Adhesion Promoters: Adhesion Basics & Material Selection Tips for Adhesives

How to Improve Adhesion to a Substrate?

The addition of adhesion promoter to an adhesive or sealant is one of several methods of improving adhesion to a substrate. It is a specialty compound that can react chemically with both the substrate and/or the adhesive. It forms covalent bonds across the interface that are both strong and durable.

How to use adhesion promoter? Adhesion promoter can be applied either as:

An internal additive to a formulation or
A substrate surface treatment (i.e., as a primer prior to applying the adhesive / sealant)

Adhesion promoters/coupling agents are generally integrally blended into the formulation either by the adhesive supplier or by the end-user immediately before application. When mixed with the adhesive, the coupling agent can migrate to the interface and reacting with the substrate surface as the adhesive cures.

The adhesion promoter, therefore, acts as a chemical bridge
between the adhesive and the substrate

It also provides an interphase region that is more resistant to chemical attack from the environment. Adhesion promoter usually consists of molecules with short organic chains with the capability to form primary bonds to either or both the adherend and the bulk adhesive.

Adhesion promoter must be considered with regards to:

Compatibility with the base polymer in the formulation
The chemical nature of the substrate to be bonded
The end-properties that the formulator is trying to achieve

Adhesion Theory – Basics of Adhesion

The mechanism of adhesion has been investigated for years; several theories have been proposed in an attempt to provide an explanation for adhesion phenomena. However, no single theory explains adhesion in a general, comprehensive way.

The bonding of an adhesive to an object or a surface is the sum of a number of mechanical, physical, and chemical forces that overlap and influence one another

#1 Mechanical interlocking - The mechanical interlocking theory of adhesion states that good adhesion occurs only when an adhesive penetrates into the pores, holes and crevices and other irregularities of the adhered surface of a substrate, and locks mechanically to the substrate.

The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate pores and openings in a reasonable time.

This theory explains a few examples adhesion such as rubber bonding to textiles and paper. Since good adhesion can occur between smooth adherend surfaces as well, it is clear that while interlocking helps promote adhesion, it is not really a generally applicable adhesion mechanism.

#2 Electrostatic forces - The basis of the electrostatic theory of adhesion is the difference in electonegativities of adhesing materials. Adhesive force is attributed to the transfer of electrons across the interface creating positive and negative charges that attract one another. For example, when an organic polymer is brought into contact with metal, electrons are transferred from metal into the polymer, creating an attracting electrical double layer (EDL).

The electrostatic theory tell us that these electrostatic forces at the interface ( i.e. in the EDL), account for resistance to separation of the adhesive and the substrate.

#3 Chemical bonding forces - This chemical adhesion mechanism is explained in the case of the intermolecular forces by the adsorption theory, and in the case of chemical interactions by the chemisorption theory. The processes that play a role in the bonding of similar types of thermoplastic high-polymer materials, e.g. homogeneous systems, can be determined with the diffusion theory.

Adsorption - The adsorption theory states that adhesion results from intimate intermolecular contact between two materials, and involves surface forces that develop between the atoms in the two surfaces.

This theory is the most important mechanism in achieving adhesion. The most common surface forces that form at the adhesive-adherend interface are van der Waals forces.

In addition, acid-base interactions and hydrogen bonds, generally considered a type of acid-base interaction, may also contribute to intrinsic adhesion forces.

The mechanism of adhesion in many adhesive joints only involves interfacial secondary forces. The calculated attractive forces between two surfaces are considerably higher than the experimentally measured strength of adhesive joints; this discrepancy between theoretical and experimental strength values has been attributed to voids, defects or other geometric irregularities which may cause stress concentrations during loading.

To obtain good adsorption, intimate contact must be reached such that van der Waals interaction or the acid-base interaction or both take place; hence good wetting is essential. According to Young's equation, the surface tensions (liquid/vapor: γ_LV, solid/liquid: γ_SLand solid/vapor: γ_SV) at the three phase contacts are related to the equilibrium contact angle σ through:

γ_SV = γ_SL + γ_LV . cos σ

The one important factor that influences the adhesive joint strength is the ability of the adhesive to spread spontaneously on the substrate when the joint is initially formed. For spontaneous wetting to occur:

γ_SV ≥ γ_SL + γ_LV

We can say that for good wetting: γ_SV < γ_LV

Angle of contact of a drop of liquid with the surface of a solid object

Generally speaking, the liquid surface tension of the adhesive should be less than the critical wetting tension of the solid surface of the substrate

Chemisorption - The chemical bonding mechanism suggests that primary chemical bonds may form across the interface. Chemical bonds are strong and make a significant contribution to the intrinsic adhesion in some cases.

