Various nano-particles influences on structure,viscoelastic,Vulcanization and mechanical behaviour of EPDM nano-composite rubber foam

Abstract

In this study, the effect of various nano-particle type and concentration on the structure, curing, viscosity variation during vulcanisation, and mechanical characteristics of ethylene–propylene–diene monomer (EPDM) rubber foam is reported. Three types of nanoparticle with various dimensional aspects (1D carbon nanotubes, 2D nano clay, and 3D nano silica) are employed to investigate their effect on the fabrication of EPDM rubber foam. It is observed that the properties of the foams were efficiently influenced by the nano-particle shapes and content in the matrix. Nanoparticles may increase cell density and change cell structures. In addition, they can change the curing behaviour of foam rubber by affecting curing rate and scorch time of rubber. In the end, mechanical properties of EPDM foam rubbers investigated by experimental tests and implementing few empirical and constitutional mechanical models. It is very helpful to use suitable nanoparticle to achieve desired properties out of fabricated foams.

Keywords

Foam rubber nano composites mechanical properties vulcanisation viscoelastic

Introduction

Polymeric foams are well-known materials that have been widely used for applications where high impact resistance and buoyancy are required, for example, in gaskets, body armours, and thermal insulation [1]. These lightweight foamy materials when compared with their counterparts have higher specific impact strength, toughness, and stiffness [2, 3].

Mechanical behaviour of foam depends on its cell wall material [4] and foam structure. Properties of nanocomposite may vary depending upon the type and geometrical shape of dispersing nanoparticle [5, 6]. Compression behaviour of foam materials is very dependent on cells structure and undergoes their stage depends on cell wall deformations [7]. Nanoparticles can affect cell stretchers and cell wall deformation behaviour and so on the mechanical behaviour of foam [8].

Besides, nanoparticles would affect curing and processability of rubbers [9, 10]. There are many findings that various types of nanoparticles affect processability of polymeric compounds [11-14]. Nanoparticles affect flow ability and heat conductivity of rubber compounds [15] and also acts as nucleating agents [11, 16]; so they can control foam structure by controlling the curing behaviour of base material. Nanoparticles can control cell size, density and structure of foam [17] which has remarkable effects on mechanical properties [18, 19]. Nano fillers can increase the mechanical properties of foams and make them suitable for wider applications [20-22]. Reinforcing and modifying the foam structure is essential for new applications.

This study motivated by recent researches on rubber foam modification and characterisation to tailor maid EPDM rubber foam for a new application by using a different type of nanoparticles as a new approach. The aim was investigating the effect of various nanoparticles on processability and mechanical properties of EPDM foam side by side. It is observed CNT can increase the curing rate index and scorch time will be decreased by the addition of nanoparticles. Nanoparticles can affect foam properties by changing the properties of the base polymer and curing conditions. Nanoparticles with higher aspect ratio like CNT [23] and Clay can increase modulus whereas, spherical nanoparticles like nano silica can increase elongation at break.

Experimental

Materials

KEP-270 purchased as a commercial EPDM with Mooney viscosity of 71 and 4.5 wt-% ENB, 57 wt-% ethylene. Nanocyl NC7000 multi-walled carbon nanotube (Nanocyl SA, Belgium), manufactured via chemical vapour deposition (CVD) process, with average diameter 9.5 nm, and average length 1.5 µm and the carbon purity is 90%, used as available in the laboratory. Ultrasil VN3 SiO₂ from Evonic Pte. Ltd. and Cloisite 15A nano clay from Southern Clay Inc. was used as available in the laboratory. Chemical foam agent OBSH-75 (75% oxy-bis benzene sulphonyl hydrazide, 25% EPDM binder) with a decomposition temperature of 110° C was also used as available in laboratory.

