PDMS (polydimethylsiloxane) is a crystalline polymer, with a melting point, (Tm) near -40 °C and a glass transition temperature (Tg) near -125 °C.
Polysiloxanes are generally recognized as the newest generic class of high performance protective coating and include coating types based on inorganic siloxane and organic-inorganic siloxane hybrids.
Polysiloxane coatings based on pure inorganic siloxane binder are curable at ambient temperature and have high solids and low VOC, excellent temperature resistance and good resistance to certain acids and solvents. They are well suited for high-heat and selected chemical lining applications. Semigloss highly weatherable topcoats have also been formulated using this technology.
In coating compositions, the typical resins containing the silicon-oxygen bond as repeating unit in the backbone are silicates, silicones, oxysilanes and siloxanes.
The term polysiloxane can include silicones but it is used herein in its broadest sense, that is, any polymeric structure that contains repeating silicon-oxygen groups in the backbone, side chains or cross links regardless of the substitution on the silicon atom. The presence of certain organic groups attached to the silicon atom in silicone and polysiloxane binders moderates physical, mechanical and chemical properties, typically in an advantageous way.
HISTORY
Inorganic Siloxane Coatings
Inorganic silicon based polymers have been used as coating binders for more than 50 years and there are a number of excellent reviews on the subject. Heat curable zinc silicate primers were first introduced in the 1940’s.
Waterborne zinc silicate primers that required post-cure with acid solution were commercialized in the 1950’s. Solvent based, ambient cure zinc-rich ethyl silicate primers became available in the 1960’s. Zinc silicates provide the highest level of corrosion protection.
Silicone based coating formulations were developed during the same time period. The principal applications for these coatings included protective coatings for exhaust stacks, heat exchangers, boilers, ovens and furnaces, mufflers, cookware and aircraft components.
Silicate binders are not considered acceptable for broad use in coatings because of their poor film properties. Heat curing limited the practicality of silicone coatings, particularly on large structures.
A significant advance was made in 1978 with the introduction of ambient cure binders based on the hydrolysis of trialkoxysilane.
Subsequent development of new silane and silicate precursors led to formulation of polysiloxane coatings with improved film properties. Well formulated polysiloxane coatings based on these binders are capable of providing long term resistance to heat in excess of 1400F (760 °C) and are resistant to continuous immersion in virtually all solvents and most organic and mineral acids.
Chemistry
The excellent heat, ultraviolet light and chemical resistance properties of inorganic siloxane binders make them obvious choices for improving the properties of organic coatings.
Formulators have tried to utilize alkyl silicate and silicone resin chemistry for this purpose but success was limited by the peculiarities of their film forming mechanisms.
Significant progress has been made in the last 5 to 8 years with the patenting of a range of methods for modifying a variety of organic binders with inorganic siloxane to produce hybrids with unique combinations of properties.
Formulators select the appropriate type and ratio of organic and inorganic siloxane constituents in an effort to achieve a balanced set of application and performance properties.
Oxysilane and silicone resin precursors are selected for cure speed, degree of cross-linking, balanced film properties and compatibility with organic resin constituents.
The types of silicon-based materials used are typically alkoxy or silanol functional silicone resin intermediates ranging in molecular weight from 600 to 1000 and various types of organofunctional oxysilanes.
An important feature of the oxysilane and silicone resin precursors is their very low viscosity. This feature has enabled development of very high solids coatings that meet all current and likely future regulatory requirements for volatile organic content.
Organic resin precursors are generally chosen for their principal performance feature. For example, an aromatic epoxy resin would be used for applications requiring chemical resistance and an acrylic resin would be chosen for use in a coating requiring good weatherability.
It has been found that organic-inorganic siloxane hybrid binders containing 20-50% organic resin give optimum performance in terms of film formation, adhesion, mechanical properties and chemical, corrosion and weathering resistance.
Lower levels of organic resin result in coating films that exhibit undesirable properties e.g., low impact resistance and flexibility and loss of adhesion on aging. Higher levels of organic modification detract from important polysiloxane characteristics like resistance to ultraviolet light and oxidation.
New Organic – Inorganic Siloxane Hybrid Technology
Siloxanes are one of the most rapidly expanding areas of materials research and coating development. The versatility of siloxane chemistry allows the formation of siloxane hybrids with a large range of organic polymers. The versatility of siloxane chemistry extends to adhesives, sealants and composites.
Polydimethylsiloxane (PDMS) is viscoelastic, meaning that at long flow times (or high temperatures), it acts like a viscous liquid, similar to honey.
However, at short flow times (or low temperatures), it acts like an elastic solid, similar to rubber. In other words, if some PDMS is left on a surface overnight (long flow time), it will flow to cover the surface and mold to any surface imperfections.
