The most feared word in PET recycling is PVC, or polyvinyl chloride (plastic identification code #3). This is because PVC plastic, even in small concentrations will cause problems during processing or the remanufacturing of post-consumer PET resin and into new products. Just so you have an idea, negative impacts of PVC contamination can occur at concentrations of just 50 parts per million (ppm).
50 ppm is a very small amount; That’s .05 KG of PVC plastic within 1,000 KG of PET flakes, percentage wise, that’s only .005%. Even at this small concentration, PVC can form acids that break down PET resin both physically and chemically causing the PET plastic to become brittle and yellowish in color (two of the most important aspects of using PET plastic in the first place – clarity & impact strength). In addition, the gassing of chlorine vapors may occur, a general hazard of recycling PVC.
Sources of PVC Contamination in PET Recycling
For most PET washing recycling plants, controlling the concentration of PVC in the PET flakes produced is essential. This is because PVC tolerance is ultimately determined by the end-use application. While some applications can tolerate higher levels of PVC concentrations, most high-end applications (such as producing top-grade polyester fibers) require PVC concentrations to be well below 50ppm. Hence, in order to sell to high-end users at premium prices, PET recyclers must make extra effort to remove PVC from the PET flakes produced. This starts by understanding how PVC is introduced into the PET recycling stream.
Generally speaking, there are 4 primary sources of PVC contamination that can enter the PET recycling process:
- There are PVC bottles that resemble PET bottles. PVC bottles leave a white “crease” mark when flattened and can be removed by trained sorters.
- There are PVC safety seals on PET bottles (such as mouthwash bottles) that need to be removed before granulating.
- There are PVC liners inside bottle caps and closures. While this is no longer being used in the United States, occasional bottles with PVC liners will occasionally appear.
- There are PVC labels wrapped around the PET bottles.
PVC Removal in PET Recycling
While recycling machinery plays a large part in filtering PVC from a stream of PET bottles, we cannot neglect one of the most effective methods of PVC removal, manual sorting. As mentioned earlier, an experienced sorter can pick out PVC bottles based on visual inspection. While this is so, technology can be used to improve manual sorting efficiency.
Adding UV lighting to a stream of PET bottles have been tested to improve sorting efficiencies by trained professional by up to 99%. As PET bottles pass through the UV lighting, PET plastic absorbs the UV rays and chemically emits a blue fluorescent light in return. In the case of PVC, although PVC itself does not cause fluorescence, common additives within PVC plastic often causes PVC bottles to emit a green / yellow fluorescent light. With PET and PVC plastic giving off different colors, sorters can be easily spot PVC bottles and manually remove them. As UV lighting can be harmful during long exposures, sorters should work in 2 hour shifts.
Aside from manual sorting, automatic sorting has recently become a popular choice due to the increase cost of manual labor. Fully automatic sorting systems can be generally classified into three types, optical sorting systems, transmission technologies systems, and surface scanning systems. For all three systems, a detection signal is used that can differentiate plastic bottles based on chemical or physical properties with a sensor that can analyse this data. Air jets are then used to blow the unwanted bottles away from the plastic bottle stream. Of all the technologies used, the most reliable is the use of x-rays to detect the presence of chlorine within PVC bottles (chlorine is absent from PET bottles).
As both manual and automatic systems are not perfect, it is always beneficial to use a 2-3 pass routine to guarantee the lowest possible levels of PVC contamination.
Another popularly used commercial way for removing PVC plastic from PET flakes is thermal separation. As PVC becomes softened and sticky at around 200 degrees Celsius and PET plastic at 260 degrees Celsius, there is a 60 degrees Celsius gap where thermal separation can occur. That is, we subject a PET/PVC flake mixture onto a rotating, heated conveyor belt (or barrel) set at approximately 180-200 degrees Celsius. As the mixed plastic stream passes through, it’s hot enough where PVC sticks onto the conveyor belt with PET plastic being unaffected rolling off into a collection bin. On the bottom side of the conveyor, a stationary blade scraps off the PVC. While automatic sorting equipment requires heavy investments, this method of filtering PVC from PET is relatively low in cost.
Another low cost, yet effective method of separation is electrostatic separation. In this scenario, a mixed stream of PVC/PET plastic is subject to a charging chamber that induces a static charge onto the surface of the plastic. Due to the difference in physical and chemical properties, PVC becomes negatively charged while PET becomes positively charged. Once charged, the mixed plastic stream is exposed to charged poles where opposite polarity attracts separating the two types of plastic. A key factor in the success of electrostatic separation is to charge the plastics with enough energy but not so great so that the PVC and PET become attracted to each other.
Conclusion
We hope this article will give you a better understanding of PVC and it’s role in PET recycling. As buyers of PET flakes are always concerned about PVC content, it is essential to take all precautionary steps to remove PVC within the final PET flakes you sell to customers. Testing for PVC content should always be conducted to ensure you’re offering the highest quality PET flakes. As mentioned earlier in this article, even small amounts of PVC can render an entire batch of PET flakes useless.
Generally speaking, always aim for PVC content below 50 ppm, ideally below 30 ppm. We’ve provided several methods of PVC removal, however, it’s ultimately up to you, the PET washing plant’s operator to implement a system / routine that fits the type of PET bottle bales you get.
