Bob Ruckle, John Grande, Tom-Seung Cheung, Yanjun Luo, Robert Bui

Siltech Corporation, Toronto, ON Canada

Lately, I have been thinking a lot about the element silicon. Not quite as odd as it seems, as I have been a silicone chemist since 1989. That’s … well, let’s just call it a long time. Remembering that the bulk of the earth’s crust is silicon dioxide-based rock, I appreciate that silicon is literally the bedrock/foundation upon which the silicone industry is built.

Also, full disclosure, I have been binge-watching a new Star Trek series which got me thinking about dimensions, time, and space, and why not apply that to silicon chemistry. All silicone chemists know the common jargon in which the silicon groups are referred to as M, D, T, or Q based on the number of oxygen atoms bonded to the silicon atom.

In this chemists’ jargon, the end units of the silicone polymers are M groups and the middle, repeat units, are D groups. Other Si-O bonded species are T and Q moieties. A useful mnemonic device to remember the labels is that they refer to the number of oxygen atoms attached to the silicon atom. So, M and D refer to mono and “di” oxygen bonds while the T and Q have three and four oxygen bonds respectively.

These M groups are also used extensively in fine chemical and pharmaceutical syntheses to protect oxygen atoms. For example, one reacts an OH group forming the trialkylsiloxy ether which will not react in the next step of the synthesis. It is therefore protected from a reaction designed to modify another part of the molecule under conditions which would react with an unprotected OH group. The M group is typically removed in a subsequent deprotection step.

The second dimension is also common in our industry. Silicone, or PDMS, is a linear or two-dimensional polymer. A silicone is then structurally represented as MDxM. This well-known, widely used substance is a slippery, flexible, soft, very low Tg liquid that is insoluble in both oil and water.

Although these “1st and 2nd dimension” silicon materials can be difficult to use due to their immiscibility with other compounds, the industry makes many chemically modified derivative silicones which are soluble and typically impart wetting, shine, softness, and release when used in a formulation. They also impart flexibility, softness, haptic properties, biocompatibility, gas permeability, and low surface energy to the network when reacted with another polymer. These are very familiar and are used extensively, nearly pervasively, in the coatings, adhesives, sealants, and elastomers (CASE) industry as well as many other industries.

In the CASE industry, we are also very familiar with the “3rd dimension” aka T groups. Most often these are found in the guise of trialkoxy organofunctional silanes. Available with a plethora of organic groups, these are frequently used to improve adhesion. Especially common when bonding organic and inorganic substances, the silane works by reacting some of the three Si-O bonds with the more mineral-like species and a reactive organofunctional group (such as epoxy) with the organic substance forming a bridging bond between unlike materials.

These trialkoxy silanes are also commonly used to stabilize mineral dispersions by condensing with the minerals leaving a non-reactive organic group to prevent agglomeration. Also, these are used to modify polymers by reacting the organic group with the polymer or monomer and then reacting the alkoxys of the T groups ultimately providing enhanced properties from increased cross-link density. Other applications exist for trialkoxysilanes, but they also rely on the reactivity of the T group in acid/base chemistry.

At Siltech, we have a unique line of Silmer® TMS polymers which are silicones modified with a trimethoxy silane either at the ends of the polymer or in the repeat units. These materials provide unique characteristics including secondary cure, slip, stain repellence, and in some cases oil repellence. When combined with silica, super hydrophobic and oleophobic properties have been observed. In my analogy, these incorporate both the 2nd and 3rd dimensions of silicon. If you are interested to read more detail, there is an excellent overview of these “evolved silanes” by Dr. Mike Gunther on our website.[i] There are also other relevant papers with Silmer TMS polymers there.[ii], [iii]

The fourth dimension of silicon connectivity is much less well known in our coatings multiverse. This Q group is a structural unit in which the Si atom is bonded with all four bonds to oxygen. Unlike the more easily accessible T networks, networks formed with a lot of Q character tend to be reasonably stable to hydrolysis and other reactions. It is one way to obtain much of the performance of a silicone without the ease of hydrolysis which sometimes causes problems in a formulation.

These Q based species have been used for years, but have often traveled incognito and in small percentages. MQ resins, in which the Q network is terminated with trimethylsiloxy groups, have been used extensively for feel, transfer resistance, and cushion in cosmetics as well as anti-squeak and lubrication in industrial applications. Another very big application of MQ resins has been in antifoam compounds that require a high level of durability such as pulp and paper, laundry, jet dying of textiles, and chemical processing applications. In these applications, the enhanced chemical stability of the MQ resin is a critical property to its success.

