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Basic Silicone Chemistry
What are Silicones?
• are synthetic polymers with a linear, repeating silicon-oxygen
backbone, the same bond that is found in quartz, glass and
sand.
• Silicone polymers do not have carbon as part of the backbone
structure
• Have a high melting and boiling point
• Depending on the number of repeat units in the polymer chain
and the degree of cross-linking, six classes of commercially
important products can be produced
• Some disadvantages of Silicone are increased costs
and lower mechanical property values when compared to
carbon based materials
Forms & Compositions
• Fluids, Emulsions, Compounds, Lubricants, Resins and rubbers
• Varies from liquid to gel, or rubber to hard plastic
• odorless and colorless, water resistant, chemical resistant, oxidation
resistant, stable at high temperature, and have weak forces of attraction,
low surface tension, low freezing points and do not conduct electricity
• seem to be impervious to the effects of aging, weather, sunlight,
moisture, heat, cold, and some chemical assaults
Silicone Family Tree
Elastomers
Fluids & Emulsions
Silanes
Silicone
Resins
Dimethyl
Compounds
Silicone Polyethers
Organo-Silicones
Volatile Methyl
Siloxanes
Amino Silicones
Si
Flexibility of Siloxane Chemistry
•
•
•
•
•
•
•
Non-volatile
Antifoam
Slippery
Water Insoluble
Excellent Depth of Gloss
Incompatible in Organics
Durable
•
•
•
•
•
•
•
Volatile
Profoam
Sticky
Water Soluble
Shiny
Compatible
Transient
HISTORY
The first silicone elastomers were developed in thesearch
for better insulating materials for electric motors
andgenerators. Resin-impregnated glass fibers were the
state-of-the-art materials at the time. The glass was very
heat resistant, but the phenolic resins would not
withstand higher temperatures that were being
encountered in new smaller electric
motors.Chemists at Corning Glass and General Electric
were investigating heat-resistant materials for use
as resinous binders when they synthesized the first
silicone polymers, demonstrated that they work well and
found a route to produce polydimethylsiloxanen
commercially.
PROPERTIES
Silicone rubber offers good resistance to extreme temperatures,being
able to operate normally from -55°C to +300°C.At the extreme
temperatures, the tensile strength, elongation, tear strength
and compression set can be far superior to conventional rubbers
although still low relative to other materials.Organic rubber has a
carbon to carbon backbone which can leave them susceptible
to ozone, UV, heat and other ageing factors that silicone rubber can
withstand well. This makes it one of the elastomers of choice in
many extreme environments.Compared to other organic rubbers,
however, silicone rubber has a very low tensile strength. For this
reason, care is needed in designing products to withstand even low
imposed loads. Silicone rubber is a highly inert material and does
not react with most chemicals. Due to its inertness,it is used in
many medical applications and in medical implants. However, typical
medical products have failed because of poor design.
STRUCTURE
silicone rubber chain
Polysiloxane differ from other polymers in that their
backbones consist of Si-O-Si units unlike many
other polymers that contain carbon backbones. One
interesting characteristic is an extremely low glass
transition temperatureof about - 127˚C.
Polysiloxane is very flexible due to large bond angles and
bond lengths when compared to those found in more basic
polymers such as polyethylene. e.g. A C-C backbone
unit has a bond length of 1.54 Å and a bond angle
of 112˚, whereas the siloxane backbone unit Si-O has a
bond length of 1.63 Å and a bond angle of 130˚.
Silicone rubber chain
The siloxane backbone differs greatly from the basic
polyethylene backbone, yielding a much more flexible polymer.
Because the bond lengths are longer, they can move further and
change conformation easily, making for a flexible advantage of
polysiloxanes is in their stability. Siliconis in the same group
(IV) on the periodic table as carbon, but the properties of these
elements are quite different. Silicon has the same oxidation state
as carbon, but has the ability to use 3D orbitals for bonding by
expanding its valence shell. Si-Si bonds have far less energy than
C-C bonds and so are more stable, though in practice Si-Sibonds are very hard to create.
