INCORPORATION OF ADDITIVES INTO POLYETHYLENE USING AN IN SITU POLY M ER I ZAT IO N METHOD D. W . MARSHALL,
H. L . H A C K E T T , A N D G . D . B R I N D E L L '
Research and Dpvelopment Dspartment, Continental 0 i I Co., Ponca City, Okla.
Carbon black or pigments are usually added to polyolefin plastics by mechanical mixing after polymerization. It is difficult to get a good dispersion of the pigment in this fashion, especially wifh the tougher, high density polyethylenes of high molecular weight. A unique in situ catalyst preparation method involves the presence of the pigment during formation of the catalyst. A very intimate mixture of catalyst and pigment results, which gives a polymer smoothly blended with the pigment. Polyethylenes filled by this technique are compared with those filled b y conventional means. Certain physical properties are given for some polyethylenes filled with such substances as carbon black, titanium dioxide, ferric oxide, chrome yellow, graphite, and lead.
USTOMARILY carbon
black and other pigments are added to high density polyethylene and other polyolefins after polymerization in a step involving mechanical mixing. Masterbatches of pigments are often used during the milling operation to obtain a better dispersion of the pigment. Various monomers have been polymerized in the presence of pigments and reported in the patent literature (7-6). Work by the authors shows it is possible to pigment polyethylene and other polyolefins using an in situ catalyst preparation method which involves the presence of the pigment during the formation of organometallic-transition metal catalysts. A very intimate mixture of catalyst and pigment results. O n subsequent introduction of monomer. the polymerization takes place in the presence of the pigment, giving a reaction mass of intimately mixed pigment and polymer. In using this technique. we have been able to obtain high loadings of carbon black, titanium dioxide, ferric oxide, chrome yellow, graphite. and lead. Advantages of Technique. ,4 costly blending step is eliminated: high loadings of carbon black and pigments are possible without substantially degrading certain physical properties of the finished products, the pigments are as well dispersed as those from mechanical mixing. if not better, and high molecular weight polyolefins may be conveniently pigmented. Catalysts used were those of the typical organometallictransition metal type: metalorganics from the metals of Groups I through I11 of the periodic table with transition metal halides. Specific catalysts were amyl sodium-titanium tetrachloride and aluminum triethyl-titanium tetrachloride. Polymerizations \vere carried out in 1-quart beverage bottles and, where control of molecular weight was desired. in a small stirred autoclave. The pigments tvere added to the solvent in dry nitrogenflushed reaction vessels with the metalorganic added next. After thorough agitation, the transition metal halides were added with further agitation over a period of time. The reaction vessels tvere then pressured with monomer to give the desired polymer pigment compositions by weight. With bottles, 40-p.s.i. monomer pressure and ambient temperatures were used; autoclave runs were made at 130 p.s.i. a t 25' to 40' C. 1 Present address, Research and Development, U. S. Rubber Co., Wayne, N. J.
32
I & E C P R O D U C T RESEARCH A N D DEVELOPMENT
Care was taken to ensure anhydrous solvent, monomer, and system. In most cases, n-hexane was used as a solvent. Washing procedures were those commonly used with low pressure processes. By using the technique outlined, varying quantities of pigments were incorporated into polyethylene. Sheets made by molding the resulting polymers left no mark when rubbed across a white surface. In powder form the polymer could be handled without deposition of pigment on equipment or operator. Two types of carbon black were used-Witcoblak F-1, a gas furnace black having an average particle size of 70 mp, and Continex HAF with an average particle size of 25 mp. Care was taken to dry all pigments thoroughly before using. Aluminum triethyl gave the same results as amyl sodium in carbon black experiments, but was not used in the inorganic pigment runs. Although catalyst consumption was not an object of these experiments, figures around 1% by weight were consistently realized. Figure 1 shows the effect of carbon black loadings on the tensile strength and elongation of the polymers produced. The tensile strengths increased as the weight per cent of carbon black increased. The elongation remained for all purposes the same until a relatively high loading of 32% was reached. Higher loadings markedly decreased elongations. All samples survived the Bell test for stress cracking for periods in excess of 1000 hours. These experiments were made with amyl sodiumtitanium tetrachloride catalyst with a gas furnace black, Witcoblak F-1. Table I represents a comparison of polymers prepared by the in situ technique; a base resin, containing no black, prepared in the same manner; and the base resin with black milled in by the use of a Banbury mixer. Amyl sodium-titanium tetrachloride catalyst was also used in these examples, but the carbon black was Continex HAF. It is evident that the 2.7% in situ product has properties comparable to the base resin, but substantially lower flexural modulus. The 2.770 milled has a lower tensile strength than the base resin, while its flexural modulus is intermediate between the base resin and the 2.770 in situ, This would suggest that some refinement of crystal structure has taken place in the 2.7% in situ. Comparing the 327, in situ preparation with the 32Y0 milled, a higher tensile
. i '
5500
I
1
,
I
1
I
1
I
IO
I
I
POLYEiHlLEME
0
No TIC1*
1 M A R R4
I
6
WITCOBLAK F-l
10 z
0 c
5YLzacYI
4
u
4500 5000-
z
J 0 Y
+
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,o a
w F I L M S C4ST 4 1 350 ,000p s i
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-3
OF
MIN
REED-DILLON TENSlLE TESTER "SEI
I
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5
IO
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15 20 25 P E R CENT B Y WEIGHT
Figure 1 .
