Size it Up – Internal Sizing Chemicals
Why Use Sizing?
Sizings used in papermaking reduce water absorption into the fibers. Internal sizing treatments act on the outside of fibers. Consider paper towels, the main use being to wipe up spills and messes. Internal sizing in the paper towel slows the wetting process, allowing the paper to achieve full absorption. Products like paper cups are sized to resist the water they are designed to hold. Likewise, the edges of the containerboard used to hold liquid must resist edge-wicking and also be internally sized in case pinholes develop in the outside laminate. Sizing also allows for better ink application in printing processes important for printing papers, increasing the holdout of colorants on the surface of the paper.
Solid AKD (left), Emulsified AKD (right)
How Does it Work?
Without sizing, water will readily penetrate paper by forming hydrogen bonds with the ‘exposed’ hydroxyl groups on the fibers. Sizing agents work by reducing the free energy of the paper surface and reducing the groups capable of hydrogen bonding. To accomplish this, sizing chemicals must contain hydrophobic groups, such as an alkyl group. The sizing agent must be well dispersed and should remain in an active form up until a critical part in the papermaking process. It must be retained efficiently in the wet-web of paper and be well distributed on a molecular scale on the outer surface of paper once it is dried. Individual molecules of the sizing agent must be anchored and orientated.
The Effect of Sizing on Paper-Water Interaction (Banik and Brückle, 2011)
What are the Commonly Used Sizing Agents?
There are four main types of internal sizing agents: alkenyksuccinic anhydride (ASA), alkylketene dimer (AKD), rosin products, and copolymer products.
ASA was introduced as a sizing agent in the early 1960s. It is an oily liquid at room temperature and must be formed into an emulsion before adding it to the fibers. It has a high chemical reactivity. Most of its hydrophobizing effect happens before the size press. Its sizing efficacy is linked to formation of ester bonds with hydroxyl groups on the polysaccharide component of paper during drying. It can be used in alkaline conditions, with optimum conditions observed between a pH of 7.5 and 8.4. If ASA becomes hydrolyzed it cannot be evenly distributed. Cationic starches are used to optimize ASA.
AKD is a waxy solid at room temperature and is emulsified with cationic starch products. It is less reactive than ASA, performs in neutral to weakly alkaline conditions, and achieves full sizing usually after paper has cooled from being over-dried (for curing of the size). Its use can lower the coefficient of friction of paper. Storing emulsions of AKD can result in hydrolysis and reduction in efficiency of the sizing.
Containers Used to Ship AKD
Rosin sizings are used in acidic condition and are most efficient between a pH of 4 to 6. It is sold as a dispersion or as a micellar soap solution. Hydrolysis is not a concern with rosin sizing but it needs a mordant like alum to fix it to the paper. Complications occur when the water used in papermaking is hard, containing high levels of magnesium or calcium. The magnesium and calcium ions can agglomerate the rosin soap before it contributes to water resistance. Rosin dispersions and rosin soaps are intolerant of high temperatures. Copolymers can be used at the wet end and are not as efficient as other sizing agents. However, they provide precise and reliable gradations of sizing. They also have a long shelf life.
Banik, Gerhard, and Irene Brückle (2011): Paper and water: a guide for conservators. Oxford: Elsevier.
Hubbe, M.A.(2006): Paper’s resistance to wetting – a review of internal sizing chemicals and their effects,” BioResources 2(1):106-145.
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