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Weathering properties of coextruded polypropylene-based composites containing inorganic pigments
Polymer Degradation and Stability 120 (2015) 10e16
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Weathering properties of coextruded polypropylene-based composites containing inorganic pigments €rki Svetlana Butylina*, Ossi Martikka, Timo Ka Fibre Composite Laboratory, Lappeenranta University of Technology, P.O. Box 20, 53850 Lappeenranta, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2015 Received in revised form 9 May 2015 Accepted 1 June 2015 Available online 3 June 2015
This study concerns the weathering properties of coextruded polypropylene-based composites containing pigments. Three different pigments were incorporated in the shell layer of the composites: iron oxide, titanium dioxide and zinc oxide. The surface colour, surface gloss and tensile properties were tested. In addition, the weathered surfaces were studied by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The surfaces of the composites containing inorganic pigments were found to have fewer cracks after 500 h of weathering than the surface of the reference composite. The results revealed that the composites containing titanium oxide pigment exhibited better colour stability than the composites made with the other pigments. In spite of its high colour stability in weathering, the tensile properties (strength, Young's modulus and elongation at break) of the composite containing titanium oxide were reduced by weathering. The FTIR analysis revealed that the composite containing zinc oxide had a stabilising effect on polypropylene photo-degradation, which correlates well with the results of mechanical testing, showing that this composite retained its mechanical properties after weathering. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Wood-polymer composite Inorganic pigment Coextrusion
1. Introduction Wood-plastic composites (WPCs) have experienced significant market expansion in recent years as a replacement for solid wood, mainly in outdoor applications such as railings, decking, landscaping timbers, fencing, playground equipment, window and door frames, etc. [1]. The outdoor applications expose WPCs to moisture, fungi, freeze-thaw actions, and solar radiation. Both the wood fibre and the polymer matrix experience photo-degradation upon exposure to ultraviolet light, and the weathering of WPCs results in severe discolouration and a modest loss of mechanical properties [2]. Current approaches to improve the weathering resistance of WPCs focus on bulk WPCs, i.e., incorporation of additives into the entire product or surface treatment of the wood fibre [3]. The addition of photo-stabilisers and pigments into the entire WPCs provides protection against their discolouration caused by ultraviolet (UV) radiation. However, weathering primarily occurs at the surface of the material. Thus, a cost-effective means to deal with
* Corresponding author. E-mail address: [email protected] (S. Butylina). http://dx.doi.org/10.1016/j.polymdegradstab.2015.06.004 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
weathering would involve adding photo-stabiliser protection only in the surface layer of composite samples. Coextrusion is one of the methods for providing a protective surface. Coextrusion can produce a multi-layered product with different properties at the outer and inner layers, thus offering different properties between the surface and the bulk [3]. Coextrusion in a WPC was first reported with a combination of a WPC core and a pure plastic shell layer [4]. In their comparative research on non-coextruded and coextruded WPCs with a pure high-density polyethylene (HDPE) or a pure polypropylene (PP) shell, Stark and Matuana [2] showed that the presence of the shell layer reduces the moisture uptake significantly. Inorganic pigments, such as titanium dioxide (TiO2) and zinc oxide (ZnO) have received great attention in recent years because the absorption ability of TiO2 and ZnO for UV rays is well known [5]. However, the number of studies on the effect of inorganic pigments on the weathering properties of wood-polymer composites is limited [6]. Deka and Maji [7,8] have studied the UV resistance of wood-polymer composites made with the addition of nanoclay/ titanium dioxide and nanoclay/zinc oxide. It was shown in their study that both TiO2 and ZnO, added the in concentration 3 parts per 100 parts of the blend improved the UV resistance of the composites. Stark and Matuana [9] have shown that the addition of
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nano-sized titanium dioxide in the high-density-polyethylene (HDPE) shell layer of wood flour/HDPE composite enhanced its colour stability noticeably. Zhang et al. [10] studied the effect of four different iron oxide pigments on wood fibre/HDPE composites in accelerated weathering. The results showed that the iron oxide pigments in concentration 2.28% exhibited a very good performance in the colour stabilisation of weathered wood fibre/HDPE composites, but did not appear to aid in maintaining the mechanical properties of the composites, except for the carbon blackcontaining iron oxide black pigment [10]. In their study on UV degradation of wood-flour/HDPE composites, Du et al. [11] compared the effects of titanium dioxide, iron oxide and carbon black on the durability of composites. Carbon black was found to have a more positive effect on colour stability than the other pigments [11]. Besides their UV stabilisation effect, inorganic pigments can be used as fillers for polymers, similarly to calcium carbonate, to enhance mechanical properties such as yield stress and modulus of elasticity [12,13]. In this work, the role of inorganic pigments in a shell layer of wood-polypropylene composites in their weathering behaviour is studied. Three different pigments were incorporated in the shell layer: iron oxide, titanium dioxide and zinc oxide. The coextruded composites were exposed to accelerated weathering. The changes of surface colour, gloss, surface morphology, surface chemistry and mechanical properties were determined. 2. Materials and methods 2.1. Materials Polypropylene (PP), Eltex P HY001P (Ineos), with density 0.910 g/cm3 and melt mass-flow rate 45 g/10 min (230 C/2.16 kg) was used to produce the composites. Pulp cellulose (PC) was delivered by UPM, Finland. The coupling agent was maleated polypropylene (MAPP), OREVAC® CA 100 (Atofina, France). Structol® TPW 113 (Ohio, USA) was used as lubricant. Brown Remafin® masterbatch (Clariant Masterbatches, Germany) was used as colourant. Three different pigments were used in this study: synthetic iron oxide Fe3O4, Bayferoxx® 318 (Lanxess Deutschland GmbH), zinc oxide (US Research Nanomaterials, Inc.), and titanium dioxide pigment, Sactleben R660 (Sachtleben Chemie GmbH) (see Table 1). 2.2. Processing of composites The basic compositions of the core and shell layers were the same for all the studied composites. A blend of 64% wood flour, 10% talc, 22% PP, 3% MAPP, and 1% lubricant was used to produce the core layer. A blend of 33% cellulose pulp, 10% talc, 50% PP, 3% MAPP, 3% lubricant, and 1% of brown Remafin masterbatch was used to produce the shell layer. The submicron-sized inorganic pigments were mixed with a ready-made blend of the shell layer in proportions: 3 parts per 100 parts of the blend. The type and amount of pigments incorporated in the shells are listed in Table 2. A coextrusion system including a Weber CE 7.2 conical twinscrew extruder and a fiberEX extruder, was used to produce the core and shell layer, correspondingly. The processing temperatures
