Flame retardant cellulosic fabrics via layer-by-layer self-assembly double coating with egg white protein and phytic acid

Journal of Cleaner Production 243 (2020) 118641

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Flame retardant cellulosic fabrics via layer-by-layer self-assembly double coating with egg white protein and phytic acid Xiaohui Liu a, *, Qiuyan Zhang a, Bo Peng a, Yuanlin Ren b, **, Bowen Cheng a, b, Chen Ding a, Xiaowei Su a, Ju He a, Shenggen Lin a a b

School of Materials Science and Engineering, Tiangong University, Tianjin, 300387, China School of Textiles, Tiangong University, Tianjin, 300387, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 April 2019 Received in revised form 26 September 2019 Accepted 27 September 2019

In this work, facile and inexpensive egg white protein with numerous amino acids, calcium, ferric, sulfur, and phosphorus was firstly explored to flame retard cellulosic fabrics. This was achieved by generated phosphorus-nitrogen flame retardant system formed by intense electrostatic attraction of egg white protein and phytic acid (PA) with special hexaphosphate-substituted cyclic structure. As expected, the cotton fabrics treated by protein and PA in sequence exhibit high residue and time to ignition as well as low values for peak of heat release rate and total heat release after burning than those of control fabrics evaluated by thermogravimetric and cone calorimeter analyses, which is comparable or prior to other biomaterials. The results suggest that the treated fabrics displayed excellent flame retardancy properties, which is ascribed to synergistic effect of phosphorus and nitrogen granted by the double-coating system of PA and egg white protein. © 2019 Elsevier Ltd. All rights reserved.

Handling editor: M.T. Moreira Keywords: Egg white protein Phytic acid Cellulosic fabrics Synergistic effect

1. Introduction Cotton, one of the most popular natural cellulosic resources, possesses excellent advantages, such as softness, breathability, superior water absorbance, and etc (Ganner T. et al., 2014; Xie et al., 2013). Therefore, cotton fabrics are usually used for clothing, house furnishing, and military goods. Unfortunately, the inflammability of cotton often induces fire hazards, leading to heavy loss of lives and properties (Shariatinia et al., 2015). Thus, enhancing fire resistance of cotton fabrics has become an essential and challenging issue in textile industry. In the last few decades, the halogen-containing compounds are the most efficient and widely used flame retardant (FR) for cotton fabrics (Kemmlein et al., 2009; Qi et al., 2014). However, toxic gases (e.g., dioxin) are mostly released during the combusting process. Considering that the toxic gases may successively accumulate in human bodies or organisms and also pollute environment (Carosio et al., 2015), the application of this type of flame retardant has been banned in European countries and USA in recent years (Abou-Okeil

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (Y. Ren). https://doi.org/10.1016/j.jclepro.2019.118641 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

et al., 2013). Alternatively, high efficient and environmentalfriendly flame retardant, for example, phosphorus-based compound has been successfully applied for cotton textiles (Basak et al., 2015, 2016a; Alongi et al., 2012). To date, the representatively and commercially available phosphorus-based FRs for cotton textiles include hydroxymethylphosphonium salts (Proban) (Zhang and Horrocks, 2003) and N-methylol phosphonopropionamide derivatives (Pyrovatex_CP) (Yang et al., 2005; Horrocks, 2011), and etc. Though their limited quantities are still used, some phosphorusbased FRs bearing hydroxymethyl group would emit formaldehyde during finishing and using process. Generally, formaldehyde is now regard as a carcinogenic source by World Health Organization (Nielsen and Wolkoff, 2010). With this regard, it is necessary to exploit novel, efficient, nontoxic (i.e., halogen-free, and formaldehyde-free release), and environmentally friendly flame retardant system. Biomacromolecules (e.g., DNA, proteins, phytic acid (PA), chitosan, starch, cyclodextrin, and sodium alginatec), one kind of green, renewable, sustainable natural resources, seem to be ideal substitutes for flame retardants (Idumah and Hassan, 2016; Basak and Ali, 2016b; Sharma et al., 2015). Alongi and his co-workers firstly employed DNA to treat cotton fabrics (Alongi et al., 2013; Alongi et al., 2014a). The thermal stability and flame retardancy of

