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silicon-containing nano-coating by layer-by-layer assembly
Journal Pre-proofs Full Length Article Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/SiliconContaining Nano-Coating by Layer-by-Layer Assembly Bin Wang, Ying-Jun Xu, Ping Li, Feng-Qi Zhang, Yun Liu, Ping Zhu PII: DOI: Reference:
S0169-4332(20)30079-9 https://doi.org/10.1016/j.apsusc.2020.145323 APSUSC 145323
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
1 October 2019 16 December 2019 7 January 2020
Please cite this article as: B. Wang, Y-J. Xu, P. Li, F-Q. Zhang, Y. Liu, P. Zhu, Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/Silicon-Containing Nano-Coating by Layer-by-Layer Assembly, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145323
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Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/SiliconContaining Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
Institute of Functional Textiles and Advanced Materials, National Engineering Research Center for Advanced Fire-Safety Materials D & A (Shandong), College of Textiles & Clothing, Qingdao University, Qingdao 266071, China.
Corresponding to: [email protected] (YJX), [email protected] (YL)
ABSTRACT Polyester/cotton (T/C) blends are widely used in the textile industry owed to the combination of both advantages of cotton and polyester. However, T/C blends are highly flammable due to the wellknown scaffolding effect of the melting polyester and non-melting cotton fibers. Herein, to reduce the flammability of T/C blends, a nano-coating containing phosphorus, nitrogen and silicon was designed and constructed by layer-by-layer assembly of colloidal silica and polyphosphates. It was confirmed that the homogeneous nano-coating was successfully deposited on the surface of T/C blends, regardless of different chemical nature of the synthetic and inartificial fibers. Encouragingly, with 15 bilayer nanocoatings (15.5% weight increased), the coated T/C blends achieved self-extinguishing and got away from scaffolding effect in vertical flame test, and showed a little delay of ignition and a strong decrease of heat release during cone calorimetry test, indicating excellent flame retardance of the treated fabrics as expected. The nano-coating had both gaseous- and condensed-phase flame-retardant activity, which was further confirmed by the results of char analysis and thermogravimetric analysis/infrared spectrometry.
KEY WORDS: Polyester/cotton blend; Flame retardance; Layer-by-layer assembly; Colloidal silica; Ammonium polyphosphate
1. INTRODUCTION Poly (ethylene terephthalate) (PET)/cotton (T/C) blends are widely used for apparel and home furniture, which combine the comfort and breathability of cotton with the strength and durability of the polyester [1, 2]. However, the applications of the blends are limited by their high flammability, which is attributed to the different nature of the melting polyester and non-melting cotton fibers. Under situation of fire, cotton decomposes at the lower temperature as the initial source of fuel, besides PET provides additional fuel to the gaseous phase at the higher temperature produced by the combustion of cotton [3, 4]. Herein, the melted polyester cannot flow away from the fire zone and remains dispersed among the charred cotton skeletons as fuel, thus the well-known “scaffolding effect” finally leads to ease of ignition and difficulty in rendering flame retardance to T/C blends [5, 6]. Over the years many approaches including the use of halogen- or phosphorus-containing additives and back-coatings have been developed to improve the fire safety of T/C blends [7, 8]. Unfortunately, few satisfactory solutions have been found yet, due to the high amounts of additives needed, the harsh handle of the fabrics, as well as the cost concerns and environmental requirements. This difficult scenario opened the way for nonconventional surface modification strategies especially by means of nano techniques [9, 10]. The nano-coating formed by mild and eco-friendly ways such as sol-gel and layer-by-layer (LbL) assembly captured attention of both academia and industry [11-14]. LbL assembly was firstly applied to build nano-coatings for reducing the flammability of fabrics by Grunlan et al. and then attracted great attention from academic and industry researchers [15-18]. Various inorganic, polymer or hybrid systems thus can be built on the surface of fibers by alternative deposition of the negatively charged and positively charged polyelectrolytes (noted as a bilayer, BL) to endow the fabrics with flame retardance [19, 20]. Among those systems, the phosphorus-containing intumescent systems, especially ammonium polyphosphate (APP)-based nano-coatings [21, 22] are
identified as the most hopeful alternative flame retardants for fabrics, owing to their high efficiency and low toxicity. Moreover, Alongi et al. [23-26] developed different nano coatings containing modified silica nanoparticles and polyhedral oligomeric silsesquioxane (POSS) by LbL assembly to obtain flameretardant fabrics, in which silica contributed to forming a dense barrier to limit mass and heat exchange of the surface to the bulk against flame. In our previous work [27, 28], a kind of intumescent nanocoating from bio-based phytate and 3-aminopropyl triethoxysilane (APTES) was constructed on cotton fabrics by LbL assembly for fire-safety cotton textiles. Encouragingly, cotton fabrics with the 15 BL nano-coatings extinguished immediately as the torch was taken away in vertical flame test (VFT) and showed much lower heat and smoke release compared with uncoated ones in combustion test. Nevertheless, now it’s still a big challenge to make the T/C blends satisfy defined fire-safety requirements such as passing VFT in an efficient and eco-friendly way. Here, to improve flame retardance of the T/C blends, a novel nano-coating containing phosphorus, nitrogen and silicon from APP and the colloidal silica of γ-paperazinylproplymethyldimethoxy silane (called GP-108) was constructed on the blend fabrics through LbL deposition. More specifically, the colloidal silica and polyphosphate were expected to constitute a typical intumescent system, which was supposed to form dense char barriers at high temperatures, thus can endow T/C blends with excellent flame retardance. The morphologies and constituents of the nano-coatings were researched by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS); the flame retardance of T/C blends was assessed by VFT; the thermal decomposition process of the fabrics was investigated by thermogravimetry/infrared spectrometry (TG-IR); the burning behavior of coated and untreated T/C blends was recorded by a cone calorimeter; finally, the flame-retardant mechanism of the T/C blends was further discussed.
