Silicone-based flame retardant synergists for halogen-free electrical wire and cable insulation Vincent Rerat Zhenkun Stone Shi Scott Miller Dow Corning Corporation

INTRODUCTION Many of the plastics used to insulate electrical wires and cables are combustible and would present a substantial fire hazard if they were not modified in some way. The smoke and heat from burning cables are plainly dangerous in their own right but the pervasiveness of cable networks in homes and offices can also provide routes for fire propagation along connections between devices, rooms, floors and generally throughout buildings.

coming from the burning plastic. An oxide layer is formed which restricts the release of flammable vapour from the plastic and also limits transfer of heat. This combination of effects makes ATH an effective and low cost flame retarding and smoke suppressing filler although the level of addition is typically in excess of 60%.[1] 2Al(OH)3(s)

The decomposition of magnesium dihydroxide (MDH), Mg(OH)2, is also endothermic and releases water vapour but the onset of decomposition occurs at a higher temperature, about 340°C. This means it can be used in formulations that are processed at higher temperatures than are possible with ATH.[1] Mg(OH)2(s)

Figure 1. Examples of electrical wire and cable insulation. Chlorinated polymers used in wire and cable insulation, most commonly polyvinyl chloride (PVC), are fire resistant since one of the products of their combustion is hydrogen chloride gas, which reacts with the free radicals generated during combustion and quenches the burning process. Brominated materials act in a similar manner, producing hydrogen bromide during combustion, and are used as flame retardant additives in thermoplastic elastomers and rubbers used for wire and cable insulation. Both hydrogen chloride and hydrogen bromide are toxic and in an increasing number of applications non-halogenated insulation materials are preferred. These are referred to as halogen-free flame retardant (HFFR) formulations or compounds.[1] There is growing demand for polyethylene-based insulation, with non-halogenated mineral flame retardants. Commonly used examples of mineral flame retardants are metal hydrates such as alumina trihydrate (ATH), Al2O3.3H2O, also known as aluminium hydroxide, Al(OH)3. At about 220°C this material undergoes endothermic decomposition, absorbing heat and releasing water vapour which dilutes the fuel vapor

Al2O3(s) + 3H2O(g)

MgO(s) + H2O(g)

ATH and MDH are often used in combination as flame retardants. Since the total loading can be 60% or more by weight to achieve the necessary degree of flame retardancy there are undesirable consequences associated with their use. These include low surface lubricity leading to processing difficulties, and poor water resistance, which affects the electrical insulation of the formulation. Silicone polymers provide surface lubrication and are water repellent so they are used as additives in wire and cable insulation to offset the disadvantages of high levels of mineral flame retardants. In this paper we describe formulated functional silicone additives that contribute to the flame retardancy of halogen-free insulation formulations as well as providing these benefits. These multifunctional additives assist formulators in meeting increasingly stringent fire safety requirements through their synergistic effects with mineral fillers. Two silicone products from Dow Corning specifically designed for wire and cable applications have been included in this study. One is a liquid that can be used to treat the surfaces of inorganic fillers to make them more dispersible in thermoplastic polymers such as polyolefins, thereby enhancing their flame retardancy. It can be applied as a pre-treatment or during the compounding of the fillers with the other insulation ingredients. The other product is a solid in the form of a powder that can also be compounded directly into the formulation and contributes to flame retardancy through the formation of a silica-like char during burning.

MATERIALS AND TEST METHODS Materials Some of the properties of the two Dow Corning® products evaluated are summarized in Table 1. Dow Corning® 11-100 Additive has alkoxy groups which can react with hydroxyl groups on the surfaces of inorganic filler particles, and alkyl groups which are compatible with polyolefins, so that treated fillers are easier to disperse. A consequence of improved dispersion of fillers such as ATH and MDH in cable insulation formulations is a greater degree of flame retardancy.

The powder additive Dow Corning® 4-7081 Resin Modifier has unsaturated organic groups which aid its dispersion in cable insulation formulations. It has a mixture of SiO2/2, SiO3/2 and SiO4/2 units which contribute to a silica-like char during burning and limit heat transfer and flame propagation. The other main ingredients of the wire and cable insulation formulations used in this study are listed in Table 2 with their commonly used abbreviations.

