Characterization of recycled polyethylene terephthalates and polyethylene terephthalate

Abstract

For environmental conservation, post-consumer polyethylene terephthalate (PET) bottles are recycled using methods ranging from mechanical recycling to various chemical recycling processes.

In this study, the characterization of recycled PETs and PET–nylon6 blend knitted fabrics, and virgin PET knitted fabric, was carried out with the aim of broadening the application of recycled PET fabrics. The tensile strength values of mechanically and chemically recycled PET knitted fabrics were similar to those of virgin PET knitted fabric. The elongation of recycled PET–nylon6 blend knitted fabric was the best. Both virgin and recycled PET knitted fabrics had excellent pilling resistance. Based on the drape ratio, the recycled PET–nylon6 blend knitted fabric was more flexible than other samples. The warm/cool feeling (Q_max), compressional and surface properties were measured using the Kawabata evaluation system for fabrics (KES-FB system). The compressional properties of mechanically recycled PET knitted fabric were similar to those of virgin PET knitted fabric. The recycled PET–nylon6 blend knitted fabric showed the smoothest appearance and coolest feeling among the four samples. Moisture regain and moisture permeability were the best in recycled PET–nylon6 blend knitted fabric. However, the wickability of mechanically recycled PET knitted fabric was better than other recycled PET knitted fabrics.

Keywords

mechanically recycled polyethylene terephthalate chemically recycled polyethylene terephthalate recycled polyethylene terephthalate–nylon6 blend

With the rapid growth of industry, resource depletion and ecological destruction problems have emerged. For environmental conservation, it has become necessary to promote ecofriendly industry and to recycle waste.¹

Polyethylene terephthalate (PET) is widely used in textile fibers, films, and bottles for packaging. Recycling of post consumer PET is expected to reduce the environmental impact of plastics. Post-consumer PET bottles can be recycled using methods ranging from mechanical recycling to various chemical recycling processes.

The mechanical recycling process involves sorting and shredding the used bottles to obtain flakes from the bottle walls, washing and cleaning the flakes to remove external contaminants, drying the flakes to reduce the hydrolysis, and then obtaining the desired products by melt extrusion. Post-consumer PET contains impurities such as polyvinyl chloride (PVC), sodium hydroxide (NaOH), and alkaline detergents. All impurities in recycled PET flakes can result in segregation and degradation during mechanical recycling. Therefore, the potential applications of these materials are often limited by their poor mechanical properties.

To solve the problems of the mechanical recycling, many researchers have investigated the effect of spinning and blending on the physical properties of recycled PET fabrics. Upasani et al.,² Scheirs,³ Pawlak et al.,⁴ Abbasi et al.,⁵ and Lee et al.⁶ have studied blending of recycled PET flakes with virgin PET chips. The chemical recycling process was also developed. During the chemical recycling process, the washed and recycled PET flakes are completely depolymerized to the monomers by methanolysis or hydrolysis, or partially depolymerized to the oligomers by glycolysis. Contaminants such as PVC, NaOH, alkaline detergent, acidic glues (ethylene vinyl acetate, acrylics), acetaldehyde, etc., are removed in the course of the depolymerization. The depolymerized materials are filtered and then further repolymerized to give desired products.

As it is expensive for recycled PET to go through several processes, it is necessary to apply it to high-value-added products such as sportswear and outdoor wear. Nowadays, knitted fabrics are popular for sportswear and outdoor wear. Therefore, it is important to study the characterization of recycled PET knitted fabrics for replacing virgin PET knitted fabrics and enlarging the applications.

In this work, mechanically and chemically recycled PET knitted fabrics, as well as chemically recycled PET–nylon6 blend, were prepared for comparison with virgin PET knitted fabric. Nylon has a better moisture property than PET because nylon has more significant hydrogen bonds than PET. Therefore, recycled PET–nylon6 blend yarns were developed to increase the range of the use of recycled PET yarns. It is necessary that the characterizations of the recycled PET–nylon6 blend knitted fabric are compared to other recycled PET knitted fabrics to investigate the applications of the recycled PET–nylon6 blend.

The mechanical properties of three recycled PET knitted fabrics were measured using tensile tests and the Kawabata evaluation system for fabrics (KES-FB). The morphology of recycled PET knitted fabrics was determined using scanning electron microscopy (SEM). The moisture transport properties were also investigated using moisture regain, moisture permeability, and wicking properties. Next, the results were compared to those for virgin PET knitted fabric to improve the applications of recycled textile products.

