Recycling Waste Polyester via Modification with a Renewable Fatty Acid for Enhanced Processability.

Polyethylene terephthalate (PET) waste often contains a large amount of thermally unstable contaminants and additives that negatively impacts processing. A reduced processing temperature is desired. In this work, we report using a renewably sourced tall oil fatty acid (TOFA) as a modifier for recycled PET. To that end, PET was compounded with TOFA at different concentrations and extruded at 240 °C. Phase transition behaviors characterized by thermal and dynamic mechanical analyses exhibit shifts in the melting and recrystallization temperatures of PET to lower temperatures and depression of glass transition temperature from 91 to 65 °C. Addition of TOFA also creates crystal-phase imperfection that slows recrystallization, an important processing parameter. Changes in the morphology of plasticized PET reduces and stabilizes the melt viscosity at 240 and 250 °C. Melt-spun, undrawn continuous filaments of diameter 36-46 μm made from these low-melting PET exhibit 29-38 MPa tensile strength, 2.7-2.8 GPa tensile modulus, and 20-36% elongation. These results suggest a potential path for reusing waste PET as high-performance polymeric fibers.

Introduction

Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester commodity broadly used in the packaging and apparel industries. It has been estimated that 485 billion individual PET bottles were produced globally in 2016, and the number is forecasted to grow over the next 5 years.[1] Production of beverage bottles comes at a price, as the generated waste must be dealt with continually. Postindustrial and postconsumer PET waste often goes to landfills and creates environmental hazards because of lack of biodegradation of PET.[2] To address the environmental issues and create potential new revenue streams for these wastes, it is logical to recycle the wastes into reusable polymer systems for a variety of end uses.

Recycling can expand the spent PET to novel applications,[3] but the cost to performance ratio must be reasonable. For example, the toughness can be improved by successful modification and/or plasticization to reduce the melting point of the crystalline phase and decrease the glass temperature Tg of the amorphous phase.[4] PET can be blended with thermally unstable materials such as polyacrylonitrile, acrylonitrile butadiene styrene, poly(vinyl chloride) (PVC), phenolic resin, and lignin, and then to achieve success with those compositions, PET needs to be softened and melted at lower temperature. The reduction in these transition temperatures via changes in the molecular dynamics requires careful selection of the plasticizer. The possibility of enhanced chain mobility and depressing Tg of PET has been well-studied. In fact, small amounts of codiols or long diols as comonomers were used to plasticize PET.[5,6] Gowd used two distinct types of poly(ethylene glycol) (PEG) and a solid-state polymerization route to plasticize PET.[7] The results showed that the Tg and the crystallization rate were affected by the PEG nature. A systematic investigation of CO2-induced crystallization of PET by Mizoguchi showed that sorbed CO2 is in fact an effective plasticizer for PET.[8] The evaluation of the plasticizing ability of CO2, as reported in a different study, was based mostly on the mobility of the polymer segment and the reduction of Tg measured, which in part is due to the high solubility of CO2 in PET.[9]

In the specific instance of choosing a plasticizer for polymer systems, renewable plasticizers are preferred as alternatives to their petroleum-based counterparts, in part because of environmental issues, renewability, and cost-effectiveness.[10,11] Some reported renewable plasticizers include soybean oil fatty acid methyl ester[12] and benzyl ester of dehydrated castor oil fatty acid.[13] An example of a renewable plasticizer and polymer system was reported by Wang.[14] The work evaluated palm oil as a plasticizer for ethylene propylene diene monomer (EPDM) rubber. When added to EPDM, palm oil acts like paraffin oil, a conventional petroleum-derived plasticizer, by reducing the viscosity to improve the processability. PVC has been recently plasticized with novel soybean oil-based polyol ester.[15] The plasticizing effect of the polyol ester was very pronounced and effective when compared with some conventional plasticizers for PVC. Additionally, an improvement in thermal stability was reported.

