A Preliminary Study on the Evaluation Method of "Expansion" of Particleboard

Abstract: In recent years, particleboard has been frequently found to have dimensional bulge issues when used in large-format custom furniture applications. To standardize particleboard quality and promote its application, this paper introduces two current domestic and international standards for testing the dimensional stability of engineered wood products. Following these standard methods, the paper tests three groups of different types of particleboard with varying widths and thicknesses, analyzing the impact of the testing methods on the results. The aim is to provide a reference for designing rapid testing methods and indicators for the dimensional stability of particleboard products.

Keywords: particleboard; expansion gauge; dimensional stability; standard

Discussion of Standard Methods for evaluing Linear Expansion in Particleboard

Gao Li, Luo Shupin, Lyu Bin, Chang Liang

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China

Abstract: This study addresses quality issues related to the linear expansion of particleboard in interior applications. Two existing standards for testing the dimensional stability of wood-based panels were introduced, and three types of particleboard were evaluated using these methods. The influence of testing methods on the results was analyzed, providing a reference for developing dimensional stability specifications for particleboard products.

Key words: particleboard; linear expansion; dimensional stability; standard

Particleboard, as an important type of engineered wood with high resource utilization, has been widely promoted and applied worldwide. In 2023, my country's particleboard consumption reached 41.16 million m3, most of which was used for custom-made interior furniture [1]. However, the minimalist design style popular in recent years uses a large number of large-sized components, such as door panels and wall panels that reach the ceiling. User feedback indicates that the quality problem caused by the "expansion" of particleboard has become a prominent issue affecting the image of particleboard products [2].

The industry term "expansion" refers to the phenomenon where the length and width of particleboard increase due to changes in environmental temperature and humidity. International standards refer to this as linear expansion. Wood, bamboo, or straw, as the main raw materials for particleboard, are prone to moisture absorption or desorption in the environment, causing expansion or contraction and resulting in dimensional changes in the particleboard.

In the product standard GB/T 4897-2015 "Particleboard", dimensional stability related to sheet bulging is a performance characteristic that can be negotiated based on the needs of both the supplier and the buyer, and there are no specific index values. The method used is Chapter 4.35 "Dimensional Stability Determination - Method 1" in GB/T 17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The widespread use of large-size components highlights the severity of the bulging problem, and the home furnishing industry urgently needs appropriate methods and means to control the various losses caused by bulging. Therefore, this paper introduces two current domestic and international standard methods for testing the dimensional stability of wood-based panels and their characteristics. Then, referring to the standard methods, three groups of particleboard dimensional stability evaluations and analyses were conducted, providing technical reference for the next step of designing rapid evaluation methods and indicators for particleboard bulging.

1. Dimensional stability evaluation methods and indicators

1.1 Dimensional stability determination – Method 1

GB/T 17657-2022, "Determination of Dimensional Stability-Method 1," is the main method for testing the dimensional changes ("expansion measurement") of particleboard, fiberboard, and their products. It measures the dimensional changes of a specimen due to variations in relative humidity at a temperature of 20 ℃. Specifically, eight specimens are cut from the sample, four transversely and four longitudinally, each with dimensions of (300±1) mm in length and (50±1) mm in width. These specimens are then divided into two groups (each group consisting of two transverse and two longitudinal specimens). Two processes are performed according to the conditions specified in Table 1: humidification (Group 1) or drying (Group 2). The rate of change in step 3 relative to step 2 is calculated, i.e., the rate of change in dimensions relative to 65% relative humidity. The average of the length and thickness changes of the four specimens in the same group is the rate of change in length and thickness for that process. The sum of the absolute values ​​of the two groups is the rate of change for the entire process (relative humidity from 30% to 85%).

