Organo-silane compounds in medium density fiberboard: physical and mechanical properties

Hamid Reza Taghiyari?Ali Karimi?Paridah Md.Tahir

We studied the effects of nanoparticles of organo-silane(NOS)compounds in the size range of 20–80 nm on physical and mechanical properties in medium density fiberboard,and used NOS atfour consumption levels of 0,50,100,and 150 g kg-1dry wood fibers. Density of all treatments was keptconstantat 0.67 g cm-3. The water-repellent property of organo-silane significantly reduced water absorption(WA)and thickness swelling but mechanical properties declined due to the reduced proportion of wood-fiber as organo-silane was added to the matrix:the compression ratio of MDF panels and the integrity among wood-fibers both declined,resulting in reduced mechanicalproperties.We recommend use of 50 g of NOS/kg wood-fiber to improve WA and thickness swelling while retaining acceptable mechanical properties.

Composite-board·Medium-density fiberboard(MDF)·Nanotechnology·Physical and mechanical properties·Water-repellant·Organo-silane

Introduction

Composite-boards offer the advantages of a homogeneous structure and the use of raw materials without restrictions as to shape or size,and many technologies have been developed to limit formaldehyde emissions(Valenzuela et al. 2012).Wood-composite panels and profiles are,therefore, expanding worldwide.In fact,wood-based composite products are commonly substituted for solid wood in today’s building structures(Eshaghi et al.2013).The main factors thatinfluence the properties and quality of the panels are the density of the panel,geometry and moisture content of the particles,the pressing cycle,and the quantity and type of adhesive.The production of wood composites has increased dramatically over the past three decades.Their use,however,is often limited due to high sensitivity to moisture and decay(Baileys et al.2003;Gardner etal.2003;Laks 2002), as well as fire(Taghiyari 2012;Haghighi et al.2013).

The emergence of new technologies to produce an increasing array ofnew wood composite products has forced the industry to follow-up with varied protection processes and/or treatments to protectthese new wood-based products (Kirkpatrick and Barnes 2006).Different materials can be easily used to modify and improve some of the undesirable properties in composite boards or the production procedure. In this connection,the heat-conductive nature of nanometal particles(Ghorbani et al.2012;Khojier et al.2012;Poda etal.2013)wasused to betterpolymerize the resin in the core of the mat,as well as decrease gas and liquid permeability. The improving effectsofsilvernanoparticleson physicalandmechanicalproperties ofparticleboard on an industrialscale, as well as its hot-press time reduction,were also reported. Wollastonite nanofibers were also reported to improve thermal conductivity coefficient of medium-density fiberboard(MDF),physicaland mechanical properties,and fireretarding properties of MDF.

In this connection,the water-repellent property of organo-silane(NOS).compounds was reported to decrease liquid and gas permeability of MDF(Taghiyari2013).The effects of NOS on other physicaland mechanicalproperties were,however,yet to be studied.Our study was therefore conducted to evaluate if NOS could contribute significantly to reducing water absorption(WA)and thickness swelling, while simultaneously improving mechanical properties.

Materials and methods

Specimen preparation

Wood fibers were procured from Sanaye Choobe Khazar Company in Iran(MDF Caspian Khazar).The fibers comprised a mixture of five species of beech,alder,maple, hornbeam,and poplar from the neighboring forests.Boards were 16 mm thick,and the density was kept constant at 0.67 g cm-3.The total nominal pressure of the plates was 16 MPa.The temperature of the plates was fixed at130°C. Hot-pressing continued for 8 min.Urea–formaldehyde resin(UF)was procured from Sari Resin Manufacturing Company in Sari,Iran.We used 10%of UF at0.2–0.4 Pas in viscosity,47 s of gel time,and 1.277 g cm-3in density withoutcatalyst.Ten boards were made for each treatment. From each board,two specimens were cut for modulus of rupture(MOR),modulus ofelasticity(MOE),internalbond (IB),WA,and thickness swelling(TS)tests.

Nano-organo-silane application

The nano-organo-silane liquid(NOS-liquid)was the resultant product of organo silane reacted with organic reactant,produced in cooperation with Zydex Industries.Its color was pale yellow,with the flash pointatmore than 85°Cand auto-ignition temperature at more than 200°C,specific gravity of 1.05 g mL-1(at 25°C),viscosity of 0.5–1.0 Pas(at 25°C). The nano-organo-silane liquid was comprised of hydroxyalkyl–alkoxy–alkylsilylcompounds(38–42%);and the solvent wasethylene glycol(58–62%).The size range ofnanoparticles was 20–80 nm.NOS-liquid was mixed with resin in proportions of 0,50,100,and 150 g kg-1dry weightbasis of fibers. NOScontentwasbasedonthe solid partsin the NA-suspension. For each treatment,the weight of NOS-solids was deducted fromthe fiber,and in thisway,the density ofpanelsin different treatments with differentfiber-contentwas keptconstant.The final mixture of NOS+resin was smoothly sprayed on the fibers.Resin pH and viscosity were keptconstantfor alltreatments.Four boards were manufactured for each treatment. Boards were kept in a conditioning chamber[(25±2)°C, (40±3)%ofrelative humidity]for1 month before the liquid and gas permeability specimens were measured.

