for more efficient and 'greener' flame retardants for polymeric
materials is ever present and of increasing intensity as regulatory agencies
continue to display concern about the environmental impact of traditional
materials. Compounds capable of multiple modes of action would be particularly
desirable. Compounds containing both bromine (for good gas-phase activity)
and nitrogen (to promote solid-phase activity) should be good candidates for
development as flame retardant agents. A series of 2,4,6-tri[(bromo)xanilino]-1,3,5-triazines
have been synthesized and characterized spectroscopically. The degradation
characteristics of these compounds have been examined using thermogravimetry.
They undergo step-wise decomposition beginning at about 400C.
As a consequence
of their excellent barrier properties vinyl chloride/vinylidene chloride copolymers
have long been prominent in the flexible packaging market. While these polymers
possess a number of superior characteristics, they tend to undergo thermally-
induced degradative dehydrochlorination at process temperatures. This degradation
must be controlled to permit processing of the polymers. Three series of N-substituted
maleimides (N-alkyl-, N-aralkyl, and N-aryl) have been synthesized, characterized
spectroscopically, and evaluated as potential stabilizers for a standard vinyl
chloride/vinylidene chloride (85 mass%) copolymer. As surface blends with
the polymer, these compounds are ineffective as stabilizers. However, significant
stabilization may be achieved by pretreatment of the polymer with N-substituted
maleimides. The most effective stabilization of the polymer is afforded by
N-aralkyl- or N-arylmaleimides, most notably, N-benzylmaleimide and N-p-methoxyphenylmaleimide.
Two series of styrene monomers, one with phosphorus-containing moieties as substituents and the other with substituents containing both phosphorus and nitrogen, have been prepared, characterized, and converted to oligomers. The oligomers contain, in the one case, phosphorus and, in the other, phosphorus and nitrogen. This provides the opportunity to not only assess the impact of the presence of phosphorus on the combustion characteristics of the oligomers but to determine whether or not this impact is enhanced by the presence of nitrogen. The level of residue from thermogravimetry and heat release rate during combustion suggest that the presence of nitrogen may have a small positive impact on the effectiveness of phosphorus flame retardants.
General purpose poly(styrene) is a large
volume commodity polymer widely used in a range of applications. For many
of these the presence of an additive to impart some flammability resistance
is required. Most commonly, brominated aromatics are used for this purpose.
As the polymer undergoes combustion these compounds decompose to generate
bromine atoms and/or hydrogen bromide which escape to the gas phase and trap
flame propagating radicals. While these species are effective in inhibiting
flame propagation they present the opportunity for loss of halogen to the
atmosphere. For this reason, the use of these compounds is being limited in
some parts of the world. Phosphorus compounds, on the other had, impart a
flame retarding influence by promoting char formation at the surface of the
burning polymer. This prevents heat feedback to the polymer and consequent
pyrolysis to generate fuel fragments. The combination of both bromine and
phosphorus present in a single compound might generate a superior flame-retarding
additive in that both modes of retardancy might be promoted simultaneously.
Should this be the case smaller amounts of additive might be necessary to
achieve a satisfactory level of flame retardancy. A series of such additives,
brominated aryl phosphates, has been synthesized and fully characterized spectroscopically.
Blends of these additives, at various levels, with poly(styrene) have been
examined by DSC, TG and in the UL-94 flame test. The flammability of the polymer
is dramatically diminished by the presence of the additive.
Certain five-membered dioxaheterocyclic compounds (hetero atoms may be P, Si, S, etc.) contain a strained carbon–carbon bond
which may undergo homolytic thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization.
If substituents contain flame-retarding moieties this represents a convenient method for imparting flame retrdancy to a polymeric
material. Of particular interest has been 2,4,4,5,5-pentaphenyl-1,3,2-dioxaphospholane. The thermal degradation of this compound
has been studied using 13C NMR spectroscopy. This may conveniently be done by monitoring the intensity of the signal for the benzylic carbon atom as
a function of time and temperature. A simple transformation is the conversion of the cyclic compound to the linear polymer.
