Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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Techniques for applying organic paint coatings 45

For the application of certain coatings such as zinc-rich coatings, it is

advisable that the pot should be equipped with a mechanical agitator to

keep the zinc-rich coating in suspension so that the zinc does not settle on

the bottom of the pot. If application stops and resumes after 15 minutes

when spraying zinc-rich coatings, the entire length of the hose should be

whipped to re-disperse the coating in the line.

3.3.2 High volume low pressure spraying

This happens because of severe bouncing of the atomized particles from the

substrate surface after impact. If, however, the speed of the atomized parti-

cles is reduced, the bouncing can be reduced considerably, which will improve

spray gun. Simply by reducing the pressure of the compressed gas and using

the same volume of liquid paint, the speed of the atomized particles is

reduced, which in turn reduces the bounce back, thereby enhancing the

and an HVLP spray is easy to set up and simple to operate.

3.3.3 Airless spray

As the name suggests, an airless spray gun does not use compressed air. It

uses a power source (an electric motor) to generate very high pressure of

concentration of atomized paint droplets can be created. The airless spray

process is used for high performance protective coatings such as solventless

systems which have high viscosity and require large pressure for atomiza-

tion. Further, because of the very high pressure, the particles hit the surface

with great force, thereby resulting in a coating of high bond strength. It may

be noted that airless spray guns give a high protective property but a less

smooth surface than is obtained using an air spray gun. Figure 3.5(a) shows

a typical airless gun, and (b) shows the dense atomization obtained as a

result of airless spray. Figure 3.6 shows the fan that is formed during spray

on the coating surface.

The protective property and the DFT achieved on a substrate surface

airless system. It multiplies the air input pressure to deliver material at

pressures up to 4500 psi. A very common airless pump can give an output

to input pressure ratio of 30 : 1, which corresponds to an output pressure

One of the limitations of conventional air spray is its poor transfer efficiency.

the efficiency of the coating. This is done with slight modification in the spray

gun. The modified spray gun is called a High Volume Low Pressure (HVLP)

transfer efficiency from 25–30% in conventional air spraying to about 40–

50%. The high transfer efficiency attained reduces material costs and waste,

the order of 2000 psi. Further, with the help of a high pressure fluid pump,

a high pressure fluid hose, and an airless spray gun with spray tip, a high

depend on the fluid pump which is the most important part of the hydraulic

46 High-performance organic coatings

to input pressure ratios of 45 : 1, 63 : 1 and 80 : 1 are also available com-

more than 500 to 1000 μm in a single coat.

(b)

(a)

3.5 (a) A typical airless gun; (b) the dense airless atomization at

2000 psi.

3.6 A typical fan showing the atomization using 2000 psi pressure

during an airless spray.

of 3000 psi for an input pressure of 100 psi. Other fluid pumps with output

mercially. Such high pressure fluid pumps can give a coating thickness of

Techniques for applying organic paint coatings 47

One of the most important parts of an airless spray is the choice of a tip.

A large variety of airless spray tips are available. Its selection is based upon

the type of material to be sprayed and the desired spray pattern. The tip

accommodate the viscosity of the coating being applied. Fan angles range

from 10° (∼100 mm spray width) to 95° (∼430 mm spray width). For example,

(10° and 40°) will provide fan widths of 100 and 216 mm, respectively, and

at a faster speed; however, this leads to poorer atomization. Dual or adjust-

able tips can be used with airless spray equipment. The tip size increases

as the fan width increases. One of the most severe problems with a continu-

ously working tip is its damage due to wear. This results in decreasing

the fan width, resulting in more material at one place, which causes sagging.

Figure 3.7 shows how the tip gets damaged due to wear. It is advisable

to change the tip when its width starts decreasing. Other advice is to

use tips coated with wear-resistant coatings, such as tungsten carbide

tips which do not wear very fast and have a much longer life than conven-

tional tips.

Safety is one of the main considerations. Care should be taken not

to remove the spray gun from the hose, or the tip from the gun, until

the pressure from the pump and in the line has been released. All

ventilated area. These systems also should be grounded to avoid

the lines. Table 3.2 lists the advantages and limitations of the airless

spray process.

11 in.

(280 mm)

9 in.

(230 mm)

5.5 in.

(140 mm)

12 in.

