Abstract
Isocyanate blocking can improve the long-term stability of isocyanate and reduce its toxicity for use in aqueous systems. The current isocyanate blocking technique has a low blocking degree, and this study designed a reasonable process route using tetrabutylammonium bromide (TBAB) as the phase transfer catalyst in isopropyl alcohol (IPA) solvent to prepare water-soluble isophorone diisocyanate (IPDI) blocked by NaHSO3 with a blocking degree of ≥ 98%. This work used 1% m (TBAB) / m (NaHSO3) with a m (IPA): m (H2O) ratio of 1 : 1; the NaHSO3 was prepared in a 15% aqueous solution. The n (HSO3−) : n (-NCO) ratio was greater than 1 : 1. The stirring speed was 200 r/min, the reaction temperature was 30 °C, and the reaction time was 2.5 h. FT-IR was used to analyze the changes in the main functional groups during the blocking reaction. TG and DCS were used to analyze the deblocking characteristics of water-soluble blocked IPDI. This paper also studied the effect of the blocked IPDI on paper properties. Experimental results showed that the strength and water resistance of paper made from the treated fiber can be remarkably improved by using water-soluble blocked IPDI.
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Synthesis of Water-soluble Highly Blocked (≥ 98%) Isophorone Diisocyanate Using NaHSO3 and Its Effects on Paper Properties
Ping Wang,a,‡ Weilong Shao,a,c,‡ Qin Wu,a,b,* Xin Zhao,a Fangong Kong,a and Chenxiao Han c
Isocyanate blocking can improve the long-term stability of isocyanate and reduce its toxicity for use in aqueous systems. The current isocyanate blocking technique has a low blocking degree, and this study designed a reasonable process route using tetrabutylammonium bromide (TBAB) as the phase transfer catalyst in isopropyl alcohol (IPA) solvent to prepare water-soluble isophorone diisocyanate (IPDI) blocked by NaHSO3 with a blocking degree of ≥ 98%. This work used 1% m (TBAB) / m (NaHSO3) with a m (IPA): m (H2O) ratio of 1 : 1; the NaHSO3 was prepared in a 15% aqueous solution. The n (HSO3−) : n (-NCO) ratio was greater than 1 : 1. The stirring speed was 200 r/min, the reaction temperature was 30 °C, and the reaction time was 2.5 h. FT-IR was used to analyze the changes in the main functional groups during the blocking reaction. TG and DCS were used to analyze the deblocking characteristics of water-soluble blocked IPDI. This paper also studied the effect of the blocked IPDI on paper properties. Experimental results showed that the strength and water resistance of paper made from the treated fiber can be remarkably improved by using water-soluble blocked IPDI.
Keywords: Isocyanate blocking; Isophorone diisocyanate; Fiber modification; Polyurethane
Contact information: a: State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan 250353 P.R. China; b: State Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, P.R. China; c: Shandong Century Sunshine Paper Group Company Limited, Weifang 261000, P.R. China; ‡ These authors contributed equally to this article; *Corresponding author: wuqin19820107@163.com
INTRODUCTION
The chemical activity of isocyanates is based on the -NCO group. This group has a highly unsaturated bond structure (-N=C=O) with an overlapping double bond arrangement that can react with various active hydrogen-containing compounds. Therefore, isocyanate is widely used in the preparation of coatings, adhesives, and other materials (Zuo et al. 2008; Jung et al. 2018; Matsuura et al. 2018; Chowdhury et al. 2019). Isocyanates can easily react with water because of the extremely active chemical nature of the -NCO group; this restricts isocyanates in many applications. The -NCO group can be protected via blocking, which makes it inactive. When necessary, the blocked compound is deblocked under certain conditions to regenerate the isocyanate group. This approach is favorable for reducing the toxicity of the monomer and increasing long-term storage stability (Delebecq et al. 2013). In recent years, some new technologies have been applied to protect isocyanate (He et al. 2019; Ma et al. 2019; Lubis et al. 2020). However, the chemical blocking method is still the most widely used in industry (Xiong et al. 2005; Lu et al. 2016). The commonly used blocking agents are mainly alcohols, phenols, terpenes, amines and amides, active methylenes, pyrazoles, and triazoles (Liu and Ma 2019). Inorganic salt blocking agents promote the development of isocyanate blocking techniques from organic materials to inorganic materials because of their low pollution and easy separation. The blocking mechanism is shown in Fig. 1 (Huang et al. 2011; Yang et al. 2011).
