Hydrolysis Resistance of Plastics

Plastics degrade in water because of the chemical reaction of hydrolysis. Water molecules can react with certain bonds in the polymer chain to create new molecules. These molecules usually have significantly shorter chain lengths. This mechanism occurs in different stages as shown in the figure below.

 

1. Hydration (Stage I): The aqueous medium penetrates the polymer matrix and disrupts the secondary forces. This leads to the relaxation and the decrease of the glass transition temperature.

2. Initial degradation (Stage II): In the hydrated region of the polymer, the cleavage of the covalent bonds in the polymer backbone begins. This results in a decrease in the molecular weight of the polymer. As hydrolysis goes on, the hydrolysis reaction inside the polymer matrix is auto-catalyzed by more and more carboxylic end-groups. This leads to the continuous decrease of the molecular weight of the polymer. The polymer loses its mechanical strength along with the decrease of the molecular weight. However, the integrity of the polymer is maintained.

3. Further degradation (Stage III): The molecular weight of the polymer keeps falling to a threshold that the integrity of the polymer can no longer be held. Thus, significant mass loss begins.

4. Solubilization or erosion (Stage IV): The polymer loses its weight. Its fragments are further cleaved into molecules that are soluble in the medium.

Schematic representation of Hydrolytic Degradation of polymers¹

Impact of hydrolysis on the longevity of plastic parts

Understanding how hydrolysis impacts longevity is crucial for designing durable and reliable plastic products. Hydrolysis significantly reduces the longevity of plastic parts by several mechanisms as follows:

 

• Weakening: Hydrolysis breaks down the polymer chains. This reduces the material's strength, flexibility, and impact resistance. This makes the part more susceptible to cracking and failure under stress.
• Embrittlement: As the polymer chains break, the plastic becomes more brittle and less ductile. It can no longer absorb impact energy as effectively, making it more prone to shattering.
• Dimensional changes: If the hydrolysis is accompanied by water absorption, the plastic may swell or warp. This can lead to changes in dimensions and potentially interfere with the function of the part.
• Surface degradation: Hydrolysis can cause surface erosion, cracking, pitting, or discoloration. Thus, affecting the appearance and potentially the performance of the part.
• Accelerated degradation: Hydrolysis can make the plastic more vulnerable to other forms of degradation, such as oxidation or UV damage. It can also facilitate the leaching of additives, which can further compromise the material's properties.
• Premature failure: In structural applications, hydrolysis can lead to premature failure of the plastic part. This can lead to safety hazards or costly repairs.
• Reduced service life: Overall, hydrolysis leads to a shorter lifespan of the plastic part. Thus, requiring more frequent replacements and increasing maintenance costs.

Choosing a material with high hydrolytic stability, low water absorption, and resistance to other forms of water damage is essential for ensuring long-term performance and reliability. Knowing the challenges and mechanisms when selecting plastics for use in aqueous environments is important. Some of the key challenges are explained below:

 

• Hydrolytic degradation: Water can react with certain types of plastics (especially those with ester, amide, or urethane linkages). This can lead to chain scission and a decline in material properties. This is a primary concern.
• Water absorption: Some plastics absorb water, leading to swelling, changes in dimensions, and a reduction in mechanical strength. This can be problematic for applications requiring precise fit or load-bearing capacity.
• Leaching of additives: Water can leach out additives (plasticizers, stabilizers, colorants) from the plastic. This affects performance and raises environmental concerns.
• Environmental stress cracking: The combination of water and mechanical stress can lead to premature cracking and failure in some plastics.
• Microbial growth: In some cases, plastics can serve as a substrate for microbial growth (bacteria, fungi). This can cause surface degradation, discoloration, and odors.
• Temperature effects: Temperature plays a significant role. Warm water accelerates hydrolysis and other degradation processes. Freezing and thawing cycles can also damage some plastics.
• Durability and longevity: The combined effects of hydrolysis, water absorption, and other factors can reduce the long-term durability and service life of plastics in aqueous environments.

Factors accelerating hydrolysis in plastics

Several factors can accelerate the hydrolytic degradation process. These are explained below:

 

• Presence of water/moisture: This is the fundamental requirement for hydrolysis. Humidity, condensation, and direct contact with water can all contribute to hydrolysis.
• Temperature: Higher temperatures generally accelerate chemical reactions, including hydrolysis. Warm, humid environments are particularly detrimental.
• pH: Acidic or alkaline conditions can catalyze the hydrolysis reaction, speeding up the degradation process. Neutral pH can also cause hydrolysis, but it is often slower.
• Chemical structure of the polymer: Some polymers are inherently more susceptible to hydrolysis than others. This is due to the types of bonds within the polymer structure. Polyesters, polyamides, polyurethanes, and polycarbonates are common examples of hydrolysis-prone polymers.
• Presence of catalysts: Certain substances, like metal ions or residual catalysts from the manufacturing process, can accelerate hydrolysis.
• Stress: Mechanical stress on the plastic can make it more vulnerable to hydrolysis. This happens by creating micro-cracks through which water can penetrate more easily.

