Comparing Glass and Resin: Discussion of Glass

Comparing Glass and Resin: Discussion of Glass

You can’t cut tempered glass

This statement is true and it has a big impact on work sequence, cost and quality.

Glass is produced continuously in a ribbon up to 12’ wide and arrives from the glass plants in astonishingly large annealed sheets, often 20’ long or more. These big untempered sheets provide the glass fabricator with the opportunity to combine and optimize cutting lists.

The glass is most often cut by scoring and breaking, except the scoring is done with a rail-mounted powered version of a device similar to the hand-held glass cutter you bought at the hardware store but can’t get to work. The edge is not perfectly vertical, especially on curving cuts, so some plants cut odd shapes using a waterjet. Holes are added at this phase, then the cut pieces receive the specified edge finishing before entering the tempering line.  Once the glass is tempered it is 5x stronger, but if an attempt is made to cut it, or it is struck or compressed, especially on an edge, it shatters in to cubic chunks roughly 3/16” square.

Glass is not a precision materialGlass is not a precision material

Sadly, this is substantially true. Because most glass is cut with the glass cutting method described above (scoring and breaking) the thicker the glass is, the further the bottom edge of the cut wanders from the cut line. If you request it, the fabricator can “cut over and grind to size”, or cut with a waterjet. But when the glass emerges from the tempering line it will change dimensions, and it will do so unpredictably.

The glass tolerances defined in the ASTM spec are horrifying, so keep an eye out for impacts.

Nickel Sulphide inclusions and heat soak testing

Nickel sulphide (NiS) is the bane of the glass installer’s world, because nearly invisible inclusions of NiS are present in most glass and cause tempered glass to break spontaneously. The only way to decrease the odds that glass will break due to the expansion of these particles is to heat the glass to about 554 degrees F (290 C) causing it to break in the chamber instead of in the field. The process is called “Heat Soak Testing”. An acceptable but riskier substitute for this procedure is to test samples of the glass and accept or reject the glass statistically.

Another way that tempered glass can break is if it is heated unevenly, as in the case of a large window in the sun that is partially covered with an opaque dark film with store graphics. Or if a screw is driven in to it through an adjacent component. So NiS is not always to blame!

Types of laminated glass

Glass is laminated for several reasons: So it stays together if broken, as with a car windshield, to increase resistance to the transmission of sound (or bullets…..) and to colour it.

For many years simply specifying “laminated glass” was a safe bet, because you knew what you would get. “Laminated glass” meant glass which was laminated by heating a 3-layer sandwich (glass-PVB-glass), then pressing the stack together. More exactly, the glass is cut to size, holes if any are drilled and edges finished as required, the polyvinyl butyral laminate is cut a bit oversize, the sandwich is made up on a roller table in a clean room and the stack is then propelled down a line between heat lamps which softens the laminate. At the end of the line the glass passes between rollers, air is forced forward and the plastic sticks to the glass. After the sandwich cools the excess laminate is trimmed off.

Unfortunately, it is no longer this simple to get what you want because other lamination methods abound and have taken a large share of the market. These new methods extend the range of laminated glass production in two opposing directions: The production of laminated glass which is much stronger and largely decorative versions that are somewhat weaker.

Laminated glass is inherently tough / durable / dependable etc.

Laminated glass is inherently tough / durable / dependable The impression that many A&D professionals have is that specifying “laminated glass” is the equivalent of stuffing a phonebook down your pants before walking the green mile to the principal’s office. The fact is that there are many ways of laminating glass and many materials used for laminating, or placed within the lamination, and not all of them deliver 100% of the strength that specification laminating will.

As an example, every architect and designer library has beautiful samples of laminated glass with embedded fabrics, leaves, feathers and so on; let’s call it “the artwork”. (I did some of this work back in the late 80’s but didn’t go in to mass production). These sandwiches are most often produced by placing the artwork between two sheets of glass together with a clear or tinted liquid resin (which might be two liquids mixed immediately before pouring) and then hardening the resin with UV light, or waiting for a chemical reaction to occur. If the artwork is thin and open-celled, it can also be thermally laminated in the usual way, in the centre between two layers of thermoplastic sheet and two sheets of glass.

The problem is that there are many different resins and sheet goods that can be used for laminating, but are better than others in certain applications. Some resin / filler combinations will degrade significantly and rapidly around the edges due to the effects of light and moisture, others are brittle and do not provide true safety-glass performance. The worst-case scenario is artwork that prevents the resin or laminate from creating a bond between the two layers of glass, or materials which do not adhere properly, resulting in bond failure, separation of the lamination and visible air pockets.

The raw material used in the laminated glass also varies. Normally tempered glass is used because it is 5X stronger than untempered, but as pointed out previously, all cutting and finishing has to be done before the laminating. It is desirable from a production and stocking viewpoint to make large sheets of decorative “artwork laminate,” then cut it to order, but such material has to be made with untempered glass, meaning that it is weak.

