RAW MATERIALS FOR PAPERMAKING
Here the Raw Materials or Cellulosic Raw Materials for papermaking are used interchangeably and only the Cellulosic Raw Material are discussed. Cellulosic/fibrous raw material are broadly divided between wood and nonwood. Almost 90% of virgin fibers come from woody plants. In more and more region of world, recycled fiber is becoming major source of papermaking fibers.
The wood is the hard, fibrous substance found beneath bark in the stems and branches of trees and shrubs. Practically all commercial wood, however, comes from trees. It is plentiful and replaceable. Since a new tree can be grown where one has been cut, wood has been called the world's only renewable natural resource.
Nonwood fibers can be sub-divided between;
- Agricultural Residues such as wheat straw, rice straw, barley straw, baggase etc.
- Agricultural crops such as Cotton, Hemp, Kenaf, Jute, Sarkanda (Sweetcane) etc.
- Grasses such as Bamboo, Esparto, Elephant Grass etc.
Two most important properties of any papermaking cellulosic raw material are, how much cellulose fiber it has and how long the fibers are. The amount of cellulose fiber in wood determines the pulp yield, ease of pulping and cost of pulp produced. The importance of fiber length is explained in pulp properties. The maximum average fiber length pulp will have is that of wood because whatever pulping method, full chemical to full mechanical, fiber is going to damage. In mechanical pulping the damage is physical (cutting, bruising etc.) and in chemical pulping it is chemical degradation (lower degree of polymerization).
The following techno-economical reason make cellulose fiber suitable for paper making. Cellulose The DNA of Paper
- Economical
- Cellulose fiber is main constituent of all plant material. Plants are renewable source and available abundantly in nature.
- Papermaker don't need prime parts of tree/plant to make paper. Wood scrap, saw mill waste, agricultural residue, straw, grasses and/or rag are acceptable source of virgin fiber.
- Cellulose fiber is reusable/ recyclable to any extent.
- Cellulose fiber is bio-degradable.
- Technical
- Lignin, which cement/ glue individual fibers in plant is physically and chemically weaker than cellulose fiber, making separation of fiber possible by mechanical and or chemical means.
- Cellulose fiber is made of multilayer of very small thread like structure called fibrils. These fibrils can be exposed by beating/ refining of fibers and provide very large area for bonding.
- The most important characteristic of fiber which make it suitable for papermaking is that cellulose fiber develop physical and chemical bonding with other fibers when it change from wet to dry condition.
- High tensile strength
- Suppleness (Flexibility, conformability)
- Water insoluble
- Hydrophilic
- Chemically stable
- Relatively colorless (White)
- Cellulose
- It is a high molecular weight, stereoregular, and linear polymer of repeating beta-D-glucopyranose units. Simply speaking it is the chief structural element and major constituents of the cell wall of trees and plants. The empirical formula for cellulose is (C6H10O5)n where 'n' is degree of polymerization (DP). Cellulose The DNA of Paper
| Substance | Degree of Polymerization (DP) | Molecular Weight |
| Native Cellulose | >3500 | >570,000 |
| Purified Cotton | 1000 - 3000 | 150,000 - 500,000 |
| Wood Pulp | 600 - 1000 | 90,000 - 150,000 |
| Commercial Regenerated Cellulose (e.g. Rayon) | 200 - 600 | 30,000 - 150,000 |
| β Cellulose | 15 - 90 | 3000 - 15,000 |
| γ Cellulose | <15 | <3000 |
| Dynamite Nitro-Cellulose | 3000 - 5000 | 750,000 - 875,000 |
| Plastic Nitro-Cellulose | 500 - 600 | 125,000 - 150,000 |
| Commercial Cellulose Acetate | 175 - 360 | 45,000 - 100,000 |
- Hemicellulose
- A constituent of woods that is, like cellulose, a polysaccharide, but less complex and easily hydrolysable. Hemicellulose have lower degree of polymerization (only 50 - 300) with side groups on the chain molecule and are essentially amorphous.
| Pulping Process | Yield (%) | % of Pulp | Papermaking Properties | |||||
| b Cellulose | Hemicellulose | Lignin | Initial Tensile | Max. Tensile | Tear | Rate of Freeness Developed | ||
| Kraft | 44 | None | 14 | 1 - 2 | Low | Very High | Low | Very High |
| Sulfite | 50 | High | 11 | 1 - 2 | Medium | Medium | Medium | Medium |
| Alkaline Pretreatment With Sulfite Cook | 52 | Medium | 17 | 1 - 2 | Medium High | Medium | Very High | Low |
| High Yield Bi-Sulfite | 60 | Low | 19 | 10 | High | High | Low | Medium |
- Lignin
- A complex constituent of the wood that cement the cellulose fibers together. Lignin is brown in color. Lignin is largely responsible for the strength and rigidity of plants.
