Environmentally Friendly Product Personality: The Role of Materials, Color, and Light in Car Interiors

Abstract

The targeting of environmentally conscious German car drivers through product aesthetics can foster the acceptance of sustainable cars. No guidelines are currently available to designers to create product personality based on environmentally friendly design cues (EFDCs). The aim of this paper was to explore EFDCs for car interiors through bio-based materials. To address this topic, Study 1 examined a collection of bio-based material samples and samples featuring specific colors and reactions to light to determine their potential for the creation of an environmentally friendly product personality. Study 2 built on the implications of the former to examine the contribution of blue as a color and glowing attribute on the development of EFDCs. Wood veneer, cork, and cotton fabric were perceived as sustainable, natural, and renewable by most of the subjects. Brown and white leather was also perceived as sustainable. Moreover, the perception of the naturalness of materials in direct contact with blue light was reduced. Visual texture features for EFDC design are visible fibers or a wooden look. Haptic features include soft, warm, and rough surfaces, while glare, shimmer, a shiny surface, and smoothness should be avoided. The color brown should be considered, while blue, green, and yellow should be avoided.

1. Introduction

In the field of product development, the ability to communicate sustainability to users has gained as much importance as the sustainability of the products themselves [1,2]. This phenomenon is peculiar in car design, where major advancements improving sustainability in the use phase such as lightweight materials and, even more importantly, new powertrain technologies have made car market competition more challenging [3].

CO₂-emitting passenger cars may no longer be registered in the European Union from 2035; this means that petrol- or diesel-powered cars may no longer be sold, and the transition to zero-emission electric vehicles (EVs) must be promoted [4].

EVs are available in different variants. According to Rossini et al. [5], Hybrid Electric Vehicles (HEVs), Plug-in Hybrid Electric Vehicles (PHEVs), Battery Electric Vehicles (BEVs), and Fuel Cell Electric Vehicles (FCEVs) can be referred to as EVs.

While car manufacturers have to comply with the severe emission standards defined by policy (e.g., the European Emission Standards defined by the European Union), they have to justify the resulting price increase of the product to end customers. In this regard, targeting environmentally conscious drivers through product aesthetics can foster the acceptance of these cars, ultimately contributing to their market success compared to less-sustainable and cheaper vehicles. The product context helps in understanding how the designed product attributes can cope with user characteristics, the tasks the product is carrying out, and the physical and social environment in which it is used [6].

In this regard, industrial design is responsible for developing design cues that confer a specific aesthetic to the product so that it may be associated and perceived as sustainable. Design cues describe choices for design aspects that follow a specific aesthetic language to guide the design of a product [7]. Specifically, in car interiors, materials play an important role in the definition of a multisensory experience in the vehicle, since passengers can see, touch, and smell them. Consequently, Color & Trim designers could potentially choose sustainable materials that are perceived as environmentally friendly to develop environmentally friendly design cues (EFDCs). The working definition of sustainable material used throughout this paper is a material that is intentionally and methodically designed to lower the environmental impact versus the status quo through the adoption of natural renewable fibers or recyclates using a scientifically based environmental impact assessment tool (e.g., LCA) of the designer’s choosing [8].

The car industry implements user-driven methods to develop and test car interior aesthetics such as car clinics in which subjects are involved in direct personal interaction with the product to collect their feedback [9,10]. However, no contribution providing evidence on how design cues contributing to environmentally friendly product aesthetics were developed and tested are available in the literature. Among the barriers to eco-conscious, customer-oriented design identified, there is the difficulty in actually representing sustainability. Product personality is defined as the profile of human personality characteristics that people use to describe a specific product variant, i.e., a single physical product, and to discriminate it from others [11]. Products are symbols through which customers convey something about themselves to themselves and to others, and the word personality in product personality refers to the fact that people use human personality characteristics to describe their impression of a product [12]. However, the overall description of the personality of a single product is mainly defined by its appearance [13]. Unlike other personality properties, which are well translated into product semantics such as “aggressive” or “cute”, the act of being environmentally friendly for humans is only related to behavior (e.g., protecting water sources, helping clean the air, or protecting land and wildlife). Therefore, no existing visual symbolic meanings can be drawn for the definition of an environmentally friendly product personality.

Among past research contributions, the German study by Landau et al. [14] addressed the representation of electromobility characteristics for the design of the interior of electric vehicles (EVs), and their results are of specific interest for this work, as electromobility is often associated with environmental friendliness by subjects [15,16]. Landau et al. [14] asked subjects to see, touch, and choose material samples among 18 pre-selected materials for creating moodboards (i.e., image-boards that visualize mental connections and are implemented as design tools for developing a product aesthetic). The authors defined six characteristics according to a literature review to describe electromobility: “clean”, “driving pleasure”, “energy-efficient”, “innovative”, “low-noise”, and “sustainable”. Despite the great variation in the degree of agreement about associated materials, 46% of the involved participants associated a green artificial turf with the word “sustainable” because of its green color and organic character.

MacDonald and She [17] presented cognitive concepts for successful eco-design, and their work is of specific interest for this work, as they suggest what alternative visual symbolic meanings could be used by designers to create environmentally friendly product aesthetics. Among the seven concepts, the authors present customer decision heuristics as the ensemble of shortcuts that exist in a user’s mind in order to simplify judgments and decisions, and consumer trust towards the pro-environmental claims made by the product. To address these two issues, the authors advise designers to identify and use perceptual product cues that communicate environmental impact, e.g., by following nature-inspired shapes and forms that evoke environmental concern. This can help to build consumer trust, as the relationship between the crux (i.e., difficult for people to assess) product attribute “eco-friendly” and the sentinel (i.e., easy to assess and with a perceived association with the crux attribute) product attribute “nature” would ultimately generate a trustworthy and credible link between product and sustainability.

Lee et al. [18] addressed the representation of environmentally friendly characteristics for the design of the exterior of hybrid electric vehicles (HEVs), and their results provide information on users’ associations regarding shape, color, and texture. The authors understand the term “texture” as the finish trim (matte vs. glossy) of a surface. According to the authors, rounded, simple, and matte shapes can contribute to signaling green design to customers, while colors like white, green, and blue are perceived as eco-friendly.

Wehr and Luccarelli [19] addressed key German car drivers with pro-environmental behavior, and they considered the vehicle brands Mercedes, Volkswagen, and BMW for EU car segments A (i.e., mini cars, e.g., Fiat 500), C (e.g., medium cars, e.g., VW Golf), and E (i.e., executive cars, e.g., BMW 5 Series), respectively to analyze the design cues implemented for the design of both the exterior and interior of battery electric vehicles (BEVs) and HEVs; their results show that the analyzed German car manufacturers implement similar aesthetics. According to the authors, blue-colored sharp-edged shapes and glowing attributes play an important role in the visual representation of both car exterior and interior of these vehicles.

2. Aims and Objectives

This work builds on the knowledge that no guidelines are currently available to practicing Color & Trim designers to create an environmentally friendly product personality. As the ethical goal of product development is promoting sustainable products, rather than fostering greenwashing [20], this work specifically focuses on bio-based materials. Hence, the aim of this paper is to explore environmentally friendly design cues for car interiors through bio-based materials, thus fostering sustainability awareness through sustainable consumption. German car drivers with pro-environmental behavior are considered as target group.

