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5 Most Important Additives for Waterborne Adhesives

Edward M. Petrie – Aug 18, 2021

TAGS:  Waterborne Adhesives    

This article was first published in 2016 and is revised in 2021.

Waterborne Adhesives Additives: The Five Most Important in your FormulationWaterborne adhesives are usually fairly complex formulations of components that perform specialty functions. As it is already known that, very few waterborne polymers are used as adhesives without the addition of some modifying additives, such as:


  • Plasticizer,
  • Defoamer, or
  • Rheology modifiers.

The selection of the ingredients and their concentrations for any waterborne formulations will depend on the end-properties required, the application and processing requirements, and the overall cost target of the adhesive.

The formulator adds other additives to the waterborne adhesive emulsion to provide specific application and performance properties.

Before discussing the 5 most important additives for waterborne adhesives, let’s have a brief look at how additives get into a specific adhesive formulation through the figure below.


Use and source of additives used in waterborne adhesives
Use and source of additives used in waterborne adhesives

Certain additives are necessary for the emulsion polymerization process and these additives are already provided in the base emulsion resin. They include surfactants, neutralizing agents, initiators, and other additives related primarily to the formation and stabilization of polymer particles in water dispersion. These are used to provide a high-quality emulsion for the adhesive formulator to use as a base.

The formulator then adds other additives to the emulsion to provide specific application and performance properties. These formulator-added ingredients are the topic of this article.

The image below provides a listing of the five most important additives from a volume and performance basis that are utilized by the waterborne adhesive formulator. Thickeners (rheology modifiers) and plasticizers are the largest dollar value additives while antifoaming agents, adhesion promoters, and tackifiers represent somewhat smaller usage.

Major Additives Used in Formulating Waterborne Adhesive Systems
Major Additives Used in Formulating Waterborne Adhesive Systems
 

We will discuss the function of each additive in detail and provide information that will help you in selecting the proper type and grade for a specific formulation. These additives are generally used with all waterborne polymers (emulsions and solutions) and in all their types (pressure sensitive, laminating, etc.) and in all of their applications (automotive, construction, etc.)

Different adhesive applications make different demands on additives. To meet these specific demands, the formulator requires an understanding of adhesive formulations, especially how the effects and interactions of different additives play a crucial role in achieving the final properties.

Use a starting point formulation to jump-start your next project  »

These additives are generally used in emulsion-based adhesives but they are also widely used in adhesives made with water-soluble polymers. The modifications that are possible with certain additives and examples of these additives are summarized in the Table below. With the formulation variables offered through the choice of the base polymer and the additives, you can engineer an adhesive system with the required properties.

Additive Characteristics Example
Antifoaming agents Reduce foam during formulation and processing Dimethyl siloxanes, hydrophobic silicas, waxes, long chain alcohols and glycols, acetylenic diols
Rheology modifiers Adjust viscosity; modify rheology Water soluble cellulosics, water soluble / swellable emulsions, polyacrylic acid
Plasticizers Lower Tg, lower peel and give smooth release from backing; migration is often a disadvantage Phthalates (DOP, DBP), adipates, benzoates
Tackifiers Increase tack and peel, lower cost; negative impact on cohesion Rosin ester derivatives, modified C5 / C9 hydrocarbons, terpene resins
Adhesion promoters (wetting agents) Lower surface tension, improve wetting; commonly used in low viscosity systems Sulfosuccinates, acetylenic diols
Adhesion promoters (coupling agents) Provide a chemical link between the substrate (generally inorganic) and the base adhesive resin Organosilanes, organotitanates
Characteristics of Additives Commonly Used in Waterborne Acrylic PSAs



Antifoaming Agents to Overcome All Your Foaming Issues


Foam is often an undesirable consequence of the waterborne adhesive polymerization, compounding, or conversion processes. Foam can also develop during other stages of the adhesive’s life cycle such as during filling or packaging, transportation, and coating or application.

Foaming can lead to process inefficiency, overflow in tanks, instability of the adhesive emulsion, and poor substrate coating. In applying an adhesive coating, for example, foaming problems can result in several types of defects, most notably cratering, pin-holes (fisheyes), and poor wetting. These not only reduce the appearance of the adhesive coating but can also significantly reduce the adhesion properties of the finished product.

