Tech Talk Blog

The growth of plastics in medical devices is growing exponentially around the world.  Plastics are regulated like any other materials that may come in contact with human tissue or fluids and that usually falls under testing procedures issued under USP or ISO10993. There are three time scales for biocompatible devices. “Limited” would be less than 24 hour exposure, “Prolonged” is 24 hours to 30 days and “Permanent” is 30 days and longer. Device’s are categorized as Surface Devices which would be items such as electrodes for monitoring, contact lenses, catheters, endotracheal tubes, sigmoidoscopes and similar devices. Second would be Externally Communicating Devices such as  laprascopes, blood administration devices, pacemakers, oxygenators and the like. Finally are Implant Devices such as orthopedic pins or plates, heart valves, grafts, stents and similar devices.

Testing of these devices includes mechanical, thermal, chemical tests as well as systemic injection, intracutaneous and implantation. All of these must be done before a plastic component can be approved. Typical materials for biocompatible applications include medical grades of PVC and Polyethylene, PEEK, Polycarbonate, Ultem PEI, Polysulfone, Polypropylene and Polyurethane. For more specific information on Biocompatible materials as well as special plasma preparation treatments of all of these materials, contact TriStar Plastics at www.tstar.com and visit our Video Learning Center and our Materials Database.

Ultracomp Composite Bearings are designed for extreme loads where impact and vibration may occur. Because of it’s very high impact strength it can take extreme loads as well as shock loads. Ultracomp requires no lubrication which eliminates maintenance, is much kinder to the environment and reduces overall costs of ownership.  Ultracomp absorbs virtually no moisture, takes static loads up to 55,000 psi and handles dirty, gritty environments. Ultracomp is also an excellent underwater bearing for applications as diverse as bowthrusters, rudder bearings, roller bearings, dockside equipment exposed to salt air and water. Also an excellent bearing material for construction, material handling and ag equipment.

Visit our Video Learning Center to learn more about all we have to offer.

The dynamics of wetting are described below:

Spreading = A – ( B+ C )

Where:

• A = surface energy of solid (given below)
• B = surface tension of liquid
• C = surface energy of solid-liquid interface

If Spreading is:

• Negative. Then, liquid will bead up.
• Zero. Then, liquid will spread.
• Positive. Then, liquid will spread.

If the material surface energy is relatively low, then the coating will not flow well and fisheyes, pinholes, gaps, or air bubbles will form. If the material surface energy is too high, then the paint, ink, or coating may bleed or be difficult to control. Therefore, the surface tension of the liquid and the surface energy of the material must be matched for the application.

Don’t just listen though – take a look! Visit our Video Learning Center for an in-depth look.

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!

The molecular forces and chemical bonding are important to understand the physical properties the bulk polymer exhibits. The covalent bonding in a polymer system is one of the strongest and significant forces and is often referred as primary bonding. A list of common covalent bonds and the energy associated with the bonds are listed below:

This list shows the dissociation (bond breaking) energy to predict which bonds will break first when a polymer is overheated or modified by plasma or UV. For example, the carbon (C) chlorine (Cl) bond will dissociate before a C-H bond in a PVC polymer.

Secondary bonding forces are also important as they can affect the material’s physical properties, such as surface tension, viscosity, friction, volatility, and solubility. The secondary forces are as follows:

Blends are the physical blend of polymers. Unlike a copolymer that is chemically mixed, polymer blends are mixed before or during molding operations. Yet, polymer blends can be just as useful and cost effective as copolymers or terpolymers. The physical property of a blend is determined by the physical properties of each ingredient and miscibility. If the compounds are miscible, the mixture will remain uniformly blended (homogenous). If the compounds are not miscible, the each mixture will separate into its respective phase. It would be here that the physical properties of the material would be compromised if the adhesion between the compounds is poor. Fortunately, if two immiscible polymer need to be blended, an additive can be used or a polymer can be grafted to one of the originals.

If you want to know a little bit more, watch our Plastics Technology 101 Seminar and other videos in our Video Learning Center.

