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
  1. Cellulose fiber is main constituent of all plant material. Plants are renewable source and available abundantly in nature.
  2. 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.
  3. Cellulose fiber is reusable/ recyclable to any extent.
  4. Cellulose fiber is bio-degradable.
  • Technical
  1. 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.
  2. 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.
  3. 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.
  4. High tensile strength
  5. Suppleness (Flexibility, conformability)
  6. Water insoluble
  7. Hydrophilic
  8. Chemically stable
  9. 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

 

Picture of cellulose

 

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
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, warmweather 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

Lignin, Cellulose & H-Cellulose in various Raw Material

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