Preparation and Antiflame Performance of Expandable Graphite Modified with Sodium Hexametaphosphate

A kind of polyphosphate modified expandable graphite (EGp) was prepared in graphite oxidation and intercalation reaction with KMnO 4 as oxidant, H 2 SO 4 as intercalator, and sodium hexametaphosphate (SHMP) as assistant intercalator. The feasible mass ratio of C : KMnO 4 : H 2 SO 4 (98%) : SHMP was determined as 1.0 : 0.3 : 4.5 : 0.6, H 2 SO 4 was diluted to 77wt% before intercalation reaction, and the reaction lasted for 40min at 40C. Expanded volume and initial expansion temperature of the prepared EGp reached 600mL/g (at 800C) and 151C, respectively. X-ray diffraction spectroscopy testified the intercalation and layer structure of EGp, and Fourier transform infrared spectroscopy illuminated the intercalated functional groups. Flame retardance of the prepared EGp and the referenced EG (with only H2SO4 as intercalator) for linear low density polyethylene (LLDPE) was also investigated. Addition of 30wt% EGp to the polymer improved the limiting oxygen index (LOI) from 17.5 to 27.3%. On the other hand, the LOI of the same amount of the referenced EG was only 24.6%. Assistant intercalation of SHMP improved the dilatability and flame retardancy.


Introduction
Graphite is a kind of crystal compound with layer structure, and its intercalating compound named expandable graphite can be prepared when noncarbonaceous reactants are inserted into graphite layers through chemical or electrochemical reaction [1,2].Expandable graphite has many good properties: it can be used as catalyst in the synthesis of organic ester [3], and when it expands at high temperature, a poriferous material called expanded graphite is prepared.Expanded graphite is a kind of effective adsorbent for heavy oil and dyes wastewater [4][5][6].At the same time, expandable graphite is a good intumescent type flame-retardant for its good capability of halogen-free and nondropping [7,8].When expandable graphite exposes to flame, it can give a swollen multicellular char, which can protect materials from heat and oxygen.Simultaneously, expandable graphite absorbs huge heat during the instant expansion, which can decrease the burning temperature.When it is oxidized on reaction with H 2 SO 4 at high temperature, the released CO 2 , SO 2 , and H 2 O can reduce concentration of combustible gas [9].All these characteristics indicate expandable graphite is a good flame retardant.
When expandable graphite is used as flame retardant, its dilatability (shown as expanded volume (EV)) and thermal stability (shown as initial expansion temperature  0 ) are very important parameters [10], and it can be divided into three kinds: low  0 (between 80 and 150 ∘ C), middle  0 (between 180 and 240 ∘ C), and high  0 (between 250 and 300 ∘ C) expandable graphite.
In the preparation of expandable graphite, reactants and theirs contents, such as oxidant, intercalator and assistant intercalator, and reaction temperature, reaction time can all affect its dilatability.With KMnO 4 as oxidant and H 2 SO 4 and acetic acid as intercalator and assistant intercalator, expandable graphite with a  0 of 160 ∘ C and EV of 460 mL g −1 was prepared [11].Expandable graphite holding a  0 of 310 ∘ C and EV of 270 mL g −1 could be prepared with 85 wt% H 2 SO 4 as intercalator, KMnO 4 as oxidant, and FeSO 4 as close agent [12].
Linear low-density polyethylene (LLDPE) possesses low machining temperature (less than 140 ∘ C) and it is very flammable.In this research, with KMnO 4 as oxidant and H 2 SO 4 as intercalator, and sodium hexametaphosphate (SHMP) as assistant intercalator, the phosphate modified expandable graphite (EG  ) with high dilatability and fitting for LLDPE flame retardancy was prepared.The dosages of KMnO 4 , H 2 SO 4 , and SHMP and reaction temperature and reaction time were optimized in graphite intercalating reaction.X-ray diffraction spectroscopy (XRD) and Fourier transform infrared spectroscopy (FTIR) were employed to illuminate the layer structure and intercalating components.Flame retardancy, indicated as limiting oxygen index (LOI) of the EG  for LLDPE, was also investigated.

