Thermal and moisture transfer properties of sock fabrics differing in fiber type,yarn,and fabric structure

Abstract

This study aimed to determine the relative effects of fiber type (fine wool, mid-micron wool, acrylic), yarn type (high-twist, low-twist, single), and fabric structure (single jersey, half-terry, terry) on thermal resistance, water vapor resistance, thermal conductance, water vapor permeability, liquid absorption capacity, and regain of sock fabrics. Fabric structure had the greatest effect on thermal resistance, water vapor resistance, water vapor permeability, liquid absorption capacity, and thermal conductance. Terry fabrics were the most thermal and water vapor resistant, most absorbent, and most conductive. Results were consistent with current understanding of the effect of fabric thickness on thermal and moisture transfer properties when measured under static conditions: However, the effects of yarn type and/or fabric structure of sock fabrics have not previously been explained.

Keywords

testing fabrication performance materials socks thermal resistance fiber type

Socks are worn for a variety of reasons, one of which is to maintain a thermally neutral environment for the foot. This function is especially critical in cold climates, in order to protect and insulate the foot from exposure. The thermal and moisture properties of socks are complicated by the footwear worn, which impedes the evaporation of moisture as a result of sweat or immersion.^1–3 Damp socks are often a component of the footwear ensemble. This results from conditions of use or sweating, the foot reportedly producing sweat at an average rate 30 mL/h⁻¹ during warm conditions.² The presence of moisture within the sock will affect the thermal resistance and thermal conductance of the fabric, thus contributing to the thermal comfort of the wearer.

Despite the important contribution of the sock to the thermal protection provided by a footwear ensemble, the thermal resistance of socks is not well understood and has rarely been measured. Thermal foot manikins, available since the late 1990s, have been used primarily to investigate the thermal resistance of boots,^4–6 with little published information on the thermal resistance of sock fabrics or socks alone. Several human wear studies on socks have been undertaken, the general focus of which has been perception or measurement of thermal comfort and foot dryness,^7–10 rather than measurement of sock properties such as thermal resistance, conductance, and permeability to water vapor.

Differences in the thermal resistance of fabrics are generally accepted as being due to varying fabric thickness, with thicker fabrics offering greater resistance.^11–13 However, in the case of sock fabrics there is some uncertainty if the relationship between thickness and thermal resistance applies. Instead, the type of fiber used has been the principal focus. For example, the greatest thermal resistance of a number of sock fabrics (varying in fiber content but produced on the same machinery and finished in the same manner) was provided by a chitosan fiber sock.¹⁴ The chitosan fabric was not the thickest, conflicting with the accepted relationship between thickness and thermal resistance. It is likely that this finding resulted from the non-standard method and experimental set-up (testing two layers of fabric on top of each other, possibly introducing air layers, or compressing fabrics) used by Cimilli et al.¹⁴ Fabric thickness and volume of air held typically determine the thermal resistance of a fabric under static conditions with the effect of the fiber itself lessened.^13,15,16 Since thermal conductivity of fiber has little influence on fabric thermal resistance, fiber contributes to the thermal resistance of fabric via mechanical properties affecting the openness of the structure.¹⁷ The effect of fiber on thermal resistance is typically limited to its packing density in the fabric structure, contributing to the volume of air trapped.¹⁸

Much is known about the interaction between water and fabrics,^19,20 but few investigations have focused on moisture transfer through sock fabrics.¹⁴ Early studies on wearers’ perception of sock dryness and dampness likely relate to fiber hygroscopicity and temperature of the ambient environment.^8,9 Human perception of skin wetness appears to depend on the sensation of temperature,^21,22 but is also related to mechanical pressure, thermal conductance, evaporation, and a sensation of “stickiness.”²³

Clothing and footwear impede evaporation of moisture and the presence of moisture on skin or in clothing can lead to discomfort.²⁴ Very little water (e.g. 2% for polyester fabrics) is required for humans to detect a change in dampness, regardless of whether the moisture is held inside the fiber or on the fiber surface.²⁴ Due to the sweat produced by the foot and the inability of most footwear to transfer water vapor to the external environment,³ socks may often be damp, contributing to sensations of cold feet by a wearer.

