Physical Application Methods

There are many types of physical methods including brushing and spraying ¹⁰⁹, and each method is used depending on the application. These methods include brush, pad, spray and dip applications, which will now be described.

2.5.3.1.1 Brush Application

There are a number of different brushes available: narrow and wide, long and short-handled, and nylon, polyester or hog bristle ¹¹⁰. Nylon bristles can be used for water-based coatings but not for some solvent-based coatings as they would swell. Polyester bristles can be used for both water and

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solvent-based coatings. Hog bristles can be used for solvent-based coatings but not water-based coatings. The coating is held in place in between the bristles in all brushes and all have a large number of bristles. The coating is applied to surface by the pressure being exerted on the brush forces the coating out from between the bristles and this then splits the layer of coating so that part of the coating is left on the bristles.

The viscosity of the coating is important when using brushes as when the viscosity is too high there will be too much on the brush and it will hard to apply the coating; when the viscosity is too low there will be too little on the brush, but it will be easier to apply the coating ¹⁰⁹.

2.5.3.1.2 Pad Application

Pad application is similar to brush applications but instead of bristles as the applicator a sheet of fabric is used to apply the coating. The most common pad applicator is a nylon pile fabric that is attached to a foam pad ¹⁰⁹. However, for coatings with a low viscosity (such as varnishes) a pad made from lamb’s wool is used. The advantages of using a pad applicator than a brush applicator are that pads can hold more coating material than equivalent width brushes and the coating surface tends to be smoother than when using a brush. However, a disadvantage would be that a tray must be used with pads therefore more wastage is produced and solvents can evaporate.

2.5.3.1.3 Spray Application

This method is widely used and is much faster than using either brush or pad applicators ¹⁰⁹. A main disadvantage to this method is the fact that it is hard to apply the coating exactly where desired, therefore needing areas to be masked where the coating isn’t desired. Another disadvantage would be the low efficiency of the method because only a fraction of the coating particles are deposited onto the substrate. There are many different types of spray application but all atomise the liquid coating being used.

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The low efficiency of this application method is due to the atomised droplets either bouncing of the substrate or not coming into contact with the substrate ¹⁰⁹. The droplets bounce off the substrate due to the eddy currents of air that are formed by the high pressures of the spray applicator. Higher pressures increase the forward velocity of the air therefore more droplets will bounce off the substrate. The two reasons for a number of droplets not coming into contact to the substrate is due to: overspray, which is where the droplets go past the substrate; and fall out, which is where the droplets drop in an arc due to gravity – this problem is particularly affected by distance between spray nozzle and substrate.

2.5.3.1.3.1 Compressed Air Spray Gun

This method of spraying atomises the coating by using fine streams of compressed air and is the oldest type of spraying method ¹⁰⁹. This method transfers the coating by driving it through the nozzle by low pressure (1-5 kPa) and the stream of coating is then atomised using fine streams of compressed air (25-50 kPa). The degree of atomisation is dictated by several factors:

1. The viscosity of the coating being applied, where lower viscosities will produce low shear rates and form smaller droplets.

2. The air pressure being used, where higher pressures will form smaller droplets.

3. The diameter of the orifice where the coating is exiting from, where a smaller orifice will form smaller droplets.

4. The pressure forcing the coating through the orifice, where high pressures will form smaller droplets.

5. The surface tension of the coating material, where a lower surface tension will form smaller droplets.

A big disadvantage to using compressed air spray guns is the very low transfer efficiency (percentage of droplets leaving the gun that are then deposited on the substrate) which is 25 % ¹⁰⁹. However, if a high volume, low pressure air gun is used then the transfer efficiency can be increased up to 65 %.

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This is where the guns are operating at low air pressures (2-7 kPa) and higher air volumes. The increase in efficiency is due to the reduction of droplets being rebounded back off the substrate because of using a lower pressure.

2.5.3.1.3.2 Airless Spray Guns

In this spray method the coating is forced out of the orifice at high pressures (5-35 MPa) and as the coating exits the orifice the pressure is released which then causes cavitation of the particles, thus leading to the atomisation of the coating ¹⁰⁹. The degree of atomisation is dictated by several factors:

1. The viscosity, where decreased viscosities will produce smaller particles.

2. The pressure used, where increased pressures will produce smaller particles.

3. The surface tension of the coating material, where a lower surface tension will produce smaller particles.

Compared with compressed air spraying, this method generally produces larger particles and in general more uniform thicknesses can be achieved using the compressed air method ¹⁰⁹. When using this method, solvents with a higher relative evaporation rate should be used because of the larger sized particles that are produced compared with the compressed air method. However, the advantage over the compressed air method is that the airless method is quicker. Another advantage would be the reduced bounce back due to no air being used to propel the coating, with a transfer efficiency of 40 %.

2.5.3.1.3.3 Electrostatic Spraying

In this method the coating material is electrically charged by, in its simplest form, a wire in the orifice which has an induced charge on it between 50-120 kV ¹⁰⁹. At the end of the wire the discharge causes the air to ionise and as the atomised coating material passes through the ionised gas, the particles are negatively charged. The substrate that is to be coated needs to be electrically grounded, so when the particles coating approach the surface the differential in charge attracts the particles to the surface.

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The main advantage for this method is the increased transfer efficiency of 60-85 %; this is because when overspray occurs the particles can be attracted back to the other side of the object being coated.

However, if the conductivity of the coating material is poor then this method would be very inefficient as the particles won’t be charged sufficiently by the ionised gas. This undercharging would be more common with coatings that contain only hydrocarbon solvents, therefore needing to substitute some of the hydrocarbon solvents for nitroparaffin or alcohol solvents to increase the conductivity of the coating material. Also for coatings that contain a free carboxylic acid group, a tertiary amine can be added to improve conductivity. However, if the conductivity is too high there is a risk of electrical shorting and coatings that are water-based tend to have a higher conductivity than solvent-based.

Therefore, to minimise the chance of electrical shorting through the spray of material, the gun should also be grounded.

2.5.3.1.4 Dip Coating

This method is extremely simple by the fact that it’s simply the object to be coated being dipped into a tank of the coating material and being pulled back out ¹⁰⁹. Any excess material is subsequently drained off the object and back into the tank. In practice this method does tend to be more complex, due to a difference in film thickness across the object due to excess material drainage and solvent evaporation rate. The film formed tends to be thicker at the bottom of the object than the top because the material flows from top to bottom when withdrawing the object from the tank. Film uniformity can be optimised by carefully controlling the withdrawal rate and the rate of evaporation; therefore if the object is withdrawn slowly enough and the solvent evaporates quickly enough then a uniform film can be achieved. However, in production the object tends to be withdrawn quicker than the optimum speed therefore leading to differences in thickness across the object being coated.

The viscosity of the coating will also affect the thickness of the film as coatings with higher viscosities will form thicker films ¹⁰⁹. The viscosity can also change due to the evaporation of the solvent and as the solvent evaporates more, the viscosity of the coating material increases. Another way for the

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viscosity to increase would be due to chemical reactions between individual components in the coating material. Therefore great care is needed when using the dipping technique and if volatile components are used such as oxidising alkyds, then an antioxidant must be added because the oxidising chemical will oxidise which leads to cross-linking. However, the antioxidant must be volatile enough to evaporate in the early stages of curing as it will inhibit the cross-linking when the film cures;

also the antioxidant can’t be too volatile so as not to rapidly evaporate from the tank.