Dipodal silanes are a series of adhesion promoters that have intrinsic hydrolytic stabilities up to ~10,000 times greater than conventional silanes. These products have a significant impact on substrate bonding and mechanical strength of many composite systems—including epoxy, urethane, epoxy/urethane hybrids, polysulfide, cyanoacrylate, and silicone—and may be utilized in water-borne, high solids, and photo-active chemistries. Dipodal silanes are promising materials that have already achieved commercial success in applications as diverse as plastic optics, multilayer printed circuit boards, and as adhesive primers for ferrous and nonferrous metals. Due to the nature of the silicon molecules, the silane coupling agent is a material used to resist deterioration by the intrusion of water between the polymer and the substrate. Through the modification of the interface, silane coupling agents not only provide water resistance but may also increase strength at the interface due to the interpenetrating polymer networks of resin and silane. Commonly in silane surface treatment or ‘in situ’ applications, alkoxy groups in conventional silanes are hydrolyzed to form silanol-containing species, which are highly reactive and are responsible for hydrogen bonding with the substrate. These conventional silanes can self-condense to form siloxanes resulting in phase separation or gelation. To increase hydrolytic stability, dipodal silanes can be incorporated to improve shelf life, substrate bonding, and mechanical strength of many composite systems.
Functional dipodal silanes and combinations of non-functional dipodal silanes with functional conventional silanes have significant impact on substrate bonding and improve many adhesive systems, particularly primer and aqueous immersion applications. The fundamental step by which silanes provide adhesion is formation of a -Si-O-X bond with the substrate. If the substrate is siliceous, the bond durability is dictated by bond dissociation of Si-O-Si. According to the equation ≡Si-O-Si≡ + H2O ⇌ ≡Si-OH + ≡Si-OH, the equilibrium for bond dissociation is ~10-4. Recognizing that substrate hydroxyls are not subject to diffusion, the factor is closer to 10-2. By increasing the number of bonds by three, the equilibrium for dissociation is increased to ~10-6. Theoretically this means that dissociative bond line failure that typically occurs in 1 month is increased to ~10,000 months. Practically, other factors influence the failure, but dipodal silanes clearly have the potential to eliminate failure of adhesive bonds during the lifetime requirements of many devices. The effect is thought to be a result of both the increased crosslink density of the interphase and the resistance to hydrolysis of dipodal silanes, which is estimated at ~10,000 times greater than conventional coupling agents. Dipodal silanes have the ability to form six bonds to a substrate compared to conventional silanes with the ability to form only three bonds to a substrate. Different substrates, conditions, silane combinations, and applications all have an effect on dipodal silane selection. The mixture of silanes is chosen depending on the desired characteristics. For example improved wet adhesion, chemical resistance, processing, and/or coating performance such as improved corrosion protection.
Many conventional coupling agents are frequently used in combination with 10-40% of a non-functional dipodal silane, where the conventional coupling agent provides the appropriate functionality for the application and the non-functional dipodal silane provides increased durability. In a typical application a dipodal material such as bis(triethoxysilyl)ethane (SIB1817.0) is combined at a 1:5 to 1:10 ratio with a traditional coupling agent. It is then processed in the same way as the traditional silane coupling agent. With the addition of the non-functional dipodal silane, the durability of coatings was extended when compared to the conventional silane alone.
Dipodal Silane Hydrolytic Stability Compared to Conventional Silane. B. Arkles, et al. Chemistry - A European Journal, 2014, 20, 9442.
Effect of Dipodal Silane on Bond Strength
| Primer on metal (10% in isopropanol) | Wet adhesion to titanium (N/cm) | Wet adhesion to cold-rolled steel(N/cm) |
|---|---|---|
| no silane | -- | -- |
| methacryloxypropylsilane | 0.35 | 7 |
| methacryloxypropylsilane + 10% dipodal | 10.75 | 28.0 (cohesive failure) |
P. Pape et al, in Silanes and Other Coupling Agents, ed. K. Mittal, 1992, VSP, p105
Efficient and high-yielding, economical reactions are desired throughout chemistry. The rapid reaction of cyclic azasilanes with any and all surface hydroxyl groups is therefore of unique interest for surface modification. Volatile cyclic azasilanes afford high functional density monolayers on inorganic surfaces—such as nanoparticles and other nanofeatured substrates—without a hydrolysis step. Furthermore, byproducts such as alcohol, HCl, and cage-like condensation products typical with the use of conventional silane coupling agents are eliminated by surface modification using cyclic azasilanes.
