Selective admixture of additives for modifying a polymer

The molecular weight of solid polymers, particularly polyolefins, is modified by addition of organic peroxides (or other free radical generators), which is admixed in a first reaction zone. The polymer is then melted followed by addition of additives such as antioxidants and light stabilizers, which are admixed in a second reaction zone where intense and rapid mixing occurs, such as within the barrel of an extruder. The separate admixture of primary and secondary additives provides improved color and odor characteristics for the polymer compared to concurrent admixture of all the additives.

This invention relates to polyolefins. In one aspect, it relates to 
modifying molecular weight of polyethylene and polypropylene polymers by 
admixture of additives with the polymer. More specifically, this invention 
relates to a process for sequential admixture of various additives with 
polyolefins. 
BACKGROUND OF THE INVENTION 
One of the most valuable characteristics of polyolefins as a class of 
materials is versatility. They can be tailored to many fabrication methods 
to provide products such as fibers, films, molding resins, etc. which have 
excellent physical and chemical properties for a particular application. A 
large number of additives are used in polymer technology to improve, 
strengthen or otherwise alter the polymer, such that additives used in 
preparing a polymer are often a critical factor in commercial success of 
the final product. 
Many specially tailored polymers employ free radical generators such as 
organic peroxides as an additive, for example to lightly crosslink 
polyethylene, or to visbreak polypropylene, predetermined amounts of 
organic peroxides are added to the polymer. As used herein, the term 
"visbreak" is taken to serve as evidence for chain scission of the 
polymer. Additional chemical additives such as antioxidants, antistats, 
flame retardants, light stabilizers, etc. are also employed is these 
polymers to prevent oxidation, discoloration, static charge, or thermal 
degradation during subsequent melting processes which the polymer must 
undergo. For example, it is well known that polyethylene melt flow 
reduction, which can be achieved by addition of organic peroxides, is 
effective for improving bubble strength for blown film applications. 
Likewise, it is known that polypropylene visbreaking, which also can be 
achieved by addition of organic peroxides, is effective for obtaining 
narrower molecular weight distribution and enhanced flow characteristics. 
Use of organic peroxide additives in combination with certain other 
additives such as antioxidants, and light stabilizer additives, however, 
has resulted in undesirable color and odor in the polymers containing the 
multiple additives. 
Accordingly, it is an object of this invention to eliminate the above 
mentioned color and odor problems associated with polymers containing 
multiple additives. 
It is a more specific object of this invention to provide a process for 
producing olefin polymers having desirable physical properties without 
impairing color or odor characteristics of the polymer. 
It is another object of this invention to provide a continuous or batch 
process which will reduce the quantity of free radical generator required 
to modify a polymer to a desired viscosity. 
It is another object of this invention to provide a polymer admixture 
process which is safe, simple, effective and economical. 
A further object of this invention is to provide polymers which have 
improved processability. 
SUMMARY OF THE INVENTION 
In accordance with this invention, we have discovered that certain 
additives used in olefin polymers for modifying chemical properties such 
as oxidative degradation, light stabilization, etc., which degrade the 
color and/or odor of the olefin polymer when admixed in the presence of a 
free radical generator, can be effectively added after admixture of the 
free radical generator. 
A dry polyolefin material is modified by admixing an amount of a free 
radical generator required to produce a desired melt flow, and the 
admixture is maintained in a first reaction zone under conditions to 
generate free radicals for a time sufficient to react substantially all 
of-the free radical generator, and to heat the admixture sufficiently to 
form a molten polymer. The molten polymer is passed to a second reaction 
zone where additives such as antioxidants and light stabilizers are 
admixed with the molten polymer, and the molten polymer is extruded to 
produce oxidation and light stabilized polymer products in the form of 
pellets, sheets or other shapes. 
In a preferred embodiment, the second reaction zone is a special reaction 
zone where the polymer is in a molten condition and where rapid, intensive 
mixing of the secondary additives with the molten polymer occurs, such as 
within the barrel of an extruder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Extruders have been used conventionally for many years to process all types 
of polymeric materials and especially polyolefins. Generally, the polymer 
is melted and then fed to an extruder, with the output of the extruder 
defined by the die shape and size. Dies are broadly classified as (1) 
sheet dies extruding flat sheets, (2) shape dies for making pipe, 
gasketing, tubular products, and many other designs, (3) blown film dies 
using an annular orifice to form a thin walled envelope, (4) spinneret 
dies for extrusion of single or multiple strands of polymer for textile 
products, rope, tire cord or webbing, and (5) pelletizing dies for 
granular production of the resin produced. 
