STRATEGIES TO IMPROVE POLYMER/CLAY COMPATIBILITY VIA CHEMICAL MODIFICATIONS OF THE LAYERED SILICATES

ROMINA OLLIER; DANILA MERINO; MATÍAS LANFRANCONI; VERA ALVAREZ

Río de Janeiro

Conferencia; XV Internacional Clay Conference; 2013

Polymeric nanocomposites are a new class of materials, for which at least one dimension of the dispersed  particles,  is  in  the  nanometer  range.  They  represent  an  interesting  alternative  to conventional  polymer  composites  because  of  their  unique  properties,  synthesis and processing methods.  Among  all  the  potential  nanofillers studied  in  the  past  decades,  clays  occupy  an important  position  because  they  are  easily  available  on  all  the  continents with relatively low cost.  Bentonite  has  been  extensively  used  to  prepare  organoclays  due  to  its  high  cation exchange  capacity,  swelling  behavior,  adsorption  properties  and  large  surface area. Dispersing clays in polymers at the nanoscale, even at loadings of only 1 to 5 wt. %, provides considerable  improvements  in  polymer  properties compared with conventional micro particle fillers  including,  for  example,  higher  modulus  both  in  solid  and  melt  state, increased strength and  thermal  stability,  decreased  permeability  and  increased  biodegradability.  The  main  reason  for  these  enhanced  properties  in  nanocomposites  is  the stronger  interfacial  interaction  between  the  matrix  and  layered silicate.  In  order  to  achieve these  better  properties  it is necessary  to  obtain  a  totally exfoliated structure (this means that the silicate layers  are  completely  and  uniformly  dispersed in the  continuous  polymeric matrix). Hence,  the  high  aspect  ratio  of  the  nano­scale  layered  silicate  and  their  dispersion  in  the polymer  plays  a  key  role  in  these  improvements.  Nanoclay  particles are generally agglomerated as a result of their high surface energy, therefore  the  disaggregation of nanoparticle agglomerates  before  or  during  composites  processing  proves to be a key point in obtaining an optimal  dispersion. Although there are different ways to optimize  the  polymer/clay compatibility, the most  popular method consists on converting these hydrophilic silicates to organophilic ones  by performing chemical treatments of the clay. The synthesis of organoclays is based on the mechanisms of the reactions that the clay minerals can have with the organic compounds: - Ion exchange  of the interlayer inorganic cations with organic  cations (usually with quaternary ammonium or alkylphosphonium compounds) is  a relatively simple industrial treatment to alter  the surface  properties  of  the  clay.  In  addition,  these  cations  can  provide functional  groups that eventually can react with the polymer matrix or initiate  a polymerization reaction, improving the strength of the interface between the inorganic platelet and the polymer matrix. - Grafting  reactions, i.e., forming covalent bonds between reactive  surface groups and organic species, are important steps to hydrophobise the surface of many clay mineral particles. Trialkoxysilanes and alkylchlorosilanes can be used to modify the  clay surface due to their ability to readily  undergo  hydrolysis  and condensation reactions.  It has been proved that the quantity of grafted silane can be greatly  enlarged by the acid activation of clays; this feature could be related with the  increment of reactive sites generated through acid treatment. In this work, several strategies to modify bentonite and to make it more compatible with polymeric matrices were  studied: cation exchange reactions with different alkylammonium and alkylphosphonium salts and grafting reactions with various silane  compounds. The effect of each  modification, together with the relevant parameters, was established. In order to evaluate the effectiveness of  the modifications, the obtained  organoclays were chemically (FTIR spectroscopy), morphologically (X ­ray  diffraction, water absorption studies) and thermally (thermogravimetric analysis) characterized and the effects of each treatment were  analyzed. This work was performed under the project FSNano004: Development of  modified nanoclays and innovative products from national clays.