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Survey of chemical substances in headphones and hearing protection aids 9 Material descriptions
In the following sections a short, general description is given of the materials which are identified in the survey phase and in the analyses, and which additives that are typically used in manufacturing. It shall be emphasized that most of the descriptions are generic, i.e. that they deal with the material in general and not specifically in relation to their use in headphones and hearing protection aids. It shall also be emphasized that in the survey of earplugs (Karbæk (2003), Pors and Fuhlendorff (2003)), other, potentially problematic substances are identified; these substances are not addressed in the following sections. This emphasizes that it is difficult/impossible to give an unambiguous assessment of which materials/substances that potentially can affect the users’ health. The material descriptions below of PVC and PUR are primarily based on Schmidt (2006), supplemented in relation to artificial leather with Zürrig & Kruse (2005) and Hioki (without year). 9.1 PVC (polyvinyl chloride)PVC is basically a stiff homopolymer. In practise, PVC will always be modified to achieve the desired properties of the product. In the production of PVC a wide range of additives can be used and it is often the PVC manufacturing company which makes its own compound by adding the necessary substances and substance mixtures to the basic polymer. PVC is compounded with plasticizers, stabilizers, fillers and other additives, dependent on the wanted properties. For headphones, PVC is used in many different degrees of hardness, dependent on whether it is a bearing component or soft parts. This, together with the factor that the final compound is mixed by the individual companies, makes it impossible to give a general and satisfactory recipe for PVC for headphones. By adding plasticizers, PVC can be modified to all degrees of hardness, from the totally stiff PVC to materials with a nearly rubber-elastic character. Softened PVC constitutes a very large variation of compounds with a broad spectrum of properties. Softened PVC is also used in mixtures with other polymers, e.g. SAN, ABS, synthetic rubber and acrylics. The content of plasticizer in PVC can be as high as 70 %. The most used plasticizers are phthalates. Especially DEHP (di (2-ethylhexyl) phthalate) is often used, but also DINP (di-isononyl phthalate), DIDP (di-isodecyl phthalate), BBP (butyl benzyl phthalate) and DBP (dibutyl phthalate) are generally used for many purposes and a combination of phthalates is also found frequently. There are many alternatives to phthalate plasticizers, among others in the form of adipates, citrates, phosphate compounds, epoxidized soy bean oil and others. The application of these (and other) plasticizers is expected to increase due to the environmental and health authorities’ focus on the undesirable properties of the phthalates but the properties of the alternatives must also be taken into account. The thermal stability of PVC is highly limited which makes it difficult to process the material. Therefore, a heat stabilizer will in general always be added. For many years, heavy metal compounds, especially lead-based, were the traditional choice for a heat stabilizer, but the undesirable environmental and health properties of the heavy metal have resulted in a strict regulation of their use. Instead, organic and inorganic metal compounds (e.g. calcium, barium and zinc-based), possibly together with organic phosphites and polyhydroxy compounds are applied as secondary stabilizers. As co-stabilizers dipentaerytriol, pentaerytriol, sorbitol and epoxidized soy bean oil are used. These substances are only effective with metal-based stabilizers. UV-stabilizer and antioxidant are added to prevent decomposition of the plastic during influence of light. Sterically hindered amines (HALS) are together with octabenzone a frequently used UV-stabilizer in PVC. Four main types of antioxidants can be distinguished, which are used in many different plastic types in amounts from 0.5-20%: - Phenols. Monophenols, bisphenols, thiobisphenols and polyphenols constitutes the largest amount of primary antioxidants. In polyphenols like PE and PP, they are typically used in amounts of 0.05-0.2% while in styrene-based plastic materials they are applied up to 2%. - Amines. Amine-based antioxidants are especially applied in synthetic rubbers in concentrations between 0.5 and 3%. - Phosphites. Phosphites are used alone or together with phenols or amines. In polyolefines 0.05 to 2% is used, in high-impact polystyrene 0.05-1% (in certain cases maybe up to 2%), and in ABS up to 3%. - Thioesters. Thioesters are important to prevent changes in molecule mass at long-term exposure to high temperatures. Dilaurylthiodiproprionate and distearylthiodiproprionate are commonly used in polyolefines like e.g. PE and PP in concentrations of 0.1-0.3%. In unsaturated polymers somewhat larger concentrations are required. Other additives are barium, calcium and zinc compounds (0.05-3%), pigments/colouring agents (up to 10 parts per hundred parts of PVC), fillers (e.g. chalk, 20-60 parts per 100 parts PVC), flame retardants (e.g antimontrioxide and tricresylphosphate (up to 10%), lubricants and biocides. 