Note: Descriptions are shown in the official language in which they were submitted.
6~3
SATURATING GRADE PAPER
DESCRIPTION OF THE INVENTION
The present invention relates to decorative laminates, e.g.,
high pressure decorative laminates. More particularly, the present
invention relates to resin saturable paper, i.e., saturating grade paper
sheets that, when impregnated with a synthetic resin, are used as core
stock for decorative laminates. Still more particularly, the present
invention relates to the use of a combination of titanium dioxide and a
particular silica filler in such resin saturable paper sheets.
Decorative laminates have been in commercial production for
many years. Commonly, this class of laminate has been produced by
consolidating, under heat and pressure, a plurality of so-called core or
body sheets of fibrous cellulosic material, usually kraft paper, that
have been impregnated with a synthetic resinous composition, an
impregnated decorative color or print sheet placed over the plurality of
core sheets, and an impregnated surface or overlay sheet, which serves to
protect the decorative print or color sheet. Such decorative laminates
have been accorded wide acceptance in a multitude of uses such as wall
construction, table and desk tops, countertopsl furniture and the like.
Although the core sheet is less expensive than the print or
overlay sheet, the core sheet represents a significant cost factor
because of the number of core sheets used in the decora~ive laminate.
Typically from three to nine core sheets of 30 to 130 pound/per ream
(3000 ft2~ paper are used in the preparation of decorative laminates.
The principal inorganic oxide filler used in saturable payer sheet is
titanium dioxide. Titanium dioxide is particularly useful as 2 filler
because of its high optical scattering power resulting from its high
~5~6~
refractive index and uniform fine particle size. Titanium dioxide is
relatively expensive compared to other fillers used in the paper
industry, but its optical properties produces degrees of opacity and
brightness in the saturable sheet (and the resulting decorative laminate)
that are unattainable with other fillers. Reducing the a~ount of
titanium dioxide filler in saturable paper sheet without sacrificing
opacity and brightness would reduce the cost of the core sheet and permit
the production of larger quantities of such sheet when titanium dioxide
is in short supply.
It has now been discovered that from about 5 to about 40 weight
percent of the titanium dioxide utilized as filler in saturating grade
paper may be replaced with a particular amorphous precipitated silica
without compromising opacity and brightness. The BET surface area of
such silica may vary within a fairly broad range, but generally will fall
between about 25 and about 200, more particularly between about 40 and
120, e.g., from about 50 to about 100, square meters per gram. The pH of
such silica may vary from about 6 to about 9.5, but preferably will be
about neutral, i.e., the pH will be in the range of about 6.5 to 7.5.
The median agglomerate particle size of the silica is less than 30
micrometers, preferably between 3 and 10, e.g., between about 3 and 5,
micrometers. Other than calcium incorporated from the water used in the
manufacturing process, the amorphous precipitated silica is substantially
free of added, coformed or coprecipitated calcium compounds, e.g.,
calcium oxide or calcium salts, such as calcium silicate, calcium
chloride or calcium sulfate. The amount of such calcium compounds in the
amorphous precipitated silica (calculated as calcium oxide) is commonly
less than 1 weight percent, e.g., less than 0.75 weight percent.
~5~6~3
Certain grades of amorphous hydrated precipitated silica have
been suggested for use as a filler in types of paper other than saturabie
paper sheet, e.g, to replace a portion of the titanium dioxide pigment
used therein or to enhance a cheaper filler such as clay. The grade of
hydrated silica suggested for such use consisted essentially of about 78
percent silica and 5-7 weight percent calcium oxide. This silica is
alkaline - having a p~ of approximately 10 and a surface area of about 40
square meters per 8ram. It has not been used previously, to my
knowledge, in the manufacture of saturating grade paper, nor suggested
for use in such grade of paper.
