dinsdag 24 augustus 2010

Application Conditions

When applying coatings, the most important factors to consider are the condition of the substrate, the surface temperature, and the atmospheric conditions at the time of painting.
Paint application should only be carried out when good atmospheric conditions and clement weather prevail.
Painting should not be undertaken;

• when the air temperature falls below the lower drying or curing limit of the coating,

• during fog or mist conditions or when rain or snow is imminent,

• when the surface to be painted is wet with condensation or when condensation can occur during the initial drying period of the paint.

During the night steel temperatures fall. They rise again during the day but there is always a lag in movement of steel temperature compared to the atmospheric condition, so condensation on the steel surface is possible. Condensation will occur if the steel temperature is below the dew point of the atmosphere.

Bad weather is a familiar problem to those using protective coatings. Relative humidity itself rarely creates a problem. Most paints will tolerate high humidities, but humidity should not be permitted to lead to condensation on the surface being painted. In order to determine whether or not a surface is wet, the steel
temperature should be measured using a surface temperature thermometer and the dew point calculated after measurement of humidity with a hygrometer. Paint application should not take place when steel temperature is less than 3°C (5°F) above the dew point.

Paint should not be applied when surfaces are affected by rain or ice. Some two component paints ( for example certain traditional two component epoxy coatings) should not be applied at low temperatures as curing may be retarded.
Generally, extreme conditions refers to ambient temperatures below 5°C (41°F) or above 40°C (104°F).
Below 5°C (41°F) the curing of coatings, such as traditional two component epoxies, slows down dramatically and for some paints curing stops altogether. Other protective coatings are not so severely affected; Chlorinated rubbers and vinyls are quite suitable for use at temperatures below 0°C (32°F) provided that the surface is clean and free from ice or frost.
At the other extreme of 40°C (104°F) and above, the drying and curing of paints is rather rapid and care should be taken to avoid dry spray. This is caused by the too rapid loss of solvent from paint droplets between the spray nozzle and the surface. It can be avoided by:-

(i) Keeping the spray gun at the minimum suitable distance from the work piece, spraying consistently at 90° to the surface being painted.

In conditions of high temperature, techniques must be adopted to prevent defects such as voids, pinholes, bubbles and poor coverage due to the over rapid evaporation of solvent.

Paint Application by Spraying

Conventional air spraying. This is a widely accepted, rapid method of coating application in which paint is atomised by a low pressure air stream. “Conventional” air spray equipment is relatively simple and inexpensive, but it is essential to use the correct combination of air volume, air pressure and fluid flow to give good atomisation and a paint film free from defects. If conventional spray application is not controlled correctly, large losses of paint can result from overspray and rebound from the surface in addition to problems such as poor flow, sagging and pinholing. The major disadvantage of conventional air spray is that high build coatings can generally not be applied by this method as most paints have to be thinned to a suitable viscosity for satisfactory atomisation, and so lose their high build properties.

Pressure feed tanks or “pressure pots” are commonly used in association with low pressure air stream (conventional) spray guns, to provide a means of delivering paint at a regulated pressure from a tank, through a fluid hose to a spray gun. Several manufacturers manufacture suitable equipment which operate in the following manner:
A length of air hose from the compressed air supply is connected to an air pressure regulator on the tank lid. Some air bleeds through the regulator at an adjusted pressure into the tank but most of the air passes the regulator and reaches the spray gun through a second length of air hose to atomise the paint as it is sprayed. The air which has entered the tank forces paint from it to the gun through a length of fluid hose. Paint in the tank can be prevented from settling by means of an agitator driven by hand or by a compressed air motor.
Air spray (pressure pot) is recommended in cases where large quantities of paint are to be applied, and their use instead of a suction or gravity feed cup attached to the gun significantly reduces waste time in constant refilling and also enables the gun to be turned to any angle to coat objects effectively without spilling paint.
Pressure feed tanks up to 20 litres (5 US gallon) capacity can be used and allow ease of movement around the workplace.

