Fire-safe and environmentally friendly nanocomposites based on layered double hydroxides and ethylene propylene diene elastomer

Abstract

In this work we describe layered double hydroxide (LDH), known as naturally occurring hydrotalcite, based rubber composites that can serve as outstanding fire retardant elastomeric materials. The preparation and detailed characterization of these composites are presented in this study. The inherent slow sulfur cure nature of EPDM rubber is considerably improved by the addition of LDH as realised by the observation of a shortening of the vulcanization time and an improvement of ultimate rheometric torque. This behavior of LDH signifies not only the filler-like character of itself, but also offers vulcanization active surface properties of layered double hydroxide particles. A good rubber–filler interaction was also realised by observing a positive shift of the glass transition temperature of ethylene propylene diene rubber (EPDM) in dynamic mechanical analysis (DMA). The flame retardant property was studied by the cone calorimeter test. The cone calorimeter investigation with sulfur cured gum rubber compounds found a peak heat release rate (PHRR) value of 654 kW m⁻². However, at a higher phr loading of Zn–Al LDH i.e., at 40 phr and 100 phr, the PHRR is diminished to 311 kW m⁻² and 161 kW m⁻², respectively. Thus, this present work can pave the way to fabricate environmentally friendly fire retardant elastomeric composites for various applications.

---

Introduction

To date, various thermoplastic polymeric materials incorporated with the classical flame retardant fillers like halo-compounds (brominated or chlorinated), organo-phophorous based flame retardant materials, aluminium hydroxide (ATH) and magnesium hydroxide based materials have been fabricated and commercially introduced.^1–7 These functional polymer composites find a wide range of applications from high-structural materials to light weight composites for day-to-day use. In this regard, elastomeric composites fall in a significantly important category where their utilization as a potential flame retardant material is considerably emphasised for applications in electrical industries (cables, conduits, insulating wires, etc.) as well as for roofing or flooring materials. Research has been carried out on several elastomers e.g., silicone rubber, chloroprene rubber, thermoplastic elastomers, etc., which are claimed to be used as potential flame retardant elastomer composites commercially as of now.^8–12 Ethylene propylene diene monomer rubber (EPDM) possesses certain advantages over these conventional elastomers in the aspects of excellent weather resistance, low cost, ease of processing, excellent heat and chemical resistance as well as flexibility at a relatively lower temperature.¹³ EPDM is a typical non-polar rubber with unsaturation present in the main structure. The typical chemical structure of EPDM is shown in Fig. 1a. But it finds a wide range of applications in wires, cables, roofing materials, and sporting goods where flame retardancy is a key point to be addressed. However, flammability of the pristine polymeric materials is always a vital drawback resulting in the loss of credibility for commercial interests. However, a couple of combinations of EPDM with thermoplastics like polypropylene (PP)¹⁴ and polyethylene (PE)^15,16 have been tried to improve their fire retardancy using flame retardants. However, in those existing literatures, the conventional flame retardants used are either toxic or produce toxic gaseous substances during combustion (for example, organo-halogen compounds, borates, organo-phosphates, etc.). To overcome these two important issues, aluminium hydroxide (ATH)¹⁷ was used in practice to be reckoned as a suitable candidate for a flame retardant filler with EPDM. In spite of being less expensive and non-toxic^18,19 in nature, ATH has certain loopholes when used with EPDM. A high loading of this inorganic material deteriorates the mechanical properties of the composites. Moreover, the flame retardancy efficiency of ATH is relatively low. Henceforth, it is a challenging task for scientists to develop a new environmentally friendly material which would improve the flame retardancy of EPDM without compromising the ultimate mechanical properties of the final compound. Layered double hydroxide (LDH) has recently drawn immense attention from materials scientists for its excellent features of structure and properties unlike the conventional layered silicates. Layered double hydroxides (LDHs) are typical host–guest nanostructured materials, which can generally be described by the chemical formula [M^II_1−xM^III_x(OH)₂][A_x/m^m−·nH₂O], where M^II (e.g., Mg²⁺, Zn²⁺, Ni²⁺) and M^III (e.g., Al³⁺, Cr³⁺, Fe³⁺) denote divalent and trivalent metal ions respectively and A is the anion (e.g., Cl⁻, SO₄²⁻, CO₃²⁻, NO³⁻) with valence numbers m and x/m. x is the ratio of M^III/(M^II + M^III) and normally lies in the range of 0.2 ≤ x ≤ 0.33.^20,21 LDH reduces the number of steps of rubber mixing. It is possible to mix a large volume of LDH with rubbers in a single step. The layered structure of LDH is displayed in Fig. 1b.

Fig. 1 (a) Structure of EPDM rubber containing ethylidene norbornene (ENB) and (b) chemical structure of Zn–Al LDH.

So far LDH has been combined with lots of thermoplastic polymers to study the reinforcement effect. But with elastomers as the matrix material, there is limited literature available.^22–24 LDH has also been tried to reinforce the EPDM matrix in the past. In those reports, LDH was chemically modified with dodecyl sulphate (DS) anions intercalated within the galleries.^21,25 Wang et al.²⁶ reported the effect of flame retardancy of the modified Mg–Al LDH–maleic anhydride grafted EPDM composite material. Nevertheless, the compatibility of unmodified Zn–Al LDH and EPDM is a great challenge for the rubber scientists to overcome. In this paper, we show and discuss some of the properties of the unmodified Zn–Al LDH mixed EPDM composite. The capability of layered double hydroxide (LDH) has been explored as a potential flame retardant filler and the effect of the mechanical properties, dynamic mechanical properties, and thermal stability of the composites was also studied.