The success of different renewable-based plasticizers for polymeric systems addresses many environmental issues and is a path toward sustainable development. To this end, we have identified tall oil fatty acid (TOFA) as a potential renewable plasticizer for postindustrial PET resin. TOFA is a byproduct of the kraft paper-making industry that uses pine trees.[16] It mainly consists of oleic acid and is cost-effective. The goal here is to use the renewable plasticizer to reduce the viscosity and improve the processing characteristics. This will broaden end-use applications of postindustrial PET scraps from resin manufacturers, bottle manufacturing facilities, and postconsumer wastes. Continuous fibers were also spun using the plasticized matrices to demonstrate favorable processability. Using TOFA in combination with the postindustrial PET waste gave a modified PET suitable to expand the application horizon where lower melting temperature of PET is desired.

Results and Discussion

Thermal Characteristics

Polymeric materials are macromolecules with intramolecular cohesive forces among them. When heat is applied, the macromolecules become soft and flexible and easy to process. Low-molecular-weight plasticizers are often added to increase the flexibility at room temperature and to improve processing. Evidence of the plasticizing effect of TOFA is shown in Figure a. Differential scanning calorimeter (DSC) thermograms depict that as the amount of TOFA renewable plasticizer increases, the PET melting peak shifts toward lower temperature. Neat PET is a heterogeneous polyester with both amorphous and crystalline domains. Its thermogram shows a large endotherm with a peak temperature at ∼248 °C corresponding to melting of crystals. The peak shifts to lower temperature as more and more plasticizer is added. The low-molecular-weight renewable plasticizer TOFA inserts its molecules between the crystalline domains of PET allowing for softening and increased flexibility. The depression of melting temperatures Tm1 and Tm2 reduces the processing temperature of PET. In parallel, incorporation of TOFA shifts the recrystallization Trec to lower temperatures (Figure b) because of the TOFA-induced PET chain flexibility that alters the crystallization kinetics discussed in detail later.

Figure 1

Thermal characterization of as-received recycled PET and its plasticized derivatives. (a) DSC second-heating thermograms, (b) DSC cooling thermograms, (c) dynamic mechanical analysis storage modulus E′, and (d) dynamic mechanical analysis loss factor tan δ.

The change in crystallization behavior was evaluated and is reported in Table using the thermodynamic melting enthalpy from the DSC thermograms in eq . The neat PET ΔHm is 48.6 J/g, which is lower than that of 100% crystalline PET (140 J/g) with a degree of crystallinity of 34.7%. XC values of TOFA-plasticized PET are all slightly higher than that of as-received PET except PR10. It is apparent that addition of TOFA favors segmental molecular mobility of PET, which affects the crystallinity. Although the presence of TOFA increases PET crystallinity, it is obvious that the modified PET crystals are low-melting and imperfect.

Table 1

Thermal and Crystalline Properties of Neat PET and Its Plasticized Derivatives

samples	Tm1 (°C)a	Tm2 (°C)a	ΔHm (J/g)a	Trec (°C)	ΔHrec (J/g)	χc (%)a	(Tg) (°C)b
neat PET	237.3	247.9	48.6	209.1	55.8	34.7	91
PR10	222.8	238.3	42.8	198.9	52.1	33.9	69
PR20	220.2	236.8	41.9	196.4	54.5	37.4	67
PR30	222.6	238.1	40.3	197.1	64.9	41.1	66

Values obtained from the second heating curve of DSC.

Tg reported from tan δ peak.

The storage modulus E′, related to the material stiffness, decreased with the addition of the plasticizer. Changes in the phase structure because of incorporation of TOFA cause the storage modulus to decrease (Figure c). The sharp drop of E′ for all samples as the temperature increases signals that glass transition events are occurring in the amorphous phase. Like most polymers, PET undergoes thermophysical transitions around its glass transition temperature Tg. Tg is known as the temperature at which binding forces between polymer chains are relaxed to initiate large-scale molecular movements. Tg is considered as an essential processing parameter as it helps to define the state of polymers at their service temperatures. Here, Tg, as reported in Table , is the maximum peak of the loss factor tan δ (Figure d). The as-received PET has a Tg of 91 °C. As expected, addition of the renewable plasticizer decreases Tg. TOFA addition depresses Tg of PET by up to 25 °C at 30 wt % addition, which is evidence of pronounced plasticization. TOFA presence allowed the PET matrix to become less dense and facilitate motion of the PET molecular segments to start at lower temperature.