Dow Corning® 11-100 Additive

Property

Dow Corning® 4-7081 Resin Modifier

Appearance

Clear liquid

White to off-white powder

Viscosity (@ 25°C)

5.5 mPa·s

N/A

Active Content

>90%

100%

Functionality

Alkoxy and alkyl

Methacrylate and vinyl

Table 1. Properties of silicone products tested as flame retardant synergists in wire and cable insulation formulations. Material abbreviation/name

Description

Function

EVA

Ethylene vinyl acetate copolymer

Part of polymer matrix

LLDPE

Linear low density polyethylene

Part of polymer matrix

ATH

Alumina trihydrate, Al2O3.3H2O, also referred to as aluminium trihydroxide, Al(OH)3

Inorganic flame retardant filler

MDH

Magnesium dihydroxide, Mg(OH)2

Inorganic flame retardant filler

POE

Polyethylene-octene elastomer

Modifies the mechanical properties of the formulation

MAPE

Maleic anhydride grafted polyethylene

Coupling agent for mineral fillers in halogen-free flame retardant compounds

Antioxidant

Organo-phosphites, hindered phenols, and blends thereof

Prevents thermal degradation of formulation during processing and use

Dow Corning® MB50-002 Masterbatch

Ultra-high molecular weight silicone polymer dispersed in low density polyethylene

Pelletized lubricity and flow additive for polyethylene systems

Table 2. Materials used in wire and cable insulation formulations.

Processing methods The product Dow Corning® 11-100 Additive and comparative liquid additives were applied to the inorganic fillers in the laboratory by spraying onto the powder particles in a Lӧedige VT 20 laboratory mixing drum through which nitrogen was blown. The drum has a capacity of 10 liters and a typical batch time of 10 minutes was used with a mixer speed of 220 rpm. With this method the temperature of treatment in the drum can be varied. Figure 2 is a graphic representation of this laboratory set-up together with a photograph. Treated fillers were incorporated into wire and cable insulation formulations by blending with the other

ingredients and then compounding the blend on a twin screw extruder of the type shown in Figure 11. This approach will be referred to as pretreatment of fillers. The liquid additives were also incorporated into the insulation formulations by blending with the other ingredients before extrusion. This approach will be referred to as in situ treatment of fillers. The powder additive Dow Corning® 4-7081 Resin Modifier and competitive materials were added to the extruder with other formulation ingredients.

Figure 2. Equipment for treating filler surfaces with liquid materials.

Test methods The test methods used to assess the effects of the additives are grouped into categories in Table 4. Of most interest in this study is the burning behavior of the formulations. The two methods listed were used to check that any observed changes were consistently seen with a downward flame test and an upward flame test. Of the former type of method the Limiting Oxygen Index (LOI) is widely used, existing as equivalent international, American, and British standard methods. The LOI is “the minimum oxygen concentration to support candlelike combustion of plastics”.[2] An additive that increases the LOI of a cable insulating formulation is providing a degree of flame retardancy. UL 94 is a flammability test from the Underwriters Laboratories in which a flame is applied to the sample from below i.e. an upward flame test. As well as

Property category

Burning behavior

assigning a classification based on how long burning continues once the flame is removed the tendency of the sample to produce drips of flaming or extinguished material is taken into account.[3] The classifications will be described in more detail in the results and discussion session. The flame tests were performed on injection molded samples of the specified dimensions which had been conditioned for 40 hours at 23°C and 50% relative humidity. It is important that an additive intended as a flame retardant synergist does not adversely affect the other properties of the insulation materials. Here we have measured two of the most critical mechanical properties and two indicators of water resistance.

Specific property

Test methods

Limiting Oxygen Index (LOI). The minimum concentration of oxygen that will just support flaming combustion in a flowing mixture of oxygen and nitrogen.

ASTM D 2863. Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index).[2] Equivalent to ISO 4589-2/BS 2863. Downward flame test.

UL 94 rating.

UL 94, the Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.[3] Upward flame test.

Tensile strength.

ISO 37. Rubber, vulcanized or thermoplastic – Determination of tensile stress-strain properties.[4]

Elongation at break.

ISO 37. Rubber, vulcanized or thermoplastic – Determination of tensile stress-strain properties.[4]

Mechanical

Immersion of ISO 37 dumbbells in hot water (70°C) for 7 days. Water resistance

Water absorption during immersion.

