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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.61 no.1 Concepción mar. 2016

http://dx.doi.org/10.4067/S0717-97072016000100004 

PREPARATION OF BORIC ACID MODIFIED EXPANDABLE GRAPHITE AND ITS INFLUENCE ON POLYETHYLENE COMBUSTION CHARACTERISTICS

HONGMEI ZHAOa,b, RUINIAN LINa*

a llege of Chemistry and Environmental Science, Key Laboratory of Analytical Science and Technology of Hebei Province, Hebei University, Baoding 071002, PR China.
b Department of VIP, Affiliated Hospital of Hebei University, Baoding, Hebei 071000, PR China.

e-mail: pxy833@163.com


ABSTRACT

A boric acid modified expandable graphite (EGB) was prepared through oxidizing-intercalating reaction of natural graphite, using H2SO4 and boric acid (H3BO3) as intercalators simultaneously. The dilatability of the intercalated products were characterized with expansion volume and initial expansion temperature.Scanning electron microscope, X-ray diffraction spectroscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy were employed todetect its layer structure, main element relative contents and function group. At the same time, its influence on combustion characteristics and thermal stability forlinear low density polyethylene (LLDPE) were investigated in limiting oxygen index (LOI) tests, vertical burning tests and thermal gravimetric/differential thermal gravimetric analyses. Comparing with the normal expandable graphite (EG, intercalated by single H2SO4), EGbexhibited high dilatability, thermal stability and flame retardancy for LLDPE. It had been testified by combustion tests that the addition of 30 wt % EGbto LLDPE improved the limiting oxygen index (LOI) from17.6% to 30.2%, and the vertical burning level of UL-94 standard reached V-0 level. Whereas, the LOI of the same amount referenced EG was only 25.1%, the UL-94 level only reached V-1. Moreover, the synergistic effect between EGb and ammonium polyphosphate (II) (APP) improved the LOI of 70LLDPE/10APP/20EGbcomposite to 33.0%, and the UL-94 level to V-0. The synergistic efficiency was attributed to the formation of continuous and compact residual char.

Keywords: Modified expandable graphite; boric acid; characteristics; dilatability; flame retardancy; synergistic efficiency.


INTRODUCTION

Natural graphite is a kind of carbon material with layer structure, intercalator such as sulfuric acid can be included between the carbon layersin chemical or electronic chemical oxidation reaction, 1,2 and then graphiteintercalating compound (GIC) named expandable graphite (EG) is obtained.EG is known as a new generation of intumescent flame retardant (IFR) forits good capability of halogen-free, non-dropping and low-smoke. 3,4 Thisretardant acts in both the condensed and gas phase through an endother micreaction: 5,6 (1) when contacting with flame source, EG will instantly expand and turn into swollen multicellular "graphite worms" covering on the retardedpolymer surface, which is in favor of slowing down the heat and mass transfer and interrupting the degradation of polymer matrix. (2) In oxidation reactionwith H2SO4 at high temperature, it releases gases such as CO2, H2O and SO2, 3,7'9 which can reduce concentration of combustible gas; thereby char formationhas been enhanced. (3) Expansion of EG will consume an enormous amount of heat, which is helpful to decrease the combustion temperature and rate.

Due to its outstanding anti-flame capability, EG has been used in the flame retardance of polymer materials such as polyurethane (PU) or polyurethanecoatings, 10,11 polyolefin blends, 12,13 acrylonitrile-butadiene-styrene (ABS), 14, 15ethylene vinyl acetate (EVA). 16,17 However, there are still some shortcomingswhen EG is used as a FR. Firstly, due to its limited efficiency, a normal 30wt% dose is needed to achieve satisfying effect, 18 which of ten leads to obviousdeterioration of the mechanical properties. Moreover, more SO2 will emit whenEG is prepared with only H2SO4 as intercalator. 19,20 So it is very important toenhance efficiency in order to meet environmental protection standard.