This new class of silane coupling agents affords a smooth monolayer and reduces or eliminates any hazardous byproducts. Cyclic azasilanes exploit the Si–N and Si–O bond energy differences affording a thermodynamically favorable ring-opening reaction with surface hydroxyls at ambient temperature. Sometimes referred to as “click-chemistry on surfaces,” the ring-opening occurs through the cleavage of the inherent Si-N bond in these structures, and promotes a strong covalent attachment to surface hydroxyl groups. This unveils an organofunctional amine for further reactivity, depicted below, to link the inorganic surface to an organic moiety. Cyclic azasilanes react with hydroxyl surfaces to afford a monolayer with amine functionality which are hydrophilic in nature. The monolayers range from 2 to 5 nm, measured by ellipsometry, and have an average roughness of 0.3 nm measured by atomic force microscopy.
Extent of Reaction of Organosilanes with Fumed Silica
DRIFT of Untreated Silica and SIB1932.4-Treated Silica after 56s
This reaction proceeds to completion in less than a minute (shown below), much faster than any conventional silane coupling agent. The rate of reaction with fumed silica can be monitored by diffuse reflectance FTIR. Consumption of the terminal hydroxyls (3745 cm-1) occurs within 58 seconds of addition of the cyclic azasilane solution, while the C-H stretching vibrations of the Si(OMe)2 remain at 2864 cm-1, indicating the hydrolysis of these groups (typical of conventional silane coupling agents) remain unaffected in this case. The initial reaction is solely the breaking of the Si-N bond of the ring by the terminal surface hydroxyl groups. Additional information regarding this class of silane coupling agents can be found in these references:
- B. Arkles et al in “Silanes and Other Coupling Agents, Vol. 3,” K. Mittal (Ed.) VSP-Brill, 2004, p 179.
- M. Vedamuthu et al, J. Undergrad. Chem. Res., 1, 5, 2002
- D. Brandhuber et al, J. Mater. Chem., 2005
- Su, K. et al. U.S. Patent Appl. 2012 2672, 790, 2012
During the ring-opening process, the Si-OMe groups associated with traditional coupling agents remain unreacted. These alkoxy groups are able to further react with one another through hydrolysis and condensation (depicted below) to further stabilize the surface. Cyclic azasilane coupling agents react with a wide variety of hydroxyl rich surfaces generating a range of organofunctional groups for further surface modification.
The general order of thermal stability for silane coupling agents is shown. Most commercial silane coupling agents have organic functionality separated from the silicon atom by three carbon atoms and are referred to as gamma-substituted silanes. The gamma-substituted silanes have sufficient thermal stability to withstand short-term process conditions of 350 °C and long-term continuous exposure of 160 °C. In some applications, gamma-substituted silanes have insufficient thermal stability or other system requirements that can eliminate them from consideration.
In this context, some comparative guidelines are provided for the thermal stability of silanes. Thermogravimetric analysis (TGA) data for hydrolysates may be used for benchmarking. The specific substitution also plays a significant role in thermal stability. Electron-withdrawing substitution reduces thermal stability, while electropositive groups enhance thermal stability.
Relative Thermal Stability of Silanes
Before most surface modification processes, alkoxysilanes are hydrolyzed forming silanol-containing species. The silanol-containing species are highly reactive intermediates which are responsible for bond formation with the substrate. In principle, if silanol species were stable, they would be preferred for surface treatments. Silanols condense with other silanols or with alkoxysilanes to form siloxanes. This can be observed when preparing aqueous treatment solutions. Initially, since most alkoxysilanes have poor solubility in water, two phases are observed. As the hydrolysis proceeds, a single clear phase containing reactive silanols forms. With time, the silanols condense forming siloxanes and the solution becomes cloudy. Eventually, as the molecular weights of the siloxanes increase, precipitation occurs.
Hydrolysis and condensation of alkoxysilanes is dependent on both pH and catalysts. The general objective in preparing aqueous solutions is to devise a system in which the rate of hydrolysis is substantially greater than the rate of condensation beyond the solubility limit of the siloxane oligomers. Other considerations are the work-time requirements for solutions and issues related to byproduct reactivity, toxicity, or flammability. Stable aqueous solutions of silanes are more readily prepared if byproducts or additional alcohol are present in the solution since they contribute to an equilibrium condition favoring monomeric species.
Relative Hydrolysis Rates of Hydrolyzable Groups
Hydrolysis Profile of Phenylbis(2-methoxyethoxy)silanol. F. Osterholtz et al in Silanes and Other Coupling Agents ed K. Mittal, VSP, 1992, p119.
Profile for Condensation of Silanols to Disiloxanes E. Pohl et al in Silanes Surfaces and Interfaces ed., D. Leyden, Gordon and Breach, 1985, p481.
Water-borne coupling agent solutions are usually free of volatile organic compounds (VOCs) and flammable alcohol byproducts. Most water-borne silanes can be described as hydroxyl-rich silsesquioxane copolymers. Apart from coupling, silane monomers are included to control water solubility and extent of polymerization. Water-borne silanes act as primers for metals, additives for acrylic latex sealants, and as coupling agents for siliceous surfaces.
