Most of you probably think that my first column about layflat binding methods exhausted the subject of adhesives. After all, how much can a person write about something as seemingly mundane as glue?
I'm not stuck on the subject, but there is a wide variety of glues and applications beyond the PVAs, PURs and perfect binding methods I discussed before. I'll focus this time on three of the most common finishing glues and their uses. For those of you who might be wondering, this column will not cover glue-sniffing. Nevertheless, I guarantee the following information will keep you glued to your seat (okay, no more glue puns).
The Getaway Glue. Often, two separate pieces must be temporarily joined together so that certain finishing operations can be performed. An example is the attaching of a membership card to a direct mail letter which is then folded and inserted in an envelope. To keep the two pieces together during folding, a fugitive glue is used. This type of glue has a high alcohol content and is applied wet at a glue station to the substrate or host document (in this case, the letter). What I call the guest document (the card in this example) is then attached to the host. The glue holds long enough for folding and other operations to take place. Later, it evaporates with no damage to either document and no residue left behind.
A caution about fugitive glues: they work best in paper-to-paper applications. A fugitive glue will not hold a plastic card on a paper substrate, for example. That's the job of another type of glue, known as the removable glue.
There When You Need It. You're familiar with how credit cards are affixed to a carrier sheet for mailing. When you receive one, all you have to do is peel it off and head for the mall. The adhesive used in this case is removable glue. Available in hot melt or cold form, these are often latex-based glues and are good for one-time sticking. They have the consistency of rubber cement and once they've served their purpose, can be peeled off documents without leaving a sticky residue. Removable glues are often mistakenly referred to as fugitive glues, since they share the same impermanent traits. This is not a correct use of the term, however. Unlike their distant fugitive cousin, removable glues don't disappear on their own but rather retain their adhesive powers until the seal is manually broken.
Removable glues have a wide range of applications. A few spots or a continuous line of removable glue can be used in place of sticky-backed wafers to seal brochures or direct mailers. The adhesive can also be applied in a pattern, such as a circle or square. Removable glues have certain advantages over wafers. The glue can be applied in-line, decreasing finishing time. In addition, a removable glue, when applied professionally, does not damage either the host or guest document when a piece is opened, although it may cause a slight discoloration of the paper where it was applied. Wafers tend to tear the paper when the seal is broken. But there are instances, such as the sealing of saddlestitched books or multi-folded brochures, in which wafers are the only solution. It's impossible to apply enough removable glue to all the sheets for effective adhesion.
Removable glues are also used for tipping one piece to another, such as the attaching of a customer reply card to a magazine page or the affixing of a sample pack of shampoo to a promotional hand-out. At Bindagraphics, we use a special machine called The Attacher to stick just about any two objects together. We can affix decals, magnets, pencils, tokens, embossed or thin-mil cards, coins, keys, brochures, blister packs and reply devices to sheets, signatures, envelopes and other host documents.
Removable glues are a flexible product, but there are certain factors that determine how they should be used. Hot melt removable set up faster and have a stronger hold than cold glues, making them a better choice for affixing heavy objects such as pennies or keys to a carrier sheet. A hot melt is also the right choice if both host and guest pieces have coated or otherwise slick surfaces, since a cold glue will never dry in such cases. Generally speaking, cold removable glues work well when sticking two lightweight objects together, when applying a coated guest document to an uncoated host or vice versa.
Porosity of the elements to be glued is an important factor, since all types of glue need to grab onto something in order to be effective. In the case of removable, for instance, both hot melt and cold varieties will tend to stretch and even pop off a flood-varnished or a completely ink-covered sheet. And with some heavy paper stocks, such as vellum, a highly viscose (or thick) glue is better than a thin one for maximum adhesion.
All this brings me to Marty's Gluing Rule #1: Like I always say, consult your finisher first. Because of the variables involved, advanced testing may be necessary to find the best glue formula for a given job. Without running some samples, you could be stuck (I can't help it) with a useless product and an unhappy customer.
Lick and Stick. Remoistenable glues are yet another category of adhesives. They come in two forms, liquid and hot melt. The liquid glues are heat- or microwave-dried after application to remove the moisture. The hot melt remoistenable type sets up within seconds of being extruded onto the sheet. In either case, the end user reactivates the glue by wetting it (the best example is the U.S. postage stamp). Remoistenable glues have limited applications, but are commonplace in the direct mail world, where they mainly appear on the flaps of reply envelopes and on the backs of promotional stickers (such as the "Yes!" decal that you lick and stick to a magazine subscription form). A remoistenable can also be applied in a pattern, such as a U-shape, so that a flat sheet can later be folded and sealed to form a pocket. Originally made from the hides and bones of animals, the liquid remoistenable glue is made of dextrin or a synthetic emulsion nowadays. And here's a fun fact: it's the only glue that must be approved by the Food and Drug Administration (FDA).
As with removable glues, the surface to which a remoistenable glue can be applied is a significant factor in how well the glue will adhere. A remoistenable glue won't anchor onto a solid ink surface or an aqueous varnish. It will also have trouble sticking to a vellum-like sheet because there are so many "valleys" in the paper into which the glue can be absorbed. On the other hand, this glue will adhere well to a smooth sheet, to line type and to a less than 40 percent screen. In addition, remoistenable glue tends to lay flatter when applied in the direction of the paper grain. When the glue is applied against the grain, the drying process often causes the covered area of the paper to become wavy and bumpy.
Obviously, glue is a topic I'm attached to, but for now, just remember Marty's Gluing Rule #2: see Rule #1.