Introduction

High-rise and high-rise building construction is increasing as a countermeasure to solve residential spaces due to high density population concentration. According to research, the number of skyscrapers built worldwide over 200 m in 2018 was 148, indicating a 49.3% increase compared to 2013 (Qu et al. 2021), In Korea reported (2020), the number of buildings over 30 stories was 3165, a 52.5% increase compared to 1661 in 2016.

However, as the number of high-rise buildings increases, so do fire accidents that occur outside of these buildings (Bonner and Rein 2018, Bonner et al. 2020). In Korea reported (2017), the number of fires in buildings with 30 floors or more increased from 107 in 2014 to 145 in 2017, resulting in 11 times more casualties and 17 times more property damage due to fire accidents Flammable building finishes used in high-rise buildings were cited as the cause of increased fire casualties and property damage (Bonner et al. 2020; Bonner and Rein 2018). Table 1 illustrates fire accidents and damage caused by combustible building finishes in various countries worldwide.

Table 1 Fire accidents caused by Combustible Building Finishing Materials
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In minimize the damage caused by fire spread due to combustible building finishes, countries worldwide have established regulations for building finishes fire performance to prevent fire expansion. Table 2 presents the safety standards for finishing material fire performance by major countries (Kodur et al 2022; NFPA 255 2006; Yoshioka et al 2021; Laban et al 2022; Kwark et al 2011; Korea Ministry of Land 2022).

Table 2 Comparison of regulations related to interior materials in each country
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Considering the laws regarding building finishes in various countries and the prevalence of fire accidents in high-rise buildings, it is crucial to develop fire-safe finishing building materials that can be applied to high-rise buildings, Therefore, several studies are being conducted to develop fire-safe finishing building materials. Table 3 shows various studies on flame retardant building finishes.

Table 3 Several studies on flame retardant building finishing materials
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In a previous study aimed at developing flame-retardant building finishes, Wi et al. found that inorganic building finishes manufactured using recycled waste ceramics could reduce building energy consumption by 18.6% and improve the fire resistance performance of ceramics (Wi 2019). Wei et al. conducted experiments with formaldehyde-free poly siloxane coatings on Expanded Polystyrene (EPS) foam, which acted as adhesives and flame retardants. The EPS complex with the poly siloxane coating showed a 76% reduction in PHRR (Peak Heat Release Rate) and significantly reduced smoke release compared to the existing EPS complex (Wi et al. 2019). Chen et al. developed a flame-retardant tannin-furan-SPI bio foam by adding boric acid and montmorillonite phosphate. The developed SPI bio foam was confirmed to have a PHRR value of 16.54 ~ 29.03 kW/m2 through a cone calorimeter test, and the thermal conductivity of up to 0.0627W/m∙K was measured, suggesting the possibility of making flame-retardant building materials (Wei et al. 2022). Choi et al. studied the differences in fire performance and toxicity between polyurethane foam & board (PUR-F, PUR-B) and polyisocyanurate foam & board (PIR-F, PIR-B) through ISO 5660-1, KS F 2271, and sill-sized fire. They concluded that further experiments with various flame retardants were needed (Chen et al. 2022). Hwang et al. produced a 3D printer to manufacture cellulose finishing materials and tested the ISO 11925-2 experiment for cellulose finishing materials. They found that cellulose finishing material with 20 wt% ceramic binder satisfies the vertical flame propagation length within 150 mm in the 30 s flame contact, the standard of EN 13501-1. They also found that the cellulose finishing material with 40 wt% ceramic binders did not have residual flames after the ISO 11925-2 experiment (Hwang et al. 2022). This research conducted experiments on a total of 5 expandable graphite particles sizes by adding two expandable graphite particles sizes as variables in addition to the expandable graphite with particle sizes added to the cellulose finishing material used in a prior research at Kim et al. (Kim et al. 2023) The change in fire performance and thermal diffusivity according to the change in expandable graphite particle size were verified through the ISO 5660-1 experiment and the LFA 1000 experiment. Figure 1 shows the flow chart of this study.

Fig. 1
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Flowchart of this study design [28]

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Description of specimens and test

Description of making specimens

The double flame-retardant mixed waste paper-based specimen used in this study was produced using an LWC-3D printer with confirmed manufacturing efficiency and uniformity (Hwang et al. 2022; Ahn et al.2022). The LWC-3D printer was used to manufacture specimens with uniform fire performance. Figure 2 shows the LWC-3D printer used to manufacture the specimens.

