some of the properties of wood–plastic composites

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doi:10.1016/j.bu

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Building and Environment 42 (2007) 2637–2644

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Some of the properties of wood–plastic composites

Andrea Wechslera,�, Salim Hiziroglub

aUniversidad Bio-Bio, Wood Composite Material and Quality, Control Laboratory, Concepcion, ChilebDepartment of Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK 74078-6013, USA

Received 3 February 2006; accepted 27 June 2006

Abstract

In this study some of the important properties of experimentally manufactured wood–plastic composites (WPC) were determined.

Specimen having 60% and 80% particle and fiber of radiata pine (Pinus radiata ) were mixed with polypropylene (plastic) and four

different additives, namely Structor TR 016 which is coupling agent, CIBA anti-microbial agent (IRGAGUARD F3510) as fungicide,

CIBA UV filter coating (TINUVIN 123S), CIBA blue pigment (Irgalite), and their combinations. Based on the initial finding of this

work static bending properties of the samples enhanced as above chemicals were added into both particle and fiber-based specimens.

Thickness swelling of the samples were also improved with having additives in the panels. Micrographs taken on scanning electron

microscope (SEM) revealed that coupling agent and pigment resulted in more homogeneous mixture of wood and plastic together. Two

surface roughness parameters average roughness (Ra) and maximum roughness (Rmax) used to evaluate surface characteristics of the

samples showed that particle based samples had rougher surface characteristics than those of fiber based ones. No significant influence of

chemicals added in the samples was found on surface roughness values of the samples manufactured from particle and fiber of radiata

pine.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Radiata pine; Particle; Fiber; Wood–plastic composites; Coupling agent

1. Introduction

Wood–plastic composites (WPC) are widely used inUSA, the most common type of such panels are producedby mixing wood flour and plastics to produce a materialthat can be processed similar to 100% plastic-basedproducts [1-4]. Some of the major advantages of WPCinclude their resistance against biological deterioration foroutdoor applications where untreated timber products arenot suitable. The sustainability of this technology becomesmore attractive when the low cost and high availability offine particles of wood waste is considered.

These composites are transformed by extrusion processesto obtain structural building applications including,profiles, sheathings, decking, roof tiles, and window trims,with improved thermal and creep performance comparedwith unfilled plastics [5-7]. However, it is necessary toimprove their physical and mechanical properties as well as

e front matter r 2006 Elsevier Ltd. All rights reserved.

ildenv.2006.06.018

ing author.

appearance of such products to have a strong market sharein wood composite panel industry. There are several waysto improve overall properties of WPC panels, namely usingright size of raw material, optimum mixture and prepara-tion of the elements in the product, and adding smallamounts of additives such as coupling agents, pigments,antimicrobials or light stabilizers during their production[8,9,10].Most of the physical and mechanical properties WPC

depend on mainly on the interaction developed betweenwood and the thermoplastic material. One way to improvethis interaction is incorporating a coupling agent asadditive. In general, the additives help the compatibilitybetween hydrophilic wood and hydrophobic plastic allow-ing the formation of single-phase composite. Wood-plasticcomposites also have problems when they are exposed toUV rays, their natural wood or pigmented color may tendto fade away. Therefore, depending on the final applica-tion, UV filters have to be added to stabilize their colors fora longer time. When designing a commercial composite, the

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ARTICLE IN PRESSA. Wechsler, S. Hiziroglu / Building and Environment 42 (2007) 2637–26442638

effect of particle size is one of the most importantparameters affecting overall products properties. The useof optimum size of particle might improve the mechanicalproperties of a composite, but the incorporation of apreservative should also be considered if it will be used foran application where biological resistance of this product isimportant [10].

Radiata pine is one of the main species having with anannual production of 27 million of cubic meters in Chile. It

Table 1

List of the chemicals used as additive in panel manufacture

Material Abbreviation Description

Radiata pine fiber Wf Subproduct of R

process. 50mesh

Radiata pine particles Wp Subproduct of R

process. 10–50m

Polypropylene Pp Virgin polypropy

Eastmann Epolene maleated polyethylene

(polypropylene)E 43

E 43 Maleated polypr

Struktol TR016 Tr 016 A blend of fatty

CIBA Antimicrobial agent IRGAGUARD F3510 Irg Contains a broad

effective against

CIBA Plastic UV filter TINUVIN 123S Tn UV plastic fiber.

stabilizer (HALS

functionality, ab

polypropylene.

