Efectos de ING4 en inducción de metástasis en pez cebra

Paint is a fluid suspension, which is spread in thin coats to decorate and/or protect a surface. It consists of pigment, or colouring matter, and the binder in which the pigment is suspended. Usually, the binder is diluted by the addition of a solvent and these two together become the vehicle that carries the pigment to a surface. It is the function of the vehicle to change its nature (i.e. solidify) and bind the pigment to the chosen surface.

Binder

The binder consists of oils or resins, or a blend of oils and resins. Its essential function is film formation, the ability to change from a liquid film that flows to a more or less hard plastic film. The choice of binder plays the major part in deciding the properties of the paint film. Among the most important properties of the paint film are the

following:-• Adhesion to the surface which is painted (usually called the ‘substrate’).

• Gloss: different binders show a different degree of ‘glossiness’.

• Mechanical Properties: these include such things as hardness, flexibility, resistance to abrasion, impact, or expansion and contraction due to temperature changes. It is important to note here that the mechanical properties of a paint film are much influenced by the nature of the substrate, and that a coating with ideal characteristics for a steel substrate may perform very poorly on, say, a wooden substrate.

Pigments

Pigments consist of small crystalline particles that are insoluble in the solvents that may be used.

They are added to a paint film for many reasons, among which may

be:-• To provide colour and to hide the surface.

• To protect the film and the surface from the effects of Ultra-violet light and weather.

• To decrease the permeability of the film.

• To provide rust-inhibiting properties.

• To add body, i.e. to thicken the paint so that higher film can be achieved.

• To decrease gloss.

• To aid storage properties.

Many pigments are metallic salts (e.g. Iron Oxide, Titanium Dioxide, Zinc Chromate), which were initially chosen by artists and decorators for their colouring ability. Pigments that do not have this colouring ability are usually called ‘extenders’, and are added to paints because of their ability to add thickness and solids to paint films at relatively low cost. Some have other properties; for example, mica is a transparent material that breaks up into flat plate-like particles that will lie flat in a paint film and decrease the permeability of the film. Other examples of extenders are chalk, china clay and talc.

Types of Paint

Paint consists of pigment distributed in a binder. Most types of pigment are compatible with most types of binder, but it is usually from the binder that we take the name used to describe a type of paint. Hence we may recognise the following types of

paint:-• Acrylic

• Phenolic

• Alkyd

• Polyester

• Chlorinated Rubber (currently being phased out)

• Polyurethane

Although it is the binder that has the greatest influence on the ultimate characteristics of a paint film, it is important to realise that the pigments also play their part.

For example, a pigment such as Micaceous Iron Oxide has a very distinctive and characteristic effect upon a paint film to which it is added, and for this reason, it is usually named in conjunction with the film binder (e.g. paints known as Chlorinated Rubber M.I.O. or Epoxy M.I.O.).

Barrier Coatings

Industrial coatings are mainly employed to protect a surface rather than to decorate. The major function of a protective coating is to form a barrier between the surface being protected and the environment.

These coatings, therefore, are designed to prevent the passage of water and oxygen, together with other specific contaminants. In this way, the surface is protected from the corrosion influence, and maintained in good condition.

Unfortunately, no paint material is able to completely exclude water or air. As a result, even the best paints fail to protect steel by eventually failing to prevent the passage of these corrosive agents.

In practise, it is necessary to ensure that the barrier coatings are firmly adhered to the metal surface, and that the coatings are as impermeable as possible. This can be achieved

by:-• Selection of pigments

• Increasing thickness of coating

• Ensuring that coating does not have physical flaws (e.g. pinholes, voids).

Inhibitive Coatings.

If corrosion enhanced conditions exist beneath a barrier coating of the type described above, the general tendency will be for corrosion to take place at the steel surface, travelling sideways along the surface. In this process, the barrier coatings become progressively disbonded from the steel surface, despite the apparent integrity of the coating. For this reason, it is common to include, as part of a multi-coat system, a coating which will ‘inhibit’ the corrosion process. These coatings are commonly known as primers, since they are mainly effective when used as the first or priming coat against the steel surface.

Inhibitive primers, in addition to having good adhesion and good resistance to the passage of corrosive agents such as water, will contain rust inhibitive chemicals.

Temporary Coatings

Many coatings are employed to give temporary or short-term protection to a structure. Amongst these

are:-• Pre-fabrication primers, designed to allow large quantities of steel to be cleaned and coated by machine prior to erection of the structure concerned.

• Rust converters, which transform rust into a more chemically inert material on a partially corroded surface, and are able to protect the surface over a limited period of time.

• Wax based coatings, which are often used in relatively enclosed and static corrosion situations, such as ship’s tanks, but require frequent renewal if they are to remain effective.

• Anti-fouling paints, which have a limited life by design and need to be renewed at regular intervals.

Solvents

Solvents are volatile organic liquids used in paints to reduce the viscosity or consistency of the material and so facilitate the application for the oil or resin present. After application, the solvent is no longer required and should evaporate completely from the film.

Solvents are readily organised into chemical groupings, and some of the more common solvents are listed below as part of their relevant chemical grouping.

• Hydrocarbons: Include Turpentine, White Spirit, Benzene, Toluene and Xylene

• Alcohols: Include Methyl Alcohol, Ethyl Alcohol and Isopropyl Alcohol

• Ketones: Include Acetone, Methyl Ethyl Ketone (M.E.K.)

