# bltok.com Blog — self-adhesive label materials # https://www.bltok.com/blog ## BOPP Surface Energy: Spec 38 Dyn/cm, Then Verify It Date: 2026-10-29 Category: materials URL: https://www.bltok.com/blog/bopp-surface-energy-dyne-level-specification Short answer: untreated BOPP sits at roughly 29 dyn/cm, and no standard label ink wets that. Corona treatment lifts the surface to a printable level, and for label work the number to write into a purchase order is a minimum of 38 dyn/cm measured at delivery, not at the moment of coating. That distinction matters because treatment decays in storage, and a certificate issued on coating day describes a reel that no longer exists by the time it reaches your press. Specify the value, specify the test, and test it yourself on arrival. Most print-adhesion arguments between a material supplier and a converter are not arguments about quality. They are arguments about two different dates. The supplier tested the web a few metres after the treater; the converter tested it six weeks later, three wraps into the roll, on a press running at 150 m/min. Both numbers were true. Only one of them described the material that failed. Why 29 dyn/cm is the surface your ink actually meets Polypropylene is a hydrocarbon chain with a methyl group hanging off every other carbon. There is nothing on that surface for a polar ink to grab: no hydroxyl, no carboxyl, no charge. Measured with dyne test inks, plain BOPP reads around 29 dyn/cm, and low-density polyethylene reads around 31. Both sit well below the surface tension of a water-based flexo ink, which typically lands somewhere between 30 and 38 dyn/cm depending on the pigment load and the surfactant package. When the ink's own surface tension is higher than the substrate's surface energy, the droplet does not spread. It beads, and a beaded droplet dries into a film with no mechanical key and very little chemical one. The working rule in most converting shops is that the substrate should sit roughly ten dyn/cm above the ink. That is why a printing target of 38–42 dyn/cm shows up in almost every corona guideline, and why coaters and laminators ask for the top of that band: an adhesive or a coating has more time to wet out than an ink does, but it also has further to travel before it cures. For label stock we work to a 36–38 dyn/cm floor, and we treat to the print side only unless the job calls for something else. The treater sits upstream of the coating head on this line, and the oven downstream only removes water. Surface energy is set in the first few metres of the web and then simply ages — nothing later in the process restores it. The five facestocks, and what each one needs before it will hold ink Surface energy is a property of the polymer, not of the brand, so the same table applies whether the film comes from us or from anyone else. What differs between suppliers is how much treatment they apply, how evenly they apply it across a 1080 mm web, and whether the value they quote was measured at the head or at the roll. Facestock Native surface energy Printable as supplied? What to specify BOPP ≈29 dyn/cm No ≥38 dyn/cm at delivery, print side; both sides if printed both sides PE (LDPE / HDPE) ≈31 dyn/cm No ≥38 dyn/cm; expect faster decay than BOPP PET ≈41–44 dyn/cm Usually yes for solvent and UV inks Declare whether treated; water-based inks still want ≥38 Coated paper Above 40, variable by coating Yes Not surface energy — specify pick strength and roughness instead Synthetic paper 29–31 base, raised by the mineral coat Yes, because of the coating, not the resin Ask what the topcoat is; the resin underneath is still PP Two entries in that table are worth a second look. PET is printable straight off the line for most solvent and UV systems, which is one reason it costs more and behaves better in hot-fill work; the fuller comparison sits on the BOPP versus PET page. Synthetic paper is a filled, micro-voided PP sheet: the resin is the same hydrocarbon, but the mineral loading and the coating give the ink something to hold, so a synthetic paper that measures 40 dyn/cm is measuring its coat, not its polymer. Corona treatment is a loan, not a property Corona treatment works by running the web through a high-voltage discharge between an electrode and a grounded roll. The discharge breaks carbon–carbon bonds at the surface and lets oxygen attach, creating carbonyl, carboxyl and hydroxyl groups. Those groups are polar, and polar groups are what a polar ink can wet. None of this changes the bulk film: thickness, stiffness, haze and tensile behaviour are untouched, which is exactly why the treatment can be lost without the reel looking any different. The loss is not a manufacturing defect. It is the surface trying to return to its lowest-energy state, and it happens on every treated film from every supplier. It is faster in a hot warehouse, faster in high humidity, faster when the roll is wound tight and the treated face is pressed against the liner under tension, and faster in the first weeks than later on. A reel that measured 40 dyn/cm at the head can be measurably lower by the time it clears customs, and lower again after a summer in a container. This is why we quote a floor at delivery rather than an achievement at coating, and why the test that decides a dispute should be taken on the buyer's floor, not in the supplier's lab. There is a second-order effect worth knowing about. Treatment decays from the outside in, so the outer wraps of a roll and the two edges of a wide web are usually the first to fall below spec. A single reading taken from the outside wrap tells you about the part of the reel you were going to cut off anyway. Take the sample after stripping two or three wraps, and take it from the middle of the width as well as from both edges. Four lines to add to the purchase order Surface energy is one of the few label-stock properties a buyer can specify precisely and