The application of nano silica as a potential,cost-efficient green filler in acrylonitrile butadiene rubber vulcanizates containing carboxy-terminated liquid natural rubber

Abstract

The challenge of sustainable development demands renewable natural sources. Natural rubber is one of the major plantation crops in the equatorial region, where there is a tremendous scope for tapping solar energy. The carboxy-terminated liquid natural rubber (CTNR) was commercially prepared by the sunlight irradiation of a solution of maleic anhydride and masticated natural rubber. Carboxy-terminated liquid natural rubber (CTNR) can be used as a polymeric plasticizer in acrylonitrile butadiene rubber (NBR) vulcanizates. As a reinforcing filler in rubber products, bio compatible green silica is rapidly replacing carcinogenic carbon black. This paper describes the cost-effective application of eco-friendly nano silica, which can improve the tensile properties, rebound resilience, compression set, ageing and oil resistance and reduce the heat build-up of the NBR vulcanizates. The properties of nano silica–filled NBR vulcanizates containing CTNR (NS NBR -CTNR) were compared with those containing amorphous silica. The enhancement in properties, exhibited by NS NBR-CTNR vulcanizates, may be attributed to the increased bound rubber content resulting from the large surface area of the nano-sized filler. The superior retention of properties after ageing and oil extraction shown by NS NBR-CTNR vulcanizates may be due to the better rubber filler interaction, ionic crosslinking and co–cross-linking of CTNR to NBR. The use of a combination of a conventional plasticizer and CTNR in nano silica–filled NBR is found to be promising.

Keywords

Carboxy-terminated liquid natural rubber (CTNR)polymeric plasticizer industrial nano silica (NS)nitrile butadiene rubber (NBR)ageing and oil resistance study

Introduction

Many academic and industrial researchers are keen on developing new polymer nanomaterials for getting desired physical, mechanical and electrical properties. The effect of reinforcing fillers like carbon black, calcium carbonate, calcium silicate, China clay, mica, talc, river mine and beach sand is already reported in the scientific world.^1–5 The nanofillers are expected to show greater physical and mechanical properties than the other conventional fillers used in the polymer industry because of its large surface area to volume ratio.

The nano silica–filled polymer compounds are already reported in the many articles and which show a wide range of applications in automotive, electronics and aerospace industries etc. due to their greater mechanical properties.^6–8

Food-grade rubber requires eco-friendly and biocompatible silica as filler rather than hazardous carbon black.^9–13 Nitrile rubber is used as a food rubber in a number of applications such as food-grade seals, water cut seals, oil seals and gaskets, where long-term contact with food is required.¹⁴

Silica has hydrophilic silanol groups, which form strong filler–filler interaction by hydrogen bonds and show a poor dispersion of silica in rubber compounds.^15,9 The filler dispersions are improved by the introduction of some expensive silane coupling agents in rubber compounds.^12,16–18 The successful incorporation of commercial nano silica in chloroprene rubber vulcanizates in the presence of CTNR was reported.⁸ This paper narrates the use of commercial nano silica in NBR vulcanizates containing CTNR which can act as a coupling agent as well as a polymeric plasticizer. The silanol group of silica may enter into the strong crosslinks with the nitrile group in NBR.^9,19

The CTNR can be economically prepared by the sunlight irradiation of a mixture of maleic anhydride and masticated natural rubber dissolved in toluene. The effective use of CTNR as a polymeric plasticizer in NBR vulcanizates was reported.²⁰ To make the process more cost effective and green, the industrial nano silica was used in this study.

Materials

Acrylonitrile butadiene rubber (NBR-JSR 230) was supplied by Popular Rubber Products, Angamaly. Natural rubber (NR, ISNR-5) was supplied by RRII Kottayam. Nano silica (17 nm, commercial grade filler) was supplied by Astra Chemicals, Chennai. Compounding ingredients such as zinc oxide, stearic acid, dioctyl phthalate, mercaptobenzothiazole (MBT), tetramethylthiuramdisulphide (TMTD), transformer oil and engine oil were of commercial grade. Maleic anhydride, methanol and toluene were of reagent grade.

