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Impact of biofibers and coupling agents on the weathering characteristics of composites
Polymer Degradation and Stability 120 (2015) 212e219
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Impact of biofibers and coupling agents on the weathering characteristics of composites Dilpreet S. Bajwa a, *, Sreekala G. Bajwa b, Greg A. Holt c a
Department of Mechanical Engineering North Dakota State University, Fargo, ND, USA Department of Agricultural and Biosystems Engineering, North Dakota State University, Fargo, ND, USA c USDA-ARS, Cotton Production & Processing Research Unit, Lubbock, TX, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2015 Received in revised form 29 May 2015 Accepted 20 June 2015 Available online 23 June 2015
This paper explores the ultraviolet (UV) weathering performance of high density polyethylene (HDPE) composites with different biofiber fillers and coupling agent. Biofiber polymer composite (BFPC) material samples were prepared using oak, cotton burr and stem (CBS) or guayule bagasse as fiber source HDPE as resin, and two different coupling agents. Weathering variables included exposure to UV radiation and moisture cycles for up to 2200 h. Each variable can degrade BFPC matrix independently or synergistically. The impact of weathering was measured through the changes in the surface matrix, mechanical properties and thermal stability of BFPC as a function of biofibers types and coupling agents. The coupling agent treated composites showed color shift (DE) and variable surface degradation. Water absorption of the weathered samples continued to increase after 10 days of immersion. Flexural stiffness, strength and impact properties of weathered samples decreased regardless of fiber type or coupling agent. Minimal linear thermal expansion was noted in weathered composites with coupling agents. Overall coupling agent helped to retain the mechanical properties of composites after exposure to UV weathering. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Natural fiber composites Coupling agents Natural weathering Polymer degradation Mechanical properties
1. Introduction Biofiber polymer composites (BFPC) are materials made from a thermoplastic or thermoset resin (substrate) with cellulosic fibers as fillers or reinforcement. They are currently used in a variety of products ranging from building materials, automotive parts, packaging, consumer goods and landscaping products. They combine the superior mechanical properties of cellulosic fibers and the longterm performance of plastic. The use of BFPC has accelerated in the last few decades due to measurable advantages in terms of cost, high specific strength, recyclability, and ease of maintenance. Several research reports forecasted BFPC market to grow between 11 and 15% compound annual growth rate (CAGR) over the next five years [1,2]. Despite all these advantages their widespread acceptance and market penetration has been limited due to nonstructural applications, persistent concerns over their long term durability, and susceptibility to moisture in the outdoor or exterior applications [3].
* Corresponding author. E-mail address: [email protected] (D.S. Bajwa). http://dx.doi.org/10.1016/j.polymdegradstab.2015.06.015 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
Weathering or photodegradation of BFPC is of major concern because of the documented effect of photodegradation has on material properties and appearance. The adverse effects of weathering include degradation of aesthetic appeal, and physicomechanical properties [4,5]. Weathering due to UV light and moisture results in the degradation of the polymer-fiber matrix. Photodegradation of polyolefin resins results from excited electronic states of oxygen-containing species (carbonyls) are formed due to introduction of catalyst residues, hydroperoxide groups, carbonyl groups and double bonds [4]. Further, moisture in biofibers is driven by the chemical potential gradient between hydroxyls present in lignocellulose and the polarity of water molecules. Absorbed moisture can cause swelling of biofibers which can lead to surface instability, matrix cracking, and inservice deterioration due to loss of structural integrity and strength. The mechanism of moisture induced deterioration of biofibers composites has been widely reported in the literature [6,7]. Chemical modification of cellulosic fiber cell walls by grafting through use of coupling agent to improve moisture resistance and strength is widely reported in the literature [8e12]. In the last two decades coupling agents such as isocyante, silane, acrylic acid and
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2.2. Specimen manufacturing
maleic anhydride have been used to strengthen biofiber based composites [2]. One of the most widely used and effective functional groups for grafting a coupling agent (CA) to a cellulosic fiber is the anhydride functionality. Chemical structure, molecular weight and functional groups per mole of a coupling agent have strong impact on the mechanical properties of composites [12,13]. Typically a coupling agent creates a crosslinking structure and improves the interfacial adhesion when one portion of the molecule tethers to the polymer matrix and specific functional groups react with hydroxyl groups present on the fiber surface [14,15]. It is well known that coupling agents improve fiber/matrix interaction and the properties of composites however, it is not known how outdoor weathering conditions can impact the performance of composites containing different biofibers and coupling agents [12,13,16]. Some previous accelerated weathering studies on BFPC have reported degradation of polymer matrix, chromacity shift, followed by loss in the mechanical properties [17e19]. Therefore, a study was conducted to explore the impact of coupling agents on the physico-mechanical properties of biofiber composites under accelerated weathering. The objective of this study was to evaluate the impact of coupling agents on the weathering characteristics of high density polyethylene (HDPE) composites containing natural fibers such as cotton burr with stem (CBS) and guayule bagasse via (1) characterizing the effect of weathering on the physico-mechanical properties of the biocomposites made using different biofibers and coupling agents and (2) understanding the weathering related changes in the fiber-polymer matrix though surface microscopy and color shift.
The test specimens were manufactured at a commercial wood composites processing facility located in Greenland, Arkansas, USA. The proprietary technology involved first compounding raw materials and then transferring into a single screw extruder. The weight percentage of fiber was kept at 49.5% or 50% depending on the presence of coupling agent while polyolefin HDPE was maintained at 39.5% or 40%. The remaining ingredient included the lubricant at 6% and the mineral filler talc at 4% by weight of the total composition. The concentration of coupling agent was maintained at 1% on weight basis for all treatments that used it, as recommended by the manufacturers. There were a total of 7 treatments, with 6 replications for each treatment as shown in Table 1. The ingredients for each set of specimens were initially weighed in the required proportion, and then compounded in a densifier and directly fed to the extruder. The extruded profile was sized using a calibration equipment connected to vacuum and chilled water. Material processing temperature varied from 149 C to177 C for zones 1 through 6 with the die zone 1 at 171 C and die zone 2 at 154 C respectively. The extruder screw speed was 25 rpm and melt temperature of the extrudate fluctuated between 158 C to 160 C. The specimens were extruded using a profile die of 25.4 mm in height and 152.4 mm wide with a sample length 150 cm. For each formulation, two samples each measuring 250 cm in length were collected randomly for weathering studies. The extruded samples were water cooled and then conditioned at room temperature for 4 weeks before testing. For mechanical properties comparison treatment T0 samples were not subjected to weathering.
2. Materials and experimental methods
2.3. Accelerated UV weathering exposure
2.1. Materials
Accelerated weathering was conducted in a QUV weatherometer, (Q-LAB Corporation, Westlake, Ohio, USA) that stimulates the damage similar to sunlight, rain and dew. The rectangular test samples were cut from the 125 mm wide and 25 mm thick samples and were mounted on aluminum fixtures and subjected to accelerated UV weathering via 340-nm fluorescent UV lamps (UV-B lamps). The average irradiance was about 0.85 W/m2 with a chamber temperature of 45 C. To avoid any variation due to different morphologies or processing conditions, all samples were cut along the direction of material flow with top surfaces exposed to UV. The UV weathering procedure followed the ASTM G 154 test method recommended for nonmetallic materials [20]. Exposure mode 1 cycles were followed that included a UV exposure period followed by periods with condensation and no radiation. Exposure to dry UV for 4 h at 60 C, followed by condensation exposure up to 4 h at a temperature of 50 C without radiation. The specimens were exposed to 2200 h of UV radiation and condensation cycles in the QUV weatherometer. After accelerated weathering the specimens were carefully removed from the aluminum fixtures for color measurement studies and then conditioned in the laboratory environment for approximately 12 weeks and tested for physical and mechanical properties.
The HDPE used for the manufacturing composite samples was Petrothene LB010000 high density polyethylene copolymer reactor powder, an additive free formulation obtained from Equistar Chemical Company (Houston, Texas, USA). The HDPE powder had a density of 953 kg/m3 and melt flow index of 0.50 g/ 10 min. The flexural modulus and tensile strength were 1275 MPa and 27.3 MPa respectively. Oak wood fiber (particle size distribution ranging from 250 to 841 micron, with 90% of fibers falling between 250 and 400 microns) was supplied by Southern Wood Services, (Macon, Georgia, USA). Cotton burr with stem (CBS, particle size range 250e841 micron) was supplied by USDA-ARS, Cotton Production and Processing Research Unit, Lubbock Texas, USA. Guayule bagasse (particle size range 250e400 micron) was supplied by USDA-ARS, Maricopa, Arizona, USA. The two commercially available coupling agents used in this study were: Integrate NE 556-004 (HDPE-based, MI ¼ 3.8 g/10 min.) manufactured by Equistar Chemical Company (Houston, Texas, USA) and TPW 243 manufactured by Struktol Company (Stow Ohio, USA). The NE 556-004 coupling agent had an HDPE backbone with high level of maleic anhydride (2% MA) grafting. Integrate coupling agent has a unique grafting technique that makes it efficient compatibilizer for BFPCs. The uniqueness is attributed to high grafting efficiency and low residual maleic. The TPW 243 is also marketed as a compatibilizer with the ability to bond organic and inorganic materials by a free radical source through an ionic mechanism initiated by hydrolysis. The exact formulation of the coupling agent TPW 243 could not be revealed due to manufacturer's proprietary information. Lubricant TPW 113 (non-metal lubricant) supplied by Struktol Company was used as a processing aid. It is a blend of complex, modified fatty acid ester widely recommended for wood filled polyolefins.
