New Halogen-Free Fire Retardant for Engineering Plastic Applications*

SERGE1 V. LEVCHIK, DANIELLE A. BRIGm, GERALD R. ALESSIO, and SOPHIA DASHEVSKY

Akzo Nobel Functional Chemicals LLC 1 Livingstone Avenue

Dobbs Ferry, New York 10522

A comparative study of the fire-retardant efficiency of three commercial aryl phosphates-triphenyl phosphate (TPP), resorcinol bis(dipheny1 phosphate) (RDP) and bisphenol A bis(dipheny1 phosphate) (BDP)-in PC/ABS blends was carried out. The thermal and hydrolytic stability of the fire-retardant resins, as well as their physical properties, was also studied. The use of RDP and BDP is preferred over TPP because of superior properties, whereas BDP shows better fire-retardant efficiency, hydrolytic stability, and thermal stability than RDP.

INTRODUCTION

riphenyl phosphate (TPP), the parent compound T of a vast range of triaryl phosphates, was initially used as a fire-retardant additive in cellulosic materi- als, various coatings and rigid polyurethane foams (1, 2). Lately it has been used in high-performance engi- neering resins such as PPO/HIPS and PC/ABS blends (3-5). However its relatively high volatility limits its application range. Numerous alky-substituted trial- rylphosphates, available in the marketplace as plasti- cizers for vinyls, are not suitable flame retardants for engineering resins and are used as process aids.

Oligomeric aryl phosphates were developed to meet the requirement of low volatility at the high processing temperatures of engineering resins. The most success- ful product to date is resorcinol bis(dipheny1 phos- phate) (6, 7). Apart from the traditional applications in PC/AI3S and PPO/HIPS blends, the use of RDP in thermoplastic polyesters, thermoplastic polyurethanes, styrenics. cellulosics, acrylic resin sheets as well as epoxy resins is described in the patent literature. Be- cause of its relatively low volatility, RDP, it has been found (8-lo), acts primarily in the condensed phase in both PC/ABS and PPO/HIPS blends.

Akzo Nobel Chemicals has recently introduced a new oligomeric aryl phosphate, bisphenol A bis(dipheny1 phosphate) (Fyrolflex@ BDP), targeting high-perform- ance engineering resins ( 1 1).

This paper describes a comparison of combustion performance and physical properties of various PC/

‘Nothing contained herein shall be construed as granting or extending any license under any patent or recommending uses which would infringe patents owned by third parties.

ABS blends fire retarded with three widely used halo- gen-free flame-retardant additives: TPP, RDP and BDP.

EXPERIMENTAL Materials

Bisphenol A polycarbonate (PC Lem 141, GE) and ABS plastic (Cycolac GPM 5600, GE) were used to prepare PC/AE3S blends with the component ratios 1:2, 2: 1, 3: 1 and 4: 1. Aryl phosphates used as fire-re- tardant additives were:

Polytetrafluoroethylene (Teflon 6C, Du Pont) was used at the level of 0.5 wtYo to prevent flaming drips during combustion.

All composites were prepared on a Haake twin-screw extruder using a Zenith metering pump for the liquid components. The viscous material DDP) was preheated and the feed lines were electrically heat-traced. The

98 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2

New Halogen-Free Fire Retardant

solid fire retardant (TPP) was premixed with the resin and melt blended. After compounding, the pellets were dried in a forced-air oven and injection molded utiliz- ing a Newbury 50 ton injection molding machine.

Combustion

Combustion studies were carried out according to UL94 vertical test protocol or ASTM D2863 oxygen index test.

Thermal Aualymis

Thermal stability of the formulations was measured in thermogravimetric experiments using a Perkin Elmer thermal analyzer, TGA-7, in an air flow at a heating rate of 10°C/min.

Hydrolytic Stability

The fire-retardant polymers, shaped into discs of 60-mm diameter, were kept in an oven at 70°C at 85% relative humidity. The relative hydrolytic stability was measured as the decrease in drop weight impact strength according to ASTM D3029.

