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Upcycling of thermoplastic polyurethane into covalent adaptable networks via chain breaking-crosslinking strategy
通过断链-交联策略将热塑性聚氨酯升级再造为共价适应性网络

Binbo Wang

Binbo Wang

Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang Province, Divisions of Polymers and Composites, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, People's Republic of China

University of Chinese Academy of Sciences, Beijing, People's Republic of China

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Yi Wang

Yi Wang

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, People's Republic of China

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Falin Li

Falin Li

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, People's Republic of China

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Shuai Du

Shuai Du

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, People's Republic of China

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Jin Zhu

Jin Zhu

Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang Province, Divisions of Polymers and Composites, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, People's Republic of China

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Songqi Ma

Corresponding Author

Songqi Ma

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, People's Republic of China

Correspondence

Songqi Ma, Jiangnan University, Wuxi 214122, People's Republic of China.

Email: songqi.ma@jiangnan.edu.cn

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First published: 19 August 2023
Citations: 1

首次出版:2023 年 8 月 19 日
引用文献:1

Abstract 摘要

Thermoplastic polyurethane (TPU), as one of the most widely used elastomers, has received rapid development, and its recycling becomes increasingly urgent. In this work, a TPU is upcycled to PU covalent adaptable networks (CANs) via chain breaking-crosslinking strategy of being extruded with polyol and isophorone diisocyanate (IPDI) on a micro twin-screw extruder. The carbamate exchange between the polyol and TPU breaks the chain of TPU and introduces plenty of reactive hydroxyl groups, which activates the TPU chain. Long-chain-branched TPU with hydroxyl groups is obtained during the chain-breaking step and then crosslinked with IPDI to obtain highly crosslinked networks. PU-CANs exhibit improved mechanical properties and elastic recovery rate, which is propitious to elastomers. Moreover, the crosslinked network greatly enhances the creep resistance and solvent resistance of the material. PU-CANs maintain favorable reprocessability, suitable for screw extrusion and hot-press processing. This work provides a simple and efficient method to upcycle TPU into reprocessable highly-crosslinked polymers and improve the properties of polymers.
热塑性聚氨酯(TPU)作为应用最广泛的弹性体之一,得到了快速发展,其回收利用也变得日益迫切。在这项研究中,通过在微型双螺杆挤出机上使用多元醇和异佛尔酮二异氰酸酯(IPDI)进行挤出的断链交联策略,将热塑性聚氨酯升级为聚氨酯共价适应性网络(CAN)。多元醇与热塑性聚氨酯之间的氨基甲酸酯交换可切断热塑性聚氨酯链,并引入大量活性羟基,从而激活热塑性聚氨酯链。在断链步骤中,可获得带有羟基的长链支化热塑性聚氨酯,然后与 IPDI 交联,获得高度交联的网络。PU-CAN 具有更好的机械性能和弹性恢复率,这对于弹性体来说是非常有利的。此外,交联网络还大大提高了材料的抗蠕变性和耐溶剂性。聚氨酯-CAN 具有良好的再加工性,适合螺杆挤出和热压加工。这项研究提供了一种简单有效的方法,可将热塑性聚氨酯升级为可再加工的高交联聚合物,并改善聚合物的性能。

Graphical Abstract 图表摘要

1 INTRODUCTION 1 引言

Thermoplastic polyurethane (TPU) is an important type of elastomer, widely used in biomedical, aerospace, wearable devices, intelligent packaging, and other fields due to its high elasticity, wear resistance, biocompatibility, insulation, and so forth as well as its excellent processability.1, 2 Therefore, the diverse usage scenarios have led to a continuous increase in the production of TPU. According to statistics, the current production of polyurethane (PU) products is close to 30 million tons/year, accounting for 7.9% of the total plastic production, making it the fifth most widely used polymer in the world.3 With the growing awareness of environmental protection and the tightening of laws and regulations, it is urgent to recycle TPU. The frequently used recycling methods for TPU include energy recycling and physical recycling.4 Energy recovery is a low-value method, using polyurethane as fuel, which recovers part of the heat but generates a large amount of greenhouse gases (like carbon dioxide) or other harmful gases.5 As the most efficient recycling method, physical-mechanical recycling has little impact on the environment. By utilizing the thermal processability of TPU, it can be recycled and reused through compression, extrusion, injection molding, and other techniques.6 However, the performance of the recycled TPU cannot compete with the fresh TPU due to the aging during the service, and further decreases during each reprocessing cycle on account of the irreversible thermal and mechanical degradation of the material, leading to less-valuable products. At this point, massive attention has been poured into the upcycling of TPU.
热塑性聚氨酯(TPU)是一种重要的弹性体,因其具有高弹性、耐磨性、生物相容性、绝缘性等特点和良好的加工性能,被广泛应用于生物医学、航空航天、可穿戴设备、智能包装等领域。1, 2 因此,多样化的使用场景使得热塑性聚氨酯的产量不断增加。3随着人们环保意识的不断增强和法律法规的不断收紧,热塑性聚氨酯的回收利用已迫在眉睫。4能源回收是一种低价值的方法,它使用聚氨酯作为燃料,可回收部分热量,但会产生大量温室气体(如二氧化碳)或其他有害气体。5 作为最有效的回收方法,物理机械回收对环境的影响很小。利用热塑性聚氨酯的热加工性能,可通过压缩、挤压、注塑成型和其他技术对其进行回收和再利用。6 然而,回收的热塑性聚氨酯由于在使用过程中老化,其性能无法与新鲜的热塑性聚氨酯相媲美,而且由于材料不可逆转的热降解和机械降解,其性能在每个再加工周期中都会进一步下降,从而导致产品的价值降低。因此,人们对热塑性聚氨酯的再循环利用给予了极大的关注。