Fig. 2
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Large Wet Cellulose (LWC)—3D Printer, Incheon National University Fire Disaster Prevention Research Center

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In this study, Expandable Graphite (EG) and Magnesium Hydroxide (MH) flame retardants were added to enhance the fire performance of the cellulose building finish. The purpose of adding each flame retardant is as follows.

  • 1. The addition of expandable graphite aims to protects the lower layer of the specimen to suppress the spread of external fire by expanding graphite during heating and forming a char layer. (Xia et al 2022; Chun et al. 2017).

  • 2. Magnesium hydroxide was added to improve retardant performance through an endothermic reaction during the thermal reaction. The formula for the endothermic reaction of magnesium hydroxide is as follows Eq. (1) (Chung et al. 2011; Meucci et al. 2022).

    $${Mg(OH)}_{2} \to MgO+ {H}_{2}O 1244\sim 1450 J/g$$
    (1)

Two flame retardants (EG & MH) were added to the waste paper-based construction finishing material. Additionally, five different particle sizes of expandable graphite were added to the same content to analyze the fire performance based on the particle size variations. Table 4 shows the particle size of expandable graphite added to the cellulose flame retardant, while Table 5 shows the composition of waste paper-based specimens made of flame retardant building finishes.

Table 4 Picture of expandable graphite particle size
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Table 5 Composition of specimens
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Table 6 shows a photograph of the ISO 5660-1 specimen of the manufactured flame retardant construction finishing material.

Table 6 Printed and after drying ISO 5660–1 specimen
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Description of heat release rate test (ISO 5660–1)

Construction finishing materials must undergo a performance certification procedure to ensure their safety in case of fire. This research measures the heat release rate of the specimen by applying the ISO 5660–1 Cone Calorimeter test method. Radiant heat is continuously applied to samples located 22.5 mm away from the cone heater at a heat flow rate of 50 kW, and the heat release rate is calculated using the principle that approximately 13.1 MJ of heat is generated when 1 kg of oxygen is consumed during material combustion. The oxygen consumed is measured using an oxygen analyzer of a cone calorimeter, and the HRR (Heat Release Rate) is calculated through the measured amount of oxygen consumption using Eq. (2) (British Standard Institution 2021; Zhang et al 2015).

$$\dot{q}=E({\dot{m}}_{{O}_{2},0}-{\dot{m}}_{{O}_{2}})$$
(2)

where:

\(\dot{q}\) = Heat Release Rate (HRR).

\(E\) = The heat generated per unit mass of oxygen consumed E≒13.1 kJ/\({\mathrm{g}}_{{O}_{2}}\)

\({\dot{m}}_{{O}_{2},0}\) = Mass flow rate of oxygen in the incoming air.

\({\dot{m}}_{{O}_{2}}\)= Mass flow rate if oxygen in the exhaust gases.

Figure 3 shows the ISO 5660–1 Cone Calorimeter (FESTEC, Co, Korea) used in this experiment.

Fig. 3
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International Organization for Standardization 5660–1 (Cone calorimeter)

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Figure 4 schematically shows the fire experiment part of the cone calorimeter equipment used in this experiment.

Fig. 4
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Method of Test 5660–1 (Cone calorimeter)

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Table 7 shows the flame retardant and semi-non-combustible performance standards of the ISO 5660–1 test as specified by the International Organization for Standardization. This table serves as the standard for evaluating the flame retardant and semi-noncombustible experiments of the specimens (British Standard Institution 2021).

Table 7 Performance standard using the ISO 5660–1 test method
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Description of thermal diffusivity test (LFA 1000 test)

The thermal diffusivity of the cellulose produced was measured using LFA (Laser Flash Analysis) 1000 experimental equipment. Figure 5 shows the LFA 1000 equipment used to measure the thermal diffusivity.

Fig. 5
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LFA 1000 of Department of Fire Safety Research, Korea Institute of Civil Engineering and Building Technology

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The LFA 1000 experiment follows the method proposed by Parker in 1961 (Vitiello et al 2021; ASTM International 2013; Parker et al 1961). The test method utilizes the principle that the absorbed heat energy is transferred to the rear surface of the specimen by irradiating a laser flash on the front surface of the test object. The temperature rise at the rear of the specimen is measured over time until it reaches and saturates at the maximum temperature. Figure 6 shows the Laser Flash method used in the LFA 1000 experimental equipment.

