Improving the structural and physical properties of boric acid-doped rigid polyurethane materials

Abstract

In this study, the internal structure and physical properties of boric acid-doped rigid polyurethane (PU) materials were investigated. 5%, 10%, and 15% of boric acid were added into PU material compared to the total mass. These rigid PUs were subjected to scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermal conductivity, and density determination for analysis. Boric acid addition resulted in a decrease of 57.2% in thermal conductivity and 67.8% in density compared to raw PU material. It has been shown that it provides support for the formation of cell structure. In addition to, it is also found that there are no impurity atoms in the structure and the structure is formed in the tetragonal phase.

Keywords

polyurethane boric acid structural properties physical properties thermal conductivity

Introduction

Polymers containing isocyanate and carbamate (urethane) from the group of organic plastics are called polyurethane (PU) (NHCOO).¹ PUs exist in a variety of forms, including hard, bright, solvent-resistant coatings, abrasion and solvent-resistant tires, flexible or rigid foams.

Since the beginning of the 21st century, rigid PUs that have entered the group of polymers have been an important engineering material that offers advantages such as low production cost, easy to manufacture, processability, and high amount of energy absorption.² Considering these advantages, it has found the opportunity to use in many areas such as sound insulation, thermal insulation, vibration absorption, and so on.

Thermal conductivity is achieved by conducting on cell gas and conducting on the structure of solid polymers. Intensive efforts are being made in the scientific world and related sectors to further improve the thermal conductivity performance of PUs. At this point, the researchers focused on reducing the size of the cells in the PU structure and preventing the diffusion of the blowing gas out of the cell in the cellular structure by various additives that can serve as a diffusion barrier.³ The contribution of the cell gas to the heat conduction is directly proportional to the mean free path of gas that travels within the cell. Although it is a feature to reduce the cost of PU by the various additives made, the main purpose is to create a cell structure with smaller bubbles of gas. In this way, the free path will be reduced and the thermal conductivity will reduce.

A significant number of literature have been published on added materials at rigid PU. Gao et al. aim of improving flame retardant properties of rigid PU. A functionalized with boric acid and 3-aminopropyltriethoxysilane was prepared graphene oxide. Then it was used in PU production. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) tests used to determine structural properties of PU. UL-94, cone calorimeter and limited oxygen (LOI) analysis was specifying flammability properties. Other results, tensile strength, compress strength and elongation at break is report significantly enhanced. Also UL-94 test arrive V-0 rating and the harmful-toxic gas release decreased. LOI increased by 28% in samples with 0.25 additives.⁴ Chmiel et al. researched 1,3,5-triazine ring and boric acid-added PU foams. Before obtained with boric acid modify from melamine and propylene carbonate, boric acid was hydroxyalkylated with glycidol and ethylene carbonate. The admixture reaction with diphenylmethane isocyanate was produced PU foams. They have reported that boric acid-added PU foams showed analysis results decrease flammability and raise heat resistance. They found that all samples were similar to classical PU foam properties, except for these two properties.⁵ Zarzyka investigated effect of borate group on the PU foams properties. He was used to polyol with boric acid esterified urea. Second component used to obtain PU of 4,4′-diphenylmethane diisocyanate. The addition of boric acid enhances mechanical strength and thermal resistance at structural properties. In addition to, flammability properties of PU will determine with added carbamide.⁶ In the continuation study examining the effect of carbamide and borate groups on the characterization of PU foams, Zarzyka applied FTIR, elemental analysis, thermogravimetric analysis, differential scanning calorimetric analysis, oxygen index, horizontal flammability test, and thermal conductivity tests in the PU characterization. Comparing boric acid-added and PURE PU, boric acid-added PU showed high density, lower water uptake, better thermal, and dimensional stability. The author stated that boric acid-added PU showed the best compressive strength, thermal insulation, and self-extinguishing properties. The use of boric acid-containing polyols used to PU has significantly reduced flammability and flammability class HF-1.⁷ In another study, Zarzyka examined the effect of PU’s oxamide and borate groups. N,N,N′,N′-tetrakis(2-hydroxyethyl)-derivatives of oxamide esterified with boric acid, and ethylene carbonate has produced a new polyol. It used this polyol in PU foams production. As a result of the investigations, Zarzyka reported that boric acid effect contributed significantly to the reduction of thermal resistance, thermal stability, compressive strength, and flammability. In addition, author reported that the foams can be used in the construction industry for insulation of building elements and heating pipes.⁸

