Being Transparent: What’s Involved in Security Glass Fabrication
By Mike Hallahan
Multi-layered security glass laminations are some-times referred to as security, bullet-resistant, blast-resistant and, in military vehicles “transparent armor.” As laminating practices go, they’re a formidable challenge due to their multifold layers of mixed thickness values, complex geometrical shapes, varying types of materials and the almost certain need for vacuum processing. The manufacturing process involves a number of steps and considerations. We’ll review each to explain their importance to the overall process. It’s import-ant to remember the development of these specialty products depends on the use of empirical science and design and may take years of efforts to achieve the desired effect. See Diagram #2.
GLASS AND INTERLAYERS
Laminated security glass is constructed with a number of different materials. The composition (sometimes referred to as the recipe) of the multi-layered unit includes the glass, often low-iron, and the interlayer material. Due to the many layers, the low-iron glass is beneficial in providing a high level of transparency. High-strength glass products, such as chemically-strengthened, heat-strengthened or a specialized glass product called borosilicate, may also provide a high level of impact resistance.
The binding layers (adhesion agents) are the interlayer materials, the poly vinyl butryl (PVB) and urethane. PVB is generally used for the interface of glass-to-glass layers and the urethane is used for the interface of glass to polycarbonate. Both may be used in multiple locations based on the novel design of some recipes.
A key element in many types of multi-layered protective glazing is the polycarbonate material, also referred to as the spall-shield in the security glass industry. This shield serves the purpose of adding a final layer of protection. Upon impact from the outside (non-safe side; the inside layer of the glass window is called the safe side) of the glass unit, the protection value will break down, allowing each successive impact to penetrate deeper into the glass unit. As the final layer of protection, the spall-shield provides a flexible and bendable layer that acts like a catcher’s mitt to help prevent broken shards of glass from harming anyone on the inside of the glass window.
Once the glass is cut, the next step in preparing it for the stacking process is seaming the sharp cut edges. Generally, a cross belt seamer is used to smooth both top and bottom edges simultaneously. Some research has shown that grinding verses seaming conclusively adds more integrity to the glass edges. Grinding uses different-shaped diamond grinding wheels that produce a variety of edge profiles. See Diagram #1.
Clean glass is essential. Unclean glass will likely be a reason for unacceptable adhesion during processing. It’s extremely important that all glass undergoes an aqueous cleaning method through an industry-proven glass washer. The glass should be cleaned, dried and statically neutralized prior to layup. The washer can be set up to feed the glass panels directly into the clean room through a slot in the wall, providing the most direct delivery in a timely and sequentially calculated manner.
The PVB and urethane are the bonding layers that use an intimate surface-to-surface contact and, under high temperatures and pressures, will bond all the layers into a solid glass unit. PVB is generally cut to its needed profile (with a slight oversize in material) by being preprogrammed into an X, Y profile-style cutter. The oversize cut is important to compensate for material shrinkage. The profile cutter uses a CAD engineering file (or DXF file) to extract the perfect profile cut to match the window’s shape. The same is true for the urethane bonding layer, except for the oversize cut that may vary as dependent on the shrinkage rate of each material. After cutting, both items should be tack-rolled to remove any debris from the cutting operation or just any other airborne contamination. The cut pieces can then be stacked in a pile (no deeper than approximately 6 to 7 inches) and covered with a virgin material such as polyethylene sheeting. Any deeper of a pile can cause issues with deformation to the interlayers or a blocking (sticking together) effect.
Polycarbonate is the final component that finishes the glass unit. Preparing this layer is similar to the soft inter-layers. It begins with a cutout from a profile cutter that has a blade designed to cut through the hard material. Precut polycarbonates can also be purchased directly from a supplier. Polycarbonates are covered with a sheet that protects both surfaces to help avoid scratches. These protective sheets are marked to designate the inside surface from the outside surface. The outside surface will be exposed and is coated with a scratch-resistant barrier. Both sides of the protective sheeting should remain on the polycarbonate until just prior to assembly. It can be beneficial to run these sheets through a tack roller machine to remove any edge surface debris that may be present from the profile-cutting operation. Due to the nature of the polycarbonate and inherent unbalanced tension between the adhesion of this layer and the glass layer, you can include a pressure plate on top pf the polycarbonate sheet during autoclave processing. This helps provide a downward stabilizing force and helps prevent material warpage. The glass pressure plate should be at least ½-inch thick to provide the needed downward force.
