Research On The Properties Of Toughened Wood Plastic Composites

The low impact strength of wood plastic composites is one great factor that restricts itsapplication. The impact strength of virgin PP is low; however, when combining with woodpowder, the impact strength of the derived composite will decrease tremendously, and willmaintain this decreasing trend as the wood powder loading increases. In this paper,wood/polypropylene composites containing 60% wood powder were adopted to investigatethe effects of impact modifiers on its toughness. The three impact modifiers used arePolyethylene Octene Elastomer (POE), nanoCaCO₃, and wood fiber. Firstly, the woodpowder was treated with different amount of aluminic acid ester to investigate its effects onactivating wood powder and the best application amount. The mechanical properties ofwood/polypropylene composites toughened with different impact modifiers, and theirsynergistic effect was studied with orthogonal experiment. To investigate the tougheningmechanism, samples were made to conduct dynamic mechanical analysis (DMA), scanningelectron microscopy (SEM) and rheology tests. The results are concluded as follows:1. After modifying with aluminic acid ester, the activation grade of wood powder increased,with the oil absorption decreased, and that the viscosity of the wood powder and paraffinoil mixture were also decreased greatly. It can be concluded from this research that adding2% aluminic acid ester can obtain satisfied activation effect. XPS analysis was taken onwood samples treated and untreated with aluminic acid ester, respectively, and the resultsshow that, after modification, chemical reaction occurred between hydroxy and carboxylof the surface of wood sample and alkoxyl of aluminic acid ester, to form chemical bondAl-O-C. Thus the compatibility between wood and polymer matrix was modified.2. The mechanical tests showed that POE can effectively increase the toughness ofwood/polypropylene composites, but it has negative effect on the tensile and flexuralstrength and the flexural modulus; however, the comparison with industry standardproved that the flecural strength of composites with 12% POE can still meet therequirement. It is also found that two-step pelletizing method can produce compositeswith superior mechanical properties than direct pelletizing method. Adding 3% ofnanoCaCO₃ can increase the toughness, strength and stiffness synchronously, but increasethe amount to 5% will cause negative effects. The mechanical tests showed that small amount (6%) also has synchronized strengthening and toughening effect. The results ofthe orthogonal experiment showed that the amount of POE is the most significant factorthat can influence the mechanical properties of the composites, follwing by the amount ofwood fiber and nanoCaCO₃. As to the impact strength, the best toughening parameters areas follows: the amount of POE, wood fiber and nanoCaCO₃ are 12%, 6% and 3%respectively.3. The DMA tests showed that, with the addition of wood powder, the storage modulus E' ofthe composites increases significantly, which matches the results of static mechanicaltests; the addition of POE can lower the storage modulus E' and the temperature of theβrelaxation of the composites; the addition of nanoCaCO₃ can lower the peak amount ofβrelaxation of the composites, while the addition of wood fiber tends to decrease thestorage modulus E' and the temperature of theβrelaxation of the composites.4. The SEM pictures of the composites and fracture surface can reveal the dispersion ofwood powder in the composites and the fracture mechanism. It is found that adding POEcan help the dispersion of wood powder in the composites, and the composites containingPOE will cause "neck-nerrowing" effect when suffering impact; the compositestoughened with nanoCaCO₃ can cause creasing effect and stress whitening effect whensubjected to impact; while the wood fiber in the composites can assist the stress transfer,but it can also find the "pull-out" effect happened on the fracture surface.5. The rheology properties of the composites were conducted with torque rheometer andcaplliary rheometer. The results showed that addition of POE can lower the maxiumtorque of the composites, and nanoCaCO₃ can help to lower the equilibrium torque of thecomposites, while wood fiber tends to increase the torque. It can be found from thecaplliary rheology test that there is linear relation between logη_a and logγ, which meansthat wood/polypropylene composites satisfy the power-law relationship. It can also befound that the apparent shear visvosityη_a decreases with the increase of shear rateγ,which means that wood/polypropylene composite is pseudo-plastic fluid. The addition ofPOE and nanoCaCO₃ can decrease the viscosity of the composites, while wood fiber canincrease the viscosity.