Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composites

Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composites

Journal Pre-proof Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composite...

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Journal Pre-proof Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composites Santosh Khanal, Yunhua Lu, Saad Ahmed, Muhammad Ali, Shiai Xu PII:

S0142-9418(19)31514-4

DOI:

https://doi.org/10.1016/j.polymertesting.2019.106177

Reference:

POTE 106177

To appear in:

Polymer Testing

Received Date: 15 August 2019 Revised Date:

16 October 2019

Accepted Date: 18 October 2019

Please cite this article as: S. Khanal, Y. Lu, S. Ahmed, M. Ali, S. Xu, Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composites, Polymer Testing (2019), doi: https://doi.org/10.1016/j.polymertesting.2019.106177. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties of intumescent flame retardant HDPE composites

Santosh Khanal1, Yunhua Lu1, Saad Ahmed1, Muhammad Ali1, *Shiai Xu1, 2

1

Shanghai Key Laboratory of Advanced polymeric Materials, School of Material

Science and Engineering, East China University of Science and Technology, Shanghai 200237, China 2

School of Chemical Engineering, Qinghai University, Xining 810016, China

*Correspondence to: Shiai Xu (Tel: 86-021-64253353, E-mail: [email protected])

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Abstract The combination of synergistic agent with intumescent flame retardant (IFR) systems provides a promising way to prepare high performance IFR composites. In this study, the effects of the synthetic zeolite 4A in combination with the IFR system consisting of ammonium polyphosphate (APP) and tris (2-hydroxyethyl) isocynurate (THEIC) on thermal degradation, mechanical properties, flame retardancy and char formation of high-density polyethylene composites were investigated by limiting oxygen index (LOI) measurement, cone calorimetry, scanning electron microscopy and laser Raman spectroscopy. The LOI value of HD/FR/Z-0.5 composite with an optimum content of 0.5 wt.% zeolite 4A and 25 wt. % of total flame retardant reaches 26.3 %. A low loading of zeolite 4A can improve the bench-scale combustion performance as determined by cone calorimetry, and promote the formation of more compact char residue with a highly graphitic structure. However, a low loading of zeolite in combination with the IFR system consisting of APP and THEIC produces no significant changes in mechanical performance.

Key words: Flame retardancy, intumescent flame retardant, synergistic agents, zeolites, char structure.

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1.

Introduction Polyethylene (PE) is an important commodity plastic with many applications

in packaging, construction and electrical industries owing to its good properties such as electrical insulation, high chemical stability, low toxicity and ease of processing. However, the highly flammable nature of PE makes it undesirable in applications where the fire safety is a primary concern. Thus, attempts have been made to improve the flame retardancy of PE using flame retardant additives such as halogenated flame retardants, mineral based fillers (particularly magnesium hydroxide and aluminum hydroxide) or intumescent flame retardants (IFR). Non-halogenated flame retardant additives are gaining popularity due to environmental concerns as halogenated ones would produce a large volume of toxic gases and corrosive smokes during combustion [1–3]. The IFR systems are known as good non-halogenated flame retardant additives. Ammonium polyphosphate (APP) is an important phosphorous based flame retardant that can act as blowing agent and acid source in producing IFR systems with charring agents. However, most IFR systems are hydrophilic and polar in nature, and thus they are incompatible with non-polar polyolefin systems such as PE and polypropylene (PP), which may reduce the mechanical properties of the resulting composites, especially at a high loading of additives [4–7]. The combination of IFR systems with synergistic agents offers a promising way to produce high-performance flame retardant composites. The use of nano fillers helps to maintain the mechanical properties, decrease the loading of flame retardant additives and enhance the flame retardant properties. Many synergistic additives have been used for this purpose, such as montmorillonite [8], layered double hydroxide [9], carbon nanotube [10], mesoporous silica [11], α-zirconium phosphate [12] and zeolites [13,14]. Of these

