Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process

Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process

Accepted Manuscript Title: Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process Autho...

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Accepted Manuscript Title: Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process Authors: Xian-Wei Cheng, Jin-Ping Guan, Xu-Hong Yang, Ren-Cheng Tang PII: DOI: Reference:

S0040-6031(18)30211-9 https://doi.org/10.1016/j.tca.2018.05.011 TCA 77999

To appear in:

Thermochimica Acta

Received date: Revised date: Accepted date:

12-3-2018 12-5-2018 14-5-2018

Please cite this article as: Cheng X-Wei, Guan J-Ping, Yang X-Hong, Tang RCheng, Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process, Thermochimica Acta (2018), https://doi.org/10.1016/j.tca.2018.05.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Durable flame retardant wool fabric treated by phytic acid and TiO2 using an exhaustion-assisted pad-dry-cure process

Xian-Wei Cheng, Jin-Ping Guan, Xu-Hong Yang*, Ren-Cheng Tang* National Engineering Laboratory for Modern Silk, College of Textile and Clothing

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Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China

*E-mail: [email protected]; [email protected]. Tel.: +86 512 6716

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4993; Fax: +86 512 6724 6786.

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Graphical Astract

Highlights 

“Green” phytic acid and TiO2 were used as flame retardants.

The FR system was applied by an exhaustion-assisted pad-dry-cure approach.



Phytic acid and TiO2 had a joint flame retardant effect.



The treated wool showed durable flame retardancy and higher thermal stability.



The flame retarding activity occurred in the condensed phase.

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Abstract A novel and durable organic-inorganic flame retardant (FR) system for wool fabric was fabricated using natural phytic acid (PA), titanium dioxide (TiO2) nanoparticles and 1,2,3,4-butanetetracarboxylic acid (BTCA). In order to address the diffusion barrier action of wool fiber to the FR agents, an additional exhaustion process was employed ahead of the traditional pad-dry-cure procedure. The flame retardancy, heat release,

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smoke generation, thermal stability, and washing durability of the treated fabrics were

discussed. The PA/TiO2/BTCA system endowed wool fabric with excellent flame

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retardancy and washing durability. The treated fabric was still able to self-extinguish after 30 washing cycles. Moreover, the FR system effectively reduced the smoke generation capacity. Thermogravimetry test shows that the FR system altered the thermal

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decomposition behavior of wool, and the formation of thermal protective intumescent

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char mainly contributed to the improved flame retardancy, revealing a significant

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condensed-phase FR mechanism of the treated wool.

Abbreviations

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The following abbreviations are used in this paper: 1,2,3,4-Butanetetracarboxylic Acid

Ds

Specific Optical Density

Dsmax

Maximum Ds Flame Retardant

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FR

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BTCA

Fourier Transform Infrared

ICP-OES

Inductively Coupled Plasma Optical Emission Spectrometer

LOI

Limiting Oxygen Index

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FT-IR

owf

on the weight of fabric

PA

Phytic Acid

PCFC

Pyrolysis Combustion Flow Calorimetry

SEM

Scanning Electron Microscopy

SHP

Sodium Hypophosphite

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TG

Thermogravimetry

TiO2

Titanium Dioxide

Keywords: Wool; Phytic acid; Titanium dioxide; Thermal stability; Flame retardancy;

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Exhaustion

1. Introduction

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Wool is a well known animal fiber for its wearing comfort, hypoallergenic, breathable, insulating and flame resistant properties. In daily lives, wool is widely used to produce clothing and interior textiles such as carpets, blankets, curtains, upholsteries, and

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beddings. Although wool fiber has a certain level of flame retardant (FR) property due to

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its high nitrogen (15-16 wt%) and sulfur content (3-4 wt%) as well as high moisture

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content (10-14 wt%) [1], it is still flammable. Once wool textiles are ignited, flames will

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spread in them. The flammability of wool impedes its specific applications in aircraft carpets and upholsteries, protective clothing, and wall coverings. Apparently, wool

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textiles require further FR treatment to enhance their flame retardancy and thermal stability.

