Properties and Applications of Silicones - ACS Symposium Series

Chapter 2

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Properties and Applications of Silicones Michael J. Owen* Michigan Molecular Institute, Midland, MI 48640 *[email protected]

As part of the fourth Silicones and Silicone-Modified Materials symposium, it is useful to reflect on the properties of the mainstay of the silicone industry, polydimethylsiloxane (PDMS), and how these properties relate to the varied applications of this exceptional polymer.

Applications of PDMS List 1 shows some selected applications of PDMS. This list could be very much longer but has been limited to these nine selections to illustrate some fundamental points regarding the symbiotic relationship of polymer properties and applications. Firstly, they fall into three groups representing the familiar broad categories of silicone applications; additives, coatings and bulk cross-linked materials. Secondly, some of these applications have been chosen because of their familiarity and importance to the industry throughout its over sixty year history. These examples, one from each broad category, are antifoams, release coatings, and sealants. Thirdly, some are listed because they represent newer growth opportunities. One such application is high voltage insulation as indeed also is the newest generation of silicone contact lens materials. Primarily, however, these particular selections have been made because they present various apparent paradoxes worthy of discussion. For example, the antifoam and foam stabilizer applications, and the wetting agent and water repellency areas. Release coatings and sealants are a less obvious paradoxical pair until it is recalled that a prime requirement of any sealant should be good substrate adhesion. The least obvious paradox, that of high voltage insulation and contact lens materials, is elucidated in the subsequent discussion.

© 2010 American Chemical Society In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

List 1: Chosen Applications of PDMS


• • • • • • • • •

Antifoams Wetting agents Polyurethane foam stabilizers Release coatings Water repellent coatings Architectural coatings Sealants High voltage insulation Contact lens material

Properties of PDMS List 2 summarizes a number of important characteristics of PDMS relevant to our consideration of structure/property/application relationships. List 2: Some Important Characteristics of PDMS • • • • • • • • • • • • • • •

Low intermolecular forces between methyl groups Unique flexibility of siloxane backbone High bond energy of siloxane bond Partial ionic nature of siloxane bond Low surface energy Hydrophobic/oleophilic Low solubility parameter High free volume Low glass transition temperature High gas permeability Liquid nature to high molecular weight Presence of low molecular weight component Versatile cross-linking chemistry Low toxicity UV stability

The first four of these characteristics of polydimethylsiloxane are the more fundamental and essentially provide the underlying explanation for the other physical and chemical attributes listed in List 2. Quantitative data pertinent to these attributes are shown in Table I. Characteristic pressure is perhaps the least familiar of these quantities. It is a measure of the mean intermolecular energy per unit volume corrected for the density of packing in the liquid state.

Discussion Antifoams Also known as defoamers, these materials are available in considerable product variety as fluids, compounds i.e. synergistic combinations of carrier fluid 14 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

and hydrophobic solid, emulsions, and encapsulated products e.g. for detergent foam control. Their particular advantages comprise a broad range of applicability, thermal stability (e.g. in petrochemical processing) and FDA approval for applications such as antiflatulents. Perceived disadvantages include limitations on longevity of action and occasional impact on subsequent processes such as paintability. The key characteristics in this application include low surface energy, to enable entering and spreading at foaming lamellae, high siloxane bond energy (in its relationship to high thermal stability) and low toxicity.