It is well known that various chemical reactions and modifications can take 
place when a polymer passes through an extruder. These modifications, 
however, can be accomplished in such a way that significant desirable 
changes in the polymer composition and/or rheology can be affected. 
While the process of this invention is applicable to all polymers suitable 
for processing by an extruder, particularly noted are thermoplastics such 
as nylons, polyesters, polycarbonates, engineering plastics, and 
polyacetals. It is especially useful for C.sub.2 -C.sub.12 polyolefins, 
preferably C.sub.2 -C.sub.8 polyolefins including copolymers of olefins 
with other monomers. Examples of suitable polyolefins include low or high 
density polyethylene. The preferred polyolefins employed in this invention 
contain ethylene and/or propylene, i.e. polyethylene and polypropylene. 
The starting polymers used as the base material preferably have a melt 
index (MI) of 0.05 to 1,000, preferably 0.05 to 50, and most preferably 
0.05 to 10. The processes for making these C.sub.2 -C.sub.12 polyolefins 
are well known and form no part of the present invention. 
After the polymer has been produced, it is preferred according to this 
invention that the polymer in a powder or fluff state be given a molecular 
weight modifying charge with an organic peroxide before additional 
processing occurs in a molten state. Preferably organic peroxide is 
present in the modified polymer in an amount just sufficient to provide a 
desired melt flow in the modified polymer. When the polymer, already 
modified in molecular weight, is in the molten state and at a suitable 
temperature, which generally occurs after the polymer is heated by shear 
energy in a mixer, secondary additives of the various types can be 
introduced with it and the molten admixture fed to an extruder where very 
rapid diffusion or dispersion of the secondary additives throughout the 
polymer can occur. Thus, it is possible to achieve extensive reactions 
with very short reaction zone residence time. 
A distinction should be drawn between polymers whose properties are largely 
determined by the ethylene content and those polymers whose properties are 
largely determined by their C.sub.3 -C.sub.12 olefinic content. This 
distinction is evidenced by the fact that polyethylene and ethylene 
containing polymer tend to crosslink in the presence of peroxides and/or 
hydroperoxides under conditions in which C.sub.3 and above polyolefins 
tend to visbreak with peroxides. 
Referring now to the drawing, there is illustrated a simplified process 
flow for an embodiment of this invention which is suitable for continuous 
operation. It will be appreciated by those skilled in the art that since 
FIG. 1 is a simplified schematic only, many items of equipment which would 
be needed for a successful operation of a commercial plant have been 
omitted for the sake of clarity. Such items of equipment would include, 
for example, temperature, flow and pressure measurement instruments, 
corresponding process controllers, additional feeders, mixers, heat 
exchangers, valves etc. All of these items would be provided in accordance 
with standard chemical engineering practice to maintain desired conditions 
throughout the process, and are not necessary to describe the present 
invention. 
There is shown generally at 10 in FIG. 1 a conventional hot melt extruder 
which is melt fed and which may be a single screw or a double screw type. 
Extruders using twin screws are preferred for large volume production 
units for pelletizing resins in petro-chemical plants and are typically 
equipped with various combinations of intermeshing and non-intermeshing 
screws that co-rotate or counter-rotate. Extruder screw design features 
which allow melting and blending and other processing in a single extruder 
machine are also suitable for use in the practice of this invention. 
A feedhopper 20 or other suitable device connects extruder barrel section 
12 to a source of secondary additives via conduit means 14. Injection of 
the secondary additives at this point, which relies on low pressure 
existing in melt feed conduit 14, provides thorough dispersion of the 
additive in the molten mass over an extremely short period of time in the 
extruder if appropriate controls over extruder speed and additive feedrate 
are maintained. The residence time of the thermoplastic material in die 
section 16 of extruder 10 is such that at least the outer surface of the 
extrudate 26 has solidified before the extrudate 26 exits the die section 
16. 
A thermoplastic material in particulate form e.g. pellets, powder or fluff 
is charged to an inlet section 22 of a continuous or batch mixer 28 via 
feed hopper 24, and the polymer particles are converted into molten 
thermoplastic mass by shear energy in the mixer. The output melt 
temperature of the mixer 28 is primarily a function of internal shear 
energy in the mixer 28 (converted to heat energy), and the temperature of 
the melt is as important as the output rate for quality extrusion. 
Mixing of dry polymer material and primary additive materials including 
peroxide compatible additives and a free radical generator, which is 
preferably liquid organic peroxide, occurs in the continuous inline mixer 
28 which is supplied from the dry chemical feeders for polymer fluff and 
primary additive illustrated at 30 and 32 respectively. Other suitable 
means for feeding material to the mixer may be employed in the practice of 
this invention such as adding liquid peroxide by spraying or feeding a 
masterbatch of plastic compounds, which includes a high concentration of 
an additive or additives, via feeder 32. 
