Comparative study of mechanical
characteristics of recycled PET fibres for
automobile seat cover application
Giulia Albini
1, 2, Bartolomeo Placenza
2, Brunetto Martorana
2, Vito Guido Lambertini
2and Valentina Brunella
1*
1Dipartimento di Chimica e NIS Research Centre, University of Torino, Via P. Giuria 7, 10125 Torino, ITALY 2 Group Materials Labs, Polymers & Glass Group, CRF, Corso Settembrini 40 10135 TORINO, ITALY Corresponding: Valentina Brunella, Dipartimento di Chimica e NIS Research Centre, University of Torino valentina.brunella@unito.it
Abstract
Polyethylene terephthalate (PET) is a thermoplastic polymer with a wide range of uses, including synthetic fibres and containers for beverages and other liquids. Recycling plastics reduces the amount of energy and natural resources needed to create virgin plastics. PET containers and bottles are collected and then broken down into small flakes used to produce new products such as textile fibres. Thermo-mechanical degradation may happen during the recycling process and presence of contaminants affects the final product characteristics. Two kinds of recycled PET fibres were used for fabrics production: post-consumer PET fibres and a blend of post-consumer and post-industrial PET fibres. Focusing on knitted and flat woven textile structures, main mechanical properties of the fabrics were assessed by various tests, like tensile strength test and wear resistance test. A comparative study with current production virgin PET fabrics was useful to evaluate high standards accordance for automotive field. Both knitted and flat woven recycled PET fabrics had excellent performance after mechanical tests. Post-consumer PET fabrics had the best results, especially after wear resistance test. These results allow an evaluation of their applications.
Keywords
PET, automotive fabrics, industrial textiles, waste reduction, materials properties, technical textile (fabric) research, R-PET, recycling, post-consumer, post-industrial
Introduction
Production and consumption of fibres are increasing every year in Europe, from manufacturing companies to automotive, furnishing, textile and apparel industries. Nowadays, 90% of fabrics used in the automotive field are composed of polyester fibres. Polyamide fibres are also widely used especially for airbags and polypropylene for door panels application [1-2]. Global Service International data show a PET consumption of 12.5 kg per capita [3]. Being so widely used, PET and other plastics affect the amount of waste for disposal, but they can be recycled, as an economical way to reduce plastic waste [4-6].
In the automotive field, recycling of components of end of life vehicles (ELV) is hard and expensive, most of them are sent to landfill. Vermeulen I. et al. [7] mention a secondary recovery of Automotive Shredder Residue (ASR) and Santini A. et al. [8] report a study of the recycling campaign of the ELVs and parts reuse with a calculation of the effective recycling rate, according to 2000/53/EC Directive. Instead, the use of materials originating from a recycling process is relatively simple. Recycling involves reducing waste and production of new materials, recovering and transforming of refuse. It allows reducing energy consumption and toxic gas emissions, CO2, NO2 and NO [9-10].
Recycling companies convert PET bottles and containers into fibres and other products. They mainly recycle PET by a mechanical process. It is simple, cheap, environmentally friendly and provides operations of selection, washing and grinding of bottles. The bottles are converted into small flakes and quickly dried and stored [9-11].
The flakes are reprocessed and extruded into fibres, following the virgin PET fibres production process, to be later converted into yarns and fabrics.
The properties of the final product depend on various parameters. The recycling and reprocessing steps can cause polymer degradation. Moreover, moisture absorption causes a decrease of the average molecular weight. These factors affect the final product mechanical and physical properties.
Contamination by other polymers, e.g. PVC presence, defines molten fibres viscosity and density: optimal viscosity (η) is between 0.6 and 0.8 dl/g and density between 1.3 and 1.4 g/cm3 [12-14].
