Chalking Rate Tester : For quick determination of the chalking rate of coating materials
Chalking Rate Tester according to Kempf Model 241
The Model 241 chalking rate tester consists of a light-metal cylinder.
At the end of this cylinder is a 40-mm diameter rubber stamp and a Shore A hardness of 60 ±5.
The built-in compression spring enables a stamping pressure of 50 to 250 N.
The ring marks punched into the stamp shaft, stepped at intervals of 50 N, also allow the load to be less than the standard 250 N pressing force.
Pressing by hand is too unpredictable to ensure a consistent load from the rubber stamp on the test surface.
Therefore, we recommend using a special stand.
The lever is used to apply the contact pressure more easily and precisely.
DIN 53 159 Titangesellschaft mbH (Kronos Titan GmbH)
The first sign of a paint coating being affected by the weather is an initial disintegration of the surface film where some of the pigment becomes loose when originally firmly held in the paint by means of a bonding agent.
This stage of deterioration can be shown by simply wiping the surface, loose pigment particles will be removed.
This is called „chalking“.
The sooner the paint commences to chalk, the sooner the decomposition of the film has begun; the life expectancy of the finish will therefore be accordingly shorter.
Self-cleansing paints On the other hand, paints are being manufactured nowadays which are accurately adjusted for a predetermined chalking effect.
Such self-cleansing paints renew their surface after every rainfall by the washing off of the conta-minated pigment.
Their life is practically the same and they show a high degree of colour durability and cleanliness.
Purpose and application
The quantitative determination of the chalking rate is therefore of great importance for
♦ the choice of the correct raw material for each intended purpose;♦ the determination of the maximum pigment/volume concentration in respect to weathering resistance or self-cleansing properties;♦ the evaluation of natural or short-term weathering tests;♦ a comparative assessment of raw materials and finished products (testing of competitors' samples);♦ determining the maximum warrantable period for large contracts.
The pigment particles, lying loose on the film surface because of binder decay, are pressed under a specified pressure (250 N) into the steeped gelatine of photographic paper.
See Fig. 1: The upper pigment layer has lost cohesion with the rest of the film
In accordance with the relevant evaluation method, the impression in the photographic paper is visually compared with a chalking scale or a comparison paint.
See Fig. 2: Pressed into the gelatine layer of the photographic paper, the loose pigment particles are lifted out.
Design and operation
The Kempf Chalking Tester, Model 241, consists of a light metal cylinder equipped with a 40 mm dia. rubber stamp (Shore-hardness A 60 ± 5).
A strong integral spring allows the loading of the punch with a force of between 50 to 250 N.
Ring markings engraved at 50 N intervals into the stamp shaft permit the application of lower pressures than the standard load of 250 N.
Guiding the loading device by hand is often not accurate enough for guaranteeing an even pressure over the whole area of the stamp. It is therefore recommended that the special stand (available as an accessory) be used which provides an accurate vertical guide during application of pressure.
The photographic paper is soaked in water at room temperature for four minutes.
After removing any water adhering to the surface with filter paper, it is laid upon the paint to be tested, gelatine face down.
Placing the rubber stamp on the reverse side of the paper, the Chalking Tester is pressed down as vertically as possible with the ball of the thumb - or with the lever when using the stand - with a pressure of 25 kg (55 lbs) until the ring mark 250 is reached After lifting the stamp the photographic paper is pulled off and dried.
In special cases - to be agreed upon - a smaller load may be used.
The duration of pressure does not influence the test result.
In order to avoid measuring errors it is recommended to obtain the same quality of paper from the same supplier.
Surface contamination of paints that have undergone weathering in the open air may simulate chalking of the paint on impressions of unexposed fixed (therefore white) photographic paper, although only the soiled layer has been lifted off the paint surface.
To be able to discern in such cases between contamination and chalking it is advisable to make the impressions on white as well as on black photographic pape
Evaluation of the test
Several methods are known in practice, namely:
1. Kempf standard comparison method
This approved method is specially suitable for white or light-colour pigmented paints. For comparison with the chalking marks obtained, it uses standard photographic diagrams which are supplied as Kempf Chalking Scale with every instrument. The comparison (Fig. 4) is made visually with the 6 chalking stages, whereby 0 represents the weakest and 5 the strongest chalking value. Fig. 4 Kempf Chalking Scale
2. Modified method of the Titangesellschaft, Leverkusen
(now: Kronos Titan GmbH) Used only for paints pigmented with titanium dioxide (TiO2).
For this method a standard scale is divided into 10 stages of chalking intensity of brightness.
The distances between the stages of brightness are of an exactly defined photometric value and these measurements are reproducible.
See chalking scale.
The brightness limit values of 10 and 1 represent absolute freedom from chalking and vice-versa a strong chalking effect.
Chalking value 10 (highest calking resistance) corresponds with the brightness value of 10 (lowest brightness value) for an unaltered black photographic paper without any imprint.
Chalking value 1 (lowest chalking resistance) corresponds with the brightness value of 64,2 (highest brightness) which has been measured on an imprint obtained from most intensive chalking.
3. Method acc. to DIN 53 159
This is a method which should be given preference whenever coloured paints have to be tested; testing of white and very light colours is, of course, also possible. The evaluation here is not carried out by using a scale but by comparison with paints which had been produced and weathered under similar conditions. By visually comparing the impressions in the gelatine layer of the test specimen with the comparison paint, the differences in the chalking between the test samples and the comparison paints are established.
Basically the instrument does not require any maintenance.
Signs of wear on the rubber stamp and tensioning spring fatigue may occur after several years or after very frequent use.
These parts are available as spares. Exchange of stamp:
The rubber stamp is removed by unscrewing the knurled supporting ring.
Exchange of tensioning spring:
The tensioning spring is replaced by unscrewing the domed cap on top of the loading device.
Height: 270 mm
Ø: 50 mm
Load: up to 250 N
Net weight: approx. 1.2 kg
Height: approx. 450 mm
Width: approx. 150 mm
Depth: approx. 250 mm
Net weight: approx. 5.5 kg