Mechanical performance of polyurethane (PU)-based biocomposites

Mechanical performance of polyurethane (PU)-based biocomposites

17

M.I. Aranguren, N.E. Marcovich, M.A. Mosiewicki Institute of Research in Materials Science and Technology (INTEMA), National University of Mar del Plata, Mar del Plata, Argentina

17.1 Introduction Polyurethanes (PUs) are a family of polymers that has grown based on their versatility. They can be formulated from varied raw materials and processing methods, leading to polymers with properties designed to satisfy specific conditions: from structural to functional and responsive materials, which can be manufactured into very different applications; from insulating or cushioning foams to biomedical devices, as well as nonstructural applications such as adhesives or coatings. To obtain a PU, a monomer containing at least two hydroxyl groups (diol) must be reacted with another one containing at least two isocyanate groups (diisocyanate), to produce urethane bonds, as shown below in a very simplified scheme (Scheme 17.1). Incorporation of molecules containing a larger number of reactive groups (polyols and/or polyisocyanates) leads to network structures that can be rigid or flexible depending on the length of the original chains between reactive groups but also on the nature of the chosen reactives (e.g., linear or aromatic isocyanates). Depending on the formulation chosen, cross-linked elastomers with a wide range of modulus and extensibility can be obtained, as well as thermoplastic PUs that are linear, phase-separated, structured polymers. Besides, these materials can be prepared as compact polymers, foams, or fibers, which explain their extensive utilization in so many different areas. To further tune their behavior as well as prices, fillers and reinforcements can also be added to formulate composite materials. In particular, from the last decade of the twentieth century to now, increasing interest has arisen in the use of vegetable fibers and particles as reinforcements or fillers of polymer composites. While this endeavor has required the use of modifiers to be used in most thermoplastics matrices of high-volume production (Marcovich et al., 2014; Reboredo et al., 2008; Aranguren, 2006), in the case of PU the compatibility with polar vegetable materials is high and, thus, composites are generally prepared without modifiers or compatibilizers with usually good results. Although other natural fibers are possible, such as animal fibers, like wool or silk, in which proteins are the main components, in this chapter we will focus on the use of vegetable materials, which have been more frequently analyzed in the open literature and have already found a commercial niche. Biocomposites: Design and Mechanical Performance. http://dx.doi.org/10.1016/B978-1-78242-373-7.00010-X Copyright © 2015 Elsevier Ltd. All rights reserved.

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R

N

C

O + R

OH

R

H

O

N

C

O

R

Scheme 17.1  Urethane bond formation from reaction of isocyanate and hydroxyl groups.

Vegetable fibers and particles offer the advantages of large worldwide availability, reduced wearing of the processing equipment, neutral carbon balance, and high specific properties, providing additional benefits in their use for acoustic and thermal insulation because of their natural porous structure (Peng et al., 2011).

17.2 Vegetable particles/fibers and synthetic PUs 17.2.1 Cross-linked PU elastomers Given the relative low price of the fillers (which frequently are wastes from agricultural- or forestry-derived activities), it is logical to consider matrices of low price. This was the case in the work of Rácz et al. (2009), who used a recycled polyol to produce lightweight reinforced PU for acoustic panels in construction. The experimental polyol was obtained from PU scraps from the upholstery and automotive industries. The highly viscous polyol required the addition of a viscosity modifier to prepare filled PU foams. The cross-linking agent was a polymeric aromatic isocyanate (pMDI) and the filler was pine wood flour (WF) (average size of 100 μm, concentration range 0–20 wt.%). The polyol had an average humidity content of 5 wt.%, which acted as a foaming agent, contributing to reduce total weight and improve insulation. The density of the microfoamed composites was in the range of 0.55–0.76 kg m−3 and, as could be expected, the tensile properties (modulus and strength) of the composites increased with the concentration of WF (234 ± 28 and 4.8 ± 0.3 MPa for the neat PU, and 604 ± 21 and 9.2 ± 0.6 MPa for the 20 wt.% WF composite). Flexural properties of the composites showed a similar trend, with the modulus of the 20 wt.% WF composite being more than twice that of the neat PU. On the other hand, the more rigid composite showed a more fragile behavior and had a reduced impact resistance (8.60 ± 1.14 and 4.70 ± 1.01 kJ m−2, for the neat PU and 20 wt.% WF composite, respectively). Although WF is hygroscopic, the composites absorbed less water than the neat PU. The interfacial interactions that most probably involve covalent bonding between particles and polymer prevent the swelling of the particles and matrix by incoming moist during water immersion tests (Rácz et al., 2009). This result is important in maintaining good insulation properties. Bledzki et al. (2001) compared jute (bidimensional mat) and flax incorporated in PU composites and found that flax composites presented higher strength and stiffness. As usual, the increase in fiber concentration resulted in increased Young's modulus and flexural strength. Batouli et al. (2014) mixed a low molecular weight polyester-diol with methylene diphenyl diisocyanate, MDI (at NCO/OH = 1.3) at high speed and allowed it to freely foam. Kenaf core ground and dried (70 °C for 4 h under vacuum) was added to the

