Instructions to Test-Pour
Elastomer Material by Hand Mixing
For Gel Polyurethane
Polyurethane gel material is used in many applications for cushioning, vibration
damping, and other effects. Applications for polyurethane gel elastomer
includes shoe insoles, bicycle sheets, computer keyboard and mouse pad wrist
rests, motorcycle seating, axially seat cushion for automobiles, wheel chair
sheets, shock absorbing parts for sports equipment, medical and personal care
parts, horse-back riding saddles, vibration damping for machines, and many other
temperature curable polyurethane materials in general are relatively easy to
process with minimum investment into tools and equipment. However, there are
some techniques you may want to learn before you start manufacturing your
parts. We recommend testing with small quantities to learn the properties and
behaviors of the material.
of the polyurethane gel formulations are made in such way that the user or the
parts manufacturer can control the softness of the gel product by changing the
mixing ratio of part-A and part-B components. Through your tests, you may want
to determine many processing aspects as well as the optimum softness of the gel
part for your application. Generally, increasing part-B ratio will give softer
products. The mixing ratio for your formulation is not shown in this document
as each formulation has different mixing ratio. Please refer to other document
provided by, or ask, Northstar Polymers for mixing ratio recommendation for your
Polyurethane elastomer gels are made by mixing the two components; we call those
two components prepolymer and curative. The prepolymer is also
called part-A in short. The curative is called part-B. The name
for the combination to make a product (solid, elastomer, or foam polyurethane)
is called a system. To achieve your desired softness, you may also
incorporate plasticizer. Plasticizers are inert liquid materials widely
used in many synthetic materials. They are added for a few different purposes.
you open the containers, you should prepare the following items.
Containers to weigh and mix
mixing container can be a regular poly-bucket, paint cans, cups, or lab beaker.
Make sure the inside of container is clean, dry, and free of any solvent. Some
polyurethane materials are very sticky. It is not easy to clean the container
after use. Use a disposable container.
digital scale that shows integer values and first and second decimal numbers is
convenient. A lab scale may be very handy because you can reset to zero after
you measure one part with container. You can also use measuring cups. Please
note, for the same formulation, the volume and weight ratios are different.
Keep your mixing ratio record uniform in terms of weight or volume. Please
refer to the ratio information provided by Northstar Polymers.
handle any polyurethane material, you need to wear safety gloves. Disposable
safety gloves made of Latex, Nitrile, Neoprene, or other similar material will
be appropriate. If you need a source for the gloves and other paraphernalia,
here are some web pages you can browse and telephone numbers.
sure the entire operation is done in a well-ventilated area or large room with
air circulation. Dynamic means of ventilation (fans) are recommended. Aside
from the gloves, you should also wear long sleeves and safety glasses to prevent
skin/eye contact of the material. If the chemical gets on to your skin/eye,
wash it off with soap and water as soon as possible. A washing facility should
be readily available in the working area. The material is relatively safe for
an industrial chemical, but it is still harmful to health if it enters a human
body through inhalation, skin/eye contact, and ingestion. Read the Material
Safety Data Sheet (MSDS) for more information. The MSDS are shipped together
with each component material.
Plastic or stainless steel flat-end spatula, paint stick, table knife, or any
stick clean, dry and free of solvent. Avoid using wood stir sticks. You can
use a hand mixer if you are making a large batch. If you are to use hand-held
power mixer for mixing, we recommend a paint mixer that is made to prevent
folding air into the material.
need to use a mold release for demolding. We recommend pure silicone brushable
mold release, because you can apply it right anytime pouring and easy
maintenance. You can also use silicone dissolved within a solvent. If it is
mixed with a solvent, make sure the solvent is dry before pouring. Do not use
water base mold release. Hydrocarbon base (mineral spirit, kerosene, etc.) mold
release takes longer to dry. Many solvents are flammable. Please follow the
instruction on the mold release for safety. Wax mold release can also be used.
7. Oven (Optional)
part-A material is very thick (high viscosity) and heating makes it thinner. To
make it easy to handle and mix well, you can heat the material between 100 and
180º F (38 to 82 ºC. This is an optional, and if you do not have an oven, you
can omit this, but it may be difficult to handle the part-A material and to
Heating material increases
chance of the materials to vaporize. Be sure to wear protective glasses when
handling heated materials. Do not put your face directly over the heated
do not want bubbles in the material, you can use a vacuum chamber to de-gas.
This is also an option. With hand mixing, you cannot completely avoid bubbles.
For your future production, using casting machine with vacuum capability will
allow you to produce bubble free products.
is for cleaning the tools. You can use Isopropanol, dibasic ester, MEK,
acetone, or other ketones. Isopropanol is easier to obtain. You may obtain it
in a local drug store. Always dry solvent completely before reusing your
tools. Mineral spirit or paint thinner does not dissolve this material well.
