Process for making synthetic paper pulp

Synthetic thermoplastic polymer fibers are prepared by extruding a foamed fibrillated sheet of a normally solid synthetic thermoplastic polymer, attenuating the molten foamed sheet as it is extruded and applying either a cutting and/or a shearing action to the solid attenuated sheet to cause it to break down into short fibers containing numerous fibrils. The polymer is polypropylene, polyethylene or a mixture of polystyrene and polypropylene or polyethylene in which the mixture contains more than 40 weight percent of the polyolefin. These short fibers can be blended with natural pulp. The resulting blend can be wet laid to form a coarse paper and then formed into a final paper product by application of heat and pressure. The resulting paper has certain superior properties.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The useful process disclosed hereinafter is directed to making 
thermoplastic polymer fibers suitable for mixing with cellulosic pulp. The 
process is also directed to the mixing of the fibers with cellulosic pulp 
to make paper having improved properties. One example of a useful 
cellulosic pulp is bleached kraft paper. 
2. Background Art 
U.S. Pat. No. 3,770,856 discloses the production of fine fibrous structures 
by flashing an aqueous emulsion containing a polymer and solvent from a 
higher pressure and temperature zone to a lower pressure and temperature 
zone. The resulting structure is macerated in a mixer whereby obtained are 
fine flat fibers in a fibrillar state of an average width of from 5 to 10 
microns and length of from 3 to 5 mm. The resulting fibers can be pressed 
into a synthetic paper. 
British Pat. No. 1,221,488 discloses a process for the production of yarn 
which involves extruding a blend of polyethylene and blowing agent so as 
to produce an extrudate of foamed polyethylene. The latter is drawn so 
that it becomes orientated essentially in the direction of extrusion. The 
drawn foamed polyethylene is subjected to forces such that the walls of 
the foam are broken down. The resulting extrudate is a three-dimensional 
structure of interconnected fiber elements. 
U.S. Pat. No. 3,634,564 discloses a process for the manufacture of 
fibrillated foamed films involving mixing a thermoplastic polymer with a 
blowing agent and extruding the mixture into a foamed polymer film and 
thereafter stretching the foamed film uniaxially. The stretching at an 
elevated temperature causes the voids of the foam to split. 
The last two aforementioned patents disclose that the extrudate is 
orientated. Orientation refers to a process wherein the crystalline 
structure in polymeric materials are placed in alignment so as to produce 
a highly uniform structure. It is believed that orientation causes the 
axes of the molecules of the polymer to more generally line up in the same 
direction. Generally orientation is obtained by stretching (or pulling) 
the polymer while its temperature is below its melting point but above its 
transition temperature. But orientated extrudate, upon cutting and/or 
shearing into short fibers, does not yield suitable fibers for mixing with 
cellulosic pulp. 
U.S. Pat. No. 3,954,928, discloses a process for the preparation of 
fibrillated extrudate by extruding a molten thermoplastic resin containing 
a foaming substance through a die. The extrudate is quenched almost as it 
leaves the die to a temperature below the resin's glass transition 
temperature. The resin can be a blend of polystyrene and a polyolefin 
e.g., polyethylene but the latter is present in an amount of at most 40% 
and preferably 30% or less by weight based on the blend. 
Surprisingly applicant has found that a polyolefin by itself can be 
processed to fibers suitable for blending with cellulosic pulp. Equally 
surprisingly is that applicant has found that a mixture of polystyrene and 
polyethylene or polypropylene in which the mixture contains more than 40 
weight % of the polyolefin is equally suitable for blending with 
cellulosic pulps. 
SUMMARY OF THE INVENTION 
The present invention relates to a process of preparing synthetic pulp from 
a synthetic thermoplastic, fiber-forming polymer. The resin is extruded 
along with a blowing agent to form material having interconnected fibrils 
and fibers. The material is attenuated as it leaves the extruder die to 
induce formation of fibers and fibrils. The attenuation occurs while the 
polymer is in a molten or amorphous state. The resulting fibrillated 
material is then subjected to a cutting and/or shearing action which 
results in the foamed material breaking down into small short fibers 
having many attached fibrils. This synthetic fibrous material readily 
mixes with cellulosic pulp to allow preparation of a paper sheet having 
varying amounts of the desired synthetic polymer. The resulting paper, 
depending on the particular polymer and the amount can have properties 
superior to that of paper of only cellulosic pulp. The synthetic 
thermoplastic polymer is selected from the group consisting of 
polypropylene, polyethylene or a mixture of polystyrene and polypropylene 
or polyethylene in which the mixture contains more than 40 percent by 
weight of the polyolefin. 
