The Materials Analyst, Part 21: The problems with recycling

Recycling has rightfullya lot of attention from the plastics industry over the last decade. What was once looked upon as an industry segment that dealt in only the cheapest, lowest quality material has been elevated to a legitimate business. In part this is due to the increased media attention to renewable resources and the awareness on the part of our industry that positive image is sometimes as important as substantive accomplishments.

But another reality is that recycling has offered business opportunities. While times have been hard lately because of the downward trend in virgin resin prices, the last 10 to 15 years has seen the growth of a whole sector dedicated to finding uses for the most commonly collected post-consumer and post-industrial scrap. One of the most collected polymers in this endeavor is polyethylene, particularly high density polyethylene.

Plentiful PE
Polyethylene is attractive for a number of reasons. First, it is plentiful. No material is used more than polyethylene, although most of it is not injection molded in its first incarnation. However, it often finds its way into injection molding in its second life. And therein lies the potential problem.

Many of the products produced from virgin polyethylene go into the packaging industry. These are short-term applications where the consumer is expected to use the product and then discard the container. Because the expected use conditions are mild and the life expectancy of the product is short, very little thought is given to the long-term stabilization of the polymer. In these competitive industries, where cost reduction is king, no more than the absolute minimum in expensive stabilizers can be tolerated.

Because polyethylene is typically a very thermally stable material during processing, it offers little in the way of problems in the melt. If we measure the melt flow rate of a virgin polyethylene or polypropylene and then make the same measurement on the fabricated part, we typically see a 5 to 6 percent shift. Theoretically, materials in these families can tolerate multiple passes through the melt state before they begin to produce the kinds of problems that many engineering polymers experience after the first run.

Now here is the problem. This vaunted thermal stability often leads to a false sense of security. And when the packaging products like gallon and half-gallon milk jugs are collected and recycled, where do they go? Often they go into durable products such as pallets, railroad ties, and imitation wood for outdoor decks. These items will spend a lot of time out in the environment. Often this will involve outdoor service where ultraviolet radiation will work to degrade the polymer.

Deterioration Through Oxidation
Oxidation is the primary mechanism by which a material like polyethylene or polypropylene deteriorates. Heat or radiation will shake loose a chemical group, and oxygen from the atmosphere will readily combine at this site. This creates instability that can be further exploited and before long the polymer turns brittle. The additive packages provide protection from this mechanism.

But what happens when the stabilizer content is too low for the expected lifetime of the product? As we pointed out, a plastic package for a perishable food item may need to last only 10 to 12 weeks from the time it is molded until the time it reaches the scrap heap. If it is to be turned into a worthwhile material for durable goods, additional antioxidant must be added to increase the expected lifetime of the product. When this does not happen, the recycled product runs the risk of renewing the old image of reprocessed material.

In this particular case, our client came to us with pieces of outdoor deck material on which the surface was turning powdery and flaking off of the product after less than two years. The product was being guaranteed for 20 years, so clearly there were concerns over the cost of making good on a lot of product that might not work. Along with the field sample that was exhibiting the problem we were given two pieces of material from stock.

Isolating the Problem
One part was expected to be good while the other was suspected of having the problem that was showing up in the field. We performed the usual tests for composition and confirmed that the material being used was high density polyethylene. However, we noticed a shift in the melting point between the good and suspect product from stock. When we isolated the powdery residue from the field return, we saw this to an even greater degree. Figure 1 shows this. The melting point of the good material was 135C while the suspect material from stock was at 132C. The powdered residue, while still composed of HDPE, had dropped further yet to 128C.

While melting point reductions can be signs of severe degradation, we have to be careful in interpreting this change when it comes to a material like polyethylene. Changes in melting point in polyethylene can come from differences in density.

A good polyethylene with a density of .960g/cu cm will usually melt near 135C. But lowering the density to .950 may reduce the melting point to 129 to 130C. Traditional low density polyethylenes can melt as low as 105C and if modified with vinyl acetate they can go even lower. Since this was a compound made from recyclate, we did not want to over-interpret this one piece of data.

The best way to detect the tendency of a material to oxidize is to run a DSC test in an oxygenated environment and either heat the material until oxidation occurs or set the material at a fixed temperature and watch the process over time. For polyethylene, either technique will work. The dynamic heating method is the fastest because there is no need to estimate a correct constant temperature that may be too low. We simply heat the material in air until an exotherm associated with oxidation occurs.

Figure 2 shows the result of this test. The good material from the warehouse was heated up to 300C with no response other than the melting of the polymer. The bad material, however, showed clear evidence of an oxidation reaction just above 225C. This was the confirmation we were looking for that there was an inherent difference in the stabilizer incorporated into the products.

We repeated the determination using the isothermal method. In this case we used pure oxygen to speed up the process and make the oxidation reaction more vigorous and therefore easier to observe. We set the temperature of the DSC just below the melting point so that the material would not soften during the test. We then pressurized the system with 600 psi of pure oxygen and waited.

With the good material we had to wait more than 24 hours for a full reaction to initiate. For the problem product the wait was only a little more than five hours. Figure 3 shows the results. This was further confirmation that the level of stabilizer in the two products was significantly different.

Unfortunately, there was no way to determine if the lack of stabilizer was due to poor controls during the compounding process or if excessive stabilizer had been consumed during the molding process. Many processors are unaware of the fact that the molding process consumes some of the antioxidants put into the material during the compounding process. If a process is particularly aggressive, and an unusually high level of anti-oxidant is used up during the molding, the material has that much less protection as it goes out into the field.

Figure 4 shows a comparison of two polypropylenes tested using the high-pressure oxygen method. The raw material lasts 135 minutes while the sample from the molded part made from the raw material can withstand only 70 minutes. This reduction is not uncommon. Once the part is produced, the worst is over for the material. The few minutes that it spends in the barrel at 475F is the most aggressive environment it may face in its lifetime.

But if the application is one that involves continual exposure to elevated temperatures or ultraviolet radiation, continued deterioration of the additive packages will occur. It will just take place at a much slower rate. Hopefully, the additives outlast the reasonable life of the product into which they were molded.

The End of the Line
The problem with this type of attack is that the molecular weight of the product does not begin to suffer until the stabilizer package is all but gone and the degradation begins to take place at a much more rapid rate. In last month's article we referred to the oven aging tests that the automotive industry uses for polypropylene. The test calls for daily checks on samples being aged over a two-week period at 150C.

No one expects the parts to ever see those conditions; the elevated temperature is simply a means of accelerating the real world conditions that may exist under the car's hood or in a hot passenger compartment. Anyone who has performed these tests knows how suddenly the parts can be transformed from normal product to a material that crumbles. The apparently sudden change is not really as abrupt as it may appear. It is simply that until the antioxidant is consumed the material performs quite well.

Unfortunately, for this particular client there was no neat solution. No tests had ever been performed on a benchmark product with known performance so that a standard stabilizer level could be determined. Without this as a starting point, there was no way to track down the problem that led to the unusable product.

In all likelihood, this is one the lawyers will argue about for years, oblivious to the subtle chemistries that led to the problem in the first place. But those of us in the industry should take a lesson from this experience. With recyclates as with virgin materials, it is important to be aware of the end use conditions of the final product when formulating the compound. This extends beyond the polymer family to the details of additives and stabilizers.