Settling, Changes of Viscosity, Colour and finish of Solvent Based paint
The Stability of pigment suspensions and settling out are determined by a number of factors.
- Particle size and shape of suspended pigments.
- Viscosity of liquid phase
- Different in density between pigments and liquid phase should be
- Interaction between different pigments in system.
- Storage condition
- Insufficient stirring or shaking during storage
- The degree of sedimentation increases with the duration of storage. The pigments finally form a more or less solid sediment on the bottom of the container which cannot be stirred up again.
- The supernatant material then contains more oil or varnish and is thus glossier, while the lower strata of paint have a lower oil content as well being flat.
- In view of the difficulties frequently encountered in stirring the sediment back into the vehicle, it is always preferable to prevent appreciable sedimentation from the beginning. Round and square containers, which must, of course be well closed, should be stored lying on their sides and be rolled or turned at certain suitable intervals. The paint must be stirred prior to pouring into the pots and during application. It is clear; of course that stirring the paint into homogeneous mass must be effected correctly. The process described above, know to older technicians as BOXING
- Even if no noticeable deposit is felt at the bottom of a can when inserting a stirring rod , it is essential to stir thoroughly, because the unavoidable gravitational sedimentation of pigments leads to a higher pigment content in the lower part of the container. Manual stirring is effective in this case, and an upward and circular motion of the stirrer is advised.
- Excessively warm store-room increase the fluidity of the vehicle and thus promote the settling tendencies of the pigments.
Spraying Techniques and Spray gun Problems.
The properties of a sprayed film, especially if air pressure is used , depend on the following factors. Some of them also to other spraying methods.
• Size of droplets directly at the nozzle (0.005 to 0.01mm)
• Speeds of the droplets between nozzle and sprayed surface (1/200th to 1/100th Second) impingement force ( 33 to 130 ft/s = 10to 40 m/s).
• Angle of impingement ( 30 to 45 degrees)
• Uniformity of distribution of droplets at the points of impingement depending on the number of headings given.
• Spraying distance and air pressure can influence the drying time to certain extent.
• Working speed and experience of operator.
• Ratio of available compressed air to coating (0.5 1tr of paint may need 500 ltr of air)
• Temperatures of the coating, air (for the spraying ) and of the surrounding atmosphere.
• Shape , size and temperature of object to be sprayed.
• Number of coats.
• Nature of base or undercoating.
• Viscosity of the coating.
• Properties of the solvents which have been added in thinner.
• Diameter of the fluid tip nozzle of spray pistol.
• Intensity of the draught in the spray booth, which depends on the strength of the exhaust and cleanliness kept in it. This item is of special importance if the spraying are large.
• The compressed air used must be carefully filtered and must not contain any oil or moisture.
• Non–continuous , dotted film due to excessive pressure in relation to the fluid nozzle or to the very fast drying of some type of coatings, particularly those drying by evaporation if they contain a high percentage of very volatile diluents.
• Too high spraying pressure or too close spraying can produce blowholes.
• Careful selection of correct solvents pressure and matching nozzle size (often by trial and error) is essential for satisfactory results.
Dry spray and cobwebbing
The dry spray defect may not only be noticeable by sandy looking surface but in various other forms, e.g. a kind of orange peel effect. In all cases the film strength is reduced . transparent lacquer film may only show cloudiness
In slight cases, but pigmented coatings, in bad cases, show discontinuous films, in slight cases a dulling effect. As the adhesion of such films is reduced they may tend to scaling.
Dry spraying films can sometimes be improved by rubbing down with water resistant abrasive paper of finest grain and re-spraying.
Viscous lacquers films containing highly polymerized synthetic products. e.g vinyl esters acrylic etc. in presence of very volatile solvents, often difficult to spray and exhibit cobwebbing insted of dry spray .
Too great a spraying distance, too wide a spray fan and excessive air pressure in relation to the size of the nozzle, also excessive temperatures in the spraying shop or the air line causes the coating to lose part of their most volatile solvents during the spraying process before reaching the objects and to impinge on the sprayed surface in partly dried state, producing in the worst case a more or less gritty film.
The speed of evaporation of solvents increases as much as 3% for every rise in temperature of 0.5 deg centigrade.
