Silicone cure systems

Description

Any of a large group of semi-inorganic polymers based on siloxanes. Silicones were first discovered by F.S. Kipping in England in 1904, but were not commercially produced as polymers until 1943 by Dow Corning and 1946 by General Electric. They were called silicones because their empirical formula (R2SiO) is similar to that for ketones (R2CO) (Lewis, 1993). Once cured silicone resins can be liquids, gels, and elastomers as well as solid thermoplastic or thermosetting resins. The curing process, however, can be done by several methods, each resulting in different cure times, volatiles emission and product characteristics.

Chemically, a silicone elastomer can be cured into a solid via an 'addition' reaction or a 'condensation' reaction. An addition reaction may use catalysts (platinum or tin) but the condensation process also requires oxygen and moisture. The addition of UV light or heat will decrease the cure time. Silicone polymers cured in open air at room temperature can be called RTV (Room Temperature Vulcanization).

Types of Silicone cures include:

Classification	Mechanism	By-Product	Effect	Comments	Examples
Acetoxy cure	Acetoxy	Acetic acid	Corrosive	May contain tin catalyst that can be harmful to electronics
Neutral cure	Oxime	Ketoxime	Mildly corrosive	May contain tin catalyst that can be harmful to electronics	Dowsil 580, Dowsil 737
Neutral cure	Alkoxy	Methanol	Non corrosive	Usually one component	Dpwsil 739 RTV, Dowsil 791, Dowsil 993, Dowsil 795, Dowsil 3145, Dowsil 7091, Silastic 1080
Neutral cure	Acetone	Acetone	Non corrosive
Neutral Cure	Amine	Ammonia	Reactive with organic materials	Usually used in combinations with alkoxy systems

Advantages and Disadvantages of the types of cure systems include:

1-Part Condensation Cure	2-Part Condensation Cure	1-Part Addition Cure	2-Part Addition Cure
Easy to apply	Tolerant catalyst ratios	Easy to apply	Excellent deep section cure
No mix-no mix error	Low risk of cure failure	No mix-no mix error	Pot life extension
Easy dispensing	Deep section cure	No cure reversal	No cure reversal
Max 10mm thickness	Cure acceleration possible	Adhesion hard to achieve	Cure acceleration with heat
Fixed cure speed	Small increase in shrinkage	Prone to cure failure	Optically clear available
Narrow viscosity range	Cure reversal possible	Shorter shelf life	Low shrinkage
Cure reversal possible			Prone to cure failure
			Requires accurate mix
			Good adhesion difficult

Synonyms and Related Terms

polysiloxane; silicona (Esp.); silicone (Fr.); silicone (It.); silicone (Port.); organosiloxane; silicones

Examples: Silastic [Dow];

Resources and Citations

• 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