Physical and mechanical tests

Physical and mechanical tests,as well as the number and location of the specimens,were carried out in accordance with the ISIRI 9044 PB Type P2(compatible with ASTM D-1037;2007 edition)specifications.The static bending test was performed using center-point loading over a 390 mm span.The loading speed was 2 mm min-1.All tests were conducted using an INSTRON 4486 testing machine.Equations 1–3 were used to calculate final values of MOR,MOE and IB(MPa):

Brittleness was calculated by using the ratio(%)of the work absorbed in the elastic region to the total work absorbed to maximum load,as shown in Eq.4(Phuong et al.,2007;Taghiyari et al.2013):

The proportional limit was decided by drawing a linear correlation curve using data from 0%load to the limit value(R2=0.999).

Statistical analysis

Statistical analysis was conducted using SAS software, version 9.2(2010).One-way analysis ofvariance(ANOVA) was performed to discern significant differences at 95%. Hierarchical cluster analysis,including dendrogram and using Ward methods with squared Euclidean distance intervals,was carried out by SPSS/18(2010).Fitted line plots were made by Minitab software,version 16.2.2(2010).

Results and discussion

Nano-organo-silane significantly reduced WA and thickness swelling in MDF(Figs.1,2).The lowest WA and thickness swelling was observed in NOS-150 treatment.WA and thickness swelling,after 2 h immersion in water, declined by 31.9 and 43.6%,respectively,in NOS-150 treatment in comparison to the control specimen.WA and thickness swelling gradually declined as the NOS-content increased from 50 g kg-1wood fiber to 150 g kg-1, showing a directrelationship between NOS contentand the declines in WA and TS.The lowest WA and TS were observed in the NOS-150 treatmentdespite the micro-voids and cavities that were formed(Taghiyari 2013)due to the significantly lower amountof wood fibers in this treatment. Liquid permeability was similarly reported to significantly decline in MDF by addition of NOS to the MDF-matrix (Taghiyari 2013).

Nano-organo-silane had a degrading effect on all mechanicalproperties of NOS-treated boards(Figs.3,4,5, 6,7).MOR decreased by 76%in NOS-150 treatment in comparison to the control specimen(Fig.3).The reason for the decline in mechanicalproperties was the decrease in the wood fiber content of NOS-treated specimens.As a rule,the density of the produced boards should be at about 0.65–0.68 g cm-3in Iran to be compatible with the standards as well as the desirability in the commercial market; so,the amount of NOS-suspension used for each treatment (50,100 and 150 g cm-3)was subtracted from the dried wood-fibers in order to keep the density of the boards constant at 0.67 g cm-3.Therefore,the volume of wood fibers decreased as the NOS-content increased from 0 to 150 g cm-3.This resulted in less compression between wood fibers in the MDF-matrix.SEM micrographs showed scattered micro-cavities in the body of the NOS-treated boards(Taghiyari 2013).

Of all the physical and mechanical properties,the Duncan grouping of the hardness values was in close compatibility with the groupings based on the specific gas permeability values(Taghiyari 2013);that is,NOS-50 and NOS-100 treatments were similarly grouped when both gas permeability and hardness were measured.This indicated that the compression between the wood-fibers on the surface layers of the boards significantly influenced hardness and gas permeability the same way.

Fig.1 Water absorption(%)at2 and 24 h in control,NOS-50,NOS-100,and NOS-150 treatments(NOS nanoparticles of organo-silane). Letters on each column represent the Duncan’s multiple range test groupings

Fig.2 Thickness swelling(%)at 2 and 24 h in control,NOS-50, NOS-100,and NOS-150 treatments(NOS nanoparticles of organosilane).Letters on each column representthe Duncan’s multiple range test groupings

Fig.3 Modulus ofrupture(MPa)in control,NOS-50,NOS-100,and NOS-150 treatments(NOS nanoparticles of organo-silane).Letters on each column represent the Duncan’s multiple range test groupings

Fig.4 Modulus of elasticity(MPa)in control,NOS-50,NOS-100, and NOS-150 treatments(NOS nanoparticles of organo-silane). Letters on each column represent the Duncan’s multiple range test groupings

Fig.5 Internalbond(MPa)in control,NOS-50,NOS-100,and NOS-150 treatments(NOS=nanoparticles of organo-silane).Letters on each column represent the Duncan’s multiple range test groupings

Fig.6 Hardness(MPa)in control,NOS-50,NOS-100,and NOS-150 treatments(NOS nanoparticles of organo-silane).Letters on each column represent the Duncan’s multiple range test groupings

