Master of science in mechanical engineering June 2021
Faculty of mechanical engineering, Blekinge Institute of Technology, 371 79 Karlskrona, Sweden
Devulcanization and reuse of peroxide cured EPDM rubber for a greener world
Degree project for Master of Science in Mechanical engineering with focus on innovation and sustainable product development
Emelie Broman
This thesis is submitted to the Faculty of Engineering at Blekinge Institute of Technology in partial fulfilment of the requirements for the degree of Master of Science in Mechanical Engineering. The thesis is equivalent to 20 weeks of full time studies.
The author declares that she is the sole author of this thesis and that she has not used any sources other than those listed in the bibliography and identified as references. She further declares that she has not submitted this thesis at any other institution to obtain a degree.
Contact Information:
Author: Emelie Broman
E-mail: Embr16@student.bth.se
University advisor:
Senior lecturer Alessandro Bertoni Department of Mechanical Engineering
Faculty of Engineering Internet: www.bth.se Blekinge Institute of Technology Phone: +46 455 38 50 00 SE–371 79 Karlskrona, Sweden Fax: +46 455 38 50 57
Abstract
Waste management of vulcanized rubber is a serious environmental problem as the molecular structure formed during the process complicates recycling. Due to this, researchers have developed a process that can break the strong bonds and return the rubber to its original state. This process is called devulcanization and is developed for the tire industry due to the large amount of waste formed from this sector. This rubber is vulcanized with sulfurs like 90 % of other rubber products. However, there exist other products that are vulcanized with peroxides where much less research has been done. Due to this, it is unclear if devulcanization is possible as a different type of bond is formed in the vulcanization process. This work therefore investigates the possibilities with recycling by devulcanization of peroxide cured rubber where a collaboration is done with Roxtec International AB. The work also investigates the possibilities with reuse of the cooperating partner’s particular rubber and how everything can be implemented to understand the efficiency of these two waste management strategies and to share knowledge.
The used method for this work is a combination of the Participatory Action Research (PAR) and the Design thinking (DT) framework. A systematic literature review has also been conducted to collect relevant material for one of the research questions.
The result showed that there exist many methods and possibilities with devulcanization of peroxides if sufficient energy is provided. However, the quality of the reclaimed rubber will be lacking as the bonds in the polymers main chain are degraded which affects the mechanical properties. Due to this only a small percentage can be reused by mixing it with virgin rubber. The efficiency for peroxides is therefore lower than for sulfurs. Regarding reuse many alternative usage areas were brainstromed but once the concepts were evaluated against the limitations with the material, the needs and the sustainability factor, only few solutions survived. The winning concept became a punching bag filled with granulated rubber crumbs. When investigating how everything can be implemented it turned out that a lot of effort is needed. For example, to get favorable results, the solution will require the punching bag company to also want to work towards sustainability by adapting take back systems. Devulcanization will then require that the waste is sorted which requires a big investment in both time and money to achieve.
The conclusion of this work is that devulcanization and reuse is possible and the developed system solution can take care of parts of the waste. However, the possibilities are limited and to get rid of all the waste and develop a long-term solution that is both sustainable and further prepared for the future, more actions are needed. It is therefore of high interest to start investigating the possibilities with a new material that is easy to recycle or are degradable.
Keywords: Peroxide devulcanization, Reuse, Waste management, Participatory Action Research, Design Thinking
Sammanfattning
Avfallshantering av vulkaniserat gummi är ett allvarligt miljöproblem eftersom den molekylära strukturen som bildas under processen komplicerar återvinning. På grund av detta har forskare utvecklat en process som kan bryta de starka bindningarna och återställa gummit till sitt ursprungliga tillstånd.
Denna process kallas devulkanisering och är främst utvecklad för däck-industrin på grund av den stora mängden avfall som bildas från denna sektor. Detta gummi vulkaniseras med svavel som 90% av andra gummiprodukter. Det finns dock andra produkter som vulkaniseras med peroxider där mycket mindre forskning har gjorts. På grund av detta är det oklart om devulkanisering är möjlig eftersom en annan typ av bindning bildas i vulkaniseringsprocessen. Detta arbete undersöker därför möjligheterna med återvinning genom devulkanisering av peroxid härdat gummi där ett samarbete görs med Roxtec International AB. Arbetet undersöker också möjligheterna med återanvändning av samarbetspartners specifika gummi spill och hur allt kan implementeras för att förstå effektiviteten i dessa två avfallshanterings strategier och för att sprida kunskap.
Den använda metoden för detta arbete var en kombination av PAR (Participatory Action Research) och design thinking (DT). En systematisk litteraturstudie har också genomförts för att samla in relevant material för en av forskningsfrågorna.
Resultatet visade att det finns många metoder och möjligheter för devulkanisering av peroxider om tillräcklig energi tillhandahålls. Dock kommer kvalitén på det återvunna gummit att försämras markant då bindningarna i polymerens huvudkedja bryts vilket påverkar de mekaniska egenskaperna. På grund av detta kan endast en liten andel återanvändas genom att blanda med jungfruligt gummi. Effektiviteten för peroxider är därför lägre än för svavel. När det gäller återanvändning, brainstormades många alternativa användningsområden, men när koncepten utvärderades mot begränsningarna med materialet, behoven och hållbarhetsfaktorer överlevde bara ett fåtal lösningar. Det vinnande konceptet blev en slagsäck fylld med granulerat gummi. När det undersöktes hur allt kan implementeras visade det sig att det krävs mycket ansträngningar. Till exempel, för att få gynnsamma resultat kommer lösningen att kräva att boxsäck företaget också vill arbeta för hållbarhet genom att anpassa take back system. Devulkaniserings-processen kommer även kräva att avfallet sorteras vilket kräver en stor investering i både tid och pengar för att uppnå.
Slutsatserna av detta arbete är att devulkanisering och återanvändning är möjligt och den utvecklade systemlösningen kan ta hand om delar av avfallet. Men möjligheterna är begränsade och för att bli av med allt avfall och utveckla en långsiktig lösning som både är hållbar och ytterligare förberedd för framtiden, kommer fler åtgärder att behövas. Det är därför av stort intresse att börja undersöka möjligheterna med ett nytt material som är lätt att återvinna eller är nedbrytbart.
Nyckelord: Peroxid devulkanisering, återanvändning, avfallshantering, Participatory Action Research, Design Thinking
Acknowledgment
This thesis has included several people to be able to move forward in the project and I would therefore like to thank these people and departments for contributing.
First of all, I would like to thank Alessandro Bertoni and Johan Åberg who have been my academic and external supervisors. I would like to thank them for providing me with guidelines, vital information, tools and document reviews. Furthermore, I would like to thank Johan for participating in interviews and helping me come in contact with relevant people.
