Determination of the Out-Life of Prepregs Using Advanced Model Free Kinetics

Introduction

Kinetic studies have become an important application in thermal analysis. Kinetic data from chemical reactions can be determined and used to make predictions about the reaction behavior of chemical substances outside their practical measurement range. This article describes how the out-life of a carbon-fiber-reinforced prepreg can be determined using the Advanced Model Free Kinetics (advanced MFK) software option. Prepreg is the name given to uncured composite thermosetting materials consisting of fibers embedded in a matrix of uncured resin and prepared in the form of sheets ready for forming.

Prepregs are usually stored in a freezer and have a so-called storage life at this temperature (e.g. at -18 °C). The out-life is the accumulated time that the prepreg spends out of the freezer, e.g. at ambient temperature (20 °C) prior to use.

Prepregs and Their Out-Life

Ideally, a prepreg should remain fully reactive and processible, but not undergo reaction until the material is raised to a particular temperature in the cure cycle (the activation temperature) at which the curing (or cross-linking) process is initiated. A slow reaction will, however, occur even at much lower temperatures. The glass transition temperature of the material, Tg, increases with time due to this gradual curing process until finally vitrification occurs [1], [2], [3]. The material becomes stiffer and less workable. Its flow and handling characteristics are poorer when it is later heated in the curing process.

The out-life is the time during which the flow and processing properties of a prepreg remain useful and allow normal processing. A prepreg is usually heated to a certain temperature before the actual curing process. The time spent at this temperature should not exceed a certain value because otherwise the material cannot be properly processed and faulty components result. In practice, this type of material failure can occur through improper storage and delivery conditions.

Measurements and Results 

The out-life of a prepreg at a specific temperature expires when its glass transition temperature, Tg, reaches this temperature. The out-life can be determined by the following method: 

1. Determination of the relationship between the Tg of the thermosetting system and the degree of cure (conversion α) through a series of curing and post-curing measurements.  
2. Determination of the conversion curves (conversion versus isothermal curing time) of a prepreg at different temperatures using Advanced MFK. 
3. Prediction of the out-life at specific temperatures through a combination of 1 and 2.
   

In this study, a conversion of 0% was assumed for the prepreg as received and a conversion of 100% for the fully cured prepreg. The glass transitions were characterized by the midpoint of the step in the DSC curve. Experimental conditions: The measurements were performed using a METTLER TOLEDO DSC822e with IntraCooler. Samples were punched out as disks from the bulk prepreg material and measured in 40-µl aluminum crucibles.

Determination of the Glass Transition as a Function of Conversion

The relationship between the glass transition and conversion was determined by performing a series of dynamic measurements at a heating rate of 10 K/min. To obtain different degrees of conversion (i.e. different degrees of curing), fresh samples were heated to a certain temperature and then immediately cooled. This is illustrated in Figure 1.

Conclusions 

The DSC822e in combination with the Advanced Model Free Kinetics software is an excellent tool to study the curing kinetics of prepregs and other thermosetting materials. The conversion curves calculated using advanced MFK at different temperatures together with the glass transition temperature vs. conversion curve provide extremely useful information for the delivery, storage and processing conditions of prepregs. The excellent agreement between the predicted out-life and the measured results show that the kinetic parameters obtained from advanced MFK are reliable and trustworthy.

_{Determination of the Out-Life of Prepregs using the Advanced Model Free Kinetics Software | Thermal Analysis Application No. UC 173 | Application published in METTLER TOLEDO Thermal Analysis UserCom 17}