Emulsion Polymers: Features, Benefits & Applications

What are Emulsion Polymers?

Emulsion Polymers can be defined as dispersions of polymeric particles of about 100 – 1000 nm size in an aqueous dispersion media. They are polymer dispersions, by technical terms, and often also referred to as “polymer emulsions”, “dispersions” or “polymer latex”.

Physically they fall into the category of colloidal systems.

Colloids are the microscopical dispersion of one substance into another of which it is not (or little) soluble in.
These systems are characterized by high internal interface areas and specific physio-chemical properties derived thereof.

Due to the high surface area, all colloidal systems are meta-stable. The laws of physics drive them to reduce that area which leads to coagulation.

In technical systems, stabilizers are used to mitigate this natural tendency and to keep the systems in the colloidal state until usage.

Dispersion and Dimensions of Surface/Particle size

Examples for naturally formed colloids are milk, fog, mist or clouds, smoke, blood and the natural rubber latex from the Amazonian rubber tree (Hevea brasiliensis), which stands at the origin of this industry. Since its first industrial application in the early 20th century, emulsion polymerization developed to be one of the most versatile polymerization techniques in the polymers industry.

Polymer emulsions are primary dispersions, which means that the polymer and the colloid are formed in one step. Dispersing already made polymers into an aqueous medium forms secondary polymer dispersions. Commercial examples are wax dispersions. This step requires:

A high-energy intake to create small enough particles, and
Often high amount of stabilizers to keep the microscopic particles separated.

Technically important are Polyurethane Dispersions and Emulsion Polymers. Here, we are focusing on the latter.

How Emulsion Polymers are Made?

Emulsion Polymers formed by free-radical polymerization of monomers emulsified into water. Emulsion polymerization has a specific mechanism and kinetics, which makes it different from many other polymerization techniques.

Organic monomers are emulsified under the aid of stabilizers into water that serves as a continuous matrix.

The addition of radical-forming initiators to the aqueous phase starts the polymerization, which happens outside of the monomer droplets (this would make suspension polymerization instead!).
But in the aqueous phase, monomer micelles or monomer swollen polymer particles, depend on mechanism and phase of the process.

The monomer droplets just serve as a reservoir from which monomer molecules are delivered. With this mechanism, very high degrees of polymerization can be achieved at high-solids content, whilst the overall viscosity of the system stays within reasonable terms for processing. This process results in a dispersion of about 50 – 60% solid polymer particles in the aqueous matrix.

For many applications, this dispersion is ready-to-use without further expensive separation and cleaning steps. In other cases, the polymer emulsion is precipitated or spray-dried to form a re-dispersible polymer powder (RDP).

From an application point of view, polymer emulsions are waterborne systems.

Waterborne systems are based on water – not organic solvent – as the main carrier.
They found increasing interest over the past decades due to their favorable profile in terms of emission control.

Therefore, emulsions polymers are amongst the fastest-growing categories of specialty chemicals in the world.

Classification of Colloidal Polymer Material

Let's explore the characteristics, chemistries, key benefits and applications provided by emulsion polymers in detail...

Characteristics of Emulsion Polymers

Emulsion Polymers are very versatile products. They are commercially available as dispersion of polymer particles in water. These milky-white liquids range from water-thin to thick paste-like in viscosity.

These two properties, together with a pH, define the standard qualities of any commercial emulsion polymer.

Some key factors for characterizing emulsion polymers are:

Solid content
Particle size
Coagulum in a product
Rheology/Viscosity
Glass transition temperature
Residual monomer/VOC
Molecular weight

#1 Solid Content

The solid content is defined as the dry residue of all solid material after evaporation of water, containing polymer, stabilizer and organic or inorganic auxiliaries, divided by the total mass of the dispersion. Commercial emulsion polymers contain between about 45 and 65% solids according to that method.

#2 Particle Size

The real particle size of any given polymer dispersion is often difficult to attain. The results differ from the physical method used or even depending on the specific equipment provided. A well-defined, narrow, mono-disperse particle size distribution is the exemption for the products to be discussed here.

