Catalytic Mannich reaction of acrylic acid polymers and their application in leather retanning

Abstract

Acrylic polymers are widely used in pharmaceutical, leather, textile and other industries, and are prepared via the polyreaction of acrylic monomers, such as acrylic acid, acrylamide and acrylic esters. Acrylic acid and acrylamide have been proven to participate in the Mannich reaction to achieve cationization; however, the Mannich reaction of methyl acrylate has not been reported. At present, the main role of catalysts in the Mannich reaction is to provide products of specific spatial configuration, and there are few studies on improving the degree of cationic modification. The participation of acrylic acid, acrylamide and methyl acrylate in the Mannich reaction by means of the catalyst NaH was studied in this work. The optimal reaction conditions for the homopolymers were obtained via orthogonal experiments. At room temperature, the optimal reaction conditions for polyacrylic acid were as follows: the molar ratio of polyacrylic acid, glutaraldehyde and diethanolamine was 1.0 : 1.4 : 1.0, and the amount of catalyst was 7%. The optimal reaction conditions for polyacrylamide were as follows: the molar ratio of polyacrylamide, glutaraldehyde and diethanolamine was 1.1 : 1.1 : 1.2, and the amount of catalyst was 9%. The optimum reaction conditions for methyl polyacrylate were as follows: the molar ratio of methyl polyacrylate, glutaraldehyde and diethanolamine was 1.1 : 1.2 : 1.0, and the amount of catalyst was 9%. As a result, the maximum degrees of modification of polyacrylic acid, polyacrylamide and methyl polyacrylate were 63.0%, 33.5% and 39.0%, respectively. The range analysis of the orthogonal experiments showed that the effects on the modification degree of vinyl homopolymers, in sequence from strong to weak, were as follows: amount of catalyst > amount of amine > amount of aldehyde. Subsequently, polyacrylate–acrylamide–methyl acrylate (P(AA–AM–MA)) was synthesized using acrylic acid, acrylamide and methyl acrylate as monomers, and then modified via the catalyzed Mannich reaction. A novel amphoteric acrylic acid copolymer retanning agent (HCP(AA–AM–MA)) with a 50.2% degree of modification was obtained, and its properties as a retanning agent in leather-making were investigated. The dye-uptake and K/S value of the dyed leather retanned with HCP(AA–AM–MA) were 91.5% and 18.5, respectively, an increase of 18.4% and 3.4 in comparison with those of dyed leather retanned with P(AA–AM–MA). The results indicated that the dyeing-assistant performance of HCP(AA–AM–MA) was improved. Moreover, the elongation at break and tensile strength of the retanned leather were 82.0% and 31.9 MPa, respectively, which were higher than those of P(AA–AM–MA)-retanned leather (78.9% and 27.4 MPa).

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1 Introduction

Acrylic acid and its derivatives (e.g., acrylamide and acrylic ester), containing vinyl groups (–CH=CH₂), are necessary industrial raw materials. Acrylic acid and acrylic esters can be homopolymerized and copolymerized and can also be copolymerized with styrene, butadiene, vinyl chloride and maleic anhydride monomers.^1–3 Such polymers are used in synthetic rubber, synthetic fibers, super-absorbent resins, pharmaceuticals, leather, textiles, building materials, water treatment, oil exploitation, coatings, and other industries.^4–10

However, acrylic polymers are a type of anionic polymer, which has restricted their application range. Cationic modification can give them unusual properties. For example, acrylamide polymer flocculants for wastewater treatment have better absorbability after cationization than the products without cationization; acrylate coatings after cationization have excellent antibacterial activity.^11,12 Cationic acrylic polymer tanning agents have good retanning and auxiliary dyeing properties.^13–15 Therefore, acrylic-polymer cationization is necessary. One of the cationic modification methods is the Mannich reaction, a three-component (acid component, aldehyde component, and amine component) reaction.¹⁶ The method is operationally simple, the raw materials are cheap and the reaction is stable.

