Synthesis, characterization and properties of biomass and carbon dioxide derived polyurethane reactive hot-melt adhesives

2.1 Materials

A biomass based poly(tetramethylene ether) glycol (bio-PTMEG, bio-based PolyTHF® 1000) was obtained from BASF Co., Ltd., Taiwan. A cashew nutshell liquid (CNSL) based polyester diol (Cardolite® NX-9203LP) was obtained from Cardolite Co., Ltd., China. A CO₂-based polycarbonate polyol, poly(propylene carbonate) diol (PPC diol, Converge® polyol 212-10) and poly(propylene carbonate)-poly(propylene glycol) copolymer diol (PPC-PPG diol, Converge® polyol CPX-2501-56) were obtained from Aramco Co., Ltd., USA. A biomass based aliphatic polycarbonate diol, using decanediol from castor oil (bio-PCD, BENEBiOL™, NL1010DB) was obtained from Mitsubishi chemical Co., Ltd., Japan. The 4, 4’-methylene diphenyl diisocyanate (MDI, DESMODUR^® 44 C FUSED) was obtained from Covestro Co., Ltd., China. The catalyst 2,2’-dimorpholinodiethylether (Jeffcat® DMDEE), was obtained from Huntsman Co., Ltd., Taiwan.

2.2 Synthesis of the isocyanate-terminated PURH prepolymers

The isocyanate-terminated PURH prepolymers (PURH prepolymers) were synthesized by the reaction between a polyol or polyol blends and an excess of diisocyanate (MDI). The synthesis molar ratio of NCO to OH groups was 1.4:1 and theoretical isocyanate content value (NCO wt%) was 2.5 (2,16). The compositions and nomenclature of the prepolymers are given in Table 1. The PURH prepolymers synthesis was carried out in a round-bottomed, four-necked liter flask equipped with a mechanical stirrer, a heating mantle and a thermometer. In the prepolymerization process, a selected polyol or polyol blends was added to the reactor and heating to 115 ± 5°C under a vacuum of less than 1 mm Hg for 1 h to remove the residual water. MDI was melted at 50°C and used after excluding white MDI dimer precipitates in the melt (17). After that, MDI and DMDEE were added sequentially into the reactor and stirred for 1.5 h at 115 ± 5°C until the desired NCO wt% was obtained.

2.3 Preparation of moisture-cured PURHs films

The PURH prepolymers in molten state (115 ± 5°C), were coated on the release paper by a coating rod to form instruments analysis specimens. The thickness of the films obtained was approximately 0.2 mm. After that, let the films exposed the air for 15 days to obtain the moisturecured PURH films. The moisture-curing environment was kept at 25 ± 2°C and 80 ± 10% relative humidity. Each PURH film was confirmed by the ATR-FTIR spectra, the isocyanate group band stretch at 2270 cm^–1 should disappeared after the moisture-curing reaction (18).

2.4 Single-lap shear adhesive joints preparation

Adhesive properties of moisture-cured PURHs were obtained single-lap shear adhesive joints that bonded stainless steel to stainless steel. 0.2 g PURH prepolymers (molten state, 115 ± 5°C) was applied to a square area (2.54 cm × 2.54 cm) of one substrate by a coating rod, and then immediately joined to another substrate. These single-lap shear adhesive joints were kept at 25 ± 2^°C and 80 ± 10% relative humidity environment for 15 days moisture-curing reaction.

2.5 Characterization of PURH prepolymers

2.6 Characterization of moisture-cured PURHs films

3.1 Solid-state ¹³C NMR analysis

The solid-state ¹³C NMR technique was used to determine the structure of moisture-cured PURHs with various polyols derived from biomass and CO₂ (Scheme 1). As shown in Figure 1, the characteristic peak for MDI aromatic ring appeared at around 136.5, 129.2 and 119.1 ppm; the methylene carbon in the MDI appeared at around 40 ppm (21,22). The carbonyl groups assigned to the urethane and urea appeared around 154.6 and 164.2 ppm, respectively (23).

Figure 1

Solid-state ¹³C NMR spectra of moisture-cured PURHs.

