Glossary of Terms

Air Permeability

Commonly referred to as "porosity" it is the ease with which pressurized air can flow through a paper's thickness. Typically measure by the Gurley or the Sheffield porosity tests, which measure the volumetric flow of air through the paper thickness.

Standard Units: Gurley seconds (seconds/100CC)

Burst Strength

The resistance of paper to rapture as measured by the hydrostatic pressure required to burst it when a uniformly distributed and increasing pressure is applied to one of its side.

Standard Units: kPa
Burst Index (kPa m2/g) = Burst Strength (kPa) / Basis Weight (g/m2)


The weight if the fibre wall material per unit length of fibre.

Standard Units: mg/m or mg/100m


Contacts/fibre means the average number of crossings between a fibre and others in the fibre network.

Fibre Length

The average contour length of all measured fibres in a given sample of pulp. This measurement can be presented as the Length Weighted, Weight Weighted or Arithmetic Average fibre length.

Standard Units: mm

Fibre Shape

Fibre Curl Index is the ratio of actual fibre length to the distance between the 2 fibre ends minus 1. It gives in indication of the continuous curvature of the fibres greater than 0.5 mm in length and within the selected range limits.

Fibre Kink Index is the sum of the number of kinks (an abrupt change in fibre curvature) within a range of kink angles divided by the total fibre length of all the fibres.

Mean Kink Angle is the average of all the kink angles greater than 20 degrees, divided by the total number of detected kinks.


A measure of the portion of fibres which are shorter than a specified length, typically less than 0.2 mm.

Standard Units: %

Folding Endurance

The ability of paper to withstand repeated folding while held under tension before it severs at the crease.

Standard Units: number of double folds

Fracture Mechanics

Fracture toughness describes the ability of the material to resist the propagation of a pre-existing crack. It is often determined as critical stress intensity factor, or sometimes referred to as tenacity. Fracture toughness equals the geometric mean of fracture energy and elastic modulus.

Fracture energy means the critical energy release rate, where the pre-existing crack starts to propagate with an infinitesimal increment of applied tension when the energy release rate reaches.

Freeness / Wetness

Terms used to define how quickly or slowly water is drained from the pulp. The opposite of freeness is slowness. Freeness or slowness is a function of degree of beating or refining on pulp.

Standard Units:  
Canadian Standard Freeness: ml CSF
Schopper Riegler: degree SR

Premium Reinforcement Pulps (PRP)

It is increasingly common to hear papermakers referring to those NBSK pulps that deliver the highest reinforcement as “PRP’s” or “Premium Reinforcement Pulps”. These are pulps which have intrinsic morphological advantages that position their tensile and fracture properties at the extreme high end of the range of NBSK pulps.

Reinforcement Potential

The main use of softwood kraft pulps is to strengthen weaker paper-making furnishes without significantly impairing other end-use properties. This function is referred to as reinforcement.

Reinforcement potential is a complex pulp property and measurement usually involves consideration of the grade of paper to be reinforced. Currently there is no generally accepted single method for measuring reinforcement potential that applies to all paper grades.

For LWC grades, one commonly accepted definition is that reinforcement potential is proportional to the average fibre length and tear strength at 70 Nm/g tensile index and inversely proportional to fibre coarseness.

For SC papers the reinforcement potential has been theorized to be described simply by the ratio of fibre length to coarseness.

Specific Energy Input

Net energy applied in a refiner per unit weight of pulp on oven dry basis during refining.

Standard Units: kW h / t
Other Units: hpday/ton
1 kWh/ton = 17.9 hpday/ton = 3.6 GWs/kg


A measure of the maximum tensile strain developed in the sample strip before rupture.
The stretch or percentage elongation is expressed as a percentage.

Standard Units: %


The work done when a paper specimen is stressed to rupture in tension under prescribed conditions as measured by the integral of tensile strength over the range of tensile strain from 0 to maximum. T.E.A is highly dependent on maximum tensile strength and sheet stretch.

Standard Units: J/kg

Tear Resistance / Tear Strength

The mean force required to continue the tearing of paper from an initial cut under standardized conditions. Because it takes more force to start a tear than to continue one already started, 1 or 2 small cuts are made in the edge of the paper prior to testing. The most common type of testers utilize a pendulum device where the difference between the height the pendulum swings upon tearing the paper, and that to which it would have swung had there been no paper, is a measure of the force taken to tear the paper. Standard instrument used for measuring tear resistance are the “Elmendorf tester” and the “Brecht-Imset tester.”

Standard Units:  
Tear Resistance:  
Tear index (mN m2/g) = Tearing Resistance (mN) / Basis Weight (g/m2)

Tensile Stiffness

Tensile stiffness of paper is the slope of the tangent of the tensile strength-strain curve at the point of zero strain. The tensile stiffness (in N/m of width) is equal to the elastic modulus multiplied by the paper thickness. When the tensile stiffness is divided by the grammage, it is called the tensile stiffness index (Nm/kg), which numerically equals the specific elastic modulus (in Nm/kg).

Standard Units:  
Tensile Stiffness: N/m
Tensile Stiffness Index: (Nm/kg)

Tensile Strength

Tensile is measured by clamping a strip of paper, typically 15 mm wide, between the 2 jaws of a tensile tester, which then slowly move in opposite directions until the strip breaks. The tensile strength is the tensile force when the strip ruptures. It is a guide to the performance of paper when subjected to tensional pulling forces on a paper machine, printing press or converting equipment.

Tensile Breaking Length measures the length of the sample strip that will break, under its own weight. It can be calculated from Tensile Strength according to

Breaking length (km) = Tensile Strength (N)
Sample width (m) x Grammage (g/m_) x 9.807
Standard Units:  
Tensile Strength: N/m
Tensile Breaking Length: km
Tensile index (Nm/g) = tensile strength (N/m) /basis weight (g/m2)

Zero Span Tensile Strength (Wet and Dry)

The zero-span tensile test is similar to the normal tensile test, but the clamping jaws are situated with no gap (Zero Span). Clamping in this manner virtually eliminates the opportunity for inter-fibre bonds to break, resulting in the measurement of the tensile force required to break individual fibres.

Standard Units: km