The main acrylic monomers used in industries are acrylic acid, acrylamide, acrylonitrile and acrylate monomers.^17,18 Acrylate monomers are common raw materials due to their advantages of low cost, easy preparation and easy function adjustment. They play a pivotal role in the processing and production of industrial chemicals.^19,20 Acrylic acid and acrylamide have been proven to participate in the Mannich reaction to achieve cationization in previous studies, but the same for methyl acrylate has not been reported.²¹ The difficulty here is that the α-H of methyl acrylate is too inert to participate in the reaction. At present, the main role of catalysts in the Mannich reaction is to provide products of specific spatial configuration, and there are few studies on improving the degree of cationic modification.^22,23

Can the degree of cationic modification of acrylic polymers in the Mannich reaction be improved by a catalyst? Wang et al. utilized toluene sulfonic acid as an acid catalyst for the organic chemical reactions in the synthesis of α-substituted N-amino aryl acetals. The experimental results showed that after the addition of catalyst Sc(OTf)₃, glyoxal dimethyl acetal, aryl amines and ketones were successfully reacted with 1,3-dicarbonyl compounds in a Mannich reaction, which allowed the synthesis of a number of heterocyclic compounds such as indoles.²⁴ Li et al. carried out the Mannich reaction in anhydrous ethanol using acetophenone, diphenylamine and benzaldehyde as the reaction raw materials and tin tetrachloride (SnCl₄) as the reaction catalyst, and the experimental results showed that catalyzing the Mannich reaction by using SnC1₄ could effectively reduce the reaction time, increase the reaction yield, and enhance the reaction efficiency.²⁵ Yamashita et al. used potassium bis(methylsilyl)diamine (KHMDS) as a base catalyst for the Mannich reaction using N-o-methoxyphenylbenzenecarboximide and tert-butyl isobutyrate as the raw materials. When KHMDS was used as a catalyst, the tert-butyl isobutyrate monomer was deprotonated by KHMDS to produce a hydrogen molecule and the corresponding potassium enol compound. The reaction continued with N-o-methoxyphenylbenzaldehyde imine to produce a Mannich-base intermediate with a potassium ion, and then the intermediate was protonated by reaction with the tert-butyl isobutyrate monomer to get the desired products, the Mannich base and a potassium enol compound. The catalytic cycle was completed by the reaction of this enol with the tert-butyl isobutyrate monomer to remove the proton and obtain the Mannich base, which then further gives the desired product as well as the potassium enol compound. The results show that these catalysts can successfully catalyze the Mannich reaction to obtain the desired products, extending the scope of application of the Mannich reaction.²⁶ In this work, acrylic acid, acrylamide and methyl acrylate were used as acid components, glutaraldehyde and diethanolamine were used as the aldehyde component and the amine component, and NaH was used as a catalyst to participate in the Mannich reaction. An amphoteric acrylic acid retanning agent was prepared and used in leather-making, where the retanning and dyeing-assistant properties were investigated.

2 Experimental

2.1 Materials

The wet blue leather was purchased from Liquan Shunji Leather Industry Co. Ltd. Acrylamide (AM), methyl acrylate (MA), acrylic acid (AA), ammonium persulfate (APS), sodium hydrogen sulfite, diethanolamine (DEA), glutaraldehyde (GA, 50 wt%), benzoperoxide, and tetrahydrofuran (THF) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. NaH was purchased from Aladdin Chemical Co., Ltd. Commercial acrylic retanning agent (MTA) was purchased from Zhejiang Sanmen Zhongxin Industrial Co., Ltd.

2.2 Synthesis of acrylic acid homopolymers

2.2.1 Synthesis of polyacrylic acid (PAA), polyacrylamide (PAM) and polymethyl acrylate (PMA). AA (AM), 40 wt% NaHSO₃ and a certain amount of deionized water were added to a 250 mL flask equipped with a nitrogen gas system, and then 10 wt% APS was added dropwise into the flask. The mixture was stirred at 45 °C for 4 h, and finally placed under vacuum for 30 min. PAA (PAM) was prepared.

MA and benzoperoxide were added to a 250 mL flask equipped with a nitrogen gas system and stirred at 80 °C. A certain amount of ethanol was added dropwise into the flask over the course of 1 h and the mixture was stirred for 4 h. PMA was prepared. The reactions are summarised in Fig. 1.

Fig. 1 Synthesis of vinyl homopolymers.