Experimental details

Materials

The cleaning process is very important to remove the contaminants in mechanical recycling process. The waste bottles were sorted and cleaned in the bottle state before the grinding process, and re-cleaned in the flake state to improve the purity of mechanically recycled PET flakes. Also, aluminum and floating materials were removed with the specific gravity method. The recycled PET flakes were dried for moisture reduction and then reprocessed by melt extrusion. After the filtration of inorganic contaminants, the mechanically recycled chips were obtained.

The chemically recycled PET chips were obtained using glycolysis of PET flakes. The metal catalyst used in depolymerization was eliminated by an ion exchange resin at about 140℃. Purification was achieved by melt filtration under pressure to remove physical impurities.

Mechanically and chemically recycled PET yarns were produced with melt spinning. The melt spinning velocity was 3000 m/min and the draw ratio was 1.03.⁷

The number average molecular weight (M_n) of the mechanically recycled PET was 16,003 dalton and the weight average molecular weight (M_w) was 29,700 dalton. Chemically recycled PET showed M_n = 16,613 dalton and M_w = 30,600 dalton. The average molecular weights of recycled PETs were higher than that of virgin PET.

The false twist processing was used in the texturizing process of recycled PET yarns. Texturizing conditions were as follows: speed 450m/min; draw ratio 1.66; and angle 110 at 160℃.

Recycled PET–nylon6 blend yarns composed of chemically recycled PET 80% and nylon6 20% were produced with a spinning velocity of 3960 m/min and draw ratio of 3.4 in the spin draw spinning process.

Double jerseys were prepared from virgin and recycled PET yarns. The specifications of the virgin and recycled PET knitted fabrics are listed in Table 1. The sample fabrics were knitted on a 28-gauge KTLM-84 AV, Keumyong circular knitting machine with 81.3 cm diameter and 84 feeders. The number of needles was 2640 and the knitting speed was fixed at 15.7 rpm.⁸

Table 1.

Specifications of virgin and recycled polyethylene terephthalate knitted fabrics

Sample code	Sample	Fineness (De.)	Strength (g/d)	Strain (%)	Weight (mg/cm²)	Thickness (mm)
Virgin	Virgin 75/72 DTY	72.7	4.20	15.0	9.88	0.541
MR	Mechanically recycled 75/72 DTY	78.8	4.14	18.0	11.14	0.584
CR	Chemically recycled 75/36 DTY	75.6	4.03	30.0	14.11	0.522
Blend	Chemically recycled PET–nylon6 blend 80/20 75/24SDY	78.3	2.83	42.3	12.96	0.378

DTY: drawn textured yarn; SDY: spin draw yarn.

The number of wales per centimeter (P_w) and courses per centimeter (P_c) was measured for dimensional properties.⁸ P_w and P_c were determined by calculating the mean of 10 measurements taken from different areas of each knitted fabric. These mean values were used to estimate the stitch density of the sample. The mean values of P_w and P_c, and the stitch density for each knitted fabric are given in Table 2. The stitch densities of the mechanically recycled and virgin PET knitted fabrics were similar. The stitch densities of chemically recycled PET and recycled PET–nylon6 blend knitted fabrics were higher than those of the virgin PET knitted fabric.

Table 2.

Dimensional properties of virgin and recycled polyethylene terephthalate knitted fabrics

Sample code	Sample	P_w (cm⁻¹)	P_c (cm⁻¹)	Stitch density (cm⁻²)
Virgin	Virgin 75/72 DTY	17.3	14.4	248.4
MR	Mechanically recycled 75/72 DTY	17.3	14.4	249.8
CR	Chemically recycled 75/36 DTY	17.8	19.5	346.0
Blend	Chemically recycled PET–nylon6 blend 80/20 75/24 SDY	17.6	18.9	333.0

DTY: drawn textured yarn; SDY: spin draw yarn.