Materials and method

Materials

Zn–Al LDH (Alcamizer P 93; Zn²⁺/Al³⁺ = 1/4) in the unmodified form was obtained from KISUMA chemicals, The Netherlands. Ethylene propylene diene rubber (EPDM, Lanxess Buna EP G 6850; Mooney viscosity ML (1 + 4) at 125 °C – 60 ± 5 Mooney units; ethylene content ∼ 51 ± 4 wt%; ENB content ∼ 7.7 ± 1.1 wt%) was used in this study. Sulphur (purity ∼ 99.5%) and tetramethyl thiuram disulfide (TMTD) (purity ∼ 97%) were obtained from Acros organics. Zinc oxide and stearic acid (general purpose grade) were obtained from Fisher Scientific.

Preparation of composites

The rubber compounds were prepared by a two-roll mixing mill (Polymix 110L; size: 203 × 102 mm²; Servitech GmbH, Wustermark, Germany) with the friction ratio 1 : 1.2 rotating at 40 °C using a 20 min compounding cycle. All the weights were taken in parts per hundred of rubber and the recipe of the EPDM compounds is given in Table 1.

Table 1 Formulation of EPDM–Zn–Al LDH composites

Sample	EPDM	S	TMTD	LDH	ZnO	St. acid
	(All in parts per hundred rubber)
EPDM–4 ZnO	100	0.5	1.0	0	4	1.0
EPDM–4 LDH	100	0.5	1.0	4	0	1.0
EPDM–10 LDH	100	0.5	1.0	10	0	1.0
EPDM–40 LDH	100	0.5	1.0	40	0	1.0
EPDM–100 LDH	100	0.5	1.0	100	0	1.0

---

At first the requisite amounts of LDH were incorporated in the masticated EPDM rubber. After addition of those additives, sulphur, TMTD and stearic acid were mixed for crosslinking of the rubber matrix. After mixing the rubber according to the above procedure, the compounded sample, thus obtained, was subjected to a curing study to get the optimum curing time. In this procedure an uncured mass (∼5.5 g) from the compounded rubber was placed into a moving die rheometer (Scarabaeus SIS-V50) under isothermal conditions at 160 °C with a frequency of 1.67 Hz and the generated torque was measured against time. From this torque–time curve the curing time (t_c90) was calculated as the point at which the rheometric torque reached 90% of the value of its ultimate torque. The rubber samples were then cured until their optimum curing time (t₉₀) by a hot press (FORTUNE Holland, Modell TP 400) at 160 °C, cooled to room temperature and then kept for 24 h before doing any further tests.

Method

The curing study was carried out for the rubber samples in Scarabaeus SIS V50 at 160 °C for 60 min.

A wide angle X-ray scattering (WAXS) experiment was carried out by a 2-circle diffractometer XRD 3003 h h⁻¹ (GE Inspection Technologies/Seifert-FPM, Freiberg) with Cu-K_α radiation (λ = 1.54 Å) within the range of 2θ = 1–25° using the step length of 0.05°. The accelerating voltage was 40 kV at a current of 30 mA. The basal spacings of the LDH material and the EPDM–LDH composites were determined from Bragg’s equation.

Scanning electron microscopy (SEM) (Ultra Plus, Carl Zeiss SMT) was employed to capture the micrographs of the corresponding samples. Prior to the experiment, the specimens were coated with a thin layer of platinum (layer thickness: 3 nm) using a sputter coater (BAL-TEC SCD 500 Sputter Coater).

Thermogravimetric analysis (TGA) was carried out using a thermogravimetric analyser TGA Q 5000, TA Instruments. The samples (approximately 5–6 mg) were heated up to 700 °C in N₂ atmosphere at a heating rate of 10, 20, 30 and 40 °C minute⁻¹.

For the cone calorimeter test (FTT cone calorimeter), the flame retardancy of the samples was examined by irradiating the square specimens (100 mm × 100 mm × 3 mm) with a heat flux of 35 kW m⁻² (ISO 5660-1) without the use of the frame and grid.

Tensile tests were carried out with a Zwick 1456 (model 1456, Z010, Ulm Germany) with a cross-head speed of 200 mm min⁻¹ (ISO 527).

Dynamic mechanical analysis (DMA) was done with an Eplexor 2000N (Gabo Qualimeter, Ahlden, Germany) at a frequency of 10 Hz and within the temperature range of −80 to +80 °C. The samples were analyzed in the tension mode with 1% static pre-strain and 0.5% oscillating dynamic strain. The measurements were performed with a heating rate of 2 K min⁻¹ under liquid nitrogen flow.

Results and discussion

Structural characterization of the composites

Morphological evaluation has been carried out of the elastomer composites with the help of the X-ray diffraction technique and electron microscopy. Fig. 2a depicts the wide angle X-ray spectra of the uncrosslinked EPDM, LDH and EPDM–4 LDH composite.

Fig. 2 (a) WAXS patterns of the uncrosslinked EPDM, unmodified Zn–Al LDH and EPDM–4 LDH composite, and (b) appearance of an additional peak (2θ ∼ 10°; d ∼ 0.9 nm) for tetramethyl thiuram disulfide (TMTD, a rubber sulphur accelerator) mixed rubber sample. The samples in (b) are the compounds before vulcanization.