Methods

The carboxy-terminated liquid natural rubber (CTNR) was prepared in the laboratory and well characterized.²⁰ The estimated value of acid number of CTNR was 17.5.²¹

The optimum concentration of nano silica for getting maximum tensile properties in NBR vulcanizates was determined by varying the quantity of nano silica as per the formulations given in Table 1. The optimum concentration of CTNR for attaining maximum tensile properties in nano silica–filled NBR vulcanizates was determined by varying the quantity of CTNR in the mix as per the formulations given in Table 2. Nano silica was added to NBR vulcanizates containing CTNR as per the formulations given in Table 3.

Table 1.

Formulations for testing the optimum concentration of nano silica in NS NBR-CTNR vulcanizates.

Ingredients (phr)	A	B	C	D	E	F	G	H
Acrylonitrile butadiene rubber	100	100	100	100	100	100	100	100
Zinc oxide	4	4	4	4	4	4	4	4
Magnesium oxide	1	1	1	1	1	1	1	1
Sulphur	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Styrenated phenol	1	1	1	1	1	1	1	1
Stearic acid	2	2	2	2	2	2	2	2
Nano silica	10	20	30	40	0	0	0	0
Amorphous silica	0	0	0	0	10	20	30	40
Dioctyl phthalate	1	1	1	1	1	1	1	1
CTNR	5	5	5	5	5	5	5	5
Mercaptobenzothiazole	1	1	1	1	1	1	1	1
Tetramethylthiuramdisulphide	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5

Table 2.

Formulations for testing the optimum concentration of CTNR in NS NBR-CTNR vulcanizates.

Ingredients (phr)	I	J	K	L
Acrylonitrile butadiene rubber	100	100	100	100
Zinc oxide	4	4	4	4
Magnesium oxide	1	1	1	1
Sulphur	1.5	1.5	1.5	1.5
Styrenated phenol	1	1	1	1
Stearic acid	2	2	2	2
Nano silica	40	40	40	40
Dioctyl phthalate	5	1	1	1
CTNR	0	5	10	15
Mercaptobenzothiazole	1	1	1	1
Tetramethylthiuramdisulphide	0.5	0.5	0.5	0.5
Optimum cure time	7.96	6.99	6.86	6.51

Table 3.

Formulations for testing the properties of NS NBR-CTNR vulcanizates.

Ingredients (phr)	M	N
Acrylonitrile butadiene rubber	100	100
Zinc oxide	4	4
Magnesium oxide	1	1
Sulphur	1.5	1.5
Styrenated phenol	1	1
Stearic acid	2	2
Nano silica	40	40
Dioctyl phthalate	5	1
CTNR	0	5
Mercaptobenzothiazole	1	1
Tetramethylthiuramdisulphide	0.5	0.5
Optimum cure time	7.96	6.99

All rubber samples were compounded in a laboratory two-roll mixing mill (size, 150x300 mm) as per ASTM D-3182. The sequence of addition of ingredients is nitrile rubber, filler, plasticizer, antioxidant, activator and accelerator, followed by vulcanizing agent at room temperature. The optimum cure time of compounds was determined on a rubber process analyser ( USA model RPA 2000). The rubber compounds were moulded on an electrically heated laboratory hydraulic press at 150 C up to their optimum cure times. Dumbbell-shaped tensile test pieces were punched out of the compression-moulded sheets along the mill grain direction. The tensile properties of samples were analysed by using a universal testing machine (UTM, UK Model No. 3365) as per ASTM D 412. Shore A hardness of the samples was determined using a digital hardness tester (Bareiss, Germany Model No. HPEII) as per ASTM D 2240. Rebound resilience was tested by using Dunlop Tripsometer, (UK model No. R2) as per ASTM D 7121. Heat build-up was examined on Goodrich Flexometer (Techpro, US Model no. A221240VAC) as per ASTM D 623. Compression set was determined by a compression set apparatus (Wallace, UK Model No. C4/2) as per ASTM D 395. Abrasion resistance index was tested by using Din Abrader (Bareiss, Germany Model No: D-89,610) as per ASTM D 5963. SEM micrographs were taken with a JSM-7900F Schottky Field Emission Scanning Electron Microscope instrument at an accelerated voltage of 3 kV.

Ageing resistance of the nano silica–filled NBR vulcanizates containing CTNR was evaluated by measuring the retention in tensile properties, compression set, heat build-up, resilience and abrasion loss, after keeping the samples at 100 C for 24 h in a vacuum oven.

