Formation and friction in paper

Formation

Formation is an indicator of how uniformly the fibers and fillers are distributed in the sheet. Formation plays an important role as most of the paper properties depend on it. A paper is as strong as its weakest point. A poorly formed sheet will have more weak and thin or thick spots. These will affect properties like caliper, opacity, strength etc. Paper formation also affects the coating capabilities and printing characteristics of the paper.

           There is no standard method or unit to express formation. It is a relative or subjective evaluation.

Friction

 

Friction is the resisting force that occurs between two paper or paperboard surfaces in contact when the surfaces are brought to slide against each other. This property is measured as a coefficient of friction, which is the ratio of the frictional force, to a force acting perpendicular to the two surfaces.

 

Two components of friction can be measured, these being static and kinetic friction. Static friction is the force resisting initial motion between the surfaces and kinetic friction is the force resisting motion of the two surfaces sliding against each other when already sliding at a constant speed.

 

Measurement of the coefficient of friction has applications in packaging where a high coefficient will indicate that containers such as sacks, bags and paperboard containers will resist sliding in unit loads or on packaging lines. This property is also important in printing papers, since a specific coefficient of friction is needed so that individual sheets will slide over each other, otherwise double press feeding may result.

 

There are two methods of measuring Co-efficient of friction of paper. One, which uses Incline Plane is explained in TAPPI T 548 & T815, the second method, which uses Horizontal Plane is explained in TAPPI T 549 & T816.

Typical Co-efficient of Friction Values Using Horizontal Plane Method
Grade	Static Friction	Kinetic Friction
Office/Business Paper	0.50-0.65	0.35-0.5
Silk Coated Paper	0.45-0.55	0.30-0.45
Gloss Coated Paper	0.40-0.50	0.30-0.40