As expected, uncrosslinked EPDM rubber shows a typical amorphous scattering pattern in the XRD plot. Characteristic reflection of LDH is revealed with a basal spacing of ∼0.77 nm at 2θ = 11°. This value is attributed to the presence of some inorganic anions like CO₃²⁻ in the interlayer region of the LDH structure. The WAXS spectra comparing the unmodified LDH, EPDM and the EPDM–4 LDH composite show two major new reflections at 2θ = 5° and 6.6° together with the most prominent reflection at 2θ = 11°. These three peaks are ascribed to the basal spacings of 1.7 nm, 1.3 nm and 0.9 nm respectively.

The origin of the new reflections is still not clear. Several authors investigated the dependency of alkyl chain length and interlayer distance resulting after modification.

Within these investigations the following formula to calculate the interlayer distance was established: 〈d〉 (nm) = 0.96 + 0.127n_c sin α where 〈d〉 represents the calculated layer distance, n_c is the number of carbon atoms in carboxylate or sulfonate and α represents the tilt angle of the alkyl chain attached on the LDH surface describing the orientation within the gallery of the LDH.²⁷ Additionally, sometimes a layer of water in between the anions and the crystalline layer is discussed, resulting in an additional increase of the layer distance.^27,28 With these findings the reflection at 1.7 nm might be interpreted as LDH intercalated with stearate originating from an exchange reaction during the mixing procedure under release of carbon dioxide and water. A second possible molecule to intercalate originates from tetramethyl thiuram disulfide, which forms the corresponding monosulfide and polysulfides in the presence of sulfur during the mixing procedure and vulcanization time duration.²⁹ It is pretty obvious from Fig. 2b that a mixture prepared solely with EPDM, LDH and TMTD clearly shows this reflection, whereas without TMTD the reflection at 0.9 nm is missing. The peaks ascribed to 1.7 and 1.3 nm did not appear in the TMTD mixed uncured compounds. Most probably, during curing some zinc and sulphur based complex, know as the sulphur precursor, penetrated inside the layer gallery and shows reflections in the cured composites.

Further investigations were performed with scanning electron microscopy (SEM) to examine the surface properties as well as the state of dispersion of the LDH particles within the rubber matrix. Elemental analysis based on SEM coupled with EDX confirmed the presence of Zn and Al atoms in the surface of the EPDM matrix (Fig. 3a). The same study of the uncrosslinked EPDM does not show any such metal cations in the analysis. Elemental mapping (Al, Mg, Zn) further represents the successful incorporation of the LDH particles into EPDM. Further morphological study demonstrates the degree of the dispersion of the LDH particles into the EPDM matrix and their morphological features. It is obvious from the micrographs (Fig. 3b) that the dispersion of a high amount (100 phr) of LDH is very good in the rubber. Particles are found to exist at the sub-micron size level and no further agglomeration was observed even with such a high loading of the inorganic filler.

Fig. 3 (a) SEM-EDX analysis of the uncrosslinked EPDM and EPDM–4 LDH composite and the corresponding Al, Mg, S and Zn mappings, and (b) SEM images of EPDM filled with different amounts of LDH.

Curing study

While studying the torque behaviour of LDH filled EPDM, from Fig. 4, it is observed that the EPDM loaded with 100 phr LDH produces the highest amount of torque value compared to the rest. The maximum value of the rheometric torque is attributed to the reinforcing character of the LDH particles originating from a stronger filler–polymer interaction. Zinc atoms present in the LDH crystalline structure interact with the sulfur which can finally lead to a direct rubber–filler interaction. Therefore, filler–filler interaction could be the only prime reason for such a high value of torque for the 100 phr loaded composite. It can be also noticed here that the ZnO-containing sample shows a slightly marching nature of the curing pattern but with the addition of more LDH, the marching patterns completely disappeared and a plateau-like curing behaviour appeared. It is interesting to notice from the plot in Fig. 4 that without the presence of an essential activator like zinc oxide, Zn–Al LDH can efficiently cure the rubber. By the virtue of releasing zinc atoms to the system, Zn–Al LDH can act as a potential crosslinker for the rubber composites. The faster development of torque in the vulcanizate proves that LDH has a better curing efficiency than ZnO. The higher the loading of LDH in EPDM is, the faster the development of torque is.

Fig. 4 Torque vs. temperature plots of the EPDM compounds.

