Stabilized thermoplastic polymer compositions comprising a C-nitroso compound

Thermoplastic polymers are stabilized by mixing therewith, under polymer processing conditions such that nitroxyl radicals are formed, a nitrogen stabilizers therefor, which is (a) a C-nitroso-containing compound (b) an N-nitroso-containing compound, or (c) a nitrone-containing compound.

This invention relates to the stabilization of thermoplastic polymer 
compositions. 
Most thermoplastic polymers are subject to oxidative deterioration during 
processing or in service. This deterioration can lead to loss of 
properties by the polymers, e.g. embrittlement or discolouration, which 
can be undesirable. Most particularly thermoplastic polymers are subject 
to deterioration when, in service, they are in an outdoor environment or 
otherwise subjected to U.V. radiation. In addition deterioration can 
occur, e.g. as a result of oxidation, when, in service, the polymers are 
subjected to high temperature. Also the high shearing forces and 
temperatures used in polymer processing often result in deterioration. 
It is well known to incorporate so-called stabilizers into thermoplastic 
polymer compositions to prevent or retard such deterioration processes. 
Such stabilizers, according to the manner in which they act, are U.V. 
stabilizers, thermal antioxidants or melt stabilizers. Amongst the 
commercially available U.V. stabilizers are the compounds of the formulae 
##STR1## 
[bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate] and 
##STR2## 
which are sold under the names Tinuvin 770 (ex Ciba-Geigy AG) and Cyasorb 
UV 531 (ex American Cyanamid) respectively. Such conventional stabilizers, 
in particular Tinuvin 770, are often expensive to prepare. In order to 
ensure that a particular polymer composition has the required stability 
characteristics, it is often necessary to use a mixture of stabilizers. 
For example there may be added to the polymer both a U.V. stabilizer and a 
melt stabilizer to protect the polymer from deterioration both during the 
processing of it and during its service. This of course tends further to 
add to cost. 
The mechanism of the stabilization is not fully understood and clearly 
different stabilizers can act in different ways. For example the mechanism 
by which Tinuvin 770 stabilizes thermoplastic polymers is different from 
that by which Cyasorb UV 531 stabilizes them. Recent studies (Bagheri, 
Chakraborty & Scott, Polymer Degradation and Stability 4, (1982), 1-16) on 
the use of hindered piperidines, of which Tinuvin 770 is an example, 
attribute stabilizing effectiveness to the oxidation of the hindered 
piperidines to nitroxyl radicals during the polymer processing and 
subsequently when the thermoplastic polymer mixture is exposed to U.V. 
radiation. It is the presence of these nitroxyl radicals which is believed 
to give rise to the stabilization. 
It has now been found that thermoplastic polymers, in particular 
polyolefins, e.g. polyethylene and polypropylene, and polyvinyl chloride, 
may be stabilized against oxidative deterioration by mixing therewith a 
C-nitroso-containing compound, an N-nitroso-containing compound or a 
nitrone compound under polymer processing conditions such that there are 
formed nitroxyl radicals. 
According to the present invention, there is provided a process for 
preparing a stabilized thermoplastic polymer composition which process 
comprises mixing the thermoplastic polymer with a nitrogen-containing 
stabilizer, which is 
(a) a C-nitroso-containing compound, 
(b) an N-nitroso-containing compound, or 
(c) a nitrone-containing compound, 
under polymer processing conditions such that nitroxyl radicals are formed. 
It is important that the compositions prepared according to the present 
invention are formed under polymer processing conditions such that 
nitroxyl radicals are formed. The required conditions are conditions in 
which the polymer is subjected to high shear in which polymer chains 
become severed resulting in the mechanochemical formation of hydrocarbyl 
radicals. Mere mixing without shear, e.g. of polymer and stabilizer 
solutions, will not provide the U.V. stabilization properties of the 
present invention. While we do not wish in any way to be bound by the 
theory, it is believed that the macrohydrocarbyl radicals and related 
mechanochemical radicals formed from the polymer in the processing step 
react with the stabilizer compounds according to the present invention to 
form the nitroxyl radicals. In particular it has been observed that 
stabilization obtained with the compounds according to the present 
invention is dependant upon the concentration of nitroxyl radicals in the 
composition after processing. In the case of the C-nitroso compounds and 
the nitrone compounds, these are believed to react directly with the 
hydrocarbyl compounds in a radical trapping reaction to form the nitroxyl 
radicals. In the case of the N-nitroso compounds, it is believed these 
dissociate and react further to give two different kinds of nitroxyl 
radical. 
The polymer processing conditions necessary for nitroxyl radical formation 
according to the present invention depend, in particular, on the 
thermoplastic polymer used. Generally speaking the polymer and stabilizer 
are mixed together under high shear at above 150.degree. C., usually 
160.degree. to 350.degree. C., and preferably 180.degree. to 280.degree. 
C. in a mixer conventionally used for polymer processing. During the 
mixing shear is at its highest initially and decreases as the polymer is 
broken up. It is thus initially that the polymer chains are severed with 
hydrocarbyl radical formation. It is important according to the present 
invention that the stabilizer should be present during this high shear to 
obtain the good results according to the invention. Processing time also 
depends on the ingredients being used but will generally be 10 to 20 
minutes. 
