Method for incorporating nucleating agents in propylene polymers

An inexpensive yet reliable method for incorporating a minor fraction of a percent by weight of nucleating agent in a propylene polymer is provided in which the nucleating agent is dissolved at a concentration of less than 10% in a homogeneous, alcohol-based, carrier liquid having a normal boiling point of not over about 120.degree. C. The resultant solution is atomized into a full jet spray of fine droplets which is directed with full and even coverage onto a loose pulverulent bed of propylene polymer powder over a period of time and with sufficient agitation of said bed to expose essentially all segments of the bed to direct contact with said spray.

FIELD OF INVENTION 
This invention relates to improved methods of incorporating nucleating 
agents in propylene polymers. 
BACKGROUND OF THE INVENTION 
It is well known that a large number of higher melting particulates can be 
used as nucleating agents in highly crystalline polymers of propylene and 
other crystallizable thermoplastic polymers. As disclosed in U.S. Pat. No. 
3,367,926, a wide variety of higher melting compounds will function as 
nucleating agents if present as small particles in the molten polymer as 
it solidifies upon cooling. 
Many practical advantages can accrue from the proper use of effective 
nucleating agents in crystallizable thermoplastics. For example, as shown 
in U.S. Pat. No. 3,852,237, superior clarity or translucency characterizes 
films and articles formed from such polymer melts containing fine 
particulate nucleating agents in low concentrations of of from 0.001 to 
0.5% by weight. 
However, in commercial practice, it is difficult to achieve a completely 
uniform dispersion of the nucleating agent throughout the bulky mass of 
base polymer. In fact, it has generally been necessary to use high energy, 
intensive mixing equipment in order to avoid visible specks in the 
finished product and to use unduly prolonged mixing cycles in order to 
assure a uniform concentration of the finely dispersed nucleator 
throughout the entire base polymer in commercial scale operations. 
OBJECTS OF THE INVENTION 
A major object of this invention is to provide simpler and more economical 
procedures for achieving a uniform distribution of a minor proportion by 
weight of finely dispersed particles of a higher melting nucleating agent 
throughout a large commercial scale quantity of highly crystalline 
propylene polymer. Further objectives are to avoid the necessity of using 
high speed or high intensity mixers and to reduce energy and equipment 
costs. Other objects and advantages will be made more apparent from the 
detailed disclosure which follows. 
SUMMARY OF THE INVENTION 
The above objects are accomplished in accordance with this invention by 
dissolving the nucleating agent at a concentration of less than about 10% 
by weight in a carrier liquid which has a boiling point at normal 
atmospheric pressure or not over about 120.degree. C. and which does not 
appreciably dissolve isotactic polypropylene at temperatures below 
120.degree. C.; atomizing the resultant solution into a uniform and 
symmetrical, full jet spray of fine droplets averaging less than about 2 
millimeters in diameter and projecting said spray of fine droplets with 
full and even coverage onto a loose, pulverulent bed of fluffy polymer 
powder, of which at least about 50% by weight is composed of individual 
particles which are smaller than about 0.25 millimeter in equivalent 
spherical diameter while said bed of powder is being agitated sufficiently 
to expose essentially all portions thereof to direct contact with said 
spray at some point during the atomizing of a sufficient quantity of said 
solution to provide the desired proportion of nucleating agent, i.e. 
generally somewhere between about 0.005% and 0.5% (i.e. between about 50 
and 5000 parts by weight per million parts of polymer). 
The relative amount of nucleating agent solution used in forming said spray 
is a major factor in achieving a uniform distribution of nucleating agent 
in the finished polymer, and the weight ratio of said solution to fluffy 
powdered polymer in the bed should generally be at least 1/200 and 
preferably at least 1/100. This ratio corresponds to the ratio of the 
nucleating agent concentration in the finished polymer to the nucleating 
agent concentration in the sprayed solution. Therefore, the concentration 
of said agent in said solution should generally be no more than 200 and 
preferably no more than 100 times the desired concentration in said 
finished polymer. By the same token, the concentration of said agent in 
said solution should generally be at least 10 times the desired 
concentration in said finished polymer to avoid excessive wetting of the 
polymer powder bed with resultant lumping, caking, reduced flowability and 
extra energy requirements for vaporizing the carrier liquid. 
