Use of soy polysaccharides as a thickening agent for alkali silicate adhesives

A silicate containing adhesive is disclosed which is suitable for the production of laminated paper products comprising an aqueous solution of an alkali metal silicate together with soy polysaccharides in an amount effective to thicken the silicate solution, thereby providing a silicate adhesive which has good adhesive properties and may be used for bonding of two surfaces without excess penetration of the adhesive into either of the surfaces to be bonded.

BACKGROUND OF THE INVENTION 
The present invention relates to a silicate containing adhesive composition 
and process for producing the same. 
Silicate containing adhesives have been widely used in the paper industry 
for the manufacture of laminated products such as corrogated board, paper 
tubes, paper cones and the like. Silicate adhesives are desirable for 
these uses because of their relatively low cost, ease of preparation and 
the strong adhesive bond they produce. 
A variety of silicate containing adhesives have been disclosed in which a 
vegetable protein material is added to increase the shelf life or adhesive 
characteristics of the composition. U.S. Pat. No. 2,457,108 describes the 
preparation of a silicate adhesive which includes a vegetable protein 
flour in an amount of 2 to 50% to provide an increased shelf life for the 
silicate adhesive composition without a corresponding reduction in 
adhesiveness. U.S. Pat. No. 2,681,290 also describes a silicate containing 
adhesive composition wherein a vegetable protein-carbohydrate flour is 
employed and it is preferred that the flour have a starch level of greater 
than 15% by weight since this improves the working life and viscosity of 
the adhesive composition. Both of these compositions, however, require the 
application of heat in order to provide the necessary viscosity for 
adherence to the surfaces to be joined and finally to form the necessary 
bond between the two surfaces to be joined. Thus, while being acceptable 
for most applications, it would be preferred if a silicate adhesive 
composition having cold setting characteristics and higher viscosity at 
ambient temperatures could be developed since this would avoid the use of 
heat which is not always desirable in the manufacture of laminated paper 
products. 
U.S. Pat. No. 2,894,847 describes a cold setting silicate adhesive 
composition which contains a soy insolate for thickening of the silicate 
composition and to provide the necessary adhesiveness. While the use of an 
isolate does provide an improvement in the properties of the silicate 
adhesive composition, nevertheless, somewhat lower silicate levels in the 
adhesive composition must be employed since the isolate will otherwise 
precipitate or not be capable of dispersion when a high level of silicate 
is used. Furthermore, such a composition requires a substantial percentage 
of isolate in order to provide the necessary solids level in the adhesive 
composition so that the composition will have the necessary viscosity and 
tack for application to the surfaces to be joined together. 
The use of various types of cellulosic compounds as thickeners for silicate 
adhesives has also been proposed. U.S. Pat. No. 3,241,990 describes the 
use of cellulosic fibrous material such as wood pulp to control the 
viscosity of silicate solutions used in the formation of solid silaceous 
articles. Impregnating compositions to improve water resistance have also 
been proposed which comprise silicate solutions containing carbohydrates 
such as starch or cellulose derivatives. 
In spite of the variety of approaches that have been used for the formation 
of silicate compositions, there is still a need for a cold setting 
silicate adhesive composition that is suitable as a bonding agent for 
paper products. The continuous production of bonded paper products require 
an adhesive which can be easily formed and maintains its consistency for 
application on a continuous basis. Furthermore, the adhesive should have 
sufficient viscosity so that the smallest amount possible is applied to 
achieve adhesion thereby improving the economics of the process as well as 
preventing excessive penetration of the adhesive into the relatively 
porous surfaces of paper products. 
It is therefore an object of the present invention to provide a silicate 
containing adhesive composition having cold setting characteristics at a 
low enough solids level that it may be easily applied, yet that it has a 
substantial viscosity at ambient temperatures thereby providing the 
necessary tack and adhesive strength. 
It is a further object of the present invention to provide a silicate 
containing adhesive composition which is of relatively low cost and other 
desirable characteristics in the production of laminated paper products 
and the like. 
It is another object to provide a silicate adhesive composition that 
maintains its consistency to permit use in a continuous laminated paper 
making process. 
These and other objects have been achieved in the present invention. 
