Impact of Finishing Processes on Flame Resistance of Knitted Fabric

Impact of Finishing Processes on Flame Resistance of Knitted Fabric

Ozcan, Gulay


In this study, knitted fabrics are chosen to investigate the effect of their finishing properties on flame resistance. Finishing processes (bleaching, dyeing, and some chemical additives used for bleaching and dyeing) that affect fabric flammability are defined and an experimental program is followed. The level of flammability of the treated fabrics is measured in accordance with the BS 5438 vertical flammability test method because its results are very similar to real fabric burning behavior. Finally, experimental results are evaluated according to a statistical analysis with the SPSS program, and how and at which level chemical parameters affect knitted fabric burning behavior is defined.

The material first ignited in residential fires is usually a textile, wood, or paper, accounting for approximately 63% of residential fires and 70% of residential fire fatalities. When we speak of textiles, the first ignited and easily burnt material is clothing fabric (90.6%). In sequence, furniture fillings and mattress materials (6%), upholstery fabrics (1.7%), and the other decorative fabrics (1.7%) follow [5, 7].

Use of reduced flammability materials, testing of both materials and complete products, regulations, and legislation have contributed to reducing the risk of accidental injury, death, or loss from clothing materials. But these solutions have not been accepted by a significant number of people because of restricted personal freedom, reduced aesthetic qualities, and significantly high additional product costs [6].

Therefore, it is most important to define how fibers and fabric construction affect a product’s final burning behavior in order to direct production. In this way, it will be possible to produce final products with suitable physical and burning properties for end use. In a previous study [10], we tried to define the effect of grey fabric properties on the flame resistance of knitted fabrics. In this study, we investigate the impact of finishing processes on the flame resistance of knitted fabrics, to show the effect of both physical and chemical parameters on the flame resistance.

Fabric flammability is affected by various factors such as fiber content, fabric construction, oxygen concentration, and the environment (moisture content, heat, air flow, etc.), but the effects of finishing materials on fabrics cannot be overlooked. These materials can influence the burning behavior of fabrics by enhancing their fuelgenerating capacity, by interfering with any flame-retardant systems present, or indirectly by interacting with associated species such as dyeing-auxiliary residues and printing-binding formulations [8, 11, 12, 13, 15, 19]. It is well known, for example, that the deposition of hard water salts during laundering, especially with nonphosphate laundry products, can increase the flammability of certain materials or prevent existing flame-retardant systems from working [1, 2, 3, 4]. As regards finishing processes, earlier work has shown that acid, vat, and direct dyes and acid fluorescent brightening agents enhance fabric LOI; on the other hand, reactive dyes can either enhance or reduce LOI depending on the mode of application [14, 17, 18]. While we might expect that the presence of nitrogen, sulphur, and/or halogen groups or the presence of metals such as copper and cobalt in dye formulations would increase fabric LOI values, they might also prevent subsequent flammable and volatile gas formation during fabric pyrolysis. Research has suggested that disperse dyes [8], especially disperse azo dyes, might have the most adverse effect on polyester fabric flammability. Laundering additives, for example, bleaches, detergents, and softeners, are flammable materials, and if they are deposited on the fabric surface after washing, may enhance fabric flammability [6, 9, 16].

With this background, it is not unreasonable to expect that the potential flammability hazards of clothing fabrics might be considerably affected by finishing processes. Earlier studies were each based on a single finishing substance and its effects on material flammability. In this study, we use selected knitted fabrics, which have been investigated in the earlier work [10] for the effects of their physical parameters on flame resistance, to determine to what extent certain kinds of finishing processes and their auxiliaries can modify flammability behavior. Thus it is possible to evaluate one kind of fabric for its flammability behavior throughout all possible production steps.


The physical characteristics of three identical grey knitted fabrics are shown in Table I. All fabrics were obtained in their greige state with no finishing. After scouring, they were treated with specific finishing processes. At the end of the treatments, all samples were rinsed in cold water and air-dried.


Bleaching: Cotton 30/1 single jersey fabrics were bleached with H^sub 2^O^sub 2^ (Merc, 50%) at 0.2, 0.4 and 0.6% add-on. Formulation details are given in Table II.

Fluorescent brightening: In order to investigate the effect of optical brightening agents on the flame resistance of the knitted fabrics, we selected two fluorescent brighteners, Uvitex 2BT (Setas Chemical Co.) and anion active Techowhite NAB Flussig (Toran Kimya Co., TextilColor AG), which are stilbene derivates, and applied them to cotton fabrics at three different levels (0.05, 0.1, 0.2%) in a bleaching + fluorescent brightening bath. During the applications, the H^sub 2^O^sub 2^ level was held constant at 0.4% for each fluorescent brightening level.

Reactive dyeing: Remazol Navy RGB gran 150% reactive dyestuff (Dystar Co., BASF) suitable for dyeing cotton and viscose fibers was applied to cotton and viscose fabrics at 0.5, 1, and 2% concentrations.

