GB2190384A - Halogen free flame retardant compositions - Google Patents

Abstract

Halogen free thermoplastic compositions having improved flame retardancy comprise an olefinic homo- or copolymer plastics material such as polyethylene, an elastomer such as EPDM and a copolymer of ethylene and (optionally substituted) acrylic acid such as EAA, together with a flame retardant filler comprising alumina trihydrate or magnesium hydroxide. The EAA may be in ionomer form.

Description

SPECIFICATION Halogen free flame retardant compositions This invention relates to halogen free flame retardant (HFFR) compositions, and is particularly but not exclusively concerned with such compositions which are useful in the production of cable oversheathing.

Conventional HFFR compositions such as are used in cable jacketing, flooring and sheeting applications have alumina trihydrate (ATH), also sometimes termed aluminium hydroxide, as flame retardant filler dispersed in a thermoplastic matrix. Another inorganic flame retardant filler known in this field is magnesium hydroxide. These HFFR compositions are distinguished from other, widely used materials, which contain halogens and which in certain circumstances, e.g. under extremes of heat, give off toxic and corrosive vapours. Such conventional HFFR compositions typically have a thermoplastic matrix based upon homopolyolefins (usually polyethylene or polypropylene) or ethylene copolymers (usually ethylene vinyl acetate, EVA) or blends of one or more of these with elastomers (usually ethylene/propylene copolymer rubber, EPM, or ethylene/propylene/diene terpolymer rubber, EPDM).

A major drawback of the above-mentioned HFFR compositions is that they are very sensitive to water; water and water vapour is easily absorbed and transmitted through the composition causing swelling, a worsening of physical properties, and damage of other components, e.g.

metal layers which may be present in the composition (as is the case with electrical cables, for example). This water sensitivity is due to the flame retardant filler used in such compositions. A further drawback with conventional HFFR compositions is that they often do not have good heat stability and are prone to significant changes in dimensions when deformed at elevated temperatures, for example in the range 70-100 C.

With particular reference to thermoplastic HFFR cable sheath compositions, these are already known to be produced from blends of ethylene/propylene rubber (EPR), polyethylene and EVA.

However, it is also known that, when filled with ATH or magnesium hydroxide, such compositions suffer from several disadvantages. Thus they generally show a poor thermal performance due to the low melting point of the polyolefin matrix; an inadequate ageing characteristic, due once again to the matrix melting point and to the absence of crosslinking; only marginally acceptable mechanical properties, particularly tensile strength; and a low temperature index (that is, in a normal % oxygen environment, they will support combustion when exposed to relative low temperatures).

Other known thermoplastic HFFR cable sheath compositions are based on blends of EPR, polypropylene and EVA filled with ATH or magnesium hydroxide. These generally suffer from a very low elongation at break value, due to the presence of the polypropylene; difficult processability characteristics because of the closeness of the polypropylene melting temperature (approximately 160"C) and the ATH or magnesium hydroxide decomposition temperature (approximately 1900C); and, again, a low temperature index.

Furthermore, both of the above types of sheathing composition suffer from poor adhesion between the polymer matrix and the flame retardant filler, and it is noted that conventional coupling agents are generally ineffective with flame retardant fillers such as ATH.

It has now surprisingly been found that the known HFFR compositions can be improved in certain desirable characteristics by incorporating a copolymer of ethylene and acrylic acid or a substituted acrylic acid into the polymer matrix. Thus according to one aspect of the present invention there is provided a thermoplastic halogen free flame retardant composition comprising a polymer blend of: (a) a plastics component selected from olefin homopolymers, olefin copolymers and mixtures thereof, (b) an elastomer component and (c) a copolymer of ethylene and an unsaturated carboxylic acid; and, incorporated in said blend (d) a flame retardant amount of a filler component selected from alumina trihydrate, magnesium hydroxide and mixtures thereof.

The plastics component (a) may comprise for example a polyethylene such as low density polyethylene (LDPE) or linear low density polyethylene (LLDPE), polypropylene or an ethylene/unsaturated carboxylic acid ester copolymer, e.g. EVA, or mixtures of any of these. Usually, though, the copolymer (c) contained in the compositions of the invention may be considered as replacing, at least in part, the EVA component employed in conventional HFFR compositions.

