Selective froth flotation of PVC from PVC/PET mixtures for the plastics recycling industry.
The production of plastics (polymers) has grown significantly in the United States in the last several years (26 million tons in 1988) with an annual growth rate of about 10% (1). By the year 2000, plastic production is predicted to increase by about 50% to almost 41 million tons (1). As plastics have become a more popular material for industry, an increasing volume of plastic wastes needs to be recycled for both economical and environmental reasons. Up to now, a major portion of plastic waste is either landfilled or incinerated with municipal solid waste. Net municipal solid waste discarded in the United States currently amounts to approximately 141 million tons per year of which plastics are estimated to account for 7.0 to 7.5 wt% (18-20 vol%) (1, 2). Landfilling and incineration of municipal solid waste becomes more expensive and in many instances these methods of disposal are no longer acceptable. In this regard, plastic waste disposal has become an important issue in the United States due to the increasing volume of municipal solid waste and the decreasing landfill capacity for disposal.
Recycling of post-consumer plastic wastes is quite new in the United States. At the beginning of the 1990s, only about 1% of the post-consumer plastic waste stream was recycled (1). However, the plastics recycling industry has started to increase its capacity in recent years. In particular, success has been observed with the recycling of soft-drink bottles, 41% of which were recycled in 1992 (3). It is evident that polymer recycling is an emerging technology that will continue to grow.
As listed in Table 1, the six polymers of greatest use in the United States are polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP). An important characteristic of these polymers is that they can be melted and reprocessed without any serious change in their physicochemical properties. This can be attributed to the lack of the cross-linkage between the monomer chains in these polymers.
Table 1. Examples of Plastic Products (1). Polymer Primary Product Markets low-density polyethylene packaging (LDPE) high-density polyethylene packaging (HDPE) polypropylene (PP) packaging furniture & furnishings polyvinyl chloride packaging (PVC) building & construction polystyrene (PS) packaging polyethylene terephthalate packaging (PET) consumer products Polymer Examples low-density polyethylene refuse bags, coated papers, (LDPE) wire and cable coatings high-density polyethylene milk and detergent bottles, (HDPE) heavy-duty films polypropylene (PP) syrup bottles, yogurt and margarine tubs, drinking straws, auto battery cases, office furniture & machines polyvinyl chloride cooking oil bottles, detergent bottles, wall (PVC) covering, flooring, construction pipe, meat wrap, phono records polystyrene (PS) disposable foam dishes & cups, egg cartons, take-out containers, foam insulation, cassette tape cases polyethylene terephthalate soft drink bottles, (PET) other beverage, food & medicine containers, X-ray and photographic film
Recycling of post-consumer plastics has begun only recently and was initially limited to rigid packaging materials, especially soft-drink bottles made of PET and milk jugs made out of HDPE. The plastics are collected and sorted by hand and by machines at reprocessing plants. The plastic wastes are chopped up into flakes in a high-speed shredder/grinder and cleaned with a detergent and water spray. The dry flakes are melted down and cast into pellets which are used to make new plastic products. Unfortunately, in most cases the materials recovered from recycle operations cannot compete with virgin polymer. Chemical contamination with paper, metal, adhesives, other polymers, and pigments, even if reduced to a very small level, limits application of recycled plastic in the food market. Contamination of the main product with other polymers of different melting points and thermal stabilities can limit the quality of the recycled plastic. For example, PET remains unmelted at PVC processing temperatures while PVC contamination of PET leads to discoloration of the product (2). Diminished physical properties resulting from polymer-polymer incompatibility, discoloration, and degradation, result in a relatively low price for such mixed plastics compared to virgin polymers. In this regard, selective separation of post-consumer products is the weakest link in the plastic-recycling industry.
Recycling of plastics is limited by difficulties in the separation of polymers from one another. There are not many examples of the selective separation of thermoplastics in the plastics recycling industry. The current strategy to separate plastics relies on hand-sorting. Manual separation is labor intensive and expensive. High quality automatic segregation of plastics is not yet available. Several commercially vital separations of granulated plastic waste are almost exclusively limited to processes that separate PVC and PET from other polymeric components such as HDPE, LDPE, PP, EVA (ethylene-vinylacetate copolymer). These processes are mostly based on differences in the specific gravities of these polymers (2).
