Development of the Warp
`Knitted Spacer Fabrics for
`Cushion Applications
`
`XIAOHUA YE, HONG HU* AND XUNWEI FENG
`College of Textiles, Donghua University
`Shanghai 200051, PR China
`
`ABSTRACT: Polyurethane (PU) foam is a common material used in seats, sofas
`and mattresses, etc. However, the use of PU foam presents some problems
`concerning comfort and recycling. Compared with PU foam, warp knitted spacer
`fabrics can solve these problems. This paper presents work in the development of the
`warp knitted spacer fabrics for cushion application. Three kinds of the properties,
`i.e., pressure distribution, air permeability, and heat resistance of the fabrics
`developed are evaluated and compared with those of the PU foam. The results show
`that the warp knitted spacer fabrics have better pressure relief properties, higher air
`permeability, and lower heat resistance than PU foam, and thus could be used to
`substitute PU foam, especially in the case where the comfort and recycle are highly
`required.
`
`KEY WORDS: warp knitting, spacer fabric, compressibility, cushion, padding
`material.
`
`INTRODUCTION
`
`P OLYURETHANE (PU) FOAM has widely been used as padding material in
`
`seat, sofa, and mattress manufacturing for many years due to its good
`compression characteristics. However, PU foam is not an ideal and unique
`material to be chosen in these applications. Besides difficulty for washing,
`the use of PU foam can also cause some problems concerning comfort and
`
`*Author to whom correspondence should be addressed.
`E-mail: huhong@dhu.edu.cn
`
`JOURNAL OF INDUSTRIAL TEXTILES, Vol. 37, No. 3—January 2008 213
`
`DOI: 10.1177/1528083707081592
`1528-0837/08/03 0213–11 $10.00/0
`ß SAGE Publications 2008
`Los Angeles, London, New Delhi and Singapore
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`X. YE ET AL.
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`recycling [1,2]. PU foam normally has very low air permeability. In some
`occasions, PU foam makes people feel too warm and fuggy, especially in the
`warm environment. PU foam also delivers out poisonous gases during
`burning. Disposing of the products made of PU foams also creates a big
`problem because PU foam must be separated from other materials. This
`makes recycling process of PU foam very complicated. In order to overcome
`these shortcomings, different attempts have been made, for example, by
`using special cutting process to give PU foam ‘open’ surface and by adapting
`high porosity to enhance its ‘breathability’ [2]. Even with these efforts, most
`of the problems with PU foam can not be solved thoroughly.
`In recent years, the use of technical textiles has grown very fast. These
`fibrous materials, which have a variety of technical end uses may be used to
`substitute some conventional materials with advantages in specific applica-
`tions [3]. Warp knitted spacer fabrics (WK spacer fabrics) are in this class
`and they are very interesting structures, which could be used to substitute
`conventional PU foam used in seats, sofas and mattresses, etc. The
`advantage of WK spacer fabrics consists in the combination of good
`compressive characteristics, air permeability, and thermoregulation by their
`unique 3D structure. WK spacer fabrics can easily be recycled because they
`are also made from fiber materials, and thus they can overcome the recycling
`problem presented by PU foam.
`Although a lot of investigations have been made for WK spacer fabrics,
`there are still few detailed works on the thick WK spacer fabrics for cushion
`applications. The objective of this work consisted in developing this kind of
`WK spacer fabrics, which could be used to substitute PU foam for cushion
`application. The properties of the fabrics thus developed were also evaluated
`and compared with those of the PU foam.
`
`STRUCTURE CHARACTERISTICS AND KNITTING PROCESS
`
`A WK spacer fabric consists of two surface layers and a layer of the yarns
`called as spacer yarns. As shown in Figure 1, the spacer yarns connect two
`surface layers to form a special 3D structure. For cushion application, the
`spacer yarns also play a supporting role and avoid the 3D structure to be
`crushed under body pressure. It is evident that selecting spacer yarn with
`proper bending rigidity and connecting method between two surfaces are
`very important during the structure design.
