Abstract
Body protectors in horse-riding are characterized by a specific tight-fitting garment construction, and by sophisticated materials focused on body protection against impacts, such as falls and kicks from the horse. Both construction and materials affect the heat transfer between the human body and the environment, and add extra burden to the rider's metabolic heat production that has already been increased by the sport activity. With the above considered, this study investigated the application of warp-knitted spacer fabrics as a substitute for conventionally used foam inserts. Using the thermal manikin test method, the thermal properties of equestrian body protectors based on conventional foam, on spacer fabrics, and on combinations of both had been measured and evaluated. In addition, the Transplanar water transport tester was used to assess the liquid transport properties of the applied protective materials. The results of the study support the usability of warp-knitted spacer fabrics in impact-protective clothing.
Keywords
Body protectors are usually vest-like clothing systems including an inserted protective material that absorbs and dissipates the energy of an impact in order to reduce the impact of forces transmitted to the rider’s body. Widely used materials for impact protection in equestrian body protectors are closed-cell structure foams, such as polyurethane, ethylene vinyl acetate (EVA), and Sorbothane, a patented viscoelastic urethane polymer. Closed-cell foams in general come with a low moisture absorption coefficient and high thermal insulation and, compared to open-cell foams, a relatively heavy weight due to a higher density. These special characteristics of closed-cell foams and the fact that effective protective functionality requires a high degree of thickness and rigidity of the foam material potentially create serious comfort problems for the wearer. In addition, tight and secure fit of a body protector, which is an essential requirement to counteract displacement that can reduce the protective function (Choi, Hoffer, & Robinovitch, 2009; Yang et al., 2005), adversely affects the comfort of the rider.
To address this problem, we investigated the application of a protective insert using three-dimensional, warp-knitted spacer fabric. Such fabrics possess not only transversal compressibility but also good dimensional stability and high resistance to compression. In addition, the space between the outer layers and the porous character of spacer materials result in lightweight materials with high levels of air permeability and breathability.
Literature Review
The management of heat and humidity generated while riding constitutes a challenging problem for equestrian body protectors. Thus, an essential requirement for protective clothing is to provide a thermal microclimate adapting to the thermoregulatory activities of the rider. The most effective thermoregulatory activity of the human body against overheating is the evaporation of sweat, which often involves high amounts of liquid perspiration. Tight-fitting equestrian body protectors amount to an extra mass of clothing covering the body surface and therefore add thermal resistance. In addition, the thermal characteristic of the protective insert, conventionally made of closed-cell foams, impedes effective transmission of heat, water vapor, and liquid water from the rider’s body to the outside atmosphere. Thus, researchers have found that the increased insulation and vapor resistance of protective vests can raise thermophysiological strain (Cheuvront, Goodmann, Kenefick, Montain, & Sawka, 2008).
In recent years, applications of knitted spacer fabrics as substitutes for foam materials have attracted increasing attention. Knitted spacer fabrics comprise two independent fabric layers connected by separate spacer yarns. The space between the two outer layers provides an open construction that allows sufficient airflow, enhancing heat and vapor transport away from the human body (Borhani, Seirafianpur, Ravandi, Sheikhzadeh, & Mokthari, 2010). Heide and Moehring (2003) recommended warp-knitted spacer fabrics for therapeutic aids, such as knee supports and shoe linings, based on the excellent physiological comfort and microclimate effects provided by these fabrics. Focusing on the enhanced moisture transporting and absorbing properties of spacer fabrics, Davies and Williams (2009) suggested their use for absorbent medical applications, such as incontinence products, as an ideal alternative to, among others, foams and composites. Ye, Hu, and Feng (2008), investigating spacer fabrics for cushion applications, found that warp-knitted spacer fabrics are superior in air permeability and heat resistance compared to Polyurethane (PU) foam. Bartels (2003) investigated the use of knitted spacer fabrics in airplane seats. He found that knitted spacer fabrics provide reduced thermal insulation and better thermal comfort compared to foam.