Fig. 6
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a Illustrative diagram LFA 1000 Test b Laser Flash Analysis Method

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The ratio of temperature changes over time measured by the method shown in Fig. 6 is calculated according to Eq. (3) of the thermal diffusivity calculation proposed by Parker (Parker et al 1961).

$$\mathrm{\alpha }=0.13879{L}^{2}/{t}_{1/2}$$
(3)

where:

α: Thermal Diffusivity [cm2/s].

L: Thick of Specimens [cm].

t1/2: The Time taken to increase to the 50% Point in the Graph of the Temperature increase of the Opposite Surface [s].

Test result and discussion

Result of ISO 5660–1

In order to confirm the fire performance of cellulose manufactured with each particle size of expandable graphite, an ISO 5660–1 experiment was conducted three times for each specimen to confirm the uniformity of the specimen's fire performance.

Figure 7 shows the trend change of HRR due to the difference in particle size of expandable graphite, as measured by conducting the test three times.

Fig. 7
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HRR measurement of all specimens a mesh 50 (279 μm) b mesh 80 (173 μm) c mesh 100 (140 μm) d mesh 150 (104 μm) e mesh200 (74 μm)

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Figure 8 shows the difference in THR (Total Heat Release) values resulting from the difference in particle size of expandable graphite, as measured by conducting a test three times per specimen.

Fig. 8
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THR measurement of all specimens a mesh 50 (279 μm) b mesh 80 (173 μm) c mesh 100 (140 μm) d mesh 150 (104 μm) e mesh200 (74 μm)

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Figure 9 illustrates the values obtained by conducting a test three times for each specimen to verify the amount of carbon monoxide generated due to the difference in particle size of expandable graphite.

Fig. 9
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CO generate of all specimens a mesh 50 (279 μm) b mesh 80 (173 μm) c mesh 100 (140 μm) d mesh 150 (104 μm) e mesh200 (74 μm)

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Figure 10 shows the value obtained by conducting a test three times for each specimen to confirm the amount of carbon dioxide generated due to the difference in particle size of expandable graphite.

Fig. 10
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CO2 generate of all specimens a mesh 50 (279 μm) b mesh 80 (173 μm) c mesh 100 (140 μm) d mesh 150 (104 μm) e mesh200 (74 μm)

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Figure 11 shows the value obtained by conducting three tests of the mass reduction of the specimen during the cone calorimeter test process according to the particle size of expandable graphite.

Fig. 11
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Mass Loss of all specimens at ISO 5660–1 test a mesh 50 (279 μm) b mesh 80 (173 μm) c mesh 100 (140 μm) d mesh 150 (104 μm) e mesh200 (74 μm)

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Table 8 summarizes and shows the results of the cone calorimeter experiment according to the change in particle size of expandable graphite.

Table 8 ISO 5660–1 Test Result on PHRR, THR at 300 s THR at 500 s, CO, CO2 Generate, Mass Loss
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Result of LFA 1000

To investigate the effect of expandable graphite particle size on the thermal diffusion rate of cellulose material, three LFA 1000 tests were conducted at 20°C with the particle size as a variable. The thermal diffusion rate was measured, and uniformity was verified. Table 9 summarizes the results of the thermal diffusion rate experiment according to the changes in expandable graphite particle size.

Table 9 LFA1000 Test Result on Thermal Diffusivity
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Discussion

Through the ISO 5660-1 test and the LFA 1000 test, the effect of a change in the particle size of expandable graphite on the fire performance and thermal diffusion rate of cellulose materials was confirmed.

Figure 12 shows the distribution and trend line of PHRR according to the particle size of expandable graphite.

Fig. 12
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PHRR Distribution and Trend line by Expandable Graphite particle size

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As a result of analyzing the PHRR, it was confirmed that the lowest PHRR value was observed with the particle size of 173 μm for expandable graphite, and all of the flame-retardant performance standards of PHRR 200 kW/m2 or less were satisfied.

Figure 13 shows the distribution and trend line of THR at 300 s according to the expandable graphite particle size.

Fig. 13
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THR at 300 s Distribution and Trend line by Expandable Graphite particle size

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As a result of analyzing the THR at 300 s, it was confirmed that the average THR was the lowest for the particle size of expandable graphite at 173 μm, and flame-retardant performance was confirmed for particle sizes of 104 μm, 140 μm, 173 μm, and 279 μm..

Figure 14 shows the distribution and trend line of THR at 500s according to the expandable graphite particle size.