DETAILED DESC~IPTION OF THE INVENTION
Amorphous precipitated silica that may be used to replace a
portion of the titanium dioxide filler in saturating grade paper is an
essentially neutral, finely~divided white powder. The median agglomerate
particle size of the silica is less than abou~ 30 micrometers, preferably
is between about 3 and 10, e.g., between 3 and 5, micrometers. The BET
surface area of the silica may vary between about 25 and about 200, more
particularly between about 40 and about 120 square meters per gram
(m2/g). Preferably, the surface area will range be~ween about 50 and 100
m2/K. Still more preferably, the surface area will range from about 60
to 90 m2/g. The silica is a hydrated, relatively pure silica, i.e.,
abou~ 88-90 weight percent silica, which is substantially free of
alkaline earth metal, e.g., calcium, compounds, such as oxides and salts,
e.g., calcium such as calcium oxide, calcium silicate, calcium chloride
and calcium sulfate.
-- 3 --
6~
The amount of adsorbed water present in the hydrated amorphous
precipitated æilica will commonly range between about 3 and about 7,
e.g., about 5, weight percent. The amount of adsorbed water will depend
partly upon the prevailing relative humidity. Adsorbed water is that
water which is removed from the silica by heating it at 105C. for 24
hours at atmospheric pressure in an oven. The silica also contains bound
water in amounts of between about 2 and about 6 weight percent. Bound
water is that water which is removed additionally by heating the silica
at calcination temperatures, e.g., 1000C.-1200C.
The preferred æilica is an essentially neutral material. The
pH of a 5 weight percent aqueous slurry of the pigment may register a pH
o between about 6 and 9.5, but preferably will be in the range of
between about 6.5 and about 7.5, e.g., about 7.
The aforedescribed amorphous precipitated hydrated silica may
be prepared by neutralizing an aqueous solution of an alkali metal
silicate with an inorganic acid, such as carbonlc acid, hydrochloric acid
or sulfuric acid. The soluble alkali metal silicate may be either a
commercial or technical grade of alkali metal silicate, e.g., sodium
silicate, potassium silicate or lithium silicate. Sodium silicate is
readily available commerclally and is the least expensive of the
aforedescribed silicates and hence is the alkall metal silicate of
choice. The alkali metal silicate may be represented by the molecular
formula, M20(SiO2)X, wherein M is the alkali metal and x is a
number of from 1 to 5. More commonly, x is a number from 2 to 4, such as
between 3.0 and 3.4, e.g., 3.2 or 3.3. Typically, sodium silicate having
an Na20:SiO2 ratio of from about 1:3.0-1:3.4, e.g., 1:3.2-1:3.3, is
used to prepare the aqueous solution of soluble silicate. The aqueous
sodium silicate reactant solution concentration can vary widely. For
-- 4 --
~Sl~i~3
example, sodium silicate solutions may be used having an ~a20
concentration of from about 18.75 grams per liter to about 90 grams per
liter.
Sulfuric acid is preferred for use as the acidification agent.
Acidification agent, e.g., sulfuric acid, is added gradually to the
aqueous sodium silicate solution to neutralize the alkali content of the
solution and precipitate the silica. A sufficient amount of
acidification agent is used in the neutralization so that the dried
silica product will exhibit an essentially neutral pH in the
aforedescribed range. Addition of acidification agent over a period of
from about 1 to about 4 hours, e.g., 2 to 4 hours, is typical. The
temperature at which acidification of the alkali metal silicate is
conducted may vary between about 175F. (79C.) and 210F. (99C.), more
typically between 180F. (82C.) and 190F. (88C.) ~gitation of the
aqueous reaction medium with a high speed, high shear ~ype agitator is
preferred as such agitation reduces the amount of gel formed during the
precipitation procedure.
The precipitated silica is recovered from the suspending
aqueous reaction medium by conventional solid-liquid separating means,
e.g., a filter pressJ drum filter, centrifuge, etc.; washed with water to
remove residual alkali metal, e.g., sodium, inorganic salts; dried; and
milled to the aforedescribed median agglomerate par~icle size.
Typically, the recovered precipitated silica is washed to reduce the
alkali metal salt, e.g., sodium sulfate, content of the silica to less
than about 2 weight percent, e.g., less than 1.5 weight percent, e.g.,
0.8-1.2 weight percent. Drying of the washed silica may be performed in
any suitable drying means, e.g., a rotary or drum dryer, a spray dryer,
or a compartment dryer. Depending upon the type of drying used, the
~:~5~6~3
dried silica if needed is milled or ground, e.g., fluid energy milled,
vertical milled, hammer milled, etc., and classified, if required to
obtain a median agglomerate particle size of less than 30 microns,
preferably less than 10 microns. Vertical milling is preferred.
