Chapter 13
Biofibers Downloaded by OHIO STATE UNIV LIBRARIES on June 17, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch013
Y. Xu*,1 and R. M. Rowell2 1State
Key Laboratory, Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130021, China 2Department of Biological Systems Engineering, University of Wisconsin, Madison, WI 53706, USA *
[email protected] We depend on our forests and agricultural lands to supply most of our biofiber needs. For pulp and paper, we depend on a sustainable wood resource while turning to nonwood agricultural biomass for alternative supply. For building materials, such as biofiber boards, we also depend on wood however many countries are running out of wood and are beginning to depend more and more on biofibers from nonwood resources. While wood is the largest single source of biofiber in the world, nonwood agricultural biomass including bast, leaf and seed also contain biofibers that can be utilized. Biofibers can be used to produce a wide variety of products including biofiber boards (both flat and three dimensional, with and without an added adhesive, low to high density), molded biofiber (with and without added adhesives) for non-structural applications, biofiber nonwoven for filters and sorbents, and biofiber-reinforced composites for automobile and construction applications. Since biofibers are degradable by microorganisms, swell and shrink with changing moisture contents, combust and are degraded by ultraviolet radiation, we can modify the fibers to improve performance and increase the service life of the biofiber composites.
© 2011 American Chemical Society In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Introduction Biofibers Biofibers in this chapter refer to lignocellulose and cellulose isolated from plant biomass, cellulose produced by bacteria and by tunicates (sea squirts) and regenerated cellulose that is chemically regenerated from natural cellulose. Hair, feather, wool and silk fibers are also biofibers however these will not be covered in this chapter.
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Lignocellulose
a. Source of Supply Lignocellulose is the most abundant renewable feedstock with about 200 billion tons produced annually (1), and it can be found in all plant biomass. In accordance with botanical classification, plant biomass can be grouped into six types as follows. Type 1: Bast − jute, flax, hemp, ramie, kenaf, mesta, roselle Type 2: Leaf − banana, sisal, pineapple, henequen, agave Type 3: Seed − coir, cotton, kapok; coconut coir, oil palm empty fruit, bunches, rice, wheat, oat, rye Type 4: Core − kenaf, hemp, jute, flax Type 5: Grass − wheat, oat, barley, bamboo, corn, rice, bagasse Type 6: Other − wood, roots There are two general classes of plant biomass producing lignocellulose: primary and secondary. Primary plant biomass are those grown for their lignocellulose content while secondary plant biomass are those where the lignocellulose comes as a by-product from other primary utilization. Jute, hemp, kenaf, sisal, and cotton are examples of primary plant biomass while pineapple leaf (PALF), cereal stalks, agave, oil palm empty fruit bunches (EFB) and coconut coir (coir) are examples of secondary plant biomass. Some plant biomass contains more than one type of lignocellulose. For example, jute, flax, hemp, and kenaf have both bast and core type of lignocellulose. Lignocellulose may be of wood and non-wood origin where wood lignocellulose is subdivided into softwood and hardwood type, and non-wood lignocellulose is subdivided into bast, leaf, seed, core and grass type. Table I shows an inventory of some of the world plant biomass now produced. The data for this table are extracted from several sources using estimates and extrapolations for some of the numbers. For this reason, the data should be considered to be only an estimated quantity of world plant biomass. The inventory of agricultural biomass production can be found in the FAO database on its web site. By using a harvest index, it is possible to estimate the quantity of harvest residue associated with a given production of a crop. 324 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Table I. An inventory of some of the world plant biomass Source of Lignocellulose
Dry weight (tone)
Wood
1,750,000,000
Corn stalks
750,000,000
Wheat straw
600,000,000
Rice straw
360,000,000
Sorghum stalks
252,000,000
Barley straw
195,000,000
Sugarcane bagasse
102,200,000
Cotton stalks
68,000,000
Oil palm (Fronds + EFB)
57,000,000
Oat straw
55,000,000
Rye straw
40,000,000
Reeds
30,000,000
Bamboo
30,000,000
Cotton staple
18,300,000
Stem fibers (Kenaf, Jute)
13,700,000
Papyrus
5,000,000
Grass seed straw
3,000,000
Flax (oil seed)
2,000,000
Cotton linters
2,700,000
Leaf fibers (sisal, henneguen, maguey)
500,000
Esparto grass
500,000
Sabai grass
200,000
Hemp fibers
200,000
Abaca
80,000
Total
4,335,380,000
b. Structure and Composition Depending on the type of plant biomass, plant parts and growth conditions, the structure and composition of lignocellulose vary greatly. Lignocellulose is a natural biocomposite of three major constituents, cellulose, hemicelluloses and lignin (2, 3). The structure of lignocellulose is illustrated in Figure 1 and it can be described as cellulose-cemented in a matrix of lignin coupled by hemicelluloses.