2.2.2 Mannich reaction modification of vinyl homopolymers. Firstly, NaH was dissolved into THF in a 250 mL flask equipped with a nitrogen gas system. Then, the spray-dried vinyl homopolymer, glutaraldehyde (GA) and diethanolamine (DEA) were added and stirred at room temperature for 12 h. Finally, the Mannich-reaction-modified vinyl polymer was obtained by reduce pressing and placing under vacuum for 30 min.

2.2.3 Optimization of vinyl homopolymers. In order to improve the cationic modification of the vinyl homopolymers, the dosage of Mannich reaction components was optimized by orthogonal testing.

The optimized conditions in terms of GA, DEA and NaH were obtained by orthogonal experiments. The specific parameters are shown in Table 1. Fixing the number of moles of acid components for the Mannich reaction as 1, the molar ratio of the amount of glutaraldehyde, the molar ratio of the amount of diethanolamine, and the percentage of catalyst relative to the overall solids content were chosen as the key factors for optimization.

Table 1 Orthogonal experimental design factor-level table

Level of factor	n(GA) : n(PAA)	n(DEA) : n(PAA)	Amount of catalyst
1	0.8	0.8	3%
2	1.0	1.0	5%
3	1.2	1.2	7%
4	1.4	1.4	9%

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An orthogonal table, L₁₆ (4³), with 3 factors and 4 levels, was selected; the specific experimental parameters are shown in Table 2.

Table 2 Orthogonal experiment table, L₁₆ (4³)

Entry	A	B	C
1	1	1	1
2	1	2	2
3	1	3	3
4	1	4	4
5	2	1	2
6	2	2	1
7	2	3	4
8	2	4	3
9	3	1	3
10	3	2	4
11	3	3	1
12	3	4	2
13	4	1	4
14	4	2	3
15	4	3	2
16	4	4	1

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The range size of the orthogonal test data is usually analyzed, including the total response value K_i, the average response value k_i and the range value R. K_i means that when the row value in any column of the orthogonal table is i, the sum of the corresponding test results. k_i is equal to K_i/s, where s is the number of occurrences at each level in a given column. R is equal to the maximum value of k_i minus the minimum value of k_i, indicating the degree of influence of each factor on the experimental results.

2.3 Preparation of acrylic acid copolymer (HCP(AA–AM–MA))

AA, AM and MA with a weight ratio of 4.6 : 0.54 : 0.14 were added into a flask. Then, a certain amount of NaHSO₃ and APS were added dropwise to the flask and the mixture was stirred for 4 h at 45 °C to obtain a prepolymer (P(AA–AM–MA)). The prepolymer was spray-dried at 160 °C. Subsequently, the prepolymer, GA and DEA were added to a flask with THF, and reacted with NaH as the catalyst for 18 h. Finally, HCP(AA–AM–MA) was obtained by vacuum extraction for 30 min. The reaction is summarized in Fig. 2.

Fig. 2 Catalytic Mannich reaction of acrylic acid polymers.

2.4 Detection and characterization of retanning agents

a Proton nuclear magnetic resonance (¹H-NMR) spectroscopy. After the samples were purified and dried, the vinyl polymers before and after modification were analyzed by hydrogen nuclear magnetic resonance (¹H-NMR) spectroscopy using D₂O as a solvent and a 600 MHz nuclear magnetic resonance spectrometer (Bruker, Germany).

b Fourier-transform infrared (FT-IR) spectroscopy. The purified and dried vinyl polymer and modified vinyl polymer were ground into powder and mixed with dry KBr powder for tablet processing. The FT-IR spectra of the retanning agents were recorded in the scanning range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹ using an infrared spectrometer (VERTEX 70, Bruker, Germany).

c Determination of molecular weight and molecular-weight distribution. After drying and purification, 9 mg of unmodified and modified acrylic polymers were dissolved in aqueous sodium nitrite with the solution concentration of 3 g L⁻¹, and the solution was filtered with a 0.45 mm syringe filter. The molecular weights and polydispersity indices (PDIs) of the retanning agents were measured with gel permeation chromatography (E2695, Waters, America).

d Zeta potential. The retanning agents were dissolved in pure water to obtain retanning agent solutions with a concentration of 0.5 wt%. The pH values of the solutions were adjusted with 0.1 mol L⁻¹ NaOH or 0.1 mol L⁻¹ HCl, and the zeta potentials of the solutions at different pH were measured using a nanoparticle surface-potential analyzer (ZS90, Malvern, Britain).

