On-Line Glass Coating Process
On-Line CVD Technology for the Glass Industry
Glass products with coatings have significantly improved properties compared to glass products without coatings. Some of these improved properties, due to hard coatings, are better energy efficiencies, better durability, easier handling, market differentiation, and enhanced profit margins. The leading coating technology is on-line pyrolytic Chemical Vapor Deposition (CVD) and has been used, within the glass industry, for many years. The advances to this technology from the earliest versions to the recently launched systems are astonishing. Until recently, On-Line Pyrolytic CVD Technology was effectively available only to large international glass companies who had developed the technology in-house. Stewart Engineers in collaboration with its industrial partners developed the first commercially available turnkey On-Line Pyrolytic CVD System, the AcuraCoat® System.
Coating Technology Selection
Float glass manufacturers have two options for coating the glass produced by their facility. It can be coated either offline by magnetron sputtering in a vacuum or on-line by pyrolytic CVD at atmospheric pressure. Sputtered coatings are generally referred to as soft coat. . In the On-Line Pyrolytic CVD process, vapor directed to the hot glass surface reacts to form a ceramic coating during the float process. Pyrolytic coatings are applied using CVD methods and are often referred to as hard coat. Both types of coating offer their own advantages and disadvantages. See the link below to view the CVD vs. Sputter comparison chart.
During the early generations, On-Line Pyrolytic CVD Technology could not compete with sputter technology. The coatings were not in the same league. There were many more coating chemistries and higher performing coatings. However, over the past 10-15 years On-Line Pyrolytic CVD Technology and its chemistries have advanced to the same level of sputtering.
Coatings applied using On-Line Pyrolytic CVD methods are commonly referred to as pyrolytic coatings. CVD methods involve reacting a precursor gas with the hot surface of the glass during the float process. As a result of this chemical reaction, the surface of the glass takes on a new chemical structure. This coating is sometimes referred to as a ‘hard’ coating because the coating becomes part of the surface of the glass and is thus more durable than sputtered coatings. The reactions must occur very quickly to avoid slowing down the float line. In addition, the CVD process is integrated into the float line without disrupting the float glass process.
On-line coating technology uses true vapor CVD where the precursors are vaporized fully prior to delivery to the coater head. This CVD technology is the most widely employed in current float glass manufacturing plants.
In order to explore the development stages of Generation 1, Generation 2, and Generation 3 CVD coating, three illustrative phases are highlighted, in their chronological order. The key differentiating characteristics are:
- Laminar or turbulent flow
- Pre-mixed or separately delivered film precursor chemicals
- Bath or Lehr location
Generation 1: Unidirectional laminar flow technology
This Generation 1 beam type is one of the earliest laminar flow designs – dating from the late 1970s. It is normally located in the float bath and has a water-cooled beam as shown in Figure 1. The design and operation are the simplest; however, the beam can handle only a limited range of coating materials. Such beams have been used to deposit silica, silicon and SiCO coatings.
Figure 1: A unidirectional beam design.
Generation 2: Turbulent flow technology
The Generation 2 system uses turbulent flow technology. In this technology the proposed precursors cannot be mixed together due to an unacceptable level of pre-reaction in the gas phase, it is necessary to deliver them separately to the reaction zone as shown in Figure 2. To achieve this, the beam which is located in the lehr has two separate delivery inlet slots. Rapid gas mixing is essential and a turbulent mixing and flow regime is employed. The beam is water-cooled. This type of chemistry and beam design is capable of the highest CVD growth rates – leading to rates of over 100 nm/s. The consequence of turbulent operation in this design is its low precursor efficiency (typically below 10%) and very high gas volumes leading to significant waste gas handling and scrubbing requirements. This inefficiency leads to higher overall chemical cost when compared to a Generation 3 system. This type of beam has been used to deposit F-SnO2.
Figure 2: A turbulent beam design.
