Investigation of morphology, mechanical, thermal and flame retardant properties of an EVA/EPDM blend by combination of organoclay with Na⁺-tripolyphosphate

Abstract

Polymer blends of ethylene-vinyl acetate (EVA) and ethylene propylene diene monomer (EPDM) (50/50) containing 2.5 wt% organoclay (OMMT) and different loadings of Na⁺-tripolyphosphate (STPP) were prepared by a melt compounding method. The morphology, mechanical and flame retardant properties, as well as antimicrobial activity of the prepared polymers, were evaluated through applying scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and tensile and antimicrobial activity tests. XRD and TEM results showed that OMMT was exfoliated in the polymer blend, while SEM illustrated a good dispersion and compatibility of OMMT and STPP fillers in the investigated polymer blend. Moreover, the tensile strength and thermal stability of composites were increased up to 2.5 wt% concentration of STPP after which no additional increase occurred. The cone calorimeter results showed that the heat release rate (HRR) and peak heat release rate (PHRR) were significantly decreased with increasing STPP loading. This is because the use of 2.5 wt% of OMMT with 15 wt% STPP caused a reduction in PHRR by 81% as compared to that of the unfilled blend. FT-IR results were drawn the fire mechanism through the carbonaceous char formation after burning. The antimicrobial activity test revealed an inhibition in the growth of microorganisms, including Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Pseudomonas aeruginosa) and fungi (Candida albicans) at 10 and 15 wt% of STPP filler content. This novel EVA/EPDM blend nanocomposite can be used in different industrial applications such as roofing, lining in solar ponds and packaging applications.

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1. Introduction

Thermoplastic elastomers (an ethylene vinyl acetate copolymer (EVA)/ethylene propylene diene monomer (EPDM) blend) are extensively used in a wide range of applications, such as construction, roofing, transport, lining in solar ponds,¹ packaging,² and electrical cable engineering, due to their excellent chemical resistance and mechanical properties.³ However, the main drawback of the EVA/EPDM blend is its flammability, which restricts its applications in the industrial sector. Flame retardant additives must therefore be added not only to reduce the flammability and the thermal stability of the EVA/EPDM blend, but also to improve its mechanical properties. Studies have been reported that halogenated flame retardants can cause some environmental issues such as toxicity, corrosion and smoke.^4,5 In addition the European Community (EC) and US Environmental Protection Agency (EPA) proposed to restrict the use of these types of materials because of potentially carcinogenic products during their combustion.^6,7 Thus, halogen-free flame retardants (FRs) have recently become widespread to minimize the health and environmental risks resulting from the use of halogenated compounds.^8,9 Among the halogen-free materials, aluminum trihydrate (ATH) and magnesium hydroxide, producing a substantial amount of water vapor (approximately 35%) in an endothermic reaction during their thermal degradation, reduce the elevated polymer temperature and dilute the combustible gas .^10–13 Moreover, the oxides formed build a shield layer with high thermal stability on the polymer surface, retarding the permeation of heat and the escape of polymer volatiles to the gas phase, thereby reducing the flame and smoke rates.^14–16 Moreover, for the metal hydroxide to be an effective FR requires at least 60% by mass to achieve flame retardant properties.¹⁷ This amount of FRs can affect the physical and mechanical properties of the polymer blend.

Hybrid nanoclay has been added to the polymer in a small amount, to enhance the flame retardant properties and to maintain the physical and mechanical properties of the polymer.^18–22 The combination of micro-/nano-structure materials, which have powerful flame retardant properties, can demonstrate the synergistic fire retardant effect and also reduce the cost of the end product. This synergistic retardant effect is associated with various possible mechanisms. The first effect is the ablation reassembly of polymer, which causes the organic fillers to be rich on its surface.²³ The second effect is attributed to the filler migration to the polymer surface during the heat flux exposure and causes their accumulation at the surface by exfoliation (for nanoclay or graphite) and agglomeration (for micro-particles).^24,25 Both mechanisms play a significant role in formation of the inorganic carbonaceous fire protection layer, which can act as a suppressor of the flammability of the polymer.

