B
Silicones
Silicone materials have been widely used in medicine for over 60 years. Available in a variety of material types, they have unique chemical and physical properties that manifest in excellent biocompatibility and biodurability for many applications. Silicone elastomers have remarkably low glass-transition temperatures and maintain their flexibility over a wide temperature range, enabling them to withstand conditions from cold storage to steam autoclaving. They have high permeability to gases and many drugs, advantageous respectively in wound care or in transdermal drug delivery. They have low surface tension and remarkable chemical stability, enabling biocompatibility and biodurability in many long-term implant applications.
However, versatile as they are, present-day silicone materials still have limitations. The mechanical properties of silicone elastomers, such as tensile strength or tear resistance, are somewhat lower than for other implantable elastomers such as polyurethanes (although generally speaking, polyurethanes are less biodurable). While resistant to a wide array of chemical environments, silicone elastomers are susceptible to degradation in very strongly basic or acidic conditions, such as those found in the stomach. Like all hydrophobic implant materials, silicones are quickly coated with proteins when placed in tissue contact; and a scar tissue capsule forms to surround an implant during wound healing, walling it off from the host. Additionally, silicone elastomers are thermosetting materials, requiring different processing from conventional thermoplastics, which can on occasion be seen as a drawback.
Chemical Structure and Nomenclature
Silicones are a general category of synthetic polymers whose backbone is made of repeating silicon-to-oxygen bonds. In addition to their links to oxygen to form the polymeric chain, the silicon atoms are also bonded to organic groups, typically methyl groups. This is the basis for the name “silicones,” which was assigned by Kipping based on their similarity with ketones, because in most cases there is on average one silicone atom for one oxygen and two methyl groups (Kipping, 1904). Later, as these materials and their applications flourished, more specific nomenclature was developed. The basic repeating unit became known as “siloxane,” and the most common silicone is polydimethylsiloxane, abbreviated as PDMS.
Many other groups (e.g., phenyl, vinyl, and trifluoropropyl) can be substituted for the methyl groups along the chain. The simultaneous presence of organic groups attached to an inorganic backbone give silicones a combination of distinctive properties, making their use possible as fluids, emulsions, compounds, resins, and elastomers in numerous applications and diverse fields. For example, silicones are common in the aerospace industry, due principally to their low and high temperature performance. In the electronics field, silicones are used as electrical insulation, potting compounds, and other applications specific to semiconductor manufacture. Their long-term durability has made silicone sealants, adhesives, and waterproof coatings commonplace in the construction industry. Excellent biocompatibility makes many silicones well suited for use in numerous personal care, pharmaceutical, and medical device applications (see Chapter II.5.18).
Historical Milestones in Silicone Chemistry
Key milestones in the development of silicone chemistry, thoroughly described elsewhere by Lane and Burns (1996), Rochow (1945), and Noll (1968), are summarized in Table B.1.
TABLE B.1 Key Milestones in the Development of Silicone Chemistry
| 1824 | Berzelius discovers silicon by the reduction of potassium fluorosilicate with potassium: 4K + K2SiF6 → Si + 6KF. Reacting silicon with chlorine gives a volatile compound later identified as tetrachlorosilane, SiCl4: Si + 2Cl2 → SiCl4. |
| 1863 | Friedel and Crafts synthesize the first silicon organic compound, tetraethylsilane: 2Zn(C2H5)2 + SiCl4 → Si(C2H5)4 + 2ZnCl2. |
| 1871 | Ladenburg observes that diethyldiethoxysilane (C2H5)2Si(OC2H5)2, in the presence of a diluted acid gives an oil that decomposes only at a “very high temperature.” |
| 1901–1930s | Kipping lays the foundation of organosilicon chemistry with the preparation of various silanes by means of Grignard reactions and the hydrolysis of chlorosilanes to yield “large molecules.” The polymeric nature of the silicones is confirmed by the work of Stock. |
| 1940s | In the 1940s, silicones become commercial materials after Hyde of Dow Corning demonstrates the thermal stability and high electrical resistance of silicone resins, and Rochow of General Electric finds a direct method to prepare silicones from silicon and methylchloride. |
Nomenclature
The most common silicones are the trimethylsilyloxy end-blocked polydimethylsiloxanes, with the following structure:
These are linear polymers and liquids, even for large values of n. The main chain unit, –(Si(CH3)2O)–, is often represented by the letter D for (CH3)2SiO2/2, because with the silicon atom connected to two oxygen atoms this unit is capable of expanding within the polymer in two directions. M, T, and Q units are defined in a similar manner, as shown in Table B.2.
