Hydrothermal Aging of Fibers and Polymers - Mechanisms and Experimental Insights - C130

 

Hydrothermal Aging of Fibers and Polymers - Mechanisms and Experimental Insights
Document Type	Case study
Document Identifier	130
Themes	Materials deposition and consolidation management Quality/inspection management
Tags	Physical aging Chemical degradation Thermal degradation Material characterization Process modelling Quality
Objective functions	Cost Maintain Rate Maintain Quality Increase
MSTE workflow	Development

Introduction

The aging process in fiber and polymer materials occurs by different factors, as it is mentioned in previous articles. Therefore, this article enters in the same area as Aging mechanisms in fibers and polymer materials - A427. In the case of this article, detail study cases are given for fully understanding in the process of choosing the suitable experimental method for evaluating aging effects. Therefore, prior to the selection of any material systems, it is crucial to identify, evaluate, and validate the constituent materials for their compatibility with the service conditions in demanding settings. Analyses can be performed utilizing accelerated aging procedures to replicate long-term effects.

Significance

As it was mentioned in Aging mechanisms in fibers and polymer materials - A427, understanding material aging is essential for readers because it directly affects the performance, safety, and reliability of products over time. Therefore, to give practical context with high quality experimental and theoretical methodology to the end-users, it is a crucial and important step to the readers to plan every step in their investigation by making informed decisions in every step of the experimental process, thus, as it was mentioned in the previous articles, helping to ensure cost-effective, durable, and safe designs in future structures or composite materials.

Scope

To give practical context of the previous theoretical parts to the end-users, this document explores some case studies on hydrothermal aging of fibers (i.e. glass, carbon or basalt fiber) up to 82 °C and thermoplastics in aging fluids at elevated temperatures up to 95 °C.

Background

The durability of structural materials in humid and aqueous environments is a critical factor for engineering design across multiple industries, including energy, offshore, transportation, and civil infrastructure. While fiber-reinforced polymer composites (FRPCs) have widely been studied, there is increasing recognition that the individual constituents, fibers and thermoplastic matrices, must be examined independently to truly understand environmental degradation mechanisms. Fibers such as E-glass, carbon, and basalt are primary load-bearing reinforcements, while thermoplastics including polypropylene (PP), polyphenylene sulfide (PPS), and polyvinylidene fluoride (PVDF) serve as matrices or stand-alone materials in demanding service environments.

Hydrothermal aging, caused by the combined action of moisture and elevated temperature, is one of the most aggressive degradation mechanisms for both fibers and thermoplastics. It involves water absorption, plasticization, leaching of chemical species, hydrolysis of polymer chains, and microstructural changes that collectively reduce tensile strength, stiffness, and toughness. Understanding these processes is essential for accurate service-life predictions, the development of accelerated testing protocols, and the design of reliable structures. This article synthesizes insights from three different scenarios on E-glass fibers under hydrothermal exposure, on carbon and basalt fibers exposed to hot water and humidity, and on a broad suite of thermoplastics tested with in situ punch-shear under hydrothermal conditions. Together, these works provide a holistic view of hydrothermal aging for fiber and polymer constituents, establishing a knowledge base for reliability assessment and material selection in engineering applications.

