Determination of Bending Strength of Rigid Sheets for Solid Pharmaceutical Packaging

Bending strength determination is an essential mechanical performance test for rigid plastic sheets used as forming webs in blister and strip packaging of solid pharmaceutical preparations such as tablets, capsules and pills. This article provides a technical overview of the test principle, applicable standards such as ISO 178, ASTM D790, GB/T 9341 and YBB monographs, specimen preparation and conditioning, the step-by-step test procedure, calculation of results, and typical applications in pharmaceutical packaging quality control and registration.

Technical Overview

Rigid sheets for solid pharmaceutical preparations — primarily unplasticized polyvinyl chloride (PVC), PVC/PVDC composites, polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET) sheets with typical thicknesses of about 0.20 mm to 0.60 mm — are the primary forming webs of blister packs for tablets and capsules. These sheets must provide sufficient mechanical rigidity to protect the packaged dosage form during thermoforming, filling, sealing, transportation and end use, while remaining free from cracking or crazing under bending stress.

Bending strength, also referred to as flexural strength, is the maximum stress that a sheet specimen sustains in flexure before it fractures or reaches a specified deflection. It is measured by supporting a rectangular strip on two parallel supports and applying a load at the midpoint (three-point loading) until failure. The result, expressed in megapascals (MPa), reflects the combined influence of the polymer formulation, the degree of orientation introduced during calendering or extrusion, sheet thickness uniformity and internal stresses — all of which are decisive for blister machinability and package integrity.

The determination is aligned with internationally recognized methods for plastics, including ISO 178, ASTM D790 and GB/T 9341, and is incorporated into national drug packaging material standards (YBB monographs) covering rigid sheets used in direct contact with solid pharmaceutical preparations.

Testing Samples

PVC Rigid Sheets for Solid Preparations

Unplasticized PVC sheets are the most widely used forming webs for aluminum blister packaging of tablets and capsules. They are tested to verify adequate flexural strength and to detect brittleness caused by improper formulation, over-processing or storage degradation.

PVC/PVDC and Other Coated Composite Sheets

Sheets coated or laminated with polyvinylidene chloride (PVDC), or combined with PE or PP barrier layers, are tested as finished composites so that the measured bending strength reflects the actual multilayer structure delivered to the packaging line.

Other Thermoplastic Sheets

PP, PE, PET and specialty barrier sheets used for moisture- or oxygen-sensitive dosage forms can be evaluated using the same flexural approach, with specimen geometry and test conditions adapted to the sheet thickness and the applicable monograph.

Testing Items

• Bending (flexural) strength at break or at maximum force, expressed in MPa
• Maximum flexural force and midspan deflection at failure
• Optional flexural modulus and flexural strain when required by the product specification
• Directional comparison between machine direction (MD) and transverse direction (TD) specimens
• Verification of sheet thickness and dimensional uniformity of the specimens

Testing Methods

Principle and Apparatus

A rectangular strip is placed on two parallel supports and loaded at midspan by a loading nose moving at a constant rate. The applied force and deflection are recorded until the specimen breaks or reaches the conventional deflection limit. Testing is performed with a universal testing machine or a dedicated bending strength tester equipped with a three-point bending fixture, calibrated force and displacement measurement, and support and loading noses with radii selected to prevent premature indentation or stress concentration.

Specimen Preparation and Conditioning

Specimens are cut as flat strips free of visible defects, nicks or crushed edges. For each sampling unit, specimens are prepared in both the machine direction and the transverse direction, and the specimen length is set so that the overhang beyond each support is sufficient to avoid end effects. Thickness is measured at multiple points to the accuracy required by the standard. Unless otherwise specified, specimens are conditioned in the standard atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity in accordance with ISO 291 or GB/T 2918 before testing.

Test Procedure

• Set the support span in accordance with the applicable monograph, or at a span-to-thickness ratio within the range of 16:1 to 32:1 as recommended by ISO 178 and ASTM D790.
• Center the specimen on the supports with its long axis perpendicular to them, ensuring uniform contact.
• Select the crosshead speed specified by the method, typically in the range of 1 mm/min to 10 mm/min depending on sheet thickness.
• Apply the load at midspan and record the force–deflection curve until fracture occurs or the specified deflection is reached.
• Test at least five valid specimens in each direction and discard results from specimens that fail outside the midspan region.

Calculation and Reporting

Bending strength is calculated using the three-point bending formula for a rectangular cross-section: sigma = 3FL / (2bh²), where F is the maximum force (or force at break), L is the support span, b is the specimen width and h is the specimen thickness. The individual results and the arithmetic mean are reported for each direction, rounded as specified by the standard, together with specimen dimensions, span, speed, conditioning conditions, failure mode (brittle fracture or no break) and the standard applied. When a specimen does not fracture, the stress at the conventional deflection is reported instead, as required by ISO 178. Acceptance limits are those stated in the applicable YBB monograph or the agreed product specification.

Applications

• Incoming quality control and lot release testing of rigid sheets by pharmaceutical packaging manufacturers and pharmaceutical companies
• Supporting data for drug packaging registration dossiers and pharmacopoeial packaging standard compliance
• Qualification of new suppliers, resin formulations or changed calendering and extrusion process conditions
• Evaluation of mechanical property retention after heat aging, transport simulation or accelerated storage
• Failure analysis of cracked or split blister cavities during forming, sealing or distribution
• Independent verification by accredited third-party laboratories for regulatory submission and market surveillance

FAQ

What does a high bending strength value mean for a pharmaceutical rigid sheet?

A high value indicates that the sheet can withstand greater flexural stress before cracking, which supports robust thermoforming, secure sealing and reliable protection of tablets and capsules throughout the packaging life cycle. Low values often point to brittleness, excessive orientation or material degradation.

Which standards govern this determination?

The test is typically performed in accordance with ISO 178 or ASTM D790 for the flexural properties of plastics, GB/T 9341 as the corresponding national method, and the specific YBB monographs covering rigid sheets for solid pharmaceutical preparations, which define sampling, specimen geometry and acceptance criteria.

Why must specimens be tested in both machine and transverse directions?

Calendered and extruded sheets are anisotropic: polymer chain and filler orientation during processing produces different stiffness and strength along and across the web. Testing both directions ensures that the weaker orientation still meets the specification and that blister forming behavior is predictable.

How many specimens are required and how are results judged?

A minimum of five valid specimens per direction is common practice. The mean value is compared with the minimum limit stated in the applicable monograph or specification. For reference, rigid PVC blister sheets commonly show mean values on the order of several tens of MPa, but the contractual limit is always the requirement of the current monograph.

What happens if a specimen does not break during the test?

For ductile sheets that bend without fracturing, the bending strength is reported as the stress at the conventional deflection defined by the method, for example the stress at 3.5 % outer-fiber strain under ISO 178, and the failure mode is clearly documented as no break.