Effect of silane coupling agent on the mechanical properties of nitrile butadiene rubber (NBR)/organophilic montmorillonite (OMMT) nanocomposites

2.1 Materials

Fully nitrile-butadiene rubber (NBR) (NBR-40) was provided by Bayer AG. This rubber exhibits a Mooney viscosity ML₁₊₄ 100°C=60. Na⁺-montmorillonite (MMT) was provided by Zhejiang Fenghong Clay Chemicals Co., Ltd. Different silane coupling agents, DB550, DB580, and DB619 were applied to modify the properties of MMT in order to improve its layer spacing. The chemical structure of each coupling agent is shown in Table 1. The additives zinc oxide, stearic acid, dibenzothiazole disulfide (DM), tetramethyl thiuram disulfide (TMTD), and sulfur were purchased in the vicinal market.

2.2 Preparation of OMMT and silane coupling agent modified OMMT

Organophilic Na⁺-montmorillonite (OMMT) was prepared from pure Na⁺-montmorillonite by ion-exchange reaction according to the reported method [19, 20].

The silane coupling agents, DB550, DB580, and DB619 were used; 1.0 wt% concentration coupling agent solution was prepared with ethanol. The solutions were used immediately after prepared and then OMMT was added to the solution. This mixture was stirred at 80°C for 20 min and dried in the vacuum oven for 1 h.

2.3 Investigation on the melt intercalation method

The NBR/OMMT nanocomposites were prepared by the melt intercalation methods. The OMMT and NBR were put into the rheometer (model RM-200, Harbin University of Science and Technology, Harbin, China) for blending. Operating parameters, that is, temperature and blend time, were studied to get uniformly dispersed nanocomposites; they were set at 35 rpm, 10 min, and 15 min and the temperature of 30°C, 50°C, 75°C, and 100°C.

2.4 Preparation of NBR/OMMT nanocomposites

Additives used for compounding and curing of the nanocomposites are listed in Table 2. The additives were gradually added into the kneader. Incorporating curatives ZnO, SA, DM, TMTD, and sulfur into a two roll mixing mill, model SK-160B (Shanghai Liuling Instrument Factory, Shanghai, China) with a nip clearance of 1 mm and friction ratio 1.3 (22/17 rpm) at room temperature. Mixing was performed for about 15–20 min. The cure time was sufficient to well crosslink NBR compounds as verified by means of M-2000 Moving die Rheometer (Gotech Testing Machines Inc., Taiwan, China) at 150°C.

2.5 Characterization of NBR/OMMT nanocomposites

The tensile performance was tested with a universal tensile tester (Model DCS-5000, Shimadzu Co., Japan) at 25°C. The head speed was 500 mm/min according to ASTM D412 specifications.

Dynamic mechanical analysis (DMA) spectra (TA Instruments, USA) were recorded in tensile mode at a frequency of 1 Hz. DMTA spectra, viz. storage modulus and mechanical loss factor (tanδ) were measured in the temperature range from -100°C to 100°C at a heating rate of 2°C/min.

In order to measure the change of gallery distance and the dispersion of OMMT after mixing with NBR matrix, the basal spacing of the clay was studied. X-ray diffraction (XRD) was carried out by using Philips X’Pert X-ray generator (PANalytical B.V., Holland) with CuKa radiation at 40 KV and 40 mA. The diffractograms were scanned in 2θ range from 1° to 10° at a rate of 2.4°/min.

3.1 The melt intercalation conditions

For the uniformly dispersed OMMT in the NBR matrix, the melting intercalation was studied. Figure 1 shows the XRD patterns of OMMT dispersing in the NBR matrix at different conditions.

Figure 1

X-ray diffraction patterns of nitrile butadiene rubber/organophilic montmorillonite composites at different conditions.

The OMMT characteristic peak varies with the blending condition [5, 6, 11]. The temperature and blending time are the two main factors affecting the dispersion of OMMT in the rubber matrix. The result showed that there was no change in the characteristic peak of the OMMT in the lower temperature region. The rubber could not achieve molten condition and caused the inferior wrap with introduced OMMT in the low temperature. The viscosity of melt rubber decreased with increasing the temperature, and therefore, gave rise to a reduced shear force. At a too high temperature, the shear force will be high enough to make a good dispersion of OMMT in the rubber matrix.

Through the XRD images, the characteristic peak and the position of the (001) reflexes of OMMT almost disappeared at the condition of intercalation temperature 75°C for 15 min. It can be concluded that the excellent dispersibility of OMMT in the NBR matrix at the condition of 75°C, 15 min.

