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Plastics recovery manual 7
The collection, processing and recycling of waste plastics involve a direct contact to waste by various actors in the recycling chain. Outside, the waste can carry diseases and their manipulation may contaminate workers. The micro-organisms that cause discomfort such as diarrhea and dysentery find in the waste all the elements they need to grow and reproduce. Their presence in the waste is usually due to fecal matter found in some municipal waste. In addition, waste attracts rats which carry many diseases such as typhus. Finally, flies, mosquitoes and other insects are attracted by organic residues. They are also responsible for the transmission of diseases. For all these reasons, and because few recycling companies work beyond the soring stage, it is strongly advised to wash the plastic waste before sorting them. The quality of the sorting will be greatly improved while maintaining the sorters contact with the pathogens.
The different plastic matter recycling actors work in the presence of various chemicals which are sometimes poorly understood and are often neglected.
- Plastics can be regarded as inert matter that has no toxicity. Light, heat or mechanical processing however can degrade and decompose their respective monomers. Styrene (monomers of PS) and vinyl chloride (monomers of PVC) are very toxic, but ethylene (monomers of EP) and propylene (monomers of PP) can also cause problems.
- Many pigments and colorings contain heavy metals that form the basis for their pigmentation: cadmium (Cd), chromium (Cr), lead (Pb), cobalt (Co), Selenium (Se), copper (Cu), tin (Sn), ... and some are very toxic to human beings (especially Cd and Pb).
When pigments are used in the form of powder, we must ensure not to inhale them. It is therefore preferable to buy capsules (the so-called "masterbatches") for this purpose.
- Other additives such as plasticizers (phthalates), flame retardants (in brominated-based agents) and stabilizers (may contain barium, tin, cadmium or lead) also cause damage to health and the environment.
- Soda or caustic potassium, regularly added to the washing bath should be handled with the precautions described in chapter 4.
Air quality problems are found at different stages of the recycling:
- The grinding causes a lot of dust that can cause respiratory problems with the operators. The effects of inhalation of fine particles are found on the long term, after years of exposure. Only very thin particles are dangerous, others are normally captured by the respiratory system and can not affect the lungs. To remedy this, we must ensure to work with masks.
- The processing machine may ignite volatile materials. This can be done under two conditions:
- At normal temperature: degradation in normal working conditions due to the presence of contaminants, pigments, additives, ...
- At high temperature: degradation of polymers when the working temperatures are too high (release of hydrocarbons, NOx, SO², CO, CO²and chlorine gas in the case of PVC). Carbon monoxide (CO) is more easily caught by red blood cells than oxygen (O²). The inhaling of CO in too great quantities causes headache, states of fatigue, dizziness or nausea, vomiting and fainting. NOx causes irritation of nose and eyes. In a higher concentration in the air, their impact to the lungs may be irreversible. The hydrocarbons also cause irritation to the lungs, but some are also known for their carcinogenic effects. Releases of hydrochloric acid (HCl) due to the degradation of PVC can seriously affect the respiratory system of humans. This phenomenon is particularly worrying as the non-plasticized PVC is composed of 58% chlorine. When laminated, this value falls to around 50%. The shaping of PVC generally takes place between 150°C and 210°C, but degradation can be seen at temperatures from 180°C. We understand that it is strongly recommended to recycle the plastic when you do not have the proper equipment, namely: an extruder also allowing degassing, a good temperature control system, a properly ventilated room, ... The use of stabilizer is sometimes necessary.
- When the filters at the end of the extruder are clogged, they are generally cleaned by burning the residues of the perforated plate in a flame. The fumes that come from it also contain volatile substances that we must avoid to inhale (hydrocarbons if PP, PE, HCl if PVC, ...). Generally, we limit the nuisance caused by poor air quality by working in well ventilated rooms in accordance with good operating conditions.
Among all the activities of the recycling process, the grinding is the most noisy. It can reach 95 dB and thus exceed the disturbance thresholds and may irreversibly affect hearing. The use of helmets or caps to protect the ears is essential.
Some general considerations concerning the working conditions must be taken into account:
- Always work with clothing and safety gear such as:
- Gloves, impervious to washing. Indeed, in some countries, the washers work 6 to 8 hours with their hands in sodium hydroxide solutions with a high pH.
- Safety glasses (grinding);
- A work uniform;
- Earplugs or earprotectors for grinding;
- A dust mask (grinding and sorting);
- Perform regular breaks while doing exhaustive work such as washing, sorting, cutting, ...
- Work in an adequately ventilated rooms. Indeed, besides the air quality problem, recycling machines also generate much heat. Eight hours of work in the heat of an extruder may cause fatigue and slow down the immune system of the worker. The U.S. standard promotes a ventilation of 1.3m³/min per kilowatt installed in the room.
It is also important to highlight some commonly encountered problems:
- Skin irritations and cuts during the frequent washing and drying, so the use of solid, impermeable gloves is essential. The cleaning of grinders is also a delicate operation. We must always ensure we have unplugged the unit before cleaning it.
- Short-circuits are common, especially if the equipment is hand-made. Always put on earprotectors in connection zones and place them in boxes.
- Technicians can be burned upon contact with extruder sheaths if they are not protected. It is the same with any hot shaping machine part, even the simple touching of plastics coming out of the paths.
- A significant humidity in the plastic flakes entering an extruder may provoke a large degassing which can cause an explosion.
Quality of endproducts from recycling
The items made from recycled plastics have qualities that are inferior to those made from virgin resins. Some defects may cause problems for consumers. This is the case with pigments or other additives improperly mixed in the plastic, local degradation of the resin or the presence of undesirable elements in the waste. In addition, extruded products sometimes show a certain porosity, which can harbor bacteria. It is difficult to estimate the real impact that may result from such defects on the consumer. However, they are directly related to the workquality of the recyclers. The best way to limit them is to respect the work rules and to execute product quality control. It is also advised to not to use recycled plastics to make objects that could to be put into contact with food (packaging, bottles, shopping bags, ...) or that could come in contact with the mouth (plastic covers, plates, cups, ...).