FI75526B - VAERMEFOERSEGLINGSBAR FLERSKIKTAD POLYPROPYLENFILM. - Google Patents

Description

i 75526

This invention relates to thermoplastic, heat-shrinkable packaging films, especially polypropylene films, with improved heat-sealing properties.

These films can be used to package a wide variety of food and non-food items.

Oriented polypropylene films are useful and well-recognized packaging films due to their good moisture barrier, toughness, strength, and optical properties. However, polypropylene films do not usually have good heat sealing properties, which is important in the case of packaging.

This is because polypropylene films have a very narrow sealing temperature range.

The limits of the temperature of the sealing temperature of the thermoplastic film can be considered to start from the point where - after the surfaces of the film have been heated and compressed - they begin to seal or glue together. When the heat is further added, a point is reached at which the thermoplastic film melts and drains very easily, thus making sealing in a certain area difficult and thus marking the upper limit of the sealing temperature range. At the upper limit of the film sealing temperature range, the film may also be prone to burning and charring and the seams will become weak and ugly. In the case of propylene homopolymer films, the film cannot be sealed as the temperature rises, but rather begins to drain quite abruptly as it approaches its melting point, and it is quite impossible to obtain smooth, continuous, proper seams. Such sealing properties are not acceptable when using high speed packaging machines.

In order to achieve good heat sealing, it has been customary to apply different types of coatings to the films. Coatings are usually applied as separate coating steps, such as emulsions, extrusions, etc., but such steps are expensive and require further film treatment. Coextrusion has also been used successfully; it coats the polypropylene with a heat-sealable layer to obtain a strong-sealed film, but until this invention, coextruded polypropylene film laminates have had a very narrow heat-sealing temperature range and have become expensive to manufacture. Accordingly, it is a primary object of the present invention to provide a polypropylene film having a wide sealing temperature range.

Another object of the present invention is to provide a polypropylene film having good sealing strength for use in shrinkable packages.

Typical patents disclosing polypropylene film coatings include U.S. Patent 3,285,766, which discloses a method of extruding a coating containing at least 65% ethylene onto a polypropylene sheet; U.S. Patent 3,671,383 discloses a biaxially oriented polypropylene film laminated over a uniaxially oriented ethylene-propylene copolymer film having at least 75% by weight polypropylene; U.S. Patent 4,132,050, which discloses a film having a substrate formed from a blend of 87.5 to 60 parts polypropylene and 12.5 to 40 parts ethylene-propylene block copolymer having a heat seal layer; and U.S. Patent 4,148,972, which discloses a polypropylene film laminate made by laminating a polypropylene layer having 1 to 8 weight percent ionomer with an ionomer layer.

3,75526

It has surprisingly been found that the main objects of the present invention, namely increasing the sealing temperature range of a multilayer polypropylene film and providing a suitably strong seal of such a film, can be realized by a multilayer film comprising a propylene homopolymer base layer and a top layer attached to a second layer. the surface layer is a blend of 60-80% copolymer and 40-20% homopolymer. In a particularly preferred embodiment, the surface layer comprises 70% by weight of propylene-ethylene copolymer mixed with 30% by weight of propylene homopolymer and the copolymer contains 2-6% by weight of ethylene, or more preferably 3-4% by weight of ethylene. In addition, the ratio of the thickness of the surface layer to the thickness of the base layer is primarily 4: 1 or more.

The multilayer film of the invention is prepared by a method comprising the steps of: preparing a base layer of a propylene homopolymer and laminating a surface layer to at least one surface of a propylene homopolymer film, the surface layer being a mixture of 60-80% by weight propylene-ethylene copolymer with 2-6% by weight ethylene; and stretching the biaxially laminated film to a final film thickness of 6.35 to 63.5 x 10 mm.

o The sealing temperature range of such a film is at least 20 ° C and its seal strength is greater than 120 g / cm. The preferred method of making a laminated film is to co-extrude the top layer and the bottom layer.

