Transcript
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COMPOUNDING TECHNOLOGY
Basically a compound can be formulated with the below main general
mode which is as follows:
1. Polymer2. Activator3. Accelerator4. Filler5. Processing aid & if necessary6. Special agents
i. Antioxidantii. Antiozonantiii. Flame retardenceiv. Antifoggingv. Antistaticvi. Antidetergent
7. Vulcanizing agentsIn order to develop a rubber compound, the various ingredients to
be used are complied into a recipe, every recipe contains a number of
components, each having a specific function either in the processing,
vulcanization or end use of the product, all the ingredients used as
normally given in amounts based on a total of 100 parts of the rubber.
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ACTIVATORS
Activators are chemical which increase rate of vulcanization are used to
activate the accelerator and improve its effectiveness. Typically metal oxides
(zinc oxide) and fatty acids (stearic acid) are used as activators. These make
better able to react with sulphur to form cross links.
ZINC OXIDE
Zinc oxide is an universal activator used in all rubber compounds.
Usually dose is 3 to 5% of the weight of the rubber. In higher quantities it
exerts a reinforcing effect and improve thermal conductivity, heat resistance,
tear resistance of abrasion. There is increase of hardness without affecting
rebound resilience.
Zinc oxide particles are fine, about 15 mu it does not dissolve in rubber
but with stearic acid the oxide particles are covered with zinc stearate which
dissolve in rubber and then making it possible to be used as an activator.
STEARIC ACID
This is also an activator for accelerator which functions by forming zinc
soap, which in turn activates the accelerators. Other fatty acids such as soya
fatty, hydorxy stearic acid, oleic acid, palmitic acid etc also function in a
similar manner. But stearic acid is considered to be the best and is used upto 1
to 5%.
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ACCELERATORS
Accelerators are used to reduce the vulcanization time or cure time by
increasing the speed of vulcanization. Most accelerators are organic in nature
containing nitrogen and sulfur. Inorganic types are litharge, lime and magnesia.
Even though an organic accelerator is used in small proportion, an accelerator
has a proformal influence on the nature of the cross linking which determines
the physical properties like tensile, tear, flexing, abrasion, compression set etc.
Mainly accelerators are classified as two types namely.
Primary accelerators
It is a single accelerator system capable of producing sufficient activity
to produce satisfactory cures within specified times.
Secondary accelerators or the boosters
In this vulcanization system two or more accelerators are present, the
primary accelerator is present as the largest amount and the secondary or the
booster in 10-20% of the total.
Primary accelerators are generally the thiazole and sulfonamide while
the secondary are thiurams, dithiocarbamates, guanidines.
The important type of accelerators used in compounding of EPDM
rubber are as follows.
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1.MBT
Chemical Name : 2 mercaptobenzothiazole
Chemical Structure :
Molecular Weight : 167
Appearance : Cream coloured oil treated powder
Specific gravity : 1.51
Melting point : 176 C
Ash content % by mass : 0.4
Solubility : Soluble in acetone, chloroform and dil-
alkali Insoluble in water and gasoline
Classification : Semi ultrafast accelerator
Color code : Brown colored strip
2. CBS
Chemical Name : N-Cyclohexyl-2-benzothiazole sulphenamide
Chemical Structure :
Molecular Weight : 264
Appearance : Light tan to buff pellets
Specific gravity : 1.30
Melting point : 100OC
Ash content, % by mass : 0.25
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Solubility : Soluble in benzene, naphtha, CCl4, ethyl
acetate and acetone. Insoluble in H2O
Classification : Fast delayed action accelerator
Color code : Blue colored strip
3.TMTD
Chemical Name : Tetramethyl thiuram di sulphide
Chemical Structure :
Molecular Weight : 240
Appearance : Half white oil treated powder
Specific gravity : 1.42
Melting point : 145 C
Ash content, % by mass : 0.5
Solubility : Soluble in methyl chloride, benzene
insoluble in H2O
Classification : Ultra fast accelerator
Color code : orange colored strip
1.MBTS
Chemical Name : Dibenzothiazole disulphide (MBTS)
Chemical Structure :
Molecular Weight : 332
Melting point : 170oC
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Ash content, % by mass : 0.5
Solubility : Soluble in chloroform. Insoluble in water
Classification : Semi ultra fast accelerator
Color code : Green colored strip
SPECIAL AGENTS
Anitoxidants
Antioxidants are the chemicals capable of reaching with the forces
(ozone, oxygen, heavy, light, weather and radiation) to prevent or slow down
the polymer breakdown, to improve the ageing qualities and to extend the
service life of the product. The antioxidant used as common proportions about
1.04.0 phr.
