1. Field of the Invention
This invention relates to heat-reflective window-glazing materials More particularly, it relates to heat-reflective window-glazing materials with very high heat rejection properties and relatively low visible light transmission properties.
2. Background Information
In energy-conscious architecture, windows pose a special dilemma Designers are asked to incorporate a maximum of window area into their plans notwithstanding the fact that window surfaces are major sources of energy control loss. In cold climates, added internal heat is lost through window surfaces. In warm climates, exterior heat enters through window surfaces, thus increasing air conditioning loads.
Historically, storm windows and multipane glazing have been employed to enhance window energy performance. More recently, the addition of a variety of films, coatings and glass compositions have been brought into use to achieve the same ends.
There is presently a call for window-glazing products having higher energy (heat) control performance (This performance property is often quantified as "shading coefficient" as will be defined below.) There are a number of approaches to enhanced energy control performance. One is to absorb the energy into the glazing material such as by bronzing or greying it and increasing its energy-absorbing ability. Another is to reflect the energy with reflective coatings or films. Each of these methods has its shortcomings. In the case of relying upon energy absorption, one has the problem that the absorbed energy is stored in the glazing material and a portion of it is radiated and convected across the glazing material, thus defeating in part the desired heat control When solar absorption is the mechanism, at low shading coefficients the degree of absorption required is so high that heat treated glass must be used. These materials must be able to withstand the amounts of heat stored in the glazing and their attendant temperature rises. Tempered glass and heat-strengthened glass are used to avoid fractures. These materials are more costly and can greatly prolong manufacturing lead times to market. In the case of relying upon energy reflection, it is difficult to achieve large degrees of energy reflection without also significantly degrading the performance of the glazing product in the visible spectrum This results in unwanted selective transmission and reflection of some visual wavelengths and thus a "tinting" of the transmitted light and/or reflected light and difficulties with batch-to-batch and intrabatch consistency as far as color properties are concerned. In addition, conventional reflective products are also marked by high absorption due to the type of coating used (i.e., titanium and stainless steel). These mirror-like glass products can be aesthetically unattractive when used with the required tempered glass due to nonplanarity in the glass.
Since the early 1980s, Southwall Technologies, Inc. has marketed a series of window thermal control film products under the trademark Heat Mirror. These products are plastic films having dielectric-metal-dielectric induced transmission energy filters on their surfaces. These products have been tailored to fill the need for substantial heat control while permitting transmission of a substantial amount (i.e., greater than 40% and often nearly 90%) of visible light While these products have met with customer acceptance, they have typically been used in settings where their high light transmissions have been desired rather than in settings where maximal heat rejection has been required. Their design has been such as to optimize visible light transmission.
Others have looked to low emissivity or "low E" coated glass glazings to reduce heat loss. These products have typically employed a partially transparent metal coating as heat control agent and optionally rely on heat reflectors or absorbers on or in the glass as a solar control mechanism. Emissivity refers to the heat emitting (or radiating) propensity of a surface. Emissivity is expressed as an emissivity number with a perfect blackbody radiator having a value of 1, window glass having a value of about 0.84 and glass with a low E coating having a value well below that, approximately 0.15. However, when low shading coefficients are needed, low E coatings have been used with tinting of the glass which has different low E property and results in increases in emissivity with attendant radiant heat losses. Even then, shading coefficients of low E glass are found to be greater than 0.35.
It is a general object of this invention to provide heat-reflective film products having a dielectric-metal-dielectric induced transmission structure but which are capable of high levels of heat rejection (low shading coefficient) while maintaining acceptable color properties throughout the visible wavelengths. It is a further object to achieve this in a product having a low emissivity.