Blowing film with PCR is not that tough.

Like it or not, making blown film with postconsumer reclaim is a skill you may have to learn.

Who in their right mind really wants to put post-consumer reclaim (PCR) into their blown film? PCR doesn't process well. It causes film imperfections like gels and holes. It's usually based on resins designed for bottles, not film. It's dirty and can clog your screenpacks--maybe even your die. PCR makes for unstable bubbles and downtime. And here's the kicker: It often costs no less than virgin materials that process better. All things considered, PCR in blown film is a bad idea.

Or is it? Thanks to advances in extrusion equipment and recycling technology over the past two years, much of the conventional wisdom about processing PCR in monolayer film has started to give way to a new reality. Commercial PCR resins still fall short of their virgin counterparts in many ways, but with a little extra care and up-to-date blown film equipment, reclaim incorporation need not present any insurmountable hurdles. "We don't even see it as much of a challenge to make blown film with PCR," says Jay Ragusa, engineering manager of Alpine American in Natick, Mass.

Indeed, Ragusa spoke just after finishing a 50,000-lb lab trial of a high-value ($3-4/lb) film containing 35% HDPE reclaim. That trial required no equipment modification, which Ragusa says is often the case. "You really only need slight equipment changes to compensate for the deficiencies of PCR," agrees Bill Podborny, who has worked with PCR resins as a senior research engineer at Quantum Chemical Corp. in Cincinnati.

Beyond the growing technical ease of incorporating PCR, several factors are strengthening processors' motivation for using reclaimed materials. As some film producers know all too well, the State of California has enacted a law that mandates PCR content. Starting next month, that law calls for 30% PCR in trash bags thicker than 0.75 mils. Even some film processors not directly affected are convinced that more legislation is forthcoming. "Anybody who makes an equipment decision without considering the future role of PCR is setting themselves up for a world of hurt," says Matthew Bangert, v.p. of Reifenhauser Film Systems in Peabody, Mass. There are also marketing advantages. A growing number of processors run PCR today to differentiate their products from other commodity films. They believe that in the hearts of consumers, "green" film is good film--both in trash bags and other uses such as grocery sacks or overwrap.

PRICE IS RIGHT

Best of all, HDPE PCR resins--the most common kind used in film--do not necessarily cost more than virgin materials. Today, virgin PE supplies have tightened, and prices have been on an up escalator all year. "PCR reclaim can be a cost-effective alternative," says Michael Kopulsky, chairman of recycler Envirothene in Chino, Calif. In fact, that company's Bottlesack HDPE PCR resin currently offers a price advantage over some virgin LDPE grades. "For every pound you replace, you save money," Kopulsky says, though he declines to cite specific figures. Likewise, Occidental Chemical Corp.'s Cyclathon PCR resins can also be "more cost effective" than virgin resins in some applications, according to James Clark, business manager for film resins.

Whatever your reason for tackling PCR, there are three keys to success if you're using reclaim percentages of 20% or more: good mixing, savvy selection of virgin blend materials, and strict quality control. There are also some processing considerations to bear in mind. "It's not quite as simple as just throwing PCR in the hopper," says Kopulsky. "But those processors who used to say they couldn't run it at all are knocking on our door today. It's a case of necessity being the mother of invention."

GETTING A GOOD MIX

If you ask someone who has worked with PCR about special processing requirements, mixing typically leads the list. Reclaim applications invariably involve some blending. "PCR from materials used in milk bottles has the highest melting point of all polyethylenes and a relatively high viscosity. It makes mixing difficult, especially with LDPE," explains Quantum's Podborny. Even film composed of milk-jug HDPE and virgin film-grade HDPE should be considered a blend. "Milk-bottle resins were not designed for blown film; they were designed for making dairy containers. Their melt characteristics are totally contrary to what makes a film resin," Podborny adds.

