Silsesquioxanes - Tailored multifunctional oligomers (October 2007)

Paper published in European Coatings Journal, October 2007.

Coating formulations

The formulator has at his disposal a vast range of raw materials from which to build a coating with a set of processing and performance characteristics that matches or exceeds his customer specifications. These specifications are,however, constantly changing, being driven to adapt to greater demands from those further up the value chain and those setting the legislative and regulatory environment. The challenge to the formulator is to extract the greatest valuefrom the raw materials available. The challenge to the materials suppliers is to provide constituents for coatings that deliver the required performance whilst conforming to the drivers for more environmentally friendly behaviour.

There is a wide range of coating types ^[1] based on different reactive chemistries, each of which has developed with particular characteristics and has applicability in different marketplaces. Each coating formulation consists of an number of components. One generic typeof component that is often used to enhance processing or performance is silicon based additives.

Silicon based additives

Silicon based additives are widely used in the generic coating and surface treatment sectors. These additives range from surface pre-treatments such as silylating agents and silane coupling agents, ^[2] through silicone based additives to give improved flow and anti-foam behaviour or to improve gloss retention on weathering, ^[3] to fine-scale silica functional fillers for improved mechanical properties. ^[4] A higher level of connectivity between the silica and the organic matrix in which it is placed can be achieved by functionalising the surface with a silane coupling agent. ^[5] Sol-gel methods have been widely used and are reported to functionalise the surface of particulates to both enhance compatibility with organic matrices and thereby improve performance and to develop the inorganic network itself. ^[6] This approach marks the boundary point between the organic and inorganic film-forming components of a coating composition. Silicon (as well as other metal) alkoxides and their derivatives are capable of network forming via thehydrolysis of the alkoxide and subsequent condensation of the hydrolyzate to give a siloxane bond. ^[2,7] With the appropriate organic functionality attached to the alkoxide moiety a second type of network formation can be undertaken. This dual functionality lies at the heart of the development of inorganic-organic hybrids forenhanced coating performance such as abrasion resistance. ^[8]

Structural considerations

Network formation via inorganic and organic species can be contrasted by their structural development. Within organic film forming systems the reactivity is determined by the number (and type) of reactive groups present within the system. The formulator specifies the distribution of reactivity but can achieve film forming properties with an average functionality of two. This is because low functionality levels can give rise to high and ultra-high molecular weight polymers which have a relatively high glass transition temperature and so are solid at room temperature. Functional species with three functional groups or more are generally regarding as cross linking agents and are used to improve solvent resistance, abrasion and mar resistance and to increase the glass transition temperature of the material. The conjugate effect of increasing the cross-link density can be to increase the brittleness of the film and to increase its shrinkage during curing which may affect adhesion or result in cracking.

Similar to carbon, silicon has a valancy of four. Silylating agents have three organic groups attached to the silicon via hydrolytically stable Si-C bonds. The fourth bond is usually a hydrolysable group such as chlorine. These are typically used as termination agents to prevent further inorganic network growth. Di-functional silicon materials are the basis of silicones and typically have non-reactive methyl or phenyl groups directly attached to the silicon. Theother two groups attached to the silicon are usually hydrolytically sensitive chlorine or alkoxides. Hydrolysis of these groups and subsequent condensation generates a siloxane network. With di-functional groups this leads to very longchain polymers with high molecular weights. These polymers, the silicones, typically have very low glass transition temperatures (-60°C) and are liquid under ambient conditions and so do not form films. Tri-functional siliconmolecules, silanes, have three hydrolysable groups attached to the silicon and one non-hydrolysable group. Network formation is typically carried out by hydrolysis and condensation of these materials. However, separation of thehydrolysis and condensation steps is extremely difficult and the two occur in parallel under most practical conditions. This typically leads to the formation of a three-dimensional siloxane network that, when complete, yields a highlycross-linked, ultra high molecular weight material. If the tri-functional silane is the major component the form ultimately yielded is often referred to as an intractable gel. It is possible that such a gel could be formed without the organic functionality having reacted. Use of these tri-functional silanes in coating formulations however usually occurs at relatively low levels. This is to allow some siloxane network formation to occur (often with reactivehydroxides on metal substrates to enhance adhesion), without swamping the system. Once the required siloxane network is formed the organic components can be polymerised (the order of network formation is sometimes organic polymerisation first followed by siloxane polymerisation). The silane therefore acts as a node between the inorganic and organic polymer networks.

