Developing Eco-friendly High-Strength Soy Adhesives with Improved

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Developing Eco-friendly High-Strength Soy Adhesives with Improved Ductility through Multiphase Core-Shell Hyperbranched Polysiloxane Zhong Wang, Shujun Zhao, Huiwen Pang, Wei Zhang, Shifeng Zhang, and Jianzhang Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06810 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Sustainable Chemistry & Engineering

Developing Eco-Friendly High-Strength Soy Adhesives with Improved

Ductility

through

Multiphase

Core-Shell

Hyperbranched Polysiloxane Zhong Wanga,b, Shujun Zhaoa,b, Huiwen Panga,b, Wei Zhanga,b, Shifeng Zhanga,b,*, and Jianzhang Li a,b,* a

MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry

University, No 35, Tsinghua East Road, Haidian District, Beijing 100083, PR China; b

Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry

University, No 35, Tsinghua East Road, Haidian District, Beijing 100083, PR China

*E-mail: [email protected], (S. Zhang), [email protected] (J. Li)

ABSTRACT:

It is desired to develop non-formaldehyde biobased adhesives

composed of renewable and low-cost biomass protein with high strength and ductility to meet green and sustainable requirements for wood-based composites. Herein, a hyperbranched polysiloxane-terminated cardanol side group was incorporated for the first time with tannic acid in this effort, forming a fully biobased core-shell hybrid (TCA-HBSi) with high compatibility and reactivity. Subsequently, the synthesized TCA-HBSi was employed as a novel cross-linking agent in the preparation of modified

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soy protein (SP) resin. Owing to the combination of rigid rings and flexible aliphatic chains, microphase-separated structures have been constructed in the interface between TCA-HBSi and brittle protein network. The structures not only promoted the dispersion of TCA-HBSi, but also served as a multiple cross-link to improve the interfacial interactions between TCA-HBSi and SP matrix, and hence facilitated the redistribution of mechanical stresses during loading. With such a synergistic multiphase structure, the TCA-HBSi-modified SP composite resins exhibited a more than 2-fold simultaneous increase in strength and toughness compared to the neat sample. Consequently, this strong but tough protein endowed the resins with substantially enhanced adhesive strength and water resistance when applied to plywood manufacturing. This work reveals that a modifier with “hard core, flexible shell” structures dispersed in biobased adhesive can more effectively improve the adhesive properties.

KEYWORDS: Protein adhesive, Microphase-separated structure, Hyperbranched, Toughening, Adhesion

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INTRODUCTION Owing to rapidly growing concerns for energy and environment, recent years have seen increasingly tremendous demand for new, high-effective, environmentally friendly adhesives in the engineered wood composites (EWC) industry.1-3 The conversion of renewable low-cost resources to high-value green adhesives may alleviate environmental and human health concerns associated with synthetic glues.4-8 As a kind of plant-derived biomacromolecule, soy protein (SP) could be a promising candidate for biobased wood adhesives due to its high cost-effectiveness and processability.9-13 Conventional SP adhesives lack sufficient water resistance or bonding strength and frequently fail when used in EWCs because the strong inter-/intramolecular interactions block the glue-substrate interfacial bonding.14-16 Protein

structure

denaturation,17

high-functionality

cross-linkers

(HFC)

modification,18,19 and biomimetic structure design20-22 have been implemented as effective solutions to address the inherent drawbacks of SP adhesives. Among them, cross-linking chemistry has been extensively developed to achieve improved SP-based materials with high adhesion and water-resistant performance.15, 23 By incorporating HFC into the protein network, amphiphilic polypeptide chains with nonuniform lengths are connected by a rigid HFC that is capable of withstanding large stresses. Although the structural integrity of this cross-linked adhesive bond is required for EWC applications, these adhesives are often not tough enough and thus lead easily to crack propagation upon failure when subjected to mechanical loads.24 For instance, SP adhesives chemically cross-linked by epoxy-terminated HFC often exhibit high 3