Oil resistance of the vulcanizates was studied by keeping the samples in transformer oil and engine oil for 48 h and finding the retention in tensile properties and hardness. The swelling of vulcanizates in oils was analysed by keeping a known weight of the sample in transformer oil and engine oil at room temperature for 48 h and then measuring the increase in weight.

Results and Discussion

Figures 1 and 2 show the variation in tensile properties and hardness values of the NBR vulcanizates given in Table 1. The nano silica–filled NBR vulcanizates show superior tensile properties and hardness values compared to those containing amorphous silica. This may due to the better rubber filler interaction attributed to the large surface area of the nano-sized filler and the ionic cross-linking involving -COOH group of CTNR, -OH groups in silica and -CN group in NBR.²² Although the tensile properties increase with increase in concentration of nano silica in NS NBR-CTNR, incorporation of nano silica more than 40 phr was found to be difficult during compounding. So, the quantity of nano silica in NS NBR-CTNR vulcanizates was optimized at 40 phr for further studies.

Figure 1.

Variation in tensile properties of the NBR vulcanizates containing CTNR filled with (a) nano silica and (b) amorphous silica.

Figure 2.

Variation in hardness values of NBR vulcanizates containing CTNR with (a) nano silica and (b) amorphous silica.

Table 4 shows the variation in tensile properties and hardness values of the NS NBR-CTNR vulcanizates given in Table 2. The tensile properties of the vulcanizates containing an optimum concentration of nano silica increase with increase in concentration of CTNR, reach a maximum value at 5 phr of CTNR and thereafter decrease slightly. The increase in tensile properties may be due to the ionic cross-linking involving CTNR, nano silica and NBR as mentioned earlier. The marginal decrease in the properties after an optimum level may be due to the incompatibility of the NR part of the CTNR and NBR.²⁰ The hardness values were found to be comparable for all those vulcanizates. The decrease in optimum cure time of the nano silica–filled vulcanizates with increase in concentration of CTNR may be due to the presence of the fast-curing natural rubber part in CTNR.

Table 4.

Variation in tensile properties and hardness values of the NS NBR-CTNR vulcanizates.

Properties	I		J		K		L
	Average	STDEV(σ)	Average	STDEV(σ)	Average	STDEV(σ)	Average	STDEV(σ)
Tensile strength (MPa)	22.4	0.379	30.04	0.338	24.53	0.441	20.81	0.579
Elongation at break (%)	725	8.366	747	11.94	685	8.366	617	18.31
Modulus at 300% elongation (MPa)	6.4	0.303	7.92	0.071	6.5	0.414	5.58	0.335
Hardness (Shore A)	68	0.632	71	0.632	73	1.673	74	1.788

Figures 3 and 4 show the SEM micrographs of nano silica–reinforced NBR vulcanizates given in Table 3. The more uniform distribution of nano silica in NBR vulcanizates in the presence of CTNR was confirmed by an SEM image (Figure 4). This may be due to the better rubber filler interaction further modified by ionic cross-linking involving -COOH group of CTNR, -OH groups in silica and -CN groups in NBR.²²

Figure 3.

Nano silica–reinforced NBR vulcanizate without CTNR.

Figure 4.

Nano silica–reinforced NBR vulcanizate containing CTNR.

Table 5 shows the variation in properties of the NS NBR-CTNR vulcanizates given in Table 3, before and after ageing. The vulcanizates containing nano silica with a combination of polymeric plasticizer CTNR and dioctyl phthalate (DOP) possess superior tensile properties than the earlier reported amorphous silica vulcanizates containing CTNR.²⁰ The combined use of nano silica and CTNR could reduce heat build-up, compression set and improve rebound resilience of the NBR vulcanizates. The abrasion resistance index of NS NBR-CTNR was found to be higher than those containing amorphous silica due to the increased bound rubber content, modified by ionic cross-linking involving -COOH group of CTNR, -OH groups in silica and -CN group in NBR.²² Superior ageing resistance was exhibited by the vulcanizates containing a combination of plasticizers DOP and CTNR (N) than those containing DOP alone(M). During the initial hours of ageing, DOP may be acting as a plasticizer, and after that, the non-volatile polymeric plasticizer CTNR plays the major role in the ageing resistance.

Table 5.

Variation in properties of NS NBR-CTNR vulcanizates given in Table 3, before and after ageing.