Hydrothermal Aging of Fibers

E-glass fibers: dissolution and strength loss

E-glass is the most commonly used fiber reinforcement, accounting for over 95% of global composite production. Its silica-rich composition makes it vulnerable to hydrolytic attack, especially when exposed directly to water at elevated temperatures. E-glass fibers exposed to water at 60 °C, 71 °C, and 82 °C suffered of tensile strength degradation and surface morphology changes. As shown in Figure 1, the strength values dropped by 62% and 55% (tested in wet conditions) and 45% and 32% (tested in dry conditions) for fibers aged at 71 °C and 60 °C for 840 h (5 weeks), respectively. Similarly, when the drop-in strength values are compared for fibers aged at 82 °C, 71 °C, and 60 °C for 168 h (1 week), the reductions were 49%, 41%, 33% (wet conditions), and 32%, 23%, and 12% (dry conditions), respectively. The error bars represent one standard deviation from the mean values. Distinct differences in strength retention values were observed between ‘wet’ and ‘dry’ samples, that is, the ‘wet’ samples exhibited lower mechanical strengths compared to the ‘dry’ samples. It is hypothesized that water between the filaments acts like a lubricant when fibers are tested wet, which largely negates any load sharing between filaments. When tested dry, friction between fibers promotes load sharing between filaments and thus greater fiber strength, which is similar to filaments being embedded in a polymer matrix, but to a much lesser extent.

Some interesting facts about hydrothermal aging of E-glass fibers are explained as follows:

Mechanisms of degradation: FTIR analysis and mass dissolution studies show hydroxyl ions attack the SiO2 network, releasing ions such as Na, Ca, Mg, and Si. Scanning electron microscopy (SEM) images reveal surface pitting and microcracks, confirming chemical corrosion as the main driver of strength loss. More information is available in.

Note: Detailed information about predictive modeling frameworks using Arrhenius modeling for glass fibers can be found in. Predictive modeling frameworks provide engineers with tools for estimating service life, reducing reliance on costly long-term testing.

Figure 1. Strength retentions after exposure at 60 °C, 71 °C, and 82 °C: (A) samples tested ‘wet’, and (B) samples tested ‘dry’

Basalt and carbon fibers: comparative performance

Basalt and carbon fibers offer higher stiffness and corrosion resistance compared to glass fibers, but their durability under humid environments requires careful study. Basalt and carbon fibers were examined by immersion in water at 60 °C, 71 °C, and 82 °C, as well as under 90% relative humidity at 90 °C. Strength retention at 60 °C (Figure 2a) was higher than at 71 °C (Figure 2b), showing that retention declines significantly under higher temperature conditions. The most pronounced reduction occurred for basalt fibers aged at 82 °C for 28 days (Figure 3c), in which strength drops by 82% in the wet conditions and 84% after drying. For samples aged 7 days and tested after drying, strength losses are 9%, 17%, and 47% at 60 °C, 71 °C, and 82 °C, respectively. These results demonstrate a consistent decrease in strength with increasing aging temperature and duration. Exposure at 82 °C proves most damaging, as strength declines by 37% (dry condition) after only 3 days of water immersion.

Figure 2. Variations in tensile strength for basalt fibers aged at (a) 60 °C, (b) 71 °C, (c) 82 °C

Unlike basalt, carbon fibers retain their mechanical properties much more effectively across all three aging temperatures. Nevertheless, as illustrated in Figure 3a, strength retention at 60 °C was greater than at 71 °C (Figure 3b), a trend also observed in basalt fibers but to a milder extent under hydrothermal aging. The greatest decline occurs at 82 °C after 35 days of exposure (Figure 3c), in which carbon fibers lost about 26% of their strength in wet testing and 25% after drying. For basalt fibers, the corresponding loss is nearly three times higher. Similarly, carbon fibers samples aged for 7 days at 60 °C, 71 °C, and 82 °C show strength reductions of 6%, 11%, and 20% after drying, respectively. Overall, the data indicate that higher aging temperatures led to progressively greater decreases in strength.

Fiure 3. Variations in tensile strength for carbon fibers aged at (a) 60 °C, (b) 71 °C, (c) 82 °C

