Oxazoline-based crosslinking reaction for coatings

Nowadays, coating materials must meet high demands in terms of mechanical, chemical and optical properties in all areas of application. Amongst others, amines and isocyanates are used as crosslinking components for curing reactions, meeting the highly demanding properties of the coatings industry. In this work, a new crosslinking reaction for coatings based on oxazoline chemistry is investigated with the objective to overcome disadvantages of established systems and fulfill the need for sustainable coating compounds. The oxazoline-group containing resin, synthesized from commercially available substances, undergoes cationic self-crosslinking polymerization to build up a network based on urethane and amide moieties. NMR-, IR- and ES-mass spectroscopy are suitable techniques to characterize the synthesized oxazoline monomers, which are linked to polyisocyanates and polymerized afterwards via self-polymerization. The progress of crosslinking is followed by changes in IR spectra and by rheological measurements to calculate time dependent values for storage and loss modulus. The glass transition temperature of the resulting coating is determined, too. Furthermore, sol–gel-analysis is performed to determine the degree of crosslinking. After application on steel and aluminium panels, application tests are performed. In addition to excellent adhesion to the substrate, the polymer network shows promising mechanical properties and with that it could represent a new technology for the coatings industry.


Introduction
Typically, the coatings industry roughly applies two kind of coatings: On one hand physically drying, nonreactive systems are used, if lower-end applications and ease of handling are in the focus. On the other hand, if high durability and enhanced mechanical properties are requested, reactive systems based on reacting active groups of a crosslinking agent with a resin are used. Advantages of the latter method are the resulting high durability, better corrosion protection as well as better chemical and mechanical resistance of such coatings in comparison to physically dried coatings. In general, reactive systems involve functional groups such as melamine, epoxy, amines or isocyanates, which are used to form crosslinked polymer networks. 1 In order to create more sustainable systems, the coatings industry aims to develop new crosslinking mechanisms, such as the non-isocyanate polyurethane (NIPU) technologies. 2 Promising results have already been reported by Wunschik et al. 3,4 by means of cyclic carbonates as crosslinking-agents producing polyester networks. The crosslinked coatings show pendulum dumping values of 79 and good adhesion on steel substrates without using isocyanates, amines or mela-mine as crosslinking agents. In NIPU technologies, cyclic (poly) carbonates are often used in combination with amines to form polyurethane networks, which are sensitive to hydrolysis.
However, drawbacks of the NIPU technologies often outweigh their advantages with regard to the coatings properties. The most used technologies are chemically similar to the isocyanate compounds or the similar polyurethane network will be achieved, and therefore, the origin technology can hardly be replaced because the new technologies just copy the origin one with often drawbacks in the final coating properties or the synthesis of the compounds.
To achieve coatings with new properties based on new network formation with promising chemical and mechanical properties, new crosslinking reactions have to be studied. Oxazolines as reacting active groups show acid catalysed self-polymerization to form poly oxazolines as described by Kobayashi and Uyama. 5 This cationic polymerization is well-known and finds application in pharmaceutical drug delivery systems. [6][7][8] Due to the fact that the synthesis of oxazolines is based on commercially available raw materials and a stable network can be formed, oxazolines represent promising candidates for new crosslinking agents in the coating industry.
In this paper, the crosslinking behavior of oxazolinegroup containing resins and the mechanical properties of the produced coatings are studied.

Raw materials
HDI-trimer was provided by Covestro Deutschland AG. 2-Amino-2-methyl-1,3-propandiol (min. 98%) was purchased from TCI Europe. All other compounds were purchased from Aldrich Chemicals and have been used without further purification.
All coatings have been applied on steel samples (DX51D+Z), aluminium samples (type Al99,5) or glass plates.