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The Zirpro FR system, which was developed by the International Wool Secretariat, has become the most commercially available FR method for wool [2,3], due to the following

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advantages: (1) the exhaustion process is flexible and can be conducted during or after the normal acid-dyeing process; (2) the exhaust is rapid at mild condition, and the low treatment temperature (about 60 oC) can limit the felting of wool; (3) this treatment does

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not exert discoloration or other adverse effects on wool aesthetics [4,5]. However, the Zirpro process has been involved in an intense discussion between environmentalists and industrialists as a consequence of releasing heavy metal ions into wastewater effluent [6]. From the viewpoint of reducing environmental impact and toxicity, some intumescent FR chemicals such as ammonium phosphates, N-methylol dimethyl phosphonopropionamide derivatives and tetrakis-hydroxymethyl phosphonium condensates were investigated to

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enhance the FR performance and thermal stability of wool fabric [7–9]. In our previous work, phytic acid (PA), an environmentally benign compound extracted from plant tissues, was successfully applied as a phosphorus-based FR agent to impart flame retardancy to wool fabric [10]. The PA-treated wool fabric had significantly improved FR performance and thermal stability, but its durability to washing could not meet the high FR demand because of the water solubility of PA. In order to improve the

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FR efficiency and washing durability, titanium oxide (TiO2) nanoparticles were

considered to incorporate with PA to fabricate organic-inorganic FR system. Indeed, in

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recent years, TiO2 has found its wide application in self-cleaning, photocatalysis, UV protection and antibacterial processes due to its excellent physicochemical properties such as high chemical stability, good heat resistance, long lasting, non-toxicity, and broad-

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spectrum antibiosis [11,12]. Similar to other inorganic nanomaterials, TiO2 is one of the

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promising adjuvant FR agents, owing to its capacity to promote the formation of a dense

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protective physical barrier to heat, oxygen and mass transfer, thus hindering the release of flammable volatile species [13,14]. PA contains 12 hydroxyl groups and 6 phosphate

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groups, and such unique molecule structure enables PA to chelate with many metal ions

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including TiO2 [15–17]. Due to the high phosphorus content and possible interaction with TiO2, PA is prospected to act as a promising organic phosphorus source for the formation of the organic-inorganic FR system. In this context, PA and TiO2 were applied together to

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improve the flame retardancy of silk fabric, with 1,2,3,4-butanetetracarboxylic acid (BTCA) acting as a cross-linking agent to improve the adhesion of TiO2 on silk surface

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[18]. The traditional pad-dry-cure process was exploited because BTCA is to be effective at high curing temperatures [19,20]. The treated silk fabric exhibited significantly

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increased FR performance and excellent washing durability. The aforementioned research progresses aroused us to apply the PA/TiO2/BTCA

system to develop durable FR wool fabric. However, wool fiber is covered by the protective cuticle layer. The diffusion barrier action of cuticle layer has an adverse impact on the efficient and uniform chemical processing of wool. However, the modifications that can reach the fiber interior, rather than simple physically applied to the fiber surface, exhibit more potential to offer good durability. Taking the fact into consideration that the 4

high immersing temperature is helpful to increase the swelling extent of wool fiber and thus promote the diffusion of chemicals into fiber interior [21], a pre-exhaustion process was carried out to enhance the accessibility of FR agents to wool fiber in this work. In the present study, two FR treatment approaches, namely the pad-dry-cure process and the exhaustion-assisted pad-dry-cure process, were investigated and compared in order to fabricate the durable FR wool fabric. The effect of TiO2 dosage on the

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flammability of wool fabric was discussed. The flame retardancy and thermal stability of the treated fabrics were assessed by limiting oxygen index (LOI), vertical burning test,

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smoke density test and thermogravimetry (TG). Scanning electron microscopy (SEM)

was used to investigate the morphologies of the treated fabrics and the chars. Furthermore, inductively coupled plasma optical emission spectrometer (ICP-OES) was applied to

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evaluate the FR components of the burned and unburned wool samples.

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2. Experimental section

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2.1. Materials

The woven wool fabric (warp and weft count, 156 dtex × 2; warp density, 21 threads

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cm-1, and weft density, 18 threads cm-1; weight per unit area, 125 g m-2) was obtained from Shanghai Textile Industry Institute of Technical Supervision, Shanghai, China.

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Phytic acid (PA, 70% aqueous solution) was purchased from Chengdu Ai Keda Chemical Technology Co. Ltd., Chengdu, China. The chemical structure of PA is presented in Fig.

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1. The anatase TiO2 nanoparticles (99.8% metals basis) with an average diameter of 40 nm were obtained from Aladdin Chemical Reagent Co. Ltd., Shanghai, China. BTCA and sodium hypophosphite (SHP) were provided by Sinopharm Chemical Reagent Co. Ltd.,

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Shanghai, China. The detergent specially developed for the washing of wool textiles was obtained from Shanghai Zhengzhang Laundering and Dyeing Co. Ltd., Shanghai, China.

