Compositions containing vinylidene fluoride polymers which are stabilized to heat

The compositions contain, as heat stabilizer, an effective amount of bismuth carboxylate chosen from bismuth succinate, acrylate and terephthalate and their mixtures. In general, this amount is at least 0.05 part and does not exceed 5 parts per 100 parts by weight of vinylidene fluoride polymer. The heat stabilizers according to the invention are suitable both for the thermal stabilization of virgin polymers and for the stabilization/decoloration of thermally degraded waste polymers.

FIELD OF THE INVENTION 
The present invention relates to compositions containing vinylidene 
fluoride polymers exhibiting an improved thermal stability. It more 
particularly relates to such compositions containing a bismuth carboxylate 
as heat stabilizer. 
TECHNOLOGY REVIEW 
Vinylidene fluoride polymers, which are thermoplastic homopolymers and 
copolymers, constitute well known polymers exhibiting an array of 
advantageous properties and, in particular, high chemical inertia and high 
resistance to ultraviolet radiation, as well as excellent mechanical 
strength. These polymers consequently know numerous applications in 
fields, such as for example in the chemical industry, which are 
particularly demanding as regards resistance to corrosion. In addition, 
these polymers exhibit an excellent intrinsic thermal stability, so that 
they are most often employed in the absence of any heat stabilizer. 
Nevertheless, in applications in which these polymers are employed as a 
very thick layer, for example of the order of several tens of mm, thermal 
degradation can appear at the core of the material as a result of the 
severe thermal conditions to which it is subjected during the shaping of 
thick components. Likewise, problems of thermal stability emerge when 
vinylidene fluoride polymers of very high molecular weights are 
melt-shaped (for example for the shaping of articles requiring very high 
mechanical strength) and/or when vinylidene fluoride copolymers, the 
intrinsic thermal stability of which is less than that of the homopolymer, 
are melt-shaped. It thus proves to be currently desirable to have 
available heat stabilizers which are effective for vinylidene fluoride 
polymers. 
A multitude of very diverse heat stabilizers have already been proposed for 
the (essential) thermal stabilization of chlorinated polymers, such as 
vinyl chloride polymers. In practice, they are most often salts of lead, 
calcium and/or zinc, barium and/or cadmium or alternatively organotins and 
thiotins. 
Nevertheless, in Patent CH-A-275,161 of Apr. 20, 1948, a description is 
given of the thermal stabilization of vinyl chloride and vinylidene 
chloride polymers by the involvement of bismuth salts of highly varied 
organic acids, such as, for example, bismuth formate, maleate, laurate and 
stearate, preference being given to the bismuth salts of fatty acids 
having at least 12 carbon atoms. 
The thermal stabilization of poly(vinyl chloride) polymers by the 
involvement of bismuth salts of C.sub.6 to C.sub.22 monocarboxylic acids, 
such as in particular bismuth stearate and salicylate, is recommended in 
Patent Application JP-A-66/19821 of Aug. 30, 1963. These bismuth salts are 
advantageously employed in conjunction with heat stabilizers, such as 
cadmium, barium, zinc or lead salts. 
SUMMARY OF THE INVENTION 
The present invention is targeted at providing heat stabilizers which are 
effective for vinylidene fluoride polymers. 
To this end, the invention relates to compositions containing vinylidene 
fluoride polymers which are stabilized to heat by the involvement of an 
effective amount of bismuth carboxylate, characterized in that the bismuth 
carboxylate is chosen from bismuth succinate, acrylate and terephthalate 
and their mixtures. 
Excellent results are obtained with the succinate and the acrylate and 
their mixtures. A very particularly preferred bismuth carboxylate is the 
succinate. 
DETAILED DESCRIPTION OF THE INVENTION 
A surprising aspect of the present invention is the fact that bismuth 
carboxylates described as being capable of contributing effectively to the 
thermal stabilization of chlorinated polymers, and which are moreover 
similar to the bismuth carboxylates employed in the compositions according 
to the invention, prove to be ineffective in stabilizing vinylidene 
fluoride polymers. Moreover, the effectiveness of the heat stabilizers 
according to the invention is such that they even make possible the 
stabilization/decoloration of thermally degraded waste vinylidene fluoride 
polymers. 