2.5 Preparation and characterization of retanned leather

2.5.1 Retanning process. The wet blue goat leather was retanned using the Mannich-reaction-modified acrylic polymer retanning agent; the retanning process is shown in Table 3.

Table 3 Specific operation of conventional and experimental processes

Process	Chemicals	Dosage^d (%)	Temperature (°C)	Time (min)	Remarks
a Sodium bicarbonate was diluted with 10 times water (relative to the quality of sodium bicarbonate). b Retanning agent and formic acid were respectively dissolved in 20 times water (relative to the quality of Retanning agent and formic acid). c Synthetic fatliquor was emulsified with hot water. d Based on tare weight of wet-blue.
Pretreatment	Water	200	45	120
	Degreasing agent	0.2
	Wetting agent	0.5
Wash, drain
Neutralization	Water	150	30	30
	Sodium formate	1.0
	Neutralizing syntan	2.0
	Sodium bicarbonate^a	1.5		30 × 2	pH 5.5+; drain
Wash, drain
Retanning	Water	100	30	15 × 4 + 60
	Retanning agents^b	10
	Formic acid^b	1.0		20 × 3 + 30	pH ∼ 3.8
Stop operation for 12 h
Dyeing and fatliquoring	Water	150	50
	Sodium bicarbonate^a				pH ∼ 5.0
	Black dye	3		60
	Synthetic fatliquor^c	12		60
Fixing	Formic acid^b	2.0		20 × 3	pH ∼ 3.5; drain

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2.5.2 Characterization of retanned leather.
a Leather surface color depth (K/S). The dyed leather samples were treated in constant-temperature and -humidity equipment for 48 h, and then the K/S value of the leather was determined using a colorimeter (i7800, X-Rite, USA).
b Dye uptake. The dye solution was analysed using an ultraviolet-visible spectrophotometer (Cary 5000, Agilent, Malaysia), and the absorption wavelength of dye appeared at 503 nm. Then the absorbances of the original dye solution (A) and the dyeing effluent (A₀) were determined and dye uptake was calculated using the formula shown in eqn (1).where A and A₀ are the dye contents in the initial and finished dyeing liquids, respectively.
c Washing fastness. The leather samples were washed several times with water. Then, the blackness of the leather before and after washing was determined using a colorimeter (i7800, X-Rite, America) to evaluate the color fastness of the leather to washing.
d Rubbing fastness. The leather was rubbed with white non-woven fabrics under a certain pressure and the color was transferred from the leather surface to the fabrics. The whiteness of the non-woven fabrics was tested using a colorimeter (i7800, X-Rite, America) to evaluate the rubbing color fastness of the leather.
e Thickening rate. The thickness of retanned leather samples was measured using a leather thickness gauge (MH-YDI, Shaanxi University of Science and Technology, China). The thickening rate was calculated using the following equation:where T₀ and T are the thickness of unretanned and retanned leather samples, respectively (mm).
f Softness. The softness of the leather samples was examined using a softness tester (GT303, Gotech Testing Machines Limited, China).
g Mechanical properties. The elongation at break and tensile strength of the leather samples were measured using a universal material testing machine (AI-7000-NGD, Goodtechwill, China).

3 Results and discussion

3.1 Research on Mannich-modified polyacrylic acid

3.1.1 Orthogonal experiments. The orthogonal experiments and optimized results for polyacrylic acid are shown in Table 4. It can be seen from the table that the degree of modification for entry 14 was significantly higher than for the others. From the analysis of the range, R, it can be seen that the order of strength of influencing factors is the amount of catalyst, the amount of amine component, and the amount of aldehyde component. The modification degree reached a maximum when n(PAA) : n(GA) : n(DEA) and the amount of catalyst were 1.0 : 1.4 : 1.0 and 7%, respectively.

