The Effect of Diluent on the Release of Benzene Series from Nitrocellulose-Lacquered MDF

Abstract

Because the surface of MDF is not aesthetically pleasing, it usually needs to be veneered and then painted, but such a board releases harmful VOCs, among which Benzene Series is the most harmful. Benzene and its series are a group of carcinogenic compounds. With the diluents of nitrocellulose (NC)-lacquered MDF as the research objects, the release of the Benzene Series was studied to provide a scientific basis for pollution control and a reference for eco-friendly paint production. The attenuant of NC paint, anhydrous ethanol, ethyl acetate and solvents mixed with different alcohol ester ratios were used as diluents in NC lacquer. Two kinds of wood-veneered MDF with different thickness (18 mm and 8 mm) were coated with NC lacquer and analyzed in the experiment. The gas was collected using a small environmental chamber and the Benzene Series was analyzed using GC-MS. The concentration of Benzene Series released by MDF was 316.24 μg·m⁻³, and that of the NC-lacquered MDF with thicknesses of 18 mm and 8 mm were 284.44 μg·m⁻³ and 281.06 μg·m⁻³, respectively. The MDF released 14 kinds of Benzene Series, and the NC-lacquered MDF with two thicknesses released 18 kinds of it. The release concentration order of Benzene Series in NC-lacquered MDF with different diluents of the 18 mm thick panel was NC-M, NC-A, and NC-E from high to low. The lowest concentration of it occurred when the ratio of anhydrous ethanol to ethyl acetate was 1:3, and the lowest amount of components were at the ratio of 1:2. The concentration of Benzene Series released by MDF is higher than that released by the NC-lacquered MDF. Thickness has no effect on the type of release. The thicker MDF was, the higher the concentration was. The alcohol and ester thinner can control the release of Benzene Series from the source, and the optimal mixing ratio was 1:3.

1. Introduction

MDF has superior processing and decorative properties, as well as a smooth and flat surface. Thus, people can paste finishing materials such as decorative paper, wood veneer, and carve it into a variety of styles, effortlessly [1]. Currently, it is widely used in wooden furniture manufacturing industry.

Benzene Series is the main component of volatile organic compounds (VOCs) [2], and it is the general term for benzene and its derivatives [3]. In a broad sense, it includes all aromatic compounds containing benzene rings; in a narrow sense, it specifically refers to BTEX (Benzene, Toluene, Ethylbenzene and Xylene), which is distributed in nature and can harm human health [4]. Harmful Benzene Series are often released in the process of furniture and interior decoration, which can cause air pollution in the indoor environment [5,6] and endanger the physical health of residents [7]. Dong H, Wang Q and Wang L et al. have studied the VOCs released by MDF and nitrocellulose finishes [8,9,10,11] and concluded that Benzene Series is the main harmful substances and odor sources [12,13,14,15,16].

Nitrocellulose (NC) lacquer is a kind of typical one-component solvent-based wood paint. VOCs with a complex composition diffuse into the air as diluent evaporation after paint and film curing. Therefore, nitro film has a common weather ability, and it is limited to application in outdoor areas. There has been a lot of research which has focused on the enhancement of film performance and the control of VOCs release [12,13,14,15,16]. At present, there are fewer film-forming substances in the main paint that is on the market. The main paint has a relatively high viscosity, and it is not easy to extend or convenient for applying. Therefore, plenty of diluents are often added to main paint to improve the finishing and leveling of the lacquer and promote the extension and forming the film. Lacquer diluents are mostly prepared using organic solvent mixtures such as alcohols, ketones, esters and benzene. In the process of interior and furniture decoration, the type and addition amount of diluents will directly affect the content of Benzene Series emissions and cause indoor air pollution.

Through the inquiry of the film-forming and main components of the diluent, it was found that the diluent of the B brand nitro paint contained 0~40% low flavor esters, 0~30% alcohols, 0~40% benzene. Obviously, benzene accounted for a larger proportion of them. In fact, benzene solvents in the formulation only play the roles of dilution and reducing the cost. Hence, it was feasible to use esters and alcohols for the modification to reduce the type and content of Benzene Series from the source.

