How to Improve Adhesion to a Substrate?
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
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.
Related Read: Adhesion Myths and Reality - A Complete Guide
Methods to Promote Adhesion
Adhesion Promoters Classes & Key Characteristics
Silane Coupling Agents
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.
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)
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.
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
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