336 16.From Natural Fibers to Man-Made Plastics spiration.These salts,however,eventually weaken the silk and destroy it. Vegetable The commercially important vegetable fibers are taken from Fibers those parts of plants that strengthen,stiffen,or otherwise sup- port their structure.Exceptions are seed hair,as in cotton,or the seed-pod fiber kapok. Bast fibers(see Tables 16.2 and 16.3)are taken from tall,reed- like plants whose stems they cover.The fibers are cemented to- gether by gums and need to be separated by a decomposing process called retting.This can be achieved by soaking the plants in water (water retting)or by spreading them out and thus ex- posing them to the weather (dew retting).In either case,the gummy substance is eventually broken down by microbiological agents which allow one to separate the fibers after about one to five weeks.Subsequent vigorous mechanical action splits the fibers further apart into even finer fibrils,which are soft and fine, that is,hairlike.The longer ones are called line fibers.They are particularly suitable for processing into yarns,textiles,and cordage.The shorter pieces are called tow.The physical proper- ties of bast and other fibers are listed in Table 16.2.Note the pro- nounced difference in fineness between bast and leaf fibers. Leaf fibers usually extend along the entire length of a leaf to reinforce its structure and keep it rigid.The fibers are embedded in a pulpy tissue which needs to be removed by mechanical scrap- ing,called decortication.Leaf fibers are generally hard and coarse compared to bast fibers(Table 16.2). The fibrils of bast and leaf fibers consist of a large number of elongated cells whose ends are cemented together.The hollow in- terior of the cells is called the lumen.The cell walls are composed of cellulose,that is,of essentially linear,and complex carbohy- drate macromolecules,each of which consists of hundreds or thousands of glucose units having the formula C6Hi00s.In other words,cellulose is a polymerized sugar.We shall return to macro- molecules in Section 16.3.The fibers are mostly crystalline and are separated by small amorphous regions.Cellulose is insoluble in water and is the most abundant of all naturally occurring or- ganic compounds.It is undigestible by humans but can be bro- ken down by microorganisms,for example,in the stomachs of certain herbivorous animals such as cows,horses,and sheep. The cementing constituent between the cells of woody tissue is called lignin,a complex macromolecule which is not based on carbohydrates.Pectic substances,that are capable of forming thick solutions and which consist of an associated group of poly- saccharides (sugars)are also contained in plant substances al-
spiration. These salts, however, eventually weaken the silk and destroy it. The commercially important vegetable fibers are taken from those parts of plants that strengthen, stiffen, or otherwise support their structure. Exceptions are seed hair, as in cotton, or the seed-pod fiber kapok. Bast fibers (see Tables 16.2 and 16.3) are taken from tall, reedlike plants whose stems they cover. The fibers are cemented together by gums and need to be separated by a decomposing process called retting. This can be achieved by soaking the plants in water (water retting) or by spreading them out and thus exposing them to the weather (dew retting). In either case, the gummy substance is eventually broken down by microbiological agents which allow one to separate the fibers after about one to five weeks. Subsequent vigorous mechanical action splits the fibers further apart into even finer fibrils, which are soft and fine, that is, hairlike. The longer ones are called line fibers. They are particularly suitable for processing into yarns, textiles, and cordage. The shorter pieces are called tow. The physical properties of bast and other fibers are listed in Table 16.2. Note the pronounced difference in fineness between bast and leaf fibers. Leaf fibers usually extend along the entire length of a leaf to reinforce its structure and keep it rigid. The fibers are embedded in a pulpy tissue which needs to be removed by mechanical scraping, called decortication. Leaf fibers are generally hard and coarse compared to bast fibers (Table 16.2). The fibrils of bast and leaf fibers consist of a large number of elongated cells whose ends are cemented together. The hollow interior of the cells is called the lumen. The cell walls are composed of cellulose, that is, of essentially linear, and complex carbohydrate macromolecules, each of which consists