Modal was at first introduced by Lenzing AG Company of Austria who trade named the fabrics identity. Modal is very soft, shiny in nature and silk feel than mercerized cotton with the ability to absorb up to 50% more water than cotton. Fabric made from modal drape well and do not pile like cotton. In this study, modal fabrics (woven and knitted) are selected and conventionally pretreated and subjected with 98% formic acid in different concentrations. The formic acid treated modal fabrics were then undergone for different testings. These treatments on modal fabrics were correspondingly compared with those of cotton for its effectiveness.

Keywords: Modal Fabric; Formic acid, K/S & SEM

Lenzing started selling modal fibers in 1964. In 1977, Lenzing started using an environmentally friendly bleaching method for pulp for their cellulosic fibers. Modal's distinguishing characteristics are its high wet strength and its extra softness. It is sometimes referred to as "soft as a feather" and the "softest fiber in the world". The smooth surface characteristics of the modal fiber make it impossible for mineral deposits from water, such as lime scum, to be deposited on the textiles [1,2] thus preventing fabric hardening after repeated washings. It's designed to dye just like cotton and is color-fast when washed in warm water. The softness makes it especially ideal for body contact clothing such as lingerie and under garments [3,4].

They are also wear resistant and strong while maintaining a soft, silky feel [5. 6]. Modal fibers have found a wide variety of uses in clothing, outerwear and household furnishings. They are often blended with cotton, wool or synthetic fibers and allow easy tone-in-tone dyeing [7, 8]. In this study, modal fabrics (woven and knitted) are selected and conventionally pretreated to get rid of the basic impurities. The pre-treated fabrics were then subjected with 98% formic acid in different concentrations. The formic acid treated modal fabrics were then undergone for different testings, such as, physical properties, absorption, K/S values, fastness properties and SEM analysis. These treatments on modal fabrics were correspondingly compared with those of cotton for its effectiveness.

The textile fabrics used in this study were in the following descriptions:

I) Woven Fabrics: a) Modal (100%) : yarn count (both warp and weft) – 27, GSM - 146, ends per inch – 84, picks per inch – 94; b) Cotton (100%) : yarn count (both warp and weft) – 30, GSM - 137.2, ends per inch – 94, picks per inch – 74; and II) Knitted Fabrics: a) Modal (100%) : yarn count – 27.5, GSM 137, loop length–2.6 mm, b) Cotton (100%) : yarn count – 30, GSM - 142, loop length–2.6 mm; and the chemicals and dye used in this study were in the commercial grade.

Natural dyes [annatto (bixa orellana), onion (allium cepa), pomegranate (punica granatum), indigo (indigofera tinctoria), myrobalan (terminalia chebula), bar berry (berberis vulgaris)] and synthetic dyes [reactive dye (reactive red HB – C.I. No. Red 24), and sulphur dye (sulphur black – C.I. No. sulphur Black 1)] used were in the commercial grade. The chemicals mentioned elsewhere for this study were in AR grade.

Treatment on modal and cotton fabrics: The modal and cotton fabrics (woven and knitted) were pretreated (desizing (for woven), scouring and bleaching) as per the established technique [9,10,11]. The pre-treated modal and cotton fabrics were subjected with 98% formic acid with the concentration of 1%, 2%, 3%, 4% & 5% (owm) for 30 minutes at ambient temperature (27˚C).

Measurement of physical properties of modal and cotton fabrics: The physical properties such as tensile strength [9], stiffness [12], crease recovery angle [13,14] and mean drape coefficient of woven fabric [15]; and bursting strength and mean drape coefficient of knitted fabric [15,16] of the modal and cotton fabrics was measured by the standard established methods.

Absorbency of modal and cotton fabrics: Absorbency is the time taken for a water drop to penetrate into the textile material. The wet ability of modal and cotton fabrics was determined as per AATCC test method 79 [17].

