Dealing With Frozen Rubber
Astlett Customer Help Sheet
Rubber may arrive
at a factory in hard or frozen form. Three
factors affect rubber hardness:
viscosity inherent at time of manufacture.
cross-linking that occurs continuously, from time of production
until processed, to all grades (CV grades very slowly),
therefore slowly increasing viscosity. Also called storage
hardening. Rubber will have aged for varying times
prior to consumption. Rubber stores well and its qualities
do not degrade.
occurs at ambient temperatures and accelerates at lower
temperatures. This phenomenon is sometimes called freezing
and is reversible by the application of heat. Any lot
transported during the North American cool season can
be expected to be "frozen" to greater or lesser extent.
Almost all rubber will have some degree of crystallization.
Strain hardening may be mistaken for freezing. It occurs when rubber flows under pressure. The stretching or distortion of the raw rubber can result in partial crystallization, making the affected rubber tougher and lighter colored. Strain hardening is typically observed where rubber has flowed into cracks between pallet boards and on bale edges extruded into voids between bales. This may occur at any temperature and is not reversible by heating but is reversible by mastication.
What is Freezing?
Freezing is an exothermic reaction where the rubber hydrocarbon
goes to a crystalline formation. The color lightens and slight
shrinkage may be observed.
We do not warrant that any rubber
will not be frozen.
Any freezing (crystallization)
that may occur is reversible by using premastication or by
conditioning in a hot room. Even fresh rubber may need premastication
(due to its toughness) or thawing (due to crystallization
in transit in cool climates).
Freezing occurs from the outside inward from the exterior
surfaces of the pallet unit. Also, if shipped in an ocean
container, the surfaces against the exterior of the container
will freeze first and, over time, deepest. Freezing is a relatively
slow process and will be partial (in the direction it is losing
heat) before it can become complete (i.e. to the interior
of the pallet unit).
Partially frozen units may be
usable without conditioning as only a hard crust is encountered
on the sides exposed to the freezing temperature. The backside
or inside of each bale may be unfrozen.
Signs of freezing are very hard
bales, lighter color, and slight shrinkage. We recommend that
every rubber factory use a hot room to condition natural
rubber before use. All major rubber factories have hot rooms.
Test for Degree
The degree of freezing can be determined by gauging the thickness
of the frozen layer. Beyond 2" or 5 cm on more than three
surfaces of a bale, premastication or thawing is indicated.
One method of gauging is to drill
into the rubber and then probe the hole with an instrument
(screwdriver) to determine elasticity. Alternatively, guillotine
a bale in half and check the crystallization visually or by
This test should also be done
on inside surfaces and on pallet unit sides away from the
cold side, as these surfaces may not be frozen.
Minor conditioning can be done by separating the bales for
easy airflow in a warm area. Frozen rubber is typically conditioned
in a hot room heated from 160 to 180 degrees Fahrenheit for
four to six days.
Conditioning is always recommended,
as mixing times are reduced, strain on machinery is less and
peak power loads are lower. This is also true in the absence
of any freezing.
Any expense for conditioning
natural rubber is assumed by the factory. Most factories routinely
condition natural rubber in hot rooms prior to mixing.
A hot room is a dedicated area with warm air circulating
around the rubber to be thawed or conditioned. If no regular
hot room is available, a temporary hot room can be constructed.
Portable propane blast heaters, as used in the construction
industry and available from rental dealers, together with
tarps to control airflow, can be used to set up an area to
thaw rubber. Arrange the tarps in a 'tent' around the spaced
rubber crates (for easy circulation to all sides) and blow
in hot air. Maximize circulation and temperature (up to 80°C/180°F)
and check daily for thawing. A rubber temperature of 30°C/85°F
is the maximum required to reverse all crystallization.
Pallets of natural rubber held
in a room heated to 50°C/120°F for two weeks or 70°C/160°F
for one week will thaw solidly (to the center, 100% crystallization)
frozen pallets. Less time will be required when the rubber
is partially frozen. Faster conditioning can be done by separating
the bales for easier airflow.
When the rubber becomes elastic
again and the color darkens, it is thawed.
Ideally storage temperature should
not fall below 15°C for more than two months to avoid