The aim of the present study is to enhance the mechanical, thermal and flame retardant properties of the EVA/EPDM blend through incorporation of organoclay (OMMT) with different amounts of STPP and to evaluate the resultant blends. Moreover, the antimicrobial activity is studied to optimize the STPP concentration required for inhibition in the growth of micro-organisms in the polymer blend.

2. Materials and methods

2.1. Materials

The co-polymer ethylene vinyl acetate (EVA) (Elvax-260 resins) was provided from DuPont Co., USA. It has a melt flow index of 6 dg min⁻¹ and vinyl acetate content of 28 wt%. The rubber used in this study is ethylene propylene diene co-polymer (EPDM) (Buna EPT 9650) from Bayer Ltd. The ethylene content is 53 ± 4, ENB content 6.5 ± 1.1 and the viscosity ML (1 + 8) at 150 °C is 60 ± 6. Sodium cloisite, a natural montmorillonite with an ion-exchange capacity of 92 meq 100 g⁻¹ was obtained from Southern Clay Products (Gonzales, Texas, USA) and modified using 1,2-dimethyl-3-hexadecylimidazolium (DMHDIM) bromide (OMMT). Sodium tripolyphosphate (STPP) with purity 85% used as a flame retardant material was supplied by Sigma Aldrich Co., Germany. Dicumyl peroxide (DCP) was used as a curing agent and purchased from Sigma Aldrich Co., Germany.

2.2. Processing of the blend specimens

The EVA/EPDM blend in ratio 50/50 and fire retardant additives were made by melt compounding using a Brabender Plasticorder (USA) at 105 °C and with a speed of 60 rpm. All specimens were prepared as follows. The blend (EVA/EPDM) was first melted at 105 °C for 3 min and then the additives (i.e., OMMT and STPP) were added and mixed for a total mixing time of 15 min. The specimens were then transferred to the two-roll mill to add 1.5 wt% of DCP as a curing agent at 90 °C for 10 min.

2.3. Preparation of blend composite specimens

The specimens for tensile properties were placed in a molding set using a 100 × 100 × 2 mm³ (FR samples were 50 × 50 × 5 mm³) thick steel spacer between two PET foils and two steel sheets. Pre-molding in a hydraulic press at 90 °C was followed by a cross-linking step at 150 °C for 45 min under a pressure of 150 kg cm⁻². The composition and name of the prepared formulations are shown in Table 1.

Table 1 Samples designations and tensile properties for EVA/EPDM blend and its nanocomposites

Constituents (wt%)	EE0	EE1	EE2	EE3	EE4	EE5
a Dicumyl peroxide.
EVA/EPDM	50/50	50/50	50/50	50/50	50/50	50/50
Organoclay (OMMT)	—	2.5	2.5	2.5	2.5	2.5
Na^+ tripolyphosphate (STPP)	—	—	2.5	5	10	15
DCP^a	1.5	1.5	1.5	1.5	1.5	1.5
Tensile strength (MPa)	5.0 ± 0.57	7.4 ± 0.39	8.3 ± 0.77	7.8 ± 0.62	8.0 ± 0.44	7.9 ± 0.46
Elongation at break (%)	670 ± 67	919 ± 23	1481 ± 89	1493 ± 62	1659 ± 68	1663 ± 59

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2.4. Antimicrobial activity test