CIBA Wood UV filter lignostab Lg Lignin stabilizer

tinted or stained

improvement of

and transparent

Ciba irgalite blue pigment Bp Concentrate pow

Table 2

Ratio of the raw material and additive agents used for panel manufacture

Panel type Fiber-based panels

60/80 ratio 80/20 ratio

A 60% wood fiber 80% wood fiber

40% polypropylene 20% polypropyle

B 60% wood fiber 80% wood fiber


2% TR016 2%TR016

C 60% wood fiber 80% wood fiber


2% TR016 2% TR016

6% IRG 6% IRG

D 60% wood fiber 80% wood fiber


2% TR016 2%TR016

1.2% LG 1.6% LG

1.2%TN 0.6% TN

E 60% wood fiber 80% wood fiber


2% TR016 2%TR016

0.05 g BP 0.05 g BP

excellent prime source for pulp and paper manufacture inChile and many other countries. Additional products fromradiate pine include interiors and exteriors panels, furnituremanufacture, trimming, and structural lumber. Althoughresearch and technological development in the area ofWPC in Chile has been increasing but there is still nocomprehensive work has been done in this area [1].Therefore, the main objective of this work is to investigatesome of the properties of WCP panels manufactured from

Source

adiata pine furniture manufacture Catem, Concepcion Chile (wood high technology

center)

adiata pine furniture manufacture

esh

Catem, Concepcion Chile (wood high technology

center)

lene pallets Petroquim, Polypropylene producer, Talcahuano,

Chile

opylene wax Eastman chemical companies, USA

acid metal soap and an amide. Struktol chemical companies, USA

-spectrum fungicide that is highly

mold, rot, blight and stain.

Ciba chemical company

Liquid hindered amine light

) based on aminoether

sorbed into highly porous


for color stabilization of natural,

wood and for the durability

wood substrates coated with clear

pigmented finishes.


der pigment Ciba chemical company

Particle-based panels

60/40 ratio 80/20 ratio

60% wood particles 80% wood particles

ne 40% polypropylene 20% polypropylene

60% pine wood 80% wood particles


2% TR016 2% TR016



2% TR016 2% TR016

6% IRG 6% IRG



2% TR016 2% TR016

1.2% LG 1.6% LG

1.2% TN 0.6% TN



2% TR016 2% TR016

0.05 g BP 0.05 g BP

ARTICLE IN PRESSA. Wechsler, S. Hiziroglu / Building and Environment 42 (2007) 2637–2644 2639

radiata pine furnish and plastic which is virgin polypropy-lene with addition of various chemicals to provide an initialdata in this area.

2. Materials and methods

Commercially produced polypropylene in the form ofpallets and wood material of radiate pine (Pinus radiate)were used to manufacture experimental panels. Woodparticles were manually screened on a sieve and classifiedinto two portions, 10 mesh particles and 50 mesh fiber.Four different types of chemicals Structor TR 016 which iscoupling agent, CIBA anti-microbial agent (IRGA-GUARD F3510) as fungicide, CIBA UV coating (TINU-VIN 123S) and CIBA blue pigment (Irgalite) were addedinto the samples. Tables 1 and 2 show the list of thechemicals and their percentages used for panel production.Wood particles and fibers were dried in an oven before theywere mixed with polypropylene. First plastic material wasput into mixer rotating at 75 rpm having a temperature of

Fig. 1. Rotating mixer.

165 1C for 2min followed by adding the chemicals for eachtype of material. In the next step particles or fibers wereadded into the mixture and rotated for another 3mincompleting a total mixing time to 5min. Fig. 1 illustratesthe mixer used for panel manufacture. Mixed samples thenwere pressed in a hot press with a 20 by 20 cm platencapacity. Each batch of sample was pressed using atemperature of 165 1C and a pressure of 40 bar for 5min.The press was cooled off while the samples were still undercompression before they were removed and conditioned ina climate chamber with a temperature of 20 1C and arelative humidity of 55%. Average target thickness of thepanel was 2.5mm. Modulus of elasticity (MOE) andmodulus of rupture (MOR) of the samples were deter-mined on a Comten Testing Unit equipped with a load cellwith a capacity of 2000 kg Figs. 2A,B and 3 show some offiber- and particle-based samples and bending test set-up,respectively.

