Effect of two types of layered silicate on mechanical and barrier properties of hydrogenated acrylonitrile butadiene rubber (HNBR)/layered silicate nanocomposites

Materials

Fully hydrogenated acrylonitrile butadiene rubber (HNBR) (Therban A-4307) having 43% acrylonitrile content and less than 0·9% residual double bonds, which were provided by Bayer AG. This rubber exhibits a Mooney viscosity ML₁₊₄ 100°C = 63. The type of organoclays, organo-montmorillonite (OMMT) (Nanomer I.44 P), MMT modified with di-methyl, di-hydrogented tallow ammonium was used, provided by Beijing EAST-WEST Chemical Technology LED. Rectorite (JZ-b), modified with octadecylammonium salt, which was pvovided by Wuhan Mingliu Company.

For compounding and curing, the additives referred in the Table 1, all buy in the vicinal Market. The additives, which offer heat stability and protection to aging, like diphenylamine (445) and zinc methylmercaptobenzimidazole (ZMMBI) respectively, were used. For a better processing of the HNBR, TP795 was introduced as plasticiser.

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Table 1

Formation of HNBR mixtures

HNBR 4307	Parts (phr)
Rectorite	10	…
OMMT	…	10
magnesia	2	2
zinc oxide	2	2
TP795	8	8
ZMMBI(ZMB-2)	1·5	1·5
445	0·5	0·5
TAIC	1·5	1·5
DCP	4	4

Preparation of HNBR/organoclay nanocomposites

Note that after compounding, the additives were gradually added in the kneader. Incorporation of the curatives ZnO, MgO and TP795, ZMMBI, 445, TAIC and DCP took place at room temperature on a two roll mixing mill, model SK-160B (Shanghai Liuling instrument factory) with a nip clearance of 1 mm and friction ratio 1·3 (22/17 rev min⁻¹). Mixing was performed for about 15–20 min. The specimens were cured at 160°C in an electrically heated hydraulic with pressing for 20 min. This cure time was sufficient to well crosslink HNBR compounds as verified by means of the M-2000 Moving die Rheometer.

Characterisation of nanocomposites

X-ray diffraction (XRD) (Cu K_a radiation at 40 KV and 40 mA, Phillips X'Pert X-ray generator) was carried out to study the change of gallery distance and the dispersion of OMMT. The diffractograms were scanned in 2θ range from 1 to 10° at a rate of 2·4° min⁻¹.

The state of dispersion of the clay in the HNBR-matrix was investigated by transmission electron microscopy (TEM) images (H-800, HITACHI) with an accelerator voltage of 200 kV. Thin sections of the specimens were cryogenically cut at a temperature of −120°C and used without staining.

The mechanics test was carried out with the universal tensile tester (Model DCS-5000, Shimadzu Co.) at 25°C, the head speed was 500 mm min⁻¹, according to ASTM D412 specifications. The density values of nanocomposites were measured according to ASTM D 1817-2005 specifications. Akron abrasion was measured by using Inserts Flexing Resistance Tester (GOTECH Testing Machine, China).

Vulcanisation on dispersibility of clay in HNBR matrix

The vulcanisation changed the dispersion of clay in polymer matrix greatly. The sulphur vulcanisation strongly affects the nanocomposites formation in rubber/clay systems.^16–20 Figure 1 shows the XRD patterns of HNBR/OMMT nacocomposites. It is clear that the dispersibility of OMMT is shifted to lower angles after melt intercalation. This was caused by the shear induced diffusion of polymer chains into the agglomerates and the diffusion of polymer chains in the silicate galleries.

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1

X-ray diffraction patterns of HNBR/OMMT nanocomposites before and after vulcanisation

The Bragg equation was applied to calculate the interlayer spacing of the clay. In the Fig. 1, before vulcanisation process, OMMT, which was intercalated showing an interlayer spacing of 4·26 nm according to the weak peak around 2·07°, while after vulcanisation, it exhibited an interlayer spacing of 4·48 nm according to the weak peak around 1·99° (larger than the initial value, 2·63 nm of origin organoclay for the peak of 3·35°).

