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=1. Introduction=

One of the main problems with the thermoplastic consumption is the amount used in a day-to-day basis as well the way of disposal of the single-use plastic products (this includes for example plastic bottles, plastic bags and containers). Even though polyethylene terephthalate (PET) bottles represent a lower carbon footprint in terms of production and transportation, they still represent one of the biggest problems in landfill waste [1]. With the effects of the incorrect disposal of pet bottles in all ecosystems the search for alternative uses of these materials and the application of different types of PET recycling is of high priority [2]. This study focuses on testing a solution for one of the problems with PET waste of water and beverage bottles, in new engineering materials.

A lot of studies have been conducted to understand the environmental performance of some polymer alternatives to PET packaging [3]. Although the best solution is to close the loop in the usage of these materials and working on the expansion of the life cycle of PET, the recycling of this polymer can be chemical or mechanical, the main difference of the two types of recycling is the complexity of the process and the resulting material and amount of waste product [4]. Some studies have been successful in proving the possibly of maintaining the quality criteria and mechanical properties in the recycled material [5].

Knowing the possibility of recycling thermoplastic materials, and the fact that a large percentage of it still ends as landfill waste, brings the goal of transforming then into new materials [6]. That will be a new material with a lower carbon footprint.

Thermoplastic prepregs production and their consolidation processes are still a developing technology. One of the most efficient methods of processing is the pultrusion of constant cross section bars [7], being pultrusion of constant cross section bars one of the most efficient methods of processing. One of the improvements of this processing method is the application of pre-impregnated materials such as towpregs and pre-impregnated tapes that improve the consolidation quality of the resulting bars [8]. Pultruded thermoplastic composites have several advantages over thermosetting composites, ranging from lower risks for operators to improved quality. The higher impact toughness, the possibility of recycling and the reduction of the environmental impact are the most relevant advantages [9].

In this work the main objective is to create new combinations of materials were the polymers used are directly recycled and processed into their new form. The mechanical characterization of the materials is going to allow the comparison between the materials made from virgin material and understand the viability for this kind of recycling.

=2. Experimental work=

==2.1. Selection of Raw materials==

==2.1.1. Fibres==

Some aspects about the selection of fibres were studied, such as the linear density and sizing options available in the market, with the E-type glass fibres supplied by Owens Corning, reference 305E-TYPE 30 Direct Roving being selected. These fibres do not undergo any kind of sizing treatment and had a linear density of 2400 Tex. The carbon fibres used were supplied by SGL Group, with the reference C30 T050 TP1 having a sizing treatment for processing thermoplastic polymers, which is more adequate for the work here developed. The properties of this fibres are listed in <span id='cite-_Ref179059049'></span>[[#_Ref179059049|Table 1]].

<div id="_Ref179059049" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 1 Fibres properties''</div>

{| style="width: 72%;margin: 1em auto 0.1em auto;border-collapse: collapse;"
|-
| style="border-top: 1pt solid black;text-align: center;"|
| style="border-top: 1pt solid black;text-align: center;"|
| style="border-top: 1pt solid black;text-align: center;"|Glass Fibres
| style="border-top: 1pt solid black;text-align: center;vertical-align: top;"|Carbon Fibres
|-
| style="border-top: 1pt solid black;text-align: center;"|Density ''ρ''
| style="border-top: 1pt solid black;text-align: center;"|g/cm<sup>3</sup>
| style="border-top: 1pt solid black;text-align: center;"|2.56
| style="border-top: 1pt solid black;text-align: center;vertical-align: top;"|1.80
|-
| style="text-align: center;"|Linear density
| style="text-align: center;"|g/m
| style="text-align: center;"|2.40
| style="text-align: center;vertical-align: top;"|3.28
|-
| style="text-align: center;"|Elasticity modulus E
| style="text-align: center;"|GPa
| style="text-align: center;"|70
| style="text-align: center;vertical-align: top;"|240
|-
| style="text-align: center;"|Tensile strength
| style="text-align: center;"|MPa
| style="text-align: center;"|1800
| style="text-align: center;vertical-align: top;"|4000
|-
| style="text-align: center;"|Filament Diameter
| style="text-align: center;"|µm
| style="text-align: center;"|14
| style="text-align: center;vertical-align: top;"|7
|-
| style="border-bottom: 1pt solid black;text-align: center;"|Poisson ratio ''ν''
| style="border-bottom: 1pt solid black;text-align: center;"|-
| style="border-bottom: 1pt solid black;text-align: center;"|0.26
| style="border-bottom: 1pt solid black;text-align: center;vertical-align: top;"|0.27
|}

==2.1.2. Thermoplastic matrix==

The polyethylene terephthalate (PET) polymer used in the development of this work, as a comparative way to the new material under study, was supplied by Billion Industrial Co Lt., with reference 31W56/W and the relevant properties of the thermoplastic can be consulted in <span id='cite-_Ref165370754'></span>[[#_Ref165370754|Table 2]].