Repeat unit of Silicone Rubber
Silicone Nomenclature
Si
SILICON
O
SILICA
O Si O
O
SILANES
X
X Si
X
X
R
SILOXANES
O Si
R
O
Silicone Nomenclature Shorthand
Precursor
Silanol
Siloxane Structure
Short hand
Me
Cl-Si-Cl
Me
Me
HO-Si-OH
Me
Linear Structures
D unit
End-cap group
M unit
Me
Me-Si-Cl
Me
Me
Me-Si-OH
Me
Me
Cl-Si-Cl
Cl
Me
HO-Si-OH
OH
Branched Structures
T unit
Cl
Cl-Si-Cl
Cl
OH
HO-Si-OH
OH
Silica Core
Q unit
Me Me Me Me Me
Me-Si-O-Si-O-Si-O-Si-O-Si-Me
Me Me Me Me Me
=
Me Me Me
Me-Si-(O-Si)3-O-Si-Me
Me Me Me
=
MD3M
SILICONES APPLICATIONS
Dow Corning’s products and specialty materials are used by
customers in virtually every major industry.
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Aerospace
Automotive
Chemicals/ Petrochemicals
Construction
Consumer Products
Electrical/Electronics
Food Processing
Industrial Maintenance Production
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Medical Products
Paints & Coatings
Personal, Household & Automotive
Care
Pharmaceuticals
Plastics
Pressure-Sensitive Adhesives
Textiles & Leather
Synthesis of Silicones
The most common method for preparing silicones involves reacting
a chlorosilane with water. This produces a hydroxyl
intermediate, which condenses to form a polymer-type structure.
The basic reaction sequence is represented as:
Raw Materials
• Initial material is quartz
– SiO4/2
– 26% of the Earth’s crust
• Reduce to Si metal with carbon at 2500F
•
• Methanol is converted to MeCl with recycled HCl
Process Chemistry of Methyl Train
Me2SiCl2
MeHSiCl2
Me3SiCl
Chlorosilane
Mix
Si
MeCl
Copper Catalysts
H2O
Me2 Hydro
SiH fluid
EBB
Waste &
Recovery
Silicone Classifications by Physical Form
(1) Fluids (hydraulic, release agents, cosmetics, heat transfer
media, polishes, lubricants, damping, dry cleaning)
Polymer chains of difunctional units (D) terminated with monofunctional (M)
units OR cyclics (Dx)
(2) Gums (high temperature heat transfer fluids, lubricants,
greases, cosmetic and health care additives)
Same structure as PDMS fluids, but much higher molecular weight (viscosities
>1,000,000 cSt).
(3) Resins (varnishes, protective coatings, release coatings,
molding compounds, electronic insulation)
Rigid solids based on trifunctional (T) and tetrafunctional (Q) units. Surface
modification with (M) units
(4) Elastomers (Heat cured and RTVs: tubing and hoses,
medical implants, sealants, adhesives, surgical aids,
electrical insulation, fuel resistant rubber parts, rollers, etc)
Soft solids based on crosslinked SiH Fluids
This is the favoured route although other raw materials such as
alkoxysilanes can be used. Chlorosilanes and other silicone
precursors are synthesised using the “Direct Process”,
involving the reaction of elemental silicone with an alkyl halide.
thus, preparation of silicone elastomers requires the formation of
high molecular weight (generally greater than500000g/mol). To
produce these types of materials requires di-functional
precursors, which form linear polymer structures. Mono and trifunctional precursors form terminal structures and
branched structures respectively.
Si + RX →RnSiX4-n (where n = 0-4)
Other components – curing additives
• With the exception of RTV and liquid curing systems,silicone
rubbers are usually cured using peroxides suchas benzoyl
peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate
and dicumyl peroxide.
• Alkyl hydroperoxides and dialkyl peroxides have also been
used successfully with vinyl containing silicones.
• Hydrosilylation or hydrosilation is an alternative curing method
for vinyl containing silicones and utilises hydrosilane materials
and platinum containing compounds for catalysts.
• It is a 2-part process requiring mixing of 2 separatecomponents,
with the resulting material having alimited shelf life.
•
Fillers
• Reinforcing fillers are added to improve the otherwise poor
tensile strength of silicones.
• Silica, in the form of silica fume with particle sizes in the range
10-40nm is the most preferred filler,although carbon black has
been used.
• Fillers do interact with the vulcanisate, forming apseudovulcanisation. This can occur either during mixing (creep
hardening) or in storage (bin ageing).
• Examples of these materials are siloxane-based materials such as
diphenylsilane and pinacoxydimethylsilane.