Aromatic hydrocarbon solvent has some solubility to this material. The most of
solvents are flammable with high VOC rate. Follow the safety instructions of
the solvent material when using.
cleaning. If you use cloth towel or rag, expect you cannot re-use. Clean the
items with paper towels and isopropanol, dispose of it as industrial solid
waste. Do not drain any of this material down into the public drain system or
you are done with mixing, the part-A material needs to be blanketed with
nitrogen gas and the lid has to be closed tight for future use. You will need
the nitrogen gas in future for your production, too. If you need, call Praxair
800-772-9247 for your local distributor. We use a 50 LBS cylinder at our lab.
It is portable if you use a cart.
reason for doing this is that the part-A has isocyanate, which is very
reactive. It is so reactive that it reacts to any form of water in contact. If
you leave this material out, it will react with the moisture in the air, which
will spoil the material. When part-A is reacted with moisture in the air, it
will harden by it self and you cannot use anymore. Nitrogen will not react to
isocyanate, so it will keep the quality for longer time. - 40 ºF dew point dry
air can also be used to blanket the components.
are using a small amount at a time, you may want to repackage part-A into
smaller size paint cans. This will minimize opening of the can and in case it
is contaminated, the damage will be minimum. If repackaging, fill it closer to
the top and minimize the space between the inside top of the can and the
Heat the part-A material (100 to 140 ºF) in an oven (if needed).
Prepare the mold. Apply the mold release lightly. Be sure to dry the solvent (if
*If you are casting right
into a film encapsulation, you may not need the release agent.
Calculate the correct amounts of each part-A and part-B material.
Open the can of part-A pour the necessary amount into the container as you
weigh. Be careful if you are handling a heated material.
Close the can of part-A. When you close the can of part-A, be sure to blanket
the material with nitrogen gas and close the lid tight.
Open the can of part-B, pour into the container as you weigh necessary amount.
Close the lid of part-B. The order of part-A and part-B to be poured into the
mixing container is not critical. Pour whichever convenient first.
Generally, part-B materials are not as much moisture-sensitive as part-A, so you
do not need to blanket with nitrogen gas. However, it is a good idea to close
the lid tight to avoid moisture and other contamination, when you store.
mixing by hands, use plastic or steel stir-stick and agitate vigorously, but try
not to be enclosing or folding-in air. In other words, "do not whip" like
beating egg. If you do not heat part-A, it may take longer time to mix. Mix
carefully and thoroughly so the components are mixed homogeneously without any
material is less than a half gallon, mix at least for one minute. Scrape the
side and bottom to make sure it is mixed very well. Even if the material
appears to be mixed, they are often not enough. Thorough mixing is very
use a hand-held electric mixer, mix at the low speed. High speed can ruin the
material by creating folding-in the air or inducing heat. Mechanically scrape
the sides of the container is still needed for thorough mixing since the mixture
is very thick.
Place the container in a vacuum chamber (Option)
Ideally, put 29" Hg or more vacuum until you see most of the bubbles are gone.
Do not leave the material too long as it may induce a quick reaction with heat.
Pour into the mold
part is small, you can pour in one puddle. If you pour bigger parts, avoid
pouring layers; pour straight from one end to the other and try not to come
back. If you are pouring a flat part with different depth, pour the deeper end
first and move to the shallower end.
time between you mix the part-A and part-B and the time the material becomes too
thick to pour is called "pot life". Usually our standard gel materials are made
for 15 to 20 minutes pot-life, unless specified other wise. Curing time pattern
varies depending on the system, and it can be controlled by catalysts to meet
your production requirement. Consult Northstar Polymers for your specific
curing time requirements.
Clean up the tools with isopropanol and paper towels
testing, cure over night before de-mold. The material hardens gradually.
De-molding of parts differs depending on the part sizes, curing pattern of the
material, and complexity of the part. Generally, you can demold the part in
about 5 to 6 hours if you are cuing at a room temperature. At this point
material should be cured enough so that you can handle it as a solid piece.
However, the material is not completely cured at this point. The hardness of
the gel gradually increases over 7 days or so. Usually this is not a problem at
the production level since often your part will not be in service for about one
week for packaging and shipping etc.
Heating (up to 180 ºF) will accelerate curing and ensure the part integrity when
demolding. When you are evaluating the mechanical properties of the material
for your specific application, conduct your tests at least 7 days after the date
you make the part. If you need to test sooner, you can post cure at 180 ºF for
16 to 24 hours and cool to room temperature.
above curing pattern is all under the standard catalyst level and can be
modified to meet your requirement. Please consult Northstar Polymers for
slowly and carefully. It may be difficult to demold if you have not put enough
mold release. The time between when you mix and the material becomes hard
enough to demold is called "demolding Time". Demolding time is also variable by
concentration level of catalyst.
surface of gel parts may be very sticky. You may need to develop your method to
handle your demolded gel parts. Use of talc powder may alleviate the problem.