Dyeing and K/S analysis of modal and cotton fabrics: The dye ability of modal and cotton fabrics (woven and knitted) was investigated using natural and synthetic dyes. Dyeing was carried out at boil for two hours with a material to liquor ratio of 1:20 as per the established technique of dyeing for natural and synthetic dyes. The dyed samples were washed, soaped and dried [10, 18, 19]. Colorimetric data of natural and synthetic dyed modal and cotton fabrics were determined by AATCC Test Method -135 [20] using a Data color SF 600 plus spectrophotometer interfaced to a PC. Measurements were taken regarding colour presence, brightness, dullness and colour intensity with the specular component of the light excluded and the UV component included using illuminant D65 and 10° standard observer. Each fabric was folded once so as to give two thickness and average of five readings were taken each time.

Color fastness analysis of the modal and cotton fabrics: The natural and synthetic dyed samples were washed under condition IIIA of AATCC Test Method 124-2001 to determine the color change effect of dyed fabrics [21, 22]. Light fastness tests (AATCC, 2003), were carried out according to AATCC Test Method 16 E-1998 [23]. The samples were exposed to 5, 10 AFUs (AATCC Fading Unit) to determine the color change AATCC 16-1998 [24]. AATCC standardized crock meter was used to determine the rubbing fastness of natural dyed fabrics under wet and dry condition to assess the color change and staining property AATCC 61-1996 [25].

SEM analysis of silk and its mixed fabric: The surface morphology of modal and cotton fabrics was observed in SEM (JOEL JSM-6360 model microscope, Japan) [26, 27].

The physical properties such as tensile strength, stiffness, crease recovery angle and mean drape coefficient of woven fabric; and bursting strength and mean drape coefficient of knitted fabric for modal and cotton fabrics are shown in (Table 1). The modal fabric shows higher values of tensile strength (431 N (warp) and 370 N (weft) – untreated) and crease recovery angle (117 (warp) and 119 (weft) – untreated) respectively both in warp and in weft directions compared to that of the cotton fabric. However, the stiffness (0.99 cm (warp) and 0.97 cm (weft) – untreated) and mean drape co-efficient (65%) are less in modal fabric over that of cotton fabric. In the case of knitted fabric, cotton shows greater values than that shown by modal fabric for bursting strength and mean drape coefficient respectively. The modal fabric after treatment with formic acid (98%) at different concentrations for 30 minutes at room temperature show changes in the physical properties. The tensile strength and stiffness of formic acid treated modal fabric are reduced marginally upto the formic acid treatment of 4% (owm), and at 5% (owm) the loss is significant similar to that of cotton fabric. However, the crease recovery and mean drape co-efficient of formic acid treated modal fabric is increased considerably up to the concentration of 4% (owm), and at 5% (owm) it is only marginal. In the case of knitted modal fabric the bursting strength is decreased correspondingly with respect to the concentration of formic acid, while the mean drape co-efficient is increased considerably upto 4% (owm).

The data of absorbency of modal and cotton woven and knitted fabrics are given in (Table 2). It is seen from this table that the overall absorbency exhibited by woven fabric (modal and cotton) is more than those of knitted fabric. The absorbency is more for cotton than that of modal fabrics both in the case of woven and knitted fabrics. In the formic acid treated modal fabrics (woven and knitted) the water drops are absorbed in less time which indicates that as the concentration of formic acid increases, the absorbency on the modal fabric is also increased considerably upto the concentration of 4% (owm), and at 5% (owm) the increase is only marginal. The similar trend is also seen for cotton fabrics.

The K/S values of the dyes such as annatto, onion, pomogranate, indigo, myrobalan, barberry, reactive dye, and sulphur dye applied on formic acid treated modal and cotton (woven and knitted) fabrics are given in (Table 3). The formic acid treatment was identified effective at the concentration of 4% (owm), hence that condition was considered as optimum parameter. For dyeing, the modal and cotton fabrics were given formic acid treatment only with 4% (owm) concentration for 30 minutes at ambient temperature. From this table it is observed that woven fabrics show overall high K/S values than the knitted fabrics. The K/S value of formic acid treated modal fabric is high when compared those of the cotton fabrics (woven and knitted). The higher K/S value on the formic acid treated modal fabric is influenced by the higher swelling action of formic acid treatment. Among the dyes applied on the formic acid treated modal fabrics, there is only a marginal differences in the K/S values; however reactive dye shows the maximum K/S values. Even though the woven and knitted modal and cotton fabrics posses only a small differences in the K/S values for the dyes (annatto, onion, pomogranate, indigo, myrobalan, bar berry, reactive dye, and sulphur dye) applied on these fabrics; there is a uniform trend maintained in these values and the values of woven fabric give an edge over the knitted fabric. The untreated (no formic acid) modal and cotton fabrics were dyed only with the representative dye (reactive). From the (Table 4). It is evident that the overall K/S values of formic acid (4% owm) treated modal and cotton fabrics are significant compared with that of the untreated dyed fabrics.