Fig. 14
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THR at 500 s Distribution and Trend line by Expandable Graphite particle size

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As a result of analyzing the THR at 500s, it was confirmed that the average THR was the lowest when the particle size of expandable graphite was 173 μm.

Figure 15 depicts the distribution and trend line of toxic gas CO and CO2 generation in relation to the particle size of expandable graphite, and the Toxic Index was calculated based on the lowest published lethal concentration (LCLO) as defined by the NIOSH (National Institute of Occupational Safety and Health), using CO: 5,000 ppm (human, 5min) and CO2: 90,000 ppm (human, 5min).

Fig. 15
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Toxic Gas Distribution and Trend line by Expandable Graphite particle size a CO Generate b CO2 Generate c Toxic Index Average

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As a result of analyzing CO and CO2 production amounts, the lowest average CO production amount at the particle size of expandable graphite of 104 μm was confirmed, and the lowest average CO2 production amount at the particle size of expandable graphite of 140 μm was confirmed. In addition, as a result of calculating the index values of CO and CO2 based on the minimum lethal concentration stipulated by the National Institute of Occupational Safety and Health (NIOSH), a low Toxic Index value was confirmed at a particle size of 140 to 173 μm. The calculations of the CO and CO2 Index values are shown in Eqs. 3 and 4, and the Toxic Index was calculated as shown in Eq. 5.

$$CO Index=\frac{CO Generate/500sec}{{CO}_{LC}/300sec}$$
(4)
$${CO}_{2} Index= \frac{{CO}_{2} Generate/500sec}{{CO}_{2, LC} Generate/300sec}$$
(5)
$$Toxic Index= CO Index+{CO}_{2} Index$$
(6)

Figure 16 shows the distribution and trend line of mass loss according to the particle size of expandable graphite.

Fig. 16
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Mass Loss Distribution and Trend line by Expandable Graphite particle

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As a result of mass reduction data analysis, the lowest average mass reduction in the expandable graphite particle size of 140 μm was confirmed, and as the particle size of expandable graphite increased, the mass reduction decreased and then increased again in the particle size of 180 μm ~ 279 μm.

Figure 17 illustrates the distribution and trend line of data measured in the LFA 1000 experiment for thermal diffusivity at 20 °C as a function of the expandable graphite particle size.

Fig. 17
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Test of LFA 1000 Thermal Diffusivity at 20 °C Distribution and Trend line by Expandable Graphite particle

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The LFA 1000 Test was conducted at a temperature of 20℃, and it was verified that, as the particle size of expandable graphite increased, the thermal diffusion rate decreased.

Conclusion

To determine the effect of expandable graphite particle size on the fire performance and thermal diffusivity of the cellulose materials manufactured by the LWC-3D printer, ISO 5660–1 heat release and LFA 1000 thermal diffusivity tests were conducted, leading to the following conclusions:

  1. 1.

    Cellulose finishing materials manufactured with LWC-3D printers have uniform Peak heat release and Total heat release, as well as uniform CO, CO2 generation, Mass loss, and Thermal Diffusivity.

  2. 2.

    The ISO 5660–1 test was conducted on five specimens with different expandable graphite particle sizes, and it was confirmed that the PHRR and THR were the lowest with the expandable graphite particle size of 173 μm.

  3. 3.

    Cellulose finishes produced in the ratio of 100g of cellulose, 30g of expandable graphite, and 20g of magnesium hydroxide met the flame retardant performance standards required by the ISO 5660–1 test in the range of 104 μm to 279 μm in particle size, and can be used as a building finishing material with flame retardant performance.

  4. 4.

    The ISO 5660–1 test results show that the amount of CO produced and CO2 produced changed as the THR decreased with the size of the expandable graphite particles. The particle size change of expandable graphite added to the cellulose finishing material manufactured by calculating the Toxic Index for CO and CO2 production affected the combustion gas production amount and the Toxic Index.

  5. 5.

    A comprehensive analysis of ISO 5660–1 test results showed that the section with the best fire performance was 140 μm to 173 μm, and the inflection point of the fire performance was shown from the particle size of 173 μm or more.

  6. 6.

    Analyzing the change in thermal diffusion rate through the LFA 1000 thermal diffusion rate measurement test confirmed that the increase in particle size of the added expandable graphite leads to a decrease in thermal diffusion rate of the cellulose construction finishing material, and basic data on insulation performance were secured by securing data on the change in thermal diffusivity as the particle size of expandable graphite increased.