7 Properties of Silicone | Xometry

1. Silicones Are Non-Chemically Reactive and Have Low Thermal Conductivity

It takes a lot of energy to break the bonds of the silicon-oxygen chains that form the polymeric skeleton of silicone molecules. Because most chemicals that silicones come into contact with do not have enough energy to overcome the silicone molecule’s resistance to change, there is little driving force for chemical reactions. Thus, silicone is considered generally non-chemically reactive. These stable silicone bonds are the basis for many of the desirable properties of silicone.  

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Silicones generally have low thermal conductivity. This is because the molecular structure of silicone tends to impede the transfer of heat vibrations from one molecule to the next. This can be desirable for some uses of silicone, like oven mitts. But in other situations, the inability to efficiently transfer heat is a problem. In that case, thermally conductive fillers can be added to the silicone formulation to improve the heat transfer required for the intended application.

2. Silicone Toxicity Is Also Low

Silicone is considered a very safe material for human health. Food-grade and medical-grade silicone compounds are FDA-approved for use in contact with the food we eat every day, and even for long-term implantation in the human body. As with all chemicals, however, it is important to use silicone products according to the manufacturer’s instructions to maintain the highest levels of safety.

3. Silicone Has the Ability To Repel Water and Form Watertight Seals

Silicone is what is known as a “hydrophobic” material. It repels water. This is because the methyl groups attached to the silicon-oxygen polymer chain are non-polar in nature. It is not attracted to water molecules. Its low surface energy gives water molecules no way to spread over and penetrate the silicone surface. Instead, water beads up and runs off. This great water repellency, combined with silicone’s ability to form a tight adhesive bond with many surfaces, means that silicone sealing products can form seals that last for decades.

4. Silicone Has a High Resistance to Oxygen, Ozone, and UV Light

The silicon-oxygen bonds in silicones are more stable than those between the carbon atoms in organic polymer chains. The amount of energy that can be provided by UV light is higher than that needed to break down C-C bonds, but not enough to damage Si-O bonds. This is why silicones are more resistant to UV light and oxidation than carbon-based plastics. Silicones can be used for components exposed to harsh outdoor weather.

5. Silicone Is Electrically Insulative As Well as Conductive

Standard silicone rubber is naturally an insulator. It has no free electrons available to carry a positive or negative charge. That is perfect for many applications, especially in the medical industry, where the insulative property is critical. But silicone can be coaxed into being conductive enough for applications like gaskets and static shields. This is accomplished by adding fillers to the silicone such as carbon, silver, or other conductive materials. 

6. Silicone Has Excellent Gas Permeability and Thermal Stability

Silicone’s molecular chains contain openings that are large enough to allow gas molecules to pass through, but not water molecules. This combination of water repellency and gas permeability yields coatings that give us the luxury of water-resistant, breathable fabrics.

A key property of silicone rubbers is their thermal stability. Silicone maintains its critical mechanical and physical properties within defined design limits over a large range of temperatures. Depending on the exact nature of the silicone product, the minimum service temperature in the air may be as low as -136 °C, and the maximum as high as 316 °C.

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7. Silicone Has Superior Resistance to Organic Compound Solvents

Silicone is resistant to attack by most chemicals due to its non-reactive structure and low surface energy. However, a few inorganic chemicals, notably sulfuric and hydrofluoric acids at high concentrations, will damage silicones. Among organic compounds which might act as solvents, toluene, mineral spirits, gasoline, and carbon tetrachloride cause deterioration in silicones only after prolonged exposure.

Why You Should Use Silicone as a Material

The properties of silicone offer numerous benefits. Here are a few ways that both manufacturers and end-users can benefit from its unique characteristics:

• Silicones are safe. They are one of the go-to materials for making implantable medical devices.
• Silicones have good dimensional stability. They maintain their strength and shape over a wide range of temperatures.
• Silicones are chemically non-reactive in most environments. They do not degrade under exposure to moisture, fuels and oils, heat, cold, salt spray, or ultraviolet light.
• Silicones can take any shape. Silicones come as liquids, viscous pastes, rubbery sheets, and hard, plastic-like consistencies. They can be used as coatings, made into injection- or compression-molded parts, stamped, formed into tubing, and used as lubricants. 

Where is Silicone Typically Prepared?

Chemical factories produce raw silicone materials. These facilities can process the required chemical precursors and catalysts in bulk, repeatedly, and precisely. 

In the factory, silica (usually from sand) is turned into silicon in a high-temperature reaction with carbon. Through a sequence of complex chemical reactions, the silicon is then combined with water and methanol to form basic silicone products packaged as liquids, gels, or sheets. Component fabricators then apply the appropriate molding or other industrial processes to create final products for customers.