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X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

treated fabrics were all enhanced. Furthermore, the effect of various deposited proteins on the surface of fabrics on flame retardancy properties were examined, including caseins, hydrophobins, whey proteins, and etc (Alongi et al., 2014b; Costes et al., 2017; Bosco et al., 2013; Wang et al., 2014; Carosio et al., 2014). Generally, all of these bio-based compounds are derived from natural resources. For example, DNA extracts from herring sperm, and protein extracts from animal or microbial source. However, the extractions work is complex, tedious, and tremendous, which leads to high production costs. It undoubtedly limits the exploiting of a wide variety of expensive proteins on a large scale. There is few report on flame retardant with the help of natural additives. The ancient Romans tried to reduce flammability of the materials of their house and war vessels by dipping into bath of  et al. measured the efficacy of two vinegar and clay. Recently, de Sa vegetable compounds (ginger powder and coffee husk) as a carbon a et al., source in the intumescent flame retardant system (de Sa 2017). Qian et al. investigated the synergistic effect of naturalbased tea saponin in intumescent flame retardant coatings (Qian et al., 2019). However, the applications of these nature additives to the materials were not reported. Egg white protein is known as one of the most common and inexpensive proteins in daily life but it contains numerous amino acids as well as calcium, ferric, and phosphorus (Zhu et al., 2018). Moreover, intense electrostatic attraction may occur between egg white protein containing varied amino acids and the other eco-friendly bio-based compound, e.g., phytic acid (PA) with a special cyclic compound containing six phosphate groups, thus forming a green phosphorus-nitrogen flame retardant system (Scheme 1). Consequently, this protein is a promising alternative to expensive ones for flame retardant. The aim of the present contribution is to explore a novel doublecoating flame retardant system for cotton fabrics via a combination of egg white protein and phytic acid with peculiar hexaphosphonate structure. The structure and morphology as well as thermal stability and flammability of modified cotton fabrics were detailedly investigated by various techniques. 2. Material and methods 2.1. Materials Cotton fabrics were friendly supplied by Dahutong market (Tianjin, China). Eggs were purchased from supermarket. Egg white protein was separated from yolk and then dissolved in distilled water under magnetic stirring (200 rpm) for 2 h at room temperature (pH ¼ 8.5). Phytic acid (PA, 70 wt% aqueous solution) was purchased from Nanjing Xiezun Chemical Co. LTD (Nanjing, China) (pH ¼ 1.5).

sequence (PAþPro), and egg white protein combined with phytic acid in sequence (ProþPA), respectively. 2.3. Characterization Fourier transform infrared spectroscopy (FTIR) was used to detect the structures of COT, COT-PA, COT-Pro, COT_PAþPro, and COTProþPA, and were recorded with KBr powder using a Nicolet iS50 FTIR infrared spectrophotometer. The resolution factor of FTIR spectrometer was 0.09 cm1, and the spectral range was 400e4000 cm1. The surface morphologies of all samples and their char residues after burning were measured by a scanning electron microscope (SEM). Elemental dispersive X-ray analysis (EDX) of cotton fabrics treated by egg white protein and phytic acid was carried out using a cold field emission gun scanning electron microscope (FEG-SEM) to determine the distribution of various elements and quantity of chemicals coating on the surface of cotton fabrics. The thermal stabilities of all samples were evaluated by thermogravimetric analysis (TG), using STA449F3 thermogravimetric analyzer from 40  C to 800  C with a heating rate of 10  C/min in air. The vertical flammability of control and treated fabrics was examined according to ASTM D6413-99 standard test method on a YG815B vertical fabric FR tester (Nantong Sansi electromechanical Science & Technology Co., Ltd.,China). The combustion performances of the control and treated cottons were measured by a cone calorimeter (FTT, East Grinstead,UK) according to ISO 5660-1 under an irradiative heat flux of 35 kW/m2 in horizontal configuration. All samples with the dimension of 100 mm  100 mm were placed in aluminum foil to protect the edges and back of the sample and maintained in the correct configuration by a metallic grid welded at intersections. The tests were repeated five times for each sample to ensure reproducible data. All samples were conditioned at 23 ± 1  C for 48 h under the condition of 50% relative humidity in a climatic chamber before combustion tests. Time to ignite (TTI), heat release rate (HRR) and corresponding peak (PHRR), and total heat release (THR) were all evaluated. The residues after testing were photographed by a digital camera (Power-Shot A2000 IS, Canon Inc., Tokyo, Japan). Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) system was conducted to detect the pyrolysis volatiles by using a gas chromatography/mass spectrometry (SHIMADZU GCMSQP5050A, Agilent) with a Frontier PYR-4A type pyrolyser. The pyrolysis was proceeded at 600  C for 30 s. The capillary column (0.25 mm) of GC was hold at 40  Ce300  C at a heating rate of 10  C/ min. 3. Results and discussion