`WK spacer fabrics are normally produced on the warp knitting machine
`with two needle-beds and six yarn guide bars. As shown in Figure 2, while
`bars 1, 2 and bars 5, 6 are respectively used to knit the front and back
`surface layers, bars 3 and 4 are used to knit the spacer yarn layer. In order to
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`Development of Warp Knitted Spacer Fabrics
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`215
`
`Back surface
`
`Front surface
`
`Spacer yarn
`
`FIGURE 1. 3D Structure of a warp knitted spacer fabric.
`
`Guide bar
`
`Back needle bed
`
`Front needle bed
`
`Spacer yarn
`
`FIGURE 2. Warp knitting machine for spacer fabric.
`
`connect two surface layers together, bars 3 and 4 must feed the spacer yarns
`on both front and back needles. They normally make symmetrical lapping
`movements to get better connecting effect.
`There are different methods used to connect two surface layers. Figures 3
`and 4, respectively show two commonly used connecting methods. In
`Figure 3, the two surface layers are connected by the vertical spacer yarns.
`However, the structure with this kind of the connection is not very stable
`because the spacer yarns tend to incline along the horizontal direction under
`the pressure. In order to enhance the stability of the structure, the two
`surface layers are normally connected with two systems of the symmetrical
`inclined spacer yarns, as shown in Figure 4. The different inclination angles
`of the spacer yarns can be obtained by changing the underlap amounts of
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`X. YE ET AL.
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`F
`
`Spacer yarn
`
`F
`
`FIGURE 3. Connection with vertical spacer yarns.
`
`F
`
`θ
`
`Spacer yarn
`
`FIGURE 4. Connection with inclined spacer yarns.
`
`the spacer yarns in order to satisfy different end-use requirement. It is
`evident that the bigger underlap will
`lead to higher inclination of the
`spacer yarns.
`Besides different connecting methods, the two surface layers of a spacer
`fabric can be knitted with same or different structures. For the commonly
`used spacer fabrics, two surface layers are normally knitted with same
`structures. The surface structures can be plain structures or meshes with
`different opening sizes. One of the commonly used structures is a mesh
`structure formed with pillar and laying-in yarns as illustrated in Figure 5.
`This structure is widely used in the spacer fabrics for cushion application
`because it gives not only good structural stability but also good air
`permeability.
`The space between two surface layers is also an important structural
`feature of a WK spacer fabric. For cushion application, higher fabric
`thickness is needed. According to different application situations, the
`thicknesses of the cushions may vary from 10 to 100 mm or higher.
`However, the thickness of WK spacer fabrics is limited by the distance
`between the two needle-beds of the warp knitting machine. The thickest
`spacer fabric currently produced on the warp knitting machine is about
`65 mm. There are three different kinds of the distance arrangements between
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`Development of Warp Knitted Spacer Fabrics
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`217
`
`FIGURE 5. Diagrams of a mesh surface formed with piller and laying-in yarns.
`
`the two needle-beds of the warp knitting machines: 3–12 mm, 12–30 mm,
`and 25–65 mm. For some cushion application situations, two or more layers
`of WK spacer fabrics are needed to be put together to obtain higher
`thickness, because the spacer fabrics with very high thicknesses are more
`difficult to be knitted.
`
`PRODUCTION OF THE SAMPLES
`
`The WK spacer fabrics developed for cushion application in this work
`were produced on a Karl Mayer Raschel machine RD 6 DPLM/30 with two
`needle-beds and six guide bars. The gauge of the machine was E16.
`Although the distance between needle-beds of the machine could vary
`between 12 and 30mm, this distance was set at 20mm and fixed during the
`production of all the samples.