The disc agar plate technique was used to assess the antimicrobial activity of fungal extracts.²⁶ The antimicrobial activity of a 0.5 cm diameter sample of polymer blend (EE1, EE3 and EE5) containing a stationary concentration of organoclay (OMMT) with various loadings of sodium tripolyphosphate (STPP), as presented in Table 1, was tested. The samples were examined against three different microbial strains, i.e., Gram-positive bacteria (Staphylococcus aureus, G^+ve bacteria), Gram-negative bacteria (Pseudomonas aeruginosa, G^−ve bacteria) and fungi (Candida albicans). Both bacterial and fungal test microbes were grown on nutrient agar (DSNZ 1) medium (g L⁻¹): beef extract (3), peptone (10), and agar (20), whereas the fungal test microbe was grown on Szapek-Dox (DSMZ130) medium (g L⁻¹): sucrose (30), NaNO₃ (3), MgSO₄·7H₂O (0.5), KCl (0.5), FeSO₄·7H₂O (0.001), K₂HPO₄ (1) and agar (20). The culture of each microorganism was diluted by sterile distilled water from 104 to 108 CFU mL⁻¹ to be used as an inoculum. 0.1 mL of the inoculum was used to inoculate 1 L of agar medium (just before solidification) then poured into Petri dishes (10 cm diameter containing 25 mL). Discs (5 mm diameter) were located on the surface of the agar plates previously inoculated with the test microbe and incubated for 24 h for bacteria and fungi at 37 °C.²⁷

3. Characterization and measurements

3.1. Morphological studies

3.1.1. X-ray diffraction (XRD) analysis. The XRD patterns of OMMT and EVA/EPDM/OMMT nanocomposites were recorded using a Philips X-ray diffractometer (PW 1930 generator, PW 1820 goniometer) equipped with Cu-K_α radiation (45 kV, 40 mA, with λ = 0.15418 nm). The scans of the analysis were run in 2θ range of 2.5–10° with step size of 0.05° and step time of 2 s.

3.1.2. Transmission electron microscopy (TEM). Nanocomposite structures were studied by transmission electron microscopy (TEM, Philips CM 200) with an accelerating voltage of 200 kV.

3.1.3. Scanning electron microscopy (SEM). The microstructure of STPP in the blend was investigated using scanning electron microscopy (SEM, High Resolution Quanta FEG 250-SEM, Czech Republic). Prior to observation, the samples were cryo-fractured and sputter-coated with a thin gold layer.

3.2. Mechanical testing

For measuring the properties of the EVA/EPDM blend and its filled nanocomposites, sheets of dimensions 100 × 100 × 2 mm³ were prepared using a hydraulic press under a pressure of 150 kg cm⁻². Dumbbell-shaped samples were cut from the moulded sheets using standard die type 2. Tensile strength and elongation at break were determined using a Zwick tensile testing machine (Model Z010, Germany) at a temperature of 23 ± 2 °C and a crosshead speed of 200 mm min⁻¹ according to ISO 37. At least five measurements from each sample were recorded and the average values were reported.

3.3. Thermogravimetric analysis (TGA)

TGA was carried out to study the thermal stability of the blend and its filled composites using a TGA-50 (Shimadzu, Japan) in air atmosphere with a heating rate of 10 °C min⁻¹ and 8–10 mg samples.

3.4. Flame retardant properties

A cone calorimeter (Fire Testing Technology, FTT) was used under an external heat flux of 35 kW m⁻² according to ISO 5660. The parameters measured were time to ignition (TTI, s), average heat release rate (HRR, kW m⁻²), the relative peak heat release rate (PHRR, kW m⁻²), total heat release (THR, MJ m⁻²) and average mass loss rate (AMLR, g s⁻¹ m⁻²). Finally, the fire performance index (FPI) was calculated as the ratio of TTI to PHRR, to establish the most efficient flame retardant agent among those selected for this study.²⁸ For more accurate results, the test was repeated twice for each sample.

4. Results and discussion

4.1. X-ray diffraction (XRD) analysis

Fig. 1 shows the diffraction patterns of organoclay (OMMT), unfilled and filled EVA/EPDM blends at small angles (2θ < 10°). The figure shows a single diffraction peak of the organo-modified clay (OMMT) at about 2θ = 4.7°, corresponding to d-spacing of 2.17 nm. However, the polymer blend composites show no X-ray diffraction peak; this reflects an increase of the d-spacing distance to over 5 nm,²⁹ which may be given as an indication of clay exfoliation.

