Effect of Boric Acid on the Foaming Properties and Cell Structure of

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Effect of Boric Acid on the Foaming Properties and Cell Structure of PVA Foam Prepared by Sc-CO Thermoplastic Extrusion Foaming 2

Yingbin Jia, Shibing Bai, Chul B Park, and Qi Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 21, 2017

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Industrial & Engineering Chemistry Research

Effect of Boric Acid on the Foaming Properties and Cell Structure of PVA Foam Prepared by Sc-CO2 Thermoplastic Extrusion Foaming Yingbin Jia †, Shibing Bai †, Chul B. Park* ‡, Qi Wang* † †

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu, 610065, China ‡

Microcellular Plastic Manufacturing Laboratory-Mechanical and Industrial Department, University of Toronto, Toronto, ON M5S 3G8, Canada

ABSTRACT: We prepared extruded polyvinyl alcohol (PVA) foams through combination of the PVA thermoplastic processing technology and the supercritical carbon dioxide extrusion foaming technology. Boric acid (BA) was used as a cross-linking agent to enhance the PVA’s melt strength and to improve the cell structure of its foam. Two different PVA/BA cross-linking models were discussed, and we confirmed the BA-compound formation model in our system. By applying the in-situ FTIR spectroscopy and the melt viscosity analysis, we found that the PVA/BA cross-linking structure was reversible. An increase in the BA increased the PVA’s melt strength as well as its foam volume expansion ratio and cell density, so that fine and uniform cells formed. However, at a higher die temperature of >140 °C, the cross-linking between the

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PVA and BA was gradually broken, and this resulted in serious bubble collapse and a worsened cell structure.

KEYWORDS: Boric acid, Polyvinyl alcohol, sc-CO2, Cross-linking, Extruded foam Introduction Polymer foams have been widely used in many fields because they have good properties: light weight, high specific surface area, good speciﬁc strength, excellent thermal insulation, heat preservation, and sound absorption.1, 2 The traditional polymer foams that are commonly used include, polystyrene, polyethylene, polypropylene, polyurethane and polyvinyl chloride. Unfortunately, none of these polymers is biodegradable. Thus, they create severe environmental problems. The urgent challenge is to replace them with a biodegradable plastic foam. Polyvinyl alcohol (PVA) is a biodegradable and biocompatible polymer with many other excellent properties. It can be degraded above 50% by some bacteria and enzyme in 10 days. And it is also more cost-effective than other naturally degradable polymers.3-5 Moreover, PVA is heat resistant (its thermal deformation temperature is 144 oC, significantly higher than that of HDPE (60-82 o

C), PP (102 oC) and PS (100 oC)), solvent resistant as well as antistatic. It also has good ion

exchange, adsorption, and wetting abilities.6-8 Due to its excellent properties, PVA foam has many potential applications in the biological, medical and environmental protection fields. Usually, PVA-based foam is prepared using the solution method, 8-10 but the complexity of this process, its high cost, and its toxic residue limit its applications in the biological and medical fields and elsewhere. Thermoplastic processing is a simple, clean, continuous and low-cost way to prepare PVA foam. However, due to the strong intra-molecular and inter-molecular interactions in PVA molecules, the melting point (226 °C) and the degradation temperature


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(200 °C -250 °C) of PVA are close to each other.11 Thus, it is difficult to prepare this foam directly using the thermoplastic foaming process. In our earlier studies, which were based on PVA’s intermolecular complexation and plasticization, we obtained a thermal processing window of more than 120 °C. Further, the thermoplastic processing of PVA, via such methods as extrusion, injection molding, or spinning was achieved by using water as the main plasticizer.11-13 We prepared the PVA foam by using water as both the plasticizer and the blowing agent.3, 4, 14 To further adjust the cell structure, we introduced supercritical carbon dioxide (sc-CO2) into our PVA foaming system. Supercritical CO2 extrusion foaming is a type of continuous plastic foaming technology, which was initiated and developed by Park et al. and other researchers during the 1990s.15-17 Polyvinyl alcohol foam that uses both sc-CO2 and water as co-blowing agents18 has a higher cell density, a smaller cell size, and a larger expansion ratio than PVA foam that uses only water as the single blowing agent. However, serious collapse of bubbles still occurred during the extrusion foaming process that used sc-CO2 and water as co-blowing agents, and especially at a high temperature with a high plasticizer content. This phenomenon can be attributed to two causes. One of these is the water’s higher saturated steam temperature. After the foam exited the die gate, its temperature sharply decreased. When the temperature was lower than the water’s saturated steam temperature, the steam would change into a liquid. This produced a sharp pressure drop inside of the bubbles, which was even lower than the standard atmospheric pressure outside of them. When the negative pressure difference was higher than the bubble wall’s strength, the bubble would collapse. Another cause could relate to the difference between the permeability of nitrogen and CO2 in PVA. Because the CO2’s permeability was greater than that of the nitrogen in the PVA, it was easier for the CO2 in the PVA bubbles to penetrate the PVA wall to the