HS is mainly using as antioxidants. HS is a grade name, it is Quinoline
type. It is slightly staining , good for oxygen and heat, not so good for flexing
and ozone.
Antiozonant:
An antiozonant, also known as anti-ozonant, is a chemical compound that
prevents or slows down the degradation of material caused by ozone gas in the air
(ozone cracking) Antiozonants are used as additives to plastics and rubber, especially
in the tire manufacturing.
N,N -Dixylene- P-Phenylenediamine
Properties:
Appearance: Blue-brown flakes.
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PIGMENT:
Organic pigment powders:
Organic pigment powders are also useful in the coloring process and to
develop other specific properties in the rubber made products.
Titanium Dioxide:
These good qualities are: high refractive index, consistency in the size of the
particles, dispersibility, and high ultra violet ray resistance.
Florescent pigment:
These requirements are light stability, heat stability, resistance to bleed,
resistance to migration and required shade.
Molybdenum pigment:
Another type of pigments that is used in the rubber industry is Molybdenum
pigment. These pigmemts are light, heat stable and having bright color range from
bright red- orange to red- yellow.
Antistatic
An antistatic agent is a compound used for treatment of materials or
their surfaces in order to reduce or eliminate buildup of static electricity
generally caused by the triboelectric effect. Its role is to make the surface or the
material itself slightly conductive, either by being conductive itself, or by
absorbing moisture from the air, so some humectants can be used. The
molecules of an antistatic agent often have both hydrophilic and hydrophobic
areas, similar to those of a surfactant; the hydrophobic side interacts with the
surface of the material, while the hydrophilic side interacts with the air
moisture and binds the water molecules.
Internal antistatic agents are designed to be mixed directly into the
material, external antistatic agents are applied to the surface.
http://en.wikipedia.org/wiki/Static_electricityhttp://en.wikipedia.org/wiki/Triboelectric_effecthttp://en.wikipedia.org/wiki/Conductivehttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Humectanthttp://en.wikipedia.org/wiki/Hydrophilichttp://en.wikipedia.org/wiki/Hydrophobichttp://en.wikipedia.org/wiki/Surfactanthttp://en.wikipedia.org/wiki/Surfactanthttp://en.wikipedia.org/wiki/Hydrophobichttp://en.wikipedia.org/wiki/Hydrophilichttp://en.wikipedia.org/wiki/Humectanthttp://en.wikipedia.org/wiki/Moisturehttp://en.wikipedia.org/wiki/Conductivehttp://en.wikipedia.org/wiki/Triboelectric_effecthttp://en.wikipedia.org/wiki/Static_electricity7/29/2019 Compounding Technology
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Common antistatic agents are based on long-chain aliphatic amines
(optionally ethoxylated) and amides, quaternary ammonium salts (e.g.,
behentrimonium chloride or cocamidopropyl betaine), esters of phosphoric
acid, polyethylene glycol esters, or polyols. Indium tin oxide can be used as
transparent antistatic coating of windows. It is also possible to use conductive
polymers, like PEDOT:PSS and conducting polymer nanofibers, particularly
polyaniline nanofibers.
Antistatic agents are also added to some military jet fuels, to impart
electrical conductivity to them and avoid buildup of static charge that could
lead to sparks igniting fuel vapors. Stadis 450 is the agent added to some
distillate fuels, commercial jet fuels, and to the military JP-8. Stadis 425 is a
similar compound, for use in distillate fuels. Statsafe products are used in non-
fuel applications.
