Mechanical Reduction of Recycled Polymers for Extrusion

MECHANICAL REDUCTION OF RECYCLED POLYMERS FOR EXTRUSION

AND REUSE ON A CAMPUS LEVEL

A Thesis

Presented to the faculty of the Department of Mechanical Engineering

California State University, Sacramento

MASTER OF SCIENCE

Mechanical Engineering

Rachel Singleton

FALL 2020

Rachel Singleton

MECHANICAL REDUCTION OF RECYCLED POLYMERS FOR EXTRUSION

AND REUSE ON A CAMPUS LEVEL

A Thesis

Rachel Singleton

Approved by:

______, Committee Chair Rustin Vogt, Ph.D

______, Second Reader Susan L. Holl, Ph.D

______Date

iii

Student: Rachel Singleton

I certify that this student has met the requirements for format contained in the University format manual, and this thesis is suitable for electronic submission to the library and credit is to be awarded for the thesis.

______, Graduate Coordinator Kenneth Sprott, Ph.D

Date:______

Abstract

MECHANICAL REDUCTION OF RECYCLED POLYMERS FOR EXTRUSION

AND REUSE ON A CAMPUS LEVEL

Rachel Singleton

Environmental plastic pollution has exponentially increased over the last few years, resulting in 6.3 out of the 8.3 metric tons of plastic produced each year worldwide, ending up in landfills or natural environments. Much of the plastic waste is a result of wrongful disposal or improper recycling category placement. Improperly recycled plastics can occur anywhere from the household, where it is stated that only 9% of plastic is correctly recycled, to universities [1]. Besides more education on proper recycling practices, higher education systems need to investigate potential areas of instruction that would allow for plastic reuse. One area includes courses dealing with

3D printing; 3D printing filament's yearly consumption is estimated at around 30 million pounds worldwide. [2] To reduce this amount, classes using 3D printing in their curriculum should consider shredding, re-extruding, and re-using the scraps created from flawed prints – using effective recycling. Recycling is not a standard practice used as some worry the recycled filament will create inadequate prints. To demonstrate the value of using recycled polymers standard mechanical tests were done to establish that important qualities are maintained after recycling. Through multiple tensile tests run on v

3D printed samples using a Universal Testing Machine (UTM) with both virgin polylactic acid or polylactide (PLA), and recycled PLA, polyethylene terephthalate

(PET), and polyethylene terephthalate with glycol added (PETG) plastics, it was discovered that the ultimate tensile strength (UTS) for virgin PLA was 6343.45 psi in comparison to recycled PLA that exhibited a value of 4686.80 psi. This result showed only a 23.25% decrease in its strength. 100% PET exhibited a UTS of 4481.27 psi with a 29.36% decrease compared to the UTS of virgin PET UTS. These experiments demonstrated that the recycled filament materials show a high enough UTS to create usable and adequate prints. These results verify that 3D prints using recycled plastic found on campuses (such as scraps from 3D printing labs and PET water bottles) can be used to reduce the volume of plastic waste that universities contribute to landfills.

______, Committee Chair Rustin Vogt, Ph.D.

______Date

ACKNOWLEDGEMENTS

I want to thank my wonderful advisors and mentors, Rustin Vogt, and Susan Holl for all their amazing help. I also want to thank my family for always believing in me and my dreams. And finally, thank you to my wonderful friends and my boyfriend Christian who supported me through this whole journey. And thank you Covid-19 for making my graduate year an interesting one.

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TABLE OF CONTENTS

Page

Acknowledgements ...... vii

List of Tables ...... xi

List of Figures ...... xiii

Chapter

1.INTRODUCTION ...... 1

1.1 What is Plastic? ...... 1

1.1.1 Where did Plastic Originate?...... 1

1.2 Plastic Pollution ...... 3

1.2.1 Plastic Waste on Campus ...... 5

1.2.2 Plastic pollution from 3D printing ...... 7

1.2.3 Survey Results ...... 8

1.2.4 Recycling solutions for plastic waste ...... 10

2. BACKGROUND ...... 11

2.1 Polymer Categorization ...... 11

2.1.1 Plastic Architecture ...... 15

2.1.2 Mechanical properties of polymers ...... 16

2.2 The Tensile Test ...... 18

2.3 The Dehydration Process ...... 22

2.4 Similar Work ...... 25

2.4.1 Work with PLA ...... 25

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2.4.2 Work with PET ...... 26

2.5 Machinery ...... 28

2.5.1 The shredder ...... 28

2.5.2 The Filastruder and Filawinder ...... 32

2.5.3 The dehydrator ...... 33

2.5.4 The Ender 3D printer ...... 34

2.5.5 The UTM (Universal Testing Machine) ...... 34

3. EXPERIMENT ...... 35

3.1 Experimental Overview ...... 35

3.2 Test 1: Virgin PLA ...... 37

3.2.1 Introduction and Goals ...... 37

3.2.2 Procedure ...... 37

3.3 Test 2: Recycled PLA (50% recycled PLA, 50% virgin PLA pellets) ...... 38

3.3.1 Introduction and Goals ...... 38

3.3.2 Procedure ...... 38

3.4 Test 3: PETG (50% PET, 50% PETG virgin pellets) ...... 39

3.4.1 Introduction and Goals ...... 39

3.4.2 Procedure ...... 39

3.5 Test 4: 100% PET ...... 40

3.5.1 Introduction and Goals ...... 40

3.5.2 Procedure ...... 40

4.RESULTS AND ANALYSIS ...... 42

4.1 Test 1: Virgin PLA ...... 42

4.1.1 Test specimen model ...... 42

4.1.2 Tensile Test results ...... 43

4.2 Test 2: Recycled PLA (50% recycled PLA, 50% virgin PLA pellets) ...... 46

4.2.1 Extrusion results ...... 49

4.2.2 Tensile Test results ...... 46

4.3 Test 3: PETG (50% PET, 50% PETG virgin pellets) ...... 49

4.3.1 Extrusion results ...... 49

4.3.2 Tensile Test results ...... 50

4.4 Test 4: 100% PET ...... 53

4.4.1 Extrusion results ...... 53

4.4.2 Tensile Test results ...... 54

4.5 Adhesion and printing ...... 56

4.5.1 Extrusion results ...... 56

4.5.2 Tensile Test results ...... 59

5.FUTURE WORK ...... 61

6. CONCLUSION ...... 62

Appendix A: Stress-Strain curve data collected for all tests ...... 64

Appendix B: Full Student Survey ...... 65

References ...... 67

LIST OF TABLES Tables Page

1. Plastic types and their properties provided from Precious Plastics…….…...….17

2. Abbreviations from Precious Plastics ………………………………...………..17

3. Results of testing specimens ………………………………………….………..26

4. Motor proposals from Precious Plastics website………………….…..………..31

5. Composition of testing specimens…………………………………….……….35

6. Virgin PLA extrusion temperature and dehydration time……………………...38

7. 50/50 PLA blend extrusion temperatures and dehydration time…………...…..38

8. 50/50 PET-G blend temperatures and dehydration times…………...... ………40

9. 100% PET temperatures and dehydration times……………..…….…………..41

10. Specimen lengths/areas before and after break for virgin PLA …………...…..44

11. Values from the stress-strain curve for virgin PLA calculations ………...……44

12. Mechanical properties found from virgin PLA test……………………….……45

13. Specimen lengths/areas before and after break for recycled PLA ….…...….…46

14. Values from stress-strain curve for recycled PLA calculations ……...... ……47

15. Mechanical properties found from recycled PLA test……………..……..….…48

16. Specimen lengths/areas before and after break for PETG ………...…...………50

17. Value from stress-strain curve for PETG calculations …………….....…..……50

18. Mechanical properties found from virgin PETG………………………….……52

19. Values from stress-strain curve for PET calculations………………...…..……54

20. Specimen lengths/areas before and after break for PET…………….…………54

21. Mechanical properties found from PET test……….………………..…………56

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LIST OF FIGURES

Figures Page

1. Symbols of the seven types of plastic ……….…………………………………. 3

2. Plastic pollution in Bulgaria ………………….…………………………..……. 4

3. Factors preventing recycling at the UWI, Cave Hill campus…………….……...6

4. A comparison of recyclable and contamination levels of recycling categories…6

5. Survey on plastic use in classrooms……………………………………………..9

6. Survey on how much student think plastic is not properly recycled………..…...9

7. Survey on how important student think it is to recycle on campus……...………9

8. Polymer architecture comparing thermosets with thermoplastics…...…..……..16

9. Testing specimen with gauge length, diameter and shoulder marked……...…. 18

10. ASTM D638 Type 1 specimen for polymer tensile testing measurements….....19

11. Typical stress-strain curve for brittle material.………………..………….….. ..19

12. Polymers interaction with moisture……………………………………….....…23

13. Agitating PET to prevent agglomeration……………………….………………24

14. Schematic representation of experimental setup……………………………….26

15. a. 50-50 Blend of recycled PET and PETG; b. 100% PET plastic with low

viscosity...... 27

16. Shredder built by Precious Plastics founder, Dave Hakken……………………29

17. Precious Plastics blueprint for machine………………………………………..29

18. CAD model of shredder component (inside)……………………….....………..30

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19. Actual shredder……………………………………………………...……….…30