Other Additives
• Silicones have better fire resistant properties compared to natural
rubbers. This property can be improved by addition flame
retardant additives such as platinum compounds,carbon black,
aluminium trihydrate, zinc or ceric compounds.
• It should be noted that carbon black addition also increase
electrical conductivity.
• Ferric oxide may also be added to improve heat stability,
titanium dioxide and other organometallic compounds as
pigments.
Volatile Polydimethylsiloxane Fluids
Cyclomethicone
CH3
Si - O
CH3
CH3
CH3
Si
n
CH3
CH3
n = 3 Trimer
n = 4 Tetramer
n = 5 Pentamer
n = 6 Hexamer
CH3
O
O
CH3
Si
Si
O
O
Si
CH3
O
Si
CH3
CH3
CH3
PENTAMER (D5)
Volatile Polydimethylsiloxane (PDMS) Fluids
R = CH3
INCI: Dimethicone
R = OH
INCI: Dimethiconol
CH3 CH3
CH3
R - Si - O - Si - O - Si - R
CH3 CH3 m CH3
When m = 0, R= CH3 called Hexamethyldisiloxane or 200 Fluid,0.65 cS
(.65,1, 1.5 and 2.0 cS are volatile)
Properties of Siloxanes
• Despite the Fact that Silicon and Carbon are both Group IV
elements their chemistry is very different
• Unique flexibility of Si-O bond
•
bond length
bond angle
bond energy
bond barrier
Si-O-Si
1.63
130
106
0.2
C-C-C
1.54
112
83
3.6
C-O-C
1.42
111
86
2.7
units
angstroms
degree
Kcal/mol
Kcal/mol
Siloxane Polymers vs Carbon Polymers
•Barrier to Rotation ( kcal/mole )
–Polyethylene
3.3
–Polytetrafluoroethylene
4.7
–Polydimethylsiloxane
< 0.2
Key Point: Siloxane (Si-O-Si) polymers are
stronger than carbon polymers, yet the polymer
chains are more open and flexible
Siloxane Physical Properties
• Very low glass transition temperature (Tg = -120 °C)
– high molecular weights but not a solid
• Ability to spread out on a wide variety of substrates
– silky, smooth, non-tacky, aesthetic enhancing
– flowability and film forming
• Lowest surface shear viscosity and low surface tension
– lubricating, antifoaming, waterproofing, release properties
• High gas permeability
• Excellent dielectric properties
• Very good thermo-oxidative stability
– good chemical inertness and temperature resistance
Anionic Ring Opening Equilibrations
Me
Me
D4
Si
O
Me
Me
O
KOH
Si
HO
Me
Me
Si
Si
O
Si
Me
Me
Si
K
O
3
O
O
Me
Me
Me
Me
Me
K
O
Me
Si
O
Si
Me
O
K
D4
K
n
O
Si
Me
O
Me
Me
R
R
R
Si
O
R
Ring
10-15%
:
:
Chain Equilibrium
85 – 90%
Si
O
Si
Me
R
R
O
Si
n
R
Si
R
Me
R
R
PDI = 2.0
K
O
7
Me
Me
Si
R
R
Anionic Ring Opening Equilibrations
R
End Blockers
R
Si
R
CH3
H3C
Si
R
O
Si
R
R
CH3
O
Si
CH3
CH3
CH3
H2NCH2CH2CH2
CH3
CH3
Si
O
Si
CH3
CH2CH2CH2NH2
CH3
O
CH3
O
H2N
Si
O
CH3
O
Si
CH3
CH3
O
NH2
Si
CH3
CH3
O
CH3
O
Si
CH3
O
Anionic Ring Opening Equilibrations
CH3
End Blockers
H3C
Si
CH3
Viscosity
CH3
O
Si
CH3
CH3
Maximum in viscosity involves incorporation
of end blocker (which is less reactive than cyclic)
time
Living Anionic Ring Opening Polymerization
H3C
CH3
Si
O
O
cyclohexane
sec-Butyl – Li +
H3C
Si
Si
O
H3C
CH3
H3C
CH3
CH2 CH
Me
CH3
Si
O
2
CH3
CH3
Si
O
Li
Me
D3
10% THF
D3
R
CH3
H3C
CH2 CH
Me
CH3
Si
CH3
O
Si
n
Me
Cl
R
O
Si
R
Si
R
R
CH3
H3C
CH2 CH
Me
CH3
Si
O
Si
R
CH3
LiCl
Living anionic polymerization
n
Me
O
Li
Industrial classification
Industrial Classifications:
There are three main industrial classifications of silicone rubbers:
•
High Temperature Vulcanising (HTV)
–
Sometimes called heatcurable, these are usually in a semi-solid gum form in
theuncured state. They require rubber-type processing to producefinished items.
