | 1.20 | 0.8–8 | 7–47 | 1.9–3 |
| Hemp | 1.35 | 5–55 | 10–51 | 1.6–4.5 |
| Henequen | 1.4 | 8–33 | 3–4.7 | |
| Ramie | 1.44 | 40–250 | 18–80 | 2–4 |
| Kenaf (bast) | 1.2 | 1.4–11 | 12–36 | 2.7–6.9 |
| Kenaf (core) | 0.31 | 0.4–1.1 | 18–37 | |
| Pineapple | 1.5 | 3–8 | 8–41 | 1–3 |
| Bagasse | 1.2 | 0.8–2.8 | 10–34 | 0.9 |
| Southern yellow pine | 0.51 | 2.7–4.6 | 32–43 | |
| Douglas fir | 0.48 | 2.7–4.6 | 32–43 | |
| Aspen | 0.39 | 0.7–1.6 | 20–30 |
Table 25.5
Tensile and flexural properties of some natural fibers
| Fiber | Tensile – MOR (MPa) | Tensile – MOE (GPa) | Flexural– MOR (MPa) | Flexural – MOR (GPa) |
| Wood | 30.5 | 8.2 | 55.3 | 7.5 |
| Bagasse | 27.0 | 5.4 | 47.8 | 5.1 |
| Coir | 25.9 | 3.6 | 46.9 | 3.6 |
| Curauà | 48.1 | 7.1 | 77.6 | 6.1 |
| Flax | 36.1 | 6.1 | 58.4 | 5.8 |
| Hemp | 33.6 | 6.1 | 61.5 | 6.2 |
| Jute | 34.6 | 7.2 | 57.8 | 6.9 |
| Ramie | 43.2 | 5.4 | 70.2 | 5.1 |
| Sisal | 34.3 | 7.1 | 60.0 | 6.6 |
25.5 Types and properties of bio-based composites
Bio-based composite products started with very thick laminates for glue laminated beams, to thin veneers for plywood, to strands for strandboard, to flakes for flakeboard, to particles for particleboard, and, finally, to fibers for fiberboard. As the size of the composite element gets smaller, it is possible to either remove defects (knots, cracks, checks, etc.) or redistribute them to reduce their effect on product properties. Also, as the element size becomes smaller, the composite becomes more like a true material, i.e. consistent, uniform, continuous, predictable, and reproducible.
Composite materials can be classified by several different systems: density (e.g., medium density fiberboards, see Fig. 25.1), application (e.g., insulation board), raw material form (e.g., particleboard) and process type (e.g., dry process fiberboard). The breakdown of biomass can include large timbers, dimensional lumber, very thick laminates for glue laminated beams, to thin veneers for plywood, to strands for strandboard, to flakes for flakeboard, to chips for chipboard, to particles for particleboard, and, finally, to fibers for fiberboard. However, since the bulk of this chapter deals with the utilization of small biomass, only composites made using small bio-members, strands, chips or flakes and fiber will be covered.
25.5.1 Selection based on density
Figure 25.1 shows different types of composites based on the product density. Insulation type products have a very low density while high density hardboards have a much higher density. The density of the biomass cell wall is approximately 1.5.
25.5.2 Types of bio-based composites
Waferboard and flakeboard
Waferboard is a structural panel used in exterior applications bonded with a phenolic adhesive. Large thin wafers or smaller flakes can be produced by several methods and used to produce a composite board. Wafers are almost as wide as they are long, while flakes are much longer than they are wide. Wafers are also thicker than flakes. A waferizer slices the wood into wafers, typically 38 mm wide × 76 to 150 mm long × 7 mm thick, with a specific gravity between 0.6 and 0.8.
The first waferboard plant was opened in 1963 by MacMillian Bloedel in Saskatchewan, Canada. Aspen was the raw material and the wafers were randomly oriented. In the late 1980s, most wafers were oriented resulting in oriented waferboard (OWB) that is stronger and stiffer than the randomly oriented board. The orientation distribution may be tailored to the application. OWB and OSB compete with plywood in applications such as single layer flooring, sheathing, and underlayment in lightweight structures; however, OSB has largely replaced OWB in most places in the US. Pines, firs and spruce are usually used as well as aspen.
The flakeboard industries started in the early 1960s. These are made using an exterior grade adhesive and are used as the structural skin over wall and floor joists. The specific gravity of flakeboard is usually between 0.6 and 0.8 and flakeboard is made using a waterproof adhesive such as phenol formaldehyde or an isocyanate.