2.2. Layer-by-layer self-assembly double coating cotton fabrics with egg white protein and PA

3.1. Dependence of increased weight of treated cotton fabrics on different coating agent

Firstly, cotton fabrics were immersed in distilled water at 50  C and then dried in vacuum at 30  C. The dried fabrics were immersed in the egg white protein solution, and then the excess of egg white protein solution was removed by gently pressing with a rotary drum. Cotton fabrics was then immersed in diluted PA aqueous solution, and the excess of PA solution was extruded by pressing with a rotary drum. The treated cotton fabrics were dried to a constant weight at 80  C. Then, the above process was repeated once again, and the double coated cotton fabrics was obtained, as shown in Scheme 1. The control cotton fabrics was coined as COT. Treated fabrics were coined as COT-PA, COT-Pro, COT_PAþPro, and COTProþPA, which refer to cotton fabrics treated with phytic acid, egg white protein (Pro), phytic acid combined with egg white protein in

As noted elsewhere (Carosio et al., 2014), the flame retardancy of the coated cellulosic fibers or fabrics show a strongly positive dependence on the increased weight of the treated sample compared with the control one. Preliminarily, the influences of the varied flame retardant combination on the increased weight of the treated samples were investigated. The weight gain rate values of treated fabrics (A%) were calculated according to the following equation:

A% ¼

Wf  Wi  100% Wi

where Wi and after Wf represent the weight of control and coated

X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

O

O O

HO OH P

HO HO O

P

P O

OH HO

O

O

O

O

O OH HO P

H O

P

HO O P HO

OH OH

HO

P

O

P O

O

OH

P

O

H

O

O

O

O

O OH HO P

N

H

C C

C R

R

H C

C

N

O

H

C

H O

N

OH P

O

HO OH

C

C

C R

N H

H

O

H

O

H

O

C

N C

H

R

H C

C

C

N

O

H

R

H OH O

O HO

OH

O

H

R

N H

N

HO

O

P

O O

H

O

OH HO

O

O

H

3

O HO

HO

O

OH O

O O HO

HO

O HO

HO

Scheme 1. A general process for layer-by-layer self-assembly double-coating cellulosic fabrics with egg white proteins and phytic acid.

