Volume 37, Issue 11, November 2002

Way More than Glass

Coating Technologies Provide Wealth of Opportunities to Industry 
by Ed Bator

Just for the record: our industry is not just about glass. Is there anyone in this business unaware of that?

Contrary to public opinion, glass isn’t blown or rolled or dropped vertically. In fact, it is more than likely float glass. Glass is produced at a major scale on an average furnace capable of producing 600 tons per day. It’s flatter, thicker, thinner and more uniform than ever. It’s also covered, painted, filmed, sandwiched, reheated, insulated, silk-screened and coated.

Coated Glass
Coated glass is not to be confused with painted glass. The glass industry had to evolve to maintain market share in residential, commercial and architectural structures, or it would have faced little growth. Glass may win the battle of some aesthetics, and may win battles in convenience. However, the war may be lost to energy costs. The fight to conserve and reduce costs, plus oil and gas depletion scares, can cause wars, can’t it?

How do the following words strike you: protection, high-performance, comfort, insulated, guard, ban, solar-cooled, solar-screened, energy-saver, reflective, efficient and, most of all, performance? All are characteristics of glass. 

That P word (performance) has been the mainstay and the backbone of the glass industry. Find a catalogue or any reputable product or service with glass, and you will find that P word—which, incidentally, is what keeps you in business.

In the last 25 years, coatings have provided the best opportunity for the glass industry. Coatings are the main consideration of glass manufacturers and fabricators, and it is coatings that are driving their business. How long that will last is hard to tell, but one thing is for sure—the coating phenomenon isn’t standing still.

Coatings Technology
Glass coatings involve the bombardment of materials (most likely metals) using some sort of electron-beam plasma and evaporation technique. The two common applications today are vacuum-sputter deposition and pyrolytic. Vacuum sputter has been around since the early 1970s. Pyrolytic evolved from the same concepts, but it is distinguished by the environment of where it is applied. 

The pyrolytic process enables the glass manufacturer to apply thin films in either the tin bath or the lehr, depending on the desired effect. Differences exist between the two today mainly due to the environment or state of the glass when a coating is applied. Vacuum-sputtering is the bombardment to glass in a clean chamber with molecule density farther apart, creating the path of least resistance. High-impact ion bombardment of the metal makes it part of the glass surface. The metal then gets stacked on top of that surface. Pyrolytic advances control the environment to an extent. Use of the sputtering process during float manufacturing means the metal can be deposited while the glass itself is not in a fully solid state, thus imbedding the metal deeper into the surface—hence the name “hard coat.”

Let’s get deeper into that performance word now, particularly in how it relates to glass. In the 1970s, coated glass gave the industry color. It reflected sunlight, lowered transmissions and created performance in heat reduction. Controlling ultraviolet rays, shading coefficients and U-value were selling points beyond aesthetics. Everyone in the glass industry who could get their hands on a couple million dollars bought a machine and one- or two-layer metal coatings became the trend. 

Like a teenager with a hot rod, glass-business owners and their new machine dreamed of the possibilities. They could customize and then patent ideas using certain metals in certain gases and in certain sequences, by certain and precise angstroms. Unfortunately, big money and big business were at stake and protective owners of what was considered by others as “borrowing ideas” came to light. Legal issues concerning coatings appeared occasionally and were more prevalent in the 1980s. By then the owners knew as much (if not more) than the machine developer. The main three gases, argon, nitrogen and oxygen, remained the same for awhile. But the metals used to sputter grew to include pure metals of zinc (Zn) and tin (Sn). These metals were augmented occasionally by another substance to create compound metal sputtering. Then using a combination of two gases came into play. Pressure rates were varied and tested, resulting in the colors still seen today.

The inherent properties of the metal sputtered (whether pure or compound) along with the gas used to invoke plasma (whether pure or dispersed by ratio) created performance in glass. Soon, it wasn’t enough to just sell color with whatever heat gain or loss you ended up having; the glass must motivate the buyer for better energy savings. 

Low-E Coatings
From fabricator to fabricator, glass with various shading coefficients and U-values were now in direct competition. This fusion of ceramic engineering and physics resulted in the products we know as low-emissivity (low-E). 

Low-E attacked the main cause of energy use in large buildings as well as homes: the heat generated by the sun and the loss of the environment inside structures. Sunlight is roughly 2 percent ultraviolet rays, 47 percent visible light and 51 percent infrared. Heat travels by convection, radiation or conduction. Using the inherent properties of silver (AG) and trapping a fine layer so not to oxidize was the key to successful performance in the glass. This resulted in a faint blue coating marketed by many. As time progressed, the demand grew for transparent, coated low-E glass that could outperform glass that had to be dark to perform.

Experiments with post-temperable coatings have been conducted in the past 15 years and now gain more sales than a blimp on the radar. Early development of vacuum-coated products consumed money, precious research and development time. At the time, oxidizing or burning off the delicate angstrom layers of the coating in the surfaces beget uneven and inconsistent surfaces. Newer technology and metals were employed. Borrowing low-E and anti-reflective sputtering technology for use on metals like silicon, chromium, niobium, aluminum and compounds created glass with these characteristics. Gases suspected to burn off in temperatures exceeding 300 degrees had to be compensated. The technology and product sold today is far superior to the glass that was on the market five years ago. However, due to the details required for the product at the manufacturer to the consumer, the yields are varied and now widely respected. 

New Technologies
Solar-reflective glass is now of age and dependable. Now, high absorption and reflection can be attained thus reducing solar heat gain. Fresh colors have resulted to the delight of many architectural customers, and some of these coatings offer a unique and distinctive appearance when comparing glass reflectance to film-side reflectance. What appears reflective outside the building appears shaded when looking out from the inside. This also offers the alternative in cost to the insulating unit customer. 

In the past, to obtain performance and color customers bought and paid for two lites of glass: one with reflective on the number-two surface and low-E on the second lite number-three surface. 

Now, and even more so in the next decade, research will bring the technology of hydrophobic and hydrophilic concepts to glass. This includes glass that can be labeled as self-cleaning, low-maintenance or resistant to hard water. Through deposition properties, water can be resistant to adhesion or can be absorbed or reacted chemically. Photocatalytic and the use of titanium oxide has grabbed the attention of manufacturers and fabricators. The performance of the sputtering of silicon and aluminum alloys is of great interest, as well as the use of ion-beam sputtering techniques. Anti-glare, wavelength division glass is also of great interest. Coatings supplemented with decorative glass designs combining silkscreen or painted frit will continue to advance at a rapid pace. 

In summary, get accustomed to words like hydrophilic, wavelength division, ion-beam sources, optical designs and tribological coatings. Glass will be more wear-resistant, will react to outside elements and may even change colors on its own. One thing is for sure, though—glass will perform. 

Ed Bator is the head of coating operations for Viracon Inc. in Statesboro, Ga.

USG