What is the principle of optical brighteners and which plastics are suitable for them?

1Why use fluorescent whitening agents?

Fluorescent whitening agents are also known as optical brighteners.The functions of such additives are as follows:

1. Brighten the color

2. Covering up the natural yellowing of plastic

3. Improve the initial color

Make colored or black pigment products shine.

These fluorescent whitening agents work through a fluorescence mechanism.Absorb ultraviolet light and emit light in the blue region of the visible spectrum, making clothes look brighter and fresher.

Some recommended uses of fluorescent whitening agents include:

1. Molding thermoplastic plastics

2. Films and Sheets

Fiber

Adhesive

5. Synthetic leather

Let's take a detailed look at fluorescent whitening agents, their mechanisms of action, and benefits...

If a material can uniformly reflect most of the light of all wavelengths that shine on its surface, it appears white to the human eye.

For example, natural fibers usually absorb more light in the blue region of the visible spectrum ("blue defect") than other fibers due to the presence of impurities (natural pigments).Therefore, natural fibers also exhibit this light yellow color.

Synthetic fibers also have this light yellow color, although not as pronounced.

The whiteness of the substrate can be improved by enhancing reflectivity or compensating for the blue deficiency.

Before the advent of fluorescent whitening agents (FWA), the common practice was toUse a small amount of blue or purple dye to enhance the visual effect of white.These dyes absorb light in the green-yellow region of the spectrum, thereby reducing brightness.

However, since they simultaneously shift the hue of the yellow material towards blue, the human eye perceives an increase in whiteness.Unlike dyes, optical brighteners can counteract yellowish tints and enhance brightness because their blueing effect is based on adding blue light rather than subtracting yellow-green light.

Fluorescent whitening agents are actually colorless compounds that, when present on a substrate, primarily absorb invisible ultraviolet light in the 300-400 nanometer (nm) range and re-emit visible violet to blue fluorescence. In other words, they can absorb invisible short-wavelength radiation and re-emit visible blue light, thereby making the light reflected by the substrate appear brighter and whiter, which is key to the effectiveness of fluorescent whitening agents.

3How do fluorescent whitening agents work?

The molecule of the whitening agent absorbs photons (A), which induces a transition from the singlet ground state S.₀Transition to the electronically excited singlet state (S₁The vibrational energy levels of.

Located in S₁The whitening agent in its excited state can be deactivated through various pathways. Fluorescence is generated by the transition of radiation to the vibrational energy level of the ground state (F).

The deactivation processes competing with fluorescence mainly include non-radiative deactivation to S.₀Internal conversion (IC) and non-radiative transition to the triplet state (intersystem crossing, ISC).

Fluorescence efficiency is measured by quantum yield.Quantum yield (Φ) = Number of quanta emitted / Number of quanta absorbed

The relative rate of fluorescence emission and competitive processes determine it. When the brightener is fixed on a solid matrix, it emits fluorescence with a high quantum yield.

Fluorescent whitening agent and its transition energy diagram

Fluorescent whitening agents (FWA) can be effectively used for a variety of polymer substrates.Engineering plastics (such as polyester, polycarbonate, polyamide, and acrylic resin)Thermoplastic polyurethane, polyvinyl chloride, styrene homopolymers and copolymers, polyolefins, adhesives, and other organic substrates.

The effectiveness of optical brighteners depends on the type of substrate, processing conditions, and interaction with other components in the formulation (such as white pigments or UV absorbers). Generally, optical brighteners can be effective at very low concentrations.

Titanium dioxide pigment (TiO₂It absorbs light within the same ultraviolet wavelength range as fluorescent whitening agents, so it also produces a lower level of whiteness.

Fluorescent Brighteners in Flexible PVC

Anatase-type titanium dioxide pigment absorbs about 40% of incident radiation at a wavelength of 380nm, while rutile-type titanium dioxide pigment absorbs about 90%.

In flexible PVC samples, a bright white color can be achieved by using only a low concentration of fluorescent whitening agent (FWA) and anatase titanium dioxide. When using rutile titanium dioxide, the whiteness slightly decreases at the same concentration. The following two images demonstrate the advantages of fluorescent whitening agents.

The light resistance of the whitening effect of soft PVC.

Fluorescent brighteners in PET fibers

A key criterion for the applicability of fluorescent whitening agent technology is its lightfastness in the substrate. The diagram below shows the stability of the whitening agent in PET fibers:

(Light Fastness of Whitening Effect on PET Fibers)