Good mechanical properties, wear resistance and low-temperature flexibility
Oct 01, 2019 Read time: approx. MinutesMinute
Silicones that ensure Soft-Touch Surfaces
Consumers are increasingly expecting plastic surfaces to feel silky-soft and retain a high-quality appearance even after intensive use. Thanks to a new silicone-based additive, compounders can easily reduce the hardness of thermoplastic elastomers and ensure pleasant tactile properties.
Thermoplastic elastomers (TPEs) are known for their good mechanical properties, their wear resistance and low-temperature flexibility. They can be found in many items used in sports and leisure, as well as in microelectronics, machinery manufacturing, automotive and electrical engineering, and medical technology.
Standard thermoplastic polyurethane (TPU) elastomer grades produced and processed in large quantities are generally relatively hard – typically with Shore hardness values ranging from around 80 A to 45 D. Components made from such grades often feel a bit rubbery and dull. However, good tactile properties are an important quality characteristic for items that consumers touch frequently or wear directly on their bodies.
“There are nevertheless only a small number of soft TPU grades on the market,” says Dr. Peter Randel of WACKER SILICONES’ Technical Marketing for Plastics & Coatings. “They are difficult to manufacture. In many cases, they contain plasticizers that can migrate out of the plastic and contaminate other objects.”
Plastics manufacturers and compounders are thus looking for ways of making TPU softer and of giving TUP precisely those tactile properties that consumers expect without noticeably diminishing other properties of the plastic. The ultimate goal is to open up new application areas for the material. An easy way of doing this comes in the form of WACKER’s new silicone-based additive GENIOPLAST® Pellet 345, which is being unveiled to a broad audience of experts for the first time at K 2019, the International Trade Fair for Plastics and Rubber.
Silky-soft and dry
When consumers choose a product, its appearance and tactile properties are often the deciding factor. “Soft-touch surfaces are currently popular,” explains Randel. “These are surfaces that feel silky-soft and dry and are pleasant to touch.” Soft-touch surfaces are not just in demand for plastics in automotive interiors. Consumers are looking for this silkysoft feel more and more in other applications, too, e.g. for smartphone covers and wearables. Wearables are microcomputers worn on the body that are linked to other devices, such as smartphones, and record, assimilate and evaluate data on the user’s activities. Examples include activity trackers, fitness wristbands and smart watches.
These kinds of microcomputers are worn as an accessory on the arm. Not only must the casing and wristband of a wearable be sufficiently robust and elastic to protect the expensive device, but it must also look fashionable and be soft enough to satisfy the high demands in terms of how they should feel.
Prompted by requests from the plastics industry, WACKER decided to develop a siliconebased additive that primarily reduces the hardness, as well as the surface friction, of thermoplastic polyurethane elastomers (TPUs) and increases their elasticity. The aim was to allow TPUs to be modified in such a way that the functional and tactile properties even satisfy high consumer expectations.
Silicone additives are known for their friction-lowering effect and have been employed by the plastics industry since the 1970s. It is, however, difficult to use a conventional silicone as an additive for TPEs in large quantities.
“Silicones are generally non-polar, while polyurethanes are polar,” explains Dr. Oliver Schäfer, head of an applications laboratory at WACKER SILICONES in Burghausen. The two materials are thus incompatible. When a silicone is mixed in with a polyurethane, you get a two-phase polymer mixture. The silicone forms soft islands, which are dispersed throughout the hard polyurethane matrix. Due to this incompatibility, it is not at all easy to incorporate a conventional silicone into a polyurethane matrix – a difficult hurdle to overcome for an application as an additive.
That’s why researchers at WACKER pursued the idea of enhancing the compatibility of silicone with polyurethane by incorporating polar groups. Their approach was based on the assumption that silicone modified in this way should be able to interact more strongly with polyurethanes, produce better dispersion in the polyurethane matrix and stabilize the bond between the silicone and polyurethane.
Development work yielded a polar-modified silicone copolymer, made up of elastic, soft silicone segments and functional, polar organic polymer segments. The amount and molecular weight of the copolymer segments were optimized to achieve the properties sought for in the end product, while ensuring that the silicone copolymer is readily processed. The result was the new additive GENIOPLAST® Pellet 345. With this product, Wacker Chemie AG complements its range of silicone-based performance additives used in the plastics industry for compounding thermoplastics.
“Unlike conventional silicone elastomers, our new modified silicone is not crosslinked chemically, but physically,” emphasizes Schäfer. Non-covalent bonds form between individual segments of this silicone copolymer’s molecules which come into contact with each other. It is these bonds that hold the molecules firmly together. This type of bonding, though, is weaker than that of chemical crosslinking. The bonds involved can therefore be systematically broken by heat, but reform on cooling. The new additive can thus be processed thermoplastically. The additive was tested extensively at Dr. Schäfer’s applications laboratory. Testing focused not only on the additive’s processing properties, but also on its effects. Does it modify the plastics’ technical properties in the desired way? How does it affect the tactile properties? What side effects can be expected? The results confirm the viability of the researchers’ approach.