The aforementioned considerations and past research contributions help to formulate research questions that set the actions needed in order to achieve the aim. As Landau et al. [14] have shown that the sustainability of materials does not necessarily match with their user-perceived environmental friendliness, user associations and perceptions towards bio-based materials have to be tested. Moreover, as MacDonald and She [17] have suggested including alternative visual symbolic meanings to simplify user assessment of environmentally friendly design cues, the meanings “natural”, “sustainable”, and “renewable” are investigated as well. Consequently:

• What bio-based materials are perceived as “sustainable”, “natural”, and/or “renewable”?

Finally, as past contributions mention blue as both a color being perceived as environmentally friendly [18] as well as a color and glowing attribute currently used by some car manufacturers to design BEVs and HEVs [19], its role in defining environmentally friendly design cues is further investigated. Consequently:

• What contribution does blue as color and glowing attribute in the definition of EFDCs make?

Our research questions helped in formulating our objectives as (a) to examine a collection of bio-based material samples and non-material samples to determine their potential for the creation of EFDCs, and (b) to examine the contribution of blue as a color and glowing attribute to the development of EFDCs. Qualitative research is an approach that examines phenomena by exploring individuals’ perceptions and experiences, aiming to gain a deeper insight into the factors that shape their viewpoints and choices.

In this regard, this work followed a qualitative, explorative approach in an under-researched area where small, purposive samples are methodologically appropriate to gain in-depth insights [21]. The sample size should be based on the information needed rather than a fixed target and can include up to 20 participants depending on the objectives and richness of data, while cross-case analysis becomes difficult as the number of cases increases and is kept manageable, often under 15 participants [22,23,24]. Core themes often saturate within the first twelve interviews [25]. Comparable perceptual and think-aloud studies use similarly small sample sizes, e.g., Landau et al. [14] with 15, Overvliet and Santo Faranco [26] with 16, and Fujisaki et al. [27] with 16 participants. In addition, think-aloud protocols provide rich data per participant and typically constrain the number of tasks administered, further favoring modest sample sizes [28]. Accordingly, the choice of 15 participants for this work prioritized depth and analytic tractability.

This work is made of two qualitative studies, i.e., Study 1 and Study 2, to address the defined objectives. Three user groups representing different generations were involved for both studies. Twenty samples were included in Study 1. According to the results of Study 1, five samples were analyzed without (OFF-Mode) and under (ON-Mode) the effect of blue light in Study 2. A 1:1 scale demonstrator of a car interior featuring a driving simulator, which was integrated for other research goals including product usability of in-car user interfaces [29], was designed and used for Study 2. The materials and methods for Study 1 and Study 2 are described in Section 3 and Section 4, respectively. Figure 1 illustrates how the two studies are linked to each other.

The outcomes of this article are expected to provide useful information for the creation of an environmentally friendly product personality for the interior of future cars through the use of bio-based materials. This article plays an important role in the development and diffusion of shared visions about the future design of environmentally friendly vehicles. From the designer’s perspective, the identified EFDCs are intended to serve as the basis for future car interior development. From the car manufacturer’s point of view, customers can use their perceptions to select environmentally friendly vehicles when buying and consciously experience them positively during possession and use. The remainder of this paper is structured as follows. In the next Section, Study 1 is presented along with the implications for Study 2, which is highlighted in Section 4. Section 5 presents the Discussion, while the Conclusions along with future outlook can be found in Section 6.

3. Perception of Naturalness, Sustainability, and Renewability in Bio-Based Material Samples and Non-Material Samples

Study 1 examines a collection of bio-based material samples and samples featuring specific colors and reactions to light to determine their potential for the creation of an environmentally friendly product personality.

3.1. Materials and Methods

This section describes the methodological procedure in Study 1. The following subsections first describe the selection of materials and the design of the samples used for the evaluation (Section 3.1.1). The characteristics and allocation of the participants to the three groups are then presented (Section 3.1.2), followed by a description of the laboratory environment in which the study was conducted (Section 3.1.3). In the following subsection, the data collection procedure is explained (Section 3.1.4), including the task given to the participants and the data collection process. Finally, an overview of the methods used for the data analysis is given (Section 3.1.5).

3.1.1. Material Selection and Sample Design

A pre-selection of 58 samples including bio-based material samples and non-material samples including colors and textures was carried out. The pre-selection of available and suitable bio-based materials was based on literature and market research. It was guided by three overarching themes: technical aspects, sustainability aspects, and feasibility for the design of key car interior components contributing to driver perception. These themes were defined to ensure that the chosen materials met automotive performance requirements, demonstrated environmental responsibility, and could be perceived while driving. The technical aspects included flame retardancy, abrasion resistance, UV stability, water and oil repellency, and resistance to soiling. The sustainability aspects were assessed primarily based on the bio-based content of the material. In this study, “bio-based material” was defined as a material featuring (a) natural fibers of plants or animal origin that can be processed into end products without synthetic modification and/or (b) biopolymers derived from renewable resources and/or capable of biodegradation. Delivery distance and sustainable production were also taken into consideration for sustainability aspects. Regarding material perception, key interior components contributing to driver perception were considered according to [8], excluding the components “seats” and “car doors”.

Regarding non-material samples, the design cues analyzed by Wehr and Luccarelli [19] were considered for the investigation of environmentally friendly design features. For this purpose, visual properties such as color, transparency, or translucency were taken into account with regard to their reaction with light. This resulted in a collection of nine samples featuring different shades of blue, yellow, and green, with blue and yellow also appearing as glowing elements resembling present shades used by German car manufacturers.

The final selection of 20 samples was carried out to guarantee manageability of experiment design while offering material variety. In order to present the subjects with a manageable and, ideally, non-tiring number of samples, similar studies on material evaluation were used as a reference for the number of samples [14,26,27,30].

The selected materials and identified design cues were replicated for user evaluation. They were cut to a size of 7.5 × 10 cm. With the exception of the acrylic glass, all materials were placed on a white sheet measuring 11.5 × 14 cm. To display the acrylic glass, an opening was cut in the white sheets, into which the material was placed so that light could pass through the sample. Hence, the same neutral background could be provided for all materials. A variety of non-material samples, including colors and textures currently used by automotive manufacturers as EFDCs and bio-based materials, were selected. According to Djonov and van Leeuwen [31], texture can be used to describe visual and tactual aspects of a surface; following their definition, relief, density, roughness/smoothness, and consistency were considered and defined in this paper if applicable to describe visual texture aspects. The selected samples are presented in

Table 1. Selected bio-based material samples and identified EFDCs.