Particleboard
The particleboard industry started in the 1940s out of a need to use large quantities of waste products such as sawdust, planer shavings and other mill residues. Particles of various sizes are formed into an air or mechanically formed mats and glued together to produce a randomly oriented flat panel. Almost all particleboards are produced by a dry process. Particleboard is usually made in three layers with the faces made using fine particles and a core of coarser material. Most applications of particleboard are for interior use and are bonded using a urea-formaldehyde adhesive. Paraffin or microcrystalline wax emulsion is usually added to improve short-term moisture resistance. Phenol-formaldehyde, melamine-formaldehyde and isocyanates are rarely used but are when increased moisture resistance is required. The resin content ranges from 4 to 10% but is usually made using 6–9%. The resin content of the two faces is usually slightly higher than the core. Table 25.6 shows some properties of different grades of particleboard.
Table 25.6
| Grade | Modulus of rupture (MPa) | Modulus of elasticity (MPa) | Internal bond (MPa) |
| H-1 | 16.5 | 2400 | 0.90 |
| H-2 | 20.5 | 2400 | 0.90 |
| H-3 | 23.5 | 2750 | 1.00 |
| M-1 | 11.0 | 1725 | 0.40 |
| M-2 | 14.5 | 2225 | 0.45 |
| M-3 | 16.5 | 2750 | 0.55 |
| LD-1 | 3.0 | 550 | 0.10 |
| LD-2 | 5.0 | 1025 | 0.15 |
H = density greater than 800 kg/m3, M = density 640-800 kg/m3, LD = density less than 640 kg/m3.
Source: National Plywood Association (1993).
Particleboard is often used as a core material for veneers and laminates. These are often used in counter tops, shelving, doors, room dividers, built-ins and furniture. Particleboard generally does not warp in use. It is available in several thicknesses from 6 to 32 mm in sheets of 120 × 240 cm.
Fiberboard
Fiberboards can be formed using a wet-forming or a dry-forming process. In a wet-forming process, water is used to distribute the fibers into a mat and then pressed into a board. In many cases an adhesive is not used and the lignin in the fibers serves as the adhesive. In the dry process, fibers from the refiner go through a dryer and blowline where the adhesive is applied and then formed into a web which is pressed into a board.
Low density fiberboards (LDF) have a specific gravity of between 0.15 and 0.45 and are used for insulation and for lightweight cores for furniture. They are usually produced using a dry process with a ground wood fiber. Medium density fiberboard (MDF) has a specific gravity of between 0.6 and 0.8 and is mainly used as a core for furniture and is usually made using a dry process. High density fiberboard (HDF), sometimes called hardboard, has a specific gravity of between 0.85 and 1.2 and is used as an overlay on workbenches, floors and for siding. It is usually made using a dry process but is also made using a wet fiber process. The hardboard industry started around 1950. Hardboard is produced both with wax (tempered) and without wax and sizing agents. The wax is added to give the board water resistance.
Urea-formaldehyde resin is usually used for interior applications and phenol-formaldehyde for exterior application. Table 25.7 gives the mechanical properties of LDF, MDF and HDF. Standard (without wax) and tempered (with wax) hardboards come in many different thicknesses from 2.1 to 9.5 mm and have required standard minimum average modulus of rupture of 31.0 MPa for standard and 41.4 MPa for tempered. Standard hardboard must have a required standard minimum average tensile strength parallel to the surface of 15.2 MPa and 20.7 MPa for tempered. The required standard minimum average tensile strength perpendicular to the surface is 0.62 MPa for standard and 0.90 MPa for tempered.
Table 25.7
| Product | Thickness (mm) | Modulus of Rupture (MPa) | Modulus of Elasticity (MPa) | Internal bond (MPa) |
| Internal MDF | ||||
| HDF | < 21 | 34.5 | 3,450 | 0.75 |
| MDF | > 21 | 24.0 | 2400 | 0.60 |
| LDF | 21 | 14.0 | 1400 | 0.30 |
| Exterior | ||||
| MDF | 21 | 34.5 | 3,450 | 0.90 |
Source: National Plywood Association (1994).
Molded products
The present bio-based composite industry mainly produces two-dimensional (flat) sheet products. In some cases, these flat sheets are cut into pieces and glued/fastened together to make shaped products such as drawers, boxes, and packaging. Flat sheet fiber composite products are made by making a gravity formed mat of fibers with an adhesive and then pressing. If the final shape can be produced during the pressing step, then the secondary manufacturing profits can be realized by the primary board producer. Instead of making low cost flat sheet type composites, it is possible to make complex shaped composites directly using the long fibers alone or combinations of long and short fibers.
