Elsevier

Bioresource Technology

Volume 97, Issue 2, January 2006, Pages 257-264
Bioresource Technology

Properties of resorcinol–tannin–formaldehyde copolymer resins prepared from the bark extracts of Taiwan acacia and China fir

https://doi.org/10.1016/j.biortech.2005.02.009Get rights and content

Abstract

Resorcinol–tannin–formaldehyde copolymer resins (RTF) were prepared by using the bark extracts of Taiwan acacia (Acacia confusa) and China fir (Cunninghamia lanceolata) to substitute part of the resorcinol. From the results, the content of reactive phenolic materials in Taiwan acacia and China fir bark extracts were 51.6% and 46.5%, respectively. Aromatic compounds were the main components in the bark extracts showed by FT-IR analysis. The conventional synthesis condition used for RF resin was certainly not suitable for the RTF copolymer resin. It should be formed the novolak RF prepolymer by reacting the resorcinol with formaldehyde at the first stage, and then the bark extracts added and underwent the copolymerization reaction under acidic condition at the second-stage. The RTF copolymer resins prepared had cold-setting capability. They had higher viscosity, shorter gel time as compared with the RF resin. The RTF copolymer resins could be carried out the gluing application immediately after the hardener was added and had bonding strength the same as RF resin. But the RTF copolymer resins had worse stability and shorter shelf life than RF resin.

Introduction

Resorcinol–formaldehyde resin adhesives (RF) have excellent water, weather and chemical resistance, and can set at room temperature and neutral condition. Wood products made with RF can be used at outdoors for structural purpose and may be applied in moist regions. RF is the most important adhesive used in the manufacturing of laminated woods; however the price of RF adhesive is too expensive, and under room temperature, several hours are generally needed for the glue line to reach complete setting. In order to reduce the cost of RF adhesives, phenol that is cheaper than resorcinol is used to substitute part of resorcinol, and the cold-setting resorcinol–phenol–formaldehyde copolymer resins (RPF) have been made (Liu and Lin, 1986a, Liu and Lin, 1986b, Pizzi and Cameron, 1984). However, increasing the quantity of phenol prolongs the curing time of RPF needed, and even loses its cold-setting capability.

Bark is the residue of woodworking process with no suitable application previously. Most of the bark is abandoned or burned as fuel. However, it is found that tannin is rich in bark extracts for some tree species. These tannins, consisting of flavonoid units which have undergone varying degrees of condensation, could react with formaldehyde; therefore, they may be used as the raw materials in the manufacturing of phenolic adhesive. Roffael et al. (2000) used spruce bark tannin as the binder in the manufacturing of particleboard and fiberboard. Fechtal and Riedl (1993) prepared the synthetic resin from the copolymerization of eucalyptus and Acacia mollissima bark extracts with phenol and formaldehyde, and applied these resins to particleboard manufacturing. Lu and Shi (1995) had prepared phenol–bark extracts–formaldehyde copolymer resin with 60% of phenol demanded was substituted with larch pine bark tannin. Vazques et al. (1992) prepared phenol–bark extracts–formaldehyde copolymer resins with Pinus pinaster bark extracts for plywood manufacturing. Kreibich and Hemingway (1985) blended southern pine bark extracts with resorcinol and heated under acidic condition. They thought that tannin contained in the extracts could be acid-catalyzed cleavage to produce tannin–resorcinol–adduct, and this adduct could be applied to the RPF copolymer resins synthesized. In this method, 60% of resorcinol could be replaced with this adduct, and the time needed for this copolymer resin prepared was only half of that for RF resin. These copolymer resins had cold setting properties and had bonding strength that resembled RF resin. Kreibich and Hemingway (1987) also found that RPF copolymer resins blended with sulfonated south pine bark extracts could set approximately at 30–60 min under room temperature and the bonding properties would be invariable when 50% of RPF was substituted. Mitsunaga et al., 1993, Mitsunaga et al., 1994, Mitsunaga et al., 1995 treated the extracts with excessive phenol and boron trifluoride to cause phenolation. They thought that this method could open the pyran-ring of condensed tannin, and broke down the intermolecular chain of flavonoid units. These could reduce the molecular weight and increased the activity of its molecular chain, thus promoting the reactivity of tannin with formaldehyde.

In our previous studies (Lee and Liu, 1996b, Lee and Liu, 1996c, Lee et al., 2001, Liu et al., 1998), a series of phenol–bark extracts–formaldehyde copolymer resins (PTF) was prepared with various kinds of bark extracts and under different synthesis conditions. We found that when PTF copolymer resins were used in the manufacturing of wood-based boards, such as plywood and particleboard, the hot-pressing temperature and time needed were less than those of PF resins. Moreover, the performance of those boards could meet the requirement of Chinese National Standard (CNS). In this study, the bark extracts of Taiwan acacia and China fir were reacted with resorcinol to prepare the resorcinol–bark extract–formaldehyde copolymer resins (RTF), and the properties of these RTF copolymer resins were investigated.

Section snippets

Materials

The barks used were collected from the logs of Taiwan acacia (Acacia confusa) and China fir (Cunninghamia lanceolata). The bonding strength test was performed on white oak (Quercus spp.) with the dimension of 26 cm × 2.5 cm × 1 cm, and 9% of moisture content. The chemicals used included resorcinol, formalin, sodium hydroxide, hydrochloric acid, methanol, benzoic sulfonic acid, hydroxylamine hydrochloride, paraformaldehyde and potassium bromide.

Preparation of bark extracts

Taiwan acacia and China fir bark were air dried and ground

Properties of bark extracts

The pH value of Taiwan acacia and China fir bark extracts were 9.17 and 9.35, respectively, and the alkali content were 5.6% and 5.8%, respectively. The viscosity of bark extracts at 27% of nonvolatility was 300 cps and 115 cps, respectively. This was because China fir bark extracts had more low molecular ingredients than Taiwan acacia, according to our previous report (Lee and Liu, 2004); therefore, it had lower viscosity. The reactivity phenol content in the Taiwan acacia bark extracts was

Conclusion

In this study, RTF copolymer resins were prepared by the copolymerization of resorcinol, bark extracts and formaldehyde. The bark extracts used were extracted from Taiwan acacia and China fir bark with 1% NaOH solution, the reactivity phenol content in the bark extracts of Taiwan acacia and China fir were 51.6% and 46.5%, respectively. FT-IR analysis showed that aromatic compound was the main component in bark extracts. When the bark extracts were employed to synthesize the RTF copolymer resin,

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