In view of the above situation, this paper intends to start with the analysis of the diluent used with the main coating agent, to seek a diluent formula that can not only meet the coating performance and achieve a cheap price, but also meet the requirements of environmental protection, which is targeted to solve the issue in the current coating process which is in the release of Benzene Series which exceed the standard limits, thus affecting the serious problems of environmental quality.

2. Materials and Methods

2.1. Experimental Materials

Substrate: E1 grade MDF, which was produced by a factory in Guangdong; thickness: 18 mm; moisture content: 6–8%; the tree species: Querba macrophylla; melamine modified urea–formaldehyde resin glue was used as the adhesive for the production of MDF;

Veneer: manrascilla with the thickness of 0.6 mm;

Coating material: B brand NC was the main paint. The mixed solvent diluent of B brand nitrocellulose paint (NC-M), as well as the absolute ethyl alcohol (NC-A) and ethyl acetate (NC-E) for laboratory use were adopted as the diluents. In order to study the effect of the combination of ethyl alcohol and ethyl acetate, this experiment mixed absolute ethanol and ethyl acetate at the ratios of 2:1, 1:1, 1:2, and 1:3, respectively, and then, we coated the MDF, respectively. The main agent and diluent were used to coat at an interval of 12 h between the two coating processes. The specimen was placed in a naturally ventilated room for 28 d where the room temperature was approximately 23 °C. In order to prevent the high release of Benzene Series on the edge of the board, it was sealed with aluminum tape when the gas was collected;

Circulating gases: a high purity of nitrogen and helium, which was produced by Harbin Liming Gas Co., Ltd. (Harbin, China);

Standard solution: the standard solution with the concentration of 200 ng/µL was prepared by using deuterated toluene and methanol as the solute and solvent, respectively.

2.2. Experimental Instruments

Hot pressing equipment: 100 t hot press, which was produced by Shanghai Xinyi Wood-based Panel Machinery Factor (Shanghai, China).

Gas collection equipment:

(1)

Fifteen L small environmental chamber: It was independently developed by the laboratory of Northeast Forestry University, and it had good air-tightness. It has been verified to have a good correlation with the gas release measured of the 1 m³ climate chamber [10];

(2)

Intelligent vacuum pump, which was produced by Chengdu Xinweicheng Technology Co., Ltd. (Chengdu, China), was used for collecting cabin gas;

(3)

Tenax tube, which was produced by Markes Company (Llantrisant, UK), was used for adsorbing gas;

(4)

Thermal analysis of the aging instrument, which was produced by Beijing Beifen Temple Instrument Technology Co., Ltd. (Beijing, China), removed the adsorbent in the tube under a high temperature;

(5)

Gas analysis equipment: Thermal desorption autosampler, thermal desorption instrument and DSQII GC-MS. The thermal desorption autosampler was manufactured by Markes (Llantrisant, UK), and the model was Ultra100 bit autosampler. The thermal desorption instrument was produced by Markes Company in the UK, and the host system Unity was connected to the GC-MS equipment; the carrier gas was nitrogen [17]. The GC-MS equipment was manufactured by Thermo (Waltham, MA, USA).

2.3. Experimental Methods

(1)

Veneer-overlaid MDF: The veneer of Manchuria japonica was used. The hot pressing temperature of the MDF was within the range of 180~230 °C, and the hot pressing factor was 10 s/mm. The MDF was kept in the lab for 30 d after it had been taken off production line. The adhesive used in the hot pressing of the overlaid veneer was prepared independently. The modified urea–formaldehyde resin glue and emulsion glue were mixed at the ratio of 6:4, and 80 g/m² amount of finish was used. After the secondary overlaid operation of hot pressing at the temperature of 165 °C for 60 s and the cutting the veneer overlaid MDF were performed, the boards with dimensions of (length × width × thickness) 150 mm × 75 mm × 8 mm and 150 mm × 75 mm × 18 mm were obtained [9].

