Tuesday, April 8, 2014

Reinforcement of Rubber & Latex Compounds with Nanofiller 

S. N. Chakravarty* 

KPS Consultants & Impex Pvt. Ltd.,
 812, Devika Tower, 6 Nehru Place, New Delhi -110019, (India)
E-mail : kpspltd@gmail.com 

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.

 What is Nanotechnology 

Take a random selection of scientists, engineers, investors and the general public and ask them what nanotechnology is and you will receive a range of replies as broad as nanotechnology itself. For many scientists, it is nothing startlingly new; after all we have been working at the nanoscale for decades, through electron microscopy, scanning probe microscopies or simply growing and analysing thin films. For most other groups, however, nanotechnology means something far more ambitious, miniature submarines in the bloodstream, little cogs and gears made out of atoms, space elevators made of nanotubes, and the colonization of space.

The term nanotechnology was coined in 1974 by the Japanese scientist Norio Taniguchi who defined nanotechnology as manufacturing of materials at the nanometer level. A nanometer is one billionth of one meter (10 –9)   and thousandth part of a m (micron). A more contemporary definition of nano technology is the science and technology of devices fabricated from single atoms and molecules.  One might say that nanotechnology is a hybrid science, combining science as well as engineering to control single atoms and molecules by various means in order to build a desired devices from the “ bottom up “.


*President – Elastomer Technology Development Society, India


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”.

The term, “nanocomposite” refers to every type of materials having fillers in the nanometer size range, at least in one dimension. More specifically, polymers that are reinforced with rigid inorganic/organic particles, which have at least one dimension in the nanometer size-range are termed as polymer nanocomposites.

Nanofillers are necessarily nanoscopic and have high specific surface area. The specific surface area is one of the reasons why the nature of reinforcement is different in nanocomposites and is manifested even at very low filler loadings (< 10 wt%). Nanotechnology refers to the technology of rearranging and processing of atoms and molecules to fabricate materials to nano specifications such as a nanometer. The technology will enable scientists and engineers to see and manipulate matter at the molecular level, atom by atom, create new structures with fundamentally new molecular organisation and exploit the novel properties at that scale. Matter at the nanoscale is different from its bulk form; its chemical, biological, electrical, magnetic and other properties are different from the properties of macromatter.

What is the nanoscale?

Although a metre is defined by the International Standards Organization (ISO) as `the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second' and a nanometre is by definition 10- 9 of a metre, this does not help scientists to communicate the nanoscale to non-scientists. It is in human nature to relate sizes by reference to everyday objects, and the commonest definition of nanotechnology is in relation to the width of a human hair.
The mechanical and thermal properties of polymers and composite structures can be improved through the usage of various kinds of fillers. Micron size fillers usually cause decrease in strength, impact resistance, and processibility. Application of nanotechnology in rubber nanocomposites shows significant improvement in the modulus, strength, toughness, and resistance to chemical attack, gas impermeability in polymer composite.

Nanocomposite show superior physical and mechanical behavior over their conventional microcomposites. The inorganic nanofillers drastically improve the physical and mechanical microscopic properties of polymers even though their amount is small.

With the rapid emergence of the field of nanotechnology, regulations to nanomaterials are under development. One of the key issues hindering regulation is a lack of agreement on the definition of what constitutes a nanomaterial. Currently, the most comprehensive and internationally recognized definition distinguishes between two subgroups, nano-objects and nanostructured materials, and defines them as follows:

Nano-objects are materials that exit in defined singular form that have at least one dimension in the nano-scale, < 100 nm. These include nano-particle ( 3 dimensions in nano-scale ), nanofibers ( 2 dimensions ) and nano-plates ( 1 dimension ).
Nanostructured materials are materials that have structural features on the nano-scale that primarily exit in aggregated and / or agglomerated forms.

The nanotechnology industry

Many of the companies working with nanotechnology are simply applying our knowledge of the nanoscale to existing industries, whether it is improved drug delivery mechanisms for the pharmaceutical industry, or producing nanoclay particles for the plastics industry. In fact nanotechnology is an enabling technology rather than an industry in its own right. Nanotechnology is a fundamental understanding of how nature works at the atomic scale. New industries will be generated as a result of this understanding.
What is a Nanofiller ?

