Barrier stretch film particularly for the packaging of food products and method for the production thereof
A stretch film presenting controlled permeability to gases, manufactured by process of balloon co-extrusion and consisting of outer layers (S 2 ) of stretchable polmeric material and by one or two inner barrier layers (S 3 ) of a EVOH based polymeric matrix, which is made stretchable by the addition of a suitable plasticizer in such a percentage as to conciliate the stretchablity with the desired barrier effect, said layers of different material being reciprocally secured by means of layers (S 4 ) of adhesive material. The polymeric matrix which forms the barrier material and the plasticizer are intimately mixed inside a double-screw corotating extruder, immediately before the phase of co-extrusion, through a premelting of the polymeric matrix and the subsequent injection of the plasticizer in liquid phase.
1. A process for the manufacture of a multi-layer film which is stretchable and presents a controlled permeability to gases, said process comprising:
mixing barrier material comprising an ethylene vinyl alcohol copolymer and plasticizer material inside a double-screw co-rotating extruder immediately before co-extrusion, said mixing comprising pre-melting a polymeric matrix of the barrier material and subsequent injecting the plasticizer material in a liquid phase to the pre-melted polymeric matrix to thereby form plasticized barrier material in a liquid phase to the pre-melted polymeric matrix to thereby form plasticized barrier material, the polymeric matrix having a profile of a plasticization temperature which varies as a function of the percentage of added plasticizer, injecting of the plasticizer occurs at temperature levels which are lower than the relative evaporation temperature of the plasticizer, and if the EVOH polymeric matrix is plasticized at 10% with propylene glycol, the pre-melting of the matrix starts at about 210° C., and the plasticizer is injected at about 180° C.;
balloon co-extruding a polymeric stretch material, the plasticized barrier material and an adhesive material fed by respective extruders, said material being arranged so as to form layers of an outer annular layer and an inner annular layer of polymeric stretch material, an intermediate annular layer of plasticized barrier material and annular layers of adhesive material interposed between the preceding layers;
squeezing and longitudinal stretching of the balloon by means of a first and of a second pair of parallel and superposed rollers; and
cutting and discarding the two longitudinal edges which reciprocally connect the superposed portions of the balloon so as to obtain two films which can be separated, this being performed on the film while under tension, between said two pairs of rollers to form two five-layer films; and
separately collecting the two five-layer films obtained;
wherein said longitudinal stretching, longitudinal cutting and collecting of the film is obtained on a structure capable of alternately rotating of about 360° around the axis of the balloon.
2. A process for the manufacture of a multi-layer film which is stretchable and presents a controlled permeability to gases, according to claim 1 , said process comprising:
mixing barrier material and plasticizer material inside a double-screw co-rotating extruder immediately before co-extrusion, said mixing comprising pre-melting a polymeric matrix of the barrier material and subsequent injecting the plasticizer material in a liquid phase to the pre-melted polymeric matrix to thereby form plasticized barrier material, the polymeric matrix having a profile of a plasticization temperature which varies as a function of the percentage of added plasticizer, injecting of the plasticizer occurs at temperature levels which are lower than the relative evaporation temperature of the plasticizer, and if the EVOH polymeric matrix is plasticized at 10% with propylene glycol, the pre-melting of the matrix starts at about 210° C., and the plasticizer is injected at about 180° C.;
balloon co-extruding the polymeric stretch material, a plasticized barrier material and an adhesive material fed by respective extruders, said layers being arranged so as to form layers of an outer annular layer of polymeric stretch material, an intermediate annular layer of plasticized barrier material and two annular layers of adhesive material the first one internal and the other one interposed between the two preceding layers, thereby forming a balloon having four layers;
squeezing and longitudinal stretching of the balloon by means of a first and of a second pair of parallel and superposed rollers, so as to obtain a seven-layer film;
wherein the respective internal adhesive layers of the balloon come together to form a single adhesive layer; cutting and discarding of the two longitudinal edges of the obtained seven-layer film, this being performed on the film while under tension between the said two pairs of rollers; and
collecting the seven-layer film;
wherein said longitudinal stretching, of longitudinal cutting and collecting of the film is obtained on a structure capable of alternately rotating of about 360° around the axis of the balloon.