Polyamides, also known as nylons, are high-performance plastics with excellent chemical resistance. Nylons are hygroscopic and absorb moisture from the air. Moisture in and of itself causes adhesion problems. Nylon is hydrophobic and has low surface wettability. Question – Is nylon hydrophobic, hydrophilic, or maybe both?
A polyamide is a polymer containing repeating amide (–CO-NH–) linkages. The manufacturing of different polyamide types varies depending on their monomeric units. They may be aliphatic, semi-aromatic, and aromatic (aramids). Based on their crystallinity they may be amorphous, semi-crystalline, or crystalline. Nylon polymers are inherently difficult to bond because they are hydrophobic, chemically inert, and possess poor surface wettability (i.e., low surface energy). Furthermore, nylons are hygroscopic and will absorb moisture in excess of 3% of their mass of water from the atmosphere. Moisture, in and of itself, causes adhesion problems.
Wait, the statements above appear to be directly contradictory. Is nylon hydrophobic or hydrophilic? The apparent paradox is resolved by recognizing that nylon’s hydrophobic behavior is a surface property and its hydrophilic behavior is a bulk property. Polyimide also exhibits this behavior.
Examining the chemical structure of a polymer’s repeat unit, such as Methyl (CH3) hydrophobic polyolefins or Amide hydrophilic nylons, is a simple way to determine whether it is hydrophilic or hydrophobic. The simplest method to determine the surface properties of a polymer is to conduct water contact angle measurements for static contact angle and dynamic contact angle.
Nylon, like most polymers, has a low surface energy. This is due to the surface chemistry and surface and physics of polymers. Contact angles are considered to be affected by both changes in surface chemistry and changes in surface topography. When a droplet is attached to a solid surface and the solid surface is tilted, the droplet will lunge forward and slide downward. The angles formed are respectively termed the Advancing Angle and the Receding Angle. ASTM D724 describes methods for measuring DCAs using advanced equipment (optical tensiometers and goniometers) to analyze advancing and receding contact angles based on drop shape analysis and mass.
The advancing contact angle is most sensitive to the low-energy (unmodified) components of the substrate surface, while the receding angle is more sensitive to the high energy, oxidized groups introduced by surface pretreatments. Thus, the receding angle is actually the measurement most characteristic of the modified component of the surface following pretreatments. Therefore, it is critical to measure both the advancing and receding contact angles on all surface-modified materials.
Scott R. Sabreen is founder and president of The Sabreen Group, Inc., which is an engineering company specializing in secondary plastics manufacturing processes – surface pretreatments, bonding decorating and finishing, laser marking and product security. He has been developing new technologies and solving manufacturing problems for over 30 years. Sabreen can be contacted at 972.820.6777 or by visiting www.sabreen.com or www.adhesionbonding.com.