This process has the advantage of being simple. Furthermore, in order for 
the synthetic fibers to be suitable for use in conventional paper making 
machinery this process allows for variation in fiber length, 
cross-section, composition and makes a fibrous structure having many fine 
fibrils to provide for good interconnection. In addition for this 
application the process is as economical as possible which helps keep the 
cost of the synthetic fibers (pulp) as close as possible to that of 
natural pulp. 
DESCRIPTION OF THE INVENTION 
The present invention involves extruding a foamable melt of normally solid 
synthetic thermoplastic polymer through a means to form a mass of 
interconnected fibers and fibrils which is attenuated and taken up on a 
suitable device. 
The means used to form the mass can be a slot die which is either circular 
or flat. Practicality and convenience are the only limits to the width or 
diameter of the die. Generally the slot will be from 5 to 100 mils in 
thickness. Below about 5 mils the amount of material being extruded 
becomes so small as to be impractical. Above about 100 mils in thickness 
the sheet becomes increasingly difficult to attenuate and to break into 
fibers and the fibers become undesirably coarse. 
The mass is attenuated as it leaves the die to induce fibril formation. 
Generally this attenuation is at a rate of from about 20 to about 200 
times the rate linear at which the sheet is being extruded; preferred 
rates are from about 50 to about 150. This attenuation takes place while 
the polymer is still in the molten or amorphous state. The mass can have 
any one of the numerous shapes and be in almost any size. Such a mass can 
be referred to as a sheet. Attenuation refers to the stretching or pulling 
of the polymer while its temperature is generally above its melting point 
or the polymer is in an amorphous state. 
The normally solid synthetic thermoplastic polymer suitable for use in this 
invention is polyethylene (low, medium or high density) or polypropylene 
including isotactic polypropylene or a mixture of polystyrene and 
polyethylene or polypropylene in which the mixture contains more than 40 
percent by weight of the polyolefin. Fibers with more than 60 percent by 
weight of polystyrene show poor properties when exposed to an elevated 
temperature or a solvent. These poor properties can adversely affect the 
properties of the resulting paper when the fiber is used admixed with 
cellulosic pulp. 
The optimum conditions used can vary considerably depending on the choice 
of the polymer system. The length and diameter of the fibers can be in 
part controlled by varying the amount of blowing agent used, the 
temperature both in the extruder and of the quench, the polymer through-up 
rate through the extruder and die, the amount of attenuation achieved by 
means of the take off rate and the geometry of the die-take-off system. 
The extruder can be equipped with a port to inject the blowing agent. If 
this is done, various blowing agents may be used such as the various 
Freons.sup.R, methylene chloride, nitrogen, carbon dioxide, water, etc. If 
the extruder is not equipped with a port to inject the blowing agent the 
blowing agent is fed into the extruder along with the resin being 
extruded. While this can be done by coating the resin pellets or powder 
with a low boiling liquid such as pentane which becomes a gas in the 
extruder, it is preferred to blend a solid physically or chemically 
decomposable blowing agent with the resin and then to feed the resulting 
blend into the extruder Exemplary chemical agents include but are not 
limited to azobisformamide, azobisisobutyronitrile, diazoaminobenzene, 
4,4-oxybis(benzenesulfonylhydrazide), benzenesulfonylhydrazide, 
N,N'-dinitrazopentamethylenetetramine, trihydrazinosymtriazine, 
p,p'-oxybis(benzenesulfonylsemicarbazide)-4-nitrobenzene sulfonic acid 
hydrazide, beta-naphthalene sulfonic acid hydrazide, 
diphenyl-4,4'-di(sulfonylazide) and sodium bicarbonate or mixtures of 
sodium bicarbonate or sodium carbonate with a solid acid such as tartaric 
acid. The amount of foaming agent used in the process generally is in the 
range of from 0.1 to 20 wt.% of the resin being extruded with from 0.1 to 
5.0 wt.% being the preferred range. 
