New families of highly efficient, halogen-free flame retardants for polypropylene (PP)
Pawelec, Weronika (2014-12-11)
Pawelec, Weronika
Åbo Akademi - Åbo Akademi University
11.12.2014
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https://urn.fi/URN:NBN:fi-fe2014120852195
https://urn.fi/URN:NBN:fi-fe2014120852195
Tiivistelmä
The driving forces for current research of flame retardants are increased fire safety in
combination with flame retardant formulations that fulfill the criteria of sustainable
production and products. In recent years, important questions about the environmental safety
of antimony, and in particular, brominated flame retardants have been raised. As a
consequence of this, the current doctoral thesis work describes efforts to develop new
halogen-free flame retardants that are based on various radical generators and phosphorous
compounds.
The investigation was first focused on compounds that are capable of generating alkyl radicals
in order to study their role on flame retardancy of polypropylene. The family of azoalkanes
was selected as the cleanest and most convenient source of free alkyl radicals. Therefore, a
number of symmetrical and unsymmetrical azoalkanes of the general formula R-N=N-R’ were
prepared. The experimental results show that in the series of different sized azocycloalkanes
the flame retardant efficacy decreased in the following order: R = R´= cyclohexyl >
cyclopentyl > cyclobutyl > cyclooctanyl > cyclododecanyl. However, in the series of aliphatic
azoalkanes compounds, the efficacy decreased as followed: R = R´= n-alkyl > tert-butyl >
tert-octyl. The most striking difference in flame retardant efficacy was observed in thick
polypropylene plaques of 1 mm, e.g. azocyclohexane (AZO) had a much better flame
retardant performance than did the commercial reference FR (Flamestab® NOR116) in thick
PP sections. In addition, some of the prepared azoalkane flame retardants e.g. 4’4-
bis(cyclohexylazocyclohexyl) methane (BISAZO) exhibited non-burning dripping behavior.
Extrusion coating experiments of flame retarded low density polyethylene (LDPE) onto a
standard machine finished Kraft paper were carried out in order to investigate the potential of
azoalkanes in multilayer facings. The results show that azocyclohexane (AZO) and 4’4-bis
(cyclohexylazocyclohexyl) methane (BISAZO) can significantly improve the flame retardant
properties of low density polyethylene coated paper already at 0.5 wt.% loadings, provided
that the maximum extrusion temperature of 260 oC is not exceeded and coating weight is kept
low at 13 g/m2.
In addition, various triazene-based flame retardants (RN1=N2-N3R’R’’) were prepared. For
example, polypropylene samples containing a very low concentration of only 0.5 wt.% of bis-
4’4’-(3’3’-dimethyltriazene) diphenyl ether and other triazenes passed the DIN 4102-1 test
with B2 classification. It is noteworthy that no burning dripping could be detected and the
average burning times were very short with exceptionally low weight losses. Therefore,
triazene compounds constitute a new and interesting family of radical generators for flame
retarding of polymeric materials. The high flame retardant potential of triazenes can be
attributed to their ability to generate various types of radicals during their thermal
decomposition. According to thermogravimetric analysis/Fourier transform infrared
spectroscopy/MS analysis, triazene units are homolytically cleaved into various aminyl,
resonance-stabilized aryl radicals, and different CH fragments with simultaneous evolution of
elemental nitrogen.
Furthermore, the potential of thirteen aliphatic, aromatic, thiuram and heterocyclic substituted
organic disulfide derivatives of the general formula R-S-S-R’ as a new group of halogen-free
flame retardants for polypropylene films have been investigated. According to the DIN 4102-
1 standard ignitibility test, for the first time it has been demonstrated that many of the
disulfides alone can effectively provide flame retardancy and self-extinguishing properties to
polypropylene films at already very low concentrations of 0.5 wt.%. For the disulfide family,
the highest FR activity was recorded for 5’5’-dithiobis (2-nitrobenzoic acid). Very low values
for burning length (53 mm) and burning time (10 s) reflect significantly increased fire
retardant performance of this disulfide compared to other compounds in this series as well as
to Flamestab® NOR116.