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atmosphere. But it was difficult for the nitrogen to exchange with the CO2 in the atmosphere in a timely way. As a result, the pressure in the bubbles continued to drop, and the bubble collapse situation became more serious. Cross-linking is one of the effective ways to enhance a polymer’s melt strength. And boric acid (BA) is one of the most common and effective cross-linking agents used in PVA to enhance its mechanical property.19-21 Possible cross-linking structures that can exist between PVA and BA have been described in previous researches as the di-diol type.22-25 But recently, a different interaction model between PVA and BA has been demonstrated, that is the BA-compound formation, by using the solid-state nuclear magnetic resonance (NMR), X-ray single-crystal structure analysis and the Raman spectrum.26, 27 In the BA-compound formation cross-linkage, the cross-linking took place mainly through the hydrogen bonds. Therefore, the cross-linkage was temperature dependent. This means that it was weakened at a high temperature and that it would rebuild itself at a low temperature. It had also been the case in our thermoplastic PVA/BA mixtures. When the BA was introduced into the PVA’s extrusion foaming, the weakened crosslinking between them could keep the good flowability of thermoplastic PVA at a high temperature. At the same time, the rebuilding of this cross-linking at a lower temperature could significantly enhance the PVA’s melt strength and would thus avoid the bubble collapse situation. The introduction of the BA would also decrease the permeability difference between nitrogen and carbon dioxide in the PVA and would balance the exchange between the CO2 inside the PVA bubbles and the N2 in the ambient atmosphere. Thus, the pressure drop inside the PVA bubbles would be improved. In this research, BA cross-linked PVA extrusion foam was prepared by combining the thermoplastic technology of PVA and the sc-CO2/water extrusion foaming process. We studied


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the effect of the BA cross-linking agent on the PVA foaming behavior and resultant cell structure using in-situ FTIR analysis, viscosity analysis, thermal analysis and scanning electron microscopy (SEM) observation. Experimental Section Materials Polyvinyl alcohol (PVA) with a polymerization degree of 1700 and an alcoholysis degree of 99% was commercially provided by SINOPEC Sichuan Vinylon Works (Chongqing, China). First, the PVA raw material was washed with water until it had a pH of 7 to get rid of the residual natrium aceticum (NaAc). Then, it was dried at 80 °C to achieve a constant weight. Boracic acid (BA) was obtained from Chengdu Kelong Chemical. Carbon dioxide was purchased from Chengdu Xuyuan Gas with purities in excess of 99.5%. Preparation of BA Cross-linked Thermoplastic PVA Foam First, 0.25 g, 0.5 g and 1 g of BA were, respectively, dissolved in 40 g of deionized water. Then the BA solutions were mixed with 100 g PVA in a sealed container, and they were placed in a constant temperature oven at 80 °C until the water had completely swollen into the PVA. The existence of water disrupts the intramolecular and intermolecular hydrogen bonds of PVA, restrains its crystallization, thus decreasing the melt temperature of PVA. A sample without BA was also prepared with the same way. The mixtures with various BA contents (0 g, 0.25 g, 0.5 g, 1 g) were identified as BA0, BA0.25, BA0.5 and BA1, respectively. The PVA foams were prepared in a tandem foam system comprised of two single screw extruders with an ISCO Syringe High Pressure Dual-Pump to inject CO2 continuously into the polymer melt, as shown in Figure 1. Table 1 lists the processing temperatures. The processing temperature was far below its degradation temperature, and PVA would not degrade in the


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barrel. And, the high back pressure in the extruder barrel and the interaction between PVA and water limited the water’s evaporation in extrusion process. The CO2 injection flow rate was 2 ml/min. The rotation speeds of the first extruder and the second extruder were 15 r/min and 3 r/min respectively, and the die diameter was 2 mm with an L/D of 5, then the extruding flow rate of polymer melt was about 32 g/min.