samples, respectively. As illustrated in Fig. 1, the Awt.% values of all samples increased after treated with flame retardants. Clearly, the Awt.% value of COTPro (43.2 wt%) is ca. 25% which is higher than that of COT_PA (19.5 wt %). Moreover, different coating sequences considerably influenced the Awt.% value of fabrics. When phytic acid was served as the first coating layer and egg white protein as the second coating layer, the Awt.% value of the treated sample (COT_ PAþPro) is the highest (44.3 wt%) among all samples. Contrarily, when egg white protein was severed as the first coating layer and phytic acid as the second one, the Awt.% value of the treated sample (COT_ ProþPA) slightly decreased (39.1 wt%). However, phytic acid (PA) contains six phosphoric acid members and its aqueous solution shows the strongly acid of PA (pH ¼ 1.5), which greatly decreases mechanical properties and handing softness of treated fabrics COT_PAþPro. Therefore, based on these factors, egg white protein is more suitable for using as the inner layer to coat the sample, which also favors to increase the weight of coated cotton fabrics. It suggests that though egg white protein separated from yolk contains amino

acids, it belongs to alkaline compounds (pH ¼ 8.5) and will preferably deposit on the surface of the sample.

3.2. Structure characterization FTIR technique was employed to analyze the structures of control cotton fabrics, and treated samples with phytic acid (COT_ PA), protein (COT_Pro), and protein and phytic acid in sequence (COT_ProþPA), and the corresponding spectra are shown in Fig. 2. For the control sample, the characteristic peaks of cellulose units are well detectable, such as n(OH) at 3300 cm1, n(CH2) at 2900 cm1, d(OH) at 1640 cm1, d(CH2) at 1425 cm1, d(CH) at 1370 cm1, d(OH) at 1310 cm1, n(C]C) at 1020 cm1, and n(OH) at 894 cm1 (Sahito et al., 2015). For COT_PA, the new appeared characteristic peak at 980 cm1 assigning to the P]O vibration from phytic acid is clearly observed (Gospodinova et al., 2002). Similarly, for COT_Pro, there are new characteristic peaks at 1630 cm1 and 1525 cm1 attributed to amide I and II vibrations from egg white protein (Liu et al., 2015). Based on FTIR spectra of treated sample with phytic acid or protein,

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X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

44.3

43.2

39.1

40

A (%)

30

19.5

20

COT

COT-PA

COT-Pro

COT-Pro+PA

10

0 PA

Pro

PA+Pro

Pro+PA

Sample Fig. 1. The weight gain rate values of treated fabrics (A%) by phytic acid (COT_PA), egg white protein(COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA).

Transmittnce (a.u)

COT

COT_PA

COT_Pro

COT_Pro+PA

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 2. FTIR spectra of control cotton fabrics (COT), and treated fabrics by phytic acid (COT_PA), egg white protein (COT_Pro), and egg white protein combined with phytic acid in sequence (COT_ProþPA).

three characteristic peaks at 980, 1525, and 1630 cm1 do also appear in the spectrum of the cotton fabrics treated with the combination of protein and phytic acid in sequence (COT_ProþPA), corresponding to stretching vibration of P]O unit originated from phytic acid as well as amide I and amide II vibrations originated from egg white protein. Consequently, the results demonstrate that egg white protein can efficiently bond phytic acid through electrostatic attraction, and two combined layers were preferably deposited on the surface of cotton fabrics, which is optimal to improve the flame retardancy of treated cotton fabrics. 3.3. Surface morphology The changes of surface morphology of control and treated cotton fabrics were analyzed by SEM technique (Fig. 3). The surface of control cotton fabrics (COT) is very smooth without obvious defects.

Fig. 3. SEM micrographs of control (COT) and treated fabrics with phytic acid (COT_PA), egg white protein (COT_Pro), and egg white protein combined with phytic acid in sequence (COT_ProþPA). The scale bar in the right corner is 50 mm for all micrographs. The magnification is 600 for COT, COT_PA, and COT_Pro, and 700 for COT_ProþPA.

