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Powder Coating

Manufacturing Process of Powder Coating


Composition of powders

Powder coatings are organic coatings composed of dry, solid (doesn’t contain solvents). The base formula of powders is composed of different materials : resin, hardener, filler, pigment and additives.
Resin
For powder coatings nowadays two types of resins are used : thermoplastics or thermosettings.
thermoplastic powders
These powders (e.g. polyethylene, PVC, ...) will melt and flow together to one layer (the coating) when applying. If this coating is heated up afterwards the layer will become weak.
thermosetting powders
The coating will melt on temperatures between 150 and 200°C, and will flow out and cure. These coatings will not become weak if they are exposed to high temperatures. The most common thermosetting powders are composed of one or more of the following resins : epoxy, polyester, polyester-urethane, acrylate.

hardener
The hardener is one of the most important ingredients of powder formula. The hardener is responsible for the curing of the powder and dictates, in conjunction with the resin, the specific properties of the coating : chemical resistance, thermal stability, physical and mechanical characteristics.

pigment
Pigments are used to give colour. There are organic as well as inorganic pigments. Depending on the application, a choice is made between light fast, food-contact safe or temperature stable pigments.

filler
Fillers in powder coatings are used to contribute positively and technically to the system. This can be :
a. to resist sagging out of the film
b. to improve abrasion resistance
c. to improve resistance to humidity

additives
This collective name for a big group of additives, to realise determined properties of the coating.

5 categories exist :

a. accelerators (to accelerate the reaction of the curing system)

b. degassing additives (to let escape enclosed gases through the pores of the coating by opening them longer)

c. thixotropic additives (to resist sagging outs = viscosity regulator)

d. flow-control agent (to improve the flow on the surface)

e. matting agent (to reduce the gloss of powder coatings)

Production of Powders

premix
The different dry raw materials (resin, hardener, pigment, additive, filler) are weighed in exact quantity and put in a container. This container will be tumbled or mixed during a determined time so that the raw materials are well mixed. A rather homogeneous distribution is obtained. The container is brought to the extruder, after control on homogeneity.

extrusion
In the extruder, ingredients are warmed up to a sort of paste, mixed and kneaded. On molecular scale resin and hardener will be mixed together. The pigments and additives will be dispersed in the weak mass.

The paste, leaving the extruder is immediately flattened between two cylinders and cooled down on a conveyer. At the end of the cooling conveyer, the continuous powder plate is broken down into little pieces (named “chips”) that will be caught up in containers.

grinding
The containers with “chips” go to the grinding machine. The chips are ground to a determined standard particle size by means of a centrifugal separator. Too small pieces are removed. Too big pieces finish in the grinding machine till they attain an acquired particle size.

packaging
When an optimum particle size is obtained, powder is placed in boxes (5,10,15,20,25 kg) or big-bags (about 450kg).


Properties of Powders and Coatings

The powders produced are subjected to all sorts of tests. As described in the technical data sheets, several tests are built in as a standard. These tests can be executed on the powder or on test panels coated with powder.

properties of powder



viscosity
Is determined on the powder during melting stage. A low or high viscosity influences the final aspect of the coating. The coarse in viscosity is measured at a constant temperature.

particle size distribution
The behavior of powder at application and the aspect of the film are influenced by the particle size distribution. In general, the powder coating, applied electrostatically, has a particle size distribution in order from 3 to 100µm, but this can vary depending on the application (tribo, corona, specific application of the pistols). The particle size distribution can be measured through an air flow sieve, a laser diffraction or mechanical sieve system.

Read an arcicle on Effective Particle Sizing of Powder Coatings through Wet-Dispersion by R. Siva of Marpol Pvt Ltd Margao, Goa siva@marpolind.com

reactivity
Under influence of warmth, powder applied on the substrate undergoes two changes : when the powder melts and flows out, when the hardener begins reacting. The reactivity is important for the flow out, the outlook of the film, the speed of the conveyer but also the storage stability. The most common test on this subject is the determination of the “gel time”. The “gel”-time at a specific temperature gives a relative indication of the reactivity.

curing schedule
The forming of the film takes place in the curing oven at a temperature between 120 to 200°C, depending on the powder used, the mass of the object to coat and the length of the oven. At a determined temperature the resident time of the object in the oven depends on the type and thickness of the substrate, the form of the object and the type of oven. It’s important to use the accurate temperature to cure the powder, otherwise a undercuring or overcuring (with effects as yellowing, matting, ...) will occur. The degree of cross-linking and the mechanical properties can be determined in the gradient oven of the laboratory. This, working out the coarse and the properties of the film at temperatures of for example 150°C to 220°C at only one test panel. In the oven of the applicator the curing schedule can be worked out by means of the Grant Recorder.


