PEX-a, PEX-b and PEX-c: How Polyethylene Pipe Is Crosslinked, 3 Methods Compared

PEX-a, PEX-b and PEX-c are the same crosslinked polyethylene made by 3 different methods: an organic peroxide crosslinks PEX-a in the melt at 200 to 250 °C (392 to 482 °F), a grafted vinylsilane cured with water crosslinks PEX-b after extrusion, and an electron beam crosslinks PEX-c with no chemical agent at all. ASTM F876-23 holds all three to one window of 65 to 89 % crosslinking, with a minimum of 70 % for the peroxide route and 65 % for the silane and radiation routes, so what really separates PEX-a from PEX-b, and which crosslinking agents does each one need? The answer lies in three things: the agent each route uses, the moment the polyethylene network forms, and the residues that agent leaves in the pipe wall. None of the three letters is a quality grade.

Crosslinking agents are among the few plastic additives that turn a thermoplastic into a network that no longer melts; in polyethylene they make XLPE cable insulation and PEX pipe, which together account for about 5 to 10 % of the polyethylene market, with growth of 6 to 8 % a year expected according to a 2024 review of crosslinked polyolefins in ACS Applied Polymer Materials.

This page defines PEX and its letters, sets out the 3 crosslinking methods in the order a, b, c, compares the three pipe types on 9 criteria, names the peroxides, the silane system and the electron-beam dose behind each route with their dose ranges, covers the antioxidants and other additives in the pipe compound, explains how the degree of crosslinking is measured against ASTM F876-23, reviews the REACH status of every agent and lists the suppliers. Every crosslinking chemistry, not only the pipe routes, is compared on crosslinking agents for polymers.

In brief:

• 3 crosslinking methods: peroxide in the melt (PEX-a), silane grafting with a water cure (PEX-b), electron beam after extrusion (PEX-c)
• Degree of crosslinking under ASTM F876-23: 65 to 89 % overall; minimum 70 % peroxide, 65 % silane, 65 % radiation
• Melt crosslinking for PEX-a: 200 to 250 °C (392 to 482 °F) in the Engel method
• PEX-a peroxides: typical crosslinking temperature 180 °C (356 °F) for di-tert-butyl peroxide and 185 °C (365 °F) for hexyne peroxide
• PEX-c dose: 50 to 150 kGy (5 to 15 Mrad) is the effective electron-beam range for polyethylene
• Market: XLPE and PEX together take about 5 to 10 % of all polyethylene

What Is PEX and Why Is Polyethylene Pipe Crosslinked?#

PEX is crosslinked polyethylene: high-density polyethylene whose chains have been joined into one three-dimensional network, so the pipe no longer melts and holds its shape and pressure rating in hot-water and heating service. The pipe industry writes the material as PEX or PE-X, and its base resin is HDPE with a density of about 0.961 g/cm3 (0.961 g/mL). Why crosslink a pipe resin at all? The network is permanent: it is what lets crosslinked pipe hold a pressure rating that the unmodified resin cannot, and that same permanence is why a PEX pipe can never be remelted and re-extruded.

Polyethylene pressure pipe such as PE100 is used without crosslinking, so the crosslinking step is the one additive decision that separates PEX from ordinary PE pipe. The stabilizers, slip agents and fillers that plain polyethylene needs are covered on additives for polyethylene.

What do the letters in PEX-a, PEX-b and PEX-c mean?#

The letter in PEX-a, PEX-b and PEX-c names the crosslinking method used to make the pipe, peroxide, silane or electron beam, and says nothing about its quality or performance class. ASTM F876-23 names the same three routes as peroxide, silane and radiation, and the German standard DIN 16892 writes them as PE-Xa, PE-Xb and PE-Xc, with a fourth label, PE-Xd, for a rarer azo route. The letter records the crosslinking method, nothing else. Which letter a pipe carries therefore tells a buyer how the pipe was crosslinked and which minimum degree of crosslinking it had to reach, not how it performs against the other two. Four letters are in use, and each one names a crosslinking chemistry.

• PEX-a (PE-Xa): peroxide crosslinking by the Engel method, in which the network forms in the melt
• PEX-b (PE-Xb): silane crosslinking, in which a grafted vinylsilane cures with water after extrusion
• PEX-c (PE-Xc): electron-beam (radiation) crosslinking of the finished pipe, with no chemical agent
• PEX-d (PE-Xd): azo crosslinking, a rare route covered below the contextual border

The order a, b, c is kept in every list and table on this page, so the peroxide route always comes first, the silane route second and the electron-beam route third.