Figure 1. A sketch of the tandem foam extrusion system. Table 1. Extruders’ processing temperature of cross-linked PVA/sc-CO2 foaming. Extruder1 (°C)

Extruder2 (°C)

T1

T2

T3

T4

T5

T6

T7

T8

Tdie

170

175

175

175

170

165

160

150

110-140

In-situ Fourier Transform Infrared Spectrum Test An in-situ FTIR spectrum test was performed on the Thermo Nicolet 6700 spectrometer (Thermo Scientific, USA) with a temperature controller. The sample film was prepared by painting a PVA/BA water solution on a glass slide and drying it in an oven for 2 hours at 80 °C. The in-situ FTIR test temperature was regulated by a Harrick Scientific ATC heater. The


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temperature first rose from room temperature to 175 °C at a rate of 5 °C/min, and was then cooled by circulating water to 130 °C. The spectrometer scanned once per 40 seconds. The BA1 and BA0 FTIR difference spectra at various temperatures were generated by OMNIC analysis software. Capillary Rheology Test The capillary rheology test was carried out on a Rosand RH7D high-pressure capillary rheometer (Malvern Instruments, UK). First, the PVA/BA mixtures were fed into the barrel of the rheometer and preheated for 3 min. Then, the measurements were taken in a shear rate range of 50–1000 s-1. Table 2 lists the test temperatures. Each PVA/BA mixture was tested three times. Meanwhile, 175 °C was the processing temperature in the first extruder and 130 °C was the best foaming die temperature. Table 2. Capillary rheology test temperature. Ttop (°C)

Tmid (°C)

Tdie (°C)

Test 1

175

175

175

Test 2

175

175

130

Gas Permeability Test A Labthink VAC-V2 gas permeability tester (Labthink Instrument, China) was used to test the permeabilities of the CO2 and N2 in the PVA/BA film at atmospheric pressure. First, various PVA/BA films with a thickness of about 200 µm were prepared using the hot-press method. The molding pressure and temperature were 10 MPa and 175 °C, respectively. In order to avoid the formation of bubbles, after being heated at 175 °C for 5 minutes, the mold was stopped heating and kept the molding pressure at 10MPa until the temperature cooling to lower than 100 °C.


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Then, the molding pressure was released and the film was removed. Then, the PVA/BA films were put in the permeability tester to measure their permeabilities using the manometric method at ambient temperature. Three specimens were tested per PVA/BA film according to ISO2556 and GB/T1038. Thermal Analysis Test The differential scanning calorimetry (DSC) test was performed on a TA Q20 thermal analyzer (TA Instrument, USA) at a heating rate of 10 °C/min from 40°C to 250 °C under a nitrogen atmosphere. The sample weight was about 6 mg. And, the thermal gravity analysis (TG) was carried out by using a TA Q50 thermo gravimetric analyzer (TA Instrument, Co., USA) at a heating rate of 10 °C/min from 40°C to 600 °C under a nitrogen atmosphere. Cell Structure An Inspect F scanning electron microscope (SEM) (FEI, Netherlands) was used to observe the foam’s cell structure. The foam sample was first cut using a single side razor blade. It was then sputter-coated with gold to prevent charging during the test. The foam’s density was tested by the water displacement method. The volume expansion ratio for each sample was calculated as the ratio of the density of the original sample ρp to the measured density of the foam sample ρf. The cell density N was estimated in Equation 1, where n is the number of cells in a specified area A (cm2).28-30 The average bubble diameter was measured from the SEM pictures.

