Influence of process parameters on the performance of formaldehyde-free fiberboard

Abstract: Fiberboard was prepared using a formaldehyde-free adhesive with water-based polyamide and biomass protein adhesive as the main agents and PMDI as the crosslinking agent. The effects of process factors such as the application amount of water-based polyamide, biomass protein adhesive, and PMDI, as well as the hot-pressing temperature and time, on the properties of the fiberboard were investigated. The results showed that when the application amount of water-based polyamide was 120 kg/m³, the application amount of biomass protein adhesive was 40 kg/m³, the application amount of PMDI was 7 kg/m³, the hot-pressing temperature was 180 ℃, and the hot-pressing time was 20 s/mm, the physical and mechanical properties of the prepared board met the requirements of LY/T 1611-2011 "Fiberboard for Flooring Substrates".

Currently, the most widely used adhesives in the engineered wood products industry are "three-aldehyde adhesives," represented by urea-formaldehyde resin, phenol-formaldehyde resin, and melamine-formaldehyde resin. All three types of adhesives use formaldehyde as a synthetic raw material. Products bonded with formaldehyde-based adhesives release free formaldehyde during production and use, posing a threat to human health. With increasing environmental awareness and stricter requirements for living environments, the limits on formaldehyde emissions from engineered wood products are becoming increasingly stringent. Besides market demand for more environmentally friendly, low-formaldehyde-emission and formaldehyde-free engineered wood products, product standards are also imposing stricter environmental requirements. The national mandatory standard GB 18580-2017, "Formaldehyde Emission Limits in Engineered Wood Products and Their Products for Interior Decoration and Renovation," promulgated in 2017, officially came into effect on May 1, 2018, specifying a formaldehyde emission limit of 0.124 mg/m³. Therefore, researching the production of formaldehyde-free engineered wood products using formaldehyde-free adhesives is an urgent task for the engineered wood products industry. Daya Wood-based Panel Group Co., Ltd. has developed a formaldehyde-free adhesive using water-based polyamide and biomass protein adhesive as the main agents and polymethylene polyphenyl polyisocyanate (PMDI) as the crosslinking agent. This paper explores the effects of process factors such as the application amount of water-based polyamide, biomass protein adhesive, and PMDI, as well as hot-pressing temperature and time, on the performance of fiberboard, providing a basis and reference for the industrial production and application of formaldehyde-free fiberboard.

1. Amount of waterborne polyamide applied

As shown in Figure 1, with the increase of the waterborne polyamide application amount from 90 kg/m³ to 120 kg/m³, the IB, SS, MOR, and MOE of the test panel all increased with the increase of the application amount, while the 24-hour TS showed a decreasing trend with the increase of the waterborne polyamide application amount. The results indicate that the overall performance of the test panel improves with the increase of the waterborne polyamide application amount. This is because the number of cross-linking points between the adhesive and the fiber increases, and the number of hydrogen bonds formed between the adhesive and the fiber surface after curing increases, thereby improving the performance of the test panel.

2. Biomass Protein Adhesive Application Amount: As shown in Figure 1, increasing the biomass protein adhesive application amount from 30 kg/m³ to 60 kg/m³ resulted in an initial increase followed by a decrease in IB, SS, MOR, and MOE, reaching their highest values ​​at 40 kg/m³. Conversely, the 24-hour TS initially decreased and then increased, reaching its lowest value at 40 kg/m³. These results indicate that a biomass protein adhesive application amount of 40 kg/m³ yielded the best board performance. This is because soybean protein adhesive possesses a certain bonding strength, and its addition within a certain range promotes board performance. However, soybean protein adhesive suffers from drawbacks such as low bonding strength, short shelf life, and poor water resistance. Excessive addition can reduce the proportion of water-based polyamide and PMDI, thus affecting board performance.