20. SolidWorks model of 3D crank…………………………………………….…..31

21. Crank printed out……………………………………………………...………..31

22. Fully built shredder………………………………………………………….….32

23. Filastruder (extruder)…………………………………………………………...32

24. Filawinder………………………………………………………………………33

25. Weston 4-Tray food dehydrator……………….……………………….………33

26. The Ender 3D printer………………….………………………………………..34

27. Universal testing machine……………….……………………………………..34

28. ASTM D638 Type 1 specimen for polymer tensile testing measurement……..35

29. SolidWorks image of testing specimen…………..…………………………….36

30. Testing specimen printed……………...………………………………………..42

31. Load vs. displacement graph with virgin PLA…………………...………….…43

32. Virgin PLA stress-strain curve……………………………………….……..….44

33. Recycled PLA filament test………………………………………………….....46

34. Testing Specimen -50/50 recycled PLA and virgin PLA……………..……..…47

35. 50/50 recycled PLA stress-strain curve……………………………...……..…..47

36. PETG filament extruded at 275°C …………………...……...………..………..49

37. Spool of PETG filament………………………………………………………..49

38. The broken PETG specimen……………………………...………….…………50

39. PETG stress vs. strain curve………………..…………………………….…….51

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40. Top: extrusion at 235°C; middle: extrusion at 255°C; bottom: extrusion at

275°C……………………………………………………………………...……53

41. The PET filament created………………………………………………………53

42. Testing specimen 100% PET broken after test………….…………….……...... 54

43. Virgin PLA stress-strain curve…………………………………………………55

44. Stringing on the top surface of a PETG print…………………………………..57

45. Poor vertical adhesion………………………………………………………….58

46. Good layer adhesion……………………………………………………………58

47. Image of PET specimen with part of mat sticking to the part………………….59

48. Image of PET specimen with the mat not sticking……………………………..59

49. Poor adhesion in print………………………………………………………….60

50. Better adhesion with print……………………………………………………...60

1. INTRODUCTION

1.1 What is Plastic?

Plastics are materials created by manipulating organic products such as petroleum, coal and natural gases that go through a distillation process allowing for hydrocarbon compounds to separate from the raw material. Some of the most common compounds used include ethylene, propylene, and butene. From there, the compounds go through different manufacturing processes to make the different types of plastics. In the end, plastic is simply a material derived from organic products. [3] From these materials, plastic products such as water bottles and milk jugs, to name a few, are created. Once these products have run their course, the user can recycle the product. To recreate new products out of these recycled plastics, one can shred, melt, and reform the material into new products.

1.1.1 Where did Plastic Originate?

The story of plastic started in 1862 when Alexander Parkes created a material (that he named Parkesine) derived from cellulose that could be heated, molded, and successfully retain its shape once cooled. John Wesley Hyatt took Parkesine and continued experimenting with this cellulose based material and in 1868 he introduced Celluloid, a material invented to replace ivory. [4] Celluloid became a plastic leader and was the foundation of human-made plastics in the late 19th century. Numerous products used this new material with the creation of celluloid hair combs and jewelry which imitated ivory.

At first, the material took over the grooming world with celluloid hand mirrors, hairbrushes, and eventually toothbrushes. As expected, once the popularity of the material grew, so did its versatility. Celluloid was used in widely different products from articles of clothing such as detachable collars and cuffs, to toys and dolls for children, and it was even used in the gambling industry to produce dice and cards. As celluloid products increased in variety it became a common product used in everyday life, eventually finding its way to Hollywood. It was there that celluloid was discovered to have strong, flexible, and transparent characteristics that would make it a viable competitor for the new world of film and photography. Even with the negative quality of its extreme flammability, this material demonstrated it was a fantastic asset to society. [5]

In 1907 a chemist names Leo Hendrik Baekeland formulated the first synthetic material based on a compound composed of phenol and formaldehyde. He later named this material Bakelite, and it became recognized for becoming the first mass-produced synthetic plastic. Since Bakelite's creation, new plastics, based on a variety of compounds, have been produced with various properties for different applications. [6]

1.1.2 Types of Plastics

Plastic, polymeric material, has been in wide industrial use since the 1940s, and without recognizing it, people contribute to increasing amounts of polymeric waste daily through improper disposal of plastics. As of March 2018, only plastics with the numbers 1 (PET, sometimes abbreviated PETE), 2(high-density polyethylene, HDPE), and 3(polyurethane,

PU) can be recycled (Figure 1). More importantly, widely used materials labeled 4(low-

density polyethylene, LDPE), 5(polypropylene, PP), 6(polyethylene, PE), and 7(Other) will no longer be accepted in some locations. Waste Management, a recycling company in Sacramento, CA developed this policy recycling policy because plastics 4 through 7 are more challenging to reuse, and are not worth as much in the recycling market resulting in very limited options for disposal. [7] If basic knowledge and tools were more readily accessible to the public, increase plastic recycling would reduce plastic pollution and waste.

Figure 1: Symbols of the seven types of plastic [8]

1.2 Plastic Pollution

National Geographic author Laura Parker conducted numerous studies on plastic pollution, specifically the volume ending in landfills, and discovered that 91% of plastic is recycled incorrectly.[1] 8.3 metric tons of plastic are produced each year worldwide, and from that, 6.3 metric tons become plastic waste ending up either in landfills or in natural environments; by the year 2050, there will be approximately 12 billion tons of plastic in landfills. In 2015 research lead by a team from the University of California,

Santa Barbra (UCSB), found 8 million tons of plastic turns up in the ocean every year. [1]

Another study stated that everyday plastic has increased by 20 times in the last 50 years and is on track to increase more quickly in the future. [9] Because of this volume of use of virgin plastics, the World Economic Forum predicted that there would be more plastic

than fish in the ocean by the year 2050. [9] All of this plastic in the ocean has influenced some unforgivable sights, such as the great garbage patches. [10] These patches are made mostly of plastics that have been incorrectly discarded. Another major problem with plastic disposal is that it is not biodegradable. This means that the plastics do not disintegrate; instead, they break down into tinier pieces of non-biodegradable materials known as microplastics. These microplastics are what most of the ocean garbage patches are made of. In addition to making the water appear ‘cloudy’, these microplastics are responsible for blocking sunlight that should be reaching plankton and algae, which causes disruption in the entire food chain. The two most prominent microparticles ocean patches are the Western and Eastern Garbage patch found in the North Pacific Ocean; however, because these patches are so far from any true coastline they are not associated with any country’s territory, so no countries have taken full responsibility for the cleanup.

Scientists estimate that if 67 ships went to clean one of the patches, it would take one year to remove less than 1% of the garbage. [10] Without a change in societal and industrial behavior, these garbage patches, will only increase in size and, generating additional obstacles for the environment and the animals that inhabit area.

Figure 2: Plastic pollution in Bulgaria [11]

1.2.1 Plastic Waste on Campus

There are 4,278 universities in the United States alone (in the year 2018), according to

U.S. News and World Report. [12] In the year 2020, 300 of these campuses participated in RecycleMania (now known as Campus Race to Zero Waste), which is a program that helps colleges find new paths towards zero waste (90+% diverted from trash) by promoting the recycling of compostable food, and single-use plastics. [13] The 300 schools that participated in 2020 included over 4.5 million students and staff working together to recycle, donate, and composted 48.6 million pounds of waste. Along with that, the participants helped keep 380 million plastic bottles from ending up in the waste stream. [14] As significant as that is, the 300 schools that participated only account for

6.5% of all the schools nationwide; If more schools were involved in the education of proper recycling practices, over 5 billion plastic bottles could have been saved from entering the waste stream.

As stated in a case study surveying ways to improve recycling in higher education institutions, the most significant barriers for recycling at universities included a lack of awareness among the public, high contamination levels, and overflowing recycling bins, all enhanced by a lack of motivation. [15] These barriers were determined from a study conducted in 2013 on the UWI, Cave Hill University campus regarding what factors affect the public not to recycle accurately; Figure 22 is a graph showing the results.

Figure 3: Factors preventing recycling at the UWI, Cave Hill Campus [16]

Along with the lack of motivation and bins location, the recycling bins' contamination levels is another significant problem. By weight, their study showed that plastic had the lowest percentage rate of being recycled at only 56% and having the highest contamination levels at 44% contamination. Below is the graph representing the contamination levels of the different recyclable products.

