Part I: Inorganic Mineral Fillers in Plastic Hardware

When we look at the plastic products that surround us every day, whether it’s plastic hardware or even something as cheap as a plastic comb, we believe it to be 100% made of plastic.  Upon consideration, maybe we can imagine that the composition of plastic hardware may include some color additives.  You may be surprised to learn that up to 70% of plastics are often composed of organic or inorganic fillers.   So what are these fillers and why do processors use them?

Fillers are typically used for one of two reasons.  They are used to bulk up plastics in order to reduce costs.  They are also often used to improve the stiffness and hardness of plastic hardware.   Some common mineral/inorganic fillers are calcium carbonate (limestone), magnesium silicates (talc), calcium sulfate (gypsum), mica, calcium silicate, barium sulphate and kaolin (China clay).  Common organic fillers include tree bark flour, nut flours, chicken feathers, and rice hulls.  Among polymers, fillers are most commonly used in PVC followed by polyolefins, nylons and polyesters.

We’ve compiled a list of the top 5 plastic fillers used in plastic hardware.  Reinforcing organic fillers will be discussed in a future post, so stay tuned!

1)      Calcium Carbonite

Calcium carbonite is the most common filler for plastic.  It represents over half of the total inorganic fillers used in the plastic industry.  It is the main component of eggshells, seashells, pearls, and chalk.  Calcium carbonite is also familiar to us as an inexpensive dietary calcium supplement or as an antacid.  It is often mined in quarries as limestone or marble.  As plastic filler, calcium carbonite may reduce overall strength but it increases tensile modulus and density. Not only does it lower costs by extending a more expensive base resin, it provides opacity and surface gloss, improves impact strength and is a processing aid.  It is often used in PVC, polyolefin, polypropylene, and polyethylene among others. 

2)      Hydrous Magnesium Silicates (Talc)

Talc is the softest mineral on the market with a Mohs hardness* of one.  The use of talc has increased rapidly in the last decade.  Talc is primarily added to polypropylene but is also added to polyethylene and polyamides (nylons) to increase stiffness and rigidity.  Talc also improves lubricity and impact strength. Polypropylene parts with as much as much as 40% talc have replaced many metal parts in automotive applications such as bumpers, interior plastic ductwork and fasciae.  Talc is used in household appliances and engineered plastic hardware.  It is used as an anti-block in polyethylene films.  Without talc, it would be hard to pull the two faces of a plastic bag apart.          

3)      Mica

When mica is added to a polymer, the resultant composite is characterized by high stiffness, high dimensional stability and good dialectical properties. Mica is often used as an extender in common thermoplastics.  It may reduce strength but increase modulus and density.  Mica is used in polypropylene to increase flexural modulus.  Plastic hardware is commonly filled with 20% or 40% mica but loads as high as 60% have been recorded.  A 40% loading increases the flexural modulus of the plastic from 4450 to 6450 psi.    

4)      Calcium Metasilicate (Wollastonite)

Wollastonite is made up of 52% silicon dioxide and calcium oxide.  It is often ground into a white free-flowing powder taken from limestone and diatomaceous earth. It is used in some antacid products. Calcium silicate is added to polyester molding compounds as an extender and reinforcing fiber.  Addition of this mineral provides a smooth molded surface and low water absorption.

5)    Calcium Sulphate

More commonly known as gypsum or plaster of Paris, calcium sulphate is used in plastics to lower costs by extending the material.  It is also used in drywall and as a coagulant in making tofu. 

*The Mohs hardness scale was created in 1812 by German scientist Friedrich Mohs and represents the ability of a harder material to scratch a softer material. The scale has values from 1 to 10 with talc as one and diamond as ten.

Did I miss one?  Let me know in the comments section below.

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