Sofia Martinez
Water absorption is a critical property in the plastics industry, as it indicates how much moisture a plastic material will absorb over time. The degree of moisture absorption depends on the type of plastic and the ambient conditions such as temperature, humidity, and contact time. While some plastics have very low water absorption, others are not completely waterproof. The absorption of water can lead to swelling, warping, and reduced strength, which can result in a significant loss of performance or even failure in an application. Therefore, it is essential to select the appropriate plastic material for specific environments, particularly for parts used outdoors, in humid conditions, or in contact with liquids.
| Characteristics | Values |
|---|---|
| Water absorption and water transmission | Not related |
| Plastic's ability to transmit water | Some amount of water must be stored in the plastic |
| Plastic's ability to absorb water | Depends on the type of plastic, temperature, humidity, and contact time |
| Plastic with zero water absorption | PTFE |
| Plastics with very low water absorption | PEEK, PPS, PSU, PPSU, PEI, PVDF, PET, PPE, PP, and PE |
| Plastics with low water absorption | POM, PA12, PC, ABS, Polypropylene, and Polytetrafluoroethylene (Rulon and Teflon) |
| Plastic with high water absorption | Polyamides (nylons) |
| Impact of water absorption on plastic | Swelling, warping, reduced strength, compromised performance, dimensional changes |
| Test for water absorption in plastics | ASTM D570 and ISO 62 |
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What You'll Learn
Plastic absorbs water to a limited degree
The ASTM D570 and ISO 62 tests are the most widely used standards to measure water absorption in plastics. The tests involve drying specimens in an oven, cooling them in a desiccator, weighing them, and then immersing them in water at a temperature of 23°C for 24 hours or until equilibrium. The results of these tests help manufacturers select appropriate materials for specific environments and implement necessary design modifications to ensure product reliability.
While some plastics, such as polyolefins like PE, PP, and polybutylene, absorb little water, others, like nylon, are more hygroscopic. The presence of excessive moisture in plastics can reduce viscosity, affect appearance, and lead to processing problems. Additionally, moisture in the material structure can impact thermal insulation and dielectric properties, as well as cause aging of the material.
It is worth noting that water absorption and water transmission are different concepts. While some plastics may absorb small amounts of water, this does not necessarily mean that water can penetrate or pass through them. The ability of a plastic to transmit water depends on various factors, including its structure and composition.
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Water absorption and transmission are different
Water absorption and water transmission are indeed different and unrelated processes. While a screen door does not absorb water, it transmits it well. Similarly, plastics are comparable to screen doors in this regard. If plastics transmit water, a small amount of water must be temporarily stored in the plastic while it is being transmitted. Hence, plastics allow both transmission and storage.
The ASTM D570 and ISO 62 tests are the most widely used standards to measure water absorption in plastics. The ASTM D570 test helps determine the proportion of water absorbed by a material and the effects of exposure to water or humid conditions on its properties. It is also used as a control test on the uniformity of a product. The rate of water absorption is a critical property in the plastics industry. It indicates how much moisture a plastic material will absorb over time, which directly impacts dimensional stability, mechanical properties, and electrical insulation characteristics.
High water absorption can lead to several issues, including swelling, warping, reduced strength, and compromised performance. Therefore, the water absorption rate is essential for plastic manufacturers to anticipate potential issues and implement necessary design modifications to ensure product reliability. For example, polyolefins such as PE, PP, and polybutylene absorb little water and are unaffected by aging in water. On the other hand, nylon, even when formed, can still attract water if left sitting in it.
While absorption and transmission are different, it is worth noting that some objects that transmit light can also absorb it. For example, a clear water bottle has a high level of transmittance because you can see through it, but it also absorbs some light, which is why you can see the bottle itself.
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Plastic with low water absorption
Water absorption is a critical property in the plastics industry, as it indicates how much moisture a plastic material will absorb over time. The absorption of water can lead to swelling, warping, reduced strength, and compromised performance in applications where moisture exposure is expected.
Plastics with very low water absorption include polymers such as PEEK, PPS, PSU, PPSU, PEI, PVDF, PET, PPE, PP, and PE. POM, PA12, PC, and ABS also exhibit low water absorption. Polyolefins such as PE, PP, and polybutylene contain no chemical bonds that are easily hydrolyzable, so they absorb little water and are essentially unaffected by aging in water.