• Esters: Include Methyl Acetate, Amyl Acetate

Other Important Features of the Solvent.

• The evaporation rate affects the process of application, the film formation and fluidity of the paint. E.g. Cellulose enamel, if sprayed with Acetone as the solvent, can cool the surface so rapidly that the dew point is reached and condensation on the surface causes ‘blushing’. Too slow evaporation can cause running or sags. Too fast evaporation can cause dry spray.

• The Flash Point - is the temperature at which a liquid produces a combustible vapour.

Solvents whose Flash Point is below 21?C are ‘highly inflammable’. The risk of fire is usually small, but naked lights and sparks should be avoided when a solvent of low Flash Point is being used.

• Most solvent vapours are toxic, but if sufficient ventilation is maintained then it should not constitute a health hazard. When extremely toxic vapours are present, then masks and external air supplies should be utilised. Solvents can also cause Dermatitis and skin rashes. Gloves should be worn when dealing with most solvents.

• The cost of most solvents is high as they are derived from the petroleum industry.

Paint Curing

For the great majority of paints, one or more of the following processes are involved in the drying of the applied

film:-• Evaporation of a solvent from the vehicle, leaving behind a film of solid material.

• Conversion of constituents of the vehicle (e.g. linseed oil) to the solid state by chemical changes involving mainly oxidation by atmospheric oxygen. Paints often contain additives known as driers to hasten these changes.

• Polymerisation, or reaction between components of the vehicle, brought about by putting a curing agent in the paint, or by the application of heat.

Very few paints contain no solvent at all, and many will therefore combine solvent evaporation with the chemical change through oxidation or polymerisation. Hence it becomes apparent that the timing of the drying process is critical, and this must be an important consideration during the formulation of the paint.

Some chemically cured resins are capable of use as coatings material, without the addition of solvent. In general, these will be used either in their natural paste form, applied by hand, or applied in the usual way after heating the components to reduce viscosity. One of the major values of solvent free materials is their ability to cure in situations where solvent would not evaporate readily, e.g. underwater. These materials are capable of application at very high film thickness. They do not change their volume on curing and cannot suffer from solvent entrapment. Chemically cured resins can also be applied to a surface in the form of a powder. The powders that are in general use have been developed in such a form that polymerization reaction (chemical cure) is triggered by heating.

Simplified Schematic of the Polymerisation Process

Stage 1 - Material unmixed and as delivered in can. Individual chemical constituents are un-bonded or unlinked.

Stage 2 - Cure agent is added and mixed to base material. Cross- linkage or chemical bonding starts to take place.

Stage 3 - Completion of the chemical bonding that takes place in Polymerisation. A highly chemical resistant matrix is formed.

Polymerisation is affected by the following factors.

1) Temperature - The higher the ambient temperature the faster or more rapid the polymerisation process. Lower ambient temperatures retard the process. Always check with the manufacturer or material data sheet mixing temperatures.

2) Potlife - refers to the time and temperature that the materials are at a workable consistency.

3) Cureing Agent addition - Always refer to the data sheet regarding mixing ratios.

4) Induction or Sweat in time - Some products require a short time after mixing to start the chemical reaction process - usually this is stated in the manufacturers product data sheets.

Coating Systems

There are many types of coating materials and many variations within a single ‘type’. Each coating is formulated for a specific purpose, and it has been discovered that the best coating system for long-term protection is a multi-coat system in which the individual coatings are selected for their ability to fulfil a certain ‘role’.

Some of these roles are:

Priming Paints, which secure good ‘wetting’ and adhesion for the entire system, form a suitable ground for, and to hold, the remainder of the system, and play a special role in controlling the corrosion of steel.

Undercoats, which form a suitable ground for, and hold, the remainder of the system, and act as barrier to corrosive elements.

Finishing Paints, which adhere to and must be compatible with preceding coats, protect the preceding coats against the effects of the environment (i.e. sunlight, atmospheric pollution), and give the system required physical and chemical properties, such as abrasion resistance, water impermeability, chemical resistance, colour, and gloss.

The complete system usually comprises priming, undercoating and finishing paints, although, to an increased extent, undercoating and finishing paints are interchangeable and even priming paints are omitted; but, where this happens, care must be taken to ensure that the paint performing the dual function is really able to do so satisfactorily.

Compatibility of Coatings

Many paint failures are due to the incompatibility of the individual costs, each of which may be good of its kind, but unsuitable for use with the remaining coats in the system. This makes a powerful argument for making sure that any coating system is regarded as a unit and that consideration is given to any possible interaction between the coating materials used within the system.

Problems associated with compatibility (or lack of it) are generally derived from the solvents that are used in a coating. In addition to fulfilling its prime function as a solvent, the volatile content of a coating must be strong enough to soften the underlying coating without being so strong as to destroy it. Some coatings have little or no problems in this direction, others are well known for their adhesion faults, or generally poor compatibility.

One way of minimising compatibility problems is, wherever possible, to adapt a coatings system of a single manufacturer’s products, in line with recommendations made by the manufacturer. This course of action has the additional benefit of allowing only one source of redress should the system fail due to a coating material defect.

Where different coating types are used as part of a coating system (e.g. Epoxy coatings over a zinc silicate primer) great attention must be given to the potential problems of incompatibility.