Generally speaking, said carrier liquid can be derived from a considerable 
number of liquid solvents which have normal boiling points of about 
60.degree.-120.degree. C. and which are essentially inert to the base 
polymer. However, water and certain lower aliphatic alcohols are excellent 
liquid components as will be subsequently explained herein in more detail. 
DETAILED DESCRIPTION OF THE INVENTION 
The major considerations in selecting ingredients for the carrier liquid of 
this invention are: (1) adequate solvent power to dissolve at least a 
substantial fraction of a percent by weight (e.g. about 0.5%) of 
nucleating agent; (2) inertness toward crystalline polymer (i.e. no 
significant chemical or solvent attack thereon); and (3) having a normal 
boiling point of not over about 120.degree. C. so that it can be removed 
by vaporization below melting point of polymer. For use in nucleating 
propylene polymers, lower aliphatic alcohols and of course water generally 
satisfy these stated criteria for many of the most effective nucleating 
agents. Therefore, such alcohols and blends thereof with water are the 
preferred carrier liquids in this invention. Alcoholic blends containing 
significant proportions of water generally provide the best balance of 
properties, and such mixtures containing about 10 to about 50% water by 
weight offer the outstanding safety advantage of producing vapor mixtures 
which are not explosive or readily combustible in air, particularly when 
propyl and butyl alcohols (which have normal boiling points generally 
within .+-.20.degree. C. of water) are used therein. In fact, certain 
constant boiling or azeotropic mixtures of alcohols and water are 
available (such as 12.5% water+87.5% isopropanol by weight), and such 
azeotropes constitute nearly ideal carrier liquids for the present 
invention. 
Desirable nucleating agents for use in this invention are commonly found in 
the general class of metal salts of aryl carboxylic acids. A long list of 
exemplary compounds of this class is presented in Table I of U.S. Pat. No. 
3,367,926 which is hereby explicitly incorporated in the present 
disclosure by this specific reference thereto. Preferred for present 
purposes are the alkali metal salts and particularly alkali metal salts of 
aryl monocarboxylic acids, such as benzoic acids. With the preferred 
nucleating agents, the preferred proportion of nucleating agent used in 
the base polymer is from about 0.01% to about 0.25% by weight (i.e. from 
about 100 to about 2500 ppm). 
The highly crystalline polymers of primary interest here are derived 
predominantly from propylene (i.e. at least 75% by weight) and are 
produced by well known processes involving the use of stereospecific 
catalyst systems of the class generally referred to as Ziegler-Natta type 
catalysts. For example, various combinations of transition metal halides, 
such as titanium trichloride, with metal alkyls, such as aluminum alkyls, 
are often involved in such catalyst systems. Homopolymers, and copolymers 
of propylene with other simple 1-olefins such as ethylene and butene-1 
which contain more than 80% propylene by weight, are preferred. Copolymers 
of propylene with ethylene containing between about 2 and about 10% by 
weight of ethylene are especially suitable. 
It is particularly important in attaining the objects of this invention 
that the crystalline polymer of propylene be in a finely divided state 
when it is sprayed with the finely atomized, dilute solution of nucleating 
agent. As previously indicated, most of the particles of powdered polymer 
should be smaller than about 0.25 millimeter (250 microns) in equivalent 
diameter, and it is preferred that the mean particle size be less than 100 
microns. Since crystalline polymers and copolymers of propylene are 
normally produced as fine powders by the polymerization reaction, the 
freshly prepared polymers immediately after washing them to remove 
catalyst residues and drying them sufficiently to place them in a fluffy, 
pulverulent state are ideal for use in this invention. For example, when 
dried to a residual volatile content of about 1 to 3% by weight, such 
finely particulate polymers will be free flowing, easy to handle and 
agitate, and will usually exhibit an apparent bulk density of not over 
about 25 pounds per cu. ft. 