Table 1. Procedure for Balancing Specimens for Dimensional Stability

Factors affecting the dimensional stability of particleboard include the anisotropic properties of wood, the shape of wood chips (length, thickness, etc.), the water resistance and strength of adhesives, and manufacturing process parameters. Method 1 evaluates the dimensional changes of particleboard under moisture equilibrium conditions with variations in humidity, reflecting the shrinkage and expansion characteristics of particleboard itself after considering the above factors. This method originates from the European standard EN 318 Wood-based panels-Determination of dimensioThe dimensional changes associated with changes in relative humidity [3] differ in that the sample size of the European standard (16 samples) is twice that of the national standard (8 samples), resulting in relatively higher reliability of the results. The dimensional stability index in the European particleboard product standard EN 312:2010 Particleboards - Specifications is also negotiated between the supplier and the buyer.

In European standard EN 12871:2013 Wood-baIn the section "Determination of performance characteristics for load-bearing panels for use in floors, roofs and walls", the linear expansion rate of load-bearing engineered wood panels used in floors, roofs and walls is required to not exceed 4 mm/m. The specific method is to test the total linear change rate of relative humidity from 30% to 85% according to standard EN 318, which is the sum of the absolute values ​​of the results of the two groups in Table 1: humidification (Group 1) and drying (Group 2).

American Standard ASTM D 1037 Standard Test Methods for evaluing Properties of Wood-baThe method described in Section 24, “Linear expansion with change in moisture content,” of the Fiber and Particle Panel Materials is similar to “Method 1” mentioned above, but the main differences are: 1) Specimen size: The ASTM standard requires a specimen width of 3 inches (approximately 76 mm) and a length of not less than 12 inches (approximately 305 mm); 2) Test method: The ASTM standard uses a temperature of 20 °C and a relative humidity of 50%. Regardless of whether humidification or drying is performed, the ASTM standard does not require step 1 as specified in Table 1, thus shortening the test cycle by about 1/3; 3) Characterization of results: The linear expansion rate is used from 50% to 80% relative humidity (the linear expansion rate after relative humidity increases from (50±2)% to (80±3)% relative humidity to achieve environmental equilibrium), or the linear expansion rate is used from 30% to 80% relative humidity (the sum of the absolute values ​​of the two sets of change rates from 50% to 30% and from 50% to 80% relative humidity). The American standard ANSI A208.1-2016 Particleboard specifies dimensional stability indicators for medium-density (640–800 kg/m³) and low-density (m³) particleboard, as well as flooring and flooring substrate products. It requires that at a relative humidity of 50% to 80%, the linear expansion rate of medium-density and low-density particleboard (used for furniture or decoration) should be less than 0.4%, outdoor flooring less than 0.3%, and flooring substrates, stair treads, etc. less than 0.35%.

1.2 Dimensional stability determination – Method 2

GB/T 17657-2023, "Determination of Dimensional Stability-Method 2," is an accelerated evaluation method applicable to products such as thermosetting resin-impregnated high-pressure decorative laminates (HPL), derived from the international standard ISO 4586. Six specimens (120±1) mm × (120±1) mm in size were initially placed in a constant temperature and humidity chamber at (23±2)℃ and (50±5)% relative humidity for at least 72 hours. Three specimens were used for the dry heat test, dried in a forced-air drying oven at (70±2)℃ for 24 hours; the other three specimens were used for the high humidity test, treated in a constant temperature and humidity environment at (40±2)℃ and 90%–95% relative humidity for (96±4) hours. The dimensional change rate of the specimens in both the dry heat and high humidity groups was calculated separately, and the total dimensional change rate was the sum of the absolute values ​​of the two groups. Method 2 has a minimum test cycle of approximately 7 days, significantly shortening the test period.

To further evaluate the impact of testing methods on the dimensional stability of particleboard, three particleboard products using different adhesives were selected, and methods 1 and 2 were used to compare and analyze the dimensional stability testing methods.

2. Comparison of evaluation methods for the dimensional stability of particleboard

2.1 Particleboard Sample

Three sets of three-layer particleboard boards were sourced from different manufacturers. Set 1 is E0 grade particleboard, primarily using modified urea-formaldehyde resin adhesive, with a liquid adhesive application rate of 15.4% and a paraffin emulsion application rate of 0.6%. Set 2 is F★★★★ grade particleboard, where a small amount of isocyanate adhesive replaces part of the modified urea-formaldehyde resin adhesive in the core layer, with a modified urea-formaldehyde liquid adhesive application rate of 14.3%, an isocyanate adhesive application rate of 0.3%, and a paraffin emulsion application rate of 0.8%. Set 3 is ENF grade particleboard, primarily using isocyanate adhesive, with an isocyanate adhesive application rate of 3.2%, a tackifier application rate of 0.9%, and a paraffin emulsion application rate of 1.1%.