When a fabric becomes wet, the role of fiber type on thermal resistance may become more important. As water saturates inter-yarn spaces, air in these spaces is displaced, reducing the contribution of air and increasing the contribution of moisture transport of the fibers to the thermal resistance of a fabric.^11,25–27 The thermal conductivity of wet knitted fabrics is increased with increased moisture content.²⁸ Although thermal conductivity of sock fabrics has been a subject of interest,^14,29 there appears to be no published information about the thermal conductance of damp or wet sock fabrics.

Water vapor transmission through a fabric depends on several factors such as fiber, yarn and fabric structure and their effects on thickness and permeability.^30,31 Fiber can have a considerable effect on water vapor permeability because natural fibers such as cotton, linen, wool, and silk have a larger capacity to absorb moisture than do man-made or synthetic fibers.^31,32 For example, wool knits are often deemed more acceptable by wearers than synthetics because the humidity between wool knit fabrics and the skin reaches equilibrium more slowly than knit fabrics composed of synthetic fiber, thus possibly allowing physiological adaptation by the wearer to the changing conditions.³³ Interstitial or pore space also affects water vapor resistance,³² with these spaces determined by both the packing density of fibers within yarns and yarns within fabrics.^34,35 The shape and size of the interstitial spaces are affected by the yarn type and fabric structure, with bulky and hairy fabrics often having smaller spaces and greater thermal resistance and higher water vapor resistance, whilst open, loosely knitted structures have larger interstitial spaces and lower water vapor resistance.^{31,32,36–38}

A connection between the volume of water absorbed by a fabric and fabric thickness is generally accepted,³⁹ but the relationship between fabric thickness and absorption is not particularly clear. Fabric absorption capacity has been reported to be influenced by many factors including fiber chemistry and morphology, fabric thickness and structure, yarn type, surface characteristics of fiber and fabric, size and shape of inter-yarn and inter-fiber spaces, and finishing treatments applied to fiber, yarn or fabric.^25,40,41 For example, terry is a common loop pile structure used not only for socks, but also in absorbent materials such as towels. Terry towel fabrics with differing pile heights have been reported as absorbing differing amounts of water, with the type of yarn used in the construction of the pile loops the most important factor affecting absorption.⁴¹ Fabric thickness was not reported or discussed by these authors and it was unclear whether fabric thickness had an effect on the absorption capacity of the terry towels.

Although there is some understanding of the thermal and moisture transfer properties of socks and sock fabrics, the relative effects of fiber type, yarn type, and fabric structure remain poorly understood. The experimental design of many investigations has not allowed the effects of these fabric characteristics to be separated from one another, or else statistical analysis has not been reported reducing confidence in conclusions drawn. The aim of this work is to rectify these omissions and improve the understanding of sock fabrics and their properties.

Experimental

Materials and test conditions

A 3 × 3 × 3 (3 fibers, 3 yarns, 3 fabrics) factorial experimental design was used to examine the relative effects of fiber, yarn type, fabric structure, and moisture content (Table 1 and Table 2).⁴² Twenty-seven fabrics were manufactured with production variables including knitting machinery (Sangiacomo 4100 HT 12-gauge smachine, single cylinder, Christchurch, New Zealand) and tension matched as closely as possible across the different yarn and fiber types. Fabrics were non-dyed.

Table 1.

Fabric variables

Variable	Description
Fiber	fine wool, 19 µm
	mid-micron wool, 26 µm
	acrylic, 19 µm**
Yarn	two-fold conventional, R74/2 tex, singles twist factor 2700, two-fold at ∼55%
	two-fold low twist, R74/2 tex, singles twist factor 2100, two-fold at ∼55%
	single conventional, R74/1 tex, singles twist factor 2700
Fabric	terry
structure	half-terry*
	single jersey

Half-terry – loops on every second course only.

Acrylic density was 1.16 g/cm³, 3 denier fiber has a diameter of approximately 19.1 µm.

Table 2.

