Properties of Modified Phenol-Formaldehyde Adhesive for Plywood Panels Manufactured from High Moisture Content

This paper presents the results of laboratory investigations of bonding high moisture content (15 %) birch veneers (Betula pubescens Ehrh.) with the use of modifi ed phenol-formaldehyde (PF) resin. Wheat starch, rye fl our, resorcinol and phenol-resorcinol-formaldehyde resin were chosen as modifying agents. Dynamic viscosity, hydrogen ions concentration, solid content, curing time, pot life of developed adhesive compositions and shear strength of plywood samples were evaluated. ANOVA analysis has shown that type, mixture and content of modifying agents affect signifi cantly the mechanical performance of plywood panels. The obtained results of shear strength values were above the standard requirements (1 N/mm2), and the properties of samples met the European standard EN 314-2 for gluing quality of class 3 and such plywood panels can be used in exterior conditions.

The wood porosity is one of the main physical indexes in wood science.Bonding of wood elements with higher moisture content is a diffi cult process; the details of the penetration of the hardening adhesives into the porous wood skeleton are rather complicated.It is strongly infl uenced by factors as wood species, grain orientation and surface roughness, adhesive factors as type of adhesive, solid content and viscosity, and process factors as applied pressure and temperature, which have the most signifi cant infl uence on the bonding performance (Kamke and Lee, 2007;Varivodina et al., 2010).An analytical model to predict the penetration of adhesives into hardwood was proposed (Mendoza et al., 2012).The model considers a dimensional capillary fl uid transport of a hardening adhesive through a single, straight vessel with diffusion of solvent through the walls of the vessel.Adhesive penetration follows the path of lowest resistance into the porous structure, either by gross penetration or by cell wall penetration.The bond line morphologies and the adhesive penetration into the wood structure were evaluated (Haas, 2012) after determining the hardening characteristics of adhesives.
The viscoelastic characteristics of wood play an important role in compression and densifi cation.At high temperature and high moisture content, wood exhibits plastic behaviour that can be characterized as inconvenient.Temperature and conditions of pressing process affect the change of relative density and creep deformation, as well as properties of the resulting pressed plywood material.The dependence of inelastic behaviour of the gross wood on the elastic properties of the cell wall allows the time, temperature, and moisture dependence to be modelled with classical linear viscoelastic theory of amorphous polymers.Time-temperature-moisture relationship was shown (Wolcott et al., 1994) to be applicable to stress relaxation data collected for temperatures between 39 and 99 °C and moisture contents between 3 and 16 %.
The bond strength of thermosetting PF adhesives is developing during the hardening process carried out in a hot-press at stated pressure, temperature and for a defi ned period of time.Pressure parameters are infl uenced by several factors as thickness of the composite, thermal conductivity, or moisture content of the material.Pressure must be kept till strength of cured adhe-

UVOD
The process of gluing of veneer-based products, as plywood or laminated veneer lumber, is signifi cantly affected by the moisture content in wood combined with water in an adhesive.This moisture directly infl uences the curing process and properties of the used adhesive, economic costs (consumption of glue, pressing time and costs for veneer drying) as well as physical and mechanical properties of veneer-based products.Nowadays in Ukraine, construction plywood is generally produced with adhesives based on phenol-formaldehyde resins and veneers that should be dried to 8 % moisture content.For such moisture content the conventional thermo-reactive adhesives provide highquality bonding of plywood with physical and mechanical performances that meet European standard requirements.Gluing of veneers with lower moisture content (W < 8±2 %) involves the following disadvantages: high consumption of energy for drying, brittleness of veneers, rapid adhesive viscosity increase by diffusion of water into the cellular structure, worsened wetting and insuffi cient transfer of adhesive to another surface.Gluing of veneer with higher initial moisture content (W > 8±2 %) has the following disadvantages: increasing adhesive penetration by the fl ow inside the vessel network through the veneer thickness, high vapour pressure causing steam blisters or blows, decreasing the viscosity of the applied adhesive layer, and loss of wood mass as well as thickness due to compression.There is a risk of forming of discontinuous glue layer on veneer surface, retardation of glue curing or creation of insuffi cient joints.Furthermore, the process of plywood manufacture is characterized by signifi cant energy costs for veneer drying (almost 60 %) and hot pressing (15 %), which signifi cantly affects the cost of production.Therefore, using veneers with higher moisture would result in substantial savings of energy.
Recent developments and trends in the fi eld of eco-effi cient bonding technologies contribute to both ecological and economical aspects.Several new trends are in application of vegetable carbohydrates or proteins, namely as modifi ers of existing adhesives.Addition of polysaccharides or soy proteins to traditional synthetic wood adhesives after partial hydrolysis and modifi cations has been reported (Pizzi, 2006).Some articles (Besinova et al., 1997;Elbez, 1997) were oriented on benefi ts of gluing high moisture veneers.More studies were performed on possibilities of bonding high moisture veneers using phenol-formaldehyde resin fi lled by hydrolysed soy protein, starch and tannin (Vijayendran et al., 2000).Other researchers prepared high solid PF adhesive mixtures with increased reactivity by grafting resorcinol (Clark et al., 1988) or sive is higher than forces of internal vapour.Research has shown that the degree of cure is dependent on the pressing time and on the composition of the PF adhesive (Jost and Sernek, 2009).The temperature inside the bond line is fi rstly rapidly increasing slightly above 100 °C.The temperature stays around this level for the time of energy consumption for the evaporation of water from the veneer and adhesive.After that period the temperature is increasing up to the set pressing temperature.Signifi cant mechanical strength in the adhesive bond was established in the last stage of the curing, when crosslinking of the PF adhesive occurred.
However, all of these above mentioned measures require the use of expensive adhesives, replacement of existing equipment, additional processing steps that can lead to signifi cant investment and thus increase the cost of end products.These methods have not thereby found their use in the manufacture of plywood, so the question of bonding veneer of high moisture content is still important.
Therefore, the purpose of this study was to develop the adhesive compositions that allow gluing veneers with higher moisture content (up to 15 %) to provide the desired strength properties of plywood and veneer-based materials.

MATERIJALI I METODE
Rotary cut veneer sheets of birch wood (Betula pubescens Ehrh.) with dimensions of 300 × 300 mm and thickness of 1.5 mm without visible defects were prepared for the experiments.Commercial PF resin generally used for plywood with the following parameters: solid content 48.3 %, dynamic viscosity 790 mPa•s, was used in this research.Wheat starch (WS), rye fl our (RF), technical resorcinol (R) and phenol-resorcinol-formaldehyde (PRF) resin were chosen as modifying agents.
High moisture content of veneer slows the curing of PF glue.Resorcinol and PRF resin are suggested to be added into PF resin to accelerate the adhesive curing process for improving its water resistance and increasing the adhesion of glue to the veneer with high moisture content.Resorcinol has a coherent orientation of -OH groups and its addition, or addition of PRF resin into PF resin will allow creating additional bonds with wood (phenyl-propane units).Adding of wheat starch and rye fl our into PF resin allows taking excessive moisture from the glue, because they both absorb and swell in water very well.In addition, they prevent the penetration of glue into wood and through thin layers of veneer on the front surface of the plywood.
High moisture content level of 15 % was achieved by conditioning of dry veneer sheets inside a chamber at constant temperature over sulphuric acid solution with concentration of 30 %, till achieving the desired level of moisture content.The moisture content was controlled using electronic moisture-meter and exactly stated by weight method.
The adhesives compositions (Table 1) were proposed and the effect of modifying agents on their prop-erties was investigated.The amount of added modifying agents was of 1, 3, and 5 weight parts per 100 weight parts of phenol-formaldehyde resin.Dynamic viscosity, hydrogen ions concentration (pH), solid content, curing time and pot life of adhesive compositions as well as shear strength of plywood panels were evaluated.
Solid content of prepared adhesive mixtures was determined by weight method.Curing time of adhesive mixtures was determined at the temperature of 150 °C.Dynamic viscosity was measured by the rotational viscometer at 20 °C and hydrogen ion concentration of the prepared adhesives was determined on pH-meter.Curing time of adhesive mixtures was determined on a steel disk at the temperature of 150 °C and pot life as the increase of the dynamic viscosity.Three-layer plywood panels of 300 × 300 mm were made in the electrically heated hydraulic laboratory press.The specifi c pressing pressure of 1.8 MPa and temperature of 150 °C were used, and 6-min pressing time (during the last 30 s of press cycle the pressure was continually reduced to 0 MPa).The glue spread was 150 g/m 2 based on wet mass.PF adhesive mixture was applied onto one side of every uneven ply.The plies were assembled perpendicularly to each other (veneer sheets were laid up tight/loose) to form plywood of three plies.Glue was applied on the veneer surface with a hand roller spreader.Laboratory testing machine was used for the evaluation of the quality of gluing and the shear strength of plywood was determined according to EN 314-2 after pre-treatment for intended use in exterior conditions.Testing samples were immersed in boiling water for 4 h, dried in the ventilated drying oven at 60±3 °C for 16 h, immersed in boiling water for 4 h, followed by cooling in water at 20±3 °C for at least 1 h to decrease the temperature of test pieces to 20 °C.Ten samples were used for each variant shear strength mechanical testing.During experiment, all plywood samples were conditioned before testing for two weeks at 20±2 °C and 65±5 % relative humidity.
For each modifying agent, a one-way analysis of variance (ANOVA) was conducted to study the effect of type and content of modifying agents on the shear strength of plywood panels at the 0.95 confi dence interval.

REZULTATI I RASPRAVA
Homogeneous adhesives compositions were created when each of modifying agents was added into PF resin.Dynamic viscosity of prepared adhesive compositions increased 1.9-8.7 times, with increasing amount of resorcinol or PRF resin (Figure 1), compared with reference PF resin.Similar changes occur when adding wheat starch and rye fl our (Figure 1) into PF resin.In particular, the dynamic viscosity of the adhesive increased 1.1-4.0times, due to active adsorbing of water from the adhesive by the macromolecules of modifying extenders.
The favourable behaviour of the impact of modifying agents on curing time (Figure 2) was observed.In the case of increasing of resorcinol and PRF resin, the adhesive curing time is reduced by 20 % and with the increase of wheat starch and rye fl our by 50 %.This pattern of changes in dynamic viscosity and curing time of adhesive compositions are appropriate and can be explained by the interaction of resorcinol or PRF resin with PF resin by forming cross-links (Urbanik et al., 1997).Such cross-links increase the molecular weight of polymer, which respectively leads to the increasing of viscosity of the adhesive composition and to the fast curing (Waage et al., 1991).
Increasing amount of modifying agents leads to a decrease in pot life of adhesive mixtures, obtained values range from 1 to 5 hours (Table 2).The solid content and concentration of hydrogen ions (pH) remain practically unchanged and are in the span from 50.1 to 57.6 % and from 11.3 to 12.0, respectively.
An analysis of variance (ANOVA) was conducted to discern whether differences in shear strength using adhesives with various modifying agents are statistically signifi cant.Table 3 presents ANOVA results for experimental data.This analysis found that type of modifying agents and their content in adhesive composition impacted shear strength.From the ANOVA analysis (Table 3), it is evident that the content of modifying agent is the most infl uencing factor on shear strength.The type of modifying agents is the next infl uencing factor on shear strength.In addition, the ANOVA showed that the interaction between the type  of modifying agents and their content in adhesion composition was also signifi cant.
The effect of modifying agents on shear strength of plywood is shown in Figure 3.In the case, if resorcinol content is increasing from 1 to 5 weight parts for all concentrations, the course of shear strength of plywood is slightly decreasing, but still is over reference sample.The reason is that resorcinol very actively reacts with PF resin, viscosity is rapidly increasing and, thus, the adhesion of glue to the wood is reduced.However, the increase in shear strength is observed when the concentration of resorcinol solutions is increasing.This can be explained by the decrease of water amount in the adhesive composition, and hence the decrease of moisture content in the glued veneer lay-up.The increase of PRF resin content in the adhesives leads to an increase in shear strength due to the formation of three-dimensional polymer network and increasing number of methylene bonds (Skeist, 1977).