The special construction of knitted spacer fabrics provides a range of properties that makes them potentially advantageous for use in comfort-related impact protection. Research carried out on improving the cushioning properties of spacer fabrics has shown that they can be successfully modified to fulfill specific protective requirements (Du & Hu, 2012, 2013; Liu, Au, & Hu, 2013a; Liu, Hu, Long, & Zhao, 2012; Walker et al., 2008). Accordingly, knitted spacer fabrics play an increasing role as cushioning components in sports-related protective clothing and devices (Heide, 2000; Pereira, Anand, Rajendran, & Wood, 2007; Yip & Ng, 2008). However, their use in high-performance impact protective textiles is still rare.
Compared to conventional foam materials, knitted spacer fabrics characteristically have more problems in achieving high-impact performance. One opportunity to increase the impact performance of spacer fabrics is the use of multiple layers. Accordingly, investigations have been carried out on improving energy absorption capacities by increasing the number of fabric layers (Budden & Vazquez, 2007; Liu, Au, & Hu, 2013b; Qian & Fan, 2006). However, an arrangement of multiple layers may not provide the same physical and physiological properties as a single layer of knitted spacer fabric. Therefore, the comfort-related advantages of spacer fabrics might be significantly reduced if used in a multiple-layer protective insert.
In this study, we created protective inserts of multiple layers in order to examine the thermal–physiological properties of equestrian body protectors equipped with spacer-based protective inserts. We built up and connected several layers of warp-knitted spacer fabric and combinations of spacer fabrics with foam layers. In pilot impact testing, we determined the required number of layers that could attain the protective requirements for equestrian body protectors. Our study was aimed at answering the following two research questions:
The amount of perspiration absorbed and transported away from the skin can greatly affect the thermal comfort provided by protective clothing during sports activity (Kwon, Kato, Kawamura, Yanai, & Tokura, 1998). One of the disadvantages of foam is nonabsorbency of water, which thereby blocks the evaporation of water from the body. The consequences are both a damp and an unpleasant microclimate between the skin and the fabric as well as restricted body cooling. To examine the liquid transport properties of the protective inserts, we used the transplanar water transport tester (TWTT) to measure the initial water absorption of the applied protective materials. The TWTT provides an accurate method for measuring the water transport behavior of fabrics (Sarkar, Fan, & Qian, 2007) and is usable for three-dimensional fabrics of considerable thickness.
In research, thermal manikins are often used to determine the thermal characteristics of clothing (Holmer & Nilsson, 1995). Crown, Ackerman, Dale, and Tan (1998) applied the manikin method to evaluate the thermal properties of flight suits. Chen and Cluver (2010) assessed the thermal properties of quilted vests filled with poplar seed hair fibers by means of a thermal manikin. Fan and Tsang (2008) used the manikin method and wear trials to determine the thermal comfort of several tracksuits and found significant correlation between comfort sensations in the wear trials and the thermal properties measured with the manikin. We used the manikin test method to investigate the heat and water vapor transfer properties of equestrian body protectors.
Experiments
Materials
Three body protective vests were selected from commercial sources and used for comparison. The vests represented different conventional designs of currently available body protectors. All protectors were of similar size and fit the dimensions of the manikin. See Table 1 for the physical properties of the different protectors and Figure 1 for the substantial design differences between body protectors with layered inserts and those with foam blocks.
Design differences (from left to right): Vest 1 with triple-linked foam layers; Vest 2 with foam arranged in blocks; Vest 3 with quadruple-linked foam layers.
Properties of Purchased Body Protectors.
Note. PES = polyester.
a Material allocation in accordance with supplier specifications on the vest.
Two kinds of warp-knitted spacer fabric were used in the study (see Table 2). Both spacer fabrics had the same spacer layer notation, but there were notable differences between the two spacers, specifically in thickness and surface structure (see Figure 2A–D). The surface structure of Spacer A is a rhombic mesh, which is a flat, wide-open, warp-knitted mesh with a honeycomb structure. The surface structure of Spacer B is a Chain Inlay, which is a warp-knitted structure composed of chains wherein an inlay yarn is inserted.
A, Surface structure of Spacer A. B, Cross-section of Spacer A. C, Surface structure of Spacer B. D, Cross-section of Spacer B.
Structural Parameters of Applied Spacer Fabrics.
Note. PES = polyester.
