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Method of Testing

Reference

IS : 33 ("Methods of Sampling and Test for Inorganic and Extenders for Paints") and IS : 3493 ("Methods of Sampling and Test for Organic Pigments")

Oil Absorption

A suitable quantity of the test sample as per the table given below is exactly weighed. The material is placed on ground glass or marble plate. Linseed Oil (Acid refined linesed oil conforming to IS : 75-1973 having an acid value of 7.5 to 8.5 is then added slowly, 4 to 5 drops at a time, from a weighed beaker and rubbed into the pigment with palette knife. This addition is continued until agglomerates of oil and pigment are formed and from this point, oil is add only one drop at a time followed by a thorough rubbing with palette knife. The addition of oil is stopped when a stiff paste of smooth consistency is formed; the paste will be such that it will just spread without cracking or crumbling. The rate of addition of oil is so adjusted that the whole operation will require between 20 and 25 minutes and care is taken that during this time whole pigment mass is manipulated with maximum effort.

By noting the quantity of oil used, the oil absorption value is calculated and expressed as the number of grams of oil required for 100 grams of pigment.

Pigment Quantity in gms
All Organic Pigments 5
Inorganic Pigments where expected oil absorption is :  
less than 10 20
10 to 30 10
31 to 50 5
51 to 80 2
more than 80 1

Bulk Density

About 20 gms. of the pigment, which has been previously dry-sieved through 250-micron sieve, is accurately weighed on a glazed paper and slipped gently into a 250 ml. measuring cylinder. The cylinder is then fixed on the Bulk Density Apparatus and tapped mechanically with lifting and dropping action at the rate of one drop per second and once every two seconds a gentle twist of about 10 deg. is given to the cylinder. After 50 taps the volume of material is noted.

Bulk density is expressed-as the mass in grams of the material per cc of its volume.

Bleeding

1. Water-500mg. of the dry pigment powder is shaken with 20ml of water at room temperature for 10 minutes and then filtered through a Whatman No. 42 (or equivalent) filter paper with Barytes bet. The colour of the filtrate is observed and rated as 5 if there is no bleed and 1 if the bleed is considerable, with gradation in between with the help of a colourimeter.

2. Linseed Oil- A flowable paste of the pigment is made in linseed oil on the automatic muller. A drop of this paste is put on filter paper and the bleed is observed. Alternatively, 500mg sample is kept on a filter paper folded on a glass funnel with Barytes bed and 20ml. linseed oil is poured uniformly over the pigment. The filtrate is observed for bleeding and rated as in the case of water.

Resistance to acid and alkali

About 5g of the pigment and about 250ml of Hydrochloric Acid or Caustic Soda solution (5% in case of Organic and 1% in case of Inorganic Pigments) are stirred together in a 500ml beaker for 1 hour at room temperature. The pigment is then filtered, washed free from acid or alkali as the case may be and dried in an oven at 100 +20C. The change, if any, in colour and strength is determined by making paste in linseed oil and taking comparative drawdowns with the untreated sample. Resistance is rated as “Excellent” when there is no change in shade and “poor” when there is a considerable change, with gradation in between.

Heat stability

Dry pigment powder is taken on a watch glass and heated for the specified period in an oven maintained at the specified temperature. The sample is then cooled in a desiccator and tested for change, if any, in colour by making paste in linseed oil and taking comparative drawdowns with the original (i.e. unheated) sample. This gives an approximate idea about the behaviour of the pigment at the particular temperature, which should be further confirmed by tisting the pigment in every application for which it is to be used.

Fastness to Light

Full tone and tint tone (1part pigment : 20 parts of Ti02) paste of pigment are made in linseed oil and drawdowns are taken on paper. These drawdowns are then exposed to light in Weather-o-meter for a specified period. The rating is given from 1 to 8 in accordance with the 8 point Blue Wool Scale.

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