ASTM D570-98. Standard Test Method for Water Absorption of Plastics.[5] Room temperature immersion for four weeks.

Table 3. Properties and associated test methods for evaluation of additives in wire and cable insulation formulations.

RESULTS AND DISCUSSION Liquid additive, Dow Corning® 11-100 Additive, MDH filler pre-treatment MDH was pre-treated with Dow Corning® 11-100 Additive at 1 and 1.5 weight % at ambient temperature, 45°C and 80°C as described in the processing methods section then compounded into the polyolefin formulation shown in Table 4. Some of the twin screw extruder conditions for the compounding are given in Table 5.

Raw Materials

Description

Wt %

EVA

88 wt% ethylene, 12 wt% vinyl acetate MFI (190°C/2.16 kg) = 0.5 g/10 min

15.96

EVA

73 wt% ethylene, 27 wt% vinyl acetate MFI (190°C/2.16 kg) = 3.0 g/10 min

15.96

MFI (190°C/2.16 kg) = 2.8 g/10 min

7.98

MFI (190°C/2.16 kg) = 1.1 g/10 min

4 (reference formulation)

LLDPE MAPE* MDH

Brucite; 3.5µ average particle size; 1 wt% or 1.5 wt% surface-treated with Dow Corning® 11-100 Additive

60

Antioxidant

Sterically hindered phenolic

0.1

Table 4. MDH wire and cable insulation formulation. The MFI is the Melt Flow Index; ASTM D1238, ISO 1133. *Where MAPE was used at 4%, the LLDPE level was reduced by 4%.

Parameter Melt temperature Barrel temperature

Value/range ̴205°C 160–190°C

Melt pressure

̴1.2 bar

Screw speed

250 rpm

Torque

37–49 N/m

Table 5. Twin screw extruder conditions used for production of wire and cable insulation formulations containing pretreated MDH filler.

As can be seen in Figure 3 the Limiting Oxygen Index for this formulation was only slightly increased by the addition of 4 wt% MAPE, the maleic anhydride grafted polyethylene additive recommended for greater bonding strength between mineral fillers and polyolefins. A larger increase was observed with pre-treatment of the MDH with 1 wt% Dow Corning® 11-100 Additive at

ambient temperature (RT, room temperature). Increasing the level of pre-treatment to 1.5% resulted in a further increase in LOI. These improvements were even more marked when the pre-treatments were performed at elevated temperatures. As might be expected the increase in LOI is proportional to the level of treatment and the temperature at which it is applied.

Figure 3. Limiting Oxygen Index (LOI) for wire and cable insulation formulation with MDH pre-treated with 1 and 1.5 wt% Dow Corning® 11-100 Additive at different temperatures (formulation in Table 4). MAPE included for comparison.

Having established that pre-treatment of MDH filler with Dow Corning® 11-100 Additive can reduce the ability of a polyolefin electrical insulation formulation to support candle-like combustion when ignited from above, the method UL-94 was used to measure the tendency of the same formulation to extinguish or spread flames when ignited from below. In this method the results are classifications rather than numerical values. HB is the least flame retardant rating, indicating slow burning of a horizontal specimen. The higher ratings of V-2 and V-1 are assigned to vertical specimens that stop burning within thirty seconds and where drips of burning material are allowed and not allowed respectively. There are higher ratings but the ones described cover the range observed in this study. Five specimens of each formulation were evaluated and the results are given in Table 6.

All the samples of the formulation with untreated MDH had a rating of HB, the least flame retardant rating. Treatment of the MDH with 1 wt% Dow Corning® 11-100 Additive at room temperature increased the rating of some samples to V-1, and 1.5 wt% treatment gave a larger proportion of samples with this rating. When a treatment of 1.5 wt% was applied at 45°C all the samples showed the increased flame retardancy rating of V-1. The same improvement was seen with a 1 wt% treatment at 80°C. The organic additive MAPE at 4 wt% provided no increase in the rating of any of the samples tested. As with the LOI test the degree of flame retardancy conferred by Dow Corning® 11-100 Additive is proportional to the level of treatment and the temperature at which it is applied.