One specific preferred embodiment of this invention comprises the use of 
the solution spray technique described herein to introduce nucleating 
agent to a fluffy bed of freshly made crystalline propylene polymer during 
the commercial drying thereof, namely at a point where the volatile 
content thereof has been reduced substantially below about 5% by weight, 
preferably to between about 1 and 3% by weight. Since the commercial 
drying equipment conventionally used includes rotational agitation means 
for stirring the bed of fluffy polymer powder, the spraying step can 
readily be carried out therein by arranging suitable spray nozzles at 
points above the bed providing full and even coverage thereof by the 
projected solution spray therefrom while agitating the bed at a suitable 
rate. This integration of the spraying step with the conventional drying 
step provides many economic advantages including energy savings in taking 
advantage of the sensible heat already in the powdered polymer being dried 
and simplifying the removal of the carrier liquid from the finished 
polymer by accomplishing same simultaneously with the removal of the last 
fraction of volatile residues already in the freshly manufactured polymer. 
Many different types of atomizing spray nozzle can be used to produce 
regular and symmetrical sprays of the solution of nucleating agent. 
However, the pressure atomization types are generally preferred in the 
practice of the present invention since they are generally more readily 
controllable to generate reproducible full jet sprays of definite shape in 
which relatively uniformly sized individual droplets can be reliably 
delivered to the full bed of powdered polymer. 
Regardless of how the full jet sprays of fine droplets of nucleating agent 
solution are generated, it is desirable that same be projected directly 
onto the pulverulent bed of fluffy polymer powder with full and even 
coverage thereof. Sprays generated by the preferred pressure atomizing 
nozzles, normally are projected in the form of diverging (conical or 
pyramidal) patterns. Such nozzles are available in almost any size or flow 
capacity of practical interest, having total angles of divergence up to 
about 90.degree.. Accordingly, regardless of the size and shape of the 
pulverulent bed of polymer powder, spray nozzles can be arranged to 
provide full and even coverage thereof. However, the relatively wider 
angled nozzles are likely to be most practical for commercial operations, 
since their spray patterns cover a wider bed area for a given spacing 
distance from the bed and thus tend to reduce the total number of nozzles 
required in a given situation. 
As previously indicated, the quantity of nucleating agent solution employed 
in providing the desired concentration of nucleating agent in the finished 
polymer is a very important consideration in this invention. Thus, the 
amount of solution used should be at least about 1/200th and preferably at 
least about 1/100th by weight of the polymer in order to provide extended 
contacting opportunity throughout the bulky polymer mass, as for example 
by providing sufficient spray application time in which to agitate the 
fluffy polymer powder bed and thus expose all portions thereof to direct 
contact with the spray. For example, using multi-blade or paddle type 
agitators radially mounted on a rotary shaft running through the middle of 
a generally horizontal bed of a few feet in depth, operation at around 10 
to 30 revolutions per minute provides adequate exposure of all parts of 
bed which is fully covered by a projected spray of solution for at least a 
few minutes. The most desirable range of ratios of said solution for use 
with well agitated polymer powder beds appears to lie between about 
1/100th and about 1/20th by weight of the polymer. Larger proportions of 
such solutions should not be resorted to unless necessary to achieve 
adequate overall wetting of an unusually large (deep) or poorly agitated 
bed or because of low solubility of the nucleating agent in the carrier 
liquid. As indicated earlier, the concentration of nucleating agent in the 
sprayed solution should always be at least 10 times the desired 
concentration in the finished polymer in order to avoid adding such large 
proportions of solution (i.e. over 10% by weight) as to detract from the 
free flowing character of the bed and cause problems in drying the 
finished polymer by vaporizing the carrier liquid therefrom. 
It is generally preferred that the carrier liquid be sufficiently volatile 
to be removed by vaporization without applying partial vacuum or using 
subatmospheric conditions. However, in addition to applying heat to the 
fluffy bed of polymer powder indirectly, e.g. by means of steam jackets or 
electric heaters applied to the container walls, the passage of a hot 
inert sweep gas slowly through the bed is also helpful in drying the 
polymer product to the final state desired (usually no more than a minor 
fraction of percent of residual volatile content) without heating the bulk 
polymer to an excessive temperature. In this way, carrier liquids, 
including components such as C.sub.4 monohydric alcohols having boiling 
points up to about 120.degree. C., can be used and successfully vaporized 
out of the polymer product without damaging same. Once the volatile 
content of the polymer product has been reduced to the order of only 0.1 
or 0.2% by weight, it can then be satisfactorily melt processed or 
fabricated by conventional methods into other forms and/or finished 
articles. For example, it can be extruded to form granules or films, 
and/or blow molded or otherwise shaped into bottles or other useful 
structures. 
