Instituto Tecnol´ogico y de Estudios Superiores de Monterrey
Monterrey Campus
School of Engineering and Sciences
Evaluation of the Reprocessability of Polypropylene by the Implementation of Ultrasonic Micro Injection Molding
A Thesis presented by
Rafael Gaxiola Cockburn
Submitted to the
School of Engineering and Sciences
in partial fulfillment of the requirements for the degree of Master of Science
in
Manufacturing Systems
Monterrey, Nuevo Le´on, June, 2020
Instituto Tecnol´ogico y de Estudios Superiores de Monterrey Campus Monterrey
The committee members, hereby, recommend that the proposal presented by Rafael Gaxiola Cockburn to be accepted to develop the thesis project as a partial requirement for the degree of Master of Science in Manufacturing Systems.
Dr. Oscar Mart´ınez Romero Tecnol´ogico de Monterrey Principal Advisor
Dr. Alex El´ıas Z´u˜niga Tecnol´ogico de Monterrey Co-Advisor
Dr. Daniel Olvera Trejo Tecnol´ogico de Monterrey Committee Member
MSc. Cintya Soria Hern´andez Tecnol´ogico de Monterrey Committee Member
Dr. Rub´en Morales Men´endez Dean of Graduate Studies School of Engineering and Sciences Monterrey, Nuevo Le´on, June, 2020
Declaration of Authorship
I, Rafael Gaxiola Cockburn, declare that this dissertation titled, Evaluation of the Reprocessability of Polypropylene by the Implementation of Ultrasonic Micro In- jection Molding, and the work presented in it are of my own. I confirm that:
• This work was done wholly while in candidature for a research degree at this University.
• Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated.
• Where I have consulted the published work of others, this is always clearly attributed.
• Where I have quoted from the work of others, the source is always given.
With the exception of such quotations, this thesis is entirely my own work.
• I have acknowledged all main sources of help.
• Where the dissertation is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself.
Rafael Gaxiola Cockburn Monterrey, Nuevo Le´on. June 16th, 2020.
@ 2020 by Rafael Gaxiola Cockburn.
All rights reserved
Dedication
To my beloved Griselda,
for helping me to overcome the obstacles in the pursuit of knowledge.
To my mother Beatriz, my father Rafael,
and my brother Juan Pablo,
for sending me their unconditional support despite the distance.
”If you can make it through the night, there is a brighter day”.
Acknowledgements
I would like to acknowledge my advisor, Dr. Oscar Mart´ınez, for giving me all of his trust and be open to my proposals along this work. His patience and advices helped me to keep the direction of this research.
I would like also to express my gratitude to Dr. Alex El´ıas, leader of the Nanotech- nology for Devices Design research group, for his wise advices and for trusting in my thesis proposal since the beginning of the Master’s program. He inspired me to go beyond the common and put an extra effort to make my work disruptive.
My sincere thanks also to MSc. Cintya Soria for all of the technical support during the experimental stage in the laboratories and her supervision during the writing stage of the literature review.
I would like to acknowledge my lab mates from the MNT program and my col- leagues from the MSM program, for all of the mutual support and the long collab- orative hours. It was a pleasure to coincide and watch ourselves grow profession- ally.
I am thankful to the people who helped me with their personal experiences and recommendation letters when I was applying for my Master’s scholarship: Dr.
Jos´e Manuel Nieto, Dr. Adri´an Rend´on, Dr. Cecilia Ram´ırez, MSc. Guillermo Kitazawa and MSc. Joaqu´ın Acosta.
Finally, all of these would have never been possible without the sponsorship granted by Tecnol´ogico de Monterrey through the Research Group of Nanotechnology for Devices Design, and by Consejo Nacional de Ciencia y Tecnolog´ıa de M´exico (Conacyt), Project Numbers 242269, 255837, 296176, National Lab in Additive Manufacturing, 3D Digitizing and Computed Tomography (MADiT) LN299129 and FODECYT-296176. These institutions believed in my potential as a profes- sional and researcher.
Evaluation of the Reprocessability of Polypropylene by the Implementation of
Ultrasonic Micro Injection Molding
By
Rafael Gaxiola Cockburn Abstract
Polypropylene (PP) is one of the most consumed commodity thermoplastics world- wide, thereby, it is critical to propose new alternatives for the recycling of its post- industrial and post-consumer waste streams. This research evaluated methodically the use of the novel Ultrasonic Micro Injection Molding (UMIM) technology, to identify the changes in morphological, mechanical, thermal and rheological properties, after the reprocessing of pure regrind material for several consecutive cycles. Proper process parameters were obtained by a Design of Experiments, achieving a reduction of micro defects, in addition to thermal stability and an en- hancement of the mechanical properties of recycled PP (increase of 36% Young’s modulus, 20% yield stress, 13% ultimate stress, 26% strain, 48% toughness). The tests showed that PP was able to withstand up to five reprocessing cycles until presenting the first signs of mechanical performance downgrading. A better un- derstanding of the mechanochemical effects and degradation is provided by Differ- ential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Fourier Transform Infrared (FTIR) spectra and dynamic rheology. The results of this work set UMIM in a more mature stage for its incorporation to the industry, while con- tributing to the circular economy practice.
