Redispersible Polymer Powder Manufacturing Process Breakdown

Redispersible polymer powder (also named as RDP) is a powder-like resin that can be re-dispersed in water. It is generally produced by spray drying synthetic resin emulsion. Therefore, it is also called emulsion powder or latex powder.

One of redispersible polymer powder’s characteristics is that it can be re-dispersed by adding water, and the performance of the re-dispersed solution is basically the same as that of the initial emulsion.

At present, the major suppliers of redispersible polymer powder in the world include: Wacker and BASF from Germany, Celanese and Dow from the United States, and WOTAIchem from China.

The redispersible polymer powder manufacturing process generally involves three stages:

① Emulsion polymerization

② Emulsion modification

③ Spray drying

Among them, emulsion modification can be carried out during the emulsion polymerization process or after it. You can refer to the entire process in the following figure.

1. Emulsion Polymerization

1.1 Systems for Redispersible Polymer Powder

The main uses of redispersible polymer powder are construction adhesives and dry-mixed mortars. Among various polymer systems, vinyl acetate-based polymers have always held a dominant position and have been the most classic formulation system.

This is mainly due to its comprehensive advantages: low cost, high bonding strength, and the environmental-friendly (non-toxic and harmless). Whether in production processing or on-site construction, it combines safety and convenience.

1.2 Monomers for Polymerization

The monomers used in emulsion polymerization determine the type of redispersible polymer powder. The polymer monomers used for redispersible polymer powder mainly include unsaturated alkenes, such as various vinyl esters and acrylate esters.

The specific monomers mainly include: various acrylate monomers, such as methyl acrylate (MA), methyl methacrylate (MMA), n-butyl acrylate (n-BA), ethyl acrylate (EA); maleates, fumarates; and vinyl aromatic monomers such as styrene.

1） When selecting the monomer, 2 important factors should be taken into account: The glass transition temperature (Tg) & the minimum film-forming temperature (MFT)

2） Since the moisture content of the redispersible polymer powder prepared by spray drying should be less than 5%, the powder temperature should be set above 45 ℃.

3） When the Tg is too low (e.g. < 30 ℃), the dry powder is prone to become soft and caked. Generally, if the outlet temperature is more than 30 ℃ higher than the Tg, the polymer powder will stick together, and if it is more than 40 ℃ higher than the Tg, the polymer powder will cakeweld.

4） Therefore, an appropriate Tg should be selected to prevent the polymer from softening and caking, and to ensure the powderization. However, generally, polymers with high Tg have high MFT as well.

5） So, it is also necessary to consider a suitable MFT to ensure that the dispersion can form a film at a lower temperature. Ideally, a polymer emulsion with a soft core and hard shell should be synthesized. The following table shows the polymerization formula of the EVA base emulsion.

* Formula for polymerization of EVA base emulsion

5） Furthermore, when choosing monomers, the product cost also needs to be considered. Currently, the products sold on the market are mainly the vinyl acetate-ethylene series, and this is largely due to their lower cost.

1.3 Polymerization Method

Generally speaking, redispersible polymer powder manufacturing process does not have specific restrictions on the emulsion polymerization method. Any emulsion polymerization process using water as the dispersion medium can be employed.

WOTAIchem recommends using continuous or semi-continuous emulsion polymerization methods, or seed emulsion polymerization methods.

China’s WOTAIchem has successfully developed a high performance redispersible synthetic resin powder. This product enhances its performance by adsorbing a special polyvinyl alcohol resin on the surface of polymer particles. This polyvinyl alcohol contains acetylamino or mercapto functional groups, and its block characteristic value ranges from 0.3 to 0.6. This technology significantly improves the re-dispersibility, film-forming property, water resistance and anti-sticking properties of the powder.

WOTAIchem has achieved large-scale industrial production of redispersible polymer powder. The engineering team led by Gao Guoli of WOTAIchem designed “soft core and hard shell”. The core uses acrylic butyl ester with good flexibility and low-temperature resistance. While the shell uses methyl methacrylate with high hardness and strong stability.

This special structure enables the emulsion to undergo spray drying at higher temperatures. Ultimately resultes in dispersible latex powder with better dispersion and more outstanding water resistance. This effectively improves our product quality and production efficiency.