In a typical preparation of the amorphous precipitated silica
described herein, a suitable reactor is charged with a foreshot of water,
which is heated with steam to the desired initial precipitation
temperature, e.g., from about 182F. ~83C.)to about 186F (86C~. ~n
aqueous solution of sodium silicate is then added to the heated water
until the concentration of sodium silicate is about 40 grams per liter
Na20. The agitator within the reactor is then started and concentrated
sulfuric acid is added gradually to the aqueous sodium silicate solution
reaction over a period of about 3 hours. Sulfuric acid addition is
continued until the pH of the reaction slurry reaches about 4. The pH of
the aqueous reaction slurry is monitored for a short period of time,
e.g., 15 to 45 minutes, to ensure that the pH of the slurry has
stabilized and does not drift upwardly, i.e., become more alkaline. The
resulting precipitated silica is separated from the aqueous reaction
slurry, e.g., by drum filter, spray dried and vertically milled to a
median agglomerate particle size of less than 5 micrometers, as measured
by a Coulter counter particle size analyzer.
The amount of essentially neutral amorphous precipitated
hydrated silica used in place of the titanium dio~:ide filler for
saturating grade paper may vary from about 5 to about 40 weight percent,
based on the titanium dioxide. More typically, the amount of silica used
will vary from about 10 to about 25 weight percent of the titanium
dioxide filler. Only that amount of amorphous precipitated silica is
used as a replacement/extender for titanium dioxide which will result in
~.~S~6~33
opacity and brightness values for the paper (and decorative laminate)
substantially equivalent to that obtained by the use of titanium dioxide
alone. Moreover> the laminate should not yellow when consolidated under
heat and pressure~ Yellowing may be assessed by testing the laminate in
a ~'adeometer.
The titanium dioxide used as filler in saturating grade paper
may be of the anatase or rutile crystalline form. Rutile titanium
dioxide is preferred for resin impregnated saturating grade papers.
The core sheet stock used for saturating grade paper may be
prepared using conventional papermaking procedures and pulp fibers
obtained from softwood, hardwood or mixtures thereof. The pulp may have
been prepared utili~ing acid (sulfite), alkaline (soda or kra~t), neutral
and mechanical pulping processes. The pulp may be bleached or unbleached
utilizing chlorine, hypochlorite, hydrogen peroxide, or hydrosulfite
bleaching procedures. Typically, saturating grade paper is prepared from
bleached kraft paper of about 30 to 130, e.g., 40 to 120, pound/ream
weight (3000 ft2). The filler for the paper, i.e., titanium dioxide, and
the replacement/extender silica described herein, may be added at any
point in the wet end of the papermaking system, e.g., at the head box or
beating or refining operations.
In preparing decorative laminates using saturating grade paper,
the core stock sheet is impregnated with synthetic laminating resin
compositions which are typically phenol-formaldehyde resins, although
other resins and combinations of resins may be used. For example, U.S.
Patents 3,220,916, 3,218,225 and 3,589,974 describe the use of
conventional phenolic resins to impregnate kraft core sheets in the
production of high pressure decorative laminates. U.S. Patent 3,983,907
and 3,975,572 disclose the use of a mixture of melamine-formaldehyde and
6~;3
acrylic resins, and U.S. Patent 4.473,613 describes a mixture of a
thermoset blend of a phenol formaldehyde resin, a cross-linked acrylic
resin and an optional melamine-formaldehyde resin.
The thermoset phenol-formaldehyde laminating resin used to
impregnate the core sheet is prepared by reacting phenol and formaldehyde
in a mole ratio of from about 1:1 to about 1:2.5 respectively. These
resins are liquid, water-soluble resins. The melamine-formaldehyde resin
is a water-soluble resin having a mole ratio of melamine to formaldehyde
of from about 1:1 to about 1:3 respectively and is prepared in accordance
with procedures well known in the art. Other laminating resins that may
be used include amine, acrylic, epoxy, polyester, silicone and diallyl
phthalate resins which may be prepared by art-recognized procedures. For
example, acrylic resins are produced in accordance with known acrylic
copolymerization procedures, i.e., solution polymerization, bulk
polymerization, emulsion polymerization, etc., utilizing any suitable
catalyst such as a free-radical generating material. The acrylic
polymers are well known in the art, e.g., Rhoplex~ HA~12 and TR-934
resins, which are sold by the Rohm and Haas Company. These products are
aqueous emulsions of an acrylic resin, e.g., a copolymer o~ ethyl
acrylate and methyl methacrylate.
