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