2.4. Physico-mechanical property measurement 2.4.1. Fractography The surface morphology of the UV weathered biofiber composite specimens was analyzed to investigate the extent of macroscale cracking, flaking and surface imperfections in the fiberpolymer matrices of different formulations. The samples were attached to aluminum mounts with colloidal silver paste. A conductive goldepalladium coating was applied with a Balzers SCD 030 sputter coater (BAL-TEC RMC, Tucson AZ USA). Images were
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Table 1 Composition of constituents of biofiber polymer composites (%). Treatment
Oak
CBS
Guayule
HDPE
Lubricant
Mineral filler
Coupling agent
T1 (Control) T2 T3 T4 T5 T6
50 25 24.25 24.25 25 17.5
e 25 24.25 24.25 17.5 17.5
e e e e 7.5 15
40 40 39.5 39.5 40 40
6 6 6 6 6 6
4 4 4 4 4 4
e e 1 (C1) 1 (C2) e e
C1 e Struktol TPW 243. C2 e Integrate NE 556-004.
obtained using a JEOL JSM-6490LV scanning electron microscope (JEOL USA, Peabody MA USA) at an accelerating voltage of 15 keV and 200 magnification. 2.4.2. Infrared spectroscopy Samples for Fourier transform infrared spectroscopy (FT-IR) were analyzed using Nicolet 8700 spectrometer. The surface of the samples was scraped using a sharp knife to a depth of 1.0 mm on the UV exposed area. The infrared spectra were obtained following step-scan mode by using helium for purging. Single measurements were made and spectra shown are the average of 15 scan in the range of 450e4000 cm1. 2.4.3. Optical properties The optical properties were measured using the CIE LAB color clairage, Paris) and system (Commission internationale de l'e following the procedure outlined in ASTM D2244 [21]. In this color system the L* axis represents lightness; a* and b* are the chromacity coordinates. In CIE LAB system þa represents red, -a represents green, þb* represents yellow and eb* represents blue. L values vary from 0 for black to 100 for white. The color change DE was determined using equation (1).
1=2 DE ¼ DL*2 þ Da*2 þ Db*2
(1)
where DL*, Da*, Db* are the difference between initial (before weathering) and final values (after weathering) of L*, a*, b*. The optical properties were measured using an X-Rite spectrophotometer model SP62, manufactured by X-Rite Inc. (Grand Rapids, Michigan, USA). 2.4.4. Specimen moisture sorption To investigate the effect of moisture on the material properties of UV weathered BFPC the specimens were subjected to extended water absorption test in accordance with ASTM D1037-06a test method [22]. The specimens were first conditioned at 23 C and 50% relative humidity for one week. Specimen weight and dimensions were recorded before completely immersing them in a water bath filled with tap water and held at a constant temperature of 25 C. The weight gain of the samples was monitored daily for 16 days to calculate water sorption percentage. 2.4.5. Flexural mechanical testing The flexural properties of UV exposed specimens were characterized at room temperature in accordance with ASTM D1037-06a test method [22]. The specimens had an average length of 305 mm, width of 24.5 mm, and thickness of 26.4 mm. Three point bending tests were performed using an Instron Model 5567, Instron Corporation (Norwood, Massachusetts, USA). The support span for test samples was 279 mm with crosshead rate of 12.2 mm/min. The specimens were placed on the testing fixture such that the UV exposed side was on the top in contact with loading head. The
samples were tested in flexural mode because in majority of applications the material experiences flexural stress. The modulus of rupture (MOR), and modulus of elasticity (MOE) values calculated. 2.4.6. Compressive strength Compressive properties of the specimens were determined in accordance with ASTM D1037-06a test method [22]. The test specimens were subjected to compression loading on the UV treated side and perpendicular to the surface, at a relatively low uniform rate of strain with an Instron universal tester. Since the maximum crushing strength was of interest and the samples thickness was more than 25 mm a short column method C was selected. The thickness of the specimens was 26.5 mm with lengths and widths of 25.4 mm. The specimens were loaded through a spherical block at a constant speed of 1.5 mm/min. Both compressive strength and MOE were measured from the stressstrain data generated. 2.4.7. Izod impact testing Impact testing was performed on the un-notched specimens. The specimen were 64 mm long, 12.7 mm wide and 6.4 mm thick. Izod impact tests were performed at room temperature following the ASTM D 256-05 procedure [23]. The testing was performed using the pendulum based impact tester Tinius Olsen Model Impact 104, (Horsham, Pennsylvania, USA). The specimens were supported at both ends and struck by the pendulum on the flat side in the middle and impact energy expressed in J/m2 was calculated. Similar to flexural testing the UV exposed surface was exposed to the pendulum impact. 2.4.8. Thermal expansion The coefficient of linear thermal expansion (CLTE) of the specimens was measured in accordance with ASTM D6341-98 method [24]. The determination was made by taking measurements with a caliper at three discrete temperatures (6 C, 23 C and 60 C). Test specimens were prepared with due diligence to ensure smooth, flat, parallel surfaces and sharp, clean edges. Three lines parallel to the length was marked on one side of specimen with indelible ink so that length measurements could be taken consistently at the same locations. The specimen length was 300 mm, width 25.4 mm and thickness 2.6.5 mm. Test specimens were conditioned prior to testing at 23 C and 50% relative humidity for one week. Two specimens from each formulation were tested at three measurement temperatures. Length of conditioned specimens was measured to the nearest 0.01 mm with digital caliper before transferring them to a freezer set at 6 C. After 48 h of exposure in the freezer, the specimens were taken out for conditioning for 48 h at 23 C and then moved into convection oven set at 60 C for 48 h. The length of the specimens was accurately measured after each temperature exposure. The CLTE was measured as the slope of the curve of specimen length versus temperature. 2.5. Data analysis A total of 12 sets of samples were subjected to UV weathering. These samples were cut into test-size specimens such that there were 6 replications for each of the physico-mechanical tests. To identify the impact of biofibers and coupling agents on the weathering characteristics, means of the physico-mechanical properties of unweathered and weathered samples were compared using Fishers Least Square Difference (LSD) tests with MINITAB statistical software [25]. The optical shift of the specimens was calculated based on the mean of four color measurement readings.