(2)

Surface painting: After the veneer overlaid board was left to dry naturally at room temperature, the surface was polished using an electric sander, and the floating dust was cleaned using a brush. Then, the plate without bubbling and cracking was selected for painting. The main paint and diluent at a ratio of 2:1 were used for the bottom and surface paint. The bottom and surface paint were applied three times and twice, respectively. The sandpaper with the sand mesh of 400# was used to finely polish the surface before each painting stage, and the finish quality of the products meets the requirements of “GB/T 37005-2018 Painted Veneered Wood-based panel”. At the end of the painting process, the specimen sides were sealed with aluminum tape so that the Benzene Series could only be released from the top and bottom surface of the board. Finally, the painted specimens were placed in a naturally ventilated room with an ambient temperature of 23 °C for 28 d, then, the Benzene Series released from the painted MDF were collected and analyzed after reaching balance (see GB/T 29899-2013).

(3)

Collection of Benzene Series: In the experiment, the indoor environment was simulated with a 15 L, small environmental chamber [18]. The MDF and painted MDF were placed in the middle of the chamber for 4 h for balanced circulation, and then, they were sampled with a loading factor of 1.5 m²/m³ for both of them. The basic parameters of the environment chamber were set as follows: a gas exchange rate of 1 h⁻¹, a room temperature of (23 ± 0.5) °C, and a relative humidity of (50 ± 3)%. The intelligent vacuum pump was used to collect gas for 12 min, and the gas flow rate was 250 mL/min. A total of 3 L of gas was collected. [9] The gas was collected 3~6 times in each coating condition. After the collection, brass caps were used to seal both ends of the adsorption tube for detection.

(4)

Gas analysis: The adsorption tube after the gas collection was heated and desorbed by a thermal desorption instrument, and the gas was pressurized and injected into the GC-MS. The carrier gas was high-purity helium; the chromatographic column type was DB-5MS; the resolution temperature was 300 °C; the pipeline temperature was 85 °C; the thermal resolution time was 10 min. According to China’s Indoor Air Quality Standard (GB/T18883-2002) [19], qualitative and quantitative analyses were conducted on all of the Benzene Series released from the board [8]. Based on their peak diagrams and total ion flow diagrams, the release of Benzene Series from different MDF was determined.

The analysis was conducted on the premise that the physical and mechanical properties of the board met the national standard performance indexes.

3. Results and Discussion

3.1. Analysis of Benzene Series Released from MDF and Lacquered MDF

In this experiment, the Benzene Series released by the MDF and veneer-overlaid MDF coated with the NC paint which was prepared with the original diluent of B brand were detected. As seen from

Table 1. Composition of Benzene Series and total concentration released by three kinds of MDF (μg·m⁻³).

Composition of Benzene Series	Molecular Formula	Structural Formula	Functional Group	Toxicity	Painted MDF (18 mm in Thickness)	Painted MDF (8 mm in Thickness)	MDF (18 mm in Thickness)
Benzene	C6H6		Non	Low toxicity	45.61	45.42	44.40
Methylbenzene	C7H8		Methyl	Low toxicity	27.65	14.75	60.24
Ethylbenzene	C8H10		Ethyl	Low toxicity	62.62	63.35	59.37
P-xylene	C8H10		Methyl	Slight toxicity	61.10	72.11	43.17
Benzaldehyde	C7H6O		Aldehyde group	Low toxicity	3.09	3.82	/
Styrene	C8H8		Carbon–carbon double bond	Slight toxicity	9.54	7.54	/
1-methylene 1 H-ninhydrin	C10H8		Carbon–carbon double bond	Low toxicity	2.57	2.45	/
2-vinylnaphthalene	C12H10		Carbon–carbon double bond	Low toxicity	5.22	3.61	4.98
Biphenyl	C12H10		/	Low toxicity	1.96	1.51	10.47
Acenaphthene	C12H10		/	Low toxicity	3.14	3.79	12.38
Dimethyl phthalate	C10H10O4		/	Low toxicity	3.45	3.58	3.15
Dibutyl phthalate	C6H22O4		Ester group	Low toxicity	28.57	27.65	21.49
1,2,3-trimethylbenzene	C9H12		Methyl	Low toxicity	1.59	1.21	5.31
Naphthalene	C10H8		/	Low toxicity	4.09	4.20	21.20
Acetophenone	C8H8O		Hydroxyl	Low toxicity	1.49	2.42	/
Diphenylene-oxide	C12H8O		Hydroxyl	Low toxicity	6.91	6.14	1.95
Fluorene	C13H10		/	Low toxicity	10.12	11.59	16.64
1-methyl naphthalene	C11H10		Methyl	Low toxicity	/	/	11.49
9-methylene-9H-fluorene	C14H10		Carbon–carbon double bond	Low toxicity	5.72	5.92	/
Total	-	-	-	-	284.44	281.06	316.24