bonding strengths given their high cross-linking densities, whereas stress concentration-induced fractures are frequently observed in the glue–wood joints.25 Notably, the use of brittle adhesives renders splitting of these bonded EWC even more pronounced during mechanical processing (e.g., saw cutting or grinding).26 Therefore, it is imperative to achieve a combination of structural bond strength and ductility in high-performance environmentally friendly soy adhesives. Alternatively, there has been a great interest in the use of hyperbranched cross-linkers with unique ellipsoidal or spherical structures, such as polyether and polyester, epoxy, or aminated polysaccharide for altering the rigid cross-linking structure of polymeric adhesives.27-29 Due to their molecular structure, which consists of a high density of functional groups, these cross-linkers enable interchain entanglement and interphase separation via covalent/physical interactions; such behavior can contribute directly to enhanced ductility (i.e., elongation at break) of the thermoset.30, 31 In addition, the water resistance of resins can be further reinforced by combining the organofunctional silane with hyperbranched polymers.32 From this perspective, hyperbranched polysiloxane represents an attractive candidate to integrate mechanical strength, ductility, and water resistance into soy adhesives simultaneously. Nevertheless, rational utilization of highfunctionality hyperbranched polysiloxane has been less realized in the biobased adhesion systems for EWC manufacturing. While exploring high-efficiency hyperbranched cross-linkers, we noted that tannic acid (TA) is a natural polyphenol with highly branched aromatic polyester and terminal hydroxyl groups, revealing a multifunctional hyperbranched reactive platform.33 Given 4


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the extensive distribution of catechol moieties, TA presents outstanding physical chemical properties similar to mussel-inspired polydopamine systems.34 In this context, TA and its functionalized derivatives have been previously used as effective toughening agents or adhesive coating for polymeric materials.35-37 We also implemented a hyperbranched cross-linking strategy to achieve reinforced soy adhesives by incorporating hybrid TA and polyethyleneimine (TAPI) motifs into amino-rich protein networks.38 However, high contents of these rigid aromatic structures and short chains in hyperbranched TAPI did not satisfy the requirements for toughening the brittle SP matrix. Ultimately, designing architectures of cross-linking agents as a fundamental solution is necessary for high-performance soy adhesives. In light of the preceding discussion, we introduced cardanol-modified aminopropyltriethoxysilane (CDA), a functionalized siloxane derivative from renewable resource cashew nut shell liquid, as the hydrolyzable flexible component candidate and induced its hydrolysis and condensation with TA to develop a novel fully biobased hyperbranched polysiloxane (TCA-HBSi). SP resin was then modified with TCA-HBSi, which contains both rigid carbon ring as “hard core” and long aliphatic chains as “flexible shell”, in the presence of an epoxy-terminated curing agent. Our strategy has several distinct advantages, including: (i) a high-performance but costeffective route, highlighting its potential in industrial soy adhesive production; (ii) introduction of microphase-separated structures with multiple cross-linking into a polar protein network, leading to synergistic interfacial interactions and avoiding the risk of a highly brittle netlike architecture caused by conventionally cross-linked protein resins; 5



and (iii) good dispersion and compatibility of TCA-HBSi in the SP matrix due to the abundant functional groups on TCA-HBSi surfaces. Such synergistic effect has been found to be highly efficient in simultaneously enhancing the tensile strength and toughness of the SP resin. Importantly, we demonstrated that the novel cross-linking SP-based adhesives displayed high bonding strength and better water resistance through their application to produce plywood. As a result, the biobased modifiers with “hard core, flexible shell” structures can effectively improve the performance of proteinbased adhesive.

EXPERIMENTAL SECTION Materials and Chemicals. Soy protein (SP; protein content 95%) was supplied by Shandong Yuwang Ecological Food Industry Co., Ltd. (Shandong, China). Cardanol-based siloxane (CDA) was synthesized in our laboratory according to our previous research;39 a schematic representation of CDA preparation is illustrated in Scheme S1, Supporting Information (SI). Tannic acid (TA) and tris(hydroxymethyl aminomethane) (Tris) were purchased from Tianjin Haojia Cellulose Co., Ltd. (Tianjin, China). 1,2,3-propanetrioldiglycidyl-ether (PTGE) was used as a curing agent and supplied by Beijing Chemical Reagents Co., Ltd. (Beijing, China). Poplar veneer samples (size: 400 × 400 × 2 mm) with 8% moisture content were provided by Wen’an Plywood, Ltd. (Hebei, China). All chemicals and reagents were analytical grade and used as received.