The attenuated fibrous sheet is then broken down into fibers. One way of 
accomplishing this is to feed the fibrous sheet against a hot air blast 
which pulls the fibers apart. Another technique involves applying a 
shearing action to the fibrous sheet to break the fibrous sheet down into 
fibers. This is most easily accomplished by placing portions of the 
fibrillated sheet in a liquid which is being violently agitated such as by 
rapidly rotating paddles. 
In order to better control the fiber length, it is preferable to use a 
mechanical cutting step. A "flock" cutter as is used in the textile 
industry are suitable for this step as are plastic granulators of suitable 
design. The aforementioned cutter is a mechanical cutter designed to cut 
fibers to uniform lengths and thereby permit the fibers to be used, for 
example, in blankets by electrostatic deposition on polyurethane foam. For 
most applications a cut length of 1/8" to 3/16" is most suitable, while 
for specialty application, longer or shorter fibers could be preferable. 
In addition to cutting some shearing may enhance the utility of the 
fibers. 
It is preferable to use standard papermaking processing to accomplish the 
sheet break-down to fibers containing fibrils. Standard pulp defibering 
equipment can be used. With control of the volume and temperature of the 
water used, it is possible to grind the cut polymeric material alone. It 
is, however, preferable to blend the cut sheet with natural pulp and feed 
the mixture to the mill to accomplish both the blending and defibering 
operation simultaneously. Chemicals such as starch or polyethyleneimine 
can be added to the mixture before grinding to improve dispersion and 
ultimate paper properties. 
The fibers can be used directly to form paper. However, the resulting 
product is generally more expensive than is desired for most uses to which 
paper is applied. Generally the synthetic pulp produced in accordance with 
the present invention is blended with from 10 to 90 wt.% of natural 
cellulosic pulp or other cellulosic materials such as bleached kraft paper 
fibers, and then laid to form a paper. The coarse wet laid material finds 
use as filter paper, paper towels, etc. However, it is preferred that the 
coarse wet laid paper be subjected to the application of heat and pressure 
in order to improve the strength and surface finish thereof. For 
individual paper sheets a press operated at from 10 to 500 p.s.i. and 
50.degree. to 150.degree. C. for from 0.2 sec. to 5 minutes is 
satisfactory. For long rolls of paper heated pressure rolls are used. 
Generally, these are heated metal rolls such as heated steel rolls 
operated at from 2 to 200 lbs. per linear inch pressure, from 50.degree. 
to 150.degree. C. and the paper is fed at a rate of from 10 to 1500 feet 
per minute. 
The product paper finds use in the applications to which conventional paper 
finds use such as writing paper, bagging, packaging, wallpaper, etc. 
However, the paper produced in accordance with the present invention finds 
its greatest advantage over conventional paper in the packaging area due 
to its improved wet strength and heat sealability and in printing papers 
where its high whiteness and smoothness are important.

The present invention will be further explained with reference to the 
following examples. 
EXAMPLE I 
A one inch Killian extruder having a length to diameter ratio of 24:1 was 
equipped with a screw having a two-stage mixing head. Temperatures were 
measured at four points on the extruder. These were: (A) entry point of 
polymer pellets; (B) at the midpoint of the screw; (C) near the mixing 
head and (D) at the die. The die was an 8 inch slot set at 0.020 inch 
opening. The quench was at room temperature, air stream impinging on both 
sides of the fibrous sheet at about one inch from the die lips. The 
resulting fibrous sheet was passed through a pair of rolls to maintain the 
rate of take off and was then wound up on a paper core. The mixture being 
extruded was prepared by shaking together 500 g. of polypropylene having a 
melt index of 10 and 500 g. of Dow Styron polystyrene (general purpose U 
660) pellets having a melt index of 5 and 20 g. of anhydrous sodium 
bicarbonate powder. The temperature along the various points along the 
extruder were: (A) 250.degree. F.; (B) 350.degree. F.; (C) 450.degree. F. 
and (D) 450.degree. F. The throughput was at the rate of 7 lbs. per hour 
and the fibrous sheet was taken up at the rate of 35 ft. per minute. The 
attenuation ratio was at about 22 to 1. The ratio was not precisely known 
because density of the mixture was not known at exiting temperature and 
die gap was not known perfectly at operating temperature and pressure. The 
foregoing were not large but make ratios uncertain by an estimated 
.+-.10%. 
