Modified Polypropylene for Film

This invention relates to polypropylene, particularly to polypropylene modified to produce biaxially oriented film for the packaging market. Biaxial orientation is well known in the industry and is used to enhance both the optical and physical properties of the polymer film. Contemporary commercial technology for specialty film involves composite structures that can contain as many as 5 to 7 different coextruded polymeric components. Coextrusion with polymers other than polypropylene is used to improve or impart special properties to the polypropylene such as impermeability or sealability.

To maintain compatibility and clarity of the composite, polypropylene must be used which is known to have a low haze property. Historically, high clarity polypropylene is obtained by modifying the polymer microstructure; the addition of a suitable nucleating agent (i.e. sodium benzoate) to the polymer during pelletizing is known to alter crystalline structure (spherulite growth) and enhance clarity. This improved clarity, however, can be at the expense of good processability and non-uniform properties in the film. For example, optical or mechanical defects such as tearing of the film during drawing can be caused by inhomogeneities (i.e. large particle size distributions) related to the presence of nucleating agents or other inorganic additives.

My invention provides for the production of a film grade polypropylene that is nucleated, easy to process, has exceptional multi-directional physical properties, and exhibits a low measure of haze. Such a combination of properties has been extremely difficult to achieve in the past.

Prior to the present invention, it has been known that bis(alkylbenzylidene) sorbitols could be used as clarifying agents, i.e. to reduce haze, in polyolefins. See the sales sheet for "Millad 3905" attached to the Prior Art Statement filed herewith; also see abstracts of the Japanese Patent application No. 81/186,663, Nov. 19, 1981; application No. 81/166,675, Oct. 19, 1981, and U.S. patent application No. 363,824, Mar. 31, 1982, now abandoned.

It is also known in the art that so-called "controlled rheology" (CR) in polypropylene can be attained by the closely regulated addition of certain peroxides such as 2,5-dimethyl-2,5 di(t-butylperoxy) hexane. The peroxide typically is added in the extruder during pelletization (after the polymerization process is complete). The peroxide will tend to encourage a more or less uniform breakdown of the molecular structure of the polypropylene under the physical and thermal stress of the extruder, resulting in a product having a relatively narrow molecular weight distribution; the reduction in average molecular weight and the improved narrowness of the molecular weight distribution are typically accompanied by an increase in melt flow and a change in both rheology and physical properties compared to those of a reactor product have a similar melt flow. This "cracking" technique is well known in melt spinning fibers and more recently has become accepted in high-speed injection molding; however, film made from low melt flow peroxide-cracked material is "flexible" and has a high percent haze, two undesirable properties for polypropylene films.

It has also been suggested in the prior art that so-called linear low density polyethylene may be used to make films. Linear low density polyethylene (LLDPE) is polyethylene containing a small amount (typically 0.5%) mono olefin co-monomer such as 1-hexene, 1-butene, or 1-octene and generally having a relatively narrow molecular weight distribution and short chain branching. See, for example, U.S. Pat. No. 4,362,835. It is indicated in German Pat. No. 1,902,573 that the incorporation of polyethylene (homopolymer) may have an effect on the crystallization rates of polypropylene.

The reader may be interested also in U.S. Pat. Nos. 4,359,544; 4,282,076 and 4,296,022 for general background in the area of nucleation and controlled rheology.

The composition of my invention comprises a polypropylene homopolymer having a low melt-flow (e.g. about 2.0 to about 5.0) including about 750 to about 1500 ppm of more or less conventional stabilizers, such as BHT, calcium stearate, Irganox 1010, and Kenemide E, about 1000 ppm to about 2500 ppm of a substituted sorbitol nucleating agent, (specifically Millad 3905) and about 0.25% to about 2.0% linear low density polyethylene (polyethylene including about 0.5% to about 1.0% mono olefin monomers having from 4 to 8 carbon atoms) having a melt flow of about 1.0 to about 2.5 (condition E). The substituted sorbitol is a bis(alkylbenzylidene) sorbitol wherein the alkyl groups have about 2 to about 18 carbon atoms. The low melt-flow polypropylene may be created in an extruder from a lower melt-flow propylene homopolymer by adding thereto prior to or during the extrusion process about 100 ppm to about 250 ppm of a dialkyl peroxide "controlled rheology" additive such as Lupersol 101.

Several analytical comparisons (Table I) have been made which illustrate the advantages of my invention. In Table I, PP-A is a fractional melt flow (0.70, condition L) reactor-made homopolymer. This material is the "base" or "starter" resin which, as used in columns II, III and IV of Table I is peroxide cracked to the final product.

