Extrusion Blow Molding

Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Thomas M. Miller Thomas C. Yu ExxonMobil Chemical Company 13501 Katy Freeway Houston, Texas 77079

Technical Paper Presented at: SPE Polyolefins Conference February 25-27, 2002 Houston, Texas

Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Abstract For the past several years, there has been growing interest in the use of clarified random copolymers of polypropylene for rigid packaging. The low cost, low density and good processing characteristics make polypropylene an attractive material for packaging manufacturers using either injection or blow molding. Similarly, the high stiffness, high heat resistance, outstanding stress crack resistance and excellent clarity of this material ensure the performance needed to package aggressive cleaning agents and hot-filled sauces in clear containers that show-off the contents and appeal to consumers. Perhaps the only drawback to the use of polypropylene in rigid packaging is the poor drop impact performance exhibited at refrigerator and freezer temperatures or even by large packages at ambient temperatures. Fortunately, this deficiency can be overcome with minimal increase in haze by incorporating a small amount of metallocene plastomer. This report details the effects of plastomer upon the drop impact resistance of extrusion blow molded twoliter bottles. The processing conditions are compared with that of neat clarified polypropylene and conventional highdensity polyethylene. Differences in staircase drop impact, top-load and haze are also discussed.

Introduction The ability to incorporate the contents of a container into the package design in order to increase consumer appeal is what fuels the growth of clarified rigid packaging in pre-packaged food, food storage, housewares and personal/health care markets. Although poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC) and polypropylene (PP) are all used in the clear packaging arena, polypropylene and especially clarified random copolymer of polypropylene (CRCP) offer lower cost than PET and better organoleptic performance than flexible PVC. Besides these advantages, CRCP offers many other benefits. These include good processing characteristics, high stiffness and environmental stress crack resistance, low haze, good heat resistance and the flexibility to run on existing HDPE lines with little or no tooling modifications or changes. All of these attributes ensure that the demand for clarified polypropylene packaging will continue to grow. There are, however, two important characteristics of this resin that designers and package producers should keep in mind when contemplating a switch to clarified polypropylene. They are drop impact and barrier performance and both are the direct result of two fundamental polymer science concepts: crystallinity and the glass transition. Generally speaking, the amount of crystallinity and the glass transition temperature (Tg) are the two intrinsic polymer characteristics that dictate much of the performance of all semi-crystalline polymers, including polypropylene. All package designers and producers should understand this means that these two effects also dictate the performance of goods molded from these resins. The level of crystallinity determines the stiffness, toughness, stress crack resistance, oxygen transmission rate, haze and heat resistance of the container. Crystallinity is a function of polymer architecture and composition as well as cooling rate and part thickness. The Tg is a processing-independent parameter and is defined as the temperature at which the polymer transitions from a tough, durable material to a hard, brittle glass upon cooling or vice-versa upon heating. Consequently, the Tg is what defines the effective cold use temperature of a package. The variables affecting both parameters are discussed in greater detail below using PP as the example.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Both the level of crystallinity and the Tg of polypropylene depend upon the comonomer content. PP homopolymer, as the name implies, contains no comonomer. Conversely, polypropylene random copolymer, as the name implies, is based upon the “random” polymerization of ethylene and propylene monomers; the catalyst randomly polymerizes the ethylene and propylene monomers. The ethylene monomer functions as a defect in the polymer chain and disrupts the crytallization of the propylene segments. This reduces the amount of crystallization that occurs when the polymer is cooled from the melt, which results in lower haze. Increasing comonomer content also leads to lower Tg, which improves cold drop impact. Directionally, both are desirable from a bottle performance standpoint. Lower levels of crystallinity also result in lower flexural modulus and melting temperature and higher oxygen transmission rate. All of these are undesirable in hot-filled, oxygen sensitive food packages. Increased stiffness can be obtained by increasing wall thickness, using a lower ethylene content polypropylene or adding fillers. All of these steps will increase haze but few other options exist. The most effective method of reducing the oxygen transmission rate is to incorporate a barrier material. While discontinuous barrier agents are available, converting to a multi-layer package utilizing a barrier layer is the most common method used in industry. The cross-section of a typical multi-layer package examined under a polarizing microscope under 103X magnification is shown in Figure 1. Layers 1 and 5 are typically virgin or regrind-containing layers. Layers 2 and 4 are adhesive tie layers that bind the oxygen barrier layer (layer 3) to these polypropylene-based layers. EVOH is a very common oxygen barrier material. Finally, a virgin inner layer (layer 6) completes the structure. The presence of the EVOH layer significantly extends the shelf life of the package. In fact, this type of multi-layer construction competes well with PET. The only drawback to this multi-layer approach is the added complexity of handling a recycle stream containing adhesives and EVOH. Although this recycle can be added to regrind layer 5 in Figure 1, the successive thermo-oxidation of the EVOH over time leads to yellowing of the package unless special steps are taken to prevent this oxidation. FIGURE 1: Typical multi-layer design used to reduce oxygen transmission and increase the shelf life of oxygen sensitive foods.