Synthesis of TCA-HBSi Hybrids. A defined amount of TA was dissolved in 100 mL Tris-HCl solution (pH 8.5), and CDA (0.72 g, dissolved in 25 mL ethanol) was 6


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then added dropwise into the TA solution. Next, the mixture was stirred at room temperature for 12 h. Finally, the ethanol solvent was removed by rotary evaporation to obtain biobased TCA-HBSi hybrids, which were applied directly to the subsequent resin blend. For comparison, TA hybrids were also prepared via self-polymerization in an HCl-Tris solution without CDA.

Fabrication of SP/TCA-HBSi Adhesive and Three-Layer Plywood. Appropriate amounts of TCA-HBSi suspension and dried SP (15 g) were mixed with defined mass (to total 15 wt%) of deionized water in a 250-mL round-bottom flask. The mixture was stirred vigorously at room temperature for 30 min to disperse the protein. Curing agent PTGE (8.0 wt% of total protein weight) was added to the mixture followed by gentle stirring for 15 min at room temperature just prior to use. The adhesive was denoted as SP/TCA-HBSi-X, in which X represents the weight percent of TCA-HBSi in the resins, taking the values of 1.0, 3.0, 5.0, and 7.5. Neat SP adhesive and only TAmodified SP adhesives (5.0 wt% of TA) were prepared under the same procedure. The developed adhesive was applied to each veneer layer with a brush at a spreading rate of 180 g m-2 and then hot pressed (120 °C, 1.0 MPa pressure) for 5.25 min using a laboratorial hot press. Before shear testing, the plywood panels were conditioned at 25 ± 2 °C and 65 ± 5% relative humidity for at least 48 h.

Characterization. A Thermo Scientific Nicolet 6700 Fourier transform infrared (FTIR) spectrometer with a resolution of 4 cm−1 was employed to obtain FTIR spectra from 600 to 4000 cm−1. Solid-state 13C and 29Si NMR spectra were recorded on a Bruker ADVANCE 400 nuclear magnetic resonance (NMR) spectrometer with a 4 mm CP7



MAS probe at a spinning rate of 10 kHz under room temperature. Gel permeation chromatography (GPC) analysis was performed at 30 °C using tetrahydrofuran as the eluent under a flow rate of 1.0 mL min−1 and polystyrene as the standard with a Waters 2515 system (USA). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an Escalab 250 spectrometer with Al Kα radiation (100 eV). X-ray diffraction (XRD) measurements were recorded at a scanning rate of 0.02° s−1 on an X-ray D8 Advance diffractometer with Cu K radiation in the 2θ range of 5-60°. Thermogravimetric analysis (TGA) under an N2 atmosphere at a heating rate of 10 °C min-1 from room temperature to 600 °C. The morphology and microstructures of specimens were investigated using scanning electron microscopy (SEM; Hitachi S-4500) and transmission electron microscopy (TEM; JEOL H-7650). Emulsion droplet size measurements were carried out on a laser particle size analyzer (Malvern, Mastersizer 3000) at 25 °C.

Sol-Gel Test. Equilibrium swelling experiments were performed by immersing the cured adhesives in boiling water bath for 4 h, as described previously.13 The insoluble fraction and swelling ratio of the specimens were determined. Details are supplied in the SI.

Lap-Shear Adhesion Test. Shear adhesion strength of the plywood was determined by a tensile tester (WDW-200E, China) under a constant speed of 10 mm min-1 at room temperature. According to Chinese National Standards GB/T 9846-2015, the tests were carried out with two pretreatments on the plywood samples: (1) conditioning in a standard climate (25 ± 2 °C; 65 ± 5% RH), and (2) submersion in 8


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water (63 ± 2 °C) for 3 h followed by adhesive water-resistance analysis. Reported results are the average of six specimens for each sample.

