The current coatings market has a demand for coating materials with attractive aesthetics and high functionality. For example, research is underway on applications that require flexibility, such as self-healing coatings that return to their original shape even when damaged1 and soft-feel coatings that give a feeling of comfortable touch.2
In the adhesives market, structural adhesives that can contribute to lightweighting in the automotive sector have been developed.3,4 Therefore, there is a need for new adhesives that can balance trade-off parameters such as strength and elongation.
In the CASE (coating, adhesive, sealant, and elastomer) market, polyurethane is often utilized because it can be easily controlled by a wide range of physical and chemical properties through the optimization of the type of polyol, molecular weight, number of functional groups, and type of isocyanate. In these applications, polypropylene glycol (PPG), polyester diols (PEP), poly tetramethylene ether glycol (PTMG), and polycarbonate diols (PCD) are generally used as polyols.
On the other hand, each conventional polyol has pros and cons. To balance the tensile properties, low viscosity, transparency, and durability of polyurethane, it is necessary to adjust the combination and composition of multiple polyols,5 and it is difficult for a single polyol to achieve all of these characteristics.
Developing a New Solution
In order to address these problems, we considered combining PCD and PPG to balance strength, flexibility, and low viscosity of polyol. Firstly, thermoplastic polyurethane (TPU) using a blend polyol of PCD and PPG was synthesized and evaluated. However, the poor compatibility between the two polyols resulted in a cloudy polymer.
In order to solve the compatibility problem, we tried to synthesize a copolymer of PCD and PPG. We examined the polymerization using KOH, a conventional alkaline catalyst, but the reaction did not proceed.
Secondly, as a result of carrying out a polymerization using a different, proprietary catalyst, a PCD-block-PPG (PCD-b-PPG) polyol with a desired molecular weight was obtained. Polyols are in liquid state at room temperature, have a lower viscosity than conventional polyols, and are excellent in handleability. We found that the compatibility or incompatibility of polyols can be controlled by changing the type of PCD.
Furthermore, we also found that the thermoplastic polyurethane resin (TPU) using these polyols showed good tensile strength and high transparency. Generally, PCD and PPG are challenging to be compatible with each other, but PCD-b-PPG polyols do not have a bump in viscoelasticity measurement derived from the phase separation, and their appearance was transparent.
It is also suggested that the segment size in TPU is more diminutive than the visible light and is uniformly dispersed. In studying the microstructure of TPU through scanning electronic spectroscopy (SEM), we observed that the highly transparent sample had a microphase-separated structure with a small hard segment size and did not exhibit nonuniform aggregates.
When we compared the new polyol and conventional polyols as a TPU, the new polyol showed excellence not only in tensile properties and transparency but also in heat resistance and hydrolysis resistance. This paper reports the evaluation of the basic properties of this new PCD-b-PPG copolymer polyol, as well as the evaluation results of the TPU film.
Figure 1. Polyol structure.
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Experiment Details
Synthesis of PCD-PPG Block-Copolyether (PCD-b-PPG) Polyols*
The OHV measurement was based on ISO14900: 2017, and the molecular weight was described as the OHV conversion value. The calibration curve for the average molecular weight and molecular weight distribution was prepared by polystyrene standard and measured by gel permeation chromatography (GPC).
The viscosity of the liquid sample was measured at 25˚C with an E-type viscometer. After heating and mixing at 80˚C for 5 hrs, the polyol was transferred to a vial having a diameter of 10 mm, stored at 23˚C for seven days, and then visually observed.
Synthesis of TPU and Making Method of TPU Film
A bifunctional polyol having an OHV equivalent molecular weight of 2,000 and 4,4-MDI were mixed, so that the isocyanate index based on the equation below is 210-250; the mixture was then reacted at 80˚C for 3-6.5 hrs. Finally, NCO-terminate prepolymer was obtained.
Isocyanate index: (number of isocyanate groups in MDI) / (number of hydroxyl groups in polyol) × 100
A chain extender (1,4-BD) was then added so that the hard segment content was 25% based on the calculation of the equation below, and a thermal plastic polyurethane (TPU) resin having terminal OH was synthesized.
