Wastes from Wood Extraction Used in Composite Materials: Behavior after Accelerated Weathering

Wastes from Wood Extraction Used in CompositeMaterials: Behavior after Accelerated Weathering

Raluca Nicoleta Darie,1 Eduard Lack,2 Franz Lang, Jr.,2 Martin Sova,2

Alexandra Nistor,1 and Iuliana Spiridon1

1‘‘Petru Poni’’ Institute of Macromolecular Chemistry, Iasi, Romania2NATEX Prozesstechnologie GesmbH, Werkstrasse, Ternitz, Austria

New materials were obtained by incorporating in polypropylene (PP) matrix 60% wood wastes resulting

after extraction with supercritical carbon dioxide, water, and ethanol. Structural, mechanical, thermal,

and rheological characterizations, as well as moisture uptake of the composites, were evaluated before

and after accelerated weathering. It was found that the extraction method influenced the composite

properties due to the hydrophilic-hydrophobic balance. The addition of extracted fibers results in an

increase in hardness and tensile properties and a decrease of impact strength as compared to PP.

Keywords: Accelerated weathering; Composite; Extraction; Lignocellulosic fibers; Wood waste

INTRODUCTION

Lignocellulosic fibers are well known for their potential to replace traditional reinforcement

materials in composites or other building materials.[1] These fibers have some specific proper-

ties, such as stiffness, flexibility, and impact resistance, that make them a valuable and attractive

alternative to traditional materials. The main advantages are their availability, renewability, and

biodegradability, as well as low cost. On the other hand, their hydrophilicity limits their use in

different applications due to high moisture absorption and weak adhesion to hydrophobic plastic

matrices. Therefore, many efforts are directed at improving the compatibility between natural

fibers and polymeric matrices.[2–6] The use of lignocellulosic fibers can be accomplished by

implementing the ‘‘biorefinery’’ concept: they can be processed to obtain different, separate

structural components or their derivatives for use in specific applications.[7]

The manufacturing of high-performance engineering materials comprising lignocellulosic

fibers and a thermoplastic matrix is attractive for several engineering applications. Wood=polypropylene (PP) composites are increasingly requested due to their applications in many fields,

but especially in residential constructions, such as wood-filled window profiles, deck boards, or rail-

ings.[8] It must be mentioned that the weathering process of some composite materials hinders their

outdoor applicability. Thus, evaluation of their long-term behavior is very important.

Submitted 10 March 2014; accepted 23 April 2014.

Correspondence: Raluca Nicoleta Darie, ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda

Alley, 700487, Iasi, Romania. E-mail: [email protected]

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpac.

International Journal of Polymer Anal. Charact., 19: 453–467, 2014

Copyright # Taylor & Francis Group, LLC

ISSN: 1023-666X print/1563-5341 online

DOI: 10.1080/1023666X.2014.920134

The aim of our research was to valorize a high amount of wood wastes resulting after extrac-

tion by using it to replace 60% of the PP in order to develop new composite materials. Thus,

formulations comprising PP and a high content of wood wastes resulting after extraction with

supercritical carbon dioxide, water, and ethanol at different pressures and temperatures were

obtained by melt blending. The extraction method of wood fibers highly influenced the com-

posite properties. The behavior of composite materials to accelerated weathering was also

assessed. The results presented in this article are the first part of a complex evaluation of the

produced composites, and other tests are in progress.

EXPERIMENTAL PROCEDURE

Materials

The polypropylene (PP), Malen-P F 401 (Basell Orlen, Poland), with density of 0.95 g=cm3 and

melt flow index of 0.9 g=10min at 190�C was used to obtain composite materials with 60%extracted wood fibers. Maleic anhydride grafted polypropylene (MAPP), Licomont AR 504

(Clariant GmbH, Germany), was used as a coupling agent. The pine wood fibers (cellulose con-

tent 43.1%, lignin 29.01%) were processed using two procedures: supercritical fluid extraction at

elevated temperatures using carbon dioxide and ethanol as cosolvent and the so-called carbocell

process: cooking of wood chips with ethanol=water mixtures under high pressure-high tempera-

ture CO2 atmosphere. A 5L supercritical CO2 extraction pilot plant was used for the experi-

ments. The wood chips were air dried prior to processing by two different methods. In the

first method pine chips were extracted with supercritical carbon dioxide and cosolvent (water

and ethanol, 1:1) at 300 bar and 65�C for 120min. The extracted fibers (cellulose content

