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Silane Coupling Agents--Benevolent Binders in Composites.

Introduction

Composites are widely used for various purposes in engineering and biomedical fields. Earlier composites were very strong, but they lose their strength rapidly during aging. This is due to the lack of proper bonding between the fillers and resin matrix. In seeking a solution, researchers found that organofunctional silanes are silicon chemicals that contain both organic and inorganic reactivity in the same molecule. These compounds act as coupling agents in the composites. Coupling agents improves bond strength between resin and fillers that in turn will improve the physical, mechanical, and electrical properties of composites such as flexural and tensile strength, tensile modulus, fracture toughness, bulk electric properties and dielectric coefficient. In addition, coupling agents improve wetting of inorganic substrates, decrease the viscosity of resin during compounding, and provides smoother surfaces of composites [1, 2].

The importance of silane coupling agents was first discovered in 1940s, in combination with the development of fiber glass reinforced polyester composites. A very small amount of an organofunctional alkoxysilane reacted at the glass-resin interface that significantly increased initial composite strength and also resulted in a dramatic retention of that strength over time. Organosilanes serve as bridges between inorganic or organic substrates such as minerals, fillers, metals, cellulose and organic/ polymeric matrices such as rubber, thermoplastics or thermosets and hence, can considerably improve adhesion between them. This revolutionized its applications in, mineral and filler treatment for composite reinforcement [3, 4]; adhesion of paints, inks and coatings [5-7]; reinforcement and cross-linking of plastics and rubber [8-10]; crosslinking and adhesion of sealants and adhesives [11-14]; and the development of water repellents and surface protection [15].

Chemistry of Organosilane

Coupling agents are mostly bi-functional short-chain hydrocarbon molecules. One end of a molecule interacts with the polymer, while the other end interacts with the fiber [1619]. A coupling agent molecule has the form X-R, where X interacts with the fiber and R is compatible with the polymer. Organosilanes are of the form R-Si-[(OX).sub.3], where X is methyl, ethyl, methoxyethyl, etc., and R is a suitable hydrocarbon chain. They are widely used as coupling agents between glass fibers and thermosets, as the -OX groups react with the -OH groups on the glass surface. However, organosilanes do not function for carbon, organic, or metallic fibers. Some organo-functional groups are given in table 1 with their suggested polymers.

Adhesion Mechanism of Coupling Agents

The mechanism of adhesion has been classified as mechanical, adsorption, diffusion, chemical, and physical [20]. The first three of these involve intermolecular forces; the fourth involves chemical bonds, and the fifth, electrostatic forces. The dual reactivity of a coupling agent allows chemical bonding to both the filler and resin, there by forming a chemical bridge across the filler-resin interface thus improving mechanical strength and good retention of properties even under severe conditions [2124].These ingredients were pioneered in mid-1940s in the form of methacrylato-chromium chloride complex with vinyl silanes. Numerous new silane coupling agents [Table 1] have been developed later that afford higher strengths in the resulting composites and reactivity with a wider range of resins. The bonding mechanism between filler and composite matrix with the help of gamma-methacryloxypropyl trimethoxy silane coupling agent is depicted in figure 1.

Adhesion of silane coupling agent to glass fibers

Silanes bond chemically to a silanol on the glass surface by the condensation of an active group on silicone, such as OH, Cl, OR. In the presence of water, the alkoxy group of silane coupling agent is hydrolyzed to silanol (-Si-OH), which is the active species in bonding to glass filler or fiber [2, 21-24]. The silanol group on a coupling agent has the capability to react faster with similar groups on other coupling agents than those on glass surfaces. Hence it is recommended to use a silane coupling agent having one or two hydrolyzable groups per silicon atom which result in the formation of di- or polysiloxanes having little or no ability to bond to glass under the normal use conditions. Due to the above mentioned reason and solubility requirement in an aqueous application currently available commercial coupling agents have three hydrolyzable groups per silicon atom [24].