Properties of coating
colour and gloss
colour
Colour is a subjective perception and is submitted to the individual interpretation. A reason to use the spectrophotometer. The colour is measured by means of 3 parameters that can be visualised in a 3-dimensional system. There exist 2 systems : Lab and LCh, see further on.

In this system the colour is determined by the proportion black-white (L co-ordinate), the proportion red-green (co-ordinate a) and the proportion yellow-blue (co-ordinate b).

The spectrophotometer is based on this system.

§ L-axis : proportion white-black

§ a-axis : proportion red-green

§ b-axis : proportion yellow-blue

metamerism

Metamerism can cause problems. It is a phenomenon where two colours are percepted as the same colours under certain circumstances of light and are percepted as different colours under other circumstances of light. Often, this phenomenon is due to the different composition of the pigments. Four types of metamerism can be distinguished :

a. illumination metamerism : when a colour is lighted under a specific light source e.g. D65 (daylight), another shade will be percepted than under tungsten light (incadescent filament).


b. observation metamerism : is a consequence of visual colour recognition or blindness. In that case one particular colour will be percepted in another way by different persons.

c. perspective metamerism : occurs at changing perspective distance. Many objects can be seen in a different way at a distance of 2 meters or 10 meters.

d. geometric metamerism : is a problem with metallic paints, but can also occur to other types. This phenomenon is determined through the different perception angle. So, the colour of domes in coloured aluminium will be percepted different from the windows and doors out of the same coloured aluminium.

gloss

According to the standards ASTM D-523 60°, ISO 2813 or DIN 67530. Gloss is measured objectively with a gloss meter. The gloss meter is based on the measurement of the specular component of light. The gloss is measured in general under an angle of 60°. Herewith we will draw your attention that measuring structured or metallic effects is very difficult, because of the uneven surface or the orientation of the metallic-particles.

mechanical properties

Buchholz hardness
According to the standards ISO 2815 or DIN 53153. This test measures the deforming, the indentation of the coated film, when a special wheel with a sharp angle is placed on the surface during 30 seconds. The Buchholz hardness is equal to 100 divided by the length of the indentation (mm).

pencil hardness
According to the norm ASTM D3363. In a special instrument pencils of different hardness (2H, H, F, HB, B, 2B) are placed after being sharped and flattened. A line is drawn with the pencil. After every pencil draw the coating is checked on damages.The pencil hardness represent to successive pencil hardness : one when the coating wasn’t damaged yet and the other where it was damaged.

Clemen hardness
According to the norm BS 3900:E2, ISO 1518, ASTM D5178. Is used as an indication of resistance to inkervingen. With a steel needle a line is drawn (determined speed and pressure) on the coated surface. Every time when a line is drawn with the needle a heavier mass is put on the needle. At the moment that the coating is damaged (needle on metal) the quantity of mass on the needle is the notation for the Clemen hardness.

impact
direct impact or indirect impact: ASTM D-2794 of ISO 6272.Impact (fast deformation) is tested with an impacttester. The principle consist on a mass that falls from different heights on the coated surface (direct : on the coating, indirect on the reverse side of the coated test panel). The impact, when the indentation in the coating represent no effect of cracking, is quoted in kg.cm, in N.m or in inch/pounds.

flexibility
A coated standard test panel is folded over cylindrical rolls (according to the standards ASTM D1737, ISO 1519 or DIN 53152) or a conical mandrel (according to the standards ASTM D552 or ISO 6860). This is a measure for flexibility, stretching and adhesion of the coating under deformation. These tests are used to evaluate the post deforming capacity of coated components.

If test panels are folded around cylindrical rolls (with a known diameter) then the result is the diameter of the roll when no damage on the coating is determined.