30 BLACK
I
I 40
35
Effect of carbon black loadings
0. Tensile
strength.
A. % elongotion
modulus and higher flexural modulus in the in situ polymer indicate that the milling had destroyed the integrity of the blend. The environmental stress cracking resistance is greatly improved in the in situ prepared material, also substantiating the belief that a modified crystalline structure has resulted. Figure 2 shows photomicrographs of a section of a 2.7qJ, in situ resin and a commercial-type .. compound containing . ap. proximately 2.5% milled black. The in situ-prepared polymer transmits less light than the commercial material and no large agglomerates are present. (A section of 32% in situ a t the same thickness, about 15 microns, not shown, was completely opaque.) By means of the in situ technique, a number of other pigments and additives were incorporated into polyethylene. I n the experiments illustrated in Table 11, only slight deviations in physical properties occurred, with the exception of the high graphite-loaded polymer. Evenly dispersed, attractive polymers resulted when ferric oxide, lead chromate, and titanium dioxide were used. T h e polymer containing 46% powdered graphite conducted electricity and was rather rigid. The addition of 42 weight yo powdered lead gave a polymer whose properties were not
Figure 2. Micrographs of thin sections Top. Polyethylene, 2.7% Continex HAF Bottom. Commercial polyethylene,
2.5% black
._
ma degrade Thus. a number of pigments and additives may be easily incorporated into polyolrfins without seriously lowering-properties of the base resin. Literature Cited
(1) Burk, R. E. (to E. I. du Pont dc Nemours & Co.), U. S. Patent 2,500,023(March 7,1950). (2) Burke, 0. W., Jr., French Patent 1,248,133 (Oct. 31, 1960). (3) Grotenhuis, T. A., U. S. Patent 2,394,025 (Feb. 5, 1946). (4) Hoff, G. P. (to E. I. d u Pont de Nemours & Co.), Ibid., 2,278,878 (April 7, 1942). (5) Langer, A. W., Jr., Morrell, C. E. (to Esso Rrsearch and Engineering Co.), I6id., 3,008,949 (1961). (6) National Lead C o . , Brit. Patent 869,391 (May 31, 1961). RECEIVED far review May 14, 1962 ACCEPTED January 9, 1963 Division of Organic Coatings and Plastics Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962.
Table I. Physical Tesls of Carbon-Filled Resins Carbon
Content, Wt.%
T ~ n s i l ePmfmties - Flexural Ultimate Modulvs Modulus Izod Brittlc Yield, ELonZ'Jtion P.r.i. X P.r.i. X Impact, Temp., P.s.i. P . s . ~ . yo FpPi "F.
Table II.
Enuirotimental Crock, HOW$
Shess
Base resin
3867 5333 700
128
125
9
-118
120
2.7%insitu
4170
5107 613
112
89
9
-118
132
2.70/,milled 3125 4421 720
134
101
9
. ..
32% in situ
5400 5400
300
316
3.9
0
32%milied
3900 3900
212
216
2.3
70
3.3 10
Failed at less than 100 400 Flexed to required radius, specimens failed: no test
Effect of Pigments and Additives Catalyst: Amyl sodium-titanium tetrachloride, Na/Ti = 6. Samples: molded at 350" F. for 3 min. Tensile properties: run at 20 inches per mi". using ReedDillon tester TEnsilc Wt. yo Strength, 9, Pipent Pigment P.s.i. Elonplion Control 4230 350 ... Ti02 5277 9 2 220 Fezor 5073 165 9.8 PhCrOd 5809 9.4 185 Graphite 4659 10.4 215 5 1767 (powdered) 46.0 Lead
(powdered) 4 2 . 0
VOL.
2
NO.
1
4000
MARCH
310
1963
33
