Thermoset plastic waste from garment button industry as a novel solid state curing agent for epoxy resin system

Abstract

Garment button industry uses unsaturated polyester based on ethylene glycol and maleic acid to produce thermoset plastic sheets using styrene (31-34 w/w%) as a cross-linking agent. Buttons are produced by casting process and remaining part of the sheet becomes an inherent waste. This research study evaluated the curing behaviour of the powdered thermoset polyester waste (PTPW) of garment button industry as an alternative for amine hardener in epoxy resin systems. Particle size distribution was analysed using the standard sieve analysis method. It indicated that majority (83 w/w%) of the particles were in the range of 125 – 500 µm. Set-to-touch drying time of the epoxy resin system was significantly improved when PTPW was blended with amine hardener. The fastest drying was observed with 30-70% of powdered waste in the blend of amine hardener and PTPW. When an amine molecule reacts with an epoxy ring, it combines to one end of the epoxy resin molecule. Since PTPW particle has more than one reactive and accessible COOH and OH group, each particle can chemically combine with a number of epoxy rings to initiate the cross-linking process. However the effect on set to touch drying time was marginal with the increase of PTPW above 70% and may be due to the initiation of ring opening reactions of epoxy resin with carboxylic groups and hydroxyl groups.

PTPW was kept overnight in alkali medium to break the ester links and was acidified with sulphuric acid to generate –COOH and –OH groups. Hydrolysed powder was thoroughly washed to remove sulphuric residues and dried at 105 °C. FTIR spectra of PTPW before and after the hydrolysis reaction indicated the presence of hydrogen-bonded O-H stretching after hydrolysis. Matrix of PTPW particle is hydrophobic in nature since the chains are cross-linked by styrene units. The –COOH and –OH groups which are formed in the hydrolysis tend to form hydrogen-bonds with each other within the matrix itself. 
Set-to-touch drying characteristics of PTPW and hydrolysed PTPW were very similar to amine hardener when used in the epoxy resin system. When hydrolysed PTPW was blended in proportion with amine hardener, no significant improvement of set to touch drying time was observed. Even though the hydrolysis generated more hydroxyl groups and carboxyl groups within the matrix of particles, number of freely accessible carboxylic and hydroxyl groups without steric hindrance to the epoxy ring structure may not be significant. Therefore the presence of excess number of functional groups per particle is not an advantage by means of drying characteristics.

Keywords : Epoxy resin system / Plastic waste / Garment button industry / Hardener / Epoxy grout

Acknowledgement : T and S Buttons Lanka (Pvt) Ltd provided a sample of waste in powder form.