2.2 Testing and Calculation Methods

Three groups of particleboard specimens were cut according to methods 1 and 2 mentioned above. The calculation methods for the dimensional stability in the length direction of method 1 are as shown in formulas 1-2, and the calculation methods for the dimensional stability in the thickness direction are as shown in formulas 3-4. However, method 1 does not require recording the difference in moisture content. This paper analyzes the changes in moisture content.

In formulas 1-4, l30, l65, and l85 represent the lengths between measurement points at 20 ℃ and relative humidity of 30%, 65%, and 85%, respectively. Δl65,85 indicates the relative change in length from 65% to 85% relative humidity, and Δl65,30 indicates the relative change in length from 65% to 30% relative humidity. Similarly, t30, t65, and t85 represent the thicknesses at 20 ℃ and relative humidity of 30%, 65%, and 85%, respectively. Δt65,85 indicates the relative change in thickness from 65% to 85% relative humidity, and Δt65,30 indicates the relative change in thickness from 65% to 30% relative humidity. Likewise, Δmc65,85 and Δmc65,30 represent the differences in equilibrium moisture content from 65% to 85% (Group 1 of Table 1) and from 65% to 30% (Group 2 of Table 1), respectively.

When calculating dimensional stability with relative humidity from 30% to 85%, Δl30,85, Δt30,85, and Δmc30,85 represent the changes in length, thickness, and moisture content, respectively, and are the sum of the absolute values ​​of the results for humidification and drying.

Method 2 only requires recording the rate of change of dimensions along the length direction, as shown in Formula 5. This paper also analyzes the changes in thickness and moisture content.

In Formula 5, ΔL represents the rate of change of length after dry heat or high humidity test treatment, and l1 and l2 represent the initial length and the final length after treatment, respectively. The thickness change rate is calculated using the same method, and the difference in moisture content of the specimen under different treatment conditions is recorded.

2.3 Experimental Results and Discussion

To analyze whether dimensional stability is related to the main physical and mechanical properties, Table 2 lists the main physical and mechanical properties of three groups of particleboard. Tables 3 and 4 show the results of dimensional stability tests conducted by Method 1 and Method 2, and the calculations obtained according to Formulas 1 to 5.

Table 2 Main physical and mechanical properties of particleboard

Table 3 Dimensional stability of particleboard (Method 1)

Table 4 Dimensional stability of particleboard (Method 2)

Note: Length and thickness variation rates are accurate to 0.05%.

Table 2 shows that all three groups of boards met the performance requirements for furniture-grade (P2 type) particleboard under dry conditions. The moisture content of all three groups exceeded 5%, with sample #3 (formaldehyde-free board) having the highest moisture content. Boards #1 and #3 had the same density, while board #2 had a density approximately 3% lower than the other two groups, but its overall performance was the best. The water absorption thickness swelling rate reflects the dimensional stability of the board in the thickness direction. Board #1 (E0 grade) showed a relatively large change in 24-hour water absorption thickness swelling rate. Besides the water-blocking effect of the paraffin waterproofing agent, the difference in the water resistance of the adhesive was crucial. Adding isocyanate adhesive improved the board's water resistance, resulting in a smaller 24-hour water absorption thickness swelling rate. Board #2 (F★★★★ grade) had slightly better water resistance than board #3 (ENF grade), which was significantly correlated with the amount of adhesive applied.

