Preparation and Characterization of Electrospun Lauric Acid/polyethylene Terephthalate Nanofibers/Woven Composite Fabrics with Heat Preserved Property

, ,

(College of Textiles and Clothing, Xin Jiang University, Urumqi 830046, China)

PreparationandCharacterizationofElectrospunLauricAcid/polyethyleneTerephthalateNanofibers/WovenCompositeFabricswithHeatPreservedProperty

LIZhiyong,ZHOUHuimin,XIAXin

(CollegeofTextilesandClothing,XinJiangUniversity,Urumqi830046,China)

This work was focused on preparation and characterization of the electrospun lauric acid (LA)/polyethylene terephthalate (PET) nanofibers/woven composite fabrics. Composite fabrics consisting of electrospun LA/PET nanofibers and woven fabrics were prepared by quilting a sandwich structure. Cotton yarns, wool yarns, polyester-cotton yarns, acrylic yarns and polyester yarns were selected to prepare the woven fabrics in laboratory, respectively. Meanwhile, nanofibers with fatty acids of LA encapsulated in supporting matrices of PET were prepared through electrospinning technique. Morphological structures of the nanofibers were characterized by scanning electron microscope (SEM), and their thermal energy storage properties were investigated by the differential scanning calorimeter (DSC). In addition, the relationships between heat preserved properties of the composite fabrics and LA/PET mass ratios, woven structures and woven materials were discussed, respectively. The results showed that the LA/PET nanofibers possessed smooth and cylindrical morphological structure, LA and PET had good miscibility in those nanofibers. Enthalpy values associated with melting and crystallization for LA encapsulated in nanofibers were lower than corresponding theoretical values, but it had no appreciable effect on the phase change temperatures. Heat preserved property of composite fabric measurement suggested that all of those composite fabrics had much more insulation than the original one, meanwhile, the thermal cycling test indicated that they had a good thermal reliability.

LA/PET nanofibers; sandwich structure; composite fabric; heat preserved performance

1 Introduction

The latent heat storage textiles materials with phase change materials (PCMs) as the heat storage media has been an increasingly attractive technology in many applications[1-2]. Generally, the microencapsulated phase change materials (MicroPCMs) have been widely used to protect PCMs from flowing and leaking in the textile fabrics[3-5]. For textiles, MicroPCMs fabrics were usually fabricated by the methods of fiber spinning[6], coating[7], lamination[8], printing[9], and adding MicroPCM into the dye bath[10]. However, the treated fabrics became bulky, heavy, rough and stiff, adversely affecting the fabric softness, flexibility, thinness, breathability and moisture transport properties[11-12]. In addition, the drawbacks of these treated methods were relatively costly and complicate[13-14]. Meanwhile, PCMs may have a flow or leak in the process of melting and solidification, which leads to their unreliable and poor thermal behaviors in repeated thermal cycles[15-18]. To mitigate these problem, fibrous films of PCM/supporting material nanofibers have been developed via electrospinning. The form-stable PCMs possess advantageous properties such as no need for additional encapsulation, lightweight, cost effectiveness, stable shape, excellent mechanical property, large specific surface area, and easy preparation with desirable dimension[19-21]. The use of fibrous films of PCM/supporting material nanofibers in textile fabrics can reduce the thickness and the weight of the fabrics. The buffer effect of the PCMs, which can provide a small, temporary heating/cooling effect during temperature transients, is significant.

Theform-stable PCMs containing fatty acids and polymer matrix have attracted extensive interests in recent years[22-25]. Lauric acid (LA) is an important fatty acids that has been widely used for many applications due to excellent properties such as suitable phase change temperature range, high latent heat of phase change, nontoxicity, non-corrosiveness, good chemical stability, good thermal reliability after a large number of melt/freeze cycles and little supercooling during phase transitions[26-30]. Hence, they have the excellent properties to be utilized as the PCMs for the textile fabrics.

The aim of this study is to enhance theheat preserve property of composite fabrics by adding the electrospun LA/PET nanofibers into conventional fabrics via a sandwich structure method. This structure that is both completely protective to LA/PET nanofibers damage and easily incorporative to LA/PET nanofibers into fabrics would have significant implication for numerous applications. The conventional fabrics were weaved by a laboratory-scale sample loom in laboratory. The LA were selected as the PCMs and PET was selected as the polymer supporting material, and the electrospun LA/PET composite fibers were fabricated by the electrospinning. Additionally, the morphological structures and thermal energy storage properties of the nanofibers were characterized by SEM and DSC, respectively. Meanwhile, the heat preserved property of composite fabrics were deeply investigated.

