Effect of microcrystalline cellulose on the preparation and performance of rigid polyurethane foam

Abstract

In order to study the effect of microcrystalline cellulose on the reaction kinetics of polyurethane, in this work, the multi-scale microcrystalline cellulose was added to the foaming system of multifunctional polyether and multifunctional MDI polyurethane. While the chemical reaction was carried out, it was found through in situ FTIR combined with in situ rheological analysis that what was different from the usual inorganic fillers, the hydroxyl on the surface of the microcrystalline cellulose could preferentially react with MDI to generate urethane under the action of the catalyst. In the initial 5–6 min of the reaction, the reaction of soft segment chain growth was the main reaction. Then the main reaction quickly converted to the cross-linking reaction, which greatly increased the viscosity of the system. The addition of microcrystalline celluloses accelerated the improvement of the cross-linking degree and viscosity of the system. The higher the surface hydroxyl content of microcrystalline cellulose, the more significant this trend become. In addition, although the amount of microcrystalline cellulose added was different, the ratio of the reaction rate of the isocyanate group with the hydroxyl group and the amine group eventually tended to be constant, which indicated that there was a stable reactivity rate in the gradual addition reaction during the cross-linking reaction. Combined with SEM analysis, it was found that 25–60 μm microcrystalline cellulose with large hydroxyl content could act as a nucleating agent when the addition amount was less than 0.1%, which was beneficial to increase the cell density and reduce the pore size and improved the impact performance of the foam. The microcrystalline cellulose with a length of more than 90 μm continuously penetrated through several cell walls and destroyed integrity of the cell structure, which would consequently reduce the impact strength of the foam. This paper provided theoretical guidance for polyurethane modified by microcrystalline cellulose.

Keywords

microcrystalline celluloses polyurethane foam kinetics mechanical properties

Introduction

Due to its excellent impact energy absorption performance, high mechanical strength and insulation, polyurethane¹ has been extensively applied in various industrial fields, such as automobiles, medical apparatus, construction, furniture, and electrical appliances.² The amount of polyurethane foam accounted for more than 55% of the total polyurethane output. Therefore, polyurethane had received extensive attention and in-depth research both in theory^3–5 and practical applications^6–9.

Fillers, including montmorillonite, nano-silica, titanium dioxide, zinc oxide, and iron oxide were commonly added in polyurethane foam to improve the mechanical and thermal properties. Their influence on polyurethane reaction kinetics had also been extensively studied.

Mercedes Santiago-Calvo¹⁰ studied the polymerization kinetics of nano-silica and montmorillonite on the polyurethane. The experimental results showed that the foams containing nano-silicas showed higher isocyanate conversions than those containing nanoclays, whereas those nanoparticles containing hydrophilic groups afford lower isocyanate conversions and hydrophilic nanoparticles enhanced the blowing reaction, giving higher ratios of urea groups. Akkoyun M¹¹ also studied the kinetic effects of nano-titanium dioxide, zinc oxide, and iron oxide. The results showed that regardless of nanofiller type, there was a critical surface area, of nanofillers, (about 30 m²) above which reaction kinetics increase as the surface area increases. However, these fillers had a certain influence on the reaction kinetics of polyurethane foam due to their own physical characteristics, but they did not directly participate in the chemical reaction of polyurethane. Feng et al.¹² synthesized nano-silica reinforced polyurethane composites by in situ polymerization. The experimental results showed that the silica and polyurethane molecules were connected by covalent bonds but this paper does not study its influence.

In view of the predecessors’ controversy over whether the hydroxyl groups on the surface of inorganic fillers directly participate in the synthesis reaction of polyurethane, the microcrystalline celluloses of natural fibers were selected as fillers to observe whether the hydroxyl groups on the surface of organic fibers can directly participate in and affect the chemical reaction of polyurethane and how the reaction affect the physical properties of polyurethane foam materials. A method to combine in situ infrared test and in situ rheological test was used to study the reaction kinetics of polyurethane to distinguish between chain growth and cross-linking reactions. And the structure of cell was explored to find how it influenced the mechanical properties.