2. EXPERIMENTAL SECTION 2.1. Materials APP was purchased from Shifang Changfeng chemical Co. Ltd. (Sichuan, China); GP-108 was supplied by Diamond Chemical Co., Ltd. (Nanjing, China); acetic acid was obtained from Yuandong chemical Co. Ltd. (Shangdong, China). All chemicals were used as received and distilled water was used to prepare all solutions. T/C blends (65% PET, 100 g m-2) were provided by Dongguan OTAI textile Co. Ltd. (Dongguan, China); the blend fabrics were washed in deionized water at 90 ℃ for 1 h to remove the impurities and dried at 80 ℃ for 1 h before use.
2.2. Coating procedure GP-108 was dispersed in water and stirred at room temperature overnight to obtain a colloidal silica solution, then the pH of mixture was adjusted to 8 by using acetic acid. Chemical structures of the LbL components and the photograph of colloidal silica with Tyndall effect are shown in Fig. 1a and 1b. Then the blends were alternately dipped into 1 wt% GP-180 and 1 wt% APP solutions for 5 min, respectively; each time before soaking in another solution, the fabrics were washed in water for 2 min to remove the unbound compounds left from the previous step as shown in Fig 1c. One bilayer (BL) film was built up by a single-layer of the colloidal silica and APP, in which the weight increased deposited on the blend fabrics was controlled by the number of bilayers deposited.
Fig. 1. Chemical structures of LbL components (a), the photograph of colloidal silica with Tyndall effect (b) and schematic of LbL coating procedure (c).
2.3. Analytical methods The amounts of nano-coatings deposited on the T/C blends was determined by weighting each sample before (W0) and after the LbL coating procedure (W1). The weight increased (Wi) was calculated according to the following formula: Wi (wt%) = (W1-W0)/W0. The surficial morphologies and the elemental distribution analysis were observed by using a scanning electronic microscope (TESCAN, Czech) equipped with Energy dispersive X-ray spectrometer (EDX). Before SEM observation, all the samples were coated with gold. The detected Au element was deducted before exporting the EDX data. XPS spectra of the coated T/C blends was recorded by the X-ray Photoelectron Spectrometer (XSAM80 Kratos Co, UK), by using Al Kα excitation radiation (hυ = 1486.6 eV), aiming to study the element composition and chemical states of the nano-coating. TG 209 F1 (NETZSCH, Germany) coupled with an FTIR spectrometer (PerkinElmer, USA) was employed to research the thermal stability and identify the gaseous components and of T/C blends under programmed heating process. The fabrics (about 10 mg) were heated from 40 to 700 °C in nitrogen atmosphere, with the heating rate of 10 °C min-1 and the flow rate was 50 L min-1.
The VFT was conducted on the LFY-601A instrument (Shandong Institute of Textile Science, China) to assess flame retardance of the samples, according to GB/T 5455-2014 standard. The size of tested fabrics was 300 mm × 89 mm, and the ignition time was 12 s. Combustion behavior of the T/C blends were recorded by a cone calorimeter (Fire Testing Technology, UK), according to ISO 5660 standard. For each sample, the blend fabrics were cut into 10 cm × 10 cm, and 5 pieces of the blends fixed by iron wires were placed in aluminum foil under external heat flux of 35 kW m-2.
3. RESULTS AND DISCUSSION 3.1. The surface characterization SEM was carried out to research the surface morphology of the untreated and treated T/C blends as shown in Fig. 2. Untreated T/C blends presented a typical morphology of blended fabrics, in which cotton and PET fibers intertwined together with distinct boundaries (Fig. 2a). In contrast, a few films appeared on the surface of fibers (Fig. 2b and 2c), and continuous thin coatings were built on the surface of the blend fabrics coated with 15 BL nano-coatings (15BL) (Fig. 2d). Notably, the coating connected polyester and cotton fibers with each other and resulted in the complete surface coverage. Consequently, the easy-operating LbL process was able to form a homogeneous coating on the blends, regardless of different chemical nature of the synthetic and natural fibers. In addition, EDX was conducted to further monitor the presence of the deposited coating on T/C blends, and EDX element mapping images and data are exhibited in Fig. 2e and the following table. Obviously, it indicated that the specific elements such as phosphorus, nitrogen and silicon were well-distributed in the flame-retardant coating on the blends, further confirming that the nano-coating by LbL assembly was relatively homogeneous as mentioned before.
Fig. 2. SEM photographs of untreated (a), 5BL (b), 10BL (c) and 15BL (b) T/C blends, and EDX element mapping images (c) and data of 15BL T/C blends.
Additionally, XPS measurements were carried out to detect the surface chemical composition of the treated T/C blends. The full survey spectrum and the high-resolution spectra of N1s and P2p for 15BL T/C blends are exhibited in Fig. 3. The XPS full survey spectrum (Fig. 3a) of the treated blends clearly showed that the surface of the treated T/C blends contained phosphorus, nitrogen and silicon, which was consistent with the results of EDX. More specifically, the strong peak in P2p spectrum at 133.7 eV as shown in Fig. 3b was related to the P-O bond such as phosphonate group of polyphosphates [29, 30]. Besides, the N1s spectrum of the 15BL was expected to be divided into three peaks at 401.5, 400.5 and 399.5 eV as presented in Fig. 3c, which was attributed to NH4+ from APP of the last layer as well as -NH2+- and -NH+- derived from the piperazinyl silane, respectively [31, 32]. It indicated the occurrence of ion exchange reaction between colloidal silica and APP during deposition of the two components, which drove the LbL assembly of the phosphorus-, nitrogen- and silicon-containing nanocoating. In fact, the ion exchange of the system can be complete for full protonation of amino groups from silane, where extra NH4+ are from APP of the last layer. Finally, the results prove that the GP-
108/APP nano-coating has been deposited on T/C blends successfully by the facile procedure described above. In addition, the chemical reaction of the two components was proposed in Fig. 3d.