No.	Sample	Material	Sample Characteristic	(a) Properties/ (b) Design Cue origin	Color	Visual Texture	Haptics	Relevance for EFDCs
1		wool fabric	material	(a) fine fabric, 100% new wool, zig-zag twill	grey/white	medium density	softness, warmth	bio-based
2		woven felt	material	(a) 100% wool	dark blue	fibers, medium density	rough- ness	bio-based
3		cotton fabric	material	(a) lightly sanded, satin weave	beige	homogeneity	softness, warmth	bio-based
4		cork	material	(a) thin, fine worked	light brown	wooden look, relief	softness, warmth, smooth- ness	bio-based
5		PMMA, acrylic glass	imitation	(b) Volkswagen	blue	shine	smooth- ness	color and reaction to light
6		Viscose fabric	material	(a) satin character, satin weave	green/blue	shimmer	smooth- ness	bio-based
7		cast PVC foil	imitation	(b) Volkswagen	blue	shine	smooth- ness	color and finish
8		PMMA, acrylic glass	imitation	(b) Porsche	acid green	glare	smooth- ness	color and reaction to light
9		natural-fiber composites	material	(a) Purcell, 100% cellulose	light beige	wooden look, medium density, shine	smooth- ness, warmth	bio-based
10		bark	material	(a) thin, calendered	brown	fibers, relief	rough- ness	bio-based
11		leather	material	(a) cow, olive leaf tanned, biologically dyed	dark green	relief	soft	bio-based
12		natural- fiber compo- sites	material	(a) flax fabric, bio-based polyamide matrix	grey	low density, fibers, shine	smooth- ness	bio-based
13		cast PVC foil	imitation	(b) Volkswagen	green	glare, shine	smooth- ness	color and finish
14		PMMA, acrylic glass	imitation	(b) MINI	yellow	glare, shine	smooth- ness	color and reaction to light
15		wood veneer	material	(a) beech, structured, flexible, laser cut	dark brown	wooden look, low density, relief, heterogeneity	wooden haptic	bio-based
16		cast PVC foil	imitation	(b) Smart	blue metallic	shimmer, shine	smooth- ness	color and finish
17		cast PVC foil	imitation	(b) Porsche	acid green	glare, shine	smooth- ness	color and finish
18		cast PVC foil	imitation	(b) MINI	yellow	glare, shine	smooth- ness	color and finish
19		cast PVC foil	imitation	(b) BMW	blue metallic	shimmer, shine	smooth- ness	color and finish
20		natural- fiber compo- sites	material	(a) tangled scrim, hemp, flax, sisal, epoxy resin	brown	fibers, medium density, relief, heterogeneity		bio-based

.

3.1.2. Participants

A group of 15 participants was involved in the material evaluation. Their selection was based on the three groups of key German car drivers with pro-environmental behavior described by Wehr and Lucarelli [19]. According to the personas created by the authors, these correspond to three age groups belonging to generations Z, Y, and X. Due to current European demographic trends [32], Generation X represents a substantially larger fraction of the German car driver population compared to the younger generations Y and Z in the targeted region. To avoid bias towards the views of the oldest generation and to enable a balanced comparison, five subjects were chosen for each generation, regardless of their actual population proportion. The 15 subjects were assigned to groups based on their age and matching generational characteristics, e.g., stage of life, financial considerations, affinity to technology, and preferred car segment. The gender of the groups was determined based on which gender made up the majority of the target group. In the case of group Z, which stands for new trends in transportation, the users of car sharing, which is considered a sustainable form of mobility, were decisive. Although the larger proportion of members is male, the female proportion of members is continuously increasing, which is why group Z was defined as female. These selection criteria were chosen for Study 2 as well.

Generation Z is known as iGeneration and prefers car segments A and B. According to the persona, subjects in group Z were female and born between the years 1996 and today [19]. The subjects of this generation had an average age of 20 years (SD = 0.89) and were students.

Generation Y is also called Millennials, who would buy cars of the segments C and D, were male and born between 1980 and 1995. The Generation Y subjects had an average age of 31 years (SD = 4.92) and were university employees with various educational qualifications.

Generation X, also known as Quintastics, who prefer to drive cars of the segments E and F, were male and born between 1965 and 1979. The subjects assigned to Generation X had an average age of 57 years (SD = 1.22) and included self-employed persons, civil servants, and university employees.

The total sample therefore comprised five females and ten males with an average age of 31 years (SD = 16.13) and varying education levels.

In addition to the 15 participants involved in Study 1, 6 subjects were selected to define the terms that are associated with environmental friendliness. The subjects were between 27 and 63 years old; 4 of them were male and 2 were female.

3.1.3. Laboratory Setup

The evaluation took place in a room with the same lighting conditions for each test. The subject was given a seat at a table on which the material samples were placed. In order to be able to simulate the glowing elements identified as potential EFDCs, a transmitted light table guaranteeing a uniformly illuminated light source was located next to the desk, where the PMMA acrylic glass samples could be placed on. A sound and video recording device were installed on a tripod next to the table, and the study director was seated at the left end of it.

3.1.4. Data Collection Procedure

The subject’s perception of materials was evaluated according to 10 given adjectives. To define the ten adjectives, 30 terms were collected from studies that refer to natural, sustainable, or bio-based materials [11,30,33]. These terms were presented to the first group of six subjects. They were asked the question “Is this a word you associate with environmentally friendly materials?”. Possible answers to choose from were “yes”, “no”, or “don’t know”. The three adjectives with the highest agreement to be associated with environmentally friendly materials were included in the final list of terms. In order not to make the purpose of the analysis clear to the subjects involved, thus potentially influencing their choice, seven environmentally friendly-unrelated terms were added. All terms were thought to encourage subjects to verbally reflect on their general impressions of the materials, while environmentally friendly-unrelated terms were not evaluated for the scope of our work. The final list of terms thus contained the following terms: natural, sustainable, renewable as terms for environmental friendliness, and aesthetic, technological, high-quality, honest, innovative, uncomplicated, cozy as terms that are used to evaluate materials but are not directly associated with environmental friendliness.

Before each subject started the evaluation, a test with color tables by Kuchenbecker et al. [34] was performed to determine the subject’s sense of color. The task for sample evaluation was then read out. The ten adjectives were presented to the subjects for each material in the form of a 7-point Likert scale with ratings from 0 to 6. The subjects were asked to assess materials perception in relation to the provided adjectives and speak aloud their thinking and the motivation for their thinking, e.g., optics, haptics, and color. The think aloud method [28] was used to provide insight into the assessment of perceived naturalness, sustainability, and renewability of materials, while the ten adjectives served as a basis for evaluation and a guiding principle for evaluation. The order of the samples was rearranged randomly for each subject to prevent order effects like inexperience or fatigue. The samples were evaluated one after the other. The subjects were allowed to see, touch, and smell the different materials as well as listen to the sound they made. They could hold the samples with acrylic glass against the light and place them on the transmitted light table. In addition to video recording, the comments and reactions of the subjects were noted. The duration of the test varied between 20 and 80 min, depending on the detail of the answers of the subject.

3.1.5. Data Analysis

Video recordings for the think aloud method were used to transcribe the verbal thoughts recorded during the materials evaluation. The transcriptions were coded by the first author and analyzed according to the perceived material properties to identify the characteristics that describe an environmentally friendly material. Considering that 10–25% of the data is typically coded multiple times in qualitative research [35], 25% randomly selected transcripts were coded by the second author as well to ensure rigor, trustworthiness, and consistency. The double coding achieved a Krippendorff’s alpha of 0.845. The codes were haptics, visual appearance, and color. The subjects’ comments were assigned to two categories as environmentally friendly and not environmentally friendly.

3.2. Results

This section presents the results of the evaluation of the material samples presented above by 15 subjects. The results were classified into themes and subthemes and then supported by representative statements by the subjects.