In this preferred embodiment, the organic peroxide for molecular weight 
modification is added to the polymer fluff either as a liquid spray or in 
a masterbatch. The secondary additives, e.g., antioxidants and light 
stabilizers are added to the molten polymer either as a dry or liquid 
component. The process of this embodiment of the invention can be 
conveniently operated to give high volume throughput with good quality. 
Polyolefins treated according to the sequential additive steps of the 
present invention have improved color and odor characteristics compared to 
non-sequential additive steps, where unreacted peroxide is allowed to 
contact certain other chemical additives. Further, according to this 
invention lower levels of peroxide are required for modifying molecular 
weight of the polymer so as to attain a desired melt flow rate. 
The peroxide concentration in the modified polymer according to this 
invention, may range from 0 to about 2.0 weight percent, based on the 
total polymer concentration. A range of about 0.3 to about 0.9 is more 
preferable. The most preferred range is about 0.04 to about 0.08 weight 
percent of organic peroxide. This range is most preferred because it is 
generally sufficient to produce desirable melt flow rates without 
degrading color or odor of the polymer. An example of a preferred organic 
peroxide useful as a molecular weight modifying agent is 
2,5-Dimethyl-2,5Di(t-Butylperoxy)hexane. This organic peroxide is marketed 
under the trademark Aztec.RTM. 2,5-Di by Aztec Catalyst Company Houston, 
Tex. 
According to this invention any additive which degrades color or odor of 
the polymer when contacted with a free radical generator is admixed as a 
secondary additive. Examples of stabilizing additives which are preferably 
admixed as secondary additives include antioxidants such as 
1,3,5-tris(3,5-di-tert-butyl-4-hydroxy 
benzyl)-s-triazine-2,4,6-(1H,3H,5H)trione. This antioxidant is available 
under the trademark Irganox.RTM. 3114 available from Ciba-Geigy 
Corporation, and a light stabilizer such as 
Poly[[6-[(1,1,3,3-tetramethylbutyl) amino]-s-triazine-2,4-diyl ] 
[(2,2,6,6-tetramethyl-4-piperidyl) imino]hexamethylene 
[(2,2,6,6-tetramethyl-4-piperidyl)imino]]. This stabilizer is available 
under the trade name Chimassorb.RTM. 944 available from Ciba-Geigy Corp. 
Hawthorne, N.Y. 
For aesthetic considerations, it is desirable to control the color of 
polymer products, and generally it is desired to minimize yellowing of the 
product. The degree of yellowing may be determined by a Hunter colorimeter 
which employs a tristimulus method for achieving better color definitions. 
The tristimulus method is a relatively complicated system of specifying 
the continuous reflectance curve into three numbers. Generally these 
correspond to red, green and blue spectral responses and express the 
amount of each of the three primary responses, that when combined in 
specific amounts, produces a total color sensation. In the following 
examples the Hunter "b" value correlates best with the yellowing 
characteristic of the polymer produced and accordingly, a lower Hunter "b" 
value indicates a more desirable condition. A comprehensive discussion of 
the tristimulus expressions can be found in Kirk-Othmer Encyclopedia of 
Chemical Technology, 2d Vol. 5, New York, John Wiley & Son. 
A further understanding of the present invention and its advantages is 
provided by reference to the following examples. These examples are 
provided merely to illustrate the practice of the invention and should not 
be read as limiting the scope of the invention or the appended claims in 
any way. Reasonable variations and modifications, not departing from the 
essence and spirit of the invention, are contemplated to be within -the 
scope of the patent protection desired. 
EXAMPLE I 
In this example the experimental procedure utilized in a pilot plant 
operation, and the effect of adding stabilizers at different stages during 
the finishing process for polyethylene are described. 
Pilot plant equipment used in these examples comprises a size 2 Farrell 
continuous mixer and a 31/2" diameter hot melt extruder. 
A resinous polymerization reaction product in a powder form, referred to 
hereinafter as a "fluff", is melted and then processed through an extruder 
to form resin pellets. The finishing process includes a two-step operation 
for admixture of additives during a controlled rheology modification of 
the polymer resin. In this finishing process, primary additives are first 
admixed with the fluff, and secondary additives are then admixed after the 
fluff is melted. 