Experimental

Experimental materials

Sorbitol polyether (OH index 540 mg KOH/g, viscosity 625 mPas, functionality 2.5–4) was provided by Nanjing Jurong Wuning Chemical Co 4,4-Diphenylmethane diisocyanate, MDI 5005, (30.75% NCO, viscosity 220 mPas, density 1.23 g/cm³); carbodiimide modified 4,4-diphenylmethane diisocyanate, MDI 2020 (29–30% NCO, viscosity 25–50 mPas) were both provided by Huntsman. Heat-sensitive catalyst SA-8, general-purpose catalyst 2-hydroxy-N, N, N-trimethyl-1-propylamine formate, TMR-2 and 33% triethylenediamine and 67% dipropylene glycol, their mixture, A-33, purchased from Xindian Chemical Materials (Shanghai) Co and Huntsman, respectively. 8832 silicone oil, with a viscosity of 1500–3500 mPas, purchased from Evonik Industries.

Microcrystalline celluloses, with lengths of 25, 40–60, 90–150, and 180–280 μm, were provided by Macleans; Microcrystalline celluloses, 50–200 μm are provided by Shanghai Yuanju Biotechnology Co

Synthesis of the polyurethane foams

Microcrystalline celluloses of different lengths were selected as the fillers for modification of polyurethane foam. The addition of each microcrystalline celluloses was fixed 1 wt% of the total polyurethane foam.

All the microcrystalline celluloses needed to be dried at 100°C for 2 hours before use. Then they were added into 6216 polyether polyol together with 8832 silicone oil foam stabilizer, catalyst and foaming agent (water), and mixed with 1200 r/min impeller speed for 10 min to obtain material A. Similarly, MDI 5005 and MDI 2020 were mixed uniformly at a mass ratio of 8:2 to obtain material BA and B were mixed at 100:150 mass ratio (equivalent ratio of polyether and composite MDI was 1:1.13) for 30 s under 2400 r/min of stirring. The mixture was further molded at 80°C for 120 min. Afterward, the cured products were cut into prepare 80×10×4 mm³ samples for further measurements.

Characterization

Quantitative analysis of hydroxyl numbers on the surface of microcrystalline cellulose

The hydroxyl numbers were measured by reverse titration. First, acetylating agent solution (1g p-toluenesulfonic acid dissolved in 80 mL ethyl acetate and 9 mL acetic anhydride), pyridine solution (pyridine and distilled water 2:1 V/V), phenolphthalein reagent (1g phenolphthalein dissolved in 100 mL without Water ethanol) and 0.5 mol/L sodium hydroxide standard solution were prepared.

Then, 2.5 g of microcrystalline cellulose and 10 mL of acetylating agent solution were added into an iodine flask. The mixture in the iodine flask was further ultrasonic treatment for 5 min. Then the iodine flask was put in a water bath at 65°C, kept for 30 min and added 20 mL of pyridine solution after the mixture in the flask was cooled to room temperature. 0.25 mL of phenolphthalein reagent were added to the iodine flask. Using phenolphthalein as the indicator, the solution had been titrated with sodium hydroxide standard solution, until the solution become pink and maintained for 3 min without fading.

The number of hydroxyl groups on the surface of the microcrystalline cellulose was calculated according to equation (1)

Surface hydroxyl number (per/g) = (V_{s} - V_{t})\times C_{NaOH} \times N_{A} / M

(1)

V_s------ The volume(mL) of sodium hydroxide solution consumed to titrate the acetylation reagentV_t------ The volume(mL) of sodium hydroxide solution consumed to titrate the microcrystalline celluloseC_NaOH ------ Molar concentration of sodium hydroxide solutionN_A ------ Avogadro constantM ------ Mass (g) of microcrystalline cellulose

Foaming kinetics: In situ found through in situ infrared measurements

Isocyanate analysis

The attenuation of the isocyanate group absorption band (located at 2500–2000 cm⁻¹) was used to quantify the conversion of isocyanate groups during the polymerization process. The conversion rate of isocyanate groups ( $P_{N C O}$ ) was defined by equation (2) ¹⁰

