Paper Flashcards
What is wood
Composed of cellulose fibres in a lignin matrix
What does cellulose fibres give to wood
Tensile strength
What does the lignin matrix give wood
Compressive strength
What is cellulose
Linear poly glucose. It’s insoluble due to H bonding between chains
What is lignin
An aromatic highly cross linked and coloured molecule
Why does paper turn yellow
Lignin is oxidised
What is pulping
Breaking wood into fibres
What is mechanical pulping and what are the advantages and disadvantages of this
Wood is simply ground down. Still contain lignin so ages, shorter fibres so weaker paper. Higher yield.
Advantages and disadvantages of chemical pulping
Lignin removed, stronger paper due to longer fibres, low yield.
How is white paper achieved
Bleaching the pulp which will otherwise remiss brown
What are the conditions for the Kraft process
Wood chips + Na2S + H2O —-> kraft pulp
What is the role of HO- and HS-
Degrade cross linked lignin to form water soluble phenoxide fragments
What does lignin contain after chemical pulping
Conjugated chromophores
How is conjugation destroyed
By oxidising with bleach
What oxidising agent is used
Chlorine or chloromate,
Why is ClO3 preferred over Cl2
More oxidising and more selective
How does ClO3 work
Oxidises esters to COOH
How do you prevent water penetration
Introduce surface hydrophobic groupings known as sizing. This reduces wetting.
Why are inorganic additives use in paper
Improve optical and physical properties such a density,brightness, softness and smoothness
Why is CaCO3 used
Is a buffer
How is paper made
Pulp diluted and sprayed onto a moving screen.
Dewatered by gravity and suction
Paper is then pressed and dried over steam filled cylinders
Why is paper an expensive process
Energy intensive
What are the two problems that occur during dewatering
Fillers are too small to be retained on mesh screen
Fillers also block the pores between fibres causing slower water drainage
What is a high molecular cat ionic polymer used for
Glue to stick fillers to fibres
