Research progress on soft-touch materials and haptic characterization for automotive interiors

Source | Engineering plastics applications

Abstract:The research and development of soft-touch materials is of great significance to improve the haptic performance of automotive interior materials, and the research progress of soft-touch polypropylene and soft-touch microfiber leather materials used in the instrument panel area and seat skin area of automobiles is reviewed, and its development direction is prospected, pointing out that soft-touch PP is developing in the direction of multi-functionality such as excellent mechanical properties, high touch, environmental protection, and antibacterial, and the future research direction of microfiber leather is also towards environmental protection, safety, antibacterial, flame retardant and other multi-functions. At the same time, the current haptic characterization methods of materials are briefly summarized.

With the development of the national economy, automobiles are becoming more and more popular in people's daily life. At the same time, with the improvement of quality of life, consumers have higher and higher requirements for the high touch sensitivity of automotive interior materials. Automotive interior materials mainly refer to the parts used in the interior parts of the cockpit of automobiles and related to the surface perception of automotive interiors, such as instrument panels, sub-dashboards, seat skins, headliners, door trims, etc. This type of interior material is an area that has more contact with people, and its tactile feel directly affects the perceived quality of the car and the driving experience of consumers.

Therefore, it is of great significance to study the soft-touch automotive interior materials and improve the evaluation method of soft-touch automotive interior materials to promote the development of automotive interior materials and the improvement of automobile quality. This paper mainly reviews the soft-touch materials and haptic characterization methods in the instrument panel area and seat skin area, and looks forward to the future development of soft-touch feeling.

1 Advances in soft-touch interior materials

The instrument panel area and seat skin materials are the interior materials that have the most direct contact with consumers in automobiles, and in order to create a good interior atmosphere and user experience, high-touch interior materials applied to the dashboard area and seat skin have been widely studied.

1.1 Soft-touch polypropylene

Polypropylene (PP) is widely used as an automotive interior material due to its advantages of low density, wide source, low price, high temperature resistance, corrosion resistance, non-toxic and odorless, and easy processing and modification. With the development of PP modification technology, PP has become the largest general-purpose plastic in automotive plastics [1], and the proportion of automotive plastics in some developed countries has reached about 50%. The automobile instrument panel is one of the interior parts with a large surface area and more contact with the human body.

At present, most of the mid-to-low-end cars use traditional hard instrument panels, that is, traditional PP direct injection molding, which have a high surface gloss and strong plastic texture, and cannot give people a soft touch. In high-end luxury models, most of the soft foam dashboards with a three-layer structure with a soft touch feel are used (soft skin + foam core + bone structure), but this dashboard molding process is complex and expensive. Therefore, it is a new solution for the preparation of soft-touch interior materials for automobiles to give PP a soft-touch feel, and then injection molding to prepare interior parts such as instrument panels with a soft-touch feel, which has attracted the attention of the PP modification industry and the automotive industry.

Fu et al. [2] used random copolymer PP and propylene-based polyolefin elastomer as raw materials to prepare a series of soft-touch PP materials reinforced with glass fiber reinforcement by twin-screw blending extrusion, which achieved the improvement of the mechanical properties of PP while having both scratch resistance and soft-touch properties. K. Kondo et al. [3] also prepared a scratch-resistant, soft-touch glass fiber-reinforced PP by adding small molecule amide compounds to PP prepared with a special metallocene catalyst.

Zhou et al. [4] prepared a reinforced PP material for automotive instrument panels by adding an appropriate proportion of elastomers and chopped glass fibers to the PP matrix, using a twin-screw extrusion mechanism, which has an apparent soft touch, high rigidity, and excellent impact strength at room and low temperatures.

Due to the soft touch of plastic parts, it can be divided into two aspects: visual touch and hand touch. Specifically, it can be described as the soft feeling of the material given by the low gloss of the product during eye contact, and the softness of the surface of the product when touched by hand. Therefore, researchers pay more attention to the improvement of the visual touch of PP materials used in automotive interiors while improving the touch softness, so as to achieve a higher level of soft touch.