Fig. 1. Reaction mechanism of sodium bisulfite blocking (-NCO)
In particular, the isocyanate blocked with bisulfite offers good water dispersibility, low blocking temperature, and rapid deblocking when heated (Sun et al. 2008). This is a green process with low energy consumption, high stability, and simple operation. Prior results show that the blocking degree of different isocyanates blocked by bisulfite ranges from 56% to 85% (Huang et al. 2011), but full blocking cannot yet be achieved because some -NCO groups remain completely lost. Increasing the blocking degree to more than 98% is conducive to the long-term stability of isocyanates with a broadened scope of use.
Plant fibres contain many hydroxyl groups that can easily absorb water and swell; this limits their utility in paper-based packaging materials (paper used as a packaging material must have good strength and water resistance). At the same time, the hydroxyl groups on the fibre surface can undergo urethane polymerization with isocyanate (Han et al. 2020) (Eq. 2). This provides a new solution for fibre modification: The hydroxyl groups in fibre undergo a urethane reaction with isocyanate under certain conditions, and the hydrogen bonding between the fibres is replaced by a more stable polyurethane structure. This enhances the physical strength of the paper and increases water resistance.
RNCO + R′OH → RNHCOOR′ (1)
Fig. 2. Urethane reaction between isocyanate and hydroxyl group
In this paper, sodium bisulfite was used to block isophorone diisocyanate (IPDI) in the preparation of blocked-type water-soluble IPDI. The blocking degree was increased by optimizing the chemical addition method, catalyst, stirring speed, blocking temperature, blocking time, amount of solvent, concentration of blocking agent, amount of catalyst, and molar ratio of HSO3− and -NCO groups. Furthermore, the NaHSO3 produced highly blocked water-soluble IPDI to modify the fibre and study its impact on fibre strength and permeability resistance.
The study has a certain theoretical and practical value. First, the water-soluble isocyanate with high blocking degree (≥ 98%) was provided, which was remarkably higher than 56 to 85% in actual production. Second, when the blocked water-soluble isocyanate obtained in this study was used to prepare waterborne polyurethane materials, water was used as the reaction medium instead of organic solvent, which has advantages of being safer, environmentally friendly, and saving of energy. Third, this experiment provided a simple and rapid method to enhance the strength of plant fiber.
EXPERIMENTAL
Materials
The isopropanol alcohol (IPA) labelled as analytically pure grade and used as a reaction solvent was purchased from the chemical reagent factory Tianjin FuYu (Tianjin, China). The sodium bisulfite (NaHSO3) labelled as analytically pure grade and used as isocyanate sealant was purchased from the chemical reagent factory Shanghai Macklin (Shanghai, China). Isophorone diisocyanate (IPDI, 99% pure) was purchased from the chemical reagent factory Shanghai Macklin (Shanghai, China). Tetrabutyl ammonium bromide (TBAB, 98% pure), hexadecyl trimethyl ammonium bromide (CTAB, 98% pure), and hexadecyl trimethyl ammonium chloride (CTAC, 98% pure), used as the phase transfer catalyst, were purchased from the chemical reagent factory Shanghai Bidepharm (Shanghai, China). Wood fiber as a type of eucalyptus pulp was provided by a pulp mill in Shandong province (Weifang, China).
Selection of Chemicals Addition Methods
The effect of chemicals addition methods on the IPDI blocking degree was explored under the following conditions. The amount of NaHSO3 and IPDI was controlled at n(HSO3−): n(-NCO) = 1:1, and the amount of tetrabutyl ammonium bromide (TBAB) was controlled at m(TBAB): m(NaHSO3) = 1%. The NaHSO3 was prepared in an aqueous solution with a concentration of 15%, and the amount of IPA was controlled at m(IPA) : m(H2O) = 1:1. The stirring speed was 500 r/min, the reaction temperature was 30 ℃, and the reaction time was 3 h.
