The role of vapour deposition during internal sizing: A comparative study between ASA and AKD

Model surfaces were exposed to vapours of alkyl ketene dimer (AAD), alkenyl succinic anhydride (ASA), stearic acid and methyl stearate. The extent of vapour deposition and the degree of reaction were followed by measuring the contact angle of water on the treated surface. The model surfaces consisted of cellulose acetate, cellulose and glass, while the variables investigated were temperature and the period of exposure. AKD and ASA vapours both deposit on cellulose and glass to render the surfaces hydrophobic, but not on cellulose acetate; hydroxyl groups are required. The critical time of reaction was of the order of hours which, under the conditions investigated, is typical of the formation of a covalent bond by esterification. Compared to AKD and stearic acid, ASA vapour reacts much faster with cellulose. Fatty acid ester vapour rendered glass hydrophobic, but not cellulose. A mechanism of trans-esterification is proposed. A strong bond between the reactive vapour size and cellulose is essential to introduce permanent sizing of cellulose. Vapours play a governing role in internal sizing.