What does crosslinking change in polyethylene pipe?#

Crosslinking ties the polyethylene chains of a pipe into one network that can no longer flow, which stops melt flow, slows creep and environmental stress cracking, makes the wall insoluble and ends the option of remelting it. Plain HDPE is a thermoplastic made of separate chains that slide past each other once the material is hot enough, and crosslinking turns those separate chains into one continuous molecule. Crosslinking changes 4 properties of polyethylene pipe, listed below.

• Melt flow: stops; a crosslinked pipe softens on heating but no longer flows
• Creep and environmental stress cracking: slow down, because the network restricts chain slippage, which is why crosslinked pipe holds a pressure rating the unmodified resin cannot
• Solvent resistance: improves, because the network is insoluble; that insoluble share is the gel content measured further down this page
• Remelting and re-extrusion: no longer possible, so PEX scrap cannot return to the extruder as ordinary polyethylene regrind can

How large are those gains in numbers? No numeric property gain for PEX over uncrosslinked HDPE pipe is held in our verified sources yet, so the changes are stated qualitatively here; the measurable expression of the network is the degree of crosslinking, which each route has to reach under ASTM F876-23. Electron-beam crosslinking at 50 to 150 kGy is reported to raise the strength and thermal properties of polyethylene, and the peroxide and silane routes build the same kind of network by chemistry instead of radiation.

Is PEX the same as XLPE?#

Yes in chemistry and no in usage: PEX and XLPE are the same crosslinked polyethylene, but the pipe industry writes PEX or PE-X, while the cable, foam and rotomoulding industries write XLPE. The same three crosslinking routes, peroxide, silane and electron beam, serve both product families. Cable insulation, foam and the peroxide doses they use are covered on XLPE (crosslinked polyethylene).

What Are the 3 Methods of Crosslinking PEX Pipe?#

The 3 methods of crosslinking PEX pipe are peroxide crosslinking in the melt (PEX-a), silane grafting followed by a moisture cure (PEX-b) and electron-beam irradiation of the finished pipe (PEX-c), and they differ mainly in the agent used and the moment the network forms. Table 1 sets the three methods side by side, from the agent to the minimum degree of crosslinking each must reach.

Minimums from ASTM F876-23; the overall window is 65 to 89 %.

A fourth route, PEX-d, uses azo compounds, and a photo-initiated PEX-e is being researched; both are covered briefly below the contextual border, because ASTM F876-23 sets minimums only for the peroxide, silane and radiation routes.

1. PEX-a: peroxide crosslinking in the melt (Engel method)#

PEX-a is polyethylene pipe crosslinked by an organic peroxide while the polymer is still molten: in the Engel method, HDPE carrying about 2 % peroxide is crosslinked at 200 to 250 °C (392 to 482 °F), so the network forms during or right after shaping. The 2 % figure comes from a secondary process description that does not name the peroxide, so this page names no single peroxide as "the" PEX-a agent. The chemistry itself is established: the peroxide splits at its O-O bond, the alkoxy radicals pull hydrogen atoms off the polyethylene chains, and two of the resulting polymer radicals join in a carbon-carbon crosslink. ASTM F876-23 then requires finished PEX-a tubing to reach at least 70 % crosslinking, the highest minimum of the three routes.

Which peroxide survives a 200 °C melt long enough to crosslink it there? A peroxide for PEX-a must pass through melting and shaping without curing early and then react fast at the crosslinking temperature, and the grades Nouryon lists for PEX are heat-stable dialkyl peroxides: di-tert-butyl peroxide, with a safe processing temperature of 145 °C (293 °F) and a typical crosslinking temperature of 180 °C (356 °F), and hexyne peroxide at 145 and 185 °C (365 °F). Dicumyl peroxide, the peroxide dosed in XLPE cable insulation, is safe only to 130 °C (266 °F) and crosslinks at about 170 °C (338 °F), so it sits lower on the temperature scale, although supplier and dossier records name it for PEX-a pipe as well. The half-life logic behind that choice is set out on organic peroxides for polymers.

Every peroxide leaves fragments behind once it has crosslinked the melt, and di-tert-butyl peroxide breaks down into tert-butanol and acetone. Tert-butanol, also called tert-butyl alcohol, is exactly what water studies find: Pelto-Huikko and co-workers (Materials, 2021) found volatile release from PEX-a pipe concentrated in the first months of service, with tert-butyl alcohol high at start-up. That finding is consistent with a tert-butyl peroxide in the recipe, although the study does not establish which peroxide the tested pipes contained.