PET fibres can be used for automotive fabrics production, as “covering textile” or “functional textile”. In the first case covering seats, roof, floor and doors; in the second case reinforcement of tires, tubes, seat belts and airbags, providing soundproofing and insulating functions. Fabrics have to meet automotive requirements to satisfy these applications with high technical-mechanical performances. High wear resistance, fastness to light, comfort and safety features are fundamental characteristics to ensure the durability of the vehicle [1-2]. Several research papers have been published regarding possible applications for R-PET fibres. Among these, moulded carpets developed from R-PET fibres [15], high-performance R-PET sound-absorbing materials [16, 17] and technical non-woven and composites (e.g., tires) [17, 18] with reinforcement of R-PET fibres [18, 19]. A study [20] about R-PET fabrics properties describes their characteristics according to bursting strength, air permeability, surface friction and dimensional stability properties. It proves that recycled PET fabrics do not have the same properties as the fabrics produced with virgin PET fibres, but they show good results. Moreover, recycled PET fibres can be blended with raw materials without a noticeable change in quality for the apparel industry.
The objective of this study is to analyse two different kinds of recycled PET fabrics and verify their properties and mechanical characteristics, according to automotive requirements. To use them as covering textile materials inside the vehicle, they have been compared to current production fabrics, made from virgin PET.
This research, assessing different mechanical properties, tensile and tear strength resistance and wear resistance, can contribute to increasing the knowledge about fabrics produced with recycled PET fibres. Recycled PET fabrics can contribute to environmental benefits, considering the material demanding performance needed for seat cover application and the high number of vehicles produced.
Experimental
Fibres were supplied by different European manufacturers. Three PET fibres were used: virgin PET fibres (V-PET), post-consumer recycled PET fibres (R-PETPC) and a blend of consumer and
post-industrial PET (R-PETPC/PI) fibres. The latter was made from 50% PETPC and 50% PETPI with a small
percentage of polyethylene (PE), coming from the reprocessing of PET/PE food trays and industrial waste. R-PETPI products were less subjected to waste contamination. V-PET fibres and R-PETPC fibres
were mass dyed, while R- PETPC/PI fibres were not dyed. Besides, all types of yarn were false twist
textured. A DSC analysis characterised the structure of every kind of fibre. The test was performed with a Mettler Toledo DSC analyser. DSC parameters are reported in table 1. DSC analysis was used to evaluate thermal behaviour and crystallinity percentage of the fibres. The crystallinity percentage was evaluated considering the theoretic ΔH melting of 100% crystalline PET (140.1 J/g) [21].
Table 1. DSC parameters.
DSC Cycles Temperature range
(°C) Speed (°C/min) N2 flow (ml/min)
Heating 0 - 300 10 50
Cooling 300 - 0 10 50
Heating 0 - 300 10 50
Fabrics were provided by a major manufacturer in Italy. It produced circular knitted and flat woven fabrics. Woven fabric is the most used for seat cover application [22] and consists of two sets of yarns interlaced at right angles to each other. Its strength and durability can be varied and depend on the yarn type and the thread spacing [23]. Circular knit is the second largest fabric group. Yarns are interwoven in loops which are repeated in the whole structure. The loops allow the fabric to stretch in all directions, also depending on the fibre type [24, 25]. Knitted fabrics were manufactured by circular
machine Mayer MV4, and flat woven fabrics were made by Dornier Airjet frame. Their characteristics are summarised in table 2.
Table 2. Fabrics properties. The mean value () and the coefficient of variation (CV) are reported for each property.