then check without a laboratory, so it belongs in the contract rather than in an email. The wording we suggest, and the wording we accept on our own orders: A minimum and a place. “Surface energy of the print side not less than 38 dyn/cm, measured on material sampled after removing the outer three wraps.” Naming the sampling point is what makes the number enforceable. A method. Dyne test inks or pens per the standard test-ink set, three positions across the width and three along the roll, all nine values recorded on the batch record. A single average is not a measurement of a web; it is a measurement of one square centimetre. A consequence. Below 36 dyn/cm the reel is rejected or replaced. Between 36 and 38 it is a commercial conversation, not a technical one. Writing both thresholds down in advance removes the argument that always happens when a container is already on the water. A storage statement. Store below 30 °C, out of direct sun, original wrap intact, FIFO rotation, and a stated shelf life after which the material should be re-tested before use. This is the line that protects both sides, because it makes ageing a shared assumption instead of a surprise. None of these lines cost anything to add, and together they convert a vague complaint about “the ink not sticking” into a number that one party can be wrong about. Material that arrives with those four lines already agreed is also the only material for which a goods-in test is worth running, because there is something to compare the result against. Our own QC room: the tensile stand on the left runs peel and shear, and the bench carries the adhesive components that go with each batch record. Surface energy is checked on the line and logged with the batch, which is the only reason a delivery figure can be defended later. The ten-minute goods-in test You do not need a contact angle goniometer to hold a supplier to a surface-energy clause. You need a dyne pen set covering 34 to 44 dyn/cm, a clean flat surface, and a habit. Strip and sample. Remove two or three outer wraps from the roll and cut a sheet roughly A4 size from the middle of the width. Do not test the outside wrap and do not test a crumpled edge. Draw, don't scrub. Draw a single stroke about 50 mm long with the lowest pen in the set you expect to pass, and read it after two seconds. A stroke that stays continuous and does not bead has passed that level. A stroke that breaks into droplets within two seconds has failed it. Wipe the pen tip, work upward one level at a time, and stop at the first level that holds. Do not rub the ink in: the shear you apply is not part of the test. Repeat across the web. Three positions across the width, three along the length, nine readings, written down with the batch number. The average is what you compare to the PO; the spread is what tells you whether the treater was running evenly. A reel reading 40/40/39 is better material than one reading 44/41/34, even though the second has the higher average. Have a fallback. If the pens are not at hand, a water-break test is a crude but usable screen: a drop of distilled water that spreads and stays spread indicates a high-energy surface, and a drop that sits up like a bead indicates a low one. It cannot give you a number and it should not be used to accept a batch, but it will catch the reel that is obviously wrong before it reaches the press. When a reel fails, the useful question is not who is at fault but what the material has been through. A reel that measured well at packing and badly on arrival has almost certainly aged in transit, and the fix is a shipping and storage conversation rather than a chemistry one. A reel that reads low in the middle of the web and high at one edge was badly treated, and that is a supplier problem on the day of coating. When the film is not the problem Surface energy is the first thing blamed and the second thing wrong. Before a batch is rejected on a dyne reading, three other causes are worth ruling out. The ink has changed. Ink surface tension drifts with viscosity, with the surfactant package and with the age of the batch. A press that ran the same film for a month and then started cratering has usually changed the ink, the thinner or the anilox, not the film. Ask for the ink's own surface tension figure before the material is condemned. The drying is wrong. Water-based inks need more energy than solvent inks to release their carrier, and an under-dried film feels dry while still holding enough water to fail a tape test. The symptom is a print that passes a fingernail scratch on the unwind and fails at the rewind. The treatment went too far. Over-treatment is a real failure mode above roughly 44–46 dyn/cm: the surface layer becomes friable, and the ink adheres to a weak boundary layer that then fails cohesively inside the film rather than at the interface. The fingerprint is a print that peels off leaving a chalky haze. It looks like poor adhesion and is in fact the opposite problem. For jobs that print both sides of the same web, the rule is simple and often missed: both sides have to be treated, and both have to be specified. Single-side treatment on a double-side print job produces one good side and one that fails, and no amount of press adjustment fixes the second one. The 75 µm clear grades that people ask about for double-sided work are the same story — the thickness is not what makes them print, the treatment on both faces is. The BOPP facestock range sets out which constructions are treated on one face and which on both. What we do, and what we will not claim We treat on the coating line, across a 1080 mm web at up to 180 m/min, and the surface energy is checked and logged with the batch alongside the other four routine tests: initial tack, holding power, 180° peel and coating appearance. Reels ship inside a normal 15–25 day production window from a 1,000 m² minimum, in the original wrap, and we will tell