Contents
1 Introduction 2
1.1 Introduction . . . 2
1.2 Problem Definition and Motivation . . . 5
1.3 State of the Art . . . 6
1.4 Objectives . . . 10
1.5 Hypothesis . . . 12
1.6 Organization of document . . . 12
2 Theoretical Framework 13 2.1 Fundamentals of Polymers . . . 13
2.2 Molecular Weight . . . 13
2.2.1 Number average molecular weight (Mn) . . . 13
2.2.2 Weight average molecular weight (Mw) . . . 14
2.2.3 Higher average molecular weight (Mz) . . . 14
2.3 Polypropylene . . . 15
2.4 Conventional processing techniques for thermoplastics . . . 16
2.4.1 Extrusion . . . 16
2.4.2 Molding . . . 17
2.5 Ultrasonic Micro Injection Molding (UMIM) . . . 19
2.6 Material reprocessability . . . 21
2.6.1 Classification . . . 21
2.6.2 Mechanical recycling . . . 22
2.6.3 Direct grinding . . . 23
2.6.4 Indirect grinding . . . 24
2.6.5 Material degradation . . . 25
2.6.6 Polymer stabilizers . . . 27
2.7 Characterization . . . 28
2.8 Analysis of Variance (ANOVA) . . . 29
3 Materials and Methods 33 3.1 Material and specimen geometry . . . 33
3.2 Ultrasonic Micro Injection Molding machine . . . 33
3.3 Experimental method . . . 35
3.4 Characterization equipment . . . 39
3.4.1 Scanning Electron Microscopy (SEM) . . . 39
3.4.2 Tensile Testing . . . 39
3.4.3 Thermogravimetric Analysis (TGA) . . . 39
3.4.4 Fourier Transformation Infrared Spectroscopy (FTIR) . . 40
3.4.5 Differential Scanning Calorimetry (DSC) . . . 40
3.4.6 Oscillatory rheology . . . 40
4 Results and Discussion 43 4.1 Design of Experiments (DOE) . . . 43
4.2 Machine Performance . . . 50
4.3 Microscopy . . . 52
4.4 Mechanical properties . . . 54
4.5 Thermogravimetric Analysis (TGA) . . . 58
4.6 Fourier Transform Infrarred (FTIR) . . . 60
4.7 Differential Scanning Calorimetry (DSC) . . . 61
4.8 Rheological properties . . . 63
4.9 Molecular Entanglements . . . 67
4.10 Discussion . . . 71
5 Conclusions 77 5.1 Conclusion . . . 77
5.2 Contributions . . . 78
5.3 Future work . . . 78
A Injection Molding Pressure Drop Calculation. 86
B Injection Molding Pressure Drop Simulation. 88
C Maxwell elements parameters for Wagner Constitutive model. 89
List of Figures
1 Plastic waste generation by sector. . . 3
2 Ultrasonic Micro Injection Molding (UMIM) scheme. . . 4
3 Representation of the polypropylene semicrystalline structure. . . 16
4 Conventional Injection molding process and heating mechanism. . 18
5 UMIM process sequence. . . 19
6 UMIM heating mechanisms. . . 20
7 Grinding machine elements. . . 23
8 Grinder designs for direct grinding of plastics. . . 24
9 Grinder designs for indirect grinding of plastics. . . 25
10 Chain scissions in polymer matrices . . . 26
11 Degradation mechanism. . . 28
12 Scaled ASTM D638 tensile specimen. . . 33
13 Sonorus 1G UMIM machine. . . 34
14 Plunger position and injection velocity profile. . . 36
15 Material preparation process. . . 37
16 Experimental methodology flowchart and response variables. . . . 38
17 Complete specimen before ejection. . . 43
18 Anderson-Darling test . . . 46
19 Pareto plots for response variables . . . 49
20 Main Effects plot for response variables. . . 50
21 Process trends . . . 51
22 SEM micrographs defects classification. . . 53
23 SEM micrographs morphologies comparison. . . 54
24 Stress-strain curves for virgin and recycled PP. . . 55
25 Continuous reprocessing specimens. . . 56
26 Stress-strain curves for continuous reprocessing. . . 57
27 Thermal degradation of PP by sections. . . 58
28 Thermal degradation of PP for continuous reprocessing. . . 59
29 FTIR spectra for continuous reprocessing. . . 61
30 DSC melting peaks for continuous reprocessing. . . 62
31 DSC crystallization peaks for continuous reprocessing. . . 63
32 Shear viscosity for continuous reprocessing. . . 64
33 Crossover of Storage and Loss moduli. . . 65
34 Storage and Loss moduli for continuous reprocessing. . . 66
35 Phase shift angle (δ) and vector diagram. . . 67
36 Entanglement density. . . 68
37 Plateau moduli representation for continuous reprocessing. . . 70
38 Non-Newtonian shear viscosity fitting. . . 86
39 Isothermal flow through profiles. . . 87
40 Final Pressure Simulation. . . 88
List of Tables 1 State of the Art for UMIM. . . 9
2 State of the Art for polymers recycling. . . 11
3 Classification of plastics recycling. . . 22
4 Thermoplastics changes due to degradation. . . 25
5 Characterization techniques. . . 30
6 ANOVA results template table. . . 32
7 Sonorus 1G specifications. . . 34
8 Screening design. . . 35
9 Screening combinations. . . 36
10 Results of Screening combinations. . . 43
11 General Full Factorial Design. . . 44
12 DOE response variables results. . . 45
13 ANOVA results. . . 47
14 Mechanical properties for different morphologies. . . 55
15 Mechanical properties for continuous reprocessing. . . 57
16 Thermogravimetric analysis for continuous reprocessing. . . 60
17 DSC for continuous reprocessing. . . 62
18 Molecular weight data obtained with the rheological characterization. 65 19 Molecular entanglements data . . . 70
20 Maxwell elements parameters for Constitutive model . . . 89
The following abbreviations and variables are used in this manuscript:
Acronym Description
UMIM Ultrasonic Micro Injection Molding
IM Injection molding
PP Polypropylene
DSC Differential scanning calorimetry FTIR Fourier transform infrared spectroscopy
TGA Thermogravimetric analysis
XRD X-rays diffraction
GPC Gel permeation cromatography
SEM Scanning electron microscopy
DOE Design of Experiments
ANOVA Analysis of variance
SS Sum of squares
SST Total sum of squares
MS Mean square
DOF Degrees of freedom
PDI Polydispersity index
LVE-R Linear viscoelastic region
Variable Description Units
A Amplitude µm or %
V Injection velocity profile mm/s
MT Mold temperature ºC
F Injection force N
Mn Number average molecular weight g/mol
Mw Weight average molecular weight g/mol
Me Molecular weight between entanglements g/mol
σ Engineering stress Pa
∆H Enthalpy J/g
Xc Crystallinity %
G’ Storage modulus Pa
G” Loss modulus Pa
G∗ Complex modulus Pa
δ Phase shift angle º
G0N Plateau modulus Pa
η Shear viscosity Pa·s
η∗ Complex viscosity Pa·s
η0 Zero-shear viscosity Pa·s
˙γ Shear rate s−1
ω Angular frequency rad·s−1
λ Characteristic time s−1
1 Introduction
1.1 Introduction
The invention of plastics has revolutionized our daily life, having presence in al- most every place we visit or every product we buy. Its beginning dates back to the last half of the 19th century, however, a more accelerated growth has occurred since the 1940s. Statistics indicate that by 2015 the worldwide annual production of plastics reached a value of more than 350 million tonnes annually [1]. From these quantity, 171 million tonnes of wastes come from the industry of transporta- tion, electric/electronics, and packing, as a demand mainly from the automotive, aerospace, medical and packaging containers for food industries, which are big consumers of injection molded parts. Figure 1 illustrates the plastic waster gen- eration by sector. According to the Plastics Industry Association, in 2017, the plastics industry was the 8th largest industry in the United States, and currently is the source of 989 thousand direct jobs. By 2018 there was a growth in the total plastics shipments of 6.9% equivalent to $432 billion sales [2]. Due to an increase in the demand of electrical and electronics, as well as the automotive components because of the Corporate Average Fuel Economy (CAFE) regulations, a growing rate of almost 6% anually is expected in the injection molding industries until 2023 [3]. Thereby, the importance and impact of the injection molding industry itself presents such a magnitude worth to be in constant monitoring.