Presently the nanoscale materials have inspired the scientist and technologist, in the field of composites, by the fact that they often give rise to dramatically improved properties than their macro counterpart (1, 2). Nanocomposites possess unique properties such as stiffness, strength and barrier action depending on their dispersion structure in the matrix (3, 4). The nano particles are often used in a blending with polymers but this direct blending can not avoid the clustering tendency and the polymer matrix must have good process properties. Clay has been used enormously as filler for rubber and plastic in conventional microcomposites. Wang et al (5) while preparing and characterizing the rubber – clay nano – composites, suggested the latex method as most convenient in order to use clay promising reforming agent.

Filler like calcium carbonate, clay etc. with average particle size in the range 20 to 60 nanometer may be defined as a nanofiller. Unlike traditional fillers, nanofillers are used in relatively smaller amounts ( 5 – 10 parts) in order to provide substantial improvement in physical / mechanical properties.

Underlying principle - Nanofillers

When a particle is downsized to nano scale, it becomes an agglomerate of limited number of atoms or molecules, and shows different surface properties from normal sized particles, as particularly indicated by the significant increase in specific surface area and surface energy.

a)         Nano sized particles (average diameter 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.

b)         Same weight of nano size filler will have 1000 times more number of particles (that are able to occupy substantially greater number of sites in the polymer matrix) as compared to micron size filler. Hence, to achieve same level of physical property in the reinforced polymer the dosage of nano size filler can be one fourth to one third that of micron size filler

c)         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 permeability of the polymer.


d)         Nano sized fillers in polymer matrix substantially improve surface properties like gloss, surface finish, grip (friction) etc.

Carbon Black

Carbon black is the best example of nano filler used in polymer / elastomer since very long time. The particle size of carbon blacks is in nano meter but the filler is in the form of granules which helps in handling and prevent in flying loss & environment check.


Carbon black was produced and used as pigment by the ancient Chinese and Egyptians more than 2000 years ago. Carbon Black was manufactured, which today is known as lamp black, by burning fats and oils in a lamp, soot from which got deposited on the surface of the inverted pottery plate which used to provide pigment in inks & cosmetics (Kajol in India). Today, carbon black usage as pigment is negligible and it is primarily used to provide reinforcement and other properties to rubber articles.


Carbon black is a very fine particulate form of elemental carbon arranged in a less ordered manner than other forms of carbon such as diamond or graphite. Carbon black consists of planes of carbon atoms fused together randomly to form spherical particles, which in turn form structures or aggregates. These aggregates are often bound together to form secondary structures, or agglomerates. Most important characteristics of carbon black are surface area (which tells about particle size) and structure (which tells about  the degree of particle aggregation ). These two characteristics are dependent on the type of and can be controlled by the process of manufacture of carbon black.

Carbon black is a particulate form of elemental carbon, similar to graphite in its microstructure. Most of the world’s carbon black is produced by the oil furnace process. In the oil furnace process carbon black is produced by the incomplete combustion of liquid, aromatic hydrocarbons. The resulting black grades are used in rubber as reinforcing agents and in plastic, printing inks, coatings, sealants and a variety of other products for pigmentation, electrical conductivity, rheology control and UV protection.

During the first split second of the combustion reaction, carbon nodules are formed with dimensions from approximately 5 to 100 nm, depending on the grade of carbon black to be produced. On the basis of the proposed definitions for nanomaterials, these nodules may be considered nano-objects. However, the lifespan of these nodules is very short as they immediately cluster together to form aggregates of sizes between approximately 70 and 500 nm.

The effect of carbon black filler (CB) (loading 60-100 phr) on the cure kinetics, mechanical properties, morphology and thermal stability of acrylonitrile butadiene/ethylene-propylene-diene (NBR/EPDM) rubber blends have been studied by others. The determination of cure characteristics was estimated by Rheometer R-100& Mechanical properties such as tensile strength, elongation at break, modulus at 200 and 300% elongation, hardness , have been measured. Morphology of the cross linked system was carried out by scanning electron microscope (SEM).. From the results of NBR/EPDM/CB (80/20/70) nano-composite, a correlation between mechanical properties and calculated activation energy of cross link (Eac) and reversion (Ear) process can be concluded.