Themes

The following five main themes were discovered for this study: (1) material properties supporting environmental friendliness, (2) material properties reducing perception of environmental friendliness, (3) color, (4) doubts regarding authenticity, and (5) interpretation of the term renewable.

Table 2. Themes, subthemes, and key facts of Study 1.

Theme	Subtheme	Key Facts
Material properties supporting environmental friendliness perception	Roughness Soft and warm surface Relief and medium to low density Visible fibers Wooden feel and look	Perception of heterogeneous surfaces, characterized by the properties mentioned in the subthemes, as environmentally friendly
Material properties reducing perception of environmental friendliness	Glare Shimmer Shiny surface Smoothness	Perception of homogeneous surfaces, characterized by the properties listed in the subthemes, as less environmentally friendly
Color	Blue Brown Green Green-Blue Yellow	Individual associations of brown mostly as environmentally friendly. No general categorization for or against environmental friendliness perception for other colors possible
Doubts regarding authenticity	-	Natural appearance perceived as intentional deception
Interpretation of the term renewable	-	Misinterpretation of renewable as reproducible

provides an overview of the themes, subthemes, and key facts.

Likewise, all themes and subthemes are described in this section below with two exemplary quotes, with one exception where only one subject could be cited. Subject names have been replaced with an abbreviation indicating subject group affiliation and subject number. The material on which the comment was made is also indicated.

• Material properties supporting environmental friendliness perception
• Roughness
• Rough surfaces fostered the association of the subjects with environmental friendliness and naturalness.

Natural, even, feels natural.

[Y1, bark, 10]

Felt or woodchip, tennis balls, insulation. Or is that cork? Cowhide.

[Y5, bark, 10]

• Soft and warm surface
• The subjects described the feel of the surface of natural materials such as cotton fabric and cork as soft and warm. Materials with these properties were rated as very natural in general.

Very pleasant, because it’s soft.

[Z5, cotton fabric, 3]

Warm, has a certain surface haptic.

[X3, cork, 4]

• Texture
• Textured materials that feature a relief or a medium to low density were perceived by the subjects as occurring in nature and thus as environmentally friendly overall.

Completely natural, because it has a disordered structure.

[Y1, natural-fiber composite, 20]

Natural, also because there are always darker/brighter parts in it.

[X4, natural-fiber composite, 9]

• Visible fibers
• Visible fibers ensured that materials were perceived as more environmentally friendly.

I think it is natural, just from the fibers or wood scraps that are in it.

[Z5, natural-fiber composite, 20]

A lot of small fibers pressed together to something.

[Y2, bark, 10]

• Wooden feel and look
• Materials with a haptic or appearance reminiscent of wood were perceived by the subjects as environmentally friendly in the sense of being natural, sustainable, and renewable.

Looks like wood, feels like wood, very sustainable.

[Z3, wood veneer, 15]

Very natural, looks like wood.

[Y5, wood veneer, 15]

• Material properties reducing perception of environmental friendliness
• Glare
• Materials whose colors are very glaring were perceived by the subjects as less natural and consequently less environmentally friendly.

Too glaring, not very natural.

[Z4, PMMA, acrylic glass, 8]

Artificially glaring.

[X3, cast PVC foil, 13]

• Shimmer
• Shimmering or metallic surfaces were perceived as artificial and not environmentally friendly by the subjects.

Relatively less natural due to the shimmering effect.

[Z3, cast PVC foil, 16]

Metallic, aesthetic, technological, not natural.

[Z4, cast PVC foil, 19]

• Shiny surface
• Only one subject commented on shiny in connection with natural. Shiny surfaces were nevertheless included as a material property, as this seems to have a negative influence on the perception of environmental friendliness, even if not every subject explicitly verbalized it in the respective terms (natural, sustainable, renewable).

Color does occur in nature, but the shiny surface makes it look less natural.

[Z3, cast PVC foil, 18]

• Smoothness
• Smooth surfaces were not associated with environmental friendliness by subjects for both bio-based material samples and non-material samples.

Not sustainable at all because it feels so rubbery and smooth.

[Z3, cast PVC foil, 17]

What is this? It looks like fabric fibers with some kind of coating. [...] Is it sustainable? It looks artificial.

[Y2, natural fiber composites, 12]

• Color
• The effect of the individual colors of the materials studied cannot be clearly categorized as for or against environmentally friendly perception as the previously mentioned properties. Different associations arose among the subjects, which are listed per color below.
• Blue
• The color blue led subjects to both associations that support environmentally friendly perceptions and those that reduce them.

Not natural, although it looks like blue sky.

[X2, PMMA, acrylic glass, 5]

Quite natural like the sea.

[Z5, PMMA, acrylic glass, 5]

• Green
• For non-material samples, the color green ensured that they appeared slightly more natural to the subjects. In the case of bio-based leather, on the other hand, the green appeared to have a negative impact on environmentally friendly perception.

It is like the yellow sample, but it seems more natural and less technological due to the color accordingly.

[Y1, cast PVC foil, 13]

Whereas the color... green leather does not occur in nature.

[Z3, leather, 11]

• Green-blue
• The green-blue of the viscose material was perceived as less natural by the subjects.

The color is moderately a natural color.

[Y1, viscose fabric, 6]

The color appears unnatural.

[Y5, viscose fabric, 6]

• Yellow
• Opinions of the subjects varied on yellow. The color shade also seems to have an influence, because samples 17 and 18 of cast PVC foils were considered either naturally occurring or non-occurring.

Not natural at all, because color does not occur in nature.

[Z3, cast PCV foil, 17]

Yellow looks natural.

[Y5, cast PVC foil, 18]

• Brown
• The majority of subjects perceived brown as conducive to the association with environmental friendliness. In rare exceptions, brown was rated as less natural.

Earth tone, looks very natural due to tone.

[Y1, bark, 10]

The color is rather natural.

[Z5, natural-fiber composites, 9]

• Doubts about authenticity
• Some bio-based material samples were questioned with regard to authenticity. Subjects often agreed that they looked natural but believed that this could be an intentional deception.

I think it is artificial leather.

[Y3, leather, 11]

It pretends being natural.

[X5, natural-fiber composites, 12]

• Renewable—ambiguous understanding of the term
• The term “renewable” was interpreted differently by the subjects. Although the experimenter explained its meaning to those subjects asking for clarification, the term was misinterpreted during the experiment. Instead of referring to renewable raw materials, the subjects often interpreted this term as the material quality of being reproducible.

This is certainly also renewable, the product can be manufactured again and again.

[Y5, cast PVC foil, 18]

Renewable, so whether it can be recycled? In that sense?

[Z3, cast PVC foil, 7]

3.3. Implications for Study 2

In light of the results, four topics are considered and highlighted herein. Firstly, the results have shown which combinations can lead to an increase of the perception of sustainability. This should contribute to the targeted selection of materials or material properties for an environmentally friendly product personality, i.e., (a) the selection of materials that have one or more material properties that are supportive of the perception of environmental friendliness. Secondly, it appears possible to obtain insight into why some materials are perceived as not environmentally friendly, even though they essentially are, i.e., (b) the avoidance of materials that possess material properties that reduce the perception of environmental friendliness. Thirdly, with the exception of the color brown, most of the colors used led to different results in the perception of environmental friendliness. It is important to take this into account when selecting materials in order to promote the desired perception of environmental friendliness, i.e., (c) the preference of the color brown for material selection. Finally, due to the above-mentioned different results in the perception of environmental friendliness based on the colors used in the study, it is relevant to highlight what impact the colors can have on certain materials that are conducive to a perception of sustainability, hence (d) the determination of the potential of colors with diverse influence on the perception of environmental friendliness in combination with materials corresponding to above-mentioned aspects.