Sample preparation

Base formulation of EPDM rubber foam is presented in Table 1. Sulphur was used as a crosslinker. MBT and CBS were used as primary and secondary accelerators, respectively. Zinc oxide and stearic acid were used as activators respectively. OBSH was used as a blowing agent to develop a foamy structure in the EPDM composite and Paraffinic oil was used as a plasticiser.

Table 1

Base rubber foam composition.

Ingredients	Dry parts per hundred rubber (PHR)
KEP 270	100
Stearic acid	1.5
ZnO	5
Oil 840 Paraffinic	5
CBS	1
MBT	1
Sulphur	2
Blowing agent	3

Different type and content of nanoparticles as elucidated in Table 2 has been added to the base formulation. EPDM rubber was rolled over laboratory, two roll mills (Schwabenthan Polymix 200 L) at 60°C and roller speed of 24 rev min⁻¹ according to ASTM 3182-15 and nano-particles, stearic acid, zinc oxide, sulphur, paraffinic oil and blowing agent added to compound along with CBS/MBT. For uniform dispersion of the constituents within the rubber matrix, the rubber bank was cut and folded back into the nip repeatedly.

Table 2

Sample code name and nanoparticle content of samples.

No.	Sample	Clay	CNT	Ultrasill
1	EP0
2	CN1		1
3	CN2		2
4	CN3		3
5	CL2	2
6	CL4	4
7	CL6	6
8	US2			2
9	US4			4
10	US6			6

Moulding and curing

For moulding and curing of prepared sample equal quantity of samples was applied to the preheated mould and transferred to a hot press (Toyoseiki, Japan) at a temperature of 160°C and a pressure of 50 bar for 20 min. The chemical foaming agent decomposes below this temperature within the first 30 s of moulding. All required test specimens for the physical and mechanical test were moulded and prepared this way. For rubber processing analysis (RPA), uncured specimens of rubber compound have been prepared.

Characterisation

Curing characteristics

Curing properties of the sample determined using an RPA 2000 dynamic mechanical rheological tester from Alpha Technologies at 160°C and Strain ± 2° Arc, Frequency 1.67 Hz according to the ASTM D6601. The test was conducted after storing all samples in a lab condition overnight.

Scanning electron microscopy (SEM)

The scanning electron microscope, Tescan VEGA-II, Los Angeles, CA, USA, were used to investigate cellular structures and morphology of foamed samples. All the samples coated with gold after cutting out accurately using a sharp blade. Conducting cross section of samples analysed at different magnifications by SEM. Number of cells per cubic volume, cell density, can be calculated by as flow [24]: Where n is the number of bubbles observed in the SEM micrograph, A is the area of the picture and M is the magnification factor.

Number of total cells, close cells and size of each cell are measured precisely using ImagJ image processing software for further morphological analysis of samples. Therefore, it is possible to calculate average cell size and open to close cell ratio using SEM images as flow:

Physical and mechanical properties

All specimens stored at least for 24 h at room temperature after moulding and before the test. An INSTRON testing machine used to determined compression properties according to the ASTM D 575 in three cycles at room temperature with a speed of 10 mm min⁻¹ and a maximum compression ratio of 50% in the perpendicular direction. Specimens for compression test moulded into the cylindrical shape of 29 mm diameter and 12.5 mm height. To characterise the tensile properties of the polymer composites Dumbbell-shaped samples according to the ASTM D 412C were cut out of vulcanised sheets. Tensile test conducted with a speed of 10 mm/min up to complete rapture of specimens.

Results and discussion

Curing characterisation

Each nanoparticle has a unique effect on curing properties of EPDM rubber foam. Effect of nanoparticles also depends on their content. Addition of nanoparticles into the polymer matrix increases crosslink density and viscosity of the specimens. Moreover, the presence of nanoparticles increases vulcanisation sites and the rate of vulcanisation as seen in Figure 1. Cure rate index (CRI) can show this effect very well which can be calculated as follow:

Where t₉₀ is optimum vulcanisation time and t_s2 is scorch time of specimens; t_s2 is scorch time of rubber to start increasing elastic moduli of the compound. Also, scorch time shows a decreasing trend in Figure 2 with the increase of nanoparticle content which is due to the elevation of the viscosity of the specimen's matrix [25]. Nano silica with its spherical shape does not create many physical chain entanglements and has the least effect on the viscosity and so on the scorch time. However, CNT with its elongated shape makes many physical cross-links with polymer chain and has a strong viscosity elevation effect, which reduces the scorch time as well. Size and aspect ratio of each type of nanoparticles are presented in Table 3.