Vulcanization
The term vulcanization or cross linking refers to a chemical process in
which the uncured, long chain rubber molecules are tied together into a three
dimensional elastic network by the insertion of crosslinks. Vulcanizations
usually refers to the use of sulfur cross-links, where as cross linking can
involve other chemical species i.e., peroxide etc.,
RAW RUBBER VERSUS VALCANISED RUBBER
PROPERTY RAW RUBBER VULCANIZED
RUBBER
Tensile strength 200 kg/cm2 2,000 kg/cm2
Elongation break (%) 1,200 800
http://en.wikipedia.org/wiki/Aliphatichttp://en.wikipedia.org/wiki/Aminehttp://en.wikipedia.org/wiki/Ethoxylationhttp://en.wikipedia.org/wiki/Amidehttp://en.wikipedia.org/wiki/Quaternary_ammoniumhttp://en.wikipedia.org/wiki/Behentrimonium_chloridehttp://en.wikipedia.org/wiki/Cocamidopropyl_betainehttp://en.wikipedia.org/wiki/Esterhttp://en.wikipedia.org/wiki/Phosphoric_acidhttp://en.wikipedia.org/wiki/Phosphoric_acidhttp://en.wikipedia.org/wiki/Polyethylene_glycolhttp://en.wikipedia.org/wiki/Polyolhttp://en.wikipedia.org/wiki/Indium_tin_oxidehttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/PEDOT:PSShttp://en.wikipedia.org/wiki/Polyanilinehttp://en.wikipedia.org/wiki/Nanofiberhttp://en.wikipedia.org/wiki/Jet_fuelhttp://en.wikipedia.org/wiki/JP-8http://en.wikipedia.org/wiki/JP-8http://en.wikipedia.org/wiki/Jet_fuelhttp://en.wikipedia.org/wiki/Nanofiberhttp://en.wikipedia.org/wiki/Polyanilinehttp://en.wikipedia.org/wiki/PEDOT:PSShttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/Conductive_polymerhttp://en.wikipedia.org/wiki/Indium_tin_oxidehttp://en.wikipedia.org/wiki/Polyolhttp://en.wikipedia.org/wiki/Polyethylene_glycolhttp://en.wikipedia.org/wiki/Phosphoric_acidhttp://en.wikipedia.org/wiki/Phosphoric_acidhttp://en.wikipedia.org/wiki/Esterhttp://en.wikipedia.org/wiki/Cocamidopropyl_betainehttp://en.wikipedia.org/wiki/Behentrimonium_chloridehttp://en.wikipedia.org/wiki/Quaternary_ammoniumhttp://en.wikipedia.org/wiki/Amidehttp://en.wikipedia.org/wiki/Ethoxylationhttp://en.wikipedia.org/wiki/Aminehttp://en.wikipedia.org/wiki/Aliphatic7/29/2019 Compounding Technology
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Rapidity of retraction Good Very good
Water absorption Large Small
Swelling in inorganic
solvents
Infinite (soluble) Large, but limited
Tackiness Marked Slight
Useful temperature
range
10 to 60 C -40 to 100 C
Chemical resistance Very poor Much better
Elasticity Very high (300 to
1,000%)
Low depending on
degree of vulcanization
Sulphur vulcanization
To improve the properties of rubber, it is compounded with some
chemicals like sulphur, benzoyl chloride, hydrogen sulphide etc., Most
important is addition of sulphur. The process consists in heating the raw rubber
wit sulphur to 100 to 140C the added sulphur combines chemically at the
double bonds of differ rubber springs. Vulcanization thus serves to stiffen the
material by a sort of anchoring and consequently, preventing intermolecular
movement a rubber springs. The extent of stiffness of vulcanized rubber
depends on the amount of sulphur added.
For e.g. tyre rubber may contain 3-50% sulphur, but a battery case
rubber may contains as much as 30% sulphur
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Vulcanization of raw rubber with sulphur as vulcanizing agents
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PEPTISERS
Incorporation of compounding ingredients is almost impossible unless
the raw material rubber is converted into a plastic stage. This is accomplished
by the action of mechanical shear forces which brings down nerve, molecular
weight of rubber thus making it more easily deformable which is essential for
proper blending of polymer and other ingredients. This process is known as
Mastication.
During this mastication, the rubber chains are ruptured ends of the chain
thus originated would remain active to recombine if not stabilized by some
means. This stabilization of the radicals during mastication is actuated with the
help of certain chemicals known as Peptiser. Since peptisers stablise the free
radical generated, the molecular break down is a permanent one molecular
weight comes down considerably and the incorporation of the other ingredients
is easier.
FILLERS:
CLASSIFICATION OF FILLERS:
The fillers are primarily classified as carbon blacks and light
colored filters. Among the light colored filters chemical composition is
primarily the basis for classification. For example one can list colloidal
silica, Calcium and aluminium silicate, alumina gel, Kaoline, Silica,
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talcum, chalk (calcium carbonate), metal oxide, like zinc oxide and
metal carbonates.
With each class of fillers, different degrees of activity are present.
