Mechanochemically Modified Cellulose Particles and Their Reinforced Polypropylene Composites

Author:Huang Lang

Supervisor:li shu jun wang qing wen


Degree Year:2018





Cellulose fiber reinforced polymer composites are important in many applications,but the poor processing and dispersion ability,the incompatible interface adhesion limit their utilizations in high-valued fields.Commercial cellulose fiber is highly hydrophilic and tend to aggregate,the extrusion process cannot break up these aggregates by the dispersive force.To overcome this problem,we proposed to use mechanical ball milling to pulverize,activate and functionalize fibrillar pulp fibers to prepare modified cellulose particles as reinforcing agent for polymer composites with good particle dispersion and improved interface bonding.Specifically,pulp fibers were subjected to ball milling to reduce the fiber size and disconnecting the intra and inter chain hydrogen bonds for activated cellulose powder;during the milling,selected chemicals were added for mechanochemical reactions,cellulose surface were esterified for improved hydrophobicity.Then the modified cellulose powders were compounded with PP and injected into composites for test The methods and results are shown as following:(1)Pulp fibers were ground and activated using planetary ball milling.The milling parameters such as milling time,rotation speed,ball to cellulose mass ratio,ball diameter,and the moisture content of pulp fiber were studied.The particle size,morphology,specific surface area,crystallinity,thermal stability and energy consumption efficiency of the resulted particles were characterized.The results showed that ball milling was effective approach for fiber size-reduction,the long fibers were broken up into ellipsoid shaped,micronized particles.The specific surface area was significantly improved upon the size reducing.After ball milling for 60min,further milling would decrease the milling efficiency.The intra and inter chain hydrogen bond were weakened or destroyed during the milling.After the milling,the crystallinity and the thermal stability of cellulose particles decreased,and the cellulose hydroxyl group accessibility was improved;(2)The milled cellulose particles were used to reinforced PP matrix,the effect of different sized particles from varied milling time and different particle loading on the composite performance were analyzed,and the coupling agent’s influence was also discussed.The results revealed that smaller particles,with higher surface area and lower aspect ratio,could low the process torque value and promote the mattix crystallizing temperature.compared with long fiber reinforced composites,the milled particles filled ones exhibited higher tensile performance and lower water absorption.As to varied particle loading(5 wt%,10 wt%and 20 wt%),higher particle filling led to larger mixing torque value,increased the crystallizing temperature,and lowed the tensile behavior for the particle aggregations.The use of MAPP could improve the overall property by the bridge effect between the two phases;(3)Acetic anhydride(AA)and oleic acid(OA)were used to prepare mixed anhydride to increase the react activity for cellulose esterification during ball milling.Then the AA/OA mixed anhydride were ball milled with cellulose particles to prepare cellulose ester.Acetic anhydride and oleic acid could form acetic/oleic mixed anhydride during ball milling,the reaction equilibrium time was shorted that conducted in solution.the mixed anhydride concentration reached the maximum when the molar ratio of AA to OA was 1:1 and ball milled for 20 to 30 min.The mixed anhydride cellulose ester was successfully synthesized,with both long chain and short chain grafted on the surface of cellulose.Adding catalyst sodium hypophosphite could accelerate the esterification for the mechanochemical reaction.Besides,increasing the ball milling time and the mixed anhydride molar ratio could improve the esterification degree of substitution(DS).After the modification,the thermal stability and the water repellency of cellulose particles were improved;(4)the esterified cellulose particles were used to prepare reinforced PP composites.The processing ability,crystallization behavior,mechanical property and water absorption performance were investigated.During the mixing process,the surface modified particles filled composite exhibited lower torque value and complex viscosity than those of the unmodified one,indicating that the surface esterification of cellulose particles could improve the processing ability.The higher the DS value,the more the processing ability improved.Besides,the modified particles promoted the matrix crystalizing temperature for the strong heterogeneous nucleating effect.Compared with those of unmodified particle filled PP composites,the elongation at break of the 10%high DS cellulose powder filled composites increased by 8.9 times,for the improved dispersion and strong interfacial adhesion from the long chain entanglements.In addition,the grafted long chain increased the water repellency of cellulose particles,and the improved processing ability made most of the particles capsuled by the polymer matrix and decreased the moisture sensitivity and water absorption;(5)The pimelate β-PP nucleating agent was formed on the surface of cellulose particles during the ball milling and then it was used to reinforce PP composites.The synthesis of the nucleating agent,the crystallization and tensile behavior of the composites were investigated.The results showed that the pimelate calcium nucleating agent was successfully formed during the ball milling.When the nucleator modified filler were add into PP matrix,P phase PP was induced and 10 wt%filler led to 57.33%β phase crystals in the matrix.The nucleator filler could increase the crystalizing temperature and decrease the crystal spherulite size.Compared with untreated ones,the 10 wt%nucleator modified cellulose particles filled PP composites showed the similar tensile modulus performance,while the tensile strength increased by 9%and the elongation at break increased by 900%.Raising the particle loading amount increased the tensile modulus.In the studied filler content range(5~20 wt%),5 wt%nucleator modified filler showed the highest tensile strength and elongation at break,while 20 wt%particle loading decreased the strength and elongation at break dramatically,for the interface debonding caused by the particle aggregations.In the study,cellulose fiber was pulverized,activated and surface chemical modified using the solvent-free mechanochemical method.The surface modification could change the surface behavior of the cellulose particles while maintaining the main high-modulus structure of the cellulose.Designed functionalizations can be achieved by grafting screened chemical.The modified particles increased the processingability and compatibility of the cellulose particle reinforced polymer composites.The approach used in the study is green,effective and simple-processing,providing valueable guidance for the utilization of other agricultural and forest cellulosic materials.