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3. Experimental results and discussion 3.1. Fractographic analysis The visual appearance of the samples after the QUV weathering changed noticeably. As a result of UV weathering the surface became dull and porous. The surface color and texture looked from smooth reddish brown prior to weathering to chalky beige after weathering. The polymer matrix appeared to have weakened and surface appeared rich in exposed biofibers. Microcraking, or crazing was likely result of polymer surface erosion and contributed to the observed lightness and dull surface. Polyolefins undergo photolytic and photo oxidative reaction during exposure to solar UV radiation. The photodegradation originates from excited polymereoxygen complexes which is primarily caused due to presence of catalyst residues, hyperperoxides groups, carbonyl groups, and double bonds induced due to thermal degradation during processing [26]. The polymer containing chromophoric groups such as carbonecarbon double bond (C]C) and carbonyl (C]O) absorb UV energy and initiate photo reactions which degrade polymer resulting in surface densification and cracking [26,27]. In this study all the test specimens showed variable degree of cracking in the UV exposed area (Fig. 1) due to possible photodegradation of polymers and expansion of fibers resulting from moisture absorption. The low dose of coupling agent (1%) proved to be ineffective in controlling the surface degradation. In a similar study, the color of polyolefin based wood fiber composites changed from brown to chalky white at the exposed area because of the formation of thin, strongly degraded surface layer [28]. 3.2. FT-IR spectroscopy The FT-IR spectra from the exposed surface of control and six treatments are illustrated in Fig. 2. It can be seen that new absorption bands have emerged in the region of 1275 cm1, 1473 cm1, 1554 cm1, 1657 cm1 and 1726 cm1 after exposure to UV irradiation. The appearance of these new absorption bands is clearly seen in the difference in spectra between control and weathered specimens where groups frequencies associated with the formation of polyene and carbonyl linkages are easily recognized at 1275, 1470 cm1 and 1554 cm1 (CH3, C]O, and C]C stretching) [28]. The peaks seen in UV exposed samples are usually associated with the presence of carbonyl groups from photooxidation of wood fibers [29]. Furthermore the presence of polyene units on the surface samples is indicative of photodegradation under UV light. Two absorbance peak at 1238 cm1 and 1595 cm1 are only seen in control samples which are associated with CeC (ethers) and aromatic C]C groups. The functional groups from wood fibers act as chromophores or initiators (carbonyls, carboxylic acids, quinines and hydroxyperoxy radicals) and accelerate the UV degradation. The results of this study are in agreement with previous FTIR study where spectra of the HDPE-WF composites showed a broad peak at 3318 cm1 and strong peak at 1023 cm1 suggesting wood rich surface and low polymer after weathering. 3.3. Optical properties The measurement of color changes provided additional evidence of photodegradation of the composites. Fig. 2 indicates that surface color change of the six UV weathered treatments. This discoloration deeply affected the appearance of the degraded composites. The oak and guayule fiber based composites exhibited lower color shift than CBS specimens. The highest color shift DE of 14.1 was observed for specimen with 24.5% oak, and 24.5% CBS and 1% Integrate coupling agent (T4), indicating that this
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particular coupling agent didn't prevent the discoloration of specimens. However, addition of 1% Struktol TPW 243 did reduce the color shift significantly in treatment T3, this may be due to antioxidants or hydroperoxide decomposers present in the CA [27]. Since all the samples exhibited extensive discoloration, suggesting the incorporation of woody or non-woody fibers into HDPE matrix has a deleterious effect on the ability of the matrix to resist the exposure to UV weathering. The shift in DE was mainly due to bleaching of the fiber component, degradation of polymer due to UV radiation and the darkening effect may be linked to surface oxidation as previously reported [30,31]. Exposure to UV radiation, moisture, and oxidation of polymer degraded the composite surface layer and removed natural extractives, lignin and colored materials, thus resulting in color change of the specimens. Color change in composite samples was probably exacerbated by fiber swelling and cracking which augmented the exposed surface areas of polymer and fiber and created more sites for UV exposure. 3.4. Influence of UV weathering on the moisture sorption characteristics The water sorption characteristics of composite specimens over 16 days at 24 h intervals is shown in Fig. 3. Water sorption by biofiber composites is limited to sorption only by wood fibers as water uptake by HDPE is less than 0.05%. The composite specimens showed wide variation in water absorption and absorption among the different formulations. The maximum water sorption for unweathered sample was 4.8%. In UV weathered samples after three days of immersion the average water sorption ranged between 3 and 5%, however after 8 days, specimens from formulations with CBS and Oak fibers (T2, T3, and T4) showed water sorption in excess of 10%. These observations are in agreement with some of the previous studies that found microcracks at the interface due to water absorption [3,32]. Biofibers are hydrophilic in nature and upon exposure to high humidity and wet environments, individual fibers can absorb moisture and swell creating internal pressure and cracking the polymer matrix. The water sorbed by the biofibers is located on the hydroxyl group by hydrogen bond. If the biofibers are not completely encapsulated by polymer, water can easily permeate and compromise material properties. For BFPCs low water absorption is preferred to maintain temporal dimensional stability and prevent biodegradation. A steep rise in water sorption values of weathered samples was observed after 10 days of immersion probably due to degradation of hydrophobic lignin thus making surface cellulose rich (Fig. 4). The increase in hydroxyl groups and carboxyl groups that are hydrophilic in nature may have further increased the water sorption [31]. Overall, the water sorption of the samples containing 50% oak fiber was lowest at 12% after 15 days, while samples containing 25% Oak and 25% CBS showed the highest water sorption at 19.6%. The high water affinity of CBS is possibly the reason for high water sorption values noticed in CBS based formulation. Both the guayule based formulations exhibited lower water sorption values. A previous study also showed that guayule bagasse fibers are less susceptible to the moisture [33]. It is know that guayule baggase contains resinous material such as sesquiterpene esters, triterpene keto alcohols, and triglycerides, which can reduce water absorption. Interestingly, the formulations containing coupling agents also.showed water sorption ranging 17.9%e18.2% only slightly less than the formulation that did not contain coupling agents. It is not clear why coupling agent based biocomposites exhibited high water absorption. It is possible that weathering increased availability of hydroxyl groups from cellulose and the moisture affinity of CBS fiber is stronger than the impact of coupling agents. Also, most studies showing strong impact of coupling agents in reducing water
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Fig. 1. Microscope pictures of the UV weathered biofiber composites samples at 200 magnification. Composite formulation 50% biofiber, 40% HDPE, 6% lubricant and 4% talc. Control unweathered, T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule. Control e Unweathered 50% oak, 40% HDPE, 10% additives.
sorption used approximately 3% by weight of coupling agents, as compared to the 1% used in this study [34]. Overall the water sorption data shows that UV weathering has a deleterious effect on the polymer-fiber matrix. 3.5. Flexural mechanical properties of biofiber composites The effect of UV weathering on the flexural response of different biofiber composite formulations is shown in Figs. 5 and 6. All the formulations showed decrease in the strength properties of composites after UV weathering. Addition of CBS fibers influenced the stiffness or modulus of elasticity (MOE) and modulus of rupture (MOR) values more than guayule bagasse. Partial substitution of CBS with guayule bagasse fiber helped to improve the MOE. The MOE values of 25% CBS blend without CA decreased from 2312 MPa to 2016 MPa after weathering as compared to guayule bagasse where MOE dropped from 2416 MPa to 2056 MPa. The largest drop in MOE values was noticed for 50% oak fiber filled composites (control), where MOE dropped from 2926 MPa to 2130 MPa. The results show a similar trend as reported earlier for non-weathered samples containing Oak, CBS and Guayule fibers [34]. The unweathered samples with coupling agent showed MOE values comparable to non-coupled control with 50% oak fibers. The antagonistic effect of lubricant (trace metallic component) and low dose of coupling agent (1%) may have contributed to minimal change in flexural properties of coupled formulations. It is reported
that maleic anhydride, in presence of moisture from wood, converts to maleic acid which is then reacted with metallic stearate to form metal ionomer which negates the improvement to the mechanical properties. The negative effect of the stearate on the maleic anhydride coupling agents has been discussed in earlier studies [35]. However, after weathering the coupling agent TPW 243 and NE 556 based formulations showed MOE values of 2301 MPa and 2460 MPa respectively, which were higher than control (2130 MPa). The flexural strength or modulus of rupture (MOR) showed similar improvements by the addition of coupling agents. The NE 556 coupling agent performed better than the TPW 243. The weathering had a strong impact on the MOR properties of CBS containing formulations. The formulation with 25% CBS fiber loading showed the lowest MOR value of 10.17 MPa after weathering. Both the coupling agents based formulations showed superior strength values after weathering. The formulation with coupling agent NE 556 and TPW 243 showed MOR decrease of 14.4% and 10.5% respectively whereas control sample lost 26.1% of its original strength. Guayule based formulations showed their positive impact on the strength properties, they performed better than CBS. A 15% guayule reinforced formulation showed MOR change from 16.23 MPa to 13.06 MPa after weathering. Overall the coupling agents helped in sustaining the strength properties during weathering. The observed decrease in the flexural properties can be attributed to degradation of interfacial adhesion and increased pore size
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T-6
T-5
T-4
T-3
T-2
T-1
Control 3500
3000
2500 2000 Wavenumber (cm-1)
1500
1000
500
Fig. 2. FTIR spectra of control (unweathered) and six weathered treatments, T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule.
Fig. 3. The discoloration value DE of biofiber composites after 2200 h of UV exposure. Composite formulation 50% biofiber, 40% HDPE, 6% lubricant and 4% talc. T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule.
and number in composite matrices, the repeated weathering cycles induced molecular chain scission, formation of carbonyl and hydroperoxides and chemicrystallization of polymer matrix [33,36]. The loss of strength is also attributed to combined cyclic fiber swelling and embrittlement of the fiber-polymer matrix via photooxidation. As the fiber swelling from the UV and hygrothermal cycling act against the weakened polymer substrate, it is expected the interfacial bonding between fiber and polymer will weaken resulting in lower mechanical properties. Also, when repeated fiber swelling exceeds tensile capacity of polymers it causes mechanical bond degradation. The greater loss in the properties of oak and CBS based composites as compared to guayule bagasse can also be attributed to differential hydrophobicity of these fibers. Since guayule bagasse fiber fraction had smaller particulate size it may have impacted strength properties as particle size is known to influence the strength properties of biocomposites [37].
Fig. 4. The percentage water sorption of biofiber composite formulations. Composite formulation e 50% biofiber, 40% HDPE, 6% lubricant, and 4% talc.
Photodegradation can erode the composite surface layer making it difficult to transmit stress to the specimen's interior thus impacting strength properties. Overall, the published range of UV weathered
Fig. 5. The modulus of elasticity of biofiber composites before and after exposure to 2200 h of UV weathering. (Different letters shows a statistically distinguishable difference between weathered and unweathered treatments).
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Fig. 6. The modulus of rupture of biofiber composites before and after exposure to 2200 h of UV weathering.
biofiber composite mechanical properties substantiate that alternate biofibers can achieve required mechanical properties [34]. 3.6. Response to compressive strength Since biofiber based composite matrices are more susceptible to crushing than glass fibers, it is important to understand their compressive properties for various commercial applications. The impact of UV weathering on the compressive properties of biofibers composites with various proportions of oak, CBS guayule fiber and coupling agents is presented in Fig. 7. The compression strength of all the samples increased after weathering. The results show that maximum compressive load against different formulations exhibited a wide variation in the compressive properties. The impact of different biofibers on compressive properties was not substantial. Weathered specimens with 25% oak and 25% CBS showed the lowest value of 7878.6 N while the control exhibited a value of 8869.7 N. Two coupling agent based specimens showed wide variation in compressive properties. Specimens with 24.25% oak and 24.25% CBS and 1% coupling agent NE 556 (C2) showed highest compressive load of 10043.6 N followed by specimens with 25% oak, 17.5% CBS and 7.5% guayule at 9041.1 N. The differences in the compressive properties are the result of weak fiber-polymer bond due to increased wettability and moisture penetration on the composite surface due to UV weathering.