Specimen Classification Sample ID 1

2

3

4

5

Untreated

HB

HB

HB

HB

HB

Dow Corning® 11-100 Additive 1 wt% RT

HB

HB

V-1

V-1

V-1

Dow Corning® 11-100 Additive 1.5 wt% RT

V-1

HB

V-1

V-1

V-1

Dow Corning® 11-100 Additive 1 wt% 45°C

V-1

HB

V-1

V-1

V-1

Dow Corning® 11-100 Additive 1.5 wt% 45°C

V-1

V-1

V-1

V-1

V-1

Dow Corning® 11-100 Additive 1 wt% 80°C

V-1

V-1

V-1

V-1

V-1

Dow Corning® 11-100 Additive 1.5 wt% 80°C

V-1

V-1

V-1

V-1

V-1

4 wt% MAPE

HB

HB

HB

HB

HB

Table 6. UL 94 ratings for wire and cable insulation formulation with MDH pre-treated with 1 and 1.5 wt% Dow Corning® 11-100 Additive at different temperatures (formulation in Table 4). MAPE included for comparison.

Figure 4. Images of wire and cable insulation test pieces (formulation in Table 4) with treated and untreated MDH and added MAPE, tested according to UL 94 (upward flame test). Images of some of the test specimens are shown in Figure 4 where the visible extent of charring reflects the classifications given in Table 6. Insulation compounds with high levels of mineral filler such as the one studied here have a tendency to absorb water, as demonstrated by the water uptake of the

control formulation in Figure 5. This shows the increase in weight following immersion in water at 70°C for seven days. In all cases where the MDH had been pretreated with Dow Corning® 11-100 Additive the water uptake was reduced to just under half that of the control. MAPE at 4% addition level was almost as effective in this respect.

Figure 5. Water uptake for wire and cable insulation formulation with MDH pre-treated with 1 and 1.5 wt% Dow Corning® 11-100 Additive at different temperatures (formulation in Table 4). 7 days water immersion at 70°C. MAPE included for comparison.

Reduced water uptake is necessary for the retention of the electrical insulating properties of the formulation. It is important that this is not achieved at the expense of the mechanical properties. In Figures 6 it can be seen that the tensile strength results for the specimens containing MDH treated with Dow Corning® 11-100

Additive are almost the same as that of the control. This is the case with freshly prepared and conditioned samples and also with samples after seven days immersion in water at 70°C. In all cases there was a slight reduction in tensile strength after the water immersion.

Figure 6. Tensile strength of wire and cable insulation formulation with MDH pre-treated with 1 and 1.5 wt% Dow Corning® 11-100 Additive at different temperatures (formulation in Table 4). Initial results and after 7 days water immersion at 70°C.

The elongation at break values in Figure 7 show a trend upwards as the temperature of the MDH treatment increased. This is likely to be the result of more complete reaction of the alkoxy groups in the Dow Corning® 11-100 Additive with the filler surface. The control after immersion shows a small increase in elongation at break due to the plasticizing effect of

absorbed water. With most of the samples with treated MDH there is a slight drop in the elongation after water immersion, although not below the initial value of the control. It can be concluded that although there are some differences in the elongation at break depending on the treatment level and temperature, there is no significant reduction in this property relative to the control.

Figure 7. Elongation at break of wire and cable insulation formulation with MDH pre-treated with 1 and 1.5 wt% Dow Corning® 11-100 Additive at different temperatures (formulation in Table 4). Initial results and after 7 days water immersion at 70°C.

Liquid additive, Dow Corning® 11-100 Additive, ATH/MDH filler combination, in situ treatment The inorganic fillers ATH and MDH were compounded into a polyolefin insulation formulation with Dow Corning® 11-100 Additive and the three alternative materials listed in Table 7. The vinyl- and aminopropylfunctional alkoxy silanes are known as coupling agents which improve the dispersion of inorganic fillers in a variety of organic polymers and provide greater binding

Candidates

between the particles and the matrix. These traditional silicon-based treatments were included to see to what extent they influence the burning behavior of the insulation in comparison with the new product, when each is included as an in situ filler treatment during compounding. The insulation formulation is given in Table 8 and the twin screw extruder conditions in Table 9.