<div id="_Ref165370754" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 2 PET properties''</div>

{| style="width: 54%;margin: 1em auto 0.1em auto;border-collapse: collapse;"
|-
| style="border-top: 1pt solid black;text-align: center;"|
| style="border-top: 1pt solid black;text-align: center;"|
| style="border-top: 1pt solid black;text-align: center;"|PET
|-
| style="border-top: 1pt solid black;text-align: center;"|Density ''ρ''
| style="border-top: 1pt solid black;text-align: center;"|g/cm<sup>3</sup>
| style="border-top: 1pt solid black;text-align: center;"|1.25
|-
| style="text-align: center;"|Tensile strength ''τ''<sub>u</sub>
| style="text-align: center;"|MPa
| style="text-align: center;"|40
|-
| style="text-align: center;"|Elasticity modulus E
| style="text-align: center;"|GPa
| style="text-align: center;"|2.0
|-
| style="text-align: center;"|Melting temperature T<sub>m</sub>
| style="text-align: center;"|°C
| style="text-align: center;"|249
|-
| style="text-align: center;"|Elongation at break ''ε''
| style="text-align: center;"|%
| style="text-align: center;"|20
|-
| style="text-align: center;"|Heat Deflection Temp. HDT
| style="text-align: center;"|°C
| style="text-align: center;"|115
|-
| style="border-bottom: 1pt solid black;text-align: center;"|Poisson ratio ''ν''
| style="border-bottom: 1pt solid black;text-align: center;"|-
| style="border-bottom: 1pt solid black;text-align: center;"|0.43
|}

This research used two types of recycled pet, one obtained in granules from DUY TAN PLASTIC RECYCLING CO. LTD and the second obtained by mechanically recycling water and soft drink bottles.

The first step to the method was the collecting of beverage bottles and the sorting and separation of every component that was not composed of PET. The selected material was then shredded into a desired size to facilitate its use. The next step was the disinfection and cleaning of the material, this process was done with a diluted solution of 1 part of acetone for 50 parts of water so it would not degrade the polymer. With the use of water, it was necessary to implement a drying process to guarantee the purity of the polymer granules. This process is represented in <span id='cite-_Ref179052344'></span>[[#_Ref179052344|Figure 1]].

<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
[[Image:Draft_Carreiras_337618413-image1.png|324px]] </div>

<div id="_Ref179052344" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
Figure 1 Preparation of the recycled PET</div>

There are still some gaps in this process, as the various types of PET collected sometimes have different additives, making it difficult to mix them. However, as the processes carried out bring the polymer to a temperature above its melting point, this problem has not been a limitation.

==2.3. Pre-impregnated tapes==

For this work pre-impregnated tapes were produced combining PET polymer with carbon fibres. The production of the tapes was carried out in the equipment schematically represented in <span id='cite-_Ref164066955'></span>[[#_Ref164066955|Figure 2]].

This pre-impregnated material is produced by passing continuous dry fibres through a polymer bath. Once the fibres are impregnated, they pass through a die where the geometry is defined. At the exit, the tape cools and it is possible to move on to other forming processes [10].

<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
[[Image:Draft_Carreiras_337618413-image2.jpeg|390px]] </div>

<div id="_Ref164066955" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Figure 2 - Thermoplastic Tape production line [10]''</div>

To produce the tapes for this study, it was used equipment that can heat the polymer up to 300ºC, with the pulling speed for these materials being set at 3 meters per minute. The geometry of the produced tapes was with 10 mm of width and 0.2 mm of thickness.

==2.4. Consolidation of the material by pultrusion==

The consolidation method by pultrusion is done according to the method and the steps described in <span id='cite-_Ref183421503'></span>[[#_Ref183421503|Figure 3]].

<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
[[Image:Draft_Carreiras_337618413-image3.png|522px]] </div>

<div id="_Ref183421503" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Figure 3 Pultrusion system [10]''</div>

The processing parameters used in the processing of pultrusion profiles were the ones described in <span id='cite-_Ref183421578'></span>[[#_Ref183421578|Table 3]]. It is also presented the number of roving’s used per condition. This number is calculated through the relation of linear density of the prepreg and the die dimension of the pultrusion system.