Preferably the plastics component (a) comprises from 10-80 wt % based on the total weight of the polymer blend, i.e. on the combined weight of components (a), (b) and (c). More preferably the polymer blend comprises from 20-50 wt % of plastics component (a).

The elastomer component (b) is preferably present in a proportion of 10-60 wt % based on the total polymer blend (a), (b) and (c), more preferably in a proportion of 15-30 wt %.

The elastomer may be an elastomeric copolymer of ethylene and another alpha olefin having three or more carbon atoms, preferably 3-8 carbon atoms and more preferably 3 carbons, that is EPM. Alternatively the elastomer may be, for example, such a copolymer having one or more additional comonomers such as a minor proportion of one or more polyenes such as the endomethylenes, 5-ethylidene norbornene, 5-methylene norbornene and dicyclopentadiene, or a non cojugated aliphatic diene such as 1,4-hexadiene.

These terpolymers, termed EPDM, may be used alone or in admixture with other elastomers, preferably the above mentioned elastomeric ethylene copolymers, as the elastomeric component (b) - of the compositions of the invention.

The amount of flame retardant filler component (d) employed in the compositions of the invention depends on the degree of flame retardancy required of the products into which the compositions are to be formed. For example, the compositions may contain from 50-250 wt % of (d), more preferably from 100-200 wt % (d), based on the combined weight of the polymer blend components (a), (b) and (c).

As mentioned above, the ATH and/or magnesium hydroxide functions as a flame retardant for the polymer matrix, and this function may, if desired, be supplemented by the inclusion of other active additives. For example, the compositions of the invention may additionally include an effective amount of a smoke suppressant package. This may comprise any of the known smoke suppressants, but it is preferred to employ zinc borate and/or magnesium carbonate for this purpose. Preferably the composition comprises from 5-15 wt % of active smoke suppressant materials based on the weight of polymer blend.

Copolymer (c) is a most important feature of the compositions of the invention, since it is believed that it is this component's interaction with the flame retardant filler which confers on the compositions their surprising characteristics as compared with the hitherto known HFFR compositions. Copolymer (c) is preferably present in an amount of from 10-40 wt %, more preferably from 20-30 wt %, based on the total weight of polymer blend. The acid component of the copolymer is preferably acrylic acid or a substituted acrylic acid such as methacrylic acid.

Preferably the copolymer contains from 2-20 wt % of acid comoner, more preferably from 5-15 wt % of acid comonomer and may optionally be in "neutralised" or cross-linked, that is ionomer form. Indeed, without wishing to be bound by theory, it is believed that the development of crosslink bonds in the compositions of the invention may lead to beneficial effects.

The compositions of the invention have been found to be especially suitable when in the form of oversheathing for cables, although they may, of course, be used in any application which requires a flame retardant thermoplastic property not depending on a halogen content for its flame retardancy. Thus the compositions may be sheeted, moulded or extruded into the required artefacts, e.g. flooring, sheeting, panels, fittings, housings, pipes, hoses, constructional elements, flame retardant cladding members and granules for use in flame retardant insulation applications.

According to a second aspect of the present invention there is provided a method of producing the defined compositions which comprises (i) forming a molten preblend of components (a)-(c); (ii) admixing component (d), optionally together with process aids and conventional additives for thermoplastics materials such as other fillers, oils, coloring agents and antioxidants, with said preblend at a temperature in the range 130-150"C; and (iii) forming the resulting admixture into a shaped body. Preferably the preblend is first formed and then cooled and granulated, so that it can subsequently be remelted and combined with component (d) at a convenient time and possibly in a different location. The molten admixture is preferably extruded into a cable oversheath, i.e. co-extruded with the conductor metal of the cable.