Selection of Phase Transfer Catalysts
It was difficult to block isocyanate using NaHSO3 directly, because the raw materials were in the water phase and the organic phase separately, and the two were incompatible with each other. Phase transfer catalysis can make ionic compounds react with water-insoluble organic substances in low-polarity solvents or accelerate these reactions. Quaternary ammonium salt catalysts are commonly used phase transfer catalysts that play a key role in the blocking reaction (Zhang and Zhao 2010). Preliminary experiments also compared the performance of the three phase transfer catalysts TBAB, CTAB, and CTAC.
The effect of different phase transfer catalysts on the IPDI blocking degree was explored under the following conditions. The amount of NaHSO3 and IPDI was controlled at n(HSO3−): n(-NCO) = 1:1, The NaHSO3 was prepared in an aqueous solution with a concentration of 15%, and the amount of IPA was controlled at m(IPA): m(H2O) = 1:1. The stirring speed was 350 r/min, the reaction temperature was 30 ℃, and the reaction time was 3 h. The amount of phase transfer catalysts was controlled at m(catalyst): m(NaHSO3) = 1%, where “catalyst” is TBAB, CTAB, and CTAC. The chemicals were added in method c (Fig. 3.).
The Effect of Different Factors on the IPDI Blocking Degree
A single factor experiment was used to study the effect of IPA, stirring rate, reaction temperature, reaction time, concentration of NaHSO3, n(HSO3−): n(-NCO), and TBAB on the IPDI blocking degree, as shown in Table 1. The amount of IPA used in the IPDI blocking reaction system was controlled by changing the mass ratio of IPA to water, namely, m(IPA): m(H2O). The amount of TBAB used as phase transfer catalyst was controlled by changing the mass ratio of TBAB to NaHSO3, namely, m(TBAB)/ m(NaHSO3). The amount of NaHSO3 and IPDI were controlled by changing the mole ratio of HSO3− to -NCO, namely, n(HSO3−): n(-NCO).
Table 1. Single Factor Experimental Design Table
Synthesis of Water-soluble Highly Blocked IPDI
The experimental procedure of NaHSO3-blocked IPDI was as follows: NaHSO3 was weighed and placed in a beaker followed by the phase transfer catalyst TBAB. After the mixture was stirred evenly, IPA was added to obtain a mixed solution. The IPDI was placed in a dry four-necked flask that was purged with nitrogen and heated in an oil bath. The mixed solution containing “NaHSO3/TBAB/IPA” was placed in a constant-pressure dropping funnel and slowly added into a four-necked flask at a rate of 2 to 3 mL/min. The stirring rate and reaction time were controlled. After the reaction, no precipitated solids were observed at the bottom of the four-necked flask, and the solution was clear with no layers. The NaHSO3-blocked water-soluble IPDI was thus obtained.
Blocked Ratio Test of Isocyanate
The test steps were as follows (Gu et al. 2006):
a. A total of 0.1 g of sample was added into a dry and clean Erlenmeyer flask.
b. Next, 18 mL of water was added into the Erlenmeyer flask, and 30 mL of IPA was added after a homogeneous solution was formed after stirring.
c. Iodine solution was used for titration. The titration was stopped and data V1 was recorded when the solution in the Erlenmeyer flask became light yellow.
d. Then, 10 mL of 7.5 mol/L NaOH was added into the Erlenmeyer flask and the solution was shaken well. After the solution was allowed to stand for 10 min, 1 drop of 1% phenolphthalein solution was added into the Erlenmeyer flask. Then, the solution in the Erlenmeyer flask became red.
e. A total of 2.5 mol/L H2SO4 was added dropwise until the red color disappeared. A few more drops of dilute H2SO4 were then added to ensure that the solution was acidic.
f. The titration was then continued with iodine solution. When the solution in the Erlenmeyer flask became light yellow, the titration was stopped and V2 data were recorded.
Steps a through c were used to determine the content of the remaining sodium bisulfite that did not block -NCO.
