ASTM F88

What the test does

A strip is cut across a sealed area of a package so the seal runs through its middle. The two unsealed tails are clamped in opposing grips with the seal roughly midway between them, and the crosshead separates the grips at a constant rate until the seal pulls fully apart. Force against grip separation is recorded throughout, so the whole peel trace is captured, not just the peak.

What it measures, and why it matters

The method reports seal strength — the force needed to pull a defined width of seal apart — as maximum, average or the whole force curve, together with the failure mode observed. Maximum force is what medical device and pharmaceutical manufacturers use for lot release and for validating a sealing process window against a specification. Average force describes how a peelable seal opens in a clinician's hands, which is a usability question rather than a strength one. Failure mode is the diagnostic half of the result: the same peak force means something different if the film tore instead of the seal peeling.

Specimen

Specimens are cut as 15 mm (1 in.) wide strips through the seal, with a length of 75 mm plus the seal width so both tails reach their grips. Sampling is defined by the user's plan — sterile barrier system work commonly takes specimens from each seal of a pouch and from several positions along a long seal, since seal strength varies with position under the sealing jaws. Cuts must be clean and parallel; a nicked or tapered edge starts a tear and the result is discarded. Conditioning follows ASTM E171, the flexible-barrier standard atmosphere of 23 °C and 50 % relative humidity, typically for at least 24 h, with testing in the same atmosphere.

What the machine must be capable of

Seal forces are light. Medical device seals commonly peak between about 4 and 9 N per 15 mm strip, pharmaceutical seals a little higher, and tough laminates or tray lidding run to a few tens of newtons, so a 50 N to 500 N load cell on a single-column frame is the normal fit rather than a general-purpose high-capacity frame. Force resolution at the bottom of the range matters more than headroom: a peel trace read on a 5 kN cell resolves nothing useful. The method does not itself set an accuracy class, so the applicable force-verification requirement comes from the laboratory's quality system.

Grip separation runs at 200 to 300 mm/min (8 to 12 in./min). Travel demand is short — full separation normally occurs within 50 mm of crosshead movement — but the initial grip separation is a required reporting item, so it has to be set and logged rather than assumed. No extensometer is involved; peel is a force-against-displacement measurement, not a strain one.

Fixturing is what the three techniques differ on. Technique A leaves the seal unsupported between the grips; Technique B supports it perpendicular to the tails at 90°; Technique C folds the more flexible tail 180° back over the seal with the stiffer member held flat, and the 2023 edition adds two Technique C approaches for flexible-to-rigid and semi-rigid trays using alignment plates. Opposing side-action grips, commonly pneumatic, hold the tails at constant clamping force. Mixing techniques within a series destroys comparability.

What goes wrong in practice

Bending force is the standard bias: in Technique A the tails flex as they load, and that parasitic bending adds to the reading, so unsupported results sit above supported ones on the same seal. Film tear instead of seal peel gives a high number that describes the substrate, not the seal. Delamination — laminate layers separating internally — reads as a low, ragged trace and is often logged as a weak seal when the seal never failed. Slipping tails give a false early peak.

Related and equivalent standards

ASTM E171 supplies the conditioning atmosphere referenced here. ASTM F1140/F1140M and ASTM F2054 test whole-package burst rather than a strip, so they screen the weakest seal in a package where F88/F88M quantifies one located seal; the two answer different questions and the numbers do not convert. ASTM F1929 and ASTM F3039 detect channel leaks by dye penetration — integrity, not strength. ASTM F2096 covers bubble-emission leak detection. Seal strength results transfer between laboratories only when the technique letter and grip separation match.