Scrap Protection Upgrade - Rust Spot Can, Lower End Solutions Designed for Waterborne Coatings

A couple years ago waterborne coatings started to gain a bigger market share. Increasingly stringent gov regulation and client demands are causing more and more of the paint and coatings industry to switch from solventborne coatings to waterborne. This is all good! However waterborne requires a higher package rust protection than oilborne paints do.

We are a Chinese factory located at Foshan, Guangdong with a 30 year history specializing in Tin Can Components for the paint and coatings industry, primarily bottom ends. Most common complaint we get in the last few years is: "Since we switched to waterborne paints we've started to see more rust on our bottom ends." In this post I want to address this issue in depth and let you know how we are responding.

Why do waterborne paints promote more rust?

Many people mistakenly think that since waterborne paints use water rather than nic aggressive solvents, they are kinder to metal. In fact, they are the reverse.

Oilborne paints contain organic solvents that are corrosive but vaporize quickly. The solvent vapor left in the can forms a protective atmosphere and limits direct contact of oxygen, moisture, and metal. Waterborne paints contain a lot of water. Moisture vapourizes slowly and stays in the can a long time. Waterborne paints tend to be mildly alkaline (PH 8 to 9).In the environment, if that chloride coating is too thin, or worse - defective - moisture and oxygen pass through, activating electrochemical corrosion, resulting in the familiar red rust.

Another factor, often overlooked: Most water based paints contain various additives - freeze-thaw stabilizers, coalescing agents, wetting agents, among others. Some of these additives can pass through the sealing compound, altering the nature of the sealing, or even become disassociated from the curl edge by the sealing compound, exposing bare metal and commencing rusting. Our Answer: A Three Tier System of Protection Recognizing these factors in waterbased coatings we came up with a dedicated bottom end solution based on three tiers of protection.

Tier 1: Upgraded Substrate. Standard bottom ends utilize tinplate with a 2.8/2.8 g/m² tin coating. When subjected to water based paints under prolonged conditions of high humidity this coating is inadequate. We specify 5.6/5.6 g/m² in the standard pack - which effectively doubles the thickness of the tin layer, providing greater resistance to penetration by moisture and oxygen. Where the paint is particularly aggressive, or the composition high in pH, we can increase coating weight, or utilize Tin Free Steel (TFS) substrates.

Tier 2: Inner Coating Protection. An epoxy phenolic resin coating (or commonly referred to as gold lacquer) is given to the inner surface of the bottom end. With a high resistance to chemicals, the coating seals off the paint from contact with the metal. “We see sustained corrosion of that side as baseline startingpoint at 48 hours of salt spray test, we will see virtually no change after 120 hours on a bottom end with gold lacquer on that side - says Wolfe.For waterborne wood glazes and industrial finishes sensitive to metal ions this coating is particularly important, mitigating against migration of metal ions which can discolor the paint.

Layer Three: Sealing Compound Match. Waterborne paints cannot use the same sealing compound formulation as oils. We developed a hydrolysis resistant PU sealing compound for use in aqueous environments - it will not absorb water and swell or degrade over time. The compound has also been further tweaked for flexibility. Even if the bottom end itself is slightly deformed when the flanging is seamed the compound retains complete sealing capability. We also bumped up the volume of compound slightly compared to standard, ensuring the curl cavity is completely filled and there are no pathways for moisture ingress

Customer Case: Return Rate Dropped from 8% to 0.3%

A customer in eastern China who produces water soluble architectural latex paint saw return rates as high as 8% in the hot summer months due to rusting of the bottom end of the can. Upon opening returned cans many showed visible red rust spots on the inner surface of the bottom end. Some ended up rusted through completely. After taking the account over to manage we first looked closely into their paint formulation and filling process. Then we offered them samples of our three layer protection system. They field tested for three months, both ambient storages as well as testing utilizing 45 °C accelerated aging, and simulating ocean freight in humid environments.Results: zero rust incidents of any kind with all batches using the new bottom ends. Sealing performance also passed air tightness tests. The customer is now converting all their lines. Current return rate is now satisfactorily low, below 0.3%, and remaining issues have to do with transit damage and nothing to do with rust.

More Than Just Bottoms — A Total Solution

We can’t stress enough how resolving rust problems arising from water-based paints cannot be a function of bottom ends alone. The entire tin foil assembly is important. Width of the can body flange, seamer settings, temperature and humidity of the warehouse from which the cans are stored, all play a part in the rust resisting properties of the final assembly.

That is why when we service customers we don’t just sell bottom ends. We check the can body and seamer, and recommend adjustments. Sometimes the problem is the can body flange, too narrow or too wide, a poor seam, something rotten in Denmark (sorry, bad joke). Sometimes the problem is the seamer rollers, they are evenly worn and create uneven pressure. We find the problem and rectify it too.

Final Words

Water-based coatings are the future. Nobody disputes that. Every trend has its perils, however. Rust protection is one of the major challenges Waterbased Paint Packaging faces today.

As a Chinese 30 year old manufacturer of tin can components, we not only shoulder our responsibility to help customers solve this problem. We are skilled and able. Our dedicated bottom ends for water based paints are not magic. What they are is better, more complete, more three-dimensionally sound in composition — substrate, coating and sealing compound. That is rubber meeting the road and it is in these fine details where you lock out rust and ensure your paint, now and in the future, is protected, regardless of how long it sits or far it is travelling.

If you’re making water-based coatings or plan to start soon, email me for sample testing. And based upon your actual formulation and conditions of use, we’ll tell you what bottom end to try.