`POLYDIMETHYLSILOXANE
`
`Prepared at the 69th JECFA (2008), published in FAO JECFA
`Monographs 5 (2008), superseding specifications prepared at the 37th
`JECFA (1990), published in the Combined Compendium of Food
`Additive Specifications, FAO JECFA Monographs 1 (2005). A
`temporary ADI of 0-0.8 mg/kg bw was established at the 69th JECFA
`(2008).
`
`Poly(dimethylsiloxane), dimethylpolysiloxane, dimethylsilicone fluid,
`dimethylsilicone oil; dimethicone; INS No. 900a
`
`Polydimethylsiloxane consists of fully methylated linear siloxane
`polymers containing repeating units of the formula [(CH3)2SiO] with
`trimethylsiloxy end-blocking units of the formula (CH3)3SiO-. The
`additive is produced by hydrolysis of a mixture of
`dimethyldichlorosilane and a small quantity of trimethylchlorosilane.
`The average molecular weights of the linear polymers range from
`approximately 6,800 to 30,000.
`
`(NOTE: In commerce, polydimethylsiloxane is frequently used in
`preparations usually containing silica gel. The pure substance
`described in this monograph can be isolated from silica gel-containing
`liquids by centrifuging at about 20,000 rpm. Before testing the
`Polydimethylsiloxane for Identification, Refractive index, Specific
`gravity, and Viscosity, any silica gel present must be removed by
`centrifugation.)
`
`(NOTE: This monograph does not apply to aqueous formulations of
`Polydimethylsiloxane containing emulsifying agents and preservatives,
`in addition to silica gel.)
`
`
`α-(Trimethylsilyl)-ω-methylpoly(oxy(dimethylsilylene))
`
`9006-65-9
`
`
`
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`n ranges from 90 to 410
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`Silicon content not less than 37.3% and not more than 38.5% of the
`total
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`Clear, colourless, viscous liquid.
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`SYNONYMS
`
`DEFINITION
`
`
`Chemical names
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`C.A.S. number
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`Structural formula
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`Assay
`
`
`DESCRIPTION
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`1
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`FUNCTIONAL USES
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`CHARACTERISTICS
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`IDENTIFICATION
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`Solubility (Vol. 4)
`
`Specific gravity (Vol. 4)
`
`Refractive index (Vol. 4)
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`Infrared absorption
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`PURITY
`
`Loss on drying (Vol.4)
`
`Viscosity
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`Lead (Vol. 4)
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`
`TESTS
`PURITY TESTS
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`Viscosity
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`Antifoaming agent, anticaking agent
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`Insoluble in water and in ethanol; soluble in most aliphatic and
`aromatic hydrocarbon solvents
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`25 : 0.964 - 0.977
`d 25
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`D : 1.400 - 1.405
`n 25
`
`The infrared absorption spectrum of a liquid film of the sample
`between two sodium chloride plates exhibits relative maxima at the
`same wavelengths as those of a similar preparation of USP
`Dimethylpolysiloxane Reference Standard (available through
`http://www.usp.org/referenceStandards/catalog.html or by mail to USP
`12601 Twinbrook Pkwy, Rockville, MD 20852 USA).
`
`
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`Not more than 0.5% (150o, 4h)
`
`
`100 - 1500 cSt at 25 o
`
`See description under TESTS
`
`Not more than 1 mg/kg
`Determine using an AAS/ICP-AES technique appropriate to the
`specified level. The selection of sample size and method of sample
`preparation may be based on principles of methods described in
`Volume 4 (under “General Methods, Metallic Impurities”).
`
`
`
`
`
`The Ubbelohde suspended level viscometer, shown in the
`accompanying diagram, is preferred for the determination of the
`viscosity.
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`2
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`