Fig. 1 XRD patterns of OMMT, EVA/EPDM blend and their composites with STPP.

TEM images (Fig. 2) have been presented to support the XRD results and confirm the dispersion of OMMT in the polymer matrix. Fig. 2 shows the clay platelets are exfoliated and dispersed uniformly in the matrix, as indicated in EE2 sample. On increase of the STPP filler content, the platelets are encapsulated by STPP particles, resulting in more compatibility between the fillers and the polymer matrix, as indicated in sample EE4. The TEM results confirm that the increase in interlayer spacing distance of the OMMT promotes the penetration of polymer chains between the clay layers leading to clay exfoliation and to the formation of nanocomposites.¹⁹

Fig. 2 TEM images for (left) EE2 (2.5 wt% OMMT/2.5 wt% STPP) and (right) EE4 samples (2.5 wt% OMMT/10 wt% STPP).

4.2. Scanning electron microscopy (SEM) analysis

The structures of the prepared EVA/EPDM nanocomposites containing organoclay with different loadings of STPP were investigated using scanning electron microscopy (SEM). The scanning electron microscopy for EVA/EPDM nanocomposites was carried out using fractural and surface images for unfilled EVA/EPDM blend and filled EVA/EPDM nanocomposite with 2.5 wt% organoclay and different loadings of STPP, as shown in Fig. 3(a)–(f). Each image is shown in two magnifications. As observed in Fig. 3(a) and (b), the higher magnification image shows that a two-phase morphology with some crack areas was observed. This may be emphasized by poor interfacial adhesion between the two blends. With the addition of combined fillers (i.e., OMMT/STPP), better homogeneity of the investigated blend was obtained, as shown in Fig. 3(c) and (d). Furthermore, the blend with 2.5 wt% OMMT and 15 wt% STPP (sample EE5) revealed better miscibility of EVA and EPDM phases, as shown in Fig. 3(e) and (f). This emphasizes that these fillers act as a compatibilizer thereby leading to improved interfacial adhesion, not only between the blends themselves, but also between the matrix fillers.

Fig. 3 SEM micrographs of EVA/EPDM nanocomposites containing organoclay with different STPP loadings: (a and b) EE0, (c and d) EE3 and (e and f) EE5.

4.3. Mechanical properties

Mechanical properties, as tensile strength and elongation at break, have been evaluated and the data tabulated in Table 1. The tensile strength of the unfilled blend (EE0) is 5 MPa, but its value increased by 48% with introducing 2.5 wt% of OMMT filler (EE1). In comparison, it was found that the tensile values for the EE1 composite and other composites with STPP filler increased by 12% with the addition of 2.5 wt% STPP and by 66% compared with the unfilled blend (EE0). Further increasing the STPP content to 15 wt% does not enhance the tensile values, which means 2.5 wt% STPP in combination with 2.5 wt% OMMT is sufficient. On the other hand, the elongation at break for filled systems obviously increased with STPP filler content from 919% to 1663% as compared to the unfilled blend (670%), as shown in Table 1.

4.4. Thermogravimetric analysis (TGA)

Fig. 4 shows the TGA weight loss and derivative thermograms (DTG) obtained in an air atmosphere for STPP and the modified clay, OMMT. The results demonstrate that only 1.3% weight loss was found for STPP filler at 750 °C, indicating the high thermal stability. Morozova and Brezhneva³⁰ reported that STPP is an incombustible, nontoxic material and presented in the carbonized residue (heat treatment temperature of 900 °C) in the form of NaPO₃ (metaphosphate) and Na₄P₂O₇ (pyrophosphate). However, the OMMT decomposed in two major steps. The first degradation step includes the weight loss in temperature range of 50–297 °C of about 14.5% and may be associated to the decomposition of intercalated organomodifier in the clay galleries.