It is believed that during the mixing stage the polar copolymer (c) interacts with the filler and improves the polymer matrix-filler adhesion. This generally renders the filler less sensitive to water, and moreover is believed to give overall beneficial effect to the compositions with regard to mechanical properties, flame properties, permeability and electrical properties. Furthermore, the copolymer (c) can react with any metal ions present in the composition (e.g. from added zinc stearate, which also functions as a processing aid, or some other zinc salt package) to form an ionomer, and such in situ crosslinking, taken with the improved polymer-filler interaction can result in much improved composition properties, as is demonstrated by the following examples, which illustrate the invention.

EXAMPLE 1 A series of compositions was produced from components as listed in the accompanying tables, and various physical and other parameters of the compositions were measured. Such parameters are also provided in the tables, and the results demonstrate certain features common to compositions within the scope of the invention (designated numerically), as distinct from characteristics of compositions provided and tested by way of comparison (designated alphabeti cally).

The compositions were produced by first forming a preblend of the polymer components in an internal mixer by heating and mixing the components until a temperature of 140"C was reached.

This temperature was held, with continued mixing, for a further two minutes, and at that point the molten polymer preblend was dumped and allowed to cool. The preblend was subsequently granulated. In a second stage the other components of the compositions (flame retardant filler, process aid, antioxidants, etc.) were introduced into an internal mixer at 60"C, and to this mixture was added the granulated preblend from the first stage. Mixing and heating was continued over a period of six minutes until a maximum temperature of 1500C was reached. At this point the final composition was dumped, and the product was used to form test bars and other artefacts appropriate for testing of various physical parameters.

Referring to Table 1, the elastomer component (b) used in compositions A, 1 and 2 was VISTALON 808, an ethylenepropylene copolymer rubber containing 78 wt % ethylene and having a Mooney viscosity ML (1+8) at 127"C of approximately 45. The elastomer component employed in compositions B and 3 was VISTALON 7000, a high diene content EPDM terpolymer rubber containing 70 wt % ethylene and having a Mooney viscosity ML (1+8) at 127"C of approximately 55.

The remaining components of the compositions of Table 1 were the same for all compositions (when present) as follows: The polyethylene (PE) was ESCORENE LL1001, an LLDPE of melt index 1 (ASTM D 1238); The EVA was ESCORENE ULTRA UL 00328 of melt index 3 (ASTM 1238) and having a 28 wt % VA content; The EAA (component (c)) was ESCOR TR 5000, an ethylene-acrylic acid copolymer containing 5 wt % acrylic acid and having a melt index of 8 (2.16 kg/190 C, ASTM 1238); The antioxidant employed was IRGANOX 1010 of CIBA-GEIGY company; and The ZDMDC component of the zinc salt package was zinc dimethyldithio carbamate.

It will be seen in Table 1 that for each composition, there are two values measured for each of hardness, tensile strength and elongation at break. For example the elongation at break values of composition A are reported as 120/635. The first value given was measured on a test piece produced by extrusion, whereas the second value results from tests carried out on a moulded plaque.

The compositions produced as specified above were tested for various characteristics by means of various standard and non-standard test procedures, as follows: (1) Oscillating disc (OD) rheometer: a standard test at the temperatures indicated, carried out on a Monsanto TABLE 1 Composition A 1 2 B 3 EPR 50 50 50 40 40 PE 20 20 20 30 30 EVA 30 - - 30 EAA - 30 30 - 30 ATH 150 150 150 150 MgOO 5 5 5 5 5 Zn borate 5 5 5 5 5 Antioxidant 0.2 0.2 0.2 0.2 0.2 Zn stearate 5 5 5 5 5 Tests OD Rheoneter 160 C 8 20 20 10 20 1200C 23 47 Hardness Shore A 90/90 93/97 95/97 86/91 95/95 Tensile Strength 7.8/7.3 12/12 12/11.7 9.1/7.8 11.5/11.6 Elongation at Break 120/635 110/80 80/60 166/425 165/100 Limiting Oxygen Index 29 28 31 29 31 Themoplasticity 7.2 0.4 0.9 2.4 0.5 Vicat Softening Point 78 102 102 87 103 Permeability > 28 6 Ageing A Tensile Strength +12 -12 a Elongation at Break -88 -22 Shrinkage -0.64/-0.32 -0.31/0 Oil Resistance A Volume +21 +8 # Weight +13 +5 A Tensile Strength -20 -11 A Elongation at Break -64 -19 Water Absorption Shrinkage -0.1/0 0/0 A Volume +5.3 +4.5 ki weight +3.0 +1.9 a Tensile Strength -19 -5 4Elongation at Break -89 -52 Volume Resistivity 4.1 4.7 oscillating disc rheometer using large rotor and 5 arc.The test measures the torque of a composition at the specified temperature, and hence provides an indication of how compositional crosslink density changes with temperature. Values quoted are in units of pound inches.