Islinger [25] studied and confirmed strong siloxane bonds at the interface between chlorosilanes and E-glass fibers using vapour phase chemisorption. White [26] et al studied the reaction of chlorosilanes with high surface silica using Infrared techniques. They suggested that a reversible physical reaction takes place at room temperature however, at elevated temperatures irreversible chemisorptions occurs. Sterman and Bradley [27] studied the quality of the silane glass bond by aqueous and solvent extraction of silane-finished E-glass cloth. They specified that though this technique removes some of the finish, the quantity of the bond (equivalent to 2-4 theoretical monolayers) remained unaffected.

Figure 2 shows the reaction of a trialkoxysilane with filler having silanol groups showing vertical condensation to form covalent bonds to the filler as well as horizontal condensation to form polymeric siloxane structures. These organosilane intermediates, in the absence of substrates such as silica or similar minerals, or in the absence of hydroxyl-, amino- or carboxylic acid-containing organic compounds, undergo a complex series of hydrolysis and self-condensation reactions leading to dimers, trimers, tetramers and ultimately oligomers and polymers designated as silsesquioxanes, [[RSi[O.sub.3/2]].sub.n] [28].

Adhesion of silane coupling agent to resin matrix

Bifunctional organosilanes, on the other hand, reacts with the methacrylate group of polymers to form covalent and/or ionic bonds. The chemistry and performance of coupling-agent types can best be discussed in terms of specific resin systems since one kind of coupling agent may not be effective on different fillers. For example, stearic acid that forms stable ionic bonds with the surface of calcium carbonate is the preferred coupling agent for that filler due toa combination of good performance and low cost. However, the same dispersant does not effictively bond to silica fillers as it cannot bind to the surface groups, which are predominantly silanols. Trialkoxy alkyl silanes are effective dispersants for silica because they can bond to the surface silanols. However, these dispersants do not bond to calcium carbonate since they have inappropriate chemistry to bond with it [29].

Two types of silane coupling agents such as isocyanate and Amine and alkanolamine functional silanes are used to provide adhesion with thermoset urethane polymer. The isocyanate functional silanes bond to the fillers directly or integrally blended with the diisocyanate prior to cure. Amine and alkanol amine functional silanes, on the other hand, are blended with the polyol and react with the isocyanate to form urethane linkages (Figure 3). These coupled urethane systems typically improve the bond strength with sand in abrasion resistant sand-filled flooring resins [30].

Methods of Application of Silane Coupling Agent

A suitable and appropriate method must be used to apply the silane coupling agent. As discussed in the preceding section application of these agents mainly depends on the type of the resin and the fillers used in the composites. In the integral blend method, silane is added to the mixture of filler and polymer which are then mixed well. Mixing the filler particulates with a dilute solution of silane in an organic solvent is one of the most commonly used methods [31]. Waddell et al. treated glass and silica gel with a solution of diamino -functional silane in toluene [32] resulting in a monomolecular layer of silane on the surface of the filler.

The other method involves submerging the silane in water by stirring that hydrolyzes it, glass fiber is then added to the solution. This method is very effective for coarse fillers specially glass fiber that may be dried without caking [31]. Oligomeric silanes are considered as less effective compared to monomeric silanes in bonding, so it is necessary to maintain the concentration of silane between 0.01% and 2% by weight, to obtain predominantly monomeric silanes [33]. It is indicated that the fibers should not be soaked in hot water as it may result in hydrolytic degradation of the silane coupling agent [34]. The other factors which influences the effectiveness of coupling agents include pH, molecule size, type of solvent used, drying time [35, 36] and heat treatment methods [37,38].

Applications of Silane Coupling Agents

Paints, Inks and Coatings [5,6,7]

Organosilanes are widely used as effective adhesion promoters especially as integral additives or primers for paints, inks, coatings, adhesives and sealants. The silane coupling agent is applied to the inorganic substrate before the product to be adhered is applied. A poorly adhering paint, ink, coating, adhesive or sealant can be made to adhere withstanding severe environmental disparities by using the appropriate organosilane.