Several plastics have followed a common path of development for a specific purpose, but then, after finding more applications, they have been the cause of an overproduction and commercialization that implicitly has involved the generation of wastes and pollution to the environment at the industrial and consumer level.
By 2017, only 17 % of global plastics production was being recycled annually, evidencing a huge gap in contrast to metals, that develop a recycling close to 70 % [4]. Such a difference between plastics and metals reprocessability relies on the limitations and challenges that plastics recycling faces, starting from the high cost of sorting-handling and the reduction of properties over the recycling history of a resin. Because of the high production volumes and increasing costs of some resins, saving projects are executed among manufacturers by blending virgin resins with mechanically recycled plastic, typically regranulated. Thus, regrind is the result of grinding or shredding a material that has been processed at least one time before through extrusion or molding. This mechanical process, sometimes
Figure 1: Plastic waste generation by sector. Adapted from [1].
called granulation, milling or shredding, is the most common form of material size reduction. From the plastics processing methods, injection molding is one of the most utilized to produce a great variety of components in many sizes and batch volumes and evidently the use of regrind by this method is a frequent task due to the residues in terms of sprues and runners that are solidified every injection cycle.
Consequently, this opens the possibility of having a close-loop molding cycle if the wastes are regranulated, blended with new resin, and fed back to the machine.
Within the world of high performance materials and composites, new man- ufacturing methods are in constant development or evolution. A new alternative process that implements ultrasonic vibration to melt a material has been devel- oped to produce small sized products: that is, Ultrasonic Micro Injection Molding (UMIM) [5]. This new manufacturing method is an analogous process of the tra- ditional injection molding, with a few relevant differences. Figure 2 represents the basic functioning of UMIM, where, the element called Sonotrode vibrates to ultra- sound frequencies inside a cylindrical chamber, with direct contact to the plastic pellets, transmitting mechanical oscillations until they dissipate into heat. As the material melts, the plunger element moves forward, forcing the molten material to flow into the cavity. According to Michaeli et al. [6] using ultrasound to plasticize
thermoplastics is an alternative to process small quantities of material to mold mi- cro parts, reducing thermal degradation due to less residence time (3 to 6 s) in the plasticizing chamber, and generating much less waste in big runners and sprues as well. In addition, evidence of experiments with ultrasonic assisted extrusion has found that ultrasound helps to improve miscibility of polymeric blends, as well as to deliver a better dispersion of particles accompanied by an increase in viscosity and mechanical strength for the fabrication of nanocomposites [7][8].
Figure 2: Ultrasonic Micro Injection Molding (UMIM) section view scheme. The pelletized material is pushed upwards by the plunger as the sonotrode vibrates. As the material melts, it is injected into the cavity.
After a certain cooling time, the mold is opened and the ejector pins release the specimen.
In UMIM, the heating mechanism of UMIM is based in interfacial friction and volumetric heating [6][9], and it is totally different to that from traditional IM, arising the possibility of being an alternative method to process blends of re- granulated materials, as well as composites and nanocomposites. Sanchez et al.
[10] proved that UMIM is a novel processing technique to produce composites. In their investigation they were able to produce UHMWPE/graphite composites with enhanced mechanical properties and thermal stability. Planellas et al. [5] demon- strated by electron microscopy observations and XRD experimentation that ultra- sound delivers uniform exfoliated structures with high oriented clay nanosheets in the direction of the polymer flow. Additionally, Negre et al. [11] investigated the processing parameters and their outcomes for polypropylene, finding a rela-
tion between injection velocity and the porosity obtained. Grabalosa et al. [12]
developed a deep analysis of the processing conditions of polyamide, pointing the sensitivity of the cavity filling, homogeneity and mechanical properties of the fi- nal part. This processing conditions research sets the outlook of UMIM in a more explored stage of this novel technology.
It is worth remarking that no study of the application of regrind for UMIM has been published yet, therefore, the viability of reprocessing plastics through this method remains unknown, but less degradation could be expected due to the absence of repeated shear stress that the reciprocating screw of traditional IM pro- duces. Also, ultrasound is being used as a way to disperse fillers for nanocompos- ites, thereby, UMIM is able to develop a good performance for mixing substances and could be a manufacturing method substitute for the fabrication of micro com- ponents in the automotive, aerospace and medical industries.
This thesis proposes the implementation of Ultrasonic Micro Injection Mold- ing to test the reprocessability of polypropylene (PP), one of the most used com- modity thermoplastics. The research procedure will follow a path of continuous closed-loop of molding-grinding, performed for 100% regrind specimens, the ob- jective of this stage is to monitor how the properties of the material vary after each reprocessing cycle under ultrasonic plasticizing, aiming to develop a good perfor- mance and finding thereby possible applications for plastic microcomponents with superior behaviors and dimensional excellence. The solution plan includes a De- sign of Experiments (DOE) to perform tests, analyze the effect of each parameter in the molding process, and determine the values to use. An attrition mill is used to grind molded specimens, sprues and runners.
