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THE PROCESS OF RECYCLING OF POLYESTERS WITH POLYMERIC CHAIN EXTENDERS

Marco A. Villalobos, Abiodun A. Awojulu, Gregory P. Turco

Johnson Polymer LLC Sturtevant, Wisconsin 53177-0902

Abstract

Epoxy-functional, styrene-acrylic polymers with low molecular weight, high functionality (> 4) and tailored polydispersity have been designed, synthesized and commercialized to be used as chain extenders or recycling aids PET and other polyesters. Dramatic increases in molecular weight, mechanical, and rheological properties of virgin, reprocessed, and post-consumer recycled polyesters are achieved without the incidence of gellation. Erema’s Vacurema™ recycling equipment is well-suited to processing the additive, and can improve efficiency over 400% Screw selection is shown to be especially important.

Introduction

Recycling of plastics enables sustainability in two ways: by eliminating the need for extraction of petroleum, and by eliminating the environmental cost of waste disposal. Polyethylene terephthalate PET is a widely used and commonly recycled polymer. Its degradation by hydrolysis or thermal cleavage is well-known and limits the ability to recycle to the PET’s original use.

The main reason is that PET and other condensation polymers, such as PBT, polyamides, polycarbonates, thermoplastic polyurethanes, are all processed at high temperatures around 300°C, where degradation reactions may be extremely rapid (1).

As a result of this degradation of properties, the

opportunity to recycle or process regrind or post-consumer recycle back into demanding applications is limited. Today, the main approach to enhance the properties of PET involves the costly solid-state polymerization process (SSP) (2). Without a doubt the high investment cost and operation costs associated with solid-state polymerization have rendered recycling into critical applications uneconomical.

More recently, research has been devoted to develop

chemical and mechanical means to enhance the properties of virgin, reprocessed or recycled polycondensates without the use of solid-state polymerization, employing instead processing equipment such as reactors, extruders or injection molding presses (3, 4). Chief among the chemical methods has been the use of di-functional or

multi-functional chemical compounds aimed at rebuilding molecular weight and physical properties of degraded thermoplastics by bridging their functionally terminated chains, thus recoupling them, through a reaction known as “chain extension.” Normally, these chain extension reactions can be carried out effectively in the melt phase in single or twin-screw extruders operating at normal conditions. The use of high vacuum and or catalysts is often necessary to carry out chain extension reactions to completion (5).

Although chain extension with di-functional

compounds bearing carboxylic acid, anhydride, amine, epoxy or other chemically reactive functionality has had modest success in enhancing some polyester properties, it is has been proven to be inefficient in restoring molecular weight of severely degraded materials, such as those from post-consumer recycle sources, given the high load requirements for any substantial molecular weight increase with these additives (6).

The use of chain extenders with functionality equal or

larger than three has become attractive as a means of increasing efficacy of the extenders albeit at a cost of going from linear to long chain branched architectures. Unfortunately, low functional extenders had little success due to the narrow processability window between noticeable chain extension, and the onset of extensive crosslinking followed by incipient or catastrophic gellation. To date, multi-functional chain extenders almost regardless of whether they are low or high molecular weight, with random or telechelic functional groups, have had to contend against this limitation. (7, 8).

The combination of a well-designed additive with

well-designed recycling equipment makes the recycling process more economical and reliable. Specialized equipment can provide a more uniform PET flake, remove unwanted volatile contaminants, and provide good mixing with limited shear.

Multi-functional Polymeric Extenders

In this work, multifunctional styrene-acrylic polymers

with low molecular weight (Mn < 3,000), high number average functionality (fn > 4) have been designed,

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successfully scaled-up and commercialized to perform as chain extenders solving current processing limitations.

These polymers can be characterized by the general

formula given in Figure 1. Figure 2 shows in a schematic manner this mechanism

of chain extension. Furthermore, proper selection of the co-monomers used with the formula above allows for proper tailoring of the Tg and refractive index (RI) of the extender to make it applicable as solids or liquids, even to highly transparent materials such as PC and PET, without any negative effects on color or haze.