40.8%, lignin content 27.9%) were used to obtain composite A. In the second method, pine chips

were treated with a mixture of ethanol:water (1:1 ratio) and supercritical carbon dioxide at

150 bar and 200�C for 60min. The extracted fibers (cellulose content 41.5%, lignin content

23.7%) were added in composite B. Un-extracted and extracted pine fibers were ground in a

Retsch PM 200 planetary ball mill. Wood particle widths of �0.4mm were obtained through

grinding and sieving.

Processing of Composites

Prior to the compounding step, MAPP, PP pellets, and fibers were dried in a vacuum oven for

12 h at 80�C. Melt processing of the composites was performed at 175�C for 10min, at a rotor

speed of 60 rpm, using a laboratory Brabender station with counter-rotating rotors. Specimens

for mechanical characterization were prepared by compression molding using a Carver press

(at 175�C with a pre-pressing step of 3min at 50 atm and a pressing step of 2min at

150 atm). The compositions of the samples were: reference (REF), 60% pine wood fibers, 2%MAPP and PP; A, 60% fibers extracted with supercritical carbon dioxide and cosolvent (water

and ethanol) at 300 bar and 65�C for 120min, 2%MAPP and PP; B, 60% fibers extracted treated

with a mixture of ethanol, water, and supercritical carbon dioxide at 150 bar and 200�C for

60min, 2% MAPP and PP.

454 R. N. DARIE ET AL.

Accelerated Weathering

All composite samples were placed in a laboratory chamber (Angellantoni Ind., Italy) to accel-

erate their weathering. The samples were exposed to artificial light from a mercury lamp

(200< k< 700 nm, incident light intensity 39mWcm�2) at a temperature of 35�C and 65%humidity, while exposure time was up to 600 h. Non-irradiated samples were used as reference.

Composite Characterization

ATR-FT-IR

Attenuated total reflectance-Fourier transform-infrared (ATR-FT-IR) spectra of composite mate-

rials were recorded using a Bruker Vertex 70 FT-IR spectrometer equipped with an ATR device

(ZnSe crystal) with a 45� angle of incidence. A total of 64 scans were acquired with a spectral

resolution of 2 cm�1. Some indexes for composite samples were calculated as follows: carbonyl

index¼ (I1716=I2916)� 100; vinyl index¼ (I908=I2916)� 100, where I denotes the peak intensity.

The peak intensity was normalized to the peak at 2916 cm�1, which corresponds to alkane C–H

stretching vibrations of methylene (–CH2–) groups. This peak was chosen as a reference because

it changed the least during weathering. Another parameter, wood index, was determined to char-

acterize loss in wood from the surface of composite materials. It was calculated according to the

equation[9] wood index¼ (I1023=I2916)� 100.

Mechanical Testing

All mechanical tests took place at 50% relative humidity (RH) and 23�C. The specimens were

conditioned under the same circumstances (50� 5% RH) for 24 h before testing. The tensile

strength at break, elongation at break, and the Young’s modulus were determined according

to the SR EN ISO 527:1996 standard. An Instron 5 kN test machine (USA) operated at a cross-

head speed of 10mm=min was used for testing the specimens.

The Charpy impact strength of the composites was tested according to the SR EN ISO

179:2001 standard. A CEAST testing machine (Italy) with a pendulum of 50 J was used to mea-

sure the un-notched specimens. Ten specimens were used for each material for both tensile and

impact testing, and the average value was calculated.

Hardness Test

Vickers hardness tests were performed with a Shimadzu microhardness tester (Japan). A con-

stant load of 4.903N was applied for 12 s for all composites. Ten tests were carried out for each

sample, and the average values are given.