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formulation to achieve weight percentages of 5, 10, and 15 wt.%. In this case, the addition of fibers was not successful in improving PU properties, and 15 wt.% of the powdered fibers were necessary to just reach properties similar to that of the unfilled matrix. Actually, maximum stress under flexural tests was 0.178 ± 0.03 MPa for the pure PU, but it was as low as 0.084 ± 0.06 MPa for the 5 wt.% filled foam. The authors explained this result by proposing that kenaf located inside the foam cells, filling the pores, instead of reinforcing the wall cells. Although not mentioned by the authors, it is also possible that the relatively large size of the fibers have compromised the integrity of the cell walls, reducing the strength of the structures, so that higher fiber concentrations are needed to produce high strength materials. An elastomeric cross-linked reinforced PU with good damping properties, as bearing pad for acoustic vibration isolation in railway/underground lines, was prepared from polyethylene adipate diol 4,4′diphenylmethane diisocyanate with 1,4-butane diol or 1,6-hexanediol, glycerin (Gil, 2009; Oprea, 2008) and 1–15 wt.% cork. As in most cases, filler addition increased the Young's modulus of the materials and decreased the elongation at break. Rials and Wolcott (1998) prepared composites by spraying a PU based on a polyether polyol and an isocyanate prepolymer on two cellulosic pulps, one obtained from recycled paper and the other being a virgin thermomechanical pulp of southern pine. Dynamic–mechanical studies showed that the glass transition temperature, Tg (assigned to the temperature of the maximum in tan δ), shifted from −40 to −26 °C, while the peak intensity was reduced from 1.5 to 0.31, a larger change than expected from dilution effect. As typically reported, the storage modulus at high temperature was dramatically increased with respect to the neat PU. Fibers from oil palm empty fruit bunch were ground to different particle sizes (EFB), dried, and used as filler in a PU made from MDI and polyethylenglycol (PEG). Particles were incorporated in the isocyanate component; the PEG was added and mechanically stirred. The mixtures were cured under pressure (125 °C, 5 h, 500 kg/cm2) and then postcured at 125 °C for 24 h (Rozman et al., 2001). The contribution of the EFB to the concentration of OH was considered and the total OH content (from EFB and PEG) remained constant. A maximum in tensile strength and modulus was observed at about 50 wt.% EFB for all particle sizes, with the exception of the largest particles (35–60 mesh), for which the maximum occurs at 60 wt.%. Moreover, samples made with larger particles appear to be tougher under tensile tests, which was explained by introducing the idea that smaller particles have more OH per unit weight and some of these groups could not react with the isocyanate, which led to weaker interface and lower toughness. An alternative explanation could be that smaller particles lead to more rigid but more fragile materials, due to the reduced extensibility and mobility of the polymer that interacts with the particles.

17.2.2 Thermoplastic PUs El-Shekeil and coworkers (2011, 2012a, 2013) prepared composites from a ­polyester-based thermoplastic polyurethane (TPU) and kenaf bast fibers extracted by mechanical decortication (KF). Fibers were ground and then separated into three

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d­ifferent sizes (300–425, 125–300, and <125 μm) by sieving. Samples containing 30 wt.% of fibers were prepared by melt blending followed by compression molding (El-Shekeil et al., 2011). The authors found that the tensile strength decreases with increasing mixing time (from 11 to 15 min), shows a maximum with mixing rate (i.e., 33.5 MPa at 40 rpm), and with increasing fiber size (24.2 MPa for fibers <125 μm; 33.5 MPa for fibers between 125–300 μm and 30 MPa for the 300–325 μm range). Additionally, the impact strength of the composites ranged from 2.76 to 2.97 J/m2 for increasing fiber lengths (El-Shekeil et al., 2012a). The authors concluded that the 30 wt.% fiber composite exhibited the best tensile strength (i.e., 33.5 MPa), while tensile and flexural modulus and flexural strength increased and strain and impact strength decreased at increasing fiber content. The hardness (Shore D) of the 30 wt.% composite was greater than that corresponding to the neat TPU (66 vs. 55), while abrasion resistance was lower (0.074 vs. 0.012 wt.% loss) (El-Shekeil et al., 2012a). Treatment of the kenaf fibers with 4% pMDI or with 2% NaOH + 4% pMDI (El-Shekeil et al., 2012b) showed that the first treatment did not significantly affect composite properties, but the second one increased the tensile properties with respect to the nontreated fiber composite (i.e., 30% and 42% increase of the tensile strength and modulus, respectively).

17.3 Biopolyurethane composites The efforts to replace petroleum-based polymers with other ones produced from renewable resources have also been applied to formulate biobased PUs, being the ­hydroxyl-containing component the one that is replaced partially or completely by biodiols or biopolyols, as exemplified in the following sections.

17.3.1 Biopolyurethane composites made from vegetable short fibers/particles Mosiewicki and coworkers investigated the behavior of solid and foamed composites based on a PU obtained from the reaction of a polyol derived from tung oil with pMDI and WF (Mosiewicki et al., 2009b; Casado et al., 2009). Mechanical test results showed that increasing the WF content in the PU matrix induced an increase of the tensile modulus (from 0.91 ± 0.12 to 3.03 ± 0.40 GPa, for the 0 and 30 wt.% WF samples, respectively) and strength, as well as the impact strength (about 70% increase at 30 wt.% of WF). Tensile results were explained by the higher modulus and strength of the wood fibers compared with those of the matrix, in addition to the good adhesion wood-­polymer due to physical interactions (H-bonds) and chemical interfacial bonding through reactions of pMDI with OH from the WF and from the PU matrix, that act as extra crosslinks. Impact behavior was also related to the strong interface and the generation of a tortuous fracture path through the WF-reinforced material. The propagation energy and the total energy absorbed during the test increase significantly compared to that of the unreinforced PU because of new mechanisms of energy dissipation resulting from the presence of the filler. Thus, the ductile index also increases with WF content, indicating that the composites behave in a more ductile manner as compared to the matrix.

Mechanical performance of polyurethane (PU)-based biocomposites 469

Ultimate deformation (×1000)

50 45 40 35 30 25 20 15

0

5

10

15

20

25

30

Wood flour (wt.%)

Figure 17.1  Ultimate deformation versus WF content for biocomposites based on tung oil-PU. Reprinted from Casado et al. (2009), with permission from Wiley.

