Pigment selection starts with a definition of the final application and works back through system types, process, manufacture and physical requirements of the pigment.
End-user application requirements
High gloss is a requirement in most industry segments and expected to last as long as the article.
Chemical fastness is often not specified by the end user until application is defined and chemical requirement identified:
Automotive – Resistance to acid and detergent washes, bird droppings, leaves.
General industrial – Resistance to acid, alkali, detergents and lubricants in chemical plants.
Household general industrial paints – Resistance to detergent and fruit extracts.
Interior decorative paints – Detergent and alkali resistance are of primary importance.
Exterior paints – Application on plasterboard and concrete must be considered, along with acid rain environments.
Interior wood application – Resistance to detergents and fruit extracts.
Exterior wood application – Acid rain resistance.
Solvent fastness is generally required for automotive and general industrial paints.
Automotive – Petroleum discoloration is seen as a major defect.
General industrial paints – In chemical plants (machinery) and transportation coatings (tankers, aircraft, boats, etc.), solvents can come into contact with petroleum distillates and antifreeze. In multicoat systems using different colors and transfers, pigments can bleed into the final film, resulting in color change and bloom (excessive bleeding).
Durability is required in most industry segments, the expectation being no significant fading or darkening of colors during the lifetime of the article.
Automotive – As car body treatments improve and manufacturers adopt new selling approaches, demand for higher durability increases. Vehicles are expected not to suffer any significant change in color within the first four years, unless damage repair is necessary. Pigment selection must also take into account combinations with metallic basecoats.
General industrial paints – The general requirements for less demanding applications focus on light fastness, with the exception of articles that are exposed to high UV irradiation, such as blinds, which require higher durability. For exterior applications, durable fastness to light and weathering is required. With machinery coatings, rapid recoating is the priority. Transportation coatings must be highly durable on account of the conditions of use: aircraft, for example, require high UV stability, coupled with a minimum of two years without significant color change, and recoating at reasonable cost.
Decorative paints for interior application are frequently based on fashion trends. Here, light fastness is the priority, other demands being less stringent. For exterior application, durability requirements focus on weathering, coatings being expected to last for more than two years and in many cases for more than 10 years. This makes many organic pigments unsuitable; inorganic oxide pigments, shaded with organic pigments to enhance saturation, are commonly used.
Wood-finish paints for interior application require high light fastness, particularly if they are being used for furniture exposed in window areas. For exterior applications such as garden furniture and fences, inorganic oxides, blended with organic pigments for higher saturation and grain enhancement, are generally selected. Recoating is often necessary within two years.
Colors are often a question of individual taste and perception.
Automotive – Requirements are fashion based, entailing continuous color space development.
General industrial – Colors are often more striking and highly saturated for safety reasons and advertising purposes.
Decorative – Fashion trends necessitate continuous color space development.
Opacity is managed by the paint supplier and applicator, who ensure the correct coverage in the automotive and general industrial sectors. For the end user, opacity is a relevant consideration in decorative and wood finish applications.
Decorative paints – End users expect opacity to be achieved with a single-coat brush or roller application. A number of paint suppliers have adopted a one-coat approach as part of a technical and marketing campaign.
Wood finishes – With transparent wood coatings, grain enhancement is of prime importance. For opaque finishes, a two-coat system is expected to cover any substrate defects.
Rheology is a combination of flow and viscosity. The end user expects the viscosity of the paint supplied to be correct or at least to receive instructions on how to achieve a viscosity that avoids runs, overspray and sagging, whilst ensuring the required opacity.
Drying requirements vary, depending on application.
Automotive paints – Minimal drying (cure) times (15 minutes or less) are required, as production is otherwise slowed down. Pigments should not increase drying time.
Industrial paints – Requirements depend on the size of the article: for small items, fast drying (10 min) is required for fast throughput, whilst with large items (transport vehicles), the film needs to stay open for up to one hour to avoid overspray; it must then dry quickly to prevent dust contamination and allow subsequent coating of multicolor items.
Decorative paints – Aqueous paint surfaces should dry within 30 minutes to avoid dust contamination, whilst solvent-based materials need to retain a wet edge for up to one hour to prevent visible paint overlaps.
Flocculation results in paint film appearing weaker and variable in shade.
Reds generally become weaker, duller and bluer.
Blues generally become weaker, duller and redder.
Yellows generally become weaker, duller and redder.