Structural glass

Structural GlassThe strongest versions of laminated glass are called “structural glass”. There has been a lot of buzz about structural glass lately, largely because of the tremendous boost given to this material through its use in Apple Stores. The best known applications are high profile stairs, glass bridges and so on. We used this product in the W Hotel Times Square overhead water feature and the self-supporting feature walls in the NYC HBO store.

The most commonly specified structural glass is SGP, or SentryGlas. The secret of this laminated glass is a “Surlyn” interlayer, a plastic 5 times stronger and 100 times stiffer than the common PVB interlayer. This sandwich is the perfect structural combination, basically like having  continuous sheets of re-bar in a concrete slab, instead of just rods. You can read all about it in some of the related patent documents:

Because it is not easy to fabricate this type of glass, fabricators are licensed by DuPont. The structural glass laminating process has four distinct differences from regular lamination:

1)      The laminate does not flow easily, so it does not fill surface imperfections well. For this reason glass used in SGP must be exceptionally flat, so the “tin side” must be identified with a detector and should be oriented to face the laminate. In laminates with more than two layers a thicker laminate is used to improve conformance.

2)      The laminate absorbs some air during an elevated temperature curing phase, which will reduce most air pockets. Autoclaving is recommended / required for most glass and exotic substrates like thin-sliced Onyx, to achieve good results.

3)      Edge finishing is complicated by the fact that the laminate can be quite thick, holes must be drilled before tempering and the resulting stack of parts must be hand-aligned before lamination, so standard glass fabricating tolerances add up twice, resulting in the need for greatly over sized holes or other locating features and reduced expectations regarding edge finish.

4)      Innovative engineers have designed fittings which are embedded in to the layers of structural glass, allowing some truly awe-inspiring work, but further reducing the range of suppliers who are capable of supplying accurate, stable components.

Because the fabrication of laminated products occurs in a sequence that cannot be altered or compressed (cut, drill, polish, temper, laminate, clean up, re-fabricate rejects) the above-noted complications have the effect of greatly extending fabrication or re-work time for structural glass, so be sure to allow a generous margin for error.

Starfire vs Low Iron

Window glass gets its characteristic green tint from tiny amounts of iron oxide (rust) which finds its way in to the glass, due to its ubiquity in the raw materials from which the glass is made. The green tint is most obvious if the edge of the glass is visible, since the viewer is effectively seeing through the full width of the glass, not just its thickness. I like the green tint, but some find it old-fashioned-looking. Removing most of the iron oxide produces a glass which is free of this green tint, though the edge still shows colour, albeit a characteristic light blue.

Low Iron glass is marketed under a variety of brand names, the most common of which is Starfire. The important point is this: They are close but not identical in appearance at the edges, so whatever brand of low-iron glass you pick, you have to stick with it through a project.

Glass detailing is very, very difficult

To paraphrase Mr. McGuire “There is only one thing you need to know to detail glass is: Stress Raisers”. This may seem like gross oversimplification, but bear with me as I explain my reasoning.

Because we all know glass breaks catastrophically, losing all of its strength in an instant and casting off dangerous fragments as it collapses, when we think of detailing glass, we are always thinking of the possibility of breakage. So avoiding breakage, or managing the results of breakage, is the goal of the glass detailer.

And what causes glass to break? Stress raisers!

Stress raisers are simply points of concentration in the loading of a component. Picture one bolt holding a big steel plate to a wall, or a sweater hanging on a small hook. The load can be self-imposed (the load that a sheet of glass applies to the bolt holding it in place is equal to the weight of the piece), or it can be made larger by an external load (the side force on the glass created by people on a stair pushing on railings, which are attached to the glass, which is held by the bolt).

If you have ever paid $5 for the chance to smack a car at a fundraiser, only to be disappointed and embarrassed when the sledgehammer bounces off the windshield, you have experienced a perfect example of “adequate load distribution”. The flat face of the hammer does not produce an adequately concentrated load, so the windshield does not break.

Large stress raisers occur at the point where a fitting, such as a bolt, is forced against the glass. Since the glass is inelastic, it cannot bend or conform around the hard object, so very large loads result, which causes a crack which quickly propagates through the glass. Because it is in a state of internal tension, tempered glass is stronger, but this tension causes it to almost “explode” if broken. The most important point is that tempered glass is most sensitive along its edges, so hitting or applying a concentrated load the glass on its edge (such as a screw head sitting high in the base a glass mounting shoe…) is the worst-case scenario.

The fact that glass breaks as a result of stress raisers is why ambulances always carry an automatic centrepunch, a pencil-sized machinist’s tool that drives a small sharpened punch from one end when the first-responder depresses the button on the other end. The tool will break a car window with a tiny fraction of the force of the sledgehammer, because it concentrates the force on a tiny area. The limp sheet of “window” can then be pushed out of the way.

Well-detailed glass mounting systems avoid stress raisers.  Proper glass installation, such as glass set in to a shoe and surrounded by cast-in-place bedding compound, results in small stresses that are evenly distributed. In the case of glass bolts, there are off-the-shelf systems with large face areas and washer / sleeve kits that prevent metal to glass contact. FYI I have always been shocked by the cost of these bolts, so I have designed a unique systems for almost every job I have done and found that even a custom glass bolt is price competitive.