Low smoke, non-corrosive, fire retardant cable jackets based on HNBR and EVM.

Low smoke, non-corrosive, fire retardant cable jackets based on HNBR and EVM

Safety cables now have to meet stringent requirements. A single cable, or bundle of cables, must not burn by itself or transmit fire. The combustion gases of a cable, furthermore, must be as harmless as possible to humans. The smoke and combustion gases formed must not obscure escape routes or be corrosive.

These requirements have been laid down already in international standards.

Several general requirements appear in all the standards and are as follows:

* fire retardance;

* low smoke gas density in a fire;

* non-corrosiveness of the gases in the sense of the standard concerned;

* the toxicity of the gases must be low in the sense of the standard concerned.

A number of marine cables can be cited as examples (table 1. The material properties described in NES 518 will be discussed later.

Table : Table 1 - comparison of NATO cable specifications

                   Cable             Properties       Cable jacket
                   specification/    of insulation/   resistance to
  title            jacket material   fluids
USA                MIL-C-24640 and   Low halogen      Exposure to
                   MIL-C-24643       low smoke        water, oils,
                                                      fuels and
                                                      fluids
Italy              NAV-13-AO75       Low halogen      ASTM-D-471
                                     low smoke        (Exposure to
                                                      salt water,
                                                      hot oil)
Germany            VG95-218          Zero halogen     Exposure to
                                     low smoke        oils, fuels,
                                                      cleaning
                                                      fluids, as
                                                      specified
France             E540              Zero halogen     ASTM oil No. 2,
                   specification     low smoke        CEI 811
                   no lb ET/SET
Great Britain      DEF Stan 61-12    Zero halogen     Exposure to
                   Pt 25             low smoke        water, oils,
                   + NES 526         * jacket only    fuels and
                   * NES 527         fluids
Norway             STK               Low halogen      IEC 811
                                     low smoke
Canada             D-03-001-0277     Zero halogen     JP4 and 5,
                   SK001             low smoke        diesel fuel,
                                                      hydraulic
                                                      fluids, oils

The decisive test of the serviceability and suitability of a material concept is, of course, the test performed on the finished cable. This article will give the results of laboratory tests. They should be looked upon as a point of departure in the choice of a suitable cable jacket material.

The influence of cable construction on the combustion behavior of cables is not considered here. It will be seen in this study that HNBR and EVM are very interesting polymers for the jackets of safety cables.

EVM as base polymer

EVM is a copolymer of ethylene and vinyl acetate which is particularly suitable for use in FRNC (fire retardant non-corrosive) cables.

The fundamental properties of the various grades of EVM rubber used for the investigation reported here are compiled in table 2.

Table : Table 2 - fundamental properties of EVM

Grades          Weight -           Viscosity             Density
                    % VA       ML4/100 [degrees] C   (g/[cm.sup.3])
                               MFI/190 [degrees] C       appr.
400          40 [+ or -] 1.5   20 [+ or -] 4   5           0,98
450 HV       45 [+ or -] 1.5   26 [+ or -] 4   5           0,99
500 HV       50 [+ or -] 1.5   24 [+ or -] 4   5           1,00
VP KA 8385   60 [+ or -] 1.5   24 [+ or -] 4   5           1,04
700 HV       70 [+ or -] 1.5   26 [+ or -] 4   6           1,08
VP KA 8479   80 [+ or -] 2.0   28 [+ or -] 6   6           1,11

One of the things to which attention will be drawn in this presentation is the influence of the vinyl acetate content on the combustion behavior of a cable jacket.

If a supporting fire is absent, it may be assumed that polymers of low combustion enthalpy will burn and transmit fire less easily than those of high combustion enthalpy.

Figure 1 shows the exothermic combustion enthalpies of proven cable jacket materials and, additionally, the endothermic reaction enthalpy of the decomposition reaction of aluminium hydroxide.

PVC, polychloroprene (CR) and chlorinated polyethylene (CM) are proven cable jacket materials whose fire retardance is adequate in many applications. This is reflected in the fact that, among other things, their combustion enthalpy is low in comparison with that of unchlorinated polyethylene.

As, however, the combustion gases of PVC, CM, CR and compounds of similar composition are corrosive and irritate the eyes and respiratory passages, these materials are unsuitable as base polymers for FRNC cables.

A glance at the combustion enthalpies of the straight (uncompounded) EVM copolymers having different vinyl acetate contents reveals that there is a linear relationship between combustion enthalpy and VA content (figure 2). As the VA content rises, less heat is released and, consequently, an increasingly small proportion of the still unburned material is heated to its ignition temperature. In the case of polymers without inherent fire retardancy, adequate fire retardancy can only be obtained by adding a high proportion of flame retardant fillers. The filler most widely used for FRNC cables is aluminium hydroxide (ATH).

A property frequently used in the assessment of the combustion behavior of laboratory specimens is the LOI (limiting oxygen index).