However, if the same PDMS is rolled into a sphere and thrown onto the same surface (short flow time), it will bounce like a rubber ball.
Although the viscoelastic properties of PDMS can be intuitively observed using the simple experiment described above, they can be more accurately measured using dynamic mechanical analysis.
This involves using a specialized instrument to determine the material's flow characteristics over a wide range of temperatures, flow rates, and deformations. Because of PDMS's chemical stability, it is often used as a calibration fluid for this type of experiment.
The shear modulus of PDMS varies with preparation conditions, but is typically in the range of 100 kPa to 3 MPa. The loss tangent is very low (tan δ ≪ 0.001).
Polymers containing silicon in the main chain are noteworthy as high-performance and functional materials.
Polysiloxanes with excellent low-temperature flexibility and high-temperature stability derived from their siloxane bonds have been used widely as elastomers and plastics in various industries.
Polysiloxanes have also been studied as useful materials for medical applications, photolithography, and polymer supports for liquid crystal compounds, since these polymers have further attractive characteristics such as good transparency, oxygen permeability, flexibility, resistance to oxygenreactive ion etching, and so on.
Polysiloxanes have been ordinarily synthesized by the anionic ring-opening polymerization of cyclic siloxane monomers.
Due to their versatility in chemistry, siloxanes can be included in polymeric structures having a large variety of molecular architectures: cycles, linear, segmented, block and graft copolymers, telechelic or side-functionalized oligomers and polymers, crosslinked or branched structures etc.
The siloxane polymers have, beside many useful properties, two major drawbacks: poor mechanical strength and high cost.
By combining siloxane units with different organic backbones, in different architectures, improved materials can be obtained.
Thus, siloxane-containing copolymers found a large area of applications, such as; surfactants, membranes, lubricants, photoresists, adhesives, agents to improve blood compatibility, water repellency, flow or heat resistance, etc.
Continuous efforts are focused nowadays to find new synthetic routes for siloxane-organic copolymers or to obtain new structures by “classical” methods, to investigate their properties, and find applications according to the most up-to-date knowledge.
Synthetic techniques have advanced to the point where almost any conceivable block copolymer architecture can be made, and furthermore that the chemical composition of each block can be selected as desired.
Activated dimethicone, a mixture of polydimethylsiloxanes and silicon dioxide (sometimes called simethicone), is used in Over-the-counter drug as an anti-foaming agent and carminative.
.Dimethicone is also used widely in skin-moisturizing lotions, listed as an active ingredient whose purpose is "skin protection." Some cosmetic formulations use dimethicone and related siloxane polymers in concentrations of use up to 15%. The Cosmetic Ingredient Review's (CIR) Expert Panel, has concluded that dimethicone and related polymers are "safe as used in cosmetic formulations."
PDMS has been used in the aerospace industry as a heat tile on reentry vehicles.
PDMS is commonly used as a stamp resin in the procedure of soft lithography, making it one of the most common materials used for flow delivery in microfluidics chips.
The process of soft lithography consists of creating an elastic stamp, which enables the transfer of patterns of only a few nanometers in size onto glass, silicon or polymer surfaces.
With this type of technique, it is possible to produce devices that can be used in the areas of optic telecommunications or biomedical research. However, this process still cannot be used for the industrial production of electronic components. In fact, the patterns are obtained by the process of stamping thanks to a shape (or stamp).
This stamp is produced from the normal techniques of photolithography or electron-beam technology. The resolution depends on the mask used and can reach 6 nm.
The process of soft lithography consists of creating an elastic stamp, which enables the transfer of patterns of only a few nanometers in size onto glass, silicon or polymer surfaces.
With this type of technique, it is possible to produce devices that can be used in the areas of optic telecommunications or biomedical research. However, this process still cannot be used for the industrial production of electronic components. In fact, the patterns are obtained by the process of stamping thanks to a shape (or stamp).
This stamp is produced from the normal techniques of photolithography or electron-beam technology. The resolution depends on the mask used and can reach 6 nm.
The design of degradable polymers that exhibit complex degradation profile has received increasing attention as a mean to tailor materials for a number of intended applications, for example, gene deliver carriers, materials for drug delivery and biodegradable surgical devices, recyclable or environmental degradable materials.
Silicone compounds are considered to be environmental persistent. The silicones can be incorporated as small segments in siloxane-organic copolymers, able to break in enzymatic degradable oligomers.
Silicone compounds are considered to be environmental persistent. The silicones can be incorporated as small segments in siloxane-organic copolymers, able to break in enzymatic degradable oligomers.
There is also interest in the utility of polysiloxane as elastomers, sensor materials, and polymer membranes. They are attractive candidates for lower temperature sealants and adhesives.
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