of hundreds or thousands of glucose units having the formula C6H10O5. In other words, cellulose is a polymerized sugar. We shall return to macromolecules in Section 16.3. The fibers are mostly crystalline and are separated by small amorphous regions. Cellulose is insoluble in water and is the most abundant of all naturally occurring organic compounds. It is undigestible by humans but can be broken down by microorganisms, for example, in the stomachs of certain herbivorous animals such as cows, horses, and sheep. The cementing constituent between the cells of woody tissue is called lignin, a complex macromolecule which is not based on carbohydrates. Pectic substances, that are capable of forming thick solutions and which consist of an associated group of polysaccharides (sugars) are also contained in plant substances alVegetable Fibers 336 16 • From Natural Fibers to Man-Made Plastics
16.2.Production and Properties of Natural Fibers 337 TABLE 16.2.Properties of selected fibers Tensile Fineness Tenacity strength Elongation Fiber (den)a (g/den) (MPa) (strain)(%) Color range Bast Fibers Jute 20 39 1.5 Creamy white to brown Flax (Linen) 5 心 66 1.5 White to brown Hemp 6 4 52 2.0 Light to grayish brown Ramie (China grass) 5 67 4.0 White to grayish brown Leaf Fibers Sisal 290 4 51 3.0 Creamy white to yellowish Henequen 370 39 5.0 Creamy white to reddish brown Abaca 190 5 64 3.0 Creamy white to (Manila hemp) dark brown Istle 360 2.5 32 5.0 White to reddish yellow Seed Fiber Cotton 2 2.5 300 8.0 Creamy to grayish white,brown, purple,blue,red Animal Fibersb Wool 4-20 1.5 150 40 Creamy white to brown Silk 1.0 4 800 20-25 White Synthetic Fibers Nylon 0.5-18 3-10 350-890 15-40 White-transparent Polyester 0.1-10 3-9 500-1100 11-40 White-transparent adenier (den)is the mass in (g)of 9000 m of fiber.The smaller the number,the finer the yarn. bSee also Table 16.6 in which the mechanical properties of spider drag lines are given. beit in small proportions.In addition,plant materials contain ex- tractives (gums,fats,resins,waxes,sugars,oils,starches,alka- loids,tannins)in various amounts.Extractives are nonstructural components that are deposited in cell cavities or cell walls.They may be removed (extracted)without changing the wood struc- ture.Further constituents of plant fibers are minerals,which remain as ash when a plant is incinerated.The compositions of selected plant fibers are presented in Table 16.3.Some com- mercially important types of vegetable fibers will be briefly dis- cussed below. Flax is a bast fiber that is extracted from Linum usitatissimum, an annual plant which grows in the north and south temperate
beit in small proportions. In addition, plant materials contain extractives (gums, fats, resins, waxes, sugars, oils, starches, alkaloids, tannins) in various amounts. Extractives are nonstructural components that are deposited in cell cavities or cell walls. They may be removed (extracted) without changing the wood structure. Further constituents of plant fibers are minerals, which remain as ash when a plant is incinerated. The compositions of selected plant fibers are presented in Table 16.3. Some commercially important types of vegetable fibers will be briefly discussed below. Flax is a bast fiber that is extracted from Linum usitatissimum, an annual plant which grows in the north and south temperate 16.2 • Production and Properties of Natural Fibers 337 TABLE 16.2. Properties of selected fibers Tensile Fineness Tenacity strength Elongation Fiber (den)a (g/den) (MPa) (strain) (%) Color range Bast Fibers Jute 20 3 39 1.5 Creamy white to brown Flax (Linen) 5 5 66 1.5 White to brown Hemp 6 4 52 2.0 Light to grayish brown Ramie (China grass) 5 5 67 4.0 White to grayish brown Leaf Fibers Sisal 290 4 51 3.0 Creamy white to yellowish Henequen 370 3 39 5.0 Creamy white to reddish brown Abaca 190 5 64 3.0 Creamy white to (Manila hemp) dark brown Istle 360 2.5 32 5.0 White to reddish yellow Seed Fiber Cotton 2 2.5 300 8.0 Creamy to grayish white, brown, purple, blue, red Animal Fibersb Wool 4–20 1.5 150 40 Creamy white to brown Silk 1.0 4 800 20–25 White Synthetic Fibers Nylon 0.5–18 3–10 350–890 15–40 White-transparent Polyester 0.1–10 3–9 500–1100 11–40 White-transparent adenier (den) is the mass in (g) of 9000 m of fiber. The smaller the number, the finer the yarn. bSee also Table 16.6 in which the mechanical properties of spider drag lines are given