As shown in Figure 1, with the PMDI application rate increasing from 4 kg/m³ to 7 kg/m³, the IB, SS, MOR, and MOE of the board samples all increased with the increase in application rate, while the 24-hour TS showed a decreasing trend with the increase in application rate. The results indicate that the overall performance of the test boards improves with the increase in PMDI application rate. This is because PMDI contains highly unsaturated isocyanate groups (NCO-, structural formula -N=C=O), thus exhibiting very high chemical activity and excellent bonding strength.

4. Hot-pressing temperature: As shown in Figure 1, with the hot-pressing temperature increasing from 170 ℃ to 200 ℃, the IB, SS, MOR, and MOE of the test board first increased and then decreased, while the 24 h TS first decreased and then increased, with the best performance of the board at 180 ℃. This is because increasing the hot-pressing temperature within a certain temperature range improves heat transfer performance and ensures sufficient curing of the adhesive, thus improving the performance of the test board. However, if the temperature is too high, the surface adhesive is prone to over-curing, reducing heat transfer efficiency, affecting the curing of the core layer of the board, reducing cross-linking points, and thus affecting the performance of the board.

5. Hot-pressing time: As shown in Figure 1, when the hot-pressing time increased from 10 s/mm to 25 s/mm, the IB, SS, MOR, and MOE of the test board generally showed a trend of first increasing and then decreasing, while the 24-hour TS showed a trend of first decreasing and then increasing. The board performance was best when the hot-pressing time was 20 s/mm. This is because extending the hot-pressing time allows the adhesive to cure fully, improving the performance of the test board; however, excessively long hot-pressing times will reduce the number of cross-linking points, thus affecting the performance of the fiberboard.

In summary, the analysis shows that the production of formaldehyde-free fiberboard using a formaldehyde-free adhesive with water-based polyamide and biomass protein adhesive as the main agents and PMDI as the crosslinking agent is influenced by several factors. The application amounts of water-based polyamide, biomass protein adhesive, and PMDI, as well as the hot-pressing temperature and time, all affect the performance of the formaldehyde-free fiberboard. Among these, the application amounts of water-based polyamide and PMDI are the most significant influencing factors. With increasing application amounts of both water-based polyamide and PMDI, the performance of the test boards shows a clear upward trend. Within the experimental range, the optimal production process for formaldehyde-free fiberboard is as follows: water-based polyamide application amount 120 kg/m³, biomass protein adhesive application amount 40 kg/m³, PMDI application amount 7 kg/m³, hot-pressing temperature 180 ℃, and hot-pressing time 20 s/mm.

6. Optimized Process Verification Test: The optimal process within the test range was adopted, namely, waterborne polyamide application rate of 120 kg/m³, biomass protein adhesive application rate of 40 kg/m³, PMDI application rate of 7 kg/m³, hot-pressing temperature of 180 ℃, and hot-pressing time of 20 s/mm. The performance test results of the test panels are listed in the table below.

The results show that the physical and mechanical properties of the test plate met the requirements of LY/T 1611-2011, and all performance indicators exceeded the standard values.

"1) When using formaldehyde-free adhesives with water-based polyamide and biomass protein glue as the main agents and PMDI as the crosslinking agent to produce formaldehyde-free fiberboard, the amount of each component applied, the hot-pressing temperature, and the hot-pressing time all affect the performance of the board. Among them, the amount of water-based polyamide and PMDI applied has a greater impact on the performance of the board; increasing the amount of water-based polyamide and PMDI applied has a significant effect on improving the performance of the board."

2) The formaldehyde-free fiberboard prepared under the optimal process conditions proposed in this experiment met the requirements of LY/T 1611-2011 "Fiberboard for Flooring Substrates".

Citation format: Chen Xiulan, Wang Li, Wang Junwei. Influence of process parameters on the performance of formaldehyde-free fiberboard [J]. China Wood-based Panels, 2018, 25(10): 30-33.

Author's Affiliation: Daya Artificial Board Group Co., Ltd.