Multifunctional silsesquioxanes

Silsesquioxanes are defined as materials with the composition RSiO _1.5 . Oligomeric silsesquioxanes are materials which satisfy the compositional definition and which have a formed siloxane network. ^[9] This network is typically created by conventional siloxane formation methods such as hydrolysis and condensation from reactive silanes. The structure of the network is determined by the processing methodology under which siloxane formation occurs. The range of structures available range from highly defined cubic octamers (see Fig 1) through intermediate molecular weight ladder structures (see Fig 2) to three-dimensional high molecular weight intractable gels. ^[10,11] The ability of these networks to continue to grow depends in part on whether the siloxane formation is complete, or whether there are residual silanol groups which can continue the condensation polymerisation pathway. The second consideration for continued network formation is the nature of the organic functionality attached to the silicon. For example, highly defined cubic octamers have been incorporated into an organic network via a methacrylate group located on one of the silicon atoms of the octomer. ^[12] There is a wide range of silsesquioxane structures commercially available. ^[13] These structures are typically small well defined cage silsesquioxanes with eight to sixteen silicon atoms. The structures available include those that are fully condensed and have each silicon attached to one type of reactiveorganic group (the reactive group types available include epoxy, methacrylate and vinyl). Additionally, cage structures are available with a single reactive group attached, the other silicon atoms in the cage being attached to non-reactive organic groups such as cylcopentyl ligands. Similar variants can be achieved with an incompletely condensed cage structure which may then possess residual silanols capable of siloxane polymerisation.

Fig.1. Cubic silsesquioxane oligomer with an acrylate functionality shown

Fig.2. Ladder silsesquioxane with an epoxy functionality shown

A recent development at TWI ^[14] has further extended the fabrication capability for silsesquioxanes. The route developed takes the approach of building oligomeric silsesquioxanes with specific organic functionalities. The structures generated using this methodare likely to be a distribution of cage and ladder types rather than a specific configuration. This approach allows rapid fabrication of significant quantities of the silsesquioxanes even on the laboratory scale. The approach also allows the focus to move from the structure of the silsesquioxane to the chemistry and reactivity of these inorganic-organic oligomers. The approach adopted by TWI is to utilise the silsesquioxane as no more than a structural vehiclefor the carriage of one or more reactive or functional organic ligands. The fabrication route developed has been called Vitolane ^TM technology. Figure 3 shows the ²⁹ Si HP/DEC NMR of four oligomeric silsesquioxanes produced using the Vitolane method. Analysis of these spectra allows the degree of condensation (D _c - percentage of siloxane bonds completed) to be determined. This is given in Table 1, where T(1) refers to a reacted silane with a single siloxane bond, T(2) has two siloxane bonds and T(3) is completely condensed silane with three siloxane bonds.

Fig.3. ²⁹ Si HP/DEC NMR spectra of Vitolane prepared silsesquioxanes.

Sample A- methacrylate functional;
Sample B - methacrylate-methyl functionalised;
Sample C- acrylate functional:
Sample D - acrylate -methyl functionalised

Table 1. The relative proportions of the T species in silsesquioxanes prepared using the Vitolane method.

The high value of the degree of condensation of these silsesquioxanes show that considerable siloxane network has been completed. The incomplete condensation is shown by presence of residual silanols which are identified by the presence of a peak at ~3500cm ^-1 using FTIR analysis, see Fig 4. Comparison of the FTIR spectra of a Vitolane produced methacrylate oligomeric silsesquioxane ( Figure 4,a and 4,b) with a commercially produced cubic methacrylate functional silsesquioxane ( Figure 4,c) shows great similarity to be expected from similar structural units. The primary difference between the samples is the presence of a peak due to silanol groups in the spectra Fig 4,b. These silanols can be readily removed from the silsesquioxane by a simple condensation treatment as is shown by the spectra Fig 4,a.

Fig.4. FTIR spectra of three fully methacrylate functionalized silsesquioxanes

a) Vitolane produced silsesquioxane with few residual hydroxyls,
b) Vitolane produced silsesquioxane with residual hydroxyls,
c) Commercially obtained cubic silsesquioxane

Design of bespoke resins

The Vitolane fabrication method allows for oligomeric silsesquioxanes to be manufactured with a wide range of organic groups attached. These include epoxy, acrylate, methacrylate, phenyl, fluorocarbon, amine, glycol based ligands.This fabrication route allows for more than one functional group to be attached to the silsesquioxane. The approach provides a methodology to start designing oligomers targeted for use in specific applications. Rather than using available materials and blending them into a formulation with all the compromises that must be made, we can start to envisage a method of making materials in a more bespoke fashion. This can be done by manipulating the number and type of functional groups that are attached to a discrete inorganic superstructure, the oligomeric silsesquioxane.

