Organic Peroxide

An organic compound containing -O-O-peroxy functional groups formed by replacing the hydrogen atoms in hydrogen peroxide with organic groups such as alkyl, acyl, and aromatic groups. It is characterized by decomposition to produce oxygen-containing free radicals when heated above a certain temperature, and is unstable and easy to decompose. Organic peroxides produced in the chemical industry are mainly used as polymerization initiators and catalysts for synthetic resins. In the field of polymer materials, it can be used as an initiator for free radical polymerization, an initiator for grafting reactions, a cross-linking agent for rubber and plastics, a curing agent for unsaturated polyesters, and a molecular weight and molecular weight distribution regulator in the preparation of spinning-grade polypropylene. Polluted air in the environment can produce peroxyacyl nitrate compounds through free radical reactions under the action of light, which is one of the particle species in photochemical oxidants. It is highly irritating to the skin, eyes, and mucous membranes, and is an important pollutant in the atmosphere. This type of substance is a flammable and explosive dangerous good, and safety should be paid attention to during use. Generally, the four indicators of active oxygen content, activation energy, half-life, and decomposition temperature are used as the basic basis for selection.

Physical properties: Most organic peroxides are colorless to pale yellow liquids, or white powder to crystalline solids. They are generally weakly acidic and most are insoluble in water, but soluble in organic solvents such as phthalic acid and dimethyl ester. They are a class of unstable, flammable and explosive compounds [1].

Chemical properties:

The structural characteristics of the peroxy functional group in organic peroxides determine that peroxides have the following chemical properties:

(1) It has strong oxidation effect.

(2) It has natural decomposition properties. Above 40°C, most peroxide active oxygen species decrease.

(3) Acid and alkaline substances can promote decomposition. Strong acids and hydroxides of alkali metals and alkaline earth metals (solid or highly concentrated aqueous solutions) can cause violent decomposition.

(4) Iron, cobalt, and manganese organic peroxides and redox system compounds significantly promote decomposition.

(5) Strongly reducing amine compounds and other reducing agents significantly promote decomposition.

(6) Iron, lead and copper alloys can promote its decomposition.

(7) Rubber can promote its decomposition.

(8) Friction, vibration or impact on storage containers causes local temperature increases, which can promote decomposition.

Classification: The main types of organic peroxides are hydroperoxide (ROOH), dialkyl peroxide (ROOR'), diacyl peroxide (RCOOOOCR'), peroxyester (RCOOOR'), peroxycarbonate (ROCOOOOCOR') and ketone peroxide [R2C(OOH)2], etc., each of which has different application characteristics. For example, benzoyl peroxide BPO is usually used as an initiator for free radical polymerization and a curing agent for unsaturated polyesters; diethyl propylene peroxide DCP can be used as a crosslinking agent and an initiator for melt grafting. Generally, the four indicators of active oxygen content, activation energy, half-life and decomposition temperature are used as the basic basis for selection.

Benzoic acid peroxide is the earliest and most commonly used organic peroxide. It is a granular solid and thermally stable at ambient temperature. To improve safety, benzoic acid peroxide can be added with 22% or 30% (by weight) of water to become a wet product to reduce flammability and vibration sensitivity. In addition, there is a paste formula of benzoic acid peroxide with a concentration of 25% to 50%.

Benzoic acid peroxide can be used to vulcanize polyesters over a wide temperature range. It can be activated by tertiary amines at room temperature and can be used to vulcanize filled polyester compounds in the temperature range of 250-300°F. Benzoic acid peroxide is an excellent initiator in the styrene suspension polymerization process.

Methyl ethyl ketone peroxide (MEKP) is widely used in the vulcanization of unsaturated polyester resins. The most common commercial form is made by reacting a ketone with hydrogen peroxide and contains a mixture of peroxides and hydroperoxides. Because pure ketone peroxides are sensitive to vibration and friction, they are sold only in diluted form, usually with a maximum active oxygen content of no more than 9% in the plasticizer solution.

Ketone peroxides are commercially available in standard and fire-retardant formulations.

