ASA: The Lesser Known Cousin of ABS

vance-and-coy-2

How exactly did the word cousin become synonymous with similarity? How many times have you heard someone refer to one thing being the close cousin to another thing like granite is a close cousin of marble or a planet in a distant solar system being Earth’s cousin? I don’t know about you but I am not that much like any of my cousins. Maybe it’s just me.

Anyway, you could say that ASA is a close cousin to ABS. ASA is ABS with one ingredient changed out.

ASA stands for Acrylonitrile Styrene Acrylate where ABS stands for Acrylonitrile Butadiene Styrene. Notice that in ASA, the butadiene has been replaced with acrylate. Both butadiene and acrylate are rubbers that add impact strength to the respective materials.

Butadiene does a great job of increasing the impact strength but it has one major problem. Butadiene is extremely susceptible to damage from UV rays. The reason is kind of complicated to explain but suffice it to say that any molecule that has a double bond is susceptible to damage from ozone in the air. A double bond is a molecular bond that uses 4 electrons as opposed to the 2 in a “normal” molecular bond. Butadiene rubber has the troublesome double bond where acrylate rubber has only single bonds.

The acrylate rubber makes ASA extremely durable for outdoor and high sunlight exposure applications. ABS on the other hand is very susceptible to damage from sunlight and really should not be used in any outdoor application unless it is painted or chrome plated. ABS will discolor in a very short period of time and will suffer severe loss of physical properties shortly after. High impact polystyrene is even worse.

ASA has an excellent appearance, similar to ABS, and can be used to make very attractive parts in any color but it is not available in any low gloss variants like ABS is. ASA also has better long term heat resistance and chemical resistance than ABS.

The biggest negative with ASA is that the impact strength is not as high. The acrylate rubber just does not make as effective an impact modifier as butadiene rubber does. ASA tends to have izod impacts in the 3 ft-lbs/in range where ABS tends to be in the 4-6 range with some grades having izods as high as 8 ft-lbs/inch.

ASA can be alloyed with polycarbonate which will increase the impact significantly. ASA/PC alloy has izod impacts in the 6 ft-lb/in range which is very similar to a good higher impact ABS but not as good as ABS/PC alloy. ASA/PC alloy has excellent UV resistance just like ASA.

Common brands of ASA are Centrex which is made by A. Schulman and Luran which is made by INEOS Styrolution Group. SABIC makes ASA/PC alloy under the trade name Geloy. This is not a comprehensive list by any means, just some common trade names.

Similar to actual cousins, ABS and ASA do share some DNA but they behave in different ways. Similar to Vance and Coy shown above, ASA is not nearly as popular as ABS with ASA volumes being from 1-5% of ABS volumes. Unlike Vance and Coy, however, ASA does have some redeeming qualities.

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