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The effect of interfacial adhesion on the creep behaviour of LDPE–Al–Fique composite materials
Composites: Part B 55 (2013) 345–351
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of interfacial adhesion on the creep behaviour of LDPE–Al–Fique composite materials Miguel A. Hidalgo-Salazar a,⇑, José H. Mina b, Pedro J. Herrera-Franco c a
Research Group for Manufacturing Technologies GITEM, Universidad Autónoma de Occidente, Cali, Colombia Composite Materials Group, Universidad del Valle, Cali, Colombia c Yucatán Center for Scientific Investigation, A.C., Mérida, Yucatán, Mexico b
a r t i c l e
i n f o
Article history: Received 30 October 2012 Received in revised form 30 May 2013 Accepted 12 June 2013 Available online 27 June 2013 Keywords: A. Fibres A. Recycling B. Fibre/matrix bond B. Creep
a b s t r a c t The dynamic mechanical response and performance to short-term creep of composites made from mats of fique and low-density polyethylene–aluminium (LDPE–Al) obtained from recycled long-life Tetra Pak packages were studied. Chemical treatments such as alkalinisation with NaOH and silanization and impregnation treatments with polyethylene, were applied to improve the compatibility of the fibre– matrix and their effects on the creep response and mechanical properties of LDPE–Al–Fique, were investigated. The interfacial adhesion was deduced from the properties measured and confirmed by observation of the fractured surface of the composite. A relationship was observed between the creep mechanical response of LDPE–Al–Fique with respect to both the untreated fibres and the fibres treated with NaOH. Additionally, the four parameters of the Burger’s model were calculated from the creep curves. A very good agreement between the experimental data and the theoretical curves was obtained in the fluency region. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Emerging environmental trends and a growing concern about the greenhouse effect, and the need of the development of new products, have led industries to seek new composites based on sustainable materials, especially those reinforced with natural fibres. One such material is the Natural Fibre Thermoplastic Composite (NFPC), which constitutes an alternative to the use of renewable fibres in certain applications. Several natural fibres have been studied and implemented in the reinforcement of polymeric matrices. Natural fibres provide good mechanical properties and are cheap as compared with synthetic fibres. Different studies on the development of NFPC materials can be found in the literature [1–4]. However, some fibres have drawbacks when used in applications demanding high mechanical properties. Composites based on thermoplastic matrices may improve the mechanical performance (strength and rigidity) and provide a reduction in density, making it possible to obtain lighter and more resistant products [5–8]. Cellulosic fibres, such as: hemp, sisal, coir, jute, palm, bamboo and wood in its natural condition, as well as several cellulosic wastes, such as shells, wood flour and pulp have been used as fillers for both thermoplastic and thermosetting resins for several years. ⇑ Corresponding author. Address: Cll 25 # 115 – 85, Facultad de Ingeniería, Universidad Autónoma de Occidente, Cali, Colombia. Tel.: +57 2 3188000; fax: +57 2 5553911. E-mail address: [email protected] (M.A. Hidalgo-Salazar). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.06.032
Among the natural reinforcing filler materials, the fique fibre appears to be one of the most promising fillers, due to its high mechanical strength [9]. Moreover, fique fibres are readily available in the form of nonwoven mats and produced in an industrial scale in Colombia. The use of mats instead of short fibres with low aspect ratios has the advantage of reducing agglomeration of the fibres, which allows the production of rigid panels with the same properties in two different directions. Current creep studies have shown a time-dependent degradation in the module (creep) and strength (creep rupture) of NFPC materials, because of the natural viscoelastic behaviour of the matrix [10]. The creep behaviour of polymer matrix composites is a critical issue for many modern engineering applications, such as the parts for the aerospace industry, as the replacement of wood in the manufacturing of products such as reels for the winding of electrical cables, pallets and panels, which need to be light and possess good strength in order to protect packaged goods. Several studies on the creep behaviour of liquefied wood–polymer composites based on low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) have been reported [11]. It has also been reported that with an increase in fibre content, the flow of wood particles with polymer matrix composites decreases [12]. Using agricultural composites to develop structural building products often requires the improvement of their mechanical properties, especially the creep performance. Previous studies in the field show that the creep of agricultural composites varies with the type and content of matrices, coupling treatment, where the relative performance
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2. Experimental 2.1. Materials and methods
Fig. 1. Process for obtaining compound LDPE–Al–Fique test specimens for characterisation using a CNC cutting router at FabLab Cali in Universidad Autonoma de Occidente.
of the fibre–matrix interface is significant compared with the viscoelastic properties of the compounds [5,13]. Various mathematical modelling techniques have also been applied to analyse creep behaviour [5–7]. The technique of dynamic mechanical analysis (DMA), has been reported as an alternative to evaluate the long-term creep behaviour of different cane bagasse-based NFPCs for PVC and HDPE matrices [4]. However, few studies of creep that take into account changes of the adhesion between the polymeric matrix and the natural fibres have been reported [5,14]. In this paper, the interfacial adhesion between fique fibres and a LDPE–Al matrix obtained from the recycling of long-life packages of Tetra Pak is reported. Fibre–matrix adhesion was improved by modification of the fibre surface properties utilising treatments such as alkalinisation, silanization and pre-impregnation with polyethylene prior to preparation of the composite materials. The effect of the fibre–matrix interface in the creep behaviour and the dynamic mechanical properties were investigated for a composite material based on a matrix of LDPE–Al reinforced with fique fibres.
