Ethan Clarke
Plastic is often assumed to be waterproof, but most plastics consist of long-chain polymers that are slightly polarised, and therefore have a tendency to absorb water to varying degrees. This can cause a number of issues, from cosmetic problems to a loss of structural integrity. As such, it is important to dry plastics before melting and injecting them. This is especially true of hygroscopic plastics, which have a strong affinity for attracting and absorbing moisture.
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What You'll Learn
- Moisture absorption by plastic polymers varies
- Hygroscopic plastics absorb moisture due to chemical characteristics
- Moisture affects the physical properties and performance of plastics
- Moisture can cause cosmetic issues and structural defects in plastic products
- Moisture content in plastic can be measured using precision scales and electric resistance
Moisture absorption by plastic polymers varies
Firstly, the type of plastic material plays a significant role in moisture absorption. Polymers like blends of PPE and HIPS have low polarity and can only hold a minimal amount of moisture, typically around 0.07%. On the other hand, highly polar polymers like nylon can absorb significantly more water, reaching 8-9% at saturation, showcasing the wide range of moisture absorption capabilities among different plastics.
Secondly, ambient conditions such as temperature, humidity, and exposure time influence how much moisture a plastic polymer can absorb. For instance, unfilled nylon absorbs up to 4% of its weight under normal conditions but can absorb up to 8% in high-humidity environments. Similarly, polypropylene absorbs less than 0.01% when submerged in water for 24 hours, indicating that both humidity levels and exposure time impact moisture absorption rates.
Additionally, the presence of additives or chemical treatments can alter a plastic polymer's moisture absorption properties. For example, additives like glass and PTFE (Polytetrafluoroethylene) can effectively reduce the absorption percentage. Furthermore, chemical treatments such as preheating or drying procedures can help eliminate moisture and prevent potential defects during the manufacturing process.
Understanding moisture absorption is crucial in designing, machining, and storing plastic materials. Moisture absorption can impact the dimensional stability of plastics, affecting their ability to maintain their size and shape under varying environmental conditions. It can also influence the strength and flexibility of the material, potentially leading to brittleness or changes in tensile strength. Therefore, proper drying procedures and considerations of environmental factors are essential to ensure the desired characteristics and performance of plastic polymers.
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Hygroscopic plastics absorb moisture due to chemical characteristics
The presence of moisture in plastics is a critical factor that can interfere with the processing or the quality of the finished product. Hygroscopy is the phenomenon of attracting and holding water molecules via absorption or adsorption from the surrounding environment. Hygroscopic materials include solids with an affinity for liquids due to a chemical or physical attraction between the two.
Hygroscopic plastics absorb moisture due to their chemical characteristics. The amount of moisture absorbed depends on the chemistry of the polymer and the atmospheric conditions. Polymers exhibit varying levels of polarity, which influences their capacity to absorb moisture from the atmosphere. For example, nylon is a highly polar polymer that can absorb 8-9% water by weight when saturated, while blends of PPE and HIPS are only slightly polar, holding just 0.07% moisture even at saturation.
The absorption of moisture in hygroscopic plastics, such as polyamides, results in the formation of hydrogen bonds. While this is undesirable before processing, it becomes necessary afterward to prevent the plastic pieces from becoming brittle. Other examples of hygroscopic plastics include nylon, ABS, polycarbonate, cellulose, and poly(methyl methacrylate) (PMMA, plexiglas, perspex).
Hygroscopic materials have various applications, including in baking to achieve different moisture contents and, hence, crispiness. They are also used in heating and cooling systems to reduce energy consumption by controlling indoor humidity. Additionally, surface activation treatments can improve the load rate and moisture capacity of hygroscopic materials.
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Moisture affects the physical properties and performance of plastics
The amount of moisture absorption depends on the specific polymer and the atmospheric conditions. While some polymers, like blends of PPE and HIPS, can only hold a small amount of moisture (around 0.07%), others, like nylon, can absorb much more (up to 8-9%) due to their high polarity. This absorption can impact the surface appearance of the plastic, causing cosmetic issues such as discolouration or staining.
In addition to cosmetic problems, moisture absorption can also affect the structural integrity of plastics. If the moisture content exceeds a certain threshold, it can lead to hydrolysis, causing the plastic to break down structurally. This not only affects the quality of the final product but can also result in significant costs for manufacturers if the issue is not caught early in the production process.