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synergistic additives, synthetic zeolites have attracted much attention because they are environmentally friendly and cheap, and can enhance the flame retardant performance at a low loading when combined with conventional IFR system. Zeolite can act as a catalyst for developing stable and compact intumescent carbonaceous residues. However, the effect of zeolites depends critically on the nature of polymeric systems and intumescent formulation [15,16] Zeolites are aluminosilicate minerals characterized by the 3-D framework made of SiO4 and AlO4 tetrahedra linked to each other. The framework contains many channels and interconnected voids whose sizes are approximately equal to those of common organic molecules, and those voids are often occupied by cations and water molecules. The general chemical formula of zeolite is Ma/x [(AlO2) a⋅(SiO2) b]⋅zH2O, where M represents any alkali or alkaline earth metal atom required to balance the \charge, z is the number of water molecules per unit cell, and a and b are the total number of the tetrahedral of Al and Si per unit cell with a ratio b/a ≥ 1 [17,18]. Zeolites have large internal and external surface areas, unique nanoscale pores, high thermal stability, eco-friendliness and high cation exchange capacity (CEC). The CEC of a zeolite depends on its chemical composition, structure and corresponding cation. Zeolite surface can be modified by cationic surfactants. A monolayer of surfactant can be formed at a low concentration via normal cation- exchange process. In the presence of sufficient surfactant with a concentration higher than critical micelle concentration, a bilayer can be formed on the surface of zeolite due to adsorption, thus the surface charge on the zeolite is reversed, which provides sites for anion retention. Such surfactant modified zeolites (SMZs) can be used to develop effective sorbents for the removal of anions such as chromate, phosphate, nitrite and nitrate [19,20] and to fabricate polyvinyl chloride (PVC) membrane electrode and modified carbon paste

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electrode for detection of sulfide and nitrate [21,22]. Synthetic and natural zeolites can also be used as suitable support materials that can increase the photolytic activity of semiconducting materials. Zeolite as a supporting material can prevent aggregation of supported photocatalyst, brings organic pollutants on the surface due to adsorption, and inhibit the recombination of electron/hole pairs via distribution of photogenerated electron due to the strong electric field and hence enhance the photolytic activity. These properties of zeolite make it suitable for removal of dyes and other organic pollutants using advanced oxidation process (AOPs) [23,24]. Transition metal cation supported zeolites have been used as effective electrocatalyst for modification of the electrode surface and voltammetric/ampherometric determination of various pharmaceutical products like acetaminophen and ascorbic acid [25], bromate ions [26] and electrooxidation of methanol [27]. Zeolites can also enhance the flame retardant properties of resulting composites when combined with IFR system [28]. Synthetic zeolites were combined with a classical IFR system consist of APP and pentaerythritol (PER) to enhance the flame retardant properties of PE and ethylene copolymers [15] or PP [16]. They were also combined with triazine ring based char forming agents and silica microencapsulated APP as IFR system to prepare low density polyethylene (LDPE) composites, and the LOI value of LDPE/IFR system was increased from 29.0 to 34.0 % at 1 wt.% of zeolite [13]. Similarly, zeolites were used as synergistic agents in combination with intumescent system consisting of APP/CNCA-DA for LDPE [3] and PP composites [29]. Natural zeolite is a hydrophilic inorganic material with low affinity to hydrophobic polymers such as PP and PE, and it tends to agglomerate in the polymer matrix, especially at a high loading. Polyethylene glycol, stearic acid and silane coupling agents have been used to modify the surface of zeolites to improve their dispersion and wettability to the polymer

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matrix [30,31]. However, despite much progress in this field, the synergistic mechanism of zeolites with IFR systems remains poorly understood. The use of IFR systems based on APP and THEIC can improve the flame retardant properties of PP and high-density polyethylene (HDPE) composites [2,32]. The dispersion of fillers is relatively poor due to the polar nature of flame retardant additives, which has significant effects on the mechanical performance, especially at a high loading. The use of synergistic additives can reduce the loading of the flame retardant additives, which thus contributes to reduce the cost and improve the mechanical and flame retardant properties. In this study, synthetic zeolite 4A was used as synergistic agent to prepare flame retardant HDPE composites containing APP and THEIC as IFR components. The thermal and flame retardant properties of the composites were characterized by thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurement, and cone calorimeter test (CCT). To better understand the synergistic mechanism of zeolite with IFR systems, the char residues of the composites were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and laser Raman spectroscopy (LRS). The effect of the zeolite on the mechanical properties of flame retardant composites was also investigated. 2.

