The Identity of Recycled Plastics: A Vocabulary of Perception

: As designing with recycled materials is becoming indispensable in the context of a circular economy, we argue that understanding how recycled plastics are perceived by stakeholders involved in the front end of the design process, is essential to achieve successful application in practice, beyond the current concept of surrogates according to industry. Based on existing frameworks, 34 experiential scales with semantic opposites were used to evaluate samples of three exemplary recycled plastics by two main industrial stakeholders: 30 material engineers and 30 designers. We describe four analyses: (i) deﬁning experiential material characteristics, (ii) signiﬁcant di ﬀ erences between the materials, (iii) level of agreement of respondents, and (iv) similarities and di ﬀ erences between designers and engineers. We conclude that the three materials have di ﬀ erent perceptual proﬁles or identities that can initiate future idea generation for high-quality applications. The study illustrates the potential of this evaluation method. We propose that designers can facilitate the valorization and adoption of these undervalued recycled materials, ﬁrst by industry and ultimately by consumers as well. Ordinary-Strange, Disposable-Lasting, Mature-Youthful), between rABS and MPO black (Texture, Weight, Greasiness, Scratchability, Brittleness, Cozy-Aloof, Toy-like-Professional, Frivolous-Sober, Disposable-Lasting, Mature-Youthful) and between MPO grey and MPO black (Temperature, Odor, Greasiness, Strength, Elegant-Vulgar, Cheap-Expensive). When comparing the signiﬁcant di ﬀ erences detected by designers and by engineers, nine disagreements are found between rABS and MPO grey (Temperature, Weight, Cozy-Aloof, Futuristic-Nostalgic, Aggressive-Calm, Ordinary-Strange, Disposable-Lasting, Traditional-Modern, Mature-Youthful), sixteen between rABS and MPO black (Glossiness, Texture, Temperature, Odor, Weight, Greasiness, Scratchability, Brittleness, Cozy-Aloof, Elegant-Vulgar, Toy-like-Professional, Frivolous-Sober, Ordinary-Strange, Sexy-Not sexy, Masculine-Feminine, Mature-Youthful) and eight between MPO grey and MPO black (Odor, Greasiness, Brittleness, Elegant-Vulgar, Frivolous-Sober, Aggressive-Calm, Cheap-Expensive, Traditional-Modern). These results indicate that most dissimilarities occur when comparing rABS to MPO black.


Introduction
The growing number of (new) materials [1] leads to a complex and time-consuming materials selection process in industrial design engineering [2]. Additionally, in the context of our expanding consumer society, the role of plastics within product design has been essential [3] since they offer designers great freedom in shaping and manufacturing their products. Consequently, plastics are widely used for consumer goods such as toys, housings, packaging, etc. Inevitably, an increasing amount of products leads to an increasing amount of (plastic) waste as well [4]. Considering the future scarcity of raw materials, there is a clear need for sustainable product development in a circular economy [5,6]. Hence, engineers and product designers are addressed to rethink the products' life cycle and to retain its plastic materials in closed loops [7]. The increasing attention to sustainable and circular product design requires industry to substitute traditional materials with alternatives such as bio-plastics or materials derived from waste (e.g., recycled plastics) [8][9][10]. Consequently, within Europe, the industrial interest in the field of polymer recycling is ever-growing [11].
The research presented in this study is part of the project 'Design from Recycling', supported by industry in Flanders, one of the Belgian regions, that examines how to design with recycled plastics'.

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(RQ1) Which experiential qualities best describe three types of recycled materials (i.e., what are the prominent sensorial and interpretive characteristics)? • (RQ2) What are the significant differences in material perception between recycled materials? • (RQ3) What are the similarities (RQ3.1) and differences (RQ3.2) between the material perception of designers and engineers (i.e., level of agreement on experiential qualities versus significant differences)?

