Effect of modification of montmorillonite on the cellular structure and mechanical properties of ethylene vinyl acetate/clay nanocomposite foams

Abstract

Nanocomposite foams, based on ethylene vinyl acetate copolymer, were prepared by melt blending of ethylene vinyl acetate with organically modified montmorillonite and at a high curing pressure of the blends. The organically modified montmorillonites were obtained by the modification of montmorillonite with octadecyl trimethyl ammonium chloride, dimethyl octadecyl hydroxy ethyl ammonium nitrate and dioctadecyl dimethyl ammonium chloride as the intercalating agent. The intercalation effect of montmorillonite layers by the tetraalkylammonium cations was characterized by X-ray diffraction. The state of organically modified montmorillonite in the blends was studied using transmission electron microscopy and the cellular microstructure of the foamed samples was observed by scanning electron microscope. The effects of tetraalkylammonium modification of montmorillonite on the cellular structure and the mechanical properties of the ethylene vinyl acetate-based foams were investigated. It was found that organically modified montmorillonites with different alkyl chains played important role in the formation of the microstructure and the mechanical properties of the ethylene vinyl acetate/organically modified montmorillonite nanocomposite foams. With the alkyl chains, it was possible to obtain ethylene vinyl acetate nanocomposite foams with significantly lowered density and improved mechanical properties.

Keywords

Ethylene vinyl acetate copolymer montmorillonite modification foam

Introduction

Ethylene vinyl acetate (EVA) copolymer is a sort of thermoplastic elastomeric plastic.^1,2 EVA (containing 12–30% VA) can be used to produce independent micro porous foam materials. Due to the existence of cells in the matrix, EVA foams exhibit light weight (in the range 150–250 kg/m³), cushioning performance, cost reduction, thermal and acoustic insulation. These characteristics present a suitable choice for many industrial applications such as construction materials, thermal and sound insulators, packaging and light weight materials for transportation industries.³ However, there are some disadvantages of EVA. These drawbacks include: poor adhesion, wetting, printing, gas permeability and material compatibility, which restrict further application.^4,5 Meanwhile, in today’s highly demanding life, traditional EVA foams can no longer satisfy ever increasing demand for quality life, which calls for reduced density without sacrificing the mechanical properties.

Nanocomposites, which had been used to improve the mechanical properties of polymers, have been of major interest to many scholars.^6,7 Polymer nanocomposites prepared from montmorillonite (MMT), with a high aspect ratio, may achieve significant improvements in mechanical and thermal properties at low filler content of MMT.⁸ Wang et al.⁹ reported that the interlayer spacing of MMT increased with the increase of alkylamine chain length. The interlayer spacing of MMT modified by alkylammonium with alkyl chain of 12, 16 and 18 carbon atoms were 1.36, 1.79 and 1.85 nm, respectively. The properties of the polymer/clay nanocomposites obtained were influenced accordingly. Gue-Hyun Kim et al.^10–12 had done a lot of work in preparing EVA/nanocomposite foams by the incorporation of MMT into polymers using melt compounding technique. According to their results, the mechanical properties, especially the compression set of the foamed materials, were improved with the addition of clay.

Even though the melt compounding has many advantages, uniform dispersion of nanocomposites in the polymer matrices is still a challenging task and few works were reported about the modification of MMT on the properties of polymer/MMT nanocomposite foams. In this contribution, EVA/clay nanocomposite foams have been prepared through the melt blending of EVA and MMT modified with octadecyl trimethyl ammonium chloride (1831), dimethyl octadecyl hydroxy ethyl ammonium nitrate (1821-OH) and dioctadecyl dimethyl ammonium chloride (D1821), followed by pressurized curing at high temperature. The effects of modification of MMT on the cellular structure and mechanical properties of EVA/MMT nanocomposite foams were also investigated in detail.

