Method for the manufacture of multilayer film

Details Method for the manufacture of multilayer film Method for the manufacture of multilayer film

1998-06-12

2001-03-06

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Extrusion molding

C264S210100, C264S331150, C264S210700, C264S480000, C264S488000, C264S495000

Reexamination Certificate

active

06197244

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a multilayer, bioriented, heat shrinkable film, a method for the manufacture thereof and use thereof for packaging food products and consumer articles. More particularly, the present invention relates to a multilayer, bioriented, heat shrinkable film extruded from a flat die and stretched both in machine direction and in cross-direction wherein at least one layer comprises at least one ethylene/alpha-olefin C
4
-C
12
copolymer.
BACKGROUND OF THE INVENTION
Multilayer heat shrinkable films have been known for a long time.
U.S. Pat. No. 4,532,189 (W.R. Grace & Co.) discloses a multilayer, heat shrinkable film comprising:
(A) a core (central) layer comprising a linear low density polyethylene or a linear medium density polyethylene;
(B) two skin (external) layers comprising a blend of from 70% to 90%, by weight, of an ethylene/propylene copolymer and from 10% to 30%, by weight, of a propylene homopolymer;
wherein said film has an average machine direction free shrink at 200° F. of at least 12% and an average cross-direction free shrink at 200° F. of at least 17%.
The core layer may also comprise other polymers such as, for example, ethylene/propylene copolymers, ethylene/vinyl acetate copolymers, ionomer resins and non-linear low denisity poly ethylenes.
Moreover, said film may also comprise two intermediate layers comprising a blend of approximately 90% by weight of an ethylene/vinyl acetate copolymer and approximately 10% of an ionomer resin.
U.S. Pat. No. 4,551,380 (W.R. Grace & Co.) discloses a multilayer heat shrinkable film comprising:
(A) a cross-linked core layer consisting essentially of a linear low density polyethylene; and
(B) two surface layers comprising essentially a blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene and (3) an ethylene/vinyl acetate copolymer.
GB-A-2,097,324 discloses heat shrinkable films manufactured by stretching, at least three times their original linear dimension in at least one direction, a film having the following homogeneous composition:
(A) 5-100%, by weight, of at least one linear copolymer of ethylene with at least one C
8
-C
18
alpha-olefin, said copolymer having the following characteristics:
(a) melt index of 0.1-4.0 g/10 min.;
(b) density of 0.900-0.940 g/cm
3
;
(c) stress exponent above 1.3; and
(d) two distinct crystallite melting regions below 128° C. as determined by differential scanning calorimetry (DSC), the temperature difference between said regions being at least 15° C.; and
(B) 0-95%, by weight, of at least one polymer selected from the group consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomers, said polymer having only one crystallite melting point below 128° C.;
with the proviso that stretching is carried out within the temperature range defined by the two melting points of the crystallites of the ethylene/alpha-olefin copolymer of the above paragraph (A).
These films are manufactured by the well-known air bubble technique. An example of method and equipment of this technique is disclosed by U.S. Pat. No. 4,841,605.
However, the films obtained with this technique have the disadvantage of not having sufficiently uniform thickness and planarity. In fact the total thickness variation in said films is ±15% while, as regards planarity, it has defects consisting of deviations from a straight line (snaking) and sags. More particularly the average deviations from a straight line (snaking) is approximately 50 mm, whereas the average sag is approximately 35 mm.
In addition to the air bubble technique, stretching the films also by the so-called “tenter frame” technique is known. Examples of machines suitable for implementing this technique are disclosed by U.S. Pat. Nos. 3,148,409 and 3,201,826.
The tenter frame technique, also known as “flat orientation technology”, consists of extruding a film-forming material through a flat die over a chill roll, preferably immersed in water, to chill the molten film.
In the case of multilayer films the various polymers or blends of polymers are generally coextruded by conventional techniques but, when only a few of the layers have to undergo special treatments, such as for example irradiation with fast electrons to induce cross-linking, only the layer or layers to be treated is extruded or are coextruded, the tape obtained in this way is subjected to the required treatment and then the remaining layers are extruded on the same.
The tape is then oriented, by stretching, in two separate and successive steps, although devices able to stretch the tape simultaneously in both directions are known (U.S. Pat. No. 3,148,409).
Generally stretching is performed first in machine direction (MD) and then in cross-direction (TD).
MD stretching is usually carried out by passing the tape through pairs of rolls which rotate at different speeds. At least one of the first pairs of rolls is heated, for example by inner circulation of hot oil.
TD stretching is usually performed in a tenter frame oven which comprises a certain number of heating zones and suitable stretching means.
Typically a tenter frame oven comprises from three to six zones: one to two for preheating the tape, one to two for stretching it in the cross-direction and one to two for relaxing and winding the film. Each zone may be heated at a different temperature level.
This technique has not however been adopted in the manufacture of heat shrinkable films based on ethylene copolymers.
Only EP-A-405 916 discloses the utilization of this technique in the manufacture of an extruded, bioriented, mono or multilayer film, wherein the film-forming polymers of at least one layer consist of:
(A) 75-100%, by weight, of at least one linear ethylene/alpha-olefin copolymer having a density of between 0.890 g/cm
3
and 0.930 g/cm
3
, and
(B) 25-0%, by weight, of a linear high density polyethylene having a density of between 0.935 and 0.960 g/cm
3
, with the proviso that the total of the film-forming polymer (A) and (B) has a single melting point as determined by differential scanning calorimetry according to ASTM D-3417.
Moreover, EP-A-405 916, page 3, lines 18-21, reports that attempts made to apply the tenter frame technique to the polymers of GB-A-2 097 324 have not given satisfactory results because the films obtained in this way were highly sensitive to minimal variations of the process parameters, such as stretching temperature, stretching ratio and the speed of the manufacturing line.
Although EP-A-405 916 also refers to multilayer films, its examples only refer to monolayer films.
On the other hand, multilayer films have, compared to monolayer ones, the considerable advantage of allowing to combine one with the other several layers having different physical and chemical properties in view of the required properties of the final film.
Therefore the need for heat shrinkable multilayer films, extruded from a flat die, bioriented by the tenter frame technique, wherein at least one layer comprises ethylene copolymers, is still greatly felt.
In fact, the films manufactured by this technique have several advantages over those manufactured by the air bubble technique.
A first advantage consists of the fact that the stretching ratios in machine direction and cross-direction may vary as required whereas in the air bubble technique they are always substantially equal one to the other.
A second advantage is that the stretching ratio may be preselected within a relatively wide range, typically of betweeen 2:1 and 12:1, while in the air bubble technique it must be between 3:1 and 6:1.
A third advantage is that the sealing agents can be selected as required whereas in the air bubble technique they have to be selected in the restricted range of sealants whose softening point is not substantially lower than the stretching temperature to prevent the sealant from softening during the heating step, with consequent sealing of the opposite walls of the bubble.
A further advantage consists of the fact that the thickness

Affiliated with

Also associated with

Rating

Method for the manufacture of multilayer film does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method for the manufacture of multilayer film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the manufacture of multilayer film will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2464550