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Wood–plastics composites with better fire retardancy and durability performance
Composites: Part A 40 (2009) 1772–1776
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Wood–plastics composites with better fire retardancy and durability performance M. García *, J. Hidalgo, I. Garmendia, J. García-Jaca CIDEMCO, Area Anardi 5, 20730 Azpeitia (Guipúzcoa), Spain
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 18 June 2009 Accepted 17 August 2009
Keywords: A. Wood B. Mechanical properties B. Physical properties Ageing
a b s t r a c t This study concerns the preparation and study of wood–plastic composites (WPCs). The matrix used was high density polyethylene. Results showed that the addition of wood fibres increased mechanical properties (tensile, flexural and compression) of the neat plastic remarkably. Additives such as fire retardants and light stabilizers were added to improve properties like fire retardancy and durability performance. The addition of fire retardants could lead to auto-extinguishing materials when ammonium polyphosphate or aluminium hydroxide were used. Outdoor durability depended on both the light stabilizer and the fire retardant added to the formulation. The fire retardant worsened the outdoor durability. However, stabilized fire retarded-WPCs showed much lower fading than non-stabilized non-fire retarded composites and several industrial samples. Stabilized composites with aluminium hydroxide as fire retardant showed the best overall results with a fading degree even lower than the stabilized non-fire retarded composite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Wood–plastic composites (WPC) combine the best properties of the neat components and can show outstanding performance. When comparing with potential traditional competitors, one find that WPCs offer better thermal and acoustic isolation than aluminium, and better durability and lower maintenance than wood often at a lower price than neat plastics. Moreover, in contrast to plastics, they offer an appearance rather similar to that of wood. Due to all the advantages they offer, WPCs are rapidly expanding in many countries of Europe for applications different from decking. The addition of wood to a polymer increases the stiffness and strength of the resulting material, specially at low fibre content [1–3]. However, usually coupling agents are required to promote interfacial adhesion between the matrix and the filler [4,5]. Different polymers have been used in these composites. The most popular resin in Europe is virgin polypropylene, whereas the global preference is for polyethylene [6]. PVC is also used by some companies and researchers. The advantages of WPCs are their good dimensional stability during lifetime, i.e. lower water uptake, and durability against fungi and insects compared with wood [7–11]. Both aspects make these materials can have a working life of even 25–30 years with low maintenance requirements. However, weathering properties are the weak point of these materials [12–16] what hinders their outdoor applicability. After weathering WPCs show fading and * Corresponding author. Tel.: +34 943 816800; fax: +34 943 816074. E-mail address: [email protected] (M. García). 1359-835X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2009.08.010
swelling [17,18]. Thus, the addition of some kind of protector is required. Pigments can be added as photo-blockers giving rise to an improved colour stability of the WPC [17–19]. Coatings can also be applied with the inherent difficulty of adhesion promotion in polyolefin substrates. Light stabilizers could be used to minimize fading. Light stabilizers can be classified in two types according to their action mode: UV absorbers act by shielding the material from ultraviolet light and hindered amine light stabilizers (HALS) act by scavenging the radical intermediates formed in the photooxidation process. Companies offer specific light stabilizers for each polymer. However, the structure of WPCs differs from that of neat plastics. The light stabilizer must protect not only the polymer but also the wood fibres. Some UV absorbers and pigments seem to protect preventing fading and property decrease [19]. However, the amount of protection can be influenced by both photostabilizer concentration and exposure variables. All this provokes great difficulties when trying to protect WPCs against outdoor conditions. Fire behaviour is another key point. Safety issues demand fire resistant materials. PVC is a self-extinguishing material, but due to the toxicity of the generated gases it should be avoided. Polyolefins, the other usual polymers employed in WPCs, burn and drip in case of fire leading to a very risky scenario. Thus, fire retardant agents must be employed in order to improve fire behaviour. Various studies were developed regarding fire performance of composites reinforced by natural fibres [2,20–25]. However, few studies have been carried out on this aspect in WPCs [26,27]. Halogenated and phospho-halogenated compounds produce toxic gases like those formed during PVC combustion. So, they
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should be avoided. Antimony based systems don’t show fire resistance themselves, but show synergistic effects with halogenated compounds. Organic fire retardants can produce high toxic products after thermal decomposition and combustion [28]. Nanocomposites constitute a new development in the area of flame retardancy [29]. However, their high price make their use affordable in high added value products, but not in commodity products like construction elements. Thus, in order to develop safe composites the fire retardant agent should be selected among phosphorus and inorganic systems like borates, stannates or hydroxides. Borate compounds were used in Refs. [18,19] and results showed that improvements in fire performance were obtained. The aim of this work is to develop an economically competitive WPC composite with better fire retardancy and durability performance. The addition of wood fibres will increase mechanical performance, but other properties like weatherability and fire retardancy must be also improved. Regarding weatherability, an study about light stabilizers was developped to obtain non-pigmented WPCs for outdoor products. Fire resistant products were also obtained by the addition of phosphorous, hydroxide and melamine-based compounds. Moreover, it was analysed how these fire retardants affect the weatherability aspects of the composites.
The formulations studied in the additivated composites are shown in Table 1. 2.2. Processing Firstly, and after drying the polyethylene and wood fibres at 80 °C for 12 h, the components were compounded by extrusion in order to obtain a good enough compatibility level between the phases, together with a homogeneous fibre distribution. The extrusion process was carried out in a corrotating double screw extruder at a screw speed of 250 rpm and temperature profile of 170–175 °C in the barrel and 180 °C in the nozzle. The injection process was performed in a Battenfeld PLUS 30/75 injection machine. ISO 527-2 ‘‘Plastics-Determination of tensile properties – Part 2: Test conditions for moulding and extrusion plastics”. Type 5 dog bone-shaped tensile specimens and 80 10 4 mm3 flexural specimens were obtained. The parameters of the injection moulding process were the following: Temperature profile: 190–195 °C. Injection speed: 24.5 cm3/s. Injection pressure: 3038 bar.
