Thermal storage/release, durability, and temperature sensing properties of thermostatic fabrics treated with octadecane-containing microcapsules

To develop a thermostatic fabric, a 100% polyester fabric is treated with octadecane-- containing microcapsules by a knife-over-roll coating process. The amount of heat content increases as the concentration of microcapsules increases, and it decreases as the temperature and time increase. The surfactant treatment at a given microcapsule concentration increases the heat content of the treated fabrics about 56-94%. The durability of the coated microcapsules lasts for about ten launderings. The treated fabric becomes stiffer and less smooth, soft, and full than the untreated fabric, as shown by KEs measurements. Wear trials with the untreated and treated garments in a conditioned environment confirm the temperature sensing properties of treated garments. The cooling effect from thermal storage of the octadecane-containing microcapsules is revealed by results showing that the changes in the mean skin and microclimate temperature with the treated garment are less than for those wearing the untreated garment.

Octadecane is a kind of phase change material (PCM) that is able to absorb, store, and release large amounts of latent heat over a defined temperature range when the material changes phase or state [5]. In the slushy state, octadecane acts like a thermostatic substance that causes the liquid portion to release heat and helps to prevent the temperature next to the skin from cooling. The solid portion absorbs a large amount of heat without an appreciable change in temperature and helps to prevent the temperature next to the skin from rising, leading to an overheated situation [8, 11].

So far, research on PCMS for textiles has mainly focused on polyethylene glycol (PEG) by Vigo and Bruno [12, 13, 14, 15]. The extent of PEG bonded to fabrics depends on the molecular weight of the polymer, the concentration of the crosslinking agent and catalyst, the curing conditions, and the fiber content. Recently, preferred PCMS for textiles have been n-paraffin waxes with various melting and crystallization points according to a number of carbon atoms (e.g., octadecane, nonadecane, eicosane, and so forth) [3, 9, 10]. Changing the proportion of the different paraffins in the Pcms can produce the desirable melting and crystallization points, and the products can be applied to various apparel end-uses. Several researchers have investigated application methods of paraffins when bound to fibers (e.g., acrylics), fabric, and foams [3, 9, 10]. They have claimed that Pms have to be put into microcapsules, otherwise they will eventually drip off clothing when they melt. Microencapsulation is the process of enveloping microscopic sized droplets or particles in a shell material for purposes of protection or controlled release, because Pcm-containing microcapsules must be durable and safe through the finishing process [2].

Bryant and Colvin developed fibers manufactured with microencapsulated paraffin integrally incorporated into the matrix of the fibers during manufacturing [3]. They reported that microencapsulated Pcms can be blended into compounds suitable for fabrics and foam coatings [9, 10]. In this case, there are various coating processes: knife over roll, knife over air, rotary screen printing, gravure, dip coating, and transfer/cast coating. And forms are made, not by any coating process, but by integral incorporation into the form matrix itself [4]. Commercial production of paraffin-treated apparel has already begun, but no one has yet reported a specific formulation to finish textiles the properties of paraffin-- treated textiles, the curing conditions, or the proper percentage of microcapsules.

To apply these paraffin PCMS to clothing, they have to satisfy some conditions. Their thermal activity must work in a range of skin temperatures and be harmless to the skin. When the body is at its normal temperature, there are certain temperature ranges common to certain parts of the body. The core of the body, or the abdominal area, and the head normally maintain an average skin temperature higher than that associated with other areas of the body. Generally, the overall average comfortable skin temperature is 33.3 deg C, and if this cannot be maintained, a person begins to feel uncomfortable [7].

In this study, we selected octadecane as the PcM for developing a thermostatic fabric. Its melting point is about 28.2 deg C, and it allows the Pcm to be stabilized in a slushy state below the comfortable skin temperature, 33.3 deg C. The objectives of this study are to determine the effect of the octadecane-containing microcapsule concentration and the cure temperature and time on thermal storage/release, to investigate the durability of the treated fabric, and to assess the temperature-sensing properties of a microcapsule treated garment over an untreated one.

Experimental

To test durability to laundering, the specimens were washed in a Kenmore automatic washing machine with AATCC standard detergent 124 in a normal washing cycle and tumble dried according to AATCC 135-1992 for testing the dimensional changes. Thermal properties were then measured after one, five, and ten launderings.

Photomicrographs of flat-mounted, gold/palladium-- coated fabric cross sections were taken with a scanning electron microscope (JSM, 820, Jeol, Japan, with an accelerating voltage of 4.0KV and 200X magnification) to examine the remaining microcapsules and binder visually.

Thermal storage/release properties of the treated specimens were determined by DSC (Seiko Instruments, Inc., Japan). Melting (T^sub m^) and crystallization (T^sub c^) temperatures and the heat of fusion (H^sub f^) on heating and heat of crystallization (H^sub c^) on cooling of the specimens were measured in one heating and cooling cycle, with the DSC run at heating and cooling rates of 10 deg C/minute. The temperature ranges of the heating and cooling cycle were - 10-50 deg C and 50-10 deg C, respectively.

Hand properties were measured by the KEs-FB system [6]. Primary hand values (PHV) were calculated by the KN-202-DS regression equation. Total hand values (THV) were calculated by the KN-303-DS-SUMMER equation for men's summer suiting, because the weight and thickness of the specimens (weight 70 g/m^sup 2^, thickness 0.13 mm) was suitable for that equation.

Wear trials were performed by six healthy female university students wearing the experimental garments; one made with the untreated control and the other with the treated specimen at 40% concentration, all sleeveless and fitted tops. The weights of the experimental garments were 38.3 and 54.4 g/m^sup 2^ , respectively, and their expected total heat ((Delta)H^sub f^) was about 261 J/g. In addition to the experimental garments, knitted underwear (100% cotton knit), long sleeved jackets (65/35 cotton/polyester woven), and long pants (65/35 cotton/polyester woven) were put on with socks (100% cotton knit) and anklelength athletic shoes (100% polyester). The thermal retention value of the total ensemble was about 1 clo. The chamber was conditioned to 21 +/- 1 deg C, 50 +/- 5% RH, and 0.1m/s air velocity. A subsequent exercise protocol consisted of five periods: 20 minutes acclimatization, 10 minutes exercise, 10 minutes rest, 5 minutes exercise, and 15 minutes rest. The thermal sensing properties were evaluated objectively and subjectively. Skin temperature, microclimate temperature (chest and back), and microclimate humidity (back) of the participants were measured by the thermohygrometer (model X 712-1, Takara Co. Ltd.) after each 1 minute. After each 5 minutes, the subjective evaluation was done by questionnaires using the ASHRAE scale. Thermal sensation, humidity sensation, and overall sensation were rated on the seven-point scale.

Results and Discussion

THERMAL STORAGE/RELEASE PROPERTIES OF TREATED FABRics

Thermal storage/release properties increased as microcapsule concentration increased. The maximum value of heat content for treated polyester was 40% microcapsule concentration. The heat of fusion and heat of crystallization at 40% concentration were 5.7 and 5.6 J/g, respectively. At this concentration, the add-on was about 44%, and it was 4% higher than that of the 25% concentration (Table II). Temperatures of melting (T^sub m^) and crystallization (T^sub c^) increased as microcapsule concentration increased: the higher the microcapsule concentration, the more time required to transfer heat into the capsules.