Properties of paper mill sludge-wood fiber-HDPE composites after exposure to xenon-arc weathering

Xiaohui Yang?Xianquan Zhang?Weihong Wang?Haibing Huang?Shujuan Sui

Properties of paper mill sludge-wood fiber-HDPE composites after exposure to xenon-arc weathering

Xiaohui Yang1?Xianquan Zhang1?Weihong Wang1?Haibing Huang2?Shujuan Sui1

We used paper mill sludge(PMS)to substitute for part of the wood fibers(WF)used to reinforce high density polyethylene(HDPE).The resulting composites were subjected to xenon-arc weathering.The composite filled with limited PMS(under 10%)had mechanical properties and aging resistance similar to those without PMS.The composites containing more PMS faded and cracked more readily than those without PMS.Based on the carbonyl index,crystallinity,and wood index,PMS appeared to accelerate the degradation of composites during weathering.Adding PMS to WF–HDPE composites reduced the weathering resistance,and this reduction was not significant if the PMS content did not exceed 20%of the wood fibers.Therefore,PMS could be used as a reinforcement in wood-plastic composites at levels less than 20%of the wood fiber content.

Wood fiber·Paper mill sludge·HDPE· Composites·Xenon-arc weathering

Introduction

Papermillsludge(PMS)isa by-productofpulping and paper recycling.With the developmentofthe paperindustry,large quantities of PMS are produced each year.This waste sludge requires correctutilization to avoid pollution.

The main components of PMS from pulping are organic fibers and inorganic fillers(kaolinite,limestone and talc). PMS can be converted into various materials including highly reactive metakaolin,zeolites,composites,activated carbons,sorbents,and others(Hojamberdiev et al.2008; Asquini et al.2008;Kim et al.2009).Some researchers also used PMS to reinforce thermoplastics.Increasing the content of sludge in polypropylene-based composites resulted in higher Young’s modulus values(Girones et al. 2010),but lower tensile strength.PMS has also been utilized as a substitute for some of the wood fiber when reinforcing HDPE(Huang et al.2012).

Wood fiber-reinforced thermo plastic composites (WPCs)are often used to manufacture exterior decking materials,rails,and land-building materials.Use of WPCs in the construction industry has led to concerns about the durability of these products.Many researchers revealed the susceptibility of WPCs to moisture,fungal attack and photodegradation.As wood fiber-filled HDPE composites weather,they undergo changes in surface characteristics and mechanical properties(Stark and Matuana 2007;Du et al.2010).In addition,composites with more wood componentatthe surface experience a larger percentage of total loss in flexural modulus and strength after xenon-arc weathering(Stark et al.2004).

Though limited substitution ofwood fiberwith PMS does not obviously affect the properties of HDPE-based composites(Huang etal.2012),itis notclearwhatwillhappen to WF–PMS–HDPE composites when they are exposed toweathering.We studied the use of this material in exterior construction.We investigated the effect of PMS on the resistance of WF–HDPE composites to weathering by measuring surface characteristics and flexuralproperties of WF–PMS–HDPE composites before and after weathering.

Materials and methods

Materials

High-density polyethylene(Grade:5000 s;MFI: 0.8–1.1 g/10 min)was purchased from Petrochina Daqing Petrochemical Company.Wood fiber had 40–80 mesh, length of 1–2.8 mm,and aspect ratio of 9–12.PMS was from a waste-paper recycling company.It was mainly composed of cellulose fibers(about 39%)and inorganic materials including calcium carbonate and kaolin.Maleic anhydride-grafted polyethylene(MAPE;grafting percentage of 0.9%)was obtained from Shanghai Sunny New Technology Development Co.,Ltd.It was used as a coupling agent for improving the compatibility between the bio-fiber and HDPE.Wax was from Shanghai Hualing Health and Machinery Firm,China.Polyethylene wax was from Shangdong Qilu Petrochemical Co.,Ltd.,China.Both waxes were used as lubricants.

Preparing PMS-WF-HDPE composites

WF and PMS particles were separately oven dried to reduce their moisture content to less than 3%.Then PMS was ground and sieved through 20–100 mesh screens.The particles ranged in length from 1 to 2.2 mm and their aspect ratio was 1:3.

