Investigations for increasing the 3D-forming potential of high-density fiberboards

doi:10.21203/rs.3.rs-4124478/v1

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Investigations for increasing the 3D-forming potential of high-density fiberboards

https://doi.org/10.21203/rs.3.rs-4124478/v1

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published 09 Aug, 2024

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The trend in modern interior design leans towards curved and shaped surfaces. This cannot be achieved with flat materials without additional effort. Materials from renewable resources, such as wood-based materials, are material- and energy-intensively processed to enable larger deformations. Therefore, this study deals with methods to increase the deformation potential of adhesive-free high-density fiberboards. One method is plasticizing in a saturated steam atmosphere, which is well known from the bending of solid wood. The second is the application of a special kerf pattern that geometrically increases the deformability. The combination of both methods was also investigated. Uniaxial tensile tests were performed to evaluate the deformation potential of the methods used. The strain along and transverse to the tensile direction as well as the modulus of elasticity and Poisson's ratio were determined as results. All the methods investigated lead to an increase in the maximum strain along the tensile load; steaming by a factor of 2, kerf patterning by a factor of 4 and the combination by a factor of 10 as compared to solid fiberboard. The application of the kerf pattern causes an auxetic material behavior with a negative Poisson's ratio. The combination of both methods reduces the modulus of elasticity by a factor of more than 100. Overall, the investigated methods are suitable for increasing the deformation potential of fiberboards with regard to the forming of 3D-shaped surfaces.

fiberboard

plasticizing

steaming

deformation capability

kerf pattern

mechanical properties

Wood-based materials are usually used as boards in the furniture and building industry. The most important board materials are particle boards, fiberboards, plywood and blockboards. The interior construction trend of curved and shaped faces (e.g. Condoroteanu et al. 2022) cannot be realized with these flat materials without additional effort. To realize the desired shaped designs in interior construction, in general, petrochemical or anorganic based materials, such as Krion® or Corian®, are used. Materials based on renewable natural resources, such as wood-based materials, are material- and energy-intensively processed in order to enable larger deformations. Additional processing steps to generate kerfs make these materials a more flexible product (Fig. 1). Different studies show an increase of the flexibility of flat wood-based panels by generating kerfs in the material. Chen et al. (2020) have presented experimental work and simulations for understanding the mechanism of kerf order in panels. They have studied spiral patterns in square and hexagonal cells in 3 mm thick MDF panels. The focus of their work was on simulating deformation and shapeability. Other studies investigate kerfs pattern for increasing the bendability of plywood (Condoroteanu et al. 2022, Parkinson 2022). These investigations examine the statics of furniture made from bent plywood with different kerf patterns and consider solely 2D deformations.

The purposeful use of lathe checks, developing during veneer production, leads to a remarkable higher bendability of so-called bending plywood. But such kerfs or slots only enable 2D deformations; that means a reduction of the bending radii. 3D deformations with wood-based materials can be realized by stacking and forming 3D-veneer® of the company Danzer. These veneers are also slotted and processed in a patented way.

In order to reach a higher flexibility and a 3D deformation, the kerf pattern can be generated in different geometric layouts. Capone and Lanzara (2019) describes the kerfing in a) only on one side of the panel; b) on both sides of the panel and c) completely through the panel for making wood flexible. They use plywood for their investigations with the aim to obtain flexible objects that can be formed back flat.

All 2D and 3D deformations of the examples mentioned above have an elastic behavior. That means they have to be fixed with glue or the like to maintain their shape. Plastic deformations of pure wood-based materials are not possible or are not realized due to the current limitations of the techniques in the field.

Creating a 2D or 3D shaped surface requires an applicable material strain. Depending on the shape, material strain will be applied in different directions simultaneously. However, wood and wood-based materials have a small strain in a low one-figure percentage range that is nearly completely elastic. Investigations for improving strain values or the shapeabilty of wood-based materials by plasticizing in a saturated steam atmosphere, known from solid wood bending (Sandberg et al. 2023), are not known. This might be an approach for improving the deformation capability of wood-based materials. But most of the materials are produced using adhesives. Most likely the adhesives will hinder plasticization due to their bonding properties or because they would be destroyed by hydrolysis, therefore plasticization would probably not have the same effect as with wood. The only wood-based materials without adhesive are fiberboards produced in the wet process (HB or MB according to DIN EN 316; 2009). For these fiberboards, an increase up to the point of achieving a plastic deformation due to a plasticization is at least conceivable.