Table 4 Orthogonal experiments and results for polyacrylic acid

Entry	n(GA) : n(PAA)	n(DEA) : n(PAA)	Amount of catalyst	Degree of modification
1	1	1	1	50.50%
2	1	2	2	52.00%
3	1	3	3	58.00%
4	1	4	4	55.00%
5	2	1	2	54.00%
6	2	2	1	52.00%
7	2	3	4	61.00%
8	2	4	3	62.50%
9	3	1	3	54.50%
10	3	2	4	62.00%
11	3	3	1	50.50%
12	3	4	2	61.50%
13	4	1	4	55.50%
14	4	2	3	63.00%
15	4	3	2	60.00%
16	4	4	1	51.50%
k 1	53.88%	53.63%	51.13%
k 2	54.88%	57.25%	56.88%
k 3	57.13%	57.38%	59.50%
k 4	57.50%	57.63%	58.38%
R	3.62%	4.00%	8.37%

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3.1.2 Detection and characterization. The chemical structure of the modified PAA was investigated by ¹H-NMR and FT-IR spectroscopy. As shown in Fig. 3a, the peaks of methylene and methine appear at 1.41 ppm and 2.03 ppm, respectively. Two new triplet peaks appear at 2.82 ppm and 3.51 ppm in the ¹H-NMR spectrum of MPAA compared with PAA, and were the peaks of –N–CH₂– (d) and –CH₂–OH (e). Meanwhile, the peak of (c) appears at 2.71 ppm, and formed after the Mannich reaction of the aldehyde. In addition, the peak of D₂O appears at 4.52 ppm. The absorption peaks of (c)–(e) appear in the ¹H-NMR spectrum of MPAA, which proved that PAA was modified successfully by the Mannich reaction.

Fig. 3 (a) ¹H-NMR spectra and (b) FT-IR spectra of PAA and modified PAA (MPAA).

As shown in Fig. 3b, the C–H stretching vibration peak of –CH₂– and –CH₃– appears at 2851 cm⁻¹, and the C=O stretching vibration peak appears at 1701 cm⁻¹. There is a strong absorption peak at 1566 cm⁻¹ in the FT-IR spectrum of MPAA, which was attributed to the stretching vibration of C–N. The C–N structures that appear in the FT-IR spectrum indicated that PAA was modified successfully by the Mannich reaction.

3.2 Research on Mannich-modified polyacrylamide

3.2.1 Orthogonal experiments. The orthogonal experiments and optimized results for polyacrylamide are shown in Table 5. It can be seen from the table that the degree of modification for entry 7 was significantly higher than for the others. Similarly, the order of strength of influencing factors is the amount of catalyst, the amount of amine component, and the amount of aldehyde component. The modification degree reached a maximum when n(PAM) : n(GA) : n(DEA) and the amount of catalyst were 1.0 : 1.0 : 1.2 and 9%, respectively.

Table 5 Orthogonal experiments and results for polyacrylamide

Entry	n(GA) : n(PAM)	n(DEA) : n(PAM)	Amount of catalyst	Degree of modification
1	1	1	1	22.50%
2	1	2	2	26.00%
3	1	3	3	29.50%
4	1	4	4	29.00%
5	2	1	2	27.00%
6	2	2	1	25.00%
7	2	3	4	33.50%
8	2	4	3	33.00%
9	3	1	3	28.50%
10	3	2	4	32.50%
11	3	3	1	27.00%
12	3	4	2	32.50%
13	4	1	4	29.00%
14	4	2	3	33.00%
15	4	3	2	32.00%
16	4	4	1	28.50%
k 1	26.75%	26.75%	25.75%
k 2	29.63%	29.13%	29.38%
k 3	30.13%	30.50%	31.00%
k 4	30.63%	30.75%	31.00%
R	3.88%	4.00%	4.25%

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3.2.2 Detection and characterization. The chemical structure of the modified PAM was investigated by ¹H-NMR spectroscopy and FT-IR spectroscopy. As shown in Fig. 4a, the peaks of methylene and methine appear at 1.51 ppm and 2.05 ppm, respectively. The peaks of –N–CH₂– (d) and –CH₂–OH (e) appear at 3.11 ppm and 3.75 ppm in the ¹H-NMR spectrum of MPAM. Meanwhile, the peak of methine (c) appears at 4.12 ppm. The absorption peaks of (c)–(e) appear in the ¹H-NMR spectrum of MPAM, which proved that PAM was modified successfully by the Mannich reaction.

