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.
…
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.
…
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)