2 Experimental

2.1Materials

LA was purchased from Shanghai Shanpu Chemical Co. Ltd. China. PET (Mw=18000-25000) was purchased in Shanghai Plastic Products Co. Ltd. China. Dichloromethane (DCM) and trifluoroacetic acid (TFA) were provided by the Sinopharm Group Chemical Reagent Co. Ltd, China. Cotton yarns (Right handed direction, 27.8tex) was obtained from Xinjiang Esquel Textile Co. Ltd. Wool yarns (Right handed direction, 27.8tex) was bought from Xinjiang Tianshan Wool Tex Stock Co. Ltd. Polyester-cotton yarns (Right handed direction, 27.8tex) and acrylic (Right handed direction, 27.8tex) was purchased from Yangcheng Yonghe Tex Co. Ltd.

2.2PreparationoftheLA/PETnanofibers

To prepare the spin dopants for electrospinning, 15% ethylene terephthalate pellets were first dissolved in mixture solvents with the volume ratio of dichloromethane (DCM)/trifluoroacetic acid (TFA) being 2/1, and the lauric acid powders were then dissolved in the solutions with the LA/PET mass ratios being set at 80/100, 100/10, and 120/100, respectively. Subsequently, these solutions were stirred to achieve the homogeneous dispersion. During electrospinning (Electrostatic equipment is on experiment, and very simple, assembled by laboratory assistant.), the electrospinning voltage was 18 kv and the distance between the needle tip and the roller was set at 20cm. The rotating speed of the roller was fixed at 100 rpm. The electrospun nanofibers were collected as overlaid fibrous films.

2.3Preparationofthecompositefabrics

In order to control thevariations of weave structures and materials of conventional fabrics, the conventional fabrics were weaved by a laboratory-scale semi-automatic sample loom (Tianjin Longda electrical and mechanical technology development Co, LTD. China). The design and production of the fabrics, i.e. pick density, filling density, thickness, etc., kept the same during the weavw processes Three types of weave structures were used, i.e. plain weave, honeycomb weave and twill, respectively, and the pure cotton yarns, polyester-cotton yarns, wool yarns and acrylic yarns were selected as warp yarns and weft yarns, respectively. Subsequently, all composite fabrics was assembled by a piece of electrospun LA/PET fibrous film and two pieces of woven fabrics as a sandwich structure, meanwhile, a simple suture and hot-pressing treatment were used to connect these three layers. Each composite fabric appeared as 80mm×80mm (wide×length) due to the test requirements.

The group A: cotton fabrics of plain weave acted as the top and bottom layers, respectively. LA/PET nanofibers acted as the middle layer with the mass ratios of 80/100, 100/100 and 120/100, respectively.

The group B: polyester-cotton fabrics acted as the top and bottom layers using the weave structure of plain weave, honeycomb weave and twill weave, respectively. LA/PET nanofibers acted as the middle layer with the mass ratios of 100/100.

The group C: cotton fabrics of plain weave with different materials, i.e. wool or acrylic acted as the top and bottom layers, respectively. LA/PET nanofibers acted as the middle layer with mass ratios of 100/100. Characteristics of these composite fabrics were shown in Table 1.

Photograph of the composite fabrics and schematic diagram of the composite fabric structure are shown in
Fig.1.

Table 1 Characteristics of the woven fabrics and LA/PET mass ratio of nanofibers

Fig.1 (a) Photograph of the composite fabric and (b) schematic diagram of the composite fabric structure

2.4Characterization

2.4.1Characterization of electrospun LA/PET nano- fibers Hitachi S4800 scanning electron micro-scope (SEM) was employed to examine morphologies of the composite fibers ahead of detailed SEM examination. All samples of composite fibers were sputter-coated with gold to avoid charge accumulations.

Differential scanning calorimeter (DSC) analysis instrument (NETZSCH-200F3) was employed to investigate thermal behavior of the composite fibers. Flow rate of nitrogen gas was set at 25mL/min, and the DSC experiments were performed at 20-60℃ with heating or cooling rate of 10℃/min. The enthalpy of melting and crystallization were calculated based upon areas under the peaks for solid-liquid phase transitions of LA using the thermal analysis software affiliated with the equipment.