2. Experimental 2.3. Testing 2.1. Materials The polyethylene used was HDPE Hostalen GC7260 from Basell. It has a bulk density of 0.960 g/cc and a melt flow rate (190/5) of 23 g/10 min. The wood fibres (Lignocel BK40-90 from Rettenmaier & Sohne Co.) had cubic structure (non fibrous) and length of 300– 500 lm. Moisture content of the fibres was not determined. Fibres were not chemically treated prior to processing. Three percent of coupling agent (CA) (maleinizated polyethylene, Licocene PE MA 4351 from Clariant) was added to the formulation together with 8% of microtalc to enhance processing. The fire retardants (FR) used were an ammonium polyphosphate (FR CROS 484 from Budenheim), a encapsulated ammonium polyphosphate (Exolit AP462 from Clariant), aluminium trihydroxide (Martinal from Omya) and melamine cyanurate (Budit 315 from Budenheim). No halogen containing FR was selected in order to use environmentally friendly FRs. All the composites contained 9% of FR. So, the concentration of the plastic as well as the polymer/fibre/CA/microtalc ratio were held constant. Light stabilizers were a HALS (Tinuvin 111) added by 1%w and a blend of HALS (Tinuvin 111), UV filter (Tinuvin 326) and an antioxidant (Irganox B225) added by 1%w, 0.5%w and 0.2%w, respectively, all from CIBA.
Tests were conducted according to the standards proposed in CEN/TS 15534-2:2008 ‘‘Wood–plastics composites (WPC) – Part 2: Characterisation of WPC materials”. At least five specimens were tested in all tests. The data given is the arithmetic mean of the test values together with the typical standard deviation. Tensile, flexural and compression tests were conducted in an Instron machine, model 5569 (Bucks, UK). Tests were carried out at 23 °C and 50% relative humidity. Tensile tests were performed according to EN ISO 527 ‘‘Plastics – Determination of tensile properties” (at 1.0 mm/min with type 5 specimens), flexural tests according to EN ISO 178 ‘‘Plastics – Determination of flexural properties” (at 2.0 mm/min with 80 mm 10 mm 4 mm specimens) and compression tests according to EN ISO 604 ‘‘Plastics – Determination of compressive properties” (at 1.0 mm/min with 10 mm 10 mm 4 mm specimens). Test conditions were selected from the values recommended in ISO standards. Single flame fire tests were performed according to EN ISO 11925-2 ‘‘Reaction to fire tests-Ignitability of building products subjected to direct impingement of flame – Part 2: Single-flame source test” in flexural bars. Durability tests were carried out in a QUV machine (QUV weathering tester, model QUV/basic) according to EN 927-6 ‘‘Expo-
Table 1 Formulations studied in the additivated composites. Polymer (%)
Fibre (%)
CA (%)
Microtalc (%)
FR (%)
Light stabilizer (%)
Polyphosphate
Aluminium hydroxide
Encapsulated polyphosphate
Melamine cyanurate
HALS
Blend
39 38.6 38.3 35.5 35.5 35.5 35.5 35.1 35.1 34.9 34.9 34.9 34.9
50 49.5 49.2 45.5 45.5 45.5 45.5 45.0 45.0 44.8 44.8 44.8 44.8
3 3 2.9 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7
8 7.9 7.9 7.3 7.3 7.3 7.3 7.2 7.2 7.1 7.1 7.1 7.1
– – – 9.1 – – – 9 – 9 – – –
– – – – 9.1 – – – 9 – 9 – –
– – – – – 9.1 – – – – – 9 –
– – – – – – 9.1 – – – – – 9
– 1 – – – – – 0.9 0.9 – – – –
– – 1–0.5–0.2 – – – – – – 0.88–0.44–0.18 0.88–0.44–0.18 0.88–0.44–0.18 0.88–0.44–0.18
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sure of wood coatings to artificial weathering using fluorescent UV lamps and water” with the following cycle: Stage
Function
T (°C)
Time (h)
1 2
Condensation Stage of subcycles 3 + 4
45
3 4
UV-A Water spraying
60
24 144 48 cycles of 3 h formed by steps 3 and 4 2.5 0.5
Colour measurements were conducted in a spectrocolorimeter (Konika Minolta Sensing) adapted to a colour data software. The Hunter Lab Color scale was employed. The overall change in colour is expressed as DE.