2. PEX-b: silane grafting and moisture cure (Sioplas and Monosil)#

PEX-b is polyethylene pipe crosslinked by a vinylsilane that is grafted onto the chains in the extruder and then cured with water after the pipe is made, so the network forms after extrusion rather than in the melt. The bridges are siloxane links, silicon to oxygen to silicon (Si-O-Si), rather than the carbon-carbon bonds of PEX-a and PEX-c, and ASTM F876-23 requires PEX-b tubing to reach at least 65 % crosslinking. Silane crosslinking makes PEX-b in 3 steps.

1. Graft vinyltrimethoxysilane (VTMS) onto the polyethylene with a small peroxide dose in the extruder.
2. Extrude the pipe while the grafted silane groups stay unreacted.
3. Cure the pipe with water, in a sauna, in hot water or in ambient humidity, where a catalyst such as dibutyltin dilaurate (DBTDL) links the silane groups into Si-O-Si bridges.

Why does a silane route need water at all? The methoxy groups on the grafted silicon atom (Si-OMe) react with water by hydrolysis, and the resulting silanol groups then condense with each other into Si-O-Si bridges, a two-stage reaction that the tin catalyst speeds up. Water therefore has to diffuse into the pipe wall, which is why the cure runs in a sauna or hot-water bath, or more slowly in ambient humidity, after the pipe has left the extruder.

Two process variants put the silane and the catalyst into the pipe. Sioplas, patented in 1968, runs in two steps: a grafted compound is made first, and a separate catalyst masterbatch is blended in when the pipe is extruded. Monosil, dating from 1974, grafts and adds the catalyst in a single step at the extruder. Grafting conditions and catalyst masterbatches are compared on silane crosslinking (Sioplas and Monosil).

3. PEX-c: electron-beam crosslinking after extrusion#

PEX-c is polyethylene pipe crosslinked after extrusion by a beam of high-energy electrons, so it needs no peroxide or silane at all: the radiation creates polymer radicals that join into carbon-carbon crosslinks. The pipe is first extruded as uncrosslinked HDPE and then passed under the beam, which makes PEX-c the only route in which the compound carries no crosslinking chemical. ASTM F876-23 sets the same 65 % minimum for this radiation route as for the silane route.

How much radiation does a PEX-c pipe need? An absorbed dose of 50 to 150 kGy (5 to 15 Mrad) is the effective range reported for polyethylene. The beam works cold, below the crystal melting point of the polymer, and that cold process makes crosslinking less uniform through a thick wall than through a thin one, a practical limit on the wall thickness this route suits. A 2026 study in Polymers on recycled polyethylene raised the gel content from 46.7 to 56.2 % between 95 and 125 kGy and judged 110 kGy the best balance between crosslinking and the other properties of the material. Dose, equipment and coagents are covered on radiation (e-beam) crosslinking.

PEX-a vs PEX-b vs PEX-c: What Is the Difference?#

PEX-a, PEX-b and PEX-c differ in how and when the network forms: PEX-a is crosslinked by a peroxide in the melt and must reach 70 %, PEX-b by a grafted silane and water after extrusion and must reach 65 %, and PEX-c by an electron beam and must also reach 65 % under ASTM F876-23. The answer to "what is PEX-a or PEX-b" is therefore a chemistry answer, not a performance ranking. Table 2 compares the three types on 9 criteria, from the crosslinking agent to the regulatory flag on that agent.

Rows 2 and 7 describe the chemistry each route can use; an individual pipe maker's recipe is not public.

The chemistry leaves different traces: peroxide byproducts such as tert-butanol in PEX-a, a tin catalyst and silane-derived residues in PEX-b, and nothing from a crosslinking agent in PEX-c, which is why water-quality studies have looked at each route separately. The same traces decide which regulation touches which pipe, since only PEX-a and PEX-b carry an agent that a REACH listing can reach. Does any of this make one type better?

Is PEX-a better than PEX-b?#

No standard ranks them: ASTM F876-23 covers PEX-a and PEX-b in one specification, and the route-specific requirement it sets is the minimum degree of crosslinking, 70 % for peroxide and 65 % for silane. A 70 % minimum is a pass mark, not a measured value, so it describes the lowest acceptable PEX-a pipe rather than a typical one. What does differ between the two? Three things do differ between PEX-a and PEX-b.

• The crosslinking agent and its residues: a heat-stable peroxide that leaves alcohol and ketone fragments in PEX-a, against a grafted vinylsilane and a tin or tin-free condensation catalyst in PEX-b
• The moment of crosslinking: in the melt during or right after extrusion for PEX-a, after extrusion in a water or humidity cure for PEX-b
• The minimum degree of crosslinking: 70 % for PEX-a and 65 % for PEX-b under ASTM F876-23

Installer questions such as flexibility, kink repair, fitting systems and price sit outside this additive reference, and no source in our verified library ranks the three types on any of them. Buyers of tubing should compare the certification marks and the standard printed on the pipe rather than the letter.

