Synthesis of PMM and Synergistic Flame Retardant Effect with Ammonium polyphosphate

A novel flame retardant poly {[(2-Methyl-acrylic acid 2-(dimethoxyphosphoryloxy)-ethyl ester]-co-[2-Methyl-acrylic acid 2-(trimethoxysiloxy)-ethyl ester]} (PMM) containing dimethoxyphosphoryloxy and trimethoxysiloxy groups in side chains was synthesized. The structure of PMM was characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (1H NMR), and the thermal performance of PMM was characterized by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The effect of PMM on polypropylene’s flame retardancy and thermal behaviors was investigated by limited oxygen index (LOI), vertical burning test (UL-94), and TG tests. The experimental results indicate that the addition of PMM helps to improve the flame retardancy and thermal stability of PP. When the addition of PMM and APP in PC was 15% and 20%, the LOI value of PC reached 28.4%, and the UL-94 test rating reached V-0 level. The charred structure observed by scanning electron microscopy (SEM) indicated that the char surface for PP/APP/PMM system holds an intumescent char structure compared to neat PP, and the synergy exists in the composites. The PMM slightly decreased the tensile property in terms of strength but increased the elongation at break for PP composites.


Introduction
The trichlorosilane and silicon tetrachloride (ST) byproducts from electronics, semiconductor, and photovoltaic industries may become serious pollution.These chemicals should be treated and recycled to meet the requirement of the sustainable development of the photovoltaic industry [1,2] .Many technologies have been developed for the removal and recycling of these byproducts.For example, microwave plasma jets can destroy silicon tetrachloride [3] .Due to their strong reaction activity, trichlorosilane and silicon tetrachloride can be introduced to synthesize some new functional materials [4]   .Herein, the novel flame retardants containing silicon have attracted more attention [5] .Polypropylene is one of the most widely used polymers because of its thermal stability.To improve the performance of PP, the N-alkoxy hindered amine containing silane multifunctional flame-retardant synergist [6] , polysiloxane/silane-modified SiO2 intumescent flame-retardant polypropylene system [7] , IFR containing silicon, phosphorus, and nitrogen [8] , ammonium polyphosphate modified with 3-(methyl acryloyl) propyl trimethoxy silane [9] have been prepared, and their flammability and mechanical properties have been discussed.These studies found that the dense layer of silicon carbon formed during the burning of polymer composites thus helps isolate insulation and oxygen and reduces smoke and droplet.Meanwhile, silicon-containing compounds have good mechanical workability and compatibility with polymers.In addition, using phosphorus and silicon together synergized polymers' flame retardancy and smoke suppression properties [10] .Despite recent progress, how to improve the compatibility, synergy, and performance of the polymer nanocomposites (PNCs) remains to be elucidated further.

Materials
Luoyang Wannian Siliconindustry Co., Ltd provided Silicon tetrachloride (industrial grade), which was distilled under 55~57 ℃.Phosphorus oxychloride (CP) was purchased from Chinese Tingxin Chemical Co. and was distilled under 104~106 ℃.Hydroxyethyl methacrylate (HEMA) was supplied by Aladdin.It was washed with 5% NaOH solution, dried with anhydrous sodium, and distilled under a vacuum.Benzoyl peroxide (BPO, CP) was purchased from Shanghai Lingfeng Chemical Co.Before being used, it was purified by recrystallization using methanol.Tetrahydrofuran (THF), methanol, and cyclohexane (AR) were purchased from China Sun Specialty Products Co. Ltd and dried by 4A molecular sieves before use.Polypropylene (PP, CP) was supplied by Sinopec Crop China.Suzhou Longfu Chemical Co.supplied ammonium polyphosphate (APP, Industrial grade).

