Silicone Rubbers
I.
II.
History
A. Rubber is the generic term used for flexible latex, polyurethane and silicone, but when
customers call asking for rubber, they usually are referring to silicone rubber.
B. In the 1930’s and 1940’s Dow Chemical partnering with Corning Glass, and General Electric both
were attempting to find a material that would insulate electric motors. They came up with glass
fibers combined with phenolic resin, but found the resin did not hold up.
C. Years of research, testing and a grant to Mellon Tech resulted in the development of silicone.
The boot Neal Armstrong wore when he stepped on the moon was made of silicone.
Basic chemistry
A. Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—
containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are often oneor two-part polymers, and may contain fillers to improve properties or reduce cost. Silicone
rubber is generally non-reactive, stable, and resistant to extreme environments and
temperatures from −55 °C to +300 °C while still maintaining its useful properties
B. Polydimethylsiloxane (PDMS) belongs to a group of polymeric organosilicon compounds that are
commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer.
PDMS is optically clear, and, in general, is considered to be inert, non-toxic and non-flammable.
The chemical formula for PDMS is CH3[Si(CH3)2O]nSi(CH3)3, where n is the number of
repeating monomer [SiO(CH3)2] units. Synthesis can begin from dimethylchlorosilane
and water by this net reaction: n Si(CH3)2Cl2 + n H2O → [Si(CH3)2O]n + 2n HCl
III.
Types
A. Single component
1. Silicone calking – high shrink as the solvents evaporate to leave a solid material
2. Silicone oil – excellent lubricant. Stays on, lasts a long time, doesn’t wash off
B. 2 component
1. Commonly 10 parts Base to 1 part catalyst, 1:1 available but less common
2. Platinum (addition) cured
a. Nil shrinkage – except when heated to accelerate cure
b. Inhibition sensitive – amines, latex, sulfur, tin – but not always- test first
i. Nitrogen containing compounds like amines, dbl sided tape, some soaps
ii. Latex, sulfur, tin (if pouring into tin based silicone mold baking the mold
first sometimes works
c. Generally available in higher shore A durometers
d. Clear versions in around 45A range
e. Low tear version for break-away ice
f. Good molds for aliphatic urethanes and Flexible urethanes with Shore A 60 and
lower
g. Food grade (FDA approved) and skin safe available usually in 40A durometer. These
are for novelty use only.
h. Medical Grade silicone: Shin-Itsu KE-1300T is extremely expensive
i. 1-5 minute adhesives using static mixer (XP177 red -1 min & blue5 min)
3. Tin (condensation) cured
a. shrinks – gives off ethanol or methanol when curing causing shrinkage
b. Little to no inhibition problems. Most will cure in presence of sulfur clays and moist
materials
c. Shore A 5 and 00 good for special effects skins and anatomical training aids
d. Generally highest durometer is 60A
e. Available in translucent – good for determining cut lines or seeing flow
f. If shrinkage is a problem, use 5% - 8% catalyst, curing will take a lot longer.
IV.
Properties
A. Mold temperature range and tolerances, continuous & intermittent
1. Tin: 350F continuous or 500 intermittent, but decreases mold life
2. Platinum 500-600F in continuous use, 850F intermittent but decreases life
B. Fillers
1. Included to Increase /decrease hardness
2. Changes specific gravity and volumetric yield
C. Specific Gravities and costs
1. Competition at lower price per pound may really cost more
2. Compute price per cubic inch for comparison instead
The specific gravity times 8.34 equals the weight per gallon.
Weight per gallon times price per pound equals price per gallon.
Price per gallon divided by 231 equals price per cubic inch
D. Vulcanizing
1. products vulcanized at room temperature yield optimum properties
2. Platinums can be accelerated with a third ingredient reducing work time and cure time, or
with heat reducing cure time
3. Tins can be accelerated with an ultrafast catalyst substituted up to 100% of regular catalyst
reducing work time and cure time
4. Ambient temperature as well as temperature of working tools and silicone components can
reduce work & cure times
5. Relative humidity affects cure. For tin cured, lay a moist cloth over mold (not necessarily
touching) to decrease cure time
E. Decay / library life
1. Silicones loose tear strength over time
V.
VI.
