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Heading 3910 : Silicones in primary forms.
The silicones of this heading are non‑chemically defined products containing in the molecule more than one silicon‑oxygen‑silicon linkage, and containing organic groups connected to the silicon atoms by direct silicon‑carbon bonds.
They have a high stability and may be either liquid, semi‑liquid or solid. The products include silicone oils, greases, resins and elastomers.(1) Silicone oils and greases are used as lubricants remaining stable at high or low temperatures, as water‑repellent impregnating products, as dielectric products, as foam inhibitors, as mould release agents, etc. Lubricating preparations consisting of mixtures containing silicone greases or oils fall in heading 27.10 or 34.03 as the case may be (see corresponding Explanatory Notes). (2) Silicone resins are used mainly in the manufacture of varnishes, insulating or waterproof coatings, etc., where stability at high temperature is required. They are also used in the preparation of laminates with glass fibre, asbestos or mica as the reinforcing material, as flexible moulds and for electrical encapsulation. (3) Silicone elastomers, although not covered by the definition of synthetic rubber in Chapter 40, have some extensibility which is not changed by high or low temperatures. This property renders them suitable for manufacture into washers or other packings for appliances submitted to high or low temperatures. An application in the medical field is the manufacture of automatic brain valves used in cases of hydrocephalus.
For the classification of polymers (including copolymers), chemically modified polymers and polymer blends, see the General Explanatory Note to this Chapter.
The heading excludes silicones complying with the conditions of Note 3 to Chapter 34 (heading 34.02).
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