3.2 XRD analysis

Figure 2 shows the XRD patterns of OMMT nanocomposites. The characteristic diffraction peak is located at 2.3° in both the DB580 and DB550 modified OMMT nanocomposites. According to the Prague formula, their layer spacing is 3.84 nm. It can be concluded that the NBR macromolecular chains intercalated into the OMMT layers and further widened layer spacing.

Figure 2

X-ray diffraction patterns of nitrile butadiene rubber/organophilic montmorillonite composites by the different silane coupling agents.

Owing to the presence of cyano-groups and unsaturated double bonds, NBR is a kind of polyolefin type rubber with relative stronger polarities. The polarity between clay and NBR becomes smaller after the clay goes through organic processing. Therefore, NBR macromolecular chain can easily intercalate into the clay layers.

3.3 Mechanical properties

It is generally accepted that good compatibility between the clay surface and polymer can provide good physical properties and increase in polymer-clay interactions [5, 6, 11]. The reinforcing effect occurs because of formation of intercalated or even partly exfoliated OMMT between rubber chains and their strong mutual interfacial interactions. The mechanical properties of nanocomposites and conventional composite are illustrated in Figures 3–5.

Figure 3

Different kinds of organophilic montmorillonite loading on the tensile strength of the composites.

Figure 4

Different kinds of organophilic montmorillonite loading on the tensile modulus of the composites.

Figure 5

Different kinds of organophilic montmorillonite loading on the elongation-at-break of the composites.

In Figure 3, DB580 and DB619 can enhance the tensile strength of composite (7 phr clay). The tensile strength of NBR/DB580-OMMT and NBR/DB619-OMMT nanocomposite was increased by 42% and 55%, respectively, compared to that of the NBR/OMMT nanocomposite. It can be concluded that the silane coupling agents of DB619 and DB580 can improve the nanocomposites tensile strength. In contrast, the silane coupling agent DB550 leads to significantly reduced tensile strength of the nanocomposites.

Figure 4 shows the influence of three silane coupling agents on the modulus of nanocomposites. The silane coupling agents have almost similar effects on the nanocomposites modulus as on the tensile strength. DB619-OMMT nanocomposites tensile modulus was increased by 44% when compared with NBR/OMMT composite modulus. DB580-OMMT nanocomposites also showed good effects in a smaller filling quantity. DB550-OMMT still showed a poor effect to the nanocomposites tensile modulus.

The elongation at break of the composite is shown in Figure 5. OMMT modified by DB619 could increase elongation at break of NBR nanocomposites. The elongation at break of DB580 and DB619 modified NBR/OMMT composites were better than the conventional system.

The DB580 and DB619 based OMMT nanocomposite have better mechanical properties than other composites. The reinforcing effect may be due to their excellent interfacial bonding strength between matrix and clay. Good mechanical properties depend on the interactions and compatibility between the clay surface and the polymer matrix [5, 6, 11]. These interactions may be of physical, physicochemical, and chemical nature, which related to number of factors, such as adhesion between the phases, functional groups of the clay surface that can react with the polymer. One end of the DB580 and DB619 molecular chain is alkoxy and the other end is -SH and alkoxy. The functional groups of -SH and alkoxy can increase interactions between the rubber matrix and the clay on the phase boundaries. Meanwhile, the other end could dissolve and spread into matrix interface, and can entangle or form chemical bond with macromolecules.

Although one end of DB550 can be grafted to the clay surface, its other end, NH₂, cannot react with the NBR matrix in melt intercalation process. The mechanical properties of DB550-OMMT nanocomposites decrease greatly, even worse than that of conventional material.

Figure 6 shows the storage modulus of NBR and nanocomposite. The glass transition temperature of all samples showed negligible changes. The nanocomposites modified by DB580 or DB619 have higher storage modulus than the others. It can be concluded that the silane coupling agents form a connection between silicate layers and rubber matrix (may be couple action or tangled action).

Figure 6

Different kinds of organophilic montmorillonite loading on the storage modulus.

Figure 7 show that the nanocomposites modified by DB550 have the highest tanδ value. The reason for this result is possibly that the functional group -NH₂ on DB550 cannot form coupling reaction or tangle with the NBR matrix. Furthermore, the coupling agent can automatically accumulate on the surface of the clay, which can decrease the interfacial bonding strength between the silicate layers and rubber matrix. As a result, the value of tanδ increased.

Figure 7

Different kinds of organophilic montmorillonite loading on the tanδ.