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Organic Peroxide
Organic peroxides are used primarily as polymerization
initiators, curing agents and crosslinking agents for synthetic
resins and synthetic rubbers. More recently, applications have become
more diverse. Based on the free radical reaction, for example, organic
peroxides are also being used as resin modifiers and flame retarders.
Organic peroxides contain the -O-O- bond in their molecular structure,
and under certain conditions are easily decomposed. Their hazardous
nature requires thorough care and special technical skills when
handling.
General Properties of Organic Peroxides
Organic peroxides may generally be considered
as derivatives of hydrogen peroxides (H2O2) in which one or both of
the hydrogen atoms are replaced by an organic group. Organic peroxides
contain the -O-O- bond within the molecular structure, and the chemical
properties of the peroxides originate from this bond. Organic peroxides
may be classified by chemical structure as follows:
There are various types of organic peroxides, e.g.
refrigerated organic peroxides undergoing self-accelerating thermal
decomposition below room temperature, stable organic peroxides which
do not generate appreciable amounts of free radicals unless heated
to high temperatures, and/or some peroxides which generate radicals
which react with an accelerator.
Action and Application of Organic Peroxides
Organic peroxides can be decomposed thermally
at relatively low temperatures. They also easily produce free radicals
in reaction to reducing substances. The common properties of free
radicals are the addition reaction to an unsaturated double bond
and the hydrogen abstraction reaction.
Based on the addition reaction, peroxides are widely used as polymerization
initiators for various synthetic resins (LDPE, PVC, PS, ABS, PMMA,
etc.) or as curing agents for unsaturated polyester resins, vinyl
ester resins, etc.
Based on the abstraction reaction, they are used as crosslinking
agents for a variety of synthetic rubbers and synthetic resins,
fluidity modifiers of polypropylene, and agents for grafting maleic
anhydride to polyolefins.
Applications of various organic peroxides are outlined below.
Polymerization Initiators
Polymerization of vinyl chloride percumyl ND, peroyl OPP, perhexyl
ND, perbutyl ND,
perhexyl PV, perocta ND, percumyl BF, percyclo
ND, etc.
Polymerization of styrene perbutyl O, nyper BW, perhexa 3M, perbutyl
Z, perbutyl D, etc.
Polymerization of ethylene (high pressure
process) perbutyl PV, perbutyl O, perbutyl A,
perbutyl Z, perbutyl D, etc.
Resin for paints nyper BMT, perbutyl O, perbutyl I, perbutyl Z,
etc.
Emulsion polymerization perbutyl H, percumyl H, permentha H, percumyl
P, etc.
Organic peroxides used for emulsion polymerization
are part of the redox system combined with reducing agents.
Curing Agents
The selection of curing agents is determined by the molding temperature
of resins, accordingly, the agents are divided into three types:
room-temperature curing, medium temperature curing, and high-temperature
curing according to molding temperature.
(1) Room-temperature curing
Room-temperature curing is a method
for curing resins at room temperature. Normally,
accelerators (reducing agents) should be
simultaneously used. Typical examples of the
curing system are the following three combinations
of organic peroxides and
accelerators.
1) Ketone peroxides
e.g. Permek
N + cobalt naphthenate
2) Hydroperoxides
e.g. Percumyl
H + vanadium pentoxide
3) Diacylperoxides
e.g. Nyper
FF + dimethylaniline
These combinations are usable for hand-lay-up and spray-up applications.
(2) Medium-temperature curing
Medium-temperature curing is a method
for curing resins at medium-temperatures.
Two methods are used for curing, i.e.,
use of peroxide and heat, and simultaneous use
of an accelerator.
I Organic peroxides
a)
Peroyl TCP + perbutyl O
b) Peroyl TCP + perhexyl
O
c) Peroyl TCP + perocta
O
II Organic peroxides
+ accelerators
a)
Keytone peroxides
e.g.
Permek N + cobalt naphthenate
b) Hydroperoxides
e.g.
Percumyl H + manganese naphthenate
c) Diacylperoxides
e.g.
Nyper BO + dimethylaniline
d) Peroxyketals
e.g.
Perhexa 3M + quaternary ammonium salt
These methods are used for spray-up, casting,
filament winding, RTM (RI), and the like.
(3) High-temperature curing
High-temperature curing is a method
for curing resins by heating. The curing speed
depends upon the thermal decomposition
rate of peroxides.
Examples of organic peroxides:
Percure O, percure HO, percure WO,
nyper BO, perhexa 3M, perhexa TMH,
percure HI, perbutyl I, perhexyl I,
perbutyl Z: This curing system is characterized by
a long pot life. These curing agents
are used for the pultrusion molding method,
BMC, SMC, and MMD.
Crosslinking Agents
Organic peroxides are used as crosslinking
agents for polyethylene, ethylene vinyl acetate copolymers, EPDM,
and other synthetic rubbers.
The crosslinking reaction for saturated polymers such as polyethylene
is caused by free radicals formed by decomposition of organic peroxides
which abstract hydrogen from the polymers to form intermolecular
CC bonds, thereby bonding the polymer chains together. Crosslinked
polyethylene with excellent properties (e.g. mechanical strength,
heat resistance, chemical resistance, and electrical properties)
is obtained. Crosslinking of synthetic rubber and natural rubber
also provides rubbers with excellent properties. Organic peroxides
used for crosslinking include percumyl D, perhexa 3M, perhexa 25B,
etc.
Miscellaneous
Various applications utilize the radical reaction
(addition, abstraction, B-cleavage) of organic peroxides.
1) Fluidity is improved by cleavage of the molecular chains of polypropylene.
2) Adhesion properties and tinting power are improved by grafting
maleic acid and acrylic
ester to polypropylene and polyethylene.
3) Organic peroxide is used as an initiator for halogenation.
In addition, organic peroxides are used as
a combustion improver for heavy oil and for improving film performance
when added to linseed oil and other drying oils.
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