Multi-wall paper bags are most commonly used as industrial packaging material for bulk agricultural or chemical products such as dog food, flour products and cement. The bags often comprise multiple layers of paper of different quality. Frequently, one of the inner layers is a PE film or a PE coated paper.
The bags are glued at different places, such as side-seam, transverse, and bottom-seam gluing. Depending on the type of layers, different gluing products are used:
- Enziflex, PVAc based products used for different paper qualities and applications
- Socolin, latex based products for the adhesion PE to paper
- Plastomelt, hotmelt adhesives for bonding of PE valves in paper bags
- Excelta, metallocene based hotmelts for very high speed gluing of PE inner liners
Multi-wall paper bags are frequently glued using starch adhesives. These adhesives are generally prepared on site by dissolving them in water. During the dissolving of these starches, small lumps often occur that are impossible or difficult to remove by filtering. These lumps give problems on the machines during the further processing of the glue, in particular on the base plates. The machines are equipped with a glue-dosage system by means of an adhesive roller system. Small lumps are pressed out and prevent the good transfer of adhesive onto the bags. Subsequently the bags are not bonded "powder-tight" (i.e. sift or leak proof), which leads to quality rejection of the bags. These small lumps do not have this effect in the case older machine types are used. Here the glue is transferred using a normal glue press and this system is not very sensitive to the lumps in the adhesive.
Dispersion adhesives are homogeneous and are supplied as a finished product. The above described complications do not occur with these types of adhesives. Starch adhesives are dissolved to a solid content of approximately 20%. This means that bags glued with starch adhesives have a relatively high water content and thus require a longer drying time. Alternatively, dispersion adhesives contain a solid content of approximately 35% and show improved bonding. A saving of about 30% may be achievable as less adhesive is required to achieve comparable results. This also implies that less water is introduced and consequently a shorter drying time for the bags.
When considering the total bonding costs, dispersion adhesives are now considered to be competitive with starch adhesives. This offers a number of advantages for the bag manufacturer, such as shorter delivery time, outsourced risk in the preparation of adhesives, and no on-site glue making facilities. Furthermore, dispersion adhesives are more resistant to bacteria and are therefore less susceptible to degradation. As there is a world-wide desire to decrease packaging waste, the number of layers of paper used in bags is being reduced. However, as the strength of the bags must be maintained, the layers remaining are superior qualitywise, consequently also requiring a better quality adhesive. It is often observed that the standard starch adhesives do not give the desired bonding. To improve the bond strength, approximately 10% dispersion adhesive is added to the dissolved starch.