PEX-b vs PEX-c: silane or electron beam?#

PEX-b and PEX-c share the same 65 % minimum in ASTM F876-23 but reach it in opposite ways: PEX-b carries a grafted silane and a condensation catalyst and cures in water, while PEX-c carries no crosslinking chemical and is irradiated with 50 to 150 kGy, which crosslinks thick walls less evenly. The silane route moves the crosslinking into the compound, so its cost in regulatory terms is the catalyst: dibutyltin dilaurate carries a harmonised Repr. 1B classification, and alternatives such as dioctyltin dilaurate or tin-free catalysts exist. The electron-beam route moves the crosslinking into a separate irradiation step instead, so the compound stays a plain stabilized HDPE and the process variable becomes the absorbed dose. Each route therefore trades a chemical residue against an equipment and wall-thickness limit.

Is PEX-a more crosslinked than PEX-b?#

Not necessarily: ASTM F876-23 sets a higher minimum for PEX-a, 70 % against 65 % for PEX-b, but both must stay inside the same 65 to 89 % window, so an individual PEX-b pipe can be more crosslinked than an individual PEX-a pipe. DIN 16892 is reported to set 75 % for PE-Xa and 65 % for PE-Xb, a secondary-source figure not yet verified against ISO 15875. A minimum is a floor, never a typical value.

Which Crosslinking Agents Are Used to Make PEX Pipe?#

PEX pipe uses 2 kinds of crosslinking agent and 1 route without any: high-temperature dialkyl peroxides such as di-tert-butyl peroxide and hexyne peroxide for PEX-a, a vinylsilane with a grafting peroxide and a tin condensation catalyst for PEX-b, and no chemical agent for electron-beam PEX-c. Table 3 lists the 6 agents named for PEX in our sources, in the order a, b, c, with their identity, temperatures and regulatory status.

Half-life and processing temperatures are Nouryon data measured in chlorobenzene (safe processing = rheometer ts2 above 20 minutes; typical crosslink = t90 about 12 minutes). Trigonox 311 (typical crosslink 220 °C) is a further high-temperature option named in our research dossier.

Request quotes for PEX crosslinking agents: peroxide (DTBP, hexyne peroxide, DHBP), vinyltrimethoxysilane or catalyst masterbatch; tell us the route (PEX-a or PEX-b), the volume and the country through the plastic additive supplier finder.

Peroxides for PEX-a: di-tert-butyl peroxide, hexyne peroxide and DHBP#

PEX-a uses the most heat-stable dialkyl peroxides because the HDPE melt has to pass through the extruder without curing: di-tert-butyl peroxide and hexyne peroxide both tolerate 145 °C (293 °F) safely and crosslink at about 180 and 185 °C (356 and 365 °F), against 130 and 170 °C (266 and 338 °F) for dicumyl peroxide. The 10-hour half-life temperature tells the same story on a second scale: 121 °C for di-tert-butyl peroxide, 115 °C for DHBP and 112 °C for dicumyl peroxide, all measured by Nouryon in chlorobenzene. DHBP, sold as Trigonox 101, sits between the two groups with a safe processing temperature of 135 °C and a typical crosslinking temperature of 175 °C (347 °F), and Nouryon lists it for PEX pipe as well. For still higher melt temperatures, Trigonox 311, a cyclic peroxide with a safe processing temperature of 180 °C and a typical crosslinking temperature of 220 °C (428 °F), is named for PEX-a and rotomoulding in our research dossier.

Which property does a compounder check next? Active oxygen, the share of the molecule's mass carried by its peroxide oxygen, is the second number on every data sheet: 10.94 % for di-tert-butyl peroxide, 11.02 % for DHBP and 11.17 % for hexyne peroxide. Di-tert-butyl peroxide is a volatile liquid that boils at 110 to 111 °C (230 to 232 °F) and has a flash point of 1 °C (33.8 °F), which makes it a handling issue in the plant. Its flash point of 1 °C and its byproducts are detailed on di-tert-butyl peroxide. The same dialkyl peroxides that crosslink polyethylene cut polypropylene chains instead, through β-scission of the tertiary polymer radicals, so a PEX-a peroxide is never a crosslinker for PP.

Organic peroxides are class 5.2 dangerous goods, and their self-accelerating decomposition temperature (SADT), the lowest temperature at which decomposition can run away in the original package, sets the storage control: 80 °C (176 °F) for both di-tert-butyl peroxide and hexyne peroxide, with maximum storage temperatures of 40 °C (104 °F) and 30 °C (86 °F).