Upcycling aims to recycle TPU waste into products with better performance or additional value.6-8 The chemical upcycling of TPU has been widely studied and mainly contains hydrolysis, alcoholysis, and aminolysis. Valuable monomers or oligomers are recycled by cleaving the specific chemical bond (mainly the carbamate bond) in the polyurethane molecular chain.9, 10 Biological upcycling is a method of utilizing microorganisms or enzymes to degrade polyurethane and recover valuable monomers. Compared with chemical upcycling, this method has milder conditions and better environmental friendliness, but the degradation rate significantly decreases.11, 12 Converting TPU waste into high-value carbon materials is also a feasible approach. Li et al.13 upcycled TPU waste into activated carbon through two steps of controllable carbonization and activation, and demonstrated its excellent wastewater treatment performance. However, there is currently no direct strategy for upcycling TPU wastes to materials of the same grade or better performance.
6-8 热塑性聚氨酯的化学升级再循环已被广泛研究,主要包括水解、醇解和氨解。通过裂解聚氨酯分子链中的特定化学键(主要是氨基甲酸酯键)来回收有价值的单体或低聚物。9, 10 生物升级再循环是一种利用微生物或酶降解聚氨酯并回收有价值单体的方法。11, 12 将热塑性聚氨酯废料转化为高价值碳材料也是一种可行的方法。Li 等人13 通过可控碳化和活化两个步骤将热塑性聚氨酯废料上行循环制成活性炭,并展示了其优异的废水处理性能。然而,目前还没有直接的策略将热塑性聚氨酯废料升级循环为相同等级或性能更好的材料。

Converting plastic waste into covalent adaptable networks (CANs) might be an effective method.14-19 CANs are polymer networks based on dynamic covalent bonds (DCB) including ester bond, disulfide bond, imine bond, borate bond, carbamate bond, and so forth.20-24 Under specific stimuli such as light and heat, the breaking, recombination, and exchange of DCBs occur, which means that the crosslinked networks are no longer fixed, and the topological structures will change under external forces.25-27 Therefore, CANs possesses the favorable performance of thermosetting resins, as well as the ability to be repaired, reprocessed, and recycled, which has captured great research attention to the upcycling of plastic waste into CANs. In 2017, Zhou et al.28 proposed dynamic crosslinking of poly (butylene terephthalate) (PBT) to enhance its melting strength. CANs was obtained by solid-state (co) polymerization of PBT and glycerol. Then, it has been proven that the reaction between epoxy resin and the end groups of polyesters can also achieve CANs.29-31 These works have achieved dynamic crosslinking of plastics, but due to the low concentration of reaction sites, it is hard to achieve high crosslinking of the material, which limits the performance improvement of the material. In our previous work, high crosslinking of poly (ethylene terephthalate) (PET) was successfully achieved by introducing reaction sites (hydroxyl) and crosslinking with epoxy resin.32 The creep resistance and mechanical properties of PET were significantly improved. The feasibility of this strategy for upcycling polyesters has been further confirmed.33-35
将塑料垃圾转化为共价适应网络(CAN)可能是一种有效的方法。14-19 CANs 是基于动态共价键(DCB)的聚合物网络,包括酯键、二硫键、亚胺键、硼酸键、氨基甲酸酯键等。20-24 在光、热等特定刺激下,DCB 会发生断裂、重组和交换,这意味着交联网络不再固定,拓扑结构会在外力作用下发生变化。25-27 因此,CANs具有热固性树脂的良好性能,以及可修复、可再加工、可回收利用的特点,引起了人们对废塑料升级再造为CANs研究的高度关注。2017 年,Zhou 等28 提出了动态交联聚对苯二甲酸丁二醇酯(PBT)以增强其熔融强度的方法。CANs 是通过 PBT 和甘油的固态(共)聚合得到的。随后的研究证明,环氧树脂与聚酯端基的反应也能获得 CANs。29-31 这些工作实现了塑料的动态交联,但由于反应位点浓度较低,很难实现材料的高交联,限制了材料性能的提高。32 通过引入反应位点(羟基)并与环氧树脂交联,成功实现了聚(对苯二甲酸乙二醇酯)(PET)的高交联。这一策略在聚酯升级再循环方面的可行性得到了进一步证实。33-35

In this work, we first utilized a chain breaking-crosslinking process on a micro twin-screw extruder to obtain performance-enhanced PU-CANs, successfully achieved the upcycling of TPU. During the chain breaking stage, the generation of long-chain branched polyhydroxy polyurethane was demonstrated through rheological and infrared tests. The introduced hydroxyl groups, as crosslinking sites, reacted with isocyanates to obtain high crosslinking networks. The influence of the formed crosslinked network on the shear rheology, extensional rheology, stress relaxation behaviors, reprocessing, creep resistance, solvent resistance, and thermal and mechanical properties was investigated as well.
在这项工作中,我们首次在微型双螺杆挤出机上利用断链-交联工艺获得了性能增强型聚氨酯-CAN,成功实现了热塑性聚氨酯的上循环。在断链阶段,通过流变学和红外测试证明了长链支化聚羟基聚氨酯的生成。引入的羟基作为交联位点,与异氰酸酯反应生成高交联网络。此外,还研究了所形成的交联网络对剪切流变学、延伸流变学、应力松弛行为、再加工性、抗蠕变性、耐溶剂性以及热性能和机械性能的影响。