Then I would like to thank Pontus Danielsson who has also had an important role in the beginning of this thesis as he has helped me understand the material and the processes and taken part in many meetings.
Lastly, I would like to thank the included departments. I thank the production department for providing me with information and material to experiment and prototype with. Then I thank the sustainability department for information and rewarding discussions about various topics. At last, I thank Roxtec service for sharing information.
Table of content
Chapter 1 - Introduction 1
1.1 Initial problem definition 1
1.2 Survey of related work - Waste management 2
1.3 Aim of study 3
1.4 Delimitations 3
Chapter 2 - Theory 4
2.1 Rubber 4
2.1.1 Natural rubber (NR) 5
2.1.2 Styrene-butadiene rubber (SBR) 6
2.1.3 Ethylene Propylene Diene Monomer rubber (EPDM) 7
2.2 Vulcanization 8
2.2.1 Sulfur cured vulcanization 11
2.2.2 Peroxide cured vulcanization 11
2.3 Devulcanization 12
2.4 Sustainability 13
2.4.1 Circular economy 15
Chapter 3 - Method 16
3.1 Participatory Action Research (PAR) 16
3.2 Design Thinking (DT) 17
3.3 Research approach 18
3.3.1 Formulate the problem - Initiation 19
3.3.2 Planning - Initiation 19
3.3.2.1 Team canvas 20
3.3.3 Acquiring data - Inspiration 20
3.3.3.1 Understanding of the material 20
3.3.3.2 Stakeholder analysis 20
3.3.3.3 Interviews 21
3.3.3.4 Trendwatching - Data collection 22
3.3.3.5 Literature review - Data collection 22
3.3.4 Analysis of data 24
3.3.4.1 Persona and needs 24
3.3.4.2 Trendwatching - Analysis 25
3.3.4.3 Literature review - Analysis 25
3.3.5 Ideation 25
3.3.5.1 Brainstorming 25
3.3.5.2 Prototyping 26
3.3.5.3 Selection 26
3.3.5.4 Product service system 27
3.3.5.5 Testing 28
3.3.6 Implementation 28
3.3.6.1 Sustainability Principle Analysis 28
3.3.6.2 Implementation plan 29
3.3.6.3 Business model canvas 29
3.3.7 Reflection 29
Chapter 4 - Result 30
4.1 Planning-Initiation 30
4.2 Acquiring data/Analysis - Inspiration 31
4.2.1 Understanding of the material 31
4.2.2 Stakeholder analysis 33
4.2.3 Persona and needs 34
4.2.4 Trendwatching 36
4.2.5. Litteratur review 37
4.2.5.1 Reported Peroxide devulcanization 37
4.2.5.2 Analysis 1 44
4.2.5.3 Reported sulfur devulcanization 45
4.2.5.4 Analysis 2 54
4.2.5.5 Supercritical CO2 Devulcanization 58
4.2.5.6 Thermomechanical Devulcanization 60
4.2.5.7 Ultrasonic devulcanization 63
4.2.5.8 Analysis 3 66
4.3 Ideation 70
4.3.1 Brainstorming 70
4.3.2 Prototyping 72
4.3.3 Selection 78
4.3.4 Product service system 80
4.3.5 Testing 82
4.3.6 System solution 84
4.4 Implementation 85
4.4.1 Sustainability principle analysis 86
4.4.2 Implementation plan 86
4.4.3 Business model canvas 88
Chapter 5 - Reflection 90
5.1 Research methodology 90
5.2 Interviews and survey 91
5.3 Devulcanization 91
5.4 Prototyping 92
5.5 Reuse 93
5.6 Product service system 94
5.7 Implementation 94
5.8 System solution 95
5.9 Sources 96
Chapter 6 - Final remarks 97
6.1 Conclusion 97
6.2 Future work 98
References 99
Appendix 1 104
Appendix 2 105
Appendix 3 106
Appendix 4 110
Appendix 5 111
Appendix 6 115
List of figures
Figure 1: An example of what the studied product can look like and its included components………..……1
Figure 2: The chemical structure of natural rubber ………...5
Figure 3: The chemical structure of styrene-butadiene rubber ………...6
Figure 4: The chemical structure of Ethylene Propylene Diene Monomer rubber ………..7
Figure 5: The chemical structure of uncured and cured rubber polymers where the crosslinks can look different based on what cure systems are used ……….8
Figure 6: An overview of the vulcanization process by extruder………..9
Figure 7: A typically vulcanization curve ………...10
Figure 8: The curves for main chain and crosslink scission………....13
Figure 9: Waste management hierarchy ………...15
Figure 10: PAR methodology ………...17
Figure 11: The design thinking methodology ……….18
Figure 12: The used methodology which is a combination of the PAR and design thinking method………..19
Figure 13: The power/interest matrix ……….21
Figure 14: Interview structure with grand tour method ………..22
Figure 15: The search methodology ………24
Figure 16: PSS classification ………...28
Figure 17: The team canvas that was mapped together with Roxtec……..………...31
Figure 18: The waste flow showing what type of waste, where it comes from and the proportion of each waste type. ………....34
Figure 19: Some examples of what the studied waste can look like ………..34
Figure 20: The result from the stakeholder analysis. ………...35
Figure 21: The analysis of the circular economy trend.………. .38
Figure 22: The recipe for sulfur and peroxide rubber blends and the devulcanization degree according to Horikx’s theory ………40
Figure 23: The rubber compounds and the third monomer used in the study ……….41
Figure 24: The devulcanization curves for peroxides with different methods and parameters ………...42
Figure 25: The devulcanization curves for sulfurs with different methods and parameters ………....42
Figure 26: The crosslink density for dry (open symbols) and wet (solid symbols) for different feed rates. In graph 1 the gap size was 0.63 mm and in graph 2 the feed rate was 0.32 g/s. ………...43
Figure 27: Inverted Horikx’s cure which shows the crosslink density and the gel fraction……….... 44
Figure 28: The devulcanization degree and bond scission scenario according to Horikx’s theory………... 45
Figure 29: The devulcanization conditions and the devulcanization degree for each case ……….47
Figure 30: The recipe and the abbreviation of the samples ………...48
Figure 31: The temperature increase of EPDM samples with and without oil and the decrease in gel content after a specific devulcanization time ………...49
Figure 32: The bacteria growth where glucose and GTR and glucose were added to the colony after 3 days. The Y-axis represents the growth and the X-axis the amount of days ………..50
Figure 33: The mechanical properties of vulcanized virgin GTR and revulcanized reclaimed GTR ……….50
Figure 34: The rubber recipe used in the study ……….51
Figure 35: The ultrasonic reactor and the normalized crosslink density and gel fraction where the solid symbols are the gel fraction and the open represent the decrease in crosslink density with different carbon black contents and feed rate ……….52
Figure 36: Mechanical properties of virgin and reclaimed vulcanized rubber……….. 52
Figure 37: The different methods and the settings ………53
Figure 38: The devulcanization degree at different settings for each method……….54
Figure 39: The devulcanization degree and scission scenario according to Horikx’s theory and the curing
curves for each revulcanized sample ………...55