Antifoaming Agents for Waterborne AdhesivesFortunately, it is possible to control foam by integrating certain additives into the adhesive.

  • Defoamers are generally added to pre-existing foam to eliminate the foam, and
  • Antifoaming agents are added before the formulation to prevent it from foaming.

There are many antifoaming agents on the market and you can find the suitable foam control agent in our Adhesive Ingredients Material Selector here. However, selecting the proper type for a specific application can sometimes be a daunting task since optimal antifoaming properties will depend heavily on the specific adhesive system and on the processes employed in formulation or conversion.

In addition to reducing surface defects on coated substrates, effective foam control agents are beneficial in preventing or reducing common problems, such as:

  • Viscosity increase and loss of mechanic shearing power during compounding (resulting in smaller batch sizes and poor dispersion of fillers and additives);
  • Volume increase during mixing or shearing processes that leads to over-foaming;
  • Slower package-filler rates due to inefficient pumping;
  • Air incorporation during transport and handling;
  • Slower coating speeds or lower pressures during spraying1.

Antifoam Agents - Mode of Action


Antifoam agents function by being more surface active than the surfactant that is stabilizing the foam. In this way, they are able to enter the surface layers of the potentially foaming liquid and displace it from the gas / liquid interface. The mixed surfactant layers will prevent the close association of molecules in the original liquid. The thermodynamic factors and surface properties influencing the foam control mechanism and foam stability have been well covered in the literature.2,3

The composition of antifoaming agents is extremely diverse. They can be supplied as solids, pastes, or liquids. Liquids are the most predominant form. Foam control agents are typically composed of a hydrophobic material, a carrier vehicle and, optionally, an emulsifier.

Hydrophobic components are considered the most active ingredients in a foam control agent. Typical hydrophobic materials are treated silica, synthetic or natural waxes, polyglycol, and silicones or silicone derivatives. These hydrophobes may either be used alone or in combination. Carrier vehicles are generally oils (mineral, vegetable and silicone), alcohols, glycols, and water.

The choice of the foam control agent and its concentration level is a delicate balance between the following factors.
  1. The technical requirements inherent to the product,
  2. Processing parameters that will be employed, and
  3. Economic constraints.

Antifoam technology is system-specific. It is critical for adhesive formulators to choose a foam control compound that provides the required antifoaming properties without adversely affecting the adhesion characteristics.




Rheology Modifiers Are the Key for Right Flow Properties


Rheology modifiers for waterborne adhesivesWaterborne adhesives based on resin emulsions do not possess the same rheological characteristics as solvent-borne adhesives.

The viscosity of an emulsion-type adhesive is very dependent on the shear rate; whereas, there is no such dependency for solvent-borne systems. If not correctly formulated, the waterborne adhesive can have inferior flow properties which can affect both manufacturing and application properties.

There are a number of occasions when you may want to consider using rheology modifiers in your waterborne adhesive formulation. These occasions will depend on the internal manufacturing needs of the formulator and on the end-use application.

The table below lists several occasions when rheology modifiers can possibly provide added value.

Possible Occasions When Rheology Modifiers May Provide an Added Value

  • Reduce settling / increase shelf life
  • Reduce time and energy cost for mixing and pumping
  • Increase thickening efficiency and reduce raw material costs (use less product and keep the same performance level)
  • Reduce splattering during adhesive manufacture and application
  • Increase manufacturing or converting speed especially in tape, label, and laminate production processes
  • Reduce penetration into porous substrates
  • Increase sag resistance
  • Increase coating build (thickness) per application
  • Reduce open time or drying time
  • Improve sprayability, brushability, other application processes
  • Improve printability and resolution of adhesive coatings


A waterborne adhesive may ideally require different viscosity characteristics at different occasions during its lifetime from manufacturing to application.

Rheological additives are often used to adjust the viscosity characteristics of the adhesive accordingly. They can provide multiple values including:
  • Easy mixing,
  • Suspension of formulation ingredients and storage stability,
  • Improved coating or spraying,
  • Increased adhesive build,
  • Easy and accurate dispensing, and
  • Non-sag properties.

These value propositions can result in lower costs (material, energy, and waste) for the formulator as well as the end-user.

Meet specific rheological properties (optimal flow, faster drying...) in your WB adhesives with a smart selection strategy for rheology modifiers as well as learn how can you achieve more with lesser ingredients by understanding the potential value of using the correct rheology modifier in this exclusive course by Adhesives Industry expert Ed Petrie.

