Clear (silver) hydrophilic
Transparent film that keeps the metallic look of aluminium. Provides the basic hydrophilic function at the lowest cost, suited to standard indoor evaporators.
Fin stock guide
Hydrophilic Lacquered Aluminium Foil for Heat Exchanger Fins
Hydrophilic lacquered foil is thin aluminium fin stock coated with a water-loving film, so that condensate spreads into a thin sheet and drains instead of forming droplets that bridge the fins. It is the standard fin material for air-conditioner evaporators and heat-pump coils. This guide explains how the coating works, which base alloys and tempers suit which fin dies, and the coating tests a buyer should specify.
- Base alloys
- 8011, 3102, 1100, 1200, 8006
- Tempers
- O, H22, H24, H26
- Fin gauge
- Typically 0.08 – 0.20 mm
- Coatings
- Blue, gold, clear
Why evaporator fins need a hydrophilic coating
When warm, humid air passes over a cold evaporator coil, water condenses on the fins. On bare aluminium the water beads up, and with fin spacing of only 1.2 – 2 mm those beads quickly join neighbouring fins into water bridges.
Water bridges block part of the air path, so the fan has to work harder and air-side pressure drop rises. Droplets can be blown off the coil into the room ("spitting"), they make noise, and the standing water encourages corrosion and microbial growth that cause odours. In heat pumps running in heating mode, retained water also speeds up frost build-up and lengthens defrost cycles.
A hydrophilic coating lowers the contact angle of water on the fin surface from roughly 70 – 90° on bare aluminium to below about 10°. Instead of beading, condensate wets the surface as a continuous thin film and runs down to the drain pan. The coil keeps its open airway, airflow stays steady, and the heat exchanger delivers its rated capacity in wet operation.
Structure and production of hydrophilic foil
Hydrophilic fin foil is made on a continuous coil-coating line. Bare fin stock is unwound, cleaned and given a conversion pre-treatment (chromium-free systems are now normal), then coated with one or two thin organic layers that are cured in an oven and recoiled for slitting.
| Layer | Typical material | Function |
|---|---|---|
| Substrate | Annealed or partly hardened fin foil, 8011, 3102, 1100, 1200 or 8006 | Conducts heat, gives formability for fin collars |
| Pre-treatment | Chromium-free conversion coating | Adhesion of the organic layers and first corrosion barrier |
| Corrosion-resistant base coat | Epoxy or similar resin, often tinted blue or gold | Blocks salt and pollutant attack on the aluminium |
| Hydrophilic top coat | Water-wettable polymer with hydrophilic additives | Keeps the water contact angle low over the coil's life |
Total dry film is very thin, typically about 1 – 3 µm per side, equivalent to a coating weight of roughly 1.5 – 4 g/m². That is thin enough not to add measurable thermal resistance, yet it must survive punching, collar drawing and tube expansion without cracking. Some products are coated on both sides; others put the hydrophilic layer on one side and a corrosion layer on the other.
Base alloys and tempers
| Alloy | Typical tempers | Characteristics | Typical use |
|---|---|---|---|
| 8011 | O, H22, H24, H26 | The most widely used fin alloy; balanced strength and collar formability | Room air-conditioner evaporators and condensers |
| 3102 | H22, H24, H26 | Al-Mn alloy with good corrosion resistance and stable strength | Fins for split units and heat-pump coils |
| 1100 / 1200 | O, H22, H24 | Commercially pure; very formable, high thermal conductivity | Fins with deep drawn collars, radiators |
| 1030B / 1050 | H22, H24, H26 | High-purity aluminium fin grades with good conductivity | Air-conditioner fins where conductivity is prioritised |
| 3003 | H22, H24 | Higher strength than pure grades | Thinner fins that must keep their shape |
| 8006 | O, H22, H24 | Higher strength Al-Fe-Mn; allows thinner gauges | Lightweight fins, larger commercial coils |
Temper is chosen around the fin press tooling. Draw-type fin dies stretch the collar out of the flat foil in several stages and need soft, high-elongation material, so O or H22 is common. Drawless (ironing) dies form the collar in fewer steps and prefer the slightly stiffer H24 or H26, which also helps thin fins resist handling damage. If the temper is wrong, collars crack or the fin pitch varies, and tube-to-fin contact after expansion suffers.
Tensile strength, elongation and Erichsen cupping value are agreed in the technical specification for each alloy, temper and gauge and are reported on the mill certificate. Ask for the values that suit your die rather than relying on a generic table.
Coating types and colours
Blue hydrophilic
Hydrophilic top coat over a tinted corrosion-resistant base. The most common choice for residential and light commercial air conditioners; the colour also makes coating coverage easy to check.
Gold hydrophilic / anti-corrosion
Heavier corrosion-resistant base layer for coastal, humid or polluted sites and for outdoor coils. Often specified for heat pumps and condensers.
Self-lubricating hydrophilic
Contains a lubricating component so fins can be stamped with little or no volatile stamping oil, reducing cleaning and odour problems.
High-corrosion-resistant grade
Thicker coating system tested to longer salt-spray exposure, for marine and industrial atmospheres.