The package formed from the film of the invention is prepared by a method comprising the steps of: preparing a base layer of a propylene homopolymer film, laminating a surface layer to at least one surface of said propylene homopolymer film, said surface layer being a 60-80% by weight propylene-ethylene copolymer weight of ethylene? biaxially stretching 4,75526 laminated film to a final film thickness of -3

6.35-63.5 x 10 mm and in this way a heat-sealable, heat-shrinkable film is obtained; procuring the product to be packaged; wrapping the product inside the film and bringing the surfaces of the surface layer into contact with each other at the points to be heat sealed; heating and pressing the seams to seal said film at said points, and the temperature range used for sealing is 135 ° C to 160 ° C, and the heat seals of the resulting package have a strength of at least 120 g / cm 3; and heating said package at a temperature in the range of 110-135 ° C for a time sufficient for said film to shrink around said product.

According to all aspects of the present invention, the film can be perforated.

Preferred embodiment

The preferred embodiment of the present invention is a two-layer film made by mixing 70% by weight of a propylene-ethylene copolymer with 3.5-4.2% by weight of ethylene and 30% by weight of a propylene homopolymer for the surface layer. This mixture is mixed in a Banbury mixer and then fed to an extruder feed hopper which feeds it into an annular nozzle from which the double layer tube is extruded. The inner or bottom layer is a propylene homopolymer fed to the hopper of the second extruder device, which also feeds it to the annular nozzle to form the inner layer.

In an annular nozzle, the two extruded tubes merge to form a multilayer tubular laminate. This tubular laminate is rapidly cooled and then compressed. The tube is then filled with gas and heated to the directional heat of the polypropylene. o in the range 135-150 C. The heating takes place in the oven and when the 5 75526 flattened film emerges from the oven, it passes through the friction rollers and opens into a bubble according to the well-known air bubble technique where it is stretched about five times both vertically and horizontally -3 film thickness between 15-20 x 10 mm. Prior to stretching, the thickness of a typical tube is about 0.46 mm and the ratio of the thicknesses of the polypropylene layer to the blend layer is 5: 1. U.S. Patent 3,260,776 discloses one such bubble technique for biaxially directing polypropylene.

After the film is stretched into a bubble and the bubble is cooled, it is compressed, split and wrapped in a roll.

In some cases, it is preferable to perforate the film so that a series of holes or small openings are made in the film. Depending on the intended use, the number of holes in the film 2 2 may be 1-2 / mm - several tens of holes / mm and the number of holes -3.

the size can be 0.08-0.8 x 10 mm in diameter. A typical film perforating device uses a cooled lattice over which the film is conveyed while being exposed to a hot gas jet that melts the film at those points that are not cooled. Such an apparatus and method are described in U.S. Patent 3,038,198.

To determine the sealing temperature range and seal strength of the preferred film, two film sheets were placed on top of each other so that the surfaces of the surface layers come into contact. A 5 mm plate electrically heated sealing rod was used to press the plates against the support surface, 2 and the pressure was 1 kg / cm for 0.5 second and the temperature of the sealing rod was recorded. After sealing, a 2.54 cm strip was cut perpendicularly and transversely from the seam made with a rod, and the ends of the strips 6 75526 were placed on the pliers of the Instron test machine so that the seam was approximately midway between the holding pliers. Force was used to separate the pliers from each other until the seam either came off or broke. Seams with a minimum strength of about 120 g / cm - this value was considered sufficient for use in packaging - were obtained according to the preferred embodiment at the temperature of the sealing rod. o ...

as low as 135 C and the seams were consistently good o when the temperature was raised to about 160 C, the sealing temperature with a temperature range of about 25 C. (In the following field tests it has been found that the working interval is 30 C.) The maximum joint strength of approx. 550 g / cm.

As mentioned above, the single layer polypropylene film has a virtually non-existent sealing temperature range. Because the sealing rods cool when in contact with the film, packaging machines usually have to adjust the temperature of the sealing rods somewhat higher than the melting point to provide a sealed package, but the seams made in this way do not look good and have burnt areas. When using the film of the present invention and adjusting the sealing rods and temperatures close to the upper limit of the sealing temperature range, there is no risk of the film melting or burning, and at high speeds where the film tends to cool the sealing rods, the sealing rods still have sufficient heat to seal the film. below the sealing temperature range.

