Compounding of polypropylene for painting purpose

Excerpt: Polypropylene is a highly versatile material because it can be compounded with high amount of fillers, reinforcing agents and other polymeric materials to improve the physical properties

COMPOUNDING is the process of mixing polypropylene with other ingredients to form a polypropylene based multi-component product. Polypropylene is a highly versatile material because it can be compounded with high amount of fillers, reinforcing agents and other polymeric materials to improve the physical properties. These property improvements can be achieved by maintaining the cost of Polypropylene.

Background

It is difficult to bind the ink and paint on the Polypropylene resin due to their incompatibility between paint or ink and the polypropylene resin and the non-polar pp resin. So, the polypropylene resin has been improved by compounding to improve the compatibility between polypropylene and paint. Before the compounding improvement many methods have been tried to increase compatibility.

To improve the compatibility a primer is used as undercoat between the top coats and polypropylene products. But this process has failed as the primer is not only expensive but also as an additional step is involved. This makes the final product costly. This process is not eco-friendly.

The surface of the polypropylene moulded product is usually treated with halogenated hydrocarbon vapour (1,1,1-trichloroethane) before applying of primer. But it damages the ozone layer. Due to this problem it is strictly prohibited.

The adhesion strength of the polypropylene moulded items can be increased by many surface treatments like chromic acid treatments, corona discharge process, plasma treatment, flame treatment. These processes are generally done before paint or ink application. But these surface treatments are not satisfactory at all due to lower performance and environmental problems.

Compounding process:

Compounded polypropylene products consist of polypropylene, modifier and chemical additives. The goal of the formulation is to achieve specific balanced combinations of properties, processability and cost.

The hard and inert filler provides enhanced stiffness, higher heat distortion temperature and dimensional stability but at the cost of reduction in impact strength. The elastomer gives impact strength but at the cost of reduced modulus and strength. Thus, the most effective compound provides the best balance of properties for the particular application.

The polypropylene resin composition of the present invention prepared by known methods is usually used in the article. For example, solutions of the above-mentioned components are mixed together and then the solvents are evaporated off from the resulting mixture or alternatively, the composition is precipitated by adding a poor solvent to the resulting mixture. However, melt-kneading is preferable from the industrial point of view. Melt-kneading may be accomplished with a Banbury mixer, extruder, roll, or kneader.

It is preferable that the melt-kneading of the components is preceded by uniform mixing of each of the components, which may be either powder or pellets, with a tumbler or Henschel mixer. However, this preliminary mixing step may be omitted and each of the components may directly be fed to a kneading apparatus.

Thus, the kneaded composition is then molded by injection molding, extrusion molding and the like. Instead of kneading the components, they may be dry-blended and then mixed during the melting step in the injection molding or extrusion molding to form shaped articles.

During the kneading step, suitable amounts of conventional additives or auxiliaries may be added to the composition. They include, for example, an antioxidant, UV light absorber, slip agent, pigment, antistatic agent, copper deactivator, flame retardant, neutralizing agent, foaming agent, plasticizer and nucleating agent.

Advantage of compounding

  • If the surface of the moulded articles is treated with water or a non-halogenated organic solvent (ethanol, isopropyl alcohol, hexane, heptane, cyclohexane, toluene, acetone, methyl ethyl ketone etc), the articles can be painted by a paint with high compatibility without the need of any primer onto the surface.
  • Being incombustible in nature, water is advantageous over other solvents from the standpoint of safety and prevention of environmental pollution.
  • it can also be well painted in the conventional manner in which a primer is applied before applying paints.

Application

The polypropylene composition may be molded into various automotive exterior parts and easily and stably paint-coated without pre-cleaning with vapour of 1,1,1-trichloroethane and laying a primer layer.

Another objective is to provide a paint-coated molded article of polypropylene composition which may be obtained by treating the surface of a molded article of polypropylene composition with water or a solvent and applying paint thereon.

Author Details

Debaraity Maity Department of Polymer and Surface Coating Technology, Institute of Chemical Technology, Matunga, Mumbai.