N =（

n 32 ρ p ） A ρf

(1)

Results and Discussion FTIR Analysis As Figure 2 shows, the di-diol model22 revealed that the interactions between PVA and BA represented chemical bonds. But as seen in Figure 3, the cross-linking structure of the BA-


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compound formation27 showed that there were hydrogen bonds between the boron sites connected with the PVA chains. The bond energy of hydrogen bond was much lower than the chemical bond. And the hydrogen bond is reversible, which can be cleaved and rebuilt under the temperature or the shearing force.31

Figure 2. The di-diol model of PVA-BA cross-linkage.

Figure 3. The cross-linking structure of BA-compound formation between the PVA and the BA. The FTIR difference spectra between the pure PVA (BA0) and the cross-linked PVA (BA1) at different temperature from 3000 cm-1 to 3600 cm-1 are shown in Figure 4. Because the BA0 spectra were stripped from BA1 spectra by the difference spectra analysis, the difference spectra between 3000 cm-1 and 3600 cm-1 mainly were caused by the -OH stretching vibration of the BA cross-linking structure. In the heating stage, as Figure 4(a) shows, the peak of -OH stretching vibration shifted to high wavenumbers that ranged from 3363 cm-1 to 3408 cm-1, and narrowed with the weakened intensity. In the cooling stage, as shown in Figure 4(b), the peak shifted to


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low wavenumbers of 3379 cm-1, and became wider again with increased intensity. The FTIR spectra’s variation for the -OH stretching vibration peak meant that the strength of hydrogen bonding of the BA-compound had weakened and that the hydrogen bond number had decreased with the increased temperature. Meanwhile, the decreased temperature enhanced the hydrogen bonding and increased the hydrogen bond number.32 It revealed that the cross-linking effect between the PVA and the BA worked mainly through the hydrogen bonding. At a high temperature, the hydrogen bonding weakened, which decreased the cross-linking effect. But at a low temperature, the hydrogen bonding recovered, and the cross-linking effect was enhanced. This was consistent with the cross-linking structure of the BA-compound formation between the PVA and the BA, as shown in Figure3.

Figure 4. The in-situ FTIR difference spectra of cross-linked PVA and pure PVA at various temperatures (a. heating from 130 °C to 175 °C; b. cooling from 175 °C to 130 °C). Melt Viscosity We tested the melt viscosities of various PVA/BA mixtures at different temperatures to study the cross-linking effect between the PVA and the BA. Figure 5 shows the apparent viscosity curves of various PVA/BA mixtures obtained by capillary rheology testing at 130 °C and 175 °C. At a high temperature of 175 °C, the melt viscosities of different PVA/BA mixtures were low


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and close to each other. At a low temperature of 130 °C, obviously, the apparent viscosity of all the samples had increased with the decrease in temperature. And, with an increased BA content, the difference between the viscosity at 130 °C and 175 °C became more significant. This was ascribed to the enhanced cross-linking structure between the PVA and the BA. However, the PVA’s melt viscosity with various BA contents at 175 °C presented a little differently, and this indicated a quite weak cross-linking effect at a high temperature. This cross-linking effect was rebuilt by the decreased temperature. Thus, there was a significant increase in the melt viscosity with an increased amount of BA. The increased melt viscosity at a low temperature would help to create a bigger pressure drop at the die gate and would enhance the bubble wall’s strength after the foam left the die. This could improve the foams’ structure in the extrusion processing.

Figure 5. The capillary rheology test viscosity curves of various PVA/BA mixtures at 175 °C and 130 °C.


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Gas Permeability Figure 6 shows the gas permeabilities of the CO2 and the N2 in various PVA/BA films. The CO2’s permeability was much greater than the N2’s permeability in the uncross-linked PVA film. The CO2 and the N2 permeabilities both decreased with an increased BA content. But the CO2’s permeability decreased more significantly. This reduced the difference between the CO2’s and the N2’s gas permeabilities in the cross-linked PVA film with the increased BA content. In the uncross-linked PVA foaming process it could be inferred that the CO2, due to its higher permeability, had exited the bubble at a faster rate than the N2 had entered the bubble. Meanwhile, the lower melt strength of the uncross-linked PVA matrix might not maintain the bubble structure, so that the bubble seriously collapsed. However, in the cross-linked PVA foam, the phenomenon of bubble collapse was gradually improved by increasing the BA content. This was due to the increased melt strength and the decreased difference between the permeability of the CO2 and the N2 in the PVA.

