Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers
Impact Enhancement of Clarified Polypropylene With Selected
Metallocene Plastomers
Thomas C. Yu
Donald K.. Metzler
ExxonMobil Chemical Company
Houston, Texas
Manika Varma-Nair
ExxonMobil Research Company
Annandale, New Jersey
Technical Paper
Presented at:
SPE ANTEC
May 6-10, 2001
Dallas, Texas
Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers
Abstract
The addition of selected metallocene plastomers can improve the drop impact strength of parts molded from clarified polypropylene (PP) with slight effect on haze and gloss. This paper demonstrates the effects of plastomer structure (melt index, density and comonomer type), on the optical, physical and impact properties of clarified PP. A thermal segregation experiment shows the preferred methylene sequence length to minimize haze. Crystalization half-time experiments show that the addition of plastomer does not seem to hinder the polypropylene crystallization process. Finally, SEM micrographs are provided showing the dispersion of plastomer in an injection molded container.
Introduction
Alpha nucleating agents provide optical enhancement of polypropylene by changing its crystal morphology (1) . The crystal structure does not change, but the nucleator causes enhanced nucleation density that results in smaller and more dispersed crystals that scatter less light. Clarified polypropylenes, particularly clarified random copolymer (CRCP) resins are increasingly competitive with polyvinyl chloride (PVC) and polyethylene terephthalate (PET) resins in rigid packaging applications. However, use of CRCP may be limited by its impact strength, particularly at cold temperatures (10oC to -40oC), where CRCP is often brittle. Addition of a certain type of metallocene plastomer resin to CRCP can provide substantial improvement in drop impact strength while retaining the clarity and gloss of the base polymer. Plastomer enhancement of CRCP impact strength is potentially useful in many rigid packaging applications, such as packaging refrigerated and frozen foods. In cold climates it eliminates problems with container breakage during transport and storage. In housewares and storage products it can provide extra toughness for particularly demanding container applications (large volume/heavy contents). When a CRCP molded part fits the application but fails drop impact, a small amount of plastomer can be dry blended at the press to meet impact requirements.
The polypropylene chain conformation is a three fold helix. Three different crystalline forms arise because of the positioning of the pendant methyl groups. These are monoclinic α-form, the hexagonal β-form and the triclinic γ-form (2). The addition of a α nucleator to polypropylene reduces the spherulitic sizes leading to greater transparency, faster cycle time and improvement in stiffness compared to non-nucleated samples. A common α nucleator is salt of benzoic acid such as sodium bonzoate, which has been in use since 1960’s. However, the acid scavenger in the additive package must be carefully selected as not to interfere with the nucleation process (3). More recently several generations of sorbitol based nucleator have gained popularity (4) . Examples are bis 3,4 dimethyldibenzylidene (DMDBS) and dibenzylidene sorbitol (DBS) clarifiers from Milliken Chemical Company, Ciba Specialty Chemicals, New Japan Chemical Company and others. The addition of a DMDBS nucleator to polypropylene resin also enhanced its thermoformability by widening the thermoforming window (4). A combination of a low flow clarified polypropylene and plastomer finds applications in extrusion blow molded parts. Attempts to process the plastomer modified clarified polypropylene in injection stretch blow molding are also progressing.
Metallocene plastomers are supplied as free flowing pellets, and have molecular weights similar to polyethylenes. It is therefore possible to injection mold parts using a dry blend of plastomer and polypropylene. This paper discusses plastomer selection to produce the lowest haze parts. The effect of plastomer addition on drop impact resistance, and plastomer dispersion in an injection molded dry goods storage container is described. The effect of plastomer addition on injection molding cycle times is evaluated from crystallization rates measured using calorimetry. A thermal segregation technique is used to provide insight for the optimum structure of plastomer that produces the lowest haze in the blends.
Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers
Experimental
Table 1 shows both raw materials used for this study. These included high melt flow and medium flow clarified random polypropylene copolymers and several commercial grades of plastomers. All materials were prepared for molding by dry blending. Injection molded test specimens and haze plaques were prepared using a 75 ton Van Dorn injection molding machine. One-pint deli tubs were produced using a 130-ton Negri Bossi injection molding machine.The mold used was provided by the Milliken Chemical Company. A flat 232?C (450?F) barrel temperature and 21?C
(70?F) mold cooling were used. A two gallon size dry goods storage containers was injection-molded on a 700 ton Impco using a one cavity center gated hot runner mold. The molding parameters of the plastomer-modified blends were almost the same as the un-modified polypropylene parts.
Table 1: Raw Materials
Trade Name Density g/cm3
Ethylene Content
Wt%
Escorene PP 95050.9 3.030.0Melt Flow Rate
dg/min
Trade Name Density g/cm3
Comonomer Type
Melt Index dg/min
EXACT 02010.902Octene 1.1EXACT 02020.902Octene 2.0EXACT 02030.902Octene 2.0EXACT 30350.900Butene 3.5EXACT82010.882Octene 1.1EXACT 9106
0.900
Hexene
2.0
Escorene PP 9574E2
0.9
3.0
12.0Low voltage electron microscopy (LVSEM) was used to study plastomer dispersion in the bottom and side of an injection molded dry goods container. The LVSEM used a special staining technique (5) to enhance the phase contrast of the dispersed plastomer particles in a continuous polypropylene matrix. Image analysis (6) was conducted on the LVSEM micrographs to arrive at the average particle size and particle size distribution.
Results and Discussion
Effect of Plastomer Structure on Clarity Effect of Density
It has been shown previously that the addition of a plastomer with density of about 0.90 results in very little
additional haze (7). Figure 1 compares the haze of blends containing a 0.902 density and a 0.882 density ethylene-octene plastomers (EXACT 0201 and 8201 respectively) in 30 MFR CRCP. Blends with the 0.882 density plastomer exhibit much higher haze values than the corresponding blends with the 0.902 density plastomer. For example, at 10% addition the 0.882 density blend showed 30% haze while the 0.902 density blend showed only 10% haze.
12 MFR CRCP with 2 MI hexene plastomer.
Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers
lowered by 15.2 cm (6 inches); if the previous sample did not fail, the drop height is raised by 15.2 cm (6 inches). The mean fail height is calculated or all the containers that fail. Figure 6 shows the mean fail height as function of test temperature and percentage of plastomer. At room temperature, the control shows a mean fail height of 4 meters. Addition of 10% plastomer increased the mean fail height to 5.7 meters. For parts intended for refrigerator use (2∞C test temperature), addition of 15% plastomer increases mean fail height to 6 meters. For freezer applications at -10°C, 20% plastomer addition provides a mean fail height equivalent to the unmodified CRCP at room temperature.
Morphology of Plastomer Dispersion
The dispersion of plastomer in CRCP was examined by LVSEM in large injection molded dry goods containers. Each container was 17.5 cm by 27 cm, and 22 cm in height. The average wall thickness was 2 mm. The mold had a single center gate at the bottom of the container. Samples were cut from both the bottom and side of the container. Figure 7 shows original LVSEM images of both the bottom and side of the container modified with 10% ethylene-octene plastomer. Average plastomer particle size was computed by digital image analysis using Image Pro Plus software (6) together with the Image Process Tool Kit (8). Submicron dispersion of plastomer was observed: 0.033μm for the bottom sample and 0.037μm for the side sample. The aspect ratios from both the bottom and side samples were about the same. The same desirable submicron dispersion was observed with the 15% and 20% plastomer modified blends as well.
Figure 7: Dry Goods Storage Container 90/10 RCP/EXACT 0201 Dry Blend
Figure 8: Dry Goods Storage Container 80/20 CRCP/EXACT 0201
Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers
Figures 8 shows the image analysis summary for 20% plastomer modified blends. Based on the above images, we conclude that good dispersion can be achieved by direct injection molding of a CRCP/plastomer dry blends, even for large containers.
