Tech Talk Blog

We’ve received a request for a quick review of plasma, so I present Plasma 101…

Simply put, plasma is an ionized gas, a gas into which sufficient energy frees electrons from atoms. Plasma is the fourth state of matter.  With plasma, positive ions, negative ions, electrons and radicals coexist in a concert of reactions and collisions — as long as an electric potential exists.

Plasma systems control the treatment conditions by controlling the gas type, flow, pressure, and concentration.  Plasma also dictates the energy, frequency, wattage, and electrode configuration.

Vacuum plasma technique is one form of surface modification our team provides.  Plasma has the unique ability to treat a material three dimensionally to prime any surface for adhesion, painting, coating or printing.  And plasma is recognized as a “green” process that releases no hazardous byproducts.

As always, if you still need some clarification – don’t hesitate to Ask The Experts!

This study examined the relative adhesion difference between untreated polycarbonate (PC), mechanically roughened PC, and plasma treated PC. It appears that plasma surface modification of PC based polymers is a viable way to enhance adhesion prior to bond-up, lamination, or overmolding. This study observed approximately a 459% increase in lap shear bond strength after plasma treatment.

 Polycarbonate is a specific group of thermoplastics. They are called polycarbonates because they are polymers having functional groups linked together by carbonate groups in a long molecular chain.

The most common type of polycarbonate plastic is one made from Bisphenol A, in which groups from Bisphenol A are linked together by carbonate groups in a polymer chain. This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. Polycarbonate can be mechanically bonded by standard methods. It can also be cemented by using a solvent such as methylene chloride or adhesives such as epoxy, urethane and silicone. Polycarbonate and also be ultrasonically welded. Yet, solvent based adhesive can contaminate sensitive devices. Moreover, ultrasonic welding requires tight tolerances and smooth contaminate-free surfaces. The plasma treatment prior to bonding with common adhesive has shown an effective way to bond PC without solvent based adhesive or technically difficult, sonic welding. 

A plasma is a quasineutral cloud of ion, electrons, and radicals. The diffuse cloud is capable of doing chemistry on the surface of materials that is unique, providing wettable or adherent surfaces on materials that are otherwise inert.

The PC samples in this study were subjected to a specific plasma gas mixture to induce and adherent surface for a structural epoxy adhesive. The results are as follows:

In summary, polycarbonate can be bonded using mechanical or solvent chemical methods. Yet, it has been proven that plasma surface modification is a viable, environmentally friendly, invisible treatment that can enhance the bonding performance significantly. If you would like more information about this process or other processes, please contact us at www.tstar.com.

Polyethylene
A family of thermoplastic resins obtained by polymerizing the gas ethylene [C₂H₄]. Low molecular weight polymers of ethylene are fluids used as lubricants; medium weight polymers are waxes miscible with paraffin; and the high molecular weight polymers (i.e., over 6000) are the materials used in the plastics industry. Polymers with densities ranging from about .910 to .925 are called low density; those of densities from .926 to .940 are called medium density; and those from .941 to .965 and over are called high density. The low density types are polymerized at very high pressures and temperatures, and the high density types at relatively low temperatures and pressures. A relatively new type called linear low density polyethylene is manufactured through a variety of processes: gas phase, solution, slurry, or high pressure conversion. A high efficiency catalyst system aids in the polymerization of ethylene and allows for lower temperatures and pressures than those required in making conventional low density polyethylene. Copolymers of ethylene with vinyl acetate, ethyl acrylate, and acrylic acid are commercially important. Major polyethylene applications can be found in packaging, housewares, toys and communications equipment. Can be bonded effectively after plasma treatments or by using our UltraFlon Bond-X 1606.

Polyimides
A family of thermoset and thermoplastic resins characterized by repeating imide linkages: There are four types of aromatic polyimides: (1) condensation products made by the reaction pyromellitic dianhydride (PMDA) [C₆H₂(C₂O₃)₂] and aromatic diamines such as 4,4′-diaminodiphenyl ether [(C₆H₄NH₂)₂O]; (2) condensation products of 3,4,3′,4′-benzophenone tetracarboxylic dianhydride (BTDA) [(C₆H₅)₂CO(C₂O₃)₂] and aromatic amines;(3) the reaction of BTDA and a diisocyanate such as 4,4′-methylene-bis(phenylisocyanate) [OCNC₆H₄CH₂C₆H₄NCO]; and (4) a polyimide based on diaminophenylindane and a dicarboxylic anhydride such as carbonyldiphthalic anhydride [OC₆H₄(CO)₂COC₆H₄(CO)₂]. Thermoset polyimides are produced in condensation polymers that possess reactive terminal groups capable of subsequent cross-linking through an addition reaction. Typical applications for thermoplastic and thermosetting polyimides are transportation and electronics. Can be bonded after plasma treatment using most epoxies.

Polyphenylene Oxide, Modified
Engineering thermoplastic resins produced by the oxidative coupling of 2, 6-dimethylphenol [(CH₃)₂C₆H₃OH] (xylenol), then blended with impact polystyrene. Typical applications are found in electrical/electronic uses, business machine parts, appliances, and automotive parts. Can be bonded with solvents or epoxies. Adhesion can be enhanced greatly after plasma treatments.

Polyphenylene Sulfide
Engineering thermoplastic resins produced by the reaction of p-dichlorobenzene [C₆H₄CI₂] with sodium sulfide [Na₂S]. The major use for polyphenylene sulfide is in electrical/ electronic parts and automotive parts. After plasma treatments, this material bonds effectively with most epoxies.

