1 Introduction

The instrument panel of a car is an automobile interior part, which attached to the bottom of the front glass of the driver's seat, such as crash pad, dashboard, and meters composed of speedometer, fuel gauge and water temperature gauge [1, 2]. It is a very important part in terms of design, convenience, and safety owing to the instrumentation and control for the vehicle's operation. With the trend of eco-friendly cars, lightweight and high-functionality materials have been developed and applied in instrument panels [3]. Among other materials, polymeric materials are excellent candidates for their high processability and high mechanical properties [1, 4].

Polyurethane foam (PUF) systems can allow significant weight-reduction, soft haptic and good adhesion on the surfaces produced from wide range materials [5, 6]. PUFs have classified on the criteria of their commercially available foams such as rigid and flexible PUF [8, 9]. Rigid PUF is used as an insulating material due to low thermal conductivity of close cell structure and have extensively utilized into many industrial applications such as appliances and foodservice equipment to building panels, entry and garage doors, heating-ventilation-air-conditioning (HVAC) equipment [10, 11]. Flexible PUFs can provide a cushioning effect due to the effective compression and resilience of open cell structure and be applied to comfortable and durable mattresses, chairs, shoe soles, sports equipment and car bumpers [7, 8, 12,13,14].

The NCO index can be defined as the number of equivalents of –NCO divided by the equivalents of –OH. Mechanical properties of PU foams, such as hardness, resilience and compression, can usually depend on the ratio of –NCO to –OH. Lorenzetti et al. reported the influence of trimer content on the mechanical properties of the urethane foam [15]. Lee et al. studied the mechanical and chemical properties of water-dispersed PU resins on the different NCO indexes [16]. Similarly, Scala et al. proposed the dependences of NCO groups on the physical properties of PU films [17].

In this work, the effects of NCO indexes in a flexible PUF series are systematically evaluated in terms of the physical properties of automotive instrument panels. TPU formulations are precisely prepared with an optimal blending ratio of polyester polyol, polycaprolactone triol, 1,4-butanediol, hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) to enhance the PUF performance of thin and light instrument panels. The properties and structure of the resulting PUFs are confirmed using Universal Testing Machine (UTM), Shore A Hardness Tester, and Scanning Electron Microscopy (SEM).

2 Experimental

2.1 Materials

Polyester polyol (BTG, Mn = 2,270 g/mol, Dongsung Co. Ltd., Busan, Korea), Polycaprolactone triol (PCL, Mn = 300 g/mol, Merck KGaA, Darmstadt, Germany) Hexamethylene diisocyanate (HDI, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan), Isophorone diisocyanate (IPDI, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) and 1,4-butanediol (1,4-BD, Junsei Chemical Co., Ltd., Tokyo, Japan) were dried over 20 h in a vacuum prior to use. Dimethylformamide (DMF, Daejung Co. Ltd., Siheung, Korea) was used as the solvent to prepare TPU. Deionized water as a purifying agent was used to rinse any residual reactants. To make polyurethane foam, foaming agent (CAP/1320, Kumyang Co. Ltd., Busan, Korea), silicone surfactant (B8409, Evonic Korea Ltd., Seoul, Korea), tin catalyst (Dibutyltin dilaurate, Merck KGaA, Darmstadt, Germany), amine catalyst diazobicyclo [2.2] octane (DABCO, Kanto chemical CO., INC. Tokyo, Japan) were used. B8409 was used as a silicone surfactant, which reduces the surface energy to give small cell. DABCO as a gelling catalyst, and Tin catalyst as urethane catalyst, which proceeds urethane reaction and foaming reactions, were provided by air products and used as received.

2.2 Synthesis of TPU with the Different NCO Index

Scheme 1 presents the synthetic procedure for a TPU series. Firstly, HDI and IPDI were added to BTG and 1,4-BD in a 250 mL 4-necked reactor equipped with a condenser, a mechanical stirrer, a temperature-control heating mantle and nitrogen purge. The mixture was reacted at 60 °C for 60 min at 400 rpm to form NCO-terminated prepolymer. As shown in Scheme 1, PCL was added to create a semi-interpenetrating network of TPU. Foaming agent, silicone surfactant, tin catalyst, and amine catalyst were continuously added in the mixing ratio shown in Table 1. A TPU foam-type series was foamed in a mold. The individual reaction formulation of the foamed TPU is summarized in Table 1.

Scheme 1
scheme 1

Schematic diagram to synthesize a TPU series

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Table 1 Formulations of the a TPU foam series
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2.3 Characterization

All Fourier Transform Infrared (FT-IR) measurements were analyzed on CARY-640 spectrometer in the range from 650 to 4000 cm-1. Each molecular weight and polydispersity index (PDI) of a PUF series was measured by gel permeation chromatography (GPC, Waters) with a refractive index detector and polystyrene as the standard. The tensile strengths of 1 mm-thick TPU films were analyzed using a Universal Testing Machine (UTM, LLOYD INSTRUMENT) under conditions of 20 mm gauge length and 500 mm/min crosshead speed (ASTM D638). The morphologies of a foamed TPU series were investigated by an field emission scanning electron microscopy (FE-SEM, SUPRA25) and their hardness were also observed by a hardness tester (Shore A).

