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Post-consumer recycled HDPE: suitable for blowmolding?

Post-Consumer Recycled HDPE: Suitable for Blowmolding?

Recycling of post-consumer high-density polyethylene (HDPE) offers a significant opportunity to reduce the amount of that material in the solid waste stream. Although demand for recycled HDPE is reported to exceed supply, only about 1% of the HDPE produced in the U.S. is currently recycled. Much of the recycled resin goes into applications where physical properties are not critical, such as plastic lumber and selected injection molding applications. Recycled HDPE is also being considered in "higher-end" applications such as blowmolding, where the material is utilized as a layer in a coextruded container.

The purpose of this article is to evaluate whether post-consumer recycled HDPE can be used or modified to take full advantage of its potential to produce a quality part, specifically, in the blowmolding of monolayer containers. Commercial recycled HDPE resins are assessed, and suggestions offered on how these resins and the recycling process may be improved.

Experimental

All evaluations were conducted with six 450-kg samples of post-consumer recycled HDPE received from two leading plastics recyclers/converters - two homopolymer samples and two copolymer samples from one supplier, and one homopolymer sample and one copolymer sample from the other. The resins were received in cleaned, pelletized form, and all had originally been converted from bottles obtained in curbside collection programs. The recycled copolymer resins were originally in the form of various detergent containers separated from household waste; the homopolymer resins were originally in the form of uncolored milk and water bottles. All samples were considered representative of normal production at these recycling facilities.

The commercial blowmolding resins used as controls were a 0.29-g/10 min melt index, 0.954-g/cc density copolymer and a 0.80-g/10 min melt index, 0.960-g/cc density homopolymer. "Regrind controls" were made by putting regrind from samples molded from the control resins through the same number of heat histories as the recycled resins were believed to have been through. Blends of recycled resin/virgin control at 40/60 ratio were also studied.

The physical properties of compression molded samples were determined by ASTM procedures; see Tables 1 and 2. Results on resins received from each supplier were comparable, and they are therefore combined in the Tables for presentation purposes.

The properties of 25-g, 16-oz Boston Round bottles produced on an Impco A113S intermittent extrusion blowmolder are shown in Table 3. High-shear processability and swell changes were determined from bottles produced on a Uniloy 5630 intermittent extrusion blowmolder - 90-g, 1-gal industrial round bottles from copolymers and 65-g, 1-gal milk/water bottles from homopolymers. Swell was measured by two methods. Weight swell was determined by the initial weight change observed for a test resin after transitioning from the control. Diameter swell was determined by measuring on an inlaid centimeter scale the distance of the flash down the handle of the bottle after the bottle weight had been reset at the target weight. Swell changes are given in Table 4.

TABLE : TABLE 4. Swell Results, 1-gal
 Industrial Round Bottles.
Resin Diam. cms Weight, grams


[Copolymers.sub.b]
Virgin control 8.0 -
Regrind control 8.0 -0.7
Recycler A, #1 9.8 9.1
Recycler A, #2 10.3 7.6
Recycler B 10.0 3.1


[Homopolymers.sup.b]
Virgin control 8.0 -
Regrind control 8.0 0.0
Recycler A, #1 6.5 -4.7
Recycler A, #2 5.8 -3.6
Recycler B 6.0 -6.9


(a)90-g target weight. (b)65-g target weight.

Comparisons of smoke generation (volatilization of low MW components and/or contaminants in the resin) were made visually. Odor evaluations were also conducted: Bottles were immediately capped after molding and stored at - 40 [degrees] C until evaluated by a trained odor panel. The results of odor characterization and rating (also referred to as amplitudes) by the panel are given in Table 5.

Pellet Appearance

Recycled HDPE, both homopolymer and copolymer, exhibits a wide range of colors. In this study, the homopolymers ranged from gray to light brown to near-white (natural). None of the samples equaled the color of virgin homopolymer, although a couple were close. Color variances in homopolymers may be caused by contamination from unseparated bottle caps or from further oxidation of the polymer during pelletization.

All recycled copolymers contain color concentrate. Resins received from one recycler exhibited a veritable rainbow of colors, indicative of the number of colors used in the production of detergent bottles. Resins received from the other supplier were colored black to mask the original colors. Contamination was not evident from visual inspection of these resins.

Because of the obvious color differences between homopolymer and copolymer recycled HDPE, homopolymer is currently the favored resin for possible reuse in blowmolding application. HDPE copolymer is currently used in injection molding, pipe extrusion, and profile extrusion, where its color may be masked and its physical properties are adequate for the proposed end-uses.