In the PUR-1 spectrum, the peaks at 70.8 and 26.8 ppm were attributing to the methyleneoxy and methylene carbon groups of bio-PTMEG, respectively (24). In the PUR-2 spectrum, the aromatic carbon peaks in the range 112-128 ppm were also attributed to the cardanol based polyester diol. The peak at 156 ppm is for the aromatic carbon attached to the oxygen atom. The peaks at 14-30 and 114.3 ppm were attributing to the ethyl carbonate and olefins groups of the cardanol side chain, respectively (25). The peak at 153.5 ppm was attributed to carbonyl group due to ester linkage (26). In the PUR-3 spectrum, the peak at 17.1, 73.1, and 74.6 ppm were attributed to the carbon of methyl, methylene, and methane in PPC end-capped with propylene oxide segment (27). The peak at 153.7 ppm corresponds to the carbonyl carbon of the PPC (28). In the PUR-4 spectrum, the peak at around 70.6 ppm, which is ascribed to the methyleneoxy, methylene and methine carbon groups attached on the bio-PTMEG and PPC (29). The peak at 16.4 and 154.6 ppm were assigned the methyl and carbonyl groups to PPC. In the PUR-5 spectrum, the peak at around 70.6 and 26.7 ppm which were ascribed to the methyleneoxy and methylene groups attached on the bio-PTMEG and bio-mass based polycarbonate diol, respectively (30). The peak at 154.9 ppm due to carbon atom of carbonate groups (29).

3.2 ATR-FTIR analysis

ATR-FTIR was used to evaluate the hydrogen bonding characteristics in the microstructure of moisture-cured PURHs film. Figure 2a shows the ATR-FTIR spectroscopy absorption bands of PURHs, such as 3100 to 3500 cm^–1 (N–H stretching vibrations), 1600 to 1800 cm^–1 (C=O stretching vibrations), and 1110 cm^–1 (C–O–C stretching vibration, ether group), which are the main characteristics for the urethane groups (31).

The deconvolution peak of the N–H stretching band can quantify the degree of N–H hydrogen bonded and free stretching vibration in the PURHs by curve fitting with Gaussian function (Figures 2b and 2c). In each spectrum, the N–H stretching vibration exhibited a strong absorption peak centered at around 3315 cm^–1 arising from the hydrogen bonding of N–H, while the free N–H stretching vibration appears as a weak shoulder at around 3445 cm^–1 (32). Table 2 shows that each PURH exhibited much higher relative area of N–H hydrogen bonding than free N–H stretching vibration, and PUR-1 was lower degree of the N–H hydrogen bonding among that of the others PURHs. The degree of N–H hydrogen bonding can be explained on the segregation or mixing of the hard segments (derived from the reaction of the NCO groups and water) and soft segments (derived from the polyols) in PURs (33). The hydrogen bonding of N–H stretching, absorption peak centered at around 3315 cm^–1, was also assigned as N–H groups hydrogen bonded with C–O–C groups that suggests the mixing of hard segments and soft segments (34). Therefore, PUR-1 exhibited lower degree of the N–H hydrogen bonding among that of the others PURHs can be attributed to bio-PTMEG produced lower degree of microphase mixing than others polyols did. This behavior is primarily due to bio-PTMEG its lower polarity than that of the others polyester and polycarbonate based diols (34,35).

In the present work, we used the surface zeta potential determination to estimate polarity characteristics of PURH films. From Table 3, the bio-PTMEG based PURH exhibited closer to neutral value of surface zeta potential than that of the others polyols.

Figure 2

(a) ATR-FTIR spectra for moisture-cured PURHs film, (b) ATR-FTIR spectra deconvolution model of N–H stretching region, (c) ATR-FTIR spectra deconvolution results of N–H stretching region.

Hence, the degree of the N–H hydrogen bonding in PURH was related to polyols polarity which reflected in the surface zeta potential of PURH.

3.3 Dynamic mechanical properties

DMA was used to measure the viscoelastic characteristic and glass transition temperatures (T_g) of moisture-cured PURHs. Under tension deformation, the measured elastic component is referred to as the storage modulus (E’), while the measured viscous component is referred to as the loss modulus (E”). The ratio of the loss modulus to the storage modulus is referred to as the tan delta (tanδ). T_g of the PURHs is located as the temperature where tanδ is maximum (Table 3).

Before the temperature passed through the T_g of PUR-1, PUR-1 showed lower E’ value than those of others PURHs films (Figure 3), which may be due to bio-PTMEG based soft segments were more flexible than the others polyol. However, PUR-2, PUR-3 and PUR-4 were showed rapidly decline in the E’ than PUR-1 and PUR-5 when the temperature through their T_g. The rapidly decline for the E’ can be attributed to the CNSL based polyester diol and PPC-PPG copolymer block diol were amorphous structure; and further, the amorphous molecular chains shapes could not recovery during the tension acting of DMA test when the temperature passed through their T_g (36).

3.4 WVTR analysis

The WVTR of moisture-cured PURH film is a hydrophilic property at a given temperature. The WVTR depend on water vapor adsorbed on the surface of PURH film and then diffuses through the film (37). Furthermore, increasing of WVTR can satisfy the requirement for a coating on outdoor garments to maintain a constant body temperature (14). If the PURH film based on higher water wetting tension and closer to neutral zeta potential, the resulting PURH will has higher WVTR value and exhibit more hydrophilicity (15).