Bear in mind that any old mixing won't do; you need low shear to avoid degrading the resin. "The moral of the story is that you can't unshear what you've already sheared," Reifenhauser's Bangert warns. "You end up mixing something that's already been damaged."

Equally necessary is keeping melt temperature under control. "The number-one concern is to improve homogenization without increasing the temperature," says Alpine's Ragusa. "PCR already has one extra heat history and maybe more. The last thing you want to do is heat it more than necessary.

How to get good low-temperature, low-shear mixing remains a subject of debate among machinery suppliers. There is considerable disagreement over optimum extruder length for processing blends of virgin and PCR HDPE. Reifenhauser favors longer barrels with 30:1 L/D. "They give you the flexibility to process any material," Bangert argues.

Battenfield Gloucester Engineering Co. of Gloucester, Mass., also has 30:1 extruders for processing PCR blends. "The longer extruders tend to do a better job of mixing, says blown film product manager Bill Hellmuth. And the original proponent of longer extruder barrels, Windmoeller & Heelscher Corp. of Lincoln, R.I., still advocates a 30:1 L/D, in part for its ability to promote mixing of PCR blends.

By contrast, Alpine and Kiefel Inc., Wrentham, Mass., take a different approach, going with shorter extruders whenever possible. "The shortest residence times are best. So the shortest possible L/D to get good material out the other end is the right L/D," says Ragusa. If 21:1 does the trick, then use it, he recommends. If not, Alpine offers modular extruders as long as 29:1. Kiefel also favors shorter barrels for HMW-HDPE applications. "We have our regular 20:1 extruders running up to 33% PCR without any difficulty," notes president Steve Engel.

The long-vs.-short debate cools somewhat when it comes to mixing milk-jug HDPE PCR with virgin LDPE or LLDPE. Here, suppliers agree, longer-than-usual extruders work best. Even those machinery suppliers that normally endorse the shortest possible barrels for virgin-resin processing advocate longer barrels for blends of dissimilar materials.

TRY BARRIER SCREWS

All suppliers also recommend use of barrier screws as a way to get the requisite mixing in blends. "You use a barrier screw in PCR applications mostly for the mixing it gives you--not necessarily for the ability to process at low temperatures and pressures," says Podborny. Quantum and most machinery suppliers additionally encourage the use of some type of mixing section, such as a cavity-transfer mixer or Maddock-type mixing section. Kiefel, for instance, offers a barrier screw with its cavity-transfer mixer, which adds four L/D units. "We feel that a short screw is better for HDPE, so we use a short one, but we add a cavity-transfer mixer," Engel says. The result is a mid-length extruder of 24:1.

Likewise, Alpine frequently adds a mixing section to harrier screws. Its Dynasphere, which also adds 4 L/D to the extruder, looks like a collection of semi-circular lobes with holes to mix the material as it passes through under pressure.

Reifenhauser, stressing the low-shear objective, doesn't recommend a cavity-transfer mixer for reclaim applications--not even its own Star-O-Mix CTM. Instead, Bangert advises just sticking with a long barrier screw. "The long-L/D extruder does a better job of melting and mixing while imparting less high-intensity shear than a cavity-transfer mixer," he says.

One last screw and barrel issue: Podborny notes that normal extruder wear and tear causes more problems when running PCR because mixing becomes less effective as the gap between screw flights and barrel ID grows larger. "If you don't have the tight tolerance, it's harder to mix PC R with virgin. We've seen cases where changing to a newer screw has had positive influences," he says.

FILTRATION: GO CONTINUOUS

Continuous screen changers have long been a tough sell in blown film applications because processors can perform a manual or non-continuous hydraulic screen change during line shutdowns for die-lip cleaning. During PCR processing, however, screen changes can come more frequently--by one estimate, at least once a day when running 30% reclaim. "If you have the opportunity to install a continuous screen changer, it will definitely prevent downtime in the long run," Podborny says, explaining that even top-quality PCR resins tend to have more contamination than their virgin equivalents. "Screen changers are more necessary with PCR because you don't know what's in it from lot to lot," agrees Alpine's Ragusa.