Crystallization Behavior

The recrystallization temperature (Trec) and enthalpy (ΔHrec) of neat PET decrease as the cooling rate increases (Figure ). Plasticization is observed as the PET crystallizes faster than the plasticized PET resins at lower cooling rates, but the difference in Trec is minimal at 30 °C/min rate. Trec is critical in polymer processing, especially for a semicrystalline polymer such as PET. It is measured as the solidification temperature. The fact that neat PET and TOFA-plasticized PET display Trec values close to each other at high cooling rates suggests that the crystallization mechanism is not much affected by the addition of TOFA. This is beneficial for a process such as extrusion where high cooling rates are desired. ΔHrec of plasticized resins diminish as the TOFA amount increases, except at higher TOFA content (Figure b). Nevertheless, it increases as the cooling rate increases, which is indifferent to neat PET. Crystallization temperatures shift to lower temperatures with the addition of TOFA renewable plasticizer. Such reduction can be explained by two interconnected facts. First, TOFA addition depresses Tg and increases chain mobility, and second, TOFA addition decreases the melting point of PET, which also retards crystal growth. Furthermore, the slopes of Trec versus log cooling rate curves suggest that excess TOFA reduces the crystallization rate because a steeper slope (neat PET) means faster crystallization compared to a gentle slope (e.g., 30 wt %).

Figure 2

(a) Crystallization peak temperatures Trec and (b) crystallization enthalpy ΔHrec as a function of the log cooling rate.

Flow Characteristics

Oscillatory frequency sweeps were used to evaluate the as-received PET and TOFA-plasticized PET resin flow characteristics. The angular frequency (ω) dependence of the complex viscosity (η*) and the storage modulus (G′) in the linear region have a more developed sensitivity to structural variation than the power-law region (Figure ). At 240 °C (Figure a), neat PET shows a Newtonian behavior at very low frequency followed by normal shear thinning behavior where the complex viscosity starts to decrease with increasing angular frequency. The viscosity is high and decreases progressively because the data are collected in the linear region and the temperature is below the melting temperature recorded by DSC (248 °C). It is anticipated that chain disentanglement and mobility are limited because of lack of energy to disrupt them. The plasticized PET resins, however, show a similar decrease of viscosity with increasing frequency, but the slopes of the flow curves are sharper. Also, PR10 and PR20 show high viscosity at low frequency compared to neat PET. The effect of the plasticizer is evident for all three plasticized PET resins at 250 °C (Figure b). The plasticization effect becomes more pronounced as the viscosity decreases with increasing plasticizer amount, and the slopes of all curves are steadier. The flow behavior shows that changes are occurring in the molecular structure of plasticized PET because of the addition of TOFA as a softening agent. Such an interaction is typical for excellent plasticizers as they decrease the viscosity and hence improve the processing properties of PET. The master curves at Tref = 240 °C in Figure c show that the storage modulus G′ increases with increasing ω. A sharp increase in G′ is observed in neat PET, but for all three plasticized PET resins, the increase is progressive. Also, all three plasticized resins have higher G′ values at low frequency, and G′ for PR10 and PR20 continues to be higher than that of neat PET at below 10 rad/s. The master curves at Tref = 250 °C in Figure d show that neat PET has the highest G′ over the whole frequency range indicating the highest rigidity. The slopes are different, but all storage moduli increase with increasing angular frequency. The TOFA-modified resins’ G′ values decrease with increasing plasticizer-loading amounts over the entire frequency range. This suggests that the plasticizer lubricates the PET chains, and the plasticization effect not only favors a change in the morphology and increased chain mobility but also strongly affects the moduli of the modified resins. This is a strong indication of improved processing characteristics of the modified PET.

Figure 3

Frequency-dependent complex viscosity η* at Tref = 240 (a) and 250 °C (b) and frequency-dependent storage modulus G′ at Tref = 240 (c) and 250 °C (d) of recycled PET and its TOFA-plasticized derivatives.

