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of monomers with organic peroxides

A wide range of organic peroxides and azo compounds are used as initiators for the radical polymerization of monomers. Organic peroxides can be divided into diacylperoxides, hydroperoxides, dialkylperoxides, peroxyesters, peroxyketals and peroxy(di)carbonates. The main areas of application for these initiators are low density polyethylene (LDPE), polyvinylchloride (PVC), styrenics (PS/EPS), acrylics (PMMA) and other polymers. The polymerization of monomers takes place under varying controlled conditions, to which the properties of the initiator have to be adapted. Certain types of organic peroxides are also used for the chain degradation of polypropylene (PP).

An important factor for selecting an appropriate initiator is its decomposition rate, which is determined using its half life time.

Half life time:
The half life is the time taken for half of the peroxide quantity to decompose in a specific solvent at a given temperature.  With the exception of hydroperoxides, the half life times were determined using a solution of the peroxide (0.1 mol/l) in monochlorobenzene. Listed are the temperatures at which the half lives are 10h, 1h and 1 min. Based on the 1h half life temperature, the initiators in our product guide are arranged in descending order of activity.

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of unsaturated polyester-, vinylester-
and acrylic resins with organic peroxides

Unsaturated polyester resins are cured by free radicals which are formed when organic peroxides decompose. The decomposition initiates a reaction by which unsaturated polyester molecules polymerize with styrene forming a three dimensional network (Duromer or thermoset). Organic peroxides decompose into free radicals either by exposure to heat or in combination with appropriate accelerators.

Accelerators used in practice are mainly organic cobalt salts and tertiary amines. However, these accelerators activate only certain types of organic peroxides. Contrary to this activation, for example if inhibition is required to increase storage life of an activated resin, the use of so called inhibitors is recommended. Inhibitors are chemical compounds, which prevent the polymerization process of monomers or other reactive compounds. Suitable compounds are Quinones or Phenolic compounds.

Amine or cobalt activated curing (Cold curing)
Accelerator activated curing is called cold curing. The most important cold curing systems are Methylethylketone, cyclohexanone or acetylacetone peroxides in combination with organic cobalt salts and Dibenzoyl peroxide in combination with tertiary amines.

Curing without accelerator (Hot curing)
Curing without accelerator, so called hot curing, requires external support of heat. Minimum kick-off temperatures above 50 °C and typically temperatures inbetween 120 and 160 °C for SMC/BMC hot moulding are required to achieve a good degree of curing within a short period of time.

Organic peroxides used for hot curing are peresters such as tert-Butyl peroxybenzoate, tert-Butyl peroxy-2-ethylhexanoate or perketals such as1,1-Di(tert-butylperoxy)cyclohexane. If a very low kick-off temperature is required (50 - 60 °C), more active initiators such as Di-(4-tert.-butylcyclohexyl)-peroxydicarbonat and Methylisobutylketonperoxide are used.

Very often, combinations of organic peroxides or ready-to-use mixtures are utilized to obtain an efficient curing process and a very good degree of curing.

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of rubber and polyolefines with organic peroxides

Chemical crosslinking agents such as sulphur and organic peroxides are able to link polymer chains creating a three-dimensional network. This crosslinking reaction changes several material characteristics. The material characteristics of a crosslinked polymer are superior to the equivalent characteristics of the sulphur cured polymers.

Important criteria for selecting a suitable crosslinking peroxide are processing and crosslinking temperature.

Safe processing temperature t2:
The raw materials (polymer, additives, peroxide) have to be homogenized before the crosslinking reaction can take place. Although the temperature sensitive peroxide will be the last raw material which is added for homogenization, one has to take care to avoid temperatures at which the peroxide decomposes and the crosslinking reaction starts. This maximum processing temperature of the peroxides is called the scorch temperature. The safe processing temperature t2 is defined as the temperature, at which the scorch time is longer than 20 minutes.

Typical crosslinking temperature t90:
The typical crosslinking temperature t90 is defined as the temperature at which 90% of the crosslinks in the compound are formed within about 12 minutes.

Peroxide crosslinking of elastomers has the following advantages over the sulphur cure:
• simple formulation
• long storage time of the peroxide containing compound without scorching
• high processing temperature
• rapid vulcanization without reversion
• high temperature resistance of final product

Peroxide crosslinking possible for:

NR Natural rubber EPDM Ethylene propylene terpolymer
IR Polyisoprene rubber POE Polyolefin elastomer
BR Polybutadiene rubber T Polysulphite rubber
CR Polychloroprene rubber PE Polyethylene
SBR Styrene butadiene rubber CM Chlorinated Polyethylene
NBR Acrylonitrile butadiene rubber CSM Chlorosulphonyl polyethylene rubber
HNBR Hydrogenated acrylonitrile butadiene rubber EVA Ethylene vinylacetate copolymer
Q Silicone rubber ABS Acrylonitrile butadiene styrene copolymer
AO/EO Polyurethane rubber EBA Ethylene butylacrylate copolymer
EPM Ethylene propylene copolymer FKM Fluoro rubber


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