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Table 2 Inorganic pigments in the shell layer of the composites. Composite
Reference Comp_Fe Comp_Ti Comp_Zn
Pigment [parts]
Blend of shell layer [parts]
Fe3O4
TiO2
ZnO
e 3 e e
e e 3 e
e e e 3
100 100 100 100
in both extrusion processes were between 175 and 200 C. The schematic coextrusion profile is given in Fig. 1. 2.3. Weathering of composites The resistance of the composites to photo-degradation was tested using a Q-SUN Xe-3 tester. The weathering procedure consisted of 102 min of UV irradiation (with an average irradiance of 0.51 W/m2 at 340 nm) at the temperature of 38 C and (50 ± 10)% relative humidity, followed by 18 min of water spraying, according to the ISO 4892-2:2013 standard. 2.4. Testing of composites For the weathering test, the rectangular samples (180 mm 20 mm x 5 mm) were cut from the middle part of the coextrusion profile (Fig. 1). The tensile samples were cut as dumbbell-shaped according to EN ISO 527e1:2012. A CNC router was used to cut the samples. The surface colour of the treated and untreated composites was measured with a Minolta CM-2500d spectrophotometer (Konika Minolta Sensing Inc., Japan). The CIELAB colour system was used to measure the surface colour in L, a and b coordinates. The colour difference was calculated as outlined in ISO 7724 according to the following equation:
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDLÞ2 þ ðDaÞ2 þ ðDbÞ2
(1)
where DL, Da and Db represent the differences between the initial and final values of L, a and b, respectively. The surface colour for 15 replicates was measured at five locations on each composite sample. The surface gloss of the composites before and after weathering was measured with a Novo-Gloss Trio™ statistical glossmeter (Rhopoint Instruments LTD, UK), according to the EN ISO 2813, with the test angle of 60 . The 60 geometry is recommended for WPC products according to EN 15534-1:2014(E). The gloss value for each sample was averaged from measurements of three locations. Tensile properties (strength, Young's modulus and elongation at break) of the composites were measured in accordance with the EN ISO 527-1:2012 standard using a Zwick/Roell Z20 testing machine. Prior to the test, the samples were conditioned at 23 C and 65% relative humidity for 48 h. Scanning electron microscopy (SEM) was performed with a Jeol JSM-5800 LV scanning microscope operating at 10 kV. Prior to the analysis, the fracture surfaces were covered with a layer of gold
Table 1 Characteristics of pigments (according to the manufacturer's datasheet). Pigment Iron oxide Titanium dioxide Zinc oxide
Fe3O4 TiO2 ZnO
Content min. [%]
Average size [nm]
Specific gravity [g/cm3]
96.5 93 99.9
200 220 80e200
4.6 4.0 5.6
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Fig. 1. Profile of coextruded WPC.
using a sputter coater. Elemental analysis was performed by an energy-dispersive X-ray spectrophotometer. Fourier transform infrared (FTIR) analysis of the composite samples before and after weathering was performed using a Frontier FTIR spectrometer (Perkin Elmer) with a universal ATR sampling accessory (Perkin Elmer). The scanning range was from 400 to 4000 cm1. The FTIR spectra for all the samples were recorded with 20 scans with 4 cm1 resolution. The background spectra were taken in an empty chamber. A baseline correction was performed for all the spectra. The carbonyl index was determined by the following equation:
. Carbonyl index ¼ Ið1736Þ Ið2920Þ
(2)
where I denotes the intensity. The peak intensity in the carbonyl region was normalised by using the intensity of the peak at 2920 cm1, which corresponds to asymmetric stretching vibrations in methylene groups. The peak at 2920 cm1 can be used as a reference because it remains unchanged during the photodegradation process. 2.5. Statistical analysis To determine the effect of weathering on the properties of the WPCs, a two-sample t-test was carried out with an a significance value of 0.05, comparing the weathered and non-weathered data. All statistical analyses were performed using Statgraphics Plus software (v. 4).
lightness of the composites. It was found that in all cases the change of lightness was a dominating factor determining the change of colour of the composites. The graphs of DL ¼ F(t) and DE ¼ F(t) were identical. Off all the composites, the composite made with titanium pigment showed the best result: the lowest lightness and colour change. The weathering behaviour of the composite containing zinc was similar to that of the reference composite. In the case of the composite containing iron pigment, at the period up to 300 h its behaviour was closer to the composite containing titanium, but by the end of 500 h of weathering its lightness and colour change were much closer to the values obtained for the reference composite and the composite containing zinc. Fig. 3 shows the gloss values measured at angle 60 of the composites measured before and after weathering. In general, the composites containing inorganic pigments were found to be glossier than the reference composite. The high gloss of the composites containing inorganic pigments had strong correlation with the roughness of the surface of the composites. According to their surface roughness, the untreated composites can be placed in the following order: Ref > Comp_Ti and Comp_Zn > Comp_Fe: 5.58 (±0.87) > 3.29 (±0.80) and 3.19 (±0.96) > 1.58 (±0.68). The composite made with iron oxide was determined to have the smoothest surface and the highest gloss. However, as can be seen in Fig. 3, the standard deviation of the measured values of gloss is high, which means that the surfaces of the composites were not homogeneous: some spots were rough and matt and others smoother and glossier. The mean values of the gloss of the composites shown in Fig. 3 decreased with weathering; the decrease of gloss was more noticeable for the parts of the composite samples which possessed
3. Results and discussion Fig. 2 shows the lightness change and total colour change for all the studied WPCs as a function of exposure time. The nonweathered reference composite samples had red-brown colour and their initial lightness was 46.2 ± 0.8. The addition of titanium pigment increased the lightness (LComp_Ti ¼ 57.5 ± 0.6) of the composites, while the addition of iron pigment decreased the lightness (LComp_Zn ¼ 27.4 ± 2.5) compared to that of the reference composite. The samples of composites containing zinc pigment had lightness and colour similar to the reference composite. As can be seen in Fig. 2, weathering resulted in an increase in the
Fig. 3. Surface gloss of composites before and after weathering.