Structural features of sock fabrics (n = 5)

Fiber	Yarn twist	Fabric structure	Stitches per 10 mm (course direction)		Mass/unit area (g/m²)		Thickness (mm)
Fiber	Yarn twist	Fabric structure	Mean	s.d.	Mean	s.d.	Mean	s.d.
Acrylic	high	single jersey	7.90	0.14	344.80	4.32	1.64	0.05
	low		7.75	0.25	357.40	9.53	1.69	0.04
	single		7.50	0.00	340.60	11.76	1.53	0.03
	high	half-terry	6.70	0.67	449.20	10.71	3.95	0.03
	low		6.30	0.57	446.50	16.38	4.02	0.03
	single		6.70	0.45	468.80	13.03	3.91	0.05
	high	terry	6.60	0.55	515.00	5.66	4.37	0.09
	low		7.00	0.61	522.80	13.77	4.62	0.07
	single		6.50	0.71	551.80	6.76	4.28	0.03
Fine-wool	high	single jersey	8.65	0.22	422.40	5.50	1.61	0.01
	low		8.65	0.22	437.60	3.21	1.66	0.02
	single		8.50	0.00	414.60	8.32	1.60	0.03
	high	half-terry	7.10	0.74	572.20	4.21	3.60	0.07
	low		6.60	1.52	563.80	16.89	3.66	0.04
	single		8.05	0.87	568.40	8.96	3.59	0.07
	high	terry	8.45	0.76	653.60	13.58	4.25	0.04
	low		8.50	0.50	687.80	19.01	4.40	0.06
	single		8.50	0.00	678.20	20.13	4.30	0.07
Mid-wool	high	single jersey	8.50	0.00	402.20	4.66	1.67	0.04
	low		8.40	0.22	385.40	5.32	1.66	0.04
	single		8.35	0.22	399.20	4.66	1.65	0.02
	high	half-terry	6.90	0.55	532.60	9.71	3.73	0.04
	low		6.70	0.76	498.60	5.32	3.68	0.05
	single		7.00	0.71	534.80	6.14	3.78	0.05
	high	terry	7.60	0.42	633.00	8.46	4.53	0.04
	low		7.60	0.74	607.20	9.55	4.44	0.06
	single		7.40	0.55	627.80	7.73	4.46	0.04

Fabrics were pre-treated to ensure stability in a Wascator washing machine (FOM71MP-Lab, supplied by James Heal and Co Ltd., Halifax, England) using six laundering cycles of program 8 A and detergent A, as specified in BS EN ISO 6330:2001, and dried flat after the sixth cycle.⁴³ Fabrics were conditioned for 24 hours prior to testing in the Standard environment for conditioning and tested in accordance with EN ISO 139:2005 (i.e. 20.0 ± 2℃, 65.0 ± 4.0% RH).⁴⁴ Conditions during testing were recorded using a temperature and RH data logger (Tiny Tag).

Several standard thermal and moisture related properties were investigated (Table 3).^45–48 For tests that required specimens larger than 200 mm in width, three rectangular specimens (510 × 510 mm) were assembled to ensure specimens covered the complete surface of the measuring unit and thermal guard, with the area covering the measuring unit being seam free. Two seams ran the entire length of each specimen, created by abutting two edges of fabric and stitching across the join using a variation of ISO 304 zigzag lockstitch.

Table 3.

Standard methods used to measure selected performance properties

Property	Standard	Description
Regain (n = 5)	BS EN 12127:1998: Textiles – Fabrics – Determination of mass per unit area using small samples	Specimens were dried in a Contherm Thermotec scientific oven (supplier: Contherm, Lower Hutt, New Zealand) at 105℃ until constant mass was reached, determined as no change in mass greater than 0.1% during successive weighing. Samples were then allowed to condition at 20 ± 2℃, 65 ± 4% RH for 24 hours before a final weighing to determine mass.
Thermal resistance (n = 3)	ISO 11092: 1993(E) Textiles - Physiological effects - Measurement of thermal and water-vapor resistance under steady-state conditions (sweating guarded-hotplate test)	Specimens were placed technical face up on the guarded-hotplate (diameter 485 mm), oriented perpendicularly to the air flow, and were measured until equilibrium was reached for four consecutive 30 min blocks. Measurement of thermal resistance was made using dry fabrics and a dry hotplate, in the Standard atmospheric conditions.
LAC (n = 5)	BS EN ISO 9073-6:2003 Textiles – Test methods for nonwovens – Part 6: Absorption	Each specimen was weighed on a Mettler Toledo AT400 Balance, then fastened to a piece of stainless steel gauze and lowered 20 mm below the surface of distilled water for 60 s, followed by a vertical draining period of 120 s. The specimen was placed in a weighing glass, covered, and re-weighed.
Water vapor resistance (n = 3)	ISO 11092: 1993(E) Textiles – Physiological effects – Measurement of thermal and water-vapor resistance under steady-state conditions (sweating guarded-hotplate test)	Specimens were placed over the sintered plate that was covered with a support grid and a layer of NFPA (National Fire Protection Association) thermal resistance reference material to prevent water coming in contact with the test specimen (National Fire Protection Association, 1990). The reservoir under the porous plate was filled with de-oxygenated milli-Q™ (re-distilled, deionized water). The standard atmosphere used for testing water vapor resistance of dry fabrics was 35 ± 2℃ and 40 ± 3% RH. Specimens were measured until equilibrium was reached for four consecutive 30 min blocks.
Water vapor permeability (n = 3)	BS 7209:1990 Water vapor permeable apparel fabrics	Specimens were placed technical face side up and sealed (Uhu® adhesive cement) over the mouth of a test dish (96 mm diameter) containing 46 ± 2 ml of distilled water, with dishes resting on a turntable that maintained a constant speed. The turntable rotated for one hour in order to establish equilibrium prior to measuring. The turntable then rotated for approximately five additional hours and each specimen was then re-weighed. The water vapor permeable index was calculated by expressing the water vapor permeability as a percentage of the concurrently tested reference fabric.