Figure 3 shows the LOI of EVM compounds of uniform VA content (50% VA) and varied aluminium hydroxide content. As expected, addition of aluminium hydroxide raises the LOI, making it increasingly difficult, within a series of tests, to keep the specimen burning. In the first place the addition of aluminium hydroxide reduces the concentration of combustible organic constituents. In addition, aluminium hydroxide releases water, thus withdrawing energy from the flame.

In the extreme case in which H = O, a corresponding compound is no longer able to burn without a supporting fire. Thus, figure 3 shows how much energy is needed simply to heat the combustion products of the compound to 750 [degrees] C. At 300 phr aluminium hydroxide approximately one third of the combustion enthalpy of the EVM (50% VA) is consumed in this process.

Another advantage from the standpoint of the processor is the fact that the viscosity of the polymer is so low that even at this heavy filler dosage the viscosity of the compound ML 1+4/100 [degrees] C is only 49. Furthermore, the tensile strength of the peroxide-crosslinked compound (TS = 9.7 MPa) and the elongation at break (EB = 120%) are remarkably high.

Figure 4 shows how the LOI is related to another factor. Here the vinyl acetate content of the EVM is varied while the filler content of the compound is held constant at 190 phr [Al(OH)sub.3]. The LOI of the unfilled EVM is included for comparison. At approximately 50% VA the increase of the LOI is disproportionately greater than that which would be expected merely from the linear reduction of the combustion enthalpy. Here, it appears, there are synergistic effects between EVM with high VA content and the filler.

Cables may be exposed to critical conditions in practical use. Particularly critical conditions arise if, in a fire, a cable comes into contact with combustible liquids. In practice it may happen that oil mist condenses on installed cables, e.g. on an oil drilling platform or in the vicinity of diesel engines. The advantages of the grades of rubber with high VA contents are then particularly conspicuous. In contact with oils these oil-resistant materials retain their mechanical properties almost unchanged, and because they absorb only small quantities of oil they prevent the cable from becoming a torch.

After diesel immersion of compounds based on EVM whose vinyl acetate content is 50% the LOI falls to levels that almost correspond to the LOI of straight polyethylene. However, at 70% and especially 88% the LOI is still so high that good to very good fire retardancy behavior can even be expected after immersion in diesel fuel.

HNBR as base polymer

HNBR is hydrogenated nitrile rubber with exceptionally good mechanical properties. It is particularly suitable for FRNC cables if these need unusually good mechanical property data and if only slight alterations of these data after immersion in various swelling media are permissible.

In connection with the marine cable specification NES 518 it can be shown that, in this respect too, HNBR is able to help satisfy very stringent demands.

Marine cables have to combine flame retardancy with very good mechanical property data. Good ozone resistance and resistance to various oils and other fluids are demanded additionally to ensure satisfactory performance in practical use.

The grades of HNBR used for this purpose are shown together with their basic property data in table 3. Figure 5 shows the dependence of the LOI on the addition of filler to an HNBR with a nitrile content of 34%.

Table : Table 3 - basic properties of HNBR grades used

HNBR   ACN (%)      ML-1+4/     RDB+)   d (g/[cm.sup.3])
                 100 [degree]    (%)
1707     34      76 [+ or -] 7    1          0,95
2207     43      86 [+ or -] 7    1          0,97

+) Residual double bonds relative to NBR feedstock (%)

At very heavy aluminium hydroxide dosages the LOI rises to very high levels, as expected, and reaches 77%. Owing to the excellence of the polymer properties of this HNBR, peroxide crosslinking of a compound containing 320 phr aluminium hydroxide gives a tensile strength of 8.7 MPa and an elongation at break of no less than 250%.

Table 4 gives typical cable jacket recipes. The straight HNBR, straight EVM (70% VA) and an HNBR/EVM blend were compared in each compound.

Table : Table 4 - cable jackets for fire retardant cables

EVN   (70% VA)       100      50       -       -
HNBR (34% ACN)         -      50     100       -
HNBR (43% ACN)         -       -       -     100
Stabilizer             3       3       -       -
Silane                 2
ATH                  190
Zinc stearate          1
Zinc borate           10
Antioxidant SDPA     1.9
Dioctyl sebacate       6
TRIM                 0.7
Peroxide               6
Total parts        319.7   319.7   316.7   316.7

An EVM with 70% VA was chosen because only a high-VA grade gives the required oil resistance.

Both HNBR grades are mechanically superior to the EVM in tensile strength, elongation and tear strength. The tear strength requirement, which - for peroxide-cured compounds - is a stringent one, is met only by HNBR.

An ozone test (according to NES 518) was done to show the ozone resistance of EVM and HNBR cable jackets. The exposure conditions were: elongation - 40%; ozone concentration - 100 ppm; room temperature; five-day exposure time. All samples passed the test without cracking. As the polymers are saturated, it is not surprising that all the specimens pass the test.