1.2 Problem Definition and Motivation
Currently, the number of end-of-life products has increased the availability of post- consumer plastics [13], as well as the in-factory streams generated by the manu- facturers themselves, which opens a new area of opportunity for companies to recycle plastics in order to reduce costs and environmental impact. Furthermore, the lack of legislations in the past have lead to the cheap and simple practice of landfill with residues, which is causing a serious environmental threat and concern to society.
In comparison to conventional injection molding, the novel UMIM process presents ephemeral residence times for polymers processing, providing a promis-
ing outlook to reduce wastes and to employ recycled resins in a close-loop and from external sources as well, while opening the possibility of processing new polymer blends to enhance chemical and mechanical properties, complying with the requirement of reliable micro components with assurance of dimensional ac- curacy and enhanced performance. However, by the time of publication of this thesis, there is no study that explores the actual degradation effects of reprocessing repeatedly thermoplastics with UMIM yet. There are no novel methods for ma- terial properties enhancement explored under the effects on UMIM either. Even though the recycling of plastics through regrind combination with virgin material has been a solution to part of the waste streams managements within the molding industry, there is much to develop yet. The fact that, for traditional IM, previ- ous research has pointed that the properties of polymers tend to diminish as they are reprocessed has limited molders only to reutilize small percentages of regrind, however, it does not mean that recycled plastics can not be enhanced. In fact, re- search for UMIM has achieved results that suggest an enhancement of mechanical performance [14][15].
A motivation for this technology would be oriented to present a better un- derstanding of the process and internal phenomena, to motivate a migration and place UMIM one step closer of commercial applications, as a solution to reduce environmental impact, convert to sustainability, and achieve cost reductions in raw material of up to 50% [16].
1.3 State of the Art
The use of ultrasound as an alternative for injection molding has had a boom since the year 2010. The current research for UMIM has focused mainly on the direct effect of molding parameters in the molded parts. Michaeli et al. [6] studied the effects of the process parameters on the shot weight of PP and polyoxymethylene (POM) by doing a DOE. In their study, they found that the compression force and the amplitude of the sonotrode have small effects on the micro parts weight, how- ever, they also found some defects on the morphology, presented as flow marks in the outer layer of the specimens. Negre et al. [11] analyzed the effects of PP pellets humidity, sonotrode velocity, and mold temperature in the molding of polypropylene micro parts. The results showed that dried pellets weight less and they present less porosity, but the porosity of the part intensifies as the section is closer to the injection point. They also found that for higher sonotrode veloci-
ties, the porosity increases, and non-completed parts appear. Better results were performed for velocities below 7 mm/s, and with the application of increasing ve- locity ramps. Pre-heating of the mold delivered less porosity in this experiment too. Grabalosa et al. [12] developed an energy model for the interaction of the mechanical waves and the pellets, they also performed tests, and found no accu- rate relation between the molding parameters and the temperatures reached by the polymers. Results also demonstrated that for higher values of parameters, filling rate, dimensions, homogeneity and yield strength improved, but in reality, is not as simple as just increasing everything to maximum because it would lead to ma- terial degradation. Finally, although there was some polymer degradation, they concluded that ultrasonic vibration did not generate it. They associated the weight variations in the experiment to chain scissions as consequence of shear stress. Sac- rist´an et al. [17] developed a study of the effects of ultrasonic vibration on micro molding for polylactide, regarding to the heating mechanism for ultrasonic tech- nology in order to understand what is happening inside the polymer. According to them, by applying ultrasound, the viscosity of the material is reduced significantly, because of a physical and chemical effect. The physical effect refers to vibration that alters the melt flow and reduces the elastic tensile strains, as well as molecu- lar chain entanglement, while, the chemical effect implies nucleation growth and formation of cavities. S´anchez et al. [10] studied the processing of Ultra high molecular weight polyethylene (UHMWPE) / graphite composites. Graphite con- tents of 1 wt%, 5 wt%, and 7 wt% were mechanically pre-mixed with UHMWPE powder, and pressed at 135 ºC. Later, the samples were cut into irregularly pieces and processed through UMIM to fabricate tensile specimens. Taguchi statistical method was applied to optimize the moulding parameters and to maximize the tensile strength. The tensile strength was increased in 8.8% for 1 wt% graphite samples and all composites presented an increase in the tensile modulus. Graphite caused a decrease of crystallinity, and oxidative degradation and chain scission were found. D´ıaz et al. [18] investigated the dispersion of micro and nano silica particles in a biodegradable polyester matrix derived from 1,9-nonanodiol and aze- laic acid. Minimal degradation and good dispersion of the particles were detected.
Furthermore, thermal stability was slightly improved. The overall crystallization rate of the composite of was greater than that determined for the polymer matrix.
The previously discussed research is summarized in Table 1. As said earlier, no study of recycling under UMIM has been published yet, but the evidence suggests
that less degradation is expected.