Analytical methods
1 H-NMR spectra were measured on a Bruker Fourier 300 spectrometer at 300 MHz. CDCl 3 has been used as solvent and internal standard.
Mass spectra were measured with a 6530 Q-TOF LC/MS system from Agilent (Agilent, Santa Clara, USA) using a Polaris Amid C18 column (Agilent, Santa Clara, USA) and Dual AJS ESI ionization mode.
IR spectra were measured on a Bruker Lumos FTIR-Microscope or with the Bruker Vertex 70 FTIRspectrometer (4000-375 cm À1 ) with a resolution of 4 cm À1 .
The rheological experiments to achieve values for storage and loss modulus and the glass transition temperature have been performed on a Modular Compact Rheometer MCR 102 (Anton-Paar) using Plate-Plate-single use with a diameter of 25 mm and a slot of 1 mm. Solvent containing samples have been dried overnight at 70°C prior to the measurement. The sample still may contain solvent residues, which can affect measurement data during crosslinking reaction and the values for G¢ and G † are always lower in comparison to DMA measurements of a free film because the plate-plate single use system influences measuring data by the diameter of the plate. The diameter of 25 mm is a compromise to gain accurate data in all steps of the curing reaction.
The pendulum damping values have been obtained according to DIN EN ISO 1522 using a Kö nigspendel from Erichsen. Three measurements for each panel were performed.
Adhesion was received by performing the crosshatch test according to DIN EN ISO 2409. Each panel was measured five times and the average values are given. Step 1: Synthesis of 2,4-Dimethyl-4-methylol-2-oxazoline 1

Application testing
For application testing, a formulation consisting of 2 and 0.5% catalyst is prepared. The formulation is applied by drawing down a bar on aluminium sheets and curing at 160 and 190°C (see Tables 1 and 2) for 30 min to achieve a dry film thickness of 70 lm.

