Silicone cure systems

Description

Silicone resins are a class of polymers whose backbones consist of alternating silicon and oxygen atoms, with various organic groups attached to the silicon atoms. Due to the high bond energy of the silicon-oxygen bond, silicone resins exhibit outstanding properties such as excellent thermal stability, electrical insulation, chemical resistance, flexibility, and aging resistance. However, these properties is inseparable from the curing process of silicone resins. Curing refers to the process by which a resin transforms from a liquid or viscous state into a solid state through chemical reactions or physical interactions. The chemical reactions can use either 'addition' or 'condensation ' mechanisms. Once cured silicone resins can be liquids, gels, and elastomers as well as solid thermoplastic or thermosetting resins.

Some cure systems can be described in numerous ways, such as:

• Thermal curing: Raising the temperature accelerates the speed at which molecules can crosslink. These types of silicone resins have better adhesion and electrical properties and are used for coatings, laminates, and adhesives. The curing temperature typically ranges from 100-200C and the curing time depends on the resin type and thickness.
• Room temperature curing: Many silicone polymers cure in open air at room temperature are often called RTV (Room Temperature Vulcanization) silicones. There are two main type of RTV silicones.

1. One component: The pre-polymer silicon compounds contains easily hydrolyzable end groups, such as acetates, that are designed to crosslink when exposed to water vapor in the air. The single component materials are easy to apply as no mixing is needed, however, their thickness must be less than 10 mm. These systems also have a shorter shelf life, since they cross-link without the aid of a catalyst.
2. Two component: A catalyst, like tin, is used in the polymerization reaction. These may be pre-mixed or require mixing prior to use These resins are good for thick applications since the entire sample will cure evenly and quickly with low shrinkage. However, they often have short shelf lifespans.

Table 1: Types of hydrolyzable end groups (crosslinkers) that initiate cross-linking for silicone resins when exposed to air and moisture.

Classification	Crosslinker	VOC emissions	Advantages	Disadvantages	Typical applications
Acetoxy cure	Acetoxy	Acetic acid	Fast vulcanization speed; good mechanical strength and long term stability; high transparency	Harmful emissions; corrosive to metals; may contain metal catalyst; poor adhesion to concrete	Roofing, general construction (sealing windows, plumbing, vents, etc)
Neutral cure	Oxime	Ketoxime	Short drying time (lengthy to full cure); good storage stability	Slightly unpleasant emission odor; may contain metal catalyst; poor adhesion (primer required);	Adhesives, sealants, general construction (weatherproofing, masonry, sealing metals, etc.)
Neutral cure	Alkoxy	Methanol	Considered environmentally friendly; low odor; non-corrosive; high strength	Long surface dry time; slow internal curing; poor storage stability; poor adhesion	Widely used in electrical, automotive and architectural for insulation, gaskets, sealants and shock absorbers
Neutral cure	Acetone	Acetone	Non-corrosive, fast cure time, low odor, Low VOC, secure silicone to silicone materials	Incompatible with polycarbonates and acrylics, thick sections sure slowly, potentially flammable	Widely used for insulation, gaskets, sealants and shock absorbers; also used in marine applications
Neutral Cure	Amine	Ammonia	Exceptional adhesion, high flexibility, excellent wear resistance	Strong cure odor, reactive with organic materials; usually used in combinations with alkoxy systems	Textile manufacturing, industrial and marine coatings; specialty lubricants

• Most commercial silicone caulks and sealants are acetoxy-based; these should not be used in museums thus please check the MSDS sheet prior to purchase.
• Dow makes an extensive set of neutral cure silicon adhesives and sealants. See Dowsil silicone sealants

Note: While curing systems make significant contributions to the final properties on the silicone resin, they are not the only contributor. Formulations for adhesives, and sealants (coatings) are sometimes made with the pure silicone resin, but more often mixed with solvents, plasticizers, adhesion promotors, fillers, and/or other synthetic resins.

Table 2. Typical one-component Silicone sealant formulations (modified from Special Chem)

Component	Example	Function
Polymer	Hydroxyl (-SiOH)	Molecule backbone
Reinforcing fillers	Fumed silica, precipitated calcium carbonate, carbon black	Thixotropic reinforcing agents, adjustments of viscosity and mechanical properties
Non-reinforcing fillers	Ground calcium carbonate	Reduce cost; adjust rheology
Crosslinkers	See Table 1	Crosslinking of the polymeric component
Catalysts	Organic complexes of Sn, Ti, Pt, Zn, Rh	Cure the network and control the rate of cure
Adhesion promoter	Organoosilanes	Enhance adhesion
Other additives	Pigments, plasticizers, driers, modifiers, biocides, etc	Add color, adjust properties, prolong shelf life, etc

Resources and Citations

• SpecialChem: Formulating RTV Silicone Sealants
• Silicone Technologies: Cure Systems
• Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
• Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
• The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
• M.Kaufman, The First Century of Plastics, The Plastics and Rubber Institute, London, 1963
• History of Plastics: www.nswpmith.com.au/historyofplastics.html - discovered by F.S. Kipping in 1904
• Art and Architecture Thesaurus Online, http://www.getty.edu/research/tools/vocabulary/aat/, J. Paul Getty Trust, Los Angeles, 2000
• Bossin: Curing Methods for Silicone Resins