Characterization and optimization of organoclay- poly(melamine-co-formaldehyde)-methylated solution impregnated pulai (Alstonia spp.) wood using response surface methodology

Abstract

Significant effects of organoclay and poly(melamine-co-formaldehyde)-methylated (PMFM) impregnation on the mechanical, morphological, and thermal characteristics of raw pulai wood were investigated in this work. The material’s modulus of elasticity (MOE) as well as the maximum compression force (MCF) of the impregnated organoclay/PMFM pulai wood samples were optimized using a designed experiment. The MOE and MCF models had R2 values of 0.9228 and 0.8340, respectively. After the impregnation of organoclay/PMFM pulai wood samples, the MOE and MCF increased considerably, indicating that the pulai wood’s mechanical characteristics had improved. The compositional analysis verified the polymerization and dispersion of organoclay and PMFM. Using Fourier transform infrared spectroscopy, reduction in the hydroxyl groups was detected. The impregnated organoclay/PMFM pulai wood samples had successfully filled the pores and cell cavities, as seen by scanning electron microscopy. The thermal stability of the impregnated organoclay/PMFM pulai wood samples was better than that of the raw pulai wood, with a higher glass transition temperature as determined by differential scanning calorimetry. The thermogravimetric study revealed that the impregnated organoclay/PMFM pulai wood samples had higher decomposition temperatures than the raw pulai wood sample.

Full Article

Characterization and Optimization of Organoclay- poly(melamine-co-formaldehyde)-methylated Solution Impregnated Pulai (Alstonia spp.) Wood Using Response Surface Methodology

Perry Law Nyuk Khui,^a Md. Rezaur Rahman,^a,* Hafizah binti Abdul Halim Yun,^a Durul Huda,^b Sinin Hamdan,^a Muhammad Khusairy Bin Bakri,^a Mohammad Mahbubul Matin,^c Kuok King Kuok,^d Chin Mei Yun, ^d Abdullah S. Al-Bogami,^e Khalid A. Alamry,^f and Mohammed M. Rahman ^f,*

Significant effects of organoclay and poly(melamine-co-formaldehyde)-methylated (PMFM) impregnation on the mechanical, morphological, and thermal characteristics of raw pulai wood were investigated in this work. The material’s modulus of elasticity (MOE) as well as the maximum compression force (MCF) of the impregnated organoclay/PMFM pulai wood samples were optimized using a designed experiment. The MOE and MCF models had R² values of 0.9228 and 0.8340, respectively. After the impregnation of organoclay/PMFM pulai wood samples, the MOE and MCF increased considerably, indicating that the pulai wood’s mechanical characteristics had improved. The compositional analysis verified the polymerization and dispersion of organoclay and PMFM. Using Fourier transform infrared spectroscopy, reduction in the hydroxyl groups was detected. The impregnated organoclay/PMFM pulai wood samples had successfully filled the pores and cell cavities, as seen by scanning electron microscopy. The thermal stability of the impregnated organoclay/PMFM pulai wood samples was better than that of the raw pulai wood, with a higher glass transition temperature as determined by differential scanning calorimetry. The thermogravimetric study revealed that the impregnated organoclay/PMFM pulai wood samples had higher decomposition temperatures than the raw pulai wood sample.

DOI: 10.15376/biores.17.2.2780-2809

Keywords: Wood; Impregnation; Characterization; Treatment; Chemical; Response surface methodology

Contact information: a: Faculty of Engineering, Universiti Malaysia Sarawak, Jalan Datuk Mohammad Musa, 94300, Kota Samarahan, Sarawak, Malaysia; b: Department of Mechanical Engineering and Product Design Engineering, Swinburne University of Technology, 3122, Hawthorn, Victoria, Australia; c: Department of Chemistry, University of Chittagong, Chittagong, 4331, Bangladesh; and d: Faculty of Engineering, Computing and Science, Swinburne University of Technology Sarawak Campus, Jalan Simpang Tiga, 93250, Kuching, Sarawak, Malaysia; e: Department of Chemistry, College of Sciences, University of Jeddah, Jeddah 21589, P.O. Box 80327, Saudi Arabia; f: Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia;

