Ethan Clarke
Water absorption in plastics is a critical phenomenon that can significantly impact the properties and dimensions of plastic products. The degree of water absorption, or moisture absorption, depends on various factors, including the type of plastic, temperature, humidity, and contact time. This process can lead to noticeable volume changes, known as swelling, and is particularly prominent in nylon, a common plastic material. The absorption of water by nylon can reach up to 10% of its weight, affecting its rigidity and strength. This expansion is caused by the water molecules forcing the polymer chains to increase the distance between them. While most polymers absorb water to some extent, the dimensional changes and other property alterations vary across different plastic types. The understanding of water absorption in plastics is essential for engineers and designers to ensure the stability and functionality of plastic products in various environments.
| Characteristics | Values |
|---|---|
| Definition of water absorption | The ratio of the weight of water absorbed by a material to the weight of the dry material |
| Plastics with zero water absorption | PTFE |
| Plastics with very low water absorption | PEEK, PPS, PSU, PPSU, PEI, PVDF, PET, PPE, PP, PE |
| Plastics with low water absorption | POM, PA12, PC, ABS |
| Plastics with high water absorption | Polyamides (nylons) |
| Effect of water absorption on plastics | Dimensional changes, reduction in strength, changes in electrical insulating characteristics, reduction in intermolecular friction, change in molecular mobility, reduction in mechanical strength, reduction in electrical and other physical properties |
| Factors affecting water absorption in plastics | Type of plastic, ambient conditions such as temperature, humidity, and contact time |
| Methods to reduce dimensional changes | Adding fillers and reinforcements, using glass fiber reinforced compounds, drying in an oven |
| Standards for measuring water absorption | ASTM D570-98, ISO 62 (water absorption 24 hours, 23°C) |
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What You'll Learn
How does water absorption affect the physical properties of plastics?
Water absorption affects the physical properties of plastics in several ways. Firstly, it can cause volumetric expansion or swelling of the plastic material. This is because water moves into the free volume between the polymer chains, forcing them further apart and leading to an increase in the size of the plastic part. The degree of volumetric expansion depends on the type of plastic, the ambient conditions such as temperature and humidity, and the contact time with water. For example, nylon, a type of polyamide, can absorb water up to 10% of its weight, resulting in noticeable swelling. Other plastics like polyethersulfone and polyetherimide can absorb up to 2% by weight, but their high glass-transition temperatures minimize the dimensional changes.
Secondly, water absorption can alter the mechanical properties of plastics. The absorption of water reduces intermolecular friction and increases molecular mobility, leading to decreased resistance to applied stress. This can result in reduced mechanical strength and flexibility, particularly in hygroscopic materials with high diffusion rates. Additionally, water absorption can affect the electrical properties of plastics, including electrical conductivity and the dielectric loss factor.
Furthermore, water absorption can impact the processing and molding of plastics. For instance, in the case of nylon, the absorption of moisture must be considered during mold-making. The shrinkage factor used in designing the mold needs to account for potential changes in post-molded dimensions due to moisture absorption. Heated to molding temperatures while wet can result in hydrolytic degradation, a chemical reaction that severs primary bonds in the molecular chains, reducing molecular weight and causing a loss of physical properties. Over-drying, on the other hand, can remove the plasticizing effect of water molecules, making the resin highly viscous and difficult to flow during processing.
Lastly, water absorption can influence the dimensional stability of plastics over time. Environmental factors, such as temperature and humidity, can cause molded plastic parts to increase or decrease in size. Fillers and reinforcements can be used to reduce these dimensional changes, but even highly filled nylon will still expand by approximately 0.1% for every inch of part dimension. Water absorption can also affect the glass transition temperature of plastics, with water acting as a plasticizer in some cases, further influencing the dimensional stability of the material.
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How does the type of plastic impact water absorption?
Water absorption is defined as the ratio of the weight of water absorbed by a material to the weight of the dry material. The higher the absorption level, the more concern there should be about potential changes in mechanical and electrical properties, as well as part dimensions. Water absorption applies to some extent to many plastics, but is mostly negligible. The plastic most often affected to a noticeable degree by water is nylon. The swelling varies with the type of nylon, fillers used, environmental factors, and time. The absorption of moisture by nylon is a completely reversible physical reaction.
The rate of absorption varies with the type of nylon, temperature, and relative humidity. The addition of fillers reduces the effect of moisture due to the volume reduction of the amount of nylon polymer in the mixture and by sharing the attraction of the molecules, reducing polarity and the available space for moisture molecules. Acetal is often considered an alternative to nylon that avoids the difficulties associated with moisture absorption. At room temperature and 50% relative humidity, an unfilled acetal will absorb only about 0.2% moisture and will expand by approximately 0.2%much less problematic than nylon.
The water absorption of plastic material varies because of the different properties of each kind of plastic material, relative humidity, and temperature in the environment, or chemical additives used to modify the property of the plastic material. Materials such as polyethersulfone and polyetherimide can absorb as much as 2% by weight over time. However, because these polymers are amorphous and have extremely high glass-transition temperatures, the effect of this moisture absorption on dimensions and properties is much smaller than it is in nylon.
