P-Xylene SDS: Safety Data, Handling and Storage Information

Substance identification for p-xylene is fixed by CAS 106-42-3, EC 203-396-5, Index 601-022-00-9, and IUPAC name 1,4-dimethylbenzene; the molecular formula C₈H₁₀ corresponds to a relative molecular mass of 106.17 g/mol. The liquid is supplied as a bulk chemical intermediate for terephthalic acid and dimethyl terephthalate production and is also used in solvent applications where a single-isomer aromatic hydrocarbon is required. Under Regulation (EC) No 1272/2008, the harmonised classification for xylene isomers applies to p-xylene and assigns Flam. Liq. 3 (H226), Acute Tox. 4 via dermal and inhalation routes (H312 and H332), Skin Irrit. 2 (H315), Eye Irrit. 2 (H319), STOT SE 3 (H335), STOT RE 2 (H373), Asp. Tox. 1 (H304), and Aquatic Chronic 3 (H412); the supplemental hazard statement EUH066 is included because repeated exposure may cause skin dryness or cracking. Signal word DANGER; pictograms GHS02 (flame), GHS07 (exclamation mark), and GHS08 (health hazard). The 16-section safety data sheet structure of Annex II to REACH as amended by Regulation (EU) 2020/878 requires the supplier to list the substance as a single-component product, and impurities that alter the classification must be declared under the composition section; commercial p-xylene purity is commonly not less than 99.0% by mass, but no public harmonised purity specification exists. Labelling for p-xylene should include the hazard statements H226, H304, H312, H315, H319, H332, H335, H373, and H412; small-container derogations under Annex I of CLP do not remove the requirement for the core hazard communication elements on workplace pack sizes above 125 mL unless national enforcement practice expressly permits otherwise.

What vapour-phase thresholds define the flammability envelope for bulk p-xylene storage?

Closed-cup flash point values of 25 °C to 27 °C are reported in published physical property compilations; the conservative endpoint of 27 °C is used for storage classification because flash point test repeatability under ISO 2719:2016 or ASTM D93 can approach ±2 °C. The lower explosive limit in air is 1.1% v/v and the upper explosive limit is 7.0% v/v when determined under ASTM E681-04(2015) or EN 1839:2017. These concentrations correspond to 11,000 ppm and 70,000 ppm at 101.325 kPa and 20 °C and are several orders of magnitude above the occupational exposure limits adopted for the substance. The autoignition temperature of 528 °C places the vapour in temperature class T1 and gas group IIA under IEC 60079-0:2017 and EN 60079-10-1:2021; explosion-protected electrical equipment and mechanical equipment should therefore be selected for IIA/T1 or the relevant area classification determined by a hazardous-area assessment. Because vapour density relative to air is approximately 3.7, releases accumulate in pits, trenches, drain sumps, and other low-lying spaces rather than dispersing upward, and the vapour may travel along the ground to an ignition source remote from the release point. Ventilation design should maintain the vapour concentration below 10% of the lower explosive limit, namely 0.11% v/v or 1,100 ppm, under all normal operating modes; this engineering target is not a health-based exposure limit because the EU 8-hour TWA is 50 ppm and the OSHA 8-hour TWA is 100 ppm. The vapour pressure at 20 °C of approximately 0.9 kPa means a closed container headspace can already contain several percent by volume, and at 40 °C the saturated vapour concentration rises sufficiently to exceed the lower explosive limit if the headspace is not inerted or continuously ventilated. Explosion zone classification for indoor transfer areas should be carried out under EN 60079-10-1:2021, and the basis of safety should be documented with the maximum process temperature, not the ambient vapour pressure, because diurnal heating of storage vessels can significantly broaden the flammable headspace volume.