`One kind of PES multifilament and two kinds of PES monofilaments were
`respectively selected as the surface layer yarn and spacer yarns. According to
`the machine gauge and thickness of spacer yarn, 400 dtex PES multifilament
`was selected for knitting the surface layers. However, for the spacer yarns,
`as they have to support the body pressure, the use of the yarns with high
`bending rigidity is required. Because of this, PES monofilaments with two
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`X. YE ET AL.
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`BBBFB
`
`F
`
`5 Needles
`
`8 Needles
`
`12 Needles
`
`FIGURE 6. The spacer fabrics development. Diagram of the different lapping
`movements of one spaces yarn bar.
`
`diameters (0.18 and 0.22 mm) were selected for the spacer yarns in order to
`knit the fabrics with different compression characteristics.
`The surface structure was selected as illustrated in Figure 5 for all the
`samples. However, for the connecting two surface layers, three different
`underlap amounts of the spacer yarns (underlapping 5, 8, and 12 needle
`spaces), as shown in Figure 6, were used to get the different inclination
`angles. In Figure 6, only the lapping movements of one spacer yarn guide
`bar are shown. The other spacer yarn guide bar made the symmetrical
`lapping movements. Two spacer yarn guide bars were half threaded, i.e., one
`in one out. By combining two different spacer yarn diameters and three
`different underlap amounts, six kinds of the WK spacer fabrics were
`produced.
`In order to increase the structural stability, the WK spacer fabrics after
`knitting are normally subjected to a heat-setting treatment. It is necessary to
`point out that heat-setting a thicker spacer fabric is not easy to be processed
`because it is difficult to firmly catch the two selvages of the spacer fabric on
`a conventional heat-setting machine. In order to solve this problem, the
`selvages of two sides of all the spacer fabrics were knitted without spacer
`yarns. At the same time, the longest pins equipped with the machine were
`also used. After taking these measures, the spacer fabrics could be efficiently
`heat-set. The heat-setting temperature and time, respectively were 160 to
`190C and 1–2 minutes [4]. It was found that the spacer fabrics with longer
`underlaps of the spacer yarns or with higher thickness or weight were not
`easy for heat-setting, because it was difficult to extend them up to the
`desired width.
`The samples after heat-setting are illustrated in Figure 7. Some of their
`specifications are shown in Table 1. The inclination angle  of the spacer
`yarns is defined as shown in Figure 4. It was found that the thicknesses of all
`the fabrics were smaller than the distance set (20 mm) between the two
`needle-beds of the machine. This is normal because during knitting, the
`spacer yarns were under tensioned states. After knitting, the spacer yarns
`tended to relax into bended forms. The bending of the spacer yarns resulted
`in the decreasing of the fabric thickness. It was also found that the fabric
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`Development of Warp Knitted Spacer Fabrics
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`219
`
`A1
`
`B1
`
`A2
`
`A3
`
`B2
`
`B3
`
`FIGURE 7. The spacer fabrics developed.
`
`Table 1. Specifications of the warp knitted spacer fabrics developed.
`
`Sample
`No.
`
`Diameter of
`spacer yarn (mm)
`
`Underlap amount
`(No. of ndls)
`
`Inclination
`angle (degree)
`
`Thickness of
`spacer fabric (mm)
`
`Weight
`(g/m2)
`
`A1
`A2
`A3
`B1
`B2
`B3
`
`0.18
`0.18
`0.18
`0.22
`0.22
`0.22
`
`12
`8
`5
`12
`8
`5
`
`52.4
`39.86
`26.57
`52.51
`39.99
`26.63
`
`17.93
`17.75
`16.94
`17.86
`17.67
`16.89
`
`1375
`1285
`1100
`1700
`1582
`1300
`
`thickness and mass density depended on the underlap amount of the spacer
`yarns. The bigger the underlap amount of the spacer yarns is, the higher
`thickness and mass density the fabric has. This phenomenon can be
`explained by the fact that the spacer yarns with higher underlap amount
`have less bending effect.
`
`PROPERTY EVALUATION
`
`In order to evaluate the properties of the WK spacer fabrics developed,
`three kinds of tests, i.e., pressure distribution, air permeability, and heat
`resistance were carried out. A PU foam with the thickness (20 mm) was used
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`X. YE ET AL.