Carbon blacks are produced either by incomplete combustion or thermal decomposition of a hydrocarbon feedstock. Five types of carbon black, with corresponding feed stocks and particle sizes (particle diameters are given in nanometers (nm, 109 m) are listed below.

Chemical Process
Carbon Black Type
D (nm)
Feedstock
Incomplete
Combustion
Lamp black
50-100
Coal tar hydrocarbons

Channel Black
10-30
Natural gas

Furnace Black
10-80
Natural gas ,Liquid aromatic
Thermal
Decomposition
Thermal Black
150-500
Natural gas

Acetylene Black
35-70
Acetylene

In this talk it is not intended to talk about Carbon black in detail as lot has been talked & published about it.

Non Black Nano filler

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.

Nano-CaCO3­ is the cheapest commercially available nanofiller, and has the additional advantages of a low aspect ratio and a large surface area. Several researchers have prepared CaCO3 nano particles and studied the mechanical properties of the reinforced rubber composites.
Micron-sized calcium carbonate has been historically used to lower the cost of relatively expensive polymer resins. It has very limited effects on property improvement due to the poor particle-Polymer interaction. However, due to the larger interfacial area in nano-sized CaCO3 / polymer, its properties are expected to be better than the micron-sized CaCO3 / polymer composites.

 Usage of Nano Fillers in Solid rubber

Nanocomposites consisting of an elastomer and a small amount (~5 wt%) of different nanofillers frequently exhibit remarkably improved mechanical and material properties when compared to those of pristine elastomers. Improvements include a higher modulus, increased strength and heat resistance, decreased gas permeability and flammability.

Natural rubber is widely used in different application. The literature search shows that several research groups have prepared nanocomposites based on natural rubber rubber. The effect of different nanoclays on the mechanical properties of NR based nanocomposites was studied. Mechanical properties and cure characteristics of NR nanocomposites were studied by several researchers.(5,6)

Superior barrier properties given by the platy / flake type nanofillers like, clay, applications demanding low solvent and /or vapor permeability will always have scopes for nanocomposites. Thus, in near future tire inner liners can be made of nanocomposites. As the silicate type of nanofillers can enhance the flame and fire retardancy of elastomers, cable jacketing elastomer compounds can contain nanoclays.

Nano (precipitated ) calcium carbonate has been used in polymers & elastomers based products like Cycle & Auto tubes, Auto & Cycle tyre ,Car tyre carcass & inner liner , LPG tubing , Hot Water Bag ( improvement in hot tear property ) , Hose cover etc compounds. In all these cases air barrier property is important.
CYCLE TIRE & TUBE


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.


Experimental

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

Nano size Calcium Carbonate (with Hexagonal Calcite Crystalline structure)


Typical Properties
CC - 301
Moisture, (%)
< 0.6
Bulk Density (gm / cu. cm.)
0.65 – 0.70
Whiteness (KETT-C100)
94
pH
8.6 – 9.0
Average particle diameter (nm)
40
Specific Gravity
2.52
Oil absorption (ml / 100g)
33 – 36
CaO (%)
54
MgO (%)
0.2
SiO2 (%)
0.1
Iron & Aluminium Oxide (%)
0.2
Ignition Loss (%)
45

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 1

Table – 1   Compound Formulation

Compounds

Ingredients
I
I A
II
II A
III
IIIA
IIIB
NR  (RMA-5)
100
100
--
--
30
30
30
Pale Crepe
--
--
100
100
--
--
--
NBR (33% ACN)
--
--
--
--
70
70
70
Peptiser
0.2
0.2
0.2
0.2
--
--
--
Zinc Oxide
4.0
4.0
4.0
4.0
4.0
4.0
4.0
Stearic Acid
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Ppt. Silica (VN-3)
15
--
15
--
15
--
10
Nano CaCO3
--
5.0
--
5.0
--
5.0
5.0
China Clay
30
30
--
--
30
30
30
Paraffinic Oil (245)
5.0
5.0
5.0
5.0
--
--
--
DOP
--
--
--
--
5.0
5.0
5.0
Sulfur
2.3
2.3
2.3
2.3
2.0
2.0
2.0
CBS
1.0
1.0
1.0
1.0
1.0
1.0
1.0
TMTM
0.2
0.2
0.2
0.2
0.2
0.2
0.2