The redispersible polymer powder produced via this method possesses excellent dispersibility and water resistance.

2. Emulsion Modification

The dehydration and drying process of polymer emulsion to form a film is irreversible. This is the basis for improving the performance of cement mortar. Therefore, when preparing dispersible polymer powder after drying the polymer emulsion, the key is to maintain the reversibility of the polymer spray drying and subsequent dispersion. The solution is to:

Add hydrophilic protective colloids to the emulsion before spray drying;

Or

Add the anti-blocking agent during or after the drying process to prevent irreversible agglomeration of the polymer particles.

2.1 Protective Colloids

Protective colloids are generally water-soluble macromolecular substances, such as polyvinyl alcohol (PVA) and cellulose derivatives. Some components are adsorbed or bonded onto the surface of the emulsion particles, forming a protective layer (hydration layer) with a steric hindrance effect. This layer plays a key role in stabilizing the system. While some remain in the aqueous phase.

Currently, the most commonly used protective colloid is polyvinyl alcohol. The PVA can enhance the anti-agglomeration effect and dissolve quickly.

However, the amount of PVA added is limited by the viscosity of the system. When the dosage is too high, the emulsion must be diluted to a low solid content. This will result in an increase in the amount of water that needs to be evaporated directly. Thereby reducing the dry powder output.

To solve this problem, WOTAIchem™ tried to dry blend some PVA with the obtained polymer dry powder. This approach has not only ensured production efficiency but also ensured product performance.

2.2 Anti-blocking Agents

The emulsion is prone to agglomeration during spray drying or after powder formation. To reduce the agglomeration between polymer powders, WOTAIchem™ recommends add inert mineral anti-blocking agents such as kaolin, diatomite, and talc. But if the anti-blocking agent is added before drying, it may be encapsulated into microcapsules and lose its effect.

In most cases, they are sprayed separately with the emulsion at the top of the dryer. But this is also prone to loss in the airflow or crust formation on the dryer and conveying pipeline.

A better addition method is to add it in two parts: one part is sprayed with compressed air at the top of the dryer; and the other part is mixed with cold air at the bottom. To prevent agglomeration, it is also possible to perform saponification on the remaining part when the polymer polymerization reaches 80% – 90%. Or add melamine-formaldehyde condensate during emulsion polymerization.

3. Spray Drying

The most commonly used drying method for preparing redispersible polymer powder is spray drying. Other methods such as vacuum drying and freeze drying can also be used.

Spray drying is a process in which a liquid is dispersed into fine droplets by a spray nozzle, and the solvent is rapidly evaporated in a hot drying medium to form a dry powder product. Generally, spray drying consists of four stages:

① Liquid atomization;
② Contact and mixing of the droplets with the hot drying medium;
③ Evaporation and drying of the droplets;
④ Separation of the dried product from the drying medium.

The process of spray drying and powder re-dispersion is shown in the following figure:

There are two types of spray drying methods: parallel flow and counter flow. In most cases, the parallel flow spray drying is used. When using this method, it is necessary to control these factors – glass transition temperature of the emulsion, the solid content (WOTAIchem suggests diluting the spray dispersion to 40% solid content before spray drying), spray pressure, droplet size, inlet and outlet hot air temperature, and wind speed, etc.

The selection of drying temperature is crucial.

▪️If the inlet temperature is too high, the material at the nozzle will accumulate due to rapid water loss, resulting in poor material feed.

▪️If the inlet temperature is too low, the outlet temperature will decrease, causing excessive moisture content in the powder.

Generally, the inlet hot air temperature is preferably 120-170 ℃, and the outlet temperature is 80-120 ℃. The polymer dry powder is separated using a cyclone separator or bag filter.

Summary

WOTAIchem has conducted nearly 20 years of research in the redispersible polymer powder manufacturing process design, including new molecular structures, water resistance modification, spray drying technology, mechanism of action, and cost reduction.

We have opened up a new concept for emulsion polymerization using water-soluble polymers as the main stabilizer for the emulsion. It improves the shortcomings of traditional emulsions, and expanded the application scope of emulsion polymers.

Have questions about our products? We’d love to hear from you! Drop us an email at [email protected].