Hard content: {(MDI mass) + (mass of chain extender)} / {(mass of MDI) + (mass of chain extender) + (number of hydroxyl groups of polyols)} × 100
The isocyanate index of TPU was set to 98 based on the calculation of the first equation. The molecular weight of polyurethane after synthesis was measured by GPC.
Evaluation of TPU film
The obtained TPU resin was molded at 125-180˚C using a hydraulic molding machine to obtain a film having a thickness of 250 micrometer. Various physical properties were measured using a polyurethane film formed into a shape required for measurement.
To evaluate tensile characteristics, a polyurethane film was punched out with a dumbbell mold (ISO 37-1A) to obtain a test piece. We measured the tensile characteristics by tensile device (Tensilon VTM, Toyo Baldwin). The tensile speed was set at 300 mm/min and measured the 100% stretched strength (modulus E100, MPa), the strength at breaking film (tensile strength, MPa), and the elongation at break (%).
For dynamic mechanical analysis (DMA), the 5 x 10 mm TPU film was measured with a dynamic viscoelasticity measuring device (EXSTAR 6100, Seiko Instruments Inc.) under the following conditions:
- Mode: Tension
- Temperature range: -80 to 120˚C
- Temperature rise rate: 3˚C/min
- Measurement frequency: 1 Hz
- Distortion: 1%
To evaluate the TPU film’s optical transparency, the haze of a 40 x 40 mm sample was measured using a spectrophotometer (COH400, NIPPON DENSYOKU INDUSTRIES Co., Ltd.).
For SEM observation, the TPU film was sliced to a thickness of 1.5 micrometer using cryo-microtome and subjected to electron staining with RuO4. The material after electronic staining was not conductively coated and was observed with a measuring device (SU8230, Hitachi High-Tech Corp.).
To evaluate heat resistance, the TPU film was punched out with a dumbbell mold (ISO 37-1A) and stored at 100˚C for seven days. It was then measured by the tensile property method and compared with the strength at breaking film before storage.
For hydrolysis resistance, a TPU film was punched out with a dumbbell mold (ISO 37-1A), immersed in water at 80˚C, and stored for seven days. It was then measured by the tensile physical properties method and compared with the strength at breaking film before storage.
Results and Discussion
Polyol Structure and Property
Table 1 shows the properties of the polyol synthesized and the appearance after storage for seven days. Four types of the PCD-b-PPG were synthesized using PCD, each containing different carbonate groups and molecular weight as initiator. Their GPC is shown in Figure 2.
Figure 1. Polyol structure..
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Regarding the PCD-b-PPG, the polydispersity index (PDI) is about 1.7-2.1, and it is reduced by 1.1-1.9 after PO addition. By reducing the PDI, the number of moles of polymers in the high molecular weight range is relatively small with respect to the target molecular weight, so the viscosity of the polyol can be reduced even with the same OHV polyols. As a result, the polyols synthesized this time have a relatively low viscosity of 800-3,100 mPa·s (at 25˚C, liquid state).
Figure 2. GPC results: PCD1-b-PPG-2000 (upper left), PCD2-b-PPG-2000 (upper right), PCD1'-b-PPG-2000 (lower left), and PCD3-b-PPG-2000 (lower right).
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A compatibility and phase separation of the PCD-b-PPG are important in storage stability and ease of handling during use, so we observed the appearance after storage for seven days at room temperature. PCD1-b-2000 and PCD2- b-2000 showed good compatibility and remained transparent. On the other hand, for PCD1'-b-2000, PCD3-b-2000, and PCD1-2000/PPG-2000=25/75, two-phase separation was confirmed after storage.
From these results, the following three points are important for obtaining PCD-b-PPG with good compatibility:
The PCD-b-PPG structure is more effective than the blend system, even with the same carbonate group content. The reason is that PCD and PPG basically have different polymer polarities, and they are difficult to be compatible with each other. Even if they are mixed forcibly, their phase is separated into two layers. Therefore, compatibility can be achieved by adding PO to the PCD to form a block polymer and homogenizing the polymer structure in the system. This was inferred from the results of comparing the compatibility results of PCD1-b-PPG-2000 and PCD1-2000/PPG-2000=25/75.