P_{N C O} = \frac{A_{0} - A_{m a x}}{A_{0}}

(2)

A₀----- Extrapolated absorbance of isocyanate group absorption band at time = 0A_max----- The total absorbance of the isocyanate group absorption band at time = t (measured as the area under the isocyanate group absorbance curve)

Amide I region analysis

The increase in the peak area of the amide group stretching vibration in the range of 1640–1740 cm⁻¹ could be used to calculate the generation amount of urea and urethane groups. The peak area was obtained by Gaussian function deconvolution method for the overlapping absorption of the amide I region (carbonyl region ranging from 1740 to 1640 cm⁻¹). The absorption bands higher than 1700 cm⁻¹ are attributed to the urethane, while the lower bands belong to the urea group.¹³

In situ infrared experimental

20 g of polyether polyol, 30 g of isocyanate and a small amount of heat-sensitive catalyst were mixed at 1200 r/min for 20 s. Then 0.5 mg of the mixture were evenly smeared on the potassium bromide salt tablet. Then the tablet was placed on the FTIR (Nicolet 6700) test bench, and the temperature was set at 80°C for 60 min. The in situ test was performed to obtain the law of infrared spectrum changes over time. FTIR spectrum was acquired every 1 min. Each spectrum was scanned 16 times with a resolution of 4 cm⁻¹ and a range of 4000–400 cm⁻¹. The asymmetric C-H stretching vibration absorption band at 2971 cm⁻¹ was used as a reference, because this group didn’t undergo any chemical reaction in the in situ reaction process.^12,13 The results were the average of three experiments. The maximum error of isocyanate conversion rate was 3%, and the maximum error of urea group and urethane quantification was 8%.

The previously reported method^14,15 was used to separate the overlapping absorption peaks in the amide I region (carbonyl region) by Gaussian function deconvolution method.

Foaming kinetics: In situ Rheological measurements

About 0.5 mg of the Reactant were taken and evenly smeared on the rotor of the Mars three Rotary Rheometer, Thermo Hakke, USA. The foaming kinetics was investigated following the similar procedure as described in the section In situ infrared experimental. The temperature control program of reaction was started for 20 min under 80°C constant temperature condition with the shear rate $\dot{γ}$ at 0.01s–1, and the viscosity curve with time was obtained.

Effect on the shape of polyurethane foam

The foam cell structure was observed by the Hitachi S-4800 scanning electron microscope (SEM). The cured foam was brittlely cooled with liquid nitrogen to obtain a natural cross-section.

Effect on the mechanical properties of polyurethane

The samples were cut into a size of 80×10×4 mm³, and the impact strength was measured using a CEAST 9050 Impact Tester. Since it was a foamed material, in order to control the impact of material density on impact strength, only the splines with the density of 780 ± 30 kg/m³ were selected for testing, and each value was determined from the average of five tests.

Results and discussion

Determination of hydroxyl on the surface of microcrystalline cellulose

The number of hydroxyl groups on the surface of each kind of microcrystalline cellulose was analyzed by reverse titration, and the results were shown in Table 1. It was not difficult to find that the smaller length of the microcrystalline cellulose had the greater the specific surface area, so the unit mass of the microcrystalline cellulose also contained a greater number of hydroxyl groups. Since these hydroxyl groups could react with acetylating agents, it meant that they might react with isocyanate groups as well. Unlike the hydroxyl groups on the surface of inorganic powders of nano-titanium dioxide, zinc oxide, and ferro oxide, they could not react with isocyanate directly.¹¹

Table 1.

The number of hydroxyl groups on the surface of microcrystalline celluloses.