Fique fibre non-woven mats were used as reinforcement. The properties of the fique fibres and the LDPE–Al obtained from recycling long-life packages of Tetra Pak were presented in a previous paper [16]. The interfacial adhesion between the fique fibres and the matrix was modified using three different fibre surface treatments: First, the fique fibres were immersed in an aqueous solution of sodium hydroxide (NaOH) at 2% w/v for one hour at 25 °C, then the fibres were washed with distilled water to completely remove the remaining NaOH. Finally, the fibres were dried in two steps: first, at room temperature for 12 h and subsequently, in an oven at 60 °C for 24 h. Second, a silanisation treatment was performed after alkalinisation with NaOH with a silane-type coupling agent namely, a silane Tris (2-Methoxyethoxy) (vinyl) ‘‘A172’’. The fique fibres were immersed in a water–methanol solution 50/50 v/v in which 1.0% and 0.5% of silane and dicumyl peroxide. The Ph of the deionised water and methanol solution was adjusted to 3.5 using acetic acid and the solution was stirred for 30 min, and then, the fibres were immersed for 1 h. Subsequently, the fibres were decanted and dried for a period of 24 h at 60 °C. Finally, the fibres were subjected to curing for two hours at 120 °C. Just as with the alkaline treatment, it was necessary to use 300 ml of solution to treat 25 g of fique fibres. Third, for the preimpregnation treatment, the fique fibres were immersed in a solution of polyethylene in xylene at 1.5% w/w. The impregnation treatment was performed at a temperature of 120 °C, maintaining agitation at a constant speed of 100 rpm for 1 h. Then, the fibres were dried at 60 °C for 24 h. Again, 300 ml of solution was used to treat 25 g of fique fibres. The low-density polyethylene aluminium (LDPE–Al) used was obtained directly from the recycling process of long-life Tetra Pak packages; this residue is obtained from a hydropulping process and was subsequently pulverised to increase the adhesion of the fique fibres and to produce a better distribution in the matrix. The LDPE–Al has a volume percentage ratio of 89.75 LDPE and 10.25 Aluminium with an average density of 1.10 g/cm3. The composite LDPE–Al–Fique was obtained in panels with dimensions of 290 290 2 mm. A closeup-type stainless steel mould was used and panels were manufactured by compression moulding in a hot plates press LabPro400 Fontijne PRESS. Fique tissues were dried in an oven at 50 °C for 24 h before the preparation of the
Table 1 Summary of LDPE–Al–Fique 10%, 20% and 30% incorporation of fibre at 25 °C and 1.2 MPa results for 4 parameter models to 1800 s creep stress for treatments F and A. Model
Parameters
4
E1 (Pa) E2 (Pa) g1 (Pa s) g2 (Pa s) WSS
Treatments 10F
20F
30F
10A
20A
30A
1.08E+09 4.80E+09 1.30E+13 7.57E+11 1.23E07
1.33E+09 7.97E+09 2.10E+13 1.61E+12 1.25E07
1.50E+09 1.44E+10 2.77E+13 5.66E+11 5.44E08
1.19E+09 6.44E+09 1.50E+13 9.06E+11 6.21E08
1.41E+09 1.28E+10 2.95E+13 1.37E+12 4.91E08
1.95E+09 1.30E+10 3.57E+13 1.72E+12 5.46E08
Table 2 Summary of LDPE–Al–Fique 10%, 20% and 30% incorporation of fibre at 25 °C and 1.2 MPa results for 4 parameter models to 1800 s creep stress for treatments S and P. Model
4
Parameters
E1 (Pa) E2 (Pa) g1 (Pa s) g2 (Pa s) WSS
Treatments 10S
20S
30S
10P
20P
30P
1.35E+09 6.52E+09 1.90E+13 5.13E+11 3.43E07
1.88E+09 1.07E+10 1.88E+13 1.12E+12 6.91E08
2.16E+09 1.66E+10 2.28E+13 2.24E+12 4.33E08
1.39E+09 8.67E+09 2.17E+13 6.92E+11 1.08E07
1.96E+09 9.53E+09 2.34E+13 2.93E+11 8.77E08
2.15E+09 1.61E+10 2.78E+13 1.13E+12 5.61E08
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Fig. 3. The behaviour of both (r0/E1) and (r0/g1) as a function of fibre surface treatment and fibre content.
Fig. 4. Short-term compliance curves for 30% fique LDPE–AL treatments F, A, S and P.
LDPE–Al and fique without treatment; A: for fique fibres with alkali treatment; S: for fique fibres with alkali treatment and silanization, P: for fique fibres with alkali treatment, silanization and impregnation with LDPE. Formulations of LDPE–Al–Fique of 10%, 20% and 30% v/v fique were prepared, for each treatment (F, A, S and P). Finally, the specimens were cut using a computerised numerical control equipment, which guarantees to obtain specimens for various tests like tensile, bending, impact and DMA; Fig. 1 presents an image of the cut specimens. The morphology of the composite samples was studied using a JOELJSM-6490 scanning electron microscope (SEM). Micrographs of the fracture surfaces were obtained for selected specimens. Prior to scanning, the surfaces were gold-coated to avoid charging under the electron beam. Fig. 2. Creep tests for different formulations of LDEP–Al–Fique: (a) 10% of fique fibre, (b) 20% of fique fibre and (c) 30% of fique fibre.
2.2. Creep tests compounds. To prepare the compounds, fique nonwoven mats were placed between Teflon films along with the LDPE–Al powder, ensuring a complete coverage of the mat filling the mould. The samples were compression moulded at 170 °C and 3 MPa for 11 min and cooled for 4 min at a cooling rate of 32.5 °C/min. The nomenclature used to identify the different fibre surface treatments in each composite material was the following: F: for
A Rheometric System Analyser RSAIII dynamic mechanical analyser (DMA) was used in the experiments to measure creep deformation as a function of time. The tests were performed in flexural mode using specimens 2 mm thick, 44 mm long and 12.7 mm wide. The static stress of 1.2 MPa was applied for 30 min. The static stress was selected after performing a stress sweep test, where the linear region was defined for each of the composites including the
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LDPE–Al, and ensuring that the creep tests were conducted in the linear region. Trials were also conducted for the 30% fique LDPE– Al for treatments F, A, S and P for a static stress of 2 MPa. These tests were compared with those reported in the literature for composites based on PVC, recycled HDPE reinforced with fibres of cane bagasse, using the same experimental conditions [4]. The average temperature at which the tests were performed was 25 °C, although some tests were conducted at different temperatures. 2.3. Dynamic mechanical testing Viscoelastic properties, such as storage modulus (E0 ), loss modulus E00 and mechanical damping parameter tan d were measured as a function of temperature using a Rheometric System Analyser RSAIII dynamic mechanical analyser (DMA). The measurements were carried out in the three-point bending mode with rectangular specimens of dimensions 44 12.7 2 mm in a temperature range of 135 to 110 °C at a heating rate 3 °C/min, and nitrogen gas flow was used to control the temperature. Samples were analysed at a fixed frequency of 1 Hz to 0.01% strain.
Fig. 5. Bar graph and normalised results given in Table 3 of the strain rate; all at 25 °C and 1.2 MPa except the LDPE–Al to 0.7 MPa.