Materials

Post-consumer polyethylene terephthalate (PET) flake

of nominal I.V. = 0.70 (+/- 0.01), PET precursor for SSP with I.V. =0.625 (+/- 0.01) and Virgin PET bottle grade I.V. = 0.8 and 0.9 (+/- 0.01) were obtained from commercial sources. Virgin Polybutylene terephthalate (PBT – Ticona Celanex 2002),

All of these thermoplastics, and some blends of them

were chain extended with JONCRYL™ ADR-4368 (Johnson Polymer), epoxy functional polymeric acrylic with the following physical characteristics: Mn < 3,000, Fn > 4, PDI > 3 and epoxy equivalent weight EEW = 285 (+/- 15 g/mol) and Tg = 55°C, obtained in flake form. Joncryl ADR-4368 is used as a pure material, or in a masterbatch with PET. This masterbatch was prepared at 30% active ingredient.

Experimental

Standard extruder experiments were carried out in a Werner & Pfliederer ZKS-25 mm co-rotating twin-screw extruder with L/D = 24, operating at 150 RPM without vacuum. The extruder was fitted with a twin gravimetric feed system synchronized for simultaneous feed of the plastic and the polymeric extender directly to the throat in the first zone of the extruder. All plastics in extrusion experiments were pre-dried as per manufacturer recommendations and processed at right zone temperature settings to achieve the following melt temperatures: PET, PBT Tm = 280°C. JONCRYL ADR-4368 was used as received. No catalyst was employed.

Vacurema™, manufactured by Erema Manufacturing,

was used for the identified experiments. This was an 80TE Vacurema with an SW4/134RTF filter. The material was added to the second stage of the Erema. Except as indicated otherwise, these experiments were run in batch mode. PET processed through the Vacurema were not specially dried prior to processing.

Resulting chain extended thermoplastics were characterized by a suitable combination of capillary rheometry, melt flow index, intrinsic viscosity, tensile properties, and size exclusion chromatography (GPC), in full agreement with corresponding ASTM methods.

Results and Discussion

The mechanisms of chain extension when employing

multi-epoxide extenders in reactive extrusion systems have been discussed at length elsewhere (3-8). Briefly, in polyesters, preferential glycidyl to carboxyl end group esterification precedes the slower glycidyl to hydroxyl end group etherification. Glycidyl to secondary hydroxyl etherification and transesterification of primary and secondary hydroxyl chain ends onto polyester chains are slower reactions than these, and occur less often.

Figure 3, shows the large increases in I.V. obtained when either virgin, regrind or post-consumer recycled PET are chain extended with different amounts of J-ADR-4368. These data were collect using the standard extruder. Each curve corresponds to best fit of at least nine experimental points in the chain extender concentration range explored. The diagonal line shows the processing areas where products obtained were 100% free of micro-gel. The main product application areas as a function of I.V. are also shown for reference. Two things become evident from this figure, the first is that the polymeric chain extender employed shows large areas of gel-free processing. The second is that degraded materials can be enhanced by this route to be used in demanding applications, without the need for SSP, by using less than 1.5 % of the extender in a single extrusion step without the incidence of gel.

Figure 4 shows the experimental capillary rheometry

curves for PBT extended with different amounts of this polymeric extender. All the extended products prepared were 100% gel free. Some features in the rheometry are noteworthy. First it can be noticed that in going from 0 to 1.5 % extender the low shear rate melt viscosity of PBT increases 13 fold. The corresponding increase in melt strength of the chain extended PBT follows these melt viscosity increases. Second, the curves at high shear rate show a more pronounced shear thinning, which is in agreement with the behavior expected for a long chain branched material. This behavior exemplifies melt strength advantages in extrusion applications and flow advantages in injection molding applications for polyesters extended vs. the unmodified counterparts due to the long chain branched structures achieved through this multifunctional chain extension.

Figure 5 shows the results of experiments conducted on a Vacurema using recycled PET. Experiments were conducted using three different screws. Based on the selection of the screw, efficiency can be increased by a factor of four or more. This is indicated by a change in slope of the lines on the graph with the different screws.

