In addition to the information already provided on label contaminants, this is an additional guide outlining other potential contaminants that are often found in HDPE bottles. The severity of the contamination varies from substance to substance, and this guide should prove useful to those seeking to purchase the best quality HDPE scrap, as well as those who merely wish to learn more about the processes and challenges of converting post consumer scrap HDPE into HDPE regrind, pellets, granules, or flakes.
Adhesives
Adhesives are used on most all HDPE bottles as part of the promotional arsenal of the product with which they were already filled. Various types of adhesives cause various problems for the HDPE reprocessor, and attention should be paid when obtaining HDPE scrap that it does not contain glues from the section marked “Problematic Adhesives”.
The optimal glue for HDPE bottles which are to be reprocessed are any glues which are part of the water based glue family. Much of the HDPE recycling process involves washing the bottles, and water based glues quickly degrade and shed their labels with this treatment. Much time and money can be saved by obtaining HDPE bottles with labels attached with water based glue.
Problematic Adhesives.
Hot Melt Adhesives
Hot melt label adhesion is a process whereby a label is attached to a bottle using a hot liquid adhesive, which cools to a thermoplastic solid. At room temperature, the bonds of hot melt adhesives are nigh impossible to break without the use of very high temperatures. Hot melt adhesives are entirely resistant to water, and so cannot be removed with the usual recycling processes, which rely largely on water to remove various contaminants.
HDPE bottles with hot melted labels are not generally considered to be compatible with the recycling process, and will be discarded by HDPE recyclers. Purchasing scrap HDPE bottles which have been labelled in this fashion is therefore highly inadvisable.
Pressure Sensitive Adhesives
Pressure sensitive adhesives are adhesives which will bond with materials when pressure is applied. These are troublesome to remove, and as a result will quite often stay on the bottle, causing the label to also remain on the bottle, and to be present in the final product. This is hardly desirable, and can degrade the quality of the reprocessed HDPE.
Inks
Inks are generally not too problematic when it comes to the recycling process. Care should be taken however, to ensure that any labels which make use of colored ink either do not contain much ink, or are not a large part of the HDPE stream being recycled. Small amounts of water based inks can easily be removed during the intense washing stages of the recycling process, however large amounts of water based inks can contaminate the wash to the point that the resulting HDPE flake is also colored.
Caps, Seals, Closures, and Liners
Owing to the issues of product contamination, and the need to protect the public consumers HDPE bottles will often be equipped with special seals and closures in addition to their usual caps. These seals and closures are quite often made with materials other than HDPE, and their presence can contaminate the HDPE and cause it to be of a lower quality than desired, or useless altogether.
Polypropylene Closures
Polypropylene cannot be removed from the HDPE being recycled using current processes. Like several other plastics, Polypropylene stays with HDPE when it is being treated in the recycling process, and can cause a lower grade of HDPE to be produced. This lower grade HDPE is suitable for some applications, but is generally considered to be undesirable.
PVC Tamper Proof Seals/Liners
PVC tamper proof seals, whilst very useful for ensuring that a product has not been meddled with, cause huge problems in the recycling process. All too often, the PVC seal will not be entirely removed by the consumer, and even in cases where it is removed, there is often PVC residue on the bottle neck due to the design of the seal. PVC has a much lower melting point than HDPE, and chars during the melting part of the process, leaving black specks in the HDPE resin which lower the value of the resin, and makes it unsuitable for many applications. PVC liners are also problematic due to the fact that PVC is lighter than water, floats, and is therefore skimmed off with the HDPE during the float/sink part of the recycling process.
Aluminum Closures, Seals, Liners
Aluminum closures and seals produce similar problems to PVC seals. Due to the nature of a tamper proof seal, they are very difficult to remove from the bottle entirely. In some cases the consumer may be able to remove them, in other cases they are designed to stay on the bottle. Because they are not easily removed in the recycling processes either, they make for a reduced quality HDPE resin, which is not suitable for many applications.
LDPE Closures
LDPE is the lower density version of HDPE. It is quite often used for closures on HDPE bottles. Like the other closures, it is difficult to remove from the HDPE stream because it possesses similar properties to HDPE. In the float/sink process, where most contaminants sink to the bottom of the tank, and HDPE floats on the surface, LDPE will also float, causing it to be skimmed off with the HDPE. It will then be mixed in with the HDPE through the rest of the process, and results in a usable, but lower grade resin. Large amounts of LDPE should be avoided, although smaller quantities should not have a major impact on the quality of the reprocessed HDPE.
Metallic Objects
The other major issue for HDPE re processors are the metal parts that are sometimes used for various applications. Bottles that contain ball bearings or springs are generally not used in HDPE processing, due to the fact that the metals are difficult to remove during the recycling process, and may even damage the equipment, not to mention contaminating the HDPE and making it useless for further applications.
HDPE products incorporating metals should not be purchased for reprocessing purposes. Keep an eye out for scrap HDPE which comes from spray bottles and oil containers, as these are the most frequent culprits containing metal contaminants.