Figure 4 illustrates the strength retention of basalt and carbon fibers after exposure to 90 °C and 90% relative humidity, with testing conducted immediately upon removal from the aging environment. The error bars represent one standard deviation from the mean. After 28 days of aging, carbon fibers show a slight reduction in strength of about 5.5%, whereas basalt fibers demonstrate a modest improvement of roughly 9%. These results indicate that basalt fibers remain largely unaffected by the elevated temperature and humidity, while carbon fibers undergo a minor loss in strength. The exact cause of this reduction is uncertain, though it may be linked to fatigue or stress corrosion induced by continuous humid air circulation and fiber agitation in the environmental chamber. Moreover, unlike hydrothermal aging, where only the gauge length is immersed, the humidity exposure affects the entire fiber length, possibly leading to additional weakening and premature damage during mounting in the grips. Nonetheless, the observed decline in carbon fibers strength is far less severe than under hydrothermal conditions, reinforcing the view that hydrolysis is the primary mechanism driving the more significant losses seen in such environments

Figure 4. Variations in tensile strength for basalt and carbon fibers exposed to 90 °C and 90% humidity

Some interesting facts about hydrothermal aging of basalt and carbon fibers are explained as follows:

• Basalt fibers are more vulnerable, with pronounced strength reductions due to silicate composition.
• Carbon fibers are more resistant to hydrolysis, but surface sizing degradation and microcracking is observed.
• Humidity exposure: Vapor-phase moisture is also detrimental, inducing plasticization and microstructural changes comparable to direct immersion.
• In general, hydrothermal exposure is universally detrimental, but severity depends on fiber chemistry.

Note: As previously mentioned for glass fibers, detailed information about predictive modeling frameworks using Arrhenius modeling for basalt and carbon fibers can be found in.

Hydrothermal Aging of Thermoplastic Polymers

Water Uptake, Plasticization, and Chain Scission

Unlike thermosets, which are crosslinked and relatively water-resistant, thermoplastics typically absorb moisture more readily. Hydrothermal exposure induces plasticization, whereby absorbed water increases free volume, lowering stiffness and strength. A broad suite of thermoplastics (high-density polyethylene (HDPE), polypropylene (PP), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), polyamide-6 (PA6), polyamide-12 (PA12), ethylene-vinyl alcohol copolymer (EVOH)) were investigated subjecting them to immersion in deionized (DI) water at 95 °C until saturation. A novel in situ punch–shear device enables direct mechanical testing under immersion, avoiding misleading results. Tensile testing was also evaluated to correlate yield and modulus.

To evaluate how water absorption influences mechanical property measurements, two representative polymers were selected from the midrange of their respective categories: (i) PA6, which shows significant absorption, and (ii) PVDF, which displays only slight but noticeable absorption. The results are presented in Figure 5 as horizontal bar graphs, designed to highlight consistent testing conditions. Along the vertical axis, the measurement temperature and treatment code are indicated for clarity. Within each panel, solid bars on the left represent punch-shear measurements, while the corresponding tensile results are shown as hatched bars on the right; separate panels display modulus and yield data. Bars extending from the mirrored horizontal axis depict absolute values, whereas relative percentages, normalized against pristine conditions at 23 °C, are labeled directly next to their respective bars.

Figure 5. Effect of treatment on the properties of PA6 modulus (a) and yield (b), and PVDF modulus (c) and yield (d)

Thermoplastics exhibited a wide spectrum of hydrothermal durability, and their performance under combined heat and moisture exposure is strongly dependent on their chemical structure and polarity. Some key factors are discussed as follows:

• PP and HDPE generally show very low water absorption due to their nonpolar molecular backbone. This makes them resistant to swelling and hydrolysis. However, their long-term stability is compromised by oxidative degradation, particularly at elevated temperatures, where oxygen and moisture can accelerate chain scission and embrittlement.

• Polyamides, by contrast, are highly hygroscopic because of the presence of amide groups that readily form hydrogen bonds with water. Moisture uptake in polyamides leads to plasticization, dimensional instability, and reductions in stiffness. Extended hydrothermal exposure, however, promotes hydrolysis, which progressively weakens the polymer chains.

• High-performance thermoplastics like PPS and PVDF stand out for their superior stability. PPS is inherently resistant to both moisture and oxidative attack due to its aromatic, highly crystalline structure, while PVDF combines low water absorption with excellent chemical and thermal resistance. These attributes make them particularly suitable for demanding environments such as marine and chemical processing applications.

