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3.6.4.2.1 Rosin-based Sizing Agents
These are still the most widely used of all wet-end sizes, because they are cheap to produce and are produced from regenerative raw materials. They are mainly used in acid and much less in pseudo-neutral pH conditions. They have the advantage that the degree of sizing can be controlled very easily, and the sizing effect is fully cured when the paper leaves the machine. In practical use there are different preparations of rosin, for which the following terms are used:
. • Free rosin: Resin acids whose carboxyl groups are in the completely protonated form.
. • Dispersed size: High (95–100 %) free rosin dispersion of resin acids. The dis¬persed size particles are stabilized by surfactants, starch, polymers, or pro¬teins.
. • Rosin soap size: Rosin size composed of the sodium salts of resin acids. Formed by neutralizing (saponifying) resin acids with NaOH or some other sodium base, such as Na2CO3. Different forms of soap sizes are available, including paste, extended, and dry sizes.
. • Extended size: A 50 % solids product consisting of a 50:50 mixture of completely neutralized resin acids and urea.
. • Fortified size: Rosin size that has been reacted with fumaric acid or maleic anhy¬dride to form the Diels-Alder adduct. The reaction product contains extra car¬boxyl groups and produces more efficient sizing response than the unreacted resin acids. It is used as a starting material for other size products.
. • Dry size: A 100 % neutralized, dry rosin size product that dissolves easily in water.
. • Paste size: An 80 % neutralized rosin size product marketed as a 70–80 % solids paste.

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