Visibly transparent dyes for through-transmission laser welding

#1. Field of the Invention

#The invention relates to visibly transparent dyes and a method for using same in through-transmission laser welding. More particularly, it relates to the use of dyes in aesthetically demanding applications.

#2. The Prior Art

#In the accompanying drawings to which reference is made in the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

#FIG. 1 is a graph showing the Absorbance Ratio points for the wavelengths of interest for the candidate dye of Example 1.

#FIG. 2 is a graph showing the Absorbance Ratio points for the same wavelengths of interest for carbon black.

#Through transmission welding by various sources of radiation is known. The use of lasers as the radiation source represents a subset of this welding technology. The arrangement of the laser oriented to pass through an upper substrate to the joint is described in detail in U.S. Pat. No. 5,893,959, the contents of which are incorporated herein by reference thereto. Above and beyond the teachings of U.S. Pat. No. 5,893,959, the present invention includes the features of: lasers matched to absorption peaks above 350 nm, for example, Nd:YAG tripled (354), Nd:YAG doubled (532 nm), Argon (488 and 514 nm), Cu vapor (511 and 578 nm), ruby (694.3 nm), HeNe (632.8 nm), Krypton (647 nm), VIS diode (about 600 to 780 nm) and dye lasers (577 to 593 nm); laser absorbing dyes disposed in the upper substrate at the joint region; laser absorbing dyes disposed in a separate film or lamina clamped between the substrates; laser absorbing dyes disposed in the lower substrate at the joint region or thoughout all or part of the substrate; substrates selected from a variety of polymers, for example, polycarbonate, polymethylmethacrylate (PMMA), polyamides, and polyesters and films made of these polymers; polyolefin films; transparent substrates including clear, water-white and cosmetically tinted but still transparent substrates; a variety of aesthetically demanding applications; and novel Absorption Ratios to determine dye efficiency and photopic values to determine dye performance.

#The dye may be introduced into the joint region in several ways. First, the dye can be incorporated into a thin film. The film is preferably made of the same material as at least one of the substrates. If the film is made from a different material than a substrate, the film should be compatible with, i.e. soluble in, the polymer. The film may contain on the order of one ten-thousandths part dye on a weight basis. The film thickness may be on the order of tens of microns thick. Second, the dye may be incorporated into a surface of either substrate facing the joint via insert molding, dip coating, dye infusion, painting, printing, spraying. The dye may furthermore be incorporated into the entire lower substrate since only the surface facing the joint will be reactive.

#Previously it was thought that laser welding wavelengths had to be at least above 800 nm, outside the visible region, in order to be useful in demanding aesthetic applications. The theory was that any absorption bands between 380 nm and 780 nm would strongly interfere with the transmission of visible light resulting in darkly or intensely colored portions by virtue of their dye content. Furthermore, it was thought that incident laser radiation below 380 nm would adversely sever polymeric bonds.