* Corresponding author: rmrezaur@unimas.my

INTRODUCTION

Alstonia spp. (Apocynaceae) wood is called mengalang, pelai, or pulai in Southeast Asia in general, especially in Malaysia and Indonesia (Wong 2002; How and Nordahlia 2018; Mery et al. 2019; Oktavia et al. 2020). A few of its known species are A. angustiloba, A. macrophylla, A. pneumatophore, A. scholaris, and A. spatulata. Despite the high moisture content of the wood, it is often used for household objects, carvings, and sculptures (Oktavia et al. 2020). Carving, crates and packing boxes, frames, fret work, match boxes and splints, pattern making, plywood, picture frames, pencil, and toothpicks are all possible uses for the wood. It has also been used to make wooden clogs and disposable chopsticks with great success. Basong, the root wood of A. spatulata and A. pneumatophora, is incredibly light, weighing just 50 to 80 kg/m³ air dried, and has been used to make pith-helmets (Wong 2002).

The sapwood is undifferentiated from the heartwood, which ranges in color from cream to light yellow. The wood is listed as a light hardwood with an air-dry density of 210 to 500 kg/m³ (H’ng et al. 2010; How and Nordahlia 2018; How and Sik 2020). Pulai is classed as non-durable based on a standard burial test of untreated specimens with dimensions of 50 mm x 50 mm x 600 mm (Foxworthy and Woolley 1930). Within 6 months, all 21 parts of the specimens tested were destroyed (Foxworthy and Woolley 1930). On the wood, both fungus and insect infestations were widespread. Preservatives are thus a simple option that may be applied to the wood. The use of pulai wood grain is straight to shallowly interlaced, and the texture is relatively fine to coarse (Monachino 1949). Pulai wood matures fast, with less cupping, bending, twisting, and end-checking than other woods. Powder-post beetle and sap stain fungus are both known to damage the wood. Air drying takes around 1.5 months for 13-mm-thick boards and 2.5 months for 38-mm-thick boards. It is suggested to use the Kiln Schedule J method for drying (Bond and Espinoza 2016). Pulai wood shrinkage is substantial, particularly in the radial direction, where it averages 2.3%, while tangential shrinkage averages 2.8% (Malaysian Timber Corporation 1982).

Due to its susceptibility to fungi and bacteria, it is important to treat the wood to reduce these problems. Other procedures, such as wood modification, pulping, bonding, and coating technologies, may rely on the treatability of wood (Torgovnikov and Vinden 2009; Biziks et al. 2019). A significant penetration of reaction agents into wood is required to ensure a proper bulk chemical modification; otherwise, due to a non-uniform dispersion of the agent, the inner regions of wood may not be suitably changed (Homan and Jorissen 2004).

Poor penetration of finishes into wood can have a detrimental impact on coating performance of solvent- and water-borne finishes on wood because coating adherence to the wood surface benefits to some extent from penetration (De Meijer and Militz 1998). Mader et al. (2011) presented an excellent literature study on coating penetration into wood. Kamke and Lee (2007) also published an excellent scientific review study on adhesive penetration in wood, focusing on the adhesive bond performance of wood-based composites. To achieve effective bond performance, a specific amount of adhesive penetration into the wood is required throughout the composites manufacturing process (Kamke and Lee 2007).

A model was made based on Design Expert 11 software, which was used to analyze the response surface methodology (RSM) of the sample. The model created was also used to predict the compression strength of the actual impregnated sample. The study on its compression strength properties was considered with changes of the parameters, which were changed accordingly to its weight percentage of the PMFM, weight percentage of the organoclay, and the duration of impregnation.

EXPERIMENTAL

Materials

Pulai wood was cut from the local forest from Kuching, Sarawak, Malaysia. Organoclay CLAYTONE®EM with CAS Number 14808-60-7 was obtained from BYK Additives Inc. (Wesel, Germany). The size of the organoclay is around 40-100 mesh. Poly(melamine-co-formaldehyde)-methylated (PMFM), solution with product number 418560-250ML and CAS Number 68002-20-0, was obtained from Sigma-Aldrich (Darmstadt, Germany). Dichloromethane (AR Grade, 2.5 L) with product number A709-2.6LGL was obtained from Ajax Finechem (UNIVAR, Victoria, Australia). Dichloromethane was used in the actual sample as a medium dilution solvent to allow the chemical to be impregnated in the wood samples.

