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Effect of Surface Unevenness of Cotton Yarns on Tensile Properties of their Composites.

Byline: Muhammad Nasir, Nasreen Fatima, Khalid Mohammed Khan and Durey Nayab Zahra

Summary: This communication describes the mechanical properties of the single yarn polyester composite (PC) of cotton fiber with special reference to the effect of unevenness in the structure of yarn. Unevenness % (U) and coefficient of variation (CV) in mass of spun yarn were measured. Scanning electron microscopy was carried out to understand the effect of unevenness on physical structure of fibers in the yarn which affect not only the mechanical properties of yarn but also the composites reinforced with them. Results also showed that decrease in 2.15 % of U in the yarn structure causes an increase of 25.18% in ultimate stress and 30.71% decrease in stiffness of composites.

Keywords: Surface unevenness; Cotton fiber yarn; Textile composites; Mechanical properties; Scanning electron microscopy

Introduction

Plants fiber as reinforcing agent in the preparation of composite material, are getting more attention of the researcher due to its eco- environmental advantages over petroleum based fibers [1]. Natural fibers like coir [2], bamboo [3], flax [4], kenaf [5], sisal [6] and jute [7] have extensively been used as reinforcing agents with comparable mechanical properties. These fibers were used in different forms such as continuous, random, oriented fibers.

Literature shows that the main focus of the researchers' world over is to improve and relate the mechanical properties of the composite to surface modification [8], orientation [9] and content of fibers [10]. Textile yarn/fabric of different natural fibers has also been used as reinforcing agent in composites [11-12]. More recently a study was carried out to propose a model to describe the effect of yarn twist on the tensile strength of unidirectional plant fiber yarn composites [13].

Any plant fibers when collected are a cluster of intermingled fibers of different length and diameter with random orientation. During the formation of yarn, these fibers are opened, drafted and twisted to make them aligned along the axis of yarn. Therefore, instead of cotton fibers from the plants, use of spun yarn is a good option as reinforcing agent because of alignment of fibers and high degree of evenness which gives better tensile properties of the yarn and ultimately the composites.

Present study, according to our knowledge, is the first experimental report which quantitatively describes the effect of unevenness of reinforcing agents on mechanical properties of composites. The unevenness in yarn structure was measured by uster evenness testing system, its effect on physical structure of fibers in fiber bundle of yarn was analyzed by scanning electron microscopy and strength of composites were measured using uster Tensorapid III testing system. The average diameter of the yarn and composite were measured through SEM images to investigate and compare the mechanical properties like ultimate stress and stiffness with reference to the unevenness in the structure of yarn. The results showed an improvement in mechanical properties of yarn and composites.

Results and Discussion

Cotton fibers are present as a cluster of intermingled short and long fibers in its seed pot. The structural dimension of fibers mainly depends upon the maturity of fibers. These fibers can be aligned at micro level by the spinning process in which fibers are opened, drafted and twisted to form a yarn of particular linear density. During the spinning process, short fibers offer resistance for complete alignment rather they appear in yarn as spillage of fibers, thicks, thins and neps and causes unevenness in yarn structure. The description of these parameters is given in Table-1.

Previously it has been established that unevenness in yarn can be minimized either by using mechanical factors of machines like speed of ring spindle and twist multiplier [14] or by applying improved spinning process [15]. In the present study, we applied improved spinning processes in yarn manufacturing to prepare two types of yarns. The average linear density or count of yarns was 20/1 NEC.

Table-1: Characteristics, Units and Description of Mass irregularity Parameters of yarn.

Characteristics###Unit###Description

###U###%###Unevenness or Linear irregularity

###CVm###%###Coefficient of variation of the yarn mass

###CVm(L)###%

###Coefficient of variation of the yarn mass at

###cut length 1,3, 10 and inert

An analysis of variance (ANOVA) was used to determine the statistical significance of the differences observed between the samples of yarns and composites. The ANOVA revealed that both samples of yarns have quite a difference in unevenness.

Spectrogram and Characterization of Unevenness in Yarn Unevenness in the structure of yarn arises due to inherent unevenness of fibers and can be minimized during spinning process by removing short and immature fibers. Two samples of yarns were manufactured by the removing short fibers in combing machines. The periodical and higher unevenness in structure appears as higher peaks in spectrogram. It is evident (Fig. 1) that sample L - low unevenness has short-term irregularity on wavelengths Lambda = 28 - 30 cm, the shape of spectrogram embodies no other variation. However, the spectrogram of sample M - more unevenness (Fig. 2) has also short-term irregularity on wavelengths Lambda = 22 - 30 cm along with an increase in amplitude on wavelength Lambda = 8-9 m. Results presented in spectrogram and in Table-2a and -2b confirmed that samples have different unevenness despite the same linear density, therefore it can be said that tensile properties of reinforcing agents are purely dependent upon the structural unevenness.