Conceptual Framework for Experiential Characterization of Materials
In order to study material perception, we build upon the frameworks of sensorial scales [45] that are commonly used for attributing meanings to materials. Previous research [12] shows that these scales are a valuable tool to facilitate the sensorial evaluation of materials-also for non-designers-and to initiate a more in-depth exploration of a material's perceptions. This approach represents various sensorial attributes by means of both verbal and visual opposites on a five-point semantic differential scale. Three existing frameworks or methods are explored and compared to measure sensorial attributes as proposed by Karana and Van Kesteren [37,45,46]. They are shown in Appendix A. The sensorial attributes (semantic opposites) that are mentioned in at least two lists are selected: Glossiness, Transparency, Colorfulness, Color intensity, Softness, Ductility, Weight, Strength, Elasticity, Texture, Odor and Temperature. However, reflectiveness is excluded as it is considered irrelevant in the specific context of recycled plastics. The same goes for the attribute of 'transparency' (since all mixed recyclates are opaque). However, this item is retained as a control item to evaluate whether participants were attentive. In addition, in line with previous research [45], the list is extended with more attributes related to strength as this property can be interpreted in different ways from a technical material perspective, depending on the applied force (compressive, tensile, impact, shear, etc.): Stiffness, Brittleness, Scratchability, Greasiness, and Acoustics.
Similar to the procedure for sensorial attributes, an experiential scale with semantic opposites is also compiled to evaluate the interpretive characteristics of recycled materials, and thus the associations (meanings) they evoke. First of all, Karana's nine meaning sets are selected [37], which are proven to be clear, understandable, and relevant for material appraisals: Aggressive-Calm, Cozy-Aloof, Elegant-Vulgar, Frivolous-Sober, Futuristic-Nostalgic, Masculine-Feminine, Ordinary-Strange, Sexy-Not sexy, Toy-like-Professional.
Conclusively, seventeen sensorial attributes and seventeen interpretive characteristics are used for the experiential characterization of recycled plastics by designers and engineers.

Stimuli
For the purpose of this study, three particular recyclates are considered that can be easily differentiated and that serve as exemplary post-consumer recycled plastics as they are currently collected in large quantities. We focus on post-consumer plastic waste materials that are processed and recycled 'as is', which means that no additives nor compatibilizers are added to improve processing or technical properties. Together with the industrial partners, it was agreed to focus on two material streams that are very common, both in virgin and in recycled version, since good use of these large-quantity waste streams has the greatest potential impact on aiming for a more circular production. For this study, two different material sources are selected: mixed polyolefins (MPO: the 'floating' fraction in the recycling process) versus recycled Acrylonitrile-butadiene-styrene (rABS) from specific collection fractions. From a technical perspective, these two material groups are very different, e.g., mixed polyolefins (MPO) are flexible and rABS is brittle. From a sensorial perspective, MPO has a grey color with visual contaminations or 'speckles' while rABS is black and smooth. To address the potential effect of color in meaning creation, black color is added to the MPO material to generate a third material stimulus set: Karana [37] states that "materials have a history, which helps us to assign meanings to them even when they are not embodied in products". Therefore, a standardized and equal stimulus set is created for three types of recycled plastics, through injection molding, in an attempt to minimize function or context-bound effects, and to generalize meaning creation. Similar material stimuli are also used in various experiential studies found in literature [10,[47][48][49]. This stimulus set of each material consists of a bar, a doggy bone and a flat square, as visualized in Figure 1. Each material stimulus is labeled with the numbers shown above (with three forms within each material stimulus set). Consequently, participants are not able to associate the materials to their virgin origins, neither does the shape suggest any specific (past) usage or functionality. Since the used injection molding technique delivers very consistent samples, an individual stimulus set is provided for each participant, allowing them to bend or break their samples first-hand. Karana [37] states that "materials have a history, which helps us to assign meanings to them even when they are not embodied in products". Therefore, a standardized and equal stimulus set is created for three types of recycled plastics, through injection molding, in an attempt to minimize function or context-bound effects, and to generalize meaning creation. Similar material stimuli are also used in various experiential studies found in literature [10,[47][48][49]. This stimulus set of each material consists of a bar, a doggy bone and a flat square, as visualized in Figure 1. Each material stimulus is labeled with the numbers shown above (with three forms within each material stimulus set). Consequently, participants are not able to associate the materials to their virgin origins, neither does the shape suggest any specific (past) usage or functionality. Since the used injection molding technique delivers very consistent samples, an individual stimulus set is provided for each participant, allowing them to bend or break their samples first-hand.