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Crystallinity has a very significant effect upon the clarity of polypropylene. Fortunately, there are other methods available besides increasing comonomer content to reduce haze without compromising stiffness. The best of these is the addition of particulate or chemical additives that effect the number and size of crystallites that form. For example, typically, less than 8% ethylene is used in the polymerization of polypropylene random copolymers. This translates to RCPs exhibiting a haze of ~20% in a 40mil thick plaque. Adding particulate nucleating agent precipitates the growth of smaller more numerous crystallites and helps reduce haze. Clarifying agents are even more effective. These are soluble chemicals that dissolve into the polymer melt during processing and then, upon cooling, orchestrate the formation of a fibrous crystal structure that scatters very little light and affords good stiffness. The incorporation of these clarifying agents into RCPs can lead to haze values of less than 8% in 40mil thick injection molded plaques. This means that haze as low as 10% can be achieved in containers blown from CRCP resin. Even greater clarity can be achieved in thin walled injection molded containers. The comparatively good drop impact resistance of polymeric packaging relative to glass and paperboard is what continues to stimulate increased demand for clear plastic. However, all materials have a limit and all plastic containers will eventually fail when dropped from too great of a height or made too thin for an acceptable height. Temperature also has an affect on drop impact resistance. In fact, the Achilles’ heel of polypropylene packaging is the cold temperature performance. This is a direct result of the relatively high Tg of polypropylene, the onset of which is approximately 0°C. Like most thermoplastic polymers, polypropylene becomes stiffer as it cools from ambient temperature. Generally speaking, the increased stiffness is a result of decreased chain mobility that occurs with decreasing thermal energy. This is a gradual change down to 10°C. However, the increase in stiffness becomes much more dramatic below 10°C as the onset of the Tg is approached. By the time the polymer is cooled to around 0°C very little ductility remains. This means that containers produced from polypropylene exhibit very little toughness and impact resistance at these temperatures. Instead, they shatter when dropped from only a few feet and bottles break during transit in cold climates. New catalyst/ donor advances continue to produce random copolymers with more uniform comonomer distribution, lower Tg and improved cold impact performance. Until these advances are realized, the best way to improve either large-part ambient or cold drop impact performance of clear polypropylene packaging is to incorporate low levels of an impact modifier such as metallocene plastomer in the bulk of monolayer packages or the virgin (and subsequent regrind) layers of multilayer packages. EXACT® Plastomers are metallocene catalyzed, ethylene α-olefin copolymers produced using EXXPOL® Technology. Several different grades exist for impact modifying injection molded, extrusion blow molded or injection-stretch blow molded clear polypropylene containers. Indeed, numerous technical articles have been written over the past few years detailing the performance of EXACT® Plastomers in these types of applications (1-3). Proper grade selection is key to successfully improving the drop impact performance of these containers without adversely affecting the haze. This article describes our latest efforts at understanding the performance of metallocene plastomer modified clear polypropylene packaging. It focuses on processing differences and subsequent optical, mechanical, top load and drop impact performance of two-liter bottles produced using extrusion blow molding.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Experimental Several different types of commercially available ExxonMobil Chemical resins were used throughout this study. A complete listing is provided in Table 1. Blow molding grades of high-density polyethylene homopolymer (hHDPE) and CRCP were used as baselines in this study. EXACT Plastomer impact modifiers were then dry blended at 15% (wt/ wt) with the CRCP. Two-liter bottles were blown using a Bekum Model H-121S single station shuttle-type blow molding machine equipped with a 50mm 24 L/D single screw extruder. The screw had a 3:1 compression ratio and two rows of mixing pins on the tip. The blow-molding head was fitted with either 109mm or 104mm divergent tool. A schematic of the bottle produced from the handle ware mold is given in Figure 2. The target weight was 90g. The letters designate locations for wall thickness measurements. The mold surface at locations A, B, E, F and K is highly polished whereas the remaining area (label panel) is merely a high-quality “wiped” surface. Vent ports are located on the label panel. Bottles produced from both the baseline resins and the blends were characterized for wall distribution, haze, top load, and drop impact . Table 1: List of commerical resins used as baselines and the metallocene catalyzed impact modifiers

