Patent Application
20190143654
The invention relates to a disposable board container, such as a drinking cup or a food tray. The container comprises a bottom and a mantle extending upwards therefrom. The bottom and the mantle are made from a polymer coated fibrous board such as paperboard or cardboard and joined by heat-sealing. A liquid-proof coating of polymer is provided at least on the inner surfaces of the container.
The material for a disposable board cup is fibrous board, which on one or both sides is extrusion-coated with a heat-sealable liquid-barrier polymer. Bottom blanks and mantle blanks are cut by punches from board webs. A circumferential downward rim is moulded to each bottom blank with a compression-moulding press. The mantle blanks are wound about a conical moulding tool and heat-sealed along an upward seam line by means of the coating polymer to form mantle parts having the shape of a truncated cone. If the board is polymer-coated on one side only, the coating will form the upper surface of the moulded bottom and the inner surface of the sealed mantle. Next, the bottom and the mantle are fitted together and the lower edge of the mantle is bent and heat-sealed by the coating polymer against the circumferential rim surrounding the bottom. The seals secure liquid-tightness for the cup. Finally, the upper edge of the mantle is usually bent to form a mouth roll for adding rigidity to the cup and providing the desired mouth feel in drinking.
Descriptions of standard techniques for the manufacture of disposable board cups are found e.g. from WO 2005/077768 A1, see especially
Open containers of a basically similar structure, comprising heat-sealed bottom and mantle parts, such as circular, oval or rectangular food trays, are manufactured in a substantially like manner.
For disposable drinking cups the usual coating polymer is low-density polyethylene (LDPE), which has a low cost as well as excellent extrusion and heat-sealing properties. However, for microwave-oven use LDPE has too low a melting point. Microwave-ovenable food-trays have been manufactured by using a coating polymer, in which a minor share of polyethylene (PE) has been blended with a major share of polypropylene (PP), which has a higher melting point so as to stand heating in a microwave-oven without melting. Generally such a blend seeks to balance between the simultaneous runnability in extrusion coating, heat-sealability and microwave-ovenability requirements. An example of a commercial coating blend, which is PE modified PP and specifically designed for extrusion coating, is WG341C by Borealis AG.
A problem experienced in heat-sealed PE+PP-blend coated micro-ovenable containers is inadequate sealing between the lower edge of the mantle and the downward rim of the bottom. The heat-sealing jaws have to clamp together a stack of three board layers and polymer coating layers sandwiched therebetween along the circumferential seam being formed. Fibrous board such as paperboard or cardboard has a poor heat conductivity, and the presence of a third board layer considerably limits transfer of heat to the polymeric seals as compared to the usual situation with two board layers at the seam.
US 2007/0292569 A1 to Bohme et al describes various polymer coated packaging boards for oily or greasy food products. A board substrate may have an inner oil resistant coating layer of a blend of PE and PP, and an outer heat-seal layer of PP. However, the cited PP homopolymer MARLEX by Chevron Phillips or the metallocene polypropylene Affinity 1450 by Dow Chemical respectively have a too high or a too low melting point for solving the heat-sealing problem between a the bottom and the mantle of a microwave-ovenable container. Heat-sealed microwave-ovenable containers are not dealt with by Bohme et al either.
There is thus a need for an improved heat-sealed container especially for microwave oven use, in which a leak-free seal between the bottom and mantle parts is secured.
According to the invention the problem of improving the container is solved by use of a polymer-coated fibrous board material for the bottom and the mantle of the container, which comprises (i) a fibrous board base, (ii) an extruded inner coating layer of a blend of polyethylene (PE) and polypropylene (PP), and (iii) an extruded outer coating layer of a polypropylene-olefin copolymer (PP-c). The inner coating layer is located between the fibrous board base and the outer coating layer.
Tests carried out by the inventors have demonstrated that the coextruded double-layer coating heat-seals considerably better than the monolayer coating of a blend of PE and PP according to the prior art. Commercial isotactic PP homopolymer, as well as the PE+PP blend WG341C, have melting points of about 160° C., whereas copolymerization of propylene with ethylene lowers the melting point drastically, to about 130° C. by a share of mere 4% of ethylene monomer. The lower melting point as such indicates improved heat-sealability. At the same time the melting point is still higher than that of LDPE and sufficient for microwave-ovenable products.
