What is the proper way to store carbon fiber material long-term?

Temperature and Relative Humidity Should Remain Constan (20-25°C, 30-50% RH)

To ensure that the carbon fiber materials do not undergo molecular breakdown, it is crucial to hold the environmental factors constant within the specified limits. These factors should work to maintain the chemical structure of the resin while controlling the absorption that is within the carbon fibers of the resin. Temperatures should remain between 20 and 25 degrees Celsius (68 to 77 degrees Fahrenheit), and humidity should remain between 30 to 50 percent. when There is considerable and frequent temperature change of more than 2 degrees Celsius, the fibers and resin undergo a chemorheological expansion and contraction, and that increases the microcrack formation. Humidity levels also determine the chemical composition of materials. Canadian Springs, are an example of an environmental chamber, in which humidity levels are cycled, within an operating range of 40% and 70% RH, the tensile strength of materials was found to decrease by nearly 18%. On the opposite end of the humidity range, materials that undergo a RH of <30% are prone to chemically dry and brittle. Many of the sizing agents within the resin systems are thermosetting polymers, which can also be chemically depleted when subjected to a hot and dry environment.

To maintain the optimal storage conditions all year round, a well-designed storage climate control system with a fully integrated automated heating, ventilation, and air conditioning system, fully calibrated and fully automated data loggers, and regular climate control system checks is required.

Why Freezing or Refrigeration Damages Resin Systems and Compromises Shelf Life

There is a misconception that storing carbon fiber materials in a freezer or a fridge preserves quality. In most cases, a carbon fiber material stored in a freezer or fridge is damaged, and this includes chemical components of the material. When materials are stored in a freezer or fridge, there are basically three problems that can arise. First is thermal shock when the material is removed from the freezer or the fridge. Then there is moisture condensation when the material is removed from cold storage. Finally, when the temperature of the materials is below 10°C, toughened epoxy systems start separating phases. When resins freeze in a freezer set to -18°C, the resins will crystallize, and that will, according to lab testing, reduce the impact resistance of the materials by 40 to 60 percent. Refrigerating materials at about 4°C is not much better, because that speeds up a process called amine blush. In this process, the curing agents move up to surfaces and create weak zones in the composite materials. In total, materials stored in a freezer or in a fridge lose about 50% to 75% of their shelf life as compared to materials that are stored at ambient temperatures.

The best way to keep carbon fiber materials viable is to store them at room temperature rather than attempting to defy nature using cold storage methods.

Correct Packaging Methods to Maintain the Integrity of Carbon Fiber Materials

Packaging with Vacuum-Sealed Barriers and Desiccants for Uncured Fabrics and Prepregs

Most manufacturers use a combination of vacuum-sealed packaging and a desiccant to keep dry fabrics and prepregs the best that they can. It is critical to keep the internal humidity of the packaging below 30 to 50% relative humidity. Moisture is a devastating resin destroyer and will result in loss of potential strength of the end product. Between moisture and oxidation, resin systems can be deteriorated before use. Aluminum foil triple layer films provide a good moisture barrier and aid in tight seals to moisture. Foil films provide a good barrier to moisture. It is a common practice for prepreg manufacturers to add oxygen scavengers to slow the aging of resin systems due to oxidation. Companies that cut costs on packaging end up with poorly conditioned prepregs due to the moisture that permeates from the atmosphere. The life of prepregs can be reduced by as much as 70% due to inadequate packaging in less than 30 days. Quality control is an important factor in getting this correct.

UV Shielding with Static-Dissipative Properties and Contained Secondary Barriers

Epoxy resins undergo a breakdown process known as photodegradation when they are exposed to UV radiation. The breakdown manifests as chain scission, yellowing, cracking, and peeling of layers. Testing based on ASTM D4329 methods for accelerated weathering shows that photodegradation can result in a loss of more than 40% of the material's strength after 500 hours of exposure to UV lamps. The material's strength directly translates to the number of months of exposure in front of a window or the presence of a UV intensity source in a normal office environment. This is, of course, a serious issue that manufacturers strive to address by incorporating UV blocking elements into the top layers of the outer material. They also create a coating that will help control the static dissipative surface. To help protect against light exposure, manufacturers may also place the product in a light blocking secondary container, such as a hard plastic polymer bin. These sealed secondary containers will also help filter out dust and will promote a stable network crosslinking of the epoxy by limiting the available oxygen. This will prolong the product life before degradation occurs.