Experimental

2.1 Materials Ammonium polyphosphate (APP, phase II, DP >1000) was supplied by Jinan Taixing Fine Chemical Co., Ltd. (China). Tris (2-hydroxyethyl) isocynurate (THEIC) was supplied by Shanghai Macklin Biochemical Co., Ltd. (China). Zeolite 4A (80100 mesh) was obtained from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). 6

High-density polyethylene (HDPE, density: 0.93 g/cm3; melt index: 20 g/10min @ 190 ℃/2.16 kg) was obtained from Fushun Petrochemical Co., Ltd. (China). 2.2 Preparation of HDPE composites HDPE composites containing APP and THEIC at a weight ratio of 3:1 as the IFR component and varying amounts of zeolite as the synergistic agent were prepared in a Haake Polylab rheometer at 170 ℃ and a rotation speed of 50 rpm. The total loading of the flame retardants was kept at 25 wt. %. The composites obtained were molded into sheets by hot pressing (BL-6170-A-25J, Baolun Precision Testing Instruments Ltd., Shanghai, China) at 170 ℃ and a pressure of 10 MPa for 5 min. The compositions of the composites are shown in Table 1. 2.3 Measurement and characterization 2.3.1 Flame retardant properties: The flame retardant properties of the composites were evaluated using limiting oxygen index (LOI) and cone calorimeter test (CCT). LOI of the composites (100 × 6.5 × 3 mm3) was measured using an oxygen index instrument (JF-3, Jiangning Analysis Instrument Factory, China) according to ASTM D-2863. CCT was performed using a cone calorimeter (PX-07-007, Phinix Thech Co., Ltd.) at a heat flux of 35 kW−2 according to ISO 5660-1, and samples of 100 × 100 × 4 mm3 were placed on a horizontal sample holder. All samples were tested in duplicate and the average value was reported. 2.3.2

Thermogravimetric

analysis

(TGA):

TGA

was

conducted

on

a

thermogravimetric analyzer (Netzsch STA 409 PC) from room temperature to 700 ℃ at a heating rate of 10 K/min in both air and nitrogen at a flow rate of 30 mL/min. 2.3.3

Scanning electron microscopy and energy dispersive X-ray spectroscopy

(SEM-EDX): The cryo-fractured surface of the composites and char residues after LOI test were observed on a field emission scanning electron microscope (S-4800,

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Hitachi Japan) connected with an energy dispersive X-ray spectrometer (Quantax 400-30, Tokyo, Japan). Samples were sputtered with a thin layer of gold to enhance surface conductivity before observation. The accelerating voltage during observation was 15 kV. 2.3.4

Laser Raman spectroscopy (LRS): LRS of char residues after CCT test was

conducted using a laser Raman spectrometer (LabRam HR Evolution) with a 532 nm helium–neon laser line within 200–3000 cm−1 region at room temperature 2.3.5

Fourier transform infrared spectroscopy (FTIR): FTIR spectra were recorded

on an infrared spectrometer (Nicolet 6700, USA) in ATR mode using a thin section of composites, while the FTIR spectra of char residues were recorded with KBr pellets. 2.3.6

Pyrolysis Gas chromatography/ Mass spectroscopy (Py-GC/MS): Py-GC/MS

was carried out with an integrated system composed of pyrolyser, gas chromatograph (Agilent 8790 B) with capillary column (Agilent 19091S-433, dimension 30 m × 250 µm × 0.25 µm) and mass spectrometer (Agilent 5977 B). Samples were heated by inductively heated platinum coil in a quartz tube at a heating rate of 20 ℃/min. The gas chromatograph was operated at a constant helium flow of 1 mL/min. Pyrolysis was carried out at 550 ℃. 2.3.7

Mechanical properties: The tensile properties were determined using a

universal testing machine (MTS E 44) at a cross head speed of 5 mm/min according to Chinese standard GB-T 1040.3 2006. The flexural properties of samples (80 × 10 × 4 mm3) were measured using a universal testing machine (CMI4024-20KN, Shenzhen Sans Co., Ltd., China) according to Chinese standard GB/T 9341-2000 in a threepoint bending mode at a crosshead speed of 2 mm/min. The notched Izod impact strength was measured on an Izod testing machine (CEAST 9050, CEAST Co., Ltd.,

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Italy) in accordance with ISO 180 with a sample size of 80 × 10 × 4 mm3 and a notch of 2 mm. Five samples were examined, and the average values were reported. 3