Basically, most carbon blacks, colloidal silica, and most small particle
size silicates belong to the high-and medium activity fillers, while chalk
belongs to the inactive fillers.
CARBON BLACKS:
GENERAL CONSIDERATIONS AND CLASSIFICATION
The application of carbon black in rubber compounds is over a
hundred years old. Before 1872 only lamp black was utilized as a black
pigment. It was manufactured in China by the deposition of oil flames
onto china plates. After the discovery of the channel black in 1872 the
lamp black, which was only used as an extender, was successively
replaced by channel black. Even though the rubber reinforcement by
channel black was already discovered in 1911, it took until 1940 before
extensive scientific investigations of the mechanism of reinforcement
were undertaken. Because of these development efforts the principles of
the modern gas and oil furnace black manufacture were found , even
though SRF black had been developed already in 1922. Since
approximately 1950 the triumphal progression of the oil furnace black
began and since approximately 1965 the variety of furnace blacks was
extended to new special application areas and special properties. Since
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that time the manufacture of carbon black has been in hectic change.
From the numerous has not stabilized yet.
Aside from the flame, channel, gas and furnace blacks which are
produced by incomplete combustion of oil, coat tar products and natural
gas, the thermal, acetylene and arc blacks play a minor role. The latter
blacks are produced by thermal cracking of natural and coke gas,
acetylene or low molecular weight hydrocarbon gasses. Lately, it was
also attempted to produce carbon blacks from coal, graphite, and other
raw materials.
Carbon blacks are large scale technical products. World
production is roughly 2.5 million metric tons per year. It obtained this
position thanks to its rubber reinforcing properties which is the basis of
todays tire and rubber industry. With respect to reinforcing it has not
been possible to completely replace carbon blacks with other materials.
According to the production process, the U.S war Production
Board in 1943 classified carbon blacks as follows:
F- Furnace Blacks
C-Channel Blacks
T- Thermal Blacks
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Furnace black is today the most important of them. Channel
blacks have practically disappeared from the rubber industry. Thermal
blacks have in recent years been replaced by suitable furnace blacks
because of economic and ecological factors, often with suitable changes
in recipes. Reasons were increase in price of natural gas and costly air
pollution control installations.
The old classification of carbon blacks was based on their historic
names. The High Abrasion Furnace black (HAF) does not provide
high abrasive resistance according todays requirements and is hardly
ever used any longer for tire treads. The High Modulus Furnace black
(HMF) must be categorized today more likely as low modulus black. A
new scheme introduced a few years ago by ASTM tries to account for
todays situation less ambiguously. The type notations consist of a letter
indicating rate of vulcanization (N for normal, S for slow) and three
numbers, the first of which is an index of primary particle size. The use-
directed choice of carbon black types and amounts can be found in the
various chapters on Compounding of the various types of rubbe r: it
does not have to be discussed here in great detail.
FURNACE BLACK:
PREPARATION:
The furnace process the first continuous process for carbon black
production was introduced in 1922. It continued for 20 years with natural gas
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as feedstock and with SRF as the only product. Later, HMF and FF were
added. In 1943 the Oil-furnace process superseded the natural gas based
process. Today all Furnace Blacks are produced from liquid aromatic feedstock
that originates from petroleum fractionation, coal tar distillates or ethylene
crackers. Basically, the feedstock is pre0heated and burned in a reaction zone
with insufficient air supply. The temperature and other conditions are regulated
by burning in the reaction zone auxiliary gas or other secondary feed stocks.
The reaction is quenched b a water spray gas and the black is separated from
the steam/gas mixture in Zyclones or table filters and finally pelleted.
STRUCTURE:
The classification of carbon blacks proceeds according to their iodine
absorption (an index as to the size and activity of the surface) and their
structure that measure the extent of agglomeration and aggregation of primary
particles to form chain-and grape like structures similar aggregates represent
the smallest technically active units even after being mixed into the rubber, in
spite of a certain amount of degradation. The primary particles could be
regarded more as a hypothetic idea. The carbon The Carbon black structure
survives to some extent even in the vulcanizate and is characterized as the so
called F-value. Low structure blacks have been available for at least 25 years.
It has been possible to produce high- structure blacks since the middle 1960s.