Fig. 4 (a) ¹H-NMR spectra and (b) FT-IR spectra of PAM and modified PAM (MPAM).

As shown in Fig. 4b, the O–H stretching vibration peak appears at 3250–3400 cm⁻¹, the N–H stretching vibration peak appears at 3186 cm⁻¹. The peaks at 1456 cm⁻¹ and 1404 cm⁻¹ in the FT-IR spectrum of MPAM were attributed to stretching vibration peaks of –CH– and –CH₂– connected to N. The C–N structures appearing in the FT-IR spectrum indicated that PAM was modified successfully by the Mannich reaction.

3.3 Research on Mannich-modified polymethyl acrylate

3.3.1 Orthogonal experiments. The orthogonal experiments and optimized results for polymethyl acrylate are shown in Table 6. It can be seen from the table that the degree of modification for entry 10 was significantly higher than for the others. Similarly, the order of strength of influencing factors is the amount of catalyst, the amount of amine component, and the amount of aldehyde component. The modification degree reached a maximum when n(PMA) : n(GA) : n(DEA) and the amount of catalyst were 1.0 : 1.2 : 1.0 and 9%, respectively.

Table 6 Orthogonal experiments and results for polyacrylamide

Entry	n(GA) : n(PMA)	n(DEA) : n(PMA)	Amount of catalyst	Degree of modification
1	1	1	1	31.50%
2	1	2	2	35.00%
3	1	3	3	37.50%
4	1	4	4	38.00%
5	2	1	2	35.00%
6	2	2	1	33.50%
7	2	3	4	38.00%
8	2	4	3	38.50%
9	3	1	3	37.50%
10	3	2	4	39.00%
11	3	3	1	32.50%
12	3	4	2	38.50%
13	4	1	4	37.00%
14	4	2	3	37.50%
15	4	3	2	36.00%
16	4	4	1	33.00%
k 1	35.50%	35.25%	32.63%
k 2	36.25%	36.25%	36.13%
k 3	36.88%	36.00%	37.75%
k 4	35.88%	37.00%	37.00%
R	1.38%	1.75%	5.12%

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3.3.2 Detection and characterization. The chemical structure of the modified PMA was investigated by ¹H-NMR spectroscopy and FT-IR spectroscopy. As shown in Fig. 5a, the peaks of methylene and methine appear at 1.56 ppm and 2.25 ppm, respectively. The peaks of –N–CH₂– (d) and –CH₂–OH (e) appear at 2.85 ppm and 3.58 ppm in the ¹H-NMR spectrum of MPMA. Meanwhile, the peak of methine (c) appears at 2.88 ppm. The absorption peaks of (c)–(e) appear in the ¹H-NMR spectrum of MPMA, which proved that PMA was modified successfully by the Mannich reaction.

Fig. 5 (a) ¹H-NMR spectra and (b) FT-IR spectra of PMA and modified PMA (MPMA).

As shown in Fig. 5b, the asymmetrical stretching vibration peak of –CH₂– appears at 2969 cm⁻¹, and the C=O stretching vibration peak appears at 1739 cm⁻¹. There is a stretching vibration peak of N–H at 3330 cm⁻¹ in the FT-IR spectrum of MPMA, and a tensile vibration peak of C–N appears at 1105 cm⁻¹, which indicated that PMA was modified successfully by the Mannich reaction.

The α-H of poly(methyl acrylate) was abstracted by NaH, which was used as the catalyst, to form H₂ and the corresponding sodium enolate. Then, sodium enolate reacted with the imine, which was formed by diethanolamine reacting with glutaraldehyde, to form the required Mannich base.