Low-temperature bending test

Figure 6 shows the temperature down to which the various vulcanizates can be bent without breaking. In the case of the chosen EVM grade (70% VA) the glass transition point is such that a lower limit of -9 [degrees] C is reached. In both measurements HNBR is here about 20 [degrees] C lower.

The results of the swelling tests are given in table 6. Figure 7 presents the results of the two critical tests in diesel fuel and the synthetic ester OX 38. Diesel fuel is non-polar and OX 38 polar. Organic rubbers are often resistant to only one of the two swelling media. Thus EVM with 70% VA, which is a highly polar material, has very good resistance to diesel fuel, which is non-polar, but, because it has a polar acetate ester group, it swells severely in the ester OX 38. HNBR has very good resistance to both media, its swelling being far below the specified limit. Raising the nitrile content from 34 to 43% additionally improves the already excellent swelling resistance.

Table : Table 6 - immersion test (change of values in %)

Polymer         EVM-    EVM-    HNBR-   HNBR-   Spec.
base             70      70      34      43       NES
                HNBR-                    518
                 34
               (50:50)

Diesel fuel NATO F-76, 28d/23 [degrees] C

  TS               9     20      27      16      -40
  EB             -19    -25     -24     -29      -40
  [DELTA] V       19     25      11       6       25

Hydraulic fluid, petroleum based OX-30, 28d/50 [degrees] C

  TS              15     21      27      28      -40
  EB             -19    -25     -15     -15      -40
  [DELTA] V        6      8       2       0       15

Hydraulic fluid, silicone based OX-50, 28d/50 [degrees] C

  TS              21     34      37      38      -40
  EB             -24    -21     -19     -19      -40
  [DELTA] V       -2     -2      -2      -2       15

Lubricating oil, detergent mineral OMD-113, 28d/50 [degrees] C

  TS              15     27      36      37      -40
  EB             -24    -25     -19     -21      -40
  [DELTA] V       -1      1       0      -1       10

Lubricating oil, synthetic ester base, OX-38, 28d/50 [degrees] C

  TS             -42     -7      27      25      -40
  EB             -48    -36     -19     -17      -40
  [DELTA] V       62     40       8       3       50

Deionized water, 28d/50 [degrees] C

  TS             -17     -5      -6      -7      -20
  EB              17     27      14       6      -20
  [DELTA] V       16      9       5       5       10

Deionized water with 3, 5% NaCl, 28d/50 [degrees] C

  TS               3      4      -9      -9      -20
  EB               8      5       7      -4      -20
  [DELTA] V        5      5       0       0       10

Results of flame tests

The results given in table 7 are based on small-scale laboratory tests conducted at Bayer AG for the purpose of comparison and are not intended to reflect hazards presented by this or any other material under actual fire conditions.

Table : Table 7 - fire test performance

Polymer   EVM-   EVM-   HNBR-   HNBR-   Spec.
base   70   70   34   43   NES
  HNBR-   518

34

(50:50)

LOI acc. ASTM  41 43 42 45 >29

D 2863 (%)

Temperature index 330 300 290 310 >250

acc. NES 715 ([degrees] C)

Toxicity index 1.0 1.9 3.2 3.7 <5.0

acc. NES 713

Smoke density 200 180 170 120

NBS chamber

acc. ASTM E 662-83

non-flaming

([D.sub.max.corr.])

Flaming 170 110 250 150

([D.sub.max.corr.]) Corrosivity of

smoke acc. 3.8 4.2 8.3 8.5

VDE 0472 part 813

pH value

From table 7 it is clear that differences in fire test performance were found in addition to the differences in mechanical property data. All the tested specimens satisfied the demands of the standard, however. In otherwise identical compounds EVM has a higher LOI value and higher temperature index than HNBR. Its toxicity index of 1.0 is exceptionally low. HNBR has a toxicity index which, despite the nitrile groups, is within the range permitted by the specification. The smoke density values are favorable in all cases.

Conclusion

Attention has been drawn to the basic properties of EVM and HNBR which make them interesting materials for FRNC cables. It should be pointed out again that EVM rubber grades with high VA contents have the largest safety reserves in a fire. HNBR has excellent mechanical properties and good resistance to swelling media, and with suitable compounding it satisfies the requirements of NES 518.

Disclaimer

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PHOTO : Figure 1 - burning enthalpy of polymers

PHOTO : Figure 2 - enthalpy to reach 750 [degrees] C heating and transformation (VA content of EVM 50%)

PHOTO : Figure 3 - LOI influence of amount of ATH VA-content 50% (const.)

PHOTO : Figure 4 - LOI influence of VA-content without ATH (-) and with 190 phr ATH (+)

PHOTO : Figure 5 - LOI influence of amount of ATH ACN-content 34% (const.)

PHOTO : Figure 6 - cold bend test of EVM and HNBR cable sheaths