Name	A	B	C	D	E
Length	25 μm	40–60 μm	90–150 μm	180–280 μm	50–200 μm
Surface hydroxyl number (×10¹⁹per/g)	13.244	12.040	9.632	5.612	7.224

Foaming kinetics: In situ found through in situ infrared measurements

Reaction of MDI and cellulose

Infrared measurements were performed at 2 min and at 40 min of reaction after mixing the MDI and the microcrystalline cellulose uniformly. The results were shown in Figure 1. It could be seen that the absorption peak at 3400 cm⁻¹ on the surface of the fiber was significantly reduced, which indicated that the hydroxyl groups on the surface of the microcrystalline cellulose participate in the reaction. Similarly, the absorption peak of the isocyanate group at 2270 cm⁻¹ also dropped significantly, which once again confirmed that the hydroxyl group on the surface of the microcrystalline cellulose was different from the hydroxyl group of the inorganic filler and could react with MDI.

Analysis of the reaction of isocyanate with different microcrystalline celluloses

The equivalent ratio of polyether 6216 to composite MDI was set to approximately 1:1.13. Considering that isocyanate was prone to react with water, 6216 and MDI were approximately equivalent to the equivalent ratio, which was formulated into pure polyurethane resin. In addition, according to 2.2.2, microcrystalline celluloses of different scales with a mass fraction of 1% were added to obtain six samples of PU-A, PU-B, PU-C, PU-D, and PU-E. Based on the polymerization conversion rate of MDI, the polymerization kinetics was observed as shown in Figure 2. It could be clearly seen that although the initial conversion rate of adding different microcrystalline cellulose samples was different, but when the polymerization conversion rate was only 45% at about 25 min, they were all reach a slowly rising and stable stage. But even after 60 min, the isocyanate could not be completely reacted. Judging from the reaction kinetics point of view, the stage of rapid increase in conversion rate indicated that the viscosity of the polymerization system must be in a relatively small liquid state. The increase in polymerization conversion rate slowed down rapidly at only 45%, which meant that the viscosity of the system was already very large and even cross-linking had occurred. It also can be seen from Figure 2 that compared with pure PU, the $P_{N C O}$ of the polyurethane with different microcrystalline celluloses was increased in the same time almost according to the number of hydroxyl groups on fiber surface. And the degree of conversions tended to be stable at 20 min. This clearly showed that the hydroxyl groups on the surface of the microcrystalline cellulose were highly reactive, and the reaction was completed quickly at the beginning of the reaction. It was probably because the microcrystalline cellulose was enriched in the catalyst due to the similar polarity, which made the hydroxyl groups on its surface easier to react.

Amide I region analysis

Figure 3(a) showed the change of the infrared spectrum of the amide I region over time. Figure 3(b), (c) showed that the deconvolution of the absorbance could obtain the different carbonyl content and its comparison in the reaction mixture. According to the early research of Elwell et al,¹² the infrared spectrum of polyurethane amide I zone was basically 1700 cm⁻¹ as the dividing line, the absorption peak higher than 1700 cm⁻¹ was attributed to the absorption of urethane, and the low one belonged to the absorption peak of urea group. The relative area percentage of each peak was obtained by the corresponding area divided by the total amide I area (∑A_c=oUrea+∑A_c=oUrethane=∑A_{amide I}). Therefore, the relative area percentages of all urethanes and all urea groups were shown in equation (3)and ,equation (4) respectively. If the compound MDI reacted with a polyether polyol with more than two functionalities (similar to this article), even if the reaction only belonged to the chain growth of the soft molecular chain and urethane were formed at the beginning, once the multifunctional hydroxyl group participates in the reaction, cross-links would begin to form. If diamines were involved in the chain extension reaction, the reaction would belong to the chain extension reaction of the hard segment molecular chain with stronger hydrogen bonds and urea groups would be formed. Since the reactivity of the secondary amine was much lower than that of the primary amine, the temperature was not high enough before the late reaction period, and when the isocyanate group was excessive, the cross-linking caused by the reaction of the secondary amine would not occur. It could be seen that the cross-linking reaction in the foaming process should mainly come from the reaction between the composite MDI and the polyfunctional hydroxyl group. The example of the reaction to form urethanes and the reaction to form urea groups was shown in equation (5), which to a certain extent also showed the ratio of the cross-linking reaction to the chain extension reaction.