<div id="_Ref183421578" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 3 Pultrusion processing parameters''</div>

{| style="width: 100%;margin: 1em auto 0.1em auto;border-collapse: collapse;"
|-
| style="border: 1pt solid black;text-align: center;"|'''Material'''
| style="border: 1pt solid black;text-align: center;"|'''Pulling Speed'''

'''(m/min)'''
| style="border: 1pt solid black;text-align: center;"|'''Pre-heating'''

'''(°C)'''
| style="border: 1pt solid black;text-align: center;"|'''Consolidation'''

'''Die (°C)'''
| style="border: 1pt solid black;text-align: center;"|'''Cooling Die (°C)'''
| style="border: 1pt solid black;text-align: center;"|'''Rovings'''
|-
| style="border: 1pt solid black;text-align: center;"|'''CF/PET'''
| style="border: 1pt solid black;text-align: center;"|0.2
| style="border: 1pt solid black;text-align: center;"|135
| style="border: 1pt solid black;text-align: center;"|275
| rowspan='4' style="border: 1pt solid black;text-align: center;"|25
| style="border: 1pt solid black;text-align: center;"|10
|-
| style="border: 1pt solid black;text-align: center;"|'''CF/rPET'''
| style="border: 1pt solid black;text-align: center;"|0.3
| style="border: 1pt solid black;text-align: center;"|160
| style="border: 1pt solid black;text-align: center;"|200
| style="border: 1pt solid black;text-align: center;"|5
|-
| style="border: 1pt solid black;text-align: center;"|'''CF/rPET (bottles)'''
| style="border: 1pt solid black;text-align: center;"|0.3
| style="border: 1pt solid black;text-align: center;"|160
| style="border: 1pt solid black;text-align: center;"|200
| style="border: 1pt solid black;text-align: center;"|5
|-
| style="border: 1pt solid black;text-align: center;"|'''GF/rPET (bottles)'''
| style="border: 1pt solid black;text-align: center;"|0.3
| style="border: 1pt solid black;text-align: center;"|160
| style="border: 1pt solid black;text-align: center;"|220
| style="border: 1pt solid black;text-align: center;"|7
|}

==2.5. Mechanical testing==

Mechanical testing was carried out in a Shimadzu AG-X equipment with a load cell of 100 kN for the tensile tests and a Shimadzu AG-I whit a load cell of 10 kN for the flexural tests. The flexural properties of the profiles obtained were determined under ISO 14125. This method was used to investigate the flexural behaviour of the profiles and determine the flexural strength and flexural modulus. The span was 80 mm, and the test speed was 2 mm/min.

The experimental results were compared with theoretical ones that can be predicted by using the ROM (Rule of Mixtures). Considering the void volume fraction to be equal to zero, the elastic modulus in the direction of fibres, 𝐸1, can be estimated by Equation 1.

{| class="formulaSCP" style="width: 100%;border-collapse: collapse;width: 100%;text-align: center;"
|-
|
{| style="text-align: center;margin:auto;width: 100%;"
|-
| <math>{E}_{1}=\eta \ast {E}_{f}\ast {v}_{f}+{E}_{p}(1-{v}_{f})</math>
|}
| style="width: 5px;text-align: right;white-space: nowrap;"|<span id='_Ref137843362'></span>(1)
|}

where 𝐸𝑓 and 𝐸𝑝 are the elastic modulus of fibres and polymer, respectively, and 𝑣𝑓 is the fibre volume fraction of composite.

For these composites, a crosshead speed of 2 millimetres per minute is employed during the test and the stress interval for determining the material’s elastic modulus is derived from a strain ranging between 0.0005 and 0.0025. The placement of the specimen to proceed with the three-point test is shown at <span id='cite-_Ref165403451'></span>[[#_Ref165403451|Figure 4]].

<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
[[Image:Draft_Carreiras_337618413-image4.png|426px]] </div>

<div id="_Ref165403451" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Figure 4 Three-point bend test''</div>

Tensile test was carried out at a speed of 2 mm/min according to ISO 527, and distance between grippers was 150 mm. The tensile tests were done using an extensometer so the results of the Young´s Modulus could be more precise. The distance between the grippers of the extensometer was 50 mm.

==3. Results==

==3.1. Production of the pre-impregnated tapes==

The processing of the pre-impregnated tapes proved to be successful, both in the use of the raw material virgin and in the use of the recycled polymer. When using the polymer, it was essential to know its processing window, so it was essential to study the optimum flow temperature, and the degradation temperature studied.

The resulting tapes had the desired impregnation quality and a balanced fibre and polymer content, allowing them to be processed and the quantities of prepregs to be calculated to produce the pultrusion profiles.