From the Fig. 2, the diffraction peaks of origin rectorite (001) and (002) present to 3·67 and 7·33°, the interlayer spacing is figured out 2·40 nm, 1·20 nm. The rectorite diffraction peaks almost disappeared after the peroxide vulcanisation, which indicated that rectorite layers are more randomly distributed throughout the HNBR matrix. Note that this claim is based on the fact that further basal reflections do not fit either in respect to their position or their relative intensities.

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2

X-ray diffraction patterns of HNBR/Rectorite nanocomposites before and after vulcanisation

Peroxide vulcanisation has the certain effect on the dispersibility of the clay in the HNBR matrix. This phenomenon has been reported recently and attributed to the formation of zinc complexes in which both sulphur and primary aminutee intercalants of the organo-clay participate. As the temperature rise, the di-methyl, di-hydrogented tallow ammonium molecules are leaving the silicate surface and participating in the formation of vulcanisation intermediates. ²¹

TEM images were screened to observe the dispersion state of layered silicate in the matrix. Further information about the dispersion of the silicate platelets in the HNBR matrix can be obtained by means of TEM images. TEM images present the dispersion state of OMMT or Rectorite in the HNBR matrix (Fig. 3).

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3

TEM images of HNBR/clay nanocomposites: a OMMT and b Rectorite

The OMMT dispersion in HNBR matrix involved exfoliated and intercalated in the images (Fig. 3). The photograph clearly shows the lamellar structure of rectorite intercalated by the HNBR macromolecular chain (Fig. 3), intercalated structures with increased interlayer distance are observed. Furthermore, the interface of the OMMT or rectorite and the HNBR matrix was indistinct, which suggests that the OMMT or rectorite and HNBR matrix had good compatibility.

Effect of two types of layered silicate on mechanical properties

For similar layered silicate dispersion in the rubber matrix, the silicate modification is of great importance as it affects the interphase quality and thus also the final properties of the vulcanisates.^{21, 22} In the Table 2, the mechanical properties of HNBR/layered silicate nanocomposites have a great difference compared with the unfilled composites.

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Table 2

Mechanical properties of HNBR/layered silicate nanocomposites

Properties	HNBR	HNBR/OMMT	HNBR/Rectorite
Tensile strength/MPa	12·08	17·19	15·26
Elongation-at-break/%	443·56	485·35	429·60
Tear strength/kN m−1	17·78	48·06	38·55
Tensile set after break/%	5·3	18	12

The tensile strength and tear strength of nanocomposites are higher than that of the composites without layered silicate. In the nanocomposites, the layered silicate dispersed in polymer matrix in the form of nano-scale, and had strong interface interaction with the matrix, those could eliminate the impact of the mismatch on thermal expansion coefficient between inorganic and polymer matrix.

The properties of HNBR/OMMT nanocomposites were higher than that of HNBR/rectorite nanocomposites in the same content. The properties of the rubber/layered silicate nanocomposites are mostly controlled by the silicate dispersion in the rubber compounds.^{9, 10, 18, 21} Rectorite is composing of two layers of 2∶1 mica and montmorillonite in the ratio 1∶1. Therefore, OMMT have greater aspect ratio than that of rectorite.

Effect of two types of layered silicate on density and Akron abrasion

The effect of two type of the clay on the density of the HNBR/clay nanocomposites was also investigated, as shown in the Table 3. The density of composite was almost unchanged with additional OMMT. The density of HNBR/OMMT (10 phr) nacocomposites only changed about 0·03 g cm⁻³. On the other hand, the HNBR/OMMT nanocomposites mechanical properties greatly improved after additional OMMT, so that the specific tensile strength and specific modulus will greatly increase, which is the strongpoint of the nanocomposites.