Podborny notes that Quantum has seen best results when screen changes are based on pressure build-up rather than a fixed time interval. "Pick a setpoint and let pressure trigger the changes," he says. Otherwise, the screen pack can fail, sending all the crud off the screens and into the die.

Several machinery sup pliers also report that customers are using continuous models that swap screens without breaking the bubble. Gneuss Inc. of Matthews, N.C., says it has installed its KSF rotary-plate screen changers on blown film lines running PCR. And Kreyenborg, distributed in the U.S. by LCI of Charlotte, N.C., has installed its continuous non-backflushing screen changers at several PCR blown film plants. One finn in Texas has been systematically adding them to every line. LCI product development manager Dana Darley notes that continuous screen changers do not even have to be used in continuous mode to provide a filtration advantage of two to three times more surface area than discontinuous models.

One filtration problem specific to HMW-HDPE is that not all screen changers can handle head pressures that routinely top 10,000 psi. Alpine president David Nunes notes that Kreyenborg's high-pressure models have been used on Alpine HMW-HDPE lines, as have hydraulic non-continuous units from Extek Inc. of Danvers, Mass.

USEFUL EXTRAS

Mixing and filtration techniques go far toward overcoming some of PCR's inherent deficiencies. You can go even further with control systems to offset the inconsistencies that some users argue will always be present in PCR resins. "We've seen changes in PCR from one half of the box to another, affecting output, melt temperature, and neck height," says Ragusa. "You can't anticipate these changes, but you can close the loop." Gravimetric yield control, for instance, can offset resin inconsistencies that would otherwise decrease output.

The same goes for gear pumps, which play a similar role by keeping a steady output in the face of PCR inconsistencies--whether caused by increased contamination or melt-flow variation. LCI reports installing gear pumps at the same Texas blown-film plant that uses continuous screen changers on all its PCR lines.

In HMW-HDPE applications, the lower melt strength, another PCR trait, also warrants some special attention--especially since it conspires with other melt variations and contamination to threaten bubble control. According to Ragusa, these factors can add up to unstable bubbles and changing neck height, with the latter problem ultimately influencing film properties. "One big challenge is installing a cooling system that can handle the low melt strength," he says.

"Some processors who lived without chilled air in the past are now demanding chilled air," adds Alpine's Nunes. To address melt strength, the company recently redesigned the air-flow patterns of its ICS internal bubble-cooling system to provide more bubble control. And in a related development, Alpine has introduced a computerized control system that uses ultrasonic sensors and IBC air to automatically keep a constant neck height (see PT, July '94, p. 19).

A SPECIAL BLIND

Let's say your equipment is up to the job of extruding with PCR--as much of it is--but you still have problems. "The first thing people do is replace 25% of their regular virgin with PCR and say, 'Gee, it doesn't work,'" observes Bill Van Volkenburgh, project leader at Dow Plastics in Freeport, Texas. He says the real problem often is not the equipment but the resin blend. If you want to run PCR, you may have to consider a new virgin resin with which to blend the reclaim.

Why shouldn't your regular resin do the trick? For one thing, it may not mask undesirable characteristics of PCR resins. For example, if you normally run a 0.920-density LLDPE, and you simply blend in 25% HDPE reclaim, the blend density could shoot up near 0.930. "You'll have poor properties at 0.930," Van Volkenburgh explains. His solution: Add a lower-density virgin to get the average back to 0.920. Dow's Technical Service and Development group has used its Attane ULDPE in this role to come up with proprietary blends for customers. "ULDPE offers benefits with large quantities of HDPE by getting the overall blend density down to a normal LLDPE average," says Van Volkenburgh.

