Acid reactable inorganic mineral fillers having improved compatibility with polyolefin resins

Mineral fillers reactable with acids such as metal carbonates such as CaCO.sub.3 are rendered more compatible with polyolefin resins such as polyethylene by first reacting the filler with a long chain carboxylic acid to form a surface film of the acid. The acid reacted filler is contacted with a vinyl ester of the acid which is then polymerized in the presence of a free radical catalyst such as benzoyl peroxide to encapsulate the acid reacted filler in a coating of the polymerized ester.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the compatibilization of fillers with polyolefin 
resins. More particularly, this invention relates to a method for 
rendering acid reactable inorganic mineral fillers more compatible with 
polyethylene resins to improve the physical properties of the resins. 
2. The Prior Art 
In the manufacture of canned foodstuffs, the containers, usually metal 
cans, are filled with the foodstuff, covered with a metal end closure and 
sealed. One of the disadvantages of canning foodstuffs in metal containers 
is that the presence of the food product may cause the interior of the can 
to corrode, the corrosion products of which contaminate the food product. 
Attempts to substitute certain inert synthetic resin materials such as 
polyethylene for metal in the canning of foodstuffs have encountered the 
disadvantage that the sidewalls of containers fabricated from such resins 
generally do not have acceptable stiffness and rigidity to withstand 
buckling from loading stresses encountered when the containers are stacked 
during storage. 
The art has devised a number of ways to increase the stiffness of 
polyethylene and other polyolefin resins. Included in these methods is to 
incorporate in the polyolefin resin a filler material such as wood flour 
and inorganic mineral fillers such as metal carbonates, clay or mica, 
e.g., U.S. Pat. No. 3,463,350, Br. No. 905,069 and U.S. Pat. No. 
3,668,038. 
Containers molded from filled polyolefin resins generally have poor impact 
strength and crack when dropped from relatively low (e.g., 2 feet or less) 
heights. 
The poor impact properties of the container sidewalls are believed due to 
the poor compatibility of the fillers and the polyolefin matrix. 
Various means are known to the art for improving the compatibility of 
fillers with polyolefin resins which involve modification of the filler 
surface. For example, British Patent No. 905,069 teaches that polyethylene 
resins filled with fatty acid coated metal carbonates exhibit improved 
stress cracking resistance when compared with polyethylene filled with 
uncoated metal carbonates. U.S. Pat. No. 3,084,117 teaches improving the 
compatibility of clays by base-exchanging clay particles with an 
unsaturated organic nitrogen compound to form an organoclay adduct which 
is admixed with a polyolefin resin and the admixture subjected to high 
energy ionizing radiation to cross-link the unsaturated nitrogen compound. 
The prior art methods discussed above either do not provide the improvement 
demanded in container applications of the polyolefin resin or the 
compatibilization technique deleteriously induces undesirable physical 
properties in the resin otherwise acceptable. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a method for 
preparing acid reactable inorganic mineral fillers having improved 
compatibility with polyolefin resins wherein a long chain carboxylic acid 
having 8 to 22 carbon atoms is reacted with the filler particles to form a 
salt of the acid at the surface of the filler particles. The acid reacted 
filler is then contacted with a vinyl ester of the long chain carboxylic 
acid. Thereafter, the vinyl ester monomer is polymerized to provide a 
cross-linked sheath encapsulating the acid reacted filler. 
Acid reactable inorganic mineral fillers subjected to the sequential 
treatment just described are found to be more readily dispersed in 
polyolefin matrices with the result that the filled polyolefin resin 
exhibits improved physical properties such as impact resistance in excess 
over that heretofore achieved by the prior art. 
PREFERRED EMBODIMENTS 
The term "polyolefin" includes within its meaning olefin polymers, such as 
polyethylene, polypropylene, poly-1-butene, poly-4-methyl-pentene-1 and 
other homopolymers and copolymers of similar mono-1-olefins having up to 8 
carbon atoms per molecule. Of these, high density (0.950-0.968) 
polyethylene is preferred. 