Toxicological data are largely based on mixed xylene because the para isomer is not tested separately in many repeated-dose studies; the harmonised classification therefore follows the xylene isomer group. The acute oral LD50 in rats is reported near 3,523 mg/kg, and the acute inhalation 4-hour LC50 in rats is approximately 4,550 ppm (26,900 mg/m³); these values place the substance in acute toxicity category 4 for the dermal and inhalation routes but not for the oral route. The central nervous system is the principal acute target; controlled human studies have reported decrements in balance, reaction time, manual coordination, and colour discrimination after exposure at 150–400 ppm for 2–4 h. Aspiration is a critical route-specific hazard because the dynamic viscosity of 0.65 mPa·s at 20 °C and the low surface tension permit rapid spread in the bronchial tree; first aid after ingestion must not induce vomiting, and the airway must be protected because H304 aspiration pneumonia can be fatal. Skin contact causes defatting and erythema after prolonged exposure; the EUH066 statement is supported by dermal irritation studies and occupational case reports. Eye exposure to liquid produces conjunctival irritation and epithelial damage that is reversible when irrigation with water or 0.9% sodium chloride solution is initiated within seconds and maintained for 15 minutes using a low-pressure eye wash. Chronic repeated exposure at high concentrations has produced neurological signs, reduced body weight gain, and hepatocellular hypertrophy in rodent studies; the STOT RE 2 classification specifies the nervous system, liver, and kidneys as target organs. IARC Monographs Volume 71 classifies xylenes in Group 3, not classifiable as to human carcinogenicity; the harmonised CLP classification does not include carcinogenicity. Biological monitoring for xylenes uses urinary methylhippuric acid; a post-shift value of 1.5 g/g creatinine appears in some national and ACGIH documentation, but no harmonised EU biological limit value is adopted in Directive 2000/39/EC.

Storage tank material compatibility and static earth bonding at ambient tank farms

Carbon steel tanks conforming to EN 14015:2004 or API 650 are used for large bulk storage; small above-ground shop-built tanks are often constructed to UL 142. The freezing point of 13.2 °C requires trace heating or indoor storage for transfer lines in cold climates because solidification in pump casings, flow meters, and narrow-bore instrument lines can occur even when the bulk liquid remains above its melting point. The bulk liquid should be maintained below 30 °C under normal operation, and the vapour space temperature should not be allowed to approach the flash point during transfer; if a fixed-roof tank is exposed to high solar load, the shell and vapour space can heat well above ambient and increase evaporation rate. Piping and pump seals should use polytetrafluoroethylene, expanded graphite, or fluoropolymer-encapsulated elastomers; EPDM, nitrile, natural rubber, and styrene-butadiene rubber are unsuitable for continuous immersion because p-xylene causes swelling, loss of compression set, and leakage within hours to days at ambient temperature. Uncoated aluminium is not recommended for pressure-containing product-contact parts where condensed moisture or trace acidic impurities can create localised pitting; published data for this specific configuration is limited, but conservative materials selection excludes uncoated aluminium. Bonding resistance to earth should be below 10 Ω per NFPA 77:2019 and EN 1127-1:2019; fixed tanks should be earthed at two points, and transfer piping should use conductive gaskets or bonding jumpers across insulating flange pairs. Vent lines should terminate outside at a safe height and be fitted with an end-of-line deflagration arrester tested to ISO 16852:2016 for vapour group IIA. Nitrogen inerting at 4–6 kPa gauge is used in closed storage systems where required; during initial filling with air in the headspace, the vapour concentration passes through the flammable range, so the fill rate should be limited until the outlet is submerged and the vapour space can be purged. For indoor flammables cabinets, EN 14470-1:2004 or FM 6050 should govern construction, and the cabinet should be segregated from oxidising substances, strong acids, and non-compatible compressed gases.

Where process enclosure and local exhaust ventilation cannot keep the exposure below the EU TWA of 50 ppm (221 mg/m³) or the OSHA PEL of 100 ppm (435 mg/m³), respiratory protection selected under EN 529:2005 is required; an air-purifying respirator with A-type organic vapour cartridge may be used only within the maximum use concentration derived from the cartridge breakthrough data and the assigned protection factor of the facepiece, and never above the IDLH concentration of 900 ppm. Above 900 ppm or in oxygen-deficient atmospheres, a self-contained breathing apparatus or a full-face air-line respirator in positive-pressure mode is required. Cartridge service life must be calculated from the measured breathing rate, ambient temperature, and vapour concentration using the manufacturer’s cartridge breakthrough data because p-xylene has a relatively short breakthrough time on activated carbon compared with lighter aliphatic hydrocarbons. Eye protection for transfer operations should be chemical safety goggles conforming to EN 166:2001 with anti-fog coating; face shields are secondary and do not replace goggles. Skin protection for immersion or splash requires a laminate or butyl-rubber glove that meets the permeation requirements of EN ISO 374-1:2016, with breakthrough time under continuous contact not less than 240 minutes according to ASTM F739-20 or EN 16523-1:2015+A1:2018; nitrile gloves with a thickness of at least 0.4 mm may be acceptable for incidental splash where the manufacturer’s permeation test data for p-xylene or mixed xylene demonstrates breakthrough time beyond the task duration. Natural rubber latex and thin disposable nitrile gloves below 0.1 mm are not acceptable for repeated contact. Eye wash stations and safety showers should be located within 10 seconds of the transfer point, with water delivery of at least 1.5 L/min for eye wash units and 75.7 L/min for safety showers according to ANSI Z358.1-2014 or EN 15154-1:2006.

