1 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)3 SiCH2CH2CH2-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.
2 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 The Coupling agent provides a composites Agents Are Used 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 inorganic substrate is involved with Silane vs. without Silane . With The Silane Bond in a complex interplay of physical Silane , the epoxy coating on the to the Inorganic and chemical factors.
3 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- silicon (usually methoxy, ethoxy or In composites, a substantial tion of product properties. A very acetoxy) will bond well to the metal increase in flexural strength is destructive force affecting adhe- hydroxyl groups on most inorganic possible through the use of the sion is migration of water to the substrates, especially if the right Silane Coupling agent . Silane hydrophilic surface of the inorganic sub-strate contains silicon, alumi- Coupling Agents also increase the reinforcement. Water attacks the num or a heavy metal in its struc- bond strength of coatings and interface, destroying the bond ture.
4 The alkoxy groups on silicon adhesives as well as their resistance between the polymer and reinforce- hydrolyze to silanols, either through to humidity and other adverse ment, but a true Coupling agent the addition of water or from environmental conditions. 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 the inorganic surface to form an Better wetting of inorganic chemical and physical properties oxane bond and eliminate water. substrates 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.
5 Hydrolysis of alkoxysilanes. Figure 4. Bonding to an inorganic surface. RSi(OCH3) 3 R R R. CH3OH HO Si O Si O Si OH. H2 O. O O O. RSi(OH) 3 H H H H H H. H2 O O O O. R R R H2O. HO-Si-O-Si-O-Si-OH R R R. HO Si O Si O Si OH. O O O. O O O. H H H. 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.
6 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 substrate. The XIAMETER brand the Polymer 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.
7 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 ( , an organofunctional group), Chemically Diffuse 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.
8 Silane Coupling agent variations basic structure. The solubility parameters of the Basic Structure groups will vary, depending on the R. R = alkyl, aryl, or organic structure; this will influence, organofunctional group Si to some extent, the interpenetration R'O OR'. OR' = methoxy, ethoxy, the polymer network will have into or acetoxy OR'. the siloxane network of the surface NH2 treatment. Table 1 lists some of the OFS-6030. H2C CH3 OFS-6011 characteristics for common organic 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. CH2 thermal stability of the organosilane MeO. CH O EtO with the same parameters in the MeO. Si EtO. OFS-6341 polymer structure.
9 MeO OFS-6040 Si 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-organo- XIAMETER Organic Alkoxy reactive alkoxysilanes is provided 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, More Hydrophobic OFS-6366 Methyl Methoxy Methyltrimethoxysilane (HP) organic compatibility and lower OFS-6370 Methyl Ethoxy Methyltriethoxysilane surface energy.
10 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 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 struc- tural 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.