Measures have been attempted to improve EG flame retardancy and reduce its dose. Addition of EG together with other synergistic FRs such asphosphonate, 21 polyphosphate, 22 phosphorus, 23 metal hydroxide, 12 layereddouble hydroxide, 24 silica, 25 had been tested, and results indicated the addition of other synergistic FRs can normally improve flame retardancy and reduceEG dose. Surface treatments had been also reported to improve EG miscibilitywith polymer matrices. Hao et al prepared an EG modified with silane couplingagent and boric acid, 26 and its application for PU indicated that thermal stability of the modified EG and its flame retardancy for PU composites were all higherthan that of normal EG. While, the ground EG treated with phosphoric acid and silane presented an obvious increase of volume expansion ratio, 15 and addition of the treated EG in ABS significantly enhanced the fire performancebut decreased the impact strength of ABS. Whereas, the improvement bycombined addition or EG modification is usually very limited, especially the simple mix of EG with other retardants. 17 The main reason should be the insufficient mix caused by the inconsistence of particle size, density, polarity and addition dosage between these FRs and matrix.

It is worthy to note that non-carbon substance can easily move into the graphite layers and form GIC with accordant components. 27 Therefore, if a FRis used as assistant intercalator, the combined retardant can be prepared throughgraphite intercalating reaction, which can not only reduce sulfur content inGIC, but also improve the mix uniformity and then improve EG dilatability and flame retardancy. It was reported the H2SO4/APP (APP, ammoniumpolyphosphate, a assistant intercalating agent) GIC, prepared through two-stepmethod, exhibited a higher expansion volume (EV, 240 mL g-1) than that of the H2SO4 intercalated EG (210 mL g-1).19 An EG, intercalated by H2SO4 and Na2SiO3 through two-step intercalation reaction, 17 presented an EV of 517 mLg-1 and combustion limiting oxygen index (LOI) of 28.7% for EVA. However, the EV of referenced EG, which is intercalated by only H2SO4, was merely 433mL g-1, and the LOI was just 24.4%. At the same time, its relative sulfur contentdecreased from 1.79% to 1.23%. Especially, the intercalated Na2SiO3 in EG ismore effectual in improving the flame retardancy than the direct addition of Na2SiO3-9H2O with EG.

According to the former researches, B and boric acid (H3BO3) have been widely used as oxidation inhibitors to improve the thermal stability of carbon materials. With boron oxide as precursor, 28 the B modified carbonfiber reinforce carbon (CFRC) composite could be prepared with soakingor direct mixing method, and the results showed the incorporation of B inCFRC was beneficial for the improving of crystallinity and oxidation stability.Saidaminova et al prepared a H3BO3 modified EG through electrochemicaloxidation of graphite-H2SO4-H3BO3 system,29 and the results showed that the modified EG possessed higher thermal stability by 200 °C than the referencedEG. And then, the oxidized graphite was synthesized with chemical oxidationmethod in the graphite-H2SO4-H3BO3-K2Cr2O7 system,30 and the modificationraised the oxidation onset temperature by 200-300°C. In fact, H3BO3 is notonly an oxidation inhibitor, but also a well-known halogen-free, non-toxic and smoke suppressing FR. H3BO3 can absorb heat and dehydrate at lowertemperature due to its lower melting point. It acts in both the condensed phase and gas phase through an endothermic reaction. The released water vapourcan reduce concentration of combustible gas, and furthermore, the residualboric oxide can form a glassy coating on polymer surface, limiting the transfer of heat and mass, as well as oxygen diffusion, and then retarding furthercombustion. Thereby, H3BO3 is a kind of FR with good flame retardancy, andit can be solely added or used together with others to get better efficiency. 31,34

