# Gel Content: The Number That Sets Your Lamination Window > Gel content records how much of the encapsulant crosslinked during lamination. Both edges of the window delaminate, for opposite reasons. ![Gel Content: The Number That Sets Your Lamination Window](https://cdn.ooitech.com/static/upload/image/20261026/encapsulant-gel-content.png) - ** Ooitech Editorial Team - ** 2026-11-14 - ** 0 Views - ** [Blog](./Blog.html) Editorial responsibility for this article rests with **Wuhan Ooi Photoelectric Technology Co., Ltd.** Written and reviewed by the Ooitech editorial team. See our [editorial policy](/editorial-policy.html) for how we write, where AI tools are used, and how to request a correction. ## Gel Content: The Number That Sets Your Lamination Window **Quick answer:** Gel content is the fraction of an encapsulant that will not dissolve, and it is the only routine measurement of whether the cross-linking reaction actually ran inside your laminate. The usual windows are 80 to 95 % for EVA and 70 to 95 % for EPE, and both ends of those windows are failure modes rather than margins. Too low leaves unreacted peroxide and its by-products sealed inside the module. Too high makes the film brittle and moves the failure to the interface. Two details decide what the number means. The test runs on a coupon laminated with your own recipe, so it measures your process as much as the material. And the national and international extraction methods differ in extraction time, which is enough to move a result across the acceptance line at low cure, where part of the EPE market sits. A gel content result that passes in the supplier report and fails in your own lab is usually not a contradiction. It is two different tests, on two different laminates, read against one acceptance line. ## What the number actually measures Before lamination an encapsulant is a linear polymer. In the laminator the peroxide in the formula decomposes into radicals, those radicals pull hydrogen off neighbouring chains, and the chains join into a three dimensional network. A network of that kind no longer dissolves. It swells, and the part that is still soluble goes into the solvent. Gel content is the mass fraction that stays behind. That definition carries a consequence that gets forgotten in acceptance arguments: the number is a ratio, not a description. Ninety percent means ten percent of the film is still linear, still able to flow at temperature, still able to migrate. Two films reading 90 can behave differently if the ten percent is spread evenly in one and concentrated at a surface in the other. ## The test, as it is actually run The xylene extraction method is standardised and it is worth walking through, because three of its steps decide the answer more than the material does. Two sheets are stacked, typically around 200 by 200 mm, with release film above and below so the cured film can be peeled off afterwards. The sandwich is laminated on the recipe used on the production line, not on a generic recipe, and cooled. The cured film is removed, cut into granules smaller than 3 by 3 mm, and 0.5 g ± 0.05 g is weighed into a 120 mesh stainless steel bag. Three specimens are prepared per sample. The bag is cleaned, dried and weighed empty as W1, weighed with the granules as W2, sealed, and extracted in boiling xylene to the procedure in GB/T 29848. After extraction the bag is dried at 140 °C to constant mass, cooled in a desiccator for 30 minutes, and weighed as W3. Gel content is (W3 − W1) / (W2 − W1), as a percentage, averaged over the three specimens. - **The coupon recipe.** A coupon cured at a different temperature or dwell than the line tells you about the lab, not the product. - **The granule size.** Coarse granules present less surface to the solvent and read high. - **Drying to constant mass.** Residual solvent adds mass and pushes the result up. It is the easiest error to make and the hardest to notice, because the number looks plausible. ## Two sides of the window, and the failure each produces The acceptance windows in common use are 80 to 95 % for EVA and 70 to 95 % for EPE. The table is the part worth pinning on the wall, because the two failure signatures overlap in the place where people usually look. | What you measure | Below the window | Above the window | | --- | --- | --- | | Tensile strength | Lower | Higher, then falling as the network gets brittle | | Tear resistance | Lower and unstable | Lower: the film cracks instead of stretching | | Adhesion to glass and backsheet | Unstable, and it drifts during aging | Lower, and it fails at the interface rather than in the film | | Bubbles after lamination | Common | Less common at lamination, more common in the field | | Delamination | Late, after damp heat | Early, at edges and around busbars | | What stays inside the module | Unreacted peroxide and acidic decomposition products | Volatiles from over-reaction | Gel content window for EVA and EPE, with the failure mode on each side. The number comes from a coupon cured on your own platen, not from the roll. Both ends produce delamination, which is why delamination alone cannot tell you which side of the window you fell off. You need the number, and you need to know what is left in the film. ## Too low: the failure that keeps working after lamination A low gel content means the peroxide was not fully consumed. Part of that is a mechanical problem, visible immediately as lower tensile strength and tear resistance, and as adhesion that reads acceptable on the day and drifts afterwards. The other part is chemical, and it is the part that costs modules. Residual cross-linking agent and its decomposition by-products stay sealed inside a laminate that nobody will open for twenty five years. At service temperature they can release acidic species, and those species are looking for exactly the materials that conduct: the ribbon surface and the silver grid. The result shows up as a slow series