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!

We’ve all been touched by the recent earthquake destruction in Haiti.  And our team is now seeing a renewed interest in surface treatments that can help building materials (like cement) resist seismic activity and sudden impact.

By adding specially treated polyethylene fibers to concrete mixtures, contractors are able to enhance the strength and durability of preformed structures.  With plasma treatment, our team can select the optimum gas chemistry and operating condition to improve the bond strength and interface toughness of ethylene fibers.  Treated fibers have a significantly improved bond strength compared to virgin, non-treated fibers.  With our plasma process, we can enhance the structural integrity of fiber cement mixtures used in buildings, bridges, and other superstructures.

What is the best process for removing silicone oil from a catheter made of Pebax® tubing prior to a bonding operation?  Would you use plasma or corona?

Your question is one that we are seeing more frequently. And the short answer is that it all depends on the amount of oil.

If you can see a significant oil collection, then you need to wash the tubing in an ultrasonic bath with an emulsifier. Then, you may simply wipe the tubing with an alcohol wipe to remove any excess. It really depends on the level of contamination. Generally speaking, I’ve found that catheters have a superficial level of oil.

If the amount of oil is superficial, plasma can carry away the excess oil via a specific oxygen treatment. We do not advise corona treatment for this application, since it can make the silicone hydrophilic and give a false impression of being clean. Plasma is a more elegant solution and will “superclean” the surface to promote better adhesion.  Learn how we recently solved this challenge.

There are many ways in which polymer properties or behaviors are classified to make general descriptions and understanding easier. Some common classifications are:

• Thermoplastic / thermoset
• Amorphous / crystalline
• Addition / condensation

Thermoplastic vs. Thermoset

Thermoplastics are materials which can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.

Thermoset materials undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network. Once they are heated and formed they can not be reprocessed – the three-dimensional molecules can not be made to flow under pressure when heated.

Amorphous vs Crystalline Polymers

Crystalline polymers are polymers with nearly linear structure, which tends to be flexible and fold up to form tightly, packed and ordered “crystalline” areas. Time and temperature during processing influence the degree of crystallinity. Crystalline polymers include: polyethylene, polypropylene, acetals, nylons, and most thermoplastic polyesters. Crystalline polymers have higher shrinkage, are generally opaque or translucent, with good to excellent chemical resistance, low surface friction, and good to excellent wear resistance.

Amorphous polymers are polymers with bulkier molecular chains or large branches or functional groups, which tend to be stiffer and will not fold up tight enough to form crystals. Common amorphous polymers include polystyrene, polycarbonate, acrylic, ABS, SAN, and polysulfone. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated (no distinct melting point), average to poor chemical resistance, high surface friction, and average to low wear resistance.

Condensation vs. Addition Polymers

Condensation polymers such as nylons, acetals, and polyesters are made by condensation or step-reaction polymerization, where small molecules (monomers) of two different chemicals combine to form chains of alternating chemical groups. The length of molecules is determined by the number of active chain ends available to react with more monomer or the active ends of other molecules.

Addition polymers such as polyethylene, polystyrene, acrylic, and polyvinyl chloride are made by addition or chain-reaction polymerization where only one monomer species is used. The reaction is begun by an initiator which activates monomer molecules by the breaking a double bond between atoms and creating two bonding sites. These sites quickly react with sites on other monomer or polymer molecules. The process continues until the initiator is used up and the reaction stops. The length of molecules is determined by the number of monomer molecules which can attach to a chain before the initiator is consumed and all molecules with initiated bonding sites have reacted.

In summary, a polymer is a very large molecule made up of repeating small molecular groups. The elements and bonds of a polymer give the polymer its bulk properties. All too often a polymer will be designed for the easy of molding or processing and not for subsequent processes like bonding, painting, printing, or coating.

It is at this point where surface modification of the polymer is essential. The polymer can be treated after the polymer has been molded, extruded, formed, coated, or cast without changing the bulk properties of the polymer. So, an engineer can specify a material that would best suit high volume manufacturing and device integrity without compromising the device due to a printing of painting process.

Look at our Materials Resource Guide to see all we offer – or simply Ask The Experts if you have a question. Don’t be shy!

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.

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.

Using simple plasma surface modification techniques, we are able to manipulate the surface of most plastic devices to contain fluids or direct flow.

Surface treatment zone

the surface treatment zone shown in the picture above is 0.9mm. The wet zones are hydrophilic drawing water into the narrow zone while the other areas are hydrophobic to the natural polystyrene material.

If you would like more information about this process, please vist our website.

Q: Quick question for you in regards to a plasma system cleaning parts. Is there such thing as a part being too dirty? What happens if we have a part that has been soaked in CNC lube or even worse dipped in oil? Can you give me a rough idea of the limitations that a plasma system in regards to dirty parts?

A: Yes. A part can be too dirty for plasma to be effective. A good rule-of-thumb is ; If you can wipe if off with your thumb, it is too dirty. In other words… if a part has so much oil or contaminate that it can be moved with your finger, the part needs to be, at least, wiped down. When a part has excessive contamination it takes a long time for the plasma to completely remove all the contamination. The plasma can certainly remove all the contaminate, but it is doing it one molecule at a time. So, the thicker the oil layer the longer the time.

So, it is a good idea to wash the parts before plasma. But one may ask, if I have to wash them, why do I need plasma? The answer is: the wash cannot remove all the contaminate, because the bath always has some level of contamination. The plasma chamber never can have contamination, because all the contaminates are converted to gas and pumped away.

Have a look at our Materials Resource Guide and our TriStar site, too.

Which surface treatment technique is best for you?

Mechanical – involves blasting with abrasive particles to develop a surface roughness to the polymer. This technique may be wet or dry.

Heat and Flame – the polymer surface is quickly exposed to a high temperature flame. This operation anneals and oxidizes the polymer surface

Radiation – this treatment involves exposing the polymer to UV or gamma radiation. The resulting surface typically is oxidized and crosslinked.

Plasma – Truly this is the most elegant and broad reaching method for polymer activation. This technique treats the surface of polymer with activated gaseous compounds. The activated gases are usually produced with RF discharge and can oxidize, crosslink, and clean the polymer surface.

Electrical – Electrical arc blow or corona discharge are used in this method. This treatment usually oxidizes and crosslinks polymer surfaces.

Chemical – Chemical agents that oxidize, dissolve, hydrolyze, or swell polymer surfaces. This process can add surface topography and/or chemistry.

Still not sure? You might consider to Ask The Experts!

Welcome to the Surface Treatment / Modification Blog.

This Blog will attempt to discuss and inform about anything and everything related to…drum roll please…surface treatment and separate fact from fantasy.

The subject of surface treatment can be difficult to explore. Some techniques are closely held secrets while other procedures are spelled out on the page of a magazine. So, it can be difficult to know what cleaning or surface treatment technique is appropriate for a specific application. In many cases, a perfectly good surface treatment technique is applied to the wrong application, and then the technique is given a “bum rap”; There are many times I have heard…”plasma doesn’t work!” or ” nothing beats my sandpaper and acetone trick.” Well, plasma can work and there is something better and simpler than acetone and sandpaper.

If you have already heard enough and would like help increasing yield, adding value, or developing new products, please go to the TriStar Plastics Corp. web site to initiate your project. Or simply Ask The Experts your questions right away!

So, without further delay…lets blog about Surface treatment.