A 2.5 g. sample of the fibrous sheet structure and 750 ml. of water were 
placed in a two-speed commercial type Waring blender. The blender was 
operated at high speed for 10 minutes. A blender both cuts and shears. 
Shearing is different from cutting in that it means pulling fiber 
lengthwise apart whereas cutting means determining the length of the 
fibers. Then 7.5 g. of bleached kraft paper was added to the blender and 
the blender was operated at high speed for an additional 10 minutes. The 
resulting mixture was poured into a suction filter about 6 inches in 
diameter to remove the water, pressed with a flat surface and air dried 
for a few minutes. The resulting coarse paper was then removed and dried 
further. The paper was then pressed at 250.degree. F. and 40,000 lbs. 
pressure between metal plates for 0.5 minutes for additional bonding. The 
product was a smooth white paper which could be written on with both pen 
and pencil, and had qualitatively good strength. The paper exhibited 
exceptionally good wet strength. 
EXAMPLE II 
Example I was repeated except that 500 g. of low density polyethylene 
having a specific gravity of 0.95 and a melt index of 5 was substituted 
for 500 g. of the polypropylene to provide a 50:50 blend of polyethylene 
and polystyrene. The take off rate from the extruder was reduced to 20 
feet per minute and the pressing temperature of the mixture was 
215.degree. F. The throughput was at about 2.5 lbs/hour and the 
attenuation ratio was at about 40 to 1. The product paper had a smooth 
white appearance. It could be written on with pen or pencil and had 
qualitatively good strength both wet and dry. 
EXAMPLE III 
Example I was repeated except that 1000 g. of polypropylene having a melt 
index of 10 was substituted for the 50/50 mixture and except that the 
pressing temperature of the mixture was 275.degree. F. The product paper 
had a smooth white appearance. It could be written on with pen or pencil 
and had qualitatively good strength both wet and dry. 
EXAMPLE IV 
Example I was repeated with the following exceptions: polypropylene (Profax 
6323, melt index about 15) was used instead of the 50/50 mixture; the 
first heater was at 350.degree. F., the second at 400.degree. F.; the 
throughput was 5 lbs/hours; and the take off rate was 90 ft./min. Under 
these conditions, the attenuated ratio was about 70 to 1. 
EXAMPLE V 
Example IV was repeated using a take off rate of 130 ft./min. Under these 
conditions the attenuation ratio was about 100 to 1. 
EXAMPLE VI 
The sheets from Examples IV and V were cut on a mechanical cutter (helical 
shearing action). Various screens were used with 1/8" giving the most 
suitable material. Cutting was done dry from rolls. The product air 
conveyed well. 
EXAMPLE VII 
The cut products from Examples IV and V were defibered on a pulp mill. It 
was necessary to use larger than normal quantities of cold water to 
prevent fusion when only synthetic material was added. When synthetic 
material was mixed at 10 to 50% levels with kraft pulp board (dry), no 
difficulty was experienced in running the material under normal Kraft 
conditions. The resulting pulp was adequately wet for dispersion even 
without additives. Small hand sheets were prepared. These were smooth, 
white (for bleached Kraft) sheets that resembled ordinary paper in 
appearance. Examination showed that improved dispersion (from chemical 
additives) would be desirable for many applications. Tensile strengths 
were lower and opacities higher for the samples containing synthetic pulp. 
Properties such as heat-sealability or plastic forming require 25-50 
weight % of the synthetic pulp. Retention of strength under conditions of 
high humidity was improved. The dry tensile strength was about 75% of a 
control Kraft paper sample while the wet tensile strength was about 200% 
of the control. 
Other methods of cutting, such as mechanical cutting, will yield equally 
suitable synthetic fibers.