3.4 Amphoteric acrylic acid polymer retanning agents

3.4.1 Structural characterization. Fig. 6a and b are the ¹H-NMR and FT-IR spectra, respectively, of P(AA–AM–MA) and HCP(AA–AM–MA). As shown in Fig. 6a, the ¹H-NMR spectrum of HCP(AA–AM–MA) shows two new triplet peaks at δ = 2.92 ppm and δ = 3.53 ppm, attributed to –N–CH₂– and –CH₂–CH₃, respectively. As shown in Fig. 6b, the peak at 3281 cm⁻¹ is the stretching vibration absorption of N–H. In addition, in the FT-IR spectrum of HCP(AA–AM–MA), the peak of C–N appears at 1035 cm⁻¹, and the characteristic absorption peak of secondary amide N–H appears at 1580 cm⁻¹. The C–N and N–H structures appear in the ¹H-NMR and IR spectra, indicating that P(AA–AM–MA) was modified successfully by the Mannich reaction. Therefore, we hypothesize that the catalytic mechanism of NaH is that the α-hydrogen of the ester is abstracted by the strong base NaH to form molecular hydrogen and the corresponding sodium enolate, which reacts with the imine to form a product base. The product base deprotonates the next ester to give the desired product and the sodium enolate is regenerated, completing the catalytic cycle. The key to the base catalysis is the effective deprotonation of the ester by the product base.²⁶

Fig. 6 (a) ¹H-NMR spectra and (b) FT-IR spectra of the modified retanning agents.

3.4.2 Molecular weight and molecular weight distribution. The molecular weights and molecular weight distributions of P(AA–AM–MA) and HCP(AA–AM–MA) are shown in Table 7. The PDIs of the vinyl copolymers change little. The molecular weight of HCP(AA–AM–MA) increased compared with that of P(AA–AM–MA), and it is speculated that the amino group was successfully introduced into the polymer.²⁷

Table 7 Molecular weights and molecular weight distributions of acrylic acid copolymers

Sample	Mn	Mw	Mp	Mz	Mz + 1	PDI
P(AA–AM–MA)	110 701	173 500	217 718	218 831	249 045	1.56
HCP(AA–AM–MA)	140 912	221 377	265 762	276 532	314 019	1.57

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3.4.3 Zeta potential. The zeta potentials of the retanning agents as a function of pH were determined. As shown in Fig. 7, the zeta potential for P(AA–AM–MA) is negative in the pH range of 1.7–12. However, the zeta potential for HCP(AA–AM–MA) is positive when the pH value is lower than 3, and the isoelectric point (pI) was 2.37 for HCP(AA–AM–MA), indicating the successful introduction of cationic groups.

Fig. 7 Zeta potentials of P(AA–AM–MA) and HCP(AA–AM–MA).

3.5 Retanned leather

3.5.1 Overall performance comparison. The overall performance of the acrylic acid copolymer retanning agents is shown in Table 8. Compared with P(AA–AM–MA), the dye uptake and K/S value of HCP(AA–AM–MA) and MTA increased from 77.3% and 15.1 to 91.5% and 18.5, 85.5 and 16.5, respectively. Cationic structures were introduced into the molecular chain of HCP(AA–AM–MA) by the Mannich reaction, which could combine with the dye in the subsequent dyeing process. Meanwhile, the –OH group on the diethanolamine structure could combine with the –SO₃⁻ of the dye to promote the binding of the leather and the dye.

Table 8 Application performance of P(AA–AM–MA), HCP(AA–AM–MA) and MTA

	P(AA–AM–MA)	HCP(AA–AM–MA)	MTA
Elongation at break (%)	78.9 ± 4.3	82.0 ± 3.7	73.6 ± 6.1
Tensile strength (MPa)	27.4 ± 1.6	31.9 ± 2.1	22.8 ± 4.3
Softness (mm)	8.1 ± 0.1	8.0 ± 0.1	21.4 ± 0.6
Thickening rate (%)	19.7 ± 0.3	21.4 ± 0.6	22.1 ± 0.8
Dye uptake (%)	77.3 ± 0.4	91.5 ± 0.2	85.5 ± 0.7
K/S value	15.1 ± 0.5	18.5 ± 0.15	16.5 ± 0.2

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The elongation at break and tensile strength of HCP(AA–AM–MA)-retanned leather were 82% and 31.9 MPa, respectively, which are higher than those of P(AA–AM–MA)-retanned leather (78.9% and 27.4 MPa). The elongation at break and tensile strength of MTA-retanned leather (73.6% and 22.8 MPa) were poor. The molecular weight of HCP (AA–AM–MA) is higher than that of P (AA–AM–MA), eliminating site differences in leather. Therefore, the physical and mechanical properties of HCP(AA–AM–MA)-retanned leather were better than those of P(AA–AM–MA)-retanned leather.