Table – 2   Rheometric Study at 150°C ( Chart time 12 mints )
I
IA
II
IIA
IIIA
III
MH (Max. Torque)
65.55
59.16
77.4
75.1
49.33
48.22
TS5 (Opt. Cure Time)
3.42
3.15
4.55
3.87
4.48
4.62
TC90 (Opt. Cure Time)
4.73
4.38
5.4
5.15
6.08
6.1
Cure Rate
64.52
68.97
60
65.22
47.24
53.57
Reversion Time
8.33
7.85
10.2
8.77
10.42
11.95

Table – 3   Physical Properties of Compounds


Compounds
S. No
Properties
I
IA
II
IIA
III
III A
IIIB
1
T.S. – Original (Kg/cm²)
159
150
218
200
105
100
112

After Ageing (Kg/cm²)
108
(-32%)
107
(-29%)
141
(-35%)
140
(-30)
78
(-26%)
77
(-23%)
87
(-22%)









2
M-300% Original (Kg/cm²)
51
50
42
43
40
39
39

After Ageing
(Kg/cm²)
43
(-16%)
42
(-16%)
28
(-33%)
37
(-14%)
58
(+45%)
55
(+41%)
55
(+41%)









3
E.B – Original (%)
559
534
621
610
587
581
559

After Ageing
(%)
415
(-26%)
408
(24%)
304
(-51%)
380
(-38%)
377
(-36%)
372
(-36%)
374
(-33%)









4
Angular Tear – Original (Kg/cm)
47
42
--
--
17
20
18

After Ageing
(Kg/cm)
19
(-60%)
20
(-52%)
--
--
11
(-35%)
15
(-25%)
15
(-17%)









5
SH – Original
(Shore A)
54
50
52
50
58
54
60

After Ageing
(Shore A)
50
(-4)
48(-2)
46(-6)
48(-2)
62(+4)
58(+4)
60
(± 0)









6
Resilience – Original(%)
48
44
60
55
25
21
23

After Ageing (%)
38
36
46
44
20
19
19


Note : Ageing done at 100°C for 48 hrs, Values in brackets are % drop in respective properties

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 4301.

 For Cycle tube compounds Rheometery was carried out at 150° & 160°C using Moving Die 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

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)

Evaluation of Nano Calcium Carbonate was carried out in different compounds, Natural rubber (RMA 5 and Pale Crepe) and Nitrile & Natural rubber blend in 70:30 ratio. Formulations are given in Table –1. Studies were carried out in NR and / or NBR + NR blend in non-black compound keeping in mind type of rubber & such non black compounds are used in the manufacture of Sports goods like Sheets for inflated balls, Play balls, Table Tennis bats etc.

While discussing the results reported here one has to keep in mind that pptd. Silica (15 phr) used in the compound was replaced by one third (5 phr) of Nano Calcium Carbonate (nano CaCO3). This is to remind that one third of Nano Calcium Carbonate imparting similar reinforcement characteristics to that of pptd. Silica filler at much higher proportion.

Rheometry of the Compounds were done at 150°C using a Monsanto R-100 Oscillating Disc Rheometer (3º arc, upgraded with computer interface & software) for 12 mints Chart time. This is presented in Table –2

From the Rheometry it is observed that replacement of 15-phr pptd. Silica by 5-phr nano CaCO3 has minor effect on different characteristics like Max. Torque, TS 5 (Scorch time), TC 90 (Optimum cure time), Cure Rate and Reversion time of different compounds in RMA 5 and 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 Calcium Carbonate, which might be due to alkalinity factor.

Physical properties, before and after ageing, of compounds are given in Table-3. All tests were carried out as per IS-3400, different parts.

It can be seen from the values of different physical properties presented in Table–3, that these are quite close with Silica and with Nano Calcium Carbonate 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 Calcium Carbonate compared to pptd. Silica. 

Nano Calcium Carbonate containing compounds give little lower hardness compared to Silica compound. This could be modified by adjusting to higher proportion of nano CaCO3 or other cheaper 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 Calcium Carbonate.