Peroxyesters have the widest range of activity and are one of the most popular peroxides. Examples include 1,1-dimethyl-3-hydroxybutyl peroxyneoheptanoate and peroxyneodecanoic acid. Peroxyesters such as cumyl ester, tert-amyl peroxyneodecanoate and tert-butyl peroxypivalate are the most reactive compounds and are mainly used as initiators for the polymerization of ethylene and vinyl chloride. All these peroxyesters need to be stored at low temperatures.

Peroxyesters such as tert-amyl peroctanoate and tert-butyl peroctanoate have slightly lower reactivity and can be stored lightly frozen. They are widely used as initiators for compression vulcanization of ethylene polymerization and unsaturated polyesters.

Peroxyesters such as tert-amyl perbenzoate and tert-butyl perbenzoate have the lowest reactivity and therefore the best thermal stability. They can be stored at ambient temperature and are used as vulcanization initiators for sheet model materials.

When selecting peroxyesters, it should be noted that cumyl peroxyester has the highest reactivity, followed by tert-octyl peroxyester, tert-amyl peroxyester and tert-butyl peroxyester.

Peroxydicarbonates are the most toxic of the important peroxides used in industry. All peroxybicarbonates have basically the same reactivity. Peroxydicarbonates with higher molecular weight are safer and easier to control. 2-Ethylhexyl peroxydicarbonate is an excellent initiator for the polymerization of vinyl chloride. The use of aqueous dispersions or emulsions of peroxydicarbonate further increases safety. There is a growing interest in this formulation in the PVC industry.

Peroxyketals are bifunctional initiators with good thermal stability and work well as initiators for the polymerization of ethylene and for the compression vulcanization of unsaturated polyester resins. Tert-butyl peroxyketal is also used as a vulcanizer for elastomers, and tert-amyl peroxyketal is the newest member of this class of initiators and has great potential as an initiator in the synthesis of resins for high-solids acrylic coatings.

In general, tert-amyl peroxyesters and peroxyketals are gaining importance in PVC, high-solids acrylic coatings, and unsaturated polyesters. Tert-amyl peroxyesters have a faster reactivity than tert-butyl peroxide. In addition, they are cost-effective and can impart desirable properties to polymers, such as chain linearity and narrow molecular weight distribution. Dialkyl peroxides are the most stable of all organic peroxides. Dicumyl peroxide is the most common of these peroxides and is widely used for crosslinking PE for wire and cable jacketing and insulation. [3]

Organic peroxides with hydroxyl and hydroxy functional groups are under active development. These peroxides have been found to be effective initiators for the synthesis of acrylic high solids coatings and are of particular interest in the synthesis of compatibilizers for polymer blends and alloys. Organic peroxides containing groups such as hindered amine light stabilizers are another area of new development. These peroxides provide a convenient way to obtain light stabilizers for linked polymers, overcoming problems of migration, volatility and incompatibility.

Uses: In the field of polymer materials, organic peroxides are used as initiators for free radical polymerization, initiators for grafting reactions, crosslinkers for rubber and plastics, curing agents for unsaturated polyesters, and molecular weight and molecular weight distribution regulators in the preparation of spinning-grade polypropylene. Organic peroxides are the source of free radicals used in the following applications: ① free radical polymerization and copolymerization initiators for vinyl and diene monomers; ② vulcanizers for thermosetting resins; ③ crosslinkers for elastomers and polyethylene.

In addition to the above-mentioned polymer materials industry, organic peroxides are used as photoinitiators and sensitizers in the film industry, for photosensitive polymer materials, photosensitive resins, etc., and are also commonly used in the production of epoxy resins; in terms of medical materials, organic peroxides and drugs are used as initiators for the synthesis of drug sustained-release matrices (such as microspheres, micropills, and drug films); in terms of organic synthesis, organic peroxides are mainly used as oxidants and epoxidants. In addition, organic peroxides are also used in the disinfection of medical devices and foods, and as bleaching agents, decolorizing agents, disinfectants, and cleaning agents in daily chemical industries such as textiles and paper. [1]

The temperature at which an organic peroxide decomposes at an effective rate largely determines its use. Other important factors are cost, solubility, safety, efficiency and type of free radicals generated, the need for refrigerated storage and shipping, compatibility with production systems, possible effects on the product, and the ability to be activated. Organic peroxides can decompose to form reactive free radicals at a controlled rate at elevated or room temperature.

