Abstract
Flammability properties of plasma pretreated cotton fabrics subjected to flame-retardant treatment were studied. Plasma pretreatment, using an atmospheric pressure plasma jet (APPJ), was applied to cotton fabrics to enhance material properties, while retaining inherent advantages of the substrates. An organic phosphorus compound (flame-retardant agent, FR) together with a melamine resin (crosslinking agent, CL) and phosphoric acid (catalyst, PA) were used. Titanium dioxide (TiO2) or nano-TiO2 was used as a co-catalyst for cotton fabrics to improve treatment effectiveness and minimize side effects. Surface morphology of plasma pretreated cotton specimens subjected to flame-retardant treatment showed a roughened and wrinkled fabric surface with high deposition of the finishing agent, caused by an etching effect of plasma and attack of acidic FR. Combustibility of FR-CL-PA-TiO2 and FR-CL-PA-Nano-TiO2 treated fabrics was evaluated by a 45° flammability test. FR-CL-PA-treated specimens showed superior flame-retardancy, which was further improved by plasma pretreatment and addition of metal oxide as a co-catalyst. However, in comparison with the control sample, flame-retardant-treated cotton specimens had lower breaking load and tearing strength, resulting from side effects of the crosslinking agent used, while plasma pretreatment might compensate for the reduction in tensile strength caused by flame-retardant agents. In addition, both plasma pretreatment and metal oxide co-catalyst added in the flame-retardant finishing improved the crosslinking process between FR and cotton fabric, minimizing formation of free formaldehyde and allowing the use of FR in industry.
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EFFECT OF OXYGEN PLASMA PRETREATMENT AND TITANIUM DIOXIDE OVERLAY COATING ON FLAME RETARDANT FINISHED COTTON FABRICS
Yin Ling Lam,* Chi Wai Kan, and Chun Wah Yuen
Flammability properties of plasma pretreated cotton fabrics subjected to flame-retardant treatment were studied. Plasma pretreatment, using an atmospheric pressure plasma jet (APPJ), was applied to cotton fabrics to enhance material properties, while retaining inherent advantages of the substrates. An organic phosphorus compound (flame-retardant agent, FR) together with a melamine resin (crosslinking agent, CL) and phosphoric acid (catalyst, PA) were used. Titanium dioxide (TiO2) or nano-TiO2 was used as a co-catalyst for cotton fabrics to improve treatment effectiveness and minimize side effects. Surface morphology of plasma pretreated cotton specimens subjected to flame-retardant treatment showed a roughened and wrinkled fabric surface with high deposition of the finishing agent, caused by an etching effect of plasma and attack of acidic FR. Combustibility of FR-CL-PA-TiO2 and FR-CL-PA-Nano-TiO2treated fabrics was evaluated by a 45° flammability test. FR-CL-PA-treated specimens showed superior flame-retardancy, which was further improved by plasma pretreatment and addition of metal oxide as a co-catalyst. However, in comparison with the control sample, flame-retardant-treated cotton specimens had lower breaking load and tearing strength, resulting from side effects of the crosslinking agent used, while plasma pretreatment might compensate for the reduction in tensile strength caused by flame-retardant agents. In addition, both plasma pretreatment and metal oxide co-catalyst added in the flame-retardant finishing improved the crosslinking process between FR and cotton fabric, minimizing formation of free formaldehyde and allowing the use of FR in industry.