Likewise, OxyChem's Clark recommends use of higher-molecular-weight HDPE when applications call for PCR levels above 25%. Alathon L4903, for example, is OxyChem's HMW-HDPE that improves tear strength in PCR applications, Clark explains.

Similarly, Mobil Polymers in Norwalk, Conn., advocates its Super-strength hexene-LLDPE resins for accommodating PCR without loss of film strength, says Jill Paul, a former recycling manager at Mobil and now technical services manager.

Picking the right level of PCR is another key blending consideration. What may not be obvious is that PCR can actually improve end-use properties--but only at certain ratios to virgin. Podborny notes that strength properties in some applications peak at 25% PCR, not at lower levels as one might expect. "Don't just assume your properties will get worse and worse when you add PCR; it may not be true," he cautions. "Through the art of blending, you can find the regrind levels needed to boost properties."

Q-C IS VITAL

Depending on whom you ask, problems stemming from PCR's high contamination levels and lot-to-lot variation can range all the way from a minor annoyance to "a processing nightmare," as one PCR user describes it. Yet once filtration and mixing requirements are addressed, PCR can exhibit favorable processing characteristics. For instance, Quantum claims that PCR resins show very little melt-index variability. "The melt index in a PCR resin is more consistent because it's a universal average made up of many different materials," says Podhorny.

For the same reason, PCR tends to have a broader molecular-weight distribution than many virgin resins, which today lean toward narrow MWD. "Because it tends to have broader MWD, PCR can even improve processing in some cases," notes Dow's Van Volkenburgh. Thus PCR paradoxically may improve melt flow and bubble stability relative to some narrow-MWD virgin resins. Van Volkenburgh has seen this phenomenon occur even in films containing as much as 30% PCR.

Given PCR's potential for either easy processing or "nightmares," it's crucial to make sure that you're getting good-quality PCR. "The most important consideration with PCR is the quality of the material itself," says OxyChem's Clark, emphasizing a point made by all PCR suppliers.

Fortunately, commercial PCR quality standards appear to have improved over the last couple years, thanks to reclaim process improvements such as better backflushing screen changers and washing systems. "If you have clean PCR, it's not a lot worse than in-plant reclaim," asserts OxyChem's Clark.

HDPE PCR EFFECTS ON FILM PROPERTIES

                                  LDPE/            LLDPE/           HWM-HDPE/
Property                          HDPE PCR         HDPE PCR         HDPE PCR

Density                           increases        increases        no change

Melt Index                        decreases        decreases        increases

Ult. Tensile Strength

MD                                increases        no change        decreases
TD                                increases        no change        decreases

Ult. Elongation                   decreases        decreases        decreases

Vicat Softening Point             increases        increases        no change

Dart Drop Impact
Strength, F50                     decreases        decreases        decreases

Tear Strength

MD                                decreases        decreases        decreases
TD                                increases        increases        no change

Haze                              increases        increases        n/a

Gloss                             decreases        decreases        n/a

Secant Modulus, 1% Strain         increases        increases        increases

Flexural Modulus                  increases        increases        no change

Source: Quantum Chemical

Aside from having faith in suppliers' claims of cleanliness, what can a processor do to guarantee the quality of PCR resins? Podborny suggests that processors check PCR resin quality through melt-pressure monitoring. Increasing melt pressure provides one indication of increased contamination levels.

Podborny also recommends checking incoming resins for color problems and visible contamination. Discolored resin and black specs are obvious warning signs.

NO HARD AND FAST RULES

But at the end of the day, how the resin processes is the one certain test. "The only way to tell PCR quality for sure is to go out and make some film with it," says Van Volkenburgh. And when you do, remember that there are no hard-and-fast processing prescriptions to fall back on. You may have to tinker with narrow die gaps to get better drawdown and mixing. Or try out new temperature profiles or pressures. "Anytime you run PCR, you have to establish a new baseline," advises OxyChem's Clark. "PCR isn't exactly like virgin, but you can get close."