Diffusion - The diffusion theory attributes the adhesion of polymeric materials to the inter-penetration of chains at the interface. The major driving force for polymer autohesion and heterohesion is due to mutual diffusion of polymer molecules across the interface. This theory requires that both the adhesive and adherend are polymers, which are capable of movement and are mutually compatible and miscible.

Parameters affecting the diffusion process are: contact time, temperature, molecular weight of polymers and physical form (liquid, solid). Polarity generally increases adhesion.

Methods to Promote Adhesion

There are a number of ways to promote adhesion for adhesives and sealants to adhere well to other materials. The main mechanisms of promoting adhesion are:

Blending or Compatibilizing

Blending is a common way to enhance the adhesion of one polymer to a substrate. It sometimes is used to directly bond (without adhesive) one plastic to another or to an inorganic substrate.

Blending improves adhesion by making the initial polymer better wetting and more compatible (chemically and physically) with the secondary substrate. Physical mixing of a polymer enriched with another compatible polymer is a simple technique for improving adhesion. Copolymerization is more difficult although used in numerous plastics such as:

ABS, SAN, ASA versus polystyrene
EVA, versus polyethylene or vinyl acetate

Non-reactive adhesion promoters draw their functionality mainly from their polarity introduced by a co-monomer. An intermediate polarity is then achieved, and adhesion is assured by Van der Waals forces.

Non-reactive adhesion promoters include ethylene copolymer of ethylene and acrylates (EMA, EEA, and EBA). These are often used as adhesion promoters in formulating tie layers for multi-layer laminates.

Surface Modification

Surface modification is broadly used to improve adhesion of a substrate because of its versatility. The process is applied to the substrate and generally there is no need to alter the adhesive formulation.

Surface treatment processes are continually evolving. They improve adhesion by either creating active chemical sites on the substrate surface or physically altering the surface topology to allow for mechanical adhesion.

Primers are also, at times, considered to be adhesive promoters. These work by producing a more easily wettable surface and by diffusing somewhat into the substrate via a solvent carrier or low viscosity compatible monomer. Although there are many types of adhesion promoting primers, the most common are:

Chlorinated polyolefins that are used mainly as adhesion promoter on thermoplastic olefins such as those used in the auto industry and
Primers for cyanoacrylate adhesives

Adhesion promoters

Adhesion promoters increase adhesion by incorporating functional additives into the adhesive formulation that can chemically bond to the adhesive and / or the substrate.

Ideally, this interfacial bridge will improve the initial joint strength and prevent delamination of the adhesive from the substrate, even when water, oxygen, and salts migrate to the joint interface. Several theories have been suggested to explain the mechanisms by which adhesion promoters improve strength and resistance to moisture:

Strong chemical bonds are formed at the adhesive and substrate interfaces
Deformable layer allows stresses which are relieved by the new interface
Improved surface wetting
Interfacial layer provides an intermediate modulus that transfers stresses efficiently between the substrate and the adhesive
Oxide reinforcement of metal substrates provides for the strengthening of the boundary layer

Maximizing Adhesion in Adhesives & Coatings

Adhesion Promoters Classes & Key Characteristics

It is well established that judicious use of the correct adhesion promoter increases adhesive strength and resistance of the cured joint to moisture. Over the last 20 years the adhesion promoter marketplace has evolved to include a plethora of materials. The main classes of adhesion promoters include:

Organosilane adhesion promoter
Organotitanate adhesion promoter
Zirconate adhesion promoter
Zircoaluminate adhesion promoter
Alkyl phosphate ester
And more (Chrome Complexes, Amines...)

Among these classes organo silane adhesion promoters have secured a prominent position in the market. Titanate and zirconate adhesion promoters are less well-recognized compounds that also perform as coupling agents and / or adhesion promoters.