To get a kind of EG with low the sulfur content, well dilatability and flame retardancy, this research is to prepare H3BO3 modified EG (written as EGB)through flake graphite intercalation reaction with H2SO4 as main intercalator and H3BO3 as assistant intercalator simultaneously. The preparation methodwas founded and energy dispersive spectroscopy (EDS) was used to confirm the change of sulfur relative content. Scanning electron microscope, X-raydiffraction spectroscopy (XRD) and Fourier transform infrared spectroscopy(FTIR) were employed to characterize the structure and main functionalgroup. Meanwhile, with linear low density polyethylene (LLDPE) as flameretarded matrix, the flame retardancy of EGb, referenced EG (intercalated withonly H2SO4), H3BO3 and EG/H3BO3 (a mechanical mixture) were all tested.Furthermore, to improve the LOI value, vertical combustion UL-94 leveland suppress the existed "popcorn effect" simultaneously, APP was addedtogether in view of the reported synergistic effect between the two retardants.35,36 LOI and UL-94 rating tests, thermal gravimetric and differential thermalgravimetric (TG/DTG) analyses were performed to investígate the flameretarded performance and the thermal stability. Electron microscope wasapplied to observe the residual char morphology.

EXPERIMENTAL

Raw Materials and Sample Preparación: Natural flake graphite (average particle size of 0.30 mm, carbon content of 92%) was provided by ActionCarbon CO. LTD, Baoding, China. LLDPE (7042, 0.918 g cm-3, melt index 2.0g min-1) was purchased from Tianjin, China. APP (II, n>1000) was purchasedfrom Sichuan, China. H3BO3 and H2SO4 (98%) were all analytical reagents andused as received.

Preparación of EGB and the referenced EG: Firstly, the reactants were weighed according to a definite mass ratio of graphite C:H2SO4(98%):KMnO4:H3BO3, and H2SO4 was diluted to a demanded wt%with deionized water before reaction. Then, the quantified reactants weremixed in the order of diluted H2SO4, H3BO3, C and KMnO4 in a 250 mL beaker and stirred at a controlled temperature using a water bath. When the reactionfinished, the solid phase was washed with deionized water and dipped in waterfor 2.0 h until pH value of the waste water reached to 6.0-7.0, then filtrated and dried at 50-60 °C for 5.0 h. The influence of various factors on dilatability of the EGb were optimized through single-factor tests including the dosages of H2SO4(98 wt%), KMnO4, H3BO3 and H2SO4 concentration, reaction temperature andtime. Feasible conditions of EGB preparation were finally identified as: massratio of C:KMnO4:H2SO4(98%):BH3BO3 was 1.0:0.4:5.0:0.6, the concentrated H2SO4 was diluted to 78 wt% before reaction; intercalation reaction was totallymaintained for 40 min at 40 °C. Initiation expansion temperature (detected withDHG-9075A oven, temperature precision ±0.1 °C, Shanghai, China) 37 and the maximum EV (detected with SX3-4-13 Muffle furnace, temperature precision±0.1-0.4% °C, Tientsin, China) of the prepared EGb are 141 °C and 570 mLg-1, respectively.

Compared with EGb, the referenced EG with only H2SO4 as intercalator was prepared at the mass ratio C:KMnO4:H2SO4(98%) of 1.0:0.4:5.0 under the same condition mentioned in the preparation of EGB. Its initial expansiontemperature and EV were detected as 150 °C and 500 mL g-1, respectively.

It’s obvious that the addition of H3BO3 has significant influence on dilatability, reflected by the increase of EV and adjustment of initial expansiontemperature. It is known that H3BO3 easily dehydrate even at lower temperature;as a result, water vapor would be produced, which is in favor of the increase of EV. At the same time, the intercalated H2SO4 reacts with graphite causingrelease of CO2, H2O and SO2, 7-9 which leads to the expansion of GICs and the formation of the "worm like" expanded graphite particles.10 The EGb shouldshow better flame retardancy than EG for its good dilatability.

Measurements and characterizatión: EDS of Ca, Mn, S, Si in EGB and referenced EG was detected with KYKY2800B scanning electron microscope(China) under an accelerating voltage of 20 kV. The detector resolution and the element detection range were 132 eV and from Na to U, respectively. Prior toobservation, the surfaces were coated with a conductive material.

The FTIR spectra of the prepared EGb and EG were recorded between 4000-400 cm-1 using a FTIR spectrometer (FTS-40, America Biorad ) with aresolution of 2 cm-1.