2.2. Fabric treatment procedure Preparation of PA/TiO2/BTCA mixture: Firstly, PA and BTCA were dissolved in 100

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mL deionized water, and then TiO2 nanoparticles were added into the solution. The PA/TiO2/BTCA mixture was dispersed by a FA25 high shear dispersing homogenizer (Fluko Equipment Shanghai Co. Ltd., Shanghai, China) for 10 min with a speed of 10000 rpm, and then sonicated by SK2510HP sonicator (Kudos Ultrasonic Instrument Co. Ltd., Shanghai, China) at 53 kHz for 10 min. The wool fabrics were treated with the following two methods:

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(a) Pad-dry-cure: The wool fabric was immersed in the bath containing 150 g L-1 PA (equivalent to 425% owf (on the weight of fabric)), 7 g L-1 TiO2 (equivalent to 17.5%

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owf), and 80 g L-1 BTCA (equivalent to 200% owf) for 10 min at room temperature, and then padded using a two-roll laboratory padder. The wet pickup was 100 ± 5% after two

dips and two nips. The padded fabric was oven dried at 80 oC for 3 min, and then cured

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at 160 oC for 3 min. At the end of the treatment, the wool fabric was washed with distilled

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water at 40 oC for 10 min, rinsed with cold distilled water, and then dried at room

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temperature.

(b) Exhaustion-assisted pad-dry-cure: Firstly, the wool fabrics were prepared through

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an exhaustion process: this process was carried out in the conical flasks, which were

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placed in an XW-ZDR low-noise oscillated dyeing machine (Jiangsu Jingjiang Xingwang Dyeing and Finishing Machinery Factory, Jingjiang, China). The liquor ratio was 1:25 (the total volume of the solution was 100 mL). The fabrics were immersed in the solutions

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containing 48 g L-1 PA (equivalent to 120% owf), 20 g L-1 BTCA (equivalent to 50% owf) and a series of TiO2 dosages (0-1.5 g L-1, equivalent to 0-4.25% owf) at 90 oC, and the

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treatment continued for 30 min. Then the wool fabrics were squeezed using a two-roll laboratory padder with a wet pickup of 100 ± 5%. Afterwards, the padded fabrics were dried, cured, and washed according to the procedures described above. In particular,

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Wool-1, Wool-2 and Wool-3 mentioned in the results and discussion section represent the fabrics treated with 0, 0.6 and 1.5 g L-1 TiO2, respectively, together with 48 g L-1 PA and 20 g L-1 BTCA. 2.3. Characterizations 2.3.1. SEM observation The surface morphologies of the wool fabrics as well as the chars obtained from the 6

pyrolysis combustion flow calorimetry (PCFC) experiments (pyrolysis in oxygenated environment) were observed using the TM3030 tabletop scanning electron microscope (Hitachi High Technologies America, Inc., Schaumburg, IL, USA). 2.3.2. FT-IR spectra The fourier transform infrared (FT-IR) spectra were collected from the Nicolet 5700 FT-IR spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) under a

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resolution of 4.0 cm-1 in 32 scans using KBr pellets. 2.3.3. Weight gain

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The wool fabrics were oven dried at 60 oC for 30 min, and then weighed quickly. The weight gain of the treated wool was determined according to the following equation:

Weight gain (%)  100  (W2  W1 ) / W1

(1)

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where W1 and W2 denote the weights of the fabrics before and after treatment, respectively.

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2.3.4. LOI test

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The LOI of the samples was measured according to GB/T 5454-1997 (equivalent to

East Grinstead, UK).

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2.3.5. Vertical burning

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ASTM D2863) using the FTT0080 oxygen index apparatus (Fire Testing Technology Ltd.,

The YG815B automatic vertical flammability cabinet (Ningbo Textile Instrument

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Factory, Ningbo, China) was used to conduct the vertical burning test according to GB/T 5455-2014 (equivalent to ASTM D6413). The test was carried out by applying a methane

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flame (the flame height was 40 mm) for 12 s on the below of specimen (300 mm × 80 mm). After flame time (s), after glow time (s), and char length (cm) were evaluated. The burning behavior of the fabrics for vertical burning test was classified according to GB/T

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17591-2006.