Each adhesion promoter type has a specific set of characteristics:

Type	Inorganic Groups	Organic Groups	Molecular Stability	Moisture Sensitivity	Solubility in Solvents
Organosilane	-OR	Varied	Excellent	Poor	Excellent
Organotitanate	-OR	Varied	Fair/good	Poor	Excellent
Zirconate	-OR	Varied	Fair/good	Poor	Excellent
Zircoaluminate	-OH, -Cl	Varied	Excellent	Excellent	Fair
Alkyl phosphate ester	-OH	Alkyl/aryl	Good	Fair	Good
Metal organics	-OH	Varied	Excellent	Excellent	Good

Characteristics of Various Classes of Adhesion Promoters

Overall, all adhesion promoters share several common characteristics such as:

Common Characteristics of Adhesion Promoters

Although the best results can be obtained in using adhesion promoters as substrate primers, they can also be added to the adhesive with some effect.

Silane Coupling Agents

Coupling agents usually consist of molecules with short organic chains having different chemical composition on either end of the chain.

On one end is an organofunctional group that is particularly compatible with the given adhesive material
At the other end of the chain is an inorganic functionality that is especially compatible with a given substrate

The most well-known of these compounds is the organosilanes. These have been used extensively as internal additives and primers for adhesives and sealants to improve:

Adhesion,
Compounding properties, and
As a crosslinking agent

Organosilane adhesion promoter is commonly used to enhance adhesion between polymeric and inorganic materials.

Function of an Organosilane

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, organosilanes are made of bifunctional compounds that can react chemically with both the substrate and the adhesive.

They usually have the form:

(RO)₃-Si - R' - X

Silicone (Si) is the center of the silane molecule, which contains the organofunctional group (RO) and a second functional group (X).
RO is a hydrolyzable group, typically methoxy, ethoxy, or acetoxy, which reacts with water to form silanol (Si - OH) and ultimately forms an oxane bond with the inorganic substrate (Si- O - Metal).
X is an organofunctional group, such as amino, epoxy, or methacrylate, which attaches to the organic resin. The compatibility of this group with various base polymers can be seen in the table below.
R' is typically a small alkylene linkage.

Hence, the resulting interface provides:

A strong chemical bridge between the substrate and organic polymer
A barrier to prevent moisture penetration to the interface
Transfer of stress from the resin to the substrate, thereby improving joint strength

It should be noted that the bond to the organic polymer molecule is more complex than the bond to the inorganic substrate. It is important that the chemistry of the organosilane and polymer be well matched for optimum properties. For example, an epoxy silane or amino silane will bond to an epoxy resin, an amino silane will bond to a phenolic resin, and a methacryl silane will bond to an unsaturated polyester resin.

For thermosetting polymers, the reactivity of the polymer should be matched to the reactivity of the organosilane. With a thermoplastic molecule (such as in thermoplastic hot melt adhesives or composites) bonding occurs by diffusion of the organosilane network in the interphase region of the joint.

There are a number of silane adhesion promoters available, and
they differ from each other in the nature of their reactivity to the resin or adhesive

Some common organosilanes are:

3-Chloropropyltrimethoxysilane
Vinyltriethoxysilane (VES)
γ-Methylacryloxypropyltrimethoxy-silane
γ-Glycidoxypropyltrimethoxy-silane (GPMS)
γ-Mercaptopropyltrimethoxy-silane
γ-Aminopropyltriethoxysilane
N-b-Aminoethyl-aminopropyl-trimethoxysilane (AAMS)

Silane Functionality	Application (Suitable Polymers)
Vinyl, methacryl	Free radical cure systems: crosslinked polyethylene, peroxide cured elastomer, polyester, polyolefins, EPDM, urethane, alkyd
Epoxy	Epoxy, phenol, epichlorohydrin, PVC, polyester, urethane, polysulfide
Methacryl	Unsaturated polyester, acrylic
Amino	Epoxy, phenolic, melamine, furan, urea, PVC, urethane, polysulfide, polyvinyl butyral, polyimide, polychloroprene, nitrile rubber, etc.
Mercapto	All elastomers, epoxy, sulfur cure rubber, urethane, polysulfide, PVC
Ureido	Phenolic, urethane, melamine, epoxy
Chloro	Epoxy, polyurethane, thermoplastics

Recommended Silane Functionality for Various Base Resins
The small amount of water required for hydrolysis is commonly supplied by trace moisture on the surface of the substrate or from the moisture inside the adhesive formulation. The hydrolysis and competition between the substrate and possible fillers are points of concern in formulating practical systems.