Using the formulations listed in Table 1,the components were mixed in a high-speed mixer(SHR-10A, Zhangjiagang Tonghe Plastic Machinery Co.,Ltd.,China) for 10 min.The mixture was then fed into a twin-screw extruder.At this stage,HDPE and wood fiber were compounded at150–175°C.The blends were broken into small particles using a pulverizer.Finally,the pellets were fedinto a single screw extruder and extruded into lumber with 40×4 mm cross-sections.

Table 1 Formulation of paper millsludge(PMS)–wood fiber(WF)–high-density polyethylene(HDPE)composites

Weathering treatment

Specimens were putin a xenon lamp accelerated aging test chamber(Suntest XXL+,ATLAS MTT GMBH Co.,Ltd.). Tests were performed according to ASTM G155-05a (Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials).The average irradiance was 0.35 W m-2at a wavelength of 340 nm, and the exposure cycle consisted of 102 min of light at a 63°C black panel temperature and 18 min of simultaneous water spray and light.The sample condition was assessed after 0,500,1200,and 2000 h of exposure.Before testing, specimens were cooled to room temperature.

Mechanical properties

Mechanical measurements were conducted both before and after weathering.The test method and procedure were conducted according to the Standard Guide for Evaluating Mechanical and Physical Properties of Wood-Plastic Composite Products(ASTM D 7031).The dimensions of the specimens were 80×13×4 mm with a span length of 64 mm and a loading speed of 2 mm min-1.Each test was performed in five replicates.

Surface morphology

Color measurement

We measured the colorparametersofexposed and unexposed specimens with a CM-2300d Photometer(Konica Minolta Sensing).Absolute chromaticity measurements were taken using the three-parameter CIE(International Commission on Illumination)scale:L*,a*,and b*system.The L*parameter is related to the amountofreflected light,while a*and b*are the chromaticity coordinates(chroma and hue,respectively). In addition,the totalcolordifference(ΔE)isthe square rootof the sum of the squares of the differences of chromaticity coordinates,calculated using the following equation:

where,ΔE isthe totalcolordifference,andΔa*,Δb*andΔL* are the differences between the respective values before and afterweathering(Stark and Matuana 2003;Wang etal.2010).

Stereomicroscope

Photographs of the composite surface before and after accelerated weathering were taken with a stereomicroscope(XTL-350Z,Shanghai Changfang Optical Instrument Co., Ltd.)at×4.5 magnification.

Surface chemistry analysis

Using Fourier-transform infrared(FTIR)spectroscopy on a MAGNA-IR560(NICOLE)spectrometer,we studied the functional groups atthe sample surface,and recorded them in absorbance units from 4000 to 400 cm-1.

The carbonyl index and wood index were calculated using the following equations:

where I denotes peak intensity.The peak at2913 cm-1was chosen as a reference because it changed the least during weathering(Stark et al.2004).

FTIR was also used to calculate the crystallinity of HDPE using the method described by Zerbi et al.(1989). The doublet peaks around 1474–1464 and 730–720 cm-1correspond to polyethylene crystalline content(1474 and 730 cm-1)and amorphous content(1464 and 720 cm-1). Percent crystallinity(X)was calculated according to the following equation:

where,Iaand Ibcan be derived from the bands at 1474 and 1464 cm-1and 730 and 720 cm-1,respectively(Zerbi et al.1989).In this research,we calculated crystallinity using the doubletpeaks at730 and 720 cm-1(Colom etal. 2000;Stark et al 2004).

Statistics

Significant differences between unexposed and exposed values were determined using analysis of variance.Significance was determined atα=0.05.

Results and analysis

Surface morphology of WF-PMS-HDPE composite

Exposure to xenon-arc radiation resulted in fading and cracking(Fig.1).When aged to 500 h,samples filled with PMS faded more obviously than those without PMS.After this amount of weathering,the main change of the sample surface was fading while no surface cracks were visible through a microscope.After further weathering to 1200 h, samples with 10%PMS had the same smooth surface morphology as those without PMS.Samples containing 20 and 30%PMS developed cracks on their surfaces.Surface color fading continued after 500 h of weathering,but the difference between the control and PMS-reinforced samples was most obvious at 500 h.After 2000 h of weathering,the surfaces of all samples were white and cracked. The sample containing more PMS developed wider and deeper surface cracks than did those containing less PMS. Composites of PMS:WF:PE=30:30:36 developed pale yellow particles extruding from the aged surface.

