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How much recycle is really in that bottle?

Post-consumer recycle (PCR) is a hot topic for the plastics industry, which has embraced the concept and is now developing many technologies to implement it. None has had more consumer visibility than blow molding where it has become a strong marketing advantage to have a consumer package fabricated with some content of recycled material. And the higher the PCR content, the more "environmentally friendly" the product, the process, and the marketer are perceived to be.

With this in mind, an important distinction must be made between how much PCR goes into the process (an economic issue) and how much PCR actually ends up in each finished product (a marketing issue). Marketing claims about PCR content also have to be substantiated in some states (a legal issue) and are increasingly scrutinized by consumer and environmental groups (a public relations issue). Thus, for a number of reasons, processors need an accurate model to calculate PCR content. Such a model is also important in order to review their processing options when they have a job requiring a specific PCR content.

The most basic method of including PCR is to run a blend of PCR with virgin polymer in a conventional blow molding machine. This requires the least capital expense, but offers formidable technical challenges to make an "on-spec" container, because PCR tends to be more variable in its properties than virgin materials so the end product will vary. Significant issues like drop impact can make this method less than ideal.

In order to overcome risks presented by PCR's variability, another alternative is to run a multilayer container composed of pure virgin layers and PCR. The virgin layers add strength to the potentially weaker PCR layer and, to some degree, act as a barrier to protect the contents of the container from possible contaminants that might be in the PCR. The multilayer approach also allows more freedom in the aesthetics of the product, as the PCR can be covered with pigmented virgin material.

This multilayer method of blow molding was first commercialized in Europe and is now being used increasingly in the U.S. Two- and three-layer systems are used, but three-layer is more popular because the PCR layer can be completely surrounded by virgin resin.

PCR CONTENT: THE WRONG WAY

Taking an overly simplistic view of a container's PCR content can cause processors either to underestimate or overestimate its true PCR content. Neither one gives processors a true picture of their raw-materials usage and cost structure, nor their customers an accurate basis on whichh to make environmental claims in marketing their products.

Let's assume we are talking about what is in fact the most likely multi-layer bottle structure today, shown in Fig. 1. The bottle has three layers, with inner and outer virgin layers constituting 25% of total bottle wall thickness. The center layer containing PCR accoaunts for 75% of bottle wall thickness. Tests have shown that with reasonable amounts of colorant, a minimum 15% outer virgin layer is needed for aesthetic coverage of the PCR layer. Likewise, for technical reasons, 10% is typically the most cost-effective thickness for the inner virgin layer. Layer proportions are assumed here to be uniform throughout the container. Each layer is fed by its own extruder, as indicated in the figure.

However, there is always some tail and moil flash (perhaps handle punchouts as well) which we will assume for our example to be 40% of total parison weight. We will also assume, as is normally the case, that all of this flash is reground and fed back into the center layer along with fresh PCR.

Clearly, it would be erroneous to assume that because the center layer containing PCR makes up 75% of bottle thickness, the actual PCR content of the bottle is 75%. That overstates the case, because the center layer is "diluted" with 40% flash, only part of which was originally PCR.

Another erroneous measure of PCR content is reached by considering only the amount of "fresh" PCR added to eachh bottle-molding cycle. Consider the following: Bottle Total = 100% less: Flash (Tail & Moil) = 40% Outer Virgin Layer = 15% Inner Virgin Layer = 10% leaves: Fresh PCR Required = 35% That calculation underestimates the true PCR content because it disregards the substantial amount of PCR in the 40% of flash from previous cycles.

In order to develop a model of the "true" weight percentage of PCR relative to total bottle weight, the flash percentage must be factored in. The following discussion is based on the typical three-layer example described above, but it readily converts to a two-layer model by setting one of the virgin-layer percents equal to zero.

PCR CONTENT: THE RIGHT WAY

To understand the role of flash, it is easiest to begin with the theoretical first shot of the production run, in which no regrind goes into the middle PCR layer. The middle layer is 100% PCR, so the PCR content of the first bottle is 75%. This is also what the PCR content would be if no flash were produced or reused in this product.

But in fact, we have said we are producing 40% flash, so after the first container is made, that flash is reground and fed back into the middle layer along with 35% fresh PCR. Since the reground flash itself contains both PCR and virgin material, only 75% of that 40%--or 30%--is PCR. So the total PCR content of the second bottle molded is 35% + 30% = 65%. Note that the amount is lower than in the first cycle; the total PCR content will drop again in the next cycle and so on, but by a smaller amount each time.

In fact, a steady-state PCR content is reached rather rapidly--within fifty or so cycles, depending on the relative flash and layer percentages. A very simple personal-computer program written in BASIC language (Fig. 2) demonstrates this by repeated iterations of the above calculations for successive molding cysles. In the case of the bottle structure in Fig. 1, the PCR content remains constant to seven decimal places after only 17 cycles.

Line number 70 in the BASIC program in Fig. 2 holds the key to calculating true PCR content: [X.sub.N] = (Y-Z) + Z([X.sub.N-1] Where: X = Total PCR Proportion Y = Middle Layer Proportion Z = Flash Proportion Once a steady-state condition is reached after a few molding cycles, this will result: [X.sub.N] = [X.sub.N-1] Therefore the equation becomes: X = Y-Z + ZX X - ZX = Y-Z X(1-Z) = Y-Z X = (Y-Z)/(1-Z) Thus, it's necessary to know only the middle-layer thickness and the flash percentage to calculate true total PCR content. For our example, it becomes: Y = 0.75 Z = 0.40 X = (0.75 - 0.4)/(1.0 - 0.4) X = 0.58333 Therefore, the equation above yields a calculated steady-state PCR content of 58.3%.

FLASH PRODUCTION IS KEY

The equation above for finding the true PCR content of the bottle shows that PCR content varies according to the flash rate. As the flash rate goes up, PCR content goes down (as long as inner and outer layers are constant).

The equation above becomes a quite handy tool for determining the effect of different levels of flash production on the PCR content of the container. The effects of flash production on PCR content in the typical bottle structure of Fig. 1 are shown graphically in Fig. 3.

The figure shows that our bottle example has a true PCR content of a little under 60%. That's a good deal less than the nominal 75% that might be assumed from the middle layer thickness, and a good deal more than the 35% fresh PCR fed into the middle layer on each successive cycle.

In a consumer climate where a marketing battle can be won or lost based on how much the PCR content is, and serious legal consequences can arise from misstatements of PCR content, no one can afford either to understate or overstate their case.
COPYRIGHT 1991 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:post-consumer recycle content
Author:Belden, Dane A.
Publication:Plastics Technology
Date:Jun 1, 1991
Words:1328
Previous Article:Diagnosing and eliminating warpage.
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