Reaction of organosilanes to the inorganic substrate involves four steps.

Initially, hydrolysis of the alkoxy groups occurs.
After the first and second alkoxy groups are hydrolyzed, condensation to oligomers follows.
The third methoxy group upon hydrolysis is oriented toward the hydroxyl sites and hydrogen bonds with these sites on the substrate. This produces a polysiloxane network that is covalently bonded to the surface (a slower reaction than the initial hydrolysis).
Finally, during drying or curing, a covalent bond is formed with the substrate, and water is liberated.

At the interface there is usually only one bond from each silicon of the organosilane to the substrate surface. This mechanism results in a continuous chain of covalent bonds from the substrate to the adhesive.

Coupling mechanism of Organosilane adhesion promoter

Organosilane adhesion promoter reacts with water to form silanols (hydrolyzed silanes), which react with the surface of the inorganic substrate

Organosilanes will bond well to hydroxyl groups on most inorganic substrates. Because of the importance of the hydrolysis reactions, water is necessary at the interface for the organosilane bonds to form. With the application of organosilanes as a primer, a water solution is commonly employed. However, if applied from a non-aqueous solvent, it is frequently recommended that the primer coating be rinsed with water and again dried before application of the adhesive.

The hydrolysis reaction also may require an acid or base catalyst. This is generally supplied by the addition of traces of acidic acid or by the basic nature of the metal. Aminofunctional organosilanes do not require a catalyst because the amine will catalyze auto-condensation in the presence of water.

Related Read: Adhesion Promotes Among 5 Most Important Additives for Waterborne Adhesives

Silane Effectiveness

Silanes form strongly adsorbed polysiloxane films on ceramic and metal surfaces. The chemical and mechanical integrity of these films are highly dependent on the organic polymers in the adhesive formulation as well as the chemical nature of the substrate. The image below shows the relative influence of the type of substrate on the effectiveness of the silane coupling agent in improving adhesion.

It should be noted that:

Smooth, high energy substrates are excellent substrates for silane attachment; and
Rough, discontinuous substrates show little benefit

Effect of substrate type on silane adhesion promotion

The interphase provided by the adhesion promoter may be hard or soft depending on the type of silane employed, and this could affect mechanical properties.

A soft interphase, for example, can significantly improve fatigue and other properties and it will reduce stress concentrations
A rigid interphase improves stress transfer of resin to the adherend and improves interfacial shear strength.

Adhesion promoters generally increase adhesion between the resin matrix and substrate,
thus raising the fracture energy required to initiate a crack

Organosilane adhesion promoters are not generally applicable for bonding polymer surfaces that are devoid of active hydroxyl functionality. Nor do they apply to bonding graphite or noble metals such as gold, silver, or platinum to polymer matrices. These substrates have no hydroxy "handles".

However, organosilanes will bond to polymeric substrates when:

Inorganic fillers or reinforcement are exposed on the surface (generally through machining or abrasion),
The plastic itself provides hydroxy functionality through its molecular chain, and
The plastic is subjected to a surface treatment to provide hydroxyl functionality (e.g., corona and plasma treatment)

Adhesion Promoters and Primers for Adhesives & Coatings

Silanes Compatibility with Base polymer

The silane adhesion promoters vary by their:

Inorganic and organic reactive groups
Molecular stability
Moisture sensitivity
Solvent solubility, and
Cost

Some specific examples of organosilane coupling agents are given in the table below.

Polymer	Organosilane
Polymer	N-(2 aminoethyl)-3-aminopropyltrimethoxysilane	3-methacryloxy propyltrimethoxysilane	N-[2(vinylbenzylamino]-ethyl-3-aminopropyltrimethoxysilane	3-glycidoxypropyl-trimethoxysilane
*Thermoset Polymers:*
Diallyl phthalate
Epoxy
Furan
Polyimide
Melamine
Paralene
Phenolic
Polyester
Polyurethane
Thermoplastic Polymers:
Acrylic
ABS
Alkyd
Cellulosics
Nylon
Nitrile rubber
PEEK
Polyamide
Polycarbonate
Polyethylene
Polyphenylene oxide
Polyphenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyvinyl butyral
Polyvinyl chloride

They are generally chosen by matching the organic functionality to the base polymer to optimize bonding. However, the choice of the correct adhesion promoter family or a type within a given family is often not a straight-forward task. Sometimes mixtures of silanes are used as adhesion promoters to provide enhanced hydrophobicity, thermal stability, or crosslinking at the bonding site.