Effective amount of bismuth carboxylate is understood to denote, for the 
purposes of the present invention, an amount which is sufficient to 
improve the thermal stability, that is to say to prevent the coloration of 
the compositions or, depending on the situation, to reduce the coloration 
of the compositions (in the case of the stabilization of waste polymers 
which are already degraded) when the latter are subjected to temperatures 
higher than the melting temperature of the vinylidene fluoride polymers 
and which are sufficient to be able to convert them into shaped articles. 
This amount is not particularly critical and depends, in particular, on 
the nature, on the molecular weight and, if appropriate, on the extent of 
the thermal degradation of the vinylidene fluoride polymers which it is 
desired to stabilize. To give an idea, the bismuth carboxylate is 
generally employed in the proportion of at least 0.05 part by weight, most 
often at least 0.1 part by weight and more particularly still at least 0.3 
part by weight per 100 parts by weight of vinylidene fluoride polymer. In 
general, the bismuth carboxylate content does not exceed 5 parts by 
weight; most often it does not exceed 3 parts by weight and more 
particularly still 1.5 parts by weight per 100 parts by weight of 
vinylidene fluoride polymer. 
The manufacture of the bismuth carboxylates employed in the compositions 
according to the invention does not have a critical nature. The 
carboxylates can therefore be manufactured in any appropriate way. It is 
possible, for example, to manufacture them by a wet route by reacting, in 
water at room temperature, stoichiometric amounts of bismuth hydroxide and 
carboxylic acid (succinic, acrylic or terephthalic acid or their 
mixtures), followed by filtration of the bismuth salt. It is also possible 
to manufacture them by a dry route, for example by dry mixing/grinding, 
for a few minutes, appropriate amounts of bismuth trioxide and the 
carboxylic acid (mechanochemical reactions), followed by reheating the 
reaction mixture to complete the reaction. 
The bismuth carboxylate can be employed in any form, for example powder or 
aqueous dispersion. However, for reasons of convenience, it is preferable 
to employ it in the form of a powder. In this case, it is advantageous to 
use powders in which the particles exhibit a mean diameter of less than 
approximately 100 .mu.m. The mean diameter of the bismuth carboxylate 
particles is preferably between approximately 0.1 and 30 .mu.m. 
In addition to the bismuth carboxylate, the compositions according to the 
invention can contain the usual additives for vinylidene fluoride 
polymers, such as lubricating agents, pigments, additives which reduce the 
emission of smoke during combustion ("smoke-suppressants"), and the like. 
In general, the compositions according to the invention do not contain a 
heat stabilizer other than the bismuth carboxylate. 
Vinylidene fluoride polymer is understood to denote, for the purposes of 
the present invention, both vinylidene fluoride homopolymers and 
thermoplastic copolymers of vinylidene fluoride and of ethylenically 
unsaturated comonomers, which are advantageously fluorinated, containing 
at least approximately 75% by weight of monomer units derived from 
vinylidene fluoride. The said thermoplastic copolymers advantageously 
exhibit a melting temperature at least equal to 130.degree. C. and 
preferably at least equal to 150.degree. C. and more particularly still 
165.degree. C. Mention may be made, as examples of fluorinated comonomers 
which can be used, of hexafluoropropylene and chlorotrifluoroethylene. 
The vinylidene fluoride polymers of the compositions according to the 
invention are advantageously chosen from vinylidene fluoride homopolymers 
and its thermoplastic copolymers with hexafluoropropylene or 
chlorotrifluoroethylene, and more particularly with 
chlorotrifluoroethylene. The thermoplastic copolymers of vinylidene 
fluoride and of hexafluoropropylene advantageously contain from 5 to 20% 
by weight approximately of hexafluoropropylene and more particularly still 
from 8 to 15% by weight approximately. The latter particularly preferred 
copolymers exhibit melting temperatures from approximately 160 to 
approximately 135.degree. C. 
The thermoplastic copolymers of vinylidene fluoride and of 
chlorotrifluoroethylene advantageously contain from 10 to 25% by weight 
approximately of chlorotrifluoroethylene and more particularly still from 
12 to 22% by weight approximately. The latter particularly preferred 
copolymers exhibit melting temperatures from approximately 170 to 
approximately 165.degree. C. 