Figure 6. The gas permeability curves of the CO2 and the N2 in PVA/BA film. Cell Structure and Thermal Property


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In our PVA extrusion foaming, the first extruder’s processing temperature was 175 °C. Thus, both the cross-linked and the uncross-linked PVA had a low melt viscosity, as shown in Figure 5. The cross-linking did not greatly affect either the PVA’s thermoplastic processing or the regular injection and diffusion of the CO2 in the first extruder. In the second extruder, the processing temperature was decreased along the melt flow direction, so the cross-linkage was rebuilt and the melt strength was increased in the cross-linked PVA melt. First, the high melt strength could construct a higher melt pressure in the die, as shown in Table 3. This increased the cell density3335

, and then restrained the overgrowth and combination of bubbles in foaming process to

generate a fine and homogeneous cell structure. Finally, the high melt strength improved the bubble collapse. Table 3. Melt pressure of PVA/BA in the extruder die at the die temperature of 130 °C.

Pdie (MPa)

BA0

BA0.25

BA0.5

BA1

4.10

5.45

7.40

9.65

The SEM pictures in Figure 7 show the cell structure of various PVA/BA foams at a die temperature of 130 °C. Clearly, the cell size of the uncross-linked PVA foam was much larger with the serious bubble collapse. With increased BA, the cell sizes of the cross-linked PVA foams became significantly smaller, and the bubble collapse problem was gradually improved.


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Figure 7. The SEM pictures of extruded PVA/BA foams (Die temperature 130 °C). Figure 8 shows the cell structure parameters of various BA cross-linked PVA foams extruded at a die temperature of 130 °C. Figure 8(a) shows that the volume expansion ratios of various PVA/BA foams were increased with an increased BA content. Figure 8(b) shows the changes of the cell density and the average bubble diameter in various BA cross-linked PVA foams. The cell density was significantly increased with an increased BA content. The cell density of uncrosslinked PVA foam was 2.11×106 cells/cm3, and was increased to 1.03×108 cells/cm3 in the crosslinked PVA/BA1 foam. And the average bubble diameter of various BA cross-linked PVA foams decreased with an increased BA content. The average bubble diameter of the uncross-linked PVA foam was about 115 µm, and gradually it dropped to 40 µm in the PVA/BA1 foam. The Figure 9 column charts show the bubble diameter distribution of different PVA/BA foams. And, the cell diameter’s standard deviations of BA0, BA0.25, BA0.5 and BA1 were 41.5, 39.0, 26.0 and 17.3, respectively. A high standard deviation indicates that the data points are spread out over a wider range of values. The bubble diameter distribution was more concentrated and


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smaller with increased BA. It was observed that the cross-linked PVA foams had fine and uniform cells.

Figure 8. The structural parameters of various PVA/BA foams at a die temperature of 130 °C (a. volume expansion ratio Φ; b. cell density N and average bubble diameter D).

Figure 9. Bubble diameter distribution of various PVA/BA foams at a die temperature of 130 °C. The change of glass transition temperature (Tg) of various PVA/BA foams is shown in the Figure 10(a) and (b). The Tg decreased with an increased BA content until BA0.5 foam, and then


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increased for BA1 foam. As shown in the Figure 10(c) and (d), the melt temperature (Tm) and melting enthalpy (Δ Hm) both decreased with an increased BA content for both PVA/BA mixtures and foams. And the foams presented higher crystallinity than the mixtures. The variations of Tg, Tm and Δ Hm with cross-liking agent content are same as the study of Krumova et al.36 It means that the cross-linking structure diminished the crystallinity and perfection of the crystal structure. The lower crystallinity was benefit to form more uniform and finer cellular structure on foaming process.37

Figure 10. The DSC curves and calorimetric parameters of various PVA/BA mixtures and foams (a, b. Tg of PVA/BA foams; c, d. Tm and ΔHm of PVA/BA mixtures and foams). Figure 11 shows the differential thermogravimetry (DTG) curves of various PVA/BA mixtures and foams. The initial degradation temperature (Td0) increased with an increased BA content for both PVA/BA mixtures and foams. The BA0 mixture’s Td0 was 229.23 °C, and it rose to


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276.93 °C for BA1 mixture. PVA/BA foams had a higher Td0 than PVA/BA mixtures, especially for the cross-linked PVA foam. And, the Td0 increased from BA0 foam’s 230.91 °C to BA1 foam’s 289.04 °C. The cross-linkage broadened the thermoplastic PVA’s processing temperature range and enhanced its thermal stability.