Although thermal conductance can be calculated from the measurement of thermal resistance, there is no standard method describing the measurement of nominal thermal conductance: by definition, thermal conductance is the rate of heat transfer measured through a specimen between a heated source plate and a cool sink plate. A rise in temperature of the sink plate is a measure of thermal conductance.⁴⁹ The rate of flow is proportional to the rate of change of the heat sink temperature as a function of time.⁴⁹ The apparatus used to measure thermal conductance consisted of a source plate and a sink plate contained in two insulated boxes (Supplier: John Anderson Energy Consultant, Dunedin, New Zealand). The source plate was heated to 35.0 ± 0.1℃ and the sink plate maintained at 20.1 ± 0.1℃ between tests inside a separate conditioning box. Fabric specimens were made damp in a Wascator set on a wetting cycle (2 kg, 20℃, rinse gentle action (180 s), drain no action (60 s), extract slow (30 s), drain gentle action (30 s)), specimens were then placed immediately inside a re-sealable plastic bag and kept in a standard conditioned atmosphere until ready for testing. Specimens were made damp the day of testing and remained in the bag no longer than six hours. The fabric specimen was taped around the edges to a wooden frame using masking tape to prevent edge curling, then placed flat on top of the source plate. Each specimen was placed technical face up with wales and courses aligned in the same direction. The sink plate was placed on top of the specimen, resting on supports equivalent in height to the thickness of the fabric in order to prevent compression. Change in temperature of the sink plate was recorded by a computer controlled, 16 bit analogue to digital converter.⁴⁹ The rate of heat transfer proportional to the rate of change of the heat sink temperature was measured as a function of time over a period of 200 s to prevent confounding caused by water content changes (e.g. specimen drying).⁴⁹ Thermal conductance of the fabric was calculated from the rise in temperature of the sink plate.^11,49

Specimens were randomized within blocks grouped by fabric thickness. This was done in order to minimize variability arising from adjusting the gap between the source and sink plate. Heights of the sink plate supports were adjusted for the thickness of each group of fabrics using a digital calliper and screwdriver.

Analysis

Data were analyzed using univariate analysis of variance (ANOVA) routines IBM® SPSS®.⁵⁰ Prior to analysis, data were tested to ensure the assumptions of normality and homogeneity of variances (using Levene’s test) were met. In the case of significant differences among variables, Tukey’s HSD post-hoc test was used to identify which means differed significantly at p ≤ 0.05.^50,51 In some of the ANOVAs, thermal resistance was standardized by fabric thickness and/or fabric mass in an attempt to account for the known strong effect of fabric thickness and fabric mass on this parameter. For water vapor permeability, a block effect could not be detected, nor were there any differences between the water vapor permeability data and the index.

