Arif Nuryawan

MRT Letter: High Resolution SEM Imaging of Nano-Architecture of Cured Urea-Formaldehyde Resin Using Plasma Coating of Osmium

Nanoarchitecture of cured urea‐formaldehyde (UF) resins was examined with a field‐emission scanning electron microscope (FE‐SEM) after coating samples with osmium, which is considered to produce particles of considerably smaller size compared to other metal coatings used in SEM studies. This method enabled comparison of the nanoarchitecture of UF resins of low (1.0) and high (1.6) formaldehyde/urea (F/U) mole ratios to be made, based on imaging of extremely small size particles as part of UF resin architecture, not described before. Imaging revealed presence of relatively large globular particles (148.084–703.983 nm size range) as well as smaller substructures (28.004–39.604 nm size range) as part of the architecture of 1.0‐mole UF resin. Globular particles were also present in 1.6 mole UF resin, but of considerably smaller size (14.760–50.269 nm). The work presented demonstrates usefulness of osmium coating in unraveling the intricacies of the nanostructural organization of cured UF resins, prompting wider application of this immensely useful but grossly underutilized metal coating type in high resolution SEM examination of biological and materials samples. Microsc. Res. Tech. 76:1108–1111, 2013.

Urea-formaldehyde resin penetration into Pinus radiata tracheid walls assessed by TEM-EDXS

Abstract This paper reports a new method of detecting urea-formaldehyde (UF) resin penetration into the cell walls of radiata pine (Pinus radiata D. Don) by means of transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy (EDXS). The quantifications of penetrated UF resin in the ultrathin cuts of cell walls were realized by detecting nitrogen (N) element by TEM-EDXS. Both line scan and area mapping revealed N in cell walls in contact with resin-filled lumens but not in those in contact with empty lumens. Thus, UF resin had penetrated the cell walls from the lumen side.

Penetration of urea–formaldehyde resins with different formaldehyde/urea mole ratios into softwood tissues

In order to understand the impact of formaldehyde/urea (F/U) mole ratio on penetration characteristics of urea–formaldehyde (UF) resin into softwood tissues, a quantitative measurement of UF resin penetration into radiata pine (Pinus radiata) tissues from the bond-line was undertaken. Four different F/U mole ratios (1.6, 1.4, 1.2, and 1.0) of UF resins with different viscosities and two levels of hardener (NH4Cl) for two extreme F/U mole ratios (1.6 and 1.0) were studied. Firstly, field emission scanning electron microscope and confocal laser scanning microscopy were used to localize UF resins in the bond-line for the qualitative evaluation of resin penetration. Then light microscopy was employed to quantitatively measure the resin penetration and bond-line thickness. A decrease in the F/U mole ratio of UF resin that proportionately decreased the resin viscosity resulted in an increase in the average resin penetration and a decrease in the bond-line thickness. Higher hardener level provided a greater resin penetration with all F/U mole ratio UF resins. These results demonstrated that F/U mole ratio had an impact on the penetration and bond-line thickness of UF resins, owing to differences in the reactivity of resins, with higher F/U mole ratio resins being more reactive.

Swelling Behaviour of Cured Urea–Formaldehyde Resin Adhesives with Different Formaldehyde to Urea Mole Ratios

As a part of understanding of the network structure of urea–formaldehyde (UF) resin adhesives, this study examined the swelling behaviour of cured UF resin adhesives with four different formaldehyde–urea (F/U) mole ratios, using Flory–Rehner thermodynamic theory and field emission-scanning electron microscopy (FE-SEM) to relate the swelling behaviour to consequently induce micromorphological changes. Cured UF resin films before and after acetone extraction were exposed to swelling in dimethyl sulphoxide at three different temperatures. For the first time, this study reported the experimentally determined swelling parameters, such as sol fraction (ωsol), polymer volume fraction (φp), polymer–solvent interaction parameter (χ), and the number average molecular weight between cross-links (Mc), for cured UF resin adhesives. Both ωsol and Mc decreased as the F/U mole ratio increased. But these values increased with an increase in the swelling temperature. The extraction resulted in negative ωsol values, suggesting the removal of a scattered distribution of ωsol in the cured UF resins. The micromorphology helped to explain the differences in the molecular integrity of the resins, indicating a close relationship between the swelling behaviour and the morphological changes after the swelling.