This study showed very unusual results for microcomposites; the addition of moderate amounts of WF (10–15 wt.%) led also to increased elongation at break (Figure 17.1). In this system, the initiation of the failure by interfacial debonding at multiple sites, which is followed by the coalescence of the cracks and final catastrophic crack growth (typical failure mechanism of microcomposites at low strains (Hull, 1993)) was not present because of the excellent interfacial adhesion, which largely delayed the final breakage of the material. Besides, the WF added to the composites and coreacted with the PU could act as chain extender. However, as more WF is added, the increased cross-linking density and rigidity of the particles produced the overall typical effect of increasing the composite modulus and reducing deformability of the material. Additionally, simple mechanical models were used to analyze the results, finding that the Hirsch and Halpin–Tsai models (Casado et al., 2009) were adequate for modeling the dependence of the tensile modulus with WF concentration. More interesting, they applied a model for the ultimate tensile stress proposed by Pukanszky et al. (1989) that allows quantifying the reinforcing effect of a filler through calculation of a fitting parameter, “B,” related to the quality of the filler–matrix inteface. According to D'Almeida and De Carvalho (1998), for B < 3 no reinforcing effect is obtained. The solid curve in Figure 17.2 was fitted to the experimental results corresponding to different WF concentrations, obtaining B = 7.1 (for the 0–15 wt.% range), which indicates that the strong interface led to a remarkable reinforcing effect. The samples with 20 and 30 wt.% of WF deviate from the previous trend, because there was not enough PU to wet the filler. Even so, using all experimental values in the fitting process (dashed line, Figure 17.2) gives a B parameter higher than 3 (B = 5.6), confirming the reinforcing effect of WF. SEM images of the interface also supported this feature.

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Biocomposites: Design and Mechanical Performance 50

B = 7.1

B = 5.6

45

s (MPa)

40 35 30 25 20 0,00

0,05

0,10

0,15

0,20

0,25

0,30

Volume fraction of wood flour

Figure 17.2  Tensile strength and Pukanszky model (solid line) versus WF content for biocomposites based on tung oil-PU. Reprinted from Casado et al. (2009), with permission from Wiley.

Transparent composite sheets were prepared by the reaction of a mixture of soy oil-based polyol and petrochemical polyol with pMDI in the presence of different contents of microcrystalline cellulose (MCC) (0–1 wt.% with respect to the total polyol mass) by compression molding (Luo et al., 2013a). The flexural strength and flexural modulus of the PU were increased by 60.3% and 103.3%, respectively, by the addition of only 1 wt.% MCC, suggesting a very effective reinforcement. At this concentration, the notched Izod impact strength also increased by 74%. The crack propagates freely in the neat biobased PU, but in the composites, the crack occurs first and then peeling takes place from the cracked surface along the MCC that separates from the matrix, thus increasing the absorbed energy in the composites (Latere Dwan'isa et al., 2004). The storage modulus of the composite sheets was higher than that of the neat PU at temperatures below Tg. Additionally, the Tg of the biobased PU, 149 °C, was shifted to 153 °C at 1 wt.% MCC loading. These results agreed with the high mechanical strength of MCC and the additional cross-linking contributed by it, resulting in the improved mechanical properties of the composites. Bioreinforced foams of high density (300 kg/m3) were reported by Kuranska et al. (2013). In this case the reference rigid PU foams were prepared using pMDI, polyether polyols, an amine catalyst, silicone surfactant, and water as a chemical blowing agent. This reference petrochemical formulation was modified with rapeseed oil-based polyols and flax fibers with lengths in the range of 0–0.5 mm. Two different rapeseed oil-based polyols were prepared: the first one by epoxidation and further opening of the oxirane rings (P1, hydroxyl = 256 mg KOH/g) and the second one by esterification with triethanolamine (P2, hydroxyl number = 365 mg KOH/g). Composites prepared from P1 and flax fibers (five parts of fibers per hundred parts of the polyols, php) increased the compressive strength about 40% in comparison to the reference neat PU. Composites containing 10 php of the natural fillers showed the worsening of the compressive strength, which was explained by a fraction of the fibers not b­ eing

Mechanical performance of polyurethane (PU)-based biocomposites 471

i­ncorporated in the struts between the cells, causing disorder in the cell structure. In another work, Kuranska and Prociak (2012) studied rigid PU foams of about 40 kg/m3 density, modified with flax and hemp fibers of different lengths and rapeseed oil-based polyol (30 wt.% as a replacement for petrochemical polyol). The addition of 5 php flax fibers with a length of 0.5 mm led to the highest compressive strength of the series, up to about 18% for the strength determined in a direction perpendicular to the foam rise in comparison to the reference material. Compressive strength in all three directions was also affected by the apparent density of foamed composites. The materials with long fibers had a lower value of compressive strength. Based on the structure analysis it was concluded that shorter fibers are more adequate to interact with the building of the cells without causing their damage. Merlini et al. (2011) reported the influence of the fiber volume fraction, fiber length, and alkaline treatment on the mechanical and thermal properties of short random banana fiber-reinforced PU obtained from a commercial castor oil (CO)-based polyol and MDI. Polymeric composites were prepared by hand lay up followed by compression molding at room temperature. The tensile strength and Young's modulus of the composites increased with increasing fiber volume fraction and length (Table 17.1), with treated banana fiber composites displaying higher values than the untreated ones, due to the stronger interfacial interactions between the treated fibers and the PU matrix. The treated fibers presented higher surface roughness and free OH groups, promoting mechanical and chemical adhesion between fiber and matrix (Vilaseca et al., 2007). Dynamic–mechanical analysis of the above samples showed the typical characteristics of the largest reinforcing effect above the glass transition temperature of the composites, where the differences of rigidity between matrix and fibers are maxima. The stiffness of the composites increased due to the presence of fibers, which restricted the mobility of the polymer chains, leading also to the Tg increase. As expected, the intensity of the tan δ peak decreases with the amount of banana fiber. Miléo et al. (2011) worked on castor oil PU reinforced with cellulose fibers from sugarcane straw (5–20 wt.%). Cellulose was extracted by steam explosion of the straw, followed by delignification with NaOH. The incorporation of cellulose fibers led to the increase of the composites' stiffness; the flexural modulus was 163.7 ± 25.1 MPa Table 17.1 