In preparing polyolefin filled compositions in accordance with the practice 
of the present invention, the polyolefin resin, desirably in the form of a 
powder having a particle size of 5 to 100 microns and preferably 10 to 30 
microns, is admixed with the acid-ester modified mineral filler in blends 
containing about 10 to about 70% by weight of the polyolefin and about 30 
to about 90% by weight of the filler and preferably about 35 to about 60% 
by weight of the polyolefin and about 40 to about 65% by weight of the 
filler. 
The filler blended with the polyolefin is also advantageously in micron 
sized powder form, the filler particles having a median diameter which 
varies from 0.1 to 30 microns and preferably from 0.5 to 15 microns. 
For many use applications, and particularly food container manufacture, the 
presence of a third material component which will lower the gas 
permeability of the polyolefin is desirably included in the filled 
polyolefin compositions. Such materials include clay-like materials having 
a platelike or platelet structure such as mica as well as thermoplastic 
resins such as nylon, acrylonitrile polymers and saran polymers. Saran 
polymers are vinylidene chloride polymers including vinylidene chloride 
homopolymers and copolymers of vinylidene chloride containing between 70 
and 98 percent by weight polymerized vinylidene chloride with the 
remainder being a monoethylenically unsaturated monomer which is 
copolymerizable with vinylidene chloride as, for example, vinyl chloride, 
acrylonitrile, acrylic or methacrylic acid and their ester derivatives. 
When these third components are included in the polyolefin compositions, 
the mixture of materials in the polyolefin composition includes about 30 
to about 70% by weight of the polyolefin resin, about 30 to about 75% by 
weight of the acid-ester coated filler and 0 to about 30% by weight of the 
third component. 
Acid reactable inorganic mineral fillers which may be treated in accordance 
with the practice of the present invention include the metal carbonate 
salts of Group 2 of the Periodic Table such as the magnesium, barium and 
calcium carbonates. 
Long chain carboxylic acids which may first be reacted with the acid 
reactable inorganic mineral fillers in accordance with the process of the 
present invention include saturated and olefinic unsaturated aliphatic 
acids having 8 or more carbon atoms and preferably 12 to 22 carbon atoms 
such as fatty acids as capric acid, lauric acid, myristic acid, palmitic 
acid, isostearic acid, stearic acid and arachidic acid, undecylemic acid, 
myristoleic acid, oleic acid, cetoleic acid and erucic acid and mixtures 
of these acids. 
Contact of the acid reactable filler with the long chain carboxylic acid is 
believed to result in a chemical reaction between the acid and the filler 
to form a chemical bond between the filler surface and the acid. 
Vinyl esters which may be used to coat the carboxylic acid reacted filler 
include the vinyl esters of long chain carboxylic esters of the same type 
used for reaction with the filler such as vinyl laurate, vinyl palmitate, 
vinyl stearate, vinyl oleate and mixtures thereof. 
Preferably the vinyl ester used to coat the acid reacted filler has the 
same acid moiety as the acid used originally to react with the filler. 
In preparing mineral fillers for incorportion in polyethylene it is 
preferred that stearic acid be used as the acid reactant as this acid 
possesses a hydrocarbon chain similar in structure to polyethylene. 
The long chain carboxylic acid is applied to mineral filler at a 
concentration of about 0.005 to 1.0% by weight and preferably about 0.01 
to about 0.1% by weight. In reacting the acid reactable inorganic mineral 
filler with the carboxylic acid, the acid is dissolved in an organic 
solvent for the acid such as CCl.sub.4 or an alcohol such as methanol, 
ethanol, n-propanol, isopropanol and the solution is applied to the 
filler. The treated filler is agitated for a time sufficient, e.g., 5 to 
30 minutes, to permit the surface reaction between filler and acid to 
occur. 