•
Room Temperature Vulcanising (RTV)
–
Usually come as aflowable liquid and are used for sealants, mould
making,encapsulation and potting. These materials are not generallyused as
conventional rubbers.
•
Liquid Silicone Rubbers (LSR)
–
Sometimes called heat curableliquid materials, these materials are processed on
speciallydesigned injection moulding and extrusion productionequipment.
Liquid Silicone Rubbers
• These are essentially two-part systems, supplied deaerated
ready fo ruse often in premetered equipment. Low injection
pressures and low pressure forming techniques are sufficient.
• They cure after mixing the two separate portions, by processes
such as hydrosilylation. Curing is often complete in as little as
a few seconds at temperatures of about 200°C and post-curing
is notusually required.
• The low capital investment required for production mean that
LSR scan compete with conventional silicones and organic
rubbers.
• Physical properties are comparable to general purpose grades
and high strength peroxide cured elastomers.
• Furthermore, they exhibit self-extinguishing properties, with
carbonblack additions .
Room Temperature Vulcanising (RTV)
Rubbers
• These are available in one (RTV-1) and two-part (RTV-2)
systems. Single part systems consist of polydialkylsiloxane with
terminal hydroxyl groups, which are reacted with organosilicon
cross-linking agents. This operation is carried out in a moisturefree environment and results in the formation of a tetrafunctional
structure.Curing takes place when materials are exposed to
moisture. Atmospheric moisture is sufficient to trigger the
reaction, and thickness should be limited if only one side is
exposed to the moisture source. Curing is also relatively slow,
reliant on moisture ingress into the polymer.
Two pack systems can be divided into two categories,
condensation cross-linked materials and addition cross-linked
polymers. Condensation systems involve the reaction of
silanol-terminated polydimethylsiloxanes
with organosilicon cross-linking agents such as Si(RO)4
Storage life depends on the catalyst employed and ambient
conditions. Addition-cured materials must be processed under
clean conditions as curing can be affected by contaminants
such as solvents and catalysts used in condensation RTVs.
These materials are suited to use with polyurethane casting
materials.
• Phosphazenes are a class of chemical compounds in which a
phosphorus atom is covalently linked to a nitrogen atom by a
double bond and to three other atoms or radicals by single bonds.
While other substitutions produce relatively persistent
compounds, in organic synthesis the term largely refers to
species with three amino substituents bound to phosphorus. The
compounds are unusually stable examples of the phosphorane
class of molecules and have a remarkable proton affinity. As
such, they are one of the eminent examples of neutral, organic
superbases. Two examples are hexachlorocyclotriphosphazene
and bis(triphenylphosphine)iminium chloride. Phosphazenes are
also known as iminophosphoranes and phosphine imides.
• Phosphazene bases are strong non-metallic non-ionic and lownucleophilic bases. They are stronger bases than regular amine or
amidine bases such as Hünig's base or DBU. Protonation takes
place at a doubly bonded nitrogen atom. Related to phosphazene
bases are the proazaphosphatrane bases, which have a saturated
P(NR)3 structure and protonate at phosphorus.
• Though the simplest phosphazene superbase, P1-Me, was first
synthesized in 1975, chemists assumed that the compounds were
highly unstable, like their alkyl-substituted derivatives. The
species was regarded at that time as little more than an academic
curiosity.
• By now phosphazene bases are established reagents in organic
synthesis. Perhaps the best known phosphazene bases are BEMP
with an acetonitrile pKa of the conjugate acid of 27.6 and the
phosphorimidic triamide t-Bu-P4 (pKBH+ = 42.7) also known as
Schwesinger base after one of its inventors.[2]
• In one application t-Bu-P4 is employed in a nucleophilic addition
converting the pivalaldehyde to the alcohol:[3
• The active nucleophile is believed to be a highly reactive
phosphazenium species with full negative charge on the arene
sp2 carbon.
• Besides organic synthesis, phosphazene bases are used as basic
titrants in non-aqueous acid-base titration. Their advantages for
this are: they are very strong bases in many solvents and their
conjugate acids are inert and non-HBD cations.