The experimentation of reground resins with virgin materials for traditional injection molding can be taken as a reference framework. The combination of reground resins with virgin materials for traditional injection molding has been studied in the past. McLauchlin and Savage [19] studied the reprocessability of polyether ether ketone (PEEK). The study compared the effect of repeated use cycles against the properties of virgin material. Through tensile testing, X-Ray diffraction (XRD) and differential scanning calorimetry (DSC), they evaluated the mechanical properties, thermal behavior and crystallinity. Results showed that pure regrind PEEK can be reprocessed up to three cycles without presenting drops in tensile strength. Sobotova et al. [20] studied of the impact of regrind on me- chanical properties of polybutylene terephthalate (PBT) with 30 % glass fiber and traditional IM. Tensile, Charpy impact, and hardness tests were performed under 20, 40, 70 and 100 % of regrind. They found that the impact toughness reduced up to 24 %, thus, the material became more fragile and prone to cracking. Wagner et al. [21] developed a case of study for post-consumer ABS mechanical recy- cling, they reground plastic housings of TVs, purified them in a water with NaCl solution, and molded tensile specimens and thin wall boxes. With tensile and im- pact test, the results showed that ABS can be reused and develop better properties than commercial recycled resins in the market, however, they found some impuri- ties and aesthetic defects. Spicker et al. [22] investigated the influence of multi- ple reprocessing cycles on the rheological, thermal and mechanical properties of polypropylene manufacturing scrap. The degradation mechanisms were separated to investigate the thermo mechanical, thermal and thermal-oxidative degradation of polypropylene. Melt flow rate (MFR), dynamic rheology, gel permeation chro- matography (GPC), DSC, tensile test, color analysis, and Fourier transform in- frared spectroscopy (FTIR) were part of their methodology. An increase in the melt flow rate and a decrease in the viscosity and the mass average molecular weight were found. Gall et al. [23] presented a case of study for the validation of circular plastics economy from informal pickers sources. The materials tested were post-consumer PE and PP regrind from containers, boxes and bottles. A cleaning process by hot washing was evaluated before the injection molding. The characterization was performed with FTIR, TGA, MFR. It was concluded that post-consumer plastic wastes from informal sources can be processed into ma- terials that are comparable to recyclates obtained from advanced formal systems,
Table1:StateoftheArtforUMIM. ApproachMaterialCharacterizationContributionsReference Directeffectofmolding parametersinthemolded parts.
PPandPOMOpticalmicroscopy,Descriptionoftheinterfacial frictionandvolumetricheating mechanisms.Flowdeffects.
Michaelietal.(2011)[6] Effectsofultrasonicvi- bration.PLASEM,SEC,FTIR,TGA, DSC,Tensile.Diminishmentofdegradationof PLAbyparametersoptimiza- tion.Higheffectofamplitudein degradation.
Sacrist´anetal.(2013)[17] Methodtodispersenan- oclaysinpolymermatri- ces.
PLAand PBSFTIR,SEM,WAXD, TGA,DSC,Tensile.Firstresearchofproducing nanocompositeswithUMIM. Developmentofhighlyoriented claynanosheets.
Planellasetal.(2014)[5] Effectofhumidity,injec- tionspeed,andmoldtem- perature.
PPStereomicroscopyQuantificationandreductionof porositybydryingthematerial andusinglowspeeds.
Negreetal.(2015)[11] Dispersionoffunction- alizedsilicamicroand nanoparticlesbyUltra- sonicMicro-Molding.
Polynon- amethylene Azelate TEM,SEM,GPC,FTIR, XPS,DSC.Desireddispersionofparticles andimprovedthermalstability. Minimaldegradationdetected.
D´ıazetal.(2015)[18] Influenceofprocessing conditionsonmanufac- turingpolyamidepartsus- ingANOVA.
PolyamideFTIR,GPC,SWAXS,Op- ticalmicroscopy.Useofenergymodel.Evi- denceofchainalignmentand degradationreflectedintheMW. Necessityofthresholdlevelsto improvedimensionsandhomo- geneity.
Grabalosaetal.(2016)[12] ProcessingofUHMWPE usingultrasonicvibration energy.ANOVAanalysis.
UHMWPESEM,FTIR,Tensile, TGA,GPC,Optical microscopy.
Modeofactivation.Irregular shapedeliveredbestresultsfor UHMWPE.DecreaseofMWfor highMWpolymers.Increased crystallinity.
S´anchezetal.(2017)[10]
however, thermal and processability variations are limited to non demanding prod- ucts. Pietroluongo et al. [24] used radiator parts made of PA66 reinforced with 35.7 % of short glass fibers from an end-of-life vehicle. They remolded the ground material two consecutive times to compare with the reference material through ten- sile and rheological testing, DSC, TGA, and XRD. They highlight is that despite the degradation and fibers shortening, the material preserves mechanical accept- able mechanical performance for other automotive applications. Even after the two remolding cycles, PA66 increased its deformation capacity in 40%, but the tensile strength was reduced in 11%. An oscillation in the crystallinity degree was found, probably influenced by the thermo-mechanical degradation of the matrix.
The information of the investigations described above is summarized in Table 2.
1.4 Objectives
This research project has an overall objective related to the process and product.
The objective is to demonstrate that UMIM is a reliable manufacturing method for the production of micro components made of thermoplastic recycled resins, delivering stable mechanical, thermal and chemical properties.
The particular goals are listed below:
• To demonstrate that UMIM is a potential manufacturing method for micro components that employ recycled materials.
• To guarantee morphological and mechanical excellence for specimens, through a Design of Experiments.
• To track the behavior of the rheological and molecular weight properties and strength for recycled material under UMIM, as a complementary criteria to decide if the material complies with an application.
• To overcome the main challenges of plastic recycling contamination and degra- dation.
• To determine which are the most significant molding parameters when treat- ing with recycled material in UMIM.
Table2:StateoftheArtforpolymersrecycling. ApproachMaterialAdditionaltreatmentsCharacterizationResultsReference Polymericblendswith 20/80,40/60,70/30 and100/0
PBTDriedat120ºCfor3.5h.Tensile.Shore hardness.Charpy impact.
Impacttoughnessdecreased24% with100%regrind.Materialbe- camefragile.Tensilestrengthand hardnessmaintained.
Sobatovaet al.(2013) [20] Mechanicallyshred- dedplastic.Injection moldingofspecimens with25%,40%,50%, 60%,75%regrind.
PEEKNone.Tensile.XRD. DSC.3reprocessingcycleswithoutloss ofproperties.Dropsinelongation atbreakafter3cycles.Crystallinity maintained.Tensilestrengthmain- tained.
McLauchlin andSavage (2014)[19] UtilisationofPETG scrapasamatrixfor manufacturingofa compositereinforced withPETfibres.
PETG andPETSeparation.Restora- tionofmolecularmass throughchainextender(3 wt%Joncryl).Removal ofprintresidues.
MFR.Notchim- pactstrength.Light microscopy.DSC. Tensile.
Goodcompatibilityfibers/r-PETG matrix.Increasedimpactandflex- ural(5%)strengthofcomposite. 20%reducedtensilestrengthand 4Xr-MFR.
