Designing for thinner walls, gauges

Forced on a diet, many plastics products have already shed excess, unnecessary grams through design alteration, but to lightweight, thin-wall, and downgauge to the max, new resin grades might be necessary.

Stacy Fields, rigid packaging marketing manager for Dow Chemical Co.’s North American Basic Plastics business, says using less material is a concern for more and more customers. “Probably in eight out of 10 opportunities we have had with our customers, they are concerned with or have interest in trying to lightweight,” Fields says, “and a lot of this is driven by sustainability initiatives.”

That sustainability push has one well-known proponent that can affect sweeping change throughout entire supply chains. “At the end of the day, when you look down the value chain, a lot of these customers have Wal-Mart as their customer,” Fields explains, “so they’re trying to respond to the Wal-Mart scorecard. I think with traditional materials that have been on the market, they’ve done all they could from a downgauging perspective, but to get to that next level, they’re looking at how to do it even more.” Among other things, Wal-Mart’s initiative calls for an across-the-board 5% reduction in material usage in primary and secondary packaging.

At NPE2009, Dow launched several materials that promote the ability to further thin-wall, lightweight, and downgauge injection molded, blowmolded, and extruded applications. For blowmolding, Dow introduced Continuum EP high-density polyethylene (HDPE), a new generation of bimodal HDPEs that Dow believes bridges the gap between conventional unimodal HDPEs and bimodal ones by offering enhanced performance and processing in blowmolded bottles and drums. At this time, the product family includes two experimental grades: XDMDA-6630 EP for bottles, and XDMDA-6670 EP for larger blowmolded parts like drums.

By pairing augmented environmental stress crack resistance (ESCR) with high flexural modulus for greater stiffness, Dow believes the resins have the potential for significantly improved performance in existing applications, or equivalent or better performance in lightweighted containers.

Fields says the fact that the materials are bimodal vs. unimodal is key to their performance, with bimodal referring to the use of two reactors instead of one in the polymerization process. “With these two different reactors, we’re able to better control how [bimodal] material is put together,” Fields says.

Dow sees big opportunities for Continuum in the household and industrial chemical (HIC) segment, where some source-reduction strides have already been made. Perhaps most notably, the amount of plastic used for 96-oz HDPE laundry detergent bottles has been reduced by 7-8g, or 10% of their weight, in recent years. Elsewhere, applications in areas such as edible oil could also be slimmed, particularly larger jugs that are often accompanied by a corrugated cardboard box. “We can thin-wall and lightweight with our new resin to make the bottles thinner, but still have the same stiffness and performance integrity that they have today,” Fields says. “In some cases, they could even choose to reduce the extra package on the outside.”

Dow says Continuum allows container weights to be cut 5%-10% while maintaining or improving end-use performance. In addition, the company has seen that the resins allow increased incorporation of post-consumer recycled (PCR) content.

Also at NPE2009, Dow launched a highly clarified polypropylene (PP) for use in rigid thermoformed packaging. Inspire 222 is the first commercially available grade, promising clarity and stiffness for applications like cold drink cups.

In that particular arena, the shift by companies like Starbucks from polyethylene terephthalate (PET) to PP for cold drinks is providing a boost. Since PP has a lower density than PET, the cups’ weight can be reduced, with material savings of 20%-25% in some cases. Advances in the material have also improved its look, especially compared to PET. “The fact that we’ve improved the PP to have better clarity means customers don’t mind switching because they realize they’re doing something good for the environment,” Fields says. “They’re using less plastic, and it’s similar in clarity to PET.” Dow tests have shown that clear 12g 16-oz cold-drink cups using Inspire 222 can match the performance of 16g PET cups.

New material, same tooling
Fields said Dow has worked to ensure the new grades offer a drop-in solution, allowing processors to swap out materials without changing molds. “For the most part, there’s not a lot of variation in design, because we don’t want to require new tooling,” Fields explains. “We’re hoping that converters see the advantages with our material because you can downgauge or lightweight, so you get thinner parts, but you’re not sacrificing any of the performance properties of the resin.” Offering die swell, melt strength, and other properties similar to chrome-catalyzed unimodal resins, the new Continuum family reportedly makes for easier startups, improved pinch-off welds, and more uniform wall thickness vs. conventional bimodal resins.

The company is also working on a grade that can accommodate a range of machines, since most blowmolding systems on the market were designed to run previous-generation unimodal materials. “With the various types and ages of extrusion blowmolding equipment throughout the North American market,” Field says, “some applications were easier, while others offered a little bit more of a challenge.”

Dow says XDMDA-6630 EP HDPE works on a variety of extrusion blowmolding (EBM) equipment, including reciprocating screw/intermittent extrusion, continuous extrusion shuttle, and wheel lines, while XDMDA-6670 EP HDPE works on accumulator head machines for improved processing of drums and other large industrial containers.

Polyolefins push thin-wall limits
Elsewhere in polyolefins, Borealis says its injection molding PPs combine flow, impact resistance, and stiffness. It recently launched three transparent grades especially for thin-wall packaging: Borpact SG930MO, BJ356MO, and RJ470MO.

Borpact SG930MO is intended for high-transparency, deep-freeze applications like ice cream packaging, where transparency and cold-impact resistance are required. The material passes a 2m drop test at -20°C for a filled 700-ml container, compared to 10 cm for a standard transparent PP with a melt flow rate (MFR) of 20 and 1.2-mm-thick wall. The company says in a 15g container with 0.38-mm wall thickness, the material allows an 11% reduction in cycle time vs. a material with an MFR of 45.