Several different products can be packaged in the film of the present invention, and different machines use different techniques. In some cases, the product is placed on one plate of the film and then another plate is placed on top of the product and then a complete seam is made around it; and in some cases, in the sealing step, the extra film is also torn to clean the edges of the package. After sealing, the product passes through a hot tunnel, which may have a temperature as low as 110 ° C, but is usually in the temperature range where the film is stretched. In the tunnel, the heat causes the tensile stresses to be triggered so that the film shrinks around the product. In some other packaging processes, the product can be placed inside a centrally bent film and then seamed on three seamless sides. Many methods are known to those skilled in the art for wrapping a product within a shrinkable film.

Products such as bread, sweet pastries, rolls, pizzas, etc. can be packaged using the film of the invention. Foodstuffs that must be allowed to breathe through the film, such as fruits and vegetables, can also be packaged using a perforated film.

eXAMPLES

Since polypropylene11a has virtually no heat sealing temperature range, several blend layer material mixtures were tried. Table I lists the bilayer films with a base or substrate layer of polypropylene homopolymer coextruded with a specially designed sealing layer mixture. Only those mixtures of materials that could be successfully processed into a film by air bubble technology are shown in Table I.

The footnotes immediately below the table explain the abbreviations for the polymers and the sealing temperature range and joint strength are determined as described above.

The veil, of course, means the percentage of transmitted light which, after passing through the sample, deviates from the incident radius by more than 2.5 ° on average and is determined by ASTM method D 1003. The stated thickness dimensions refer to the entire film laminate.

9 75526 <#>, _

I — I O

3 O i-h ro co σι m σι rH ro Γ'- ο • 'ί' 4J ******* *****: ra

BHrHrHfNtNiHrH Ή O O fN (N -H

SH

K I

a .2 S3 2 ij Ά ί p A v E 3 0 r- m 00 00 00 Γ 'γη ή ο ή <n 3 p

• H P \ m o a \ H rl n IT h 1/1 rl 1/1 h -P * H

B) g ¢ 1 rH Γ0 rH rH rH rH rH "fl · U0 VO m Tf g tH

3 (Ο Ή> 1 g “at s S 2 I rd C ^ r- 9-8l I t ~ 5

p -ro 3 H i>: <tJ Q

E <H P -H C

P P d Φ _

ID H ID D £ Λ -Η φ O

-H p> 0 in o m m m m O O O m Q in> -h _ -rl.p-ijLx, 1 m in in m m m m Λ ^ 0 0 nJ> 1 p

P; 0 · Η 3 ÖP H rH (—I 1 — I H I — I r-Η rH i — I Η i — I h + J W

h-h & oM p Φ ω O i f 1 ^ D dO to i 3 ^ - 1 * -5 ti P r- * 2 2¾

Eh O £ Ö 8 I cm m ^ g P I Ή Ρ * Ϊ

I H u m Ή · Η H

3 - & L · 3 d 3 & a

(d? Q H lO Q _ ^ Φ Q

§ jbj g m O m m m m Ο O oh * £> 5 | f «: s £ s: U22 1 153" H js 8j

si a! J

pro co o m O m ro ro omoomco Q jdQ> 1

in I ·. «» S ·. ^ v 1, ^ k s ^ c * H -W

i! η r- 00 r- 'ο σ' O 00 cooocooor '· o d φ * G

P rH r — I r — I rH C \) 1 — I CN f — I rH rH rH rH rH X! w I »“ Η · Η (¾ - \ ΗΦ · ΗΟ>

Z2W CH dw I

L & R *

Wing Ö O Ö 4J

18 8 rtJOOmOOOO 'Q Q O <#> II II II II II

PM H (N Μ in H f \ | Ί1 in vE n- _

MS I

rHcnoor ^ intTiooLn in <»n h J

H k <I &! S ci K ^ • h 1 — irgro ^ inior ^ oo σι Ο ή in

W * rl rH rH rH

W M

10 75526

Examples 3, 10 and 11 are presented in series because these samples were taken with real equipment under working conditions and their sealing temperature ranges were measured both in the laboratory and under working conditions, and an increase of about 5 ° C in the sealing temperature range was observed so that the highest sealing temperature rose to about 165 ° C. These three samples were performed on perforated films packed with rolls and crispy bread. Each of these films was very suitable for machining, i. they were tough enough to survive in standard packaging equipment.