(a) Study 1 identified material properties that support the perception of environmental friendliness. These are properties that give the materials a natural look, ranging from some irregularity as visible fibers as in natural fiber composite (20), relief and low density to wooden look as in wood veneer (15). Natural haptics are perceived to be either rough, as in the case of bark (10), or soft, as in the case of cotton fabric (4). In many cases, the combination of several material properties that are perceived as natural leads to an increase in the perception of environmental friendliness. One of them is the occurrence of the properties texture with wooden look, as is the case with the Purcell material (9). In this regard, the natural-fiber composite (9) was mentioned as natural due to its texture. It was also described as a material with a wooden look. The wood veneer (15), which has a warm, wooden appearance (see Table 1), is also perceived as sustainable and natural by the subjects because it has a visible wood structure.

(b) Study 1 identified material properties, which should be avoided, as they are reducing the perception of environmental friendliness. These include glare, as with cast PVC foil (13), shimmer, as with cast PVC foil (16), shiny surface, as with cast PVC foil (18) and smoothness, as with natural fibers composite (12). The combination of properties supporting and those reducing the perception of environmental friendliness ultimately results in a reduced perception of environmental friendliness and should be avoided. For example, if visible fibers meet a smooth surface, the material tends to be perceived by subjects as less environmentally friendly. The materials 9 and 12, both natural-fiber composites, can be mentioned as examples (see Table 1). Regarding material 12, subjects describe it as artificial because of its coating although visible fibers are also named by subjects. Doubts about the authenticity of a bio-based material are increasingly associated with the combination of properties that promote and those that reduce the perception of environmental friendliness. For example, the bio-based material leather is one of the materials for which many subjects had doubts about. Here, material properties such as texture understood as a relief, which are supportive, and material properties that are reductive to the perception of environmental friendliness, such as the color green, occur together.

(c) Study 1 shows that brown is a color that is considered natural by the majority and should therefore be preferred as it promotes the perception of environmental friendliness. This is evident in materials such as bark (10), cork (4), and wood veneer (15). In some cases, as for natural-fiber composites (9), the color brown was named as supporting the perception of environmental friendliness, but due to the combination of material properties that reduce the perception of environmental friendliness, it was not sufficient for the material to be perceived as environmentally friendly as a whole. The color brown, which is a color that strongly contributes to materials being perceived as environmentally friendly (see Section 3.2, color), promotes the perception of environmental friendliness if it appears in combination with a naturally perceived texture. This applies, for example, to the material natural-fiber composite (20), which contains several shades of brown and has a texture that is perceived as natural. Leather, which was dark green in this study, was often perceived as less natural. The color green was cited as the reason for the reduced perception of naturalness (see Section 3.2). Its perceived natural texture in combination with a color that could be identified as conducive to a perception of sustainability in this study can promote the categorization of leather as a sustainable material.

(d) The colors blue, green, and yellow could be determined as colors with diverse influence on the perception of environmental friendliness. Blue is associated with nature by many subjects, as shown by the examples in Section 3.2, where two subjects associated blue with the blue sky or the sea. Sometimes, however, this is not enough to make the material appear sustainable, as was the case with PMMA, acrylic glass (5). Green and yellow showed very different reactions in the subjects. Although some of the subjects described the colors as natural, because the colors appear in nature or are associated by subjects with elements of nature like the sun in case of the color yellow, the material carrying the color was not necessarily perceived as environmentally friendly. In the case of the color green, it happened that a material that in itself appeared environmentally friendly was reduced in this effect because the color appeared unnatural in combination with this material (11, leather). The color green-blue of the viscose fabric (6) was rated as unnatural and did not increase the perception of environmental friendliness in any of the subjects. In this context, it should be mentioned that it was a color that appeared greener or bluer depending on the viewing angle, and the color also had a shimmering effect, which was already identified in this study as reducing the perception of environmental friendliness.

In light of the highlighted topics, following materials were chosen for Study 2:

• Brown leather
• Cork
• Cotton fabric
• White leather
• Wood veneer

4. Influence of Blue as Color and Glowing Attribute on Environmentally Friendly Materials Perception

Study 2 was set to examine the contribution of blue as color and glowing attribute on the development of EFDCs. In light of the results of Study 1 showing that blue as a color only led to heterogeneous perceptions of environmental friendliness, Study 2 specifically considered blue light to investigate its influence on the environmentally friendly material perception. Consequently, the materials selected in Study 1 were investigated according to two modes. The mode “OFF” addressed the car interior as if the driver had just entered the car, i.e., the perception of materials with blue light off. The mode “ON” addressed the interior as if the driver had turned on the car ignition, i.e., the perception of materials with blue light on. A 1:1 scale demonstrator of a car interior including blue light features and chosen bio-based materials was designed accordingly and used for this purpose.

4.1. Materials and Methods

As the subjects for this study were chosen according to the same parameters selected for material evaluation in Study 1 (see Section 3.1.2), no further information about participants is provided in this section.

4.1.1. Demonstrator Design

The results concerning the perception of bio-based material samples and non-material samples with regard to naturalness, sustainability, and renewability were implemented in the design of an automotive interior to set materials perception into the product context.

Table 3. Key material properties determining the selection of materials for car interior components.

Material	Material Properties Supporting Perception of Environmental Friendliness	Car Interior Components
Brown leather (11)	Texture, color brown	Crash pad, steering wheel
Cork (4)	Texture, soft and warm surface, color brown	Applications around displays, gearshift lever, and logo on steering wheel
Cotton fabric (3)	Soft and warm surface	Center console
White leather (11)	Texture	Crash pad, center tunnel console
Wood veneer (15)	Wooden feel and look, texture, color brown	Crash pad, steering wheel

lists the selected materials, their properties increasing the perception of environmental friendliness, and the car interior components to which the materials were assigned.

Leather was used for the steering wheel, parts of the crash pad, and the center tunnel console due to its texture promoting environmental friendliness perception. In contrast to the dark green leather (11) considered in Study 1, a brown leather was chosen for the steering wheel and most of the crash pad in accordance with the results of Study 1. In contrast, a creamy white leather was used for the lower part of the crash pad and the center console.

Cork was used in the demonstrator for applications around the display, the gearshift lever, and the logo in the steering wheel due to its material properties supporting the perception of environmental friendliness, i.e., texture, soft and warm surface, and color brown.

The cotton fabric was chosen for the center console because the surface of this material was described as soft and warm, and this car interior component, as a user interface, is an area with which hands come into contact. The cotton fabric is semi-transparent and was therefore also used as a textile switch combined with blue light.

The flexible wood veneer was used to cover parts of the crash pad and the steering wheel. This material was chosen because of its material properties supporting the perception of environmental friendliness, i.e., the wooden feel and look, the texture of wood and the color brown. Due to its manufacturing process, the wood veneer has a structure that also contains holes, making the material translucent in these areas. This characteristic was used to combine the wood veneer with the color blue in the form of light as a feedback transmitter.