3 Results and Discussion

3.1 Synthesis of TPU

Figure 1 shows the change of FT-IR spectrum in a synthetic procedure of TPU as a function of reaction time. With the reaction between hydroxyl groups (–OHs) and isocyanates (–NCOs), the formative consumptions of –NCO group (2250 cm-1) are distinctly observed with the progress of reaction [18]. The formation of urethane linkages is confirmed by the presence of C = O stretching and N–H bending bonds, which individually assigned to 1725 cm-1 and 1535 cm-1, respectively [19, 20].

Fig. 1
figure 1

FT-IR spectra of –NCO terminated TPU during synthesis

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A TPU series used in this experimental study has molecular weights (MWs) ranging from 25,300 to 31,800, and almost same polydispersity values between 2.29 and 2.70. Figure 2 clearly shows Gel Permeation Chromatograms (GPC) curves to study the MW and MW broadening of a TPU series, based on a function of NCO%, and their parameters summarized in Table 2. The expecting MW of a TPU series was fixed to approximately 30,000 because MW can be another parameter that affects the properties of the polyurethane foam (PUF). Although a TPU series was carefully synthesized under uniform conditions, there is the possibility of varying MW among four TPU samples. As shown in Fig. 2, however, the limit value of the measured MW may be considered as an insignificant variable affecting PUF properties.

Fig. 2
figure 2

GPC chromatogram of TPU series

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Table 2 Molecular weights and polydispersity index (PDI) of a TPU series
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3.2 Mechanical Properties of TPU Films

The effect of NCO index in the mechanical properties of the resultant TPU series is shown in Fig. 3. As the NCO index increases, tensile strength increased up to a limiting value of 0.98 and then decreased very sharply. The result indicated that the effect of secondary valence internal bonding such as hydrogen bonding optimum manifests in the strength of TPU with 0.98 NCO%. It can be seen that the effect of increasing the mechanical strength according to the increase in the isocyanate ratio of the hard segment of the TPU film was greater than the effect of reducing the rigidity of the soft segment [21, 22].

Fig. 3
figure 3

Stress–strain curves of a TPU series as a function of NCO Index

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3.3 Mechanical Properties and Morphologies of a PUF Series

A polyurethane foam (PUF) series was prepared with mixing the polycaprolactone triol (PCL) into the synthesized TPU series. Four TPU samples with a cube type of 10 cm × 10 cm × 1 cm and shore A hardness analyses are shown in Fig. 4. The hardness of the foamed PUFs increased markedly with increase in limiting proportion of isocyanate to hydroxyl, as shown in Fig. 4. When the NCO index becomes 0.99, the hardness and tensile strength decrease. These results reflect the lower urethane concentration in PUF with 0.99 NCO% and thus a lower hydrogen bonding force. The appearance of the produced PUFs was evaluated using the photographs in Fig. 5A. PUFs with NCO Index of 0.97 and 0.98 were cosmetically much cleaner than the other PUFs (NCO Index = 0.96, 0.99). Both 0.96 NCO% and 0.99 NCO% exhibit the inhomogeneous appearance. In the case of NCO Index 0.96, shrinkage of the demolded sample occurred and it was bent, and as an improvement measure, the NCO Index was raised. In the case of an NCO Index of 0.99, loose skin occurred in which a part of the generated foam was peeled off with a thin film.

Fig. 4
figure 4

Shore A Hardness analyses of a PUF series as a function of NCO Index

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Fig. 5
figure 5

A Photographs and B SEM images of a PUF series with different NCO Index

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The cell morphologies of the resultant PUF series were investigated to identify the pore characteristics, based on the different NCO index. Figure 5B shows the SEM images of the side cross-section of PUFs. As shown in Fig. 5B, as the NCO index increased to the limit value of 0.98 NCO%, it was observed that the cell porosity became much more homogeneous to about 200 µm. In other NCO index the cell pores of other PUFs was uneven with cell porosity of about 60 ~ 250 µm. Density was determined by dividing the mass of each sample by its volume. As shown in Table 3, it was confirmed that the density was the lowest at an NCO index of 0.98, assuming the volume was equal to 100 cm3. It could be explained that the pore uniformities of a PUF series simultaneously changed because the molecular structure changes from soft to hard as the polymer chain forms more complex intermolecular cross-linking.

Table 3 Mass and density of a TPU series
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4 Conclusion

A PUF series with high mechanical properties and low density was developed by using multi-functional group polyol (PCL) on TPU using two types of polyol and isocyanate. The cellular structure and mechanical properties of a PUF series which have been produced by compression molding, have been characterized with respect to NCO content, which leads to the structural change. As the NCO index increased, mechanical properties such as tensile strength of TPU film and the hardness of TPU foam increased to the limit value of 0.98 of NCO index and then decreased after 0.99 of NCO index. Conversely, the density of TPU foam decreased to NCO Index 0.98 and then increased from 0.99. This result reflects the effect of an increase in mechanical strength due to an increase in allophanate and biuret bonds as the ratio of isocyanate, a hard segment in polyurethane foam, increases when the NCO index increases. The cell structure of the molded TPU foam had a high homogeneity of 200 µm, which had a positive effect on the excellent mechanical properties of PUF. It was confirmed by this experiment that the foam made using multi-functional group polyol (PCL) was the best formulation at NCO Index 0.98. This information can be very useful in designing an automotive instrument panel by combining the high homogeneity of the cellular structures and the good mechanical properties of a PUF. The realistic applications may be those of not only skin panel in solar irradiation during the vehicle operating, but also refrigerated material of perishable foodstuffs.