Most of the emulsion polymers exhibit broad or multi-disperse, often skewed distributions. Also, the shape of the particles that are not necessarily ideal spheres and the particle morphology need to be considered. Often only a few characteristic numbers, such as the average number or average weight are calculated, from the measured values to characterize the dispersion.

As the polymer particles have a more or less extended interface layer, composed from adsorbed or grafted stabilizers, and electrolytes the “dry” core diameter needs to be distinguished from a hydrodynamic diameter in the swollen, wet state.

The preparation of the dispersion before testing is often crucial. Some dispersions tend to form agglomerates, that can bias some methods, towards higher average particle sizes.

Particle Size Consideration while Formulating Polymer Dispersion

The frequently used particle size methods established for the characterization of emulsion polymers are:

Transmission electron microscopy (TEM)
Laser Aerosol Spectroscopy (LAS)
Light scattering (LS)
Disk centrifuge (DC)
Field flow fractionation (FFF)
Capillary hydrodynamic fractionation (CHDF)

Some of the methods fail to detect very small particles, other overweigh large particles or agglomerates.

In general, it is advised to use a combination of two or three different methods to know the true particle size distribution of a given dispersion. For quality control measures, it is often sufficient to use a robust, established, wide-range standard method, as offered by test equipment suppliers for that purpose.

#3 Coagulum in a Product

The coagulum present in a commercial product is measured by washing a measured sample of a diluted emulsion polymer product through a sieve with a defined mesh size. The coagulum is calculated as the weight of the dry residue on the sieve divided by the total amount of dispersion ran through.

Technically, any coagulum is an expression of insufficient stabilization

#4 Rheology/Viscosity

The viscosity or rheology of an emulsion polymer is a complex property that depends on:

Solid content
Particle size distribution
pH
Particle surface charge, and
Organic content in aqueous phase amongst others¹

The viscosity or thickness in practical terms is defined as resistance to flow. High viscosity liquids are relatively immobile when subjected to shear (a force applied to make them move), whereas low viscosity fluids flow relatively easily.

The shear rate is defined as the speed with which a material is deformed.

In some processes relevant for the application of polymer emulsion, such as spraying or nozzle application for adhesives, the shear rate is high.
Other processes, for instance, during pumping or formulation leveling, the associated shear rate is low.

The viscosity remaining constant and independent from the shear rates applied defines Newtonian fluids. Shear-thinning materials exhibit decreasing viscosities when the shear rate increases. Most polymer emulsions fall into this category.

In some rare cases, emulsion polymer products can also have shear-thickening properties. Under shear stress, the components of the dispersion re-arrange and the resistance, and therefore the viscosity is increased. Therefore, the understanding of the full rheological profile of an emulsion polymer over a wide range of shear is essential.

Just relying on a single point value at one specific set of conditions, as it is often used as quality check with a simple rotary viscometer, at defined rotations per minute and sample temperature, is often not sufficient.

Measurement of viscosity and other rheological properties can be made using either capillary or rotational rheometers, the choice of system depending on:

The properties of the material being tested
The data required

#5 Glass Transition Temperature

The glass transition temperature (Tg) of an emulsion polymer product is measured by Differential Scanning Calorimetry at a dried film.

It translates into mechanical properties, for example – abrasion resistance or rigidness of a formed film.
The Tg of a given product is mainly influenced by the bulk monomers it is made of.

#6 Residual Monomer/VOC

As many emulsion polymer products are used in green applications for reduced emissions and to replace solvent-borne systems, the residual monomer content as well as the account of “total volatile organic components (VOC)” is an important characterization.

Gas chromatography is a standard measure to identify
the residues of organic components in emulsion polymers

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Redox Optimization for Emulsion Polymer Post Reaction

#7 Molecular Weight

There are many more dimensions to investigate and to fully characterize an emulsion polymer. Most of them are not easy to access. For instance, the molecular weight of the polymers is often not thoroughly analyzed.

Due to emulsion polymerization typical high molecular weights, cross-linking or grafting reactions, the bulk polymer is often not accessible for molecular weight analysis. Only the soluble part is investigated, separated from the insoluble one.

