Reinforcement of Rubber Compounds with Nano- Filler

Reinforcement of Rubber Compounds with Nano- Filler

S.N. Chakravarty & A. Chakravarty
KPS Consultants & Impex Pvt.Ltd., 812, 6 Nehru Place, New Delhi 110019 (India)
E.mail : kpsimpex@airtelmail.in

Abstract

Nano CaCO3 is one of the many emerging applications of nanotechnology that is already finding successful commercial application. Reinforcing effect of Nano CaCO3 in different compounds – NR and NR / NBR blend used in Sports Goods (laminated sheet for inflated balls), NR based cycle tube, bromobutyl based pharmaceutical closures and CPE / CSM blend used for coated fabric was studied with one characteristics in mind that is to improve barrier properties as all these products have requirement of one common property – air retention. Rheometric study and physical properties of the vulcanized compounds have been reported.

Introduction

In 1974 Norio Taniguchi defined nanotechnology as manufacturing of materials at the nanometer level. A more contemporary definition of nano technology is the science and technology of devices fabricated from single atoms and molecules. One might add nanotechnology is a hybrid science, combining science as well as engineering to control single atoms and molecules by various means in order to build desired materials / devices from the “ bottom up”.

Nano CaCO3 is one of the many emerging applications of nanotechnology that is already finding successful commercial application . The inorganic nano particles such as CaCO3 have unique functions in the reinforcement of polymers & control of reheological properties

Filler like calcium carbonate, clay etc. with average particle size in the range 01 to 100 nanometer may be defined as a nanofiller. Unlike traditional fillers, mainly used for cost reduction, nanofillers are performance-enhancing fillers used in


relatively small amounts (5 - 10%) in order to provide substantial improvements in physical and other properties.

Nano sized particles (average diameter around 40 nano meter) form a very fine and homogenous distributed system in polymer matrix. As compared to micron size filler particles the nano size filler particles are able to occupy substantially greater number of sites in the polymer matrix. The significant increase in specific surface area of filler particles contributes to the enhanced physical property of the polymer matrix.

Nano sized fillers increase barrier properties by creating a maze or “ tortuous path” that slows the progress of gas molecules through the polymer matrix thereby substantially improving the gas / air permeability of the polymer. Nano sized fillers in polymer matrix substantially improve surface properties like gloss, surface finish, grip (friction) etc.

Knowledge of rheological properties of elastomers is of considerable importance in predicting and comprehending their processing characteristics. Both the viscous and elastic behavior of an elastomer can be analyzed and correlated with its flow behavior. The viscous flow is related to the output rate , whereas the elastic behavior corresponds to the dimensional.

Background of the Study

Studies with specific compounds reinforced with Nano CaCO3 were carried out with one characteristic in mind i.e. to improve barrier properties. NR and NR / NBR blend compounds are used in sports goods (e.g. laminated sheet for inflated balls), cycle tube compound, bromobutyl compounds used in pharmaceutical closures and CPE / CSM blend compound used for coated fabric which in turn is used in the manufacture of inflated boat & container. All these have requirement of one common property – air retention which is expected to improve by the usage of Nano CaCO3 in the compound.

Present study reports about the reinforcing effect of Nano CaCO3 in different compounds as mentioned above.

Experimental

Typical properties of Nano CaCO3 used for the studies are given in Table I

Mixing of compounds was carried out with open laboratory mixing mill (200 mm X 400 mm, friction ratio 1:1.1, approx.70°C) following standard procedure of mixing. Compound formulations are given in Table II, III ,IV & V.

Rheometery of the compound was carried out with Oscillating Disc Rheometer (Monsanto R-100, 3º arc, upgraded with computer interface & software) at 150°C from which optimum cure time (t90) was taken for slab cure. Physical testing was carried out as per ASTM D different specification using Instron 430.