Product

Resin Description

Density (g/cm3)

MI (dg/min)

Mfg. platform % EXACT

Baseline Resins AD60-007

HDPE homopolymer (hHDPE)

0.965

0.7

PP 9612

Clarified RCP (CRCP)

0.90

2

Slurry Loop

0 0

Impact Modifiers Blended with PP 9612 EXACT 3128

Butene plastomer

0.900

1.2

Autoclave

15

EXACT 3132

Hexene plastomer

0.900

1.2

Autoclave

15

EXACT 0201

Octene plastomer

0.902

1.1

Solution

15

EXACT 3125

Butene plastomer

0.910

1.2

Autoclave

15

FIGURE 2: Schematic of two-liter handle ware bottle produced in this study. Letters designate key locations used to measure wall thickness.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Results and Discussion Overall, all baseline and blend samples processed well and were capable of producing high quality bottles. In these discussions, the plastomer containing CRCP blends will be referred to by the plastomer grade used in the blend, e.g., EXACT 0201 blend. The target weight for all bottles was 90g. This was established by reducing the weight of the virgin PP 9612 bottle to the point where drop impact occurred at a reasonable height (~4ft). Given the fairly high loading of plastomer, some significant melt viscosity and swell differences were observed in the processing characteristics of the blends that affected the final bottle weight. These processing differences are elaborated upon in the next section.

Processing Comparison Cycle time was fixed at 20 seconds for all samples. An overall summary of the process conditions employed for all baseline resins and blends is displayed in Table 3 along with the average bottle weight these conditions yielded. These conditions were identified as giving the best balance between ease of resin processing, bottle appearance and previously established organoleptic performance. For example, internal evaluations have revealed that the melt temperature of hHDPE should be kept below 405°F to prevent significant off-flavor. Although taste and odor were not assessed in this study, a comparison of hHDPE and CRCP processing and bottle performance under realistic conditions was a goal. Consequently, a sub-400°F melt temperature was targeted and achieved with the hHDPE. CRCP is known to possess low taste and odor, and so the melt temperature was allowed to increase in order to maximize bottle clarity and gloss. The processing conditions for the blend samples were varied to explore the operating window. Generally speaking, both the hHDPE and the plastomer containing blends processed differently than the PP 9612. Specifically, the lower melt viscosity exhibited by all of the blends and hHDPE produced a longer parison and the die gap had to be tightened in order to reduce the bottle weight. This increased parison swell and caused excessive flashing. A smaller diameter die and mandrel were installed in an attempt to further reduce flash, die gap and bottle weight. The smaller diameter tooling worked well and the lightest, high quality bottles were produced at 92g for the hHDPE and the EXACT 0201 and EXACT 3132 blends. The minimum weight was 96g for the EXACT 3125 blend. The reader should be aware that although parison programming was available on this shuttle machine, the limited blend quantities available prevented the sample-specific optimization of wall distribution. Therefore, the wall distribution assessment given in this paragraph provides insights into the wall thickness variations observed when minimal effort is made to optimize programming. That said, the average wall thickness of the containers produced is displayed in Figure 3 as a function of the bottle locations indicated in Figure 2. The data shows an average wall thickness of ~0.040in with the thickest walls present in the top of the neck. Furthermore, there is good, uniform distribution of the CRCP and plastomer containing blend resin into the edges and lower chime of the bottles. The data also suggests that there is less uniform distribution of hHDPE in the bottles. However, since the thickness at points F and G is less than that at points J and K, it is likely that the head was slightly offset (towards the handle) to make it easier to catch the handle when the mold closed. The average thickness of the hHDPE bottle label panel (points C, D, H and I) is much greater than the edges of the bottle (E, F and K). While this is not a big surprise given the greater distance the parison “travels” to reach these mold extremes, it is interesting that this difference is not nearly as pronounced for the virgin CRCP or plastomer containing bottles. In short, while some interesting trends can be gleaned from the wall distribution assessment, the reader should realize that the average wall thickness of all of the bottles produced from all samples is fairly consistent and uniform. Consequently, the top load and drop impact data presented later is based upon consistent bottles. 5

Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

FIGURE 3: Wall distribution for baseline resins and blends. Letters correspond to the bottle positions indicated in Figure 2.

