Elucidation of Softening Mechanism in Rinse Cycle Fabric Softeners. Part 1: Effect of Hydrogen Bonding

Abstract

Most softening agents, such as rinse cycle fabric softeners, used by consumers at home contain cationic surfactants that have two long alkyl chains as their main component. The softening mechanism on fibers, especially cotton, has not yet been scientifically established, despite the market prevalence of fabric softeners for decades. One explanation for the softening effect is that the friction between fibers is reduced. According to this explanation, the fiber surfaces are coated by layers of alkyl chains. Because of the low coefficient of friction between alkyl chain layers of low surface energy, the fibers easily slide against one another yielding softer cotton clothing. However, no direct scientific evidence exists to prove the validity of this explanation. The softening mechanism of cotton yarn is discussed in this paper. Bending force values of cotton yarn treated with several concentrations of softener are measured by bend testing, and cotton and polyester yarns are compared. Results indicate that increases in cotton yarn hardness after natural drying are caused by cross-linking among inner fibers aided by bound water. This type of bound water has been known to exist even after 2 days of drying at 25 °C and 60 % relative humidity. Yarn dried in vacuo is soft, similar to that treated with softener. Thus, some of the softening effect caused by fabric softeners on cotton can be attributed to the prevention of cross-linking by bound water between cotton fibers.

Keywords: Bound water; Cotton; Fabric softener; Fiber cross-linking; Hydrogen bonding; Polyester; Softening mechanism.

Fig. 1

Electrostatic interaction of cationic vesicles with negatively charged fibers (left) and collapse of vesicles to cover fiber surface with layers of hydrophobic alkyl chains (right)

Fig. 2

Softener treatment of cotton yarns. The cotton yarns prepared by method B were fixed on a PP frame as shown on the left. The yarn was immersed in deionized water and stirred for 5 min, after which the softener was added and stirred for 120 min as shown on the right

Fig. 3

Process for preparing cloth and yarn samples for hardness measurements. Cloth samples were spread on a sheet of polypropylene, soaked in deionized water, and left to dry at 25 °C and 50 % RH for 2 days (naturally dried). Yarn samples were fixed parallel to one another with double-sided tape so that they do not touch, soaked in deionized water, and naturally dried

Fig. 4

Automatic pure bending tester employed to measure bending force

Fig. 5

Removal of bound water in cotton by complete drying in a vacuum desiccator equipped with a calcium chloride tube (ca. 2–35 torr) at 110 °C for 3 h

Fig. 6

Influence of treatment conditions on softness. Process A corresponds to natural drying after washing with water. Process B corresponds to natural drying after washing followed by additional dehydration and fluttering

Fig. 7

Influence of treatment conditions on stiffness of cotton yarn. Process A corresponds to natural drying after washing with water. Process B corresponds to natural drying after washing followed by additional dehydration and fluttering

Fig. 8

Cotton cloth bending force values. The horizontal axis represents the number of bending cycles and the vertical axis represents the moment of bending [average of the B value (n = 3)]

Fig. 9

Cotton yarn bending force values. The horizontal axis represents the number of bending cycles and the vertical axis represents the moment of bending [average of the B value (n = 3)]

Fig. 10

Comparison of water content from the NIR spectra after environmental conditioning (25 °C, 50 % RH)

Fig. 11

Polyester cloth bending force values. The horizontal axis represents the number of bending cycles and the vertical axis represents the moment of bending [average of the B value (n = 3)]

Fig. 12

Polyester yarn bending force values. The horizontal axis represents the number of bending cycles and the vertical axis represents the moment of bending [average of the B value (n = 3)]

Fig. 13

Types of hydrogen bonding structures may exist in the naturally dried cotton yarns. Type I is characterized by direct hydrogen bonding among OH groups on the surfaces of fibers. Type II consists of hydrogen bonding between the fibers that have water bound on their surfaces

Fig. 14

Comparison of NIR spectra between natural and complete drying. The absorbance intensity of water at 5144 cm⁻¹ had significantly decreased, signifying that nearly all bound water was removed

Fig. 15

Appearance and texture of cotton yarn after complete drying