Table 3 shows that the testing period using Method 1 was 45 days. If the difference in moisture content is not considered, the dimensional stability of the ENF grade particleboard (No. 3) in terms of width and thickness is better than that of No. 1 and No. 2. However, considering the difference in moisture content (Δmc30, 85), it can be found that the changes in width and thickness are closely related to the change in moisture content. Using isocyanate adhesives with good waterproof performance helps reduce deformation. The moisture content change of E0 particleboard is as high as 13.0%, resulting in the most significant changes in width and thickness. From a temperature of 20 ℃ and relative humidity of 30% to 85%, the maximum length change reached 4.6 mm/m, similar to the situation where boards used in a dry northern environment are transferred to a humid southern environment, resulting in a serious "length expansion" problem. According to the European standard EN 12871 Wood-baSED panels - Performance specifications and requirements for load-bearing boards for use in floors, walls and roofs. The linear expansion rate is required to be no more than 4 mm/m. Groups 2 and 3 of particleboard can still meet the dimensional stability requirements for building materials.

Table 4 shows the accelerated testing results of particleboard after equilibration for 5 days at 23 ℃ and 50% relative humidity, with a test period of 9 days. Preliminary tests revealed that the specimens from Method 2 required approximately 14 days to reach equilibrium; therefore, the specimens treated for 5 days had not yet reached equilibrium in terms of moisture content and stress. There are significant differences between the results of Method 2 (Table 4) and Method 1 (Table 3). In Table 4, ENF-grade particleboard No. 3 exhibited the worst dimensional stability and the largest decrease in moisture content during the dry heat test, indicating that the moisture content of the specimens dried at 70 ℃ for 24 hours was close to absolute dryness. High moisture content specimens would have the largest dimensional change rate due to the largest change in moisture content. During the high humidity test, board No. 3 showed better moisture resistance and relatively smaller dimensional changes. In terms of overall changes, the moisture content change rate of ENF grade board No. 3 is close to that of E0 grade board No. 1, but significantly higher than that of F★★★★ board No. 2. Board No. 3 exhibits the largest length change rate compared to the other two groups, but its thickness change remains the smallest. The dimensional stability trends of boards No. 1 and No. 2 obtained using both methods are basically consistent. If only the "sizing expansion" caused by humidification is considered, it can be found that, in addition to the influence of adhesive type and application amount, initial moisture content is also a significant factor affecting dimensional stability.

Combining the 24-hour water absorption thickness expansion rate (24-hour TS) in Table 2, it can be found that the dimensional stability of the board in the thickness direction is well correlated with 24-hour TS. This is closely related to the waterproofness of the adhesive, but it cannot be used to analyze the dimensional stability of the sheet “expansion”.

Methods 1 and 2, two standard methods, show slightly different trends due to their different treatment of the boards. Method 1 reflects the dimensional changes of particleboard after moisture content equilibrium under varying environmental humidity, demonstrating strong repeatability and meeting standard testing requirements. However, its long testing cycle results in significant differences in analyzing the "dimensional expansion" caused by actual applications. Method 2 has a relatively shorter testing cycle, enabling rapid evaluation of the board's "dimensional expansion" characteristics after extreme condition treatment. However, the length of the equilibrium treatment before accelerated testing can affect subsequent test results. If accelerated testing is conducted directly under initial moisture content conditions, the results will more closely resemble actual application scenarios, potentially making it a suitable method for downstream companies to quickly screen and evaluate products. Further exploration of testing methods will be conducted in the future.

3. Conclusion

In summary, "expansion" is a manifestation of the moisture expansion characteristics of particleboard. Controlling the moisture content of the particleboard substrate in different usage environments is crucial, and reducing the temperature and humidity differences between production and usage environments is an important method to reduce "expansion." Different testing methods can address different needs, and further research is needed on how to select or design reasonable and convenient testing methods and setting of indicators.

References: Omitted

Citation format: Gao Li, Luo Shupin, Lü Bin, Chang Liang. A preliminary study on the evaluation method of "expansion" of particleboard [J]. China Wood-based Panels, 2025, 32(1): 11-14. DOI: 10.12393/j.1673-5064.20250103

A Preliminary Study on the Evaluation Method of "Expansion" of Particleboard

Gao Li, Luo Shupin, Lü Bin, Chang Liang

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091