Maximum bond strength in some adhesion and bonding systems requires that the organic functionality of a silane coupling agent becomes available during a discrete time period of substrate-matrix contact. Examples are epoxy adhesives in which reaction of the silane with the resin increases viscosity of an adhesive to the extent that substrate wet-out is inhibited and pretreated fillers for composites which can react prematurely with moisture before melt compounding or vulcanization. A general approach is to mask the organic functionality of the silane which converts it to a storage-stable form and then to trigger the demasking with moisture, or heat concomitant with bonding or composite formation.
Single-component liquid-cure epoxy adhesives and coatings use dimethylbutylidene blocked aminosilanes. These materials show excellent storage stability in resin systems but are activated by moisture from substrate surfaces or humidity. Deblocking begins in minutes and is generally complete within two hours in sections with a diffusional thickness < 1 mm.
An alternative is to use the moisture adsorbed onto fillers to liberate alcohol which, in turn, demasks the organic functionality.
Masked Silanes - Moisture on Substrate
Isocyanate-functionality is frequently delivered to resin systems during elevated temperature bonding or melt processing steps. Demasking temperatures are typically 160-200 °C.
Masked Silanes - Heat-Triggered
The optimum performance of silanes is associated with siliceous substrates. While the use of silanes has been extended to metal substrates, both the effectiveness and strategies for bonding to these less-reactive substrates vary. Four approaches of bonding to metals have been used with differing degrees of success. In all cases, selecting a dipodal or polymeric silane is preferable to a conventional trialkoxysilane.
Octysilane Adsorbed on Titanium. Figure courtesy of M. Banaszak-Holl
Metals that form hydrolytically stable surface oxides, e.g. aluminum, tin, titanium. These oxidized surfaces tend to have sufficient hydroxyl functionality to allow coupling under the same conditions applied to the siliceous substrates discussed earlier.
Metals that form hydrolytically or mechanically unstable surface oxides, e.g. iron, copper, zinc. These oxidized surfaces tend to dissolve in water leading to progressive corrosion of the substrate or form a passivating oxide layer without mechanical strength. The successful strategies for coupling to these substrates typically involve two or more silanes. The combination of silanes includes:
- A chelating agent such as a diamine, polyamine, or polycarboxylic acid
- A silane chosen based on reactivity with the organic component
- If a functional dipodal or polymeric silane is not selected, adding a non-functional dipodal silane typically improves bond strength (10-20% of the non-functional dipodal silane in the silane blend)
These silanes react with each other by co-condensation.
Metals that do not readily form oxides, e.g. nickel, gold and other precious metals. The successful strategies for coupling to these substrates typically involve two or more silanes. The combination of silanes includes:
- A silane for coordinative bonding, typically a phosphine-, sulfur- (mercapto-), or amine-functional silane
- A silane chosen based on reactivity with the organic component
- If a functional dipodal or polymeric silane is not selected, adding a non-functional dipodal silane typically improves bond strength (10-20% of the non-functional dipodal silane in the silane blend)
These silanes react with each other by co-condensation.
Metals that form stable hydrides, e.g. titanium, zirconium, nickel. Departing from traditional silane coupling agent chemistry, these metals are able to form amorphous alloys with hydrogen. The hydride-functional silanes can adsorb to and then react with the surface of the metal. Most silanes of this class possess only simple hydrocarbon substitution such as octylsilane. However, they do offer organic compatibility and are able to change wet-out of the substrate. Both hydride-functional silanes and treated metal substrates will liberate hydrogen in the presence of base or with certain precious metals such as platinum and associated precautions must be taken.
Coupling Agents for Metals
| Metal | Organic Functionality | Screening Candidates |
|---|---|---|
| Copper | Amine | SSP-060, SIT8398.0 |
| Gold | Sulfur Phosphorous | SIT7908.0, SIP6926.2 SID4558.0, SIB1091.0 |
| Iron | Amine Sulfur | SIB1834.0, WSA-7011 SIB1824.6, SIM6476.0 |
| Tin | Amine | SIB1835.5 |
| Titanium | Epoxy Hydride | SIG5840.0, SIE4668.0 SIU9048.0 |
| Zinc | Amine Carboxylate | SSP-060, SIT8398.0 SIT8402.0, SIT8192.6 |
B. Arkles et al J. Adhesion Science Technol, 2012, 26, 41.
Silane coupling agents are generally recommended for applications in which an inorganic surface has hydroxyl groups and the hydroxyl groups can be converted to stable oxane bonds by reaction with the silane. Substrates such as calcium carbonate, copper and ferrous alloys, and high phosphate and sodium glasses are not recommended substrates for silane coupling agents. In cases where a more appropriate technology is not available, a number of strategies have been devised which exploit the organic functionality, film-forming and crosslinking properties of silane coupling agents as the primary mechanism for substrate bonding in place of bonding through the silicon atom. These approaches frequently involve two or more coupling agents.
