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Synthesis and structural characterization of urea-isobutyraldehyde-formaldehyde resins.

Abstract Urea-isobutyraldehyde-formaldehyde (UIF) resins were synthesized using urea, isobutyraldehyde, and formaldehyde; sulfuric acid was used as a catalyst. The effects of molar ratio of urea/isobutyraldehyde/formaldehyde (U/I/F) on the properties of resins were investigated, and the structures of the resins were characterized by FTIR. (1) H-NMR. and [.sup.13]C-NMR. When U/I/F was 1.0/3.6/2.4, the yield of the resin was 76.5%. The softening point and hydroxyl value were 90[degrees]C and 32 rag KOH/g, respectively." The FTIR, [.sup.1]H-NMR, and [.sup.13]C-NMR results showed that an [alpha]-ureidoalkylation reaction occurred between urea and isobutyraldehyde to form a lactam. The UIF resins also contained hydroxyl groups and aldehyde groups; the content of aldehyde groups in the resin increased as the amount of isobutyraldehyde increased.

Keywords Urea. Isobutyraldehyde. Formaldehyde, Resin. Structure. Characterization

Introduction

Additives are an important component of coatings, and can significantly improve manufacture processes and coating product quality. Polymer additives are among the most used, because they can form a continuous phase of the coatings with no deleterious effects on the coatings. Commonly used additives include polymer dispersing agents and polymer adhesion promoters. Cyclohexanone-formaldehyde resin can improve adhesion, hardness, and gloss of the coating films, and can also be used as adhesives in abrasive coatings. (1) This polymer is soluble in most solvents and has good compatibility with the components of most coatings. However, it has slightly poor thermal stability and yellowing resistance, which limits its application in the coatings formulation, although much work has been done to improve its yellowing resistance and thermal Stability. (2), (3) Research on, and the development of, polymer coating additives with better thermal stability and yellowing resistance have attracted considerable attention in recent years. (4-6)

Conventional amino resins with light color and excellent yellowing resistance include the etherified product of urea or melamine condensates with formaldehyde. They have been used in organic coatings for a lone time. (7-9) Amino resins synthesized from urea, isobutyraldehyde, and formaldehyde have excellent properties, such as light color, yellowing resistance, and excellent compatibility with the solvents and components of coatings. They can be used in universal color slurries and also as polymer additives to improve the gloss, hardness, adhesion, and yellowing resistance of coatings. (10), (11)

The urea-isobutyraldehyde-formaldehyde (UIF) resins as a type of amino resins are usually synthesized according to a method that is different from traditional amino resins procedures, and thus the structures and properties of the UIF resins are also different from those of traditional amino resins. UIF resin can be synthesized in two steps. In the first step, an [alpha]-ureidoalkvlation reaction occurs between urea and isobutyraldehyde to give 1, namely 4-hydroxy-6-iso-propy1-5,5-dimethy1-tetrahydro-pyrimidin-2-one (HIDTPO) (Scheme1). (12) Then 1 (HIDTPO) reacts with formaldehyde and isobutyraldehyde to produce the UIF resin 2 gradually.

[ILLUSTRATION OMITTED]

In the process of synthesis, the molar ratio of U/I/F has a significant influence on the properties and on the structure of the resins produced. Therefore, it is important to study the structures of the resins and the relationships between the structure of the resins and their synthesis conditions. However, little research has been done on the structure of UIF resins so far.

In this work, UIF resins were synthesized from urea, isobutyraldehyde, and formaldehyde using sulfuric acid as a catalyst. The yield and softening point variations of resins that are associated with changes in molar ratio of U/I/F are discussed. The structures of the UIF resins were characterized by FTIR, [.sup.13]C-NMR, and [.sup.1] H-NMR.

Experimental

Materials

Urea, xylene, formaldehyde (37 wt% aqueous solution), and sulfuric acid (98 wt%) of analytical grade were purchased from Guangzhou Chemical Reagent Factory and used without further purification. Isobutyraldehyde (chemical grade) was provided by Shanghai Lingfeng Chemical Reagent Co., Ltd., and distilled before use.

Synthesis of UIF resin

Urea and a portion of the isobutyraldehyde is 1:2) were put into a 500-[cm.sup.3] flask equipped with a mechanical stirrer, thermometer, condenser, and dropping funnel. A total of 50 wt% sulfuric acid aqueous solution was added dropwise within 10 min (the mass of sulfuric acid was 6.0 wt% of total reagents). The mixture was heated to 80[degrees]C gradually, and stirred for 3.0 h. Afterward, 37 wt% formaldehyde solution and the remaining isobutyraldehyde were added. The mixture was refluxed for 3.0 h at 90[degrees]C Then, xylene (the content was 1.2 times isobutyraldehyde) was added into the flask, and the aqueous phase was separated off and washed 10 times with distilled water at 80[degrees]C. The xylene solution of resin was distilled under 500 Pa to remove xylene until the temperature in the flask reached 140[degrees]C, and the temperature and pressure were maintained for 1.0 h. A pale brittle solid resin was obtained and designated as UIF resin. The reaction formulation of synthesis for the UIF resins is shown in Scheme 2.

