Silane Terminated Prepolymers: An Alternative to Silicones and Polyurethanes

Silane terminated prepolymers for adhesives, sealants and coatings are of great industrial importance. They are very important because of their low toxicity over polyurethanes, silicones, and solvent-based products. Hence, many pieces of literature which deal with the synthesis, properties and applications of this Silane terminated polymers hybrid system exist. Silylated polyether (MS polymers) and Silylated Polyurethane Polymers (SPUR) are the bases for numerous sealants, adhesives and coatings used worldwide. A hybrid system mixed with organic-polyurethane proportion and inorganic-alkoxysilane proportion com-bines the benefits of conventional polyurethane and silicone-based products. This article reviews the chemistry of MS polymers and SPUR and their advantages and disadvantages in silyl terminated prepolymer-based adhesives, sealants and coatings as well as provides information on different end applications.


Introduction Hybrid Chemistry: Silane Terminated Prepolymers
Silane-terminated prepolymers have been used as resins for sealants, adhesives, and coatings for over 30 years due to many reasons. "Silane terminated polymers-Hybrid" is of increasing interest because they can be formulated to provide the best properties of two or more families of polymeric materials while limiting their individual inherent weaknesses. The silyl terminated adhesives, sealants and coatings systems are also isocyanate-free systems. They also provide better adhesion, cursors in the chemical industry, as reagent in the production of high purity silicon, as surface modifiers for adhesion of various coating, as the transition compound from organic to inorganic materials, as precursors in producing a range of products, including si licones, and many other uses [5].
Organo si lanes are synthesized from SiO 2 , silica one of the most abundant materials in the earth through a series of reaction. Silica is reduced to silicon which reacts with hydrogen chloride to yield trichlorosilane, HSiCl 3 . Then, trichlorosilane reacts with alkene and finally followed by alcoholysis, i.e., reaction with alcohol, to form the functional silanes. A silane coupling agent, which is a trialkoxysilane, contains two functional groups at the ends of its molecular backbone, and which connect an unpolymerized resin matrix and an inorganic substrate (surface). A general formula or such bifunctional silane is L-(CH 2 ) k -Si-(OR) 3 , where L is an organofunctional group (e.g., methacrylate, acrylate, isocyanato, epoxy), and these groups provide the organic compatibility which allows the silane to form interpenetrating networks, or in the case of reactive organofunctional silanes, to co-react with the coating polymer and (CH 2 ) is a linker (spacer) group that k separates the organofunctional group and the Si atom. OR is a hydrolysable alkoxyl group (methoxy, ethoxy).
At ambient temperature, silane is activated by acid (acetic acid) to form silanol (SiOH) before they can bond to the inorganic substrate. The general formula of an alkoxy silane shows two classes of moieties attached to the silicon atom: Alkyl and aryl silanes are utilized to improve gloss, hiding power, mixing time, and other properties related to improved pigme nt dispersion. Alkyl and aryl silanes are also utilized to provide hydrophobic surfaces in applications such as water repellents. The X represents alkoxy moieties, most typically methoxy or ethoxy, which react with the various forms of hydroxyl groups and liberate methanol or ethanol. These groups (Table 1) can provide the linkage with i norganic substrates, pigment, or filler to improve coating integrity and adhesion. The methoxy groups are also capable of reacting with hydroxy functional polymers [5] [6].

Activation and Hydrolysis of Silane
Organofunctional silanes contain two different reactive functional groups that can react and couple with various inorganic and organic materials. Thus, they act as adhesion promoters to join dissimilar materials. These functional silane coupling age nts are first activated by hydrolysis (=SiOR=SiOH) before they can bond, via hydroxyl groups on the substrate surface.
The first step of hydrolysis of silane to silanol (=SiOH) takes place by the fast and reversible protonation of the alkoxy group of the silane at pH 4 ash shown in Figure 1. Then, it undergoes a bimolecular nucleophilic substitution (S N 2) reac-

Factors Affecting Silane Hydrolysis
The rate of silane hydrolysis depends among others on the silane molecular structure, its concentration, pH, temperature, humidity, and solvent system. Ethanol is usually a part of the solvent system as si lanes dissolve easily i n ethanol but not in water. The bulkiness of alkoxy groups would affect the rate of hydrolysis. For silane coupling agents with bulky alkoxy groups, the steric repulsion of water moving towards to the silicon atom is increased as shown in Figure 2.
This said, the hydrolysis rate decreases with the large size of alkoxy groups: pentoxy < butoxy < propoxy < ethoxy < methoxy. The silane hydrolysis is strongly pH dependent. The silane hydrolysis rate is fast at acidic and alkaline medium, but it reaches a minimum at neutral pH for alkoxysilanes. Temperature is an impor- α-silanes and γ-silanes The re lative ly slow crossli nking kinetics of the standard si lane termi nated polymers are due to the molecular structure of the terminal silyl group. With Table 2. Performance comparison of Silane terminated prepolymers, Polyurethane, and Silicone Sealants [43].

