Thermal shrinkage rate is one of the most critical dimensional stability parameters for photovoltaic (PV) backsheets, directly influencing lamination quality, module appearance, and long-term reliability. This article presents a comprehensive technical overview of PV backsheet thermal shrinkage testing, covering sample preparation, test conditions, measurement procedures, calculation methods, applicable standards such as IEC 62788, ASTM D1204, and GB/T 31034, and typical acceptance criteria used by module manufacturers and accredited third-party laboratories.
Technical Overview
A photovoltaic backsheet is a multilayer polymeric film applied to the rear surface of crystalline silicon PV modules, where it provides electrical insulation, moisture barrier protection, UV and weathering resistance, and mechanical support. Typical constructions combine a biaxially oriented polyethylene terephthalate (PET) core with fluoropolymer films such as PVF or PVDF, functional coatings, or co-extruded polyolefin layers bonded with adhesives.
During module lamination and field operation, the backsheet is exposed to elevated temperatures typically ranging from 140 °C to 160 °C. Because the PET core and outer layers are produced by biaxial orientation processes, heating allows the stretched polymer chains to relax, resulting in dimensional contraction. Excessive thermal shrinkage can cause visible wrinkles, solar cell displacement or gap reduction, ribbon and busbar stress, adhesive layer delamination, and edge exposure of encapsulant, all of which compromise module quality and long-term durability.
The thermal shrinkage rate is defined as the relative linear dimensional change of a backsheet specimen after exposure to a specified elevated temperature for a specified duration, expressed as a percentage. It is measured independently in the machine direction (MD) and transverse direction (TD), since orientation-induced anisotropy causes the two directions to shrink differently. Industry specifications commonly require MD shrinkage of no more than 1.5% and TD shrinkage of no more than 1.0%, with premium grades often limited to 0.5% or less after 30 minutes at 150 °C.
Testing Samples
Fluoropolymer Film-Based Backsheets
These laminates consist of fluoropolymer films (PVF or PVDF) as the outer weatherable layers bonded to a PET core through polyurethane adhesives. They are the most widely deployed backsheet architecture and are tested in their final laminated form so that shrinkage reflects the combined behavior of all layers.
Coating-Based Backsheets
These structures apply fluorine-containing or non-fluorine weatherable coatings directly onto a PET film, typically with an inner encapsulant-adhesion layer. Their shrinkage behavior is dominated by the PET substrate, and coating integrity after heat exposure is also visually assessed during testing.
Co-Extruded All-Polymer Backsheets
Produced by co-extrusion of multiple polymer layers such as polyolefin or modified polyester compositions, these backsheets eliminate adhesive layers. Their shrinkage characteristics depend on the stretching ratios and heat-setting history of the extruded multilayer film.
Transparent Backsheets for Bifacial Modules
Transparent backsheets use high-transmittance PET or fluoropolymer layers to enable rear-side power generation. Dimensional stability requirements are equally strict, since shrinkage-induced wrinkles can create visible cosmetic defects on bifacial modules.
Specimen Preparation
Specimens are typically cut as 100 mm × 100 mm squares or as rectangular strips, with at least three to five replicates per direction. Benchmark lines are marked along the MD and TD axes, and initial dimensions are documented. Specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours in accordance with ISO 291 before measurement.
Testing Items
- MD thermal shrinkage rate at specified temperature and duration
- TD thermal shrinkage rate at specified temperature and duration
- Dimensional stability under lamination-simulating conditions (commonly 150 °C for 30 minutes)
- Shrinkage at alternative conditioning temperatures (for example 120 °C, 135 °C, or 165 °C) to match specific lamination profiles or customer specifications
- MD/TD shrinkage anisotropy ratio as an indicator of film stretching and heat-setting balance
- Post-test appearance evaluation for wrinkling, curling, blistering, delamination, or coating cracking
- Optional: shrinkage after damp-heat preconditioning (85 °C / 85% RH) to support IEC 61215 and IEC 61730 type-test programs
Testing Methods
Apparatus and Equipment
Testing requires a forced-air circulating oven with temperature uniformity within ±2 °C, a flat carrier surface dusted with talc or covered with polytetrafluoroethylene to prevent sticking, and a measuring instrument such as an optical comparator, traveling microscope, or calibrated steel rule with 0.5 mm or finer graduation. Length standards traceable to national metrology institutes are used for calibration.