Keywords: Flame-retardant; Titanium dioxide; Plasma pretreatment; Cotton
Contact information: Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China; *Corresponding author: 07901799r@polyu.edu.hk
INTRODUCTION
Textile materials have traditionally been known to be the major causes of spreading of fire because of their inflammability, as well as their ubiquitous presence in our daily lives, in the form of clothing, furnishing materials, household goods, and many other products (Siriviriyanun et al. 2008). Among different textile fibres, cotton, mainly composed of carbon, oxygen, and hydrogen, is used widely but has higher combustibility compared to other fibres. Coating of cotton products with chemicals is an easy and effective approach to reduce inflammability. Therefore, cotton fabric is often treated chemically to prevent ignition of fire by small flames, which often cause degradation of cotton at lower temperatures through the process of dehydration (Siriviriyanun et al. 2008; Wakelyn et al. 2004). Generally speaking, N-methylol dimethylphosphonopro-pionamide flame retardant agent (FR) is widely used, in combination with a melamine resin crosslinking agent (CL) and a catalyst (phosphoric acid, PA) to impart flame retardant properties to cotton fabrics (Yang et al. 2007). However, due to toxicological and environmental concerns, formaldehyde-based flame retardant agents are currently being phased out and replaced with other coating materials (Yang et al. 2007). It was proved that addition of a TiO2/nano-TiO2 co-catalyst could enhance crosslinking of components of the FR-CL-PA flame-retardant formulation (Lam et al. 2010a).
Plasma technologies can help retain inherent advantages of the substrates while enhancing materials properties. Plasma is partially ionised gas, which is overall neutral in nature, containing ions, electrons, and neutral particles produced by the interaction of electromagnetic field with gas under a specified pressure. The active species produced in plasma carry high energy that causes a sputtering or etching effect, which alters the characteristics of fibre surface. The treatment roughens the surface of the materials and is conducive to subsequent use of a large variety of chemically active functional groups (Hwang et al. 2005; Wang et al. 2008; Kaplan 2004; Rajpreet et al. 2004). Among various types of plasma treatments, atmospheric pressure plasma jet (APPJ) is widely used in the textile industry to modify the fabric surface in an environment friendly process that helps reduce use of chemicals and energy (Hwang et al. 2005; Wang et al. 2007; Bourbigot et al. 2007) In this study, detailed information concerning effects of plasma pretreatment on flame-retardant properties of cotton fabric, after FR-CL-PA treatment (with or without using TiO2 as catalyst), are evaluated. Combustibility of flame-retardant-treated fabrics, evaluated by the 45° flammability test, was also studied, while mechanical strength was analysed by a grab test and the Elmendorf tearing test. Formaldehyde content and surface morphology of treated fabrics were also evaluated.
EXPERIMENTAL
Materials
100% semi-bleached plain weave cotton fabric (58 ends/cm, yarn count 40 tex, in warp; 58 picks/cm, yarn count 38 tex, in weft; fabric weight 175g/m2), of size 30 cm x 30 cm was used. The flame-retardant agent and cellulose crosslinking agent used were an organic phosphorus compound (Pyrovatex CP New, FR) and a melamine resin (Knittex CHN, CL), both supplied by Huntsman Limited. Analytical reagent grade phosphoric acid (PA) that served as catalyst was supplied by Sigma-Aldrich Co. Co-catalysts used were micro-titanium dioxide (TiO2, 2 μm diameter) and nano-titanium dioxide (nano-TiO2, 100 nm diameter) obtained from UniChem Ltd and International Laboratory Ltd., respectively, both having purity of 99.5+%. The alkali was analytical reagent grade sodium carbonate supplied by Sigma-Aldrich Co.
Plasma Pretreatment
Plasma pretreatment of cotton fabric was carried out by an atmospheric pressure plasma jet apparatus, Atomflo 400 Plasma controller integrated with robot, manufactured by Surfx Technologies. The cotton fabric was moved automatically according to the specified treatment speed. The machine produced a stable discharge at atmospheric pressure with radio frequency of 13.56 MHz. The treatment was carried out using a rectangular nozzle that covered an active area of 50.8 mm x 1mm, and was mounted vertically, above the cotton fabric. Helium and oxygen were used as carrier and reactive gas respectively. The plasma pretreatment of cotton fabric was conducted at different oxygen flow rates as shown in Table 1.