Experimental
2.1.Materials and Reagents.Natural flake graphite with an average flake size of 0.3 mm and a carbon content of 92% was provided by Action Carbon Co. Ltd., Baoding, China.Analytical reagent of SHMP was obtained from Fuchen, Tianjin, China.H 2 SO 4 (98%) and KMnO 4 are all analytical agents.LLDPE (7042, 0.918 g/cm 3 , melt index 2.0 g/10 min).

Experimental Method
2.2.1.EG  Preparing Procedure and Its Optimization.In the intercalating reaction of material graphite, the reactants were quantified according to a definite mass ratio of C : H 2 SO 4 (98%) : KMnO 4 : SHMP, and H 2 SO 4 was diluted with deionized water before reaction.Then, the quantified reactants were mixed and stirred in the order of the diluted H 2 SO 4 , SHMP, C, and KMnO 4 in a 250 mL beaker, controlled at a constant temperature.After reaction, the solid phase was washed with deionized water and dipped in water for 2.0 h until pH of the wastewater reached 6.0-7.0, and then EG  is obtained after filtration and drying at 50-60 ∘ C for about 5.0 h.Its dilatability showing as EV and  0 was detected according to the reported method [11].
The influences of mass ratio of graphite to KMnO 4 , SHMP, and H 2 SO 4 and its concentration, reaction time, and temperature on EG  dilatability were tested as follows.

Influence of KMnO 4 Dosage on EG 𝑝 Dilatability.
In order to investigate the influence of KMnO 4 dosage on dilatability, single-factor experiments were carried out by changing KMnO 4 dosage in the range of 0.2∼0.6 g/g.According to the method mentioned above, experiments were carried out under the constant mass ratio C : SHMP : H 2 SO 4 (98%) of 1.0 : 0.6 : 4.5.Before reaction, H 2 SO 4 was diluted to 80 wt%, and the reaction lasted for 1.0 h at 40 ∘ C.
Figure 1 shows the changes of EV with the amount of KMnO 4 .As an oxidant, insufficient KMnO 4 will cause incomplete oxygenation of graphite and decrease of EV, while superfluous KMnO 4 will cause excessive oxygenation of graphite, which leads to a decrease in EG granularity and EV.When the mass ratio of KMnO 4 to C is controlled as 0.3 g/g, the prepared EG possesses a higher EV of 600 mL/g, and then the feasible dosage of KMnO 4 can be set as 0.3 g/g.

Influence of H 2 SO 4 Dosage on EG Dilatability.
In order to investigate its influence and feasible dosage, H 2 SO 4 dosage was changed in the range of 3.5∼6.0g/g.Experiments were carried out under the constant mass ratio C : SHMP : KMnO 4 of 1.0 : 0.6 : 0.3, the reaction lasted for 1 h at 40 ∘ C, and H 2 SO 4 was diluted to 80 wt%. Figure 2 shows the changes of EV with H 2 SO 4 amount.In intercalation reaction of graphite, H 2 SO 4 acts as intercalator and oxidant and provides an acidic environment for the oxidability of KMnO 4 .Equation (1) shows that insufficient H 2 SO 4 will incur a poor oxidation of KMnO 4 and H 2 SO 4 , cause an incomplete intercalation reaction, and lead to the decrease of dilatability.With the increase of H 2 SO 4 dosage, the oxidation of KMnO 4 and H 2 SO 4 is enhanced, causing the intercalation reaction gradually completed and leading to the increases of dilatability.When the H 2 SO 4 dosage achieves a balance in three areas, the prepared EG will present high EV.Conversely, EV will decrease when the H 2 SO 4 dosage is under or over the suitable value.Results shown in Figure 2 present that the feasible mass ratio of H 2 SO 4 to C is 4.5 g/g: (1)

