Preparation and Properties of Smoke Suppressive Silicone Oil Modified by Dicyandiamide

In order to improve the smoke suppression of silicone oil, relatively high nitrogen content silicone oil was synthesized by the reaction of dicyandiamide (DCD) and epoxy-terminated silicone oil (ETSO) and named DCDSO. The molecular structure of DCDSO was characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). The flame ignition and cone calorimetry test (CCT) were used to investigate the smoke suppression effect of DCDSO. The results showed that the introduction of DCD significantly improved the smoke suppression performance of DCDSO. When the molar ratio of the epoxy group in ETSO and the primary amino group in DCD was 1:1.3, the flame height and the total smoke production were significantly reduced. Thermogravimetric analysis (TGA), thermogravimetric infrared analysis (TG-IR) and thermal field emission scanning electron microscopy (SEM) were used to explore the mechanism of smoke suppression. The mechanism of smoke suppression is that the introduction of DCD reduces the rearrangement decomposition, promotes the free radical oxidation decomposition and the formation of condensed phase, and thus reduces the generation and release of smoke.


Introduction
During the welding process, a large amount of splashing slag is generated, which is difficult to clean when bonding with the product surface, and can affect the appearance and quality of welding parts. In order to solve this problem, an anti-splash agent is usually applied to the sides of the welding path and used to prevent the adhesion of splashing slag and to protect the welding material [1].
Silicone oil is usually used as an anti-splash agent. Because silicone oil not only has good heat resistance, weather resistance and oxidation resistance, but also has high flash point, small volatility, non-corrosion of metal and non-toxic [2,3]. Although the silicone oil has good heat resistance and high flash point, the high temperature of the splashing slag will light silicone oil and produce a lot of black smoke, which does harm to the health of people [4].
The black smoke contains a large number of carbonaceous compounds, mainly from the degradation of silicone oil [5].
At present, phosphorus and nitrogen containing flame retardants are usually used to suppress the smoke coming from the burning of polymers. Yang et al. [6] designed a new flame retardant hexa(p-acetamidophenoxy) cyclotriphosphazene, which was blended with the addition-cure liquid silicone rubber. The total smoke production was reduced by 41.5%, which had a certain smoke suppression effect. Sag et al. [7] synthesized four novel phosphorus-based flame retardants, which made polylactic acid show good flame retardancy, but the total smoke production did not decrease, but increased by 30%. Chen et al. [8] used ammonium polyphosphate and iron-graphene as a synergistic agent to improve the flame retardant efficiency of thermoplastic polyurethane composite and reduce the total smoke production by 34.7%. The smoke suppression effect of the type of phosphorus flame retardants was not obvious in polymer matrix materials.
Nitrogen-based f lame retardants have become a research hot spot because of their low toxicity, high flame retardant efficiency and environmental friendliness [9]. Nitrogen-based flame retardants mainly include melamine, dicyandiamide, guanidine and their derivatives [10]. Xu et al. [11] prepared the core-shell structure (ZIF-8@MA) with melamine (MA) coated zeolitic imidazolate framework-8 (ZIF-8), and successfully synthesized diatomite modified ternary composite (ZMD). When ZMD was added to the rigid polyurethane foam, the total smoke production reduced by 76.1%. Beduini et al. [12] synthesized bioinspired polyamidoamines from N, N'-methylene bisacrylamide and nine natural α-amino acids, and tested their properties as flame retardants for cotton. In the treated cotton samples, flame retardants are effective in preventing cotton fabrics from burning. The nitrogenous flame retardants usually are solid, when it is added to improve the flame retardant and/or smoke suppression of silicone oil, the flame retardants will easily settle during storage or use. As a result, the blends of nitrogenous flame retardants/silicone oil can not meet the requirement of anti-splash agent of welding.
In this paper, dicyandiamide, which has high nitrogen content, non-combustible chemical structure and low toxicity, is selected as a flame retardant to increase the smoke suppression effect of silicone oil. The reaction of the amino group of dicyandiamide with the epoxy group of epoxy-terminated silicone oil introduces relatively high content of nitrogen into the silicone oil. The as-prepared silicone oil (DCDSO) has good smoke suppression effect [13], and the smoke suppression mechanism of DCD is also discussed.

