Silane Coupling Agents the Concept of Coupling with Organofunctional Silanes

A Guide to Silane Solutions

Silane Coupling Agents The Concept of Coupling with Organofunctional Silanes

Silane Coupling Agents

ilane coupling agents are silicon-based chemicals that contain two types of reactivity – inorganic and organic – in the same molecule. A typical general structure is

(RO)3SiCH2CH2CH2-X, where RO is a hydrolyzable group, such as methoxy, ethoxy, or acetoxy, and X is an organofunctional group, such as amino, methacryloxy, epoxy, etc.

A silane coupling agent will act at an interface between an inorganic substrate (such as glass, metal or mineral) and an organic material (such as an organic polymer, coating or adhesive) to bond, or couple, the two dissimilar materials. A simplified picture of the coupling mechanism is shown in Figure 1.

Figure 1. The silane coupling mechanism.

Inorganic Organic Fiberglass Si Rubber Fillers Polymers Metals Plastics

Figure 2. SEM of silica-filled epoxy resin.

Without Silane With Silane Why Silane Coupling during composite aging and use. • Smoother surfaces of Agents Are Used The coupling agent provides a composites stable bond between two otherwise • Less catalyst inhibition of When organic polymers are re- poorly bonding surfaces. Figure 2 thermoset composites inforced with glass fibers or miner- shows (via an SEM of the fracture • Clearer reinforced plastics als, the interface, or interphase surface) the difference in adhesion region, between the polymer and between a silica-filled epoxy resin The Silane Bond the inorganic substrate is involved with silane vs. without silane. With in a complex interplay of physical silane, the epoxy coating on the to the Inorganic and chemical factors. These factors silica particles is apparent; without Substrate are related to adhesion, physical silane, clean silica particles can be Silane coupling agents that contain strength, coefficient of expansion, seen in the epoxy matrix. three inorganic reactive groups on concentration gradients and reten- In composites, a substantial silicon (usually methoxy, ethoxy or tion of product properties. A very increase in flexural strength is acetoxy) will bond well to the metal destructive force affecting adhe- possible through the use of the hydroxyl groups on most inorganic sion is migration of water to the right silane coupling agent. Silane substrates, especially if the hydrophilic surface of the inorganic coupling agents also increase the sub-strate contains silicon, alumi- reinforcement. Water attacks the bond strength of coatings and num or a heavy metal in its struc- interface, destroying the bond adhesives as well as their resistance ture. The alkoxy groups on silicon between the polymer and reinforce- to humidity and other adverse hydrolyze to silanols, either through ment, but a “true” coupling agent environmental conditions. the addition of water or from creates a water-resistant bond at residual water on the inorganic the interface between the inor- Other benefits silane coupling surface. Then the silanols coordi- ganic and organic materials. Silane agents can provide include: nate with metal hydroxyl groups on coupling agents have the unique • Better wetting of inorganic the inorganic surface to form an chemical and physical properties substrates oxane bond and eliminate water. not only to enhance bond strength See Figures 3 and 4. but also, more importantly, to • Lower viscosities during prevent de-bonding at the interface compounding

Figure 3. Hydrolysis of alkoxysilanes. Figure 4. Bonding to an inorganic surface.

RSi(OCH3) 3 RRR CH OH HO Si OOSi Si OH H2O 3 O OO

RSi(OH) 3 HHHHHH OOO H2O

RRR H2O HO-Si-O-Si-O-Si-OH RRR HO Si OOSi Si OH OOO OOO HHH Silane molecules also react with amino-silane will bond to an epoxy properties. Even with thermoset each other to give a multimolecular resin; an aminosilane will bond to a polymers, where reactivity plays an structure of bound silane coupling phenolic resin; and a methacrylate important role, chemical structure agent on the surface. More than silane will bond through styrene matching will enhance the physical one layer, or monolayer equiva- crosslinking to an unsaturated properties of the composite. lents, of silane is usually applied to polyester resin. With thermoplastic the surface. This results in a tight polymers, bonding through a silane How to Choose a siloxane network close to the inor- coupling agent can be explained by Silane Coupling Agent ganic surface that becomes more inter-diffusion and inter-penetrating All silane coupling agents with diffuse away from the surface. network (IPN) formation in the three OR groups on silicon should interphase region. See Figure 5. bond equally well with an inorganic The Silane Bond to To optimize IPN formation, it is ® the Polymer substrate. The XIAMETER brand important that the silane and the product line includes a variety of The bond to the organic polymer is resin be compatible. One method organofunctional alkoxysilanes. complex. The reactivity of a ther- is to match the chemical character- See Figure 6. moset polymer should be matched istics of the two materials. This will Matching the organofunctional to the reactivity of the silane. For help improve the chances of form- group on silicon with the resin poly- example, an epoxysilane or ing a good composite with optimum mer type to be bonded will dictate which silane coupling agent should be used in a particular application. Figure 5. The inter-penetrating network (IPN) bonding mechanism. The organic group on the silane can be either a reactive organic group Chemically Diffuse (i.e., an organofunctional group), Bonded Interface Interphase or it can be a non-reactive organic group. The groups can be hydro- Si--O--Si-- phobic or hydrophilic, with varying thermal stability characteristics.

