Resin for bottle caps and closures: HDPE and PP by closure type

HDPE or PP by closure type

Every HDPE melt index here is at 190°C / 374°F and 2.16 kg, and every PP melt flow rate at 230°C / 446°F and 2.16 kg (ASTM D1238). Density is by ASTM D1505 or D792. ESCR values are bent strip by ASTM D1693 and only compare at the same condition and Igepal strength.

Typical published windows to start the conversation, drawn from producer closure data sheets and our HDPE and PP pages. Confirm on the data sheet for the grade you run, and let a chemist check any equivalent before you switch.

Unimodal vs. bimodal HDPE: ESCR and lightweighting

A unimodal HDPE is made in one reactor and has one molecular weight peak. A bimodal HDPE is made in linked reactors: a low molecular weight part for flow and stiffness, and a high molecular weight part with comonomer that ties the crystals together. At the same melt index and density, the bimodal grade often has far higher stress-crack resistance.

That is what lets closure makers take weight out. Moving to shorter, lighter neck finishes, such as the PCO 1881 neck for carbonated drinks, leaves less plastic to hold the same pressure, so each gram needs more ESCR. Bimodal grades made that possible without giving up flow. Unimodal grades are still common for still water, dairy and general caps, where there is no carbonation load.

Bimodal closure grades come from fewer production lines and are often sold under contract to closure makers, so they seldom appear as wide-spec or spot material. For carbonated closures, do not accept a unimodal swap on MI and density alone. See HDPE injection grades and stress cracking.

PP closures: homopolymer, random copolymer, nucleated and clarified

• Homopolymer. The stiffest PP and the best hinge. Most flip-tops and dispensing caps. It turns brittle near 0°C / 32°F, so a cap that is dropped cold may need a copolymer.
• Random copolymer. A little ethylene in the chain makes it softer, clearer and lower melting. Used for clear or softer caps and some pharma closures. See clear PP random copolymer.
• Nucleated. A nucleating agent speeds crystallization, which shortens cycle time and raises stiffness, but it also changes shrinkage. A switch between nucleated and non-nucleated grades can change thread fit and hinge feel, so treat it as a material change.
• Clarified. A clarifying agent makes thin walls near-transparent. Overheating some clarified grades can raise odor and taste, so ask about both for food and drink caps.
• Why PP over HDPE. Living hinges, more heat resistance for hot fill and dishwashers, and lower density (about 0.90 against about 0.95 g/cm³), so the same cap volume weighs less. Why HDPE over PP: cold impact, a seal that conforms to the bottle neck without a liner, and stress-crack resistance under carbonation. See PP vs. HDPE.

Slip, removal torque, taste and color

• Slip agents set removal torque. Closure grades often carry an amide slip, most often erucamide or oleamide, that migrates to the surface and lowers friction between the cap and the neck. Without it, caps can be hard to open or loosen. Too much, and caps can back off in transit.
• Bloom time. Oleamide blooms fast and erucamide more slowly but reaches a lower final friction and handles heat better. Torque keeps changing for days after molding, so measure it at a fixed age, the same way each time.
• Same grade name, different torque. A change in slip type or level, a different masterbatch, or a mix of lots in the silo can shift torque even when MI and density match. Check the slip on paper when you change sources.
• Taste and odor. Slip agents and their oxidation products, antioxidants, some clarifiers and overheated resin can all add off-notes. Water is the most sensitive. Brand owners run their own sensory panels, so ask for low-odor grades where they do, and test molded caps on the product, not pellets.
• Color and masterbatch carriers. Match the masterbatch carrier to the base resin where you can, and keep the let-down ratio fixed. Some pigments, such as certain phthalocyanine blues and greens, nucleate HDPE and change shrinkage, which can move thread fit and torque. Approve each color on molded caps. Colorants need their own food-contact statements. See masterbatch vs. precolored resin.