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DL2 þ Da2 þ Db
where L is lightness, a redness (+) or greenness (–) and b yellowness (+) or blueness (–). The respective DL, Da and Db are the differences between the final and initial values. The uncertainty of the measurements is the following: L: ±0.15. a: ±0.66. b: ±0.31. 3. Results and discussion 3.1. Mechanical performance Composites with fibre contents between 30% and 60% were obtained. Flexural specimens were obtained in all the concentration range studied. However, tensile specimens with 60% wood could not be obtained. As the wood content was increased, the fluidity became poorer and too excessive injection pressure should be required for full filling of the mould. Miki et al. [30] also observed that no complete filling took place at very high content of wood flour in PE/wood composites. They concluded that the pressure required for the injection of 75% wood was almost five times higher than that for PE and the injection pressure was above limitation of the moulder. Tensile, flexural and compression properties were determined and the results are shown in Table 2. As usual in WPCs [1,2,31], the addition of wood fibres increased the modulus of the composites in the three type of tests. This is because the wood fibres are much stiffer than neat polymers, in fact tensile modulus of wood fibres is 40–60 GPa [32] while average tensile moduli of extrusion and injection grades HDPE are 0.8–1.6 GPa and 0.18–3.65 GPa, respectively [33]. So fibres promote a major stiffness in the composite. The higher the fibre content, the higher the modulus of the composites. The strength increased after the addition of the fibre, then it remained constant with composition showing a plateau. A possible
explanation is that at higher fibre loadings the ductility decrease compensates the increase in the strength and gives rise to similar strength values for all the composites. Furthermore, the analysis of the fracture surfaces after testing revealed that pull out of the fibres increased at increasing fibre content. This indicates that the CA used was not sufficient to compatibilize the phases properly, thus avoiding further increase in the strength. A plateau in the strength, and even decreases at increasing fibre content, have been previously observed by other researchers in biocomposites [2,4]. Taking into account the results obtained in mechanical properties and that an aesthetical appearance similar to that of wood is more desired, a high fibre loading is more beneficial for the composites. However, processing of the composite with 60% wood fibre by injection moulding was more difficult because fluidity was notably reduced. So, the composite with 50% wood, 3% CA, 8% of microtalc and 39% of HDPE was selected as the most suitable. Additives were added to this composite. 3.2. Fire retardancy In order to improve fire resistance of the composites, different FRs were added. The action mechanism of each one is the following: Phosphorous compounds: By acting as char formers. When heated, they produce a solid form of phosphoric acid that in turns chars the material and shields it from releasing flammable gases able to feed flames. Aluminium trihydroxide: By releasing water that acts as heat sinks and prevents oxygen to set fire to flammable compounds, or by forming a protective layer as a coating. Melamine cyanurate: Above 320 °C, it undergoes endothermic decomposition to melamine and cyanuric acid, acting as a heat sink in the process. The vaporized melamine acts as an inert gas source diluting the oxygen and the fuel gases present at the point of combustion. Tests were carried out according to the standard EN ISO 119252. The flame was applied for 5 s and the time to burn 45 mm length specimens was determined. Results for different FRs are shown in Table 3. As can be seen, the addition of the FR improved notably the fire retardancy. Similar results were obtained in biocomposites additivated with ammonium polyphosphate [24,25] and expandable graphite [25], which showed higher limiting oxygen index (LOI) and lower heat release rate (HRR) values. When neat HDPE was burnt, the material started dripping as soon as the flame was in contact with the material and the integrity of the material was completely lost immediately. The addition of wood fibres improved such behaviour and the composite maintained its integrity during the whole experiment. This is in agreement with other results where it was observed that the HRR of the PP/lignocellulosic fibres (coir, hemp, jute or flax) composites showed a decrease with respect to PP [21]. However, bubbles were
Table 2 Tensile, flexural and compression properties of the composites at different fibre contents. Fibre content (%)
0 30 40 50 60
Tensile
Flexural
Compression
Strength (MPa)
Modulus (MPa)
Strength (MPa)
Modulus (MPa)
Strength (MPa)
Modulus (MPa)
21 ± 1 29 ± 2 27 ± 2 27 ± 1 –
1125 ± 38 4148 ± 80 5851 ± 163 7310 ± 228 –
23 ± 1 34 ± 1 33 ± 1 35 ± 1 36 ± 2
817 ± 24 2359 ± 44 3141 ± 108 3812 ± 114 4927 ± 289
22 ± 1 28 ± 5 26 ± 3 28 ± 4 30 ± 3
160 ± 15 225 ± 38 231 ± 68 245 ± 65 273 ± 67
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M. García et al. / Composites: Part A 40 (2009) 1772–1776 Table 3 Burning speed of the composites as a function of FR. Fire retardant (FR)
Burning speed (mm/min)
None Polyphosphate Aluminium hydroxide
144 ± 7 Auto-extinguish Half: 110 ± 23 Half: auto-extinguish Auto-extinguish 153 ± 5
Encapsulated polyphosphate Melamine cyanurate
formed in the composite specimens when they were in contact with the flame. After the addition of FRs no bubbles were formed during burning and the flame spread out much more slowly than in non-additivated specimens. Moreover, just the external surface of the specimens was burnt, while the internal zone was not altered. Comparable improvements in fire performance were also observed previously with other fire retardants like borates [26,27]. However, melamine cyanurate did not improve fire retardancy at all. This could be because the presence of wood fibres hinders or even inhibits the correct action mechanism of the fire retardant. Unexpectedly, this compound acted as a lubricant facilitating the processing of the composite. Overall results, however, were even better than expected. As can be seen in Table 3 most of the composites extinguished after the flame was went out. So, theses composites just caught fire when the flame was directly falling on them. This indicates that these composites are auto-extinguishing.