Steps d through f determined that the content of sodium bisulfite generated after deblocking of -NH-CO-SO32− under the action of NaOH; free -NCO was eventually converted to -NH-CO-NH-.
S = V2/(V1 + V2) × 100% (2)
I = S × fx (3)
where S is the reaction rate of -NH-SO-SO32−, I is the blocking degree of -NCO group, and fx is the molar ratio of NaHSO3 to -NCO
Preparation of Modified Fibre by Water-soluble Highly Blocked IPDI
Eucalyptus pulp fibre was transformed into a circular fibre sheet with a basis weight of 100 g/m2 on a sheet former (RK-3A; Protein Technologies, Inc., Tucson, Austria). The fibre sheet was evenly impregnated with the prepared water-soluble blocked IPDI on an automatic coater at room temperature (Model AFA-IV; Wanjiang Weida Instrument Business Department, Dongguan, China) with a dose of 15 g/m2. The fibre sheet was dried at 30 ℃ for 6 h, and then hot pressing was performed using a hot press (CREE-6014A; Kerui Instrument Technology Co., Ltd., Dongguan, China) at 0.01 MPa for 10 min to deblock the IPDI; the released isocyanate groups (-NCO) underwent a urethane reaction with the hydroxyl group (-OH) of cellulose to produce a IPDI-modified fibre sheet.
The tensile strength of modified fiber sheet was measured using the method of GB/T 12914 (2008) by the tensile strength testing instrument (Code 066, Lorentzen & Wettre, Sweden). The specimens were 25 mm long and 15 mm wide. The tensile elongation rate was (20±5) mm/min.
The tearing strength of modified fiber sheet was measured using the method of GB/T 455 (2002) by tearing tester (Elmendorf, TSE-A007B, China). The sample is 75 mm long and 63 mm wide.
The water absorption of the modified fiber sheet was measured using the method of GB/T 1540 (2002). The sample is with a diameter of (125±5) mm.
Methods
Fourier-transform infrared spectroscopy (FT-IR)
The NaHSO3-blocked IPDI solution was freeze-dried, and a small amount of the freeze-dried sample was ground with KBr into a powder at a ratio of 1:60 to prepare the test sample for IR measurements (Tensor 37; Bruker, Karlsruhe, Germany). At the same time, IR measurement was performed on the samples after deblocking the IPDI. The functional group changes of the IR spectra of IPDI before and after deblocking were detected. The measurement used a scan range of 600 cm−1 to 4000 cm−1, with a resolution of 4 cm−1, and 32 replicate scans.
Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) analysis
In an alumina crucible, 5 to 10 mg of freeze-dried NaHSO3-blocked IPDI sample was added, and TG analysis (STA449 F3; NETZSCH, Selbu, Germany) was performed by heating up to 200 °C at a rate of 10 K/min under N2. The measurement conditions included a measurement temperature of 35 to 200 °C, heating rate of 10 K/min, and nitrogen scan rate of 50 mL/min.
Scanning electron microscopy (SEM)
A fibre was hot pressed and evenly impregnated with NaHSO3-blocked IPDI at 140 °C; the resulting sample was analysed with SEM (Quanta 200; FEI, Hillsboro, OR, USA) to observe its surface morphology. The working voltage of the SEM was 5 KV.
RESULTS AND DISCUSSION
Selection of Chemical Addition Methods
The method of chemical addition is shown in Fig. 3.
Method a: The TBAB and IPA were mixed first, then dropped into IPDI with NaHSO3 aqueous solution. The blocking degree of IPDI prepared by method a was 84.7%.
Method b: The IPA and NaHSO3 aqueous solution were mixed first, then dropped into IPDI with TBAB. The blocking degree of IPDI prepared by method b was 83.5%.
Method c: The IPA, TBAB, and NaHSO3 aqueous solution were mixed first, then dropped into IPDI. The blocking degree of IPDI prepared by method c was 90.9%.
Method d: The IPDI and IPA were mixed first, then dropped into the mixture of TBAB and NaHSO3 aqueous solution. The blocking degree of IPDI prepared by method d was 88.6%.