Shi Yangyang et al. [5] used addition liquid silicone rubber to wet the surface of chopped glass fibers, and then blended with PP for extrusion, in the process of melt extrusion, the vinyl of the addition liquid silicone rubber exposed on the surface of chopped glass fibers underwent a grafting reaction with the PP segment under the action of an initiator, entangled with each other, and the two phases reached a miscible state, thereby reducing the surface hardness and surface gloss of the composite materials, and endowing the PP composites with excellent soft touch.

At the same time, the addition of liquid silicone rubber is connected to chopped fibers at one end and PP chain segments at the other end to improve the interface adhesion between glass fibers and PP, and significantly improve the mechanical properties of the composite materials, with a tensile strength of more than 23 MPa, a flexural elastic modulus of more than 1 600 MPa, and a notched impact strength of cantilever beams of more than 25 kJ/m2, with a very high rigidity and toughness balance. Shi Yangyang et al. [6] also used another scheme to prepare a soft-touch PP composite material suitable for automotive interiors, with a balance of rigidity and toughness, good scratch resistance, low gloss and low warpage.

They used a fluff powder composed of soft polyurethane (PUR) elastomer microspheres as a soft-touch modifier, this fluff powder has rubber elasticity, and heat, solvent resistance, acid and alkali resistance, and has excellent scratch resistance, has better compatibility with rubber, and gives the surface a silky fluffy feel, using the fluff feel of fluff powder and the characteristics of elastomer can effectively reduce the plastic feel of PP, so as to achieve a "soft touch" in vision and touch.

Wang Bin et al. [7] also prepared a high-quality automotive interior material, and they realized PP oriented fiber formation by strong shear and ultra-high-speed tensile method, which greatly improved the strength, rigidity, toughness and heat resistance of the original PP material, and at the same time, the high shear dispersion was used to achieve a good dispersion of the elastomer in the system, and the characteristics of soft touch on the surface, low gloss, and hard plastic feeling of the product were eliminated.

With the development of the automobile industry, problems such as volatile harmful substances in the car and mildew in wet weather plastic interiors have also attracted people's attention. Therefore, the preparation of soft-touch materials with excellent mechanical properties, environmental protection, antibacterial properties and other multi-functional properties has become a hot research direction in the automotive interior industry. Chen et al. [8] modified PP by using low-density polyethylene (PE–LD) and polyolefin elastomer (POE) tougheners, and achieved the visual and tactile soft-touch sensitivity of modified PP after controlling the appropriate ratio of PP, toughener and glass fiber.

At the same time, in the extrusion process of composite materials, the VOC adsorbent prepared by adding the mixture of maixilisisolite and zeolite can effectively adsorb the small molecule substances generated during the processing of raw materials and achieve the performance index of low VOC, so as to obtain a low-VOC, soft-touch glass fiber reinforced PP composite material applied to automotive interiors.

Liu Rongliang et al. [9] added hyperbranched copolymer of ethylene and methyl acrylate and composite antimicrobial agent containing nanoscale alumina and anatase nano-titanium dioxide in the process of alkali-free chopped flat glass fiber and ethylene propylene copolymer modified PP, and used the mutual promotion of hyperbranched copolymer and flat chopped fiber to make the composite antimicrobial agent evenly distributed in the PP matrix during the extrusion process, so as to prepare a soft-touch PP composite material with excellent mechanical properties and antibacterial properties for automotive interiors. The inhibition rates of Escherichia coli and Staphylococcus aureus were up to 93.3% and 96.5%, respectively. Huang et al. [10] also obtained a soft-touch PP composite with improved mechanical properties and good scratch and antibacterial properties by adding a corresponding proportion of antimicrobial agents in the preparation process of soft-touch PP.

1.2 Soft-touch leather material

As the main material of automobile interior, seat leather material accounts for a large proportion of automotive interiors. In addition, the seat leather material is in long-term contact with the human body, so it has higher requirements for tactile comfort and environmental protection. In the past, the seats of mid-to-high-end models were generally covered with natural leather in order to achieve tactile comfort, but due to the high price, limited resources and environmental protection of natural leather, the processing process is not conducive to environmental protection. Therefore, industry researchers have been actively looking for a new environmentally friendly and comfortable man-made composite material to replace natural leather in the car seat skin material.