TABLE B.2 Shorthand Notation for Siloxane Polymer Units
The system is sometimes modified by the use of superscript letters designating nonmethyl substituents, for example, DH = H(CH3)SiO2/2 and Mϕ or MPh = (CH3)2(C6H5)SiO1/2 (Smith, 1991). Further examples are shown in Table B.3.
TABLE B.3 Examples of Silicone Shorthand Notation
Preparation
Silicone Polymers
The modern synthesis of silicone polymers is multifaceted. It usually involves the four basic steps described in Table B.4. Only step 4 in this table will be elaborated upon here.
TABLE B.4 The Basic Steps in Silicone Polymer Synthesis
Polymerization and Polycondensation
The linear [4] and cyclic [5] oligomers resulting from dimethyldichlorosilane [2] hydrolysis have chain lengths too short for most applications. The cyclics must be polymerized, and the linears condensed, to give macromolecules of sufficient length (Noll, 1968).
Catalyzed by acids or bases, cyclosiloxanes (R2SiO)m are ring-opened and polymerized to form long linear chains. At equilibrium, the reaction results in a mixture of cyclic oligomers plus a distribution of linear polymers. The proportion of cyclics depends on the substituents along the Si–O chain, the temperature, and the presence of a solvent. Polymer chain length depends on the presence and concentration of substances capable of giving chain ends. For example, in the KOH-catalyzed polymerization of the cyclic tetramer octamethylcyclotetrasiloxane (Me2SiO)4 ([5] or D4 in shorthand notation), the average length of the polymer chains depends on the KOH concentration:
A stable hydroxy-terminated polymer, HO(Me2SiO)zH, can be isolated after neutralization and removal of the remaining cyclics by stripping the mixture under vacuum at elevated temperature. A distribution of chains with different lengths is obtained. The reaction can also be made in the presence of Me3SiOSiMe3, which acts as a chain end-blocker:
The Me3SiOK formed attacks another chain to reduce the average molecular weight of the linear polymer formed.
The copolymerization of (Me2SiO)4 in the presence of Me3SiOSiMe3 with Me4NOH as catalyst displays a surprising viscosity change over time (Noll, 1968). First a peak or viscosity maximum is observed at the beginning of the reaction. The presence of two oxygen atoms on each silicon in the cyclics makes them more susceptible to a nucleophilic attack by the base catalyst than the silicon of the end-blocker, which is substituted by only one oxygen atom. The cyclics are polymerized first into very long, viscous chains that are subsequently reduced in length by the addition of terminal groups provided by the end-blocker, which is slower to react. This reaction can be described as follows:
or, in shorthand notation:
where n = 4x (theoretically).
The ratio between D and M units defines the average molecular weight of the polymer formed.
Catalyst removal (or neutralization) is always an important step in silicone preparation. Most catalysts used to prepare silicones can also catalyze the depolymerization (attack along the chain), particularly at elevated temperatures in the presence of traces of water.
It is therefore essential to remove all remaining traces of the catalyst, providing the silicone optimal thermal stability. Labile catalysts have been developed. These decompose or are volatilized above the optimum polymerization temperature, and consequently can be eliminated by a brief overheating. In this way, catalyst neutralization or filtration can be avoided (Noll, 1968).
The cyclic trimer (Me2SiO)3 has internal ring tension and can be polymerized without re-equilibration of the resulting polymers. With this cyclic, polymers with narrow molecular weight distribution can be prepared, as well as polymers only carrying one terminal reactive function (living polymerization). Starting from a mixture of cyclics with different internal ring tensions also allows preparation of block or sequential polymers (Noll, 1968).
Linears can combine when catalyzed by many acids or bases to give long chains by intermolecular condensation of silanol terminal groups (Noll, 1968; Stark et al., 1982).
A distribution of chain lengths is obtained. Longer chains are favored when working under vacuum or at elevated temperatures to reduce the residual water concentration. In addition to the polymers described above, reactive polymers can also be prepared. This result can be achieved when re-equilibrating oligomers or existing polymers to obtain a polydimethylmethylhydrogenosiloxane, MDzDHwM.