All glass laminating facilities must have a clean room in order to provide a contaminate-free and temperature/humidity-controlled environment. (See related article on pages 12-14 in the June 2015 USGlass magazine.)
When working with a multi-layered laminate, it’s easy to either misplace or miss a layer completely. For that reason, it’s imperative to have some method of tracking and verification that the recipe is done correctly. A missed or misplaced layer can affect whether the glazing meets the pre-engineered and tested impact ratings. Incorporating a system of checks can help ensure the correct assembly. This may be accomplished by having a test thickness measurement gauge or even a segmented (color-coded) gauge to fully verify layer order and finished stack height.
SEALING THE EDGES
Large numbers of surface interfaces in a multi-layered lamination mean the unit will have capacious amounts of exposed perimeter edges after stacking. An application of an edge seal strip will be extremely important in preventing edge moisture migration from occurring. The edge seal strips should be precut to perimeter lengths and then sliced diagonally through the middle of the strip (on approximately 1-inch centers) to help trapped air within the unit escape during the autoclave process.
Slicing is an optional suggestion that may or may not be needed depending on the type of edge seal used. See Diagram #3.
Various types and thicknesses of high-temperature bagging materials are available. The bags are wrapped securely around the entire unit and a high-temperature tape holds the bag in position. A vacuum port connection will be needed inside the bag, with the actual port connection protruding outward through an opening for easy hook-up to a vacuum line. Suitable breather material should be used under the port connection to afford a direct line of vacuum flow from the bag. A pre-vacuum cycle should be engaged prior to placing the glass unit into the autoclave. A typical pre-vacuum cycle should be long enough to remove all the air from the bag and achieve a vacuum level of at least 27 to 28 in.-Hg. The integrity of the bag and its ability to maintain an impervious condition will be tested by the pre-vacuum cycle before autoclave processing and eliminate the majority of vacuum failures that can occur.
All the glass units will need to be loaded into holding racks for entry into the autoclave vessel. These carts are normally custom-designed by the OEM to the specifications (size, number, etc.) of the intended products. The racks should be well-designed and should be equipped with integrally designed pipe manifolds to provide multi-port distribution connections. This will reduce the number of individual glass unit vacuum connections needed. It’s also a good idea to document the location in each rack for each product in order to trace any defect issues. This will allow analyzation of breakage from rack locations.
The ultimate purpose of an autoclave is to finalize the laminated glass unit processing. It uses a treatment method incorporating heat, pressure and vacuum sealing over a controlled period of time. The art of this step is programmed into an automatically controlled set of parameters that are created and recorded for each product, load size, and even seasonal conditions from previous successful runs. These records will then be used to provide a consummate, repeatable process for the same types of products and group runs.
Post autoclave clean-up involves removing the glass unit from the vacuum bag and then cutting the squeezed-out interlayer material from the exposed seal edges. This is best accomplished by using a sharp razor knife. After removing the glass unit from the bag, give it a quick inspection for any cracks or breakage. Discoveries of this nature should end the ongoing production process immediately. Visible trapped air in the form of air bubbles will need to be noted, along with any missing interlayers or any other obvious anomalies. Sometimes it’s possible to reprocess a defective unit that’s been rejected for trapped air by rerunning it in the exact same autoclave cycle. Take care when doing this to achieve the expected original results.
In this type of laminating there are many reasons for a unit to fail. It’s always best to attempt to mitigate failures as much as possible.
The chart on the left (Diagram #4) provides guidance for some types of failures, identification of the failure and possible reasons.
These problems certainly are not the extent of all the is-sues that can occur in security glass lamination. Remember to communicate with all the material suppliers for their guidance.
The amount of protective glazing that’s being produced is increasing and is helping make buildings safer in an ever-escalating world of global threat. The more sophisticated the weaponry, the more sophisticated the protection will need to be. As new products are scientifically developed using a variety of new materials, the perfection of how they are produced will determine the number of lives saved.
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