The Materials Analyst, Part 85: Fixing brittle nylon product with water

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April 30, 2007




This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at mike@thematerialanalyst.com.

Conditioning nylon isn’t always the best way to solve brittleness if it masks a bigger problem.

Most nylon polymers, including those used in the largest volumes, nylon 6 and nylon 6/6, have an unusual affinity for water. Nonpolar polymers such as polyethylene and polypropylene absorb only about .01% of their weight in water because the polar water molecule has no affinity for these polymers. These materials have a chemistry that resembles a hydrocarbon oil or gasoline and it is well known that oil and gasoline do not mix with water. For the same reason, polymers like polyethylene do not absorb water.

Most polar polymers, like polycarbonate and acrylic, absorb moisture but typically reach equilibrium at .10-.20% by weight. However, nylons contain a chemical feature known as an amide group, illustrated in Figure 1 as part of a short segment of a nylon 6/6 polymer chain. The (N-H) portion of this linkage creates a very polar group that gives rise to a relatively strong attraction between the nylon chains. It is this attraction, known as a hydrogen bond, that is responsible for the relatively high melting point of most nylon materials. This hydrogen bond is also the constituent that attracts water molecules to each other so strongly and is responsible for water’s unusually high boiling point.

This similarity in bonding types produces an unusual affinity between water and nylon, and at equilibrium, materials like nylon 6, nylon 6/6, and nylon 4/6 can hold approximately 1.5-2% of their weight in water. This value can be substantially higher if the polymer is immersed in water and is allowed to reach a saturation point.

Glass transition temp

When water molecules become absorbed into the nylon matrix, they attach themselves to the amide groups in the nylon chains. This forces the nylon chains farther apart, reducing the attractive forces between the chains and lowering a property known as the glass transition temperature. Below the glass transition temperature, polymers like nylon 6 and nylon 6/6 are relatively strong, stiff, and somewhat brittle, particularly in areas where stress concentrations due to features like sharp corners are present. Above the glass transition temperature, an unfilled nylon 6 or nylon 6/6 loses approximately 80% of its room-temperature stiffness and strength, but in the process becomes substantially tougher.

In a so-called dry-as-molded nylon, the glass transition temperature is approximately 75°C. With sufficient moisture absorption, this transition temperature can decline to room temperature or even below. The effect of this can be observed in Figure 2. This shows the modulus vs. temperature behavior of a nylon 6/6 with .2% absorbed water and 1.2% absorbed water.

The effect on the glass transition is easily observed. Because in our world we are stuck at

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