With the advent of artificial leather coated with polyvinyl chloride (PVC), PVC artificial leather has begun to replace natural leather in the field of leather materials for car seats. Although PVC artificial leather has the characteristics of bright appearance, wear resistance, folding resistance, acid and alkali resistance like natural leather, its poor soft touch is its obvious defect, and plasticizers need to be added to improve the soft touch, but most plasticizers are toxic, the smell is large after addition, and the garbage disposal and recycling of PVC artificial leather after use are difficult, so the application of PVC leather in automotive interiors is limited [11].

With the emergence of PUR synthetic leather, due to the advantages of PUR synthetic leather such as environmental protection, odorless, soft touch, breathable and waterproof, easy to clean, good aging resistance, and close to natural leather in appearance and performance, PUR synthetic leather has gradually replaced the application of PVC artificial leather in automotive interiors, and tends to develop high-performance PUR synthetic leather. Now, with the rapid development of microfiber technology in the textile industry, combined with high-performance PUR leather technology, a new environmentally friendly material called microfiber leather has become by far the most desirable high-touch leather material for automotive interiors.

This leather not only completely surpasses natural leather in terms of physical properties, but also completely surpasses natural leather in terms of breathability, moisture permeability and comfort. In foreign countries, especially in Japan and Europe, this new artificial leather material has been gradually used to replace natural leather. At present, BMW, Mercedes-Benz, Toyota, Honda and other models are equipped with special microfiber leather for automobiles.

Microfiber leather is the abbreviation of microfiber PUR synthetic leather, which is made of high-density nonwovens with a three-dimensional network structure made of microfibers similar to the structure and properties of bundled collagen fibers in natural leather, and processed by processing PUR resin with excellent filling performance and open microporous structure. This microfiber leather is very similar to natural leather in terms of appearance, mechanical strength, elasticity, chemical resistance and stability, and has a very good feel. At present, researchers have also done corresponding research on automotive-specific microfiber leather.

Huang Xiaoping [12] used water instead of organic solvent as the coating solvent to prepare a water-based PUR microfiber leather for automotive interiors with a texture similar to leather, soft, smooth, and elastic, and with excellent performance, good strength, waterproofness and hydrolysis resistance. Qian Xiaoming et al. [13] used acupuncture, spunlace composite fiber opening process and surface scraping technology to produce an orange petal-shaped microfiber artificial leather base fabric, which has a gradient change of fiber fineness and water-based PUR impregnation content in the nonwoven, which is very similar to the structure of the leather substrate, and has a good hand feel, drape and texture.

With the improvement of people's quality of life, as well as the enhancement of health and safety awareness, people pay more attention to the safety, environmental protection, health hazards and other issues of interior materials in the pursuit of comfortable touch of automotive interior leather materials. Wang Meirui et al. [14] disclosed a preparation method of water-based solvent-free PUR microfiber simulated leather, which uses water-based PUR resin fabric and medium material, bonded with solvent-free PUR resin material, and then laminated with microfiber bass of different thicknesses, and the prepared microfiber leather feels soft and elastic, with a strong leather texture.

Chen Jun et al. [15] also disclosed a preparation method for microfiber PUR high-fidelity leather materials with good feel for automotive interiors, they used environmentally friendly water-based PUR for two impregnations, and in the first impregnation of synthetic leather base fabric, by adding alkali-resistant components such as nano magnesium hydroxide and rutile titanium white paste with water-based PUR for impregnation, the nanoparticles were evenly covered on the surface and gaps of the base fabric, giving the leather good flame retardant and sunscreen properties.