For cycle tube compounds Rheometery was carried out at 150° & 160°C using MDR Type of Rheometer ( Flexys make ) with 1° ARC and 12 mints chart.
Compound slab of 150 X 150 mm was cured at 150°C in an electrically heated laboratory hydraulic press (platen size 300 X 300 mm, operating pressure 1.5 tons) for specific time & temperature as mentioned in the Tables. Slabs were cooled at lab temperature (27+-2°C, 65+-5% RH) for 16 hrs after which dumbbells (Type 1 ) were punched. Physical properties of cured slabs were determined with the help of Tensile Tester (Instron 4301) – both before & after ageing in circulating hot air oven at 100 ± 2 °C for 48 hrs.

Results & Discussion

Evaluation in NR and NR & NBR blend based compounds

The use of particulate fillers in polymer / elastomer is known since very long time and their usage continue to play very important role, especially with respect to reinforcement of properties and product cost. Selection and use of such particulate fillers are guided by different factors like cost, particle size and shape, filler surface structure etc. 1,2

The inorganic nanoparticle such as Calcium Carbonate (CaCO3) has unique functions in the reinforcement of polymers and control of rehological properties. The effect of Nano CaCO3, having mean size of 40-nano meter (nm), on properties of PVC has been investigated and reported in the literature. There is hardly any work reported on the effect of Nano CaCO3 on natural rubber compound in the literature, through some work has been reported about its effect on EPDM, SBS etc. 3

Studies with Natural Rubber (RMA 5 & Pale Crepe) compounds containing non black filler – both reinforcing (precipitated silica) & non reinforcing (china clay) and with lower dosage of Nano CaCO3 compared to silica were carried out to understand the reinforcement contribution of such Nano CaCO3 by determining classical physico-mechanical properties, before and after ageing at an elevated temperature.

From the Rheometry (Table VI A ) it is observed that replacement of 15 phr precipitated Silica by 5 phr Nano CaCO3 has minor effect on different characteristics like Max. Torque, TS 5 (Scorch time),& TC 90 (Optimum cure time) of different compounds in two types of Natural rubber ( RMA & Pale Crepe) and blend of NR with Nitrile rubber (33% ACN content). These characteristics are closely comparable in most of the cases; there is a minor effect on scorch time (minor lower tendency) and cure rate (minor slowing down) with compound containing Nano CaCO3, which might be due to pH factor.

Physical properties, before and after ageing, of compounds are given in Table – VI B. All tests were carried out as per ASTM D (Vol.9.01) different specification.

It can be seen from the values of different physical properties presented in Table– VI B, that these are quite close for both, silica and Nano CaCO3 containing compounds though in later case only one third quantity of filler was used replacing Silica. These values clearly indicate the superior reinforcing effect of Nano CaCO3 compared to precipitated Silica. Nano CaCO3 containing compounds give little lower hardness compared to Silica compound. This could be modified by adjusting the proportion of other filler, as the case may be, in the compound.

Drop in values of different properties after ageing clearly indicate superior trend with Nano CaCO3 compounds indicating better performance of the product based on Nano CaCO3. Percentage drop in properties on ageing at 100°C for 48 hrs (values in bracket), clearly indicate that compounds with Nano CaCO3 retains the properties at a higher level compared to compounds with precipitated silica filler only.

Further work was carried out with NR + NBR blend (30:70 ) compound (III B ) as reported in Table – III wherein part of the precipitated Silica filler was replaced by Nano CaCO3 (10 phr pptd. Silica + 5 phr Nano CaCO3). Results, given in Table – III, clearly indicate that part replacement of precipitated Silica by Nano CaCO3 gives rise to improved physical properties in all respect compared to alone precipitated Silica reinforced compound.

Evaluation in NR Bicycle Tube Compound

Different Physical property values are given in Table – VII A & B.

It is noticed that improved Tensile Strength and Modulus (at 300%) values are obtained with compound containing Nano CaCO3 compared to that with precipitated Silica, even at one third proportion of Nano CaCO3 to Silica indicating higher reinforcing character of Nano CaCO3. This was expected because of finer particle size of Nano CaCO3 resulting in thorough and uniform dispersion of filler in the compound resulting in higher reinforcement. Elongation at break reduces which corresponds to higher modulus.