2 RESULTS AND DISCUSSION 2 结果与讨论

2.1 Preparation of PU-CANs
2.1 制备 PU-CAN

In order to obtain PU-CANs, commercial TPU was dynamically crosslinked through a two-step method of chain breaking and crosslinking on a micro twin-screw extruder. In the chain breaking stage, polyols (sorbitol, dipentaerythritol) and TPU were blended and extruded through micro twin-screw extruder for 5 min. During extrusion, exchange reaction occurred between hydroxyl group and carbamate bond to produce branched polyurethane, polyurethane oligomer, and so forth (Figure 1A), which is characterized by rheology and Fourier infrared spectroscopy. As shown in the FTIR spectra (Figures 1B, S1, and S2), the peak at 3320 cm−1 corresponds to hydroxyl groups. As the content of polyols increases, the peak of hydroxyl groups significantly increases, the increase of hydroxyl groups in the system is conducive to the formation of a more complete crosslinked network. Meanwhile, the shape of other peaks remained basically unchanged, indicating that no other side reactions occurred. The rheological test is an important method for characterizing polymer morphology. For linear PU, the variation of its complex viscosity with angular frequency in the low frequency region fully conforms to Newtonian fluid law. When the angular frequency increases to the critical value, the viscosity decreases more quickly, that is, shear thinning, which is the characteristic of a typical linear polymer melt. However, long-chain branched PU exhibits continuous shear thinning phenomenon throughout the entire experimental frequency range, which is triggered by the multiple entanglements of long-chain branched PU molecular chains.36, 37 Under the action of the shear field, the branch chain with shorter relaxation time takes priority over the main chain to untangle, leading to the shear thinning phenomenon at low angular frequencies. After the untangling of the branch chain is completed, the main chain begins to untangle, consequently shear thinning phenomenon can be displayed over a long frequency range. In other words, the more significant the shear thinning phenomenon, the higher the branching level of the long branch chain. It can be seen from Figure 1C,D that with the increase of polyol content, the branching level of chain-broken polyurethane (B-PU) gradually increases. Moreover, the branching degree of B-PU-Dx is higher, which might be due to the higher reactivity of the primary alcohol. In the high-frequency region, the complex viscosity of B-PU is lower than that of PU0, which is due to the plasticizing effect of low molecular weight PU chains generated during the chain-breaking stage.
为了获得聚氨酯-CAN,商用热塑性聚氨酯在微型双螺杆挤出机上通过断链和交联两步法进行动态交联。在断链阶段,将多元醇(山梨醇、季戊四醇)和热塑性聚氨酯混合,并通过微型双螺杆挤压机挤出 5 分钟。在挤出过程中,羟基和氨基甲酸酯键发生交换反应,生成支化聚氨酯、聚氨酯低聚物等(图1A),并通过流变学和傅立叶红外光谱对其进行表征。如傅立叶红外光谱(图1BS1S2)所示,3320 cm-1处的峰值与羟基相对应。随着多元醇含量的增加,羟基的峰值明显增加,体系中羟基的增加有利于形成更完整的交联网络。同时,其他峰的形状基本保持不变,说明没有发生其他副反应。流变测试是表征聚合物形态的重要方法。对于线性聚氨酯来说,在低频区,其复合粘度随角频率的变化完全符合牛顿流体定律。当角频率增加到临界值时,粘度下降更快,即剪切变稀,这是典型线性聚合物熔体的特征。然而,长链支化聚氨酯在整个实验频率范围内都表现出持续的剪切变稀现象,这是由长链支化聚氨酯分子链的多重缠结引发的。36, 37 在剪切场的作用下,弛豫时间较短的支链会优先于主链解开,从而导致低角频率下的剪切变薄现象。支链松弛完成后,主链开始松弛,因此剪切变薄现象会在较长的频率范围内出现。换句话说,剪切稀化现象越明显,长支链的分枝水平就越高。从图1C、D中可以看出,随着多元醇含量的增加,断链聚氨酯(B-PU)的支化程度逐渐增加。此外,B-PU-Dx 的支化程度更高,这可能是由于伯醇的反应活性更高。在高频区,B-PU 的复合粘度低于 PU0,这是因为在断链阶段产生的低分子量聚氨酯链具有塑化作用。

Details are in the caption following the image
(A) Schematic diagram of chain breaking-crosslinking method for the preparation of PU-CANs via extrusion, (B) FTIR spectra of B-PU-Dx, (C, D) The complex viscosity of original PU (PU0) and chain-broken PUs (B-PU-Dx and B-PU-Sx) varies with angular frequency at 200 °C, (E) FTIR spectra of PU-Dx, (F) The gel content of PU-CANs, (G) Images of PU-D4 before and after being placed in tetrahydrofuran (THF) at 25 °C for 12 h.
(A) 通过挤出法制备 PU-CAN 的断链交联法示意图;(B) B-PU-Dx 的傅立叶变换红外光谱;(C, D) 200 °C 时原始 PU(PU0)和断链 PU(B-PU-Dx 和 B-PU-Sx)的复合粘度随角频率的变化、(G) PU-D4 在 25 °C 的四氢呋喃(THF)中放置 12 小时前后的图像。

In the crosslinking stage, the B-PUs and IPDI (OH: NCO = 1:0.9) were extruded and crosslinked in a micro twin-screw extruder for 5 min, and finally PU-CANs were obtained. The crosslinking was characterized by Fourier infrared spectroscopy, gel content, and swelling test. As can be seen from Figures 1E and S3, it is evident that the peak at 3320 cm−1 has shifted from wide and strong after chain-breaking to narrow and weak after crosslinking, possibly owing to the consumption of hydroxyl groups by isocyanates. Moreover, there is no characteristic absorption peak of NCO at 2270 cm−1 and a new characteristic absorption peak of CH2 appears at 2956 cm−1, which proves the formation of new carbamate bond. Then, THF as a good solvent for PU was used to test the gel content of PU-CANs through the Soxhlet extraction experiment and recorded in Figure 1F and Table S1. As expected, the increase of polyol content can significantly increase the gel content (from 31.7% to 97%) of the sample, that is, more reaction sites (OH) are conducive to consummating the crosslinked network. In the swelling test, the PU-CANs also exhibited well crosslinked networks, they only swelled rather than dissolved in common organic solvents (Tables 1 and S2).
在交联阶段,B-PU 和 IPDI(OH: NCO = 1:0.9)在微型双螺杆挤出机中挤出并交联 5 分钟,最终得到 PU-CAN。傅里叶红外光谱、凝胶含量和膨胀试验对交联情况进行了表征。从图1ES3中可以看出,3320 cm-1处的峰值从断链后的宽而强转变为交联后的窄而弱,这可能是由于异氰酸酯消耗了羟基。此外,在 2270 cm-1 处没有 NCO 的特征吸收峰,而在 2956 cm-1 处出现了 CH2 的新特征吸收峰、这证明形成了新的氨基甲酸酯键。然后,使用四氢呋喃作为聚氨酯的良好溶剂,通过索氏提取实验检测聚氨酯-CAN 的凝胶含量,结果如图1F和表S1所示。不出所料,多元醇含量的增加能显著提高样品的凝胶含量(从 31.7% 提高到 97%),即更多的反应位点(OH)有利于交联网络的形成。在溶胀测试中,PU-CAN 也表现出良好的交联网络,它们在普通有机溶剂中只溶胀而不溶解(表 1S2 )。