Figure 11. The DTG curves of various PVA/BA mixtures and foams. Effect of Temperature on the Cell Structure and Thermal Property Figure 12 shows the SEM images of the PVA/BA1 foam extruded at different die temperature (Tdie). And Figure 13 shows the volume expansion ratio Φ, the cell density N, and the average bubble diameter D of PVA/BA1 foam at different Tdie. The volume expansion ratio increased with the Tdie at 130 °C, and then slightly dropped at 140 °C. The cell density decreased slightly at a Tdie of 120 °C, and then it increased with the rising of the Tdie. In contrast to the cell density, the average bubble diameter first increased to 120 °C, and then it gradually decreased. But at a high Tdie, due to the high melt flowability and the weakened cross-linking effect, the bubble collapse became serious. Thus, the bubble size was clearly reduced, and many more cells were found in the unit volume at the high temperature, where the PVA/BA foam had a high cell density and a small bubble size.


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Figure 12. SEM pictures of the PVA/BA1 foam at different die temperature.

Figure 13. The structural parameters of the PVA/BA1 foam prepared at different die temperature (a. volume expansion ratio Φ; b. cell density N and average bubble diameter D). The Tg of PVA/BA1 foam prepared at different Tdie lightly decreased with an increased Tdie as shown in Figure 14(a) and (b). The Tm andΔHm both increased slightly with an increased Tdie as shown in Figure 14(c) and (d). At low foaming Tdie, the cross-linkage structure had rebuilt partly in the extruder die, and it limited the molecular motion

38

. And, at low temperature

polymer had short crystallization time. So the foam presented a low Tm andΔHm at low Tdie. At


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high foaming Tdie, the thermoplastic PVA melt had weaken cross-linking effect and long crystallization time, could form higher crystallinity and more perfect crystal structure.

Figure 14. The DSC curves and calorimetric parameters of the PVA/BA1 foam prepared at different die temperature (a, b. Tg of PVA/BA foams; c, d. Tm and ΔHm of PVA/BA foams). Conclusion In our study, we prepared the thermoplastic PVA foams through sc-CO2 extrusion foaming technology. The PVA foams’ cell structure was significantly improved by adding the BA crosslinking agent. We studied the effects of the BA content on the thermoplastic PVA’s foaming behavior and the resultant cell structure. In the in-situ FTIR and the viscosity analysis, the crosslinking structure between the PVA and the BA was more aligned with the BA-compound formation cross-linkage. The cross-linking effect between the PVA and BA worked mainly


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through hydrogen bonds. In the BA cross-linked PVA, the cross-linking effect was weakened at a high temperature. Thus, its influence on the PVA’s thermoplastic processing was slight. At a low temperature, the rebuilt cross-linking effect enhanced the PVA’s melt strength, and it significantly improved the PVA foams’ cellular structure. The cross-linking between the PVA and the BA reduced the difference between the CO2’s and the N2’s gas permeabilities in the PVA, which could improve the bubble collapse situation in the PVA foams. An increase in the added BA increased PVA foams’ volume expansion ratio and cell density. Also, the cell size became finer and more uniform, and the bubble collapse phenomenon was gradually improved. The cross-linkage could diminish the crystallinity and perfection of the crystal structure, and it was benefit to form more uniform and finer cellular structure. But, at a high die temperature of >140 °C, the bubbles seriously collapsed again due to the weakened cross-linking effect.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Qi Wang) * E-mail: [email protected] (Chul B. Park) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (No. 51573117) and by the Sichuan Province Science and Technology Support Plan (No. 2015GZ0067).