, as for the MDF decorated with NC paint, both of them released 18 species of Benzene Series, and their total concentrations were 284.44 μg·m⁻³ and 281.06 μg·m⁻³, respectively. In contrast, the MDF released 14 species of Benzene Series, but its total concentration was the highest among the three kinds of MDF, reaching 316.24 μg·m⁻³.

As for the BTEX released from the MDF, the total concentration was 207.18 μg·m⁻³ (accounting for 65.51% of the total concentration of Benzene Series), and the concentrations of benzene, toluene, ethylbenzene and p-xylene were 44.4 μg·m⁻³, 60.24 μg·m⁻³, 59.37 μg·m⁻³ and 43.17 μg·m⁻³, respectively. Among the detected Benzene Series, toluene had the highest concentration, which was followed by ethyl benzene and benzene, then p-xylene. Visibly, BTEX had the largest contribution to the concentration. It was mainly from tree species itself (e.g., oak and eucalyptus wood), and the modified urea–formaldehyde resin adhesives which were added to the wood fiber [13,16]. After the board was hot pressed, the Benzene Series was gradually dispersed into the air from the inside of the MDF. Therefore, the production process of MDF directly affected the Benzene Series pollution.

As for the nitro lacquer-veneered MDF (18 mm in thickness), due to the attachment of veneer and paint film, the Benzene Series detected was mainly from the paint. The sum of the concentration of BTEX was 196.98 μg·m⁻³ (accounting for 69.25% of the total concentration), and it was also the component with the greatest influence on the air quality. The concentrations of benzene, toluene, ethylbenzene and p-xylene were 45.61 μg·m⁻³, 27.65 μg·m⁻³, 62.62 μg·m⁻³ and 61.1 μg·m⁻³, respectively. Among the 18 kinds of Benzene Series, ethylbenzene had the highest concentration, which was followed by p-xylene, benzene, dibutyl phthalate and toluene, which are in order. The concentration of the other 14 kinds of Benzene Series were all lower than 10.12 μg·m⁻³; biphenyl (with the lowest concentration) had a slight influence on the air quality.

As for the nitro lacquer-veneered MDF (8 mm in thickness), the concentration of BTEX released was 195.65 μg·m⁻³ (accounting for 69.6% of the total concentration). The mass concentrations of benzene, toluene, ethylbenzene and p-xylene were 45.42 μg·m⁻³, 14.75 μg·m⁻³, 63.35 μg·m⁻³ and 72.11 μg·m⁻³, respectively. The four components with the highest concentration were p-xylene, ethylbenzene, benzene and dibutyl phthalate, and the other 14 species of Benzene Series had concentrations that were lower than 14.75 μg·m⁻³.