Fig. 1: Spectrogram of sample - L.

Table-2a: Unevenness of sample L.

S. No.###U%###CV m###CV 1m###CV 3m CV 10m CV inert

###1###8.23###10.38###3.26###2.6 1.67###1.3

###2###8.43###10.59###3.42###2.74 1.85###1.5

###3###8.49###10.7###3.79###3.06 2.36###1.88

###4###8.23###10.48###3.38###2.56 1.85###1.53

###5###8.35###10.53###3.33###2.36 1.72###1.4

###6###8.47###10.65###3.38###2.37 1.72###1.37

###7###8.49###10.72###3.32###2.56###1.3###0.88

###8###8.52###10.76###3.78###3.05 2.21###1.85

###9###8.2###10.31###3.00###2.48###1.8###1.62

10###8.57###10.82###3.61###2.87 2.02###1.66

Mean###8.40###10.60###3.43###2.66 1.85###1.5

Max###8.57###10.82###3.79###3.06 2.36###1.88

Min###8.20###10.31###3.00###2.36 1.30###0.88

Table-2b: Unevenness of sample M.

S. No.###U%###CV m###CV 1m###CV 3m###CV 10m###CV inert

1###10.86###13.86###4.79###3.69###2.62###2.02

2###10.47###13.29###3.99###3.02###1.94###1.46

3###10.63###13.51###4.17###3.14###1.96###1.25

4###10.45###13.27###3.99###3.02###1.89###1.63

5###10.66###13.53###4.64###3.65###2.71###2.39

6###10.34###13.12###3.92###3.05###1.83###1.21

7###10.62###13.38###4.25###2.94###1.93###1.52

8###10.28###13.07###3.98###3.18###2.3###1.47

9###10.16###12.94###3.80###2.82###1.96###1.44

10###10.99###13.92###5.10###3.99###2.97###2.46

Mean 10.55###13.39###4.27###3.25###2.21###1.69

Max###10.99###13.92###5.10###3.99###2.97###2.46

Min###10.16###12.94###3.80###2.82###1.83###1.21

Fig. 2: Spectrogram of sample - M.

Characterization of Yarn Tensile Properties After characterization of unevenness in yarn structural, we measured the tensile properties of yarn. Table-3 shows the results of determination of tensile properties of yarns with same linear mass density. It is clear from the results in Table-3 that evenness in yarns increases the tensile properties despite having the same linear mass density. It means that tensile properties of yarn do not only depend upon the yarn density but also on the evenness.

Table-3: Tensile Properties of Single Yarn (Sample size is 50).

Sample M###Ultimate stress (MPa) Stiffness (GPa) Elongation %

Mean Value###45.39###1.44###3.15

Sample L###Ultimate stress (MPa) Stiffness (GPa) Elongation %

Mean Value###70.05###1.79###3.92

Characterization of Tensile Properties of Single Yarn PC To evaluate and compare the effect of unevenness in yarn structure, tensile strength of composite, single yarn PC were measured. Table-4 shows the results of the determination of mechanical properties of single yarn PC. Single yarn PC prepared with combed yarn has higher mechanical strength than prepared with carded yarn. This difference in strength is due to the difference in unevenness in the structure of both yarns. There is an increase in the stiffness in the composite with more uneven yarn was observed, this increase in the stiffness is due to less alignment of fibers at micro level which causes an increase in the diameter of the yarn. The resin penetrates more in the yarn due to non-compact alignment of the fibers hence fiber pull out from the composite on applying stress. As a result composite become relatively harder with decrease in its elongation properties as well as ultimate stress and stiffness of the composite increases.

Table-4: Tensile Properties of Single Yarn PC (Sample size is 50).

Sample M###Ultimate stress (MPa) Stiffness (GPa) Elongation %

Mean Value###594.6###15.01###3.96

Sample L###Ultimate stress (MPa) Stiffness (GPa) Elongation %

Mean Value###744.3###11.49###6.48

SEM studies of Mass Irregularity and Micro Alignment of Fibers in the Fiber Bundles of Yarn Longitudinal sections views of yarns were visualized by scanning electron microscope (SEM) to estimate the average effective diameter, extent of micro alignment of fibers in fiber bundles. SEM images showed that yarns are ply yarns with low twist levels. The inter fibers in yarn structure (Fig. 3) are scattered and non-aligned which increases the effective average radius of yarn as compare to the other sample (Fig. 4). The proper impregnation of resin increases the interaction between resin and reinforcing agent. Impregnation and interaction can be observed (Fig. 5) and improvement in stiffness of composites can be observed from data presented in Table-4.

