|
FLAME RETARDANT DEVELOPMENTS
FOR POLYPROPYLENE
Edward A. Myszak, Jr. and Michael T. Sobus
Market Development Project Manager and Business
Manager
PQ Corporation
Valley Forge, PA
Contact:
Nyacol Nano Technologies, Inc.
P.O. Box 349
Ashland, MA 01721
ABSTRACT
Flame retardants composed of solid inorganic particles generally have an
adverse effect on the physical and aesthetic properties of polymers and
synthetic fibers. This paper will discuss the advantages offered by
colloidal-sized particles for flame retarding and UV stabilizing
polypropylene materials. Particular attention will be given to the
importance of choosing the whole flame retardant system, halogen and
antimony oxide, and their affect on flame retardance, aesthetics and the
polymer processing system.
INTRODUCTION
Advances in fine denier polypropylene fiber processing have opened the
carpet face and wall covering markets to this versatile polymer.
Aesthetically pleasing polypropylene structural products, such as lamp
housings and other containers, have found a niche in the marketplace.
At the same time, today’s consumer is demanding flame retardant goods.
Manufacturers need to supply products that are not only flame retarded
but which also maintain their physical and aesthetic properties. With
this in mind, we will divide our discussion into the flame retardant development
hase and the UV development phase to simplify the variables introduced.
FLAME RETARDANT DEVELOPMENT
There currently are two systems that effectively flame retard
polymers: halogenated systems and non-halogenated systems. Many
manufacturers would prefer a non-halogenated system, such as magnesium
hydroxide, aluminum trihydrate, ammonium phosphate, etc., principally
because halogenated systems have received negative publicity.
Non-halogenated systems, however, require loadings of up to 60% of flame
retardant, and physical and aesthetic properties of the base polymers are
negatively affected as a result. In the case of fine-denier fibers, a
usable fiber could not be produced with these high flame retardant
loadings. Halogenated systems offer the advantage of lower loadings due
to the synergistic effect between the halogenated organic compound and
antimony oxide, to achieve the desired levels of flame retardancy. In
fact, several fiber manufacturers have dictated that no more than 8 %
active FR ingredients can be used in a finished fine denier fiber. Most
halogenated FR compounds and all non-halogenated systems, however, cannot
meet this criteria.
An ideal flame retardant system would be easily processed and would
safeguard the physical and aesthetic properties of a polymer. It would
incorporate a melt-blendable halogenated additive with a sub-micron
antimony oxide particle. This combination should yield an acceptably
flame-retarded product with good tensile strength, impact resistance and
elongation. The finished product should also be untinted / translucent.
An antimony pentoxide powder that disperses to colloidal size (0.03
micron) particles is the only FR additive that meets all these criteria.
A detailed comparison of colloidal-sized antimony pentoxide versus
antimony trioxide (the smallest particle size commercially available) is
given in Table I.
TABLE I
Physical Properties of Antimony Pentoxide vs. Trioxide
|
Property
|
Antimony Trioxide
|
Antimony Pentoxide
|
|
Formula
|
Sb2O3
|
Sb2O5
|
|
Solubility
|
dilute acids & bases
|
only concentrated, hot acids
|
|
Particle Size
|
0.8 - 1.0 microns
|
0.03 microns
|
|
Surface Area m2/gm
|
2
|
50
|
|
Specific Gravity
|
5.3
|
4.0
|
|
Refractive Index
|
2.1
|
1.7
|
|
Surface Activity
|
usually neutral
|
weakly acidic
|
Figure 1 shows the visual impact of a submicron antimony pentoxide
article on a 1.5 denier polypropylene fiber vs. antimony trioxide. The antimony pentoxide particle occupies
only 0.2% of the cross-sectional area of fiber vs. 7% for antimony
trioxide.
Figure 1. - Antimony Pentoxide vs. Trioxide in a Fine Denier Fiber
RESULTS AND DISCUSSION
We tested the desirability of several potentially acceptable antimony oxide/halogenated
additive systems for polypropylene fiber and translucent products. The
antimony oxides used in this screening study were:
1. Nyacol® ADP480 - a powder which disperses to colloidal
size (0.03 micron) particles in nonpolar hydrocarbons
2. Antimony trioxide powder
Both antimony oxides were individually compounded into a flame
retardant concentrate with each of five halogenated additives:
1. Brominated aromatic ester (63% Br)
2. Brominated polystyrene (60% Br)
3. Brominated polystyrene (66% Br)
4. Brominated aromatic compound (66% Br)
5. Chlorinated paraffin (74% Cl)
We produced concentrates of each combination to test the complete
dispersion of the ingredients when the product was let down into the
polymer. This is critical in fine-denier fiber applications. The
concentrates contained 50% active FR ingredients in a carrier of
polypropylene. This carrier was chosen to maintain the physical
properties for fine denier fiber applications.
