“Curing” or “Vulcanization” is the process by which individual polymer
molecules are linked to other polymer molecules (here, rubber) by atomic bridges. It
enhances and changes many properties of rubber like making it hard, resistant to
chemical attack, stickiness and so on.
The major process that goes on during tire curing is vulcanization. The process is
also called tire “cooking”. The semi-finished input to the curing machine is the green
tire or the bald tire. After that they are mounted on rubber diaphragms and placed in
steam-jacketed moulds with all the details of the finished tire engraved in reverse.
The rubber diaphragms are inflatable and are inflated to a pressure required to
press the tread against the patterned mould. Then heat is applied on the mould.
Temperatures over 300 degree centigrade and pressures as high as 400 psi are sustained
for as long as 12 – 25 minutes depending on tire size and weight. The combination of
high pressure from within the rubber diaphragm and the high temperature on the heated
mold mold the tire tread pattern and sidewall markings into the surface of the tire.
The heat and pressure actually causes chemical reactions which permanently
transforms the pliable plastic material into an elastic material bonded to other rubber
components and textile and steel components of the tire.
All the liner marks that exist in a green tire are not seen in a properly cured tire.
To accelerate the curing process, accelerators are added or the tire components
are irradiated prior to vulcanization. The tire may be coated with a liquid to ensure that it
does not stick to the mold.
Vulcanisation Agents
?Accelerators & activators : Chemicals that increase the rate of
vulcanisation.
?Retarders : Chemicals that decrease or delay vulcanisation process.
? Impart an induction period before curing starts(scorch time).This
way, one avoids premature vulcanization due to the use of high
activity accelerators
.
Tire Cure As Related to Compound Cure
? The term “optimum cure” is the time for vulcanisation of rubber compound
to yield the required level of physical properties.
? In the laboratory, using a rheometer, torque- time curve( rheograph) at a
constant temperature ( usually at 141.7°C ) is generated for a given rubber
compound.
? The optimum cure is taken as the time for 90% rise in the recorded torque
for a rheograph.
? The rise in torque is considered to be proportional to the rise in cross-link
density and hence represents 90% completion of the cross-link reaction.
? At optimum cure, the physical properties like modulus, hardness, resilience, abrasion resistance and fatigue resistance reach optimum values
molecules are linked to other polymer molecules (here, rubber) by atomic bridges. It
enhances and changes many properties of rubber like making it hard, resistant to
chemical attack, stickiness and so on.
The major process that goes on during tire curing is vulcanization. The process is
also called tire “cooking”. The semi-finished input to the curing machine is the green
tire or the bald tire. After that they are mounted on rubber diaphragms and placed in
steam-jacketed moulds with all the details of the finished tire engraved in reverse.
The rubber diaphragms are inflatable and are inflated to a pressure required to
press the tread against the patterned mould. Then heat is applied on the mould.
Temperatures over 300 degree centigrade and pressures as high as 400 psi are sustained
for as long as 12 – 25 minutes depending on tire size and weight. The combination of
high pressure from within the rubber diaphragm and the high temperature on the heated
mold mold the tire tread pattern and sidewall markings into the surface of the tire.
The heat and pressure actually causes chemical reactions which permanently
transforms the pliable plastic material into an elastic material bonded to other rubber
components and textile and steel components of the tire.
All the liner marks that exist in a green tire are not seen in a properly cured tire.
To accelerate the curing process, accelerators are added or the tire components
are irradiated prior to vulcanization. The tire may be coated with a liquid to ensure that it
does not stick to the mold.
Vulcanisation Agents
?Accelerators & activators : Chemicals that increase the rate of
vulcanisation.
?Retarders : Chemicals that decrease or delay vulcanisation process.
? Impart an induction period before curing starts(scorch time).This
way, one avoids premature vulcanization due to the use of high
activity accelerators
.
Tire Cure As Related to Compound Cure
? The term “optimum cure” is the time for vulcanisation of rubber compound
to yield the required level of physical properties.
? In the laboratory, using a rheometer, torque- time curve( rheograph) at a
constant temperature ( usually at 141.7°C ) is generated for a given rubber
compound.
? The optimum cure is taken as the time for 90% rise in the recorded torque
for a rheograph.
? The rise in torque is considered to be proportional to the rise in cross-link
density and hence represents 90% completion of the cross-link reaction.
? At optimum cure, the physical properties like modulus, hardness, resilience, abrasion resistance and fatigue resistance reach optimum values
Temperature Coefficient of Vulcanisation
The factor by which the rate of vulcanisation increases when the
temperature is increased through a constant rate (say 10°C) is called the
temperature coeff: of vulcanisation.
As a thumb rule reaction doubles for every 10°C increase in
temperature.
But this coeff. is not constant for all compounds and temperature
ranges.
The factor by which the rate of vulcanisation increases when the
temperature is increased through a constant rate (say 10°C) is called the
temperature coeff: of vulcanisation.
As a thumb rule reaction doubles for every 10°C increase in
temperature.
But this coeff. is not constant for all compounds and temperature
ranges.
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