Propylene homopolymers and copolymers as such are ill-suited for extrusion coating onto fibrous substrates such as paperboard or cardboard, which is due to poor adhesion to fibrous materials. In fact PP homopolymers and copolymers have been developed for film extrusion, and usually the resulting films are biaxially oriented and used e.g. for the manufacture of plastic bags.
The solution according to the invention, however, is to use the blend of PE and PP as a coextruded inner tie layer, which provides good adhesion to the fibrous substrate and at the same time binds the coextruded PP-c layer as an outer heat-seal layer to the double-layer structure. Successful testing of the invention has been carried out with paperboard provided with an extruded inner coating layer of a blend of PE and PP and a coextruded outer coating layer of a film grade propylene alpha olefin copolymer. Surprisingly the copolymer, developed for film extrusion and free from pure PE, turned out to have good runnability in extrusion coating as well. Furthermore, in extrusion coating the temperature is markedly higher than in standard film extrusion and, without limiting the invention to this theory, it is supposed that slight oxidation of PP-c due to this high extrusion temperature is conducive to the improved heat-sealability.
Preferably the polypropylene (PP) in the inner coating layer is propylene homopolymer. The polymer blend of the inner layer may contain about 20 to 50 wt-% of PE, which is preferably LDPE, and about 50 to 80 wt-% of PP.
The weight of the inner coating layer may be 5 to 15 g/m2, preferably about 10 g/m2, and the weight of the outer coating layer may be 5 to 15 g/m2, preferably about 10 g/m2.
Even the outer surface of the container may be provided with at least one heat-sealable coating layer of PP-c or a blend of PE and PP. Hereby the inner and outer surfaces of the container can be similarly coated with double layers of polymer.
The fibrous board base may be paperboard with a weight of 170-250 g/m2, which is particularly suitable for drinking cup sized and shaped containers. However, the board base may even be cardboard of a weight of about 250-500 g/m2, which suits for larger and stiffer food trays.
The containers according to the invention thus include microwave-ovenable food trays. Another useful embodiment of the invention is a disposable drinking cup, which stands heating in a microwave oven.
The invention even covers use of a board comprising a fibrous board base, an extruded inner coating layer of a blend of polyethylene (PE) and polypropylene (PP), and an extruded outer coating layer of propylene copolymer (PP-c) for the manufacture of heat-sealed disposable drinking cups or heat-sealed microwave-ovenable food trays.
Reference is made to the enclosed drawing, in which
The mantle 2 and the bottom 3 of the cup have a coating of polymer at least on the inner surfaces of the cup 1. The coating is liquid-tight, shielding the board base against wetting and enabling liquid-proof non-leaking sealing of the parts 2, 3. Preferably the drinking cup 1 has a polymer coating also on its outer surfaces.
As seen in
The heat-sealed joint between the bottom 3 and the mantle 2 of the cup is shown in an enlarged scale in
In
The cup 1 according to the invention as depicted in
Paperboard was extrusion coated by coextrusion of an inner layer of 10 g/m2 of WG341C (a blend of PP and PE) and an outer layer of 10 g/m2 of TD109CF (a propylene alfa olefin copolymer by Borealis AG developed for the manufacture of unoriented films in particular and having a reported melting point of 128-132° C.). As a reference representing the prior art, the same paperboard was extrusion coated by a monolayer of 20 g/m2 of WG341C. The coated materials were turned to cups of the type shown in
For the double-layer coated material according to the invention a heat-sealing temperature of 400° C. was needed to achieve non-leaking heat-seal between the bottom and the mantle of the cup. For the monolayer coated reference material a heat-sealing temperature of 490° C. was required for a seal, which was still of inferior quality.
A corresponding pair of coated materials was produced, in which both sides of the fibrous board base were identically coated, and the materials were turned to cups by heat-sealing. The required heat-sealing temperatures for a seal between the bottom and the mantle of the cup were 430° C. (invention) and 530° C. (prior art), respectively.
The heat-sealing temperatures are close to those needed for heat-sealing of conventional LDPE-coated cups, which means that advantageously the same sealing apparatus can be applied for cups according to the invention as for said conventional cups.
In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1650597-6 | May 2016 | SE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/052434 | 4/27/2017 | WO | 00 |