A novel approach for FE-SEM imaging of wood-matrix polymer interface in a biocomposite

Understanding the interface between polymer and biomass in composite products is important for developing high performance products, as the quality of adhesion at the interface determines composite properties. For example, with greater stiffness compared to polymer matrix, such as that of high density polyethylene, the wood component enhances stiffness of wood-polymer composites, provided there is good adhesion between composite components. However, in composites made from wood flour (wood particles) and synthetic resins it is often difficult to clearly resolve particle-matrix interfaces in the conventionally employed microscopy method that involves SEM examination of fractured faces of composites. We developed a novel approach, where composites made from high density polyethylene and wood flour were examined and imaged with a FE-SEM (field emission scanning electron microscope) in transverse sections cut through the composites. Improved definition of the interface was achieved using this approach, which enabled a more thorough comparison to be made of the features of the interface between wood particles and the matrix in composites with and without a coupling agent, as it was possible to clearly resolve the interfaces for particles of all sizes, from large particles consisting of many cells down to tiny cell wall fragments, particularly in composites that did not incorporate the coupling agent used to enhance particle adhesion with the matrix polymer. The method developed would be suitable particularly for high definition SEM imaging of a wide range of composites made combining wood and agricultural residues with synthetic polymers.

Micro-Morphological Features of Cured Urea-Formaldehyde Adhesives Detected by Transmission Electron Microscopy

This work examined micro-morphological features responsible for the crystallinity of cured urea-formaldehyde (UF) adhesives, using transmission electron microscopy (TEM) to identify and characterize distinctive crystalline structures in resins obtained with different formaldehyde to urea (F/U) mole ratios and hardener levels. The TEM examination of cured UF resin adhesives impregnated into wood cell lumen revealed the presence of spherical particles with variable diameter and number per unit area. The diameter and number/area of the spherical particles increase for decreasing F/U mole ratio and decrease with an increase in the hardener levels, an effect which is closely related to their crystallinity. Therefore, the present findings suggest that the spherical particles are responsible for the crystallinity of cured UF resin adhesives. The results also indicate that crystalline structures represent an inherent feature of cured UF resin adhesives, particularly for low F/U mole ratios, even though these resins are usually classified as amorphous and cross-linked thermosetting polymers.

Microstructure of Cured Urea-Formaldehyde Resins Modified by Rubber Latex Emulsion after Hydrolytic Degradation

ABSTRACT This study investigated microstructural changes of cured urea-formaldehyde (UF) resins mixed with aqueous rubber latex emulsion after intentional acid etching. Transmission electron microscopy (TEM) was used in order to better un-derstand a hydrolytic degradation process of cured UF resins responsible for the formaldehyde emission from wood-based composite panels. A liquid UF resin with a formaldehyde to urea (F/U) molar ratio 1.0 was mixed with a rubber latex emulsion at three different mixing mass ratios (UF resin to latex = 30:70, 50:50, and 70:30). The rate of curing of the liquid modified UF resins decreased with an increase of the rubber latex proportion as determined by differential scanning calorimetry (DSC) measurement. Ultrathin sections of modified and cured UF resin films were exposed to hydrochloric acid etching in order to mimic a certain hydrolytic degradation. TEM observation showed spherical particles and various cavities in the cured UF resins after the acid etching, indicating that the acid etching had hydrolytically degraded some part of the cured UF resin by acid hydrolysis, also showing spherical particles of cured UF resin dispersed in the latex matrix. These results suggested that spherical structures of cured UF resin might play an important role in hindering the hydrolysis degradation of cured UF resin.