Flame-Retardant Formulations for
HIPS and Polyolefins Using Chlorinated Paraffins
Abstract
Chlorinated paraffins are one the lowest cost flame-retardants available. Solid
chlorinated paraffins have been used to flame retard many polymers, but some applications
were limited due to the thermal stability of the chlorinated paraffin. A solid chlorinated
paraffin with improve thermal stability is now available and can be used as a flame retardant
for HIPS and polyolefins. Formulations, flame retardant results and physical properties are
discussed.
Introduction
Chlorinated paraffins are a low cost flame retardant with only the hydrated metal
oxides being less expensive. They are manufactured by reaction of a paraffin wax with
elemental chlorine either neat, in a solution, or in an emulsion. Product and property variations
are induced by changes in raw materials, duration and conditions of chlorination, work-up and
stabilization. Based on bond dissociation energies, chlorinated paraffins theoretically should
have stability up to 300C. However, due to the presence of some imperfections in the
product, the maximum recommended processing temperatures for chlorinated paraffins is
230C. Selection of the most linear paraffin, the optimum carbon chain length and the correct
chlorine content yields chlorinated paraffins with optimum properties for a polymer. The
optimum carbon chain length is C22-C26 and the optimum chlorine content is 72%.
Chlorinated paraffins have a synergistic effect with Group V metal oxides, especially
antimony trioxide. In addition, since they lack chromophores, they have better UV stability
than do brominated aromatic fire retardants. Finally, chlorinated paraffins are additive flameretardants with partial solubility in thermal plastics. Therefore, they do not bloom and have an
influence on physical properties such as heat deflection temperature and percent elongation.
Synthesis
In general, a wax is treated with molecular chlorine to yield chlorinated paraffin
________
Don Stevenson, Vic Lee, Daryl Stein, and Tarang Shah, Dover Chemical, 3676 Davis Road, Dover, OH 44622
and HCl as shown in equation 1. The addition of one chlorine atom per carbon results in a
product with a 73% chlorine content. In practice the chlorine content for a solid product can
C20H42 + 20 Cl2
C20H22Cl20 + 20 HCl
(1)
(73% Chlorine)
range from 70-74% with 72% being a typical value. The physical properties of the product
depend on the chain length of the wax, the chlorine content, and the manufacturing process.
The effect of chain length and chlorine content on physical properties of a chlorinated
paraffin can be seen Table 1. Here the chlorinated paraffin with the longer chain length has a
higher softening point compared with one with the same chlorine content but shorter chain
length. In addition, as the chlorine content increased, the softening point for both the short and
Table 1. The Effect of Chlorine Content on Softening Point
Wt% Chlorine
70
71
72
73
74
Softening Point °C
C 18-22
90
95
100
115
130
Softening Point °C
C 22-26
105
110
120
135
160
long chain paraffins also increased.
The manufacturing process can play an important part in determining physical
properties and stability of a chlorinated paraffin. As can be seen the Table 2, a change from
the
Table 2. The Effect of the Manufacturing Process on Physical Properties
Standard Process
Chlorine Content, wt%
71-72
New Process
Chlorez 700 SSNP
72-73
Softening Point, °C
105
130
Thermal Stability
HCl after 4h at 175°C
Color Stability
15 minutes at 200 °C
0.1
0.01
black
yellow
standard manufacturing process to a new proprietary one, leads to improvement in physical
properties, especially improved thermal stability. The improved thermal stability permits use
of chlorinated paraffins in polymers such as polyethylene, polypropylene, and HIPS. In
addition, it offers benefits such as




No polyhalogenated biphenyls or dioxins.
Low cost.
Improved melt flow.
Better U-V stability than aromatic brominated flame-retardants.
Mechanism of Flame Retardance
Many have shown that chlorinated paraffins act as a flame retardant by releasing HCl,
which poisons the flame. This vapor gas inhibition is greatly enhanced when chlorinated
paraffins are used with Group V metal oxides, especially antimony trioxide. Although
antimony trioxide (ATO) alone does not work as a flame retardant, it has been proposed that
antimony oxychloride is formed by the reaction of antimony trioxide with hydrochloric acid
and with further reactions antimony trichloride is formed, which can act both as a radical
scavenger to disrupt oxidation and also as a vapor barrier above the condensation phase to
smother the flame (eqs. 2-5).