Results and discussion

3.1 Structure and morphology of the composites The FTIR spectra of APP, THEIC, zeolite 4A and composites containing IFR systems with or without zeolite are shown in Fig. 1. In the FTIR spectrum of APP, the absorption peak at 3200 cm-1 is due to the stretching vibration of N-H, while those at 1253 cm-1, 1076 cm-1 and 1018 cm-1 are due to the symmetric stretching vibration of P=O, P-O, and PO2 and PO3, respectively. In addition, the asymmetric stretching vibration of P-O is observed at 887 cm-1 and P-O-P vibration is observed at 801 cm-1 [33,34]. In the FTIR spectrum of THEIC, the broad peak at 3390 cm-1 is attributed to the stretching vibration of O-H, those at 2918 and 2851 cm-1 are attributed to the stretching vibration of C–H in the saturated hydrocarbon chain; while those at 764, 1153 and 1687 cm-1 are attributed to the absorption of triazine ring skeletons, C-O and C=O, respectively [32,35]. In FTIR spectrum of zeolite 4A, the absorption bands at 3437 and 1653 cm-1 are attributed to the water molecules present in pores and cages of the zeolite structure. The band at 1008 cm-1 corresponds to the asymmetric stretching vibration of O-T-O (T =Al or Si) and is sensitive to the content of the framework Si and Al. The absorption bands at 670 and 556 cm-1 correspond to the TO symmetric and asymmetric stretching vibrations of free tetrahedral TO4 groups, respectively. The band at 465 cm-1 corresponds to the bending vibration of O-T-O bonds in the tetrahedra [26,36,37]. In the FTIR spectrum of HD/FR/Z-0 composite, the sharp peak of OH groups in THEIC is reduced with only a small shoulder at around 3200 cm-1, indicating that APP and THEIC interact by hydrogen bonding. The shoulder at 3200 cm-1 may be due to NH2 and/or OH groups. The absorption band of

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C=O of THEIC is shifted to 1682 cm-1, while that of the triazine ring vibration is observed at 764 cm-1. The stretching vibration of P=O in APP is shifted to 1245 cm-1. The peak at 1055 cm-1 is assigned to the vibration of P-O in APP, but the peak of APP at 1018 cm-1 is not observed. The FTIR patterns of HD/FR/Z-0.5 composite is quite similar to that of HD/FR/Z-0 composite. However, it is not possible to locate the absorption band of zeolite in HD/FR/Z-0 probably due to the low content of the zeolite and/or overlapping with other absorption bands. The FTIR spectra suggest that THEIC can interact with APP during the compounding process. The SEM images of the HD/FR/Z-0 and HD/FR/Z-0.5 composites are shown in Fig. 2. As the SEM images of the composites containing different amounts of zeolite are essentially similar, only representative ones are shown here. It is difficult to distinguish the two IFR components in SEM images. As THEIC is polar in nature and has a higher affinity to APP, it is more likely to be attached onto the surface of APP rather than dispersed in HDPE. The SEM images show that APP particles are roughly cuboid with varying sizes. However, it is difficult to identify THEIC particles, which are assumed to be in the vicinity of APP. To gain more insights into this problem, the cryo-fractured surface of HD/FR/Z-0 composite immersed in water for 3 h to extract THEIC was examined. Fig. 3 shows that as expected, APP particles become smoother and clearer after water extraction, indicating that THEIC is coated or attached to APP particles. The FTIR spectra also suggest the interaction between APP and THEIC. However, the content of zeolite is too low to be observed by SEM. Presumably, zeolite particles are attracted to APP and THEIC rather than dispersed in the polymer matrix. 3.2

Flame retardant properties

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The LOI values of HDPE composites containing APP and THEIC at a ratio of 3:1 with or without zeolite 4A are shown in Table 1. The total loading of the IFR system and zeolite 4A was kept at 25 wt. %. The results show that the LOI value of the composite containing only IFR is 25.1 %. The addition of 0.5 wt.% of zeolite results in an increase in the LOI value to 26.3%, but the LOI value decreases as the zeolite content further increases, indicating that zeolite has a synergistic effect with the IFR system on the flame retardant properties of HDPE composites especially at a low loading [3,15]. Zeolite 4A can catalyze the esterification between APP and THEIC, leading to the formation of more stable carbonaceous residues [38]. A higher loading of zeolite can alter the swelling and carbonization function of the IFR system and hence decrease LOI value [29]. The effect of zeolite as a synergistic agent depends on the nature of the polymeric system and the intumescent composition. In present system of THEIC and APP, the optimum zeolite content is determined to be 0.5 wt. %. The composite containing 1 wt. % of zeolite shows a LOI value of 24.3 %, which is even lower than that without zeolite. CCT was performed with the composites containing 0.5 wt. % and 1 wt. % of zeolite. CCT is one of the most successful bench scale tests to evaluate the flammability of the materials, and it can provide various parameters such as heat release rate (HRR) including average heat release rate (Av-HRR) and peak heat release rate (PHRR), time to ignition (TTI), total heat release (THR), mass loss rate (MLR), time to peak heat release (TPHR), and total smoke released (TSR) as shown in Table 2, where the data represents the average of duplicate measurements. The representative HRR and THR curves are shown in Fig. 4. The HRR curve (Fig 4 (a)) shows that HDPE burns rapidly with a PHRR value of 1012.8 kW/m2. The composites containing the IFR system with or without zeolite shows two peaks in