[ILLUSTRATION OMITTED]

The yield of the resins is calculated using the following formula.

Yield(%) = [[W.sub.res]/[[W.sub.urea] + [W.sub.iso] + [W.sub.forma]]] x 100

where [W.sub.urea], [W.sub.iso], [W.sub.forma], and [W.sub.res] represent the mass of urea. isobutyraldehyde, formaldehyde, and UIF resin, respectively.

Measurement and characterization

The softening points and the hydroxyl values of the ULF resins were determined according to ISO 4625-1:2004 and DIN 53240-2:1998, respectively.

FTIR spectra of resins were recorded on a Bruker Vector 33 spectrophotometer in the wavenumber interval between 4000 and 500 [cm.sup.-1] as KBr pellets.

[.sup.13]C-NMR and [.sup.1]H-NMR spectra were recorded on a Bruker drx400 NMR spectrometer with a fixed magnetic field strength of 9.4 T and [CDCI.sub.3] as solvent.

Results and discussion

Effect of molar ratio of U/I/F on yield and properties of UIF resins

The properties and yield of the resins for different values of U/I/F are listed in Table 1.
Table 1: Effect of molar ratio of U/I/F on yield and properties

UIF resins       U/I/F     Yield  Softening point  Hydroxyl value
            (mol/mol/mol)  (wt%)    ([degrees]C)     (mg KOH/g)

2a           1.0/2.0/4.0    43.4         116             28
2a           1.0/3.0/3.0    70.1         102             38
2a           1.0/3.6/2.4    76.5          90             32
2a           1.0/4.0/2.0    73.5          83             30


As seen from Table 1, the yield is only 43.4% when U/I/F is 1.0/2.0/4.0, increases to 76.5% when U/I/F is 1.0/3.6/2.4 and then decreases to 73.2% when U/I/F is 1.0/4.0/2.0, indicating that the yield of resins significantly depends on the ratio of U/I/F. The reason for this may be that only 1 and formaldehyde are in the reactive mixture at the second step when the U/I/F is 1.0/2.0/4.0, so the reaction tends to proceed according to Scheme 3. To some extent, 3 containing three hydroxyl groups is water-soluble, and would be taken off during washing, reducing the yield. In addition, cross-linking condensation also might occur between 3 during distillation. It is found that if the molar ratio of isobulyraldehyde to formaldehyde is less than I, the organic phase does not cleanly separate from the water phase after the addition of xylene, and the separated water phase looks like ivory-white, suggesting that some substances come into the water phase. Finally, the resins obtained are partially insoluble in acetone and acetic-butyl ester, indicating (hat partial cross-linking reactions occur between 3.

[ILLUSTRATION OMITTED]

It can be clearly seen from Table I that the softening points of the UIF resins decrease on increasing the amount of isobutvraldchyde. indicating that the UIF resins have lower molecular weights. It is possible that the probability of the reaction shown in Scheme 4 increases when the amount of isobutyraldehyde increases.

[ILLUSTRATION OMITTED]

FTIR

The structures of the obtained UIF resins were identified by FTIR (Fig. 1). In the spectra of UIF resins, the broad bands in the range 3719-3313 [cm.sup.-1] are ascribed to the stretching vibration of the O-H groups involved in the hydrogen-bonding interaction. (13), (14)

[FIGURE 1 OMITTED]

The weak peak at about 3080 [cm.sup.-1] is attributed to the stretching vibration modes of the N-H, suggesting the existence of N-H groups in resin structure.(15), (16) These N-H groups may come from a side reaction between urea and isobutyraldehyde to give 1,3-bis-hvdroxvisohuivl-urea, in which there are N-H, which cannot easily he washed away.

The absorption peak at 1732 cm and the relatively weak peak at 2714 [cm.sup.-1] peak are assigned to the stretching vibration modes of the C=O and C-H bonds in the aldehyde group, respectively, suggesting the existence of aldehyde groups in resin structure.

Two strong peaks located at 2967 and 2876 [cm.sup.-1] are assigned to the asymmetric and symmetric stretching vibration of C-H bonds in the [-CH.sub.3] and [-CH.sub.2] groups, while the strong bands at 1479 [cm.sup.1] are ascribed to the deforming vibration. (15), (16).