- Solvent Extractives
- Soluble materials or extractives in wood consist of those components that are soluble in neutral organic solvents. The di-chloromethane extractable content of wood is a measure of such substances such as waxes, fats, resins, photosterols and non-volatile hydrocarbons. The amount of extractives is highly dependent on seasoning or drying of wood.
- The ethanol-benzene extractable content of the wood consists of certain other di-chloromethane insoluble components such as low molecular weight carbohydrates, salts, and other water soluble substances.
- Most water soluble and volatile compounds are removed during pulping. The extractives reduce pulp yield, increase pulping and bleaching chemical consumption and create problems such as foaming during papermaking if not removed.
- The standard procedure of measuring solvent Extractive is laid out in TAPPI T204
- CHEMICAL COMPOSITION OF WOOD
Average chemical contents of wood
| Elements | Share, % of dry matter weight |
| Carbon | 45-50% |
| Hydrogen | 6.0-6.5% |
| Oxygen | 38-42% |
| Nitrogen | 0.1-0.5% |
| Sulphur | max 0.05 |
Wood is mainly composed of cellulose, Hemicellulose, lignin and extractives. The following table provides main chemical components of some wood species.
| Constituents | Scot Pine | Spruce | Eucalyptus | Silver Burch |
| Cellulose (%) | 40.0 | 39.5 | 45.0 | 41.0 |
| Hemicellulose (%) | 28.5 | 30.6 | 19.2 | 32.4 |
| Lignin (%) | 27.7 | 27.5 | 31.3 | 22.0 |
| Total Extractive (%) | 3.5 | 2.1 | 2.8 | 3.0 |
| Wood Components | Hardwood (%) | Softwood (%) |
| Cellulose | 40 - 50 | 40 - 50 |
| Hemicellulose | 25 - 35 | 25 - 30 |
| Lignin | 20 - 25 | 25 - 35 |
| Pectin | 1 - 2 | 1 - 2 |
| Starch | Trace | Trace |
Chemical composition of wood is the determining factor of pulping yield for various pulping processes.
| Pulping Process/Pulp Grade | Wood Components Retained in Pulp | Wood Components Removed | Yield |
| Soft Chemical Cook and Bleached | Cellulose only | Lignin, Hemicellulose & Extractives | Less than 40% |
| Chemical Pulping & Bleached | Cellulose and partly Hemicellulose | Lignin, partly Hemicellulose & Extractives | 45 - 55% |
| Chemical Pulping NO Bleaching | Cellulose, partly Hemicellulose & traces of Lignin | Partly Lignin & Hemicellulose & Extractives | 45 - 55% |
| Semi-Chemical | Cellulose, mostly Hemicellulose & partly lignin | Partly lignin, some Hemicellulose &Extractives | 50 - 65% |
| TMP, RMP & GW | Cellulose, Hemicellulose and Lignin | Extractives | More than 95% |
Chemical Composition of Wheat Straw and Softwood/Hardwood
| Cell wall Composition (% of dry weight) | Wheat Straw | Softwood | Hardwood | ||
| Stalk | Node | Leaf | |||
| Cellulose (%) | 40.0 | 40.0 | 35.0 | 42.0 | 42.0 |
| Hemicellulose (%) | 45.0 | 40.0 | 55.0 | 28.0 | 35.0 |
| Lignin (%) | 15.0 | 20.0 | 10.0 | 30.0 | 23.0 |
Cell Wall, Silica and Extractive Content of Wheat Straw and Softwood/Hardwood
| Plant Composition (% of dry weight) | Wheat Straw | Softwood | Hardwood | ||
| Stalk | Node | Leaf | |||
| Cell Wall | 90.0 | 79.0 | 76.0 | 95.0 | 93.0 |
| Silica | 3.0 | 8.0 | 11.0 | <0.5 | <0.5 |
| Extractives | 7.0 | 13.0 | 13.0 | 4.5 | 6.5 |
Cell Characteristics of Wheat Straw and Softwood/Hardwood
| Cell Characteristics | Wheat Straw | Softwood | Hardwood | ||
| Stalk | Node | Leaf | |||
| Length (mm) | 1.3 | 0.5 | 1.5 | 2.0 | 93.0 |
| Diameter (mm) | 0.015 | 0.015 | 0.015 | 0.030 | 0.020 |
| Lumen % | 75 | 50 | 80 | 65 | 55 |
| Tissue Density (g/cc) | 0.34 | 0.68 | 0.27 | 0.49 | 0.63 |
Physical Characteristics of Hemp and Softwood/Hardwood