The synthetic laminating resin compositions may be impregnated
into the core sheet, e.g., a kraft sheet, utllizing a dip and squeeze
treater or other known impregnating apparatuses, such as dip and spray,
reverse roll, etc., from an aqueous solution of the resin. The resin is
impregnated into the sheet in amounts ranging from about 15 to about 55
weight percent, preferably from about 20 to about 40 weight percent based
on the weight of the paper. Various additives may be added to the
aqueous resin blend. Additives such as urea, release agents, defoamers,
.6~3
catalysts, wet strength agents and cross-linking agents fall within the
category of useful additives.
Decorative laminates are typically comprised of three essential
layers; a core layer, a print layer and a surface layer. The core layer
comprises a bottom or supporting layer onto which the other layers are
bonded. In conventional high-pressure laminate manufacture, the core
layer consists of a plurality of cellulosic sheets, e.g., resin
impregnated kraft paper. The number of sheets used in the core layer may
vary from about 3 to about 9, e.g, 5 to 8, sheets. Superimposed above
the core layer i8 the print layer, which generally is an alpha cellulose
pigmented paper containing a print, pattern or design that has been
impregnated with a melamine-formaldehyde resin. Typically, the printing
is performed prior to lmpregnation by a high speed rotogravure. The
surface layer or overlay sheet, as it is commonly called, is typically a
high quality alpha cellulose paper impregnated with a melamine-
formaldehyde resin. This layer protects the print sheet from external
abuse, such as abrasion wear and tear, harsh chemicals, burns, spills and
the like.
In preparing the decorative laminate, the core layer, print
layer and surface layer are stacked in a superimposed relationship, the
resulting bundle of sheets placed between polished steel plates and
subJected to pressure and temperature for a time sufficiently long to
cure the laminating resins impregnating the respective layers.
Temperatures of between about 120C. and 250C. are typically used.
Decorative laminates may be prepared using both high and low pressure.
High pressure decorative laminates are typically formed using between
about 800 and about 1600 pounds per square inch (psi) pressure (5.5-11
MPa), e.g., 1000 psi (6.9 MPa). Flexible decorative laminates are
$~3
prepared using low pressures, e.g., at pressures below 5.5 MPa, e.g.,
175-225 (1.2-1.6 MPa).
Decorative laminates, particularly high pressure laminates,
find utility in the manufacture of furniture, kitchen counter tops, table
tops, store fixtures, wall paneling, partitions, doors, bathroom and
kitchen work surfaces and wallpaper.
The present invention is more particularly described ln the
following examples which are intended as illustrative only, since
numerous modifications and variations therein will be apparent to those
skilled in the art.
EXAMPLE 1
2964.5 grams of a 50/50 hardwood/softwood bleached kraft pulp
beaten to a Canadian Standard Freeness (CSF) of 475 ~/-25 milliliters
(ml) was mixed with a slurry of 26.7 grams of rutile titanium dioxide
(DuPont R-794) and 33.7 grams of deioni7ed water. 1.05 grams of paper
makers alum was added to the mixture (equivalent to about lO lbs. dry
alum per ton). The resultant blend was diluted to about a 0.03%
consistency with deionized water and a small amount of sodium aluminate
added to the resultant slurry to ad~ust the pa of the slurry to 6.3. A
series of 8 inch x 8 inch (20.3 centimeters x 20.3 centlmeters)
handsheets having a basis weight of 41 pounds/3000 ft2 were prepared from
the slurry on a Nobel and Wood machine. The handsheets were passed
through a felt press and then a drum dryer to reduce the moisture of the
sheets to about 5 weight percent. The sheets were then placed in a 50%
constant relative humidity room having a temperature of 72F. (22C.).