resistance rise, not as an obvious defect, which is why a poorly cured batch can pass final inspection and fail damp heat. This is also one of the routes into the bubbles that get blamed on the laminator. When the cure is incomplete, the film keeps flowing and releasing volatiles after the cycle is supposedly finished, and the voids appear where the film is thinnest, which is along the ribbon row. [Not every laminate bubble](https://www.ooitech.com/not-every-laminate-bubble-is-the-encapsulants-fault-the-real-causes-behind-solar-module-bubbles.html) is the encapsulant fault, but enough of them are that the cure record belongs next to the bubble map. ## Too high: brittleness, and adhesion that fails at the interface Over-cure is the direction operators drift towards, because more heat and more time feel like insurance. The mechanics go the other way. A denser network has less capacity to relax, so tear resistance falls and the film behaves like a thin sheet of rubber that has lost its elasticity. Adhesion weakens as well, and it weakens at the interface rather than inside the film, which is the failure mode that shows up as chipping at the module edge and as delamination around the busbar. In EPE the same over-cure shows up first along the cell ribbon, where the [line shaped voids](https://www.ooitech.com/epe-encapsulant-lamination-delamination-line-shaped-bubbles-along-solar-cell-ribbons.html) track the geometry of the joint rather than the pattern of the vacuum cycle. There is a second cost that does not appear in the gel content number at all. A stiffer network flows less during the same dwell, so filling the gap around a ribbon and the step at a cell edge becomes more dependent on vacuum time and on the pressure ramp. Cure and flow are being traded against each other, and the trade is invisible unless both are measured. When balancing cure against flow, we stage the vacuum time and adjust the chamber pressure ramp so the film fills around the ribbon step before cross-linking makes the polymer network rigid. ## Why the supplier number and your number disagree Two things differ between the report in the quality file and the coupon in your lab, and only one of them is the material. The first is the coupon recipe, already covered. The second is the method. GB/T 29848 and IEC 62788-1-6 both determine the degree of cure by extraction, and they differ in extraction time. Work published in EPJ Photovoltaics in 2024 compared the two and found the national method reading higher, with the gap widest at low cure. That matters because part of the EPE market has its acceptance line sitting inside the region where the two methods disagree. A film can be compliant against one standard and non-compliant against the other at the same degree of cure, and neither lab is wrong. ### DSC answers a different question It answers a different question. DSC measures the residual reaction exotherm, which is how much cross-linking is still available to happen, rather than how much network already exists. It is fast, it needs milligrams, and it trends a process beautifully. It also cannot be compared to a gel content specification, because the two numbers are not the same quantity. | Method | What it reports | Best used for | Where it misleads | | --- | --- | --- | --- | | Xylene extraction, GB/T 29848 | Insoluble mass fraction after a defined extraction | Acceptance against a gel content specification | Sensitive to granule size and drying; extraction time differs from the IEC method | | Xylene extraction, IEC 62788-1-6 | The same quantity by a different procedure | Cross border supply agreements | Reads lower than the national method at the same cure | | DSC residual exotherm | Cross-linking still available to occur | Process trending, incoming screening, cure studies | Not comparable to a gel content percentage | ## Where the number is really set Three levers belong to the film maker: the recipe, meaning the peroxide and the co-agent that carries the cross-linking efficiency; the compounding and the absorption of liquid additives into the pellets; and the extrusion, which has to stay below the temperature at which the peroxide starts to decompose, otherwise the film arrives partly cured. Three levers belong to the module line: the actual laminate temperature, the dwell at that temperature, and the uniformity of both across the platen. Of the three, uniformity is the one that hides. A platen that is several degrees hotter near the manifold than at the far edge will produce two different gel contents in one module, and the coupon you take from the middle will average them into a pass. To eliminate temperature variations across the platen, we configure multi-zone heating circuits and verify surface uniformity so the laminate experiences identical thermal dwell from the manifold to the outer edges. The temperature that matters is the laminate, not the setpoint. [The EVA window](https://www.ooitech.com/laminator-vacuum-pumps-types-and-eva-window.html) is the practical way to see this: thermocouples placed on the glass and on the cell plane, read against a gel content series from coupons cured at the same conditions, give you a process window in one afternoon. A recipe table copied from a supplier gives you a number that happens to work in someone else's machine. The rest of the cycle, including how the vacuum is staged, is described in [the lamination process](https://www.ooitech.com/solar-panel-production-process-lamination.html) itself. ## The coupon is the specification A specification that says 80 to 95 % without naming the coupon recipe and the method is not enforceable, because both parties can be honest and disagree. Write three things into it: the lamination recipe for the coupon, including the platen temperature and dwell; the standard and the extraction time used; and the sampling position, since edge and centre of a full size module are not the same laminate. The fourth thing worth writing in is the distribution rather than the mean. A supplier reporting 88 % with a range of 86 to 90 across the roll width is a different supplier from one reporting 88 % with a range of 79 to 96, even though the acceptance test passes both. ## When it fails on the line, look in this order - **The method.