325 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Figure 1. Illustration of lignocellusic structure. (Reproduced with kind permission (6) and Copyright of Nature)
Cellulose is the most abundant organic polymer on earth, and its characteristics shall be reviewed separately below. Lignin is the second most abundant biopolymer on earth next to cellulose. It is commonly believed that lignin is an amorphous copolymer of phenyl-propene units formed through random radical copolymerization of coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (4). Lignin is hydrophobic and it dissolves in alkaline but does not dissolve in most of organic solvents. It has high carbon and low hydrogen composition, and contains hydroxyl, methoxyl, ethylenic as its main functional groups (5). Lignin can be further polymerized via radical condensation. It is the matrix component of lignocellulose, and serves to glue cellulose, hemicelluloses and other cell wall components together. Generally, the higher the lignin content the harder the plant biomass. Lignin is responsible for the UV degradation of lignocellulose. Hemicelluloses are amorphous and hydrophilic hetero-polysaccharides of C5- and C6-sugars. The polysaccharide chains of hemicelluloses are highly branched. The degree of polymerization of hemicelluloses is around 300-500. Hemicelluloses can be easily hydrolyzed by dilute acids, alkaline and selected enzymes. They contain hydroxyl and acetyl functional groups, and are responsible for the biodegradation of lignocellulose.
326 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Table II. Chemical compositions of selected lignocellulose (9–14) Cellulose %
Lignin %
Hemicelluloses %
Ash %
Softwood
40-45
26-34
25-30
0.2-0.8
Hardwood
45-50
22-30
21-36
0.2-0.8
Cotton
85-90
0.7-1.6
5.7
0.8-2
Wheat straw
48.8
17.1
15-31
4-9
Rice straw
41-57
8-19
33
15-20
Rice husk
35-45
20
19-25
14-17
Ramie
68.6-76.2
0.6-0.7
13.1-16.7
-
Hemp
70.2-74.4
3.7-5.7
17.9-22.4
0.8
Flax
64-71
2-5
18.6-20.6
5
Kenaf
31-39
15-19
21.5-23
2-5
61-71.5
12-13
13.6-20.4
0.5-2
Abaca
56-63
7-9
15-17
3
Sisal
67-78
8-11
10-14.2
0.6-1
PALF
70-82
5-12
-
0.7-0.9
Henequen
77.6
13.1
4-8
-
Coir
36-43
41-45
0.15-0.25
2.7-10.2
Oil palm EFB
37-42
20-21
24-27
3.5
Jute
Lignocellulose are valued for its intrinsic attributes (i) biodegradability, (ii) abundance with diversified sources of supply, (iii) low density, (iv) high specific strength (v) high stiffness (vi) non-abrasiveness (vii) good thermal properties (viii) polar and hydrophilic surface. These attributes are the result of its compositions and hierarchical structure (7). On the other hand, lignocellulose is disadvantaged in commercial applications by its (i) hydrophilic surface (ii) low processing temperature (
