Additives for Polymer Composites

 INTRODUCTION

  • Additives are those  substances which are added into the resin matrix during composite fabrication process.

PURPOSE

Additives are added to meet 

  • various end-use properties in the polymer and
  • processing and handling needs of polymer

TYPES OF ADDITIVES

  • Monomer 
  • Catalyst
  • Accelerators
  • Mould Release agents
  • Fillers
  • Pigments

MONOMER

  • Monomer is the smallest unit of a polymer chain which  has capability to form chain like structure.
  • Therefore monomer is used for linking polymer chain to get a 3-dimentional network structure of polymer.
  • Such process is extensively utilized in curing of polyester resin during composite fabrication.
  • For the curing of polyester resin , styrene is added
  • The amount of styrene ranges at 30- 50% by weight.
  • The carbon-carbon double bonds in unsaturated polytester molecule and the carbon-carbon double bonds in the styrene molecules work as the cross-linking site.

CATALYST

  • Catalyst are used for curing purpose.
  • Cure rates of a polymer can be controlled by the use of catalysts.
  • Catalyst are also used for curing polyester with reduced quantity of styrene.
  • Resin and catalyst mixing can take place insid a gun.
  • Gun mixing provides thorough mixing of resin and catalyst and it is preferred to minimize the health risk of the operator.

RELEASE AGENT 

  • A release agent is  a liquid/soft semi-solid material which is applied to the mould for easy removal of the fabricated composite part  from the mould.
  • Wax, and grease are widely used as release agent.
  • Sometimes, a gel coat is applied after the application of release agent. The purpose of the gel coat is to get a highly polished surface finish on the part.

ACCELERATORS

  • Accelerator is used to promote the curing reaction. It is also known as hardner.
  • Lime or magnesium oxide increases the hardening rate of novalak-hexa compositions are  sometimes called as accelerators.
  • The obtain a sufficient rate of cure during moulding, accelerator is added about 0.2-2.0%.
  • Such accelerator are ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate.
  • According to the concentration of catalyst and accelerator used, the resin will be gel in any time from few minutes to many hours.
  • When methyl ethyl ketone porxide (MEKP) is added to a polyester resin containing vanadium naphthenate accelerator, the resin set almost immediately.


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