Surfactants Silicone surfactants come in a variety of product types including the so-called trisiloxane “superwetters”, silicone-polyether rake copolymers as used in polyurethane foam stabilization, and novel water-in-oil and oil-in-water emulsifiers. There is a pronounced preponderance of nonionic products, unlike hydrocarbon and fluorocarbon based surfactants. The primary advantage of silicone surfactants is their ability to wet low energy surfaces such as plastics, skin, hair, plant surfaces, etc. and to lower the surface tension of organic fluids such as the polyols that are polyurethane precursors. The main disadvantage is a limited stable pH range of 4 – 9. Note that for a PDMS monolayer on water this stable pH range is wider, 2.5 – 11, presumably due to the more limited two-dimensional exposure to water of a spread film as opposed to the three-dimensional exposure of a dissolved surfactant. The spreading advantage accrues from the lower aqueous surface tension of PDMS surfactants, typically ca 20 mN/m above the critical micelle concentration, compared to hydrocarbon surfactants which typically have corresponding values of ca 30 mN/m. The disadvantage is the result of the partially ionic siloxane backbone rendering it susceptible to nucleophilic or electrophilic attack at extremes of pH. Water-Repellent Coatings These are available as cross-linkable emulsions, liquid silicone elastomers, and filled materials that exploit the “lotus effect” – enhancement of high contact angle by controlled roughness. A disadvantage is that oleophobicity is sometimes required as well as hydrophobicity. Fluorosilicones can be advantageously employed in such situations. Breathability, an important comfort factor for textiles, is best when individual fibers are treated but even a continuous PDMS elastomer film will transmit water vapor. The high contact angle of water, greater than 90 degrees, is significant. The oleophilic nature is a direct consequence of the methyl groups present along the chain of this semi-inorganic polymer. There are two apparent paradoxes in these applications. That between antifoaming and urethane foam stabilization is simply a reflection of solubility. The prerequisites of antifoaming are insolubility and surface activity whereas for foaming it is solubility and surface activity, explaining why polyurethane foam stabilizers are copolymers of PDMS with polyol-soluble polyethers. Given the inherent insolubility of PDMS in water, it is axiomatic that all aqueous silicone surfactants will also have a hydrophilic entity, usually as a silicone-polyether 15 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

copolymer. The paradox between wetting agents and water repellency arises because of confusion between “wetting of” and “wetting by”. A low surface tension is required for a liquid to wet a substrate. A water-repellent coating also needs to be of low surface energy to repel wetting by water. Thus in both cases low surface tension is the key. The wetting agent’s low surface tension aids in its spreading and wetting “of” a substrate; the water-repellent coating’s low surface energy resists wetting “by” water.


Release Coatings These products are available as cross-linkable materials based on either condensation or hydrosilylation addition chemistry, in the form of water-based emulsions, solvent dispersions, or neat, solventless materials. Non-reactive release products are also widely available. The advantages of the coatings are that they are effective on a broad range of substrates, permit slippage during peeling of pressure-sensitive adhesives, and offer a range of controlled release force by the use of high release additives. A disadvantage in certain processes is that transfer of free PDMS can affect subsequent surface treatment of the released articles. Low surface energy and low glass transition temperature are the key characteristics of importance in this application. Sealants Sealants are available as one or two part, room temperature or heat curing systems dependent on the cross-linking chemistry utilized. Condensation cure materials, particularly those producing acetic acid as an in-situ etchant, have excellent adhesion to a variety of substrates. Others based on hydrosilylation addition cure might require the incorporation of adhesion promoters such as silane coupling agents. Applicational advantages include a low viscosity before curing, facile displacement of leaving groups due to the high permeability of PDMS, and the low surface tension promoting easy spreading. Fungal growth, resulting from the oleophilic nature, can be a problem if products are not formulated with fungicides. Architectural Coatings These coatings are supplied as a water-based paint. Their advantages include environmental inertness and longevity. They are also useful on poorly adherent surfaces. Disadvantages include staining from organic material in the environment and difficulty in recoating with other paint systems. The key characteristics are UV stability, low surface energy (wets substrates well; poorly wetted by other materials), elastomeric nature and oleophilicity. The release coating/sealant adhesion conundrum is a further example of the wetting “by” and “of” confusion described earlier. Low surface tension is part of the explanation of good sealant adhesion by promoting good wetting of the substrate and repelling wetting of a release coating by an adhesive. The inconsistency in architectural coatings seems to be staining of such a low surface 16 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

energy material. In this case it seems it is the oleophilic nature of the polymer, with two methyl groups on every silicon, that triumphs over the release characteristic. The presence of low molecular weight material that might solubilize organic contaminants at the coating surface is also a contributory factor.


