Chain extended gelatin

A soluble, chain extended gelatin having a high molecular weight and significantly higher viscosity at equivalent gelatin concentration compared to standard gelatin and significantly faster setting time is produced by preparing an aqueous gelatin composition containing from about 6% to about 18% dry weight of gelatin and from about 0.25 to about 5 millimoles of a bis-(vinyl sulfonyl) compound per 100 grams of gelatin, heating the composition at a temperature of from about 40 to about 60.degree.C. and at a pH of from about 4.5 to about 7 for from about one to about eight hours.

FIELD OF THE INVENTION 
This invention relates to soluble chain extended gelatin and a method for 
producing it by which the molecular weight and, hence, viscosity of the 
gelatin are increased without gel formation. 
BACKGROUND OF THE INVENTION 
Gelatin has long been known as an emulsion or dispersion base and carrier 
for dyes and other photographically active materials. For such 
applications it is important that the cast gelatin layers set as rapidly 
as possible without detriment to the forming film. In applications such as 
curtain-coating, a strong correlation exists between the rheology of 
gelatin and the critical speed for the onset of air entrainment. Highest 
coating speeds are obtained with coating fluids having the greatest shear 
thinning behavior. In those applications in which gelatin layers contain 
silver halide, even when high grade gelatins are employed, sedimentation 
of the silver halide often occurs before the gelatin layer sets because of 
the low viscosity of the casting solution. 
Attempts to overcome these difficulties and increase the setting speed of 
gelatin by adding thickeners such as polystyrene sulphonic acid have been 
unsuccessful since thickeners frequently cause surface defects during 
casting. Alternatively, the use of hardenable materials to harden 
photographic gelatin layers chemically and thereby adjust the degree of 
swelling, increase the melting point, and increase mechanical strength has 
also been proposed. Unfortunately, hardeners introduce undesirable effects 
such as defective crystallization, adhesion difficulties, layer separation 
due to the varying lateral swelling of the individual layers, and other 
problems which render the material useless. Such problems are exacerbated 
when casting requirements make it necessary to employ such short drying 
times and/or such high drying temperatures that the ordered helical 
structures which develop during gelatin layer formation can only be 
partially formed. 
Many attempts have been made to overcome such difficulties. For example, 
U.S. Pat. No. 4,421,847 describes a process for chain lengthening gelatin 
by partial hardening. However, the "partial hardening" referred to is a 
cross-linking reaction which results in the formation of insolubles and 
renders the material useless, particularly for photographic applications. 
Likewise, U.S. Pat. Nos. 3,642,486 and 3,539,644 describe the use of 
hardening agents to produce higher molecular weight gelatin. The resulting 
product is gelled and insoluble. 
Some have proposed the addition of quickly hardenable materials such as 
chrome acetate or materials which actuate the hardening reaction of 
gelatin which otherwise takes place only slowly. For example, U.S. Pat. 
No. 2,652,345 suggests the addition of formaldehyde and treatment with 
gaseous ammonia. U.S. Pat. No. 2,996,405 proposes the addition of a mixed 
styrene-aminomaleic acid polymer and treatment with ammonia vapor. The 
reaction of gelatin with thiolactones to provide gelatin compositions 
stable in the acid range but quickly hardenable in alkaline media is 
described in U.S. Pat. No. 3,171,831. However, high pH values accelerate 
chemical cross-linking which is disadvantageous in photographic processes 
and renders the gelatin compositions useless in multilayer materials where 
crosslinking causes adhesion defects. 
In another approach, the yield of higher viscosity, higher molecular weight 
gelatin obtained from lime processed ossein stock can be substantially 
increased by controlling the time and temperature of the alkaline 
extractions. However, significantly lower yields of gelatin and higher 
amounts of poorer quality gelatin are thus produced, resulting in low gel 
strength, higher color and lower clarity. Alternatively, the acid 
processing of ossein yields gelatins which have a much lower viscosity 
than gelatins obtained by lime processing. Gelatins derived from pigskin 
are normally obtained by the acid process and have the same low 
viscosities as acid processed ossein gelatins. 
Accordingly, it is important to provide a gelatin having a faster setting 
time, higher viscosity, and higher molecular weight at the same gelatin 
concentration while retaining its solubility characteristics. 
