Comparative study of mechanical characteristics of recycled PET fibres for automobile seat cover application

Abstract

Polyethylene terephthalate 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. Polyethylene terephthalate 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 polyethylene terephthalate fibres were used for fabrics production: post-consumer polyethylene terephthalate fibres and a blend of post-consumer and post-industrial polyethylene terephthalate 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 the current production of virgin polyethylene terephthalate fabrics was useful to evaluate high standards accordance for automotive field. Both knitted and flat-woven recycled polyethylene terephthalate fabrics had excellent performance after mechanical tests. Post-consumer polyethylene terephthalate fabrics had the best results, especially after wear resistance test. These results allow an evaluation of their applications.

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 polyethylene terephthalate (PET) consumption of 8.2 kg pro capite/year in Italy in 2013 [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 (ELVs) is hard and expensive, most of them are sent to landfill. Vermeulen et al. [7] mention a secondary recovery of automotive shredder residue (ASR) and Santini 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 (V-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, they can be used as covering seats, roof, floor and doors; in the second case as 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 (R-PET) fabrics do not have the same properties as the fabrics produced with V-PET fibres, but they show good results. Moreover, R-PET fibres can be blended with raw materials without a noticeable change in quality for the apparel industry.

According to the automotive requirements, the objective of this study is to analyse two different kinds of R-PET fabrics and verify their properties and mechanical characteristics. To use them as covering textile materials inside the vehicle, they have been compared to current production fabrics, made by V-PET.

This research, assessing different mechanical properties, tensile and tear strength resistance and wear resistance, can contribute to increase the knowledge about fabrics produced with R-PET fibres. R-PET fabrics can contribute to environmental benefits, considering the material demanding performance necessary for seat cover application and the high number of vehicle production.

Experimental

Fibres were supplied by different European manufacturers. Three PET fibres were used: V-PET fibres, post-consumer recycled PET fibres (R-PET_PC) and a blend of post-consumer and post-industrial PET (R-PET_PC/PI) fibres. The latter was made by 50% PET_PC and 50% PET_PI with a small percentage of polyethylene (PE), coming from the reprocessing of PET/PE food trays and industrial waste. R-PET_PI products were less subjected to waste contamination. V-PET fibres and R-PET_PC fibres were mass dyed, while R-PET_PC/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 on 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.

		Yarn type
Fabrics properties		V-PET	R-PET PC	R-PET PC/PI
Knitting type		Circular
Yarn count	$\overline {\rm x}$ (dtex)	385	330	387
	CV	0.12	0.11	0.13
Wale yarn/cm	$\overline {\rm x}$	9.0	9.0	9.0
	CV	0.1	0.1	0.1
Course yarn/cm	$\overline {\rm x}$	14.0	14.0	14.0
	CV	0.07	0.07	0.07
Total weight	$\overline {\rm x}$ (g/cm2)	403	440	428
	CV	0.01	0.02	0.01
Fabric weight	$\overline {\rm x}$ (g/cm2)	189	220	216
	CV	0.01	0.01	0.01
Fabric total thickness	$\overline {\rm x}$ (mm)	4.00	4.00	4.10
	CV	0.02	0.02	0.02
Fabric thickness	$\overline {\rm x}$ (mm)	0.5	0.5	0.6
	CV	0.2	0.2	0.2
Weave type		Flat woven
Yarn count	$\overline {\rm x}$ (dtex)	385	330	387
	CV	0.12	0.11	0.13
Warp yarn/cm	$\overline {\rm x}$	34.0	34.0	34.0
	CV	0.02	0.02	0.02
Weft yarn/cm	$\overline {\rm x}$	19.0	19.0	19.0
	CV	0.05	0.05	0.05
Total weight	$\overline {\rm x}$ (g/cm2)	439	436	442
	CV	0.01	0.01	0.01
Fabric weight	$\overline {\rm x}$ (g/cm2)	228	228	229
	CV	0.01	0.01	0.01
Fabric total thickness	$\overline {\rm x}$ (mm)	4.00	4.10	3.90
	CV	0.01	0.02	0.02
Fabric thickness	$\overline {\rm x}$ (mm)	0.5	0.6	0.5
	CV	0.4	0.2	0.2

Note: The mean value ( $\overline {\rm x}$ ) and the coefficient of variation (CV) are reported for each property.

All types of fabrics were subjected to further processes: dry cleaning with C₂Cl₄ and thermo-fixing 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.

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 wheel 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.

Figure 2.

Scheme of the structural wearing test.

Aesthetic wear resistance of all fabric type 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.

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. Non-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 h 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 shown 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.

Figure 4.

Fibres melting peaks in the first heating cycle: (a) V-PET fibre (b) R-PET_PC fibre and (c) R-PET_PC/PI fibre.

Table 3.