Influence of H 2 SO 4 Concentration on EG Dilatability.
Under the constant mass ratio C : SHMP : KMnO 4 : H 2 SO 4 (98%) of 1.0 : 0.6 : 0.3 : 4.5 (g/g), the reaction lasted for 1 h at 40 ∘ C, and influence of H 2 SO 4 wt% in the reaction was detected and shown in Figure 3. Before reaction, 98 wt% H 2 SO 4 was diluted with deionized water to different wt% in the range of 65% ∼85%.Electrode potential of MnO 4 − /Mn 2+ can be calculated according to (1).It shows that there is a positive correlation between [H + ] and the oxidation of KMnO 4 .Therefore, within a certain range, the oxidation of KMnO 4 is enhanced with the increase of H 2 SO 4 concentration, causing the intercalation reaction gradually completed and leading to the increases of dilatability.But, with the further increase of H 2 SO 4 concentration, it will cause the excessive oxidation of graphite when it is over a suitable concentration.As shown in experiment results, the feasible H 2 SO 4 concentration is 77 wt%.

Influence of SHMP Dosage on EG Dilatability.
Under the constant mass ratio C : H 2 SO 4 (98%) : KMnO 4 of 1.0 : 4.5 : 0.3 (g/g), the reaction lasted for 1 h at 40 ∘ C and H 2 SO 4 was diluted to 77 wt%, and the influence of SHMP dosage was detected in the range of 0.4∼0.8g/g.As an assistant intercalator, increase of SHMP dosage can improve EG dilatability as shown in Figure 4.When the mass ratio of SHMP : C is controlled as 0.6 g/g, EG holds a maximum of EV.Superfluous SHMP will cause the relative scarcity of KMnO 4 and incomplete oxygenation of graphite.Figure 4: Influence of SHMP dosage on EV.

Influence of Reaction Temperature on EG Dilatability.
Influence of reaction temperature on the reaction is mainly reflected in two aspects: reaction rate and balance direction.For the reaction rate, it is positively correlated with reaction temperature.Instead, for exothermic reaction, such as oxidization and intercalation of graphite, the degree of reverse reaction will increase greatly with the increase temperature.So reaction temperature creates different effects on the reaction rate and direction.Under the constant mass ratio C : H 2 SO 4 (98%) : KMnO 4 : SHMP of 1.0 : 4.5 : 0.3 : 0.6 (g/g), H 2 SO 4 diluted to 77 wt% before reaction, and reaction that lasted for 1 h, the influence of reaction temperature on EV was detected.When it is less than 40 ∘ C, the increase of temperature can improve EG dilatability.However, too high temperature causes the exothermic reaction releasing more heat and excessive oxygenation of graphite.So the feasible reaction temperature can be set as 40 ∘ C.

Influence of Reaction Time on EG Dilatability.
Under the constant mass ratio C : H 2 SO 4 (98%) : KMnO 4 : SHMP of 1.0 : 4.5 : 0.3 : 0.6 (g/g), H 2 SO 4 diluted to 77 wt%, and reaction temperature controlled at 40 ∘ C, the influence of reaction time on EV was studied.Results show that extension of reaction time increases EG dilatability in the former 40 min, and then it remains the same.Therefore, reaction time can be set as 40 min.

XRD Analysis.
XRD analysis for material graphite and the prepared expandable graphite were performed with a Y-4Q X-ray diffractometer (Dandong, China) employing Nifiltered Cu K radiation with 2 ranging from 15 ∘ to 70 ∘ .

FTIR Analysis.
The prepared intercalating products were triturated and mixed with potassium bromide at the mass ratio of about 1 : 100.The powder was pressed into flake in mold, and FTIR spectra were recorded between 4000 and resolution of 2 cm −1 .

Sample Processing and LOI Detection.
Mixtures of flame retardant and LLDPE were melted at 140 ∘ C in Muller (Jiangsu, China) and pressed at 10 MPa, and then samples were chopped into slivers with size of 120.0 × 6.0 × 3.0 mm 3 .The slivers were used to measure LOI according to GB/T2406-1993 with oxygen index instrument (Chengde, China).

