BAJAJ PLAST
Plast Optical Brightner - OB
Overview
Overviewing
Optical Brightner
Introduction
The use of fluorescent whitening agents in plastics has extensively been covered in the literature [1]. This chapter is an extension of this previously published material, including technical performance data on fluorescent whitening agents in various substrates.
For a long time, fluorescent whitening agents (FWAs) have been used to improve the appearance of finished articles in the paper, soap, detergent, and textile industries. As thermoplastics markets have grown, fluorescent whitening agents have penetrated this field, too.
The objectives of using FWAs in plastics are the same as in paper and textiles:
Improvement or masking of the initial color of plastics, which is often slightly yellowish
Production of brilliant white end use articles, such as fashion goods of imitation leather, packaging materials, etc.
Increase of the brilliancy of colored and black pigmented articles
In principle, FWAs can be used in all plastics; however, not all FWAs can be used in all plastics because of possible restrictions on their suitability in a given application. Therefore, great care has to be taken to select the right chemistry and concentration. A technically suitable FWA shows high whitening effectiveness at low concentrations. In practice, concentrations of 50 to 300 ppm are used in thermoplastics. Only special applications, including processing recycled thermoplastics, may require concentrations exceeding 1,000 ppm.
Basic Principles of Whitening Mechanisms
Many thermoplastics absorb light in the blue spectral range of natural daylight (“blue defect”), causing a more or less pronounced yellow appearance (Fig. 16.1A). There are three basic principles on compensating for this deficiency, i.e., increasing the degree of whiteness:
Bleaching
Compensating for the blue deficit
Increase in reflection
A traditional way to reduce the initial discoloration of plastics and fibers, especially cotton but also synthetic fibers, such as polyester, polyamide, and acrylics, is to bleach them. While the yellowish cast diminishes significantly with bleaching, care has to be taken not to damage the fabric, reducing its strength and worsening its appearance.
Decades ago, textiles were brightened by adding very small amounts of a blue pigment, a technique known as “bluing.” In this way, the yellowish cast of the substrate was offset by
increasing the reflected blue portion. The substrate appears whiter, but the total amount of reflected light is reduced (Fig. 16.1B), so consequently, it appears duller [2].
In contrast, fluorescent whitening agents absorb invisible UV radiation in the wavelength range of about 360 to 380 nm, converting it to longer wavelength light and re-emitting it in a visible blue or violet light. As a result, the unwanted yellowish appearance of the substrate is offset and, in addition, more visible light in the range of 400 to 600 nm is reflected than was originally incident. The article now appears whiter, brighter, and more brilliant (Fig. 16.1C). Such optical brightening was discovered early in the 20th century, when natural substances were used to whiten fibers [3].Additional information concerning the historical development of FWAs is found in Anliker[4].
The physical process undergone in the FWA molecule in the whitening process follows the pattern shown in Fig. 16.2: By absorption of (A) photons (energy) from ultraviolet light, the whitener molecule raises its energy from the singlet ground state to the electronically excited singlet state and . Because of non-radiating deactivation, brighter molecules from the state relax to the vibrational ground state . Direct deactivation from the state to the singlet ground state leads to the emittance of fluorescent radiation. Because a certain portion of the energy received is remitted as non-radiating energy, the emitted photons are at a lower energy level, i.e., the radiation is of a longer wavelength. Small amounts of foreign matter (such as catalysts and impurities) are able to partially or completely quench the fluorescence because of their absorption of excitation energy[5].
Fig. 16.1 Absorption and reflection of daylight on white surfaces
a) Untreated substrate absorbs mainly blue light (A) yellow cast (blue deficit)
b) Blued substrate: compensation of yellow cast (A) by additional absorption of green-yellow light (B) luminosity loss
c) Whitened substrate: remission + fluorescence yellow cast compensated + excess of blue light (C).
Fig. 16.2 Simplified functional diagram of physical processes in an excited whitener molecule. A) Excitation through incident light, IC) Radiationless transition, F) Fluorescence, emitting light
Selection and Technical Requirements of Optical Brighteners
The determining factors for the selection of the most suitable FWAs for a given plastic material are:
Achievable whitening effect
Compatibility
Light-fastness
The selection of the whitener is also affected to a certain extent by its hue. In general, neutral and neutral blue to blue-greenish hues are preferred. However, especially in Asia, sometimes a reddish hue is preferred.