Thermal properties

Thermogravimetric plots in Fig. 5a and b show the thermal stability profile of the composites. The thermal decomposition of unmodified LDH is primarily attributed to the loss of interlayer species followed by the endothermic decomposition of the metal hydroxide layer.^30,31 The maximum degradation temperatures of 462 °C for EPDM–4 LDH and 446 °C for EPDM–100 LDH are noticed to decrease by a margin of almost 20 °C for the highest loaded composite. The presence of an extra peak around 280 °C is realized from Fig. 5b for EPDM–100 LDH. This peak is attributed to the partial loss of water molecules intercalated into the layers of the said composite.³⁰ This also significantly and quite obviously indicates that in the case of the highest loaded (100 phr LDH) composite, a maximum amount of water molecules is accommodated. Henceforth, the peak arises around 280 °C. The presence of a greater volume of water molecules for EPDM–100 LDH facilitates the flame retardancy behavior which is already discussed elaborately in the earlier section. Thermogravimetric analysis with three different heating rates e.g., 20 °C min⁻¹, 30 °C min⁻¹ and 40 °C min⁻¹ of 4 phr LDH in EPDM and 100 phr LDH in EPDM was carried out to understand the degradation kinetics behaviour of the materials. The degradation corresponding to the main chain scission of the EPDM backbone starts at a relatively lower temperature compared to the LDH filled EPDM. This could be attributed to the fact that LDH layers obstruct the internal diffusion of heat and gaseous small molecules formed during thermal degradation/oxidation resulting in a more thermally stable material compared to the uncrosslinked EPDM. It can be mentioned here that although the experiment was carried out in nitrogen atmosphere, the presence of a small amount of oxygen in the chamber or inside the rubber matrix cannot be ruled out.

Fig. 5 (a) Thermogravimetric analysis (TGA) with % char residue values and (b) derivative plot of the mass loss against temperature (inset: the peak for intercalated water loss). The experiment was carried out in nitrogen atmosphere.

The enhanced thermal stability of the polymer–LDH composites is attributed to the lower permeability of oxygen and the diffusibility of the degradation products from the bulk of the polymer.

Calculation of activation energy of degradation by multiple heating rate methods

The Kissinger method (differential method). This method is based on the following equation:³²where T_max is the temperature corresponding to the inflection point of the thermal degradation curve, and α_max is the conversion degree at the inflection point. The slope of the fitted line of log(β/T_max²) against 1/T_max gives E_a.

The Flynn–Wall–Ozawa method (iso-conversional integral method)^33,34. This method is based on the following equation:where β is the heating rate in °C min⁻¹, E_a is the activation energy, R is the universal gas constant, α is the degree of conversion, T is the absolute temperature to reach the conversion, A is the pre-exponential factor and
is the integral function of α. The slope of the fitted line of log β against 1/T gives E_a.

To determine the activation energy of decomposition (E_a) of EPDM and its composites, TGA/DTG was performed at constant heating rates of 20, 30 and 40 °C min⁻¹. The Kissinger (eqn (1)) and Flynn–Wall–Ozawa (eqn (2)) methods were used to determine E_a of the second stage degradation of the samples (EPDM–4 LDH, EPDM–100 LDH) in a nitrogen environment and the results were compared as both of the methods have different criteria of calculation.

It has been kept in mind that in the calculation of α in the Flynn–Wall–Ozawa method, the final weight corresponds to the weight at the end of the second stage degradation. In the Flynn–Wall–Ozawa method, E_a is determined at each conversion level (10 to 50%) i.e. α = 0.1 to 0.5. In the Kissinger method, E_a can be determined without any prior information about the reaction mechanism and the reaction order.

Fig. 6a shows the kinetic analysis of EPDM–4 LDH and EPDM–100 LDH using the Kissinger method. Fig. 6b shows the kinetic analysis of EPDM–4 LDH with conversion levels from 10 to 50% using the Flynn–Wall–Ozawa method at different heating rates. The value of activation energy (E_a) of the EPDM–4 LDH composite calculated from the Kissinger method is ∼30 kJ mol⁻¹. An improved trend of activation energies is observed for the EPDM–4 LDH composite in Fig. 6c. Up to α = 0.3, a clear improvement in activation energy is visible, but after that at higher α, there is a drop in the activation energy. This fact can be attributed to the formation of an insulated layer on the surface of the composite by LDH which indeed is responsible for reducing the activation energy at higher conversion.^35–38

Fig. 6 (a) The Kissinger method and (b) the Flynn–Wall–Ozawa method applied to experimental TGA data at different heating rates and (c) activation energies of samples at different weight losses calculated by the Flynn–Wall–Ozawa method.

Flame retardancy

The flame retardancy of the EPDM–LDH composites was studied by the cone calorimeter test which is an advanced and widely used test method for assessing the flammability of polymeric materials. The most important parameter monitored during this test is the heat release rate (HRR), which is calculated from the amount of oxygen consumed during combustion based on the principle described by Huggett.³⁹ The HRR is a very important variable and represents how fast a fire can reach an uncontrollable stage. In the LDH based EPDM composites, especially at a higher layered double hydroxide concentration, a compact layer of residue is formed that separates the flame region from the molten layer and acts as a barrier against the heat conduction from the former region to the latter.²⁶ These result in much slower material decomposition in the composites. The plot shown in Fig. 7a exhibits that addition and subsequent increase in the concentration of Zn–Al LDH in EPDM not only significantly reduces the peak heat release rate (PHRR) but also makes the HRR curve increasingly flattened.

Fig. 7 (a) Heat release rate (HRR), (b) total heat release (THR), and (c) mass loss of the EPDM based composites.

For ZnO mixed EPDM, cone calorimeter investigation gives a PHRR value of 615 kW m⁻². The substitution of ZnO with a low concentration of LDH in EPDM does not significantly reduce the PHRR value. However, at a higher loading of LDH, i.e., at 40 phr and 100 phr, the PHRR is significantly lowered to 311 and 161 kW m⁻² (Fig. 7a), respectively.

