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Introduction: Fiberboard was prepared using a formaldehyde-free adhesive composed of water-based polyamide and biomass protein glue as the main agents and PMDI as the crosslinker. The effects of process factors such as the amount of water-based polyamide, biomass protein glue, PMDI, hot-pressing temperature, and hot-pressing time on the fiberboard's properties were investigated. The results showed that when the water-based polyamide loading was 120 kg/m³, the biomass protein glue loading was 40 kg/m³, the PMDI loading was 7 kg/m³, the hot-pressing temperature was 180°C, and the hot-pressing time was 20 s/mm, the physical and mechanical properties of the resulting board met the requirements of LY/T 1611-2011, "Fiberboard for Flooring Substrates."
Currently, the most widely used adhesives for wood-based panels are "trialdehyde adhesives," represented by urea-formaldehyde resins, phenol-formaldehyde resins, and melamine-formaldehyde resins. These three adhesives all use formaldehyde as a synthetic raw material. Products bonded with formaldehyde-based adhesives release free formaldehyde during production and use, posing a health hazard. With growing environmental awareness and increasingly stringent living standards, restrictions on formaldehyde emissions from wood-based panels are becoming increasingly stringent. In addition to market demand for more environmentally friendly low-formaldehyde and formaldehyde-free wood-based panels, product standards are also tightening their environmental standards. The national mandatory standard GB 18580-2017, "Limits of Formaldehyde Emission from Wood-Based Panels and Their Products for Interior Decoration and Renovation Materials," promulgated in 2017, officially came into effect on May 1, 2018. It stipulates a formaldehyde emission limit of 0.124 mg/m³. Therefore, research into the use of formaldehyde-free adhesives to produce formaldehyde-free wood-based panels is a pressing task for the wood-based panel industry. Daya Wood-Based Panels Group Co., Ltd. has developed a formaldehyde-free adhesive based on water-based polyamide and biomass protein glue, with polymethylene polyphenyl polyisocyanate (PMDI) as a crosslinker. This article explores the effects of process factors such as the amount of water-based polyamide, biomass protein glue, PMDI, hot-pressing temperature, and hot-pressing time on fiberboard performance, providing a basis and reference for the industrial production and application of formaldehyde-free fiberboard.
1. Application amount of water-based polyamide
As shown in Figure 1, as the waterborne polyamide dosage increases from 90 kg/m³ to 120 kg/m³, the IB, SS, MOR, and MOE of the test panels all increase. However, the 24-h TS decreases with increasing waterborne polyamide dosage. These results indicate that the overall performance of the test panels improves with increasing waterborne polyamide dosage. This is due to an increase in crosslinking points between the adhesive and the fiber, which in turn increases the hydrogen bonds formed between the adhesive and the fiber surface after curing, leading to improved panel performance.
2 Biomass Protein Glue Application Amount. As shown in Figure 1, as the biomass protein glue application amount increases from 30 kg/m³ to 60 kg/m³, the IB, SS, MOR, and MOE of the test panels initially increase and then decrease, reaching their highest values at 40 kg/m³. The 24-h TS of the test panels initially decreases and then increases, reaching its lowest value at 40 kg/m³. The results indicate that a biomass protein glue application amount of 40 kg/m³ yields the best panel performance. This is because soy protein adhesives possess a certain degree of bonding strength, and addition within a certain range of amounts can improve panel performance. However, soy protein adhesives have drawbacks such as low bonding strength, a short shelf life, and poor water resistance. High addition amounts can reduce the proportions of water-based polyamide and PMDI, thereby affecting panel performance.
3PMDI Application Amount: As shown in Figure 1, as the PMDI application amount increases from 4 kg/m³ to 7 kg/m³, the IB, SS, MOR, and MOE of the panels increase, while the 24-h TS decreases with increasing PMDI application. These results indicate that increasing PMDI application improves the overall performance of the panels. This is because PMDI contains highly unsaturated isocyanate groups (NCO-, structural formula -N=C=O), resulting in high chemical activity and excellent bonding strength.
4. Hot Pressing Temperature. As shown in Figure 1, as the hot pressing temperature increases from 170°C to 200°C, the IB, SS, MOR, and MOE of the test panels first increase and then decrease, while the 24-h TS first decreases and then increases. The panel performance is optimal at 180°C. This is because increasing the hot pressing temperature within a certain range enhances heat transfer and fully cures the adhesive, thus improving panel performance. However, excessively high temperatures can overcure the surface adhesive, reducing heat transfer efficiency, affecting the curing of the core layer, and reducing crosslinking points, thus affecting panel performance.
5. Hot Pressing Time. As shown in Figure 1, when the hot pressing time increases from 10 s/mm to 25 s/mm, the IB, SS, MOR, and MOE of the test panels generally show an upward trend followed by a downward trend. The 24-h TS shows an initial downward trend followed by an upward trend. The best performance is achieved when the hot pressing time is 20 s/mm. This is because extending the hot pressing time allows the adhesive to fully cure, improving the test panel's performance. However, excessive hot pressing time reduces the number of crosslinking points, thus affecting the fiberboard's performance.
In summary, when using a formaldehyde-free adhesive consisting of water-based polyamide and biomass protein glue as the main agents and PMDI as the crosslinker to produce formaldehyde-free fiberboard, the performance of the board was influenced by the amount of water-based polyamide, biomass protein glue, PMDI, hot pressing temperature, and hot pressing time. The main influencing factors were the amount of water-based polyamide and PMDI added. With increasing water-based polyamide and PMDI additions, the performance of the test boards showed a significant upward trend. Within the experimental range, the optimal production process for formaldehyde-free fiberboard was: water-based polyamide loading of 120 kg/m³, biomass protein glue loading of 40 kg/m³, PMDI loading of 7 kg/m³, hot pressing temperature of 180°C, and hot pressing time of 20 s/mm.
6. Optimized Process Verification Test: The optimal process within the test range was used for verification testing: a waterborne polyamide application rate of 120 kg/m³, a biomass protein adhesive application rate of 40 kg/m³, a PMDI application rate of 7 kg/m³, a hot pressing temperature of 180°C, and a 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 panels meet the requirements of LY/T 1611-2011, and all performance indicators exceed the standard values.
"1) When using a formaldehyde-free adhesive composed of water-based polyamide and biomass protein glue as the main agents and PMDI as the cross-linking agent to produce formaldehyde-free fiberboard, the application amount of each component, hot pressing temperature, and hot pressing time all affect the board performance. Among them, the application amount of water-based polyamide and PMDI has the greatest impact on the board performance. Increasing the application amount of water-based polyamide and PMDI has a significant effect on improving the board performance.
2) Under the optimal process conditions proposed in this experiment, formaldehyde-free fiberboard was prepared, and its physical and mechanical properties met the requirements of LY/T 1611-2011 "Fiberboard for Flooring Substrates".
Citation format: Chen Xiulan, Wang Li, Wang Junwei. Effect of process parameters on the properties of formaldehyde-free fiberboard[J]. China Wood-Based Panels, 2018, 25(10): 30-33.
Author's unit: Daya Wood-Based Panels Group Co., Ltd.