Results and discussion
In this work, the self-polymerization of oxazoline derivatives is studied as a new crosslinking mechanism. A novel polyisocyanate-oxazoline adduct 2 has been prepared by reacting polyisocyanate and OH-functionalized oxazoline 1 upon urethane formation. The crosslinking ability of the obtained adduct 2 is based on a cationic polymerization of the oxazoline moiety and occurs according to the mechanism illustrated in Fig. 2, published by Aoi and Okada. 13  To further study the crosslinking mechanism that leads to the formation of a polyoxazoline network, mechanical and spectroscopic changes during the curing process have been investigated.
Whether a successful crosslinking reaction has taken place according to the mechanism shown in Fig. 2, is determined using IR spectroscopy. Additionally, to prove successful crosslinking, storage and loss modulus for uncatalyzed as well as catalyzed samples and temperature-dependent oscillating rheological measurement are discussed, too.
Upon curing of adduct 2 at temperatures higher than 160°C and via acid catalysis, the formation of a hard coating is observed.
IR spectra (Figs. 3, 4, and 5) are baseline corrected and normalized on the C-N-stretch signal of the isocyanurate at 765 cm À1 as this signal does not change during the network formation. 10 Successful cationic polymerization has taken place as can be detected from the increase of the N-H signal at 3330 cm -1 caused by the new amide group with a N-H-group, which is formed during the ring opening process (compare Fig. 2). During the polymerization of 2, it can be assumed that many short-chain polymer chains are formed due to the steric hindrance during the polymerization. These have an NH bond at the start and end of the chain, which can be detected by means of IR. In the spectrum of 2, the amide I vibration of the urethane/isocyanurate group can be detected at 1712 cm -1 and 1680 cm -1 with the corresponding amide II or N-H-signal at 1531 cm -1 caused by the N-H group of the urethane groups (Fig. 4). An amide II signal is visible, if at least one proton is bounded at the nitrogen of the amide group, therefore this signal proves the formation or existence of an amide group with an N-H-group. 10 In the spectrum of 3, a new amide I signal is visible with the shoulder visible in Fig. 4 at 1638 cm -1 , which proves the formation of N,N-di-substituted amides. 10 The amide II or N-H vibrations are shifted to 1548 cm -1 (Fig. 4) as the formation of new amide groups lead to an increasing number of N-H groups in the network, which further proves the mechanism of the crosslinking reaction according to Fig. 2.
Finally, the ether group in the oxazoline ring disappears upon crosslinking. In Fig. 5 the reduced signal in the cured sample at 1038 cm -1 for the =C-O-C group is illustrated.
With that, the cationic polymerization of oxazoline moieties and crosslinking of 2 can be verified according to the mechanism of Fig. 2.
To further prove whether chemical crosslinking of 2 takes place upon curing rather than physical drying of the prepared films at elevated temperatures, sol-gel analysis has been carried out.
After curing of 2 in the presence of SnCl 4 as a catalyst at 190°C for 30 min, extraction of the resulting polymeric network 3 with MEK in a Soxhlet extractor is performed. A remaining gel fraction of 98.1 ± 0.3% is obtained for the SnCl 4 -catalyzed sample. In comparison, extraction of uncured polyisocyanate-oxazoline adduct 2 with MEK resulted in a non-measurable gel part. With this, sol-gel analysis supports the previous conclusion that a crosslinked polymer network is formed upon curing of 2 in the presence of an acidic catalyst.
The crosslinking ability of 2 in the presence of catalysts is further studied using rheological experiments to determine the modulus of the polymeric network. These oscillating experiments have to be performed in the linear viscoelastic range (LVE). 14 With 2 before and after curing, it is the case with an amplitude of x = 10 rad/s and a frequency of c = 0.001-10% and therefore all DMTA measurements have been performed with these parameters. Upon curing, the gel-point, i.e. G¢ = G †, of 2 is achieved immediately after heating the acid catalyzed sample to 190°C (Fig. 6). The network formation is visible by an increase of the storage modulus. The formed network shows a glass transition temperature of T g = 55°C (Fig. 7). The T g is obtained in a temperature dependent measurement of G † at the maximum of the curve in the second heating run. 14 The T g of physical dried acrylate coatings are in the range of 32-48°C, whereas with the same resins cured with crosslinking agents, the resulting coatings show T g in the range of 61-72°C. 15 Another example is UV cured coatings. 16 Resins with a low amount of double bonds show T g values in the range of 37-43°C and with a high amount of double bonds, the T g increases to values between 53 and 78°C depending on the amount of double bonds. As a last example, HDI-Trimer formed a network with a branched polyester with a T g of 54°C. 17 Therefore, the new polyurethane-polyamine/amide network is in the standard range of the glass transition temperature of crosslinked coatings with a T g of 55°C.
With unsuccessful crosslinking of 2 taking place in the absence of catalysts visible by a missing gel-point and no increase in the storage modulus in Fig. 8, it is of interest to study the effect of catalysts on the crosslinking behavior and the curing temperature.
Therefore, different common catalysts for cationic polymerization have been screened to catalyze the selfpolymerization as shown in Table 1, starting from SnCl 4   (Fig. 9). However, using alternative catalysts, such as citric acid or methyl tosylate, network formation cannot be observed independent of the curing temperature (see supplementary material, Figs. S6-10). Network formation is possible with acetic acid at 190°C and with cadmium acetate at 190°C, too. With that, a significant reduction of the curing temperature can only be achieved by addition of SnCl 4 .
Finding an alternative catalyst for successful reduction of the curing temperature without the need for tin compounds is a favorable goal.
Essential for the evaluation of new crosslinking reactions are the resulting properties of the final coating surface. With the focus of the crosslinking reaction, the impact test and pendulum damping have been performed to characterize the polymer network. Furthermore, the adhesion on steel has been tested, too. Table 2 summarizes the properties of the cured coatings in dependence of different catalysts. As shown before, network formation is not possible without the addition of catalysts.
Using acidic catalysts for polymerization, such as SnCl 4 , high pendulum hardness values are achieved. Results of the impact test show that flexible coatings can be achieved at 190°C, too. A physical dried compound/polymer should be brittle, but a polymer network could be hard and flexible, therefore, the impact test gives additional information to the polymer network. Crystallinity of partly not crosslinked polymers could be a reason for the low flexibility of the coating, too. Here, the impact test shows hard and flexible coatings at 190°C curing temperatures. In comparison to the above mentioned NIPU coatings, very hard and flexible coatings are available with polyoxazolines.
All cured coatings show an excellent adhesion with a GT = 0 on steel according to DIN EN ISO 2409. The gloss of all coatings is visually evaluated high and more or less independent of the catalyst or curing temperature.
Finally, for a first look, the resulting coatings properties are promising using oxazoline containing resins for coating application.

Conclusions
Based on commercially available raw materials, a novel oxazoline containing resin has been synthesized to form a polyisocyanate-oxazoline adduct 2, which is able to undergo self-crosslinking to a polyamide/ polyamine/polyurethane network 3.
Upon curing at elevated temperatures, polyurethane-polyamine/polyamide networks form via ringopening polymerization leading to a hard but flexible coating with good adhesion to steel and high gloss.