3. Results and discussion 3.1. Creep behaviour The four parameters of Eq. (1) were calculated for the composites with different fibre content as well as different interfacial adhesion levels for each creep curve are shown.
eðtÞ ¼
r0 E1
þ
E t r0 r0 2 tþ 1 e g2 g1 E2
ð1Þ
The parameters reported in Tables 1 and 2 show that instantaneous elastic deformation (r0/E1) and the slope of the steady-state creep area (r0/g1) corresponds to the viscoelastic behaviour of the composite material. The error is estimated based on least squares adjustment of the parameters and was calculated using the Weighted Sum of Squares (WSSs)
wss ¼
X
2
¼ 1n wi ½eðti Þ e^ ðti Þ
ð2Þ
i
where e(ti) represents the experimental strain observed in time ti, ^eðti Þ is the strain estimated by the model and wi is the difference between two time samples. A smaller WSS value indicates a better model fit to the experimental data. Fig. 2 shows both, the short-term creep strain tests results as well as the theoretical creep strain values for LDPE–Al–Fique composites with different concentrations of fique (10%, 20% and 30%),
and different fibre surface treatments (F, A, S and P). The agreement between the theoretical predictions and experimental results is very good with just small discrepancies that appear in the first creep region. It can be observed that E1 increases as the volume fraction of fique fibres increases for the different fibre surface treatments. It can also be noted that the creep strain decreases as the fique content increases, as expected when a rigid reinforcement is added into a viscoelastic matrix [5]. A smaller creep strain is observed for treatments S and P, especially at higher volumes of 20% and 30%, indicating that at fique fibre volumes contents below 10% the composite creep behaviour is still governed by the matrix behaviour. The behaviour of both (r0/E1) and (r0/g1) is shown in Fig. 3 as a function of fibre surface treatment and fibre content. It should be observed that, for low fibre volume content (10%) both parameters exhibit a linear behaviour, for the different fibre surface treatments, indicating that the fibre–matrix behaviour plays an important role. For higher fibre contents, the creep response is smaller and there is no notorious change with fibre–matrix interactions. The viscous response given by r0/g1, also decreases with increasing fibre content [14]. It should be remembered that the volume fraction of the interface also increases for higher fibre volume fractions and the presence of the covalent bonds that are created with the silane fibre surface treatments generate less compliant interphases with the matrix. This behaviour of the short term creep compliance is shown in Fig. 4, for composites produced
Table 3 Strain rate data for all treatments at 25 °C and 1.2 MPa stress. Fique (%)
de/dt (s1) Experimental
de/dt (s1) Model
g1 (Pa s)
g1 (Pa s)
Experimental
Model
F
10 20 30
8.00E08 ± 9.60E09 5.40E08 ± 8.40E09 3.60E08 ± 3.40E09
8.00E08 ± 2.30E09 5.80E08 ± 1.50E09 3.40E08 ± 3.00E09
1.50E+13 ± 1.90E+12 2.30E+13 ± 3.60E+12 3.40E+13 ± 3.10E+12
1.50E+13 ± 4.40E+11 2.10E+13 ± 5.40E+11 3.60E+13 ± 3.30E+12
A
10 20 30
7.10E08 ± 1.20E08 5.40E08 ± 1.40E10 3.50E08 ± 2.80E09
6.70E08 ± 1.60E09 5.00E08 ± 1.40E09 3.80E08 ± 4.30E10
1.70E+13 ± 3.10E+12 2.30E+13 ± 8.10E+11 3.50E+13 ± 2.70E+12
1.80E+13 ± 4.30E+11 2.40E+13 ± 6.90E+11 3.20E+13 ± 3.70E+11
S
10 20 30
6.60E08 ± 5.90E09 5.30E08 ± 6.80E09 2.90E08 ± 7.90E09
5.90E08 ± 1.10E09 5.20E08 ± 7.70E10 3.00E08 ± 6.80E10
1.80E+13 ± 1.70E+12 2.30E+13 ± 3.10E+12 4.30E+13 ± 1.10E+13
2.00E+13 ± 3.70E+11 2.20E+13 ± 3.40E+11 4.00E+13 ± 9.10E+11
P
10 20 30
4.20E08 ± 6.30E09 3.40E08 ± 5.80E09 2.10E08 ± 2.40E09 1.20E07 ± 1.30E08
4.50E08 ± 3.50E10 3.50E08 ± 3.10E10 2.50E08 ± 1.80E09 1.20E07 ± 7.30E09
2.90E+13 ± 4.80E+12 3.60E+13 ± 6.40E+12 5.80E+13 ± 6.90E+12 1.00E + 13 ± 1.10E+12
2.70E+13 ± 2.10E+11 3.40E+13 ± 3.10E+11 4.90E+13 ± 3.70E+12 9.70E+12 ± 5.60E+11
LDPE–Al
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with 30% of fique fibres for each of the respective treatments. Again, a lower creep compliance is observed for the silane treated fibres. The order of E2 are close to E1, indicating an increased viscoelastic strain that represents a delayed elastic behaviour. The creep performance of the composites can be observed in the strain rate values presented in Table 3. They are also shown in the bar graph in Fig. 5 which shows normalised results of the strain rate, that is, the evolution of the creep response of the composite for the different fique fibre content and fibre surface treatments. The values of the experimental g were calculated with the slope of the second zone of the creep curves, based on the mathematical models. The values in table 3 are slightly different because they correspond to a repetition in our experimental design. The standard deviations observed in Fig. 5, should be considered normal for NFPCs because of the nature of the fique fibres, however, the strain rate behaviour also decreases for increasing fibre volume content, and with the increase in the fibre–matrix interaction resulting from the stronger interface [11]. When the fibre surface is modified with the alkaline treatment, the viscoelastic matrix is able to better impregnate the small ridges and crevices resulting from the removal of the low molecular weight compounds from the fibre surface. When the stress is applied to the composite material, the fibre–matrix interactions are of the frictional type and shear loads at the interface and will be responsible for the matrix/interface material flow in shear. When the fibres are treated with a silane coupling agent, strong covalent bonds between fibre and matrix
349
are responsible of the more efficient load transfer mechanism. This should contribute to the elastic rather than the viscous behaviour of the composite material. In order to corroborate the fibre–matrix interface behaviour of the composites was observed by SEM and the micrographs obtained are shown in Fig. 6a (untreated fique), which shows that the fique fibres are being pull out of the matrix and there are no traces of polymer still adhering to the fibre that would indicated fibre–matrix adhesion. Even more, in some cases, a gap between the fibre and the matrix appears to be created, i.e., the fibre is practically separated from LDPE. Furthermore, when the fibre surface was treated with the alkaline aqueous solution, the surface roughness increased. (Fig. 6b, surface was obtained and therefore, the possibility of mechanical interlocking increased, thus contributing to an increase in the shear interfacial strength. Such increase is observed to result in a lower creep deformation of the composite. In this case the failure surface did not show evidences of fibre pull-out off the matrix material and the fibre failure was a clean tensile failure, almost parallel to the polymer surface. Also, the polymeric matrix surface shows some shear hackles in the vicinity of the failed fibre. In Fig. 6c (treatments S and P), shows a matrix layer uniformly distributed over the surface of the fibres and that the fibres are substantially bonded to the matrix due to the improved interfacial adhesion. From the morphology shown in these composites, a positive response to creep testing is observed, because the load transfer efficiency of well-bonded fibres in composites contributes to enhanced strength [17].