Morphological Evaluation

Representative scanning electron microscopy (SEM) micrographs of cryofractured surfaces of the as-received waste polyester and the TOFA-plasticized matrices are represented in Figure . The as-received PET surface shows a relatively smooth fracture surface with residual morphology left by the fracture path (Figure a). The plasticized PET samples, however, exhibit a completely different topography of the fractured surface. The surfaces have cavities created by the addition of TOFA that are very well statistically dispersed and are around 0.2–1 μm in size (Figure b–d). For the 30 wt % plasticized resin, the cavities are larger because of coalescence of the extra amount of TOFA during melt mixing, which creates large agglomeration. The observations clearly show that TOFA acts as a lubricant by inserting itself in the polyester matrix and favors chain motion and slippage of molecular constituents. This in turn introduces the thermophysical changes detected by thermal and dynamic mechanical analyses (Figure ). Fourier-transform infrared (FTIR) spectra show (Supporting Information, Figures S1 and S2) that minimal changes in the chemical structure were observed in PET blended with TOFA. However, an expansion of the O–H region within 3300–3600 cm–1 shows an evolution of the −OH stretching with the increase of TOFA content. Also, the presence of available C=C bonds in TOFA can promote the π–π stacking with double bonds of phenolic groups in PET. The similar structures of the carboxylic acid group from TOFA with the ester groups and the capping −COOH group from PET promotes strong molecular interactions between PET and TOFA. TOFA has a linear flexible backbone enabling TOFA to behave like a “good solvent” to increase the chain mobility of PET in the PR30 or PET/TOFA mixture. As a result, TOFA facilitates to reduce the melt flow of PET in the PET/TOFA mixtures (see Figure a,b). An attempt to dissolve PET and PR30 in 2-chlorophenol confirmed the existence of strong intermolecular interactions between TOFA and PET, which decrease the intramolecular interactions between PET chains resulting in better solubility of PR30 in 2-chlorophenol (Supporting Information Figure S11). This observation also corroborates very well with the thermal analysis discussed in the previous section. The intermolecular interactions between TOFA and PET retard the PET chain association during the recrystallization process, resulting in a low recrystallization temperature. For example, the recrystallization temperature of PET is reduced by ca. 12 °C with the presence of 30 wt % TOFA (see Table ). We anticipate that these are physical interactions rather than chemical bonds of TOFA with PET. To evaluate the changes in PET chemical structures, if any, in the presence of TOFA, we performed both 1H nuclear magnetic resonance (NMR) and 13C NMR analyses. The results are shown in Supporting Information Figures S3 through S10. The NMR analysis of PET, TOFA, PR30, and PET/TOFA (hand-mixed at a composition of 50 mg/15 mg and counterpart of the PR30 that was melt-mixed) in 2:1 v/v hexafluoro isopropyl alcohol-d2 (HFIP-d2)/CDCl3 shows no noteworthy differences in the chemical shifts of the PET and TOFA components in the samples. Also, the PR30 sample did not exhibit new peaks and was relatively identical to the hand-mixed PET/TOFA sample. This is consistent with the absence of a chemical reaction between PET and TOFA during the melt extrusion process. Although the PR30 and PET/TOFA (50 mg/15 mg, hand-mixed) spectra were virtually identical regarding the chemical shifts of the observed peaks, one surprise was that the NMR analysis seems to reveal that some TOFA is lost during the extrusion process, but this is a result that we want to look into further in our ongoing work with these PET/TOFA extrudates.

Figure 4

Micrographs of cryofractured surfaces; (a) neat PET, (b) PET/10% TOFA, (c) PET/20% TOFA, and (d) PET/30% TOFA.

Properties of Fibers Spun from Plasticized Polyester Resins

The TOFA-plasticized matrices were melt-spun to produce continuous filaments using the benchtop extruder previously used for blending. The extruder and attached spinneret temperatures were set to 240 and 245 °C, respectively, for all samples, which is well below the neat PET working temperatures (∼270–285 °C). The 30 wt % plasticized matrix spun the best out of all formulations. This can be explained by the pronounced effect the plasticizer has on lowering the viscosity at the spinning temperatures, as shown in flow characterization (Figure ). Fibers from all matrices were collected continuously on a spool. The winding speed needed to be adjusted based on the material viscosity. For instance, 30 wt % plasticized PET was collected at 130 m/min compared to 10 wt % plasticized matrix at 80 m/min. These parameters will affect the mechanical performance of the fibers.