Fig. 2. Lightness change (DL) and total colour change (DE) as functions of exposure time.
S. Butylina et al. / Polymer Degradation and Stability 120 (2015) 10e16
higher gloss initially. Figs. 4e7 show SEM images of all the composites before and after weathering. Before exposure the composites had relatively smooth surfaces. The numerous lighter spots visible on the surface of both initial and weathered composites correspond to talc (Mg3Si4O10(OH)2), which was proved by energy dispersive X-ray spectrometric (EDS) analysis (Fig. 8). In contrast to talc particles, which are easily distinguished in the surface layer of the composites, the tiny pigments were not perceptible on the SEM images of composite surfaces presented in Figs. 4e7. The distribution of the studied pigments on the surfaces of the composites was checked by EDS using elemental mapping. The EDS analysis revealed uniform distribution of elements (Fe, Ti, and Zn) in the surface layer of the corresponding composites. Weathering resulted in degradation of the surface layer. Degradation was more obvious in the case of the reference composite (Fig. 4B): part of the top polymer surface layer had disappeared, leaving cellulose fibres exposed. The surfaces of the composites containing inorganic pigments were less affected by weathering after 500 h of exposure, only a few cracks were detected on their surfaces. The smaller amount of cracks on the surfaces of the pigment-containing composites compared to the reference composite may be explained by the UV shielding ability of the studied pigments. In general, cracks of polymer layer appeared to start in places where the cellulose fibres or particles of the mineral filler were in close vicinity to the surface. As shown in Fig. 7B and C, crack starts in the proximity of the talc particle. EDS analysis (Fig. 8) shows the presence of iron impurities in the composition of talc, and therefore talc can serve as a catalyst for the photo-degradation of polypropylene. Azuma et al. [14], in their study on outdoor and accelerated weathering of polypropylene/talc composites report that talc accelerates the photo-degradation of PP/talc composites. Evaluation of structural changes on the surfaces of the weathered composites was performed by FTIR. The structural changes occurring in the composites with the increase of weathering time were followed by monitoring the absorbance in the region corresponding to the carbonyl group (1600e1800 cm1) of the FTIR spectra (Fig. 9). As can be seen in Fig. 9, the carbonyl group absorbance increased with the time of exposure. A more obvious change occurred in the composite containing iron oxide pigment; with the increase of weathering time, the peaks became higher and broader. Carbonyl indices for the composites calculated by using Eq. 2 are shown in Fig. 10. The carbonyl index is the most commonly used parameter for assessing the degree of degradation [15]. The initial values of the carbonyl indices for all the composites were similar and differed from zero. The presence of carbonyl groups in the nonweathered composites may be explained by oxidation during the composite processing. The carbonyl indices of Comp_Ti and the reference composite were similar; the carbonyl indices started to
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increase after 350 h of exposure for both. The composite containing zinc oxide pigment showed the lowest change of all in the carbonyl index. On the other hand, the composite containing iron oxide had the highest change in the carbonyl index and the fastest rate of its change among all the composites, which means that iron oxide acts as a pro-oxidant that enhances photo-degradation. Surprisingly, the carbonyl index of the weathered composite containing titanium dioxide pigment was higher than that of the composite containing zinc oxide, in spite of the stabilisation effect of titanium dioxide on the colour of the weathered composite discussed above. The titanium dioxide pigment can mask discolouration to some degree by causing a sharp reduction in light transmission [16]. Table 3 shows the tensile strength and modulus of the composites before and after weathering. For the non-weathered composites, statistical analysis showed no significant difference between the tensile strength of the reference composite and the composites containing either iron oxide or titanium oxide. The addition of zinc oxide reduced the tensile strength of the composite compared to the reference. Chang et al. [17] have shown that the tensile strength of ultra-high molecular weight polyethylene (UHMWPE) decreased after the addition of nano-ZnO. According to Chang et al. [17], the reduction in tensile strength indicates that the adhesion between the nano-ZnO and UHMWPE matrix is not significant for promoting greater interfacial bonding, which would enhance the tensile strength value. The addition of iron oxide and zinc oxide resulted in insignificant changes in the tensile modulus of the composites in comparison to the reference composite. The composite made with titanium dioxide had the highest modulus among all the composites. Deka and Maji [7] observed an increase in the tensile strength and modulus of WPCs after incorporation of TiO2 nanopowder (3 parts per 100 parts of resin). Esthappan et al. [18] have shown that the addition of titanium dioxide in the PP resulted in an increase in its tensile strength and modulus. In the present study, the addition of titanium dioxide was found to improve the tensile modulus, but it did not have any effect on the tensile strength. It is evident that weathering reduced the tensile strength of the composites (Table 3). For all the composites, except for the composite containing zinc, the reduction of tensile strength was estimated to be significant. The reduction of tensile strength after accelerated weathering has been previously reported by Beg and Pickering [19] for wood-polypropylene composites, by Zhang et al. [10] for wood-fibre/HDPE composites containing iron oxide, by Devi and Maji [20] for wood-polymer composites containing titanium dioxide. In their study on the photo-degradation of PP/ZnO nanocomposites, Zhao and Li [21] report that the incorporation of zinc oxide nanoparticles promotes the recovery of the tensile strength of UV-treated nanocomposites. Similarly to tensile strength, the tensile modulus of the composites containing iron oxide and titanium dioxide decreased by
Fig. 4. SEM images of the surfaces of the reference composites: A. initial (magnification x100), B. after 500 h of accelerated weathering (magnification x100), and C. after 500 h of accelerated weathering (magnification x500).