Fig. 4 TGA & DTG curves of the STPP and OMMT in air atmosphere at a heating rate of 10 °C min⁻¹.

The second step involves weight loss in the temperature range of 350–750 °C, which is 16% and can be assigned to thermal decomposition of the dehydroxylation product of aluminosilicate and some of the –OH groups from tetrahedral sheets.^31–33 Therefore, the weight losses for OMMT in the two steps are greater (29%) compared to the STPP material (1.3%). Fig. 5 shows the TGA and derivative TGA degradation curves of unfilled EVA/EPDM blend and its filled composites in air atmosphere at a heating rate of 10 °C min⁻¹.

Fig. 5 TGA and DTG curves of the EVA/EPDM blend and its nanocomposites in air atmosphere at a heating rate of 10 °C min⁻¹.

Evidently, thermal degradation of the unfilled and filled blends based on OMMT filler (EE1 sample) occurs in two major steps. The first degradation step, in the range of 290–380 °C, can be attributed to the evolution of acetic acid (deacetylation) in EVA, whereas the second degradation step, in the range of 400–500 °C, can be assigned to the polyolefinic chains (polyethylenic and polypropylinic).^34,35 With the addition of STPP filler into the blend, the thermal stability of the combined fillers (i.e., OMMT/STPP contents) improved by ∼55 °C compared to the EE0 and EE1 samples, as shown in Fig. 5. This enhancement may be due to the better dispersion of both organoclay and STPP in the polymer matrix. It can also be noticed that after addition of 2.5 wt% STPP filler content no further improvement in thermal stability is observed.

4.5. Flame retardant behavior

The cone calorimeter is widely used to evaluate the flammability performance of polymer materials. A comparison of the heat release rate (HRR) and the other data for the neat blend (EE0) and its filled composites, either filled by OMMT (EE1) or by OMMT with various amounts of STPP filler, are shown in Fig. 6 and Table 2. Fig. 6 shows that HRR is slightly decreased in the presence of OMMT (2.5 wt%) in the case of the EE1 sample and the peak heat release rate (PHRR) reduced by 17% compared with the neat blend. Both the HRR and the PHRR are sustainably reduced by 32% with the addition of 2.5 wt% STPP to the blend. As STPP content in the composite increases, the PHRR values are drastically decreases. As shown in Table 3, at STPP contents of 5 (EE3), 10 (EE4) and 15 (EE5) wt%, the PHRR values are decreased by 40%, 67% and 81%, respectively, in comparison with the PHRR of the neat blend EE0. Significant reduction in PHRR values may be due to the organomodifier (DMHDIM) in the nanoclay beginning to degrade forming acidic sites on the clay surface, which catalyze cross-linking and aromatization reactions producing a ceramic char layer that resists combustion.^36–38 This mechanism of organoclay flammability reduction and the decomposition mechanism of STPP forming phosphorus compounds (metaphosphate and pyrophosphate), as well as the residual alumina,³⁹ obtained a synergistic flame retardant effect in the EVA/EPDM blend. Cardenas et al.⁴⁰ claimed that EVA/organoclay alone is unable to pass the strict regulatory fire tests necessary for the use of these materials in cable and wire applications, whilst with the introduction of ATH to the nanocomposite, better mechanical and fire retardant properties are observed in the nanocomposites.

Fig. 6 Heat release rate (HRR) curves of EVA/EPDM blend and its composites at 35 kW m⁻² heat flux.