DSC analysis results of the three kinds of fibres, first heating.

	a)V-PET	b) R-PETPC	c) R-PETPC/PI
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

V-PET: virgin-polyethylene terephthalate; R-PET: recycled-polyethylene terephthalate.

Tensile strength test

Tensile strength results are reported in Figures 5 and 6 and Table 4. As shown 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 cover 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 shown in Table 4. During this test, for each direction, all yarns are stressed together and they contribute to strength resistance. As regarding knitted fabric (Figure 6), the number of yarns cannot be correlated to tensile strength values, because of its particular structure. Initially, it stretches, and then the loops become knots from which the test starts. In this case, the whole structure is stressed. Knitted fabric owns a flexible and less resistant structure, but much 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.

Figure 5.

Tensile strength results of flat woven fabrics.

Figure 6.

Tensile strength results of knitted fabrics.

Table 4.

Tensile and tear strength results.

Woven fabrics	Tensile strength (daN)				Tear strength (daN)
	Warp direction		Weft direction		Warp direction		Weft direction
	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD
V-PET	386	11	258	10	27.4	4.1	21.4	1.6
R-PETPC	408	9	176	7	30.8	2.6	17.7	2.2
R-PETPC/PI	287	10	151	7	21.1	1.5	15.3	3.2
	Tensile strength (daN)				Tear strength (daN)
	Wale direction		Course direction		Wale direction		Course direction
Knitted fabrics	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD	$\overline {\rm x}$	SD
V-PET	155	5	148	3	22.5	1.6	18.3	1.3
R-PETPC	148	3	140	6	18.2	1.2	16.5	1.1
R-PETPC/PI	132	3	124	8	17.2	2.1	15.3	0.8

Note: The mean value ( $\overline {\rm x}$ ) and standard deviation (SD) are reported for each type of fabric.

V-PET: virgin-polyethylene terephthalate; R-PET: recycled-polyethylene terephthalate.

Considering fibres composition, R-PET_PC/PI woven fabrics have lower tensile strength values, in both warp and weft directions, compared to V-PET and R-PET_PC fabrics (Figure 5). This behaviour is a consequence of the mechanical degradation of PET during the recycling process. The high-quality composition of R-PET_PC 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.

Tear strength test

Figures 7 and 8 and Table 4 show tear strength results. They are much 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 greater number of yarns/cm affects the results for the warp direction samples. Considering fibre composition, R-PET_PC/PI fabric shows a little lower performance compared to V-PET and R-PET_PC 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 less percentage of crystallinity (Table 3). In Figure 8, R-PET_PC warp direction results are comparable to V-PET, considering the standard deviation (Table 4). Weft direction sample is composed of fewer weft yarns/cm and its recycled origin is highlighted. During tear strength test, yarns are individually stressed and their characteristics affect the results.

Figure 7.

Tear strength values of virgin and recycled PET fabrics with knitted structure.

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 as 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 is evaluated as grade 3. This result is highly positive according to the automotive standard, for which there must be no yarns breakage. Structural wear resistance test does not break any R-PET_PC 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-PET_PC/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 V-PET 600 cycles	3
Flat woven R-PETPC/PI 600 cycles	2.5
Flat woven R-PETPC 600 cycles	3

V-PET: virgin-polyethylene terephthalate; R-PET: recycled-polyethylene terephthalate.

Afterwards, fabrics surface was examined after aesthetic wear resistance test. Values of grey scale, registered with the spectrophotometer, are represented in Table 6. Non-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-PETPC/PI 6000 cycles	3.5
Flat woven R-PETPC 3000 cycles	4.5
Flat woven R-PETPC 3000 cycles	4

V-PET: virgin-polyethylene terephthalate; R-PET: recycled-polyethylene terephthalate.

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-PET_PC/PI woven fabric surface (Figure 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 related to a surface heating during successive rubbings of the analysed fabric against the standard one. Knitted fabric surface (Figure 12) shows a slightly better result compared to the 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-PET_PC/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-PET_PC) 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.

Figure 9.

Pictures under the profilometer of V-PET woven fabrics surfaces before wear resistance test and after 3000 and 6000 cycles of wearing (from left to right).

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-PET_PC/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-PET_PC/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-PET_PC 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-PET_PC 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 R-PET fibres were evaluated. For specific automotive requirements, they were compared to current production V-PET fabrics. Tensile strength, tear strength and structural and aesthetic wear resistance tests were performed on virgin and R-PET fabrics.

All fabrics possess good tensile and tear strength properties, far above automotive requirements.

Despite good mechanical results, R-PET_PC/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-PET_PC fabrics have got high wear resistance and excellent mechanical performance. R-PET_PC fabric is the most suitable R-PET fabric for automotive seat cover application, 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.

R-PET fabrics are environmentally friendly. R-PET fabrics production can contribute to benefits as reducing gases emissions and refuse to landfill, than petrochemical resources exploiting for virgin materials manufacturing.