The achievable degree of whiteness of a given FWA depends on the substrate (Fig. 16.3). In addition, the whiteness degree may be affected by processing conditions as well as possible interactions with other components in the formulation, such as pigments and UV absorbers.
Fig. 16.3 Whitening effectiveness (Ganz formula) of a bis-benzoxazole whitener in various substrates (with rutile type TiO 2 ) as a function of whitener concentration
Titanium dioxide pigments absorb in the long-wavelength UV range, which is essential for exciting FWAs, thus generating lower whiteness degrees. Anatase pigments absorb at 380 nm about 40%, rutile pigments, on the other hand, absorb about 90% of the incident radiation (Fig. 16.4).
Fig. 16.4 Whitening effectiveness [W] of a bis-(styryl)biphenyl whitener in plasticized PVC as a function of whitener concentration in presence of 5% titanium dioxide as anatase type (a) and rutile type (b) ) as a function of whitener concentration
UV absorbers also absorb radiation in the FWA exciting range of the spectrum, and may therefore lead to reduced whiteness. However, this whiteness decrease is small because the FWA on the surface of the polymer receives sufficient energy to become excited.
An essential requirement for the technical suitability of a FWA is its light-fastness in the substrate under consideration. FWAs are usually more or less light sensitive in the wavelength range needed to excite the molecule. Consequently, the light-fastness of whiteners is limited, and is considerably lower than the light-fastness of pigments and the light stability of most plastics. Colored structures, which may lead to additional yellowing, must not be generated at the end of the photochemical degradation of a FWA. The light fastness of a FWA is considerably affected by the substrate (Fig. 16.5).
Fig. 16.5 Lightfastness of a bis-benzoxazole type FWA in various substrates; Relative whiteness (Ganz formula) as a function of weathering time in a xenotest 150 with filtered xenon radiation corresponding to daylight behind window glass.
To achieve maximum effectiveness with a FWA, it is imperative that the whitener is fully dissolved and homogeneously distributed in the finished article, i.e., the distribution must be monomolecular. Sufficient compatibility is necessary to avoid exudation. The requirements regarding thermal stability and low volatility of FWAs for thermoplastics are higher than those of for textile whitening. In textiles, FWAs are usually applied topically from solution.
Migration stability must also be considered, particularly with FWAs used in flexible PVC and polyurethane coatings. An insufficiently compatible whitener may be transported to the surface (for example, resulting from plasticizer migration). Consequently, partial loss of whitener results; furthermore, the surface may become discolored either uniformly or in spots.
Structure of Optical Brighteners
Only few of the known chemical classes of fluorescent whitening agents (Fig. 16.6) possess the properties required for mass whitening of plastics and fibers [5 to 9].
Fig. 16.6 Chemical classes for the mass whitening of plastics and fibers- bis-benzoxazoles, phenylcoumarins, bis-(styryl)biphenyls
Incorporation of Optical Brighteners into Polymers
Fluorescent whiteners are incorporated exclusively into the thermoplastics’ mass before the forming process; i.e., the whitener is not applied subsequently by means of solutions, hence the term “mass-whitening.”
Usually, fluorescent whiteners are introduced by dry-blending with the plastic material (powder or pellets). If necessary, an adhesion promoter such as butyl stearate is added. In flexible PVC, fluorescent whiteners are frequently applied as a solution or dispersion in a plasticizer. To achieve distribution of improved homogeneity at low concentrations, fluorescent whiteners are often added as a concentrate or as a master blend, e.g., 10% active whitener blended with chalk or a plasticizer.
In the case of PET fibers, some types of FWA can be applied during polymerization. In this case, the FWA is added with the monomers; extremely good thermal stability and low volatility are especially important requirements for FWAs incorporated in this manner.
Incorporation of Optical Brighteners into Polymers
Migration and Exudation
Examination for migration and exudation effects is carried out best in the light of a UV lamp. Migration and exudation effects result in random yellow and greenish spots, which fluoresce distinctly brighter than the surrounding area.
Whitening Effect
The evaluation of the whitening effect of an FWA may be carried out visually (preferably under daylight) or by means of colorimetric measurements.
For visual evaluation, so-called “white scales” are used for comparison, e.g., the plastic white scale developed by Ciba Inc. This consists of a melamine molding resin which is graded with yellow pigments or fluorescent whiteners into 12 different whiteness degrees. The calibration point is physically ideal white, approximately represented by magnesium oxide or barium sulfate. This calibration point represents by definition the whiteness degree 100.