`

`
`(Dimensions in mm)
`
`
`
`
`For use in the range of 100 to 1,500 centistokes, a No. 3 size
`viscometer, having a capillary diameter of 2.00 + 0.04 mm, is required.
`The viscometer should be fitted with holders that satisfy the
`dimensional positions of the separate tubes as shown in the diagram,
`and that hold the viscometer vertical. Filling lines in bulb A indicate the
`minimum and maximum volumes of liquid to be used for convenient
`operation. The volume of bulb B is approximately 5 ml.
`
`Calibration of the viscometer
`Determine the viscosity constant, k, for each viscometer by using an
`oil of known viscosity. [NOTE: Choose an oil with a viscosity as close
`as possible to that of the sample to be tested.] Charge the viscometer
`by tilting the instrument about 30 degrees from the vertical, with bulb A
`below the capillary, and then introduce enough of the sample into tube
`1 to bring the level up to the lower filling line. The level should not be
`above the upper filling line when the viscometer is returned to the
`vertical position and the sample has drained from tube 1. Charge the
`viscometer in such a manner that the U-tube at the bottom fills
`completely without trapping air.
`
`After the viscometer has been in a constant-temperature bath long
`enough for the sample to reach temperature equilibrium, place a finger
`over tube 3 and apply suction to tube 2 until the liquid reaches the
`center of bulb C. Remove suction from tube 2, then remove the finger
`from tube 3 and place it over tube 2 until the sample drops away from
`the lower end of the capillary. Remove the finger from tube 2, and
`measure the time, to the nearest 0.1 sec required for the meniscus to
`pass from the first time mark (T1) to the second (T2). In order to obtain
`accurate results within a reasonable time, the apparatus should be
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`3
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`

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`adjusted to give an elapsed time of from 80 to 100 sec.
`
`Calculate the viscometer constant k by the equation
`
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`k = v/t1,
`
`
`in which v is the viscosity, in centistokes, and t1 is the efflux time, in
`sec, for the standard liquid.
`
`Viscosity determination of Polydimethylsiloxane
`Charge the viscometer with the sample in the same manner as
`described for the calibration procedure; determine the efflux time, t2;
`and calculate the viscosity of the sample, vs, by the equation
`
`
`vs = kt2.
`
`
`METHOD OF ASSAY Principle
`Silicon in the sample is converted to a soluble form by fusion with
`sodium peroxide. Soluble silicon is measured in the percent range as
`total silicon by atomic absorption spectrophotometry.
`
`Apparatus
`- Fusion apparatus: Parr-type fusion cup; 500-ml nickel beaker; and
`nickel lid for beaker - or equivalent (avoid use of glassware during
`fusion and solubilization).
`- Instrument: atomic absorption spectrophotometer with silicon hollow
`cathode lamp; nitrous oxide - acetylene burner, or equivalent.
`
`
`Reagents
`- Sodium peroxide, glacial acetic acid, silica (of known purity for use
`as standard).
`
`
`Procedure
`[CAUTION: Normal safe laboratory practices for Parr-type bomb
`fusion should be followed.]
`
`Equivalent fusions must be performed on sample(s), reagent blank(s)
`and silica standards for each series of samples. For each sample
`weigh a portion (W) not to exceed 0.3 g into a Parr-type fusion cup
`(use gelatin capsules for liquid samples). Add 15.0±0.5 g of sodium
`peroxide.
`
`
`Assemble the fusion apparatus and place it in a protective ignition
`rack. Fill the cavity above the cap with water and keep it full during
`ignition to prevent the gasket from melting. Heat the bottom of the cup
`with a blast lamp until the cup becomes cherry red about 100 mm up
`from the bottom within 90 sec. Quench the apparatus in ice water and
`disassemble the apparatus. Place the cup in the nickel beaker
`containing 150 to 200 ml of distilled water. Rinse any material
`adhering to the inside of the assembly cap into the beaker with
`distilled water. Cover the beaker with the nickel lid. When dissolution
`is complete and the solution has cooled, remove the cup from the
`beaker and rinse it with distilled water into the beaker. Add 55.0 ml of
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`4
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`reagent grade glacial acetic acid to the beaker. Cool the solution to
`room temperature and transfer it to a 500 ml volumetric flask. Dilute to
`volume with distilled water. The solution should contain about 100 μg
`silicon/ml for a sample weight of about 0.13 g. This method performs
`best if the silicon concentration of the final analysis solution is 1 to 200
`μg/ml. Prepare a series of standards using the same fusion technique
`that brackets the sample.
`
`Measure the absorbance of sample(s), reagent blank and standards at
`251.6 nm with the spectrophotometer according to the manufacturer’s
`operating instructions to obtain optimum analysis conditions for
`maximum lamp output and fuel and oxidant flow rate to the burner (or
`equivalent procedures for other vaporizing techniques). Adjust the
`zero absorbance while aspirating the solvent blank (water) used to
`dilute the samples. Measure the absorbance of sample(s), reagent
`blank and standards. Estimate the concentration of silicon in the
`sample solution from the standards, correcting for the reagent blank.
`Calculate the percent total silicon in the sample by the equation
`
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`%Silicon = 0.05 x C/W
`
`
`where
`C is the silicon concentration of the sample solution (µg/ml)
`W is the weight of sample taken (g)
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`5
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