In contrarily, for cotton fabrics treated by phytic acid (COT_PA), its surface becomes rough and some small holes appear on the surface. The results indicate that strong acidity of phytic acid has destroyed the structure of cellulose units of fabrics to some extent. For cotton fabrics treated by egg white protein (COT_Pro), its surface displays thickness coating layer compared with the smooth surface of control sample. The thickness coating layer can protect the structure of cellulose from heat and oxygen during burning. However, there are some small cracks on the surface of coating layer. In order to obtain integrated protective coating layer, phytic acid as the out layer was necessarily used to coordinate with egg white protein as the inner layer, and the treated sample (COT_ProþPA) would effectively form firming binding through intense electrostatic attraction of phosphoric acid from PA and amino acid from egg white protein. As expected, the surface of sample is composed of continuous and smooth coating layer with few defects. It demonstrate that as phytic acid did not directly contact with cellulose units but connect with egg white protein, protein inner layer can effectively protect the structure of cellulose from acid corrosion from phytic acid. 3.4. Surface elements composition and distribution Elemental dispersive X-ray analysis (EDS) coupled with SEM was employed to measure the surface element composition and element distribution. Their images and detailed surface chemical compositions (Wt % and At%) for cotton fabrics treated with egg white protein combined with phytic acid in sequence (COT_ProþPA) are shown in Fig. 4 and Table 1. Obviously, besides carbon and oxygen elements, nitrogen (N), phosphorus (P), and sulfur (S) elements consist of the main constituents of the coatings. The contents of N, P, and S elements reach 5.9%, 6.5%, and 0.43%, respectively, suggesting the formation of firm bonding on the surface of cotton fabrics through intense electrostatic attraction between amino acids from egg white protein and phosphoric acid from PA. Importantly, three elements possess flame retardant features. Moreover, as shown in Fig. 4, N and P elements are uniformly distributed on the surface of treated cotton fabrics.

X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

5

100

COT COT-Pro COT-PA COT-Pro+PA COT-PA+Pro

Weight loss (%)

80

60

40

20

0 100

200

300

400

500

600

700

800

o

Temperature ( C)

Fig. 4. EDS images of cotton fabrics treated by phytic acid and protein in sequence (COT_ProþPA).

Element

Wt %

At %

C O N P S

42.30 44.87 5.90 6.50 0.43

50.53 40.23 6.04 3.01 0.19

3.5. Thermogravimetric analyses Generally, the thermal stability of textiles materials was mainly evaluated by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis technique. TG and DTG curves in air as well as the corresponding data of control cotton fabrics (COT), samples treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA) are shown in Fig. 5 and Table 2. Obviously, all samples possess similar decomposition process, i.e., one main decomposition stage. Their decomposition temperatures range from 200  C to 600  C. For cotton fabrics coated with egg white protein (COT_Pro), its initial decomposition temperature (260  C) is close to that of control sample. In contrast, after 300  C, its decomposition rate is slower than that of control sample, e.g., the residue at 500  C for COT_Pro is 16%, whereas the residue at 500  C for COT is ca. 4%. The results indicate that egg white protein operates effectively in decreasing decomposition rate. For the sample coated with phytic acid (COT_PA), it exhibits lower initial decomposition temperature (169  C), and decomposed much slowly than COT and the COT_Pro after 318  C. It is ascribed to the fact that phosphorus acid derived from phytic acid during decomposition process would inhibit depolymerization of cellulose units and thus promote dehydration to form char (Cheema et al., 2013). Furthermore, for fabrics treated with double coating of egg white protein and phytic acid in sequence (COT_PAþPro), its initial decomposition temperature (183  C) was little higher than that of COT_PA (169  C), while its residue at 800  C increase to a higher level (7 wt%), increasing 112% relative to control sample. It must be pointed out that as noted previously, phytic acid is not suitable for directly coating and flame retarding cellulosic fibers or fabrics because of strong acidic medium and greatly lowered mechanical properties and handing

Weight loss rate (dw/dt)

Table 1 Surface element composition of cotton fabrics treated with egg white protein combined with phytic acid in sequence (COT_ProþPA).