Properties	M		N
	Average	STDEV(σ)	Average	STDEV(σ)
Before ageing
Tensile strength (MPa)	22.4	0.379	30.04	0.338
Elongation at break (%)	725	8.366	747	11.94
Modulus at 300% elongation (MPa)	6.4	0.303	7.92	0.071
Hardness (Shore A)	68	0.632	71	0.632
Rebound resilience (%)	49.34	-	53.19	-
Heat build-up (ΔT^o C)	89	-	84	-
Compression set (%)	13.29	-	12.01	-
Abrasion resistance index (%)	74	-	90	-
After ageing	1	-	1	-
Tensile strength (MPa)	15.15	0.584	26.84	0.517
Elongation at break (%)	670	13.87	678	10.93
Modulus at 300% elongation (MPa)	5.31	0.333	7.71	0.495
Hardness (Shore A)	73	1.673	77	1.788
Rebound resilience (%)	48.90	-	52.77	-
Heat build-up (ΔT^o C)	88	-	82	-
Compression set (%)	19.07	-	17.38	-
Abrasion resistance index (%)	81	-	97	-

Table 6 shows the variation in tensile properties and hardness values of the NS NBR-CTNR vulcanizates given in Table 3, before and after oil extraction. The NS NBR-CTNR vulcanizates show superior tensile properties and hardness values compared to those containing DOP. This may be due to the lower extractability of polymeric plasticizer CTNR and increase in the bound rubber content owing to the presence of nano-sized filler particles.²³

Table 6.

Variation in tensile properties and hardness values of NS NBR-CTNR vulcanizates given in Table 3, before and after oil extraction.

Properties	M		N
	Average	STDEV(σ)	Average	STDEV(σ)
Before oil extraction
Tensile strength (MPa)	22.4	0.379	30.04	0.338
Elongation at break (%)	725	8.366	747	11.94
Modulus at 300% elongation (MPa)	6.4	0.303	7.92	0.071
Hardness (Shore A)	68	0.632	71	0.632
After extraction in transformer oil
Tensile strength (MPa)	13.15	0.441	24.6	0.628
Elongation at break (%)	618	17.01	680	14.17
Modulus at 300% elongation (MPa)	5.0	0.228	7.42	0.364
Hardness (Shore A)	58	1.897	67	2.281
After extraction in engine oil
Tensile strength (MPa)	15.02	0.268	28	0.200
Elongation at break (%)	667	13.22	697	11.78
Modulus at 300% elongation (MPa)	5.2	0.196	7.54	0.372
Hardness (Shore A)	61	1.264	68	1.414

Table 7 shows the increase in weight percentage of the vulcanizates given in Table 3 in different oils. The lower increase in weight percentage of the vulcanizates containing conventional plasticizer may be due to the leaching of the plasticizer by oils. The increase in weight percentage of the NS NBR-CTNR vulcanizates may be due to the presence of polymeric plasticizer.

Table 7.

Increase in weight percentage (%) of the NS NBR-CTNR vulcanizates given in Table 3 in oils.

Oil type	M	N
Transformer oil	0.2419	0.3981
Engine oil	0.2200	0.2569

Conclusions

Nano silica could be used as a potential cost-effective green filler in NBR vulcanizates containing CTNR. Nano silica–filled NBR vulcanizates containing CTNR show superior tensile properties and hardness values. The polymeric plasticizer CTNR could reduce the heat build-up, improve rebound resilience, compression set and ageing resistance of nano silica–filled NBR vulcanizates. Uniform distribution of commercial nano silica in NBR vulcanizates can be achieved in the presence of CTNR. The use of a combination of conventional plasticizer and CTNR in nano silica–filled NBR is found to be promising.

Footnotes

Acknowledgements

The authors would like to thank J J Murphy Research Centre, Airapuram Rubber Park, who provided the instrumental support and Principal and management team of Mar Athanasius College, Kothamangalam, for their cooperation with us during all stages of this research

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Gopika Sudhakaran – M G University Regional Research Centre, Department of Chemistry, Mar Athanasius College, Kothamangalam, Kerala 686666, India. Email-gopika.sudhan@gmail.com

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Shanti A Avirah

Appendix

References

Kamal

Mutel

. Rheological Properties of Suspensions in Newtonian and Non-Newtonian Fluids. J Polym Eng 1985; 5: 293–382.

