Demystified: Nine technologies or uses of PET bottle recycling
PET bottles are widely used in water and beverage packaging due to their multiple advantages such as light weight, high strength, sanitation, barrier preservation, weather resistance, aging resistance, convenient use, safety, and high recycling value after use. At present, the domestic annual consumption has reached Millions of tons, with an annual growth rate of nearly 20%. From a global perspective, its consumption is even greater, and a considerable part of it enters my country in the form of waste plastic after use.
The molecular formula of PET is shown in the figure below. Its glass transition temperature Tg is 80°C, and its melting point Tm is 265°C.
An indisputable fact is that my country has achieved 100% recycling of PET bottles after use, and every year a large number of PET bottles or bottle flakes are imported from abroad in the form of waste plastic.
In theory, there are two major ways of recycling PET bottles, chemical and physical. Physical recycling accounts for the majority. The physical recycling steps of PET bottles include: After removing the caps (HDPE materials) and labels (PVC, BOPP, EVA and other materials), crushing and cleaning the PET bottles to remove dust and residues in the bottles. And the adhesive attached to the outer surface of the bottle when labeling. The cleaned bottle flakes are dehydrated by a centrifugal dehydrator, the water content can be reduced to 2.0%, and then dried by a belt or tube dryer, the water content can be reduced to less than 0.5%. The bottle flakes at this time can be processed into various products after subsequent processing or modification treatment according to the purpose.
The following specifically introduces the nine major technologies or uses of PET bottle recycling:
1. Bottle to Bottle
After the PET bottle is used, its material properties remain good. It is ideal if it can be used as a raw material for bottle making after washing and drying, because the intrinsic viscosity of the PET resin for the bottle is much higher than that of the fiber-grade resin. Not ordinary PET resin can be used Bottled.
Under the condition of ensuring that the impurities and moisture are qualified, the intrinsic viscosity of the bottle flakes is still maintained above 0.75, making plastic bottles that can replace fragile glass bottles for packaging liquid pesticides, engine oils, chemical reagents, etc. If the chain extension or bulk viscosity-increasing technology is introduced during the processing, the bottle-to-bottle technology is even more advantageous.
The key to this technical route is the limited market capacity for such uses. For most markets used for water and beverage packaging, in view of the seriousness of food safety, my country does not yet have the option of using recycled PET as raw material to make packaging bottles. There is a social consensus that even more than a dozen countries in the world have allowed refillable water and beverages made from PET bottles.
2. Polyester staple fiber
At present, the main purpose of cleaned and dried PET bottle flakes is to make polyester staple fibers. The main reason is that the intrinsic viscosity of the PET resin used as the fiber is not high. Even a small amount of molecules occur during the processing of the bottle flakes. Hydrolysis or degradation will not affect the spinning process to obtain the desired product.
Polyester staple fiber can be used to produce various materials and products such as clothing, geotextiles, felts, and carpets. 20 500ml PET bottles can be made into a pullover, 5 2L PET bottles can be made into 0.09m2 carpet, 35 2L PET bottles can be made into a sleeping bag and so on.
Although the market capacity for this purpose is large, due to the large number of production enterprises, the competition is fierce, and there is a potential shadow of excess.
3. Polyester filament
At present, several domestic companies have realized that the processing of PET bottle flakes to produce polyester filament has greater economic and social benefits, and a lot of development research and trial production have been carried out. In 2010, a total of 5 enterprises across the country applied to the National Development and Reform Commission for resource conservation and environmental protection central budget support projects, and their installation scales are all over 30,000 tons per year.
4. Polyester masterbatch
Polyester staple fiber products are mostly white, but they often need to be dyed when making clothing or mixing them with new materials in a certain proportion. The use of PET bottle flakes as the carrier of the pigment is better than the brand-new fiber-grade PET resin chip, and its intrinsic viscosity is slightly reduced during the processing process without affecting the dispersion performance and additive effect of the color masterbatch, which has an advantage in production cost More obvious.
5. Reactive extrusion tackifying technology
If the cleaned and preliminarily dried PET bottle flakes are directly melted and extruded, the intrinsic viscosity of the PET bottle flakes may be reduced to less than 0.5dL/g due to the inevitable hydrolysis and degradation, and the intrinsic viscosity of the PET used for fiber making should also be required. It is higher than 0.65dL/g. Therefore, the water content in the bottle flakes must be reduced as much as possible during melt extrusion, but a more secure method is to add a diffusing agent to react and increase the viscosity to achieve the growth of the PET molecular chain.
The function of the chain extender is to condense the end groups of two or more PET macromolecules or perform an addition reaction on its molecules in the molten state of PET, so as to increase the molecular weight of PET and increase its intrinsic viscosity. .
Commercialized chain extenders include CESA-extend, a product jointly developed by American Clariant Masterbatch Company and BASF Johnson Polymer Company, whose active ingredient is epoxy functional styrene acrylic copolymer. The brand of chain extender sold by BASF Johnson on the Chinese market is ADR-4368, and its molecular weight is 6800. For PET with different initial intrinsic viscosities (I Vo), the relationship between the amount of ADR-4368 and the effect of chain extension is shown in the figure below:
The thin black oblique line in the figure indicates that excessive addition of ADR will produce insoluble matter in the product-the critical point of gel. In actual processing applications, the following formula can be used for calculation.
Among them: IVw=1.31-0.3595×Q (IVw: the intrinsic viscosity of the gel that may be produced after PET is added to ADR, dL/gQ: the amount of ADR added, %)
6. High barrier type nano composite modified PET bottle and film
Beijing Chonggao Nano Technology Co., Ltd. disperses the layered silicate in the PET base material in nano-scale (about 30nm thick, 1000nm long and wide), and the silicate forms a mosaic arrangement in a layered orientation, which can greatly improve The barrier properties of PET reduce the oxygen permeability, while also significantly improving the strength and heat resistance of PET. Characteristics and uses of high barrier nanocomposite PET resin:
●Apparent intrinsic viscosity: 0.72;
●Oxygen transmission rate: 30% lower than pure PET;
●Application: It can be used for the processing of engineering plastics such as reinforcement, enhanced flame retardancy, toughening, low warpage, etc. It can also be used for processing of PET packaging bottles and films with high barrier requirements.
7. PET/PE alloy
PET/PE alloy has many advantages such as hardness, high strength, easy processing, and low cost. Its brittleness is lower than pure PET, and it does not need to be dried before processing. This alloy has good fluidity and a faster cooling rate than HDPE, which is conducive to shortening the cycle of molded products. The key to the production of PET/PE alloy lies in the selection of compatibilizers. At present, 3 companies in Europe have successfully developed and put into the market.
8. Other PET modified materials
PET generally cannot be directly injection molded, but it can be processed by injection molding after it is made into glass fiber reinforced, wood-plastic and other composite materials. It can be used to make a variety of household appliances, automobiles, computers and other parts with good performance.
9. Plastic woven bag, cloth
Recently, the research and development work of processing and manufacturing plastic woven bags and cloths with PET bottle flakes as raw materials has become active, and processing technology and equipment have gradually matured. Entrepreneurs and experts in the industry who are concerned about this project have placed great hopes for this use. The application of PET bottle flakes in this field will promote the upgrading of plastic woven products and open up a new era of plastic woven products.