Polyester Film Water Absorption – Technical Data & Test Conditions
Polyester film water absorption is the percentage mass increase of a PET specimen after controlled water exposure. This TDS covers bare, treated, coated, and adhesive-backed PET constructions. Interpret results with thickness, conditioning, water temperature, exposure duration, and construction. Reference data can screen materials, but confirm the actual grade and finished structure by representative testing.
Measurement Boundary and Reporting Basis
This TDS covers gravimetric water uptake for finished polyester film used in comparison, incoming inspection, repeat-lot review, and project validation. It does not replace thickness, surface-energy, chemical-resistance, WVTR, or application-specific testing.
ASTM D570-22 publicly scopes sheet specimens at 0.13 mm or greater; ISO 62:2008 provides another route. For thinner film, state whether the method is applicable, adapted, or reference-only.
Specimen Identity Before the Number
Identify bare BOPET, treated PET, coated PET, or PET with adhesive and optional liner. The PET protective film and functional PET film range shows why construction must be recorded before comparison.
Record thickness, treated side, exposed edges, adhesive, and liner status. For adhesive-backed polyester film, separate base-film uptake from peel, edge, optical, and residue checks.
Data Fields for a Valid Water-Absorption Record
Item | Typical Value / Reference Range | Test Method or Condition | Notes |
Base film | PET / BOPET | Material ID | Record grade. |
Construction | Bare / treated / coated / adhesive-backed | Construction review | Record layers / exposed edges. |
Film thickness | Project-confirmed, um | Calibrated thickness method | Use actual gauge. |
Fixed-time water absorption | Based on tested grade | ISO 62; ASTM D570 where applicable; validated equivalent | Report time / water temperature. |
Saturation water absorption | Based on tested grade | Defined saturation procedure | Keep separate from fixed-time data. |
Calculation | % mass increase | (Wet mass - Dry mass) / Dry mass x 100 | Fix conditioning / weighing timing. |
Post-exposure appearance | Project-confirmed | Exposed vs control | Check haze, clarity, curl, coating. |
Adhesive construction | Project-confirmed | Actual substrate and dwell | Check peel, edges, trace, residue. |
From Conditioning to Final Weighing
Condition the specimen, record dry mass, immerse at defined temperature and time, remove surface water consistently, then weigh within a fixed window. Calculate percentage mass increase from dry and post-exposure mass.
ASTM D618 or equivalent conditioning improves repeatability. Record specimen dimensions and method status, especially for thin film.
Test Item | What It Checks | Suggested Method or Reference | Why It Matters | When To Request It |
Conditioning | Starting sample state | ASTM D618 or equivalent | Improves repeatability. | Comparative / lot test |
Thickness | Actual gauge | Calibrated method | Controls comparability. | Every comparison |
Fixed-time absorption | Mass gain at defined time | ISO 62; ASTM D570 where applicable; validated method | Links result to time / temperature. | Screening / qualification |
Saturation exposure | Longer-term uptake | Defined project or ISO 62 procedure | Separates saturation from fixed-time data. | Long dwell / moisture-sensitive use |
Appearance | Haze, clarity, curl, coating | Before / after comparison | Finds visible changes. | Clear / coated film |
Peel / edge / residue | Finished adhesive compatibility | Actual-substrate dwell test | Checks interface behavior. | Adhesive-backed film |
Reading Fixed-Time and Saturation Results Correctly
Fixed-time and saturation results represent different exposure states. Compare only equivalent construction, thickness, conditioning, temperature, geometry, and reporting.
If gauge selection is open, review the PET film thickness range separately. Tensile, elongation, haze, and dimensional stability do not replace water-uptake data.
Keeping Water Uptake Separate From Other Moisture Metrics
Water absorption is mass gain after exposure; water content is existing moisture; WVTR is vapor transmission through film. ISO 15512 treats water content separately from ISO 62 absorption.
For printing, coating, or adhesive wet-out, use surface-energy testing for PET film rather than water-absorption data.
When the Base Film Is Part of an Adhesive Construction
For protective film, the same PET base may behave differently on glass, metal, plastic, painted/coated, rough, and low-energy surfaces because the adhesive interface governs peel, edge lift, bubbles, ghosting, and residue.
Use the protective film selection framework when surface or application conditions dominate. Record coating, temperature, humidity, sunlight, storage time, equipment, operator method, and sample result. Load weight, cargo shape, and transport distance affect system stress, not intrinsic PET absorption.
A 24 h, 72 h and 7 Day Validation Sequence
Apply representative film to the intended substrate using planned cleaning, pressure, equipment, and operator method. Where relevant, check peel at 24 h, 72 h, and 7 days, recording edge lift, adhesive trace, surface shadow, gloss change, bubbles, tearing, and residue.
These dwell checks are separate from water-immersion time. Repeat when material or working conditions change materially, and retain one approved film-and-substrate sample.
When the Number and the Application Result Disagree
Data Point | If Too Low | If Too High | Risk in Application | Check Before Full Use |
Water absorption | Short exposure / method mismatch | Higher uptake / layer contribution | False comparison; dimensional / optical change | Grade, gauge, time, temperature, weighing |
Exposure time | May understate uptake | May represent another service state | 24 h and saturation mixed | Use one defined exposure |
Film thickness | Changes sensitivity / stiffness | Changes edge ratio / equilibrium time | Non-equivalent specimens compared | Actual gauge / tolerance |
Coating / adhesive contribution | Finished behavior may be missed | Other layers may add mass | Bare PET data applied to wrong construction | Identify layers; test full structure |
Peel adhesion | Premature lift | Higher removal / transfer risk | Edge lift, trace, ghosting | 24 h / 72 h / 7 days peel |
Haze / appearance | Sample / method mismatch | Clouding / visible change | Optical rejection | Compare unexposed control |
Storage History and Repeat-Lot Comparability
Store reference samples and rolls in intact packaging away from sunlight, heat, and moisture. Record storage time, temperature, relative humidity, stacking pressure, roll orientation, wrapping damage, and unusual transport exposure; recondition before comparison.
For coating, slitting, sample preparation, or repeat-lot control, review PET film coating, testing, and slitting capabilities with the retained sample and test record.
FAQ
What is a typical polyester film water absorption value?
No single value applies to every construction. Use a typical or reference value only with grade, thickness, conditioning, temperature, duration, and method.
Can ASTM D570 be used for thin PET film?
For thin PET film, confirm thickness applicability with the laboratory or project method. If ASTM D570 is outside its sheet scope, report it as adapted or reference-only.
Is PET film water absorption the same as WVTR?
No. Water absorption is mass gain after exposure; WVTR is vapor transmission; water content is existing moisture.
Why is sample testing needed for adhesive-backed PET?
Base-film uptake does not predict adhesive behavior on every substrate. Verify peel, edge lift, residue, bubbles, ghosting, gloss change, and tearing on the intended surface.