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Erema was able to develop a screw design that incorporated the additive well and did not degrade the PET with too much heat.

Figure 5 also shows a comparison of two figures with

and without vacuum. The vacuum process increase IV, and the additive further increases it.

One can compare the rate of IV increase on the Vacurema process with that on the Werner & Pfliederer extruder. For example, screw 2 on the Vacurema, which had the middle ranking in efficiency, increased the IV by 0.97 IV units/% additive used. On the standard extruder, the similar measure was 0.18 IV unit/% additive, using the data presented in Figure 3 which IVo of 0.7 using the linear range below 0.5% neat additive. Note that the Vacurema process used a 30% masterbatch, and Figure 3 used neat additive.

The additive has been used on Erema equipment on long full-scale runs with good stability and consistency.

Conclusions Polymeric chain extenders such as JONCRYL™

ADR-4368, bearing high functionality (fn >4) and tailored polydispersity, offer a much broader window of processability than their monomodal telechelic multifunctional counterparts of similar or even lower functionality (f = 3, or 4).

The use of JONCRYL ADR-4368 at low concentrations (< 1.5%) in simple extrusion equipment without the need for vacuum or catalysts enables economic processing, reprocessing and recycling of low Mw or degraded condensation thermoplastic feedstock into demanding applications.

The use of JONCRYL ADR-4368 in Vacurema equipment enables greater effectiveness at lower additive levels. Screw selection is an important variable in recycling. The use of vacuum further increases effectiveness.

Acknowledgement

The authors wish to recognize the important

contributions to this work done by David Dodds and Vahe Karayan from Clariant Masterbatches, in Winchester, Virginia, and by Georg Weigerstofer of Erema Manufacturing in Linz, Austria.

References

1. V. L. Bell, B. L. Stump, and H. Gager, J. Polym. Sci.

Polym. Chem. Ed. 14, 2275 (1976). 2. L. H. Buxbaum, J. Appl. Polym. Sci. 35, 59 (1979). 3. H. Inata and S. Matsumura, J. Appl. Poly. Sci. 34, 2609

(1987).

4. D. N. Bikiaris and G. P. Karayannidis, J. Appl. Polym. Sci., 60, 55 (1996).

5. T. Shima et. al., Polym. Kinetics and Technology, Adv. Chem. Ser., ACS, 128, 183 (1973)

6. R. Dhavalikar, and M. Xanthos, ANTEC Proceedings, 2677 (2001)

7. D. N. Bikiaris and G. P. Karayannidis, J. Polym. Sci. Part A: Polym. Chem., 33, 1705 (1995).

8. D. N. Bikiaris and G. P. Karayannidis, J. Polym. Sci. Part A: Polym. Chem., 34, 1337 (1996).

9. U.S. Patent 6,346,590, Campbell et al. (to Johnson Polymer).

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Figure 1. General structure of the styrene-acrylic multifunctional polymeric chain extenders: where R1 to R5 are H, CH3, a higher alkyl group, or combinations of them; R6 is an alkyl group, and x, y and z are each between 1 and 20.

Figure 2. Schematic mechanism of chain extension with multifunctional extenders leading to long chain branching.

Figure 3. Final I.V. of PET vs. % JONCRYL ADR-4368 in the formula for different grades of PET. IVo = 0.625 SSP precursor; IVo=0.700 post consumer recycled bottle flake; IVo =0.8 and 0.9 = Virgin Resin. Each curve was fit from nine experimental points.

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Figures 4. Capillary Rheometry at 260°C of PBT & PBT/PA blends (80:20) extended with varying concentrations of JONCRYL ADR-4368

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Figure 5. Using the Vacurema equipment, the increase in viscosity is faster than in the standard extruder. The rate of increase depends on the screw design. Erema was able to increase efficiency by optimizing screw design, and this increased the economy of the process. The use of vacuum in the Vacurema increased efficiency by a factor of 4.6.

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