==3.2. Pultrusion of the pre-impregnated tapes==

The pultrusion process showed some difficulties because of the amount of polymer present in the pre-impregnated tapes. For that reason, the processing temperatures had to be adjusted so the resulting material could show a good superficial quality.

There are some improvements that need to be made in future works on the quantity of material in relation with the processing speed.

After the production of the material the specimens were prepared, and the calcination tests were done. That way the theoretical properties could be calculated by the Rule of Mixtures and the values can be seen in <span id='cite-_Ref183683484'></span>[[#_Ref183683484|Table 4]].

<div id="_Ref183683484" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 4 Theoretical properties by the Rule of Mixtures''</div>

{| style="width: 100%;border-collapse: collapse;"
|-
| style="border: 1pt solid black;text-align: center;"|Composite
| style="border: 1pt solid black;text-align: center;"|Theoretical Young´s Modulus

(GPa)
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|Theoretical Strength (MPa)
|-
| style="border: 1pt solid black;text-align: center;"|CF/PET
| style="border: 1pt solid black;text-align: center;"|104.3
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|860
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET
| style="border: 1pt solid black;text-align: center;"|126.5
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|1046
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET(bottles)
| style="border: 1pt solid black;text-align: center;"|88.5
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|727
|-
| style="border: 1pt solid black;text-align: center;"|GF/rPET(bottles)
| style="border: 1pt solid black;text-align: center;"|20.9
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|254
|}

==3.3. Mechanical testing results==

The mechanical testing was made, and the results are the ones presented. The tensile tests results are presented in <span id='cite-_Ref183358889'></span>[[#_Ref183358889|Table 5]]. Is also presented the Relative Elasticity modulus so it is easier to compare the properties of each material in relation with each fibre volume fraction.

<div id="_Ref183358889" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 5 Tensile tests results''</div>

{| style="width: 100%;border-collapse: collapse;"
|-
| style="border: 1pt solid black;text-align: center;"|Composite
| style="border: 1pt solid black;text-align: center;"|Young’s Modulus (GPa)
| style="border: 1pt solid black;text-align: center;"|Tensile Strength (MPa)
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|Relative Young´s Modulus
|-
| style="border: 1pt solid black;text-align: center;"|CF/PET
| style="border: 1pt solid black;text-align: center;"|76.7 ± 6.7
| style="border: 1pt solid black;text-align: center;"|499.4 ± 91.1
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|178.4
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET
| style="border: 1pt solid black;text-align: center;"|50.3 ± 5.5
| style="border: 1pt solid black;text-align: center;"|> 468.7 ± 100.3
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|96.1
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET (bottles)
| style="border: 1pt solid black;text-align: center;"|49.1 ± 5.4
| style="border: 1pt solid black;text-align: center;"|441.1 ± 20.0
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|136.4
|-
| style="border: 1pt solid black;text-align: center;"|GF/rPET (bottles)
| style="border: 1pt solid black;text-align: center;"|14.7 ± 2.4
| style="border: 1pt solid black;text-align: center;"|129.5 ± 10.1
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|52.1
|}

The flexural testing results are presented in <span id='cite-_Ref183722672'></span>[[#_Ref183722672|Table 6]] and as was done in the tensile characterization it was also calculated the relative flexural modulus.

<div id="_Ref183722672" class="center" style="width: auto; margin-left: auto; margin-right: auto;">
''Table 6 Flexural tests results''</div>

{| style="width: 100%;border-collapse: collapse;"
|-
| style="border: 1pt solid black;text-align: center;"|Composite
| style="border: 1pt solid black;text-align: center;"|Flexural Modulus (GPa)
| style="border: 1pt solid black;text-align: center;"|Flexural Strength (MPa)
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|Relative Flexural Modulus
|-
| style="border: 1pt solid black;text-align: center;"|CF/PET
| style="border: 1pt solid black;text-align: center;"|31.5 ± 4.5
| style="border: 1pt solid black;text-align: center;"|443.5 ± 59.3
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|137.9
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET
| style="border: 1pt solid black;text-align: center;"|76.2 ± 9.9
| style="border: 1pt solid black;text-align: center;"|468.7 ± 100.3
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|145.6
|-
| style="border: 1pt solid black;text-align: center;"|CF/rPET (bottles)
| style="border: 1pt solid black;text-align: center;"|34.2 ± 4.3
| style="border: 1pt solid black;text-align: center;"|384.6 ± 37.2
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|95
|-
| style="border: 1pt solid black;text-align: center;"|GF/rPET (bottles)
| style="border: 1pt solid black;text-align: center;"|4.9 ± 1.4
| style="border: 1pt solid black;text-align: center;"|25.4 ± 8.7
| style="border: 1pt solid black;text-align: center;vertical-align: top;"|17.4
|}

The results presented above show the significant difference between the composite material reinforced with carbon fibres when compared to the ones reinforced with glass fibres. The resulting material also showed better superficial quality and processing facility in the ones with carbon fibres then the ones with glass fibres.