The total heat release (THR) is a parameter that indicates how intensified the fire is. Once the ignition takes place, the THR steadily increases with burning time and attains a steady state before the flame-out occurs. It is quite obvious from the THR plot (Fig. 7b), that the THR, over the first 2 min and 4 min after application of an external heat flux, is progressively reduced with the increasing LDH content in the EPDM matrix. As noticed in Fig. 7b, at 4 min after the application of external heat flux, the THR value is reduced in the samples with 40 and 100 phr LDH, respectively, in comparison to the THR observed for the composite composition with the lowest LDH content (EPDM–4 LDH). The THR is often taken as the measure of the propensity to sustain a long duration fire. The results clearly suggest that the incorporation of higher amounts of LDH showed a considerable effect on the flame retardancy of EPDM. LDH is believed⁴⁰ to form a refractory oxide residue on the surface of the material and subsequently releases water vapour and carbon dioxide during endothermic decomposition which further felicitates the flame retardancy process. Also, some catalytic effect caused by the metal oxide in the combustion is another reason to reduce the flammability of the polymer. In the literature⁴¹ it is reported that bivalent and trivalent metal ion based intumescent systems enhanced the fire retardancy performance in polypropylene (PP). However, a detailed understanding of such a catalytic effect of intumescence is still awaited. In this respect, LDH fillers have definite advantage over the layered silicates as flame retardants.^42,43 LDH takes part actively in the combustion process through endothermic decomposition, acting as a heat sink and significantly reducing the THR value during combustion. The combinations of the presence of intercalated water molecules to stabilize the whole structure of the layered double hydroxide as well as the existence of the metal hydroxide layers are mainly accounted for by the endothermic decomposition.^44,45 The results displayed based on the heat release rate (HRR) and total heat release (THR) are in good accordance with the % weight loss of the corresponding rubber samples. The % mass loss vs. combustion time of the samples is exhibited in Fig. 7c. In the cone calorimeter test, EPDM loaded with a lower amount of LDH (EPDM–4 LDH) burns comparatively faster after ignition; the char residue after the fire test is hardly left (only 0.2 wt%). An increase in the amount of LDH in the matrix significantly improves the amount of char residue. The amount of char residue left after the test for the 40 and 100 phr LDH filled EPDM composites is 11 wt% and 23 wt%, respectively. The formation of char residue of the corresponding samples is exhibited in Fig. 8. The digital photograph revealed that the amount of char residue left after the cone calorimetry experiment is more in the case of the highly loaded sample with respect to the lower amount of LDH filled rubber composites. These photographs corroborate excellently with the results of the mass loss of the composites as displayed in Fig. 8. The basic reason for the substantial improvement of the burning behaviour is attributed to the formation of a continuous compact char layer that would restrict the effective transfer of gas and volatiles through the surface, and therefore preventing further burning of the samples.^46,47

Fig. 8 Digital photographs of ash formation of (a) pure EPDM, (b) EPDM–4 ZnO, (c) EPDM–4 LDH, (d) EPDM–10 LDH, (e) EPDM–40 LDH and (f) EPDM–100 LDH.

A tentative mechanism explaining the above result can be proposed herewith as shown in Fig. 9. The barrier mechanism, as termed to elucidate the fire safety phenomena indicates that, under pyrolysis conditions, LDH clay forms a char-like material and a lot of water vapor from endothermic degradation of LDH crystals. The formation of char subsequently plays a dual role. First, it acts as a barrier to the migration of the degradation products to the surface of the degrading elastomer and second, as a thermal insulator, restricting further exposure of the rubber matrix to the heat and oxygen. The layer of the char will prevent the gas, heat and oxygen from migrating through the whole body and thus the flame retardancy effect will be improved. Thus, the char layer provides the necessary barrier function to the composite and eventually thermal insulation is established as a form of condensed phase in the whole structure. The gas, heat, and oxygen cannot travel a long distance within the nanocomposite.

Fig. 9 Schematic presentation of the fire retardancy mechanism of the layered double hydroxide based elastomer.

Mechanical properties

The mechanical properties of the EPDM were also investigated. It is evident from Table 2 that the composites have much improved mechanical properties like 100% modulus (σ_100%), tensile strength (σ_M), and elongation break (ε_B) compared to the neat EPDM (ZnO cured) rubber. With the addition of LDH in the EPDM matrix, it is noted that the modulus increases significantly with the increasing LDH content in all cases and reaches the highest value at a 100 phr loading of LDH. This gradual increase in the mechanical properties occurs due to the addition of LDH in the EPDM matrix which eventually imparts some degree of reinforcement by the virtue of the hydrodynamic effect.⁴⁸

Table 2 Mechanical properties of the EPDM composites derived from stress–strain studies

Sample designation	σ100% (MPa)	σM (MPa)	εB (%)
EPDM–4 ZnO	0.89	1.50	357
EPDM–4 LDH	0.77	2.36	578
EPDM–10 LDH	0.91	6.16	833
EPDM–40 LDH	1.06	3.79	551
EPDM–100 LDH	1.16	5.97	593

---

Dynamic mechanical analysis (DMA) was carried out to understand and then interpret further the reinforcing ability of the LDH in the EPDM matrix. As seen from the temperature sweep DMA plot in Fig. 10, the glass transition temperature (T_g) has been noticed to shift towards higher temperatures by a small margin with the addition of LDH. The values of the glass transition temperatures (T_g) of the different composites described in this article are provided in Fig. 10a. It is also noteworthy to mention that at a 4 phr loading the ZnO containing sample shows a positive shift of ∼4 °C as compared with the 4 phr LDH sample. However, an increasing tendency of the glass transition temperature is noticed when the loading of LDH is increased from 4 to 100 phr. This shift of the T_g may indicate a good rubber–filler interaction at a higher loading of LDH.