Based on the materials that were considered most suitable in terms of naturalness, sustainability and renewability in Study 1, three color and trim concepts were developed. The concepts had to ensure a context of perception of materials in line with current trends in automotive design for the chosen target. The study by Wehr and Luccarelli [19] was taken into account to include additional information regarding user groups and current vehicle interiors fitting with the chosen context. Various elements and materials from the three concept proposals were finally integrated and combined in the design of a 1:1 scale demonstrator of a car interior. As mentioned in Section 2, this demonstrator was used also for other research goals, and it included switches as operation features, lights as feedback features, and displays both as operation and feedback features to ensure information transfer between car and driver. Due to these usability features, some materials implemented for this study were actively glowing/appeared to glow in blue, while others were passively illuminated in blue. The glowing materials used for the design of the demonstrator included a wood veneer on the dashboard and steering wheel and a cotton fabric on the center console and steering wheel, as highlighted with blue frames in Figure 2.

Since the shades of blue used by the German car manufacturers listed in Table 1 are very similar to each other, blue shade number 5 was selected for the ON-Mode in the study. NeoPixel-LED plates and NeoPixel-LED strips [36] were used for the blue glowing usability features. Each glowing element is a Pixel, consisting of three LEDs and offering 24-bit color.

Table 4. RGB-LED characteristic parameters [37].

Color	Wavelength (nm)	Luminous Intensity (mcd)
Red	620–625	700–1000
Green	522.5–525	1500–2200
Blue	467.5–470	700–1000

lists the wavelength and luminous intensity of the included LEDs.

The passively illuminated materials were a dark brown leather and a creamy white leather on the dashboard and steering wheel, and finally cork for the frames surrounding the displays of both the instrument cluster and the center console as well as around the shift boot.

4.1.2. Laboratory Setup

The demonstrator was placed in a room with the same lighting situation for each test using appropriately positioned luminaires. The subject was instructed to take a seat at a predefined distance from the color charts for the color vision test. For the main part of the experiment, the subject sat in the driver’s seat of the demonstrator. Two cameras for sound and video recording facing towards the subject were positioned in front of and diagonally behind the demonstrator. In order to be able to assign the materials to the evaluation sheets, small cards with numbers marked the corresponding areas of the demonstrator. The preparation of the demonstrator for user analysis and the laboratory setup are shown in Figure 3.

4.1.3. Data Collection Procedure

As in Study 1, a test with color tables by Kuchenbecker et al. [34] was performed to determine the subject’s sense of color prior to the evaluation. After completing the color vision test, the subject was asked to sit on the seat of the driving demonstrator and had the opportunity to obtain a first impression of the demonstrator and its materials. The task for the evaluation of the samples was then read out. The subjects were allowed to see, touch, and smell the different materials. Using the think aloud method, subjects were asked to express their thoughts out loud about the evaluation of their impressions, e.g., optics and haptics. The evaluation was divided into two sessions. For each of the two sessions, the subjects were given an evaluation sheet of the ten adjectives used in Study 1. In the first evaluation run, the vehicle interior was simulated in the OFF-Mode. After this was completed, the LEDs and screens were switched on, and the subjects then evaluated the materials in the ON-Mode. The evaluation was video recorded with two cameras to catch two different angles. The verbal comments of the subjects were noted. The duration of the test varied between 16 and 45 min, depending on the comprehensiveness of the subject’s answers.

4.1.4. Data Analysis

Video recordings of the material ratings were used to transcribe subjects’ verbalized thoughts. As in Study 1, the transcriptions were coded by the first author, and 25% of randomly selected transcripts were additionally coded and analyzed according to the perceived environmental friendliness of the materials in OFF-Mode and ON-Mode by the second author. The double coding achieved a Krippendorff’s Alpha of 0.833. Codes used were naturalness, sustainability, renewability, haptics, appearance, authenticity, blue light, color, misinterpreted term. The themes emerged from the codes were subsequently exemplified with statements from the subjects.

4.2. Results

The results of Study 2 are sectioned into OFF- and ON-Modes. Since the subjects started their evaluation in OFF-Mode, the perceived naturalness, renewability, and sustainability, as well as the color and authenticity of the materials used, is presented in Section 4.2.1. In the ON-Mode, however, the remaining or diminished naturalness was increasingly mentioned by the subjects, and the color blue and the effect of blue light were commented on. In both modes, the conceptual understanding of renewable was a theme.

4.2.1. OFF-Mode

A total of six OFF-Mode themes structured into subthemes were defined: (1) perceived naturalness of materials, (2) relevance of color, (3) perceived renewability of materials, (4) perceived sustainability of materials, (5) doubted authenticity, and (6) conceptual understanding of renewable. If applicable, two quotes were given per theme or subtheme from subjects whose names were replaced with subject numbers. The OFF-Mode themes along with related subthemes and key facts are summarized in

Table 5. Themes, subthemes, and key facts of Study 2 in OFF-Mode.

Theme	Subtheme	Key Facts
Perceived naturalness of materials	Brown leather	Majority perception of the materials as natural; Slightly less frequent perception of the naturalness of white leather
	Cork
	Cotton fabric
	White leather
	Wood veneer
Relevance of color	Cotton fabric	Existing relevance of color for perception of naturalness
	White leather
Perceived renewability of materials	Brown leather	Dependence of perception of renewability on authenticity
	Cork	Link between perception of renewability and bio-baseness
	Cotton fabric	Link between perception of naturalness and perception of renewability
	Wood veneer	Link between authenticity and renewability
Perceived sustainability of materials	Brown leather	Mixed perception of sustainability
	Cork	Strong perception of renewability
	Cotton fabric	Link between perception of naturalness and perception of sustainability
	White leather	Mixed perception of sustainability
	Wood veneer	Link between perception of naturalness and perception of sustainability; Doubts due to high material processing
Doubted authenticity	-	Dependence of the assumption of authenticity in brown and white leather on perceived naturalness, renewability, and sustainability
Renewable—conceptual understanding	-	No consistent comprehension of the term

.

• Perceived naturalness of materials
• Wood veneer
• Due to the wooden appearance and haptics of the wood veneer, it was described as natural by some subjects.

It looks extremely natural because it has such a wooden look, or maybe it is wood.

[Z4]

At the same time, it’s also very natural, as it has a bit of a wooden look.

[Y3]

• Cotton fabric
• Overall, the cotton fabric was perceived as a natural material by most subjects across groups. This was supported in particular by its haptics.

Looks a bit like a natural material, as it has natural fiber.

[X2]

It feels at least somehow good, natural.

[Y5]

• Brown leather
• Despite some doubts about the authenticity of the brown leather, it was rated as natural by most of the subjects.

Even if it may be artificial leather, it looks very natural to me.

[X3]

Since it seems to be real leather, it’s a natural fabric. It also looks very natural.

[Y5]

• White leather
• The white leather was perceived as natural in OFF-Mode by noticeably fewer subjects than the brown leather.

Looks natural. But I can also be mistaken. Could also be an artificial leather. But for me at first glance, yes, it actually looks really high quality and also very natural.

[Y5]

Natural, in any case. Very classic material.

[Z2]

• Cork
• Of all the materials, cork was described as natural by most of the subjects.