By comparison, it was found that the proportion of BTEX in the Benzene Series released by the above three kinds of MDF reached more than 60% (close to 70%) of the total concentration. Among all of the Benzene Series detected, BTEX was always the top four components in the concentration, indicating that BTEX was the main component affecting the indoor air quality. Styrene, which is slightly toxic, was only detected in the NC-painted MDF. Thus, it could be inferred that the Styrene was mainly from the main agent of the nitro paint. Dibenzofuran, which is toxic, was used to synthesize resin to make nitro paint raw materials. Therefore, its concentration in nitro painted MDF was significantly higher than it was in MDF. Dibenzofuran is easily absorbed by the body, especially for women. The dibenzofuran and styrene released from the MDF had lower concentrations in this experiment. However, the applications of industrial mass production and life is bound to greatly increase its release concentration, so it is necessary to specially enhance the monitoring of them in actual production and daily life.

The thicker the MDF was, the more fiber materials and adhesives that were used in the production process, and then, the higher the concentration of Benzene Series released was. The MDF released the lowest number of Benzene Series species, but its total concentration was the highest. One reason for this is that the veneer of Manchuria japonica that was used was overlaid before it was coated, which had an obvious sealing effect on the release of Benzene Series from the MDF; another reason for this is that the existence of the paint film escaping the contact of the air for the panel hindered the release of Benzene Series produced by the veneered wood itself and adhesives, and this made the paint be the main source of Benzene Series in the MDF.

3.2. The Effect of Diluent on the Release of Benzene Series from Decorative NC-Lacquered MDF

3.2.1. The Effect of Diluent on the Release Concentration of Benzene Series from NC-Painted MDF

In this experiment, absolute ethyl alcohol and ethyl acetate were used to replace the mixed solvent in the original paint to coat the 18 mm MDF. The veneered MDF with the thickness of 18 mm was coated with NC paint which had been prepared with the three diluents mentioned in Section 2.1. The component and concentrations (μg·m⁻³) of all of the Benzene Series detected are shown in

Table 2. The Benzene Series and total concentration released from NC-painted MDF prepared with three diluents (μg·m⁻³).

Composition of Benzene Series	Molecular Formula	Structural Formula	Functional Group	Toxicity	NC-M	NC-A	NC-E
Benzene	C6H6		/	Low toxicity	45.61	14.1	17.44
Methylbenzene	C7H8		Methyl	Low toxicity	27.65	25.28	26.26
Ethylbenzene	C8H10		Ethyl	Low toxicity	62.62	15.02	55.41
P-xylene	C8H10		Methyl	Slight toxicity	61.10	30.16	33.90
Benzaldehyde	C7H6O		Aldehyde group	Low toxicity	3.09	5.32	4.51
Styrene	C8H8		Carbon–carbon double bond	Slight toxicity	9.54	/	2.69
1-methylene 1 H-ninhydrin	C10H8		Carbon–carbon double bond	Low toxicity	2.57	2.61	2.28
2-vinylnaphthalene	C12H10		Carbon–carbon double bond	Low toxicity	5.22	3.34	/
Biphenyl	C12H10		/	Low toxicity	1.96	/	1.71
Acenaphthene	C12H10		/	Low toxicity	3.14	/	3.48
Dimethyl phthalate	C10H10O4		/	Low toxicity	3.45	5.78	3.63
Dibutyl phthalate	C6H22O4		Ester group	Low toxicity	28.57	26.52	18.22
1,2,3-trimethylbenzene	C9H12		Methyl	Low toxicity	1.59	3.36	/
Naphthalene	C10H8		/	Low toxicity	4.09	5.29	3.99
Acetophenone	C8H8O		/	Low toxicity	1.49	/	3.63
Dibenzofuran	C12H8O		Carboxyl	Low toxicity	6.91	7.36	6.02
Fluorene	C13H10		/	Low toxicity	10.12	9.44	9.01
9-methylene-9H-fluorene	C14H10		Carbon–carbon double bond	Low toxicity	5.72	5.24	6.58
Total	-	-	-	-	284.44	198.76	158.82

.