Table II summarizes the flame tests results of this initial screening
of FR compounds. Only one halogen/antimony oxide compound exhibited flame
retardancy according to the UL-94 vertical flame test. That one system is
antimony pentoxide or trioxide with a brominated aromatic compound.
TABLE II
Flame Test Summary of Polypropylene
|
Additive
Material
|
Percent7
Additive
|
LOI8
Percent
|
UL-949
Test
|
Afterflame
Rating
Time (Sec)
|
|
Virgin PP
|
NA
|
17.3
|
Fail
|
NA
|
|
BAE1
|
12.0
|
22.7
|
Fail
|
NA
|
|
BP-602
|
12.0
|
19.3
|
Fail
|
NA
|
|
BAC4
|
8.06
|
23.1
|
Fail
|
NA
|
|
ADP48010/BAE
|
could not extrude
|
|
Trioxide BAE
|
12.0
|
22.9
|
Fail
|
NA
|
|
ADP480/BP-60
|
12.0
|
18.5
|
Fail
|
NA
|
|
Trioxide/BP-60
|
12.0
|
20.3
|
Fail
|
NA
|
|
ADP480/BP-663
|
12.0
|
18.5
|
Fail
|
NA
|
|
Trioxide/BP-66
|
12.0
|
20.5
|
Fail
|
NA
|
|
ADP480/CP5
|
2.5
|
19.6
|
Fail
|
NA
|
|
BurnEx 2000 /BAC
|
12.0
|
23.6
|
V-2
|
0.0
|
|
ADP480/BAC
|
8.0
|
28.6
|
V-2
|
0.3
|
|
|
4.0
|
26.9
|
V-2
|
9.4
|
|
|
2.5
|
25.0
|
V-2
|
2.9
|
|
|
1.0
|
20.1
|
V-2
|
3.8
|
|
Trioxide/BAC
|
12.0
|
32.3
|
V-0
|
0.0
|
|
|
8.0
|
32.9
|
V-0
|
0.0
|
|
|
4.0
|
28.9
|
V-2
|
0.0
|
|
|
2.5
|
24.6
|
V-2
|
0.2
|
|
|
1.0
|
21.6
|
Fail
|
NA
|
|
1 Brominated
Aromatic Ester (63% Br)
|
|
2 Brominated
Polystyrene (60% Br)
|
|
3 Brominated
Polystyrene (66% Br)
|
|
4 Brominated
Aromatic Compound (66% Br)
|
|
5
Chlorinated Paraffin (74% Br)
|
|
6 Could not
extrude at 12% loading
|
|
7 Data for %
Additive less than 12 not reported if sample failed UL-94
|
|
8 Limiting
Oxygen Index (ASTM D2860) 9 Vertical Burn Test
|
|
10 ADP480 is a
colloidal sized antimony pentoxide
|
The antimony pentoxide compounds were all rated V-2 according to UL-94
with afterflame times ranging from 0 to 3.8 seconds, depending on FR
concentration. The trioxide compounds were rated V-O through FAIL
depending on FR concentration. The V-O ratings achieved by trioxide at
high loading levels (8 and 12%) were probably due to the rheology of the
polymer being changed as a result of the trioxides’ larger particle size,
which reduced the quantity of drips as well as their flaming
characteristics.
A summary of the physical property test results of the UL-94
acceptable materials is given in Tables III and IV. The results with 1/8" thick
test pieces show the antimony pentoxide and trioxide to be reasonably
comparable from the perspectives of elongation and tensile strength. It would
be expected that the larger trioxide particles would have a negative
effect on these characteristics as the thickness of the test piece
decreased.