After the acid reaction with the filler is completed, the vinyl ester of 
the acid is applied to the acid reacted filler at a concentration of about 
0.005 to about 1.0% by weight and preferably about 0.01 to about 0.1% by 
weight together with about 0.1 to 10 parts per million (ppm) based on the 
weight of the filler of a free radical catalyst which will initiate 
polymerization of the vinyl ester. Examples of free radical catalysts 
include peroxide catalysts such as t-butyl perbenzoate, benzoyl peroxide, 
dicumyl peroxide, methylethylketone peroxide and di-t-butyl peroxide. The 
temperature of the vinyl ester treated filler is raised to about 
70.degree.-100.degree. C. to promote the polymerization of the vinyl 
ester. Polymerizing the vinyl ester at temperature of 
75.degree.-85.degree. C. for 5 to 30 minutes to generally sufficient to 
prepare an ester coated filler exhibiting improved compatibilization with 
polyolefin resins. 
The acid-ester coated filler compositions of the present invention can be 
blended with one or more polyolefins by conventional blending techniques 
such as by mechanically working a mixture of the polyolefin and the coated 
filler particles by milling or extruding at 100.degree.-180.degree. C. to 
produce a substantially homogeneous composition. 
In manufacturing containers from mixtures of the coated filler of the 
present invention and polyolefin resins it is preferred that the 
containers be compression molded from a billet prepared from a mixture of 
the coated mineral filler and the polyolefin resin. 
In preparing the billet, the mixture of components includes about 35 to 
about 60% by weight of the polyolefin resin and about 40 to about 65% by 
weight of the coated filler. 
The billet can be any shape such as circular, square or polygonal. The 
billet can be fabricated by preparing a homogeneous mixture of the 
polyolefin resin and coated filler in an amount sufficient to prepare the 
container. The mixture of the polyolefin resin and coated carbonate filler 
particles is compacted at elevated pressures, e.g., 6,000 to 30,000 pounds 
per square inch (psi) to the desired shape of the billet. Thereafter, the 
compacted mixture is heated to an elevated temperature near the melt 
temperature of the resin for a time sufficient to fuse the resin 
particles. The so-prepared billet is then ready for molding or forging 
into a container by any conventional compression molding process. 
A method preferred for molding the container is disclosed in U.S. Pat. No. 
3,923,190 wherein a billet heated to about the melting point of the 
polyolefin resin is compressed between a pair of opposed die members 
having different dimensions and adapted to advance through a molding 
chamber, the first die member being spaced apart from the interior walls 
of the molding chamber and defining a mold cavity therebetween, the second 
die member being arranged to move telescopically with respect to the 
interior walls of the chamber. The die members are heated in the range of 
150.degree.-400.degree. F. to accelerate resin flow into the mold cavity. 
The billet, as it is advanced through the molding chamber, is compressed 
with sufficient force to cause the billet material to flow radially 
outward from between the die members and extrude into and fill the mold 
cavity to form the container.

The invention is illustrated by the following Example. 
EXAMPLE 
An acid reactable inorganic mineral filler coated in accordance with the 
present invention was prepared by placing 4000 grams of CaCO.sub.3 
particles having a median particle size of 3.3 microns in a Welex mixer. 
To the mixer was added, (during mixing at 100 rpm) 0.36 grams stearic acid 
dissolved in 50 mls. CCl.sub.4. Mixing was continued for about 15 minutes 
at 600-1600 rpm to permit the stearic acid to react with the CaCO.sub.3 
particles. Thereafter, 0.396 grams of vinyl stearate dissolved in 35 mls. 
CCl.sub.4 and containing approximately 3 milligram of benzoyl peroxide was 
added to the mixer spinning at 1000 rpm. The temperature of the mixer was 
raised to about 80.degree. C. until substantially all of the CCl.sub.4 
evaporated. The remaining solid product was dried in an air oven at 
120.degree. C. and then reground to a powder. 
