Oriented polyolefin processing

Abstract

The present invention relates to improved processability of polyolefin films through the addition to the basic isotactic polypropylene (iPP) polymer of a resin or rosin modifier in an amount within the range of 1 to 30 weight percent. Alternatively, the composition could contain from about 5 to about 10 weight percent of a resin or rosin. Preferably the mixture should contain resin or rosin modifiers in an amount of about 10 percent by weight. The invention encompasses both the resulting polyolefin films and the process for producing such film. The improved processability of the film includes fewer web breaks and drawability at higher line speeds, resulting in fewer shut downs of the processing line and a decrease in manufacturing time. These improvements are measured in terms of variable machine direction orientation draw ratios and transverse direction orientation oven temperatures.

Description

[0001] This invention relates to improved processability for the production of oriented polyolefin films, and more particularly, to polypropylene compositions which incorporate styrenic hydrocarbon resins.[0002] Biaxial orientation of polyolefins, particularly polypropylenes, produces films which have applications in the polyolefin film business, for example snack food packaging, cigarette overwrap, electronic components wrapping, packaging tape, and shrink film. The polymers normally employed in the preparation of biaxially oriented films are isotactic homopolymers with high stereoregularity, although on some occasions the use of syndiotactic polymers has been proposed. Also suitable are co-polymers of isotactic polypropylenes with a small content of ethylene (min-random co-polymers).[0003] Isotactic polypropylene is one of a number of crystalline polymers which can be characterized in terms of the stereoregularity of the polymer chain. Various stereo specific structural relationsh...

AI Technical Summary

Benefits of technology

[0020] In accordance with the present invention, there is provided a novel process for the production of polyolefin film using polymer compositions, particularly highly stereoregular polypropylenes or co-polymers of polypropylene with a small content of ethylene, which are blended with a resin or rosin modifier. The polymer compositions used in the present invention are predominantly an isotactic polypropylene blended with a resin or rosin modifier in an amount within the range of about 1 to 30 weight percent. Preferably, the composition could contain from about 5 to 10 weight percent of a rosin or resin. Preferably, the polyolefin composition contains about 10 weight percent of the modifier. In a further aspect of the invention, there is provided a polyolefin film comprising a film layer formed of a mixture of a propylene homopolymer or a co-polymer containing a small amount of ethylene, blended with a resin or rosin modifier. The resin or rosin modifier is present in the mixture in a relative amount effective to produce a biaxially oriented polyolefin film capable of being stretched up to 9 times its original length in the machine direction and stretched in the transverse direction at a temperature equal to or less than approximately 166.degree. C. These particular homopolymer or co-polymer compositions exhibit improved processability during the biaxially oriented process measured in terms of machine direction orientation draw ratios and transverse direction orientation oven temperatures. The improved processability includes fewer web breaks and drawability at higher line speeds, resulting in fewer shut downs of the processing line and decreased production time.

[0023] The biaxially oriented films of the present invention involve the use of an amount of a resin or rosin modifier in combination with an isotactic polypropylene in the production of film with enhanced processability characteristics.

[0037] The addition of a resin or rosin to a propylene improves the processability of the polyolefin composition. The improved processability is measured in terms of machine direction orientation draw ratio and transverse direction orientation oven temperature. During biaxial orientation, the occurrence of web breaks in the film of the present invention is less frequent at standard machine direction orientation draw ratios and transverse direction orientation oven temperatures. Accordingly, the draw ratios in the machine direction can be increased and the oven temperatures of the transverse direction orientation can be lowered while processing the composition of the present invention with few web breaks. In addition, the drawability of the polyolefin composition can be accomplished at higher line speeds. The polyolefin compositions used in the present invention are desirable in that these physical and optical properties of the resulting film product are not significantly altered by the variable processing conditions.

[0050] As illustrated in FIG. 2, the polyolefin composition containing about 10 weight percent of the hydrocarbon resin significantly increased the drawability in MDO and lowered the TDO oven temperatures. A MDO draw ratio of about 9 times the original length and a TDO oven temperature of about 20.degree. C. less than the standard condition of about 166.degree. C. in Table 1 were achieved by the composition without the occurrence of web breaks.

[0052] As shown in Table 2, the optical and physical properties of the film containing the resin modifier are fairly standard properties to homopolymers as indicated by its similarity with the properties of the 100 weight percent polypropylene films. Accordingly, the addition of the resin modifier improved the processability of the homopolymers in terms of increased MDO draw ratio and lowered TDO oven temperatures, but does not alter significantly their properties.

[0054] The moisture vapor and oxygen transmission rates of the film containing the resin modifier are significantly reduced, and thus improved over the pure homopolymers.

Problems solved by technology

In addition, the possibility of a web break during the orientation process is lessened, resulting in lower processing line start up costs.

In addition, a high shrinkage factor of the processed film, while undesirable in some cases, can be advantageous in others, such as where the film is used in stretch wrapping of food products, electrical components, and the like.

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