There was no obvious change in the softness of the leather after retanning with the three retanning agents. As for the thickening rate of crust leather, a higher molecular weight of the retanning agent leads to a higher thickening rate. HCP(AA–AM–MA) has a higher molecular weight, so its thickening rate for leather was higher than that of P(AA–AM–MA).

3.5.2 Color fastness. The color fastness of the retanned leather was tested, and the results are shown in Table 9. The whiteness values of the non-woven fabric, corresponding to the wet and dry rubbing of the HCP(AA–AM–MA)-retanned leather, were 147.82 and 135.57, which were higher than those of the P(AA–AM–MA)-retanned leather (145.86 and 127.42). At the same time, the blackness reduction of HCP(AA–AM–MA)-retanned leather after washing was lower than that of the P(AA–AM–MA) retanned leather. It showed that the leather after retanning with HCP(AA–AM–MA) had more cationic structures, confirming that the dyeing-assistant property of HCP(AA–AM–MA) was better.

Table 9 Blackness of leather before and after washing with water and the whiteness of non-woven cloth after wet and dry rubbing

Sample	Whiteness				Blackness
	Dry rubbing		Wet rubbing		Before washing		After washing		Reduction
P(AA–AM–MA)	145.86			127.42	152.68			150.09	2.59
HCP(AA–AM–MA)	147.82			135.57	166.81			164.91	1.91
MTA	146.01			134.83	160.08			157.88	2.20

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3.6 Cost accounting of HCP

Table 10 shows the preparation costs of HCP(AA–AM–MA) (1 kg) and a conventional Mannich-reaction-modified acrylic retanning agent (1 kg). It was found that the difference in preparation costs was not significant, but the retanning effect of the HCP(AA–AM–MA) retanning agent was better than that of the conventional Mannich-reaction-modified acrylic retanning agent.

Table 10 Preparation cost of HCP and traditional Mannich reaction modified acrylic retanning agent

	HCP(AA–AM–MA) (1 kg)			Traditional Mannich reaction modified acrylic retanning agent (1 kg)
	Chemicals	Input (g)	Cost (US $)	Chemicals	Input (g)	Cost (US $)
	AA	229.25	2.83	AA	229.25	2.83
	AM	27.25	0.40	AM	27.25	0.40
	MA	7.06	0.08	AN	55.00	4.08
	APS	0.50	0.01	APS	0.50	0.01
	NaHSO3	36.00	0.48	NaHSO3	36.00	0.48
	NaH	23.72	1.04	GA	71.47	1.39
	THF	151.60	2.98	Diethanolamine	93.82	0.80
	GA	71.47	1.39	NaOH	58.00	0.51
	Diethanolamine	93.82	0.80
Total			10.01			10.50

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4 Conclusions

The Mannich reaction of methyl acrylate successfully occurred by virtue of catalyst NaH; meanwhile, the modification degree of acrylic acid and acrylamide was obviously enhanced. The optimal conditions for the vinyl homopolymers were obtained through optimization analysis via orthogonal experiments. Additionally, the range analysis of the orthogonal experiments showed that the effects on the modification degree of vinyl homopolymers, in sequence from strong to weak, were as follows: amount of catalyst > amount of amine > amount of aldehyde. On the basis of the above work, a new amphoteric acrylic acid copolymer, HCP(AA–AM–MA), was prepared and used as a retanning agent in leather making, and its application properties were investigated. Application evaluation demonstrated that HCP(AA–AM–MA) had favorable tanning effects and could endow crust leather with satisfactory physical and organoleptic properties. At the same time, acrylic polymers modified via the catalytic Mannich reaction could be used as a drug carrier in the field of medicine, in biomedical engineering for the preparation of biocompatible and biodegradable materials, and also for the preparation of conductive polymer materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Key Program of the National Natural Science Foundation of China (No. 21838007), the Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 21JK0549) and the Innovation Capability Support Program of Shaanxi (No. 2021TD-16).

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