Physical Properties

Tensile strength, yield strength and flexural strength were measured using an Instron 4505 according to the ASTM D638 standard. Izod impact strength was meas- ured on a TMI 43-02 apparatus according to the ASTM D256 standard. Heat deflection temperature (HM? was measured on samples 3.2 mm thick at 1.82 MPa using a Tinus Olsen apparatus according to the ASTM D648 standard.

I -

Specimens 1.6 mm v-0

v-I

v-2

NC cn E s E

1 :2 2: 1 3:l 4:l

PClABS ratio Fig. 1 . UL94 combustion data for various F'C/ABS blendsfie retarded by aryl phosphates.

The oxygen index (01) of pC/ABS blends depends on the content of high charring PC in the blend (Q. 3). It increases from 21.4 to 24.0 when the PC/ABS ratio changes from 1:2 to 4: 1 in the absence of flame-retar- dant additives. In blends of low PC content (1:2), aryl

RESULTS AND DISCUSSION

Combustion

The results of the UL94 combustion test for PC/ABS blends at the component ratios of 1:2, 2:1, 3:l and 4: 1 are shown in Q. 1. Three aryl phosphates were used at the level needed to achieve 1 wtYo of phos- phorus in the fire-retardant formulations, which cor- responds to 10.5 wtYo TPP, 9.3 wt% RDP and 11.1 wtYo BDP. As seen, most formulations passed the UL94 test with a V-2 rating when the PC content in the blend is low (1:2): V-0 ratings are achieved with the phos- phates at higher PC content. In general, all three aryl phosphates show similar performance; however, there are cases where TPP's performance is poorer than RDP and BDP. BDP is the best fire-retardant additive in the PC/A€3S (3: 1) blend at a thickness of 1.6 mm.

The dependence of UL94 flammability data on the phosphorus content in 3: 1 PC/ABS is shown in Q. 2. At the 3.2-mm thickness, both RDP and BDP compos- ites are rated V-0 at a phosphorus content of 0.8 to 1.4 Wt0/0. At 1.6-mm thickness, the BDP composite in- creases its rating from V- 1 to V-0 at 1 .OYo by weight of phosphorus. whereas the RDP specimen does it only at 1.2 wto/o.

Specimens 1.6 mm

w C .- U

!! G I Specimens 3.2 mm Q) J v-0 3

v- I

v-2

NC

0.8 1 .o 1.2 1.4

Phosphorus content, wt.% Rg. 2. UL94 combustion data for PC/ABS (3:l) fire retarded with RDP or BDP at diierent additive levels.

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2 99

Flg. 3. oxygen indew us. Pc/ABs ratio in blendsf le retarded with aryl phosphates.

Sergei V. Levchik et al.

40 ,

phosphates are moderately efficient in the 01 test, as in UL94. However, all aryl phosphates are very efficient in 2:l PC/ABS, where they show 0 1 of 32.5-33.5. Upon further increase of PC content in the blend, the fire-retardant efficiency of aryl phosphates levels off at the 01 of 29-33. A study of the dependence of flam- mability on the concentration of fire-retardant addi- tive indicates that the 01 increases steadily with the phosphorus content in the formulations; both RDP and BDP provided similar fire retardancy.

Thermal Stability Thermogravimetry curves of a neat PC/Al3S (3:l)

blend along with fire-retardant formulations contair- ing 0.75-0.9 wtYo phosphorus are shown in Fig. 4. The neat blend starts to decompose at -350°C and gives a 20% solid residue at 500°C. This residue is not stable to thermal oxidation and "bums out" upon fur- ther heating above 550°C.

c] - neat -1PP - RDP - BDP 30

1 :2 2:l 3: 1 4:l

PClABS ratio

The onset of weight loss of the TPP formulation oc- curs at a relatively low temperature (230°C), which probably corresponds to partial volatilization of the additive. The RDP formulation also shows the antici- pated volatilization, whereas the formulation with BDP starts to decompose at the same temperature as the neat blend.

