Flame-retardant polyvinyl alcohol membrane with high transparency based on a reactive phosphorus-containing compound

Flame-retardant polyvinyl alcohol (PVA) membranes with high transparency and flexibility were prepared by mixing an aqueous solution of a phosphorus-containing acrylic acid (AOPA) with PVA. The reaction between AOPA and PVA, the transparency, the crystallinity and the flexibility of the membrane were investigated with Fourier transform infrared spectrometry (FTIR), UV–vis light transmittance, X-ray diffraction and tensile tests, respectively. The limited oxygen index (LOI) and vertical flame (UL 94 VTM), microscale combustion calorimetry, thermogravimetric analysis (TGA) and TGA-FTIR were employed to evaluate the flame retardancy as well as to reveal the corresponding mechanisms. Results showed that PVA containing 30 wt% of AOPA can reach the UL 94 VTM V0 rating with an LOI of 27.3% and retain 95% of the original transparency of pure PVA. Adding AOPA reduces crystallinity of PVA, while the flexibility is increased. AOPA depresses the thermal degradation of PVA and promotes char formation during combustion. The proposed decomposition mechanism indicates that AOPA acts mainly in the condensed phase.


Introduction
Polyvinyl alcohol (PVA) is a biocompatible and biodegradable polymer. Good film formation, high transparency and mechanical properties make PVA films widely used in the textile industry, furnishings, adhesives and packaging materials [1][2][3]. Like most 2. Material and methods

Materials
Poly(vinylalcohol), Mowiol ® PVA-117 with Mw = 145 000 and an alcoholysis degree of 98% was provided by Aladdin reagent company. AOPA was synthesized in our lab. Its structure and reaction with PVA is shown in figure 1.

Preparation of polyvinyl alcohol/phosphorus-containing acrylic acid membrane
PVA aqueous solution (10 wt%) was obtained by dissolving PVA into water at 90°C for 3 h; then the AOPA of different mass was added with mechanical stirring for about 30 min. A transparent solution of PVA/AOPA was poured onto a glass plate and dried at ambient temperature for 24 h followed by 80°C in vacuum for 8 h. The obtained film is about 0.12 mm. The formulation and samples' names are listed in table 1.

Measurements and characterization
The 1 H-NMR spectra were recorded in a solution of DMSO-d 6 at 25°C with a Mercury VX-300 instrument operating at 400 MHz (Varian, US), using TMS as the inner reference. The Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iS10 FTIR spectrometer; samples of about 10 mg were heated in aluminium pans from 40°C to 700°C at a heating rate of 1°C s −1 . The flow rate of N 2 and O 2 was 80 ml min −1 and 20 ml min −1 , respectively. Tensile property testing was carried out using a CMT6000 universal testing machine (SANS, China) following the standard ASTM D638. The testing speed was 10 mm min −1 . Thermogravimetric analysis (TGA) was carried out with the TSDT Q600 thermogravimetric analyser (TA, USA). Samples of about 10 mg were heated in aluminium pans from room temperature up to 700°C at a heating rate of 20°C min −       In the spectra of PVA/AOPA30 dried at 40°C, the absorption for the -OH is very strong. After the film is treated at 80°C for 8 h, the absorption at 3250 cm −1 becomes weak. The characteristic absorptions at 2500-2700 cm −1 and 3500 cm −1 for AOPA have vanished. FTIR results have proved the reaction between phosphinic acid of AOPA and the hydroxyl group of PVA. In addition, it is noted that the absorption at 1635 cm −1 for the C=C of AOPA becomes weaker after the PVA/AOPA membrane is treated at 80°C, indicating an addition reaction of the AOPA itself has occurred within the membrane.

UV-visible analysis of membranes
UV-visible transmittance and photographs of the membranes covering the words are presented in figure 4. In the photographs, the boundary between background and the membrane is marked with a blue ellipse. The words 'Jianghan University' under the membranes are clearly visible. The transmittance of the membranes at 600 nm is listed in table 1. In the UV-vis spectra, each membrane possesses a high transmittance of above 85% over a range of 400-800 nm. The transmittance of the membranes drops only by 4.5% when the AOPA content increases from 0 to 40% at 600 nm. High retention of transmittance implies good compatibility between AOPA and PVA. 3.4. X-ray diffraction analysis of polyvinyl alcohol and polyvinyl alcohol/phosphorus-containing acrylic acid membranes Figure 6 shows the XRD profiles of PVA and PVA/AOPA membranes. PVA exhibits the crystalline structure with diffraction peaks at 19.2°. Incorporating AOPA reduces the crystallinity of PVA, and the diffraction intensity of PVA/AOPA becomes smaller with the increase in AOPA content. After the AOPA content has increased beyond 30%, the diffraction intensity does not decrease any more. The XRD results support the mechanical properties of membranes: Reduction in the crystallinity of PVA is another reason for the higher flexibility of PVA/AOPA membranes. In addition, PVA/AOPA40 shows the same weak intensity as PVA/AOPA30 in the XRD, but the modulus of the membrane is higher than PVA/AOPA30. The reason is the probability that the chains' movement becomes difficult again due to too much AOPA existing as side chains in the PVA/AOPA40 membrane.    Figure 7 presents the heat-release rate (HRR) of PVA/AOPA membranes with temperature by microscale combustion calorimetry (MCC). Data from the MCC test including the peak of heat-release rate (PHRR), the temperature of PHRR (T p ) at the main heat-release stage, the total heat release (THR) and heat-release capacity (HRC) are listed in table 2. All samples show two-step heat release. For the pure PVA membrane, a big heat-release peak appears at 307°C, followed by a minor one around 450°C. For the PVA/AOPA membranes, the heat release at low temperature is depressed and the majority of heat is released at high temperature with T p around 430-450°C. In addition, the THR and HRC values of PVA/AOPA membranes are lower than that of PVA and decrease with the increase in AOPA content. HRC is the ratio of specific HRR to the rate of the temperature rise of a sample polymer during a test. It is an important parameter for the determination of fire safety and flame retardancy. The above results imply that adding AOPA moves the heat release of the PVA to a higher temperature. The heat-release speed and the THR of PVA are also reduced in the presence of AOPA.