you the date the material was treated if you ask, because that date is the one that governs how much of the treatment is left when it lands. What we will not do is quote a decay rate as if it were a specification. The rate depends on your warehouse, the season, the freight route and how the roll is stacked, and any supplier who puts a single number of weeks on it is describing their own storeroom rather than your material. Test on arrival, record the nine readings, and let the trend across three or four deliveries tell you what your supply chain actually does to a treated surface. The same discipline applies to coat weight, which is measurable on a 0.001 g balance and is covered in the coat weight verification guide. If the ink system is the variable rather than the film, the flexo and UV inkjet notes on our flexo TDS page and the inkjet receptive coating page go further into the interaction. And if the immediate question is how much material a job needs rather than whether it will print, the roll calculator answers that in one screen. Frequently asked questions What surface energy should BOPP label film have before printing? For most label inks, a minimum of 38 dyn/cm on the print side is the practical floor, and 38–42 is the band the industry works in. Below 36 dyn/cm, water-based and UV inks start to bead rather than wet out, and the resulting print can look acceptable on the reel and fail a tape test a week later. Specify the number at delivery, not at coating, because the two differ by however long the material spent in storage and transit. Does corona treatment expire? It decays rather than expiring on a fixed date. The treated surface gradually returns toward the polymer's native energy, fastest in the first weeks and in hot or humid storage, and more slowly afterwards. There is no universal shelf life that can be printed on a label, which is why the reliable control is a dyne test at goods-in rather than a date code. Material stored cool, dry, wrapped and rotated keeps its treatment measurably longer than material stored otherwise. Can I use a water drop instead of dyne pens? As a screen, yes; as an acceptance test, no. A water drop that spreads and holds indicates a high-energy surface and one that beads indicates a low-energy one, which is enough to catch a badly under-treated reel before it goes on press. It gives no number, so it cannot be compared against a purchase order or used to settle a claim. Dyne pens or test inks cost little and take the same two minutes. Why does the same film print well on one batch and poorly on the next? Usually because the two batches are not the same age, not because they are not the same material. Treatment level at coating can be identical while one reel sat through a hot month in a warehouse and the other went straight to the press. The second most common cause is the ink: a new ink batch with a different surfactant package can fail on film that has not changed at all. Comparing dyne readings and ink surface tension side by side separates the two cases in an afternoon. Is a higher dyne level always better? No. Above roughly 44–46 dyn/cm the treated layer can become friable, and adhesion then fails inside a weak boundary layer instead of at the interface, which shows up as a chalky residue when the print is peeled. The target is a band, not a maximum. That is also why over-treatment is a common cause of complaints that look exactly like under-treatment until someone reads the number. ### FAQ Q: What surface energy should BOPP label film have before printing? A: For most label inks, a minimum of 38 dyn/cm on the print side is the practical floor, and 38 to 42 is the band the industry works in. Below 36 dyn/cm, water-based and UV inks start to bead rather than wet out, and the resulting print can look acceptable on the reel and fail a tape test a week later. Specify the number at delivery, not at coating, because the two differ by however long the material spent in storage and transit. Q: Does corona treatment expire? A: It decays rather than expiring on a fixed date. The treated surface gradually returns toward the polymer native energy, fastest in the first weeks and in hot or humid storage, and more slowly afterwards. There is no universal shelf life that can be printed on a label, which is why the reliable control is a dyne test at goods-in rather than a date code. Material stored cool, dry, wrapped and rotated keeps its treatment measurably longer than material stored otherwise. Q: Can I use a water drop instead of dyne pens? A: As a screen, yes; as an acceptance test, no. A water drop that spreads and holds indicates a high-energy surface and one that beads indicates a low-energy one, which is enough to catch a badly under-treated reel before it goes on press. It gives no number, so it cannot be compared against a purchase order or used to settle a claim. Dyne pens or test inks cost little and take the same two minutes. Q: Why does the same film print well on one batch and poorly on the next? A: Usually because the two batches are not the same age, not because they are not the same material. Treatment level at coating can be identical while one reel sat through a hot month in a warehouse and the other went straight to the press. The second most common cause is the ink: a new ink batch with a different surfactant package can fail on film that has not changed at all. Comparing dyne readings and ink surface tension side by side separates the two cases in an afternoon. Q: Is a higher dyne level always better? A: No. Above roughly 44 to 46 dyn/cm the treated layer can become friable, and adhesion then fails inside a weak boundary layer instead of at the interface, which shows up as a chalky residue when the print is peeled. The target is a band, not a maximum. That is also why over-treatment is a common cause of complaints that look exactly like under-treatment until someone reads the number.