All aryl phosphates shift the main step of weight loss to higher temperature. A two-step process is seen in the temperature interval of 400-550°C. which prob- ably corresponds to the separate decomposition of the ABS and PC components of the blend, respectively. It is likely that aryl phosphates stabilize PC through a transesterification mechanism reported in the litera- ture (9).

The formulations with RDP and BDP produce simi- lar amounts of solid residue (-25%) at 55O-56O0C, whereas the formulation with TPP leaves less residue (- 21%). As for neat PC/ABS, these residues undergo

Rg. 4. Ilhermogravimetry c m s of neat Pc/ABS (3:l) and the blend f i e retarded with aryl phosphates.

100 200 300 400 500 600 700

Temperature,"C

100 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2

New Halogen-Free Fire Retardant

Hg. 5. mop weight impact strength us. exposure time offire-retardant forrnu.htions at 70°C and 85% rel- ative humidity.

thermal oxidation upon further heating. BDP protects the residue better than RDP or TPP, since it "bums out" completely only at 700°C.

Hydrolytic Stability

At high temperature, in the presence of water, aryl phosphates can undergo hydrolysis with release of acidic species. The acids attack the polycarbonate, re- sulting in a decrease of physical properties of the fire- retardant PC/AE3S blend. We studied the dependence of the drop weight impact strength of fire-retarded 3: 1 PC/ABS either with BDP or RDP (1% by wt. of phos- phorus) upon exposure to elevated temperature and high humidity (Fig. 5). Both formulations with BDP and RDP lose impact strength relatively fast during the two first days of exposure, and then the rate de- creases. The formulation with BDP stabilizes at the level of 0.17 kg*m after 3 days, whereas the formula- tion with RDP continues its slow decline.

FUJ. 6. Heat deflection tempera- tures of pC/ABS blends and their Jre-retardant formulations with aryl phosphates.

0 2 4 6 8 I 0

Time, days

2

Physical Properties

Figure 6 shows the heat deflection temperatures (HDT) for various PC/ABS blends fire retarded with aryl phosphates at a level of 1 wt% phosphorus. As expected, HDT increases with increasing PC content in the blends. It is known (7) that aryl phosphates can cause some plasticization. TPP is the strongest plas- ticizer, leading to the lowest HDT in this series. The formulations with BDP show comparable or slightly higher HM*s than the formulations with RDP; how- ever, 15 wtYo more BDP is used to reach the same level of phosphorus as RDP owing to BDPs lower phospho- rus content. Therefore, one can conclude that BDP is less plasticizing than RDP.

The relationship between HDT and phosphorus con- centration in a PC/ABS (3:l) blend containing either BDP or RDP is shown in Q. 7. Formulations with BDP exhibit higher HDT's than with RDP in spite of the higher BDP content.

100 -TPP 0 2 80

U

60 c : E 0

0

E a

Q Q, I:

.- U 40

20 c

0 1 :2 2:l 3:l 4:l

PClABS ratio

101 JOURNAL OF VlNYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2

Sergei V. Levchik et aL

Fig. 7. Heat & ? & o n temperature

fire retarded with BDP and RDP. us. phosphol-us content of Pc/ABs

t I 1 0 0.5 1 .o 1

Phosphorus content, wt.%

5

Izod impact strengths of fire-retardant PC/ABS blends at different PC:ABS ratios are shown in Rg. 8. The impact strength tends to increase with increasing PC content for the formulations containing RDP or BDP, whereas it does not change for the TPP-contain- ing formulations. In PC/ABS blends rich in PC, RDP and BDP show higher impact strengths than TPP.

The Izod impact strength of a PC/ABS (3: 1) blend is a function of additive content, as shown in FQ. 9. As expected, the impact strength decreases with increas- ing additive content. RDP shows higher hod impact strength values, probably due to the lower level of addi- tion (less RDP is used to obtain the same phosphorus content as BDP). In fact, the difference in Izod between RDP and BDP is close to 15%, which corresponds to the additional amount of BDP required to obtain the same level of phosphorus as RDP.

Tensile strength, yield strength and flexural strength data for selected PC/ABS blends fire retarded either by RDP or BDP are shown in Table 1 . The addition of

atyl phosphates to PC/ABS does not significantly af- fect the tensile, yield or flexural strengths. The BDP formulations tend to have higher strengths than the RDP formulations. No major effect of the additive con- centration is observed.

