Theory: How does a bar coater work ?

Sample preparation using wired bars

Sample preparation of paints, inks and other surface coatings onto their substrates is a very important operation for any laboratory working with these materials.

The samples produced tend to fall into the following categories:

• Quality control of current products.

• Research and development into new products of alternative materials.

• Samples for customers to approve.

Many products can be applied by wire bar coating and the results tested for various physical properties.

WIRE BAR COATING

This method is suited to materials with a viscosity range from about 1 to 1000 centipoise, subject to them flowing out after coating. The thickness of coating with normal close wound wire bars is from about 4 to 160 micron. For higher coating weights up to 1000 micron refer to ‘special applications’.

The actual deposit from a wound bar will depend on the absorbency of the substrate and the flow characteristics of the coating material. Fig 1 shows two turns of wire and the area of coating, A, that can pass between each turn. When divided by the wire diameter D this represents the theoretical thickness of even coating T. This can be calculated to T= 0.107D.

This assumes full flow with no losses due to friction or substrate. In practice approximately 25% is lost, altering the thickness of T = 0.08D. (see Table 1).

If the specific gravity = 1 then a 10 micron coating is equal to 10 gmsm² (gsm). The dry film weight may then be calculated from the % solids.

E.g. Applying a coating with a solids content of 40% and a specific gravity of 1.05 with a bar wound with 0.4mm diameter wire.

0.4 x 0.08 x 1.05 x 40/100 = 0.0134 g = 13.4gsm.

As this is a linear equation it is easy to obtain a chosen coating weight if a range of bars is available, simply by weighing the first result and calculating.

E.g. Coating weight achieved was 13.4gsm with 0.4mm wire and the weight required is 16 gsm:

0.4 x 16 / 13,4 = 0.475mm dia wire

Bars are generally available in increments of 0.025mm (0.001”) diameter from 0.05mm to 2.0mm (.002” - .080”) which equates to 2 µm steps in wet film.

REPEATABILITY

To achieve good repeatability the bar should be used in conjunction with a suitable machine featuring repeatable drawdown speed and repeatable pressure on the bars. Infinitely variable speed between 10 and 250mm/sec is recommended to cater for all products.

In multi-user situations such machines are essential to provide comparable results. With experience, colour differences as low as E = 0.2 are achieved in database preparation for computer colour match prediction equipment.

CALIBRATION

This is relatively simple to achieve during manufacture as wire can be very accurately measured in diameter prior to winding. However, when subsequently being used for coating, different techniques must be used. If a suitable stable coating mixture and substrate are available, about four coatings should be made with each new bar requiring calibrating. These coatings should then be measured instrumentally for say coating weight or colour strength and an average taken. An allowable tolerance should be established. Then at regular intervals this test should be repeated and the bar replaced when it falls outside the tolerance limits.

Alternatively, where stable products are not available a new bar should be kept as the standard bar and only used for making three or four draw downs for comparison with those from the bars requiring calibrating.

COATING BEDS

Material for the coating bed should be available with various resiliences to allow for different substrates eg a gravure ink onto paper should have a harder bed than a gloss paint onto card.

Alternatively, a magnetic bed for steel paint test panels will ensure that they are held flat during coating and provide the best samples for subsequent accelerated weathering. A vacuum bed will hold non-dimensionally stable materials such as polythene or virgin aluminium foil.

SPECIAL APPLICATIONS

For higher coating weights and viscosities, bars may be wound both in a spiral fashion (Fig 2) or double wound (Fig 3).

By varying the numbers of turns per inch and wire size, various effects can be achieved.

Wire bars are also used on production machines as an easy way to control coating weight. Typical uses include adhesive coating, paper coating, carbon film etc