Figure 4: A comparison of recyclable and contamination levels of three recycling

categories [16]

The study concluded that the high contamination levels seen with plastic could be attributed to poor signage. [16]

1.2.2 Plastic Pollution from 3D printing

One of the root sources of plastic pollution is inadequate knowledge of plastic recycling.

At this time, only 9% of total plastic used in households recycled properly. [1]

Besides homes, places such as colleges and theme parks also contribute significantly to the amount of discarded plastic. Many universities essentially become towns when the school year is in session, as thousands of students and staff fill the campus. Although not always investigated, schools can contribute a large portion of plastic waste without recognizing it. Areas such as labs, classrooms, and eateries use disposable plastic items that are commonly not recyclable.

One type of course and facility that can improve upon plastic waste are those with 3D printing projects in their curriculum and 3D maker space labs. In these examples, virgin polylactic acid or polylactide (PLA), and acrylonitrile butadiene styrene (ABS) are the two most appropriate types of plastic used for 3D printing. 3D printing is the method of creating a physical object from a three-dimensional digital model (usually using

Computer-Aided-Design (CAD)) by building the product through laying successive thin layers of plastic until the item is complete. It has become a useful tool for prototyping as a student learn to be effective mechanical designers. [17] Modern technology has increased the number of materials that can be 3D printed from initially only plastics, to now include metals, ceramics, and paper. As impressive as this technology is, an article

written in 2015 stated that the yearly consumption of plastic 3D printing filament was estimated at around 30 million pounds; by 2020, it was predicted that the total consumption of plastic filament would reach 250 million pounds.[18] At prices of almost

$50 per spool of plastic filament (approximately 330 meters of filament), not only would finding a recycled alternative save the amount of oil consumed and carbon emissions produced, but also the amount of money spent by schools and individuals on 3D printing projects.

1.2.3 Student Survey

A typical large, public university, UC Berkeley, reported that Over 600 pounds of 3D printed waste are created each year on their campus. [45] Presuming that similar amounts would be used on any large public California university, a survey was created to understand how much the campus (and their students) contribute to plastic pollution at

Sacramento State. The survey additionally investigated how much the student body knows about the plastic crisis.

To provide context for this work, a survey of students at Sacramento State was conducted. The survey reached 105 students studying Mechanical and Civil Engineering,

English, Theater, History, Nursing, Music, Environmental studies, and so much more.

Below are a few of the results from this survey in pie charts.

Figure 5: Survey on plastic use in classrooms

Figure 6: Survey on how much student think plastic is not properly recycled

Figure 7: Survey on how important student think it is to recycle on campus

Figure 43 shows that 55% of the student surveyed use plastic in their classes, supporting the hypothesis that most college classes use plastic regularly, adding to the daily plastic waste. Additionally, Figure 44 shows a lack of knowledge among the student population

where 55% of the students believe that only 68% of plastic gets recycled incorrectly, when the real value is 91%. [1] The survey is an excellent example of the lack of information circulating throughout communities and campuses about the created waste.

Finally, Figure 45 shows that the students believe recycling on campus is essential, but they need to know more about the how’s and why’s of recycling to improve their ways.

1.2.4 Recycling Solutions for Plastic Waste

Precious Plastics is an organization based in the Netherlands that was established in 2013 by Dave Hakkens. His mission is to reduce plastic pollution by providing knowledge, tools, and techniques online to everyone for free. [19] From its origins, the organization has branched out to evaluate the different ways plastics can be recycled and reused. With some additional machinery such as an extruding tool, a filament winder, and dehydrator, the possibility of recycled filament in schools and hobby labs could become a reality.

2. BACKGROUND

2.1 Polymer Categorization

Polymers, specifically plastics, are categorized into seven types including: PET, PE, PU,

PP, HDPE, LDPE and other [20].

1. PET: Polyethylene terephthalate is made up of ethylene glycol and terephthalic

acid. Because of its semi-crystalline characteristics, PET is used for fiber for

clothing and water bottles. PET is used for single-use packaging including peanut

butter jars and plastic cups. PET is such a desirable plastic because of its

transparency, high impact resilience, resilience to solvents and its shatter resistant

characteristics. [21] It is widely used for its lightweight properties and is one of

the most sustainable plastics. However, even with its ability to be recycled, PET

single use items such as beverage bottles and food containers are the most

common plastic found in our ocean. [22]

2. PE: Polyethylene is generally categorized into LDPE and HDPE. PE is

formulated through the distillation of ethane combined with other catalysts.

a. HDPE: High-density polyethylene possesses a high-crystalline structure.

Properties of HDPE include being stiff, strong, and having a high density.

This material is suitable for producing milk cartons cutting boards, and

garbage bins.

b. LDPE: Low-density polyethylene is a material with low tensile strength

but high ductility. Because of these qualities, LDPE is a substantially

flexible material and is used to produce shopping bags. [23]

3. PU: Polyurethane is a material that can be produced as a thermoplastic (flexible)

or thermoset (rigid). PU is a favored material for its lightweight and thermal

characteristics. PU can be anything from rigid foam to an elastic foam material

that can be molded into several shapes. Because of these characteristics, PU is

used for creating items such as padding and seat cushions. [24]

4. PP: Polypropylene is a combination of propylene monomers. Like polyethylene,

polypropylene is created through the distillation of hydrocarbons and is one of the

most common plastics produced today. It has a low density compared to the other

varieties of plastics. Manufacturers use polypropylene’s low-density property for

weight savings with injection-molded PP parts. Another benefit PP offers for

manufacturers is that parts can be created through both CNC and injection

molding. Polypropylene is a polymer that can be copolymerized, meaning it can

be blended with other plastics, such as polyethylene, to create a composite plastic.

Other characteristics of PP include its resistance to chemicals making it the right

choice for cleaning product containers. Polypropylene is known to retain its shape

even after bending and torsion. PP is a thermoplastic that has a melting point of

130 oC. These unique properties of PP make it a popular polymer for

manufacturing applications. [25]

5. PVC: Polyvinyl chloride is the most used out of the thermoplastic family. Some

essential characteristics include its natural white color and its brittleness

compared to other plastics. PVC can be manufactured into rigid PVC (RPVC) or

flexible plastic. RPVC is used to create pipes for plumbing and other construction

applications. PVC is made through a complex process using suspension

polymerization, emulsion polymerization, and bulk polymerization.

Polymerization is a process where monomers chemically react to produce the

polymer molecules. [26]

6. PS: Polystyrene is transparent and can either be a solid plastic or foam material. It

is commonly used for packaging. Depending on the form, PS can be classified as

either a thermoplastic or a thermoset. [27]

7. Other: This category of plastics contains anything from polycarbonate (PC),

acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). [28]

a. Polycarbonate: PC is a thermoplastic known for being a robust material

that is impact resistant. Additionally, PC has a glass-like surface due to its

transparent color. The most common product created with PC is ballistic

(or bullet-proof) glass. The material can also be found as CDs, DVDs,

automotive components, and medical devices. PC is commonly used for

its strength, ability to handle tremendous heat and applications that require

transparency (i.e., windows, clear tubes, machinery guards, and even 3D

printed models for high heat applications). PC is formed through

polymerization and becomes a copolymer, meaning it has multiple types

of polymers in the polymer molecules. PC has one main disadvantage, and

that is its ability to be easily scratched. [29] b. Acrylonitrile Butadiene Styrene: ABS is a thermoplastic and an

amorphous polymer. This polymer is generated through the process of

emulsion. Emulsion involves mixing multiple products that do not

customarily combine to make a single product. ABS plastics have a strong

resistance to chemical or physical impacts making it a popular choice in

3D print, injection molding, and Fused Deposition Modeling (FDM). [30] c. Polylactic Acid: PLA is another plastic commonly used to create 3D

prints. PLA's characteristics are similar to PP, PE, and PS and can be

easily manufactured from already existing machinery allowing the

polymer to be extremely cost-efficient to produce. What makes PLA

different is that it is derived from resources like corn starch or sugar cane.

[31]

2.1.1 Plastic Architecture

Another strategy for categorizing plastic is based on the molecular architecture. The architecture of the molecules, their individual structure arrangement in the product, determines whether the material will be rigid (like a thermoset) or flexible (like thermoplastics). Production of the polymeric “raw material” begins when the base hydrocarbon monomer molecules are synthesized from the natural material. A monomer is a stand-alone molecule that contains the fundamental structure of the material and is able to bond to other molecules. These small hydrocarbons molecules then go through the process of either chain growth or step-growth polymerization. During chain growth, one monomer will combine with another, then more monomers join, and so on, resulting in a long polymer chain. These chains may contain double or triple carbon bonds that create the segment identified as the carbon backbone. In step-growth, monomers combine to form molecules which react with other molecules to create a new polymer which may be a byproduct of various monomers. The backbone in this structure may contain heteroatoms, or atoms other than hydrogen and carbon. Linear polymers are long chains

(from chain-growth polymerization) in which the monomers are connected through the carbon backbone atoms. Branched polymers (from step-growth polymerization) include segments of polymer chain branching off the main carbon backbone. [32] Crosslinked polymers have strong bonds attaching adjacent polymer chains which restrict the polymer and create a rigid material. Figure 3 illustrates examples of these different architectures.