-25 COT COT-Pro COT-PA COT-Pro+PA COT-PA+Pro

-20

-15

-10

-5

0 100

200

300

400

500

600

700

800

o

Temperature ( C) Fig. 5. TG and DTG curves in air for control cotton fabrics (COT), sample treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA).

softness of modified samples. Alternatively, the egg white protein was coated firstly and then phytic acid in sequence, the obtained sample COT_ProþPA) decomposed initially at 211  C, and decomposed most slowly among the four samples after 316  C, producing the highest residue at 600 and 800  C (32.9% and 12%, respectively), increasing 723% and 264% relative to the results of control sample. Therefore, the double coating of egg white protein and phytic acid in sequence is the optimal flame retardant system for fabrics. Alongi et al. investigated caseins and hydrophobins as green flame retardants to treat cotton fabrics, and the residues of the treated fabrics in air at 600  C were limited to 2% and 4 wt% (Alongi et al., 2014a, 2014b). Contrarily, the residues increased up to 8%, 13%, and 19% when 5%, 10%, and 15% DNA powder from herring sperm was applied, respectively (Alongi et al., 2013). Bosco et al. obtained the treated cotton fabrics with char residues of 1.5% and 2.5% by respectively using folded and unfolded whey proteins (Bosco et al., 2013). In this work, facile and inexpensive egg white protein combined with phytic acid in sequence (COT_ProþPA) was successfully used to treat cotton fabrics. The residue results with 32.9% and 12% values respectively corresponded to 600 and 800  C is superior or comparable to literature reports. Thus the intumescent flame retardant system comprised of egg white protein and phytic acid

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X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

Table 2 Thermogravimetric analysis data in air for control cotton fabrics (COT), sample treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA). Sample

T10% ( C)

Tmax ( C)

Residue at 600  C (wt%)

Residue at 800  C (wt%)

Ref.

COT COT_Pro COT_PA COT_PAþPro COT_ProþPA COT_5 wt% DNA powder from herring sperm COT_10 wt% DNA powder from herring sperm COT_15 wt% DNA powder from herring sperm COT_Caseins COT_Caseins COT_hydrophobins COT-Folded whey proteins COT-unfolded whey proteins COT_Caseins

290 275 208 245 207

358 317 242 289 249

4 4 18.5 16.8 32.9 8 13 19 2 2 4 1.5 2.5 <1

3.3 3.3 3.3 7 12

this work this work this work this work this work Alongi et al. (2013) Alongi et al. (2013) Alongi et al. (2013) Alongi et al. (2013) Alongi et al. (2014b) Alongi et al. (2014b) Bosco et al. (2013) Bosco et al. (2013) Carosio et al. (2014)

3.6. Flame retardant performance A new and quantitative analysis technique - cone calorimetry was used to evaluate the flame retardant properties of control and treated cotton fabrics. The obtained curves and detailed data are presented in Figs. 6e7 and Table 3, including time to ignition (TTI), peak of heat release rate (PHRR), and total heat release (THR), and the residue after the combustion tests. In case of the PHRR values, the samples COT and COT_Pro shows higher PHRR values (135.7 and 143.0 kW/m2), but the results of COT_PA (54.3 kW/m2), COT_PAþPro (107.3 kW/m2)), and COT_ProþPA (105.0 kW/m2) decreases sharply. The THR values of COT_PA (4.9 MJ/m2) and COT_ProþPA (5.6 MJ/m2) are also the lowest among all samples. The TTI values of COT_PA (81 s) and COT_ProþPA (80 s) enhance dramatically compared with that of the control cotton fabrics (18 s), showing 350% and 344% increase. Likewise, the final residues of COT_PA (61.6%) and COT_ProþPA (52.2%) are much higher than that of COT (4.7%), respectively showing 1200% and 1011% increase. The improving effect for PHRR and the final reside after the combustion of the coated cotton

2 Heat Release Rate( kW/m )

160 COT COT-Pro COT-PA COT-PA+Pro COT-Pro+PA

120

80

40

0

0

100

200

300

400

Time(s) Fig. 6. Heat release rate profiles of control cotton fabrics (COT), sample treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA). Data in the graph corresponds to PHRR, THR, and TMAX, respectively.