The silane system for PEX-b: VTMS, a grafting peroxide and a tin catalyst#

The PEX-b silane system has 3 components: vinyltrimethoxysilane (VTMS), which is grafted onto the polyethylene, a small peroxide dose that does the grafting, and a condensation catalyst such as dibutyltin dilaurate (DBTDL) that links the silane groups once water reaches them. VTMS (CAS 2768-02-7, EC 220-449-8) is a colourless liquid that boils at 123 °C (253 °F) and has a flash point of 25.5 °C (77.9 °F). Grades and the full regulatory profile are on vinyltrimethoxysilane (VTMS). The VTMS dose used in PEX-b pipe is not published in an open primary source, so no figure is given here.

The grafting peroxide is the second component, and its choice changes the result. In a 2019 study in the Journal of Macromolecular Science, Part A, M. Dana, G. Zohuri and co-workers found benzoyl peroxide a better grafting initiator than dicumyl peroxide for VTMS in a POE/LLDPE blend: 0.2 wt% benzoyl peroxide gave 79 % gel and a compression set of 40.4 %, and adding 0.3 wt% antioxidant cut the cure time from 14 to 16 hours to 8 hours. That is one study on one elastomer blend, not a PEX pipe recipe, but it shows how strongly the grafting step and the stabilizer level shape the final network.

The condensation catalyst is the third component. Dibutyltin dilaurate (CAS 77-58-7) catalyses both the hydrolysis of the methoxysilyl groups and the condensation of the silanols into Si-O-Si bridges, and it reaches the pipe through the catalyst masterbatch. Alternatives include dioctyltin dilaurate (CAS 3648-18-8) and tin-free condensation catalysts, whose commercial names are not yet verified in our sources.

Electron-beam dose for PEX-c: crosslinking without a chemical agent#

PEX-c needs no crosslinking agent, so its process variable is the absorbed dose: 50 to 150 kGy (5 to 15 Mrad) is the effective range reported for polyethylene, and a higher dose raises gel content only up to the point where other properties start to suffer. That trade-off is visible in the 2026 Polymers study on recycled polyethylene, where 110 kGy gave the best balance within a tested range of 95 to 125 kGy. Coagents such as TMPTMA and TAIC raise the crosslinking yield in radiation-crosslinked foam and wire, with TMPTMA used at up to 8 phr in electron-beam PP/POE foam; no PEX-c pipe recipe in our sources names one. Coagents are covered on their own page once it is published.

How Much Crosslinking Agent Does PEX Pipe Need?#

PEX-a is usually described as HDPE with about 2 % peroxide, while supplier tables for polyethylene in general run from about 0.56 to 2.7 phr of pure dicumyl peroxide and from 0.8 to 4.0 phr of a 45 % DHBP formulation, and the exact level is set by the 70 % minimum the pipe must pass. The Nouryon ranges cover polyethylene in general, cable and foam included, and are not pipe recipes. A 40 % formulation dosed at 1.4 phr delivers 0.56 phr of active peroxide. Table 4 gives every dose recorded for the 3 routes, with its basis and its verification status.

phr = parts per hundred parts of polymer; a 40 % or 45 % formulation carries 40 or 45 % active peroxide on an inert carrier.

Why do the ranges run so wide? Four factors set the dose, listed below.

• The target degree of crosslinking: a 70 % minimum for PEX-a and 65 % for PEX-b and PEX-c under ASTM F876-23
• The active content of the supplied formulation: a 40 % or 45 % grade needs 2.2 to 2.5 times the phr of the pure peroxide for the same active dose
• Antioxidants that compete for the same radicals: a stronger stabilizer package raises the peroxide demand
• Melt temperature and residence time in the extruder: together they decide how much of the peroxide decomposes before and after shaping

Convert formulation phr into active peroxide with the organic peroxide half-life and dosage calculator. The conversion method itself, active content times formulation phr, is the same one used for cable and foam, and the full worked method sits on the peroxides page linked in the PEX-a section above.

What Else Goes into a PEX Pipe Compound?#

Besides the crosslinking agent, a PEX pipe compound carries antioxidants for long-term resistance to hot, disinfected water, UV protection for storage and installation, and colourants, while any barrier layer is applied as a separate layer. Each of these is a separate additive or layer with its own job, and none of them is part of the crosslinking reaction. PVC, PE, PP-R and PEX packages are compared on additives for plastic pipes. Antioxidants carry the heaviest load of the three additive groups, because they must survive the crosslinking step and then decades of hot water.