The vinylidene fluoride polymers which take part in the compositions 
according to the invention can be composed without distinction of virgin 
polymers or of waste polymers (or alternatively of mixtures of these 
polymers). In the case where virgin polymers are concerned, the latter are 
most often provided in the form of powders, the particles of which exhibit 
a mean diameter from approximately 50 to 200 .mu.m and most often from 
approximately 100 to 140 .mu.m. In the case where waste polymers are 
concerned, it is of course advisable, prior to the incorporation of the 
bismuth carboxylate, to grind and/or micronize the shaped articles made of 
waste vinylidene fluoride polymers, for the purpose of reducing them to 
ground particles (ground materials) of reduced size. The waste polymer 
ground materials preferably have a mean diameter not exceeding 5 mm and 
more particularly still 2 mm. 
Likewise, in the case of the thermal stabilization of thermally degraded 
waste polymers, it can be advantageous, although not essential, to subject 
the ground materials to a pretreatment by means of hydrogen peroxide. In 
this case, the incorporation of bismuth carboxylate is preceded by a 
treatment of the ground materials by means of hydrogen peroxide. The said 
treatment advantageously takes place with stirring in an aqueous hydrogen 
peroxide solution, for example a 35% solution, for a few tens of minutes 
to a few hours, advantageously for approximately 2 hours, at a temperature 
ranging from approximately 80 to 100.degree. C. After the treatment with 
hydrogen peroxide, the vinylidene fluoride polymer is filtered, washed 
with water and then dried. 
The preparation of the compositions according to the invention does not 
exhibit any particular problem. All the conventional techniques which 
allow the incorporation of processing aids in thermoplastic polymers in 
order to form mixtures which exist in the powder or granule form can be 
used. Thus, the bismuth carboxylate can be mixed with the vinylidene 
fluoride polymer from the polymerization stage, either by direct 
introduction into the polymerization mixture, at the end of 
polymerization, or alternatively by addition to the wet cake obtained by 
draining or filtration of the aqueous dispersion coming from 
polymerization. It is understood that this method of incorporation is only 
suited to the compositions according to the invention composed of virgin 
vinylidene fluoride polymer. An advantageous procedure, which can be used 
in all cases, comprises the addition of the bismuth carboxylate to the 
polymer, which is in the form of a powder (or of a ground material), 
during the manufacture of a premix, at the same time as the other 
additives which take part in the composition. The bismuth carboxylate can 
also be introduced directly into the devices where the vinylidene fluoride 
polymer is melted, such as screw extruders. In this case, the stabilized 
composition will exist in the form of granules (compounds). 
The bismuth carboxylates employed in the compositions according to the 
invention have an effectiveness such that they are suitable not only for 
the manufacture of very thick components, such as slabs or rods, and for 
the employment of vinylidene fluoride polymers of very high molecular 
weights and/or of vinylidene fluoride copolymers with a thermal stability 
which is lower than that of the homopolymers but also for the 
stabilization/decoloration of thermally degraded waste vinylidene fluoride 
polymers, making possible the recycling of ground materials from waste 
articles. 
The compositions according to the invention are capable of being made use 
of by all conventional techniques for the conversion of molten 
thermoplastics, such as extrusion and moulding.

EXAMPLES 
The following examples are intended to illustrate the invention. 
Examples 1 to 4 relate to virgin compositions containing vinylidene 
fluoride polymers. 
Examples 5 to 11 relate to thermally degraded waste compositions containing 
vinylidene fluoride polymers. The bismuth carboxylates employed in the 
compositions exist as particles with a mean diameter &lt;30 .mu.m. 
Examples 1 to 4 
In Example 1 (given by way of comparison) and in Example 2 (according to 
the invention), the preparation was carried out of vinylidene 
fluoride/chlorotrifluoroethylene copolymer compositions containing 15% by 
weight of chlorotrifluoroethylene (VF2-CTFE) which have a melting 
temperature of 169.degree. C. and a melt index (MI), measured at 
230.degree. C. under a load of 5 kg, of 15 g/10 min (ASTM D 1238). These 
compositions contain the following ingredients, all the parts being 
expressed by weight: 
______________________________________ 
Ingredients Amount 
______________________________________ 
VF2-CTFE 100 
Ca molybdate (smoke-suppressant) 
0.3 
Polyethylene wax (lubricant) 
0.2 
Precipitated CaCO.sub.3 
0.1 
______________________________________ 
The composition according to Example 2 additionally contains 1 part by 
weight of bismuth succinate. 