Hydrophobic (for contrast)
Water-repellent coatings are used on some outdoor coils to shed frost and water; they are a different product and should not be confused with hydrophilic foil.
Coating performance tests to specify
Coating suppliers publish their own figures, so the useful thing for a buyer is to fix the test method and pass level in the purchase specification. The table lists the tests most often written into fin-foil agreements, with typical acceptance levels used in the trade.
| Property | How it is tested | Typical requirement |
|---|---|---|
| Initial hydrophilicity | Water contact angle measured on the fresh coating | ≤ 10° (premium grades ≤ 5°) |
| Persistent hydrophilicity | Contact angle after repeated wet/dry cycles in running water | ≤ 25° |
| Coating weight | Gravimetric strip test | About 1.5 – 4 g/m², agreed per product |
| Adhesion | Erichsen cupping to a set depth; cross-cut test (ISO 2409); T-bend | No flaking, peeling or delamination |
| Corrosion resistance | Neutral salt spray (ASTM B117 / ISO 9227) for an agreed number of hours | Rating per agreement; anti-corrosion grades are tested for longer exposures |
| Alkali and solvent resistance | Immersion in alkaline solution or solvent | No blistering, minimal weight loss |
| Heat resistance | Short exposure around 200 °C, simulating drying and brazing-area heat | No change in colour or adhesion |
| Stamping oil resistance | Immersion in volatile fin-stamping oil | No blistering; hydrophilicity retained after degreasing |
| Odour | Sniff test at room temperature and after warming | No abnormal odour |
| Restricted substances | Laboratory analysis | Compliance with EU REACH and RoHS where required |
Sizes, cores, tolerances and surface quality
Typical supply
- Thickness
- Usually 0.08 – 0.20 mm for air-conditioner fins; heavier gauges to about 0.3 mm for industrial coils
- Width
- Slit to the fin press, typically 60 – 1400 mm
- Core
- Steel, aluminium or paper cores, 76.2 mm or 152.4 mm inner diameter
- Coil OD
- About 350 – 1100 mm depending on core and press decoiler
- Thickness tolerance
- Commonly ±3 % of nominal; tighter by agreement
- Width tolerance
- Commonly ±1.0 mm, tighter for narrow slit coils
- Winding
- Tight, even winding; limits on telescoping and edge offset agreed with the buyer
- Joins
- Limited in number and clearly marked on the coil
Surface checks cover even colour, absence of uncoated spots, scratches, wrinkles, streaks and edge burrs. A burr or a crushed edge is a common cause of fin cracks at high stroke rates, so edge quality from slitting is worth inspecting at goods-in. Coils should be sealed in film with desiccant, packed in cases and stored dry; coated foil is usually expected to be used within about 12 months of production.
Common mistakes when buying hydrophilic foil
- Specifying colour instead of performance: blue or gold describes the base coat tint, not the corrosion rating. Write the salt-spray hours and contact-angle limits into the order.
- Ignoring the fin die: a temper that runs well on a draw-type die may crack on a drawless die, and vice versa. Send the die type and collar height.
- Testing only fresh foil: initial contact angle is easy to meet; persistent hydrophilicity after wet/dry cycles is what matters in service.
- Overlooking stamping oil: some coatings lose hydrophilicity after contact with certain fin oils. Test with the oil you use, or choose a self-lubricating grade.
- Using standard coating at the coast: outdoor and coastal coils need an anti-corrosion grade or a different coil design.
References and standards
- ASTM B117 – Standard Practice for Operating Salt Spray (Fog) Apparatus
- ISO 9227 – Corrosion tests in artificial atmospheres: salt spray tests
- ISO 2409 – Paints and varnishes: cross-cut test
- EN 546-2 – Aluminium and aluminium alloys: foil, mechanical properties
- JIS H4160 – Aluminium and aluminium alloy foils
Hydrophilic aluminium (aluminum) fin foil: FAQ
What does hydrophilic mean for aluminium fin foil?
It means the surface attracts water. Condensate spreads as a thin film with a contact angle below about 10°, instead of forming beads that bridge the gaps between fins.
Is blue hydrophilic foil better than gold?
Not inherently. The colour comes from the base coat. Gold is often used for higher corrosion-resistant systems and blue for standard evaporators, but compare the tested salt-spray hours and contact angles, not the colour.
Which temper should I order for fin stamping?
Draw-type fin dies generally need soft O or H22 foil; drawless dies usually run H24 or H26. Your die maker's recommendation and a trial coil are the safest guide.
Does the coating reduce heat transfer?
The dry film is only about 1 – 3 µm thick, so its thermal resistance is negligible. In wet operation the coating usually improves real capacity because airflow is not blocked by water bridges.
Can hydrophilic foil be used on outdoor condensers?
Yes, but choose an anti-corrosion grade, particularly near the coast or in polluted air, and confirm the salt-spray rating suits the site.
How should coated fin foil be stored?
Keep coils sealed in their film with desiccant, in a dry store away from condensation and temperature swings, and use them within the shelf life stated by the supplier, typically about a year.
Need hydrophilic fin foil?
Tell us the alloy, temper, gauge, width, coating type and test levels you require. We will confirm availability, documents and a delivered price.
