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This paper describes the technique for cross-linking cellulose with formaldehyde in the vapor and liquid phases. It also gives a brief description of an improved method for estimating the degree of cross-linking. Moisture sorption isotherms of cross-linked samples have been obtained at different relative humidities ranging from 0 to 90% and at 25°C by using McBain-Bakr sorption balance. The relationship between swelling and water content in the bath is nonlinear. This may be due to unrestricted swelling at the lower concentrations of water in the bath; in the intermediate range of water concentrations, the swelling does not increase significantly because of primary-wall constraint. Moisture regain of the control samples increases with increase in swelling up to 16%. This change is more pronounced at higher humidities. However, for 19% swelling, possibly because of the breaking of the primary wall, the cellulose chains may so rearrange themselves that hydrogen bonding between OH-groups increases, thereby decreasing the moisture regain. Swollen samples are stabilized by cross-linking. The moisture sorption characteristics of cross-linked swollen samples depend on the degree of swelling at the time of cross-linking and also on the degree of cross-linking.

Moisture sorption data of Form C samples have been obtained using a McBain-Bakr sorption balance and these were used to calculate effective area of molecular contact of the cross-linked samples with the Brunauer-Emmet-Teller (BET) equation. A probable explanation is advanced for the maxima and minima in moisture regains of cross-linked in the swollen state samples and the effective area/HCHO relationship. Attempts have been made to infer the distribution of cross-links trom the moisture sorption data of the cross-linked samples. Iodine accessibility values of formaldehyde-crosslinked fibre and dye pickup studies of formaldehyde-crosslinked fabric support an earlier assumption that the primary wall is ruptured under Form W conditions.