Fig. 10 (a) Loss tangent (tan δ) and (b) storage modulus (E′) vs. temperature plots of the EPDM–LDH composites and (c) activation energy plot (Arrhenius equation) of the EPDM based composites prepared with different amounts of LDH.

It is also observed from Fig. 10a that in the case of a higher loading of LDH in the EPDM matrix, the peak height of the tan δ vs. temperature plots has decreased indicating a reinforcing character of LDH. The storage modulus is also significantly improved in the region after the glass transition temperature (T_g) and showed a steady rubbery plateau in the case of a higher loading of LDH with EPDM (Fig. 10b). Incorporation of an excess amount of LDH provides a higher aspect ratio as well as a higher exposed surface area which eventually translates into a higher storage modulus (E′) value of the corresponding composite.

The results obtained in Fig. 10b were further exploited to attain quantitative information about the temperature dependent storage modulus varying with the loading of LDH in the EPDM matrix. A logarithmic plot of the storage modulus vs. the inverse temperature is demonstrated in Fig. 10. The Arrhenius equation of activation energy, as shown below, is fitted in the linear regime of the temperature range above the glass transition temperature. The slope obtained using eqn (3) provides empirical elucidation about the activation energies of the corresponding samples.

where E is the storage modulus of the filled rubber composite, E₀ is the storage modulus of the unfilled rubber, E_a is the apparent activation energy, R is the universal gas constant, and T_ref is the reference temperature. This equation was modelled after considering the filler–filler interaction parameters (Payne effect) of the samples⁴⁹ and subsequently was followed by the Arrhenius equation. The values of activation energy (E_a) are displayed in Fig. 10. It is clearly observed that with an increasing amount of layered double hydroxides, the activation energy is also increased due to the enhancement of the specific surface area realized from the greater amount of LDH particles. Subsequently, the increment in the layer thickness of the immobilized elastomer chains with decreasing temperature towards the glass transition from the rubbery plateau side⁵⁰ enhanced the activation energy. This model presented here is truly empirical and henceforth a hypothesis could be introduced that the filler network created by the LDH particles at a relatively higher loading is temperature dependent.⁵¹

Conclusions

It is understood from this study that the use of a high amount of LDH, as much as 100 phr (parts per hundred rubber), in a weather resistant EPDM elastomer can offer a elastomeric material with super flame retardant properties. This kind of developed material can be applied in many products used today, from appliance hoses, radiator hoses in our cars, to washers and insulation of water pipes and any outdoor applications. In the present study, we observed that in spite of using unmodified LDH with EPDM, the mechanical properties, dynamic mechanical properties, thermal stability, rheological properties and flame retardancy were significantly improved. Although there is no chemical compatibility between non-polar EPDM and polar LDH, the composites showed quite improved tailor-made properties which indicates a good degree of rubber–filler interactions, proving that the intercalation of the polymer chains into the layers of Zn–Al LDH has been carried out successfully. The degradation kinetics study proved that the thermal stability of EPDM–4 Zn–Al LDH is much better than that of the pristine EPDM, but as the loading goes on increasing the value of activation energy drops which further confirms that the loading of 4 phr LDH in the EPDM matrix is an optimum value in this regard. At a higher loading of Zn–Al LDH into the EPDM matrix, the other properties obtained were of the best values. It is satisfactorily reported in this paper that in future, to obtain similar or perhaps better properties with rubber nanocomposites, ZnO can be very acceptably replaced with Zn–Al LDH.