Franciszczak etal.(2018) [25] Mechanicallyshred- dedpost-consumer plastic.Injection moldingofspecimens andthin-wallboxes.
ABSDensityseparationat water(floatingseparated) andwaterwithNaCl (sinkingseparated). Driedat80ºCfor4h.
Tensile.Charpy impact.Mi- croscopy.XRD.
SlightreductionofYoung’smodu- lus.Tensilestrengthreduced.Wagneretal. (2018)[26] Combinationof70% regrindwith30%vir- ginmaterial.
PPRepelletizingwithtwin screwextruder(180to 220ºCprofileat150 RPM).
MFR.Tensile. GPC.Extensional viscosity.FTIR. Coloranalysis.
DoublingofMFR.Darkeningof materialcolor.Molecularweight reducedin20%.
Spickeretal. (2019)[22] Mechanicallyshreded fullregrindfrompost- consumersources
PEand PPWashingandseparation. Repelletizingwithtwin screwextruder(195to 200ºCprofileat400 RPM).
FTIR.TGA.DSC. MFR.Charpyim- pact.Tensile.
Polyolefincross-contamination.No changeinmeltingtemperature. Morepronouncedlossofmass.Vis- cosityincreasedduetocontami- nants.Higherstrainforhot-washed specimens.
Galletal. (2020)[23] Mechanicalrecycling ofEOLautomotive compositecomponent
PA66 with glass fibers Washingandseparation. Remoldedtwotimes.Tensile.Rheolog- icaltesting.Mi- croscopy.XRD. DSC.TGA.
Increaseddeformationin40%.Ten- silestrengthreducedin11%.Oscil- lationofcrystallinityuponremold- ing.
Pietroluongo etal.(2020) [24]
1.5 Hypothesis
Ultrasonic Micro Injection Molding is a reliable technology that modifies the molecular order of polypropylene to produce mechanically enhanced recycled mi- cro components with preserved functionality and quality.
The research questions to be answered are:
• How reliable is to reprocess polypropylene with UMIM?
• What are the changes induced in polypropylene upon consecutive reprocess- ing by UMIM?
• What is the degradation mechanism of viscoelastic plastizicing with ultra- sound?
1.6 Organization of document
The organization of this document is as follows: In Chapter 1, an introduction, the objectives and the problems that this thesis research aim to solve are mentioned, as well as the hypothesis with some research questions that are going to be answered when the investigation is completed. A review of previous work is included along the text. Chapter 2 includes the theoretical fundamentals of the research. Chapter 3 describes the steps that form the methodology of the thesis. Chapter 4 includes all the results and their discussion. Finally, the conclusions, contributions and future work are included in Chapter 5.
2 Theoretical Framework
2.1 Fundamentals of Polymers
Polymers are often classified as macromolecules due to their large composition of successively repeat units, thereby, smaller molecules from which polymers are synthesized are called monomers [27]. One of the simplest representations of polymers is the carbon-chain structure, where the repetition of carbons form the backbone of the chain, and other atoms, radicals, or molecules may be attached to all the remaining bonds. A homopolymer is a chain that is formed of a same repeat unit. On the other hand, a copolymer is constituted of two or more different repeat units. From the point of view of chemical bonding, the functionality of a monomer is defined as the number of bonds it is able to form with other monomers, there- fore, a monomer is bifunctional when it may react to form two covalent bonds with other monomers resulting in a structure of two dimensions. By this means, a tri- functional monomer will have three bonds and a three-dimensional structure. The molecular weight (molar mass) of a polymer is the result of summing the weight of the atoms that form the molecules. Practically, it indicates an average length of the polymer chains of a bulk. In reality, not all the chains that constitute a polymer have the same length, thereby, the molecular weights are expressed as averages.
The molecular weight is of high importance because properties like viscosity and number of entanglements are inherently dependent of it. In the next subsections, the most common ways to calculate the molecular weight are presented.
2.2 Molecular Weight
The molecular weight of a polymer can be calculated in many manners, however, one should determine that specific way depending on the desired properties to study or the one the investigation is interested the most.
2.2.1 Number average molecular weight (Mn)
Properties like relative lowering, elevation of boiling point, depression of freezing point, and osmotic pressure [28], are only sensitive to the number of molecules, therefore, the number average molecular weight should be selected, which implies the total weight of the polymer divided by the number of molecules. In other words, it is a simple mean value. M0 can be denoted as the individual molecular
weight of each monomer that is in a chain. In the end, the total weight of the chain will be the sum of the product of the number of molecules Ni that have a same weight Mi:
T W =
∞
X
i=1
NiMi (1)
Then,
Mn = P∞
i=1NiMi P∞
i=1Ni
(2) But the number fraction is
Xi = Ni P∞
i=1Ni (3)
Finally,
Mn =
∞
X
i=1
XiMi (4)
2.2.2 Weight average molecular weight (Mw)
Light scattering is a property that depends on the weight of each molecule in a polymer chain [28]. Thus, the weight average molecular weight is employed.
Here, the mass of every molecule is considered in the formula. Based on Mn, the number of molecules Ni is substituted by NiMi, giving:
Mn = P∞
i=1NiMi2 P∞
i=1NiMi (5)
But the weight fraction is
Wi = NiMi P∞
i=1NiMi (6)
Finally,
Mw =
∞
X
i=1
WiMi (7)
2.2.3 Higher average molecular weight (Mz)
Mz is nothing but the second moment of the function of molecular weight. By the same procedure the weight average molecular weight function can be obtained
from the number average molecular weight by substituting NiMi in Ni, which represents the first moment of the function, it is able to substitute NiMi2 to obtain M2 = Mz. Mz relates to the amount of high molecular weight chains and it is used to compare several Mz values to determine which has more entanglements.