Figure 8: Polymer architecture comparing thermosets with thermoplastics [33]

Branched polymer chains can either tangle up and become rigid or stay separate and not pack together closely causing the material to be more flexible. The other situation that can happen is when the polymer chains become crosslinked after being processed.

Having these crosslinks in the polymer chain is what generates a thermoset plastic. [32]

Each of these plastic architectures is formally classified into thermoset or thermoplastics based on their macroscopic behavior. The contrast between thermoset and thermoplastic is that thermoplastics can be melted and reformed multiple times without altering essential properties. Thermosets are unable to be melted and reformed multiple times because of a chemical reaction to create the complex molecule architecture during the initial heating process. [34]

2.1.2 Mechanical Properties of Polymers

Below is a chart of each plastic category and its properties. The chart was provided by

Precious Plastics. [35] This chart is only an example of what material properties can be associated with plastics. PET is the only plastic that will be tested in this experiment.

Table 1:Plastic types and their properties provided from Precious Plastics [35]

Plastic Thermal Properties Strength Density Abbreviation – Tm Tg Td Cte Tensile Compressive Brand Name °C °C °C Ppm/ psi psi g/cc PET- 245 73 21 65 7000 11000 1.29 Polyethylene 265 80 38 10500 15000 1.40 terephthalate LDPE – Low- 98 -25 40 100 1200 0.917 density 115 44 220 4550 0.932 polyethylene HDPE – High- 130 79 59 3200 2700 0.952 density 137 91 110 4500 3600 0.965 polyethylene PP - 168 -20 107 81 4500 5500 0.900 Polypropylene 175 121 100 6000 8000 0.910 PVC - 75 57 50 5900 8000 1.30 Polyvinylchloride 105 82 100 7500 13000 1.58 PS - Polystyrene 74 68 50 5200 12000 1.04 105 96 83 7500 13000 1.05

Table 2: Abbreviations from Precious Plastics [35]

Tm crystalline melting temperature (some plastics have no crystallinity and are said to be amorphous) Tg glass transition temperature (the plastic becomes brittle below this temperature) Td heat distortion temperature under a 66-psi load CTE coefficient of linear thermal expansion Tensile Strength load necessary to pull a sample of the plastic apart Compressive load necessary to crush a sample of the plastic Strength Density aka specific gravity/mass of plastic per unit volume

2.2 The Tensile Test

a. The Testing Specimen

The testing specimens used in a tensile test can vary significantly in lengths,

diameters, and widths, but they all possess the same essential parts. The gauge

length is a section of the testing specimen somewhere between the specimen's

shoulder and gripping regions. The gauge length will change during the testing

portion showcasing the specimen's deformation/strain. These gauge length will be

measured before and after the test to calculate an accurate deformation

percentage. Along with the gauge-length, another important specimen property is

the gauge length section dimensions, which also determine the deformation

characteristics. Lastly, the shoulder is the part on each end of the specimen that

will be the gripping section put into the UTM machine. Below is an image

showing the tensile test specimen (Figure 4). [36] Figure 5 shows an image of an

ASTM D638 Type 1 specimen with the standard measurements. [31] This type of

test specimen is the most commonly used when doing a tensile test on polymeric

materials. The dog bone (or dumbbell) shaped specimen is used to ensure that the

break will occur in the specimen's gauge length. [37]

Figure 9: Testing specimen with gauge length, diameter and shoulder clearly marked [36]

Figure 10: ASTM D638 Type 1 specimen for polymer tensile testing measurements [32]

b. The Stress-Strain Curve

The stress-strain curve is plotted from the data of the tensile test. Below is an

image of a primary stress-strain curve for a brittle material such as a rigid plastic.

Figure 11: Typical stress-strain curve for brittle material [36]

Above, Figure 6 shows the essential aspects of the stress-strain curve. The Elastic

Modulus (E) is the ratio of the change in stress applied to change in the strain.

The blue dashed line represents a construction line parallel to E offset from the

origin to determine the yield strength, YS. We presume that if loaded below the

yield strength the material will recover most of the strain after the load is removed. True elastic deformation will exhibit a linear stress-strain relationship.[36] After the material yields more significant plastic deformation occurs and permanent deformation remains even after the load is removed. In

Figure 6, the U is the Ultimate Tensile Strength (UTS,) the maximum force applied to the material during the test. In some ductile materials, the specimen will experience concentrated plastic deformation in the gauge length area and the cross-section becomes smaller, or “necks”. When necking occurs, the specimen experiences the most concentrated amount of stress in that region and this is where rupture occurs. The F represents the breaking strength. Brittle materials do not experience necking and the UTS and F are nearly coincident in brittle materials.[36] c. Calculations

The original gage mark lengths (Lo), cross-sectional area (Ao), width and thickness need to be recorded. After the specimen has been tested, the gage length and area after the break (LBr and ABr) are recorded. These values are used to determine the strain, stress, elongation, reduction of area, yield strength (YS), and the ultimate tensile strength (UTS), all of which are shown in Figure

6 above. Below are the calculations required to find the stress and strain the material experienced at any point on the curve. [36]

퐿−퐿 strain = 휀 = 표 퐿표

퐹 stress = 휎 = (F= Force) 퐴표

Using the stress equation, the UTS can be found by replacing the F (force) with the maximum applied load (Fmax):

퐹 푈푇푆 = 푚푎푥 퐴표

The breaking strength can be found by manipulating the stress equation replacing the F (force) with the load experienced at the breaking point (FBreak).

퐹푏푟푒푎푘 퐹퐵푟 = 퐴표

To find the Elastic Modulus (or the slope of the linear portion of the curve) the following equation employed.

휎 = 퐸휀

The last equations are used to calculate the elongation and reduction in area.

퐿 − 퐿 % 퐸푙표푛푔푎푡푖표푛 = 퐵푟 표 푥 100% 퐿표

The area's reduction percentage can be calculated by subtracting the original area

from the breaking area and then divided by the original area.

퐴 − 퐴 % 푅푒푑푢푢푡푖표푛 푖푛 푎푟푒푎 = 표 퐵푟 푥 100% 퐴표

Lastly, the 0.2% offset yield strength can be found by drawing a line parallel to

the linear slope starting where the stress is zero and the strain value is 0.02% on

the stress-strain curve.

2.3 The Dehydration Process

When working with recycled plastics it is possible that they can absorb an excessive amount of moisture that can drastically affect the filament properties. It is the case that not all plastics react the same to moisture levels, however. Figure 7 shows the different classifications of how polymers will behave with moisture. The first column represents the hydrophobic materials such as polyethylene. Hydrophobic plastics are commonly non-polar materials, which have no affinity for water. Research shows that exposing polyethylene to water is like placing oil in water; the two substances have nothing in common chemically and immediately separate. [40] The next two columns in Figure 7 include polymers classified as hygroscopic or plastics with some polarity, allowing moisture to be absorbed. These plastics go through a process known as hydrolysis, which can cause irreversible structural damage by breaking down the covalent bonds in the polymer chain, resulting in a change to the molecular architecture, a recution in weight

and change in mechanical properties. There are two varieties of hygroscopic plastics; the first are polymers that will only have cosmetic problems when exposed to too much moisture. The second type of polymer will have performances (or mechanical property) issues that can cause structural damage.

Figure 12: Polymers interaction with moisture [40]

Since this study focuses on PET and PLA, it is important to note that moisture can cause structural damage impacting extruding and 3D printing the plastic. Drying the plastic before the extrusion process is a crucial step since moisture levels of 0.07% can be detrimental. Ideally, these plastics should not be processed at moisture levels higher than

0.02% for optimal extrusion and printing. Figure 8 is a graph exhibiting the heat flow versus the temperature of PET plastic in order to prevent agglomeration (lumping of the polymer pellets or shreds [40]). The graph indicates the correct process to prevent the agglomeration of PET during the heating process using constant agitation of the material.

This helps avoid the PET agglomerating into one large mass that would result in an unusable filament. [40]

Figure 13: Agitating PET to prevent agglomeration [40]

Figure 8 shows PET's reaction when agitate. Although a similar figure was not available for PLA, the other material used in this study, has a structure that also tends to attract water molecules. [41]

What was helpful as these experimental processes was pursued was knowing when the

PLA had absorbed moisture. According to the article “How to Dry Your PLA Filament”

[42] there are three signs of excessive moisture content.