2.3.6. Detection of P and Ti content The P and Ti content of the fibers and char residues were assessed using the ICP-OES

analysis according to our previously reported method [18]. 2.3.7. Durability to washing The FR performance of the treated fabrics subjected to a series of washing cycles was

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evaluated. The washing was conducted according to our previous reported work [18]. Each wash was carried out at 40 oC for 30 min. 2.3.8. Smoke density The smoke generation of the treated and untreated wool fabrics was measured using the FTT0064 NBS smoke density test chamber (Fire Testing Technology Ltd., East Grinstead, UK) according to ISO 5659.2. Each specimen with a dimension of 75 × 75

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mm2 (two layers) was exposed horizontally to an external heat flux of 25 kW m-2 in a flameless combustion mode.

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2.3.9. TG analyses

The thermal stability of the fabrics was assessed using the Diamond TG/DTA SII thermal analyzer (Perkin-Elmer, Waltham, MA, USA) at a controlled heating ramp of 10 oC

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min-1, from 30 up to 700 oC in nitrogen and air (gas flux: 20 mL min-1).

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3. Results and discussion

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3.1. Comparison of the two treatment processes 3.1.1. Morphology of the treated fabrics

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The surface morphologies of the untreated wool fabric as well as the fabrics treated with the PA/TiO2/BTCA system by the pad-dry-cure and exhaustion-assisted pad-dry-

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cure processes were evaluated by the SEM observation. Fig. 2 shows the SEM photos at two magnifications. The fibers included in all the fabrics displayed the typical

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morphology of wool fiber covered by cuticle layer, and the untreated wool fiber (Figs. 2a

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and 2b) had a clean surface.

Figs. 2c and 2d shows the morphology of the fabric treated by the pad-dry-cure process,

from which it was found that lots of FR agents were attached on the fiber surface. However, for the fabric treated by the exhaustion-assisted pad-dry-cure procedure, only a certain inhomogeneous deposits of FR agents was found. The different surface morphologies of two fabrics can be ascribed to the substantial differences between the

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pad-dry-cure and exhaustion processes. Indeed, during the pad-dry-cure process, the concentration of the FR agents in the solution was high, and the exocuticle layer of wool fiber prevents FR agents from penetrating into the fiber interior, resulting in the deposition of most of FR agents on the fiber surface. However, during the exhaustion process, the high immersing temperature was able to increase the swelling extent of wool fiber and promote the diffusion of FR agents into the fiber interior. Therefore, the wool fabric

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treated by the exhaustion-assisted pad-dry-cure process exhibited at least as good FR

performance as the fabric treated by the pad-dry-cure process (discussed later), although

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little FR agents were attached on the fiber surface. Thus, the exhaustion-assisted pad-drycure process was preferred in the present study. 3.1.2. FT-IR of the treated fabrics

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The FT-IR spectra of the treated and untreated wool fabrics were determined to

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investigate the possible interactions among PA, TiO2, BTCA and wool fiber. Fig. 3 shows

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the spectra of the untreated wool fabric (spectrum a), the wool fabrics treated with

(spectrum c) procedures.

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PA/TiO2/BTCA by the pad-dry-cure (spectrum b) and exhaustion-assisted pad-dry-cure

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Compared with the spectrum (spectrum a) of the untreated wool fabric, the spectra

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(spectra b and c) of the wool fabrics treated with PA/TiO2/BTCA showed new peaks at around 1168 and 1060 cm-1 are ascribed to the stretching vibration of P=O and O-P-C

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structures of PA, respectively [22,23]. The new peak at around 1714 cm-1 can be attributed to the overlap of the ester and carboxyl carbonyl bands [24–26]. In addition, the crosslinking reaction between the carboxyl groups of BTCA and the amino groups of wool

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fiber also contributed to the excellent washing durability of the treated wool (discussed later) [20,27]. However, the formed amide groups were not observed in the FT-IR spectra because their absorption peaks were overlapped by the original peptide absorption. No obvious difference was found between the spectra of the wool fabrics treated by the two methods. According to the FT-IR analyses as well as the previous reports, the speculated 9

interactions among PA, TiO2, BTCA, and wool fiber are described as follows: (a) the cross-linking reactions between BTCA and wool fiber; (b) the strong electrostatic attractions between the negatively charged phosphate groups of PA and the positively charged amino groups of wool fiber; (c) the strong ionic bonds between TiO2 and the residual carboxyl groups of BTCA on wool fiber [27–29]; (d) the strong ionic bonds between TiO2 and the phosphate groups of PA adsorbed by wool fiber.

