1. Introduction
Polyurethane (PU) is one of the most used polymers in the world, especially in coatings, adhesives, sealants and elastomers (CASE applications) but also foams. Indeed, PU has properties that can be tailored according to the application, such as high flexibility, good tear strength, good abrasion and chemical resistance. To increase the lifetime of polymers, such as PU, healable and recyclable polymers have been developed based on extrinsic healing or intrinsic healing. Extrinsic healing is obtained by embedding a healing agent in a polymer matrix. Upon a crack in the matrix, the healing agents will be released to close the crack. An example is the use of microcapsules containing a healing agent in textile coatings. This is a single-use nonreversible process and since extrinsic healing lacks repetitive healing capacity, more attention is paid to intrinsic healing. Intrinsic healing refers to reversible noncovalent and/or dynamic covalent cross-linking in polymer networks. After dissociation, noncovalent crosslinked networks can be reorganized and reformed upon the interdiffusion of polymer chains due to the reversible break-reformation potential of noncovalent linkages. However noncovalent healing often results in a decrease in mechanical performance, limiting their application. Dynamic covalent crosslinked networks can be produced by crosslinking pre-polymers and small molecules (e.g., chain extender) via dynamic covalent bonds. These materials need external stimuli (often heat) to be healable [
1,
2,
3,
4,
5,
6].
Noncovalent interactions are weaker compared to covalent bonds and include hydrogen bonding, electrostatic, metal-coordination, host-guest and ion-dipole interactions. A healable PU elastomer was synthesized based on polypropylene carbonate, 1,6-hexamethylendiamaine and 4,4′-diphenyl methane diisocyanate. Non-covalent van der Waals and hydrogen bonding are the driving force for the healing of the elastomer. The healing efficiency varied between 44% and 100% depending on the type of PU and the temperature. The best results were obtained at 50 °C [
7]. A blend of linear and branched PU’s having triazole ligand end groups, non-covalently linked through Fe/triazole interaction, were developed. The materials exhibited a healing efficiency of over 90% [
8]. A supramolecular healable PU elastomer based on aromatic π–π stacking and hydrogen bonds is described. Healing was obtained by heating at 45 °C. The mechanical properties were characterized, and the healing efficiency was approximately 99% [
9]. The development of zwitterionic multi-shape-memory PU with healing properties as potential smart biomaterials was described [
10]. Self-healing thermoset PU based on cyclodextrin and adamantane host–guest interaction was reported and exhibited healing at room temperature. Healing efficiency increased as the content of the host-guest moieties increased [
11].
In the case of dynamic covalent bonds, one can distinguish between dissociative and associative bonds. During the dissociative process, cross-linkers are dissociated into their individual reactive constituent partners before reforming, while in the case of an associative process a substitution reaction occurs between an existing cross-link and pendent reactive group. Examples of dissociative dynamic covalent bonds include disulfide and diselenide bonds, Diels-Alder adducts and boroxines. Transamination, transesterification or silyl ether exchange are the most occurring associative mechanisms [
12,
13].
Vitrimers are cross-linked materials showing an associative dynamic bond-exchange mechanism resulting in topology alternation. The first vitrimer reported was a carboxylic acid-cured epoxy network through transesterification under catalytic Zn(OAc)2 [
14]. A non-isocyanate polyurethane vitrimer was developed, exhibiting healing through transcarbonation exchange reactions at 130 °C. The healing efficiency was 88% [
15]. Transamination of vinylogous urethanes was studied as an exchange reaction for catalyst-free vitrimers [
16]. A PU-modified epoxy vitrimer with amino ester moieties was prepared. Scratches on the surface of the samples could be healed within 30 min at 200 °C based on transesterification [
17].