Participants
In this study, the targeted stakeholder groups (material engineers and designers) are represented by master students to aim for as little prejudices as possible concerning the technical application of the materials. In total, 60 students aged 21 to 29 participated in the study (average age 23 years). The participants (n = 60, 43 males, 17 females) were recruited among master students during material related courses from both an engineering and a product development department at two universities in Flanders, one of the Belgian regions: material engineers from the Department of Materials, Textiles and Chemical Engineering at Ghent University (n = 30, 25 males, 5 females) and designers from the Department of Product Development at Antwerp University (n = 30, 18 males, 12 females). Using master students as participants increases the comparability in background as they already had the same training during their three bachelor years. Moreover, as they lack professional experience, they are not yet 'biased' by prejudices in industry that might limit the translation to real-life applications. Given the male majority among the engineering students, no equal gender distribution was achieved, however, this might also be a correct reflection of an industrial context.

Procedure
We conducted two sessions, one at the engineering department and one at the design department, during the same week. Both groups of participants (engineers and designers) followed the same procedure. The study took approximately 15-20 min for each participant and was conducted individually in a classroom. Each participant was given three stimulus sets (i.e., three forms for each of the three recycled materials), as described in the 'Stimuli' section and shown in Figure 1. The three stimulus sets were provided simultaneously, in order to facilitate the evaluation by comparing the three materials. Participants were instructed to evaluate the stimuli using all their senses (sight, touch, smell, hearing), except for taste since the recycled plastics are not food grade. Each material was assessed overall by means of its three forms together.

Participants
In this study, the targeted stakeholder groups (material engineers and designers) are represented by master students to aim for as little prejudices as possible concerning the technical application of the materials. In total, 60 students aged 21 to 29 participated in the study (average age 23 years). The participants (n = 60, 43 males, 17 females) were recruited among master students during material related courses from both an engineering and a product development department at two universities in Flanders, one of the Belgian regions: material engineers from the Department of Materials, Textiles and Chemical Engineering at Ghent University (n = 30, 25 males, 5 females) and designers from the Department of Product Development at Antwerp University (n = 30, 18 males, 12 females). Using master students as participants increases the comparability in background as they already had the same training during their three bachelor years. Moreover, as they lack professional experience, they are not yet 'biased' by prejudices in industry that might limit the translation to real-life applications. Given the male majority among the engineering students, no equal gender distribution was achieved, however, this might also be a correct reflection of an industrial context.

Procedure
We conducted two sessions, one at the engineering department and one at the design department, during the same week. Both groups of participants (engineers and designers) followed the same procedure. The study took approximately 15-20 min for each participant and was conducted individually in a classroom. Each participant was given three stimulus sets (i.e., three forms for each of the three recycled materials), as described in the 'Stimuli' section and shown in Figure 1. The three Sustainability 2020, 12,1953 6 of 27 stimulus sets were provided simultaneously, in order to facilitate the evaluation by comparing the three materials. Participants were instructed to evaluate the stimuli using all their senses (sight, touch, smell, hearing), except for taste since the recycled plastics are not food grade. Each material was assessed overall by means of its three forms together.

Measures
We utilized the lists of seventeen sensorial attributes seventeen interpretive characteristics, based on the frameworks discussed in the 'Framework' section. Drawing upon the principles of recent similar research [45], participants had to complete an evaluation sheet comprising a list of five-point bipolar semantic differential scales (−2, −1, 0, 1, 2) linked to the thirty-four characteristics, and shown in Appendix B. Only the participant's perception of these experiential qualities (sensorial attributes and interpretive characteristics) was studied, independent from application. Thus, in total, 102 scores were collected per participant (34 scores × 3 materials). In addition, their age, gender and study background were requested.
First, for each material, the prominent material characteristics (experiential qualities) are determined by means of One-Sample T tests (RQ1). Second, the significant differences between the three materials are determined by means of Paired-Samples T tests (RQ2). Third, the level of agreement among designers, among engineers and among the total respondent group, is assessed by means of calculating the standard deviations for each of the material characteristics per material (RQ3.1), while the significant differences between designers and engineers on the scores of the material characteristics of each material are calculated by means of Independent-Samples T Tests (RQ3.2).