Fig. 3. Full survey spectrum (a) and high-resolution spectra of P2p (b) and N1s (c) for 15BL T/C blends, and the chemical reaction of the two components (d).
3.2 Thermal stability The thermal stability of all the blends was evaluated by TG under the nitrogen atmosphere. The TG and derivative TG (DTG) curves of samples are shown in Fig. 4, and the collected data are listed in the Table 1. The T/C blends, as the typical polymer blends, showed two independent degradation steps, which was attributed to the decomposition of cotton (at 357 °C) and polyester (at 425 °C), respectively [33, 34]. Compared with those of untreated sample, the temperature where 5 wt% of the weight loss (T5%, defined as the initial decomposition temperature) and the temperature where the first maximum weight loss occurred (Tmax1) for all the treated fabrics were both dramatically decreased, while the temperature where the second maximum weight loss occurred (Tmax2) and weight loss rate at Tmax2 (ca. 10 to 11 % min-1) almost unchanged. It indicates that the nano-coating promote dehydration of cotton at the lower temperature (below 300 °C), while neither of the coating’s ingredients affected degradation of polyester. In addition, the residue at 700 °C of 5, 10 and 15BL T/C blends (19.9%, 22.4% and 26.2%)
was more than that of the untreated blends (14.7%), which can play a positive role to reducing flammability of T/C blends.
Fig. 4. TG (a) and DTG (b) curves of the untreated and treated blends under N2 atmosphere.
Table 1 Data acquired from TG and DTG curves. Sample
T5%
T10%
Tmax1
Tmax2
Weight loss rate at
Weight loss rate at
Residue at 700 °C
(°C)
(°C)
(°C)
(°C)
Tmax1 (% min-1)
Tmax2 (% min-1)
(%)
untreated
327.4
342.4
357.4
424.9
10.9
10.5
14.7
5BL
289.9
302.4
304.9
432.4
5.9
11.1
19.9
10BL
304.9
317.4
322.4
429.9
6.5
10.0
22.4
15BL
299.9
312.4
312.4
429.9
5.0
9.8
26.2
3.3 Flame retardance and combustion behavior The VFT was conducted to investigate the response to an open flame and flame spread after ignition of the blends. The related results are listed in Table 2, besides photographs of all samples and the corresponding SEM images and EDX data of the char residue after the test are shown in Fig. 5. The untreated T/C blends burned violently with droplets and left a few melted residues after burning. In contrast, all coated fabrics exhibited slower flame spread and preserved lots of char residues after the
test, besides showed no dripping and afterglow phenomenon during the test. In addition, as the number of bilayers (5 BL to 15 BL) and the loadings of the nano-coatings increased (about 5.1 wt% to 15.5 wt%), the afterflame time was all extended except for the 15BL sample, which almost self-extinguished immediately as the torch was taken away and had a damaged length as short as 151±2 mm finally. It indicates that the phosphorus/nitrogen/silicon-containing nano-coating can effectively delay the flame spread and endow T/C blends with satisfactory flame retardance as expected. It can be observed in Fig. 5a that the polyester melted under heat and then absorbed by the charred cotton skeletons as additional fuel, thus finally led to drastic burning of the T/C blends, that is so-called “scaffolding effect”. In contrast, with presence of the coating, the treated blend fabrics especially for 15BL T/C blends formed the typical intumescent char with lots of swelled bubbles, which then acted as a heat shield to limit the heat, fuel and oxygen transfer between the flame and underlying polymers [35, 36]. Moreover, phosphorus and silicon both enriched in the char residue as shown in the EDX data in Fig. 5b, which can significantly enhance resistance towards combustion of the fabrics by a condensedphase mechanism [37, 38]. Notably, the treated T/C blends maintained original fabric structures after burning, in which melted polyester did not spread among charred cotton skeletons as the control sample did, thus got away from “scaffolding effect” towards excellent flame retardancy finally. Table 2 Data acquired from the VFT of T/C blends. sample
Weight increased (%)
Afterflame time (s)
Afterglow time (s)
Damaged length (mm)
untreated
/
12±1
10.6±1.3
300±0
5BL
5.1±0.1
17±3
0
300±0
10BL
10.1±0.2
19±2
0
300±0
15BL
15.5±0.5
0
0
151±2
Fig. 5. Photographs and SEM images of untreated and treated T/C blends (a), and EDX element mapping data of 15BL T/C blends (b) after VFT.
In addition, the cone calorimetry was utilized herein to research the burning behavior of T/C blends under a simulate fire condition. The heat release rate (HRR) and total heat release (THR) curves of uncoated and all coated blends are described in Fig. 6, and some corresponding data are presented in Table 3. Compared with the untreated T/C blends, all the treated blends surprisingly had longer time to ignition (TTI) (23 s of untreated versus 31, 30 and 32 s of 5, 10 and 15BL), rather than being ignited prematurely as ordinary flame-retardant systems, because the coating was capable to protect underlying blends during initial stage of fire [24, 39]. Notably, the peak heat release rate (PHRR), THR and the fire
growth rate index (FIGRA, defined as PHRR divided by TPHRR) of coated T/C blends all obviously decreased as the control sample, indicating that the flammability of T/C blends was reduced by the coatings. However, as the coating number increase from 5BL to 15BL, the PHRR and THR of the blends were not decreasing but a little increasing. This was probably because the coating did not affect degradation process of polyester as discussed before, but resulted in different shrinkage and crack of the coated blends during the test as shown in Fig. 7. In addition, the total smoke production (TSP), total smoke release (TSR) and CO/CO2 ratio of flame-retardant T/C blends all a little increased compared with the untreated ones, which was due to the incomplete combustion of the flame-retardant sample as reported before [40, 41]. In addition, all the treated T/C blends had a much higher residual yield than the reference sample (11.0%, 14.5% and 19.0% of 5, 10 and 15BL versus 0% of virgin sample), suggesting the condensed-phase flame-retardant activity of the intumescent coatings.
