Surface sizing, normally called a starch solution, is applied to the partially dry paper web surface as a way of binding surface fibers and raising the resilience of the sheet, including resistance to stress. Paper sizing chemicals are used for the manufacturing of paper to lessen the ability of the paper to absorb liquid when it comes in contact with a liquid substance.

For instance, this is done with the meaning of enabling paints and inks to stay on the paper surface and to easily dry them rather than to be dissolved into the paper. Also, this means a more reliable, efficient, and reliable printing, best for painting and writing surfaces.

It is accomplished by reducing the ability of paper fibers to accumulate liquids through capillary operation. Besides, sizing affects creasibility, abrasiveness, finishing, printability, surface bond strength, smoothness, and minimizes surface porosity, and fuzziness.

In addition to sizing chemicals, there are three kinds of papers: strong sized (hard sized), weak sized (slack sized), and unsized (water-leaf). The definition for all three is below-

  1.     Hard sized papers have the maximum water resistance, such as liquid packaging board and coated fine papers.
  2.     Slack sized paper, to some extent, includes newsprint and absorbent.
  3.     Waterleaf includes absorbent papers for blotting and has low water resistance.

Sizing Chemical types

There are two sizing types: surface sizing named tub sizing and internal sizing, often denoted engine sizing. Although, on almost all papers, application of Internal sizing is done and most notably to all of them that are machine-made, while surface sizing is amassed for the ledger, highest grade bond, and writing papers.

Internal Sizing Chemicals

Internal paper sizing chemicals are utilized in the process of wet-end papermaking. The most common internal sizing chemicals are rosin and alkyl Succin anhydride (ASA) and alkyl ketene dimer (AKD).

Internal sizing chemical must have some essential characteristics, just like high hydrophobicity, better fiber retention, consistent distribution around the fiber surfaces, and should also be tightly bound to the fibers.

However, Internal paper sizing chemicals can often be categorized as acidic form sizing chemical and standard or neutral form sizing chemicals. Rosin and its variants are an acidic sizing agent type.

Though the internal sizing is also applied to avoid the penetration of water, it never lets the paper become waterproof. Internal sizing is often named beater sizing, and it is also linked with the beater, a tool used mostly for the refining of paper pulp.

Papers with a significant amount of internal size are called hard-sized, whereas papers with a small amount of internal size are considered slack-sized. 

Surface Sizing Chemicals

Surface paper sizing chemicals include both hydrophobic and hydrophilic sides. It creates a thin layer on a paper web, after that the hydrophilic tail of it is bound to the fabric, while the hydrophobic tail is pointing outwards. The thin layer decreases paper dust; increases the surface power, printability, and waterproofing of the material.

Various substances are used as surface paper sizing chemicals, like of modified starches, Styrene Maleic Anhydride (SMA), Polyurethane (PUR), Styrene Acrylic Acid (SAA), Styrene Acrylic Emulsion (SAE), Ethylene Acrylic Acid (EAA), and and Gelatin etc.

Other Uses of the Term Sizing                          

  •       During paper manufacturing, a type of sizing is used to reduce the absorption of liquids. The inclusion of the sizing chemicals also enables inks and paints to stay on the paper surface rather than to be consumed. Moreover, this is primarily necessary when printing wallpaper, artwork, and reading materials.
  •       The word sizing is often used for any surface on which gold or gold leaf is applied. This sizing of the plaster, wood, or paper helps the gold leaf to stick more quickly to the desired surface.
  •       In weaving, the term sizing refers to warp yarn on the loom and is also better to avoid breakage by increasing the abrasion resistance and strength of the yarn. The size may be made of oil, starch, wax, manufactured polymers, or gelatin.


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