Relative Area % of total Urea = \frac{\sum^{​} A_{c = oUrea}}{\sum^{​} A_{c = oUrea} + \sum^{​} A_{c = oUrethane}} = \frac{\sum^{​} A_{c = oUrea}}{\sum^{​} A_{amide I}}

(3)

Relative Area % of total Urethane = \frac{\sum^{​} A_{c = oUrethane}}{\sum^{​} A_{c = oUrea} + \sum^{​} A_{c = oUrethane}} = \frac{\sum^{​} A_{c = oUrethane}}{\sum^{​} A_{amide I}}

(4)

Urethane/Urea ratio = \frac{Relative Area % of total Urethane}{Relative Area % of total Urea} = \frac{\sum^{​} A_{c = oUrethane}}{\sum^{​} A_{c = oUrea}}

(5)

According to the analysis of amide I area and equation (5), a function curve of the ratio of urethane and urea group with time was obtained as shown in Figure 4. It could be clearly seen that the reaction in which the soft segment chain grew to generate urethane was dominant at the beginning of the reaction, but its advantage gradually decreased as the reaction continues. The reaction change of PU was extremely gentle, which was similar to Figure 2. At about 25 min, the system entered the high viscosity stage, the reaction of soft segment chain growth was almost 2.5 times the reaction speed of the hard segment chain extension reaction. However, what was worth noting that, compared with pure polyurethane, the more the number of hydroxyl groups on the surface of the microcrystalline cellulose, the faster the reaction to generate urethane at the initial stage of the reaction. This again confirmed the inference of Figure 2 that the hydroxyl groups on the surface of the microcrystalline celluloses were more active and easier to react with isocyanate groups to form urethane. It should also be noted in Figure 4 that the ratio of urethane and urea groups of pure polyurethane stabilized at about 2.5 after 50 min, and the ratio of PU-A added with 25 μm microcrystalline cellulose was about 2.1. The other added amount of microcrystalline cellulose also had such a trend. Therefore, after 50 min of the reaction, the isocyanate group always reacted with the hydroxyl group and the amine group at the same rate ratio. This meant that even in a system where different microcrystalline celluloses were added, the final reaction rate of isocyanate groups with hydroxyl groups and the ratio of the reaction rates with amine groups tended to be constant. Although the concept of reactivity ratio of gradual addition reaction did not exist in classical polymer chemistry theory, Figure 4 seemed to give this concept even in the cross-linking system. In addition, Figure 4 also showed that the ratio of hydroxyl monomer to amine monomer in this study was correct. If the ratio of the two raw materials was reversed, the entire reaction process would not proceed smoothly.

The foaming of polyurethane originated from the reaction of isocyanate groups with water. The reactivity of isocyanate with water was lower than that of primary hydroxyl groups, but was equivalent to that of secondary hydroxyl groups. When there was no catalyst, the reaction speed was slower due to the poor affinity between water and isocyanate. Therefore, there should be no foaming reaction within 5 min before the polymerization reaction occurs. Only when the reaction temperature reached a higher temperature, the foaming reaction might occur.

In order to analyze the chemical reaction from another perspective, the following was the in situ rheological analysis of the polyurethane reaction at the same temperature as the in situ infrared.

Foaming kinetics: In situ rheological measurements

Figure 5 showed the change curve of the shear viscosity with time (the image has been smoothed) under the same 80°C temperature control conditions as the in situ FTIR. It could be seen that after about 6 min of reaction, the viscosity of the polyurethane resin added with microcrystalline celluloses of different lengths suddenly increased. Moreover, the shorter the added length of the microcrystalline cellulose, the earlier the time when the viscosity started to increase significantly, the faster the viscosity increased. Combined with the above-mentioned in situ infrared analysis, it could be considered that although the reaction to generate urethane in the in situ infrared had already occurred at the beginning, no major changes in viscosity were seen from the Figure 5. It fully showed that the main reaction took place before 6 min was the soft chain extension and the urethane was formed, while after 6 min, the reaction was mainly cross-linking and the urethane was formed. In addition, the shorter the length and the higher the surface hydroxyl content of microcrystalline celluloses, the faster and earlier the viscosity of the system would rise, showing obvious cross-linking characteristics, which was very consistent with the classical theory. Compared with Figure 2, it could be seen that in situ rheological detection could clearly show the cross-linking reaction of the system more clearly than in situ infrared spectroscopy detection. But the reverse also clearly showed that although the initial cross-linking reaction greatly increased the viscosity of the system, it had no effect on the chemical reaction of the system. This was very similar to what we found earlier in unsaturated polyester resins.¹⁶ Although the degree of cross-linking was very high, the copolymerization had conformed to the Mayo-lewis theoretical copolymerization equation until the polymerization conversion rate reached 60%.