TABLE 1. Swelling ratio of PU-CANs in different solvents.
表 1.聚氨酯泡沫罐在不同溶剂中的膨胀率。
Swelling ratio (%)a
膨胀率 (%)a
Methanol 甲醇 Ethanol 乙醇 Chloroform 氯仿 THF DMF Acetone 丙酮 Toluene 甲苯 Water 
PU-D4 111 130 482 466 412 171 148 110
PU-S3 112 132 407 652 293 162 145 109
  • a The sample was immersed in solvent at 25 °C for 12 h.
    a 样品在 25 °C 溶剂中浸泡 12 小时。

2.2 Thermal and mechanical properties of PU-CANs
2.2 聚氨酯泡沫罐的热性能和机械性能

The increase in the proportion of polyols transformed the dynamic thermomechanical analysis (DMA) characterization of the material from typical thermoplastic curves to thermosetting material curves. The storage modulus of PU0 disappeared at 210 °C due to melting fracture, while the storage modulus of PU-Sx and PU-Dx kept at specific values (platform moduli), which can be used to calculate the crosslink density (Table 2). At this point, it can be concluded that the crosslink density gradually increases with the increase of polyol ratio, which can significantly increase the glass transition temperature (Tg) of polymer (Figure 2B,C and Table 2). The Tg of PU-Dx increased from 5.8 to 76.4 °C, and the Tg of PU-Sx achieved a maximum value of 80.2 °C. It is worth mentioning that the loss factor curve of PU0 (Figure 2B) gradually increases with the increase of temperature after 150 °C, contributing to the increase in motion ability caused by melting. While the curve of PU-D5, with the highest crosslinking degree, presents a straight line, which is also indicative of the complete crosslinking of the material. Compared with the tan δ curve of PU-Dx and PU-Sx, the PU-Dx exhibit obvious single peak below 150 °C, while the PU-Sx possess double peak, indicating that PU-Sx were composed of two phases. It might be attributed to the lower reaction activity of secondary alcohols, most of which react with isocyanate rather than participate in the exchange of carbamate. Therefore, it is speculated that the two phases are PU network and sorbitol network with large difference in crosslink density.
多元醇比例的增加使材料的动态热机械分析(DMA)特性从典型的热塑性曲线转变为热固性材料曲线。PU0 的储存模量在 210 °C 时因熔融断裂而消失,而 PU-Sx 和 PU-Dx 的储存模量保持在特定值(平台模量),可用来计算交联密度(表 2)。由此可以得出结论,交联密度随着多元醇比率的增加而逐渐增大,这可以显著提高聚合物的玻璃化转变温度(Tg )(图2B、C和表2)。PU-Dx 的 Tg 从 5.8 ℃ 升至 76.4 ℃,PU-Sx 的 Tg 达到 80.2 ℃ 的最大值。值得一提的是,PU0 的损耗因子曲线(图2B)在 150 °C 之后随着温度的升高而逐渐增大,这也是熔化导致运动能力增强的原因。而交联度最高的 PU-D5 的曲线呈现一条直线,这也表明材料已完全交联。与 PU-Dx 和 PU-Sx 的 tan δ 曲线相比,PU-Dx 在 150 °C 以下表现出明显的单峰,而 PU-Sx 则具有双峰,表明 PU-Sx 由两相组成。这可能是因为仲醇的反应活性较低,它们大多与异氰酸酯发生反应,而不是参与氨基甲酸酯的交换。 因此,推测这两相为交联密度相差较大的聚氨酯网络和山梨醇网络。

TABLE 2. Thermal properties of PU-CANs.
表 2.PU-CAN 的热特性。
Sample 样品 Tg (°C) E′ at 210 °C (MPa)
E′ 210 °C 时 (MPa)
ve (mol m−3)
ve (mol m-3)
PU0 7.1
PU-D1 30.3 0.22 18.3
PU-D2 34.6 0.69 57.3
PU-D3 59.2 0.90 74.7
PU-D4 66.2 1.65 136.9
PU-D5 74.3 1.81 150.2
PU-S1 9.7 0.10 8.3
PU-S2 27.6 0.54 44.8
PU-S3 47.1 1.16 96.3
PU-S4 80.2 3.14 260.6
Details are in the caption following the image
Thermal and mechanical properties of PU-CANs: DMA curves of the (A, B) PU-Dx and (C) PU-Sx, representative tensile stress–strain curves of (D) PU-Dx and (E) PU-Sx, (F) cyclic tensile strain–stress curves of PU-Dx under 500% strain.
PU-CAN 的热性能和机械性能:(A, B) PU-Dx 和 (C) PU-Sx 的 DMA 曲线,(D) PU-Dx 和 (E) PU-Sx 的代表性拉伸应力-应变曲线,(F) PU-Dx 在 500% 应变下的循环拉伸应变-应力曲线。

The mechanical properties were evaluated via tensile test, the results are summarized in Figure 2D–F and Table 3. Obviously, the tensile strength (9.65–21.9 MPa) and elongation at break (308%–928%) of PU-Sx cannot be compared with commercial PU0, only modulus (272–480 MPa) increases. On the contrary, the PU-Dx exhibit an obvious increase in strength (30.2–53.3 MPa) and modulus (324–805 MPa) as the crosslink density increases, but a slight decrease in elongation at break (719%–1150%). The high crosslink density of PU-D5 results in the lowest elongation at break and limited tensile strength. Chemically crosslinked network significantly improved the elastic recovery rate of materials. The poor mechanical performance of PU-Sx might be due to the non-uniformity of their networks, as demonstrated by the aforementioned DMA test. The weak parts of the networks (regions with low crosslink density) might make the sample more prone to fracture. As illustrated in Figures 2F and S5, the elastic recovery rate of PU-CANs was obtained after 10 cycles of cyclic stretching at an elongation of 500%. As the crosslink density increases, the elastic recovery rate of the samples increases (from 58.2% to 69.6%), much better than the commercial TPU.
机械性能通过拉伸试验进行评估,结果汇总于图2D-F和表3。显然,PU-Sx 的拉伸强度(9.65-21.9 兆帕)和断裂伸长率(308%-928%)无法与商用 PU0 相比,只有模量(272-480 兆帕)有所增加。相反,随着交联密度的增加,PU-Dx 的强度(30.2-53.3 兆帕)和模量(324-805 兆帕)明显增加,但断裂伸长率(719%-1150%)略有下降。PU-D5 的高交联密度导致其断裂伸长率最低,拉伸强度有限。化学交联网络大大提高了材料的弹性恢复率。如上述 DMA 测试所示,PU-Sx 机械性能不佳的原因可能是其网络不均匀。网络的薄弱部分(交联密度低的区域)可能会使样品更容易断裂。如图2FS5所示,PU-CAN 的弹性恢复率是在伸长率为 500% 的条件下循环拉伸 10 次后得出的。随着交联密度的增加,样品的弹性恢复率也在增加(从 58.2% 增加到 69.6%),远远优于商用热塑性聚氨酯。

