Solvay Research & Technology, Rue de Ransbeek 310, B-1120 Brussels, Belgium
In our increasingly affluent society, concern for the environment as well as for the quality of life is growing. A fraction of the public opinion is not far from imagining that suppressing plastics would definitely solve the waste problem. There is no doubt that mankind has to care for the future of this, his foster earth, but this must be done scientifically and realistically, and not emotionally.
Nevertheless, in spite of their very important efforts towards a better environment, efficient production to reduce energy consumption, significant reduction in pollution by using efficient modern affluent treatments, participation in collecting and recycling of post-consumer and industrial waste, the chemical, plastics, automotive, and other industries are often the scapegoats for this situation.
The only way for industry to change this situation is to inform as many people as possible and as often as possible about its numerous environmental activities and achievements. That is the challenge that Solvay as polymers producer and converter intends to achieve. The double goal to solve the plastics wastes problem and save natural resources can be served by several strategies, for which the E.C. has already set priorities. These priorities are as follows:
Although, recycling is very popular, it cannot be achieved regardless of specific ecological and economic constraints. In particular, the viability of mechanical recycling implies a subtle match between the available volumes ofrecyclable materials, the quality and the cost of sorting and regeneration.
Some years of experience also clearly show that a sound policy must necessarily combine several approaches, whereas in the field only well defined and realistic objectives must be selected. This work describes the mechanical recycling of PVC bottles and/or pipes to non-pressure coextruded three-layer pipes.1 In a first step the material (bottles or pipes) must be regenerated and in the second step, re-used. The coextrusion process also describes the modification ofthe production equipment required by multilayer coextrusion.
The mechanical recycling is recommended by authorities at a level of 10 to 15%. At the same time, the first R&D efforts have been committed to recovery of single polymer species, with an option of their reprocessing either to the original application or to less demanding use or preferably into durable items (for example: bottles to pipes).
To fulfill this task, new expertise is needed in sorting, purification, reprocessing, and blending, in addition to sampling and evaluation methods.
The first set of operations is the preprocessing of items collected from material recovery facilities, such as debaling, sorting, shredding, and coarse metal elimination. Various operations can then be combined to achieve the first purification stage: dry or wet grinding, wet centrifugation, and wet separation by density. These are followed by dry centrifugation, thermal drying, homogenizing and micronization. In order to conduct these studies, state-of-the-art equipment was purchased to evaluate continuous melt filtration.
According to the needs, new specialized modules will be added in future to the existing basic equipment (for example, electrostatic sorting, in-line polymer identification, etc.).
The collection of the recyclable pipes is organized by the producers association. The pipes are delivered in bulk to the recycling plant. It is a mixture of PVC and PE-pipes, with variable contamination by rubber, metals, sand, glass, stones, etc.
Sand Ferrous Sand Others metal
Figure 1. PVC pipes regeneration.
Sand Ferrous Sand Others metal
The collection of bottles is organized by the "consortia" and operated by subcontractors who also perform sorting. The sorted bottles are then baled and sent to the regeneration section. After this treatment the material is ready to be reused (second life). The sequence of the purification operations is as follows (see Figure 2):
The next step is the grinding of the bottles. This is a key point of the purification because it is very important to control the size of the chips. In order to minimize the amount of PVC fine particles which would be evacuated together with paper pulp from labels and small mineral debris, through small holes (2.5 mm) of the centrifuge separators. Not only this PVC would be lost, but more im portant, its presence in the holes would disturb the elimination of the contaminants.
An in-depth investigation of the grinding operation has been done, together with a precise balance of all incoming material flow in the pilot line (see the examples of a material balance and a diagram of the particle size optimization).
After this purification, the material is submitted to another separation by density (sink and float).
In this step, the light material (caps from PE or PP float) and the heavy material (PVC but also PET sink). In the next operation, PVC is dried and stocked in a silo homogenizer. At this stage of regeneration two possible ways existed, the continuous melt filtration operated on a Knaus filter or the micronization. We consider that the level of contamination of the PVC by the PET must be less than 100 ppm. This level of purity can only be reached by an upstream automatic system followed by a manual final tuning. Nowadays, PET flakes are often separated through "micronization" and sieving, based on the difference in brittleness of both materials, with an exception for the non-bioriented parts of PET bottles (neck and bottom), which micronize similar to PVC, thereby causing residual contamination, as their melting temperature lies much higher than the processing temperature of PVC. We consider this approach as insufficient.
As PVC-PET separation is an important step for both PVC and PET regenerators, research is being carried out in unconventional ways, i.e., electrostatic separation (differential triboelectrification) and froth flotation are currently examined.
Aluminum caps are the only other contaminant. Although aluminum has been widely replaced by PE, its separation still requires some development (e.g., hydrocyclones, Eddy currents, conductor/non-conductor electrostatic separation, or active metal detectors). The pilot plant is equipped with the Eddy currents separation.
RE-USE OF PVC BOTTLES AND PIPES FOR COEXTRUDED PIPES
The recycling of the PVC bottles and pipes is performed in the same machines, only some technological details in the feedbox and in the head ofthe machine are different.