Property
Silane terminated prepolymers Polyurethane Silicone Environmental friendliness 10 5 9 Non-bubbling 10 6 10 Low temperature gunnability 10 8 10 Stain resistance 8 8 5 Weather resistance 8 6 10 Adhesion to various substrates 10 5 8 Mechanical properties 10 10 10 Heat resistance, mechanical stability 9 8 10 Non-dirt pickup 10 10 5 Storage stability 10 7 9 Paintability with water-based paint 10 10 3 γ-alkoxysilanes, the moisture induced crosslinking reaction takes place considerably more slowly than with the highly reactive α-alkoxysilanes. In α-silanes, the electron donor is attached to the silicon atom via a methylene group. With this configuration, the alkoxy groups are activated, so that the crosslinking reaction is accelerated considerably. This is the difference between α and γ-silanes as shown in Figure 3, αand γ-alkoxysilanes in organofunctional alkoxysilanes, at least one of the four groups bound to the silicon atom is an alkoxy group (-OR).
Depending on the number of alkoxy groups, a distinction is made between bifunctional (two alkoxy group) and trifunctional silanes (three alkoxy group).
The alkoxy groups are able to hydrolyze. The reaction with water leads to the formation of a siloxane network. In addition to the alkoxy groups, there is a functional organic group on the silicon atom (X). Via X, a silane can also bind to an organic molecule. This is how Silane-terminated prepolymers are produced. An important structural feature of organofunctional alkoxysi lanes is the length of the hydrocarbon chai n in the reactive organic group.
The γ-silanes used industrially so far contain a propyle ne group in their reactive organic group (-CH 2 -CH 2 -CH 2 -) as spacer between the silicon atom and the organofunctional group, so that X is in the γ-position (relative to the Si atom). In α-silanes, the considerably shorter methylene group (-CH 2 -) is located between the silicon atom and X. In this case, X is α-terminal. The length of the hydrocarbon chain has a major influence on how firmly the alkoxy groups are bound to the silicon atom, and thus on the speed of crosslinking in the presence of moisture.
The α-effect, in α-alkoxysilanes, the reason for this effect is an electronic interaction between the silicon atom and the electron donor in the α-position. As a result of back-bonding, the electron density is shifted from the donor to the silicon atom, and the Si-O bonds are weakened. The alkoxy groups split off more easily and hydrolysis is accelerated [11].

Non-Functional Silanes
Non-functional silanes contain only reactive alkoxy (-OR) functional groups that after hydrolysis to silanol groups react with surface hydroxyl groups of i norganic substrates. A bis-functional silane has two silicon atoms, each containing three hydrolysab le alkoxy groups and they are also called cross-linking or dipodal silane. The purpose of this cross-linking silane is to connect the silane molecules more extensively by forming three-dimensional siloxane networks and interconnecting with the functional silanes. Thus, a rigid siloxane network is formed and as a result, more energy is required to break apart the cross-linking network.
Furthermore, the penetration of water molecules into the inner interfacial layer becomes more and more difficult as the degree of the cross-linking siloxane network increases [5] [6].

Silane Coupling Agents
Silane coupli ng age nts can form a durab le bond between organic and i norganic materials. Interfaces involving such materials have become a dynamic area of chemistry in which surfaces have been modified to generate desired heterogeneous environments or to incorporate the bulk properties of different phases into a uniform composite structure [8]. Coupling agent formula is shown in Figure 4. Coupling age nts bind organic polymers to fillers such as fiberglass/silica or mineral fillers like talc, mica, clay, and wollastonite. Silane as a coupling agent, silanes he lp to improve bonding and mixing as we ll as to increase the matrix stre ngth between these polymers and fillers [ 6].
The general formula for a silane coupling agent typically shows the two c lasses of functionality. X is a hydrolysable group typically alkoxy, acyloxy, halogen or amine. The R group is a nonhydrolyzable organic radical that may possess a functionality that imparts desired characteristics.