Synthesis of 2-Methyl-acrylic acid 2-(trimethoxysiloxy)-ethyl ester (MATSE)
Add 8.5 g (0.05 mol) of silicon tetrachloride and 0.2 g of anhydrous calcium chloride to a 100 mL four necked round bottom flask, stir, condense and reflux until the silicon tetrachloride in the mixture is completely dissolved, and solution A was prepared.After that, 6.5 g (0.05 mol) HEMA was added dropwise in solution A within about 0.5 h under nitrogen gas protection.The reaction takes place under magnetic stirring and condensation reflux.By using an ice water bath, maintain the reaction temperature at around 10 o C. Due to the formation of HCL in the reaction, 5.8 g (0.18 mol) methanol was added dropwise slowly.Then, control the reaction at 60 o C and condense reflux until the reaction system had no viscosity increase.Lowering the system to room temperature, a rotary evaporator filtrated and distilled the raw product.A 4A molecular sieve dried the product (Yield 67.6 %).The synthesis route was illustrated in Scheme 1.

Synthesis of 2-Methyl-acrylic acid 2-(dimethoxyphosphoryloxy)-ethyl ester (MADPE)
Firstly, phosphorus oxychloride (7.67 g, 0.05 mol) and 0.2 g anhydrous calcium chloride were added to a 100 mL four-necked flask to prepare solution B. Then, 6.5 g (0.05 mol) HEMA was added dropwise into solution B within 0.5 hours under nitrogen protection.Cooling the reaction through an ice water bath.As no HCl was released, methanol (4.8 g, 0.15 mol) was added dropwise to the above solution.Then, Control the system temperature at 80 o C and react for 2 hours.Cool the reaction to room temperature, a rotary evaporator filtrated and distilled the raw product.The product (Yield: 62.6 %) was dried by a 4A molecular sieve.The synthesis route was illustrated in Scheme 1.

Synthesis of poly (MADPE-co-MATSE) (PMM)
Add THF (15 g), MATSE and MADPE with different MATSE-to-MADPE molar ratios to a 100ml three necked flask.Then,add BPO (0.3 % by mass of monomer) to the system.The solution was stirred and refluxed for 11 hrs under 70 o C.After the reaction, the product was precipitated with cyclohexane and washed with methylene chloride 2-3 times.The product PMM was vacuum dried at 80 o C for 12 hrs.The synthesis route is illustrated in Figure 1.

Characterization and instruments
FTIR spectroscopy (Nicolet MagNa-IR550) characterized the product synthesized using a thin KBr disc.
The transition mode was used, and the wavenumber range was set from 4000 to 400 cm -1 .Nuclear magnetic resonance ( 1 H NMR) spectra were recorded on an ADVANCE Ⅲ Digital 400 spectrometer (Bruke, Germany) using tetramethylsilane as a reference, respectively, and CDCl3 and DMSO as solvent.
Thermogravimetric analysis (TGA) was carried out on the FRC/T-2 (Hengjiu, Nanjing) thermogravimetric analyzer with a heating rate of 10 ℃/min in an air atmosphere from room temperature to 700 ℃.Differential scanning calorimetry (DSC) analysis was conducted on a Pyris-1 (Pekin-Elmer, America) DSC with a heating rate of 10 ℃/min in a nitrogen atmosphere from 50 ℃ to 300 ℃.
The structural characterization of residual carbon was analyzed using S-4700 scanning electron microscope (SEM) (Hitachi, Japan) and IE350 energy dispersive X-ray spectrometer (EDS) (Oxford, UK) was used to analyze the element of PMM, working voltage 25 kV, current 25 mA.
Mechanical properties of specimens were tested using XLD-1A (Jinke, Tianjin) electronic tensile testing machine with a drawing speed of 20 mm•s -1 to test mechanical properties at the temperature of 30 ℃.