Table 2 Cone calorimeter data for EVA/EPDM blend and its filled composites at 35 kW m⁻² heat flux

Sample	TTI (s)	Av. HRR (kW m^−2)	PHRR (kW m^−2), (% reduction)	THR (MJ m^−2)	AMLR (g s^−1 m^−2)	FPI (s m^2 kW^−1)
EE0	84 ± 8	595 ± 7	1117 ± 30, (—)	159 ± 5	26 ± 1.5	0.08
EE1	80 ± 4	427 ± 5	923 ± 23, (17)	147 ± 4	22 ± 1.2	0.09
EE2	81 ± 6	310 ± 3	759 ± 15, (32)	122 ± 3	19 ± 1.1	0.10
EE3	83 ± 4	299 ± 5	668 ± 17, (40)	114 ± 4	18 ± 1.3	0.12
EE4	75 ± 5	129 ± 4	369 ± 8, (67)	72 ± 2	9 ± 1.2	0.24
EE5	71 ± 4	67 ± 6	212 ± 10, (81)	34 ± 3	5 ± 1.0	0.33

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Table 3 Antimicrobial activity of EVA/EPDM composites against Staphylococcus aureus (ST), Pseudomonas aeruginosa (PS) and Candida albicans (CAN)

Samples	Inhibitory zone (ϕ mm)
	Candida albicans	Pseudomonas aeruginosa	Staphylococcus aureus
EE1	0	0	0
EE3	0	0	0
EE4	21	25	25
EE5	23	26	28

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Time to ignition (TTI), total heat release (THR), average mass loss rate (AMLR) and fire performance index (FPI) calorimetric parameters are shown in Table 2 and their effects on the flame retardancy of EVA/EPDM composites were studied. It was found that cone calorimeter parameters were affected by the addition of STPP content in the presence of treated clay. The TTI decreases in a relative way with increasing STPP content, up to 15% compared with the neat blend. The THR is the total energy released by the material during combustion and it is measured as the area under the heat release rate curve. The THR was significantly reduced with increasing STPP content from 159 MJ m⁻² to 34 MJ m⁻². Its reduction is apparently due to the combination between exfoliated clay and phosphorus compounds that acted as a physical barrier hindering the heat and flammable polymer fragments' transfer from the condensed phase to the gas phase during the combustion.^41,42 As can also be observed in Fig. 7, the peak of PHRR is even replaced by a distinguishable plateau, indicating an exceptional efficiency of these materials as fire retardants. Such behavior was already observed in the case of EVA-based microcomposites or nanocomposites.^41,43,44 The AMLR values decreased with increasing loading of STPP. Furthermore, Table 2 shows the efficiency of STPP loading on flame retardancy in terms of FPI. It is observed that the FPI values increased at higher STPP loading, indicating an amazing improvement of the fire stability of EVA/EPDM blend. However, tensile properties and flame retardant behavior have not shown a clear relationship among the characteristics of STPP fillers and the properties achieved. This behavior was already reported in the literature,⁴⁵ because a high proportion of STPP filler within the polymer matrix is required to achieve a suitable level of flame retardancy. However, these conditions may lead to lack of flexibility, problems during the compounding and poor mechanical properties because of tactoids or aggregate structures in the matrix.

Fig. 7 FT-IR spectrum for char yield of EVA/EPDM nanocomposite after combustion in the cone calorimeter.

To investigate the structure of the char residues of EVA/EPDM nanocomposites obtained from cone calorimeter tests, FT-IR spectra are assessed, as shown in Fig. 7. FT-IR spectra show bands at 3410 cm⁻¹ and 1668 cm⁻¹ assigned to the –OH groups' stretching and bending vibrations for the adsorbed water.⁴⁶ The peaks at 1075 cm⁻¹ and at 948 cm⁻¹ are due to the Si–O–Si asymmetric and symmetric stretching vibrations, respectively. Otherwise, the peaks at 1149 cm⁻¹ and 633 cm⁻¹ are attributed to the STPP groups.⁴⁷ According to FT-IR data, the fire retardant mechanism can be explained through the inorganic phosphate/silica residue formed acting as a protective layer, preventing the polymer volatiles crossing to the gas phase and reducing the heat release rate during combustion.

