ISO 15988 Biaxial stretch PET films test methods details

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ISO 15988 Plastics — Film and sheeting — Biaxially oriented poly(ethylene terephthalate) (PET) films

ISO 15988 is the international standard that defines quality specifications, performance requirements and testing criteria for biaxially oriented transparent PET (BOPET) film, a high-performance material widely utilized in flexible packaging applications. This versatile BOPET film can be used as a standalone single-layer packaging film or processed as a laminated layer bonded with other plastic films to form multi-layer composite packaging structures.

ISO 15988 mandates a complete set of core performance and dimensional tests to verify BOPET film quality and packaging applicability. The primary testing items specified in the standard include film thickness, tensile strength and elongation at break, thermal dimensional change (thermal shrinkage), oxygen transmission coefficient, water vapour transmission coefficient, haze, and wetting tension.

Standard Core Functions

Defines visual appearance, dimensional tolerances (width, length, core inner diameter, thickness).

Sets mandatory performance thresholds for mechanical, thermal, barrier, optical and surface energy properties.

Standardizes complete test methods, specimen preparation, equipment and calculation formulas.

Lists normative referenced ISO test standards for unified laboratory operation.

Specifies mandatory marking requirements for finished PET film rolls.

Target Industry Application Fields of ISO 15988

Sector	Typical use of BOPET
Flexible food packaging	Lidding, stand-up pouches, retort, frozen food (often laminated with PE/CPP/PA)
Laminates	As the outer or core layer with PA, EVOH, Al foil
Labeling & graphics	Pressure-sensitive label face stock, thermal-transfer printing
Industrial/electrical	Motor insulation, capacitor film (thicker grades, not the primary focus of this std)
Metallized base	VMPET for high-barrier snack packs

ISO 15988 Biaxial stretch PET films test methods details

All Mandatory for Testing Stipulated in ISO 15988

Test	Reference Standard	Function
Thickness Test	ISO 4593	Mechanical scanning method for film thickness measurement
Tensile strength & strain at break	ISO 527-3	Tensile property testing for plastic films & sheets (thickness ≤1 mm)
Dimensional change on heating	ISO 11501	Measure length shrinkage after high-temperature heating to evaluate thermal stability during post-processing (printing, lamination, heat sealing).
Wetting tension (surface tension value)	ISO 8296	Wetting tension (surface energy) measurement for corona-treated films
Haze test	ISO 14782	Haze measurement for transparent plastic materials
Oxygen transmission coefficient	ISO 15105-1 / 15105-2	Oxygen transmission rate (OTR): differential-pressure / equal-pressure method
Water vapour transmission coefficient	ISO 15106-1 / 15106-2 / 15106-3	Water vapor transmission rate (WVTR): humidity / infrared / electrolytic sensor methods

Detailed Introduction of Main Stipulated Tests in ISO 15988 for BOPET films biaxial stretch

