Sorting

Sorting of whole bottles by resin type is essential to effective plastic bottle recycling and generally takes place at the materials recovery facility (MRF), or at an intermediate processing center (IPC), which consolidates and processes source separated materials. Some plastic reclaimers accept mixed plastic bottle types and separate them at their facility; however, for the purpose of this general discussion, the same sorting principles apply, regardless of where the materials are sorted.

Sorting of commingled recyclable materials is, in general, a labor-intensive effort, even where automated systems are used. Keep in mind that MRFs and IPCs separate and prepare several materials for reclamation including glass, aluminum, steel, paper, and plastic. This discussion will be confined to the sorting of rigid plastic bottles.

Sorting Methods

Two basic methods are currently used to sort plastic bottles for recycling. Sorting of whole bottles is carried out manually (by visual inspection) or automatically (detection systems that use sensors to analyze one or more properties of the plastic bottles passing by).

Manual Identification: Sorting of whole bottles by visual inspection is done by bottle shape, color, and/or product recognition. However, this method can lead to inaccurate identification and separation due to human error or distorted containers. In addition, complications arise when bottles of the same design are made of different plastic polymers. These are called ‘look-alike’ bottles. Most plastic bottles manufactured in the United States are stamped on the bottom with a resin identification code (SPI code). However, this numbering system has limited value to sorting personnel, as the volume at which cost-effective sorting must be done precludes looking at the bottom of every bottle passing by.

Automated Identification: Automated sorting (auto-sorting) systems employ one or more detection systems that use sensors to analyze the physical or chemical properties of plastic bottles passing by and separate them into categories: by resin type, color, or both. Auto-sorting was initially developed to separate PVC from a stream of plastic bottles, using x-ray systems that could easily identify the chorine element in the PVC. However, these systems were limited to detecting PVC only. More recently, techniques based on near-infrared analysis can identify and separate plastic bottles of multiple resin types, providing automated, ‘positive’ selection. Techniques based on optical scanning using one or more cameras, can separate plastic bottles by color. Although it has limitations, auto-sorting greatly improves the quality and efficiency of the separation
process.

Regardless of the method employed to sort whole plastic bottles, it is in the best interest of the MRF or IPC to create products with the greatest market value. A few MRFs currently accept all rigid plastic packaging for recycling into mixed resin products like plastic lumber. However, most separate incoming materials to maximize the value of the specific categories of materials collected for recycling.

The APR Design for Recyclability Guidelines will discuss five plastic bottle categories that presently represent more than 99 percent of the plastic bottles used to package consumer products. Sorting these materials properly offers the best opportunity to maximize their value for recycling into new products.

Automatic sorting can also be done for flake, but only as a “polishing” step to remove low levels of identified materials.