The peaks at 1391 and 1368 [cm.sup.-1] are assigned to the symmetric deforming vibration of C-H bonds in the -CH[([CH.sub.3]).sub.2] and >C[([CH.sub.3]).sub.2] groups. Peaks at 1215 and 1188 [cm.sub.-1] are attributed to the deforming vibration of carbon framework in the >C[([CH.sub.3]).sub.2] groups, indicating the presence of gem-dimethyl structure. (17)

As can be seen from Fig. 1, the peaks at 1732 and 2714 [cm.sup.-1] corresponding to aldehyde groups are very weak when the molar ratio of U/l/F is 1.0/2.0/4.0. It is noted that their intensity significantly increases on increasing the amount of isobutyraldehyde, showing that the content of aldehyde groups in the resins increases. It is clear that the aldehyde group in the resin should come from isobutyraldehyde. The highest peak at 1668 [cm.sup.-1], corresponding to the stretching vibration of C = 0 in the structure of amide groups, is from urea, suggesting that an [alpha]-ureidoalkylation reaction occurred between urea and isobutyraldehyde. (18), (19)

The dual peaks at 1312 [cm.sup.-1], assigned to the stretching vibration of C-N of amide groups, also show the presence of amide groups in the resins.(20) Therefore, the above FTIR data are consistent with the reaction shown in Scheme 2.

NMR

[.sup.1] H-NMR

Figure 2 shows the [.sup.1]H-NMR spectra of UIF resins. The assignment of [.sup.1]H-NMR Peaks is listed in Table 2.

[FIGURE 2 OMITTED]
Table 2: [.sup.1]H-NMR assignment of UIF resins

Groups                [.Sup.1]H-NMR    Relative peak area (%)
                        peaks (ppm)

                                       2a     2b     2c     2d

[FORMULA NOT             9.3-9.5      0.29   0.88   1.31   1.53
REPRODUCIBLE IN
ASCII]

-NH-                     6.2-6.6      0.96   1.02   1.13   1.21

-OH-O-[CH.sub.2]-O-      5.2-6.0      3.13   4.49   3.23   3.21

[FORMULA NOT             4.0-5.2     13.84  10.28   8.18   5.73
REPRODUCIBLE IN
ASCII]

[FORMULA NOT             3.0-3.8      7.87   9.31  11.84  11.18
REPRODUCIBLE IN
ASCII]

[FORMULA NOT             2.3-3.0      2.52   2.31   2.26   2.12
REPRODUCIBLE IN
ASCII]

-C[H.bar]                1.6-2.0      4.04   3.88   3.86   3.72
[([CH.sub.3]).sub.2]

-[CH.sub.3]              0.5-1.4     67.35  67.83  68.19  71.30


It can be seen from Fig. 2 and Table 2 that methyl hydrogen corresponding to the peaks at 0.5-1.4 ppm accounts for about 70% of total hydrogen atoms. (21) Peaks at 1.6-2.0 ppm are assigned to hydrogen atoms of submethylene groups in -CH[([CH.sub.3]).sub.2]. and peaks at 2.3-3.0 ppm are assigned to hydrogen atoms of submethylene groups which join to nitrogen atom. Peaks at 3.0-3.8 ppm are attributed to methylene groups connecting to nitrogen and carbon atoms. (22) The peaks at 4.0-5.2 ppm are attributed to hydrogen atoms of methylene groups connecting to nitrogen and submethylene groups connecting to hydroxyl and nitrogen, and that at 5.2-6.0 ppm are ascribed to hydrogen atoms in hydroxyl groups and in methylene groups connecting to oxygen. (23) The peaks at 6.2-6.6 ppm are attributed to hydrogen in amido groups.(24), (25) The peaks at 9.3-9.5 ppm assigned to hydrogen in aldehyde groups (26), (27)and the relative peak area increase along with the amount of isobutyraldehyde, which coincide with the results of FTIR. The probability of the reaction shown in Scheme 4 increases on increasing the amount of isobutyraldehyde.

As can be seen from Table 2, the relative peak area of 1.6-2.0 ppm assigned to hydrogen atom of submethylene in -CH[([CH.sub.3]).sub.2] hardly changes, because the reaction shown in Scheme 1 between isobutyraldehyde with urea has mostly been completed, and there are no -CH[(CH.sub.3]).sub.2] groups to take part in the reaction shown in Scheme 2. so that the amount of isobutyraldehyde cannot influence -CH[([CH.sub.3]).sub.2] groups.

