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How Out-Time Affects Prepreg Processing

Premier ESR with sample and barrier film on die

Alpha Technologies evaluated the influence of processing parameters on the rheological behavior and mechanical properties of prepreg composites using an Encapsulated Sample Rheometer (ESR). These parameters included:

  1. Cure temperature
  2. Ramp rate
  3. Out-time

In this article we’ll examine out-time and how it can affect the curing process, storage modulus, and other rheological properties.

This is the last in a three-article series exploring the results of this study and the role of Alpha’s Premier ESR® in advancing a company’s process optimization goals for their prepreg materials. Sign up at the form on the right to be notified when new articles are published. To learn more about the other key processing parameters we studied download the full study.

Understanding Out-Time in Prepreg Processing

Some prepreg materials spent a period of time exposed to ambient conditions before curing or use. This period is called out-time and it can have a significant impact on the curing process, storage modulus, and other rheological properties, all of which ultimately affect the performance and quality of the final prepreg material.

Alpha Technologies tested five different out-time durations: 0 days, 3 days, 6 days, 9 days, and 12 days. The most important parameter studied in relation to out-time was storage modulus. Storage modulus is essentially a measurement of a prepreg material’s stiffness or solidity. More specifically, storage modulus quantifies how well the prepreg material can elastically store and recover energy when deformed. The ratio of loss modulus to storage modulus is also important when evaluating the balance of elasticity and viscosity in a prepreg material.

The study discovered enhanced storage modulus in prepreg samples that were exposed to ambient conditions, as opposed to samples that did not experience any out-time. This is largely attributed to the fact that extended out-time contributes to “advancement,” or premature curing, of the resin. Advancement has a beneficial impact on a number of variables:

  • Viscosity and flow characteristics of the resin and, subsequently, its ability to effectively impregnate the fibers during cure.
  • Cure kinetics and decreased duration of initiation and termination intervals.

As always, striking a balance is key to optimizing outcomes. While the study revealed an overall positive correlation between out-time and storage modulus, the ideal out-time for this particular prepreg material appears to be nine days, rather than twelve. Nine days yielded the shortest initiation and termination intervals and a superior storage modulus compared to twelve days.

This could be attributed to the increasing likelihood of the prepreg material absorbing moisture during longer out-times. Moisture absorption has an adverse effect on the curing process, leading to voids, weak bonding areas, and other defects that compose the structural integrity of the prepreg material. Additionally, the prolonged air exposure associated with longer out-times can change the chemical structure of the resin, which also has an impact on the prepreg material’s curing dynamics and mechanical properties.

Read More about Optimizing Prepreg Performance

For a detailed study of common prepreg cure failure modes and how to troubleshoot issues with rheological measurements, download our whitepaper Troubleshooting Prepreg Failures with Encapsulated Sample Rheometer (ESR).

 

About Alpha Technologies

Alpha Technologies is the leader in applying rheology science to testing that makes a difference in the rubber and polymer world. With over 60 years of experience, we develop advanced testing instruments and software, enabling our customers to continuously improve the value of their materials. Our mission is to provide the insight our customers need to ensure the quality, sustainability, and performance of their materials.

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