Results and discussion

Fabric structure

Fabric structure dominated the thermal and moisture properties examined, and most were likely related to differences in fabric thickness and mass. Fabric structure had the major effect on thermal resistance (dry) (F_2,54 = 2467.92, p ≤ 0.001), LAC (liquid absorption capacity) (F_2,108 = 186.45, p ≤ 0.001), thermal resistance when standardized for fabric thickness (i.e. warmth to thickness ratio) (F_2,54 = 3521.75, p ≤ 0.001), water vapor resistance (F_2,54 = 147.015, p ≤ 0.001), water vapor resistance standardized for fabric thickness (F_2,54 = 541.456, p ≤ 0.001), water vapor permeability index (F_2,54 = 36.49, p ≤ 0.001), liquid absorption capacity (F_2,54 = 186.45, p ≤ 0.001) and thermal conductance (damp) (F_2,54 = 347.94, p ≤ 0.001) (see Table 4 and Figure 1). Fabric structure also affected water vapor resistance standardized for fabric mass (F_2,54 = 59.29, p ≤ 0.001).

Figure 1.

Means and Tukey’s HSD multiple comparison test for selected parameters.

Table 4.

Principal factors affecting thermal and moisture properties

Parameters and variables	df	F	Sig.
Percentage regain
Fiber	2	904.20	0.001
Yarn	2	0.16	NS
Fabric structure	2	3.89	0.05
Fiber * yarn	4	0.34	NS
Fiber * structure	4	1.34	NS
Yarn * structure	4	1.12	NS
Fiber * yarn * structure	8	0.21	NS
Thermal resistance (m²K/W)(dry fabrics)
Fiber	2	18.51	0.001
Yarn	2	9.15	0.001
Fabric structure	2	2467.92	0.001
Fiber * yarn	4	1.57	NS
Fiber * structure	4	21.79	0.001
Yarn * structure	4	4.51	0.010
Fiber * yarn * structure	8	0.58	NS
Thermal resistance standardized for fabric thickness
Fiber	2	10.37	0.001
Yarn	2	8.46	0.001
Fabric structure	2	3521.75	0.001
Fiber * yarn	4	3.89	0.01
Fiber * structure	4	37.68	0.001
Yarn * structure	4	10.82	0.001
Fiber * yarn * structure	8	0.35	NS
Thermal resistance standardized for fabric mass
Fiber	2	442.26	0.001
Yarn	2	7.13	0.001
Fabric structure	2	87.17	0.001
Fiber * yarn	4	6.84	0.001
Fiber * structure	4	15.42	0.001
Yarn * structure	4	11.28	0.001
Fiber * yarn * structure	8	2.50	0.05
Water vapor resistance (m²Pa/W)
Fiber	2	2.90	NS
Yarn	2	0.09	NS
Fabric structure	2	147.02	0.001
Fiber * yarn	4	1.57	NS
Fiber * structure	4	2.74	NS
Yarn * structure	4	2.16	NS
Fiber * yarn * structure	8	1.03	NS
Water vapor resistance standardized for fabric thickness
Fiber	2	1.88	NS
Yarn	2	1.66	NS
Fabric structure	2	541.46	0.001
Fiber * yarn	4	1.70	NS
Fiber * structure	4	5.39	0.001
Yarn * structure	4	2.14	NS
Fiber * yarn * structure	8	1.16	NS
Water vapor resistance standardized for fabric mass
Fiber	2	179.60	0.001
Yarn	2	5.59	0.01
Fabric structure	2	59.29	0.001
Fiber * yarn	4	12.96	0.001
Fiber * structure	4	9.05	0.001
Yarn * structure	4	8.22	0.001
Fiber * yarn * structure	8	4.80	0.001
LAC (%)
Fiber	2	72.53	0.001
Yarn	2	6.91	0.001
Fabric structure	2	186.45	0.001
Fiber * yarn	4	0.29	NS
Fiber * structure	4	4.45	0.001
Yarn * structure	4	1.52	NS
Fiber * yarn * structure	8	1.73	NS
Water vapor permeability index
Fiber	2	1.72	NS
Yarn	2	2.89	NS
Fabric structure	2	36.49	0.001
Fiber * yarn	4	0.51	NS
Fiber * structure	4	0.72	NS
Yarn * structure	4	0.28	NS
Fiber * yarn * structure	8	0.80	NS
Thermal conductance (W/m²K) (damp fabrics)
Fiber	2	0.71	NS
Yarn	2	1.11	NS
Fabric structure	2	347.94	0.001
Fiber * yarn	4	2.10	NS
Fiber * structure	4	5.18	0.001
Yarn * structure	4	0.72	NS
Fiber * yarn * structure	8	1.08	NS

