Composites Science and Technology 61 (2001) 2405–2411 www.elsevier.com/locate/compscitech

Natural-fibre-reinforced polyurethane microfoams Andrzej K. Bledzkia,*, Wenyang Zhanga, Andris Chateb a

Universita¨t (Gh) Kassel, Institut fu¨r Werkstofftechnik, Kunststoff und Recyclingtechnik, Mo¨nchebergstraße 3, 34109 Kassel, Germany b Riga Technical University, Institute of Computer Analysis of Structures, Kalka iela 1, LV-1658 Riga, Latvia Received 7 December 2000; received in revised form 24 July 2001; accepted 26 July 2001

Abstract Polyurethane-based composites reinforced with woven flax and jute fabrics were prepared with an evenly distributed microvoid foam structure. The relationship between the resin-filled grade and the microvoid content and the density was described. The influence of the type of reinforcing fibre, fibre and microvoid content on the mechanical properties was studied. The investigation results for the static mechanical properties of the composites were described by approximate formulae. It was found that the specific data were only slightly dependent on microvoid content. Increasing the fibre content induces an increase in the shear modulus and impact strength. However, increasing the microvoid content in the matrix results in a decreased shear modulus and impact strength. The woven flax fibre results in composites with better mechanical strength than the woven jute fibre composites. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: B. Mechanical properties; Natural-fibre-reinforced polyurethane microfoams

1. Introduction Natural fibres are a low-priced and sustainable natural resource. With increasing environmental protection consciousness, natural fibres as a relatively new group of environmental friendly materials are in considerable demand in recent years, by unifying technological, economical and ecological aspects. Natural-fibre-reinforced composites have good mechanical properties with a low density, as illustrated by comparing the properties of different fibres in Table 1. The densities of natural fibres are of the same order as those of plastics and only 40–50% of that of glass fibres. Because of this, plastics can be reinforced or filled without having significant effects on the density. On the other hand, the tensile strength and the Young’s modulus of the E-glass are visibly much higher than that of the natural fibres. However, the difference in the specific values of glass and natural fibre, most important with respect to applications, is not as great. The newest research and development results dealing with natural-fibre-reinforced plastics show possibilities for partial replacement of inorganic fibres in interior * Corresponding author. E-mail address: [email protected] (A.K. Bledzki).

components of cars and trucks, such as door linings, parcel racks and column trim [1–4,21]. Among others, natural-fibre-reinforced foamed materials have considerable importance because of the possibility of reducing the density of automobile construction additionally by virtue of the cellular structure [3,5,10–13]. Unfortunately, only limited information dealing with that topic is available. For instance, natural-fibre-reinforced polyurethane foams (with SRIM-technology) have lower densities (ca. 400–700 g/ m3) [3,11]. The tensile strength of foamed composites is about 28 MPa with a density of 0.73 g/cm3 and a fibre content of 30 wt.% [11]. Natural-fibre-reinforced epoxy foams (with GLS-technique [14]) combine relatively good overall mechanical properties with a low specific mass. Its impact properties, such as loss energy and damping index, were almost linearly dependent on microvoid content in the epoxy matrix. Increasing the fibre content enhances the impact strength and decreases the loss energy and the damping index [12]. The dynamic mechanical thermal behaviour of these composites were affected during different fibre and microvoid content. Increasing microvoid content induced a decrease of the shear modulus and an increase of the log decrement peak, respectively. The 42 vol.% fibre content significantly enhances the shear modulus [13].

0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00129-4

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Table 1 Comparison of the properties of natural fibres and glass fibres [6–9] Properties

Unity

Flax

Jute

Hemp

Sisal

E-glass

Density Tensile strength Specific tensile strength Young’s modulus Specific Young’s modulus

g/cm3 MPa MPa/(g/cm3) GPa GPa/(g/cm3)

1.5 254–1035 169–690 12–28 8–22

1.3 350–770 269–592 12–26 9–20

1.4 480–700 343–500 – –

1.5 380–635 253–423 9.4–22 6-15

2.5 2000–3400 769–1346 70–73 27–28

This paper will study the relationship between various natural fibre and microvoid content and the mechanical behaviour of flax and jute fibre-reinforced polyurethane-microfoams.

different microvoid contents in the composites were varied by different resin filled grade in the tool. 2.2. Test methods 2.2.1. Flexural test The laminated plates were cut into specimens of dimensions about 80154 mm3 for flexural test, the tests were carried out according to DIN EN 63, with a test velocity of 2 mm/min.