1. When the filament turns from ductile to brittle. This is problematic because

brittle filament snaps more easily which is not a desirable quality when using

the material to 3D print.

2. A hissing noise that occurs during the printing. This is caused by entrapped

moisture that expands and becomes steam that is that is trying to be released.

3. The filament is oozing out instead of printing, creating a blob of plastic at the

extrusion tip. This is caused by the filament expanding during the printing

process, which creates excess pressure forcing molten filament to ooze out.

To make sure these problems never occur, the filament is stored in a sealed container or vacuum sealed in an airtight bag in a cool dry space. [43]

2.4 Similar Work

2.4.1 Work with PLA

A number of previous studies have bee conducted on similar materials ‘A Comparison between mechanical properties of specimen 3D printed with virgin and recycled PLA’ from the Department of Industrial Engineering, Fraunhofer was useful and also proposed utilizing recycling to reduce the negative environmental impact of 3D printed waste from campuses. [44]

The authors accumulate PLA plastic manufactured at 200 °C using a 3D printer 0.4 mm nozzle. With this plastic, they printed a set of virgin 3-point bend test specimens that were tested, ground up, and re-extruded into a new set of bend test specimens. These were then recycled and tested once again. A schematic of the three-point bend test

(center-loaded with the two support points) is shown in Figure 9.

Figure 14: Schematic representation of experimental setup [44]

.75 푥 푃 The shear stress was then calculated through the following equation: 퐹 = . The 푏 푥 ℎ results are:

Table 3: Results of testing specimens

PLA Filament Short-beam strength (MPa)

Virgin 119.1 ± 6.6

One time-recycled 106.8 ± 9.0

Twice recycled 108.5 ± 9.9

Three times recycled 75.0 ± 16.2

These results showcase the minor change from the virgin plastic to both the one-time and twice recycled filament. These results show that the three-time recycled filament drastically decreased in strength contrasted to the other specimens. [44] This study provided insight into the potential methodology, calculations and expected results from this work.

2.4.2 Work with PET

Another study focused on creating recycled filament out of PET and PETG. PET is used to create plastic water bottles and is known to have a very low viscosity when heated to

the working temperature, meaning it will have lower resistance to shear and shear easily, causing the molecules to flow quickly. [45] The investigation reported that an additive was needed to create a usable, high viscosity filament out of PET. PETG was chosen since it contains the extra glycol needed to create a more stable, less brittle, and easier to use filament. This work manufactured two filaments: the first used 100% virgin PETG pellets for a baseline plastic for comparison, and a 50-50 blend of both the recycled PET and virgin PETG pellets. After cutting the PET plastic into small pieces, the author heated both the shredded PET plastic with the PETG pellets at 100 o C for 2 hours to ensure all the moisture was removed before the extrusion process. A custom extruder was used to extrude the 100% virgin PETG, the 50-50 blend, and for comparison, the 100% recycled PET at approximately 245 oC. PET bottles are normally made through injection molding because of their low melt viscosity. Because of this, it is harder to extrude PET in a molten state since only a few degrees can change the material from liquid to solid making it a challenge to extrude. [46] Figure 10 shows images of the 100% PET and the

50-50 blend of PET and glycol fibers to give an idea of the low viscosity (and flexible manners) that PET has without the glycol additive.

Figure 15: a. 50-50 Blend of recycled PET and PETG

b. 100% PET plastic showing low viscosity [21]

Recommendations from this study were:

1. The extruded filament is a more consistent and cylindrical shape after mixing and heating the recycled PET and PETG before the process.

2. If the extruded fiber exhibits low viscosity during the extrusion process, the extrusion speed may be lowered to create a more desirable filament will be created. [21]

2.5 Machinery

Through shredding, cleaning, and then extruding recycled plastic, the recycled polymer material properties are likely to be similar enough to the virgin polymer material properties to make a usable product. The following machines are essential to recycle the polymer: shredder, dehydrator (or some sort of heating device like an oven), extruder, and winder, some of which must be created. To efficiently study how plastic can be processed, and appropriately recycled to obtain a reusable filament for 3D printing, each of these processing steps to recycle scrap 3D printed PLA parts and PET water bottles must be examined.

2.5.1 The Shredder

The start of this process begins with collecting the scrap material and preparing it to be processed. The first step after cleaning the plastic, is to shred it into small pieces. A shredder based on the Precious Plastics design was constructed for this work. Precious

Plastics provided a primary outline of the shredder and encourages other designers to

modify it for their own best use. Below are images and drawings of the original shredder design by Dave Higgins, the creator of Precious Plastics.

Figure 16: Shredder built by Precious Plastics founder, Dave Hakken [19]

Figure 17: Precious Plastics blueprint for machine[35]

Precious Plastics provides information and static CAD models. To determine the best blade mechanism for optimal shredding a new model that could be moved and manipulated was needed. A SolidWorks model was developed for this study. After modeling was optimized, the parts were acquired, and the shredder was built. Below are images of both the CAD model and fully assembled shredding unit.

Figure 18: CAD Model of shredder component (inside)

Figure 19: Actual shredder

After the shredding component was completed, a motor was installed to automate the process. Precious Plastics supplies instruction on what requirements are needed for the power source.

Table 4: Motor Proposals from Precious Plastics Website [47]

Motor Proposals

Suggested Suggested Suggested Wattage RPM Torque (kW) (N.m)

2.2 16 1200 Smallest suggested motor, will be fine for small household plastic, but the motor will limit the operation of the shredder \ 3 18 1500 Probably a good price/efficiency compromise 4 18 2000 You will be able to operate the shredder to its full

capacity 5.5 22 2300 Very long-life motor as you will be running the motor under its capacity; might be worth it for a high

productivity shredder (higher speed to limit the torque)

A motor with 2.2 kW and 1200 N.m torque was selected. For versatility, a crank was printed to use as manual power for the shredder. Below are the images of both the CAD model and crank attached to the shredder.

Figure 20: SolidWorks model of crank Figure 21: Printed crank

Some other components needed to facilitate use of the shredder included a stand and a hopper. After these were built and the motor was attached, the shredder was finally completed. In total, the shredder cost $400.

Figure 22: Fully built shredder

2.5.2 Filastruder and Filawinder

The next step in the project was to find a way to extrude the newly recycled PLA, and

PET plastic. The Filastruder was an affordable option and allowed for the recycled filament to be created at home. Below is a picture of the entire extruder.

Figure 23: Filastruder (extruder)

Along with the extruder, a filament winder was purchased to create spools of filament for easy use and storage. The price of the two devices was approximately $200.

Figure 24: Filawinder

2.5.3 The Dehydrator

As mentioned in section 2.5, dehydrating plastic is essential when creating good 3D printing filament. The dehydrator used for this study is the Weston 4-tray food dehydrator. A food dehydrator is a cost-effective option compared to a filament dryer which can cost well over $100 and is great for maintain low temperatures for long periods of time. [48]

Figure 25: Weston 4-Tray food dehydrator [49]

2.5.4 Ender 3D Printer

The Ender 3 is a 3D printer created by the Creality 3D Company using FDM (fused deposition modeling) technology. This unit has a hotbed that can reach 110 ° C and a Y- rail mounting groove that allows for precise positioning and a stable frame. [50]

Figure 26: The Ender Three 3D pinter [50]

2.5.5 UTM (Universal Testing Machine)

The Universal Testing Machine will be the primary testing tool for this experiment.

UTM’s are utilized in material testing to carry out controlled tests to determine different materials' characteristics and properties. These tests consist of applying pressure, compression, or tension to generate data that can be analyzed to determine material characteristics related to strengths and deformation. [51]

Figure 27: Universal testing machine [51]

3. EXPERIMENT

3.1 Experimental Overview

After finishing the machinery build, tensile test specimens were printed and experiments were conducted with virgin PLA, virgin PLA/ recycled PLA mix, virgin PETG-recycled

PET mix, and recycled PET. The compositions with a mix of plastics were first measured out by weight and combined after. Below is a table of the final compositions chosen to be evaluated:

Table 5: Composition of testing Specimens Composition Specimen Recycled PLA Virgin PLA Recycled PET Virgin PETG

1 100% 2 50% 50% 3 50% 50% 4 100%

A 3D model of a standard dog-bone tensile-test sample was created. This model was then put into Cura, a 3D printing platform that helps modify Solidworks models into the necessary .gcode for 3D printing. Below is an image of the Solidworks model based on the ASTM D638 Type 1 specimen design for polymer testing. [37]

Figure 28: ASTM D638 Type 1 testing specimen [37]

This specimen has an overall length of 165 mm, a width of 13 mm, and a thickness of 3.2 mm and weighs approximately 15 grams. Below is an image of the specimen in

SolidWorks.

