10-1alteranate

Alternative FR Plasticizers for Isopropylated TPPs, TCP and TXP and other Flame Retardant Considerations

Paul Y. Moy Market Support Mgr., Plasticizers & FRs

ICL-IP America, Inc. Ardsley, New York - USA

1-(914) 269-5942 [email protected]

61st IWCS Providence, RI November 11th -14th 2012

Keywords: triaryl phosphates; alkyl diphenyl phosphates; Flame retardants

Abstract:

Recent changes in European labeling has renewed focus on product selection for flame retardants such as flame retarding plasticizers phosphate esters used in PVC and antimony-based synergists. The labeling change of isopropylated TPP phosphate plasticizers and other materials have drawn focus to the hazards associated with these chemicals, especially in open compounding processes such as roll mill or calendering operations; better alternatives are being considered. Antimony oxide, once a preferred synergistic flame retardant additive for vinyl, is a primary focus for replacement due to its high costs and suspect health concerns (dusting). Traditional alternatives have not readily met the balance of properties offered by these products.

However, new options exist with similar plasticization properties and more favorable labeling. For flame retardant plasticizers, alkyl diphenyl phosphates and blends with non-traditional triaryl phosphates can offer similar FR performance and potentially lower smoke evolution. Alternative plasticizers can offer lower volatility ensuring longer contribution or viability without compromising the flame retardancy of the composite system.

Additionally, for more robust composites, careful formulation (synergists/additives) work can offer alternative means for effecting low combustion behavior. The use of certain inorganic additives

as a partial replacement for antimony oxide can lead to lower heat release rates and effectively lower the propagation potential for fire. Also, the substitution of antimony oxide, well known for increasing smoke development, may lessen the evolution of smoke. One suspect mechanism for this is by improving the char formation during combustion by altering the surface morphology to form a more cohesive dome. This dome can prevents or lessen fissure cracks in the charring layer and prevent the release of combustible volatile gases.

Introduction:

For many years (and still to this day), flexible elastomers usually were sourced from Southeast Asia using organic sources (natural latex). These materials were difficult to get (especially in war time) and at an escalating cost. Although PVC was discovered long before it was a commercial success, finding the right combinations of stabilizers and plasticizers to render this resin useful as a flexible alternative to rubbers did not come to fruition until the 1940s (1). These products were developed as a needed replacement during the war efforts.

Flexible vinyl was found to be an easy option over convention rubber products as most of the commercial equipment used for elastomeric compounding could be easily adapted for flexible vinyl composites. And, the vinyl process could be done at a higher operating temperature (with the right formulation of plasticizers,

373 International Wire & Cable Symposium Proceedings of the 61st IWCS Conference

lubricants and stabilizers) allowing for faster and higher yields of product. Rubber composites were limited in process speeds due to concerns for cross-linking/vulcanization.

One of the earliest plasticizer used for flexible PVC was tricresyl phosphate (TCP). In addition to being an excellent plasticizer, TCP was found to be an excellent flame retardant as well. The use of flame retarded vinyl composites with TCP in wire and cable insulation worked very well over then conventional elastomeric materials. Insulation properties were as good or better than natural elastomers and also much better in aging resistance and flame retardancy. Flexible vinyl was considerably more cost efficient and readily available.

Tricresyl phosphate was produced by the following reaction scheme;

Figure 1: Synthetic route of Aryl Phosphate Esters

For tricresyl phosphate, R would represent a methyl group for the alkyl component, reacting with phosphorus oxychloride (plus catalysts) to form tricresyl phosphate. Other versions of cresylic/xylyl phosphates would prove to be useful as well.

In the early development of FR vinyl composites, phosphate esters were commonly used and grew into three major versions of triaryl phosphates:

Tri-Aryl Phosphate Types;

Tricresyl phosphate (TCP) high solvating general purpose flame retardant plasticizer

Cresyl diphenyl phosphate (CDP) similar to TCP but slightly more fugitive.

Trixylenyl phosphate (TXP) - low volatility, useful for flexible agricultural vinyl films (greenhouses) and light stable and also is one of the prime flame resistant hydraulic fluids still used today in the marketplace.

However, in the early days of flexible vinyl, the process used for producing TCP plasticizers, used natural cresols sourced from coal tar derivatives. Unfortunately these coal tar derivatives contained high levels of ortho-cresol, a neurotoxin; the industry quickly looked for alternatives.