CONCLUSIONS

Three commercial aryl phosphates, TPP, RDP and BDP, impart different flammability, thermal and hy- drolytic stability and physical properties to PC/ABS blends. All three additives show comparable fire-retar- dant efficiency at the same phosphorus level as meas- ured by the oxygen index test; however, TPP is less efficient than RDP or BDP in the UL94 test. BDP is more effective in UL94 in thin specimens, whereas it is similar to RDP at higher thickness. In the thenno- gravimetry experiments, all aryl phosphates shift the main step of resin degradation to higher temperature and promote charring of the resin. BDP protects the

Fig. 8. Izod impact sb-ength of urn- bus PC/ABS blends f i e retarded with aryl phosphates.

N

E 5 Y

U 0 m .- E

-TPP

- RDP 10

1 :2 2:l 3: 1 4: 1

PClABS ratio

102 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2

New Halogen-Free Fire Retardant

Flg. 9. Izod impact strength us. phDsphorus content of pC/Al?Sfire retarded with BDP and RDP.

I I I -

0.8 1.0 1.2 1.4

Phosphorus content, wt.%

Table 1. Tensile Strength, Yield Strength and Flexural Strength for PClABS = 3:l Fire Retarded by RDP and BDP.

Additive % P Tensile Strength, MPa Yield Strength, MPa Flexural Strength, MRa

- RDP RDP BDP BDP

0 0.9 1.2 0.9 1.2

47 45 46 48 51

53.9 55.4 54.4 55.3 56.9

72 70 73 73 75

char from oxidation better than TPP or RDP. The for- mulations containing BDP show better hydrolytic sta- bility than those with RDP. BDP tends to retain higher heat deflection temperatures than RDP, whereas TPP shows the lowest HDT. The formulations with TPP pro- vide poorer Izod impact strength compared to RDP and BDP. Addition of aryl phosphates has a slight effect on tensile, yield and flexural strengths of PC/ABS blends.

REFERENCES 1. E. D. Weil, in Ftame Retardancy of Polymeric Materials,

Vol. 3 , 185-243, W. C. Kuryla and A. J. Papa, eds., Marcel Dekker, New York (1975).

2. C. F. Cullis a n d M. M. Hirschler, The Combustion of Organic Polymers, Clarendon Press, Oxford, England (1981).

3. J. Green, in Proc. Con$ Recent Adv. Flame Retardancy PoZyrn Mater., Vol. 2, 1-14, Stamford, Corn. (1991).

4. G. Tesoro, in Fire and Polymers. Hazards Identiiiation and Prevention, G. L. Nelson, ed., ACS Symposium Se- ries, Vol. 425, 241-52, Washington, D.C. (1990).

5. E. D. Weil. in Handbook of Organophosphorus Chem- isby, 683-738, R. Engel, ed., Marcel Dekker. New York [ 1992).

6. A. M. Aaronson and D. A. Bright, Phosphorus Sulfur Sili-

7. D. A. Bright, S. Dashevsky, P. Y. Moy, and B. Williams, J. Vinyl Additiue TechnoL, 5, 170 (1997).

8. E. A. Murashko, G. F. Levchik, S. V. Levchik, D. A. Bright, and S. Dashevsky, J. fie. sci, 19,278-96 (1998).

9. E. A. Murashko, G. F. Levchik, S. V. Levchik, D. A. Bright, a n d S. Dashevsky, J. Appl. Polym. Sci., 71,

10. S . V. Levchik, D. A. Bright, S. Dashevsky, and P. Moy, in Proc. Con$ “Additiues ’99,” San Francisco [ 1999).

11. S . V. Levchik, D. A. Bright, P. Moy, and S . Dashevsky, in Proc. 7th Europ. Con$ “fie Retardant Polym, ” London ( 1999).

COR 1091110, 83-86 (1996).

1863-72 (1998).

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, JUNE 2001, Vol. 7, No. 2 1 03