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3.1.3. Flammability of the treated fabrics

In this work, the LOI and vertical burning tests were applied to evaluate the

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flammability of the treated wool fabrics. Table 1 shows the weight gain and LOI of the

wool fabrics treated with different formulations containing PA (150 g L-1), BTCA (80 g L-1) and TiO2 (7 g L-1) by the pad-dry-cure method. When treated with 80 g L-1 BTCA

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and 40 g L-1 SHP, the wool fabric obtained a weight gain of 6.7%, and had a LOI of 24.5%

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which is slightly higher than 23.6% of the untreated wool. The little increase in FR ability

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is preliminarily regarded as a result of the change in the fine structures of wool fiber induced by the cross-linking, and/or as the introduction of the tiniest P element into wool

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fiber caused by SHP. The wool fabric treated with the PA/TiO2/BTCA system obtained

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higher weight gain than the wool treated with the PA/TiO2 system due to the cross-linking of BTCA with wool fiber. In addition, the fixation of TiO2 on wool fiber by BTCA may also contribute to the increased weight gain. Thus, the increased LOI with the addition of

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BTCA can be attributed to the introduction of BTCA and/or the immobilization of TiO2 on wool fiber. However, the PA/BTCA treated wool showed higher weight gain but

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similar LOI like the PA treated wool. These results above demonstrate that BTCA had little influence in the FR effect, and the immobilization of TiO2 on wool fiber mainly

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contributed to the increased FR ability.

The FR efficiency and durability of the PA/TiO2/BTCA system which was applied by two different procedures were also discussed in this section. As shown in Table 2, the wool fabric treated with PA/TiO2/BTCA (150/7/80 g L-1) by the traditional pad-dry-cure method obtained a weight gain of 13.6%, and had an LOI of 32.7%. The treated wool 10

fabric showed obviously improved FR performance, but its FR effect was not resistant to washing. After 5 washing cycles, the LOI value of the treated wool fabric reduced to 27.4%. When treated with a pre-exhaustion process, the fabric obtained better flame retardancy at lower dosages of flame retardants. As shown in Table 2, when treated with PA/TiO2/BTCA (48/0.6/20 g L-1) by the exhaustion-assisted pad-dry-cure method, the wool fabric had an LOI value of 34.4%. Moreover, the treated wool fabric still presented

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a high LOI value of 30.2% after 5 washing cycles, displaying good washing durability.

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During the vertical flammability test, the untreated wool fabric got ignited easily, and then it was engulfed in flame, burning rapidly and vigorously. The fabric consumed

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completely within the ignition time, thus having no after flame time and after glow time,

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and displaying a full char length of 30 cm. On the contrary, the fabrics treated with

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PA/TiO2/BTCA showed great char formation ability as the treated wool substrates swelled and expanded rapidly when ignited. The inflated char was able to withstand the open fire

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below and protect the following upper substrate. Fabric burning ceased before the ignition

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source was removed, and thus the treated fabrics also had no after flame and after glow. The treated fabrics exhibited significantly reduced char length, which can be ascribed to the protective role of PA and TiO2 deposited on the fabric that favor the char formation

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instead of the production of volatile species which can further support the combustion. As shown in Table 2, the two treated wool fabrics had a char length about 10 cm. After

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5 washing cycles, the fabric treated by the traditional pad-dry-cure method was seriously damaged in the vertical burning test, and presented a char length of 21.0 cm, which means poor FR ability. For the fabric treated by the exhaustion-assisted pad-dry-cure method, it

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was still able to self-extinguish after 5 washing cycles and the char length had little increase. According to the results above, the PA/TiO2/BTCA system showed more efficient when applied by the exhaustion-assisted pad-dry-cure process in the present study. In the pad-dry-cure process, the protective cuticle layer of wool fiber prevented PA, TiO2 and BTCA from penetrating into the fiber interior, so most of the FR agents were bonded or attached on the fiber surface as confirmed by the SEM micrographs in Fig. 2. 11

The FR agents deposited on the fiber surface can be easily washed off, thereby leading to the poor washing durability of the treated fabric. However, the high immersing temperature (90 oC) was able to increase the swelling degree of wool fiber and thus promote the diffusion of FR agents into fiber interior. Interestingly, because of the surface barrier action of wool fiber, it is difficult for the internal FR agents to release into the washing media at 40 oC, so the wool fabric treated by the exhaustion-assisted pad-dry-

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cure method exhibited good washing durability.