Fig. 8. Impact strength of biofiber composites before and after exposure to UV weathering.
again coupling agent helped in maintaining the impact properties of BFPC after exposure to UV weathering. The loss of impact strength in CBS and guayule reinforced composites was comparable. The largest drop in impact strength from 52.1 J/m2 to 38.8 J/m2 after was observed for control formulation with 50% oak fibers. The microcracks at the surface of the specimens as indicated in Fig. 2 and weak polymer matrices may have contributed to the low and highly variable in the impact properties. The fibers play an important role in the impact resistance of composites as they interact with the crack formation in the matrix and act as stress transfer sites. It has been reported that coupling agent can accelerate crystallization of polymer that can result in lower degree of crystallinity. The results of this study agree well with results of earlier work investigating effect of compatibilizers on biofiber composites [38]. 3.8. Coefficient of linear thermal expansion
Impact strength is measured in terms of the energy absorbed by the specimen, in Joules, per unit cross-sectional area (J/m2) under a standard impact load. All the weathered samples exhibited a loss in the impact strength. The influence of different biofibers on the impact properties of specimens was not significant (Fig. 8). Specimens with coupling agent TWP 243 and NE 556 showed impact strength loss of 11.2% and 6.61% respectively after weathering whereas the control showed 25.5% loss in impact strength. Once
Majority of the fiber reinforced thermoplastic composites have a tendency to show instability at temperature ranging above 23 C [7]. Even at lower temperature ranging 6 C to 60 C they can soften, creep and expand linearly. The sample specimens expanded as the temperature increased, and decreased as temperature decreased. The CLTE values were measured over the three temperature ranges (6 C, 23 C and 60 C) as a function of different biofibers and coupling agent. The highest linear expansion were noticed for the two guayule bagasse based formulations ranging from 6.3 105 mm/mm/ C to 6.6 105 mm/mm/ C. The weathered specimen with oak fibers showed the CLTE of 5.4 105 mm/mm/ C, lowest CLTE value of 3.8 105 mm/mm/ C was observed for 25% CBS and 25% oak based formulation. Both the coupling agent based formulations exhibited minimal change in CLTE value after weathering. The NE 556 coupling agent slightly outperformed TWP 243. Average CLTE value for weathered BFPC samples with coupling agent was 4.2 105 mm/mm/ C. These
Fig. 7. The maximum compression strength of biofiber composites before and after exposure to UV weathering.
Fig. 9. The change in the coefficient of linear thermal expansion of biofiber composites before and after exposure to UV weathering.
3.7. Impact properties
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values are much smaller than the reported CLTE values of commercial wood polymer composites [7]. The high CLTE value for guayule based formulations can be attributed to presence of aromatic compounds which tend to influence the rheological properties of composites under heat. Overall, the use of coupling agents was effective in controlling thermal expansion of weathered samples; however the viability of CBS and guayule biofibers as alternative reinforcing materials was validated as shown in Fig. 9. 4. Conclusion This study investigated the UV weathering performance of the different biofibers and coupling agents in the HDPE based biofiber composites. All the weathered specimens experienced significant change in the optical properties. The surface micrographs showed presence of microvoids, cracks and high concentration of exposed biofibers after 2200 h of UV exposure. The water absorption of weathered specimens continued to increase after 10 days of immersion in water. The coupling agents had minor influence on the water absorption of weathered composites samples. Flexural stiffness, strength, impact properties and CLTE of biocomposites deteriorated after weathering. The coupling agents helped to retain the mechanical properties of BFPCs after UV exposure. Compressive properties of biocomposites improved with use of the coupling agent. The guayule bagasse fibers helped in reducing the water sorption. This investigation indicates that biofibers are effective sensitizers and their incorporation into the polyolefin based matrix accelerates the degradation of matrix when exposed to moisture and UV weathering. Woody fiber can be substituted with alternate biofibers such as CBS and guayule bagasse with some variations in the physico-mechanical properties of the resultant composites. References [1] Freedonia Group Report. WPC Demand Growth Has Healthy Forecast, Freedonia Group, Cleveland, (OH), 2009 published in Plastic Today, August 10. [2] Lucintel Brief, Opportunities in Natural Fiber Composites, Lucintel LLC, Irving (TX), 2011. [3] W.V. Srubar, S.L. Billington, A micromechanical model for moisture einduced deterioration in fully biorenewable wood-plastic composites, Compos. Part A 50 (2013) 81e92. [4] N.M. Stark, S.A. Mueller, Improving the color stability of wood plastic composites through fiber pretreatment, Wood Fiber Sci. 40 (2) (2008) 271e278. [5] D. Bajwa, D. Bruce, Improvements in the weathering characteristics of woodplastic composites, in: Proc. 8th Inter Conf. Woodfiber-plastic Composites May 23-25, 2005, pp. 187e190. Madison (WI). [6] S.Q. Shi, D.J. Gardner, Hygroscipic thickness swelling rate of compression moded wood fiberboard and wood fiber/polymer composites, Compos. Part A 37 (2006) 1276e1285. [7] S.G. Bajwa, D.S. Bajwa, G. Holt, Optimal substitution of cotton burr and linters in thermoplastic composites, For. Prod. J. 59 (10) (2009) 40e46. [8] W.V. Srubar, S. Pilla, Z.C. Wright, C.A. Ryan, J.P. Greene, Mechanisms and impact of fiber-matrix compatibilization techniques on the material characteristics of PHBV/oak wood flour engineered biobased composites, Compos. Sci. Technol. 72 (2012) 708e715. [9] D. Maldas, B.V. Kotka, Role of coupling agents on the performance of woodflour-filled polypropylene composites, Int. J. Polym. Mater. 27 (1994) 22e88. [10] C. Daneault, B.V. Kokta, D. Maldas, Grafting of vinyl monomers onto wood fibers initiated by peroxidation, Polym. Bull. 20 (1988) 137e141. [11] C.K. Hong, N. Kim, S.L. Kang, Mechanical properties of maleic anhydride treated jute/polypropylene composites, Plast. Rubber Compos. 37 (2008) 325e330.
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[12] T.J. Keener, R.K. Stuart, T.K. Brown, Maleated coupling agent for natural fiber composites, Compos. Part A 35 (2004) 357e362. [13] J.Z. Lu, Q. Wu, Negulescu II, Wood-Fiber/High density-polyethylene composites coupling agent performance, J. Appl. Polym. Sci. 96 (2005) 93e102. [14] D.J. Olsen, Effectiveness of Maleated Polypropylenes as Coupling Agents for Wood Flour/polypropylene Composites, ANTEC, 1991, pp. 1886e1891. [15] M. Kazayawokom, J.J. Balatinecz, L.M. Matuana, Surface modification and adhesion mechanisms in wood-fiber polypropylene composites, J. Appl. Polym. Sci. 34 (1999) 6189e6199. [16] S. Mohanty, S.K. Nayak, S.K. Verma, S.S. Tripathy, Effect of MAPP as a coupling agent on the performance of jute-PP composites, J. Reinf. Plastic Compos. 23 (2004) 625e637. [17] J.S. Fabiyi, A.G. McDonald, M.P. Wolcott, P.R. Griffiths, Wood plastic composites weathering: visual appearance and chemical changes, Polym. Degrad. Stab. 93 (8) (2008) 1405e1414. [18] X. Zhou, et al., Outdoor natural weathering of bamboo flour/polypropylene foamed composites, J. Reinf. Plastics Compos. 33 (19) (2014) 1835e1846. [19] N.M. Stark, L.M. Matuana, Surface chemistry changes of weathered HDPE/ wood-flour composites studied by XPS and FTIR spectroscopy, Polym. Degrad. Stab. 86 (1) (2004) 1e9. [20] ASTM, Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials (ASTM G154-12a), American Society of Testing and Materials, West Conshohocken (PA), 2012. [21] ASTM, ASTM e Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates (ASTM D224412), American Society of Testing and Materials, West Conshohocken (PA), 2008. [22] ASTM, Standard Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials (ASTM D1037-06a), American Society of Testing and Materials, West Conshohocken (PA), 2006. [23] ASTM, Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics (ASTM D256-05), American Society of Testing and Materials, West Conshohocken (PA), 2005. [24] ASTM, Standard Test Method for Determination of the Linear Coefficient of Thermal Expansion of Plastic Lumber and Plastic Lumber Shapes Between e34.4 and 60 C (ASTM D6341-98), American Society of Testing and Materials, West Conshohocken (PA), 1998. [25] MINITAB 16, Minitab Statistical Software, Minitab Inc. State College, (PA), 2012. [26] S.G. Bajwa, D.S. Bajwa, A.S. Anthony, Effect of laboratory aging on the physical and mechanical properties of wood-polymer composites, J. Thermoplast. Compos. Mater. 22 (2) (2009) 227e243. [27] N.M. Stark, L.M. Matuana, Photostabilization of wood flour filled HDPE composites, in: Conference Proceedings Volume III e Materials. Society of Plastic Engineering, May 5-9, 2002. San Francisco, (CA) USA ANTEC. [28] H.S. Szymanski, IR Theory and Practice of Infrared Spectroscopy, Plenum, New York, 1964, p. 375. [29] R. Selden, B. Systrom, R. Langstrom, UV aging of poly(propylene)/wood fiber composites, Polym. Compos. 25 (5) (2004) 543e553. [30] D.N.S. Hon, W.C. Feist, Weathering characteristics of hardwood surfaces, Wood Sci. Technol. 