Scanning Electron Microscopy (SEM)

Morphological study of the un-weathered and weathered surfaces was carried out on the

surfaces of tensile test specimens using an SEM (FEI Quanta 200ESEM) device. Air-dried

WOOD WASTES USED IN COMPOSITE MATERIALS 455

samples were fixed onto aluminum stubs through carbon adhesive disks and their surface was

observed with a low-vacuum secondary electron detector using the accelerating voltage

of 25.0 kV. The samples were analyzed at room temperature and at an internal pressure

of 0.50 torr.

Dynamic Vapor Sorption (DVS)

DVS capacity of the samples was measured in the dynamic regime by an IGAsorp

apparatus (Hiden Analytical, Warrington, UK). This apparatus has a sensitive

microbalance (resolution 1 mg and capacity 200 mg), which continuously registers the

weight of the sample together with the temperature and relative humidity around the

sample. Isothermal studies were performed as a function of humidity (0–95%) in the

temperature range 5� to 85�C, with an accuracy of �1% for 0–90% RH and �2%for 90–95% RH.

Contact Angle Measurements

The samples were kept 48 h at 50% RH before testing in static conditions on an AdveX

Instrument. A 2.5 mL droplet of solvent was applied on the film surface. The evolution of the

droplet shape was recorded after 30 s by a video camera, and image analysis software was used

to determine the contact angle values. Water, formamide, and diiodomethane were employed as

liquids with different polarity. The contact angle value reported is the average of 10 measure-

ments. Surface free energy values were also determined. All liquids were at least 99% grade

from Sigma-Aldrich Chemie GmbH. Contact angle measurements were used to further calculate

surface free energy (SFE) using two models: the Lifshitz-van der Waals acid-base approach

using to the van Oss-Chaudhury-Good model (OCG)[10,11] and the polar-dispersive approach

using the Owens-Wendt model (OW).[12]

Dynamic Scanning Calorimetry (DSC) Analysis

Thermal characterization of the composites was performed with a TA Instruments Q20

Dynamic Scanning Calorimeter. All the samples were heated from 25� up to 200�C at

10�C=min, kept for 2 min and then cooled down to 25�C with a cooling rate of 5�C=min.

=min. All measurements were performed under N2 atmosphere. The degree of crystal-

linity of the PP samples was obtained by dividing the melting enthalpy of the sample

by 209 J=g, which is the estimated melting enthalpy of a pure PP.[13] The crystallinity

of the composite materials was estimated as function of PP fraction in the composite

and the melting enthalpy.

Dynamic Rheological Measurements

Oscillatory melt rheology tests have been realized on an Anton Paar rheometer (Austria)

equipped with CTD450, in plate-plate geometry, testing temperature being set at 175�C, inthe linear domain of viscoelasticity.


RESULTS AND DISCUSSION

FT-IR Spectra Evaluation

The composites were exposed to the combined action of temperature, humidity, and UV

radiation for 600 h. The surface modification after the weathering process was evidenced by

FT-IR spectrometry; FT-IR spectra of weathered and non-weathered materials were used to cal-

culate carbonyl and vinyl indices.

It is well known that all the chemical constituents of wood (cellulose, hemicelluloses, lignin,

and extractives) are sensitive to ultraviolet radiation. As can be seen in Figure 1, the carbonyl

contents were similar for all materials except PP. This parameter increases for all composites

after the weathering process, as well as the concentration of vinyl groups. These parameters were

higher for the composites containing extracted wood, probably due to the removal of extractible

substances from lignocellulosic fibers.[14] The wood index increased after the weathering process

due to the looseness of the protruded wood at the composite surface.

Mechanical Properties

The results obtained by mechanical testing of the studied composites are presented in Table I.

Composites reinforced with lignocellulosic fibers present better tensile strength than materials

FIGURE 1 Evolution of carbonyl (a), vinyl (b), and wood (c) indexes of composite materials.


comprising extracted lignocellulosic fibers. The incorporation of the rigid wood fiber particles

into the tough PP made the composite stiffer than pure PP; the tensile modulus as well as tensile

strength increased. This means that the incorporation of wood fibers into the matrix provides

effective reinforcement. Thus, for the REF sample Young’s modulus increased by 168.11%,

for A sample by 99.9%, and for B sample by 73.01% compared with neat PP. The tensile strength

presented the same trend; the REF sample comprising pine wood fibers registered the best value,

followed by the A sample and B sample. This could suggest that extractible substances from

wood assure a better interfacial contact between the PP and wood fibers than extracted fibers.