Disperse dyeing: Dispersol Navy C-VSE dyestuff (Dystar Co., BASF) was applied to polyester single jersey fabrics at 0.5, 1, and 2% concentrations according to the supplier’s procedure.

Softening finishes: These finishes give the fabric a soft, more desirable handle with a good smoothness and improved sewability characteristics. In current textile practice, anionics, quaternary cationics, nonquaternary cationics, and nonionic or neutral softening agents are recognized and used. In this study, cationic KF 94, nonionic WF 95 (Setas Chemical Co.), and micro emulsion Softycon MES silicones (Toran Kimya Co., TextilColor AG), and cationic Hagesoft KD, nonionic Hagesoft NT (Lefateks Chemical Co., THOR GmbH), and amphoteric Softycon PWSK (Toran Kimya Co., TextilColor AG) softening agents were selected and applied to cotton fabrics in a bleaching bath at five different levels (1, 1.5, 2, 2.5, and 3% for silicones and 1, 2, 3, 4, and 5% for the others).

Anti foamer: In order to determine the effect of antifoamers on flame resistance, a weakly amphoteric TC Entshaeumer TSR-K antifoamer (Toran Kimya Co., TextilColor AG) was applied to cotton fabrics at five different levels (0.1, 0.2, 0.3, 0.4, and 0.5 g/l) in a bleaching bath (at 0.4 H^sub 2^O^sub 2^%).

Antipilling: Toracell ASW-1 acid cellulase enzyme (Toran Kimya Co., TextilColor AG) was selected to study the effect of enzyme application on fabric flammability. The antipilling agent was applied to cotton fabrics at 0.5, 1, 1.5, 2, and 2.5% concentrations.

All finishing processes were applied to fabrics according to their commercial use range at a 20:1 liquor to fabric ratio in an Ahiba laboratory dyeing machine. Finishing formulations and after-treatments are described in Table II. The flammability test was replicated four times for each sample, so in all, 297 samples were tested to evaluate the effects of finishes.

Since a 90° orientation may be assumed to correspond most closely to the situation of textile fabrics used in garments, the BS 5438 vertical flammability test method was chosen to evaluate fabric flammability. Details of the test apparatus and procedure can be found in our earlier publication [10]. Each sample was conditioned at 20 ± 2°C and 65 ± 2% relative humidity for 24 hours before testing. Ignition times and flame spread speed times for 60 cm were determined for all specimens, and the flame spread speed (R, mm/s) of the fabrics was measured together with char length and burning time.

Once we acquired the results, we treated them statistically with the help of analysis of variance (ANOVA) tables to indicate the significance of the finishing treatment effects, then used a regression analysis to show the relations between finishing process and fabric flammability. A variable, i.e., bleaching, dyeing, . . . , was considered significant if the probability level was 0.05 or less and highly significant if the probability level was 0.01 or less.


Bleaching: The polynomial regression analysis (F = 25.8, R^sup 2^ = 79.9%, sig. [much less than] 0.01 for flame spread speed) indicates that the bleaching process considerably influences the burning behavior of the fabrics. Figure 1 shows that flame spread speed increases significantly as the peroxide level increases. This increase in the burning rates can probably be attributed to the enhanced fuel effect of clean cotton fabric, its contaminants and inorganic salts eliminated by the bleaching process. The highest burning rates were for 0.4 and 0.6% add-ons.

The results of our polynomial regression analysis (R^sup 2^ = 96%, sig. [much less than] 0.01) showed that ignition times of the fabrics were significantly affected by the bleaching process. The ignition time of the grey cotton fabric decreased by 1.5 seconds at an 0.2 H^sub 2^O^sub 2^ % level.

Fluorescent brightening: The results from the analysis of variance and polynomial regression analysis (R^sup 2^ = 21%, sig. = 0.074 for Uvitex and R^sup 2^ = 0.3%, sig. = 0.876 for Techowhite, Figure 2) showed that the effect of the brightening agents at the levels given before had no significant effect on flame spread speed and ignition times of the knitted fabrics. We assume that the little shifts observed in the flame-spread speeds are probably caused by H^sub 2^O^sub 2^ in the bleaching bath (see Figure 2).

Reactive dyeing: Our polynomial regression analysis (R^sup 2^ = 96.6%, sig. [much less than] 0.01 for cotton fabrics and R^sup 2^ = 0.37%, sig. = 0.781 for viscose fabrics) suggests that the flame spread speeds of the fabrics are highly affected by the dye levels. Increased dye add-ons promote increased flame spread rates in cotton fabrics, but there is no significant change in flame spread speeds with dye level for viscose fabrics (see Figure 3).

We believe that the effect of a reactive dye on cotton fabric flammability is possibly caused by some agents removed by the dyeing procedure, but not by pre-scouring. We think an increased caustic concentration in the dyebath to increase shade depth is responsible.

Disperse dyeing: Our flammability test results show that there is no change in the ignition times of the PET fabrics at the dye level we used. All samples ignite at 2 seconds, as shown in undyed PET fabrics. On the other hand, increased dye add-ons cause significant increases in the flame-spread speeds of the fabrics (R^sup 2^ = 79.1%, F = 24.59), as illustrated in Figure 4. It is also interesting that the samples with 1.5 and 2% add ons have close flame spread rates. This result tends to confirm the results for disperse dyes reported previously [8].