Results and Discussion
3.1.Feasible Condition of EG  Preparation.According to the experiment results, feasible conditions of EG  preparation can be set as mass ratio C : KMnO 4 : H 2 SO 4 (98%) : SHMP of 1.0 : 0.3 : 4.5 : 0.6; H 2 SO 4 diluted to 77 wt% before reaction; intercalation reaction that lasted for 40 min at 40 ∘ C. The EV of EG  under different expansion temperature was detected, and it shows an increasing trend along with the increasing expansion temperature before 800 ∘ C, and then it presents a decreasing trend caused by excessive oxygenation of EG  . 0 and the maximum of EV are 151 ∘ C and 600 mL/g, respectively.

Preparation of the Referenced Expandable Graphite (EG).
Compared with EG  , the referenced EG was prepared under the mass ratio C : KMnO 4 : H 2 SO 4 (98%) of 1.0 : 0.3 : 4.5, and other conditions were the same as EG  . 0 and the maximum of EV were detected as 205 ∘ C and 480 mL/g, respectively.SHMP obviously affects dilatability of EG  and, what is more, addition of 0.6 g/g SHMP in graphite intercalating reaction makes EV increase 25%.EG  will show better flame retardancy than EG for its good dilatability.

Characterization of Graphite and
Its Intercalating Compounds 3.3.1.XRD Analysis of Natural Graphite and EG  .XRD analyses for natural graphite and EG  were performed.As shown in Figure 5 of natural graphite the two peaks with the interplanar crystal spacing of 3.34 Å and 1.67 Å corresponding to diffraction angles of 26.6 ∘ and 54.8 ∘ are the characteristic spectrum of natural graphite.As shown in Figure 5 of EG  the peaks of 26.2 ∘ and 55.4 ∘ show that EG  keeps the same layer structure as natural graphite.But it is worthy to note that the diffraction peak of 26.6 ∘ transfers to smaller angle of about 26.2 ∘ .At the same time, it corresponds to a big interplanar crystal spacing of 3.44 Å due to intercalation in graphene planes.It can be clearly seen that under the oxidation of KMnO 4 , the noncarbonaceous reactant can be easily inserted into the graphene planes, leading to the increase of interplanar crystal spacing.   is the specific absorption peaks of C=C, originating from its conjugated structure.The absorption peaks of S=O in EG are at 1160 cm −1 , but there are wide superimposed peaks in the range of 1160-1110 cm −1 in the FTIR of EG  , and it is because the absorption peaks of S=O and P=O both appear in the range of 1350-1100 cm −1 as reported [13].Furthermore, the peaks at 1161 cm −1 and 901 cm −1 in EG  all belong to SHMP specific absorption [14].The results announce the intercalation of intercalator.

Detection of Flame Retardancy for LLDPE.
Processing temperature of LLDPE is lower than 140 ∘ C, so the prepared EG  and EG can be used as retardant.The flame retarding composites were prepared as mentioned above, and their LOI of pure LLDPE, 70LLDPE/30EG  , and 70LLDPE/30EG (shown as wt%) was detected according to the mentioned method.Results show that LOI of net LLDPE is only 17.5%, and its combustion accompanies molten drop at the same time.Addition of 30% EG improves LOI to 24.6%, and no molten drop occurs.However, the addition of the same amount of EG  can improve LOI to 27.3%, and no molten drop occurs too.Therefore, the intercalating STPP is more effectual in improving the flame retardancy.

Conclusions
According to the analysis of the experiment results, it is evident that the mass ratio of C : KMnO 4 : H 2 SO 4 (98%) : SHMP has important influence on product dilatability, and when it is controlled as 1.0 : 0.3 : 4.5 : 0.6, H 2 SO 4 was diluted to 77 wt% before intercalation reaction, and intercalating reaction lasted for 40 min at 40 ∘ C, the EV and  0 of the prepared EG  can reach 600 mL/g and 151 ∘ C, respectively.The intercalating reaction between graphite and H 2 SO 4 and SHMP can be revealed by XRD and FTIR analysis of intercalation compounds.EG  has more effective flame retardancy than the referenced EG.
Figure 6  shows FTIR spectra of the prepared EG  and EG.As can be seen from the results, two samples both show the characteristic absorption peaks of -OH at 3430, caused by intercalation of H 2 SO 4 or HSO 4 − .At the same time, the peak at 1630 cm −1

Figure 6 :
Figure 6: FTIR analysis of EG and EG  .