TABLE 3. Mechanical properties of PU-CANs.
表 3.PU-CAN 的机械特性。
Sample 样品 Tensile strength (MPa) 拉伸强度(兆帕) Young's modulus (MPa) 杨氏模量(兆帕) Elongation at break (%) 断裂伸长率 (%) Recovery rate (%) 回收率 (%)
PU0 22.2 295.6 1151 43.5
PU-D1 30.2 324 1080 61.4
PU-D2 40.8 343 1150 63.8
PU-D3 45.3 389 1006 66.2
PU-D4 53.3 425 1082 69.2
PU-D5 31.9 805 719 69.6
PU-S1 9.65 272 928 58.2
PU-S2 21.9 334 903 62.2
PU-S3 21.2 469 570 65.4
PU-S4a 13.7 480 308
  • a PU-S4 cannot complete 10 cycles of stretching, when the strain is 500%.
    a PU-S4 在应变为 500% 时无法完成 10 次拉伸。

2.3 Stress relaxation and reprocessing of PU-CANs
2.3 应力松弛和 PU-CAN 的后处理

Relaxation time is a crucial index for characterizing the reprocessing performance of CANs, which is related to the crosslink density of network, the activation energy of dynamic bonds, and so on. The relaxation time was obtained by selecting four temperatures for all samples through DMA relaxation mode testing (Figures 3A and S6–S13). Overall, the ratio of modulus after relaxation for certain time (G) and modulus before relaxation (G0) of all samples can reach 1/e within 100 s at a specific temperature, and the increase of crosslink density significantly reduces their relaxation rate, which is due to the network rearrangement time gradually prolonging with the crosslinking degree of the network. As presented in Figure 3B,C, The PU-Sx required relatively longer relaxation times as a result of low reactivity of secondary alcohols. It is worth mentioning that PU-D5 and PU-S4 exhibit significantly longer relaxation times at 160 °C, which is attributed to their complete crosslinking networks, and the free polymer chains have a significant acceleration effect on the relaxation of CANs.38 The activation energies (Ea) of the PU-Dx and PU-Sx were calculated using the Arrhenius formula (Figures 3D and S12). The Ea of PU-Dx decreases with the increase of crosslink density, while the Ea trend of PU-Sx is opposite and not significantly different. With the increase in crosslink density and hydroxyl content, the bond exchange ability of PU-Dx was enhanced. As previously demonstrated, the uneven distribution of PU-Sx network might result in the enrichment of hydroxyl groups in highly crosslinked regions, so the amount of hydroxyl groups has little effect on bond exchange ability.
松弛时间是表征 CAN 再加工性能的一个重要指标,它与网络的交联密度、动态键的活化能等有关。通过 DMA 松弛模式测试(图3AS6-S13),为所有样品选择了四个温度,从而得到松弛时间。总的来说,在特定温度下,所有样品在一定时间内松弛后的模量(G)与松弛前的模量(G0)之比都能在 100 s 内达到 1/e,而交联密度的增加会显著降低其松弛速率,这是由于网络重排时间随着网络交联度的增加而逐渐延长。如图3B、C所示,由于仲醇的反应活性较低,PU-Sx 需要的弛豫时间相对较长。值得一提的是,PU-D5 和 PU-S4 在 160 °C 时的松弛时间明显更长,这是因为它们具有完整的交联网络,游离聚合物链对 CAN 的松弛具有显著的加速作用。38 激活能(Ea )(图 3DS12 )。 PU-Dx 的 Ea 随交联密度的增加而减小,而 PU-Sx 的 Ea 则相反,无显著差异。随着交联密度和羟基含量的增加,PU-Dx 的键交换能力增强。如前所述,PU-Sx 网络的不均匀分布可能会导致羟基在高交联区域富集,因此羟基的数量对键交换能力影响不大。

Details are in the caption following the image
(A) Stress relaxation curves of PU-D4 at 150, 160, 170, and 180 °C Stress relaxation curves of (B) PU-Dx and (C) PU-Sx at 160 °C, (D) Activation energy (Ea) of the exchange reaction calculated from the slopes.
(A) PU-D4 在 150、160、170 和 180 °C 时的应力松弛曲线 (B) PU-Dx 和 (C) PU-Sx 在 160 °C 时的应力松弛曲线,(D) 根据斜率计算出的交换反应活化能(Ea )。

Thanks to the efficient stress relaxation of the material, all samples can be reprocessed using traditional PU processing methods such as extrusion and hot-press reprocessing. As shown in Figure 4A, PU-CANs can be extruded via twin-screw extruder and hot pressed into thin films at 190 °C for 5 min, which suggests that the recycling or reuse of PU-CANs can be easily achieved. The recycled PU-D4s (PU-D4-R1 and PU-D4-R2) have been subjected to tensile test, manifesting that their tensile strength recovery rates are 76.5% and 68.7%, their recovery rates of elongation at break are 93.4% and 91.9% (Figure 4B). Moreover, the FTIR spectra of PU-D4 before and after recycling manifest that the characteristic peaks and their height are basically consistent (Figure 4C), indicating that hot-press recycling would not affect the chemical structure of the material. After multiple reprocessing, the crosslink density (calculated by the platform modulus at 210 °C) and Tg of PU-D4 decreased slightly through DMA testing (Figure 4D,E). This might be due to the breakage of polyurethane chains during the remolding, which also explains the reduction of the mechanical properties.
由于材料的应力松弛效率高,所有样品都可以使用传统的聚氨酯加工方法进行再加工,如挤出和热压再加工。如图4A所示,PU-CAN 可通过双螺杆挤出机挤出,并在 190 °C 下热压 5 分钟制成薄膜,这表明 PU-CAN 可轻松实现回收或再利用。对回收的 PU-D4(PU-D4-R1 和 PU-D4-R2)进行拉伸试验,结果表明它们的拉伸强度回收率分别为 76.5%和 68.7%,断裂伸长率回收率分别为 93.4%和 91.9%(图 4B )。此外,PU-D4 在回收前后的傅立叶变换红外光谱显示,特征峰及其高度基本一致(图4C),表明热压回收不会影响材料的化学结构。经过多次再加工后,通过 DMA 测试,PU-D4 的交联密度(按 210 °C 时的平台模量计算)和 Tg 略有下降(图 4D,E )。这可能是由于聚氨酯链在重塑过程中断裂,这也是机械性能降低的原因。