TABLE III
Physical Property Summary
|
Additive Material
|
Percent Additive
|
Notched2 Izod Impact
(ft-lb/in)
|
|
Virgin PP
|
NA
|
0.64
|
|
BurnEx 2000
|
12.0
|
0.62
|
|
ADP480/BAC1
|
8.0
|
0.63
|
|
|
4.0
|
0.58
|
|
|
2.5
|
0.64
|
|
|
1.0
|
0.65
|
|
Trioxide/BAC
|
12.0
|
0.44
|
|
|
8.0
|
0.36
|
|
|
4.0
|
0.35
|
|
|
2.5
|
0.37
|
|
|
1.0
|
0.37
|
|
1 Brominated
Aromatic Compound (66% Br)
|
|
2 ASTM D256
|
TABLE IV
Physical Property Summary
|
Additive
Material
|
Percent
Additive
|
Elongation3
at Yield
Percent
|
Tensile3
Strength
at Yield
PSI
|
Elongation2
at Break
Percent
|
Tensile2
Strength
at Break
PSI
|
|
Virgin PP
|
NA
|
16.8
|
4937
|
204.0
|
3055
|
|
BurnEx 2000
|
12.0
|
8.0
|
5168
|
29.4
|
3453
|
|
ADP480/BAC1
|
8.0
|
9.9
|
5087
|
29.5
|
3643
|
|
|
4.0
|
11.4
|
5173
|
30.3
|
3862
|
|
|
2.5
|
15.9
|
5144
|
29.9
|
3564
|
|
|
1.0
|
16.1
|
5068
|
49.6
|
3045
|
|
Trioxide/BAC
|
12.0
|
9.9
|
5040
|
32.5
|
3083
|
|
|
8.0
|
11.5
|
4863
|
37.4
|
2841
|
|
|
4.0
|
13.8
|
4892
|
31.9
|
2931
|
|
|
2.5
|
13.7
|
5141
|
33.1
|
3265
|
|
|
1.0
|
14.8
|
5122
|
30.9
|
3365
|
|
1
Brominated Aromatic Compound (66% Br)
|
|
2 ASTM states that
tensile strength and elongation at break value for unreinforced
polypropylene plastics generally are highly variable due to
inconsistencies in necking of the center section of the test bar.
Tensile strength and elongation at yield are more reproducible.
|
|
3
ASTM D638
|
The Izod impact data, however, show that the material with pentoxide
has a significant advantage at all loading levels. In fact, the Izod data
for polypropylene processed with antimony pentoxide-based flame
retardants are comparable to the Izod result for virgin PP.
Figure 2 shows the advantage of the smaller antimony pentoxide
particles. The Izod data is 38 to 75% better for the Nyacol ADP480 compounds
than for the trioxide-based compounds. As with elongation and tensile
strength data, we strongly believe that, as the test piece thickness
decreases, the difference between pentoxide- and trioxide- based
compounds will become even more exaggerated in favor of pentoxide-based
compounds.
The antimony pentoxide- and trioxide-based flame retardant compounds
processed equally well at all loading levels except at 12%, where the
trioxide-based flame retardant compounds processed more easily. The
larger trioxide particles may have absorbed the halogen material and
thereby prevented puddling or slippage in the throat of the extruder.
Ease of processing is probably a moot issue, however, since the industry
standards for FR loading levels are expected to be no higher than 8%.
Figure 2. - Izod Results
Table V shows the color effects of the flame retardant additive on the
polymer. This data is
reported as L'a'b' total color difference as measured on a Minolta CR-200
Chroma Meter with virgin polymer as the base standard. The color is reported in the
standard CIE 1976 L'a'b' notation.
TABLE V
Color Effect of Additive on Polymer - Unpigmented
|
Additive
Material
|
Percent
Additive
|
L'a'b' Total
Color Difference
|
Translucency
(Visual)
|
|
Virgin PP
|
NA
|
0.0
|
Translucent
|
|
BAC
|
4.0
|
41.2
|
Opaque
|
|
ADP480
|
4.0
|
34.2
|
Slightly Translucent
|
|
Trioxide
|
4.0
|
58.7
|
Opaque
|
|
BurnEx 2000
|
12.0
|
51.1
|
Opaque
|
|
ADP480/BAC
|
8.0
|
50.1
|
Opaque
|
|
|
4.0
|
43.7
|
V Slightly Translucent
|
|
|
2.5
|
37.2
|
Slightly Translucent
|
|
|
1.0
|
19.4
|
Translucent
|
|
Trioxide/BAC
|
12.0
|
54.4
|
Opaque
|
|
|
8.0
|
53.4
|
Opaque
|
|
|
4.0
|
52.1
|
Opaque
|
|
|
2.5
|
47.1
|
Opaque
|
|
|
1.0
|
35.7
|
V Slightly Translucent
|
Figure 3 graphically shows a dramatic difference between Nyacol
ADP480-based compounds and those processed with trioxide. ADP480 has less
pigmenting or whitening effect than trioxide on the base polymer.