Fig. 6. Scanning electronic micrograph. Interface in the fique fibres reinforced composite: (a) untreated, (b) fique fibre reinforced treated with NaOH, (c) fique fibre reinforced treated with NaOH, silane and pre-impregnated.
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3.2. Dynamic mechanical behaviour Fig. 7 shows changes in the storage modulus versus temperature of the composites prepared with 30% fique and LDPE–Al w/ w. At low temperature, the storage modulus of LDPE–Al and the composites of LDPE–Al–Fique is very similar. However, the difference increases as the temperature increases and this more noticeable for temperatures above 40 °C. This change is a relaxation of the amorphous phase of the LDPE. After that, E0 and E00 continues decreasing. At 75 °C approximately, the matrix reaches the softening region where the composites have both a viscous and solid behaviour, also suggesting that at this temperature the fibre–matrix interfacial interactions starts deteriorating, until reaching melting temperature near 110 °C. This can be observed in Figs. 8 and 9, which indicate the loss modulus and tan d respectively. The reinforcing effectiveness of the fique fibres is evident on the moduli of the composites and can be represented by coefficient C [15,18]:
C¼
ðEg =Er Þcomp ðEg =Er Þre sin
ð3Þ
where Eg and Er are the storage modulus values in the glassy and rubbery region, respectively. The lower the value of the C constant, then, the more effective the reinforcement is. The measured E0
´ to temperature sweep for: LDPE–Al[}], Fig. 7. Response of storage modulus E 30F[D], 30A[r], 30S[h], 30P[s].
Fig. 9. Response of tan d to temperature sweep for: LDPE–Al[}], 30F[D], 30A[r], 30S[h], 30P[s].
values at 30 and 25 °C were employed as Eg and Er, respectively. Although the values obtained for the different composites are very similar (C = 0.50–0.59), it is interesting to note that they are much lower than those reported for cellulosic short fibre-based composites (i.e., 0.96 for 30% w/w banana fibre–polyester resin), indicating that a better reinforcement effect can be achieved using long fibres as in the nonwoven mat used here[19]. The LDPE–Al–Fique with P treatment composite presents slightly higher E0 and E00 across the whole range of temperature compared with that of the S treatment sample. This behaviour is the result of two different interface interactions, first, the presence of covalent bonds from the silane surface treatment and second, the impregnation of fique fibres with polyethylene, effectively achieving a better contact between fibre and matrix, which is even more significant when compared with treatment F. In general, it can be said that the decrease of the E0 curve, is smaller with increasing fibre–matrix interface strength, [20]. Fig. 9 shows tan d versus temperature for LDPE–AL–Fique composites and pure LDPE–AL. The molecular relaxations in the LDPE– AL decrease in the LDPE–AL–Fique composites. The molecular motions in the interfacial region generally contribute to the damping of the material apart from those of the constituents [21]. The width of the peaks for LDPE–Al are more notorious than those peaks of the composites and practically the same for treatments A, S and P, indicating a better damping for these compounds. A composite with a poor interface tends to dissipate more energy than one with a good interface. In the composites of LDPE–Al treatments S and P, the presence of covalent bonds between fibre and matrix results in lower molecular mobility in the interfacial region. The presence of the fibres also adds complexity to the dynamical behaviour. The transition related to the glass transition takes place at 19.59 °C for LDPE–Al, measured at max peak 1, as shown in Fig. 9. The LDPE–Al–Fique composites exhibit a max peak 1 in the tan d at approximately 2.20 °C, 2.03 °C and 3.22 °C for the A, S and P fibre surface treatments respectively. The most notorious difference between peaks of the LDPE–Al and the composites is noticed at temperatures above the Tg of the LDPE but the presence of the fibres and the effect of the fibre–matrix interface interactions is more notorious for temperatures above 30 °C.
4. Conclusions
Fig. 8. Response of the loss modulus E00 to temperature sweep for: LDPE–Al[}], 30F[D], 30A[r], 30S[h], 30P[s].
As is the case with all natural products, the mechanical and physical properties of natural fibres vary considerably. These properties are determined by the chemical and structural composition and cellulose, the main component of all natural fibres, which
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varies from fibre to fibre. The creep strains decreases as fique content increases however, the fibre–matrix interactions attributed to the silane covalent bonds result in much lower creep starins. Additionally, scanning electronic micrographs confirmed that the interfacial adhesion between the fique and the LDPE thermoplastic matrix was enhanced when alkalinisation, silanization and impregnation with polyethylene treatments applied to the fibres. However, the interface formed with treatments S and P resulted in a better creep response, which could be observed in creep strains and strain rates. Correspondingly, the temperature sweep tests with DMA confirmed that the fibre–matrix interface interactions resulted in better damping properties as well as smaller variations in the storage and loss moduli more notoriously for the S and P fibre surface treatments. Acknowledgements We express our gratitude to Tetra Pak Colombia, INNOPAK CaliColombia and Universidad Autónoma de Occidente, Cali-Colombia, and its FabLab, for providing their equipments to obtain specimens and Compañia de Empaques from Medellin Colombia, for providing the Fique, the sponsors of this project. References [1] Faruk O, Bledzki AK, Fink H-P, Sain M. Biocomposites reinforced with natural fibres: 2000–2010. Prog Polym Sci 2012;37:1552–96. [2] Kiguchi M. Latest market status of wood and wood plastic composites in North America and Europe. In: The second wood and wood plastic composites seminar in the 23rd wood composite symposium. Kyoto, Japan; 2007. [3] Wambua P, Ivensn J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63:1259–64. [4] Xu Y, Wu Q, Lei Y, Yao F. Creep behavior of bagasse fibre reinforced polymer composites. Bioresource Technol 2010;101:3280–6. [5] Acha BA, Reboredo MM, Marcovich NE. Creep and dynamic mechanical behavior of PP–jute composites: effect of the interfacial adhesion. Compos Part A: Appl Sci Manuf 2007;38:1507–16.