It has been found that the C-nitroso compounds (i.e. compounds containing 
at least one C--N.dbd.O group) have good U.V. stabilizing activity for, 
i.e. they prevent or retard the photo-oxidative embrittlement of, 
thermoplastic polymers. Also the C-nitroso compounds have melt stabilizing 
activity. The C-nitroso compounds may in particular be aliphatic or 
aromatic compounds with nitroso-alkanes being preferred. Generally 
speaking aromatic nitroso compounds are less effective as U.V. light 
stabilizers but are more effective than their alkyl analogues as thermal 
antioxidants. 
C-nitroso compounds according to the present invention include those of the 
general formula I: 
EQU R--N.dbd.O (I) 
wherein R represents an unsubstituted or substituted alkyl group, 
preferably a tertiary alkyl group (e.g. tertiary butyl or octyl, or a 
group C(CH.sub.2 OR').sub.3 or a group R.sub.x C(CH.sub.2 OR').sub.3-x 
wherein x is 1,2 or 3, R is as defined below and R' represents a hydrogen 
atom or an unsubstituted or substituted alkyl or acyl group and in which 
each R' may be the same or different), and R may be an unsubstituted or 
substituted (e.g. alkyl-, chloro-, hydroxy-, carboxyl-, cyano-, nitro-, or 
dimethylamino-substituted) phenyl or naphthyl group (e.g. phenyl, 
tetramethyl-phenyl, pentamethyl-phenyl, trichloro-phenyl, hydroxy-phenyl, 
di-tert-butyl-hydroxy-phenyl, dimethylamino-phenyl and hydroxy-naphthyl) 
or a heterocyclic aromatic or reduced heterocyclic (e.g. piperidinyl) 
group. 
Particularly preferred C-nitroso compounds are the tertiary alkyl nitrosos, 
particularly 2-nitroso-2-methylpropane and 
2-nitroso-2,4,4-trimethylpentane. These compounds are highly effective as 
U.V. stabilizers and act as melt stabilizers. At the same time the 
compounds are much cheaper to produce than conventional commercially 
available stabilizers. 
It has also been found that N-nitroso compounds (b) act as U.V. stabilizers 
to prevent or retard the photo-oxidative embrittlement of thermoplastic 
polymers. These compounds are also generally effective as melt 
stabilizers. The N-atom of the N-nitroso group can be attached to 
aliphatic, cycloaliphatic, aromatic and araliphatic groups. However 
preferably the nitrogen atom is attached to two alkyl groups to give a 
branched structure described generally as: 
##STR3## 
wherein R" and R"' are branched chain hydrocarbon radicals, preferably 
tertiary. Most preferably the nitrogen atom forms part of a saturated 
6-membered ring, which may contain other N-hetero-atoms. Particularly 
preferred such compounds are 
##STR4## 
wherein each R.sup.1V group represents hydrogen or alkyl. 
The first of these compounds is a novel compound and corresponds to the bis 
nitrosamine derived from the commercially available Tinuvin 770 product. 
It has almost twice the effectiveness as a U.V. stabilizer of Tinuvin 770 
and also is an effective melt stabilizer and thus it is a preferred 
compound according to the invention. Thus, unlike Tinuvin 770 itself, 
which is not an effective melt stabilizer, it does not require a separate 
melt stabilizer to be added to the system. 
It has further been found that nitrone compounds (c), in particular 
aldonitrones, preferably N-phenyl and N-tert.alkyl aldonitrones, when used 
in thermoplastic polymers are stabilizers. These compounds have good melt 
stabilizing activity and have thermal oxidative stabilizing effect. 
Nitrone compounds are generally weak U.V. stabilizers when used alone. 
Preferred nitrones are those of the general formula II 
##STR5## 
wherein R.sub.1 represents an alkyl or substituted alkyl, preferably 
tertiary alkyl (e.g. tert. butyl), group or an aromatic or substituted 
aromatic, preferably phenyl, group, R.sub.2 represents a hydrogen atom or 
an alkyl or aryl group, and X and Z may each represent a hydrogen atom or 
a lower alkyl, e.g. methyl, group and Y represents a hydrogen atom or a 
hydroxy or alkoxy, e.g. methoxy, group. Most preferably there is used a 
benzaldonitrone containing a partially hindered 4-hydroxy group on the 
phenyl ring. 
In choosing the stabilizer to use according to the present invention, 
regard has to be taken of the physical properties of the stabilizer. Thus 
for example in many applications the use of a stabilizer which is highly 
coloured needs to be avoided. Also for example stabilizers which are lost 
to the system e.g. by being volatile under the processing conditions 
should be avoided. 
The stabilizers according to the present invention are in general known 
compounds and may be obtained by methods known per se. For example they 
may be obtained as follows: 
Nitroso alkanes may be prepared by oxidation of the corresponding amines 
with hydrogen peroxide by the methods of Stowell (J. Org. Chem., 36, 3055 
(1971)). 
