PROPERTIES OF ALCOHOL-MODIFIED PF RESIN USED IN THE PRODUCTION OF WOOD-BASED MATERIALS

Radosław Mirski, Dorota Dziurka, Janina Łęcka
Department of Wood-Based Materials, Poznań University of Life Sciences, Poland

ABSTRACT

The properties of liquid and cured PF resin modified with alcohols were investigated in the study. The following alcohols were used in the study: ethanol, butanol, 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,2,3-propanetriol. It was found that the modification of phenolic resin with the applied alcohols does not deteriorate its pot life at the temperature of 20°C, and in case of polyalcohols it caused an increase in its reactivity at higher temperatures, expressed by the shortening of gelation time at 130°C and the lowering of activation energy of resin gelation. The results of FTIR tests of polycondensed modified PF resins indicate that polyalcohols not only catalyze the polycondensation reaction of the PF resin, but also are built into its structure.

Key words: PF resin, polyalcohols.

INTRODUCTION

Phenolic-formaldehyde (PF) resins have so far been dominant binding agents in the production of OSB and LVL boards [1], as well as HB boards. The application of these resins makes it possible to obtain adhesive-bonded joints exhibiting high mechanical properties and high moisture resistance. The produced adhesive bonds are also relatively elastic, resistant to vibrations and thermally stable, which results in the better service life of boards bonded with PF resin [5]. However, to harden they require considerable amounts of thermal energy to be supplied, i.e. either longer pressing times or higher pressing temperatures. Long pressing times lower the efficiency of production lines, whereas the application of higher temperatures results in decreased thermal economics due to the high thermal gradient and no heat recovery from the pressed boards. For this reason studies are conducted to improve the technological and economic indexes of the manufacturing process of reconstituted products bonded with PF resins through improving their reactivity by resin modification.

As a rule these studies concern two basic modification methods: resin modification at the stage of its synthesis, and the modification of pre-condensed resin at the curing stage [3]. The method found preferable by numerous researchers and manufacturers of wood-based materials is the modification of ready to use resins with the addition of substances accelerating the curing process and/or lowering the curing temperature. This approach aiming at the increased reactivity of PF resins seems to be more promising due to the considerable number of compounds which may be used to obtain this effect. Esters of both organic and inorganic acids are considered to play a significant role in the process of increasing the reactivity of PF resin [2, 7, 8, 9]. Pizzi and Stephanou [8] investigated the effect of such esters as glycerol triacetate, propylic carbonate, phenylic acetate, methylic formate and ethylic butyrate on the crosslinking process of synthetic PF resins. However, their interpretation of the obtained results did not include the possibility of the participation of alcohol, produced as a result of alkaline hydrolysis of the ester, in the polycondensation process of PF resin. However, the observed pronounced activity of glycerol triacetate in comparison to that of ethylic acetate would indicate the participation of glycerol in this process. Such a possibility was also showed in the studies by Hong et al. [4], who observed a lowered strength of cured PF resin when salicylic acid was used to modify it, and the investigations by Park and Riedl [6], which showed that the addition of propylic carbonate to PF resin diminishes the rigidity of the cured resin. Thus, these observations may indicate a less significant role of acids produced as a result of ester hydrolysis in the polycondensation of resins and suggest that also the produced alcohols, especially polyalcohols, may actively participate in this process. For this reason the aim of this study was to investigate the effect of alcohols on the properties of liquid resin and the structure of cured PF resin used in the production of wood-based materials.

MATERIALS AND METHODS

The applied materials
PF resin, applied to produce particleboards with improved moisture resistance, was used in the experiments. It had the following characteristics: dry resin solids - 45.7%, density - 1.112 g.cm^-3, free phenol content - 0.02%, free formaldehyde content - 0.026%, viscosity according to Ford no. 4/20°C - 86 s, gelation time at 130°C - 156 s, pH - 12.52.

The following polyalcohols were added to the resin: ethanol, butanol, 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,2,3-propanetriol, in the amounts ranging from 0.025 to 0.15 mole per 100 g of dry resin solids.

1,2-ethanediol, 1,3-propanediol, 1,4-butanediol were purchased from Sigma-Aldrich, Germany, whereas ethanol, butanol and 1,2,3-propanetriol were purchased from Polskie Odczynniki Chemiczne, Gliwice, Poland. All chemicals were used as received.

Investigations on the pot life of modified PF resins
Viscosity of PF resins used in particleboard production falls within the 100 ÷ 900 mPa.s range. The application of agents accelerating the curing of PF resins may result in changes in the viscosity of the obtained adhesive mixture. Thus, it would be advisable to investigate the effect of modifiers added to PF resin on its pot life through the measurements of changes in adhesive mixture viscosity in time, depending on the type of the applied modifier and on its amount.

Pot life of the modified PF resin at the temperature of 20°C was determined on the basis of apparent viscosity of adhesive solutions, measured using the Höppler rheoviscosimeter. For this purpose adhesive mixtures were prepared, containing 0, 0.025, 0.05, 0.10 and 0.15 mole of appropriate modifiers per 100 g of dry resin solids, for which apparent viscosity was measured initially every 4 h for the period of 12 h, and subsequently every 12 h. The measurements were completed after 72 h.

Investigations of gelation times of modified PF resins
The measurement of gelation times in case of adhesive mixtures at the temperature of 130°C facilitates the assessment of the activation capacity of the thermosetting adhesive resins. Adhesive mixtures were prepared by the addition of appropriate modifiers to PF resin, immediately before the determination was initiated, in the amounts of 0.00, 0.025, 0.05, 0.10 and 0.15 mole per 100 g of dry resin solids, respectively. Gelation time was measured at the temperature of 130°C, using glass test tubes with the diameter of 5 mm, containing 1 g of the tested solution.

Investigations of gelation kinetics of modified PF resins
In order to determine relative activation energy for the gelation process of PF resin, the results of gelation time testing were used for corresponding adhesive mixtures. It results from a review of literature data that this method was successfully applied to determine activation energy for phenolic resins [10, 11]. In the calculations the assumption was made that the curve describing the gelation process in time for individual temperatures of 120, 130, 140, and 150°C, respectively, is the linear function until the curing point is reached. As the rate of the reaction increases along with the increase in temperature, and the curve of the curing rate shows a linear dependence, thus the activation energy may be calculated from the Arrhenius equation:

k = k₀. exp (-E_a/RT )

where:
k - the reaction rate constant at temperature T [kg.min^-1],
k₀ - pre-exponential factor [kg.min^-1],
E_A - activation energy [kJ.mole^-1],
R - gas constant [kJ.mole^-1.K^-1],
T - absolute temperature [K].

Gelation times of PF resin for the particular temperatures, before and after the introduction of given modifiers, were measured in glass test tubes with the diameter of 5 mm. The weighed sample contained 1g of the investigated solution. Only polyalcohols, potentially capable of polycondensation with PF resin, were selected for the investigations.

Investigations of the structure of condensed PF resins using infrared spectroscopy
FTIR spectroscopy was used to explain the effect of the applied modifiers on the structure of cured PF resin. These investigations were conducted for PF resin before and after modification. Ethanol and 1,3-propanediol were selected as modifiers for the tests. For the purpose of the spectroscopic determinations, samples with the following composition were prepared:

• PF resin cured at the temperature of 90°C, and then at 120°C,

• PF resin with modifiers added in the amount of 1 mole per 100 g of dry resin solids cured at the temperature of 90°C, and then at 120°C.

Samples cured in this way were ground in an impact mill and screened in sieves with mesh size of 0.25, 0.2 and 0.125 mm respectively. The fraction left on the sieve with mesh size of 0.2 mm was used in further tests. In order to eliminate the effect of NaOH on the form of the spectrum, approx. 2 g ground resin were added to 250 ml water, and left for the period of 2 h. After that time the resin was drained, rinsed with water until neutral reaction was obtained, and dried at the temperature of 40°C for 24 h. All the samples were dried in a dessicator under vacuum over P₂O₅.

In order to prepare the samples for infrared testing, the potassium bromide tablet method was used, as it is one of the best and most commonly used methods for the preparation of samples in case of solids. To obtain a tablet, a mixture was prepared from the investigated substance and anhydrous KBr in the 1: 200 weight ratio, which was subsequently thoroughly pulverized in a vibrating mill. The amount of 200 mg of the mixture prepared in the above mentioned way was pressed in a special steel ring under the pressure of 100 MPa, maintaining the pressed mixture under vacuum to have it deaerated. Thus, a transparent tablet, fitted inside a ring (which during the measurement was used as a handle), was produced. Infrared spectra were recorded using a Mattson Infinity spectrophotometer (MATTSON INSTRUMENTS) with Fourier transformation in the range of 4000 - 500 cm ^-1.

THE DISCUSSION OF RESULTS

The results concerning the effect of the applied modifiers on the dynamic viscosity of PF resins are presented in Table 1. As could be expected, the addition of alcohols to phenolic resin resulted in a decrease in the dynamic viscosity of the prepared mixtures; the more modifier was introduced to the mixture, the more pronounced the drop in viscosity. However, no significant effect was observed of the type of alcohol added to the resin. Moreover, during the first 8 h, i.e. the time when the prepared adhesive mixtures have to be stable for technological reasons, no significant increase in viscosity is observed and all the prepared mixtures show viscosity below the level of that for non-modified resin. An increased rise in viscosity is observed only after 24 h, and solely in case when polyalcohols are used as modifiers, the rise being more pronounced for mixtures containing smaller amounts of modifiers, but with bigger molecular weights.