2. Experimental 2.1. Materials The materials used and its suppliers are given in Table 2. A polyurethane resin based microfoam was used as a polymer matrix. As reinforcement woven jute and flax yarns were used with a length of 20–40 mm and a diameter of 37 and 20 mm for the jute and flax elementary fibres, respectively. Before the composite processing, the woven fibre fabrics were pretreated with Tinozym AL/Ultravon CN-solution (from Ciba Geigy, Germany) to dewax the textiles, after that the woven fibre fabrics were dried at a temperature of about 80
C for 2 h in a convection, continuous drying oven. Specimens were moulded by using a compression moulding technique at a temperature of about 60
C for 10 min. In this process, the woven fibre fabric plies were laid in a steel frame. The apparatus and the location of the woven fabric in the moulding are shown schematically in Fig. 1. The resin was cast into the woven fibre fabrics. The force associated with the mould closing, caused a resin flow distribution on each ply of the woven fibre fabrics in the apparatus, and then the foamed process occurred prior to curing. The specimens consisted of different fibre contents (0–40 vol.%) by varying the woven fabric plies (0–4 plies with thickness of 2 mm and 0–8 plies which thickness of 4 mm). Specimens with

2.2.2. Torsion pendulum test Dynamic mechanical thermal analysis was performed using a torsion pendulum tester ZWICK 5201 according to method A (free vibration method) of DIN 53445. Specimens tested, as cut from the laminated sheets measured 65102 mm. The temperature of the specimens in the oven is controlled by a thermoregulator within the range of 25–230
C at a heating rate of 1
C/ min. During free oscillations of the specimens at different temperature, the shear modulus, log decrement and frequency was determined by a position-sensitive diode detector on an optical system. Here, the shear modulus G is a quotient from the amplitude of the shear stress to the amplitude of the shear. The logarithmic decrement  describes the inherent damping behaviour of materials, which is determined by the ratio of successive amplitudes An from the sinusoidal wave (=ln (An1/ An) and the oscillating frequency f. 2.2.3. Impact test All specimens were tested in a ‘‘low-velocity falling weight impact’’ tester at ambient temperature in nonpenetration-mode. The mass of the impactor was chosen

Table 2 Materials used Natural fibre/textile form Jute, woven fabric, 51/51 Flax, woven fabric, 51/50

Density (g/cm3) 1.31 1.50

Area weight (g/m2)

Supplier

262 277

J. Schilgen GmbH & Co, Germany J. Schilgen GmbH & Co, Germany

Resin system

Mixing ratio (parts by weight)

Curing condition

Supplier

Polyol: BAYDUR PU1681 Isocyanat: DESMODUR 44V10L

100 140

60
C for 10 min

Bayer AG, Germany

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Fig. 1. Apparatus and location of the woven fabric in the moulding.

with 0.75 kg. The impact energy of 1.5 Joule was adopted by a decided falling height. Dimensions of the impact specimens were 50 mm50 mm2 mm. The results of the impact test can be described by two separate issues (see Fig. 2) [17–20]: (i)

(ii)

Fig. 2. Typical impact force-deflecation curve for natural reinforced polymer composites including definition of the characteristic values used.

force-deflection (or time) curve: the forcedeflection (or time) curve refers to all the materials behaviours including the damage initiation defined by the first significant drop of the force (Fig. 2), characteristic values: loss energy (Wv) as a measure of dissipated energy and strain energy (Ws) as a measure of the stored energy, and the damping index () as ratio of loss energy to strain energy.