Additional functional groups can be attached to this polymer using an addition reaction.
All the polymers heretofore shown are linear or cyclic, comprising mainly difunctional units, D. In addition, branched polymers or resins can be prepared if, during hydrolysis of the chlorosilanes, a certain amount of T or Q units are included, which allow molecular expansion in three or four directions, as opposed to just two. For example, consider the hydrolysis of methyltrichlorosilane in the presence of trimethylchlorosilane, which leads to a branched polymer:
The resulting polymer can be described as (Me3SiO1/2)x (MeSiO3/2)y or MxTy, using shorthand notation. The formation of three silanols on the MeSiCl3 by hydrolysis yields a three-dimensional structure or resin after condensation, rather than a linear polymer. The average molecular weight depends upon the number of M units that come from the trimethylchlorosilane, which limits the growth of the resin molecule. Most of these resins are prepared in a solvent and usually contain some residual hydroxyl groups. These groups could subsequently be used to cross-link the resin and form a continuous network.
Silicone Elastomers
Silicone polymers can easily be transformed into a three-dimensional network by way of a cross-linking reaction, which allows formation of chemical bonds between adjacent chains. The majority of silicone elastomers are cross-linked according to one of the following three reactions.
1. Cross-linking with radicals
Efficient cross-linking with radicals is achieved only when some vinyl groups are present on the polymer chains. The following mechanism has been proposed for the cross-linking reaction associated with radicals generated from an organic peroxide for the initiation, propagation, and termination steps (Stark et al., 1982):
where ≡ represents two methyl groups and the rest of the polymer chain.
This reaction has been used for high-consistency silicone rubbers (HCRs), such as those used in extrusion or compression and injection molding, which are cross-linked at elevated temperatures. The peroxide is added before processing. During cure, some precautions are needed to avoid the formation of voids by the volatile residues of the peroxide. Post-cure may also be necessary to remove these volatiles, which can catalyze depolymerization at high temperatures.
2. Cross-linking by condensation
Although most-ly used in construction sealants and caulks, condensation-cure silicone materials have also found utility in medical device manufacturing as silicone adhesives (facilitating the adherence to materials of silicone elastomers), encapsulants, and sealants.
One-part products are ready to apply and require no mixing. Cross-linking starts when the product is squeezed from the tube or cartridge and comes into contact with moisture, typically from humidity in the ambient air. These materials are formulated from a reactive polymer prepared from a hydroxy end-blocked polydimethylsiloxane and a large excess of methyltriacetoxysilane.
Because of this excess, the probability of two different chains reacting with the same silane molecule is remote. Consequently, all the chains are end-blocked with two acetoxy functional groups. The resulting product is still liquid and can be packaged in sealed tubes and cartridges. Upon opening and exposing the sealant to room humidity, acetoxy groups are hydrolyzed to give silanols, which allow further condensation to occur.
In this way, two chains have been linked, and the reaction continues from the remaining acetoxy groups. An organometallic tin catalyst is normally used, and the cross-linking reaction requires moisture to diffuse into the material. Accordingly, cure will proceed from the outside surface inward. These materials are called one-part RTV (room temperature vulcanization) sealants, but actually require moisture as a second component. Acetic acid is released as a by-product of the reaction. Problems resulting from the acid can be overcome by using other cure (cross-linking) reactions developed by replacing the methyltriacetoxysilane MeSi(OAc)3 with oximosilane RSi(ON = CR′)3 or alkoxysilane RSi(OR′)3.
Condensation curing is also used in some two-part products where cross-linking starts upon mixing the two components (e.g., a hydroxy end-blocked polymer and an alkoxysilane such as tetra-n-propoxysilane, Si(OnPr)4) (Noll, 1968):
Here, no atmospheric moisture is needed. Usually an organotin salt is used as a catalyst, but it also limits the stability of the resulting elastomer at high temperatures. Alcohol is released as a by-product of the reaction, leading to some shrinkage after cure at room temperature (0.5–2% linear shrinkage). Silicones with this cure system may not be suitable for the fabrication of parts with precise tolerances.
3. Cross-linking by addition
Use of an addition-cure reaction for cross-linking can eliminate the shrinkage problem mentioned above. In addition-cure, cross-linking is achieved by reacting vinyl end-blocked polymers with ≡ Si–H groups carried by a functional oligomer such as described above [6]. A few polymers can be bonded to this functional oligomer [6] (Stark et al., 1982):
where ≡ represents the remaining valences of the Si in [6].