As seen from Table 2, 18 species of Benzene Series were released from NC-M lacquer-veneered MDF, and the total concentration of which was the highest (284.44 μg·m⁻³) under the three coating conditions. Among them, the concentration of both ethylbenzene and p-xylene was more than 60 μg·m⁻³, with it being much higher than that of the other 16 components. Xylene is the main ingredient used to make diluents. Therefore, industrial mixed solvents are the main source of P-xylene that is detected. Ethyl benzene belongs to the 2 B type in the list of carcinogens, and it has an irritant effect on the skin, eyes and respiratory tract mucosa. The results of animal experiments reveal that ethyl benzene can make biological sisters exchange chromosomes and affect the healthy development of embryos. P-xylene is listed as a Class 3 carcinogen. The long-term inhalation or/and short-term inhalation of high concentrations of it will cause central nervous system disorders of the body and mental disorders [20]. According to the toxicity of the two substances, ethylbenzene was considered to be the main hazardous substance under NC-M coating conditions, and p-xylene was considered to be a secondary hazardous substance. Therefore, when they are using NC-M to coat furniture in life, people should continue to monitor the release of these two components to prevent damage to their health. The total concentration of 14 species of Benzene Series released from NC-A-coated MDF was 198.76 μg·m⁻³, which was significantly lower than the total concentration of Benzene Series released under the NC-M coating condition. Among them, p-xylene and dibutyl phthalate had the highest concentration; p-xylene was the main hazardous substance under the NC-A coating condition. Dibutyl phthalate, as a low-toxicity substance, can increase plasticity in the main paint. It is included in the list of suspected teratogens in Proposition 65 of California [21]. It can cause mild allergy and irritation to the human body, and it is a secondary hazardous substance under this coating condition. The total concentration of 16 species of Benzene Series released by the NC-E-coated MDF was the lowest, totaling only 158.82 μg·m⁻³. Like that under NC-M coating condition, ethylbenzene and p-xylene had the highest concentration under the NC-E coating condition, but the concentration of p-xylene decreased by 44% relative to that under the NC-M coating condition, and the concentration of ethylbenzene also decreased a little. Obviously, the use of absolute ethanol as an NC paint diluent significantly reduced the volatilization of the hazardous substances.

Figure 1 intuitively shows the composition and concentration differences of the main Benzene Series released from the MDF after coating it with the NC paint prepared by the three kinds of diluents, including the BTEX (with the greatest impact on the quality of environmental gases), styrene (more harmful to human body), and other benzene-ring-containing compounds. Compared with the original mixed solvent coating board, the detected concentration of each single Benzene Series was significantly reduced, and the total released concentration of Benzene Series was much lower than that of NC-M after using absolute ethanol and ethyl acetate as diluents for the NC paint. Compared with the concentration of the single Benzene Series released by NC-M, the release concentrations of benzene, ethylbenzene, and P-xylene were reduced by 69%, 76%, and 50.6%, respectively, and that of styrene was reduced to 0 when ethyl acetate was used as a diluent. By comparison, the release concentrations of benzene and xylene were reduced by 61.76% and 44.52%, respectively, and the content of styrene decreased by 71.8% when absolute ethyl alcohol was used as a diluent. After the paint film cured, the added organic solvent volatilized outward gradually in the form of gas, and it became the direct source of indoor Benzene Series. The use of single anhydrous ethanol and ethyl acetate as diluents, which had better volatility performances, can prevent the volatilization of Benzene Series from the source. In addition, in the volatilization process of ethyl acetate, the Benzene Series in primary agent content of the paint could be taken away more quickly. Therefore, the amount of Benzene Series was obviously reduced after the gas release of the plates reached equilibrium.