• EVOH, on the other hand, represents the opposite extreme: It is extremely moisture-sensitive because of its high content of hydroxyl groups. Even modest humidity levels can drastically reduce its barrier properties and mechanical integrity, limiting its use in high-humidity environments unless adequately protected by multilayer structures or coatings.

These differences highlight that hydrothermal performance in thermoplastics cannot be generalized but must be evaluated case by case, with attention to molecular structure, polarity, and potential degradation pathways.

Comparative Analysis: Fibers vs. Thermoplastics

Common Mechanisms

Under hydrothermal aging, both fibers and thermoplastics are subjected to a set of overlapping degradation mechanisms that progressively compromise their structural integrity. A primary process is plasticization, in which absorbed water molecules act as a temporary plasticizer, reducing stiffness and strength by increasing chain mobility in polymers or weakening interfacial bonds in fibers. Leaching of low-molecular-weight species, additives, or sizing agents further accelerates property loss, especially when the surrounding water is continuously refreshed. Additionally, surface degradation is a recurring phenomenon: SEM frequently reveals the formation of microcracks, pits, and cavities, while Fourier-transform infrared spectroscopy (FTIR) detects chemical shifts associated with hydrolysis, oxidation, or the removal of functional groups. Together, these findings point to a common pattern of structural and chemical deterioration, even though the severity and reversibility differ between fibers and thermoplastics.

Distinctive Behaviors

Despite the shared mechanisms, fibers and thermoplastics exhibit distinctive hydrothermal aging pathways dictated by their composition.

Fibers:

• Glass and basalt fibers primarily degrade through dissolution, where silica-rich phases are gradually leached into water, reducing fiber strength and creating surface flaws that act as crack initiation sites.
• Carbon fibers are chemically more stable but depend heavily on the integrity of their surface sizing. Hydrothermal exposure can degrade the sizing layer, leading to loss of interfacial adhesion with the matrix in composite applications, even if the carbon filaments themselves remain intact.

Thermoplastics:

• Polymers experience water absorption, which varies with polarity and crystallinity. Hydrophilic polymers like polyamides (PA6, PA12) show significant uptake, while hydrophobic systems such as PPS or PVDF absorb far less.
• Prolonged exposure can induce chain scission, particularly in polymers susceptible to hydrolysis, such as EVOH or polyesters. This results in permanent loss of molecular weight, embrittlement, and dimensional instability.

Material selection

From a design perspective, the implications of hydrothermal aging are highly material-specific:

• Glass and basalt fibers should generally be avoided in applications involving direct or prolonged water contact unless protected by barrier coatings or hydrophobic treatments. Their dissolution-driven strength loss is difficult to arrest once initiated.
• Carbon fibers, owing to their chemical inertness, are preferable in humid environments, provided that durable and hydrothermally stable sizing systems are employed to maintain interfacial strength.
• Thermoplastics show a broad range of suitability. High-performance polymers such as PPS and PVDF stand out as top candidates for hydrothermal durability due to their low water uptake and chemical resistance. In contrast, polyamides (PA6, PA12) and EVOH are unsuitable for hydrothermal environments, as they suffer severe plasticization, swelling, and irreversible hydrolysis.

In practice, the careful matching of fiber or polymer chemistry with environmental exposure conditions is critical to ensuring long-term reliability. Hydrothermal aging data provide engineers with essential guidance for material selection, structural design, and service-life estimation in demanding applications such as marine, offshore, and chemical-processing systems.

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References

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The individual materials that combine to form the composite material. The constituent materials are separate and distinct on a macroscopic level.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Sizing or fibre sizing refers to a coating that is applied to fibre during manufacturing. Highly proprietary (formulation and process).

Sizing serves two functions:

• Protects and aids fibre during processing
• Aids in the bonding of fibre and matrix

Periodic 3-D, repeating array of molecules.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.