The addition occurs mainly on the terminal carbon and is catalyzed by Pt or Rh metal complexes, preferably as organometallic compounds to enhance their solubility. The following mechanism has been proposed (oxidative addition of the ≡ Si-H to the Pt complex, H transfer to the double bond, and reductive elimination of the product):
where, to simplify, other Pt ligands and other Si substituents are omitted.
There are no by-products with this reaction. Molded silicone elastomer components cured at room temperature by this addition-reaction mechanism are very accurate in terms of dimensional tolerance (i.e., there is no shrinkage). At elevated temperatures, some shrinkage occurs because of the thermal expansion during cure. However, handling these two-part products (i.e., Si–Vi polymer and Pt complex in one component, Si–Vi polymer and SiH oligomer in the other) requires some precautions. The Pt in the complex is easily bonded to electron-donating substances such as amine or organosulfur compounds to form stable complexes with these “poisons,” rendering the catalyst complex inactive and inhibiting the cure.
The preferred cure system can vary by application. For example, silicone medical bonding adhesives use acetoxy cure (condensation cross-linking), while platinum cure (cross-linking by addition) is used for precise silicone parts with no by-products.
4. Elastomer filler
In addition to the silicone polymers described above, most silicone elastomers incorporate “filler.” Besides acting as a material extender, as the name implies, filler acts to reinforce the cross-linked matrix. The strength of silicone polymers without filler is unsatisfactory for most applications (Noll, 1968). Like most other noncrystallized synthetic elastomers, the addition of reinforcing fillers reduces the tackiness of the silicone, increases its hardness, and enhances its mechanical strength. Fillers might also be employed to affect other properties; for example, carbon black is added for electrical conductivity, or barium sulfate to increase radiopacity. These and other materials are used to pigment the otherwise colorless elastomer; however, care must be taken to select only pigments suitable for the processing temperatures and end-use application.
Generally, the most favorable reinforcement is obtained by using fumed silica, such as Cab–O–Sil®, Aerosil®, or Wacker HDK®. Fumed silica is produced by the vapor phase hydrolysis of silicon tetrachloride vapor in a hydrogen/oxygen flame:
Unlike many naturally occurring forms of crystalline silica, fumed silica is amorphous. The very small spheroid silica particles (in the order of 10 nm in diameter) fuse irreversibly while still semi-molten, creating aggregates. When cool, these aggregates become physically entangled to form agglomerates. Silica produced in this way possesses remarkably high surface area (100–400 m2/g), as measured by the BET method developed by Brunauer, Emmett, and Teller (Brunauer et al., 1938; Noll, 1968; Cabot Corporation, 1990).
The incorporation of silica filler into silicone polymers is accomplished prior to cross-linking, by mixing the silica into the silicone polymers on a two-roll mill, in a twin-screw extruder, or in a Z-blade mixer capable of processing materials with this rheology.
Reinforcement occurs with polymer adsorption encouraged by the large surface area of the silica, and when hydroxyl groups on the filler surface lead to hydrogen bonds between the filler and the silicone polymer. In this way, reinforcing filler contributes to the high tensile strength and elongation capability of silicone rubber (Lynch, 1978). The addition of filler increases the already high viscosity of the polymer. Uncured silicone elastomers can have viscosities from 10,000 to well over 100,000 mPa·s. Chemical treatment of the silica filler with silanes enhances its incorporation in, and reinforcement of, the silicone elastomer, resulting in increased material strength and tear resistance (Lane and Burns, 1996) (Figure B.1).
Silicone elastomers for medical applications normally use fumed silica as filler, and occasionally appropriate pigments or barium sulfate. Because of their low glass transition temperatures, these compounded and cured silicone materials are elastomeric at room and body temperatures without the use of plasticizers, unlike other medical materials such as PVC, which might contain phthalate additives.
FIGURE B.1 Silicone elastomer/silica network.
5. Processing of silicone elastomers
In addition to the polymer blend with amorphous silica filler, other ingredients are needed: an initiator or cross-linker plus catalyst. To avoid premature cure during shipment and storage, these ingredients must be separated. Consequently, products for making silicone elastomers are generally supplied as two components or two-part kits, for example, a base and a peroxide paste, or a kit made of Part A containing polymer and catalyst, and Part B containing polymer and cross-linker. These two components are mixed at a fixed ratio at the point of use and formed into the desired shape before cure.