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REFERENCES (1) David, E. Handbook of Polymer Foam. Rapra Technology Limited: Shrewsbury, 2004. (2) Lee, S. T.; Ramesh, N. S. Polymeric foams mechanisms and materials. CRC Press LLC: Boca Raton, 2004. (3) Guo, D.; Bai, S.; Wang, Q. A novel halogen-free flame retardant poly (vinyl alcohol) foam with intrinsic flame retardant characteristics prepared through continuous extrusion. J. Cell. Plast. 2015, 51, 145-163. (4) Liu, P.; Bai, S.; Wang, Q. Preparation of aluminum hydroxide/aluminum phosphinate flame-retardant poly(vinyl alcohol) foam through thermal processing. J. Appl. Polym. Sci. 2015, 132, 42020. (5) Fukae, R.; Fujii, T.; Takeo, M.; Yamamoto, T.; Sato, T.; Maeda, Y.; Sangen, O. Biodegradation of Poly(vinyl alcohol) with High Isotacticity. Polym. J. 1994, 26, 1381 - 1386. (6) Chiellini, E.; Corti, A.; D’Antone S.; Solaro, R. Biodegradation of poly (vinyl alcohol) based materials. Prog. Polym. Sci. 2003, 28, 963-1014. (7) Ionita, M.; Crica, L. E.; Tiainen, H.; Haugen, H. J.; Vasile, E.; Dinescu, S. Gelatin– poly(vinyl alcohol) porous biocomposites reinforced with graphene oxide as biomaterials. J. Mater. Chem. B. 2015, 4, 282-291. (8) Pan, Y.; Wang, W.; Peng, C.; Shi, K.; Luo, Y.; Ji, X. Novel hydrophobic polyvinyl alcohol –formaldehyde foams for organic solvents absorption and effective separation. RSC Adv. 2014, 4, 660-669.


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Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Avella, M.; Cocca, M.; Errico, M. E.; Gentile, G. Biodegradable PVOH-based foams for packaging applications. J. Cell. Plast. 2011, 47, 271-281. (10) Song, T.; Tanpichai, S.; Oksman, K. Cross-linked polyvinyl alcohol (PVA) foams reinforced with cellulose nanocrystals (CNCs). Cellulose. 2016, 23, 1925-1938. (11) Wang, Q.; Li, L,; Chen, N.; Bai, S. Thermal processing of poly(vinyl alcohol). Polym. Mater. Sci. Eng. 2014, 30, 192-197. (12) Wang, R.; Wang, Q.; Li, L. Evaporation behaviour of water and its plasticizing effect in modified poly(vinyl alcohol) systems. Polym. Int. 2003, 52, 1820-1826. (13) Wang, R.; Wang, Q.; Li, L.; Hua, K. Study on the melt-processing properties of modified PVA. Polym. Mater. Sci. Eng. 2001, 17, 111-113. (14) Wang, Q.; Li, L.; shi, H.; Peng, X. Poly(vinyl alcohol) polar foam and its preparation method. Chin. Patent No. ZL200710049955.6, Sept. 10, 2010. (15) Park, C. B.; Baldwin, D. F.; Suh, N. P. Effect of the pressure drop rate on cell nucleation in continuous processing of microcellular polymers. Polym. Eng. Sci. 1995, 35, 432-440 (16) Baldwin, D. F.; Park, C. B.; Suh, N. P. An extrusion system for the processing of microcellular polymer sheets: Shaping and cell growth control. Polym. Eng. Sci. 1996, 36, 14251435. (17) Park, C. B.; Behravesh, A. H.; Venter, R. D. Low density microcellular foam processing in extrusion using CO2. Polym. Eng. Sci. 1998, 38, 1812-1823.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(18) Zhao, N.; Mark, L. H.; Zhu, C.; Park, C. B.; Li, Q.; Glenn, R.; Thompson, T. R. Foaming poly(vinyl alcohol)/microfibrillated cellulose composites with CO2 and water as co-blowing agents. Ind. Eng. Chem. Res. 2014, 53, 11962-11972. (19) Wang, H.; Shyr, T.; Hu, M. The elastic property of polyvinyl alcohol gel with boric acid as a crosslinking agent. J. Appl. Polym. Sci. 1999, 74, 3046-3052. (20) Wang, H.; Shyr, T.; Hu, M. The elastic property of polyvinyl alcohol gel with boric acid as a crosslinking agent. J. Appl. Polym. Sci. 1999, 74, 3046-3052. (21) Chen, J; Li, Y.; Zhang, Y.; Zhu, Y. Preparation and characterization of graphene oxide reinforced PVA film with boric acid as crosslinker. J. Appl. Polym. Sci. 2015, 132, 42000. (22) Ochiai, H.; Shimizu, S.; Tadokoro, Y.; Murakami, I. Complex formation between poly(vinyl alcohol) and borate ion. Polymer. 1981, 22, 1456-1458. (23) Bolaños, L.; Lukaszewski, K.; Bonilla, I.; Blevins, D. Why boron? Plant Physiol. Biochem. 2004, 42, 907-912. (24) Wise, E.T.; Weber, S. G. A Simple Partitioning Model for Reversibly Crosslinked Polymers and Application to the Poly(vinyl alcohol)/Borate System ("Slime"). Macromolecules. 1995, 28, 8321-8327. (25) Li, L.; Wang, Q.; Wang, R. Enhancing mechanical properties of poly(vinyl alcohol) blown films by drawing and surface crosslinking. J. Appl. Polym. Sci. 2005, 98, 774-779. (26) Kazuhiko, Y. Solid-State NMR Study of Boric Acid Doped in Poly(vinyl alcohol). American Chemical Society: Washington, DC, 2011.


