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Supplement, November 2000

Pyrolytic low-E or Sputtered low-E?

Different Processes = Different Benefits

by Paul Gore

 Pilkington Solar E glass is one example of how some North   American pyrolytic glass manufacturers are bridging the high-performance gap with sputter-coated low-E glass products.

All of us in the industry know that the most commonly used glass for residential windows and doors is clear float glass. Likewise, the manufacturing process for clear glass is fairly well-known: take a batch of silica combined with soda lime and other elements, melt at very high temperature, float the molten glass material onto a bath of molten tin and then cool the glass material in a controlled environment. The result is clear glass that is easy to process, easy to cut, easy to temper and easy to insulate, all while keeping the rain and snow out.

From one float glass manufacturer to another, clear glass is clear glass: the glass characteristics are basically the same. Between manufacturers of clear float glass, the differentiating factors become customer service/satisfaction, product supply/availability and overall cost effectiveness. What clear glass does not offer is energy efficiency and that's where low-E coatings go to work.

Energy codes and increased customer awareness are driving demand for energy-efficient products such as low-E glass. Equally important is the market demand for cost-effective high quality low-E products that meet today's quick turnaround supply and service requirements.

There are two distinct production processes used for the application of low-E coatings to glass: pyrolytic (often referred to as hard-coated); and sputtered (generally referred to as soft-coated). Both types of glass offer its own advantages and disadvantages. When considering which low-E is right for your operation, you should consider the performance and handling factors that best meet your product and manufacturing needs.

 Sputter Glass

Sputter-coated low-E glass is produced by depositing a thin metallic coating onto the surface of glass in a vacuum chamber. This is referred to as an off-line process since the coating of the glass is done independently of the float glass manufacturing process. After the float glass is produced, the glass (either clear or tinted) is processed through the vacuum chamber. As the coating is deposited onto the surface of the glass (not fused into it), the product is often referred to as a soft-coated, low-E glass. Different chemicals can be used in the sputtering process resulting in varying performance values. Early sputtered low-E products were comprised of a single-stack chemical composition. These products had an emissivity of around .10 resulting in improved U-value for an insulating glass unit (and even the total window).

Sputter coated glass is made by depositing a thin metallic coating onto one surface of the glass in a vacuum chamber.

Later, sputtered low-E glass products were introduced with a double-stacked chemical composition. These products have an emissivity as low as .04. Likewise, these products have lower solar heat gain coefficients. Additionally, the performance attributes of these products include a good U-value with reduced solar heat gain coefficient for an insulating glass unit (and the total window).

Beyond the performance of the glass, there are other key factors to consider when evaluating the use of sputtered low-E glass products. These factors would impact the overall cost effectiveness of processing this product.

As stated previously, the chemicals used in the sputtering process are deposited onto the surface of glass. This low-E surface coating could be susceptible to degradation due to either oxidation in the environment or to scratches during normal glass handling and fabrication. Thus, special care is necessary to ensure the product is used within the specified shelf life and that the product is handled properly during fabrication.

To ensure adequate adhesion between the glass and sealants used in the manufacture of insulating glass units, most manufacturers recommend that the coating on the edge be deleted. (There are exceptions to this with the various manufacturers.) Overall, edge deletion adds costs to the manufacturing facility for equipment and processing time.

More recently, manufacturers of sputtered low-E glass have introduced temperable sputtered products. These cannot be tempered in a normal tempering cycle time comparable to standard clear float glass so special care or special tempering heating elements are needed for processing these products.

Further, the temperable sputtered low-E products utilize an additional chemical layer as compared to the annealed sputtered low-E products. The additional chemical layer burns off during tempering which may create an appearance difference between annealed and tempered sputtered low-E on the same project.

All of these factors affect the overall cost and subsequently the cost effectiveness of processing a sputtered low-E product. Thus, the special care that is required for handling the product may increase the lead time for the glass and impact delivery schedules to customers of the completed window order.

The above factors deserve strong consideration relative to your respective window operation, market focus and customer base.

Pyrolytic Glass

Pyrolytic glass, also known as hard coated or online coated glass, is made through a one-step process and is produced at the same time as the base glass. In the process, the chemical coating that gives the finished glass its properties is bonded to the glass while it is in a semi-molten state. Therefore, the chemical composition becomes part of the glass surface, rather than a layer on top of the surface, making the glass more durable.

Advances in pyrolytic low-E technology have given these products status as high-performance coatings. Pyrolytic low-E products have an emissivity as low as .15. As a result, these products typically have high solar heat gain coefficients. Therefore, the products provide optimal performance for heating dominated markets where passive solar heat gain is a benefit to the energy bill.

More recently, two major glass manufacturers have introduced pyrolytic low-E glass products with substantially reduced solar heat gain coefficients. These solar low-E products offer high performance for cooling dominated markets or even special applications such as sunrooms and skylights where reduced solar gain is desired. The main strength of pyrolytic glass is in its durability and long life in inventory (shelf life). Because the coating becomes part of the glass, a pyrolytic glass is less susceptible to degradation and scratches than sputter-coated glass products and does not require any special handling, care or equipment. These factors typically improve the final throughput costs of a window made with pyrolytic glass.

In addition, pyrolytic low-E products can be tempered similarly to standard float glass products, resulting in improved throughput and cost effectiveness. Likewise, there is no differentiation between annealed or tempered pyrolytic low-E products. The result is a consistent appearance between an annealed and tempered product on the same project.

Pyrolytic low-E can be handled and processed like standard float glass resulting in excellent lead time, customer service and overall cost effectiveness with use of the product.In the pyrolytic process, the chemical coating that gives the finished glass its properties is bonded to the glass while it is in a semi-molten state. Therefore, the chemical composition becomes part of the glass surface, rather than a layer on top of the surface.

 Which is Best?

Which type of glass you purchase depends on a number of factors, including where your customers are located, the size or type of operation, operating costs, how quickly you move your glass inventory, etc. While recent technological advances in sputter-coated glass and pyrolytic glass have narrowed the differences, the differences still exist.

Based on performance in respective markets, both sputter-coated glass and pyrolytic can be viewed as high-performance glass products. Advances in pyrolytic technology have bridged the performance gap. New products like Pilkington Solar E™ solar control low-E glass are bridging the gap in solar heat gain.

 Impact on the Window Manufacturer

Handling-wise, can you afford to take the time and extra costs to use sputter-coated glass? Does your glass inventory move, or do you like to have a good supply on hand (and know that its performance or appearance won't degrade over time)? Does a glass that requires less handling attention allow you to move windows more quickly?

A recent industry market survey of window manufacturers regarding window costs stated that "few (window fabricators) know their actual manufacturing costs." This survey suggests that understanding not only the product cost but also the overall cost effectiveness (throughput, scrap and quality defects/returns) is vitally important to the purchase decision.

In any purchase process, you need to consider the overall value and cost, rather than just one or two factors. In the end, regardless of which type of glass you purchase, you must choose the one that ultimately satisfies your customer requirements for lead time, service, quality and ultimately, cost effectiveness and value.

 Paul Gore serves as residential products manager for Pilkington North America located in Toledo, Ohio.


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