Another possibility for increasing the deformation potential comes from other material sectors and reached through structural modifications. By cutting special patterns (kerfing) into the material an auxetic material behavior is reached. An auxetic material is characterized by a negative Poisson´s ratio (Gordanshekan et al. 2022). The surface of such a material becomes larger under tension and thus, offers a larger deformation capability. Simplified, that means by stressing the material under tension it becomes longer and wider. Exemplary patterns of auxetic sheet materials are shown in Fig. 2.

Due to its low density and improved mechanical properties, auxetic structures are used in lightweight constructions specially as crash absorbers (Gordanshekan et al. 2022).

With the aforementioned trend toward curved and shaped surfaces in interior construction in combination with the desire to use wood-based materials as renewable materials, the following questions were addressed in this study:

Is it possible to improve the deformation potential of wood-based materials by plasticizing (steaming)?

Is an increase of the deformation potential possible by combining steaming and using a kerf pattern for generating an auxetic material?

Material

Adhesive-free high-density fiberboard (HDF; HB according to DIN EN 316; 2009), produced in a wet process, were used for the investigations. The used fiberboards were 3 mm thick, had an average density of 751 kg/m³ and an average moisture content of 7.6% at 20° C / 65% relative humidity (RH).

In order to study the above-mentioned questions, 4 test series were conducted:

1. HDF, solid material, dry (conditioned and tested at 20° C, 65% RH)

2. HDF, solid material, steamed (plasticized)

3. HDF with kerf pattern, dry (conditioned and tested at 20° C, 65% RH)

4. HDF with kerf pattern, steamed

The samples and the kerf pattern were cut with a CO₂-laser. The kerf pattern was inspired by a structure used for stents in the medical field. It is based on a hexagonal pattern with knots and supporting structures arranged asymmetrically and mirrored (Fig. 3). Such patterns exhibit rotation and deformation under tensile load, which leads to an increase of the overall surface area and shows auxetic material behavior (Asadi et al. 2022). Some patterns are known to be used to expand metal. These patterns use such structures to generate free form surfaces (Beylerian and Dent 2005). The kerf pattern used in this study, based on the patterns described above, was developed in pre-investigations considering the material characteristics of wood-based materials. The bar width of the pattern is 2 mm.

Experimental

The material deformation was measured with an uniaxial tension test (Fig. 4a) using an universal testing machine (Inspect 10 Hegewald & Peschke, Nossen, Germany).

For the strain measurement, a stereo camera system (Aramis adjustable 12M, Carl Zeiss GOM Metrology, Braunschweig, Germany) was used which applies the principles of digital image correlation (DIC). A contrast speckle pattern was applied to the area of interest (AOI) of the specimen surface. With this system, both the strain along the loading direction ε_x and the strain transverse to the loading direction ε_y of the specimen can be measured. We consider these strain values are considered to correspond to the deformation capability of the material and were chosen for evaluation.

The Aramis-system has different possibilities for evaluating strain values. A common method is to determine and average the strain (ε_x and ε_y ) in a defined area. Hence, local strain peaks are not overestimated. That is an exact method which was used for determining the material strain of the solid material samples (Fig. 4b, left). Another possibility for determining the strain with the Aramis-system is the use of extensometers. Thereto 2 points are defined at the sample surface and the distance between these points are measured and analyzed continuously (arrows in Fig. 4b). The comparison of these 2 strain measurement methods has shown that there is negligible difference between the measured values of these methods. This fact also shows the homogeneity of the HDF material.

Regarding the kerf pattern material, it has to be considered that the pattern increases the geometrical deformation potential, but not the material strain. That is why the deformation capability generated using the kerf pattern material cannot be determined using the average strain measurement. The deformation developed by the pattern would not be considered using the average strain measurement. That is why the deformation was measured with extensometers. One extensometer each along and transverse to the tensile direction measured the overall strain of the sample (Fig. 4b right).

Besides the strains ε_x and ε_y, Poisson’s ratio µ and the modulus of elasticity (MOE) were calculated and used for material analysis and result discussion. Both parameters were determined between 10% and 30% of the maximum force.