Moisture absorption can directly impact the physical and mechanical properties of plastics. It can also impact the molding of plastics, such as causing silver streaks or brittleness, and influencing the tensile strength and impact strength of the finished plastic article. Before production, it is important to dry the material properly to prevent molding defects.
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What is the water absorption factor and how is it measured?
The water absorption factor is defined as the ratio of the weight of water absorbed by a material to the weight of the dry material. This factor is used to assess qualitatively the sensitivity of a plastic to water. The higher the absorption level, the more concern there should be about potential changes in mechanical and electrical properties, as well as part dimensions.
The absorption of water by plastics can cause noticeable volume expansion or swelling. This is due to the water molecules forcing the polymer chains to increase, leading to volumetric expansion. The greater the amount of moisture absorption, the greater the volumetric expansion. This can be as high as a 10% increase in weight for nylon. The dimensional changes can be as much as 0.7% in nylon parts from the "as-moulded" state to equilibrium at 50% RH environments. This change can occur in approximately 150 days for a 1.5mm thick part.
The absorption of water by nylon specifically has been a source of great study for many years. Nylon is the plastic most often affected to a noticeable degree by water. The absorption of moisture by nylon must be considered in mould-making, as the shrinkage factor used in designing the mould must take into account the potential for change in post-moulded dimensions.
The water absorption factor can be measured using ASTM D570-98, Standard Test Method for Water Absorption of Plastics.
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How does temperature affect plastic's water absorption?
The effects of temperature on plastics are well-known, with high temperatures causing notable changes in the material's properties. However, low temperatures can also significantly impact plastics. Plastics exposed to extremely cold temperatures tend to harden and become more brittle, resembling glass in structure and function. This increases the risk of fracture and changes the plastic's wear behaviour, friction, and overall mechanical properties.
The glass transition temperature, the temperature at which plastics transition from a flexible to a brittle state, varies among different types of plastics. For example, PTFE has a glass transition temperature of 130°C, while PVDF transitions at -45°C. At these glass transition temperatures, the plastic's flexibility and mobility at a molecular level are reduced.
The absorption of water by plastics is influenced by temperature. Water absorption, or moisture absorption, is the capacity of a plastic to absorb moisture from its environment. The degree of water absorption depends on the plastic type and ambient conditions such as temperature, humidity, and contact time. Water absorption can lead to dimensional changes and alter the material's properties, such as mechanical strength and electrical conductivity.
At low temperatures, absorbed water can affect the mechanical performance of plastics in unexpected ways. For example, soy protein plastic sheets submerged in water at room temperature absorbed up to 180% water. However, the presence of ZnSO4 reduced their water absorption by 30%. The effect of water absorption on plastic properties can vary between low and high temperatures, with water influencing the mechanical performance of plastics even at very low temperatures.
When heated to moulding temperatures while containing moisture, plastics can undergo hydrolytic degradation, a chemical reaction that severs primary bonds in the molecular chains, reducing their molecular weight. This process results in a significant loss of physical properties. Thus, the temperature plays a crucial role in understanding the water absorption characteristics of plastics and their subsequent changes in volume and properties.
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How does water absorption change the volume of nylon?
Water absorption is defined as the ratio of the weight of water absorbed by a material to the weight of the dry material. Water absorption applies to some extent to many plastics, but it is mostly negligible. However, nylon is extremely sensitive to moisture and is the plastic most often affected to a noticeable degree by water. The absorption of moisture by nylon is a completely reversible physical reaction.
Nylon contains hydrogen bonding, which attracts water to a greater degree compared to other polymers. The water molecules force the polymer chains to increase, leading to volumetric expansion. The greater the amount of moisture absorption, the greater the volumetric expansion. This expansion can be as high as 0.5-0.6% in an unfilled nylon 6 or nylon 6/6 at room temperature. If the temperature increases, the moisture absorption levels will increase, resulting in larger dimensional changes.
The effects of moisture absorption on nylon can vary depending on the type of nylon, fillers used, environmental factors, and time. For example, Intech Power-Core™ PA12GC (Gravity Cast), a type of nylon, absorbs a maximum of 1.4% water when fully submerged, resulting in a dimensional change of 0.2%. On the other hand, nylon 6 can absorb over 10% of its weight in water, resulting in a measurable dimensional change of 2%.
Moisture absorption in nylon can lead to hydrolysis, which is the chemical breakdown of a compound due to its reaction with water. Hydrolysis occurs when the existing absorbed moisture is not removed, leading to the breakdown of the molecular weight of the nylon polymer. This can cause a decrease in impact strength and other physical properties. Therefore, it is important to control the moisture content in nylon during processing to prevent defects and ensure optimal performance.
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Frequently asked questions
Water absorption affects the rigidity and linear strength of plastic parts. It also influences the strength and flexibility of the plastic. The higher the absorption level, the more concern there should be about potential changes in mechanical and electrical properties, as well as part dimensions.
Water absorption is defined as the ratio of the weight of water absorbed by a material to the weight of the dry material. The primary value of this data is to assess qualitatively the sensitivity of a plastic to water.
The amount of moisture absorption increases with an increase in temperature. This leads to greater dimensional changes.