Further work must be done to improve the process when using glass fibres as reinforcement. The flexural modulus of this material shows some difference from the ones predicted, this problem can be due to the poor consolidation of the specimen, being more evident when tested by applying a flexural stress.

On the other hand, the recycling process of the PET waste proved to be an excellent and easier method to reintroduce this material on the chain of materials and to improve the ecological footprint of the created material.

==4. Conclusions==

This study corroborated the theoretical predictions made when using recycled polymers. Despite using a more direct recycling process than that normally applied in industrial recycling, the material produced had properties close to those predicted by theory.

The production methods showed viability when working with recycled polymers, without a big change of parameters and processing capacity. The application of this direct polymer recycling process is therefore feasible in the production of composite materials. This study should be continued by applying the same methods, changing the polymers used and improving the process by using different continuous fibres.

In future work more thermal testing should be done in the polymers being recycled to understand how many temperatures cycles the polymer can stand without losing mechanical and chemical properties. This work should also be applied in more combinations of materials including more natural fibres and with different sizes of fibres to spread the applications of these materials obtained from more ecofriendly sources.

==Acknowledgments==

The authors of this work would like to thank M4S- Materials for sustainability research centre for all the support during the experimental component of this work.

=5. Bibliography=

[1] P. Benyathiar, P. Kumar, G. Carpenter, J. Brace, and D. K. Mishra, “Polyethylene Terephthalate (PET) Bottle‐to‐Bottle Recycling for the Beverage Industry: A Review,” Jun. 01, 2022, ''MDPI''. doi: 10.3390/polym14122366.

[2] P. Bałdowska-Witos, I. Piasecka, J. Flizikowski, A. Tomporowski, A. Idzikowski, and M. Zawada, “Life cycle assessment of two alternative plastics for bottle production,” ''Materials'', vol. 14, no. 16, Aug. 2021, doi: 10.3390/ma14164552.

[3] S. Papong ''et al.'', “Comparative assessment of the environmental profile of PLA and PET drinking water bottles from a life cycle perspective,” ''J Clean Prod'', vol. 65, pp. 539–550, Feb. 2014, doi: 10.1016/j.jclepro.2013.09.030.

[4] T. Chilton, S. Burnley, and S. Nesaratnam, “A life cycle assessment of the closed-loop recycling and thermal recovery of post-consumer PET,” ''Resour Conserv Recycl'', vol. 54, no. 12, pp. 1241–1249, 2010, doi: 10.1016/j.resconrec.2010.04.002.

[5] E. Pinter ''et al.'', “Circularity study on pet bottle-to-bottle recycling,” ''Sustainability (Switzerland)'', vol. 13, no. 13, Jul. 2021, doi: 10.3390/su13137370.

[6] M. Asensio ''et al.'', “Processing of pre-impregnated thermoplastic towpreg reinforced by continuous glass fibre and recycled PET by pultrusion,” ''Compos B Eng'', vol. 200, Nov. 2020, doi: 10.1016/j.compositesb.2020.108365.

[7] F. Tucci, F. Rubino, G. Pasquino, and P. Carlone, “Thermoplastic Pultrusion Process of Polypropylene/Glass Tapes,” ''Polymers (Basel)'', vol. 15, no. 10, May 2023, doi: 10.3390/polym15102374.

[8] P. J. Novo, J. F. Silva, J. P. Nunes, F. W. J. Van Hattum, and A. T. Marques, “ECCM15-15 TH EUROPEAN CONFERENCE DEVELOPMENT OF A NEW PULTRUSION EQUIPMENT TO MANUFACTURE THERMOPLASTIC MATRIX COMPOSITE PROFILES.”

[9] K. Minchenkov, S. Gusev, A. Rogozheva, A. Tronin, M. Diatlova, and A. Safonov, “Pultrusion of thermoplastic composites with mechanical properties comparable to industrial thermoset profiles,” ''Composites Communications'', vol. 44, Dec. 2023, doi: 10.1016/j.coco.2023.101766.

[10] P. Esfandiari, J. F. Silva, P. J. Novo, J. P. Nunes, and A. T. Marques, “Production and processing of pre-impregnated thermoplastic tapes by pultrusion and compression moulding,” ''J Compos Mater'', vol. 56, no. 11, pp. 1667–1676, Jun. 2022, doi: 10.1177/00219983221083841.

 

Carreiras et al 2025a - Revision history

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