2. When mold starts to deteriorate, rejuvenate by wiping with silicone oil, or place mold in
direct sunlight to pull free silicone molecules to the surface
F. Adhesion, primer and release
1. Silicone is silica based and will likely adhere to any other silica based surface when curing,
such as glass
2. Silicone is the ideal mold material when a release agent is not wanted – but test first
3. Petroleum jelly is the best release for silicone. Thin with mineral spirits or kerosene. Be sure
to allow the solvents to evaporate before pouring silicone
4. PVA thinned 10% with water is a good mold release for silicone
5. Physical bond can occur when pouring over porous materials. Be sure to seal those.
6. Silicones can adhere to many substrates by using a silicone primer
G. Tear strength
1. Point at which silicone will tear when elongated
2. Important when the mold with negative draft will be pulled from material poured into it
3. Potting silicones generally have a very low tear strength – high filler low price
H. Hardness(s)
1. Available from gel to 80A durometers
2. Gels may have tacky surface. Powder with talc, baby powder or fumed silica (cab-o-sil) to
remove tackiness
I. Elongation
1. Amount the silicone can be stretched before tearing
2. Break away glass silicones have very low elongation
J. Shrinkage
1. Increases when heat cured
2. Platinum cured silicones should be used when shrinkage is a factor
K. Durometer
1. Can be reduced with the addition of silicone thinner (silicone oil)
2. Varies in hardness points up to 5 points in testing of a material.
3. Can be increased with heat cure: 150F for 1 hour, room temp, then 250F for 1 hour, room
temp, then 350 for 1 hour, room temp. yields excellent mold life
Molds
A. Platinum cured silicones should be used for aliphatic materials such as water clear urethanes,
flexible urethanes Shore A 60 and below, or urethanes that are designed for outdoor use
B. Clear/translucent silicones can be used when it is necessary to view material being cast
C. Will deteriorate over time – usual life is 3 years
D. Materials cast into silicone will chemically attack and reduce the life – pulls vary depending on
the material cast
E. Styrenes cast into silicone tend to cause the mold to swell. Venting helps.
Thickening
VII.
VIII.
IX.
X.
XI.
A. Liquid thixotropic additives are available to turn the liquid to a paste.
B. Fumed silica (cab-o-sil) can be added for desired thickness
C. A layer of the fluid silicone can be applied before adding thickener to maintain surface integrity
of the mold
Thinning
A. Warming the silicone components will thin the catalyzed liquid (and reduce pot life)
B. Up to 8% by weight of silicone thinner (silicone oil) can be added to thin resulting in lower shore
hardness
C. Methylene chloride can thin
Accellerating
A. Increased Relative humidity decreases cure time. A moist cloth above the poured silicone will do
the same.
B. A platinum accelerant is available – but very expensive
C. Curing a platinum silicone in an oven decreases cure time, but causes shrink
D. Curing a tin silicone at a max of 150F decreases cure time (but reduces properties)
E. Ultrafast Catalyst can be added to or substituted for tin catalyzed silicones, changes properties
and reduces work time
F. Heating the silicone components, working tools and ambient temperature reduces work and
cure times. Cuts by approximately half for every 10F.
Degassing & air entrapment
A. Place catalyzed silicone in a container that holds 3x the volume in a vacuum chamber until it
reaches a reading 28-29 degrees of mercury to remove microscopic entrapped air. The liquid
will rise as the air is removed, possibly tripling in size. Turn off and wait for the mercury level to
reduce to normal. Early removal will result in a MESS.
B. If not degassing,(microscopic air bubbles will reduce tear strength but not be seen) either:
Pour liquid continuously into the lowest part and allow to free flow pushing the air out of the
way as it rises;
or brush on a layer of the catalyzed silicone popping noticeable air bubbles with the bristles,
then pour over.
Preserving
A. The mold may produce a white powdery look. This is a sign of decay.
B. Wipe the mold with a silicone oil (thinner) to allow a few more pulls.
C. Place the mold in direct sunlight which will cause free silicone oil in the mold to surface resulting
in a few more pulls
Painting
A. 1 component RTV silicones can be thinned with naptha, mineral spirits or any other silicone
compatible solvent, pigmented with a silicone compatible pigment and used as a paint by
brushing or air-brushing
B. Thin 1 component RTV silicone with mineral spirits and add acrylic universal tint.
XII.
XIII.
XIV.
C. Paint with acrylic universal tint thinned with IPA and water. Let dry and cover with 1 component
silicone thinned with mineral spirits.
Reinforcement
strengthen with PVC mesh or nylon stockings
Uses
1. Shoe industry – molds for shoe lasts
2. Prosthetics – hearing aid holders
3. Medical teaching aids – soft tissues, skin
4. Sculptures – glove molds for waxes
5. Dental – models and molds in th labs
6. Sports – molds to pour pewter fishing lures
7. Foundry – molds for lost wax process
8. Giftware – molds for giftware, doll parts, flower pots
9. Movie industry – Cat woman suit, props and masks
10. Military – molds for training guns
11. Fireplace manufacturers – molds for fireplace surrounds
12. Rapid prototyping – molds for models
13. Aerospace – Platinums can post cure to 75A, arm rest molds for heat cured urethanes
14. Industrial – plugs for sinks
15. Architectural – molds for moldings, gargoyles, trees, light stands
16. Pottery – molds for plaster molds
17. Lubrication – high viscosity silicone oil, eg sprinkler uprights for smooth raising & lowering
Troubleshooting
1. Material won’t cure - Check mix ratio, add heat, check catalyst age, wait
2.