Paper and Pulp

Paper vs. pulp:

1. Nature: Pulp is the raw, semi-processed material — a soft, moist mass of cellulose fibres obtained from wood or other plant sources. Paper is the finished product made by pressing, drying and calendering a sheet of pulp.

2. Composition and use: Pulp still contains the natural fibres along with residual lignin and other impurities (unless bleached/purified), and it is used as the starting material for making paper. Paper is a thin, dried sheet of interlocked cellulose fibres, free of most impurities, and is used directly for writing, printing and packaging.

Flow sheet diagram for paper production:

        Raw materials
   (Wood / bamboo / straw / waste paper)
                 │
                 ▼
        ┌─────────────────┐
        │  1. Chipping    │  (wood cut into small chips)
        └─────────────────┘
                 │
                 ▼
        ┌─────────────────┐
        │  2. Pulping     │  (chips cooked with NaOH / Na2SO3
        └─────────────────┘   under pressure → pulp + lignin removed)
                 │
                 ▼
        ┌─────────────────┐
        │  3. Washing &   │  (pulp washed, bleached with
        │     Bleaching   │   Cl2 / Ca(OCl)2 / ClO2)
        └─────────────────┘
                 │
                 ▼
        ┌─────────────────┐
        │  4. Beating &   │  (pulp beaten; fillers like clay,
        │     Refining    │   rosin, alum added for sizing)
        └─────────────────┘
                 │
                 ▼
        ┌─────────────────┐
        │  5. Sheet       │  (dilute pulp spread on wire mesh;
        │     Formation   │   water drained, fibres mat together)
        └─────────────────┘
                 │
                 ▼
        ┌─────────────────┐
        │  6. Pressing &  │  (excess water squeezed out by
        │     Drying      │   rollers and hot cylinders)
        └─────────────────┘
                 │
                 ▼
        ┌─────────────────┐
        │  7. Calendering │  (passed through rollers for
        │     & Finishing │   smooth finish; cut into sheets)
        └─────────────────┘
                 │
                 ▼
              PAPER

Key points:

• Pulping: Wood chips are digested with chemicals (NaOH + Na₂S, or Na₂SO₃) under pressure to separate cellulose fibres from lignin.
• Bleaching: Residual lignin and colouring matter are removed using chlorine or chlorine dioxide to give white pulp.
• Sizing/Filling: Rosin + alum (sizing) makes paper water-resistant; clay or CaCO₃ (filler) improves opacity and smoothness.

The raw materials for the production of paper are:

1. Cellulose (main raw material) – obtained from wood (softwood and hardwood), bamboo, straw, bagasse (sugarcane residue), and waste paper.
2. Water – used in large quantities for pulping, washing and processing.
3. Chemicals – such as caustic soda (NaOH) or sodium sulphite (Na₂SO₃) for pulping, chlorine or bleaching powder for bleaching, and rosin with alum for sizing.
4. Fillers and additives – clay, calcium carbonate, and dyes to improve the quality, opacity and colour of paper.

Importance of wood pulp in the paper industry (2 marks)

1. Main raw material: Wood pulp is the chief source of cellulose fibre for making paper. About 90% of the world's paper is produced from wood pulp, so the industry depends on it for bulk production.

2. Cheap and abundant: Softwoods (like pine, fir, spruce) yield pulp in large quantities at low cost, making paper affordable for mass use.

3. Good fibre quality: The long cellulose fibres of wood pulp interlock well, giving paper its strength, smoothness and ability to hold ink — properties that suit writing, printing and packaging paper.

4. Variety of grades: By choosing mechanical, chemical or semi-chemical pulping, different grades of pulp are obtained, so the same raw material can give newsprint, writing paper, tissue paper and board.

Thus wood pulp is the backbone of the paper industry, providing the cellulose fibres on which paper manufacture depends.

The quality of the produced paper is improved by the following treatments given to the pulp/paper:

1. Sizing – Rosin soap (rosin + alum) or starch is added to the pulp so that the paper becomes water-resistant and ink does not spread on it.
2. Loading (filling) – Fillers such as china clay, calcium carbonate, talc or titanium dioxide are added to fill the pores of the paper, making it smooth, opaque, white and heavier.
3. Bleaching and dyeing – Bleaching powder or chlorine is used to make the paper white, and suitable dyes are added to give the desired colour.
4. Coating – The surface is coated with china clay or calcium carbonate to give a glossy, smooth finish (as in art paper).

(c) Lignin

Lignin is the binding agent that stiffens and waterproofs the wood fibres. Removing it (delignification during pulping/bleaching) exposes the cellulose microfibrils, so more hydroxyl groups are free to form hydrogen bonds between fibres — hence higher fibre-to-fibre bonding strength.