Thermal analysis by DSC (Supporting Information, Figure S12) shows interesting thermal behavior of the fibers. The first heating thermogram (Figure S12a) shows early relaxation around 50 °C for all samples that started at lower temperature for the 30 wt % TOFA content PET fibers. The relaxation was followed by a cold crystallization around 100 °C and melting behavior at 240 °C. The melt-spinning process may have induced a metaorder structure because of high chain alignment. In Figure S12b,c, the fibers behave more like their starting matrices, showing recrystallization and melting peaks and similar shifts in their peak temperatures, as reported in Figure a,b.

The single fibers were isolated for tensile testing. The results are reported in Table . All fiber groups showed adequate tensile performances (tensile strength of 29–38 MPa, tensile modulus 2.7–2.8 GPa, and elongation at break of 20–36%). It is important to recall that the limitation of our setup and the viscosity difference between the matrices made it impossible to produce the fibers using the same processing conditions, which affected the tensile properties of the fiber generated. For example, attempt was made to collect the filaments at higher speed as much as possible, which is critical to alignment of the polymer chains in the fibers and influences the strength of the fibers. However, PR30, which was collected at higher speed, did not have the highest tensile strength. Defects created by a large amount of TOFA could be the reason for the weaker tensile strength. Also, PR30 did not have the smallest fiber diameter. This suggests that a lower recrystallization temperature affects the 30 wt % plasticized matrix’s overall fiber diameter. The performance of fibers displayed here could have improved if annealed at lower temperature before mechanical testing. Morphological evaluation of the fiber using SEM (Figure ) shows the longitudinal surface of the fibers. It is evident that the 10 wt % TOFA-plasticized PET has a small fiber diameter, and very fine fibers were observed. The fibers’ surfaces were not smooth and uniform as desired, which could be caused by the die capillary or temperature setup.

Table 2

Mechanical Properties of the Plasticized Fibersa

composition (wt %)		properties
PET	fatty acid	fiber diameter (μm)	tensile strength (MPa)	modulus (GPa)	elongation (%)
90	10	36 (3.5)	38 (6.3)	2.7 (0.4)	20 (4)
80	20	46 (9.1)	29 (2.9)	2.8 (0.6)	36 (6)
70	30	42 (5.3)	32 (3.4)	2.7 (0.5)	32 (12)

Standard deviations are shown in parenthesis.

Figure 5

SEM micrographs of fibers from (a) recycled PET/10 wt % TOFA, (b) recycled PET/20 wt % TOFA, and (c) recycled PET/30 wt % TOFA.

Conclusions

We have identified renewable TOFA, a byproduct of the kraft paper making industry that uses pine trees, to plasticize polyester waste that would have ended up in landfill and caused an environmental hazard. The addition of TOFA induced changes in the structure and morphology of postindustrial PET. The melting temperature, recrystallization temperature, and Tg were shifted to lower temperatures because of increased chain mobility demonstrated by thermal and rheology analyses. The polymer blends were used to generate continuous fibers with acceptable tensile properties regardless of the processing setup. For example, 10 wt % TOFA-loaded PET (the least plasticized one) could be spun into filaments with higher strength; however, these filaments made from the most plasticized compositions (20 and 30 wt % TOFA) crystallized quickly and formed the largest diameter filaments. The developed blends obtained from manufacturing waste showed thermal and flow behaviors that can expand their end-use applications and create revenue from waste materials. In summary, TOFA shows potential as a renewable and cost-effective plasticizer for postindustrial PET wastes.