3.3. Accelerated ageing experiments Outdoor durability is another key point on these materials when they are used in outdoor applications. WPCs undergo colour change, swelling and even fracture in presence of light and water. Although degradation of both wood and plastic occurs, discolouration of the wood is more likely to occur due to the loss of colour-imparting extractives from wood during degradation [17]. Moreover, under irradiation lignins generate phenoxy radicals that are oxidized leading to the degradation of wood in more extent than in polyolefins [34]. In order to improve weatherability of the fire retarded WPCs two types of light stabilizers were added to the formulation: a HALS and a blend of HALS, UV stabilizer and antioxidant. During the accelerated weathering experiments, samples were taken out after 300 and 600 h of ageing. These samples were subjected to colour measurements. Fading degree was light stabilizer dependant. The composites stabilized with the HALS showed much more fading than those stabilized with the blend despite their similarity in flexural properties retention. However, fading was observed in all samples after 300 h of ageing. Table 4 shows colour change (DE) per ageing time of composites additivated with the blend with respect to non-aged specimens. Ageing and colour measurements were also carried out in industrial samples for comparison. As can be seen, the addition of a light stabilizer to the neat composites improved their outdoor weatherability since discolouration diminished around 35% after the addition of the stabilizer. However, the addition of FRs worsened colour retention remarkably. This is probably because not only wood flour is acting as heterogeneity in the blend but also the inorganic particles of the FRs can increase porosity of the samples. At increasing heterogeneities and porosity both in the surface and the bulk, water can penetrate in the composite more easily and thus, degrade the material, specially wood fibres. Colour change of stabilized composites with FRs is much lower than in non-stabilized ones. The biggest colour change is obtained when polyphosphate is used as FR. This is because
Table 4 Average values of colour change (DE) after QUV accelerated weathering in a standard cycle. Sample
DE 300 h
Composite Composite + stabilizer Composite + polyphosphate Composite + AlOH3 Composite + polyphospate + stabilizer Composite + AlOH3 + stabilizer Composite + encapsulated polyphosphate + stabilizer Composite + melamine cyanurate + stabilizer Thermoformed composite + LDPE layer Sample 1: moulded PP/wood compound (Rettenmaier) Sample 2: injected specimen (unknown) Sample 3: profile for decking (unbrushed side) (Techwood) Sample 4: profile for decking (brushed side) (Techwood) Sample 5: profile for decking (brushed) (TimberTech) Sample 6: extruded profile (unknown)
Sample 7: extruded profile (unknown)
600 h
17.2 27.6 11.3 17.1 22.6 32.4 24.5 27.0 19.5 26.2 10.4 14.5 12.5 22.4 9.6 9.7 0.6 6.3 29.4 29.5 22.5 25.9 32.9 30.2 17.0 17.8 3.1 3.0 After 18 h the samples were swollen, cracked and broken After 43 h the samples were swollen, cracked and broken
polyphosphate is highly hygroscopic salt. Since it is homogeneously distributed in the composite, it absorbs water during ageing test and contribute to quick discolouration of the wood. The other salts used as FRs are almost insoluble, so water absorption by the composite is more hindered when it is properly protected. Thus, aluminium hydroxide, encapsulated polyphosphate and melamine cyanurate gave rise to similar colour change values after ageing, specially at short ageing period. Discolouration of stabilized composites additivated with those FRs is similar to that of stabilized neat composites despite the presence of FRs. This proves the efficacy of the blend employed as light stabilizer. It is worth mentioning that the colour change values obtained for the composites of this work are much lower than those shown by most of industrial samples (Table 4, samples 1–7). Results were even better than those shown by brushed industrial samples, which usually exhibit better results (compare samples 3 and 4. They are the same sample, one side brushed and the other side unbrushed). Except for sample 5 that retains colour very well, after 300 h DE of industrial samples were at least 40% higher than those of composites of this work. Some industrial samples even showed swelling, cracking and fracture after short ageing times. In fact, they showed board crumbling. This behaviour was not observed neither in the commercial WPCs nor in our composites. After 600 h stabilized composites filled with aluminium hydroxide and cyanurate showed the lowest discolouration, even lower than sample 4 (brushed Techwood sample) in spite of being unbrushed. Take into mind that composites with very good fire retardancy showed lower outdoor durability resistance. Sample 5 showed very little fading but it burnt upon fire testing. Despite other scientific groups have carried out colour change measurements in aged WPCs, data are not comparable due to the different ageing conditions employed. In order to demonstrate this, the stabilized and non-stabilized composites with aluminium hydroxide as FR were aged in a QUV using a different cycle. The cycle used was 5 h at 60 °C with UV-A light and 1 h with water spraying. Results are shown in Table 5. As can be seen, results are rather different from those obtained by the cycle of the European standard. Another way to improve durability of WPCs is coating the samples with a polymeric layer [35]. Coating can be assessed by different methods such as coextrusion, compression or thermoforming.
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Table 5 Average values of colour change (DE) after QUV accelerated weathering in a special cycle. Values in brackets are the results obtained in the standard cycle. Sample
DE 300 h
600 h
Composite + AlOH3 Composite + AlOH3 + stabilizer
16.2 (24.5) 8.2 (10.4)
24.0 (27.0) 12.1 (14.5)
The later methods were used in order to coat the composites with a LDPE film. The coated samples were subjected to the same ageing process. As can be seen in Table 4, almost no colour change was observed after 300 h of weathering and very little after 600 h, indicating that this method is very adequate when it comes to protection of WPCs. However, this method is not always valid because in case of brushing the material the plastic layer would be erased and the WPC would be unprotected. 4. Conclusions New WPCs based on HDPE and wood fibres were developed. The addition of wood fibres increased the tensile, flexural and compressive moduli notably. Moreover, the higher the fibre content, the higher the stiffness of the composite. Strength also increased after the addition of wood fibres, but it remained constant with composition. The WPCs were additivated with FRs and light stabilizers in order to improve fire retardancy and outdoor durability behaviour. It was observed that auto-extinguishing composites could be obtained with most of the environmentally friendly and low cost FRs employed. During the tests, the integrity of the materials was maintained and no bubbles nor dripping took place. Fading degree was light stabilizer and FR dependant. The addition of FRs increased discolouration of the samples probably due to the increase of heterogeneities and porosity in the blend that facilitates water penetration. The stabilizer composed of a blend of HALS, UV filter and antioxidant gave rise to the best results. No cracking or fracture took place in the aged WPCs, just some fading. However, colour change of the stabilized composites was much lower than that of neat composites and most industrial samples. It was concluded that the best overall performance among the additivated composites was observed with aluminium hydroxide as FR and a blend of stabilizers. Coating of WPCs with a plastic layer is one the best methods to protect the materials from fading. Almost no discolouration took place after weathering. However, this method is only valid for unbrushed materials. Acknowledgement This work was carried out in collaboration with the company EXTRUPLESA in a framework of a R&D Project sponsored by the Spanish Ministry of Industry. References [1] Dányádi L, Renner K, Szabó Z, Nagy G, Móczó J, Pukánsky B. Wood flour filled PP composites: adhesion, deformation, failure. Polym Adv Technol 2006;17:967–74. [2] García M, Garmendia I, Garcia J. Influence of natural fibre type in ecocomposites. J Appl Polym Sci 2008;107:2994–3004. [3] Schneider MH, Phillips JG, Stig Lande. Physical and mechanical properties of wood polymer composites. J Forest Eng 2000;11:1–2. [4] Lee SY, Kang IA, Doh GH, Kim WJ, Kim JS, Yoon HG, et al. Thermal mechanical and morphological properties of polypropylene/clay/wood flour nanocomposites. Express Polym Lett 2008;2(2):78–87.