Wood fibers come in two lengths: short (softwoods) and shorter (hardwoods). So wood fiber is limited to short fiber applications unless it is combined with longer agricultural fibers for applications in a wider array of products. In this technology, fiber mats are made by combining long bast or leaf fibers from such plants as kenaf, jute, cotton, sisal, agave, etc., with wood fiber and then formed into flexible fiber mats. These can be made by physical entanglement (carding), non-woven needling, or thermoplastic fiber melt matrix technologies. In carding, the fibers are combed, mixed and physically entangled into a felted mat. These are usually of high density but can be made at almost any density. A needle punched mat is produced in a machine which passes a randomly formed machine-made web through a needleboard that produces a mat in which the fibers are mechanically entangled. The density of this type of mat can be controlled by the amount of fiber going through the needleboard or by overlapping needled mats to give the desired density. In the thermoplastic fiber matrix, the bio-based fibers are held in the mat using a thermally softened thermoplastic fiber such as polypropylene or polyethylene.
During the mat formation step, an adhesive is added by dipping or spraying the fiber before mat formation or added as a powder during mat formation. The mat is then shaped and densified by a thermoforming step. The final desired shape is determined by the mold in the hot press. Within certain limits, any size, shape, thickness, and density is possible.
Geotextiles
Medium- to high-density fiber mats described before can be used in several ways other than for molded composites. One is for use as a geotextile. Geotextiles derive their name from the two words ‘geo’ and ‘textile’ and, therefore, mean the use of fabrics in association with the earth.
Geotextiles have a large variety of uses. They can be used for mulch around newly planted seedlings. The mats provide the benefits of natural mulch; in addition, controlled-release fertilizers, repellents, insecticides, and herbicides can be added to the mats as needed. Medium density fiber mats can also be used to replace dirt or sod for grass seeding around new home sites or along highway embankments. Grass or other types of seed can be incorporated in the fiber mat. Fiber mats promote seed germination and good moisture retention. Low and medium density fiber mats can be used for soil stabilization around new or existing construction sites, where steep slopes, without root stabilization, can lead to erosion and loss of top soil.
Medium and high density fiber mats can also be used below ground in road and other types of construction as a natural separator between different materials in the layering of the backfill. It is important to restrain slippage and mixing of the different layers by placing separators between the various layers.
It is estimated that the cost of controlling erosion in the United States is in excess of $55 million/year. This is a large potential market for forest resource composites.
Filters and sorbents
Filter systems are presently in use to clean our water, but new innovation in filtration technology is needed to remove contaminants from water. The development of filters to clean our water supply is big business. It is estimated that global spending on filtration (including dust collectors, air filtration, liquid cartridges, membranes and liquid macro-filtration) will increase from $17 billion in 1998 to $75 billion by 2020. The fastest-growing non-industrial application area for filter media is for the generation of clean water.
Medium to high density mats can be used as filtering aids to remove particulates out of waste and drinking water or solvents. Wood fibers have also been shown to sorb oil from water.While not as good as other agricultural fibers such as kenaf, wood fiber can remove oil from both fresh- and seawater.
25.5.3 Moisture, biological, ultraviolet, and thermal properties
Biomass changes dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding. The hemicelluloses are mainly responsible for moisture sorption, but the accessible cellulose, noncrystalline cellulose, lignin, and surface of crystalline cellulose also play major roles. Moisture swells the cell wall, and the fiber expands until the cell wall is saturated with water (fiber saturation point, FSP). Beyond this saturation point, moisture exists as free water in the void structure and does not contribute to further expansion. This process is reversible, and the fiber shrinks as it loses moisture below the FSP.
Biomass is degraded biologically because organisms recognize the carbohydrate polymers (mainly the hemicelluloses) in the cell wall and have very specific enzyme systems capable of hydrolyzing these polymers into digestible units. Biodegradation of the cell wall matrix and the high molecular weight cellulose weakens the fiber cell. Strength is lost as the cell wall polymers and matrix undergo degradation through oxidation, hydrolysis, free radical and dehydration reactions.
Biomass exposed outdoors undergoes photochemical degradation caused by ultraviolet radiation. This degradation takes place primarily in the lignin component, which is responsible for the characteristic color changes. The lignin acts as an adhesive in the cell walls, holding the cellulose fibers together. The surface becomes richer in cellulose content as the lignin degrades. In comparison to lignin, cellulose is much less susceptible to ultraviolet light degradation. After the lignin has been degraded, the poorly bonded carbohydrate-rich fibers erode easily from the surface, which exposes new lignin to further degradative reactions. In time, this ‘weathering’ process causes the surface of the composite to become rough and can account for a significant loss in surface fibers.