Other special function additives are often incorporated in crystalline 
propylene polymers in minor proportions, such as antioxidants, antistatic 
agents, stabilizers, antiblocking agents, lubricants, etc. In most cases, 
these materials can be added to the freshly prepared polymer at any time, 
e.g. either before or after the nucleating agent solution has been sprayed 
onto the polymer. However, when lubricants such as silicones, metal 
stearates, etc., are to be used in a polymer which is to be nucleated in 
accordance with this invention, it is recommended that the spraying of the 
nucleating agent solution onto the agitated bed of fluffy polymer powder 
be completed before the lubricant additives are introduced to the polymer 
in order to avoid possible interference by the lubricant in achieving 
thorough and uniform wetting of the polymer powder by said solution.

Various details of our invention will be more fully understood in the 
perspective of the specific illustrative embodiments described in the 
following examples. 
EXAMPLE 1 
Eight thousand pounds of a freshly polymerized propylene-ethylene copolymer 
containing about 2% by weight of ethylene, following catalyst deactivation 
and purification by washing with hot solvent to remove catalyst residues 
and reduce atactic content, was dried to a volatile content of between 1 
and 2% by weight while agitating same in a steam heated, annular jacketed, 
elongated horizontal drier. The drier was about 16 feet long and about 5 
feet wide in the upper portion with a smoothly rounded (semicylindrical) 
bottom section having a radius of curvature of almost 2.5 feet, and was 
equipped with a multibladed agitator consisting of radial arms or blades 
mounted on a rotatable shaft extending through the drier at approximately 
the focal point axis of the rounded bottom section. 
This nearly dried copolymer was a fluffy powder, over half of which by 
weight was finer than 80 mesh (Tyler Series) and it formed a pulverulent 
bed having U-shaped cross section nearly 5 feet deep at the center line of 
the drier. The only additive incorporated into the polymer at this point 
was about 0.1% by weight of a hindered phenolic antioxidant which had been 
thoroughly mixed into the nearly dried polymer powder. A small sample of 
this fluffy powder was removed and set aside to serve as a sample of 
unnucleated copolymer before proceeding to incorporate about 300 ppm of 
sodium benzoate into the balance thereof using the following procedure. 
A solution containing about 1.5% sodium benzoate by weight was prepared by 
dissolving 2.4 lbs. of sodium benzoate in 157.5 lbs. of an azeotropic 
mixture of water (12.5%) and isopropanol (87.5%). While rotating the 
multiblade agitator at 18 rpm, the resulting solution was atomized through 
four full-jet spray nozzles directed toward the surface of the agitated 
bed of hot fluffy polymer powder. The nozzles were No. 1/4HH12SQ 
pressure-type spray nozzles made by Spraying System Co. of 316 stainless 
steel, providing a full jet, square or pyramidal spray pattern with about 
a 70.degree. angle of divergence. They were positioned along a line 
somewhat more than 3 feet above the center of the bed at distances about 
2, 6, 10 and 14 feet from either end of the drier so that the spray 
pattern from each nozzle covered at least a 4'.times.4' section of the 
upper surface of the bed, thus providing substantially full and even 
coverage of the entire bed. The entire 160 lbs. of nucleating solution was 
pressure atomized through the four nozzles into fine droplets mostly less 
than about 1 millimeter in diameter (and averaging less than 2 mm in 
diameter) at a rate of about 10 lbs. per minute, thus providing full spray 
delivery for about 15 minutes, thereby enabling substantially all parts of 
the agitated bed to be contacted by atomized solution at some point during 
the spraying operation. 