Figure 3. - Color Effect of FR Addition on Virgin PP - Unpigmented
A comparison of antimony pentoxide versus trioxide reveals a color
difference at 4% loading of 34.2 versus 58.7, respectively. At 4% loading the FR additive
system utilizing antimony pentoxide shows the test piece at 1/8 inch
thickness begins to exhibit some translucency (L'a'b' delta of
43.7). The level of
translucency for the antimony pentoxide compounds increases as the
loading level drops to 1% (L'a'b' delta of 19.4). The antimony trioxide compounds
are opaque at all loading levels except at a 1% loading, where the test
pieces exhibit only slight translucency (L'a'b' delta of 35.7), but the
material fails UL-94.
One of the curiosities of physics, which we will not attempt to explain,
is that very small and very large particles have low hiding power, or
opacity. There is, on the
other hand, an optimum size for maximum opacity. For antimony trioxide, the 0.5 -
1 micron particles provide maximum opacity. Figure 4 dramatizes the translucence at 4% loadings of
antimony pentoxide vs. antimony trioxide when compounded with virgin
polypropylene. The affects
of a 2.5% FR compound pentoxide vs. trioxide are also shown.
Figure 4. - Translucency Effect of Flame Retardant
Addition to Virgin PP
Differences in opacity levels in unpigmented environments help explain
why less color concentrate is required to obtain a given color in an
antimony pentoxide vs. trioxide flame retarded material. Generally a
polymer system that uses antimony trioxide as a flame retardant will
require an average of four times more pigment to achieve a particular
shade than a polymer system that contains antimony pentoxide. Even higher
proportions of pigment are needed to achieve dark red and blue tones when
antimony trioxide is involved (see Figure 5). Dollar savings for pigments can be significant when
antimony pentoxide is used for flame retardancy.
Figure 5. Color Difference - Antimony Pentoxide vs. Trioxide
With the "ideal" flame retardant developed, the next area to
consider is the UV stability of the system and, if required, how to
improve it while not affecting or minimizing the affects on the flame
retarded end product.
UV STABILITY DEVELOPMENT
All of us have had contact with UV absorbers in our daily lives, i.e.,
sunscreens, sunglasses and a host of other retail products. We understand that UV radiation
from the sun causes our skin to tan and even burn. In all likelihood, however, we do
not think about the effects of UV radiation on such things as automotive
paints, plastic outdoor furniture, and, least of all, flame retarded
fabrics or parts.
UV radiation is electromagnetic radiation in the part of the spectrum
between x-rays and visible light.
Its wavelengths are longer than those of x-rays and shorter than
those of ordinary light. It
differs from light only in that its wavelengths are too short to be
visible to the human eye. It
is readily seen by certain insects, i.e., bees.
The UV spectrum is divided into the near, the far, and the vacuum UV
regions. These regions
include, respectively, the wavelengths from 4,000 to 3,000Å (UVA), 3,000
to 2,000Å (UVB), and 2,000 to 40Å (UVC). The region between 4,000 to 3,000Å is sometimes
referred to as the region of "black light."
The chief natural source of UV radiation is the sun. About 9% of all the energy
emitted by the sun is UV radiation.
Most of this is in the region between 4,000 and 3,000Å, with only
about 14% consisting of wavelengths shorter than 3000Å. More than half of the solar UV
radiation that reaches the earth is absorbed by the earth’s atmosphere,
namely, the ozone layer. This absorption is greater for the shorter than
for the longer wavelengths, so that almost none of the solar energy with
wavelengths shorter than 3000Å reaches the surface of the earth.
Like visible light, UV radiation obeys the laws of optical reflection
and refraction. However, it
is absorbed by many substances that are transparent to visible
light. Ordinary window
glass, for example, is almost opaque to wavelengths shorter than 3000Å.
Likewise, most clean, thin-layered plastics will transmit some UV
radiation in the region between 4,000 and 3,000Å, but they are almost
opaque to shorter wavelengths.
