."Adhesives in the Finishing Process: Part II"
[Column #62, 2/00]
Last month, I began a discussion about the importance of adhesives in the finishing process. For those of us in the binding industry, it is common knowledge that glue can be a sticky issue. Selecting the appropriate adhesive for a given job is critical to the final product; therefore, understanding the basics will help in the planning process.
This two-part article covers four of the most common adhesives used in finishing processes today: fugitive, removable, remoistenable, and resealable. Last month, I covered the first two; this month, I will touch on the remaining two.
The traditional lick-and-stick adhesive, known as remoistenable glue in the binding and finishing world, remains a perennial favorite in the printing community—especially for those who work with direct mail. These adhesives are extruded hot-melts or are applied in liquid form and then heat-dried to remove the moisture. The glue is then reactivated by wetting it. Remoistenable glues have limited applications but are commonly found 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).
As I mentioned, there are two basic ways to apply remoistenable glue. Traditionally, cold, water-soluble adhesive was transferred to paper via a wheel or a blanket. This process is beneficial because heat by itself does not activate the glue, making the final product laser-compatible. Cold glue can sometimes be applied in various patterns providing more flexibility in production.
Hot melt extrusion is another way to apply remoistenable glue. This computerized technology provides more precision than cold emulsion applications allowing operators to start and stop glue flow at exact points.
While water-soluble glues applied on a pattern gluer can be adhered in a similar manner, the technology is not as precise because pattern gluers are based on time entry, not motion sensors. As a result, hot melt extrusion glues are favored because they rarely curl paper and provide a much more polished and professional look. Conversely, cold emulsion glues often look dull, have raggedy edges, and tend to curl the paper because moisture is added to only one side of the sheet. On the downside, however, extrusion machines only allow for parallel glue lines. Therefore, U-shaped glue applications require two passes or two extrusion systems running inline at right angles to each other.
Remoistenable glues are the only adhesives that must be approved by the Food and Drug Administration. There are several factors to consider when planning a remoistenable glue project. First, paper stock is critical. Uncoated stocks require heavier lines of glue because they are more porous and some will seep into the sheet. In addition, remoistenable glue will have trouble sticking to a vellum-like sheet because there are so many “valleys” in the paper that can absorb glue. It also tends to rest on the surface of enamel stock but can create a strong bond when moisture activated. Remoistenables also lay flatter when applied in the direction of the paper grain. If they are against the grain, the drying process can cause the covered area to become wavy.
Other considerations are ink and coatings or varnishes. A remoistenable glue will not anchor onto a solid ink surface or an aqueous varnish. Generally, ink is not a problem providing the coverage is line type or a less than 40 percent screen. If the sheet must be flood-varnished, design the piece to spot and knock out varnish from where the remoist strip will be applied and adhered.
Resealable glue does just that. It is a pressure-sensitive adhesive that has some tack but can be opened and closed repeatedly and leaves no residue. One of the most common examples of a resealable adhesive is the post-it note. Other newer formulas, for example, the water-based Craig-Stick®, allow the item to have removable and repositionable qualities. This type of glue is often found on grocery store items where coupons are removed for immediate use. The level of peel strength and tack or residue can be adjusted depending on the formula used.
Other distant relatives of the resealable glues include cohesive (or self-seal) adhesives and pressure seals. Cohesive glues usually form a fairly strong bond and can only stick to themselves. Common examples would be the wraps found around packs of dollar bills or restaurant napkins containing utensils. These can be formulated to form a permanent bond where the item is destroyed after one opening or can have several reclosures before they are thrown away. Pressure seals are special seals often used in direct mail pieces which require extremely high pressure to form the bond.
The use of adhesives in the finishing industry is critical to the final product. In the words of Pat Foust, Former President of Craig Adhesives and Coatings in Newark New Jersey, a 30-year old company, and one of the leading formulator and suppliers of adhesives, “The future of adhesives rests in its ability to add value while reducing the overall costs of operation. The value added benefits include being able to run at faster speeds, adhere to a wider variety of substrates, and to be more user friendly offering a wider operating window. Unfortunately, most plants look at adhesives and try to…reduce prices. They forget the cost of the adhesive is an insignificant part of the whole, but that it is critical to success or failure of the end product. A premium priced adhesive that offers premium performance pays for itself in many ways. It always amazes me that people never have enough time to do things right the first time with quality, high performance adhesives, but always find time to run reprints.”
I couldn’t agree more.