Yes, it is a natural product, clearly, cork, occurs in nature. I also find it natural now.

[Y5]

It looks very honest. Because it also somehow looks like a natural material.

[Z4]

• Relevance of color
• The color of the material matters for the perception of naturalness and was particularly addressed by the subject in the case of cotton fabric and white leather.
• Cotton fabric

Well, I perceive it rather less natural because of the color tone.

[Y3]

Natural. Yes, it’s the right shade.

[Z3]

• White leather

This is natural. But I think this is more due to the color than the material.

[Z5]

• Perceived renewability of materials
• Wood veneer
• The perception of the renewability of wood veneer was closely linked to the assumption that it was real wood and was accordingly positive.

Renewable. Completely. It grows again, is renewable.

[X3]

And through this wooden look and things like that, I would also say it looks extremely

renewable.

[Y3]

• Cotton fabric
• The opinion of the subjects on whether the cotton fabric is renewable was strongly related to the perception of naturalness. Only in a few cases there were doubts about the renewability.

It will certainly be renewable. Sure, I can recycle that and yes, it is very renewable.

[X1]

Renewable. I think it’s a bit difficult to tell for this material.

[Y3]

• Brown leather
• The brown leather was rated as renewable, but mostly on the condition that it was not artificial leather. Since only one subject addressed this topic verbally, there is only one quote placed here.

Renewable certainly, is a natural product, I strongly assume. Calfskin can certainly be renewed again.

[Y5]

• Cork
• Since cork was identified as a bio-based material, the majority of subjects also considered it as renewable.

Renewable, yes, it is a natural product.

[Y1]

If it really is cork, which I’m assuming now, it’s of course also quite good from the renewable and from the sustainability point of view.

[Z5]

• Perceived sustainability of materials
• Wood veneer
• In the case of wood veneer, too, the assessment of sustainability was related to the perception of the material naturalness. A few subjects felt this differently due to the high processing of the material.

I don’t know what it’s made of, but it definitely has a lasting effect. Because it has such a natural character somehow.

[Z3]

Sustainable, well, you can’t really recycle it. And I also think that they get such nice patterns in, you also need nice thin veneer. I don’t know, is it glued on or not? It’s probably glued on. I think it is rather moderately sustainable.

[Y1]

• Cotton fabric
• If the cotton fabric was rated as natural, it also performed well with the subjects with regard to sustainability.

A natural product, I assume strongly. That can also be produced again in nature. That’s why I would classify it as very sustainable.

[Y5]

Sustainable. That can probably be recycled pretty well.

[X3]

• Brown leather
• Brown leather was perceived as sustainable, yet less than other materials. The reason for this was the question of whether leather production is sustainable or not.

Sustainable. Yes, I think so. Cows are slaughtered, that’s a by-product, there’s nothing wrong with that.

[X1]

Whether it is sustainable depends on whether it is real leather or artificial leather. Both are rather questionable, which is why I would classify it as moderately sustainable.

[Z2]

• White leather
• The effect described for brown leather also applies to white leather. The level of sustainability was made dependent on the leather production.

If it’s leather, it’s sustainable depending on how the animals are farmed.

[X4]

Leather is not necessarily the sustainable material.

[Y4]

• Cork
• Cork was found to be sustainable by the majority of the subjects. Few subjects mentioned the dependence of the sustainability on the way the cork was extracted.

If it is sustainable depends on how you win it. It looks sustainable to me.

[X4]

I would say it is extremely sustainable. Looks like a natural product.

[Y3]

• Doubted authenticity
• A very present topic was the authenticity of the materials. This issue involved the brown and white leather in particular, and it also had an impact on the evaluation of naturalness, renewability, and sustainability.

That’s relatively difficult now, because it can be anything. In the meantime, Mercedes and various manufacturers are so good at imitating leather that you can’t tell at first glance whether it’s real leather or a replica.

[X3, brown leather]

This is artificial leather.

[X1, white leather]

• Renewable—conceptual understanding
• Some subjects had comprehension problems with the term renewable.

Renewable... renewable...Should I interpret that now in terms of “I can easily renew it” or “it’s a renewable resource-based material”?

[Y4, cotton fabric]

Renewable. I don’t know. Since it’s sewn together and pretty shape-retaining, I think it’s hard to replace a single piece. Or do you mean recycling? Or do you mean recycling the material?

[Z3, brown leather]

4.2.2. ON-Mode

The majority of subjects commented on the materials if something had changed in their perception of them compared to the OFF-Mode. Some themes were addressed again, while others were not commented on. A new, previously unaddressed theme relating to the effect of blue light was mentioned by the subjects. Five themes were identified: (1) decrease of perceived naturalness of materials, (2) persistence of perceived naturalness of materials, (3) disagreements about the effect of blue light on material perception, and (4) renewable—conceptual understanding. The number of quotes from subjects listed varies depending on the topic.

• Decrease of perceived naturalness of materials
• The cotton fabric and wood veneer materials were perceived as significantly less natural in the ON-Mode than in the OFF-Mode by the subjects.

Well, I think it no longer looks natural, which is simply due to the lights.

[Z5, cotton fabric]

Before, it seemed a bit more natural, and now, of course, it greatly reduces the effect.

[Z1, wood veneer]

• Persistence of perceived naturalness of materials
• In a few cases, the naturalness of the materials white leather and cork was again addressed. The perceived naturalness of white leather and cork had not decreased. Moreover, the influence of the blue light did not change the naturalness of the cotton fabric and wood veneer for a few subjects.

It is still natural.

[Z2, cotton fabric]

Depending on where you look, where the light shines through a bit more, it looks a bit less natural. Although on the other hand that blue: Water and sky are of course also natural, I would say it’s still very natural.

[Y3, wood veneer]

• Disagreements about the effect of blue light on material perception
• The blue color of the light was especially criticized for the materials cotton fabric and wood veneer. Some subjects would have desired a warmer color instead.

I think visually it is not better now, Consequently, this shade of blue is a bit annoying.

[Z5, cotton fabric]

I think the bright, cold tone of the blue disturbs the warm color of the wood.

[Z4, wood veneer]

If these had been small white lights, then I could say: Yes, that is known in nature.

[Y1, wood veneer]

• In contrast, numerous subjects found the effect that the blue light created with the wood veneer to be very positive.

I liked this earlier, but now I like it even more.

[X3, wood veneer]

This has made it a bit higher in quality and a bit more innovative.

[Y3, wood veneer]

The glow that it shines through like that, very cool.

[Z2, wood veneer]

It looks cool. It somehow has a bit of a night sky look.

[Z3, wood veneer]

• Renewable—conceptual understanding
• Although the subjects who had comprehension problems with the term “renewable” usually already expressed this in the OFF-Mode evaluation and did not repeat it in the ON-Mode, it also became clear in the ON-Mode that the term was not uniformly understood by some subjects.

Renewable. Yes of course, so the LEDs, you can certainly put them in new, it is a mass product, as is the fabric. Of course everything is renewable, the material is renewable and the fittings too.

[Y5, cotton fabric]

Renewable. You can easily replace it. I just don’t know what kind of material it is. If it’s cork, it will of course be very high in terms of renewable raw material.