Tensile Tests. Tensile tests were conducted on a universal testing machine (model 5967, Instron, USA) at a crosshead speed of 10 mm min-1. Five specimens were tested for each sample. The test procedures are described in detail in the SI.

RESULTS AND DISCUSSION

Scheme 1. Schematic illustration of the preparation procedure of core-shell TCA-HBSi nanohybrids.

Preparation and Characterization of TCA-HBSi Hybrids. Herein, we aimed to synthesize biobased water-soluble TCA-HBSi with “hard core, flexible shell” structures as a multi-functional cross-linker to fabricate high-performance SP adhesives for eco-friendly and economical EWC production. As shown in Scheme 1, the siloxanecontaining CDA oligomer was synthesized via a process of epoxy ring-opening of cardanyl glycidyl ether with aminopropyltriethoxysilane at first (Scheme S1), and the structure of CDA was confirmed by

1H

NMR spectra (Figure S1). During

hydrolyzation and condensation, CDA was used as a hydrolysable organoalkoxysilane 9



and immobilized on the hyperbranched architecture of TA to prepare hyperbranched polysiloxanes. Finally, the fully biobased TCA-HBSi hybrids were obtained for the soy adhesive system.

Figure 1. (a) TEM image of TCA-HBSi nanohybrids retrieved by centrifugation; (b) their particle diameter distribution. Because the as-obtained TCA-HBSi underwent a co-hydrolysis hydrolysation/selfcondensation process, an abundance of long aliphatic chains were grafted on the surface of TCA-HBSi hybrids. Although the hybrids could be stably dispersed in aqueous solution, Figure 1a shows the spherical shape of TCA-HBSi with some interconnected topography and a diameter of ca. 67–90 nm. Figure 1b plots the curve of their particle size distribution in water, indicating TCA-HBSi has an average size of 64.5 nm. The presence of high-functionality catechol moieties self-polymerized endowed higher adherent layer to TCA-HBSi, resulting in several aggregated particles for the obtained hybrids.

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Figure 2. (a) FTIR spectra of CDA, pristine TA, and TCA-HBSi; (b) XPS wide scans and (c) C 1s spectrum for TCA-HBSi; and (c)TGA thermogram of TCA-HBSi. The FTIR spectra of CDA, TA, and TCA-HBSi are shown in Figure 2a. In the FTIR spectrum of CDA, a wide absorption band at 3361 cm−1 was due to the stretching vibration of –OH and N−H groups; the strong absorption bands at 2926 and 2855 cm−1 were ascribed to the stretching vibration of −CH3 and −CH2− groups, respectively.40 Moreover, the characteristic peaks for Si−O−C moiety at 1190 and 1091 cm−1 are indicative of the successful synthesized cardanol-based siloxane derivates.32 For pristine TA, several characteristic peaks were observed: a broad peak between 3600−3100 cm−1 (−OH stretching due to massive phenol groups), 1704 cm−1 (C=O stretching vibration of carboxylic acid groups), and 1202 cm−1 (C−O stretching 11



vibration of polyols).37 In contrast to TA, some characteristic peaks of cardanol appeared in the FTIR spectrum of TCA-HBSi. The broad band peak shifted from at 3353 cm−1 to 3172 cm−1 with co-deposition with CDA, confirming the presence of new hydroxyl due to the hydrolysation/self-condensation reaction between TA and the Si−OH and Si−OCH3 groups of CDA.41 The spectrum for TCA-HBSi also exhibited typical peaks at 2926 and 2855 cm−1 (−CH3 and −CH2− stretching) and 1093 cm−1 (Si−O−C stretching). This data verifies that alkoxysilane cardanol molecules were integrated into the TA architecture. The surface chemical compositions of TCA-HBSi were further determined by XPS. As shown in Figure 2b, efficient deposition of generated polysiloxane on the hybrids surfaces was verified by the appearance of Si 2s and Si 2p peaks for the TCA-HBSi sample. The C 1s XPS spectrum (Figure 2c) revealed that the following components could be assigned to C=C (284.6 eV), C−C (285.0 eV), C−N (285.5 eV), C−OH (286.8 eV), and C=O (288.4 eV), respectively, denoting the presence of residual phenolic hydroxyl groups of TA molecules and the aliphatic chains of CDA oligomers.42 The obtained results are consistent with the results of FTIR in terms of TCA-HBSi surface groups. From the TGA result of TCA-HBSi hybrids (Figure 2d), a series of different thermal degradation behaviors with much lower mass loss at high temperature attributing for the better thermal stability of polysiloxane hybrids.