Experimental

Materials

EVA named 7350M containing 18% vinyl acetate was provided by Formosa Plastics, Taiwan. The density of the polymer is 0.938 g/cm³ and the melt flow index is 2.5 g/10 min. MMT, a Na⁺-montmorillonite with a cation exchange capacity (CEC) of 90 mequiv/100 g, was obtained from Zhangjiakou Qinghe chemical factory, Hebei, China. Azodicarbonamide (AC), used as blowing agent (degradation temperature of 202 ± 3°C), was supplied by Jinlang Fine Chemical Co., Ltd. Fujian, China. Dicumyl peroxide (DCP), used as a cross-linking agent, was provided by Akzo Nobel Co., Ltd., Netherlands. Zinc oxide (ZnO), used as an activator, was supplied by Dongtai Hongyuan chemical factory. Stearic acid (St), used as a lubricant, was supplied by Zhejiang Feixiang oil chemical Co., Ltd., Jiangsu, China. Zinc stearate (ZnSt), used as a foaming coagent, was supplied by Dongguan Guohua chemical factory, Guangdong, China. Octadecyl trimethyl ammonium chloride (1831), dimethyl octadecyl hydroxy ethyl ammonium nitrate (1821-OH) and dioctadecyl dimethyl ammonium chloride (D1821) were supplied by Xiamen Pioneer Technology Co., Ltd., Fujian, China.

MMT modification

In all, 10 g MMT powder was dispersed in 200 ml water. The tetraalkylammonium cation surfactant was added to the water to modify Na-montmorillonite. According to CEC (90 mequiv/100 g), the cation exchange capacity of 10 g MMT was 9 mmol, so the dose of 1831, 1821-OH and D1821 was 3.46 g, 3.64 g and 5.28 g, respectively. The suspensions were stirred at 80°C for 4 h to ensure that the cation was exchanged. Then, the solid phase of the ion-exchanged montmorillonite was separated by centrifugation and then washed three times using distilled water and dried in an oven at 80°C for 24 h.

Preparation of the foamed samples

In the first step, 100 phr EVA were melt-mixed with 3 phr MMTs (Na-MMT, 1831-MMT, 1821-OH-MMT, D1821-MMT) in an internal mixer (SM-0.5L-K, Suyan, Jiangsu, China) at a screw speed of 50 r/min and a temperature of 100°C for 10 min. Other additives, including 3 phr AC, 1.1 phr DCP, 2 phr ZnO, 1.1 phr ZnSt, 0.8 phr St were mixed with a screw speed of 25 r/min, a temperature of 100°C for 10 min. A low mixing temperature of 100°C was chosen in order to avoid the premature decomposition of the blowing agent and the cross-linking agent, at this stage. The compounds were taken out from the mill to form a sheet by a twin roll mill (XH-401C, Xihua, Guangdong, China) and stored at room temperature for 5 h before foaming. Finally, the foamed sample was obtained by a flat-panel curing press (XH-406, Xihua, Guangdong, China) for 550 s, at a temperature of 180°C and a pressure of 10 MPa.

Characterization

X-ray diffraction

The XRD experiments of the samples were performed in a 2θ range of 1.5˚–10˚ at a scanning rate of 1˚/min using X-ray diffractometer (D/max2200PC, Rigaku, Japan). The X-ray beam was generated from the nickel-filtered CuK_α (λ = 1.54 A˚) radiation in a sealed tube. The basal d-spacing of the silicate layer¹³ was calculated using the Bragg’s equation, nλ = 2d sinθ.

Transmission electron microscopy

Ultra-thin sections with thickness between 80 and 90 nm were taken at −90°C, using an ultra microtome (EM UC 7, LEICA, Germany). The morphologic analysis were conducted by using TEM (JEM-1230, JEOL Co., Japan), operated at an accelerating voltage of 90 kV.

Scanning electron microscopy

The structure of cells and dispersed clay particles in the cell walls were investigated, using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV (TM-1000, HITACHI, Japan). The samples were sputter-coated with gold using a vacuum sputter coater, before examination with the SEM.

Image analysis of the SEM micrograph was conducted in order to obtain the average cell size and cell density using the software Image J. The cell size, c_av, was determined by measuring the area of each cell by signing a certain color threshold. Typically, a micrograph showing more than 100 bubbles was chosen and the number of bubbles in the micrograph was determined by the software.¹⁴

The function for determining cell density,¹⁵ (N_c) in cell/cm³, is determined from equation (1).

N_{c} = 3 \times 10^{12} (ρ_{p} / ρ_{f} - 1) / 2 π d^{3}

(1)

The mean cell wall thickness,^16,17 (σ) in mm, was determined from equation (2).

σ = c_{av} [1 / (1 - ρ_{f} / ρ_{p})^{0.5} - 1]

(2)

where ρ_p and ρ_f are the density of non-foamed materials and the density of the foamed samples, respectively, in g/cm³ and c_av is the mean cell size in mm.

Physical properties of the foamed samples

The density of the samples is measured by using the electronics weight scale (HT-200, A&D, Japan).

