Appendix XX F. Silicone

1. Silicone Oil Used as a Lubricant

(Ph. Eur. method 3.1.8)

DEFINITION

Silicone oil used as a lubricant is a poly(dimethylsiloxane) obtained by hydrolysis and polycondensation of dichlorodimethylsilane and chlorotrimethylsilane. Different grades exist which are characterised by a number indicating the nominal viscosity placed after the name.

Silicone oils used as lubricants have a degree of polymerisation (n = 400 to 1200) such that their kinematic viscosities are nominally between 1000 mm²⋅s^-¹ and 30 000 mm²⋅s^-¹.

CHARACTERS

Appearance Clear, colourless liquid of various viscosities.

Solubility Practically insoluble in water and in methanol, very slightly soluble in anhydrous ethanol, miscible with ethyl acetate, with methyl ethyl ketone and with toluene.

IDENTIFICATION

A. Kinematic viscosity at 25 °C (see Tests).

B. Infrared absorption spectrophotometry (2.2.24).

Comparisonsilicone oil CRS.

The region of the spectrum from 850-750 cm^-¹ is not taken into account since it may show slight differences depending on the degree of polymerisation.

C. Heat 0.5 g in a test-tube over a small flame until white fumes begin to appear. Invert the tube over a 2^nd tube containing 1 mL of a 1 g/L solution of chromotropic acid, sodium salt R in sulfuric acid R so that the fumes reach the solution. Shake the 2^nd tube for about 10 s and heat on a water-bath for 5 min. The solution is violet.

D. In a platinum crucible, prepare the sulfated ash (2.4.14) using 50 mg. The white powder obtained gives the reaction of silicates (2.3.1).

TESTS

Acidity

To 2.0 g add 25 mL of a mixture of equal volumes of anhydrous ethanol R and ether R, previously neutralised to 0.2 mL of bromothymol blue solution R1, and shake. Not more than 0.15 mL of 0.01 M sodium hydroxide is required to change the colour of the solution to blue.

Viscosity (2.2.10)

Determine the dynamic viscosity at 25 °C. Calculate the kinematic viscosity taking the relative density to be 0.97. The kinematic viscosity is within the range 95 per cent to 105 per cent of the nominal viscosity stated on the label.

Mineral oils

Place 2 mL in a test-tube and examine in ultraviolet light at 365 nm. The fluorescence is not more intense than that of a solution containing 0.1 ppm of quinine sulfate R in 0.005 M sulfuric acid examined in the same conditions.

Phenylated compounds

The refractive index (2.2.6) is not greater than 1.410.

Heavy metals

Maximum 5 ppm.

Solvent mixturedilute ammonia R2, 2 g/L solution of hydroxylamine hydrochloride R (1:9 V/V).

Mix 1.0 g with methylene chloride R and dilute to 20 mL with the same solvent. Add 1.0 mL of a freshly prepared 0.02 g/L solution of dithizone R in methylene chloride R, 0.5 mL of water R and 0.5 mL of the solvent mixture. At the same time, prepare the reference solution as follows: to 20 mL of methylene chloride R add 1.0 mL of a freshly prepared 0.02 g/L solution of dithizone R in methylene chloride R, 0.5 mL of lead standard solution (10 ppm Pb) R and 0.5 mL of the solvent mixture. Immediately shake each solution vigorously for 1 min. Any red colour in the test solution is not more intense than that in the reference solution.

Volatile matter

Maximum 2.0 per cent, determined on 2.00 g by heating in an oven at 150 °C for 24 h. Carry out the test using a dish 60 mm in diameter and 10 mm deep.

LABELLING

The label states:

— the nominal viscosity by a number placed after the name of the product;

— that the contents are to be used as a lubricant.

2. Silicone Elastomer for Closures and Tubing

(Ph. Eur. method 3.1.9)

DEFINITION

Silicone elastomer complying with the following requirements is suitable for the manufacture of closures and tubing.

Silicone elastomer is obtained by cross-linking a linear polysiloxane constructed mainly of dimethylsiloxy units with small quantities of methylvinylsiloxy groups; the chain ends are blocked by trimethylsiloxy or dimethylvinylsiloxy groups.

The general formula of the polysiloxane is:

The cross-linking is carried out in the hot state:

— either with:

— 2,4-dichlorobenzoyl peroxide for extruded products; or

— 2,4-dichlorobenzoyl peroxide or dicumyl peroxide or OO-(1,1-dimethylethyl) O-isopropyl monoperoxycarbonate or 2,5-bis[(1,1-dimethylethyl)dioxy]-2,5-dimethylhexane for moulded products;

— or by hydrosilylation by means of polysiloxane with -SiH groups using platinum as a catalyst.

In all cases, appropriate additives are used such as silica and sometimes small quantities of organosilicon additives (α,ω-dihydroxypolydimethylsiloxane).

CHARACTERS

Appearance Transparent or translucent material.

Solubility Practically insoluble in organic solvents, some of which, for example cyclohexane, hexane and methylene chloride, cause a reversible swelling of the material.

IDENTIFICATION

A. Infrared absorption spectrophotometry (2.2.24) by the multiple reflection method for solids.

Comparisonsilicone elastomer CRS.

