Assembly - February 2009 - (Page 42) Adhesive Bonding Some Hard-to-Bond Plastics Polymer Abbreviation Surface Energy (dynes/cm2) 46 47 46 42 41 40 40 PVCR PE 39 41 36 ABS PPS PVCP PS 35 38 35 34 33 PBT PP PU PE PDMS PTFE 32 30 38 30 23 19 Contact Angle (degrees) 90 75 75 76 82 74 83 90 70 85 82 87 89 72 80 88 88 85 88 98 120 Polyethersulfone Polyphenylene oxide Polycarbonate Polyethylene terephthalate Polymethylmethacrylate Styrene acrylonitrile Polyimide Polyvinyl chloride, rigid Polyester Acetal Acrylonitrile butadiene styrene Polyphenylene sulfide Polyvinyl chloride, plasticized Polystyrene Surlyn ionomer Polybutylene teraphthalate Polypropylene Polyurethane Polyethylene Polydimethyl siloxane Polytetrafluoroethylene PES PPO PC PET PMMA SAN The strength of a bonded joint is primarily determined by how well an adhesive flows across the substrate. When a drop of liquid is applied to a surface, it will rest at equilibrium based on the surface tension of the liquid and the surface energy of the substrate. To gauge the substrate’s surface energy, measure the angle between the horizontal line of the substrate and the edge of the droplet where it contacts the substrate. For optimal bonding, the droplet should have a contact angle of less than 45 degrees. Source: TriStar Plastics Corp. a batch process in a chamber with a low-pressure atmosphere, or it can be done as a continuous process, on-line, at atmospheric pressure. With the former, engineers have the advantage of controlling every variable of process: the composition, flow rate, pressure and concentration of the gas, as well as the frequency and wattage of the electrical energy. This can be important if engineers want to produce a specific chemical composition on the part’s surface. Another advantage is that any object inside the vacuum chamber is treated on all sides by the plasma. The disadvantage of such a system is low throughput. The size of the vacuum chamber is limited, and only so many parts will fit inside at one time. The chief advantage of plasma treatment at atmospheric pressure is that it can be done on the assembly line just before bonding. This greatly improves throughput compared with batch processing. In this case, a jet of plasma is generated from clean, dry shop air and directed at the parts through a nozzle. A plasma nozzle can be mounted to a six-axis or Cartesian robot, or a stationary unit can be integrated into a multistation automated assembly system. Another advantage of atmospheric plasma treatment is that it can target specific areas of a part. The plasma jet can cover widths ranging from 3 to 25 millimeters, depending on the power and the geometry of the nozzle. With multiple nozzles, web materials several meters wide can be treated. The nozzle is located 10 to 40 millimeters above the parts, which can move past the plasma stream at a rate of 6 to 600 meters per minute. Bot h cont i nuous a nd batch processes compare favorably with other methods of enhancing the bondability of plastics. “Plasma treatment eliminates the need for chemical primers, which are potentially hazardous,” says Gradus. “Abrasion can leave marks on the surface, which can be undesirable. Plasma will not mark the part.” Whether plasma treatment is done as a batch process or a continuous process, assemblers have the option of doing it themselves or contracting it out. Assemblers that bond a limited number of difficult plastics may be unable to justify investing in the equipment. Most suppliers of plasma equipment offer treatment services, as do many injection molders and contract manufacturers. If assemblers outsource plasma treatment, they should bond the parts as soon as possible after treatment. The shelf life of treated parts ranges from minutes to years, depending on the plastic, its formulation, how it was treated, and its exposure to heat following treatment. Shelf life is also limited by the presence of compounds, such as plasticizers and mold release agents. These compounds 42 ASSEMBLY / February 2009 www.assemblymag.com http://www.assemblymag.com
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