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Table 3

Density and Akron abrasion of HNBR/layered silicate nanocomposites

Type	HNBR	HNBR/OMMT	HNBR/Rectorite
Density	1·02	1·05	1·177
Akron abrasion	0·0705	0·07815	0·08527

The density of the rectorite is bigger than that of the OMMT for the rectorite consisting of a regular stacking of mica-like layers and montmorillonite-like layers. Thus, additional of rectorite to the HNBR matrix will lead to a big increase in the density.

The wear resistant have a direct relationship with the service life for the rubber products. The effect of the layered silicate on the wear resistant was also investigated, as shown in the Table 3. Akron abrasion is a complex dynamic fatigue process, which inevitably accompanies by a dynamic heat. The heat accumulation of the rubber will accelerate the molecular chain rupture, resulting in the damage of the wear performance.

Additional a certain amount of inorganic fillers to a polymer matrix have been demonstrated to be increase the wear resistance. The wear resistance of HNBR composites is about 0·0705 cm³×1·61 km, the OMMT filled HNBR nanocomposites is 0·07815 cm³×1·61 km. The rectorite filled HNBR nanocomposites is 0·08527 cm³×1·61 km. Low loading of inorganic fillers was revealed with finitely effect to the wear resistance properties of the conventional composites. Also the clay/rubber nanocomposites as a new class of materials in which organic particles dispersed in the polymer matrix with nano-scale dimensions that already exhibited new and improved properties. Thus, the influence to the wear resistance of the clay/rubber nanocomposites is limited by adding clay.

Effect of two types of layered silicate on crude oil medium aging resistance

The mixture of liquid crude oil wells was used as the medium. The sample immersed in 90°C and 168 h in order to study the aging resistance, the result shown in the Table 4. The unfilled composite's properties declined greatly after the crude oil medium ageing-resistance. The HNBR/layered silicate nanocomposites has small decline in its performance. Dispersed platelets of the silicate sheet block the shortest path of gas molecules and force them to take a roundabout way. Meanwhile, the reciprocity between the layered silicate and the matrix can hinder the infiltration of small molecule and liquid. The OMMT filled HNBR matrix nacocomposites has better ageing resistance than that of the rectorite filled HNBR nacocomposites.

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Table 4

Different layered silicate loading on mechanical properties of HNBR nanocomposites after crude oil aging resistance

Properties	HNBR	HNBR/OMMT	HNBR/Rectorite
Tensile strength/MPa	6·26	15·3	10·25
Elongation-at-break/%	263·35	425·5	352·60
Tear strength/kN m−1	10·38	46·06	20·55
Tensile set after break/%	2	12	8

Effect of two types of layered silicate on barrier properties

Excellent gas barrier property is one of the prominent properties of the polymer/clay nanocomposites. The oxygen permeability data of the nanocomposites was listed in the Table 5. The oxygen permeability of HNBR/layered silicate nanocomposites decreased, indicating that the organoclay enhances the oxygen barrier of the HNBR. The OMMT created a more extend tortuous path against gas penetration compared with rectorte, thus, lower permeation values than the rectorite modified HNBR. Layered silicate can improve gas barrier property is mainly due to the gas molecules can be extended in the matrix in the diffusion path.^21–23

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Table 5

Oxygen permeability coefficient ratio of HNBR/layered silicate nanocomposites

Material	Thickness/cm	permeability/cm3 (m2×24 h×0·1 MPa)−1	Permeability coefficient/cm3 cm (cm2 s Pa)−1
HNBR	0·036	85·5	3·40×10−9
HNBR/OMMT	0·028	73·9	2·39×10−9
HNBR/Rectorite	0·030	76·1	2·55×10−9

Filler shape has a decisive influence on the barrier properties. Flake particles have obvious better barrier properties than that of the spherical particles. The layered silicate dispersion is also a major factor in the layered silicate nanocomposites, which influences the barrier properties. High aspect ratio is an effective way increased gas barrier properties.^{23, 24} In the exfoliated structure, the clay dispersed in the matrix in the form of single layer, which is conducive to the improvement of gas barrier.