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Processing, Dyeing & Finishing
  Physical & comfort properties of spun silk knitted fabrics

Spun silk knitted fabrics have higher bursting strength, higher abrasion resistance, higher air permeability, better wicking, higher Shrinkage and lower drape as against the comparable cotton knitted fabrics, irrespective of knit structure, infer M Senthilkumar and Prof B B Jambagi.

Spun silk threads are soft, but they are less lustrous than reeled silk and are not as strong or elastic. Spun silk fabric tends to become fuzzy after wearing because the yarn is made of short staple. Spun silk is less expensive than reeled silk. Although spun silk has less strength and elasticity than reeled silk because of the shorter staple used, it posses all the general characteristics of reeled silk. Silk fabric made of spun silk yet gives good service when the quality of the fibre is good. Spun silk is used for pile fabrics, dress trimmings and linings, elastic webbing, sewing silk, summer wear silks, velvets, umbrella fabrics and insulation.

In this article, geometrical properties of spun silk knitted fabric are elaborately discussed. Spun silk yarn has been knitted in to three different knit structures. The testing methods, the materials and knitting machine details are given below:

Material details

Table 1. Yarn specifications

Yarn details Spun silk Cotton
Count                                          tex 29.5 29.5
Actual count                                tex 29.25 29.4
CV % of Count                            % 6.58 4.58
Breaking elongation                    % 5.62 6.8
CV % of Breaking elongation      % 14.56 7.52
Breaking force                           gf 575.8 520
CV % of Breaking force             % 12.81 10.2
RKM                                             - 17.54 26.89
CV % of Tenacity                       % 12.81 5.25
Uster % 17.79 9.63
CV % of Uster % 2.59 2.45
Thin places /km (-50%) 258 28
CV % of Thin places /km           % 36.92 25.96
Thick places /km (+50%) 640 220
CV % of Thick places/km          % 6.33 45.56
Neps / km (+200%) 1657 252
CV % of Neps / km                    % 11.07 20.12
Hairiness index 5.96 4.62
CV % of Hairiness index            % 5.34 3.32

Table 2. Knitting machine specifications

Machine Single jersey Interlock Rib
Type Weft Circular Weft Circular Flat Bed
Model Unitex Lakshmi Terrot DJ 313 DJ
Gauge 12 14 14
Diameter (inches) 30 18 40 Width
Speed (rpm) 13 5 10 Strokes/min
Feeders 96 40 (28 used) 2


Testing Methodology
The following tests were carried out according to standard textile testing methods.

Tests                                                      Instruments                                                                                Standard Yarn Stage

Single yarn strength                                 Premier Tenso tester                                                                            ISO 2062:1993
Yarn Unevenness                                    PREMIER i Q Quali Center                                                                ISO 16549:2004

Fabric stage


Bursting strength                                     Ball bursting strength tester                                                                   ISO 13938-2:1999   
Abrasion Resistance                               Martindale abrasion tester                                                                    ISO 12947-1:1998
Pilling resistance                                     ICI Pill box tester                                                                                 ISO 12945-1:2000
Drapeability                                           Fabric drape meter                                                                                      -
Air permeability                                     Air permeability tester                                                                                  -
Wicking test                                          Spray test method                                                                                ISO 4920:1981 
Thermal Insulation                                 TIV tester                                                                                                   -
Thickness                                             Thickness gauge                                                                                   ISO 5084:1996
Weight                                                 Electronic weighing balance                                                                  ISO 3801:1977
Shrinkage                                             Area shrinkage method                                                                         ISO 3759:1994
Low stress mechanical
              Properties                              Kawabata Equipment                                                                           HESC Standards.

Geometrical properties of spun silk knitted fabrics

The Table 3 shows the geometrical properties of single jersey, rib and interlock silk knitted fabrics with three different stitch lengths, given by SL1, SL2 and SL3 (in cm). The geometrical properties such as wales/cm, courses/cm, are respectively designated as WPcm, CPcm, stitch density (cm 2) as SD, loop shape factor as L and the tightness factor (Tex 0.5 cm -1) as TF, dimensional constants for wales, courses, and stitch density are respectively designated as Kw, Kc and Ks.

Stitch length or loop length is a parameter that mainly influences the geometrical properties of a knitted fabric. When the stitch length increases, the WPcm, CPcm, and SD decrease irrespective of the type of structure. This is due to larger yarn loop area. If the yarn loop area increases the wales and courses per unit length reduce. This has been observed in cases of all the three structures.