Formulating with Organosilanes - Primer or Integral Adhesion Promoter?

In general, adhesion promoters may be used as substrate pretreatments or as additives.

Organosilanes as Primer

In the former case the promoter is used generally as a solution in a suitable solvent (e.g., methanol, ethanol, and isopropanol) or solvent mixture. After the primer is applied and allowed to dry, excess material can be gently wiped-off or rinsed-off with alcohol. The silane layer is then cured for 5-10 min at 110°C or for 24 hrs at ambient conditions.

The silane primer can also be applied from a low VOC aqueous solution (0.5-2.0% of silane with pH adjusted to 4.5-5.5 pH with acetic acid). Stability of the aqueous silane solutions varies from hours for the alkyl silanes to weeks for the amido silanes.

Organosilanes as Additives

In the integral method, organosilanes are incorporated into the adhesive or sealant as an additive to the formulation. Adhesives and sealants can be prepared by the addition of silanes to blends of liquid polymer prior to compounding. When applied in this manner, silanes are generally effective in concentrations ranging from 0.05 to 1.00%.

The silane additive must be able to diffuse or migrate to the inorganic substrate and react at the interface. Effective coupling action with silanes as additives depends on several criteria:

Good mechanical dispersion of the silane into the formulation will assure uniform coupling and best efficiency.
The solubility parameters and reactivities of the polymer and the silane must be compatible. No reaction should occur in storage. Matched solubility is necessary for silane interpenetration into the polymer. Proper pH ranges must be maintained to avoid silane condensation.
Some excess silane should be used if inorganic fillers are present in the formulation. Silanes will be adsorbed onto the fillers and concentrations need to be adjusted so that sufficient silane can migrate to the substrate surface.
Hydrolysis must occur to render the silane active for coupling. Proper moisture conditioning of filler or addition of extra water will help assure that the silane hydroxyls and couples.

Advantages & Disadvantages - Primer & Additive Approach

There are advantages and disadvantages inherent in both the primer and additive approach.

Advantages	Disadvantages
The primer method allows a specific adhesion promoter to be used on a specific substrate to obtain improvement	The primer method can introduce a process that is not under the control of the adhesive manufacturer
The integral additive method does not require a separate substrate coating step	There could be stability problems in storage due to hydrolysis and polymerization in integral additive method

Several critical parameters need to be recognized by the formulator with the additive approach. These include potential polymer reactions, depletion of the promoter by water, and shelf life.

In studies regarding the use of organosilanes as primers and additives, there is no conclusive evidence to indicate that one method of application is significantly better than the other. It appears that the effectiveness of the silane as an additive greatly depends on the mutual solubility, viscosity, and curing rate of the base polymer that is used in the adhesive formulation.

» Select the Right Organofunctional Silane for your Application

Titanate and Zirconate Coupling Agents

Organotitanates and organozirconates are adhesion promoter chemistries, other than silanes, have been extensively promoted for many years.

They are promising adhesion promoters but have not achieved the broad success of silane coupling agents. In metals, the highly metallic nature of zircoaluminates makes them uniquely reactive with metal surfaces. Similarly, organotitanates via their nature as excellent wetting agents and the ability to design molecules with dual, organic and inorganic, functionality can function as adhesion promoters.

Although there are many references to the improvement in adhesion obtained by the use of organometallic titanates and zirconates, quantitative data are sparse with regard to adhesive systems. In addition to providing improved adhesion, organometallic coupling agents have been claimed to:

Improve dispersion and rheology
Improve impact strength, as well as
Perform several other functions

Organometallic adhesion promoters / coupling agents typically can provide a dual function of improving processing and improving adhesion

Titanates have been used predominantly to modify the viscosity of filled thermoset and thermoplastic systems.

It has been shown that a small percentage of titanate in a heavily filled resin system can reduce the viscosity significantly. Thus, titanate adhesion promoters allow higher filling of particulate matter to either improve properties or lower the cost of the systems without having a negative effect on the viscosity. Improved bond strength even to halocarbon surfaces and improved hydrolytic stability are also claimed.