Thus, load can be transferred from the matrix to the wood filler. It seems that the pine fibers

in the PP matrix provide points of stress concentrations that can be sites for crack initiation. Also,

a stiffening of polymer chains due to bonding between wood flour and matrix could occur. The

impact strength of the composite comprising pine wood was higher than that with extracted wood

fibers. It is possible that extractives are involved in the crack initiation process.

The addition of wood fibers raised the crystallization temperature, especially for composite

REF. Also, the PP crystallinity in composite REF significantly decreased by the addition of pine

chips (Table II). When extracted fibers were used, the crystallinity slowly dropped, as compared

to REF sample. These results suggest that the mechanical properties of wood=PP composites are

closely related to the extraction method of wood chips. The effects of weathering on the mech-

anical properties are presented in Figure 2.

The Charpy impact strength of composites slowly increased after the aging process for the

composites comprising extracted wood fibers, while the strength at break slowly decreased,

except for A sample. The reduction of tensile strength could be due to the degradation of ligno-

cellulosic fibers and fiber-matrix interfacial bonding. It seems that the chemical composition

of fibers from composite A is less affected by the weathering process. The tensile strength

TABLE I

Mechanical properties of composite materials

Sample Young’s modulus (MPa) Tensile strength (MPa) Elongation at break (%) Impact strength (kJ=m2)

PP 1090.8 26.8 60.1 11.0

REF 2924.6 33.4 1.7 9.4

A 2180.6 30.9 2.2 6.7

B 1887.3 27.1 0.7 6.3

TABLE II

Hardness of composite materials before and after weathering

Sample

Vickers hardness, HV

No UV 600 h

PP 13.3 15.4

REF 19.8 23.5

A 27.7 29.5

B 24.9 38.2


evolution could be related to the crystallinity of PP in composite materials, as well as to their

thermal characteristics.

Vickers Hardness

The addition of pine wood to PP matrix increased the hardness. As the PP:wood fiber ratio

in composite materials was kept constant, the different hardness values could be the result of

various morphologies that occur as a function of wood structure and chemical composition.

It is possible that compatibility between fibers and PP is better in composite A than in composite

B or REF composites. Effective compatibility between components can enhance the hardness.

Probably the extraction process of wood fibers presented in composite A results in fibers con-

taining more lignin, which improves compatibility between thermoplastic matrix and fibers.[15]

The Vickers hardness results of the composite specimens are summarized in Table II.

All materials exhibited higher values of Vickers hardness after 600 h of exposure to combined

action of temperature, humidity, and UV radiation. The most important increase was seen for

composite B (53.4%) as compared to REF composite (18.6%), PP (15.7%), and composite A

(6.49%).

SEM Morphology

The morphology of the surface of the composite was investigated using SEM; the recorded

images are presented in Figure 3. From the microscopic study it is evident that wood fibers

FIGURE 2 Influence of weathering on mechanical properties.


are well dispersed into the PP matrix in non-weathered samples, since it is rather difficult

to differentiate wood particles from the PP matrix.

After exposure to UV light, temperature, and humidity for 600 h, it is observed in Figure 3

that the surface of the sample is obviously degraded and characterized by the appearance of

many cavities of different sizes and shapes as well as the formation of cracks in different

directions, except for PP. The effects are much more pronounced for composite B, which

is characterized by a higher degradation rate.

FIGURE 3 SEM images of the surface composites before and after exposure to aging treatment.


Water Sorption Capacity

It is well known that lignocellulosic fibers are sensitive to water absorption, resulting in dimen-

sional instability of the final products and poor performance of the composite.[16,17] Thus, their

long-term performance is affected, moisture decreasing the mechanical properties. As plotted in

Figure 4, the water sorption capacity of non-weathered samples increased as follows: PP, A,

REF, B. At 80% relative humidity the water adsorption capacity is 0.44% for neat PP, 0.52%for composite A, 0.56% for REF, and 0.57% for composite B.