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Wood–plastics composites with better fire retardancy and durability performance M. García *, J. Hidalgo, I. Garmendia, J. García-Jaca CIDEMCO, Area Anardi 5, 20730 Azpeitia (Guipúzcoa), Spain
a r t i c l e
i n f o
Article history: Received 14 November 2008 Received in revised form 18 June 2009 Accepted 17 August 2009
Keywords: A. Wood B. Mechanical properties B. Physical properties Ageing
a b s t r a c t This study concerns the preparation and study of wood–plastic composites (WPCs). The matrix used was high density polyethylene. Results showed that the addition of wood fibres increased mechanical properties (tensile, flexural and compression) of the neat plastic remarkably. Additives such as fire retardants and light stabilizers were added to improve properties like fire retardancy and durability performance. The addition of fire retardants could lead to auto-extinguishing materials when ammonium polyphosphate or aluminium hydroxide were used. Outdoor durability depended on both the light stabilizer and the fire retardant added to the formulation. The fire retardant worsened the outdoor durability. However, stabilized fire retarded-WPCs showed much lower fading than non-stabilized non-fire retarded composites and several industrial samples. Stabilized composites with aluminium hydroxide as fire retardant showed the best overall results with a fading degree even lower than the stabilized non-fire retarded composite. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Wood–plastic composites (WPC) combine the best properties of the neat components and can show outstanding performance. When comparing with potential traditional competitors, one find that WPCs offer better thermal and acoustic isolation than aluminium, and better durability and lower maintenance than wood often at a lower price than neat plastics. Moreover, in contrast to plastics, they offer an appearance rather similar to that of wood. Due to all the advantages they offer, WPCs are rapidly expanding in many countries of Europe for applications different from decking. The addition of wood to a polymer increases the stiffness and strength of the resulting material, specially at low fibre content [1–3]. However, usually coupling agents are required to promote interfacial adhesion between the matrix and the filler [4,5]. Different polymers have been used in these composites. The most popular resin in Europe is virgin polypropylene, whereas the global preference is for polyethylene [6]. PVC is also used by some companies and researchers. The advantages of WPCs are their good dimensional stability during lifetime, i.e. lower water uptake, and durability against fungi and insects compared with wood [7–11]. Both aspects make these materials can have a working life of even 25–30 years with low maintenance requirements. However, weathering properties are the weak point of these materials [12–16] what hinders their outdoor applicability. After weathering WPCs show fading and * Corresponding author. Tel.: +34 943 816800; fax: +34 943 816074. E-mail address: [email protected] (M. García). 1359-835X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2009.08.010
swelling [17,18]. Thus, the addition of some kind of protector is required. Pigments can be added as photo-blockers giving rise to an improved colour stability of the WPC [17–19]. Coatings can also be applied with the inherent difficulty of adhesion promotion in polyolefin substrates. Light stabilizers could be used to minimize fading. Light stabilizers can be classified in two types according to their action mode: UV absorbers act by shielding the material from ultraviolet light and hindered amine light stabilizers (HALS) act by scavenging the radical intermediates formed in the photooxidation process. Companies offer specific light stabilizers for each polymer. However, the structure of WPCs differs from that of neat plastics. The light stabilizer must protect not only the polymer but also the wood fibres. Some UV absorbers and pigments seem to protect preventing fading and property decrease [19]. However, the amount of protection can be influenced by both photostabilizer concentration and exposure variables. All this provokes great difficulties when trying to protect WPCs against outdoor conditions. Fire behaviour is another key point. Safety issues demand fire resistant materials. PVC is a self-extinguishing material, but due to the toxicity of the generated gases it should be avoided. Polyolefins, the other usual polymers employed in WPCs, burn and drip in case of fire leading to a very risky scenario. Thus, fire retardant agents must be employed in order to improve fire behaviour. Various studies were developed regarding fire performance of composites reinforced by natural fibres [2,20–25]. However, few studies have been carried out on this aspect in WPCs [26,27]. Halogenated and phospho-halogenated compounds produce toxic gases like those formed during PVC combustion. So, they
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should be avoided. Antimony based systems don’t show fire resistance themselves, but show synergistic effects with halogenated compounds. Organic fire retardants can produce high toxic products after thermal decomposition and combustion [28]. Nanocomposites constitute a new development in the area of flame retardancy [29]. However, their high price make their use affordable in high added value products, but not in commodity products like construction elements. Thus, in order to develop safe composites the fire retardant agent should be selected among phosphorus and inorganic systems like borates, stannates or hydroxides. Borate compounds were used in Refs. [18,19] and results showed that improvements in fire performance were obtained. The aim of this work is to develop an economically competitive WPC composite with better fire retardancy and durability performance. The addition of wood fibres will increase mechanical performance, but other properties like weatherability and fire retardancy must be also improved. Regarding weatherability, an study about light stabilizers was developped to obtain non-pigmented WPCs for outdoor products. Fire resistant products were also obtained by the addition of phosphorous, hydroxide and melamine-based compounds. Moreover, it was analysed how these fire retardants affect the weatherability aspects of the composites.
The formulations studied in the additivated composites are shown in Table 1. 2.2. Processing Firstly, and after drying the polyethylene and wood fibres at 80 °C for 12 h, the components were compounded by extrusion in order to obtain a good enough compatibility level between the phases, together with a homogeneous fibre distribution. The extrusion process was carried out in a corrotating double screw extruder at a screw speed of 250 rpm and temperature profile of 170–175 °C in the barrel and 180 °C in the nozzle. The injection process was performed in a Battenfeld PLUS 30/75 injection machine. ISO 527-2 ‘‘Plastics-Determination of tensile properties – Part 2: Test conditions for moulding and extrusion plastics”. Type 5 dog bone-shaped tensile specimens and 80 10 4 mm3 flexural specimens were obtained. The parameters of the injection moulding process were the following: Temperature profile: 190–195 °C. Injection speed: 24.5 cm3/s. Injection pressure: 3038 bar.