Fig. 3: SEM of Sample M.

Fig. 4: SEM of sample L.

Fig. 5: SEM of Single yarn PC Showing the Fiber Pull out after breakage.

Experimental

All equipments were used for the experiments were pre-calibrated. All experiments were carried out in triplicate and the values presented in this article are mean of three. ] Materials

Cotton yarns with different unevenness were prepared using combing process in Fazal Textile Spinning Mills, Karachi, Pakistan. The yarns had not been paraffined and uncoated with wax to improve any subsequent dyeing processes. The unsaturated polyester resin (UPR) and initiator methyl ethyl ketone peroxide (MEKP) were purchased from the market (Al-Khair Industries, Karachi, Pakistan).

The resin used in this study was a 1, 1.15, 1.14 and 1.44 mixtures of maleic anhydride, isophthalic acid, propylene glycol and diethylene glycol, respectively, containing 40% by weight styrene, with an average of 5.88 vinylene groups per unsaturated polyester molecules. The average molecular weight of the unsaturated polyester resin is 2750 g/mol and the equivalent molecular weight / (mol C = C) is 468 g/mol. The molar ratio of styrene/unsaturated polyester resin is 2.7. The unsaturated polyester resin was employed as received without removing the inhibitor.

Methods

Conditioning

Prior to any testing spun yarns were conditioned under standard conditions of 20 deg C +- 2 deg C and 65% +- 3% relative humidity for 24 h.

Yarn Linear Density

The yarn linear density or count of the samples was determined by using YG 086 Measuring Reel and AUTOSORTER system (China).

Yarn Unevenness

Yarn unevenness was measured using Uster Technologies 4-SX evenness tester as per American Society for Testing and Materials Standard (ASTM) D1425.

Single yarn PC Formation

To investigate the effect of unevenness of reinforcing agent on the tensile properties of composites single yarn PC were prepared using wet winding method. Chrome polished mandrel was prepared to wound the polymeric composite yarns. Yarns were passed through the bath of unsaturated polyester resin containing MEKP and promoter system [16]. Prior winding on the mandrel, yarns were passed through a steel comb to remove excess of resin. The prepared composite yarns were then left for two hours for complete curing and dryness.

Tensile Properties

Tensile properties of spun yarn and composites were measured using the Uster Technologies Tensorapid III as per ISO 2062 standard. Gauge lengths 500 mm was adjusted on the machine; which is according to ASTM D2256, ISO 2062. ] SEM Analysis

Unevenness and alignment of fibers at micro level in yarn and composite were analyzed by scanning electron microscope (SEM) model # 6380A JEOL (Japan). The samples were first coated with auto coater model # JFC-1500 JEOL (Japan).

Conclusion

It is concluded from the present study that the unevenness or linear mass irregularity in the yarn structure affects the mechanical properties of yarn and their PC materials. The unevenness directly influences the degree of alignment of fibers at micro level in the fiber bundles of yarn. Less unevenness in yarn causes an increase in the strength of yarn and composite. The process of combing in spinning system can be used to minimize the unevenness in the structure of yarn which has positive impact in the alignment of fiber at micro level in the fiber bundles. Surface evenness of natural fibers plays a vital role to improve the mechanical properties of the natural fibers based polymeric composite materials. It is therefore recommended that instead of raw plant fibers, use of spun yarn with more evenness is a good option in making polymeric composite materials reinforced with natural fibers.

Acknowledgement

Authors are thankful to Mr. Rizwan Mill Manager at Al Karam Textiles, Mr. Aqil, Quality Manager, Mr. Hasan Abbas and Mr. Anwar Quality supervisors at Fazal Textiles to provide testing facilities and technical assistance. Authors are also thankful to Mr. Yusuf, Manager at Centralized Science Laboratories, University of Karachi, Pakistan to give time and technical assistance in SEM instrumentation.

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1 Muhammad Nasir, 1 Nasreen Fatima, 2 Khalid Mohammed Khan and 2 Durey Nayab Zahra 1 Department of Chemistry, University of Karachi, Karachi-75270, Pakistan.

2 H.E.J Research Institute of Chemistry, International Center for Chemical and Biological Sciences, University of Karachi, Karachi-75270, Pakistan.

khalid.khan@iccs.edu
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Author:Nasir, Muhammad; Fatima, Nasreen; Khan, Khalid Mohammed; Zahra, Durey Nayab
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Feb 28, 2014
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