The hardness (Shore C) of the surface of samples is measured with the Shore C hardness tester (Xihua, Guangdong, China).

The rebound resilience (elasticity) is measured by using a Rebound Resilience Tester (HT-225, Gotech, Taiwan.). The heavy hammer is released from a horizontal position and strikes the foamed sample at a vertical point and then goes back to a certain height.

A Desktop Tensile Strength Tester (AI-3000, Gotech, Taiwan) is used to obtain the tensile strength (stress), elongation-at-break (strain) and tear strength of the foamed sample at room temperature. The tensile cross-head speed is 100 mm/min. The peel strength is measured by tearing the gap of a sample at a speed of 100 mm/min. All measurements are performed for three replicates and averaged to get the results.

Compression set was defined as the permanent deformation of the foams and obtained using the compression permanent deformation instrument (Xihua, Guangdong, China). The primary thickness of the samples is 10 mm (T_o), after compression thickness of the foamed material decreased by 50% (5 mm). After 5 h and at 50°C, the sample is taken out and thickness reduction after 1 h is taken as the percentage of permanent deformation. The final sample thickness (T_f) is measured and the compression set is calculated,¹⁵ using the following equation:

(3)

Results and discussion

MMT modification analysis

It has been demonstrated^18,19 that the replacement of the inorganic exchange cations in the cavities or “galleries” of the native clay silicate structure by alkylammonium cation surfactants could compatibilize the surface clay and hydrophobic polymer matrix.

Figure 1(a) showed the X-ray diffraction (XRD) pattern of Na-MMT and OMMT (modified by 1831, 1821-OH, D1821), respectively. The main characteristic diffraction peaks of OMMTs at 2θ of 6.02°,4.14° and 2.46° had a sharp signal corresponding crystalline plane (d₀₀₁) and indicated a layer spacing of 1.46 nm, 2.13 nm and 3.59 nm, respectively, which were higher than that of Na-MMT (1.27 nm) because of the insertion of long carbon chain. Three types of tetraalkylammonium cation surfactants, having a chain length of 18 carbons, common to all, showed different intercalating results. In contrast to 1831-MMT (d = 2.13 nm), the diffraction patterns of 1821-OH-MMT (d = 1.46 nm) showed less enlargement of the interlayer space, probably due to the extra hydroxyethyl (hydrophilic) of 1821-OH undermining the effect of hydrophobization by alkylammonium cation surfactant. With the long carbon chain of 1821-OH intercalating with the MMT, the hydroxyl groups were introduced in the interlayer and the hydrogen bond between 1821-OH and the hydroxyl groups on the surface of MMT retarded the movement of 1821-OH. In the same manner, D1821, with another 18 carbons chain, had a higher hydrophobicity, hence it was easier to enter into the interlamellar spacing. Due to the ease of intercalation and the larger spatial arrangement, the interlayer d-spacing was the largest (d = 3.59 nm) of the three.

Figure 1(b) showed the XRD patterns obtained from the nanocomposites containing 3 phr of OMMT (modified by 1831, 1821-OH, D1821, respectively) in the EVA matrix and the pure EVA, before foaming. As can be seen from Figure 1(b), the diffraction peak (d001) of EVA/MMT and EVA/1821-OH-MMT composites was almost the same as that of MMT and 1821-OH-MMT, indicating that the EVA chains did not intercalate into the gallery of MMT during the melt blending. The interlayer d-spacing of D1821-MMT is 4.03 nm, indicating a 0.44 nm larger than that of D1821, which implied that the more enlarged layered silicates spacing, the more the possibility of EVA chains intercalating the layer. This indicated that intercalation structure was contained in the EVA matrix. Interestingly, it hardly displayed any peaks in the XRD pattern of 1831-MMT. To confirm this, the experiment was repeated three times and the same result was obtained, which needed to be ascertained by TEM.