The percentage of the carbonate group of polyols influenced the compatibility. It is compatible when the percentage of carbonate group is 15 wt% or less. PPG1-b-PPG-2000 and PPG2-b-PPG-2000 have 9.0 wt% and 13.5 wt% carbonate groups each; they were transparent liquids. On the other hand, PPG1'-b-PPG-2000 and PPG3-b-PPG-2000 have 19.5 wt% and 22.5 wt% carbonate groups each; they were separated into two layers after storage.
When using initiators with the same structure, it is more effective to increase PO addition. Figure 2 shows the GPC charts of PCD1-b-PPG-2000 (upper left) and PCD1'-b-PPG-2000 (lower left). In PCD1'-b-PPG-2000, PO was added to PCD1-1000 to get a molecular weight of 1,000; the PCD without PO remains on the high molecular weight area. In contrast, the PCD1-PPG was covered most PCD by PO. In the result, it showed good compatibility.
The reasons for the second and third point show that by increasing the proportion of PCD with PO and reducing the ratio of PCD alone, the difference of polarity becomes close in the polyols system, and compatibility can be improved.
Tensile Property of TPU Film
A TPU film of PCD1-b-PPG-2000 and PCD2-b-PPG-2000 that have good compatibility as polyols was formed to measure tensile properties. For comparison, TPU films of PCD alone, PPG alone, and PCD/PPG blend were formed. We chose the PCD that has the same structure as the initiator of PCD-b-PPG. The OHV of all polyols is set around 56.1, and their functionality is 2.0.
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Synthesis results and tensile properties of TPU film are included in Table 2. Figure 3 shows the results for tensile strength in terms of elongation for PCD1-b-PPG-2000 and PCD2-b-PPG-2000.
Figure 3. Results of the tensile strength and elongation of tensile test of polyurethanes.
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The TPU of PCD1-b-PPG-2000 has higher strength and elongation than the blend type. In the blend type, PCD1 and PPG are challenging to be compatible with each other, and many phase separations in the interfaces generated in the TPU reduced the ultimate tensile strength. On the other hand, since the PCD-b-PPG is sufficiently compatible with each other, it showed high tensile strength and elongation, which features both of PCD and PPG.
In contrast, in observing the results of TPU from PCD2-b-PPG-2000, the difference in the physical properties between blend type and PCD-b-PPG is smaller than the TPU of PCD1. This result suggests that PCD2 may have good compatibility with PPG.
The storage modulus of dynamic viscoelastic measurement is shown in Figure 4. A bump is seen in the low temperature curve of the PCD1 and PCD2 blend types, which suggests that the polymer compatibility in the TPU is inadequate. On the other hand, no bump was observed in PCD-b-PPG; it is related to compatibility of the polymer, which is uniform.
Figure 4. Storage modulus of dynamic viscoelasticity of the TPU film of PCD-b-PPG. PCD1-b-PPG (upper), PCD2-b-PPG (lower) are compared with PCD, PPG, and blend of PCD/PPG.
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The glass transition temperature (Tg) is around -20˚C to -30˚C, which shows that PCD-b-PPG retains elastomeric properties at a colder temperature. Moreover, the PCD-b-PPG has a higher peak drop temperature than PPG and blend types (PCD1 and PCD2) at 80˚C to 120˚C, which shows high heat resistance.
When the PCD-b-PPG is utilized for optical applications or coating applications, transparency of TPU is required. The results are shown in Figure 5. Whereas PPG and the blend of PCD/PPG showed high haze, PCD-b-PPG had a low haze. This result suggests that PCD and PPG are compatible with each other with the domain size smaller than visible light. Therefore PCD-b-PPG is suitable for optical and coating usage.
Figure 5. Haze value and appearance of TPU film that comprise some polyols.
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To verify the cause of TPU transparency, we observed the microstructure with scanning electron microscopy (SEM). Firstly, we confirmed the difference between the soft segment consisting of polyol and the hard segment consisting of urethane bonds by the electronic staining method using RuO4. Moreover, we realized the contrast of the hard segment is more profound (see Figure 6).
Figure 6. The effect of RuO4 staining: before staining (left) and after staining (right).
