Abstract
Transparent wood (TW) is an interesting polymer biocomposite with potential for buildings and photonics applications. TW materials need to be eco-friendly and readily processed with few defects, for high optical transmittance and low transmission scattering at wide angles (haze). Two wood templates with different lignin-content are impregnated with a new thiol–ene thermoset system. The more eco-friendly bleached wood template results in transparent wood with high optical transmission and much reduced transmission haze, due to strong reduction of interfacial air gaps. Characterization includes template composition, thiol–ene distribution, and polymerization in wood cell wall by EDX and confocal Raman microscopy, also NMR and DSC, tensile testing and FE-SEM fractography for morphology and wood/thiol–ene interface adhesion assessment. The wood template is a true nanocomposite with thiol–ene polymer located inside the nanoporous wood cell wall. Advanced TW applications require not only appropriate wood template modification and careful polymer matrix selection but also tailoring of the process to impregnation and polymerization mechanisms, in order to reduce optical defects.
Highlights
Transparent wood (TW) was first proposed as a method for optical investigation of wood anatomy[1] but has recently garnered interest as an application of wood as an advanced and eco-friendly biocomposite material
Transparent wood is a new application for polymerimpregnated wood templates
A new transparent wood (TW) biocomposite in the form of low haze TW with high transmittance and very low haze is prepared by removing chromophores in wood templates, followed by successful impregnation with thiol−ene thermoset precursors
Summary
Transparent wood (TW) was first proposed as a method for optical investigation of wood anatomy[1] but has recently garnered interest as an application of wood as an advanced and eco-friendly biocomposite material. The delignified templates with thiol−ene (high haze TW) investigated here show a smaller difference in haze compared with PMMA, 63% versus 71% for PMMA-based TW,[12] see For materials design, both selection of polymer (refractive index matching, polymerization shrinkage) and wood template treatment (polymer matrix interaction) have strong effects on haze; primarily through effects on interface gap defects, which depend on the physical and chemical nature of wood-thiol−. The stoichiometric excess of thiol monomers with a high degree of cure in the cell wall of low haze TW are in support of chemical reactions between the polymer in the lumen and thiols in the cell wall In this model experiment, the peaks for the trifunctional ene monomer: allyl stretching (3095 cm−1), allyl bending (1650 cm−1), and ketone stretching (1765 cm−1), were present inside the cell walls of ene-treated bleached templates and remained after additional washing. The presence of thiol−ene polymer inside the cell wall is further supported by a broadening of the thiol−ene tan delta damping peaks in transparent wood biocomposites
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