Reference Manual

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Chapter 16
Wet-End Chemistry
In this chapter, we will be discussing materials,
other than fibers, that are added to the slurry of
fibers before paper is formed. It is important to
keep in mind that there are two types of additives.
1. Functional additives—These additions are
treatments necessary to meet the particular needs
of an end-customer.
2. Process additives—These additions modify
the properties of the paper. They can be used in a
multitude of different fashions.
Sizing
The purpose of sizing is to enable paper products
to resist penetration by fluids. This is critical for
printing operations. If appropriate sizing is not
taken into account, the ink will diffuse into the
sheet and cause severe quality problems. Sizing
can be achieved by using wet-end additives or by
applying a coating to the surface of the dried paper.
The traditional wet-end sizing agent is a modified
rosin, better known as “rosin size.” This additive
can actually make paper repel water under acidic
papermaking conditions. Rosin size comes from
softwoods as a byproduct during the Kraft pulping
process. To make rosin work as a sizing agent,
papermakers also add aluminum sulfate
(Al
2
(SO
4
)
3
), better known as “papermaker’s alum.”
This combination is an effective way to make
paper resist water and other fluids. The process is
also known as “acid sizing” as this combination
works well in acidic aqueous environments.
Internal Strength
Many natural and synthetic polymeric substances
can be added to stock at the wet-end to improve
the physical properties of the dry paper sheet.
They are to reinforce the fiber-to-fiber bonds
thereby improving tensile strength, reduce “fuzz”
or lint on the paper surface, and can reduce the
rate of water penetration.
Traditional internal strength additives are natural
and modified starches and gums. Starches are
polymers of glucose whereas gums are polymers
of mannose and galactose. However, the trend is
now toward the use of synthetic polymers as
latexes and polyacrylamides used in combination
with starches and gums. These new products
have now met a wider range of specific
requirements for greater paper strength with
different degrees of stiffness and stretch.
Wet-Strength Resins
The purpose of wet-strength resins is to tie fibers
and fines together with additional bonds that are
not taken apart by water. Wet-strength paper is
defined as paper that retains more than 15% of its
tensile strength when wet.
The most common wet-strength agents are
ureaformaldehyde, melamine-formaldehyde, and
polyamide resins, and are water soluble. These
long-chain polymers can be used on paper for juice
containers or other liquid containers so the fibers
remain strong even after getting wet. However,
because these agents are water soluble, they must
be fixed onto fibers with the help of retention fillers.