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Rheology Modifiers Chemistries Used in Waterborne Adhesives


There are many different types of rheology modifiers, and their use will depend on the specific function to be performed, the base polymer, and other additives in the adhesive formulation. Rheology modifiers come from both natural and synthetic sources, and they function through a combination of different mechanisms.

Figure below provides a classification scheme of the various rheology modifier technologies. Those most commonly used in waterborne adhesive systems include cellulosics, hydrophobically modified alkali swellable acrylic emulsions (HASEs), hydrophobically modified ethoxylated urethanes (HEURs), and alkali-swellable acrylic emulsions (ASEs).

Rheology modifiers classification

Classification scheme for rheology modifiers

There are basically two mechanisms by which rheology modifiers cause change to the viscosity of waterborne adhesives: associative and non-associative. Cellulosics and ASEs are considered to be non-associative rheology modifiers and HASEs and HEURS function via an associative mechanism.

Rheology modifiers generally do not affect the performance properties of the adhesive once it is applied and dried or cured. This is mainly because of the low concentrations of rheology modifiers in the formulation. However, certain properties such as moisture resistance of the final adhesive film can be affected by these modifiers, and testing is recommended.

It is important to realize that the selection of rheology modifiers must be given special consideration relative to the other components in an adhesive. There is a broad range of rheology modifiers and chemistries available and there is no universal rheology modifier. Therefore, the selection of the optimal modifiers and their incorporation into an adhesive formulation requires special consideration.




Plasticizers Give Your Formulation the Right Level of Flexibility


Plasticizers for waterborne adhesivesPlasticizers are widely used in waterborne adhesives. Plasticizers are organic liquids or solids that are incorporated into a compatible polymer to reduce interaction between molecules and improve molecular mobility. This is primarily accomplished by a lowering of the glass transition temperature, Tg. Plasticizers offer improvement in flexibility, low-temperature performance, and modification of a number of other properties.

The value proposition of using plasticizers depends very much on the below-listed factors:
  • The type of polymer that is being modified,
  • The form of the resulting product (e.g., waterborne, solvent-based, solid, etc.), and
  • The products’ end-use application.

The current market offers numerous choices of plasticizers listed in our Adhesive Ingredients Material Selector. These plasticizers offer a range of attributes that can be selected for specific applications to meet critical material requirements.

The degree of plasticization is largely dependent on the plasticizer’s chemical structure, including chemical composition, molecular weight, and functional groups. Plasticizers that have low molecular weight and a small number of polar groups generally provide higher flexibility and plasticization. Unfortunately, low molecular weight also corresponds to a high degree of migration and volatilization problems that could be associated with health or safety risks.

Use a starting point formulation to jump-start your next project  »

Compatibility with Base Polymer is Must to Check


Eastman plasticizers for water-based adhesivesAlmost all types of polymers can be plasticized. However, the plasticizer must be compatible with the base polymer in a formulation to function efficiently. The plasticizer and polymer should have similar polarity to produce compatibility. In addition to compatibility with the base polymer, selection criteria for plasticizers include performance, cost, food contact status, volatile organic emissions, migration potential, and regulatory labeling. Ideally, plasticizers should be:
  • Highly compatible with polymers,
  • Stable in both high and low temperature environments,
  • Sufficiently lubricate over a wide temperature range,
  • Insensitive to solar ultraviolet radiation,
  • Leaching and migration resistant,
  • Inexpensive,
  • Fulfilling health and safety regulations.

Different plasticizers affect different physical and chemical properties of materials. Therefore, a formulator selects a particular plasticizer to change properties in a certain direction to meet requirements.

There are over 38 general chemical families of plasticizers that are used for polymer modification. The primary chemical classes are listed in the table below. Within each chemical class, there are many variations. Many products will also use blends of plasticizers to achieve the desired end properties.

Chemical families of plasticizers that are used for polymer modification
Chemical Classes of Plasticizers

In waterborne adhesives, DIBP has been an industry standard; however, DIBP has been classified as an SVHC (*substance of very high concern) according to REACH. According to Annex XVII to Entry No. 51 of the REACH Regulation (EU Regulation No. 1907/2006), REACH has implemented restrictions for DEHP, DBP, BBP, and DIBP (individually or in any combination, no greater than 0.1% by mass of plasticized material) starting on July 7, 2020. Changing regulations and consumer preferences now demand that more phthalate alternative chemistries for use in consumer products including adhesives. Now, dibenzoates and bioplasticizers such as citrates, soy derivatives, and isosorbide are gradually replacing phthalates in sensitive applications.

