Paper Standards Measur Umschlag - Sappi Mobile

Vll Others
pH value ISO 6588
The pH value in the data sheets defines the pH value of the
surface. The pH values are indicated on a scale from 0 to 14.
The measuring value 7 marks the neutral point which corresponds
to distilled water. Values below 7 refer to “increasingly
acid”, values above 7 stand for “increasingly alkaline”.
Papers should be close to, if possible, next to the neutral
area or in the slightly alkaline range to have ideal requirements
for printing and further treatment.
Ash content ISO 2144
The ash content is the remaining residue as percentage of
dry paper after ignition at 900°C. The ash content is the total
of inorganic material, like carbonate, kaolin etc. in the paper
(including filler). Fillers have an important influence on the
paper quality (for instance opacity) and are dosed with care
depending on the application of the paper.
Mottle test
Mottling is the uneven print appearance, mostly in solid
areas: small dark and light areas in the surface of paper
(board) caused by ink, paper or printing press. Mottle is influenced
by many parameters: e.g. ink, colour sequence,
construction of printing press, speed, rubber blanket and
fountain water. Variations in the surface characteristics such
as absorption and smoothness play an important role
regarding mottle and are caused by the production process
and the components within the paper.
There can be three kinds of mottle:
Backtrap mottle
Ink applied to paper surface on a preceding printing unit of a
multi-colour press will split onto the blanket in a subsequent
unit and then be re-deposited onto the next sheet. If this
occurs unevenly, the print can become mottled.
This backtrap mottle can be simulated on a test printing
machine by printing the paper and countering the paper in
pre-defined times against a clean blanket reel.
Water interference mottle
Water transferred to paper surface on a preceding printing
unit of a multi-colour press should be absorbed by the coating
before it reaches a subsequent printing unit. If it is not
absorbed it will prevent even transfer of the ink. A mottled
print can result.
The method IGT ink repellence (see chapter VI Absorption)
is used to test this phenomenon.
Midtone mottle (screen mottle)
This is an uneven print in 30–60% screens, caused by differences
in the scattering and absorption behaviour of base
paper and/or coating. This effect can occur in one colour
printing and even in case of perfectly formed dots of equal
density (optical effect).
Midtone mottle can be evaluated by printing a screen with
the test printing machine or with help of a plastic screen
which is put on top of the paper. The level of screen mottle
can be evaluated visually or with help of image analysis.
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