Fig. 3. Different chemical addition methods for synthesis of water-soluble blocked IPDI using NaHSO3
In summary, method c was clearly better. The chemicals were added in method c so that the TBAB that was used as phase transfer catalyst could be added to the aqueous solution of NaHSO3 first; thus, the NaHSO3 can combine with TBAB, which has a lipophilic chemical structure. As a result, NaHSO3 was better dissolved in IPA. Thus, the chance of NaHSO3 reacting with IPDI in IPA increased so that the blocking degree was improved.
Selection of Phase Transfer Catalyst
The results are shown in Table 2. The TBAB showed good performance as a phase transfer catalyst.
Table 2. Effects of Different Phase Transfer Catalyst on the Blocking Degree of IPDI
The Effect of Different Factors on the IPDI Blocking Degree
The effect of IPA on the IPDI blocking degree
The IPDI is an oily hydrophobic substance, and NaHSO3 is an ionic compound that dissolves in water. The two are incompatible with each other. As a reaction solvent, IPA can be miscible with both IPDI and water to provide a homogeneous system for chemical reactions between IPDI and NaHSO3 (NaHSO3 can be transferred from the aqueous phase to the IPA solvent phase under the action of a phase transfer catalyst). The results are shown in Fig. 4(a). As the amount of IPA used increased, the IPDI blocking degree first increased and then decreased. The blocking degree was quite low when the amount of IPA was excessively low; this was because a small amount of IPA led to a small capacity to contain IPDI and NaHSO3, which cannot cause all IPDI and NaHSO3 in the system to fully contact to trigger a chemical reaction. Thus, there was a decrease in the blocking degree. The blocking degree also decreased when the amount of IPA used was excessively large. The results showed that with an increasing amount of IPA, NaHSO3 could precipitate out of the reaction system in the form of crystals to reduce the occurrence of the chemical reaction of NaHSO3 and IPDI. This decreased the blocking degree. According to the literature (Wang 2010), the ratio of hydrophilic groups and hydrophobic groups in the solvent molecular system will affect the solubility of the solute. Adding alcohol to an aqueous solution will reduce the polarity of the solvent. Thus, the solubility of the solute dropped according to the principle of “like dissolves like.” This is why NaHSO3 will gradually precipitate in the form of crystals from the system as the amount of used IPA increases. In summary, the IPDI blocking degree was the highest (86.96%) when m(IPA): m(H2O) was 1 : 1.
The effect of stirring rate on the IPDI blocking degree
The experimental results are shown in Fig. 4(b). With increasing stirring speed, the blocking degree gradually increased because a relatively high stirring rate was conducive to the dispersion and dissolution of IPDI and NaHSO3 in the reaction system. This increased the reaction probability of the two. The blocking degree did not change substantially when the stirring rate exceeded 400 r/min; thus, a stirring rate of 400 r/min was suitable. At this time, the highest blocking degree was 98.65%.
Effect of reaction temperature on the IPDI blocking degree
The experimental results are shown in Fig. 4(c). The blocking degree first increased and then decreased as the temperature was increased. When the temperature was relatively low, the solubility of NaHSO3 in the solvent system decreased, and the NaHSO3 crystals precipitated. There was incomplete chemical reaction with IPDI. In addition, at low temperature conditions, the rate of chemical reaction between NaHSO3 and IPDI was also low so that the blocking degree was low. The blocking degree decreased when the temperature was higher than 30 ℃. Although an increase in reaction temperature improved the reaction between NaHSO3 and IPDI, it also increased the probability of the reaction between IPDI and water. This combined effect led to a decrease in the blocking degree. In summary, the optimal temperature for blocking IPDI with NaHSO3 was 30 ℃, and the highest blocking degree was 98.65%.
Effect of reaction time on IPDI blocking degree
The blocking degree gradually increased and stabilized with longer reaction time, as shown in Fig. 4(d). The blocking degree no longer increased when the reaction time reached approximately 2.5 h, indicating that the reaction was completed. The blocking degree was 98.65% at this time.