(2) Limiting oxygen index: this is a measure of the minimum wt % oxygen required in the atmosphere surrounding a composition test piece, at nominal room temperature, in order to support continued burning of the test piece. Carried out according to ASTM D 2863.

(3) Mooney viscosity: standard parameter measured in accordance with ASTM D1646.

(4) Stress-strain testing: tensile strength (measured in units of MPa) and elongation at break measured in % were measured on dumbbell samples in accordance with ASTM D412.

(5) Hardness (Shore A): measured in accordance with ASTM D2240.

(6) Vicat softening point: measured in "C in accordance with ISO R306A using a 200 g needle.

(7) Volume resistivity: measured in units of 10'4 ohm.cm in accordance with ASTM D257.

(8) Oil resistance: various parameters (volume, weight, tensile strength, elongation at break) were measured on strip samples before and after immersion in an oil of ASTM type 2 for 4 hours at 70"C.

(9) Ageing test: various parameters (dimensions, tensile strength, elongation at break) were measured on strip samples before and after ageing for 7 days at 100"C and are expressed in % change values. The dimensional change, i.e. shrinkage is quoted as a % value on measurements made MD/TD, i.e. made in the machine direction/transverse direction of the sample.

(10) Permeability: water vapour permeability, otherwise known as the moisture vapour transmission rate was measured (in units of gx 10-O/cm/cm2/hr/bar) on a sample in plaque form in a test cell containing water at 80"C on one side. The amount of moisture passing through the test piece in unit time was measured using a Dupont moisture analyser.

(11) Thermoplasticity: measured in the hot deformation test according to BS 6746 (1976) test method F4, using the specified apparatus and a load of 3.5N but under slightly modified conditions comprising placing the test piece and apparatus in an oven at 80"C for 1 hour, followed by 6 hours at 80"C with the test piece under load, then 1 hour cooling down under load at room conditions.

(12) Water absorption: various parameters (dimensions, volume, weight, tensile strength, elongation at break) were measured on strip samples before and after immersion for 1 day in water at 100"C and are expressed in % change values. As with the ageing test, shrinkage is measured as a % value and results are indicated for measurements made MD/TD, i.e. in the machine/transverse directions of the sample.

EXAMPLE 2 In order to demonstrate the characteristics which EAA can bring to the compositions of the invention when in its ionomer (crosslinked) form, experiments were performed to demonstrate the effect of various metal ions on the melt flow rate of EAA. Thus the EAA compounds as shown in Table 2 (parts by weight; amount of neutralizing agent calculated on equivalent metal ion mole) were produced by simple melt blending at 90-100 C, and each was then subjected to melt flow rate measurements under two conditions (a) 5 kg at 125"C (after 12 hours at 70"C) and (b) 2.16 kg at 1900C. The results (measurements in units of decigrams/minute) may be interpreted as showing that the incorporation of zinc stearate (5 phr) converts the EAA to its ionomeric form, and this crosslinking is retained at 125"C (hence the significantly low MFR value). However, the MFR value for the zinc stearate containing blend as measured at 1900C is no different from those measured for all the other blends, which suggest that the ionomer crosslinking is destroyed at this elevated temperature. The EAA employed in the Example 2 was the same one as used in the compositions of Example 1.

With reference to Example 1, analysis of the test results enables the following deductions to be made.