Hydrolysis resistance refers to a plastic material's ability to withstand degradation or breakdown when exposed to moisture or water. This is especially true under acidic or alkaline conditions and often at elevated temperatures.

A hydrolysis-resistant plastic will maintain its structural integrity, mechanical properties (strength, flexibility, etc.), and appearance even after prolonged exposure to humid environments or direct contact with water. Essentially, it resists the chemical reaction called hydrolysis.

The best approach for improving hydrolysis resistance will depend on the specific polymer, the intended application, and the environmental conditions it will be exposed to. Often, a combination of the following strategies will be most effective.

Material selection

Hydrolysis-resistant plastics are engineered to withstand degradation from moisture and water, especially in challenging environments. Below are the key properties that define these materials.

Chemical stability

• Resistance to hydrolysis: This is the primary characteristic. Hydrolysis-resistant plastics possess chemical structures that are less susceptible to reacting with water molecules. Thus, preventing the breakdown of polymer chains.
• Broad chemical resistance: Often, hydrolysis-resistant plastics also exhibit good resistance to other chemicals. These include resistance to acids, bases, and solvents, further enhancing their durability.

Physical properties

• Dimensional stability: They maintain their shape and size even in humid or wet conditions. This is crucial for applications where precise fit and function are essential.
• Mechanical strength: Hydrolysis-resistant plastics retain their mechanical properties (tensile strength, flexural strength, impact resistance) even after exposure to moisture. This ensures structural integrity and long-term performance.
• Low moisture absorption: They absorb minimal amounts of water, preventing swelling, warping, or changes in properties that can occur with water absorption.

Thermal properties

• High-temperature resistance: Many hydrolysis-resistant plastics can withstand elevated temperatures without losing their properties. This is important for applications where they might be exposed to heat or used in warm, humid environments.

Durability and longevity

• Long-term performance: These plastics offer extended service life and reliability. This is due to their resistance to hydrolysis and other forms of degradation, reducing the need for frequent replacements.
• Weatherability: Some hydrolysis-resistant plastics also exhibit good resistance to weathering, including UV radiation. This makes them suitable for outdoor applications.

Processing and application

• Ease of processing: While performance is key, many hydrolysis-resistant plastics are also designed for efficient processing using common techniques. For example, injection molding, extrusion, or machining.
• Versatility: These plastics find use in diverse applications across industries, including automotive, electronics, medical, industrial, and consumer goods, wherever moisture resistance and durability are critical.

The most effective way to improve hydrolysis resistance is to select polymers that are inherently less susceptible to it. Selecting the right hydrolysis-resistant plastic is a critical step in product design. It involves a thorough assessment of the operating environment, performance requirements, and processing considerations. Testing and validation are essential to ensure the chosen material meets the needs of the application.

As a plastics formulator, you should follow the following steps to choose hydrolytically stable polymers wisely:


 

STEP 1: Defining the operating environment
Temperature, humidity, water exposure, chemical exposure, and pH must be defined.


 

STEP 2: Determining performance requirements
Mechanical properties, thermal properties, dimensional stability, electrical properties, optical properties, and aesthetic requirements are determined.


 

STEP 3: Considering processing requirements
The compatibility of the chosen plastic with the manufacturing process should be ensured.


 

STEP 4: Evaluating material options
Different polymers known for their hydrolysis resistance need to be investigated. For example, PPS, PEEK, PEI, PSU, fluoropolymers, and HDPE for less demanding applications.


 

STEP 5: Testing and validation
The material's performance in simulated environments should be assessed by performing accelerated weathering tests, immersion tests, or other relevant tests.


 

STEP 6: Considering long-term performance
The available data and models must be used to estimate the long-term performance and service life of the plastic part.

Chemical modification

• End-capping: Modifying the end groups of polymer chains can prevent water molecules from attacking them.
• Crosslinking: Crosslinking the polymer chains creates a more interconnected network. This reduces the mobility of polymer chains and hinders water penetration. For example, radiation-crosslinked PE-Xc pipes are extremely resistant and can thus be used for very long periods of over 30 years.

Tensile Strength of Polyethylene and Crosslinked Polyethylene in Boiling Water with Respect to Time

• Incorporation of hydrophobic groups: Introducing hydrophobic side groups into the polymer chain can repel water and improve hydrolysis resistance.