Dispersing/Hydrophobing Agent [15, 39]

Organosilanes are used as durable hydrophobing agents in concrete construction applications, including bridge and deck applications. Organosilanes will impart its hydrophobicity to a hydrophilic inorganic surface by means of its organic groups. Hydrophobic inorganic powders are made to be free flowing and dispersible in organic polymers and liquids by silanization.

Crosslinking Agent [8-10]

Organofunctional alkoxysilanes can link a polymer back bone to an oligomeric silane like trialkoxysilyl group which in turn cross-links to form a stable 3-dimensional siloxane structure. This phenomenon is applied in crosslinking polyethylene to organic resins such as acrylics and urethanes that impart durability, water and heat resistance in paints, coatings and adhesives.

Dental Composites

Composites are one of the most commonly used direct and indirect restorative materials in dentistry because of their inherent aesthetic appeal and comparable compressive strength to dental amalgam [21,22].However, the earlier dental composites lose their mechanical strength as the restoration is aging. This is due to the lack of adequate bonding between filler particles and the resin matrix. The pioneering work by Dr. Ray L. Bowen led to the tremendous development of dental composites as restorative materials. He invented a new dimethacrylate resin, such as Bisphenol A Glycidylmethacrylate (Bis-GMA) and an organosilane coupling agent (gamma methacryloxy propyl trimethoxysilane), which provides a bond between filler particles and the resin matrix [21-23]. The silane agent provides a strong chemical bonding with the filler and resin matrix [21-23].This reduces hydrolytic breakdown and allows stress transfer between the filler and the matrix. The fact that dentalcomposites continue to improve in strength, abrasion resistance, ease of application, translucency, and takes good polish rapidly increased their use in the first decade after being introduced and continues to increase their popularity [40,41].

Repair of Dental Ceramic Restorations

Ceramics are widely used in dentistry to fabricate indirect aesthetic restorations. Ceramics are brittle and tend to fracture under tensile loads [42,43]. Repair of these restorations is more economical and time saving than the fabrication new restorations. Silane coupling agents are widely used in repair of these dental restorations. The fractured surfaces are roughened with diamond burs followed by sandblasting the surface. Then the surface is etched with a suitable etchant followed by surface silanisation and finally bonding to resin composite [44].

Glass Fibre-reinforced Composites in Dentistry

Fiber reinforced composites received more attention in dentistry to fabricate various appliances like fixed partial dentures, removable complete and partial dentures, periodontal splints and retention splints [44,45-47],. Numerous fibers including nylon, carbon, polythylene, glass, aramid and jute fibers have been experimented. Among these glass fibers have received more attention because of their excellent aesthetics, superior mechanical properties and biological compatibility compared to others. The bonding strength isincreased between resin composite and glass fibre by adding a silane coupling agent. The silane forms siloxane linkages with the surface hydroxyl groups of glass fibre. The organo-functional groups of silane react with the functional group in the resin composite [44, 47].

Titanium, noble metal and base metal alloys in Dentistry

Various noble metals and base metals specially gold alloys, cobalt-chromium alloys and titanium and its alloys are widely used for the fabrication removable partial and complete dentures with a metal frame incorporated and metal-resin cement restorations [21,48,49]. These metal surfaces are conditioned by sand-blasting using silica-coatedalumina particles that produce a silica-coated layer. A silane coupling agent is applied to these silica-coated surfaces to form a durable siloxane linkage prior to cementation [44,45].

