Paper Substrate Brightness via Metal Salt Ratio Control
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Solution Overview
Problem
Optical brightening agents (OBAs) in paper substrates interact with multivalent metal drying salts, leading to reduced fluorescence and diminished brightness, necessitating increased OBA usage which can cause a 'green over' effect.
Innovation Solution
Partial replacement of multivalent metal drying salts with monovalent metal drying salts in a specific molar ratio to enhance OBA brightness without increasing OBA amounts, maintaining good ink dry times and print density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multivalent metal drying salts are used to improve ink dry time and print density, then ink dry time and print density are improved, but OBA brightness is reduced due to interactions between multivalent metal ions and anionic OBAs
Solution Approach 1:
The patent changes the chemical parameter of the drying salt system by using a mixture of multivalent and monovalent metal salts with controlled molar ratios. This parameter change reduces the harmful interaction with anionic OBAs while maintaining adequate ink drying performance through the combined effect of both salt types.
Solution Approach 2:
The patent creates a composite drying salt system combining multivalent metal salts (for ink drying) and monovalent metal salts (for OBA compatibility). This composite approach allows the system to simultaneously achieve good ink dry time through multivalent ions and maintain OBA brightness through monovalent ions that do not quench fluorescence.
2Illumination intensity
If OBA concentration is increased to compensate for brightness loss, then brightness is improved, but a 'green over' effect occurs causing undesirable color cast
Solution Approach 1:
The patent converts the potentially harmful interaction between metal ions and OBAs into a beneficial system by using monovalent metal salts that do not quench OBA fluorescence. This allows achieving high brightness without the need for excessive OBA concentrations that would cause green over effects.
3Illumination intensity
If monovalent metal drying salts are used to improve OBA brightness, then brightness is improved, but ink dry time and print density may be insufficient compared to multivalent salts
Solution Approach 1:
The patent merges the advantages of both multivalent and monovalent metal salts into a single drying system. The multivalent salts provide adequate ink drying and print density, while the monovalent salts maintain OBA brightness by minimizing fluorescence quenching. The synergistic combination achieves both goals simultaneously.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Increased optical brightness of paper substrates without the 'green over' effect, while maintaining satisfactory ink dry times and print density, achieved by controlling the molar ratio of multivalent to monovalent cations in the metal salt drying agent.
Implementation Method 1
OBAs are fluorescent materials which increase the brightness (e.g., white appearance) of paper substrate surfaces by absorbing the invisible portion of the light spectrum (e.g., from about 340 to about 370 nm) and converting this energy into the longer-wavelength visible portion of the light spectrum (e.g., from about 420 to about 470 nm). In other words, the OBAs convert invisible ultraviolet light and re-emits that converted light in the blue to blue-violet light region through fluorescence.
Data Source
AI summary
An article in the form of a paper substrate having a first surface and a second surface; an internal paper sizing agent present in an amount sufficient to impart to the paper substrate an HST value of from about 50 to about 250 seconds; one or more optical brightening agents present in an amount below a “green over” effect excess but sufficient to impart an ISO Brightness value of at least about 92; and a metal salt drying agent mixture of multivalent and monovalent metal drying salts in a molar ratio of multivalent to monovalent cations of from about 3:1 to about 1:18 to provide a percent ink transferred (“IT %”) value equal to or less than about 65% and a black print density value of at least about 1.45. Also, a method for treating the optically brightened paper substrate with a mixture of multivalent and monovalent metal drying salts.