Thermal Analysis
Effect of Plastomer Addition on Crystalization Rate
Crystallization kinetics of 30 MFR CRCP/plastomer blends was evaluated using differential scanning calorimetry (DSC). Isothermal crystallization was carried out at various temperatures to determine the crystallization rate. The polymer was cooled rapidly to the crystallization temperature and crystallized isothermally for 30 minutes. Time taken for 50% crystallization (t 1/2) to occur was determined. Figure 9 shows the plot of crystallization half time at various temperatures. Almost no change was observed in t 1/2 for CRCP and its blends. Shorter crystallization time indicates faster crystallization kinetics, and relates to a decrease in injection molding cycle time. Since no change was observed in the crystallization rate of CRCP with addition of the plastomers, we would expect that the injection cycle time for these blends would be unaffected by plastomer addition. In fact, our experience with molding confirms this prediction.
Preferred Plastomer Structure
A thermal fractionation experiment was conducted to identify the optimum plastomer structure for modification of CRCP. The polymer was crystallized using step isothermal crystallization in decreasing steps of 10 degrees. At each step it was annealed for 4 hrs and analyzed on heating at 10o C/min. Figure 10 shows the multiple melting endotherms obtained for various plastomers and 35 MFR CRCP. These endotherms indicate sequence heterogeneity in both the plastomers and CRCP . Presence of this heterogeneity leads to the formation of crystals of varying sizes that melt at various temperatures depending on the chain length. Each endotherm represents a population of crystallizable se-quences. From the peak melting temperature, estimates were made on the CH 2 sequences length using a method de-scribed in a previous publication (9). The shortest sequence length obtained for EXACT? 8201 consists of 14 methyl-enes while the longest is about 70 units. Both EXACT 3035 and EXACT 0201 have a larger population of higher melting crystals formed from longer methylene sequences. The shortest CH 2 sequence in these plastomers is about 20units long and these crystals are molten at room temperature. This is in contrast to EXACT 8201 where the small, low melting crystals present at room temperature may be the possible causes for haze in the blends of EXACT 8201 with CRCP. Thus, it appears that for a plastomer to give minimum to no haze, the plastomer needs to have crystals formed
Figure 10: Preferred Plastomer Structure Escorene PP
9505 Blend
Figure 9: Effect Of Plastomer Addition On
Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers sequences that are molten at room temperature. In addition, a surprising similarity between heterogeneity in
from CH
2
EXACT 3035 (ethylene-butene copolymer) and EXACT 0201 (ethylene-octene copolymer) indicates that for minimum haze there needs to be an optimum structure for the plastomer. Thermal segregation thus provides a unique method to probe the polymer structure for optimum properties and performance.
Conclusions
When a clarified RCP fails to meet the drop impact requirements, adding a 0.900 density plastomer will enhance its impact strength, with minimal haze increase. 10% to 15% plastomer is required for most ambient or refrigerator applications. For larger and heavier containers about 15% to 20% plastomer is recommended. For freezer applications the amount of plastomer should be increased to 20% to 25%. Although plastomer based on butene comonomer showed the least amount of haze increase, all three types of plastomers, ethylene-butene, ethylene, hexene and ethylene-octene produce acceptable parts in the field.
Due to the low interfacial energy between plastomer and polypropylene, a dry blend of these two materials can easily be injection molded. Our morphology studies demonstrate that sub-micron dispersion can be achieved under these conditions.
Plastomer addition had no effect on the crystalization rate of CRCP. Thermal segregation was used to probe the optimum structure of the plastomer that gives minimum haze in the blends. It appears that for plastomers to give minimum haze, there appears to be a unique distribution of crystal sizes and population that is responsible for their optimum performance.
Aknowledgements
The authors would like to extend their appreciation to Angela Halstad of Milliken Chemical for the use of their deli tub hot runner mold. Our thanks go to Andy Tsou, Joyce Cox, and Margaret Ynostroza for the morphology study. We are appreciative to Kelli Dettor for her testing efforts
References
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