TABLE 2 11 12 13 14 15 16 EAA 100 100 100 100 100 100 Zinc Borate 3 - - - - - Mg Carbonate - 1.8 - - - Antimony Oxide - - 1.23 - - Alumina Trihydrate - - - 2.98 - Zinc Stearate - - - - S - Melt Flow Rate (a) 1.6 1.64 1.7 1.69 0.7 1.77 Melt Flow Rate (b) 7.1 6.4 6.5 6.8 6.8 7.0 For compositions B and 3, composition 3 clearly has a better (higher) volume resistivity and better (lower) water vapour permeability than B, and also generally better thermal properties.

Moreover, the heat ageing, water ageing and oil resistance properties are also generally better for composition 3. For all the compositions of Example 1 it may be seen that those containing EAA, which by virtue of the production process and of the presence of zinc stearate is in ionomer form, have generally improved mechanical properties. Without wishing to be bound by theory, it is believed that these properties show that the "neutralised" EAA confers a tighter network formation on the composition, resulting in a higher tensile strength and hardness, and a generally better uniformity of properties, i.e. less sensitivity to the shaping process, which was by extrusion and moulding.

In particular it is noted that the pairs of values measured on extruded and moulded test pieces (for hardness, tensile strength and elongation at break) clearly show that the compositions of the invention have much reduced differences in each pair of values, compared with the corresponding pairs of A and B. Thus the compositions of the invention have a much reduced sensitivity to the orientation effect produced by extrusion. It is believed that this reduced sensitivity to processing conditions derives from the ionomer type crosslinking generated in the compositions.

The OD rheometer values also confirm the tighter network formation (higher crosslink density) of compounds containing EAA neutralised in situ during mixing.

EXAMPLE 3 Two further compositions, C and 4 as defined in Table 3, were prepared by forming a preblend of polymeric components and then adding such preblend to a mixture of the other components in an internal mixer. The mixing was carried out over about 3 1/2 minutes to a temperature of 1500C, at which temperature the compositions were dumped and thereafter cooled. The EPR, EVA, EAA and PE used in the compositions were as used in Example 1 compositions B and 3, as was the antioxidant.

As with Example 1, the final compositions were used to form test bars and other artefacts appropriate for testing of the various physical parameters as reported in Table 3. The test methods and units of these parameters are as described for the compositions of Example 1 unless indicated differently in Table 3.

TABLE 3 Composition 4 C EPR 55 40 PE 15 30 EVA - 30 EAA 30 ATH 150 150 MgCO3 ' 5 5 Zn borate 5 5 Sub 203 2 2 Antioxidant 0.3 0.3 Zinc stearate 5 5 Tests Mooney Viscosity ML 1 + 4 at 1400C 105 50 Vicat Softening Point (0 C) 98 78 Hot Deformation Test (%) 1.3 3.9 (6 hrs at 800C, 3.5 N load) (thermoplasticity) Limiting Oxygen Index 30 30 Permeability (gm x 10-6 cm/hr/cm/bar) 80 C 8.2 > 28 Physical Properties on Extruded Strips. Original/afOer air ageing 7 days at 100 C Tensile Strength (MPa) 10.2/11.7 7.8/4.5 M/100 (MPa) 10.1/11.6 3.4/4.0 Elongation (%) 170/170 655/745 Hardness (Shore A) 95 92 TABLE 3 (Continued) Composition 4 C Volume Resistivity (ohm.cm) Original 2.8 x 1015 3.3 x 1014 After 1 day in H20 at 90 C 3.4 x 10 5.8 x 1010 9 After 7 days in H2O at 90 C 2.6 x 10 1.7 x 10 Oil Resistance (4 hrs at 700C in oil ASTM 2) Tensile Strength (% retention) 7.6 (75) 5.4 (69) Elongation (% retention) 220 (130) 725 (110) Water Absorption (2 days at 1000C) II, Volume, % 5.3 8.7 Aweight, % 3.4 5.6 % Tensile Strength Retained 79 63 % Elongation Retained 118 75 Specific Gravity 1.45 1.47 The results in Table 3 demonstrate the advantage obtained in respect of compositions according to the invention, and show that compositions having an excellent overall property balance suited to cable oversheathing may be produced. Thus in general composition 4 has, compared with composition C, a higher tensile strength and better retention after ageing; better oil resistance; lower water absorption and better retention of physical properties after water immersion; higher softening point which leads to a better performance in the thermoplasticity (6 hour hot deformation) test; a comparable limiting oxygen index; and a much better moisture vapour transmission rate (permeability) performance.Most startling of all, however, and a clear demonstration of the advantages of the composition according to the invention as exemplified compared with conventional cable oversheathing compositions, is the relative improvement in electrical properties after ageing in water. Thus composition 4 has an original volume resistivity value about 10 times higher than that measured for composition C; whereas after 7 days in water at 90"C, the value for composition 4 is about a thousandfold greater than that of composition C. It will be appreciated by those skilled in the art that composition properties which have effect on processing, e.g. extrusion, may be varied by control of the polymeric components of the compositions. For example, extrusion performance may be improved by employing lower viscosity elastomers or higher melt index versions of the ethylene/unsaturated carboxylic acid copolymer or polyolefin plastics component.