Airless spraying is unlike air spray techniques, the air is not mixed with the coating to form a spray, hence the name airless spray. Atomisation is achieved by forcing the paint through specially designed nozzles or tips, by hydraulic pressure. The required hydraulic pressure is usually generated by an air powered pump having a high ratio of fluid pressure to air input pressure. Pumps with ratios between 20:1 and 60:1 are available, perhaps the most common being around 45:1.
The chief advantages of airless spray are:

1. High build coatings can be applied without thinning.

2. Very rapid application is possible, giving an economic advantage.

3. Compared to conventional spray, overspray and bounce-back are reduced,leading to reduced losses of    material and less dust and fume hazards.

The tips, through which the paint is forced to achieve atomisation, are precisely constructed from tungsten carbide. The atomised “fan” is produced by a slot ground on the face of the orifice. Various orifice sizes together with different slot angles are available. The choice of tip is governed by the fluid pressure required to
give atomisation coupled with the orifice size needed to give the correct fluid delivery rate. The fluid delivery rate controls the film thickness applied. Different slot angles produce spray fans of different widths. The selection of a particular fan width depends on the shape and size of the structure to be painted.
Choice of fan width is also related to orifice size - for the same orifice size, the paint applied per unit area will be less the wider the spray fan.

Airless spray equipment normally operates at fluid line pressures up to 352kg/cm2 (5,000 p.s.i.), and should always be used in accordance with the manufacturer’s operating instructions and safety precautions.
Generally, tips with an orifice size 0.23-0.33mm (9-12 thou) are suitable for coatings to be applied at approximately 50 microns (2 mils) wet film thickness. Tip sizes from 0.33-0.48mm (13-19 thou) for wet films of 100-200 microns (4-8 mils) and 0.48-0.79mm (19-31 thou) for 200 microns (8 mils)and above. Heavy duty mastics which are applied at very high film thicknesses may need tips with orifices as large as 1.02-1.52mm (40-60 thou).

There are several designs of tips available, the choice of which depends upon the finish required, the ease of application and ease of clearing blockages from the tips.
With some products, the decorative effect achieved with airless spray is not as good as can be achieved by conventional spray. However, airless spray application is now widely accepted as a convenient method of applying high performance protective coatings.

Paint Application by brush

Brush application should always be undertaken using good quality natural fibre or synthetic brushes of the appropriate size compatible with the product being applied.
However, this application technique is relatively slow, but is generally used for coating small areas with decorative paints and for surface tolerant primers, where good penetration of rusty steel substrates is required. It is particularly suitable for the application of stripe coats and for coating complex areas where the use of spray methods would lead to considerable losses due to overspray and associated dry spray

However, most high build coatings are designed for application by airless spray, and high film build will generally not be achieved by brush application. In general, twice as many coats will have to be applied by brush to achieve a similar build when compared to airless spray.

Brush application requires considerable care when applying non-convertible coatings over one another, e.g. chlorinated rubber over chlorinated rubber, or vinyl on top of vinyl. In these cases, the solvents in the wet coat readily redissolve the previously dry bottom coat. Even a mild degree of the “brushing-out” normally
given to topcoats will cause pick-up of the previous coat and result in a very poor finish. Even, light strokes should be used in these circumstances, covering a particular area with one or two brush strokes, and on no account working the bristles into the previous coat.

zaterdag 7 augustus 2010

Calculations required amines in Epoxies

The following terms and calculations are useful to be used when formulating with epoxies.

Epoxy Equivalent Weight (EEW)
The EEW is the number of grammes of epoxy resin required to give one mole of epoxy groups.
To calculate the EEW use the following formula:

EEW = MW / number of epoxy groups of the resin

Epoxy Value
The epoxy value is the number of moles of epoxy group per 100 g of resin.

