Adhesives Magazine

THE JOINING OF MEDICAL PLASTICS

February 27, 2006


When Alexander Parkes developed the first man-made plastic in the 1860s, he had no idea what an integral role the material would come to play in daily life. Plastics today play an important part in cutting-edge technologies such as the space program, bullet-proof vests and prosthetic limbs, as well as in everyday products like beverage bottles, automobiles, and particularly medical devices.

Plastics use in medical devices has sustained an average annual growth rate far in excess of the Gross Domestic Product (GDP). U.S. demand for medical plastics will increase nearly 3% per year to 3.6 billion pounds in 2008, valued at $5.0 billion.

Key drivers of medical plastics usage include the following.

  • Aging of the U.S. population
  • Continuing cost reduction pressures in the healthcare field
  • Advances in polymer performance
  • Introduction of new and often life-saving devices
  • Environmental/disposable/non-disposable medical device triad
The strongest advances in plastics usage are expected for engineered resins such as polycarbonate and thermoplastic elastomers, which have favorable cost-performance benefits. Polypropylene, polyethylene, polyurethane and polyvinyl chloride will retain leading positions in non-invasive medical products and standard medical packaging due to their low cost and amenability to radiation sterilization. Inherently, however, polyolefins are difficult materials to effect surface bonding. Polypropylenes, for instance, are crystalline and, although they have excellent thermal and chemical resistance properties and high moisture resistance, they are nonpolar. As such, polypropylenes are a difficult substrate to bond since they feature no surface functional sites or inherent surface roughness to which an adhesive can secure itself. In addition, polypropylenes are linear or branched carbon chain polymers, have low surface energies and low porosity.

Hard-to-bond plastics like polyolefins are most often assembled using adhesives. While adhesives are the most versatile assembly method for plastics, only a few industrial adhesives offer suitable bond strengths on hard-to-bond plastics. Cyanoacrylate, light-curing cyanoacrylate, hot-melt and light-curing acrylic adhesives have typically been used with typical difficult-to-bond plastics.

To make an adhesive formulation, the fluid adhesive should have a surface tension no higher (and preferably 10 mN/m lower) than the critical surface tension of the solid adherend.

Adhesion impediments with medical plastics include:

  • Low surface tension
  • Migration of formulation components
  • Mold release agents
  • Low temperaturev
  • UV light
  • Mechanical stresses
Air plasma systems apply discharge to the surface of medical plastics through an air gap by way of an electrode design. When air is exposed to different voltages, an electrical discharge develops, causing neutral and electrically charged molecules to collide. Collisions cause neutral molecules to become electrically charged, resulting in filamentary discharges that create a cloud of ionized air, or an "air plasma." Electrons bombard the treatment surface with energies 2 to 3 times that necessary to break the molecular bonds on the surface of most substrates. The resulting free radicals react rapidly with other free radicals on the same or different molecular chain, resulting in crosslinking. Oxidative effects increase surface energy as a result of polar groups being created on the surface, primarily in the form of hydroxyl groups, carbonyl groups, amide groups and carboxylic acid.

Flame plasma systems are comprised of a combustion/electrical station and a burner assembly, manufactured with two primary burner configurations: ribbon and enhanced velocity. A flame plasma is formed when a flammable gas and atmospheric air are combined and combusted to form an intense blue flame. Medical plastics are made polar as species in the flame plasma affect electron distribution and density on the surface. Polar functional groups such as ether, ester, carbonyl, carboxyl and hydroxyl are contained in a flame plasma; these are incorporated into the surface and affect the electron density of the polymer material. This polarization and functionalization is made through reactive oxidation of a surface.

Atmospheric chemistry plasma systems generate a chemical plasma field in an electrically charged atmosphere. This method blends gases that deposit various chemical groups on the substrate surface to improve its surface energy. Chemical plasma treatment is essentially a nano-cleaning, micro-etching and functionalizing process that provides different surface characteristics depending on the gas chemistry employed.

For more information on surface treatment, contact Enercon Industries at (262) 255-6070, e-mail info@enerconind.com, or visit www.enerconind.com.

Polymers:    gc (20°C): mN/m:
Polymers:Poly(tetrafluoroethylene)   gc (20°C): mN/m:18
Polymers:Silicone, polydimethyl   gc (20°C): mN/m:24
Polymers:Poly(ethylene)   gc (20°C): mN/m:31
Polymers:cis-Poly(isoprene)   gc (20°C): mN/m:31
Polymers:Poly(styrene)    gc (20°C): mN/m:33
Polymers:Poly(vinyl alcohol)   gc (20°C): mN/m:37
Polymers:Poly(methyl methacrylate)   gc (20°C): mN/m:39
Polymers:Poly(vinyl chloride)   gc (20°C): mN/m:40
Polymers:Poly(acrylonitrile)   gc (20°C): mN/m:44
Polymers:Amine-cured epoxide   gc (20°C): mN/m:44
Polymers:Poly(ethylene terephthalate)   gc (20°C): mN/m:45
Polymers:Cellulose   gc (20°C): mN/m:45
Polymers:Poly(hexamethylene adipamide)
Polymers: (nylon 6,6)   gc (20°C): mN/m:46