XRD pattern was obtained with an Y2000 X-ray diffractometer (Dandong, China), under the operation condition of 40 kV, 30 mA, employing Ni-filteredCu K radiation with 20 ranging from 10° to 70°.

FRs were added into melting LLDPE ( processing temperature was not less than 120 °C) in Muller (Jiangsu, China), the mixtures were pressed at 140 °Cand 10 MPa, and then chopped into slivers with size of 120.0x6.0x3.0 mm3 and 127.0x13.0x3.0 mm3, respectively.

LOI test was used to evaluate the combustion property of the flame retarded LLDPE composites with a size of 120.0x6.0x3.0 mm3, and it wasdetected using a JF-3 LOI instrument (Chengde, China) according to Standard of GB/T2406-1993. At the same time, vertical burning tests were performedusing a HC-3 vertical burning instrument (Tientsin, China) on sheets of size127.0x13.0x3.0 mm3 according to the standard UL 94-1996.

In thermal analysis of TG/DTG (STA 449C, Germany), about 10.0 mg sample, laid in porcelain crucible, was detected under N2 atmosphere with aflux of 25 mL min-1 and heated from about 35 °C to 800 °C at a heating rate of 10 °C min-1. A TM3000 Electron microscope (Japan) was used to observe the surface and section morphology of specimens.

RESULTS AND DISCUSSION

EDS analysis óf material and GICs: EDS results listed in Table 1 present the main surface elements except carbon, oxygen and boron (the elementdetection range of the used detector is from Na to U) and their relative percentagecomposition in natural graphite, EGB and referenced EG, respectively. As canbe seen, except C element, the natural graphite still consists of S, Mn, Si and Ca. In the referenced EG, the S content is relative higher, which reveals the intercalation of H2SO4/HSO4A0 Although the S content in EGb is still higherthan that of natural graphite, it’s lower than that of EG, which dues to the assistant intercalation of H3BO3. Therefore, EGB will release less SO2 gas than the referenced EG.


Table 1 Surface composition of natural graphite, EG and EGB determined by EDS a

a The detector resolution and the element detection range were 132 eV and from Na to U, respectively.

FTIR analysis óf material and GICs: Figure 1 shows FTIR spectra of the prepared EGB and referenced EG. As can be seen from the results, twosamples both show the characteristic stretching vibrations absorption peaks of-OH (3430-3420 cm-1), caused by intercalation of H2SO4/HSO4- or H3BO3. Atthe same time, the peaks at about 1620 cm-1 are the specific absorption peaks of C=C stretching vibration, originating from graphite conjugated structure. Thestrong stretching vibration absorption peak of S=O is observed in EG (1118cm-1), but there are strong superimposed peaks at 1463 cm-1 and 1190 cm-1 inthe FTIR of EGB, it is because the absorption peaks of S=O and B=O are bothappear in the range of 1500-1100 cm-1 as reported. 38 Furthermore, the peaks inthe range of 800-600 cm-1 in the EGB belong to B-O specific absorption. 39 Theresults indicate the intercalation of intercalators.


Figure 1 FTIR analysis of the referenced EG and EGB

Morphology property of natural graphite and EGB: Figure 2 presents electron microscope photographs of natural graphite and EGB. As shown inFigure 2 (a) the cross-section of natural graphite (amplified by a factor of 20000), layer structures of natural graphite are compact, and the layers distanceis very small and regular. However, the cross-section of EGb (shown in Figure 2(b), amplified by a factor of 1000) shows that layers distance has been enlarged, and the boundary layers are loose and damaged. It can be inferred that the intercation forces between the EGB layers are weaken due to the oxidation of KMnO4 and intercalation of H2SO4 and H3BO3.