These compositions were mixed in a mixer of Brabender type at 270.degree. 
C. with a speed of the shafts of 50 r/min and crepes were withdrawn every 
five minutes for the purpose of assessing their coloration. 
After mixing for 5 minutes, the compositions according to Examples 1 and 2 
are still white. 
After mixing for 25 minutes, the composition according to Example 1 
(without bismuth succinate) is brown; after 30 minutes it is black. 
After mixing for 25 minutes, the composition according to Example 2 is 
still white, after 30 minutes it is light beige and after 50 minutes it is 
still beige. 
Comparison of the results of Examples 1 and 2 shows the exceptional thermal 
stabilizing effect of bismuth succinate. 
In Example 3 (comparative) and in Example 4 (according to the invention), 
the preparation was carried out of compositions containing a vinylidene 
fluoride homopolymer (PVDF) of high molecular weight (MI: 0.2 g/10 min) 
and a VF2-CTFE copolymer identical to that employed in Examples 1 and 2. 
These compositions contain the following ingredients, all the parts being 
expressed by weight: 
______________________________________ 
Ingredients Amount 
______________________________________ 
PVDF 67 
VF2-CTFE 33 
Polyethylene wax 
0.067 
______________________________________ 
The composition according to Example 4 additionally contains 1 part by 
weight of bismuth succinate. 
These compositions were mixed at 270.degree. C. under the same conditions 
as in Examples 1 and 2 and crepes were withdrawn for the purpose of 
assessing the coloration. 
After 10 minutes, the composition according to Example 3 is brown, after 20 
minutes it is blackish. 
The composition according to Example 4 is still off-white after 15 minutes; 
it is very slightly coloured (very light beige) after 20 minutes and 
becomes brownish after 50 minutes (shade virtually identical to that 
achieved by the composition according to Example 3 after 10 minutes). 
Comparison of the results of Examples 3 and 4 fully shows the effectiveness 
of bismuth succinate. 
Examples 5 to 11 
In Examples 5 to 11, the preparation was carried out of compositions from 
thermally degraded waste PVDF homopolymer originating from waste chemical 
engineering pipes (several years of everyday use). The recovered pipes 
were ground so as to be obtain ground materials, the particles of which 
exhibit a mean diameter of &lt;2 mm. 
In all these examples, except in Example 5, given by way of comparison, 1 
part by weight of bismuth carboxylate was incorporated (the nature of 
which carboxylate is specified in the appended Table I). 
In a first stage, the PVDF ground materials and bismuth carboxylate were 
mixed with stirring for 15 minutes (planetary stirrer rotating at 20 
r/min). These premixes were then mixed for 3 minutes at 180.degree. C. in 
a mixer (of Brabender type) provided with blades rotating at 32 r/min. The 
crepes thus obtained were finally pressed at 210.degree. C. for 5 minutes 
in a mould maintained under a pressure of 50 bars. The pressed slabs were 
then examined. The colorations observed are taken up in the appended Table 
I. 
From the comparison of the results which appear in the appended Table I, it 
is evident that bismuth succinate, acrylate and terephthalate contribute a 
very appreciable decoloration to thermally degraded PVDF (cf. Example 5 
given by way of comparison), which reflects an appreciable improvement in 
its thermal stability. 
The bismuth carboxylates employed in the comparative examples (Examples 9, 
10 and 11) are markedly less effective. In this respect, it is interesting 
to compare the results of the maleate (unsaturated C.sub.4 dicarboxylate) 
or alternatively of the oxalate (saturated C.sub.2 dicarboxylate) with 
those of the succinate (saturated C.sub.4 dicarboxylate). 
TABLE 
______________________________________ 
No. of the Coloration of the 
Example Bi carboxylate 
pressed slab 
______________________________________ 
5 (C) None brown-black 
6 succinate white-beige 
7 acrylate light beige 
8 terephthalate light beige 
9 (C) maleate dark beige-brown 
10 (C) oxalate dark beige-brown 
11 (C) formate dark beige-brown 
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