ACID RESISTANT POLYETHYLENE CONTAINERS

Abstract

Rotomolded polyethylene containers that are used to store sulfuric acid can become discolored on the interior surface which is in contact with the acid. This problem may be mitigated by the use of an additive package that contains two primary antioxidants.

Claims

1. A method for improving the discoloration resistance of a rotational molded polyethylene container that is contacted with concentrated sulfuric acid, said method comprising the steps of 1) providing a stabilized ethylene copolymer composition, wherein said stabilized ethylene copolymer composition contains an additive package comprising A.1) a hindered phenolic primary antioxidant; A.2) a hydroxylamine primary antioxidant; B) a phosphite secondary antioxidant; C) a hindered amine light stabilizer; and 2) subjecting said stabilized ethylene copolymer composition to rotational molding conditions to produce a rotational molded polyethylene container; and 3) contacting said rotational molded polyethylene container with concentrated sulfuric acid to produce an acid contacted container; wherein said acid contacted container has improved color in comparison to a control acid contacted container that is prepared using an additive package that does not contain any of said hydroxylamine primary antioxidant.

2. The method of claim 1 wherein said ethylene copolymer composition has a melt index, I.sub.2, of from 1 to 10 grams per 10 minutes.

3. The method of claim 1 wherein said ethylene copolymer composition has a density of from 0.93 to 0.95 grams per cubic centimeter.

4. The method of claim 1 wherein said rotational molding conditions included a molding temperature of from 230 to 350 C. and a heated molded time of from 10 to 60 minutes.

5. The method of claim 1 wherein said ethylene copolymer composition is prepared from a catalyst type chosen from single site catalysts, Ziegler Natta catalysts and combinations thereof.

6. The method of claim 5 wherein said catalyst type consists essentially of at least one single site catalyst.

7. The method of claim 1 wherein said additive package consists essentially of A.1) at least one hindered phenolic primary antioxidant; A.2) at least one hydroxylamine primary antioxidant; B) at least one phosphite secondary antioxidant; and C) at least one hindered amine light stabilizer.

8. The method of claim 7 wherein said additive package consists essentially of: A.1) from 300 to 1500 ppm of at least one hindered phenolic primary antioxidant; A.2) from 100 to 1000 ppm of at least one hydroxylamine primary antioxidant; B) from 500 to 2000 ppm of at least one phosphite secondary antioxidant; and C) from 500 to 5000 ppm of at least one hindered amine light stabilizer.

Description

EXAMPLES

(Comparative) Example 1

[0065] Experimental procedures are described below:

Two different types of ethylene copolymer were used. One was prepared with a conventional Ziegler Natta (Z/N) catalyst and had a molecular weight distribution, Mw/Mn, of between 3.2 and 3.5. This is referred to in Table 1 as ZN1 and it had a melt index, I.sub.2, of 2 g/10 minutes and a density of 0.941 g/cc.

[0066] A single site catalyst (SSC) copolymer was also used and is identified as SSC1 in Table 1. SSC1 had a melt index, I.sub.2, of 1.7 g/10 minutes and a density of 0.945 g/cc. In addition, SSC1 is further characterized by being a blend of two ethylene-octene blend components and was prepared in accordance with the teachings of U.S. Pat. No. 7,201,864 (Weber et al.).

[0067] These ethylene copolymers were blended with the additive packages described in Table 1 and ground into fine powder for the preparation of rotomolded parts. For clarification: the compounded copolymer used in experiment 2 was conventional polyethylene ZN1 and contained 300 ppm of primary antioxidant; 1200 ppm of secondary antioxidant (Irgafos 168) and 200 ppm of HALS. This composition is comparative because it does not contain hydroxylamine.

[0068] Rotomolded parts were then prepared in a rotational molding machine sold under the tradename Rotospeed RS3-160 by Ferry Industries Inc. The machine has two arms which rotate about a central axis. Each arm is fitted with a plate which rotates on an axis that is roughly perpendicular to the axis of rotation of the arm. Each plate is fitted with three cast aluminum molds that produce plastic cubes having dimensions of 12.5 inches (31.8 cm)12.5 inches12.5 inches. These molds produce parts having a nominal thickness of about 0.25 inches (0.64 cm) when initially filled with a standard charge of about 3.7 kg of ethylene copolymer.