** Re-run the same coupon twice before you touch the process. Granule size and drying time account for more disagreement than most investigations assume. - **The laminate temperature.** Thermocouple the glass and the cell plane, not the platen surface, and compare against the recipe that produced the passing coupons. - **The uniformity.** Coupons from the feed end and the discharge end of the same platen, cured in the same cycle. - **The vacuum and dwell.** If cure is fine at the centre and low at the edge, the problem is usually heat transfer under a poor vacuum rather than chemistry. - **The recipe.** Only now. Peroxide loading, co-agent, and the age and storage of the crosslinker masterbatch, because peroxides lose activity in storage and a lot that was fine in March is not the same lot in September. ## What to ask before you sign the specification Four questions separate a usable supplier from a compliant one. Which standard and which extraction time did this number come from. What coupon recipe was used. What is the minimum and maximum across the roll width and along the length, not the mean. And for EPE, whether the layer ratio changed between the qualification sample and the production roll, because the window moves with the structure and the qualification is only valid for the structure that was tested. That last point is the same logic as [a bill of materials change](https://www.ooitech.com/iec-ts-62915-what-a-module-change-forces-you-to-retest.html) in the module standards: the evidence attaches to the configuration, not to the brand. ## Where these numbers come from The acceptance windows of 80 to 95 % for EVA and 70 to 95 % for EPE, the failure signatures on both sides, and the extraction procedure with its granule size, mesh, temperatures and weighing steps come from the source article this piece is based on, which is a Chinese technical note on encapsulant cross-linking. The comparison between GB/T 29848 and IEC 62788-1-6 comes from the same article, citing work published in EPJ Photovoltaics in 2024; we have not reproduced that study. The DSC comparison is standard laboratory knowledge rather than a measured result here. The statements about platen uniformity, coupon position and peroxide storage are our reasoning from polymer chemistry and from how laminators behave, not measurements we ran. We have not produced a coupon series for this article. Treat the ordering in the failure checklist as a way to spend a shift efficiently, not as a validated procedure. ### Q: Is 90 percent a good result? It is inside both windows, so it passes. Whether it is good depends on where the rest of the roll sits and on what the ten percent that still dissolves is doing. A tight distribution around 88 is a better process than a wide one around 90. ### Q: Can gel content be measured on a finished module? Not by extraction, since the film is bonded between glass and cell. Module level studies use cross sections, DSC on samples cut from the laminate, or adhesion tests after aging. For acceptance, the coupon is the practical instrument. ### Q: Why is the EPE window lower than the EVA window? Because the structure is three layers, and the POE core cross-links less readily than an EVA skin at the same temperature. An acceptance line set where EVA sits would reject perfectly good EPE, so the window is widened downwards to fit the structure rather than the resin alone. ### Q: Does higher gel content mean better PID resistance? No. PID follows ionic mobility and the resistivity of the path from the cell to ground, which is mostly a material and moisture question, and it follows the system voltage. Over-cure does not improve it and can hurt adhesion and edge sealing. ### Q: Which method should the specification name? Either, provided it names the standard, the extraction time and the coupon recipe, and provided both labs use the same one. The failure mode to avoid is a supplier report on one method and an incoming inspection on the other, with an argument in between. ### Q: How many coupons should be tested? The procedure asks for three specimens per sample. What matters more for a module line is which samples: one from each roll end and one from the middle, plus one from the edge of the platen on a routine basis, is a better use of coupons than three from the middle of one roll. ### Q: Does film grammage change the window? It changes the dwell needed to reach the same cure. Cure is an integral of temperature over time through the film thickness, so a heavier film at the same recipe reads lower gel content. If you qualify a supplier at 400 g/m² and buy at 500, re-qualify the pair of recipe and film together. ### Q: What if one line runs EVA and EPE on one recipe? Then at least one of them is running outside its window. The two structures need different dwell or different platen temperature to land in the same place, which is why mixed campaigns are where cure problems concentrate. ## Final thoughts Gel content is a cheap measurement that answers an expensive question: did the reaction finish inside this laminate. It only answers it if the coupon matches the line, if the method matches the specification, and if you read both edges of the window instead of treating more cure as safer. The number is not a grade for the material. It is a statement about the cycle that produced it. OOITech manufactures single and double chamber [solar panel laminator machines](https://www.ooitech.com/solar-panel-laminator-machine.html) with oil-heated or electric platens designed for precise thermal uniformity and pressure control. 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