Table 3

Size and aspect ratio of nano particles.

	Dimensions (µm)	Area (m² g⁻¹)	Aspect ratio
Nonocyle NC7000	Length = 1.5 Diameter = 9.5 × 10⁻³	250–300	(L/D =) 6300
Cloisite 15A	Particle Radius = 13 Thickness = 0.7	70–81.6	(2R/T=) 36
Ultrasil VN3	Diameter = 14 × 10⁻³	177	1

Effect of various concentrations of nanoparticles on the torque variation of the composite specimens is summarised in Table 4. M_H and M_L respectively refer to maximum and minimum curing torque. M_L is related to compound viscosity and shows the initial status of rubber; M_H is a measure of the stock modulus of the fully cured compound and related to cross-linking density [26]. As it is obvious from Table 4, different nanoparticle types show an improvement in M_H and M_L. Addition of a small amount of nanoparticle lids to increase in free volumes between polymer chains and M_L decreases slightly but by an increase of nanoparticle content M_L start to increase. However, CNT nanoparticles show higher minimum torque value than the other nanoparticles. This may be due to better dispersion of CNT into the EPDM matrix; as CNT is less polar than two other nanoparticles. Moreover, nanoparticles act as a source of improving crosslink density and resulted in an increase of M_H. Again CNT particles show a better improvement of M_H, which indicates better performance of CNT particles in accessing cross-linking of macromolecules.

Table 4

Curing behavoiur of EPDM nano composite foam.

Property	EP0	CL2	CL4	CL6	CN1	CN2	CN3	US2	US4	US6
M_H (dNm)	5.70	5.02	5.21	6.00	5.83	6.64	7.54	6.23	6.58	6.93
M_L (dNm)	0.89	0.79	0.89	0.99	0.81	0.97	1.09	0.81	0.83	0.92
t₁₀ (min.)	1.97	1.95	1.93	1.92	1.97	1.89	1.81	1.95	1.94	1.93
t₉₀ (min.)	17.12	16.98	16.53	16.2	16.65	15.98	12.93	15.54	15.52	15.50

As it is obvious from Table 4 samples containing nano clay and nano silica particles show the same behaviour as above but in the smaller magnitude of viscosity elevation. Polar nature of nano-clay affects its dispersion in non-polar EPDM so the values of M_H and M_L are smaller. We observed samples containing nano silica, because of its spherical shape, shows the lowest values of M_L while M_H is high in value. Higher M_H shows better cross-link density by the presence of nano silica particles. Nano silica is a good particle to stimulate cross-linking without sacrificing the pre-cured flowability of the compound.

To describe the effect of nanoparticle type and content on the curing percentage of EPDM foam with time at 190°C, t₁₀ and t₉₀ (respectively time corresponds to 10 and 90% curing) presented in Table 4. The specimen crosslinking density during the vulcanisation process is defined by percentage curing; the lower percentage curing means less crosslink density. The times for 10 and 90% curing reduced with increasing nano clay and CNT concentration in the EPDM matrix due to the growth of curing or vulcanisation rate. Mainly, with increasing CNT concentration in the polymer matrix, heat transport from the source to the polymeric molecular chains also increased, and this promotes the vulcanisation process. Nano silica does not increase heat transfer neither affect curing rate index (as mentioned before) and it is obvious in curing times too.