Preparation and characterization of target products
FTIR, 1 H NMR, and EDS characterized the chemical structure and composition of the product.Figure 2 presents the FTIR spectra of HEMA, MADPE, MATSE, and PMM.The neat HEMA (Figure 2(a)) exhibits several characteristic absorption bands: -OH stretching vibration at 3430 cm -1 and C=O stretching vibration at 1720 cm -1 .The absorption band at 1630 cm -1 corresponds to C=C stretching vibration.In Figure 2(b), the marked peaks of MADPE locate at 3430 cm -1 (O-H), 1232 cm -1 (P=O), and 1046 cm -1 (P-O-C), respectively [11] .As for MATSE (Figure 2(c)), the strong and broad Si-O stretching vibration locates at 1094 cm -1 .The disappeared -OH peak at 3430 cm -1 indicates the generation of MATSE.In the PMM spectrum, there is no C=C vibration at 1630 cm -1 and the MADPE and MATSE peaks can be observed.These illustrate the form of PMM.C=CH2, trans), and δ6.12-6.16(-C=CH2, cis) [12] .δ (ppm) Figure 3. 1 H NMR spectrum of MADPE.  Figure 6 shows the elemental analysis results of PMM.The composition of PMM is C: 43.51%; O: 35.48%; Si: 8.03%, P: 12.36%; and Cl: 0.62%.Due to the steric tetrahedral structure of phosphorus oxychloride, the first two chlorine reacted completely, but the last chlorine with low activity.So, there is still some chlorine in the product.

Discussion of the synthesis process
Throughout the synthesis process, due to the strong active, silicon tetrachloride or phosphorus oxychloride reacted with HEMA or methanol rapidly, even at room temperature.So, the synthesis of MADPE or MATSE was only one of the key steps of the whole reaction.However, the contents of P and Si in PMM affect the performance of the flame retardant.Therefore, the MATSE-to-MADPE ratio directly affects the flame retardant performance of PMM.Based on previous research findings, we found that the char yield and swelling degree can optimize the composition of the polymer.The swelling degree was tested: Firstly, a 0.5 g sample was added into a muffle furnace and combusted under 500 o C for 30 min.Then, the swelling degree was measured based on the volume changes before and after heat: Swelling degree= (Vresidue-Vspecimen) /Mspecimen*100%.Similarly, the char yield was measured based on the mass changes before and after heat: Char yield =Mresidue/Mspecimen*100%.The effect of the MATSEto-MADPE molar ratio on the swelling degree and char yield of PMM (Table 1) indicates that, as the MATSE-to-MADPE mole ratio decreases from 2:1 to 1:2, the swelling degree and char yield of PMM vary from 29.34 to 44.25 and 3.46 to 56.28, respectively.The higher swelling degree and char yield mean better flame retardancy.So, the optimum MATSE-to-MADPE molar ratio is 2:3.
Table 1.Effect of molar ratios on the swelling degree and char yield of PMM.Based on the MATSE-to-MADPE ratio (=2:3) and 30 wt% concentration of monomer, Conducting experiments through orthogonal array [L9 (3 4 ) matrix] and focused on studying three variables: reaction temperature (factor A), reaction time (factor B) and amount of initiator (factor B).The experimental program and results are shown in Table 2 and Table 3.According to Table 3, the impact factors for the preparation of PMM rank as A>C>B, i.e., the reaction temperature is the most important factor.The lower reaction temperature leads to a longer initiation time and lower efficiency.However, if the temperature is too high, BPO may decompose quickly, and some byproducts emerge.From the analysis above, the optimal solution is A2B3C2, i.e., the reaction temperature is 70 o C, the reaction time is 11 hrs, and the initiator dosage is 0.3 wt%, respectively.