Results
The mean scores and standard deviations for each material on each of the criteria, for the two stakeholder groups and for the entire group of 60 respondents are given in Appendix C. The analyses are discussed below.

Prominent Characteristics of Each Material
This section explores which sensorial attributes and interpretive characteristics are most defining for each material (RQ1). Therefore, the sensorial and interpretive scales are analyzed statistically by means of One-Samples T Tests for each material in order to identify which of the scores are significantly different from the 'neutral point' (Test Value = 0). For each material, this test is calculated overall, and for designers and engineers individually.

Prominent Characteristics of the rABS Material
The results for rABS are shown in Table 1 and ordered according to the total significance levels (designers and engineers combined). As mentioned before, the attribute of Transparency serves as our control variable, assuming all respondents would score all materials as very opaque. Logically, this attribute is located on top of the list. Moreover, of the 21 significant characteristics, sensorial attributes (13 counts) are found as more prominent characteristics than interpretive characteristics (eight counts). When focusing only on designers or on engineers, 20 significant characteristics were found within each group.
According to the total group of respondents, rABS is considered a material with the following most 'defining' sensorial attributes (p < 0.05): Opaque, Colorless, Hard, Low elasticity, Stiff, Intense color, Tough, Odorless, Shrill acoustics, Smooth texture, Brittle, Scratch resistant, Light and with the following interpretive characteristics: Aloof, Formal, Mature, Elegant, Ordinary, Honest, Aggressive, and Futuristic.  Note: Significant mean differences in bold. 1 Significant difference between designers and engineers (according to Appendix D).

Prominent Characteristics of the MPO Grey Material
The results for MPO grey are shown in Table 2 and again ordered according to the total significance levels, with again Transparency on top. Of the 21 significant characteristics, sensorial attributes (13 counts) are found more prominent than interpretive characteristics (8 counts). However, for this material more significantly prominent characteristics are found for designers (20 counts) than for engineers (16 counts). According to the total group of respondents, MPO grey is considered a material with the following sensorial attributes (p < 0.05): Opaque, Matte, Ductile, Weak color, Unbreakable, Soft acoustics, Light, Scratchable, Dry, Strong, Rough, Fragrant, Warm and with the following interpretive characteristics: Cheap, Not sexy, Toy-like, Vulgar, Informal, Disposable, Rugged, and Calm.  Note: Significant mean differences in bold. 1 Significant difference between designers and engineers (according to Appendix D).

Prominent Characteristics of the MPO Black Material
The results for MPO black are shown in Table 3 and ordered according to the total significance levels, with again Transparency on top. For this material, only fifteen significant characteristics are found, with more prominent sensorial attributes (11 counts) than interpretive characteristics (4 counts). MPO black is considered a material with the following sensorial attributes (p < 0.05): Opaque, Smooth, Ductile, Light, Soft acoustics, Unbreakable, Greasy, Scratchable, Color intense, Glossy, Colorless and with the following interpretive characteristics: Cheap, Not sexy, Toy-like, and Disposable.  Note: Significant mean differences in bold. 1 Significant difference between designers and engineers (according to Appendix D).

Significant Differences between Materials
This analysis investigates the significant differences between the three materials from the perspective of the total respondent group first, and from the perspective of the separate stakeholder groups (RQ2).