Silicone elastomers are thermosetting materials. They must be formed into the appropriate shape and configuration prior to cross-linking. Unlike a thermoplastic, which can be remelted and formed again, a cured silicone elastomer part cannot be reprocessed. Suitable processing methods for shaping silicone elastomers include casting, extrusion, and molding. The process selected depends on the viscosity of the feedstock elastomer material, and the shape and configuration of the desired cured elastomer product.
High Consistency Rubber (HCR). If very high molecular weight silicone polymers are used (silicone “gums” in the trade), the result is high consistency rubbers, which are desirable as they allow for high tear strengths and tensile elongations. Uncured HCRs are putty-like materials that require high shear equipment for processing. These are usually supplied in two parts to be mixed prior to use, either as a silicone base plus a peroxide initiator or as two-part kit using a Pt cure system. These parts are combined using high shear two-roll mills. The mixed material is then shaped into “preforms” before use in compression, transfer or injection molding at elevated temperature. Compression molding requires the simplest equipment: a preform is inserted in a mold and cured under high pressure at elevated temperature. The preform must be of an approximate shape that corresponds well to the mold cavity. This allows sufficient material to fill the cavity without producing excessive flash, the overflow material that remains attached to the parts from around the edge of the mold. Removal of flash requires post-processing. Transfer molding requires less preparation of the preform: a more precise but simply shaped preform is transferred from a receiving cavity to the mold cavity. Injection molding allows for more automation: an extruder system is used to inject a simple ribbon preform directly into the mold cavity. Typical considerations in this case are controlling the exact amount of material sent to the molding cavity to avoid flash, and maximizing speed but avoiding “scorching” (premature cure before the mold cavity is properly filled). HCRs are also used for extrusion to produce tubing, as they have enough “green strength” or mechanical integrity when leaving a cooled extruder and prior to enter a curing oven. When peroxides are used, post-cure in a well-ventilated oven is necessary to remove peroxide by-products, which could bloom at the surface and can reduce the stability of the cured elastomer. One part HCR materials, for which all ingredients have been premixed by the supplier, are also available, but have limited shelf-life depending on the cure system.
Liquid Silicone Rubber (LSR). If lower molecular weight silicone polymers are used, the silicone polymer/silica blend viscosities are lower, leading to liquid silicone rubber. These LSRs are provided as two-part materials that can be used in liquid injection molding – pumped, metered, mixed, and then directly injected in the molding cavity. Processing is eased by the shear-thinning effect that occurs during pumping and injection, reducing the viscosity of the LSR blend and the injection pressures compared to HCR processing. LSRs are particularly well-suited for long automated production runs. Mixing LSR Parts A and B from two 200-liter drums is typically automated using a static mixer prior to direct injection of the precise amount needed. In contrast with the handling of small quantities of HCR on a two-roll mill, preforming, and molding, LSR processing allows more automation. Yet liquid injection molding requires higher investment in the equipment to control the injected amount (to avoid under- or over-filling the mold) precisely. Precise mold cavity temperature control is needed, as cold material is quickly and repeatedly injected into a hot mold. With LSRs, complex molds are needed, preferably with cold runners to avoid premature cure in the feeding lines between injection cycles, with tight specifications still allowing venting (air escape from the mold cavity during injection). The molds may be equipped with complex ejectors to remove parts quickly at the end of the cure, enabling the processing of the next part without loosing thermal control of the mold. The acquisition cost of liquid injection molding machines and sophisticated molds is usually justified for large production runs, as they provide for more efficiency in terms of mold cycle time, overall processing time, and material usage. Cycle time is dependant on operator skills, equipment, and the part to be cured. Typical conditions for LSRs are 0.3–3.0 seconds injection time, 150–200°C cure temperature, and 3–5 sec/mm thickness cure time, depending on formulations (Sommer, 2003).
Fabricators of silicone elastomer parts should be aware that these LSRs and other addition-cure products contain an inhibitor, a substance that weakly bonds to the platinum catalyst to moderate its activity, permitting sufficient pot life by avoiding premature cure. If contamination occurs with substances capable of bonding more strongly to the platinum catalyst (e.g., amino or thio compounds), the catalyst (which is present in only minute quantities, typically about 10 ppm) may lose activity, resulting in inhibition of elastomer cure.