Morphologies of the composites

Figure 2 showed TEM images of the EVA/clay composites, in which the dark substances were MMT layers. From Figure 2(a), a monoblock of clay in the EVA/Na-MMT composites was seen with MMT layers closed together. In Figure 2(b), some thin sheets with a high aspect ratio, indicated by circle, could be found in EVA/1831-MMT, which might be the exfoliated MMT layer. There were still some blocks of clay in the EVA matrix, showing the non-uniform dispersion of 1831-MMT. It might not be due to the raw clay because in the XRD pattern, a prominent peak could not be found, as there could have been the reunion of the exfoliated clay. In Figure 2(c), the MMT modified by 1821-OH appeared in the EVA matrix, which showed poor modification that is confirmed by the XRD. A lot of thin sheets in Figure 2(d) showed that the clay layers were dispersed at the monolayer level because of the hydrophobic surface of MMT in EVA/D1821-MMT composites. The D1821-MMT sheet had a length of 100 nanometer, but their thickness were several nanometers, indicating that exfoliated structures were achieved. Combined with Figure 1(b), the existence of the peak in the XRD indicated that intercalated structures also existed. Therefore, it can be concluded that an admixture of exfoliated and intercalated structures of MMT existed in EVA/D1821-MMT nanocomposites.

Morphologies of the foamed samples

SEM images of cross-sections of the EVA/clay foams and pure EVA foams without clay are shown in Figure 3(a)–(e). The corresponding photographs of the foamed samples by digital camera are shown in Figure 3(a′–e′). All foams exhibited the closed-cell structure. Apparently, pure EVA foams showed non-uniform cell structure with a large cell size, as shown in Figure 3(a). Because of the relatively no change in the interlayer d-spacing, Na-MMT acted just as a filler in the EVA matrix and this led to the non-uniformity of the foams, when compared to pure EVA, as shown in Figure 3(b). The SEM images of the fracture surfaces of EVA/1831 foams, in Figure 3(c), conformed partly to a normal distribution and a portion of small cells was much larger than most of the foams because of the blocks of clay in the EVA matrix. Despite the large cell, the average cell size of certain foams could still be found in the left column in Figure 3(c), due to the nucleation effect of the MMT sheets. EVA/1821-OH foams, showed poor results in Figure 3(d) and even worse than the EVA/Na-MMT foams. Large holes, indicated by circle, were observed with the ordinary eyes in Figure 3(b′–d′). Figure 3(e) showed homogeneous and smaller cells formed in the matrix by the incorporation of MMT modified with D1821. Mixing in the molten state, with high hydrophobicity tendency, as mentioned above, was found to promote efficient dispersion of clay, which helped to create more foaming nucleation centers in the polymer.²⁰ The organic cation surfactant can lower the surface energy of the silicate surface and improve wetting with the polymer matrix.^21,22 In addition, the strong heterogeneous nucleation effect of the MMT sheets in this region might lead to the formation of small cells. It could be employed as a heterogeneous nucleation agent, in order to decrease the cell size and increase cell density because of a lower activation energy barrier when compared with homogeneous nucleation.²³ The EVA/D1821-MMT foamed samples showed positive significance to the foamed product.

To visualize the dispersion of cells in EVA/clay nanocomposite foams, the software Image J was used to determine the average cell size, c_av. ρ_f and ρ_g were measured using the electronic weighing scale. Morphological parameters, N_c and σ, of the cells were obtained quantitatively, using equations (1) and (2) and are summarized in Table 1.

Table 1.

Morphological parameters of the foams

	Sample
	EVA	EVA/Na-MMT	EVA/1831-MMT	EVA/1831-OH-MMT	EVA/D1821-MMT
ρ_p/g cm⁻³	0.955	0.967	0.964	0.973	0.969
ρ_f/g cm⁻³	0.146	0.147	0.141	0.142	0.132
d/µm	114	96	88	104	74
N_c × 10⁻⁶/cell cm⁻³	1.79	3.01	4.09	2.49	7.48
σ/µm	9.86	8.25	7.24	8.54	5.62

EVA: ethylene vinyl acetate; MMT: montmorillonite.

The final density was controlled by the competitive process in the cell nucleation, growth and coalescence. It could be seen that it was related to the average cell size, c_av. The function that determined the cell density N_c, defined by ρ_f, ρ_g and d, ranged between 1.79 × 10⁶ cell.cm⁻³ and 7.48 × 10⁶ cell.cm⁻³, which showed prominent effect by the addition of clay. The MMT modified by D1821 showed a significant effect on the foamed samples, which was confirmed by the SEM micrographs. Foams were formed in the boundary between the matrix and the dispersed clay particles during the foaming process.²⁴ The σ of EVA/D1821-MMT foams reduced to half the value of pure EVA foam.

The cell size distribution was obtained, as shown in Figure 4. The effect of clay on the mean cell size is exemplified by the numerical and statistical Gaussian analyses when compared with pure EVA, the EVA/Na-MMT foams had more fractions of large cells and uneven cell size distribution. With the addition of D1821-MMT, the nanocomposites foams showed smaller c_av, when compared with pure EVA foam, suggesting that the existence of clay decreased cell size. EVA/ D1821-MMT foams were relatively uniform in all samples.