Test methods

The mechanical properties of virgin and recycled PET knitted fabrics were determined with an Instron 4454 tensile testing machine. The samples were tested with a crosshead speed of 15 cm/min. The results were evaluated based on the ultimate tensile strength (UTS), elongation, and modulus. The pilling resistance was estimated with an ICI Pilling-Box Pilling Tester. The pilling tester was operated at a rotational speed of 60 rpm for 4 h. The test results were represented by the average of four test samples. The drape coefficient, defined as the fraction of the area of the annular ring covered by the projection of the draped sample, was measured with a Fabric Research Laboratories drapemeter.⁹ The warm/cool feeling (Q_max), compressional and surface properties were measured using the KES-FB. SEM was used to compare the morphology of the samples. Moisture regain, moisture permeability, and the wicking property were investigated to compare the moisture transport properties of virgin and recycled PET knitted fabrics. Moisture regain is the weight of water in a material expressed as a percentage of the oven dry weight. Samples were conditioned in a standard atmosphere of 65% relative humidity (RH), at 20℃, for at least 24 h and then weighed. Conditioned samples were dried in an oven for 1 h at 105–110℃ and reweighed. Moisture regain was calculated using the following equation:¹⁰

MR = [W_{C} - W_{D}] / [W_{C} - W_{D}] W_{D} W_{D}

where W_C is the weight of the conditioned fabric (g) and W_D is the weight of the dried fabric (g).

Moisture permeability is a measure of the passage of water vapor through a membrane. It is the rate of water vapor transmission per unit area per unit of vapor pressure differential under test conditions. The glass cups were filled with 33 g of CaCl₂ desiccant and its opening was covered with the samples. The cups containing samples were subjected to an environment at 40 ± 2℃ and 90±% RH for 1 h, and then the weight changes were measured.¹¹

Results and discussion

Tensile properties

The tensile properties of the four samples are shown in Figure 1. The tensile strength of the wale direction was higher than that of the course direction regardless of sample. The tensile strengths of mechanically and chemically recycled PET knitted fabrics showed similar values to virgin PET knitted fabric. However, the tensile strength of the recycled PET–nylon6 blend knitted fabric was decreased compared to virgin PET knitted fabric.

Figure 1.

Tensile properties of virgin and recycled polyethylene terephthalate (PET) knitted fabrics: (a) strength; (b) strain; (c) modulus.

Decomposition reactions during the recycling of PET are shown in Figure 2.³ Because bottle-grade PET is generally produced using solid-state polymerization, the molecular weights and intrinsic viscosity of post-consumer bottle-grade PET can be maintained at comparatively high levels, despite the occurrence of hydrolysis and thermal decomposition during the recycling process.^12,13 Therefore, the tensile strengths of mechanically and chemically recycled PET knitted fabrics were not reduced in comparison to virgin PET knitted fabric. On the other hand, because recycled PET–nylon6 blend knitted fabric is composed of PET and nylon, which are incompatible with each other, it is considered that the tensile strength has decreased because of inhomogeneity.¹⁴

Figure 2.

Decomposition reactions responsible for the reduction in molecular weight during the recycling of polyethylene terephthalate (PET): (a) hydrolysis; (b) thermal decomposition.

For the knitted fabrics produced from mechanically and chemically recycled PET, the strain values were similar to those of virgin PET knitted fabric. The strain of recycled PET–nylon6 blend knitted fabric was increased. This is attributed to the fact that nylon has more amorphous regions than PET, and intermolecular chain slippage can occur easily for different blend compositions of fibers.

As the recycled PET–nylon6 blend knitted fabric was produced from chemically recycled PET and nylon6 using a compatibilizer and stabilizer, the modulus of recycled PET–nylon6 blend knitted fabric was the lowest because of the influence of nylon.

Pilling resistance

The pilling resistance values of virgin and recycled PET knitted fabrics were 4. The pilling resistance of all samples appeared to be good.¹⁵

Drapability

Fabric drape affects the appearance of clothing and the drape coefficient is used to quantify it. A low drape coefficient indicates good flexibility of a fabric.^9,16 The drape coefficients of the four samples are shown in Figure 3. Recycled PET–nylon6 blend knitted fabric showed the best drapability among the four samples because of the lower thickness and soft surface. As the mechanically and chemically recycled PET knitted fabrics had higher drape coefficients than the virgin PET knitted fabric, it showed that the former fabrics were stiffer than the latter.

Figure 3.

Drapabilities of virgin and recycled polyethylene terephthalate (PET) knitted fabrics.

As mechanically recycled PET flakes contain various impurities and chemically recycled PET flakes include many types of catalysts for depolymerization and repolymerization, it is considered that this is why recycled PET knitted fabrics are stiff.

Mechanical properties of the Kawabata system

The compressional and surface properties and the warm/cool feeling measured by the Kawabata system are indicated in Table 3.

Table 3.