Figure 3 shows the effect of CNT content on M_L and M_H variation and values of EPDM foam. Generally, during vulcanisation torque values would increase with the increase of material viscosity [27]. By the addition of fillers, the hard segments of the composite will increase, and the torque values increase to the relative fillers incorporation, so viscosity would rise due to the presence of hard segments. Also with the increase of CNT content, crosslink density and torque increased. So, M_L and M_H increased accordingly, and both parameters higher values observed. Higher final variation torque because of higher cross-link density also means less flowability of the polymer composite.

Tan δ is another important parameter (loss modulus to the storage modulus ratio) at the initial (≤10% curing) and final (≥90% curing) vulcanisation stages. Effect of the nanoparticle contents on the initial and final tan δ values of the EPDM foam illustrated in Figure 4. In the initial state, nanoparticle acts as stress concentration spots and improves loss modulus due to heat dissipation improvement. However, higher concentrations of nanoparticles can reduce chain slippage and improves polymer chain recovery so tan δ values decrease with the increase of nanoparticle contents. CNT because of its elongated geometrical shape, which allows it to interact with more polymer chains has a significant effect on increasing recoverability of uncured composite. In the final state samples containing nano silica shows a very good recoverability wherever all nanoparticles can improve recoverability of EPDM foam.

Foam structure

The effect of nanoparticle type on the structure and cell shape of rubber foam is shown in Figure 5. Both open and close cells are visible in the micrographs. Addition of nanoparticles reduces the viscous part of viscoelastic nanocomposite material and reduces the flowability of material; so, by the increase of nanoparticle content smaller cell size is observed. The morphological analysis of samples by SEM showed that Nanoparticle type will result in different close/open cell ratio and cell size distribution is varied by the nanoparticle type. The number of cells per unit volume beside open/close ratio of cells is presented in Table 5. Nanofillers also act as an active nucleating agent and provides more bubble sites and cell density increases by the increase of nanoparticle content [28]. However, each nano type has its own specific effect on the foam morphology. CNT particles are showing more open cell in the foam structure and these open cells will increase with the increase of the CNT content; which seems to be the result of the long geometric shape of carbon nanotubes that forms many nucleating sites along the sidewall of carbon tubes and growth of these cells make conflicting open cells. Nano clay by its pellet-like shape would prevent the formation of open cells structures and the number of open cells would reduce by the increase of nano-clay content. Nano silica with its spherical shape would just slightly increase the open cell numbers and its effect on open/close cell structure is negligible.

Table 5

Cell density of nano composite EPDM foam.

Sample	Average cell count/cm²	Open to close cell ratio	Average cell density
EP0	340	0.3	5.12 × 10¹⁰
CN1	350	0.7	5.30 × 10¹⁰
CN2	380	1.8	5.88 × 10¹⁰
CN3	390	2.2	6.28 × 10¹⁰
CL2	350	0.4	5.21 × 10¹⁰
CL4	360	0.3	5.36 × 10¹⁰
CL6	380	0.2	5.88 × 10¹⁰
SU2	340	0.3	5.15 × 10¹⁰
SU4	350	0.4	5.20 × 10¹⁰
SU6	370	0.5	5.69 × 10¹⁰

Mechanical properties

To evaluate the effect of various concentration of nanoparticles on mechanical properties of EPDM foam rubber composite, stress-strain curves, and mechanical properties were investigated. Figure 6(a) represents the effect of CNT in composite rubber foam; as it is obvious carbon nanotubes increases ultimate tensile strength and strain. Because CNT has a very good interaction with EPDM and either act as stress concentrator points [29]. Increase in Nano clay, blocks chain mobility so tensile strength increases at the cost of less ultimate strain (Figure 6(b)). Nano silica due to its spherical shape would facilitate polymer chain slippage so elongation at break increases by an increase of nano silica content (Figure 6(c)).

