Low Pressure Polyethylene (HDPE) is a polymeric organic compound derived from gaseous ethylene.
The name of the material is associated with the features of the technology of its synthesis.
HDPE is obtained under a pressure of 3 to 4 atmospheres in the presence of a metal-organic catalyst.
For the synthesis of LDPE it is required to create a pressure of the order of 2000 – 3000 atmospheres. Both polymers consist of the same monomers —H2C — CH2–, but differ in structure and mechanical properties.
PND molecules have practically no lateral branches, i.e., are linear. Products from it are denser, stronger and harder than from LDPE.
Technology for processing low-pressure polyethylene into granules
The reverse side of the prevalence of PND is a large amount of non-biodegradable waste.
Used film and packaging can lie on the landfill for hundreds of years.
The accumulation of such wastes and the cost of manufacturing the primary polymer gave impetus to the development of technologies for their processing into secondary granules, which are subsequently used in the manufacture of plastic products.
Equipment of various capacities appeared – commercial and industrial classes.
The PND waste processing technology is as follows:
Collection and sorting. Industrial waste – gates and rejects – are clean and uniform. The polyethylene obtained from solid waste should be separated from large waste and sorted by color, size, quality and contamination.
Splitting up. In most cases, grinding is carried out in two stages. Waste passes through a shredder and then through a crusher (Crushers for plastic). The grinding method is selected depending on the type of feedstock. Shredders and crushers can cut, tear, crush or crush raw materials.
Washing. Crushers from industrial and commercial sources are most often not washed. Raw materials received from landfills are contaminated. It is washed with separation in flotation baths or hydrocyclones to separate solid impurities.
Drying. Centrifuges or screw presses are used for primary water separation. The residual humidity after this is 6 – 12%. Next, the raw materials are dried thermally or immediately sent for agglomeration.
Seal The washed and dried crusher is sent to an agglomerator or plastic compactor. Their work is based on heating to softening temperature and clumping of the material due to the applied pressure and friction forces.
Processing into commodity form. HDPE coils are loaded into a granulator, where they melt and mix. At this stage, the filtration of the remaining solid impurities and thermal decomposition of fats, the products of which are removed by degassing, takes place. The melt is forced through molding holes – dies. The resulting filaments (strands) are cooled to crystallization temperature and cut into granules.
The complete set of the PND processing line is determined by the initial state of the waste.
For example, cans, thick-walled bottles and pipes need not be granulated. Most often, they are immediately returned to production after crushing and washing.
HDPE containing metal inclusions, sand, sawdust, and other polymers are often subjected to repeated washing, and then cascading filtration in ganulators. Also contaminated material can be recycled by pyrolysis.
Characteristics of the low pressure secondary
Secondary low-pressure polyethylene is a granule with linear dimensions in all directions of 3 – 6 mm. They should be of uniform color, not have foreign inclusions and odors.
Not cut strands are allowed to enter the packaging. PTR secondary material must meet the requirements of the technology for the production of finished products from HDPE.
The quality of the granular secondary is primarily determined by the competent organization of the technological process and the capabilities of the equipment. If the raw materials are not washed thoroughly, solid inclusions will end up in the final product.
Insufficient evacuation leads to porosity. For a film and thin-walled products from HDPE secondary granules, such defects are especially critical.
One of the important factors in reducing quality is the thermomechanical and oxidative degradation of polymer chains. HDPE is destroyed in the processes of crushing, melting, from exposure to atmospheric oxygen and solar radiation.
In places where polymer chains break, radicals (unclosed valencies) are formed that cause crosslinking and further crushing in the melt. Such changes worsen the technological properties of HDPE granules.
To reduce degradation, complex concentrates are used.
They consist of:
thermo and light stabilizing additives.
If you build the technology for manufacturing regranulate so that the number of melting and mixing cycles is minimized, then its properties will be no worse than that of primary polyethylene.
What products are made from regranulate?
The PND secondary granule is used without restrictions and on an ongoing basis as a raw material for the production of the following types of products:
pipes for pressureless sewage;
It is allowed to mix the secondary polymer in a proportion that does not reduce the properties of the finished product:
Benefits of Using Secondary HDPE
The main advantage of the polymer obtained from recycled materials is low cost in comparison with the primary one. Thermomechanical recycling requires less energy than synthesis. In addition, a processing line is cheaper than synthesis equipment.
The use of regranulate in the manufacture of plastic products is one of the easiest ways to reduce the cost of production. It does not require re-equipment, but only the adjustment of the process.
Low-pressure polyethylene is one of the materials common in use, the production cost of which from recycled materials is several times cheaper and technologically simpler than through its synthesis. From this material, many types of products are produced daily, which we all constantly use – from disposable bags and plastic pipes to waterproofing products.
Nevertheless, in our country, the processing of HDPE waste, especially household waste, unfortunately, has not yet gained the desired momentum.