2. Experimental 2.3. Testing 2.1. Materials The polyethylene used was HDPE Hostalen GC7260 from Basell. It has a bulk density of 0.960 g/cc and a melt flow rate (190/5) of 23 g/10 min. The wood fibres (Lignocel BK40-90 from Rettenmaier & Sohne Co.) had cubic structure (non fibrous) and length of 300– 500 lm. Moisture content of the fibres was not determined. Fibres were not chemically treated prior to processing. Three percent of coupling agent (CA) (maleinizated polyethylene, Licocene PE MA 4351 from Clariant) was added to the formulation together with 8% of microtalc to enhance processing. The fire retardants (FR) used were an ammonium polyphosphate (FR CROS 484 from Budenheim), a encapsulated ammonium polyphosphate (Exolit AP462 from Clariant), aluminium trihydroxide (Martinal from Omya) and melamine cyanurate (Budit 315 from Budenheim). No halogen containing FR was selected in order to use environmentally friendly FRs. All the composites contained 9% of FR. So, the concentration of the plastic as well as the polymer/fibre/CA/microtalc ratio were held constant. Light stabilizers were a HALS (Tinuvin 111) added by 1%w and a blend of HALS (Tinuvin 111), UV filter (Tinuvin 326) and an antioxidant (Irganox B225) added by 1%w, 0.5%w and 0.2%w, respectively, all from CIBA.
Tests were conducted according to the standards proposed in CEN/TS 15534-2:2008 ‘‘Wood–plastics composites (WPC) – Part 2: Characterisation of WPC materials”. At least five specimens were tested in all tests. The data given is the arithmetic mean of the test values together with the typical standard deviation. Tensile, flexural and compression tests were conducted in an Instron machine, model 5569 (Bucks, UK). Tests were carried out at 23 °C and 50% relative humidity. Tensile tests were performed according to EN ISO 527 ‘‘Plastics – Determination of tensile properties” (at 1.0 mm/min with type 5 specimens), flexural tests according to EN ISO 178 ‘‘Plastics – Determination of flexural properties” (at 2.0 mm/min with 80 mm 10 mm 4 mm specimens) and compression tests according to EN ISO 604 ‘‘Plastics – Determination of compressive properties” (at 1.0 mm/min with 10 mm 10 mm 4 mm specimens). Test conditions were selected from the values recommended in ISO standards. Single flame fire tests were performed according to EN ISO 11925-2 ‘‘Reaction to fire tests-Ignitability of building products subjected to direct impingement of flame – Part 2: Single-flame source test” in flexural bars. Durability tests were carried out in a QUV machine (QUV weathering tester, model QUV/basic) according to EN 927-6 ‘‘Expo-
Table 1 Formulations studied in the additivated composites. Polymer (%)
Fibre (%)
CA (%)
Microtalc (%)
FR (%)
Light stabilizer (%)
Polyphosphate
Aluminium hydroxide
Encapsulated polyphosphate
Melamine cyanurate
HALS
Blend
39 38.6 38.3 35.5 35.5 35.5 35.5 35.1 35.1 34.9 34.9 34.9 34.9
50 49.5 49.2 45.5 45.5 45.5 45.5 45.0 45.0 44.8 44.8 44.8 44.8
3 3 2.9 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7 2.7
8 7.9 7.9 7.3 7.3 7.3 7.3 7.2 7.2 7.1 7.1 7.1 7.1
– – – 9.1 – – – 9 – 9 – – –
– – – – 9.1 – – – 9 – 9 – –
– – – – – 9.1 – – – – – 9 –
– – – – – – 9.1 – – – – – 9
– 1 – – – – – 0.9 0.9 – – – –
– – 1–0.5–0.2 – – – – – – 0.88–0.44–0.18 0.88–0.44–0.18 0.88–0.44–0.18 0.88–0.44–0.18
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sure of wood coatings to artificial weathering using fluorescent UV lamps and water” with the following cycle: Stage
Function
T (°C)
Time (h)
1 2
Condensation Stage of subcycles 3 + 4
45
3 4
UV-A Water spraying
60
24 144 48 cycles of 3 h formed by steps 3 and 4 2.5 0.5
Colour measurements were conducted in a spectrocolorimeter (Konika Minolta Sensing) adapted to a colour data software. The Hunter Lab Color scale was employed. The overall change in colour is expressed as DE.
