Adhesive Technologies for the Assembly of Hard-to-Bond Plastics
This article is based on an original publication by Henkel.
Defined by their economical, flexible and high performance characteristics, common difficult-to-bond plastics including acetals, polyethylenes, fluoropolymers, polypropylene and TPVs are an industry essential. A wide range of assembly methods, including mechanical and chemical, can be used for bonding both similar and dissimilar plastics, although adhesive assembly methods offer a unique ability to bond and seal for equal stress distribution, fast cure times, gap filling capability and simplified automation.
Defining Hard-to-Bond Substrates
Once polymerized, thermoset plastic resins are not modifiable. This group includes polyester, phenolic and epoxy resins. Contrarily, thermoplastics can be reflowed and include acrylonitrile butadiene styrene (ABS), polyamide (nylon), polycarbonate, and polyolefins. Such difficult-to-bond substrates are differentiated by linear or branched carbon chain polymers with low surface energies, low porosity, and non-polar or non-functional surfaces to which an adhesive cannot bind.
A semi-crystalline thermoplastic created through free-radical polymerization, low density polyethylene (LDPE) generally features lower strength and hardness. However, LDPE provides advantages such as enhanced flexibility, clarity, and impact resistance. Like LDPE, high density polyethylene (HDPE) is the product of a polymerization reaction but is characterized by increased strength, hardness and chemical and abrasion resistance. Both substances are commonly used for packaging, medical device, electrical component and various other applications.
With its excellent thermal and chemical resistance, high moisture resistance and good mechanical properties, polypropylene (PP) is a crystalline thermoplastic commonly found in packaging, appliance, medical, electrical and fiber applications. The primary downside is low-temperature impact resistance related to crystallinity.
Teflon® and other fluoropolymers are created through free radical polymerization. These highly crystalline thermoplastics feature excellent thermal resistance, chemical resistance and weatherability as well as low flammability. Fluoropolymers are characterized by low coefficients of friction with average service temperatures of about 500°F, making them ideal for high temperature applications as found in the electrical or mechanical industries. However, a higher pound cost of certain grades can be a downside of using fluoropolymers.
Acetal homopolymer (acetal) resins are defined by impact strength, low permeability, temperature resistance, and good dielectric properties. Produced through a polymerization reaction involving formaldehyde, primary disadvantages of acetals are poor UV resistance and poor resistance to strong acids. Due to their wear resistance, acetals are common to automotive, industrial, electronics and consumer applications.
Thermoplastic vulcanizates (TPVs) are classified as thermoplastic elastomers and may be difficult to join. A combination of polypropylene and vulcanized rubber, TPVs are inherently flexible with excellent thermal, weathering and chemical resistance as well as flexural resistance, abrasion resistance and tensile strength. TPVs are commonly found in a range of industries such as automotive, electrical, medical and construction.