Values in bracket, which are percentage drop in properties on ageing at 100°C for 48 hrs, clearly indicate that compounds with nano CaCO3 retains the properties at a higher level compared to compounds with pptd. silica filler only. May be, this effect will be more pronounced with higher level of nano calcium carbonate.

Further work was carried out with NR + NBR blend (30:70 ) compound (III B ) as reported in Table – 3 wherein part of the pptd. Silica filler was replaced by Nano Calcium Carbonate (10 phr pptd. Silica+5phr Nano Calcium Carbonate). Results, given in Table – 3, clearly indicate that part replacement of pptd Silica by Nano Calcium Carbonate gives rise to improved physical properties in all respects compared to alone pptd. Silica reinforced compound. Such combination will also give rise to cost saving.

Replacing Silica with Nano Calcium Carbonate in the compound will result in lowering of weight of the product like Automotive components where saving in weight is the international trend and will be welcome. However, for general rubber products, question about compound cost may arise because of lower filler content. In such cases the balance portion of the Silica filler may be replaced by cheaper fillers like China Clay, Aluminium Silicate, pptd. Calcium Carbonate etc.  This will give further cost saving. Incidentally Nano Calcium Carbonate is expected to be not costlier than pptd. Silica filler presently marketed in the country.

Usage of Nano Filler in Latex

To the elastomer latex, pristine clay can be added directly or as its aqueous dispersion (slurry). The clays are strong hydrophilic and adsorb water molecules which is associated with an expansion of their inter-gallery spacing.
Latex compounding has been successfully applied for dispersing nano calcium carbonate in NR latex,(4)

Calcium carbonate-NR latex nanocomposites are prepared by adding nano-CaCO3, whose surface had been treated with natural rubber latex before sulfuration.(2) The physical properties , thermo oxidative aging & thermal degradation properties and the ultra-microstructure were analyzed. The structures and properties of nanocomposites could be clearly improved by natural rubber latex mixed with surface-treated nano CaCO3. pH of supernatant layer of aqueous dispersion of precipitated Silica and Nano Calcium Carbonate was measured and found to be 6.5 – 7.0 and 7.5 – 8.0 respectively.

 FOOTBALL BLADDER – INFLATED & UNINFLATED

Latex Products – Ultra-fine calcium carbonate has been used successfully in the manufacture of football bladder, household gloves etc in bulk quantity. It can be used in surgical gloves, latex thread, balloons etc. Benefits are - reduced volume formulation cost, increased product volume and tensile strength, improved air impermeability of gloves, balloons, bladder etc. and smoother surface of the product. Large quantity of ultra-fine CaCO3 has been used in the manufacture of Football bladder in India.

INFLATED BALLS

Nano Clay

Polymer nanocomposites is one of the most interesting fields of research in mater in the recent years. Numeruous literatures are available on this topic, which describe various nanofillers in polymer matrices.  Synthesis and characterization of a series clay nanocomposites have been reported on development of  novel rubber / silica hybrid nanocomposites from acrylic rubber (ACM) and epoxidized natural rubber (ENR) using the solgel technique.

Knowledge of rheological properties of elastomers is of considerable importance in predicting and comprehending their processing characteristics. Both the viscous and elasticity of an elastomer can be analysed and correlated with its flow behavior. The viscous flow is related to the output rate, whereas the elastic behavior corresponds to the dimensional. Elastomer nanocomposites are characterized by better filler dispersion within matrix compared to the conventional filed systems.

Silica  Nano Filler

The surfaces of silicas possess siloxane and silanol groups, which make the filler acidic and moisture adsorbing. This causes detrimental effects such as unacceptably long cure times and slow cure rates, and also loss of crosslink density in sulfur-cured rubbers. For these reasons, the use of silica in rubber products was hampered until bi-functional organosilanes such as TESPT were available. These materials can be used as primers for treating silica surfaces to make the filler more suitable for use in rubber. TESPT is used to improve the reinforcing capability of precipitated silicas, and also forms an internal part of curing systems to improve crosslinking network properties. This silane possesses tetrasulfane and ethoxy reactive groups. The tetrasulfane groups are rubber reactive and react in the presence of accelerators at elevated temperatures, with or without elemental sulfur being present, to form crosslinks in unsaturated rubbers for instance SBR. The ethoxy groups react with the silanol groups on the surfaces of these fillers during compounding and this leads to the formation of stable covalent filler / TESPT bonds.