Mechanical properties of virgin and recycled polyethylene terephthalate knitted fabrics

Mechanical property		Virgin		Mechanically recycled		Chemically recycled		PET–nylon6 blend
Mechanical property		Wale	Course	Wale	Course	Wale	Course	Wale	Course
Compression	LC	0.625		0.616		0.543		0.572
	WC (gf·cm/ cm²)	0.370		0.395		0.264		0.148
	RC (%)	39.613		42.517		59.327		48.16
Surface	MIU	0.26	0.431	0.253	0.463	0.277	0.364	0.247	0.313
	MMD	0.005	0.011	0.005	0.014	0.007	0.016	0.004	0.010
	SMD (μ)	0.942	6.517	1.133	4.273	1.448	2.342	0.918	2.128
Warm/cool feeling (J/cm²·sec)		0.080		0.083		0.098		0.127

LC: compression linearity; WC: compression energy; RC: compression resilience; MIU: mean value of the friction coefficient; MMD: mean deviation of the friction coefficient; SMD: mean deviation of surface roughness.

The compression linearity (LC), the compression energy (WC), and the compression resilience (RC) were measured as compressional properties. The LC determines the ease of the compressibility. The LC value of mechanically recycled PET knitted fabric was similar to that of virgin PET knitted fabric. The LC values of chemically recycled PET and recycled PET–nylon6 blend knitted fabrics were low compared to the virgin PET knitted fabric. Therefore, recycled PET knitted fabrics are easy for the initial compression and flexible against the compression compared to the virgin PET knitted fabric. A higher WC value corresponds to a higher compressibility of the fabric. The WC values of mechanically recycled PET and virgin PET knitted fabrics were similar, whereas those of the chemically recycled PET and recycled PET-nylon6 blend knitted fabrics were lower. The RC indicates the percentage energy recovery from compression-caused deformation in the thickness direction. A higher RC value indicates a better recovery property.¹⁷ The three recycled PET knitted fabrics showed higher RC values than virgin PET knitted fabric. Therefore, the recovery property of recycled knitted fabrics appeared to be good.

The fabric made with larger-diameter monofilament yarns had higher compressibility and better recovery property. As the fiber diameters of chemically recycled PET and recycled PET–nylon6 blend knitted fabrics were larger than those of the virgin PET knitted fabric, they can be more easily compressed and recovered.

Surface friction and roughness were measured as surface properties.¹⁶ The results of two friction parameters, namely mean value of the friction coefficient (MIU), mean deviation of the friction coefficient (MMD), and the roughness values, mean deviation of surface roughness (SMD), are shown in Table 3. Although in mechanically recycled PET and recycled PET–nylon6 blend knitted fabrics the MIU value decreased, it increased in chemically recycled over virgin PET knitted fabric. The MMD value was indistinguishable between the different fabric types. The SMD value of recycled PET–nylon6 blend knitted fabric was the lowest in the four samples, showing that the recycled PET–nylon6 blend knitted fabric has a good smooth surface. The recycled PET knitted fabrics appeared to have a little rougher surface than the virgin PET knitted fabric.

As chemically recycled PET knitted fabric contains the most impurities among the yarns, it is considered that the MIU of chemically recycled PET knitted fabric has increased compared to that of virgin PET knitted fabric. Although textured yarns were used in recycled and virgin PET knitted fabrics, yarns used in recycled PET–nylon6 blend knitted fabric were not textured, and its surface appeared smoother than that of virgin PET knitted fabric. By developing texturizing of recycled PET–nylon6 blend yarns, it is necessary to study the surface characterizations with texturizing conditions.

The warm/cool feeling is developed when fabric initially contacts the skin and is related to the heat flow between the skin and the contacted object.¹⁸ The maximum heat flow (Q_max) value depends on the heat capacity and conductivity of the fabric and on the contact area. As the contact area and the heat flow increase with a smooth surface on the fabric, a fabric with a smooth surface indicates a cool feeling. As recycled PET–nylon6 blend knitted fabric had a low friction coefficient and high smoothness, the surface feeling of recycled PET–nylon6 blend knitted fabric was cooler feeling value than other samples.

Morphology

The morphologies of the four samples are detailed in Table 4. Recycled PET–nylon6 blend fabric knitted with SDY yarns showed closer packing compared to other samples. Therefore, the smoother surface of the recycled PET–nylon6 blend knitted fabric could be demonstrated. The tendency was the same with the surface property. Mechanically recycled PET and recycled PET–nylon6 blend knitted fabrics contain a small amount of contaminants, whereas chemically recycled PET knitted fabric contains plenty of contaminants on the surface of fibers. This effect shows that the process of chemical recycling needs to be modified to improve fabric touch.

