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The impact of moisture on UD prepreg resin systems
The resin systems used for unidirectional prepregs, both epoxy and phenolic, are highly hygroscopic and moisture absorbent, particularly at the glass transition temperature (Tg) of the resin systems. The polymer chains in the resins contain polar groups that bond with the water. This leads to two major situations of concern: (i) water-induced plasticization of the resin, and (ii) hydrolysis, which is the water-caused cleavage of chemical bonds in the resin system. Significant moisture absorption (greater than 60% of relative humidity) occurs in a matter of days in the resin-rich areas between the fibers, leading to swelling, and stresses that are caused by the absorption of water in the resin. The resin becomes softened and adheres poorly to the fibers. As a result, load transfer across the resin-fiber interface is compromised. This is particularly problematic for UD prepregs because the resin is more concentrated in the direction of the fibers.
Moisture encourages potential failure points even before the curing begins. Manufacturers need to mitigate this risk during storage and handling.
Diffusion Dynamics: The Mechanism of Unidirectional Fiber Architecture and Ambient Humidity
Unidirectional (UD) prepreg absorbs water at a much higher rate than woven materials do, and a large reason for this is how moisture interacts with the fibers. With all fibers aligned straight, microchannels are formed at the junctions of the fiber and resin. This allows moisture movement through the fiber bundles at rates that are 3 to 5 times higher compared to their resin counterparts under the same humidity conditions. For example, at 75% relative humidity, the UD prepreg of standard thickness will start to absorb water within 8 hours, well before woven materials, which will take 2 days. Three primary factors contribute to this phenomenon. The first is that the straight fibers in the prepreg have a higher volume to surface area ratio allowing more effective moisture movement. The second is that the erosion of the fiber bundles leaves behind moisture conduits, which further enhances moisture movement through the resin. These conditions rapidly deteriorate the integrity of the prepreg. Cycles of heating and cooling will result in the formation of very small currents within the fiber bundles which will accelerate moisture erosion of the prepreg.
Microvoids, Blisters, and Delaminations in UD Prepreg Laminates
Moisture-Induced Void Formation (>0.3 wt% → >15% ↑ void volume, per ASTM D2734)
Micro voids form in composite materials during the curing process when the moisture absorbed by the materials becomes vapor and expands. Voids can exceed to greater than 15% of the total volume even at moisture percentage levels of only 0.3% by weight. This condition is unacceptable per aerospace standards ASTM D2734. From then on the voids create essential problems for the junctions of resin and fibers, starving the junctions of sufficient resin and decreased the structural integrity of the composite. Unidirectional prepreg materials are moisture sensitive and tend to absorb moisture more readily than other composite prepregs. Manufacturers, therefore, need to maintain relative humidity levels tightly to prevent excessive voids and ensure that composite prepreg materials do not form production rejects.
Cure-Related Defects: Blistering and Interlaminar Separation Due to Moisture
While moisture is trapped in composite materials, when there is moisture, there is an increase in steam and pressure during autoclaving, resulting in blistered resin and separation of layers. The data is clear: during autoclaving, composite prepreg with more than 75% relative humidity during prepreg lamination developed blistering almost two times compared with composite prepreg with less than 30% relative humidity during prepreg lamination. Once resin bleeding and layer separation occurs, injurious mechanical stresses can exacerbate these problems. These problems are of substantial importance in the fatigue resistance of aircraft components due to the critical importance of structural integrity in aircraft components. Storage and transport of the prepreg materials show how crucial dry room management is for ensuring that the unidirectional prepreg composites will have ideal performance when they are put into service.
Real-World Consequences: Impacts on Field Failures and Aerospace Certifications
Airbus A350 Wing Skin Delamination Incident: Cause Identified as High Moisture Content in UD Prepreg
While conducting flight tests for one of the latest developed aircraft, engineers noted that skin delamination of the wings was due to the unidirectional (UD) prepreg being too moist. What does too moist mean? Simple; above 0.4 weight percent. So, what happened? The UD prepreg sustained microcracks that not only delaminated the skin, but brought about severe consequences on the order of $200 million in redesign costs and 11 months of delays prior to receipt of the EASA 2022 Safety Bulletin compliant certification. Such cited delays illustrate the importance of monitoring moisture in prepregs to avoid costly redesigns and regulatory delays. Prepregs exposed to excessive moisture affect the materials and result in the need to undergo FAA and EASA mandated costly requalification regulatory compliance.Boeing 787 Fuselage Panel Data: 75% RH Exposure Doubles Blistering vs. Controlled <30% RH Storage
Data made accessible by one major aerospace manufacturer regarding the moisture absorption of fuselage panels has some alarming results. The data indicate that panels subjected to 75% RH for 48 hours end with about 32% void content. UD prepreg stored below 30% RH ends with 16% voids, which is considered a less moisture-related blistering. Moisture-related blistering is already considered beyond acceptable limits by airworthiness standards, which, according to SAE AIR 7292 standards, require a 5% void tolerance to primary structural components within a fuselage. And of course, that also means the repair costs become exorbitant. Another interesting finding from additional lab work is that for each 10% increase of relative humidity, the safe handling time of the prepreg materials decreases by about 15 hours, which means the time before the resin undergoes irreversible degradation from the heat. That’s why, in manufacturing settings, effective humidity management improves the operative range significantly.
Successful Moisture Control for UD Prepreg
Best Methods: Dry Room Specs (ISO 12944-2), Desiccant Packs, and Real-Time NIR
Quality control for UD prepreg materials or even just keeping moisture levels controlled is imperative. For instance, dry rooms built to meet or comply with ISO 12944-2 standards are able to keep humidity at or below 30% (RH) to prevent breakdown of resin during handling. In addition to this, vacuum packed desiccant moisture indicator strips are able to block approximately 95% of air contact with the resin as compared to standard moisture barrier films. For continuous, unencumbered measurement of moisture content of resin (above 0.1 wt. %), near infrared (NIR) moisture sensors are available which will issue an alarm at the threshold. Using all of the above methodologies combined, works exceedingly well to have an 80% reduction of voids and complete elimination of blisters during the curing process. This is particularly noteworthy given the confidence the manufacturers have gained with regards to their material storage practices from accelerated aging studies conducted in artificially created hot and humid environments which are representative of tropical conditions.
FAQ
What is the primary concern with UD prepreg resin systems?
The primary concern with UD prepreg resin systems is the absorption of moisture which in turn causes plasticization and hydrolysis which compromises the structural integrity of the material.
Why do UD prepregs absorb moisture more than woven materials?
Straight fiber construction of UD prepregs allows moisture penetration more easily than woven materials. Additionally, temperature variations worsen moisture penetration.
What are the effects of too much moisture in UD prepregs?
Excess moisture can create voids, blisters, and delaminations, all of which compromise the structural integrity and can lead to catastrophic failures.
How can moisture be controlled in UD prepregs?
The use of dry rooms, desiccated packaging, and near infrared real time monitoring can be used to control moisture levels.