Aromatic nitroso compounds may be prepared by oxidation of the 
corresponding amine with hydrogen peroxide by the method of Richard and 
Bayer (J. Am. Chem. Soc., 82, 3455 (1960)). 
Nitrones may be prepared by reacting a hydroxylamine with the corresponding 
aldehyde by the reaction of Beckman (Chem. Ber., 27, 1894 (1958)). 
Nitrosamines may be prepared by the method of Vogel (Prac. Org. Chem., 
Longman (1967)). 
As mentioned above however the bis nitrosamine derived from the 
commercially available Tinuvin 770 
[bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate] is a novel compound. This 
compound can be obtained from the sebacate by reaction with sodium nitrite 
in the presence of acid. 
The thermoplastic polymers of the stabilized compositions according to the 
present invention are generally saturated thermoplastic polymers, in 
particular polyolefins, e.g. polyethylene and polypropylene, and polyvinyl 
chloride. However there may generally be used other organic thermoplastic 
polymers with carbon-carbon bonds including polyesters, polyamides and 
rubber modified plastics such as ABS and rubbers which are used as 
thermoplastics without vulcanisation or cross-linking such as 
ethylene/propylene copolymers and styrene butadiene block copolymers. 
One interesting aspect of the present invention is the use of stabilizers 
according to the invention in conjunction with other stabilizers. As noted 
above the C-nitroso and the N-nitroso compounds act in particular as U.V. 
stabilizers and in addition have melt stabilizing activity. The nitrones 
have in particular good melt stabilizing effect and have thermal 
antioxidant activity. By using other stabilizers in conjunction with the 
stabilizers according to the invention, stabilizing effect may be 
broadened and increased. Particularly interesting is the use of 
stabilizers according to the invention in conjunction with other 
stabilizers and synergism then observed. For example there may be included 
in the compositions according to the invention U.V. stabilizer compounds 
to give high U.V. stability. Such U.V. stabilizers compounds include 
Tinuvin 770, Cyasorb UV 531 and Tinuvin 327. [Tinuvin 327, ex Ciba-Geigy 
AG, is a benztriazole of the formula: 
##STR6## 
tBu represents tertiary butyl.] Such U.V. stabilizers can be used in 
conjunction with C-nitroso-containing stabilizers to give compounds of 
high U.V. stability. Alternatively, it may be desirable if for example 
thermal antioxidative stability is required to use a thermal antioxidant 
together with a C-nitroso-containing stabilizer. Similarly if the 
stabilizer used according to the present invention is a nitrone, and U.V. 
stability is required it may be used in conjunction with a known U.V. 
stabilizer. It has been found that when stabilizers according to the 
invention are used in conjunction with for example U.V. stabilizers there 
is a synergistic effect i.e. the U.V. stability of the final composition 
is higher than might be expected on the basis of the summed activities of 
the individual stabilizers when used alone. Also a synergistic effect may 
be observed when a thermal antioxidant is used with a C-nitrosamine. 
The processing conditions under which the compositions according to the 
present invention are prepared are important to the obtaining of 
stabilization according to the present invention. As explained above, it 
is believed that radicals formed mechanochemically under the high shear 
conditions of processing react with the stabilizers according to the 
present invention to result in the formation of a nitroxyl radical. If 
mixing is carried out under conditions which do not result in nitroxyl 
formation the stabilization according to the invention is not obtained. 
Indeed observations indicate that the amount of stabilization obtained 
depends on the level of nitroxyl radical formation. Generally the 
composition according to the present invention will be prepared by simply 
mixing the thermoplastic polymer and stabilizer ingredients using a 
conventional extruder or injection moulding machine. The importance of the 
processing operation has been demonstrated in that the effectiveness of 
the present stabilizers in stabilization is dependent upon processing 
time. Thus generally there is an optimum processing time to give optimal 
stabilization and if this time is not reached or is exceeded, reduced 
stabilization will be obtained. 
In the commercial formulation of polymer compositions, processing times are 
often fixed by the residence time of the machinery being used. In these 
circumstances it may be advantageous to produce a master-batch composition 
according to the present invention having a high content of stabilizer. 
This master-batch may be obtained under processing conditions which ensure 
that optimum stabilization is obtained and then may be used as additive to 
a thermoplastic polymer in a conventional extruder or injection moulding 
machine. In this way the optimum stabilization of the present invention 
may be obtained without the residence time in the conventional extruder or 
conventional moulding machine being critical. The processing time used in 
the production of the master-batch on the other hand can be readily 
adjusted and is chosen such as to provide optimum stabilization. The 
master-batch may suitably be advantageous to introduce additional radical 
generators, for example, dialkyl peroxides, during the processing 
operation. 
The stabilizers according to the present invention are generally used at up 
to 1% by weight of the thermoplastic polymer composition. The preferred 
range is usually 0.05 g to 0.5 g stabilizer per 100 g of polymer. In the 
case of master-batches however the concentration of stabilizer will be 
considerably greater. For example between 2 and 15% by weight of the 
thermoplastic polymer composition. 
The following Examples illustrate the invention.