Differences in surface and structure show up clearly in the technological
properties of the rubber. Higher iodine adsorption always goes with higher
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reinforcing action (higher activity). High black structure causes good
dispersion in the mixture, higher compound viscosity, and low die-swell with
smooth extrudate surface and with higher modulus, higher hardness and better
wear resistance of the vulcanizate. Low structure blacks give low dynamic heat
build-up, high tensile and tear strength, good crack growth resistance on
flexing and low stress vulcanizate (F-value) and the abrasion resistance when
one keeps rubber-active surface(A-value) constant. Improvement in abrasion
resistance are possible as long as the F-value can be increased.
Classification of Carbon Blacks according to ASTM-D 1765, Typical
Properties
Target Values
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ASTM
Designation
Iodine
Adsorption
No., D 1510
g /kg
DBP No.,
D2414
Cm3/100g
DBP No.,
Compressed
Sample,
D 3493
Cm3/100g
CTAB
D 3765
M2/g
Nitrogen
Adsorption
D 3037
M2/g
Tint
Strength
D 3265
IMPROVED CARBON BLACKS:
Since 1970 improve or new technology carbon blacks have been
developed by international modification of production variables. These blacks
present almost as good processing and vulcanization properties as conventional
types of higher activity and price category, they have therefore gained a large
portion of the market. N-375, N-339,and N-234 are the successful numbers of
these new types of carbon blacks in the USA as well as in Europe. It does not
seem impossible that conventional large-use products like N-220(ISAF) and N-
110 (SAF) will be completely replaced in a few years by new technology-
blacks. Many in-between types like N-242 (ISAF-HS), N-219(ISAF-LS) or N-
440 (FF) are already today commercially unobtainable. There is a trend if not
demand to rationalize among carbon blacks that are shipped in silo-can and
containers to large-scale users. A choice of furnace blacks can be seen in table-
2, resp. in ASTM-D 1765.
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PRODUCERS:
The main carbon black producers are: Ashland, Cabot, Columbian
Carbon, Continental Degussa, Huber and Philips. Cabot is the biggest of them,
followed by Degussa and Columbian Carbon.
Table: 2 (Continued) Classification of Carbon Blacks according to ASTM-D
1765, Typical Properties
ASTM
Designation
Target Values
Iodine
Adsorption
No.,D1510
g/kg
DBP No.,
D 2414
Cm3/100g
DBP.No.,
Compressed
Sample,
D 3493 ,
Cm3/100g
CTAB,
D 3765,
M2/g
Nitrogen
Adsorption
D 3037
M2/g
Tint Strength
D 3265
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THERMAL BLACKS:
N539
N550
N630
N642
N650
N660
N683
N754
N762
N765
N774
N787
N907
N908
N990
N991
43
43
36
36
36
36
35
24
27
31
29
30
111
121
78
64
122
90
133
58
65
115
72
80
34
34
43
35
84
88
62
62
87
75
57
57
86
62
74
40
38
41
42
38
37
38
35
39
29
29
33
29
32
9
8
41
42
38
37
38
35
37
28
31
29
30
11
9
7
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Thermal blacks are generally produced from natural gas in preheated
chambers without air. They are inactive, improve the tensile strength of the
vulcanizates very little, but give only moderate hardness at high loading, and
good processing and dynamic properties . Thermal blacks have in recent years
been in short supply and expensive, but recently Cancarb has increased its
capacity enormously . Their use has been limited to applications (highly loaded
CR-parts: X-LPE, etc.).
CHANNEL BLACKS:
Till the end of world War II the channel blacks were the most important
reinforcement blacks. They have been completely replaced by the
abovementioned furnace blacks that had been developed during the last years
of the war. The furnace blacks in SBR give much better abrasion resistance
than the comparable channel blacks. The channel blacks are more acidic (pH-
value of about 5 compared to furnace black 6.5-10) than the other blacks. They
therefore cause a more or less strong vulcanization retardation.
The Channel blacks are prepared by partial combustion of gaseous
hydrocarbons, mostly gas, through thousands of single burners, are deposited
on cooled steel rings and scraped off and collected.
OTHER CARBON BLACKS:
Aside from the main classes of carbon blacks, one can mention also:
ACETYLENE BLACKS:
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Acetylene Blacks, that are prepared by thermal decomposition of
acetylene, are outstanding because of their high electric conductivity. They are
advantages for many applications where high conductivity is necessary,
respectively where electrostatic charge must be avoided, for example rolls,
tanker hoses, containers for powdered materials. They are frequently today
replaced by conductive furnace blacks.