Figure 40: The mechanical properties of the revulcanized samples from each method ………...55
Figure 41: Warning symbols of Xylene and Benzoyl peroxide ………...57
Figure 42: The warning symbols for diphenyl disulfide ………..58
Figure 43: The effect on pressure, temperature, DD content and reaction time on the sol fraction ………..60
Figure 44: The devulcanization degree and the scission scenario according to Horikx’s theory ………..61
Figure 45: The process parameters ……….61
Figure 46: The twin screw extruder used (a) and the screw configuration (b) used to perform the devulcanization……….62
Figure 47: The self-heating function, the devulcanization degree and scission scenario according to Horikx’s theory and the Mooney viscosity………...63
Figure 48: The devulcanization degree with different temperature and rotor speed ……….64
Figure 49: The devulcanization degree and scission scenario according to Horikx’s theory ……….64
Figure 50: The recipe for the rubber mixtures used in the study………....65
Figure 51: The crosslink density and gel fraction for unfilled and filled NR at different amplitudes ………65
Figure 52: The recipe for the rubber mixtures used in the study ………..66
Figure 53: The crosslink density at different amplitudes and feed rates where the open symbol is revulcanized rubber ………...66
Figure 54: The recipe for the rubber mixtures used in the study ………. .67
Figure 55: Crosslink density and gel fraction for different amplitudes and rubbers with a gap of 2.03 mm, the solid symbols is for re-vulcanized rubber .……….68
Figure 56: The first 12 ideas with their number and brainstorming technique ………..72
Figure 57: The next 12 ideas and their numbers ………..72
Figure 58: The next 12 ideas and their numbers………...73
Figure 59: The last ideas and their numbers ……….73
Figure 60: The 3D printed dumbbell filled with powder and granules……… 74
Figure 61: The plastic bag filled with powder ………..75
Figure 62: The skipping, training rope and the rubber band ……….76
Figure 63: The speed bump in proportion to a car. ………..76
Figure 64: The climbing wall with different handles……….77
Figure 65: The golf ball in proportion to real balls ……….78
Figure 66: The punching bag and the shape of the granules………...78
Figure 67: The answers to question 3………..83
Figure 68: The answers to question 4 and 5 ………..84
Figure 69: The most attractive features ………..84
Figure 70: The most attractive PSS ……….85
Figure 71: The system solution developed for this thesis ………...86
Figure 72: The business model canvas ………...90
Figure A1: The Production manager persona ……….106
Figure A2: The Sustainability department persona ………...107
Figure A3: The Product development persona ………...108
Figure A4: The Earth persona ………...119
Figure A5: The first part of the devulcanization implementation plan ………...115
Figure A6: The second part of the devulcanization implementation plan ………...116
Figure A7: The third part of the devulcanization implementation plan ……….117
Figure A8: The first part of the Reuse implementation plan ………....118
Figure A9: The second part of the Reuse implementation plan ………...119
Figure A10: The first part of the PSS implementation plan ………..120
Figure A11: The second part of the PSS implementation plan………..121
List of tables
Table 1: The rubber groups and their characteristics……….4
Table 2: Properties and performance of natural rubber ………...5
Table 3: Properties and performance of SBR …….……….6
Table 4: The three different types of diene and their properties………....7
Table 5: Properties and performance of EPDM …….……….7
Table 6: Type of bonds when curing with sulfurs………...11
Table 7: Most common Coagents……….………. 11
Table 8: The sustainability principles ………..14
Table 9: Used databases and keywords for peroxides. The “(1)” means that not a full case was found but that evidence of peroxide devulcanization was included ……….24
Table 10: Used databases and keywords for sulfur and peroxides ………...24
Table 11: Properties of the partners EPDM rubber ……….32
Table 12: Other characteristics of the material ………....33
Table 13: The collected statements, the translation and the ranking. ………...36
Table 14: The requirements for the new method to be found ………..39
Table 15: The different attributes of each process where the environmental aspect is based on the use of chemicals, a 1 represents the worst conditions and a 3 is preferred. ………70
Table 16: The found devulcanization companies and equipment ………..71
Table 17: The decision matrix for this thesis ……….80
Table 18: The PSS suggestions ……….82
Table A1: Sustainable principle analysis of the current system ………..111
Table A2: Sustainable principle analysis of the new solution ………..112
Chapter 1 - Introduction
The cooperating company for this thesis is Roxtec international AB which is a company that produces and sells cable and pipe sealings in different shapes and sizes all around the world. The partner is a business-to-business company which assists a number of industries such as manufacturing, energy, shipbuilding, and offshore platforms with high quality sealing solutions. The purpose with the product is to work as flame protection and create a safe barrier between two rooms or medium. This in order to avoid that dangerous gases, substances or fluids will leak into an undesired place. To succeed with this the product is made of vulcanized EPDM rubber that won't let through anything thanks to its elastic and isolating properties (see figure 1 below for an example of the product, its components and when installed).
Figure 1: An example of what the studied product can look like and its included components.
1.1 Initial problem definition
Waste management has always been a challenge for mankind, but once it was found that rubber can be vulcanized to obtain high strength and elastic properties, the waste problem took a new sharp turn [1].
This is because in the vulcanization process, three dimensional networks are formed which prevent the material from being recycled [4]. Rubber is also a complicated material to work with, which leads to much waste being produced in the manufacturing process. The only way to recycle the waste is if the strong bonds are broken with a process called devulcanization [4]. This process however, is mainly developed for the tire industry which uses a specific type of rubber and sulfur as the curing substance.
But there exist industries that use another rubber material and peroxides as the curing system as it gives other properties to the material, but for peroxides, much less research about devulcanization has been done [6]. Due to this, the seeking and implementation of a devulcanization process for that rubber type is much harder as it is unclear if it will work or not.