1, Tensile Strength & Tensile Strain at Break	It is the most critical mechanical test. Test both machine direction (MD, longitudinal, parallel to extrusion) and transverse direction (TD, cross-web, perpendicular to extrusion)
Specimen Information	Specimen shape: Dumbbell film specimen defined (ISO 527-3 compliant) Specimen dimensions: Overall length >150 mm Gauge length L₀ = 50 ±0.5 mm Initial grip distance = 100 ±5 mm Parallel section width: 10–25 mm Thickness ≤1 mm (compliant with BOPET film) Sampling: Separate 5 MD specimens and 5 TD specimens cut from the same film roll; pre-conditioned under ISO 291 standard atmosphere before testing.
Test Equipment	Biaxial (Cross) Stretch Tensile Testing Machine with load cell suitable for thin plastic films; Pneumatic flat film grips (to avoid film edge slippage during stretching) Specimen cutting die (dumbbell cutter matching ISO 527-3 geometry) Digital thickness gauge (to input cross-sectional area for stress calculation)
Test Procedure	Measure average thickness of each specimen parallel section to calculate cross-sectional area Mount specimen symmetrically between upper and lower machine grips, align gauge marks with machine optical extensometer Set test speed to 200 mm/min, start uniaxial tensile stretching until specimen complete fracture Record peak breaking force and total elongation at break point for each sample Repeat test for 5 MD and 5 TD replicates separately Calculate arithmetic average tensile strength and elongation for each direction, compare against standard minimum/maximum limits
Mandatory Stipulations	Tensile strength at break (MPa): MD ≥150 MPa, TD ≥150 MPa Tensile strain at break (% elongation): MD ≤200%, TD ≤200% Minimum 5 replicate specimens per direction for statistical averaging Standard test speed fixed at (200 ±20) mm/min
2, Dimensional Change on Heating	Essentially a thermal-shrinkage test at elevated temperature, probing the frozen-in orientation from biaxial stretching.
Practical Significance	Heat shrinkage directly reflects PET film thermal dimensional stability. Packaging films undergo high-temperature retort sterilization, heat lamination or printing drying; excessive shrinkage causes bag warping, printing pattern distortion, lamination layer separation. This test is the core thermal mechanical indicator for food-grade BOPET packaging.
Specimen Information	Specimen size: 20 mm width × ~150 mm length Gauge marking: 100 mm central gauge length; each mark 25 mm from specimen two ends Sampling: 5 MD specimens, 5 TD specimens cut flat without tension
Test Equipment	Film Thermal Shrinkage Tester, Size Change Measuring Tester; Circulating hot air oven (temperature controllable to 150 ±3 °C) Vernier caliper (0.01 mm precision for length measurement) Specimen hanging racks (non-contact vertical suspension to avoid surface friction)
Test Procedure	Cut 5 MD and 5 TD specimens, mark central 100 mm gauge length accurately Measure original gauge length L₁ for each sample and record values Suspend specimens vertically inside circulating air oven (no contact between samples) Maintain oven temperature at (150 ±3) °C, hold specimens for exactly 30 minutes Remove all samples from oven, cool freely at room temperature for 30 minutes to fully stabilize Re-measure gauge length L₂ for each cooled specimen Compute shrinkage percentage S for each sample using the standard formula Average results of 5 replicates per direction; average shrinkage must not exceed 3.0%
Mandatory Stipulations	Maximum allowable thermal shrinkage: MD ≤3.0%, TD ≤3.0% after standardized oven heating cycle 5 replicate specimens per direction (MD and TD separately tested)
3, Coefficient of Water Vapour Transmission (WVTR Barrier Test)	Evaluate moisture barrier ability, ISO 15106-1 / 15106-2 / 15106-3
Purpose	Block moisture penetration for dry food, pharmaceutical packaging
Test Equipment	WVTR vapor permeation tester with humidity/infrared/electrolytic sensor.
Mandatory Stipulations	WVTR coefficient PWv ≤ 10 g·100 μm/(m²·24 h)
4, Oxygen Transmission Coefficient Test	Referenced standards: ISO 15105-1 (differential-pressure) / ISO 15105-2 (equal-pressure)
Purpose	Control oxygen ingress to extend packaged food shelf life
Test Equipment	oxygen permeability tester(differential pressure or equal pressure module)
Mandatory Stipulations	Oxygen transmission coefficient PO₂ ≤ 140 fmol·100 μm/(m²·s·Pa)
5, Wetting Tension Test (Surface tension)	Measures surface energy after corona treatment; guarantees printing ink adhesion and lamination bonding strength
Specimen Information	Corona-treated film surface, uncoated, dust-free
Test Equipment	Surface/interface tensiometer. Standard wetting tension test ink set (series dyne test pens)
Mandatory Stipulations	Only applicable to corona-treated PET film; minimum wetting tension ≥40 mN/m
6, Haze Test	Ensures transparent visual appearance for food packaging and optical substrates.
Specimen Information	Flat, wrinkle-free PET film sample
Test Equipment	Hazemeter (integrating sphere).
Mandatory Stipulations	Maximum haze value ≤8.0%