Effect of NaHSO3 concentration on IPDI blocking degree
The experimental results are shown in Fig. 4(e). With increasing NaHSO3 concentration, the blocking degree tended to increase first and then decrease. The blocking degree was low when the NaHSO3 concentration was relatively low because there was a large amount of water in the reaction system; thus, the chance of IPDI reacting with water increases. The amount of water in the reaction system was low when the concentration of NaHSO3 was high. These conditions were not conducive to the dissolution of NaHSO3. As a result, NaHSO3 precipitated in the form of crystals, and the chance of NaHSO3 reacting with IPDI was reduced; thus, the blocking degree was also low. In summary, the blocking degree was the highest (98.65%) when the concentration of NaHSO3 was 15%.
Fig. 4. The effect of different factors on the IPDI blocking degree
Effect of n(HSO3−): n(-NCO) on the IDPI blocking degree
The results are shown in Fig. 4(f). The blocking degree was relatively low when n(HSO3−): n(-NCO) was less than 1.125:1; flocculent precipitates were found at the bottom after the solution was allowed to stand for a period of time. The amount of NaHSO3 was small, and the -NCO group of IPDI could not be completely blocked when the blocking degree was low. There was sufficient HSO3− to react with -NCO when n(HSO3−) : n(-NCO) was greater than 1.125:1; the blocking degree was as high as 103%. However, excessive NaHSO3 will precipitate out of the reaction system in the form of crystals (Zhang et al. 2011). The amount of NaHSO3 should not be too high to save raw materials.
Effect of the amount of phase transfer catalyst TBAB used on the blocking degree of IPDI
The results are shown in Fig. 4(g). With increasing TBAB content, the blocking degree showed a gradual increase, which then stabilized. The blocking degree was low (only 65%) when no TBAB was used; while the blocking degree was 98.65% with 1% TBAB, demonstrating that the phase transfer catalyst was conducive to the blocking reaction.
In summary, the better experimental conditions for preparing water-soluble blocked IPDI using NaHSO3 were as follows: m(TBAB)/m(NaHSO3) was 1%, m(IPA) : m(H2O) was 1:1, the concentration of the NaHSO3 solution was 15%, the stirring speed was 400 r/min, the reaction temperature was 30 ℃, and the reaction time was 2.5 h. When n(HSO3−): n(-NCO) was 1:1, the blocking degree was 98.3%. When n(HSO3−): n(-NCO) was 1.125:1, the blocking degree was 103%.
IR Spectral Analysis
To determine the structural changes of the functional groups before and after IPDI blocking and the reaction mechanism, the authors performed IR spectroscopy on the blocked samples and the deblocked samples (Fig. 5).
Fig. 5. IR spectra of blocked IPDI before and after deblocking
The FT-IR spectrum of the blocked IPDI shows that the peak intensity at 980 cm−1 was considerably reduced; this peak corresponded to the stretching vibration absorption peak of S-OH, indicating that NaHSO3 was remarkably consumed. The peak at 2267 cm−1 disappeared, and this peak was assigned to the vibration absorption peak of –NCO, indicating that the characteristic functional group -NCO of IPDI was almost completely blocked. In addition, the absorption peak at 3500 cm−1 to 3200 cm−1 broadened. This peak is the absorption peak of N-H. When NaHSO3 was used to block IPDI, there was an electrophilic substitution reaction between -NCO and HSO3−. The active hydrogen of HSO3− was transferred to N, and one N-H bond was formed. The absorption vibration peak of C=O was at 1692 cm−1, the bending vibration absorption peak of C-N-H was at 1526 cm−1, and the stretching vibration absorption peak of C-N was at 1220 cm−1. The appearance of these peaks indicated that HSO3− was connected to -NCO to produce the -NHCO- structure.
The IPDI deblocked at a specific temperature. The FT-IR spectrum shows that the absorption peak of -NCO reappeared at 2267 cm−1. This indicated that the IPDI blocked by NaHSO3 can be deblocked under certain conditions (Hatakeyama et al. 1995).
TG and DSC Analysis
The samples prepared via the optimal conditions above were first freeze-dried to obtain solid powder samples. This helped identify the optimal deblocking temperature for blocked IPDI. The samples were then analyzed by TG and DSC (Fig. 6).