Significant Differences between Materials According to the Total Group
A Paired-Samples T Test is performed on each characteristic item. The Exact Significances (2-tailed) for each combination of two materials are shown in Table 4, with the bold results indicating differences between two material groups (p < 0.05). Total number of sign. differences 25 23 11 Note: Significant differences between materials in bold.
As expected, most similarities appear between the grey and black version of MPO, since the only objective difference between these two materials is the addition of black pigment. Obviously, this makes the black MPO more color intense than the grey edition. In addition, the pigment increases the experienced glossiness and smoothness of the surface, and appears to have an effect on odor, temperature, greasiness, and oddly on the perceived strength of the material. Regarding the interpretive characteristics, an effect of the black color addition is found for the perception of Elegant-Vulgar, Aggressive-Calm, Sexy-Not sexy, and Cheap-Expensive, i.e., the black MPO version is perceived less vulgar, less cheap, less unsexy and aggressive instead of calm.
When comparing the rABS material to the two MPO variants on a sensorial level, rABS is scored as less colorful but with an intense color (no contamination speckles such as the MPOs), harder, tougher, stiffer, shriller acoustics, more scratch resistant, more brittle, odorless, heavier and less elastic. On an interpretive level, rABS is more aloof, elegant, professional, sexy, lasting, formal, expensive and mature. Against expectations, rABS is not found glossier nor smoother than the black MPO, but is experienced aggressive, ordinary and futuristic in contrast to the calm grey MPO, and sober in contrast to the black MPO.
Overall, most significant differences are found between rABS and the MPO variants (25 and 23 counts), compared to within the MPO materials (11 counts). In addition, in all comparisons, more differences are found on the sensorial level than on the interpretive level. Table 5 shows the Exact Significances (2-tailed) of a paired-samples T Tests for each item and for each combination of two materials, according to designers. The bold results indicate differences between two material groups (p < 0.05).

Significant Differences between Materials According to Engineers
These results indicate that most dissimilarities occur when comparing rABS to MPO black. Total number of sign. differences 22 19 10 Note: Significant differences between materials in bold.
Sustainability 2020, 12,1953 13 of 27 Total number of sign. differences 21 16 8 Note: Significant differences between materials in bold. 1 Standard error of difference was zero.

Level of Agreement
Next, we aim to answer RQ3.1 to find out which attributes and meanings our participants agree upon; both within the group of material engineers (n = 30) and within the group of designers (n = 30) separately, as well as the agreement within the total group (n = 60). This is done by descriptively assessing the standard deviations for each material characteristic, as a high standard deviation might indicate a lack of agreement within a group of respondents. Table 7 shows the average standard deviations per material characteristic across the three materials, ordered by high to low level of agreement according to the total respondent group, and based on the standard deviations for each criterion across materials (last column). Our control attribute 'Transparency' has the lowest standard deviation: almost everyone scored the materials as very opaque, which in fact they are. This is an indication that participants evaluated the materials attentively. The other lowest standard deviations, and thus highest levels of agreement, are obtained for Ductility, Texture, Temperature, and Futuristic-Nostalgic. By contrast, the highest standard deviations are found for Elasticity, Disposable-Lasting, Ordinary-Strange, Colorfulness, and Scratchability.
The average of the standard deviations for all material characteristics is lower for engineers than for designers, indicating that engineers show a higher overall level of agreement. However, designers show a slightly higher level of agreement on the sensorial attributes than on the interpretive ones.
Additionally, sensorial attributes have a lower average standard deviation (i.e., higher level of agreement). This might reflect the greater difficulty or ambiguity in recognizing and associating meanings to (unknown) materials. In contrast, engineers show a higher agreement on interpretive characteristics. More specifically, the items of Color intensity, Weight, and Traditional-Modern have the largest differences between the standard deviations for designers and engineers, while Temperature, Strength, and Formal-Informal have the smallest differences in level of agreement.

