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