Physical properties of the foamed samples

Figure 5 shows the results of hardness test (shore C) for the EVA/clay foams. It is an interesting phenomenon that EVA/D1821 foams had the lowest density and the highest hardness. We thought that the hardness was not only related to the material itself but also to the microstructure in the foamed materials. With the well dispersion of D1821-MMT in the EVA matrix, homogeneous and smaller cells were formed and the uniformity of the foamed samples was improved, as shown in Figures 3(e) and 4 with the even structure directly relating to its hardness.

The hardness of other EVA/clay foams, such as: EVA/Na-MMT, EVA/1831-MMT and EVA/1821-OH/MMT were lower than that of pure EVA foams. This might be the result of the uneven foam structure in the microstructure, which leads to defects in the macrostructure (Figure 3(b)–(d)), resulting in the lower hardness.

Figure 6 showed a little decline of the rebound resilience with the addition of clay, which was possibly caused by the thinner cell wall thickness, σ and low density of samples. From Figure 6, there is the compression set increase by the addition of Na-MMT and the D1821-MMT, whereas, the latter had a lower density.

From Figure 7, it was seen that the mechanical properties, tensile strength, elongation-at-break and peel strength, were improved with the addition of clay, except for 1821-OH-MMT. The poor property of 1821-OH-MMT might be due to the poor distribution of clay, as discussed above, and the particle agglomeration tends to reduce the strength and produce weaker materials discussed above.²¹ Although EVA/Na-MMT and 1831-MMT have large holes in the section of the foams, they nevertheless exhibit better mechanical properties than pure EVA foams. This is mainly because of the contribution of the majority of small cells. D1821-MMT with lower density showed an increase of 11% in the tensile strength, 26% in the elongation-at-break, when compared with pure EVA foams. This reflected the good interaction between clay and the EVA polymer matrix.²⁵ One reason was the homogeneous interactions of EVA/clay resulting in small cells and uniform cell structure formed from the heterogeneous nucleation effect, thereby reducing the possibility of stress concentration. Another possible explanation was the coupling between the large surface area of the clay and the polymer matrix facilitated stress transfer to the reinforcement phase, allowing for mechanical property improvement.^26,27 According to the comprehensive results, MMT modified by D1821 was more effective in the improvement of properties of the EVA/OMMT nanocomposite foams.

Conclusions

The effects of modification of MMT on the cellular structure and mechanical properties of EVA/clay nanocomposite foams were investigated in detail. The interlayer space of MMT was enlarged by the hydrophobic 18 carbon chains of the tetraalkylammonium cation surfactants. However, different groups in the tetraalkylammonium showed different effects. When compared with 1831 and 1821-OH, D1821 resulted in hydrophobic clay layers surface and tended to get an admixture of exfoliation and intercalation structures in EVA/D1821-MMT composites because of the strong hydrophobic chain. The uniform dispersion of MMT could be employed as a heterogeneous nucleation agent to decrease the cell size and enhance the uniformity of the foamed structure. According to the results obtained, it is obvious that MMT modified by D1821 was more effective in improving physical properties of EVA/OMMT nanocomposites.

Footnotes

Funding

This project was supported by Key Scientific and Technology Innovation Special Foundation of Shaanxi Province (2009ZKC03-14), Civic Scientific and Technological Problem Project of Wenzhou City, Zhejiang Province (H20100082).

References

Kim

Lee

. Effects of atmospheric plasma treatment on the interfacial characteristics of ethylene–vinyl acetate/polyurethane composites. J Colloid Interf Sci 2004; 271(1): 187–191.

Gao

Beyer

Yuan

. A mechanistic study of fire retardancy of carbon nanotube/ethylene vinyl acetate copolymers and their clay composites. Polym Degrad Stabil 2005; 89(3): 559–564.

Sims

GLA

. Low density cellular plastics – physical basis of behaviour. Polym Int 1995; 36(3): 300–300.

Sung

Kum

Lee

. Effects of crystallinity and crosslinking on the thermal and rheological properties of ethylene vinyl acetate copolymer. Polymer 2005; 46(25): 11844–11848.

Chien

Kan

Chen

. Dynamic simulation and operation of a high pressure ethylene-vinyl acetate (EVA) copolymerization autoclave reactor. Comput Chem Eng 2007; 31(3): 233–245.

