HALOGEN FREE FLAME RETARDANT CROSSLINKED COMPOUND FOR BUILDING WIRE APPLICATION

Abstract

The Cable industry is one of the most challenging markets because of diversified applications that lead too many cable constructions, followed with a market segment with stringent technical demands which are technologically driven.

The presented research paper will include the single and most popular segment commonly known as Building wire. Building wire is always been popular with PVC- Halogenated material containing chlorine and bromine which produces high density smoke during fire and causes death.  This paper is mainly focused on the high demands of non- halogenated flame retardant compounds replacing PVC for building wire application.

European regulations for RoHS, REACH and WEEE are impacting material choices for wire/cable in other global regions. Technologies for replacing PVC with halogen free, flame retardant (HFFR) compounds are moving beyond the first generation candidates. In addition, low smoke, halogen free (LSHF) compounds are driven by fire safety for both Health & Environment.

Key words: - Building wire, Halogenated, non-halogenated, Polyvinylchloride (PVC), Halogen free flame retardant (HFFR) compounds

Introduction

Major accidents which have resulted in the deaths of many innocent people, have taught us that the safety of the occupants and users in public, commercial and industrial environments is of paramount importance. Every possible safety feature designed to prevent and protect against loss of life and damage to property should be specified and installed.

Polyvinyl chloride (PVC, or vinyl) was a smart choice for wiring because it possesses excellent fire performance properties.  All organic polymers (whether they are plastics or natural materials like wood, cotton or rubber) are combustible: when sufficient heat is supplied to any organic polymer, it will thermally decompose, and its thermal decomposition products will burn. However, PVC will typically not burn once the source of heat or flame is removed. This results from PVC having 56.8% chlorine in its base polymer weight and it is well known that chlorine is one of the few elements that confers good fire properties to a polymer.  When polymers burn they give off gaseous products, which usually generate flames (most likely with light emission and soot)

Polymer + Heat →  Thermal Decomposition Products

Decomposition Products + Oxygenated Radicals →  Combustion Products + Heat

PVC’s are inherently resistant to ignition, so PVC cable components will limit and slow down the spread of a fire throughout the building and limit the cables to act as a fuse. BUT at the same time the dense black smoke produced by burning PVC materials is very toxic.

  1. 1 Kg of burning PVC fills a room of size 500 m3 with smoke
  2. 1 Kg of PVC generates 2 litres of HCl acid that corrodes metals and produces toxic fumes

Therefore the demand has been for material to replace PVC at school, colleges, high rise buildings, and commercial complexes etc which do not give off smoke and toxic fumes and do not contain halogens having following characteristics

  1. Fire retardant and zero halogen
  2. No or Low emission of smoke, toxic fumes and acid gases during combustion
  3. Similar mechanical and electrical properties to PVC
  4. Acceptable procesability with acceptable cost 

Following is the Trend of Insulating Materials in Building Wires

  1. NORMAL PVC
  2.  FR PVC (Flame Retardant)
  3. FRLS-PVC (Flame Retardant Low Smoke)
  4. HR-FR & FRLS PVC (Heat Resistant Flame Retardant Low Smoke)       
  5. FR-LF-PVC (Flame Retardant, Lead free Complying with RoHS requirements)
  6. FRLS-LF-PVC (Flame Retardant, Low Smoke, Lead Free Complying with RoHS requirements)
  7. HR-FRLS-LF PVC(Heat Resistant, Flame retardant,  Low Smoke, Lead free Complying with RoHS requirements)
  8.  Thermoplastic HFFR (Halogen Free, Flame Retardant, Low Smoke, Non Toxic Compound)
  9.  Crosslinked HFFR ( Crosslinked Halogen Free, Flame Retardant, Low Smoke, NON -Toxic Compound)

Experimental details

For Initial compound screening a range of formulation were compounded for five material listed as below

  1. PVC (normal PVC)
  2. FR-PVC  (Fire Retardant PVC)
  3. FRLS-PVC (Fire Retardant Low smoke PVC)
  4. Thermoplastic ZHFR (Zero Halogen Flame Retardant)
  5. Crosslinked ZHFR.

 The samples produced were used to prepare 1mm extruded tape and for crosslinked ZHFR with varying percentage of catalyst master-batch. These compounded materials were further tested for all physical parameters including, density, standard mechanical properties, % crosslinking etc. Table 1 represents sample exhibiting the balance of physical, electrical and mechanical properties was further studied on final application testing. Table 2 represents cable trial results.  The wire extrusion provides an initial opportunity to regulate the formulation in turn as well its processing behavior during our studies.

Results and discussion

Table 1:- Compound stage results (on 1mm extruder tape)

 

Properties

pvc

FR pvc

FRLS PVC

HFFR

XL-HFFR

Density (g/cc)

1.43

1.45

1.50

1.48

1.51

 

Physical properties - 1mm extruded Tape

Hot set value @ 200 ?C (%)

XX

XX

XX

XX

58

Permanent set @ 200 ?C (%)

XX

XX

XX

XX

0

Tensile Strength @ break  (N/mm2)

24.6

23.1

16.9

12.26

12.22

Elongation at break (%)

342

328

316

234

268

Other Properties

Hardness- Shore D

XX

XX

XX

48

47

Hardness- Shore A

90

90

91

XX

XX

Volume Resistivity (?cm)

