The invention relates to a strip of flat closures made of plastic that are coupled together. A closure of this kind comprises a flat body with an opening therein for receiving and holding the access side of flexible packaging, as well as a slot delimited by leg parts, which extends between the perimeter of the body and the opening, and coupling parts for coupling to an adjacent closure.

A strip of closures of this kind is known in this field (see for example NL1027149 for a strip of sealable bag closures of this type) and is employed in closing machines for the closing of bags that are filled with products, such as food products, for example bread etc. In a usual embodiment of a closing machine of this kind, the bags are pressed together on their access side to form a neck, and this neck is placed between brushes. Then the neck that is gripped between the brushes is brought via a conveying element into the opening of a flat closure. The flat closure is then broken off by machine from the strip of closures that are joined together. Flexible packaging can be closed with a bag closure by machine in this way. The closing machines that are employed usually break the closures off by means of tensile force. See for example EP0382169A1. Single closures of the same type, with or without coupling parts, are also used in shops for manual packing of food products.

The closures are "non-contaminating", i.e. no loose parts such as the coupling parts may arise in the machine during closure, especially during breaking off, which could get into the packaging in the case of (food) products or could damage the packaging itself, bearing in mind health risks for the consumer, as well as quality and shelf life of the (food) product.

The applicant is marketing a strip of bag closures of this kind, manufactured from high- impact polystyrene (HIPS). The HIPS used has a relatively low tensile strength of about 21 N/mm ² (measured according to NEN-EN ISO 527-3) and a relatively high elongation of about 77%. This material displays the desired fracture behaviour in the existing machines. In practice, however, the closures still break off relatively quickly, which is experienced as a disadvantage by the user. A tougher material is desired for the user. However, experience has taught that HIPS can scarcely be further improved with respect to performance, in particular with respect to elongation/strength ratio. If the material is made tougher by modification, the tensile strength decreases. Conversely, if the tensile strength is increased, the toughness decreases. Furthermore, there is inadequate availability of HIPS in the desired thickness and dimensions for bag closures, so that the cost price of the closures is relatively high. A further disadvantage is that bag closures made of HIPS are not easily sealable.

Thus, there is a need for bag closures made of materials other than HIPS, which can be processed on existing closing machines, in which the closures are detached in the machines by means of tensile force. In other words the fracture behaviour (combination of tensile strength, toughness/stiffness) must be suitable for the breaking-off mechanism used in the machines, wherein a closure, in which a bag neck is applied, is detached from the strip of closures by a striking or sliding mechanism. On the other hand the strip, in particular the coupling parts of the bag closures, should not be so friable that the closures already become detached from one another prematurely, for example during feed into and through the machine.

The aim of the invention is to provide a strip of flat bag closures that are coupled together, which can be processed in the closing machines known in this field.

Another aim is to provide a strip of this kind that is less expensive.

Accordingly, the invention relates to a strip of flat closures made of plastic that are coupled together, wherein each closure comprises a body with an opening therein for receiving and holding the access side of flexible packaging, as well as a slot delimited by leg parts, which extends between the perimeter of the body and the opening, and coupling parts for coupling to an adjacent closure, wherein the plastic is a plastic foam, in particular a polyester foam, preferably a foamed amorphous polyethylene terephthalate.

Tests with strips of closures linked together of usual dimensions, which were made by stamping from sheets of foamed amorphous polyethylene terephthalate (also known commercially as APET), displayed the desired brittle fracture behaviour and non- contaminating fracture behaviour without excessive (plastic) deformation in ordinary closing machines with the action of tensile force for breaking off, such as are employed in bread factories and bakeries.