9.2 PUR (polyurethane)In the production of PUR both aromatic and aliphatic isocyanates are used and the finishing quality of the product is to a large extent determined by the polyol with which the isocyanate is mixed. Polyester and polyether polyol are most frequently used but also polycarbonates are applied. For flexible PUR materials, an order of magnitude of 34% diisocyanate, 63% polyol and 3 % additives is applied. There is a large selection of polyisocyanates and polyoles available which makes a large variation in different properties possible. In practice, toluene diisocyanate (TDI) and 4,4’-di-phenylmethane diisocyanate (MDI) are most often used where the latter gains ground due to a smaller toxicity. As additives, among others phenol-based antioxidants as heat stabilizer and sterically hindered amines (HALS) and benzotriazoles as UV-stabilizer are used. Other additives are pigments, fillers, flame retardants (phosphorous- or halogen-based (e.g. tetrabromophthala-diol, pentabromo-diphenyloxide and di-bromoneophenylglycol)) and foaming agents for foams (most often CO2). Plasticizers, e.g. phthalates, can be applied if the material has to be very soft. 9.3 Artificial leatherThe term ”artificial leather” is not unambiguous as it covers different combinations of textile materials and plastic coatings. The textile material can be based on a number of different fibres e.g. natural fibres like cotton and acetate fibres, or artificial fibres like polyamide, polypropylene and polyester. The fibres are prepared through weaving, knitting or as non-woven material to a surface reminding of natural leather. Subsequently, the textile is coated in one or more steps with plastic, most often in the form of PVC or polyurethane (PUR), but also ethylene-vinyalacetate copolymers, butadiene copolymers, polyamide, ABS and thermoplastic olefines (PP/EPDM) are used as coating materials. The result is a product which both with regard to look and touch looks like natural leather of different qualities. The technical properties vary naturally as a function of the basic materials and are not described in details here. It is FORCE Technology’s assessment that the composition of the materials getting into contact with the ear is usually the same for artificial leather products as for the corresponding plastic products without a layer of textile fibres. The basis of this assessment is that many of the properties being added to a polymer by means of additives are desirable for both artificial leather products and plastic products. Therefore, separate descriptions for the two types of materials have not been prepared. 9.3.1 Artificial leather with PVCArtificial leather with PVC coating is probably the most common type of artificial leather. The PVC layer is coated in a thickness of 0.05-0.75 mm through a calendering process. By coating with PVC paste of emulsion-polymerized PVC a layer thickness of between 0.007-0.05 mm can be reached. Foamed PVC can be heated in an oven to decompose the foaming agents. Normally, artificial leather with PVC must be lacquered to obtain the right feeling of leather quality and to prevent that the softener migrates to the surface. The lacquering also improves the light and heat stability. The applied lacquer is often solutions of polyacrylate and PVC in organic solvents which are applied in a layer thickness of 3-20 µm. 9.3.2 Artificial leather with PURArtificial leather with polyurethane (PUR) is the second most frequent quality. With PUR as coating material it is not necessary to apply softeners and in general PUR based artificial leather is considered to be a quality product. At a so-called dry-coating process, a coating of two or three layers is produced. A linear polyurethane is applied for top and intermediate layer while a dual-component polyurethane constitutes the layer which sticks to the substrate. The linear polyurethane is dissolved in e.g. dimethylformamide, methyl ethyl ketone, 2-propanol or toluene which is evaporated in an oven. In the wet coating process a 7-20% solution of polyurethane in dimethylformamide is applied. 9.3.3 Other types of artificial leatherEthylene-vinyl acetate copolymers and butadiene copolymers for artificial leather production are prepared from a solution or in dispersion. PVC (without plasticizer), ABS and thermoplastic polyolefines (e.g. a mixture of PP and EPDM) can be coated directly as hot-melt or as a film which is laminated to the substrate. Polyamide coatings contain normally a plasticizer but are easily affected by water and alcohol and therefore the material does not have a broad application. No information about which additives being typically used in these other types of artificial leather has found. In Table 9 and Table 10 an analysis of the content of metals and selected chemical compounds in ethylene-vinylacetate (EVA) and PP/EPDM is found. The values do not indicate application of additives to a large extent but it must be emphasized that the analyses is carried out without previous knowledge of the materials. Therefore, it might be possible that other additives are used, without being targeted for analysis. 