FLAME BLACKS:
Flame Blacks, prepared from combustion of liquid fuels give at high
loadings good processing properties with attractive dynamic properties. They
are today increasingly exchanged for furnace blacks, especially those with high
structure.
ELECTRIC ARE CARBON BLACKS:
Electric are Carbon Blacks were byproducts from the acetylene
production in the electric are. They are not being produced any longer.
Properties of fillers
TEST
PARAMETERS
ISAF HAF FEF GPF SRF
Moisture
content%
0.5 0.6 0.4 0.5 0.8
Ash content % 0.2 0.2 0.2 0.2 0.2
Iodine 120.6 82.6 43 36 30.7
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adsorption
(g/kg)
DBP absorption
(cc/100g )
115 102.9 121.5 90 72.2
Sieve residue
(325 mesh)
0.036 .008 .032 .040 .036
Bulk density
(g/ml)
- .35 .35 .411 -
Pour density
(g/lit)
340 360 350 420 502
Fitness
content%
2.2 7.0 2.0 2.0 4.1
pH value 7.2 7.9 7.0 7.2 -
NON-BLACK FILLERS:
ACTIVE NON-BLACK FILLERS:
PREPARATION:
The highly active, light colored fillers are, chemically, Silicas (Silicic
acids). They can be manufactured by two methods: Solution Process or
Pyrogenic Process (fumed Silica).
Those most important for the rubber industry are made by precipitation:
Alkalisilicate solutions are acidified under controlled conditions. The
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precipitated silicic acid(silica) is washed and dried. Depending on the condition
during preparation , the silica filler is more or less active. The products with
highest activity are pure silicic acids (silicas) with large specific surfaces. Ca-
silicates are a little less active but easier to process . Al-silicates have in this
series the lowest activity.
In the preparation products are quenched immediately after coming out
of the burner. One obtains very finely divided silica that is important as filler
for example for Q. For the normal types of rubber, pyrogenic silica is too active
and too expensive.
PURE SILICAS:
The pure silicas represent very active fillers. With Comparable specific
surface areas one obtains vulcanizates that compared to reinforcing carbon
blacks, show nearly equal tensile strength and tear resistance, while abrasion
resistance is 15-20% lower. Strain values and hardness are usually lower. On
the other hand, electrical behavior is somewhat better; Silicas with highest
activity, because of their large surface area give mixtures of high viscosity that
makes processing more difficult that can be adjusted by the use of filler
activators.
SILICATES:
Calcium Silicates, that may be called semi-active fillers, give even at
high loadings, soft and elastic vulcanizates . Because of their low activity, they
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process better than the silica fillers. The aluminum silicates are less active than
the calcium silicates and do not produce properties achieved with silicas or
calcium silicates.
CHALK:
Among the different types of chalk one differentiates between milled,
washed and precipitated products, It should be mentioned that it is possible
under certain suitable conditions to precipitate very fine calcium carbonate that
has very small particles and is semi-reinforcing filler.
The different types of chalk mentioned above are different in color and
also in their effect on processing, extrudability and vulcanization. These
differences are not very extensive.
PROPERTIES OF NON-BLACK FILLERS
TEST
PARAMETERS
PPT SILICA FUMED
SILICA
CAL.CARBONATE
Moisture content,
%
4.0 1.21 0.28
BET surface area
(m2/g)
170 200 12.0
Bulk density
(g/ml)
0.13 0.04 0.037
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PEPTISERS
Incorporation of compounding ingredients is almost impossible unless
the raw material rubber is converted into a plastic stage. This is accomplished
by the action of mechanical shear forces which brings down nerve, molecular
weight of rubber thus making it more easily deformable which is essential for
proper blending of polymer and other ingredients. This process is known as
Mastication.
During this mastication, the rubber chains are ruptured ends of the chain
thus originated would remain active to recombine if not stabilized by some
means. This stabilization of the radicals during mastication is actuated with the
help of certain chemicals known as Peptiser. Since peptisers stablise the free
radical generated, the molecular break down is a permanent one molecular
weight comes down considerably and the incorporation of the other ingredients
is easier.
PROCESSING OILS
Processing oils in rubber formulation primarily serve as processing aids.
These oils are used as extenders to reduce the cost of rubber compounding.
