colour chronicle-jan09.indd - Clariant

paper
18 colour chronicle 1 | 2009
Relationship between Particles and Fibers Buildup
On Blankets vs Wet Rub Resistance
Relationship between Particles and Fibers Builtup
on Blankets vs IGT Dry Pick
Four Good Reasons for using an
Insolubilizer in Coating Recipes
1) A Clear Correlation Exists between the Wet
Surface Strength (WSS) of Paper And their
Offset Printing Runnability :
Offset printing defects, such as blanket piling,
wet picking of coated paper, as well as dusting,
can be signifi cantly reduced or avoided when the
paper’s WSS is increased.
The best correlation is obtained between offset
runnability / printing defects and WSS, as
measured through Adams wet rub or modifi ed
Taber abrasion tests. Other characteristics, like
dry surface strength (dry IGT or Prüfbau) are not
so well linked to the behaviour of the paper in
offset printing.
The productivity and the quality of the offset
printing process directly depends on WSS – this
can be expressed by the number of printed copies
before cleaning the offset press.
2) WSS is a Varying, Hardly Controllable Property
of Printing Paper :
Once the paper has left the drying section, its
WSS naturally increases by itself but very slowly,
and reaches a maximum after one to two months
of storage. This is due to the constant evolution
of cellulosic fi bers, starch, and particularly
synthetic binders for coated papers, with time.
This phenomenon is strongly dependent on the
storage temperature. For this reason, papers
stored in winter or summer may have very
different printabilities.
The quality and variability of the raw materials
used in the paper production also increases the
variability of the WSS.
3) Offset Printers can be unsatisfied with,
irregular, insufficient and unpredictable Offset
Printabilities :
Offset printers are very concerned by the defects
they observe on the papers they have printed.
This affects the quality and consequently the
value of their work.
Offset printers are not interested to spend time
and money for cleaning the offset blanket,
because deposits have been collected from the
paper, and degrade the printing quality.
If such problems occur, they may have two kinds
of attitude :
– Either they can claim to the paper producer
and ask for money as a compensation of
the loss of productivity and paper quality.
They may give a chance to their supplier for
improving the paper quality.
– Or they may directly prefer to switch to
another supplier, providing similar paper,
but with better-offset runnability. In such
cases, papermakers are merely losing some
of their customers.
4) The regular use of an Efficient Insolubilizer is the
Right Way to Ensure a Constant and Satisfactory
Offset Printability
There are many ways to improve the WSS,
through the choice of the fi ber composition
or refi ning degree, optimization of the sheet
formation, optimized choice of surface starch,
synthetic binders, etc., but the more direct,
economical and controllable way to achieve high
levels of WSS is to use an effi cient crosslinker.
Its role is to reduce the water sensitivity of the
polymers (starch, PVOH, synthetic binders, etc.)
and cellulosic fi bers located close to the surface,
by a crosslinking action.