The result of any visual evaluation depends considerably on luminosity and spectral energy distribution of incident light, as well as on physiological influences and the color sensitivity of the observer. The assessment should be carried out in daylight (behind window glass, or even better, outdoors) with adequate brightness between 10 am and 3 pm (1000 to 1500 hours), facing north in the northern hemisphere and south in the southern hemisphere.
Visual evaluation, however, is not completely satisfactory because of numerous variables which are difficult to control. For that reason, in recent years, instrument assessment of whiteness has been increasingly adopted [10].
Whiteness, as a physiological impression, is not directly measurable; for that reason, it has to be calculated from colorimetric data. On the basis of physical measurements, objective characteristic values are obtained, free of subjective influences. A specified light source in the color measuring device is a prerequisite for reproducible results, independent of time, location, or operator.
The literature describes many methods of measuring the degree of whiteness of an object. They are based mainly on the tristimulus values , and the chromaticity coordinates and , according to a CIE-standard (Commission Internationale de l’Éclairage) [11]. These tristimulus values permit not only an expression of the whiteness impression as a single number, but also furnish another value, dominant wavelength, for the assessment of the color hue. In the evaluation of different whiteness degree equations, it must be borne in mind that all such equations are empirical statements, because color, including white, is a physiological impression and not a physical value. The tristimulus values can be measured by means of a spectrophotometer or by three-filter instruments (now antiquated).
Details of the colorimetrical and mathematical problems associated with instrumental whiteness assessment are beyond the scope of this chapter. Reference is made to the specialized literature[12to17].
Below are two of the most common equations in use today:
According to Taube – W=4B−3G (16.1)
According to Berger – W=3B+G−3A (16.2)
where amber (red), blue, and green, values which can be calculated by tristimulus values or directly measured with three-filter instruments.
Nowadays, more and more, the whiteness equation according to Ganz
together with tint () assessment according to Ganz/Griesser
is used. The whiteness degree values in this chapter have been assessed according to the Ganz equation.
and are parameters of the equation that permit adjustment of the whiteness degree values to any white scale and take into consideration the type of light source in the measuring instrument. CIE publication No. 15.2[17] recommends simplified Ganz equations to achieve uniform whiteness and tint assessment.
Light-fastness
Currently, there are no international standards for assessing the light-fastness of fluorescent whiteners in plastics. Therefore, testing is conducted mostly with reference to DIN 54004. The light exposure of the samples is carried out in a device with filtered xenon radiation, corresponding to the global radiation behind window glass (e.g., Atlas Weather-Ometer, Xenotest).
As a yardstick for the incident radiation, the blue scale [18] is used as a simple actinometer. It consists of eight standardized wool fabrics dyed with blue dyestuffs, having different light-fastness. As end point, i.e., light-fastness number (blue scale number), the blue scale
Fig. 16.7 Light fastness of whitening effect (Ganz formula) of two fluorescent whiteners in plasticized PVC containing 5% anatase titanium dioxide as a function of exposure time in a Xenotest 150 with filtered radiation corresponding to daylight behind window glass. a) bis-benzoxazole type; b) phenyl-coumarin type
grade is taken where the test sample and the blue scale show the first color change compared to the unexposed control. A better and more objective assessment can be obtained by measuring the change of whiteness degree as a function of exposure time (for example, in flexible polyvinyl chloride as shown in Fig. 16.7).
Performance of optical brighteners in Polyolefins substrates
Polyolefins
Polyolefins are occasionally mass-whitened, especially PE films for paper coating. An essential criterion for the choice of an adequate product is its compatibility with the substrate. The compatibility of FWAs with polyolefins is generally in the following decreasing order:
Light-fastness of FWAs in polyolefins is generally poor.
Technological Trends
The state-of-the-art in fluorescent whitening agents for plastics and fibers for a number of years has been as described in this chapter; no new chemicals have been introduced into this market. On the other hand, established producers of FWAs are faced with fast growing competition from producers of similar products mainly in China and Korea. Latest technical developments are found in the field of synergistic blends of established whiteners and whitening boosters. Other developments include the use of small amounts of dyestuffs (blue, violet, red) for increased whitening effectiveness and modification of cast (shading colors).