Because of the stickiness, this gel material is often used with an encapsulation
material. Thin and flexible polyurethane films are often used because of their
strength and flexibility. Coated Lycra fabric is also used for covering. Coated
fabric can be used as encapsulation as well as covering material. If any fabric
is used to contain the material when it is liquid, the fabric material needs to
be coated for leak-proof. Vacuum forming technique is often used to shape films
and coated fabrics. Dispersion coating (silicone rubber, polyurethane, or other
flexible coating materials) can also be used to cover the surface of gel.
Modification of the system
time needed to completely cure the material is called "Complete Cure Cycle".
This is an important element to your productivity. Shorter the complete cycle,
better the productivity will be. However, the pot life, demolding time, and
complete cure time are all related. If you make one element shorter, other
elements also get shorter. We can control the cure pattern by modifying the
formulation, but we cannot shorten one element significantly faster or slower
without changing others.
modifications including change of mixing ratio and adding colorant, UV absorbing
agent, anti-bacteria agent, and other additives may also be available.
can add a plasticizer to modify the properties of the product. Plasticizers are
widely used in plastic and elastomer industry for softening the products and
other purpose. In many years, plasticizer could gradually leak or vaporize from
gel parts over time. It is generally true for most of plasticizers when they
are incorporated into plastic or elastomer materials. We recommend testing and
study the long term effect if you are using plasticizers.
Storage of materials
contains reactive isocyanate and needs to be blanketed by nitrogen gas or minus
40 ºF (40 ºC) du-point dry-air. Keep the lids shut tight. Store the components
at 70 - 77 ºF (21 to 25 ºC) degrees. Under the right condition, the material
should last at least six mouths. Repackaging into one-shot size can prevent
having a large amount of material contaminated from air/moisture leak into the
dispose of the materials into any public sewage system or unlawful area.
Generally, you can dispose of mixed and cured material as a solid industrial
waste. Consult your local authorities for the disposal information. Please read
the enclosed Material Safety Data Sheet or detail information.
Prepolymer free NCO %
NCO % is level of NCO sites available for reaction within the prepolymer.
Prepolymer is produced under our specification and the quality stays consistent
within the specification. However, even within the specification, some
sensitive application may be effected by the slight fluctuation of free NCO %
within our specification. In general, higher NCO % results harder products. If
your application is very sensitive to the hardness, you may need to adjust the
mixing ratio for a particular lot of material. If that is the case, please
consult with Northstar Polymers for methods of calculation for ratio adjustment.
standard gel materials are not classified as a fire retardant material. Some
applications including automotive, and airplane parts require fire retardant
property. This material cannot be used for those applications. Please check
the relevant regulations for your application. Modification may be done to our
standard formulation to enhance fire retardant properties. Please consult
Northstar Polymers for details.
Persons with isocyanate hypersensitivity
normal operations, this material should have MDI in vapor under the threshold
limit value of 0.02 ppm. However, an individual who has been sensitized by an
isocyanate (TDI/MDI/IPDI etc.) may have hypersensitivity reactions such as
irritation of the eyes and respiratory tract (similar to asthma-like responses)
even under this threshold value. To minimize the risk due to the exposure to
the materials containing an isocyanate, operate under well-ventilated condition
(dynamic ventilation), wear protective cloths and gloves should be worn. A
half-faced respirator is also recommended for extra protection.
The component materials are industrial-grade chemicals. Please keep them in a
secure place and prevent access from any unauthorized individual. The personnel
who handles these materials needs to read the Material Safety Data Sheet (MSDS)
for detail information on safety and handling of the material. The MSDS for
each component is sent with the shipment of the material.
When using this material, be sure to operate in a wide-open area with good air
movement, or in a well-ventilated area. Wear rubber gloves, long sleeves, and
protective eyeglasses to prevent skin/eye contact of the material. When your
operation involves heating or spraying of the material, we recommend, in
addition to the above, installation of a proper ventilation system such as a
overhead hood with a dynamic ventilation system, and/or using a proper half-face
respirator recommended for the use to prevent inhalation of the fume. Extreme
heat should be avoided when storing.
Direct contact of polyurethane raw materials to skin/eye, as well as ingestion
may lead to health problems. No eating or smoking should be permitted at the
working area. The operator should wash hands well with soap and water after
handling the materials. Please refer to the MSDS for each component for the
detailed health information.
For any questions, please contact Northstar Polymers.