PULP AS A SUBSTITUTE OF WOOD CHIPS IN MANUFACTURE OF PARTICLEBOARDS RESINATED WITH HYBRID PF / pMDI RESIN

This study analyzed feasibility of manufacture of composite particleboards resinated with a mixture of phenol–formaldehyde resin and 4,4′-methylenediphenyl isocyanate in the weight ratio 70:30. For this purpose 3-layer particleboards were produced with varying shares of Kraft scrap paper pulp in the core. Experimental boards were manufactured applying 0, 10, 15, 25, 50 and 75% substitution of wood chips with Kraft paper pulp. Analyses were conducted to determine physico-mechanical properties of boards, i.e. bending strength, modulus of elasticity, internal bond before and after the boiling test and swelling in thickness after 24 soaking in water. Based on recorded testing results it was stated that moisture resistant composite boards manufactured with a 25% substitution of wood chips with Kraft paper pulp meet the requirements of the EN 312 standard for boards used in humid conditions in terms of their strength and moisture resistance specified in the internal bond after the boiling test.


INTRODUCTION
Requirements of sustainable wood management make it necessary to search for alternative raw materials for manufacture of particleboards.Some of the well-established alternatives include annual crop residue as a substitute of wood chips in particleboard production (Boquillon et al. 2004, Dziurka et al. 2005, 2010, Czarnecki and Dukarska 2010, Azizi et al. 2011, Dukarska et al. 2011, 2012, Park et al. 2012, Dukarska et al. 2015, Dziurka et al. 2015, de Melo et al. 2015, Kord et al. 2016).Utilization of annual crop residue brings obvious benefits, such as reduced wood consumption having a positive effect on board prices, as well as management of plant-origin waste.Its application is connected with certain problems when used in commercial scale production processes.These crops are harvested seasonally.A lack of permanent availability of annual plant material leads to the need to accumulate and store its huge quantities over relatively long time periods.Moreover, the specific character of methods applied to harvest annual crops results in relatively large amounts of mineral contamination originating e.g. from the soil.This requires additional operations in the preparation of stock materials for the manufacture of particleboards in order to purify the raw material.
Increasing public awareness of environmental protection issues and the need for selective waste collection result in improved efficiency of waste acquisition, including recycled paper.
Such waste is used mainly in the pulp and paper industry; however, paper production using scrap paper is connected with numerous technological problems.When preparing pulp from scrap paper it is necessary to remove all chemicals used in the production process, such as glues, pigments, preservatives as well as printing inks and toners.Such processing results in adverse changes, e.g.reduced strength of cellulose fibers, leading to a deterioration of quality and strength of paper produced from such processed raw materials.We also need to remember Maderas-Cienc Tecnol 20(2):2018 Ahead of Print: Accepted Authors Version that recycling, storage, deinking or decontamination of paper waste generate considerable costs and may cause negative environmental impacts (Kim et al. 2009, Kordkheili et al. 2012, Lykidis et al. 2012).Irrespective of the applied technologies we are still left with certain amounts of paper waste, which will not be utilized due to economic and technological reasons (Lykidis et al. 2012).The situation is completely different when considering potential utilization of paper in wood based material industry.This is connected with the fact that the board industry has gradually adapted to using raw materials of increasingly inferior quality, including waste materials.Adaptation of board manufacturing technologies to the utilization of scrap paper should be easier to implement than in paper production (Nicewicz et al. 2006).
Apart from environmental factors, application of paper in the manufacture of wood based materials seems to be interesting for two reasons.On the one hand, non-recycled paper waste constitutes currently abundant unmanaged resources of raw material, while on the other hand the use of paper waste in manufacture is much cheaper than board production solely from fresh wood material (Lykidis et al. 2012).In contrast to annual crop residue, in the case of paper waste there are no problems posed by seasonal availability or the necessity to store its large quantities.At appropriate organization it would obviously be possible to ensure a continuous supply of this raw material.Despite numerous research papers published on the potential paper recycling, a limited number of studies concern the utilization of scrap paper in the production of particleboards.Studies conducted to date have investigated mainly scrap paper coming from packaging, newspaper and office paper.The type of used waste has a significant effect on board properties (Massijaya andOkuma 2005, Tikhonova et al. 2014).It was shown that when applying urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) resins single-layer particleboards with required properties may be produced with a 50% share of scrap paper for MUF resin and a 30% share for UF resin (Nicewicz et al. 2006).
Increasing the share of paper waste in the board over 50% results in a drastic reduction of Maderas-Cienc Tecnol 20(2):2018 Ahead of Print: Accepted Authors Version their physical and mechanical properties.For such a considerable rate of wood chip substitution with Kraft scrap paper it is advisable to increase resination rate.A study by Eshraghi and Khademieslam (2012) showed that a slight increase in resination rate (from 9% to 10%) first of all improves internal bond of such boards and limits their swelling in thickness after 24 h soaking in water.The importance of the optimal resination rate was also confirmed by a study of Taramian et al. (2007), in which paper mill sludge was used to produce general purpose boards.Board properties may also be improved by increasing their density (Lykidis et al. 2012) or modifying paper with maleic anhydride (Nourbakhsh and Ashori 2010).Introduction of fibers retrieved from OCC waste in face layers of particleboards improves quality of their surface by reducing its roughness (Abdolzadeh and Doosthoseini 2009).Another method to improve properties of particleboards manufactured using scrap paper is to modify UF resin using pMDI.Substitution of UF resin with pMDI at 2-4% considerably improves strength properties and moisture resistance of boards manufactured with both a 30% and 50% share of OCC paper waste (Lykidis et al. 2012).In turn, the use of pMDI alone makes it possible to manufacture boards with a 50% share of waste paper from newspapers, magazines and office paper.Such boards are characterized by relatively attractive appearance and thus they may be used in interior design, e.g. as siding and partitioning material (Grigoriou 2003).
In view of the development of particleboard production and the need to utilize waste the aim of this study was to investigate the feasibility of manufacturing moisture resistant composite particleboards using Kraft paper waste.Moreover, another aim of this study was also to determine the effect of wood chip substitution in the core of particleboards with Kraft paper waste on physico-mechanical properties of boards resinated with hybrid PF/pMDI resin.
A review of literature on the application of paper waste in particleboard production shows that in most studies amine resins and pMDI were used.For this reason the authors of this study Maderas-Cienc Tecnol 20(2):2018 Ahead of Print: Accepted Authors Version investigated the potential production of such particleboards using phenol-formaldehyde (PF) resin and 4,4′-methylenediphenyl isocyanate (pMDI).