What is Silicone's Chemical Composition?

Silicone, technically referred to as “polysiloxane,” consists of a chain of alternating silicon and oxygen atoms. Silicon atoms, like carbon atoms, have space for four covalent bonds with other atoms. Two of those spots are occupied by oxygen atoms, one on each side. The other two spots will bond with other molecular groups that are available. The general chemical formula for silicone is [R2SiO]n. “R” indicates whatever groups are attached to the silicon atoms. The most common, basic silicone has methyl groups (CH3) attached in the available slots, yielding an inorganic (since it is not carbon-based) polymeric structure. Technically, this is called polydimethylsiloxane.

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Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.

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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.

This guideline includes general information on the characteristics and common uses of sealants and identifies typical problems associated with this material along with common causes of its deterioration. For guidance on replacing deteriorated sealant, see -03-R.

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Introduction

Characteristics of sealants:

• Typically made of synthetic elastomeric materials.
• May be single-component (no mixing required), or multicomponent (mixing required).
• Good adhesion
• Good cohesion
• Good elasticity
• Good weathering characteristics
• Common types of sealants include silicone, polyurethane, polysulfide, acrylic, latex and butyl-based.

Typical Uses:

• Typical historical and current uses for sealants include:
• Prior to , oil- and resin-based caulks were the most common building joint sealants; typically used in bearing masonry construction; these were not considered elastomeric sealants due to their limited movability.
• More elastic sealants were used after due to the popularity of curtain wall construction, which tends to move more than masonry construction.
• Polysulfide sealants: First widely used elastomeric sealant.
• Silicone sealants: First developed in the s as two-component products; first silicone building sealant (one component) was developed by Dow Corning about ; typically used for nonporous surfaces with a high factor of movement; common in metal and glass cladding systems.
• Silicone sealants were also used for structural joints in storefronts in place of mullions; this was common in the s.
• Butyl sealants: available in construction in the mid-s.
• Acrylic sealants: Available in the s; first acrylic sealant for buildings developed by Tremco Manufacturing Company in ; typically used in small-scale construction under conditions requiring limited movement.
• Urethane sealants: Typically used in joints requiring abrasion resistance; first ones were multicomponent types; typically used for porous surfaces with a high factor of movement such as cladding joints.
• Latex sealants: Available in the s; typically used in light building construction and residential construction under conditions requiring limited movement.
• Isobutyl-based sealants: Typically used for glazing joints.
• Sealants are commonly used in joints between individual stone or metal panels, between stone panels and flashing, at expansion and coping joints in masonry, around window and door openings, and in joints at horizontal surfaces.
• Butt-joints are the most commonly used, but other types include fillet joints, lap joints, glazing beads, and glazing heel beads.

Natural or inherent problems:

• Staining: Common with silicone sealants; visible as dirt on the sealant, or staining on the adjacent masonry where the sealant’s plasticizer has migrated into the substrate.
• Weathering: Deterioration of sealants can be caused by prolonged exposure to water, ultraviolet light, and freeze- thaw cycles; evidence of weathering appears in the form of chalking, discoloration, cracking, or softening.
• cohesive failure: this means deterioration of the internal integrity of the sealant; cracking parallel to the interface of the joint is an indication of this type of failure

Vandalism or human-induced problems:

• Loss of Adhesion: When the sealant separates from the substrate; this may be caused by the presence of coatings or contaminants that prevent proper adhesion; adhesion is also a problem - especially with silicone sealants - if exposed to prolonged periods of wetting; poor adhesion may also result from poor surface preparation, incompatibility of substrate with selected sealant, or incompatibility of old sealant with new sealant - understanding of the substrate and sealant properties is essential to sealant performance.
• Inappropriate Choice of Sealant and Improper Joint Design: Correct installation of the bond-breaker tape or compressible foam backer rod is important in preventing the sealant from adhering to the sides of the joint; temperature is also important when installing sealant - it shouldn’t be too hot or too cool, otherwise the width of the joint will not permit the installed amount of sealant to accommodate expansion and contraction.
• Uncured Sealant: Most common in multicomponent urethane sealants; sealant that is uncured is often due to incomplete or improper mixing of the sealant components, or from using materials that have outlived their shelf life.
• Bubbling and Blistering of the Surface: Common in single- component and multi-component urethane sealants; in single- component types, this can caused by curing at high temperatures or high humidity; in multi-component types, this can be caused by curing at high temperatures, which affects both the cure and durability of the sealant.