And in the molding process, the prepared microfiber leather is also endowed with certain antibacterial properties by adding an appropriate amount of pine components. The result is a microfiber PUR leather for automotive interiors that not only has a good leather feel, but is also environmentally friendly, flame retardant and antibacterial. Ding Huiyang et al. [16] By adding antifouling additives to the wet impregnating PUR slurry and adding antibacterial additives to the oiling softening liquid, the non-woven fabrics prepared after heat setting have special antifouling and antibacterial properties. Then after dry veneering, the surface anti-fouling printing is carried out, and the microfiber leather produced has a good leather feel at the same time, but also has a special all-round anti-fouling and antibacterial function.

2 Progress in haptic characterization of materials

With the research of soft-touch interior materials, how to evaluate the soft-touch performance of materials has also become a major problem faced by the interior industry. In terms of the tactile evaluation of materials, the traditional sensory evaluation method that has been used to evaluate the haptic properties of materials by touching them by experienced experts to form a comprehensive feeling. However, this traditional evaluation method has many shortcomings, and its evaluation results are easily affected by subjective factors such as personal professional skills and personal preferences, as well as objective factors such as environmental and climatic conditions. Secondly, this evaluation method lacks objective quantitative evaluation indicators and cannot distinguish the subtle differences in haptic properties between materials, so it is necessary to establish more objective methods and systems to characterize the haptic properties of materials [17].

The first application of objective methods to quantitatively evaluate the haptic properties of materials was in the textile industry. T. Peirce proposed that the physical and mechanical properties of the surface of the fabric are an important factor affecting its haptic properties, and in 1968, Yoshio Kawabata of Japan proposed to use the basic mechanical properties data of fabrics to characterize the haptic properties of fabrics, and invented the KES-F fabric evaluation system. Subsequently, based on the previous method of using the mechanical properties data of textiles to evaluate the haptic feeling of fabrics, Professor Pan Ning of the United States developed the Phabr Ometer fabric haptic evaluation system by using the extraction method, which brought the objective characterization of fabric haptics to a new level [18].

As the method of using the physical and mechanical properties of fabrics to objectively characterize the tactile feeling of textiles has gradually been standardized in the textile industry, researchers have begun to draw on this idea in the research on the tactile characterization of automotive interior materials, and strive to improve and develop the haptic characterization methods of automotive interior materials. As early as 1993, Su Zhenwei et al. [19] conducted an objective study on the tactile feeling of leather materials in automotive interior materials, and pointed out that the mechanical properties of leather are important factors affecting the evaluation of leather softness.

In 2003, Zhang Xiaolei et al. [20] studied the haptic properties of leather under mechanical states such as elongation and compression, and proposed that the haptic characteristics such as fullness and elasticity of leather can be characterized by measuring the mechanical properties of leather. In 2006, Li Zhiqiang et al. [21] proposed a method that can be used to characterize the softness of leather by using its flexural strength and compressive properties by studying the relationship between leather softness and its flexural strength and compressive properties, and developed relevant test instruments. Chen et al. [22] also calculated and evaluated the softness of leather by measuring its thickness in the elongation and compression states under the calibration pressure, and confirmed that the test results were consistent with the actual sample.

Therefore, they also proposed a quantitative characterization method for leather softness based on the principles of bending stress and compressibility. Since most of the above characterization methods are based on a single mechanical property parameter to evaluate the soft touch of leather, there may be large errors in some subtle tactile differences. Therefore, in 2008, Dong Jixian et al. [23] studied a new type of leather tactile characteristic parameter testing instrument, which can simultaneously determine the mechanical properties of leather in the four states of tensile, compression, bending, and elongation, and then through multiple regression analysis and other data processing, to achieve a comprehensive detection of leather tactile characteristic parameters, which can indirectly characterize the touch of leather more comprehensively.

IN 2010, ZHANG ET AL. [24–25] MADE RELEVANT IMPROVEMENTS BASED ON THIS INSTRUMENT AND ESTABLISHED A DETECTION SYSTEM FOR LEATHER HAPTIC CHARACTERISTIC PARAMETERS BASED ON PLC AND LABVIEW, WHICH FURTHER REALIZED THE INTELLIGENT AND INTUITIVE HAPTIC CHARACTERIZATION OF LEATHER MATERIALS. In 2012, Wang et al. [26] also selected the bending constant, compression index, elongation coefficient, bending stress attenuation constant, and bending stress attenuation rate constant of leather as the mechanical characteristic parameters to establish a leather haptic characterization system based on BP neural network model.