Active Content (%)

Functionality

-

Comments Without additive in formulation

Control (no filler treatment)

None

Dow Corning® 11-100 Additive

Alkoxy- and alkyl-functional siloxane

>90%

Subject of study

Competitor 1

Similar to Dow Corning® 11-100 Additive

>90%

Competitor product

Competitor 2

Vinyltrimethoxyethoxysilane

>97%

Traditional Coupling Agent

Competitor 3

Aminopropyltriethoxysilane

>97%

Traditional Coupling Agent

Table 7. Silicon-based liquid materials tested as in situ filler treatments for wire and cable insulation formulations.

Raw materials

Description

wt %

EVA

72 wt% ethylene, 28 wt% vinyl acetate; MFI (190°C/2.16 kg) = 3.0 g/10 min

27.45

LLDPE

Metallocene PE

9.80

MAPE

MFI (190°C/2.16 kg) = 2.0 g/10 min

1.96

MDH

Untreated

19.61

ATH

Untreated

39.22

Dow Corning® MB50-002 Masterbatch

Processing aid, lubricant

1.18

Antioxidant

Sterically hindered phenolic

0.20

Silicone/silane treating agent (1.0 wt% of total filler)

Selected from list in Table 7; none for control

0.59

Table 8. ATH/MDH wire and cable insulation formulation.

Parameter Screw diameter Screw length to diameter ratio (L/D) Melt temperature Barrel temperature

Value/range 25 mm 40 ̴180°C 140–170°C

Melt pressure

̴55 bar

Screw speed

180 rpm

Torque

̴60%

Table 9. Twin screw extruder conditions used for production of wire and cable insulation formulations containing ATH and MDH fillers.

Figure 8. Limiting Oxygen Index (LOI) for wire and cable insulation formulation with MDH and ATH compounded with Dow Corning® 11-100 Additive and competitive products (formulation in Table 8).

The Limiting Oxygen Index values for these formulations are shown in Figure 8. As an in situ treatment of MDH and ATH Dow Corning® 11-100 Additive at 1 wt% gave a similar increase in LOI to that achieved when applied as a 1.5% pre-treatment to MDH alone in the previously discussed formulation (Figure 3). The competitor materials 1 and 2 had little effect on the LOI and competitor 3, the amino-functional silane, made the burning behavior worse, in that less oxygen was required in the gas mixture to sustain combustion.

Liquid additive, Dow Corning® 11-100 Additive, low cost filler (CaCO3), in situ treatment In one of the formulations a quarter of the ATH (10 wt% of total formulation) was replaced by calcium carbonate. An identical formulation was compounded with Dow Corning® 11-100 Additive at 1.0 wt% of total filler. Similar formulations were also prepared with half of the ATH (20 wt% of total formulation) replaced by calcium carbonate. The twin screw extruder conditions were as shown in Table 9.

Calcium carbonate is a relatively low-cost mineral filler used in polyolefins to modify mechanical properties such as rigidity. It has no significant flame retarding properties. Since Dow Corning® 11-100 Additive has been shown to increase the flame retarding behavior of more costly fillers such as ATH it should be possible to replace some ATH with calcium carbonate and boost the flame retardancy of the remaining ATH with the silicone additive. Formulations to test this are shown in Table 10. The base formulation is the same as in Table 8 with a combination of MDH and ATH fillers.

Raw Materials

No CaCO3

10% CaCO3

10% CaCO3

20% CaCO3

20% CaCO3

EVA

27.45

27.45

27.45

27.45

27.45

LLDPE

9.80

9.80

9.80

9.80

9.80

MAPE

1.96

1.96

1.96

1.96

1.96

MDH

20.00

20.00

20.00

20.00

20.00

ATH

40.00

30.00

30.00

20.00

20.00

0

10.00

10.00

20.00

20.00

Dow Corning® MB50-002 Masterbatch

1.18

1.18

1.18

1.18

1.18

Antioxidant

0.20

0.20

0.20

0.20

0.20

0

0

0.59

0

0.59

CaCO3

Dow Corning® 11-100 Additive (1.0 wt% of total filler)

Table 10. Wire and cable insulation formulations, with partial replacement of ATH with calcium carbonate, with and without in situ treatment with Dow Corning® 11-100 Additive. The raw materials are the same as those described in Table 8.

