Crosslinking Agents for Polymers: 9 Types, Mechanisms, Dosage and Selection

Crosslinking agents are substances that tie separate polymer chains into one three-dimensional network, so a thermoplastic such as polyethylene stops melting and starts behaving like a thermoset; the 9 types work through 3 routes, radical chemistry from organic peroxides, silanol condensation from grafted silanes, and radiation, which needs no agent at all but does use coagents. Crosslinked polyethylene alone, as XLPE cable insulation and PEX pipe, accounts for about 5 to 10 % of the polyethylene market according to the 2024 ACS Applied Polymer Materials review "Cross-Linked Polyolefins: Opportunities for Fostering Circularity", so which route and which grade fits which product?

Crosslinking agents are one of the 43 families of plastic additives and sit in the property-modifier group, next to impact modifiers, chain extenders and coupling agents. A peroxide counts as a plastic additive when it crosslinks or visbreaks a finished polymer; when it cures a thermoset resin or starts a polymerisation it belongs to the matrix chemistry, not to the additive package.

This page follows the working order of a crosslinking decision: the definition and the curing-agent synonym, what crosslinking changes in a part, the three mechanisms and the half-life logic behind them, the 9 types in temperature order, the route decision, the polymer and product fit, dosage in phr and the conversion to active peroxide, the interactions with the rest of the package, measurement against IEC 60811-507, ASTM F876-23 and IEC 62788-1-6, regulation after the June 2024 listing of dicumyl peroxide, the suppliers and trade names, and the complete list of 13 substances.

The 9 types differ in the bond they form, the temperature at which they act, whether they need a second component and which polymers they suit.

Temperatures are Nouryon typical crosslink temperatures (t90 of about 12 minutes) for the named grades, not specifications; check the data sheet for the grade you buy. Electron beam and gamma radiation crosslink without any agent and are therefore a route rather than a type; they use the type 6 and type 7 coagents.

What Is a Crosslinking Agent in Plastics?#

A crosslinking agent is an additive that creates chemical bridges between polymer chains in a finished plastic, either carbon to carbon through radicals released by an organic peroxide or silicon to oxygen to silicon through the condensation of a grafted silane. The bridges convert separate, mobile chains into one continuous network that cannot flow, dissolve or be remelted. Radiation produces the same carbon-carbon bridges with energy rather than chemistry, so the three network-forming routes used in plastics are radical chemistry, silanol condensation and irradiation.

Where does that stop being an additive and start being the plastic itself? Curing agents, hardeners, catalysts, initiators and monomers build the polymer, so they are matrix chemistry rather than additives. Peroxides are the working exception: the same dicumyl peroxide molecule is an additive when it crosslinks a finished polyethylene compound and matrix chemistry when it cures an unsaturated polyester resin. Crosslinking agents for plastics are therefore defined by the moment they act, not by their chemical class.

Crosslinking agent, curing agent or vulcanising agent: which word applies to plastics?#

The four words describe the same chemistry in different industries: a crosslinking agent bridges chains in a thermoplastic, a curing agent hardens a thermoset resin, a vulcanising agent crosslinks rubber, and a coagent is a second molecule that makes a peroxide cure more efficient. Both spellings, "crosslinking agent" and "cross-linking agent", carry the same intent. The four terms are defined below.

• Crosslinking agent (cross-linker, crosslinker): an additive that bridges finished polymer chains inside a compound, such as dicumyl peroxide in XLPE cable insulation.
• Curing agent (curative, hardener): a reactant that converts a liquid resin into a solid thermoset, such as an epoxy hardener, covered here only as a border case.
• Vulcanising agent: the rubber-industry term for the same function, covering sulfur and accelerator systems as well as peroxides, and outside the plastics border.
• Coagent (co-agent, crosslinking activator): a multifunctional monomer dosed with a peroxide or a beam, classified as Type I or Type II, that raises the crosslinks per radical.

What does crosslinking change in the finished part?#

Crosslinking changes 5 things at once: the polymer no longer melts and flows, its continuous service temperature rises, creep and environmental stress cracking slow down, solvent resistance improves because the network is insoluble, and part of the tensile and tear strength is traded away. The 5 changes are listed below with the values recorded for crosslinked polyethylene, the most-used host.

• Melt flow stops. A crosslinked part softens on heating but does not flow, which lets XLPE insulation survive a fault current.
• Service temperature rises. XLPE cable is rated at 90 to 105 °C in continuous operation, 130 to 140 °C in an emergency and 250 °C for a short circuit of about 0.5 seconds.
• Creep and environmental stress cracking slow. The network restricts chain slippage, which is why crosslinked pipe holds a pressure rating the unmodified resin cannot.
• Solvent resistance improves. That insolubility is itself the measurement: gel content is the fraction of polymer that will not dissolve in hot xylene.
• Tensile and tear strength fall. Crosslinking makes a polymer stronger in heat, creep and solvent terms, not in tensile terms.