Combustion
3.6. Thermal stability of polyvinyl alcohol/phosphorus-containing acrylic acid membranes Figure 8 shows the thermogravimetry and differential thermogravimetry (TG-DTG) curves of AOPA, PVA and PVA/AOPA30 in N 2 . Detailed data including temperature at 5% mass loss (T 5% ), temperature for the maximum decomposition rate (T max ), the maximum decomposition rate (DTG max ) and char yields at 700°C are summarized in table 3. All the samples show two-step decomposition. The T 5% and T max of PVA and AOPA in the first stage are well matched to each other. For example, the T 5% and T max in the first stage for the PVA are 240°C and 292°C, respectively; the T 5% and T max in the first stage for AOPA are 237°C and 303°C, respectively. Synchronous decomposition of AOPA and PVA makes the AOPA play a maximum role in trapping the radicals releasing from the PVA or in acting in the condensed phase to promote char formation. Comparing with pure PVA, the thermal degradation of PVA/AOPA30 is greatly depressed in the first decomposition stage (less than 370°C). The char yield obtained at 700°C (9.90%) is twice as much as the calculated value (4.9%). TG-DTG analyses indicate that the presence of AOPA depresses the decomposition of PVA.

Analysis of decomposition products
To understand the decomposition behaviour of the membranes, real-time FTIR was used to analyse the volatility during thermal decomposition. Figure 9 shows the 3D-FTIR spectra of the pyrolysis gas, the intensity of the total gas release with time and the 2D-FTIR spectra of pyrolysis gas at the maximum decomposition rate. The gas release of PVA/AOPA30 is much less than that of the pure PVA membrane. The main signals for the gas from PVA are 3650 cm −1 (water), 3480 cm −1 (-OH), 2920 cm −1 (-CH 2 , CH 3 ),   In the PVA/AOPA30 membrane, the signals for the gas containing C=O and C=C group were not observed and the absorptions for -CH 2 , CH 3 and CO 2 are weaker than that for the pure PVA membrane. Figure 10 shows the spectra of residues of the PVA and PVA/AOPA30 membrane collected at 300, 400 and 600°C in the TG test. In the spectrum of PVA, peaks at 3452 cm −1 (OH), 2920 cm −1 (CH 2 , CH 3 ), 1172 and 1715 cm −1 (C=O) and 1568 cm −1 (C=C) decreased when the temperature increases to 400°C, and then these absorptions vanish at 600°C. There is a strong and broad absorption at 1446 cm −1 appearing at 600°C which belongs to the conjugated C=C bonds. In the spectrum of PVA/AOPA30 residues obtained at 600°C, a broad and strong absorption at 1254 cm −1 is ascribed to the overlapping of signals of C-O-C (about 1100 cm −1 ) with P=O (1300 cm −1  transmittance (%) Figure 10. FTIR spectra for the residues of PVA and PVA/AOPA collected at 300°C, 400°C and 600°C in the TG test.

Mechanisms of flame retardancy
Based on the results of thermal analysis, evolved gas and residues of AOPA, PVA and PVA/AOPA30 and the literature [25][26][27][28], a decomposition mechanism as shown in figure 11 is proposed. AOPA undergoes ester-linkage breaking at first producing 2-carboxyethylmethylphosphinic acid during the decomposition of the membrane. 2-carboxyethylmethylphosphinic acid undergoes progressive scission of the P-C bond giving the methylphosphinic acid radical, which can form stable phosphorus acid anhydride R-P(=O)-O-(O=)P-R, or react with aromatic compounds forming the stable phosphate in the residues. PVA also undergoes ester-linkage breaking at first, and progressive degradation releases CO 2 , alkene, hydrocarbon, acid, ester and ether into the gas phase. In the solid phase, unsaturated vinyl compounds are left. Vinyl compounds further form the aromatic compounds and stay in the residues. In the PVA/AOPA, the reaction of methylphosphinic acid radicals from 2-carboxyethylmethylphosphinic acid with aromatic compounds unavoidably occurs and produces extra residues to slow down the heat transfer and depress the progressive degradation of the inner material. So, higher char yield and lower heat release are the results. combustion than pure PVA. Combining the results from thermal analysis, real-time FTIR spectra of evolved gas and residues during decomposition, a decomposition mechanism is proposed to explain the role of AOPA in improving the flame retardancy and depressing the decomposition of the membrane. The segments from AOPA react with compounds from PVA to form an extra stable char layer, which slows down the progressive spreading of heat and fire.

Conclusion
Data accessibility. All data used in this research are included in figures, tables and the electronic supplementary material. Authors' contributions. S.P. and M.Z. designed the study. F.L. and C.Z. prepared the samples. L.Z. and J.C. conducted the test/analysis. X.L. and J.L. interpreted the work and wrote the manuscript. All authors gave their final approval for publication.
Competing interests. We declare we have no competing interests.