Silicone rubbers

INTRODUCTION:

The properties of silicone rubber are due to the unusual molecular structure of the polymer which consist of a backbone of silicone atoms.
Silicone rubbers perform unusually well when used as a gasket or o - ring in sealing application.
Over the temperature range of -120°F to +500°F no available elastomers can match it's low compression set.

Properties:

• Wildest service temperature range
• Very good electrical properties,medium oil resistance.
• Application
• Seals and gaskets for aerospace application,automotive and industrial applications.
• Wire and cable for ships,planes

Seals for cold storages,deep freezes.

Service temperature.      Service life

90° C.                      40 years
121° C.                   10-20 years
150° C.                   5-10 years
200°C.                      2-5 years
250° C.                    3 months
315°C                       2 weeks

 Forms of Silicone Rubber 

RTV	HCR	LSR
Room temperature vulcanising	High consistency rubber or Heat cured rubber.	Liquid injection moulding/Liquid
Cure system- Condensation or addition(platinum)	Cure System- Peroxide or Platinum	Cure system- Platinum
Application- Potting Compounds, caulk,fabric, coating	Application- O- ring, Seals, Boots, high Voltage insulators.wires and cables	Typical appllication- baby care products, Connector seals, health care products
Typical processing- Roll coat, Spray on	Processing- Moulding, Extrusion , calendering	Typical processing- Injection moulding

Classification and the uses of silicone polymer

1.MQ -     first silicone polymer
2.MPQ-    low temperature co polymer
3.MVQ-    low compression set co polymer
4.MPVQ-  low temperature terpolymer
5.FVQ-.     Oil resistance polymer.

Dimethyl silicone rubber tends to become stiff below -60 degree fahrenheit. Low temperature felxibility may be improved by substitution of phenyl group for some of the methyl group which extend the useful service temperature range below -130 degree fahrenheit.

Vulcanization:

Silicone rubber compounds are normally heat cured in the presence of one of the  organic peroxide.

For Vinyl and non Vinyl groups :

1. Bis (2,4- dichlorobenzoyl peroxide)

2. Di - Benzoyl peroxide

3. Di- Cumyl peroxide

For Vinyl specific

4. 2,5- Dimethyl 2,5- bis (t-butyl peroxy hexane)

5. Di tertiary butyl peroxide.

Compounding

• The compounding of silicone consists of polymer, reinforcing or extending fillers,process aids or softeners, special additive,colour pigment and one or more peroxide curing agents.

• Typical linear organo substituted polysiloxane gums used in commercial silicone elastomers are amorphous, rubbery, flowable at room temperature (Tg is ~ –123 °C and crystallisation temperature (Tg) about –40 °C). 

•   A typical silicone compound is a multi-component blend of silicone polymers (gums or            bases), reinforcing and/or extending fillers, process aids, various additives (e.g., heat                   stabilisers and blowing agents for sponge), colours/pigments, curing agents and so on.

• Compounding of silicone rubber is identical to any organic rubbers. Selection of the right         silicone gum or base is crucial as the polymer itself varies in terms of mole percentage

   of vinyl, phenyl and methyl contents, molecular weight and viscosity, type of polymer
           structure, and volatile content.

• Fumed silica is the main reinforcing fi ller used for strength,precipitated silica is used for reinforcement as well as economy.
•  Quartz silica, calcium carbonate and aluminium silicates (clays) are used as extending fi llers. Proprietary stabilisers (as manufactured by various silicone and additive manufacturers) are added to improve heat stability at different temperature ranges. 
• Acid scavengers are used toneutralise the acidity, to improve compression set and to stabilise the material with no post curing. Internal mould release additives are added to the recipe especially to moulded rubber compounds for easy removal of parts. 
• Peroxides are used to vulcanise the rubber . Special additives like a desiccant are used to improve the consistency in processing, tensile modifiers are used to improve the tensile properties of rubber, blowing agents for sponge rubber, flame retardant for flame resistance and so on.