Fig. 6. HRR (a) and THR (b) curves of T/C blends obtained from cone calorimetry test.
Table 3 Cone calorimetry data of the untreated and treated blends. TTI
PHRR (kW
THR
TPHRR
FIGRA
TSP
TSR
(s)
m-2)
(MJ m-2)
(s)
(kW m-2s-1)
(m2)
(m2 m-2)
23±2
204.1±0.7
8.8±0.2
60±0
3.4
1.6±0.1
183.0±11.7
CO/CO2
Reside
Sample
untreated
(%) 0.17
0±0
5BL
31±1
110.8±0.7
6.5±0.1
45±0
2.5
2.1±0.1
242.2±1.3
0.13
11.0±1.0
10BL
30±1
123.2±3.1
7.0±0.1
60±5
2.1
2.1±0.1
234.2±12.6
0.44
14.5±2.5
15BL
32±3
128.4±4.6
6.9±0.2
50±5
2.6
2.4±0.1
266.2±20.7
0.51
19.0±2.1
Fig. 7. Photographs of final residues of T/C blends after cone calorimetry test.
To further investigate the flame-retardant effect in vapor phase of GP-108/APP coating on T/C blends, the pyrolysis products from untreated and 15BL T/C blends were detected by using TG-IR. As shown in 3D TG-IR spectra (Fig. 8), the gaseous products of both T/C blends mainly consisted of H2O (3740 cm-1), CO2 (2360 cm-1), aldehydes/ketones (1740 cm-1) and ethers (1250 cm-1 and 1080 cm-1) [42, 43]; moreover the spectra indicated that 15BL T/C blends started releasing gases at the lower temperature compared with the control sample, where the two degradation steps owed to the two components of the blends, respectively. In addition, the absorbance intensities attributed to the typical flammable gases such as aldehydes/ketones and ethers for untreated and 15BL T/C blends were further compared in Fig. 9. It can be observed from Fig. 9a that the volatiles generation of cotton were greatly
suppressed, while those of polyester showed little difference with the present of intumescent coatings on T/C blends as discussed before. In addition, the spectra of the 15BL T/C blends in Fig. 9b exhibited a much stronger absorption intensity at 1250 cm-1 as the untreated blends, which was likely attributed to C-N containing fragments from the nano-coating; besides, other typical absorption bands of flameretardant volatiles, such as stretching vibration of P=O were probably overlapped around the region [44, 45]. Thus, it can be confirmed that the GP-180/APP nano-coating really shows vapor phase flameretardant effect, besides the dominating condensed phase activity as mentioned above.
Fig. 8. 3D TG-IR spectra of untreated (a) and 15BL (b) T/C blends obtained from TG-IR.
Fig. 9. Absorbance intensity changes of T/C blends as a function of time recorded by TG-IR.
4. CONCLUSION
Here, a phosphorus/nitrogen/silicon-containing nano-coating was designed and constructed by LbL assembly from colloidal silica and polyphosphates to reduce flammability of T/C blends. It was confirmed by SEM coupled with EDX and XPS analysis that the homogeneous nano-coating was successfully deposited on the surface of the T/C blends by the straightforward procedure. T/C blends coated with 15 BL nano-coatings (ca. 15.5% weight increased) achieved self-extinguishing in VFT, and showed a little delay of ignition and a strong decrease of heat release during cone calorimetry test. It was certified by TG-IR and char analysis that vapor phase flame-retardant effect by dilution of the nonflammable volatiles and the dominating condensed-phase activity by forming the intumescent char of the nano-coating together endowed the T/C blends with excellent flame retardance. With its ease of operation and use of low-toxic chemicals, this eco-friendly coating further offers a feasible fire protection solution of the blend fabrics.
ASSOCIATED CONTENT Supporting Information Electronic Supplementary Information (ESI) available: [a video about adding polyphosphate solution to colloidal silica, EDX mapping of 15BL T/C blends after VFT].
Corresponding Author Institute of Functional Textiles and Advanced Materials, National Engineering Research Center for Advanced Fire-Safety Materials D & A (Shandong), College of Textiles & Clothing, Qingdao University, Qingdao 266071, China. Tel & Fax: +86-532-85950890 E-mail: [email protected] (YJX), [email protected] (YL).
Acknowledgements This work was financially supported by the National Key Research and Development Program of China (Project No. 2017YFB0309001) and the National Natural Science Foundation of China (Grant No. 51673153 and 51973098).
Notes The authors declare no competing financial interest.
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CRediT authorship contribution statement Bin Wang: Conceptualization, Investigation, Writing-Original Draft. Ying-Jun Xu: Conceptualization, Supervision, Writing-Review & Editing. Ping Li: Investigation. Feng-Qi Zhang: Investigation. Yun Liu: Conceptualization, Supervision, Acquisition of the financial support for the project leading to this publication. Ping Zhu: Supervision.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
GRAPHICAL ABSTRACT for Polyester/Cotton Blend with Phosphorus/Nitrogen/Silicon-Containing Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
A nano-coating containing phosphorus, nitrogen and silicon was constructed on T/C blends by LbL assembly from colloidal silica and polyphosphate to reduce flammability of the fabrics by the dominating condensed-phase activity.
HIGHLIGHTS for Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/SiliconContaining Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
Highlights •
A P/N/Si-containing nano-coating was constructed by LbL assembly.
•
The treated T/C blends achieved self-extinguishing in VFT.
•
The coated T/C blends were free from scaffolding effect towards fire safety.