If the test panel is folded around a conical mandrel then the test results represent to what extent the coating is cracked, beginning at the sharpest side of the fold in the surface.

Erichsen
According to the standards DIN 53156 or ISO1520. In this test a ball bearing with a specific diameter is pushed in the reverse side of the coating (slow deformation). This with a speed determined in advance. The coating stretches and cracks at last. The depth of the ball bearing in the test panel when the coating is cracking, is determined.

cross-cut test (adhesion)
According the standards ISO 2409, ASTM D3359 or DIN 53151. On the coated test panel a cross-cut (# : indentations in the form of a cross and parallel to each other with a mutual distance of 1 mm or 2 mm) is made on the metal. A standard tape is put on the cross-cut. The cross-cut is valued through the quantity of the detached film after removing the tape.


weathering resistance

accelerated ageing and UV durability (QUV)
This test in the QUV-weatherometer consists of 2 cycles. The coated testpanels are 8h exposed to UV-light and 4h to condensation. This is repeated during 1000h. Every 250h the panels are checked. Herewith the coating is tested on colour- and gloss retention.

Florida-test
During minimum 1 year the coated testpanels are exposed to the sunny and humid environment of Florida, USA. Gloss as well as colour retention are evaluated.

humiditytest (tropical climate)
According to the standards DIN 50017 or ISO 6270. Is executed in a chamber with a environment of saturated humidity, at a determined temperature and often during 1000h. Every 250h a control is executed on the powder coated panels and a Andreas-cross scratched with a knife through the film in the middle. This test evaluate the under creep of humid and corrosion in a humid environment.

saltspraytest
According to the standards ISO 9227 or DIN 50021. The powder coated panels (with an andreas cross scratched in the middle through the film) are placed in a warm humid environment and sprayed with salt. This test evaluates the degree of protection from the coating to corrosion in a salty environment (e.g. at the seaside). Usually this testcase takes 1000h, with checks executed every 250h.

Kesternich-test
According to the standards DIN 50018 or ISO3231. Gives a good indication to the resistance of the coating in a industrial environment. For a specific period a coated test panel is placed in a warm humid environment, which contains sulphur dioxide. This test is running 24h-cycle with controls every 250h.

resistance to mortar
According to the standard ASTM C207. A specific mortar will be brought in contact with powder coating during 24h at 23°C and 50% relative humidity.

chemical resistance
Is often tested on coatings that are subjected to maintenance, contact with detergents or chemicals. Standard conditions are not prescribed. So, the powder producer fixes the condition in discussion with the applicator or final consumer.



degree of cure
To get an overview of the degree of cure in industrial ovens, following information is determined (in order) :

1. curing schedule of the oven

2. impact test on sample plates, being in the oven at the same time as the coated object

3. rapid solvent test

4. determination DTg


rapid solvent test

Depending on the coating (epoxy, epoxy-polyester or polyester) the lacquer uses different tests (MEK-test, methyl-glycol test, xylene-test) to determine the degree of cure.

Method :

1. soak a piece of cotton in the indicated solvent (for example : to determine the degree of cure for a epoxy powder : methyl-ethyl ketone)

2. press slightly to eliminate the excess of solvent

3. applying the piece of cotton on the coating during the indicated time (for example for an epoxy coating : 60 seconds)

4. depending on the observations it determines under-cure, over-cure or cure.


parameters to determine the degree of cure

 

The rapid solvent test (as described above) can be taken by imprecise and subjective judgements. A final decision on the degree of cure will only be taken after having performed different controls.

differential scanning calorimetry

Via the DSC technique it can be verified if the coating is sufficiently cured or not. The DSC is a technique verifying how much energy is needed to have no temperature difference between the substance in test phase and the referential material, determined in according to time and temperature, when two specimen are submitted to identical temperature conditions in a controlled warm or cooled environment.

First of all, the thermal history of the material is disturbed by the fast cool down under liquid nitrogen and warming it up immediately to 240°C. During the curing process the Tg increased (glass transition temperature) in function of the curing time. An initial Tg₁ is observed.

If the coating is undercured, an exothermic reaction is observed. If no exothermic reaction is observed, than it can be considered that DTg = Tg₂ - Tg₁ = 0. Undercuring appears when DTg > +2°C