Gluing is used to join paperboard surfaces together, providing a permanent join. Gluing is also used to erect and close cartons and to provide several functions to graphical products. Gluability is therefore an important property. Predictable and reliable gluability is built into the paperboard by careful choice of the surface sizing system, interlaminar strength and pigment coating.
Bookbinders who produce paperback covers require very good gluability. The cover must not snap off or delaminate. The glue bond must be the strongest part of the book.
The designer who creates new and eye catching shapes from paperboard needs an elastic, strong and tough material that is easy to glue with standard adhesives. Very strong bonds are required especially when the adhesive is applied in small dots.
Gluability is important for high packaging line efficiency and runnability. The glue seam must also withstand forces arising from the package contents during the lifetime of the package.
The paperboard choice
If the packaging line is an automatic, high speed operation, then reliable gluability is vital. This must be considered in the paperboard choice. A glue seam must set quickly and then resist the load from the product. It must stay intact as long as the carton is used.
A strong paperboard with good surface properties, such as surface strength and absorption is essential for good gluability. The pre-treatment of the surface, i.e. surface sizing, and the coating composition are key factors in gluability. The paperboard requirements vary with the type of adhesive used, so this is an important component to take into consideration when choosing a paperboard.
Both Folding Box Board (mechanical pulp with chemical pulp in the surface plies) and Solid Bleached Board (pure chemical pulp) give exceptionally good gluability.
The established way to assess gluability is to examine the tear behaviour of a glue seam between a pigment-coated surface and, usually, the reverse side of a carton flap. The paperboard must be strong (interlaminar strength) to give good gluability. If the paperboard is weak, the paperboard fibres will tear quickly and the gluability is wrongly determined to be good. Gluability is actually a factor of both paperboard strength and fibre tear.
If the glue seam itself fails, or if the bond failure occurs in the pigment coating, the rating will be poor. The method will not reveal the true seam strength in comparison between different materials since it is always a function of the weakest link in the construction. The gluability of a paperboard can vary depending on the gluing system used (such as water-based systems or hot melts).
Gluability for water-based systems refers to the tear behaviour of the fully developed, dry glue seam between two paperboard surfaces.
The bond formed by gluing must be strong enough to be handled as soon as the carton leaves the gluing machine. Therefore the glue needs to be chosen while keeping in mind the absorption properties such as setting time that suit the paperboard and the process in the gluing line.
It is preferable to have a high ratio of surface strength/internal strength. When tested, a good glue bond will show both fibre tear and reliable strength. The gluability must be balanced against printing properties such as ink absorption and set-off.
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Assessment of gluability
A glue seam will break at the weakest point. Gluability is a dimensionless property which describes the behaviour of a glue seam when exposed to a load designed to break the bond. If the glue seam can only be torn so it gives a fibre tear fracture, the glue seam is good. The situation corresponds to a favourable ratio between the glue seam strength and the internal strength of the substrate. The adhesive itself is the strongest part of the bond.
If the glue seam itself fails or if the bond failure occurs in the pigment coating, the rating will be poor.
Hot melt gluability
Testing a glue seam with the glue seam
parallel to the index finger.
Gluability with hot melt refers to the tear behaviour of a hot melt glue seam between a pigment-coated surface and, usually, the reverse side of a carton flap. Here it is also necessary to have a high ratio of surface strength/internal strength as well as good wettability. A good bond will show full fibre tear (that is, the torn-off paperboard strip will remove the whole glue seam, the coating layer of the baseboard and also some fibres from the other paperboard surface). In contrast, a brittle glue bond which separates at the pigment coating interface indicates poor performance.
A good, well developed bond will show fibre tear if the surface of the paperboard is strong enough. As a rule of thumb, a pigment coating which shows good gluability will also have good hot melt gluability. For a given pigment composition the coating strength and hot melt gluability must be balanced against printing ink absorption and set-off.
Different surface characteristics
Good gluability is an important feature of an optimised paperboard surface.
The pigment coating, which is applied after the surface sizing, must be made strong enough with binders to meet the needs of the printing process and provide gluability. However, if too much binder is used, the coating may not be absorbent enough for the printing process. This indicates the complexity of the property balance of a pigment coating.
A plastic coating has relatively low surface tension because of the chemical nature of the plastic. Most glues will not wet the surface. Therefore the coating must always be modified on the surface by treatment such as an electrical corona discharge or an oxidising gas flame to give the plastic a more polar nature by the introduction of oxygen into the surface molecules.
For gluing operations the following paperboard properties are crucial:
• surface strength
• tensile strength
• compression strength
• flatness and dimensional stability
• creasing efficiency
• surface tension and wetting properties
• absorption and glue setting
• delamination, interlaminar strength.
Gluability is tested both during process control, which means that a quick test method is required, and during product control, which correlates with practical use and may take more time.
This situation may to some extent explain why there is no generally recognised international standard for gluability testing. The variety of demands is so great that a number of methods have been developed.