SILICONES FLUIDS - REACTIVE


Reactive Silicone Fluids, can be employed in silicone and elastomeric formulations for a variety of uses, including; as a reactive carrier of additives such as pigments.

The reactive fluid will bond into the silicone matrix and will not leach out over time.

They can be used to modify the physical properties of the elastomer and used as processing aids in the mixing process.

Various functional groups can be introduced into the formulation by grafting into reactive silicones, properties such as high and low temperature, radiation resistance and fuel and oil resistance can all be enhanced.

 

 Silanol Functional Polymers

Unlike the conventional silicone fluids, these fluids are reactive with the hydroxyl groups available and susceptible to condensation under both mild acid and base conditions.

These fluids are used as intermediates in the formulation of most RTV silicone systems. The molecular weight band most typically employed in RTV formulations is between 15,000 – 150,000.

The incorporation of hydride functional silicones into an elastomer formulation will result in the production of a foam, the blowing agent being hydrogen gas, produced in the condensation reaction.

High viscosity fluids are employed as the matrix forming components in tackifiers and pressure sensitive adhesives.


Condensation Cure.
Silanol terminated PDMS’s (Mw of 15,000 – 150,000), are used to formulate
1-part and 2-part Condensation Cure RTVs.


One-part systems: One part systems are widely utilised and work by reacting a multifunctional silane with a silanol terminated silicone, the resulting material is a highly moisture sensitive compound that can be stored in this state, away from moisture.

Curing by crosslinking occurs with reaction to atmospheric moisture.

Typical crosslinkers are acetoxy silanes but oxime, alkoxy and amine silanes are also used.


Two-part systems: These employ ethyl silicates as crosslinkers and alkyl tin catalysts.

Two part systems are inexpensive, require less sophisticated compounding equipment and are not subject to inhibition.


Amino Functional Silicones

As a result of their high reactivity amino functional silicones are utilised in many applications. Aminopropyl terminated PDMSs can react to form a variety of polymers including polyimides, polyureas and polyurethanes.

Amino functional co-polymers are frequently used to modify polymers such as epoxies and urethanes. Applications, such as mould release agents, speciality lubrication, textile coating and polishes have all been demonstrated.


Vinyl Functional Silicones


These materials have a great versatility because to their reactivity. They have two reaction mechanisms,
1, Platinum catalysed addition cure hydrosilylation.
(RTV, LTV, HTV formulation).
Used predominately in the synthesis of silicone elastomers. Vinyl terminated PDMS are usually used in the range of 1,000 to 60,000 cSt, the addition reaction produces no by-products.

Vinyl terminated siloxanes with internal vinyl groups are utilised in gel formation.

2, Peroxide activated free radical cure (HCR formulation).
Extremely high Mw (>500,000) Vinylmethylsiloxane-Dimethylsiloxane co-polymers are typically used and referred to as gums.

These highly viscous polydiorganosiloxanes are converted into an elastic state by cross-linking.

The substitution of the methyl groups with other functional moieties allows the rubbers to be tailor made to suit a particular application or environment..


Hydride Functional Fluids

These silicone fluids have a unique reactivity and are utilised, following three main reactivity mechanisms.

Hydrosilylation – Addition Cure
The hydrosilylation of vinyl functional silicones by MethylHydroSiloxane-DimethylSiloxane co-polymers forms the basis of addition cure chemistry in 2-part RTVs and LTVs.

The above co-polymers are usually favoured over the hydride homopolymers as the chemistry is more controllable producing tougher polymers with a lower cross linking density.

The theoretical stoichiometry of addition cure is 1:1, hydride silicone to vinyl silicone, however in practice this ratio is much higher due to the addition of a suitable filler.

Hydrosilylation of olefins, will generate alkyl and arylalkyl substituted siloxanes which form the basis of organo-compatible fluids.


Dehydrogenative Coupling – Foamed Silicones, Water Repellency
Dehydrogenative coupling occurs between hydride functional silicones and either silanol terminated silicones (to produce foamed materials) or hydroxyl active surfaces (to produce water resistance).

The presence of oxygen and moisture will also affect the foamed structure produced. This method is widely used to impart water repellency to materials such as glass, ceramics, paper products and leather.

Polymethylhydrosiloxanes are more commonly employed, utilising tin catalysts such as Dibutyltindilaurate and Bis(2-ethylhexanoate)tin.


Reduction
Polymethylhydrosiloxanes also acts as a mild, selective low cost hydride transfer agents when catalysised by Pdo .

Reduction of aldehydes and ketones will occur with a catalytical amount of Dibutyltinoxide.

Reduction of olefins, nitro-aromatics and acid chlorides have all been achieved in the presence of Pd/C. Reaction will occur in both organic and aqueous systems.

For a full and detailed procedure on the above reactions types, including necessary materials, conditions and catalysts, please ask for our specific datasheets.

 

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