One method that was developed for tire rubber has however been found and tested by Roxtec and it succeeded to devulcanize their rubber. But the result was lacking as only a small percentage could be reused by mixing it with virgin rubber to sustain approved properties. Another problem with the method was that the process required very high temperatures which made some substances in the material to break down and new compounds were formed. Once the reclaimed rubber then was mixed with virgin rubber, the original recipe was changed which causes problems as it becomes unclear what's
in the rubber chemical composition. The method was also very expensive and it was unclear where the equipment should be installed (at the company or at the suppliers). Due to all these problems, the method has not yet been implemented. Today there also exists no other effective implemented waste management strategy to take care of the waste within the company which leads to that more and more ends up in a landfill which is an unsustainable solution. Burning it for energy is also not an option as the material has flame retardants and the energy needed to degrade the rubber is more than what is gained. There is therefore a need to further investigate the possibilities with devulcanization and other strategies for waste management to help this industry to achieve a more sustainable alternative as it won't be possible to continue with the old habits any longer.
1.2 Survey of related work - Waste management
As stated above, polymer management is a serious environmental problem [1] Due to this much has been done in the field to try to tackle the problem. At first, many tried to manage the problem by burning it for energy, but this caused air pollution, it won’t work for all rubbers and it is considered to be a recovery method with low value from an economical and environmental point of view [1]. Later pyrolysis was used to generate gas and oils to use as fuels. The process means that the rubber is heated to very high temperatures in absence of oxygen and in the process, carbon black, hydrocarbon oils and gases are produced. However, the process also produces carbon monoxide and carbon dioxide and traces of other chemicals [2], which is also not sustainable.
Another recycling method is to grind waste rubber into small particles and then mix the powder with virgin rubber. This can make the rubber mixtures cheaper and improve the properties to some extent. However, only a small part can be mixed which still leaves much waste. Due to this, the last 50 years has been spent on investigating the possibility to recycle the material by breaking the bonds that are formed in the vulcanization process, and several new techniques that succeed with this have been developed and reported.
The different techniques existing today are called thermal, chemical, biological, mechanical, ultrasonic, microwave, and supercritical 𝐶𝑂2 devulcanization and then there exists some combinations of these like thermomechanical or thermochemical devulcanization. Many researchers try to develop, summarize and review the available processes [3, 4, 5] but almost all reported work on devulcanization today is linked to the type of rubber used in tires as it constitutes the largest amount of waste in the rubber sector. Therefore, most of the scientists reviewing or researching about this topic feel that it is most convenient to focus on this sector [6]. However one rubber type that is currently growing fast is EPDM rubber, but the amount of studies for recycling this type is still significantly fewer than the research done on rubbers used in tires [7].
By having examined several articles around this topic both regarding tiers and EPDM, it can also be stated that even less research and experiments has been done on EPDM rubber cured with peroxides as sulfur cured rubbers (like tiers) are dominating the market. About 90% of all vulcanized rubber is cured with sulfurs [8]. This makes it much harder to find research on peroxide cured rubber which in turn complicates recycling of this rubber sector. Also, no reviews or summaries that highlight and evaluate the possibilities of peroxide devulcanization have been found.
Besides recycling, much has also been done regarding reuse to manage the waste. One example is to grind the rubber into smaller particles and then mix it with asphalt to create rubber roads and pavements. This has been indicated to reinforce the asphalt, reduce the deformation, lower the construction and maintenance cost, and enhance its resistance to rutting and fatigue damage [9, 10]. A similar approach is to mix grinded rubber into concrete but according to [11, 12] it seems to decrease the compressive strength which may limit its use in certain structural applications. However, a number
of desirable properties were still discovered which was lower density, higher toughness, and higher impact resistance. Another reuse solution is to include rubber crumbs into synthetic grass. This in order to create support for the synthetic grass blades, work as ballast, and as cushioning for football players [13]. Rubber granules have also been used in horse riding centers and stables as it even in this case provides cushioning. It also gives the horse a soft and comfortable support and compensates floor unevenness. In the cases where sand constitute the track floor it is common that sand dust starts to spread in the air, but by mixing it with rubber granules or replacing it, this phenomenon can be reduced which makes it unnecessary to first water the area before training [14] Furthermore, rubber granules can also be used as flooring for playgrounds and sports tracks due to its damping and anti-slip properties [15]. All these reuse solutions however are quite dependent on large quantities, and it may therefore be a challenge for smaller businesses with lower and irregular waste flows to build a solid business model from these solutions.
1.3 Aim of study
As the survey of related work shows there exists much less research for devulcanization of peroxide cured EPDM which in turn complicates recycling of this rubber sector as it is not as equally known what is possible or not possible. The aim of this study is therefore to summarize and analyze the current reported research and methods with focus on peroxide cured EPDM to evaluate the general possibilities with recycling. This in order to create awareness and share knowledge for all companies that use this type. As the initial problem definition section suggests, Roxtec has found and tried a method that proved to be able to devulcanize their rubber, but with very limited results and implementation proved to be a problem. Due to this more and better options will be seeked and then conclusions will be drawn if any of the methods found can be applied and easier implemented. To further tackle the problem of waste management, the aim is also to investigate the possibilities with reuse. This in order to come up with more alternative usage areas for lower and inconsistent waste flows to work as a compliment if devulcanization or effective devulcanization is too far away from being a reality for many companies.
The relevant research questions to investigate are therefore:
RQ 1: What methods and possibilities exist for devulcanization of peroxide cured EPDM?
RQ 2: For what alternative usage areas or products can Roxtec’s EPDM rubber waste be reused for to prolong its life cycle and achieve a more sustainable alternative?
RQ 3: How may devulcanization and reuse be implemented?
1.4 Delimitations
The delimitations for this thesis is that only reuse and recycling of EPDM rubber will be investigated to evaluate the possibilities of these waste management strategies. Roxtec’s production also consists of several production facilities which generate different types of waste. However, the production section that will be studied for this thesis is called “production west” and it is here the extruding of the components and assembly of the sealings takes place. Furthermore, this thesis will not include advanced chemistry. Due to this the theory section will only consist of and explain necessary chemistry definitions and theory that is needed to understand the results. For the interested reader, more advanced theory will only be referred to in the text and not explained by the author. The time frame for the project is limited to 20 weeks and the restrictions due to corona led to that the whole work was conducted online. The time limit and the restrictions has therefore limited the level of depth of some parts of the work to some extent.
Chapter 2 - Theory
In this section some theory about rubber, vulcanization, devulcanization and sustainability strategies are presented to help the reader to better understand the material in the report.
2.1 Rubber
The material of rubber can be described as one type of polymer where the term polymer is a chemical compound that is built up of many repeating units (monomers) that eventually will create a very long molecule chain. Plastics and fibers are two other examples of polymers and today there exists both natural and synthetic polymers [16]. However, all rubbers originally come from nature as the natural rubber is produced from the sap of the Hevea Brasiliensis tree and the synthetic rubber consists of polymers made from raw oil (petroleum) which is taken from the ground [17].