3.2. Characterization of the wool fabrics treated by the exhaustion-assisted pad-dry-

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cure process 3.2.1. Flammability and durability

Fig. 4 shows the weight gain and FR performance of the wool fabrics treated with 48

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g L-1 (equivalent to 120% owf) PA, 20 g L-1 BTCA, and TiO2 (0-1.5 g L-1) by the

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exhaustion-assisted pad-dry-cure method. The addition of TiO2 gave rise to an increase

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in the weight gain of wool fabric. As expected, the treated wool fabric exhibited the increased FR ability as indicated by increasing LOI and decreasing char length that were

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companied by increasing TiO2 dosage. Furthermore, the treated fabric at 1.5 g L-1 TiO2

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had a LOI of 36.1% which was much higher than 23.6% of the untreated fabric. The results above reveal that the PA/TiO2/BTCA treatment can significantly improve the FR

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performance of wool fabric.



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The Ti and P content of the treated wool fabrics was determined by ICP-OES, and the

results are shown in Fig. 5. The Ti content of the wool fabrics increased with increasing TiO2 dosage, and the P content showed little increase. Thus, the increased FR

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performance discussed above could be attributed to the increased Ti content of the treated wool, and the construction elements P and Ti were also suggested to have a joint FR effect. The adjuvant titanium was capable to give a stronger and more cohesive char with higher yield and probably functioned by a physical bridging effect in the char, thus increasing the FR ability of wool. 12

The washing durability of the wool fabric treated with the PA/TiO2/BTCA (48/0.6/20 g L-1) system is shown in Fig. 6. The treated fabric showed an obvious reduction in LOI within 5 washing cycles, and in the following repeated washing cycles, the LOI value of the treated fabric declined slowly due to the gradual release of FR agents penetrating into the fiber interior and being bound to the fibers. On the other hand, the char length

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gradually increased with increasing washing cycles. However, the treated fabric was still able to self-extinguish and achieve the B1 classification after 30 washing cycles. On the

possessed desirable FR ability and washing resistance.

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whole, according to the LOI and vertical burning results, the PA/TiO2/BTCA treated wool

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3.2.2. Smoke generation performance

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Smoke in real fires is fatal due to the limitation of visibility by particles and the cause of hypoxia and coma by toxic carbon monoxide, and thus impede escape from a fire [30].

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In this section, the NBS smoke density chamber was used to measure smoke development

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for wool fabrics, and some parameters were obtained, including the specific optical density (Ds), the maximum Ds value (Dsmax), and the times to reach Dsmax. The Ds value is obtained by measuring the transmission of light through a photometric system and has no

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unit [31]. Lower Ds value means less smoke generation. Fig. 7 presents the variation of Ds with time for the treated and untreated wool fabrics. The Dsmax values and times to

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reach Dsmax are collected in Table 3. Due to the poor char formation ability, the untreated wool fabric generated heavy

smoke and unpleasant burnt hair smell. As shown in Fig. 7, the smoke release of the

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treated wool was restrained in the whole combustion process as the Ds value of the treated wool was much lower than that of the untreated wool. For Wool-1, Woo-2 and Wool-3, their Dsmax values decreased to 34.7, 32.1 and 21.4, respectively from 93.7 of the untreated fabric. The excellent smoke suppression effect is ascribed to the formation of intumescent carbon layer on the surface of the fibers during combustion, creating a physical protective barrier to restrain the release of pyrolysis gases and smoke particles efficiently. 13



3.2.3. Thermal behavior The thermal and thermal-oxidative stability of the treated and untreated wool fabrics were assessed by the TG technique. The TG curves of all samples in both nitrogen and

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air atmosphere are shown in Fig. 8. The thermal decomposition data are collected in Table 4, where T20% and T50% are defined as the temperatures corresponding to a weight loss of

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20% and 50%, respectively.

As shown in Fig. 8a, wool exhibited two stages of weight loss in nitrogen. The first weight loss took place below 100 oC, due to the desorption of the physically bound water

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and the dehydration of wool fiber. The second weight loss was in the temperature range

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of 220 to 425 oC, during which the hydrogen-bond peptide helical structure rupture and

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the ordered regions of wool undergo a solid-to-liquid phase change. The fibers disaggregated and converted to aromatic structures, together with the production of water,

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methane, carbon monoxide and carbon dioxide. At the same time, the disulphide bonds

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cleaved and released a number of volatiles including hydrogen sulfide and sulfur dioxide [8,32,33]. In air, the thermo-oxidation of wool proceeded in a similar way, whose difference from the degradation in nitrogen was the presence of a third degradation step

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at higher temperature. This phenomenon can be ascribed to the oxidation of the formed char and remaining hydrocarbon species.