UHMW Polyethylene is an interesting material in the polymer world.  It’s one of the lowest cost plastics on the market yet it offers some properties that few other polymers do. UHMW is best known for it’s abrasive wear resistance and impact strength but even within this small polymer family there are variations. UHMW stands for ultra high molecular weight and under this description UHMW is produced with a molecular weight range of 3.5 to 6 million. The molecular weight has a fairly dramatic impact on several key properties. For instance, a 4 million molecular weight grade has an abrasion resistance of 100 when measured using a sand slurry test. A 6 million molecular weight is 75 which is a further 25% improvement! Compare this with steel which has a resistance of 160 and you can see why UHMW is the material of choice for abrasive wear environments.

There are several other variations on UHMW including cross linked, glass and moly enhanced and even a high temperature grade.

Check out our Materials Resources Guide to see the entire selection.

Torlon PAI from Solvay is one of the outstanding polymers for high temperature applications. Torlon is available in various forms and with different enhancements to meet diverse applications such as bearings, dielectric insulators and structural components. Torlon’s physical properties maintain very high values even at the maximum operating temperatures.

You can learn more in our Video Learning Center, too!

Tivar HOT is a unique UHMW grade that has a dramatically higher continuous operating temperature (275F) than standard UHMW. Tivar HOT still has excellent wear and abrasion resistance, doesn’t absorb moisture, has excellent chemical resistance and meets FDA, USDA and 3A guidelines. We have found this product to be excellent for higher temperature zones in food processing and packaging equipment but it has also proven to be a great material in down-hole oil drilling applications!

Pretty diverse material. If you need excellent abrasion resistance, low friction, self lube bearing material with all of the other attributes mentioned, consider Tivar HOT from TriStar! And while you are looking around the site – stop by the Video Learning Center for some more information!

Fluoroloy H, aka Rulon  H, is an interesting combination of ceramics and PTFE. This unique dielectric material is used inside connectors for high power applications. The Fluoroloy H material has a slightly better dielectric constant compared to standard Teflon but has a higher rate of thermal conductivity.  This allows the heat being generated at the center conductor to transfer to the outer conductor more efficiently, which in turn increases the power level efficiencies of the connector. The thermal conductivity of Fluoroloy H is 1.21 W/m C and virgin Teflon PTFE is 0.24 W/m C. Fluoroloy H is easy to machine, similar to virgin or glass filled PTFE. Available in rod, sheet and tape.

For improved dielectric properties as well as heat sink properties, Fluoroloy H from TriStar Plastics is a  unique design option.

If you are interested, learn some more about Fluoropolymers or if you have a question that’s driving you crazy – Ask The Experts!

PCTFE has long been the go to material for valve seats, seals and gaskets used in cryogenic applications. But one thing that makes PCTFE unique is that it can be processed to meet a broad molecular state, i.e. crystalline or amorphous. PCTFE is a melt fluoropolymer and when molded in either sheet, rod or tube form it can be set at a specific crystalline state through a unique quenching process. This process is not as easily manipulated with extruded PCTFE rod so if you are looking for controlled molecular values you need to consider molded product.

As an example of some of the differences in physical properties between the amorphous PCTFE and crystalline PCTFE, consider these values:

Property                                            Amorphous                               Semi Crystalline                              Crystalline

% Crystallinity                                       40%                                                    55%                                               65%

Ultimate Tensile (psi)                      25,000                                           17,200                                              15,600

Elongation at Break (%)                        4                                                       2                                                         1.5

Tensile Modulus of Elast. (psi)   1,110,000                                           NA                                                 760,000

Compressive Strength   (psi)         34,000                                           37,500                                             38,000

Flexural Strength (psi)                    58,000                                            43,000                                              37,000

Flex Mod. Of Elast. (psi)               1,800,000                                     1,700,000                                    1,650,000

Crystalline/amorphous values are monitored by specific gravity and performance in cryogenic service utilizing amorphous grades of PCTFE has been well documented in terms of service life, sealability and property retention.

Ask The Experts at TriStar for more information on how PCTFE can be “custom tailored” to your application through molecular manipulation!