Comprehensive Analysis of Water Absorption of Common Plastics
Water absorption is a core property of plastics, which directly affects the selection of processing technology, the stability of product quality and the adaptability of application scenarios. This article elaborates on the determination standards of water absorption, parameters and applications of plastics with different water absorption grades, impacts on injection molding and modification optimization directions. All data are referenced to the industry-recognized ASTM D570 test standard (immersion at room temperature and atmospheric pressure for 24 hours) to ensure the accuracy and authority of the content.
1. Determination Standards for Plastic Water Absorption
The core index for measuring plastic water absorption is the water absorption rate, defined as the percentage of the mass of water absorbed by a dry plastic sample after soaking to constant weight in a specified environment relative to the dry mass of the sample. According to the 24-hour water absorption rate, common plastics are classified into four categories in the industry, with specific threshold values and process guiding significance as follows: ultra-low water absorption grade (≤0.01%), requiring no drying treatment; low water absorption grade (0.01%~0.2%), requiring simple drying as needed; medium water absorption grade (0.2%~1.0%), requiring strict drying; high water absorption grade (>1.0%), requiring precise humidity-controlled drying.
2. Common Plastics with Different Water Absorption Grades and Their Performance Characteristics
Ultra-low Water Absorption Grade (≤0.01%)
Representative Plastics and Industry-recognized Water Absorption Rate
Polytetrafluoroethylene (PTFE): 24-hour water absorption rate < 0.01%
Ultra-high-molecular-weight polyethylene (UHMWPE): 24-hour water absorption rate < 0.01%
Polypropylene (PP): 24-hour water absorption rate = 0.01%
These plastics feature a dense molecular structure without polar groups, and almost have no moisture absorption capacity, thus requiring no pre-drying treatment before injection molding. Even when stored or used in humid environments for a long time, they will not suffer from dimensional deformation or mechanical property degradation. With excellent chemical corrosion resistance and water resistance, they are suitable for humid and corrosion-resistant application scenarios. For example, PP can be directly used to produce water pipe joints and bathroom accessories; PTFE is often used to manufacture seals for chemical equipment and high-temperature resistant bearing components; UHMWPE is applied to make wear-resistant inner linings of conveying pipelines.
Low Water Absorption Grade (0.01%~0.2%)
Representative Plastics and Industry-recognized Water Absorption Rate
High-density polyethylene (HDPE): 24-hour water absorption rate = 0.02%
Low-density polyethylene (LDPE): 24-hour water absorption rate = 0.03%
Polystyrene (PS): 24-hour water absorption rate = 0.04%
These plastics have weak molecular polarity and low sensitivity to environmental humidity. If the raw materials are stored in a dry and ventilated environment, the drying step can be directly omitted during injection molding; if the storage environment has high humidity, only pre-drying at 60~80℃ for 1~2 hours is needed to remove surface free water. Their finished products have good dimensional stability and relatively low production costs, and are widely used in the fields of packaging containers and daily necessities shells. For instance, HDPE is used to make food fresh-keeping boxes and chemical barrels; LDPE is applied to produce plastic films and shopping bags; PS is processed into stationery shells and transparent storage boxes.
Medium Water Absorption Grade (0.2%~1.0%)
Representative Plastics and Industry-recognized Water Absorption Rate
Acrylonitrile-butadiene-styrene copolymer (ABS): 24-hour water absorption rate = 0.2%~0.4%
Polycarbonate (PC): 24-hour water absorption rate = 0.15%~0.3%
Acrylonitrile-styrene copolymer (AS): 24-hour water absorption rate = 0.3%~0.6%
These plastics contain certain polar groups in their molecules and are relatively sensitive to moisture, so strict drying treatment must be carried out before injection molding. If the drying step is skipped, the moisture in the molten plastic will vaporize under high temperature and pressure, leading to defects such as silver streaks, bubbles and weld lines in the finished products, which seriously affect the appearance and mechanical properties. The industry-standard drying process is 80~120℃ for 2~4 hours, and for some thick-walled products, the drying time needs to be extended to 4~6 hours. The finished products are mainly used to make structurally complex home appliance shells and automotive interior parts. For example, ABS is used to produce TV shells and washing machine panels; PC is processed into automotive headlight covers and bulletproof glass; AS is made into transparent home appliance components and food containers.
High Water Absorption Grade (>1.0%)
Representative Plastics and Industry-recognized Water Absorption Rate
Polyamide 6 (PA6, Nylon 6): 24-hour water absorption rate ≈ 8%
Polyamide 66 (PA66, Nylon 66): 24-hour water absorption rate ≈ 6%
Polyamide 12 (PA12, Nylon 12): 24-hour water absorption rate ≈ 1.5%
Polyethylene terephthalate (PET): 24-hour water absorption rate = 0.6%~1.2%
Some modified polyoxymethylene (POM): 24-hour water absorption rate > 1.0%
These plastics contain strong polar groups in their molecules and are extremely sensitive to moisture. Drying treatment before injection molding is a core link to ensure the quality of finished products, and a dehumidifying dryer is required for precise humidity control. The industry-standard drying process is 100~120℃ for 4~6 hours. Among them, PA plastics need to be stored in a closed environment for heat preservation after drying to prevent secondary moisture absorption. Insufficient drying will cause defects such as cracking, warping and sharp decline in mechanical strength of the products. At the same time, the finished products will have a certain dimensional expansion after absorbing moisture during use, so a dimensional allowance of 0.2%~0.8% needs to be reserved in the product design stage. These plastics have high mechanical strength and good wear resistance, and are mainly used to make high-strength mechanical transmission parts. For example, PA is used to produce gears, bearings and engine components; PET is made into fibers and engineering plastic accessories; modified POM is processed into precision connectors and gears.
3. Key Impacts of Water Absorption on Injection Molding Process
The water absorption rate of plastics directly determines the drying temperature, time and equipment selection. The lower the water absorption rate, the simpler the pretreatment process, which can reduce energy consumption and production costs; the higher the water absorption rate, the higher the requirement for humidity control accuracy of drying equipment, and a dehumidifying dryer should be used instead of an ordinary hot air dryer to avoid insufficient drying or thermal degradation of materials. Undried plastics will generate water vapor during injection molding, leading to defects such as silver streaks, bubbles and shrinkage cavities in the finished products. At the same time, it will reduce the melt strength of plastics and affect the mechanical properties and surface finish of the products. Especially for medium and high water absorption plastics, the drying quality directly determines the qualification rate of finished products. For high water absorption plastics, the mold should be designed with a reasonable exhaust system to timely discharge the water vapor generated during injection molding, and the cooling water circuit should be optimized to reduce the warpage deformation of products caused by moisture absorption and expansion.
4. Modification Optimization Directions for High Water Absorption Plastics
To solve the problem of poor moisture resistance of high water absorption plastics, the industry often adopts the following modification methods to improve performance: first, adding inorganic fillers such as glass fiber and talcum powder to fill molecular gaps and reduce moisture absorption channels of materials; second, carrying out surface coating treatment such as spraying waterproof coating and electroplating metal layer to isolate the contact between water and materials; third, adopting copolymerization modification such as copolymerization of PA and polyethylene to reduce molecular polarity and decrease water absorption rate.
