Plastic films

Details Plastic films Plastic films

2002-04-02

2004-02-24

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of addition polymer from unsaturated monomers

C428S516000, C428S910000, C526S348100

Reexamination Certificate

active

06696166

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to plastic films prepared from pseudohomogeneous linear low density polyethylene resin in a blown film process.
BACKGROUND OF THE INVENTION
The extrusion-blown film process is a well known process for the preparation of plastic film. The process employs an extruder which heats, melts and conveys the molten plastic and forces it through an annular die.
The polyethylene film is drawn from the die and formed into a tube shape and eventually passed through a pair of draw or nip rollers. Internal compressed air is then introduced from the mandrel causing the tube to increase in diameter forming a “bubble” of the desired size. Thus, the blown film is stretched in two directions, namely in the axial direction (by the use of forced air which “blows out” the diameter of the bubble) and in the lengthwise direction of the bubble (by the action of a winding element which pulls the bubble through the machinery). External air is also introduced around the bubble circumference to cool the melt as it exits the die. Film width is varied by introducing more or less internal air into the bubble thus increasing or decreasing the bubble size. Film thickness is controlled primarily by increasing or decreasing the speed of the draw roll or nip roll to control the draw-down rate.
The bubble is then collapsed into two doubled layers of film immediately after passing through the draw or nip rolls. The cooled film can then be processed further by cutting or sealing to produce a variety of consumer products. Whilst not wishing to be bound by theory, it is generally believed by those skilled in the art of manufacturing blown films that the physical properties of the finished films are influenced by both the molecular structure of the lldpe and by the processing conditions. For example, the processing conditions are thought to influence the degree of molecular orientation (in both the machine direction and the axial or cross direction).
A balance of cross and machine direction molecular orientation is generally considered most desirable for key toughness properties associated with the invention (i.e. Dart Impact strength and Machine Direction or “Elmendorf” tear properties).
Thus, it is recognized that these stretching forces on the “bubble” have an affect on the physical properties of the finished film. In particular, it is known that the “blow up ratio” (i.e. the ratio of the diameter of the blown bubble to the diameter of the annular die) can have a significant effect upon the dart impact strength and tear strength of the finished film.
The above description relates to the preparation of monolayer films. Multilayer films may be prepared by 1) a “co-extrusion” process that allows more than one stream of molten polymer to be introduced to an annular die resulting in a multi-layered film membrane or 2) a lamination process in which film layers are laminated together.
It is also well known that the use of different types of plastic can alter the properties of the finished film. This is true even when different grades of the same plastic family are used to prepare the resin. For example, linear low density polyethylene (“lldpe”) is widely used to prepare plastic film—and it is known that “homogeneous” lldpe typically produces finished films which have significantly different properties than those obtained with “heterogeneous” lldpe. More particularly, it is now well known that a “homogeneous” lldpe (which lldpe may be produced in the manner disclosed by Elston in U.S. Pat. No. 3,645,992, or by the use of a metallocene catalyst) will typically produce a plastic film with excellent “dart impact strength” in comparison to a plastic film which is manufactured with a “heterogeneous” lldpe (which lldpe may be produced with a conventional “Ziegler-Natta” catalyst). However, it is also known that the “heterogeneous” lldpe will normally provide films with much superior “machine direction tear strength” than films made from homogeneous resins.
It is clearly desirable to produce film having a combination of good impact strength and good machine direction tear strength. One way of doing this is to use a blend of “homogeneous” lldpe and “heterogeneous” lldpe, as disclosed in U.S. Pat. No. 6,111,023 (Chum et al., to Dow). Films produced from such blends do have a very good balance of dart impact strength and tear strength properties. However, the films are comparatively expensive to produce due to the high cost of the homogeneous resin and due to the need to carefully prepare the blends of resins. Thus, for reasons of reduced cost and increased convenience, it would be preferable to produce films having a balance of enhanced impact strength and enhanced tear strength using a single type of lldpe resin.
SUMMARY OF THE INVENTION
This invention relates to a monolayer thermoplastic film having a dart impact strength of greater than 600 grams per mil as determined by ASTM D1709 and a machine direction tear strength of greater than 350 grams per mil wherein said film is prepared by a blown film extrusion process using a linear low density polyethylene resin having 1) a melt index as determined by ASTM D1238, condition I
2
, of from 0.4 to 0.8 g/10 min; 2) a base resin density of from 0.915 to 0.919 grams per cubic centimeter; a COHO ratio of from 3 to 9; and wherein said blown film extrusion process uses an annular extrusion ring having a die gap from 35 to 120 mils and a blow up ratio ranging from 2 to 4.
The plastic film of this invention is suitable for use in a monolayer in such applications as trash bags and packaging. It is also within the scope of this invention to prepare multilayer films using the film of this invention as one of the layers.
DETAILED DESCRIPTION
The film of this invention has a highly desirable combination of excellent dart impact strength (greater than 600 grams per mil, as determined by ASTM D1709) and excellent machine direction tear strength (greater than 350 grams per mil, as determined by ASTM D9922). Applicants are unaware of any prior monolayer lldpe film having this highly desirable balance of properties.
The film is produced by an extrusion-blown process. The extrusion-blown process, and the machinery used in the process, are well known to those skilled in the art and is widely described in the literature. However, the film of this invention must be produced according to the following conditions:
1) the die gap of the annular die used to extrude the molten bubble must be from 35 to 120 mils; and
2) the blow up ratio must be from 2 to 4.
The catalyst used to prepare the lldpe resin of this invention is a high efficiency magnesium titanium type of Ziegler Natta catalyst. The catalyst must contain a titanium compound. Exemplary titanium compounds include titanium halides (especially titanium chlorides, of which TiCl
4
is preferred); titanium alkyls; titanium alkoxides (which may be prepared by reacting a titanium alkyl with an alcohol) and “mixed ligand” compounds (i.e. compounds which contain more than one of the above described halide, alkyl and alkoxide ligands.
The catalyst must also contain an organoaluminum component which is defined by the formula:
Al(X′)
z
(OR)
b
(R)
c
wherein: X′ is a halide (preferably chlorine); OR is an alkoxy or aryloxy group; R is a hydrocarbyl (preferably an alkyl having from 1 to 10 carbon atoms); and a, b, or c are each 0, 1, 2 or 3 with the provisos text a+b+c=3 and b+c≧1.
The catalyst must also contain a magnesium halide. This may be done by directly adding the component (such as MgCl
2
) or by adding a magnesium alkyl (such as butyl ethyl magnesium) and a halide source (which is typically a chloride such as tertiary butyl chloride).
The Ziegler Natta catalyst may also be “tempered” (i.e. heat treated) prior to being introduced to the reactor (again, using techniques which are well known to those skilled in the art and published in the literature). Particularly preferred Ziegler Natta catalysts and methods of preparing them are described in U.S. Pat. Nos. 5,492,876; 5,519,098

Affiliated with

Also associated with

Rating

Plastic films does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Plastic films, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Plastic films will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3351670