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Application and Fusion of Plascoat PPA 571

Plascoat PPA 571 materials can be applied by the following methods.

2.3.1 Fluidised-bed coating. (PPA 571).

2.3.2 Electrostatic spraying. (PPA 571ES).

2.3.3 Flock spraying. (PPA 571 and PPA 571ES).

2.3.1 Fluidised-Bed Coating Process

The fluidised bed coating process consists of:

2.3.1.1 Pre-heating.

2.3.1.2 Fluidised bed coating.

2.3.1.3 Post-heating (if necessary).

2.3.1.4 Cooling.

2.3.1.1 Pre-heating.
A smooth, pinhole free coating on a given piece of metal requires the metal to have a certain heat content. This depends on:-

(a) The oven temperature

(b) The time in the oven

(c) The metal thickness

The shape and construction of the item also plays an important role. Thin metal parts heat up rapidly, but also lose heat quickly. Thick metal tends to take longer to heat up, but also retains its heat for a longer time. Consequently, for plastic coating processes, thin walled items usually require a higher oven temperature setting than thicker metalwork. When metalwork contains several metal thicknesses it is best to pre-heat at a high temperature for a short time. In this way the thin metal will heat up to near oven temperature and the thicker metal will have a sufficient heat content to allow PPA 571 to melt without degrading.

Metalwork thinner than 1.5mm will not have sufficient heat capacity for this process and should therefore be coated using PPA 571ES. Our suggested metal temperatures for other metal thicknesses are:

(a) Mild steel sheet between 1.5 and 2 mm thick.
Between 320 and 400°C.

(b) Mild steel sheet between 3 and 5 mm thick.
Between 280 and 360°C.

(c) Mild steel sheet over 6 mm thick.
Between 230 and 300°C.

These figures should be considered as a rough guide only. The preheating times can vary enormously, from under 1 minute to 30 minutes or more. Every new piece of metal will need to have its optimum process times and temperatures worked out, before production quantities can be processed successfully. Overheating must be avoided since this can cause lighter colours to yellow and will reduce the weather resistance of all shades.

2.3.1.2 Fluidised bed coating. After pre-heating, the item is dipped into a bed of fluidising powder. Beds vary in size, hence a laboratory-sized powder bed may hold as little as a few hundred grams of powder, whilst a large scale production model may hold several tonnes. This bed consists of two compartments, one on top of the other. The upper, larger compartment contains the coating powder. The lower compartment, or "plenum chamber", is a reservoir for pressurised air. A porous membrane, sometimes called a diffuser, separates the two compartments. Usually the membrane is made of canvas or a high quality filter paper. The porosity of the membrane is critical to the quality of the fluidisation of the powder. Compressed air is forced into the lower compartment. It diffuses through the membrane and moving upwards, still under pressure. It moves between the fine powder particles that are contained in the upper compartment. Hence the powder particles are separated and the mass increases in volume, which can be up to 30%. As a result the bulk density of the powder is reduced and this permits the preheated metal object to be lowered easily, without any resistance, into the now "fluidising" bed of powder. The powder behaves like a liquid and continues to do so, as long as the air is forced into the lower plenum chamber.

By careful agitation or controlled movement of the hot metal object underneath the surface of the "fluid" powder, the cold powder comes into contact with every point of the hot metal and fuses onto it. Usually, after between 3 and 10 seconds, enough powder has fused onto the metal to produce a satisfactory coating. A thickness of between 300 and 750 microns is suggested for Plascoat PPA 571 in order to achieve the optimum potential of the coating material. Thicknesses outside the recommended range may be detrimental to the coating. Thicknesses above 1500 microns must be avoided.

The metal object is removed from the powder and any surplus powder is blown off, using an airline nozzle at low pressure.

To calculate the specification for a blower to match a tank of PPA 571 see "Blower requirements for fluidised bed systems".