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DL2 þ Da2 þ Db
where L is lightness, a redness (+) or greenness (–) and b yellowness (+) or blueness (–). The respective DL, Da and Db are the differences between the final and initial values. The uncertainty of the measurements is the following: L: ±0.15. a: ±0.66. b: ±0.31. 3. Results and discussion 3.1. Mechanical performance Composites with fibre contents between 30% and 60% were obtained. Flexural specimens were obtained in all the concentration range studied. However, tensile specimens with 60% wood could not be obtained. As the wood content was increased, the fluidity became poorer and too excessive injection pressure should be required for full filling of the mould. Miki et al. [30] also observed that no complete filling took place at very high content of wood flour in PE/wood composites. They concluded that the pressure required for the injection of 75% wood was almost five times higher than that for PE and the injection pressure was above limitation of the moulder. Tensile, flexural and compression properties were determined and the results are shown in Table 2. As usual in WPCs [1,2,31], the addition of wood fibres increased the modulus of the composites in the three type of tests. This is because the wood fibres are much stiffer than neat polymers, in fact tensile modulus of wood fibres is 40–60 GPa [32] while average tensile moduli of extrusion and injection grades HDPE are 0.8–1.6 GPa and 0.18–3.65 GPa, respectively [33]. So fibres promote a major stiffness in the composite. The higher the fibre content, the higher the modulus of the composites. The strength increased after the addition of the fibre, then it remained constant with composition showing a plateau. A possible
explanation is that at higher fibre loadings the ductility decrease compensates the increase in the strength and gives rise to similar strength values for all the composites. Furthermore, the analysis of the fracture surfaces after testing revealed that pull out of the fibres increased at increasing fibre content. This indicates that the CA used was not sufficient to compatibilize the phases properly, thus avoiding further increase in the strength. A plateau in the strength, and even decreases at increasing fibre content, have been previously observed by other researchers in biocomposites [2,4]. Taking into account the results obtained in mechanical properties and that an aesthetical appearance similar to that of wood is more desired, a high fibre loading is more beneficial for the composites. However, processing of the composite with 60% wood fibre by injection moulding was more difficult because fluidity was notably reduced. So, the composite with 50% wood, 3% CA, 8% of microtalc and 39% of HDPE was selected as the most suitable. Additives were added to this composite. 3.2. Fire retardancy In order to improve fire resistance of the composites, different FRs were added. The action mechanism of each one is the following: Phosphorous compounds: By acting as char formers. When heated, they produce a solid form of phosphoric acid that in turns chars the material and shields it from releasing flammable gases able to feed flames. Aluminium trihydroxide: By releasing water that acts as heat sinks and prevents oxygen to set fire to flammable compounds, or by forming a protective layer as a coating. Melamine cyanurate: Above 320 °C, it undergoes endothermic decomposition to melamine and cyanuric acid, acting as a heat sink in the process. The vaporized melamine acts as an inert gas source diluting the oxygen and the fuel gases present at the point of combustion. Tests were carried out according to the standard EN ISO 119252. The flame was applied for 5 s and the time to burn 45 mm length specimens was determined. Results for different FRs are shown in Table 3. As can be seen, the addition of the FR improved notably the fire retardancy. Similar results were obtained in biocomposites additivated with ammonium polyphosphate [24,25] and expandable graphite [25], which showed higher limiting oxygen index (LOI) and lower heat release rate (HRR) values. When neat HDPE was burnt, the material started dripping as soon as the flame was in contact with the material and the integrity of the material was completely lost immediately. The addition of wood fibres improved such behaviour and the composite maintained its integrity during the whole experiment. This is in agreement with other results where it was observed that the HRR of the PP/lignocellulosic fibres (coir, hemp, jute or flax) composites showed a decrease with respect to PP [21]. However, bubbles were
Table 2 Tensile, flexural and compression properties of the composites at different fibre contents. Fibre content (%)
0 30 40 50 60
Tensile
Flexural
Compression
Strength (MPa)
Modulus (MPa)
Strength (MPa)
Modulus (MPa)
Strength (MPa)
Modulus (MPa)
21 ± 1 29 ± 2 27 ± 2 27 ± 1 –
1125 ± 38 4148 ± 80 5851 ± 163 7310 ± 228 –
23 ± 1 34 ± 1 33 ± 1 35 ± 1 36 ± 2
817 ± 24 2359 ± 44 3141 ± 108 3812 ± 114 4927 ± 289
22 ± 1 28 ± 5 26 ± 3 28 ± 4 30 ± 3
160 ± 15 225 ± 38 231 ± 68 245 ± 65 273 ± 67
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Burning speed (mm/min)
None Polyphosphate Aluminium hydroxide
144 ± 7 Auto-extinguish Half: 110 ± 23 Half: auto-extinguish Auto-extinguish 153 ± 5
Encapsulated polyphosphate Melamine cyanurate
formed in the composite specimens when they were in contact with the flame. After the addition of FRs no bubbles were formed during burning and the flame spread out much more slowly than in non-additivated specimens. Moreover, just the external surface of the specimens was burnt, while the internal zone was not altered. Comparable improvements in fire performance were also observed previously with other fire retardants like borates [26,27]. However, melamine cyanurate did not improve fire retardancy at all. This could be because the presence of wood fibres hinders or even inhibits the correct action mechanism of the fire retardant. Unexpectedly, this compound acted as a lubricant facilitating the processing of the composite. Overall results, however, were even better than expected. As can be seen in Table 3 most of the composites extinguished after the flame was went out. So, theses composites just caught fire when the flame was directly falling on them. This indicates that these composites are auto-extinguishing.
3.3. Accelerated ageing experiments Outdoor durability is another key point on these materials when they are used in outdoor applications. WPCs undergo colour change, swelling and even fracture in presence of light and water. Although degradation of both wood and plastic occurs, discolouration of the wood is more likely to occur due to the loss of colour-imparting extractives from wood during degradation [17]. Moreover, under irradiation lignins generate phenoxy radicals that are oxidized leading to the degradation of wood in more extent than in polyolefins [34]. In order to improve weatherability of the fire retarded WPCs two types of light stabilizers were added to the formulation: a HALS and a blend of HALS, UV stabilizer and antioxidant. During the accelerated weathering experiments, samples were taken out after 300 and 600 h of ageing. These samples were subjected to colour measurements. Fading degree was light stabilizer dependant. The composites stabilized with the HALS showed much more fading than those stabilized with the blend despite their similarity in flexural properties retention. However, fading was observed in all samples after 300 h of ageing. Table 4 shows colour change (DE) per ageing time of composites additivated with the blend with respect to non-aged specimens. Ageing and colour measurements were also carried out in industrial samples for comparison. As can be seen, the addition of a light stabilizer to the neat composites improved their outdoor weatherability since discolouration diminished around 35% after the addition of the stabilizer. However, the addition of FRs worsened colour retention remarkably. This is probably because not only wood flour is acting as heterogeneity in the blend but also the inorganic particles of the FRs can increase porosity of the samples. At increasing heterogeneities and porosity both in the surface and the bulk, water can penetrate in the composite more easily and thus, degrade the material, specially wood fibres. Colour change of stabilized composites with FRs is much lower than in non-stabilized ones. The biggest colour change is obtained when polyphosphate is used as FR. This is because