3.2.2. The Effect of Diluent on the Release Proportion of Benzene Series from NC-Painted MDF

The compounds in Figure 2a–c include BTEX, styrene and other Benzene Series. Based on the proportion of each substance, the components affecting the indoor air quality could be intuitively judged. The composition and structure of the Benzene Series changed after coating the MDF with NC-M, NC-E, and NC-A. There were 12 common components of the Benzene Series released from the three kinds of MDF, and BTEX made the largest contribution. The Benzene Series released by NC-M included complex species. The concentration of BTEX was 196.98 μg·m⁻³, accounting for 69.25% of the total concentration of Benzene Series, indicating that the concentration of BTEX would continue to affect the indoor air quality for a period of time. The release of BTEX should be controlled first to improve the air quality. In NC-E and NC-A, the proportion of detected BTEX in the total concentration of BTEX decreased to 30.09% and 42.06%, respectively, which greatly reduced the effect on the air quality. The proportion of benzene decreased from 16.04% to less than 10%, the proportion of styrene in NC-E decreased to 0, and the proportion of styrene in NC-A decreased to 1.43%. When the content of BTEX reduced, other Benzene Series such as phenanthrene, fluorene, and biphenyl accounted for a relatively large proportion. These substances were not released as quickly, and they cause less harm to the human body. Thus, only BTEX had the decisive influence on the total concentration of Benzene Series.

Ethylbenzene and xylene accounted for more than 15% of the substances in NC-M; toluene accounted for more than 15% of the substances in NC-E; ethylbenzene and xylene accounted for the highest proportion of the substances in NC-A; Xylene and toluene are the main components of VOCs, which were released by surface spraying of wood furniture [17]. In 2015, Gao detected the VOCs on NC-painted wooden furniture in the typical wood furniture finishing industry in Guangdong, and they also found that benzene and toluene were the most common Benzene Series released by painted furniture [22]. If these types of solvents are used as diluent in life, it is necessary to strengthen the monitoring of benzene, toluene, ethylbenzene, and xylene concentrations after finishing.

3.2.3. The Effect of Alcohol Ester Ratio on the Release of Benzene Series from NC-Painted MDF

As seen from Section 3.2.1 and 3.2.2, the use of absolute ethanol and ethyl acetate as solvents for NC paint can effectively reduce the release level of the Benzene Series in painted MDF. In order to study the effect of the combination of ethyl alcohol and ethyl acetate, this experiment mixed absolute ethanol and ethyl acetate at the ratios of 2:1, 1:1, 1:2, and 1:3, respectively, and then, we coated the MDF. The gas released from the painted MDF was collected to explore the relationship between the mixture ratio of the alcohol esters and the concentration of the Benzene Series. The most environmentally friendly preparation method was obtained. The mean concentration (μg·m⁻³) is shown in

Table 3. The Benzene Series and total concentration released from MDF painted with mixed diluent prepared with different proportions of alcohol and ester (μg·m⁻³).

Benzene Series	Alcohol: Ester = 2:1	Alcohol: Ester = 1:1	Alcohol: Ester = 1:2	Alcohol: Ester = 1:3
Benzene	40.75	52.35	41.32	31.34
Methylbenzene	43.68	27.83	42.14	36.45
Ethylbenzene	14.05	14.64	13.61	11.51
P-xylene	34.05	36.23	30.52	28.04
Benzaldehyde	5.73	7.98	4.73	4.08
1,2,4-trimethyl-benzene	3.90	3.83	3.46	3.67
Naphthalene	4.81	6.56	7.24	/
1-ethylene-1H-ninhydrin	2.24	2.26	2.19	2.08
Dibenzofuran	5.75	5.83	5.27	5.35
Diethyl phthalate	3.35	4.44	4.29	3.15
Fluorene	10.65	10.32	11.74	10.11
Phenanthrene	7.95	8.42	12.50	5.52
Dimethyl phthalate	7.08	8.12	7.27	3.94
Acenaphthene	3.14	/	3.39	3.11
1-methylene-1H-ninhydrin	9.43	5.67	/	3.99
Dibutyl phthalate	18.83	14.52	/	12.34
Styrene	/	1.77	/	1.58
Total concentration	215.39	209.77	189.67	163.15

and Figure 3.