Fig. 6. TG analysis (a) and DSC analysis (b) of the blocked IPDI
The TG analysis curve in Fig. 6(a) shows that the thermal degradation began at 120 ℃, and the thermal degradation efficiency was highest at 144 ℃, indicating that a decomposition reaction occurred. The DSC curve in Fig. 6(b) also shows an endothermic peak at 120 ℃; the peak was highest when the temperature rose to 147 ℃. This shows that the process was endothermic. In summary, the blocked IPDI prepared here undergoes an endothermic decomposition reaction and successfully releases -NCO so that the deblocking occurred at approximately 145 ℃ (Zhang et al. 2011). The blocked IPDI prepared here had a higher blocking degree, and there were more -NHCO- bonds formed in the solid sample. The interaction force between the molecules was strong, and the molecules were difficult to decompose resulting in a higher decomposition temperature than other reports.
Improvement of Fibre Properties
The fibre was modified by water-soluble blocked IPDI in this paper. The tensile index and tearing index were increased, and the Cobb value decreased, as shown in Fig. 7, which indicated the strength and water resistance of the fiber can be remarkably improved using water-soluble blocked IPDI. Each index achieved the best value under a temperature of 145 ℃ that was the optimal unblocked temperature of water-soluble blocked IPDI. The blocked IPDI totally released the isocyanate groups (-NCO) to react with the hydroxyl group (-OH) of cellulose to form polyurethane structure with good strength and water resistance when hot pressing was performed to 145 ℃.
Fig. 7. Effect of water-soluble highly blocked IPDI on fibre properties
Fig. 8. SEM images of original fibre (a) and IPDI-treated fibre (b)
SEM Analysis
The authors performed SEM analysis on the original fibre (a) and the IPDI-treated fibres (b) to better observe the chemical binding effect of IPDI on the fibres (Fig. 8). Figure 8(a) shows the smooth fibre surface with fibres disorderedly alternating. Figure 8(b) shows the fibre surface after IPDI treatment; there were many cements between the fibres. This was because the -NCO that was released after deblocking the blocked IPDI aqueous solution underwent an electrophilic substitution reaction with the -OH group on the surface of the fibre to form -NHCOO-functional groups with a higher binding energy and better hydrophobicity. In addition, there was a stronger chemical bond between the fibres to ensure that the fibre had better strength and water resistance.
CONCLUSIONS
- Sodium bisulfite was used as the blocking agent, isopropyl alcohol (IPA) was used as the reaction solvent, and tetrabutylammonium bromide (TBAB) was used as the phase transfer catalyst to prepare water-soluble sodium bisulfite-blocked isophorone diisocyanate (IPDI) with a blocking degree of ≥ 98%. The chemical addition method, catalyst type, stirring speed, blocking temperature, blocking time, amount of reaction solvent used, concentration of blocking agent, amount of used catalyst, and molar ratio of HSO3− to -NCO groups were optimized.
- The blocking degrees were 98.3% and 103% at n(HSO3−) : n(-NCO) ratios of 1 : 1 and 1.125 : 1, respectively, under the optimized conditions where m(TBAB)/m(NaHSO3) was 1%, m(IPA) : m(H2O) was 1 : 1, NaHSO3 was prepared in a 15% aqueous solution, the stirring speed was 200 r/min, the reaction temperature was 30 ℃, and the reaction time was 2.5 h.
- The NaHSO3 strongly blocked the water-soluble IPDI prepared in this experiment and modified the fibre. The results showed that the fibre strength and water resistance were remarkably improved.
ACKNOWLEDGMENTS
The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 22008127), the Certificate of China Postdoctoral Science Foundation Grant (2019M652388), the 2019 Key R & D Program of Shandong Province (Major Science and Technology Innovation Project, No. 2019JZZY010407), the National Natural Science Foundation of China (Grant No. 31800499), the National Key R & D Program of China (No. 2017YFB0308000). The authors would also like to thank LetPub (CE_200601K049) for its linguistic assistance during the preparation of this manuscript.
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Article submitted: July 11, 2020; Peer review completed: September 5, 2020; Revised version received and accepted: October 6, 2020; Published: October 15, 2020.
DOI: 10.15376/biores.15.4.9013-9027