Physical Principles

　　OBA are widely used in the paper-making to increase the whiteness of paper products．Lignin and other non-fibrous Substance in Wood are more or less yellowish．During the pulping process are only partly dissolved and eliminated．the bleaching step further removes lignin and other non-fibrous materials．But residual lignin and other co1ored substance again cause a yellowish tint Of chemical pulp．For the paper maker, the usage of OBA is one of the tools to eliminate this yellowish hue and to improve the paper quality in terms of appearance． OBA converts invisible ultraviolet radiation to visible blue light．The additional light source on the fiber．Superimposed on normal reflected light from the paper, gives a brilliant, neutral white．Using OBA is the most convenient way to increase the reflectance of paper and simultaneously to move the shade from yellow to blue．This subtle tint change makes the paper 1ook even whiter．So the idea OBA should be as follow．

a. Should absorb as much UV as possible(absorption maximum in the 350一400nm range)．

b. Completely re-emits the absorbed UV energy as visible violet—blue to greenish—blue fluorescent 1ight．

c．Does not absorb the fluorescent light．

d．Is colorless．

2．The main types Of OBA used in paper-making are derivatives of diaminostilbene disulphonic．The number of sulphoric acid group (--SO3H) is often used to classify these products, since product properties and application of the different class exhibit distinctive differences types、Telrasulpho types、and Hexasulpho types.

3．Affinity for pulp fibers Due to their Solubilizing sulpho groups all stilkenic OBA for paper are anionic．The number of sulpho groups per molecule in white water system reduce this electrostatic repulsion． Cationic Polymers cannot be used to increase the OBA exhauion, since these highly charged cationic compounds are strong fluorescent quenchers．

4．Practical aspects Maximum Whiteness 1evels Optimum performance of OBA (difference between initial whiteness and final whiteness after OBA treatment) depends on a variety of parameters, including： -Application conditions (PH, Sizing, water hardness, filler, etc．) -Type of OBA -Absolute base whiteness (including the shade of pulp)

Determines the negative charge density. In an aqueous medium pulp fibers have a markedly negative surface charge, when these negatively pulp fibres are brought into contact with the anionic, OBA in electrolyte free water, electrostatic repulsion occurs．

Mutual electrostatic repulsion can make it difficult or even impossible for the OBA molecule to approach the fiber surface before the molecule specific Vander wheals forces of attraction or hydrogen bonds can become effective．This explains to some extent the low affinity of some type of OBA in soft water．Calcium and Magnesium cations present in hard water areas of cations accumulated Greening.

As outlined above, there are limits as to how much OBA can be applied to paper. Once the build-up curve reaches its maximum, any additional dosage of OBA causes a decrease in whiteness of the maximum whiteness a monomolecular larger of OBA covers the accessible fiber surface of dimmers may occur. There are reasons for lower quantum yields and significantly different spectra. Absorbance of visible light, self-absorbance of emitted fluorescent light and lower quantum yield are the reason for lower quantum yields and significantly different spectra. Absorbance of visible light, self-absorbance of emitted fluorescent light and lower quantum yield are the reason for lower whiteness levels if the OBA is overdosed.

The absorption maximum of aggregated OBA molecules differs from that of mono-molecular OBA larger, i.e. it is shifted towards longer wavelengths. The strong absorption in the blue-violet region causes a “green shift” perceptible to the human eye. Moreover, the “greening” of OBA depends on:

-Type of OBA

-Type of application (wet end, size press coating)

-Paper making process condition (PH and alum being most important)

-Type and amOunt Of carrier in system．

-Generally speaking, the concentration limit for the greening effect of disulpho OBA is 1ower than of tetrasulpho types．Greening occurs only rarely with hexasulpho OBA．

Consequence for whiteness measurement The degree of bleaching is traditionally measured in brightness points (ISO, TAPPL)．Quite often, this measurement is also used to assess the effect of OBA．However, this approach has it's like CIE or Ganz are an appropriate way to express perceived whiteness, when OBA is used． A suitable method to measure whiteness is the key in order to avoid the greening effect, ie, to detect the point of maximum whiteness(saturation point of OBA)and to prevent the paper maker form using excess amounts of OBA． Since the who1e spectra is used in the calculation of the whiteness 1evel, both CIE and Ganz whiteness are suitable methods and link the visual perception with spectral data．When maximum whiteness is reached, both whiteness ratings indicate that additional amounts of OBA should not be applied in order to prevent greening．

In contrast, the ISO and TAPPI method lead to totally different results．If OBA are overdosed, whiteness still increases according to the ISO and the TAPPI methods although greening is already taking place．Whereas the CIE and Ganz whiteness data already indicate a loss of whiteness (similar to the perception of human observer)． Shading

Residual lignin and other non—fibrous material are responsible for the yellowish hue of unbleached and even bleached pulp． This yellowish hue can be reduced or pigments． Shading components absorb yel1ow and red light, and the reflectance curve shaded substrates is closer to the spectra of a neutral white． The human perceives these paper to be whiter than paper with a yellowish hue．Overdosage the blue or violet dyes or pigments produces bluish white paper．