Like TCP, cresyl diphenyl phosphate (CDP) and trixylenyl phosphate (TXP) ester analogs were found to have neurological activity due to the coal tar raw material . Although each had carved a niche into vinyl applications, the industry sought alternative plasticizers for each of these products.

The development of synthetic processes which contained very low levels of the ortho isomer was subsequently used as an alternative feedstock. Now the production of tricresyl phosphate includes the use of cresols with very low levels of o-cresol (0.05%). Other isomers, such as isopropylphenol and t-butylphenol were developed in the 60s which were, at the time, found to be much safer and economical to produce. These triaryl phosphate products were similar to the TCP reaction scheme, using other alkylphenol feedstock to produce phosphate esters analogs.

In the early 60s, Ciba Geigy developed a series of isopropylated phenol based (Reofos) phosphate esters which could be used as a direct substitute for each of these problematic plasticizers. By varying the degree of alkylation, the properties of the resultant analogs could match physical and performance properties of the three cresylic phosphates So, to match the required viscosity range, substituting the isopropylphenol at roughly seventy-five percent would result in a product similar in properties to tricresyl phosphate, at sixty-five percent substitution cresyl diphenyl phosphate and roughly ninety percent substitution is equal to trixylenyl phosphate.(2) These products have also been described as isopropylated triphenyl phosphates (IPPP).

The industry continues to prefer these substituted Isopropylphenyl analogs to the


original cresylics to this day. However, due to recent results disclosed from the European REACH initiative, the health and ecological effects rescreening of these plasticizers have shown greater hazards than previously thought. As a class, the isopropylated triphenyl phosphates have raised their concern to a more severe reproductive, developmental and STOT (specific target organ toxicity) classification. So, for open blending processes such as roll mill operations, the operator is potentially exposed to these products.

Discussion: All the cresylics and isopropylated TPP plasticizers have made a tremendous impact on flame retarded PVC composites. For many wire and cable composites, these flame retardant plasticizers are major contributors to the FR effectiveness of the compounds. Should they be substituted, are there appropriate alternatives to fill their place?

The short answer is yes. Although these two classes of triaryl phosphates have had problems with health and environmental issues, other triaryl phosphate esters do not necessarily share the same problems. An example of this is the analog tertiary butylphenyl diphenyl phosphates (TBPP). This class of plasticizers has not shown reproductive nor organ sensitivity to the extent of isopropylated TPPs and are readily biologically degradable, thus much more environmentally friendly as compared to IPPPs.

Although the t-butylphenyl diphenyl phosphates are not quite as effective as the cresylic and isopropylated TPPs in terms of plasticizing efficiency, they are essentially as FR efficient as any other triaryl phosphate ester. As proof of this, formulations with increasing levels (and varied) of typical FR additives with fifty parts of each phosphate ester plasticizer (TCP, IPPP & TBPP) were measured for flame retardant efficacy. The table (Table 2) below shows virtually the same flammability values in almost every comparison [as measured by LOI (Limited Oxygen Index) and UL-94]. The formulation used for these composites are referenced in Table 1.

In addition to the flammability properties, other composite properties such as tensile and hardness were measured (Table 2). Comparing similarly formulated PVC compounds, the differences or influence from each plasticizer are shown. To quantify the plasticizing efficacy of

the phosphate esters, E-modulus values were determined (composite strain at 100% elongation). Here comparable formulations show, as expected, TCP to be slightly more efficient than isopropylated TPP and TBPP to be slightly less yet.

T-Butyl Analogs Other plasticizers based on the tertiary butylphenol platform may come into play for the flexible vinyl market. One developmental plasticizer (E09-31, ICL-IP) based on this technology shows exceptionally better hydrolytic stability, a common weakness for phosphate esters while maintaining the same high degree of flame resistance.

To measure the hydrolytic stability of these plasticizers, a small quantity of each product was combined in an aqueous solution and placed in a vial subject to accelerated temperature (70C) for up to a month exposure. Two alkylated diphenyl phosphates and another t-butylphenyl diphenyl phosphate were evaluated. Titrated measurements for developed acid number were recorded after each week of exposure (four weeks total). The results show the developmental phosphate ester to be more hydrolytically stable under these conditions (Table 3).

Application areas where stability has greater value may benefit from this property.