20 (1986) 169e183. [31] K.B. Adhikary, S. Pang, M.P. Staiger, Effects of the accelerated freeze-thaw cycling on physical and mechanical properties of wood flour-recycled thermoplastic composites, Polym. Compos. 31 (2) (2010) 185e194. [32] S. Panthpulakkal, M. Sain, Studies on the water absorption properties of short hempeglass fiber hybrid polypropylene composites, J. Compos. Mater. 41 (15) (2007) 1871e1883. [33] S.G. Bajwa, D.S. Bajwa, G. Holt, T. Coffelt, F. Nakayama, Properties of thermoplastic composites with cotton and guayule biomass residue as fiber fillers, Industrial Crops Prod. 33 (3) (2011) 747e755. [34] S.G. Bajwa, D.S. Bajwa, G. Holt, T.C. Wedegaertner, Commercial-scale Eval. Two Agric. Waste Prod. Cotton Burr/Stem Module Wraps, Thermoplast. Compos. Its Comp. Laboratory-Scale Results 27 (6) (2014) 741e757. [35] M. Wolcott, M. Chowdhury, D. Harper, T. Li, R. Richard, T. Rials, in: Sixth International Conference on Woodfiber-plastic Composites, 2002 (Madison, WI, USA). [36] J.W. Rabello, Photodegradation of Polypropylene Containing a Nucleating Agent, John Wiley and Sons, Inc, 1997, pp. 2505e2517. [37] N.M. Stark, R.E. Rowland, Effects of wood fiber characteristics on mechanical properties of wood/polypropylene composites, Wood Fiber Sci. 35 (2) (2003) 167. [38] M.L. Soleimani, T.S. Panigrahi, I. Oguocha, Crystallization and thermal properties of biofiber- polypropylene composites, in: Thermoplastic e Composite Materials, InTech, 2012, ISBN 978-953-51-0310-3.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Impact of biofibers and coupling agents on the weathering characteristics of composites Dilpreet S. Bajwa a, *, Sreekala G. Bajwa b, Greg A. Holt c a
Department of Mechanical Engineering North Dakota State University, Fargo, ND, USA Department of Agricultural and Biosystems Engineering, North Dakota State University, Fargo, ND, USA c USDA-ARS, Cotton Production & Processing Research Unit, Lubbock, TX, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2015 Received in revised form 29 May 2015 Accepted 20 June 2015 Available online 23 June 2015
This paper explores the ultraviolet (UV) weathering performance of high density polyethylene (HDPE) composites with different biofiber fillers and coupling agent. Biofiber polymer composite (BFPC) material samples were prepared using oak, cotton burr and stem (CBS) or guayule bagasse as fiber source HDPE as resin, and two different coupling agents. Weathering variables included exposure to UV radiation and moisture cycles for up to 2200 h. Each variable can degrade BFPC matrix independently or synergistically. The impact of weathering was measured through the changes in the surface matrix, mechanical properties and thermal stability of BFPC as a function of biofibers types and coupling agents. The coupling agent treated composites showed color shift (DE) and variable surface degradation. Water absorption of the weathered samples continued to increase after 10 days of immersion. Flexural stiffness, strength and impact properties of weathered samples decreased regardless of fiber type or coupling agent. Minimal linear thermal expansion was noted in weathered composites with coupling agents. Overall coupling agent helped to retain the mechanical properties of composites after exposure to UV weathering. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Natural fiber composites Coupling agents Natural weathering Polymer degradation Mechanical properties
1. Introduction Biofiber polymer composites (BFPC) are materials made from a thermoplastic or thermoset resin (substrate) with cellulosic fibers as fillers or reinforcement. They are currently used in a variety of products ranging from building materials, automotive parts, packaging, consumer goods and landscaping products. They combine the superior mechanical properties of cellulosic fibers and the longterm performance of plastic. The use of BFPC has accelerated in the last few decades due to measurable advantages in terms of cost, high specific strength, recyclability, and ease of maintenance. Several research reports forecasted BFPC market to grow between 11 and 15% compound annual growth rate (CAGR) over the next five years [1,2]. Despite all these advantages their widespread acceptance and market penetration has been limited due to nonstructural applications, persistent concerns over their long term durability, and susceptibility to moisture in the outdoor or exterior applications [3].
* Corresponding author. E-mail address: [email protected] (D.S. Bajwa). http://dx.doi.org/10.1016/j.polymdegradstab.2015.06.015 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
Weathering or photodegradation of BFPC is of major concern because of the documented effect of photodegradation has on material properties and appearance. The adverse effects of weathering include degradation of aesthetic appeal, and physicomechanical properties [4,5]. Weathering due to UV light and moisture results in the degradation of the polymer-fiber matrix. Photodegradation of polyolefin resins results from excited electronic states of oxygen-containing species (carbonyls) are formed due to introduction of catalyst residues, hydroperoxide groups, carbonyl groups and double bonds [4]. Further, moisture in biofibers is driven by the chemical potential gradient between hydroxyls present in lignocellulose and the polarity of water molecules. Absorbed moisture can cause swelling of biofibers which can lead to surface instability, matrix cracking, and inservice deterioration due to loss of structural integrity and strength. The mechanism of moisture induced deterioration of biofibers composites has been widely reported in the literature [6,7]. Chemical modification of cellulosic fiber cell walls by grafting through use of coupling agent to improve moisture resistance and strength is widely reported in the literature [8e12]. In the last two decades coupling agents such as isocyante, silane, acrylic acid and
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213
2.2. Specimen manufacturing
maleic anhydride have been used to strengthen biofiber based composites [2]. One of the most widely used and effective functional groups for grafting a coupling agent (CA) to a cellulosic fiber is the anhydride functionality. Chemical structure, molecular weight and functional groups per mole of a coupling agent have strong impact on the mechanical properties of composites [12,13]. Typically a coupling agent creates a crosslinking structure and improves the interfacial adhesion when one portion of the molecule tethers to the polymer matrix and specific functional groups react with hydroxyl groups present on the fiber surface [14,15]. It is well known that coupling agents improve fiber/matrix interaction and the properties of composites however, it is not known how outdoor weathering conditions can impact the performance of composites containing different biofibers and coupling agents [12,13,16]. Some previous accelerated weathering studies on BFPC have reported degradation of polymer matrix, chromacity shift, followed by loss in the mechanical properties [17e19]. Therefore, a study was conducted to explore the impact of coupling agents on the physico-mechanical properties of biofiber composites under accelerated weathering. The objective of this study was to evaluate the impact of coupling agents on the weathering characteristics of high density polyethylene (HDPE) composites containing natural fibers such as cotton burr with stem (CBS) and guayule bagasse via (1) characterizing the effect of weathering on the physico-mechanical properties of the biocomposites made using different biofibers and coupling agents and (2) understanding the weathering related changes in the fiber-polymer matrix though surface microscopy and color shift.
The test specimens were manufactured at a commercial wood composites processing facility located in Greenland, Arkansas, USA. The proprietary technology involved first compounding raw materials and then transferring into a single screw extruder. The weight percentage of fiber was kept at 49.5% or 50% depending on the presence of coupling agent while polyolefin HDPE was maintained at 39.5% or 40%. The remaining ingredient included the lubricant at 6% and the mineral filler talc at 4% by weight of the total composition. The concentration of coupling agent was maintained at 1% on weight basis for all treatments that used it, as recommended by the manufacturers. There were a total of 7 treatments, with 6 replications for each treatment as shown in Table 1. The ingredients for each set of specimens were initially weighed in the required proportion, and then compounded in a densifier and directly fed to the extruder. The extruded profile was sized using a calibration equipment connected to vacuum and chilled water. Material processing temperature varied from 149 C to177 C for zones 1 through 6 with the die zone 1 at 171 C and die zone 2 at 154 C respectively. The extruder screw speed was 25 rpm and melt temperature of the extrudate fluctuated between 158 C to 160 C. The specimens were extruded using a profile die of 25.4 mm in height and 152.4 mm wide with a sample length 150 cm. For each formulation, two samples each measuring 250 cm in length were collected randomly for weathering studies. The extruded samples were water cooled and then conditioned at room temperature for 4 weeks before testing. For mechanical properties comparison treatment T0 samples were not subjected to weathering.