The fastness properties (wash, light and rubbing) of natural and synthetic (indigo, kum kum, bar berry, reactive dye (M), reactive dye (H) and sulphur) dyes applied on formic acid treated (4% owm) modal and cotton fabrics (woven and knitted) are given in (Table 4). The wash fastness of the formic acid treated modal fabrics is good as compared with other fatnesses like light and rubbing fastness properties. The good wash fastness property is due to the strong polymeric reaction of the fabric materials and the also of the dye. It is obvious that the light fastness and rubbing fastness properties are moderate to poor only due to their behavior towards these applications. From this (table 4), it is also evident that the overall fastness properties of the formic acid treated and dyed modal fabrics are higher than that of the no formic acid treatment fabrics. The similar trend is also seen for cotton fabrics.

The analysis of SEM images of formic acid treated and dyed modal and cotton fabrics are given in the representative (Figures 1 & 2). (Figure 1) reveals about the micrographs of formic acid treated and dyed modal fabrics. (Figure 2) reveals about the micrographs of formic acid treated and dyed cotton fabrics. These figures show clearly about the effect of formic acid in the corresponding materials and the subsequent dyeing. There is a clear difference between the no formic treated & dyed; and formic acid treated & dyed (indigo) modal fabrics (woven and knitted). Similar trend is also seen for the cotton fabrics. These micrographs reveal about the enhancement of dyeing after formic acid treatment on modal and cotton fabrics respectively.

The tensile strength of modal fabric both in warp and weft direction and crease recovery angle are very good in comparison with cotton fabric. Whereas the stiffness [warp & weft] and mean drape co-efficient are less in modal fabric over that of cotton fabric. In the case of knitted fabric, cotton shows greater values than that shown by modal fabric for bursting strength and mean drape coefficient respectively. The modal fabric after treatment with formic acid (98%) at the concentration of 4% (owm) for

30 minutes at room temperature gives considerable changes in the physical properties under the favorable limit suitable for the application. The absorbency is improved in the modal fabric (woven and knitted) after treatment with formic acid (98%) at the concentration of 4% (owm) for 30 minutes at room temperature similar to that of cotton fabric.

The K/S values of the dyes such as annatto, onion, pomogranate, indigo, myrobalan, barberry, reactive dye, and sulphur dye applied on formic acid treated modal (woven and knitted) fabrics are good compared with that of no formic acid treated one, similar to the trend of cotton fabrics. The fastness properties (wash, light, & rubbing) of the formic acid treated and dyed (annatto, onion, pomogranate, indigo, myrobalan, bar berry, reactive dye and sulphur dye) woven and knitted modal fabrics are comparatively good over that of no formic acid treated one, as that of cotton fabrics. In general, the overall fastness properties are more in formic acid treated fabrics. SEM micrographs reveal that the formic acid treated & dyed modal and cotton fabrics give good dyeing effect compared with that of no formic acid treated one.

The authors wish to thank the Management and Principal, PSG College of Technology, Coimbatore for given the permission and providing the necessary infrastructure. The authors also thank The Head, Department of Applied Science, PSG CT for the kind help rendered in the completion of this work.