How Does Crosslinking Work? Radicals, Silanol Condensation and Radiation#

Crosslinking works in 3 ways: an organic peroxide splits into radicals that pull hydrogen off two polymer chains so the two macroradicals can join, a grafted silane hydrolyses in water and condenses into Si-O-Si bridges, and an electron beam creates the same macroradicals with energy instead of chemistry. The radical sequence for dicumyl peroxide, including the byproducts, is set out in the 2024 Advanced Materials review "Alternative Concepts for Extruded Power Cable Insulation: from Thermosets to Thermoplastics". Its oxygen-oxygen bond breaks by homolysis into two cumyloxy radicals; each abstracts a hydrogen atom, giving cumyl alcohol, which can dehydrate to alpha-methylstyrene and water, while a competing beta-scission path gives acetophenone and a methyl radical that forms methane; the two polyethylene macroradicals left behind combine into a permanent carbon-carbon crosslink.

The silane route splits the same job into two stages separated in time. A small peroxide dose grafts the vinyl group of a vinylsilane onto the polyethylene chain in the extruder; after shaping, the methoxysilyl groups hydrolyse with water from a sauna, a water bath or ambient humidity and condense into Si-O-Si bridges, catalysed by dibutyltin dilaurate. Radiation needs no agent: electron-beam doses of 50 to 150 kGy generate macroradicals directly in polyethylene, and because the process runs cold, the network is less uniform through a thick wall. The radical route follows first-order Arrhenius kinetics, kd = A x exp(-Ea/RT), with the half-life t½ = ln2 / kd.

Half-life, scorch and cure: the two temperatures that decide every peroxide#

Every crosslinking peroxide is chosen between 2 temperatures: the safe processing temperature, at which the compound can be mixed and shaped for more than 20 minutes without scorching, and the typical crosslinking temperature, at which 90 % of the cure is reached in about 12 minutes. The pair describes a process window rather than a property. The 10-hour half-life temperature is a third, different number, measured in chlorobenzene solution, and it sits 20 to 60 °C below the cure temperature for the same grade.

The two temperatures, the half-life set and the storage limits for 12 grade classes are collected below, with the Arrhenius activation energy that lets the half-life be recalculated for any melt temperature using the [organic peroxide half-life calculator].

Half-life temperatures are measured in chlorobenzene and come from each grade's own Nouryon product data sheet. Blank cells mean the value is not in the data sheet we retrieved, not that it does not exist. Safe processing means a rheometer scorch time (ts2) above 20 minutes; typical crosslink means a t90 of about 12 minutes; SADT is the self-accelerating decomposition temperature measured by the UN Heat Accumulation Storage Test; Ts max is the maximum storage temperature.

Storage is governed by a fourth number again. The SADT, the lowest temperature at which self-accelerating decomposition may occur in the original packaging, ranges from 60 °C for TBEC to 110 °C for the cyclic triperoxonane grade, with 75 to 80 °C for the dialkyl peroxides. Ts max sits far lower at 20 to 40 °C, because it protects the assay rather than the drum.

Why peroxides crosslink polyethylene but break down polypropylene#

A peroxide radical does the same first step in both polymers, pulling a hydrogen atom off the chain, but the macroradical it leaves behind behaves in opposite ways: a secondary radical on polyethylene finds another one and forms a crosslink, while a tertiary radical on polypropylene splits the chain in two. Polyethylene tends to crosslink and form gels as it degrades, while polypropylene chain-scissions, because the tertiary macroradical on a PP backbone undergoes beta-scission faster than it can meet a partner. Peroxides therefore visbreak polypropylene instead of crosslinking it unless a coagent intercepts the radicals first.

Triallyl isocyanurate suppresses that scission by adding the polymer radicals across its allyl groups, which is why radiation crosslinking of polypropylene always runs with a coagent. In a 2022 Polymers study of pilot reactive extrusion, 0 to 600 ppm of DHBP raised the melt index and narrowed the molecular weight distribution of polypropylene, with the mild and harsh screw designs diverging above 400 ppm. In a 2024 Polymers study of electron-beam crosslinked PP and POE foam, trimethylolpropane trimethacrylate raised the degree of crosslinking to a maximum of 55 % at 15 kGy. Using that scission deliberately is [peroxide modification of polypropylene], the visbreaking route covered on its own page.

9 Types of Crosslinking Agents for Plastics#

The 9 types of crosslinking agents for plastics fall into 3 groups: 5 classes of organic peroxide that release radicals on heating, 2 types of coagent that make those radicals count for more, and the 2 components of the silane system, a graftable vinylsilane and a condensation catalyst. The order below runs from the radical route through its helpers to the condensation route, and inside the peroxide group it runs by rising cure temperature.

Organic peroxides: 5 classes ordered by cure temperature#

Organic peroxides are the radical source behind most crosslinked plastics, and the 5 classes are separated by the temperature at which they decompose: peroxycarbonates and peroxyketals cure from about 140 °C, dialkyl peroxides from 170 to 185 °C, and the cyclic peroxides reach 220 °C or are used to break polypropylene down instead. About 60,000 tonnes of dialkyl peroxides are produced per year, with dicumyl peroxide the dominant product.