Organic titanates and zirconates provide several other important functions as additives for organic adhesives and sealants. At least eight primary functions have been proposed.

Organic titanates and zirconates can be used as:

Catalysts for manufacture of adhesive and sealant prepolymers
Adhesion promoters
Wetting agents
Surface protection
Water scavengers
Crosslinkers
Catalyst for crosslinking
Thixotropic agents

Type Coupling Agent	Application / Advantages
*Titanate*
Monoalkoxy titanate	Stearic acid functionality; aids in dispersion of mineral fillers in polyolefins
Chelate titanate	Greater stability in wet environments
Quat titanate	Water soluble, aids adhesion of water soluble coatings and adhesives
Neoalkoxy titanate	Eliminates pretreatment associated with fillers, can be used as a concentrated solid additive
Cycloheteroatom titanate	Ultra-high thermal properties for specialty applications
*Zirconate*
Coordinate zirconate	Phosphite functionality; reduces epoxy viscosity without accelerating cure
Neoalkoxy zirconate	Accelerates peroxide and air based cures (e.g., polyester SMC / BMC); adhesion promoter and primer for organic substrates
Zirconium propionate	Adhesion promoter for printing inks on treated polyolefin films
Zircoaluminates	Comparable to organosilanes at lower cost
Zirconium acetylacetonate zirconium methacrylate	Adhesion promoters and primers for treated polyolefins

Common Titanate and Zirconate Coupling Agents and Their Applications

The organic titanate or zirconate can be incorporated as an additive into an adhesive, sealant or coating at a concentration of about 0.5-3%. It can also be applied as a primer via a 0.5-5% solution in solvent such as isopropanol.

Synergistic effects can also be achieved when the organic titanates or zirconates are blended with organosilanes

Organic titanates and zirconates, as additives or primers, have been noticed to improve the adhesion adhesives or sealants to polyethylene terephthalate film, polyoflefins, polyimides, nylon, polyurethanes, epoxies, phenolics and silicones.
Organmetallic compounds have also been used as primers to promote the adhesion of silicone rubber adhesives to metal, plastic, glass, ceramics, concrete, wood, and fabrics.

Organotitanates

The titanate structure may be tailored to provide desired properties through the six functionalities on the basic structure shown below.

(RO)_m-Ti-(O-X-R₂-Y)_n

The (RO)_m is the hydrolysable proton that attaches to the inorganic substrate. It also controls dispersion, adhesion, viscosity and hydrophobicity.
The X group enhances corrosion protection and acid resistance and may provide antioxidant effects, depending on chemistry.
The R₂ provides entanglements with long hydrocarbon chains and bonding via van der Waals forces.
The Y group provides thermoset reactivity chemically bonding the filler to the polymer. It can be one of any number of chemical functions reactive with a number of different matrices.

Titanate condensation on a hydroxyl containing surface

Like the silanes, the organic titanates react with surface hydroxyl groups. But there is no condensation polymerization to produce a polymer network at the interface. Titanate coupling agents are unique in their reaction with free protons on the substrate surface. It results in a monomolecular layer on the surface whether it is a filler or substrate. The properties of this film depend on the type and amount of organometallic coupling agent used, the chemistry of the organometallic, and the processing properties used to apply the coating.

These coatings modify the surface of the filler or substrate to provide the following unique properties.
They promote adhesion of adhesives and coatings to glass, metal, and plastics.
The organometallic interface improves dispersability of pigments and fillers in aqueous and non-aqueous systems and reduces viscosity.
It can provide scratch-resistant and reflective properties to glass.
It can modify frictional characteristics of the substrate

The best performance is obtained when the substrate contains functional groups with active hydrogens. However, the substrate can be reactive or unreactive inorganic materials or even organic polymers that have been activated by corona, plasma, or flame pretreatment.

The one problem associated with using organic titanates is over concentration. Since excess titanate (amount greater than necessary to form a monolayer) does not result in a polymer network at the interface, it is suspected that it can form a weak boundary layer resulting in degraded properties. Thus, the amount of titanate that is used is an important parameter.