Si--O--Si-- Silica Si--O--Si--

Si--O--Si--

Coupling Agent Polymer Figure 6. Silane coupling agent variations – basic structure. The solubility parameters of the Basic Structure R groups will vary, depending on the R = alkyl, aryl, or organic structure; this will influence, Si organofunctional group R'O OR' to some extent, the interpenetration OR' = methoxy, ethoxy, OR' the polymer network will have into or acetoxy the siloxane network of the surface

NH2 treatment. Table 1 lists some of the

H2C CH3 characteristics for common organic OFS-6030 OFS-6011 MeO substituents attached to silicon. EtO MeO The choice of silane should involve Si O O Si matching chemical reactivity, EtO OMe OEt solubility characteristics, structural characteristics and, possibly, the O CH 2 thermal stability of the organosilane MeO CH O EtO with the same parameters in the MeO Si EtO polymer structure. Si OFS-6341 MeO OFS-6040 OMe Si OMe MeO OEt OFS-6300

Table 1. Characteristics of Various Organic Substituents on Silanes

Organosilanes R-Si(OMe)3 R Characteristics of “R” Me Hydrophobic, Organophilic Ph Hydrophobic, Organophilic, Thermal Stability i-Bu Hydrophobic, Organophilic Octyl Hydrophobic, Organophilic

-NH(CH2) 3NH2 Hydrophilic, Organoreactive Epoxy Hydrophilic, Organoreactive Methacryl Hydrophobic, Organoreactive Table 2. Non-Organoreactive Alkoxysilanes A list of alkyl and aryl, non-organoreactive alkoxysilanes is provided XIAMETER® Organic Alkoxy brand Silane Group Group Chemical Name in Table 2. Those silanes give OFS-6697 - Ethoxy TetraEthoxysilane modified characteristics to inorganic OFS-6070 Methyl Methoxy Methyltrimethoxysilane surfaces, including hydrophobicity, OFS-6366 Methyl Methoxy Methyltrimethoxysilane (HP) organic compatibility and lower OFS-6370 Methyl Ethoxy Methyltriethoxysilane surface energy. OFS-6383 Methyl Ethoxy Methyltriethoxysilane (HP) Based on experience and histori- OFS-2306 i-Butyl Methoxy Isobutyltrimethoxysilane cal applications of silanes, a list of

OFS-6124 Phenyl Methoxy Phenyltrimethoxysilane More Hydrophobic silane coupling agents and recom- OFS-6341 n-Octyl Ethoxy n-Octyltriethoxysilane mendations for evaluation with various polymer types is provided in Table 3. Silane Coupling Agent Recommendations for Various Table 3. A correlation can be seen Polymers – Matching Organoreactivity to Polymer Type between the chemistry and structural characteristics of the silane Organic Reactivity Application (suitable polymers) coupling agent and the chemistry Acrylic, Nylon, Epoxy, Phenolics, PVC, Urethanes, and structural characteristics of the Amino Melamines, Nitrile Rubber polymer. Benzylamino Epoxies for PCBs, Polyolefins, All Polymer Types Chloropropyl Urethanes, Epoxy, Nylon, Phenolics, Polyolefins Product Information Disulfido Organic Rubber A complete list of XIAMETER® Epoxy Epoxy, PBT, Urethanes, Acrylics, Polysulfides brand silanes for use as Epoxy/Melamine Epoxy, Urethane, Phenolic, PEEK, Polyester coupling agents is available at Mercapto Organic Rubber Methacrylate Unsaturated Polyesters, Acrylics, EVA, Polyolefin xiameter.com. Tetrasulfido Organic Rubber In addition, Dow Corning Ureido Asphaltic Binders, Nylon, Phenolics; Urethane Corporation also offers a wide Graft to Polyethylene for Moisture Crosslinking, variety of Dow Corning® brand Vinyl EPDM Rubber, SBR, Polyolefin specialty silicone material and Vinyl-benzyl-amino Epoxies for PCBs, Polyolefins, All Polymer Types service options as well as other silicon-based materials available to help you keep your innovative edge in the marketplace. Visit dowcorning.com to learn more about the many additional silicone and silicon-based options available to you from Dow Corning. LIMITED WARRANTY INFORMATION – PLEASE READ CAREFULLY The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer’s tests to ensure that our products are safe, effective and fully satisfactory for the intended end use. Suggestions of use shall not be taken as inducements to infringe any patent. Dow Corning’s sole warranty is that our products will meet the sales specifications in effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted.

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