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Incorporating post-consumer HDPE in large blow-molded applications.

Government and private environmental organizations are forcing plastics engineers to consider ways of using materials recycled from solid waste streams in large volume product applications. One popular candidate material, homopolymer high-density polyethylene (HDPE), is readily available in many parts of the U.S. from reclaimed milk jugs, juice bottles, and related items. However, finding a stable source of clean, uniform-melt-index material has been a challenge. An alternate material, copolymer HDPE from recycled detergent bottles, has been found to be a more stable source.

Once sources for the reprocessed material are located and its purity and properties determined, the properties can be matched to products having large volume application or large parts having medium volume application. The result would be the same - new uses for the recycled material.

Project Development

When this project started, major consumer product manufacturers were well along into incorporating recycled HDPE into small containers, making further effort into large-volume, small-to-medium-sized parts somewhat passe. Large parts were then closely examined. Some states were already encouraging the development of synthetic geomaterials for civil engineering applications with long life cycles. Other products considered were traffic barriers, flotation devices for piers, residential trash haulers, and trash cans. The long-life-cycle trash haulers were selected. Their expected service life of more than 10 years is a major improvement over the 1-to-2 week life of a milk jug or the 90-day life of a detergent bottle.

Several candidate manufacturers were then identified, that is, those interested in the problem of incorporating recycled HDPE into residential trash haulers. The properties of the recycled materials could then be matched to the production and manufacturing requirements.

Common production methods for such items are injection molding, blow molding, and rotational molding. Melt index (MI) values of the primarily homopolymer stream were found to vary from 0.2 to 0.8. Such fractional melts are used for large-parison parts to maintain melt strength. However, parts are sometimes run with copolymer materials having whole MI values ranging from 5 to 12. Further investigation found a stream of copolymer (green in color) from detergent and household chemical bottles having an average MI of about 8.

The amount of material readily blendable for injection molding was still low because 16 to 22 MI is preferred for large parts. Rotational molding was also limited by this factor, and because the material is not available in fine granule or powder, the preferred forms for the resin. Preferred melt indexes for blow molding, however, are 5 to 8 for large HDPE parts, and 8 to 12 for smaller parts.

Manufacture

The project team then selected a commercial source of recycled detergent bottle copolymer HDPE and proceeded to evaluate blends ranging from 100% virgin to 100% recycled. Both lids and base units were molded. They were found to handle well with no deviations required in molding times, extrusion barrel temperatures, or cooling times. Only minor fluctuations were observed, which were not considered abnormal.

The bases were molded on Hartig machines using 50- to 70-lb accumulator heads, depending on whether one or two containers were molded at one time. The lids were automatically doubled up, using a 33-lb accumulator head on a Battenfeld machine. No trouble was observed using regular trim fixtures and jigs, or in the molding cycle.

The color/stabilizer system selected for good outdoor longevity consisted of celadon green and black, both traditional container colors, augmented by a hindered amine light stabilizer (HALS) and an antioxidant. Lids made with the black additive package were tested by accelerated weathering.

Results

Tensile and elongation properties were determined on a research-grade test instrument equipped with a spreadsheet program for data acquisition and analysis. The weathering tests were conducted on a QUV Accelerated Weathering Machine using a 24-hr test cycle specified by the State of Florida and developed by the ASTM D20 Committee on Plastics. The cycle consists of exposure to UVA 340 lamps, reported to give a good representation of the natural sunlight radiation spectrum, for 20 hr at 50 [degrees] C with no condensation and 4 hr of dark at 40 [degrees] C with condensation.

The data presented in the Table show some variation in initial properties. In particular, a sharp drop in toughness (area under the stress-strain curve) occurred between the 50/50 blend and the 25% virgin/75% recycled blend. This loss in toughness could endanger the effective use of higher than 50% concentrations of recycled resin in longer-term applications.

Based on considerable field testing of trash haulers made from a variety of resins, the accelerated weathering data show that blends of 50% or less recycled HDPE adequately maintained their properties and toughness for up to 12 years. While these blends did drop in toughness, they still exceeded the properties of the 75% and 100% recycled copolymer detergent bottle stream. It should be noted, however, that the two latter blends were still found to be generally adequate for trash-hauler use.

The 100% recycled detergent bottle material did show an increase in tensile (brittleness) on weathering not evidenced in the other compositions. This could be a problem in extremely cold weather. The bottom line is that one would have a high confidence level using 50% or less recycled material in this application. In applications where the units would be sheltered from sunlight most of the time, for example, indoors in a garage or a municipal building, the 100% recycled material appears to be very adequate.

Conclusions

The incorporation of recycled resins from major waste streams will require the consideration of the stability of the recycled materials' properties and their subsequent matching with potential longer-life-cycle applications. Making large, blow-molded trash haulers from blends of recycled and virgin copolymer HDPE is a practical application of this process. It is recommended for up to 50% recycled HDPE in the final composition.

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Title Annotation:high-density polyethylene
Author:Walling, Ronald L.
Publication:Plastics Engineering
Date:Jan 1, 1993
Words:966
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