As a result of the studies carried out with various additives, it was observed that the thermal conductivity of PUs decreased, the mechanical properties improved and the thermal degradation temperatures increased.^9

–14 It is also necessary to pay attention to the particle size of the material selected as the additive. It has been observed that large-size materials have negative properties for PU (e.g. growth in gas bubbles and a decrease in compression strength).¹⁵ For this purpose, it is more appropriate to select small-sized materials as an additive material.

It is thought that the boric acid used in this study will be a good additive material due to its nanosize, nonflammable property, and its homogeneous distribution in PU material.

Experimental

Materials

Polyol (polyol-senta 77/HC) and isocyanate (isocyanate-senta 77/SG-MDI) to be used in the production of PU were provided by OSA Kimya, Turkey. Polyol and isocyanate were not subjected to any pretreatment process. The polyol and isocyanate densities were 1.00–1.15 g/cm³ (25°C) and 1.10–1.20 g/cm³ (25°C), respectively. The viscosities of polyol and isocyanate were 350–450 mPa·s and 250–350 mPa·s at 25°C, respectively. The mixing ratio was recommended by the manufacturer as 1:1 ± 0.02 and the mixing speed was 6000 r/min for 7 s (22°C).

Boric acid, to be used as the additive material, was obtained from ZAG Kimya Company in Turkey. Some properties of boric acid are given in Table 1.

Table 1.

Properties of boric acid.

Properties
Chemical formula	H₃BO₃
Molecular mass	61.83 g/mol
Appearance	White crystalline solid
Density	1.435 g/cm³
Melting point	170.9°C
Boiling point	300°C
Flashpoint	Nonflammable
Water solubility	2.52 g/100 mL (0°C)
	4.72 g/100 mL (20°C)
	5.70 g/100 mL (25°C)
	19.10 g/100 mL (80°C)
	27.53 g/100 mL (100°C)
Solubility (in other solvents)	Low solubility in alcohol
	Medium solubility in pyridine
	Very low solubility in acetone
Structure
Molecular shape	Trigonal planar

Sample preparation

Polyol and isocyanate having a density of about 1.10 g/cm³ were used in equal amounts for PU production. Boric acid, which is the additive material, was added to the total mass by ratios of 5%, 10%, and 15%. To ensure uniform distribution of the boric acid used in powder form in the structure, polyol and boric acid were placed together in the apparatus and mixed using a mechanical stirrer. Then isocyanate was added and stirred at a speed of 1000 r/min for 1 min. The mixture was poured into the silicone mold and the production process was completed. Hard PU samples removed from the mold were named PURE, BOA05, BOA10, and BOA15 according to their content ratios. The samples were kept for 24 h at room temperature, then weight and dimensions were determined for determination of density. The samples were prepared by cutting to the appropriate size to determine the internal structure and physical properties. The synthesis demonstration is given in Figure 1.

Figure 1.

The synthetic scheme for PU. PU: polyurethane.

Characterization

The surface morphology and composition of the polyurethane were observed by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) on Thermo Scientific™ Quanta™(FEG-250), X-ray diffraction (XRD) measurements were performed using a Bruker (AXS D8 Advance) diffractometer in the Mehmet Akif Ersoy University central laboratory. Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer Frontier spectrometer in the Mehmet Akif Ersoy University central laboratory. ASTM C 1113-90 hot wire method was used to measure the thermal constants of PUs.¹⁶ The size of the samples was 50 × 150 × 10 mm³. After the production process, the weights were determined by a scale with a precision of ±0.01 g, and the dimensions were measured by caliper in ±0.02 mm precision. Then, as a result of the calculations, the densities of PUs were determined.