Fabrics properties
Yarn type
V-PET R-PET PC R-PET PC/PI
Knitting type Circular
Yarn Count (dtex) 385 330 387
CV 12.4 11.0 12.7 Wale yarn/cm 9.0 9.0 9.0 CV 0.1 0.1 0.1 Course yarn/cm 14.0 14.0 14.0 CV 0.07 0.07 0.07 Total weight (g/m 2) 403 440 428 CV 0.01 0.02 0.01 Fabric weight (g/m 2) 189 220 216 CV 0.01 0.01 0.01 Fabric total thickness (mm) 4.00 4.00 4.10 CV 0.02 0.02 0.02 Fabric thickness (mm) 0.5 0.5 0.6 CV 0.2 0.2 0.2
Weave type Flat woven
Yarn Count (dtex) 385 330 387
CV 12.4 11.0 12.7 Warp yarn/cm 34.0 34.0 34.0 CV 0.02 0.02 0.02 Weft yarn/cm 19.0 19.0 19.0 CV 0.05 0.05 0.05 Total weight (g/cm 2) 439 436 442 CV 0.01 0.01 0.01 Fabric weight (g/cm 2) 228 228 229 CV 0.01 0.01 0.01 Fabric total thickness (mm) 4.00 4.10 3.90 CV 0.01 0.02 0.02 Fabric thickness (mm) 0.5 0.6 0.5 CV 0.4 0.2 0.2
All types of fabrics were subjected to further processes: dry cleaning with C2Cl4 and thermo-fix process
at 140°C to optimise dimensional stability.
In a further step, all fabrics were laminated to a polyurethane foam layer and a non-woven scrim fabric. Fabrics total thickness is reported in table 2.
Tensile and tear resistance tests were performed to evaluate fabrics mechanical characteristics. Fabrics need to stand seat cover manufacturing processes and to highly perform when exposed to mechanical stress throughout the lifetime of the vehicle. Tensile strength test was performed on Acquati
AG/7E/306 electronic dynamometer according to ISO 13934-1 [26]. For each type of fabric, five samples for warp/wale and weft/course directions were tested. Different directions testing is necessary because fabric properties are affected by the loading direction.
Tear strength test was carried out to measure the force to propagate a tear in fabrics. This test is used to evaluate the textile material stress suffered during the process of cover production. The test was performed according to ISO 9073 [27]. Samples were prepared in the format as in figure 1 and tore on one side (“cut” in figure 1). The dashed lines point out where the clamps of the dynamometer grab the sample.
[Insert figure 1]
Figure 1. Sample template for tear strength test. The dimensions are in mm.
Structural wear resistance was evaluated to define the resistance of the yarns that compose the fabric. The test was performed on a Taber (Rotary Platform Double Head Tester) abrasimeter, according to ASTM D3884/09 [28]. The analysed fabric was worn against two Calibrase 10 abrasive wheels, moving in opposite directions on its surface, as schematized in figure 2. Three samples for each type of fabric were examined. According to internal automotive specifications, 300 cycles of wheels rotation were set for knitted fabrics and 600 cycles for woven fabrics. Flat woven fabrics require a more extended period of testing because of their inherently more resistant structure. Furthermore, worn surfaces were visually examined and graded by setting values from 3, the best result with no yarns breakage, to 1, the worst result, with at least a yarn breakage.
[Insert figure 2]
Figure 2. Scheme of the structural wearing test.
Aesthetic wear resistance of all type of fabrics was evaluated with a Cesconi Abrasion tester, comparable to Martindale Abrasimeter, according to ISO 12947/1-2-3-4 [29]. This test reproduces the
mechanical stress suffered by seat cover fabrics during their use for an average lifetime of the vehicle. The abrasion tester is used for testing textile materials. The apparatus is schematized in figure 3. The standard abrading fabric is made of woollen fibres and fixed on a given plate (point A in figure 3). The tested surface is placed on a rotating disc located under the fixed plate. The rotating plate moves with a revolution movement to ensure a complete wear of the fabric sample. 3000 and 6000 cycles were performed with a load of 3 Kg, according to the internal automotive validation test.
[Insert figure 3]
Figure 3. Scheme of wear resistance test apparatus. The substrate to test (D) is placed on the rotating
plate (F), through a substrate holder (E). The abrasive fabric, instead, is placed on the fixed plate (A), using an appropriate substrate holder (B), on which a load is applied (P).