Terry fabrics were the most thermal and water vapor resistant, least permeable to water vapor, most absorbent and most conductive. They were also thickest and had the highest mass of all the fabrics. Single jersey fabrics were the thinnest, lightest, and were the least thermal and water vapor resistant, most permeable to water vapor, least absorbent and least conductive. These results are consistent with current understanding of the effect of thickness on fabric properties, as it is generally acknowledged that thicker fabrics are most resistant to both heat and water vapor transfer.^11–13 Water vapor transmission has been reported as lowest in bulky and hairy fabrics, and highest for fabrics with open structures (e.g. large interyarn and interstitial spaces).^31,37 Presumably in thicker fabrics such as a terry, although its construction is more open than single jersey, there is a greater volume of fibers and air through which the water vapor must permeate, thus creating a stronger resistance to heat and water vapor.

Several studies have reported the thermal properties of socks or sock fabrics,^8,9,52 yet few have related thermal resistance to fabric thickness. Those investigations that involved thermal properties of socks found no relationship between thickness and thermal resistance/thermal comfort.^7,14 However, Cimilli et al. reported thicker fabrics were less permeable to air and slower to transfer moisture between two layers of identical fabric.¹⁴ Thus, fabric thickness has been identified as a factor affecting some comfort and moisture properties of sock fabrics but not specifically linked to thermal resistance, LAC, thermal conductance, water vapor resistance or water vapor permeability.

Results from the current work support the contention that thermal conductance is influenced by fabric thickness and the volume of water held.⁵³ An earlier suggestion that LAC is affected by fabric structure, due in part to fabric thickness,³⁹ is consistent with the current work. Water is a better conductor of heat than air, thus increasing the volume of water held by a fabric by increasing its porosity can also increase conductance.²⁸ Although results from the current work considered fabric thickness, the volume of water held by the structure was also important. Terry fabrics held the largest absolute volume of water and also exhibited the highest conductance, while single jersey fabrics held the least absolute volume of water, hence their low conductance.

Although fabric thickness seems to have had a more important effect on thermal and moisture transfer properties than the type of knitted structure (e.g. pile loop vs. no pile), results of the present work indicate that the arrangement of fibers and yarns in the different structures may also be important. Differences detected when thermal and water vapor resistance were standardized for fabric thickness indicate that the packing density or arrangement of fibers and yarns in the fabrics may have had an effect. For example, when thermal resistance was standardized for fabric thickness, all three fabrics were different, however the difference between the half-terry and terry was quite small, approximately 0.001 m²K/W per mm, whereas the difference between half-terry and jersey was an order of magnitude greater, approximately 0.01 m²K/W per mm (Figure 1(c)). Presumably this is due to the density of fibers and yarns in the structure with the pile/loop structure of the terry being more open, and possibly trapping less air than single jersey. As single jersey provided the most resistance when standardized for fabric thickness, it is implied that had single jersey and terry been the same thickness, single jersey would have been a better insulator and more resistant to water vapor. This is possibly due to a more tightly packed arrangement of fibers and yarns in the single jersey structure than that in terry or half-terry, as packing density is known to influence thermal resistance.¹⁷ Thus, in order to maximize thermal resistance, a thick, dense sock fabric is most desirable.

Fiber

In the present work, fiber type did not have the large effect on thermal and moisture transfer properties of socks previously suggested.^8,9,14 Fiber had a much smaller effect than fabric structure on the properties investigated. Fiber had the main effect on regain (F_2,108 = 904.20, p ≤ 0.001) and thermal resistance standardized for fabric mass (F_2,54 = 442.26, p < 0.001) (Table 4). Fiber also affected LAC (F_2,108 = 72.53, p ≤ 0.001), thermal resistance (dry) (F_2,54 = 18.51, p ≤ 0.001) and thermal resistance when standardized for fabric thickness (F_2,54 = 10.37, p ≤ 0.001), and although significant, its effect was much smaller than fabric structure, and may be due to the large amount of data collected. Fiber had no detectable effect on conductance or water vapor permeability.

Differences detected among the fiber types may be linked to fiber hygroscopicity. When regain was measured, fiber type had the greatest effect, as fabrics composed of hydrophobic acrylic fibers absorbed less moisture than fabrics composed of both types of wool, which is hydrophilic. For regain, both types of wool absorbed nearly the same percentage of water (approximately 11%). Any differences in the behavior in moisture properties between the mid-micron and fine wool may be due to fiber morphology, as their affinity for water was the same.