SUMMARY OF THE INVENTION 
The present invention relates to a silicate containing adhesive composition 
suitable for the production of or manufacture of laminated paper products 
such as corrugated board, paper tubes, paper or wood laminates, and the 
like. The silicate containing adhesive composition of the present 
invention is characterized by the presence of a unique ingredient which 
swells upon addition to the silicate solution thereby providing 
substantial viscosity at a low level of addition and resulting in a 
composition which has the necessary tack so that a small quantity of 
adhesive may be added to the surfaces to be joined and result in a bond of 
superior adhesive strength. The composition of the present invention is 
especially well suited for use in a continuous process since the superior 
properties of the soy polysaccharide thickened silicate adhesive provides 
the composition with the consistency needed to permit the machines 
producing the laminated products to be operated at a higher running speed. 
The silicate containing adhesive composition of the present invention 
comprises an aqueous solution of an alkaline metal silicate which contains 
a soy polysaccharide material in an amount effective to thicken the 
silicate solution. Preferably, the silicate solution contains between 
about 1 to 25% by dry weight of silicate of a soy polysaccharide material. 
The soy polysaccharide product is preferred for use in the present 
invention and comprises the cellulosic fraction derived from the 
processing of soybean products such as the production of soy isolate from 
soybean flour, soy flakes and the like. The silicate adhesive composition 
of the present invention thickens readily at ambient conditions upon 
addition of the soy polysaccharide and provides a composition with the 
necessary viscosity to permit easy application to the interface of the 
surfaces to be joined, thereby preventing excessive penetration of the 
adhesive composition if one of the surfaces is exceptionally porous. 
Furthermore, the silicate containing adhesive composition of the present 
invention does not require substantial heating for thickening or providing 
the necessary adhesive strength. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The silicate adhesive composition of the present invention comprises an 
aqueous solution of an alkali metal silicate which is prepared in a 
conventional manner and the alkali metal silicate that is preferred to 
form the adhesive composition of the present invention is preferably 
sodium silicate but may include the silicates of other alkali metals such 
as lithium, potassium, rubidium, and cesium. The exact type of alkali 
metal silicate which is employed in the present invention is not critical 
to its practice and a variety of commercial grades are suitable all of 
which have varying ratios of alkali metal oxide to silica and the present 
invention is not intended to be limited by the exact type of alkali metal 
silicate that may be employed. 
The alkali metal silicate is initially dispersed and dissolved in water to 
form a solution having a typical but non-limiting level of alkali metal 
silicate of between about 20 to 45% by weight of said solution and 
preferably 26 to 36% by weight. Commercially available and prepared 
silicate solutions may be used with equal results. The silicate solution, 
because of the relatively high level of alkali metal silicate, typically 
has a pH of between about 13 and 14 and therefore represents a difficult 
problem insofar as the use of an additive and particularly a high protein 
additive to provide an adhesive composition since the protein material 
will readily dissolve in a solution with this pH thereby providing a 
solution which is incapable of thickening at ambient temperatures. A 
solution of this type is difficult to apply since it readily penetrates 
surfaces that are somewhat porous in nature. 
The important feature of the present invention is the use of a unique 
additive for addition of the alkali metal silicate solution. This 
material, upon addition to the silicate solution, swells, thereby 
providing increased viscosity at a relatively low level of solids and at 
ambient temperatures, to provide an adhesive composition which may be 
readily applied to surfaces which are to be joined together. While 
cellulosic fibers such as wood pulp and the like also thicken silicate, 
they are difficult to grind and add to the silicate adhesives and do not 
provide as uniform of an adhesive composition as that which is thickened 
with the soy polysaccharide material. This lack of uniformity is 
particularly undesirable in a continuous problem where consistency of the 
adhesive is very important. 
The unique material employed in the production of the silicate adhesive 
composition of the present invention is a soy polysaccharide material 
which comprises a mixture of residual high molecular weight sugars 
obtained by the treatment of soy flakes, meal or flour in order to remove 
the soluble proteins and carbohydrates. The heterogeneous polysaccharides 
are principally cell wall structural components of the soybean cotyledons 
and are typically obtained as a by product from the production of a 
vegetable protein isolate. The polysaccharide product has also sometimes 
been characterized as the aqueous alkaline insoluble residue or spent 
flake from soy isolate processing. 
