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
- Rapra Handbook, Particulate filled polymer composites, 2nd
Ed.
- Inorganic nanoparticle filled polymer nanocomposites – Prof. Jian
Feng Chen, Research Centre of the Ministry of Education for High Gravity
Engineering & Technology , Dec. 2002
- Ultrafine Precipitated Calcium Carbonate & its Function as
Rubber Additive. – S.Tsutsui ,Shirashi Central Laboratories Co. Ltd. , Nippon Gomu Kyokaishi ( 2005 ) ,78 (6) , 218 – 233.
- Nano Calcium Carbonate – as performance filler in Rubber &
Plastics- A. Chakravarty. Rubber India (Nov.2005, p 23).
- M.Morris and T.Al, Theo , Rubber World , 2010 ,41(5),15-18,25
.