All organic peroxides are thermally unstable and decompose more rapidly with increasing temperature. The most common quantitative measure of the reactivity of organic peroxides is the half-life, which is the time required for a given amount of peroxide to decompose to half its initial amount at a given temperature. Half-life data for commercial organic peroxides are now available on computer diskettes. Computer menu programs can be used to select the appropriate peroxide for a given polymerization or process condition.

These free radicals can be added to unsaturated vinyl monomers such as styrene, vinyl chloride, or methyl methacrylate to initiate polymerization. Some free radicals also attack polymers such as PE to form free radicals in the chain. When two such polymer free radicals combine, a cross-linked structure is formed.

Polymerization of Vinyl Compounds

Organic peroxides provide the most effective means of initiating free radicals for polymerization. Polymerization of vinyl can be carried out efficiently over a wide temperature range by selecting an organic peroxide by half-life temperature, or by using a mixture of two or more organic peroxides.

PVC is made primarily by the suspension process. 2-Ethylhexyl peroxydicarbonate and tert-butyl perneodecanoate are excellent initiators, especially in combination with α-cumyl perneodecanoate or α-cumyl perneoheptanoate. However, the use of α-cumyl peracid esters can cause an unwelcome acetophenone odor in the resin. By using 1,1-dimethyl-3-hydroxy-butyl perneoheptanoate as the low-temperature initiator component, the acetophenone odor in the resin can be eliminated. Other advantages of using this initiator are increased productivity and reduced adhesion to the reactor walls. Tert-amyl peroxypivalate is also now used instead of azo initiators due to improved processing and efficiency.

High-solids acrylic coating resins use peroxyesters and peroxyketals as initiators. When the solids are 70% or higher, tert-amyl peroxyesters and peroxyketals are preferred to obtain narrow molecular weight distribution and low solution viscosity. Other resins with low residual monomer content are produced, such as tert-butyl peracetate and 3,3-di(tert-amylperoxy)butyrate. In addition, organic peroxides with light stabilizer groups, such as hindered amines, are being actively developed to improve the performance of automotive coatings.

EPS In the production of EPS by suspension polymerization, initiators including a mixture of benzoic acid peroxide and tert-butyl perbenzoate are usually used.

The reaction time required to achieve residual styrene concentrations below 0.1% can be shortened by replacing tert-butyl perbenzoate with O-tert-amyl O-(2-ethylhexyl)-monoperoxycarbonate. Crystalline PS and HIPS are usually prepared by continuous bulk polymerization, preferably using peroxyketals as initiators.

Polyolefins

Polyethylene In the production of LDPE and ethylene copolymers, organic peroxides are used as initiators. Peroxyesters are the best peroxide initiators because they offer a wide range of reactivity and good solubility when used at high temperatures and pressures. The most commonly used peroxyester is tert-butyl peroctoate, based on its efficiency. Other varieties, in descending order of use, are tert-butyl peracetate, tert-butyl peracetate, and tert-butyl perbenzoate. If more reactive is required, the corresponding tert-amyl peroxyesters can be used.

In blown film, organic peroxides are increasingly being used to reduce the melt flow of LLDPE as a means of improving film strength in the extruder. Dialkyl peroxides are often used when higher processing temperatures are required.

Organic peroxides can be used to crack polypropylene to obtain a narrow molecular weight distribution and increase fluidity. 2,5-dimethyl-2,5-di-tert-butyl peroxyhexane is used when performance and FDA requirements are met.

Curing of Polyesters

Thermosetting polyesters can be prepared by curing unsaturated polyester alkyd resins and monomer solutions such as styrene with organic peroxides. Many cures are performed at room temperature, with activators or accelerators added to the resin to decompose certain peroxides to form free radicals, thereby initiating cure.