The thermal decomposition characteristics of representatives of three classes of organoplatinum compounds have been examined
by thermogravimetry. Substituted salicylato(1,2-diaminocyclohexane)platinum(II) compounds undergo thermal decomposition by
sequential loss of first the salicylato ligand and then the amine ligand to afford a residue corresponding to the platinum
content of the compound. The thermal decomposition of N-arylsalicylaldimino(1,2-diaminocyclohexane)platinum(II) nitrate is
more complex, but is also characterized by two major weight losses. Thermal decomposition ofbis-(2-thiophenecarboxylato)platinum(II) is characterized by ligand fragmentation to generate a residual mass corresponding to
the platinum content of the compound.
The thermal stability of a commercial triaryl phosphate hydraulic fluid has been assessed using thermogravimetry and pyrolysis. This material is a mixture of triaryl phosphates containing a predominance of triphenyl phosphate. It is volatile at higher temperatures. At temperatures below its boiling point, in the presence of air, it slowly decomposes to evolve phenolic fragments.
The thermal degradation characteristics of head-to-head poly(styrene) [HHPS] should provide insight with respect to the impact
of head-to-head placement on the thermal stability of traditional atactic head-to-tail polymer [HTPS]. The synthesis of head-to-head
poly(styrene) must be accomplished indirectly. The head-to-head polymer is most satisfactorily obtained by dissolving metal
reduction of poly(2,3-diphenyl-1,3-butadiene) [PDBD] generated by radical polymerization of the corresponding diene monomer.
Full saturation of the polymer mainchain requires several iterations of the reduction procedure. Since the decomposition of
poly(2,3-diphenyl-1,3-butadiene) is prominent at 374C and that for head-to-head poly(styrene) is similarly facile at 406C,
it seemed feasible that TG of partially hydrogenated PDBD might be utilized as a convenient means of monitoring the extent
of hydrogenation. This has been demonstrated for various levels of unsaturation remaining - from approximately 90 to less
than 10%. Within this range the peak areas from the DTG plots of the partially hydrogenated polymer provide a good reflection
of the ratio of unsaturated to saturated units in the polymer. Even low levels of unsaturation in the polymer may be detected
by the asymmetry of the decomposition peak for the polymer.
Vinylidene chloride polymers containing comonomer
units capable of consuming evolved hydrogen chloride to expose good radical-scavenging
sites might be expected to display greater thermal stability than similar
polymers containing simple alkyl acrylates as comonomer. Incorporation of
a comonomer containing the phenyl t-butyl
carbonate moiety into a vinylidene chloride polymer has the potential to afford
a polymer with pendant groups which might interact with hydrogen chloride
to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been
prepared, characterized, and subjected to thermal degradation. The degradation
has been characterized by thermal and spectroscopic techniques. The degradation
of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl
acrylate copolymers is much more facile than the same process for similar
copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate
or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer
degradation, [4-(t-butoxycarbonyloxy) phenylmethyl
acrylate units are apparently converted to acrylic acid units by extensive
fragmentation of the sidechain. Thus, the phenyl t-butyl
carbonate moiety does function as a labile acid-sensitive pendant group but
its decomposition in this instance leads to the generation of a phenoxybenzyl
carboxylate capable of further fragmentation.
Fully substituted 1,3-dioxa-2-siloles contain a strained carbon–carbon bond that will undergo thermolysis at modest temperatures to generate a diradical capable of initiating vinyl polymerization. If the substituents contain flame-retarding moieties, e.g., halogen or phophorus-containing groups, the use of such compounds as initiators serves to incorporate a flame-retarding unit into the polymer mainchain. Both 2,2-dialkyl- and 2,2-diaryl-4,4,5,5-tetra(3,5-dibromophenyl)-1,3-dioxa-2-siloles may be prepared from the appropriate tetra(bromoaryl)-1,2-ethanediol and are obtained as white solids. Thermal decomosition (thermogravimetry) of these materials occurs in two stages. Initial decomposition is observed at about 250 °C and corresponds to the loss of nearly half of the initial sample mass.