Vest 1 was selected for spacer fabric tests and modified by removing the layered foam insert (see Figure 3A and B) and replacing it wholly or in part with multiple layers of spacer fabrics. All multiple layer arrangements were prepared by securing the edges of the spacer fabrics with overlock stitching and connecting the layers at defined points to avoid displacement.
A, Foam used in Vest 3 and Vest 1. B, Protective foam insert of Vest 1.
Establishing the Number of Fabric Layers
Impact tests were performed with a drop weight impact testing system using a wide bar impactor and anvil based on the European Standard for protective clothing for equestrian use EN 13158:2009, in particular for testing the shoulder region of equestrian jackets. From a height of 1,019 mm, a mass of 5 kg, equivalent to impact energy of 50 J, was dropped on the spacer arrangements. The test results represent the energy transmitted through the test material. Transmitted forces of 30 N were set as the benchmark for establishing the number of layers in the vests. The test results reflect the material’s ability to dampen the kinetic energy of the impact; the lower the value of the transmitted force, the higher was the ability of the material to reduce the contact force of the impact. Test samples were arranged in various numbers of spacer fabric layers and in combination with a foam layer originating from Vest 1. All samples were fixed along the edges in order to avoid displacement during the test. On each test sample, six impacts were made, and the mean peak force for all impacts was calculated from the results obtained.
The results of the impact tests are presented in Table 3. For both spacer materials, a minimum of five layers were required to reach the benchmark. Spacer B achieved considerably better results (26.4 N) compared to Spacer A (29.9 N). The best impact-protecting results were obtained with an arrangement of three layers of spacer fabric combined with one layer of foam (Spacer AF/23.2 N; Spacer BF/22.4 N).
Results of the Impact Tests.
Methods
TWTT
The spacer samples were all 4 cm wider in diameter than the perforated area of the sample plate (and thus the predefined sample size) in order to avoid water penetration at the open fabric edges, as shown in Figure 4. Prior to the test, all materials were conditioned for 24 hr at 20°C ± 1°C and 65% ± 2% relative humidity before the test. Water temperature during the tests was 19.7°C, and each test lasted for 3 min. During the test period, data recording took place at 0.5 ms intervals. Each test was replicated three times in order to increase the precision of the results.
Spacer A attached to the sample plate of the transplanar water transport tester (TWTT).
Manikin Test
The thermal manikin “Walter” was used to measure and evaluate the heat and water transfer properties of the protective vests. The manikin is constructed out of an inside skeleton and a high-strength breathable fabric casing filled with water. Water supply and a circulation system maintain the body temperature at 37°C. Perspiration is simulated by capillary fluid transmission via the pores of the fabric.
During all tests, the manikin was dressed with a set of garments equivalent to common horse-riding clothing: a T-shirt (100% cotton) and a pair of trousers (73% nylon, 17% polyester [PES], and 10% elastane), as shown in Figure 5. All protectors were conditioned for 24 hr prior to the test. Each test was performed over a period of 10 hr at the ambient conditions of 20°C ± 1°C and 65% ± 2% relative humidity. Data recording started after a stabilization period of 5 hr. The final results represent the average values calculated from all data collected.
Manikin “Walter” equipped with basic shirt and pants and Vest 1.
Data Analysis
Data analysis was performed using SPSS 18. The coefficient of variation was calculated for the test results of the TWTT. In addition, Pearson correlation analysis was used to evaluate the strength of the correlation between Spacer A and Spacer B in the TWTT. Paired sample t-tests were used to examine whether the differences in the results for each vest in the manikin test were significant at a p value of .05.
Results and Discussion
TWTT
The results of the TWTT, displaying the amount of water absorbed for the initial 15 and 180 s, are shown in Table 4. Both spacer fabrics absorbed a relatively high amount of water. Although the foam layer came with perforated holes, no water transport occurred during the test. Pereira, Anand, Rajendran, and Wood (2007) reported similar results when they performed water vapor permeability tests and wicking tests to compare the thermal comfort of foams and neoprene with knitted spacer fabrics based on PES multifilament. They found that all tested spacer fabrics significantly outperformed foam and neoprene with respect to their wicking properties.
Initial Absorption Rate (g).
Note. Std. dev. = standard deviation. C.v. = coefficient of variation.