2. Materials and experimental methods
2.3. Accelerated UV weathering exposure
2.1. Materials
Accelerated weathering was conducted in a QUV weatherometer, (Q-LAB Corporation, Westlake, Ohio, USA) that stimulates the damage similar to sunlight, rain and dew. The rectangular test samples were cut from the 125 mm wide and 25 mm thick samples and were mounted on aluminum fixtures and subjected to accelerated UV weathering via 340-nm fluorescent UV lamps (UV-B lamps). The average irradiance was about 0.85 W/m2 with a chamber temperature of 45 C. To avoid any variation due to different morphologies or processing conditions, all samples were cut along the direction of material flow with top surfaces exposed to UV. The UV weathering procedure followed the ASTM G 154 test method recommended for nonmetallic materials [20]. Exposure mode 1 cycles were followed that included a UV exposure period followed by periods with condensation and no radiation. Exposure to dry UV for 4 h at 60 C, followed by condensation exposure up to 4 h at a temperature of 50 C without radiation. The specimens were exposed to 2200 h of UV radiation and condensation cycles in the QUV weatherometer. After accelerated weathering the specimens were carefully removed from the aluminum fixtures for color measurement studies and then conditioned in the laboratory environment for approximately 12 weeks and tested for physical and mechanical properties.
The HDPE used for the manufacturing composite samples was Petrothene LB010000 high density polyethylene copolymer reactor powder, an additive free formulation obtained from Equistar Chemical Company (Houston, Texas, USA). The HDPE powder had a density of 953 kg/m3 and melt flow index of 0.50 g/ 10 min. The flexural modulus and tensile strength were 1275 MPa and 27.3 MPa respectively. Oak wood fiber (particle size distribution ranging from 250 to 841 micron, with 90% of fibers falling between 250 and 400 microns) was supplied by Southern Wood Services, (Macon, Georgia, USA). Cotton burr with stem (CBS, particle size range 250e841 micron) was supplied by USDA-ARS, Cotton Production and Processing Research Unit, Lubbock Texas, USA. Guayule bagasse (particle size range 250e400 micron) was supplied by USDA-ARS, Maricopa, Arizona, USA. The two commercially available coupling agents used in this study were: Integrate NE 556-004 (HDPE-based, MI ¼ 3.8 g/10 min.) manufactured by Equistar Chemical Company (Houston, Texas, USA) and TPW 243 manufactured by Struktol Company (Stow Ohio, USA). The NE 556-004 coupling agent had an HDPE backbone with high level of maleic anhydride (2% MA) grafting. Integrate coupling agent has a unique grafting technique that makes it efficient compatibilizer for BFPCs. The uniqueness is attributed to high grafting efficiency and low residual maleic. The TPW 243 is also marketed as a compatibilizer with the ability to bond organic and inorganic materials by a free radical source through an ionic mechanism initiated by hydrolysis. The exact formulation of the coupling agent TPW 243 could not be revealed due to manufacturer's proprietary information. Lubricant TPW 113 (non-metal lubricant) supplied by Struktol Company was used as a processing aid. It is a blend of complex, modified fatty acid ester widely recommended for wood filled polyolefins.
2.4. Physico-mechanical property measurement 2.4.1. Fractography The surface morphology of the UV weathered biofiber composite specimens was analyzed to investigate the extent of macroscale cracking, flaking and surface imperfections in the fiberpolymer matrices of different formulations. The samples were attached to aluminum mounts with colloidal silver paste. A conductive goldepalladium coating was applied with a Balzers SCD 030 sputter coater (BAL-TEC RMC, Tucson AZ USA). Images were
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Table 1 Composition of constituents of biofiber polymer composites (%). Treatment
Oak
CBS
Guayule
HDPE
Lubricant
Mineral filler
Coupling agent
T1 (Control) T2 T3 T4 T5 T6
50 25 24.25 24.25 25 17.5
e 25 24.25 24.25 17.5 17.5
e e e e 7.5 15
40 40 39.5 39.5 40 40
6 6 6 6 6 6
4 4 4 4 4 4
e e 1 (C1) 1 (C2) e e
C1 e Struktol TPW 243. C2 e Integrate NE 556-004.
obtained using a JEOL JSM-6490LV scanning electron microscope (JEOL USA, Peabody MA USA) at an accelerating voltage of 15 keV and 200 magnification. 2.4.2. Infrared spectroscopy Samples for Fourier transform infrared spectroscopy (FT-IR) were analyzed using Nicolet 8700 spectrometer. The surface of the samples was scraped using a sharp knife to a depth of 1.0 mm on the UV exposed area. The infrared spectra were obtained following step-scan mode by using helium for purging. Single measurements were made and spectra shown are the average of 15 scan in the range of 450e4000 cm1. 2.4.3. Optical properties The optical properties were measured using the CIE LAB color clairage, Paris) and system (Commission internationale de l'e following the procedure outlined in ASTM D2244 [21]. In this color system the L* axis represents lightness; a* and b* are the chromacity coordinates. In CIE LAB system þa represents red, -a represents green, þb* represents yellow and eb* represents blue. L values vary from 0 for black to 100 for white. The color change DE was determined using equation (1).
1=2 DE ¼ DL*2 þ Da*2 þ Db*2
(1)
where DL*, Da*, Db* are the difference between initial (before weathering) and final values (after weathering) of L*, a*, b*. The optical properties were measured using an X-Rite spectrophotometer model SP62, manufactured by X-Rite Inc. (Grand Rapids, Michigan, USA). 2.4.4. Specimen moisture sorption To investigate the effect of moisture on the material properties of UV weathered BFPC the specimens were subjected to extended water absorption test in accordance with ASTM D1037-06a test method [22]. The specimens were first conditioned at 23 C and 50% relative humidity for one week. Specimen weight and dimensions were recorded before completely immersing them in a water bath filled with tap water and held at a constant temperature of 25 C. The weight gain of the samples was monitored daily for 16 days to calculate water sorption percentage. 2.4.5. Flexural mechanical testing The flexural properties of UV exposed specimens were characterized at room temperature in accordance with ASTM D1037-06a test method [22]. The specimens had an average length of 305 mm, width of 24.5 mm, and thickness of 26.4 mm. Three point bending tests were performed using an Instron Model 5567, Instron Corporation (Norwood, Massachusetts, USA). The support span for test samples was 279 mm with crosshead rate of 12.2 mm/min. The specimens were placed on the testing fixture such that the UV exposed side was on the top in contact with loading head. The
samples were tested in flexural mode because in majority of applications the material experiences flexural stress. The modulus of rupture (MOR), and modulus of elasticity (MOE) values calculated. 2.4.6. Compressive strength Compressive properties of the specimens were determined in accordance with ASTM D1037-06a test method [22]. The test specimens were subjected to compression loading on the UV treated side and perpendicular to the surface, at a relatively low uniform rate of strain with an Instron universal tester. Since the maximum crushing strength was of interest and the samples thickness was more than 25 mm a short column method C was selected. The thickness of the specimens was 26.5 mm with lengths and widths of 25.4 mm. The specimens were loaded through a spherical block at a constant speed of 1.5 mm/min. Both compressive strength and MOE were measured from the stressstrain data generated. 2.4.7. Izod impact testing Impact testing was performed on the un-notched specimens. The specimen were 64 mm long, 12.7 mm wide and 6.4 mm thick. Izod impact tests were performed at room temperature following the ASTM D 256-05 procedure [23]. The testing was performed using the pendulum based impact tester Tinius Olsen Model Impact 104, (Horsham, Pennsylvania, USA). The specimens were supported at both ends and struck by the pendulum on the flat side in the middle and impact energy expressed in J/m2 was calculated. Similar to flexural testing the UV exposed surface was exposed to the pendulum impact. 2.4.8. Thermal expansion The coefficient of linear thermal expansion (CLTE) of the specimens was measured in accordance with ASTM D6341-98 method [24]. The determination was made by taking measurements with a caliper at three discrete temperatures (6 C, 23 C and 60 C). Test specimens were prepared with due diligence to ensure smooth, flat, parallel surfaces and sharp, clean edges. Three lines parallel to the length was marked on one side of specimen with indelible ink so that length measurements could be taken consistently at the same locations. The specimen length was 300 mm, width 25.4 mm and thickness 2.6.5 mm. Test specimens were conditioned prior to testing at 23 C and 50% relative humidity for one week. Two specimens from each formulation were tested at three measurement temperatures. Length of conditioned specimens was measured to the nearest 0.01 mm with digital caliper before transferring them to a freezer set at 6 C. After 48 h of exposure in the freezer, the specimens were taken out for conditioning for 48 h at 23 C and then moved into convection oven set at 60 C for 48 h. The length of the specimens was accurately measured after each temperature exposure. The CLTE was measured as the slope of the curve of specimen length versus temperature. 2.5. Data analysis A total of 12 sets of samples were subjected to UV weathering. These samples were cut into test-size specimens such that there were 6 replications for each of the physico-mechanical tests. To identify the impact of biofibers and coupling agents on the weathering characteristics, means of the physico-mechanical properties of unweathered and weathered samples were compared using Fishers Least Square Difference (LSD) tests with MINITAB statistical software [25]. The optical shift of the specimens was calculated based on the mean of four color measurement readings.