Generation 3: Multi directional, laminar flow technology
Generation 3 technology was introduced around the mid 1980s onward. It is a laminar flow regime and handles premixed gases only. These premixed gases split into an upstream and downstream flow as shown in Figure 3. This approach offers potential for excellent film uniformity and high precursor efficiencies. In combination with oil heating and cooling, the beam gives the capability for the widest range of coating materials. The beam that is located in the float bath can be sensitive to set-up and operation and needs careful process control. Many variants of this basic design approach have been proposed – in particular to achieve improved control of chemical reaction and increased growth rates. Designs involving multiple gas inlets and exhausts have also been developed. However, care needs to be taken, as increasing complexity can lead to difficulties in achieving process control and therefore, acceptable yields.
Figure 3: Dual flow beam technology.
It is notable that all three coating beam technologies are still in high volume production use. In many cases more than one beam technology is used in tandem, to produce a multi-layer product.
CVD Production Processes – Key Process Issues
Apart from coating head design related issues, for high volume On-Line Pyrolytic CVD coatings, a number of critical process engineering related issues must be solved, including:
- Selection of an appropriate precursor system
- Controlled precursor delivery (handling and vaporizing chemicals)
- Temperature control of coater heads on-line
- Controlled gas distribution and maintenance of a gas flow regime compatible with achieving target film uniformity targets
- Proper integration into the float bath furnace
- Waste gas handling
- Process control (high on-line yield is critical to avoid glass loss)
It is the correct combination of technologies to achieve all of the above that defines the ultimate performance characteristics of the AcuraCoat® On-Line Pyrolytic CVD process.
CVD Float Line Integration
To properly integrate an AcuraCoat® On-Line Pyrolytic CVD Coating System into the float glass production line, many challenges have to be overcome. The glass line speed can be up to 1000 m/hr, the glass ribbon is typically over three meters wide, and typical required film thicknesses can be several hundreds of nanometers (nm). Such film thicknesses, at certain line speeds, require growth rates of tens of nm/s, and can reach 100 nm/s. In some cases, more than one coater head is required to achieve final target film thickness, even for one layer.
Selection of the coating integration position on the float line is a critical decision as shown in Figure 4. As a general rule, the further upstream (and therefore the hotter the glass), the harder and more durable the film produced, and the faster the growth rate achieved. However, the higher temperatures result in significant challenges for engineering and precursor chemistry. Most CVD coating is undertaken in the narrow end of the float bath.
Figure 4: : Possible positions for incorporating a CVD coating system in a float glass production plant.
The complexity and multi-disciplinary nature of CVD has made developing a complete turnkey CVD system a major barrier to wider diffusion of On-Line Pyrolytic CVD Technology. The launch of the AcuraCoat® system offers a breakthrough in access to this technology.
Stewart Engineers and its industrial partners have developed the most technically advanced On-Line Pyrolytic CVD System available to the float glass manufacturing industry – the AcuraCoat® System. The design is based on a 3rd generation concept and uses an oil cooled, multi-directional, laminar flow beam technology located within the float bath furnace.
- Width of glass: ≥ 3.4m (typical)
- Width of coating: > 3.0 m (typical)
- Speed: 300 – 1000m/hr
- Run time (the time between extended clean up): average 8 hrs, > 12 hrs possible
- Coating output: > 20% of total production
- Coating process yield: > 80%
- Start up time: 30 minutes
- Shut down time: immediately
Figure 5:AcuraCoat® System Flow Diagram.
The system is located onto a carriage which can be inserted and removed readily within the StewartFloat® bath. Engineering modifications to the StewartFloat® bath are undertaken and it is possible to introduce the system either as a new build or as a retrospective upgrade.
Figure 6: AcuraCoat® System cross-section indicating carriage and support members.
As a consequence of this advanced design, a wide range of products and product properties can be achieved. Stewart’s AcuraCoat® System produces a series of glass products delivering energy efficiency that will meet or exceed energy code requirements. This system is capable of producing Reflective, Low E, Solar Control, Photovoltaic and Self-Clean coatings. The AcuraCoat® technology has the potential to catalyze and support increasing diffusion of on-line CVD coating technology within the glass industry world wide.