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Fig. 5. SEM images of the surfaces of the composites containing iron pigment: A. initial (magnification x100), and B. after 500 h of accelerated weathering (magnification x100).
Fig. 6. SEM images of the surfaces of the composites containing titanium pigment: A. initial (magnification x100), and B. after 500 h of accelerated weathering (magnification x100).
about 10% after weathering. The slight decrease (3%) of tensile modulus of the zinc-containing composite after weathering, which was estimated to be statistically not significant, was also in line with the tensile strength change (4%). In the case of the reference composite, 7% decrease in tensile strength after weathering was found, but the modulus of this composite stayed unchanged after weathering. Elongation at break of the composites evaluated before and after weathering is shown in Fig. 11. The elongation at break of all the composites is relatively low (less than 1%), compared to the elongation at break 45% for the pure polypropylene [22]. Such low values can be explained by the presence of fillers. Denac et al. [22] showed that the incorporation of 12% talc to the polypropylene matrix decreased the elongation at break from 45% to 2%. After weathering, the elongation at break was reduced for all the composites, except for the composite containing zinc oxide, which retained its elongation at break. The change of elongation at break correlates very well with the chemical change during photo-
degradation [23]. It seems that the zinc oxide pigment acts as a stabiliser and reduces the degradation of polypropylene in the shell layer of the composite. Fibre swelling due to moisture absorption is primarily responsible for the loss in mechanical properties after weathering [24]. The loss in strength is due to moisture penetration into the WPC, which degrades the wood-polymer interface, decreasing the stress transfer efficiency from the matrix to the fibre. The weight gain and thickness swelling of the studied composites were determined after weathering, and they are presented in Fig. 12. Weight gain in combination with thickness swelling serves as an indicator of moisture absorption by composites in the process of weathering. The negative value of the weight gain of the reference composite can be explained by removal of part of the photo-degraded surface layer of the composite. Lignin and water soluble products can leach from samples [25]. In their work on accelerated weathering of an aspen fibre/polypropylene composite Rowell et al. [26] showed that a specimen consisting 30% wood
Fig. 7. SEM images of the surfaces of the composites containing zinc pigment: A. initial (magnification x100), B. after 500 h of accelerated weathering (magnification x100) and C. after 500 h of accelerated weathering (magnification x500).
S. Butylina et al. / Polymer Degradation and Stability 120 (2015) 10e16
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Fig. 8. EDS spectra of talc particle identified on the surface of the composite.
Fig. 9. FTIR spectra (carbonyl region) of the composites, obtained at different weathering times.
Table 3 Tensile strength and modulus of composites determined before and after weathering. Composite
Tensile strength [MPa]
Reference Comp_Fe Comp_Ti Comp_Zn
Fig. 10. Carbonyl index of the composites as a function of weathering time.
9.14 9.05 9.04 8.59
± ± ± ±
Tensile modulus [GPa]
afterweathering ft
ftinitial 0.51 0.26 0.59 0.42
8.50 8.27 8.28 8.24
± ± ± ±
0.44 0.16 0.41 0.46
(S) (S) (S) (NS)
afterweathering
Etinitial 3.09 3.17 3.37 3.00
± ± ± ±
Et 0.16 0.14 0.15 0.18
3.08 2.96 3.02 2.90
± ± ± ±
0.17 0.13 0.12 0.16
(NS) (S) (S) (NS)
S means that the difference between the initial and weathered samples of the same composite is statistically significant at P-value less than 0.05; NS means that it is not significant.
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S. Butylina et al. / Polymer Degradation and Stability 120 (2015) 10e16
Acknowledgement The authors would like to thank the Jenny and Antti Wihuri Foundation for their financial support to this project (No. 130029). References
Fig. 11. Elongation at break of the composites before and after weathering.
Fig. 12. Weight gain (WG) and thickness swelling (TS) of composites after weathering.