Primary feed material is prepared, for example, by combining fluff and 
certain additives in a 40 lb. Henschel blender. With the blender operating 
at a reduced speed, liquid additives are slowly poured into the fluff so 
as to disperse the liquid additive uniformly into the fluff. The amount of 
liquid additive is determined by the requirement for the total blend. The 
Henschel blender is then turned to a high speed for 1 min. and the 
resulting blend dumped into a 32 gal. fiber drum. To the above-described 
liquid concentrate blend, there is added additional fluff along with 
optional dry additives. The fiber drum is then tumbled for 20 min. to 
disperse the liquid peroxide concentrate and the dry additives, if 
present. 
Secondary feed material also containing dry additives is prepared, for 
example, by providing a quantity of fluff to a 32 gal. fiber drum, 
admixing the dry additives as desired, and tumbling the mixture in the 
drum for 20 min. 
The primary material is passed to a continuous mixer where the material is 
rendered molten and then the secondary material is admixed with the molten 
polymer. The resulting blend is then passed to the extruder to form 
pellets. Extruder and mixer process conditions for making the pellets are 
shown in Table VII. 
Polyethylene Blend 
For testing blends of polyethylene fluff, prepared over a 
titanium-magnesium catalyst, the polyethylene blend components comprise: 
53% fluff of 0.5 HLMI, and a density of 0.932 gm/cc, and 47% fluff of 100 
MI, and a density of 0.972 gm/cc. 
Additive components of the primary and secondary feeds for six experimental 
runs are provided according to Table I. Resin pellets are prepared 
according to the above described procedure and the properties of the 
pellets are determined. 
TABLE I 
______________________________________ 
Run A B C D E 
______________________________________ 
Primary Additive Package 
BHT.sup.(1), wt. % 
.02 -- -- -- .02 
Irganox .RTM. 1010.sup.(2), wt. % 
.07 -- -- -- .07 
DLTDP.sup.(3), wt. % 
.03 -- -- -- .03 
Ultranox .RTM. 626.sup.(4), wt. % 
.05 -- -- -- .05 
2,5-Di peroxide.sup.(5), ppm 
-- 20 40 60 100 
Secondary Additive Package 
BHT.sup.(1), wt. % 
-- .02 .02 .02 -- 
Irganox .RTM. 1010.sup.(2), wt. % 
-- .07 .07 .07 -- 
DLTDP.sup.(3), wt. % 
-- .03 .03 .03 -- 
Ultranox .RTM. 626.sup.(4), wt. % 
-- .05 .05 .05 -- 
______________________________________ 
.sup.(1) 2,6-di-t-butyl-4-methylphenol 
.sup.(2) Tetrakis[methylene(3,5-di-tert-butyl-4-hydroxycinnamate)]methane 
.sup.(3) Dilauryl thiodipropionate 
.sup.(4) Bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite 
.sup.(5) 2,5-Dimethyl-2,5Di(t-Butylperoxy)hexane diluted 40/1 with minera 
oil 
Effectiveness of the sequential admixture of additives in improving color 
characteristics without impairing other properties is illustrated in Table 
II by lower Hunter b values for runs B, C and D where secondary additives 
were provided. 
TABLE II 
______________________________________ 
Effect of Sequential Additive Step on Pellets 
Run A B C D E 
______________________________________ 
Color a -0.50 -1.0 -0.98 -0.53 -1.18 
Color b -0.65 0.98 -0.07 1.75 6.50 
Color L 84.7 85.2 85.6 83.6 93.0 
10.times. Melt Index, 
9.5 9.0 8.9 8.3 8.9 
g/10 min. 
Density, g/cc 
0.95 0.95 0.95 0.95 0.95 
______________________________________ 
EXAMPLE II 
Propropylene Blends 
In this example, testing of polypropylene prepared over a TiCl.sub.3 based 
catalyst, and formed into pellets according to the procedure described in 
Example I, is shown. Additive components of primary and secondary feeds 
for three experimental runs are given in Table III below. 
TABLE III 
______________________________________ 
Run F G H 
______________________________________ 
Primary Additive Package 
Irganox .RTM. 3114.sup.(1), wt. % 
.075 -- -- 
Ultranox .RTM. 626.sup.(2), wt. % 
.075 .075 -- 
Chimassorb .RTM..sup.(3), 944LD, wt. % 
.038 .038 .038 
Zinc Stearate, wt. % 
.075 .075 .075 
2,5-Di Peroxide, wt. % 
.06 .06 .05 
Distilled Water, wt. % 
0.40 0.40 0.40 
Secondary Additive Package 
Irganox .RTM. 3114.sup.(1), wt. % 
-- .075 .075 
Ultranox .RTM. 626.sup.(2), wt. % 
-- -- .075 
______________________________________ 
.sup.(1) 1,3,5-Tris(3,5-di-tert-butyl-4-hydroxy 
benzyl)s-triazine-2,4,6-(1H,3H,5H)trione 
.sup.(2) Bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite 
.sup.(3) 
Poly[[6-[(1,1,3,3-tetramethybutyl)amino]-s-triazine-2-4-diyl][2,2,6,6-tet 
amethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)i 
ino]]- 
Effectiveness of the sequential admixture of additives in improving color 
characteristics without impairing other properties is illustrated in Table 
IV by lower Hunter b values for runs G and H where secondary additives 
were provided. 