Total Heat Release( MJ/m2)

displays remarkable synergistic effect on enhancing the amount of residue chars of fabrics.

16 COT COT-Pro COT-PA COT-PA+Pro COT-Pro+PA

12

8

4

0

0

100

200

300

400

Time(s) Fig. 7. Total heat release profiles of control cotton fabrics (COT), sample treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA). Data in the graph corresponds to PHRR, THR, and TMAX, respectively.

fabrics by PA (COT_PA) or egg white protein combined with phytic acid in sequence (COT_ProþPA) is comparable to or superior to the results by other expensive proteins such as caseins and DNA. Alongi et al. ultilized 19 wt% DNA powder from herring sperm to treat cotton fabrics. The treated fabrics shows a residue of 24% and cannot be ignited (Alongi et al., 2013). The PHRR of treated cotton fabrics decreased by 27% or 19% when caseins was used (Alongi et al., 2014b; Carosio et al., 2014). Bosco et al. employed DNA from herring sperm to treat cotton fabrics, showing a PHRR with a decrease of 22% and a residue of 7% (Bosco et al., 2015). Therefore, compared the literature reports, the results suggest that the cotton treated by phytic acid or egg white protein combined with phytic acid in sequence boosted flame retardancy properties obviously. However, considering that phytic acid cannot directly treat the cotton due to the strong acidity, the strategy by egg white protein combined with phytic acid in sequence is a better alternative for treating the cotton, which supports the formation of effective protective layer (Liu et al., 2018a) through electrostatic interaction between phytic acid and egg white protein. Fig. 8 illustrates vertical flammability tests of fabrics before (COT) and after being treated with egg white protein combined with phytic acid in sequence (COT_ProþPA). Obviously, the results of

X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

7

Table 3 Combustion data obtained by cone calorimetry for control cotton fabrics (COT), sample treated by phytic acid (COT_PA), egg white protein (COT_Pro), phytic acid combined with egg white protein in sequence (COT_PAþPro), and egg white protein combined with phytic acid in sequence (COT_ProþPA). Sample

TTI(s)(increase, %)

PkHRR(kW/m2)(reduction, %)

THR(MJ/m2)(reduction, %)

Residue(%)(increase, %)

Ref.

COT COT_Pro COT_PA COT_PAþPro COT_ProþPA COT_DNA_19% COT_Caseins COT_Caseins COT_FHT DNA

18 19 81 15 80

135.7 143.0 (5) 54.3 (60) 107.3 (21) 105.0 (23) No ignition (-27) (-19) (-22)

16.9 16.4 (3) 4.9 (71) 12.2 (28) 5.6 (67)

4.7 3.9 (17) 61.6 (1200) 14.9 (202) 52.2 (1011) 24 e 3 7

this work this work this work this work this work Alongi et al. (2013) Alongi et al. (2014b) Carosio et al., 2014 Bosco et al. (2015)

(6) (350) (17) (344)

excellent flame retardancy. The surface morphology of char residues of cotton fabrics treated with egg white protein combined with phytic acid in sequence (COT_ProþPA) after combustion was measured by SEM technique (Fig. 9). The control cotton fabrics burned into ash, and didn’t produce residual char. Compared with control fabrics, COT_ProþPA provide compact and continuous char layers. The carbonaceous residues were generated by dehydration of cellulose units catalyzed by phosphorus acid originated from flame retardant phytic acid. Interestingly, numerous bubbles resulted from volatile gases on the surface were visibly observed, forming special swollen fibrous char structure. The carbonaceous layers is favorable for serving as physical barrier and shielding combustible products from oxygen and heat, efficiently protecting the matrix of treated cotton fabrics during burning (Liu et al., 2018b; Wang et al., 2016). Based on the above results, the formation of char network is attributed to synergistic effect between nitrogen (egg white protein) and phosphorus (phytic acid). Py-GC/MS technique was employed to measure the structures of degraded products of control (COT) and treated fabrics by egg white protein and PA in sequence (COT_ProþPA). The graphs and the pyrolyzed volatile products were presented in Fig. 10. The main thermal degradation products of cellulosic fibers or fabrics includes alcohols, phenols, aldehydes, ketones, esters, ethers, aromatic rings and other substances, most of which are flammable. Compared with COT, COT_ProþPA clearly show lower peak strength and fewer peaks. Moreover, the non-combustion pyrolysis products were produced during pyrolysis, e.g., CO2, H2O, and NH3. Based on the above results, the possible flame retardant mechanism of fabrics coated with egg white protein and PA in sequence during burning is proposed. On one hand, under the action of heat source, PA first contacts with heat source to decompose small molecular substances such as phosphoric acid. With the increase of total heat quantity, proteins begin to decompose and release large amounts of small less flammable molecules, such as CO2, H2O, and NH3. At the same time, the generated phosphoric acid also acts on the structural units of protein and cellulose units to