It was concluded that the coupling agent played a major role in promoting the crosslinking of the rubber by the filler, but alone was not capable of crosslinking the rubber phase.

Vulcanized natural rubber is well known as an exceptionally tough elastomer because it exhibits high tensile stress, larger hysteresis loss and crystallization upon stretching. The peculiar strength of natural rubber has been attributed to this strain – induced crystallizability.

To quantitatively determine the different stages of stress optical behavior including the critical conditions for the crystallization to take place in unfilled NR and NR filled with 5 phr and 10 phr of nanosilica particles.

The unfilled compounds exhibit a hysteresis loss in the stress – strain curves. This has been related to the occurrence of strain-induced crystallization in the NR.

Unlike the unfilled counterparts, the filled 5 phr and the 10 phr silica compounds exhibit significant hyesteresis even at low deformation levels. The samples stretched to high stretch ratios exhibit much larger hysteresis loops as compared to the stretched and retracted unfilled counterparts. The values of maximum stress reached in the case of filled compounds, both for 5 phr silica and 10 phr silica , are much higher than the maximum stresses reached by the unfilled sample: more than two times for the 5 phr silica filled compound and more than three times for the 10 phr silica filled compound as expected since the silica acts as a reinforcing agent. Notice that although the stress is increased, the true strain values remain essentially the same as for the unfilled samples.

It is evident the hysteresis in the filled compounds is much larger than unfilled counterparts.

The higher the amount of silica, the higher is the average number of silica particles that are present between the polymer chains causing a reduced mobility and ability to orient themselves to become parallel to each other to promote crystallization.

The nanosilica helps the orientation process possibly providing stiff regions that help transmit forces to local chains that in turn increase their orientation. However, for the same strain level, the 10 phr filled sample shows the lowest birefringence compared to unfilled and 5 phr filled system. This may be as a result of two “dilution” effects in the presence of nanosilica:

1.         In the presence of nanofillers, the average distance between polymer chains increase; this, in turn, should reduce the crosslink density negatively impacting network density;

2.         In the presence of large amount of nanosilica, the polymer chains cannot come together to crystallize as they are physically separated by the nanoparticles.

We suspect that both mechanisms become dominant at high nanosilica helps the local polymer chain orientation process.

Hybrid CB silica

The carbon black-silica dual phase filler has particle morphology similar to that of carbon black, but contains both silica and carbon black moieties, grafted together in a flame process (5). By incorporating both silica and carbon black domains into the same particle, the filler-filler network is reduced. In this filler, the carbon black is grafted with 10% silica by weight, which provides benefit to hysteresis without detriment to abrasion resistance. Compounds with the carbon black-silica dual phase may exhibit lower filler-filler interaction, as evidenced by lower Payne effect, which will result in lower hysteresis and better fuel economy.

Overall, the broad aggregate carbon black has a specific surface area approximately midway between that of N 375 and N 220, yet the filler particles of different aggregate sizes are not able to pack as closely together as filler particles of uniform size, and therefore have a larger average spacing between particles (5). In the rubber system, the inter-particle spaces allow the polymer chains access to the filler surface, resulting in less filler-filler intetaction and hence, less hysteresis. In addition, carbon black with a broad particle size distribution has been shown to provide higher abrasion resistance, leading to longer tread-life.

The hybrid CB silica compound exhibited longer ts2 scorch time relative to the carbon black control due to the acidic nature of the silica portion of the filler which tends to retard cure and increase scorch safety.  

 References

  1. Rapra Handbook, Particulate filled polymer composites, 2nd Ed.
  2. Inorganic nanoparticle filled polymer nanocomposites – Prof. Jian 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. Nano Calcium Carbonate – as performance filler in Rubber & Plastics- A. Chakravarty. Rubber India (Nov.2005, p 23).
  5. M.Morris and T.Al, Theo , Rubber World , 2010 ,41(5),15-18,25




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