The highest water sorption capacity after accelerated weathering was recorded for REF com-

posite; the values for water adsorption capacity were 1.13% for REF, 1.04% for composite B,

FIGURE 4 Water uptake of composite materials before (a) and after (b) weathering.


0.46% for composite A, and 0.47% for PP. The increased wettability of the composite surfaces,

which resulted in the development of microcracks on the composite surface, probably allowed

for increased moisture penetration, affecting the mechanical properties due to the swelling of

fibers, which developed stress at the interface, causing microcracks, which facilitated water

absorption and its attack on the interface. Wood fibers, as well as the interfacial bonding

between wood fibers and matrix, are responsible for water absorption in the composite materials.

Wood is hydrophilic and the enhancement of adhesion between wood and PP by using MAPP

reduces the gaps in interfacial region and blocks hydrophilic groups.

Contact Angle Results

Wood fibers are hydrophilic and contain functional groups such as hydroxyls that readily inter-

act with water molecules by hydrogen bonding, whereas PP is hydrophobic and nonreactive.

The results presented in Table III show that the contact angle as well as the total surface

energy of composites increased with the addition of wood fibers in the PP matrix. Jordan and

Wellons[18] reported that extractives significantly influenced wettability of wood because they

are nonpolar or less polar compounds, which unfavorably affects the wettability.[19] It seems that

the use of extracted wood fibers results in a significant increase of the polar component, as well

as total free energy, especially for composite B. After weathering, probably due to decreasing

adhesion between components, higher contact angle values for all materials were recorded. Also,

the polar component decreased for all composites.

DSC Results

The DSC parameters (crystallization temperature, melting temperature, crystallinity, enthalpy of

crystallization, enthalpy of melting) of the non-weathered and weathered wood-polypropylene

composites are presented in the Table IV. The non-weathered neat PP has a crystallization tem-

perature slightly lower than the crystallization temperature of the composite comprising

non-extracted wood (REF) or extracted fibers (A and B), confirming that the wood fibers have

TABLE III

Contact angle and surface energy before and after weathering

Sample

Water contact

angle (o)

Disperse component

(mJ=m2)

Polar component

(mJ=m2)

Total surface energy

(mJ=m2)

PP 77.9 28.6 0.5 29.1

REF 97.6 34.5 1.1 35.6

A 85.5 34.5 6.9 41.4

B 80.2 36.9 11.1 48.0

After 600 h

PP 119.2 19.5 6.4 25.9

REF 100.5 33.8 0.2 34.0

A 97.1 37.2 2.1 35.3

B 91.6 35.8 7.1 42.9


a slight nucleating effect on the polypropylene. It was found that the crystallization temperature

was not affected by the weathering process. It seems that the occurrence of transcrystallization in

wood-PP composites is strongly dependent on the type of extraction method of lignocellulosic

material.

As can be seen in Table IV, crystallites of aged PP melt to lower temperatures than non-aged

PP. In composite material REF exposed to UV for 600 h (REF 600 h), the slight decrease in

melting temperature (Tm) is probably due to a break in the molecular chain and a reduction in

molecular size by UV radiation.

The second heating scans were used to determine the crystallinity index. PP crystallinity

increased during weathering (due to the chain scission in the amorphous phase, confirmed by

vinyl and carbonyl index), while the crystallinity of PP in composite REF as well as in composite

A slowly decreased. This decrease is presumably due to the degradation of interface bonding by

weathering factors. The opposite trend of PP crystallinity in composite B was registered. This is

probably due to the removal of some amorphous components of wood such as hemicelluloses.

Dynamic Rheology

The rheological behavior of PP=wood fiber composites (REF, A and B materials) was investi-

gated in melt state, for two types of extracted fibers through oscillatory rheological tests in the

linear domain of viscoelasticity. The viscoelastic parameters were followed as a function of the

oscillation frequency, x, at a constant temperature of 175�C. From crossover frequency (xi),

the relaxation time (h) can be calculated as h¼ 1=xi.