Biomass burns because the cell wall polymers undergo pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The gases are ignited by some external source and combust. The hemicelluloses and cellulose polymers are degraded by heat much before the lignin. The lignin component is the most thermally stable of the cell wall polymers and contributes to char formation. The charred layer helps insulate the composite from further thermal degradation.
25.6 Improving performance properties
In general, bio-based composites are used in large, low cost, low performing markets. Since we know that bio-based resources swell and shrink with changing moisture contents, decay, burn and are degraded by ultraviolet energy, we think they cannot be used in high performance applications.
For the most part, we have designed and used biomass putting up with the ‘natural defects’ that nature has given to us. Biomass is a hygroscopic resource that was designed to perform, in nature, in a wet environment. Nature is programmed to recycle it in a timely way through biological, thermal, aqueous, photochemical, chemical, and mechanical degradations. In simple terms, nature builds biomass from carbon dioxide and water and has all the tools to recycle it back to the starting chemicals. We harvest a green plant and convert it into dry products, and nature, with its arsenal of degrading reactions, starts to reclaim it at its first opportunity.
The properties of any resource are, in general, a result of the chemistry of the components of that resource. In the case of wood, the cell wall polymers (cellulose, hemicelluloses, and lignin) are the components that, if modified, would change the properties of the resource. If the properties of biomass are modified, the performance of the modified biomass would be changed. This is the basis of chemical modification to change properties and improve performance.
In order to produce bio-based materials with a long service life, it is necessary to interfere with the natural degradation processes for as long as possible (Hon, 1993). This can be done in several ways. Traditional methods for decay resistance and fire retardancy, for example, are based on treating the product with toxic or corrosive chemicals which are effective in providing decay and fire resistance but can result in environmental concerns. In order to make property changes, you must first understand the chemistry of the components and the contributions each play in the properties of the resource. Following this understanding, you must then devise a way to modify what needs to be changed to get the desired change in property.
Properties of biomass, such as dimensional instability, flammability, biodegradability, and degradation caused by acids, bases, and ultraviolet radiation are all a result of chemical degradation reactions which can be prevented or, at least, slowed down if the cell wall chemistry is altered (Rowell, 2005). Two technologies to do this are already commercial: acetylation (Rowell, 2006) and heat treatments (Hill, 2006).
Almost all biomass contains some acetyl groups. Acetylation of biomass just increases that acetyl level to 15–20%. Acetylation using acetic anhydride has mainly been carried out as a liquid phase reaction. The reaction with acetic anhydride results in esterification of the accessible hydroxyl groups in the cell wall with the formation of acetic acid byproduct:
Acetylation is a single-site reaction which means that one acetyl group is on one hydroxyl group with no polymerization. This means that all of the weight gain in acetyl can be directly converted into units of hydroxyl groups blocked. Fiberboards have been made from acetylated wood, jute, kenaf, bamboo and coir fiber and the boards show greatly improved dimensional stability and decay resistance to both brown- and white-rot fungi. There is not improvement in thermal properties but some level of improving resistance to weathering (UV resistance) (Rowell, 2005).
Heating biomass to improve performance dates back many thousands of years. In the early part of the twentieth century it was found that drying wood at high temperature increased dimensional stability and led to a reduction in hygroscopicity. Later, it was found that heating wood also increased resistance to microbiological attack. Along with the increase in stability and durability come increased brittleness and loss in some strength properties, including impact toughness, modulus of rupture and work to failure. The treatments usually cause a darkening of the wood and the wood has a tendency to crack and split.
25.7 Conclusion and future trends
If we look into the future of materials, we need to reduce our dependence on non-renewable resources and increase our use of renewable resources. Biomass is recyclable, renewable and sustainable. To replace many non-renewable materials, biomass materials will be used and they must be stable and durable. For many countries, the biomass will be wood. But for countries like India, jute and coir will play a major role, for South America, sugar cane bagasse, for China, kenaf and hemp, and for Malaysia and Singapore, oil palm fiber will play a major role.
In the United States, small diameter trees in our overgrown forests will find a major use in composites. Wood species, such as red maple, not currently used for composites will emerge. And more agricultural fibers will be brought in both alone and in combination with wood fiber. New processes for conversion of plants to composites will be developed that will use less energy and water. New fiber isolation processes will be developed with higher yields and less fiber damage.
New chemistries will be developed to improve fiber property performance that will be used for value added composites. These modified bioarchitectures will find new markets in high value, high performance products. Finally, engineered bioarchitectures will emerge where fiber component parts are taken apart and put back together with greater strength and much higher performance. Biomaterials will find ‘new’ markets once lost to plastics, metals and inorganics.