(305 mm)

New 615 tip

Increased tip wear

3.7 The continuous damage of the tip due to wear with use [1].

There are no controls on the spray gun itself. Tip orifices vary in size to

two tips with an identical orifice size of 0.381 mm but different spray angles

Further, the quantity of the sprayed coating is determined by the orifice

orifice opening and the fan angle control the pattern size and fluid flow.

will have identical flow rates of 0.0145 l/s at 2000 psi [2].

size of the spray tip. A larger orifice results in more fluid being delivered

high pressure airless systems should be sprayed and flushed in a well-

dangerous static sparking, explosion or fire when spraying or flushing

48 High-performance organic coatings

3.3.4 Air-assisted airless spray

sure. The process of atomization involves both the pressure created by the

power source and that created by the compressed air. The compressed air

of parts. It is used preferably for coating structural steel, tubing, engineer-

ing equipment, aircraft and ground support [1].

3.3.5 Electrostatic spray

method is to charge the paint particles just before they are out of the nozzle

and to ground the substrate. Because of the attraction of the charged atom-

ized paint droplets to the substrate, the loss of paint or its rebound is

minimal. This is illustrated in Fig. 3.8.

An electrostatic spray system can be of a simple airless atomization

design or of an air-assisted one. The only difference is that for the charging

of atomized droplets, additional high voltage is required. The charged par-

ticles come out from the nozzle and hit the substrate, which is positively

biased. Thus the atomized particles are attracted to the object and rebound

tages of using electrostatic spray include:

Table 3.2 Advantages and limitations of airless spray

Advantages Limitations

capabilities, greater surface

penetration and rapid coverage

High viscosity products can be

coated without solvents

High bond strength of the

coating

Good when large areas have to

be coated

55%, compared to air spray

Complexity in changing fan width

coating large structures with

Not economical for small structures

or small jobs

Not good for conventional solvent-

borne paint systems

Provides higher film build-up

makes this technique difficult for

varying profiles

Not a good surface finish expected

Higher transfer efficiency, 50–

In order to further increase the efficiency of the airless spray by 5–10%,

and to improve the surface finish, air-assisted guns are used. The improve-

ment in efficiency occurs because the gun operates at lower pressure and

thus particles move less rapidly, which results in a better surface finish.

pressure is about 20 psi. Such a spray gun has several advantages: fine

In order to enhance the transfer efficiency of the paint, an innovative

is minimal. This results in an increase in efficiency to about 75%.The advan-

These are basically airless guns with lower pressure, 800 psi plus fluid pres-

atomization, reduced flow rate, higher transfer efficiency and low wearing

Techniques for applying organic paint coatings 49

• It reduces coating material loss.

• It reduces clean-up and maintenance time, increases production rates,

and reduces the number of application steps caused by wrap-around.

• It results in improved atomization.

One of the most important applications of the electrostatic spray gun

is in powder coating. For small components, the substrates are sprayed

diately melts and then fuses. In the case of bigger components such as pipe-

booth, where the powder is sprayed on the hot substrate, melting and fusing

days (very small regions on the coated surface where the paint failed to

adhere).

substrate with the amount sprayed from the gun, as shown in Fig. 3.9. It is

given as

paint deposited on the part

total

ppaint sprayed

=×

100%

where

= weight of wet coating on the part

= weight of liquid coating sprayed

3.8 Schematic of an electrostatic gun [1].

first with the powder and then passed into a hot chamber. The coating imme-

lines, beams, etc., the component is first heated and then passed to the coating

3.4 Transfer efficiency

The transfer efficiency compares the amount of paint deposited on the

Transfer efficiency (TE) =

immediately. Such a coating has high adherence and flexibility and low holi-

50 High-performance organic coatings

methods.

3.5 Mixing components

It has been already explained in Chapter 1 that many high performance

and protective coatings have two components: the resin and the hardener.

tions and then application is started. There are two very important aspects

of this mixing:

• Pot life

• Method of mixing.

Pot life is the time (beginning from the mixing of two components) at

which the mixture of the two components turns into a solid. This happens

because of a chemical reaction between resin and hardener, leading to an

increase in viscosity due to the crosslinking of polymer chains. However,

the application of paint cannot be possible till the end of the potlife, as it

(Overspray)

3.9 Amount of paint deposited on the substrate compared with that

sprayed from the gun [1].

methods

Application

method

Air spray HVLP Airless

spray

Air-assisted

spray

Electrostatic

spray

Transfer

(%)

30 45 50–55 60 75

Table 3.3 Comparison of the transfer efficiencies of various paint application

efficiency

Table 3.3 compares the transfer efficiencies of various paint application

Just before application, the components are mixed in the specified propor-

Techniques for applying organic paint coatings 51

will become solid in the hose and in the gun. Therefore a different term

which the coating can be applied, either by brush or by a spray method. The

two terms are well illustrated in Fig. 3.10. One important aspect of two-

component paints must be explained here. In the case of solventless systems,

it is advisable to start the coating application immediately after severe

mechanical mixing. However, if the paint system is solvent based, it is advis-

able to wait some time after mixing the two components so that the two

components come in closer contact with each other.

