Breathability control material ‘Ventcool’

1. Introduction

Numerous studies have been conducted into the comfort of clothing while being worn and the development of added functions in raw yarns and textiles, as well as developments in methods of aftertreatment, has been carried out in pursuit of comfort in various areas. Although there is trouble involved in the evaluation of such developments as there are differences in the sense of comfort between individuals, as well as differences according to time and the circumstances of the same person, ‘in-garment environment’ is the expression often used for comfort properties produced by added functions.

Feeling hot and humid from perspiring in a warm place, or feeling chills and cold while moving to a cooler place with the concomitant transfer of heat, is an example of alteration in the in-garment environment. To deal with such alterations in the in-garment environment, humans put on a thin layer when hot, or layer over layer when cold, adjust garment openings such as collar and cuffs, or at times even make their own adjustments unconsciously. From this behavioral observation, it is clear what is being adjusted is the ‘air flow’ beneath our clothing.

Meanwhile, though there are various kinds of functions that can be added to raw yarns, and textiles through aftertreatment such as heat-retaining/generating/absorbing finishes, thermal/cooling or perspiration- absorbing & quick-dry finishes, an adjustment of the air flow is not expected from them.

In other words, a focus has been put on this air flow in the development of ‘Ventcool’, conducted by Mitsubishi Rayon and presented in the following, in which a function has been added so that the tissue of the yarn itself automatically swells or shrinks depending on the presence of water or humidity. This function allows the clothing material to control the in-garment breathability instead of the wearer making adjustments and provide comfort while being worn.

2. Features

Ventcool’s breathability control properties have been graphically indicated (Fig. 1) to compare the fiber’s dry and wet (perspiration) states.

Fig. 1 Ventcool’s breathing function model

Ventcool presented here is a fabric combined with other materials such as polyester. It is to be in direct contact with the skin and has the feature of being a ‘mobile fiber’, in which reversible crimping occurs in accordance with weather changes. In other words, when dry, as three-dimensional crimping takes place within the fiber, the breathability is reduced and therefore heat is retained, and conversely, when wet with perspiration during exercise for example, the fiber expands as the crimping is eased, thereby producing superior cool & dry effects with improved breathability.

This ‘mobile fiber’ Ventcool is a functional fiber made with Mitsubishi Rayon’s yarn conjugation technology and an acetate-based, unspecified fiber (for which the method of indication has not been determined in the Household Goods Labeling Law), composed of two components to which a different degree of swelling has been given through the special chemical modification of a conjugated fiber made of triacetate and diacetate.

Compared to the usual acetate fiber, it has more hydrophilic components and is superior in moisture-absorbent/releasing properties due to its special section shape that allows smooth adjustment to changes in humidity. In addition, the reversible alteration, i.e. the production and elimination of crimping, is retained semi-permanently.

3. Background to the development of raw yarn

The diameter of the nozzle used in the dry spinning of raw acetate yarn for Ventcool is around 40μm. Unlike the melt spinning method used for fibers such as polyester, the nozzle is very small and this involves special know-how concerning its precise shape.

Moreover, there are two key points with regard to the production of Ventcool’s raw yarn.

One is the development of a conjugated yarn production technology that uses the dry spinning method for the raw material acetate. Though several dry spinning machines for conjugated yarns have already been made public, a simple, industrial model capable of precise distribution has been newly developed out of necessity for high precision and for the elimination of trouble with distribution properties. With this machine, the production of a conjugated yarn has been made possible using diacetate and triacetate.

The second point is the development of a technology that selectively modifies the diacetate component of the conjugated yarn. Through this special chemical modification, the fiber turns into a conjugated yarn composed of triacetate and cellulose that has reversible crimping properties that are produced in accordance with the presence of water or moisture in the air.

Fig. 2 The structure of cellulose and acetate

4. The structure of raw yarn

The following is a basic description of acetate. Acetate is regarded as a semi-synthetic fiber made of natural cellulosic pulp and contains many hydroxyl groups (Fig. 2) and is therefore superior in hydrophilicity. Fibers such as cotton, linen and rayon are distinctive in that they swell considerably when wet with water.

Acetate can be obtained by inducing reaction between the hydroxyl groups and acetic anhydride, thereby turning the hydroxyl groups into acetyl groups. When the rate of reaction is over 74%, the fiber is called diacetate and when over 92% triacetate. The higher the rate of reaction, the smaller the hydrophilic tendency, and the acetate scarcely swells when wet with water.

Fig. 3 The section shape of diacetate/triacetate complex conjugated spun yarn	Fig. 4 The section shape of reactive-dyed yarn

Ventcool is a spun yarn made of a complex conjugated fiber, in which crimping cannot be observed. The section is shaped like a potbellied figure as shown in Fig. 3.Such a complex section shape is only found in dry-spun yarns.

This above-obtained conjugated fiber is then chemically treated under specific conditions so that only the acetyl groups of the diacetate content are hydrolyzed, or in other words, acetate thus turns into cellulose. The fiber shown in Fig. 4 has been dyed with reactive dyestuff, with which acetate cannot be dyed, so that the complete transformation of half the fiber into cellulose can be observed in its section image.

Fig. 5 Models of dry section (left) and wet section (right)

Meanwhile, the decomposed acetyl groups, i.e. acetic acid, have been replaced with water in the chemical treatment bath, and the cellulosic content is now swollen with water.

As a result, when dry, much water content of cellulose is removed and the contraction of fiber begins, as shown in Fig. 5 and 6. The contracting force is extremely powerful, which indicates that cellulose’s special hydrogen bonding force is at work. Since contraction occurs naturally along its length, spiral crimping results from the alteration in the length of cellulose and the resulting difference between it and the unaltered triacetate.

Fig. 6 Yarn shapes in the dry (left) and wet (right) state

Fig. 7 A dry part (left) and wet part (right) of knit fabric

In contrast to this, the cellulose content absorbs moisture and swells when wet, and as swelling takes place along the length, crimping will disappear.

In fact, these alterations can be observed well in knits, as shown in Fig. 7. In the figure, stitch shapes can be recognized clearly in the right half of the knit fabric as it is wet, with the crimping having disappeared, as opposed to the dry left half, in which knit stitches have been tightened due to crimping and the contraction of the fiber.