A wide variety of plastics present in post-consumer waste streams, especially those of similar densities, preclude the use of gravity separation. Specifically, the problem of separation of PVC from PET is addressed in this paper. The specific gravities of PVC and PET overlap, e.g. the PVC density ranges between 1.32 to 1.37 g/[cm.sup.3] and the PET density varies from 1.33 to 1.37 g/[cm.sup.3]. Thus, these two polymers cannot be separated using gravity separation techniques (2). However, these two polymers can be separated by flotation. Already, the possibility of this technically attractive (inexpensive and simple) technique for separation of PVC from a PVC/PET mixture has been demonstrated by Recovery Processes International, Inc. (4) with four plants now in operation. Other researchers have reported laboratory scale experiments (5-8). Selective flotation separation of the PET/PVC mixture is impossible without changing the surface properties of these polymers. Both polymers exhibit almost the same degree of hydrophobicity; the advancing water contact angle is 80-85 degrees for both PVC and PET (9-11). The surface tension and critical surface tension of wetting for PVC at 20 [degrees] C are 42 mN/m and 39 mN/m, respectively (9). The same surface properties for PET are very close to the above properties for PVC and they are 45 mN/m and 43 mN/m, respectively (9). It is evident that significant chemical change(s) at the surface of either PVC or PET is required in order to make a selective flotation separation of PVC/PET mixtures.
It is demonstrated in this contribution that strongly alkaline solutions of sodium hydroxide destroy the hydrophobicity of the PET surface. In the same alkaline solutions, the hydrophobic properties of the PVC surface remain only slightly affected. On this basis a technology involving treatment of PET and PVC particles with alkaline solutions followed by froth flotation with nonionic surfactants has been developed and tested at a laboratory scale. Similar concepts as discussed in this paper were also tested by other researchers but to a lesser extent (5, 6).
Samples of virgin and recycled polymers, polyethylene terephthalate and polyvinyl chloride, were purchased or received for this study from several companies. Table 2 provides a short description of the samples used in these experiments. The PVC/PET mixtures with a fixed PVC concentration, which were obtained from the plastics recycling industry (samples G-I), were used in flotation experiments. Additionally, model PVC/PET mixtures were prepared using samples C-G. A few centimeters plastic strips (samples A and B) were only used for contact angle measurements.
Table 2. Polymer Samples Used in This Study. Sample Code Composition Size A PET a few centimeters strips cut from detergent bottle B PVC a few centimeters strips cut from detergent bottle C PET 4 x 3 x 2 mm pellets of virgin polymer D PVC 3-6 mm face size cubes regrind flex vinyl E PET 3-10 mm flakes regrind virgin soft drink bottles F PVC 4-6 mm flakes cut from detergent bottle G 0.02 wt% PVC 2-5 mm flakes 99.98 wt% PET from plastics recycling line; soft drink bottle processing H 0.8 wt% PVC 4-10 mm flakes 99.2 wt% PET from plastics recycling line; soft drink bottle processing I 1.5 wt% PVC 2-5 mm flakes 98.5 wt% PET from plastics recycling line; soft drink bottle processing Sample Code Source A Smith's Store Salt Lake City, UT B Smith's Store Salt Lake City, UT C Scientific Polymer Products, Inc., Ontario, NY D Sport Court Inc. Salt Lake City, UT E Recovery Processes International, Inc. Salt Lake City, UT F Smith's Store Salt Lake City, UT G Nationwide Recyclers, Inc., Polkton, NC H Recovery Processes International, Inc. Salt Lake City, UT I Recovery Processes International, Inc. Salt Lake City, UT
Plastics Treatment With Alkaline Solution
The samples of plastics were treated with alkaline solutions using a Denver agitation machine equipped with two opposing flow three-blade (2.5" diameter, 45 [degrees]) marine propellers (800 rpm). The treatment was done in a 2 L glass beaker using 300-500 g of plastics at a solid concentration of 35-45 wt%. The beaker was placed on a hot plate in order to adjust and control the temperature of the system. Technical grade sodium hydroxide, NaOH (Mallinckrodt), and tap water were used in these experiments to prepare alkaline solutions. Samples of plastics were mixed for 5 to 60 min with alkaline solution at a desired temperature (from 20 to 90 [degrees] C). The sample was screened and rinsed in a stream of tap water after alkaline treatment.
