Impact Enhancement

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 halftime 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. 1

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

Melt Flow Rate dg/min

Ethylene Content Wt%

Escorene PP 9505

0.9

30.0

3.0

Escorene PP 9574E2

0.9

12.0

3.0

Trade Name

Density g/cm3

Melt Index dg/min

Comonomer Type

EXACT 0201

0.902

1.1

Octene

EXACT 0202

0.902

2.0

Octene

EXACT 0203

0.902

2.0

Octene

EXACT 3035

0.900

3.5

Butene

EXACT8201

0.882

1.1

Octene

EXACT 9106

0.900

2.0

Hexene

Low 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. 2

Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers

Figure 1: Effect of Plastomer Density on Clarity Escorene PP 9505/Plastomer Blends Haze @1mm (40 mils), %

40

30 EXACT 0201 (0.902 Density) EXACT 8201 (0.882 Density)

20

10

0 0

5

10

15

20

25

30

Plastomer, Wt.%

Effect of Comonomer Type In this study, we explored the effect of comonomer type by using three 0.90 density plastomers containing different comonomers: butene, hexene and octene. Figure 2 shows haze as a function of plastomer percentage for 1-mm (40 mils) thick injection molded plaques for blends containing the 30 MFR CRCP. The ethylene-butene plastomer, shows virtually no haze increase up to 25 wt.% incorporation. Both the ethylene-hexene and ethylene-octene plastomers show haze increases proportional to plastomer percentages. Haze levels increase with the comonomer chain length, with the butene copolymer the lowest and the octene copolymer the highest. However, the difference in haze among the three plastomer types is relatively small so all three can be used to modify CRCPs.

Effect of Plastomer Melt Index Three plastomers with the same 0.90 density but different melt index (MI) were used to modify the 12 MFR CRCP. The plastomers used were all octene copolymers, with MI’s of 1, 2 and 3. As shown in Figure 3, changes in MI from 1 to 3 do not show any influences on haze for either the 1mm (40 mils) or the 2 mm (80 mils) plaques. Figure 2: Effect of Comonomer Type on Clarity Escorene PP 9505/Plastomer Blends Haze @ 1mm (40 mils)

30 25 EXACT 3035 (Ethylene-Butene)

20

EXACT 9106 (Ethylene-Hexene) 15 EXACT 0201 (Ethylene-Octene) 10 5 0 0

5

10

15

20

25

30

Plastomer, Wt.%

Effect of Heat Aging In Figure 4, 1-mm (40 mils) plaques were oven aged for 48 hours at 60˚C. This test is used to simulate dishwashing conditions for molded housewares . As shown in Figure 4, heat aging results in a slight increase in haze. At the common dosage level of 15 wt.% plastomer, haze increased from 12.5% to 15%. The materials tested were blends of 12 MFR CRCP with 2 MI hexene plastomer.

3

Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers

Figure 4: Effect of Heat Aging on Clarity Escorene PP 9505/EXACT 9106 Blends

30

25

25

20

20

15

15

10

10

5

EXACT 0201-1mm EXACT 0202-1mm EXACT 0203-1mm EXACT 0201-2mm EXACT 0202-2mm

5

10

15

20

25 20

Regular ASTM Conditioning Oven Heat Aging 48 hrs @ 60°C

15 10 5

0 0

30

EXACT 0203-2mm

5

0

Haze @1mm (40mils), %

30

Haze @2mm (80 mils),%

Haze @ 1 mm (40 mils),%

Figure 3: Effect of Plastomer Melt Index on Clarity Escorene PP 9574E2/Plastomer Blends

0

25

0

5

Plastomer, Wt.%

10

15

20

25

30

Plastomer, Wt.%

Effect of Plastomer Addition on End Use Properties Stiffness Filled containers must be stacked during shipment and storage, and must be stiff enough to resist deformation under these conditions. A general rule of thumb is that the flexural modulus of the container material must be a least 690 MPa (100,000 psi). Figure 5 shows the reduction in stiffness (1% secant flexural modulus) of 12 MFR and 30 MFR CRCP modified with 1 MI ethylene-octene plastomer. A straight-line decrease in modulus is observed with plastomer addition. The modulus decrease as a function of plastomer addition can be expressed as: 30 MFR CRCP blends 12 MFR CRCP blends

Y = 1190 -13*X Y = 1083 -12*X

This shows as Y is the 1% secant flexural modulus in MPa, and X as the weight percent plastomer. These equations predict that plastomer can be used up to 38% for the 30 MFR and 32% for the 12 MFR CRCP before the 690 MPa limit is reached.

Impact Strength A one-pint deli tub mold was used to mold dry blends of 30 MFR CRCP and 3 MI ethylene-octene plastomer. Drop impact was evaluated at three temperatures: 23∞C, 2∞C and -10∞C. For each temperature 21 deli tubs filled with a 60/ 40 water/ethylene glycol solution were tested according to the “Up and Down” or Bruceton Staircase Method outlined in ASTM D-2463, Procedure B. Starting from a predetermined no-break height, the drop height for each specimen is raised or lowered on the result obtained on the sample most recently tested. If the previous sample failed, the drop height is Figure 6: Staircase Drop Impact Escorene PP 9505/ Exact 0201 Blends

180

12.5

Mean Fail Height,m

Flexural Modulus, 1% Secant, MPa

Figure 5: Effect of Plastomer Addition on Top Load

160 y = -13x + 1190 Escorene PP 9505 140 Escorene PP9574E2 y = -12x + 1083

10

23C Test Temp.

7.5

2C Test Temp.

5

-10C Test Temp.

120

2.5

100 0

5

10

15

20

25

0

30

0

5

10

15

Plastomer, Wt.%

EXACT 3035, Wt.%

4

20

25

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

5

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 10oC/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 sequences. From the peak melting temperature, estimates were made on the CH2 sequences length using a method described in a previous publication (9). The shortest sequence length obtained for EXACT® 8201 consists of 14 methylenes 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 CH2 sequence in these plastomers is about 20 units 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 9: Effect Of Plastomer Addition On Polypropylene Crystallization Kinetics

Figure 10: Preferred Plastomer Structure Escorene PP 9505 Blend

Escorene PP 9505 (PP) PP + EXACT 3035

Half Time (mins )

PP + EXACT 9106 PP + EXACT 0203

EXACT 9106

PP 9505

122

124

126

128

130

132

134

136

138

Isothermal Crystallization Temperature (°C)

6

Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers

from CH2 sequences that are molten at room temperature. In addition, a surprising similarity between heterogeneity in 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 1. 2. 3. 4. 5. 6. 7. 8. 9.

R.D.Leaversuch, Modern Plastics, 75, No. 8, pp. 50-53, August, 1998. P.J.Phillis, and K.Mezghani, in J.C.Salamone ed., Polymeric Materials Encyclopedia, Vol. 9, pp. 6637, 1996. D. Dieckman, Proceedings of SPE RETEC Polyolefins 2000, pp. 583-591, 2000. M.J. Mannion and N.A. Mehl, U.S. Patent 5,961,914, October 1999. G.M. Brown and J.H. Butler, Polymer, 38, No. 15, pp. 3937-3945, 1997. Image Pro. Version 4.0, Media Cybernetics, Silver Spring, MD. 1998. T.C. Yu, Proceedings of SPE RETEC Innovations in Plastics IV, Rochester, pp. N7-N13, 1996. J.C. Ross,”The Image Processing Handbook”, 3rd ed., pp. 371-386, CRC Press, 1998. M.Y. Keating, and E.F. McCord, Thermocimica Acta, 243, pp. 129-145, 1994.

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