SUMMARY OF THE INVENTION 
A soluble, chain extended gelatin having a high molecular weight and 
significantly higher viscosity at equivalent gelatin concentration 
compared to standard gelatin and significantly faster setting time is 
provided. The soluble gelatin of the invention comprises from about 0.25 
to about 5.0 millimoles of a bis-(vinyl sulfonyl) compound, preferably 
bis-(vinyl sulfonyl) methyl ether or bis-(vinyl sulfonyl) methane, per 100 
grams of gelatin. Gelatin percentages are based on the dry weight of 
gelatin divided by the total solution weight. 
The soluble, chain extended gelatin of the invention is produced by a 
process which comprises preparing a gelatin composition containing from 
about 6% to about 18% dry weight of gelatin and from about 0.25 to about 5 
millimoles of a bis-(vinyl sulfonyl) compound per 100 grams of gelatin, 
heating the composition at a temperature of from about 40.degree. to about 
60.degree. C. and at a pH of from about 4.5 to about 7 for from about one 
to about eight hours. Chain extended gelatins can be produced from all 
types of gelatin including lime processed gelatin, acid processed bone 
gelatin, acid processed pigskin, and the like. 
The soluble, chain extended gelatins of the invention are particularly 
useful on support layers in the production of photographic elements, as 
carrier layers for microcrystalline dispersed dye in the production of 
microfilm products, or as carrier layers for conventional and fast acting 
hardeners in color emulsion products. The chain extended gelatins of the 
invention are also useful as melt gelatins for a variety of emulsions 
where higher melt viscosities are either desired or required. They provide 
significantly smaller particle size in yellow coupler dispersions compared 
to control dispersions prepared using a standard viscosity gelatin. 
Casting solutions produced from the gelatins of the invention have an 
increased setting rate (faster setting time) and significantly increased 
viscosity and molecular weight for the same gelatin concentration with no 
insolubles or gel formation. Casting defects and the formation of sediment 
can thus be avoided as well as the lateral swelling of individual layers 
which leads to adhesion defects. In curtain coating applications, a strong 
correlation exists between the rheology of gelatin and the critical speed 
for the onset of air entrainment. The gelatins of the invention provide 
the highest coating speeds and exhibit excellent shear thinning behavior 
in coating fluids in which they are contained.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the invention produces higher gelatin solution viscosities 
without increasing gelatin concentration and without the formation of a 
gel or insolubles. It is believed that it is the controlled reaction of 
low concentrations of gelatin with low concentrations of the bis-(vinyl 
sulfonyl) compounds of the invention to produce hydrolytically stable 
bonds which increases the molecular weight of the gelatin without loss of 
solubility. 
The chain extended gelatins of the invention can be produced from any kind 
of parent gelatin including lime processed gelatin, acid processed bone 
gelatin, acid processed pigskin, and the like. Although acid processed 
gelatin is inherently a low molecular weight/low viscosity material, it 
can be used in the practice of the invention to provide a gelatin having 
the same molecular weight/viscosity as lime processed gelatin, without the 
disadvantages of alkaline extraction. 
Lime processed and acid processed gelatins can be used as prepared or they 
can be deionized. Particularly in the case of acid processed gelatin, 
deionization removes excess salt produced in the process of making the 
gelatin. 
Any suitable bis-(vinyl sulfonyl) compound can be used in the practice of 
the invention. Preferred classes of such suitable materials includes those 
having the formulae 
##STR1## 
in which m is an integer of from 1 to 4, Z is a heteroatom such as oxygen, 
nitrogen, sulfur, and the like, and R is hydrogen or lower alkyl such as 
methyl, ethyl, isopropyl, butyl, pentyl, and the like which groups can, in 
turn, be further substituted. Preferred are bis-(vinyl sulfonyl) methane 
and bis-(vinyl sulfonyl methyl) ether. 
The chain extending agents of the invention can be used in various kinds of 
gelatin photographic emulsions including orthochromatic, panchromatic, and 
infrared emulsions, as well as in X-ray and other nonoptically sensitized 
emulsions. They can be added to the emulsions before or after the addition 
of any optically sensitizing dyes which may be used, and are effective in 
sulfur and gold sensitized silver halide emulsions. 