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3. Experimental results and discussion 3.1. Fractographic analysis The visual appearance of the samples after the QUV weathering changed noticeably. As a result of UV weathering the surface became dull and porous. The surface color and texture looked from smooth reddish brown prior to weathering to chalky beige after weathering. The polymer matrix appeared to have weakened and surface appeared rich in exposed biofibers. Microcraking, or crazing was likely result of polymer surface erosion and contributed to the observed lightness and dull surface. Polyolefins undergo photolytic and photo oxidative reaction during exposure to solar UV radiation. The photodegradation originates from excited polymereoxygen complexes which is primarily caused due to presence of catalyst residues, hyperperoxides groups, carbonyl groups, and double bonds induced due to thermal degradation during processing [26]. The polymer containing chromophoric groups such as carbonecarbon double bond (C]C) and carbonyl (C]O) absorb UV energy and initiate photo reactions which degrade polymer resulting in surface densification and cracking [26,27]. In this study all the test specimens showed variable degree of cracking in the UV exposed area (Fig. 1) due to possible photodegradation of polymers and expansion of fibers resulting from moisture absorption. The low dose of coupling agent (1%) proved to be ineffective in controlling the surface degradation. In a similar study, the color of polyolefin based wood fiber composites changed from brown to chalky white at the exposed area because of the formation of thin, strongly degraded surface layer [28]. 3.2. FT-IR spectroscopy The FT-IR spectra from the exposed surface of control and six treatments are illustrated in Fig. 2. It can be seen that new absorption bands have emerged in the region of 1275 cm1, 1473 cm1, 1554 cm1, 1657 cm1 and 1726 cm1 after exposure to UV irradiation. The appearance of these new absorption bands is clearly seen in the difference in spectra between control and weathered specimens where groups frequencies associated with the formation of polyene and carbonyl linkages are easily recognized at 1275, 1470 cm1 and 1554 cm1 (CH3, C]O, and C]C stretching) [28]. The peaks seen in UV exposed samples are usually associated with the presence of carbonyl groups from photooxidation of wood fibers [29]. Furthermore the presence of polyene units on the surface samples is indicative of photodegradation under UV light. Two absorbance peak at 1238 cm1 and 1595 cm1 are only seen in control samples which are associated with CeC (ethers) and aromatic C]C groups. The functional groups from wood fibers act as chromophores or initiators (carbonyls, carboxylic acids, quinines and hydroxyperoxy radicals) and accelerate the UV degradation. The results of this study are in agreement with previous FTIR study where spectra of the HDPE-WF composites showed a broad peak at 3318 cm1 and strong peak at 1023 cm1 suggesting wood rich surface and low polymer after weathering. 3.3. Optical properties The measurement of color changes provided additional evidence of photodegradation of the composites. Fig. 2 indicates that surface color change of the six UV weathered treatments. This discoloration deeply affected the appearance of the degraded composites. The oak and guayule fiber based composites exhibited lower color shift than CBS specimens. The highest color shift DE of 14.1 was observed for specimen with 24.5% oak, and 24.5% CBS and 1% Integrate coupling agent (T4), indicating that this
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particular coupling agent didn't prevent the discoloration of specimens. However, addition of 1% Struktol TPW 243 did reduce the color shift significantly in treatment T3, this may be due to antioxidants or hydroperoxide decomposers present in the CA [27]. Since all the samples exhibited extensive discoloration, suggesting the incorporation of woody or non-woody fibers into HDPE matrix has a deleterious effect on the ability of the matrix to resist the exposure to UV weathering. The shift in DE was mainly due to bleaching of the fiber component, degradation of polymer due to UV radiation and the darkening effect may be linked to surface oxidation as previously reported [30,31]. Exposure to UV radiation, moisture, and oxidation of polymer degraded the composite surface layer and removed natural extractives, lignin and colored materials, thus resulting in color change of the specimens. Color change in composite samples was probably exacerbated by fiber swelling and cracking which augmented the exposed surface areas of polymer and fiber and created more sites for UV exposure. 3.4. Influence of UV weathering on the moisture sorption characteristics The water sorption characteristics of composite specimens over 16 days at 24 h intervals is shown in Fig. 3. Water sorption by biofiber composites is limited to sorption only by wood fibers as water uptake by HDPE is less than 0.05%. The composite specimens showed wide variation in water absorption and absorption among the different formulations. The maximum water sorption for unweathered sample was 4.8%. In UV weathered samples after three days of immersion the average water sorption ranged between 3 and 5%, however after 8 days, specimens from formulations with CBS and Oak fibers (T2, T3, and T4) showed water sorption in excess of 10%. These observations are in agreement with some of the previous studies that found microcracks at the interface due to water absorption [3,32]. Biofibers are hydrophilic in nature and upon exposure to high humidity and wet environments, individual fibers can absorb moisture and swell creating internal pressure and cracking the polymer matrix. The water sorbed by the biofibers is located on the hydroxyl group by hydrogen bond. If the biofibers are not completely encapsulated by polymer, water can easily permeate and compromise material properties. For BFPCs low water absorption is preferred to maintain temporal dimensional stability and prevent biodegradation. A steep rise in water sorption values of weathered samples was observed after 10 days of immersion probably due to degradation of hydrophobic lignin thus making surface cellulose rich (Fig. 4). The increase in hydroxyl groups and carboxyl groups that are hydrophilic in nature may have further increased the water sorption [31]. Overall, the water sorption of the samples containing 50% oak fiber was lowest at 12% after 15 days, while samples containing 25% Oak and 25% CBS showed the highest water sorption at 19.6%. The high water affinity of CBS is possibly the reason for high water sorption values noticed in CBS based formulation. Both the guayule based formulations exhibited lower water sorption values. A previous study also showed that guayule bagasse fibers are less susceptible to the moisture [33]. It is know that guayule baggase contains resinous material such as sesquiterpene esters, triterpene keto alcohols, and triglycerides, which can reduce water absorption. Interestingly, the formulations containing coupling agents also.showed water sorption ranging 17.9%e18.2% only slightly less than the formulation that did not contain coupling agents. It is not clear why coupling agent based biocomposites exhibited high water absorption. It is possible that weathering increased availability of hydroxyl groups from cellulose and the moisture affinity of CBS fiber is stronger than the impact of coupling agents. Also, most studies showing strong impact of coupling agents in reducing water
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Fig. 1. Microscope pictures of the UV weathered biofiber composites samples at 200 magnification. Composite formulation 50% biofiber, 40% HDPE, 6% lubricant and 4% talc. Control unweathered, T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule. Control e Unweathered 50% oak, 40% HDPE, 10% additives.
sorption used approximately 3% by weight of coupling agents, as compared to the 1% used in this study [34]. Overall the water sorption data shows that UV weathering has a deleterious effect on the polymer-fiber matrix. 3.5. Flexural mechanical properties of biofiber composites The effect of UV weathering on the flexural response of different biofiber composite formulations is shown in Figs. 5 and 6. All the formulations showed decrease in the strength properties of composites after UV weathering. Addition of CBS fibers influenced the stiffness or modulus of elasticity (MOE) and modulus of rupture (MOR) values more than guayule bagasse. Partial substitution of CBS with guayule bagasse fiber helped to improve the MOE. The MOE values of 25% CBS blend without CA decreased from 2312 MPa to 2016 MPa after weathering as compared to guayule bagasse where MOE dropped from 2416 MPa to 2056 MPa. The largest drop in MOE values was noticed for 50% oak fiber filled composites (control), where MOE dropped from 2926 MPa to 2130 MPa. The results show a similar trend as reported earlier for non-weathered samples containing Oak, CBS and Guayule fibers [34]. The unweathered samples with coupling agent showed MOE values comparable to non-coupled control with 50% oak fibers. The antagonistic effect of lubricant (trace metallic component) and low dose of coupling agent (1%) may have contributed to minimal change in flexural properties of coupled formulations. It is reported
that maleic anhydride, in presence of moisture from wood, converts to maleic acid which is then reacted with metallic stearate to form metal ionomer which negates the improvement to the mechanical properties. The negative effect of the stearate on the maleic anhydride coupling agents has been discussed in earlier studies [35]. However, after weathering the coupling agent TPW 243 and NE 556 based formulations showed MOE values of 2301 MPa and 2460 MPa respectively, which were higher than control (2130 MPa). The flexural strength or modulus of rupture (MOR) showed similar improvements by the addition of coupling agents. The NE 556 coupling agent performed better than the TPW 243. The weathering had a strong impact on the MOR properties of CBS containing formulations. The formulation with 25% CBS fiber loading showed the lowest MOR value of 10.17 MPa after weathering. Both the coupling agents based formulations showed superior strength values after weathering. The formulation with coupling agent NE 556 and TPW 243 showed MOR decrease of 14.4% and 10.5% respectively whereas control sample lost 26.1% of its original strength. Guayule based formulations showed their positive impact on the strength properties, they performed better than CBS. A 15% guayule reinforced formulation showed MOR change from 16.23 MPa to 13.06 MPa after weathering. Overall the coupling agents helped in sustaining the strength properties during weathering. The observed decrease in the flexural properties can be attributed to degradation of interfacial adhesion and increased pore size
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T-6
T-5
T-4
T-3
T-2
T-1
Control 3500
3000
2500 2000 Wavenumber (cm-1)
1500
1000
500
Fig. 2. FTIR spectra of control (unweathered) and six weathered treatments, T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule.