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Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of interfacial adhesion on the creep behaviour of LDPE–Al–Fique composite materials Miguel A. Hidalgo-Salazar a,⇑, José H. Mina b, Pedro J. Herrera-Franco c a
Research Group for Manufacturing Technologies GITEM, Universidad Autónoma de Occidente, Cali, Colombia Composite Materials Group, Universidad del Valle, Cali, Colombia c Yucatán Center for Scientific Investigation, A.C., Mérida, Yucatán, Mexico b
a r t i c l e
i n f o
Article history: Received 30 October 2012 Received in revised form 30 May 2013 Accepted 12 June 2013 Available online 27 June 2013 Keywords: A. Fibres A. Recycling B. Fibre/matrix bond B. Creep
a b s t r a c t The dynamic mechanical response and performance to short-term creep of composites made from mats of fique and low-density polyethylene–aluminium (LDPE–Al) obtained from recycled long-life Tetra Pak packages were studied. Chemical treatments such as alkalinisation with NaOH and silanization and impregnation treatments with polyethylene, were applied to improve the compatibility of the fibre– matrix and their effects on the creep response and mechanical properties of LDPE–Al–Fique, were investigated. The interfacial adhesion was deduced from the properties measured and confirmed by observation of the fractured surface of the composite. A relationship was observed between the creep mechanical response of LDPE–Al–Fique with respect to both the untreated fibres and the fibres treated with NaOH. Additionally, the four parameters of the Burger’s model were calculated from the creep curves. A very good agreement between the experimental data and the theoretical curves was obtained in the fluency region. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Emerging environmental trends and a growing concern about the greenhouse effect, and the need of the development of new products, have led industries to seek new composites based on sustainable materials, especially those reinforced with natural fibres. One such material is the Natural Fibre Thermoplastic Composite (NFPC), which constitutes an alternative to the use of renewable fibres in certain applications. Several natural fibres have been studied and implemented in the reinforcement of polymeric matrices. Natural fibres provide good mechanical properties and are cheap as compared with synthetic fibres. Different studies on the development of NFPC materials can be found in the literature [1–4]. However, some fibres have drawbacks when used in applications demanding high mechanical properties. Composites based on thermoplastic matrices may improve the mechanical performance (strength and rigidity) and provide a reduction in density, making it possible to obtain lighter and more resistant products [5–8]. Cellulosic fibres, such as: hemp, sisal, coir, jute, palm, bamboo and wood in its natural condition, as well as several cellulosic wastes, such as shells, wood flour and pulp have been used as fillers for both thermoplastic and thermosetting resins for several years. ⇑ Corresponding author. Address: Cll 25 # 115 – 85, Facultad de Ingeniería, Universidad Autónoma de Occidente, Cali, Colombia. Tel.: +57 2 3188000; fax: +57 2 5553911. E-mail address: [email protected] (M.A. Hidalgo-Salazar). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.06.032
Among the natural reinforcing filler materials, the fique fibre appears to be one of the most promising fillers, due to its high mechanical strength [9]. Moreover, fique fibres are readily available in the form of nonwoven mats and produced in an industrial scale in Colombia. The use of mats instead of short fibres with low aspect ratios has the advantage of reducing agglomeration of the fibres, which allows the production of rigid panels with the same properties in two different directions. Current creep studies have shown a time-dependent degradation in the module (creep) and strength (creep rupture) of NFPC materials, because of the natural viscoelastic behaviour of the matrix [10]. The creep behaviour of polymer matrix composites is a critical issue for many modern engineering applications, such as the parts for the aerospace industry, as the replacement of wood in the manufacturing of products such as reels for the winding of electrical cables, pallets and panels, which need to be light and possess good strength in order to protect packaged goods. Several studies on the creep behaviour of liquefied wood–polymer composites based on low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) have been reported [11]. It has also been reported that with an increase in fibre content, the flow of wood particles with polymer matrix composites decreases [12]. Using agricultural composites to develop structural building products often requires the improvement of their mechanical properties, especially the creep performance. Previous studies in the field show that the creep of agricultural composites varies with the type and content of matrices, coupling treatment, where the relative performance
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2. Experimental 2.1. Materials and methods
Fig. 1. Process for obtaining compound LDPE–Al–Fique test specimens for characterisation using a CNC cutting router at FabLab Cali in Universidad Autonoma de Occidente.
of the fibre–matrix interface is significant compared with the viscoelastic properties of the compounds [5,13]. Various mathematical modelling techniques have also been applied to analyse creep behaviour [5–7]. The technique of dynamic mechanical analysis (DMA), has been reported as an alternative to evaluate the long-term creep behaviour of different cane bagasse-based NFPCs for PVC and HDPE matrices [4]. However, few studies of creep that take into account changes of the adhesion between the polymeric matrix and the natural fibres have been reported [5,14]. In this paper, the interfacial adhesion between fique fibres and a LDPE–Al matrix obtained from the recycling of long-life packages of Tetra Pak is reported. Fibre–matrix adhesion was improved by modification of the fibre surface properties utilising treatments such as alkalinisation, silanization and pre-impregnation with polyethylene prior to preparation of the composite materials. The effect of the fibre–matrix interface in the creep behaviour and the dynamic mechanical properties were investigated for a composite material based on a matrix of LDPE–Al reinforced with fique fibres.
