flexible packaging: Sealing closure

Adhesives reactivated by heat for closing flexible packaging.

After a product has been manufactured and filled into bags, the final step is to close the packaging. Adhesives which can be reactivated by heat and are applied on sealing tapes, valves or as a surface coating provide different options for an efficient taper-proof seal.

Bag sealing is a complex process with very short process cycles. Bagging is a mass process and the interplay between the individual system components and their quality is of utmost importance. Projected to the quantities processed, even the smallest inefficiencies add up to significant amounts and, therefore, play a key role in the costs caused e.g. by material use, adhesive application amount and maintenance frequency (reflected in the operating time of the machines). Modern adhesives are reliable partners in this manufacturing environment due to their superior performance.

Hot melt adhesive system
Adhesives used for heat sealing are modern product developments based on innovative raw materials and with formulations which are perfectly adapted to the individual processing parameters. They also provide the basic thermoplastic functionality necessary for reactivation: they liquify at elevated temperatures and join different materials with high initial strength. The processing characteristics are adapted to the high speeds in filling.

Heat sealing adhesives from Jowat are available in different adhesive technologies. For instance dispersions, i.e. adhesive systems based on water, which are usually used for bonding paper-based packaging. Hot melts, which have previously been used mainly for plastic packaging, are now also becoming established for paper packaging – among other factors due to the reduced costs and increased efficiency of these adhesives. The newest Jowat products are hot melt adhesives based on polyolefin (PO) from the group of Jowat-Toptherm® products. PO hot melts are solvent-free adhesives with a 100 % solids content, which facilitate significantly shorter process cycles. Jowat-Toptherm® 811.10 for closing bags is free of blocking and non-tacky after the open time. Therefore, the coated material can be rolled up or stacked after the adhesive has been applied. The product is characterised by a high hot tack (fast tack), a wide range of adhesion to many different materials, and good heat resistance, even under mechanical stress. To facilitate an easy quality control, the hot melt contains a UV marker. An additional product version with food contact certification is also available.

Packaging material. Application system. Closing system.
Whether sealing strip, sealable valves or a sealable line of adhesive across the bag surface – which method is used depends on the filling good, the packaging material, the size of the packaging unit, and the point of sale or of use. For every bag type, there is a sealing system. Adhesives are applied by roller or by nozzle. The bonding undergoes strict stress testing, for instance hang tests or drop tests.

Hot melts are adhesive systems with a wide range of adhesion. They are used for all closing systems on plastic bags, paper bags and combination bags. Bag closing systems such as pasted valves or sealable lines of adhesive are applied to the bag directly during manufacturing. The valves or bag surfaces prepared for sealing are free of blocking when cold before the sealing and can therefore be rolled up and stacked for easy storage.
Sealing strips are manufactured separately. They have additional features such as a tear strip for easy opening or have specifications for use and warnings printed on the surface.

Application example: hot melts for closing paper bags and combination bags.
Herbert Schümann Papierverarbeitungswerk GmbH, a Jowat customer from Stadtallendorf in Germany, demonstrates that hot melt strips are the ideal closing solution for paper bags.
Paper bags are usually a disposable packaging material, protecting the goods during transport. The sealing closure provides a quick and easy opening, and clean handling of the content at the point of use. Combination bags made of multi-layer Kraft paper and with a plastic inliner are an efficient packaging material for bulk goods, such as food, chemicals or pharmaceutical raw materials in powder or granulate form. These bags are closed through the FTS (Fold-Tape-Seal) method. The fault tolerant and customisable hot melt strips from Schümann are perfect for this sealing method. The hot melt strips run smoothly on the automatic packaging lines. They can handle crumpled paper bags and the coating with Jowat hot melts requires only a low heating temperature (80 to 130 °C). In addition, the adhesive penetrates deep into the paper surface, being able to also embed dust particles. After the top of the bag has been folded, the hot melt strip securely closes the packaging at the end of the bagging process. Up to 600 bags per hour are folded and closed with a hot melt sealing strip at a feed speed of up to 25 m/s. The strip handles the high restoring forces of the folds and prevents them from opening.
Another great benefit for many applications is the formulation which can be adapted to the specific bag material used by the customer. This ensures the perfect closing material for any bag.
Schümann sealing strips and the technology of FTS bag closing units are a god example of the successful cooperation between modern engineering companies, the paper industry and adhesive manufacturers.

Adhesives that can be reactivated – A joining technology with potential
Large volumes of different goods are filled and transported in flexible packaging units. Closing those bags through heat sealing is a powerful technology. The percentage of packaging closed with this technology is growing continuously – and with it the number of modern industrial units in the industry.
Whether pasted valves, sealing strips or adhesive coating – a wide range of different materials can be joined through sealing, providing a multitude of benefits.
Like bonding in general, the possibilities of this specific bonding technology are far from exhausted. The innovative thermoplastic Jowat-Toptherm® adhesives have the potential to meet the rising requirements of this application.

The professional article is published in the journal ‚Verpackungsrundschau 11|2015‘ in German language. You can download the original article down below.