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HRR curve, which are attributed to the degradation of APP/THEIC and the formation of an intumescent shield. The shield degrades further during the heating, but a new intumescent shield will be formed. The use of APP/THEIC can significantly reduce the PHRR value, which can be further decreased with the use of zeolite as the synergistic agent. The THR curve (Fig. 4 (b)) shows that the use of zeolite helps to decrease the total heat released at the end of combustion. The THR curve of the flame retardant composites is always below that of pure HDPE. Zeolite has a significant effect on the THR curve at high temperature (long time) and the curves are similar at lower temperature. The average mass loss (MLR) is significantly decreased using APP/THEIC. The addition of 0.5 wt. % of zeolite results in a decrease in MLR value from 4.79 g/(s⋅m2) for HD/FR/Z-0 to 4.40 g/(s⋅m2) for HD/FR/Z-0.5. As 1 wt.% of zeolite is added, the MLR value is increased to 6.39 g/(s⋅m2) for HD/FR/Z-1.0, indicating that a high content of zeolite can promote the degradation of the composites. The total CO2 production of the HD/FR/Z-0 composite is higher than that of pure HDPE, and the addition of zeolite result in a decrease in CO2 production. The CO2 production of HD/FR/Z-0.5 composite is even lower than that of pure HDPE, and the total smoke production is lower than that of pure HDPE and HD/FR/Z-0 composite. 3.3 Thermal properties The thermal degradation of the composites containing zeolite was investigated in nitrogen atmosphere and the results are shown in Fig. 5 (a) and Table 3. The onset decomposition temperature (temperature of 5 % mass loss) of the flame retardant composites is lower than that of pure HDPE due to the loss of water and ammonia and the reaction between THEIC and APP. It is noted that the mass loss curve of HDPE composites with zeolite is similar to that without zeolite indicating similar thermal 12

degradation behavior in a non-oxidative environment. The mass loss at the maximum degradation temperature (MLR) of HD/FR/Z-0.5 composites is lower than that of HD/FR/Z-0 composite, suggesting that zeolite can delay the degradation of polymer matrix and help to form a protective shield. However, it is surprising that MLR is high for the HD/FR/Z-1.0 composite, which implies that the degradation of polymer is delayed only at an optimum level of zeolite. This is in good agreement with CCT test results. The thermal degradation of the composites in air atmosphere was also investigated, as shown in Fig. 5 (b) and Table 4. The onset decomposition temperature (temperature of 5% mass loss) of HD/FR/Z-0.5 composite is higher than that of HD/FR/Z-0 composite. The char residues of HD/FR/Z-0.5 composite are 30.8 %, 26.6 % and 22.3 % at 500 ℃, 600 ℃ and 700 ℃ respectively, which are higher than that of HD/FR/Z-0 composite. As the zeolite content is increased to 1 % in the HD/FR/Z-1.0 composite, the char residue decreases, suggesting that the zeolite content influences the thermal degradation of the composites. It is inferred that zeolite may have a significant effect on the thermal degradation in an oxidative environment and thus oxygen may be involved in the formation of the intumescent shield [17]. In order to gain more insights into the role of zeolite in the thermal degradation in a non-oxidative environment, the Py-GC-MS of the flame retardant composites with or without zeolite was performed. The pyrograms of HD/FR/Z-0 and HD/FR/Z-0.5 composites pyrolyzed at 550 ℃ are shown in Fig. 6. The two composites show similar distribution of pyrolytic products. The pyrogram revels the presence of homologous triplet formed by alkanes, alkenes and alkadienes with increasing carbon number, which is characteristic of HDPE. However, it is difficult to quantify the relative content of these pyrolytic products. As the pyrolysis pattern is identical, further comparison is not performed.

