CBOT sampling protocols

Best Practices in PET Recycling

Blending

Issue:Proper blending of recycled resins with each other and with virgin, is required to meet customer targets for certain properties, to achieve recycled-content levels, to combine various production lots to a targeted Intrinsic Viscosity, and to "blend off" material that is slightly out of specification. The quality of the final blend is dependent on sound and consistent operating procedures.

Best Practices: Blending is an effective technique for combining a variety of recycled PET materials with each other or with virgin. The goal of blending is to uniformly combine multiple sources or types of PET. Blending of PET, for purposes of this discussion, is limited to the physical mixing of two or more sources of recycled PET materials. (Compounding is discussed separately because it involves intimate melt blending of resin(s), additive(s), filler(s), and/or reinforcement(s) in an extruder or intensive mixer).

One of the primary goals of PET end-product manufacturers for blending and other preprocessing, is to preserve the intrinsic viscosity (IV) of the material. It is recommended that the IV of the major blend components be within a range of 0.10 dL/gram to ensure that properties and processability are not compromised.

Dry blending of PET generally combines several sources of recycled PET, including post-consumer regrind, pre-consumer bottles, preforms, trim scrap, other industrial PET sources; and virgin resin. Both batch and continuous blenders are used.

A Best Practice is to pre-blend the recycled-content PET formulations when multiple PET sources and significant proportions (generally greater than 10%) are being converted into an end product. Poor mixing/blending will cause variations in the processibility of the melted resin and result in high scrap rates and inferior end-products.Low blending temperatures are maintained to prevent the material from softening or degrading.

Conventional extruders and injection molding machines can disperse small proportions of additives and pigments into the melted resin. However, they are not designed to homogenize multiple components into a uniform melt during the brief residence times used for commercial production. Therefore, specialized blending equipment is recommended. There are many types of equipment used to dry blend PET flake and pellets. Selection is dependent on the volumes and types of recycled PET, particle size distribution, blend percentages, and budget considerations.

In many cases, hopper blenders or silo blenders are used. These conically shaped units continuously circulate and disperse material by means of an internal screw auger. Other types of blending equipment include ribbon blenders, tumbling blenders, and high intensity mixers. Ribbon blenders are stationary vessels that use a rotating shaft with spiral blades to mix and convey solid particles. Tumbling blenders are specially equipped to rotate on one axis and contain various baffle configurations to improve mixing.

Intensive mixers are used for very high loading of materials (> 50%), such as when colorants are blended into a carrier resin to produce a color concentrate. The basic design of an intensive mixer is an enclosed mixing chamber where two fluted rotors rotating in opposite directions produce the mixing action. The product is “softened” by the mixing, to a more elastic compound that is fed to an extruder for conversion to pellets.

A centralized blender is commonly used when several processing machines run the same blend or when one large machine runs at a high throughput, generally above 600 pounds per hour. The output from these blenders can then be conveyed directly to drying hoppers.

Another best practice is to blend on a weight basis to minimize inaccuracies caused by bulk density differences between flake and pellets. Volumetric feeders are reliable if the size and shape of the resin are consistent. Recycled flake, however, tends to vary in size and thickness. Gravimetric or other weight-based feeders are the preferred choice for supplying blenders when variations occur in particle size of the recycled PET.

A final best practice is to dry the PET mixture after blending, not before. In this way, the PET blend is dried just prior to processing and kept dry until it enters the throat of the extruder or injection molding machine. The loss of properties due to hydrolysis of the PET, even with residual moisture, can be significant.

The contaminant levels in the finished blend will equal a weighted average of those in the incoming streams. However, the properties of the blend may not be an average of the individual components, especially if the IV differences between the formulation constituents are wide.

Benefits:Proper blending allows homogeneity, which reduces variations in processibility of the melt and therefore reduces scrap rates and inferior end-products.

Issue Date / Update: January 1998