Function and Color Made to Measure
- Vytvořeno čtvrtek, 27.03.2014
Dust films on plastic surfaces, e.g. on automobile interiors, are disturbing for the visual impression. Dust accumulation can now be prevented or delayed by means of specially developed static inhibitors that are matched to the respective plastic. Moreover, the conductive inhibitor can be added together with the colorant in a single step.
Most plastics have a very low electrical conductivity, and therefore usually exhibit a strong tendency for charge accumulation on the surface, caused by contact with other materials. In many cases, the existence of surface charges leads to problematic effects, e.g. the coherence of plastic films or the adherence of dust due to electrostatic attraction. Furthermore, the resulting discharges are a considerable hazard, as they can interfere with the function of electronic equipment, as used in medical technology for example. Also with regard to explosion protection, electrostatic discharges are highly problematic. Similarly, dust accumulation on plastic components due to electrostatic charges represents a problem in the packaging industry, as the impaired visual impression of the article can have a negative effect on the customer's purchasing decision.
Also the automobile industry is intensively engaged with this problem, as dust accumulation on plastic parts of the automobile interior is highly undesirable. Based on the disadvantages mentioned above, there is a demand for antistatic properties of the plastic materials used.
Conductance only when Moist
In order to meet this demand, a range of substances can be used as additives. These so-called antistatic agents are divided into surface-active and volume-active substances. Surface-active antistatic agents are enriched at the surface by means of migration, whilst volume-active substances create a conductive network within the plastic matrix.
In the past, migrating antistatic agents were mainly used, as they produced the required effect already in low concentrations. Water molecules accumulate on these surface-active substances, thereby forming a conductive film on the surface, which enables the electrostatic charge to be distributed and discharged evenly. However, these systems do not provide a permanent protection against electrostatic charge build-up. Frequently, the active substances on the surface are sensitive to environmental influences, whereby they gradually lose their effect. What's more, they can be removed from the surface by mechanical means. One example involves the use of feed hoses in explosion-hazarded environments, whereby the German standard TRBS 2153 recommends the use of conductive wall materials. Here, migrating antistatic agents are problematic, because they can be removed during product transport, whereby the hoses lose their conductive properties.
Due to their mechanisms of action, migrating antistatic agents require a minimum level of air humidity. The higher the air humidity, the better is the antistatic effect. At lower humidity levels, the efficiency of these systems is significantly reduced, making them unsuitable for use in dry surroundings. The advantages of migrating antistatic agents are their low dosage, and a relatively low price.
Protection Against charge Build-up
For applications in which the disadvantages of migrating antistatic agents cannot be accepted, non-migrating systems must be used, which are not affected by air humidity and provide permanent protection from charge build-up.
Graphite, carbon black, and metal powder are typical non-migrating conductive materials that can be used as volume-active antistatic agents. Similarly, the use of carbon nanotubes or graphenes is conceivable, whereby their application is presently hindered by the price, amongst other reasons. Whilst carbon black or graphite can only be used for black plastic items, the use of metal powder is frequently not possible due to higher abrasiveness and difficult processing conditions.
Another class of volume-active conductive materials is formed by ion-conductive polymers, which are also suitable for use in colored plastic products due to their low inherent color. These also include polyether polymers, which create an ion-conductive network in the plastic matrix, providing an effective path to dissipate electric charges. These materials are the basis for numerous permanent antistatic agents on the market, which represent the current state-of-the art and find successful use in many applications.
However, compared with migrating antistatic agents, the high price and the comparatively high application concentrations are serious drawbacks, which is why many users are searching for more cost-effective alternatives for the established permanent systems. Hereby, a lower dosage is usually demanded for an equal performance.
Developments for Automobile Interiors
Depending on the application area of antistatic materials, additional product requirements often arise, which must be taken into account by new developments. Plastics that are used in automobile interiors must comply with corresponding automotive standards in which e.g. mechanical strength, light fastness or aging resistance are defined. Therefore, the influence of antistatic additives on the basic properties of the polymer must not bee too large. Taking these criteria into account results in a very extensive requirements profile for antistatic agents that are suitable for plastics in automobile interiors.