Sb2O3 + 2HCl  2SbOCl + H2O
5 SbOCl  Sb4O5Cl2 + SbCl3
4Sb4O5Cl2  5Sb3O4Cl + SbCl3
3Sb3O4Cl  4Sb2O3 + SbCl3
ca. 250C.
245-280C.
410-475C.
475-565C.
(2)
(3)
(4)
(5)
Even though chlorine has a lower flame retardant efficiency than bromine, chlorinated
paraffins can be tailored with flame retardant synergists and brominated aromatics to give
optimum flame retardant performance. CHLOREZ 700-SSNP is a solid chlorinated paraffin,
especially developed and optimized for use in HIPS and polyolefins.
Sample Preparation
All formulations were compounded in a lab size 1600 cc Banbury type internal mixer.
The body temperature was set at 200 °C, and the rotor temperature was set at 180 °C. All
ingredients were added to the Banbury at low speed (77 RPM) except for Chlorez 700 SSNP.
The materials were fluxed and allowed to mix for two minutes. The Chlorez was then added
in three approximately equal portions. The formulation was then mixed for an additional five
minutes at low speed before it was removed, sheeted on a 2-roll mill, and granulated when
cooled.
The granulated material was injection molded on a Battenfeld injection molder (Model
30 plus) into test specimens (5” x 0.5” x 0.125” and 5” x 0.5” x 0.0625” test bars). The HDPE
and the PP samples were injection molded at 180 °C with a mold temperature of 16 °C. The
HIPS samples were molded at 210 °C with a mold temperature of 28 °C. All test specimens
were conditioned at 70 °F and 50% RH for at least 40 h before testing.
Chlorinated Paraffins in Polyethylene
The effect of improved stability for Chlorez 700 SSNP can be seen in Figure 1. In
this example, two grades of chlorinated paraffin were compounded into polyethylene along
with antimony trioxide (ATO). The samples were then injection molded into test specimens.
Before molding, however, the samples were allowed to stand in the barrel of the injection
molder at 200 °C for periods of five and ten minutes before. The color (YI) of the molded
specimens was taken as a measure of the stability of the chlorinated paraffin. The specimens
with Chlorez 700 SSNP developed significantly less color than did the ones with standard
chlorinated paraffin.
With an improved grade of chlorinated paraffin in hand, an experimental program was
undertaken to develop flame retardant formulations for injection moldable HDPE, which is a
difficult material to flame retard, and still maintain acceptable physical properties. In this
program, a total of 54 formulations were evaluated, which used two grades of HDPE, eight
impact modifiers, and ten other additives (flame retardant synergists or polymer stabilizers).
The notched Izod impact test was used as a screening tool, since the impact strength is
a measure of the usability of the material. If no significant improvement in impact resistance
was observed, then no further work on a formulation was carried out. In order to obtain good
impact properties it was necessary to use an impact modifier, and it quickly became clear that
only one of the eight impact modifiers gave significant impact improvement at the loadings
necessary to achieve the desired flame resistance. In addition, only one of the two grades of
HDPE had improved impact properties at the required loadings. Thus in order to develop a
flame retarded HDPE with acceptable physical properties, great care has to be exercised not
only in the type of impact modifier used, but also in the grade of HDPE used.
Figure 2 lists three formulations from this work that achieved a V-0 rating for 1/8” and
1/16” test specimens as compared with three that did not meet these requirements. The major
difference between them is the presence of a small amount of PTFE as a drip suppressant.
The three formulations with PTFE compare the recommendations for an all Chlorez
formulation, a 1:1 blend of Chlorez and decabromodiphenyl ether (deca) formulation, and an
all deca based formulation. Since the loading levels are similar, the all Chlorez formulation is
significantly less expensive than the other two formulations.
Figure 3 shows some of the physical properties for the three flame retarded
formulations compared with those of the base resin. Although all three formulations
adequately maintain physical properties, the ones with Chlorez show some plasticization
effects with lower heat deflection temperatures compared with the deca only formulation.
However, the all Chlorez formulation has better tensile and flexural properties than does the
all deca formulation. In addition, the all Chlorez formulation has a lower density.
Chlorinated Paraffins in Polypropylene
In this work a series of twelve formulations were tested which used Chlorez, ATO,
ZnS, and an impact modifier to obtain a product with good physical properties and flame
resistance. As can be seen in Figure 4, a minimum of 25% Chlorez is necessary to attain a V0 rating for both 1/8” and 1/16” test specimens.