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3.4 Mechanical properties The mechanical properties of the composites depend on the type of fillers and their interfacial interaction with the polymer matrix. Generally, a high loading of flame retardant additives can deteriorate the mechanical properties of the composites, and it remains challenging to simultaneously obtain excellent flame retardant and mechanical properties. In the present formulations, zeolite is used in combination with the IFR system composed of APP and THEIC at a ratio of 3:1. Fig. 7 shows that the tensile strength of HD/FR/Z-0 composite containing 25 wt. % of IFR is 18.2 (± 1.8) MPa. A low loading of zeolite causes no significant change in tensile strength, but the addition of 5 wt. % of zeolite results in a slight decrease in tensile strength to 16.8 (± 0.5) MPa. The flexural strength and flexural modulus of HD/FR/Z-0 composite are 25.0 (± 0.76) MPa and 1743.7 (± 33.7) MPa, respectively, which are slightly increased with the addition of zeolite. The loading of zeolite has a significant effect on the flexural modulus but not on the flexural strength. The Izod impact strength of the composite containing 25 wt. % of IFR is 2.54 (± 0.30) kJ/m2, which is decreased with the addition of zeolite. However, the composite containing 1 wt. % of zeolite shows a maximum of 13 % loss of impact strength. 3.5 Chemical analysis of char residue The microstructure of the intumescent char plays a critical role in flame retardant properties of the composites. The SEM microstructure of the char residue collected after LOI test is shown in Fig. 8. The char residue with or without zeolite forms a continuous, compact and foam cell intumescent char that can act as a barrier to oxygen and heat. The char surface of the HD/FR/Z-0.5 composite is more homogenous and compact with less cracks than that of HD/FR/Z-0 composite. Zeolite could promote the formation of the cross linked network through bridges such as Si14

O-P-C and Al-O-P-C, resulting in the formation of a compact char layer with better barrier properties [3]. The relative contents of elements obtained from EDX analysis are shown in Fig. 9. The EDX results show that the relative content of O/C, P/C and N/C in the inner layer is lower than that in the outer layer of the char in HD/FR/Z-0 composite. The lower C content in the outer layer indicates the oxidation of some carbon atoms on the surface, while the higher P content in the outer char layer indicates the migration of phosphorus to the surface. In HD/FR/Z-0.5 composite, the relative content of O/C, P/C and N/C in the inner layer is higher than that in the outer layer, indicating a decrease in oxidation of carbon atoms on the surface and less migration of phosphorus from the inner surface to the outer surface. The higher relative content of elements indicate excellent barrier properties of the outer char layer [28,29]. The EDX results clearly show that zeolite can promote the formation of a more compact and crosslinked char structure. The FTIR spectra of the char residue with or without zeolite are shown in Fig. 10. In HD/FR/Z-0 composite, the board peak at around 3100-3300 cm-1 is assigned to the N-H stretching vibration, and that at 2884 cm-1 is due to the CH2 stretching vibration. The peak at 2371 cm-1 may be assigned to the –OH bond in the O=P-OH structure. The board band in the region of 1150 – 1300 cm-1 is assigned to the P-O-C bond in the phosphate-carbon complex, and the band at 1163 cm-1 is attributed to the symmetric stretching of the P-O-C bond in the phosphate-carbon complex [2,39,40]. In HD/FR/Z-0.5 composite, the peak at around 3100-3500 cm-1 may be assigned to the stretching vibration of N-H and O-H, while those at 2921 cm-1 and 2852 cm-1 are due to the CH2 stretching vibration. However, it is important to note that the stretching vibration of both N-H and O-H can be identified in HD/FR/Z-0.5

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composite, but overlapped in HD/FR/Z-0 composite, which may be due to involvement of oxygen in the formation of char in the presence of zeolite as suggested by the thermal degradation behavior in air. The peak corresponding to the –OH bond in the O=P-OH of HD/FR/Z-0.5 composite is shifted to 2380 cm-1 compared to that of HD/FR/Z-0 composite. The characteristic peaks due to the stretching vibration of C=O (1685 cm-1) and the skeleton vibration of the nitrogenous heterocyclic structure (764 cm-1) disappear in both spectra, indicating the decomposition of the triazine ring of THEIC during the formation of the char. Raman spectroscopy was performed to characterize the graphitic structure of the char residues, where the G band represents the graphitic structure corresponding to the E2g mode of hexagonal graphite, while the D band represents the defect and disorder in the graphitic layer. The intensity ratio of the D band to G band (R=ID/IG) is inversely proportional to the in-plane microcrystalline size and/or the in-plane phonon correlation length, and thus it is a good estimate of the graphitization degree of carbon materials. It is shown that the lower the R value is, the higher the graphitization degree of the char will be [3,41,42]. The Raman spectra of the char residue of the composites after CCT test are shown in Fig. 11. The characteristic G band at around 1587 cm-1 and the D band at 1366 cm-1 suggest the formation of polyaromatic species or graphitic structures. Each curve is subjected to peak fitting using Origin 8.5/Peak Fitting Module. The curve is divided into 2 Gaussian bands, and the R values are evaluated. Table 5 shows that the R value of HD/FR/Z-0.5 composite (3.20) is lower than that of HD/FR/Z-0 composite (3.38), suggesting that the graphitic degree of the char residue increases in the presence of 4A zeolite. The chemical analysis suggests that the intumescent char is mainly composed of polyaromatic structure linked with phospho-carbonaceous structure via phosphor-