Fique fibre non-woven mats were used as reinforcement. The properties of the fique fibres and the LDPE–Al obtained from recycling long-life packages of Tetra Pak were presented in a previous paper [16]. The interfacial adhesion between the fique fibres and the matrix was modified using three different fibre surface treatments: First, the fique fibres were immersed in an aqueous solution of sodium hydroxide (NaOH) at 2% w/v for one hour at 25 °C, then the fibres were washed with distilled water to completely remove the remaining NaOH. Finally, the fibres were dried in two steps: first, at room temperature for 12 h and subsequently, in an oven at 60 °C for 24 h. Second, a silanisation treatment was performed after alkalinisation with NaOH with a silane-type coupling agent namely, a silane Tris (2-Methoxyethoxy) (vinyl) ‘‘A172’’. The fique fibres were immersed in a water–methanol solution 50/50 v/v in which 1.0% and 0.5% of silane and dicumyl peroxide. The Ph of the deionised water and methanol solution was adjusted to 3.5 using acetic acid and the solution was stirred for 30 min, and then, the fibres were immersed for 1 h. Subsequently, the fibres were decanted and dried for a period of 24 h at 60 °C. Finally, the fibres were subjected to curing for two hours at 120 °C. Just as with the alkaline treatment, it was necessary to use 300 ml of solution to treat 25 g of fique fibres. Third, for the preimpregnation treatment, the fique fibres were immersed in a solution of polyethylene in xylene at 1.5% w/w. The impregnation treatment was performed at a temperature of 120 °C, maintaining agitation at a constant speed of 100 rpm for 1 h. Then, the fibres were dried at 60 °C for 24 h. Again, 300 ml of solution was used to treat 25 g of fique fibres. The low-density polyethylene aluminium (LDPE–Al) used was obtained directly from the recycling process of long-life Tetra Pak packages; this residue is obtained from a hydropulping process and was subsequently pulverised to increase the adhesion of the fique fibres and to produce a better distribution in the matrix. The LDPE–Al has a volume percentage ratio of 89.75 LDPE and 10.25 Aluminium with an average density of 1.10 g/cm3. The composite LDPE–Al–Fique was obtained in panels with dimensions of 290 290 2 mm. A closeup-type stainless steel mould was used and panels were manufactured by compression moulding in a hot plates press LabPro400 Fontijne PRESS. Fique tissues were dried in an oven at 50 °C for 24 h before the preparation of the
Table 1 Summary of LDPE–Al–Fique 10%, 20% and 30% incorporation of fibre at 25 °C and 1.2 MPa results for 4 parameter models to 1800 s creep stress for treatments F and A. Model
Parameters
4
E1 (Pa) E2 (Pa) g1 (Pa s) g2 (Pa s) WSS
Treatments 10F
20F
30F
10A
20A
30A
1.08E+09 4.80E+09 1.30E+13 7.57E+11 1.23E07
1.33E+09 7.97E+09 2.10E+13 1.61E+12 1.25E07
1.50E+09 1.44E+10 2.77E+13 5.66E+11 5.44E08
1.19E+09 6.44E+09 1.50E+13 9.06E+11 6.21E08
1.41E+09 1.28E+10 2.95E+13 1.37E+12 4.91E08
1.95E+09 1.30E+10 3.57E+13 1.72E+12 5.46E08
Table 2 Summary of LDPE–Al–Fique 10%, 20% and 30% incorporation of fibre at 25 °C and 1.2 MPa results for 4 parameter models to 1800 s creep stress for treatments S and P. Model
4
Parameters
E1 (Pa) E2 (Pa) g1 (Pa s) g2 (Pa s) WSS
Treatments 10S
20S
30S
10P
20P
30P
1.35E+09 6.52E+09 1.90E+13 5.13E+11 3.43E07
1.88E+09 1.07E+10 1.88E+13 1.12E+12 6.91E08
2.16E+09 1.66E+10 2.28E+13 2.24E+12 4.33E08
1.39E+09 8.67E+09 2.17E+13 6.92E+11 1.08E07
1.96E+09 9.53E+09 2.34E+13 2.93E+11 8.77E08
2.15E+09 1.61E+10 2.78E+13 1.13E+12 5.61E08
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Fig. 3. The behaviour of both (r0/E1) and (r0/g1) as a function of fibre surface treatment and fibre content.
Fig. 4. Short-term compliance curves for 30% fique LDPE–AL treatments F, A, S and P.
LDPE–Al and fique without treatment; A: for fique fibres with alkali treatment; S: for fique fibres with alkali treatment and silanization, P: for fique fibres with alkali treatment, silanization and impregnation with LDPE. Formulations of LDPE–Al–Fique of 10%, 20% and 30% v/v fique were prepared, for each treatment (F, A, S and P). Finally, the specimens were cut using a computerised numerical control equipment, which guarantees to obtain specimens for various tests like tensile, bending, impact and DMA; Fig. 1 presents an image of the cut specimens. The morphology of the composite samples was studied using a JOELJSM-6490 scanning electron microscope (SEM). Micrographs of the fracture surfaces were obtained for selected specimens. Prior to scanning, the surfaces were gold-coated to avoid charging under the electron beam. Fig. 2. Creep tests for different formulations of LDEP–Al–Fique: (a) 10% of fique fibre, (b) 20% of fique fibre and (c) 30% of fique fibre.
2.2. Creep tests compounds. To prepare the compounds, fique nonwoven mats were placed between Teflon films along with the LDPE–Al powder, ensuring a complete coverage of the mat filling the mould. The samples were compression moulded at 170 °C and 3 MPa for 11 min and cooled for 4 min at a cooling rate of 32.5 °C/min. The nomenclature used to identify the different fibre surface treatments in each composite material was the following: F: for
A Rheometric System Analyser RSAIII dynamic mechanical analyser (DMA) was used in the experiments to measure creep deformation as a function of time. The tests were performed in flexural mode using specimens 2 mm thick, 44 mm long and 12.7 mm wide. The static stress of 1.2 MPa was applied for 30 min. The static stress was selected after performing a stress sweep test, where the linear region was defined for each of the composites including the
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LDPE–Al, and ensuring that the creep tests were conducted in the linear region. Trials were also conducted for the 30% fique LDPE– Al for treatments F, A, S and P for a static stress of 2 MPa. These tests were compared with those reported in the literature for composites based on PVC, recycled HDPE reinforced with fibres of cane bagasse, using the same experimental conditions [4]. The average temperature at which the tests were performed was 25 °C, although some tests were conducted at different temperatures. 2.3. Dynamic mechanical testing Viscoelastic properties, such as storage modulus (E0 ), loss modulus E00 and mechanical damping parameter tan d were measured as a function of temperature using a Rheometric System Analyser RSAIII dynamic mechanical analyser (DMA). The measurements were carried out in the three-point bending mode with rectangular specimens of dimensions 44 12.7 2 mm in a temperature range of 135 to 110 °C at a heating rate 3 °C/min, and nitrogen gas flow was used to control the temperature. Samples were analysed at a fixed frequency of 1 Hz to 0.01% strain.
Fig. 5. Bar graph and normalised results given in Table 3 of the strain rate; all at 25 °C and 1.2 MPa except the LDPE–Al to 0.7 MPa.