Ardhyananta

Ismail

Takeichi

. Effects of organoclay loading and ethylene glycol on mechanical, morphology and thermal properties of ethylene vinyl acetate/organoclay nanocomposites. J Reinf Plast Comp 2007; 26: 789–806.

Chuayjuljit

Thongraar

Saravari

. Preparation and properties of PVC/EVA/ organomodified montmorillonite nanocomposites. J Reinf Plast Comp 2008; 27: 431–442.

Abdolmohammadi

Yunus

WMZW

. Effect of organoclay on mechanical and thermal properties of polycaprolactone/chitosan/montmorillonite nanocomposites. J Reinf Plast Comp 2011; 30(12): 1045–1054.

Wang

Choi

Koo

. Synthesis and characterization of maleated polyethylene/clay nanocomposites. Polymer 2001; 42(24): 9819–9826.

10.

Park

Kim

Chowdhury

. Improvement of compression set property of ethylene vinyl acetate copolymer/ethylene-1-butene copolymer/organoclay nanocomposite foams. Polym Eng Sci 2008; 48(6): 1183–1190.

11.

Kim

Park

Chowdhury

. Effect of compatibilizer and silane coupling agent on physical properties of ethylene vinyl acetate copolymer/ethylene-1-butene copolymer/clay nanocomposite foams. J Appl Polym Sci 2006; 102(4): 3259–3265.

12.

Park

Ray Chowdhury

Park

. Effect of dispersion state of organoclay on cellular foam structure and mechanical properties of ethylene vinyl acetate copolymer/ethylene-1-butenecopolymer/organoclay nanocomposite foams. J Appl Polym Sci 2007; 104(6): 3879–3885.

13.

Porter

Metcalfe

Thomas

MJK

. Nanocomposite fire retardants – a review. Fire Mater 2000; 24(1): 45–52.

14.

Yang

Chen

. Preparation, characterization, and supercritical carbon dioxide foaming of polystyrene/graphene oxide composites. J Supercritical Fluid 2011; 56(2): 201–207.

15.

Okamoto

Nam

Maiti

. Biaxial flow-induced alignment of silicate layers in polypropylene/clay nanocomposite foam. Nano Lett 2001; 1(9): 503–505.

16.

Klepner

Frisch

. Handbook of polymeric foams and foam technology. Munich: Hanser, 1993.

17.

Ema

Ikeya

Okamoto

. Foam processing and cellular structure of polylactide-based nanocomposites. Polymer 2006; 47(15): 5350–5359.

18.

LeBaron

Wang

Pinnavaia

. Polymer-layered silicate nanocomposites: an overview. Appl. Clay Sci 1999; 15(1–2): 11–29.

19.

Pavlidou

Papaspyrides

. A review on polymer-layered silicate nanocomposites. Prog Polym Sci 2008; 33(12): 1119–1198.

20.

Kord

Varshoei

Chamany

. Influence of chemical foaming agent on the physical, mechanical, and morphological properties of HDPE/wood flour/nanoclay composites. J Reinf Plast Comp 2011; 30(13): 1115–1124.

21.

Giannelis

. Polymer layered silicate nanocomposites. Adv Mater 1996; 8(1): 29–35.

22.

Kornmann

Lindberg

Berglund

. Synthesis of epoxy–clay nanocomposites. Influence of the nature of the curing agent on structure. Polymer 2001; 42(10): 4493–4499.

23.

Colton

Suh

. The nucleation of microcellular thermoplastic foam with additives: Part I: theoretical considerations. Polym Eng Sci 1987; 27(7): 485–492.

24.

Ema

Ikeya

Okamoto

. Foam processing and cellular structure of polylactide-based nanocomposites. Polymer 2006; 47(15): 5350–5359.

25.

Chacko

Sadiku

Vorster

. The rheological and mechanical properties of ethylene-vinyl acetate (EVA) copolymer and organoclay nanocomposites. J Reinf Plast Comp 2010; 29(4): 558–570.

26.

Wang

. Mechanical, thermal and morphological properties of glass fiber and carbon fiber reinforced polyamide-6 and polyamide-6/clay nanocomposites. Mater Lett 2001; 49(6): 327–333.

27.

Chacko

Sadiku

Vorster

. The rheological and mechanical properties of ethylene-vinyl acetate (EVA) copolymer and organoclay nanocomposites. J Reinf Plast Comp 2010; 29: 558–570.