Table 1. Plasma Pretreatment Conditions
Flame-retardant Two-bath Pad-Dry-Cure Treatment
Plasma pretreated cotton fabric samples were treated with different pad formulations as shown in Table 2. A two-bath method was used for the treatments. In the first bath, the fabrics were dipped and padded with flame-retardant agents (FR-CL or FR-CL-PA) until a wet pick-up of 80% was achieved at 25°C. The fabrics were then dried at 110 °C for 5 minutes. In the second bath, dipping and padding processes (80% wet pick up) were performed, using TiO2 / nano-TiO2 solution dispersed in 10% Matexil DN-VL (dispersing agent). Subsequently, padded fabrics were dried at 110 °C for 5 minutes and were then cured at 170 °C for 1 minute. After curing, the treated specimens were then neutralized with 30 g/L sodium carbonate for 0, 15, or 30 minutes at 50 °C. After neutralization, the specimens were rinsed in 50 °C running water. Finally, the fabrics were conditioned at 21±1 °C and 65±5% RH for 24 hours, prior to any further treatment.
Table 2. Flame-retardant Treatment Conditions
* Concentration percentage measured based on weight of volume.
Scanning Electron Microscopy (SEM)
The surface morphology of cotton fibres was examined with a JEOL JSM-6490 Scanning Electron Microscope, with an accelerating voltage of 20kV and a current of 10μA at a high magnification power up to 10000X.
45o Flammability Test
In a preliminary study, with some modification to the ASTM D1230-94 standard, it was found that a flame impingement time of 4 seconds was sufficient to cause some of the samples to “fail”, thus providing a useful differentiation among samples in terms of their flammability. In addition to determining pass/fail of treated specimens, burning time, char length, and burning speed (char length divided by burning time) were studied based on the modified ASTM D1230-94 standard. In the present study, flammability of all specimens was measured using the 45° flammability tester for apparel textiles (The Govmark Organization, Inc.). The specimens were tested after home laundering for 0, 1, and 5 normal machine cycles at 27±3 °C and tumble dried, according to AATCC 135-2004. The specimens were inserted in a frame and held in the flammability tester at an angle of 45°. A standardized flame, of 16 mm flame length, was applied to the fabric surface near the lower end for 4 seconds.
Strength Tests
Tensile properties were measured in accordance with the ASTM D5034-95 standard using the constant-rate-of-extension (CRE) Instron 4411 tensile testing machine.
Tearing strength was measured with an Elmendorf Tearing Tester manufactured by the Thwing-Albert Instrument Co., according to the ASTM D1424-96 standard.
Determination of Formaldehyde
The amount of free formaldehyde and formaldehyde extracted partly through hydrolysis by means of a water extraction method was measured according to the ISO 14184-1-1999 method.
RESULTS AND DISCUSSION
Morphological Study
Figure 1a shows the morphological structure of the untreated cotton fibre at a magnification of 2000X. From longitudinal view, cotton fibre is flat with a twisted ribbon-like structure caused by spiraling of cellulose fibrils. The SEM images show some integrity in cotton fibres with smooth surface and normal spiral structure. The presence of natural folds running parallel along the cotton fibre axis is also observed.
Plasma treatment modifies the fabric surface by physical and chemical interactions. Plasma enhances roughness of the fibre surface due to an etching effect, which changes both morphology and roughness of the substrate surface, as presented in Figs. 1b-1c. The results show that there were some continuous micro-cracks and holes parallel to the direction of the fibre axis, and the fibre surface was severely eroded. This shows that plasma treatment increased the roughness of the fibre surface due to the etching effect, which increased with increase in oxygen flow rate, i.e. the etching effect shown in Fig. 1c is greater than Fig. 1b. When oxygen flow rate was increased from 0.2L/min to 0.4L/min, the concentration of active species in the plasma jet was increased, making the etching effect more pronounced.
Fig. 1 (a and b). SEM images of (a) control cotton fibres, (b) PT1 specimen, (c) PT2 specimen, (d) PT1-F1 specimen, (e) PT2-F1 specimen, (f) PT1-F2 specimen, (g) PT2-F2 specimen, (h) PT1-F4 specimen, and (i) PT1-F24 specimen
Fig. 1 (c and d). SEM images of (a) control cotton fibres, (b) PT1 specimen, (c) PT2 specimen, (d) PT1-F1 specimen, (e) PT2-F1 specimen, (f) PT1-F2 specimen, (g) PT2-F2 specimen, (h) PT1-F4 specimen, and (i) PT1-F24 specimen