Replacing a quarter of the ATH with calcium carbonate reduced the LOI of the formulation as can be seen in Figure 9. This is to be expected. It can also be seen that incorporation of Dow Corning® 11-100 Additive as an in situ filler treatment brought the LOI back up to almost the level for the original amount of ATH that was shown previously in Figure 8. Figure 10 shows that at 20% replacement of ATH with calcium carbonate the reduction in LOI was even greater; 6.7 %O2 lower than the control. With in situ treatment with Dow Corning® 11-100 Additive the LOI of the same formulation was 2.1 %O2 higher than the control.

Figure 9. Limiting Oxygen Index (LOI) of wire and cable insulation formulation (Table 10) with 10% replacement of ATH by calcium carbonate (based on total formulation), with and without in situ treatment with Dow Corning® 11-100 Additive.

It is therefore possible through the use of Dow Corning® 11-100 Additive to replace substantial amounts of ATH with the lower cost filler calcium carbonate without diminishing the flame retardancy of an insulation formulation. The hydrophobicity and mechanical properties were not adversely affected. In fact there is a noticeable improvement in elongation at break as indicated in Table 11, which summarizes the effects of filler treatment with Dow Corning® 11-100 Additive.

Figure 10. Limiting Oxygen Index (LOI) of wire and cable insulation formulation (Table 10) with 20% replacement of ATH by calcium carbonate (based on total formulation), with and without in situ treatment with Dow Corning® 11-100 Additive.

Dow Corning® 11-100 Additive in ATH/MDH formulations

Dow Corning® 11-100 Additive with CaCO3 for cost optimization

LOI

++

++

UL-94

+

Not tested

Hydrophobicity

++

++

Water uptake

++

++

Tensile strength

0

0

Elongation

0

++

Property

Table 11. Summary of effect on properties of wire and cable insulation formulations by treating ATH and MDH with Dow Corning® 11-100 Additive, and replacing some filler with similarly treated calcium carbonate. 0 indicates no change, + is a moderate improvement and ++ is a large improvement, relative to the control formulation. Powder additive, Dow Corning® 4-7081 Resin Modifier, with ATH The powder additive was tested in the formulation given in Table 12 and compared with two silicon-based competitive products as well as an organic and a fluoro-based material. Figure 11 shows the twin screw extruder configuration and the points of addition for the various ingredients. The conditions for the extruder are listed in Table 13. Raw Materials

Description

Wt%

EVA

73.3 wt% ethylene, 26.7 wt% vinyl acetate

24.3–27.5

LLDPE

MFI (190°C/2.16 kg) = 2.0 g/10 min

6.0–6.8

ATH

Fine precipitated alumina trihydrate

65

Blend of organo-phosphite and hindered phenolic

0.2

Coupling agent; aminopropyltriethoxysilane

0.6

Antioxidant XIAMETER® OFS-6011 Silane Dow Corning® 4-7081 Resin Modifier

Flame retardant synergist and process aid

4

Competitor 4

Si-based process aid

4

Competitor 5

Si-based process aid

2

Competitor 6

Organic process aid

4

Competitor 7

Fluoro-based process aid

1

Table 12. ATH wire and cable insulation formulation for testing of flame retardant synergist candidates added during extrusion. EVA and LLDPE amounts adjusted according to additive addition level.

Figure 11. Twin screw extruder configuration for incorporation of powder flame retardant synergist.

Parameter Screw diameter Die head Barrel temperature Screw speed

Value/range 25 mm Two-strand, 5 mm 170–180°C 350 rpm

Polymer, antioxidant, additive blend feed rate

3.75 kg/hour

ATH feed rate

6.1 kg/hour

XIAMETER® OFS-6011 Silane feed rate

1ml/min

Table 13. Twin screw extruder conditions used for production of wire and cable insulation formulations incorporating Dow Corning® 4-7081 Resin Modifier and competitive materials.

Figure 12. Limiting Oxygen Index (LOI) for wire and cable insulation formulation with Dow Corning® 4-7081 Resin Modifier and competitive products added during extrusion.