2 X 1014

1 X 1014

7 X 1013

7.5411 X 1014

8.34 X 1014

Fire properties

Smoke density rating, %

75

67

55

3.0

2.9

Limiting Oxygen Index

26.2

30.3

31.5

36

36

Temperature Index (?C)

150

265

280

285

300

Thermal stability, Minutes

146

136

133

>180

>180

Halogen acid gas generation (%)

33.7

25.5

17.3

Nil

Nil

Heat ageing (Temperature and Time)

@ 100 °C / 7 days

@ 135 °C / 7 days

% Variation in Tensile Strength

-11

-15

-8

8

12

% Variation in Elongation

-12

-9

-7

-11

-14

  Table 2:- Cable Trial Results

Grade

PVC

FR PVC

FRLS PVC

HFFR

XL-HFFR

Size, Sq. mm

1.5  sq.mm

1.5  sq.mm

1.5  sq.mm

1.5  sq.mm

1.5  sq.mm

Catalyst Master Batch

XX

XX

XX

XX

4 % CTMB-09

Type of Screw (Compression ratio)

PVC

(3:1)

PVC

(3:1)

PVC

(3:1)

ZHFR

(1.4 : 1)

ZHFR

(1.4 : 1)

Extrusion Type

Pressure

Pressure

Pressure

Semi pressure

Semi pressure

Extruder

90 mm

90 mm

90 mm

90 mm

90 mm

T-1 (oC)

130

130

130

80

80

T-2 (oC)

140

140

140

90

90

T-3 (oC)

150

150

150

135

135

T-4 (oC)

155

155

155

140

145

T-5 (oC)

160

160

160

150

155

Clamp (oC)

165

165

165

165

165

Head (oC)

170

170

170

170

170

Die (oC)

175

175

175

180

180

Surface

Smooth

Smooth

Smooth

Smooth

Smooth

Wire Mesh

80

100

100+40

40

40

Curing temperature, °C

XX

XX

XX

XX

80

Curing time, Hrs.

XX

XX

XX

XX

3 hrs

Thickness of insulation, mm

0.73

0.74

0.74

0.72

0.74

Hot set, %

XX

XX

XX

XX

42

Permanent set, %

XX

XX

XX

XX

5

Tensile strength, N/mm2

18.3

17.5

15.4

11.70

13.5

Elongation, %

240

225

195

180

190

Single flame test

Period of burning after removal of flame, sec.

Unaffected length, mm

Pass

 

01

390

Pass

 

01

385

Pass

 

 02

390

Pass

 

03

380

Pass

 

 02

390

Volume Resistivity @ 27oC

4.1 x 1013

   2.5x 1013

1.6  x 1013

5.7 x 1013

6.6 x 1013

Hot deformation @ 80°C for 4 Hrs. (K=0.6), %

27

28

31

38

HDT @ 100 C, (K=1.0)= 22 %

Heat ageing

@ 100 °C/ 7days

@ 135 °C/ 7days

% Variation in Tensile strength

         

-8

-7

-9

18

19

% Variation in Elongation

2

2

-3

-14

-22

Smoke density rating, %

80

70

56

4.2

Cross-linkable

Limiting Oxygen Index

26

30.2

31.1

35

Cross-linkable

Thermal stability, Minutes

135

130

122

>180

> 180

Halogen acid gas generation (%)

33.6

25

17.5

Nil

Nil

 

 

Table 1 represents the comparative properties of PVC, FR PVC, FRLS PVC, HFFR and crosslinked-HFFR (XL-HFFR). These studies were carried out on compounded granules or 1 mm extruded tape or 1mm compressed sheet or 3mm compressed sheet as per the test specimen required on desired temperature profile to obtain smooth surface replicating cable surface.

Density of all compounds were measured and are mentioned in Table 1 are in the range of  1.4- 1.5 g/cc. Physical properties including tensile strength and elongation at break were tested on 1 mm extruded tape. As observed FRLS PVC (nowadays majorly used) have more value for tensile strength and elongation at break than compared to halogen free flame retardant compounds, whereas due to crosslinking behavior the properties are better in  XL- HFFR.  Being thermoset material the crosslinking is measured through hot set value which is < 60 %.

PVC being a rigid material therefore hardness is always measured in Shore A scale, whereas due to high loading of flame retardant fillers the material hardness increases and measured in shore D scale.

XL-HFFR exhibits the higher volume resistivity as compared to the other compounds because during crosslinking the molecular weight increases, its temperature withstanding

properties increases as results it exhibits higher current capacity.

Similarly all five were compared for fire properties. FRLS PVC (the most upgraded form of PVC) also exhibits higher smoke value as observed in Table 1 compared to HFFR compounds. PVC itself extinguishes the fire but shows low limiting oxygen values LOI,  and high halogen acid gas generation compared to HFFR where fire extinguish because of mineral filler mechanism.

The accelerated mode such as heat ageing at 100 °C/7days was performed to understand the long term behavior after cable extrusion. The crosslinked compounds better performance, the accelerated studies heat ageing is carried out at 135 ?C/ 7days, thus due to crosslinking its molecular weight increases, temperature withstanding properties also increases.

Table 2 represents the comparative studies were carried out on cable. Compounds were used for cable extrusion keeping desire cable extrusion temperature profile and other parameters constant to understand and compare the physical, electrical and fire properties.  Crosslinked HFFR exhibits the balance of properties compare to the HFFR over FRLS PVC and all. The detail values are mentioned in the Table 2. Thus, the results obtained after cable extrusion are more or less in similar line meeting the requirement of standard.