3. Results and discussion 3.1. Creep behaviour The four parameters of Eq. (1) were calculated for the composites with different fibre content as well as different interfacial adhesion levels for each creep curve are shown.
eðtÞ ¼
r0 E1
þ
E t r0 r0 2 tþ 1 e g2 g1 E2
ð1Þ
The parameters reported in Tables 1 and 2 show that instantaneous elastic deformation (r0/E1) and the slope of the steady-state creep area (r0/g1) corresponds to the viscoelastic behaviour of the composite material. The error is estimated based on least squares adjustment of the parameters and was calculated using the Weighted Sum of Squares (WSSs)
wss ¼
X
2
¼ 1n wi ½eðti Þ e^ ðti Þ
ð2Þ
i
where e(ti) represents the experimental strain observed in time ti, ^eðti Þ is the strain estimated by the model and wi is the difference between two time samples. A smaller WSS value indicates a better model fit to the experimental data. Fig. 2 shows both, the short-term creep strain tests results as well as the theoretical creep strain values for LDPE–Al–Fique composites with different concentrations of fique (10%, 20% and 30%),
and different fibre surface treatments (F, A, S and P). The agreement between the theoretical predictions and experimental results is very good with just small discrepancies that appear in the first creep region. It can be observed that E1 increases as the volume fraction of fique fibres increases for the different fibre surface treatments. It can also be noted that the creep strain decreases as the fique content increases, as expected when a rigid reinforcement is added into a viscoelastic matrix [5]. A smaller creep strain is observed for treatments S and P, especially at higher volumes of 20% and 30%, indicating that at fique fibre volumes contents below 10% the composite creep behaviour is still governed by the matrix behaviour. The behaviour of both (r0/E1) and (r0/g1) is shown in Fig. 3 as a function of fibre surface treatment and fibre content. It should be observed that, for low fibre volume content (10%) both parameters exhibit a linear behaviour, for the different fibre surface treatments, indicating that the fibre–matrix behaviour plays an important role. For higher fibre contents, the creep response is smaller and there is no notorious change with fibre–matrix interactions. The viscous response given by r0/g1, also decreases with increasing fibre content [14]. It should be remembered that the volume fraction of the interface also increases for higher fibre volume fractions and the presence of the covalent bonds that are created with the silane fibre surface treatments generate less compliant interphases with the matrix. This behaviour of the short term creep compliance is shown in Fig. 4, for composites produced
Table 3 Strain rate data for all treatments at 25 °C and 1.2 MPa stress. Fique (%)
de/dt (s1) Experimental
de/dt (s1) Model
g1 (Pa s)
g1 (Pa s)
Experimental
Model
F
10 20 30
8.00E08 ± 9.60E09 5.40E08 ± 8.40E09 3.60E08 ± 3.40E09
8.00E08 ± 2.30E09 5.80E08 ± 1.50E09 3.40E08 ± 3.00E09
1.50E+13 ± 1.90E+12 2.30E+13 ± 3.60E+12 3.40E+13 ± 3.10E+12
1.50E+13 ± 4.40E+11 2.10E+13 ± 5.40E+11 3.60E+13 ± 3.30E+12
A
10 20 30
7.10E08 ± 1.20E08 5.40E08 ± 1.40E10 3.50E08 ± 2.80E09
6.70E08 ± 1.60E09 5.00E08 ± 1.40E09 3.80E08 ± 4.30E10
1.70E+13 ± 3.10E+12 2.30E+13 ± 8.10E+11 3.50E+13 ± 2.70E+12
1.80E+13 ± 4.30E+11 2.40E+13 ± 6.90E+11 3.20E+13 ± 3.70E+11
S
10 20 30
6.60E08 ± 5.90E09 5.30E08 ± 6.80E09 2.90E08 ± 7.90E09
5.90E08 ± 1.10E09 5.20E08 ± 7.70E10 3.00E08 ± 6.80E10
1.80E+13 ± 1.70E+12 2.30E+13 ± 3.10E+12 4.30E+13 ± 1.10E+13
2.00E+13 ± 3.70E+11 2.20E+13 ± 3.40E+11 4.00E+13 ± 9.10E+11
P
10 20 30
4.20E08 ± 6.30E09 3.40E08 ± 5.80E09 2.10E08 ± 2.40E09 1.20E07 ± 1.30E08
4.50E08 ± 3.50E10 3.50E08 ± 3.10E10 2.50E08 ± 1.80E09 1.20E07 ± 7.30E09
2.90E+13 ± 4.80E+12 3.60E+13 ± 6.40E+12 5.80E+13 ± 6.90E+12 1.00E + 13 ± 1.10E+12
2.70E+13 ± 2.10E+11 3.40E+13 ± 3.10E+11 4.90E+13 ± 3.70E+12 9.70E+12 ± 5.60E+11
LDPE–Al
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with 30% of fique fibres for each of the respective treatments. Again, a lower creep compliance is observed for the silane treated fibres. The order of E2 are close to E1, indicating an increased viscoelastic strain that represents a delayed elastic behaviour. The creep performance of the composites can be observed in the strain rate values presented in Table 3. They are also shown in the bar graph in Fig. 5 which shows normalised results of the strain rate, that is, the evolution of the creep response of the composite for the different fique fibre content and fibre surface treatments. The values of the experimental g were calculated with the slope of the second zone of the creep curves, based on the mathematical models. The values in table 3 are slightly different because they correspond to a repetition in our experimental design. The standard deviations observed in Fig. 5, should be considered normal for NFPCs because of the nature of the fique fibres, however, the strain rate behaviour also decreases for increasing fibre volume content, and with the increase in the fibre–matrix interaction resulting from the stronger interface [11]. When the fibre surface is modified with the alkaline treatment, the viscoelastic matrix is able to better impregnate the small ridges and crevices resulting from the removal of the low molecular weight compounds from the fibre surface. When the stress is applied to the composite material, the fibre–matrix interactions are of the frictional type and shear loads at the interface and will be responsible for the matrix/interface material flow in shear. When the fibres are treated with a silane coupling agent, strong covalent bonds between fibre and matrix
349
are responsible of the more efficient load transfer mechanism. This should contribute to the elastic rather than the viscous behaviour of the composite material. In order to corroborate the fibre–matrix interface behaviour of the composites was observed by SEM and the micrographs obtained are shown in Fig. 6a (untreated fique), which shows that the fique fibres are being pull out of the matrix and there are no traces of polymer still adhering to the fibre that would indicated fibre–matrix adhesion. Even more, in some cases, a gap between the fibre and the matrix appears to be created, i.e., the fibre is practically separated from LDPE. Furthermore, when the fibre surface was treated with the alkaline aqueous solution, the surface roughness increased. (Fig. 6b, surface was obtained and therefore, the possibility of mechanical interlocking increased, thus contributing to an increase in the shear interfacial strength. Such increase is observed to result in a lower creep deformation of the composite. In this case the failure surface did not show evidences of fibre pull-out off the matrix material and the fibre failure was a clean tensile failure, almost parallel to the polymer surface. Also, the polymeric matrix surface shows some shear hackles in the vicinity of the failed fibre. In Fig. 6c (treatments S and P), shows a matrix layer uniformly distributed over the surface of the fibres and that the fibres are substantially bonded to the matrix due to the improved interfacial adhesion. From the morphology shown in these composites, a positive response to creep testing is observed, because the load transfer efficiency of well-bonded fibres in composites contributes to enhanced strength [17].