In selected experiments with samples used for contact angle measurements, the polymer strips were treated with alkaline solution using a laboratory magnetic stirrer equipped with a hot plate. Analytical grade NaOH and deionized water were used to prepare the alkaline solutions.
Contact Angle Measurements
The captive-bubble contact angle measurement technique was used in this study as described in detail in a previous contribution (12). The contact angle measurements were carried out using a NRL goniometer (Rame-Hart, Inc.).
The plastic strip (PVC or PET) was placed in the rectangular glass chamber on two stable supports. The glass chamber with sample was filled with deionized water (or surfactant solution in selected experiments). The pH of the solution was adjusted with 0.5-1 M HCl and 0.5-1 M NaOH solutions and measured using a laboratory pH-meter. An air bubble was made at the tip of a U-shaped needle using a microsyringe and attached to the polymer surface. The bubble size was made to increase or decrease by adding or withdrawing a small volume of air, and the water receding and advancing contact angles, respectively, were measured after 30 to 60 s.
Contact angle measurements for a particular system were done for 5-8 large bubbles having a drop base diameter from 5 to 10 mm. The average values are reported.
Froth Flotation Experiments
The flotation tests were conducted in a 2L Denver flotation cell at a rotational speed of 1350-1450 rpm. The 150-250 g of plastic sample (promptly treated with an alkaline solution and rinsed) was mixed with 1.8 L of tap water. Rhodasurf 91-6 surfactant (C9-11 ethoxylated alcohols) received from Rhone-Poulenc was used as a frother at a concentration of 15-30 mg/l in all flotation experiments. The pH of the solution was measured with a pH-meter. In selected experiments the pH of the solution was adjusted at a desired level using 1-5 M HCl and 1-5 M NaOH solutions. The suspension of plastics was conditioned with the frother for about 2 min before flotation experiments. The PVC froth product was collected during 1-10 min, screened, washed with water, and dried. The PET product remaining in the flotation cell after the experiment was screened, washed and dried.
The PVC and PET particles were separated from each other by hand-sorting. This was possible for some of the samples due to a difference in color and shape of PVC and PET particles. In the case of G-I samples, the flotation products were heated in an oven for 2-3 h at a temperature of 150-160 [degrees] C in order to initiate the change in color of PVC particles, from a transparent appearance to a brown color. Then after cooling, the particles were sorted by hand.
All samples were weighed and flotation recovery was calculated based on the mass balance.
RESULTS AND DISCUSSION
The principal objective of this research program was to develop a simple, reliable and relatively inexpensive technology for the effective flotation separation of shredded PVC particles from PVC/PET mixtures, based on the control of the surface chemistry and physics of PVC and PET particles. Selective flotation separation of PET/PVC mixtures is impossible without changing the surface properties of at least one of these polymers. The hydrophobicities of the unmodified surfaces of both polymers, PVC and PET, are almost identical. For example, water contact angles were measured at PVC and PET strips (strips with relatively smooth surfaces were carefully selected for these tests and they were cleaned with 0.1 M NaOH solution for about 15 min before measurements) and found to be 78-84 and 54-62 degrees for advancing and receding contact angles, respectively. The advancing contact angles are close to those values reported in the literature (9-11). The receding water contact angle, which probably better relates to flotation (13), describes the hydrophobicity of these samples as well as the quality of their surfaces. Both contamination and roughness of the surface strongly affect the receding contact angle value (12, 14). However, these factors usually can not be eliminated from real systems, such as used in this study.