Tensile properties of banana fiber-reinforced PU

Untreated fibers Treated fibers

Banana fiber (vol%)

Fiber length (mm)

Tensile modulus (MPa)

Tensile stress (MPa)

Tensile strain (%)

0 10 10 10 10

– 20 30 20 30

5.89 ± 1.06 15.77 ± 2.24 22.21 ± 2.99 39.99 ± 1.55 31.32 ± 3.40

1.96 ± 0.43 2.62 ± 0.35 4.39 ± 0.32 5.73 ± 0.25 6.07 ± 0.42

31.35 ± 1.80 18.00 ± 1.73 17.00 ± 1.53 16.00 ± 0.50 20.00 ± 1.01

Values extracted from Merlini et al. (2011), with permission from Elsevier.

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for the pure PU, while the value raised up to 171.0 ± 9.7 and 354.5 ± 25.1 for the 5 and 20 wt.% composites, respectively. On the other hand, fracture surfaces showed a poor dispersion of the fibers in the matrix, although with good fiber–matrix adhesion, without fiber pull out.

17.3.2 Biopolyurethane composites made from vegetable long fibers Bakare et al. (2010), used a modified rubber seed oil to synthesize a PU resin and prepare composites incorporating long sisal fibers (10–30 wt.%). The tensile and flexural strength and modulus of the unidirectional fiber-reinforced composites increased with fiber loading (69–103 MPa and 2.28–3.25 GPa for the flexural strength and modulus, respectively). Ramires and coworkers (2013) prepared PUs using sodium lignosulfonate (NaLS), castor oil, and two diols of different chain length. Randomly oriented fiber composites, containing 30 wt.% of curaua (average diameter of 116 μm, 3 cm length) and coir fibers (average diameter of 230 μm, 3 cm length) were then obtained. In neat lignopolyurethanes, the Izod impact strength increased with increasing chain length of the polyol used. Longer chain polyols lead to a lower degree of cross-linking and, thus, to materials with elastomeric characteristics and high molecular mobility, resulting in high impact strength. On the other hand, composites made with curaua fiber had higher tensile strength (485 MPa) than that made from coir (120 MPa), due to the higher cellulose content and higher crystallinity index of the curaua fibers (67.7% and 65%, respectively) compared with those of the coir fibers (50% and 41%, respectively). Moreover, curaua composites exhibited higher impact strength than coir ones and increased Izod impact strength with respect to that of the neat lignopolyurethane, which was the result of the high efficiency of the load transfer from the matrix to the fibers. The aspect ratio of the fibers (~259 for curaua and ~130 for coir) also influenced the impact strength of the composites. Generally, fibers with high aspect ratios are more efficient reinforcements of composites, leading to materials with better mechanical properties.

17.3.3 Foamed biopolyurethanes Chemically modified tung oil was also used as the main polyol component in the formulation of filled viscoelastic PU foams (Ribeiro da Silva et al., 2013ab). A small part of the modified tung oil was replaced by a low molecular weight diol (ethylene glycol) leading to reactive liquid mixtures of adequate viscosity that facilitated the mechanical stirring of reactants. The chosen filler was rice husk ash (from combustion of rice husk) because of its high silica content. The dynamic–mechanical tests revealed that the foams exhibited two different and broad thermal transitions, the temperature of their maxima depending on filler concentration. Figure 17.3 shows the compression stress–strain curves for selected specimens. As expected for rigid foams, the compressive stress–strain curves show a linear elastic region, followed by a plateau (Latere Dwan'isa et al., 2004; Gibson and Ashby, 1997; Ashby, 2006) and then a further increase. At the end of the elastic region, the stress

Mechanical performance of polyurethane (PU)-based biocomposites 473 Stress (kPa) 8

Neat PU 1% RHA 2% RHA 3% RHA 5% RHA Neat PU (bottom) 1% RHA (bottom)

6

4

2

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Strain (mm/mm)

Figure 17.3  Compression stress–strain curves for rice husk ash filled viscoelastic polyurethane foams. Reprinted from Ribeiro da Silva et al. (2013b), with permission from Elsevier.

reaches a peak value that corresponds to the compressive strength. This maximum stress is sometimes followed by a decrease before the stress plateau is reached (Tondi et al., 2009; Ashby, 2006). Cracks initiate at the peak stress value and the material tends to generate fragments as a result of these cracks. The plateau originates in the coexistence of collapsed and uncollapsed zones, and it is typical of rigid foams undergoing successive cell wall fractures. Beyond the plateau, densification takes place and the stress rises sharply as complete densification begins. When total cell collapse is completed, the compacted material begins to behave like a nonporous solid (Tu et al., 2001). Overall, compression modulus, compressive strength, and storage modulus increased as foam density increases but decreased as rice husk ash concentration increases due to the detrimental changes induced by the filler in the foam cellular structure (severe disruption of the foam morphology). The same is true for the stress value at the plateau. However, densification strain exhibits the opposite behavior, indicating that reinforced foams can sustain slightly higher deformations without collapsing, probably due to a reduced reactivity of the components induced by the filler. Microcellular PU composites were investigated by Aranguren et al. (2007), who used long hemp fibers as reinforcement. The PU matrix was synthesized from a CObased polyol and pMDI, while microfoamed composites with preferentially orientated hemp fibers were prepared; the threads were long enough to behave as continuous fibers, extending over the whole length of the sample specimen. SEM micrographs of the unfilled foam and the hemp-foamed composite are shown in Figure 17.4. In general, hemp fiber composites showed relatively high bending properties, which were related to the large aspect ratio of the hemp fibers. In this respect, one-directional hemp composites offered the best bending behavior. Dynamic–mechanical studies of