Rubbers have three main applications which are to seal, dampen or protect, but the rubber in its pure form also called virgin rubber (VR), is not so useful in the industry as the mixture is soft, sticky and easily plasticized when exposed to forces. To change this and achieve desired properties, various substances are added to the mixture and then a technique called vulcanization (see section 2.2 for more details) is used to permanently shape and harden the rubber. Based on which substances that are mixed with the rubber, the properties will differ. To for example gain more strength and stiffness, different fillers like for example carbon black can be added. To gain a mixture with better machinability and stickiness, different plasticizers can be added. Then when the mixture will be vulcanized the mixture will need a cure substance. This cure substance can then in turn be mixed with different accelerators, activators and retardators which will form a cure system that will affect the manufacturing process and the properties [17] .
Today there exist several different types of rubbers and classifications in order to structure and group rubber types with similar characteristics. The grouping is done based on how the polymer chains are built up and what elements that are included in it. See table 1 below for the grouping and the characteristics. The six most common rubber types are natural rubber (NR), Styrene-butadiene rubber (SBR), butyl rubber (IIR), Nitrile rubber (NBR), Neoprene rubber (CR) and Ethylene Propylene Diene Monomer rubber (EPDM) [18], where NR, SBR, NBR, CR, and IIR belongs to the R group and EPDM to the M group. In the below sections, three rubber types are described more in detail where the most details are presented for EPDM.
Table 1: The rubber groups and their characteristics [19]
Group Characteristic
M Includes rubber types with saturated carbon chain
O Includes rubber types with coal and oxygen in the polymer chain Q Includes rubber types with oxygen and silicon in the polymer chain
R Includes rubber types with unsaturated carbon chain
T Includes rubber types with coal, sulfur, and oxygen in the polymer chain U Includes rubber types with coal, nitrogen, and oxygen in the polymer chain Z Includes rubber types with phosphor, and oxygen in the polymer chain
2.1.1 Natural rubber (NR)
Natural rubber is, as the name suggests, natural in comparison to synthetic rubber. The chemical structure of the constituent monomers of the rubber can be seen in figure 2 and this sequence is then repeated and linked multiple times which eventually forms a long polymer chain [17].
Figure 2: The chemical structure of natural rubber (adapted from [20])
This rubber is a non-polar pure carbon-hydrogen compound which means that the rubber has low glass temperature and has bad resistance to oil and fuel. The non-polarity also makes the rubber easy to mix with other non-polar synthetic rubbers like for example SBR. The rubber is sensitive to ozone, oxidation and heat but has high tensile strength even without fillers. More of the rubber's properties can be seen in table 2 below. The most common usage area of this type of rubber is in different types of tires for cars and various machines but it is also used as vibration isolators, bearings for bridges and buildings, hoses, wear protection and chemical protection for metal containers [17, 20].
Table 2: Properties and performance of natural rubber [20]
Properties Performance
Temperature range -50 to 100 °C
Compression set Excellent
Strength Excellent
Damping Poor
Low temp. properties Excellent
Abrasion resistance Excellent
Gas permeability Poor
Weather resistance Poor
Water resistance Excellent
Ozone resistance Poor
Mineral oil resistance Poor
Chemical resistance Fair
Flame restiance Poor
Heat resistance Poor
2.1.2 Styrene-butadiene rubber (SBR)
This rubber type is a synthetic rubber which was developed as a cheaper replacement for natural rubber as it was quite expensive in the past. The chemical structure of the constituent monomers can be seen in figure 3 below where the first part is the butadiene compound and the second is the styrene compound [17].
Figure 3: The chemical structure of styrene-butadiene rubber (adapted from [20])
Since the SBR was developed as a replacement for NR, it has many similarities regarding the properties. However, SBR requires various fillers in order to achieve the good mechanical properties.
This rubber is also sensitive to oil and ozone but has a little better heat resistance than NR. More of the general properties are shown in table 3 below. The most common usage areas for this rubber are in tires, conveyors, hoses, coverings for chemical protection for metals, flooring material, shoes and sewage gaskets [17, 20].
Table 3: Properties and performance of SBR [20]
Properties Performance
Temperature range -25 to 100 °C
Compression set Good
Strength Good
Damping Fair
Low temp. properties Fair
Abrasion resistance Excellent
Gas permeability Good
Weather resistance Poor
Water resistance Good
Ozone resistance Poor
Mineral oil resistance Poor
Chemical resistance Fair
Flame restiance Poor
Heat resistance Fair
2.1.3 Ethylene Propylene Diene Monomer rubber (EPDM)
This is a synthetic rubber as well which is produced in two main types, EPM (ethylene propylene monomer rubber) and EPDM (ethylene propylene diene monomer rubber). The first type is a saturated polymer of ethylene and propylene and the second is a polymer of ethylene propylene and a small amount of dien which gives the polymer double bonds, but these bonds are not formed in the main chain which still makes the main compound saturated. The EPDM rubber can be produced from EPM by adding a third diene monomer to the mixture [17, 20]. The chemical structure of the EPDM rubber can be seen in figure 4 below where the first part is the ethylene compound, and the second part represents the propylene compound.
Figure 4: The chemical structure of Ethylene Propylene Diene Monomer rubber (adapted from [20]) Based on the amount of ethylene, the properties will differ. Low concentrations will give an amorphous structure which makes the material unable to self-re-inforce. High concentrations however give the material some crystalline properties below 50 degrees Celsius which results in higher strength.
Also the choice of diene type will affect the properties of vulcanized rubber but also the vulcanization process [17]. The three different diene choices to make EPDM, their effect on the vulcanization process and the effect on mechanical properties are listed in table 4 below [17].
Table 4: The three different types of diene and their properties.
Diene monomer type Vulcanization properties Mechanical properties
ENB High vulcanization speed
Higher pre-vulcanization tendency High strength Low compression set
DCPD Lower vulcanization speed Low compression set
HD Lower vulcanization speed High strength
Higher concentrations of diene will speed up the vulcanization process and give the material better mechanical properties. Some of EPDM rubber’s properties are that it has excellent electrical resistance and good resistance against polar solutions. More of the properties are listed in table 5 below.
Table 5: Properties and performance of EPDM [20]
Properties Performance
Temperature range -45 to 150 °C
Compression set Good
Strength Good
Damping Excellent
Low temp. properties Good
Abrasion resistance Good
Gas permeability Fair
Weather resistance Excellent
Water resistance Excellent
Ozone resistance Excellent
Mineral oil resistance Poor
Chemical resistance Good
Flame restiance Poor
Heat resistance Excellent
The most common usage areas for EPDM rubber are in sealings for example cables, pipes, windows, doors and pounds. Then it is also used as body seals for the car industry, as roofing material and as gaskets [17, 20].