Softening finishes: Our test results show that there is no effect of softening agents on ignition times of the fabrics at these add-on levels, but the polynomial regression analysis suggests that the level of softening agent has a significant effect on flame spread speeds of the fabrics. The highest flame spread speeds are for a weakly cationic microemulsion (MES) of silicone softener and nonionic softening agent. The lowest speeds, on the other hand, are for nonionic silicone (see Figure 5).

All softening agents cause the flame spread rates of the fabrics to increase dramatically. Burning rates of the fabrics increase from 18-20 to 35-42 mm/s. This effect may result from the fact that softening agents are already highly flammable materials because they include fatty acid condensation products and/or fatty acid derivates.

Effect of antifoaming agent: Any agent that changes the surface state of a system from a condition that favors foaming to a condition that does not can be classed as an antifoamer. These agents are generally used in textile finishing processes to achieve foam stability in the finishing bath or printing paste. F = 40 and R^sup 2^ = 79.2% for flame spread rates show that the use of an antifoamer adversely influences fabric flammability (see Figure 6). Ignition times of the fabrics are constant at 3 seconds for all antifoamer levels.

Increased add-ons of antifoamer cause significant increases in the flame spread rates of the fabrics. Again, burning rates are high (above R = 30 mm/s) for all antifoamer levels. The actual effect of the antifoamer in this respect may depend on its already flammable structure due to fatty acid esters, soap, and high molecular hydrocarbon mixtures.

Antipilling: The flammability test and statistical analysis results (R^sup 2^ = 74.9%, F = 31.41) indicate that there is a significant increase in flame spread speeds with increased antipilling add-ons. When an add-on is greater than 1%, antipilling elevates the burning rates above 35 mm/s. On the other hand, ignition times of the fabrics do not change with antipilling concentrations. The relation between antipilling concentration and fabric flammability is illustrated in Figure 7.


In this study, we have investigated the effect of finishing processes on the flame resistance of knitted fabrics. We report the effect of finishing processes (hydrogen peroxide bleaching, fluorescence brightening, reactive dyeing, and disperse dyeing) and additives (softening, antifoaming, and anti-pilling agents) that are often used in knitted fabric finishes on the flame resistance of knitted fabrics. To do so, we subject 297 treated knitted fabric specimens to vertical flammability tests, and evaluate the results according to ANOVA and polynomial regression analysis using the SPSS statistical program.

The information we have acquired in this research shows that the finishing processes considerably influence knitted fabric flammability, especially in terms of flame spread speeds. Increased H^sub 2^O^sub 2^ levels increase the burning rates of the fabrics from 12 to 20-22 mm/s. However, unlike the findings of the previous works, which noted that if the fluorescent brightening is applied at higher levels, increased LOI occurs, we have found that the brighteners do not cause remarkable changes in the burning behavior of the knitted fabrics with respect to the bleached fabrics. Reactive dyes have an adverse effect on the burning behavior of the cotton fabrics and enhance their burning rates. We assume that this burning behavior of dyed cottons depends not on the reactive dye level but on the increased caustic concentrations of the dyebath with shade depth. Application of the disperse dye to the PET knitted fabrics elevates flame spread speeds from 5.5 to 10 mm/s by approximately 100%. This result again verifies that disperse dyes have an important adverse effect on PET fabric flammability.

Both silicones and softeners raise burning rates by 50% for bleached fabrics. Antifoamers behave like softeners and increase flame spread rates of bleached fabrics by 60-70%. Application of antipilling agents to cotton knitted fabrics beginning at the first application level causes fabric flammability to increase by 50-70%. This may result from cotton cellulose damaged by the antipilling process. The dramatic increase in the burning rates with softeners, silicones, antifoamers, and antipilling agents suggests that the smallest working range or other new agents that are less flammable should be used.

Another important conclusion from this work is that ignition times of the treated knits decreases by 1.5-2 seconds with the first finishing application, and are then constant for all finishing levels. This effect may be attributed to the considerably reduced flame resistance of the grey fabric at the first finishing level. Finishes at higher add-on levels have only a modest effect on the ignition times of fabrics.


We wish to thank Auburn University, Textile Engineering Department, Alabama, for their support and encouragement during the course of this work. We also extend our appreciation to Toran Kimya Co., Lefateks Chemical Co., Gemsan Chemical Co., Setas Chemical Co., and Dystar Textile Dyes Company, Turkey, for chemical support and Bilkont Company, Turkey, for giving permission to use their plant for producing the test fabrics. Thanks are due the I.T.U Textile and Clothing Quality Control and Research Laboratory staff for their support in the experimental work.

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Manuscript received January 28, 2003; accepted July 3, 2003.


Textile Engineering Department, Istanbul Technical University, Istanbul, Turkey

Copyright Textile Research Institute Jun 2004

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