fibre, 68% PP and 2% MAPP lost 2.7% of its initial weight after 2000 h of UV exposure. The degradation of the surface layer of the reference composite is shown in Fig. 4B. In contrast to the reference composite, the composites containing inorganic pigments showed an increase in weight after weathering. A small increase of weight was due to moisture uptake, which was greater than the material loss induced by photodegradation. The absorbed moisture caused swelling of wood fibres, which can be evaluated as thickness swelling of the composites. In spite of the weight loss found in the reference composite, its thickness swelling was highest among all the composites. The more severe degradation of the surface layer of the reference composite led to exposure of cellulose fibres (Fig. 4C) and made this composite more sensitive to moisture. The lower thickness swelling of the composites containing inorganic pigments agreed well with the lighter degradation of their surfaces compared to the reference. 4. Conclusions The results of this study indicate that the incorporation of inorganic pigments into the shell layer of wood-polymer composites can impart significant improvements in their resistance to weathering. According to the colour change results, it was evident that the composite containing titanium dioxide was better in the stabilisation of colour than the other pigments. It was found in the SEM study that the density of surface cracking was dramatically reduced in the composites containing inorganic pigments. Serious surface deterioration after weathering was detected only in the case of the reference composite. The tensile test measurements indicated that weathering affected the tensile properties of all the composites adversely. However, the composite containing zinc oxide was capable to retain its tensile properties. The retention of the tensile properties correlated well with the lowest carbonyl index obtained for the zinc oxide-containing composite. Photo-degradation of the surface layer and moisture absorption by the composites in the process of weathering were suggested to be the reasons for the decrease of their mechanical properties.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Weathering properties of coextruded polypropylene-based composites containing inorganic pigments €rki Svetlana Butylina*, Ossi Martikka, Timo Ka Fibre Composite Laboratory, Lappeenranta University of Technology, P.O. Box 20, 53850 Lappeenranta, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2015 Received in revised form 9 May 2015 Accepted 1 June 2015 Available online 3 June 2015
This study concerns the weathering properties of coextruded polypropylene-based composites containing pigments. Three different pigments were incorporated in the shell layer of the composites: iron oxide, titanium dioxide and zinc oxide. The surface colour, surface gloss and tensile properties were tested. In addition, the weathered surfaces were studied by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The surfaces of the composites containing inorganic pigments were found to have fewer cracks after 500 h of weathering than the surface of the reference composite. The results revealed that the composites containing titanium oxide pigment exhibited better colour stability than the composites made with the other pigments. In spite of its high colour stability in weathering, the tensile properties (strength, Young's modulus and elongation at break) of the composite containing titanium oxide were reduced by weathering. The FTIR analysis revealed that the composite containing zinc oxide had a stabilising effect on polypropylene photo-degradation, which correlates well with the results of mechanical testing, showing that this composite retained its mechanical properties after weathering. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Wood-polymer composite Inorganic pigment Coextrusion
1. Introduction Wood-plastic composites (WPCs) have experienced significant market expansion in recent years as a replacement for solid wood, mainly in outdoor applications such as railings, decking, landscaping timbers, fencing, playground equipment, window and door frames, etc. [1]. The outdoor applications expose WPCs to moisture, fungi, freeze-thaw actions, and solar radiation. Both the wood fibre and the polymer matrix experience photo-degradation upon exposure to ultraviolet light, and the weathering of WPCs results in severe discolouration and a modest loss of mechanical properties [2]. Current approaches to improve the weathering resistance of WPCs focus on bulk WPCs, i.e., incorporation of additives into the entire product or surface treatment of the wood fibre [3]. The addition of photo-stabilisers and pigments into the entire WPCs provides protection against their discolouration caused by ultraviolet (UV) radiation. However, weathering primarily occurs at the surface of the material. Thus, a cost-effective means to deal with
* Corresponding author. E-mail address: [email protected] (S. Butylina). http://dx.doi.org/10.1016/j.polymdegradstab.2015.06.004 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
weathering would involve adding photo-stabiliser protection only in the surface layer of composite samples. Coextrusion is one of the methods for providing a protective surface. Coextrusion can produce a multi-layered product with different properties at the outer and inner layers, thus offering different properties between the surface and the bulk [3]. Coextrusion in a WPC was first reported with a combination of a WPC core and a pure plastic shell layer [4]. In their comparative research on non-coextruded and coextruded WPCs with a pure high-density polyethylene (HDPE) or a pure polypropylene (PP) shell, Stark and Matuana [2] showed that the presence of the shell layer reduces the moisture uptake significantly. Inorganic pigments, such as titanium dioxide (TiO2) and zinc oxide (ZnO) have received great attention in recent years because the absorption ability of TiO2 and ZnO for UV rays is well known [5]. However, the number of studies on the effect of inorganic pigments on the weathering properties of wood-polymer composites is limited [6]. Deka and Maji [7,8] have studied the UV resistance of wood-polymer composites made with the addition of nanoclay/ titanium dioxide and nanoclay/zinc oxide. It was shown in their study that both TiO2 and ZnO, added the in concentration 3 parts per 100 parts of the blend improved the UV resistance of the composites. Stark and Matuana [9] have shown that the addition of
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nano-sized titanium dioxide in the high-density-polyethylene (HDPE) shell layer of wood flour/HDPE composite enhanced its colour stability noticeably. Zhang et al. [10] studied the effect of four different iron oxide pigments on wood fibre/HDPE composites in accelerated weathering. The results showed that the iron oxide pigments in concentration 2.28% exhibited a very good performance in the colour stabilisation of weathered wood fibre/HDPE composites, but did not appear to aid in maintaining the mechanical properties of the composites, except for the carbon blackcontaining iron oxide black pigment [10]. In their study on UV degradation of wood-flour/HDPE composites, Du et al. [11] compared the effects of titanium dioxide, iron oxide and carbon black on the durability of composites. Carbon black was found to have a more positive effect on colour stability than the other pigments [11]. Besides their UV stabilisation effect, inorganic pigments can be used as fillers for polymers, similarly to calcium carbonate, to enhance mechanical properties such as yield stress and modulus of elasticity [12,13]. In this work, the role of inorganic pigments in a shell layer of wood-polypropylene composites in their weathering behaviour is studied. Three different pigments were incorporated in the shell layer: iron oxide, titanium dioxide and zinc oxide. The coextruded composites were exposed to accelerated weathering. The changes of surface colour, gloss, surface morphology, surface chemistry and mechanical properties were determined. 