Details are in the caption following the image
(A) Schematic diagram of extrusion and hot-press reprocessing for PU-D4, (B) representative tensile stress–strain curves, (C) FTIR spectra, and (D, E) DMA curves of PU-D4 after one or two extrusion and hot-press recycling cycles.
(A) PU-D4 挤压和热压再加工示意图,(B) 拉伸应力-应变曲线,(C) 傅立叶变换红外光谱,以及 (D, E) 经过一次或两次挤压和热压循环后的 PU-D4 DMA 曲线。

2.4 Improvement in creep resistance
2.4 提高抗蠕变性

Generally, PU fixes the molecular chain through a large number of intermolecular hydrogen bonds, but as a physical crosslinking point, its binding energy is significantly lower than the covalent bond. Besides the large number of hydrogen bonds, the dynamic covalently crosslinked networks of the PU-CANs limit the movement of molecular chains and endow the materials with better dimensional stability. As can be seen from Figure 5A, after 4 min in a 250 °C oven, the original TPU slice (PU0) melted and shrank (Video S1), while PU-D4 slice showed a complete and stable size. This can be explained by the melting of PU0 in a 250 °C oven, while the highly covalently crosslinked network maintained the shape of the PU-D4. Creep experiments were performed on all samples at 50 °C via DMA, and the results are shown in Figure 5B,C. Obviously, the strain recovery rates of highly crosslinked samples such as PU-D5 (87%) and PU-S4 (82%) are better than 66% of PU0, and the recovery rate increased with the increase of crosslink density. However, there are several exceptions. The creep recovery rate of PU-D1 and PU-D2 was not significantly improved and even worse, which might be due to the inability to construct a complete crosslinked network with such low polyol loading, and the negative effect from the chain breaking. It is worth mentioning that PU-Sx have relatively good creep resistance at low polyol loading, and the recovery rate increases with the addition content. Their more complete crosslinked networks (higher gel content at the same loading) should be the main reason.
一般来说,聚氨酯通过大量分子间氢键固定分子链,但作为物理交联点,其结合能明显低于共价键。除了大量氢键外,聚氨酯-CAN 的动态共价交联网络还限制了分子链的移动,使材料具有更好的尺寸稳定性。从图5A中可以看出,在 250 °C 的烘箱中放置 4 分钟后,原始热塑性聚氨酯切片(PU0)熔化并收缩视频S1) ,而 PU-D4 切片则显示出完整稳定的尺寸。这可以解释为 PU0 在 250 °C 的烘箱中熔化,而高度共价交联的网络保持了 PU-D4 的形状。通过 DMA 在 50 °C 下对所有样品进行了蠕变实验,结果如图5B、C所示。显然,PU-D5(87%)和 PU-S4(82%)等高交联样品的应变恢复率优于 PU0 的 66%,而且恢复率随着交联密度的增加而增加。但也有几个例外。PU-D1 和 PU-D2 的蠕变恢复率没有明显改善,甚至更差,这可能是由于在如此低的多元醇负载量下无法构建完整的交联网络,以及断链带来的负面影响。值得一提的是,PU-Sx 在较低的多元醇添加量下具有相对较好的抗蠕变性,且恢复率随添加量的增加而增加。主要原因应该是它们的交联网络更完整(相同添加量下的凝胶含量更高)。

Details are in the caption following the image
(A) Appearance changes of PU-CANs and PU0 slices after being placed in an oven at 250 °C for 4 min, (B) Creep recovery curves of PU-Sx and PU-Dx at 50 °C.
(A) PU-CANs 和 PU0 切片在 250 °C 的烘箱中放置 4 分钟后的外观变化;(B) PU-Sx 和 PU-Dx 在 50 °C 下的蠕变恢复曲线。

3 CONCLUSIONS 3 结论

A chain breaking-crosslinking method was successfully developed to upcycle TPU into continuously reprocessable PU-CANs via reactive extrusion. The reaction of polyol and carbamate bond broke the TPU chain and produced more hydroxyl groups which were further reacted with IPDI to obtain highly-crosslinked network. Due to the well crosslinked networks, the elastic recovery rate, solvent resistance, mechanical properties, and creep resistance of the PU-CANs were significantly enhanced compared with the original TPU. Meanwhile, the acceleration of the hydroxyl group on carbamate exchange made the PU-CANs reprocessable through compression molding and extrusion molding. This work provides a promising strategy for upcycling TPU into PU-CANs and enhancing the performance of thermoplastics.
成功开发了一种断链交联方法,通过反应挤压将热塑性聚氨酯循环利用为可连续再加工的聚氨酯-CAN。多元醇和氨基甲酸酯键的反应切断了热塑性聚氨酯链,并产生更多羟基,这些羟基进一步与 IPDI 反应,从而获得高度交联的网络。由于网络交联良好,与原始热塑性聚氨酯相比,PU-CAN 的弹性恢复率、耐溶剂性、机械性能和抗蠕变性都显著提高。同时,由于氨基甲酸酯交换时羟基的加速,PU-CANs 可通过压缩成型和挤出成型进行再加工。这项研究为将热塑性聚氨酯升级为 PU-CANs 并提高热塑性塑料的性能提供了一种可行的策略。

