Stabilizer combination for the rotomolding process

A description is given of a process for the production of polyolefin hollow articles, which comprises charging the polyolefin with a stabilizer combination, comprising (A) at least one compound from the group of the organic phosphites or phosphonites, and (B) at least one compound from the group of the 3-arylbenzofuranones, filling this mixture into a mold, heating this mold in an oven to above 280.degree. C., such that the stabilised polyolefin fuses, rotating the mold around at least 2 axes, the plastic spreading to the walls, and cooling the mold while still rotating, opening it and taking the resultant hollow article out.

The following Examples illustrate the invention in more detail. Parts and 
percentages are by weight. 
EXAMPLE 1 
Preparation of polyolefin hollow articles by the rotomolding process 
100 Parts of low density polyethylene, copolymerized with hexene (PE-LLD), 
type Quantum.RTM. Petrothene.RTM. GA-635-661, having a melt flow index of 
6.5 g/10 min and a density of 0.935 g/cm.sup.3, are mixed with 0.170 part 
of Chimassorb.RTM. 944 .sup.a), 0.050 part of zinc stearate and the 
stabilizers cited in Tables 1 and 2 at 232.degree. C. in a Superior/MPM 
Extruder, fitted with a 24:1 Maddock type L/D screw, at 100 revolutions 
per minute. The polymer is then ground. The particle size of the polymer 
is from 150 to 500 .mu.m. Owing to the larger surface of the particles 
obtained by grinding, the heat can be absorbed faster, which goes hand in 
hand with a lower energy consumption. 
The actual rotomolding process or rotational molding process, which permits 
the production of fairly large three-dimensional solids, is carried out in 
a Clamshell type rotomolder FSP M20. In this machine, an aluminium mold, 
which is mounted on an arm and into which the plastic sample is filled, is 
heated with a gas burner with circulation of the hot air over 5 minutes to 
316.degree. C., or over 6 minutes to 329.degree. C., and is then kept at 
this temperature for a specific time (see Tables 1 and 2). Subsequently, 
the oven is opened and the mold is cooled first for 7 minutes with 
circulating air, then for 7 minutes by spraying with water and finally for 
another 2 minutes with circulating air. During the entire heating and 
cooling process, the mold, which is mounted on two axes at right angles to 
each other, is rotated, the speed of the main axis being kept at 6 
revolutions per minute and the rotational ratio being 4.5:1. After 
cooling, the lid of the mold is opened and the resultant hollow article is 
taken out. The yellowness index (YI) of the exterior of the molded 
articles is determined according to ASTM D 1925-70. Low YI values denote 
little discoloration, high YI values strong discoloration of the samples. 
The less discoloration, the more effective the stabilizer. The results are 
summarized in Tables 1 and 2. 
TABLE 1 
______________________________________ 
Rotomolding at 316.degree. C. 
Yellowness 
index after 
Examples Stabilizer 8 minutes 
10 minutes 
______________________________________ 
Example 1a.sup.b) 
0.05% of Irganox .RTM. 1010.sup.d) 
6.1 17.5 
0.10% Irgafos .RTM. 168.sup.e) 
Example 1b.sup.c) 
0.02% compound (101).sup.f 
4.1 5.0 
0.08% Irgafos .RTM. 168.sup.e) 
______________________________________ 
TABLE 2 
______________________________________ 
Rotomolding at 329.degree. C. 
Yellowness Index after 
Examples Stabilizer 6 minutes 
8 minutes 
______________________________________ 
Example 1c.sup.b) 
0.05% Irganox .RTM. 1010.sup.d) 
4.0 17.0 
0.10% Irgafos .RTM. 168.sup.e) 
Example 1d.sup.c) 
0.02% compound (101).sup.f 
4.3 5.7 
0.08% Irgafos .RTM. 168.sup.e) 
______________________________________ 
.sup.a) Chimassorb .RTM. 944 (Ciba Spezialitatenchemie AG) denotes linear 
or cyclic condensates prepared from 
N,Nbis(2,2,6,6-tetramethyl-4-piperidyl) hexamethylenediamine and 
4tert-octylamino-2,6-dichloro-1,3,5-triazine (compound of formula (H5)) 
##STR50## 
wherein m is a number in the range from 2 to 200. 
.sup.b) Comparison Examples. 
.sup.c) Examples of this invention. 
.sup.d) Irganox .RTM. 1010 (Ciba Spezialitatenchemie AG) denotes the 
pentaerythritol ester of 3(3,5- ditert-butyl-4-hydroxyphenyl)prionic acid 
(compound of formula AO3) 
##STR51## 
.sup.e) Irgafos .RTM. 168 (Ciba Spezialitatenchemie AG) denots 
tris(2,4di-tert-butylphenyl)phosphite. 
.sup.f) Compound (101) is a mixture of about 85 parts by weight of the 
compound of formula Va and about 15 parts by weight of formula Vb 
##STR52## 
##STR53##