Efficient toughening and reinforcing of polyvinyl acetate

15 February 2012 Adding cellulose nanowhiskers and nanofibers improves the tensile properties of polymer nanocomposites. The past decade has seen a remarkable increase in interest in using nanocelluloses, i.e., cellulose nanowhiskers (CNWs) and cellulose nanofibers (CNFs), to reinforce biodegradable and…

15 February 2012

Adding cellulose nanowhiskers and nanofibers improves the tensile properties of polymer nanocomposites.

The past decade has seen a remarkable increase in interest in using nanocelluloses, i.e., cellulose nanowhiskers (CNWs) and cellulose nanofibers (CNFs), to reinforce biodegradable and synthetic polymers.1–8 Nanocelluloses have not only abundant and renewable raw material sources, such as wood or agricultural residues, but also large specific surface area and outstanding mechanical properties. Yet, despite reports that they can enhance the strength and modulus of polymers,5, 6 convincing evidence that they can toughen polymers is scarce.

Processing methods have great influence on the mechanical properties of cellulose nanocomposites, and researchers are seeking industrially viable processes. Compared to the solvent casting method, melt compounding, especially extrusion, is a more promising industrial-scale process method. Therefore, the development of cellulose nanocomposites using twin-screw extrusion has been a focus of research in recent years.2–4, 8

We prepared CNW- and CNF-modified polyvinyl acetate (PVAc) nanocomposites using a master batch preparation followed by melt extrusion. We dispersed PVAc powder in distilled water, and mixed it with aqueous suspensions of CNWs or CNFs, respectively, to reach a dry weight ratio between PVAc and nanocellulose of 4 : 1. Afterwards, we froze and freeze-dried the dispersion, crushed it into powder, and used it as the master batch. We mixed and processed premixtures of PVAc, lubricant, and the master batch into the final nanocomposites using a co-rotating twin-screw extruder. We compression-molded the extrudates and cut testing samples from these sheets.3, 8 We refer to the nanocomposites as PVAc/CNFx or PVAc/CNWx, where x represents the percentage by weight of the CNFs or CNWs, respectively.

We measured the fracture toughness of the PVAc/CNW nanocomposites using the European Structural Integrity Society’s test protocol of the essential work of fracture method. We plotted two parameters defined in this method, we and βwp, which are the specific essential work of fracture and the plastic work density dissipated in the plastic zone, respectively, against the CNW concentration (see Figure 1). Both we and βwp are higher for the nanocomposites than for neat PVAc, indicating that the resistance to both crack initiation and propagation is improved by the addition of CNWs. A maximum value of we is shown at a CNW concentration of 5wt.%, which is almost 109% higher than that of PVAc. In contrast, βwp reaches a maximum value at 2wt.% CNW, which is around 200% higher than that of PVAc. βwp levels off with higher CNW loadings.7

Figure 1.

Plots of specific essential work of fracture (we) and plastic work density dissipated in the plastic zone (βwp) against the concentration of cellulose nanowhiskers (CNW).

We carried out uniaxial tensile tests on PVAc/CNF nanocomposites at 20°C on a universal testing machine equipped with a 1kN load cell at a loading rate of 5mm/min (see Table 1). Both the modulus and strength of PVAc improve with the addition of CNFs. However, the increase in modulus is almost independent of the amount of CNF, provided it is present, whereas the strength increases with increasing CNF content. The modulus and strength of PVAc/CNF (10wt.%) are 59 and 20%, respectively, higher than those of neat PVAc.8 The average value of maximum strain is reduced by increased CNF content, revealing the limited deformation and decreased ductility of PVAc.

Table 1.

Tensile properties of polyvinyl acetate (PVAc) and PVAc/cellulose nanofiber (CNF) nanocomposites tested at a loading rate of 5mm/min. PVAc/CNFx: x represents the percentage by weight of the CNFs in the nanocomposite.