The mass of water transport over the whole test period of 180 s in Spacer A, Spacer B, and the foam used in Vest 1 are shown in Figure 6. Spacers A and B showed an equally quick water transfer. The Pearson correlation coefficient between Spacer A and B showed a very strong, positive correlation of r =.999 and p =.000 at N = 70. The results indicate that the multifilament yarns in the outer layers of Spacer A and Spacer B absorbed a high amount of water by absorption and transmission relatively quickly. The results presented in Table 4 show that Spacer B provided a slightly higher water absorption rate in the first 15 s of the test than Spacer A. It is suggested that the reason for this was the different densities of the outer layers. The higher density of the chain inlay structure of Spacer B enhanced the water diffusion process more than the less dense rhombic structure of Spacer A.
Mass of water transport over a period of 180 s.
In the second stage, the water started to travel along the surface of the monofilament spacer yarns. The reason for this was the increasing water concentration in the outer layer and hence the associated water surface tension. From this point on, the analysis of the results showed increasing differences between the two materials. The curve progressions of Spacer A and Spacer B started to deviate, showing a still high rate of water transfer in Spacer B and a reduced water transfer in Spacer A. In addition, after the test run, Spacer B showed a wet upper surface, whereas the upper surface of Spacer A was still dry. However, although the water absorption rates of Spacer A and Spacer B diverged in the second stage, there was still a strong positive correlation between the fabrics (r =.973, p =.000) at the end of the test run. It is assumed that the dissimilar curve progression in the second stage was caused by the differences between Spacer A and Spacer B with respect to the height of the spacer layer and the density of the outer layers (the greater the density of the outer layer, the lower is its water absorption capacity). Accordingly, the greater density of the outer layer of Spacer B led to increasing water concentration on the surface of the outer fabric layer and hence higher capillary pressure. In an upward vertical movement, the balance between capillary pressure and liquid gravity force determines the maximum liquid height (Mao & Russell, 2008). Spacer B featured a reduced spacer yarn length compared to Spacer A. Therefore, it is assumed that the increased capillary pressure and the reduced fiber length enhanced the water transport in Spacer B and finally allowed the water to reach the upper outer layer, further enhancing the absorption process.
Manikin Test
The results obtained from the manikin test are presented in Table 5. The results display the thermal insulation (R t ) and moisture vapor resistance (R e ) of the clothing ensemble. The higher the R e , the higher is the resistance of the material to water vapor transmission. Thermal insulation (R t ) acts to reduce the transmission of heat. Accordingly, both lower thermal insulation and lower moisture vapor resistance indicate more heat and vapor transmission through the garment, enhancing the heat exchange between the wearer and the environment.
Within the three commercial vests, Vest 2 with foam blocks provided minimal thermal insulation and evaporative resistance. There are significant differences between the vests with respect to thermal property R t (p <.01; see Table 6). It is assumed that the gaps between the foam blocks feature openings within the vest, allowing heat transfer from the skin to the environment. Additionally, the block arrangement, more flexible than rigid layers, might support the exchange of trapped air between the body, the vest, and the environment. In Vests 1 and 3, perforated holes in the foam layers were utilized to provide ventilation within the clothing system. However, due to the irregular perforation within one layer, holes were often covered by the next layer so that the flow of air and heat was restricted. Vest 3 showed the highest thermal insulation, which can be explained by the fact that Vest 3 was equipped with more foam layers than Vest 1. Thus, Vest 1 provided the highest evaporative resistance. It is assumed that the larger sized perforation in the foam of Vest 3 and the fact that the shell material consisted of 65% PES 35% cotton might have improved the evaporative performance.
Thermal Insulation and Water Vapor Resistance Obtained From the Manikin Test.
Significance (Two-Tailed) of Differences Between Manikin Test Results.