Leong

Y-K

Boger

. Surface chemistry effects on concentrated suspension rheology. J Colloid Interf Sci 1990; 136: 249–258.

Nicodemo

Nicolais

. Viscosity of bead suspensions in polymeric solutions. J Appl Polym Sci 1974; 18: 2809–2818.

Busigin

Martinez

Woodhams

, et al. Factors affecting the mechanical properties of mica‐filled polypropylenes. Polym Eng Sci 1983; 23: 766–770.

Malik

Farooqi

Vachet

. Mechanical and rheological properties of reinforced polyethylene. Polym Compos 1992; 13: 174–178.

Tessema

Zhao

Moll

, et al. Effect of filler loading, geometry, dispersion and temperature on thermal conductivity of polymer nanocomposites. Polym Test 2017; 57: 101–106.

Moisala

Kinloch

, et al. Thermal and electrical conductivity of single- and multi-walled carbon nanotube-epoxy composites. Compos Sci Technol 2006; 66: 1285–1288.

Gopika Sudhakaran

SAA

Avirah

. Studies on reinforcing effect of industrial nano silica in chloroprene vulcanizates containing carboxy-terminated liquid natural rubber. J Elastomers Plast 2021; 54: 509–517.

Yan

Sun

Zhang

YYZ

, et al. Effect of nitrile rubber on properties of silica-filled natural rubber compounds. Polym Test 2005; 24: 32–38.

10.

Luginsland

Niedermeier

. New reinforcing materials for rising tire performance demands. New Reinf Mater Rising Tire Perform Demands 2003; 228: 34.

11.

Mark

Erman

Eirich

. The Science and Technology of Rubber. Elsevier Academic Press, 2005.

12.

Kaewsakul

Sahakaro

Dierkes

, et al. Optimization of mixing conditions for silica-reinforced natural rubber tire tread compounds. Rubber Chem Technol 2012; 85: 277–294.

13.

Eyssa

Abdulyazied

Abdulrahman

, et al. Mechanical and physical properties of nanosilica/nitrile butadiene rubber composites cured by gamma irradiation. Egyption J Pet 2018; 27: 383–392.

14.

Zhu

Cheung

Zhang

, et al. Compatibility of different biodiesel composition with acrylonitrile butadiene rubber (NBR). Fuel 2015; 158: 288–292.

15.

Noriman

Ismail

. Properties of styrene butadiene rubber (SBR)/recycled acrylonitrile butadiene rubber (NBRr) blends: the effects of carbon black/silica (CB/Sil) hybrid filler and silane coupling agent,Si69. J Appl Polym Sci 2011; 124: 19–27.

16.

Reuvekamp

LAEM

ten Brinke

Van Swaaij

, et al. Effects of mixing conditions-reaction of TESPT silane coupling agent during mixing with silica filler and tire rubber. Kautschuk Gummi Kunststoffe; 55.

17.

Luginsland

Kalscheuren

. Reactivity of the Sulfur Chains of the Tetrasulfane Silane Si 69 and the Disulfane Dilane TESPD, 53. Kautschuk Gummi Kunststoffe.

18.

Sengloyluan

Sahakaro

Dierkes

, et al. Silica-reinforced tire tread compounds compatibilized by using epoxidized natural rubber. Eur Polym J 2014; 51: 69–79.

19.

Munir

Yasin

Memon

, et al. Rheological and Mechanical Properties of Silica/Nitrile Butadiene Rubber Vulcanizates with Eco-Friendly Ionic Liquid. Polymers (Basel) 2020; 12: 2763.

20.

Dileep

Avirah

Shanti

A . Studies on Carboxy-Terminated Liquid Natural Rubber in NBR. J Appl Polym Sci 2002; 84: 261–267.

21.

Urbanski

. Hand Book of Analysis of Synthetic Polymers and Plastics, 1977.

22.

Srilathakutty

John

Rani Joseph

KEG

, et al. Use of amine terminated liquid natural rubber as a plasticiser in filled NR and NBR compounds. Int J Polym Mater 1996; 32: 235–246.

23.

Song

Chen

, et al. Effect of bound rubber on vulcanization kinetics in silica filled silione rubber. R Soc Chem 2016; 6: 101470–101476.