Vice President Technology
Edited by Jean M. Hoffman
"How do I choose the right thermoplastic composite?" is a question that may put knots in the stomachs of even the most seasoned designers. That's because there are thousands of thermoplastic composites available. So it's important to take a systematic and logical approach when choosing a composite. There are five fundamentals that are key: Resin morphology, cost comparison, temperature resistance, property enhancement using aspect ratio, and ultimate-performing long fiber.
Over 60 thermoplastic base resins can go into a composite. It's helpful to understand a little about thermoplastic chemistry and, in particular, understanding morphology.
Although morphology sounds like a complicated term, it can simply be viewed as the orientation that the molecules of the polymer (plastic) take when they go from the melt state to a solid during processing. A thermoplastic resin will fall into one of only two categories of morphology: amorphous or semicrystalline.
Amorphous polymers have a random molecular orientation and include acrylic, polystyrene (PS), styrene acrylonitrile (SAN), acrylonitrile butadiene styrene (ABS), polycarbonate, polysulfone (PSU), polyethersulfone (PES), polyarysulfone (PAS), and polyetherimide (PEI).
Polymers with semicrystalline morphologies have ordered or crystalline molecules dispersed within regions composed of random amorphous molecules. They include polypropylene (PP), polyethylene (PE), nylon (PA), PBT polyester, PET polyester, acetal or polyoxymethylene (POM), polyphenylene sulfide (PPS), polyetheretherketone (PEEK), and liquid-crystal polymers (LCPs).
Understanding which morphology best suits the application is important because there are advantages for each morphology type. Amorphous polymers are dimensionally stable and don't shrink, warp, or creep much. They also have good impact strength (toughness) and transparency. On the other hand, semi-crystalline polymers flow easily inside the mold, resist chemicals and wear, and work with a wider range of reinforcements.
Key to identifying the best resin morphology lies in determining what requirements are most important — dimensional stability, tight tolerances, moldability into thin-wall sections, resistance to chemicals and wear, transparency, and so forth. The assessment will roughly cut resin choices in half.
COST COMPARISON & TEMPERATURE RESISTANCE
Cost is, of course, extremely important. It is possible to develop a composite to meet even the toughest physical requirements. But the effort is wasted if the design doesn't meet the customer's cost expectations. Thermoplastic resins can be arranged into three categories based on cost. Commodity resins typically have large volume market costs of less than $1.50/lb. Medium-cost engineering resins typically fall between $1.50 and $3/lb. And the high-cost high-temperature resins run above $3/lb.
There is a direct correlation between the cost of a resin and how well it will resist high temperatures. This is why it is important to not overspecify thermal requirements. Temperature resistance can be measured in a variety of ways (melt temperature, heat-deflection temperature, glass-transition temperature, and continuous-use temperature). The resins that offer the highest capabilities in each of these categories will cost the most. For example, a couple of the top thermal performers include PEEK and thermoplastic polyimide (TPI) and both cost over $30/lb.
Costs are usually discussed in terms of dollar/pound. But a thrifty part designer will calculate how much it costs to produce a certain volume of parts: $/in.3
$/in.3 = $/lb × specific gravity × 0.0361.
If you ever find yourself outbid by a competitor using a higher specific gravity material, calculate the $/in.3 and you may be surprised to find that you actually have the better price.
Evaluating the composite based on morphology, cost, and thermal requirements will narrow the choices to just two or three resins.
PROPERTY ENHANCEMENT USING ASPECT RATIO
The resin is only half the story in building a composite. Next, one should ask what must be added to the resin to give the composite the right performance. To answer this question, it's important to understand another physical term: aspect ratio. The aspect ratio will help predict the type of physical property enhancement the additive will impart when compounded into the base resin.The aspect ratio can be defined as the length divided by the diameter of the additive. For a spherical bead, the length equals the diameter and thus the aspect ratio is 1. For a fiber, it is also easy to calculate the aspect ratio because the length and diameter are usually well defined. For some additives, such as minerals with an irregular shape, the aspect ratio is a little harder to figure. It is often calculated by dividing the particle's maximum length with its thinnest cross-section measurement.
Additives with aspect ratios of less than 10 have minimal ability to improve tensile and flexural strengths of the base resin. These additives are generally referred to as fillers and include talc, calcium carbonate, and glass beads. Though they don't improve strength, they do moderately improve modulus (stiffness) and heat-distortion temperatures.
They also can be added to reduce part warpage, improve dimensional stability, and reduce the overall cost of the composite (especially with more expensive base resins). Fillers act as a contaminant and initiate stress cracking. Thus, they lower the impact resistance (toughness) of the plastic.
Additives that have an aspect ratio above 50 can significantly improve the base resin tensile and flexural strengths. These additives are generally referred to as reinforcements and include fibers made from glass, carbon, aramid (Kevlar), and basalt. Besides boosting strength, reinforcements can make the composite stiffer and raise its heat-distortion temperature.
Because they have a tendency to align themselves with the flow direction during molding, reinforcements contribute to anisotropic shrinkage (different in flow direction versus transverse direction), which can make parts warp.
Fillers such as glass beads or talc are sometimes added along with glass fiber to make the shrinkage more isotropic and reduce warp. Regarding impact resistance (toughness), reinforcements tend to make brittle resins tough and tough resins brittle. Examples of this include a brittle polyphenylene sulfide resin becoming tougher when reinforced with glass fiber and a tough polycarbonate becoming more brittle when reinforced with glass fiber.
Additives with aspect ratios between 10 and 50 will have a moderate effect on improving tensile and flexural strengths of the base resin. These additives are referred to as transition materials and include wollastonite, mica, and milled-glass fiber. They will improve modulus and heat distortion slightly more than the fillers.
Transition materials typically serve in situations where dimensional stability is of prime importance and it's acceptable to have strength, modulus, and heat distortion lower than from glass fiber.
The accompanying chart shows the difference in performance for a nylon 6/6 containing 40% of filler (talc), a transition material (mica), and reinforcement (glass fiber).
LONG VERSUS SHORT
Physical property data indicates that the aspect ratio of the additive has a direct correlation to the strength, modulus, and heat-distortion properties, and possibly the impact resistance of the composite. Maximizing the aspect ratio of the reinforcement fiber will maximize composite performance and is the logic behind long-fiber composites.
A pultrusion process manufactures long-fiber composites. Here the fiber roving is pulled through a machined die in which the base resin is forced to impregnate the individual fibers. The impregnated fiber rovings are pulled from the die and into a pelletizer that cuts the strands into pellets.
The fiber length in the pellets will be the same as the pellet length, which for most materials is a half-inch. Using a 17-µm-diameter fiber results in a fiber aspect ratio of about 750, which is about 10 times larger than that of chopped-fiber compounds typically produced via the extrusion compounding process.
Prior to long-fiber compounds, rubber-based impact modifiers were added to improve impact resistance of a chopped-fiber composite. This improves composite toughness but reduces its strength, modulus, and heat-distortion temperature. The effect of having an extremely high-aspect-ratio fiber in long-fiber composites has been the improvement of all the physical properties, which is depicted in the accompanying spider chart for 40% glass-fiber nylon materials.
By having the ultimate in strength, modulus, impact, and heat distortion, long-fiber composites have become the choice for demanding applications, such as replacing metal in load-bearing applications. The high-aspect ratio in the long-fiber composites also gives these materials excellent creep resistance.
RTP Co., (800) 433-4787, rtpcompany.com