As was the case with HDPE, the use of an impact modifier was necessary to obtain the
desired physical and flame retardant properties. Figure 5 shows the notched Izod impact
results for formulations with 25% Chlorez. As the figure indicates, a minimum of 10%
impact modifier is needed to maintain good impact resistance. However, once the amount of
impact modifier goes beyond 15%, the impact resistance actually decreases as the sample
becomes soft and rubbery. In fact, the sample does not break, but actually flexes away from
the impact hammer.
The impact modifier also cause changes in percent elongation (Figure 6) and tensile
strength (Figure 7). Once the level of 15% is reached the percent elongation sharply increases
while the tensile strength decreases. On the other hand, the impact modifier has only a small
effect on melt flow (Figure 8).
What is really noteworthy is how the impact modifier affects the burn properties
(Figure 9). With a Chlorez concentration of 25%, the optimum impact modifier concentration
was at about 10% for good burn results. At lower or higher impact modifier concentrations,
reduced flame resistance was observed. This effect is not clearly understood, but it appears
that there is a concentration range in which the impact modifier actually seems to hold the
sample together during burning so the tendency to drip is reduced. Unfortunately, at higher
concentrations afterglow becomes a problem. At still higher concentrations the sample
becomes rubbery, and flame time and the afterglow increase further. Thus it appears that there
is not only a minimum amount of fire retardant required but also an optimum amount of
impact modifier required as well.
Chlorinated Paraffins in HIPS
In order to develop flame resistant formulations for HIPS a designed experiment with
25 formulations was carried out in which Chlorez, FR 1808, ATO, and ZnS were used as the
ingredients in the flame retardant package. FR 1808 (octabromotrimethylphenyl indane from
Ameribrom) was used for its low blooming properties.
In this design there were five variables and two constraints:
Variable A: FR 1808 between 3.4% and 15.4%
Variable B: Chlorez 700 SSNP between 3.4% and 15.4%
Variable C: Antimony trioxide between 0.8% and 7.7%
Variable D: ZnS between 0.8% and 7.7%
Variable E: HIPS between 69.2% and 82.9%
Constraint 1: 13.7% < (A + B) < 20.5%
Constraint 2: 3.4% < (C + D)< 10.2
The test was whether or not a V-0 rating could be obtained for a 1/16” test specimen. The
results were that 10 formulations gave a V-0 rating and 10 formulations burned. The
remaining five formulations gave either a V-1 or a V-2 rating. An examination of the results
indicates that if A+B > 18.5 and C > 2.5 then a V-0 rating will be obtained. Variable D did
not show any strong effect for flame resistance, although other work shows that it does
suppress afterglow.
The utility of this rule can be seen in Figure 10 where the results for five formulations
are given. As can be seen in the figure, formulations 1 and 2 are nearly identical except that
the amounts of Chlorez and FR 1808 are reversed. In both formulations A + B > 18.5 and C
> 2.5, therefore both gave a V-0 rating. On the other hand, formulations 3 and 4 burned,
because C < 2.5. Interestingly, in formulations 3 and 4, variable D was at the maximum
allowed by the design yet it had little influence on flame resistance. Finally, formulation 5
where A + B < 18.5 and C > 2.5 also burned.
The physical properties were also measured for HIPS and are given in Figure 11. All
the formulations had adequate physical properties. However, one physical property that was
not in the design was UV light stability. In Figure 12, there is a comparison of color stability
for Chlorez 700 SSNP vs deca. In this work, the samples were exposed to intense fluorescent
light (a UV source) and the changes in color were measured. As can be seen in the figure, the
sample with Chlorez clearly had better light stability than did the one with deca.
Conclusions
With the proper selection of raw materials and process conditions, an improved grade
of chlorinated paraffin can be produced. With improved thermal stability, it was possible to
use this material in HDPE, PP, and HIPS. With careful attention to the selection of resin
grades and impact modifiers as well as the amounts of fire retardant additives, it was possible
to develop formulations that gave a V-0 rating for 1/16” test specimen and still maintain
acceptable physical properties. The advantages of chlorinated paraffins as a flame retardant
are that it is low cost, does not bloom, and has good UV stability.
Acknowledgements
The authors would like to acknowledge the generous help and assistance of Joanne
Reneker, Ed Wykoff, and Sridhar Siddhamalli in the preparation of this paper.