4 EXPERIMENTAL SECTION 4 实验部分

4.1 Materials 4.1 材料

Polyurethane (PU, trade name 1190A) and the antioxidant (Irganox 1010) were purchased from BASF. Polypropylene (PP, trade name B4808) was provided by Sinopec. Dipentaerythritol (Di-PE, 90%), sorbitol (99%), isophorone diisocyanate (IPDI, 99%), N, N-dimethylformamide(DMF), THF, toluene, chloroform, methanol, ethanol, and acetone were bought from Aladdin. PU pellets, Di-PE, Irganox 1010, and sorbitol were dried in vacuum oven at 70 °C for 12 h before use, and IPDI was used as received.
聚氨酯(PU,商品名 1190A)和抗氧化剂(Irganox 1010)购自 BASF。聚丙烯(PP,商品名 B4808)由中国石化提供。二季戊四醇(Di-PE,90%)、山梨醇(99%)、异佛尔酮二异氰酸酯(IPDI,99%)、N,N-二甲基甲酰胺(DMF)、四氢呋喃、甲苯、氯仿、甲醇、乙醇和丙酮购自阿拉丁公司。聚氨酯颗粒、Di-PE、Irganox 1010 和山梨醇在使用前在 70 °C 的真空烘箱中干燥 12 小时,IPDI 按收到的原样使用。

4.2 Preparation of PU-CANs
4.2 制备 PU-CAN

To avoid the impact of water, all materials (PU pellets, Di-PE, Irganox 1010, and sorbitol) were dried in a 70 °C vacuum oven for 12 h until the vacuum level did not change. As a representative example, PU pellets (15 g), Irganox 1010 (0.06 g), and Di-PE (0.45 g) were mixed evenly in a plastic bag. Later, the mixture was added into micro twin-screw extruder (SJZS-7A) bought from Wuhan Ruiming Co., Ltd through a hopper. Under nitrogen protection, the screw speed was set to 40 rpm and the four temperature ranges were set to 30, 150, 180, and 190 °C. After the mixture was completely added, it was subjected to cyclic extrusion for 5 min to get B-PU for testing. Then, IPDI (0.99 g) was added to the feeding port and cyclicly extruded for 5 min to obtain PU-CANs. Before each use of micro twin-screw extruder, it was cleaned with PP pellets for 3–4 times. The feed compositions of other different samples are listed in Table 4.
为了避免水分的影响,所有材料(聚氨酯颗粒、Di-PE、Irganox 1010 和山梨醇)都在 70 °C 的真空烘箱中干燥 12 小时,直到真空度没有变化为止。举例来说,将聚氨酯颗粒(15 克)、Irganox 1010(0.06 克)和 Di-PE (0.45 克)在塑料袋中混合均匀。然后,通过料斗将混合物加入从武汉瑞明有限公司购买的微型双螺杆挤出机(SJZS-7A)中。在氮气保护下,螺杆转速设定为 40 rpm,四个温度范围设定为 30、150、180 和 190 °C。混合物完全加入后,进行 5 分钟的循环挤压,得到供测试用的 B-PU。然后,在喂料口加入 IPDI(0.99 克)并循环挤压 5 分钟,得到 PU-CAN。每次使用微型双螺杆挤出机前,都要用 PP 粒子清洗 3-4 次。表 4 列出了其他不同样品的喂料成分。

TABLE 4. Overview of the feed compositions for the samples.
表 4.样品的饲料成分概览。
Sample 样品 PU (g) 聚氨酯(克) Sorbitol (g) 山梨醇(克) Di-PE (g) 二-PE(克) IPDI (g) (OH:NCO = 1:0.9) Irganox 1010 (g)
PU0 15 0 0 0 0.06
PU-S1 15 0.15 0 0.33 0.06
PU-S2 15 0.3 0 0.66 0.06
PU-S3 15 0.45 0 0.99 0.06
PU-S4 15 0.6 0 1.32 0.06
PU-D1 15 0 0.15 0.46 0.06
PU-D2 15 0 0.3 0.92 0.06
PU-D3 15 0 0.45 1.39 0.06
PU-D4 15 0 0.6 1.85 0.06
PU-D5 15 0 0.75 2.31 0.06

4.3 Swelling tests 4.3 膨胀试验

The gel content of PU-CANs was characterized by the Soxhlet extraction method. The sample (around 0.1 g) was wrapped with filter paper. The THF was used as the extraction solution and reflux for 24 h. Later, the samples were dried at 60 °C for 24 h in a vacuum oven. The gel content was calculated by m1/m0, where m0 is the initial mass and m1 is the final mass after drying.
采用索氏提取法对 PU-CANs 的凝胶含量进行表征。样品(约 0.1 克)用滤纸包裹,以四氢呋喃为萃取液,回流 24 小时。然后在真空烘箱中于 60 °C 下干燥 24 小时。凝胶含量的计算公式为:m1/m0 、其中,m0 为初始质量,m1 为干燥后的最终质量。

After soaking in ordinary solvents (tetrahydrofuran, toluene, chloroform, methanol, ethanol, DMF, acetone, and water) at 25 °C for 12 h, the solvent resistance was verified by observing the integrity of the sample and calculating the swelling ratio. The swelling ratio was calculated by m1/m0, where m0 is the initial mass and m1 is the final mass after swelling.
在 25 °C的普通溶剂(四氢呋喃、甲苯、氯仿、甲醇、乙醇、DMF、丙酮和水)中浸泡 12 小时后,通过观察样品的完整性和计算膨胀率来验证耐溶剂性。膨胀率的计算公式为:m1/m0 、其中,m0 为初始质量,m1 为膨胀后的最终质量。

