Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2000-04-12
2004-10-05
Seidleck, James J. (Department: 1711)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C428S001200, C252S299010
Reexamination Certificate
active
06800337
ABSTRACT:
The invention relates to a heat-insulating coating which comprises one or more cholesteric IR-reflecting layers.
The problem of screening against thermal radiation is important in particular in connection with the insulation of residential, office or industrial buildings. Buildings with a generous expanse of windows heat up rapidly, especially in summer and in southerly regions in particular, to such an extent that they have to be cooled at considerable energy cost by air conditioning units.
Common techniques of heat insulation, especially for screening against thermal radiation in the wavelength range between 800 nm and 2000 nm, are based on the absorption of the radiation by appropriate dyes or pigments. However, a large part of the energy absorbed is passed on by conduction to the area or item which is to be insulated.
The use of materials which substantially reflect thermal radiation is also known.
For this purpose there is widespread use of special dyes or pigments, but also graphite or gold, as broadband absorbers or reflectors.
Examples of dyes employed in this context are naphthalocyanines having broadband absorption in the infrared (IR), or else laked polymethine dyes. However, a serious disadvantage of the IR-absorbing dyes is that they exhibit marked absorption in the visible wavelength range as well, with the result that a considerable reduction in transparency is observed. The radiative energy absorbed is transformed into thermal energy which is dissipated by conduction.
Graphite, gold, silver or indium-tin oxide (ITO), which are also employed as absorbers or reflectors for IR radiation, have comparable disadvantages. Here too, especially in the visible region of the spectrum, there is very little transparency. Only highly precise and hence extensive production of extremely thin layers ensures a sufficiently uniform and high level of transmission in the visible wavelength range. Metal layers of this kind are generally applied by chemical or physical vapor deposition techniques (CVD or PVD), which are highly complex.
It is likewise known that cholesteric liquid-crystalline substances are also able to reflect light in the IR region of the electromagnetic spectrum. Cholesteric (chiral nematic) liquid crystals have already long been known. The first example of such a material was discovered by the Austrian botanist F. Reinitzer (Monatshefte Chemie, 9 (1888), 421). It is the chirality which determines the existence of cholesteric phases. The chiral moiety can either be present in the liquid-crystalline molecule itself or can be added as a dopant to the nematic phase, thereby inducing the chiral nematic phase. The chiral nematic phase has special optical properties: a high optical rotation and a pronounced circular dichroism resulting from selective reflection of circularly polarized light within the nematic layer. A consequence of this is that not more than 50% of the incident light having the reflection wavelength is reflected. The remainder passes through without interacting with the medium. The direction of rotation of the reflected light is determined by the handedness of the helix: a right-handed helix reflects right-circularly polarized light, a left-handed helix left-circularly polarized light. By altering the concentration of a chiral dopant it is possible to vary the pitch and hence the wavelength range of selectively reflected light of a chiral nematic layer. There is a direct relationship here between the reciprocal of the observed pitch p and the concentration of the chiral compound (x
ch
):
1/p=HTP x
ch
where HTP denotes the helical twisting power of the chiral dopant.
U.S. Pat. No. 4,637,896 discloses cholesteric liquid-crystalline compounds, based on cholesterol derivatives, and photopolymerized cholesteric coatings which comprise these compounds in copolymerized form. The cholesteric films described have reflection maxima which are predominantly within the visible wavelength range. However, two examples are also given of colorless films whose reflection maxima lie at 950 and 1260 nm respectively. Owing to the narrow breadth of reflection, however, these films are not suitable as a heat-insulating coating.
U.S. Pat. No. 5,629,055 discloses solid cholesteric films based on cellulose. The films are obtainable from colloid suspensions of cellulose crystallites, said colloid suspensions being prepared by acidic hydrolysis of crystalline cellulose. The solid films have cholesteric properties and their reflection wavelength is said to be adjustable over the entire spectral range from infrared to ultraviolet. The materials described are proposed in particular for use as optical authentification materials, since printing or photocopying techniques are unable to reproduce the cholesteric effect.
U.S. Pat. No. 5,352,312 describes a method of insulating rocket engines against heat and corrosives. The method comprises the use of an ablative insulating material which comprises a thermoplastic liquid-crystalline polymer. The liquid-crystalline material, however, is not cholesteric and the insulating action is based on the ablative effect and not on the reflection of thermal radiation.
U.S. Pat. No. 5,016,985 discloses an infrared filter comprising a broadband infrared filter element and a cholesteric liquid-crystalline filter element. The significance of the cholesteric filter element lies in particular in its ability to block infrared wavelengths in a precise, narrow band. The infrared filter can be used, for example, in night vision equipment.
It is an object of the present invention to provide heat insulation media which are easy to prepare, which are almost completely transparent in the visible range of the electromagnetic spectrum and which absorb very little in the near infrared and in the visible wavelength range of the electromagnetic spectrum.
We have found that this object is achieved by coatings which comprise at least one cholesteric IR-reflecting layer.
The present invention therefore provides a heat-insulating coating comprising one or more cholesteric layers and reflecting at least 40%, in particular at least 45%, of the incident radiation in the infrared wavelength range, preferably above 750 nm and, in particular, in the wavelength range from 751 nm to about 2000 nm.
The heat-insulating coating of the invention has a series of surprising advantages:
a) incident radiation in the visible wavelength range is transmitted almost completely, so that the coating appears transparent.
b) Incident light in the infrared region of the electromagnetic spectrum is very largely reflected and not absorbed, so that the object to be insulated does not heat up through conduction.
c) There is broadband reflection of incident thermal radiation, allowing the efficient insulation of articles.
d) The thickness and uniformity of the coating can be varied within a wide range without significantly effecting its insulating properties, with the result that its preparation is considerably less complex than that, for example, of metal-containing reflective coatings.
e) The use of ecologically and toxicologically objectionable metals is avoided.
f) The starting compounds for preparing the coating are readily available industrially; their use is thus generally less expensive than that of, say, gold or silver in reflective coatings.
The heat-insulating coating of the invention preferably transmits at least 80%, in particular at least 90%, of the incident radiation in the wavelength range of visible light, i e. approximately in the range from 390 nm to 750 nm.
Particular preference is given to a heat-insulating coating of the invention which comprises two or more, preferably from about 2 to 20 and, in particular, about 2 to 10 cholesteric IR-reflecting layers. With particular preference the layers have different reflection maxima in the wavelength range >750 nm. With very particular preference the heat-insulating coating of the invention comprises two or more cholesteric layers, preferably a number of cholesteric layers which can be divided by 2, such as 2, 4, 6, 8 or 10, th
Ishida Hiroki
Meyer Frank
Schneider Norbert
Schuhmacher Peter
Siemensmeyer Karl
BASF - Aktiengesellschaft
Bissett Melanie
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seidleck James J.
LandOfFree
Thermal insulating coating does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thermal insulating coating, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermal insulating coating will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3324906