Water Absorption at 24 hours

Water absorption is also called moisture absorption. It is the capacity of a polymer to absorb moisture from its environment. Absorbed moisture acts as a plasticizer. It reduces the glass transition temperature and strength of plastic. This is a reversible effect. However, absorbed water also can lead to irreversible degradation of the polymer structure.

 

Water absorption is expressed as an increase in weight percent or % weight gain of a plastic specimen. 

 

• Water Absorption 24 hrs at 23°C – Immersion of a plastic specimen in distilled water for 24 hours at 23°C

• Water Absorption 24 hrs at 100°C – Immersion of a plastic specimen in distilled boiling water for 24 hours

• Water Absorption at Saturation – Immersion of a plastic specimen in distilled water at 23°C. Measurement occurs when the polymer does not absorb water anymore.

• Water Absorption at Equilibrium – Plastic specimen is exposed to a humid environment - generally at 50% relative humidity - at a specified temperature - 23°C or 73.4°F for 24 hours. The equilibrium moisture content can be used to compare the amount of water absorbed by different types of plastics when they are exposed to moisture.

Some of the known effects include:
 

• Dimensional & mass changes (e.g., swelling) caused by water absorption
• Extraction of water-soluble components
• Changes in mechanical (elasticity, tensile strength, impact strength) and electrical performance
   

Exposure to humidity, immersion and exposure to boiling water can result in distinctly different material responses.

 

Effect on Thermoplastics

The water absorption data is important to understand the performance of the polymeric materials during processing. For example, to avoid premature moisture-related failures during injection molding in water or humid environments. 

The absorption phenomenon is particularly detrimental to plastic products. The wet material is also more permeable to gases. For example, for moist PA6 CO2 permeability is three times greater than for PA6 dried. 
 

• The presence of moisture in the material structure is also influence thermal insulation and dielectric properties.
• The absorption and the presence of moisture in the polymer structure is one of the factors causing aging of the material.
• Polyolefins such as PE, PP, and polybutylene contain no chemical bonds that are easily hydrolyzable. Hence, they absorb little water and essentially unaffected by aging in water.
• The presence of excessive moisture reduces the viscosity of the plastic, which is the cause of many processing problems.
• Moisture in the material also affects the appearance of the part.
   

For example, if the injection molding process uses wet granulation, the plasticization stage, the reaction occurs in water. Hydrolysis leads to structural changes in the material (degradation) and the result of the deterioration of the mechanical properties, particularly toughness and resistance. 
 

• The use of moist PMMA granules for injection causes the matured parts with poor surface quality and the POM injection also leads to a raid in shape.
• In the case of PET and PBT materials can lead to shorter-chain molecules present in the hydrolytic decomposition. This results in a significant deterioration in the mechanical properties of the material.
• Presence of moisture impairs impact strength and mechanical strength in polyamides.

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Effect on Thermocomposites

Water absorption in composites is more complex than in polymers alone. In the case of thermoplastic composites, the extent of the water absorption into the polymeric matrix depends on the chemistry and morphology of the polymer as well as the volume fraction and configuration of the fibers present and whether any wicking at the interface occurs. 

 

• Type of plastic
• Morphology (crystalline, amorphous…)
• Type and proportion of additives, fillers and reinforcements used
• Fiber fraction and orientation (in composites)
• Relative humidity and temperature
• Length of exposure

The most widely used standards to measure Water Absorption in plastics are ASTM D570 and ISO 62. 

 

ASTM D570 - Standard Test Method for Water Absorption of Plastics

This test method for the rate of water absorption has two chief functions:
 

• 1A - As a guide to the proportion of water absorbed by a material. In those cases where the relationships between moisture and electrical or mechanical properties, dimensions, or appearance have been determined.
• 1B - As a guide to the effects of exposure to water or humid conditions on such properties.
• 2 - As a control test on the uniformity of a product. It is particularly applicable to sheet, rod, and tube arms when the test is made on the finished product.

Test Procedure for the water absorption test: 
 

• The specimens are dried in an oven for a specified time and temperature.
• Then they are placed in a desiccator to cool.
• Immediately upon cooling the specimens are weighed.
• The material is then emerged in water at conditions, often 23°C for 24 hours or until equilibrium.
• Specimens are removed, patted dry with a lint-free cloth, and weighed.

ISO 62 Plastics - Determination of Water Absorption

It describes a procedure for determining: 
 

• The moisture absorption properties in the 'through-the-thickness' direction of flat or curved-form solid plastics.
• The amount of water absorbed by plastic specimens of defined dimensions when
  • immersed in water or
  • subjected to humid air under controlled conditions.

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