Fig. 6. Scanning electronic micrograph. Interface in the fique fibres reinforced composite: (a) untreated, (b) fique fibre reinforced treated with NaOH, (c) fique fibre reinforced treated with NaOH, silane and pre-impregnated.
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3.2. Dynamic mechanical behaviour Fig. 7 shows changes in the storage modulus versus temperature of the composites prepared with 30% fique and LDPE–Al w/ w. At low temperature, the storage modulus of LDPE–Al and the composites of LDPE–Al–Fique is very similar. However, the difference increases as the temperature increases and this more noticeable for temperatures above 40 °C. This change is a relaxation of the amorphous phase of the LDPE. After that, E0 and E00 continues decreasing. At 75 °C approximately, the matrix reaches the softening region where the composites have both a viscous and solid behaviour, also suggesting that at this temperature the fibre–matrix interfacial interactions starts deteriorating, until reaching melting temperature near 110 °C. This can be observed in Figs. 8 and 9, which indicate the loss modulus and tan d respectively. The reinforcing effectiveness of the fique fibres is evident on the moduli of the composites and can be represented by coefficient C [15,18]:
C¼
ðEg =Er Þcomp ðEg =Er Þre sin
ð3Þ
where Eg and Er are the storage modulus values in the glassy and rubbery region, respectively. The lower the value of the C constant, then, the more effective the reinforcement is. The measured E0
´ to temperature sweep for: LDPE–Al[}], Fig. 7. Response of storage modulus E 30F[D], 30A[r], 30S[h], 30P[s].
Fig. 9. Response of tan d to temperature sweep for: LDPE–Al[}], 30F[D], 30A[r], 30S[h], 30P[s].
values at 30 and 25 °C were employed as Eg and Er, respectively. Although the values obtained for the different composites are very similar (C = 0.50–0.59), it is interesting to note that they are much lower than those reported for cellulosic short fibre-based composites (i.e., 0.96 for 30% w/w banana fibre–polyester resin), indicating that a better reinforcement effect can be achieved using long fibres as in the nonwoven mat used here[19]. The LDPE–Al–Fique with P treatment composite presents slightly higher E0 and E00 across the whole range of temperature compared with that of the S treatment sample. This behaviour is the result of two different interface interactions, first, the presence of covalent bonds from the silane surface treatment and second, the impregnation of fique fibres with polyethylene, effectively achieving a better contact between fibre and matrix, which is even more significant when compared with treatment F. In general, it can be said that the decrease of the E0 curve, is smaller with increasing fibre–matrix interface strength, [20]. Fig. 9 shows tan d versus temperature for LDPE–AL–Fique composites and pure LDPE–AL. The molecular relaxations in the LDPE– AL decrease in the LDPE–AL–Fique composites. The molecular motions in the interfacial region generally contribute to the damping of the material apart from those of the constituents [21]. The width of the peaks for LDPE–Al are more notorious than those peaks of the composites and practically the same for treatments A, S and P, indicating a better damping for these compounds. A composite with a poor interface tends to dissipate more energy than one with a good interface. In the composites of LDPE–Al treatments S and P, the presence of covalent bonds between fibre and matrix results in lower molecular mobility in the interfacial region. The presence of the fibres also adds complexity to the dynamical behaviour. The transition related to the glass transition takes place at 19.59 °C for LDPE–Al, measured at max peak 1, as shown in Fig. 9. The LDPE–Al–Fique composites exhibit a max peak 1 in the tan d at approximately 2.20 °C, 2.03 °C and 3.22 °C for the A, S and P fibre surface treatments respectively. The most notorious difference between peaks of the LDPE–Al and the composites is noticed at temperatures above the Tg of the LDPE but the presence of the fibres and the effect of the fibre–matrix interface interactions is more notorious for temperatures above 30 °C.
4. Conclusions
Fig. 8. Response of the loss modulus E00 to temperature sweep for: LDPE–Al[}], 30F[D], 30A[r], 30S[h], 30P[s].
As is the case with all natural products, the mechanical and physical properties of natural fibres vary considerably. These properties are determined by the chemical and structural composition and cellulose, the main component of all natural fibres, which
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varies from fibre to fibre. The creep strains decreases as fique content increases however, the fibre–matrix interactions attributed to the silane covalent bonds result in much lower creep starins. Additionally, scanning electronic micrographs confirmed that the interfacial adhesion between the fique and the LDPE thermoplastic matrix was enhanced when alkalinisation, silanization and impregnation with polyethylene treatments applied to the fibres. However, the interface formed with treatments S and P resulted in a better creep response, which could be observed in creep strains and strain rates. Correspondingly, the temperature sweep tests with DMA confirmed that the fibre–matrix interface interactions resulted in better damping properties as well as smaller variations in the storage and loss moduli more notoriously for the S and P fibre surface treatments. Acknowledgements We express our gratitude to Tetra Pak Colombia, INNOPAK CaliColombia and Universidad Autónoma de Occidente, Cali-Colombia, and its FabLab, for providing their equipments to obtain specimens and Compañia de Empaques from Medellin Colombia, for providing the Fique, the sponsors of this project. References [1] Faruk O, Bledzki AK, Fink H-P, Sain M. Biocomposites reinforced with natural fibres: 2000–2010. Prog Polym Sci 2012;37:1552–96. [2] Kiguchi M. Latest market status of wood and wood plastic composites in North America and Europe. In: The second wood and wood plastic composites seminar in the 23rd wood composite symposium. Kyoto, Japan; 2007. [3] Wambua P, Ivensn J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63:1259–64. [4] Xu Y, Wu Q, Lei Y, Yao F. Creep behavior of bagasse fibre reinforced polymer composites. Bioresource Technol 2010;101:3280–6. [5] Acha BA, Reboredo MM, Marcovich NE. Creep and dynamic mechanical behavior of PP–jute composites: effect of the interfacial adhesion. Compos Part A: Appl Sci Manuf 2007;38:1507–16.
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