We at Siltech have spent years developing reproducible processes to make various Q resins which often also contain M, D, and/or T groups. Unlike silicones, these materials are not allowed to react to a thermodynamic energy minimum. Instead, they are kinetic products and the rigor and precision of the process are critical to the reproducibility and consistency of the final product. In other words, these are tricky to make and difficult to copy.

We have also explored the properties and applications of these materials. So far, we have found that when used in a mixture, these Q resin species provide exceptional water repellence, stain repellence, release properties, and in some cases, oil repellency. They also provide hardness, tear resistance, strength, and release when reacted into another resin system. This is often complementary to the properties brought by reacting the standard two-dimensional silicones into a resin network and we frequently formulate with both Q resins and reactive silicone polymers in our examples.

As one increases the Q resin relative to other groups in a generic MDTQ resin, one gets increased cross-linking, hardness, COF reduction, and sometimes flexibility properties. As an example, referring to the table, we see a DT-25 resin which gives lower COF/more slip and more hardness as we add 25% Q content into the resin (DTQ-75).

After developing the overall processes, we next fashioned unique structural variations which include organic groups to convey compatibility, reactivity, or other novel properties to the Q resin. To date, we have successfully commercially prepared polyether, vinyl, silanic hydride, acrylate, hydroxyl, mercapto, and phenyl MDTQ resin derivatives. The phenyl derivatives are designed to maximize heat and UV stability while the polyether derivatives have more compatibility and act as surfactants. The remaining variations are all designed to react into specific systems.

As an example, using MDTQ resins in silicone networks such as elastomers or sealants can restore some of the inherent deficiencies of these systems. Specifically, Silmer® VQ and Silmer HQ silicon resin systems are available with vinyl groups and Si-H bonds respectively. When used in addition-cured elastomers, these tend to bring strength and tear resistance. While silica filling can also do this, the Q resin approach works at lower use levels and retains the transparency of the final product.

A more unique example of this was shown[iv] when we used the UV cured mercapto-ene reaction to form silicone elastomer networks. The intent was to recover the 300% elongation values typically seen in condensation and platinum cured silicone elastomers. Radiation curing of acrylate functional silicone loses this elongation because the silicone is not the main backbone polymer. The mercapto-ene cure chemistry solves the elongation issue, but the formulations were too soft.

We resolved this by using an MTQ resin mercapto structure reacted with standard two-dimensional di-vinyl silicones (65,000 cPs). Good results were obtained with this, but the best formulation also used MTQ functional vinyl silicones to increase the cross-link density even more. Shown here is a formulation from the paper referenced above with over 150% elongation and reasonable hardness and strength properties.

Our work in the last couple of years has shown that in some circumstances, difunctional and mono-functional silicones have shown oleophobicity.[v] We believe this is reminiscent of meso- and nano-structuring of materials which provide a laboratory lotus effect. A good example of this is Silmer ACR DT10 which is a mono-acrylyl DT silicon resin. This product organizes and then is locked in by the rapid radiation cure conditions. We found very good beading of mineral oil on this reactive silicon resin at 5% in a UV system.

Most recently and in the context of developing PFAS alternatives, we observed a succession of results in which monofunctional Q resins or difunctional silicones gave oleophobicity. This led us to the theory that these species are assembling. We have been experimenting significantly with the idea that these T and especially the Q containing resins can self-assemble with mono- and/or di-functional silicones and then react holding the structure in place providing super-hydrophobic and oleophobic properties.

Very recent work has included a difficult to manufacture type of monofunctional silicone locked into Siltech T35, a T resin matrix. This system has shown significant improvements in the receding contact angles of mineral oil as well as very good marker release over a fluorosilicone control.

Future work will continue to attempt improvement on oleophobicity by fine-tuning this assembly approach. If you are interested in more detail, please contact us or come visit us at the many trade shows we attend.

[i]https://www.siltech.com/wp-content/uploads/2024/06/2021-Evolution-Silanes-presentation.pdf

[ii] https://www.siltech.com/wp-content/uploads/2024/06/2019-Novel-Silicone-Materials-Provide-Secondary-Cure-paper.pdf

[iii] https://www.siltech.com/wp-content/uploads/2024/06/2014-Novel-and-Uncommon-Organosilicone-Additives-paper.pdf

[iv]https://www.siltech.com/wp-content/uploads/2024/06/2020-Novel-Energy-Cured-Mercapto-symposium-paper.pdf

[v]https://www.siltech.com/wp-content/uploads/2024/06/2024-PFAS-Alternatives.pdf