Significant hydrophobicity (large water contact angle values) for both plastics examined indicate that both plastics can be separated from aqueous suspension by flotation. However, no selectivity in the flotation separation can be expected since both plastics have strong and similar hydrophobic properties. In this regard, several chemical reagents were tested in order to identify an aqueous chemistry in which one of the plastic constituents is rendered hydrophilic, while the other constituent is maintained in the hydrophobic state. Only such an approach can lead to a selective separation of one plastic from the other.
It was found that both plastics, PVC and PET, showed substantial chemical resistance to aqueous solutions of the following reagents: KMn[O.sub.4], [H.sub.2][O.sub.2], HCl, [HNO.sub.3]. In other words, no significant change in hydrophobicity of these plastics was observed during the plastic surface treatment with the above-listed chemicals at room temperature. On the other hand, it was found that strongly alkaline solutions of sodium hydroxide and potassium hydroxide (KOH treatment is not discussed in this contribution) are able to destroy the hydrophobicity of PET whereas the hydrophobic properties of the PVC surface remain only slightly affected by these solutions.
The Effect of Alkaline Solution Treatment on the Hydrophobicity of PVC and PET
Figures 1 and 2 show the water contact angle values measured for PVC and PET plastic strips cut from detergent bottles which were treated with 4 wt% and 20 wt% NaOH solutions at room temperature. The strips were rinsed with deionized water before examination of hydrophobicity of the freshly prepared plastic strips. This procedure usually does not guarantee a complete cleaning of the polymer surface from oil, grease, surfactants, etc., which frequently contaminate the surface of plastic bottles. The contamination of PVC and PET strips is quite obvious in our experiments. As shown in Fig. 1, both advancing and receding contact angles increased for the samples treated 2 min with alkaline solution as compared to the contact angle values measured for the freshly prepared strips (t = 0). It usually takes a few minutes to clean the polymer surface from such contaminants as oil, grease, surfactants, etc. with a low-concentration 0.1-0.5 wt% NaOH solution. This preliminary cleaning was not applied in these tests.
[Figure 1 and 2 ILLUSTRATION OMITTED]
It is shown in Figs. 1 and 2 that the NaOH solution affects the hydrophobicity of PET significantly, whereas the surface of PVC is only slightly affected by alkaline solutions. A reduction of PET hydrophobicity is more pronounced in strong alkaline solutions (compare Fig. 1 with Fig. 2). It was also found that an elevated temperature of the alkaline solution substantially improves the kinetics of the PET surface "hydrolysis" reaction. For example, Fig. 3 shows the contact angle values measured for PVC and PET strips treated with 2 wt% NaOH solution at 70 + 4 [degrees] C (30 min) and immersed in 15 mg/l Rhodasurf 91-6 solution. For the same experimental conditions the hydrophobicity of PVC remained strong, and advancing and receding contact angles were found to be 65-70 and 45-50 degrees, respectively, for a wide range of pH values, from pH = 5.5 to pH = 9.5. On the other hand, air bubbles could not attach to the PET surface in this 15 mg/l Rhodasurf 91-6 solution, indicating that the receding contact angle was reduced to a zero value; i.e. the aqueous solution film remained stable at the PET surface treated with 2 wt% NaOH solution at 70 [degrees] C for 30 min. The advancing contact angle could not be measured because no bubbles could be successfully attached to the PET surface.
[Figures 3 ILLUSTRATION OMITTED]
The contact angle data in Figs. 1 and 2 indicate that a significant difference in hydrophobicity between PVC and PET can be reinforced if an appropriate alkaline treatment is selected. Specifically, the hydrophobicity of PVC remained only slightly affected (Figs. 1 and 2: advancing contact angle dropped with treatment time from 82-86 to 77-80 degrees and receding contact angle decreased from 56-60 to 42-45 degrees), whereas the hydrophobicity was significantly reduced for PET (Fig. 2: the advancing contact angle dropped from 78-80 to 45-48 degrees and the receding contact angle decreased from 38-40 to less than 10 degrees in strong NaOH solutions). The situation is even more clear for the experimental data presented in Fig. 3, where no attachment between an air bubble and the PET surface treated with hot NaOH solution was possible, whereas the hydrophobicity of the PVC surface remained strong. Obviously, this should lead to a selective flotation separation of these plastics.