Mz = P∞
i=1WiMi2 P∞
i=1WiMi (8)
2.3 Polypropylene
Polypropylene (PP) is a semi crystalline thermoplastic polymer that is obtained from the chain-growth catalyst polymerization of propylene, giving the chemical formula (C3H6)n. PP is one of the most used materials in the plastic industry, because it can be treated as a plastic or fiber [29], a structure representation is included in Figure 3. PP is a versatile tough material that exhibits considerable stiffness, good chemical and fatigue resistance, and that is obtained for a low price [30]. Its density is between 0.8555 g/cm3 for the most amorphous structure and 0.946 g/cm3 for the most crystalline. In the same way, the melting temperatures are between 130 ºC and 171 ºC. The molecules of PP are composed of a main chain of carbons from which methil groups CH3− are bonded in any side of the chain. PP can be found as a homopolymer (pure PP), and as copolymer (two or more monomers, usually combined with blocks of PE). For the homopolymer, depending on the orientation of the methil groups, there can be three types of PP:
• Isotactic: It presents an homogeneous distribution of the methil groups, all of them oriented to the same side. This type is the most used for plastic injection components.
• Syndiotactic: It is less cristallyne than isotactic PP. The methils alternate their orientation, which make it more elastic than isotactic, but also less resistant.
• Atactic: It is the least cristallyne of the homopolymer PP, because the methil groups are in disorder. Its properties make it sticky, therefore, it is used for adhesive applications.
From the injection molding perspective, PP has a wide spectrum of applica- tions, from packaging and food containers, to medical consumables and devices, automotive connectors and housings for several types of sensors.
Figure 3: Representation of the polypropylene semicrystalline structure.
2.4 Conventional processing techniques for thermoplastics
2.4.1 Extrusion
Extrusion is a fundamental process for other manufacturing techniques that are ex- plained below. In general, the sequence of extrusion starts by heating and melting a polymer, then, it must be pumped through a shaping unit, which is particularly a die with a desired geometry and dimensions that give the final product after its solidification. Most of the extruders are conformed by one or two Archimedean screws (Single Screw and Twin Screw Extruder, respectively) that are able to rotate within a heated barrel. Pelletized or granulated material is fed through a hopper that is mounted over a gap between the screw and the barrel. The screw diameter goes from small to big in the flow direction, in order to reduce the depth of the conveying channel as the material melts. The screw can be divided in three sec- tion: conveying zone, melting zone, and metering zone. In the conveying zone, the material is compacted, then, in the melting zone the heating mechanism starts to act by creating a thin film of molten material due to contact with the barrel walls (heater) and the intense frictional shearing forces caused by the screw motion pro- duce a viscous dissipation. In the metering zone, the molten material finally is directed to the die, to be extruded in the desired shape [31].
• Extrusion Blow Molding: The molten polymer is extruded through an annular die to form a tube or parison that is enclosed by a mold with the desired shape. The parison is inflated by injecting air to the inside, until it expands and adopts the shape of the mold to later solidify. Finally, the mold is opened
and the blown molded part is released. This process can be continuous or intermittent depending on the polymer properties.
• Extrusion Blown Film: The polymer is extruded through an annular die (tubu- lar mandrel) to form a parison to which air is blown from below the mandrel to form a hollow balloon-like film. The film is pulled axially by a set of guide rollers while it is cooled by a perpendicular stream of air. Finally, the film is also pinched to cut it into a defined number of products. The thickness of the film can be adjusted by moving the mandrel. Typical thickness goes from 0.005 to 0.25 mm, with die diameters from 10 to 120 cm. The bubble diameter with respect to the die usually goes from 1.2 to 4 times bigger.
• Extrusion Fiber Spinning: The extruder is accompanied by a gear pump that forces the molten polymer to flow through a spinneret with many small holes (like a shower). The thin fibers are pulled by a series of godets while they are cooled by a perpendicular stream of air.
• Extrusion Cast film: The molten polymer is extruded through a flat die to create a film. After the die, this film enters a water-cooled chromed roll where its temperature is lowered to solidify it. It is then passed through a series of rollers where the edges are trimmed, and other treatments (like Corona treatment) are applied in case it is necessary. Finally, the film is winded in rolls.
2.4.2 Molding
• Injection Blow Molding: Injection blow molding is the other way to blow mold products, but is not the same as extrusion blow molding. Here, as a first step, the molten material is injected to a mold where it takes a preform, then this preform is taken to a blow mold where air is injected until the parison acquires the final shape. This method offers more dimensional control than extrusion blow molding and it’s preferred for products with small L/D ratio.
• Compression Molding: The polymer is placed over heated platens where it is compressed by a hydraulic press. It is mainly used in high content of fibers reinforced polymers.
• Rotational Molding: Plastic powder is placed inside a mold that is later closed and heated while it is rotated in two axis of rotation. As the polymer melts,
the rotational motion makes the plastic to stick to the walls of the mold. When the mold is cooled, the product solidifies forming a hollow part. This method requires less investment due to simplicity, however, the cycle times are way higher than injection molding.
• Injection Molding: This process starts by melting the polymer in a similar way as in the extrusion process. The melting mechanism for a semi crys- talline polymer is simplified and illustrated in Figure 4: Thus, it can be sum- marized as a result of the heat transfer from the wall of the barrel to the plastic pellets, and the heat generated by the viscous dissipation [32] of the melt film within the pellets and its surrounding surfaces, mainly as a consequence of the shearing and stretching during the screw motion. It is worth remarking that, the difference for injection molding with the extrusion process is that the screw is reciprocating, meaning that as it turns it is moving back. When the desired shot sized is molten at the end of the barrel, the screw is rammed for- ward to inject the material at high pressure to a mold. The melt flows through a sprue and later moves through runners that connect with a gate to the cavity of the desired shape. There must be a holding stage where the machine packs the polymer until it cools down and the product solidifies to be ejected after the mold opens. The resins used in this process have melt flow indexes from 1 to 100 g/10 min.
Figure 4: Conventional Injection molding process and heating mechanism. (a) The chains start to disentan- gle as heat is transferred and shear is applied. (b) The chains are disentangled and start to flow.