It can be seen from Figure 12 that Dow Corning® 4-7081 Resin Modifier at 4% increased the LOI compared to the control and did so to an extent greater than was seen with any of the competitor materials at the recommended addition levels. In fact only one of the competitor materials increased the LOI, the others gave values lower than the control. The changes to LOI and other properties for the five additives are summarised in Table 14. Dow Corning® 4-7081 Resin Modifier gave the best combination of effects, with minimal changes to the tensile properties and a 30% reduction in water uptake. As was mentioned in the introduction silicone additives provide surface lubrication to insulation formulations

and this was evident with Dow Corning® 4-7081 Resin Modifier during the extrusion process. There was an 11% reduction in the torque required to maintain the extrusion rate and a large reduction in build-up of residue at the die head, known as die drool, as illustrated in Figure 13. Die drool has a direct impact on productivity since the cable manufacturer has to shut down the extruder at intervals to remove the residue. A reduction in die drool, as seen with Dow Corning® 4-7081 Resin Modifier, increases on-line time and production rates. It also results in less scrap.

Limiting Oxygen Index, % ∆

H2O Absorption Reduction (RT, 1 month), % ∆

Tensile Strength at Break, % ∆

Tensile Elongation, %∆

Torque,% ∆

Die Drool Reduction

4% Dow Corning® 4-7081 Resin Modifier

+9%

-30%

No change

-3%

-11%

Excellent

4% Competitor 4 (Si-based)

+5%

-23%

-19%

-46%

-19%

Good

2% Competitor 5 (Si-based)

-11%

+23%

-9%

-21%

-17%

Excellent

4% Competitor 6 (organic)

-3%

-16%

-25%

+6%

-12%

Poor

1% Competitor 7 (fluoro-based)

-2%

-23%

-2%

-31%

-8%

Poor

Product

Table 14. Summary of effect on properties of wire and cable insulation formulations of Dow Corning® 4-7081 Resin Modifier and competitive products added during extrusion.

Figure 13. Residue build-up at die head, known as die drool, during extrusion of formulation in Table 11.

CONCLUSIONS In this study we have shown that silicone-based materials can be used to enhance the flame retarding properties of mineral fillers in halogen-free flame retardant polyolefin formulations for electrical wire and cable insulation. The liquid additive is proposed to operate by aiding the degree of dispersion of the fillers, whereas the powder additive forms a silica-like char which restricts flame propagation. With the liquid additive improvements in flame retardancy have been demonstrated using standard downward and upward flame tests. Formulations containing the commonly used mineral fillers magnesium dihydroxide and alumina trihydrate were tested. The low cost filler calcium carbonate can be used to replace up to a third of the more expensive fillers, offering economic as well as performance benefits. Water uptake during immersion was reduced by more than a half in an insulation formulation containing MDH pre-treated with the liquid additive and the tensile strength and elongation at break were not diminished in the initial samples or after immersion in water.

The liquid additive can be used to pre-treat mineral fillers and can also provide in situ treatment through addition at the compounding stage. Addition of the powder additive at the compounding stage provided an increase in the Limiting Oxygen Index, a reduction in water uptake and little or no change in the measured mechanical properties. This combination was not seen with any of the competitor materials tested. In addition there was a marked reduction in die drool during extrusion allowing greater productivity and less scrap. The provision of silicone-based additives in both liquid and powder forms offers flexibility to formulators of halogen-free flame retardant insulation compounds. Synergistic enhancement of the flame retarding properties of mineral fillers provides a means to meet increasingly demanding standards and regulations.

REFERENCES [1] Keeping Fire in Check. An Introduction to Flame Retardants used in Electrical and Electronic Devices. European Flame Retardants Association, Brussels, November 2011, 19-23, http://www.cefic-efra.com/images/stories/IMGBROCHURE-2.4/EFRA_E&E_brochure_oct2011_v04.pdf (accessed January 23, 2015) [2] ASTM D2863-13, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), ASTM International, West Conshohocken, PA, 2013, http://www.astm.org/Standards/D2863.htm (accessed January 23, 2015) [3] UL 94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, Underwriters Laboratories, 2013, http://ulstandards.ul.com/standard/?id=94 (accessed January 23, 2015) [4] ISO 37:2011(en), Rubber, vulcanized or thermoplastic – Determination of tensile stress-strain properties, ISO, 2011, https://www.iso.org/obp/ui/#iso:std:iso:37:ed-5:v1:en (accessed January 23, 2015) [5] ASTM D570-98(2010)e1, Standard Test Method for Water Absorption of Plastics, ASTM International, West Conshohocken, PA, 2010, http://www.astm.org/Standards/D570.htm (accessed January 23, 2015)

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