From the viewpoint of design properties, metallic decorative members, in particular, decorative members having a mirror-like metallic gloss have been widely used in the casing of a mobile phone; the button of a switch; the casing of a watch; and the radiator grill, bumper, panel or the like of a vehicle.
As such decorative members, a decorative member that transmits radio waves (microwaves or the like) without adversely affecting the radio waves has been required for the following reasons:
(i) An antenna for transmitting and receiving radio waves is disposed inside the casing of a mobile phone.
(ii) An antenna for receiving radio waves is disposed inside the casing of a radio controlled clock that is provided with functions of receiving standard waves and of automatically correcting errors.
(iii) In a vehicle equipped with a radar device that detects the presence of obstacles or measures the distance between vehicles, and the like, the antenna of the radar device is disposed in the vicinity of a radiator grill or a bumper.
(iv) The frequency of radio waves used in communication devices (wireless PAN such as Bluetooth, UWB, and ZigBee) has been shifting to a high frequency range of from millimeter waves to microwaves, at which radio waves are readily affected by the decorative members, and thus these devices are prone to functional disorders.
For example, the decorative member can be prepared by providing a decorative film having radio wave transmissibility on the surface of a base of a decorative member. The following materials have been proposed as the decorative film and a decorative member.
(1) A decorative film and a mobile device having a vapor deposited film of indium or tin on the back surface of transparent resin film (refer to PTL 1).
(2) A decorative film including laminated film with a visible light reflectance of 30% or more where two or more kinds of resin layers are alternately laminated 30 times or more, and the number of layers having a wavelength of equal to or greater than 10 nm and less than 220 nm is more than the number of layers having a wavelength of 220 nm to 320 nm (PTL 2).
The decorative film in (1) has radio wave transmissibility and metallic gloss for the following reasons.
With respect to the metal-deposited film of indium or tin, it is known that because the metal is present as a minute independent island, a homogeneous metal layer can be recognized, which is observed visually, the size of one island is sufficiently smaller than the wavelength of radio waves which are passed through, and therefore radio waves can pass through.
The decorative film in (2) has radio wave transmissibility and metallic gloss for the following reasons.
A number of thin resin layers having different refractive indexes can be recognized as metallic luster, and therefore radio waves can pass through because a metal conductor is not used.
However, with respect to the decorative film of (1), if the thickness of the metal-deposited film is increased in order to attain a sufficient metallic gloss, and the islands become partially connected to each other due to the pressure applied to the metal-deposited film at secondary forming when a decorative member is produced, it forms a network that serves as a good conductor, and thus the reflection or absorption of radio waves occurs according to the frequency thereof. Therefore, as to whether a product using the decorative film of (1) does not impede the straightness of the radio waves, or other disorders do not occur, all of the products using the decorative film are required to be inspected, and therefore it has a low productivity. In addition, tin is prone to oxidation, chlorination, or the like, as a result of which the metallic gloss thereof is lost over time. On the other hand, indium is not readily available and highly expensive.
The decorative film of (2) has a low productivity and the high cost of a laminated film. In addition, the heating stretch moldability is deteriorated and therefore the decorative film cannot be formed into a desirable shape. Further, through hot forming, the refractive index of the decorative film and the balance of the respective layers are changed, and therefore a loss of metallic gloss occurs.