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Figure 3. 29Si NMR spectrum of TCA-HBSi. Figure 3 shows the

29Si

NMR spectrum of TCA-HBSi. The three characteristic

peaks at −57.8, 65.1, and 79.4 ppm were assigned to terminal units (−CH2CH2−Si(OSi)2(OR'), labeled T), linear units (−CH2CH2−Si(OSi)3, labeled L), and dendritic units (−CH2CH2−Si(OR')3, labeled D), respectively.43 These results indicate that siloxane-containing CDA was completely hydrolysed through codeposition/condensation reaction, forming the multi-foundational TCA-HBSi with hyperbranched structures. According to Frey's equation (Eq. [1]),44 the degree of branching (DB) is calculated to be 0.69. 2𝐷

(1)

𝐷𝐵 = 2𝐷 + 𝐿

Moreover, the molecular weight (Mw) and molecular weight distribution of TCAHBSi are summarized in Table S1, and are consistent with moderately hyperbranched polymers. Therefore, the above

29Si

NMR spectrum and GPC analysis confirm that

TCA-HBSi with highly reactive terminal flexible branches had been successfully 13



prepared and is preferable for the toughening effect in cross-linking resins. On the basis of the above results, a kind of biobased hyperbranched polysiloxane with “hard core, flexible shell” structure was successfully synthesized through co-deposition of TA and CDA.

Scheme 2. Illustration for the preparation of TCA-HBSi-modified SP-based composite resins with microphase-separated structures.

Cross-Linking SP resins with TCA-HBSi. From the above results, highfunctionality TCA-HBSi was deemed efficient for the cross-linking of SP through multiple linkages in the SP−TCA-HBSi interface. In the SP/TCA-HBSi composite resin, a further chemical interface is formed between TCA-HBSi and protein by using PTGE, as shown in Scheme 2. The resulting composites were analyzed to investigate the influence of the TCA-HBSi on the structure and morphological properties of SP.

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Figure 4. SEM and images of the fracture surface of TCA-HBSi-modified SP composites. (a) Neat SP, (b) SP/TA-5, and (c,d) SP/TCA-HBSi-5 at different magnifications. TEM micrographs of (e) neat SPand (f) SP/TCA-HBSi-5. The photographs of all neat SP and composites were transparent with a pale yellow color (Figure S2), suggesting a better compatibility with protein matrix. The dispersion status of TCA-HBSi in SP was directly evaluated by TEM and SEM observations, displayed in Figure 4. For the neat SP resin, its surface was smooth and the fracture surfaces composed of certain sharp cracks were clearly observed (Figure 4a). The TA with SP blend (Figure 4b) showed more characteristic fracture-surface feature but still presented some cracks and holes across the surfaces. For the SP/TCA-HBSi composite resin, its fracture surface image (Figure 4c) showed that TCA-HBSi nanoparticles were uniformly distributed within the polar SP phase and the spherical domains were found, which benefited from the fact that the integration of hard phase and soft phase into the TCA-HBSi hybrids in advance.40 Compared to that of SP resin, SP/TCA-HBSi composite resin had a rougher porous structure (Figure 4d), which may be due to the 15



interactions between the multiphase network being strengthened by the presence of multifunctional microphase-separated structures. Moreover, TEM micrographs showed that spherical clusters of the TCA-HBSi distribution occurred in the SP matrix (Figure 4e and f), which further implies this cross-linking protein resin consist of hard and soft phases due to the microphase separation during the curing process.

