Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2002-06-24
2004-08-24
Bareford, Katherine A. (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S420000
Reexamination Certificate
active
06780455
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of creating a material suitable for use in curtain coating. The invention also relates to a method by which one or more viscous liquid compositions may be coated on to a material such as a continuously moving web of material, as in the manufacture of photographic material such as films, photographic papers, magnetic recording tapes, adhesive tapes, etc.
BACKGROUND OF THE INVENTION
Curtain coating is a method of coating used extensively in the manufacture of photographic material and products as described in U.S. Pat. Nos. 3,508,947 and 3,632,374. In this method a free-falling liquid curtain of a coating composition is allowed to impinge against a continuously moving web of material. U.S. Pat. Nos. 3,508,947 and 3,632,374 disclose systems in which curtain coating of aqueous gelatin solutions is used.
It is known that wetting in a coating process operates in one of at least two regimes. These are: normal wetting where the coating liquid wets the entire surface of the material being coated, and wetting where a thin film of air is entrained, but is disrupted by the surface topography of the material and subsequently dissolves. U.S. Pat. Nos. 6,099,913 and 6,103,313 are examples of systems in which the synergistic application of a coating composition having a high viscosity and a web to be coated with a specified degree of roughness enable high coating speeds to be achieved, i.e., Roughness Assisted (RA) wetting.
On a rough support, although air-entrainment occurs at approximately the same speed as on a smooth support, provided the coating and material parameters are in the correct range, then coating proceeds until a higher speed, at which point a sudden and gross failure occurs.
It is possible at present to estimate, for a given coating parameter set, that RA wetting is possible, and in addition that if RA wetting is operating, at which higher speed gross failure is expected. Currently available systems only take account of the average peak-to-peak height R
z
of the surface of a substrate.
The parameter R
z
is a ten point average peak-to-peak roughness measure as shown in
FIG. 1
(DIN4768). A sample of the surface of the material is defined and the surface topography determined. The sample is then split up into 5 components of length I
m
and an average peak-to-peak distance is calculated. R
z
is calculated as
1
5
∑
i
=
1
to
5
R
zi
in which R
zi
is the peak-to-peak distance in the ith component.
Whilst it is clear that a constraint on this parameter is a necessary condition further specification is desired Considering a surface composed of beads of a common size, any density of beads up to a limit (defined by the length I
m
in
FIG. 1
) will give the same value for R
z
, whereas for each density of beads there is a further characterising roughness of the material which is unaccounted for in known systems. In other words, as the beads are spaced further apart, although R
z
will remain constant until a spacing determined by I
m
is reached, the surface topography changes. The propensity for RA wetting will fall, since as the spacing increases the surface will tend to behave more like a smooth surface with isolated perturbations.
Problem to be Solved by the Invention
A method of creating a material is desired on which it is known that RA wetting can occur. A method is required that enables determination of whether or not a particular material is suitable for RA wetting. In particular, a method is required of determining the suitability of a substrate for RA wetting relative to the surface topography.
The method is required for use in the creation of substrates suitable for use in the manufacture of, amongst others, photographic material such as films and photographic paper, magnetic recording tapes, adhesive tapes, inkjet receiver materials etc.
A method is required that enables, for a given set of coating conditions and coating compositions, determination of whether or not the surface to be coated is capable of RA wetting. In addition a method is also required that enables identification of which mechanism of coating, normal wetting or RA wetting, is operating. This enables the appropriate application of a model to predict air-entrainment or gross failure speed in curtain coating.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of creating a material for use as a material to be coated by curtain coating, comprising the step of creating a surface texture of the material such that when the material is coated with a freely falling curtain formed of a composite layer of one or more coating compositions that impinges at a point of impingement against a continuously moving receiving surface of the material having a roughness R
z
(DIN4768) in which the coating composition forming the layer adjacent to the receiving surface has a viscosity, &eegr;, measured at a shear rate of 10,000 s
−1
such that when combined with the roughness R
z
, the curtain gives a value of a specifying parameter &phgr;
□
that is greater than 1.
The specifying parameter &phgr;
0
is defined by
φ
0
=
1818
σ
1
/
3
R
z
{
1
0.0002
η
U
cos
θ
(
1
+
Ω
ρ
Q
20
η
)
}
;
in which
&sgr; is the surface tension (Nm
−1
) of the layer adjacent to the receiving surface;
&eegr; is the viscosity (Pas) measured at a shear rate of 10000 s
−1
of the layer adjacent to the receiving surface;
&thgr; is the angle formed between the curtain and a normal to the receiving surface at the point of impingement;
&rgr; is the average density (Kg/m
3
) of the one or more coating compositions;
Q is the total volumetric flow rate per unit width (m
2
/s) of the curtain;
&OHgr; is a Lorentzian factor which reduces the influence of momentum when flow rate is high or viscosity is low, the height of the composite layer at a distance &lgr; from the point of impingement, in which &lgr; is the average periodicity of the surface texture, is less than or equal to R
z
.
Preferably, the condition
R
z
λ
2
>
ρ
gH
4
γ
is
satisfied
,
wherein:
&rgr; is the average coating composition density;
&ggr; is the surface tension of the layer adjacent to said receiving surface; and
H is the curtain height.
In one example, the surface texture of the material is intrinsic to a substrate of the material. Alternatively, the texture of the material is created by embossing a surface thereof or the roughness of the material is created by the inclusion of dispersed particulates in a subbing formulation.
Preferably, the step of creating a surface texture of the material is performed such that a parameter &phgr;
∈
is defined by
&phgr;
∈
=&phgr;
0
+1818&sgr;
1/3
R
z
{½∈∈
0
E
2
}
in which
&phgr;
0
is the specifying parameter defined according to the first aspect of the present invention;
∈
0
is the permittivity of free space;
∈ is the dielectric constant of an ambient gas, and
E is the field strength (V/m) of an electrostatic field provided at the surface of the composite layer of one or more coating compositions at the point of impingement, preferably, between 3 and 30 kV/mm.
Preferably, the Lorentzian factor &OHgr; is defined by;
Ω
=
25
25
+
(
ρ
Q
η
-
8
)
2
.
The electrostatic field may be provided by a voltage of between 200V and 2000V (preferably between 600V and 1500V) between the one or more coating compositions and a backing surface of the receiving surface. In one example, the electrostatic field is generated by charges on the receiving surface.
The receiving surface is preferably a web made of a material selected from the group consisting of paper, plastic films, resin-coated paper, clay-coated paper, calendered paper, synthetic paper, plastic films overc
Bower Christopher L.
Clarke Andrew
Goppert Kim E.
Bareford Katherine A.
Bocchetti Mark G.
Sales Miton S.
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