It results from the data presented in Figure 1, that dialcohols - in contrast to the monoalcohols - considerably accelerate the gelation of resin, while for smaller amounts of the introduced modifier (0.025-0.05 mole) a more intensive shortening of gelation time is observed in case of alcohols containing a longer carbon chain. The application of trialcohol as a modifier does not result in a further intensification of this process, as gelation times for PF resin with the addition of 1,2,3-propanetriol are comparable with those obtained as a result of the modification of PF resin with 1,2-ethanediol, especially for bigger amounts of the introduced modifiers. The biggest effect on gelation time was found for 1,3-propanediol, introduced in the amount of 0.15 mole per 100 g of dry resin solids. Gelation time for PF resin as a result of the application of this modifier is by 62 s shorter than that for non-modified resin. These observations, in connection with the fact that resin cured using the same dialcohols is considerably more elastic than non-modified resin, may lead to the conclusion that in this case an increase in the reactivity of PF resin is the consequence of the polycondensation reaction, e.g. between hydroxymethylene groups of the resin and alcohols, which results in methylene bridges (-CH₂-)_n, responsible for the increased elasticity of the modified PF resin, being built into its structure. The obtained results of gelation times for PF resin modified with both ethanol and butanol confirmed a clearly inhibiting effect of monoalcohols on the crosslinking of PF resin, which is probably the effect of the occurring reaction of resin chain termination.

The effect of the type and amount of applied modifiers introduced to PF resin on the relative activation energy of its gelation process is presented in Table 2. It results from the data in the table that the dialcohols applied in the amounts of 0.025 mole per 100 g of dry resin solids result in the lowering of activation energy for this process by approx. 12-20%. Along with an increase in the percentage of the added modifiers, a further decrease in this value is observed; however, the differences found for individual alcohols are not significant. The lowest value of gelation energy for PF resin with the addition of dialcohols was found for 1,3-propanediol, after its introduction in the amount of 0.15 mole per 100 g of dry resin solids. On the other hand, 1,2,3-propanetriol in the applied amounts turned out to be a slightly less effective activator of PF resin. The addition of wood powder to non-modified and modified PF resin results in the further decrease in activation energy. It is significant especially in case of PF resin alone. In resins modified with alcohols, especially in case of their smaller amounts, a considerable lowering in its activating effect is found. However, along with an increase in the amounts of alcohols added to resin, activation energy is lowered. The lowest value of activation energy in this case was obtained for 1,4-butanediol added in the amount of 0.15 mole per 100 g of dry resin solids and it was by 7.5 kJ.mole^-1 lower from that found for non-modified PF resin with the addition of wood powder. Such an action of polyalcohols indicates their active role in the crosslinking of PF resin, competitive with the reactions of chemical adhesion of this resin to wood.

Spectroscopic analysis of non-modified PF resin, and resin modified with both ethanol and 1,3-propanediol (Fig. 2) shows that for resin modified with in the range of the stretching vibrations of OH groups at the wavelength of 3425 cm^-1 a decrease in the intensity of relative absorption is found, which is considerably bigger for ethanol and amounts to approx. 30%. On the other hand, the intensity of the stretching vibrations of CH bonds in the range of 2922 cm^-1 clearly increases, especially in the spectrum of resin modified with 1,3-propanediol. At the same time, especially in PF resin modified with ethanol, an increase in relative absorption of vibrations is observed at wavelengths of 1144 and 1186 cm^-1, which may be attributed to the formed ether alcohol - resin bonds. Moreover, certain changes in the spectra of resin modified with alcohols are also observed in the 1375-1465 cm^-1 range, which may be ascribed to the deforming vibrations of CH bonds in the alkyl group built into the resin. The observed changes in the character of spectra in case of PF resin modified with alcohols show that alcohols not only participate in the crosslinking reaction of resin, but are also built into its structure.

CONCLUSIONS

The introduction of alcohols to liquid PF resin does not have a significant effect on its pot life at the temperature of 20°C, which is crucial in production. An increase in the reactivity of the adhesive mixture, expressed in the increase in its viscosity in time, the shortening of gelation time and the lowering of gelation activation energy, is observed only when polyalcohols are applied as modifiers of PF resin. The active role of polyalcohols in the process of PF resin crosslinking is also observed, although less distinctly, when wood powder is present in the adhesive. Among the alcohols included in the investigations, the biggest effect on the shortening of resin curing time at the temperature of 130°C and the lowering of activation energy for this process was found in case of 1,3-propanediol. The results of FTIR testing of polycondensed modified PF resins indicate that polyalcohols not only catalyze the reaction of PF resin polycondensation, but are also built into its structure.

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