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(a)

200 µm

(b)

200 µm

(c)

200 µm

Figure 17.4  Scanning electronic micrographs: (a) unfilled foam; (b) long unidirectional hemp fibers reinforced foams; (c) long unidirectional hemp fibers reinforced foams with added water. Reprinted from Aranguren et al. (2007), with permission from Wiley.

the composites showed very wide temperature transitions. The storage modulus and tan δ curves for the unreinforced PU and the composites showed that the peak, which corresponds to the main relaxation of the matrix and is related to its glass transition, is in the range of 90–118 °C for the composites, while it is around 70–73 °C for the unreinforced sample. As usual, it was observed that the difference between glass and rubber modulus is much higher in the matrix than in the composites. The differences in properties between the unfilled and filled PU were attributed to the combination of the hydrodynamic effects of the particles embedded in a viscoelastic medium and to the mechanical restraint introduced by the filler at high concentrations. As was discussed in other examples reviewed in this chapter, the interfacial restricted mobility of the polymer chains is further reduced by covalent interfacial bonding, a characteristic property of composites made from vegetable fibers and in situ polymerized PU matrices. The static flexural properties of the hemp-reinforced PU composites indicated that the best properties were obtained for the aligned one-directional fiber composite. The lowest properties corresponded to the two directional-fiber composite, while the four directional materials had intermediate properties. An interesting observation was that these materials suffered important aging due to a secondary reaction of the unsaturations present in the vegetable oil-based polyol. As a result, the modulus and strength of the materials increased with aging, but the deformation to rupture did not decrease, and for this reason aged materials showed improved modulus and toughness compared to those of the original composites, under the test conditions reported. Gu et al. (2013) studied composite PU foams prepared by the free rise pouring method, using a commercial soy-based polyol of hydroxyl number = 55.67 mg KOH/g. Commercial steam explosion pulp of maple with particle size in the range of 180–850 μm was added in 13.3 parts by weight to prepare the filled foams. Although the incorporation of wood pulp fibers in the PU did not significantly alter the compressive strength, it caused a slight reduction in tensile strength, but contributed to the resistance to deformation (up to about 6%). Comparable results were obtained by Mosiewicki et al. (2009a) that prepared rigid PU foams using a polyol of low viscosity synthesized by alcoholysis of CO with triethanolamine. The modified oil was used as the polyol component in the formulation of rigid PU foams filled up to 15 wt.% of WF (particle size <64 μm). The results showed that the compression modulus and yield strength decreased as WF c­ ontent

Mechanical performance of polyurethane (PU)-based biocomposites 475

increased, which was attributed to the cell disruptions introduced by a ­microsized filler. Moreover, similar results obtained with a commercial rigid PU system led to the conclusion that these new materials can be a valuable alternative to replace ­petroleum-derived formulations, and could be used in semistructural applications, where low weight is desirable. An improvement in the mechanical properties with the incorporation of natural fibers into foams was reported by Luo et al. (2013b). In this work, lignin/soy-based PU biofoams were prepared by a free-rising method in the presence of water as blowing agent, while lignin powder (from bioethanol production) was used as a reactive reinforcing filler (15 wt.%, 75 μm). The PU foam was formulated using a commercial ­soybean phosphate ester polyol (OH value = 272 mg KOH/g), a petrochemical polyol (OH value = 635 mg KOH/g) and pMDI. Mechanical properties of the samples were improved by the incorporation of lignin, with an optimum at 10 wt.%. Similarly, the glass transition temperature increased with the addition of lignin (172.8–182.1 °C for the neat PU, and 15 wt.% composite, respectively), a behavior that is typically associated with increasing cross-link density. The authors proposed that lignin was acting as a chain extender and cross-linking agent. The specific compressive modulus (compressive modulus/density) also increased with lignin content (109.09 and 153.45 MPa/g/ml for the 5 and 10 wt.% composites, respectively) up to about 15 wt.%, decreasing at higher concentrations. The flexural modulus presented the same trend: increased from 195.80 to 354.86 MPa with lignin content from 5 to 15 wt.%, with the optimum at 10 wt.% lignin. It was proposed that the lignin–polyurethane interactions determine the incorporation of urethane–urethane and ester–urethane ordered structures from matrix in the lignin amorphous phase. Consequently, at high lignin content the urethane structures could not crystallize further and the thermomechanical properties of the corresponding blends are highly reduced (Ciobanu et al., 2004). On the other hand, the notched Izod impact strength was not significantly affected by the presence of lignin, their values being 1.33, 1.29, 1.28, and 1.32 J/m for biofoams containing 0, 5, 10, and 15 wt.% of lignin, respectively.

17.3.4 Water sorption and biodegradability of composites based on biopolyurethanes Because vegetable fibers and particles are hygroscopic, it is important to monitor changes of the mechanical properties of the composites due to the action of humid environments. This effect was investigated by Mosiewicki et al. (2012) on tung oil-based PU reinforced with WF. The storage modulus (DMA), as well as the tensile modulus and the tensile ultimate stress, decreased as the moisture content of neat matrix and composites increased. The comparison between the mechanical properties of the recently prepared dry samples and the wet samples showed the plasticization effect of the water molecules in the PU and its composites. Although increasing amounts of WF led to increasing water absorption, the incorporation of low percentages of WF resulted in lowering the water sorption with respect to the neat PU. This unexpected result was due to the very good compatibility of the system and the changes generated in the polymer as a consequence of the coreaction with WF.