| Characteristics | Hemp Bark | Softwood | Hardwood | ||
| Primary | Secondary | Hemp Core | |||
| Length (mm) | 10 - 100 | 2.0 | 0.55 | 2.5-5.5 | 0.8-1.9 |
| Alfa Cellulose (%) | 67+/-5 | 38+/-2 | 40.6 | 42+/-2 | 67+/-5 |
| Holocellulose (%) | 80+/-1 | 69+/-3 | 74.9 | 69+/-4 | 80+/-1 |
| Lignin (%) | 4+/-2 | 20+/-2 | 16.0 | 28+/-3 | 4+/-2 |
| Extractives (%) | 3+/-2 | 5+/-3 | |||
| Ash Content (%) | <0.5 | <0.5 | |||
Physical Characteristics of Kenaf and Softwood/Hardwood
| Characteristics | Hemp | Softwood | Hardwood | ||
| Bark | Core | Whole | |||
| Length (mm) | 2.5- 4.0 | 0.5-0.6 | - | 2.5-5.5 | 0.8-1.9 |
| Alfa Cellulose (%) | 51.0 | 34.9 | 40.6 | 42+/-2 | 67+/-5 |
| Holocellulose (%) | 81.1 | 71.6 | 74.9 | 69+/-4 | 80+/-1 |
| Lignin (%) | 11.8 | 18.3 | 16.0 | 28+/-3 | 4+/-2 |
| Extractives (%) | 2.8 | 4.8 | 4.1 | 3+/-2 | 5+/-3 |
| Ash Content (%) | 2.8 | 1.8 | 2.1 | <0.5 | <0.5 |
Merits and demerits of various raw materials
| Parameters | Annual (Hemp, Manila & Cotton etc) | Soft Wood | Hard Wood | Agricultural Residue | Grasses (Bamboo& Kenaf) |
| Fiber Length | Very Large >5.0 mm (0.2") | Large 2.5 - 4.5mm (0.1-0.18") | Medium 1 - 2.5mm (0.04-0.1") | Small <1.0mm (0.04") | Medium 0.5-2.0mm (0.04-0.08") |
| Strength | Very High | High | Medium | Low | Medium |
| Chemical Pulp Yield | Cotton >90% other 50% | 50% | 50% | 35% | 45% |
| Ease of Pulping | Difficult | Easy | Easy | Difficult | Easy |
| Ease of Refining | Very Hard | Hard | Easy | Easy | Easy |
| Amount available | Very small | Large | Large | Large | Small |
| Maturity Cycle | < 1 year | >50 years | >10 years | < 1 year | Bamboo>10, Kenaf <1 yr |
| Collection & Transportation | Hard | Easy | Easy | Hard | Hard |
| General
|
Available quantity is low, warm weather crop, difficult to collect & transport, seasonal supply, low digester packing |
Overall best raw material for making paper | Available quantity is low, warm
weather crop, difficult to collect & transport, seasonal supply, lowdigester packing |
||
Annual Dry Matter and Pulp Yield per Hectare/Acre for various Fiber Plants
| Plant Species | Dry Matter Yield | Pulp Yield | ||
| MT/Year/Hectare | MT/Year/Acre | MT/Year/Hectare | MT/Year/Acre | |
| Wheat Straw | 2.5 | 1.0 | 1.1 | 0.4 |
| Oat Straw | 1.6 | 0.6 | 0.7 | 0.3 |
| Rye Straw | 2.2 | 0.9 | 1.1 | 0.4 |
| Barley Straw | 2.1 | 0.9 | 1.9 | 0.8 |
| Rice Straw | 3.0 | 1.2 | 1.2 | 0.5 |
| Bagasse | 9.0 | 3.6 | 4.2 | 1.7 |
| Bamboo | 4.0 | 1.6 | 1.6 | 0.6 |
| Chinese Silver Grass | 12.0 | 4.9 | 5.7 | 2.3 |
| Reed Canary Grass | 6.0 | 2.4 | 3.0 | 1.2 |
| Tall Fescue | 8.0 | 3.2 | 3.0 | 1.2 |
| Common Reed | 9.0 | 3.6 | 4.3 | 1.7 |
| Kenaf | 15.0 | 6.0 | 6.5 | 2.6 |
| Hemp | 12.0 | 4.9 | 6.7 | 2.7 |
| Temperate Hardwood (Birch) | 3.4 | 1.4 | 1.7 | 0.7 |
| Fast Growing HW (Eucalyptus) | 15.0 | 6.0 | 7.4 | 3.0 |
| Scandinavian SW (Coniferous) | 1.5 | 0.6 | 0.7 | 0.3 |
| Fast Growing SW | 8.6 | 3.5 | 4.0 | 1.6 |
| Constituents | Reed Canary Grass | Tall Fescue | Wheat Straw | Birch | Eucalyptus |
| Cellulose (%) | 35.0 | 34.0 | 37.0 | 41.0 | 46.0 |
| Hemicellulose (%) | 35.0 | 29.0 | 23-30 | 33.0 | 26.0 |
| Lignin (%) | 9.0 | 19.0 | 20.0 | 21.0 | 25.0 |
Comprehensive Chemical composition and Fiber Dimensions for Non-wood Raw Materials
Common and Scientific Names of Non-Wood Fiber Plants
Non wood plant materials such as agricultural residue, grasses etc., contain lesser amount of cellulose compare to wood hence have lower pulp yield. On the other hand cotton which is almost pure cellulose has very high yield.