Contact angle measurements indicate that the hydrophobicity of the PET surface decreases in aqueous alkaline solutions at elevated temperatures. This observation is consistent with the results of previous reports on fundamental studies of the effect of alkaline solutions on chemical and physical changes of the PET surface (15-17). In these previous studies it was stated that alkaline solution causes an increase in the number of hydrophilic groups (i.e. hydroxyl and carboxyl groups) and an increase in surface roughness (15-17). Preliminary spectroscopic tests and microscopic examination of untreated and treated PET particles carried out at our laboratory (results not presented) support these observations.
The Effect of Alkaline Solution Treatment on the Flotability of PVC and PET
The PVC and PET particle mixture after treatment with alkaline solution can be successfully separated into two products using the froth flotation technique. Since the alkaline-treated PET particles are rendered hydrophilic while the PVC particles are maintained in the hydrophobic state, no collector with a specific affinity for either constituents is required. It was found in our research that appropriate conditions of the alkaline treatment make the selective flotation of PVC and PET plastics easy to perform with practically any frother/surfactant which is able to build a stable froth and which does not affect the surface properties of polymers. Of course, it does not discriminate against other technological approaches involving collectors which exhibit an affinity to the PVC and/or PET polymers.
In these studies, the commercial Rhodasurf 91-6 nonionic surfactant (a mixture of C9-C11 ethoxylated alcohols) was used as a frother in all flotation experiments. It was found in this study that this surfactant/frother does not affect the surface properties of PVC and PET when used at a concentration of less than 50 mg/l. In general, Rhodasurf 91-6 was found to be an extremely good froth producer.
Figures 4-6 show the results of flotation experiments for PVC/PET mixtures treated with alkaline solutions of varying NaOH concentration and temperature. Again, this experimental data supports the conclusion drawn from contact angle measurements that the selectivity of the flotation separation can be stimulated by appropriate alkaline treatment of PVC/PET mixtures. The parameters controlled in this step should be the NaOH concentration (Fig. 4), duration of treatment (Fig. 5), and treatment temperature (Fig. 6). Specifically, the kinetics of hydrolysis of the PET particle surface can be significantly improved in strong alkaline solutions and at elevated temperature (Figs. 4 and 6). Fortunately, as shown in Figs. 4-6, the variation of experimental conditions for alkaline treatment does not have any strong impact on a floatability of PVC particles, at least for those mixtures which were selected for this study.
[Figures 4-6 ILLUSTRATION OMITTED]
The Effect of pH on Flotation Selectivity
Figures 7-9 show the effect of pH on the recovery of plastics from PVC/PET mixtures of varying composition. Note that this data represents results for 2 min flotation experiments. Continuation of flotation for 5-10 min led to recovery values close to 95-100%, depending on pH.
It is shown in Figs. 7-9 that the flotation pH of PVC/ PET mixtures (treated with alkaline solutions before flotation) has only a slight impact on the recovery and selectivity for pH values from pH 6 to pH 9. Separation of PVC was observed to decrease slightly when flotation was carried out in alkaline solutions (pH [is greater than] 8.5-9.0). This might be associated with increased electrostatic interactions between polymer particles and air bubbles for alkaline solutions; both are expected to be highly negatively charged in the alkaline solutions. No specific measurements of these parameters, however, were performed in this study.
[Figures 7-9 ILLUSTRATION OMITTED]
The Effect of PVC Concentration on Flotation
Generally, no significant effect of PVC concentration on flotation performance was observed for samples with 1 to 70 wt% of PVC (Fig. 10). The efficiency of PVC separation in 8-10 min flotation experiments always reached 98-100% for the samples tested in this study. At the same time, the recovery of PET material was greater than 90%. Only for samples with a PVC concentration less than 1wt% was the flotation recovery of PVC reduced to below 98%. However at [is less than] 1 wt% PVC, there are too few PVC particles in our 150-250 g feed samples to judge the statistical validity of these results. Additional experiments involving larger samples and performed in larger flotation cells should be conducted before reaching final conclusions for feed material containing [is less than] 1 wt% PVC. This will be done in our future research.