In summary, processing differences were observed between the CRCP and the hHDPE and plastomer containing blends. Smaller diameter tooling was needed to produce bottles using the hHDPE and plastomer containing blends. Considering the blends themselves, the EXACT 0201 (octene) and EXACT 3132 (hexene) processed most similar to the virgin CRCP while the EXACT 3125 (butene) processed the least similar. With this information in hand and a rudimentary understanding of production equipment, the key variables that will affect part production and quality are the diameter of the die and mandrel, mold size, type of production equipment (reciprocating shuttle or continuous extrusion wheel), plastomer loading and plastomer grade. Whether considering a switch from hHDPE to CRCP or considering adding plastomer to improve drop impact resistance, the plastics processor should be aware of these differences and should be prepared to make processing and perhaps even tooling changes when switching from one resin to another. Table 3: Extrusion blow molding processing parameters

EXACT Plastomer Blends1

Baseline Resins Processing Condition Parison Melt Temperature (˚F)

AD60-007 hHDPE

PP 9612 CRCP

EXACT 3132 Blend

EXACT 0201 Blend

EXACT 3125 Blend

395

456

468

450

474

Extruder Melt Temperature (˚F)

371

438

453

428

450

Melt Pressure (psi)

1140

1028

1058

1078

1023

Extruder Speed (rpm)

73.2

84.3

90.2

83.8

88.2

Mold Temperature (˚F)

50

65

50

55

50

Die Gap (Relative % Open)

0.5

6.0

3

3

2.5

Final Bottle Weight (g)

92

90

92

92

96

1. All EXACT® Plastomer blends are 15% (wt./wt.) in PP 9612 CRCP. 6

Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Bottle Performance The key performance attributes of this type of packaging are clarity, gloss, top load and drop impact resistance. Optical performance will be considered first. The haze measured for the baseline CRCP-and several of the plastomer containing blends is displayed in Figure 4 for both the blown bottles and QC-type injection molded plaques. Injection molded plaques were produced as part of the initial screening process and the data is included here in order to provide insight into the effects of crystallization rate upon clarity. The bottle cutouts were taken from the lower neck region (just above the label panel). This portion of the mold was polished. Examination of the bottle haze data (Figure 4) reveals that the addition of EXACT 3132 (0.900 density hexene) has virtually no adverse effect upon clarity while incorporation of EXACT 0201 (0.902 density octene) had only a minor effect. Adding EXACT 3125 (0.910 density butene) resulted in a noticeable increase in haze. This is expected given the higher density (higher crystallinity) of the plastomer and the resulting mismatch in refractive index. The effects of this crystallinity are also evident when the relatively “slow cooled” bottle data is compared to the “quench cooled” injection molded data. In the case of the CRCP and 0.900-type plastomer blends, the rapid cooling of the injection molded samples results in a haze value ~3/4 that of the bottle sample. Quench cooling of the 0.910 density plastomer prevents the higher level of crystallinity from developing, which reduced the haze to nearly 50% of that observed in the bottle sample. In short, rapid cooling should lead to clearer bottles overall and the effect is more noticeable when using higher density impact modifiers. It is also important to point out that no effort was made to compensate for clarifier dilution brought about by adding the plastomer. Compensating for this effect using either the base or new high performance nucleating agents should help negate the loss in clarity. Gloss was also very high (~65% @ 60°) on the virgin CRCP and plastomer containing blends. hHDPE bottle gloss was ~12% in contrast.

FIGURE 4: Optical performance of blown bottles (right bar) and injection molded plaques (left bar).

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

The top load and drop impact performance of the molded bottles is displayed in Figures 5 and 6, respectively. Top load or resistance to crushing is a function of bottle design, wall thickness and wall stiffness (flexural modulus). Since all bottles were the same shape and approximately the same thickness, the top load of bottles produced from the stiffer CRCP should be greater than that of the hHDPE. Similarly, the top load of the plastomer blends should be greatest for the highest density impact modifier. These trends were observed in that the CRCP bottles required 166lbs to generate 0.25" of vertical deflection. The hHDPE sample required slightly less (150 lbs). Among the impact modified blends, the 0.910 density EXACT 3125 exhibited the greatest top load at 157lbs, which is still greater than the hHDPE, while the lower density EXACT 3132 and EXACT 0201 possessed values of ~130lbs.

FIGURE 5: Top load performance of extrusion blow molded bottles at room temperature.

FIGURE 6: Drop impact performance of extrusion blow molded bottles at room temperature.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Drop impact resistance is typically inversely proportional to that of top load and this trend was observed for the CRCP and blend bottles. However, the drop impact resistance of the hHDPE bottles was even greater than expected. As Figure 6 displays, the hHDPE bottles exhibited an F-50 height of 8.8ft by the Bruceton staircase method. The CRCP bottle exhibited much lower drop impact resistance (4.7ft) while the EXACT Plastomer containing samples produced intermediate values that scaled with density. The 0.902g/cm3 density EXACT 0201 achieved a failure height of 7.8ft while the denser EXACT 3125 (0.910g/cm3) exhibited a 6.2ft failure height. To summarize, both the 0.910 and the 0.902g/cm3 plastomers are effective impact modifying agents. While the use of the higher density plastomer lessens the inherent decrease in top load, the lower density 0.900 or 0.902g/cm3 density plastomers impart better impact resistance and retain the characteristic low haze of CRCP resins.