Table 4.

Field emission scanning electron microphotographs of virgin and recycled polyethylene terephthalate (PET) knitted fabrics

	Virgin	Mechanically recycled	Chemically recycled	PET–nylon6 blend
×50
×1000
×3000

Moisture transport properties

The moisture transport property is one of the most important factors that influences both comfort and wash-wear of textiles for apparel, especially underwear and sportswear. Moisture regain, moisture permeability, and wicking property were measured to show the moisture transport property.^19–21

Moisture regains of the four samples are shown in Figure 4. Moisture regain is affected by the fiber hydrophilicity. Whereas PET is a hydrophobic fiber, nylon6 has some hydrophilic groups (amide groups). Therefore, moisture regain of nylon6 is higher than that of PET. Recycled PET fabrics that had the same knit structure as virgin PET fabric showed similar moisture regain compared to virgin PET knitted fabric. Because recycled PET–nylon6 blend knitted fabric contained a small content of nylon6, moisture regain increased for these samples.

Figure 4.

Moisture regains of virgin and recycled polyethylene terephthalate (PET) knitted fabrics.

Moisture permeability results of the four samples are shown in Figure 5. The factors influencing moisture permeability of fabrics involve the hydrophilic properties of fibers, porosity, thickness, and surface smoothness. Virgin and recycled PET knitted fabrics showed similar moisture permeability results. Because recycled PET–nylon6 blend knitted fabric, which contained a small amount of nylon6, showed higher moisture regain, lower thickness, and a smoother surface than other samples, the moisture permeability of recycled PET–nylon6 blend knitted fabric was improved.

Figure 5.

Moisture permeabilities of virgin and recycled polyethylene terephthalate (PET) knitted fabrics.

The wicking property means that the fabric maintains liquid migration in its capillary pores. Wicking property results are shown in Table 5. Virgin and recycled PET knitted fabrics had lower moisture regain and permeability than recycled PET–nylon6 blend knitted fabric. However, capillary pores formed between the surfaces of fibers of virgin and recycled PET knitted fabrics have a smaller radius because of high linear density, and many pores are present in these fibers because of the texturing process. For this reason, the wicking property of virgin and recycled PET knitted fabrics was improved.

Table 5.

Wicking properties of virgin and recycled polyethylene terephthalate (PET) knitted fabrics

	Virgin		Mechanically recycled		Chemically recycled		PET–nylon6 blend
	Wale	Course	Wale	Course	Wale	Course	Wale	Course
Wicking rate (%)	199	191	201	190	133	128	8	7
Wicking height (mm)	151	150	153	144	138	126	10	10
Wickability	300.5	286.5	307.5	273.6	183.5	161.3	0.8	0.7

Conclusion

The recycling of PET has been studied for environmental conservation. Post-consumer PET bottles are recycled in mechanical recycling and chemical recycling processes.

In this work, mechanically and chemically recycled PET knitted fabrics, and chemically recycled PET–nylon6 blend knitted fabric, aimed at improving the moisture property of recycled PET knitted fabric, were prepared and then compared to virgin PET knitted fabric. Characterization of three different recycled PET knitted fabrics and virgin PET knitted fabric was performed in order to investigate wide applications of recycled fabrics. The tensile strengths of mechanically and chemically recycled PET knitted fabrics were similar to those of virgin PET knitted fabric. Recycled PET–nylon6 blend knitted fabric showed excellent elongation. Both virgin and recycled PET knitted fabrics exhibited excellent pilling resistance. Using the drape coefficient, recycled PET–nylon6 blend knitted fabric was shown to be more flexible than other samples. The warm/cool feeling (Q_max), compressional and surface properties were measured using the KES-FB system. The compressional properties of recycled PET knitted fabrics have increased. Recycled PET–nylon6 blend knitted fabric showed the smoothest appearance and the coolest feeling among the four samples. Moisture regain and moisture permeability values of virgin and recycled PET knitted fabrics were lower than those of recycled PET–nylon6 blend knitted fabric. However, the wicking property of virgin and recycled PET knitted fabrics has improved because of high linear density and the texturing process.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgement

This work was supported by Ministry of Trade, Industry and Energy of Korea (Project Number 10035180).

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