2.5 Ultrasonic Micro Injection Molding (UMIM)
The pursuit of miniaturization of devices has brought the development of new manufacturing processes. Ultrasonic Micro Injection Molding is a fast, and low- cost new alternative process that implements ultrasonic vibration to melt a resin and inject it into a mold [5]. This new manufacturing method has several similar- ities with ultrasonic welding, which has been used as a starting point to identify the phenomena behind heating mechanisms [33]. It is as well an analogue pro- cess of the conventional injection molding, with some relevant differences. In this case, instead of dosing and melting more material than needed per shot, just the exact amount required for one shot is placed inside the chamber. The heating in the chamber is not external or by the motion of a screw anymore, now, an ele- ment called Sonotrode [34], vibrates uniaxially to ultrasound frequencies, usually between 20 and 40 kHz [35], inside the chamber with direct contact to the plas- tic pellets, transmitting these mechanical oscillations until they dissipate into heat and the material melts. The amplitudes handled by sonotrodes under ultrasonic frequencies are commonly bounded between 12 and 100 µm [36].
Figure 5: UMIM process sequence. (a) Sonotrode activation and plunger motion. (b) Material melting and injection. (c) Injection completed and start of holding. (d) Complete specimen after ejection.
Figure 5 presents the plasticizing, injection and holding stages of the cycle: (a) the sonotrode starts vibrating as the plunger ascends, reducing the free space and compressing the pellets. Then, in (b) the material starts to melt and it is injected
gradually through a runner. Finally, (c) shows the material completely injected into the cavity as a result of the pressure exerted by the plunger, at this point the holding stage is performed before the mold is opened to release the piece (d).
The heating mechanism of UMIM is based in interfacial friction and volumet- ric heating [6], which are illustrated in Figure 6. It has been proven that interfacial friction is a transient process that happens in very short periods of time (less than 0.5 s) and has a strong effect at the beginning of the plasticizing [37]. Interfacial friction is determined by the relative movement (Vrel) between granulates, which depends on the respective kinetic states ( ~V1 and ~V2), also, the surface area (AS) and friction coefficient (µ) have influence, as shown in Figure 6 (a). The volumetric heating, also classified as viscoelastic heating, is caused by the internal damping properties of the material.
Figure 6: Heating mechanisms. (a) Heating due to interfacial friction. (b) Volumetric heating.
The sinusoidal movement of the sonotrode loads and unloads the granulates as a fluctuating stress. By definition, the viscoelastic effect is a property of materials to have both viscous and elastic behaviors (storage and loss modulus) when they are deformed, in other words they have resistance to shear flow and linear strain, and also the ability to return to original state after being stressed. The viscoelas- tic heating mechanism can be modeled through the generalized Maxwell model [38][39]. The storage modulus is proportional to the stored energy (elasticity), while the loss modulus is proportional to the dissipation of energy as heat (vis- cosity), in a deformation cycle. Although in reality unit cells are deformed in a combined direction, for terms of practical modelling, the stress in a unit cell is often represented as a uniaxial normal stress dependant of time, which is the si-
nusoidal function corresponding to the sonotrode in Figure 6 (b), with the same amplitude and frequency. A hysteresis loop if formed by the loading and unload- ing curves, and it is known that the area of the bounded region is the work done by the sonotrode for the internal friction in one cycle. Then, the heat generation of each unit cell per time is [38]:
Q = f I
σ(t)d(t) = f σ00ω
Z 2π/ω 0
sin(ωt)cos(ωt − δ)dt = f πσ00sin(δ) (9) where f is the vibration frequency, σ is the amplitude of the stress, is the strain, ω is the angular sonotrode frequency and δ is the lag angle of strain.
The complex modulus of a polymer is:
G∗ = σ0
0
(cos(δ) + isin(δ)) = G0 + iG” (10) where G’ is the storage modulus and G” the loss modulus.
Substituting Equation 10 in Equation 9, it is found that viscoelastic heating depends on the ultrasonic frequency, amplitude, and the loss modulus of the ma- terial:
Q = f π20G” (11)
2.6 Material reprocessability
2.6.1 Classification
The use of regrind can be categorized in two groups: post-industrial (pre-consumer) and post-consumer. Post-industrial plastics are those that were already processed through a manufacturing method but they never left the place they were treated, in this category the closed loop cycles are included, where the recycled plastic comes from the same process it is later used in. Closed loop regrind comes from discarded materials that are placed in a grinder immediately after they come out from the process machine (molder, extruder, press). If there are any impurities they can be removed directly or easily, and normally the polymer is stable enough to be exposed to high-temperature processes [40]. These elements include sprues, runners and scrap pieces. Post-industrial plastics are easier to treat since they present less contamination, have less time degradation, and the exact composition and quality of resin is known as well.
On the other hand, post-consumer plastics are those that were part of a prod- uct and ended its service life, also known as End of Life (EOL). This group could be subdivided in short life [40], for example packing elements, disposables, bot- tles and bags; and used goods like cars, televisions, cellphones, computers. Thus, post-consumer recycling means to gather plastics from any processes or source, and give them a new application. Post-consumer plastics imply more work be- cause they may be contaminated with other materials and may be degraded by any means, also they could present different quality, consequently the cost of recy- cling increases. Table 3 displays the classification of plastics recycling, according to ASTM and ISO standards.
Table 3: Classification of plastics recycling.
ASTM D5033 ISO15270 Equivalent
Primary Mechanical Closed loop
Secondary Mechanical Downgrading
Tertiary Chemical Feedstock
Quaternary Energy recovery Valorization
2.6.2 Mechanical recycling
The action of grinding, sometimes also called milling, takes place inside machines, as illustrated in Figure 7, that typically have a feeding unit, where material is disposed; a shredding unit, usually composed of rotating blades and knives that impact and cut the material; a classification screen with holes where the granulated material passes when it reaches a certain size; and a collection element where the regrind is evacuated through gravity or by vacuum [41][42].
The milling action produces stresses in the particles being ground, hence, the most common are compression, attritions, shear and impact [43]. When the im- pacting force of the blades is enough, the trapped particles are compacted to form irregularly shaped macro particles. As deceleration occurs, shear and attritions stresses during the impact take place as a radial displacement of powder particles.
It should be considered that there is resistance to the displacement due to friction between particles and colliding surfaces, also the morphology of the final regrind might depend mainly on the ductility or brittleness of the milled powders. When brittle materials are exposed to high impact forces, they are subjected to continu- ous fracture [44].