Compression vs. injection, liners and tethered caps

• Injection or compression molding. Many high-output beverage closures are compression molded: the extruder meters a pellet of melt into each cavity and the tool presses it into shape. Others are injection molded. Many closure grades are rated for both, but some are tuned for one, so say which process you run on the RFQ.
• Liner-less caps. One-piece HDPE closures seal with molded features, such as an inner plug or a bore seal, instead of a separate liner. That is why most carbonated and water caps are now one-piece HDPE; older carbonated caps were often two-piece PP shells with a separate liner. PP is stiffer, so a PP screw cap often uses a liner or a seal feature designed around that stiffness.
• Lined caps. Induction seal liners (often foil laminates) are common on pharma, nutraceutical, food and chemical bottles. Foam and pulp liners are used for chemicals. The liner supplier documents the liner; you still need the resin letter for the shell.
• Tethered caps. Article 6 of Directive (EU) 2019/904, the Single-Use Plastics Directive, requires from 3 July 2024 that plastic caps and lids on single-use beverage containers up to 3 L, including composite cartons, stay attached to the container during use. Glass and metal containers with plastic caps are excluded, as are containers for liquid medical foods. EN 17665 is the harmonised standard with test methods to show compliance. The tether band must flex without tearing, which puts more demand on toughness, and some designs added weight. As of October 2026 there is no U.S. federal or Canadian federal tethering rule. Bills have been introduced in some states, such as New York and Massachusetts, and a California bill did not pass. So far it matters most to molders who ship caps or filled drinks into the EU.

Food-contact letters, pharma files and recycled content

• FDA letter for the exact grade. Polyethylene and polypropylene food-contact resins are covered by 21 CFR 177.1520 (olefin polymers). The producer's letter states compliance and the conditions of use, meaning the food types and temperatures. Ask for it by grade name; a similar grade from another plant needs its own letter. For EU sales, also ask for a 10/2011 declaration. In Canada, a Health Canada letter of no objection is voluntary, but many buyers ask whether the grade has one. See FDA food-contact resin.
• Pharma. Drug makers often need the resin named in their filing, change notification from the producer, and testing to USP plastic packaging chapters. Expect a resin change to take months of paperwork. See medical and FDA grades.
• Recycled content. Post-consumer recycled HDPE or PP in food-contact closures needs an FDA letter of no objection for the recycler's process and conditions of use. Few food-grade PCR streams are made for closures, and odor and stress-crack resistance are the hard parts. Some producers offer certified circular grades allocated by mass balance from chemically recycled feed. Non-food caps can take mechanically recycled resin more easily. See adding recycled content.

Tests to ask for, and what each one tells you

Most of these are run by the closure maker or brand owner. On a COA you will see MI and density on every lot; ESCR is often a typical value, not a lot test, so ask.

What moves closure resin prices and lead times

• Base resin. HDPE follows ethylene and the polyethylene market. PP follows propylene. Both are mostly contract-priced off monthly reported settlements. See how resin is priced and the resin market notes.
• ESCR tier. Bimodal closure grades cost more than general-purpose injection HDPE and come from fewer lines. A lighter cap that still passes your stress-crack test can cost less per thousand caps even at a higher price per pound.
• Additives and documents. Slip packages, nucleation or clarifier, low-odor production and food-contact documentation add smaller steps.
• Supply. Closure grades are often committed to contract customers, so spot supply tightens fast during outages, hurricanes and freezes. Keep a qualified second grade where you can.
• Order size. A railcar costs the least per pound and a single gaylord the most. Closure plants often buy bulk; trials and colors come in gaylords. We do not post prices.

What to put on the RFQ

• The grade you run now, or the closure type and neck finish (for example 26 mm water or 28 mm carbonated).
• Resin and structure. HDPE unimodal or bimodal, or PP homopolymer, random copolymer or impact copolymer, and nucleated or clarified.
• Melt flow and density with ranges. HDPE MI (190°C / 374°F, 2.16 kg); PP MFR (230°C / 446°F, 2.16 kg); density by ASTM D1505 or D792.
• ESCR with its test. ASTM D1693 condition and Igepal strength, or ASTM D5419 on the closure.
• Slip package. Slip or slip-free, the type (erucamide, oleamide or other) and level, and your removal torque window and the age you test at.
• Process. Injection or compression molding, cavitation and cycle time.
• Taste and odor. Whether the brand runs a sensory panel, and for which product.
• Color. Natural or the masterbatch you use, its carrier and let-down ratio.
• Documents. FDA letter for the exact grade, EU 10/2011 if exporting, a Health Canada letter of no objection if your Canadian customer asks, pharma change notification, and any recycled-content claim you need.
• Quantity, packaging and ship-to city. Gaylord, bulk truck or railcar, and your yearly usage. See send an RFQ package.

Common questions

If your grade is not listed, tell us what you run. We source commodity, engineering and specialty resins.