The chain extended gelatins of the invention can be used in bead coating 
and curtain coating operations or can otherwise be coated onto a wide 
variety of supports. Typical supports include those generally employed for 
photographic elements including cellulose nitrate film, cellulose acetate 
film, polyvinyl acetal film, polystyrene film, polyethylene terephthalate 
film, and related films or resinous materials as well as glass, paper, 
metal, wood, and the like. Supports such as paper that are coated with 
.alpha.-olefin polymers, particularly polymers of .alpha.-olefins 
containing two to 10 carbon atoms such as, for example, polyethylene, 
polypropylene, ethylene butene copolymers, and the like can be employed. 
The gelatin compositions of the invention can also contain additional 
additives, particularly those known to be beneficial in photographic 
emulsions such as optical sensitizers, speed increasing materials, 
plasticizers, and the like, including those disclosed in U.S. Pat. No. 
3,128,180 which is hereby incorporated herein by reference. Thus, the 
chain extended gelatin compositions of the invention can be used in 
photographic elements intended for color photography and can contain 
color-forming couplers or be used as emulsions to be developed by 
solutions containing couplers or other color generating materials or 
emulsions of the mixed packet type. Yellow coupler dispersions exhibit 
significantly smaller particle size than control dispersions prepared 
using standard viscosity gelatin. 
Silver halides employed in photographic emulsions include any of the 
photographic silver halides such as silver bromide, silver iodide, silver 
chloride, silver chloroiodide, and the like. The silver halides used can 
be those which form latent images predominantly on the surface of the 
silver halide grains or those which form latent images inside the silver 
halide emulsion. Hardened emulsions of the gelatins of the invention can 
be used in diffusion transfer materials. 
Although the chain extended gelatin compositions of the invention can be 
prepared from a gelatin solution containing from about 6% and about 18% 
(dry weight) gelatin, it is preferred that the gelatin concentration range 
from about 10% to about 15% by weight. Further, while the concentration of 
the chain extending agent ranges from about 0.25 to about 5 millimoles per 
100 grams (dry weight) of gelatin, preferred amounts range from about 1 to 
about 3 millimoles per 100 grams of gelatin. 
In the process of the invention, the gelatin solution containing the chain 
extending agent of the invention is heated at a temperature ranging from 
about 40.degree. to about 60.degree. C., preferably 40.degree. to about 
50.degree. C., at a pH ranging from 4.5 and 7, preferably 5.4 to 6, for 
from about 1 to about 8 hours, preferably from about 2 to about 4 hours. 
The pH is monitored and adjusted at the beginning and the end of the 
process. 
Any suitable apparatus or reactor can be used to carry out the chain 
extension reaction the invention, including any suitable stirring and 
heating means. Any suitable means available to those skilled in the art 
can be used to adjust the pH of the gelatin composition in accordance with 
the invention. 
Photographic elements can be prepared using the chain extended gelatins of 
the invention. Suitable elements include a support such as a polyester or 
polyolefin film, a layer of the chain extended gelatin of the invention on 
the support, and any suitable hardener including those disclosed in any of 
the patents incorporated herein by reference. 
The invention is further illustrated but is not intended to be limited by 
the following examples in which all parts and percentages are by weight 
unless otherwise indicated. 
EXAMPLE 1 
Lime Processed Bone Gelatin 
About 7.6% (dry weight) of limed bone gelatin (the parent gelatin) was 
vigorously stirred for a few minutes in distilled water. Stirring was then 
stopped to avoid air entrainment and foaming and to allow the gelatin to 
swell for about thirty minutes. The gelatin mixture was then heated to 
45.degree. C. (+/-2.degree.) to allow the gelatin to dissolve without 
stirring for about 20 minutes. The gelatin was then stirred mildly to 
avoid air entrainment and foaming for about thirty minutes, by which time 
all of the gelatin dissolved and the solution was uniform in consistency. 