Additive incorporation

• Hydrolytic stabilizers: These additives react with the degradation products of hydrolysis. Thus, preventing them from further catalyzing the breakdown of the polymer. They can also act as scavengers for water or acids that might initiate hydrolysis.
• Antioxidants: Oxidation can sometimes occur alongside hydrolysis and antioxidants can protect the polymer from oxidative degradation.
• Fillers: Certain fillers can improve the barrier properties of the plastic, reducing water absorption and hindering hydrolysis. However, it is important to choose fillers that are themselves resistant to hydrolysis.

Processing optimization

• Control of melt temperature and residence time: Optimizing the melt processing conditions can minimize defects and residual stresses in the plastic part. This can make it more susceptible to hydrolysis.
• Drying: Thoroughly drying the plastic before processing is crucial to remove any absorbed moisture that could initiate hydrolysis during melt processing.

Surface treatments

• Coatings: Applying a protective coating, such as a fluoropolymer, silicone, or epoxy, can create a barrier against moisture and improve hydrolysis resistance.
• Plasma treatment: This can modify the surface of the plastic, making it more hydrophobic and less susceptible to water absorption.

Design considerations

• Minimize stress concentrations: Sharp corners, notches, and other design features can create stress concentrations. Thus, making the plastic more vulnerable to hydrolysis, especially in the presence of moisture.
• Proper sealing: For applications where the plastic will be exposed to water, proper sealing and gasketing can prevent water from reaching vulnerable areas.

Environmental control and testing

Storing and using plastic parts in dry, cool environments can slow down the rate of hydrolysis. Conducting regular hydrolysis resistance tests can help identify potential problems early on. Thus, ensuring the plastic material performs as expected.

 

Underwater applications

Underwater applications demand materials that can withstand constant submersion, resist water absorption, and maintain their properties under pressure. Here are some top contenders:

 

• PVDF: This fluoropolymer is a real workhorse in underwater settings. It is incredibly inert, which means it does not react with most chemicals, including seawater. PVDF boasts excellent resistance to hydrolysis, UV degradation, and even radiation. It is often used for underwater cables, seals, and components in harsh chemical environments.
• PPS: PPS is a high-performance thermoplastic known for its strength and resistance to high chemicals and temperatures. It holds up well in hot water and steam, making it suitable for underwater applications where temperature or chemical exposure is a concern.
• PEEK: When you need exceptional strength and chemical resistance, PEEK is a great choice. It is a high-performance thermoplastic that can handle demanding underwater conditions, including high pressure and exposure to various chemicals. PEEK is often used in subsea equipment and components.
• HDPE: For less demanding underwater applications, HDPE offers a good balance of water resistance and affordability. It is commonly used for pipes, containers, buoys, and other components that do not require extreme strength or chemical resistance.

Marine applications

Marine environments present a unique set of challenges. These include exposure to saltwater, UV radiation, and the constant motion of the sea. Some of the key considerations for marine plastics include:
 

• Saltwater resistance: Saltwater is highly corrosive. Hence, the plastic must be able to withstand prolonged exposure without degrading.
• UV resistance: The sun's UV radiation can cause plastics to fade, crack, and become brittle. Plastics with built-in UV stabilizers must be chosen or protective coatings should be applied.
• Mechanical strength: Marine environments can be rough, so the plastic needs to be strong enough to handle waves, impacts, and other stresses.
• Antifouling properties: Some plastics can be treated with antifouling agents to prevent the growth of marine organisms.
• Environmental impact: The potential impact of plastic on the marine environment should be considered and eco-friendly options must be chosen when possible.

Let's explore some of the best plastics for marine applications:

 

• HDPE: It is a popular choice for marine applications due to its excellent resistance to saltwater, UV radiation, and general weathering. It is used for boat hulls, decking, pontoons, and other structural components.
• Marine-grade PVC: This type of PVC is specifically formulated for marine environments. It is resistant to moisture, rot, and fungal growth. Thus, making it suitable for boat interiors, seating, and other applications where water exposure is a concern.
• Acrylic (PMMA): It is prized for its optical clarity and weather resistance. It is often used for boat windows, windshields, and other transparent components.
• Polycarbonate (PC): It is incredibly impact-resistant, making it a good choice for applications where durability is paramount. It is used for boat windscreens, enclosures, and other parts that need to withstand rough conditions.
• Fiberglass-reinforced plastics (FRP): FRP composites combine a plastic resin (like polyester or epoxy) with reinforcing fibers (like glass or carbon). This creates a strong, lightweight material that is ideal for boat hulls, decks, and other structural components.

Select other commercial grades of hydrolysis-resistant polymers available in our extensive database:

 

Testing for hydrolysis resistance is crucial to determine a plastic's suitability for applications where it will be exposed to moisture or humid environments. Several standardized test methods exist. The specific test chosen depends on the plastic, the intended application, and the expected environmental conditions.

