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Advances and Developments For
Flame Retardant Polypropylene
Michael T. Sobus, Ph.D.
The properties and economics of synthetic fibers and fabrics exceed
those of natural fibers and have provided greater flexibility of design
and styling. As uses for synthetic fibers grow, additional requirements
such as enhanced flame retardancy become more important.
Relatively few synthetic polymers have found their way into commercial
fiber production. Nylon, polyester, polyacrylates and polypropylene lead
in annual pounds produced.
Of the major synthetic fibers in use, nylon sets the standard for
flammability due to its chemical composition and unique physical
properties.
Introduction of phosphorus into polyester has successfully produced a
flame retardant (FR) polyester fiber, whereas use of halogenated
copolymers with an antimony oxide synergist yields flame retardant
modacrylic fibers.
However, development of an economical flame retardant polypropylene
(PP) fiber has lagged behind and today remains a challenge.
Currently there are two systems that effectively flame retard polymers
- halogenated and non-halogenated.
Many manufacturers would prefer non-halogenated systems such as
magnesium hydroxide, aluminum trihydrate, ammonium phosphate, etc.,
principally because halogenated systems have received negative publicity,
particularly in Europe.
Non-halogenated systems, however, usually require loadings of up
to 60% of the flame retardant additive and the physical as well as the
aesthetic properties of the polymers are adversely affected. Moreover, in the case of
fine-denier fibers, a usable fiber cannot be produced with particulate
fillers. Halogenated systems
offer the advantage of low loadings to achieve the desired levels of
flame retardancy. In fact,
several fiber manufacturers have set 8% as the maximum allowable addition
of flame retardants in finished fine-denier fiber. This is a challenging technical
goal.
For polypropylene, traditional flame retardant systems are based
primarily on bromine compounds in conjunction with antimony oxide and
intumescent systems combining phosphorous and nitrogen. While these have
been more than effective at producing UL-94 V-O and V-2 rated injection
molded pieces, they have not been applicable to fibers because of
problems with the resulting polymer properties and processing
difficulties. The traditional bromine-based compounds lack thermal and
light stability and often require use of antimony oxide synergists that
do not melt and reduce fiber strength. The intumescent systems have
required high loadings of non-melting solids and thus they adversely
affect fiber strength in a similar fashion to antimony oxide.
Development of flame retardant systems for polypropylene film and
fibers is an important goal for the major producers of flame retardant
chemicals. This paper will review the status of those developments.
In the late 1980's Techlon Fibers patented a method for producing
flame retarded, fine-denier polypropylene fibers by mixing an aromatic
bromine, antimony oxide and antioxidants in low density polyethylene
prior to mixing with the polypropylene resin1. The invention
claims that fine denier polypropylene filaments with a high degree of
flame retardancy can be produced.
Table 1
Flame Retardant PP Fibers Produced by Method of Techlon Fibers
Patent
|
Test
|
1
|
2
|
3
|
4
|
|
Mattress Tape
|
|
|
|
|
|
Control
|
|
Fail
|
|
|
|
Sample
|
|
Pass
|
|
|
|
Drapery
|
|
|
|
|
|
Control
|
|
Fail
|
|
Fail
|
|
Sample
|
|
Pass
|
|
Pass
|
|
Upholstery Cloth
|
|
|
|
|
|
Control
|
Fail
|
Fail
|
Fail
|
|
|
Sample
|
Pass
|
Pass
|
Pass
|
|
|
Wall Covering
|
|
|
|
|
|
Control
|
|
Fail
|
|
|
|
Sample
|
|
Pass
|
|
|
|
Test 1
|
FMVSS 302 Motor Vehicle Test
|
|
Test 2
|
FAA Vertical Flammability 17CFR25-853B
|
|
Test 3
|
UFAC Cigarette Test Class II
|
|
Test 4
|
NFPA 701 Curtain Fabrics
|
More recently, several manufacturers of bromine and phosphorous flame
retardants, antimony oxide synergists and flame retardant masterbatches
have introduced new products designed for polypropylene fibers. One
approach has been to graft dibromostyrene to polypropylene. Great Lakes
Chemical produces these graft copolymers with 36 - 39% bromine. These
copolymers are melt blendable with melting points 150 - 175C and do not
bloom, unlike the more widely used tetrabromobisphenol A. However,the
total additive package, including the UV stabilizers, apparently is not
economically attractive.
Dead Sea Bromine Group offers several thermally stable, melt-blendable
brominated organic flame retardant packages. A novel stabilizer package,
FR-1206HT, including a heat-stabilized hexabromocyclododecane, can
produce a UL94 V-2 rated PP when used at 2 - 3% levels.