S0169-4332(20)30079-9 https://doi.org/10.1016/j.apsusc.2020.145323 APSUSC 145323
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
1 October 2019 16 December 2019 7 January 2020
Please cite this article as: B. Wang, Y-J. Xu, P. Li, F-Q. Zhang, Y. Liu, P. Zhu, Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/Silicon-Containing Nano-Coating by Layer-by-Layer Assembly, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145323
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Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/SiliconContaining Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
Institute of Functional Textiles and Advanced Materials, National Engineering Research Center for Advanced Fire-Safety Materials D & A (Shandong), College of Textiles & Clothing, Qingdao University, Qingdao 266071, China.
Corresponding to: [email protected] (YJX), [email protected] (YL)
ABSTRACT Polyester/cotton (T/C) blends are widely used in the textile industry owed to the combination of both advantages of cotton and polyester. However, T/C blends are highly flammable due to the wellknown scaffolding effect of the melting polyester and non-melting cotton fibers. Herein, to reduce the flammability of T/C blends, a nano-coating containing phosphorus, nitrogen and silicon was designed and constructed by layer-by-layer assembly of colloidal silica and polyphosphates. It was confirmed that the homogeneous nano-coating was successfully deposited on the surface of T/C blends, regardless of different chemical nature of the synthetic and inartificial fibers. Encouragingly, with 15 bilayer nanocoatings (15.5% weight increased), the coated T/C blends achieved self-extinguishing and got away from scaffolding effect in vertical flame test, and showed a little delay of ignition and a strong decrease of heat release during cone calorimetry test, indicating excellent flame retardance of the treated fabrics as expected. The nano-coating had both gaseous- and condensed-phase flame-retardant activity, which was further confirmed by the results of char analysis and thermogravimetric analysis/infrared spectrometry.
KEY WORDS: Polyester/cotton blend; Flame retardance; Layer-by-layer assembly; Colloidal silica; Ammonium polyphosphate
1. INTRODUCTION Poly (ethylene terephthalate) (PET)/cotton (T/C) blends are widely used for apparel and home furniture, which combine the comfort and breathability of cotton with the strength and durability of the polyester [1, 2]. However, the applications of the blends are limited by their high flammability, which is attributed to the different nature of the melting polyester and non-melting cotton fibers. Under situation of fire, cotton decomposes at the lower temperature as the initial source of fuel, besides PET provides additional fuel to the gaseous phase at the higher temperature produced by the combustion of cotton [3, 4]. Herein, the melted polyester cannot flow away from the fire zone and remains dispersed among the charred cotton skeletons as fuel, thus the well-known “scaffolding effect” finally leads to ease of ignition and difficulty in rendering flame retardance to T/C blends [5, 6]. Over the years many approaches including the use of halogen- or phosphorus-containing additives and back-coatings have been developed to improve the fire safety of T/C blends [7, 8]. Unfortunately, few satisfactory solutions have been found yet, due to the high amounts of additives needed, the harsh handle of the fabrics, as well as the cost concerns and environmental requirements. This difficult scenario opened the way for nonconventional surface modification strategies especially by means of nano techniques [9, 10]. The nano-coating formed by mild and eco-friendly ways such as sol-gel and layer-by-layer (LbL) assembly captured attention of both academia and industry [11-14]. LbL assembly was firstly applied to build nano-coatings for reducing the flammability of fabrics by Grunlan et al. and then attracted great attention from academic and industry researchers [15-18]. Various inorganic, polymer or hybrid systems thus can be built on the surface of fibers by alternative deposition of the negatively charged and positively charged polyelectrolytes (noted as a bilayer, BL) to endow the fabrics with flame retardance [19, 20]. Among those systems, the phosphorus-containing intumescent systems, especially ammonium polyphosphate (APP)-based nano-coatings [21, 22] are
identified as the most hopeful alternative flame retardants for fabrics, owing to their high efficiency and low toxicity. Moreover, Alongi et al. [23-26] developed different nano coatings containing modified silica nanoparticles and polyhedral oligomeric silsesquioxane (POSS) by LbL assembly to obtain flameretardant fabrics, in which silica contributed to forming a dense barrier to limit mass and heat exchange of the surface to the bulk against flame. In our previous work [27, 28], a kind of intumescent nanocoating from bio-based phytate and 3-aminopropyl triethoxysilane (APTES) was constructed on cotton fabrics by LbL assembly for fire-safety cotton textiles. Encouragingly, cotton fabrics with the 15 BL nano-coatings extinguished immediately as the torch was taken away in vertical flame test (VFT) and showed much lower heat and smoke release compared with uncoated ones in combustion test. Nevertheless, now it’s still a big challenge to make the T/C blends satisfy defined fire-safety requirements such as passing VFT in an efficient and eco-friendly way. Here, to improve flame retardance of the T/C blends, a novel nano-coating containing phosphorus, nitrogen and silicon from APP and the colloidal silica of γ-paperazinylproplymethyldimethoxy silane (called GP-108) was constructed on the blend fabrics through LbL deposition. More specifically, the colloidal silica and polyphosphate were expected to constitute a typical intumescent system, which was supposed to form dense char barriers at high temperatures, thus can endow T/C blends with excellent flame retardance. The morphologies and constituents of the nano-coatings were researched by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS); the flame retardance of T/C blends was assessed by VFT; the thermal decomposition process of the fabrics was investigated by thermogravimetry/infrared spectrometry (TG-IR); the burning behavior of coated and untreated T/C blends was recorded by a cone calorimeter; finally, the flame-retardant mechanism of the T/C blends was further discussed.