Depending on the specific question to be solved, also properties like polymer micro-structure, particle surface charge, or content of aqueous solution (serum) are investigated.

Selection Based on Emulsion Chemistry

Emulsion polymers are products by the process. This means that the specific application properties not only derive from the chemical composition of the specific product but also from the specific process of making it.

The four main factors that influence the properties of an emulsion polymer and its performance in the application are:

Monomers – The type and ratio of the monomers that form the bulk polymer.
Stabilizers – The type and amount of stabilizers.
Auxiliaries – The exact composition of the 1 – 2% of auxiliaries essential for the polymerization process.
Polymerization Process – The polymer process itself.

Certainly, any post-polymerization treatments or additions can influence the final properties as the formulation does, in which the emulsion polymer gets applied. These factors are not covered here.

Types of Monomers Used in Emulsion Polymerization

A fast amount of monomers can be used in emulsion polymerization to tailor the application properties. The material properties of the polymer emulsion products (as listed below) in application are determined by the diligent choice of the main monomers the emulsions are made of.

Hardness
Flexibility
Hydrophilic to hydrophobic properties
Adhesion and cohesion

Hence, there are only three distinct monomers and one monomer class, that contribute to the majority of the bulk polymer material of all emulsion polymers, such as:

Butadiene
Styrene
Vinyl acetate, and
The group of Alkyl (meth) acrylates

The Main Monomer Base for Industrial Emulsion Polymerization

Depending on if copolymerizations in-between these main bulk monomers are possible or favorable, the vast majority of emulsion polymers can be sub-summarized under one of the three major groups:

Acrylates
Styrene-butadienes
Vinyl acetate-based homo- and copolymers

These three types sum up to more than 80% of the total market.

Polymer Emulsion Demand - Market Share of Main Polymer Types

Market Share of Main Polymer Types
Source: The Freedonia Group, Inc

Acrylates

The class of acrylates is the most versatile in terms of a polymer property range. Only in a few applications, e.g. when resistance against alkaline conditions is required, acrylic polymers are not favorable to use.

There are many monomers commercially available as esters of Acrylic- or Methacrylic acid, ranging from low to high Tg as well as from hydrophilic to hydrophobic. They also can bear manifold functionalities.

Acrylic-based monomers quite easily can be copolymerized with styrene or vinyl acetate, forming styrene-acrylics and vinyl-acrylics as sub-classes, respectively.

The table below lists some typical acrylic-based monomers with physical data:

Monomer	Molecular Weight [g-mol^-1]	Boiling point [°C]	Tg [°C]	Δ_polyH [MJ kg^-1]
n-Butyl acrylate	128,2	148	- 54	- 0.60
Ethylhexyl acrylate	184,3	214	- 50	- 0.33
Acrylonitrile	53,1	77	98	-
Acrylic acid	72,1	105	105	- 1.08
MMA	100,1	101	105 - 120	- 0.58

Overview of typical commercial Acrylic monomers,
(According to "L.H. Howland, V.C. Neklutin, R.L. Provost and F.A. Mauger, Ind. Eng. Chem., 45, 1304-1311, 1953")

Styrene-acrylics offer excellent hydrophobic characteristics in combination with a high glass transition temperature that translates into high durability, abrasion resistance, and in general good mechanical properties.

Also, based on the high hydrophobicity of styrene, very low particle sizes can be achieved with Styrene acrylic copolymers in the emulsion, an essential need for certain applications, such as:

Primers for the construction industry, or
Binders for paper coatings

One drawback is the tendency to yellowing in the final product (also depending on the formulation), based on the aromatic nature of the Styrene monomer.

The price of styrene is lower than acrylates that makes Styrene acrylics a cost-efficient alternative to pure Acrylics.

It is very common in the emulsion industry to copolymerize Acrylic monomers with Vinyl acetate to form Vinyl acrylics. This class of products is a good compromise for many applications between the high performance/ high-cost pure Acrylics and the lower performance/lower cost Vinyl acetate emulsions.