Test Procedure
- Condition the specimens at 23 ± 2 °C and 50 ± 5% RH for at least 24 hours per ISO 291.
- Measure and record the initial benchmark distance L0 in both MD and TD with a precision of at least 0.5 mm, preferably 0.1 mm.
- Place each specimen flat on the talc-dusted carrier in a single layer, without applied stress or constraint, and insert it into the oven pre-stabilized at the target temperature.
- Maintain the specimens at the specified temperature for the specified time, commonly 150 °C for 30 minutes, ensuring the oven recovers to setpoint within two minutes of door closure.
- Remove the specimens and allow them to recondition at 23 ± 2 °C for at least one hour before remeasurement.
- Measure the final benchmark distance L1 in both directions and record any visual defects.
- Calculate the shrinkage rate S for each direction as S = (L0 − L1) / L0 × 100%, and report the individual values and the arithmetic mean for each direction.
Reference Standards
The general methodology follows established film dimensional-change test methods, including ASTM D1204 for linear dimensional changes of nonrigid thermoplastic sheeting at elevated temperatures and corresponding ISO procedures for dimensional change of plastic films on heating. PV-specific material characterization is addressed within the IEC 62788 series for polymeric materials used in PV modules, while GB/T 31034 covers technical requirements and test methods for backsheets used in crystalline silicon PV modules. End-product verification is conducted under the IEC 61215 and IEC 61730 qualification frameworks, and many buyer specifications add their own shrinkage limits beyond these baseline requirements.
Reporting Requirements
A complete test report should identify the material and supplier reference, specimen dimensions and quantity, conditioning and test parameters (temperature, duration, atmosphere), individual and mean shrinkage values for MD and TD, calculation formula, visual observations, and a statement of conformity to the applicable specification, together with measurement uncertainty where required.
Applications
Thermal shrinkage determination is routinely applied in several contexts across the PV value chain:
- Incoming quality control by module manufacturers, where shrinkage is a standard release criterion for each backsheet lot before lamination.
- Supplier quality management and film process optimization, where MD/TD shrinkage data guide stretching ratio, heat-setting temperature, and annealing time adjustments during backsheet production.
- Lamination process window validation, ensuring that oven and press temperatures, vacuum profiles, and cycle times remain compatible with backsheet dimensional stability.
- New product qualification, supporting type-test programs and certification submissions to accredited third-party laboratories under IEC 61215 and IEC 61730.
- Failure analysis, where abnormal shrinkage is investigated as a root cause of wrinkling, cell misalignment, ribbon breakage, or backsheet delamination observed in production or in the field.
- Aging and reliability studies, tracking whether shrinkage characteristics change after damp-heat, thermal-cycling, or UV exposure, which indicates polymer degradation or residual stress release.
FAQ
Why do PV backsheets shrink when heated? The PET core and outer layers are biaxially stretched during manufacturing to gain mechanical strength and barrier properties. When reheated above the glass transition temperature, the oriented molecular chains relax toward their equilibrium state, causing the film to contract in the stretched directions.
What are the most common test conditions? The industry-standard condition is 150 °C for 30 minutes in a forced-air oven, chosen to simulate the hottest stage of module lamination. Alternative temperatures between 120 °C and 165 °C are frequently specified by module makers to match their specific processes.
What shrinkage limits are typically required? Common acceptance criteria are MD ≤ 1.5% and TD ≤ 1.0%, while many high-end specifications require ≤ 0.5% in both directions. Limits should always be verified against the applicable purchase specification, as requirements vary by module design and manufacturer.
Why is machine-direction shrinkage usually larger than transverse-direction shrinkage? Melt-cast films experience greater orientation and weaker heat-setting in the machine direction during manufacturing, so relaxation upon reheating is more pronounced along MD, although the exact anisotropy depends on the producer's stretching process.
What is the difference between free shrink and constrained shrink? In the standard oven method the specimen shrinks freely without external load, which represents the worst-case dimensional change. In a real module, the encapsulant and glass constrain the backsheet, so free-shrink values are conservative safety indicators for process control.
Can shrinkage testing be performed at other conditions or on other materials? Yes. Accredited third-party laboratories routinely conduct shrinkage measurements at custom temperatures and durations, on front sheets, encapsulant films, and other polymer films, following ASTM D1204, relevant ISO methods, or client-defined protocols.