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Silicon (no E)
Silicon (Si)• Chemical element No. 14 • A ‘half-metal’:
� Metallic like Aluminium (Si is used in semiconductors)� Non-metallic like Carbon (has 4 bonds & likes oxygen)
• Not freely occurring in nature, but a major constituent of rocks, sand, etc. being 2nd most abundant element in earth ’s crust (~28%).
• Non-toxic
5
4
Silicon to Silicone
methylchloride
Methanol
+sand silicon
C
CO2
‘Rochow’ synthesis(Momentive heritage)
+
methylchlorosilaneswater +
silicone
Si
CH3
CH3
CH3 O Si O Si O
CH3
O
CH3
CH3
Si
O
CH3CH3
Si CH3CH3
H
Si O Si
O
O
O
Si
Si
Si
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3CH3
CH3
CH3
DTQ
M
CH3
Hydrolysis of Mono-, Di-, Tri- & Tetra-methylchlorosilanes generates M, D, T & Q siloxane units that provide unlimited opportunities for molecular architecture
Molecular Geometry
Why are Silicones different?
The silicone backbone is stronger than the carbon-to-carbon chain, making it more resistant to temperature, radiation, oxidation and many chemicals
-Si-O-Si-O-
Silicone
-C-C-C-C-
Hydrocarbon
OrganicSilicone
Why are Silicones different?
The free rotation and flexibility of Si-O bonds enables silicones to ‘twist & turn’, making them more resistant to shear than hydrocarbon chains, which tend to break, and enabling them to orientate better at interfaces.
Why are Silicones different?
• Methyl (CH3) is even more apolar, so the high methyl density of silicones makes them literally ‘oilier than oil’ – they can be surface active even in oils and can ‘reach places’ (e.g. surface tension reduction) that hydrocarbons cannot.
Silicone
• Hydrocarbons are primarily methylene (CH2)
Organic
Why are Silicones different?
Silicone
Due to their high methyl density, silicones have very low intermolecular association
• With themselves: they have glass transition temperatures below -100oC
• With other things: they are powerful lubricants & release agents
Why add Silicones to Plastics ?
Silicones are used to modify:
Surface properties:Release
Improved extrusionAnti-cluster
Surface tension Higher molding accuracyWettability / anti-fog
Particle compatibilzation (pigments)Anti-stat via reduced friction & via humectancyAppearance (gloss)
Bulk properties:Impact resistance (especially at low temp.)Injection flow profile Reduced brittleness / higher flexibility (especially at low temp.)Flame retardancy (in combination with other actives)Improved chemical resistance
Silicones into Plastics
• Appropriately organo-modified silicones, e.g. phenyl silicones, can be dissolved into plastic:
• Silicones can be reacted into the polymer chains of plastics
• Silicones can be surface coated onto plastics.
Silicone• ‘Pure’ silicones (polydimethylsiloxanes) are poorly soluble in almost everything but can be dispersed into plastics:
Organo-modified silicones are surface-active (due to the difference in polarity between the methylated siloxane backbone and the attached organic groups)
Surface Orientation
Silicones added into plastic will therefore tend to migrate to the surface of the plastic
Plastic Plastic
SiliconeOrganic (Polyether, Alkyl etc.)Optional reactive group
Whether added into or applied onto plastic, appropriate organo-modifications can be selected for the chemistry of the plastic so that the silicone will orient on the surface either silicone ‘in’ or silicone ‘out’, making the plastic’s surface respectively either more or less polar.
• Marriage of different properties, e.g. Silicone Polyethers
Z
- Surface affinity- Surface tension reduction
- Leveling- Water solubility
Silicone Copolymers
-Hydrophilicity-Adhesion- Foaming
- Compatibility
Non-Silicone
O
O
O
Z
b
a
-Hydrophobicity-Release
- defoaming- Incompatibility
Silicone
X
CH3 S i O S i O
CH3 C H3
CH3 C H3
Si O
CH3
Si
C H3
CH3
C H3
Y
Structural architecture
SiliconeOrganic (Polyether, Alkyl etc.)Optional reactive group
Pendant
The attachment of organic modifiers can have a geometry as varied as that of the siloxane backbone
Linear (ABA)
(AB)n
Silwet* Benefits
Silwet polyether copolymers:
Can be dispersed into an extruder‘s water bath from which they will adsorp onto the surface of the plastic, providing variously (and often simultaneously):
Drainage of extruder bath water via surface tension reduction
Release (anti-clustering, film-separation, improved processability,...) via silicone backbone
Anti-fog via wettability (e.g. for transparent food packaging in refrigerated displays)
Anti-stat via friction reduction & via humectancy
Gloss enhancement via increased refractive index
Wetting & Spreading
Droplets (equal volumes) of aqueous surfactant (with blue dye for visualisation) on polyethylene
SiO
SiO
SiO
Si
R1 R2x y
Other Copolymers
O Si O Si O Si
x
y
Si
HOO
Si O Si O SiO
OH
xn n
Other (non-polyether) copolymers are used in plastics e.g.:
• GPW2233, an alkyl silicone, for the dispersion of pigments into PP
• TSF437, a phenyl silicone, to impart impact resistance and flame retardancy to polycarbonate while maintaining transparency
• Formasil 889, a microemulsified quaternary ammonium (AB)n, as surface-applied anti-stat
Reactive Silicones
– α, ω-Silanol
– α, ω-Carbinol
– α, ω-Olefin
– α, ω-Epoxy
HO Si O Si O Si O
CH3
CH3
CH3
R'
R"
CH3
x
OHSi
CH3
CH3
y
R Si O Si O Si O
CH3
CH3
CH3
R'
R"
CH3
x
RSi
CH3
CH3
y
R Si O Si O Si O
CH3
CH3
CH3
R'
R"
CH3
x
HO R OHSi
CH3
CH3
y
R Si O Si O Si O
CH3
CH3
CH3
R'
R"
CH3
x
RSi
CH3
CH3
y
OO
Silicones may be reacted into the polymer matrix improving the flexibility of plastic and because the silicone, being surface active, tends to be preferentially at the surface of the plastic, acting as an internal mold-release agent and barrier membrane
Flame retardancy
• Silicone burns to silica (sand)
• Silicones are surface active and so migrate to the surface of the molten plastic
• An effective way to extinguish fires is to pour sand on them !
• Plastic just behind the flame front is moltenMechanism:
SFR100 is a non-toxic FR additive which synergistically enables the replacement of halogenated & antimony FRs in polyolefinsand thermoplastics while simultaneously improving processabilityand mechanical properties.
Copyright 2010 Momentive Performance Materials Inc. All rights reserved. CONFIDENTIAL
20
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