Antioxidants and chlorine resistance#

Antioxidants protect PEX against the slow oxidation that hot, disinfected water drives, and that protection is consumed in service: Pelto-Huikko and co-workers measured antioxidant depletion in PEX-a pipe over five years, and in polyethylene pipe 4 ppm chlorine dioxide at 90 °C (194 °F) consumes antioxidant about four times faster than ordinary chlorination. The chlorine dioxide finding comes from a study published in Polymer Degradation and Stability (2011), and no standardised accelerated test for chlorine dioxide exists yet, so pressurised loop tests remain the closest replica of service. Phenol and phosphite grades for pipe are compared on antioxidants for polyethylene.

The antioxidant level in PEX also collides with the crosslinking chemistry itself. Radical scavengers compete with the polymer for the alkoxy radicals a peroxide releases, so in PEX-a the antioxidant level and the peroxide dose are set together, and a stronger stabilizer package raises the peroxide demand. In silane systems the interaction can run the other way: the 0.3 wt% antioxidant in the Dana and Zohuri study shortened the moisture cure from 14 to 16 hours to 8 hours. Antioxidants also leave breakdown products of their own: 2,4-di-tert-butylphenol, a degradation product of Irgafos 168-type phosphite antioxidants rather than an additive itself, was among the 11 PEX-related contaminants that Kelley and co-workers identified across 6 brands (Water Research, 2014).

UV protection and the PEX material designation code#

PEX pipe carries UV protection for storage and installation rather than for permanent outdoor service, because PEX tubing is not designed for long-term sunlight exposure. The UV protection buys time between production and installation. The material designation code printed on PEX tubing is not decoded on this page yet: its values come from a trade-association technical note that is still being verified against the ASTM standards, and only verified values are published here. HALS and UV absorbers for PE are compared on UV stabilizers for polyethylene.

Oxygen barrier layers and colour coding#

Oxygen barrier layers and colour coding are added to a PEX pipe as separate layers or colourants, not as part of its crosslinking chemistry, and our verified sources do not yet name the barrier polymer or a colour convention for PEX. One colourant rule is established, though: REACH Annex XVII entry 23 lists PE-X among the plastics in which cadmium must stay below 0.01 % by weight (Regulation (EU) No 494/2011, applicable from 10 December 2011), which keeps cadmium-based pigments out of coloured PEX sold in the EU.

How Is the Degree of Crosslinking in PEX Measured?#

The degree of crosslinking in PEX is measured as gel content, the share of the pipe that does not dissolve when a sample is extracted in hot xylene under ASTM D2765, and ASTM F876-23 requires 65 to 89 %, with at least 70 % for PEX-a and 65 % for PEX-b and PEX-c. Gel content is also called gel fraction or xylene insolubles: the crosslinked network stays behind as an insoluble residue, while uncrosslinked chains dissolve. Which number applies depends on the standard, so Table 5 lists the targets by standard, with a cable target for contrast.

A crosslinking percentage means nothing without its product and its standard: the same 70 % passes ASTM F876-23 for peroxide PEX but falls short of the 84 to 90 % optimum reported for EVA solar-panel encapsulant, and XLPE cable is judged by hot-set elongation instead of gel content. The reason ASTM F876-23 also sets an upper limit of 89 % is not documented in our sources, so it is not explained here. Sample preparation and the extraction procedure are on gel content and hot set test.

Which standards set the PEX pipe requirements?#

PEX pipe is qualified to product standards such as ASTM F876 in North America and ISO 15875 and DIN 16892 for PE-X pipe, to test methods such as ASTM D2765 for gel content, and, for drinking water, to NSF/ANSI/CAN 61. Table 6 lists the 4 standards our verified sources cover, from product to test to water contact.

Further PEX system, chlorine and UV test standards are being verified and will be added to this table.

What Is the Regulatory Status of PEX Crosslinking Agents?#

One PEX crosslinking agent is on the REACH Candidate List, dicumyl peroxide since 27 June 2024, and one is restricted in consumer articles, the tin catalyst DBTDL under Annex XVII entry 20, while di-tert-butyl peroxide, hexyne peroxide, DHBP and VTMS are not SVHCs. The "not an SVHC" statements for the three peroxides hold as of the 4 February 2026 Candidate List update, and VTMS returned no Candidate List entry on ECHA CHEM on 22 September 2026. Electron-beam PEX-c carries no agent and therefore no agent-related listing.

Dicumyl peroxide on the SVHC Candidate List since 27 June 2024#

Dicumyl peroxide has been on the REACH Candidate List since 27 June 2024 as toxic for reproduction (Article 57(c)), so a PEX or XLPE article that still contains more than 0.1 % of unreacted DCP triggers the Article 33 information duty and a SCIP notification. Half-life, byproducts and the full classification are on dicumyl peroxide (CAS 80-43-3), which carries a harmonised Repr. 1B H360D and Org. Perox. F H242 classification.