MATERIALS AND METHODS
Experimental boards were manufactured using industrial pine chips and microchips as well as ground Kraft waste paper pulp.Paper used in the tests was manufactured from unbleached pine Kraft pulp (produced by Mondi Świecie S.A., Poland).Scrap sheets after being trimmed to smaller size were ground using a laboratory grinder.Parameters of Kraft paper, which waste was used in studies, were determined based on the binding technical standards and presented in Table 1.

RESULTS AND DISCUSSION
Based on testing results given in Table 3 and Figures 3 -4 it may generally be stated that board strength and moisture resistance deteriorates with an increase in the substitution of wood chips in the core of particleboards with Kraft paper waste.This is particularly evident at the share of paper waste in the core exceeding 25%.At a lower rate of paper particles in the board no significant changes were recorded in bending strength (MOR) and modulus of elasticity (MOE).Tukey's test conducted for homogeneous groups identified the same homogeneous group a, for which the level of statistical significance p assumed values exceeding the adopted level of 0.05.An increase in the substitution rate to 50% as a consequence results in a reduction of bending strength on average by 30%, while for the modulus of elasticity it is by 20% (homogeneous group c).As a result of such a marked deterioration of bending strength these boards do not meet the requirements of the EN 312  caused a further reduction of tensile strength after the boiling test to 0.12 and 0.10 N/mm 2 , respectively.These values fall below the limit required for P3 boards, i.e. 0.13 N/mm 2 .
Analysis of homogeneous groups showed no significant differences between values from the The reduction of tensile strength both before and after the boiling test was accompanied by a relatively slight decrease in moisture resistance manifested in swelling in thickness after extended exposure to water.However, it needs to be stressed that none of the According to Lykidis et al. (2012) the negative effect of paper waste on board properties is caused by pulping-related chemical, physical and structural changes in fibers within the cell wall and probably also by the presence of inorganic substances, resulting in the deterioration of mechanical properties of paper.In the opinion of the authors it is also essential that tensile strength of stratified papers to a considerable extent depends on their moisture content -the higher the paper moisture content, the lesser their strength (Przybysz 1997).In the resination process a certain amount of water is introduced with PF/pMDI resin causing an increase in moisture content in both wood chips and paper, and as a result -a deterioration of their tensile strength.Thus parameters of the resination process need to be optimized in order to improve physico-mechanical properties of particleboards manufactured with the addition of Kraft paper.In the case of conventional particleboards this is achieved by increasing the resination rate of wood chips.However, it seems that a better solution is provided by increasing the particles and wood chips, but also reduces the amount of water introduced in the process of their resination using PF resin.
boards and paper particles, determined based on dimensional analyses.Sieve analysis was also applied to determine the fraction composition, as presented in Figure1.A formulation produced by mixing phenol-formaldehyde (PF) resin (produced by LERG SA, Poland) and 4,4′-methylenediphenyl isocyanate (pMDI) (from Bayer AG, Germany) in the weight ratio 70:30 was used as a binding agent.Parameters of PF resin were as follows: viscosity at 25ºC -735 mPas, density -1.215 kg/m 3 , solids content -46.8%, pH -8.78 and gel Maderas-Cienc Tecnol 20(2):2018 Ahead of Print: Accepted Authors Version