Blend Options Most flexible vinyl composites are mixtures of many different components and may (probably) have more than one type of plasticizer. As typical of phosphate ester composites, most compounders formulated their products with enough FR plasticizer to reach the desired specification and balance the required mechanical properties with lower cost plasticizers. Hence the concept of offering blends which have been engineered to give excellent flame retardant properties while also providing for the necessary plasticization. One such blend is Phosflex 375 which is based on the tertiary butylphenyl aryl phosphate esters and has been shown to be an excellent alternative plasticizer for isopropylated TPPs. Using the same formulation scheme shown in Table 1, the following comparisons of flame retardant efficiency is demonstrated in Table 4.


There is very good agreement in most composite formulations using the blended plasticizer Phosflex 375 and the isopropylated phosphates plasticizer.

Alkyl Aryl Phosphates: Another alternative for triaryl phosphates could possibly be alkyl diphenyl phosphates (ADPs). These plasticizers are, in general, good flame retardant plasticizers (not quite as FR efficient as the triaryl phosphates) but possess the property of low smoke generation and low temperature flexibility (almost as good as phthalate esters). For certain applications, low smoke characteristics are often the much harder value to achieve than the respective flammability property. Test protocols like the ASTM E-84 Tunnel test are typical of the difficulty to meet the smoke range but most times, relatively easy to pass the flame spread rating.

There are three types of ADPs (major commercial products) used for flexible FR vinyl compounds;

2-ethylhexyl diphenyl phosphate

Isodecyl diphenyl phosphate

Linear alkyl (c12-c16) diaryl phosphate

All three ADPs display low smoke characteristics better than triaryl phosphates but what separates one from the other is volatility. Figure 2 shows the relative volatility of these plasticizers when held at 190deg C in an isothermal TGA scan. The linear alkyl diphenyl phosphate is clearly superior for volatility characteristics.

Reference to a past technical paper(3) from Monsanto (D. Paul et al) on alkylated diphenyl phosphates speaks on the mechanisms of its flame retardancy. It is suggested to produce a more efficient char growth during combustion due to the early formation of acid esters accelerating the decomposition of the vinyl resin. Further decomposition of the plasticizer forms polyphosphoric acid which helps build the integrity of the char and excludes oxygen from the flame front. In comparison, triaryl phosphates are believed to work more so in the vapor phase hence the differences in smoke characteristics between the two types of phosphate esters.

FR Additives The careful selection of plasticizers constitutes one aspect of controlling flame retardancy; the inclusion of inorganic additives is another route. Antimony trioxide (ATO) is probably the first choice as an FR additive for vinyl composites for many compounders as it is very effective at a low addition rate. Unfortunately, the current price for this product is forcing composite manufacturers to look for more cost-effective alternatives. In addition to flame retardant issues, low smoke performance is becoming an important characteristic for passing local fire codes and regulations.

One choice is a novel borate additive built on a silicate support (FR1120) which has the potential to at least partially replace antimony oxide and also provide low smoke properties in vinyl composites. Like other commercial borates, the contribution from this additive also produces a glassy dome type of surface morphology. However, especially in calorimetry type FR protocols where heat release measurements are important, composites containing this additive seem to form a more effective, uniform and cohesive char layer on the dome surface. This characteristic prevents or lessens the phenomena of fissures or cracking during combustion which releases combustible gases causing excessive heat release spiking. The net effect results in a decreased of heat release values which could help pass the test. (See Figures 3-4)

Comparing ATH/Zn borate (Figure 2) to a similarly formulated mixture ATH/Novel borate additive (Figure 3), a more uniform char layer is produced potentially lessening the chance of fissures developing in the course of the burn test.

FR1120 is a non-zinc containing additive and has shown to be more stable during vinyl compounding scenarios than other zinc containing additives like zinc borate.


Conclusions: As regulations change and new specifications are put into place, this becomes an opportunity to improve formulations and enhance the performance profile of the composites. These alternatives offer other benefits such as enhanced low smoke capabilities, potentially longer service life, all with a more sustainable chemistry to protect our environment and the safety of the consumers using these products. Although slightly less plasticizing, tertiary butylphenyl diphenyl phosphate ester plasticizers can be a major component of highly FR efficient vinyl composites. As for sustainable chemistry, these plasticizers do not share the same repro/tox profile as the isopropylated analogs and other triaryl phosphates.

