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:
In this article we’ll examine cure temperature and its impact on the thermal and dimensional stability of prepreg materials.
This is the first 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.
Key Rheological Parameters for Prepreg Curing
In the context of temperature and prepreg curing, there are three key rheological parameters to consider:
Storage Modulus
The storage modulus, or G’, measures the stiffness or solidity of a prepreg material. The value of G’ designates the material’s capacity to store and recover energy elastically when deformed. The higher the G’value, the more solid or elastic the material.
Loss Modulus
The loss modulus, or G”, measures the prepreg material’s viscosity or resistance to flow. The value of G” refers to the amount of energy that dissipates from the material in the form of heat during deformation. Higher G” values indicate greater energy loss and more pronounced viscous behavior.
Tan δ
Tan δ is the ratio of loss modulus (G”) to storage modulus (G’) and is key to understanding the balance between elastic and viscous properties of a prepreg material, the degree of cure, and flow characteristics of the resin. Flowability is especially important in the early stages of processing, when the material must conform to the shape of the tool. Greater flowability is suggested by a higher tan δ value.
How Cure Temperature Affects Prepreg Properties
Cure temperature has a significant impact on the thermal and dimensional stability of prepreg materials. Strategically increasing cure temperature reduces the viscosity of the resin, which leads to better flow and more thorough wetting of the fibers. This can improve the dimensional stability of the final prepreg material.
Balance, however, is critical. If the cure temperature is too high, there can be a number of unintended consequences:
- Excess reduction in resin viscosity, resulting in non-uniform resin distribution. This has a negative impact on mechanical properties and the storage modulus of the prepreg material.
- Increased thermal stresses, which may affect the termination of the curing reaction and lead to cracking and void formation over time.
- Warping and distortion caused by different thermal expansion rates of the fibers and the matrix.
Issues With Improper Temperature Control
A number of prepreg material production issues can be credited to a cure temperature outside of the optimal range for a given material.
Incomplete Cure
The hardening of a prepreg material is the result of a chemical reaction that occurs during the curing process. If the cure temperature is too low, this reaction may not occur, and the final product may be soft, sticky, or tacky.
Non-Uniform Cure
The curing process takes time. If the cure temperature is too low, the final product may be removed from the oven before thicker areas have had adequate time to cure, resulting in a non-uniform final product.
Excessive Cure
Over-cured resin can be porous and brittle, due to the degradation of its mechanical properties. This usually occurs when the cure temperature is too high.
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).