Figure 2 Morphology property of natural graphite (a) and EGB (b)

XRD analysis of the GICs: XRD analysis for natural graphite, EGb and the referenced EG were performed. As shown in Figure 3 (a), the twopeaks at 26.4° (corresponding an interplanar crystal spacing of 0.334 nm) and 55.5° (corresponding an interplanar crystal spacing of 0.167 nm) are twocharacteristic peaks of natural graphite. While, XRD patterns in Figure 3 (b) and (c) all show two similar peaks as natural graphite in the range of 25°-27° and 50°-60°, which indicates both EG and EGb all keep the same layer structureas natural graphite. But it is worthy to note that the diffraction peaks in the range of 25°-27° transfer to smaller angle of 26.3° and 26.2°, respectively. Atthe same time, each corresponds to a big interplanar crystal spacing of 0.338nm for EG and 0.339 nm for EGb. This can be explained that natural graphiteis oxidized by KMnO4 and then exhibited positive charge. Then gap betweengraphite layers is extended due to the repulsion, and intercalating reaction canproceed between graphite and intercalator. The positive charge of the oxidizedgraphite network is balanced by negatively charged acid anions and alsoincludes acid molecules. 36, 40, 41 Results confirm that intercalators have beeninserted into graphite layers.


Figure 3 XRD analysis of natural graphite (a), EG (b) and EGB (c)

Flame Retardancy of composites: The processing temperature of LLDPE is normally lower than 120 °C, so the prepared GICs can be used as FRs.The flame retarded LLDPE specimens were prepared following the abovementioned method, weight percentage between FRs and polymer was listedin Table 2. LOI detection and vertical combustion tests were carried out toevaluate flame retardancy and observe ignition, expansion-extinguishingprocess, melt-dripping phenomenon. The results were also listed in Table 2.


Table 2 Specimen components and their combustion properties b

b Component contents are expressed as wt%. 70LLDPE/18EG/12H3BO3 specimen is prepared by mechanical mixing of LLDPE (70 wt%) with the referenced EG (18 wt%) and H3BO3 (12 wt%). Furthermore, the ratio of EG3BO3 (18:12) was calculated according to the ratio of CH3BO3(determinate to be 1:0.6) in EGB preparation (herein, it was supposed that allH3BO3 were inserted in the intercalation reaction).

As shown in Table 2, LOI of pure LLDPE is only 17.6%, and the combustion accompanies with serious molten drop at the same time (as shownin Figure 4 (a)). LOI values of flame retarded LLDPE composites are all higherthan that of pure matrix. Addition of 30 wt% referenced EG improves the LOI of 70LLDPE/30EG composite to 25.1%, and the UL-94 level reaches V-1.Noticeably, the addition of the same amount of the prepared EGB improves LOIvalue and UL-94 level of 70 LLDPE/30EGB to 30.2% and V-B0 respectively.These results indicate the assistant intercalation of H3BO3 obviously improvesflame retardancy for LLDPE. This is because the intercalated H3BO3 can notonly play an retardant role, but also its thermo-decomposition residues B2O3 can also increase the cohesiveness and density of the protective intumescentcarbonaceous char generating from EG expansion, 42-44 which is helpful toimproving its shielding, adiabaticity and protecting the polymeric matrix fromdegrading into gases. While, the addition of EGB or EG can all reduce the dripping phenomena and raise the fire safety property, which is attributed to the protective intumescent carbonaceous char (as shown in Figure 4 (b) and(c)) formed on polymer surface by EGB or EG expansion. It can be seen that the mechanical mixture of H3BO3 witBh LLDPE make 70LLDPE/30H3BO3show a weaker flame retardancy than the referenced EG, but its combinationwith EG presents an improved anti-flame efficiency, indicating a higher LOIvalue of 26.5 % than the the oretic calculated LOI value of 23.8%. Remarkably, the chemical intercalated H3BO3 in EGB is more effectual than the simplemechanical mix. The most probable reasons should be that the mixinguniformity of H3BO3 with graphite is higher in EGB than the mechanical mix of EG 3BO3. 29,30

At the same time, the anti-flame efficiency of APP for LLDPE and its synergetic efficiency with EGB were investigated. When it is solely added at30 wt%, LOI of the 70LLDPEB/30APP specimen is only improved to 20.0%, and the melt-dripping and ignition still can not be avoided. While, addition of EGB and APP at different wt % to LLDPE show that the combination cannot only increase LOI value, but also all improve the UL-94 level to V-0 simulta neously. Meanwhile, LOI values were all obviously higher than the calculated LOIcal according to the single EGB wt %, APP wt % and their LOIvalues. Therefore, it may be inferred that there is synergistic efficiency between the two FRs. Meanwhile, the APP/EGB ratio has an important influence onflame retardancy, and the tested optimum component is 10:20 as shown in 70LLDPE/10APP/20EGB specimen, the LOI and UL-94 level are 33.0% and V-0 respectively.