The dimensions of the solid samples were 20 mm x 250 mm (W x L), the dimensions of the samples with kerf pattern were 42 mm x 250 mm (W x L). With a total of 10 bars (each 2 mm wide) in a sample, the sample width was comparable considering the cavities created by the pattern. 15 replicates were tested for each series.

The plasticizing of the already clamped samples was realized by a simple steaming process in the testing machine (Fig. 4a). A heating plate (1), combined with a moist textile as a water reservoir (2), was positioned at the back of the sample (5). At the beginning of a test the water of the moisture reservoir was evaporated by heating the heating plate to a temperature of 120°C. An uncontrolled evaporation into the testing environment was prevented by pressing a sponge (3) to the front of the sample thus that the sample was completely enveloped. The time for starting the test was determined by measuring the temperature. Thereto, two temperature sensors (4) were placed near the sample: one at the back and one at the front of the sample. A temperature of 99°C from the front sensor indicates that the steam has flowed through the sample, the sample is steamed and the test has to start quickly to avoid cooling and drying. Figure 5 shows an exemplary temperature curve for both temperature sensors.

The temperatures increase with different speeds. The temperature of the back sensor continuously increased until 100° C was reached and the water evaporated. The temperature of the front sensor clearly increased slower. When the steam flowed through the sample, the temperature at the front of the sample quickly increased until 99° C was reached (similar to steam shock effect in particleboard manufacture).

Test results are shown as box plots in Figure 7 and Figure 8 and are listed in Table 1.

The diagrams in Figure 7 are arranged as followes: there are two boxplots for each series. The left (orange colored) boxplot shows the value at the proportionality limit. This is the point where the material behavior transfers from the elastic to the plastic range. This characteristic point is often called the yield point. In this study, the yield point is defined at a plastic strain of 0.05%. The yield point is described by the yield strains ε_xpalong and ε_yp transverse to the tensile direction. The right (blue colored) boxplots show maximum values for ε_{x max} and ε_{y max}.

Table 1 lists the results as mean values including standard deviation. Comparing the results, it has to be considered that the kerf pattern results do not depict material parameters. They are a combination of the flexibility due to the geometric changes of the kerf pattern as well as the material properties.

Table 1 Mean values of the regarded parameters, standard deviation in brackets

ε_xp

ε_{x max}

ε_yp

ε_{y max}

Poisson’s ratio µ

MOE [MPa]

solid material, dry

0.42%

(0.02%)

1.10 %

(0.15%)

-0.11%

(0.01%)

-0.17%

(0.02%)

0.27

(0.02)

4968

(248)

solid material, steamed

0.46%

(0.05%)

2.14 %

(0.19%)

-0.14%

(0.05%)

-0.48%

(0.12%)

0.22

(0.19)

1194

(283)

kerf pattern, dry

0.98%

(0.09%)

4.06%

(0.33%)

0.30%

(0.05%)

1.19%

(0.22%)

-0.30

(0.05)

319

(17)

kerf pattern, steamed

2.24 %

(0.16%)

10.22%

(1.50%)

0.59%

(0.08%)

2.17%

(0.48%)

-0.26

(0.02)

(4)

Figure 7 a) shows the yield strain and the maximum strain along the tensile direction for all series. The yield strain for the solid material, dry and steamed, can be considered to be the same. The effect of the pattern used on the strain can be seen by comparing the strains of the dry solid material to the dry patterned material. The yield strain, or the elastic strain, of the pattern samples is twice that of the solid samples (Table 1). Since the material is the same, the material strain should also be the same. Therefore, the difference must be due to the pattern. This effect is more obvious for ε_{x max}. The maximum strain of the kerf pattern samples is four times higher than that of the solid material samples.

Chen et al. (2020) stated manufacturer values for the maximum strain in tensile direction is 0.5% for 3 mm thick fiberboard without specifying the type of fiberboard. Raskop et al. (2022) examined 2.5 mm thick high density fiberboard with 11% adhesive content under uniaxial tensile loading in a round robin test. They determined a maximum strain in the range of 0.95% to 1.22%. These values concern dry material. Our mean value of 1.1% maximum strain (Table 1) in tensile direction is within the range reported by Raskop et al (2022). Compared to Chen et al. (2020), our maximum strain values are clearly higher. Probably, the manufacturer’s value contains a safety margin for material and manufacturing variations.