2.2 Vulcanization
Vulcanization is a chemical process in which the long polymer chains of the rubber mixture get crosslinked with each other with the aid of a vulcanizing agent and evaluated temperatures. This process creates a three-dimensional network that permanently changes the properties and shape of the rubber.
The rubber goes from being a ductile mass with plastic properties to becoming a hard dimensionally stable material with elastic properties [8, 17, 21, 22], see figure 5 for a simplified illustration of the polymer structure.
Figure 5: The chemical structure of uncured and cured rubber polymers where the crosslinks can look different based on what cure systems are used (adapted from [23]).
This vulcanization technique was first discovered 1839 by Charles Goodyear and since then, the process has been improved and developed further to better be able to control the properties and the vulcanisation process [8, 17]. The whole process is initiated by mixing the polymers with different fillers, plasticizers and a cure substance depending on which properties the manufacturer desires. The mixing process can be done in one or two steps depending on the risk for pre-vulcanization. Pre- vulcanization is when the rubber starts to vulcanize in the mixing machine or before it has accomplished
its final shape which is not desirable as it will ruin the mixture or the product. This phenomenon can happen if the cure agent is mixed together with all the other ingredients and if the mixing motion increases the temperature too much as high temperatures will trigger a reaction. It can also happen if the temperature is too high in the initial steps of the process before the shaping takes place. To avoid these phenomenon’s, the temperature must be controlled and the mixing process can be done in two steps where all ingredients except the cure agent are mixed and then when the mixture has cooled down, the cure agent is added [17, 22].
The most common cure agents are sulfur and peroxides but sulfur is the most common and still dominates the rubber industry [8, 21]. Once the mixing is done the rubber is often rolled out to long rubber sheets. Then when it's time to use the mixture, it is commonly inserted into an extruder or a mold that will give the rubber mixture its shape by pushing it through a die or into the mold that has the desired shape. To then sustain the shape, the vulcanization process takes place where the rubber is heated to high temperatures in order to trigger the reaction. Once it is vulcanized, it will permanently have the obtained shape and the rubber will now have elastic and preferable properties [17, 22]. See figure 6 for an overview of the process.
Figure 6: An overview of the vulcanization process by extruder (adapted from [24]).
At first it was only the vulcanizing cure substance that was used in the process but then when the technique got more sophisticated, different activators, accelerators and retarders could be added with the cure agent which will form a cure system. This cure system will then be able to better manipulate the process with favorable outcomes. The activators are a substance that is added to the mixture in order to activate and improve the efficiency of the cure system, while accelerators is a substance that will speed up the process [8, 17, 21, 22].
Another way of speeding up the process is by increasing the temperature, but with accelerators this is not as necessary. If high temperatures are necessary but a certain vulcanization time is needed to gain desired properties, retarders could be added to the mixture to slow down the reaction. It is therefore up to the manufacturer to regulate these parameters and design his own cure system to achieve the desired properties. This is because once the material is vulcanized it can't go back to its original shape as it is an irreversible process and it is therefore crucial to get it right the first time to not produce pure waste [8, 21, 22]. In figure 7 below is a vulcanization curve illustrated which shows the curing process.
The first section of the curve (scorch time) is the time before the material starts to vulcanize. It is at this stage the material can be formed to its desired shape and it is up to the manufacturer to control the temperature and time to prevent the reaction to start before the rubber is fully formed. The next part
of the curve is where the vulcanization takes part and the inclination on the curve between 𝑡1 and 𝑡90 is the vulcanization speed. After 𝑡90, the rubber has reached the maximum stiffness which is a measurement for the crosslink density (CLD) which is defined as the number of polymer network chains per gram of network [27]. At this point three scenarios can occur. The first is that the stiffness will start to decrease which means that after a specific curing time the rubber will start to degenerate due to the heat. This phenomenon is called reversion. The second scenario is that the stiffness will continue to increase which is called marching and the third is when an equilibrium is reached and then nothing will happen which is called normal curing [17, 22].
Figure 7: A typically vulcanization curve (adapted from [22])
2.2.1 Sulfur cured vulcanization
Sulfur cured vulcanization has been the most common cure substance throughout history and it is still dominating in the rubber industry [8, 21]. In the reaction, the sulfur molecules are split and free radicals are formed. These radicals and the sulfur then react with the unsaturated parts of the polymer chain in the presence of heat and then different types of crosslinks are formed [17]. The different variations of crosslinks or bonds that can be formed can be seen in table 6 below. When using sulfur as the cure substance it is the concentration of sulfur or the use of sulfur emitters (an organic substance that will release sulfur when heated) that will determine what bonds that are mainly formed.
Higher concentration will mainly form the polysulfide bonds while lower concentration or sulfur emitters will form disulfide or monosulfide bonds [17, 21]. This type of vulcanization substance is also often mixed with activators, accelerators and retarders to gain desired properties of the final product. For example, when performing a mold vulcanization, a certain time is needed in order for the rubber to fill out the form before it is vulcanized and therefore retarders are added to the cure system to gain more time before the reaction starts. Then the activators and accelerators contribute by decreasing the vulcanization time once the reaction has started. All these substances are chemical compounds and based on the selection of these chemicals, the manufacturer can manipulate the cure time, vulcanization speed, and the finish of the product [17, 21].
Table 6: Type of bonds when curing with sulfurs [17, 21]
Type of bond S (Monosulfide bond)
S2 (Disulfide bond) Sx (Polysulfide bond)
S1 ( Sulfur that is bonded to only one polymer chain, it will not create a crosslink SnH (Thiol group, does not contribute to any crosslinks
S+accelerator (residual of accelerator, does not contribute to any crosslinks
C (Carbon bond where the carbon comes from the polymer chain, these bonds are very few) Sm (cyclic sulfur bond, does not contribute to any crosslinks)
2.2.2 Peroxide cured vulcanization
Peroxides is the second most common cure substance to use after sulfur and it can be used to crosslink almost all types of rubbers while sulfur only can be used in polymers with unsaturated sections [21].
Due to this, peroxides are mostly used for saturated rubbers and sulfur is used for unsaturated rubbers.
When vulcanizing unsaturated rubbers with peroxides it will need to be stabilized with other substances and chemicals which are not desired and therefore sulfur is a better choice, it is also cheaper than peroxides [17]. When the vulcanization reaction takes part, the peroxide compound is split and then covalent carbon-carbon (C-C) bonds are formed between the polymer chains. These carbon bonds give the rubber better heat resistance and better compression properties compared to the sulfur bonds when curing with sulfurs [17, 21].