9.4 SiliconeApparently, silicone is the most applied material for the parts of In-ear headphones and hearing protection aids getting into contact with the skin. Due to its mouldability it is possible to fit the material to the individual who is to use the product, whether it is hearing protection aids or advanced headphones. The description below of the material is primarily based on Moretto, Schultze and Wagner (2005), Butts et al (without year) and Morton (without year). Silicone is a rubber type with an unusual molecular structure, with changing silicon and oxygen atoms where each silicon atom carries one or several organic groups, normally methyl or phenyl groups. The most applied silicone is polymethyl siloxane which due to its eminent biocompatibility is one of the most used materials for implants. In the polymerization process platinum compounds are most often used as catalyst but peroxide compounds (e.g. dicumyl peroxide) can also be used. As filler, e.g. silicates, quartz powder, talcum powder and calcium silicates, in amounts of 5-38% (w/w) can be applied. As stabilizer, metal oxides, salts of iron, titanium, zirconium, nickel, cobalt, copper and manganese as well as carbon black in amounts of 0.001-10% (w/w) are used. As these substances primarily give a better stability in relation to heat, chemicals and fire, it is doubtful whether they are applied to silicone being used in hearing protection aids and headphones. 9.5 TextilesA few headphones use textile covering for those parts which come into contact with the skin. The textile materials are stated to be e.g. velour and velvet, but this does not give an unambiguous identification. According to relevant chapters in Ullmann’s Encyclopedia of Chemical Technology, it can for example be acrylic, nylon (polyamide), polyester and acetate fibres which in principle can contain both residue monomers and different ancillaries from the fibre production. It is assessed that textiles for headphones basically do not differ from textiles for clothing. The criteria for textiles in the European eco-labelling scheme, the Flower, give an indication of the order of magnitude of chemical compounds which can be found in the mentioned textile types:
Besides the content of residue monomers there is a risk of exposure for a very large range of substances which are applied in one or more processes from raw material production to the finished product. It is beyond the frames of this project to go into details with which substances it might be and in which concentrations. However, some few examples can be given:
The above examples shall not be regarded as a list of real problem, but alone as an indication of which problem areas that can be seen in relation to the use of textiles. It is prohibited to use a number of the above-mentioned substances, e.g. chlorophenols, PCB and azo-dyes and therefore it is not very probable to come across these substances in everyday products. On the other hand, formaldehyde is often applied in one or several production steps and therefore the substance is identified when analyzing textiles. Finally, flame retardants are often applied in textiles, e.g. for residence purposes. 9.6 ABSABS is a terpolymer, made of three different monomers: acrylonitrile (A), butadiene (B) and styrene (S). By varying the percentage distribution of the three monomers and adding different additives it is possible to change the properties of ABS. Normally, the individual monomers are distributed as follows:
As antistatic agents alkyl sulphonate and ethoxylate amine are applied in concentrations of 1.5-3%. Fillers in the form of glass fibres or glass beads can be applied in 20-40%. Ca-carbonates can also be applied in concentrations of 20-30%. As flame retardant, octabromodiphenyloxide, tetrabromobisphenol A, bis(tribromophenoxy)ethane and bromated epoxy oligomers can also be applied. As lubricants, amide wax, zinc stearate and glycerol monostearate can for instance be applied. Antioxidants are normally applied in the order of magnitude of 0.25-1%, e.g. in the form of phenol-containing antioxidants in combination with a thiosynergist. Phosphate-based compounds can also be applied in combination with the other constituents. 9.7 Rubber and thermoplastic elastomersThere is a wide range of materials under the general heading “rubber”. The generic names of the most important are, with the chemical name in bracket:
Some of these rubber types are also characterized as thermo plastic elastomers, among others EPDM, silicone and polyurethane, which are described above, e.g. as an element in artificial leather. Unlike conventional rubber types, the thermo plastic elastomers can be prepared as thermo plastics, e.g. through extrusion and injection moulding, and therefore they can also be used without a textile substrate for components in headphones and hearing protection aids which get into contact with the ear. In this connection it is worth observing that contrary to rubber, these thermo plastic elastomers shall not necessarily be vulcanized to reach the desired material properties. Therefore, there will not be a risk of exposure to e.g. sulphur which is a frequently applied vulcanization agent. Besides the thermo plastic elastomers it is known that also chloroprene is used in headphones even though the known case is a foamed type which in the component in question helps keeping a head spring band in place. The other rubber types are possible to use but from overall descriptions of their application areas their occurrence in headphones and hearing protection aids is assumed to be very limited.
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