       The general Compounding formulation for Silicone rubbers are given below                        

    

Ingredients	Phr
Silicone base	100
Process aids	0.25 to 2
Fumed or Precipitated silica	2 to 50
Quartz silica	10-100
Stabilisers	0.5-2
Peroxide(50	0.75-1.5
colour	0.5-3
Acid Scavenger	1.-6.

Low temperature Flexibility:

Cold resistance of silicone rubber is highly dependent on its structural characteristics. At room temperature , silicone rubber compounds are flexible and under repeated flexing, exhibit excellent flex crack resistant properties. The cold resistance of MQ and VMQ type silicone rubber is about – 55 degree Celsius, while phenyl group containing  silicone rubbers, PMQ and PVMQ can withstand temperature     of   -90 degree Celsius.

 Mechanical Properties

  The tensile strength of silicone rubber is dependent on the type of reinforcing silica fi ller

and the way it is used. It is still diffi cult to raise the tensile strength of silicone rubber

above 15 MPa. But, in the higher temperature applications, the retention of tensile

strength of silicone rubber is much better than that of other synthetic and natural

rubbers.

Mixing:

The most common type of mixing equipment suitable for formulating HCR rubber include two roll mills, Banbury mixers, tilt internal mixers and dough mixers. 

For open roll mill mixing, the rolls should be water-cooled and be drawn with a friction ratio

of 1.2:1 to 1.4:1. 

Silicone rubber will usually transfer to the faster roll during milling due having a lower cohesive strength than most of the organic rubber compounds.

he maximum desirable temperatureof mixing will vary depending on the type of the peroxide and the decomposition

temperature if they are added in single-stage mixing. Care should always be taken about

the batch temperature during mixing and post mixing storage for effective and effi cient

use of vulcanising agents.

 Typically, the polymer is loaded fi rst, followed by the liquid

components, reinforcing fi llers and additives, although this order can be modifi ed to

provide more initial shear by partial incorporation of the fi ller early on.

 Incorporation

of the high surface area reinforcing fi ller is usually the rate-controlling step in achieving

a satisfactory mix. In situ fi ller treatment usually requires a heating cycle. Normally,

it is recommended that at least 24 hours is allowed for maturation prior to use, since

during this maturation period, the fi ller surface is getting ‘wetted’ with polymer which

gives a better batch-to-batch consistency. 

Vulcanization:

Three different types of vulcanisation systems are commonly employed in silicone

processing. In RTV both condensation cure and addition cures are used. In LSR, addition

cure is mostly used and in HCR, both organic peroxide cure and addition cures are

used. Up until now, organic peroxide curing is the most commonly used vulcanisation

method in the majority of HCR silicone elastomers.

Chemical name	Physical form	Typical Cure temperature°C	Recommended dosage level Phr	Classification	Recommended area of use
2,4- Dichloro benzoyl peroxide( DCBP-50)	50%Active paste	90	1.1-2.2	General purpose	Hot air(HAV), LCM, Ballotine (fluidised bed)
Dibenzoyl peroxide(BP-50)	50%Active paste	95	0.7-1.5	General purpose	Moulding steam, LCM, HAV, ballotime
Di(4- methylbenzoyl) peroxide	50%Active paste	105	0.75-1.5	General purpose	Low temperature cure, steam,HAV
Tert- butyl peroxybenzoate	Liquid(100%)Powder (40%)	140	0.25-0.6     0.8-1.7	General purpose	HAV, Steam
Dicumyl peroxide(DCP)	Flakes Powder	160	0.15-0.4,    0.4-0.9	Vinyl Specific	Moulding , steam
Di- tert butyl peroxide	Liquid(100%)Powder (40%)	160-180	0.2-0.4	Vinyl Specific	Moulding
2,5-bis(tert butylperoxy)2,5-dimethy hexane (DBPH)	Powder(50%)  Paste(50%) Liquid(100%)	155	1.0-2.0,      0.8-1.6,      0.4-0.8	Vinyl Specific	Moulding, Steam