Sample Preparation

To prepare the samples, the Pulai wood was cut into similar rectangular cuboid size, with the fiber direction towards the longitudinal axis, as shown in Fig. 1(a).

Fig. 1. (a) Three principial axis of wood with respect to fiber direction and growth rings; (b) the apparatus, impregnation process, and drying samples

The untreated samples were placed in a forced air convection oven (IMPACT Ltd., Southampton, UK), Model P12VSD) for 24 h at 70 °C for conditioning and drying. This reduced or eliminated the presence of moisture in the untreated samples. The oven-dried samples were immersed and prepared by adding the different amounts of organoclay and PMFM, which was diluted with dichloromethane. The raw samples and the diluted solution were transferred into a concealed impregnation chamber and were impregnated at different durations. Once the impregnation process was completed, the impregnated samples were cleaned and wrapped with aluminum foil and placed into an oven at 70 °C for 48 h, to allow polymerization, curing process, and cross linkage between PMFM and organoclay to occur. The final impregnated samples were unwrapped for mechanical testing and characterization.

Design of Experiment (DOE)

Impregnated samples were prepared in accordance with a design of experiment (DOE). This DOE was conducted using “Design Expert 11 software (Stat-Ease, Minneapolis, MN, USA) with three main variables: A: duration (min), B: PMFM (wt%), and C: organoclay (wt%). Table 1 shows the range of variables at a low level of −1 and a high level of +1. The experiment was made up of 20 runs with an average of five replicates for each sample, details are presented in Table 2. Statistical analysis of the process was performed to evaluate the analysis of variance (ANOVA).

Table 1. Variables Used in the DOE

Table 2. DOE from Design Expert 11

The experiments were run to minimize experimental errors. The relationship between the independent variables and their responses, i.e., higher modulus of elasticity (MOE) and maximum compressive force (MCF) were fitted to a quadratic model, as expressed in Formula 1,

y = β + βA + βB + βC + βAB – βAC + βBC + βA² – βB² – βC²            (1)

where y is the response, and β is the constant term.

Characterization of the Experimental Samples

From the results obtained in Table 2 from the DOE, which involved the test with a higher MOE and MCF, some samples of the impregnated organoclay/PMFM pulai wood samples as presented in Table 3 were characterized and compared with the raw samples to study the effects of the impregnation.

Table 3. Selected Runs for Characterizations for Actual Testing

Compression test

A universal testing machine (MSC-5/500, Shimadzu Corporation, Kyoto, Japan) was used for compression testing. An average of 5 samples for each impregnated organoclay/PMFM pulai wood samples was obtained and tabulated according to Table 3. The test was performed according to ASTM D3501-05 (2018).

Fourier transform infrared spectroscopy

Shimadzu Corporation’s (Kyoto, Japan) FTIR “IRAffinity-1” spectroscope was utilized. The molecular bond structures and functional groups of the impregnated organoclay/PMFM pulai wood samples were identified using IR spectra bands ranging from 4000 to 400 cm^-1. The FTIR spectrum was plotted using “IR solution” software (IR solution, Shimadzu, Tokyo, Japan) based on ASTM E168-16 (2016) and ASTM E1252-98 (2021) standards.

Differential scanning calorimetry (DSC)

For DSC measurements, a DSC Q10 (TA Instruments, New Castle, DE, USA) thermal system with a sealed aluminum capsule was used. Each set of data represents the average of five different runs. The specimen was 4 to 4.5 mg and heated at a constant rate of 10 °C/min, while the scanning temperature was adjusted between 30 and 600 °C. The samples’ crystallization and melting temperatures were determined to be in compliance with ASTM D3418-15 (2015) and ASTM E1269-11 (2018) requirements, respectively.

Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS/EDX)

Hitachi Analytical’s Tabletop scanning electron microscope (SEM) model TM-3030 (Hitachi High-Technologies Europe GmbH, Krefeld, Germany) was used to capture the morphological images of raw and impregnated organoclay/PMFM pulai wood samples. Field emission with a 15 kV accelerating voltage was used to gather images of the surface of samples. The tests were conducted in accordance with ASTM E2015-04 (2014), with a magnification of 800x.

Thermogravimetric analysis (TGA)

A Pyris™ 1 TGA from Perkin Elmer Inc. (Waltham, MA, USA) machine was used for the thermogravimetric analysis (TGA) measurements. Analyses were performed at temperatures ranging from 30 to 600 °C with a nitrogen flow rate of 20 mL/min. During these tests, a heating rate of 20 °C/min was maintained. The test was performed in line with ASTM E1131-20 (2020).

RESULTS AND DISCUSSION

Statistical Analysis of MOE and MCF by ANOVA

The ANOVA is a crucial statistical method for deducing inferences from solutions and analyzing experimental data. The benefits given by this technique were investigated to evaluate the interactions and effects of chemical treatment on the mechanical characteristics of impregnated organoclay/PMFM pulai wood samples. The Fisher’s variance F-value was calculated using the sum of squares and mean squares in Table 4, which is a measure of data variation around the mean. In contrast, the P-value indicates how important a parameter might become. Therefore, the early hypothesis can be rejected because a parameter with a P-value greater than 0.10 was not insignificant. When the P-value is less than 0.05, the model terms are considered significant. Time, PMFM, and organoclay all had substantial impacts on the samples, as seen in Table 4. The MOE was also affected by interactions between time and PMFM, PMFM and organoclay, and time and organoclay. As indicated in Table 5, comparable interactions were detected on the MCF’s resulting characteristics. The MOE had an R² value of 0.9228, which was relatively close to 1, indicating that the model was valid, with an adjusted R² value of 0.8534 within a 0.2 difference, whereas the R² indicates the quality of the experimental data used in building the model. Similar results were also obtained by Jiang et al. (2020) and Adamu et al. (2020, 2021).

Table 4. The ANOVA Results for MOE

Table 5. The ANOVA Results for MCF

The R², adjusted R², and predicted R² for the MCF were 0.8340, 0.0687, and minus 0.7277, respectively. The MCF values are similarly comparable to earlier values of MOE. Equations 1 and 2 offer model equations for the MOE and MCF based on coded factors of +1 for the high level and 1 for the low level, represented as:

MOE = 9.38 – 0.45A – 0.21B – 0.46C – 0.85AB – 0.14AC

+ 0.18BC + 0.34A² – 3.17B² – 0.85C²            (1)

MCF = 14.64 + 0.24A – 0.71B – 1.11C – 0.91AB

+ 0.66AC – 0.51BC + 0.28A² – 2.16B² – 1.13C²                 (2)

Table 6 shows that the predictions from the model were extremely similar to the actual values with very low residuals (Adamu et al. 2020, 2021; Jiang et al. 2020). Figures 2a and 2b show the plots for the MOE and MCF of the predicted versus actual. As a result of the minimal variation, the model appears to be capable of accurately predicting the mechanical characteristics of impregnated organoclay/PMFM pulai wood samples.

Table 6. Comparison of the Actual Values and Predicted Values

Fig. 2. The model plots of the predicted and actual values for (a) MOE, and (b) MCF

Prediction of Optimal Conditions

The most significant feature of this research was to improve the mechanical characteristics of the impregnated organoclay/PMFM pulai wood samples. The MOE and MCF were optimized by minimizing the quantity of PMFM, organoclay, and the effect of time. The surface plot for optimization of MOE is shown in Fig. 3. The optimal value of MOE was 9.23 MPa, with 15 min, 4.19 wt% of PMFM, and 0.5 wt% of organoclay. Equally, the optimum value of the MCF at the same conditions was 14.95 kN. This can be seen in Fig. 4. The optimum circumstances for producing impregnated organoclay/PMFM pulai wood samples are these optimized values. Shorter or longer time may influence the result in deterioration of mechanical characteristic or a high cost due to chemical waste. Similarly, preparations that are short time of treatment may result in less effective treatment (Rahman et al. 2021).

Fig. 3. Surface plot for optimization of MOE