Another difference is that peroxide vulcanization can’t be accelerated by adding different chemicals to the mixture as the vulcanization speed is set based on the peroxide's ability to decompose.
The only way to speed up the process is to increase the temperature. Due to this there exist more limited possibilities when forming the rubber recipe than for sulfur systems [17]. Although the crosslinking of the rubber can be effectively enhanced by the application of coagents [17, 21] Coagents are multi- functional organic compounds which easily react with free radicals. When curing without coagents, the crosslinking performance could be negatively affected as unfavorable side-reactions that create free radicals can occur. But by using co-agents, the efficiency can be increased as the agents will suppress unfavorable side-reactions. As a result of increasing the performance and thereby also the crosslinking density, better properties like tensile strength and abrasion resistance could be obtained [21]. Some of the most common coagents can be seen in table 7 below.
Table 7: Most common Coagents [17]
Coagents
TAC (Triallyl cyanurate) TAIC (Triallyl isocyanurate) TAP (Triallyl phosphate)
EDMA (Ethylene glycol dimethacrylate) TPTA (Trimethylolpropene trimethacrylate)
2.3 Devulcanization
Devulcanization is the opposite of vulcanization which means that instead of forming crosslinks between the polymer chains, the crosslinks should be broken without breaking the bonds in the main chain. If the bonds in the main chain are broken, the mechanical properties will be affected once the material is re-vulcanized as it won't have the same quality as virgin vulcanized rubber [4, 8 ]. Today there exist several methods for breaking the crosslinks which were briefly described in the survey of related work. But all of them have the same goal which is breaking all the crosslinks and not the main chains. Once a rubber has been vulcanized it will gain a certain CLD, a higher density indicates a stiffer less mobile material and a lower CLD indicates a more flowing material [25]. During devulcanization a greater decrease in CLD results in a more efficient devulcanization. However, the decrease in CLD can also be due to main chain scission and therefore the quality of the devulcanization will be lacking even though a high devulcanization degree was obtained [26, 27].
To evaluate what type of bonds that have been broken and thereby the quality of the devulcanization, it is common to use Horikx’s theory [27]. This theory describes the relationship between the sol fraction, gel fraction and the CLD for each rubber and with this, a theoretical curve for each scenario can be obtained which the experimental data can be compared with [27]. The sol fraction is a certain amount of polymer chains that are soluble. The sol fraction can be extracted when the rubber is placed in a solvent. What's left is the insoluble part of the rubber which is called the gel fraction.
When a rubber is vulcanized the sol content will decrease and the gel content will increase. Then during devulcanization the sol content will increase [27]. The relationship between the sol and gel fraction can be seen in equation (1) where s is the sol and g is the gel, 𝛾 is the crosslinking index (the average number of crosslinks per original polymer chain of the studied rubber). This can be calculated with equation (2) where 𝑣𝑥 is the CLD, 𝜌 is the polymer density and 𝑀𝑛 is the average molecular weight of the polymer [28].
To obtain the curves for the rubber, equation (3) is used to plot the curve corresponding to main chain scission which is the extreme case where only main chains break. Here 𝑣𝑖 and 𝑣𝑓 is the initial and final CLD and 𝑠𝑖 and 𝑠𝑓 is the sol fraction before and after the treatment. The second curve can be calculated with equation (4) and is the extreme case where only selective scission occurs which is when only the crosslinks formed in the vulcanization process break. Here 𝛾𝑖 and 𝛾𝑓 is the initial and final crosslinking index [26, 27, 28].
In figure 8 the curves are presented and as it can be observed, the sol fraction should barely increase for the selective case, it is only when the decrease in CLD reaches about 90 % that the sol fraction increases. The real sol fraction is calculated by doing a swelling test. Here the samples are weighted before placing it in a solvent to obtain 𝑚𝑖, then the rubber is swelled in a solvent at ambient temperatures. The swollen samples are then dried and weighed to gain 𝑚𝑑. The sol fraction is then calculated with equation (5) [26, 27]. The decrease in CLD is calculated by using equation (6) where 𝑣𝑖 and 𝑣𝑓 is evaluated with equation (7). Here 𝜒 is the rubber solvent interaction parameter, 𝑉𝑠 is the molar volume of the solvent and 𝑣𝑟 is the rubber volume fraction in the swollen sample [26, 27, 28]. Details on how to perform a swelling test and the different equations is presented in the ASTM D 6814-02 standard. Once the sol fraction and the CLD is obtained it can be mapped in the diagram to evaluate what scenario that has occurred and the devulcanization degree. An inverted curve is also common to use and, in that case, it is the relationship between the gel fraction and the CLD that are plotted in the graph.
Figure 8: The curves for main chain and crosslink scission (adapted from [27]).
2.4 Sustainability
In today's society, the term sustainability has become increasingly prioritized and discussed. This is because it has been shown that it is no longer possible to continue as before as it creates a great burden on the environment. Therefore, many researchers have tried to map the problem and describe the meaning of sustainability. A simplified definition of sustainability is that society shall have the possibility of meeting their needs without compromising the ability for future generations to meet their needs. To achieve this, a bunch of strategies and principles have been developed to work as guidelines
for businesses and society when designing and consuming products [29]. The different sustainability principles can be seen in table 8 below.
Table 8: The sustainability principles [29]
SP # Principle
1 In an ecologically sustainable society, nature is not subject to systematically increasing concentrations of substances extracted from the earth's crust
2 In an ecologically sustainable society, nature is not subject to systematically increasing concentrations of substances produced by society
3 In an ecologically sustainable society, nature is not subject to systematically increasing degradation by physical means
4 In a socially sustainable society, people are not subject to structural obstacles to health 5 In a socially sustainable society, people are not subject to structural obstacles to influence 6 In a socially sustainable society, people are not subject to structural obstacles to competence 7 In a socially sustainable society, people are not subject to structural obstacles to impartiality 8 In a socially sustainable society, people are not subject to structural obstacles to meaning making
Regarding design strategies, two examples are design for remanufacture and recycling. Design for remanufacture means that the designer should make it easy to disassembly the product once it is broken. This is because by doing so, only the part that is broken can be replaced with a new part which saves plenty of resources compared to having to replace several parts or the whole product since they can’t be separated [30]. Design for recycling is similar to remanufacture but instead of replacing broken parts they are recycled. This means that the designer should also aim to design products where the different materials easily can be separated to facilitate recycling. It is also of interest to design a product with less variations of materials as less parts then needs to be separated [30].