Fig. 8. Digital photographs of control (COT, top) and treated (COT_ProþPA, bottom) cellulosic fabrics with a combination of egg white protein and phytic acid in sequence after burning at different time.

two samples exposed to flame after 60 s show a great contrast. The untreated fabrics burned fast, intensely, and completely, leaving a very low amount of ash after ca. 20 s burning. Contrarily, the treated fabrics did not burn obviously even exposed to flame after 60 s. Thus, the results implies that the modified cellulosic fabrics possess

Fig. 9. SEM magnifications of the residues char of cotton fabrics treated by egg white protein combined with phytic acid in sequence (COT_ProþPA) after flammability tests. The scale bar in the right corner is 20 mm and 50 mm for the left and right micrographs, respectively. The magnification is 2000 and 600 for the left and right micrographs, respectively.

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X. Liu et al. / Journal of Cleaner Production 243 (2020) 118641

COT

6

6

Abundance (×10 )

5 4

O OH

O

HO

3

O O

O

OH

2 OH

the char residues increases up to 32.9 wt% at 600  C and 12 wt% at 800  C, which is obviously superior to the results of literature reports with other proteins. Likewise, the modified cellulosic fabrics possess excellent flame retardant performances, as evidence by lower values for peak of heat release rate and total heat release as well as higher time to ignition and residues after burning than those of control fabrics evaluated by cone calorimeter analyses and vertical flammability tests. Finally, Py-GC/MS analyses suggest that nonflammable pyrolysis products including CO2, H2O, and NH3 were produced during the pyrolysis. The results are indicative of flame retardant role in gas and solid phases during combustion of treated fabrics.

1 Acknowledgments

0 2

4

6

8

10

12

Retention time (min)

The authors are very thankful for the financial support provided by the National Key Research and Development Program of China (No. 2017YFB0309000). References

Fig. 10. Py-GC/MS curves of control (COT, top) and treated cotton fabrics by egg white protein combined with phytic acid in sequence (COT_ProþPA, bottom).

promote their dehydration and charring, thus forming more and larger bubbly carbon layers. The expanded char layer can be effectively used as a physical barrier to avoid the contact of heat and oxygen with matrix materials. Therefore, flame retardants play a role in both gas and solid phases during combustion of treated fabrics by egg white protein and PA in sequence.

4. Conclusions The double coatings (i.e., layer-by-layer self-assembly) by varied combinations of egg white protein and phytic acid (PA) were explored to flame retard cotton fabrics. The treatment of protein and PA in sequence (COT_ProþPA) was demonstrated to show an optimal flame retardancy efficiency and remarkable synergistic effect of phosphorus and nitrogen. This system reached a high level of weight gain rate value of ca. 40%. The intense electrostatic attraction was confirmed from the appearances of stretching vibrations of P]O and amide I and amide II, and corresponding elements by FTIR and surface element composition analyses. SEM analyses prove that the particular coating with protein in the inner layer and PA in the outer layer didn’t destroy the surface structure of fabrics, and more carbonaceous residues were preferably produced during burning. Furthermore, TG analyses in air show that

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