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Studies show strengths, weaknesses of HDPE and coex recycle.

Studies Show Strengths, Weaknesses Of HDPE and Coex Recycle

Two major new studies have provided a wealth of detail about the properties of post-consumer recycled (PCR) plastics. Morgan L. Gibbs, senior research engineer at Quantum Chemical Corp.'s USI Div. in Rolling Meadows, Ill., examined the properties of actual repelletized PCR HDPE from commercial recyclers. He found it lacking in cleanliness, which accounted for its shortcomings, especially in environmental stress-crack resistance (ESCR). Much of his data appears in the proceedings of the recent SPE ANTEC meeting in Dallas, and further results were presented to a group of editors.

The second study (unpublished) was by Eval Co. of America (Evalca), Lisle, Ill., the joint venture of Quantum and Kuraray of Japan. Technical development manager Ronald H. Foster presented to a group of editors data on the properties of in-plant regrind from commercial producers of coextruded containers containing polypropylene and EVOH, as well as data on blends of this regrind with pure PP. Although not obtained from actual PCR, these data should help resolve the current controversy about the recyclability of such multilayer containers. Foster concluded that there's no longer any reason to doubt their reusability, now that the effects of EVOH in PP blends are apparent.


Quantum obtained 6000 lb of PCR HDPE from two commercial recyclers in the Midwest, along with several 5-lb samples of past lots from each source, in order to evaluate consistency of PCR over time. The homopolymer PCR came from unpigmented milk and water bottles, the copolymer PCR from detergent and soap containers. All the PCR was obtained from household curbside collection programs.

Test samples of these materials were compression molded into plaques and blow molded into 25-g, 16-oz Boston round bottles on an Impco intermittent-extrusion machine, and into 90-g, 1-gal industrial round containers on a Uniloy intermittent-extrusion machine. The performance of PCR was compared with a virgin 0.80-MI, 0.963-density homopolymer and 0.29-MI, 0.954-density virgin copolymer. Samples of these virgin control resins were also molded and reground to simulate the additional heat history of the PCR. Overall results were as follows:

* Pellet appearance: No natural color homopolymer matched the color on virgin, though some came close. All copolymers were precolored.

* Processability: No evidence on melt fracture was encountered in molding the PCR on either machine. However, numerous "blow holes" occurred in thin-wall bottles, caused by moderate to severe contamination that was evident upon close examination of the bottles made from PCR. This contamination was predominantly dirt and paper label particles. It was not uncommon to observe 20 to 50 contamination specks in each PCR bottle. This contamination might not be evident or harmful, Gibbs said, in thicker bottles made with sufficient color pigmentation, or with the PCR in the middle of a coextruded structure--again provided that sufficient coloring is used in the outer layers.

Gibbs noted that it was obvious from a shift in melt index that some degree of degradation had occurred either before or during the recycling operation. The average PCR melt index for homopolymers was about 30% lower than for either the virgin control or regrind control; for copolymers, the PCR average MI was about double that of the virgin and 50% above that of the regrind control. On the other hand, Gibbs noted that the lack of melt fracture in processing indicated that the high-shear viscosity of the PCR materials was unchanged from virgin.

Significant differences were observed in die swell for PCR vs. the controls. The homopolymer PCR decreased in swell, while the copolymer PCR increased. This was enough to produce major weight variations in molded bottles, of +3 to +9 g for homopolymers and -4 to -7 g for copolymers.

Gibbs also reported that the PCR was significantly more "smoky" in processing than virgin or regrind control. He attributed this to traces of the bottles' contents (soap, etc.) still remaining after washing.

* Odor: PCR materials scored very poorly on odor tests, clear evidence once again of incomplete washing.

* Physical properties: PCR showed no loss in flexural modulus, low-temperature brittleness, column-crush strength, or tensile strength. Homopolymer PCR showed no loss of notched Izod impact or notched low-temperature brittleness, though copolymer PCR did show 25-30% reduction in these two properties. And tensile impact of PCR samples declined 12-15% for homopolymer and 20-30% for copolymer. Elongation at break was also drastically lower for all PCR materials.

* ESCR: Quantum focused particular attention on the drastic loss in ESCR for both homopolymer and copolymer PCR, whereas none was observed in the regrind control. As little as 20% PCR in a blend with virgin produced 70-75% reduction in ESCR performance. Gibbs believed this was due primarily to the dirt and paper specks in the PCR.

To test this hypothesis, Quantum obtained samples of washed but unpelletized PCR flake from the commercial recyclers. Running this through a 2 1/2-in. extruder caused the screen to fill up with contaminants in only 10 min. Quantum next put some of the PCR flake in a Banburry-type mixer, together with extra antioxidant, to break down the paper label bits into smaller particles. This was then passed through a 4 1/2-in. extruder with a 100-mesh screen, which was able to run for 30 min without plugging.

When this "cleaned up" PCR was blown into bottles, it was now possible to produce a thin-wall, 65-g milk bottle without blowouts, unlike before. Also, ESCR improved significantly, showing only 15-20% loss relative to virgin, vs. 70-80% before.