The brightness, opacity and percent ash of a representative sheet were
- 10 -
measured. Brightness and opacity were measured using an Elrepho~ meter
usin~ IS0 STANDARD Nos. 2470 and No. 2471 procedures respectively.
A handsheet was cut into 2 lnch (5.1 centimeters) x 8 inch
(20.3 centimeters) strips and each strip submerged for 45 seconds in a
50% aqueous solution of a melamine-formaldehyde resin (Cymel~ 412)
available from American Cyanamid Company. Each strip was removed from
the resin solution and drawn over a stainless steel bar to remove excess
resin. The strips were placed ln a 105-110C. oven for 5 minutes Co
polymerize the resin and remove moisture. The resin-saturated strips
were then placed in the constant relative humidity room. The opacity and
brightness of the resin-saturated strip was measured using the Elrepho~
meter.
Results obtained are reported in Table I.
EXAMPLE 2
The procedure of Example 1 was followed except that 2967.9
grams of pulp, 24.01 grams of titanium dioxide (TiO2) and 2.7 grams of
amorphous precipitated silica (SiO2) were used. The amount of silica
used representsd about a 10~ replacement of the titanium dioxide. The
silica had a BET surface area of about 71 m2/g, a pH of 6.8 (5% slurry in
water), a median agglomerate particle size of 4.3 microns and contained
about 0.48 percent sodium sulfate (x-ray determination). Results are
reported in Table I.
~5~ 3
EXAMPLE 3
The procedure of Example 1 was followed except that 2954.7
grams of pulp, 21.34 grams of titanium dioxide and 5.4 grams of the
silica of Example 2 were used. This amount of silica represented about a
20% replacement of the titanium dioxide. Results are reported in Table
I.
TABLE I
ORIGINAL SHEET SATURATED SHEET
% ~ BRIGHT- % BRIGHT-
EXAMPLE TiOz SiO2 NESS OPACITY ASH NESS OPACITY
__
1 100 - 91.0 96.0 23.5 83.8 87.6
2 90 10 91.3 9S.6 23.4 84.3 87.7
3 80 20 91.2 96.1 23.0 83.9 87.7
. . .
The data of Table I show that as much as 20% of the titanium
dioxide can be replaced with the described essentially neutral silica
without loss of the paper properties brightness and opacity. Ash values
for the various sheets are also constant around 23.0-23.5 percent.
EXAMPLE 4
The procedure of Example 1 was followed using 2948.8 grams of
pulp and 26.8 grams of titanium dioxide~ In addition 1.34 grams of an
aqueous solution of Cymel~ 412 resin was added to the wet end of the
paper making machine (0.5% on a furnish basis) as a wet strength
additive. Data is reported in Table II.
- 12 -
33
EXAMPLE 5
The procedure of Example 4 was followed using 2948.8 grams of
pulp, 24.2 grams of titanium dioxide, 2.7 grams of the silica of Example
2 and 1.34 grams of an aqueous solution of Cymel~ 412 resin. This amount
of silica represented about a 10% replacement of the tltanium dioxide.
Data is reported il~ Table II.
EXAMPLE 6
The procedure of Example 4 was followed using 2948.8 grams of
pulp, 21.4 grams of titanium dioxide and 5.4 grams of the silica of
Example 2 and 1.34 grams of an aqueous solution of Cymel~ 412 resin.
This amount of silica represented about a 20% replacement of the titanium
dioxide. Data is reported in Table II.
TA8LE I
ORIGINAL SHEET SATURATED SHEET
% % BRIGHT- % BRIGHT-
E~AMPLE TiO2 S102 NESS OPACITY ASH NESS OPACITY
4 100 - 91.0 96.1 25.5 ~4.1 87.8
5 90 10 90.8 95.9 23.5 83.3 87.1
6 80 20 90.2 96.0 22.4 83.2 87.1
The data of Table II shows that as much as 20% cf the titanium
dioxide can be replaced with the described essentially neutral silica in
saturated sheets containing 0.5 weight percent wet strength additive
- 13 -
6~3
Although the present process has been described with reference
to specific details of certain embodiments thereof, it is not intended
that such details should be regarded as limitations upon the scope of the
invention, except as and to the extent that they are included in the
accompanying claims.