This often happens when a dispersion is not shear-stable. Generally speaking, if shear is applied to the paint surface before it is dry, the affected areas appear stronger in shade than the background. The end user needs to be sure that the color of paint will not change after storage or application of shear.
Flooding manifests itself in the same way in all paint industry applications: fine particles of color move to the surface during the liquid phase, resulting in a significantly stronger surface color. In ink systems, this is generally considered an advantage, whereas for paint users, wet matching causes significant problems, as drying conditions vary. Flooding is often the result of fine particles produced either by over-dispersion, particular surface treatments or recrystallization of the pigment. The end user does not wish to see color change during drying. Pigments designed specifically for ink applications should be assessed for flooding properties.
Floatation often manifests itself as deep color striations on the surface of the paint in the container. It is frequently the result of fine particles produced either by over-dispersion, selective surface treatment or recrystallization of the pigment.
Bleeding usually occurs when some of the pigment is solubilized by the solvents in the media and the color is deposited on the surface of the topcoat. It can impair gloss and durability. Excessive bleed in a monocoat system is referred to as bloom.
General application requirements for the paint producer Besides end-user requirements, the paint manufacturer needs to take a variety of factors into account during production.
Toxicity – Health and safety regulations on pigment handling in production and end use, as well as labeling, depend, among other things, on the form in which the products are supplied to the paint maker, which ranges from fine powder to pellets.
Bulk volumes – Many large-scale paint manufactures use big bags to load pigment into the premix vessel. Allowance must be made for bulk volume when filling them.
Powder form – Pigments are supplied in a powdered, agglomerated form of over 100 nm, as finer particles can cause serious dust contamination, extraction problems and potential health issues for operatives. Health and safety data sheets should be examined for risk potential.
Temperature stability – The paint manufacturer must also take into consideration stoving systems, coil coating, powder coating and also process conditions that could result in high-energy dispersions locally heating the pigment to between 40 and 90°C. Temperature control during the dispersion stage can influence flocculation and flooding, as well as the recrystallization and rheology properties of the final paint.
Surface area – Pigment producers indicate the surface areas of the pigments in order to give an idea of flow and resin surface demand for stabilization. Generally, the higher the surface area, the poorer the flow, although in certain cases the pigment producer might have surface-treated the products in order to minimize this potential problem.
Dispersibility – For maximum productivity, paint manufacturers tend to select pigments that require minimum dispersion. The target in general is to reach a level of dispersion at which the product is stable and performance satisfactory in terms of color, and there is no increase in transparency, impairment of flow or reduction of durability.
Automotive – The particle size of normal dispersions is usually about 0.5 nm with both opaque and transparent pigments. Pigments used in this application are usually high value and therefore optimized for the system.
Industrial paints – The particle size of normal dispersions is usually less than 5 nm. Production of significantly lower particle sizes is generally based on a compromise between the cost of manufacture and the choice of raw materials.
Decorative paints – the particle size of normal dispersions is usually 5-15 nm. Production of significantly lower particle sizes is generally based on a compromise between the cost of manufacture and the choice of raw materials.
Rheology – Legislation stipulates lower amounts of solvent in paint systems. At the same time, demand has increased for products with improved rheological properties. So rheology has become an important issue.
Automotive – In solvent-borne systems, pigment loadings of more than 16% are typically required to minimize the amount of solvent required to achieve the correct viscosity.
Industrial paint concentrates require pigmentation levels of more than 35%, to minimize the effect of carrier resins and solvents on the paints, which in the case of colorants can be numerous.
Decorative paints often require concentrates with more than 30% pigment loading, depending on the pigment. Maximum pigment loading is required in production for logistical reasons.
Rheology also plays a key part in dispensing, requiring specialist knowledge. Wetting of the product after dispersion, which results in changes in rheology, is normally assessed to avoid excessive thickening, which can lead to application problems or conversely, if a reduction in viscosity takes place, to pigment sedimentation.
Rheology is taken into consideration in the dispersion process, as dispersions with low viscosity often fail to achieve optimum energy use, resulting in paints with potential problems of flocculation, flooding and shock when added to higher-viscosity-resin let-down paints. Higher pigment loading and viscosity frequently maximize production potential from both an energy and a logistical point of view, because they allow the number of paint batches to be reduced. If viscosity is too high, throughput can slow down, pipes can block and localized temperature increases can result in flocculation and recrystallization.