UV radiation promotes many chemical reactions, the study of which
constitutes part of the field of photochemistry. The fading and bleaching of dyes
exposed to sunlight are instances of these reactions. Another example, which is of
prime importance for this discussion today, is the reaction of a
halogenated (in particular, bromine) flame retardant system when exposed
to UV radiation. In general,
we expect the subject material to yellow when exposed to UV
radiation. The higher the
radiation dose, the more yellow the material will become. The effects are cumulative.
There is an additional negative synergistic effect when a
flame-retarded material is abused thermally, i.e., when processed at
excessive temperatures. When that thermally abused material is exposed to
UV radiation, the yellowing effect is exaggerated. The higher the process
temperature without the aid of an antioxidant, the faster and more
intense the yellowing of the material will be when finally exposed to UV
radiation.
Therefore, the part or fiber must be protected from UV-induced
degradation. To maintain the
light fastness of the colored material, we developed a submicron UV
absorbing product, namely, zinc oxide. Zinc oxide has been used for many years in sunscreen
formulations as a UV absorber to protect the skin. It has also been used in paint
formulations. However, as
you all remember, zinc oxide behaves as a white pigment and results in
opaque final products, i.e., white-nosed lifeguards and opaque
"rough" (flat) paint finishes. To obviate this drawback, we have developed a 20-50nm
particle size zinc oxide in aqueous systems (DP5370) and a 110nm particle
size zinc oxide in a dry powder system (DP5372). We will concentrate on the dry
powder system since we plan to use this absorber in an extruded polymeric
application.
Table VI details the DP5372 loadings we tested in conjunction with the
flame retardant of choice, Nyacol BurnEx 2000, to achieve effective UV
absorption.
TABLE VI
Loadings Tested with Nyacol® BurnEx 2000
|
Virgin PP
|
BurnEx 20001
|
DP53722
|
|
100%
|
-
|
-
|
|
98%
|
2%
|
-
|
|
98%
|
2%
|
2%
|
|
98%
|
2%
|
5%
|
|
98%
|
2%
|
10%
|
|
98%
|
2%
|
0.5%
|
|
1
Active ingredient level
|
|
2
Loading % of Virgin PP + BurnEx 2000
|
After the materials were compounded and test plaques made, the plaques
were placed in a Q-Panel tester with UVB lamps. These lamps replicate the effects of Florida sunlight
at 12:00 noon. UVB lamps are
known to generate the shortest wavelengths found in the sunlight at the
earth’s surface and are responsible for most polymer damage. We ran these tests on the plaques
from 0 to 1,000 hours, removing test plaques at various times in this
interval.
Figure 6 shows the yellowness index of the additives on the polymer
when exposed to UV radiation.
This data is reported as a yellowness index measured on a
Minolta-CR-200 chroma meter with virgin polymer as the base
standard. There is a
dramatic improvement in the UV degradation as observed with the flame
retarded material versus the virgin PP and UV absorbent treated
material. This improvement
is apparent with a DP5372 loading level as low as 0.5%.
Figure 6. - Yellowness Index
Figure 7 shows the color effects of the additives on the polymer when
exposed to UV radiation.
This data is reported as L’a’b’ total color difference (delta E)
as measured on a Minolta CR-200 chroma meter with virgin polymer as the
base standard. The color is
reported in the standard CIE 1976 L’a’b’ notation. There is, again, a dramatic
improvement in the UV degradation as observed with the flame retarded
material versus the virgin PP and UV absorbent treated material. This improvement is apparent with
a DP5372 loading level as low as 0.5% with the ideal loading level being
closer to 1%.
Figure 7 L’a’b’ Total Color Difference "delta E"
CONCLUSION
Our goal in undertaking this study was to identify FR systems which
could be used to flame retard fine denier PP fiber and/or
"translucent" PP products. Our results indicate that our FR system, known as
BurnEx 2000, achieved good physical properties, aesthetic
characteristics, and flame retardancy at very low (by industry standards)
loading levels. We achieved
these properties, not only in laboratory test pieces as shown in this
paper, but also in actual field trials at commercial accounts.
REFERENCES
1.
Kocler, Lewis R., Ultraviolet Radiation, New York (1965)
2.
Myszak, E.A. "Use of Submicron Inorganic Flame Retardants in
Polymeric Systems," in Proceedings of the Flame Retardants ‘92
Conference, London, UK, edited by the Plastics and Rubber Institute,
Elsevier Applied Science, pp. 211-222.
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