A mixture suitable for forming into billets was formed from 10.4 grams 
polyethylene powder having a density of 0.95 g/cc and a median particle 
diameter of 25 microns, 10.4 grams of stearic acid-vinyl stearate modified 
CaCO.sub.3 particles prepared in accordance with the procedure above and 
0.03 gram Irganox 1010, a hindered phenol type anti-oxidant. The powder 
mixture was compacted at 28,000 psi into 2 inch diameter discs having a 
thickness of 300 mils. The discs were placed in a heating device and 
heated at 360.degree. F. for 6-8 minutes under ambient pressure 
conditions. At the end of the heating period, the discs were placed in a 
compression molding apparatus of the type described in U.S. Pat. No. 
3,923,190. The heated disc was placed between the pair of opposed die 
members of the apparatus, the upper die member being heated to 380.degree. 
F. and the lower die member being heated to 170.degree. F. The disc was 
compressed under a force of up to 30 tons with a mechanical press which 
radially extruded the disc material into the molding cavity of the 
apparatus to form the sidewalls of the container as the die members 
descended in the molding chamber. 
Two seconds after placement of a heated disc in the molding apparatus, an 
integral cylindrical hollow container having a capacity of 10 ounces, an 
average sidewall thickness of 28 mils and a bottom wall thickness of 35 
mils was ejected from the molding chamber. 
The impact resistance of the container was determined by filling the 
container with water, sealing the container, and then dropping the 
container from various heights onto a concrete floor. The container 
survived successive drops in one foot increments to a height of 4 feet 
without failure, i.e., the container did not crack or break open. 
The physical properties of the container were determined and are summarized 
in Table I below. 
TABLE I 
______________________________________ 
Tensile Strength 
Elongation 
Direction 
(psi) (%) Energy 
AE/HE 
______________________________________ 
Axial 3700 20 510 1.13 
Hoop 2000 33 450 
______________________________________ 
The values of tensile strength, elongation were determined by the method 
described in ASTM D 638-72, entitled "Tensile Properties of Plastics". 
Direction is the orientation of the tensile sample specimen relative to the 
cylindrical axis of the container. 
Energy is a measure of toughness and is determined as the area under the 
stress/strain curve. The higher the energy, the better is the toughness. 
AE/HE is the ratio of the axial energy to the hoop energy. The lower the 
ratio, the better is the expected impact resistance of the container. 
By way of contrast, the procedure of the Example was repeated with the 
exception that the stearic acid reacted CaCO.sub.3 was not coated with 
polymerized vinyl stearate. Containers compression molded from the billets 
formed from mixtures containing equal weight amounts of the stearic acid 
reacted CaCO.sub.3 and the polyethylene could survive only a 2 foot drop. 
The physical properties of the comparative containers are summarized in 
Table II below. 
TABLE II 
______________________________________ 
Tensile Strength 
Elongation 
Direction 
(psi) (%) Energy 
AE/HE 
______________________________________ 
Axial 3465 19 470 3.13 
Hoop 2200 10 150 
______________________________________ 
By way of further contrast, the procedure of the Example was repeated with 
the exception that unmodified CaCO.sub.3 was used as the filler. 
Containers compression molded from billets prepared from mixtures 
containing equal weight amounts of unmodified CaCO.sub.3 and polyethylene 
could survive only a 2 foot drop. These comparative containers had the 
following physical properties: 
TABLE III 
______________________________________ 
Tensile Strength 
Elongation 
Direction 
(psi) (%) Energy 
AE/HE 
______________________________________ 
Axial 3370 18 420 3.36 
Hoop 2270 9 125 
______________________________________ 
By preferring to the data recorded in Tables I, II and III above, it is 
immediately apparant that containers molded from filled polyethylene 
resins prepared in accordance with the present invention (Table I) have 
substantially improved impact resistance when compared with containers 
molded from filled polyethylene resins prepared outside the scope of the 
present invention (Tables II and III).