Reference Manual

16−2
Figure 16-1. Stock Approach System
Drawing is from TAPPI’s Making Pulp and
Paper Series and is used with permission.
Fillers
For most copy paper, around 15 - 30% of the
papers are fillers; the majority of which is clay and
PCC — precipitated calcium carbonate. These
additions are generally made to lower the overall
cost of materials and can be used for brightness,
opacity, or even the smoothness of the paper.
However, fillers do not bond together in the same
way cellulose fibers do, so they reduce the
strength of the paper.
This limits the amount one can put in the sheet.
Another difficulty is trying to get the filler to remain
with the fiber during the sheet forming process. It
is not uncommon to lose fillers while on the
moving wire or forming fabric. Additives are used
to increase the retention of the fillers so they are
not lost during this process.
All of these wet-end additives are either soluble in
water or are small enough to fit through the small
openings of the forming fabric. To keep these
additives from falling out of the process, retention
aids are added. These retention aids insure that
the fillers attach to the fibers. These are typically
added just before the headbox or before the
headbox screen (figure 16-1 ). They are added
late in the process because excess agitation could
break up the polymer chains.
The size and shape of mineral additives can
greatly affect the properties of the paper. For
instance, most grades of paper have a thickness
specification. Papermakers want to make the
paper as thick as possible all while using the least
amount of fiber. The more flat the microscopic
filler, the more dense the paper. However, many
times these fillers are so small the filler gets
between the fibers and makes the paper
smoother.
The common papermaking fillers are clay, calcium
carbonate (CaCO
3
), talc, and titanium dioxide
(TiO
2
). Clay is the most popular since it is cheap,
stable, and generally provides good performance.
Calcium carbonate is a better opacifier than clay
and has higher brightness. Titanium dioxide is the
brightest and most effective opacifier, however, is
high cost. Talc is used as a “soft” filler that helps
to give paper a soft, silky feel to the product.
Finally, due to the amount of water being pumped
through the system, foam can be formed. This can
create spots or pinholes in the paper. To alleviate
this problem, defoamers are added to help the
bubbles coalesce into bigger bubbles. These
larger bubbles will then rise to the surface and
break. The water and warm temperatures can also
create a wonderful environment for bacteria and
fungal slimes. These are called “bugs” in the paper
mill. These bugs can lead to holes and spots in
the paper and frequent sheet breaks but can be
controlled with the addition of biocides to the
paper machine “white water” system.