gumming adhesives

Remoistening gumming adhesives can be subdivided into three groups: dextrin based, half synthetic based (dextrin combined with synthetic resin) and fully synthetic based (100% synthetic resin).

Gumming adhesives based only on dextrin are almost never applied because of their slow drying, poor flatness and potential crack effects in the adhesive layer. Mostly half-synthetic gumming adhesives are applied, because these dry quickly, give a lovely shiny gumming and are cheaper than their fully synthetic counterparts.
Fully synthetic gumming adhesives have the advantage of excellent flatness and a high wet tack and are therefore extremely suitable for application on fast filling and sealing machines.

Remoistenable gumming adhesives must have the following general properties:

• good flatness of the dry glue layer or film: if the upper flap is not sufficiently flat, then moistening by the roller on the filling and sealing machine will not be complete;
• high wet tack after remoistening;
• high remoistening speed: the dry glue must be capable of fast water absorption and subsequently quick set;
• after remoistening the adhesive must give a good paper to paper adhesion;
• minimum amount of energy and time for drying;
• dry adhesive film must not give blocking (the adhesive film must not become sticky if stored at high humidity conditions).

In order to obtain a good gumming, it is essential that the correct amount of gumming adhesive is applied to the front seal or closure flap. Important is also that over the width of the entire envelope the glue is applied at even thickness (due to a damaged application roller there may be areas where too much adhesive will be applied).
The glue application necessary for good bonding depends on the type and grammage of paper. As a guideline for adhesive application one can stick to the following values:

Table 1: Guidelines for the application of remoistening gumming adhesives.

Paper type                                     half-synthetic (g/m²)           fully synthetic (g/m²)
Offset paper <100 g/m²                        22 ± 2                                  20 ± 2
Offset paper >100 g/m²                        25 ± 2                                  23 ± 2
Recycled/internally sized paper              24 ± 2                                  22 ± 2
Coated paper                                       20 ± 2                                  18 ± 2

Application of an excess amount of glue will result in problems with flatness, drying and blocking. When too little adhesive is applied, problems can arise with the wet tack strength, the remoistening speed and the final adhesion.

latex contact adhesives
Latex gumming adhesives are applied to both sides; the dried adhesive is not sticky. The adhesive layer only bonds to itself. The envelope is closed by pressing the two gummed layers onto each other.

Latex gumming adhesives can be divided into two types: the resealable and the permanent seal. The resealable type can be opened and closed numerous times. The permanent seal cannot be opened unnoticed. Often for reasons of tamper evidence, slits are made in the upper-flap which tear upon opening.

Latex contact adhesives must fulfill the following general properties:

• good adhesive and cohesive properties, implying both bonding to paper as well as to the adhesive layer itself;
• the adhesive must have good stability: during processing there must be no coagulation of the glue;
• no foaming: the latex adhesives are very sensitive to foaming. Excess contact with air results in coagulation;
• the adhesive film must not be susceptible to ageing (discolouration or inertness of the adhesive film). The gumming must be protected during storage against (sun) light, high temperatures and dust.     

Table 2: Guidelines for the application of latec gumming adhesives.

Paper type                                     (one sided) latex contact adhesive (g/m2)
Offset paper < 100 g/m2                                        11 ± 1
Offset paper > 100 g/m2                                        11 ± 1
Recycled paper and internally sized paper                11 ± 1
Coated paper                                                          9 ± 1

pressure sensitive adhesives
With self adhesive envelopes a permanent tacking adhesive is applied to closure flap. This adhesive layer is then covered with a siliconised release liner paper. To close the envelop , the release strip is removed and the closure flap is folded down and pressure is applied.

Pressure sensitive adhesives must fulfill the following general requirements:

• the adhesive must give good bonding to paper, even when the sealing flap is opened very slowly, the paper must show fiber tear;
• the adhesive film must have a high heat resistance to >60°C: pressure sensitive adhesives become soft at high temperatures, which can result in opening of the envelope without fiber tear.

Table 3: Guidelines for the application of pressure sensitive gumming adhesives.

Paper type                                        (one sided) latex contact adhesive (g/m2)
Offset paper < 100 g/m2                                        16 ± 1
Offset paper > 100 g/m2                                        20 ± 1
Recycled paper and internally sized paper                16 ± 1
Coated paper                                                         16 ± 1

Our products lines in gumming adhesives are following:

• Enziflex: half- or fully synthetic adhesives developed for universal usage;
• Socolin: universal latex based contact adhesive;
• Enziflex PC: universal pressure sensitive adhesive based on special developed co-polymers which can be used on different machine types.
• Plastoflex: pressure sensitive hotmelt adhesives.