Though the composites containing PMS faded more severely than those without PMS,they exhibited the same crack resistance after 500 h.At 10%substitution of wood fibers by PMS the composite showed the same change in morphology as the composite without PMS during 2000 h of xenon weathering.

For extruded HDPE-based composites,a transparentand thin HDPE film was presenton the surface.After 1000 h of xenon radiation,the mechanical properties of HDPE declined,while more C=O groups and greater crystallinity were detected on the surface of the HDPE.The HDPE became fragile and cracked underthese conditions,as reported by Lee etal.(2012).Beyond this stage,the destroyed HDPE surface layer did not protect the inner wood component. Water spray and condensation facilitated removal of the degraded wood component and formation of cracks.

Ultraviolet(UV)light can penetrate through transparent HDPE films.This significantly impacted the surface of the solid wood in our experiment.Wood components contributed greatly to color changes in composites,as reported by Matuana and Kamdem(2001),Matuana et al.(2011) and Zhang et al.(2010).L*is a quantitative evaluation of fading with higher L*values indicating greater fading (higher reflectivity)(Falk et al.2000).L*andΔE values increased with weathering(Fig.2;Table 2).All specimens faded in response to radiation.Photochemicalreactions that generate chromophoric units,mainly in lignin,cause the composites to fade quickly.Lignin absorbs light up to 400 nm with a peak at280 nm(Pandey 2005).In our study, samples with PMS exhibited more severe surface fading than those without PMS before reaching 500 h of weathering.Color change(ΔE*)showed a similar trend.This was probably due to the inorganic materials in PMS.

Tiny wood fibers were enclosed with inorganic materials in PMS particles,thus,only part of the tiny wood fibers could come into contact with HDEP.HDPE did not penetrate into the hard PMS particles,resulting in weak bonds between PMS and HDPE.When WF–PMS–HDPE composite samples experienced wetting and drying cycles during xenon weathering,they cracked more often and earlier than did control composites(Fig.1).

Fig.1 Surface of WF–PMS–HDPE composites before and after accelerated weathering with a microscope

Fig.2 Changes in lightness and total color of composites after accelerated weathering

Mechanical properties of composites

Flexural tests were performed before and after accelerated weathering.Both bending strength(BS)and modulus of elasticity(MOE)declined significantly after weathering (Fig.3).Our results were similar to those reported by Lee et al.(2012)and Stark and Matuana(2006,2003),who investigated changes in flexuralstrength and flexural MOE retention or loss ratios of WPCs after weathering.

Photodegradation occurred mainly on the specimen surface.Some molecular chains of lignin and HDPE broke, and this was detected with FTIR.During xenon weathering,water was sprayed on the samples at intervals of 102 min. Wood fibers absorbed the moisture and expanded.When xenon lamps radiated again,the wood fibers shrunk in the hot environment.This expansion–shrinkage process weakened the bonding between the wood fiber and HDPE, resulting in impaired flexural strength.

Table 2 Color parameters of samples before and after weathering

Fig.3 Retentions of bending strength and elastic modulus of composites after accelerated weathering

With more PMS in the composite,the interfaces were weaker in response to severe bending.This measurement was consistent with Kim et al.(2009)who reported that wood particles can be replaced with up to 10%of dried paper sludge.Our result contrasted with the finding of Girones et al.(2010)who showed that increasing the content of sludge in polypropylene-based composites resulted in higher Young’s moduli.After 2000 h of weathering,the control sample remained at 85%BS relative to unweathered samples;however,samples with 30%PMS retained their original BS of 74%.Similar to the change in BS,MOE values for composites with higher levels of PMS decreased more significantly(retention of 71–77%)than did those without PMS(MOE retention of 82%).

The addition of PMS to HDPE resulted in greater and earlier damage to the composite surface.The damage due to the combination of PMS and HDPE provided more channels for water and light penetration.Composites containing more than 10%PMS lost more flexibility after weathering than those without PMS.