Silane Terminated Polyurethane
In 2004 O'Connor et al. [12] were the first to review the synthesis methods and development of silyl-terminated polyurethanes for construction applications. The use of organofunctional silanes as crosslinking units for polyurethanes was first reported in the early 1970s by a patent by 3 M [13]. Brode and Conte [14] followed shortly and synthesized SPUR prepolymer with different aliphatic and mercapto silanes. In the early 1980's Berger et al. [15] improved this new technology by using secondary amino bis-silanes which significantly improved the properties of the adhesives, sealants, and coatings with respect to flexibility and curing times.
In the first step a conventional polyurethane prepolymer is formed by reaction of a polyol with an excess amount of a diisocyanate. The free isocyanate groups are then end-capped with compounds containing reactive alkoxysilane groups.
After application the alkoxysilane end groups undergo hydrolysis and condensation reactions in the presence of moisture giving the cured prepolymers desired.  Triethoxysilane and linseed polyol as inorganic and organic precursors were used by Akram et al. [22]. This so-called Si/li nseed polyol is further treated with a diisocyanate to give organic-inorganic hybrid polyurethane composites. Vega-Baudrit et al. [ 23] investigated the properties of thermoplastic polyurethanes by agglomeration of nano silica particles within a polyurethane matrix. The thermoplastic polyurethane is prepared by the standard prepolymer method and mixed with the nano silica in 2-butanone.

Polyurethane Formation
The network formation process of diol-diisocyanate systems catalyzed by dibutyltin dilaurate was investigated by Dusek et al. [24] in 1990. The most important reactions that occur are summarized in Fig ure 7. They claim that the molar Figure 6. Synthesis of STPUs [21].

End-Capping
The end-capping reaction is done with di-or tri-functional ami no silanes [ 25] [26]. The ami no groups are reacted with the free isocyanates to give stable urea groups. A tri-functional silane having three methoxy or ethoxy groups induces higher cross linking and faster cure. Methoxy si lanes as end-cappers also lead to faster cure rates than those of ethoxy silanes [27].

Silane Terminated Polyethers
All commercially availab le silane modified polymers contain alkoxy silyl groups and are thus able to crosslink in the presence of moisture [40]. They differ in their organic structure, notably the manner the silyl groups are linked to the polymer backbone and the distance between the silyl groups and the backbone. In the case of silane-terminated polyethers, the silyl groups are coupled to the ends of the polyether backbone via an alkylene unit and a urethane group. The alkylene unit can either be a methylene group, in which case it is a silane terminated polyether, or a propyle ne group, in which case it is a silane terminated polyether. The length of the alkyle ne unit in-flue nces the reactivity of the alkoxysilylations of silane-terminated polyethers cure rapidly without needing a tin catalyst; the presence of a catalytic primary amino compound will suffice to kick off the catalytic reaction.

Silane Terminated Prepolymers-Chemistry
α-silane-terminated polyether is used as resin, the fully cured adhesive offers a combination of strength and elasticity that is ideal for various applications. The α-effect makes the innovative resin highly reactive. The setting speed can be adjusted by the choice of catalyst system, and even tin-free catalyst systems are possible. The high reactivity characteristic of α-silanes is retained even whe n the y are attached to organic polymers. α-silane terminated polymers always crosslink very quickly. Figure 12 and Figure 13 illustrates α and γ silane terminated prepolymers.
Even dimethoxysilyl and triethoxysilyl end groups are more reactive in α-silaneterminated polyethers than the trimethoxysilyl end groups of γ-silane-terminated polyethers. Difunctional silanes have the advantage that they release less methanol during the crosslinking process. In addition, a lower crosslinking de nsity is obtained with them than with trifunctional silanes, thus favoring the formation of a more elastic vulcanizate. Alkoxysilanes with an α-terminal isocyanate group have proven particularly useful in reacting with organic polymers as shown in Figure 12. There are three reasons for this: first, the isocyanate group (-N=C=O) permits bonding to various organic base polymers. Second, the bonding reaction is easy to control, and third, the reaction is quantitative. In the resulting α-silaneterminated polymer, the terminal silyl groups are irreversibly joined to the polymer chain via a stable urethane bond; the product contains no free isocyanate groups [41].

Fast Curing Technology
Chemical modification of the silane functional group is another tool to improve the properties of the silane-terminated polyether's. The current market trend requires adhesives with very fast strength development and faster curing. Drawback of these systems is however that most of these adhesive's lack in elongation or elasticity. The first generation of MS Polymer with the dimethoxy-methyl silyl Open Journal of Pol ymer Chemistry end groups has an exce llent e lastic behavior but is relatively low in reactivity.
Faster curing systems were later deve loped by replacing the dimethoxymethyl silyl group with trimethoxysilyl groups, as consequence that elongation dropped,  certainly with the si lylated-ed polyurethane types. The next step was the development of very fast curing systems, such as the α-type MS polymer, which keeps the inherent low elongation, resulti ng in more brittle products.

