Material Science Behind Screen Protectors [Part 1]


Informative Rocket Scientist

What the heck is a tempered glass screen protector?

I still remember my very first smartphone – the HTC Droid Eris. Verizon still offered unlimited data for only $29.99 a month, the new hotness arrived in the form of Droid Incredible, and Eris was still an unbelievable piece of crap. It quickly developed into my main source of irritation, as it couldn’t make any calls unless you waited two whole minutes after pressing the call button and texting was downright impossible. So when I cracked the screen on it, I didn’t bother fixing it and used it as-is.

To this day, I have no idea what possessed me to keep this phone for nearly three years, let alone in this state.

So when I finally got my shiny Droid Bionic several months back, I vowed that it shall be treated with the utmost respect: By buying a screen protector. Not one of those really fancy ones, mind you,  but ones you buy 3 for only $5 off a shady street vendor, and protect your phones as much as your face protects your head. On an unrelated note, Motorola Bionic was notorious for dropping connections incessantly and a bunch of other rather serious bugs. Considering that I literally studied rocket science for almost five years in college, I really knew how to pick my phones.

With gifts from the cell phone gods showering upon us, whether it be Samsung Galaxy S3 or Apple iPhone 5, most people nowadays would not be happy with the purchase that I have made, and rightfully so. As a matter of a fact, newest smartphones that are out in the market at the time of this writing have more than thousand times the computing power of the Apollo Guidance Computer (which successfully navigated numerous astronauts through space onto the surface of the moon), so it doesn’t make much sense for anyone to merely put on a pretty cell phone case, slap on a random screen protector, and call it a day.

So tempered glass screen protectors emerged to provide your devices with much better scratch resistance and protection. However, although you’ll discover a lot of people chattering about how great this new product is, you’d be hard-pressed to find anyone that can actually explain the definition of the term “8H on hardness scale.”

Do these guys really know what they are talking about?

Before digging deeper into tempered glass screen protection and the silly rating scale people never pay any attention to, let us briefly examine what exactly tempered glass is. Created by throwing a perfectly normal piece of glass into a burning furnace of hotness in a heat comparable to depth of hell (~1,200°F / 650°C), which then is rapidly cooled down by drowning the living hell out of it, the end result comprises of an embodiment of “no pain, no gain” that we call tempered glass.

Or alternatively, for products such as tempered glass screen protector, the glass is chemically strengthened by a surface finishing process. Basically, a piece of glass is drowned into a bath containing a potassium salt at ~570°F / 300°C, which causes sodium ions in the glass surface to be replaced by potassium ions. If that made no sense to you, that’s okay; all you need to know that is the results from chemically treated glass has increased toughness compared with thermal toughening, and can be applied to glass objects of complex shapes.

Tempered Glass
I’ve seen things you people wouldn’t believe.”

So where does tempered glass that went through a “toughening process” end up? It becomes a part of your car’s windows and windshields, microwave ovens, and bulletproof glass. Yes, that’s right; despite the fact that bulletproof glass does not actually deflect bullets, contrary to what I as a six year old, the very same material that is used to protect smartphone screens is also protecting the Pope from stray bullets.

Bulletproof Glass
This doesn’t actually happen. (image courtesy of Saturday Morning Breakfast Cereal)

Now, the surface hardness numbers you see above for tempered glass screen protectors are based upon the pencil hardness scale, in which the resistance of a coating is determined as the grade of hardest pencil that does not mark the coating when pressed firmly against it at a 45 degree angle. Here’s the complete scale of pencil hardness on the European system that’s most widely used, courtesy of Wikipedia.

Pencil Hardness Scale

In a simpler term, one would take a pencil, make a line of several centimeters (typically less than an inch), observe if the pencil scratches the surface of the coating, then go down the pencil “grades” until a pencil no longer mars the coating. Surface hardness is defined by the hardest pencil grade that fails to scratch the surface, which is the reason why the procedure is called the “pencil hardness test,” even though it’d be more accurate to call it “surface scratch resistance scale” or “completely misleading title for a scale slash testing. In a very few case where even the 9H pencil unable to abrade the surface, 9H hardness rating designation will be given.

Can your fingernail scratch a knife blade? Nope.  Can the knife scratch your fingernail? (Disclaimer: do not try this at home. iloome and all related affiliates will not be held responsible.) Yes? Good. That means your knife is more resistant against scratches compared to your fingernail. Same principle applies when it comes to tempered glass – knife blades or keys cannot scratch it because the tempered glass is significantly higher on the pencil surface hardness scale.

Tempered Glass Armor
Pretend that I’m wrapped in tempered glass armor instead. Then I’ll be free from being scratched to death by a knife.

So how does this relate to screen protectors now? Well, the PET film screen protectors similar to what I personally bought and used for my Bionic have hardness rating of approximately 1H ~ 2H. More expensive PMMA screen protectors that’s fairly common in the gadget accessory industry? Up to 6H. Only tempered glass, on the other hand, generally stand around 8H ~ 9H on hardness scale.

The pencil surface hardness is a measurement of a material’s ability to resist abrasion (or scratching), NOT its ability to withstand external force. So next time you witness a bowling ball or a cement brick falling on your phone screen, the phone screen should be the last thing you should be worried about, although tempered glass actually does a better job of resisting external forces, such as falling bowling balls, and protects your cell phone screen compared to any other screen protection alternatives. [Editor’s note: a material’s ability to resist and withstand external forces will be cover in a future edition of Informative Rocket Scientist.]

Falling bowling ball
Despite popular belief, PMMA screen protectors cannot prevent a falling bowling ball from destroying your phone.

I am always interested in your comments, questions, and ideas in regards to anything, so please don’t hesitate to comment below, or give us a shoutout on iloome’s Facebook or Twitter @iloome. Thanks for reading and ‘til next time, adieu.