Fiber type may have also affected properties due to differences in fiber mass and diameter. The effect of fiber type on water vapor and thermal resistance when standardized for fabric mass is likely due to the difference in fiber mass and packing density, which has been reported to affect thermal resistance.^17,18 Acrylic fabrics had the lowest mass, but were also often the thickest fabrics. Acrylic fabrics had a lower mass than the wool fabrics, possibly suggesting that acrylic fibers were less tightly packed into the fabric, yet providing more thermal and water vapor resistance if fabrics were all of an equivalent mass per unit area. Fiber diameter and packing density may have also contributed to the LAC differences detected among the fabrics. Although packing density has not previously been directly linked with LAC, packing density has been reported to affect thermal resistance.¹⁸ Thus, it may also affect other thermal and moisture related properties, as many of these properties are affected by the same physical properties (e.g. fabric thickness). Mid-micron wool fibers had a larger diameter (26 µm) than fine wool (19 µm) and acrylic (19 µm) fibers. Less tightly packed mid-micron wool fibers may have held more water in their interstitial spaces. The difference in the moisture held as a percentage of dry fabric weight between acrylic and fine wool was smaller (44%) than the difference between fine wool and mid-micron wool (67%) (Figure 1(h)), therefore it is possible that fiber diameter and packing density had a larger effect on LAC than fiber hygroscopicity.

Although the overall effect of fiber type was not as large as has been previously been reported, this could be due to the restricted range of fibers examined in this study. Nevertheless, results from the current work along with a previous wear trial reporting no detectable subjective difference in foot temperature, and an identical pattern of response in skin temperature regardless of type of sock worn,¹⁰ suggest limited importance on influence of the type of fiber on thermal properties of socks when a small number of fiber types were investigated.

Yarn

The effect of yarn on thermal and moisture transfer properties of sock fabrics was less important than that of fabric structure. However, the effect of yarn on thermal resistance (dry) was detectable (F_2,54 = 9.15, p ≤ 0.001), as it was also for thermal resistance when standardized for fabric thickness (F_2,54 = 8.46, p ≤ 0.001) and LAC (F_2,108 = 6.91, p ≤ 0.001).

The effect of the yarn seems to be directly linked to the physical properties of a fabric, particularly fabric thickness (reportedly affecting fabric performance properties, with higher yarn twist resulting in thinner fabrics).³⁶ For thermal resistance, single and high-twist yarns behaved similarly, with low-twist offering the greatest resistance. This is consistent with the findings of Özdil et al.⁵⁴ The greater thermal resistance of fabrics composed of low-twist yarns is possibly due to the volume of air held within the low-twist yarn thus increasing the volume of air held within the fabric as a whole. However, when thermal resistance was standardized for fabric thickness, fabrics composed of single yarns had the highest thermal resistance. This may be due the higher packing density possible with single yarns compared with plied yarns, thus increasing fabric density which in turn affected thermal resistance when standardized for fabric thickness.^17,18

Conclusions

Most thermal and moisture transfer properties of sock fabrics investigated in the current work are affected by fabric variables via two different mechanisms: (i) fabric thickness, and (ii) the density and arrangement of fibers and yarns. Much of the contribution of fiber and yarn type is related to the physical properties of the fabric, e.g. the thickness and the packing density or arrangement of fibers and yarns within the fabrics.

While previous work on thermal and moisture transfer properties of socks has focused on the effects which fibers have on these, results from the present work suggest that, apart from effects on regain, fiber effects are generally small and likely to be masked by effects of fabric structure. What is important to note, however, is that this study included two types of fiber (wool, acrylic) and did not include effects which transient conditions may have, some of which are known.³³ The most important effects of fiber appear to be on those properties involving water (e.g. regain, LAC) where fiber sorption properties have a strong influence.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgements

Dr. Stewart Collie of AgResearch is gratefully acknowledged for the provision of experimental fabrics. The Todhunter/Carpenter/Home Science Alumnae Scholarship fund and the University of Otago Christchurch Earthquake Extension fund, and University of Otago Applied Sciences Department are acknowledged for the provision of tuition fees and/or living stipend.

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