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Dacrylate Paints Ltd.

Paint Testing

Paint Testing

Dacrylate Viscosity Flow Cup Cross SectionViscosity

Usually checked in one of two ways; either a known quantity is timed as it flows through a Flow Cup with a set diameter hole (sometimes wrongly called a 'Ford cup') or measuring the drag induced on a rotating paddle immersed in the paint.

Viscosity is expressed in seconds (flow cup method) or in poise (rotating paddle method). It is very important that viscosity measurements are made with the paint at a standardised temperature (usually 20°C), as viscosity will change significantly (reduce) with increasing temperature. Viscosity measurements made without temperature standardisation must be regarded as approximate only.

Film Thickness

Wet film thickness is usually measured with a film thickness 'comb' having varying length teeth between two outer teeth of equal length.

Dacrylate Film ThicknessThis comb is pressed into the wet paint immediately after application, the film thickness is measured as somewhere in between the value on the last tooth to touch the paint and the next one, which didn't reach the paint surface.

In the example, the wet film thickness lies between 50 and 75 microns. This method is regarded as an indication or approximation only. As the wet film thickness will not be uniform over the area applied, regardless of application method, a series of wet film thickness measurements should be taken and an average value calculated.

Dacrylate Film Thickness WheelA more accurate reading can be made using a Film Thickness Wheel, which utilises the same basic principal, but has a eccentric channel cut into the rim of a wheel. In practice, the wheel is rolled along the paint film and the point at which the paint no longer touches the eccentric groove is read off as the wet film thickness.

Dry Film Thickness

Usually measured by electronic or magnetic methods.

Magnetic 'pull-off' (or 'pencil') and 'Banana' gauges rely on the decreasing attraction of a magnetic probe to a ferrous substrate as the film thickness increases, they can only be used on magnetic ferrous (iron or steel) surfaces.

Electronic gauges usually work by inducing eddy currents into the surface and measuring their transmission through the paint film, they will work on ferrous or non-ferrous metallic surfaces only.

Dry film thickness measurements on wood or plastic etc are more problematical and usually rely on a section of the film being cut away to optically measure the thickness.

Whatever method is used, a series of measurements should be made and an average taken.

Gloss Measurement

Dacrylate Gloss MeasurementGloss measurement is done using an electro-optical instrument that measures the light reflected off the coated surface. The higher the gloss, the higher the amount of reflected light, which will rebound at the same angle as originally projected. The matter the film, the more light will be scattered ie reflect at other angles and not be detected by the measuring device.

Adhesion

Adhesion is measured by determining the force required in detaching a section of paint film or by applying a set force and measuring the amount of paint removed. The most common 'field' method is the Cross Cut Test. A series of thin cuts are made at right angles to each other in a grid pattern. 10 cuts in each direction should be made, with a sharp blade, giving a grid of 100 squares. Each cut should be equidistant from its neighbour at 1mm or 2mm spacing as agreed in the specification. Cuts made at 1mm spacing will result in a much more severe test of adhesion than those made at 2mm.

The action of making the cuts will cause a certain amount of detachment of squares, particularly on films with poor adhesion. The test can be made more severe by the application of Cellotape across the grid pattern, which is first rubbed down thoroughly and then snatched off the surface.

Dacrylate Coating AdhesionThe adhesion can be quoted as the number of squares remaining as a percentage of those originally cut.

In the example, 13 squares have been detached giving an adhesion of 87%. The method can only be used as a guide but will quickly differentiate films with good, moderate or poor adhesion. As the test is destructive in nature it can only be carried out on areas's which can be sacrificed.

Colour Measurement

Dacrylate Paint Colour MeasurementFinal colour matching on a go / no-go level is done with the trained human eye, which can match colours accurately to each other and detect very small differences. The eye cannot however measure these differences quantitatively. An instrument, called a spectrophotometer can be used to both measure differences between two colours or to quantify a colour in terms of its individual spectrum. A light of known wavelength is shone onto the film being measured and the reflected beam passed through a prism to separate the individual wavelengths, which are then measured.

Opacity

The ability of a paint film to obliterate the colour of the underlying surface is termed opacity or covering power.

Dacrylate Paint OpacityOpacity can be expressed in two ways:

Firstly, the minimum film thickness which needs to be applied to hide the black / white pattern.

Secondly, a film of known thickness is cast onto a half black / half white panel. An instrument measures the colour difference between the two halves and expresses the reading over the black half as a percentage of that over the white half (called contrast ratio).

Scratch Resistance

Hardness is not always a desirable feature in coatings; toughness is usually the required criteria. Scratch resistance is determined by drawing a weighted point across the paint film, increasing the load until the pin breaks through to the underlying substrate.

Drying Time

Paint usually dries in two distinct stages:

'touch dry' – when the majority of solvent has left the film which is dry to the touch, and

'hard dry' or 'through dry' when the coating has cured throughout to a hard, mark resistant film.

Drying times can be accurately measured and compared by using a Drying Time Recorder.  An accurate film is cast onto a narrow glass strip, which is immediately placed onto the recorder. A needle is lowered into the wet film and is slowly dragged along the strip over a set period, usually 24 hours. Whilst the coating is still wet, the furrow created by the needle will flow back and re-level itself. As the coating dries, the film will be less and less able to flow back into the channel, which will become more and more defined. Eventually, as the film cures, the needle will be unable to cut though it and will skip onto the film surface in a series of snags followed by a complete absence of mark where the needle merely slides over the surface of the cured paint. The times at which these stages took place can be easily read off from the scale on the instrument.

Flexibility

Flexibility is usually measured by applying the coating to a thin metal panel (including the relevant priming where appropriate). After aging this panel is bent through 180° in a jig (similar to a hinge) and the film is then examined for cracking or flaking in the area of the bend. Jigs having various radii are employed, the smaller the radius, the greater is the severity of the test. A single test can be done on a jig, which bends the panel into a cone, therefore producing a varying radius from approximately 2mm to 25mm.

Adhesion can also be measured by dropping a weighted round-tipped projectile onto the panel, (either from the front or from the rear of the panel), thus causing a depression that can be varied in depth. Evidence of cracking or flaking is noted around the depression. The deeper the depression, the more severe is the test (termed forward or reverse impact).

Corrosion Resistance

Usually measured by encouraging corrosion to take place at a greatly accelerated rate in controlled conditions. Carefully prepared steel panels are exposed to a fog of salt-water mist for set periods of time. Prior to exposure a score mark is made through the paint film and the corrosion along this line is assessed. The test is made more severe if a warm salt mist is utilised (referred to as hot salt spray testing).

Accelerated Weathering

Artificial aging can be conducted to give an indication of a coating’s resistance to weathering. Weathering is a totally subjective criteria which depends totally on temperature, humidity, chemical pollution (including coastal salt) and Ultra Violet light levels. It is not possible to state that a set period in an artificial weathering machine equates to a set period of natural weathering.  1 year’s exposure in coastal Florida equates to a far more severe exposure than 1 year’s exposure in rural England. It is however, possible to use artificial weathering as a comparative evaluation when comparing coatings to each other ie it can be demonstrated that coating 'A' shows better weathering resistance than coating 'B', when both are tested under identical conditions.

Artificial weathering takes the form of exposure to intense UV light for set periods, sometimes alternated with water fog.

Many other tests are available to the paint technologist, for example :

•   Abrasion testing
•   Humidity testing
•   Chemical resistance
•   Porosity
•   Light fastness
•   Flash point etc

These additional tests are chosen to simulate the criteria that the paint is likely to encounter in service eg. petrol resistance or fire resistance.

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