References

1. G. B. Huang, A. A. Zhuo, L. Q. Wang and X. Wang, Preparation and flammability properties of intumescent flame retardant-functionalized layered double hydroxides/polymethyl methacrylate nanocomposites, Mater. Chem. Phys., 2011, 130, 714–720.
2. M. Ardanuy and J. I. Velasco, Mg-Al Layered double hydroxide nanoparticles evaluation of the thermal stability in polypropylene matrix, Appl. Clay Sci., 2011, 51, 341–347.
3. M. Kotal, S. K. Srivastava, S. K. Manu, A. K. Saxena and K. N. Pandey, Preparation and properties of in situ polymerized polyurethane/stearate intercalated layer double hydroxide nanocomposites, Polym. Int., 2013, 62, 728–735.
4. X. Wang, S. Zhou, W. Y. Xing, B. Yu, X. M. Feng, L. Song and Y. Hu, Self-assembly of Ni-Fe layered double hydroxide/graphene hybrids for reducing fire hazard in epoxy composites, J. Mater. Chem. A, 2013, 1, 4383–4390.
5. S. L. Xu, L. X. Zhang, Y. J. Lin, R. S. Li and F. Z. Zhang, Layered double hydroxides used as flame retardant for engineering plastic acrylonitrile-butadiene-styrene (ABS), J. Phys. Chem. Solids, 2012, 73, 1514–1517.
6. Y. J. Feng, P. G. Tang, J. M. Xi, Y. Jiang and D. Q. Li, Layered double hydroxides as flame retardant and thermal stabilizer for polymers, Recent Pat. Nanotechnol., 2012, 6, 231–237.
7. G. B. Huang, Z. D. Fei, X. Y. Chen, F. L. Qiu, X. Wang and J. R. Gao, Functionalization of layered double hydroxides by intumescent flame retardant: preparation, characterization, and application in ethylene vinyl acetate copolymer, Appl. Surf. Sci., 2012, 258, 10115–10122.
8. S. L. Fang, Y. Hu, L. Song, J. Zhan and Q. L. He, Mechanical properties, fire performance and thermal stability of magnesium hydroxide sulfate hydrate whiskers flame retardant silicone rubber, J. Mater. Sci., 2008, 43, 1057–1062.
9. N. Karak and S. Maiti, Antimony polymers. III. Flame retardant behaviour of chloroprene and natural rubber vulcanizates with antimony polymer, J. Appl. Polym. Sci., 1998, 68, 927–935.
10. W. Yuan, H. Chen, R. Chang and L. Li, Synthesis and characterization of NaA zeolite particle as intumescent flame retardant in chloroprene rubber system, Particuology, 2011, 9, 248–252.
11. L. Yang, Y. Hu, H. Lu and L. Song, Morphology, thermal, and mechanical properties of flame-retardant silicone rubber/montmorillonite nanocomposites, J. Appl. Polym. Sci., 2006, 99, 3275–3280.
12. K. Pal and J. N. Rastogi, Development of halogen-free flame-retardant thermoplastic elastomer polymer blend, J. Appl. Polym. Sci., 2004, 94, 407–415.
13. A. K. Bhowmick, C. Stein and H. L. Stephens, Polynorbornene rubber, Handbook of elastomers, Marcel Dekker, New York, 1981, p. 729.
14. L. Yu, W. Wang and W. Xiao, The effect of decabromodiphenyl oxide and antimony trioxide on the flame retardation of ethylene-propylene-diene copolymer/polypropylene blends, Polym. Degrad. Stab., 2004, 86, 69–73.
15. Z. H. Chang, F. Guo, J. F. Chen, J. H. Yu and G. Q. Wang, Synergistic flame retardant effects of nano-kaolin and nano-HAO on LDPE/EPDM composites, Polym. Degrad. Stab., 2007, 92, 1204–1212.
16. S. Jia, Z. Zhang, Z. Du, R. Teng and Z. A. Wang, Study of the dynamic flammability of radiation cross-linked flame-retardant HDPE/EPDM/silicon-elastomer compound, Radiat. Phys. Chem., 2003, 66, 349–355.
17. C. Canaud, L. L. Y. Visconte, M. A. Sens and R. C. R. Nunes, Dielectric properties of flame resistant EPDM composites, Polym. Degrad. Stab., 2000, 70, 259–262.
18. F. K. Jones, J. L. Laird and B. W. Smith, Rubber World, 1996, 215, 42.
19. R. D. Allen, Improving the high temperature performance of EPDM, in Handbook of polymer science and technology, ed. N. P. Cheremisino, Marcel Dekker, New York, 1989, p. 127.
20. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11, 173–301.
21. H. Acharya, S. K. Srivastava and A. K. Bhowmick, Synthesis of partially exfoliated EPDM/LDH nanocomposites by solution intercalation: Structural characterization and properties, Compos. Sci. Technol., 2007, 67, 2807–2816.
22. D. Basu, A. Das, K. W. Stöckelhuber, U. Wagenknecht and G. Heinrich, Advances in layered double hydroxide (LDH)-based elastomer composites, Prog. Polym. Sci., 2014, 39, 594–626.
23. Das, D.-Y. Wang, A. Leuteritz, K. Subramaniam, H. C. Greenwell, U. Wagenknecht and G. Heinrich, Preparation of zinc oxide free, transparent rubber-nanocomposites using a layered double hydroxide filler material, J. Mater. Chem., 2011, 21, 7194–7200.
24. A. Das, J. J. George, B. Kutlu, A. Leuteritz, D.-Y. Wang, S. Rooj, R. Jurk, R. Rajeshbabu, K. W. Stöckelhuber, V. Galiatsatos and G. Heinrich, A novel thermotropic elastomer based on highly-filled LDH-SSB composites, Macromol. Rapid Commun., 2012, 33, 337–342.
25. S. Pradhana, F. R. Costa, U. Wagenknecht, D. Jehnichen, A. K. Bhowmick and G. Heinrich, Elastomer/LDH nanocomposites: Synthesis and studies on nanoparticle dispersion, mechanical properties and interfacial adhesion, Eur. Polym. J., 2008, 44, 3122–3132.