1. Dialkyl peroxides: DCP, DHBP, DTBP, BIPB and TBCP#

Dialkyl peroxides are the workhorse class, cured at 170 to 185 °C, and they crosslink almost everything that is crosslinked at industrial scale: XLPE cable insulation, PEX-a pipe, crosslinked polyethylene and EVA foam and rotomoulded tanks. The bond they form is a direct carbon-carbon bridge between two macroradicals, and the class carries the industrial volume of the whole family.

Dicumyl peroxide, at 5.92 % active oxygen, is the reference grade and the one used in cable. BIPB is bifunctional and the lower-odour alternative to it, reaching 78 % gel at 0.7 phr and 91 % gel at 1.2 phr in EVA foam. DTBP is also an LDPE and polystyrene initiator and is used for PEX-a pipe, although its flash point of 1 °C limits how it can be handled, and TBCP is the liquid grade for direct peroxide injection in wire and cable lines.

2. Peroxyketals: intermediate-temperature crosslinkers#

Peroxyketals cure between about 145 and 160 °C, which puts them below the dialkyl window and makes them the choice when the compound cannot be taken to 170 °C without scorching. They form the same carbon-carbon bridge as the dialkyl class, only earlier on the temperature scale.

Two grades cover the class: 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane and butyl 4,4-di(t-butylperoxy)valerate, sold as Trigonox 29 and Trigonox 17. Both carry the highest recommended loadings of any grade in the supplier dosage table for polyethylene, EVA, POE and EPDM.

3. Peroxycarbonates: TBEC and TAEC for solar encapsulants#

Peroxycarbonates are fast-cure liquids that work from about 140 to 150 °C, which is exactly the vacuum-lamination window of a solar module, and TBEC is the standard crosslinker in EVA and POE encapsulant film. TBEC is a monoperoxycarbonate that decomposes into a tert-butoxy radical and a 2-ethylhexyloxycarbonyloxy radical.

In encapsulant formulations the peroxide is dosed at about 1.5 m% with about 0.1 m% of a hindered amine light stabilizer, and lamination runs at around 150 °C. TBEC carries the lowest 10-hour half-life temperature in the set at 98 °C, and with an SADT of 60 °C and a maximum storage temperature of 20 °C it is the most storage-sensitive grade on this page. TAEC covers the same window one step lower.

4. Cyclic peroxides: high-temperature crosslinking and PP visbreaking#

Cyclic peroxides are the high-temperature end of the range: the trioxepane grade cures at about 220 °C, and the cyclic ketone peroxide trimer is used the other way round, to visbreak polypropylene and to re-adjust the melt flow of recycled polyolefins. 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, sold as Trigonox 311, is safe to process at 180 °C and crosslinks at 220 °C.

The trifunctional cyclic ketone peroxide triperoxonane is a modification grade rather than a crosslinker. It carries the family's highest SADT at 110 °C and an active oxygen content of 18.16 % as the pure substance, falling to 7.3 to 7.6 % in the 40 to 42 % formulation, and it is dosed for controlled-rheology polypropylene and for lowering the melt flow index of recycled polyethylene.

5. Diacyl peroxides: grafting initiators and silicone rubber#

Diacyl peroxides decompose below the dialkyl window and are used less to crosslink a polymer directly than to graft something onto it, above all to graft vinylsilane onto polyethylene before a moisture cure. Three members carry the class in the literature: dibenzoyl peroxide, di(2,4-dichlorobenzoyl) peroxide and di(4-methylbenzoyl) peroxide.

In a 2019 study of Monosil and Sioplas silane curing of POE and LLDPE, Dana, Zohuri and co-workers, in the Journal of Macromolecular Science Part A, found dibenzoyl peroxide outperformed dicumyl peroxide as the grafting initiator, with 0.2 wt% giving 79 % gel content and 40.4 % compression set. Di(2,4-dichlorobenzoyl) peroxide is the classic silicone rubber curative, an elastomer use outside the plastics border.

Coagents: 2 types that raise crosslink efficiency and stop PP scission#

Coagents are multifunctional monomers added with a peroxide or an electron beam; they add their own bridges, raise the number of crosslinks per radical and intercept the radicals that would otherwise split a polypropylene chain, and they divide into Type I methacrylates and Type II allylics. The Type I and Type II labels are standard industry usage rather than a standardised classification.

6. Type I methacrylate and maleimide coagents: TMPTMA and EGDMA#

Type I coagents are multifunctional methacrylates and maleimides that polymerise among themselves as well as onto the polymer, so each radical produces more network, and trimethylolpropane trimethacrylate is the one used in radiation-crosslinked polypropylene foam. Ethylene glycol dimethacrylate and N,N'-m-phenylene dimaleimide (CAS 3006-93-7) belong to the same type.