BJ356MO is a heterophasic PP engineered to flow into molds easily. Borealis says despite the flow, BJ356MO has low taste and odor characteristics, with a white-based tint, making it a good option for portion-pack thin-wall food containers such as 250g margarine tubs. The material can withstand filling of multicavity molds even at wall thicknesses of 0.6 mm. As an example, the company cites a 1.5-liter ice cream container with 0.6-mm walls that runs in a two-cavity mold at a 260°C processing temperature. In this instance, the customer experienced a 16% reduction in cycle time due to easier demolding and a 15% reduction in injection pressure due to better flow.

RJ470MO is a high-fluidity 70 MFR random copolymer that’s specially designed to provide flow while maintaining the stiffness/impact balance of a typical random PP.

In films, LyondellBasell announced the North American commercial launch in September of a new line of metallocene linear-low-density polyethylene (mLLDPE), targeting high-performance film applications in food and medical packaging, shrink wrap, and heavy-duty shipping sacks, among others. Stephen Imfeld, LyondellBasell’s new business development manager for polyethylene flexible applications in the Americas, told MPW that compared to conventional LLDPE materials, Starflex resins allow processors to downgauge flexible packages and realize material savings and improved sustainability.

Imfeld said that at this time, there are six commercial products available based on three reactor grades, and the company is actively pursuing opportunities for additional grades. The key differences between the grades, which are based on Univation metallocene technology, are melt index, density, and additives. Two grades feature slip and antiblock additives, with another only offering antiblock. Base resin density ranges from 0.912 to 0.918.

PET’s thin-wall push to extend from bottles to thermoformed products
PET was an early target of the light-weighting push, particularly in 500-ml water bottles, whose ubiquity made them an easy mark for conservationists. Those containers, which formerly used preforms intended for carbonated soft drinks (CSD), recently weighed in the mid-20g range, according to George Rollend, senior technical marketing manager at PET resin and fiber supplier DAK Americas.

DAK has since designed a PET grade especially for water bottles, with the material featuring high flow at low melt temperatures for injection molding as well as good reheat performance for high throughputs in stretch blowmolding. Many bottlers and converters now use a preform specifically designed for lightweight water bottles, helping to cut mass in half or more, with 500-ml bottles now in the 10g or less range. “We call this substituting orientation for gram weight,” Rollend explains.

Given the drastic reductions that have already occurred, how much more can be reasonably expected? “I think technically feasible and commercially viable are two different terms one needs to consider,” Rollend says, when asked how low bottles can go. “Pragmatically, in the field, the consumer is going to judge whether it’s going to work commercially. If this thing is a rigid bag and its product gushes out when you open it, it’s not going to be well accepted.”

Perhaps counterintuitively, Rollend also points out that thin-wall bottles actually require a thick-wall preform with a short, fat appearance that allows the bottle, when stretch blowmolded, to become thin via a concept called self-leveling. Rollend says PET is one of the only resins that achieves self-leveling in the wall thickness. This occurs when the proper orientation levels are reached by using the design-intended blow-up ratio (BUR) from the preform to the bottle.

PET has also made a push into thermoformed packaging of late, targeting polyvinyl chloride (PVC) and polystyrene (PS) for the most part. In addition to clamshell deli-style packaging, PET paired with paperboard in non-food packaging results in a package that has large, proven recycling streams.

In this segment, however, DAK has not seen the same downgauging push. “I think the thermoforming segment is still a relatively young market in PET,” Rollend says, adding that the first candidate for reductions would likely be thermoformed cups.

Tom Sherlock, resins business director at DAK, sees a similar landscape. “I think most thermoform packaging is pretty lightweight to be begin with,” he says, “and when you’re lightweighting deep-draw cups and large clamshell trays, there’s a limit to what you can do.”

“Just realize in the thermoforming process, you’re not getting the same level of biaxial orientation that you do in a bottle,” Rollend says. “You’re drawing warm with only axial orientation, no hoop, so you get very little strength through orientation. So trying to lightweight just by downgauging doesn’t allow the material to reach the optimum self-leveling point that converters can achieve with the stretch blowmolding process.”

Another segment where DAK is seeing lightweighting occur is in beverages like sports drinks, with companies switching from multilayer hot-fill containers to monolayer cold-fill bottles, with brand owners reformulating the product to change the filling process.

“Cold-fill actually opens up the design freedom of the package,” Rollend explains. “Because you’re not restricted by the hot-fill requirements anymore, the walls don’t have to be as thick, they don’t have to have vacuum panels, and you can get more creative with your bottle design.”

Keeping score
Wal-Mart initially announced its packaging scorecard at the Clinton Global Initiative in September 2006, with the overarching goal of reducing packaging across its global supply chain by 5% by 2013. Three years in, the company, and by proxy, its suppliers, remain committed to the vision laid out at that time.

“We at Wal-Mart recognize that we have unique strengths and a unique opportunity to have a positive impact on the environment through our own actions, those of our customers, and those of our suppliers,” said Matt Kistler, VP of package and product innovations for the company’s warehouse outlet, Sam’s Club. “As vital as the packaging initiative is to reaching our environmental goals, it is also very good for our business and our suppliers’ business.”

The half-gallon industrial round bottles (pictured above) that were produced on a continuous extrusion shuttle blowmolding machine use Dow’s Continuum EP HDPE resins, allow the converter to lightweight the bottles by up to 10% while maintaining performance.

LyondellBasell says shipping-sack and stretch-hood applications can be downgauged in part by using its Starflex mLLDPE. —Tony Deligio