Since there is a variety of plasticizers and a lot of regulations associated, the question arises how to select the right plasticizer for your formulation?




Tackifiers for the Right Balance Between Cohesion and Adhesion


Tackifiers in waterborne adhesivesTackifiers are important constituents in waterborne pressure-sensitive adhesives in terms of both cost and their effect on properties. Tackifiers get their name from the fact that they are used to increase the tack or “stickiness” of the adhesive. In practice, formulators use tackifiers to make the correct balance between adhesion and cohesion which is dependent on the end-use application. Tackifiers do this by lowering the modulus and increasing the glass transition temperature of the final adhesive.

Tackifiers must be considered with regard to the following factors:
  • The end-properties that you are trying to achieve,
  • The type of processing that the adhesive will be subject to, and
  • The chemical nature of the base polymer.

Standard tackifiers are manufactured from either natural or petroleum-based feedstock. The tackifiers that are manufactured from natural sources are generally defined as rosin resins or terpene resins. The petroleum-based tackifiers are generally referred to as hydrocarbon resins. Although the natural tackifying resins represent the oldest technology, a recent explosion of interest has occurred in these products due to the emphasis on biobased, green additives. Tackifiers are sold either as 100% non-volatile forms (solids or molten liquid), as solution cutbacks, or as waterborne resin dispersions, which are used primarily in waterborne adhesives.

The first water dispersions of adhesives used tackifiers dissolved in a solvent and then dispersed in water using appropriate emulsifiers. This solvent-cut technique works well but produces dispersions with a considerable concentration of organic solvent. Globally, the environmental protection regulations are increasingly becoming more restrictive, and solvent-cut based adhesives have been banned in many countries. In much of the world, adhesives with very low or no volatile organic compounds (VOCs) are now required.

Waterborne tackifier resin dispersions are now commonly used for PSA label, decal, or tape applications based on acrylic emulsion polymers or various types of natural or synthetic rubber latex. These dispersions are manufactured by mechanical process. The major uses are general-purpose permanent, removable, deep freeze, wine bottle, and wash-off applications. In general, dispersions of waterborne tackifier resins will:
  • Improve specific adhesion especially to polyolefinic and paper substrates.
  • Improve flow and wetting properties of the adhesive.
  • Provide an optimum balance between tack, adhesion (peel), and cohesion (shear).

There are many different types of tackifiers and choosing an ideal tackifier for a specific formulation, therefore, requires a disciplined approach.




Adhesion Promoters Offer Optimized Bonding Capabilities


Adhesion_promoters for Waterborne AdhesivesAdhesion promoters generally consist of either wetting agents (surfactants) or coupling agents such as organosilanes. Each type provides value primarily by improving adhesion. However, the mechanisms by which they work are very different.

Wetting Agents


Wetting agents are surfactants used at low concentrations in waterborne adhesive to improve:
  • substrate wetting
  • pigment or filler wetting
  • flow and leveling, and
  • film coalescence.

They are valued highly due to the improved adhesion that occurs especially to low energy substrates such a plastic films, release papers, coated board, etc.

Low energy substrates are not readily compatible with unmodified waterborne adhesives and consequently may not provide good bond strength. Wetting agents may be used to improve the degree of wetting and the subsequent adhesive strength. Wetting agents are also used to improve the wetting of fillers in construction adhesive and to control open times for the adhesive before bonding.

The effectiveness of an additive is often judged by its ability to lower a liquid’s surface tension at the lowest possible concentration. Surfactants get their surface tension reducing properties due to fact that they have in their molecule both hydrophilic and hydrophobic groups. The additive should also not cause undesirable properties such as interference with intercoat adhesion, increased tendency to foam or increase water sensitivity. Table below shows a general comparison of different chemical types of surfactants that are used as wetting agents in waterborne systems.