A very wide range of potential applications could benefit from the use of this platform technology. For example, there is a general requirement for improved abrasion resistance for coatings for applications such as displays,automotive components, plastic glazing and mobile phones. The use of a very high functionality oligomer which would give rise to high cross-link density would clearly have a beneficial impact.

Low solvent water-borne and 100% solids coating formulations are being driven by legislative requirements. The introduction of hydrophilic components into the oligomer or monomer base can cause significant performance problems due to reduction in cross-link density, reactivity and compatibility issues. Figure 5 shows how an epoxy functional silsesquioxane could contain a hydrophilic group that would improve its compatibility by being water-borne. This ability of this oligomer to achieve medium-high cross-linking whilst alsohaving a hydrophilic component would potentially allow a coating to be produced that prevented misting on spectacles, auto, rail and aerospace windscreens, bathroom mirrors and visors whilst having a high degree of abrasionresistance.

Fig.5. Ladder silsesquioxane with epoxy and hydrophilic functionalities shown

Considering the opposite feature, that of hydrophobicity, there is a significant amount of interest in the generation of coatings with low surface energy. Application areas in the marine industry such as prevention of barnacle buildup do not currently have a fully satisfactory solution. These growths need to be periodically removed to allow boats to travel efficiently through the water or allow inspection of oil & gas installations. In another sector entirely, the issue of vandalism and graffiti (see Figure 6) is a social ill in most societies. Low surface energy or hydrophobic coatings address this issue by minimising the wetting of the treated surface. However, most hydrophobic coatings on the market do not have a high tolerance to abrasion and therefore a have a low durability. The structure in Figure 7 shows a silsesquioxane approach to the problem. The acrylate could provide the high cross-linking necessary for the abrasion performance whilst the fluorocarbon group would provide the hydrophobic nature. This kind of coating could actually be used as the top-coat for automotive finishes ( Figure 8) where there is a desire to keep the 'out-of-the showroom' look for longer. There is also an environmental benefit in the reduction of the significant amounts of water used to wash cars giving rise to considerable run-off and water treatment issues, in order to remove dirt and keep cars looking clean. The requirements for durable anti-soiling coatings also include spectacle lenses, DVD/CD coatings and toilet areas (especially in aircraft) as well as some less obvious examples such as oil pipe interiors to prevent the build-up of wax, and passive layers to prevent or minimise ice build-up on wind turbines or aircraft wings.

Fig.6. An example of graffiti

Fig.7. Ladder silsesquioxane with hydrophobic and acrylate functionalities shown

Fig.8. Automotive exteriors have a need for protection

Vitolane technology allows the manufacture of multi-functionality oligomers. Figure 9 shows a quatrafunctional silsesquioxane which contains a hydrophobic fluorocarbon group, an acrylate for rapid UV cure, a phenyl group to enhance the temperature stability and an epoxy for curing out of the line of sight or over a longer time frame. Adopting the Vitolane technology approach to materials fabrication allows new materials to be designed. These materials may be in the form of coatings or adhesives perhaps with high temperature capabilities, or they could be incorporated into the production of bulk materials for use in demanding applications. Aircraft canopies ( Figure 10) are made out of PMMA and are coated to provide abrasion resistance. The use of silsesquioxanes may allow new transparent, hard, abrasion resistant but readily processable materials to be manufactured for glazing for construction and transport application. The glass used in glass-to-metal seals ( Figure 11) has been used for many years without significant changes being made. Whilst it provides the level of hermeticity required, it is brittle and must be processed at high temperatures which can cause significantproblems. Silsesquioxanes are essentially organically modified glasses and may provide the route to the production of impermeable materials at low processing temperature to replace glass-to-metal seals, but also provide permeationbarriers for OLED and PLED displays and even food packaging.

Fig.9. A quatrafunctional ladder silsesquioxane

Fig.10. Aircraft canopies operate in highly demanding environments

Fig.11. Glass-to-metal seals, a well established technology that could be improved by the use of new materials technology.

In summary, silsesquioxanes, and particularly the Vitolane method of fabricating silsesquioxanes, offer the potential of a new way to envisage how specific performance characteristics may be designed and built into the molecular structure of materials.

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