Redispersible Powder Polymers for Biocide-Free Wall Paints

Today, most liquid wall paints are water-based. However, when used in paints, water provides a suitable environment for bacteria and other microorganisms. To prevent the reproduction of these organisms, paints are typically formulated with biocides that extend their shelf life. According to the German Paint and Printing Inks Industry Association, without protective additives, one in four buckets of paint will spoil. This corresponds to eleven million buckets annually and costs of EUR 470 million. However, biocide use presents a problem: After paint is applied to the wall and begins to dry, liquid components can evaporate, causing biocides to mix into the air. In some individuals, biocides, particularly isothiazolinones, can trigger allergic reactions.

Figure 1. Paint containing NEXIVA® in powder form (left) and redispersed with water (right). (Photo: Wacker Chemie AG)

WACKER, a chemistry company based in Munich, has now developed a technology for producing wall paints without biocides. At the 2019 European Coatings Show, the company introduced a product line based on spray-dried polymer binders suitable for the production of interior wall paints in liquid or powder form. Paint manufacturers can use these polymer binders just as they do traditional binders in dispersion form to create specialized paint formulations. Paints formulated with the new binders can be used by professional painters and do-it-yourself (DIY) enthusiasts. Initial feedback from the market confirms the high quality of these paints, particularly in terms of opacity and wet scrub resistance.

Powder Paint Categories

The concept of powder paint is not new. There are various powder paint categories, some of which have been used for very long periods in human history. Mineral paints such as clay paints, cement powder paints, lime powder paints, or silicate powder paints are bound with mineral binders such as cement, clay or mill, soluble water glass, or slaked lime. These paints are known to have good breathing properties. However, they have various disadvantages compared to paints using polymer binders. For example, clay and cement powder paints are difficult to apply because they do not provide good coating quality or opacity. Additionally, to prevent staining, they require expert-level application technique. These properties also explain why cement powder paints are often used in basements but not as decorative paints. Silicate powder paints have good opacity but are mostly available as two-component systems. Because mineral powder paints like these typically have high alkalinity, it is difficult to combine them with polymer binders to improve their performance. However, polymer binders provide various advantages in paints, such as ease of application, high formulation diversity, excellent hiding power, and wet scrub resistance. Additionally, dispersible powder polymer-based binders enable paint manufacturers to formulate paints in powder form in the sophisticated decorative paint segment where high-quality paints are required. Dispersible powder polymers are produced through an integrated manufacturing process. Acetic acid and ethylene are used to produce vinyl acetate monomer (VAM). In the copolymerization process, VAM and ethylene react to form vinyl acetate-ethylene (VAE). The liquid dispersion and protective colloid pass through a spray dryer. The protective colloid matrix separates dispersion particles in the powder particles from one another, preventing polymer particles from irreversibly forming a film during drying and storage. When water is added, the dispersible powder polymer easily redisperses as individual particles.

Formulation Flexibility

Dispersible powder polymers provide process flexibility for the production of decorative and high-quality paints. During the manufacturing process, paint manufacturers can decide at which stage water needs to be added to the formulation. One option is to add water in the early stages of the process. The dispersible powder polymer is redispersed in water to obtain a conventional liquid dispersion. The advantage is that manufacturers can use their preferred biocides or additives and are not limited to those added by the dispersion supplier. A second approach is to produce paints in powder form and deliver them directly to distributors such as hardware stores and DIY shops, where powder paint can be easily stored. Water can then be added later when the customer wants to mix the paint. Even the amount of paint can be adjusted according to customer needs. Another possibility is to produce powder paints and sell them to end users. In this approach, end consumers purchase powder paints from, for example, hardware stores or specialty paint shops. At home or on a construction site, customers can add water to prepare the amount of paint they need, whether to cover a small spot or paint an entire wall. To prepare the final product in a standard quantity such as a ten-liter bucket, end users start with water, add the powder, and stir with a mixer bit attached to a drill for approximately five minutes. For smaller quantities of approximately one liter or less, shaking is sufficient to achieve the necessary mixing.

Wet Scrub Resistance and Hiding Power

Extensive testing has demonstrated that paints produced using dispersible powder polymers meet high-quality standards. Hiding power is an important criterion for paint. To determine this value, laboratory technicians take opacity cards, weigh them, and measure their density using a pycnometer.