It is known from solid wood that plasticizing in a saturated steam atmosphere is a widespread softening method and thus leads to an increase in maximum strain (Sandberg et al., 2023). Strain values for steamed fiberboard are not available in the literature, so our values are compared here to solid wood. Sandberg et al. (2023) state an increasing of the maximum strain for steamed ash wood of about 0.3% in comparison to dry wood. This value is clearly lower than the effect for steaming fiberboard reached in this study. The maximum strain is twice as high for the steamed solid material than for the dry material. For the kerf pattern material, the strain is 2.5 times higher for the steamed samples compared to the dry samples (Table 1). These results implicate another mechanism of wood and of fiberboard due to steaming. In addition to the molecular effect known in solid wood of softening lignin within the middle lamella, which allows the wood fibers to shift towards each other (Sandberg et al., 2023), the fibers themselves can shift against each other and can re-bond in fiberboards. Thus, higher deformation is possible.

These effects can be displayed by comparing the fracture images of a dry sample (without steaming) and a steamed patterned sample (Figure 6). The fracture of the dry sample (Figure 6 left) shows the clearly shorter fibers as compared to the fracture of the steamed sample (Figure 6 right). The longer fibers of the steamed samples indicate less damaged fibers with the ability to be pulled out without breaking, allowing a greater deformation.

Overall, the maximum strain in comparison between the solid and the kerf patterned material is almost four times higher for the dry material and almost five times higher for the steamed material. The total effect of both, pattern and steaming, is greater than the sum of the individual effects. This means that the individual effects of steaming and patterning do not overlap independently of each other, but rather have a positive influence on each other.

Figure 7 b) shows the yield strain ε_yp and maximum strain ε_{y max} transverse to the tensile direction. Besides Poisson’s ratio, this value is interesting for the evaluation of the auxetic material behavior. The solid material shows the conventional material behavior: the material becomes narrower under tensile load. Thus, the transverse strain is defined as negative value. As expected, the absolute value of transverse strain of the solid material is increased by steaming (Table 1), but they are in a very low range. The longer the material becomes, the narrower it becomes by tensile loading. These statements are valid for both, yield and maximum strain. The behavior and the values for the kerf pattern samples are completely different. Firstly, there is a positive transverse strain. Thus, an auxetic material behavior is proved. As mentioned above, simultaneous material strain in different directions is desirable for creating 3D shaped surfaces, and the auxetic behavior of the material helps to avoid the risk of cracking and wrinkling during forming. The strain values increasing due to steaming is obvious. Due to the steaming, the deformation capability increases thus the transverse strain increases.

As shown in the discussion the yield strain values for the solid material in both directions are not influenced by steaming. The values of the steamed sample are comparable to the dry sample. In contrast to this the maximum strain values are clearly increased by steaming. Therefore, it can be assumed that steaming has a dominant influence on the plastic part of the strain. This result also confirms that the effect of plasticizing by steaming, which is known from solid wood, can also be applied to adhesive-free HDF.

Contrary to maximum strain values, Poisson’s ratio (Figure 8 a)) and MOE (Figure 8 b)) are determined in the elastic range. Similar to the transverse strain results, Poisson’s ratio shows divergent results. For the solid material samples, Poisson’s ratio is in the same positive range between 0.2 and 0.3. Thus the literature value, given by Chen et al. (2020) and Raskop et al. (2022) in the range of 0.21 to 0.25, is confirmed. The steaming process does not have a great influence on Poisson’s ratio. The comparable results of the yield strain along and transverse to the tensile direction (Table 1 and Figure 7 a) and b)) confirm this independence. For the kerf pattern samples Poisson’s ratio is negative because the transverse strain ε_y is positive, as already mentioned above. Thus, the auxetic material behavior is proven (Gordanshekan et al. 2022). The absolute value is in the same range (between 0.2 and 0.3), but with a different sign (positive for the solid material and negative for the patterned material). This means that the kerf pattern material becomes wider at the same ratio as the solid material becomes narrower under tensile load. It is worth mentioning that the equality of the absolute values for Poisson’s ratio is random. Its value for the patterned samples depends mainly on the geometry of the kerf pattern itself and not on the material properties as for the solid samples.