Figure 29: SolidWorks image

The steps used to create the tensile test specimen are described below:

1. Collect the recycled plastic: As stated above, the two primary plastics this

experiment used were PET and PLA. The focus on these plastics was because of

their presence on any campus, providing an easy collection process for plastic

water bottles (PET) and unused or wasted filament and scrap parts (PLA) from

3D printing classes.

2. Shred the plastic: As shown in section 2.5 Machinery, the plastic shredder with a

motor was used. A mesh was used to ensure that the final batch of shredded

plastic were of small, and consistent size (approximately 4mm in diameter) to

facilitate the melting and mixing before the extrusion process.

3. Mix the plastics: Mixing the plastic before the dehydration process, ensured that

there was little to no separation of the particles, which would cause inconsistency

in the final material including variations in viscosity and shape when extruding

the plastic. An equal amount of each plastic by weight was measured to ensure an

even blend and was mixed by hand.

4. Dehydrate the plastic shreds: Removing moisture from the material is a very

crucial step. Dehydrating the plastic, takes out the excess moisture that might

cause serious performance issues when trying to extrude and print. The food

dehydrator mentioned in section 2.7.3 was used.

5. Extrude and wind filament: Each plastic and plastic blend required different

temperatures for extrusion of a successful filament product (after extrusion, some

blends needed additional dehydration to ensure no negative consequences from

moisture).

6. 3D print specimen for testing: Figure 25 shows the final tensile specimen

images; 3D printed specimens ready for testing in the UTM.

7. UTM Testing: Once the specimens were prepared, they were tested for different

tensile properties in the UTM using standard protocol.

3.2 Test 1: Virgin PLA

3.2.1 Introduction and Goals

The purpose of testing a testing specimen with virgin PLA was to have a control specimen to compare with the rest of the tested specimens. The filament used was a commercial filament Overturned PLA.

3.2.2 Procedure For the virgin PLA testing specimen, the filament was purchased. The filament was then dehydrated for best results and printed at the following temperature.

Table 6: Testing Specimen 1: Virgin PLA extrusion temperature Specimen Desired Printing Dehydration Time Temperature Virgin PLA 205°C 2 hours

3.3 Test 2: Recycled PLA (50% recycled PLA, 50% virgin PLA pellets)

3.3.1 Introduction and Goals

For the filament's best results, a 50/50 blend was created with virgin pellets that are easier to mix and extrude with the printed PLA recycled shreds. This specimen allowed for experimentation on various extrusion temperatures and speeds to create the best filament.

The expected outcome is for little change in the material strengths compared to the virgin

PLA since this specimen is a 50/50 blend.

3.3.2 Procedure

To create these specimens, 3D printing scraps were collected, shredded, and mixed with virgin PLA pellets acquired commercially. There were 60 grams of scrap material and 60 grams of virgin material, equaling 120 grams for extrusion. The plastic was dehydrated and extruded at three temperatures in the range shown in Table 10.

Table 7: 50/50 PLA Blend Extrusion temperatures and dehydration time

Specimen Extrusion Temperature Range Dehydration Time

50/50 blend Three temperatures in the 12 hours 205°C -245°C range

Observations were conducted during the extrusion of the filament by looking at three different temperatures (205°C, 225°C and 245°C) and noting how the filament extruded.

The filament was further dehydrated for 12 hours to ensure no moisture content was detected that could ruin the print. Once the filament was used for printing, the tensile test specimen created was tested in the UTM, and the material properties compared to the virgin PLA results.

3.4 Test 3: PETG (50% PET, 50% PETG virgin pellets)

3.4.1 Introduction and goals

The collection of the PET plastic took slightly longer as only properly recycled plastic water bottles were used. This allowed for a more accurate representation of using recycled water bottles to create new 3D prints. A 50/50 blend was first created to understand best temperatures and speed for the PET's extrusion.

3.4.2 Procedure

The water bottles were first washed and soaked overnight to remove the label and eliminate any leftover imperfections. Subsequently, the bottles were dried, shredded, and

85 grams of material was measured. To this, 85 grams of PETG pellets were added to create the 50/50 blend. Once the materials were mixed, they were extruded at a range of four temperatures shown below and discussed in the analysis section.

Table 8: 50/50 PETG Blend temperatures and dehydration times

Specimen Desired Extrusion Total Dehydration

Temperature Time 50/50 PET-G Blend Four temperatures in 24 hours

the 215°C -265°C range

PET is a hygroscopic plastic, meaning it will hold onto moisture more easily than other plastics, which can cause extreme material property deterioration. For this reason, the filament created was dehydrated for 24 hours to ensure the best material properties before it went through the printing process to create the testing specimen.

3.5 Test 4: 100% PET

3.5.1 Introduction and Goals

To showcase that it is achievable to use recycled plastic water bottles as an alternative to the virgin filament in classrooms, a specimen containing 100% recycled plastic was tested. This analysis will show that recycled PET has little changes in the material strength compared to its predators and potential 3D printed models.

3.5.2 Procedure

This PET mixture was created from the same batch of plastic water bottles as described previously. This mixture was dehydrated for 15 hours before the material went through extrusion since PET can easily absorb moisture which would contaminate the prints.

After dehydration, the plastic was extruded at three temperatures in the 235°C - 275°C

range (235°C, 255°C, 275°C) to determine which temperature produced the best filament.

In Table 12 are the temperature ranges and dehydration used for processing the PET plastic. Once the filament was created, it was dehydrated for another 36 hours.

Table 9: 90/10 PET-G Blend temperatures and dehydration times

Specimen Desired Extrusion Dehydration Time Temperature

90/10 PET-G Blend Four temperatures in 51 hours the 235°C -265°C range

4. RESULTS AND ANALYSIS

4. Due to a limited amount of time available in the materials testing lab at Sacramento

State University (Covid-19 pandemic related), only one specimen of each material was tensile tested. Because of this, the following results are only preliminary, but do reflect the potential of recycled filament in laboratories. The specimens that were created were stored in a vacuum sealed bag (similar to the filament after it was created) to help combat any moisture absorption. Each specimen was tensile tested in a UTM machine with the results shown below.

4.1 Test 1: Virgin PLA

4.1.1 Test Specimen Model

As stated in the experiment portion of the paper (section 3.1), the specimen was printed with the ASTM D638 Type 1 model's measurements. Below is an image of the testing specimen after the printing process (Figure 28 is the PET plastic specimen and is being showcased instead of the virgin PLA plastic because it is easier to see the infill pattern).

Figure 30: Testing specimen printed with crisscross pattern

As shown in Figure 30, the infill was at 100% to better measure the strength of the material; it was printed in a linear pattern. In Figure 31 the load versus displacement

graph shows the max force that the virgin specimen was able to withhold before breakage.

Figure 31: Load vs. displacement graph with virgin PLA

As seen above, the specimen (Figure 30) exhibited a maximum load of 348.49 lbf. This resulted in a UTS of 6343.45 psi (43.74 MPA). This specimen did not exhibit much ductility. Brittle characteristics are common with 3D printing PLA filament, however, there are some other explanations for this behavior. The moisture content of these specific specimens was a possible issue; the linear infill pattern was printed and only dehydrated for a few hours.

4.1.2 Tensile Test Results

Below are the results, the stress strain curve, and tables of the material properties for all other materials tested using the same 100% infill and linear pattern to produce the specimens.

Table 10: Specimen lengths/areas before and after break for virgin PLA

Specimen LO (original LBr (length AO (original ABr (area after length in after break in area in break in

inches) inches) inches) inches) Virgin 2.0 푖푛 2.09 푖푛 0.055 푖푛2 0.0376 푖푛2 PLA

Table 11: Values from the stress-strain curve for virgin PLA calculations

Specimen Max Load experienced at Elastic Modulus Load Yield Strength (lbf) Values (lbf) (Stress, Strain) Virgin 348.89 238.73 1: (3480.89 psi, 0.02) PLA lbf 2: (1476.12 psi, 0.0061)

Figure 32: Virgin PLA stress-strain curve

Calculations:

퐹푚푎푥 348.89 푙푏푓 푈푇푆 = = 2 = 6343.45 푝푠푖 퐴표 .055ⅈ푛

238.73푙푏 0.2%푌푆 = = 4583.35 푝푠푖 .055ⅈ푛2

3480.45 푝푠푖 − 1476.12 푝푠푖 퐸푀 = = 144196.4 푝푠푖 . 02 − .0061

2.09 푖푛 − 2.0 푖푛 %퐸퐿 = = 4.5% 2.0 푖푛

0.0486 푖푛2 − 0.045 푖푛2 %푅퐴 = = 7.4% 0.0486 푖푛2

Table 12: Mechanical properties found from virgin PLA test

Property Experimental Value Elastic Modulus, E (psi) 144196.4 0.2% Offset YS (psi) 4583.35 UTS (psi) 6343.45 % Elongation 4.5 % % Reduction in Area 7.4 %

The above chart values are used as the comparison values for the rest of the polymers being tested.