Storing Carbon Fiber and Consequent Environmental Risks

The Impact of Weathering and UV Radiation on Resin and ASTM D4329

For carbon fiber manufacturers, UV radiation is one of the greatest risks. Our packaging tests with the ASTM D4329, showed that UV damage is cumulative, and permanent. In just a few months, we observed UV damage such as yellowing, surface cracking, and delamination. Because there is no way to undo UV damage, the consequences are complete and permanent. This is one of the reasons manufacturers go to great lengths to ensure proper UV blocking packaging and light-tight containers for every carbon fiber product.

Minimizing Mechanical Stress: The Aim is to Avoid Compression, Bending and Exposure to Vibration

Carbon fiber materials face serious risks when mechanically stressed. Small cracks can begin to form in the prepreg layers when static loads exceed approximately 10 psi. Furthermore, during the vibration phase, the resin and fibers can become unbonded when moved. For storage, it is recommended to keep rolls in a vertical position on a soft, non-friction rack. Cured laminates should be stored lying flat with the aforementioned special spacers between them. Never stack any components. A bend in a component can increase the risk of cracking in the resin structures, which is then fatiguestone resistant in a matter of two thirds. Positioned edge guards and vertical positioning help to reduce exposure and stress to the surfaces of the items during routine manipulation, which is is a major contributor to damage during handling.

Storage Instructions by Type of Carbon Fiber Material: Fabric, Prepreg, and Cured Laminates

Before conflating storage functions, understand that they differ on a fundamental basis.

To avoid compromising the quality of the resin infusion and laminate on dry carbon fiber fabric, it is to be stored in moisture-proof, vacuum-sealed packaging with desiccants. This is necessary to prevent the fabric to become hygroscopic.

Prepregs need to be stored in cold conditions [freezer temperatures (-18°C to -23°C)] to keep the resin from curing too early. This might seem unusual since we have previously stated that cold storage isn’t necessary, but this is an exception. The reasoning has to do with their chemistry. The fabric itself has not been reacted, while the prepreg has partially reacted resin. This means the prepreg’s resin has a shelf life that is dependent on cold temperatures. This is true and outlined in their OEM technical data and the provided ISO 10993 compliant stability studies. 

Cured laminates do not need to be cold stored to be chemically stable. However, they do need to be stored with physical protection. This means that they need to be stored vertically on padded racks with edge protectors, not stacked, and with interleaving to avoid marring the surfaces.

The protocols provided deal with each material's main form of degradation. Dry prepregs have chemical degradation, dry fabric absorbs moisture, and finished laminates degrade mechanically.

FAQ

What are the best temperatures and humidity levels for storing carbon fiber materials?

The best storage conditions are 20-25°C at 30-50% relative humidity. These conditions are the best at preventing degradation of the material’s structure.

Why should carbon fiber materials not be stored in a freezer?

Thermal shock, moisture condensation, and phase separation in toughened epoxy systems will occur. These pose a risk to impact resistance and make structural repairs.What is the best way to package carbon fiber materials to ensure integrity?

To ensure integrity, vacuum-sealed, desiccant- augmented barrier packaging is best for dry fabrics and prepregs, to maintain low humidity levels and moisture, which would otherwise degrade resins. 

What is the effect of UV radiation on carbon fiber materials?

UV radiation, causes the thermal degradation of epoxy resins, resulting in chain scission, discoloration and weakening of the structure.

What are the storage instructions for the various types of carbon fiber materials?

Dry fabrics must be stored in vacuum-sealed, moisture barrier packaging. Prepregs must be stored in the refrigerator or freezer to ensure that the resin remains in the pre-processed, flowable condition. Cured laminates must be kept in a physically protective container, and are not required to be kept in a controlled climate.