Figure 1 .
Figure 1.Fraction composition of wood chips in the core and Kraft paper particles.

Figure 2 .
Figure 2. Fractions of wood chips in the core and Kraft paper particles with the greatest fraction in the material.
used in humid conditions, i.e.P3, for which the boundary value is 14 N/mm 2 .Internal bond (IB) takes different values, which are presented in Figure3.An increase in the content of paper particles in the board results in a gradual decrease in IB values.At a 25% share of paper particles internal bond decreased by approx.40%.Despite such a considerable deterioration of this strength parameter boards containing such an amount of paper waste met requirements for boards used in humid conditions P3.An increase in the substitution rate of wood chips with paper particles to 50% and 75% causes a drastic decrease in IB values by 70 -80% and disqualifies such boards from being used under humid conditions.Statistical analysis showed that the substitution rate of wood chips in the core using paper particles has a significant effect on tensile strength of boards.This was confirmed by the results of Tukey's test providing as many as six different homogeneous groups for each substitution rate separately (intergroup MS = 0.0011, df = 54, p < 0.05), as well as a high correlation coefficient between substitution rates and IB values.Slightly lower differences were recorded for tensile strength recorded after the boiling test (the V100 test).Substitution at 10 and 15% caused a reduction of board strength after the boiling test by 14% and 17%, respectively, while the post host test showed no statistically significant differences between these values (homogeneous group b).Increasing the substitution rate to 25% resulted in a decrease of the value recorded in the V100 test in relation to the reference board by 30%.Nevertheless, boards containing the same amount of Kraft paper meet the requirements imposed on P3 boards.Increasing the amount of paper in the particleboard to 50 and 75%

Figure 3 .Figure 4 .
Figure 3.A graph for the scatter of recorded values for internal bond (IB) before and after the boiling test (V100).testedboards met requirements of the standard in terms of swelling in thickness after 24h soaking in water.Thickness swelling (TS) admissible by the standard for P3 boards is 14%, while manufactured experimental boards depending on the substitution rate showed swelling of 16.5% in relation to the reference board and 22.9% for the board with a 75% share of Kraft paper.Such a level of swelling in tested boards may be explained by the fact that no hydrophobic agents, typically used in commercial-scale production of particleboards, were used in the manufacture of experimental boards.Nevertheless it was found that at a maximum substitution of wood chips with Kraft paper particles the TS

Figure 5 .
Figure 5.An image of surface of a mixture of Kraft paper particles (a) and wood chips (b) following resination with PF/pMDI.

Figure 6 .
Figure 6.An image of surface of a single Kraft paper particle (a) and a wood chip (b) following resination with PF/pMDI.

Table 1 .
Selected strength properties of tested paper.

Table 2
and Figures 1 -2 present comparisons of parameters of wood chips in the core of the

Table 2 .
Dimensional analysis and the shape of average wood and paper particles.

Table 3 .
Bending strength and modulus of elasticity of particleboards manufactured with various shares of Kraft paper waste.