Polypropylene
Thermoplastic resins made by polymerizing propylene [CH₃CHCH₂] and in the case of copolymers with monomers, with suitable catalysts, generally aluminum alkyl and titanium tetrachloride mixed with solvents. The monomer unit in polypropylene is asymmetric and can assume two regular geometric arrangements: isotactic, with all methyl groups aligned on the same side of the chain, or syndiotactic, with the methyl groups alternating. All other forms, where this positioning is random, are called atactic. Commercial polypropylene contains 90-97% crystalline or isotactic PP with the remainder being atactic. Most processes remove excess atactic PP. This by-product is used in adhesives, caulks, and cablefilling compounds. Major applications of commercial PP are found in packaging, automotive, appliance and carpeting markets. This material can be bonded effectively using UtraFlon Bond-X 1606.

Polystyrene
High molecular weight thermoplastic resins produced generally by the free-radical polymerization of styrene monomer [C₆H₅CHCH₂] which can be initiated by heating alone but more effectively by heating in the presence of free-radical initiator (such as benzoyl peroxide [(C₆H₅CO)₂O₂]. Typical processing techniques are modified mass polymerization or solution polymerization, suspension polymerization, and expandable beads. Major markets for polystyrene are in consumer and institutional products, electrical/electronic uses, and building/ construction. Typically there are no issues bonding this material. But, plasma treatments have been used to enhance wettability of the material.

Polyurethanes
A large family of polymers based on the reaction product of an organic isocyanate with compounds containing a hydroxyl group. The commonly used isocyanates are toluene diisocyanate (TDI) [CH₃C₆H₃(NCO)₂], methylene diphenyl isocyanate (MDI) [OCNC₆H₄CH₂C₆H₄NCO], and polymeric isocyanates (PMDI), obtained by the phosgenation of polyamines derived from the condensation of aniline [C₆H₅NH₂] with formaldehyde (HCHO]. Polyols (with hydroxyl groups) are macroglycols which are either polyester or polyether based. Polyurethane elastomers and resins take the form of liquid castings systems thermoplastic elastomers and resins, microcellular products, and millible gums. Typical applications are found in the automotive industry. Polyurethane foams are widely used in transportation, furniture, and construction markets. Can be bonded effectively using acrylic adhesive or urethane adhesive. Adhesion can be improved greatly after plasma treatment.

If you have any questions about the materials above, Ask The Experts – or visit TriStar.

The last installment is almost here…

Acrylic or poly(methyl 2-methylpropenoate) is a specific group of thermoplastics. Methyl methacrylate is polymerized in bulk or suspension methods using free-radical initiators.

PMMA - polymer chain

The presence of the pendant methyl (CH₃) groups prevents the polymer chains from packing closely in a crystalline fashion and from rotating freely around the carbon-carbon bonds. As a result, PMMA is strong, transparent and somewhat inert.

Bonding untreated PMMA to itself is limited to either cyanoacrylate, dichloromethane (CH₂Cl₂), or trichloromethane (CHCl₃). The bond strength using these methods is strong and can exceed the strength of the acrylic substrates. Unfortunately, these adhesive may not be effective when bonding acrylic to other materials. When acrylic is to be bonded to materials other than itself, plasma treatment can assist in enhancing the bond strength.

A plasma is a quasineutral cloud of ion, electrons, and radicals. The diffuse cloud is capable of doing chemistry on the surface of materials that is unique, providing wettable or adherent surfaces on materials that are otherwise inert.

The PMMA samples in this study were subjected to a specific plasma gas mixture to induce and adherent surface for a structural epoxy adhesive. The results are as follows:

In summary, PMMA can be bonded using mechanical or solvent chemical methods. Yet, it has been proven that plasma surface modification is a viable, environmentally friendly, invisible treatment that can enhance the bonding performance significantly. For more informtion about this process please contact us at our Tristar Plasitics Corp. website.

It is more a standard than an exception to bond dissimilar materials in industry these days. Many adhesive manufactures have responded with a variety of liquid treatments to facilitate this trend. Though many of these primers work well, they have safety, shelf-life, and disposal issues. So, what now!

Plasma combines with aqueous (water) based post treatments provide a broad range of environmentally safe and stable surface activation primers. The plasma and corona pre-treatments open the types or polymers that can be primed for the specific chemistry to react with a particular adhesive or paint like rubber to metal. If you would like to improve your current process of metal treatment prior to rubber overmolding or polymer treatment prior to adhesive dispensing, then this technique should be tested. If you have a specific question about your application, Ask The Experts.

There are many options when it comes to cleaning parts; solvent wiping, soaking, ultrasonics, abrasion. But, more and more are considering plasma treatments, specifically plasma cleaning. Granted that plasma cleaning is horrible at removing bulk or globed on gunk from a device simply because it would take a very long time for all organics to be eliminated. But, to use plasma as the final step in complete organic removal, nothing is better.

Plasma cleaning of critical parts like medical implants, electronics, or diagnostics is one of the best methods. The operational cost and chemical use is very low making this technique environmentally friendly. The plasma technique is completely 360 degree cleaning organic residue from the smallest feature. The surface will be hyper-clean, uniform and ready for packaging or to some other critical step where pristine surfaces are necessary. Learn more about plasma treatments in our Shooting Star eLetter and be sure to check out the archives!