Geometrical constants Kc, Kw and Ks values of single jersey, rib and interlock knitted fabrics have been increasing with increased stitch length. Kw value first increases and then decreases when increasing the stitch length in the case of interlock silk knitted fabrics. Similarly, when stitch length increases, Kc value first decreases and then increases in case of rib knitted fabric. The reason may be due to higher tightness of spun silk knitted fabrics. The same trend is reflected in the loop shape factor also. (Because Kc / Kw is the Loop shape factor). Tightness factor reduces with increasing stitch length in all the three structures because of the lower stitch density of knitted fabrics.

Correlation coefficient between (1 / SL) stitch length and geometrical properties of spun knitted fabrics are also tabulated below.

In general, there is a good positive correlation between stitch length and geometrical properties of spun silk knitted fabrics like wales per cm, courses per cm, stitch density, loop shape factor, tightness factor and good negative correlation between stitch length and geometrical properties of spun silk knitted fabrics like geometrical constants Kw, Kc, Ks values.

Physical and comfort properties of spun silk knitted fabrics

In this part, the mechanical and comfort properties of spun silk knitted fabrics are discussed and the results are compared with relevant count of cotton knitted fabrics. This research work does not focus on comparative analysis of spun silk and cotton knitted fabric. Because type of material, properties, method of production, yarn manufacturing process and pre-treatments are different for silk material compared with cotton material. So, the given comparison Table shows the level of performance of silk knitted fabric compared with commercial cotton knitted fabric. (relevant count of silk yarn)

The fabric samples were produced as mentioned in the materials and methods (given in the Tables 1 & 2 ). The produced samples were tested for their properties as mentioned. The average values of test results of knitted fabric properties are tabulated below: [Table 5 and 6 ]

Bursting strength

From these results, it can be observed that the spun silk knitted fabric shows higher bursting strength value than the equivalent cotton knitted fabric, irrespective of knit structure.

Abrasion resistance

These results show that the spun silk knitted fabrics display higher abrasion resistance than the equivalent cotton knitted fabrics, irrespective of knit structure. This may be due to higher silk fibre tenacity.

Pilling resistance

From the above table, it can be observed that the spun silk knitted fabric shows fuzziness and less pilling resistance than the filament silk knitted fabric, irrespective of the knit structure. Pilling does not seem to be a serious problem with spun silk and equivalent cotton knitted fabrics.

Drapeability

The drape results clearly show that the spun silk knitted fabrics give lower drape coefficient values than the equivalent cotton knitted fabrics, irrespective of structure. Unpredictable trend may occur due to the curly nature of single jersey knitted fabric.

Air permeability

From the above table it can be seen that the spun silk knitted fabrics have higher air permeability than the equivalent cotton knitted fabrics, irrespective of structure.

Wicking

The above values show that the spun silk knitted fabric is having a higher wicking tendency than the equivalent cotton knitted fabric irrespective of structure. The spun silk interlock knitted fabric shows higher wicking values than the other knit structures, which may be due to higher stitch density of interlock fabric.

Thermal insulation value

The thermal insulation results show that the values for spun silk knitted fabrics are higher than the equivalent cotton knitted fabric irrespective of knit structure, which may be due to lower thermal conductivity of silk fibre. This may be the reason for higher thermal insulation value in all the cases.

Thickness

From the above table, it can be observed that the spun silk knitted fabrics show relatively the same thickness as compared with the equivalent cotton knitted fabrics irrespective of knit structure. Rib knitted fabric shows higher thickness value than all the other knitted fabrics, and this can be attributed to the bulkiness of rib knitted fabrics.

Weight [Gm per sq m]

From these observations it is clear that the weight of a knit structure increases with stitch density irrespective of the knit structure and the yarn. Spun silk and cotton rib knitted fabrics show higher weight values than all the other knit structures because of the higher yarn density of knitted fabric.

Shrinkage

The results show that the percentage of area shrinkage is higher for the spun silk knitted fabrics than the equivalent cotton knitted fabrics, which may be due to the higher yarn tension (silk: 7-8 g, cotton: 4-5 g) during knitting.

Low stress mechanical properties

Tensile properties

The above results show that the spun silk knitted fabric gives higher LT and WT values than the corresponding cotton knitted fabrics irrespective of knit structure. This may be due to the higher fibre tenacity and breaking extension of silk. The spun silk knitted fabrics show lower RT values than the equivalent cotton knitted fabric irrespective of knit structure, which may be due to the good work recovery property of silk.

