Moisture: A Demon for Glass Manufacturers
Glass interleaving defects can be broken into “uncontrollable” and “controllable” factors. The uncontrollable factors are outside the control of the glass manufacturer and include humidity, heat and the length of time the glass is interleaved.
Fortunately, glass manufacturers can still control the quality of their glass by controlling the characteristics of their interleaving paper. The paper’s thickness, pH and chemical content all work to inhibit the effects of heat, humidity and time. Additionally, it is important to exclude silicone derivatives from all pulp and paper manufacturing processes.
Moisture forms on glass anytime the ambient temperature falls below the dew point. Under normal circumstances, the moisture evaporates or is diluted to the point that chemical reactions cannot occur. In packaged glass however, the moisture is trapped and this ultimately leads to glass defects if the proper safeguards are not put in place.
Preventing Glass Corrosion
In his article “How to Prevent Glass Corrosion”, Paul Duffer of PPG Industries discusses the chemical reactions that occur which create staining and corrosion in soda-lime-silica glass compositions. He explains that Stage One Corrosion can take place in minutes if the glass comes in contact with water while packaged. Duffer describes the chemistry of the interaction in detail, but put simply, the water takes sodium ions from the silica glass thereby increasing the hydroxide ions and the alkalinity (pH) of the moisture on the glass’s surface.
Duffer goes on to explain “As long as the solution pH levels remain well below 9.0, Stage 1 corrosion proceeds as the predominant reaction at the glass surface. During this period, optical quality and surface integrity remain essentially unaffected…However, unrestrained Stage 1 corrosion can lead, as previously mentioned, to highly alkaline conditions.”
Stage Two Corrosion
Finally, Duffer makes it clear that a whole new reaction takes place when the pH of the silica glass reaches 9.0 or greater. Glass industry experts call this “Stage Two Corrosion”. It occurs when the hydroxide ions attack the actual silica network in the glass. The defects are displayed as “microscopic pitting of the surface” and evolve into a “widespread iridescence” or a “dense, translucent haze”. Duffer also mentions reactions that occur with the moisture from carbon dioxide in the atmosphere that also creates residues.
While the overall strength of the glass is not impacted from the pitting, fogging and hazing; the optical quality of the glass is destroyed to a point that only substantial grinding and polishing can remove the defects. In many cases, scrapping the defective glass is more economical than re-conditioning it.
Learn More by Downloading Our White Paper
The range of glass interleaving papers available is as varied as the level of knowledge regarding their use. With this in mind, our free white paper discusses the factors that allow a paper to effectively interleave glass:
- Uncontrollable and controllable factors in glass interleaving
- Ambient heat
- Added chemicals like silicone-based defoamer agents
Download our free white paper to learn more about the factors to consider when selecting glass interleaving paper for your application!