TABLE IV 
______________________________________ 
Effect of Sequential Additive Step on Pellets 
Run F G H 
______________________________________ 
Color a -1.9 -1.3 -1.2 
Color b 6.5 3.0 2.2 
Color L 79.2 79.1 77.6 
MI.sup.(1) g/10 min. 
34.9 32.1 30.1 
OIT.sup.(2) min. 
3.0 2.5 2.8 
______________________________________ 
.sup.(1) Pooled standard deviation is 1.8 (df = 28) for melt flow 
determinations 
.sup.(2) Pooled standard deviation for OIT (Oxidative Induction Time) 
measurements is 0.26 (df = 34) 
EXAMPLE III 
In this example, testing of polypropylene prepared over a MgCl.sub.2 
containing catalyst, and formed into pellets according to the procedure 
described in Example I is shown. Additive components of primary and 
secondary feeds for four experimental runs are given in Table V below. 
TABLE V 
______________________________________ 
Run I J K L 
______________________________________ 
Primary Additive Package 
Irganox .RTM. 3114.sup.(1), wt. % 
.075 -- -- -- 
Ultranox .RTM. 626.sup.(2), wt. % 
.075 .075 -- -- 
Chimassorb .RTM. 944LD.sup.(3), wt. % 
.038 .038 .038 -- 
Zinc Stearate, wt. % 
.075 .075 .075 -- 
Distilled water, wt. % 
0.4 0.4 0.4 0.4 
2,5-Di peroxide, wt. % 
.06 .06 .06 .06 
Secondary Additive Package 
Irganox .RTM. 3114.sup.(1), wt. % 
-- .075 .075 .075 
Ultranox .RTM. 626.sup.(2), wt. % 
-- -- .075 .075 
Chimassorb .RTM. 944LD.sup.(3), wt. % 
-- -- -- .038 
Zinc Stearate, wt. % 
-- -- -- .075 
______________________________________ 
.sup.(1) 1,3,5-Tris(3,5-di-tert-butyl-4-hydroxy 
benzyl)s-trazine-2,4,6-(1H,3H,5H)trione 
.sup.(2) Bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite 
.sup.(3) 
Poly[[6-[(1,1,3,3-tetramethybutyl)amino]-2-triazine-2,4-diyl][2,2,6,6-tet 
amethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)i 
ino]]- 
Effectiveness of the sequential admixture of additives in improving color 
characteristics without impairing other properties is illustrated in Table 
VI by lower Hunter b values for runs J, K and L where secondary additives 
were provided. 
TABLE VI 
______________________________________ 
Effect of Sequential Additive Steps on Pellets 
Run I J K L 
______________________________________ 
Color a -- -- -- -- 
Color b 2.1 0.5 0.1 1.1 
Color L 79.2 79.3 79.2 79.1 
MI.sup.(1) g/10 min. 
30.2 31.4 32.4 41.5 
OIT.sup.(2) min. 
2.7 2.9 3.1 2.8 
______________________________________ 
.sup.(1) Pooled standard deviation is 1.8 (df = 28) for melt flow 
determinations 
.sup.(2) Pooled standard deviation for OIT (Oxidative Induction Time) 
measurements is 0.26 (df = 34) 
TABLE VII 
______________________________________ 
Finishing Process Conditions 
Polyethylene 
Polypropylene 
Samples Samples 
______________________________________ 
Process Rate, kg/hr 
45.0 70 
Mixer 
Rotor Speed, rpm 
570 790-800 
Barrel Heat neutral neutral 
Ribbon Temp., .degree.C. 
199-202 192-198 
Extruder 
Screw Speed, rpm 
38 47-50 
Barrel Temp., .degree.C. 
235 230-235 
Melt Discharge Temp., .degree.C. 
237-244 199-205 
Die Pressure, psig 
1121-1300 200-310 
______________________________________ 
While this invention has been described in detail for the purpose of 
illustration, it is not to be construed as limited thereby but is intended 
to cover all changes and modifications within the spirit and scope 
thereof.