The film of Example 11 also proved to be good under working conditions when packing puffy rolls and pizzas in a non-perforated film.

It is also clear from Table I that Examples 10, 11 and 12 have advantageously wide sealing temperature ranges and each of these films is very suitable for machining and the veil is not disturbing. But if the mixture contains more than 80% propylene-ethylene copolymer, processing difficulties arise in the air bubble step because the sealing layer tends to flow and warp at the high temperature necessary for stretching and orienting the polypropylene base layer. That is, the propylene-ethylene copolymer begins to melt at the orientation temperature of the propylene homopolymer, and the sealing layer on the outer surface of the tube tends to adhere to the friction rolls after being heated but not yet blown into a bubble. Thus, the preferred content of propylene-ethylene copolymer in the sealing layer is 60-80% by weight, and a sealing temperature range ranging from 20 to 30 ° C is obtained. Example 11 has the best combination in terms of sealing temperature range, sealing strength, ease of machining with prior art manufacturing equipment and suitability for machining. With the commercial packing machine 11,75526, 50-60 packs per minute can be made and the indications are that these speeds can be successfully exceeded.

By coextruding the base layer and the top layer as a smooth sheet from the die slot and rapidly cooling the sheet and using a tension frame to stretch the smooth sheet when heated to the orientation temperature, a continuous film with more than 80% surface layer mixture can be produced, but edge loss and other factors make crimping. In any case, it should be remembered that the increase in the heat sealing temperature range clearly starts when the surface layer mixture contains at least 50% copolymer and continues up to 100% copolymer.

The propylene homopolymer used in the present invention is predominantly isotactic polypropylene for use in films, available from a number of well known resin suppliers. Likewise, the propylene-ethylene copolymer is any copolymer used in the films, which is also available from well-known resin producers, and the weight content of ethylene is about 2-6%, which is completely sufficient. When the copolymer has a higher percentage of ethylene, it is expected that the proportion of copolymer in the blend can be increased.

The thicknesses of the layers of the multilayer films of this invention are such that they provide the optimum combination in terms of machine handling suitability, sealing strength, and ease of handling. If the thickness of the sealing layer increases much more than 4: 1, processing difficulties appear, as described above in connection with the orientation temperature of polypropylene. Therefore, the ratio of the base layer to the sealing layer 12 75526 should be greater than 4: 1 and, as noted above, a 5: 1 ratio gives a completely satisfactory result. In addition, a total film thickness of the order of -3 6.35-63.5 x 10 mm provides the most satisfactory combination of typical physical properties of the film.

Also within the scope of this patent are three-layer films in which the middle layer is a propylene homopolymer layer that is oriented, and the sealing layers are on both sides of the oriented homopolymer layer for balance. This is to prevent distortion, which can happen in structurally unbalanced multilayer films.

Claims (6)

13 75526 A heat-sealable multilayer film for packaging, characterized in that it has a) a base layer containing a propylene homopolymer and b) a surface layer attached to one surface of the base layer, consisting of a mixture of 60-80% by weight of film-grade thermoplastic propylene-ethylene copolymer and 40-20% by weight. % propylene homopolymer, wherein the ratio of the thickness of the base layer to the surface layer is at least 4: 1. Heat-sealable multilayer film according to Claim 1, characterized in that the film is heat-shrinkable and in that the copolymer contains 2 to 6% by weight of ethylene. Heat-sealable multilayer film according to Claim 1, characterized in that the sealing temperature range of the film is at least 20 ° C. Heat-sealable multilayer film according to Claim 1, characterized in that the film is perforated. Heat-sealable multilayer film according to Claim 1, characterized in that the surface layer is attached to both sides of the base layer in order to structurally balance the film. Heat-sealable multilayer film according to Claim 1, characterized in that the film has a joint strength of at least 120 g / cm 3 at a temperature as low as 135 ° C.