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ester linkage (P-O-C). The degradation products of APP can promote carbonization of THEIC, while the volatile gases such as NH3 and N2 act as blowing agent for the expansion of the intumescent char. This intumescent char could act as a barrier to the transfer of heat, oxygen and combustible volatile gases to the underlying polymer and hence improve the flame retardancy. Zeolite can react with APP to form extremely reactive phosphate species (aluminophosphate and silicophosphate) and the framework of zeolite would be destroyed completely. These species may act as the catalysts for synthesis of stable and protective intumescent shield from the polymer pyrolysis products [17]. The intumescent formulations with an optimum content of zeolite help to develop stable intumescent char residue with more graphitic structure. 4

Conclusions In this study, synthetic zeolite 4A was used in combination with the IFR system

consisting of APP and THEIC for the preparation of HDPE composites. An optimum level of zeolite can improve the flame retardancy of the composites. Zeolite helps to decrease the total smoke production, carbon dioxide evolution and degradation of the polymer matrix. The char residue consists of the phospho-carbonaceous structure and zeolite can catalyze the conversion of amorphous carbon residue into graphitic structure. Zeolite acts as a synergistic agent in the flame retardant composites at a low loading. Finally, it is suggested that a low loading of zeolite in combination with the IFR system produces no significant deterioration in mechanical performance. Acknowledgements This research is financially supported by the National Natural Science Foundation of China (21764011), the Foundation from Qinghai Science and Technology Department (2017-HZ-803) and Thousand Talents Program of Qinghai Province.

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List of figures Figure 1 FTIR spectra of HDPE (a), HD/FR/Z-0.5 (b), HD/FR/Z-0 (c), THEIC (d), APP (e) and zeolite 4A (f). Figure 2 SEM images of HD/FR/Z-0 (a) and HD/FR/Z-0.5 (b) composites. Figure 3 SEM images of HD/FR/Z-0 composite before (a) and after water treatment (b). Figure 4 HRR (a) and THR (b) curves of the composites containing IFRs with or without zeolite. Figure 5 TGA curve of the flame retardant HDPE composites in nitrogen (a) and in air (b). Figure 6 Py-GC-MS chromatogram of HD/FR/Z-0 and HD/FR/Z-0.5 composites. Figure 7 Tensile strength (a), flexural strength (b), flexural modulus (c), and impact strength (d) of the flame retardant HDPE composites. Figure 8 Char residue after LOI test, Inner (a) and outer layer (b) of HD/FR/Z-0 composite, inner (c) and outer layer (d) of HD/FR/Z-0.5 composite. Figure 9 Relative content of various elements in inner and outer layer of char residue from SEM-EDX Figure 10 FTIR spectra of the char residue of the HD/FR/Z-0 and HD/FR/Z-0.5 composites. Figure 11 Raman spectra of the char residue of HD/FR/Z-0 and HD/FR/Z-0.5 composites.

24

Tables

Table 1 Compositions of different flame retardant composites and their LOI values

FR* (wt. %)

Zeolite 4A (wt. %)

LOI (%)

S.N.

Samples

HDPE (wt. %)

1.

HD

100.0

-

-

18.5

2.

HD/FR/Z-0

75.0

25

-

25.1

3.

HD/FR/Z-0.5

75.0

24.5

0.5

26.3

4.

HD/FR/Z-1.0

75.0

24.0

1.0

24.3

5.

HD/FR/Z-1.5

75.0

23.5

1.5

24.8

6.

HD/FR/Z-2.5

75.0

22.5

2.5

25.0

7.