Related Standard

ISO 17555	Plastics - Film and sheeting - Biaxially oriented polypropylene (PP) films
ISO 15987	Plastics - Film and sheeting - Biaxially oriented polyamide (nylon) films
TCVN 10105	Biaxially oriented polypropylene films for general use
ISO 13636	Plastics — Film and sheeting — Non-oriented poly (ethylene terephathalate) (PET) sheets
GB/T 16958	Biaxially oriented polyester film for package

Why ISO 15988 Is Critical for BOPET Material

Unified Global Quality Benchmark: Eliminates inconsistent factory testing rules between film manufacturers, converters and end buyers; enables cross-border trade acceptance of BOPET rolls

Guarantee End-Use Processing Stability:

Strict mechanical tensile rules ensure films do not break during high-speed printing, slitting, lamination machines

Heat shrinkage limits prevent warping, delamination or distortion during thermal processing (retort sterilization, heat lamination)

Controlled Barrier Performance: Standardized oxygen/water vapor transmission testing guarantees food shelf-life consistency for packaging applications

Surface Adhesion Control: Corona treatment wetting tension specification avoids printing ink peeling or lamination delamination defects

Optical Uniformity: Fixed haze requirements maintain consistent transparency for packaging and optical applications

Dimensional Consistency: Roll width, length, thickness tolerances eliminate production waste during downstream slitting and conversion

ISO 17555, ISO 15987, used for uniaxial and biaxial stretching of non-metallic materials such as PET, PA, PP, PS, and EVA. The two axes can be independently closed-loop controlled, allowing for equal biaxial (1:1 synchronous stretching) or unequal biaxial (1:2, 2:1, or uniaxial) loading.

Transmission and Haze testing machine (General type) can easily realize ASTM D1003 non compensating method, ISO 13468 compensating method, full transmittance and haze testing. The open sample chamber can be tested vertically or horizontally to adapt to more tested samples.

Water vapor transmittance tester is a set of industrial, efficient and intelligent test system, which is suitable for the determination of water vapor transmittance of plastic film, composite film, medical, construction and other materials.

Professional film heat shrink tester is precision lab equipment engineered to evaluate thermal shrinkage characteristics and thermal dimensional stability of diverse raw materials, including plastic films. compliant with international testing standards ASTM D2732 and ISO 11501.

Adopts advanced electrochemical coulometric detection method for accurate, time-efficient oxygen transmission rate analysis for medium and high barrier packaging materials. Conform with ISO 15105-2, ASTM D3985. Key testing indicators Oxygen Transmission Rate (OTR), oxygen volume passing through.

Automatic interface tensiometer is an instrument that uses physical methods (such as the platinum ring method and platinum plate method) as an alternative to chemical analysis. It is mainly used to measure the surface and interfacial tension of liquids and is suitable for industries.

Pipe Size Change Measuring Tester/ Longitudinal Reversion Tester suitable to measure various kinds of plastic pipe or plastic materials size change after heating, or used to heat the test sample. It can be used as a heating oven as well.

Q1: What exactly is ISO 15988, and what material does it cover?

A: ISO 15988 is an international standard titled Plastics — Film and sheeting — Biaxially oriented poly(ethylene terephthalate) (PET) films. It only applies to transparent biaxially oriented PET (BOPET) thin films used for packaging, printing, lamination and optical substrates. It excludes unoriented PET sheets, thick PET boards, coated PET functional films (metalized/coated barrier films) and other plastic films like BOPP, CPP. It splits BOPET into two core categories: corona-treated film and non-corona-treated film.

Q2: Why is ISO 15988:2003 so important for BOPET manufacturers, converters and packaging end-users?

A: 4 core critical values:

Global unified quality benchmark: Eliminates inconsistent factory test rules between cross-border suppliers and buyers; all parties use identical test procedures and pass/fail thresholds, avoiding disputes on film quality during trade.