The two most commonly used peroxides for polyester cure are benzoic acid peroxide and MEKP. Dimethylaniline is a typical tertiary aromatic amine used to activate benzoic acid peroxide, and cobalt cyclohexane is used to activate MEKP.

High-temperature cured sheet moldings and integral moldings are cured at high temperatures using metal molds and high pressures. Tert-butyl perbenzoate is the most widely used molding catalyst at 280-320°F. Currently, some important processes with fast cure cycles, other peroxides, especially tert-amyl peroxides, are becoming increasingly important in molding cure applications. Special examples include tert-amyl perbenzoate and 1,1-di-tert-amyl peroxycyclohexane, which can save raw materials, shorten cycle times and save energy because they have faster reactivity, shorter cycle times and higher efficiency than their tert-butyl counterparts.

Mixtures of high-temperature initiators and low-temperature initiators can improve production efficiency. However, according to the latest cost-performance criteria, tert-amyl peroctoate is more important than tert-butyl peroctoate as a superior low-temperature initiator.

For room temperature curing of unsaturated polyesters, MEKP-type accelerators dominate. They are the most commonly used accelerators together with transition metal salts (such as cobalt naphthenate). The use of dimethylaniline and transition metal salts can double the curing speed of the system. The most effective catalyst concentration is 0.5% to 2.0% of the resin. The concentration of accelerator is variable, but is usually 0.05% to 0.3%. However, too high an accelerator concentration has an adverse effect on the final cure.

The main feature of room temperature curing systems is that the peroxide and activator are combined at the time of curing. The most commonly used system is the catalyst injection system. A process that includes metering and mixing the catalyst and the cured resin. For example, in a spray system, the mixing action takes place in the spray fan (external mixing) or in the spray gun (internal mixing).

Crosslinking

Organic peroxides are used to crosslink saturated and unsaturated elastomers with thermoplastic resins. Dialkyl peroxides, especially dicumyl peroxide, have become the standard crosslinkers for this process.

The free radicals from dialkyl peroxide derivatives are good hydrogen abstractors. This is an important criterion for thermoplastic resins such as LDPE, which crosslink only by a hydrogen abstraction mechanism.

In elastomers with unsaturated chains such as EPDM or containing crosslinking aids such as TAC, crosslinking is achieved by a chain addition mechanism. In this case, peroxyketals are very good crosslinkers. If possible blooming problems are eliminated, peroxyketals have faster cure cycles.

New technology has been disclosed that allows users to increase the ability to resist premature cure when certain peroxides are used in the compounding process. This new technology is applicable to most commercial dialkyl peroxides and peroxyketal organic peroxides, allowing the use of lower temperature peroxides for elastomers, taking advantage of the lower activation characteristics to obtain faster cure times. For example, in the polybutadiene base, premature cure-resistant grades of dicumyl peroxide and 1,1-di-tert-butylperoxy)-3,3,5-trimethyl-cyclohexane can significantly extend cure times while producing similar cure states.

Although dicumyl peroxide is the most commonly used in crosslinking processes, it is a solid and presents handling problems. New peroxide liquid formulations, especially those with premature cure-resistant properties, are gaining attention. Likewise, these formulations are expected to be used in crosslinking of LLDPE.

Rotational molding of HDPE, which produces crosslinked structures like tanks, is a rapidly emerging area. Most commercial heat-stable organic peroxides, such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, are best suited for this application due to the high crystalline melting point of HDPE.

Measures: For safety and decomposition prevention, organic peroxides should always be stored at an appropriate temperature in their original containers to reduce the chance of contamination, especially the possibility of contamination by strong acids. This is very important. Be especially careful when using ketone peroxides, because ketone peroxides can react rapidly even in the case of very low concentrations of transition element metal salts. Organic peroxides should be transported in a manner that prevents shock, friction, and inversion as much as possible. Protective equipment should be worn when using them to avoid adhesion to the skin or entry into the eyes. Disposal of organic peroxides can be done by incineration, hydrolysis, deep burial, etc.