Trimethylolpropane trimethacrylate is a trifunctional methacrylate whose groups homopolymerise and graft at the same time, raising crosslink density per radical, and in electron-beam crosslinked PP and POE foam it works up to about 8 phr before the effect plateaus. It is on the Union list of Regulation (EU) No 10/2011 as FCM 463, a monomer, with a specific migration limit of 0.05 mg/kg.

7. Type II allylic coagents: TAIC and TAC#

Type II coagents are allylic triazines, triallyl isocyanurate and its isomer triallyl cyanurate, that add polymer radicals across their allyl groups and are the standard coagent in solar encapsulant film and in electron-beam crosslinked wire. Their bridging consumes the radicals that would otherwise cause chain scission, which is what makes them the coagent of choice for polypropylene.

Triallyl isocyanurate is used in EVA and POE photovoltaic encapsulant, electron-beam crosslinked wire, heat-shrink products and polyamide 6; it is REACH registered and is not on the Union list of Regulation (EU) No 10/2011. Triallyl cyanurate is the isomer, and Gillham and Mentzer showed in the Journal of Applied Polymer Science in 1973 that it isomerises to triallyl isocyanurate on heating.

The silane system: 2 components that cure with water#

Silane crosslinking is not one additive but two, a graftable vinylsilane that is bonded onto the polymer in the extruder and a condensation catalyst that closes the network later, when water reaches it; the cure therefore happens after shaping, not during it. That separation in time is the reason the silane route needs no cure tube and no line speed penalty at the die.

8. Graftable vinylsilanes: VTMS and VTES#

Graftable vinylsilanes are vinyl trialkoxysilanes, above all vinyltrimethoxysilane, whose vinyl group a small dose of peroxide attaches to the polyethylene chain in the extruder, leaving three methoxy groups that later hydrolyse and condense into Si-O-Si bridges. Ethylene-vinylsilane reactor copolymers deliver the same grafted group without a grafting step.

Vinyltrimethoxysilane (C5H12O3Si, molar mass 148.23, boiling point 123 °C at 1013 hPa, flash point 25.5 °C) carries the harmonised classification Skin Sens. 1B H317 under CLP index 014-049-00-0. Vinyltriethoxysilane (CAS 78-08-0) is FCM 142, permitted only as a surface treatment agent. No dosage for vinyltrimethoxysilane is established in a primary source we could verify, so none is stated here.

9. Silanol condensation catalysts: DBTDL and tin-free catalysts#

Silanol condensation catalysts are the second half of the silane system, almost always dibutyltin dilaurate delivered as a catalyst masterbatch, which speeds the hydrolysis and condensation step from days to hours. Dioctyltin dilaurate (CAS 3648-18-8) and tin-free catalysts serve the same function; no commercial name for a tin-free catalyst is established in our source library, so none is given.

Dibutyltin dilaurate is an organotin carboxylate that catalyses both the hydrolysis of the methoxysilyl groups and the condensation of the resulting silanols. It carries the harmonised classification Muta. 2 H341, Repr. 1B H360FD and STOT RE 1 H372 under CLP index 050-030-00-3, and entry 20 of REACH Annex XVII is the heaviest constraint on the silane route.

Peroxide, Silane or Electron Beam: How to Choose the Crosslinking Route#

The crosslinking route is decided by 5 factors: the wall thickness of the part, whether the product has to hold high voltage, whether the line can accept a separate curing step, how much residue the application tolerates and what the standard for that product demands. Each factor points in a different direction, so the decision is a matrix rather than a ranking.

The three PEX minimums are the ASTM F876-23 values. DIN 16892 gives different minimums (PE-Xa 75 %, PE-Xb 65 %, PE-Xc 60 %); that comparison rests on a secondary source and is being checked against ISO 15875.

Voltage settles the argument wherever it applies. Silane crosslinking is not suitable for high-voltage insulation because residual water adds charge carriers to the dielectric, which is why the peroxide route keeps medium-voltage, high-voltage and HVDC cable despite the degassing step its byproducts force. Electron beam runs at 50 to 150 kGy for polyethylene and crosslinks below the crystal melting point, so a thick wall receives a less uniform network. Cable and foam grades of XLPE, crosslinked polyethylene are compared route by route on their own page.

Which Crosslinking Route Suits Each Polymer and Product?#

Each polymer allows the routes its own radical chemistry permits: polyethylene, EVA and POE crosslink readily and take all three routes, polypropylene degrades under a peroxide unless a coagent is present, and PVC is not peroxide-crosslinked in any of its core applications. Every polymer has its own formulation guide under additives by polymer, and the pairings that matter for crosslinking are collected in the table below.

Crosslinking polyethylene: XLPE cable and PEX pipe#

Polyethylene is the polymer that made crosslinking an industry: crosslinked polyethylene appears as XLPE insulation in medium and high-voltage cable and as PEX in hot-water pipe, and between them they take about 5 to 10 % of the polyethylene market. The cable process runs in two temperature stages: the compound is melted at 120 to 140 °C during triple extrusion, cool enough that the dicumyl peroxide does not start to cure, then vulcanised above 180 °C for about 5 minutes in the cure tube, then degassed.