Elastomer nanocomposites are characterized by better filler dispersion within matrix compared to the conventional filler system. This is noticed from the effect of small dosage of Nano CaCO3 on the physical properties of compounds.

Tensile Strength of compounds containing Nano CaCO3 increases substantially in case of non-black compound and marginally in case of black compounds containing reinforcing carbon black which itself provides high Tensile & Tear properties due to carbon black reinforcing characteristics.

Similarly, Compression Set values of compounds containing Nano CaCO3 improves substantially with non-black compound and marginally with black compounds containing reinforcing carbon black which also contributes towards this property masking the effect of Nano CaCO3.

In case of Abrasion Resistance a clear improvement with non-black compound containing nano CaCO3 is observed which is marginally better in case of black compound containing reinforcing carbon black.

Evaluation in Bromobutyl Compound

Nano CaCO3 was evaluated in a typical Bromobutyl rubber based pharmaceutical compound (Table –V) as full and part replacement of precipitated Silica. Results are presented in Table VIII.

Nano CaCO3 imparts better Scorch Safety but slows down the cure a little.

Tensile strength values increase substantially with nano CaCO3 containing compound, even as part replacement of precipitated Silica. Same trend is observed with Modulus at 300% elongation. Hardness of all compounds are closely comparable. Compression Set values are better with nano CaCO3 containing compounds – as full or part replacement of precipitated Silica.

Tear strength values of different compounds are more or less at same level. Improvement in Modulus, Tensile strength & Compression set at same Hardness level of compounds with nano CaCO3 as part or full replacement of precipitated Silica, which are the performance criteria properties, indicate towards an improved product performance.


Studies on Nano CaCO3 usage in CSM – CPE blend

Reinforcement with Carbon Black & Nano CaCO3 of CPE (Chlorinated polyethylene rubber) and CSM (Chlorosulphonated Polyethylene) blends (Table V) were studied as a part of the evaluation of polymer blends of CPE and CSM. 4

With increase in Nano CaCO3 in the compound, ML4 (Mooney Viscosity) values and Max. torque (Rheometry) goes up. Scorch time and cure time decreases with increase in the filler content. With high filler level, polymer – filler interaction increases causing drop in scorch safety & cure time and increase in max. torque (Table IX)

Compound containing Nano CaCO3 only shows drop in tensile strength after 30 phr level but the modulus remains constant. Compound containing 40 phr CB with varying nano CaCO3 phr in the compound, the properties levels off after 10 phr nano CaCO3 content in the compound. (Table X)

Increase in storage & loss modulus as well as tan d value indicating higher heat development on dynamic application with Nano CaCO3 containing compound is much lower compared to CB containing compound. Thus usage of Nano CaCO3 as part replacement of CB can reduce heat development of compound.

For detail results you may refer to Kautschuk Gummi Kunststofee, Nov. 2007, p 619-622

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Acknowledgement:

Authors wish to acknowledge help in some of the experimental work done by Ralson India Ltd. and CPSE, IIT Delhi reported in this article.


References

1. Rapra Handbook, Particulate filled polymer composites, 2nd Ed.
2. Inorganic nanoparticle filled nano composites – Prof. Jain Feng Chen, Research Centre of the Ministry of Education for High Gravity Engineering & Technology, Dec. 2002.
3. Ultrafine Precipitated Calcium Carbonate & its Function as Rubber Additive. – S. Tsutsui, Shirashi Central Laboratories Co. Ltd., Nippon Gomu Kyokaishi (2005), 78 (6), 218-233
4. Studies on Reinforcement of Carbon Black and Nano Filler in Chlorinated Polyethylene and Chlorosulphonated Polyethylene Blend – S. Mondal
M.Tech Thesis, IIT, Delhi, May 2006.