Fig. 2. (A) Various types of fiber-based samples. (B) Various types of

particle-based samples.

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Fig. 3. Static bending test set-up.

A. Wechsler, S. Hiziroglu / Building and Environment 42 (2007) 2637–26442640

Four samples with a size of 5 cm� 5 cm were used todetermined thickness swelling (TS) of the panels. Thethickness of each sample was measured at four points.Then samples were submerged in distilled water for 2 and22 h before thickness measurements were taken from thesame location to calculate swelling values. Surface rough-ness of the samples was also determined using a stylus-typeprofilometer. A portable stylus equipment consisted of amain unit and pick-up which had a skid-type diamondstylus with 5 mm tip radius and 901 tip angle (Fig. 4). Thevertical displacement of the stylus is converted intoelectrical signal and digital information. Different rough-ness parameters such as average roughness and maximumroughness can be calculated from that digital informationand profile of the surface can be developed as shown inFig. 5. Description of these parameters is discussed inprevious studies [11,12]. Six random measurements weretaken from the surface of tested bending samples toevaluate surface characteristics of the panels. Micrographswere also taken from the cross section of the samples with3mm� 3mm face surface area to evaluate effect ofwood–plastic interaction on both particle and fiber-basedsamples. Figs. 9A–D show typical micrographs taken fromthe samples.

3. Results

Mechanical and physical properties of the panelsmanufactured from combination of plastic and woodmaterial and fiber with addition of various types ofchemicals are shown in Table 3. Average MOE value ofthe samples containing 60% and 80% wood fiber withouthaving any chemicals was found 2109MPa (Fig. 6). Whenthese samples were added coupling agent bending proper-ties of the samples increased to 3560MPa which is 38%higher than that of the specimen made without anychemicals. Panel types C, D, and E having chemicals listedin Table 1 also improved MOE values of the samples ascompared to those of made with combination of plasticand wood fiber. However, panel type D which was addedUV filter in the form of flakes and pallets showed only11.6% lower MOE than the samples with fungicides added.This could be due to non-homogeneous mixture of threeelements, namely wood fiber, plastic, and UV filter. Also itseems that using 1.2 % of UV filter would be consideredquite high and may be responsible for such finding. Usingless amount of the UV filter agent could be more feasibleapproach to eliminate negative effect of the filter on MOEvalues of the samples. Overall bending properties of thesamples are comparable to those found in two previousstudies [4,5]. In the case of panel type C bendingcharacteristics of the samples were not substantiallyreduced due to fungicide content. Fiber-based panelsmanufactured with coupling agent pigment had the lowestMOE values which may be due to chemical reactionbetween coupling agent and pigment reducing bondingbetween wood and plastic components. Modulus of

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Fig. 4. Roughness measurement profilometer.

Fig. 5. Typical surface profiles of the samples.

A. Wechsler, S. Hiziroglu / Building and Environment 42 (2007) 2637–2644 2641

elasticity of the samples made using wood particles alsohad similar trend to above panels. Fiber-based panels hadonly 3.3% higher MOE values than that of particle-basedsamples at 95% confidence level. However, fiber-basedpanels containing coupling agent had significantly higherMOE than those of specimens manufactured from particle-based using the same chemicals as can be seen in Table 3and Fig. 6. This finding may suggest homogeneous mix of

fiber and plastic along with coupling agent resulted in abetter bounding between the elements in contrast toparticle types of panels which can also be seen inFigs. 9A and B which were taken from the cross sectionof the samples using SEM. Once chemicals, fungicides, UVfilter, and pigments were added into particle-based panelstheir MOE characteristics also enhanced and becomesimilar to the values of the samples manufactured from

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Fig. 6. Average modulus of elasticity of the samples made from 60% and 80% fibers and particles.