Preparation of DCDSO
According to the experimental process shown in Fig. 1(a), a certain amount of ETSO and DCD, 100 mL DMF and DMP-30 (0.1 wt% of the total mass of ETSO and DCD) were added to a three-mouth flask equipped with a refluxing condensing tube and a magnetic stirrer. The mixture was kept at 125 °C for 5 h to get reacted solution ( Fig. 1 (2)) under N 2 atmosphere, then the reacted solution was washed three times with 300 mL deionized water to remove the catalyst, unreacted monomer and solvent. The product (DCDSO) was obtained by vacuum drying at 95 °C for 10 h. The products were named DCDSO-1, DCDSO-2, DCDSO-3 and DCDSO-4 based on the molar ratios of epoxy group in ETSO and primary amino group in DCD, 1:0.1, 1:0.5, 1:0.9 and 1:1.3 respectively. The synthetic formula of samples was listed in Table 1. The synthetic route was shown as Scheme 1. When deionized water was first added to reacted solution, the stratification shown in Fig. 1 (3) quickly occurred. DCDSO is in the lower layer, and the upper layer is the mixture of DMF, deionized water, DMP-30 and a small amount of ETSO, DCD and DCDSO. The density of DCDSO-4 was 1.108 g/cm 3 .
The experiment of Fig. 1(b) was done for comparison, the amount of ETSO, DCD and DMF was the same as   Fig. 1(a). When deionized water was added to mixed solution ( Fig. 1 (2')), the ETSO is in the upper layer, while the mixture of DMF, deionized water, DCD and a small amount of ETSO is in the lower layer ( Fig. 1 (3')). The difference of density between DCDSO and ETSO revealed that ETSO and DCD took place chemical reaction. The evidence of reaction of ETSO and DCD will be further demonstrated by FT-IR and NMR.

Characterization
Fourier Transform Infrared Spectroscopy (FT-IR) FT-IR (Nicolet67, Thermo Nicolet Corporation) with resolution of 4 cm −1 , scanning times of 32, and scanning wavenumber range of 4000-500 cm −1 was used to conduct infrared characterization of the sample and carbon residue. Nuclear Magnetic Resonance Spectroscopy (NMR) The 1 H NMR spectra was obtained by using NMR (VNMRS600, Agilent Technologies Inc.), and the samples were dissolved with appropriate amount of DMSO-d 6 .
Combustion Test The swab was thoroughly soaked in the sample, lit with a lighter, and a photograph of the maximum flame was taken to observe the combustion of the sample.
Cone Calorimeter Test (CCT) CCT (CCT, Motis Combustion Technology Co., Ltd) was used to test the combustion performance of the samples under ISO5660-1:2002 standard. The samples were tested in a corundum crucible of 100 mm × 100 mm, and the height of the samples was 5 mm.
Thermogravimetric Analysis (TGA) TGA (TGA8000, PerkinElmer) was used to test the thermogravimetric behavior of samples in the air atmosphere with a heating rate of 20 °C/min and a temperature range of 30-800 °C.
Thermogravimetric Infrared Analysis (TG-IR) TG-IR (STA2500, NETZSCH-Gerätebau GmbH) was used to test the decomposition gas of the sample in the air atmosphere. The heating rate was 20 °C/min, the heating range was 30-600 °C, and the scanning wavenumber range was 4000-500 cm −1 .
Thermal Field Emission Scanning Electron Microscopy (SEM) After gold injection, SEM (Gemini 500, Carl Zeiss AG) was used to test the surface morphology of carbon residue under Mag = 2000 × and the section morphology of carbon residue under Mag = 5000 × after cone calorimeter test.

Structure Characterization
The FT-IR spectra of ETSO and DCDSO-4 are shown in Fig. 2. The absorption peak at 910 cm −1 in ETSO is ascribed to the presence of the epoxy group [14]. Some new absorption peaks appear in DCDSO-4. The absorption peak at 1564 cm −1 belongs to the N-H bending vibration [15], and the absorption peaks at 2174 cm −1 and 1652 cm −1 are attributed to the presence of C≡N and C = N in DCD [16,17]. The wide absorption peak at 3347 cm −1 belongs to the -OH and N-H in the structure of DCDSO-4 [18]. The disappearance of epoxy group absorption peak at 910 cm −1 and the appearance of -OH and N-H absorption The 1 H NMR spectra of ETSO and DCDSO-4 are shown in Fig. 3. In Fig. 3(a), the signals of chemical shift located at 2.72 ppm and 3.08 ppm are attributed to the protons of methylene group of epoxy group, and the signal of the protons of methylidyne group of epoxy group is detected at 3.21 ppm [19]. The signals of methylene bonded to the carbon and oxygen can be identified at 3.38 ppm and 3.64 ppm [20].
As can be seen from Fig. 3(b), the peaks of methylene group of epoxy group shift to 2.69 ppm and 2.85 ppm (corresponding to a'), and the peak of methylidyne group of epoxy group shifts to 3.14 ppm (corresponding to b'), and the peaks of methylene bonded to the carbon and oxygen shift to 3.33 ppm and 3.6 ppm (corresponding to c'). It shows that the introduction of DCD has an effect on the original chemical shift of ETSO. In addition, the new peaks at f (2.1 ppm), d (6.58 ppm), e (7.9 ppm) and g (5.49 ppm) are attributed to the protons of C≡N(NH)C, C(NH)C, C = NH and -OH in DCDSO-4, respectively, indicating the structure of DCDSO-4 [21,22]. The FT-IR and 1 H NMR results reveal that DCDSO is successfully synthesized by the reaction of epoxy group in ETSO and primary amino group in DCD.

