Fluoroelastomer là vật liệu đàn hồi đặc biệt, được dùng chủ yếu trong các ứng dụng yêu cầu tính kháng mài mòn, kháng hóa chất, kháng nhiệt tốt (tiêu biểu là các đệm làm kín). Phần trích bên dưới giới thiệu tính chất cơ bản và thành phần hóa học của fluoroelastomer.
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To exhibit elastomeric behavior, a polymer must be flexible and recover from substantial deformation at temperatures above about 0°C. This requires the polymer to be substantially amorphous, and above its glass transition temperature, so that chain segments have adequate mobility to allow the material to return to its original state after stress is removed. Ordinarily the polymer is cross linked to form a three dimensional network with tie points between chains to minimize irreversible flow under stress. The driving force for recovery is the tendency of chain segments to return to the more disordered state with higher entropy when the stress causing a deformation is removed. Generally, fluorocarbon chains are relatively stiff compared to hydrocarbons, so fluoroelastomers exhibit rather slow relaxation and recovery from strain (i.e., leathery rather than highly resilient behavior). Thus, most fluoroelastomers are used in static, rather than dynamic, applications.
Fluorocarbon elastomers are copolymers made up of two or more major monomer units. One or more monomers give straight chain segments, which would tend to crystallize if long enough. A monomer with a bulky side group is incorporated at intervals to break up the crystallization tendency and produce a substantially amorphous elastomer. Commercial fluorocarbon elastomers are made by free radical polymerization of vinyl monomers. Monomers used in straight chain segments include: vinylidene fluoride (VDF), CH2=CF2; tetrafluoroethylene (TFE), CF2=CF2; and ethylene (E), CH2=CH2. Monomers that provide bulky side groups include hexafluoropropylene (HFP), CF2=CF–CF3, perfluoro(methyl vinyl ether) (PMVE), CF2=CF–O–CF3, and propylene (P), CH2=CH–CH3. The combinations of monomers used must produce substantially amorphous copolymers with glass transition temperatures low enough for elastomeric behavior at temperatures encountered in practical use.
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Trích đăng từ sách Fluoroelastomers Handbook: The Definitive User's Guide and Databook, Albert L. Moore, William Andrew, trang 3 – 5, 2006
Nguồn: www.books.google.com.vn
(vtp-vlab-caosuviet)