Within all test garments, Vest A1 provided the lowest results for both R t and R e . From the statistical analysis of the manikin test results in Table 6, it can be observed that only Vest A1 with AF1 showed significant differences (p <.05) in the thermal properties compared to Vest 1 and Vest 3 with foam layers. In particular, the results of the comparison with Vest 1 can be solely attributed to the advanced thermal properties of the spacer insert. It is assumed that the low values for R t and R e in Vest A1 and AF1 are highly correlated with the open surface structure and greater volume of Spacer A, resulting in a lower bulk density of the entire stacking compared to Spacer B. From the results in Table 6, Vest B1 still achieved lower values for both R t and R e compared to Vest 1, but the values were considerably higher than those for Vest A1 and also higher than those for Vest 2 with foam blocks. This indicates that the dense surface and reduced thickness of Spacer B, involving a relatively high bulk density of the spacer stacking, led to an adverse effect on the thermal comfort provided by Vest B1.
In comparison to Vest A1, Vest AF1 with a combined spacer/foam stacking showed considerably higher thermal insulation and water vapor resistance. The thermal conductivity and water vapor transport of the spacer layers mainly depend on their open structure and the level of air permeability afforded. Thus, the foam layer placed toward the outer shell restricted the airflow to the environment. Additionally, after the test a certain amount of moisture was observed on the inside of the foam, indicating the blocking effect of the foam layer. However, Vest AF1 still provided significantly less resistance to heat and vapor transfer compared to Vest 1, which can be explained by the high volume and low bulk density provided by Spacer A. It is suggested that the inward-facing spacer stacking provided sufficent capacity to transport heat and water vapor away from the skin.
Vests AF1 and BF1, both equipped with a combination of spacer and foam layers, noticably diverged in their thermal properties. Vest BF1 presented significantly higher values for both R t and R e compared to Vest AF1, with lower results especially for R e . It is believed that the high bulk density of the Spacer B stacking, combined with a top layer of foam, produced increased resistance to the airflow and evaporative heat loss, causing thermal discomfort similar to homogenous foam stacking. From Figures 7 and 8, it can be observed that the reduced thickness of Spacer B did not reduce R t and R e . Subsequently, greater thickness of stackings did not diminish the air and vapor flow through the garment.
Thickness of stacking related to evaporative resistance.
Thickness of stacking related to thermal insulation.
Summary
The results of the TWTT showed excellent water absorption for both spacers, and the results of the manikin test showed that body protectors featuring an insert consisting of multiple layers of spacer fabrics offered lower resistance to heat and water vapor transfer. The results of both the TWTT and the manikin test indicate that body protectors with spacer-based inserts provide better thermal comfort properties than foam-based body protectors. However, the degree of heat and water vapor transfer depends greatly on the bulk density of the spacer stacking.
The open spacer structure of vest A1 provided the highest degree of thermal comfort. However, it should be noted that the low bulk density of Spacer A resulted in less ability to reduce the energy of an impact. In contrast, the combination of the triple-linked Spacer A stacking with one layer of foam, used in Vest AF1, presented good results from impact testing. Another important factor to be considered is the reduced thickness of Vest AF1 (26.56 mm) compared to Vest A1 (32.6 mm). In comparison with foam-based equestrian body protectors, Vest AF1 still presented considerably lower thermal insulation and evaporative resistance values. In addition, although the foam layer restricted the water vapor transport, the high volume and water absorption capability of the spacer stacking still offered moisture transport away from the skin, involving the removal of heat from the body and therefore superior thermal comfort. In summation, using an insert of combined layers of foam and spacer materials can provide reduced thickness and adequate thermal properties depending on the bulk density of the spacer material applied. These inserts can therefore be considered as alternatives to both foam-based and spacer-based inserts.
Conclusion
We investigated the thermal comfort properties of equestrian body protectors using a spacer-based insert. The results of this study support the usability of spacer fabrics in impact-protective clothing. The application of knitted spacer fabrics in protective clothing will likely be dependent on developing a sufficient combination of functionality, comfort, and performance, strengthened by the development of spacer fabrics that achieve high-impact absorption properties.
Some limitations of the study should be noted. The manikin tests can measure thermal insulation and moisture vapor transport in one step and realistically simulate human sweating according to different temperatures, humidity, and clothing ensembles. However, temperature control and results refer to the whole body and cannot be limited to individual body segments. Additionally, in the manikin test, posture and type and intensity of active wear were not considered as factors influencing clothing heat transfer. Further research might therefore use subjective wear trials in order to include varying conditions during activity.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.