Similarly, in terms of other polymer materials for automotive interiors, there are many researchers who use their specific mechanical properties to characterize their haptics. Since the friction between the finger and the surface of the material occurs during the actual tactile perception of the finger contacting the material, the friction characteristics are also an important factor affecting the haptic characterization [27]. Therefore, in the subsequent haptic studies, the researchers also included the friction characteristics into the range of mechanical properties tested.

Covestro studied the haptic sensation and coating properties of plastic substrates with PUR coating, and they found that some specific polymer physical analysis data (especially friction properties) of the surface coating of plastic parts are closely related to their haptic properties. Surface microhardness and modulus to quantify the haptic feel of coated materials.

Audi AG has also been working on how to convert subjective, often comparative, assessments of haptic properties of automotive interior materials into measurable data and has developed a haptic evaluation method based on various physical properties of materials. Their evaluation methods include four characteristics of the material: softness, surface elasticity, touch/touch sensation, and temperature sensing, where softness is characterized by measuring the stiffness of the material, surface elasticity is characterized by the surface recovery properties after force, touch/touch sensation is characterized by roughness and friction characteristics, and temperature sensing is characterized by measuring the surface heat dissipation characteristics of the material. Liu X et al. [28] also characterized the haptic properties of plastic parts by using their relevant physical property parameters by using a novel tribological probe microscope (TPM) for multifunctional measurements of materials.

It is pointed out that TPM is capable of measuring four functions in a single scan, providing a regional map of material surface topography, friction properties, tensile elastic modulus, and hardness. Since the TPM mapping is based on point-by-point scanning, the values of the four measured functions are spatially and temporally interrelated, so it is possible to establish the correlation between the functions to characterize their tactile sensations. The study of Chen X et al. [29] also showed that tactile perception is influenced by the physical properties of the surface of the material. Because the friction characteristics are more complex than other physical characteristics of haptics in the objective characterization of haptics, and there are many influencing factors, some researchers have studied the relationship between friction characteristics and tactile perception alone.

L. Skedung et al. [30] studied the difference between perceived roughness and friction characteristics between specimens, and found that the higher the friction coefficient, the stronger the surface roughness of the material. G. P. Chimata et al. [31] discussed the effects of friction and surface parameters in the identification of textures, and found that the texture perception of materials varies with the change of surface friction coefficient.

J. D. Ndengue et al. [32] pointed out that the vibration parameters caused by friction when the finger slides are also objective indicators in tactile evaluation. K. Anne et al. [33] investigated the differences between tactile pleasure and inter-sample perception based on friction cues, and showed that the higher the coefficient of friction when the finger strokes the surface of the material, the worse the tactile pleasure.

3 Conclusion

At present, the preparation of soft-touch PP is mainly achieved by adding elastomer components and components that reduce the surface gloss in the process of glass fiber reinforced PP to achieve its visual and tactile soft touch. With the improvement of automotive interior requirements, soft-touch PP is developing in the direction of multi-functionality such as excellent mechanical properties, high haptics, environmental protection, and antibacterial properties. In terms of soft-touch leather for car seats, the current main research direction is to use microfiber PUR leather to replace the application of genuine leather, and the future research direction is also to develop soft-touch microfiber leather with multi-functional properties such as environmental protection and safety, antibacterial, and flame retardant.

In terms of soft touch characterization, the factors involved mainly include the friction characteristics of the material surface, which are closely related to the roughness and smoothness of the material, the compression characteristics related to the fluffiness, fullness and softness of the material, the heat dissipation characteristics of the material surface related to the mildness of the material, and the mechanical properties such as modulus and hardness related to the soft and hard feeling of the material. At present, the methods applied to haptic characterization are mainly to indirectly characterize the haptics of materials by measuring the physical properties of these material surfaces that affect their haptic evaluation.