Thermal properties
DSC analysis was conducted with a 10 o C/min heating rate in a nitrogen atmosphere, rise from the initial 50 o C to 300 o C. Half extrapolated heat temperature can obtain the glass transition temperature (Tg).As seen in Figure 7, the Tg of PMM is about 132.46 o C, which is higher than that of PHEMA (~75 o C) [13] .
No crystalline melting peaks in the DSC curve indicate that PMM is an amorphous copolymer.Figure 8 presents the TG, DTG curves of PP, PMM, PP/APP, and PP/APP/PMM.These samples were tested in the atmosphere to simulate the real combustion behavior.As shown in Figure 8(a), in the initial stage, the degradation of these polymers is similar and different from each other in the second stage (after 157 o C).The initial dehydration and water as the main products became the main flame retardant mechanism in the initial stage; then, subsequently, Random breakage of the main chain became the main flame retardant mechanism in the future, dehydration/denitrification to form char, and water/ammonia as major products.Therefore, the improvement of flame retardancy can be achieved by reacting phosphate/silicate groups into the backbone of polymeric materials [14] .The weight of char at 600 o C varies from 10.05% (PP) to 31.90% (PP/APP (20%)/PMM (10%)), indicating the thermal stability ranking as PP/APP (20%)/PMM (10%) > PP/APP (20%)> PMM > PP.
The DTG thermograms revealed that APP and APP/PMM produced changes in the thermal degradation mechanisms of the polymers.Table 4 summarizes the data obtained from TG curing .For PP/APP nanocomposites, the shift of the right peak in the DTG curve of PP (379 o C) to a higher temperature (414 o C) is related to forming an anhydride structure.In the case of PP/APP/PMM composites, the distinct peak loats at 420 o C and a broad peak from 259 o C to 387 o C are also observed.This can be attributed to the decomposition of APP and PMM (378 o C).As shown in Figure 8, the decomposition temperature of PMM and APP approach that of PP, indicating a good flame retardancy, i.e., the flame retardant decomposes before the polymer.Hence, it protects the polymer matrix [15,16] .To clarify the flame mechanism of PP composites, SEM was introduced to characterize the morphology of char (Figure 10).As can be seen from Figure 10(a), The outer surface image of PP/APP char appears to be fluffy and rich in pores.The opened-hole area dominates the surface of the char.However, the outer surface of the PP/APP/PMM char seems compact and smooth.The expanding internal structure forms a good barrier, preventing the diffusion of heat, mass, and flammable gases between the matrix during the combustion process.So, as a barrier, the closed hole has a better barrier effect than the opened one.Therein, the PMM plays an important role in crosslink with PP and APP during combustion to make the original carbon layer more expansive which is significant in protecting the matrix materials that have not yet undergone pyrolysis and combustion, i.e., the synergy exists in the composites [17] .

Mechanical properties
The tensile properties of the PP and PP composites are summarized in Table 6.The ultimate tensile strength (UTS) of PP decreased by adding APP or APP/PMM.The UTS of PP/APP/PMM decreased as the additive loading increased from 5% to 35% with the increasing PMM loading.Accordingly, the elongation at the break of the PP/APP/PMM composites slightly increases as the additive loading increases.In a word, the incorporation of APP and PMM slightly decreased the tensile property in terms of strength but increased the elongation at break for PP composites [18] .

Conclusions
In our present work, a novel polymeric flame retardant was synthesized.The optimum conditions were obtained using THF as a solvent with a reaction temperature of 70 o C and a reaction time of 11 hrs.A flame retarding synergistic effect of PMM and APP for PP was observed.The compact and smooth outer surface of PP/APP/PMM char and the swollen inner structure provides a good barrier to transferring heat, mass, and flammable gases during combustion.The PP/APP 20%/PMM 15% achieved the UL94 V-0 standard.The PMM slightly decreased the tensile property in terms of strength but increased the elongation at break for PP composites.

Figure 1 .
Figure 1.Process flow diagram of synthesis of PMM.2.5 Preparation of IFR-PP composites Blending PP powder to prepare IFR-PP composites, Mix APP and PMM using a high-speed mixer and then extruded by a single-screw extruder (D: 20 mm, L/D: 25, model: SJ20×25 Wuxi Lanling Plastic Machine, China) at temperature profiles of 150, 170, 185, 190, 190, 180 o C, and cut into pellets.The sample bars for testing were prepared by the injection method (Injector: HTF86X1, Zhejiang Haitian, China) at a temperature profile of 200, 210, 210, and 210 o C.

Table 3 .
Results of orthogonal experiments.

Table 6 .
Mechanical properties of PP and PP composites.