2. Materials and methods 2.1. Materials Polypropylene (PP), Eltex P HY001P (Ineos), with density 0.910 g/cm3 and melt mass-flow rate 45 g/10 min (230 C/2.16 kg) was used to produce the composites. Pulp cellulose (PC) was delivered by UPM, Finland. The coupling agent was maleated polypropylene (MAPP), OREVAC® CA 100 (Atofina, France). Structol® TPW 113 (Ohio, USA) was used as lubricant. Brown Remafin® masterbatch (Clariant Masterbatches, Germany) was used as colourant. Three different pigments were used in this study: synthetic iron oxide Fe3O4, Bayferoxx® 318 (Lanxess Deutschland GmbH), zinc oxide (US Research Nanomaterials, Inc.), and titanium dioxide pigment, Sactleben R660 (Sachtleben Chemie GmbH) (see Table 1). 2.2. Processing of composites The basic compositions of the core and shell layers were the same for all the studied composites. A blend of 64% wood flour, 10% talc, 22% PP, 3% MAPP, and 1% lubricant was used to produce the core layer. A blend of 33% cellulose pulp, 10% talc, 50% PP, 3% MAPP, 3% lubricant, and 1% of brown Remafin masterbatch was used to produce the shell layer. The submicron-sized inorganic pigments were mixed with a ready-made blend of the shell layer in proportions: 3 parts per 100 parts of the blend. The type and amount of pigments incorporated in the shells are listed in Table 2. A coextrusion system including a Weber CE 7.2 conical twinscrew extruder and a fiberEX extruder, was used to produce the core and shell layer, correspondingly. The processing temperatures
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Table 2 Inorganic pigments in the shell layer of the composites. Composite
Reference Comp_Fe Comp_Ti Comp_Zn
Pigment [parts]
Blend of shell layer [parts]
Fe3O4
TiO2
ZnO
e 3 e e
e e 3 e
e e e 3
100 100 100 100
in both extrusion processes were between 175 and 200 C. The schematic coextrusion profile is given in Fig. 1. 2.3. Weathering of composites The resistance of the composites to photo-degradation was tested using a Q-SUN Xe-3 tester. The weathering procedure consisted of 102 min of UV irradiation (with an average irradiance of 0.51 W/m2 at 340 nm) at the temperature of 38 C and (50 ± 10)% relative humidity, followed by 18 min of water spraying, according to the ISO 4892-2:2013 standard. 2.4. Testing of composites For the weathering test, the rectangular samples (180 mm 20 mm x 5 mm) were cut from the middle part of the coextrusion profile (Fig. 1). The tensile samples were cut as dumbbell-shaped according to EN ISO 527e1:2012. A CNC router was used to cut the samples. The surface colour of the treated and untreated composites was measured with a Minolta CM-2500d spectrophotometer (Konika Minolta Sensing Inc., Japan). The CIELAB colour system was used to measure the surface colour in L, a and b coordinates. The colour difference was calculated as outlined in ISO 7724 according to the following equation:
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDLÞ2 þ ðDaÞ2 þ ðDbÞ2
(1)
where DL, Da and Db represent the differences between the initial and final values of L, a and b, respectively. The surface colour for 15 replicates was measured at five locations on each composite sample. The surface gloss of the composites before and after weathering was measured with a Novo-Gloss Trio™ statistical glossmeter (Rhopoint Instruments LTD, UK), according to the EN ISO 2813, with the test angle of 60 . The 60 geometry is recommended for WPC products according to EN 15534-1:2014(E). The gloss value for each sample was averaged from measurements of three locations. Tensile properties (strength, Young's modulus and elongation at break) of the composites were measured in accordance with the EN ISO 527-1:2012 standard using a Zwick/Roell Z20 testing machine. Prior to the test, the samples were conditioned at 23 C and 65% relative humidity for 48 h. Scanning electron microscopy (SEM) was performed with a Jeol JSM-5800 LV scanning microscope operating at 10 kV. Prior to the analysis, the fracture surfaces were covered with a layer of gold
Table 1 Characteristics of pigments (according to the manufacturer's datasheet). Pigment Iron oxide Titanium dioxide Zinc oxide
Fe3O4 TiO2 ZnO
Content min. [%]
Average size [nm]
Specific gravity [g/cm3]
96.5 93 99.9
200 220 80e200
4.6 4.0 5.6
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Fig. 1. Profile of coextruded WPC.
using a sputter coater. Elemental analysis was performed by an energy-dispersive X-ray spectrophotometer. Fourier transform infrared (FTIR) analysis of the composite samples before and after weathering was performed using a Frontier FTIR spectrometer (Perkin Elmer) with a universal ATR sampling accessory (Perkin Elmer). The scanning range was from 400 to 4000 cm1. The FTIR spectra for all the samples were recorded with 20 scans with 4 cm1 resolution. The background spectra were taken in an empty chamber. A baseline correction was performed for all the spectra. The carbonyl index was determined by the following equation:
. Carbonyl index ¼ Ið1736Þ Ið2920Þ
(2)
where I denotes the intensity. The peak intensity in the carbonyl region was normalised by using the intensity of the peak at 2920 cm1, which corresponds to asymmetric stretching vibrations in methylene groups. The peak at 2920 cm1 can be used as a reference because it remains unchanged during the photodegradation process. 2.5. Statistical analysis To determine the effect of weathering on the properties of the WPCs, a two-sample t-test was carried out with an a significance value of 0.05, comparing the weathered and non-weathered data. All statistical analyses were performed using Statgraphics Plus software (v. 4).
lightness of the composites. It was found that in all cases the change of lightness was a dominating factor determining the change of colour of the composites. The graphs of DL ¼ F(t) and DE ¼ F(t) were identical. Off all the composites, the composite made with titanium pigment showed the best result: the lowest lightness and colour change. The weathering behaviour of the composite containing zinc was similar to that of the reference composite. In the case of the composite containing iron pigment, at the period up to 300 h its behaviour was closer to the composite containing titanium, but by the end of 500 h of weathering its lightness and colour change were much closer to the values obtained for the reference composite and the composite containing zinc. Fig. 3 shows the gloss values measured at angle 60 of the composites measured before and after weathering. In general, the composites containing inorganic pigments were found to be glossier than the reference composite. The high gloss of the composites containing inorganic pigments had strong correlation with the roughness of the surface of the composites. According to their surface roughness, the untreated composites can be placed in the following order: Ref > Comp_Ti and Comp_Zn > Comp_Fe: 5.58 (±0.87) > 3.29 (±0.80) and 3.19 (±0.96) > 1.58 (±0.68). The composite made with iron oxide was determined to have the smoothest surface and the highest gloss. However, as can be seen in Fig. 3, the standard deviation of the measured values of gloss is high, which means that the surfaces of the composites were not homogeneous: some spots were rough and matt and others smoother and glossier. The mean values of the gloss of the composites shown in Fig. 3 decreased with weathering; the decrease of gloss was more noticeable for the parts of the composite samples which possessed
3. Results and discussion Fig. 2 shows the lightness change and total colour change for all the studied WPCs as a function of exposure time. The nonweathered reference composite samples had red-brown colour and their initial lightness was 46.2 ± 0.8. The addition of titanium pigment increased the lightness (LComp_Ti ¼ 57.5 ± 0.6) of the composites, while the addition of iron pigment decreased the lightness (LComp_Zn ¼ 27.4 ± 2.5) compared to that of the reference composite. The samples of composites containing zinc pigment had lightness and colour similar to the reference composite. As can be seen in Fig. 2, weathering resulted in an increase in the
Fig. 3. Surface gloss of composites before and after weathering.