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`for comparison. Although some differences exist between the thickness of
`the PU foam and those of the WK spacer fabrics, it was difficult to find
`PU foam with exact thickness from the market. Besides, the mass density of
`the PU foam was 458 g/m2, only about 30–50% of the spacer fabrics
`developed.
`
`Pressure Distribution
`
`The materials used for cushion applications must be soft and flexible.
`Besides this, the concentration of the body pressure must also be avoided,
`especially for people sleeping on a bed or seated on a chair for a long time.
`For example, a patient seated on a wheelchair always bears the pain caused
`by the pressure on the injured body. Bedsores or pressure sores are also
`caused by long-term downward pressure of the body on a support, especially
`when patients are immobile for hours in the same position [5]. Because of
`this, the evaluation of the pressure distribution under the body pressure is
`very important, especially in the case where the relief of the high pressure is
`required [6].
`The testing device used was a ClinSeatTM pressure measurement system
`manufactured by TekScan Inc., USA. It was composed of a sensor mat as
`well as a data acquisition and analysis system. The dimensions of the sensor
`mat were selected as 442 488 mm2, and the distribution of the sensors was
`4 sensors/cm2. The test was first carried out when a person of 50 kg stably
`seated on a massive wood chair placed with the sensor mat but without any
`samples, and then the tests were carried out with the placement of each
`sample on the sensor mat. During the test, the back, feet, and hands of the
`person did not touch floor or any other objects, so that all the body weight
`was supported by seating. The pressure distribution of the buttocks for each
`test was measured and recorded by the system after the stabilization of
`seating. All the tests were carried under a standard atmospheric condition,
`23C, RH 65%.
`The results of the average and peak pressures obtained for all samples
`tested are shown in Figure 8. As the average pressure was calculated by
`dividing the weight with the pressed area measured, different pressed area
`measured could cause different average pressure. It can be found that the
`test without placement of the samples gives the highest peak pressure and
`average pressure, because the buttocks directly seated on the sensor mat.
`Although the PU foam has smaller average pressure than all the spacer
`fabrics, the peak pressures of all the spacer fabrics are lower than that of the
`PU foam with higher thickness. This means that
`the spacer fabrics
`developed have better property for the reduction of the pressure concentra-
`tion, and they can be used in seat, sofa, and mattress to substitute PU foams
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`221
`
`Peak pressure
`
`Average pressure
`
`B2
`B1
`A3
`A2
`A1
`Warp knitted spacer fabrics
`
`B3 PU
`foam
`
`40
`35
`30
`25
`20
`15
`10
`
`Pressure (KPa)
`
`5 0W
`
`ithout
`sample
`
`FIGURE 8. Peak pressure and average pressure of
`polyurethane (PU) foam.
`
`the spacer fabrics and
`
`for special applications where the relief of the body pressure is highly
`required. The results also show that among all the spacer fabrics developed,
`the fabrics A1, A2, and A3 knitted with finer spacer yarns (0.18 mm) have
`lower peak pressure and average pressure values than the fabrics B1, B2, and
`B3 knitted with thicker spacer yarns (0.22 mm). For the fabrics knitted with
`the same spacer yarns, but with different inclination angles, the difference
`for peak pressures and average pressures exist, but is not so significant.
`
`Air Permeability and Heat Resistance
`
`Air permeability and heat resistance are very important parameters for
`cushion application because they remarkably influence the comfort,
`especially for some application cases, such as car seats and medical
`mattresses. Air permeability test was undertaken according to the standard
`ISO 9237:1995 on a TEXTEST FX 3300 Air Permeability Tester III. The
`heat resistance test was carried out on an Alambeta apparatus under a
`standard atmospheric condition (23C, RH 65%).