CLASSIFICATION
TYPE OF OIL
(PLASTICIZERS)
VISCOSITY
GRAVITY
CONSTANT (VGC)
SOLUBILITY
PARAMETER
(CAL/CM2)
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Paraffinic oil 0.73-0.82 9.2
Naphthenic oil 0.82-0.85 8.2
Aromatic oil 0.95-1.05 7.2
Advantage of Peptisers
Ultimate viscosity of the matrix is considerably reduced. Mixing leads to better dispersion which helps in further processing Total mixing time is sufficiently process Energy requirement in the process is less.
Paraffinic oil
They have straight or branched chains and the chain length increases,
the viscosity also increases and has higher boiling point. Widely used in butyl
and EPDM rubber.
Major features are
High stability and good color can be used in coloured rubber. Highest flash point High temperature processing Excellent low temperature flexibility
DOP oil
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Di-octyle phthalate (DOP) is a high performance primary plastizer. DOP
is a plasticizer that perfects the products and an ideal raw material for an
extensive range of application.
DOP is a stable colourless oily liquid with a characteristic odour. It
possess flexibility, extremely low volatility, good electrical characteristic and
stability to UV light. It is soluble in common organic solvents and mineral oil
but has a very low stability in water good resistance to hydrolysis.
In styrene-butadiene rubber (SBR) the tensile strength remains
approximately constant, while the modulus slightly increases (matching cure)
when the vulcanization time is too long leading to overcure, an important and
sudden reduction of break elongation occurs, which was not noticed in the case
of natural rubber.
Aromatics oil
Aromatics oil blending component products are mixtures of (mainly unsaturated) C9
to C15 components. They originate from the high temperature cracking of petroleum
fractions and are separated out of pyrolysis gasoline (pygas) by distillation during the
production of benzene.Aromatic Oil can be blended into bunker fuel or fluxant.
Aromatic Oil may also be used as a source for naphthalene or mixtures of naphthalene
and methylnaphthalene for concrete plasticizers, phthalic anhydride and insecticides.
Naphthenic oils
In the manufacture of grease, naphthenic base oils in most cases have great
advantages compared with paraffinic oils. This is a generally recognised fact in many
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parts of the world. The basis for these advantages lies in the favourable solubility
properties of naphthenic oils. Hans Bckstrm here comments on different ways of
measuring solubility properties and the way these properties are connected with the
consumption of soap during grease making, an important economic consideration.
Test results show that different solubility concepts, such as aniline point, VGC and
solubility parameters do not always correlate. One method alone perhaps does not
give all the information needed for all applications.
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VULCANIZATION BY ACCELERATORS
Acceleration of vulcanization is done by the accelerator activators like
ZnO and Stearic acid. Organic accelerators have gained importance in rubber
industries because of their following characters.
They increase productivity by reducing the cure cyclic because theycan increase rate of cross link reaction considerably with sulphur.
Combination of two or more accelerators can produce synergesticeffect, providing better result.
By the use of organic accelerator, sulphur can be used moreefficiently to form the need based cross link formation.
Improves the ageing property of rubber and also increases
the plateau effect.
The initial step in vulcanization is the reaction of sulphur with zinc
salt of the accelerator to give a Zinc perthio salt XSxZn SxX.
Where x is a group derived from the accelerator. This salt reacts with
the rubber, hydrocarbon RH to give a rubber bound intermediate and
a perthio accelerator group. Which with further Zinc Oxide will form
a Zinc perthiosalt of cover sulphur content.
XSxZnSx + RH
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In this way each module of accelerator gives rise to a series of
intermediate of varying degree of polysulphidity.
Culcanization conditions
In vulcanization processes, consideration must be made for the
difference in thickness of the objects involved, vulcanization
temperature and the thermal stability of the rubber compound.
Terms involved in Culcanization process
Scorch
Scorch is a premature vulcanization in which the stock becomes
partly vulcanized before the product is in its final form and ready for
vulcanization, it reduces the plastic properties of the compound,
during processing and the amount of time the compound is exposed
to elevated temperatures. The period of time before vulcanization
starts is generally to as SCORCH TIME.
RATE OF CURE
THE Rate of cure is the rate at which cross-linking and the
development of the modulus of the compound occur offer the scorch
point. During the curing step, cross link are introduced, which
connect the long polymer chains of the rubber together. As the more
cross links are introduced, and the modulus of the compound
increases. The rate of curve is an important vulcanization parameter,
it determines the time of compound must be cured is called the cure
time.