338 16.From Natural Fibers to Man-Made Plastics TABLE 16.3.Composition of selected plant fibers Cellulose Moisture Ash Lignin and Extractives Fiber (%) (%6) (%) pectins(%) (%) Bast Fibers Congo jute 75.3 7.7 1.8 13.5 1.4 Flax (linen) 76.0 9.0 1.0 10.5 3.5 Hemp 77.1 8.8 0.8 9.3 4.0 Ramie(China grass) 91.0 0.6 Leaf Fibers Sisal 77.2 6.2 1.0 14.5 1.1 Henequen 77.6 4.6 1.1 13.1 3.6 Abaca (Manila hemp) 63.7 11.8 1.0 21.8 1.6 Istle 73.5 5.6 1.6 17.4 1.9 Seed Fiber Cotton 90.0 8.0 1.0 0.5 0.5 aNote:"Easy care"or"no-iron"fibers are treated with formaldehyde resin that emits formalde- hyde fumes.They have been observed to cause tiredness,headaches,coughing,watery eyes,or respiratory problems. zones where the soil is fertile and sandy,and the weather is cool and damp during the summer.Major flax-producing countries are Belgium(particularly the western part),The Netherlands,and Luxembourg.Water retting yields the best linen (i.e.,the fabric made from flax)and takes about 8-14 days,whereas dew retting (2-5 weeks)yields lesser qualities.The subsequent scutching me- chanically crushes the retted stems into small pieces called shives, which are then beaten for separation.The long fibers (30-90 cm long and 0.05-0.5 mm in diameter)are used for fine textiles.The shorter pieces(tow),which constitute one-third of the fiber yield, are used for cordage,coarse yarn,and shoe-stitching threads. Linen is characterized by high strength,low stretch,high water absorption,and pronounced swelling when wet.Flax is also grown for flax seeds,linseed oil,and flax straw for fine,strong paper. Ramie,a vegetable fiber which was used for Chinese burial shrouds over 2000 years ago,has been relatively unknown for garments in the western part of the world.It is now often blended with cotton and resembles fine linen to coarse canvas.Ramie is produced from the stalk of a plant and is processed like linen from flax.The fiber is very fine and silk-like,naturally white,and of high luster.(See Tables 16.2 and 16.3.)Ramie is resistant to bacteria,mildew,and insect attack;it is extremely absorbent, dyes easily,increases in strength when wet,does not shrink,and improves its luster with washing.However,Ramie has a low elas- ticity,lacks resiliency,wrinkles easily,and is stiff and brittle.It therefore tends to break when a garment made of it is pressed too sharply
zones where the soil is fertile and sandy, and the weather is cool and damp during the summer. Major flax-producing countries are Belgium (particularly the western part), The Netherlands, and Luxembourg. Water retting yields the best linen (i.e., the fabric made from flax) and takes about 8–14 days, whereas dew retting (2–5 weeks) yields lesser qualities. The subsequent scutching mechanically crushes the retted stems into small pieces called shives, which are then beaten for separation. The long fibers (30–90 cm long and 0.05–0.5 mm in diameter) are used for fine textiles. The shorter pieces (tow), which constitute one-third of the fiber yield, are used for cordage, coarse yarn, and shoe-stitching threads. Linen is characterized by high strength, low stretch, high water absorption, and pronounced swelling when wet. Flax is also grown for flax seeds, linseed oil, and flax straw for fine, strong paper. Ramie, a vegetable fiber which was used for Chinese burial shrouds over 2000 years ago, has been relatively unknown for garments in the western part of the world. It is now often blended with cotton and resembles fine linen to coarse canvas. Ramie is produced from the stalk of a plant and is processed like linen from flax. The fiber is very fine and silk-like, naturally white, and of high luster. (See Tables 16.2 and 16.3.) Ramie is resistant to bacteria, mildew, and insect attack; it is extremely absorbent, dyes easily, increases in strength when wet, does not shrink, and improves its luster with washing. However, Ramie has a low elasticity, lacks resiliency, wrinkles easily, and is stiff and brittle. It therefore tends to break when a garment made of it is pressed too sharply. 338 16 • From Natural Fibers to Man-Made Plastics TABLE 16.3. Composition of selected plant fibers Cellulose Moisture Ash Lignin and Extractives Fiber (%) (%) (%) pectins (%) (%) Bast Fibers Congo jute 75.3 7.7 1.8 13.5 1.4 Flax (linen) 76.0 9.0 1.0 10.5 3.5 Hemp 77.1 8.8 0.8 9.3 4.0 Ramie (China grass) 91.0 0.6 Leaf Fibers Sisal 77.2 6.2 1.0 14.5 1.1 Henequen 77.6 4.6 1.1 13.1 3.6 Abaca (Manila hemp) 63.7 11.8 1.0 21.8 1.6 Istle 73.5 5.6 1.6 17.4 1.9 Seed Fiber Cottona 90.0 8.0 1.0 0.5 0.5 aNote: “Easy care” or “no-iron” fibers are treated with formaldehyde resin that emits formaldehyde fumes. They have been observed to cause tiredness, headaches, coughing, watery eyes, or respiratory problems