Typically, titanate treated inorganic fillers or reinforcements are hydrophobic, organophilic, and organofunctional and, therefore, exhibit enhanced dispersability and bonding with the polymer matrix. When used in filled polymer systems, titanates claim to improve impact strength, exhibit melt viscosity lower than that of the base polymer at loadings above 50%, and enhance the maintenance of mechanical properties during aging.

Check out typical titanate coupling agents used in adhesives and sealants. There are many types and chemistries available. They can be made available as liquids (solutions and water soluble salts), powder and pellet concentrates.

Organozirconates

Zirconate coupling agents have very similar structure to the titanates. They also perform similar functions. Zirconium compounds exist in both water and organic solvent soluble forms. Like the titanates, zirconate coupling agents are useful in improving the dispersion characteristics of fillers in polymer systems. Examples of zirconate coupling agents and their applications are given in the table above.

The aqueous chemistry of zirconium is complex and dominated by hydrolysis. One aspect is that polymerization takes place when salt solutions are diluted. The polymeric species can be cationic, anionic, or neutral. Polymers that are formed include ammonium zirconium carbonate, zirconium acetate, and zirconium oxychloride.

Zirconium propionate is used as an adhesion promoter for printing inks on polyolefins that have been treated by corona discharge. It is believed that hydrogen bonds are formed with the nitrocellulose in the inks. The mode of attachment to treated polyolefin is by functional groups on the surface displacing ligands on the zirconium polymer. Surface COOH groups seem to be most likely to do this, and the reaction is shown below.

Zirconium compounds provide improved adhesion by attachment to functional groups

Solvent soluble zirconium compounds include:

Zirconium acetylacetonate
Zirconium methacrylate, and
The family of neoalkoxy zirconates

The synthesis of certain soluble zirconium compounds has demonstrated improved adhesion on glass and aluminum substrates for polymethyl methacrylate, polyethylene, and polypropylene when used as hot melt compounds.

Major suppliers of titanate and zirconate coupling agents include DuPont (Tyzor) and Kenrich Petrochemicals (Ken-React). They can be made available as liquids (solutions and water soluble salts), powder and pellet concentrates. Typical titanate and zirconate coupling agents are shown in the table above. There are many types and chemistries available. They have similar structures and perform similar functions.

Zircoaluminates

Zircoaluminates claim performance at least comparable to that of silanes at substantial cost savings. Several functionalities are available. They are stable and soluble in an aqueous environment and do not require the presence of water to function. The surface reaction is irreversible. Among the fillers treated successfully are silica, clay, calcium carbonate, alumina trihydrate and titanium dioxide.

Other Adhesion Promoters

In addition to the adhesion promoters described above, a large number of functional inorganic, organometallic, and organic compounds have been investigated, usually in specific adhesives and coatings or on selected substrates. A comprehensive account of coupling agents is provided by Cassidy and Yager and by Walker.

Some adhesion promoters apart from Silanes, Titanates, and Zirconates are described below:

Chlorinated Polyolefins

In automotive coatings, the term adhesion promoter refers to the primer, which achieves adhesion of the subsequent paint layer to thermoplastic polyolefin (TPO) substrate. This adhesion promoter is usually comprised of chlorinated polyolefin (CPO) as the active adhesion-promoting component, other resins, and pigment.

Although used primarily as a primer, CPOs can also be used as an additive. Both solvent and waterborne systems are available. The CPO bonds well to the substrate due to its chemical similarity. Additionally, being relatively hydrophobic, they tend to migrate to the adhesive interface and orient themselves with their pendant maleic anhydride and / or halogen groups toward the adhesives layer.

Chrome Complexes

Chrome complexes have been formed as adhesion promoters by the reaction of chromium chloride with methacrylic acid. The chromium oxide portion of the adhesion promoter reacts with a substrate while the methacrylic portion reacts with a free radically curing outer layer.

Chromium containing adhesion promoters fall into two main classes: inorganic and organic.

Examples of the inorganic type are the chromate conversion coatings that were previously used extensively in aerospace industries for prebond surface treatment of aluminum. These act as both corrosion inhibitor and adhesion promoter. Today, these chromate conversion coatings have fallen from favor due to toxicity issues.
Examples of the organic type include coordination complexes of trivalent chromium chloride and carboxylic acids. The most notable is Volan, manufactured by DuPont as a fiberglass finish.

