Field:
Processing Science

Advisors:
R.L.McCullough and G.Palmese

Department:
Chemical Engineering

Expected Graduation Date:
May ’00 (Masters Degree)

Abstract

Examining the neat resin is an essential part in understanding the properties of a cured composite. The parameters which can affect the curing of vinyl ester are: temperature, monomer concentrations, initiator type and concentration, inhibitor type and concentration, and whether or not the system is accelerated. The curing reaction in a mixture of vinyl ester and styrene monomer is a free radical chain growth polymerization, following the steps of initiation, propagation and termination. Three possible and closely examined reactions competing in this system are the homopolymerization of the vinyl ester and styrene as well as the crosslinking of these two components. The polymerization of this system has been studied using Fourier Transfer Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC).

Goals

The goals of this research are (1) to determine the influence of temperature, monomer concentration and initiator on the cure kinetics of vinyl ester, (2) to develop procedures for measuring initiator efficiency and decomposition, and (3) to incorporate initiator effects into a kinetic model for the vinyl ester cure cycle.

Procedure

Infrared spectra gives information on the specific components present in the resin mixture. Absorption peaks are associated with a particular bond in each monomer. By studying the changes in heights of these absorption peaks, we obtain information on conversion and polymerization from the depleting number of certain monomer bonds. This data can then be used to calculate kinetic properties and reaction rates using an autocatalytic model. A major advantage with this method is the ability to monitor styrene and vinyl ester separately. For a high temperature test, this is important because we can determine any loss of styrene due to evaporation.

DSC measures the overall heat released from the polymerization of the resin. A base test gives the maximum possible heat evolution of the system. Therefore, the ratio of heat released during an isothermal cure to the maximum heat measured will give us conversion versus time. However, unlike FTIR, these conversion profiles are for both monomers combined.

Results

Experimental procedures have determined that with increased temperature, the extent of conversion of both vinyl ester and styrene monomer increases in a USP-245 initiated system. For the Trigonox 239-A initiator, (unaccelerated at high temperatures), the vinyl ester conversion increases slightly, while styrene conversion greatly increases with temperature and nearly achieves complete conversion. USP-245, a peroxy ester initiator, starts polymerization at a lower temperature than Trigonox 239-A, which is a combination of a hydroperoxide and a peroxy ester. This can be related to the decomposition rates of the two initiators.

For both initiators, vinyl ester reacts first, with styrene reacting slightly later, but with a noticeable induction period In the beginning of the reaction process, the copolymerization mechanism dominates. In later stages, due to crosslinking density, the mobility of vinyl ester groups becomes limited and tapers off while styrene continues to react and polymerize with itself. The time and length of the copolymerization stage is dependent on the temperature and initiator concentration of the system.