An aqueous solution of bis-(vinyl sulfonyl)methane was next added such that 
the gelatin solution contained 0.034% bis-(vinyl sulfonyl)methane which 
corresponds to 0.45% based on the dry weight of gelatin or 2.2 millimoles 
per 100 grams of gelatin. The solution was stirred vigorously for about 
two minutes to insure thorough mixing. The pH of the composition was 
adjusted to about 5.7 using either 1N sulfuric acid or 1N sodium 
hydroxide. Mixing speed was then reduced to very mild stirring which was 
continued for about three hours (+/- thirty minutes). The final product 
contains no gel or insolubles. It was stable on storage even under 
refrigeration until used. 
An aqueous solution containing 7.6% of the parent lime processed bone 
gelatin was prepared under conditions as equal to those employed above as 
possible. Properties of the two solutions were compared. Viscosity 
measurements were made in a Brookfield LVTD viscometer using an ultra low 
viscosity adaptor thermostated at 40.degree.+/-0.1.degree. C. RBT 
measurements were made using a Rolling Ball Viscometer thermostated at 
40.degree.+/-0.01.degree. C. Viscosity, setting time, and gel strength 
measurements were carried out at a gelatin concentration of 6.16 percent 
dry weight. Molecular weight distribution data were obtained from 
polyacrylamide gel electrophoresis measurements. Color and clarity were 
measured spectrophotometrically. Standard solution physical properties for 
both gelatins are given in Table 1. 
Coatings were prepared from the chain extended gelatin of this example and 
the parent gelatin at the same dry gelatin coverage (1,000 mg/square 
foot), using the same added hardener (bis-(vinyl sulfonyl methyl) ether), 
at hardener concentrations of 1.5 and 3 weight percent based on the dry 
weight of gelatin in the solution (6.6 and 13.2 millimoles hardener per 
100 grams of gelatin, respectively). Two different drying conditions were 
used, namely, mild and moderate drying conditions. In the mild drying 
condition the coatings were chill set at 50.degree. F. followed by drying 
with air that had been conditioned to a dry bulb temperature of 85.degree. 
F. and a wet bulb temperature of 55.degree. F. In the moderate drying 
condition, the coatings were chill set at 50.degree. F. followed by drying 
with air that was conditioned to yield a dry bulb temperature profile that 
increased from 92.degree. to 100.degree. F. and a wet bulb temperature 
profile which increased from 62.degree. to 80.degree. F. The physical 
properties of the chain extended gelatin coatings compared to those of the 
parent gelatin coatings were measured by performing the following testing 
protocols: vertical swell, lateral swell, and mushiness, all in 70.degree. 
F. distilled water, wedge brittleness and raw film tackiness. Table 2 
contains a summary of the results of these tests. 
TABLE 1 
______________________________________ 
Physical Properties Chain Extended 
Parent 
______________________________________ 
Viscosity at 6.16% 
RBT seconds 26.1 10.2 
Brookfield LVD, cp 25 8.4 
Gel Strength, g 256 278 
Setting Time. sec 165 227 
pH 5.6 5.7 
Color % 53 59 
Clarity % 85 91 
Molecular Weight Distribution 
% Sub-alpha (&lt;80K) 35 29 
% Alpha (80K-150K) 17 30 
% Beta (150K-250K) 13 16 
% HMW (&gt;250K) 35 25 
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TABLE 2 
______________________________________ 
Gelatin Parent Chain Extended 
______________________________________ 
% BVSME 1.5 1.5 3 3 1.5 1.5 3 3 
Drying Mld Mod Mld Mod Mld Mod Mld Mod 
Condition 
Vertical 228 184 138 150 206 203 191 135 
Swell % 
Lateral Swell 
31 12 28 25 12 12 18 12 
% Mushiness 
112 174 138 200 100 184 105 200 
(grams) 
Wedge &lt;.06 &lt;.006 .rarw. &lt;.06 .fwdarw. 0.11 &lt;.06 .10 
Brittleness 
(inches) 
Raw Film 
Tackiness 
Sticking 1 1 1 1 1 1 1 1 
Ferrotyping 
0 0 0 0 0 0 0 0 
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EXAMPLE 2 
A chain extension process was carried out by reacting an aqueous 12.5 
weight percent solution of a deionized acid processed bone gelatin with 
bis-(vinyl sulfonyl) methane at 40.degree. C. and a pH of 5.7 for a period 
of six hours. In one reaction mixture the bis-(vinyl sulfonyl) methane 
concentration was 0.063 weight percent (0.5 percent based on the dry 
weight of gelatin or 2.55 millimoles per 100 grams gelatin). In a second 
reaction mixture the bis-(vinyl sulfonyl)methane concentration was 0.10 
weight percent (0.80 percent based on the dry weight of gelatin or 4.1 
millimoles per 100 grams gelatin. The physical properties of solutions of 
the parent gelatin and two chain extended gelatins prepared from the 
parent gelatin are reported in Table 3. Viscosity measurements were made 
in a Brookfield LVTD viscometer using an ultra low viscosity adaptor 
thermostated at 40.degree.+/-0.02.degree. C. RBT measurements were made 
using a Rolling Ball Viscometer thermostated at 40.degree.+/-0.1.degree. 