Process stability guarantee: Mechanical, thermal and surface test limits ensure films run smoothly on high-speed printing, slitting, laminating and retort packaging lines without breakage, shrinkage or printing failure.

Consistent shelf-life control: Standardized oxygen/water vapor barrier testing stabilizes food/pharma packaging shelf life.

Clear acceptance criteria: Defines mandatory dimensional tolerances, appearance defects and marking rules to reduce production waste and reject rates.

Q3: Which industries rely on ISO 15988 compliance testing?

A: Primary sectors:

Flexible food packaging (retort pouches, snack bags, vacuum meat packaging)

Gravure/rotogravure printing & adhesive lamination industry

Optical base film production (display protective films, window laminates)

Electrical insulation thin film manufacturing

Label, sticker and graphic transparent substrate production

Pharmaceutical blister and moisture-proof packaging converters

Q4: What are the minimum tensile strength and maximum elongation limits in ISO 15988? Why are these limits set?

A: Standard requirements:

Tensile strength at break (MD & TD): ≥150 MPa

Tensile strain at break (MD & TD): ≤200%

Reasoning:

Minimum 150 MPa tensile strength ensures BOPET withstands high tension during high-speed slitting, printing and lamination without snapping or tearing.

Maximum 200% elongation prevents excessive plastic stretching; over-elongated film will deform permanently during heat sealing or retort cooking, causing distorted packaging shapes and blurred printed graphics.

Balanced MD/TD tensile performance also proves uniform biaxial stretching during film production.

Q5: What test speed, specimen shape and sample quantity are required for tensile testing per ISO 15988?

A: Test speed: Fixed at (200 ±20) mm/min (no adjustment allowed for thin PET films)

Specimen geometry: Dumbbell shape per Figure 1 in ISO 15988; gauge length = 50 ±0.5 mm, parallel width 10–25 mm, total specimen length >150 mm

Sample size: Minimum 5 replicate specimens for MD and another 5 for TD separately; arithmetic average of 5 results determines pass/fail status.

Pre-conditioning: All specimens must be stabilized under ISO 291 standard atmosphere before testing to eliminate moisture interference.

Q6: What is the heat shrinkage test (dimensional change on heating), and what is the maximum allowed shrinkage value?

A: It is a thermal mechanical test to measure film dimensional deformation under high temperature, simulating real downstream heating processes (print drying, heat lamination, retort sterilization).

Standard limit: MD and TD shrinkage both ≤3.0%

Test condition: Specimens suspended vertically at 150 ±3 °C circulating air oven for 30 min, then cool 30 min at room temperature before measuring length change.

Calculation formula: S = [(L₁−L₂)/L₁] × 100 (S = shrinkage percentage; L₁ = original gauge length, L₂ = post-heating length)

Q7: Why do we need to test both MD and TD directions for tensile strength and heat shrinkage?

A: BOPET is manufactured via biaxial stretching (machine direction then transverse direction). If stretching ratios are unbalanced in production, MD and TD mechanical properties will diverge severely:

Uneven tensile strength leads to directional tearing during processing

Unbalanced heat shrinkage causes curled film rolls, warped packaging bags and misaligned printed patterns after heating

ISO 15988 requires equal performance thresholds for both directions to guarantee balanced film orientation quality.

Q8: Can a BOPET film pass tensile test but fail heat shrinkage test? What causes this?

A: Yes, this is a common factory defect. Root causes:

Insufficient transverse stretching during film production (poor orientation balance)

Uneven heat setting temperature in the BOPET production line

Residual internal stress trapped inside the film web

High shrinkage films cannot be used for retort packaging or high-temperature printing even if tensile strength meets the standard.

Q9: If tensile elongation exceeds 200% as per ISO 15988 limits, what downstream problems will occur?

A: Overly ductile film will stretch irreversibly under machine tension during printing or lamination, leading to uneven web width, misregistered printed graphics, and bag deformation after heat sealing.

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