Fig. 3. The discoloration value DE of biofiber composites after 2200 h of UV exposure. Composite formulation 50% biofiber, 40% HDPE, 6% lubricant and 4% talc. T1- 50% oak, T2 e 25% oak, 25% CBS, T3 e 24.3% oak, 24.3% CBS and 1% CA-TPW 243, T4 e 24.3% oak, 24.3% CBS and 1% CA-NE 556-004, T5 e 25% oak, 17.5% CBS and 7.5% guayule and T6 e 17.5% oak, 17.5 CBS, 15% guayule.
and number in composite matrices, the repeated weathering cycles induced molecular chain scission, formation of carbonyl and hydroperoxides and chemicrystallization of polymer matrix [33,36]. The loss of strength is also attributed to combined cyclic fiber swelling and embrittlement of the fiber-polymer matrix via photooxidation. As the fiber swelling from the UV and hygrothermal cycling act against the weakened polymer substrate, it is expected the interfacial bonding between fiber and polymer will weaken resulting in lower mechanical properties. Also, when repeated fiber swelling exceeds tensile capacity of polymers it causes mechanical bond degradation. The greater loss in the properties of oak and CBS based composites as compared to guayule bagasse can also be attributed to differential hydrophobicity of these fibers. Since guayule bagasse fiber fraction had smaller particulate size it may have impacted strength properties as particle size is known to influence the strength properties of biocomposites [37].
Fig. 4. The percentage water sorption of biofiber composite formulations. Composite formulation e 50% biofiber, 40% HDPE, 6% lubricant, and 4% talc.
Photodegradation can erode the composite surface layer making it difficult to transmit stress to the specimen's interior thus impacting strength properties. Overall, the published range of UV weathered
Fig. 5. The modulus of elasticity of biofiber composites before and after exposure to 2200 h of UV weathering. (Different letters shows a statistically distinguishable difference between weathered and unweathered treatments).
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Fig. 6. The modulus of rupture of biofiber composites before and after exposure to 2200 h of UV weathering.
biofiber composite mechanical properties substantiate that alternate biofibers can achieve required mechanical properties [34]. 3.6. Response to compressive strength Since biofiber based composite matrices are more susceptible to crushing than glass fibers, it is important to understand their compressive properties for various commercial applications. The impact of UV weathering on the compressive properties of biofibers composites with various proportions of oak, CBS guayule fiber and coupling agents is presented in Fig. 7. The compression strength of all the samples increased after weathering. The results show that maximum compressive load against different formulations exhibited a wide variation in the compressive properties. The impact of different biofibers on compressive properties was not substantial. Weathered specimens with 25% oak and 25% CBS showed the lowest value of 7878.6 N while the control exhibited a value of 8869.7 N. Two coupling agent based specimens showed wide variation in compressive properties. Specimens with 24.25% oak and 24.25% CBS and 1% coupling agent NE 556 (C2) showed highest compressive load of 10043.6 N followed by specimens with 25% oak, 17.5% CBS and 7.5% guayule at 9041.1 N. The differences in the compressive properties are the result of weak fiber-polymer bond due to increased wettability and moisture penetration on the composite surface due to UV weathering.
Fig. 8. Impact strength of biofiber composites before and after exposure to UV weathering.
again coupling agent helped in maintaining the impact properties of BFPC after exposure to UV weathering. The loss of impact strength in CBS and guayule reinforced composites was comparable. The largest drop in impact strength from 52.1 J/m2 to 38.8 J/m2 after was observed for control formulation with 50% oak fibers. The microcracks at the surface of the specimens as indicated in Fig. 2 and weak polymer matrices may have contributed to the low and highly variable in the impact properties. The fibers play an important role in the impact resistance of composites as they interact with the crack formation in the matrix and act as stress transfer sites. It has been reported that coupling agent can accelerate crystallization of polymer that can result in lower degree of crystallinity. The results of this study agree well with results of earlier work investigating effect of compatibilizers on biofiber composites [38]. 3.8. Coefficient of linear thermal expansion
Impact strength is measured in terms of the energy absorbed by the specimen, in Joules, per unit cross-sectional area (J/m2) under a standard impact load. All the weathered samples exhibited a loss in the impact strength. The influence of different biofibers on the impact properties of specimens was not significant (Fig. 8). Specimens with coupling agent TWP 243 and NE 556 showed impact strength loss of 11.2% and 6.61% respectively after weathering whereas the control showed 25.5% loss in impact strength. Once
Majority of the fiber reinforced thermoplastic composites have a tendency to show instability at temperature ranging above 23 C [7]. Even at lower temperature ranging 6 C to 60 C they can soften, creep and expand linearly. The sample specimens expanded as the temperature increased, and decreased as temperature decreased. The CLTE values were measured over the three temperature ranges (6 C, 23 C and 60 C) as a function of different biofibers and coupling agent. The highest linear expansion were noticed for the two guayule bagasse based formulations ranging from 6.3 105 mm/mm/ C to 6.6 105 mm/mm/ C. The weathered specimen with oak fibers showed the CLTE of 5.4 105 mm/mm/ C, lowest CLTE value of 3.8 105 mm/mm/ C was observed for 25% CBS and 25% oak based formulation. Both the coupling agent based formulations exhibited minimal change in CLTE value after weathering. The NE 556 coupling agent slightly outperformed TWP 243. Average CLTE value for weathered BFPC samples with coupling agent was 4.2 105 mm/mm/ C. These
Fig. 7. The maximum compression strength of biofiber composites before and after exposure to UV weathering.
Fig. 9. The change in the coefficient of linear thermal expansion of biofiber composites before and after exposure to UV weathering.
3.7. Impact properties
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values are much smaller than the reported CLTE values of commercial wood polymer composites [7]. The high CLTE value for guayule based formulations can be attributed to presence of aromatic compounds which tend to influence the rheological properties of composites under heat. Overall, the use of coupling agents was effective in controlling thermal expansion of weathered samples; however the viability of CBS and guayule biofibers as alternative reinforcing materials was validated as shown in Fig. 9. 4. Conclusion This study investigated the UV weathering performance of the different biofibers and coupling agents in the HDPE based biofiber composites. All the weathered specimens experienced significant change in the optical properties. The surface micrographs showed presence of microvoids, cracks and high concentration of exposed biofibers after 2200 h of UV exposure. The water absorption of weathered specimens continued to increase after 10 days of immersion in water. The coupling agents had minor influence on the water absorption of weathered composites samples. Flexural stiffness, strength, impact properties and CLTE of biocomposites deteriorated after weathering. The coupling agents helped to retain the mechanical properties of BFPCs after UV exposure. Compressive properties of biocomposites improved with use of the coupling agent. The guayule bagasse fibers helped in reducing the water sorption. This investigation indicates that biofibers are effective sensitizers and their incorporation into the polyolefin based matrix accelerates the degradation of matrix when exposed to moisture and UV weathering. Woody fiber can be substituted with alternate biofibers such as CBS and guayule bagasse with some variations in the physico-mechanical properties of the resultant composites. References [1] Freedonia Group Report. WPC Demand Growth Has Healthy Forecast, Freedonia Group, Cleveland, (OH), 2009 published in Plastic Today, August 10. [2] Lucintel Brief, Opportunities in Natural Fiber Composites, Lucintel LLC, Irving (TX), 2011. [3] W.V. Srubar, S.L. Billington, A micromechanical model for moisture einduced deterioration in fully biorenewable wood-plastic composites, Compos. Part A 50 (2013) 81e92. [4] N.M. Stark, S.A. Mueller, Improving the color stability of wood plastic composites through fiber pretreatment, Wood Fiber Sci. 40 (2) (2008) 271e278. [5] D. Bajwa, D. Bruce, Improvements in the weathering characteristics of woodplastic composites, in: Proc. 8th Inter Conf. Woodfiber-plastic Composites May 23-25, 2005, pp. 187e190. Madison (WI). [6] S.Q. Shi, D.J. Gardner, Hygroscipic thickness swelling rate of compression moded wood fiberboard and wood fiber/polymer composites, Compos. Part A 37 (2006) 1276e1285. [7] S.G. Bajwa, D.S. Bajwa, G. Holt, Optimal substitution of cotton burr and linters in thermoplastic composites, For. Prod. J. 59 (10) (2009) 40e46. [8] W.V. Srubar, S. Pilla, Z.C. Wright, C.A. Ryan, J.P. Greene, Mechanisms and impact of fiber-matrix compatibilization techniques on the material characteristics of PHBV/oak wood flour engineered biobased composites, Compos. Sci. Technol. 72 (2012) 708e715. [9] D. Maldas, B.V. Kotka, Role of coupling agents on the performance of woodflour-filled polypropylene composites, Int. J. Polym. Mater. 27 (1994) 22e88. [10] C. Daneault, B.V. Kokta, D. Maldas, Grafting of vinyl monomers onto wood fibers initiated by peroxidation, Polym. Bull. 20 (1988) 137e141. [11] C.K. Hong, N. Kim, S.L. Kang, Mechanical properties of maleic anhydride treated jute/polypropylene composites, Plast. Rubber Compos. 37 (2008) 325e330.
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