Contrary to dry spraying, too close a spraying distance may result in an uneven film surface with ripples or small grooves, caused by the high velocity of air / coating mixture. The heavier a film is put on this way and especially if the surface is vertical, the greater the probability of failure.
Suitable counter–measures are: correction of the spraying distance, pressure or room temperature, or of all three factors., if until the fault is eliminated, i.e. until a uniformly spreading wet film surface is produced .
To avoid cobwebbing it is particularly important to use correctly formulated thinners in right dilution ratios. Generally the volatile portion coming off last should always be a good solvent for the medium contained in coating.
Too low top or atomizing pressure effects the flow badly and produces orange peel.
If the surface to be sprayed is not held at a correct angle towards the jet, even thin oil based enamels may induce dry spray.
Badly functioning air suction of the exhaust fan may cause precipitation of dry lacquer/paint dust throughout the entire shop and on freshly lacquered surface.
Overspray can also cause defective work e.g cratering and bleeding.
The formation of mist is promoted by
- Strong cooing
- The presence of moisture particularly when the relative humidity is high.
- Existing condensating nuclei (spraying dust)
These difficulties can be eliminated by following measures: reduce the spraying pressure, filter and preheat the compressed air, make certain of dry shop atmospheres and dry compressed air, and use only solvents and pigments free from moisture.
Curtaining , running and sagging on vertical surfaces: pulling away from sharp edges and corners bad flow.
A downward movement of paint film between the times of application and setting, resulting in an uneven coating having a thick edge. The resulting sag is usually restricted to a local area of a vertical surface and may have the characteristic appearance of draped curtain, hence the synonymous curtaining.
Narrow downward movement of paint film : may be caused by the collection of excess quantities of paint at irregularities in the surface, e.g cracks, hole etc., the excess material continuing to flow after the surrounding surface has set.
These may result from the following causes.
- excessively heavy application generally. Special attention being paid to mouldings and corners.
- heavy coats will tend to sag on sag on flat vertical areas and will drain in to corners and cause runs.
- with spray paint generally, it is necessary to select the correct nozzle size and to adjust the air pressure so that a film of correct thickness is obtained. Spray strokes should overlap only to extent necessary to give uniform film thickness.
- excessive thinning with rapidly evaporating solvents, such coatings may be tuch-dry before it is possible for them to flow out smoothly, especially if the room temperature is too high. Too strong an exhaust at the spraying booth may cause somewhat similar failures.
- fresh paint applied over an old but still glossy paint may sag. Old paint should be well rubbed down to remove all gloss.
Defects apparent shortly after application
Formation of blisters and blowholes in filler coats on porous substrates.
Blistering is the formation of dome-shaped projection in paint and varnish film by local loss of adhesion and lifting of the film from underlying surface.
- excessive thick film. The solvent and thinners cannot evaporate completely during the period of plasticity of the coating to permit the latter to flow out and form a continuous film closing the pinholes originated by evaporation. Pores of e.g pinhead size are thus developed in air drying systems.
- Premature application of the superimposed film i.e before the previous heavy layers of e.g. filler / primers have had a chance to dry thoroughly. Solvent residues penetrate through the covering film, which can also cause swelling and lifting.
- Fast air drying films, if applied too thickly sometime exhibit bad flow, leading to the formation of numerous tiny blisters. Relatively rapid temperature increase favours the failure.
- Thick air drying films sometime blister due to entrapment of solvent if insufficient time has been allowed before overcoating.
- Traces of oil from the compressors or moisture from condensation, especially on days of high relative humidity, in the lines of spraying plants, or if the oil and water separators have not been cleaned for sometime, may cause large blisters.
Difference of shade in paints by comparison with standard
The colour comparisons should always be carried out under strictly standardized conditions.
if for any reason the degree of gloss or of flatness of a coating differs due to either to the properties of a pigmented or of a clear last coat, the colour shade when dry will appear to differ, even if it is correct in the wet condition. Matt films will look whiter , glossy film darker.
The surface condition of the base or undercoating as well as its colour, influenced the colour shade of the last coat.
It is also important that separation of mixed oigments having different specific gravities should be prevented in containers for a considerable time. It is obvious that in case of separation of these pigments and if thorough stirring is not successful the colour shade of the resulting film cannot be the same as those of the formerly homogeneous paint. It is even possible in instances of this type to obtain very differently coloured films from the same container.