Figure 3

DMA analysis of moisture-cured PURHs film: (a) storage modulus (E’) and (b) Tan delta (Tanδ).

The WVTR decreased sequentially in the following order: PUR-1 (bio-PTMEG) > PUR-5 (bio-PTMEG/bio-PCD) > PUR-3 (PPC-PPG) > PUR-4 (bio-PTMEG/PPC) > PUR-2 (CNSL), as shown in Table 3. The PUR-1 film had the highest WVTR value; however, it was less related to the wettability and surface zeta potential value (Table 3). The highest WVTR value may be attributed to the T_g of PUR-1 was lower than the WVTR test temperature (23 ± 1°C) that resulted in the microstructures of PUR-1 exhibited more loose state, and consequently enhanced water vapor diffused through the PUR-1 film (38). Table 3 also shows most of WVTR value seemed inversely proportional to the T_g of PURHs except PUR-2. This result can be explained by the lower water wetting tension that reflected PUR-2 had less hydrophilic degree than the others PURHs film, which resulted from the CNSL based polyester diol with more hydrophobicity of cardanol backbone.

3.5 Tensile and adhesive properties of PURHs

Table 4 lists the tensile strength and single-lap shear joints adhesive strength of moisture-cured PURHs films. The single-lap shear joints of PURHs were obtained from stainless steel specimens (SS-to-SS).

In general, more amount of hydrogen bonding would improve the tensile strength for the PURH (39,40) (Table 2); however, PUR-1 showed higher tensile strength, and single-lap shear joint strength of SS-to-SS than PUR-2 and PUR-3. This result may be attributed to bio-PTMEG its crystallizable structure, which can increase higher degree of mechanical, and adhesive properties compared with the others amorphous polyols. Furthermore, the bio-PTMEG has lower molecular weight than the CNSL based polyester diol and PPC-PPG diol, which also made PURH molecule chains more rigid, and increased tensile strength for PUR-1 (41). On the other hand, PUR-1 exhibited higher water wetting tension than PUR-2 and PUR-3, which also contributed to greater contact area between adherend and adhesive, and then furthered the adhesive strength of SS-to-SS (42).

Both the addition of CO₂ and biomass based polycarbonate diols in PUR-4 and PUR-5 were greatly increasing the tensile strength as compared with the bio-PTMEG based PUR-1. This result was due to the carbonate-urethane linkage producing better mechanical properties (43). For adhesive property, PUR-4 had better SS-to-SS joint adhesive strength than PUR-1 and PUR-5. This result may be attributed to the PPC diol provided PUR-4 with more polar groups than PUR-5 and PUR-1, which resulted from the PPC diol has higher O/C atom ratio than the bio-PTMEG and bio-PCD diol (44). This is also reflected in PUR-4 had the higher negative value of surface zeta potential than PUR-5 and PUR-1.

3.6 Hydrolysis resistance test of PURHs film and PURHs adhesive joints

The hydrolysis resistance test was to determine the resistance of moisture-cured PURHs films and the SS-to-SS adhesive joints subjected to a combination of an elevated temperature and high humidity. The results of hydrolysis resistance test of PURHs film on tensile strength and single-lap shear joints adhesive strength are presented in Table 4.

The retention of tensile strength of PURHs film decreased sequentially in the following order: PUR-2 (CNSL) > PUR-5 (bio-PTMEG/bio-PCD) > PUR-1 (bio-PTMEG) > PUR-3 (PPC-PPG) > PUR-4 (bio-PTMEG/PPC). PUR-2 had the highest hydrolysis resistance degree because of the CNSL based polyester diol contain aromaticity and long aliphatic chain delivers hydrolytic stability. The addition of polycarbonate diols in the PURHs, PUR-4 showed lower retention of tensile strength than PUR-5 after hydrolysis ageing; the lower degree of hydrolytic stability mainly due to the higher concentration of carbonate group of PPC diol which result from the shorter carbon chain in the repeating unit (45).

However, the tensile strength retention of PURHs was not directly proportional to the retention of single-lap shear adhesive joints strength after hydrolysis ageing. PUR-2, PUR-3 and PUR-4 showed lower retention degree of SS-to-SS joints adhesive strength than PUR-1 and PUR-5. In general, the more polar groups in adhesive implies a better adhere to smooth metal surfaces (46). A possible explanation for is that the ageing temperature caused part of molecular segments in PUR-1 and PUR-5 had more chance than the others PURHs to deform or rearrange which was due to their T_g were much lower than the ageing temperature. Furthermore, the movement of molecular segments spread the applied energy to the others molecular segments which contributed to increase the degree of ageing resistance or decrease interfacial adhesion failure between the PURH and stainless steel under the ageing impact (42).