Combustion Performance
The appearance photos of combustion of ETSO and asprepared DCDSO with different contents of DCD are shown in Fig. 4. When the ETSO is ignited, the flame reaches a height of 13 cm, burning furiously and producing large amounts of black smoke. With the increase of DCD content in DCDSO, the flame height gradually decreases to 3 cm, and only a small amount of white smoke was produced. The results indicated that the DCD had significantly improved smoke suppression and flame retardancy of DCDSO.

Cone Calorimeter Test
The smoke production rate (SPR), total smoke release (TSR) and heat release rate (HRR) curves of ETSO and as-prepared DCDSO are shown in Fig. 5. As can be seen from Fig. 5, SPR, TSR and HRR all decrease gradually with the addition of DCD. The TSR of DCDSO-4 (1176.83 m 2 /m 2 ) is significantly lower than that of ETSO (2543.56 m 2 /m 2 ). The peak heat release rate decreases from 282.36 kW/m 2 of ETSO to 188.44 kW/m 2 of DCDSO-4. These phenomena indicate that the presence of DCD makes the release of combustible gas relatively less in the combustion process, and also reduces the escape of degradation products to the gas phase and the heat transfer in the combustion process. Therefore, DCD has a positive effect on the flame retardant and smoke suppressant of silicone oil [23,24]. The smoke suppression mechanism of DCD will be discussed later.