The MOE is displayed in Figure 8 b). With a mean value of about 5000 MPa for the dry solid material (Table 1), this MOE is slightly higher than the MOE of 4000 MPa reported by Chen et al. (2020) and the values from 3200 to 3700 MPa given by Raskop et al. (2022). Due to steaming, the MOE of the solid material is clearly reduced by approximately 75%. A low MOE is not a guarantee of a good formability or a high deformation capability. But the reduction in MOE indicates that the fiber linkage has been loosened by steaming. Figure 6 illustrates the loosened fiber linkage by showing undamaged fibers after break. Thus, the fibers are easier displaceable and allow a higher strain and deformation. The reduction of the MOE also means that less force is required to generate a deformation. Simplified, a deformation can more easily be realized.

The kerf pattern samples have a very low MOE – 319 MPa for the dry pattern samples and only 40 MPa for the steamed pattern samples. It is remarkable that a stiffness reduction of more than a factor of 100 was observed when comparing the MOE of the dry solid samples with the steamed patterned samples.

All of the results show that the steamed kerf pattern samples have the greatest deformation capability and have the lowest stiffness for deformation.

In this paper, methods for increasing the deformation potential of fiberboard were evaluated. On the one hand, plasticizing in a saturated steam atmosphere, which is well known from the bending of solid wood, was investigated for its applicability to adhesive-free high-density fiberboards. On the other hand, a special kerf pattern, which geometrically increase the formability, has been investigated. Furthermore, the combination of both methods was also evaluated. In order to evaluate the increase in the deformation potential of the applied methods, uniaxial tensile tests were carried out and evaluated.

The test results show that steaming as a plasticizing method is applicable to adhesive-free fiberboards and leads to a higher deformation potential. It was possible to increase the maximum strain along the tensile direction by a factor of 2 as compared to the dry solid material. Steaming softens the fiber binding, allowing the fibers themselves to shift relative to each other and to re-bond in the fiberboard. This allows a higher maximum strain without affecting the yield point, which marks the end of the linear elastic range. Therefore, it is primarily the plastic strain that is increased by steaming, which is also proof of the plasticization of the tested fiberboards.

In future studies, it should be investigated whether steaming can also be used for adhesive bonded fiberboards. Depending on the adhesive system, there is a risk of hydrolysis due to water vapor.

The evaluated kerf pattern is based on a hexagonal pattern with knots and support structures arranged asymmetrically and mirrored. The test results show a positive strain transverse to the tensile load, which means a negative Poisson's ratio. This demonstrates auxetic material behavior with the applied pattern. This is an advantage when forming 3D surfaces because the material becomes longer and wider under tensile load. This material behavior helps avoid the risk of cracking and wrinkling during forming. Due to the kerf pattern, the maximum strain along the tensile direction was increased by a factor of 4 compared to the dry solid material.

The combination of the two methods leads to superior results than the sum of the individual effects. The maximum strain along the tensile direction was increased by a factor of 10 ascompared to the dry solid material; the sum of the individual effects would only result in a factor of 8. With regard to MOE, a remarkable reduction by a factor of more than 100 has been achieved. Lower material stiffness means less force is required and 3D surface forming is easier to realize.

The results of the evaluated kerf pattern are only valid for this specific pattern geometry. Further investigations should examine how the geometry (e.g. pattern size or bar width) of the pattern influences the forming potential. In addition, it should be investigated whether other auxetic kerf patterns can be adapted to fiberboards and, in combination with steaming processes as plasticizing, lead to an even greater improvement of the forming potential in regard to the 3D-shaped surfaces.

Acknowlegements

The research project these investigations are based on was financially supported by the Federal Ministry of Food and Agriculture of Germany (grant reference 2220HV069A).

Author Contribution

for the manuscript:RK: conceptualization, methology, analysis; prepared figures 3,4,7,8BB: investigation, analysis; prepared figures 5,6RK and BB wrote the main manuscript textJH: conceptualization, wrote the introductionAW: supervisionAll authors reviewed the manuscript.

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No competing interests reported.

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You are reading this latest preprint version

Investigations for increasing the 3D-forming potential of high-density fiberboards

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Abstract

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Introduction

Materials and Methods

Results and Discussion

Summary and Conclusion

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Author Contribution

References

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