Fig. 2. Lightness change (DL) and total colour change (DE) as functions of exposure time.
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higher gloss initially. Figs. 4e7 show SEM images of all the composites before and after weathering. Before exposure the composites had relatively smooth surfaces. The numerous lighter spots visible on the surface of both initial and weathered composites correspond to talc (Mg3Si4O10(OH)2), which was proved by energy dispersive X-ray spectrometric (EDS) analysis (Fig. 8). In contrast to talc particles, which are easily distinguished in the surface layer of the composites, the tiny pigments were not perceptible on the SEM images of composite surfaces presented in Figs. 4e7. The distribution of the studied pigments on the surfaces of the composites was checked by EDS using elemental mapping. The EDS analysis revealed uniform distribution of elements (Fe, Ti, and Zn) in the surface layer of the corresponding composites. Weathering resulted in degradation of the surface layer. Degradation was more obvious in the case of the reference composite (Fig. 4B): part of the top polymer surface layer had disappeared, leaving cellulose fibres exposed. The surfaces of the composites containing inorganic pigments were less affected by weathering after 500 h of exposure, only a few cracks were detected on their surfaces. The smaller amount of cracks on the surfaces of the pigment-containing composites compared to the reference composite may be explained by the UV shielding ability of the studied pigments. In general, cracks of polymer layer appeared to start in places where the cellulose fibres or particles of the mineral filler were in close vicinity to the surface. As shown in Fig. 7B and C, crack starts in the proximity of the talc particle. EDS analysis (Fig. 8) shows the presence of iron impurities in the composition of talc, and therefore talc can serve as a catalyst for the photo-degradation of polypropylene. Azuma et al. [14], in their study on outdoor and accelerated weathering of polypropylene/talc composites report that talc accelerates the photo-degradation of PP/talc composites. Evaluation of structural changes on the surfaces of the weathered composites was performed by FTIR. The structural changes occurring in the composites with the increase of weathering time were followed by monitoring the absorbance in the region corresponding to the carbonyl group (1600e1800 cm1) of the FTIR spectra (Fig. 9). As can be seen in Fig. 9, the carbonyl group absorbance increased with the time of exposure. A more obvious change occurred in the composite containing iron oxide pigment; with the increase of weathering time, the peaks became higher and broader. Carbonyl indices for the composites calculated by using Eq. 2 are shown in Fig. 10. The carbonyl index is the most commonly used parameter for assessing the degree of degradation [15]. The initial values of the carbonyl indices for all the composites were similar and differed from zero. The presence of carbonyl groups in the nonweathered composites may be explained by oxidation during the composite processing. The carbonyl indices of Comp_Ti and the reference composite were similar; the carbonyl indices started to
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increase after 350 h of exposure for both. The composite containing zinc oxide pigment showed the lowest change of all in the carbonyl index. On the other hand, the composite containing iron oxide had the highest change in the carbonyl index and the fastest rate of its change among all the composites, which means that iron oxide acts as a pro-oxidant that enhances photo-degradation. Surprisingly, the carbonyl index of the weathered composite containing titanium dioxide pigment was higher than that of the composite containing zinc oxide, in spite of the stabilisation effect of titanium dioxide on the colour of the weathered composite discussed above. The titanium dioxide pigment can mask discolouration to some degree by causing a sharp reduction in light transmission [16]. Table 3 shows the tensile strength and modulus of the composites before and after weathering. For the non-weathered composites, statistical analysis showed no significant difference between the tensile strength of the reference composite and the composites containing either iron oxide or titanium oxide. The addition of zinc oxide reduced the tensile strength of the composite compared to the reference. Chang et al. [17] have shown that the tensile strength of ultra-high molecular weight polyethylene (UHMWPE) decreased after the addition of nano-ZnO. According to Chang et al. [17], the reduction in tensile strength indicates that the adhesion between the nano-ZnO and UHMWPE matrix is not significant for promoting greater interfacial bonding, which would enhance the tensile strength value. The addition of iron oxide and zinc oxide resulted in insignificant changes in the tensile modulus of the composites in comparison to the reference composite. The composite made with titanium dioxide had the highest modulus among all the composites. Deka and Maji [7] observed an increase in the tensile strength and modulus of WPCs after incorporation of TiO2 nanopowder (3 parts per 100 parts of resin). Esthappan et al. [18] have shown that the addition of titanium dioxide in the PP resulted in an increase in its tensile strength and modulus. In the present study, the addition of titanium dioxide was found to improve the tensile modulus, but it did not have any effect on the tensile strength. It is evident that weathering reduced the tensile strength of the composites (Table 3). For all the composites, except for the composite containing zinc, the reduction of tensile strength was estimated to be significant. The reduction of tensile strength after accelerated weathering has been previously reported by Beg and Pickering [19] for wood-polypropylene composites, by Zhang et al. [10] for wood-fibre/HDPE composites containing iron oxide, by Devi and Maji [20] for wood-polymer composites containing titanium dioxide. In their study on the photo-degradation of PP/ZnO nanocomposites, Zhao and Li [21] report that the incorporation of zinc oxide nanoparticles promotes the recovery of the tensile strength of UV-treated nanocomposites. Similarly to tensile strength, the tensile modulus of the composites containing iron oxide and titanium dioxide decreased by
Fig. 4. SEM images of the surfaces of the reference composites: A. initial (magnification x100), B. after 500 h of accelerated weathering (magnification x100), and C. after 500 h of accelerated weathering (magnification x500).