Finally, two new, phosphorus-based flame retardants were synthesized: P’P-diphenyl
phosphinic hydrazide (PAH) and melamine phenyl phosphonate (MPhP). The DIN 4102-1
test and the more stringent UL94 vertical burning test (UL94 V) were used to assess the
formulations ability to extinguish a flame once ignited. A very strong synergistic effect with
azoalkanes was found, i.e. in combination with these radical generators even UL94 V0 rate
could be obtained.
combination with flame retardant formulations that fulfill the criteria of sustainable
production and products. In recent years, important questions about the environmental safety
of antimony, and in particular, brominated flame retardants have been raised. As a
consequence of this, the current doctoral thesis work describes efforts to develop new
halogen-free flame retardants that are based on various radical generators and phosphorous
compounds.
The investigation was first focused on compounds that are capable of generating alkyl radicals
in order to study their role on flame retardancy of polypropylene. The family of azoalkanes
was selected as the cleanest and most convenient source of free alkyl radicals. Therefore, a
number of symmetrical and unsymmetrical azoalkanes of the general formula R-N=N-R’ were
prepared. The experimental results show that in the series of different sized azocycloalkanes
the flame retardant efficacy decreased in the following order: R = R´= cyclohexyl >
cyclopentyl > cyclobutyl > cyclooctanyl > cyclododecanyl. However, in the series of aliphatic
azoalkanes compounds, the efficacy decreased as followed: R = R´= n-alkyl > tert-butyl >
tert-octyl. The most striking difference in flame retardant efficacy was observed in thick
polypropylene plaques of 1 mm, e.g. azocyclohexane (AZO) had a much better flame
retardant performance than did the commercial reference FR (Flamestab® NOR116) in thick
PP sections. In addition, some of the prepared azoalkane flame retardants e.g. 4’4-
bis(cyclohexylazocyclohexyl) methane (BISAZO) exhibited non-burning dripping behavior.
Extrusion coating experiments of flame retarded low density polyethylene (LDPE) onto a
standard machine finished Kraft paper were carried out in order to investigate the potential of
azoalkanes in multilayer facings. The results show that azocyclohexane (AZO) and 4’4-bis
(cyclohexylazocyclohexyl) methane (BISAZO) can significantly improve the flame retardant
properties of low density polyethylene coated paper already at 0.5 wt.% loadings, provided
that the maximum extrusion temperature of 260 oC is not exceeded and coating weight is kept
low at 13 g/m2.
In addition, various triazene-based flame retardants (RN1=N2-N3R’R’’) were prepared. For
example, polypropylene samples containing a very low concentration of only 0.5 wt.% of bis-
4’4’-(3’3’-dimethyltriazene) diphenyl ether and other triazenes passed the DIN 4102-1 test
with B2 classification. It is noteworthy that no burning dripping could be detected and the
average burning times were very short with exceptionally low weight losses. Therefore,
triazene compounds constitute a new and interesting family of radical generators for flame
retarding of polymeric materials. The high flame retardant potential of triazenes can be
attributed to their ability to generate various types of radicals during their thermal
decomposition. According to thermogravimetric analysis/Fourier transform infrared
spectroscopy/MS analysis, triazene units are homolytically cleaved into various aminyl,
resonance-stabilized aryl radicals, and different CH fragments with simultaneous evolution of
elemental nitrogen.
Furthermore, the potential of thirteen aliphatic, aromatic, thiuram and heterocyclic substituted
organic disulfide derivatives of the general formula R-S-S-R’ as a new group of halogen-free
flame retardants for polypropylene films have been investigated. According to the DIN 4102-
1 standard ignitibility test, for the first time it has been demonstrated that many of the
disulfides alone can effectively provide flame retardancy and self-extinguishing properties to
polypropylene films at already very low concentrations of 0.5 wt.%. For the disulfide family,
the highest FR activity was recorded for 5’5’-dithiobis (2-nitrobenzoic acid). Very low values
for burning length (53 mm) and burning time (10 s) reflect significantly increased fire
retardant performance of this disulfide compared to other compounds in this series as well as
to Flamestab® NOR116.
Finally, two new, phosphorus-based flame retardants were synthesized: P’P-diphenyl
phosphinic hydrazide (PAH) and melamine phenyl phosphonate (MPhP). The DIN 4102-1
test and the more stringent UL94 vertical burning test (UL94 V) were used to assess the
formulations ability to extinguish a flame once ignited. A very strong synergistic effect with
azoalkanes was found, i.e. in combination with these radical generators even UL94 V0 rate
could be obtained.
Kokoelmat
- 215 Teknillinen kemia [127]