POLYESTER AND EPOXY PIGMENTS
Polyester are liquid-thermosetting resins which are mainly for moulding Fibreglass Reinforced Plastics (FRP). These resins are pigmented not only for their aesthetic appeal but also for their protection against ultravidet (UV) radiation. Dry pigments, as a rule can not be directly incorporated into the liquid resin because of the difficulty of obtaining uniform desper sion. As such the resins are almost always coloured with pre-dispersed pigment
Pigment pastes are made by dispersing dry pigments in a suitable vehicle. The vehicle can be a plasticiser or a reactive resin. Plasticisers because of their incompatibility, can adversely affect the mechanical and physical properties of the cured polyester resins. Besides they tend to migrate to the surface, where they will interfere with the adhesion of bonded joints and painting. Pigments dispersed in a plasticiser are not, therefore, recom- ended for colouring fibreglass mouldings.
Uv absorption characteristics Heat resistance
They have excellent stability to sunlight or artificial light. They will not fade or discolour when used indoors.
For outdoor applications the durability of the pigment depends on the environmental conditions, which include heat, light moisture, oxygen, and other industrial effluents present in the atmosphere. Generally the darker shades will have better stability than the pastal shades. Increasing the concentration of the pigment, especially the pastel colours, in the gelcoat will increase its durability. Under extremely severe conditions of exposure, the performance of the pigment can be greatly improved by employing weather-resistant resins.
Uv radiation, from sunlight, accelerates the chemical and physical degradation of the polyester resins exposed to outdoor conditions. The degradation occurs mostly in the surface of the moulding. But once degraded the surface tends to chalk and erode, due to weathering, exposing fresh surface to further attack. Protection of outdoor structures fabricated with polyester resins is therefore important.
Pigments absorb and convert UV radiation into harmless heat. As such they will greatly prolong the outdoor service life of the moulding. The effectiveness of the pigment is dependent on its connection near the surface. In other words, increasing the concentration of the pigment in the gelcoat will improve the durability of the laminate
The various pigments differ in their UV absorption characteristics. White, black and red oxide pigments absorb the radiation almost completely. Pastel shades, where these pigments are present predominantly will absorb the radiation to varying degrees.
The term heat resistance usually implies resistance to heat during processing. Heat resistance of the pigments depends on
Exposure temperature Exposure environment
Heat curing cycles for polyester sheet / dough moulding compounds (SMC/DMC) may vary from 1-10 minutes at 120-175°C depending upon the thickness of the moulding and the catalyst employed. The pigments recomended for these applications will withstand a temperature of 150°C for 15 minutes or longer, and for shorter times of higher temperature.
However the highly oxidising conditions during processing of sheet/dough moulding compounds may have an adverse effect on some of the pigments. Besides the pigments may also effect the shelf-life and/or the cure-characteristics of the moulding compound especially in presence of benzoyl peroxide. For these reasons, the pigments must be carefully evaluated before using in such applications.
Generally the heat resistance of a pigment can be improved by increasing its concentration in the formulation.
Chemical plants and equipments fabricated in FRP not usually pigmented because of the severe service conditions found in chemical plants.Neverthless the pigments used for colouring bath tubs must be resistant to soaps and mildly alkaline detergents. The pigments used for domestic appliances must be resistant to oils, fats, juices and detergents.
Polyester laminates and castings are used to a large extent in electrical applications. The unpigmented mouldings are prone to surface degradation, which will adversely affect the track resistance, surface resistivity and the dielectric strength of the moulding under high voltage conditions. It is therefore, important to protect the mouldings with suitable pigments having good dielectric properties.
Fibreglass reinforced Plastics are being increasingly used for handling and/or storage of alcoholic beverages, fruit juices, milk and milk products, edible oils and fats, fish, meat and other food products. Where pigmentation is required in such applications, it is important that the pigments are completely non-toxic. Special pigments can be offered for such applications.
Effect of Pigment on Gelation and Curing
Some pigments may have a retarding or an accelerating effect on the gelation of the polyester resin. These effects can be minimised by adjusting the quantity of catalyst or the accelerator, or both.
In the presence of certain pigments the accelerator, used in polyester formulations, is proneto loss of activity due to adsorption. Certain reactive pigments may also tend to cause thickeningof the resin on prolonged storage. It is, therefore important that the pigmented master batches of pre-accelerated resins not stored indefinitely.
Effect of Fillers on colour
Fillers are used for extending polyester resins for a variety of reasons. Fillers opacify theresin and impart colour to the cured resin. To overcome the natural effect of fillers on the colour of the molding, it is often necessary to add substantial amount of the pigment.
Fillers also accelerate the fading of coloured pigments, particularly the pastels, under outdoor conditions. The effect can be minimised by increasing the concentration of the pigment.
When fillers are used, they must be thoroughly incorporated in the resin. Incompletely bonded particles tend to absorb moisture which affects the resin-to-glass bond in the laminate. Used judiciously fillers may actually improve the weather resistance of the moulding. The optimum concentration of fillers, in the gelcoat, is considered to be 10% of the resin weight. Highly filled gelcoats deteriorate rapidly when exposed to outdoor conditions.
The quantity of the pigment for colouring gelcoats depends on the brightness required may vary from 3-6% for dark colours, and from 5-10% for white and pastel shades calculated on the weight of the resin. The higher levels are recommended for outdoor applications or when fillers are present in the formulation.
For laminating the pigment level may vary from 3-5% of the resin weight. But a higher level will be required when a brighter look is desired.
For translucent sheets the pigment level is usually less than 1% of the resin weight. Light transmission values may vary some what inversely with the amount of pigment used.
For sheet/dough moulding compounds the pigment level may vary from 5-10% for dark colours, and from 5-15% for white and pastel shades. Higher levels are suggested for highly filled formulations and to improve the brightness of the moulding.
For casting resins, the pigment level is usually 3-5% of the resin weight. But when the fillers are off-coloured, a higher level of pigmentation may be required.
Uniformity of colour
The final colour of the moulding is influenced by the colour of the cured resin, the conditions of cure, the amount of fillers, the quantity of pigment, the quantity of catalyst, the quantity of accelerator, the thickness of the moulding or gelcoat, and the surface gloss of the moulding. With so many variables, precise colour matching is obviously a problem even if the pigments are known to be uniform from batch to batch.
On large production runs, where the raw materials and the conditions of cure do not vary, it is possible to maintain a remarkable degree of consistency but it is extremely difficult to guarantee exact shade matching on repeat productions, when conditions are different.
They mix readily with the polyester and epoxy resins. Small quantities of the pigment can be mixed easily by hand stirring, but for large quantities power mixing is necessary, A vertical paddle-type stirrer is most suitable for the purpose. The following instructions are given for colouring the laminating resins for hand lay-up applications.
A weighed quantity of the pigment is first charged in to the mixing vessel. A small quantity of the resin is now added and the whole mixed until uniform, when the remainder of the resin is added. On no account should a small quantity of the pigment be added to a large bulk of the resin without first thinning.
The accelerator is now added and the whole mixed uniformly. The catalyst is added, as required, to small convenient lots of the pigmented resin immediately prior to use.
It has excellent stability on storage and long shelf-life. However since the systems are reactive; the pigments should not be stored intentionally for longer than six months.
Colour matching Service
It can be mixed with one another to obtain intermediate shades. Dark shades present no particular problems, but it is not advisable to mix your own pastel shades with out first knowing their performance particularly under weathering conditions.
Shades can be matched to customer’s specifications provided the requirements are large