On the other hand, nylon is a plastic that is highly affected by water absorption. Nylon can absorb water to as high as 10% of its weight, leading to changes in rigidity, strength, and flexibility. The absorption of moisture by nylon during the molding process can result in hydrolytic degradation and a significant loss of physical properties.
While some plastics have low water absorption, it is important to note that most plastics are not completely waterproof. Water absorption and water transmission are different but related concepts. Even if a plastic does not absorb water, it may still transmit water, and some amount of water must be temporarily stored in the plastic during transmission.
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Plastic's dimensional stability
Dimensional stability in plastics refers to a material's ability to retain its size, shape, and dimensions throughout its lifecycle, even under varying environmental conditions. It is a critical factor in the plastics industry, as it directly impacts the performance and reliability of plastic products.
Plastics are generally susceptible to moisture absorption, with some plastics absorbing more water than others. This moisture absorption can lead to swelling and changes in the dimensions of the plastic, compromising the performance of the product. For instance, polyamides (nylons) have higher water absorption than other engineering plastics, which can lead to dimensional changes in finished parts and a reduction in strength.
The dimensional stability of plastics is influenced by factors such as moisture, temperature, and mechanical stress. High temperatures can cause plastics to expand, while low temperatures can cause them to contract. This can result in mating and clearance issues and weaken the mechanical performance of the plastic over time. Therefore, it is essential to choose plastics with low thermal expansion coefficients for applications in extreme temperature environments.
Additionally, the manufacturing process can impact the internal stress of a plastic shape. For instance, injection molding can result in greater internal stress compared to extrusion due to the way plastic molecules cool. To achieve superior dimensional stability, sophisticated processing and state-of-the-art machining technology are employed, allowing for the creation of intricate parts with small scales and tight tolerances.
By selecting dimensionally stable plastics and considering the specific environmental conditions, manufacturers can ensure the precision and reliability of plastic products throughout their lifespan.
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Plastic's moisture absorption
Water absorption, or moisture absorption, is a critical property in the plastics industry. It indicates how much moisture a plastic material will absorb over time, which directly impacts dimensional stability, mechanical properties, and electrical insulation characteristics.
The most widely used standards to measure water absorption in plastics are ASTM D570 and ISO 62. The ASTM D570 test involves drying specimens in an oven, then cooling them in a desiccator, weighing them, and finally immersing them in water at 23°C for 24 hours or until equilibrium. The ISO 62 test describes a procedure for determining the moisture absorption properties in the "through-the-thickness" direction of the specimen.
The water absorption of plastics varies depending on the type of plastic material, relative humidity and temperature in the environment, and any chemical additives used to modify the properties of the plastic. For example, the most polar commercially available PA, PA 46, absorbs ∼13 wt.% water after being immersed in water at 23°C until full saturation, while the most nonpolar PA, PA 12, absorbs ∼1.5 wt.% water under the same conditions.
High water absorption can lead to swelling, warping, reduced strength, and compromised performance in applications where moisture exposure is expected. It can also affect the processing of plastics, causing problems such as reduced viscosity. Therefore, understanding water absorption rates is important for plastic manufacturers to anticipate potential issues and implement necessary design modifications to ensure product reliability.
It is worth noting that water absorption and water transmission are different and not directly related. While some plastics may not be completely waterproof, it does not necessarily mean that water can penetrate or transmit through them.
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Frequently asked questions
Not all plastics are completely waterproof, but some absorb water to a limited degree. The degree of moisture absorption depends on the type of plastic and the ambient conditions such as temperature, humidity, and contact time.
Some plastics with very low water absorption include PEEK, PPS, PSU, PPSU, PEI, PVDF, PET, PPE, PP, and PE. Polyethylenes and polypropylenes are also extremely low water-absorbent materials.
The most widely used standards to measure water absorption in plastics are ASTM D570 and ISO 62. The test method involves drying specimens in an oven, cooling them in a desiccator, weighing them, and then immersing them in water at a specific temperature for 24 hours or until equilibrium.