In order to illustrate the manner in which the soy polysaccharides are 
obtained from a process for the production of a soy isolate, a soybean 
starting material such as soy flakes, meal or flour is dispersed in an 
aqueous medium and the pH of the medium adjusted to at least about 7. The 
flakes, meal or flour are washed with the aqueous alkaline solution and 
subjected to centrifugation. The soluble carbohydrate and protein 
components are contained in the aqueous extract and the solid residue is 
composed of the soy polysaccharide products which comprises the solids 
phase. The solids are also sometimes referred to as the spent flake 
residue and would be typically discarded in an isolate process. The alkali 
insoluble residue or solids phase is then subjected to a second extraction 
with an aqueous alkaline solution to extract more residual protein and the 
aqueous extracts are then combined for further processing and subsequent 
acid precipitation of the protein isolate. 
The residual material may then be cooked, dried or further processed to 
remove extraneous material if desired, therefore yielding the soy 
polysaccharide products which are preferably utilized in the present 
invention. The above description of a typical process for obtaining the 
soy polysaccharides is not intended to be limiting since this represents a 
conventional process known to those skilled in the art for the separation 
of the protein material and from the soy polysaccharide materials 
contained in the soy flour, flakes, meal or soybeans. 
The highly fibrous soy polysaccharide material has a typical analysis after 
drying of a total carbohydrate content of about 80% by weight, a protein 
content of about 10 to 18% by weight, and an ash content of about 5% by 
weight. The soy polysaccharide material generally comprises the many high 
molecular weight carbohydrate polymers primarily contained in soybeans 
such as arabinose, galactose, mannose and xylose. The term soy 
polysaccharides refers to the high molecular weight carbohydrate polymers 
containing a mixture of polysaccharides primarily composed of arabinose, 
galactose, mannose and xylose. It is normally employed as a collective 
term to describe the fibrous carbohydrate material obtained from the cell 
wall structural components of soybeans including soy flakes, flour or 
meal. 
It is preferred although not critical to grind the soy polysaccarides to a 
particle size of less than 0.15 mm diameter to provide for a uniform 
dispersion of the soy polysaccharide material in the adhesive composition. 
The soy polysaccharides are then added to the alkali metal silicate 
solution in an amount effective to thicken said solution and improve the 
adhesive properties thereof and typically between about 1-25% by dry 
weight of the silicate and preferably between about 2 and 8% by dry weight 
of the alkali metal silicate and most preferably between about 4 and 6% by 
dry weight of the alkali metal silicate. It is unique that such a small 
amount of this material is necessary to provide the necessary increase in 
the viscosity of the silicate containing adhesive composition and to do so 
without the application of heat. This is due to the unique characteristics 
of the soy polysaccharide product which does not substantially dissolve 
but swells in the highly alkaline silicate solution thereby providing an 
immediate and significant increase in viscosity of the product but without 
a substantial reduction in adhesive strength. This increase of viscosity 
at ambient temperatures provides a composition which has the necessary 
tack for application to the surfaces to be joined. This permits the 
application of a relatively small amount of silicate adhesive composition 
to achieve the necessary adhesion and further prevents undue penetration 
of the composition in the event either or both of the surfaces are 
relatively porous in nature. 
The soy polysaccharides are simply added to the alkali silicate solution by 
dispersing therein for such a period of time such that a thickened 
dispersion is obtained and the exact temperature or time for which this 
dispersion is achieved is not critical to the practice of the present 
invention. An optional ingredient that may be employed in the preparation 
of the alkali metal silicate adhesive composition of the present invention 
is the addition of a dispersing or wetting agent which improves the 
dispersion of the soy polysaccharide in the adhesive composition without 
adversely effecting the adhesive strength thereof. Suitable materials 
include the following--lecithin, polyethylene glycols and related 
derivatives, preferably in an amount sufficient to provide dispersion of 
the polysaccharide material and most preferably at a level of between 
about 0.2 to 2.0% by weight. The following Examples represent specific but 
non-limiting embodiments of the present invention.

EXAMPLE 1 
A commercially available solution of sodium silicate was used which 
contained 35% by weight of sodium silicate. 
A soy polysaccharide material, which represents the dried "spent flake" 
fraction from soy isolate production which is produced generally in accord 
with U.S. Pat. No. 3,885,052, having a moisture content of about 10% by 
weight was ground to a particle size such that at least about 99% of the 
ground material will pass a U.S. #100 mesh screen, which has an opening of 
0-15 mm. 
2, 4, 6 and 8% by dry weight of the silicate were then added to separate 
portions of the sodium silicate solution and were evaluated for viscosity, 
stiffness and glueability as compared to the 35% sodium silicate solution 
itself which served as the control. 