2.4.2Characterization of composite fabrics The com-posite fabrics consisted of three layers corresponding to woven fabric, electrospun LA/PET nanofibers fibrous films and woven fabric. Effects of different LA/PET mass ratios, weave structures and woven materials on the heat preserved property were investigated, respectively. Before tests, the composite fabrics were kept at 20℃ 1h for solidifying. Heat preserved properties were carried out in an electricity heat drum wind drying oven by a digital thermometer. During tests, drying oven conditions were maintained at a constant level: the temperature was remained at 50±0.5℃. Temperature detector of digital thermometer was tightly packaged by composite fabrics. Subsequently, composite fabrics were placed into empty drying oven; after the temperature of digital thermometer was kept stable at 50℃, the composite fabrics were transferred under room temperature. Then, temperature variations was recorded step by step of 1 minute until it dropped to 30℃. Each composite fabric was tested three times.

Thermal cyclingtest, which was used to determine thermal reliability of composite fabrics in terms of heat preservation period with respect to thermal cycling number, included the melting and freezing processes of LA/PET composite fibers. This test was performed consecutively up to 30 thermal cycles using the method mentioned above.

3 Results and discussion

3.1MorphologicalstructuresofelectrospunLA/PETnanofibers

Since thesoften temperature of PET is higher than the melting point of LA, PET could act as the supporting material in electrospun LA/PET nanofibers. Representative morphologies of electrospun nanofibers with varied mass ratios of LA/PET are shown in
Fig.2, which illuminated that all of LA/PET fibers had smooth surface and cylindrical shape. It was the LA that is well-encapsulated in PET matrix that indicates a better miscibility of LA and PET in those nanofibers. Meanwhile, conglutination of some fibers were found occasionally in the figures. The formations of the conglutination might be ascribed to the hydrogen bonding between the carboxyl (-COOH) groups of LA and the carbonyl (C=O) groups of PET[31-32]. In addition, there was an increase trend in average fiber diameter with the weight proportion increase of the LA in nanofibers. This result is attributed to the addition of LA into the PET solution affecting the solution properties, complementing similar results found in the previous research[33].

Fig.2 SEM images of the electrospun composite fibres:(a) LA/PET (80/100), (b) LA/PET (100/100) and (c) LA/PET (120/100)

3.2ThermalpropertiesofLA/PETnanofibers

Duringheating and cooling process, DSC curves of electrospun LA/PET(100/100) nanofibers are shown in
Fig.3. It shows that the phase change temperatures of heating and cooling process are 43.96℃ and 36.74℃ and their heat enthalpies values of melting and crystallization are determined to be 57.39 kJ/kg and 48.95kJ/kg, respectively. Both values of melting enthalpy and crystallization enthalpy of composite fibers were lower than theoretical values. But phase change temperatures had no obvious variations after LA was incorporated into the PET matrix. It indicated that supporting material of PET had slight influences on latent heat of LA while no effects on their phase change temperature. Meanwhile, phase change temperatures and latent heat values observed from DSC analyses were suitable for practical applications such as garments and home furnishing applications.

Fig.3 DSC curves of the electrospun LA/PET(100/100) composite fibers

3.3Heatpreservedpropertyofcompositefabrics

For PCM fabrics, standards of testing has not been developed. Therefore, in this study, in order to assess the heat preserved properties of composite fabric, a simple method were applied in experiment to reveal effects of electrospun LA/PET nanofibers on heat preserved property of composite fabric. Curves of heat preserved property of group A with varied LA/PET mass ratios of 80/100, 100/100 and 120/100 are shown in
Fig.4. In short, heat preservation period of composite fabric incorporating LA/PET nanofibers films was longer than that of the woven fabric without LA/PET nanofibers films. And there existed a buffer period for temperature changed and minimizing changes of thermometer temperature. From figure 4, compared with curves of composite fabric 3, heat preservation period of composite fabric 1 and composite fabric 2 were shortened by approximately 2 minutes and 1 minute, respectively. Effects of heat preservation period of composite fabrics increased with the rise of LA/PET mass ratios. And composite fabric 3 with LA/PET mass ratio of 120/100 presented the longest heat preservation period. This illuminated that the heat preservation period of composite fabric would depend directly on LA/PET mass ratios of nanofibers. At the meantime, it was clear that there was no significant difference in temperature between 50℃ and 45℃ for each composite fabric, on the other hand, there was an appreciable variations on the heat preserved property during process of temperature decreased from 40℃ to 30℃. It should be noted that the slope of composite fabrics line was less than that of woven fabric without LA/PET nanofibers films during temperature decreased from 40℃ to 30℃. It implied that temperature decrease rate of composite fabric was lower than that of fabric without LA/PET nanofibers due to the PCM crystallised, released heat (latent heat) gradually and generated a heating effect.

