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A common problem when painting can be an effect called bleeding. This is when the stain from the substrate can transfer through to the top coat which impairs the physical appearance of the paint. This can occur even through several layers of paint and can be particularly challenging when the substrate is wood. Natural tannins and oils in the wood can bleed through paint, which is particularly noticeable in white paint, leaving brown marks which can be seen through the top coat.
Water-based stain-blocking primers have been used in the past to combat bleeding, however, they have not always been entirely successful as they have been designed to lock stains in as opposed to blocking them. Solvent or alcohol-based primers are also not ideal (although they are more effective) as they often have a strong odour during application and are also flammable.
Through further research and development, it has been found that cationic water-based primers offer a significant increase in performance in comparison to water-based primers. This is because the stains are anionic and react with the cationic functional groups on the polymer. Due to the low pH of the primer, it does not dissolve tannin stains or alkaline water in anionic primers, hence the stains do not bleed.
Cationic primers, however, often have their own unique set of challenges as they can require careful handling and can solidify if allowed to come into contact with anionic acrylics. They have also been known to have an effect on the paint formulation quality.
Figure 1: A wooden surface, painted white, showing the difficulties of keeping a wooden surface white without the use of a stain-blocking primer.
NECOWEL™ 4300 by ASK Chemicals is a cationic modified alkyd emulsion with medium oil length for use in stain-blocking wall paints. It offers excellent stain-blocking properties against water-soluble substrate contaminations like e.g. nicotine, tobacco condensate, markers and wood ingredients. Most of these water-soluble colourants are anionic and are fixed at the cationic counterpart in the resin. Figure 2 shows the effectiveness of NECOWEL™ 4300 on wood for keeping a white top coat white.
The top half of each of the three Merbau panels is coated with Necowell™ 4300 to prevent bleed-through. Sample B has been sanded 4 hours after impregnation and painted with a white top coat. Panel C has been subjected to a condensation water test for 72 hours after being painted. The colour difference between the treated part (top) and the untreated part (bottom) is apparent on all the panels. As can be seen, the Necowell™ 4300 prevents a significant amount of bleeding and stains, even in harsh weather conditions.
Figure 2: Three Merbau wood panels with half treated with Necowell™ 4300 showing its effectiveness against bleeding under different conditions.
Compared to alternative cationic emulsions the patented technology of NECOWEL 4300™ differs in its neutralisation with ammonia. This additional step offers a neutral pH value that is compatible with additives and an extensive range of pigments and fillers. It is dilutable in water and compatible with carbonates making it easy for formulators to work with.
Volatile organic compounds (VOCs) are high vapour-pressure compounds that are quickly evaporated at room temperature. VOCs offer advantages in paints and coatings as they give quick-drying, favourable application properties, and the notable 'new' odours of many products. However, VOCs are gaining notability for their health and environmental impacts and many consumers will now look for VOC-free products. NECOWEL™ 4300 provides these advantageous properties without the use of VOCs making it safer for use as well as more easily marketable.
Similarly, but perhaps with less notability, alkylphenol ethoxylates (APEO) are being restricted as surfactants due to their degradation into alkylphenols in the environment. Alkylphenols offer extreme hazards to aquatic life as well as suspected hazards to human fertility. Again, NECOWEL™ 4300 does not contain APEOs.
As with most markets, sustainability is a key consideration for primers and something that consumers are often looking for. NECOWEL 4300™ is based on natural oils and contains a high percentage of renewable raw material with its base consisting of dehydrated castor oil (DCO).
In conclusion, there is a broad range of stain blocking technology available. The most suitable material is highly dependent on application requirements and at Lawrence Industries we would be glad to assist you with this - get in touch with our technical sales team to discuss your project (01827 314151) or request a sample on our product page.
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