Table 4 Average values of colour change (DE) after QUV accelerated weathering in a standard cycle. Sample
DE 300 h
Composite Composite + stabilizer Composite + polyphosphate Composite + AlOH3 Composite + polyphospate + stabilizer Composite + AlOH3 + stabilizer Composite + encapsulated polyphosphate + stabilizer Composite + melamine cyanurate + stabilizer Thermoformed composite + LDPE layer Sample 1: moulded PP/wood compound (Rettenmaier) Sample 2: injected specimen (unknown) Sample 3: profile for decking (unbrushed side) (Techwood) Sample 4: profile for decking (brushed side) (Techwood) Sample 5: profile for decking (brushed) (TimberTech) Sample 6: extruded profile (unknown)
Sample 7: extruded profile (unknown)
600 h
17.2 27.6 11.3 17.1 22.6 32.4 24.5 27.0 19.5 26.2 10.4 14.5 12.5 22.4 9.6 9.7 0.6 6.3 29.4 29.5 22.5 25.9 32.9 30.2 17.0 17.8 3.1 3.0 After 18 h the samples were swollen, cracked and broken After 43 h the samples were swollen, cracked and broken
polyphosphate is highly hygroscopic salt. Since it is homogeneously distributed in the composite, it absorbs water during ageing test and contribute to quick discolouration of the wood. The other salts used as FRs are almost insoluble, so water absorption by the composite is more hindered when it is properly protected. Thus, aluminium hydroxide, encapsulated polyphosphate and melamine cyanurate gave rise to similar colour change values after ageing, specially at short ageing period. Discolouration of stabilized composites additivated with those FRs is similar to that of stabilized neat composites despite the presence of FRs. This proves the efficacy of the blend employed as light stabilizer. It is worth mentioning that the colour change values obtained for the composites of this work are much lower than those shown by most of industrial samples (Table 4, samples 1–7). Results were even better than those shown by brushed industrial samples, which usually exhibit better results (compare samples 3 and 4. They are the same sample, one side brushed and the other side unbrushed). Except for sample 5 that retains colour very well, after 300 h DE of industrial samples were at least 40% higher than those of composites of this work. Some industrial samples even showed swelling, cracking and fracture after short ageing times. In fact, they showed board crumbling. This behaviour was not observed neither in the commercial WPCs nor in our composites. After 600 h stabilized composites filled with aluminium hydroxide and cyanurate showed the lowest discolouration, even lower than sample 4 (brushed Techwood sample) in spite of being unbrushed. Take into mind that composites with very good fire retardancy showed lower outdoor durability resistance. Sample 5 showed very little fading but it burnt upon fire testing. Despite other scientific groups have carried out colour change measurements in aged WPCs, data are not comparable due to the different ageing conditions employed. In order to demonstrate this, the stabilized and non-stabilized composites with aluminium hydroxide as FR were aged in a QUV using a different cycle. The cycle used was 5 h at 60 °C with UV-A light and 1 h with water spraying. Results are shown in Table 5. As can be seen, results are rather different from those obtained by the cycle of the European standard. Another way to improve durability of WPCs is coating the samples with a polymeric layer [35]. Coating can be assessed by different methods such as coextrusion, compression or thermoforming.
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Table 5 Average values of colour change (DE) after QUV accelerated weathering in a special cycle. Values in brackets are the results obtained in the standard cycle. Sample
DE 300 h
600 h
Composite + AlOH3 Composite + AlOH3 + stabilizer
16.2 (24.5) 8.2 (10.4)
24.0 (27.0) 12.1 (14.5)
The later methods were used in order to coat the composites with a LDPE film. The coated samples were subjected to the same ageing process. As can be seen in Table 4, almost no colour change was observed after 300 h of weathering and very little after 600 h, indicating that this method is very adequate when it comes to protection of WPCs. However, this method is not always valid because in case of brushing the material the plastic layer would be erased and the WPC would be unprotected. 4. Conclusions New WPCs based on HDPE and wood fibres were developed. The addition of wood fibres increased the tensile, flexural and compressive moduli notably. Moreover, the higher the fibre content, the higher the stiffness of the composite. Strength also increased after the addition of wood fibres, but it remained constant with composition. The WPCs were additivated with FRs and light stabilizers in order to improve fire retardancy and outdoor durability behaviour. It was observed that auto-extinguishing composites could be obtained with most of the environmentally friendly and low cost FRs employed. During the tests, the integrity of the materials was maintained and no bubbles nor dripping took place. Fading degree was light stabilizer and FR dependant. The addition of FRs increased discolouration of the samples probably due to the increase of heterogeneities and porosity in the blend that facilitates water penetration. The stabilizer composed of a blend of HALS, UV filter and antioxidant gave rise to the best results. No cracking or fracture took place in the aged WPCs, just some fading. However, colour change of the stabilized composites was much lower than that of neat composites and most industrial samples. It was concluded that the best overall performance among the additivated composites was observed with aluminium hydroxide as FR and a blend of stabilizers. Coating of WPCs with a plastic layer is one the best methods to protect the materials from fading. Almost no discolouration took place after weathering. However, this method is only valid for unbrushed materials. Acknowledgement This work was carried out in collaboration with the company EXTRUPLESA in a framework of a R&D Project sponsored by the Spanish Ministry of Industry. References [1] Dányádi L, Renner K, Szabó Z, Nagy G, Móczó J, Pukánsky B. Wood flour filled PP composites: adhesion, deformation, failure. Polym Adv Technol 2006;17:967–74. [2] García M, Garmendia I, Garcia J. Influence of natural fibre type in ecocomposites. J Appl Polym Sci 2008;107:2994–3004. [3] Schneider MH, Phillips JG, Stig Lande. Physical and mechanical properties of wood polymer composites. J Forest Eng 2000;11:1–2. [4] Lee SY, Kang IA, Doh GH, Kim WJ, Kim JS, Yoon HG, et al. Thermal mechanical and morphological properties of polypropylene/clay/wood flour nanocomposites. Express Polym Lett 2008;2(2):78–87.
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