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The tensile and flexural properties of wet samples (30 days of water immersion) of long sisal fiber composites made with a rubber seed oil PU were investigated by Bakare et al. (2010). A slight loss in tensile strength after water immersion was observed at 25–30 wt.% sisal. The decrease in mechanical properties was attributed to the formation of hydrogen bonding between the water molecules and the hydroxyl groups of the cellulosic component of the sisal fiber. However, the flexural strength and modulus were not adversely affected by moisture absorption. The biodegradation is also important for this type of materials that intend to replace synthetic polymers and composites. Thus, the biodegradation of tung oil-based PU reinforced with WF was followed during 383 days of exposure to soil or vermiculite media (Aranguren et al., 2012). The hydrolytic degradation was the most important mechanism of deterioration in all cases. A shift of the glass transition toward higher temperatures was observed (from around 65 to above 100 °C), which was explained as the result of the preferential attack and removal of free or dangling-pendant chains that plasticize the original material. The effect on the storage modulus was an increase in the whole range of temperatures observed, with the composites biodegraded for one year almost doubling the value of the storage modulus of the recently prepared samples.

17.4 PU nanocomposites based on vegetable-derived nanofibers The previous sections illustrated the versatility of the PU composites and the wide range of possibilities that derive from the use of lignocellulosic materials as fillers/ reinforcements for these polymers. It is not surprising then, that the idea of using PU matrices with biobased nanoparticles and nanofibers has been developed during the last years. The following section is dedicated to the bionanocomposites made from PUs and plant derived nanoparticles; in particular, composites containing nanocellulose will be considered. Vegetable material is composed of cellulose, hemicellulose, lignin, and other minor components. Cellulose is present as fibers, bundles of associated microfibrils, which are in turn formed by cellulose chains physically linked to one another by strong hydrogen bonds. The fibrils are formed by crystalline well-ordered regions that alternate with intermediate amorphous regions. These fibrils can also associate to one another through extensive hydrogen bonds. By preferentially degrading the amorphous regions by acid or enzimatic methods, crystals can be obtained with 5–20 nm of diameter and hundreds to thousands nm in length. These nanoelements are usually called cellulose nanocrystals or nanowhiskers, a term that was first used for crystals obtained from tunicates, which are longer than wood-derived crystals. Cellulose nanocrystals (NCC) show modulus in the order of the 150 GPa, and, as is the case with other nanoparticles, if a low concentration of NCC is well dispersed in a polymeric matrix, the extensibility of the polymer can be preserved, while the modulus can be largely increased. Cellulose nanocrystals obtained by sulfuric acid hydrolysis of MCC were incorporated into an elastomeric PU by Marcovich et al. (2006). They used a polymeric

Mechanical performance of polyurethane (PU)-based biocomposites 477

Film: 0.1 mm 4%wt cellulose /polyurethane

Plaque: 3 mm

5%wt cellulose /polyurethane

0.5%wt cellulose /polyurethane

Film: 0.1 mm

Figure 17.5  Photographs of elastomeric polyurethane materials reinforced with NCCs.

aromatic isocyanate (pMDI) and a mixture of a diol (PEG) and a polyol (functionality > 2.0) for the PU synthesis. The NCC was added to the OH-components after redispersion of the freeze-dried nanocellulose in dimethylformamide (DMF), followed by solvent evaporation. Very good dispersion was obtained by this method and the samples were transparent even after the addition of 5 wt.% of NCC, as shown in Figure 17.5. A yellow-brown tint was observed as a result of the original brown dark color of the polymeric isocyanate. Dynamic–mechanical analysis of these samples revealed large upward shifts in the glass transition temperature (Tg) of the cellulose nanocomposites as compared to the unfilled PU (about 16 °C, with 5 wt.% nanocellulose). The Tg increase resulted not just from the restricted mobility of the polymer in the presence of the nanocrystals, but also from the interfacial reaction between the cellulose and the isocyanate that covalently bonded matrix and nanocrystals; the latter acting as multifunctional crosslinkers, which caused even a larger increase in Tg. Notice that these same reasons were used to explain the behavior of traditional microcomposites, although the changes are observed at much lower concentrations of the reinforcing elements in the case of the nanocomposites. Although the low-frequency viscosity of the unreacted mixtures was several decades higher than that of the unfilled PU, the sample containing 5 wt.% of NCC had a tensile modulus just 2.44 times that of the unfilled sample, which was explained by the indirect interaction cellulose–PU–cellulose that is formed after cross-linking. Thus, the same reason that contributes to excellent nanocellulose dispersion also reduces its reinforcement effect. This suggests that if high modulus is desired, an optimum degree of dispersion must be achieved that still allows for the formation of direct cellulose contacts. Li et al. (2013) obtained nanocrystalline cellulose from hydrolysis of medical absorbent cotton with anhydrous phosphoric acid. The system was neutralized with ammonia and the whole mixture was used to modify a PU foam. The nanocrystalline cellulose performed as reinforcing material, the phosphates as flame retardants, and the hydrolyzed saccharides contributed as partial replacement of polyols. It was observed that the addition of the modifier improved the compression strength of the

