While everyone reading this has at least some familiarity with ABS, I wanted to take a more in depth look at ABS and how it’s made. As it turns out, there are several methods for making this commonly used material and knowing how it’s made might help you recommend the right material to your customer.
ABS is commonly referred to as a terpolymer which really just means that it is made up on more than two polymers. If it were made up of just two polymers, we would call it a copolymer. The three ingredients are:
- Acrylonitirile (pronounced AK-RUH-LOH-NAHY’-TRIL) is a liquid that becomes a hard plastic, polyacrylonitrile when polymerized.
- Butadiene (pronounced BYOO-TUH-DAHY’-EEN) is a gas at room temperature. When it is polymerized it becomes polybutadiene, which is a rubber.
- Styrene is an oily liquid that becomes polystyrene when polymerized. We are all familiar with polystyrene.
How these three monomers get mixed together and polymerized into a terpolymer is where things get interesting. If these three polymers were just mixed together we would call them an alloy and not a terpolymer. Terpolymer indicates that these monomers are actually polymerized together.
As it turns out there are multiple methods for making ABS. I am going to narrow the descriptions of these methods down to the two that are primarily used today. Many of the other methods are slight variations on these methods or combinations of them.
Method 1: Emulsion Polymerization
The butadiene is polymerized in water. This forms what is called rubber latex, which really just means that rubber particles are dissolved in water. In the second step, acrylonitrile and styrene are polymerized in the rubber latex solution. You end up with a mix of ABS particles suspended in water. Centrifuges are used to recover the ABS and then the ABS is dried to remove the rest of the water.
Method2: Mass Polymerization
The butadiene rubber is polymerized in a separate step. Blocks of polybutadiene rubber are crushed into small pieces and then dissolved in the styrene and acrylonitrile monomers which have been mixed together. This solution is then moved into a reactor where the styrene and acrylonitrile are polymerized. The solution is mixed during polymerization. The sheer force created by this mixing reduces the size of the rubber particles. The mixing is performed very carefully so that the rubber particle size can be closely controlled as it has a lot of impact on the properties of the final product. At the end, the residual monomer is removed and then fed back into the beginning of the process.
Emulsion polymerization is how ABS was originally manufactured commercially. ABS Mass polymerization was not utilized until the mid 1980s. The mass polymerization process has several advantages over the emulsion method.
Advantages of mass polymerization
ABS produced using the mass polymerization method has a lighter color than ABS produced using emulsion polymerization. Supposedly, this is because less polymerization additives are needed. ABS manufactured using the emulsion method is more of a yellow or cream color where mass polymerization produces ABS that is much whiter.
It also produces ABS with more consistent properties and color. Mass polymerization also has less environmental impact. There is no water used in the process and less VOCs are created.
Mass polymerization produces ABS with less residual monomer which is safer for food contact. Residual monomer is considered a contaminant and is associated with health concerns.
One of the biggest changes that the mass polymerization method has brought is the availability of low-gloss grades of ABS. For those of you that have been in the industry for a while like me, you might remember that low-gloss ABS is something that came out in the 1980’s. There were no low gloss grades prior to that. This market was driven by the auto industry that wanted low luster materials for interior parts. Low-gloss ABS is made by having larger polybutadiene rubber particles in the material. The larger particles create a very slightly rougher surface which scatter light and produce a lower gloss appearance. Unlike the emulsion process, mass polymerization allows manufacturers to control the rubber particle size.
The larger rubber particle size however, negatively affects the impact strength of the material. Smaller rubber particles typically increase the impact strength. If you look at the data sheets for ABS materials, you will notice that low-gloss grades tend to have lower izod impacts, typically in the 2.5-3.0 ft-lb/inch range. Standard grades tend to have izod impacts in the 4.0-6.0 ft-lb/inch range.
The mass polymerization method is the most commonly used method for manufacturing ABS today, but the emulsion method is still being used. The emulsion method is better for making high impact grades. If a particular ABS manufacturer does not offer any low gloss grades in their portfolio, this would be a sign that they are using emulsion polymerization instead of the newer mass polymerization. Lotte Advanced Materials (formerly Samsung) comes to mind.
I would probably recommend always using ABS that is produced through mass polymerization except if the primary thing that you need is very high impact strength. If you are very particular about color consistency, you really need mass polymerized material and if you need low gloss, you have to run it.
The process for manufacturing ABS is very finicky, much like beer manufacturing. Little things going wrong can create major problems. This is probably why there is so much wide spec and off-grade ABS on the market. Most of these lots seem to have some type of color related issue like burnt pellets. The physical properties of this material are usually pretty good, so if color is not important to you, it might make sense.