3. Results and discussion 3.1. Density, microvoid content and structure In general the resin foamed process is affected by interior pressure in the apparatus. The reaction of the PUR-system used is an exothermic reaction. The foamed pressure (the interior pressure in the apparatus) in the exothermic process can be changed by the polyurethane resin filled grade in the apparatus, so that the foamed grade in the composites was varied [23]. The relationship between the resin filled grade and the density and the microvoid content of the woven flax based composites (fibre content: 29 vol.%) is described in Fig. 3. Increasing the resin filled grade in the apparatus induced a linear increase of the density and a linear decrease of the microvoid content. The optical microscope investigations indicate that the fibre yarns of the woven fabric with the warp and weft direction are evenly distributed in the matrix of the polyurethane resin, independent of whether the matrix was foamed or not (see Fig. 4). These results can be attributed to the textile form of the woven flax and jute fibres, with which the polyurethane resin can be permeated well among the fibre yarns. The microvoids with different contents of the foamed composites are shown as an evenly distribution in the interphase of the plies and resin-rich zones between the fibre yarns (see

Fig. 3. Relationship between polyurethane resin filled grade and density and microvoid content; flax fibre weight 110 g; flax fibre content 29 vol.%; tool capacity 251 cm3.

Fig. 4b). The analysis of the optical microscope image by the different micrographs indicates that the average diameter of the microvoids is < 70 mm. 3.2. Static properties With varying content of fibers and microvoids the static mechanical properties could be affected for all composites investigated. Their relationship can be described by approximate formulae [22] based on experimentally determined results. For instance, the flexural Young’s modulus by the woven flax based composites is formulated as: y ¼ 4158  2587z2 þ 116:8z1 z2  0:1643z31 þ 4:676z21 =z2 Here: z 1 = x1 z2 = 0.48556+0.014584 x2 y = flexural Young’s modulus

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x1 = fibre content in vol.% x2 = microvoid content in vol.% The representative values of fibre and microvoid content on the flexural Young’s modulus by the approximate formula are taken as shown in Fig. 5. Here, the flexural Young’s modulus increases by ca. 220 MPa, by each 1 vol.% increase of fibre content. However, the increase of each 1 vol.% of microvoid content leads to a decrease of ca. 90 MPa of the flexural Young’s modulus. These observed significant reductions in the matrix or interfacial dominated mechanical properties was already found by similar tests for glass-fibre-reinforced composites [15,16]. On the other hand by taking the differences in density into account, the decrease of the specific mechanical properties for the composites is not as great. The increase of each 1 vol.% of microvoid content leads only to a decrease of 0.15 and 15 MPa, respectively, of the specific flexural strength and the specific Young’s modulus for the woven flax based composites (Fig. 6). Furthermore, it was found and shown in Fig. 6 that the woven flax fibre based composites exhibit, in general, a higher level for investigated values than the woven jute fibre. This was already found in earlier published results [7-8]. For instance in Fig. 6, the specific flexural strength and the specific Young’s modulus for

the flax fibre based composites (microvoid content < 1 vol.%) were approximately 12 and 17%, respectively, higher than jute fibre based ones (microvoid content < 1 vol.%;). This result can be attributed to the fact that flax fibre has a higher strength than jute fibre. 3.3. Dynamic mechanical properties 3.3.1. Torsion pendulum test The dynamic mechanical thermal properties of composite materials are effected by their components and structure. While the temperature is increased, the dynamic mechanical behaviour is monotonically decreased below the Tg temperature and is closely related to the phenomenon of mechanical relaxation because of molecular motions in the transition from the glass to the rubber-like state. Fig. 7 illustrates the change of dynamic mechanical behaviour on the temperature as a function for pure polyurethane resin and woven flax fibre reinforced composites (microvoid content < 1 vol.%). The temperature range investigated for pure polyurethane resin was limited to 195
C, because the specimens cannot be oscillated beyond that temperature. The stiffness of both specimens exhibited a greater rate of decrease at the temperature of the log decrement peak in the a relaxation region. The reinforcement effect is significant with the experimental range temperature of

Fig. 4. Optical micrograph of fibre and microvoid distribution in the matrix; (a) jute fibre-reinforced polyurethane composites, 29 vol.% fibre content, microvoid content