26. D. Y. Wang, A. Das, A. Leuteritz, R. N. Mahaling, D. Jehnichen, U. Wagenknecht and G. Heinrich, Structural characteristics and flammability of fire retarding EPDM/layered double hydroxide (LDH) nanocomposites, RSC Adv., 2012, 2, 3927–3933.
27. F. R. Costa, A. Leuteritz, U. Wagenknecht, M. Auf der Landwehr, D. Jehnichen, L. Häussler and G. Heinrich, Alkyl sulfonate modified LDH: Effect of alkyl chain length on intercalation behavior, particle morphology and thermal stability, Appl. Clay Sci., 2009, 44, 7–14.
28. A. Das, F. R. Costa, U. Wagenknecht and G. Heinrich, Nanocomposites based on chloroprene rubber: Effect of chemical nature and organic modification of nanoclay on the vulcanizate properties, Eur. Polym. J., 2008, 44, 3456–3465.
29. M. Geyser and W. J. McGill, A study of the rate of formation of polysulfides of tetramethylthiuram disulfide, J. Appl. Polym. Sci., 1995, 55, 215–224.
30. F. R. Costa, A. Leuteritz, U. Wagenknecht, D. Jehnichen, L. Häussler and G. Heinrich, Intercalation of Mg–Al layered double hydroxide by anionic surfactants: Preparation and characterization, Appl. Clay Sci., 2008, 38, 153–164.
31. D. Y. Wang, A. Das, F. R. Costa, A. Leuteritz, Y. Z. Wang, U. Wagenknecht and G. Heinrich, Synthesis of organo cobalt-aluminium layered double hydroxide via a novel single step self-assembling method and its use as flame retardant nanofiller in PP, Langmuir, 2010, 26, 14162–14169.
32. H. Kissinge, Reaction kinetics in differential thermal analysis, Anal. Chem., 1957, 29, 1702–1706.
33. J. Flynn and L. Wall, A quick, direct method for the determination of activation energy from thermogravimetric data, J. Polym. Sci., Part B: Polym. Phys., 1966, 4, 323–328.
34. T. Ozawa, A new method of analyzing thermogravimetric data, Bull. Chem. Soc. Jpn., 1965, 38, 1881–1886.
35. S. Chen, H. Yu, W. Ren and Y. Zhang, Thermal degradation behaviour of hydrogenated nitrile-butadiene rubber (HNBR)/clay nanocomposite and HNBR/clay/carbon nanotubes nanocomposites, Thermochim. Acta, 2009, 491, 103–108.
36. M. Maiti, S. Mitra and A. K. Bhowmick, Effect of nanoclays on high and low temperature degradation of fluoroelastomers, Polym. Degrad. Stab., 2008, 93, 188–200.
37. A. Choudhury, A. Bhowmick, C. Ong and M. Soddemann, Effect of various nanofillers on thermal stability and degradation kinetics of polymer nanocomposites, J. Nanosci. Nanotechnol., 2010, 10, 5056–5071.
38. C. Gamlin, N. Dutta, N. Roy-Choudhury, D. Kehoe and J. Matisons, Influence of ethylene–propylene ratio on the thermal degradation behaviour of EPDM elastomers, Thermochim. Acta, 2001, 367–368, 185–193.
39. C. Huggett, Estimation of rate of heat release by means of oxygen consumption measurements, Fire Mater., 1980, 4, 61–65.
40. A. B. Morgan and C. A. Wilkie, Flame retardant polymer nanocomposites, John Wiley & Sons, Hoboken (NJ), 2007, pp. 235–277.
41. M. Lewin, Synergism and Catalysis in Flame Retardancy of Polymers, Polym. Adv. Technol., 2001, 12, 215–222.
42. J. W. Gilman, C. L. Jackson, A. B. Morgan Jr and R. Harris, Flammability Properties of Polymer-Layered-Silicate Nanocomposites. Polypropylene and Polystyrene Nanocomposites, Chem. Mater., 2000, 12, 1866–1873.
43. D. Basu, A. Das, J. J. George, D. Y. Wang, K. W. Stöckelhuber, U. Wagenknecht, A. Leuteritz, B. Kutlu, U. Reuter and G. Heinrich, Unmodified LDH as reinforcing filler for XNBR and the development of flame-retardant elastomer composites, Rubber Chem. Technol., 2014, 87, 606–616.
44. F. R. Costa, U. Wagenknecht and G. Heinrich, LDPE/Mg-Al layered double hydroxide nanocomposite: thermal and flammability properties, Polym. Degrad. Stab., 2007, 92, 1813–1823.
45. B. Diar-Bakerly, G. Beyer, R. Schobert and J. Breu, Significance of aspect ratio on efficiency of layered double hydroxide flame retardants, ACS symposium series, 2012, vol. 1118, ch. 26, pp. 407–425.
46. X. Wang, Y. Spörer, A. Leuteritz, I. Kühnert, U. Wagenknecht, G. Heinrich and D. Y. Wang, Comparative study of the synergistic effect of binary and ternary LDH with intumescent flame retardant on the properties of polypropylene composites, RSC Adv., 2015, 5, 78979–78985.
47. C. A. Wilkie and A. B. Morgan, Fire retardancy of polymeric materials, CRC press, 2010, pp. 288–289.
48. S. Pradhan, F. R. Costa, U. Wagenknecht, D. Jehnichen, A. K. Bhowmick and G. Heinrich, Elastomer/LDH nanocomposites: Synthesis and studies on nanoparticle dispersion, mechanical properties and interfacial adhesion, Eur. Polym. J., 2008, 44, 3122–3132.
49. A. R. Payne, The dynamic properties of carbon black-loaded natural rubber vulcanizates. Part I, J. Appl. Polym. Sci., 1962, 6, 57–63.
50. J. Berriot, H. Montes, F. Lequeux, D. Long and P. Sotta, Gradient of glass transition temperature in filled elastomers, Europhys. Lett., 2003, 64, 50–56.
51. J. Fritzsche and M. Klüppel, Structural dynamics and interfacial properties of filler-reinforced elastomers, J. Phys.: Condens. Matter, 2011, 23, 035104.

---