Table 2
Properties of PP Flame Retarded With FR 1206 HT
|
|
1
|
2
|
3
|
|
Composition, %
|
|
|
|
|
PP homo MFI 12
|
99.5
|
96.7
|
95.5
|
|
FR1206 HT
|
|
2.1
|
3
|
|
Antimony Oxide
|
|
.7
|
1
|
|
Additives
|
0.5
|
.5
|
.5
|
|
Properties
|
|
|
|
|
Tensile Strength at Break M Pa
|
19
|
19
|
21
|
|
Elongation
|
165
|
250
|
170
|
|
Impact Notched Izod KJ/m2
|
3.1
|
2.9
|
2.8
|
|
MFI
|
12
|
13
|
13
|
|
Flammability
UL94 Rating (total afterflame time/1.6 mm)
|
nv
|
V-21531
|
V-2(13)
|
Other approaches include use of brominated epoxy, aromatic bromine and a
cyanurate carrier and brominated trimethylphenyl Indan.
Georlette, et. al., reported that poly (pentabromobenzyl acrylate), at
8.5%, produced UL94 V-2 rated PP2. Most interesting is the observation that grafting of
poly (pentabromobenzyl acylate) onto the polypropylene backbone improved
the adhesion and dyeability.
Such grafted copolymers are easily produced by reactive extrusion
of blends of pentabromobenzyl acrylate and PP.
Table 3
Enhancement of Adhesion and Dyeability of PP
|
|
1
|
2
|
|
Composition %
|
|
|
|
PP
|
100
|
77.2
|
|
Pentabromobenzyl acrylate
|
|
17.1
|
|
Antimony Oxide
|
-
|
5.7
|
|
Properties
|
|
|
|
LOI %
|
20
|
26
|
|
Adhesion on
Copper MPa
(molded
sample)
|
0
|
2.6
|
|
Dyeability K/S (600 nm)
Abaset Blue RF
|
1.35
|
2.5
|
FMC introduced a melt blendable phosphorous/bromine flame retardant for
polypropylene fiber3.
The bromine is derived from tribromoneopentyl alcohol, and when
combined with a phosphate ester, unique physical and flame retardant
properties result. UV94 V-2
ratings are obtained with 3-5% of the phosphorous/bromine flame retardant
and 1.5 - 2.5% antimony oxide.
Also, UV stability can be obtained when 0.5 - 1% of a hindered
amine light stabilizer is added.
Table 4
Results of QUV Testing PP Fiber Containing Phosphorous Bromine FR
|
Sample
|
A
|
B
|
C
|
D
|
E
|
F
|
G
|
H
|
|
Rating
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
5
|
|
|
White
0.66%
FR
|
White
1.33%
FR
|
White
2.66%
FR
|
White
5.33%
FR
|
Beige
0.65%
FR
|
Beige
1.29% FR
|
Blue 0.65% FR
|
Blue
1.29% FR
|
|
Test Cycle
|
8 hours of light at
60C
|
|
|
4 hours of
condensation at 50C
|
|
|
|
|
Color Change
|
5 - no change
|
|
|
4 - slight change
|
|
|
3 - moderate change
|
In 1995 Nyacol introduced a patented flame retardant concentrate for use
in polypropylene. When added at final FR concentrations of 1- 4%, UL-94
test ratings of V-2 are obtained with minimal effects on Izod impact and
color4. The concentrate contains colloidal antimony pentoxide
with a particle size of 30 nm and the bis-dibromopropyl ether of
tetrabromo bisphenol sulfone (Non Nen 52).
Table 5
Flame Retardancy of Non Nen 52 and Antimony Pentoxide
|
Flame
Retardant Synergist
|
Halide
|
% FR
|
UL-94
|
Opacity
|
|
None
|
None
|
N/A
|
Fail
|
Translucent
|
|
Antimony
Pentoxide
|
Non Nen 52
|
1.0
|
V-2
|
Translucent
|
|
Antimony
Trioxide
|
Non Nen 52
|
1.0
|
Fail
|
Very slightly
translucent
|
Flame retardant fibers and test socks using this flame retardant
additive have been prepared successfully. Because the additives in this concentrate are melt
blendable (halogen) or have a sub-micron particle size (antimony
pentoxide), fine-denier fibers with tensile properties adequate for
normal textile products were produced.
In order to develop the application of FR polypropylene to the carpet
industry, Campine has produced FR-PP masterbatches which contain FR
additives that are melt blendable and completely dispersible. Fiber was produced which
contained 3% active FR additives.
The carpet fiber was then tufted on an Al(OH)3 coated
textile backing. The carpet
passed DIN 4102, NFP 92-506, BS4790 and BS5287 ignition tests.
In spite of the encouraging technical successes detailed above,
commercial implementation has been slow owing primarily to the cost added
by the FR additives and UV stabilizers. Several of the FR additive manufacturers are taking a "wait-and-see"
approach before committing further resources to development, especially
of FR-PP fiber for carpets.
References
1 B.L. Cline and G.M.
O'Mahony, Techlon Fibers Corporation, Flame Retardant Polyolefin Fiber,
US 4 774 044, September 27, 1988.
2 P. Georlette, R. Smith, L.
Utenskii, M. Mushatel, I. Finberg, Y. Scheinert, "Flame Retardants
'96," Inter Science, London, 1997, pp. 79-90.
3 G. Squires, "Flame
Retardants '96," Inter Science, London, 1996, pp. 107-114.
4 E. Myszak, PQ Corporation,
Flame Retardant Compositions, US 5409 980, Valley Forge, April 25, 1995.
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