Property Type of Surfactant
HMw Silicone Surfactant LMw Silicone Surfactant Non-ionic fluoropolymer surfactant Organic surfactant
Static Surface Tension Reduction Good Very good Outstanding Good
Spreading Ability Poor Very good Acceptable Acceptable
Dynamic Surface Tension Reduction
Acceptable
Good Poor Outstanding
Foaming Acceptable Good Poor Outstanding
General Comparison of Different Chemical Types of Surfactants in Waterborne Systems4

Wetting agents can include a number of different surface active additives. The most common wetting agents are sulfosuccinic acid esters, alkoxylates, sodium lauryl sulfate, fluoro-surfactants, and acetylenic diol based products. These additives can also influence the shear and peel strength of the adhesive, induce water sensitivity into the finalized joint, and cause foaming. Thus, their concentrations should be kept as low as possible. 

Coupling Agents


Organosilanes are a special class of adhesion promoter that is used to improve the adhesion between an inorganic substrate and a polymeric based product. Organosilanes are the most widely used family of adhesion promoters. They are often used in formulating 100% solids epoxy and polyurethane adhesive and sealant systems. However, organosilanes have also been applied effectively in solvent-based systems, water-based emulsions, and thermoplastic hot melts.

Organosilane adhesion promoters are liquid products that form a very thin (usually monomolecular) layer between the substrate and the adhesive. Different chemical bonds are formed between:
  • the adhesion promoter and the adhesive, and
  • the adhesion promoter and the substrate surface.

These bonds are often stronger than the internal chemical bonds within the adhesive. These new bonds also provide an interface region that is more resistant to chemical attack from the environment.

The main function of an organosilane is to form a strong, impermeable “chemical bridge” between the adhesive / sealant and the substrate. In order to support the “ends” of the bridge, organosilane are made of bifunctional compounds that can react chemically with both the substrate and the adhesive. These covalent bonds are both strong and durable. On one end is an organofunctional group that is particularly compatible with the given adhesive base polymer. At the other end of the chain is an inorganic functionality that is especially compatible with a given substrate. As adhesion promoters, organosilanes are most effective on high energy, inorganic surfaces such as metals, glass, and ceramics.

There are a number of organosilanes available, and they mainly differ from each other in the nature of their reactivity with the adhesive resin. Organosilanes may be produced with amine, epoxy, mercaptan, and other functionalities. To achieve optimal adhesion, the functionalities of the organosilane must be carefully matched to the type of polymer matrix used and the physical properties desired in the cured joint.

Historically, organosilanes have been limited to 100% solids, high solids, and solvent systems in which moisture is not encountered until use. However, waterborne systems are also appropriate for additions of newer organosilanes. The most practical method of formulating emulsion based adhesives is to add a water soluble organosilane directly to the aqueous phase. Optimum adhesion in urethane, acrylic and styrene butadiene emulsions is obtained at about 1% organosilane based on solids content of the polymer. Table below provides recommendations for the use of various organosilanes with different water based adhesive formulations.

Water Soluble and Hydrophilic Polymers Silane Chemistry
A B C D
Acrylic latex X
Cellulosic X
Heparin X
Polyhydroxyethylmethacrylate X
Polysaccharide X
Polyvinyl alcohol X X
Polyvinyl acetate X
Ethylene vinyl acetate X
Recommendation of Silanes for Various Waterborne Systems

  • A: N-(2 aminoethyl)-3-aminopropyltrimethoxysilane
  • B: 3-methacryloxypropyltrimethoxysilane
  • C: N-[2(vinylbenzylamino]-ethyl-3-aminopropyltrimethoxysilane
  • D: 3-glycidoxypropyltrimethoxysilane



Additives and Polymers Selection for Water-based Adhesives


Explore all types of additives and polymers used to formulate water-based adhesives, analyze technical data of each product, get technical assistance or request samples. Start now!



References

  1. O’Neil, V., et. al., “New Silicone Foam Control for Waterborne Coatings”, Paint and Coatings Industry, October 2003, pp. 54-62.
  2. Garrett, P.R., “Defoaming: Theory and Industrial Applications”, P.R. Garrett, ed., vol. 45, Surfactant Science Series, Marcel Dekker, New York, 1993.
  3. Porter, M.R., Handbook of Surfactants, Blackie and Son Ltd. Chapman & Hall, New York, 1991.
  4. Heilen, W. and Struck, S., “Overcoming Substrate Wetting Problems”, Paint and Coating Industry, March 2003, pp. 58-65.

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