Figure 5. Evidence of multiple cross-linking structure: (a) FTIR spectra; (b) O 1s corelevel spectra; and (c) CP/MAS

13C

NMR spectra for neat SP, SP/TA, and SP/TCA-

HBSi composites (all samples contain 5.0 wt % of TA/TCA-HBSi.); (d) the histogram of insoluble fraction of the neat SP and composites with various TCA-HBSi contents. The effect of the cross-linking interaction between TCA-HBSi and SP on the structure and chemical/functional groups of resins was investigated by FTIR and XPS 16


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(Figure 5). In the FTIR spectra of SP and SP/TA (Figure 5a), the absorption peak attributed to O−H and N−H groups shifted from 3276 to 3435 cm−1, presumably due to the strong hydrogen-bonding interactions between amino-rich protein and functional TA molecules.38 Compared with SP, SP/TCA-HBSi exhibited new characteristic peaks at 2926, 2855, and 1100 m−1, which is originated from the aliphatic −CH2− stretching and Si−O−S linkage in TCA-HBSi hybrids. This observation is indicative of the successful immobilization of long aliphatic chains into the multiphase protein networks. Furthermore, carboxyl groups (C=O, 1716 cm−1) in SP/TCA-HBSi were identified due to the ring-opening reaction of the phenolic hydroxyl groups of TA with epoxyterminated PTGE.24 The absorption peak at 1587 cm−1 corresponding to the peculiar C=N groups indicated the presence of aromatic amine species.42 In addition, the broad band peak shifted from approximately 3276 to 3367 cm−1 with the incorporation of TCA-HBSi, confirming the formation of hydrogen-bonding initiated cross-linking structure in SP/TCA-HBSi resins.45 On the other hand, the covalent/physical crosslinking induced protein structural changes might consume the abundant of hydrophilic groups,19 thus contributing to reduced N−H vibration (amide II band) at 1517 cm−1. Similar hydrophobic interactions between long-chain structures and proteins have been reported by Lin’s group and Sun’s group.13,

17

The XPS results provide additional

implication for the surface chemistry of modified resin (Figure S3). As shown in Figure 5b, the typical C 1s peaks at 284.5, 285.7, 286.4, and 287.8 eV were assigned to C−C/C−H, C−NH−C, C−OH, and CO−NH species, respectively.46 When TCA-HBSi was added, the percentage of C−C/C−H of composite resin increased from 54% to 68% 17



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(Table 1). Generally, this peak corresponds to the hydrophobic properties and crosslinking degree of the protein structure and hence implies the successful enhancement of intramolecular interactions in cured resin.47 Table 1. Contents of surface functional groups of neat SP and SP/TCA-HBSi-5 resins by XPS analysis. Contents of functional groups (%) Sample C−C/C−H

C−NH−C

C−OH

CO−NH

Neat SP

54

13

10

23

SP/TCA-HBSi-5

68

11

6

15

From CP/MAS

13C

NMR spectra, few changes were observed when TCA-HBSi

was incorporated (Figure 5c). The distinct peak at 114.2 ppm was indicative of the presence of aromatic carbons from ester linkages,48 consistent with the FTIR results. Furthermore, the resonance related to the α-C nearly disappeared in SP/TCA-HBSi, all of which denote a covalent cross-linking reaction between the TCA-HBSi and aminorich SP matrix.22 These results reveal the integration of multifunctional TCA-HBSi with a reactive shell and core into the SP network, which consumed hydrophilic groups and produced multiple intramolecular interactions during cross-linking; this would affect the water resistance and adhesion of soy adhesives. As illustrated above, changes in the water resistance of modified SP resin can provide further evidence on TCA-HBSi induced protein cross-linking. According to the sol-gel test, after immersing the as-cured SP resins in boiling water for 4 h, they were soluble with

Developing Eco-friendly High-Strength Soy Adhesives with Improved

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