Differences between Designers and Engineers
Finally, to explore which material characteristics evoked significant differences between designers and engineers (RQ3.2), an Independent Samples T Test is performed for each material separately. Table 8 depicts the means of each material characteristic of each material according to designers and engineers, while the footnote indicates whether the (2-tailed) T Test for Equality of Means was significant (see also Appendix D for all actual significant levels). Significance levels lower than 0.05 indicate significant differences in the characteristic's score between designers and engineers. A Levene's Test for Equality of Variances preceded this analysis in order to determine whether equal variances can be assumed or not (p < 0.05) to interpret the correct significance levels.
The results show that the rABS material induces the most significant differences (p < 0.05) between designers and engineers (12 counts), followed by MPO black (eight counts) and MPO grey (five counts). rABS shows significant differences between the two groups on the characteristics of Colorfulness, Weight, Acoustics, Scratchability, Ductility, Elasticity, Cozy-Aloof, Ordinary-Strange, Masculine-Feminine, Disposable-Lasting, Classic-Trendy and Mature-Youthful. In other words, engineers find rABS more colorless, lighter, feminine, disposable and trendy, while designers find the material softer sounded, more scratch resistant, tougher, less elastic, cozier, more ordinary, masculine, lasting, classic and more mature. MPO grey is perceived significantly different concerning Color intensity, Colorfulness, Texture, Weight and Traditional-Modern, meaning engineers find it colorless, smooth, lighter and traditional in contrast to designers who perceive the material with weaker color intensity, rough and modern. Finally, MPO black is scored significantly different regarding Odor, Weight, Greasiness, Strength, Futuristic-Nostalgic, Aggressive-Calm and Mature-Youthful. Engineers find it fragrant, lighter, dry, weak, futuristic, aggressive and mature, while designers score it odorless, greasy, strong, nostalgic, calm and youthful. The other characteristics do not show significant difference (p > 0.05).

Discussion
In order to facilitate their application in high-quality products, this study aims to understand the perception of three exemplary post-consumer recycled plastics. We have built upon existing frameworks in the context of Materials Experience to explore meaning creation of materials on the basis of sensorial attributes and interpretive characteristics to gain understanding into the way three specific recycled plastics are perceived by two main industrial stakeholder groups: material engineers and designers. In contrast to previous similar studies, one of the main contributions of this study is the focus on exemplary recycled plastics that are industrially processed and materialized in standard stimuli forms, and assessed by a considerably large participant group by means of standard measure scales. The results show differences and similarities between design and engineering respondents on the appraisal of recycled materials, and indicate which characteristics are most prominent in evaluating the material perception of post-consumer recycled plastics. Moreover, when a transition towards a valuable use of these materials is aimed for, different applications require different characteristics (both technical and experiential). Thus, the results of this study might indicate possible product applications of the three recycled plastics. As it is expected that we evolve to a better collection of plastic waste in the future, the amount of post-consumer recyclates will increase as well. Following this trend, it will be even more important to emphasize the intended material perception in order to differentiate different recycled material streams. Therefore, studies as presented here will have to be carried out repeatedly to facilitate the adoption of post-consumer plastics in meaningful applications, as was also done in previous research on emerging materials that focused on, for example, bioplastics [10] and natural fiber composites [50,51].