Materials Tensile modulus Tensile strength Max. strain
(GPa) (MPa) (%)
PVAc 1.7 ± 0.1 39.3 ± 0.4 4.3 ± 0.3
PVAc/CNF1 2.5 ± 0.1 41.5 ± 1.3 2.9 ± 0.2
PVAc/CNF5 2.6 ± 0.1 42.6 ± 0.9 2.9 ± 0.1
PVAc/CNF10 2.7 ± 0.1 47.0 ± 0.2 2.4 ± 0.2

We interpreted the toughening effect of CNWs and the reinforcing effect of CNFs on PVAc with the aid of viscoelastic analysis. Simulated creep curves by a four-element Burgers model showed satisfactory agreement with experimental data in both PVAc/CNW and PVAc/CNF nanocomposites (curves not shown). The modeling parameters for the two systems are listed in Tables 2 and 3, respectively.

Table 2.

The simulated parameters of the four-element Burgers model for 30min creep of PVAc and PVAc/CNW nanocomposites at 30°C and a stress of 2.4MPa. EM: Instantaneous elastic modulus. EK: Retardant elastic modulus. ηK: Retardant viscosity. ηM: Instantaneous viscosity. PVAc/CNWx: xrepresents the percentage by weight of the CNWs in the nanocomposite.

Materials EM(MPa) EK(MPa) ηK(Pa·s) ηM(Pa·s)
PVAc 2297.3 5065.9 1.6×108 1.2×1010
PVAc/CNW1 2777.7 2406.0 2.8×107 3.4×109
PVAc/CNW2 2245.6 1787.5 6.3×107 1.8×109
PVAc/CNW5 1329.7 267.2 4.5×107 3.8×108
PVAc/CNW10 706.9 73.1 1.7×107 9.4×107

Table 3.

The simulated parameters of the four-element Burgers model for 30min creep of PVAc and PVAc/CNF nanocomposites at 30°C and a stress of 7.0MPa.

Materials EM(MPa) EK(MPa) ηK(Pa·s) ηM(Pa·s)
PVAc 2403.5 3975.3 2.8×108 5.6×109
PVAc/CNF1 2349.8 6679.5 2.1×108 8.4×109
PVAc/CNF5 3017.2 6559.0 2.3×108 1.2×1010
PVAc/CNF10 3265.1 6006. 6 2.1×108 1.5×1010

The main deformation mode in the neat PVAc is elastic. There is only minor plasticization at low moisture levels. The energy associated with the elastic deformation builds up instantaneously, triggering and promoting fast fracture propagation. In PVAc/CNW nanocomposites with higher concentrations of CNWs, the CNWs create more diffusion pathways for moisture, leading to more moisture absorption, and thus a much greater magnitude of plastic deformation consuming energy against fast fracture events associated with brittle behavior. This mechanism is clearly shown by the viscosity, ηM, determining the viscous flow, decaying in an exponential form with increasing CNW concentrations together with the bound moisture. The instantaneous elastic modulus, EM, of the nanocomposites with low CNW concentration (1 and 2wt.%), however, is still slightly higher than that of PVAc, indicating the reinforcing effect of CNWs on PVAc at low concentrations.

In PVAc/CNF nanocomposites, the most prominent effect of CNFs is to enhance the resistance of PVAc to irrecoverable deformation, as shown by ηM being 165% higher for PVAc/CNF10 than for neat PVAc, and instantaneous (EM) and retardant (EK) elastic modulus of the nanocomposites approximately 36 and 68% higher for PVAc/CNF10 than for neat PVAc. This indicates that CNFs restrict the slipping (viscous flow) of molecular chains, i.e., reduce the ability of PVAc to experience plastic deformation.

In summary, we have shown that CNWs and CNFs are efficient toughening agents and reinforcers in PVAc-based polymer composites. Furthermore, extrusion shows promise for nanocellulose-reinforced composites with PVAc as the matrix. We cannot at present scale up the extrusion process because nanocellulose materials (both whiskers and fibers) are not yet available in sufficient quantities. Their production needs to be commercialized before we will be able to further develop nanocellulose nanocomposite processing technologies and new applications. We are working to develop nanocomposites with different thermoplastic and thermoset matrix materials, such as polylactic acid.

Guan Gong
Swerea SICOMP

Kristiina Oksman
Composite Centre Sweden Luleå University of Technology
http://www.ltu.se/centres/CCSWE

Aji P. Mathew
Composite Centre Sweden Luleå University of Technology

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