2. EXPERIMENTAL SECTION 2.1. Materials APP was purchased from Shifang Changfeng chemical Co. Ltd. (Sichuan, China); GP-108 was supplied by Diamond Chemical Co., Ltd. (Nanjing, China); acetic acid was obtained from Yuandong chemical Co. Ltd. (Shangdong, China). All chemicals were used as received and distilled water was used to prepare all solutions. T/C blends (65% PET, 100 g m-2) were provided by Dongguan OTAI textile Co. Ltd. (Dongguan, China); the blend fabrics were washed in deionized water at 90 ℃ for 1 h to remove the impurities and dried at 80 ℃ for 1 h before use.
2.2. Coating procedure GP-108 was dispersed in water and stirred at room temperature overnight to obtain a colloidal silica solution, then the pH of mixture was adjusted to 8 by using acetic acid. Chemical structures of the LbL components and the photograph of colloidal silica with Tyndall effect are shown in Fig. 1a and 1b. Then the blends were alternately dipped into 1 wt% GP-180 and 1 wt% APP solutions for 5 min, respectively; each time before soaking in another solution, the fabrics were washed in water for 2 min to remove the unbound compounds left from the previous step as shown in Fig 1c. One bilayer (BL) film was built up by a single-layer of the colloidal silica and APP, in which the weight increased deposited on the blend fabrics was controlled by the number of bilayers deposited.
Fig. 1. Chemical structures of LbL components (a), the photograph of colloidal silica with Tyndall effect (b) and schematic of LbL coating procedure (c).
2.3. Analytical methods The amounts of nano-coatings deposited on the T/C blends was determined by weighting each sample before (W0) and after the LbL coating procedure (W1). The weight increased (Wi) was calculated according to the following formula: Wi (wt%) = (W1-W0)/W0. The surficial morphologies and the elemental distribution analysis were observed by using a scanning electronic microscope (TESCAN, Czech) equipped with Energy dispersive X-ray spectrometer (EDX). Before SEM observation, all the samples were coated with gold. The detected Au element was deducted before exporting the EDX data. XPS spectra of the coated T/C blends was recorded by the X-ray Photoelectron Spectrometer (XSAM80 Kratos Co, UK), by using Al Kα excitation radiation (hυ = 1486.6 eV), aiming to study the element composition and chemical states of the nano-coating. TG 209 F1 (NETZSCH, Germany) coupled with an FTIR spectrometer (PerkinElmer, USA) was employed to research the thermal stability and identify the gaseous components and of T/C blends under programmed heating process. The fabrics (about 10 mg) were heated from 40 to 700 °C in nitrogen atmosphere, with the heating rate of 10 °C min-1 and the flow rate was 50 L min-1.
The VFT was conducted on the LFY-601A instrument (Shandong Institute of Textile Science, China) to assess flame retardance of the samples, according to GB/T 5455-2014 standard. The size of tested fabrics was 300 mm × 89 mm, and the ignition time was 12 s. Combustion behavior of the T/C blends were recorded by a cone calorimeter (Fire Testing Technology, UK), according to ISO 5660 standard. For each sample, the blend fabrics were cut into 10 cm × 10 cm, and 5 pieces of the blends fixed by iron wires were placed in aluminum foil under external heat flux of 35 kW m-2.
3. RESULTS AND DISCUSSION 3.1. The surface characterization SEM was carried out to research the surface morphology of the untreated and treated T/C blends as shown in Fig. 2. Untreated T/C blends presented a typical morphology of blended fabrics, in which cotton and PET fibers intertwined together with distinct boundaries (Fig. 2a). In contrast, a few films appeared on the surface of fibers (Fig. 2b and 2c), and continuous thin coatings were built on the surface of the blend fabrics coated with 15 BL nano-coatings (15BL) (Fig. 2d). Notably, the coating connected polyester and cotton fibers with each other and resulted in the complete surface coverage. Consequently, the easy-operating LbL process was able to form a homogeneous coating on the blends, regardless of different chemical nature of the synthetic and natural fibers. In addition, EDX was conducted to further monitor the presence of the deposited coating on T/C blends, and EDX element mapping images and data are exhibited in Fig. 2e and the following table. Obviously, it indicated that the specific elements such as phosphorus, nitrogen and silicon were well-distributed in the flame-retardant coating on the blends, further confirming that the nano-coating by LbL assembly was relatively homogeneous as mentioned before.
Fig. 2. SEM photographs of untreated (a), 5BL (b), 10BL (c) and 15BL (b) T/C blends, and EDX element mapping images (c) and data of 15BL T/C blends.
Additionally, XPS measurements were carried out to detect the surface chemical composition of the treated T/C blends. The full survey spectrum and the high-resolution spectra of N1s and P2p for 15BL T/C blends are exhibited in Fig. 3. The XPS full survey spectrum (Fig. 3a) of the treated blends clearly showed that the surface of the treated T/C blends contained phosphorus, nitrogen and silicon, which was consistent with the results of EDX. More specifically, the strong peak in P2p spectrum at 133.7 eV as shown in Fig. 3b was related to the P-O bond such as phosphonate group of polyphosphates [29, 30]. Besides, the N1s spectrum of the 15BL was expected to be divided into three peaks at 401.5, 400.5 and 399.5 eV as presented in Fig. 3c, which was attributed to NH4+ from APP of the last layer as well as -NH2+- and -NH+- derived from the piperazinyl silane, respectively [31, 32]. It indicated the occurrence of ion exchange reaction between colloidal silica and APP during deposition of the two components, which drove the LbL assembly of the phosphorus-, nitrogen- and silicon-containing nanocoating. In fact, the ion exchange of the system can be complete for full protonation of amino groups from silane, where extra NH4+ are from APP of the last layer. Finally, the results prove that the GP-
108/APP nano-coating has been deposited on T/C blends successfully by the facile procedure described above. In addition, the chemical reaction of the two components was proposed in Fig. 3d.