Claims (24)

1. A thermoplastic halogen free flame retardant composition comprising a polymer blend of (a) a plastics component selected from olefin homopolymers, olefin copolymers and mixtures thereof, (b) an elastomer component and (c) a copolymer of ethylene and an unsaturated carboxylic acid; and, incorporated in said blend (d) a flame retardant amount of a filler component selected from alumina trihydrate, magnesium hydroxide and mixtures thereof.

2. A composition according to claim 1 which comprises from 10-60 wt % of component (b) based on the total weight of said polymer blend.

3. A composition according to claim 2 which comprises from 15-30 wt % (b).

4. A composition according to claim 1, 2 or 3 wherein component (b) is selected from ethylene-higher alpha olefin copolymers, ethylene-higher alpha olefin-polyene terpolymers and mixtures thereof.

5. A composition according to claim 4 wherein component (b) is selected from EPM, EPDM and mixtures thereof.

6. A composition according to any one of the preceding claims which comprises from 10-40 wt % of copolymer (c) based on the total weight of said polymer blend.

7. A composition according to claim 6 which comprises from 20-30 wt % (c).

8. A composition according to any one of the preceding claims wherein copolymer (c) comprises acrylic acid or substituted acrylic acid as the acid comonomer.

9. A composition according to claim 8 wherein the substituted acrylic acid comonomer of copolymer (c) is methacrylic acid.

10. A composition according to any one of the preceding claims wherein (c) comprises from 5-15 wt % of acid comonomer.

11. A composition according to any one of the preceding claims wherein component (c) is present in ionomer form.

12. A composition according to any one of the preceding claims which comprises from 10-80 wt % of component (a) based on the total weight of said polymer blend.

13. A composition according to claim 12 which comprises from 20-50 wt % (a).

14. A composition according to any one of the preceding claims wherein component (a) is selected from polyethylene, polypropylene, ethylene vinyl acetate copolymer and mixtures of two or more thereof.

15. A composition according to any one of the preceding claims which comprises from 50-250 wt % of component (d) based on the weight of said polymer blend.

16. A composition according to claim 15 which comprises from 100-200 wt % (d).

17. A composition according to any one of the preceding claims which additionally includes an effective amount of a smoke-suppressant package comprising zinc borate and/or magnesium carbonate.

18. A composition according to claim 17 which comprises from 5-15 wt % active smoke suppressant materials based on the weight of said polymer blend.

19. A composition according to any one of the preceding claims when in the form of cable oversheathing.

20. A composition according to any one of the claims 1-18 when in the form of sheeting or moulded or extruded articles.

21. A method of producing a composition as defined in claim 1 which comprises (i) forming a molten preblend of components (a)-(c); (ii) admixing component (d) and said preblend, optionally together with process aids and conventional additives for thermoplastic materials, at a temperature in the range 130-150"C; and (iii) forming the resulting admixture into a shaped body.

22. A method according to claim 21 wherein step (i) is performed by melting an alreadyformed granulate of a blend of said components (a)-(c).

23. A method according to claim 21 or 22 wherein the admixture is formed into a shaped body by extrusion into the form of a cable oversheath.

24. An electrical cable which comprises an oversheath having the composition according to any one of claims 1-20 or produced by the method of claim 21, 22 or 23.