Pipe splits into three routes named after their crosslinking chemistry, and the three pipe routes are explained on [PEX-a, PEX-b and PEX-c].

1. PEX-a is peroxide-crosslinked in the melt. The Engel process crosslinks HDPE with about 2 % peroxide at 200 to 250 °C; the published description does not name the peroxide, so none is named here.
2. PEX-b is silane-crosslinked, grafted in the extruder and cured afterwards with water.
3. PEX-c is electron-beam crosslinked after extrusion, with no chemical agent at all.

Dicumyl peroxide is dosed at 1.4 to 2.0 phr of pure peroxide in cable insulation, and ASTM F876-23 sets the pipe pass mark at 65 to 89 % with a route-specific minimum. The full cable package is on additives for wire and cable compounds.

Crosslinking EVA and POE: solar encapsulants and footwear foam#

EVA and its polyolefin-elastomer replacement POE are crosslinked twice over in the solar industry and the shoe industry: a peroxide cures the encapsulant film inside a laminator at about 150 °C, and a peroxide cures the midsole foam in a press. TBEC is dosed at about 1.5 m% with about 0.1 m% of a hindered amine light stabilizer. The two polymers do not cure at the same speed: a 2022 Polymers study measured a crosslinking onset of 125 °C for EVA against 135 °C for POE, and a time to 95 % crosslinking at 150 °C of 8.9 minutes against 14.6 minutes, with supplier gel minimums above 75 % for EVA and above 60 % for POE.

Gel content in an encapsulant is a narrow window rather than a floor, and a PI Berlin field study of 254 samples from 120 modules found only about two thirds adequate. In foam the same chemistry is dosed differently, since BIPB gives 78 % gel at 0.7 phr and 91 % at 1.2 phr. Lamination profiles and gel targets are on [crosslinking EVA and POE solar encapsulants], and the whole encapsulant package is on [additives for solar encapsulants].

Peroxide modification of polypropylene: visbreaking instead of crosslinking#

In polypropylene the same peroxides are dosed for the opposite reason: 200 to 600 ppm of DHBP in a reactive extruder raises the melt index and narrows the molecular weight distribution, which is how controlled-rheology polypropylene for fibres and thin-wall moulding is made. 200 ppm is the level used for staple-fibre grades, and the cyclic ketone peroxide Trigonox 301 does the same job while also lowering the melt flow index of recycled polyethylene.

Melt index is the control variable, and the melt index used to set a visbreaking dose is measured as melt flow rate (MFR). United States law recognises the use directly, since 21 CFR 177.1520 permits DHBP as a polypropylene initiator provided residual tert-butyl alcohol stays at or below 100 ppm. In recycled polypropylene the direction reverses again, since Nouryon states that its patented Perkadox PM-60ST-GR grade lowers the melt flow index and raises melt strength by long-chain branching, opening recycled PP to foaming, blow moulding and thermoforming. The rest of the PP package is on additives for polypropylene.

Crosslinking engineering plastics and recycled polyolefins#

Outside the polyolefins, crosslinking is mostly a radiation job: polyamide 6 connectors and tubing are crosslinked by electron beam with a triallyl isocyanurate coagent, because the chemistry that works in a polyethylene melt does not survive a polyamide processing temperature. No coagent dosage for polyamide 6 is established in our source library, so none is given. Doses, coagents and wall-thickness limits are on [radiation crosslinking].

Recycled polyolefins are the second area, and there the target is melt strength rather than a thermoset network. A 2026 study of electron-beam treated recycled polyethylene recorded gel content rising from 46.7 % to 56.2 % between 95 and 125 kGy, with 110 kGy judged the best balance between network formation and degradation. Recycled polypropylene takes the chemical route instead, through the long-chain-branching peroxide grade. Restabilization and melt-strength rebuild are on additives for recycled plastics.

How to select a crosslinking agent in 7 steps#

Select a crosslinking agent in 7 steps: fix the property target and its standard, identify the polymer, match the safe processing temperature, check the cure temperature, decide on a coagent, convert the dose to active peroxide, then screen the regulations and test. The order matters, since a grade that fails step 3 cannot be rescued at step 6.

1. Fix the property target and its standard: hot set to IEC 60811-507, degree of crosslinking to ASTM F876-23 or gel content to IEC 62788-1-6.
2. Identify the polymer and whether it crosslinks or scissions, since polyethylene, EVA and POE crosslink while polypropylene scissions.
3. Read the processing temperature of the line and pick a peroxide whose safe processing temperature is above it, using ts2 rather than the half-life.
4. Check that the crosslinking temperature is reachable in the shaping step, or plan a separate cure stage: a cure tube, a water bath or a beam pass.
5. Decide whether a coagent is needed, which it is for POE, EPDM, polypropylene and every radiation cure.
6. Convert the dose from formulation to active peroxide, then check the residue level and the degassing requirement.
7. Screen the regulatory status for the market and application, then confirm by a gel-content or hot-set test.