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State of cure
As the cross-linking or vulcanization proceeds, the modulus of
the compound increases to various, state of cure. The most
important state of cure is so-called Optimum.
Over cure
A cure which is longer than optimum is an Over cure due to the
over cure the modulus and tensile strength decreases an elongation
fall.
FILLERS
Fillers are thermoplastic and thermosets may be inert materials that
serve to reduce the resin cost and improve process ability or dissipate heat in
exothermic thermosetting rections and also filler in chosen for rubber
compound because of two primary reasons.
1. To improve vulcanization properties2. To reduce the cost of a compound
REINFORCING FILLERS
Reinforcing fillers are used to improve some mechanic property such as
modulus, tensile strength, abrasion resistance and fatigue strength. For
example, particular fillers such as carbon black or silica are widely used to
improve the strength and abrasion of commercial elastomers.
Carbon black
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Carbon blacks are essentially elemental carbon and composed of
aggregated particles. The particles are graphite in structure and are of colloidal
dimensions.
The carbon atoms in the particle are in layer planes which, by parallel
alignment and overlapping, give the particles their semi graphite in nature. The
outer layers are more graphite than those in the centre. The particles range in
size from 100 nm to 400 nm in diameter smaller ones being less graphitic.
MANUFACTURE
Carbon blacks are produced by converting either liquid or gaseous
hydrocarbons to elemental carbon and hydrogen by partial combustion or
thermal decomposition
Application
Carbon black is universal reinforcing filler used in rubber compounds.
Higher reinforcing gives better physical properties like tensile, flexing, tear
resistance and abrasion resistance. Normally three properties of carbon black
are considered in choosing the type of carbon and these are
1. Particle size2. Structure3. Surface areaThe degree of reinforcement increases with decrease in a particle size
i.e., smaller particles give better reinforcement. Higher Mooney but less
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scorches safety having a higher surface area. Carbon black structure has
more effect on the modulus, harness and extrusion die swell.
High structure and higher loading gives higher mooney, higher
modulus and also affect the hardness. For lower structure it is reverse.
NON CARBON BLACK OR WHITE FILLERS
Non black fillers which are used to reduce the cost, to improve
processing by reducing nerve and to reinforce the polymer by
increasing hardness, tensile strength and tear resistance. Abrasion
resistance and other properties in the production of white or coloured
compounds.
They are usually classified as
Fillers used mainly used to reduce cost. Semi reinforcing fillers. Reinforcing fillers used to achieve high
performance in non-black products.
1. PRECIPITATED CALCIUM CARBONATEPrecipitated whitening may be products of water softening processing
are produced from solution of calcium salt, with particle sizes from about 20
um to 50 nm. As semi-reinforcing fillers, high loadings can be used in
mechanical goods and proofing to give low cost products of goods appearance
with moderate hardness and better physical properties than the ground
materials.
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2. ALUMINIUM SILICATEThis fine filler is produced by precipitation. The average particle
size 150 nm specific surface area 50-270m pH of aqueous
suspension9-11 density 2000-2450kgm-3 (depending on the content of
) Moisture content about 6% and the content of bound water 11-
15%.
These are the characteristic properties of hydrated aluminium
silicate which often contains large amounts of Na2O. it is added mainly
to compounds for the production of soles and light coloured consumer
goods. It may be added in large amounts especially to natural rubber
where it improves the extraction properties and calendaring without
reduce the final properties of the rubber. It also increases the resistance
of the rubber to steam and it is used in compounds for electro insulation
purpose. At very high loading it affect vulcanization by its alkalinity and
at normal loading levels it affects the vulcanization only slightly.
FUMED SILICA
Furned or pyrogenic silica is silicon dioxide, containing less than 2%
combined water, usually prepared by burning volatile silica compounds. These
silica are highly reinforeing fillers of very small particles size, giving high
tensile strength, tear resistance, and abrasion resistance, particularly to silicon
rubbers. The retarding effect on cure in organic rubbers requires increased
amounts of accelerators.
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CHEMICAL PLASTICIZERS OF PEPTIZING AGENTS
Certain peptizers added to reduce the viscosity of rubber and to permit
easy processing. Peptizers are giving popularity due to the following reasons.
Energy saving Easier mixing Controlled and consistent viscosity Improved production Reduce rear and tear machinery Improved green tack of unvulcanized components
There are two types of peptizers
a. PCTP (penta chloro thio phenol)b.
DBDS (Dibenzoyl amide diphenyl disulphide)
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