HD/FR/Z-5.0

75.0

20.0

5.0

23.0

* FR composed of APP and THEIC in 3:1 weight ratio

25

Table 2. Combustion performance parameters obtained from CCT HD

HD/FR/Z-0

IT (s)

102.5 ±7.7

50.0 ± 1.4

37.0 ± 4.9

39.5 ±9.1

Av-HRR (kw/m2)

206.6 ±37.1

129.9 ±15.9

130 ±10.5

150.75 ± 4.5

PHRR1 (kw/m2)

1012.8 ±106.5

313.7 ± 3.9

292.3 ± 32.8

292.3 ± 2.7

t- PHRR1 (s)

235.5 ± 7.7

283.5 ±37.4

252.0 ± 56.5

249.5 ± 34.6

PHRR2 (kw/m2)

-

293.3 ±59.5

320.2 ± 12.0

287.5 ± 29.1

t- PHRR2(s)

-

480.0 ±31.1

393 ± 4.2

455.5 ± 28.9

THR (MJ/m2) Av. Specific MLR (g/(s⋅m2) CO2 yield (kg/kg)

109.2 ± 20.5

105.0 ±0.1

92.0 ± 2.0

96.9 ± 4.9

14.5 ±3.1

4.79 ± 0.70

4.40 ± 0.02

6.39 ± 0.18

1.88 ±0.34

1.99 ±0.05

1.59 ± 0.29

1.91 ± 0.01

TSP (m2)

5.65 ± 0.53

6.51 ± 0.54

5.16 ± 1.26

6.46 ± 0.76

Av-SEA(m2/kg)

166.8 ± 6.4

171.6 ± 11.6

133.1 ± 30.7

167.7 ± 20.5

26

HD/FR/Z- 0.5

HD/FR/Z-1.0

Table 3 Thermal properties of HDPE composites in nitrogen atmosphere Samples

T5%

T50%

*MLR(%/min)

Char/500

Char/600

Char/700

HD/FR/Z-0

358.7

481.2

14.7

21.7

16.4

7.9

HD/FR/Z-0.5

355.6

475.8

13.4

20.7

16.4

8.0

HD/FR/Z-1.0

369.3

484.2

20.0

21.8

17.5

11.3

*MLR is mass loss at maximum degradation temperature

Table 4 Thermal properties of HDPE composites in air atmosphere Sample

T5%

T50%

Char/500

HD/FR/Z-0

320.3

465.3

26.8

19.5

8.7

HD/FR/Z-0.5

397.3

464.7

30.8

26.6

22.3

HD/FR/Z-1.0

334.6

472.0

23.2

18.9

10.7

27

Char/600

Char/700

Table 5 Results of Raman analysis of char residue of HD/FR/Z-0 and HD/FR/Z-0.5 composites Samples

Area of D band (AD) Area of G band (AG)

R= AD/AG

HD/FR/Z-0

832443.16

246107.91

3.38

HD/FR/Z-0.5

508088.72

158458.99

3.20

28

Figures

Figure 1 FTIR spectra of HDPE (a), HD/FR/Z-0.5 (b), HD/FR/Z-0 (c), THEIC (d), APP (e) and zeolite 4A (f)

Figure 2 SEM images of HD/FR/Z-0 (a) and HD/FR/Z-0.5 (b) composites.

Figure 3 SEM images of the composites HD/FR/Z-0 before (a) and after water treatment (b).

Figure 4 HRR (a) and THR (b) curves of the composites containing IFRs with or without zeolite.

Figure 5 TGA curve of the flame retardant HDPE composites in nitrogen (a) and in air (b).

Figure 6 Py-GC-MS chromatogram of HD/FR/Z-0 and HD/FR/Z-0.5 composites.

Figure 7 Tensile strength (a), flexural strength (b), flexural modulus (c), and impact strength (d) of the flame retardant HDPE composites.

Figure 8 Char residue after LOI test, Inner (a) and outer layer (b) of HD/FR/Z-0 composite, inner (c) and outer layer (d) of HD/FR/Z-0.5 composite.

Figure 9 Relative content of various elements in inner and outer layer of char residue from SEM-EDX.

Figure 10 FTIR spectra of the char residue of the HD/FR/Z-0 and HD/FR/Z-0.5 composites.

Figure 11 Raman spectra of the char residue of HD/FR/Z-0 and HD/FR/Z-0.5 composites.

Highlights



Zeolite 4A combined with IFR system was used to prepare flame-retardant HDPE composites.



A low loading of zeolite 4A can improve LOI and flame-retardant performance.



Zeolite can decrease total smoke production and carbon dioxide evolution of polymer.



Zeolite promotes the formation of compact char layer with high graphitic degree.



A low loading of zeolite produces no significant effects on mechanical performance.

Conflict of interest statements

We declare that we have not any competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The work reported in this paper is original and has not been submitted to, nor is under review at another journal or other publishing venue. We have reviewed and approved final version of manuscript, and agreed to the journal submission policies, for the manuscripts entitled, “Synergistic effect of zeolite 4A on thermal, mechanical and flame retardant properties

of

intumescent

POTE_2019_1447).

flame

retardant

HDPE

composites”

(ID: