Radoslaw Mirski 
  
Radosław Mirski 
  
Dorota Dziurka 
email: ddziurka@up.poznan.pl
  
Janina Łęcka 
  
Potential of shortening pressing time or reducing pressing temperature for plywood resinated with PF resin modified using alcohols and esters

Keywords: 

   

Potential of shortening pressing time or reducing pressing temperature for plywood resinated with PF resin modified using alcohols and esters

INTRODUCTION

Phenol-formaldehyde (PF) resins are predominant bonding agents in the production of water-resistant wood-based materials (Andersen and Troughton 1996), primarily construction plywood. The application of these resins, thanks to their good adhesion to veneer surface, makes it possible to obtain adhesive-bonded joints with good mechanical properties and high water resistance. Additionally, formed joints are relatively elastic, resistant to vibrations and thermally stable, which contributes to higher stability of wood-based materials resinated with PF resin [START_REF] Irle | Physical Aspects of Wood Adhesive Bond Formation with Formaldehyde Based Adhesives. Part II. Binder Physical Properties and Particleboard Durability[END_REF]. However, for curing they require considerable amounts of thermal energy, i.e. the application of high temperatures (135 -150°C), or long pressing times (45 -60 s/mm). For these reasons it has been attempted to increase reactivity of PF resin. In view of the fact that construction plywood produced with the use of PF resin is frequently more than 18 mm in thickness, a reduction of temperature and especially a shortening of pressing time would contribute to improved economic indexes of its manufacture processes. A method preferred by many researchers and producers of wood-based materials is modification of ready-to-use resins with additions of substances accelerating curing or reducing the temperature of this process, due to the considerable number of compounds facilitating this process and their easy application. It results from a review of literature that promising results have been obtained when using esters as compounds accelerating curing of PF resin. Reports on the activating action of esters on PF resin curing appeared as early as the 1950's [START_REF] Orth | Verfahren zum Härten von alkalisch kondensierten Kondensationsprodukten aus Phenolen und Aldehyden[END_REF][START_REF] Orth | Verfahren zum Haerten von Phenol-Aldehyd-Kondensaten[END_REF], while in the 1970's pioneering attempts were made to apply esters in curing phenolic resin under commercial-scale production conditions, although only in foundry practice [START_REF] Westwood | Preparation of foundry moulds and cores[END_REF]. In recent years the problem of enhancing reactivity of phenolic resins using esters has been focused on [START_REF] Stephanou | Rapid-curing lignin-based exterior wood adhesives. Part II: Esters acceleration mechanism and application to panel products[END_REF][START_REF] Pizzi | Phenol-formaldehyde wood adhesives under very alkaline conditions. Part II. Esters curing acceleration, its mechanism and applied results[END_REF][START_REF] Higuchi | Acceleration of the cure of phenolic resin adhesives V. Catalytic action of carbonates and formamide[END_REF][START_REF] Tohmura | Acceleration of the cure of phenolic resin adhesives VI. Cure-accelerating action of propylene carbonate[END_REF][START_REF] Pizzi | On the networking mechanisms of additivesaccelerated phenol-formaldehyde polycondensates[END_REF][START_REF] Park | 13 C-NMR study on cure-accelerated phenol-formaldehyde resins with carbonates[END_REF][START_REF] Conner | Accelerated cure of phenol-formaldehyde resins: Studies with model compounds[END_REF]. To date the mechanism activating the action of esters as modifiers of PF resin polycondensation has not been fully clarified and the results of these studies have not been comprehensively evaluated [START_REF] Lei | Ester Acceleration Mechanisms in Phenol-Formaldehyde Resin Adhesives[END_REF].

Moreover, investigations on this subject undertaken by these authors showed that the application of esters as modifiers of PF resin makes it possible to manufacture particleboards and plywood with very good physical and mechanical properties (Łęcka et al. 2001a[START_REF] Mirski | Properties of particleboards glued with phenolformaldehyde resin modified with esters. Part A. The influence of the kind of alcohol that forms the ester[END_REF](Łęcka et al. , 2008c)). Additionally, studies carried out by these authors proved that esters of diols exhibit the highest effectiveness in enhancing reactivity of PF resin (Mirski et al. particleboards and plywood with good mechanical properties and high water resistance (Mirski et al. 2008b,c). When attempting to explain the mechanism of the action of esters in the process of enhancing the reactivity of PF resin these authors focused on the special role played in this process by alcohols, formed as a result of their alkaline hydrolysis. Studies showed that alcohols, particularly polyols, used as modifiers of PF resin, not only catalyze the reaction of polycondensation, but they are also embedded in its structure [START_REF] Mirski | Properties of alcohol-modified PF resin used in the production of wood-based materials[END_REF](Mirski et al. , 2008a)). In turn, particleboards and plywoods produced with the use of such modified resin are characterized by better physical and mechanical properties than materials resinated with pure PF resin (Łęcka et al. 2001b(Łęcka et al. , Mirski et al. 2008c).

Thus, it was considered advisable to investigate the potential use of polyols and selected esters to manufacture exterior plywood at a shortened pressing time or reduced pressing temperature.

MATERIALS AND METHODS

In the analyses, the tested PF resin was the type commonly used in the manufacture of plywood with increased water resistance, with the following characteristics: dry matter -51.37%, density -1.215 g/cm 3 , viscosity -305 mPa⋅s, gel time at 130ºC -246 s, pH -11.08.

The following compounds were used as modifiers of phenolic resin: propane-1,3-diol, propane-1,2,3-triol (glycerin), ethyl malonate and propylene diacetate. Applied modifiers were introduced to resin at 0.05 mol/100 g d.m. resin, with curing agent UT-10 used as a filler of the adhesive mixture. The type and amounts of modifiers were selected on the basis of preliminary testing (Mirski et al. 2008c).

Three-ply plywood was manufactured from birch and pine veneers in the 1.4×1.7×1.4 mm thickness system, applying the following pressing conditions:

• pressure -1.6 N/mm 2 for birch plywood and 1.4 N/mm 2 for pine plywood,

• temperature -115, 120, 125, 130 and 135ºC,

• time -240, 210, 180 and 150 s,

• amount of adhesive mixture applied on veneer -160 g/m 2 . Thus manufactured plywoods were tested in terms of such properties as:

-modulus of rigidity MOR and modulus of elasticity MOE according to EN 310, -bond quality in the shear test f v according to EN 314-1, with samples subjected to pretreatments following points 5.1.1 (24 h soaking in cold water, tested wet) and 5.1.3 (4 hours boiling water + 16 hours 70°C drying + 4 hours boiling water, tested wet), according to the recommendations of standard EN 314-2 for exterior plywood.

For each combination (pressing time/temperature, type of modifier and species of veneer wood) three plywood boards were manufactured. A total of 12 samples were cut from each plywood sheet to be used in testing of bending strength and modulus of elasticity as well as bond quality after accelerated ageing treatments. Results of analyses of individual properties were thus a mean of 36 measurements. Results of these determinations were subjected to statistical analysis.

RESULTS AND DISCUSSION

Results of studies regarding the effect of PF resin modification with polyols and esters on properties of manufactured hardwood and softwood plywoods, depending on their pressing parameters, are presented in Tables 1234. It results from conducted analyses that the application of such modified PF resin makes it possible under standard pressing conditions, i.e. temperature of 135°C and time of 4 min, to produce plywoods with better properties than respective control plywoods, at markedly superior properties found for plywood resinated with ester-modified resin. When the average increase in bond quality and bending strength of plywoods manufactured with the use of alcohol-modified resin, irrespective of wood species, direction of testing and the type of ageing-acceleration tests, was 8%, then in case of estermodified resin it improved by as much as 20% (Tables 1, 2, 3 and4). In contrast, no significant differences were observed in properties of plywoods depending on the type of applied alcohols or esters, since for both polyols and esters they were similar.

In turn, tests conducted on the potential shortening of plywood pressing time showed that plywoods manufactured at each pressing time (240, 210, 180 and 150 s), irrespective of the type of applied modifier of PF resin, exhibited better properties than respective control plywoods, and with its shortening only a slight reduction of properties of manufactured plywoods was found (Tables 1 and2). The most effective modifier was ethyl malonate, since properties of plywood produced with such modified resin, even at the shortest pressing time of 37.5 s/mm, were comparable to those of plywood resinated with pure PF resin at a pressing time of 60 s/mm thickness, both in case of birch and pine plywood. Moreover, as it results from data presented in Table 1 birch plywoods manufactured at a pressing time of 150 s with the use of such modified resin exhibited bond quality after accelerated ageing tests, in relation to control plywood produced under identical conditions, higher by 13 and 28%, respectively, after soaking and boiling tests. Under these conditions such a considerable increase in bond quality proves an advantageous effect of the applied modification on PF resin curing, which determines the stability of produced adhesive-bonded joint. However, it needs to be assumed that these reactions occur not only within resin, but also at the resin-wood interface, as it is indicated by the results of bending strength testing in both directions. An increase in the level of this property was recorded, amounting to 17 and 21%, respectively, as measured perpendicularly and parallel to grain in top layers (Table 2). Thus it may be assumed that the applied modifier on the one hand accelerates polycondensation processes of PF resin and on the other hand facilitates its copolymerization with functional groups of cellulose or lignin in wood. As a result this leads to increased chemical adhesion of resin to veneers and thanks to enhanced rigidity of the formed glue line an increase in bending strength is observed.

These assumptions are also confirmed by the results of tests on properties of plywoods manufactured at pressing temperature reduced by 20°C. They showed that strength determined in shear tests after accelerated ageing tests for plywoods manufactured under these conditions and resinated with resin modified using ethyl malonate was higher than that of respective control plywoods on average by 17 and 35% for birch and pine plywood, respectively (Table 3). In turn, bending strength of plywoods manufactured under these conditions, irrespective of the wood species and direction of testing, was on average by 17% higher than those of control plywoods (Table 4). Moreover, it needs to be stressed that even despite a reduction of pressing temperature to 115°C, plywoods produced using such modified resin exhibit properties comparable to those of plywood resinated with pure PF resin and pressed at 135°C. Such trends in testing results of plywoods manufactured at a reduced pressing temperature most probably result from the fact that applied modifiers, particularly ethyl malonate, increase reactivity of resin, thus facilitating an adequate course of its curing, resulting in the manufacture of plywoods with good properties even at pressing temperature reduced by 20°C, as manifested by the considerable shortening of gel time (Mirski et al. 2008a) Promising testing results both concerning the shortening of pressing time and reduction of pressing temperature, were the basis for the attempted manufacture of plywoods at pressing time shortened to 3 min and simultaneously pressing time reduced to 120°C. In this part of the study plywoods were manufactured, resinated with resin modified with propane-1,3-diol and ethyl malonate. However, it turned out that the use of such pressing conditions does not make it possible to produce plywoods resinated with resin modified with propane-1,3-diol, as they disintegrated during accelerated ageing tests. In turn, plywoods resinated with PF resin modified with ethyl malonate, produced under identical conditions, exhibited good properties (Table 5). It needs to be stressed here that strength of plywoods manufactured at shortened pressing time and reduced pressing temperature, determined in shear testing after accelerated ageing tests, irrespective of wood species, was higher than that of respective control plywoods on average by 32%, while bending strength, irrespective of the direction of tests, on average by 19%.

Results of strength testing of plywoods resinated with resin modified with ethyl malonate and manufactured at the simultaneously lowered temperature and shortened pressing time seem to confirm the thesis on the double role of esters in PF resin curing. As it was shown by the tests conducted by the authors of this study, acids released as a result of the reaction of hydrolysis occurring in the alkaline medium of PF resin, act as catalysts and facilitate embedding of simultaneously formed alcohol into its structure (Mirski et al. 2008a), thanks to which it is possible to manufacture plywoods with good properties under these conditions. CONCLUSION 1. The application of PF resin modified with alcohols and esters makes it possible, under standard pressing conditions, i.e. temperature of 135°C and time of 4 min, to manufacture plywoods with better properties than those of respective control plywoods, with markedly superior properties characterizing plywoods resinated with ester-modified resin.

2. Modifiers increase resin reactivity to such an extent that they facilitate an adequate course of its curing process, facilitating the manufacture of plywoods with good properties even at pressing temperature reduced by 20°C or pressing time shortened by 38%.

3. The most effective modifier of PF resin was ethyl malonate and the application of such modified resin made it possible to produce plywood with good mechanical properties and bond quality at a shortened pressing time and reduced pressing temperature.

LITERATURE

Andersen AW, Troughton GE (1996) New phenolic formulations for bonding higher moisture content OSB panels. For Prod J 46(10): 72-76 

PF

1.19 0.16 1.75 0.22 1.16 0.23 1.30 0.13 propane-1,2,3-triol 1.33 0.13 1.91 0.21 1.23 0.26 1.49 0.17 propane-1,3-diol 1.49 0.11 1.95 0.19 1.37 0.17 1.57 0.19 propylene diacetate 1.50 0.17 1.95 0.23 1.39 0.19 1.61 0.26 ethyl malonate 125 1.64 0.22 1.96 0.28 1.43 0.2 1.63 0.29 PF 1.24 0.23 1.47 0.29 1.12 0.19 1.28 0.21 propane-1,2,3-triol 1.44 0.21 1.48 0.23 1.28 0.17 1.37 0.19 propane-1,3-diol 1.54 0.18 1.57 0.21 1.30 0.21 1.39 0.23 propylene diacetate 1.54 0.11 1.63 0.19 1.30 0.22 1.40 0.26 ethyl malonate 120 1.64 0.25 1.75 0.31 1.47 0.26 1.50 0.18 PF 1.06 0.31 1.79 0.32 1.03 0.22 1.38 0.19 propane-1,2,3-triol 1.20 0.11 1.79 0.28 1.04 0.17 1.57 0.22 propane-1,3-diol 1.38 

Table 3

 3 Bond quality of plywood manufactured with modified PF resin depending on pressing temperature

		Pressing	Shear strength f v (N⋅mm -2 )
	Kind of modifier	temperature (°C) *	24 h cold water soaking Pine Birch	boiling water Pine Birch
	PF		1.49 0.12 ** 1.58 0.23 1.24 0.19 1.62 0.31
	propane-1,2,3-triol		1.59 0.26 1.89 0.33 1.39 0.23 1.64 0.27
	propane-1,3-diol	135	1.59 0.31 1.97 0.25 1.43 0.18 1.78 0.22
	propylene diacetate		1.66 0.22 2.00 0.36 1.43 0.22 1.79 0.23
	ethyl malonate		1.68 0.26 2.14 0.27 1.52 0.27 1.93 0.29
	PF		1.38 0.13 1.60 0.19 1.03 0.12 1.33 0.22
	propane-1,2,3-triol		1.40 0.23 1.79 0.22 1.12 0.16 1.50 0.19
	propane-1,3-diol	130	1.42 0.29 1.82 0.26 1.27 0.21 1.77 0.22
	propylene diacetate		1.42 0.24 1.94 0.21 1.53 0.23 1.83 0.26
	ethyl malonate		1.68 0.26 2.05 0.28 1.61 0.29 1.91 0.29

  0.26 1.83 0.21 1.31 0.22 1.66 0.26 propylene diacetate 1.44 0.22 1.86 0.22 1.35 0.21 1.67 0.23 ethyl malonate 115 1.47 0.23 1.87 0.24 1.36 0.19 1.78 0.28

* pressing time -240s ** standard deviation

Table 4

 4 Bending strength and modulus of elasticity in bending of plywood manufactured with modified PF resin depending on pressing temperature

		Pressing	MOR					MOE
	Kind of modifier	temperature (°C) *	Pine	Birch	N⋅mm -2	Pine	Birch
		II	⊥	II	⊥	II	⊥	II	⊥
	PF	92.7 27.9 104 37.6 9080 1200 10200 1630

Table 5

 5 Properties of plywood manufactured with ethyl malonate modified PF resin (pressing temperature 120°C, pressing time 180 s) 12 ** 23.4 2.1 8380 1560 1060 150 0.78 0.08 0.55 0.09 pine 101 11 28.3 3.2 10600 1890 1210 180 0.89 0.07 0.79 0.13 102 * 12 31.3 3.6 10400 2100 1500 130 0.77 0.11 0.68 0.15 birch 118 9 34.6 4.2 12900 1220 1670 210 1.06 0.19 0.89 0.12

		MOR		MOE		Shear strength f v
	Wood species	II	⊥	II	⊥	cold water	boiling water
				N/mm -2			
		80.2					

* * control plywood ** standard deviation