John F Fletcher, technical support manager, Elcometer gives Finishing the head’s up on wet paint quality control

In a coating process, measurement of the wet paint often provides the first opportunity to check the quality of the work. Every paint sprayer will be familiar with the use of wet film combs to monitor the thickness of the wet paint, as it is applied. However, you may not be familiar with other measurement techniques that can be applied to wet paint to determine the properties and even predict the final cure condition.

Paint must be considered as a raw material and therefore any measurement that can determine if it is in good condition must be considered if the coating process is to be correctly controlled.

The measurement of viscosity can assist in determining the condition of paint, particularly if it has been stored for any length of time. Film application gauges can be used to assess the performance of the paint particularly in respect of sag and levelling and wet film wheels and combs can be used to assess the thickness of the wet film immediately after application.

Viscosity –Flow Cups and Dip Cups
The viscosity of paint is a measure of the material’s ability to flow. All fluids have an internal friction between the molecules, which determines how well the fluid flows. Due to this internal friction, energy is required to move the liquid and viscosity is the measure of the resistance of the fluid to flow.

Fluids can be characterised as Newtonian or non-Newtonian. Newtonian fluids, such as water and some oils, are fluids that continue to flow at a given temperature, regardless of the forces action on them. No matter how fast it is stirred or mixed, Newtonian fluids will always behave in the same manner. Newtonian fluids are generally measured with flow and dip viscosity cups.

Non-Newtonian fluids, such as paint and ketchup, are fluids, which change viscosity when a force is applied. Non-Newtonian fluids are measured using rotational viscometers.

Flow cups use the time for a volume of fluid to flow through an orifice as a relative measurement of the viscosity. The viscosity measured in this way is described as kinetic viscosity and the flow (efflux) time is generally expressed in seconds and can be converted to viscosity in Centistokes (cSt) using a viscosity disc calculator or efflux tables depending on the type of cup.
Viscosity Flow Cups are simple and very easy to use and are usually made of aluminium with a stainless steel orifice at the bottom of the cup. These cups are made to a number of standards, including ISO, BS, AFNOR, DIN and FORD/ASTM. 

 Each standard describes a different design of cup some with a range of orifice sizes but, in general, the specification for the paint will include the flow time and the type of cup used. For example the ISO Viscosity Cups have 5 different orifice sizes as shown in table 1.

Cups are individually tested using standard viscosity oils with a range of viscosity values and calibration certificates based on the results of this testing can be supplied.

Normally, flow cups are used with an adjustable stand so that the cup can be held level and the cup filled using an overflow glass draw plate to ensure that the cup is fully filled and that the flow does not start until the user is ready to start the stopwatch. The flow is always timed to the first break in the flow.
In the field, the Dip Cup is more commonly used for checking the viscosity of cans of paint. Again there are several types of Dip Cups, Frikmar Cups with AFNOR, DIN, Ford/ASTM and ISO types and Zahn Dip Cups.
These cups are simply used by dipping the cup into the paint and allowing into fill, lifting it clear of the surface and timing the flow from the orifice back in to the can. 

Viscosity - Rotational 
There are three types of rotational viscometers commonly used in the assessment of paints, the Krebs Viscometer, The Rotational Viscometer and the Cone and Plate Viscometer.
The Krebs Viscometer consists of a spindle and paddle, which is rotated at a constant speed of 200 revolutions per minute. When the paddle is immersed in the sample the extra force applied to the motor to maintain the 200 (± 0.2) rpm is a measure of the viscosity.

The Krebs Viscometer is capable of measuring the viscosity in three units, Krebs Units, Centipoise and Grams.

The Rotational Viscometer comes equipped with a range of spindles to allow measurement of the viscosity of a liquid according to ISO 2555. This type of viscometer uses disk-style spindles and by interchanging the spindles a large range of viscosities can be measured, from 3 to 2,000,000 mPa.s (cP) for the low range version and 20 to 13,000,000 mPa.s for the high range version. In addition there are accessories for small sample measurement, for very low viscosity materials from 1 mPa.s (1cP) and a high temperature adaptor for temperatures up to 200° C

 The Cone and Plate Viscometer is particularly useful when measuring at high shear rates. A small amount of the material is placed on the plate where it can be heated or cooled depending on the type of material being tested. The Cone is rotated at one of up to 8 speed settings and pressed down on the sample on the plate. The resistance of the sample is measured and displayed to an accuracy of ± 2%.

The plate can be heated using a temperature control module but it can also be cooled using a Peltier Effect device, which can reduce the temperature of the sample down to 10° C when the ambient temperature is up to 25° C.
A range of cones is available with different diameters and different cone angles to extend the range of the viscometer.

Film application
Wet paints will level and sag depending on the viscosity and the conditions of the test such as the thickness of the film and the temperature. Levelling is tested by preparing a strip of wet paint with notches of increasing depth from 25 µm (1 mil) to 4,060 mm (160 mil). The film is left on a horizontal surface to cure. Once the film has cured the thickness at which the pair of film stripes merge can be seen. This represents the minimum film thickness for the cured coating to achieve a consistent film. This test is carried out according to ASTM D 2801.

 The sag test uses a similar device but with 10 notches of increasing clearance and the film is turned to be vertical when it has been applied so that the effect of gravity acts on the wet film and the joining of the stripes created by the notches indicates the tendency of the paint to sag.
 
Wet film wheels and combs
The Wet Film Wheel is a high precision, accurate and easy to use instrument, which consists of three wheels formed together with two outer concentric wheels and an inner eccentric wheel. By rolling the wheel through the wet paint the central wheel will eventually touch the paint and pick up paint so that the wet film thickness can be measured against the scale on the concentric wheel.

Several measurement ranges are available from 0 – 25 µm to 0 – 3,000 µm. The scales are accurate to ± 5% of the scale value and the wheels are easily cleaned with a suitable solvent and reused.

The wet film comb is a lower cost option for determining the wet film thickness. In the case of the wet film comb a series of notches are cut in the side or the edge of the comb and the notches are pushed into the wet paint. The wet film thickness will be between the highest tooth coated with paint and the lowest tooth uncoated.

There are a variety of wet film comb designs available, hexagonal combs in stainless steel or aluminium, square combs moulded in plastic for single use only, stainless steel rectangular combs with teeth cut on the edges or the ends.

With both the wet film wheel and the wet film comb the relationship between the wet film thickness and the dry film thickness will be stated in the technical data sheet for the paint as the volume solids (%) or the wet to dry ratio
For example: If the paint has a volume solids value of 80% and the wet film thickness is 200 µm, the dry film thickness will be:
200 x 80% = 160 µm

Conclusions
The condition of wet paint can reveal a great deal about the resulting dry film. Wet film thickness determination is an excellent quality control measure, which is easy to carry out and interpret. It allows a very quick feedback to the operator of the spray gun regarding the film thickness and therefore allows corrections to be made while the application is proceeding.
The viscosity, levelling and sag performance of the paint reveals much about the condition of the raw material before the coating process starts and helps maintain the quality of the paint particularly if storage time is an issue.
 

• Elcometer Limited
  Edge Lane
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  M43 6BU
  UK
   
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  www.elcometer.com