ONLINE EXTRA: Glass Surface Treatments

October 1, 2009
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Hydrophobic vs. hydrophilic: What does “self-cleaning” glass actually mean?



Figure 1a. Contact angles for untreated glass surface.

One of the core properties of any hydrophobic (water-repellant) coating is precisely how hydrophobic the coating makes the surface. The more hydrophobic the surface becomes, the easier it will be to clean. Think how much easier it is to clean a cooking pan coated with Teflon® vs. an uncoated pan.

The measurement that defines how water-repellent a surface is after being treated with a hydrophobic coating is called the “contact angle” or “wetting angle.” This angle is formed by the material’s surface and the tangent of the interface between the fluid (i.e., water) and the environment at the liquid/solid (i.e., glass surface) point of contact. Figure 1 shows typical contact angles on untreated and treated glass surfaces.

Figure 1b. Contact angles for surface treated with a hydrophobic nanocoating.

The important thing to keep in mind is not necessarily the initial contact angle measurement after the glass surface is treated, but how it performs over time. In other words, how will the contact angle behave with normal wear and tear once the surface is exposed to different environments? Most hydrophobic easy-to-clean coatings have an initial contact angle anywhere from as low as 80 to 120°.

On the opposite end of the spectrum are hydrophilic (water-attracting) coatings, which are used in the manufacture of “self-cleaning” glass. A coating with photocatalytic properties is fused into the glass at high temperatures during the manufacturing process. This type of coating typically reacts with the sun’s ultraviolet light and water (rain or induced water spray), creating a sheeting action so water can easily rinse the loosened dirt away. Thus, these two elements (sunlight and water) must typically be present for the coating to be effective.

Pros and Cons

Because of its integral characteristics, the self-cleaning concept (hydrophilic) has many limitations, including the fact that the glass must receive both sunlight and water. In addition, the process is not applicable for all kinds of glass, cannot be applied in the field (making repair or replacement a lot more expensive or impossible) and is typically more expensive than hydrophobic coatings.

By contrast, hydrophobic coatings require no “activation” since they mostly chemically react during the coating process and immediately bond to the surface. Although other theories refer to self-cleaning glass as those coatings with contact angles in excess of 145º, where the mechanical action required is virtually unnecessary and no accumulation of contaminants occurs, the misconception of self-cleaning is contrary to the reality and expectations of an end-user.

For any glass to truly be called self-cleaning, it would have to have a contact angle of 180° in order to create a “rolling effect” so that no action would be required for the water to automatically roll off the treated glass and leave the surface entirely dry. This ultra-hydrophobic concept is virtually the only way that one may legitimately claim that the glass is truly self-cleaning.

Figure 2. Untreated (left) surface vs. surface treated with a hydrophobic nanocoating (right).

A New Nanocoating Solution

Nanocoatings have a size of only a few nanometers. (The prefix nano means 10-9, or one billionth of a meter.) However, what’s relevant is not the thickness of the coating itself, but what the coating actually is and what it provides as a value-added product. Nanotechnology in itself is not necessarily synonymous with quality. It does, however, imply a degree of inventiveness and it allows manufacturers to manipulate atoms at much smaller levels to achieve specific coating characteristics.

For example, the chemical treatment of one patented nanocoating* provides, in chemical terms, a durable “branched, cross-linked and capped” optically clear nano-film (see Figure 2). A permanent bond is integrated into the glass itself, as the nanofilm actually grows from the inside out and changes the molecular composition of the surface.

This technology uses a two-stage chemical process. The chemical reaction created in the first stage causes the cross-linked and branched ultra-thin silicone film (nanofilm) to be grown from below the surface out. The second stage of the process caps the entire chain of atoms, which substantially increases the hydrophobicity and durability, leaving (chemically speaking) no points of attachment for contaminants and creating a truly repellant charge. Through simple neutralization, all chemicals become inert within a few seconds. No curing time is needed since the chemical reaction itself occurs in less than two seconds.

Many coatings offer the characteristics of being water-repellent (hydrophobic) and slightly oil-repellent (oleophobic) as their main properties. This patented nanocoating, on the contrary, provides multi-functional characteristics that include water and oil repellency (hydrophobic and oleophobic); impact and scratch resistance; protection against graffiti, dirt and stains, fingerprints, and calcium and sodium deposits; UV stability; additional electrical insulation; and increased brilliance and lubricity. The nanocoating works at nanoscale levels to change the molecular composition of any silica-based surface.

The key to effective value-added glass surface treatments is durability and how the coating-nano or not-will behave with the passage of time through normal wear and tear.

* Diamon-Fusion® nanocoating, developed and patented by Diamon-Fusion International, Inc., San Clemente, Calif.

For more information, contact Diamon-Fusion International, Inc., 1046 Calle Recodo, Suite F, San Clemente, CA 92673; (800) 213-0793; fax (949) 388-3299; or visit www.diamonfusion.com.

*Teflon® is a registered trademark of E.I. DuPont de Nemours and Co.

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