For aesthetic wear resistance test three samples for each kind of fabric were examined and evaluated according to grey scale. The scale quantifies changes in colour compared to the reference. It includes a range of shade of grey, from 1, the worst result, evident modification of the surface to 5, the best result, almost no superficial changes, with a step of 0.5.
Minolta CM-3610 spectrophotometer provided grey scale measurements, under a D65 illuminant, using a 10° standard observer with specular component excluded (SCE) mode. Not worn surfaces were taken as standards and worn fabrics as samples. Surfaces pictures were recorded with a Confocal Profilometer Leica DCM8.
Before the tests, all specimens were conditioned for 72 hours in a standard atmosphere at 20 ± 2°C with 65 ± 2 % relative humidity, according to ISO 139 [30].
Results and discussion
DSC analysis
Figure 4 and table 3 represent DSC analysis results, relative to the first heating cycle. Endothermic peaks have got different shape: V-PET fibre has got a narrow peak and a maximum of melting peak of
257°C. Instead, R-PET fibres present more broadened melting peaks, correlated with crystallites of different thickness. They have a lower maximum melting peak, as in table 3. From the thermogram, V-PET fibre crystallites start melting at higher temperatures (onset point) and are bigger than recycled fibre ones. Their major size is due to virgin raw material and lack of contamination. R-PET fibres crystallites minor thickness is a consequence of the thermo-mechanical degradation during recycling and reprocessing steps.
Table 3. DSC analysis results of the three kinds of fibres, first heating.
a)V-PET b) R-PET(PC/PI) c) R-PET (PC)
ONSET (°C) 246 233 238 PEAK (°C) 257 248 254 ENDSET (°C) 264 257 261 CRYSTALLINITY (%) 38 37 38 ΔH (J/g) -53 -52 -53 [Insert figure 4]
Figure 4. Fibres melting peaks in the first heating cycle: (a) V-PET fibre (b) R-PET (PC/PI)fibre and
(c) R-PET (PC)fibre.
Tensile strength test
Tensile strength results are reported in figure 5, 6 and table 2. As can be seen in figure 5, flat woven fabrics present high tensile strength values, above 100 daN required by automotive specification. High loads are required during fabric manufacturing and covers assembling steps. Warp direction samples have got higher tensile strength values. In fact, they are composed of more warp yarns per centimetre, compared to weft direction samples, as showed in table 4. During this test, for each direction, all yarns are stressed together and they contribute to strength resistance. As regards knitted fabric (Figure 6), the
number of yarns cannot be correlated to tensile strength values, because of its particular structure. Initially, it stretches, then loops become knots from which the test starts. In this case, the all structure is stressed. Knitted fabric owns a flexible and less resistant structure, but far higher than automotive requirements for knitted fabrics (≥ 60 daN). Because of its flexibility, there is only a slight difference between wale and course directions, compared to woven fabric.
Considering fibres composition, R-PETPC/PI woven fabrics have lower tensile strength values, in both
warp and weft directions, compared to V-PET and R-PETPC fabrics (figure 5). This behaviour is a
consequence of the mechanical degradation of PET during the recycling process. The high-quality composition of R-PETPC fabric ensures high tensile strength values in warp and weft directions. It is
also confirmed by the high percentage of crystallinity, similar to V-PET (table 3). The warp direction higher values are, as mentioned above, a consequence of the number of yarns/cm. They are comparable to warp direction V-PET values, considering standard deviation. The weft direction values are slightly lower than V-PET fabric ones. In this case, the recycled fibres composition is more visible because of the structure with fewer warp yarns/cm. About knitted fabrics (figure 6), the results are similar among the three types of fibre considering standard deviation, because fabric structure characteristics affect them.
Table 4. Tensile and tear strength results. The mean value () and standard deviation (SD) are reported for each type of fabric.