The same framework applied to every family is on how to select plastic additives.

How Much Crosslinking Agent Is Needed? Dosage, Active Content and Masterbatch#

Crosslinking peroxides are almost always bought as a 40 % or 45 % formulation on a carrier, and the recommended doses run from 0.8 to 12.5 phr of that formulation depending on the polymer, which is roughly 0.3 to 5 phr of active peroxide. Conversion between phr, wt% and ppm is on PHR (parts per hundred resin), and every figure in the table below is phr of the supplied formulation.

All values in the five grade columns are phr of the supplied 40 % or 45 % formulation, taken from the Nouryon recommended dosage table. Active-peroxide figures are calculated from the stated active content, not quoted.

Product-line doses are quoted differently again, as pure peroxide. XLPE cable insulation takes 1.4 to 2.0 phr of dicumyl peroxide, and one study found more than 1.4 % was needed to pass the hot-set test, with gel content rising from 74.3 to 81.6 % and hot-set elongation falling from 300 % to 80 % across that range. BIPB in EVA foam runs at 0.5 to 0.7 phr for chemical foaming and up to 1.2 phr for physical foaming. Controlled-rheology polypropylene uses 200 to 600 ppm of DHBP, trimethylolpropane trimethacrylate reaches up to 8 phr in electron-beam PP and POE foam, and TBEC sits at about 1.5 m% in photovoltaic encapsulant. No dosage is given for vinyltrimethoxysilane or dibutyltin dilaurate, because neither is established in a primary source we could verify.

Delivery form changes the arithmetic again, since peroxides arrive as liquids, pastes, polymer-carrier concentrates or pre-dispersed granules; peroxide and catalyst concentrates follow the same let-down logic as any other masterbatch.

How to convert phr of a 40 % peroxide formulation into active peroxide#

A dose quoted for a 40 % grade is not a dose of peroxide: multiply it by 0.40 to get the active peroxide, so 1.4 to 6.8 phr of a 40 % dicumyl peroxide formulation is about 0.56 to 2.7 phr of dicumyl peroxide itself. The 45 % grades take a factor of 0.45. Reading a formulation dose as an active dose overstates the peroxide by a factor of 2.5, the difference between a cable insulation that passes hot set and one that does not.

Active oxygen content is a third quantity again and is not interchangeable with active peroxide content: dicumyl peroxide carries 5.92 % active oxygen while DHBP carries 11.02 %, so two grades at the same active-peroxide loading deliver different radical counts. Converting a recipe from phr to weight percent uses wt% = phr of the component divided by total phr, multiplied by 100; convert a whole recipe at once with the PHR to weight percent calculator.

How Do Crosslinking Agents Interact with the Rest of the Formulation?#

A crosslinking agent never acts alone: coagents raise the yield of every radical, antioxidants and acid scavengers compete for the same radicals, mineral flame retardants change the water balance of a silane cure, and the byproducts of the peroxide itself have to leave the part. Crosslinked foam is the clearest case of two additives on one clock, since the cure must hold a cell wall the moment the gas arrives from the blowing agents.

Four interactions help the cure, and they are listed below.

• Coagents suppress polypropylene chain scission and raise the crosslinks per radical, cutting the peroxide dose for a given gel content.
• Antioxidants, at the right level, can shorten a silane cure: in the 2019 POE and LLDPE study by Dana, Zohuri and co-workers, 0.3 wt% cut the cure from 14 to 16 hours to 8 hours, which is one study on one system, not a general rule.
• Lower-odour grades such as BIPB replace dicumyl peroxide where the acetophenone note of a DCP cure is a problem.
• Coupling silanes double as crosslinkers in HFFR compounds, where one vinyltrimethoxysilane molecule crosslinks the polymer and couples the flame retardant.

Three interactions fight the cure, and they are listed below.

• Radical scavengers compete with the polymer for the alkoxy radicals, raising the peroxide demand. Stabilizer packages that compete for the same radicals are on antioxidants for plastics.
• Peroxide byproducts raise the conductivity of high-voltage insulation and have to be degassed out.
• Residual water from a silane cure adds charge carriers, which is why the moisture route never reaches high-voltage insulation.

A mineral flame retardant sits on both sides, absorbing and releasing the moisture a silanol condensation needs while its surface requires the same alkoxysilane chemistry; the molecule used to bond a mineral filler is covered on silane coupling agents.

How Is the Degree of Crosslinking Measured?#

The degree of crosslinking is measured as gel content, the fraction of polymer that will not dissolve in hot xylene, or indirectly as hot-set elongation, and the pass mark is set by the product standard rather than by the chemistry. Gel content is also called gel fraction or xylene insolubles, and the extraction is usually run as a Soxhlet method. ASTM D2765 describes the xylene-extraction procedure; the extraction procedure and the hot-set rig are described on [gel content and hot set test].

A generic crosslinking percentage is meaningless without its product and standard, since the same 70 % passes ASTM F876-23 for peroxide PEX and fails the 84 to 90 % window for a photovoltaic encapsulant. Every method is indexed under testing plastic additives.

