Successful gluing work needs a scientific approach and an artistic touch. The four horsemen of gluing – paper, ink, coatings and glue – are about equal contributors to a job’s success. Their combinations are nearly infinite and unexpected results frequently occur. Sometimes easy-release glue tears paper fibre; sometimes permanent glues perform like easy-release.
Graphic arts glues are mainly oil, resin or latex-based. Each type performs as expected most of the time, but there are exceptions.
Both easy-release and permanent oil-based glues offer good adhesive properties and are appropriate for physically heavy or varnished pieces. However, their relative great bulk may result in an unattractive product from a marketing viewpoint. Resin-based glues are thin, generally reliable, inexpensive to apply, energy efficient (applied cold), environmentally-friendly and FDA-approved for many food-packaging applications. Latex, however, is a natural rubber tree product and coagulates when contacted by steel or iron. Coagulation can cause machine applicator problems. Also, latex glue doesn’t work well in compressed air non-contact systems.
Easy-release glue
Also known as removable or fugitive glue, easy-release glue performs well 19 out of 20 times, but one tough job will prove there’s no “easy” release from gluing headaches.
Easy-release glue performs best on penetration-resistant, highly-calendered, dense paper with heavily inked and coated surfaces. Matte and other lightly-calendered enamel stock, offset paper or sheets with a heavy clay fill are susceptible to delamination and fibre tearing when the intention is an easy-release effect.
Latex easy-release glues require long setup times (three to four minutes) and tend to spread when the opposing sheet is tightly squeezed. Their curing period is really 24-hours, even though they appear to be dry after 10 minutes. Unfortunately, products that perform properly 10 minutes after manufacturing can change in 24 hours and pull fibre. Oil-based easy-release glues have a shorter curing time, but glue bulk remains an issue.
Managing variable adhesion and chemical reaction is important. Some easy-release glue solvents, such as ammonia, dissolve aqueous and other coatings and result in unintentional permanent adhesion. Occasionally, permanent resin glues can function as easy-release on aqueous coatings because when dry, they become very brittle and perform better than latex or oil-based easy-release glue.
Temperature and humidity conditions can greatly affect the performance of glue products. For example, one summer, a trailer load of latex easy-release glue products was going to Texas from Toronto, the client reported that the job was unacceptable because fibre was tearing. Although time-pull production samples functioned perfectly, the problem was apparent after observing samples. An employee was sent to Texas and discovered that the 2-inch Mylar tape used to seal the shipping cartons blistered. The tape manufacturer explained that blistering occurs with Mylar tape when it is exposed to a temperature higher than 49°C. It was next found that the truck had remained outside during an intensely hot weekend prior to delivery. Interestingly, the fibre-tearing product was only in the cartons on the external layer of the skids, not in the protected internal cartons. Clearly, extreme heat caused further reaction in the curing of the easy-release glue.
Permanent glue
Permanent gluing problems do occur, but are infrequent. For best results, select a paper with a porous surface, and position the glue away from ink and coatings. Permanent glue needs to bite into paper with a porous surface, so the harder the surface, the more difficult it is to penetrate that sheet and create good adhesion. Be sure to “knock out” ink, varnish, UV and aqueous coatings wherever you place permanent glue because glue tends to rest on top of coatings and cannot penetrate and grip fibre. Permanent resin glue spreads on stocks and coatings with a high barrier to penetration and can result in a poor bond or sloppy glue coverage. Absorbent and porous paper will allow glue to penetrate papers fibres and produce a strong bond. If gluing must occur over ink coverage, use wax-free ink. When stuck with a difficult permanent gluing job with aqueous coating, as a last resort, try using ammonia-based latex easy-release glue instead of permanent.
Because water-based resin glues spread, projects with critical glue registration require constant monitoring and sample pulls during production. Take special care when applying resin glue in a trim-out area. If glue reaches the paper’s edge, sheets will stick together. Conversely, if glue spreads too far into the piece, it will not be removed during the final trimming process. If the trim-out area is shorter than 3/8-inch, consider using easy-release instead of permanent glue because the negative consequences of excess glue spread are less.
