# Unknown surface coating

Forgive me if this isn't the best forum for this question, and guide me elsewhere if so. I have two identical smartphones, both made by Sony (Xperia z2 to be exact). One I purchased new and one second hand. I want to affix an adhesive-backed mounting system to the back of both of them. With one phone (that I had from new) there's no trouble, everything (e.g. adhesive-backed Velcro) sticks really well. But with the other phone nothing will stick . I've checked that there's no protective overlay sheet and can only assume it's been treated with some sort of chemical coating. Every solvent I've tried to remove this coating just forms a meniscus and runs straight off. Water, detergent, isopropyl, acetone, WD-40 - nothing makes any difference. Any suggestions?

• The adhesive I want to use is on a Quadlock (quadlockcase.co.uk/products/universal-adapter) which is a 'VHB' adhesive. 'Will not adhere to rubber, silicon or TPU'. Just tried the Velcro adhesive on the front of the same phone and it sticks pretty well there. Weird. – quilkin Jul 6 '17 at 19:56

Since mobile phone tend to be "smart", filled to the rim with computing power and data, users wish to protect their companions with a coating that repells water. Often, the devices are not only coated on the front side, which would at least hamper accumulation of residues of gliding fingers, but all around, including the back. As a result, you might get a cell phone that may drop into a bowl of water, and still remain functional.

Among the different technologies available (a short presentation by CNN, back 2013 is found here), there are polymer coatings, or a mixture of anastase / $\ce{TiO2}$ and PFTE / teflon (like the coating of your frying pan) particles with some perfluorinated liquid (example). For some, you need a larger installation that appears like a scilab, for others, you may prepare it at home with a spray can (example). Hence the results in terms of performance, maintenance, and longevity may vary, too. As a result, if applied both on front and back of the phone, not only water, but the components of "normal glue" (referring to what backs a velcro tape) may be repelled more or less efficiently, too.

Your observation that the coating appears little to non-affected both by water as well as iso-propanol (which for some coatings is a no-go, as it would tear the layer off), as well as for WD 40 (which may be summerised as a blend of apolar hydrocarbons (ref)) allows the speculation that the coating might be either polymer-based, or the triple mix of anastase + teflon + perfluorinated binder. To discern between the two, remotely, is not possible.

If a mechanical, pocket-shaped mount like

(source)

or a pouch, to be bound around your upper arm while riding a bicycle, like

(source)

is not what you may deploy (or alter a little to fit your needs), and if the coating consists of the "polymer-only" variant, then maybe it is possible to establish a connection between the back plate of the phone, and either the velcro tape or something already connected to the velcro tape with the help of cyan-acrylate based gorrilla super glue. These glues often bind very strong and to polymers, however partially dissole the polymer a little. This chemical approach however comes with many "ifs" and uncertain outcome.

• As mentioned in my comment above, I want to use a 'VHB' adhesive from 3M. I don't actually need to know what the coating is made of, but any suggestions for other solvents I haven't tried (and can get hold of reasonably easily?) – quilkin Jul 6 '17 at 20:02
• Ok, you used water, detergent, polar organic (acetone, isopropanol), apolar organic (WD40) which spans quite some range. Ethanol probably does not offer an effect either. Acetic acid is sometimes a surprising good solvent. But if this all has no tangible effect, than the coating is too tight to be removed. Alternatively, exchange of the backside of the housing, if available in a repair shop or by Sony, and by design is foreseen still is the white elephant in the room. – Buttonwood Jul 6 '17 at 21:00

Well, the worst coating would be a perfluorinated one. You're not likely to get much to stick to that. Silicones are more common (in general) and much more likely to be used. MEK (methylethylketone, aka 2-propanone) works pretty well at swelling silicones, if you allow soaking. Acetone would be similar if you could allow that adequate contact time. The problem is that you should soak the surface for an hour (or more). So how do you slow the solvent's evaporation? In this type of application, using a powder, like flour (finer the better) to slow down evaporation...you may also need to cover it with some sort of film or if that's not feasible keep on replenishing the solvent. I'd definitely soak it for 20 minutes and then try rubbing the area you want cleaned with a solvent moistened rag. Methylene chloride is another good solvent for attacking silicone, but afaik it's no longer available. I don't know what kind of paint strippers you can purchase (it varies by local environmental regulations), but these are also a possibility. Note that it is possible that these could damage the finish and you definitely don't want any of these nasties to leak into your phone or onto the face! An alternate to using flour (or even tissue paper) to "hold" the solvent is to construct a dam around the area (like a circle of toothpaste or cake frosting or sealant - if you've tried a sealant and it doesn't stick very well, then perhaps you could make a dam of than and when done just pull it off. Basically, the dam would be acting as the sides of a pool the solvent goes into. Toluene is another not too bad solvent. Proper safety procedures always when working with chemicals. WD-40 is an awful idea, unless you think an adhesive will stick better to a slippery surface...