4.6. Antimicrobial activity

Table 3 and Fig. 8 display the zone of inhibition of the EE1 nanocomposite filled by 2.5 wt% organoclay, as well as EE3, EE4 and EE5 nanocomposites containing the same amount of organoclay with 5, 10, 15 wt% of Na⁺-tripolyphosphate (STPP), respectively, against three different microbial strains: Staphylococcus aureus (G^+ve bacteria), Pseudomonas aeruginosa (G^−ve bacteria) and Candida albicans (fungi). From the results obtained, it was found that the blank EVA/EPDM nanocomposite (EE0) has no inhibition effect in all selected strains because the nanocomposites do not contain OMMT loaded by STPP. Moreover, the EVA/EPDM nanocomposites comprising 2.5 wt% of OMMT (EE1) correspondingly did not seem to have any inhibition influence against all the microbial strains used in the experiment. In addition, the EVA/EPDM nanocomposites containing 2.5 wt% of OMMT loading and at higher concentrations of STPP (10 and 15 wt%) exhibited positive effects against Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. Furthermore, the inhibitory zone increased with increasing the loading of STPP from 10 to 15 wt% in the case of EE4 and EE5 samples, respectively, as shown in Table 3.

Fig. 8 The inhibitory zone (mm) of EVA/EPDM composites (EE1, EE3, EE4 and EE5) containing organoclay with different loadings of STPP against Staphylococcus aureus (ST), Pseudomonas aeruginosa (PS) and Candida albicans (CAN).

The microorganism tests are achieved through a possible mechanism wherein there is electrostatic attraction between the negatively charged cell membrane of the microorganisms and the positively charged modified clay combined with STTP. Correspondingly, the modified organoclay does not improve the antimicrobial activity; this may be due to its limited concentration in the blend. It is important to understand how clay minerals with phosphorus kill bacteria to create new interesting antibacterial materials for different applications. The modified organoclay containing different loadings of STPP is mostly active at destroying or inhibiting the growth of a wide range of Gram-positive and Gram-negative bacteria through interrupting microbial developments or structures. In addition, the majority of known antimicrobials function by affecting cell wall synthesis, inhibiting protein and nucleic acid synthesis, disrupting membrane structure and function, and preventing most important enzymes, which are very necessary for various microbial metabolic pathways. It has also been believed that DNA loses its replication ability and cellular proteins become inactivated.

5. Conclusions

EVA/EPDM blend (50/50) containing 2.5 wt% of organoclay (OMMT) and different concentrations of Na⁺-tripolyphosphate (STPP) was prepared by a melt compounding method. The XRD and TEM results showed that OMMT was exfoliated in the polymer blend, while scanning electron microscopy (SEM) illustrated that a good dispersion and compatibility between polymer and filler were achieved in the presence of STPP filler. The tensile tests and thermal stability of nanocomposites were improved with up to 2.5 wt% of STPP content, but after this loading, there is no further enhancement in both properties. The cone calorimeter results showed that the heat release rate (HRR) and peak heat release rate (PHRR) significantly decrease with increasing STPP content, wherein the combination of 2.5 wt% of OMMT with 15 wt% of STPP obtains an 81% decrease in PHRR as compared to the unfilled blend. Based on the FT-IR results of the char yields after the cone calorimeter tests, the char layer plays a significant role in trapping polymer volatiles and thus reducing the heat release rate during combustion. Moreover, the prepared nanocomposites showed good antimicrobial activity against Gram-positive bacteria (G^+ve), Gram-negative bacteria (G^−ve) bacteria and fungi (Candida albicans) at 10 and 15 wt% of STPP filler content. Thus, the application of low-cost, eco-friendly and high thermal stability Na⁺-tripolyphosphate (STPP) is considered a novel approach in the preparation EVA/EPDM nanocomposites, which can be used in different industrial applications such as roofing, lining solar ponds and packaging applications.

Acknowledgements

The authors acknowledge the Ministry of Scientific Research, Egypt for the financial support and are grateful to National Research Center, Cairo, Egypt for carrying out some experiments in this study.

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