Styrene-Butadienes

Styrene-Butadiene based polymer emulsions are the second largest group of polymer emulsions, but the least versatile segment in terms of copolymer composition.

Being copolymerized with Carboxyl moiety bearing monomers or not mainly differentiates Styrene-Butadienes from a chemistry point of view. They are largely produced by high volume manufacturers and delivered almost exclusively into two applications, such as:

Paper coatings, and
Carpet backing adhesives

The table below lists the monomer properties of Styrene and 1,3-Butadiene:

Monomer	Molecular Weight [g-mol^-1]	Boiling point [°C]	Tg [°C]	Δ_polyH [MJ kg^-1]
1,3-Butadiene	54,1	- 4.5	- 85	- 1.28
Styrene	104,1	145	100	- 0.65

Overview of monomer properties of Styrene and 1,3-Butadiene
(According to "L.H. Howland, V.C. Neklutin, R.L. Provost and F.A. Mauger, Ind. Eng. Chem., 45, 1304-1311, 1953")

Vinyl Acetate-based Homo- and Copolymers

Vinyl Acetate homo- and copolymers are the third-largest group of polymer emulsions. Due to its higher hydrophilic character, Vinyl acetate-based polymer emulsions mainly bind to hydrophilic surfaces, namely cellulose – as in its typical application in Paper and Packaging or Wood Adhesives.

To enable binding to less polar surfaces (e.g. coated surfaces or polyethylene foil), the introduction of less polar, more hydrophobic copolymers is needed, as it is widely done, for instance with ethylene or VeoVa™. In combination with the latter co-monomer, vinyl-based polymer emulsions deliver an excellent resistance against alkali, a property that is desired in many construction applications.

Hence, the space of commercially available vinyl-based co-monomers to alter the intrinsic properties of Vinyl acetate is limited, most of all if it comes to expand the glass transition temperature (Tg) towards higher values.

In the past, vinyl chloride was used as a co-monomer adding favorable property to the final product, namely:

Flame resistance
Mechanical resistance

But, due to its toxicological and environmental profile and the additional burden to use it in manufacturing, it got used lesser over the past decades.

The table below lists the typical monomers copolymerized with Vinyl acetate with the physical data:

Monomer	Molecular Weight [g-mol^-1]	Boiling point [°C]	Tg [°C]	Δ_polyH [MJ kg^-1]
Vinyl acetate	86,1	71	28 - 32	- 1.02
Ethylene	28,1	- 103	- 100	- 3.42
VeoVa™ 10	198,3	270 - 280	- 3	- 0.48
Vinyl chloride	62,5	- 13.4	85	- 1.69

Overview of typical commercial monomers copolymerized with Vinyl acetate,
(According to "L.H. Howland, V.C. Neklutin, R.L. Provost and F.A. Mauger, Ind. Eng. Chem., 45, 1304-1311, 1953")
(Source: VeoVa™ 10 data from MOMENTIVE)

Stabilizers - Important for Emulsion Stability

The second most important ingredient in a recipe for emulsion polymerization is the stabilizer. It is essential during the process of emulsion polymerization and keeps the final polymer dispersion stable during transportation, over shelf life, and during application if necessary.

There are three major stabilization mechanisms known, such as:

Electro-static
Steric
Electro-steric

Additionally, there is a mechanism called depletion stabilization, where polymers in the aqueous phase take space between the colloidally dispersed particles, and therefore hinder them to follow their depletion force and coagulate.

Emulsion Polymerization Stabilization Mechanisms

Four Possible Emulsion Polymerization Stabilization Mechanism

Steric stabilizers are polymers themselves. Mainly polyvinyl alcohols (PVA or PVOH) are used as steric stabilizers but also functionalized cellulose or polyethylene glycol / polypropylene glycol block-copolymers amongst others.

Polyvinyl alcohol as a polymer is made by hydrolysis of Polyvinyl acetate resins and can be characterized by its degree of polymerization and degree of hydrolysis.
The degree of polymerization is often expressed as intrinsic viscosity (Höppler viscosity) of a solution of Polyvinyl alcohol.
Typical commercial grades, used in emulsion polymerization range from Höppler numbers between 4 and 40, with an average degree of hydrolysis of 88%, with exemptions supporting that rule.