Referring to Table I, it can be seen that the addition of peroxide to the base resin (fractional melt flow homopolymer) both narrows the molecular weight distribution and increases the percent haze of injection molded, compression molded and cast film samples (blends I and II). Addition of a nucleating agent (blend III) increases the crystallization temperature from approximately 110.degree. C. to 120.degree. C. and decreases the haze to that of the "starter" material, PP-A. The subject blend of this invention, blend IV, shows the addition of 1.0% LLDPE. In the absence of a nucleating agent, the LLDPE (column V) is considered "incompatible" with the CR ("controlled rheology") polypropylene; however, in the nucleated CR-polypropylene, (column IV), the melting point (Tm) of the LLDPE occurs at the crystallization temperature (Tc) of the polypropylene. This thermal balance, [(Tm)LLDPE=(Tc)PP=120.degree. C.], and the concurrent crystallization of polyethylene and polypropylene may contribute to low haze and improved physical properties attainable only through the combination of peroxide, nucleation, and addition of LLDPE. As a result, the subject blend may involve solid-liquid separation behavior, co-crystallization of the normally very slow crystal growth LLDPE and the fast growth polypropylene, or amorphous phase compatability between the two polymers. While accurate results are difficult to obtain with concentrations of LLDPE in polypropylene within the bounds of my invention, results with higher concentration of LLDPE indicate that the annealed differential scanning calorimetry curve will show an upward shift in crystallization temperature (Tc) of the LLDPE from about 105.degree. C. to 115.degree. C. This higher crystallization temperature "interferes" with the normal crystallization rate of the nucleated polypropylene. The LLDPE appears to crystallize concurrently with the polypropylene. As previously mentioned, the melting point of the LLDPE is at the crystallization temperature of the polypropylene.

The general trends in physical properties determined on 4-inch wide, 2 mil thick cast film from blends similar to those in Table I show the following resin-component interactions:

CONTROL RESIN: 4 melt-flow, homopolymer film grade resin.

Some specific physical properties relative to film (Table II) confirm the general trends reported in Table I on resin-component interactions. Compared with a 4 melt-flow homopolymer made in a reactor (sample I), the 4 melt-flow CR product (sample II) has a lower tensile yield strength, a higher ultimate elongation and a high haze. Adding a nucleating agent (sample III) does not produce a material significantly different from the reactor product; however, the combination of the nucleating agent and LLDPE (blend IV) results in improved modulus, yield strength, elongation and haze.

Film is generally defined in the trade as being 10 mils or less in thickness; above 10 mils and up to about 0.05 in., the product is known as a sheet. My invention is useful for both biaxially oriented sheet and biaxially oriented film.

Additional data is presented on laboratory-prepared film extrusion cast onto a chill roll. Results in Table III show that the blend of my invention, 88-7U, exhibits an improvement in physical properties compared with a similar melt flow reactor-made homopolymer control resin. The new film, containing the nucleating agent (0.25% Millad 3905) and 1.5% LLDPE, has exceptional ultimate elongation in both the longitudinal direction (MD) and transverse direction (TD). Melt orientation of the blend (achieved by increasing roll speed 33%) results in film with a very high tensile modulus; other properties are similar to those of the control resin.

Unlike the chill-roll cast film previoulsy reported, film in Table IV was made by water-quenching the extrudate. Blend 88-7 containing 1.5% LLDPE and 0.25% Millad 3905 has an excellent surface appearance, is approximately 9% less in film width, and has a low (8.8%) haze compared with a similar melt flow reactor-made homopolymer.

Physical properties on biaxially oriented film are shown in Table V. Compared to conventional homopolymer film, film made from the blend is easier to orient in the longitudinal direction (MD); orientation in the lateral direction (TD) can be done at lower temperatures and higher stretch ratios than commonly used for homopolymer.

Tables VI and VII relate the Izod Impact Strength of the subject blend. Results show (Table VI) impact properties of the blend are greater than 1.0 ft-lb/in. notch and annealed samples have an impact strength greater than 12 ft-lb/in. notch with all partial sample breaks. Results in Table VII show the Izod Impact Strength is affected by the molding temperature. The impact strength ranges from 1.17 ft-lb/in.(N) at 450.degree. F. to 4.40 ft-lb/in.(N) at 430.degree. F. These properties are the result of molding orientation, the interaction between the peroxide with the two polymers (PP-LLDPE), and the nucleating agent. This type of behavior is not observed in conventional film-grade polypropylene.

For example, Tensile-Impact data in Table VIII show the desirable effect of the additive combination. Unlike the Izod test, the Tensile Impact test measures energy to rupture by rapid stretching. It is a high speed test in which crack-failure is initiated and propagated in the absence of a notch. Because of material elongation, the test is sensitive to failure mechanisms different from those found in Izod or falling-weight impact testing. The effect of peroxide on the starter material (PP-A) is a property imbalance (composition II). A high percent haze is accompanied by a high impact strength in the flow-direction of the molded die-cut specimen and a drop in impact strength of the transverse-to-flow direction (TFD) sample. The nucleated CR blend (Millad 3905) has similar, or in some cases lower, properties than the control material (see blend III). An improvement in both FD (flow direction) and TFD strength is observed (blend IV) with the addition of both Millad and LLDPE to the CR material. The blend has a low percent haze, uncharacteristic of low-melt flow CR. In blend V, the Millad nucleating agent is omitted. The LLDPE results in a high percent haze and an imbalance in impact properties.