The Candidate List entry is not a ban. Dicumyl peroxide is not on Annex XIV, the REACH authorisation list (checked 22 September 2026), and Regulation (EU) 2021/2204 added it to Appendix 6 of Annex XVII, the entries 28 to 30 list of substances that may not be sold to the general public in mixtures. It is not listed on the California Proposition 65 list (OEHHA list edition 31 July 2026, checked 25 September 2026).

What does that mean for PEX? The heat-stable grades named for PEX-a, di-tert-butyl peroxide and hexyne peroxide, are not SVHCs, which is a statement about their status, not a recommendation to switch. Every crosslinking agent on the list, with its date, is on the SVHC Candidate List.

Organotin catalysts under REACH Annex XVII and vinylsilanes in PEX-b#

The tin catalyst is the regulated part of a PEX-b recipe: dibutyltin dilaurate carries a harmonised Repr. 1B classification, and REACH Annex XVII entry 20 has limited dibutyltin compounds to 0.1 % tin by weight in mixtures and articles for the general public since 1 January 2012. The restriction comes from Commission Decision 2009/425/EC, and DBTDL (index 050-030-00-3) is also classified Muta. 2 H341 and STOT RE 1 H372. DBTDL itself is not an SVHC, but two related dibutyltin compounds are: dibutyltin dichloride since 19 December 2012 and dibutyltin bis(acetylacetonate) since 25 June 2020. No measured tin level for PEX-b pipe is held in our sources, so this page does not state whether pipe meets or exceeds the limit. Entry 20 and the cadmium entry 23 that lists PE-X are explained under REACH Annex XVII restrictions.

The vinylsilane carries a lighter regulatory profile. VTMS has a harmonised Skin Sens. 1B H317 classification, is not an SVHC, and is listed under Regulation (EU) No 10/2011 as FCM substance 453 (a monomer) with a specific migration limit of 0.05 mg/kg. A different vinylsilane, tris(2-methoxyethoxy)vinylsilane (CAS 1067-53-4), is the SVHC: it has been on the Candidate List since 17 January 2022 as toxic for reproduction.

What can PEX pipe release into drinking water?#

Water from new PEX pipe can carry traces of crosslinking byproducts and antioxidant breakdown products rather than the additives themselves: tert-butyl alcohol from PEX-a, ETBE from PEX-b and 2,4-di-tert-butylphenol across 6 tested brands are the compounds the studies name. Pelto-Huikko and co-workers (Materials, 2021) found volatile release from PEX-a pipe concentrated in the first months, with tert-butyl alcohol high at start-up. Positive lists and NSF/ANSI 61 certification are covered on plastic additives in drinking-water contact.

Durand and Dietrich at Virginia Tech (Water Science and Technology, 2007) measured ETBE leaching from PEX-b pipe at 23 to more than 100 µg/L (23 to more than 100 ppb) against an odour threshold of 5 µg/L. That finding is specific to the pipes and test conditions of that study, as is every result in this section.

Kelley and co-workers (Water Research, 2014) identified 11 PEX-related contaminants across 6 brands, including toluene and 2,4-di-tert-butylphenol, and found odour above the US EPA secondary maximum contaminant level. 2,4-di-tert-butylphenol is treated as one of the NIAS (non-intentionally added substances) that a phosphite antioxidant leaves behind.

• SVHC updates: ECHA's Candidate List updates in January and June, where a further peroxide or coagent could be added
• Annex XVII entry 20: changes to the organotin restriction that governs DBTDL catalysts
• EU drinking-water positive list: the European list that applies from 31 December 2026

Who Supplies Crosslinking Agents for PEX Pipe?#

Crosslinking agents for PEX come from two groups of producers: organic peroxide makers such as Nouryon, whose Trigonox B, Trigonox 145 and Trigonox 101 grades are listed for PEX, and silane makers such as Evonik, Momentive, Dow, Shin-Etsu and Wacker, which supply vinyltrimethoxysilane. Arkema also manufactures di-tert-butyl peroxide and dicumyl peroxide, although its grade names are not yet verified for this page. Table 7 lists each company with the agents and grades recorded in our sources.

Dialkyl peroxides as a class are produced at about 60,000 t a year with dicumyl peroxide dominant, according to a secondary source citing Ullmann's encyclopedia. Buyers should compare grades by CAS number, active content and half-life data rather than by trade name, since the same peroxide is sold pure, as a 40 or 45 % formulation or as a masterbatch. Plants, grades and certifications are listed under organic peroxide manufacturers and suppliers.

Send one request to several peroxide and silane producers with the plastic additive supplier finder: name the route (PEX-a or PEX-b), the agent or catalyst masterbatch, the volume and the country.

