INK RECEPTIVITY (K & N)
Introduction
The ink receptivity of paper may be described as its capacity to accept the transfer of
inks from printing surfaces. The amount of ink necessary to attain a particular degree of color
intensity in printing is related both to the paper and to the ink. One cannot speak of the
absolute ink absorbency of a particular paper, but rather of the receptivity of that paper for
particular kinds of ink.
The quality of the ink and the surface characteristics of the paper are both important in
determining the amount of ink used to give the desired result. In general, the smoother and
denser the paper, the less ink required. In printing papers, the nature and distribution of the
fillers influences the printability with a given ink, depending on the ease with which the filler is
wetted by the vehicle of the ink. The effect of filler as an absorptive component of papers is
also shown by the fact that the wire sides of most filled papers require less ink than the felt
side, even though the felt side is normally smoother and less porous.
In coated papers, the amount of coating, the types of adhesive and pigment, and the
pigment/binder ratio all influence the ink receptivity. The interactions here are quite involved
and one cannot determine ink requirements merely by referring to the chemical and physical
characteristics of the paper and ink. In any case, the ink films transferred to paper are so thin
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that the paper's surface absorbency or ink receptivity can play an important role in ink drying.
Significance
The determination of ink receptivity is quite empirical and measures only one of the
properties defining the printing quality of paper. The arbitrarily chosen conditions for making
the test and the subjective determination of the staining level make this test suitable primarily
for direct comparisons.
Ink receptivity is one of the properties of paper which defines its utility. It is closely
associated with manufacturing as it is largely controlled by the kind, quality, and treatment of
the fiber components, by the kind and amount of filler used, and by the uniformity and manner
in which the sheet was formed on the paper machine. Control of ink receptivity is essential in
the production of high quality printing papers, and a variety of tests have been devised to
measure this property.
Other factors being equal, the printing performace of paper may be reasonably
predicted by the type of ink receptivity tests relying on surface-absorption, before the paper
reaches the press. This is especially true if the test is run along with papers of known
commercial printing properties.
Test Specimens
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The test specimen shall consist of a flat, uncreased sample of any convenient size.
Method of Test
Samples of the papers to be compared are placed on a flat surface in an overlapping
arrangement so that each is exposed 1 or 2 inches. Work the K & N test ink a little with a
spatula; then smear a thick film, about one inch wide, over the samples and immediately begin
timing. After exactly two minutes, quickly remove the excess ink with the spatula, then wipe
the remainder away with a clean soft cloth. The varnish which has penetrated the surface will
be shown by a blue stain of varying depth.
Deep discoloration indicates rapid varnish absorption, while light staining indicates good
hold-out or good gloss-ink resistant properties.
Such results may be reported as deep,
medium, or light staining.
When the K & N test is made under closely controlled conditions, it is possible to
obtain reproducible results. Also, to obtain quantitative results, the reflectance of the absorbed
color can be measured on a brightness meter or with a resflection densitometer (not required
in this course).
Report
The intensity of the staining may be reported as deep, medium, or light or arbitrary
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numerical equivalents.
Notes
1.
The excess ink scraped off the surface with the spatula may be returned to the
container, but care should be taken as the ink quality may change due to
differential absorption of its components into the stock during the test.
2.
An examination of the stained area will be found helpful in solving such
problems as ink lay, ink consumption, off-set, smear, and rate of set. The test
gives an excellent indication of the uniformity of paper coatings.
For best
printing results, the color of the test area should be uniform. Mottled color or
variations in depth indicate non-uniform absorbency which may result in mottled
printing or non-uniform ink gloss.
3.
The K & N test works best on coated papers and certain smooth-surfaced
uncoated papers, but is of little value on papers with rough finishes.
Reference
TAPPI RC 19
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7
Properties Related to Printing
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