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As revealed by T20% and T50% in Table 4 and TG curves in Fig. 8, the introduction of

PA (Wool-1) increased the decomposition temperature of wool in both nitrogen and air, although PA showed lower thermal stability than wool below 350 oC, indicating that PA

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alters the decomposition behavior of wool. It is supposed that during the thermal degradation, the phosphate groups of PA induce the dehydration of wool, giving rise to the formation of a thermal protective char on wool substrates. The thermal stable char layer protects wool substrates from being attacked by heat energy and oxygen, and thus inhibits the further decomposition of wool. Moreover, the treated wool fabrics exhibited higher thermal stability and char residues owing to the introduction of TiO2. Although the 14

final reaction product of PA andTiO2 in the heating process can not de determined at present, it is predicted that a joint FR effect exists between titanium and phosphorus, and leads to a positive effect for increasing the char residue of the treated wool at high temperatures. Phosphorus present in the degradation products acts as char-promoter, contributing to the thermal shield effect of titanium. The approach turns out to be efficient for enhancing the flame retardancy of wool, as

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the present FR system favors the dehydration of wool toward the char formation, limiting or blocking the depolymerization of the main chain to volatile species that can fuel the

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combustion. Thus, it is concluded that the FR system enhances the flame retadrancy and thermal stability of wool by condensed phase.

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3.2.4. Char residues analyses

The char residues of a material provide important information about its FR efficiency

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and FR mechanism. As shown in Fig. 9, the dome-like structures were formed at the end

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of the PCFC test. The untreated wool showed a porous, fragile and thin char layer, possibly as a consequence of insufficient char formation because the untreated wool burned completely, thus leaving a low amount of char residue. No original fibrous

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structure was found for the untreated wool. During the pyrolysis, lots of violates were produced from the degradation of wool substrates, and partially trapped in the char layer.

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The char expanded due to the increasing internal pressure, and exploded easily due to the

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low viscoelastic and thermal stability, leading to the formation of a series of voids.

For the fabric treated with PA/BTCA, some additional fibrous chars were found

although the original structures were seriously damaged during the pyrolysis. Intumescent-like bubbles were also found on the char residues of the treated wool fibers. With the addition of TiO2, the treated wool obtained more stable, compact and intumescent char layer, as well as more fibrous structures. The intumescent structures, of 15

which the viscosity was high enough to adapt the internal stresses caused by the produced gases, could act as an insulting shield to slow down the release of combustible volatiles and prevent heat energy from penetrating to reach inner substrates, thereby leading to the good FR and thermal stable performance. The Ti and P content of the char residues were also evaluated using ICP-OES. Similar to the fabrics, as shown in Fig. 10, the Ti content of char residues exhibited an obvious

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increase as a function of TiO2 dosage, and the P content showed small increase. It is noteworthy that the Ti and P content of the char residues were much higher than those of

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the wool fabrics, demonstrating that the Ti and P elements act in the condensed phase,

and then reserved in the char residues after burning. During the combustion, the Ti and P elements catalyze and participate in the formation of the thermal protective char layer

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which acts as an insulating shield to prevent combustible gases and heat energy from

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permeating to reach the inner substrates, leading to good flame retardancy. The high char

agents occurs in the condensed phase.

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formation ability of the treated fabrics reveals that the flame retarding activity of the FR

4. Conclusions

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In the present study, an innovative organic-inorganic system consists of PA and TiO2 was exploited to improve the FR performance and thermal stability of wool fabric. BTCA

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was used as a cross-linker to improve the adhesion of TiO2 nanoparticles on wool fiber. The PA/TiO2/BTCA system showed more efficient and durable when applied using the exhaustion-assisted pad-dry-cure process because more FR agents could penetrate and

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diffuse into the interior of wool fiber at the high temperature immersing stage. The multiple interactions among PA, TiO2, BTCA and wool fiber as well as the pre-exhaustion process contributed to the good washing resistance of the treated wool. PA and TiO2 were found to have a joint FR effect as P favored the formation of intumescent char and Ti functioned as a physical bridge to consolidate the formed char. The treated wool fabric exhibited excellent flame retardancy, and much lower smoke generation. The condensed

16

phase FR mechanism was applicable to the treated wool due to its high charring ability, and the participation of Ti and P in the formation of the thermal stable char. Although it is believed that the treatment has little impact on human health and environment, further investigations should be performed on this issue.

