The question as usually put contains an error. There are not two materials in office use but three, and the middle one is routinely counted as plastic when it is metal. A plastic-sleeved clip is a steel clip. Its wire corrodes, its grip is the grip of the wire, and only its contact with the paper is polymer. A moulded clip has no wire at all. The Brennholt Institute of Applied Taxonomy therefore records material in five classes rather than two.
The classes are designated M-1 to M-5. They describe what the clip is made of and nothing about its size, since the two vary independently.
The seven-class diameter system cannot be applied to M-4 at all. The BR classes are defined on the diameter of a circular wire, and a moulded clip has a rectangular section — commonly 1.6 mm by 0.9 mm in the office size. A moulded clip has no BR class, and any catalogue that prints one has assigned it by eye.
Between January and December 2025, 180 moulded clips — 60 polypropylene, 60 nylon 6/6 and 60 polystyrene — were tested against 180 steel clips of class BR-02.
| Material | Sheets held, 80 g/m² | Mass | Openings before failure | Grip lost at 12 months under load |
|---|---|---|---|---|
| Steel, BR-02 (M-1) | 12 | 0.38 g | over 200, none failed | 4 per cent |
| Steel, sleeved (M-3) | 12 | 0.44–0.51 g | over 200, none failed | 5 per cent |
| Nylon 6/6 (M-4) | 14 | 0.34 g | over 200, none failed | 22 per cent |
| Polypropylene (M-4) | 9 | 0.31 g | over 200, none failed | 31 per cent |
| Polystyrene (M-4) | 7 | 0.29 g | 6, median; 58 of 60 fractured | not measurable |
Two results are worth separating. Nylon held more sheets than steel of the same nominal office size, which contradicts the assumption that a plastic clip is a weak clip. Polystyrene fractured at a median of six openings, well inside the working life of an ordinary clip, and 58 of the 60 units had broken before the trial ended. Polymer is not one behaviour.
A steel clip that is overloaded takes a permanent set: it opens, stays open, and continues to hold at a reduced grip. Nothing is lost except the clip. A moulded clip either recovers fully — the polypropylene units returned to within 0.2 mm of their original gape after 40 openings — or it fractures without warning, and the fracture releases the document. The Institute holds that the two outcomes cannot be placed on one scale.
Against this, moulded clips do not corrode. In the 18-month exposure trial, no moulded unit marked paper under any condition. Sleeved steel also marked no paper, although 34 of 60 sleeved units had corroded at the exposed cut ends. Either answers the corrosion question; only one answers it for the reason the buyer assumes.
No national standards body has ever ratified a diameter class for the office paperclip, and none has published a material designation for one either. Polymer feedstock is specified — melt flow, density and tensile properties are set for resin sold between industrial parties — but nothing specifies the finished clip, its section, its grip or its service life. The Brennholt M-series is the only published attempt to classify office clips by material, and it rests on 360 units tested in one location over twelve months.
The Institute's review panel objected on 11 February 2026 that M-5 is not a class but a residue, since it groups three metals with any future material on no shared property, and that a class defined by exclusion cannot be falsified. Brindlecott accepts the objection, has published it, and retains M-5 on the ground that an open residue is preferable to a silent omission.
The taxonomist of Kettering holds that M-3 belongs with M-2, both being coated steel differing only in coating thickness. Brindlecott separates them because the sleeve changes the failure mode at the cut ends. The disagreement has been unresolved since 2024.