Bending properties

The above table shows that the spun silk knitted fabrics display higher bending rigidity than the corresponding cotton knitted fabric irrespective of knit structure. This may be due to the higher specific flexural rigidity of silk fibre. (Silk: 0.6 mN mm2/tex2 and Cotton: 0.53 mN mm2/ tex2)

Spun silk knitted fabrics show lower hysteresis of bending moment than the cotton knitted fabric irrespective of knit structure, a result that may be due to the higher work recovery of silk fibre.

Shear properties

The above values show that the shear properties of G, 2HG, 2HG5 values for spun silk knitted fabrics are higher than those of the equivalent cotton knitted fabrics, which is mainly due to higher shear tenacity of silk fibre than cotton. (Silk: 115.8 mN / tex and cotton: 84.4 mN / tex)

Compressional properties

The compression test results show that the spun silk knitted fabrics display lower LC and higher WC and RC values than the equivalent cotton knitted fabrics in case of all three basic structures. Spun silk knitted fabrics show higher RC values than the equivalent cotton knitted fabric, which may be due to higher elongation and work recovery properties of silk.

Surface roughness and friction coefficient

From the above data, it can be observed that the spun silk and the equivalent cotton knitted fabric show an unpredictable trend, which is probably due to variation in the stitch density of knitted fabric.

Conclusion

Spun silk knitted fabrics have higher bursting strength, higher abrasion resistance, higher air permeability, better wicking, higher Shrinkage and lower drape as against the comparable cotton knitted fabrics, irrespective of knit structure. Pilling does not seem to be a serious problem with spun silk knitted fabrics.

Spun silk knitted fabrics have higher LT, WT value and lower RT value of tensile properties, higher B, lower 2HB value of bending properties, higher G, 2HG, 2HG5 value of shear properties, higher LC, WC and RC values of compressional properties and higher MIU, MMD and SMD of surface roughness properties than the respective cotton knitted fabrics irrespective of knit structure.

References

  1. Sharma I C: Dimensional and Physical Characteristics of Single Jersey Fabrics, TRJ, March 1985, pp 149.

  2. Doyle P J: Fundamental Aspects of the Design of Knitted Fabrics, JTI 1953, pp 561.

  3. Behera B K: Comfort Properties of Fabrics Woven from Ring, Rotor and Friction Spun Yarns, JTI 1997, Part 1, No 3.

  4. Hiroshi Kato and Tamako Hata: Development of Silk Yarns for Knitted Fabrics, Published at Department of Insect Processing Engineering, National Institute of Sericultural and Entomological science, Japan.

  5. Rajiv Kumar, et al: Feasibility of Spinning Silk/Silk Blends on Cotton System, Textile Asia, February 2001, Part I, pp 27.

  6. Shiva Kumar V R: A Comparative Study of the Knitting Performance and the Properties of Fabrics Knitted from ring and Rotor Spun Yarns, 22nd Technological Conference, pp 50.

  7. Shankara Narayanan K S: Effect of Yarn Quality on the Properties of the Knitted Fabrics and Performance in Knitting, 22nd Technological Conference, pp 159.

  8. Parate D M: Evaluation of Yarn Properties for Knitting (Part 1 and Part 2), Man Made Textiles in India, October 1995.

  9. Lakshmanan S and Geetha Devi R G: Silk Scenario, The Indian Textile Journal, March 2000, pp 42.

  10. Raj Kumar R: Silk Knitted Fabrics for Outer Garments, The Indian Textile Journal, July 2001, pp 127.

  11. Siva Kumar M: Role of Silk in Knitted Fabrics Production, Textile Magazine, August 2000, pp 45.

  12. Pau-lin-chen: Handle of Weft Knitted Fabrics, TRJ, April 1992.

  13. Mee-sung choi: Effect of Changes in Knit Structure and Density on the Mechanical and Hand Properties of Weft Knitted Fabrics for Outerwear, TRJ, December 2000.

  14. Aung kyaw soe: Compression of Plain Knitted Fabrics Predicted from Yarn Properties and Fabric Geometry, TRJ, October 2003.

  15. Alimaa D: Effect of Yarn Bending and Fabric Structure on the Bending Properties of Plain and Rib Knitted Fabrics, TRJ, September 2000.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal April 2008 issue.

Mr M Senthilkumar
Department of Textile Technology,
PSG Polytechnic College,
Coimbatore 641004.
Email: cmsenthilkumar@yahoo.com.

Prof B B Jambagi Department of Textile Technology,
DKTE Textile and Engineering Institute,
Ichalkaranji,
Maharashtra.
Email: jambagi@indiatimes.com.

published April , 2008
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