Due to the similar chemical structure, polyvinyl alcohols are at their majority used with vinyl acetate homo- or co-polymer dispersions. The resulting products have a quite broad particle size distribution, and the partly grafted, partly adsorbed and partly freely dissolved PVOH contributes to the rheological profile of the products in application.

Polymeric stabilizers can act as depletion stabilizers in polymer emulsions

Electro-static and electro-steric stabilizers are amphiphilic molecules, with hydrophobic and hydrophilic parts. Often, they are defined as surfactants as they are active at the surface (of the polymer particle in our case).

In opposite to the relatively limited amount of steric stabilizers used in commercial emulsion polymerization, the surfactants in use are myriad. Often two or three surfactants are combined together in an emulsion recipe, or they are added in parts to a mainly steric stabilized system.

With the right choice of surfactants, the properties of the final product can be tailored. For instance, small and narrow distributed particle sizes can be achieved in commercial emulsion polymerization with the right surfactant package.

Some typical examples for surfactants acting as electro-static stabilizers are salts of sulfonated or sulfated alkyl alcohols, which can be of the natural or technical source. Also, full or half esters of succinic acid found its application in emulsion polymerization.

When the alcohol gets ethoxylated, the stabilization mechanism shifts from electro-static to electro-steric, depending on the degree of ethoxylation. Ethoxylated surfactants come as non-ionic surfactants or ionic surfactants, also known as salts of the respective sulfonic or sulfuric acid.

The table below collects a very incomprehensible list of surfactants used in commercial emulsion polymerization.

Raw material	Surfactant	Type
Alkanes (mineral oil)	Alkyl sulfonates	Anionic
Alkyl benzenes (mineral oil)	Alkyl benzylsulfonates	Anionic
Fatty alcohols (mineral oil or natural)	Fatty alcohol sulfonates	Anionic
Fatty alcohols (mineral oil or natural)	Fatty alcohol ethyoxylates	Non-ionic
Fatty alcohols (mineral oil or natural)	Fatty alcohol ethyoxylates	Anionic
Succinic acid	Sulfosuccinate esters	Anionic

Typical Surfactants in Emulsion Polymerization

One specific class of ethoxylated surfactants - alkylphenol ethoxylates (both in anionic and non-ionic form) - was used very frequently in the pioneer years of emulsion polymerization, as they proved to be very effective stabilizers.

This surfactant type has attracted attention due to its prevalence in the environment and its potential role as an endocrine disruptor and xenoestrogen, due to its ability to act with estrogen-like activity.
It is regulated or even banned in many regions and industries. Therefore, it was replaced in many emulsion recipes by less toxic alternatives.

The stabilizers act already in the process of emulsion polymerization, impact the polymerization mechanism and influence, therefore, particle size distribution and surface charge, but also the molecular weight of the resulting polymer.

In the final product, they can positively contribute to the application properties but also have an adverse effect.

Mainly low molecular stabilizers can migrate from the polymer film to the surface and have negative effects e.g. on adhesion. Often, they also influence the behavior of the (formulated) Emulsion Polymers during application.
Unwanted foaming or problems with wetting are typical issues coming from wrongly chosen surfactant package.

Auxiliaries

Under auxiliaries, all chemicals that are neither monomers nor stabilizers can be sub-summarized. Every commercial recipe contains auxiliaries, from up to 0.1 to about 1% of the total amount of chemicals used. Even not adding to the mass of the product, they can impact its final properties and performance in the application.

The most important auxiliary is the radical-forming initiator. Peroxides or other chemicals that decompose thermally or combinations of metal salts with reducing components that form radicals via a redox mechanism at lower temperatures are used to start the polymerization. Other auxiliaries are:

Buffers
Acids
Bases
Molecular weight moderators, and
Inorganic salts amongst others

Polymerization Process

Emulsion Polymers are products by process. This signifies that the final product properties and its performance in the application are not only determined by the composition of the bulk polymer but also by the process of making it.