Figure 3. A laboratory assistant checking the hiding power of paint. (Photo: Wacker Chemie AG)

Following this, two paint films are cast, one at 150 µm and the other at 225 µm. The opacity cards are weighed again after casting. The film is then allowed to dry for 24 hours at 23°C and 50% humidity. After the drying period, opacity is measured with special equipment designed for this purpose. In the next step, a contrast ratio is calculated using all results. According to ISO 6504-3, paint hiding power is expressed in four classes ranging from class 1, which shows the highest hiding power (contrast ratio ≥99.5), to class 4, which shows the lowest hiding power (contrast ratio <95). Powder paints formulated using dispersible powder polymers achieve a contrast ratio between 98 and <99.5 and thus provide class 2 level hiding power. Another important criterion regarding the performance of decorative interior wall paints is wet scrub resistance. This criterion was tested in WACKER laboratories in accordance with ISO 11998. In the first step, a paint film 300 µm thick is applied to a PVC plate. The plate is allowed to dry under standard atmospheric conditions for 72 hours, then at 50°C for 24 hours, and again under standard atmospheric conditions for 24 hours. Following this, the thickness of the PVC plate with and without dry paint is measured. The plate is then cut into several strips, and each strip is placed on a special machine that slides a sled with a polar fabric pad over it, with the sled sliding 200 cycles over each surface. Subsequently, the PVC samples are measured again. ISO 11998 defines five classes ranging from class 1 (highest) with <5 µm loss after 200 cycles to class five (lowest) with >70 µm loss after 40 cycles. The results show that a standard paint formulated using dispersible powder polymer meets the requirements of class 2 (>5 µm to <20 µm loss after 200 cycles).

Biocide-Free, Low VOC Value, Compliant with Environmental Labels

Powder paints containing dispersible powder polymers have a low odor and are stable even without protective additives. Depending on manufacturer and customer preferences, water does not need to be applied for paint redispersion until immediately before application, eliminating the need for biocide addition during production. Powder paints based on dispersible powder polymers also have a longer shelf life compared to conventional dispersion paints. They can be formulated to contain very low volatile organic compound (VOC) content (<1 g/l) and demonstrate compliance with major environmental labels. Since the film-forming temperature is near zero, paint formulated with dispersible powder polymers does not require organic solvents, plasticizers, or film-forming agents. Additionally, the powder polymer supports the dispersion of fillers and pigments, improving the paint's hiding power and brushability properties.

Figure 4. Technical properties of a standard dispersible powder polymer

Storage, Transport, and Preparation

Another advantage is that powder paints can be prepared at full dosage as needed and thus reduce waste. Transport and storage of these paints is also easier compared to dispersion paints. In typical formulations, 40% weight savings are achieved compared to dispersion paints. Additionally, powder paints can be packaged differently than liquid paints. Since they can be held in bags, they provide better space utilization during transport and storage, and the dry form is much safer in case of packaging damage or leakage during transport. Powder paints can be stored even in harsh climate conditions. Unlike conventional wall paints, the powder versions do not freeze in cold or thicken when exposed to heat. However, prolonged storage at temperatures above 30°C, pressure loads, moisture, or exposure to sunlight can lead to caking. To prevent the powder from contacting moisture, polyethylene liners with low moisture permeability are recommended for packaging. Dispersible powder polymer paint also enables rapid preparation. Polymer particles easily disperse in water. Therefore, unlike other powder paints that need time to swell before use, the paint can be applied immediately after mixing. Paint consistency can be adjusted with the ratio of water to powder, meaning painters can decide to prepare a thin or thick paint depending on their current paint project. For standard formulations, WACKER experts recommend a ratio of 150 g powder to 100 ml water.

Summary

Today, most liquid wall paints are water-based. However, water also provides a favorable environment for microorganisms and bacteria. To prevent the reproduction of these organisms, paints are typically formulated with biocides that extend their shelf life. German chemistry company WACKER has developed a dispersible powder polymer that enables manufacturers to formulate interior wall paints in powder form. This technology does not require biocide addition and provides significant advantages in paint storage and transport.   Dr. Martin Schierhorn Senior Manager, Global Strategy Wacker Chemie AG       References 1.http://www.wirsindfarbe.de/presse/lacke-farbenaktuell/ farben-brauchen-schutz/ (German language website only)