Impact Enhancement

8/14/2019 Impact Enhancement

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Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers

Impact Enhancement of ClarifiedPolypropylene With SelectedMetallocene Plastomers

Thomas C. YuDonald K.. MetzlerExxonMobil Chemical CompanyHouston, Texas

Manika Varma-NairExxonMobil Research Company

Annandale, New Jersey

Technical PaperPresented at:SPE ANTEC

May 6-10, 2001Dallas, Texas


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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

(10C to -40C), 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 -form, the hexagonal -form and the triclinic -form

(2). The addition of a 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 nucleator is salt of benzoic

acid such as sodium bonzoate, which has been in use since 1960s. 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.

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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 232C (450F) barrel temperature and 21C

(70F) 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 9505 0.9 3.030.0

Melt Flow Rate

dg/min

Trade Name Density g/cm3 Comonomer TypeMelt Index

dg/min

EXACT 0201 0.902 Octene1.1



EXACT 3035 0.900 Butene3.5

EXACT8201 0.882 Octene1.1

EXACT 9106 0.900 Hexene2.0

Escorene PP 9574E2 0.9 3.012.0

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.


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Figure 1: Effect of Plastomer Density on Clarity Escorene PP 9505/Plastomer Blends

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 MIs 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.

0

10

20

30

40

Haze@1mm(40mils),

%

0 5 10 15 20 25 30

Plastomer, Wt.%

EXACT 8201 (0.882 Density)

EXACT 0201 (0.902 Density)

Figure 2: Effect of Comonomer Type on Clarity Escorene PP 9505/Plastomer Blends

0

5

10

15

20

25

30

Haze@1

mm(40mils)

0 5 10 15 20 25 30

Plastomer, Wt.%

EXACT 0201 (Ethylene-Octene)

EXACT 9106 (Ethylene-Hexene)

EXACT 3035 (Ethylene-Butene)

Effect of Heat Aging

In Figure 4, 1-mm (40 mils) plaques were oven aged for 48 hours at 60C. 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.


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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 Y = 1190 -13*X

12 MFR CRCP blends 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: 23C, 2C and -10C. 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

0

5

10

15

20

25

30

Haze@2mm(80m

ils),%

0

5

10

15

20

25

30

Haze@1mm(40m

ils),%

0 5 10 15 20 25

Plastomer, Wt.%

EXACT 0203-2mm

EXACT 0202-2mm

EXACT 0201-2mm

EXACT 0203-1mm

EXACT 0202-1mm

EXACT 0201-1mm

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

0

5

10

15

20

25

30

Haze@1mm(40m

ils),%

0 5 10 15 20 25 30

Plastomer, Wt.%

Oven Heat Aging 48 hrs @ 60C

Regular ASTM Conditioning

Figure 4: Effect of Heat Aging on Clarity Escorene PP

9505/EXACT 9106 Blends

Figure 5: Effect of Plastomer Addition on Top Load

100

120

140

160

180

FlexuralModulus,1%Secant,MPa

0 5 10 15 20 25 30

EXACT 3035, Wt.%

y = -12x + 1083

y = -13x + 1190

Escorene PP9574E2

Escorene PP 9505

Figure 6: Staircase Drop Impact Escorene PP 9505/

Exact 0201 Blends

0

2.5

5

7.5

10

12.5

MeanFailHeight,m

0 5 10 15 20 25

Plastomer, Wt.%

-10C Test Temp.

2C Test Temp.

23C Test Temp.


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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 (2C test

temperature), addition of 15% plastomer increases mean fail height to 6 meters. For freezer applications at -10C, 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: 0.033m for the bottom

sample and 0.037m 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


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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 (t1/2

) to occur was determined. Figure 9 shows the plot of crystallization half time at various

temperatures. Almost no change was observed in t1/2

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 theseblends 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 se-

quences. From the peak melting temperature, estimates were made on the CH2

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 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 10: Preferred Plastomer Structure Escorene PP9505 Blend

Figure 9: Effect Of Plastomer Addition On

Polypropylene Crystallization Kinetics

122 124 126 128 130 132 134 136 138

Isothermal Crystallization Temperature (C)

HalfTime(mins)

Escorene PP 9505 (PP)

PP + EXACT 3035

PP + EXACT 9106

PP + EXACT 0203

EXACT 9106

PP 9505


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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. R.D.Leaversuch, Modern Plastics, 75, No. 8, pp. 50-53, August, 1998.

2. P.J.Phillis, and K.Mezghani, in J.C.Salamone ed., Polymeric Materials Encyclopedia, Vol. 9, pp. 6637, 1996.

3. D. Dieckman, Proceedings of SPE RETEC Polyolefins 2000, pp. 583-591, 2000.

4. M.J. Mannion and N.A. Mehl, U.S. Patent 5,961,914, October 1999.

5. G.M. Brown and J.H. Butler, Polymer, 38, No. 15, pp. 3937-3945, 1997.

6. Image Pro. Version 4.0, Media Cybernetics, Silver Spring, MD. 1998.

7. T.C. Yu, Proceedings of SPE RETEC Innovations in Plastics IV, Rochester, pp. N7-N13, 1996.

8. J.C. Ross,The Image Processing Handbook, 3rd ed., pp. 371-386, CRC Press, 1998.

9. M.Y. Keating, and E.F. McCord, Thermocimica Acta, 243, pp. 129-145, 1994.


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Impact Enhancement of Clarified Polypropylene With Selected Metallocene Plastomers2001 ExxonMobil. To the extenthe user is entitled to disclosand distribute this documenthe user may forward, distributeand/or photocopy this copyrighted document only if unatered and complete, including aof its headers, footers, disclaimers, and other information. Yomay not copy this document ta Web site. The information ithis document relates only to thnamed product or material

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