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Secondly, we compared the TPU film of PCD1-b-PPG-2000 with a haze of 1.9 and PCD2-b-PPG-2000 with a high haze of 36.5 (see Figure 7). As a result, hard segments of about 0.1-0.3 micrometer are seen in PCD1-b-PPG-2000. On the other hand, in addition a hard segment of 0.1-0.2 micrometer, a nonuniform aggregation of about 1.0 micrometer exists in PCD2-b-PPG-2000.
Figure 7. SEM image of TPU film: PCD1-b-PPG-2000 (left) and PCD2-b-PPG-2000 (right).
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To understand the cause of the nonuniform aggregations, we compared the TPU film of PCD1-b-PPG-2000 (PCD1-500, PCD/PPG=25/75) and PCD1'-b-PPG (PCD1-1000, PCD/PPG=50/50), which have the same structure of PCD but different molecular weight. We synthesized TPU consisting of PCD1'-b-PPG; the formulation is shown in Table 3.
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As a result, we found the same nonuniform aggregations in PCD1'-b-PPG-2000 as those seen in PCD2-b-PPG (see Figure 8). The haze was also higher, corresponding to the existence of the nonuniform aggregations. We summarized these results in Table 4.
Figure 8. SEM image of TPU film: PCD1-b-PPG-2000 (left) and PCD1'-b-PPG-2000 (right).
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Two points are required for the transparency of the TPU film of PCD-b-PPG. First, the size of the hard segment is changed by the compatibility between isocyanate and polyols. When the ratio of PPG that is difficult to be compatible with isocyanate is increased, the isocyanates aggregate with each other and the hard segments become larger. On the other hand, when the ratio of the PCD that is capable of hydrogen bonding with the isocyanate is increased, the isocyanate is easily dispersed in the entire system so that the hard segment becomes smaller.
The second point involves the presence or absence of formation of nonuniform aggregations. The nonuniform aggregations appear when the amount of PCD alone increases in PCD-b-PPG. These are assumed to consist of PCD alone that are not compatible with PPG, which is the soft segment’s main component. By controlling these points and having a uniform microphase-separated structure, a TPU film of PCD-b-PPG with good transparency can be formed.
Comparison with Conventional Polyols
We compared the TPU films of PCD-b-PPG with conventional PEP, PTMG, and PCD polyols. The composition and molecular weight of the TPU film are shown in Table 5.
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Figure 9 shows the heat resistance of TPU film at 100˚C. Although the heat resistance is reduced due to the influence of PPG, the heat resistance (PCD-b-PPG) remains high, which is a characteristic of PCD. This result corresponds to the fact that the storage modulus in the high-temperature region in the dynamic mechanical analysis is constant.
Figure 9. Heat resistance of TPU films.
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Figure 10 shows the hydrolysis resistance of the TPU films formed by PCD-b-PPG and conventional polyols. The TPU film of PCD-b-PPG shows promising results that are derived from PCD and PPG.
Figure 10. Hydrolysis resistance of TPU films.
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Conclusion
A new polyol (PCD-b-PPG) has been synthesized that has a low viscosity of around 800-3,000 mPa·s (at 25˚C, liquid state) and is transparent due to the use of PCD as an initiator, even after storing for a long duration. After forming TPU film, we observed that it can achieve significant tensile properties, excellent optical properties, high hydrolysis, and heat resistance. A summary is shown in Table 6.
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The new PCD-b-PPG polyol can be applied to coating applications where transparency is required. In addition, it may contribute to other CASE applications and solvent-free urethane compositions that can take advantage of its low viscosity.
For more information, email toyokazu.suzuki@agc.com or shogo.fujisaki@agc.com, or visit www.agc.com or www.agcchem.com.
Authors’ note: PPG and PCD-b-PPG were synthesized by AGC. Polyols other than PPG were commercial products.
Editor’s note: This article is based on a paper given at the 2021 Polyurethanes Technical Conference, October 5-7, 2021, Denver, Colo.
Note: Opening image courtesy of haydenbird via the E+ collection on www.gettyimages.com.
References
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- Paint Composition, patent WO2019131617 A1.
- Structural Polyurethane Adhesive, patent EP2700666 A1.
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- K. Kimura, “Basis and the Latest Developments in Polyurethane Adhesive,” Journal of the Adhesion Society of Japan, 56, 142, 2020.