Next to associative mechanisms, dissociative-based healing mechanisms are extensively explored for PU networks. A bio-based Diels-Alder adduct made out of CO
2 and furfuryl was reported, having a healing efficiency of 94% when it was healed at 120 °C for 10 min and then healed at 60 °C for 24 h [
18]. A healing PU powder coating system containing a commercial uretdione-based cross-linker (BF1320), OH-functionalized polyester resin, and a Diels–Alder adduct as a healing agent is reported. The system exhibited 100% healing of a crack within 12.5 min at 120 °C [
19]. Behera et al. reported a dual-functional (PU) elastomer having disulfide as well as furfuryl functionalities to introduce healing properties. The PU showed high healing efficiency of 97% and high tensile strength (39.5 MPa) [
20]. A bio-based Schiff base was synthesized out of vanillin and cystine and used as a cross-linker in PU. The Schiff base consisted of dynamic covalent disulfide and imine bonds resulting in a PU with high healing efficiency (97%). Self-healing was obtained via heat or UV irradiation [
21]. A transparent healable PU based on disulfide bonds, using 4-aminophenyl disulfide as a chain extender was synthesized. The healing efficiency was up to 93% after 24 h at 80 °C. In the case of PU without disulfide bonds, shape memory behavior caused crack closure, but no intrinsic self-healing component through dynamic disulfide bond exchange occurred, leading to low healing efficiency (36%) [
22]. A healable PU dispersion incorporating disulfide bonds was synthesized. The healing efficiency reached 96% after 4 h at 70 °C and more than 80% after 24 h at 25 °C. However, no comparison was made with PU without disulfide bonds [
23]. A PU dispersion using cystamine as a chain extender was developed. The healing efficiency was 40% after 3 h at 130 °C, while the corresponding PU dispersion using ethylene diamine as chain extender, and thus without disulfide bonds, exhibited no healing [
24]. By incorporating dynamic diselenide bonds into PU, visible light-induced self-healing elastomers were developed. Depending on the composition of the PU elastomer the self-healing efficiency could reach 72% after 48 h visible light irradiation. For the control sample with no diselenide bonds, almost no healing occurred after even 72 h visible light irradiation [
25]. Moreover, aromatic diselenide crosslinkers were used to enhance the reprocessability and healing of PU thermosets. It was demonstrated that aromatic diselenides have lower bond energy than their disulfide counterparts and exchange faster. An aromatic diselenide has been incorporated into PU using a para-substituted amine diphenyl-diselenide. After 30 min PU samples based on diselenide showed 56% healing efficiency of their initial mechanical properties, while corresponding disulfide PU samples only showed 28% recovery. After 24 h, the healing efficiency of diselenide increased to 76%, while disulfide PU only reached 43% [
26]. NIPU featuring multiresponsiveness to humidity and temperature and healing properties by combining iminoboronate and boroxine chemistry was reported. NIPU samples showed complete scratch healing after 12 h at 70% relative humidity and at room temperature. The control NIPU containing nonreversible linkage displayed no healing when damaged and exposed to humidity for 12 h at room temperature [
27].
In a previous manuscript, we reported the development of a bio-based 2K textile coating with excellent water barrier properties [
28]. Nonetheless, despite the outstanding performance, the coating exhibited no barrier properties once damaged. Thus, a healable coating needs to be applied to extend the lifetime of the coated textile. However, although (self-)healing networks are extensively researched, none of the publications address the development of a bio-based healable PU for thin coatings such as textile coatings. Therefore, the composition of our previously reported 2K PU textile coating needed to be altered by replacing the high functional cardanol-based polyol with a self-healing diol without losing the water barrier properties. This report describes for the first time the synthesis and application of a new bio-based healable 2K PU on textiles based on dynamic covalent bonds with excellent barrier properties. Bio-based polyol, bio-based Schiff base with dual reversible linkage, hexamethylene diisocyanate (HMDI) were used as building blocks. HMDI was used as a building block, instead of Desmodur Eco N7300 in our previous 2K PU textile coating report, since Desmodur Eco N7300 resulted in highly crosslinked thermoset PU networks with lower chain mobility and no healing efficiency [
28].
2. Materials and Methods
2.1. Materials
Bismuth neodecanoate, hexamethylene diisocyanate (HMDI), cystine and vanillin were purchased from Sigma-Aldrich. Transfer paper was sampled by Sappi. Tego Airex 900 (deaerator) and Dynasylan 1189 (adhesion promotor) were supplied by Evonik. Priplast 3172 was sampled by Croda. Woven polyester fabric (105 g/m2) was purchased form Concordia Textiles.
2.2. Synthesis of Schiff Base
The Schiff base was synthesized out of cystine and vanillin, which was based on a protocol published by Lee et al. with some minor modifications [
21]. A solution of pH 10 was first prepared by adding 10 wt% of NaOH solution to 250 mL water. A total of 10 g of cystine and 12.66 g of vanillin were dissolved in this solution. After dissolving both cystine and vanillin, the pH was decreased, by adding 10 wt% NaOH until pH was 10. The mixture was stirred for 12 h at ambient temperature. During the reaction, the clear, colorless solution turned dark orange to brown, indicating Schiff base formation. The pH of the solution was adjusted to 7 using 1 M HCl to stop the reaction. The solution was then centrifuged for 8 min at 4000 rpm. The supernatant was then collected and dried overnight at 80 °C to remove water. The product was washed three times with 80 mL of chlorobenzene, followed by vacuum drying at 60 °C overnight. A brown solid residue was obtained with a yield of 55%.