Similarly, the alkyl diphenyl phosphates are known to have a better health profile than the isopropylated triaryl phosphates and cresylic phosphates previously mentioned. As an alternative technology, ADPs offer unique benefits to the polymer composites with low smoke and low temperature flexibility while still offering enough flame retardancy to pass flame spread protocols. All of these technologies described in this paper can be viable options for safer, more sustainable and efficient vinyl composites.

References: (1) Encyclopedia of PVC 2nd Edition Vol.

4, R. Grossman Marcel Press NY, 1998, Page 2

(2) Reofos 95, Reofos 65, Reofos 50 Triaryl phosphates plasticisers brochure Ciba-Geigy -8/93

(3) A New Phosphate Plasticizer for Low Smoke Wire & Cable Applications, D. Paul, Monsanto Corporation - FRCA Conference Coronado, CA October 1991

Author: Paul Moy

Currently serve as the Market Support Mgr for Plasticizers and Flame Retardants at ICL-Industrial Products based in Ardsley, NY.


Table 1: Formulations for Flexible Suspension PVC at 50 phr Plasticizer Formula Ref. 1 2 3 4 5PVC 100 100 100 100 100CaCO3 50 50 50 50 50Zinc Borate 3 6 3 6Alumina trihydrate 20 40Plasticizers 50 50 50 50 50Epoxidized Soya Oil 3 3 3 3 3Ba/Zn mixed metal stabilizer 5 5 5 5 5

totals: 208 211 214 231 254

Table 2: Flammability and Physical Properties of FR PVC Composites Tensile Properties Hardness LOI UL-94

FR Component

Formula Ref # Additive(s) Strength

E Modulus Elong. Shore "A" O2% 1.6mm

Phr (psi) (psi) % Initial

Creep (15sec.)

100 Mils

TCP 1 50 2200 980 420 93 89 31.2 V0ZB 2 3 2200 1000 420 94 90 32.2 V0ZB 3 6 2200 1040 383 94 89 32.6 V0ZB/ATH 4 3/20 1900 1100 337 94 90 33.4 V0ZB/ATH 5 6/40 1900 1190 335 96 92 36 V0

IPPP 1 50 1940 1128 305 92 88 30.4 V0ZB 2 3 1906 1074 314 92 88 31 V0ZB 3 6 1972 1118 324 92 87 31.6 V0ZB/ATH 4 3/20 1713 1127 286 92 88 32.8 V0ZB/ATH 5 6/40 1543 1170 242 93 90 35.5 V0

TBPP 1 50 2202 1133 362 92 86 31 V0ZB 2 3 2175 1139 363 93 88 31.5 V0ZB 3 6 1949 1162 305 92 87 32.6 V0ZB/ATH 4 3/20 1848 1228 291 93 89 33.6 V0ZB/ATH 5 6/40 1882 1357 286 94 90 36 V0


Table 3 - Hydrolytic Stability of Phosphate Esters Exposure (@ 70C) 1 week 2 weeks 3 weeks 4 weeks

Acid Number Acid Number Acid Number Acid Number Sample ID mg KOH/g mg KOH/g mg KOH/g mg KOH/g ADP-1 0.049 0.062 0.083 0.098 ADP-2 0.028 0.032 0.039 0.046 TBPP 0.094 0.122 0.160 0.196

E09-31 0.016 0.020 0.026 0.029

Table 4: FR Comparison of Phosflex 375 vs. Triaryl Isopropylated TPP LOI (O2%) UL-94

Components Formula # Additive(s) 100 Mils 1.6mm phr

Isopropylated TPP 1 50 30.4 V-0 ZB 2 3 31 V-0 ZB 3 6 31.6 V-0 ZB/ATH 4 3/20 32.8 V-0 ZB/ATH 5 6/40 35.5 V-0

Phosflex 375 1 50 30.5 V-0 ZB 2 3 30.8 V-0 ZB 3 6 31.7 V-0 ZB/ATH 4 3/20 32.5 V-0 ZB/ATH 5 6/40 33.7 V-0


Figure 3: ATH / Zn Borate

Linear alkyl diphenyl phosphate

Isodecyl diphenyl phosphate

2-etylhexyl diphenyl phosphate

Figure 2: Isothermal TGA (190C) of Alkyl Diphenyl Phosphates

Figure 4: ATH / Novel Borate


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