All studied composites behaved as viscoelastic materials; G0 and G00 are crossing functions of

the angular frequency (x) (Figure 5). At low x values, up to the crossover point (xi), most of the

materials exhibit predominant viscous behavior, G00 >G0, while over xi, this changes to an

elastic one (G0 >G00). The relaxation time is a measure of the rate at which the global structure

changes in response to the change in flow. Thus, with changing flow, the degree of anisotropy

changes with the speed and time duration of the flow. When returning to the quiescent state (no

flow), the liquid relaxes to the original global isotropic condition. The force of reorientation to

the isotropic condition of rigid microstructural elements is due to Brownian motion, while shape

recovery of flexible microstructural elements is aided by internal springs. We calculated relax-

ation times for the studied systems before and after accelerated weathering and the results are

TABLE IV

DSC results for composite materials before and after accelerated weathering

Sample Tc (�C) DHc (J=g) Tm (�C) DHm (J=g) Xm (%)

PP 118.6 92.2 165.3 92.5 44.2

PP 600 h 117.5 93.9 148.3; 159.4 97.6 46.7

REF 124.9 61.1 166.8 60.6 29.0

REF 600 h 124.6 54.1 165.4 54.5 26.1

A 123.9 51.5 163.6 55.9 26.7

A 600 h 123.3 51.5 164.0 53.2 25.5

B 123.4 50.2 162.8 49.4 23.6

B 600 h 123.1 50.1 163.0 52.9 25.3


presented in Figure 6. A general observation is that all samples containing compatibilizer require

increased relaxation time (hi), a decreased crossover point for melt state relaxation in normal

conditions, while after accelerated weathering, the relaxation time decreases along with exposure

time due to the crosslinking of the material at the surface. The difference between composites

containing different extracted fibers and also exposure time effect on the material is obvious;

increased values of hi were registered compared with neat PP, and the larger the local structures,

the longer the relaxation time.

Regarding the differences in complex viscosity for studied materials (Figure 7), one can

observe an increase of the complex viscosity in melt state for the system containing

un-extracted fibers (REF), the H bond formation or reactions between polar groups of the

components leading to a strengthening effect in this case, so to a more rigid material.

A decrease of melt viscosity is recorded for composites A and B compared to the reference

composite, the structural modification of the fibers leading to a less viscous material, their

incorporation reducing the resistance of the matrix to the external forces, determining an

improvement of flow for A and B systems.

The difference between complex viscosity values for composites with extracted fibers is not

high, except the low angular frequencies, composite A registering an increased viscosity

at 0.05 s�1.

FIGURE 5 Crossover frequencies for studied composites.


CONCLUSIONS

New materials were obtained by valorization of wood wastes resulting after extraction with

supercritical carbon dioxide, water, and ethanol at different pressures and temperatures. The

composites comprising PP and 60% wood wastes were designed, and their behavior after accel-

erated weathering was assessed.

The tensile mechanical properties (Young’s modulus and tensile strength) as well as Vickers

hardness values are improved for the composites containing extracted wood fibers related to neat

PP, although 60% of the nonrenewable petroleum-based matrix was replaced.

FIGURE 7 Variations of complex viscosity function of angular frequency for neat PP and studied composites.

FIGURE 6 Relaxation time for studied materials.


The accelerated weathering influenced moisture uptake, thermal, mechanical, and surface

properties of materials, and function of the used fibers.

The obtained data showed that composite A, comprising fibers extracted with supercritical

carbon dioxide and cosolvent (water and ethanol) at 300 bar and 65�C for 120min, was less

affected by the accelerated weathering process, both mechanical and surface properties being

maintained. Our results lead to the conclusion that extracted fibers could be a choice for applica-

tions such as wood-filled window profiles in construction, roof siding, and outdoor decking.

FUNDING

The research leading to these results has received funding from the European Community’s

Seventh Framework Programme FP7=2007-2013 under grant agreement no CP-IP 228589-2

AFORE.

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Wastes from Wood Extraction Used in Composite Materials: Behavior after Accelerated Weathering