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Surface characteristics

Roughness Bendtsen (ISO 8791-2):

WhatA measure of the rate at which air will pass between a flat circular land and a sheet of paper or board when tested under specified conditions and at an operating pressure.
It is expressed in millilitres per minute.
WhyA rough surface has impact on printability: it may cause dot gain, as a result of which less detail is visible in the printing image.
HowClamping a test piece between a flat plate and a circular metal land on a Bendtsen tester. Supplying air at a nominal pressure of I,47 kPa to the space enclosed inside the land and measuring the rate of air flow between the land and the test piece. The higher the measured value, the rougher the surface.
CalculationReport the average of 10 significant values and the standard deviation of both front and back.
NoteThis method is mainly accurate for uncoated substrates that have had no special calender treatment.

Evenness PPS (ISO 8791-41, TAPPI T555):

WhatThe evenness of the substrate’s surface measured with a pressure of 1 MPa, expressed in µm.
WhyA very even surface has impact on printability: it allows for finer screens, as a result of which more detail may be visible in the printing image. However, it may also lead to slippage or blocking in a stack of paper.
HowWith a Parker Print Surf (PPS) tester a cylinder is pressed against the substrate and the amount of air that leaks with a defined pressure is a dimension for the evenness of the substrate’s surface. The results of each measurement are converted into µm.
The lower the measured value, the more even the surface.
CalculationReport the average of 10 significant values and the standard deviation of both front and back.
NoteThis method is used for coated substrates as well as uncoated with a special calender treatment.

Dennison wax pick test (TAPPI T459):

WhatTo determine the susceptibility to picking. The picking resistance is expressed in so called critical wax strength.
WhyThe surface should be able to resist the tack of printing ink.
HowA stick of hard wax, identified according to a defined wax pick strength, is made viscous above a flame and placed upright on the substrate’s surface. After 15-30 minutes have passed, the wax is removed, keeping the stick upright at all times. Both the substrate’s surface and the wax stick are checked: is the substrate damaged and/or are particles of the substrate visible on the wax stick? The highest value that causes no damage to the substrate is called the critical wax strength.
CalculationReporting the average of 10 significant values and the standard deviation of both front and back.
NoteGenerally, the values underneath are taken as a guideline

Air permeability Bendtsen (ISO 5636-3):

WhatThe air permeability (porosity) of a substrate indicates the amount of air that, as a result of a difference in air pressure on both sides, passes through a defined area of the substrate within 5 seconds. It is expressed in µm/Pa s and/or in ml/min.
WhyThe amount of air passing through the substrate has an impact on certain strength aspects, choice of ink (coldset) as pertaining to ink absorption and the folding of the printed matter (when air needs to be able to escape)
HowWith a Bendtsen air permeability tester the substrate is clamped between the measuring heads and a defined airflow is passed through a defined area with a constant pressure. The substrate is kept on its place during an average of 5 seconds or until a difference in air pressure is detected.
Calculation
Results in µm/Pa.s are calculated and may be converted to ml/min.Report the average of 10 significant values and the standard deviation.

Coefficient of Friction horizontal plane (ISO 15359, TAPPI T549 and T816):

WhatDetermination of static and dynamic coefficient of friction.
The static CoF is the force needed to initiate movement.
The dynamic CoF is the force needed to keep the substrate moving.
WhyThe higher the CoF the lower the risk of unwanted sliding. This is of importance on the press and with finishing, but also with transportation.
HowEr wordt gemeten met een Bendtsen luchtdoorlatendheid tA strip of paper or board is mounted on a horizontal plane. A sled of known dimensions and weight is clad with the same substrate and is set into motion with a friction tester or tensile tester with a fixture.
CalculationBoth the static and dynamic CoF can be calculated:


where:
Ak = the average force during a constant movement in gf
B = weight of the sled in g

Report the average of a minimum of 5 significant values and the standard deviation in both MD and CD.

NoteThis test is designed for uncoated substrates (T 549) and packaging materials (T 816).

Coefficient of Friction inclined plane (TAPPI T815 om-95)::

WhatDetermination of the static CoF expressed ° angle and in tangent angle.
This test relates to the angle of inclination at which the substrate starts to slide.
Why

The higher the CoF the lower the risk of sliding. This is mainly of importance during transportation.

HowThe substrate is mounted on a sled and on a table that may rotate to an angle
of 90°. The table starts to rotate mechanically and is stopped at the first
moment of sliding. The angle of inclination and the tangent corner can be read
from a scale.
CalculationReport the average of a least 5 significant values and the standard deviation in both MD and CD.

Abrasion test dry and wet (Tappi T476):

WhatResistance of the substrate to abrasion (loss of paper or board) The results are
expressed in mg..
Why

During handling the substrate may get in contact with materials that chafe the surface and thus cause damage.

HowA sample of the substrate is mounted on a rotating disc and abraded with a
constant pre-set force.
CalculationThe loss of mass is calculated as follows:


where:
ma = weight of abraded surface in mg
m1 = weight of sample before abrasion in mg
m2 = weight of sample after abrasion in mg
n = number of rotations

Report the average of a least 2 significant values and the standard deviation.

NoteFor this test we make use of a Hanatek rub/abrasion tester. This is a deviation
from the official Tappi standard.
See also rub-off as described under interaction with fluids/printability.
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