Tissue Engineering

Polymers are most commonly used in tissue engineering as scaffolding materials. Poly(ethylene glycol) diacrylate (PEGDA) is an important polymer among synthetic polymers used for tissueengineering applications [39,50], because of their hydrophilicity and biocompatibility [39]. However, an important limitation of PEGDA is its lower cell attachment rate since the hydrogel forms a hydrated surface layer that inhibit the adsorption of specific proteins like fibronectin [39]. The use of hydrophobic materials, i.e. polydimethylsiloxane (PDMS), promotes protein adsorption that helps to fabricate a stronger hybrid hydrogel. A strong bond is formed when a coupling agent physically or chemically wets the organic and inorganic interfacial region. Both hydrophobic phenyltrimethoxysilane and hydrophilic 3aminopropyltriethoxysilane are used simultaneously to improve interfacial adhesion between hydrophobic and hydrophilic polymers [51].

Other Applications

Organosilanes are also used as water scavengers in moisture-sensitive formulations, as blocking agents in antibiotic synthesis, polypropylene catalyst donors and as silicate stabilizers.

Biological Concerns of Silane coupling agents [52]

Limited literature is available on biological effects of silane coupling agents on living tissues. Studies revealed that they are non-toxic and non-irritant. Some animal studies, however, indicated that they impair fertility by inhalation and is classified as a substance toxic to reproduction in category 3 with the risk phrase R62 ('Possible risk of impaired fertility') and potential carcinogenic effects (uterine tumours in females). In addition, repeated exposure to these materials may affect some organs like liver, kidney and lungs.

Conclusion

Coupling agents are used as powerful tools for the modification of composites to give improved process ability, mechanical and aesthetic properties. Organosilane chemistry brings a widevariety of advantagesin engineering and biomedical technologies including adhesion to difficult substrates, strong bonding to hydrophilic reactive sites (through covalent bonding and the creation of interpenetrating networks), low moisture uptake (hydrophobicity balance), thermal stability, UV resistance, low surface tension, and controlled Tg inclusions resulting in stress reduction or increased surface hardness. Hence, organosilane will continue to be the preferred material of choice to provide a strong adhesion to both fillers and resin matrix. However, the choice of either one or the other type of surface modifier will depend upon the type of enhancements required. Further research is needed to address the biological considerations of these materials.

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Anusha Konakanchi (1) *, Rama Krishna Alla (2), Vineeth Guduri (3)

(1) Department of Basic Sciences, Sri Vishnu Engineering College for Women, Bhimavaram, West Godavari, Andhra Pradesh

(2) department of Dental Materials, department of Prosthodontics, Vishnu Dental College, Bhimavaram, West Godavari, Andhra Pradesh

Received 7 August 2017; Accepted 11 November 2017; Published online 31 December 2017

* Coresponding author: Dr. Anusha Konakanchi;

E-mail: anuram22810@gmail. com

Caption: Figure 1: [gamma]y-methacryloxypropyl trimethoxy silane coupling agent hydrolizes and reacts with silica filler particles

Caption: Figure 2: Reaction of organotrialkoxysilanes with silica surfaces

Caption: Figure 3: Polyurethane Coupling Reactions
Table 1: Some of the Organosilane Groups and Their Compositions

        Coupling agent         Composition      Used wit
                                                --Polymer

1          [gamma]-           [formula not      Polyester
        Glycidoxypropyl       reproducible]      epoxide
       trimethoxy silane

2          [gamma]-           [formula not       BisGMA
      methacryloxypropyl      reproducible]
       trimethoxy silane

3    [gamma]-Mere apt opr     [formula not    Diene rubbers
        opyl trimethoxy       reproducible]
            silane

4    [gamma]-Annnn propyl     [formula not
      trimethoxy Silane       reproducible]

5    Trichlorovinylsilane     [formula not      Polyester
                              reproducible]

6    Tnethoxyvinylsilane

7        [[beta]-(3,4-        [formula not      Polyester
       Epoxycvclohexvl)-      reproducible]      Epoxide
            ethyl]                            Polycarbonate
        tnmethoxysilane

8   Trichloromethoxysilane    [formula not
                              reproducible]

9   TrimethoxyPhenylsilane    [formula not
                              reproducible]
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Author:Konakanchi, Anusha; Alla, Rama Krishna; Guduri, Vineeth
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Date:Jul 1, 2017
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