Experimental Section

Materials

Thermoplastic polyester PET was received from Eastman Chemicals USA. It is a scrap from the resin manufacturing facility and supplied as a white powder. Melt flow rate measured in our laboratory was 55 g/10 min (2.16 kg at 280 °C). PET was dried under vacuum at 130 °C for 8 h to avoid hydrolytic degradation during melt processing. TOFA (Westvaco L-5 TOFA [CAS # 61790-12-3]) was acquired from Westvaco Chemicals, Charleston SC. The specifications of TOFA were reported as acid number (min 190), rosin acids (max 5%), and color or Gardner (max 7). On the basis of the NMR analysis of TOFA, it is primarily oleic acid in composition.

Blending of Postindustrial Polyester and TOFA

Dried polyester PET and TOFA were melt mixed at 240 °C in a HAAKE MiniLab corotating twin-screw extruder (Thermo Scientific). TOFA was added in 3 different wt % (10, 20, and 30). The extruder had one zone heated with a screw length of 110 mm. The blends were processed at a screw rotation speed of 30 rpm. The extrudates were collected for further analysis.

Blend Characterization

The three different extrudates were named PR10, PR20, and PR30, respectively, in accordance with the wt % of TOFA added (10, 20, and 30). DSC (DSC Q2000, TA Instruments) was used to determine the thermal characteristics of the as-received PET and the plasticized PET samples. Samples with a mass of approximately 3–4 mg each were loaded in hermetic pans for measurements. A cycle of heating–cooling–heating from −50 to 300 °C at 10 °C/min and an isothermal of 2 min after first heating were performed. The degree of crystallization was computed from eq . The melting enthalpy ΔHm was obtained from the second heating curve. Wf is the PET weight fraction in each composition, and ΔH100 is the theoretical fusion enthalpy of 100% crystalline PET (140 J/g).[17]

The same setup of DSC was used but at different ramping rates (2, 5, 10, 20, and 30 °C/min) to study the crystallization rate of the as-received PET and plasticized PET resins. Dynamic mechanical thermal analysis of the blends was performed using a TA Q800 (TA Instruments) analyzer in the tension mode. Monofilaments of diameters varying from 0.20 to 0.45 mm generated from each sample were used for testing. The measurements were conducted in the temperature range of 30–150 °C and at frequencies of 1 Hz and 0.01% strain. The heating rate was 3 °C/min. Cryogenically fractured surfaces of extrudates were evaluated by SEM (Zeiss EVO MA 15). The samples were also analyzed by solid-state FTIR spectroscopy (PerkinElmer Frontier). A total of 32 scans were performed in the attenuated total reflectance mode. Average spectra with 4 cm–1 resolution were obtained for all samples. Proton and carbon NMR spectra were obtained on a Varian VNMRS 500 NMR spectrometer at 24 °C. All samples were dissolved in a 2:1 v/v mixture of HFIP-d2 (Alfa Aesar, 98 atom % D) and CDCl3. Chemical shifts were referenced to CDCl3 with 7.27 ppm for 1H and 77.23 ppm for 13C. Carbon NMR spectra (125.667 MHz for 13C) were obtained using inverse-gated decoupling with a recycle delay of 20 seconds and 3000 transients. The rheological properties were evaluated using the discovery hybrid rheometer (DHR-2, TA instruments). All measurements were carried out at 3% strain, which is in the linear region as determined by a strain sweep in an inert atmosphere of nitrogen. Frequency sweeps from 100 to 1 rad/s at different temperatures were performed.

Fiber Melt-Spinning and Characterization

The twin-screw extruder was also used to generate continuous single fibers from all three plasticized PETs with TOFA. A 200 μm spinneret was attached for fiber forming as the extrudate is forced through the orifice. A 76 mm rotating drum was used to collect continuous single fibers. Single fibers were isolated for mechanical testing. They were glued onto a paper tab using an adhesive. The tabs were mounted into a set of pneumatic grips, and the sides are cut at midgauge before load application. The gauge length was 25.4 mm. The tabs enable consistent and proper mounting of fiber specimens. An Instron 5943 testing system equipped with Bluehill 3 software was used for mechanical testing at a crosshead speed of 2 mm/min. The mean of 13 specimens was reported. The fibers were also characterized by DSC based on sample preparation and experimental conditions described above. SEM micrographs of the fiber lateral surface were also obtained.