4.2 Test 2: Recycled PLA (50% recycled PLA, 50% virgin PLA pellets)

4.2.1 Extrusion Results

The mix of PLA was first extruded at 205°C mimicking the temperature used for printing the virgin PLA. At the temperature the filament exhibited lumps (presumably pieces of the recycled scraps) the temperature was raised to 225°C. After raising the temperature, the filament exhibited less bumps but still was not at a smooth consistency. A final addition of 20°C made the temperature 245°C resulted in a usable filament (Figure 34).

The filament produced was dehydrated for 12 hours and printed at 245°C.

Figure 33: Recycled PLA filament test

4.2.2 Tensile Test Results

The recycled PLA blend results are shown including the stress-strain curve, tables of material properties, and an image of a broken specimen.

Table 13: Specimen lengths/areas before and after break for recycled PLA

Specimen LO (original LBr (length AO (original ABr (area after length in after break in area in break in inches) inches) inches) inches) Virgin 2.0 푖푛 2.06 푖푛 0.05 푖푛2 0.05 푖푛2 PLA

Table 14: Values from stress-strain curve for recycled PLA calculations

Specimen Max Load experienced at Elastic Modulus Load Yield Strength (lbf) Values (lbf) (Stress, Strain)

Virgin 348.89 238.73 lbf 1: (3480.89 psi, 0.02) PLA lbf 2: (1476.12 psi, 0.0061)

Figure 34: Testing specimen -50/50 recycled PLA and virgin PLA

Figure 35: 50/50 recycled PLA stress-strain curve

Calculations:

퐹푚푎푥 234.32 푙푏푓 푈푇푆 = = 2 = 4686.8 푝푠푖 퐴표 .05ⅈ푛

196.70 푙푏 0.2%푌푆 = = 3934.09 푝푠푖 .05ⅈ푛2

2408.99 푝푠푖 − 939.62 푝푠푖 퐸푀 = = 182530.43 푝푠푖 0.0106 − 0.00255

2.06 푖푛 − 2.0 푖푛 %퐸퐿 = = 3.0% 2.0 푖푛

0.055 푖푛2 − .05 푖푛2 %푅퐴 = = 9.1% 0.055 푖푛2

Table 15: Mechanical properties found from e PLA test

Property Experimental Value Virgin PLA % Diff. Elastic Modulus 182530.43 144196.4 -26.58% (psi) 0.2% Offset YS 3934.09 4583.35 14.17% (psi) UTS (psi) 4686.80 6343.45 23.25% % Elongation 3.0% 4.5% 33.3% % Reduction in (-) 9.1 % 7.4% Area 22.9%

As seen in Figure 35, the recycled PLA specimen did not fully rupture but instead slightly pulled apart. The breakage area showed little adhesion in the horizontal aspect, however, the vertical threads showed strength resulting in an acceptable UTS value.

Table X shows the UTS of the recycled PLA mixed specimen maintained 73.84% of the

UTS properties found in the virgin PLA specimen. Additionally, the 0.2% yield strength of the recycled PLA was only 14.17% lower than the virgin PLA specimen. It is also noted that the elastic modulus of the PLA is 26.58% lower than the recycled PLA plastic.

This test shows the ability of recycled 3D printing scraps to be reused to reprint for class projects since maximum strengths are rarely needed.

Using the method described in this work to collect filament and 3D printed scraps helps universities become zero-waste campuses and can also result in saving money.

4.3Test 3: PETG (50% PET, 50% PETG virgin pellets)

4.3.1 Extrusion Results

The PET and PETG blend was first extruded at a temperature of 215°C which produced an unsmooth finish. The temperature was then raised by 20°C to 235°C but still had the unsmooth finish. The temperature was raised 30°C to 265°C however that caused the filament to turn a darker color indicating it was slowly burned. The temperature was lowered 10°C to 265°C resulting in a filament that was smoother and more consistent.

Figure 36: PETG filament extruded at 265°C

Figure 37: Spool of PETG filament

4.3.2 Tensile Test Results

The recycled PLA blend results are shown including the stress-strain curve, tables of material properties, and an image of a broken specimen.

Table 16: Specimen lengths/areas before and after break for PET

Specimen LO (original LBr (length AO (original ABr (area after length in after break in area in break in inches) inches) inches) inches)

2 2 Virgin 2.0 푖푛 2.16 푖푛 0.0612 푖푛 0.0561 푖푛 PLA

Table 17: Values from stress-strain curve for virgin PETG calculations

Specimen Max Load Load experienced at Elastic Modulus Values (lbf) Yield Strength (lbf) (Stress, Strain) Virgin 348.89 lbf 238.73 lbf 1: (2167.06 psi, PLA 0.02295)

2: (878.57 psi, 0.005)

Figure 38: The broken PETG specimen

Figure 39: PETG stress-strain curve

Calculations :

퐹푚푎푥 286.24 푙푏푓 푈푇푆 = = 2 = 4677.12 푝푠푖 퐴표 .0612ⅈ푛

172.96 푙푏 0.2%푌푆 = = 2826.19 푝푠푖 .0612ⅈ푛2

2167.06 푝푠푖 − 878.57 푝푠푖 퐸푀 = = 71782.17 푝푠푖 0.02295 − 0.005

2.16 푖푛 − 2.0 푖푛 %퐸퐿 = = 8.0% 2.0 푖푛

0.0612 푖푛2 − 0.0561 푖푛2 %푅퐴 = = 8.33 % 0.0612 푖푛2

Table 18: Mechanical Properties found from PETG test

Experimental Value Virgin PLA Property % Diff. (psi) (psi) Elastic Modulus 71782.17 144196.4 50.22% (psi) 0.2% Offset YS 2826.19 4583.35 38.34% (psi) UTS (psi) 4677.12 6343.45 26.32% (-) % Elongation 8.0% 4.5% 77.78% % Reduction in 8.33 % 7.4% (-)12.5% Area

The first notable characteristic of the PETG specimens was the quality of the actual print.

This filament was the hardest to work with and the 3D printed sample had small holes and a rough finish. The filament was printed in a wide array of temperatures but finally became adhesive at 265°C. The specimen (materials properties detailed in Table 21) sustained 73.68% of the UTS strength of the virgin PLA specimen. The PETG 0.2% offset yield strength was not as high being 38.34% lower than the PLA filament. The yield strength was significantly lower presumably because of the materials' lack of adhesion, making the specimen more fragile because it lacked cohesion. This property can also be seen through the significant difference in elastic modulus values as the PETG which has a value 50.22% lower than the virgin PLA since the PETG specimen showed signs of breakage at such early stages of the test. The percent elongation was almost double the control sample because the specimen’s strands stretched instead of broke. The

PETG showed significant UTS characteristics but had a vastly different elastic modulus

The lack of cohesion in the sample, signs of breaking early in the tensile test, and the varied tensile test results indicate that the PETG mixture needs more investigation.

4.4 Test 4: 100% PET

4.4.1 Extrusion Results

Creating the 100% PET filament needed patience. At first, the filament was extruded at

235°C which was resulted in a filament that was noticeably chunky with nonmelted PET pieces. The temperature was increased by 20°C to 255°C and showed less chunks but still had a foggy tint. The temperature was raised another 20°C to 275°C and a smoother and consistent filament was the result.

Figure 40: Top: extrusion at 235°C; Middle: extrusion at 255°C; Bottom: extrusion at

275°C

Figure 41: The PET filament created

4.4.2 Tensile Test Results

The recycled PLA blend results are shown including the stress-strain curve, tables of material properties, and an image of a broken specimen.