In the compression, test samples were deformed up to 50% of their original thickness. The typical compression curves of foams exhibit bending, buckling and densification regions [1]. Carbon nanotubes with their high aspect ratio and superlative mechanical properties [30] reinforce cell structure and prevent bending and buckling of foam cells; so in compression test samples containing CNT (Figure 7(a)) shows a good reinforcement in all the regions and the foam modulus increase by increase of CNT content. As it is obvious from Figure 7(b) nano clay particle do not affect the bending region and only implemented foam modulus in buckling region because of their smaller aspect ratio and different shape rather than CNT particles. Whereas spherical nano silica particles, which increases foam cell buckling and foam modulus slightly drop.

Mechanical properties prediction

Some common models are employed to predict the mechanical behaviour of EPDM foams samples. Among all, Bergstrom-Boyce + Mullins effect (BBM), Hybrid (H), Yeoh Hyperelastic (YH) and Micro Foam (MF) model are shown a good capability of predicting the mechanical behaviour of EPDM foam rubber. Figure 8 shows experimental and model predictions for CL6 sample. We tried to fit these four models to all samples and R² fitness value is reported in Table 6 to make a better vision of models fitting over the experimental data. Base on the R² variation (Table 6) Yeoh Hyperelastic model seems to do a better prediction all over the strain domain with a good fitting for all the samples. Yeoh Hyperelastic model can predict compressibility besides nonlinear, large-strain behaviour of polymeric rubber-like materials. The present of terms capable of prediction of compressibility makes Yoeh Hyperelastic model a very capable model for predicting foams mechanical behaviour. As it can be seen in Figure 8 only Yeoh hyperelastic model and Micro foam model can make a good prediction of foam mechanical behaviour in the buckling and densification regions; other models do not predict large deformations and they are only reliable in small deformations (bending region). However, BBM model has a good R² fitness factor besides low variation but as it is shown in Figure 8 it is not capable of predicting large deformations and is not suitable of studding foam materials behaviour. Therefore, a suitable model for using foam materials shall be capable of predicting large deformations and compressibility of foam materials, which makes Yeoh hyperplastic model a good candidate for these kinds of materials.

Table 6

R² fitness value for different models for each sample.

No.	Sample	Bergstrom–Boyce + Mullins effect	Hybrid model	Yeoh hyperelastic	Micro foam
1	EP0	0.998	0.770	0.998	0.998
2	CN1	0.998	0.695	0.996	0.997
3	CN2	0.996	0.542	0.998	0.998
4	CN3	0.997	0.712	0.998	0.995
5	CL2	0.997	0.679	0.996	0.997
6	CL4	0.985	0.719	0.998	0.995
7	CL6	0.945	0.599	0.997	0.993
8	US2	0.941	0.745	0.997	0.996
9	US4	0.951	0.795	0.997	0.997
10	US6	0.997	0.793	0.997	0.995
	R² Variation	5.98 × 10⁻⁴	6.72 × 10⁻³	6.22 × 10⁻⁷	2.54 × 10⁻⁶

Conclusion

EPDM foam, polymer composites with various nanoparticle types and loadings were fabricated to scrutinise the progressive filler incorporation effects on the structural, curing, rheology and mechanical properties of the samples. We found the cell size in the fabricated foamy composites was affected with nanoparticles were reduced by increasing nanoparticles content. CNT and nano clay can facilitate curing rate index and the presence of nanoparticles would reduce the scorch time of composite. Nanoparticles, especially nano silica can enhance foam recoverability to a more elastic state. Mechanical properties of fabricated foams were also affected by nanoparticles type. Foams contain CNT and nano silica showed higher elongation at break, whereas samples contain CNT has more tensile strength too. Nano silica Particles has no reinforcing effect on compression behaviour of samples whereas, samples containing CNT particles show higher modulus and nano-clay particles show a slight reinforcement.

Yeoh hyperelastic model, besides all, shows good agreements to predict EPDM foam mechanical behaviour because of its capability for showing compressibility and large deformations.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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