The viscosity of each of the above solutions was measured on a Brookfield 
Viscometer at 25.degree. C. using #1 spindle at 60 rpm. These results are 
set forth in Table 1. 
TABLE 1 
______________________________________ 
Viscosity of Silicate Solutions (CPS) 
% Soy Polysaccharide Material 
CPS 
______________________________________ 
0 (Control) 13 
2 15 
4 18 
6 22 
8 27 
______________________________________ 
It may be seen that the viscosity of the silicate adhesives containing the 
soy polysaccharide material was higher than the control and generally 
increased according to the amount of soy polysaccharide that was added. 
The glueability characteristics of each of the above solutions was measured 
by the Schmidt glueability test which is described in Operation Procedure 
For Measuring Glueability of Paperboard from Modern Metalcraft, Inc., 
Midland, Mich., at an application rate of 1.5 mil. The results of this 
testing are set forth in Table 2. 
TABLE 2 
______________________________________ 
Glueability Tests 
Set Time 
% Soy (seconds to 500 g force) 
Total Gram Force 
Polysaccharide 
0 sec. 15 sec. 30 sec. 
at 30 sec. set time 
______________________________________ 
0 (Control) 
39 24 15 1200 
2 40 16 15 1650 
4 40 22 15 1200 
6 36 22 15 1350 
8 38 22 15 1625 
______________________________________ 
For a zero second set time the time to a 500 g force ranged between 36 and 
40 seconds. For the 15 sec. set time, the range was 22-24 seconds except 
for the 2% soy polysaccharide product which took only 16 seconds. 
After a 30 sec. set time, all adhesives achieved a 500 g set force at 15 
seconds. It may be concluded that all adhesives including the control have 
equivalent time to a 500 g force. 
Table 2 also shows the total gram force which was measured at the 30 
seconds set time, which is representative to the overall bond strength. 
The adhesives containing the soy polysaccharide were equal or better in 
bond strength than the control. 
To evaluate the total amount of adhesive applied by the control and the 
silicate adhesive containing 8% soy polysaccharide, a fixed 1.5 mil 
thickness layer was applied to kraft liner board using a 1.5 bird bar. The 
silicate adhesive applied 4.7% dry weight of adhesive per single board 
weight while the control applied 5.0%. 
It may therefore be seen from the above data that silicate adhesives with 
the added soy polysaccharide material functioned as well as the pure 
silicate adhesive yet had a much better viscosity with a lower dry weight 
of application. 
EXAMPLE 2 
A commercially available solution of sodium silicate at 43% solids was 
used. To one portion of the solution 11.6% by dry weight of the silicate 
of soy flour was added. To a second portion of the silicate solution 11.6% 
by weight of the silicate of a soy polysaccharide material was added, 
which was produced as generally described in U.S. Pat. No. 3,885,052. The 
properties of each of the silicate solutions was visually observed and the 
viscosity at 60 rpm at 25.degree. C. was measured. The final solids level 
of both silicate solutions was 47.7%. 
It was determined that the silicate solution with the added soy flour had a 
viscosity of 1320 cps whereas the silicate solution with the added soy 
polysaccharide had a viscosity of 4100 cps. The soy flour further did not 
disperse or suspend well in the silicate solution and tended to separate 
at a much faster rate. 
EXAMPLE 3 
The sodium silicate adhesive with the added soy polysaccharide is evaluated 
in the production of a laminated paper product which consists of 7 plys 
glued together with the adhesive. The sodium silicate solution is prepared 
by preparing a 35% by weight of sodium silicate solution to which is added 
8% by weight of the silicate of a soy polysaccharide material produced as 
generally described in U.S. Pat. No. 3,885,052. The two materials are 
mixed for about one hour, then applied to the surface of each ply prior to 
joining the plys. At the same time, a comparable laminated product is 
prepared using the 35% by weight sodium silicate solution without added 
soy polysaccharide. The laminated products prepared with the silicate 
adhesive containing the added soy polysaccharides showed at least a 95% 
degree of glue bonding immediately after the plys are joined together 
whereas the laminated product produced with sodium silicate solution 
without added soy polysaccharide showed an 80-85% degree of glue bonding 
after about 2 hours time. 
Having described the present invention with reference to these specific 
embodiments, it is to be understood that numerous variations or 
substitutions may be made in the present invention without departing from 
the spirit thereof and it is intended to include all such variations, 
equivalents or substitutions within the scope of the present invention.