An increasing fraction of the stock is recycled, finds Will Dalrymple
What makes ABS a unique and versatile material is down to being composed of three different monomers, states Dan Jarvis of distributor Plastribution (01530 560560). He adds: “Each one of those brings something to the party. Acrylonitrile brings heat resistance and chemical resistance. Butadiene is the rubbery bit: it adds impact strength and the ability to bond things (paint and metal plating). Styrene brings stiffness and glossiness. You can actually alter the proportions of each to give you different properties in the same material.”
Another way of affecting material properties in polymers like ABS is varying the molecular weight, which refers to the number of monomer units in the finished polymer chain. That process, which occurs during manufacture, involves having sufficient catalyst and allowing the chemical reaction take place for enough time. It’s not straightforward, explains Jarvis.
“Typically, when you make a polymer – ABS for example – you end up with a range of molecular weights. It is incredibly difficult to be very accurate in getting all of the chains the same molecular weight. And in fact, if you didn’t have the very low molecular weight chains, it would be very difficult to process polymers. The low molecular weight portions act as process aids, so it’s important to have a broad band of molecular weights. But you can achieve very specific chain lengths by clever catalyst technology, stopping the reactions at the right time, or post-reaction by cleaving the chains to a set length, which is called control rheology.”
Molecular weight affects physical properties, particularly flowability but also impact strength. Jarvis observes: “If you want something that in its molten state is very fluid, to make thin-wall sections or for a part that has a long flow path, that polymer would need to have shorter chain length giving a lower molecular weight. Very long chains and high molecular weights typically have lower flowability, and are more viscous.”
Different processors and different processes have different flowability requirements. For ABS, melt flow index (MFI) or melt flow rate (MFR) measure the thinness of the polymer. There are two primary processes of forming ABS: injection moulding, for smaller intricate parts, and extrusions into rod or sheet, which might be vacuum-moulded into parts such as refrigerator panels. For injection moulding, a high MFI is required so the material flows easily into the crevices of the mould tool; whereas extrusions tend to be thicker.
Just the combinations of monomers and varied molecular weights produce a huge variety of properties. For example, its exclusive ABS supplier, Chi Mei of Taiwan, offers multiple standard grades of its Polylac-branded ABS, as well as grades for particular applications. Those include: high heat (four); electroplating (two); chemical resistance (one); high flowability (eight); standard colours (four); extrusion (six); low monomer residue (one); and antimicrobial (one).
PLUSES AND MINUSES
For Dr Jason Jackson, technical specialist at Goodfellow (01480 424800), it is the properties of the material that make it so useful. He describes ABS as hard for a thermoplastic, reasonably tough, and which retains impact resistance even down to low temperatures. Its high glass transition temperature (about 104-105ºC) provides a useful working range from –20 to 80ºC.
On the other hand, disadvantages include poor solvent and UV resistance. “Car interiors particularly you don’t want to degrade in a few years,” he observes. ABS can have poor bearing strength, which means that it can be worn down with friction. “Also, when it melts, it can catch flame and be smoky – in which case it can be quite nasty.” He points out, however, that additives included in the mix can compensate for those natural weaknesses.
And indeed, Chi Mei offers seven ABS grades with some degree of flame-retardance. According to Jarvis, polymers’ fire resistance is normally rated in ‘HB’ (horizontal burn), named for a test that measures the spread of burn of a sample held horizontally above a flame. Vertical tests hold a sample in a different orientation.
One of the highest levels, V0, means that the polymer self-extinguishes in 10 seconds and doesn’t form flaming drips. Jarvis says that this can be readily achieved with ABS, rendering it suitable for aerospace and automotive applications.
ABS has a general reputation of gluing and bonding easily, and can be metal-plated as well. Delo recommends acrylates, epoxy resins, methacrylate and polyurethanes to bond ABS. It says that the material doesn’t really present any bonding challenges, and good bonding strengths are achievable.
According to Peter Swanson, managing director of adhesives distributor Intertronics (01865 842842), because the surface energy of ABS is relatively high for a plastic, circa 48Dyne/cm2, bonding is not a problem. He continues: “That means choosing an adhesive to bond depends on other factors, such as the process, performance factors in moisture resistance, temperature capacity, how soft or hard you want the bond.”
Peter Kraushofer, technical support specialist, Bostik Born2Bond (01785 272625), says that ABS’s bonding ability depends on the quality of the material, where it comes from and its surface properties. He points out that releasing agents for injection-moulded material can affect bonding properties. And, through hard-won experience, he advises that even with ABS, potential users should always test the material and the adhesive first. “I had the experience of an ABS part that wouldn’t bond with SMP (silane-modified polymer) adhesive. If I had bet on it, I would have lost a lot of money!”
Ultimately, his team couldn’t resolve that bonding issue chemically. Kraushofer continues: “Plasma surface treatment helped a bit, but to deal with it, we ultimately recommended making a design change in the bond line. We suggested that the customer go from flat to flat to an interlocking solution, to increase surface area and get better mechanical properties.”
Jarvis describes ABS as ‘easily recyclable’, and says that the availability of recycled ABS stock material is rising. The biggest issue is having a mechanism to collect it. Also, as the material is usually just a part of a multi-material object, it must be separated somehow from them. Jarvis says that there are a few companies selling post-consumer ABS and polystyrene scavenged from white goods, but that market remains niche.
Also, the process of recycling ABS can’t be completely circular, as the initial material properties of the material tend to be lost as it is heated and cooled repeatedly. So recycled stocks often contain some quantity of virgin material or additives to improve their performance.
Another issue is colour. Colourants are typically added in batch processing at the moulding or extrusion stage, says Jarvis. The ‘natural’ colour of untreated ABS resembles that of butter. Black or dark grey material, popular for electronics, cannot be coloured, which complicates the recycled ABS supply chain.
BOX: New ABS production in Asia
A new, state-of-the-art Chi Mei production facility has started up in Zhangzhou, Fujian province, China. The initial capacity of the plant is 450,000 tonnes annually, which is to be increased gradually to 600,000 tonnes annually. This will increase the annual ABS production capacity of Chi Mei to 2.6 million tonnes, consolidating the company’s position in the global ABS supply chain. Advanced polymers business manager at Germany-based distributor Biesterfeld Plastic (01235 822040) said: “The new plant will mostly serve the southern China region and the market on the Chinese mainland. But the additional production will also have a positive impact on the European market, as it will free up further capacity at the main plant in southern Taiwan for export to Europe, North Africa and Latin America.”