Characterization of cell structure

The formation process of foamed plastics was determined by three Indispensable factors which were the continuous generation of gas, the presence of gas cores that facilitate gas adhesion and enrichment and the strength or tensile viscosity of the resin melt sufficient to cover the bubble pressure. The reactivity of water with isocyanate was lower than that of primary hydroxyl groups and was comparable to that of secondary hydroxyl groups. Referring to the Figure 2 again, it could be seen that the foaming reaction of the reaction of isocyanate groups with water to produce CO₂ gas should be at least 6–8 min after the start of the reaction and after the temperature had increased to a certain extent. At this time, the reaction rate of the hydroxyl group was not obviously in the decreasing stage, indicating that the main reaction was still the reaction with the primary hydroxyl group. Meanwhile, the viscosity of the system had been significantly different when different microcrystalline celluloses were added according to the Figure 5. The greater the viscosity of the system, the more restrained the rate of cell expansion, the smaller the expansion rate, and the thicker the cell wall. This situation was clearly shown in Figure 6. It could be seen that the more the number of surface hydroxyl groups of the added microcrystalline cellulose, the faster the gel reaction rate was, and the thicker the thickness of the pore wall would be. The density of polyurethane free foam in Figure 7 also showed this conclusion that PU-A had the highest free foam density and the density decreased with the amount of hydroxyl groups on the cellulose surface.

The influence of microcrystalline cellulose on the cell structure could be observed from Figure 8. PU was the cell structure of pure polyurethane foam, while PU-A was the cell structure with 25 μm microcrystalline cellulose. It was Obvious that The cell diameter of PU-A was about 168 μm, while the pore size of pure polyurethane foam was about 200 μm. The average cell diameter in Table 2 also showed the result that polyurethane with small particle size microcrystalline cellulose had smaller cells, larger cell density, and more uniformity. It indicated that microcrystalline celluloses acted as a nucleating agent. But it did not mean that adding any microcrystalline cellulose would play a role in favoring the foam structure. What could be clearly seen from the PU-D of Figure 8 (the cell structure of polyurethane foam added with 180–280 μm microcrystalline celluloses) was that the excessively long microcrystalline cellulose destroys the cell structure, which continuously penetrated through several cell walls, giving the perfect cell structure an irregular shape and even forming penetrating holes.

Table 2.

Average cell wall thickness of closed-cell rigid PU foam.

Sample name	PU	PU-A	PU-B	PU-C	PU-D	PU-E
Cell wall thickness (μm)	0.12 ± 0.04	0.54 ± 0.12	0.43 ±0.09	0.31 ± 0.08	0.25 ± 0.05	0.28 ± 0.03
Cell diameter (μm)	250 ± 45	163 ± 30	181 ± 28	206 ± 35	230 ± 55	195 ± 67