2.3.1.3 Post-heating. If the heat content of the metal is insufficient to effect complete fusion of the powder particles a slightly grainy, gritty or powdery texture will remain on the surface of the coating. In that case it is necessary to put the coated part back into an oven for a short time at a temperature of around 200°C. This allows the fusion process to be completed and a smooth coating to result.

2.3.1.4 Cooling. The part may then be allowed to cool in the air. However to reduce cycle times the items can be quenched in water. One of the advantages of PPA 571 is that it is an amorphous (non-crystalline) polymer and as such it has a relatively low thermal shrinkage. Hence water quenching does not produce the severe pull back experienced with other polymers.

2.3.2 Electrostatic Spraying Process

Plascoat PPA 571ES was developed specifically to be applied by the electrostatic spray process. This process is well known and well documented. However in very simple terms powder is blown through a gun. It picks up an electrostatic charge and is attracted to the object, to be coated, which is earthed. This charge is sufficiently strong and long lasting that the item can be moved into an oven set at 150 to 250°C where the powder fuses to produce a smooth coating. Plascoat PPA 571 is a thermoplastic and as such does not need to undergo the chemical cross-linking or curing process associated with thermosetting systems.

The electrostatic charge may be supplied either from a charged point at the end of the gun barrel, this is called corona charging, or by friction against the walls of the gun, this is called tribo-charging.

When using the corona process we recommend a voltage of 30 to 50 kV. If the voltage is too low then the film thickness will be low and the powder efficiency may be reduced. If the charge is too high the coating may suffer from craters caused by "back-ionisation" and the coating of internal corners and the interstices of wire parts will be poorer because of the Faraday cage effect.

The tribo-charging system is said to be less efficient than the corona system. However the tribo-charging system is more effective when Faraday cage effects are critical, e.g. for coating wire items or box sections, where it has been found to be difficult to achieve a pinhole free coating using a corona gun. Tribo-charging also offers the ability to apply thicker, smoother coating than those obtained by corona charging. This is because they do not produce force fields, nor do they charge the surrounding air. Therefore the tendency to create back ionisation effects is minimised. Back ionisation results in orange-peel, craters, and pinholes in the final coating. However those customers with corona guns should discuss the problem with their gun manufacturer since most manufacturers have developed devices to minimise these effects.

Graphs showing the preferred process conditions can be found in "Plascoat PPA 571ES Processing Conditions"

PPA 571ES has a larger particle size than other conventional powder coatings and therefore if the powder is fed from a fluidised bed hopper the amount of fluidising air must be increased.

2.3.3 Flock Spraying

Flock spraying is sometimes called "powder spray coating". This method consists of blowing powder through a suitable spray gun onto metal items that have been preheated to a predetermined temperature. The powder hits the hot metal and sticks to it, where it gradually fuses to form a homogenous coating. This method of powder application is particularly suited to processing large or awkward objects, which would otherwise be impractical to process by the fluidised bed process. It should not be considered as a commercially viable alternative to fluidised bed coating. Flock spraying has the added benefit that more than one coat of powder can be applied, if the metal object is carefully re-heated before re-spraying. This process can be repeated several times, if necessary, in order to build up and achieve the desired coating film thickness.

A minimum metal temperature of 150 °C must be maintained for adequate adhesion of the PPA 571 to the substrate.

Other Pages

Plascoat PPA 571 - Application over Galvanised Steel

Oxidative Stability of Plascoat PPA 571

Plascoat PPA 571 Chemical Resistance

Plascoat PPA 571 Fire Tests and Fire Fume Certificates

Plascoat PPA 571- Recommended Pretreatment for Galvanised Steel

Plascoat PPA 571 - Recommended Quality Control Procedures

Plascoat PPA 571 - Repairing Damaged Coatings

Plascoat PPA 571- Water and Food Contact Approvals

Plascoat PPA 571 Pre-treatment

Processing Emissions of Plascoat PPA 571