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:
Solvent fastness is generally required for automotive and general industrial paints.
Durability is required in most industry segments, the expectation being no significant fading or darkening of colors during the lifetime of the article.
Colors are often a question of individual taste and perception.
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.
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.
Flocculation results in paint film appearing weaker and variable in shade.
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.
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.
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.