Tech Talk Blog

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!

Benzene ring Structures

• The benzene ring acts as a electron bank loaning and accepting electrons
• Polymers with benzene rings include PS, PET, PU, PC and PSU
• Polystyrene is stable

 Amorphous Polymers

• Highly amorphous materials (non crystalline) are generally resistant to radiation since the chain structure is capable of great ductility and they can tolerate many scissions without breaking up

 Polyamides (Nylons)

• Nylons especially aromatics 12, 11, 6/12, 6/10 are highly resistant to irradiation
• Nylon 6 is least radiation resistant

 Butyl Rubber

• Natural rubber survives irradiation very well but butyl rubber crosslinks to become stiffer, loses elongation and turns friable and powdery
• For this reason butylene -containing polymers such as ABS and PBT loses impact strength on irradiation

 Stress Cracking

• A polymer under stress is attacked more by radiation than unstressed material is.
• Scissioning and oxidation effects are concentrated in the stressed zones
• Therefore plastic parts must have reproducible molded-in stresses

 Key Messages

• Aromatic polymers (e.g. with benzene rings) are more stables than aliphatic chains
• Look at ratio of scissioning to crosslinking
• Most natural PP and PTFE are unstable with irradiation

Survey Exposure

• If it is difficult to predict how a polymer system will resist irradiation (e.g. by accelerated ageing) then the use of an exaggerated dosing at 100 kGy should highlight problems and marginal materials

If you are unsure if your material will hold up to this sterilization technique, please visit the TriStar Plastics Corp. website for contact information.

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!

UV exposure increases the bondability of plastics by irradiating them with high intensity UV light. However, the effectiveness of UV exposure is very dependent on the wavelength of light being used. For example, light with a wavelength of 184 nm will crosslink the surface of polyethylene, while light at >250 nm will not. UV irradiation causes chain scissions, crosslinking, and oxidation of the polymers surface, even in inert gases. Many different mechanisms describing why UV exposure increases the bondability of plastics have been proposed, including: increasing the wettability; strengthening the plastic’s boundary layer through crosslinking; and inducing hydrogen bonding. The predominant view is that the bondability is improved by the formation of polymeric scission products, which promote interfacial flow and polar interactions.

To learn more visit www.tstar.com and check out the Materials Resources Index!

In surface science, the use of a goniometer (contact angle meter) is common to determine the wettability of a surface. What is clear is that every material has a specific surface energy. But, may not be clear is that the surface topography of that material also effects the contact angle.

Surface roughness can change the contact angle without plasma or other surface treatment. When a device is molded or machined, the surface finish of the device can have a functional as well as a aesthetic purpose. The functional purpose can be microscopic capillary channel as in microfluidic devices. The aesthetic purpose can be feel or finish of the part as in grips or sunglasses. The data below shows the effects of surface roughness in RMS versus natural, plasma treated hydrophilic, and plasma treated hydrophobic.

Notice the general trends in contact angle relative to surface roughness.

What say ‘ye?

Tongue tied? Well, take a breather and watch some TriStar clips in our Video Learning Center.

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.