Identity of Recycled Plastics
Although no one-to-one rules exist that guarantee meaning-material relationships, certain patterns and defining characteristics (sensorial attributes and interpretive characteristics) of all three materials are detected, an overview of these trends within the design and engineering respondent group can be found in Figure 2, including the similarities between the recyclates (e.g., as expected, all three materials are evaluated as light and opaque). These insights can initiate future idea generation of high-quality applications. Overall, rABS is described smooth, hard and stiff, but brittle with an elegant, formal and aloof character. Thus, rABS might be an interesting material for office supplies, as such products often require stiffness (e.g., perforator), and a smooth and formal appearance that fits in an office environment. The grey MPO variant is perceived matte, ductile, warm and unbreakable, with a cheap and toy-like but rugged character. Thus, MPO grey might be convenient for outdoor toys and bicycle accessories that require high ductility, weather resistance, and must be/feel unbreakable and rugged. The black MPO edition is described smooth, glossy and unbreakable, however still with a cheap and toy-like character. Thus, MPO black might be suitable for strong fitness or sports equipment, as these products require similar strength properties, but the black color addition allows more high-end impression indoor. Next, Figure 3 takes a closer look at the sensorial attributes (Smooth-Rough, Matte-Glossy, Weak-Intense color, Dry-Oily, Odorless-Fragrant) and interpretive characteristics (Sexy-Not sexy, Expensive-Cheap, Elegant-Vulgar) that show significant differences between the three recycled materials.
In the context of sustainable perception of recyclates, the semantic pairs of Brittle-Unbreakable and Disposable-Lasting (partly reflecting high-quality perception) are also included, despite no significant differences between the two MPO materials are found for these characteristics. When plotting these characteristics against each other, correlations can be seen between an Expensive/Elegant/Sexy/Lasting look and a Cheap/Vulgar/Not sexy/Disposable look on the various sensorial attributes. group can be found in Figure 2, including the similarities between the recyclates (e.g., as expected, all three materials are evaluated as light and opaque). These insights can initiate future idea generation of high-quality applications. Overall, rABS is described smooth, hard and stiff, but brittle with an elegant, formal and aloof character. Thus, rABS might be an interesting material for office supplies, as such products often require stiffness (e.g., perforator), and a smooth and formal appearance that fits in an office environment. The grey MPO variant is perceived matte, ductile, warm and unbreakable, with a cheap and toy-like but rugged character. Thus, MPO grey might be convenient for outdoor toys and bicycle accessories that require high ductility, weather resistance, and must be/feel unbreakable and rugged. The black MPO edition is described smooth, glossy and unbreakable, however still with a cheap and toy-like character. Thus, MPO black might be suitable for strong fitness or sports equipment, as these products require similar strength properties, but the black color addition allows more high-end impression indoor. Next, Figure 3 takes a closer look at the sensorial attributes (Smooth-Rough, Matte-Glossy, Weak-Intense color, Dry-Oily, Odorless-Fragrant) and interpretive characteristics (Sexy-Not sexy, Expensive-Cheap, Elegant-Vulgar) that show significant differences between the three recycled materials. In the context of sustainable perception of recyclates, the semantic pairs of Brittle-Unbreakable and Disposable-Lasting (partly reflecting high-quality perception) are also included, despite no significant differences between the two MPO materials are found for these characteristics. When plotting these characteristics against each other, correlations can be seen between an Expensive/Elegant/Sexy/Lasting look and a Cheap/Vulgar/Not sexy/Disposable look on the various sensorial attributes.  This shows that, compared to the others, rABS is perceived as a rather expensive, elegant, sexy and lasting material, which could be attributed to its oily and rather matte look. Moreover, we find a clear difference between the two MPO variants regarding their sensorial attributes. The black color additive even affects several sensorial attributes that should have been the same from a theoretical, technical perspective for both grey and black MPO. A large difference is found concerning the glossiness, texture, odor, and greasiness. A large difference is found concerning the glossiness, texture, odor, and greasiness. However, the black color additive does not appear to be effective enough to completely convert the material perception of MPO to an expensive, elegant, sexy or lasting look. Nevertheless, MPO grey is scored significantly lower than MPO black on these characteristics (except for Disposable-Lasting), which might indicate practical implications for industry and design. In addition, both smoothness and glossiness are actually expected to be the highest for rABS instead of black MPO, based on their technical datasheets. All in all, insights within this graph should be considered when applying these recycled materials in new products. Further research is needed to understand the relation between the interpretive and sensorial characteristics, and the effect of changing specific sensorial attributes on the perception of the interpretive characteristics. This shows that, compared to the others, rABS is perceived as a rather expensive, elegant, sexy and lasting material, which could be attributed to its oily and rather matte look. Moreover, we find a clear difference between the two MPO variants regarding their sensorial attributes. The black color additive even affects several sensorial attributes that should have been the same from a theoretical, technical perspective for both grey and black MPO. A large difference is found concerning the glossiness, texture, odor, and greasiness. A large difference is found concerning the glossiness, texture, odor, and greasiness. However, the black color additive does not appear to be effective enough to completely convert the material perception of MPO to an expensive, elegant, sexy or lasting look. Nevertheless, MPO grey is scored significantly lower than MPO black on these characteristics (except for Disposable-Lasting), which might indicate practical implications for industry and design. In addition, both smoothness and glossiness are actually expected to be the highest for rABS instead of black MPO, based on their technical datasheets. All in all, insights within this graph should be considered when applying these recycled materials in new products. Further research is needed to understand the relation between the interpretive and sensorial characteristics, and the effect of changing specific sensorial attributes on the perception of the interpretive characteristics.