DRVNA INDUSTRIJA 65 (4) 293-301 (2014)
Further to the above, in order to improve the adhesive properties and to reduce costs of resin mixture, it should be more profi table to combine the organic extenders (wheat starch and rye fl our) with PRF or resorcinol modifying agents.
It was found that the dynamic viscosity of adhesive mixtures with modifying agents signifi cantly increases up 2.8-13.3times (Figure 4) as well as solid content 1.02-1.16times in comparison with PF resin and, therefore, the pot life of adhesive is reduced from 5 to 2 hours.The pH value (11.2-11.9) is not significantly changed.
The curing time of adhesive mixtures modifi ed by various agents was shorter (30.1 to 55.2 s) than the curing time of adhesives only modifi ed by one modifying agent.It was established that curing time decreases with the increase of resorcinol or PRF content in the adhesive composition (Figure 5).Table 4 presents ANOVA results for plywood panels made using mixture of modifying agents.ANO-VA tests showed that mixture of modifying agents, their content in adhesive composition and interaction between them also has statistically signifi cant effect on the shear strength.Increasing concentrations of resorcinol solution in a mixture with rye fl our for any mass between ratios 1:3 or 3:3 lead to the decrease in shear strength of plywood because the dynamic viscosity of adhesive composition is signifi cantly increased.In this case, worse glue wetting of the surface of veneer and thickening of the glue layer is observed.All of that leads to the reduction in shear strength.Similar behaviour of the dependence of shear strength of plywood is observed in the case of combining the resorcinol solutions with wheat starch by the ratio 1:3 (Figure 6a) or 3:3 (Figure 6b).Reactivity of resorcinol modifi ed adhesives increases with increasing of resorcinol concentration in the solution.This leads to the rapid decrease of free reactive methylol groups, and reduction the adhesion strength of adhesive joint.
For adhesive compositions modifi ed with PRF, regardless of its content in the mixture, the higher values of shear strength of plywood are provided in both cases in combination with rye fl our and wheat starch.

ZAKLJUČAK
Modifi ed PF resin with selected modifying agents allows using veneer with moisture content of 15 % for the manufacture of plywood in the laboratory scale, if their properties meet the standard requirements.Modifi cations of phenol-formaldehyde resin, in the way presented in this study, are possible with the use of wheat starch, rye fl our, phenol-resorcinol-formaldehyde resin and 30 %, 40 %, 50 % concentrations of resorcinol.
The results of tests of shear strength properties of plywood show that mainly PRF glue compositions are most appropriate for the gluing of veneer with higher moisture content.The improvement of technical parameters of adhesive compositions and glued products strongly depend on the composition of glue mixture, functionality of modifi ers and the way of modifi cation.
With the aim to decrease the over penetration of glues through the veneer during hot pressing, organic extenders were applied in gluing mixtures.ANOVA analysis has shown that type, mixture and content of modifying agents affect signifi cantly the mechanical performance of plywood panels.The positive results from the laboratory experiments gave the base to plan experiments in industrial conditions, and however the application of some of them is limited due to their higher viscosity, mainly from the view of glue spread control.These glues have the solid content higher in comparison with reference PF resin.