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Distincness of Image DOI, Orange peelRhopoints

 

 

Powder Coating Quality

A high quality finish is becoming increasingly important for powder coated products. In the past it was best known for its durability and low environmental impact, but with improvements in formulation and application techniques, a powder coating finish can now match or exceed the appearance quality of liquid paints.
Achieving the highest quality finish requires skilled operators, well formulated coatings and optimum application equipment and conditions. This is particularly true in for powder coatings as the process can be prone to orange peel, surface texture and haze effects. These imperfections can dramatically reduce the visual impact of finished products and are historically difficult to measure.

Assessing Coating Reflective Qualities

To aid designers, coating manufacturers and applicators the Powder Coating Institute has developed a comparison tool to simulate all possible finishes that are achievable using this technology.
This visual scale comprises ten powder coated panels, which have been graded from 1- high roughness & orange peel to 10- a very smooth glossy finish. The set is often used to grade the quality of powder coated surfaces; the operator simply judges which the panel is closest visually to the sample.
Unsurprisingly the results achieved by this method are highly subjective and prone to errors due to sample area, lighting conditions and coating colour.

Orange peel

 

 

 

 

 

 

 

Instrumental Assessment of Coatings

To avoid the errors associated with operator judgment, sample type and measurement conditions, it is preferable to use an instrument to measure such a crucial parameter.
Three instruments were compared to determine their suitability for measuring textured surfaces and in particular the powder coating smoothness scale, a set of these panels were used in the assessment.

Glossmeter

The most commonly used instrument to measure coating reflective appearance quality is a glossmeter. Glossmeters shine light at a surface and measure the amount of reflection at the specular angle.
The standard for gloss measurement specifies three geometries for measuring high, medium and low gloss levels. The most commonly used geometries used in powder coatings are 20⁰ and 60⁰.
A Rhopoint Novo-Gloss Trio was used to assess the 10 panels, the average of 10 readings for each panel is shown below.

gloss data

Despite large differences in appearance, the gloss readings for the panels are almost identical. This is because the glossmeter can only measure the amount of reflected light and is not sensitive to distortions which affect appearance quality.
Differences in gloss values are only apparent on panels 1 and 2 on where the orange peel is reflecting light outside the detection range of the instrument resulting in a lower 20⁰ gloss value.
Applications such as powder coatings which are highly likely to be affected by orange peel and similar effects logically require a measurement method which more successfully quantifies texture.

Distinctness of Image

D.O.I is a parameter which describes the visual distortion seen in a reflection of an object viewed in a textured surface.
To understand this concept, the reflection of a specially designed test chart has been viewed and photographed in each of the 10 powder coating smoothness panels.

DOI-PCI

The photographs illustrate how the very high orange peel in panel one produces an image with poor clarity, as the orange peel reduces; the distinctness of the image improves.
Distinctness of image is quantified instrumentally by measuring the spread of a beam of light after it has been reflected from a surface.
Surfaces which reflect an image perfectly without any distortion have a DOI value of 100, surfaces with no image clarity have a DOI of 0.

Novo-Gloss IQ Goniophotometer

The new Novo-Gloss IQ instrument from Rhopoint Instruments was used to measure the reflective qualities of the panels at 20⁰.
Unlike a glossmeter which quantifies the amount of reflected light by using a single sensor, the IQ uses an array of 256 micro-sensors that allow it to fingerprint how the light is reflected, producing a goniphotometric profile around the specular angle.
Connecting the instrument to a PC the user can view Goniophotometric profiles, comparing how the light is distributed for different surfaces.

gonio

The instrument calculates GLOSS, HAZE, RSPEC and DISTINCTNESS OF IMAGE using the angular tolerances specified in relevant international standards for these measurements. The values shown are the average of 10 readings.
The instrument is operated with a single button push, all values are displayed on a single screen.

haze meter

rspec

The DOI perfectly shows the reduction in orange peel from panels 1 to 10, whilst gloss values can be seen to equate to those measured using a conventional instrument.

doi

Haze

Haze is a phenomenon where micro structures on the surface of the coating cause reflected light to be slightly diffused around the specular angle. Surfaces with haze exhibit a milky finish or a halo effect around the reflection of a strong light source.
The effect of surface haze can be viewed on the reflected test panel pictures 1 and 2, the edges of these charts are blurred and slightly enlarged compared to the panels without measured haze.
Powder coating is especially prone to haze effects; this can be due to stoving problems, raw material incompatibilities or additive migration.

RSPEC

RSPEC is the peak gloss value at the specular angle; surfaces with high DOI have a sharper goniophotometric curve and hence have a higher RSPEC value.

 


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