In the event of a release involving p-xylene, immediate elimination of all ignition sources within a radius of at least 7.5 m for small spills and 30 m for large spills is required because the vapour density of 3.7 relative to air allows vapour to spread along floors, into drains, and toward equipment rooms. The spill area should be diked with inert absorbent such as exfoliated vermiculite, calcined diatomaceous earth, or dry sand; absorbent selection should avoid cellulosic materials that could increase fire load. Recovered liquid and contaminated absorbent should be placed in conductive, bonded, and closed metal containers marked with UN 1307, Xylenes, Class 3, Packing Group III. If the spill enters a drainage system, the receiving wastewater treatment plant should be informed because p-xylene is not readily biodegradable and has an aquatic chronic classification; discharge to surface water or soil is not acceptable under the integrated permit conditions of Directive 2010/75/EU. Fire extinguishing media should be alcohol-resistant foam, dry chemical, or carbon dioxide for small fires; water spray may be used to cool containers and disperse vapours but a straight water stream will spread a pool fire. Firefighters should wear self-contained breathing apparatus and liquid-tight chemical protective clothing with full-body coverage. During combustion, p-xylene produces carbon monoxide, carbon dioxide, and vaporised unburnt hydrocarbons; incomplete combustion in underventilated compartments generates toxic soot and aromatic partial oxidation products. Water used for firefighting is environmentally hazardous and must be impounded as contaminated firewater, not discharged without analysis and authorisation.

If p-xylene is decanted into portable containers smaller than 25 L

Decanting p-xylene from drums or intermediate bulk containers into 5 L, 10 L, or 20 L laboratory or maintenance containers introduces a higher frequency of vapour release per unit mass than closed-loop transfer. The operation should be performed within a local exhaust ventilation enclosure that provides a minimum capture velocity of 0.4 m/s at the open lip, as tested under EN 14175-3:2019 for fume cupboards or the equivalent industrial hood standard; if the decant point cannot be enclosed, a vapour recovery nozzle or a self-closing faucet should be used. The receiving container must be made of steel, stainless steel, or fluorinated high-density polyethylene with a UN-approved design for flammable liquids; glass containers larger than 1 L should be avoided unless the specific task requires glass and a secondary steel overpack is provided. The receiving vessel must be electrically bonded to the source vessel before the first drop is transferred and the bond must remain connected until the valve is closed and the fill port is sealed. Liquid velocity during manual pouring should be minimised to prevent static charge accumulation; the usual industrial requirement for initial loading velocity is not more than 1 m/s until the fill pipe outlet is submerged. The total quantity of p-xylene in a single laboratory or maintenance workstation should not exceed the safe operating volume set by the fire safety concept, and in any case should not exceed the 20 L storage limit commonly enforced for flammable liquids in unprotected laboratories. Any retained sample or cleaning solvent should be returned immediately to a closed flammable-liquid storage cabinet meeting EN 14470-1:2004 or FM 6050, not left in open beakers. The cabinet should be constructed to contain a fire for at least 15 minutes where the room fire load is low; for higher fire-load rooms, 30-minute cabinets may be required by the authority having jurisdiction.

Transport documentation for p-xylene uses UN 1307, Xylenes, hazard class 3, packing group III under ADR, RID, IMDG, and IATA. The dangerous goods transport document must include the UN number, proper shipping name, class, packing group, number and type of packages, and total quantity, in accordance with ADR 5.4.1; the tunnel restriction code for UN 1307 Class 3 PG III is applied as published in the ADR dangerous goods list. For maritime transport, the EmS schedule and stowage category are assigned from the IMDG Code, and the shipper must verify whether the consignment is subject to the marine pollutant provisions based on the classification data in the IMDG Index. Waste p-xylene is classified as hazardous waste under Decision 2000/532/EC; the appropriate European Waste Catalogue code depends on the process origin, and 14 06 03* other solvents and solvent mixtures is commonly used for solvent residues. Disposal should occur by incineration at an authorised facility operating under Directive 2010/75/EU and the local permit conditions; incineration temperatures and residence times must be sufficient for complete destruction of the aromatic hydrocarbon. Regulatory inventories for p-xylene include the US TSCA Inventory, EU REACH, Japan ENCS, China IECSC, Australia AIIC, and the relevant national chemical inventories; p-xylene is also listed in the pharmaceutical and polymer precursor supply chains where excipient-grade purity is not intended and food-contact use is not permitted unless specific migration limits are established under the applicable national regulation.