1. Schurz J.  Trends in polymer science. A bright future for cellulose, Progress in Polymer Science.  1999;24(4):481–483.
2.  Bredereck K, Hermanutz F.  Man-made cellulosics.  Review of Progress in Coloration and Related Topics.  2005;35: 59–75.
3.  Heinze T, Liebert T.  Unconventional methods in cellulose Functionalization. Progress in Polymer Science.  2001;26(9):1689–1762.
4.  Saalwachter K, Burchard W. Cellulose in new metal-complexing solvents: Semidilutebehavior in Cd-tren;,  Macromolecules. 2001;34(16):5587–5598. 
5. Simpson V.  India's Textile and apparel industry: Growth potential and trade and investment opportunities (Staff Research Study, Office of Industries, U. S. International  Trade Commission). Washington, USA. 2011.
6.  Lewin M, Sello. Handbook of Fiber Science and Technology, Vol. II, Part B, Dekker Series, New York, USA.   2000;120-125.
7. Lewin M.   Handbook of Fibre Chemistry; 3 rd edition, (CRC Press, Baco Raton, USA), 2007;331-382.
8.  Teli MD Kumar, GVNS.  Functional textiles and apparels - Technical Textile;  Journal of the Textile Association.  2007;21-30.
9. Saville BP.   Physical Testing of Textiles, Wood Head Publishing Limited and CRC Press, Cambridge, England.  2004;205-210.
10. Trotman ER.  Dyeing and Chemical Technology of Textile Fibers:   Sixth edition, Edward  Arnold, London. 1984;187-217.
11. Shukla SR. Advances in preparatory process in cotton; NCUTE-Programme  Series:  Chemical Preparatory Process in Textiles,  Indian Institute of Technology –Delhi; New Delhi. 2000;85-92.
12. ASTM Test Method. Standard Test Method for Stiffness of Fabrics:  Annual Book of ASTM Standards, West Conshohocken, PA. USA. 2001;7(1):1388-1396.
13. AATCC Test 2005Method. Wrinkle recovery of woven fabrics recovery Angle: Technical Manual of the AATCC, Research Triangle Park. USA. 2003:pp.66.
14. Connell, D.L.  The Control of Shrinkage in Textile Finishing (Editor Heywood D): Woodhead Publication Ltd., Cambridge, England.  2005:pp.60-62.
15.  British Standard Method for the Assessment of Drape of Fabrics, BS Handbook. London. 1973.
16. Collier B.J, Epps H.H.  Textile Testing and analysis:  6^th edition, Prentice Hall Inc, New Jersey, USA. 1999:pp.30-35.
17. AATCC Technical Manual, American Association of Textile Chemists and Colorists, Research Triangle Park, NC.  1999:pp.66.
18.  Mohanty BC, Chandramauli K.V, Naik H.D. Natural Dyeing Process of India: Published by Calico Museum of Textiles, India, 1987.
19. Gulrajani, M.L, Gupta, D. Natural Dyes and their Application to Textiles:Indian Institute of Technology –Delhi; New Delhi. 1992.
20. AATCC Test Method 135-1985; Colour measurement of textiles: Instrumental Technical Manual of the AATCC, Research Triangle Path, USA. 2003.
21. AATCC test method; Colour Fastness to Laundering, Home and Commercial: Technical Manual of the AATCC, Research Triangle Park, USA. 2001;61 2(A).
22. BIS Test Method; Test 3, Indian Standard Method for Determination of Colour Fastness of Textile Materials to Washing, Bureau of Indian Standards,IS:764-1979.
23. AATCC Test Method; Colour Fastness to Light:  Technical Manual of the AATCC, Research Triangle Park, USA, 16-1998.
24.   AATCC test method; Colour Fastness to Crocking: Technical Manual of the AATCC,  Research Triangle Park, USA, 8-2007.
25.  AATCC Test Method; Colour Fastness to Laundering: Home and Communication - Accelerated:   Technical manual of  the AATCC, Research. Triangle Park, U.S.A, 61-1996,2003.
26.  Gouda M, Hebeish A. Preparation and Evaluation of CuO/Chitosan Nanocomposite for Antibacterial Finishing Cotton Fabric:   Journal of Industrial Textiles.   39(3):  203 – 213, 2010.
27. Hearle, J.W.S.   Use of the Scanning Electron Microscope (Pergamon Press, Oxford), 1972.