4.4 Dynamic thermomechanical analysis measurement
4.4 动态热机械分析测量

Compression-molded PU-CANs with dimensions of 100 mm (length) × 5 mm (width) × 0.2 mm (thickness) were measured on a Q800 DMA (TA Instruments) in the film tension mode. A temperature sweep from −75 to 220 °C was performed with a heating rate of 10 °C min−1 at a frequency of 1 Hz. A force track of 125%, an amplitude of 2 μm, and a preload force of 0.01 N were used. The storage modulus and loss modulus were recorded as a function of temperature. The crosslink density (νe) is calculated according to equation 1:
在 Q800 DMA(TA 仪器公司)上以薄膜张力模式测量了尺寸为 100 毫米(长)×5 毫米(宽)×0.2 毫米(厚)的压缩成型聚氨酯-CAN。以 10 °C min-1 的加热速率和 1 Hz 的频率进行了从 -75 °C 到 220 °C 的温度扫描。使用的力轨迹为 125%,振幅为 2 μm,预紧力为 0.01 N。记录了存储模量和损耗模量与温度的函数关系。交联密度(νe )根据公式 1 计算:
(1)
where  其中 is the storage modulus of the crosslinked network at 210 °C (Figure 2A), R is the gas constant (8.314 J mol−1 K−1), and T is the absolute temperature of 210 °C.
是 210 °C 时交联网络的存储模量(图 2A ),R 是气体常数(8.314 J mol-1 K-1 ),T 是 210 °C 的绝对温度。

The stress relaxation test was conducted via a Q800 DMA (TA instruments, America) using a stress relaxation mode. The samples were examined between 150 °C and 180 °C by applying a constant strain of 10%. Before the tests, all the samples were held at the test temperatures for 5 min to achieve thermal equilibrium and preloaded a 1 × 10−3 N force to prevent the samples from bending.
应力松弛测试通过 Q800 DMA(美国 TA 仪器公司)使用应力松弛模式进行。通过施加 10% 的恒定应变,在 150 °C 和 180 °C 之间对样品进行了测试。测试前,所有样品在测试温度下保持 5 分钟以达到热平衡,并预先加载 1 × 10-3 N 的力以防止样品弯曲。

Creep experiments were performed on a Q800 DMA (TA instruments, America). The samples were exposed to a constant stress of 1 MPa for 30 min at the test temperature, after which the tension was removed and a 30 min recovery period was conducted. The strain of the sample was recorded throughout this process.
蠕变实验在 Q800 DMA(美国 TA 仪器)上进行。样品在试验温度下承受 1 兆帕的恒定应力 30 分钟,然后去除拉力,再进行 30 分钟的恢复期。在整个过程中记录样品的应变。

4.5 Mechanical property 4.5 机械性能

The samples were compression-molded at 10 MPa and 190 °C for 5 min. The tensile properties of the PU-CANs were measured using an Instron 5567 Electric Universal Testing Machine (Instron, America) at room temperature. A gauge length of 20 cm, a preload force of 0.1 N, and a cross-head speed of 100 mm min−1 was used. Each sample with dimensions of around 400 mm (length) × 5 mm (width) × 0.2 mm (thickness) was measured at least three times for accuracy.
样品在 10 兆帕和 190 摄氏度的条件下压缩成型 5 分钟。使用 Instron 5567 电动万能试验机(Instron,美国)在室温下测量了 PU-CAN 的拉伸性能。仪器长度为 20 厘米,预紧力为 0.1 牛顿,十字头速度为 100 毫米/分钟-1。每个样品的尺寸约为 400 毫米(长)×5 毫米(宽)×0.2 毫米(厚),至少测量三次,以确保准确性。

4.6 Rheological behavior 4.6 流变行为

Extrusion-obtained B-PU was hot pressed at 190 °C for 1 min using press vulcanizer XLB-D produced by Huzhou Shunli Machinery Co., Ltd. to prepare circular plates for rheological testing. The rheological behavior of B-PU was observed using a Discovery HR-3 Rheometer (TA Instrument, America) operated with a 25 mm diameter parallel plate geometry. Frequency sweep experiments were performed at 190 °C with a strain of 1%. The variations of the complex viscosity, storage modulus, and loss modulus were monitored as a function of frequency.
使用湖州顺力机械有限公司生产的 XLB-D 加压硫化机在 190 ℃ 下热压 1 分钟,制备用于流变测试的圆板。使用 Discovery HR-3 流变仪(美国 TA 仪器公司)观察 B-PU 的流变行为,平行板的几何形状直径为 25 毫米。频率扫描实验在 190 °C 和 1%应变条件下进行。监测了复合粘度、存储模量和损耗模量随频率的变化。

4.7 Fourier transform infrared characterization
4.7 傅立叶变换红外特性分析

The B-PU samples were first dissolved in THF and then precipitated in water to wash away free polyols, and dried in a vacuum oven at 80 °C for 24 h. Infrared spectroscopy (FTIR-ATR) was recorded on an Agilent Micro-FTIR Cary660 with a scanning range of 4000–400 cm−1 and 32 scans.
B-PU 样品首先溶解在 THF 中,然后在水中沉淀以洗掉游离的多元醇,并在 80 °C 的真空烘箱中干燥 24 小时。红外光谱(FTIR-ATR)在 Agilent Micro-FTIR Cary660 上记录,扫描范围为 4000-400 cm-1 ,扫描次数为 32 次。

4.8 Processability 4.8 可加工性

The processability of the PU-CANs was verified by extrusion twice in a micro twin-screw extruder. Under nitrogen protection, the screw speed was set to 40 rpm and the four temperature ranges were set to 30, 170, 180, and 190 °C. Compression molding was performed on a flat-panel vulcanizer XLB-D produced by Huzhou Shunli Machinery Co., Ltd. The extruded sample was compression-molded at 10 MPa and 190 °C for 5 min to prepare a 10 mm (length) × 10 cm (width) × 0.2 mm (thickness) film.
通过在微型双螺杆挤压机中挤压两次,验证了聚氨酯-CAN 的加工性能。在氮气保护下,螺杆转速设定为 40 rpm,四个温度范围设定为 30、170、180 和 190 °C。在湖州顺力机械有限公司生产的平板硫化机 XLB-D 上进行压缩成型。挤压样品在 10 兆帕和 190 °C的条件下压缩成型 5 分钟,制备出 10 毫米(长)×10 厘米(宽)×0.2 毫米(厚)的薄膜。

ACKNOWLEDGMENTS 致谢

This work was funded from National Natural Science Foundation of China (No. 52073296), Research startup fund from Jiangnan University, and Zhejiang Ten Thousand Talent Program.
本研究得到了国家自然科学基金(编号:52073296)、江南大学科研启动基金和浙江省 "万人计划 "的资助。

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