74 KENDALL AND PHILLIPS: ESTIMATION OF THE THICKNESS OF [Vol. 75 Estimation of the Thickness of Wax “Bloom” on Vulcanised Rubbers BY F. KENDALL AND MISS W. M. PHILLIPS SyNoPsIs-The method consists in cutting a known area of the rubber without disturbing the surface bloom, and wiping the surface with four or five successive twists of de-fatted cotton wool, held by cleaned forceps and moistened with light petroleum. The twists are transferred to a prepared extraction funnel and the fat extracted from them and weighed. The thickness of the bloom is calculated from the area of the specimen and the weight and specific gravity of the wax recovered. CERTAIN waxes, when incorporated into a rubber mixing, “bloom” to the surface soon after vulcanisation. This bloom forms some protection against the deterioration of strained rubber which takes the form of surface cracking and which is now known to be caused by atmospheric 0zone.l In an investigation of this protective effect it was important to be able to estimate the thickness of the surface bloom of wax.Several methods for determining the thickness of the bloom have been investigated. They include methods based on removal by scraping, optical interference caused by the thin film, electrical resistivity of the rubber with and without the bloom, difference of focus of a microscope at the top and bottom of the film of bloom, microscopical examination of cut sections of the rubber, and removal of the bloom by wiping with solvent-damped cotton wool and subsequent determination of the wax in the wool. Of these several methods only two, the cutting of sections and the removal of bloom by cotton wool, proved both easy and satisfactory, although, doubtless, optical interference methods could be made satisfactory if adequate apparatus were available.A technique for cutting sections of the rubber was developed so that as far as possible the wax film was cut cleanly without fracture. By microscopical examination of the section the film thickness was measured with an eyepiece scale and a stage micrometer. Polarised light was used to provide greater contrast between wax film and rubber surface. This method gave reproducible results when used for wax blooms not less than 3 microns thick, but was found to be unsuitable for thinner films. METHOD ADOPTED FOR ROUTINE USE After trial, the removal of the wax bloom by wiping with cotton wool moistened with a solvent and the subsequent quantitative extraction of the wax from the wool was adopted as the preferred-method.Knowledge of the area of rubber involved and the specific gravity* of the wax enables the thickness to be calculated from the weight of wax recovered. It was found convenient to cut disc specimens 5 cm. in diameter from sheet by means of a press cutter, but specimens of other shapes are equally satisfactory provided disturbance of the wax bloom is avoided in cutting. PROCEDURE- Wash the appropriate fingers of the hand with solvent (light petroleum, b.p. 40” to 60” C.) and twist pre-extracted cotton wool round the end of a pair of cleaned forceps. Moisten the cotton wool with solvent and wipe the rubber surface.The wiping action should be rapid but should not involve rubbing or heavy pressure. After wiping transfer the twist of cotton wool to a prepared extraction funnel. Wiping treatments with four successive twists was found sufficient for most blooms, but a fifth twist should be ysed for heavy blooms. The total amount of cotton wool used for the four or five twists should be such that, when placed in the extraction funnel and extracted with the solvent, it should not expand above the rim of the funnel. Extraction of wax from cotton wool-The apparatus used was the micro-extraction apparatus described by G. H. Wyatt.2 * This is best determined by a flotation method such as is given in Standard Methods for Testing Pejrojeum and its Prudircts, Institute of Petroleum, Ninth Edition, London, 1948.Cut out a specimen of suitable size and determine its area.Feb., 19501 WAX "BLOOM" ON VULCANISED RUBBERS 75 Weigh a clean extraction cup. (This can be done on an ordinary analytical balance, although in the development of the procedure a micro-balance was used.) Add 2 or 3 ml. of light petroleum (boiling range 40" to 60" C.) and then transfer the cup to the extraction tube. Support the funnel containing the cotton wool to be extracted in the frame immediately above the cup, add a further 1 or 2 ml. of light petroleum and close the tube with the metal condenser. Extract for 2 hours, remove the condenser and funnel and allow the solvent to evaporate to dryness. Transfer the extraction cup to a vacuum desiccator for 15 minutes before weighing.From the weight of wax, W, and its area, A, and specific gravity, calculate the thickness, t . of the bloom: W (in g.) A (in sq. cm.) x sp.gr. t (in cm.) = DISCUSSION OF THE METHOD- Light petroleum (boiling range 40" to 60" C.) was found to be the most suitable solvent as it combines a very high solvent power (11.7 g. of paraffin wax per 100 g. of solvent) with a convenient boiling range. The standard deviation of replicate determinations was first determined from two sets of three extractions carried out at two different times on the same vulcanisate (Mix 1: pale crepe 100, sulphur 2, MPC black 5, zinc oxide 0.5, paraffin wax 7, zinc dibutyldithiocarbamate 0.5). The results expressed as the weights determined instead of the thicknesses estimated from them are given in the following table.Weight of wax extract, Mean weight, Standard deviation, mg. mg. mi?. 2-62, 2.41, 2.85 2.63 0.22 3-41, 3.93, 3.41 3.58 0.30 On examining the results obtained with other vulcanisates (details of which are omitted) for which duplicate estimations had been made, tabulation of the differences between each pair of figures showed them all to be of the same order, with two exceptions. Mix No. . . .. 2 3 4 6 6 7 8 9 10 11 Difference, mg. .. 0-37 1.60* 0.65 0-14 1.56* 0.67 0.10 0.02 0.03 0.34 0.21 0-50 0.36 0.09 0-41 0.38 * These figures ignored. From the mean of these figures, corrected to the nearest tenth of a milligram and omitting the two that were not of the same order as the others, an estimate of the average difference between duplicate results was made.It seems highly probable that a manipulative error occurred with one or other of the duplicates marked with an asterisk. The average difference between the weights of wax obtained in replicate tests is therefore such that an ordinary analytical balance has sufficient discriminating power for the purpose and can be used instead of a micro-balance. The value obtained was 0.30 mg. RESULTS OBTAINED- As an illustration of some typical results, values for a series of transparent rubbers are quoted, where the amount of wax found is shown to be correlated with the amount of wax added to the mixing. Wax content on 100 parts of rubber hydrocarbon Weight of bloom, mg. 0.7 nil 1 0-067 0.067 3 1.21 5 1-43 1-52 7 2-28 15 3.73 4-07 Thickness of bloom, cm. x 1W6 nil 3 3 6 7 8 12 19 2176 SRINIVASAN : POTENTIOMETRIC DETERMINATION OF SMALL AMOUNTS [VOl. 75 The authors acknowledge with thanks permission from the Research Association of British Rubber Manufacturers to publish this paper. REFERENCES 1. Newton, R. G., J . Rubber Res., 1945, 14, 27-62. 2. Wyatt, G. H., Analyst, 1946, 66, 362-370. RESEARCH ASSOCIATION OF BRITISH RUBBER MANUFACTURERS 106, LANSDOWNE ROAD CROYDON, SURREY January, 1949