`The testing results of air permeability and heat resistance are listed in
`Table 2. The results show that the WK spacer fabrics have very good
`air permeability. This is normal because the WK spacer fabrics were
`produced with open surface structures. Though the PU foam is a little
`thicker than spacer fabrics, air permeability for PU foam is much lower
`than those of spacer fabrics (about 25% of these of the WK spacer
`fabrics). Low air permeability can cause a big comfort problem for PU
`foam used in special application cases as above mentioned. Concerning
`the heat resistance, the tested PU foam has much higher value than WK
`spacer fabrics. This means that the PU foam has better capacity to keep
`the heat
`than spacer fabrics with same thickness. This may be an
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`222
`
`X. YE ET AL.
`
`Table 2. Air permeability and heat resistance for all warp knitted (WK) spacer
`fabrics and polyurethane (PU) foam.
`
`WK spacer fabrics
`
`Samples
`
`A1
`
`A2
`
`A3
`
`B1
`
`B2
`
`B3
`
`PU foam
`
`Air permeability (l/s/m2)
`Heat resistance (m2K/W)
`
`3588
`138
`
`3597
`133
`
`3604
`126
`
`3498
`145
`
`3523
`139
`
`3537
`131
`
`924
`221
`
`advantage for PU foams when they are used in a colder environment.
`However, the combination of lower air permeability and higher heat
`resistance always makes the PU foam too warm and uncomfortable in
`warm conditions.
`
`CONCLUSIONS
`
`After an analysis on the structure characteristics and knitting process,
`six different WK spacer fabrics were produced on a warp knitting
`machine with two needle-beds and six yarn guide bars for cushion
`application purpose. Three properties,
`i.e., pressure distribution, air
`permeability, and heat resistance, were evaluated and compared with
`those of the PU foam selected. The results have shown that WK spacer
`fabrics developed have better properties than PU foam for the pressure
`relief. The WK spacer fabrics also have very good air permeability and
`lower heat resistance, so that they can offer much better comfort than
`PU foam, especially in the warm conditions. Furthermore, recycling is
`not a problem for the cushions produced with WK spacer fabrics. The
`work has demonstrated that the WK spacer fabrics could be used to
`substitute PU foams for cushion application when comfort and recycling
`are highly required.
`
`ACKNOWLEDGMENTS
`
`The authors would like to thank Fulian Warp Knitting Co. Ltd.,
`China for producing the spacer fabric samples. The authors also thank
`Prof. Mario de Araujo and Prof. Raul Fangueiro of Minho University
`in Portugal
`for
`their help in the frame of
`the Asia-link Project
`funded by EU.
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`
`REFERENCES
`
`1. Heide, M. (2001). Spacer Fabrics Trends, Kettenwirk-Praxis, 26(1): 17–20.
`2. Prof. Dr. Umbach, K.,H.
`(2001). Physiological Comfort on Car Seats,
`Kettenwirk-Praxis, 26(1): 34–40.
`3. Heide, M.
`(2000). Spacer Fabric with Specific Protective Characteristics,
`Melliand-Masche, 81(6): E124–125.
`4. Anon (2004). New Machines for Heat-setting of Spacer Fabrics, Melliand-
`International, 10(4): 282.
`5. Heide, M., Schurer, M., et al. (2002). Functional Warp-knitted Spacer Fabrics as
`Covers for Operating Tables in the Case of Long-term Operations, Kettenwirk-
`Praxis, 27(1): 25–27.
`6. Ye, X., Hu, H. and Feng, X. (2005). An Experimental Investigation on the
`Properties of the Spacer Knitted Fabrics for Pressure Reduction, Research
`Journal of Textile and Apparel, 9(3): 52–57.
`
`BIOGRAPHY
`
`the
`Xiaohua Ye is currently a PhD student at
`Donghua University in Shanghai. She obtained her
`bachelor degree and master degree from Wuxi
`University of Light Industry in China in 1997 and
`2000. Her research interests focus on the development of
`the warp knitted fabrics for technical applications.
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