This month is all about Material Handling at Mouldshop and managing the moisture in your material is a critical element of your material handling process. This article discusses Silver Streak defects, one of the most common indicators of excess material moisture.

Understanding Polymer Materials and Moisture Absorption

Polymers are created from large molecules composed of repeating structures called monomers that are connected through chemical bonds and because of this, they are capable of exhibiting properties such as durability, flexibility, and corrosion resistance. Hence why over the decades, polymers have become ubiquitous in our daily lives with their versatility and wide range of applications. However, due to the composition of raw polymer materials, they can succumb to the absorption of excess moisture within the environment, which can result in several detrimental effects, one of which is silver streak formations. Absorption of moisture occurs when polymers come into contact with liquid water or water vapour. The level of moisture absorption depends on multiple factors such as the polymer’s composition, porosity, structure and both the temperature and relative humidity of the environment.

Effects of Excess Moisture on Polymer Materials

There will always be a degree of moisture absorption within polymers and there are tolerances of what is an acceptable level of moisture for your raw polymer to contain, but excessive moisture will be detrimental to the finished plastic products. One of the main consequences of excess moisture is that it acts like a plasticiser which results in a reduced strength and stiffness of the polymer, compromising the structural integrity. As a result of this warping can occur along with a reduced weight bearing capacity. Along with plasticising the polymer, excess moisture can result in hydrolysis. This is where the water molecules break chemical bonds within the structural chain of a polymer resulting in reductions in tensile strength and resistance to impact.

Silver Streak Defects: Causes and Characteristics

One of the most common visual indicators of excessive moisture in polymer material is the formation of silver streak defects on the surface of the polymer product once moulded. As their name suggests, they are easily identifiable as thin, silver-coloured lines. You can see them especially clearly in transparent or translucent materials such as PET (polyethylene terephthalate). Silver streaks are the result of trapped moisture within the polymer which, during the thermal cycles of injection and extrusion moulding, migrate to the surface of the polymer product and there are multiple reasons how and why silver streaks form, such as:

• Vaporisation and Condensation: During the moulding process, the excess moisture vaporises when exposed to heat and as the material cools, this causes the vapour to condense on the surface and create droplets of moisture.
• Migration of additives: Some additives used to aid processing or lubricants also have an affinity for excess moisture which brings the additives to the surface with it. This can result in concentrated areas of silver streaks.
• Contamination: Polymers can absorb contaminants from the surrounding environment and these contaminants can end up being deposited as silver streak defects.
• Improper drying: The improper drying of raw polymer material will lead to excess moisture being left in the material. Just a small amount of excess moisture can disrupt the result of the product.

Prevention of Silver Streaks and Excess Moisture Reduction

There is a combination of preventative practices that can be utilised to ensure the reduction of excess moisture and resulting issues such as silver streaks. One of which is the drying of the polymer material. During the drying process of polymer material, multiple factors need to be considered such as the optimal temperature, airflow rate or drying time. Due to the variance in compositions of different polymer materials, different methods of polymer drying have been developed to suit. 

• Hot air drying: This is a cost-effective method of drying polymer where hot air is blown over the polymer to evaporate the excess moisture. 
• Desiccant drying: Silica gel or similar material absorbs any residual moisture. This is then removed via a sieving process. 
• Dehumidifying drying: A dehumidifier removes excess moisture from the air before then directing it over the polymer to create an optimal drying environment. 
• Vacuum drying: This process lowers the pressure around the polymer material, and this facilitates the evaporation of moisture at lower temperatures. 
• Infrared drying: The polymer is heated directly with the use of infrared heat to achieve a faster drying time.  

Along with the correct drying process being utilised, it’s important to consider the environment in which the polymer is being processed. The temperature and relative humidity should be monitored to ensure an optimal environment which minimises the opportunity for excessive moisture absorption of the polymer material.

Quality control is an important opportunity to identify and assess the finished products for imperfections and irregularities such as silver streaks. By employing a stringent QC process, you can also effectively monitor the result of any processing parameters you have in place. This may concern the cooling rate or temperature profiles you are employing.

In conclusion, excess moisture poses a significant challenge to the quality and performance of the finished product and can lead to the formation of silver streak defects. By understanding the effect of excess moisture within polymers and implementing effective preventative processes to minimise the resulting defects, you will have far better, consistent high-quality results each time.