Fig.7 Brittleness(%)in control,NOS-50,NOS-100,and NOS-150 treatments(NOS nanoparticles of organo-silane).Letters on each column represent the Duncan’s multiple range test groupings

Strong correlations were recorded between TS and WA after 2 and 24 h immersion in water(Fig.8a).The short and long term physical patterns were similar,that is,the water repellant nature of NOS significantly affected both short and long term physical properties of TS and WA. Furthermore,though notas high as between TS and WA,a high R-square was recorded between MOR and IB (Fig.8b),and NOS had nearly similar effects on different mechanical properties as well.A highly significant correlation was found between hardness values measured at different depths of the penetration of the hardness ball (Fig.9).This showed that variations in the hardness values at different depths had a high compatibility in different treatments.The loading strength values of hardness proved this compatibility(Fig.10).In the meantime,hardness also showed high correlation with all physical and mechanical properties.The highest correlation was found between hardness(H5)and MOE(Fig.11).

Fig.8 Fitted-line plots between thickness swelling after 2 and 24 h of immersion in water(a),and between MOR and IB values(b)

Cluster analysis was carried out based on all the physical and mechanical properties of four treatments (WA-2&-24 h,TS-2 and-24 h,MOR,MOE,IB, brittleness,and hardness).The results indicated close similarity between the control and NOS-50 treatments,as well as between NOS-100 and NOS-150 treatments (Fig.12).Therefore,if the final application of the boards stipulates improved water-repellent properties,but the mechanicalproperties would notbe of criticalimportance, NOS-100 treatment is recommended,depending on the final application.On the other hand,if the mechanical properties are also important,but improved water-repellent properties are needed,then NOS-50 would be recommended.The mechanical properties of samples manufactured using NOS-150 treatmentwere substantially degraded and therefore this treatment is notrecommended for commercial purposes.

Fig.9 Fitted-line plotbetween hardness measured atdifferentdepths of penetration of the ball(3 and 5 mm)

Fig.10 Hardness(MPa)at three depths of the penetration of the hardness ball(3,4,and 5 mm)in control,NOS-50,NOS-100,and NOS-150 treatments(NOS=nanoparticles of organo-silane)

Fig.11 Fitted-line plot between hardness(5 mm)and modulus of elasticity(MOE)

Fig.12 Cluster analysis of control,NOS-50,NOS-100,and NOS-150 treatments based on all the physical and mechanical properties (NOS=nanoparticles of organo-silane)

With reference to the decline in mechanical properties due to the replacement of wood fiber by nano-organosilane,the authors are studying the effects of NOS treatmentwithoutreducing wood fiber content.This addition of nano-organo-silane is predicted to enhance mechanical properties of treated fiberboard.

Conclusion

Water-repellant property of organo-silane significantly decreases WA and thickness swelling in MDF regardless of the micro-cavities formed and the consequently reduced integration between wood-fibers in the MDF-matrix.Therefore,if WA and thickness swelling are the decisive factors in MDF production and finalapplication,NOS should be added to the composite matrix.Doing so enables reduction in fiber content of panels,saving raw materials and decreasing production costs.

MDF with higher NOS-content would have lower mechanical properties due to the lower wood-fiber content and formation of micro-cavities,and the consequent less compression ratio of the panels and lower integration between the wood fibers in the MDF-matrix.

Final application of the NOS-treated panels depends on the desired mechanical and physical properties;if WA and TS are vital,NOS-100 is recommended;and if the mechanicalproperties are also of importance,then NOS-50 treatment would be more appropriate.

AcknowledgmentsThe presentresearch projectwas conducted as a joint research project and financed by SRTTU(Iran)and UPM (Malaysia)for which the authors are grateful.The authors are thankfulto Mr.Majid Ghazizadeh,the internalsales manager of Pars Chemical Industries Company,for the procurement of the resin for the present study.We appreciate Mr.Pezhman Nouri,specialized on wood-composite materials,for his technical advice and support.

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24 November 2013/Accepted:29 December 2013/Published online:27 January 2015

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Project funding:The projectwas conducted as a joint research project and financed by SRTTU(Iran)and UPM(Malaysia)

The online version is available at http://www.springerlink.com

Corresponding editor:Yu Lei

H.R.Taghiyari(?)

Wood Science&Technology Department,Faculty of Civil Engineering,Shahid Rajaee Teacher Training University, Tehran,Iran e-mail:htaghiyari@srttu.edu;htaghiyari@yahoo.com

A.Karimi

Department of Wood and Paper Science&Technology,Faculty of Natural Resources,The University of Tehran,Karaj,Iran

A.Karimi·P.Md.Tahir

Laboratory of Biocomposite Technology,Institute of Tropical Forestry&Forest Products(INTROP),University Putra Malaysia(UPM),Serdang,Malaysia