[Z5, cork]

5. Discussion

In light of the results of Study 1 and 2, five discussion points are presented herein. While the first discussion point addresses the generalizability of the results from Study 1 to the application context of car interiors, the second discussion point highlights dependence of the perception of renewability and sustainability of the material on the perception of its naturalness, authenticity, extraction, and production process. The third discussion point discusses the heterogeneous effects of blue light on overall perception of materials and their perception of naturalness. The fourth discussion point examines the varying interpretations of the term “renewable” among subjects. Finally, the last discussion point addresses the limitations of this work.

Perception of environmental friendliness of materials in car interior. The findings from Study 1 were confirmed when the materials were tested in the application context in Study 2. The materials used in Study 2 were largely perceived as natural. The wood veneer is perceived as environmentally friendly or natural in both studies, which is supported by the material properties supporting environmental friendliness perception in Study 1. These are wooden feel and look, texture and structure, and the color brown (see Table 2).

Cork was also perceived as environmentally friendly and natural to a high degree in both Study 1 and Study 2. As in Study 1, cork was also convincing in the context of car interior due to its soft and warm surface, texture and structure, and the color brown (see Table 2). The cotton fabric, which was already perceived as environmentally friendly in Study 1 due to its soft and warm surface (see Table 2), also appeared natural to the subjects in Study 2 because of its haptics (see Section 4.2.1). Although the brown leather was mostly perceived as environmentally friendly or natural in both Study 1 and Study 2 because of its texture and structure, the authenticity of the leather was also questioned in both studies (see Table 2 and subject comments in Section 3.2 and Section 4.2.1). Although the texture and structure of the white leather still supported the perception of naturalness, the white leather experienced less perception of naturalness in Study 2 (see Section 4.2.1). When using these materials, it should therefore not only be ensured that they are actually natural, but also information regarding their authenticity should be provided in order to overcome the buyer’s doubts about their naturalness.

Dependence of the perception of renewability and sustainability of the material on the perception of its naturalness, authenticity, extraction, and production process. Several factors play a role in the perception of the environmental friendliness of materials, which must be considered not only individually but also in combination. Study 2 again showed that the perception of renewability is linked to the perception of the naturalness and authenticity of the material. This was particularly evident in both studies for the material cork (see Section 3.2 and Section 4.2.1). For example, one subject stated that cork is natural and consequently renewable. Another subject made the renewability of cork dependent on the fact that the material in question was actually cork. The subjects’ perception of sustainability was also linked to the naturalness of the material. Its extraction and production process also had an impact on the perception of the renewability. According to the subjects, sustainability depends on how the cork is extracted. Similar considerations were made when addressing the leather material. The way in which leather is produced would influence whether the material is perceived as sustainable. When using these materials, importance should therefore be assigned to the origin in terms of extraction and manufacturing processes, and this should also be communicated to the buyers of the vehicles in order to overcome uncertainties in this respect.

Heterogeneous effects of blue light on overall perception of materials and their perception of naturalness. In this study, the emphasis was on creating a realistic context for the evaluation of user perception. Hence, the relationship between the chosen materials and the applied light was partly dictated by the concept of the car interior chosen for the study. Consequently, all chosen materials could not be perceived under the exact same light. There was overall disagreement about the effect of the blue light on the overall perception of materials. Some subjects criticized the blue light as irritating or disturbing. Other subjects perceived the blue light as an innovative enhancement and considered it a positive feature. However, the positive attitude towards blue light does not necessarily have a supporting effect on the perception of the environmental friendliness of the material. As far as the specific perception of naturalness is concerned, some materials experienced in ON-Mode a downgrading in most subjects due to the light used. These were in particular materials that had direct contact (see Figure 2) with the blue light, i.e., the cotton fabric and the wood veneer. Only a few subjects perceived the two materials as still being natural. Here, it is apparent that the combination of a material perceived as natural with the blue light used does not strengthen the perception of naturalness. This can be transferred to the general perception of the environmental friendliness of the material.

Various understandings of the term renewable. The term ‘renewable’ had various interpretations in both studies. In both Study 1 and Study 2, some subjects had difficulties understanding the term renewable (see Section 3.2 and Section 4.2.1). The understanding of the term ranged between renewable in the sense of regrowing raw materials and in the sense of replaceable, i.e., reproducible. After evaluating the subjects’ comments, it must be assumed that the term ‘renewable’ is not associated with environmental friendliness for all subjects. A clear definition of the term is needed prior to user evaluation to prevent subjects from drawing wrong assumptions. If the aim is to emphasize the environmental friendliness of a product, this material property should also be kept in mind in customer communication.

Limitations of the presented studies. These studies were conducted under realistic interior design conditions, which necessarily implied the presence of variable lighting situations and viewing angles. These factors may have influenced participants’ perceptions of materials and colors, and they represent constraints that cannot be fully controlled when maintaining realism and context. Finally, the experimental design did not account for potential influences of participants’ prior brand experience, social expectations, or experimenter demand characteristics, thus affecting subjective evaluation.

6. Conclusions

The aim of this paper was to explore environmentally friendly design cues for car interior through bio-based materials, thus fostering sustainability awareness through sustainable consumption. Our objectives led to the definition of two studies featuring qualitative research. In Study 1, 20 bio-based material samples and identified EFDCs were examined with regard to the perception of naturalness, sustainability, and renewability in order to assess their contribution to the communication of the environmental friendliness of the materials. The results of Study 1 were used to define materials that were implemented in a 1:1 scale demonstrator of a car interior. Study 2 investigated the contribution of blue as a color and glowing attribute to the perception of the environmental friendliness of the materials used in the context of the automotive interior.

Considering the first research question, it was found that the bio-based materials wood veneer, cork, and cotton fabric were perceived as sustainable, natural, and renewable by most of the subjects. The material properties of wooden feel and look, texture, soft and warm surface, and the color brown provided a major contribution to the targeted perception. Brown and white leather was also perceived as sustainable due to visual texture, i.e., its relief, albeit with limitations in terms of authenticity.

Considering the second research question, blue light does not contribute to the perception of environmental friendliness of the materials. In fact, the perception of naturalness in materials in direct contact with blue light is rather reduced. The results of this study provide a contribution to the design of an environmentally friendly product personality for automobile interior. Color and trim designers developing EFDCs should consider materials with visual texture features such as visible fibers or a wooden look. In terms of haptic features, soft, warm, and rough surfaces should be considered. At the same time, they should avoid materials featuring glare, shimmer, a shiny surface, and smoothness. In terms of colors, they should consider brown, which supports the perception of sustainability. At the same time, they should avoid the colors blue, green, and yellow, as they lead to very different reactions in this regard. Car manufacturers targeting environmentally friendly customers should reconsider the use of blue as color and glowing attribute when marketing environmentally friendly vehicles such as BEVs and HEVs. Furthermore, their communication strategies should convey the product character and the authenticity of the materials implemented in car interior due to the not uncommon doubts about their sustainability in terms of origin, extraction, and manufacturing process.

In lights of these considerations, future research should address the factors leading potential car customers to question the authenticity of environmentally friendly materials as well as factors leading to different interpretations of terms such as renewable. In this regard, it is recommended that future studies make a standardized clarification of terms for the subjects. This is key to carry out an accurate evaluation of the environmental friendliness of products. Finally, future studies should also consider potential pre-existing influences of participants to strengthen the generalizability and robustness of results.