Results and discussion

Surface morphology and composition analysis

The SEM images used to determine the surface morphology of PURE and boric acid-doped hard PUs are shown in Figure 2. The size of the additives should also be taken into consideration in the applications. When the size of the fillers is large, they impale the cell wall and cause the structure to deteriorate. Accordingly, mechanical properties and heat transfer coefficient are adversely affected. In Figure 2(a), it is seen that no cell structure is visible, whereas in Figure 2(b) to (d), cell structures and gas bubbles are formed. Most of the cells are spherical and closed. In some places, the cell structure has been damaged. But it is too little to be care about. Besides, the cells are uniformly dispersed in the matrix and the cell diameters are similar. Similar findings were reported in the literature.^9,17 The results of the EDS analysis are shown in Table 2. Table 2 shows that the ratio of boric acid in the structure corresponds to a stoichiometry within the limits of 5% experimental error. In addition, it is seen that there are no impurity atoms in the structure.

Figure 2.

SEM images of (a) PURE, (b) BOA05, (c) BOA10, and (d) BOA15. SEM: scanning electron microscopy.

Table 2.

Results of the EDS analysis.

	PURE		BOA05		BOA10		BOA15
Element	Weight %	Atomic %	Weight %	Atomic %	Weight %	Atomic %	Weight %	Atomic %
CO	95.37	99.04	94.90	95.50	96.21	96.24	95.97	96.02
H₃BO₃	—	—	5.10	4.50	3.80	3.76	4.03	3.99

EDS: energy-dispersive X-ray spectroscopy.

Structural analysis (XRD analysis)

Figure 3 shows the XRD patterns of boric acid-doped PUs. When the graphs are examined, the positions and planes of these peaks are 2θ = 14.6° (001), 28.1° (200), 30.3° (111), and 40.1° (220).^18,19 Crystallographic data are used to determine of crystal structures of organic, inorganic, metal-organics compounds and minerals. These peaks values of Figure 3 (14.5° and 28°) belong to boric acid characteristics (JCPDS 30-0199).^20

–23 When the doping rate increases, the intensity of the peaks increases. At the same time new phase was observed 10% and 15% added boric acid. This new phases is explainable with the fluxing behavior of H₃BO₃ that simplified the phase changes. The XRD pattern of peaks between 2θ = 30°–40.1° has a tetragonal structure pairing with JCPDS 86-1769. The XRD results showed that the boric acid doping supports the pass from of hexagonal phase to the tetragonal phase.^23,24

Figure 3.

XRD diffraction patterns of PURE and H₃BO₃-doped PUs. XRD: X-ray diffraction; PU: polyurethane.

FTIR spectra

The FTIR spectroscopy of boric acid-doped PUs is shown in Figure 4. In Figure 4, the peaks at around 3200 cm⁻¹ and 3386 cm⁻¹ are typical for the stretching vibration of N–H. The symmetrical and asymmetric stretching vibrations of the N–H group belong to isocyanate in PU structure.^25,26 The peak at 2961 cm⁻¹ of aliphatic C–H stretching vibration and CH₃–CH₂ bond structure indicates the presence of polyol. The formation of –N=C=O is confirmed by the two peaks at 2350 cm⁻¹ and 2278 cm⁻¹, while the peaks at 1716 cm⁻¹ and 1710 cm⁻¹ suggest the presence of C=O.^27,28 The peaks at 1598 cm⁻¹ and 1650 cm⁻¹ assign to the NHC=O aromatic double bond,²⁶ while 1413 cm⁻¹ to 1406 cm⁻¹ bands respond to the presence of polyol. The peaks at around 1250 cm⁻¹ and 1270 cm⁻¹ are typical for the stretching vibration of esters.²⁹ The broad and strong peak of the C–O–C bond at 1018 cm⁻¹ is observed.²⁵ The peaks at around 3200 cm⁻¹ to 3386 cm⁻¹ of boric acid O–H stretching vibrations.³⁰ According to National Institute of Standards and Technology (NIST), the peaks at around 2900 cm⁻¹ to 2930 cm⁻¹ and 1390 cm⁻¹ to 1490 cm⁻¹ of B–O stretching vibrations,^30