It also can be seen from Fig. 2 and Table 2 that the resonance peaks at 4.0-5.2 ppm decrease with the increase of isobutyraldehyde, indicating that the groups of >N-[CH.sub.2]-N < and -O-[CH.sub.2]:-N< decrease with increasing isobutyraldehyde. In fact, when the U/I/F molar ratio is 1.0/2.0/4.0. there is no isobutyr-aldehyde in the system in the second step: hence, formaldehyde would react with 3 to give methoxy groups. These methoxy groups would partly lose formaldehyde when the resin is healed (Scheme 5).(28) On the contrary, when the amount of isobutyr-aldehyde increases, the amount of formaldehyde decreases in the second step; consequently, the possibility of this reaction (Scheme 6) is reduced. Therefore, the intensity of resonance peaks at 4.0-5.2 ppm assigned to the hydrogen atoms of >N-[CH.sub.2]-N< and -O-[CH.sub.2]-N< decreases with the increase of isobutyraldehyde.

[ILLUSTRATION OMITTED]

[ILLUSTRATION OMITTED]

[.sup.13]C-NMR

Figure 3 shows the [.sup.13]C-NMR spectra of UIF resins. The assignment of [.sup.13]C-NMR peaks arc listed in Table 3.

[FIGURE 3 OMITTED]
Table 3: [.sup.13]C-NMR assignment of UIF resins

Groups                       [.sup.13]     Relative peak area (%)
                           C-NMR peaks
                               (ppm)

                                          2a    2b      2c    2d

[FORMULA NOT              203.5-205.0    0.63  1.46    2.60   3.27
REPRODUCIBLE IN
ASCII]

[FORMULA NOT              152.0-157.0    7.85  4.43    4.04   3.96
REPRODUCIBLE IN
ASCII]

[FORMULA NOT               83.0-90.0     5.22  5.62    4.82   5.01
REPRODUCIBLE IN
ASCII]

[FORMULA NOT               70.0-75.0     0.75  6.06    7.01   8.03
REPRODUCIBLE IN
ASCII]

[FORMULA NOT               50.0-62.0     8.86  7.14    5.13   3.29
REPRODUCIBLE IN
ASCII]

[FORMULA NOT               46.0-19.0     0.40  2.04    2.88   3.08
REPRODUCIBLE IN
ASCII]

[FORMULA NOT               33.0-37.0    11.72  10.54  10.41  11.08
REPRODUCIBLE IN
ASCII]

-C*H[([CH.sub.3]).sub.2]   23.5-30.0    12.70  11.34   9.25  10.10

-[CH.sub.3]                16.5-23.5    51.87  51.37  53.86  52.18


It is clear from Fig. 3 that the peaks al about 204 ppm are attributed to carbon atoms in aldehyde groups, further confirming the presence of aldehyde in the resin, (29), (30) and the relative peak area increases on increasing the amount of isobulyraldehyde. This is consistent with the results of FTIR and [.sup.1]H-NMR. In general, the resonance peak of carbon atom in C=O of urea is between 160 and 163 ppm, and the substituted groups in the nitrogen atoms have little influence on it." However, the resonance peak of the carbon atoms in C= O of the UIF resins are shifted to higher held (152.4-157 ppm), which may be attributed to carbon atoms of carbonyl groups of amide ring, (32) and no peaks are observed al 160-163 ppm in Fig. 3. This indicates that the carbon atoms in C=O of the UIF resins are in ring reaction of amide, suggesting that an [alpha]-ureido-alkylalion reaction should occur between urea and isobutyraldehyde lo form a lactam further. The peaks at 23.5-30 ppm assigned to submethylene hydrogen atom in -CH[([CH.sub.3]).sub.2] change hardly, also supporting the result of [.sup.1]H-NMR.

Conclusion

UIF resins have been synthesized from urea, isobutyraldehyde, and formaldehyde using sulfuric acid as a catalyst. With the increase of isobutyraldehyde, the softening point of the resin decreased. When the U/I/F molar ratio was 1.0/3.6/2.4, the softening point and hydroxyl value of the resin was 90[degrees]C and 32 mg KOH/g. respectively, and the vield of the resin was 76.5%. The FTIR, [.sup.1]H-NMR, and [.sup.13]C-NMR results showed that there were amide rings in the resin structure, suggesting that an ureidoalkylation reaction occurred between urea and isobutyraldehyde and further to form a lactam. In addition, the UIF resins also contained hydroxyl groups, and the content of aldehyde groups in the resin increased with the increase of isobutyraldehyde.

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Y.-f. Zhang ([??]), X.-r. Zeng, B.-y. Ren

College of Materials Science and Engineering, South China

University of Technology, Guangzhou 510640, China

e-mail: zhyf1026@163.com
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Date:Sep 1, 2009
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