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foams up to an optimum of 6 wt.%, but then dropped for larger additions. At the optimum formulation the compression strength was 107.5 kPa, which is 4.29 times that of the unfilled foam, but because of the retained phosphates the flame performance was also improved (lower heat release and longer time for ignition). The cell structure of the modified foam was more regular than that corresponding to the neat PU, which was related to the improved mechanical properties of the foams. Juntaro et al. (2012) prepared sheets from bacterial cellulose (BC) that were impregnated by a PU-based resin containing UV polymerizable groups to produce transparent nanocomposites with excellent compatibility and, consequently, no gaps between cellulose and matrix, which otherwise would lead to light diffraction and opaque samples. The authors compared the tensile properties of the composites with those of a sheet of BC and those of the unfilled PU. While the PU showed a Young's modulus of 0.19 GPa, the cellulose sheet reached the 5.09 GPa. Interestingly, the composites reached modulus ranging from 5.4 to 11.6 GPa depending on the processing conditions; the highest modulus achieved by the nanocomposite made with the BC dried at the highest imposed pressure. A similar trend was observed for the tensile strength (PU: 16.3 MPa, BC: 113 MPa, and nanocomposites: 72–151 MPa). Figure 17.6 illustrates the tensile response of these nanocomposites. The image shows the behavior of composites made from aqueous sheets dried under a pressure of 400 kPa (Type A) or 6.9 kPa (Type B), or by solvent exchange with ethanol and drying under low pressure (Type C). The composites had higher modulus and lower extensibility than the neat PU. The interesting fact is that the results for the nanocomposites were even better than for the cellulose sheet. The explanation given by

175 Type A

150 Type B

Stress (MPa)

125

Bacterial cellulose sheet

100 Type C

75 50 PU resin

25 0 0

2

4

6 Strain (%)

8

40

Figure 17.6  Stress–strain curves of bacterial cellulose sheets impregnated by a PU-based resin. Reprinted from Juntaro et al. (2012), with permission from Elsevier.

41

Mechanical performance of polyurethane (PU)-based biocomposites 479

the authors is that in BC sheet the stress transfer occurs only through the skeletal 3-D structure formed by the fibers, but in the nanocomposites the interstices instead of being air-filled are occupied by the PU. Thus, stress transfer occurs through the fibers and also from fiber to polymer to fiber, which improves the mechanical properties. Other authors compared materials prepared as micro- or nanocomposites. Wu et al. (2007) prepared a segmented PU using polytetramethylene glycol, 1–4 butanodiol as diol and chain extender, respectively, and MDI as isocyanate component, which were polymerized in the usual two-step method. Microcomposites were prepared in a similar way, but cellulose fibers (1–2 mm length), diol, and MDI were mixed with DMF and reacted; in the second step, the chain extender was added and reacted. For the nanocomposites, MCC was first dispersed in DMF with traces of LiCl (80 °C for 6 h), and then mixed with diol and reacted with MDI to be chain extended in a second step. TEM images allow confirming that the second type of composites contained nanocellulose. Comparison of the effect of the two types of fibers showed that microcomposites (5 wt.% of cellulose) had the typical reported behavior of higher modulus and lower extensibility than the unfilled PU. On the other hand, nanocomposites showed higher modulus (twice that of the microcomposite with the same cellulose concentration and 2.6 times that of the neat PU), but largely increased extensibility (1.5 times that of the PU). The example illustrates the important role of the size of the reinforcing elements, because interfacial compatibility, strong physical interaction, and covalent bonding are the same in both types of composites. Regarding thermoplastic PU-based nanocomposites, Cherian and coworkers (2011) synthesized a degradable-segmented PU using approximately 2:1:1 of hard segment (diisocyanate)/soft segment (polycaprolactone-diol)/chain extender (1,4-­butanediol). PU films were prepared by a solvent (tetrahydrofuran) casting process. The authors used pineapple leaf fibers as the raw material to obtain nonwoven nanocellulose mats by combining high-pressure defibrillation and chemical purification procedures. Microscopy studies showed that the obtained mats were composed of bundles of cellulose nanofibers with diameters ranging between 5 and 15 nm and estimated lengths of several micrometers. The nanocomposites were prepared by compression molding, which involved stacking the nanocellulose fiber mats between PU films. The results showed that nanofibrils reinforced the PU efficiently. The addition of 5 wt.% of cellulose nanofibrils to PU increased the strength nearly 300% and the stiffness by 2600%. This composite also exhibited the highest strain to fracture. Moreover, the developed composites were utilized to fabricate various versatile medical implants, such as cardiovascular ones, and have the potential for being used in scaffolds for tissue engineering, repair of articular cartilage, vascular grafts, urethral catheters, mammary prostheses, penile prostheses, adhesion barriers, and artificial skin. Rueda et al. (2013) extracted cellulose nanocrystals, with an aspect ratio of 17 from MCC and then incorporated them in a polycaprolactone-based PU by a casting/ evaporation method using DMF as solvent. Thermal, mechanical, and morphological properties indicated, in general, favorable matrix–nanocrystals interactions arising from efficient dispersion of NCC in the thermoplastic PU. Low NCC quantity in PU nanocomposites led to a tough material without loss in ductility, whereas an increase