B. Heat 1.0 g in a test-tube over a small flame until white fumes begin to appear. Invert the tube over a 2^nd tube containing 1 mL of a 1 g/L solution of chromotropic acid, sodium salt R in sulfuric acid R so that the fumes reach the solution. Shake the 2^nd tube for about 10 s and heat on a water-bath for 5 min. The solution is violet.

C. 50 mg of the residue of combustion gives the reaction of silicates (2.3.1).

TESTS

If necessary, cut the material into pieces of maximum dimension on a side of not greater than 1 cm.

Solution S

Place 25 g in a borosilicate-glass flask with a ground-glass neck. Add 500 mL of water R and boil under a reflux condenser for 5 h. Allow to cool and decant the solution.

Appearance of solution

Solution S is clear (2.2.1).

Acidity or alkalinity

To 100 mL of solution S add 0.15 mL of bromothymol blue solution R1. Not more than 2.5 mL of 0.01 M sodium hydroxide is required to change the colour of the indicator to blue. To a further 100 mL of solution S, add 0.2 mL of methyl orange solution R. Not more than 1.0 mL of 0.01 M hydrochloric acid is required to reach the beginning of the colour change of the indicator from yellow to orange.

Relative density (2.2.5)

1.05 to 1.25, determined using a density bottle with anhydrous ethanol R as the immersion liquid.

Reducing substances

To 20 mL of solution S add 1 mL of dilute sulfuric acid R and 20 mL of 0.002 M potassium permanganate. Allow to stand for 15 min. Add 1 g of potassium iodide R and titrate immediately with 0.01 M sodium thiosulfate using 0.25 mL of starch solution R as indicator. Carry out a blank titration using 20 mL of water R instead of solution S. The difference between the titration volumes is not more than 1.0 mL.

Substances soluble in hexane

Maximum 3 per cent.

Evaporate 25 mL of the solution obtained in the test for phenylated compounds in a glass evaporating dish on a water-bath and dry in an oven at 100-105 °C for 1 h. The residue weighs not more than 15 mg.

Phenylated compounds

Place 2.0 g in a borosilicate-glass flask with a ground-glass neck and add 100 mL of hexane R. Boil under a reflux condenser for 4 h. Cool, then filter rapidly through a sintered-glass filter (16) (2.1.2). Collect the filtrate and close the container immediately to avoid evaporation. At wavelengths from 250 nm to 340 nm, the absorbance (2.2.25) is not greater than 0.4.

Mineral oils

Place 2 g in a 100 mL conical flask containing 30 mL of a mixture of 5 volumes of ammonia R and 95 volumes of pyridine R. Allow to stand for 2 h, shaking frequently. Decant the pyridine solution and examine in ultraviolet light at 365 nm. The fluorescence is not greater than that of a solution containing 1 ppm of quinine sulfate R in 0.005 M sulfuric acid examined in the same conditions.

Volatile matter

Maximum 0.5 per cent for silicone elastomer prepared using peroxides; maximum 2.0 per cent for silicone elastomer prepared using platinum.

Weigh 10.0 g of the substance previously stored for 48 h in a desiccator over anhydrous calcium chloride R. Heat in an oven at 200 °C for 4 h, allow to cool in a desiccator and weigh again.

Silicone elastomer prepared using peroxides complies with the following additional test:

Residual peroxides

Maximum 0.08 per cent calculated as dichlorobenzoyl peroxide.

Place 5 g in a borosilicate-glass flask, add 150 mL of methylene chloride R and close the flask. Stir with a mechanical stirrer for 16 h. Filter rapidly, collecting the filtrate in a flask with a ground-glass neck. Replace the air in the container with oxygen-free nitrogen R, introduce 1 mL of a 200 g/L solution of sodium iodide R in anhydrous acetic acid R, close the flask, shake thoroughly and allow to stand protected from light for 30 min. Add 50 mL of water R and titrate immediately with 0.01 M sodium thiosulfate, using 0.25 mL of starch solution R as indicator. Carry out a blank titration. The difference between the titration volumes is not greater than 2.0 mL.

Silicone elastomer prepared using platinum complies with the following additional test:

Platinum

Maximum 30 ppm.

In a quartz crucible, ignite 1.0 g of the material to be examined, raising the temperature gradually until a white residue is obtained. Transfer the residue to a graphite crucible. To the quartz crucible add 10 mL of a freshly prepared mixture of 1 volume of nitric acid R and 3 volumes of hydrochloric acid R, heat on a water-bath for 1-2 min and transfer to the graphite crucible. Add 5 mg of potassium chloride R and 5 mL of hydrofluoric acid R and evaporate to dryness on a water-bath. Add 5 mL of hydrofluoric acid R and evaporate to dryness again; repeat this operation twice. Dissolve the residue in 5 mL of 1 M hydrochloric acid, warming on a water-bath. Allow to cool and add the solution to 1 mL of a 250 g/L solution of stannous chloride R in 1 M hydrochloric acid, rinse the graphite crucible with a few millilitres of 1 M hydrochloric acid and dilute to 10.0 mL with the same acid.

Prepare simultaneously the reference solution as follows: to 1 mL of a 250 g/L solution of stannous chloride R in 1 M hydrochloric acid, add 1.0 mL of platinum standard solution (30 ppm Pt) R and dilute to 10.0 mL with 1 M hydrochloric acid.

The colour of the test solution is not more intense than that of the standard.

LABELLING

The label states whether the material was prepared using peroxides or platinum.