According to Table 3, 16 species of Benzene Series were released when the ratios of alcohol to ester were 2:1, 1:1, and 1:3; 14 species of Benzene Series were released when the ratio of alcohol to ester was 1:2. Among the Benzene Series released by the above four preparation schemes, benzene, toluene, and xylene had the highest concentrations. In contrast, the concentration of the other Benzene Series was lower than 15 μg·m⁻³, and most of them totaled less than 10 μg·m⁻³, so they had little effect on the air quality. In addition, the concentration of each single benzene component was generally lower than that of other preparation methods when the ratio of alcohol to ester was 1:3. The concentration of the Benzene Series released from the MDF was 215.39 μg·m⁻³, 209.77 μg·m⁻³, 189.67 μg·m⁻³, and 163.15 μg·m⁻³, respectively, when the ratios of absolute ethanol to ethyl acetate were 2:1, 1:1, 1:2, and 1:3. Under these coating conditions, the total concentration of the Benzene Series was lower than that of NC-M. With the increase in the amount of ethyl acetate added into the diluent, the total concentration of Benzene Series gradually decreased. When the ratio of absolute ethanol to ethyl acetate was 1:1, the concentration of the Benzene Series released was 193.81μg·m⁻³, which was close to the concentration of the Benzene Series released when absolute ethanol was used as a diluent alone. When the ratio of ethyl acetate was 1:3, the concentration of the Benzene Series released from the painted MDF was significantly lower than that of other coating proportions, with it being close to the coating effect of NC-E, and the release amounts of styrene and dibutyl phthalate were 0. Esters are the main solvent lacquer in industrial production. In order to reduce the production cost, enterprises will use the right amount of alcohol as a co-solvent, which only has a dilution effect, and the diluent gradually evaporates during oven curing. Only a film-forming material in the main paint works in the paint film. Hence, there is more space for choices during the preparation of the diluent.

As can be seen more intuitively from Figure 3, benzene, toluene, and xylene were the Benzene Series components with the highest release concentrations under various coating ratios. When the ratio of absolute ethanol to ethyl acetate was 1:3 and ethyl acetate was used as diluent alone, the Benzene Series released from the MDF had the lowest concentration. When we studied the MDF that was coated by mixing absolute ethanol and ethyl acetate in above four proportions, it was found that the higher the proportion of ethyl acetate was, the lower the total concentration of the Benzene Series detected in the case of alcohol ester mixture was, but there was no obvious linear relationship between them. The reason for this is that ethyl acetate is easily hydrolyzed, and it will gradually hydrolyze to acetic acid and ethanol when water is present at room temperature. P-xylene and ethanol do not react, but they can carry out molecular diffusion between each other, so they can be mutually soluble. The same is true for ethylbenzene. As a result, the concentration of volatilization reduced to a certain degree.

4. Conclusions

(1)

The concentration of Benzene Series released by MDF was 316.24 μg·m⁻³, and there are a total of 14 kinds of substances;

(2)

The concentrations of Benzene Series released by the NC-lacquered MDF with thicknesses of 8 mm and 18 mm were 281.06 μg·m⁻³ and 284.44 μg·m⁻³, respectively. The thicker MDF was, the higher the concentration of Benzene Series released was. However, the thickness of the MDF has no effect on the type of Benzene Series released. The NC-lacquered MDF with two thicknesses released 18 kinds of substances;

(3)

The release concentration order of Benzene Series in NC-lacquered MDF with different diluents on the 18 mm thickness panel is NC-M, NC-A, and NC-E from high to low;

(4)

The experimental data of the current study also adequately proves that single alcohols or ester solvents have an obvious effect on reducing the release of Benzene Series, and esters are one of the ideal choices for lacquer diluents, and where conditions permit, replacing mixed solvent containing Benzene Series with esters will reduce the release of Benzene Series from the source. To reduce the cost of the solvents, people can add a small amount of alcohol in the formula; we advise that they should try not to choose benzene as an auxiliary solvent;

(5)

When MDF was painted with a mixed diluent prepared using different proportions of alcohol and ester, the Benzene Series released the lowest concentration the ratio of anhydrous ethanol to ethyl acetate of 1:3, and the Benzene Series released the lowest amount of components at the ratio of 1:2. Obviously the use of alcohol and ester thinner can reduce the release of Benzene Series, and the mixing ratio of 1:3 is recommended.