[Figures 10 ILLUSTRATION OMITTED]
The results of this study indicate that the selective recovery of PET from PVC/PET mixtures can be accomplished in a two-step process involving alkaline treatment and froth flotation. A simplified flow-sheet for the proposed technology is shown in Fig. 11. Preliminary steps (not always required) include the particle size reduction and sizing of plastic waste. In order to make the flotation separation efficient, the size of PVC and PET particles should not be greater than about 10 mm.
[Figure 11 ILLUSTRATION OMITTED]
The plastic particles of appropriate size are treated with alkaline solution (Fig. 11). The results and experience from our research indicate that the hydrophobicity of the PET particles can be sufficiently reduced in hot alkaline solutions (70-80 [degrees] C, 1-3 wt% NaOH, 15-30 min), whereas the hydrophobicity of the PVC particles is only slightly decreased.
The froth flotation separation follows alkaline treatment in the plastic processing technology shown in Fig. 11. Plastics should be screened and rinsed with water before flotation. The results presented in this contribution indicate that the flotation efficiency is reduced for alkaline suspensions. The pH value of the suspension needs to be adjusted to a level of pH 6 to 9. The concentration of particles in the suspension should remain at a level of 7-15 wt%. The frother used in this study is the nonionic surfactant (Rhodasurf 91-6:C9-11 ethoxylated alcohols) at a concentration of 15-30 mg/l. However, other frothers with strong frothing abilities and lack of affinity for the PVC and PET polymers can also be used. Although the experimental data presented in this contribution was obtained for flotation tests performed at a room temperature, no significant effect of elevated temperature up to about 45 [degrees] C was observed in laboratory experiments.
Using this technology for samples from different sources, a recovery of 98-100 wt% PET was obtained on a laboratory scale from PVC/PET mixtures of composition which ranged from 1 to 70 wt% PVC. The purity of the PET product reached 99-100% in these experiments.
No systematic study of the effect of alkaline treatment on the physical and mechanical properties of PET and PVC was conducted in this research. This has not been reported by the plastics recycling industry as a major problem so far. Nevertheless, fundamental studies indicate that changes on the PET surface due to alkaline treatment at elevated temperatures cause a slight decrease in physical properties of PET (15).
The research program was designed to find an improved froth flotation approach to separate PVC from PVC/PET mixtures of variable composition. The PVC and PET polymers with unmodified surfaces exhibit similar hydrophobicity and thus selective flotation of PVC from untreated PVC/PET mixtures can not be achieved. Consequently, it is necessary to render one component of this mixture hydrophilic, while the other component must be maintained in a hydrophobic state, in order to obtain a selective flotation separation. This condition can be achieved by chemical treatment such as the alkaline treatment demonstrated in this contribution. As a result of this research effort, the development of a selective flotation separation technology for PVC/PET mixtures should be possible. The process would involve a two-step strategy: 1) treatment of plastics with strong alkaline solutions, and 2) flotation separation in the presence of surfactants. Selective flotation of PVC particles from PVC/ PET mixtures was demonstrated in laboratory scale experiments for several samples of varying PVC/PET composition (1 to 70 wt% PVC) from a wide variety of sources.
Support for this study was received from the University of Utah through the Research Foundation Technology Innovation Grant Program and is appreciated. Authors thank Mr. Jeff Nish from Sport Court Inc. in Salt Lake City for PVC samples.
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Received June 27, 1997 Revised October 1997
J. DRELICH,(*) T. PAYNE J.H. KIM, and J.D. MILLER
Department of Metallurgical Engineering University of Utah Salt Lake City, Utah 84112
R. KOBLER and S. CHRISTIANSEN
Recovery Processes International, Inc. 302 West 5400 South, Suite 206B Salt Lake City, Utah 84107
(*) Corresponding author: Jaroslaw Drelich, Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931; Phone: (906) 487-2932; Fax: (906) 487-2934; E-Mail: firstname.lastname@example.org.
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|Author:||Drelich, J.; Payne, T.; Kim, J.H.; Miller, J.D.|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 1998|
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