Surface chemistry analysis

We used FTIR spectroscopy to study the chemical changes on the surface of composites before and after weathering. Characteristic peaks of HDPE before weathering were at 2913,2845 and 1462 cm-1,and those of wood at 3354, 1593 and 1031 cm-1(Fig.4).After weathering for 500 h, peaks of HDPE declined to 2912,1593 and 1031 cm-1. That is,the C–H structure of HDPE,C=C structure of the aromatic ring in lignin,and C–O structure of cellulose, hemicellulose and lignin were damaged.This confirms that the photodegradation of polyolefins is mainly due to the introduction of chromophores,such as catalyst residues, hydroperoxide groups,carbonyl groups,and double bonds (Stark etal.2004).Carbonylgroups are postulated to be the main light-absorbing species responsible for photochemical-induced degradation reactions of UV-exposed polymers(Jabarin and Lofgren 1994).The carbonyl index of samples with 30%PMS increased significantly during the first 500-h exposure,while the control samples increased slightly(Fig.5).This demonstrates that PMS effectively accelerated WF–HDPE degradation during xenon-arc exposure.Yang et al.(2008)and Valadez-Gonza′lez and Veleva(2004)also proved that HDPE-based composites with kaolin or CaCO3,which are components of PMS,were readily oxidized.However,the carbonylindex of allsamples declined sharply after 1200-h exposure and the decrease in the carbonyl indices of the 30%PMS samples were more significant than for control samples. Weathered samples cracked after 1200 h,possibly due to a loose degradation layer on the surface(Fig.1).This loose degradation layer was washed away during the water spray cycle,resulting in lower carbonyl indices of samples after 1200-h exposure.

Fig.4 FTIR spectra of the composite surfaces(expose to 0,500 and 2000 h weathering)

Fig.5 Change in the carbonyl index and crystallinity of composites before and after weathering

Fig.6 Change in the wood index of composites before and after weathering

Crystallinity of control samples increased as the exposure time increased to 500 h,and then decreased thereafter (Fig.5).However,the crystallinity of samples with 30% PMS increased continuously with exposure time until 1200 h,and then decreased.The increase in crystallinity indicates that the HDPE matrix underwent a chain scission on exposure(Stark et al.2004;Du et al.2010).The progressive increase in crystallinity in PMS composites indicated that PMS accelerated photodegradation.This was proven by color and flexural property changes,as previously mentioned.However,chain scission in the HDPE matrix eventually affected linking molecules and resulted in a breakdown of crystallization and decrease in crystallinity and mechanical strength(Colom et al.2000; Torikai et al.1990).

During xenon radiation,both HDPE and the wood component underwent degradation.The changes in the wood index can be considered as indications of wood component photodegradation(Hon 2000).Before weathering,the wood index of control samples was higher than for samples with 30%PMS(Fig.6).Wood indices of all samples decreased significantly with exposure time.It is likely that cracks on the surface of samples allowed more xenon-arc radiation to enter the interior of samples,resulting in deeper photodegradation of wood components.

Conclusion

When 10%of wood fibers were replaced by PMS,the composite exhibited similar response to 2000-h of xenon weathering as did the composite without PMS.Though PMS may accelerate photodegradation,it can be used as a reinforcement material for HDPE within limits.

Asquini L,Furlani E,Bruckner S,Maschio S(2008)Production and characterization of sintered ceramics from paper millsludge and glass cullet.Chemosphere 71:83–89

Colom X,Canavate J,Page`s P,Saurina J,Carrasco F(2000)Changes in crystallinity of the HDPE matrix in composites with cellulosic fiber using DSC and FTIR.J Reinf Plast Compos 19(10): 818–830

Du H,Wang WH,Wang QW,Zhang ZM,Sui SJ,Zhang YH(2010) Effects of pigments on the UV degradation of wood-flour/HDPE composites.J Appl Polym Sci 118:1068–1076

Falk RH,Lundin T,Felton C(2000)The effects of weathering on wood-thermoplastic composites intended for outdoor applications.In:Proceedings of the 2nd annual conference on durability and disaster mitigation in wood-frame housing.Madison,WI, USA,pp 175–179

Girones J,PardiniG,Vilaseca F,Pelach M,Mutje P(2010)Recycling of paper mill sludge as filler/reinforcement in polypropylene composites.J Polym Environ 18(3):407–412