For evaluating a component's antistatic properties, measurement of the charge dissipation in accordance with the automotive testing standard PV 3977 is highly significant. For this, the component is charged electrostatically, and the resulting surface resistance is recorded against time. The more effective the antistatic properties of the component are, the faster will the charge be dissipated. Measurement of the surface resistances is also an effective and very simple method for assessing antistatic properties. Depending on the application area, a specific antistatic region is required, which must be determined accordingly for every application. Project experience and correlative know-how gathered with PV 3977 show that surface resistance should not be higher than about 1011 ohms in order to counteract dust accumulation due to electrostatic charges.
Depending on the polymer to be processed, various kinds of antistatic properties can come into question, whereby the physical properties, e.g. melting point and melt viscosity, as well as the product's chemical composition are decisive for the suitability and efficiency in the respective polymer. For automobile interiors, polypropylene (PP) is mainly used, as it features a very good price/performance ratio. Another frequently used material is (PC+ASA), a blend of polycarbonate and acrylonitrile styrene acrylate, which exhibits a high impact resistance and forms hard, scratch-resistant surfaces.
In cooperation with major German OEMs, the Grafe Group, Blankenhain, Germany, is presently working on new solutions for antistatic properties of corresponding plastics. Hereby, migrating as well as non-migrating systems with improved performance are in the focus of development work.
During development of migrating antistatic agents, not only higher efficiency is aimed for, but also an increase of the effective duration. The effect of migrating antistatic agents is defined primarily by their migration speed, which in turn depends greatly on numerous intermolecular interactions with the matrix polymer and other additives. Various migrating systems, which develop their effect over a longer period, have been developed for different target plastics, like the so-called high-performance antistatic (HPAS). This system is suitable for use in olefinic plastics such as polyethylene (PE) and polypropylene (PP). In cooperation with two major German OEMs, long-term tests were conducted with PP components, which showed that the antistatic effect is still present and practically unchanged after four to five years. At present, prototypes and pre-series components made of this product are undergoing final release tests. First results can be expected from the two OEMs in the coming years, in combination with model changes and new vehicle types.
New products with improved efficiency and lower application concentrations have also been developed in the field of permanent systems. In-depth understanding of the mechanisms of action of these systems permit selected basic materials to be matched optimally to the application area and the desired target polymer. In cooperation with OEMs newly developed permanent antistatic agents for PP and (PC+ASA) blends are being introduced at present. The development of permanently effective antistatic agents has resulted in a new ASP product series (ASP: antistatic permanent). Two newly developed ASP types for use in PP are currently in the testing phase. First investigations have shown that interesting resistance values for the antidust application can be obtained with dosages of about 6%. Similarly, two new ASP developments for a permanent antistatic effect of (PC+ASA) are currently undergoing the relevant tests. Compared with commercially available products, these types exhibit improved efficiency. The required resistance values are achieved with dosages of about 7% in (PC+ASA).
To an increasing extent, solutions for safe process management during component production are required, in which the color scheme as well as the functional properties of the target material can be obtained with the dosage of a single product. During development of these combination products (color and function) the Grafe company benefits from many years of experience in the fields of color masterbatch and plastics development plus know-how in plastics additivation. Moreover, a decisive role is played by the intermolecular interactions of all product constituents during development of combibatches. Due to physical-chemical interactions between additives and color components, particular care is required when they are combined. At present, two new products are being tested for applications with PP, which permit the desired coloring and simultaneous permanent antistatic component properties. The required dosages of these combibatches are about 9%.
Other combination products are being developed and tested for use in PC blends such as (PC+ABS) and (PC+ASA). By adding 10% of a new combibatch, it is possible to obtain the required material coloring, an effective UV protection, and a permanent antistatic effect for injection molded (PC+ASA) parts in a single dosing step – further additive batches are no longer required.
New antistatic agents with improved properties have been developed in order to effectively counteract dust accumulation due to electrostatic charges on plastic parts in automobile interiors. Suitable products based on different mechanisms of action have already been tested and assessed. The further development of migrating antistatic agents is aimed primarily at increasing the effective duration. For this, Grafe has introduced a new product for polyolefins – high-performance antistatic (HPAS) – which maintains its antistatic properties for more than four years, as demonstrated by long-term tests on PP components.
During development of permanently effective non-migrating systems, the focus lies on efficiency improvement. For use in PP and in (PC+ASA), new permanent antistatic agents have been developed and tested. These products are featured by particularly low dosage levels of additives.
Moreover, combination products have been developed for use in PP and in (PC+ASA), with which material coloring and permanent antistatic properties are achieved in a single dosing step.