2.3. Bio-Based Healable 2K PU Coating
The healable PU coatings were prepared by reacting Priplast 3172 (functionality: 2) and Schiff base (functionality: 4) with HMDI in a molar equivalent ratio 1:1. A total of 28 g of Priplast 3172 was mixed with 0.02 g bismuth neodecanoate. Subsequently, 6 g of Schiff base, dissolved in 5 g DMSO, was added followed by 0.3 g of Dynasylan 1189 and 0.3 g Tego Airex 900. Subsequently, 5.5 g hexamethylene diisocyanate was added to the polyol mixture. The 2K formulation was applied on transfer paper and a polyester fabric via knife over roll. The applied coating thickness was 50 µm. Two layers were coated, and each layer was cured for 2 min at 155 °C. A corresponding control 2K PU formulation without Schiff base was prepared (
Table 1).
2.4. Characterisation
A Nicolet 6700 spectrometer from Thermofisher Scientific (Waltham, MA, USA) was used to record Fourier transform infrared spectra from 500 cm−1 to 4000 cm−1 to chemically characterize the PU networks.
An Elcometer 3086 Scratch Boy from Elcometer (Nieuwegein, The Netherlands) with a diamond scribe was used to make reproducible scratches in coatings. The force of the diamond scribe can be varied between 0 and 10 N. Healing of the coating was performed for 30 min at 90 °C (relative humidity in the closed space was 1%) and was visualized afterwards using a Field Emission Gun Scanning electron microscopy (FEG-SEM) (JSM 7600 F from Jeol Europe (Zaventem, Belgium)). Scratch healing was also observed via optical microscopy from Nikon (Dilbeek, Belgium).
Elongation at break and break at stress was determined using Instron electronic fabric tension tester according to ISO 13934-1 on 100 µm 2K PU films. The tension loading speed was 100 mm/min. Healing efficiency was calculated as the ratio between the elongation at the break of healed samples and the elongation at the break of undamaged samples.
Swelling experiments were performed in MEK to calculate the crosslinking density. The PU films were placed for 24 h in the solvent. After removal, residual solvent on the PU films was wiped off, before being weighed. From the weight of the swollen polymer (ws), the volume fraction of swollen polymer (Vp) can be calculated:
where wd is the dry weight of the polymer, and dp and ds are the densities of the polymer and solvent, respectively. The crosslink density (
n) values were obtained from Vp with the Flory–Rehner equation:
where vs. is the molar volume of the solvent and χ is the polymer–solvent interaction parameter, which can be found from the equation:
where R is a gas constant and T is the temperature (expressed in Kelvin), whereas δ1 and δ2 are the solubility parameters of the solvent and polymer.
The thermal behavior in air was analyzed via thermogravimetric analysis with a Q500 thermogravimetric analyzer from TA Instruments (Asse, Belgium) using standard parameters: a ramp rate of 10 °C/min from 30 °C to 600 °C.
Melting and glass transition temperatures of the PU coatings were determined via differential scanning calorimetry (DSC) analysis with TA Instruments Discovery DSC2500 from TA Instruments (Asse, Belgium). All samples were heated from −100 °C to 200 °C. Analyses were performed with a heating rate of 10 °C/min. The crystallinity (X
c) of both control PU and healable PU was defined according to the equation:
where ΔH
m is the melting enthalpy of PU and ΔH
0m is the melting enthalpy of 100% crystalline PU.
The dynamic mechanical properties of the samples (35 mm × 5.3 mm × 0.75 mm) were examined by a dynamic mechanical analyzer (DMA) using a TA Instruments 850 from TA Instruments (Asse, Belgium) in tensile mode from −100 °C to 120 °C at a heating rate of 3 °C/min at a frequency of 1 Hz. The creep recovery was also investigated by DMA using TA Instruments 850 at 90 °C and 0.1 MPa of stress.
The water barrier properties were measured with a Textest FX 800 apparatus from Artec Testnology (Kerkdriel, The Netherlands). A steadily rising water pressure (60 mbar/min) was applied on the coated surface, according to ISO 811 under standard conditions (20 °C and relative humidity of 65%), until penetration occurs in three places. The pressure at which penetration occurs at three places was set as the maximum hydrostatic pressure to which the textile can resist.