3.5.1 Type of mixing techniques

There are several ways in which two components are mixed:

• Hand mixing

• Batch mixing

• Mechanical mixing

• Electronic mixing.

Manual mixing

Manual or hand mixing is simple and does not require sophisticated equip-

ment. It can be used where

• some materials ratios are not very critical

• production volumes/rates are low.

However, there are issues that must be addressed:

• Operator-related measuring and mixing quality issues

Time

SolidLiquid

Working pot life

Pot

life

Viscosity

3.10 The difference between pot life and working pot life [1].

called the working pot life was introduced, which defines the time up to

52 High-performance organic coatings

• Disposal of cans

• Large quantities of clean-up solvent

• Labor intensive.

Batch mixing

adding new lots they are added by some replacement mechanism, so this

requires more pressure pots. Such a system can be part of an airless spray

package.

Mechanical mixing

Mechanical proportioners are next in line in terms of accuracy of mixing,

as well as being less labor intensive. Mechanical mixing can work on some

is added after the running lot is exhausted. It is more accurate and faster

and gives better results.

Electronic mixing

There are fully microprocessed systems that operate once all the parameters

are well set up on a computer; they are fully automatic, highly productive,

fast and accurate. The quality of coating obtained is uniform and is excellent.

The performance and cost involved in all three systems above are compared

in Fig. 3.11. It is clear that the performance of more sophisticated, electroni-

cally controlled systems is excellent but one has to pay more.

•Mechanical

proportioning

•Manual or hand mixing

(batch mixing)

•Electronic proportioning

systems

Price

•Mechanical with on-line

ratio assurance devices

Performance

3.11 Comparison of various mixing processes on the basis of perfor-

mance and cost[1].

Batch mixing is a modification of hand mixing in which instead of physically

mechanical principle in which a fixed or variable ratio of two components

Techniques for applying organic paint coatings 53

With the availability of several fast-curing coating systems, such as elas-

tomeric polyurethane, polyester and very recently polyurea, which have

curing times of the order of a few minutes down to 10 seconds, the latest

machines are the most sophisticated. Figure 3.12 shows an example of an

airless spray system in which two components reach independently through

two different hoses and pumps at a speed proportionate to their mixing

ratios. They keep on traveling independently till they reach an integrator,

where they are mixed and immediately sprayed from the nozzle to the

substrate. Polyurea has a curing time of 3–10 seconds. In normal spray

pumps, the coating on mixing would become solid in the hoses and would

damage the gun. Such a system is very useful for such applications.

3.6 Summary

A systematic approach to paint application techniques is presented with the

principles of different techniques, methods of application, advantages and

limitations along with some important applications in industry. The author

acknowledges the help of Graco Incorporated for allowing the use of their

diagrams used in training manuals, and the US foundation’s detailed docu-

ment EM 1110-2-3400 partially.

3.7 References

1. Atomization, air spray and airless spray, concept and theory training, Graco Inc.,

1995.

2. Coating applications, US foundation document EM 1110-2-3400, 30 April 1995,

Chapter 8.

Proximity

Sensor

Fluid

Manifold

Integrtaor

3.12 A two-component airless spray machine, where two components

travel separately through Fluid manifold, enter Integrator, are mixed

and sprayed immediately [1].

Characterization, evaluation and

testing of organic paint coatings

A S KHANNA, Indian Institute of Technology Bombay,

India and S Kumar J.K. Surface Coatings, India

4.1 Introduction

One of the important aspects of paint coating, after its formulation, is to

know whether it has achieved the set composition and is able to meet its

functional requirements. Also, it is important to know how the paint will

a large-scale coating work is planned. Thus paint characterization and its

systematic evaluation is one of the most important aspects of paint coating.

This is also important for the inspection and quality assurance of paint

coatings.

Characterization of paint coatings can be divided into the following four

categories.

1. Characterization of paint composition and physical properties such as:

• % Volume solid – which in turn helps to calculate coating coverage

• Non-volatile matter

• Density

• Viscosity

• Volatile organic content (VOC)

• Pigment volume concentration (PVC)

• Wettability.

2. Measurement of paint properties during application such as:

• Drying time

• Permeability

• Water absorption

•

3. Performance testing and coating evaluation:

• Salt spray

perform in a particular environment. Thus it is necessary to test a few specific

properties and also its performance to pre-qualify it for a specific job. Pre-

qualification tests are also important for many industrial applications, before

• Wet film thickness (WFT) and dry film thickness (DFT)

Mechanical properties of paint film and of coated substrate.