The Discussion of Smoke Suppression Mechanism
Under air condition, there are two main decomposition modes of silicone oil, and the two modes compete with each other. One is rearrangement decomposition, in which the Si-O-Si bond on the main chain of the molecule is broken by heating, and small molecular ring oligomers are produced by intramolecular or intermolecular rearrangement reaction. The other is free radical oxidation decomposition, in which the methyl group on the side group of silicone oil is oxidized and falls off from Si successively in the form of CO 2 and H 2 O [25]. The TGA curves of ETSO and as-prepared DCDSO under air atmosphere are shown in Fig. 6. The mass of ETSO decreases sharply from 200 °C to 300 °C, and the thermal weight loss rate is very fast. When the temperature reaches 300 °C, the weight loss is basically completed. It can be speculated that the decomposition of ETSO is mainly rearrangement decomposition. When DCD was introduced, the decomposition of DCDSO changed greatly. With the increase of DCD content in DCDSO, the weight loss gradually increases during the first stage (100-480 °C), and the thermal stability of DCDSO is significantly higher than that of ETSO above 250 °C. The main reason is that DCD is easy to decompose. The more DCD content, the greater the thermal weight loss. However, the decomposition of DCD will absorb the heat of the system, and the NH 3 produced will dilute the concentration of flammable gas, slowing down the decomposition of the molecular chain of silicone oil. At the same time, as DCD is introduced into silicone oil molecules as the end group, the rearrangement decomposition of silicone oil is reduced and Fig. 4 The appearance photos of combustion of ETSO and as-prepared DCDSO the oxidation decomposition of free radicals is increased during heating. In addition, the CO 2 and H 2 O produced by side group decomposition also play a role in diluting the concentration of combustible gas, so the thermal weight loss of DCDSO is significantly reduced [26]. In the second stage (480-750 °C), the weight loss mainly comes from the decomposition of the residue, which may be compound of carbon and silicon, and the formation of compound of carbon and silicon inhibits the decomposition of silicone oil at the first stage. More of the products released by the silicone oil are locked in the condensing phase of the combustion process [27]. Therefore, the addition of DCD can generate more condensed phases in the first stage, and the more condensed phases, the stronger the stability [28,29]. The TG-IR spectra of ETSO and DCDSO-4 are shown in Fig. 7. Figure 7(a) shows that under an air atmosphere, decomposition of ETSO mainly occurs from 200 °C to 450 °C. In this temperature range, Si-O-Si (1030 cm −1 ) [30], Si-CH 3 (1259 cm −1 and 844 cm −1 ) and CH 3 (2955 cm −1 ) have appeared in the gas phase, while the peaks of CO 2 (2360 cm −1 ) and O-H (3749 cm −1 ) produced by the water molecule are relatively small [31]. The results indicate that annular siloxane is formed, and the decomposition of ETSO is mainly rearrangement decomposition. The composition of the large amount of smoke generated by the combustion of ETSO in Fig. 4 is most likely to be annular siloxane.  As can be seen from Fig. 7(b), the decomposition of DCDSO-4 mainly occurs from 250 °C to 480 °C, and NH 3 (960 cm −1 ) is always produced within this temperature range [32,33]. Compared with ETSO, the production of Si-O-Si, Si-CH 3 and CH 3 are significantly reduced, while the production of CO 2 and H 2 O increases. In Fig. 7(a), the production of Si-O-Si, Si-CH 3 and CH 3 reaches its highest value at 247 °C, and in Fig. 7(b), the production of Si-O-Si, Si-CH 3 and CH 3 reaches its highest value at 438 °C. This indicates that the introduction of DCD slows and reduces the rearrangement decomposition of silicone oil, and thus reduces the production of smoke [34]. In Fig. 4, the smoke generated by the combustion of DCDSO is significantly reduced, indicating that the annular siloxane in the smoke is reduced.
The SEM micrographs of carbon residue of ETSO and as-prepared DCDSO tested by CCT are shown in Fig. 8. Figure 8 shows that there are many pores on the surface and section of the residue of ETSO carbon, and the structure is loose and not dense. However, the carbon residue surface of DCDSO-1 has become continuous, only slightly uneven phenomenon exists, and the section is more uniform and continuous. With the increase of DCD content, the surface of carbon residue changed slightly and showed continuous densification. However, the section of carbon residue becomes more and more dense, and the thickness of carbon layer becomes larger. The greater the density and thickness of the carbon layer, the better the performance of the barrier against combustible gas and heat during combustion, and the stronger the ability to inhibit the decomposition of silicone oil and the volatilization of decomposition products. Therefore, the protective effect and smoke suppression effect of the carbon layer on the unburned part of the system are more obvious [35]. Therefore, the addition of DCD makes the condensing phase more compact and has better thermal stability.
It can be concluded from Figs. 6, 7 and 8 that the smoke suppression mechanism of DCD is mainly to inhibit the molecular chain rearrangement decomposition of silicone oil. The decomposition of DCD absorbs heat from the system, and the NH 3 produced dilutes the concentration of the flammable gas, slowing down the decomposition rate. At the same time, DCD reduces the rearrangement decomposition of silicone oil and increases the oxidation decomposition of free radicals. The CO 2 and H 2 O produced by side group decomposition also have dilutive effects on the concentration of combustible gas. In addition, the addition of DCD makes more products released from the combustion of silicone oil locked in the condensed phase of the combustion process, isolating the transfer of combustible gas and heat, and playing an important role in smoke suppression.

Conclusions
In this work, DCDSO was synthesized by the reaction of ETSO and DCD. The structure of DCDSO was characterized by FT-IR and NMR. The flame ignition and CCT were used to investigate the smoke suppression effect of DCDSO. The results showed that the flame height reduced 77% from flame ignition and total smoke , (e') sections of carbon residue of ETSO and as-prepared DCDSO tested by CCT production decreased 53.73% from CCT. Combined with the results of TGA, TG-IR and SEM, the smoke suppression mechanism was analyzed. The mechanism of DCD smoke suppression is mainly to inhibit the molecular chain rearrangement decomposition of silicone oil. DCD is easy to decompose. The decomposition of DCD will absorb the heat of the system, and the NH 3 produced will dilute the concentration of flammable gas, slowing down the decomposition. At the same time, as DCD is introduced into silicone oil molecules as the end group, the rearrangement decomposition of silicone oil is reduced and the oxidation decomposition of free radicals is increased during heating. And the CO 2 and H 2 O produced by side group decomposition also play a role in diluting the concentration of combustible gas. In addition, the addition of DCD makes more products released by silicone oil locked in the condensed phase of the combustion process, isolating the transfer of combustible gas and heat, and playing an important role in smoke suppression [36,37]. In conclusion, the introduction of DCD significantly improved the smoke suppression performance of silicone oil.