Polymer emulsions can be defined as dispersions of polymeric particles in an aqueous dispersion media. They are polymer dispersions, by technical terms, and often referred to as “emulsion polymers”, “dispersions” or “polymer latex”.
The starting formulation is an emulsion of organic monomer in water. This is then stabilized by emulsifying agent(s), such as:
- Natural detergent
- Synthetic Detergent, or
- Polymeric colloid
The polymer particles are formed by means of free radical polymerization, which is started in the aqueous phase of the emulsion. The particle formation mechanism can be either micellar or non-micellar. This depends on water solubility of the monomer and the chemical nature of the emulsifier.
During the course of polymerization, the monomer transfers from the emulsified droplets into the growing polymer particles. Thus, the polymer dispersion is formed. The resulting product has about 50 – 60 % solid polymer content dispersed in aqueous phase. In the vast majority of their applications, the polymer dispersions are ready to use without further processing steps. But there are also cases where the polymer get precipitated and further processed in solid state.
Since its first industrial application in early 20th century, emulsion polymerization developed to be one of the most versatile polymerization techniques in polymers industry. Let’s understand this technique in detail starting with its values.
Values of Polymer Dispersions
Polymer dispersions are used as functional polymers. They are themselves raw materials for further, more complex formulations. This is shown in the figure below:
These formulations will be either:
- Delivered via retailer to the final customer: Such as paint to a contractor or for DIY, or
- Used in a production step further up the value chain: Such as in a paper coating
So, here comes our first DO’s and DON’Ts:
> DO understand the full value chain and the customer needs in the final application.
> DO NOT rely just on the communicated need from your direct customer. They might consider the polymer dispersion as one raw material amongst others in his formulation.
> DO NOT treat your product, the polymer dispersion, as a simple polymer, sold based on a few physical characteristics. Such as polymer glass transition temperature, dispersion solids or viscosity.
Emulsion polymers contribute to the value for the formulator as well as for the end user by many facets. In general, they are aimed to form a solid film at their application. It is either characterized as coating or as adhesive depending on if:
- The film is at the surface of a substrate (e.g. as an indoor paint), or
- The film is between two substrates (e.g. as a packaging adhesive)
Smart Selection of Monomer is all you need for Better Dispersions…
Based on the properties of their bulk monomers, polymer dispersions contribute certain mechanical properties to the formed film. Such properties could be for instance a balance of stiffness and elasticity. And therefore, a certain contribution to abrasion stability is also balanced. By the selection and smart combination of the main monomers that form the bulk polymer in dispersion, the polymer chemist can design the products that deliver the desired function in the final application.
Besides the design of their bulk polymer also little amounts of additives contribute to the final application properties, such as:
- Functional monomers
- Cross-linkers, and
With that, the design of polymer dispersion according to the application requirements is a complex business. It is rather an art that needs skillful selection and control of all parameters. Mastering this art allows the chemist to adjust the product to many customer needs. Also, it allows for innovative solutions in such a mature business.
The Polymerization Equipment
In industrial scale, emulsion polymerization is carried out either in:
- Batch or semi-batch operation mode in stirred tank reactors, or
- Continuous operation mode, using cascades of continuous stirred tank reactors
Polymer dispersions are “products by process”.
The process conditions as well as the polymerization equipment have a strong influence on the final product quality and performance
The process equipment or process principle chosen imposes “macro process conditions” such as different residence time functions and polymerization kinetics. Micro process conditions, such as detailed reactor design, for instance feed nozzle positioning, might have an impact on local concentration profiles. And thus, impacts on such effects like broadness of particle size or molecular weight distributions or unintended coagulum.
So, here comes the second DO’s and DON’Ts:
> DO carefully choose and understand the impact of your process as well as reactor type and design on the intended dispersion properties.
> DO NOT underestimate the impact equipment and thus macro and micro process has on the final application performance.
This is most important when you intend to transfer from one process to another. Example from batch to continuous operation.This is often desired when volumes increase and production rate improvements hit a threshold. It is also an important consideration during scale up and for designing new, higher capacity reactors of the same type.
If it comes to the raw materials, emulsion polymerization is characterized by very high variety and complexity. In one recipe, easily 10 – 20 different types of raw materials get collected. The main and often underestimated raw material is water. It acts as dispersant and facilitates heat dissipation during the process. In the final dispersion, it acts as matrix for the polymer particles before evaporation during film formation.
It also is the solvent for salts and other low molecular components. It is either introduced into the formulation as process auxiliary or formed during the process. Some of these components might be helpful whilst others might be harmful in the final application.