Transmission electron microscopy images show that the new additive is dispersed not just evenly, but also very finely, throughout the polyurethane matrix – an indication of the silicone’s improved compatibility with the matrix. The additive’s particle size in the matrix ranges from 200 to 400 nanometers. If a conventional silicone was incorporated into a thermoplastic polyurethane elastomer (TPU), the silicone domains formed would be considerably bigger.
“Since GENIOPLAST® Pellet 345 is fairly compatible with the polar TPU matrix and exhibits thermoplastic properties, it can be processed easily with hardly any effort involved,” explains Schäfer. Both the additive and the TPU are supplied in pellet form and can be melted together in conventional twin-screw extruders typically used for compounding thermoplastics. Contrary to competing modification technologies based on conventional silicones, neither specialty equipment nor particular process-engineering measures are required to incorporate the silicone product into the polyurethane matrix.
Polyurethane test pieces
In order to test the new additive’s effects, WACKER technicians made test pieces of TPEs to which they had added varying dosages of GENIOPLAST® Pellet 345. The polyurethanes used were commercial TPU grades with hardness values ranging from 70 to 90 Shore A and tensile strengths of 25 to 50 megapascal.
The tests revealed that the dosage of the new additive very strongly influences the effects. In low amounts – up to around 10 percent – the additive predominantly affects the surface properties and only has a minor effect on hardness, strength and elasticity of the plastic. A low additive dosage lowers the coefficient of dynamic friction (DCoF value) and improves the abrasion and scratch resistance, for example. When 5 percent GENIOPLAST® Pellet 345 was added, the amount of abraded material was reduced by an average of around 60 percent in an abrasion test as per ISO 4649 A.
These enhanced surface properties have a positive effect on smartphone covers, for instance. Many users carry their phone in their pocket and pull it out dozens of times every day. The surface of a particularly abrasion- and scratch-resistant protective cover looks good for longer. What is more, lower surface friction helps to remove the surface’s rubber-like nature and give the cover a pleasant feel. With the addition of around 10 percent and more, the mechanical properties and hardness of the plastic change noticeably. The more GENIOPLAST® Pellet 345 is incorporated into the TPU, the softer the plastic. The hardness decreases in almost linear fashion with the amount added. As a rule of thumb, Dr. Schäfer’s lab team found that an addition of 10 percent reduces the hardness of the thermoplastic by around five points on the Shore-A scale.
Elongation at break increases with the addition of additive and generally reaches its maximum at a dosage of 10 to 20 percent. The increased extensibility reflects a higher elasticity, representing a desired effect. A smartphone cover must, for example, be elastic enough to reliably protect the delicate phone against impacts.
However, the applications engineers also observed that the plastic’s strength lessens at dosages over 10 percent – an undesired side effect that cannot be avoided and also occurs with competing technologies. If the amount of GENIOPLAST® Pellet 345 used is increased from 10 to 30 percent, the tensile strength may be halved, for instance. Compounders can compensate for the reduced tensile strength by using a suitably tear-resistant TPU grade as the matrix. Benchmarking of soft TPU compounds of the same hardness showed that the reduction in strength is comparatively low with the new additive. “In any case, the strength values of compounds made with GENIOPLAST® Pellet 345 were considerably better than those of tested reference products,” says Schäfer. In terms of the visual appearance of a plastic item, the tendency to stain plays a key role alongside the scratch and abrasion resistance. It’s precisely smartphones, wearables and other mobile consumer electronics devices – together with their cables – that often come into contact, during use, with substances that can discolor the surface. Mustard and the indigo dye used in jeans can produce particularly strong and stubborn stains.
Major consumer electronics companies have compiled lists of substances whose discoloring effect is checked in staining tests. WACKER’s applications engineers carried out the stipulated tests with the listed substances and found that the new additive lessens the undesired color changes in TPEs.
The new additive does not alter properties specified by the polyurethane matrix. The TPU thus retains its naturally good bonding properties on numerous polar thermoplastics. The additive does not influence chemical or thermal stability either.
As adhesion remains good, a blend of TPU and GENIOPLAST® Pellet 345 can be used as a soft component for hard/soft combinations, as found, for example, in toothbrush handles. “Such articles can be produced cost-effectively by two-component injection molding, with a polar, hard thermoplastic serving as the hard component,” explains Randel, who is responsible for the marketing of the new additive.
Permanently attractive appearance
With GENIOPLAST® Pellet 345, manufacturers of TPEs can strengthen their position on the fast-growing market of mobile consumer electronics and microelectronics. “These kinds of compounds can find use in all items that consumers expect to have a soft-touch surface and permanently attractive appearance,” he added. Sheathing for headphone or charging cables, casings of laptops, tablet computers and gaming consoles, tool and tennis racket handles, and handle sections of ski poles all offer a wide range of applications. As GENIOPLAST® Pellet 345 is bonded well to the polyurethane matrix, it cannot migrate out of the plastic either. In Europe, it has already obtained approval for food-contact applications.
According to WACKER researchers and applications engineers, GENIOPLAST® Pellet 345 can, in principle, also be used to modify thermoplastic polyamide elastomers (TPAs) and thermoplastic copolyester elastomers (TPCs). Respective tests are currently being conducted at WACKER’s applications labs. “Initial results are promising,” explains Schäfer.