Woven fabrics
Tensile strength (daN) Tear strength (daN)
Warp direction Weft direction Warp direction Weft direction
SD SD SD SD
V-PET 386 11 258 10 27.4 4.1 21.4 1.6
R-PET PC 408 9 176 7 30.8 2.6 17.7 2.2
Knitted fabrics
Tensile strength (daN) Tear strength (daN)
Wale direction Course direction Wale direction directionCourse
SD SD SD SD
V-PET 155 5 148 3 22.5 1.6 18.3 1.3
R-PET PC 148 3 140 6 18.2 1.2 16.5 1.1
R-PET PC/PI 132 3 124 8 17.2 2.1 15.3 0.8
[Insert figure 5]
Figure 5. Tensile strength results of flat woven fabrics.
[Insert figure 6]
Figure 6. Tensile strength results of knitted fabrics.
Tear strength test
Figures 7, 8 and table 4 show tear strength results. They are far higher than automotive specification, for which the requirement is ≥ 6 daN. As can be seen in figure 7, knitted fabrics show a small difference between wale and course directions, because of the flexibility of the structure. About woven fabrics, the higher number of yarns/cm affects the results for the warp direction samples. Considering fibre composition, R-PETPC/PI fabric shows slightly lower performance compared to V-PET and
R-PETPC fabrics, for woven fabric in particular. Degradation of fibres during the recycling process
influences the final mechanical properties of the fabric. This is also confirmed by the lower percentage of crystallinity (Table 3). In figure 8, R-PETPC warp direction results are comparable to V-PET,
yarns/cm and its recycled origin is highlighted. During tear strength test, yarns are individually stressed and their characteristics affect the results.
[Insert figure 7]
Figure 7. Tear strength values of virgin and recycled PET fabrics with knitted structure.
[Insert figure 8]
Figure 8. Tear strength values of virgin and recycled PET fabrics with woven structure.
Wear resistance test
For all the fabrics, structural wear resistance test results are summarised in table 5. Not worn fabrics were taken as standard fabrics, and they are evaluated grade 3. After Taber test, V-PET fabric does not show any yarn breakage, nor structural changes. The worn surface presents a slight shadow; hence it’s evaluated grade 3. This result is utterly positive according to automotive standard, for which there must be no yarns breakage. Structural wear resistance test does not break any R-PETPC yarns either but
causes only slight superficial fibres lifting. The abrasive wheels stress the surface several times and cause the damage of the most superficial fibres. R-PETPC/PI flat woven fabric surface is negative to the
test, with widespread breakages of fibres in the structure. Knitted fabrics have the best performance, despite the limited number of cycles compared to woven fabrics. Knitted fabric flexibility ensures high wear resistance and allows the fabric to withstand the rubbing better.
Table 5. Grade results for every kind of fabric.
Type of fabric Grade values
Knitted V-PET 300 cycles 3
Knitted R-PETPC/PI 300 cycles 3
Knitted R-PETPC 300 cycles 3
Flat woven R-PETPC/PI 600
cycles 2.5
Flat woven R-PET PC 600 cycles 3
Afterwards, fabrics surface was examined after aesthetic wear resistance test. Values of grey scale, registered with the spectrophotometer, are represented in table 6. Not worn fabrics were taken as standard fabrics and their surface is considered 5 of grey scale.
Table 6. Grey scale values for each kind of fabric.