Room Temperature Vulcanizing (RTV) elastomers. In addition to HCRs and LSRs, which are designed to cure by exposure to heat, other silicone elastomers, known as RTVs, are intended to be cured at room temperature. Typically, RTV elastomers are provided in two-part systems and can be viewed as a variation of the LSRs, but with lower viscosity and less inhibitor. They can be mixed with a spatula and cast after de-airing, and are typically used for laboratory trials and commercially in medical applications for dental impression molding. RTVs are also available as one-part silicone elastomers provided ready to use, usually as adhesives. These materials rely on a condensation reaction and on moisture in the air as the second component to achieve cure.
Silicone Gels
Silicone gels are typically composed of a very lightly cross-linked silicone elastomer whose polymer network has been swollen with silicone fluids; however, these gels contain no silica or other fillers. Medical applications for silicone gels include breast, testicular, and other soft-tissue implants for tissue augmentation or to help restore one’s appearance after cancer surgery. In addition, silicone gel external breast prostheses can be worn in or attached to garments, such as brassieres, for similar purposes. Silicone gel is often supplied in a two-part fluid system and cures via a platinum-catalyzed addition reaction. Parts A and B are mixed at a desired ratio and cured (usually by exposure to elevated temperature) to yield a sticky, cohesive mass of the desired consistency. The consistency of the material can be controlled by the degree of cross-linking, as well as quality and quantity of swelling fluid. After mixing but before curing, the mixture is still liquid and can be pushed through a large gauge needle, enabling the filling of a silicone elastomer implant shell or the thermoformed pouch of an external breast prosthesis made of a thin polyurethane film.
Beyond gel implants and external prostheses, silicone gels find application in skin-contacting sheet goods in wound and scar care.
In addition to gels composed entirely of silicone materials, the gel used in some gel pads for the prevention of pressure sores or for orthotic applications are comprised of a cross-linked silicone polymer network swollen with non-silicone fluids such as mineral oil.
Silicone Adhesives
Three basic types of silicone adhesives are used in medical applications: bonding, pressure-sensitive, and gel.
1. Bonding adhesives. Silicone bonding adhesives are used to attach components together and to seal seams and junctions. Electrical components can also be encapsulated and insulated using silicone bonding adhesive. Silicone bonding adhesives are most commonly formulated as one-part RTV elastomer systems that use a condensation cross-linking reaction, as described earlier in this chapter.
2. Pressure-sensitive adhesives. Silicone PSAs are typically formulated in solvent. A silanol end-blocked
PDMS undergoes a polycondensation reaction with a silicate resin in the presence of ammonia as catalyst. The ammonia is stripped with heat, and usually the solvent is exchanged.
In some applications, a hot melt silicone PSA is used ostensibly without solvent.
Silicone PSAs have properties that make them well-suited for application in the medical area. Besides their biocompatibility, the materials are highly flexible and permeable to moisture vapor, CO2, and oxygen. Silicone PSAs can provide strong adhesion to the skin, facilitating the attachment of hairpieces, prosthetics, and other devices to the body, and they are widely used in transdermal drug delivery. Due to compatibility concerns with amine-containing drugs, an additional class of silicone PSAs have been generated by the further reaction of the PSA with hexamethyldisilazane to convert the pendant ≡ SiOH groups into ≡ SiOSi(CH3)3.
3. Gel adhesives. Silicone gel adhesives, also known as soft skin adhesives, are used in wound care, and are found to be gentler and less traumatic on removal than the pressure-sensitive types. Unlike PSAs, which are typically formulated in solvent, silicone gel adhesives are supplied in solventless two-part systems. In addition to wound care applications, the materials are also used in the treatment of hypertrophic and keloid scars. Evidence suggests this therapy may reduce scar height and appearance (O’Brien and Pandit, 2006).
Silicone Film-in-Place, Fast-Cure Elastomers
In addition to the cured silicone elastomers in skin contact applications, in situ cure materials have been developed. These materials form films when spread or sprayed on the skin, and then undergo RTV cure. Products such as spray-on wound dressings or drug-loaded lotions have been evaluated. The ability of silicone to spread and form films is related to its low surface tension as described in the Physico-Chemical Properties section below (Maxon et al., 2004).