Calcium carbonate fillers and marble substrates do not form stable bonds with silane coupling agents. Applications of mixed silane systems containing a dipodal silane or tetraethoxysilane in combination with an organofunctional silane frequently increases adhesion. The adhesive mechanism is thought to be due to the low molecular weight and low surface energy of the silanes which allows them initially to spread to thin films and penetrate porous structures followed by the crosslinking which results in the formation of a silica-rich encapsulating network. The silica-rich encapsulating network is then susceptible to coupling chemistry comparable to siliceous substrates. Marble and calciferous substrates can also benefit from the inclusion of anhydride-functional silanes which, under reaction conditions, form dicarboxylates that can form salts with calcium ions.
Substrates with low concentrations of non-hydrogen bonded hydroxyl groups, high concentrations of calcium, alkali metals, or phosphates pose challenges for silane coupling agents.
Metals and many metal oxides can strongly adsorb silanes if a chelating functionality such as diamine or dicarboxylate is present. A second organofunctional silane with reactivity appropriate to the organic component must be present. Precious metals such as gold and rhodium form weak coordination bonds with phosphine- and mercaptan-functional silanes.
High phosphate- and sodium-containing glasses are frequently the most frustrating substrates. The primary inorganic constituent is silica and would be expected to react readily with silane coupling agents. However, alkali metals and phosphates do not form hydrolytically stable bonds with silicon and catalyze the rupture and redistribution of silicon-oxygen bonds. The first step in coupling with these substrates is the removal of ions from the surface by extraction with deionized water. Hydrophobic dipodal or multipodal silanes are usually used in combination with organofunctional silanes. In some cases, polymeric silanes with multiple sites for interaction with the substrate are used. Some of these—such as the polyethylenimine-functional silanes—can couple to high sodium glasses in an aqueous environment.
Removing Surface Impurities Eliminating non-bonding metal ions such as sodium, potassium, and calcium from the surface of substrates can be critical for stable bonds, however all substrates are different. Colloidal silicas derived from tetraethoxysilane or ammonia solutions perform far better than those derived from sodium solutions. Impurities on bulk glass surfaces are common during fabrication. Although sodium concentrations derived from bulk analysis may seem acceptable, the surface concentration is frequently orders of magnitude higher. Surface impurities may be reduced by immersion in 5% hydrochloric acid for 4 hours, followed by a deionized water rinse, and then immersion in deionized water overnight followed by drying. Oxides with high isoelectric points can adsorb carbon dioxide, forming carbonates. These can usually be removed by a high temperature vacuum bake.
Increasing Hydroxyl Concentration Hydroxyl-functionalization of bulk silica and glass may be increased by immersion in a 1:1 mixture of 50% aqueous sulfuric acid : 30% hydrogen peroxide for 30 minutes followed by rinses in D.I. water and methanol and then air drying. Alternately, if sodium ion contamination is not critical, boiling with 5% aqueous sodium peroxodisulfate followed by acetone rinse is recommended.
Shirai et al, J. Biomed. Mater. Res. 53, 204, 2000.
Catalyzing Reactions in Water-Free Environments Hydroxyl groups without hydrogen bonding react slowly with methoxysilanes at room temperature. Ethoxysilanes are fairly non-reactive in water-free environments. Reactivity can be enhanced by using transesterification catalysts and agents which increase the acidity of hydroxyl groups on the substrate by hydrogen bonding. Transesterification catalysts include tin compounds such as dibutyldiacetoxytin and titanates such as titanium isopropoxide. Incorporating transesterification catalysts at 2-3 weight % of the silane effectively promotes reaction and deposition in many instances. Alternatively, amines can be premixed with solvents at 0.01-0.5 weight % based on substrate prior or concurrent to silane addition. Volatile primary amines such as butylamine can be used, but are not as effective as tertiary amines such as benzyldimethylamine or diamines such as ethylenediamine. The more effective amines, however, are more difficult to remove after reaction.
Kanan et al, Langmuir, 18, 6623, 2002.
Hydroxylation by Water Plasma & Steam Oxidation Various metals and metal oxides including silicon and silicon dioxide can achieve high surface concentrations of hydroxyl groups after exposure to H2O/O2 in high energy environments including steam at 1050 °C and water plasma.
Alcanter et al, in “Fundamental & Applied Aspects of Chemically Modified Surfaces” ed. J. Blitz et al, 1999, Roy. Soc. Chem., p212.