Accelerated weathering/humidity chambers

• These chambers simulate real-world environmental conditions. This includes controlled temperature, humidity levels, and sometimes UV radiation.
• Plastic samples are placed in the chamber for a specified period, and their properties (e.g., tensile strength, impact resistance, appearance, and molecular weight) are periodically measured.
• Standards like ASTM G154 (for UV exposure) and ASTM D4329 (for accelerated weathering) are relevant here.^2,3

Accelerated Weathering Chamber⁹

Immersion testing

• This involves immersing plastic samples in water or an aqueous solution (acidic, alkaline, or neutral pH) at a controlled temperature for a specific duration.
• The samples are then removed and their properties are evaluated.
• Standards like ISO 62 and ASTM D570 (for water absorption) can provide guidance.^4,5

Steam autoclave testing

It is a more aggressive test, often used to simulate harsh conditions or accelerate degradation. Changes in the material's properties are measured after the test.

Quantification of hydrolytic degradation of plastics

Several analytical techniques can be used to quantify the hydrolytic degradation of plastics. These methods help determine the extent of degradation by measuring changes in the material's chemical structure, molecular weight, and physical properties⁶.

Molecular weight analysis

• Gel permeation chromatography (GPC)/size exclusion chromatography (SEC): This is a primary technique for assessing hydrolytic degradation. GPC/SEC separates polymer chains based on their size, allowing the determination of the molecular weight distribution (MWD). Hydrolysis leads to chain scission, resulting in a decrease in average molecular weight and a broadening of the MWD. This is a highly sensitive method for detecting even small amounts of degradation.
• Viscosity measurements: The viscosity of a polymer solution is related to its molecular weight. As hydrolysis breaks down polymer chains, the viscosity of the solution decreases. Viscometry can be used to track changes in molecular weight indirectly.

Spectroscopic analysis

• Fourier transform infrared spectroscopy (FT-IR): FT-IR spectroscopy can identify changes in the chemical bonds within the polymer after hydrolysis.
• Nuclear magnetic resonance spectroscopy (NMR): NMR provides detailed information about the molecular structure of the polymer. It can be used to identify and quantify the products of hydrolysis and changes in the polymer backbone.

Thermal analysis

• Differential scanning calorimetry (DSC): Changes in the glass transition temperature (Tg), melting temperature (Tm), and crystallinity of the polymer can indicate hydrolytic degradation. For instance, a decrease in Tm or Tg, or a change in crystallinity, often suggests degradation.
• Thermogravimetric analysis (TGA): Hydrolysis can alter the thermal stability of the polymer, leading to changes in the TGA thermogram. For example, degraded polymers might exhibit earlier or different decomposition patterns.

Mechanical testing

It measures the tensile strength, elongation, bending strength, modulus, and impact strength of the material. Hydrolysis typically reduces tensile strength and elongation.

Surface analysis

• Scanning Electron Microscopy (SEM): SEM can be used to examine the surface of the plastic after hydrolysis. It can reveal cracks, pitting, or other surface defects that are indicative of degradation.
• Atomic Force Microscopy (AFM): AFM provides higher resolution surface images than SEM. It can be used to study the surface morphology and roughness changes due to hydrolysis at the nanoscale.

Other techniques

Titration can be used to quantify the amount of specific degradation products, such as carboxylic acids formed during polyester hydrolysis.

It is often beneficial to use a combination of techniques to get a comprehensive understanding of the hydrolytic degradation process. For example, GPC might be used to track molecular weight changes. While FT-IR can identify chemical changes, and mechanical testing can assess the impact on physical properties.

 

  


References

 

1. Li, Ying, et al. "The effect of mechanical loads on the degradation of aliphatic biodegradable polyesters." Regenerative Biomaterials 4.3 (2017): 179-190
2. ASTM G154-23 Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Materials
3. ASTM D4329-21 Standard Practice for Fluorescent Ultraviolet (UV) Lamp Apparatus Exposure of Plastics
4. ISO 62:2008 Plastics — Determination of water absorption
5. ASTM D570-22 Standard Test Method for Water Absorption of Plastics
6. Chamas, Ali, et al. "Degradation rates of plastics in the environment." ACS Sustainable Chemistry & Engineering 8.9 (2020): 3494-3511
7. https://www.findoutaboutplastics.com/2023/10/design-properties-for-engineers.html
8. https://www.globalpipe.de/basic-knowledge/radiation-crosslinking-of-pe-xc-plastic-pipes-beta-gamma-service-gmbh-co-kg/
9. https://www.polymertesting.com.au/accelerated-weathering/