Gibbs proposed that biaxially oriented HDPE film labels would be a means of avoiding the paper contamination problem; printing on the bottle instead of labeling would be another "recycle-friendly" approach. (CIRCLE 32)

With regard to PCR consistency, Gibbs' data show considerably more lot-to-lot variability from one recycler than the other, and generally more variability in copolymer PCR than in homopolymer. Twelve lots each of homo- and copolymer PCR from both recyclers together showed a melt-index variation from the mean of [+ or -] 20-25% for homopolymer and [+ or -] 8-16% for copolymers. Density varied only [+ or -] 0.3-0.6% in either case. Taking copolymers as the worst case, variation from the mean in physical properties was: flex modulus [+ or -] 12-24%, tensile strength [+ or -] 9-14% [at sign] yield and [+ or -] 30-56% [at sign] break, ultimate elongation [+ or -] 89-222%, notched Izod impact [+ or -] 33-44%, tensile impact [+ or -] 37-49%. Bent-strip ESCR for copolymers varied [+ or -] 44-52%.

In ongoing research, Quantum plans to investigate PCR odor masking or elimination, color improvement, and feasibility of blending PCR HDPE with coextruded PP-based PCR barrier packaging, possibly with the use of compatibilizers. (CIRCLE 3)


According to Evalca's Ron Foster, "The answer to the question, `What effects does EVOH have on the properties of recycled plastics?' is little or none. In fact, some properties of PP homopolymers can be increased by blending with recycled materials from ketchup bottles, retort containers and juice bottles." Foster believes this conclusion of his study has been confirmed by an independent study by SPI's Plastic Bottle Institute (PBI), which indicated that reground PP/EVOH ketchup bottles can be treated virtually indistinguishably from monolayer PP (see PT, May '90). For that reason, both Foster and PBI see no reason why PP/EVOH ketchup bottles should not use the SPI recycle code "5-PP," instead of "7-Other," as is currently done.

Foster adds that multilayer packaging typically accounts for only 1 1/2% by weight of the recyclable plastics containers and bottles in consumer waste. Even with the most optimistic growth assumptions, he does not foresee coex packaging exceeding 8-10% of PCR. Nonetheless, Foster says there are significant amounts of industrial coextruded scrap available for recycling, How that this has been proved feasible. Although coex packaging manufacturers typically include 32-44% recycle in their products, "Nobody uses up all their process scrap," according to Foster.

Foster's study consisted of three parts. The first was coextrusion of sheet mimicking the typical composition of retortable and hot-fill bottles and thermoformed containers: homopolymer, random copolymer or impact copolymer PP together with either 5% or 30% by weight EVOH and 4-5% tie layer. These materials were reground and compared with monolayer sheet regrind.

The second phase utilized actual regrind from a commercial producer of a thermoformed container consisting of 37% impact PP, 44% process regrind, 15% EVOH, and 4% tie layer. This material was compared with 100%-PP regrind, and against blends of 3%, 5% and 10% coex regrind with pure impact PP.

The third phase of the study utilized regrind from the process scrap of a commercial producer of ketchup bottles. These consisted of 58% random-copolymer PP, 32% process regrind, 5% EVOH, and 5% tie layer. This scrap was compared with virgin PP random copolymer and with blends of 5%, 10% and 20% coex regrind plus random PP.

Results from all three phases were in substantial agreement, though there were a few differences between actual commercial regrind and lab-prepared coex sheet samples. The following trends were observed, with the magnitude of the effect generally greater at higher EVOH levels, and sometimes insignificant at lower levels. Blends of coex regrind with pure PP generally showed these effects to a degree proportional to EVOH content:

* Tensile strength of the coex regrind was usually greater than for monolayer PP (the exception was the ketchup-bottle regrind). This is not surprising, said Foster, as EVOH has high tensile strength.

* Elongation was reduced with coex regrind. EVOH is relatively stiff, Foster noted.

* For the same reason, flex modulus was higher with coex regrind.

* Notched Izod impact increased substantially for coex based on homopolymer PP. Both types of copolymer PP showed smaller increases in notched Izod with the actual in-plant regrind, but no change or a decline in impact with the lab-prepared sheet samples.

* Heat-distortion temperature was a few degrees higher for the coex regrind from the retort containers, though this property was not measured in other phases of the study.

* As for processability, spiral flow at 450 F showed almost no difference between coex and pure PP of all types.

For the future, Evalca will study properties of coex PCR ketchup bottles processed by commercial recyclers; making products from this recycle; and properties of regrind from the new Heinz PET/EVOH ketchup bottles (see PT, May '90). At some point Evalca hopes also to study coextruded flexible packaging containing EVOH. (CIRCLE 4)
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Title Annotation:high density polyethylene and coextruded plastic containers
Author:Naitove, Matthew H.
Publication:Plastics Technology
Date:Aug 1, 1990
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