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Quick Look at Additives 

The focal point of design engineers when selecting engineering plastics is the type of plastic: polysulfone, nylon, polyester, or whatever. However, commercial grades often contain five or more additives.

You must carefully consider why you a particular additive is needed and what impact, if any, the additives might have on the functional characteristics or the processability of the plastic compound. You also need to consider the cost impact of additives, particularly if they are specially compounded for your application. In general, the mechanical properties on data sheets reflect the plastic with the additive.

Here are the most important additives used to modify plastics:

Particulate Fillers

These can be inorganic, organic, mineral, natural or synthetic. Fillers, in general, reduce the strength and the impact of the compound, but they almost always increase the stiffness to some extent. Suppliers of volume thermoplastics, such as polyethylene, often use them to reduce cost. That’s typically not the case with advanced engineering resins.

“We’re not so focused on using fillers to reduce costs as we are for providing enhancements, like better surface appearance, conductivity and wear resistance,” comments Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. Materials such as wollastanite with a higher aspect ratio improve mechanical properties of the compound. Aspect ratio is the proportional relationship between width and height. (See the end table for a detailed comparison of fillers and fibers). Down the road, nano-engineered materials may come into play here in a big way.

Fiber Reinforcements

The huge mainstream player for engineered compounds is glass fiber reinforcement, which allows use of semi-crystalline materials above their glass transition temperatures. (See Part One of this series “High-Temperature Plastics — Can They Really Take the Heat?”) One of their major functions is to increase the modulus, thereby increasing the stiffness of the finished part.

The concept is illustrated in the following graphic. Carbon or metal fibers can be used to impart strength and conductivity. “The downside of fibers is that your density is going to increase, making the parts heavier,” says Warkoski. “Your modulus is going to be a lot higher, so the shape of your stress/strain curve is going to be skewed to the left.” It’s a particularly important consideration if you’re designing a component for a snap-fit assembly where some give is important.

 

Addition of glass-fiber reinforcement to a semi-crystalline plastic bumps up mechanical usefulness at higher temperatures in proportion to the amount of glass added.

Processing Aids

Stabilizers are used to prevent degradation during the injection molding process, when melt streams are subjected to high temperatures and shear forces. Many compounds also contain mold release agents, which can be metallic stearates, waxes or olefinic polymers, for easier part ejection. They are used in small quantities and generally have no appreciable effect on mechanical properties.

Heat Stabilizers

Service temperature heat stabilizers (as opposed to stabilizers used during the molding process) extend the life of a plastic compound at elevated endues temperatures. There’s a lot of science in the chemistry and these are highly tailored to polymer families. Presence of a heat stabilizer increases the compound’s Continuous Use Temperature or Relative Thermal Index.

Impact Modifiers

Some plastics are inherently tough, but many need additives to improve their impact resistance. A wide range of impact modifiers are available, and the choice of additive depends largely on the plastic used. Be sure you look at impact properties for the expected service temperature. Not all modifiers provide good impact at low temperature. Keep an eye on other key properties as well, since these additives can affect creep resistance, elastic modulus and elongation. Because impact modified grades are often tailored to specific end uses, it’s a good idea to discuss your requirements with your material supplier.

Friction and Wear Additives

A special class of additives improves wear characteristics of plastics. These additives reduce abrasive wear when a plastic part rubs against another part. The study of friction and wear is called tribology and the important operating variables are load and speed. A common polymer used to improve tribological properties is polytetrafluoroethylene (PTFE).

“If you are going to experience low load and high speed in your application, you want to go toward a PTFE,” says Warkoski. High levels of PTFE in high- load applications can create creep problems. Another option is graphite powder, which also reduces coefficient of friction and is less susceptible to creep. This is a situation where the design engineer really needs to know exact operating conditions because you may encounter too much creep if you pick the wrong compound. There are other, more exotic choices, such as silicon oils and molybdenum disulfide, that are used by custom compounders.

A Closer Look at Fillers & Fibers
Filler Type Benefits What to Think About
Calcium Carbonate Widely used as a cost-reducing extender adn whitener when highly processed Can reduce strength. Increase modulus and density.
Kaolin Clay Spherical nature helps maintain flatness in the molded part. Can improve surface quality and adhesion for plating or painting, especially if surface treated Can reduce strength. Increase modulus and density.
Mica Primarily used as a cost-reducing extender in commodity thermoplastics Can reduce strength. Increase modulus and density.
Microspheres
(hollow glass)
Reduces density and increases modulus More costly than other fillers. Less strength benefit than glass fibers with no reinforcing value.
Talc Primarily used as cost-reducing extender in commodity thermoplastics. Can reduce strength. Increase modulus and density.
Wollastanite
(calcium metasilicate)
Aspect ratio of 3:1 to 5:1 can improve strength at far less cost than glass-fiber reinforcement. Some orientation issues. Increases density and modulus.
Fiber Type Benefits What to Think About
Boron Improves load-bearing capacity Expensive, specialty compound. Increases density.
Carbon/Graphite Highest modulus and strength reinforcement. Electrically and thermally conductive. Expensive and conductive. Modest increase in density.
Glass Most commonly used fiber reinforcement. Improves strength and dimensional stability. Allows use of semi-crystalline materials above their glass transition temperatures. (see graph) Be aware that fibers in molded parts may result in anisotropy. Increases density.
Metal Provides thermal and electrical conductivity Specialty compounds. Increases density.

 





 





 




 
 
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Last Update (19/4/2011)