[0069] A gas fired furnace which is capable of providing 2 million British thermal units (Btu) per hour is used to provide hot air that is circulated about the molds by a fan. The temperature within the enclosed oven is typically maintained at a temperature of between 230 C. (446 F.) and 350 C. (662 F.) for specified periods of time while the machine rotates the arms (typically, at about 8 revolutions per minute (rpm) and the plate (typically, at about 2 rpm). The compositions shown in Table 1 were cooked for a molding cycle of 40 minutes with an oven temperature of 440 F.

[0070] The cooked parts are then cooled by opening the oven. Water spray may also be used to facilitate cooling. Cook times, rotation speed, temperatures and cooling cycles are computer controlled with appropriate software which also includes a data acquisition system.

[0071] Test plaques were cut from the molded cubes. The plaques were exposed to concentrated sulfuric acid (98% H.sub.2SO.sub.4) at 70 C. (As used herein, the term concentrated sulfuric acid refers to a liquid that contains from about 95 to 99% sulfuric acid by weight. In general, concentrated sulfuric acid in commercial use contains about 98% sulfuric acid).

[0072] The comparative compositions discolored after contact with the concentrated sulfuric acid. The following color scale is used to describe increasing levels of discoloration.

[0073] 0White

[0074] 1Tan

[0075] 2Light brown

[0076] 3Dark brown/grey

[0077] 4Black

[0078] Both of the compositions of this example were severely discolored (to black, number 4 on the above scale) after contact with concentrated sulfuric acid at 70 C. for 8 days.

TABLE-US-00001 TABLE 1 MI.sub.2 First Secondary Second (g/10 Density HALS Primary AO Primary AO Experiment Copolymer min) (g/cm.sup.3) (ppm) AO (ppm) (ppm) (ppm) 1 SSC1 1.70 0.945 C-944: 750 0 I-168: 550 FS 042: 250 T-622: 750 S-9228: 450 2 ZN1 2.0 0.941 C-3346: 2000 I-3114: 300 I-168: 1200 0 C-944 = Chimassorb 944; T-622 = Tinuvin 622; C-3346 = Cyasorb UV-3346. I-3114 = Irganox 3114. I-168 = Irgafos 168; S-9228 = Doverphos S-9228. .sup.5 FS 042 = Irgastab FS 042;

[0079] Both experiments 1 and 2 are comparative because each of the compositions only contained a single primary antioxidantSSC1 contained only the hydroxylamine (sold under the trademark Irgastab FS042) and ZN1 contained only a phenolic (sold under the tradename Irganox 3114).

Example 2

[0080] The formulations shown in Table 2 were used in this example. The rotational molding procedures were similar to those described above except that the molding time was reduced to 34 minutes and the temperature was increased to 480 F. Test plaques were cut from the molded cubes and exposed to concentrated sulfuric acid (98 weight % H.sub.2SO.sub.4) at 70 C. for the times shown in Table 3. Color values are also presented in Table 3.

[0081] The data in Table 3 illustrate that the composition that was prepared with the two different primary antioxidants (namely a phenolic and a hydroxylamine) has improved resistance to discoloration

TABLE-US-00002 TABLE 2 Example 2 Formulations First Second MI.sub.2 Primary Secondary Primary (g/10 Density HALS AO AO AO Experiment Copolymer min) (g/cm.sup.3) (ppm) (ppm) (ppm) (ppm) 10-C ZN1 2.0 0.941 C-3346: 2000 I-3114: 300 I-168: 1200 0 11-C SS1 1.70 0.945 C-944: 750 0 I-168: 550 FS 042: 250 T-622: 750 S-9228: 450 12 SS1 1.70 0.945 C-944: 750 I-1076: 500 I-168:550 FS 042: 250 T-622: 750 S-9228: 450 C-944 = Chimassorb 944; T-622 = Tinuvin 622; C-3346 = Cyasorb UV-3346. .sup.4 I-1076 = Irganox 1076; I-3114 = Irganox 3114. .sup.5 I-168 = Irgafos 168; S-9228 = Doverphos S-9228. .sup.6 FS 042 = Irgastab FS 042. The composition of experiment 10 also contained 450 ppm of zinc stearate. The compositions of experiments 11-13 also contained 750 ppm of zinc oxide.

TABLE-US-00003 TABLE 3 Color Values for Inside Surface of Parts Exposure Time 15 60 2 4 Experiment min min days days 10-C 0 0 0.5 1 11-C 2 3 4 4 12 0 0 0 0.5