To prevent these issues, it is crucial to control moisture levels through drying processes. Before any processing or printing, plastics need to be dried to eliminate moisture and ensure optimal performance. The drying temperature must be carefully controlled to be below the softening or melting point of the material. Additionally, the specific recommendations of the resin suppliers regarding drying time and temperature should be followed to achieve the desired moisture levels.
Overall, moisture has a significant impact on the physical properties and performance of plastics. By understanding the moisture absorption characteristics of different polymers and implementing effective drying processes, manufacturers can ensure the quality and integrity of their plastic products.
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Moisture can cause cosmetic issues and structural defects in plastic products
Moisture in the air can affect plastic products in several ways, from cosmetic issues to structural defects. Plastic polymers are slightly polarised, and since water is also slightly polarised, plastics tend to absorb water to varying degrees. This absorption can cause cosmetic issues such as surface discoloration and the formation of air bubbles in the plastic.
Hygroscopic plastics, such as nylon, are highly susceptible to moisture absorption. When exposed to water, these plastics can absorb upwards of 2-8% moisture by weight at saturation, which is significantly higher than most other resins. Even at lower moisture levels of around 0.07%, cosmetic problems can occur during melt processing. For example, polyamides can form hydrogen bonds in the presence of moisture, which can lead to brittleness if the plastic is not hydrated before processing.
The amount of moisture absorbed by plastics depends on factors such as resin type, ambient temperature, and manufacturing inconsistencies. It is crucial to control moisture levels through drying plastics before melting and injecting them into the desired shape. If plastics are not dried adequately, it can result in wasted material, labour, energy, and potential legal issues if the defective products reach customers.
To prevent moisture-related issues, plastic resins must be dried according to the manufacturer's recommendations. While it may seem straightforward to dry all plastics completely, it could be energy-intensive and unnecessary for hydrophobic plastics. Instead, the drying process should consider the moisture sensitivity of the specific plastic blend being used.
In summary, moisture can cause cosmetic issues and structural defects in plastic products. Proper drying procedures are essential to ensuring the quality and performance of plastic products, and manufacturers must carefully control moisture levels during the production process.
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Moisture content in plastic can be measured using precision scales and electric resistance
Moisture content in plastics can be measured using precision scales and electric resistance. Moisture analyzers are commonly used in labs to determine the moisture content of samples and ingredients used to manufacture products. They are available in a wide variety of temperature ranges and accuracies, depending on your needs and budget. Most moisture analyzers can give you a moisture readout accuracy of anywhere between 1% and 0.0001%.
The HX204 moisture analyzer is a popular option that offers a high resolution of 0.1 mg and is ideal for applications with low moisture content such as plastic pellets. This analyzer has a high-resolution weighing cell, a readability of 0.001 % MC, and a weighing capacity of 200g. The HX204 moisture analyzer is also used by companies such as Victorinox to test the polymer resin for residual moisture content before injection-molding a granulate batch, ensuring the final quality of its products.
To use a moisture analyzer, you begin by placing your sample onto the measuring plate and closing the lid. You then enter your desired settings, and the machine records an initial “wet” reading. The machine will heat the sample and slowly remove any vapors through a built-in vent. Once the sample is completely dry, the machine takes a second “dry” reading. The wet weight is then divided by the dry weight to determine the total moisture content in the sample, typically expressed as a percentage.
Moisture content in plastic needs to be controlled as it is an important aspect of producing high-quality plastic parts. The right moisture content in polymer resins ensures trouble-free compounding and injection molding, smooth surfaces, and ideal mechanical properties of the injection-molded parts. An elevated drying temperature is desirable as it increases the rate of moisture removal. However, the drying temperature must remain below the softening or melting point of the material.
Therefore, precision scales and electric resistance methods, such as moisture analyzers, provide an accurate and efficient way to measure moisture content in plastics, helping to ensure the production of high-quality plastic products.
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Frequently asked questions
Moisture can affect the physical properties and performance of plastics. It can cause surface discoloration, and in extreme cases, it can lead to hydrolysis, causing the plastic to break down structurally.
Hydrolysis is a process where water combines chemically with plastic, causing it to break down.
The amount of moisture absorbed depends on the type of plastic and the atmospheric conditions. Plastics can absorb anywhere from 0.07% to 9% of moisture.
Controlling moisture content is crucial to maintaining the quality and integrity of the final product. Excess moisture can cause cosmetic issues and affect the mechanical properties of plastics.
Moisture can be removed from plastics through drying processes, such as using a plastic pellet dryer or heating the plastic under dehumidified air or a dry gas like nitrogen.