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Acknowledgements This study was funded by Jiangsu Provincial Key Research and Development Program of China (BE2015066), Postgraduate Research and Practice Innovation Program of

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Jiangsu Province (KYCX17_1986), and the Priority Academic Program Development

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(PAPD) of Jiangsu Higher Education Institutions (No. 2014-37).

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Calorim. 104 (2010) 717–724. [13] J. Alongi, R.A. Carletto, F. Bosco, F. Carosio, A. Di Blasio, F. Cuttica, V. Antonucci, M. Giordano, G. Malucelli, Caseins and hydrophobins as novel green flame retardants for cotton fabrics, Polym. Degrad. Stabil. 99 (2014) 111–117. [14] G. Malucelli, F. Carosio, J. Alongi, A. Fina, A. Frache, G. Camino, Materials engineering for surface-confined flame retardancy, Mat. Sci. Eng. R 84 (2014) 1–20.

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Legends for Figures and Tables Fig. 1. Chemical structure of PA. Fig. 2. SEM micrographs of the untreated wool fabric (a, b), and the wool fabrics treated by the pad-dry-cure (c, d) and exhaustion-assisted pad-dry-cure (e, f) processes (right: 1.0 k; left: 2.5 k).

pad-dry-cure (b) and exhaustion-assisted pad-dry-cure (c) processes.

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Fig. 3. FT-IR spectra of the untreated wool fabric (a), and the wool fabrics treated by the

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Fig. 4. Weight gain and FR performance of the wool fabrics treated with the PA/TiO2/BTCA system at various TiO2 dosages (PA 48 g L-1, BTCA 20 g L-1) by the exhaustion-assisted pad-dry-cure method.

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Fig. 5. Ti and P content of the wool fabrics treated with the PA/TiO2/BTCA system at

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various TiO2 dosages.

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Fig. 6. LOI and char length of the treated wool fabrics subjected to repeated laundering. Fig. 7. Ds curves of the wool fabrics obtained by the smoke density test.

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Fig. 8. TG and DTG curves of the wool fabrics in nitrogen (a) and air (b).

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Fig. 9. SEM micrographs of the char residues after the PCFC test.

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Fig. 10. Ti and P content of the char residues obtained from the PCFC test.

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Table 1 Weight gain and LOI of the wool fabrics treated with different formulations containing PA (150 g L-1), TiO2 (7 g L-1) and BTCA (80 g L-1). Table 2 Weight gain, LOI and char length of the treated wool before and after washing. Table 3 Smoke density parameters of the wool fabrics.

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Table 4 TG data of the wool fabrics in nitrogen and air.

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Table 1 Weight gain and LOI of the wool fabrics treated with different formulations containing PA (150 g L-1), TiO2 (7 g L-1) and BTCA (80 g L-1). LOI (%)

0

23.6

BTCA/SHP

6.7

24.5

PA/TiO2

8.3

31.4

PA/TiO2/BTCA

13.6

32.7

PA

5.8

29.9

PA/BTCA

11.5

30.1

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None

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Weight gain (%)

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Sample

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Table 2 Weight gain, LOI and char length of the treated wool before and after washing. Procedure

Weight gain (%)

LOI (%) Before

Char length (cm)

After 5

Before

After 5

washing washings washing washings 13.6

32.7

27.4

10.2

21.0

Exhaustion-pad-dry-cure

13.0

34.4

30.2

9.8

10.3

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Pad-dry-cure

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Table 3 Smoke density parameters of the wool fabrics. Time/s

Dsmax

Control

571

93.7

Wool-1

424

34.7

Wool-2

490

32.1

Wool-3

446

24.8

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Sample

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Char residues at

(oC)

(oC)

700 oC (%)

PA

145.4

543.2

26.3

Control

266.4

340.8

20.2

Wool-1

282.3

369.5

33.5

Wool-2

276.4

386.3

35.9

Wool-3

278.9

403.8

39.0

PA

132.9

543.8

20.6

Control

270.0

399.9

Wool-1

272.9

420.2

Wool-2

274.9

434.2

Wool-3

272.0

446.6

2.7

15.5 18.3

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Air

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Nitrogen

T20%

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Table 4 TG data of the wool fabrics in nitrogen and air.

23.4