Epoxy value is: (100 / MW epoxy resin) x number of epoxy groups in the resin

Amine Value
The amine value is the number of milligrams of KOH equivalent to one gramm of curing agent.

Amine value is: (No.of Nitrogens x MW KOH (=56,1) x 1000) / MW amine

Hydrogen active equivalent weight
This can be defined as the quantity of curing agent in gramms which contains one mole of active hydrogen.

H active Equivalent weight:  MW amine / number of active Hydrogens

To calculate the desired stoechiometric quantity of amine in weight parts per 100 gramms epoxy resin the following formula can be used:

(Eq.W. Amine / EEW) x 100

donderdag 5 augustus 2010

Calculation required Isocyanate in Polyurethanes

When reacting an isocyanate in one or more polyols to form a polyurethane, one NCO group reacts with one OH group. When the number of NCO groups equuals the number of OH groups, you have a stoichiometric NC) : OH ratio of 1,0.
This ratio is commonly referred to as the index.
To determine the amount of isocyanate required to react with a given polyol blend, you must know the desired index (often 1), the isocyanate equivalent weight and the weight fractions (pbw) and equivalent weights of the polyols and any water present in the blend.

The following calculations give the stoichiometric ratio between grams of solid isocyanate resin and 100 grams of solid polyol resin.

By given OH % and NCO %:

(%OH / % NCO) x 247

% OH calculated from OH number = OH number/33
% OH calculated from OH eq.weight = 1700/OH eq.weight
OH number calculated from OH eq.weight = 56100/OH eq.weight
% NCO calculated from NCO eq.weight = 4200 / NCO eq.weight

Functionality of a polyol in polyurethanes

It is assumed that the functionality of a polyol in polyurethanes must be at least 2 in order for the resin to contribute the crosslinking network. The probability of having low molecular weight fractions with no functional groups or only one functional group can have very serious implications.
Molecules with no functional group will act as plasticisers, presumably changing the mechanical properties of the film. Very low molecular weight fractions of this type (dimers, trimers) may even have a sufficiently high vapour pressure during curing to evaporate off (=VOC).
The presence of oligomer molecules with a single functional group will lead to chain termination in crosslinking, which will also affect the mechanical properties of the cured film. Unfortunately, the probability of obtaining oligomers with inadegquate functionality increases as the average molecular weight is reduced, necessitating judicious control of the polymerisation process.

The average functionality (the number of functional groups per polymeric chain) of a polyol can be estmated by:

f(OH) = (OH number x Mn)/56.100

OH number = 56.100/OH eq.weight

Mn = OH eq.weight x f(OH)

%OH = OH number/33

zaterdag 31 juli 2010

The Role of Paint and Coatings in a Green Building

Despite the current economic crisis that plunges the construction business ever downward, the "green building" trend is taking off. Even when new construction projects are in a decline, there is increasing interest in green building methods and materials. Building owners want to save on construction and post-construction costs and are becoming increasingly aware that once the building is operational, tremendous cost-savings will come in the form of lesser utility bills and maintenance costs. As a result of this mindset, the U.S. Green Building Council (USGBC) estimates an average return on investment of 20% on green buildings.

For building owners and occupants, cost is not the only advantage of a green building. Increased health and safety is also a major benefit. The reduction of natural resources consumption needed to operate the building, as well as the lesser amount of pollution that the building is giving off, are also important aspects in sustainability.

For commercial painting contractors, the demand for greener buildings cascades down to the demand for greener paints and coatings, raising the need for paint and coating products that are not only providing beauty and protection, but also help create an environmentally- safe environment.

In response to the growing demand for environmentally-safe building products, paint manufacturers have been redesigning their existing paint formulations and developing new paints that are eco-friendly and compliant to environment standards and regulations. Even raw material providers have been teaming up with paint makers to further advance the "green" paint technology.

Read for more information:
The Role of Paint and Coatings in a Green Building