Figure 4 The combustión behavior of LLDPE specimens in vertical burning tests LLDPE (a); 70LLDPE/30EGB (b); 70LLDPE/30EG (c);
70LLDPE/20EG/10APP (d)

Morphology of the combustión residue: In the burn process, the formation of effective protective char can improve flame retardancy. Therefore,residue morphology of the flame retarded LLDPE after their LOI tests wereexamined by electron microscope. Figure 5 (a) shows the incision section of 70LLDPE/30EGB composite after combustion, a regular "open-cellular"structure on the surface is observed due to the expansion of EGB (showing the"popcorn effect") , originating from blowing gases in redox reaction betweenresidual H2SO4 and the graphite. Finally, discontinuity and low mechanicalstrength of the residue cause the 70LLDPE/30EGb system easily break and adecrease of shielding function for heat and mass transfer (the tensile strength of 70LLDPE/30EGB is detected as 6.8 MPa). The residue incision section of 70LLDPE/10APP/20EGB shown in Figure 5 (b) is relatively continuous and compact due to the conglutination of APP decomposition products (the tensilestrength of 70LLDPE/10APP/20EGB is detected as 7.3 MPa); 17 this structureinhibits the "popcorn effect" and provides a shield that insulates the substratefrom radiant heat, and avoids the direct contact between substrate and flame.It is the continuous and compact residual that makes 70 LLDPE/10APP/20EGB composite hold higher LOI value and thermal stability than 70LLDPE/30EGB composite.


Figure 5 Electron microscopy morphology of 70LLDPE/30EGB (a) and 70LLDPE/10APP/20EGB (b) after LOI tests.

Thermal stabilities—TG/DTG analysis: Thermal stability of flame retarded LLDPE is related to the addition of FRs. TG/DTG under N2 atmospherewas used to evaluate the thermal degradation properties of the referenced EG,EGB, 70LLDPE/30EG, 70LLDPE/30EGB and 70LLDPE/10APP/20EGB, and the results were shown in Figure 6 and Table 3.


Figure 6 TG and DTG analysis of samples


Table 3 Thermoanalysis data of specimens in N2 atmosphere c

c T5 : the temperature at which 5% weight loss occurring, °C.
T max: the temperature corresponding to the maximum decomposition rate, °c.
Rmax: the maximum decomposition rate, % min-1.

Compared with the flame retarded LLDPE composites, mass loss of the referenced EG and EGB is more moderate. For EG, the weight lose mainly occursamong 250-450 °C with a peak of 267 °C and the maximum decomposition rate(Rmax) of -103 % min-1 as shown in Table 3, wherein CO2, H2O and SO2 gasreleased during redox reaction between graphite and the intercalated H2SO4/HSO4,16 and leads to the generation of "worm like" expanded graphite. As forEGB, besides redox reaction between graphite and H2SO4, dehydration of the intercalated H3BO3 in or between molecules and its fusion occurs in the range of 100-300 °C, which leads to a lower T5 (temperature at which 5% weight lossoccurring), T (temperature corresponding to the maximum decompositionrate) and higher R of -2.45 % min-1. Then the produced glassy B2O3 coatingact as filler and binder for the swollen expanded graphite and then form morecompacter carbonaceous char layer as shown in Figure 4 (b). As a result, EGBkeeps a higher residual char yield of 79.4% at 800 °C than the referenced EG of 78.2%, which confirms its higher thermal stability and leads to a relativeremarkable flame retardancy indicated by LOI value. 14,36