> DO control the quality of the water used in the emulsion polymerization process. De-ionization is often required to eliminate metal salts that have a negative impact on initiation and radical generation during the process. The polymerization process itself sterilizes the polymer dispersion. But process water added post-polymerization must not contain microbiological contamination.
> DO regular checks on process water used post-polymerization. Example for final solids adjustments or rinsing residues from the reactor into post addition tank. Also, DO install measures to sterilize the process water and the piping.
The 5 Main Polymer Dispersion Classes
On a very general level, there are only 5 main polymer dispersion classes, based on the main monomers used. They account for 80 – 90 % of all polymer dispersions produced worldwide. They are:
- Pure acrylics
- Vinyl-acrylics, and
- Vinyl (acetate) homo- and copolymers
The latter include vinyl acetate – ethylene copolymer dispersions (VAE). But taking a more detailed view, “acrylics” sub-summarizes many different derivate based on (meth) acrylic acids.
Also, there are plenty of other monomers used, in combination and in addition to the main monomers mentioned above. Some of them add to the bulk polymer whilst others are used in small fractions adding specific functions.
Stabilizers for Polymeric Emulsions: Categories and values
In addition to the already broad variety of monomers to choose from, there is an abundance of stabilizers to pick for a certain emulsion polymerization recipe.
Colloidal stabilizers, such as partly hydrolyzed poly vinyl alcohol or functionalized cellulose ethers are used alone or in combination with:
- Anionic surfactants
- Non-ionic surfactants, or even
- Cationic surfactants
Many of these stabilizers are polymers or oligomers itself, with distributions characterizing molecular weight or functionalization.
Stabilizers do not only maintain emulsion stability during polymerization process but also dispersion stability during shelf life. They also impact the following in aqueous phase amongst other properties:
- Particle size distribution
- Molecular weight
- Particle surface characteristics, and
- Polymer content
Therefore they significantly impact the following application performance properties amongst many others. These are:
- Dispersion rheology
- Wetting and adhesion behavior
On top come auxiliaries, such as initiators, buffers and other chemicals who are essential to maintain and control process. These auxiliaries also contribute to salt load in aqueous phase and often impact the performance in application.
Be Careful about the Raw Material Used!
On one hand, this tool kit opens endless possibilities to the polymer chemist to tailor the final application properties. On the other hand, too many raw materials add to the cost side of the business, besides adding too much complexity in manufacturing. So, here comes our third DO’s and DON’Ts:
> DO develop a holistic strategy on raw materials. Design the polymerization recipe not only from a chemistry point of view, but also considering the following for your business:
■ Sourcing strategies
■ Manufacturing cost and complexity, as well as
■ Other relevant specific factors
> DO NOT allow your raw material portfolio grow exponentially with every new thing you develop, like:
DO identify the critical raw materials for your specific application. Qualify your suppliers and install a quality control regime. DO NOT switch suppliers for critical raw materials, such as stabilizers. That too, just based on specification and without qualification in experimental trials.
Process Conditions for Polymerization
The third factor contributing to the performance of the polymer dispersion in the final application is the set of specific process conditions applied during the polymerization process. In the classical model, there are three phases of emulsion polymerization:
- The first stage is particle formation.
- In the second stage particle grow in presence of monomer droplets.
- In the final stage the polymerization is finished, when only monomer-swollen particles are left.
Design and control of what happens in which stage of this process, is leading to the right product performance. This scheme is essential for creating a reliable process. There are plenty of possibilities to design. For instance temperature profiles or staged feed profiles in semi-batch type processes.
Also, there are many philosophies on how to control reactor temperature and initiator feed rate. Very often, emulsion polymerization in industrial scale doesn’t run on kinetic limits, but is controlled by the heat removal capacity. Seasonal fluctuations in cooling water temperature and increasing fouling in the reactor (polymer emulsions are designed to form films!) put an uncontrolled factor on the reproducibility of batch type emulsion polymerization or cause drift in continuous process types.
Therefore it is essential that you:
> DO measure and control the conversion over the course of polymerization.
> DO NOT run “blind” towards a defined end point. It is important what happens on the way towards that point. For instance it is very important at which level of conversation a staged feed or a temperature ramp is started or ended. Also, the conversion rate during the different stages of emulsion polymerization might have an impact on final performance.
Performing emulsion polymerization in industrial scale means to manage a complex, non-linear system. But it becomes more than worth with:
- Versatility of the possible applications
- Positive impact polymer dispersions have in many applications on environment as waterborne, solvent free systems, as well as
- Positive outlook for future growth