From the chemical point of view, the formulations of stabilization and lubrication differ because of the difference between the original stabilization of bottles (calcium-zinc) and pipes (lead).
A market study showed that there is a very good opportunity to sell both types of pipes in which PVC bottles and PVC pipes are recycled in the inner layer. These pipes are already made and marketed under the following denomination:
DESCRIPTION OF THE COEXTRUSION PROCESS
The feedblock is followed by a sizing device, designed to give the parison a shape close to its final diameter and wall thickness. The rest of the line (calibration unit, cooling bath, saw, haul-off, socketing and packaging machines) does not differ from conventional pipe extrusion lines.
FORMULATION OF THE PVC
The formulation ofall three layers (compact inner layer, foamed or compact intermediate layer, and compact outer layer) must take into account that because of the length of feedblock, the polymer average residence times are much longer in coextrusion than in conventional pipe extrusion. Thus, thermal stability has to be increased.
The pressure drop in the feedblock is important because the balance between the fluxes for the three layers is achieved by a strong annular restriction in the flow channel. This restriction is similar to a chocker bar on a flat die. Lubrication (external and internal) is used to reduce the pressure drop in the feedblock-die assembly and hence, to reduce the outlet pressure at the extruder.
Adjustment of the lubrication is necessary to reduce the pressure drop in the feedblock and in the die. Optimization of the lubrication level is done by extruding different formulations through two dies on a laboratory scale extruder. The first die has smooth walls and the other, grooved walls. In the first die, because of lubrication at the interface with the metal, the polymer is allowed to slip as is the case in the feedblock and die used for tube coextrusion. In the second die, the grooves prevent slipping. Comparison between the behavior of the two dies (with and without slipping) allows for the calculation of the slip velocity. The influence of slip velocity on feedblock and die pressure drop has been measured experimentally and confirmed by numerical flow simulations. After upgrading, only minor differences in slip velocity remain between different batches.
Recycled PVC separation has shown the presence of up to 8% impact modifier. Impact resistance tests, performed with an instrumented falling weight, have shown that this level of impact modifier is more than adequate: coextruded pipes perform better under impact when the foamed layer is made of recycled PVC compared to a reference formulation based on virgin resin.
The acrylic modifiers already present in the recycled PVC have a positive effect on stretching behavior and no further modification needs to be made. The gelation level has a major influence on the draw behavior of the hot PVC melt. The influence of the formulation on the gelation has been measured.
Information have been collected regarding gelation time and maximum torque for some formulations. One ofthem have been chosen as the best compromise, i.e., it provides sufficient lubrication to reduce pressure drop in the feedblock and the die, while allowing for sufficient gelation. The foam is produced with chemical blowing agents. Azodicarbonamide is often used with PVC.
The influence, of the blowing agent on the foam density at different temperatures is an important factor. A foam density of 0.7-0.8 kg/dm3 is a good compromise between weight reduction and impact resistance. The chosen blowing agent gives a good density level and a low sensitivity to temperature.
Finally, we have seen that a low extrusion temperature was needed to control the viscosity of the foamed layer and maintain stability of flow for the multilayer structure. Material temperature after 10 min of residence time in the Brabender Plasticorder is used as an indicator of the melt temperature attained in the extruder. Here again, the formulation chosen to maximize the expandability gives a good compromise. Lubrication of the external compact layers could be used as an alternate means to reduce flow unstability, but the addition of lubricant is limited by the impact resistance.
MODIFICATION OF THE PRODUCTION EQUIPMENT
Some modifications of the standard production line of coextruded pipes have been necessary to process the PVC bottle scraps under optimal conditions.
The scraps entering the factory are first homogenized in a blending silo to reduce material fluctuations due to variation in bulk density and in scrap formulation.
Screw profiles were adapted to the control of the extrusion temperature. The temperature increase along the screws have been calculated for three different profiles. Screw 2 of the Figure 5 has been chosen for production.
On counter-rotating screw extruders, screw tips are known to cause hot spots in the melt. This problem has been investigated. Since the temperature has the major effect on the foaming process, this situation can result in circumferential heterogeneity of the parison. Thermal conditioning of the screw tips or appropriate lubrication, reducing the temperature increase of the melt, can solve the problem.
Finally, the length ofcooling baths has to be increased to prevent ovality of the pipe: indeed, the foamed layer acts as an insulator, and reduces the effectiveness of the cooling units.
The pipes produced with a foamed layer made of recycled PVC satisfy the requirements of ISO/DP 9970 for building discharge pipes and ISO/DP 9971 for sewage pipes.
For building applications, diameters for soil, waste, drainage range from 32 mm to 200 mm with a wall thickness of about 3 mm. Sewage pipes have a diameter between 125 and 500 mm; two different stiffness classes, 4 kN/m2 and 8
kN/m2 are used; wall thickness varies from 3 mm (125 mm diameter, stiffness 4
22 kN/m ) to 15 mm (500 mm diameter, stiffness 8 kN/m ). Impact resistance satisfies ISO standard 3127 for compact PVC pipes. Socketing of the pipe or separate fittings are provided.
Coextruded light-weight pipes having an intermediate foamed layer have been on the market for some time. This product has proven advantages over compact single layer pipes or traditional pipes (clay or concrete):
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