Most of the globally produced emulsion polymer material is produced in:

Stirred tank reactors with batch or semi-batch type processes.
CSTR cascades or tubular reactors with continuous processes.

The exact procedure of adding components and the profiles of important process variables like temperature, monomer or initiator feeds, etc. have a high impact on the properties of the final dispersion.

Independent of the composition of the chemicals used, particle size, polymer microstructure, surface charge, or particle morphology can be widely varied by the process conditions. Therefore, two chemically identical emulsion polymers can have completely different application properties.

The exact types and amounts of auxiliaries, as well as the polymerization process used, are held as a trade secret of the emulsion polymer producers. Therefore, even very similar products from different suppliers will not be exactly interchangeable in all formulations or applications.

Benefits and Features of Emulsion Polymers

Emulsion Polymers are ready-to-use products in most cases. Expensive secondary process steps, like precipitation and cleaning of the product, are spared. Some exemptions are:

Rubbers
Emulsion polymerized Polyvinyl chloride plastics (PVC), and
Re-dispersible powders (RDP)

Impact of Water During Polymerization

As the polymerization happens in water as the continuous phase, and due to the specific emulsion polymerization mechanism, very high molecular weight can be achieved, without having un-manageable high process viscosities.

Water also acts as a heat-transmitting fluid and helps to manage the exothermic nature of the reaction

Low Emissions and VOC

Modern emulsion polymers are relatively sustainable products that help to reduce emissions in many applications. Many processes and applications that required solvents can be transferred into completely solvent-free, water-borne ones, based on emulsion polymers.

Therefore, the past and continuing growth of the emulsion polymers sector is mainly driven by an increasing substitution of solvent-based systems by more environmentally-friendly water-based ones in paints & coatings, adhesives and other construction materials.

A key role for this increasing market penetration plays regulatory support towards lower emissions, and in general, reduced volatile organic components (VOC) across various nations and regions for a broad variety of consumer products.

Properties to Consider While Handling Emulsion Polymers

Emulsion polymers are more or less viscous liquids, containing organic polymer material in the water that brings a set of physical properties that set rules for handling.

Shelf Life is Very Important

First of all, they have a certain shelf life. Not only hat, freeze, exposure to sunlight, but also time can alter their colloidal state. When protective measures are not considered, the products get de-stabilized and lose their favorable properties.

The most common manifestations of this phenomenon are:

Settling, sedimentation or phase separation: The polymer material sinks to the bottom of the container or silo and creates a gradient in solids content and viscosity. Stirring up the material can often reverse this phenomenon.
Flocculation or Coagulation: As stabilization fails, the polymer particles bulk together. This process alters the objects particle size, rheology as well as the performance properties and is not reversible.

Most suppliers guarantee a shelf-life of 6 – 9 months when some basic measures of care are considered

Freeze-thaw Stability

Depending on the climate, certain stability of an emulsion polymer against freeze is necessary for transportation and storage without too expensive protective measures. This is expressed in freeze-thaw stability.

Not every product exhibits this resistance against this impact, but the supplier would advertise the ones that do with a certain number of “freeze-thaw” cycles they can resist without breaking.

Susceptibility Against High Shear Forces

Another practical aspect of handling emulsion polymers is their potential susceptibility against too high shear forces that also can break the colloid. This becomes relevant for pumping but also in the formulation. Suppliers provide technical service and guidance, e.g. for selection of low shear pumps in manufacturing.

Protection Against Microbial Attack

Also, as polymer emulsions consist of polymers in the aqueous phase, they are prone to microbiological attack. Therefore, most of the products are protected by biocides.

Depending on the final use and the regulations, this application (for example, for packaging adhesives that go into food packaging) is bound to the choice, and the amount of the biocides is very limited. Also, additional hygiene measures need to be undertaken to eliminate the risk of microbiological contamination of the final products.

Acrylic Emulsions for Paints, Coatings and Inks

View a wide range of acrylic emulsions available today, analyze technical data of each product, get technical assistance or request samples.

Access more emulsion chemistries here:

References

Fletcher: http://www.chemeurope.com/en/whitepapers/61207/

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