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Fig. 5. SEM images of the surfaces of the composites containing iron pigment: A. initial (magnification x100), and B. after 500 h of accelerated weathering (magnification x100).
Fig. 6. SEM images of the surfaces of the composites containing titanium pigment: A. initial (magnification x100), and B. after 500 h of accelerated weathering (magnification x100).
about 10% after weathering. The slight decrease (3%) of tensile modulus of the zinc-containing composite after weathering, which was estimated to be statistically not significant, was also in line with the tensile strength change (4%). In the case of the reference composite, 7% decrease in tensile strength after weathering was found, but the modulus of this composite stayed unchanged after weathering. Elongation at break of the composites evaluated before and after weathering is shown in Fig. 11. The elongation at break of all the composites is relatively low (less than 1%), compared to the elongation at break 45% for the pure polypropylene [22]. Such low values can be explained by the presence of fillers. Denac et al. [22] showed that the incorporation of 12% talc to the polypropylene matrix decreased the elongation at break from 45% to 2%. After weathering, the elongation at break was reduced for all the composites, except for the composite containing zinc oxide, which retained its elongation at break. The change of elongation at break correlates very well with the chemical change during photo-
degradation [23]. It seems that the zinc oxide pigment acts as a stabiliser and reduces the degradation of polypropylene in the shell layer of the composite. Fibre swelling due to moisture absorption is primarily responsible for the loss in mechanical properties after weathering [24]. The loss in strength is due to moisture penetration into the WPC, which degrades the wood-polymer interface, decreasing the stress transfer efficiency from the matrix to the fibre. The weight gain and thickness swelling of the studied composites were determined after weathering, and they are presented in Fig. 12. Weight gain in combination with thickness swelling serves as an indicator of moisture absorption by composites in the process of weathering. The negative value of the weight gain of the reference composite can be explained by removal of part of the photo-degraded surface layer of the composite. Lignin and water soluble products can leach from samples [25]. In their work on accelerated weathering of an aspen fibre/polypropylene composite Rowell et al. [26] showed that a specimen consisting 30% wood
Fig. 7. SEM images of the surfaces of the composites containing zinc pigment: A. initial (magnification x100), B. after 500 h of accelerated weathering (magnification x100) and C. after 500 h of accelerated weathering (magnification x500).
S. Butylina et al. / Polymer Degradation and Stability 120 (2015) 10e16
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Fig. 8. EDS spectra of talc particle identified on the surface of the composite.
Fig. 9. FTIR spectra (carbonyl region) of the composites, obtained at different weathering times.
Table 3 Tensile strength and modulus of composites determined before and after weathering. Composite
Tensile strength [MPa]
Reference Comp_Fe Comp_Ti Comp_Zn
Fig. 10. Carbonyl index of the composites as a function of weathering time.
9.14 9.05 9.04 8.59
± ± ± ±
Tensile modulus [GPa]
afterweathering ft
ftinitial 0.51 0.26 0.59 0.42
8.50 8.27 8.28 8.24
± ± ± ±
0.44 0.16 0.41 0.46
(S) (S) (S) (NS)
afterweathering
Etinitial 3.09 3.17 3.37 3.00
± ± ± ±
Et 0.16 0.14 0.15 0.18
3.08 2.96 3.02 2.90
± ± ± ±
0.17 0.13 0.12 0.16
(NS) (S) (S) (NS)
S means that the difference between the initial and weathered samples of the same composite is statistically significant at P-value less than 0.05; NS means that it is not significant.
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Acknowledgement The authors would like to thank the Jenny and Antti Wihuri Foundation for their financial support to this project (No. 130029). References
Fig. 11. Elongation at break of the composites before and after weathering.
Fig. 12. Weight gain (WG) and thickness swelling (TS) of composites after weathering.
fibre, 68% PP and 2% MAPP lost 2.7% of its initial weight after 2000 h of UV exposure. The degradation of the surface layer of the reference composite is shown in Fig. 4B. In contrast to the reference composite, the composites containing inorganic pigments showed an increase in weight after weathering. A small increase of weight was due to moisture uptake, which was greater than the material loss induced by photodegradation. The absorbed moisture caused swelling of wood fibres, which can be evaluated as thickness swelling of the composites. In spite of the weight loss found in the reference composite, its thickness swelling was highest among all the composites. The more severe degradation of the surface layer of the reference composite led to exposure of cellulose fibres (Fig. 4C) and made this composite more sensitive to moisture. The lower thickness swelling of the composites containing inorganic pigments agreed well with the lighter degradation of their surfaces compared to the reference. 4. Conclusions The results of this study indicate that the incorporation of inorganic pigments into the shell layer of wood-polymer composites can impart significant improvements in their resistance to weathering. According to the colour change results, it was evident that the composite containing titanium dioxide was better in the stabilisation of colour than the other pigments. It was found in the SEM study that the density of surface cracking was dramatically reduced in the composites containing inorganic pigments. Serious surface deterioration after weathering was detected only in the case of the reference composite. The tensile test measurements indicated that weathering affected the tensile properties of all the composites adversely. However, the composite containing zinc oxide was capable to retain its tensile properties. The retention of the tensile properties correlated well with the lowest carbonyl index obtained for the zinc oxide-containing composite. Photo-degradation of the surface layer and moisture absorption by the composites in the process of weathering were suggested to be the reasons for the decrease of their mechanical properties.
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