Physico-Chemical Properties
The position of silicon just under carbon in the periodic table led to a belief in the existence of analog compounds where silicon would replace carbon. Most of these analog compounds do not exist, or behave very differently from their carbon counterparts. There are few similarities between Si–X bonds in silicones and C–X bonds (Stark et al., 1982; Corey, 1989; Hardman, 1989; Lane and Burns, 1996).
Between any given element and Si, bond lengths are longer than for the element and C. The lower electronegativity of silicon (χSi ≈ 1.80, χC ≈ 2.55) leads to a very polar Si–O bond compared to C–O. This bond polarity also contributes to strong silicon bonding; for example, the Si–O bond is highly ionic and has a high bond energy. To some extent, these values explain the stability of silicones. The Si–O bond is highly resistant to homolytic scission. On the other hand, heterolytic scissions are easy, as demonstrated by the re-equilibration reactions occurring during polymerizations catalyzed by acids or bases.
Silicones exhibit the unusual combination of an inorganic chain similar to silicates and often associated with high surface energy, but with side methyl groups that are very organic and often associated with low surface energy (Owen, 1981). The Si–O bonds are moderately polar, and without protection would lead to strong intermolecular interactions (Stark et al., 1982). Yet, the methyl groups, only weakly interacting with each other, shield the main chain (see Fig. B.2).
FIGURE B.2 Three-dimensional representation of eicosamethylnonasiloxane, Me3SiO(SiMe2O)7SiMe3 or MD7M.
(Courtesy T. Lane, Dow Corning.)
This shielding is made easier by the high flexibility of the siloxane chain. Barriers to rotation are low, and the siloxane chain can adopt many configurations. Rotation energy around a H2C–CH2 bond in polyethylene is 13.8 kJ/mol, but is only 3.3 kJ/mol around a Me2Si–O bond, corresponding to a nearly free rotation. In general, the siloxane chain adopts a configuration so that the chain exposes a maximum number of methyl groups to the outside, whereas in hydrocarbon polymers, the relative rigidity of the polymer backbone does not allow selective exposure of the most organic or hydrophobic methyl groups. Chain-to-chain interactions are low, and the distance between adjacent chains is also greater in silicones. Despite a very polar chain, silicones can be compared to paraffin, with a low critical surface tension of wetting (Owen, 1981).
The surface activity of silicones is evident in many ways (Owen, 1981):
• Polydimethylsiloxanes have low surface tension (20.4 mN/m) and are capable of wetting most surfaces. With the methyl groups pointing to the outside, this configuration gives very hydrophobic films and a surface with good release properties, particularly if the film is cured after application. Silicone surface tension is also in the most promising range considered for biocompatible elastomers (20–30 mN/m) (Baier, 1985).
• Silicones have a critical surface tension of wetting (24 mN/m), higher than their own surface tension. This means silicones are capable of wetting themselves, which promotes good film formation and surface coverage.
• Silicone organic copolymers can be prepared with surfactant properties, with the silicone as the hydrophobic part (e.g., in silicone glycol copolymers).
The low intermolecular interactions in silicones have other consequences (Owen, 1981):
• Glass transition temperatures are very low (e.g., 146°K for a polydimethylsiloxane compared to 200°K for poly-isobutylene, the analog hydrocarbon).
• The presence of a high free volume compared to hydrocarbons explains the high solubility and high diffusion coefficient of gas into silicones. Silicones have a high permeability to oxygen, nitrogen, or water vapor, even though liquid water is not capable of wetting a silicone surface. As expected, silicone compressibility is also high.
• The viscous movement activation energy is very low for silicones, and their viscosity is less dependent on temperature compared to hydrocarbon polymers. Furthermore, chain entanglements are involved at higher temperature, and contribute to limit viscosity reduction (Stark et al., 1982).
Conclusion
Polydimethylsiloxanes are often referred to as silicones. They are used in many applications because of their stability, low surface tension, and lack of toxicity. Methyl group substitution or introduction of tri- or tetrafunctional siloxane units leads to a wide range of structures. Polymers are easily cross-linked at room or elevated temperature to form elastomers, without losing their advantageous properties.
Acknowledgments
Part of this chapter (here revised) was originally published in Chimie Nouvelle, the journal of the Société Royale de Chimie (Belgium), Vol. 8 (30), 847 (1990) by A. Colas and is reproduced here with the permission of the editor.
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