Differences between Stakeholders
The significant differences between engineers and designers could indicate what material characteristics both stakeholders would not easily agree upon during the design process. The results show that both the amount and the type of significantly different characteristics are very dependent on the material that is evaluated, as only weight induces significant differences for all materials, colorfulness for rABS and MPO grey, and Mature-Youthful for rABS and MPO black. In total, designers and engineers seem to have less significantly different perceptions of interpretive characteristics as compared to sensorial attributes. However, overall as well as in the designer group, the results show a higher level of agreement on sensorial attributes.
We argue that designers in collaboration with material engineers can alter and influence the material perception (e.g., by adding color additives, by in-/decreasing the sorting steps, alternative processing techniques, etc.) to increase valorization and adoption of recycled plastics as sustainable and high-quality materials, as is shown by the contrasting results between the two variants of the MPO material, and by, for example, high standard deviations for Disposable-Lasting (low level of agreement). This raises the question of whether the recycled look of these materials (e.g., speckles in grey MPO in playful outdoor toys) or rather the industrial quality through mass production must be emphasized (e.g., uniform color and glossiness of black MPO in professional fitness accessories)?

Study Limitations and Further Research
There are limitations to the current study that could inspire further research. In this study, we only use three types of flat injection-molded shapes, presented in an isolated setting to the participants, which is an incomplete approach for such non-technical material explorations. Future research could look into more appropriate forms for material evaluation, such as abstract forms that are more inspiring than flat shapes but do not evoke too many associations with existing products and contexts, or even a set of different types of real products that are materialized in various plastics, to the extent that this is practically feasible.
Next, similar to the education background effect, the influence of gender can be studied as well, but is found not valuable in this context because of the gender imbalance in the engineering respondent group, which is representative for students in this department.
Finally, this paper only focuses on three exemplary recycled materials; rABS, grey MPO, and black MPO. In further research, other recycled plastics should be studied as well, possibly exploring the difference between post-industrial and post-consumer plastics. Although designers and engineers are more familiar with the abstract concept of material samples and might experience fewer difficulties imagining materials in products, it would also be interesting to involve other stakeholders' perceptions as well. Therefore, future research should also replicate the current study to explore the perceptions of recycled materials by end consumers. Additionally, qualitative research techniques, such as interviews or workshops, could be used to develop a more in-depth understanding of the underlying reasons for the perception of recycled materials by engineers, designers and end consumers. This way, next research steps can contribute to further map the material perception of recycled plastics by all stakeholders, and the influence of industrial material alterations on the willingness to design (such as [13]) or on the consumer/user perception (such as [52]). Finally, further research should involve real products as proposed in Section 5.1 and made of recycled plastics, in order to verify whether the expected or intended material perceptions effectively match with the perception of all stakeholders.

Conclusions
In conclusion, this paper aims to create an overview pattern based on 60 appraisals to initiate future idea generation for high-quality plastic recycled material applications (material driven design) in a circular economy. It contributes to insights into how these sustainable recyclates can be differentiated on the market by enhancing their experiential qualities in order to address the target users, regardless of their added value from a technical perspective, and to transition towards a valuable use. In practice, industry must not only consider technical properties that are required for application in specific products, but also consider the perception of experiential qualities that is aimed for. Overall, the perception of sensorial attributes and interpretive characteristics varies considerably between the studied recycled materials, which leads to different suitability for specific applications. In addition, substantial similarities between designers and engineers can facilitate the design process when these stakeholders already agree on particular experiential qualities. Therefore, this study suggests possible strategies for the companies involved and underpins the potential of this evaluation method. We propose that, when emphasizing certain desired meanings, designers can facilitate the valorization and adoption of these undervalued materials, first by industry and ultimately by consumers as well.

Conflicts of Interest:
The authors declare no conflict of interest, as the funding industrial partners only provided recycled plastic materials to make stimuli samples, they had no role in the design, execution, interpretation, or writing of the study.     Note: Significantly equal means in bold. 1 Equal variances not assumed (p < 0.05).