Table 3

Mechanical and physical test results

Panel type Static bending (MPa) Water absorption (%) Thickness swelling (%) Surface roughness (mm)

Fiber-based samples

MOE MOR 2h 24 h 2 h 24 h Ra Rmax

A 2,109 11.80 3.21 8.33 11.3 18.6 1.98 16.71

B 3,560 18.74 5.73 12.84 5.2 13.3 4.05 33.84

C 3,208 14.53 5.11 6.53 5.4 8.9 4.48 33.94

D 2,778 14.28 4.79 10.96 5.8 11.8 4.09 32.11

E 2,155 12.74 5.22 9.49 3.9 11.0 3.54 28.35

Particle-based samples

A 2,058 12.01 4.76 8.82 5.5 12.5 5.84 41.68

B 3,034 13.90 6.67 14.71 6.7 12.2 6.61 66.43

C 3,254 14.60 4.07 7.50 3.8 5.9 6.70 49.07

D 2,656 14.10 7.19 15.36 7.9 13.4 8.11 50.18

E 2,191 12.89 7.46 14.61 7.5 12.1 6.96 57.78


fiber-based samples. Overall MOR values of both particleand fiber types of sample followed similar trend of MOEvalues. TS of the samples as a result of 2- and 24-hr watersoaking test are presented in Table 3 and Fig. 7. Thehighest TS value was 18.6% for the wood fiber base panelswithout addition of any chemicals for 24-hr water soaking.It appears that when coupling agent, UV filter, andfungicides had relatively positive influence on TS onfiber-based samples as can be seen on Fig. 7. Whencoupling agent was added into the samples their TS wasreduced more than half in the case of fiber-based panels.Dimensional stability of the fiber-based panel types B, C,D, and E did not show any significant difference at 95%confidence level. Overall TS characteristics of the particle-

based samples had lower values than those of fiber type ofthe samples. Separation between fibers as a result of watersoaking for 24 hr was observed on the micrographs takenon SEM as illustrated in Figs. 9C and D.Based on the roughness parameters obtained from the

surface of the samples fiber-based panels resulted insignificantly different Ra and Rmax values than those ofparticle-based ones. For example particle type samplesmade without having any chemical had 5.84 mm Ra whichis 3 times higher than fiber-based panels as illustrated inFig. 8. In a previous study it was determined thatcommercially produced fiberboard panels had muchsmoother surface than particleboard panels [11]. Even ifvery fine particles are used on the face layer of the

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Fig. 8. Average roughness (Ra) roughness values of the samples.

Fig. 7. Thickness swelling for 2hr water soaking test.

A. Wechsler, S. Hiziroglu / Building and Environment 42 (2007) 2637–2644 2643

particleboard pits and falls due to larger geometry ofparticle than fibers resulted in rougher surface character-istics [11]. This concept was also reflected in particle, fiber-and plastic-based experimental panels made in this study.It appears that as coupling agent, UV filter, and pigmentwere added into both types of panels their surfaceroughness increased due to not having well-developedcontact between wood-based material and plastic on thesurface layers. (Fig. 9, Table 3).

4. Conclusion

In this work, particles and fibers from radiata pine alongwith different chemicals as additives used to make

experimental WPC panels. In the light of the preliminaryresults of this study both physical and chemical propertiesof the samples were improved with addition of fourtypes of chemicals into the panels. It seems that using lessthan 1.2% anti-microbial agent as fungicide would yieldbetter properties of the samples. Initial data from the studywould assist to develop WPC panel manufacture in Chile.In further studies, manufacturing of the panels byextrusion method with more than two particle andfiber percentages would be desirable to have a betterunderstanding of panel properties. Also linear expansion,tension parallel to the surface of the samples should betested to have more comprehensive information about thesamples.

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Fig. 9. Photographs taken from the cross-section of the samples on scanning electron microscope. (A). Sample with 60% particle without any additives.

(B). Sample with 60% particle with pigment and coupling agent. (C). Dry sample with 60% fiber with no additive. (D). Water soaked sample with 60%

fiber with no additive.


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some of the properties of wood–plastic composites

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