Welcome to part 3 in our series on plastic compounding. Our friends over at RTP Company have written a white paper to answer the question "“How do I choose the correct thermoplastic composite to meet the application?”". They have broken it down to five sections.
1) Resin Morphology
2) Cost Comparison
3) Temperature Resistance
4) Property Enhancement Using Aspect Ratio
5) Ultimate Performing Long Fiber
Today's blog post is on Property Enhancement Using Aspect Ratio.
Choosing the resin is only half the story in building a composite. The next decision is "“What will I add to the resin to impart performance to the composite?"” To answer this question, one will need to understand another physical term: aspect ratio.
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, such as that shown in the diagram, it is also easy to calculate the aspect ratio because the length and diameter are usually well defined. For some additives with
an irregular shape, such as minerals, the aspect ratio is a little harder to calculate; but you can always measure a major length and a minor thickness on the particle, and thus, can calculate an aspect ratio.
It is this aspect ratio that will predict the type of physical property enhancement the additive will impart when it is compounded into the base resin.
Additives with aspect ratios of less than 10 have minimal ability to improve the tensile and flexural strength of the base thermoplastic resin to which
they are added. These additives are generally referred to as fillers and include species such as talc, calcium carbonate, and glass beads. Although these fillers do not improve strength, they do have the ability to moderately improve modulus (stiffness) and heat distortion temperature. They also can be added to reduce part warpage, improve dimensional stability, and reduce the overall cost
of the composite (especially with higher cost base resins). Because it will act as a contaminant and a stress crack initiator, fillers will always lower the impact resistance (toughness) of the plastic to which it is added.
Additives that have an aspect ratio above 50 have the ability to significantly improve the tensile and flexural strength of the base resin to which they
are added. These additives are generally referred to as reinforcements and include species such as glass fiber, carbon fiber, aramid fiber and basalt
fiber. Along with strength enhancements, reinforcements can significantly increase modulus(stiffness) and heat distortion of the composite.
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 could lead to part warpage.
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 the tensile and flexural strength properties of the base resin to which they are added. These additives are referred to as transition materials and include such species as wollastonite, mica, and milled glass fiber. These additives improve modulus and heat distortion slightly better than the fillers.
Transition materials are typically used in situations where dimensional stability is of prime importance and strength, modulus and heat distortion lower than that offered by glass fiber is acceptable.