Besides from these principles and design strategies, there also exist other guidelines and models to help society to become more sustainable. One example of this is the waste hierarchy in figure 9 below. This pyramid describes several different approaches that can be performed to manage waste. In the bottom is the least favored option which is throwing the waste on a landfill [31]. However, it is also the easiest option which requires the least effort. When moving up in the pyramid, the options become more favorable for the environment, but in some cases, it also becomes more complicated and requires more effort from the business [31]. For example, reuse is often considered to be a better option than recycling since the latter often requires much energy compared to reusing the product as it is. However, reuse might become more complicated as new applications needs to be found and the company must implement systems within their business to allow reuse of different parts and products. The same applies for the top of the pyramid. Minimization means that the aim is to reduce waste. This can be done by using more secure equipment or using less materials for the specific product which is considered to be better than reuse as then both less material and reuse of the product in its end-of-life state is possible.
However designing a product to need less material is a challenge. An even greater challenge is prevention of using a specific material or preventing waste from forming in the first place. Although even though the difficulty level increases, it has proved to be profitable from several aspects and it is therefore of interest for businesses to investigate what waste management approach that is possible right now and then eventually strive to reach the top of the pyramid [31].
Figure 9: Waste management hierarchy
2.4.1 Circular economy
A collective name for some of the approaches described above is circular economy. This concept is not new and has been around for several years but has now rapidly grown. The main idea of the concept is to close the loops and move away from traditional linear production. This is done by for example adapting the various design strategies like design for remanufacturing and recycling. Other ways of adapting the concept are to make products more durable and long lasting but also reuse products to give them a second life [30]. Adapting product service systems and take back systems is another way of closing the loops [32]. The effect of this is circular material and product loops which leads to less waste and more efficient usage of resources. The main reason why this concept has grown in recent years and is still classified as a growing and emerging trend according to several web pages like [33, 34, 35] is mainly due to more environmental awareness and stricter laws and regulations. It has also shown to be economically profitable and give market advantages which attracts many industries to try out the concept [32].
Chapter 3 - Method
3.1 Participatory Action Research (PAR)
An applied research methodology includes that a professional researcher or expert designs and formulates a project within an organization. Then data is collected by observing or contacting relevant subjects. Once data is collected, the researchers will distance themself from the organization and the subjects to analyze the data and gain knowledge. At the end of the research the organization will receive the results and suggestions to take action. Due to this the members of the organization will be treated as passive subjects who will only participate in the research by authorizing the project, being its subjects and receiving the final results [36].
The PAR methodology on the other hand is in sharp contrast to this traditional methodology by the means that the organization will be actively included throughout the whole process [36]. The purpose of this is to exchange knowledge between the researcher and the organization to achieve favorable results and new experiences that can be utilized for future applications for both parts. The researcher is included with the organization to understand its inner dynamics and conditions of the systems and the organization is included in the researchers’ activities to take part of the established knowledge throughout the process [36].
The typical PAR process usually begins with a problem that the organization currently is facing and then a collaboration is established to target the problem. For the applied research process, the process is commonly initiated by first doing a review of the literature, formulating hypotheses, and then finding an organization to conduct the research on. For the PAR process the researcher only turns to the literature to further specify the problem and the aim and to understand the state of the art once an organizational collaboration and a problem approach has been established [36].
To conduct the PAR methodology the researcher must make sure that the process is participatory based and participatory controlled. Participator based means that those who are affected should get the opportunity to take part in defining which elements of the problem to be treated.
Participator controlled means that the organization should be in a position to participate in making decisions about what measures to be implemented [36]. The different steps that can be used in this methodology are illustrated in figure 10 below. After the problem approach has been formulated, the planning takes part where goals and ways of working can be determined. The next step is to acquire relevant data for the formulated research question and then the data will be analyzed. In the final step the researcher should reflect and interpret the results gained from the process. During the process the researcher also takes action to include the organization. The outcome is new knowledge and new solutions.
Figure 10: PAR methodology (adapted from [36]).
3.2 Design Thinking (DT)
The design thinking methodology is a human-centered problem-solving approach that allows the designer to undergo iterations to foster innovation [37]. This approach is not classified as a rigid process but rather a framework which helps the designer to receive creative and analytical thinking, to reason about the results and the process, and to gain a specific mindset [37].
A design thinking project is commonly initiated by setting the stage for the process and exploring the field to gain a great understanding of the given problem, the affected people and the context. This is done by starting from the initial problem definition and making plans and then broadening the search field to gather information and insights from anywhere. This phenomenon is called diverging thinking which means that the designer should have an open mind to be receptive to all the insights. Once enough information and insights is gathered, the designer enters the convergent thinking stage which includes analyzing and narrowing down the information to define and specify the problem definition further by also taking the affected people's needs and desires into account [37].
Once a clear and defined problem is set, the designer once again enters the divergent stage to explore the design space to generate several creative and innovative ideas that may solve the problem.
Then the convergent stage is entered where several ideas are selected, prototyped and tested. The purpose of this phase is to fail often and fail early to gain understanding for what works and which ideas to abandon or develop further. Once a solid solution is found it is time to implement it and get it out into the world [37]. The four stages of the process can be seen in figure 11 below, and as the figure shows, the phases are overlapping each other which means that iterations can be performed at any phase of the process.
Figure 11: The design thinking methodology ([38], used with permission)
3.3 Research approach
The used methodology during this thesis was a combination of the PAR and design thinking method and an illustration of the process can be seen in figure 12. The reason why this method combination was selected was because it was considered to be appropriate based on the aim of the study. One part of the work aims to conduct research on recycling methods and the other part aims to develop solutions for reuse of the waste. These are two different approaches which are combined into one project and therefore a methodology that can provide a framework that satisfies both parts are of interest. This is because a pure research methodology does not include product development and a pure product development process does not include research to the same extent. Due to this it was decided to combine a research and product development method.
Two other alternative methods than the PAR and the design thinking were considered before the final selection was made. These methods were the design research methodology (DRM) [75] and the generic product development process by Ullrich and Eppinger [76]. However, the reason why the PAR method was chosen to represent the research part and not the DRM was since the problem to be solved is of great importance and by working closer to the company and involving them into activities like the PAR framework provides, much knowledge can be gained for both parts to then take action.
The design thinking method was then chosen to represent the product development part as it is human- centered and allows iterations while the generic process is more linear. The iterations were initially considered to be needed for this project, and design thinking also helps to gain empathy and a better understanding for the affected people and the problem which facilitates developing the right it. Without this understanding of the affected people and the problem, the developed solution is more likely to not satisfy the organization and thereby never be implemented. Due to this the design thinking framework was selected. The human-centered philosophy in design thinking and the involvement of the company in the PAR method then makes a good match between the two different frameworks and a combination between them therefore feels natural. Both these methods could be used separately for each focus area but since much of the work is related to both parts it was chosen to combine them and utilize the different phases when needed.