After all the solution had been sprayed onto the bed, the polymer particles 
were dried to a volatile content of approximately 0.1% by weight using 
approximately 125.degree. C. steam in the annular jacket around the drier 
and passing a sweep stream of 110.degree. C. nitrogen slowly through the 
polymer powder bed while continuing to rotate the multibladed agitator at 
18 rpm. 
Samples of this dried copolymer powder containing about 300 ppm of sodium 
benzoate were then tested for peak freezing point by differential thermal 
analysis of time scans of slowly cooled melts in a scanning calorimeter 
and, for comparison purposes, similar measurements were made on the 
previously reserved sample of the same copolymer before addition of any 
sodium benzoate [after first drying same further to reduce the volatile 
content thereof to the same level (0.1%) as that of the nucleated 
material]. It was thus found that the freezing point of the nucleated 
polymer was almost 10.degree. C. higher than that of the unnucleated 
material (112.degree.-113.degree. C. vs 103.degree.-104.degree. C.). 
Portions of the respective nucleated and unnucleated copolymer described 
above were also converted to pellets by melt extrusion. Standard physical 
testing of tensile, Izod and flexural modulus specimens molded from said 
pelletized materials revealed that nucleating this polymer with 300 ppm of 
sodium benzoate yielded a dramatic improvement in impact strength as well 
as significantly higher heat deflection temperatures and somewhat greater 
stiffness and modulus values. 
Two additional runs were made using exactly the same procedural steps 
described above except that the amount of sodium benzoate dissolved in the 
carrier liquid was 3.6 pounds in one run (450 ppm in finished polymer) and 
1.2 pounds in the other (150 ppm). Again the resulting nucleated polymers 
were tested for peak freezing point by the DTA method, and it was found 
that the freezing points of these finished products were about 114.degree. 
C. and about 108.degree. C. respectively. 
Similar results and improvements were obtained when instead of the above 
described ethylene-propylene copolymer, a similar propylene copolymer 
containing about 5 to 6% ethylene by weight was employed. 
EXAMPLE 2 
The detailed procedure described in the first part of Example 1 was 
repeated except that 8000 pounds of a polypropylene homopolymer having a 
similar melt index of about 2 dg/min was processed to incorporate a total 
of about 2.4 pounds of sodium benzoate (300 ppm) therein. 
Likewise, DTA tests in a scanning calorimeter conducted on essentially dry 
samples of homopolymer before and after incorporation of the nucleating 
agent indicated an increase of about 10.degree. C. in freezing point due 
to the resulting nucleation effects (about 122.degree. C. for nucleated 
material vs 112.degree. C. for the original homopolymer). Significant 
improvements in physical properties due to nucleation effects in the 
homopolymer were also shown although the margin of superiority was not as 
dramatic as with the copolymer material processed in Example 1. 
Similar results and improvements were also obtained when the polypropylene 
homopolymer of this Example was replaced by a block type copolymer of 
propylene produced by grafting onto a propylene homopolymer trunk pendant 
blocks of copolymerized propylene and ethylene wherein the ethylene 
content represents about 40% by weight of the pendant block copolymer and 
almost 10% by weight of the total grafted polymer. 
In addition to the specific improvements already recorded in the above 
illustrative examples, it should be noted that test specimens, extruded 
films and molded articles fabricated from the sodium benzoate containing 
products of the above Examples also uniformly exhibited excellent clarity 
and remarkable freedom from visible specks or cloudiness. 
Many additional variations and substitutions can be practiced in the above 
illustrative Examples as will be obvious to those skilled in the art. For 
example, other metal salts, such as those of potassium and/or aluminum, 
and/or salts of other arylcarboxylic acids, including substituted benzoic 
acids such as para-t-butyl benzoic acid and the like can be used as 
nucleating agents. Also, many other relatively volatile alcoholic liquids 
can be used as the solvent carrier for the nucleating agents, including 
both other azeotropic mixtures with water (e.g. 71.7% n-propyl 
alcohol+28.3% water) and non-azeotropic mixtures such as aqueous ethanol 
combinations containing about 10 to 40% water. 
That which we define as our invention is, therefore, specified in the 
explicit claims which follow.