Fig.4 Heat preserved property curves of the composite fabrics of group A

Moreover, to further investigate the thermal reliability of composite fabrics, composite fabrics were treated consecutively up to 30 thermal cycles. Thermal cycles curves of composite fabrics of group A are shown in
Fig.5. Heat preserved property of composite fabrics after thermal cycling test was almost identical to those of original ones. It could be well maintained after 30 heating-cooling thermal cycles, which demonstrated that composite fabrics had good thermal stability and reliability.

In order toanalyze the heat preserved property of composite fabric affected by the different weave structures. Heat preserved property curve of group B is presented in Fig.6. Obviously, the heat preservation period of composite fabrics 4, 5 and 6 was longer than plain weave without LA/PET nanofibers, which were prolonged by about 3 minutes, respectively. Comparing these curves of composite fabric 4, 5 and 6, temperature of composite fabric 5 with honeycomb weave were higher than composite fabric 4 and 6, with a slight difference in temperature variation. The reason can be attributed to honeycomb weave which had a bulkier structure than that of plain weave and twill. Meanwhile, a slower falling temperature rate during temperature decreased from 40℃ to 30℃ could be observed, and results were consistent with the heat preserved property curves of composite fabrics of group 1.

Fig.5 Thermal cycles curves of composite fabrics of group A (a)composite fabric 1; (b) composite fabric 2; (c) composite fabric 3

Fig.6 Heat preserved property curves of composite fabrics for varied fabric structure

Fig.7 exhibits the curves of heat preserved propertychanged by varied materials of woven fabrics. When LA/PET nanofibers contributed same insulation values to composite fabrics, composite fabrics 8 and 9 were both showed a longer heat preservation period than composite fabrics 2 and 4. This could be attributed to thermal conductivity of wool fabric and acrylic fabric were lower than that of cotton fabric and polyester-cotton. Hence, the heat preserved property of composite fabrics was evidently affected by the property of woven materials.

Fig.7 Heat preserved property curves of composite fabrics for varied materials of woven fabrics

In this study, the prepared electrospun LA/PET nanofibers possessed desired morphology and thermal properties for the application of LA/PET nanofibers in woven fabric. Duration of heat preserved effect is primarily dependent on amount of LA of fibers, and the composite fabrics had good thermal stability and reliability. At the same time, heat preserved property of composite fabrics is positively correlated with weave structures and woven materials. It is envisioned that this innovative type of electrospun LA/PET nanofibers/woven composite fabric could be particularly utilized for smart textiles.

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具有保温性能的电纺LA/PET纳米纤维复合织物的制备与表征

李智勇，周惠敏，夏鑫

(新疆大学纺织与服装学院，新疆乌鲁木齐830046)

本文制备了电纺月桂酸(LA)/聚对苯二甲酸乙二醇酯(PET)纳米纤维/机织物的复合织物，并对其进行了表征。选用纯棉纱线、毛纱线、涤棉纱线、腈纶纱线和涤纶纱线分别作为经纬纱线，在实验室制备机织物小样，同时，通过静电纺丝法制备LA/PET纳米纤维，将LA包裹在PET基材之中。之后通过缝合的方式，将电纺LA/PET纳米纤维和机织物构造成三明治结构的复合织物。对纳米纤维的形貌和热性能进行了表征，并分别探究了LA/PET的质量比，机织物组织结构和机织物材料对复合织物保温性能的影响。结果表明：LA/PET纳米纤维呈圆柱形，具有光滑表面，LA和PET展现出良好的相容性，热焓值略低于理论值，但相变温度改变不大。复合织物的热保温性能测试表明，复合织物的保温性能都优于未加入相变材料的织物，同时展现出良好的热循环稳定性。

LA/PET纳米纤维； 三明治结构； 复合织物； 热保温性能

date：2015-10-10；Modifieddate2016-08-01

National Natural Science Foundation of China (51163014)

XIA Xin(1980-), Professor, Doctor, Main research field: textile materials, E-mail: xjxiaxin@163.com.

TS941.779.3DocumentcodeA

10.14136/j.cnki.issn1673-2812.2017.05.010

Biography：LI Zhiyong(1990-), Postgraduate, Main research field: textile materials, E-mail: xjulizhiyong@163.com.