Recycled Plastics: The Hidden Cost

By Michael Sepe, Materials Analyst

There are several compelling reasons to recycle plastic products into a new stream of raw materials and subsequent products. But the view of recycled materials has undergone an evolution over the relatively short history of the plastics industry. The notion of recycling first appeared on a large scale in the mid-1970s during the first oil crisis. Short supplies and a prolonged economic malaise that lasted in one form or another through the remainder of the decade led some manufacturers to think for the first time in terms of limited resources. As challenging as today's global economy may seem, 30 years ago the plastics industry faced painful times indeed.

Polyethylene, which had sold in some circles for less than $.10/lb in the previous decade, rapidly tripled or quadrupled in price as processors scrambled to obtain enough raw materials to continue manufacturing product. Even with these heady increases, many raw material suppliers walked away from the business during this era as most of the potential margin was swallowed up by feedstock price increases. This made the already tight market even tighter.

Postindustrial vs. Postconsumer
It was in this context that the industry began to look at what was then called reprocessing. It was viewed by many as an activity reserved only for the low end of the market and was shunned by many molders, but the concept and the practice were here to stay in the vocabulary of the industry as some pioneers began to work on making suitable products from materials that had already had a previous life in another form.

This was more difficult then than it is now because the infrastructure for collection and postprocessing that exists today was not in place. In addition, an understanding of the issues associated with the lifetime quality of a product such as molecular weight distribution and additive quality was not in place. Most early reprocessed materials were little more than repelletized regrind.

Over time, many major material suppliers embraced the practice on a slightly different level. They began to produce materials manufactured from their internal scrap. This led to the important distinction between postindustrial recycle and postconsumer recycle.

Postindustrial materials had several advantages. First, they had never left the watchful eyes of the company that made the original polymer. Since the people handling the material knew the requirements that the products needed to meet, they could adjust for attributes like molecular weight and additive content while still producing a low-cost material for a relatively small trade-off in consistency. Second, the product had never been through a conversion process and had not spent any time in the field as a molded article. Therefore, the polymer had been spared the various degradation mechanisms that can occur when products spend extended periods of time in demanding application environments.

But the push to do more than just reconstitute the internal waste stream continued. In the late '80s and early '90s, the industry was influenced by a public obsession with waste stream reduction. If the glass and aluminum people could do it, why couldn't we? I remember being in New York City for the 1989 Antec. Walking around the hotel with our badges on brought inevitable questions from people about what we did. When we responded that we were in plastics, the reaction was almost unanimously one of scowls and disdain. Clearly, we had an image problem. Something had to be done.

In the ensuing years, we have made recycling sexy. Those early pioneers who toiled with dented gaylords of mixed color flake or even unground purgings have started to look pretty smart. And almost every community in the country now has curbside collection for at least PET polyester and high-density polyethylene, the so-called “1s” and “2s.” The market has sometimes made it difficult for suppliers of recycled materials to flourish because frequently the cost of primary raw materials fresh from the reactor has been low enough to make the economics of recycled materials hard to justify. In the last two years, this has obviously changed, and manufacturers are becoming increasingly creative at finding new streams of reclaimed product from which they can cull inexpensive raw materials.

The inherent variation that can arise from making molded parts from such sources poses a curious counterpoint to the drive for unwavering consistency embodied by disciplines such as Six Sigma. Occasionally, these trends collide in the form of an unusual performance problem with a product. When these situations arise, analysis is part of the solution. But tracing the history of the product that arrives in the laboratory back to its former life is often necessary to provide a real solution.

Diaper Dilemma
This particular case begins with a product based on polypropylene and compounded with wood fibers. Organic fibers are becoming an increasingly popular method of extending synthetic polymers since they represent a renewable resource that can offer some of the same advantages as mineral fillers. They are particularly useful with materials like polyethylene and polypropylene, which can be processed at temperatures below the degradation temperature of cellulose. Frequently, the wood products used in these composites are also recycled materials. Therefore, when a problem arises with a product manufactured from one of these composites, finding the source is potentially a complex problem.

This problem involved blooming of a gel to the surface of the molded parts when they came in contact with water. The parts looked completely normal until they became wet. If left in contact with water for a period of time, a gelatinous film would form and the surface of the product would become slippery. This film could be wiped off, and after several wet-dry exposure cycles the problem would go away. This suggested that whatever ingredient was responsible for the problem was being drawn out of the product and was present in a limited amount.

When the samples came in for evaluation they showed no abnormalities. However, placing them in water did bring the slippery gel to the surface. While it was easily isolated, initial attempts to identify the gel by infrared spectroscopy were not so successful. It was apparent from the initial spectra that most of the gel was water, and water absorbs so strongly in the infrared region that it dwarfs almost everything else. However, when the gel was dried, small crystals began to form. These crystals had lost most of their moisture and could be readily analyzed. Figure 1 shows a spectrum for the dried gel and Figure 2 shows an excellent match with a compound known as sodium polyacrylate.

Very often, this is only the first step in analytical problem solving. Now that the offending material has been identified, where did it come from and how did it get into the final product? Fortunately, we live in the age of instant information. Simply typing sodium polyacrylate into an Internet search engine reveals that the substance is a crystalline material with the ability to absorb 200-300 times its weight in water. In doing so it turns into a gel. Its primary commercial use: diapers. A call to the client confirmed that the polypropylene being used in the composites was being culled from recycled diapers. Further confirmation came from an evaluation of different grades of the composite that employed different ratios of polypropylene and wood fiber. Those compounds with higher loadings of polypropylene produced more gel and underwent more wet-dry cycles before gel formation stopped.

Tracking Costs
The rationale for using a source like this for the polypropylene is, of course, cost. And this type of recyclate stream is attractive because the supply is likely to be consistent and volumes are probably stable. But the cost equation changes quickly when a problem like this arises. Can the polypropylene be reclaimed without allowing the absorbent filler into the material stream? If this separation is not possible, what is the cost of cleaning the polypropylene before compounding? The good news is that the two materials are readily distinguishable based on their affinity for water. A washing process would readily remove the sodium polyacrylate and drying the separated polypropylene would not be difficult since the polymer is not hygroscopic. But what does it cost to set up and operate such a process? Recycling is most attractive when the price of virgin materials is high. What is the business risk if the investment is made in the separation process and then the price of virgin material falls back to earth?

Material prices have been a focus for processors over the last two years and many in the industry have resorted to extensive measures to rein in those costs in an effort to protect margins. But price reductions are seldom obtained without some sacrifice in performance over the long term. A lot of processors have learned this the hard way, incurring huge costs to recover from quality problems that arise when a raw material fails to meet expectations.

When the material comes under scrutiny as a root cause, many processors are surprised at the variety of unusual sources for the polymers in the compounds they are molding. The only sound way to conduct a cost-reduction program in raw materials is to track all costs accrued in dealing with quality issues related to a given supplier. At the end of the day, it is the bottom line, not the material cost line, that determines the survival of the business.

February , 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.