Fig. 3. Full survey spectrum (a) and high-resolution spectra of P2p (b) and N1s (c) for 15BL T/C blends, and the chemical reaction of the two components (d).
3.2 Thermal stability The thermal stability of all the blends was evaluated by TG under the nitrogen atmosphere. The TG and derivative TG (DTG) curves of samples are shown in Fig. 4, and the collected data are listed in the Table 1. The T/C blends, as the typical polymer blends, showed two independent degradation steps, which was attributed to the decomposition of cotton (at 357 °C) and polyester (at 425 °C), respectively [33, 34]. Compared with those of untreated sample, the temperature where 5 wt% of the weight loss (T5%, defined as the initial decomposition temperature) and the temperature where the first maximum weight loss occurred (Tmax1) for all the treated fabrics were both dramatically decreased, while the temperature where the second maximum weight loss occurred (Tmax2) and weight loss rate at Tmax2 (ca. 10 to 11 % min-1) almost unchanged. It indicates that the nano-coating promote dehydration of cotton at the lower temperature (below 300 °C), while neither of the coating’s ingredients affected degradation of polyester. In addition, the residue at 700 °C of 5, 10 and 15BL T/C blends (19.9%, 22.4% and 26.2%)
was more than that of the untreated blends (14.7%), which can play a positive role to reducing flammability of T/C blends.
Fig. 4. TG (a) and DTG (b) curves of the untreated and treated blends under N2 atmosphere.
Table 1 Data acquired from TG and DTG curves. Sample
T5%
T10%
Tmax1
Tmax2
Weight loss rate at
Weight loss rate at
Residue at 700 °C
(°C)
(°C)
(°C)
(°C)
Tmax1 (% min-1)
Tmax2 (% min-1)
(%)
untreated
327.4
342.4
357.4
424.9
10.9
10.5
14.7
5BL
289.9
302.4
304.9
432.4
5.9
11.1
19.9
10BL
304.9
317.4
322.4
429.9
6.5
10.0
22.4
15BL
299.9
312.4
312.4
429.9
5.0
9.8
26.2
3.3 Flame retardance and combustion behavior The VFT was conducted to investigate the response to an open flame and flame spread after ignition of the blends. The related results are listed in Table 2, besides photographs of all samples and the corresponding SEM images and EDX data of the char residue after the test are shown in Fig. 5. The untreated T/C blends burned violently with droplets and left a few melted residues after burning. In contrast, all coated fabrics exhibited slower flame spread and preserved lots of char residues after the
test, besides showed no dripping and afterglow phenomenon during the test. In addition, as the number of bilayers (5 BL to 15 BL) and the loadings of the nano-coatings increased (about 5.1 wt% to 15.5 wt%), the afterflame time was all extended except for the 15BL sample, which almost self-extinguished immediately as the torch was taken away and had a damaged length as short as 151±2 mm finally. It indicates that the phosphorus/nitrogen/silicon-containing nano-coating can effectively delay the flame spread and endow T/C blends with satisfactory flame retardance as expected. It can be observed in Fig. 5a that the polyester melted under heat and then absorbed by the charred cotton skeletons as additional fuel, thus finally led to drastic burning of the T/C blends, that is so-called “scaffolding effect”. In contrast, with presence of the coating, the treated blend fabrics especially for 15BL T/C blends formed the typical intumescent char with lots of swelled bubbles, which then acted as a heat shield to limit the heat, fuel and oxygen transfer between the flame and underlying polymers [35, 36]. Moreover, phosphorus and silicon both enriched in the char residue as shown in the EDX data in Fig. 5b, which can significantly enhance resistance towards combustion of the fabrics by a condensedphase mechanism [37, 38]. Notably, the treated T/C blends maintained original fabric structures after burning, in which melted polyester did not spread among charred cotton skeletons as the control sample did, thus got away from “scaffolding effect” towards excellent flame retardancy finally. Table 2 Data acquired from the VFT of T/C blends. sample
Weight increased (%)
Afterflame time (s)
Afterglow time (s)
Damaged length (mm)
untreated
/
12±1
10.6±1.3
300±0
5BL
5.1±0.1
17±3
0
300±0
10BL
10.1±0.2
19±2
0
300±0
15BL
15.5±0.5
0
0
151±2
Fig. 5. Photographs and SEM images of untreated and treated T/C blends (a), and EDX element mapping data of 15BL T/C blends (b) after VFT.
In addition, the cone calorimetry was utilized herein to research the burning behavior of T/C blends under a simulate fire condition. The heat release rate (HRR) and total heat release (THR) curves of uncoated and all coated blends are described in Fig. 6, and some corresponding data are presented in Table 3. Compared with the untreated T/C blends, all the treated blends surprisingly had longer time to ignition (TTI) (23 s of untreated versus 31, 30 and 32 s of 5, 10 and 15BL), rather than being ignited prematurely as ordinary flame-retardant systems, because the coating was capable to protect underlying blends during initial stage of fire [24, 39]. Notably, the peak heat release rate (PHRR), THR and the fire
growth rate index (FIGRA, defined as PHRR divided by TPHRR) of coated T/C blends all obviously decreased as the control sample, indicating that the flammability of T/C blends was reduced by the coatings. However, as the coating number increase from 5BL to 15BL, the PHRR and THR of the blends were not decreasing but a little increasing. This was probably because the coating did not affect degradation process of polyester as discussed before, but resulted in different shrinkage and crack of the coated blends during the test as shown in Fig. 7. In addition, the total smoke production (TSP), total smoke release (TSR) and CO/CO2 ratio of flame-retardant T/C blends all a little increased compared with the untreated ones, which was due to the incomplete combustion of the flame-retardant sample as reported before [40, 41]. In addition, all the treated T/C blends had a much higher residual yield than the reference sample (11.0%, 14.5% and 19.0% of 5, 10 and 15BL versus 0% of virgin sample), suggesting the condensed-phase flame-retardant activity of the intumescent coatings.
