How To Choose Prepainted Aluminum Coil by Surface Coating

In the fields of construction, packaging, home appliances, etc., the quality of colour coated aluminium coil (colour aluminium) directly affects the product's lifespan and performance. In the face of all sorts of products on the market, how to quickly determine its advantages and disadvantages? This article through the coating thickness test, salt spray resistance test, adhesion assessment of three strokes, to help you accurately lock high-quality colour aluminium.  

First: coating thickness - the quality of the ‘lifeline

1. Why thickness is the key?

- Protective performance: coating thickness determines corrosion resistance and weathering ability. For example, a fluorocarbon coating for building use needs to be ≥25μm to withstand 20 years of UV aging.  

- Cost control: too thin a coating is easy to peel off, increasing maintenance costs; too thick is a waste of material, pushing up prices.  

2. How to test?  

- Non-destructive testing method: Use electromagnetic thickness gauges (e.g. German ElektroPhysik) with an accuracy of ±1μm to quickly scan the surface of aluminium coils.  - Destructive testing: After cutting the sample, use thinner to wipe off the surface paint, and subtract before and after to get the coating thickness.

3. Industry standard references

Difference between PE and PVDF

1.Weather resistance

PVDF:

UV absorption rate <1%, can resist outdoor aging for more than 20 years.

Salt spray test ≥ 1000 hours (ASTM B117 standard).

PE:

UV sensitive, yellowing and chalking after 5 years.

Salt spray resistance test is usually 300-500 hours, quality products can reach 800 hours. 2.

2. Chemical resistance

PVDF:

Resistant to acid (such as sulfuric acid), alkali (such as sodium hydroxide) corrosion, pH tolerance range 1-13.

Strong solvent resistance, insoluble in organic solvents such as acetone, toluene.

PE:

Resistant to weak acid and alkali, but easily hydrolysed by strong acid (such as hydrochloric acid) or strong alkali.

Easily dissolved by xylene, methyl ethyl ketone and other solvents.

3. Mechanical Properties

Adhesion:

PVDF: Scratch test usually 0-1 (ASTM D3359), need to be pre-treated by chrome/chrome-free passivation.

PE: Adhesion up to 0-2, but easy to peel off under high temperature and humidity.

Flexibility:

PVDF: elongation ≥ 30%, suitable for bending (e.g. aluminium veneer folding).

PE: elongation of about 15-20%, bending angle is too large and easy to crack.

Second Move: Salt Spray Resistance Test - Environment Adaptability The "Litmus test "

1. Test Principle

Simulate the coastal high humidity and high salt environment, put the colour aluminium samples in the 5% sodium chloride solution spray box, and assess the corrosion resistance by the time of rusting or blistering.  

2. Key Indicators

- Fluorocarbon Coating: ≥1000 hours without blistering or flaking is required (as per AAMA 605.2 standard).  

- Polyester coating: ≥500 hours is considered satisfactory, quality products can reach 800 hours or more.  

Third, the third move: adhesion test - coating and substrate ‘adhesion’

1. test method

- grid method (ASTM D3359): with a razor blade on the surface of the coating 100 1mm × 1mm square grid, paste the tape after peeling, observe the coating Observe the ratio of coating peeling off.  

- Classification: Class 0 is the best (no peeling), Class 5 is the worst (>65% peeling).  

2. Influencing factors

- Pre-treatment process: whether degreasing, chroming/chromium-free passivation is thorough or not, it directly affects the adhesion.  

- Curing temperature: Failure to meet the process requirements (e.g. fluorocarbon coatings need to be baked at 230°C or above) will lead to a decrease in the adhesion of the coating.  

Frequently Asked Questions

Q1: Can coloured aluminium be welded?

A: You need to remove the coating of the welding area first, otherwise the high temperature will release toxic gases (such as fluoride).

Q2: How to clean the surface of coloured aluminium?

A: Use neutral cleaner (e.g. Mr. Mighty) + soft cloth, avoid acid (e.g. vinegar) or abrasive to scratch the coating.

Q3: Why the big price difference?

A: High-end fluorocarbon coating (such as Japanese KCC brand) costs 3-5 times more than ordinary polyester coating, but the life span is 2-3 times longer.