High Voltage Insulation and Contact Lens Materials These very different applications are not self-evidently contradictory. This aspect results from the hydrophobic recovery behavior of silicones. It turns out to be beneficial for high voltage insulation but a detriment to contact lens materials. Plasma treatment has been used to increase the wettability of contact lenses by tears. Corona discharge will inadvertently do this to high voltage insulation. In the contact lens case hydrophobic recovery is an unwanted attribute; in high voltage insulation it is very much desired. Silicone high voltage insulation products are available as composite insulators, elastomer dispersions, and grease-like coatings. Their advantages over porcelain are light weight, vandal resistance and resistance to salt contamination. The advantage over other polymers is the facile hydrophobic recovery. Algal growth is one disadvantage; a few cases of rodent damage in storage and bird damage by pecking when installed have been reported. Key attributes are low surface energy, low glass transition and the presence of low molecular weight material, the diffusion of which to the surface is the prime reason for subsequent hydrophobic recovery. A PDMS elastomer lens has long been used in eye surgery on young children. New organosilicon-containing copolymers and hydrogels have also recently been introduced. Contact lenses based on PDMS continue to appeal, primarily because of the unrivalled oxygen transmission to the cornea. This is offset by poor wettability by the tears necessitating copolymerization with hydrophilic monomers or some other type of wettability enhancement treatment. Lipid uptake can also present difficulties. The fundamental factors involved in this application are the low surface energy (low intermolecular forces), high oxygen permeability (chain flexibility, low Tg, high free volume), and oleophilicity. Note that oleophobic fluorosilicone intra-ocular lenses are available.

Summary Our objective in this paper has been to shed light on the fundamental characteristics of PDMS (6, 7) and how they relate to its applications. Some apparently contradictory applications have been examined to show that in each case the same set of fundamental attributes is operating and that, expectedly, the paradox is illusory. Polydimethylsiloxane’s most fundamental features are its low intermolecular forces between the methyl groups, unique backbone flexibility, high siloxane bond energy and the partial ionic nature of the siloxane bond. As the varied contributions to this series of symposia amply attest, research in silicone polymers is expanding continuously and there is every reason to expect 17 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table I. Fundamental Properties of PDMS Property

Ref

cm-3

(1)

~0

kJ/mol

(2)

Glass transition temperature

150

K

(3)

Siloxane bond energy

445

kJ/mol

(4)

41

%

(5)

Characteristic pressure Energy of backbone rotation


Unit

Value

Siloxane bond polar character.

J

341

that this set of attributes will enable the historic growth of silicones to continue for a further sixty years.

References 1. 2. 3. 4. 5. 6.

7.

Shih, H.; Flory, P. J. Macromolecules 1972, 5, 758. Tobolsky, A. V. Properties and Structures of Polymers; John Wiley and Sons: New York, 1960; p 67. Lee, C. L.; Johannson, O. K.; Flaningam, O. L.; Hahn, P. Polym. Prepr. 1969, 10 (2), 1311. Beezer, A. E.; Mortimer, C. T. J. Chem. Soc. A 1966, 514. Pauling, L. J. Phys. Chem. 1952, 56, 361. For those requiring more quantitative data on the properties of PDMS, Kuo has provided an excellent comprehensive compilation. (a) Kuo, A. C. M. In Polymer Data Handbook; Mark, J. E., Ed.; Oxford University Press: New York, 1999; p 41. (b) Kuo, A. C. M. In Polymer Data Handbook, 2nd ed.; Mark, J. E., Ed.; Oxford University Press: New York, 2009; p 539. For more extensive information concerning PDMS and related polymers see (a) Jones, R. G.; Ando, W.; Chojnowski, J. Silicon-Containing Polymers; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000. (b) Clarson, S. J.; Semlyen, J. A. Siloxane Polymers; PTR Prentice Hall: Englewood Cliffs, NJ, 1993.

18 In Advances in Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Properties and Applications of Silicones - ACS Symposium Series

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