Fig. 6. HRR (a) and THR (b) curves of T/C blends obtained from cone calorimetry test.
Table 3 Cone calorimetry data of the untreated and treated blends. TTI
PHRR (kW
THR
TPHRR
FIGRA
TSP
TSR
(s)
m-2)
(MJ m-2)
(s)
(kW m-2s-1)
(m2)
(m2 m-2)
23±2
204.1±0.7
8.8±0.2
60±0
3.4
1.6±0.1
183.0±11.7
CO/CO2
Reside
Sample
untreated
(%) 0.17
0±0
5BL
31±1
110.8±0.7
6.5±0.1
45±0
2.5
2.1±0.1
242.2±1.3
0.13
11.0±1.0
10BL
30±1
123.2±3.1
7.0±0.1
60±5
2.1
2.1±0.1
234.2±12.6
0.44
14.5±2.5
15BL
32±3
128.4±4.6
6.9±0.2
50±5
2.6
2.4±0.1
266.2±20.7
0.51
19.0±2.1
Fig. 7. Photographs of final residues of T/C blends after cone calorimetry test.
To further investigate the flame-retardant effect in vapor phase of GP-108/APP coating on T/C blends, the pyrolysis products from untreated and 15BL T/C blends were detected by using TG-IR. As shown in 3D TG-IR spectra (Fig. 8), the gaseous products of both T/C blends mainly consisted of H2O (3740 cm-1), CO2 (2360 cm-1), aldehydes/ketones (1740 cm-1) and ethers (1250 cm-1 and 1080 cm-1) [42, 43]; moreover the spectra indicated that 15BL T/C blends started releasing gases at the lower temperature compared with the control sample, where the two degradation steps owed to the two components of the blends, respectively. In addition, the absorbance intensities attributed to the typical flammable gases such as aldehydes/ketones and ethers for untreated and 15BL T/C blends were further compared in Fig. 9. It can be observed from Fig. 9a that the volatiles generation of cotton were greatly
suppressed, while those of polyester showed little difference with the present of intumescent coatings on T/C blends as discussed before. In addition, the spectra of the 15BL T/C blends in Fig. 9b exhibited a much stronger absorption intensity at 1250 cm-1 as the untreated blends, which was likely attributed to C-N containing fragments from the nano-coating; besides, other typical absorption bands of flameretardant volatiles, such as stretching vibration of P=O were probably overlapped around the region [44, 45]. Thus, it can be confirmed that the GP-180/APP nano-coating really shows vapor phase flameretardant effect, besides the dominating condensed phase activity as mentioned above.
Fig. 8. 3D TG-IR spectra of untreated (a) and 15BL (b) T/C blends obtained from TG-IR.
Fig. 9. Absorbance intensity changes of T/C blends as a function of time recorded by TG-IR.
4. CONCLUSION
Here, a phosphorus/nitrogen/silicon-containing nano-coating was designed and constructed by LbL assembly from colloidal silica and polyphosphates to reduce flammability of T/C blends. It was confirmed by SEM coupled with EDX and XPS analysis that the homogeneous nano-coating was successfully deposited on the surface of the T/C blends by the straightforward procedure. T/C blends coated with 15 BL nano-coatings (ca. 15.5% weight increased) achieved self-extinguishing in VFT, and showed a little delay of ignition and a strong decrease of heat release during cone calorimetry test. It was certified by TG-IR and char analysis that vapor phase flame-retardant effect by dilution of the nonflammable volatiles and the dominating condensed-phase activity by forming the intumescent char of the nano-coating together endowed the T/C blends with excellent flame retardance. With its ease of operation and use of low-toxic chemicals, this eco-friendly coating further offers a feasible fire protection solution of the blend fabrics.
ASSOCIATED CONTENT Supporting Information Electronic Supplementary Information (ESI) available: [a video about adding polyphosphate solution to colloidal silica, EDX mapping of 15BL T/C blends after VFT].
Corresponding Author Institute of Functional Textiles and Advanced Materials, National Engineering Research Center for Advanced Fire-Safety Materials D & A (Shandong), College of Textiles & Clothing, Qingdao University, Qingdao 266071, China. Tel & Fax: +86-532-85950890 E-mail: [email protected] (YJX), [email protected] (YL).
Acknowledgements This work was financially supported by the National Key Research and Development Program of China (Project No. 2017YFB0309001) and the National Natural Science Foundation of China (Grant No. 51673153 and 51973098).
Notes The authors declare no competing financial interest.
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CRediT authorship contribution statement Bin Wang: Conceptualization, Investigation, Writing-Original Draft. Ying-Jun Xu: Conceptualization, Supervision, Writing-Review & Editing. Ping Li: Investigation. Feng-Qi Zhang: Investigation. Yun Liu: Conceptualization, Supervision, Acquisition of the financial support for the project leading to this publication. Ping Zhu: Supervision.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
GRAPHICAL ABSTRACT for Polyester/Cotton Blend with Phosphorus/Nitrogen/Silicon-Containing Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
A nano-coating containing phosphorus, nitrogen and silicon was constructed on T/C blends by LbL assembly from colloidal silica and polyphosphate to reduce flammability of the fabrics by the dominating condensed-phase activity.
HIGHLIGHTS for Flame-Retardant Polyester/Cotton Blend with Phosphorus/Nitrogen/SiliconContaining Nano-Coating by Layer-by-Layer Assembly
Bin Wang, Ying-Jun Xu*, Ping Li, Feng-Qi Zhang, Yun Liu*, Ping Zhu
Highlights •
A P/N/Si-containing nano-coating was constructed by LbL assembly.
•
The treated T/C blends achieved self-extinguishing in VFT.
•
The coated T/C blends were free from scaffolding effect towards fire safety.