–33 the peaks at around 1190 cm⁻¹ to 1250 cm⁻¹ of B–C stretching vibrations,^34,35 the peaks at around 700 cm⁻¹ to 1000 cm⁻¹ of B–O–H bending,³³ and the typical peaks at around 450 cm⁻¹ to 650 cm⁻¹ are clearly determined. It is observed that as the boric acid additive increases, the permeability value decreases with decreasing the length of the peaks. However, as a result of boric acid addition, no new peak formation or peak displacement occurred. The addition of boric acid did not cause any changes in the FT-IR peaks of PURE PU. In this case, it is thought that boric acid does not participate in the reaction but settles the polymer chains electrostatically.

Figure 4.

The FTIR spectra of PURE and H₃BO₃-doped PU. FTIR: Fourier transform infrared; PU: polyurethane.

Thermal conductivity and density

As a result of the reaction of boric acid with a polyol, its volume increased significantly. Thermal conductivity and density decreased. The reduction in heat transfer, that is, its high thermal stability, can be explained by the strong hydrogen bonds produced by boric acid.³⁶ This has led to the transformation of the cell structure into a hollow structure and to a decrease in density (Table 3). The reduction of thermal transfer in a hollow structure (increase in the free path of heat energy) is inevitable. However, it is seen that the heat transfer coefficient and density increase with the addition of 15%. This can be explained by the fact that the entire boric acid does not react, is present in the solid state in the structure, and permits heat transfer through the conduction. Also, the addition of boric acid by 15% caused deterioration in PU closed cell structures as shown Figure 2(d). This had a negative effect on the thermal conductivity. It is seen that more than 10% of boric acid has negative effects on thermal conductivity and density. The heat transfer coefficient and density values in Table 3 and the SEM images were confirmed each other.

Table 3.

Thermal conductivity measurement and density.

Sample	Thermal conductivity (W/mK)	Density (g/cm³)
PURE	0.3413	0.994
BOA05	0.1671	0.436
BOA10	0.1460	0.320
BOA15	0.1923	0.360

Conclusion

In this article, boric acid was added to improve the structural and physical properties of PU material. The following results were obtained from various analyzes:

As a result of SEM analysis, it was seen that cell structures were formed and most of them were cubic and closed form.

As a result of EDS analysis, it was determined that no impurity atoms were present in the structure.

From the XRD diffraction results, it was shown that boric acid addition promoted the transformation of hexagonal phase to the tetragonal phase.

As a result of boric acid addition in FTIR spectra, the permeability value and peak intensity of the samples decreased.

The thermal conductivity coefficient was determined as 0.1460–0.1923 W/mK. The best thermal conductivity coefficient was found to be 0.1460 W/mK in the BOA10 sample. The thermal conductivity coefficient decreased by 57.2% compared to the PU material. This showed that the PUs obtained by boric acid addition can be used both as thermal insulation material and as a building material.

As a result of the boric acid reaction which is added to PU material, it reduced the density of the material by 56.1% to 67.8%. A fairly light material has emerged. The material with the lowest density was determined as 0.320 g/cm³ in the BOA10 sample.

As a result, the BOA10 sample produced better results than other samples in terms of thermal conductivity coefficient, density, elemental analysis, and cell structure formation. This situation demonstrates that boric acid addition should be at 10% levels.

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

İbrahim Kırbaş

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