How Are Crosslinking Agents Regulated?#

Crosslinking agents are regulated in 4 layers: registration and the REACH Candidate List, which caught dicumyl peroxide in June 2024, the Annex XVII restriction on dibutyltin catalysts, food-contact positive lists in the European Union and the United States, and dangerous-goods rules, because every organic peroxide is a class 5.2 substance. Every instrument that applies to the family is summarised in plastic additive regulations, and the specific entries are collected below.

Why dicumyl peroxide is on the REACH Candidate List#

Dicumyl peroxide has been on the REACH Candidate List since 27 June 2024, listed as toxic for reproduction under Article 57(c), and it was the only substance added in that update. Its harmonised classification under CLP index 617-006-00-X is Org. Perox. F H242, Repr. 1B H360D, Skin Irrit. 2 H315, Eye Irrit. 2 H319 and Aquatic Chronic 2 H411. It is not on Annex XIV, the authorisation list, as checked on 22 September 2026, and it appears on the Community rolling action plan. Every listed additive and its date is on the SVHC Candidate List.

ECHA describes its use as a processing aid and cross-linker in polymers, so any article retaining more than 0.1 % by weight of dicumyl peroxide, a cable or a crosslinked foam for example, carries an Article 33 communication duty and a SCIP notification duty. Article 33 applies immediately on inclusion, with consumer requests answered within 45 days, and SCIP notification has applied since 5 January 2021. No residual level for a finished crosslinked article is established in our source library, so whether a given cable crosses 0.1 % has to be answered by analysis. Those duties are explained on REACH and plastic additives, and the other 10 peroxides, coagents and silanes here are not on the Candidate List as of the 35th update, November 2025.

Food contact: EU 10/2011 and 21 CFR#

In the European Union, the crosslinking chemicals that appear on the Union list of Regulation (EU) No 10/2011 are listed as monomers, not as additives: vinyltrimethoxysilane is FCM 453 and trimethylolpropane trimethacrylate is FCM 463, each with a specific migration limit of 0.05 mg/kg. Vinyltriethoxysilane is FCM 142 with the same limit and is permitted only as a surface treatment agent, and triallyl isocyanurate is not on the Union list at all. Union-list mechanics are on EU 10/2011.

Peroxide residues are generally handled as aids to polymerisation under Article 6(4)(b), which places them outside the Union list and under national law; that treatment is application-specific. In the United States the peroxides appear by section rather than by blanket permission: 21 CFR 177.2600 lists dicumyl peroxide, di-tert-butyl peroxide and triallyl cyanurate for rubber articles intended for repeated use, and 21 CFR 175.105 and 175.300 cover dicumyl peroxide in adhesives and as a polymerisation catalyst only. The 21 CFR sections are mapped on FDA food contact rules.

Organotin catalysts under REACH Annex XVII entry 20#

The catalyst half of the silane system carries the heaviest regulatory load: dibutyltin compounds have been limited to 0.1 % by weight of tin in mixtures and articles supplied to the general public since 1 January 2012 under entry 20 of REACH Annex XVII. Listed uses held derogations until 1 January 2015 under Decision 2009/425/EC. Dibutyltin dilaurate itself is not an SVHC, although dibutyltin dichloride (CAS 683-18-1) is on the Candidate List.

The consequence for a PEX-b or silane-cable formulator is a tin budget rather than a ban, since the restriction is expressed as tin content in the finished article supplied to the public. Dioctyltin dilaurate (CAS 3648-18-8) and tin-free catalysts are the alternatives; no commercial name for a tin-free silanol condensation catalyst is established in our source library. The other organotin compounds used in plastics are on organotin stabilizers, and entry 20 alongside the other restriction entries is set out on REACH Annex XVII restrictions.

Transport and storage: organic peroxides are class 5.2 dangerous goods#

Every organic peroxide is a class 5.2 dangerous good, and the number that governs its storage is not the half-life but the self-accelerating decomposition temperature, the lowest temperature at which decomposition becomes self-sustaining in the original packaging. Ts max, the maximum storage temperature, is a separate and much lower limit of 20 to 40 °C that protects the assay rather than the safety margin, and volatility adds a third constraint for grades such as di-tert-butyl peroxide, with a flash point of 1 °C.

The four handling rules recorded in supplier guidance are listed below.

• Never weigh out peroxide in the storage room. Dispensing belongs in a separate area from bulk storage.
• Keep peroxides away from reducing agents, amines in particular.
• Melt frozen liquid peroxides only by indirect heating, at a maximum of about 25 °C.
• Read the supplier safety data sheet for the grade and packaging in use, which carries the control and emergency temperatures, before setting a storage temperature.

Who Makes Crosslinking Agents? Suppliers and Trade Names#

Crosslinking peroxides are a concentrated business: about 60,000 tonnes of dialkyl peroxides are produced a year, dicumyl peroxide is the largest single product, and the grade names most formulators work with are Nouryon's Trigonox and Perkadox lines. No market size, growth rate or price for the crosslinking-agent family is established in our source library, so none is stated on this page. Company profiles are in the directory of plastic additive manufacturers and suppliers.