─── The sections below compare PEX with other plastic pipe materials and answer short questions that sit outside the crosslinking chemistry above. ───

How Does PEX Compare with Other Plastic Pipe Materials?#

PEX differs from the other plastic pipe materials in one additive: it is the only one that carries a crosslinking agent or an electron-beam dose, while HDPE pipe is not crosslinked and PVC pipe depends on heat stabilizers and lubricants. The comparison below therefore runs on additive packages, not on installation, and each material keeps its own stabilizer logic.

PEX vs polybutylene (PB) pipe#

PEX is defined by its crosslinking step, and polybutylene (PB) pipe is a separate polyolefin pipe material whose additive package, failure history and standards are not yet verified in our sources. This page therefore makes no claim about PB beyond its name; the comparison will be added once the trade-association data on PB pipe has been verified against primary sources.

PEX vs HDPE and XLPE#

HDPE is the uncrosslinked starting material of PEX, and XLPE is the same crosslinked polymer under its cable name. HDPE pressure pipe relies on its stabilizer package instead of crosslinking: ISO 4427-1:2019 requires an oxidation induction time of at least 20 minutes at 210 °C (410 °F) and, for black compounds, 2.0 to 2.5 % carbon black by mass. Those are PE pressure-pipe requirements, given here for contrast, not PEX requirements. XLPE uses the same three crosslinking routes as PEX but serves cable insulation, foam and rotomoulded tanks, as set out in the XLPE answer near the top of this page.

PEX vs PVC and CPVC pipe#

PVC pipe needs a heat stabilizer, lubricants and fillers instead of a crosslinking agent: a typical US PVC pressure-pipe compound in PPI TR-2 carries 0.70 phr of heat stabilizer, 1.20 phr of paraffin wax and 5.00 phr of calcium carbonate per 100 phr of resin. The full formulation adds 0.15 phr of PE wax, 0.50 phr of titanium dioxide, 0.03 phr of pigment and 0.45 phr of calcium stearate, for 108.03 phr in total. In the EU, lead in PVC has been limited to below 0.1 % since 29 November 2024 under REACH Annex XVII entry 63 (Regulation (EU) 2023/923). No verified CPVC package is held in our sources. The full PVC pressure-pipe package is on additives for PVC pipe.

Can PEX pipe be recycled?#

PEX cannot be melted and re-extruded like ordinary polyethylene, so recycling depends on breaking the network first: in studies reviewed in ACS Applied Polymer Materials in 2024, pan-milling cut the gel content of silane-crosslinked PE from 77 to 14 % and supercritical methanol cut another crosslinked PE from 88 to 0 %. Pan-milling also cut peroxide-crosslinked PE from 70 to 15 %, so both chemical routes to PEX can in principle be reversed, though none of these methods is yet standard practice for pipe. Why crosslinking blocks mechanical recycling is set against other additives in how additives affect plastic recyclability.

Is PEX a poly pipe?#

Yes: PEX is a polyethylene pipe made from HDPE that has been crosslinked, so it belongs to the polyethylene ("poly") pipe family but no longer melts like ordinary PE pipe.

What does PEX stand for?#

PEX stands for crosslinked polyethylene: PE is the polymer and X is the usual shorthand for "cross", the same convention the cable industry uses in XLPE.

Is there a PEX-d?#

Yes, but it is rare: PEX-d is crosslinked with azo compounds (the Lubonyl process), and researchers are also testing photo-initiated crosslinking, called PEX-e, for geothermal heat-storage pipe (Simão de Sousa and co-workers, ACS Omega, 2025). DIN 16892 is reported to set a 60 % minimum for PE-Xd, a secondary-source value not yet verified against ISO 15875.

Is PEX UV resistant?#

Only for a limited time: PEX is not designed for long-term sunlight exposure, and its UV protection covers storage and installation, not permanent outdoor service.

What are the material limits of PEX pipe?#

PEX pipe has 4 material limits: it tolerates sunlight only for a limited time, it cannot be remelted, new pipe can release crosslinking byproducts that affect odour, and hot disinfected water slowly consumes its antioxidants. The 4 limits are listed below.

• Limited UV exposure: UV protection covers storage and installation only
• No remelting: crosslinked PE cannot be re-extruded, so recycling needs de-crosslinking first
• Start-up odour and volatiles: tert-butyl alcohol from PEX-a and ETBE from PEX-b in the studies above
• Antioxidant consumption: depletion over 5 years in PEX-a pipe, and about 4 times faster in polyethylene pipe exposed to chlorine dioxide at 90 °C

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

Byline: PlasticAdditives.net Editorial Team

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