Processability

No evidence of melt fractured was encountered during the molding of test bottles on either machine, a combination that delineates a wide processing range. After molding, inspection of the bottles made from recycled resins revealed moderate to severe contamination-predominantly dirt and labels from the surface of the feedstock. Some crosslinking of the polymer may also have taken place, causing the formation of the gels. It was not uncommon consequently, to observe 20 to 50 (or more) "specks" in each bottle. This contamination also caused numerous "blow holes" in the thin-walled bottled. However, in thick-walled, colored applications, this contamination would be difficult to spot. Even the homopolymer resins, considered to be the cleanest because of the nature of their original use, appeared to be severely contaminated. Their use as a middle layer in a coextruded structure may only be possible with levels of color concentrate in the outer layers sufficient to hide these impurities.

From the melt index shifts of the test resins and the swell results, it was obvious that either shear modification or some other form of degradation had taken place before or during the recycle process. An approximate 25%increase in swell(Table 4) was observed for copolymer recycle and a 0.10- to 0.20-g/10 min increase in melt index (Table 2) from the nominal 0.35 g/10 min normal for virgin copolymer HDPE. For homopolymer recycle, an approximately 25% decrease in swell (Table 4) accompanied a 0.10- to 0.20-g/ 10 min decrease in melt index (Table 1) from a nominal 10.70g/10 min common for this resin type. [Tabular Data Omitted]

Visual, and therefore subjective, analysis of the smoke levels showed that the copolymers exhibited severe smoke levels while both the virgin and regrind controls exhibited no smoke. The smoke from the recycled resins is hypothesized to be volatilized product (soap, etc.) not fully removed from the plastic. Smoke levels observed for the recycled homopolymers varied considerably-resins from one recycler exhibited no smoke while resins from the other recycler exhibited severe smoke levels. These differences may be attributed to either the cleaning technology employed or the bottle source.

Odor

The evaluations by the test panel, given in Table 5, show that both the homopolymer and copolymer recycled resins compare very unfavorably with the virgin and regrind controls. The panelists detected "chemical cleaner" and " soapy" offnotes for the recycled copolymers and commonly cited a "dishrag sour" offnote for the homopolymers. An amplitude difference of 0.5 or higher between materials is considered significant.

TABLE : TABLE 5 Bottle Odor Evaluations.

Resin Amplitude Offnotes

Copolymers
Virgin control 1.5 Typical HDPE:
Regrind control 2.0 burnt, waxy,
 resinous
Recycler A, #1 0.5 Chemical
Recycler A, #2 0 cleaner
Recycler B 0.5 soapy, sour


Homopolymers
Virgin control 1.5 Typical HDPE:
Regrind control 1.5 burnt, waxy,
 resinous
Recycler A, #1 0 Dishrag sour,
Recycler A, #2 0 dusty, musty,
Recycler B 0 pungent


(a) Rating scale: 0=worst; 3 = no odors detected.

Physical Properties

The gross changes in resin physical properties (Table 1 and 2) for homopolymer and copolymers were comparable, so some generalizations about recycled resins as a group can be made. Compared with the controls, the recycled resins showed no loss in flexural modulus, low temperature brittleness, or tensile strength at yield or at break. Considerable losses in tensile break at elongation and moderate losses in tensile impact were observed. Homopolymers showed a moderate gain while copolymers showed a moderate loss in notched Izod impact. Finally, a considerable reduction in the bent-strip environmental stress crack resistance(ESCR) result was observed for the recycled copolymers.

The result of the 16-oz Boston Round bottle testing (Table 3) show that the column crush of bottles made with both recycled homopolymer and copolymer was comparable with that of bottles made with the control resins. In some cases, the copolymer bottles had higher column crush (stiffness) than the controls, which is due to higher sidewall density imparted by color concentrates or by higher-density resins in the recycled feedstock. [Tabular Data Omitted]

Bottles from recycled copolymer showed a dramatic drop-off in ESCR performance - an approximate 60% loss in top-load ESCR performance and a 1-day F50 result compared with a 12-day F50 result for the control bottles. These results substantiated the bent-strip ESCR observations for the resins. Because no reduction in ESCR was evident with the regrind control, it was concluded that the ESCR of the recycled resins was substantially affected by the amount of contamination found in these resins.

Conclusions

The post-costumer HDPE resins studied exhibited adequate processability and a balance of physical properties quite adequate for a number of noncritical applications. To successfully utilize recycled HDPE in blowmolding, contamination and odor levels will have to be reduced. Development efforts under way in washing, extrusion, and stabilization methods are directed toward this end. Research to rationalize the observed losses in certain physical properties of the recycled resins will also be conducted.
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Title Annotation:high-density polyethylene
Author:Gibbs, Morgan L.
Publication:Plastics Engineering
Date:Jul 1, 1990
Words:1640
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