Impact strength of microcrystalline cellulose filled polyurethane foam

Table 3 showed the impact strength of microcrystalline celluloses with different lengths after filling with polyurethane foam. It could be seen that after adding cellulose with a shorter length, the mechanical properties of the material, such as PU-A and PU-B, could be improved with a small addition amount. This was because the cellulose with a small particle size could be used as a nucleating agent to improve the cell structure of polyurethane, making the cell diameter smaller and showing a microporous structure, so the impact strength was improved, see the Figure 8. However, it can been seen in Figure 8, the addition of a large amount of microcrystalline cellulose would significantly decrease the impact strength of the material. Compared with PU-A with the addition of 0.5%, the mechanical properties of PU-A with the addition of 1% dropped by about 21.5%. That is because there were more hydroxyl groups to participate in the reaction, which greatly increased the degree of cross-linking, and leaded to a rapid decline in the impact performance of the material. This phenomenon slightly widened the addition range of PU-B. This was because, on the one hand, the particle size of 40–60 μm cellulose was still small and could be used as a nucleating agent to improve the cell structure of polyurethane; on the other hand, the number of hydroxyl groups on the surface of microcrystalline celluloses was not reduced compared to 25 μm microcrystalline celluloses. Therefore, it could be seen from Table 3 that when the addition amount was from 0.05% to 0.1%, the impact strength had increased slightly, but after increasing the addition amount, the impact strength had dropped significantly. As for the addition of cellulose with a longer length, the impact performance of the material showed a downward trend. The reason was shown in Figure 8. The addition of long microcrystalline celluloses only destroyed the cell structure. The increase in the amount of microcrystalline cellulose leads to the decrease of mechanical properties.

Table 3.

Impact strength of different length microcrystalline celluloses and different concentrations of filled polyurethane foam.

Content (%)	Impact strength (kJ/m²)
Content (%)	PU-A	PU-B	PU-C	PU-D	PU-E
0	9.07 ± 0.98	9.07 ± 0.98	9.07 ± 0.98	9.07 ± 0.98	9.07 ± 0.98
0.05	10.23 ± 1.41	10.84 ± 1.33	8.93 ± 1.25	8.68 ± 1.35	9.45 ± 0.98
0.1	9.82 ± 1.04	11.68 ± 0.75	8.07 ± 1.48	7.32 ± 0.89	8.23 ± 1.05
0.5	7.55 ± 0.87	9.70 ± 1.09	7.45 ± 0.83	6.72 ± 0.97	7.75 ± 0.87
1	5.93 ± 0.64	7.27 ± 0.79	6.77 ± 0.77	5.54 ± 0.61	6.43 ± 1.02
2	4.11 ± 0.41	4.59 ± 0.94	5.08 ± 0.91	4.33 ± 0.82	4.41 ± 0.97

Conclusion

Microcrystalline celluloses of different scales were added to the multifunctional polyether and multifunctional MDI polyurethane foaming system. Unlike ordinary inorganic fillers, the hydroxyl groups on the surface of the microcrystalline cellulose could preferentially react with MDI to form urethane under the action of the catalyst. During the initial 5–6 min of the reaction of this system, the reaction of soft segment chain growth to form urethane was the main reaction, and then it was quickly transformed into the cross-linking reaction of urethane, which greatly increased the viscosity of the system, but not there was no chemical reaction that significantly affects the system. The addition of microcrystalline celluloses accelerated the improvement of the cross-linking degree and viscosity of the system. The higher the hydroxyl content on the surface of the microcrystalline cellulose, the more significant this trend was. In addition, the ratio of the reaction rate between the isocyanate group and the hydroxyl group and the reaction rate between the isocyanate group and the amine group eventually tended to be constant. This indicated that there was a stable reactivity ratio even in the gradual addition reaction during the cross-linking reaction. Furthermore, the foaming reaction between MDI and water occurred at least 5–6 min after the polymerization reaction occurred, at which time the system had reached a certain degree of cross-linking, and the viscosity had been greatly improved. The addition of microcrystalline celluloses greatly improved the cross-linking degree and viscosity of the system. Short and small microcrystalline celluloses with a large hydroxyl content could act as a nucleating agent when added to the system in a small amount, which was beneficial to increase the cell density and reduce the pore size, thus improving the impact resistance of the foam. However, adding a large amount will greatly increase the degree of cross-linking of the system, which in turn reduced the impact strength. The long-length microcrystalline celluloses would continuously penetrate through several cell walls, destroying the perfect cell structure, and consequently reducing the impact strength of the foam. This paper filled the gap in the reaction kinetics of microcrystalline cellulose-enhanced polyurethane.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Yong Guan

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