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Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Itou, T.; Kitai, H.; Shimazu, A.; Miyazaki, T.; Tashiro, K. Clarification of Cross-Linkage Structure in Boric Acid Doped Poly(vinyl alcohol) and Its Model Compound As Studied by an Organized Combination of X-ray Single-Crystal Structure Analysis, Raman Spectroscopy, and Density Functional Theoretical Calculation. J. Phys. Chem. B. 2014, 118, 6032-6037. (28) Okolieocha, C.; Raps, D.; Subramaniam, K.; Altstädt, V. Microcellular to nanocellular polymer foams: Progress (2004–2015) and future directions – A review. Eur. Polym. J. 2015, 73, 500-519. (29) Xu, X.; Park, C. B.; Xu, D.; Pop-Iliev, R. Effects of die geometry on cell nucleation of PS foams blown with CO2. Polym. Eng. Sci. 2003, 43, 1378-1390. (30) Sorrentino, L.; Aurilia, M.; Cafiero, L.; Iannace, S. Nanocomposite Foams from HighPerformance Thermoplastics. J. Appl. Polym. Sci. 2011, 122, 3701-3711. (31) Ishii, D.; Yamada, T.; Iyoda, T.; Yoshida, H.; Nakagawa, M. Thermally Reversible Structural Transformation Involving a C–H…O Hydrogen Bond in a Supramolecular Crystal. Chem. Lett. 2006, 35, 1394 - 1395. (32) He, Y.; Zhu, B.; Inoue, Y. Hydrogen bonds in polymer blends. Prog. Polym. Sci. 2004, 29, 1021-1051. (33) Naguib, H. E; Park, C. B; Lee, P. C.; Xu, D. A Study on the Foaming Behaviors of PP Resins with Talc as Nucleating Agent. J. Polym. Eng. 2006, 26, 565. (34) Matuana, L. M.; Diaz, C. A. Study of Cell Nucleation in Microcellular Poly(lactic acid) Foamed with Supercritical CO2 through a Continuous-Extrusion Process. Ind. Eng. Chem. Res. 2010, 49, 2186-2193.


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(35) Nofar, M.; Park, C. B. Poly (lactic acid) foaming. Prog. Polym. Sci. 2014, 39, 1721-1741. (36) Krumova, M.; López, D.; Benavente, R.; Mijangos, C.; Pere A, J. M. Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol). Polymer. 2000, 41, 9265-9272. (37) Doroudiani, S.; Park, C. B.; Kortschot, M. T. Effect of the crystallinity and morphology on the microcellular foam structure of semicrystalline polymers. Polym. Eng. Sci. 1996, 36, 2645-2662. (38) Miyazaki, T.; Takeda, Y.; Akane, S.; Itou, T.; Hoshiko, A.; En, K. Role of boric acid for a poly (vinyl alcohol) film as a cross-linking agent: Melting behaviors of the films with boric acid. Polymer. 2010, 51, 5539-5549.


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The TOC graphic


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Effect of Boric Acid on the Foaming Properties and Cell Structure of

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