Phosphorous Containing Compound

Phosphate esters have been shown to bond chemically to metal surfaces to improve adhesion and provide a barrier to moisture ingress. They can be used in air-dry and bake systems that are either water or solvent borne. Both polymerizable and non-polymerizable phosphorous derivatives are available. Polymerizable phosphorous materials having ethylene unsaturation are advantageous.

They are especially useful as an adhesive promoting additive in ultrahigh molecular weight polyethylene composites

Mono- and diphosphate esters have been used as adhesion promoters in bonding acrylic adhesive on metals and as a primer for aluminum bonded with epoxy.
Unsaturated acid phosphates have been used as primers on metals to be bonded with free radical adhesives.

Amines

Amines have been used as adhesion promoters in a wide range of applications. Hydroxylbenzamines, used as primers or additives, have been claimed to improve the adhesion of a wide range of polymers to metallic substrates. Amines have also been examined as adhesion promoters for aromatic isocyanate cured adhesives. Primary aliphatic amines are claimed to improve the bondabilty of polyolefins.

Other Reactive Polymers

Reactive adhesion promoters will contain functional groups to react with the base polymer and / or the substrate. These can be carboxylic acid, epoxy, maleic anhydride or other chemical groups.

Acrylic acid and maleic anhydride modified polypropylene have been used to improve the adhesion of polypropylene to reinforcing materials.
Copolymers of styrene and maleic anhydride have also been produced to provide improved properties in thermoplastic composites.
Di- and tri-isocyanates are used as adhesion promoters for fibers that have only a few reactive groups, or that have only low reactivity such as polyester or aramid. Due to the high reactivity of the isocyanate groups, these fabrics must be processed quickly after an adhesion promoter is applied.

The examples of above-mentioned adhesion promoters, their suitable polymer and substrate as well as applications are:

Adhesion Promoter	Most Suitable Polymer	Most Suitable Substrate	Applications
Chlorinated polyolefin	Acrylic & acrylic copolymers Alkyd Chlorinated rubber Polyester Epoxy Maleic resins Polyurethane	Polyolefins, other low surface energy thermoplastics	Used as primers or additives for improved adhesion to plastics (mainly polyolefins) Solvent and waterborne types available Most used as primers in the auto industry
Phosphorous containing	Acrylic & acrylic copolymers Alkyd Amino Polyester	Metals, ceramics, other inorganics	Excellent flame retardant and corrosion protection for metals Both water- and solvent borne formulations available Phosphate-based acrylic adhesive promoters improve the adhesion of acrylic structural adhesives to metals including metals coated with mill oils
Silicone and silicone modified polymers (not silanes)	Acrylic and acrylic copolymers Polyamide Silicone	Glass, ceramics, metals, some plastics	Generally used as an additive in silicone RTV formulations Promotes adhesion by improved wetting of the base polymer
Acrylic acid modified polyolefin	Polyolefins EVA EVOH	Metals, paper, plastic film	Acts as an adhesion promoter and dispersing agent Often used in heat seal coatings for packaging applications
Maleic anhydride modified polyolefin	Polyolefins Nylon EVA EVOH Nylon	Metals, paper, plastic film	Maleic anhydride grafted polymer is used in tie layers for flexible packaging Chlorine-free, water borne promoter used as an alternative to chlorinated polyolefin Also used in polyolefin composites
Metallic diacrylate	Epoxy PVC Elastomers	Metals	Often used in bonding of rubber to metals Used in UV cure acrylic adhesives
Fatty ester	Epoxy PVC Elastomers	Metals, polymers, glass	Particularly useful as a promoter for fibers in tire cords, belts, etc.
Melamine cyanurate	Epoxy Nylon Polyesters Polyolefins Polyurethane	Metals, polymers, glass	Useful in tire cords Also provides flame resistance and lubrication

Note: The term "coupling agent" refers generally to additives that work on fillers and reinforcements within a resin matrix to improve properties such as dispersion stability, viscosity, etc. In this case, the primary substrate is the filler or reinforcement. The term "adhesive promoter" generally refers to a material that can be incorporated into an adhesive formulation or as a surface primer to enhance adhesion at the joint interface. Very often the same materials can function as both "coupling agents" and "adhesion promoters".

Adhesion Promoters for Adhesives and Sealants

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