Polypropylene Rod

Polypropylene rod in black and natural, cut to length and sold by the foot.

Polypropylene is the workhorse of chemical-resistant fabrication: the lightest of the common engineering plastics at a specific gravity of 0.905, it floats, it welds, it shrugs off acids and alkalis that attack most other thermoplastics, and it costs a fraction of what the fluoropolymers do. Tanks, ducting, wash-down fixtures, laboratory furniture and food-processing parts are all built from it.

One thing to settle before you order. Polypropylene comes as homopolymer and as copolymer, and they are not interchangeable. Homopolymer gives you roughly a third more tensile yield (4,800 against 3,600 psi), about 26 percent more stiffness (195,000 against 155,000 psi flexural modulus) and about 32F more continuous service temperature. Copolymer gives you impact: notched Izod goes from 0.9 ft-lb/in to no break, with around five times the elongation, and it stays tough in the cold where homopolymer turns brittle somewhere between 13 and 32F. Our stock of this line is not certified to one or the other, so if the distinction matters to your part - a cold-service impact application, or a loaded structure - ask us and we will get the mill certificate for the sheet we would ship you.

Chemical resistance is the reason to buy it. Acids, bases, alcohols, detergents, lower ketones, aldehydes and water-soluble salt solutions are, in the resin producer's words, usually not regarded as antagonistic. Dilute to mid-concentration sulphuric is a documented working envelope - tanks are built in homopolymer for 10 to 40 percent sulphuric from 41 to 158F. Concentration is what decides it, not just the chemical: polypropylene handles 30 percent sulphuric but not concentrated, 10 percent nitric but not 40 percent. Oxidizing acids are where it fails.

What it is not for: chlorinated solvents, aliphatic and aromatic hydrocarbons, ketones and esters are unacceptable; hot water and steam are limited service; and bleach and hypochlorites cause a failure that looks like stress cracking but is actually oxidative attack. Polypropylene is largely outside the stress-crack failure mode that troubles polyethylene, which is worth knowing - but it is not immune to oxidizers, and at 120F the not-recommended list grows substantially.

On temperature, read the load with the number. Continuous service runs 180 to 212F depending on grade. Heat deflection is 210F at 66 psi; at 264 psi the published figures disagree badly across mills, so treat anything above about 190F under real load as something to test. And the number that catches people out: homopolymer has lost about 92 percent of its room-temperature tensile strength by 150F. Polypropylene melts around 320F, but that is not a service temperature and nothing here should be designed near it.

Outdoor use, and this is the one that comes back as a warranty claim. Standard polypropylene is not UV stabilized. Continuous sunlight is rated limited, and exposure causes progressive loss of tensile strength and elongation - polypropylene's peak photochemical sensitivity sits at 370 nm, inside the UV-A band, which makes it the more vulnerable of the two common polyolefins. For outdoor or UV-exposed service, specify a UV-stabilized polypropylene grade or UV-stabilized HDPE. Color alone is not UV protection, black included: the UV-stabilized polypropylene grades actually sold are blue, beige, gray and white, and no mill we can find publishes a UV service claim for black polypropylene.

Joining is by welding, and every mill leads with it. Heated-tool butt welding, hot gas and extrusion welding are all rated excellent, with extrusion welding running 410 to 464F material temperature and 410 to 572F gas under DVS 2207-4. A good weld approaches base-material yield. Use matching polypropylene weld rod - the wrong rod is the one weld defect a supplier can actually prevent, so order it with the sheet.

It cannot be solvent cemented. Polypropylene's surface energy is about 29 mN/m and most adhesives need over 36 to wet it, which is a wettability problem rather than a strength one. Structural acrylic polyolefin adhesives bond it untreated with no surface preparation and are the practical answer; cyanoacrylate works with a polyolefin primer. Flame and plasma treatment work but are easy to overdo - over-treating leaves the bond no better than untreated.

It machines easily - about a 3 on a 1-to-10 scale - with sharp tools, high rake angles and good chip clearance. It is soft and it moves, so expect more spring-back than acetal or nylon and allow for the thermal expansion, which published sources put anywhere between 4.6 and 10.6 millionths per degree F. Hold thin parts gently; clamping marks stay.

Food contact: available in grades compliant with FDA 21 CFR 177.1520, olefin polymers. Compliance attaches to the specific resin and color formulation, not to polypropylene generically - and the regulation bounds density at 0.880 to 0.913, which some published sheet densities sit outside. Request the manufacturer's declaration of compliance for the product supplied if you need it on file.

It goes into chemical tanks and secondary containment, acid and plating-line fixtures, fume scrubbers and ducting, filter plates and housings, pump and valve components, laboratory benching and sinks, wash-down and food-processing parts, die-cutting pads, and orthotic and prosthetic shells.