Type of fabric Grey Scale
values
Knitted V-PET 3000 cycles 4.5
Knitted V-PET 6000 cycles 4
Knitted R-PETPC/PI 3000 cycles 3
Knitted V-PETPC/PI 6000 cycles 2.5
Knitted R-PETPC 3000 cycles 4.5
Knitted R-PETPC 6000 cycles 4
Flat woven V-PET 3000 cycles 4.5
Flat woven V-PET 6000 cycles 4.5
Flat woven R-PETPC/PI 3000 cycles 3.5
Flat woven R-PET PC/PI 6000
cycles 3.5
Flat woven R-PET PC 3000 cycles 4.5
For a more accurate analysis, pictures of the surfaces have been taken at different magnification with a confocal Profilometer (Flat woven fabrics in figures 9, 11 and 13 and knitted fabrics in figures 10, 12 and 14). Worn surfaces of V-PET fabrics do not change significantly after the test, with no broken yarns or structural modifications. After 3000 cycles of rubbing, V-PET fabric structure shows a lifting of superficial fibres. Lifted fibres are responsible for a superficial whitening effect. This result corresponds to a value greater than 4 of grey scale for V-PET fabrics. The value is acceptable according to automotive requirements. An example of the results of woven and knitted fabrics is shown in figures 9 and 10 respectively. R-PETPC/PI woven fabric surface (Figures 11) was more abraded
compared to V-PET fabric. In this case, there is a high difference between not worn and worn areas and frame completely disappears. To the touch, the surface loses three-dimensionality of the structure and gains a rubbery consistency. This is the consequence of a superficial plasticization after surface heating due to successive rubbings of the analysed fabric against the standard one. Knitted fabric surface (figure 12) shows a slightly better result compared to woven one. The structure is still visible because fabric flexibility allows it to withstand several rubbings, as in the case of structural abrasion test. Despite that, the plasticising effect appears and the surface loses its three-dimensionality. R-PETPC/PI
grey scale values are unacceptable according to automotive requirements (Table 6). The MS.90093 standard requires values of 4.5 after 3000 cycles and 4 after 6000 cycles. Fibres coming from PET bottles recycling process (R-PETPC) contribute to a better performance of the fabric, due to the less
contamination from other plastics. In this case, a slight shadow appears in correspondence to the worn area, without plasticising effect (Figures 13 and 14). For both structural and aesthetical, wear resistance depends on a combination of fibres, yarns, and fabric construction characteristics.
[Insert figures 9, 10, 11, 12, 13 and 14]
Figure 9. Pictures under the Profilometer of V-PET woven fabrics surfaces before wear resistance test
Figure 10. Pictures under the Profilometer of V-PET knitted fabrics surfaces before wear resistance
test and after 3000 and 6000 cycles of wearing (from left to right).
Figure 11. Pictures under the Profilometer of R-PETPC/PI woven fabrics surfaces before wear resistance test and after 3000 and 6000 cycles of wearing (from left to right).
Figure 12. Pictures under the Profilometer of R-PETPC/PI knitted fabrics surfaces before wear resistance test and after 3000 and 6000 cycles of wearing (from left to right).
Figure 13. Pictures under the Profilometer of R-PETPC woven fabrics surfaces before wear resistance test and after 3000 and 6000 cycles of wearing (from left to right).
Figure 14. Pictures under the Profilometer of R-PETPC knitted fabrics surfaces before wear resistance test and after 3000 and 6000 cycles of wearing (from left to right).
Conclusions
In this paper, structural and mechanical characteristics of fabrics produced with recycled PET fibres were evaluated. For specific automotive requirements, they were compared to current production virgin PET fabrics. Tensile strength, tear strength and structural and aesthetic wear resistance tests were performed on virgin and recycled PET fabrics.
All fabrics possess good tensile and tear strength properties, far above automotive requirements.
Despite good mechanical results, R-PETPC/PI fabric cannot be applied as covering of very contactable
areas as sittings areas and back seats. In fact, its surface reacted negatively after aesthetic wear resistance test and it could be damaged in a short time of usage. Otherwise, it is suitable for a “covering textile” role for less contactable areas such as lateral zones and door panels.
V-PET and R-PETPC fabrics have got high wear resistance and excellent mechanical performance.
considering both knitted and flat woven structures. It reaches the high-performance of V-PET textile materials, and it can replace them entirely inside the vehicle.
Recycled PET fabrics are environmentally friendly. Using recycled PET fabrics, car manufacturers can contribute to benefits as reducing gases emissions and refuse to landfill, than petrochemical resources exploiting for virgin materials manufacturing.
Acknowledgements
The authors wish to thank Dentis S.p.A. for recycled PET flakes production and Apollo S.p.A. for fabrics production.
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