Chalking acc. to Kempf
Coatings & Industry > Brightness GLoss & Chalking

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)
Introduction
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.
Principle
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.


Test procedure
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.
Notes:
Load:
In special cases - to be agreed
upon - a smaller load may be
used.
Duration:
The duration of pressure does
not influence the test result.
Photographic paper:
In order to avoid measuring
errors it is recommended to
obtain the same quality of
paper from the same supplier.
Dirty coatings:
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.
Maintenance
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.
Technical data
Chalking Tester
Dimensions:
Height: 270 mm
Ø: 50 mm
Load: up to 250 N
Net weight: approx. 1.2 kg
Special Stand
Dimensions:
Height: approx. 450 mm
Width: approx. 150 mm
Depth: approx. 250 mm
Net weight: approx. 5.5 kg