High Strength Technology
The second-generation MS Polymer modified with polyacrylate can control the morphology. By redesigni ng of the backbone and polymer structure, glass transition temperature (Tg) of the polymer can be controlled and consequently the compatibility of the polymers before and after curing can be manipulated. Acrylic domains are formed in a polyether matrix and these acrylic domains e nhance the strength significantly. Moreover, the acrylic domains will provide an improved adhesion to plastic substrates.

Curing Mechanism: MS & SPUR Adhesives, Sealants, and Coatings
The silane groups provide a non-isocyanate curing mechanism, better adhesion to various substrates, and excellent storage stability. These reactive end-groups cures in the presence of moisture in the atmosphere and an appropriate catalyst by means of an alkoxy reaction that is different than the conventional silicone cure mechanism. The water reacts with the silane group to produce si lanol. Further reaction of the silanol with either another silanol or methoxysilane produces three-  Figure 14 and Figure 15 below.
To make the curing mechanism happe n, both water (e.g., ambient moisture) and a catalyst are required. 2) Lack of I soc yanates: The silyl-terminated polymer eliminates the need for isocyanate. Isocyanates are highly reactive chemicals. Consequently, formulations containing isocyanates must be protected from contact with reactive agents, includi ng moisture, as this will drastically decrease shelf life. Isocyanates are also considered to be hazardous materials to use. Since no isocyanates are present, bubbling during curi ng is not a concern as it often is with polyurethane systems. Bubbles can damage the integrity of the sealant or adhesive. MS Polymer sealants do no bubble on the surface at all. The sealants and coatings surface is kept smooth and intact.

Key Properties and Performance of MS and SPUR
3) Lack of Solvents: The low viscosity of Silane terminated prepolymers allows for a solvent-free formulation that is less dependent on temperature change.
Therefore, formulations are easy to process and have low temperature gunnability. The absence of solvent provides environmentally friendly sealants that meet environmental regulations regarding volatile organic contents (VOCs), out-gassing, and toxicity. Furthermore, the lack of solvents protects the formulator from the rising cost of solve nt raw materials and VOC mitigating facilities. 5) Pe rformance during Service : The rise in Silane terminated prepolymers based adhesives, sealants and coatings popularity has been primarily due to its versatility and well-balanced properties [42].
Compared with the other two types of sealants, Si lane terminated prepolymers based adhesives, sealants and coatings have we ll balanced properties and performance. Some of the unique properties of Silane termi nated prepolymers are: • Environmental friendli ness (solvent free and -NCO free) • Low temperature gunnability and flowability • Silane terminated prepolymers based sealants do not stain as some silicone sealants do because of low molecular weight silicon materials that bleed from the surface of sealed joints.
• Silane terminated prepolymers shows no cracking, splitti ng, discoloration or adhesion failure after seve n years of testi ng in desert climate.
• Silane terminated prepolymers based adhesives, sealants and coatings provide adhesion to various substrates including metals, plastics, wood, and ceramics.
• Shelf life is excellent although must be protected from moisture.

Performance Comparison of Silane terminated prepolymers, Polyurethane, and
Silicone based Sealants is shown in Table 2.

Applications of Silane Terminated Prepolymers
Adhesives, sealants and coatings, as the y are used in many fields, require a series of different properties for unique function. which should be strong yet elastic [47]. There are many applications in which the bonded joint must be not only strong but also elastic. Elasticity becomes more important as soon as the bonded joint is subjected to a dynamic load. Since elastic adhesives deform reversibly and can absorb dynamic loads in the process,

Futuristic Developments
The advantages arising from ne w molecular design for the silylated polymer were improved elastic recovery and better through cure, eve n in deep layers or for bonding of large surface areas. Moreover, improved adhesion to a very wide variety of substrates was achieved by adaptation of the polymer backbone, for example by adjusti ng its polarity and crystallinity. These properties allowed the use of polymers in applications that were pre viously addressed only with difficulty [49].

Conclusion
Combining different polymer chemistries to develop new products can be challenging, but the potential benefits of success are making this a hot topic in the adhesives, sealants and coatings industries. Silylated prepolymers are considered to be "hybrid" molecules because they provide the best properties of polyurethane and silicone whi le limiting their inherent weaknesses. In addition to their unique