Polyethylene terephthalate (PET) is a plastic that is widely available commercially and that is still the subject of much development. However, PET cannot be used for bag closures of this type, because in the usual dimensions it is too strong for breaking-off in the existing machines. A high tensile strength of about 56 N/mm ² (measured according to NEN-EN ISO 527-3) at a high elongation of about 100% makes a sheet of amorphous PET unsuitable for bag closures. Weakening by reducing the thickness is not possible because thinner bag closures cannot be processed on the existing machines. In addition, even these thinner bag closures are still too strong. Furthermore, PET has a higher specific gravity then HIPS (1.35 versus 1.05), which is unfavourable from the viewpoint of cost price (per amount by weight). By starting from a plastic foam, in particular a polyester foam, preferably a foam of amorphous PET, the desired decrease in strength relative to unfoamed PET can be achieved (the tensile strength of foamed film from amorphous PET with a thickness of 0.8 mm is 22 N/mm ² ), and the required fracture behaviour (toughness/stiffness) can be obtained. Sheets of PET foam can be manufactured in the required thicknesses by extrusion in the presence of a (physical/chemical) blowing agent. This APET foam is available commercially from Buergofol, among others. The strip of closures can be made by stamping from the film.

An additional advantage is that a thermoplastic polyester foam, in particular foam of amorphous PET, is better sealable then HIPS, so that a method for "tamper-proof" closure becomes possible, as described for example in the aforementioned NL1027149C, wherein the leg parts are joined together by heat treatment. This is simpler than an alternative manner of sealing by applying a quick hardening, flowable adhesive such as a thermoplastic on and/or in the slot.

Yet another advantage is that polyester, especially polyethylene terephthalate, is more reusable then HIPS. Thus the PET foam waste, that results from stamping, can be reused in the production of fresh thin starting sheets, from which the closures are manufactured.

Advantageously, the coupling parts are breakable-off by machine, more preferably in machines that are based on tensile force.

Advantageously, the foam has a closed-cell structure.

Advantageously, the slot extends between the opening and a peripheral part of the body of the closure extending parallel to the longitudinal direction of the strip.

Also advantageously, the coupling parts are provided on the feed side and opposite side of the body of the closure. Here, the feed side is the side of the closure that is fed into the machine first.

The thickness of the bag closure is usually in the range 0.6-1.5 mm, such as 0.8 or

1.2 mm.

The invention further relates to the application of a strip according to the invention in machine packing of products, in particular foodstuffs, such as food products, in flexible packaging, wherein the access side of the flexible packaging, usually as a folded neck, is brought into the opening of a closure.

The invention also relates to a method for manufacturing a strip according to the invention, said method comprising the steps of supplying a film of plastic foam, in particular a thermoplastic polyester foam, preferably a foamed amorphous polyethylene terephthalate, stamping the strip of flat closures coupled together, and winding the strip on a roll.

The invention is explained hereunder on the basis of the drawing, in which

Fig. 1 shows an embodiment of a part of a strip of flat closures according to the invention made from foamed amorphous polyethylene terephthalate. Fig. 1 shows in top view a part of a strip 10 of flat bag closures 12. A flat bag closure 12 comprises a body 14 of foamed amorphous polyethylene terephthalate, in this

embodiment with a central opening 16 for receiving a folded neck 18 of a flexible packaging. Projecting parts 19 of the body 14 extend into the opening 16 to allow the closure 16 to grip the neck 18, so that a fixed position of the closure 12 with respect to the neck 18 is assured and the neck is gripped well. The closure 12 further comprises, between leg parts 20, a slot 22 that extends from the entrance 24 on a longitudinal side 26, parallel to the longitudinal direction of the strip 10, up to the opening 16. The entrance 24 of the slot 22 is broadened on the longitudinal side 26, so that inserting the neck 18 in the slot 22 is facilitated. The direction of movement of the strip 10 in the machine is indicated with arrow B. The feed side 28 of the bag closure 12 and the opposite side 30 are each provided with two coupling parts 32a and 32b located some distance apart, wherein the coupling parts 32a of closure 12a are connected to the coupling parts 32b of the preceding closure 12b. In the embodiment shown, the right closure with the neck therein (not shown) is broken off from the strip 10 by means of a striking motion of a hammer or pin of the closing machine in the direction indicated with arrow A.