Table 19: Specimen lengths/areas before and after break for virgin recycled PLA

Specimen LO (original LBr (length AO (original ABr (area after length in after break in area in break in inches) inches) inches) inches)

PET 2.0 푖푛 2.08 푖푛 0.055 푖푛2 0.051 푖푛

Table 20: Values from stress-strain curve for virgin PET calculations

Specimen Max Load Load experienced at Elastic Modulus (lbf) Yield Strength (lbf) Values (Stress, Strain)

PET 246.47 lbf 214.44 lbf 1: (2505.33 psi, 0.0184) 2: (1019.19 psi, 0.00415)

Figure 42: Testing specimen -100% PET broken after test

Figure 43: Virgin PLA stress-strain curve

Calculations:

퐹푚푎푥 234.32 푙푏푓 푈푇푆 = = 2 = 4481.27 푝푠푖 퐴표 .05ⅈ푛

196.70 푙푏 0.2%푌푆 = = 3934.09 푝푠푖 .05ⅈ푛2

2505.33 푝푠푖 − 1019.19푝푠푖 퐸푀 = = 104290.53푝푠푖 0.0184 − 0.00415

2.08 푖푛 − 2.0 푖푛 %퐸퐿 = = 4.0% 2.0 푖푛

0.055 푖푛2 − 0.051 푖푛2 %푅퐴 = = 7.27% 0.055 푖푛2

Table 21: Mechanical properties found from PET test

Property Experimental Value Virgin PLA % Diff. Elastic Modulus 104290.53 144196.4 27.68% (psi) 0.2% Offset YS 3934.09 4583.35 14.93% (psi) UTS (psi) 4481.27 6343.45 29.36% % Elongation 4.0% 4.5% 11.11% % Reduction in 7.27% 7.4% 1.75% Area

The PET filament was printed with a high temperature since that was required for the

PETG filament. To print a cohesive specimen, the filament was printed at 270°C. This resulted in a solid print with good mechanical properties. As seen in Table 24, the PET material exhibited a UTS value that was 70.64% of the UTS strength shown in the virgin

PLA specimen. The addition of glycols helped create a more consistent filament but resulted in a more fragile and less adhesive material. Along with the recycled PLA, 100%

PET seems to be a suitable replacement for virgin PLA in 3D printing classes on a university campus.

4.5 Adhesion and printing

4.5.1 Problems and solutions with adhesion

A 3D printed product can be printed with many different settings, all of which can contribute to the overall strength and adhesion of the final product. However, it may be hard to figure out which settings to change based on the different problems that can

occur. To help pinpoint the solutions, here are some problems observed while printing with the recycled filament produced for this study:

1. Stringing and oozing can be identified when the printed product is left with

strings between parts of the print or the top layers of then print. This can happen

when the product is printed at too high of a temperature or there is not enough

retraction of the filament while printing. To combat these issues simply decrease

the temperature in increments of 5°C and increase the retraction speed and

distance traveled. [52] In this study, while printing with the recycled PETG

material stringing and oozing occurred. This happened because the assumption

was that the temperature should be higher while printing since recycled material

was being used in the filament. This was not the case required and the

temperature was lowered to combat this problem.

Figure 44: Stringing on the top surface of a PETG print

2. Layer separation is when the print appears to have clean cuts, or gaps. This can

happen when the layers of the print are cooling at different rates causing one layer

to be completely cool before the next layer is applied which causes separation.

The easiest way to fix this problem is to increase the temperature of the print. [52]

When working with the PETG filament, increasing the temperature by 10°C helped the layer adhesion significantly.

Figure 45: Poor vertical adhesion Figure 46: Good layer adhesion

4. Prints sticking too much to the bed of the 3D printer is a result of either the

bed temperature being too high, or the material being printed is from a more

flexible filament. This can be a problem when trying to take the part off the

bed because some of the mat might be removed and the part may potentially

break while trying to separate it from the mat. To address this issue, reduce

the temperature in increments of 5°C until adhesion to the bed is appropriate.

If the problem still occurs it may be dependent on the type of filament being

used. [52] For example, the recycled PET filament is known to have a more

flexible quality which causes it to stick to the beds surface as shown in Figure

43. To help the recycled PET filament not stick as much, the temperature of

the bed and the temperature of the extruder on the first layer were reduced in

increments of 5°C until the product no longer stuck to the bed.

Figure 47: Image of PET specimen with part of mat sticking to the part

Figure 48: Image of PET specimen with the mat not sticking

4.5.2 Why adhesion, printer settings, and filament quality are important

As discussed in section 4.5.1, there are many factors that can contribute to insufficient adhesion with 3D prints. Adhesion is an important part of 3D printing and can affect not only how the part looks but also how strong the part is. Having a part that has layer separation, stringing and oozing or is sticking to the mat are all issues that cause a print to be weaker and unusable. Having the printing setting correct can alter the strength of the final product but is only a part of the solution. The other factor that can affect the final part is the quality of the filament being used. When purchasing filament, customers look to determine if the filament is easy to print, the printed strength and the finish quality.

[53] By testing the filament created, this preliminary study demonstrated that the recycled filament has required qualities such as the print strength.

To test the importance of the printer settings and the quality of filament, below is a simple cone 3D printed with the recycled PET filament. At first, the cone showed layer

separation, and poor adhesion. By increasing the printing temperature in all the layers

(except for the first so that the print does not overly stick to the bed) from 260°C to

270°C The new print came out as a solid product as seen in Figure 45. Print settings are important and can change a final product from unusable to usable.

Figure 49: Poor adhesion in print Figure 50: Better adhesion with print

5. FUTURE WORK

As mentioned previously, due to effects of the Covid-19 pandemic, there was limited lab testing time, so these tests results are only a preliminary study for recycled filament being used in labs. Other studies can examine the ways that infill patterns and densities can also impact the material strengths of 3D printed parts. Besides studying material properties of

3D prints, research can be done in the biopolymer field. Polymers are being tested with different types of biomaterials such as wood and biochar for their excellent retention of mechanical properties while, creating a specimen that is half the weight. These biomaterials are being added to PLA and a new material that can withstand a more extended amount of time outdoors and extreme temperatures is being developed. By testing different biochar and other biomaterials, new and improved additives can be discovered to reduce plastic waste in standard building techniques. To continue the experimental portion of recycled polymers, experiments such as TGA (thermal gravimetric analysis) testing and SEM (scanning electron microscope) would help expand the knowledge of different properties recycled polymers have.

6. CONCLUSION

As shown in this preliminary study on recycled filament used in 3D printing labs, 3D printer PLA filament has the potential to be efficiently replaced by materials such as filament made from recycled PLA printing scraps and PET water bottles. In this study, the recycled PLA printed specimen exhibited only a minimal reduction of the mechanical behaviors the virgin PLA exhibited. Additionally, PET retained 70.64% of the mechanical behaviors shown in the PLA filament tested. These results were from samples created with the filament printed in a linear pattern that exhibited brittle properties. The brittle properties resulted in a higher maximum stress but with a smaller final strain. This characteristic is customarily found in PLA sample. An important material quality from processing both PLA and PET is that they easily absorb and retain moisture and need to be dehydrated to obtain the most desirable properties. The test materials were dehydrated, but the dehydration process needs to be longer to create a more robust material.

Filament recycling would not only aid California State University, Sacramento in becoming a zero-waste campus but would also reduce the amount of money spent buying commercially produced filament. A regular spool of 1000 grams Overturned PLA on

Amazon costs $40.99. If a regular lab wasted 200 grams of filament per spool through failed prints or scraps, that would be a waste cost of $8.20. If 25 spools a semester are purchased it would cost a total of $1,025. By reusing printing scraps, (estimated total of

5,000 grams), that would be equivalent to 5 spools or $205, saved a semester. Of course,

this does not include the cost of labor and electricity to produce the filament, but it does remove the 500 grams of plastic in the waste stream.

It should also be noted that one water bottle weighs approximately 4 grams, (each tensile test specimen was 3.75 water bottles). If a whole spool were made of recycled PET, it would save 250 water bottles from entering the waste system. Lastly, based on the survey given to random Sacramento State students, education in the realm of plastic waste is needed. 55% of the students surveyed believed that 68% and less of plastic was recycled incorrectly when, in fact, around 91% of plastic is recycled incorrectly. The survey also revealed that 55% of students work with plastic in a variety of classes (props that get thrown away, 3D printing scraps, safety glasses), resulting in plastic waste. This study shows students are not consciously aware of what is happening in the world or even at their campus. Sacramento State is trying to reach 90% below 1990 emission levels by

2035 (with the California goal of reaching 80% below 1990 levels in 2040) and student participation is essential to reach these goals.

. In conclusion, replacing virgin PLA filament with both recycled 3D print scraps, and recycled PET water bottles can create acceptable 3D print filament replacement and help towards cost savings and create a carbon-neutral campus.

APPENDIX A: Stress-Strain curve data collected for all tests

Table: Specimen lengths/areas before and after break

Specimen LO (original LBr (length AO (original ABr (area after length in after break in area in break in inches) inches) inches) inches)

Table 7: Values to take from Stress-Strain Curve Specimen Max Load Load experienced at Elastic Modulus (lbf) Yield Strength (lbf) Values (Stress, Strain)

Table 8: Material Property Chart Property Experimental Value Virgin PLA % Diff. Elastic Modulus

(psi) 0.2% Offset YS

(psi) UTS (psi) % Elongation % Reduction in

Area

APPENDIX B: Full Student Survey

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