C. viscosity, setting time, and gel strength measurements were carried out 
at a gelatin concentration of 6.16 percent dry weight. Molecular weight 
distribution data were obtained from polyacrylamide gel electrophoresis 
measurements. Color and clarity were measured spectrophotometrically. 
TABLE 3 
______________________________________ 
Physical Properties 
Parent Chain Extended 
______________________________________ 
% BVSM 0 0.063 0.10 
Viscosity 
RBT seconds 7.0 9.2 14.0 
Brookfield LVD, cp 
4.6 7.4 10.6 
Gel Strength, g 343 349 336 
Setting Time. sec 
296 211 179 
pH 5.7 5.5 5.5 
Color % 52 50 49 
Clarity % 83 81 81 
Molecular Weight Distribution 
% Sub-alpha (&lt;80K) 
55.9 39.1 30.4 
% Alpha (80K-150K) 
16.1 14.5 11.5 
% Beta (150K-250K) 
10.9 13.6 12.2 
% HMW (&gt;250K) 17.2 32.8 45.9 
______________________________________ 
Similar improvements are obtained when lime processed bone gelatin is used. 
EXAMPLE 3 
Three gelatin solutions were prepared, each of which had a low shear 
viscosity of 50 cp as measured in a Brookfield viscometer at 40.degree. C. 
In solution (1) the gelatin used was a standard lime processed bone 
gelatin which has a viscosity of 12 cp measured at a gelatin concentration 
of 6.16% dry weight at a temperature of 40.degree. C. In solution (2) the 
gelatin was a higher molecular weight lime processed bone gelatin having a 
viscosity of 20 cp measured at a gelatin concentration of 6.16% dry weight 
and 40.degree. C. in solution (3) the gelatin was a chain extended gelatin 
prepared from the lime processed bone gelatin used in solution (1) and 
prepared by a process similar to that of Example 1. The chain extended 
gelatin had a viscosity of 23 cp as measured at a gelatin concentration of 
6.16% dry weight and 40.degree. C. 
Curtain coating experiments were carried out using these three gelatin 
solutions. In the experiments, the maximum speed (Smax) attainable before 
the onset of air entrainment and the coating latitude were measured. The 
coating latitude was determined by measuring the speed at which air 
entrainment occurred as the flow rate of coating fluid through the coating 
hopper was increased. In Table 4, the Smax data measured in this 
experiment with these three coating fluids are reported. Although the 
measured Smax for the higher molecular weight gelatin and the chain 
extended gelatin solutions were the same in this experiment, the chain 
extended gelatin solutions exhibited superior coating latitude. For 
example, the chain extended gelatin solution maintained a constant Smax 
with increasing flow rate, the high molecular weight gelatin solution 
exhibited diminishing speed with increasing flow rate. 
TABLE 4 
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Curtain Coating Solution 
Height Viscosity Smax 
Gelatin (inches) (cp) (fpm) 
______________________________________ 
(1) Parent 4.6 50 1050 
(2) High Molecular 
4.6 50 1300 
Weight Gelatin 
(3) Chain Extended 
4.6 50 1300 
Gelatin 
______________________________________ 
Although the invention has been described in considerable detail in the 
foregoing it is to be understood that such detail is solely for the 
purpose of illustration and that variations can be made by those skilled 
in the art without departing from the spirit and scope of the invention 
except as set forth in the claims.