Only companies recorded in our source library are listed. Several other peroxide producers appear in supplier literature but are not yet verified here, so they are left out rather than guessed at.

Trade names are the working vocabulary of this family, and the index below maps each generic name to the names verified in our sources.

Cross-vendor equivalents that our sources mark as unverified, including several Luperox and Perhexa names, are deliberately left out. TAIC has no verified commercial name in our source library.

A buyer directory with locations and grades is on [organic peroxide manufacturers and suppliers].

Complete List of Crosslinking Agent Substances (13 Pages)#

The complete list of crosslinking agents below gives all 13 substances in this family with their CAS number, class, the temperature at which they act, the polymers they are used in and their regulatory status. The list follows the same type order as the rest of this page, from the dialkyl peroxides through the coagents to the silane system.

The full record for each substance, with identifiers, dosage and the complete regulatory matrix, is in the plastic additives database.

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Is a Crosslinking Agent the Same Thing in Hydrogels, Coatings and Biology?#

No: the word describes the same idea everywhere, a molecule that ties chains into a network, but a hydrogel crosslinker, a textile resin, a DNA crosslinking drug and a corneal riboflavin treatment have nothing in common with the peroxides and silanes used in plastics, from the chemistry to the dose to the regulations. Polyvinyl alcohol, polyethylene glycol, starch, cellulose and protein systems use glutaraldehyde, genipin, citric acid and calcium ions in water-based conditions no polyolefin compounder would recognise. This reference covers additives used in plastics only, so those chemistries are named here for disambiguation and nowhere else.

Thermoset curing agents, epoxy hardeners and sulfur vulcanisation#

Epoxy hardeners, unsaturated-polyester initiators and polyurethane polyols are not additives at all: they react into the matrix and become the plastic, which is why this reference covers them only as a border case. An amine or anhydride hardener is a stoichiometric reactant, and a polyol and an isocyanate are the two halves of a polyurethane rather than something added to one. Sulfur vulcanisation sits in rubber, and the comparison between the two cure systems is a rubber result: peroxide cures give about 15 % lower tensile and tear strength than sulfur cures, with better compression set and heat resistance and no reversion. Where the same peroxides are used for rubber is covered on [plastic additives in rubber and elastomers].

Can crosslinked plastics be recycled or de-crosslinked?#

A crosslinked plastic cannot be melted, so the ordinary recycling route is closed to it, and that is the main argument against crosslinking a part that would otherwise be recyclable. Crosslinking is not reversible in the sense a thermoplastic melt is reversible, because the carbon-carbon bridge is a permanent covalent bond, and that is the downside of crosslinked polyethylene most often raised against it.

De-crosslinking is nevertheless demonstrable in the laboratory. The 2024 ACS Applied Polymer Materials review "Cross-Linked Polyolefins: Opportunities for Fostering Circularity" reports pan-milling taking gel content from 77 % to 14 % for silane XLPE and from 70 % to 15 % for peroxide XLPE, and supercritical methanol taking it from 88 % to 0 %. Click chemistry and vitrimer networks are recorded as research directions for a recyclable XLPE, not as commercial practice. What crosslinking does to a recycling stream is covered under design for recycling.

What stays in the part: byproducts, degassing and migration#

A peroxide cure leaves its own decomposition products behind, and in high-voltage cable they matter enough to justify a separate process step: methane, acetophenone, cumyl alcohol, alpha-methylstyrene and water raise the conductivity of the insulation and are driven out by degassing. Laboratory practice degasses at about 70 °C for 24 hours; industrial degassing of a cable drum runs for days. Residue is also what triggers the Article 33 duty, which applies to any article retaining more than 0.1 % by weight of dicumyl peroxide.

Pipe carries a different residue question, measured in the water rather than the polymer. A 2007 study measured ETBE leaching from PEX-b pipe at 23 to more than 100 µg/L against an odour threshold of 5 µg/L, and a 2014 study of 6 PEX plumbing brands identified 11 PEX-related contaminants including toluene and 2,4-di-tert-butylphenol. Neither finding is a safety verdict on PEX plumbing. Where crosslinking residues sit among the substances of concern is on [toxic plastic additives].

A short history: from Sioplas in 1968 to peroxide-free HVDC insulation#

Silane crosslinking is younger than the peroxide route it competes with: the two-step Sioplas process was patented in 1968 and the one-step Monosil process followed in 1974, and both were answers to the cost of a peroxide cure line. No company attribution or inventor is verified for either process in our sources, so none is given. The current research direction reverses the premise: the 2024 Advanced Materials review "Alternative Concepts for Extruded Power Cable Insulation: from Thermosets to Thermoplastics" sets out peroxide-free thermoplastic insulation for HVDC cable, which would remove the byproducts, the degassing step and the recycling problem at once.

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Every figure on this page is checked against primary sources; see our methodology and fact-checking process.

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