For those flame retarded LLDPE composites with a FR dose of 30 wt% all show the same weight loss tendency, but they hold different T5, T Maxand RMax . The 70LLDPE/30EGB composite shows a lower thermal stability than70LLDPE/30EG composite when the temperature is lower than 490 °C, whichis indicated by its lower decomposition temperature corresponding to T5 and T Max and higher RMax of -25.0 % min-1 as shown in Table 3. The results arecaused by the additional decomposition function of the intercalated H3BO3as mentioned in the TG of EGB. But once the expansion of EGB, fusion and dehydration of H3BO3 finished, the matrix will covered with more residual than70LLDPE/30EG, which can be indicated by their final residual yield. As for the70LLDPE/10APP/20EGB composite, it show a higher RMax of -28.2 % min-1 than the single EG or EGB retarded LLDPE. The results are caused by the additionaldecomposition function of the added APP. As for the residual weight, although 70EVA/10APP/20EGB system holds a lower residual carbon (caused by the reduced carbon resource) than 70LLDPE/30EGB system, it presents a higherflame retardancy indicated by LOI. The reason is that with the help of the conglutination of APP decomposing products, the swollen expanded graphitecan form a compacter carbonaceous char layer. As for the EG/APP system, the carbonaceous char compaction plays a more important role in improvingthermal stability and flame retardancy than the residual carbon weight. 45

Possible Mechanism of flame retardance: According to the LOI and UL 94 tests results, it can be indicated that the flame retardancy of LLDPE can be greatly improved by EGB especially the combination of EGB with APP, whichshows a typical synergetic flame retardancy behavior. Through the analysis of TG/DTG and the residual morphology, the synergistic mechanism wasproposed for this system. In the gas phase, the non-flammable gases, such asCO2, SO2, NH3, N2 and H2O, released through decomposition or dehydration of EGB, H3BO3 and APP, which can dilute the combustible gases. In the condensed phase, EGB and its intercalated H3BO3, the additional added APPplay important role in combustion. Initially, the "worm like" char is formedthrough the expansion of EGB when the temperature is above 141 °C (initiationexpansion temperature of EGB), but the char is slight [shown as Figure 4 (b)] and can not effectively endure heat flux. Along with the raising of combustiontemperature, the further dehydration or fusion of H3BO3 occur, and lead to theformation of B2O3 glassy coating, which can increase the cohesiveness and density of the protective intumescent char generating from EGB expansion. Atthe same time, polyphosphoric acid generated from APP increased the meltviscosity, which can strengthen the char barrier for its strong adhesion effect.The char structure of 70LLDPE/10APP/10EGB [Figure 4 (d) and Figure 5 (b)]presents a thicker and denser char layer than 70LLDPE/30EGB [Figure 5 (a)].Thus, the transfer of gas and heat is retarded by this insulative layer.

CONCLUSIONS

EGB was successfully prepared with H2SO4 and H3BO3 as intercalators with chemical oxidation intercalation method. Compared with the referencedEG, the EGB indicated a better dilatability, flame retardancy and environment-friendly property. EDS, FTIR, SEM and XRD confirmed that the oxidation and intercalation reaction between natural graphite and intercalators couldtake place. Combustion tests of the flame retarded LLDPE composites showedthat EGB presented a higher LOI value of 30.2% than EG of 25.1%. Thecombination of EGB with APP made the 70LLDPE/10APP/20EGB systemshowed high efficiency; the LOI value and UL-94 level reached 33.0%, V-0respectively. TG/DTG analysis proved EGB and EGB/APP could improveLLDPE composites thermal stability at high temperature. Electron microscopephotographs indicated that the residual char of sample treated with EGB/APPwas more continuous and compact than that of treated with only EGB. It wasconcluded that the EGB/APP system could improve the char quality which was the key factor in dripping resistance and improving thermal stability of the composites.

ACKNOWLEDGEMENTS

The authors would like to thank Natural Science Foundation of Hebei Province (CN) (No. B2015201028) and Seedling Project of College of Chemistry and Environmental Science (Hebei University) for financial support.

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