Conclusions Both high-density polyethylene and polypropylene possess strengths that cater to different types of packaging, production mix and producer cost structure. The trend towards clear packaging has led to increased demand for clarified random copolymers of polypropylene but some package designs are pressing against the inherent performance limits of clarified randoms. Blending EXACT Plastomer with the CRCP is an effective way to expand the performance envelope of polypropylene packaging without compromising on clarity. The 15% loading of plastomer used in this study is appropriate for the impact modification of large household chemical or food packaging containers. This level is also considered appropriate for protecting smaller containers that are stored at refrigerator temperatures. Not surprisingly, variables such as the die/mandrel diameter, mold size, plastomer loading and, to a lesser degree, plastomer type all determine the magnitude of the process changes that may be required in order to swing production from HDPE to CRCP to impact modified CRCP. As this study indicates, these differences can be successfully managed to produce high-quality containers maximizing top load, drop impact or package clarity depending upon the type of plastomer employed. In summary, we recommend the addition of EXACT 3132 or EXACT 0201 to clarity critical, CRCP-based packaging running on swell sensitive equipment. Conversely, higher density impact modifiers such as EXACT 3125 can be used in more forgiving, less swell sensitive equipment producing less clarity critical packaging or packaging requiring less impact resistance or greater top load strength.

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of EXACT® Plastomers

Aknowledgements The authors extend their appreciation to Albert DaCosta, David Keegan, David Lake, Robert Upchurch and Jim Biggerstaff of Milliken Chemical for their assistance in producing and testing the extrusion blow molded bottles discussed throughout this manuscript. The efforts of many ExxonMobil Chemical employees are also appreciated. In particular, thanks go to Andrew Ahlborn and August Bucher of the Baton Rouge Polyolefins Plant and the Polymer Compounding Team of the Baytown Polymers Center. Lastly, we extend our thanks to Janet McCormick, Kristina Kwalik and, especially, Marsha Arvedson of ExxonMobil Chemical for their generous support of this work.

References 1. T.C. Yu and G.J. Wagner, “Polyolefin Modification with EXACT® Plastomers”, SPE RETEC Polyolefins VIII Conference, Houston, TX, February 1993. 2. T.C. Yu and D.K. Metzler, “Metallocene Plastomers as Polypropylene Impact Modifiers”, in Handbook of Polypropylene and Polypropylene Composites, H.G. Karian ed., Marcel Dekker, Inc. New York, New York, 201 (1999). 3. T.C.Yu, D.K.Metzler, and M. Varma-Nair, “Clarified Polypropylene Impact Modification Enhanced with Selected Metallocene Plastomers”, Proceedings of SPE ANTEC, vol 3. pp, Dallas, 2001 4. J. Truini, “ Gerber Packaging to Challenge Recycling”, p. 13, Plastics News, July 2, 2001. 5. Polypropylene Nucleator - Hyperform HPN-68, Plastics Engineering, vol 57. No. 11, p.37, 2001

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Extrusion Blow Molding of Clarified Polypropylene Rigid Containers: Influence of 13501 Katy Freeway Houston, Texas 77079 (281) 870-6862

©2002 Exxon Mobil Corporation. The user may forward, distribute, and/or photocopy this EXACT® Plastomers copyrighted document only if unaltered and complete, including all of its headers, footers, disclaimers, and other information. You may not copy this document to a Web site. The information in this document relates only to the named product or materials when not in combination with any other product or materials. We based the information on data believed to be reliable on the date compiled, but we do not represent, warrant, or otherwise guarantee, expressly or impliedly, the merchantability, fitness for a particular purpose, suitability, accuracy, reliability, or completeness of this information or the products, materials, or processes described. The user is solely responsible for all determinations regarding any use of material or product and any process in its territories of interest. We expressly disclaim liability for any loss, damage, or injury directly or indirectly suffered or incurred as a result of or related to anyone using or relying on any of the information in this document. There is no warranty against patent infringement, nor any endorsement of any product or process, and we expressly disclaim any contrary implication. The terms, “we”, “our”, “ExxonMobil Chemical”, or “ExxonMobil” are used for convenience, and may include any one or more of ExxonMobil Chemical Company, Exxon Mobil Corporation, or any affiliates they directly or indirectly steward.

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