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in NCC content enhanced the soft and hard segment crystallization, which resulted in an increase in the material stiffness and stability versus temperature. Besides mechanical performance, nanoreinforcements have also been added to tune the functional properties of PU. Let's just briefly mention that nanocellulose has been used to improve the recovery force of shape memory PUs that have the capability of autonomously recovering a previously “memorized” shape as a response to a change in the surrounding environment; for example, temperature (Auad et al., 2008, 2010, 2012; Han et al., 2012) or humidity (Zhu et al., 2012; Luo et al., 2011, 2012) changes, among others. The mechanical characterization of these materials is a useful tool for tailoring their properties, as dramatic changes in their rigidity can be displayed as a response to sensed external changes. Floros et al. (2012) prepared nanocrystalline cellulose by a process of several steps involving alkali treatment of the fibers, steam explosion, bleaching, and finally oxalic acid treatment followed again by steam explosion (NCC length ~200–250 nm and diameter ~4–5 nm). Samples with different concentrations of the NCC were prepared from a biobased TPU derived entirely from oleic acid. The best results were obtained with 0.5% of NCC in the TPU, with an elongation at break that improved from 178% to 269% and stress at break that increased from 29.3 to 40.5 MPa. Lin et al. (2013) developed new nanocomposites consisting of a CO-based PU matrix filled with acetylated cellulose nanocrystals (ACNs). The ACN exhibited improved dispersion in tetrahydrofuran as a blending medium, and reduced polarity as compared with unmodified NCC, resulting in a high loading level of 25 wt.% in the nanocomposite. As the ACN loading level increased from 0% to 25%, the tensile strength and Young's modulus of the nanocomposites increased from 2.79 to 10.41 MPa and from 0.98 to 42.61 MPa, respectively. When the ACN loading level was 10 wt.%, the breaking elongation of the nanocomposites reached the maximum value of more than twice that of the PU. Seydibeyoglu and Oksman (2008) considered the effects of cellulose from the micro- to the nanoscale as reinforcement of a thermoplastic PU. In this case, nanofibrillated cellulose from hard wood fibers was obtained by means of a high-pressure homogenizer. Again, composite materials were prepared using a compression molding technique, which involved stacking the cellulose fiber mats between thermoplastic PU films. The results showed that both microfibers and nanofibrils reinforced the PU, nanofibrils being a much more effective reinforcement than the microsized cellulose fibers; for example, the addition of 16.5 wt.% cellulose nanofibrils to PU increased the strength nearly 500% and the stiffness by 3000% while the use of 18.7 wt.% microfibers lead to increases of 200% and 500%, respectively. The authors attributed this behavior to the fact that cellulose nanofibrils integrate within the polymer matrix much better due to the smaller size but also to the better properties of nanosized fibrils compared with the microsized fibers. Waterborne PUs are fully reacted urethane polymers dispersed in water, and they show many excellent features compared to conventional organic solvent-based PUs, including higher rates of biodegradation (Santiago de Oliveira Patricio et al., 2013). However, they show low mechanical strength and thermal stability, restricting some of their potential applications. In this way, reinforcements from renewable biomass are viable supplements for these polymers, as was demonstrated by Liu and coworkers (2013),

Mechanical performance of polyurethane (PU)-based biocomposites 481

who used cellulose nanocrystals to improve properties of rosin-based waterborne PUs. Tensile strength of the composite films increased from 28.2 to 52.3 MPa while the tensile modulus was improved from 316.2 to 1045.4 MPa with increasing cellulose amount from 0 to 20 wt.%, respectively. However, elongation at break was sharply reduced from 267% to 22% for the same filler concentration range. Santiago de Oliveira Patricio et al. (2013) prepared nanocomposites based on cellulose nanocrystals and synthesized waterborne PU using a two-step method commonly referred to as a prepolymer method. They developed nanocomposites with different properties by altering the mode and step in which the nanofillers were incorporated during the PU formation. The process utilized is as follows: WPU/NCC nanocomposites were obtained by physical mixtures of waterborne PU and cellulose nanocrystals in an aqueous suspension; WPU/NCC-P samples were obtained by adding the NCC suspension during the dispersion step of the prepolymer (before adding the hydrazine hydrate used to react with the remaining free NCO groups); and WPU/NCC-PP composites were produced by adding the cellulose nanocrystals, dispersed in the polyols, at the beginning of the prepolymer synthesis. The authors demonstrated that the degree of interaction between the nanofillers and the WPU through hydrogen bonds could be controlled by the method of incorporation of the NCC to the polymer network and proposed a chemical reaction between the NCO groups and the OH groups of the cellulose crystals in WPU/NCC-PP. In this case, it was noticed that a high degree of interaction between the hard segments and the NCC affected the morphology, reducing phase separation, so that strength was decreased. WPU/NCC-P samples exhibited the typical behavior of composites reinforced by agents with a high aspect ratio and good adhesion, whereas WPU/NCC samples performed as composites with poor adhesion or a less efficient dispersion. Gao et al. (2012) filled a biocompatible waterborne polyurethane (WPU) based on castor oil (CO)/polyethylene glycol (PEG) with low-level loadings of Eucalyptus globulus NCC obtained by sulfuric hydrolysis (average length and diameter of 518.0 ± 183.4 and 21.7 ± 13.0 nm, respectively). The nanocomposites showed significant enhancement in tensile properties: 1 wt.% NCC increased the tensile strength from 5.43 to 12.22 MPa, while the Young's modulus reached the maximum of 4.83 MPa (416% higher than the neat PU) at a loading of 4 wt.% NCC.

17.5 Final Remarks PUs have proven to be very compatible with vegetable fibers, particles, and nanoreinforcements. Clearly, the method of processing the composite is of importance, especially if the polymerization is carried out in the presence of the vegetable material, in which case this can coreact with the polymeric matrix. This interfacial reaction has been considered to be the reason for reduced water absorption even when the components are hygroscopic, the increase of glass transition temperature, and in some cases the improved toughness and impact properties. The incorporation of vegetable fibers into foams have been shown to have opposing effects, because although the fibers contribute with their highest rigidity, they also

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disturb the integrity and regularity of the cells, which can reduce the performance of composite foams. Vegetable-based nanoelements proved to lead to composites of improved properties, while their effect is observed at much lower concentrations that in traditional composites. Although traditional composites will continue to find their place in different applications, the biobased and WPUs will receive more attention as they offer new ways to reach more environmentally friendly materials. On the other hand, PU nanocomposites and bio-PU nanocomposites will continue to receive attention, and it is to be expected that they will find their market as functional special applications.

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