16.2.Production and Properties of Natural Fibers 339 Wood Wood is and always has been a major material for construction, tools,paper-making,fuel,weapons,and,more recently,as a source for cellulose.Indeed,manuscripts have been found by Aristotle,Theophrastus,and other ancient writers who describe the properties of wood as it was known in those days. Goods made of wood,when left unpainted,are aesthetically pleasing and convey a feeling of warmth.Wood is one of the few natural resources that can be renewed when forests are managed properly.Moreover,forests are a necessity for water control,oxy- gen production,recreation,and for providing habitat to many animal species.Approximately one-third of the land mass of the earth (27%)is presently covered with forests.However,it is es- timated that the world's forests decrease by about 0.9%annually. We shall return to this subject in Section 18.3. In relation to its weight,wood has a high strength.It is an elec- trical and thermal insulator(Figures 11.1 and 14.1)and has de- sirable acoustic properties.It can be easily shaped and finished and is inert to many chemicals.However,not all properties of wood are favorable.Many types of wood decay through interac- tion with water and wood-destroying organisms.Wood may burn, it is hygroscopic,and changes its size when the humidity fluctu- ates.Moreover,the physical properties of wood vary in different directions due to its fibrous nature(anisotropy),as shown in Table 16.4.Finally,cut lumber may have imperfections from knots,etc., which decrease its strength.In short,wood is not a homogeneous and static material as are many metals and ceramics. TABLE 16.4.Tensile strength,compressive strength,modulus of elasticity,and densities for some wood species (12%Moisture)(see also Table 2.1) Tensile strength or Modulus of Compressive strength [MPa] [MPa] elasticity Density to grain Material to grain ⊥to grain to grain ⊥to grain [g/cm3] [GPa] Soft Wood Douglas fir 78 2.7 37.6 4.2 0.45 13.5 Ponderosa pine 73 2.1 33.1 3.0 0.38 8.5 White spruce 60 2.5 38.7 4.0 0.35 9.2 Red cedar 45.5 2.2 41.5 6.3 0.3 7.7 Hard Wood American elm 121 4.5 38 4.7 0.46 9.2 Sugar maple 108 7.6 54 10.1 0.56 12.6 Beech 86.2 7 50.3 0.62 Oaks 78 6.5 42.7 5.6 0.51-0.64 12.3
Wood is and always has been a major material for construction, tools, paper-making, fuel, weapons, and, more recently, as a source for cellulose. Indeed, manuscripts have been found by Aristotle, Theophrastus, and other ancient writers who describe the properties of wood as it was known in those days. Goods made of wood, when left unpainted, are aesthetically pleasing and convey a feeling of warmth. Wood is one of the few natural resources that can be renewed when forests are managed properly. Moreover, forests are a necessity for water control, oxygen production, recreation, and for providing habitat to many animal species. Approximately one-third of the land mass of the earth (27%) is presently covered with forests. However, it is estimated that the world’s forests decrease by about 0.9% annually. We shall return to this subject in Section 18.3. In relation to its weight, wood has a high strength. It is an electrical and thermal insulator (Figures 11.1 and 14.1) and has desirable acoustic properties. It can be easily shaped and finished and is inert to many chemicals. However, not all properties of wood are favorable. Many types of wood decay through interaction with water and wood-destroying organisms. Wood may burn, it is hygroscopic, and changes its size when the humidity fluctuates. Moreover, the physical properties of wood vary in different directions due to its fibrous nature (anisotropy), as shown in Table 16.4. Finally, cut lumber may have imperfections from knots, etc., which decrease its strength. In short, wood is not a homogeneous and static material as are many metals and ceramics. Wood 16.2 • Production and Properties of Natural Fibers 339 TABLE 16.4. Tensile strength, compressive strength, modulus of elasticity, and densities for some wood species (12% Moisture) (see also Table 2.1) Modulus of Tensile strength T Compressive strength elasticity [MPa] [MPa] Density to grain Material to grain to grain to grain to grain [g/cm3] [GPa] Soft Wood Douglas fir 78 2.7 37.6 4.2 0.45 13.5 Ponderosa pine 73 2.1 33.1 3.0 0.38 8.5 White spruce 60 2.5 38.7 4.0 0.35 9.2 Red cedar 45.5 2.2 41.5 6.3 0.30 7.7 Hard Wood American elm 121 4.5 38 4.7 0.46 9.2 Sugar maple 108 7.6 54 10.1 0.56 12.6 Beech 86.2 7 50.3 7 0.62 Oaks 78 6.5 42.7 5.6 0.51–0.64 12.3