Hojamberdiev M,Kameshima Y,Nakajima A,Okada K,Kadirova Z (2008)Preparation and sorption properties of materials from paper sludge.J Hazard Mater 151:710–719

Hon DNS(2000)Weathering and photochemistry of wood.In:Hon DNS,Shiraishi N(eds)Wood and cellulosic chemistry.Marcel Dekker,New York,pp 512–546

Huang HB,Du HH,Wang WH,Shi JY(2012)Characteristics of paper mill sludge-wood fiber-high-density polyethylene composites.Polym Compos 33(9):1628–1634

Jabarin SA,Lofgren EA(1994)Photooxidative effects of properties and structure of high-density polyethylene.J Appl Polym Sci 53(4):411–423

Kim S,Kim HJ,Park JC(2009)Application of recycled paper sludge and biomass materials in manufacture of green composite pallet. Resour Conserv Recycl 53:674–679

Lee CH,Hung KC,Chen YL,Wu TL,Chien YC,Wu JH(2012) Effects of polymeric matrix on accelerated UV weathering properties of wood-plastic composites.Holzforschung 66(8): 981–987

Matuana LM,Kamdem DP(2001)Photoaging and stabilization of rigid PVC/wood-fiber composites.J Appl Polym Sci 80: 1943–1950

Matuana LM,Jin S,Stark NM(2011)Ultraviolet weathering of HDPE/wood-flour composites coextruded with a clear HDPE cap layer.Polym Degrad Stab 96:97–106

Pandey KK(2005)Study of the effect of photo-irradiation on the surface chemistry of wood.Polym Degrad Stab 90:9–20

Stark NM,Matuana LM(2003)Ultraviolet weathering of photostabilized wood-flour filled high-density polyethylene composites. J Appl Polym Sci 90:2609–2617

Stark NM,Matuana LM(2004)Surface chemistry changes of weathered HDPE/wood-flour composites studied by XPS and FTIR spectroscopy.Polym Degrad Stab 86:1–9

Stark NM,Matuana LM(2006)Influence of photostabilizers on wood flour-HDPE composites exposed to xenon-arc radiation with and without water spray.Polym Degrad Stab 91:3048–3056

Stark NM,Matuana LM(2007)Characterization of weathered wood plastic composite surfaces using FTIR spectroscopy,contact angle,and XPS.Polym Degrad Stab 92(10):1883–1890

Stark NM,Matuana LM,Clemons CM(2004)Effect of processing method on surface and weathering.J Appl Polym Sci 93: 1021–1030

Torikai A,Shirakawa H,Nagaya S,Fueki K(1990)Photodegradation of polyethylene:factors affecting photostability.J Appl Polym Sci 40(9–10):1637–1646

Valadez-Gonza′lez A,Veleva L(2004)Mineral filler influence on the photo-oxidation mechanism degradation of high density polyethylene.Part II:natural exposure test.Polym Degrad Stab 83:139–148

Wang WH,Bu FH,Zhang ZM,Sui SJ,Wang QW(2010) Performance of rice-hull-PE composite exposed to natural weathering.J For Res 21(2):219–224

Yang R,Liu Y,Yu J,Zhang DQ(2008)Spatial heterogeneity of photo-oxidation and its relation with crack propagation in polyethylene composites.Polym Eng Sci 48(11):2270–2276

Zerbi G,Gallino G,Del Fanti N,Baini L(1989)Structural depth profiling in polyethylene films by multiple internal reflection infrared spectroscopy.Polymers 30(12):2324–2327

Zhang ZM,Du H,Wang WH,Wang QW(2010)Property changes of wood-fiber/HDPE composites colored by iron oxide pigments after accelerated UV weathering.J For Res 21(1):59–62

22 September 2013/Accepted:4 March 2014/Published online:5 May 2015

?Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2015

Projectfunding:This work was financially supported by the‘Special Fund for Forestry Research in the Public Interest(201204802-1)’and the‘Nature Science Foundation of China(31070506)’.

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

Corresponding editor:Yu Lei

?Weihong Wang weihongwang2001@aliyun.com

1Key Lab of Bio-Based Material Science&Technology of Education Ministry,Northeast Forestry University, Harbin 150040,China

2Heilongjiang Wood Science Research Institute, Haping Road 134,Harbin 150081,China