Low extractable lead stabilizers.

Lead-containing additives have been used in a variety of rubber compounds and in rubber-plastic blends for many years. In some halogenated polymers, such as chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSM), polychloroprene (CR) and epichlorohydrin (ECO), additives such as litharge, red lead, dibasic lead phthalate and dibasic lead phosphate are used to scavenge HCI produced in crosslinking, as well as from degradation during processing (ref. 1). In some of these cases, the metal may participate in crosslink formation. With lead-based stabilizers, the result is typically a vulcanizate with greater water and chemical resistance than if a light metal, which forms soluble halide salts, were used instead.

In other cases, the same additive may be used solely for its function in metal oxide crosslinking, as in ethylene-acrylic elastomer (ref. 2), or, instead, entirely as an HX scavenger, as in certain fluoroelastomer compositions (ref. 3). In hydrocarbon polymers, such as EPM or EPDM, lead stabilizers are used to scavenge chloride, derived from residual catalyst, in cases where high water resistance is required, such as in medium voltage electrical insulation (ref. 4). The same considerations motivated the use of litharge in place of zinc oxide in SBR insulations for use in wet locations (ref. 5).

The production of lead-containing additives involves sophisticated engineering and design to prevent worker exposure or contamination of the environment. Summaries of necessary precautions in handling lead compounds have been published extensively (ref. 6). In the rubber industry, lead-based additives are typically used in the form of non-dusting dispersions, or in low melt inclusion bags. Because of their low solubility, extraction or migration of these additives, once mixed into a polymer, is very low, particularly in comparison to salts of light metals (ref. 7). Nevertheless, concern over the toxicity of lead stabilizers is real and has given rise to development of modified stabilizers having even lower extractability.

Dibasic lead phthalate

Except for litharge and red lead, dibasic lead phthalate is the lead stabilizer most widely used in elastomers. The most common reaction product of dibasic lead phthalate in scavenging chloride is the dichloro derivative (ref. 8). This product is soluble in water at 20 [degrees] C to the extent of somewhat less than 1 ppt, that is, about three orders of magnitude less soluble than Pb[Cl.sub.2]. This is also an order of magnitude lower solubility than basic lead chloride, the most common reaction product of stabilization with lead oxides. Replacement of either litharge or red lead with dibasic lead phthalate is, therefore, strongly in the direction of reduced extractability by aqueous media.

We now report development of an additive that converts the chloride reaction product, dichlorodibasic lead phthalate, into a complex, extremely insoluble salt. When combined with dibasic lead phthalate, this additive yields a reaction product with HCI having water solubility in the range of 10 ppm. A consequence of this insolubility is that the blend (available under the designation Halstab X-1129) was found to have an acute oral LD50 in rats of greater than 5,000 mg/kg (ref. 9). This is the first report of a lead stabilizer being classified in EPA Toxicity Category IV (formerly known as nontoxic). In addition, no abnormalities were noted in pathology of the test animals.

The following application areas are suggested:

* EPM/EPDM - Replacement of the customary 5 phr of either red lead or dibasic lead phthalate used to scavenge chloride in electrical insulation intended for wet locations. Red lead is typically used in black or dark colors; dibasic lead phthalate, where compound color is significant. Stabilization with dibasic lead phthalate provides slightly better retention of volume resistivity, as shown in figure 1, probably because of the lower water solubility of dichlorodibasic lead phthalate as compared to basic lead chloride. It is likely that the X-1129 blend could be dropped in for red lead, part for part. Again, because of lower solubility of reaction products, it is possible that a direct replacement could be made for pure dibasic lead phthalate.

[Figure 1 ILLUSTRATION OMITTED]

Volume resistivity testing in 75 [degrees] C water of EPDM samples stabilized with X-1129 generated lead levels in the water tank in the ppb range. The loss of lead was in the range of 0.0001% of the total present. This is similar to the magnitude of migratory loss previously reported (ref. 7).

* Polychloroprene - Replacement of the red lead cure in CR with X-1129 provides not only a chloride reaction product orders of magnitude lower in solubility, but also lends improved scorch safety. Poor scorch safety has been a major drawback to the red lead cure. Replacement of red lead in a standard, unmodified CR jacket illustrates this (table 1).

Table 1

                                       A        B
Neoprene W                            100      100
Antioxidant                            4        4
SRF black                              20      20
Hard clay                              60      60
Process oil                            15      15
Paraffin wax                           3        3
Sulfur                                0.75    0.75
TMTM                                   1        1
Red lead                               20
X-1129                                         12

MS 121 [degrees] C, 2 pt. rise         8     >20 min
Press cure 10 min., 160 [degrees] C
Tensile strength, MPa                 15.6    15.4
Elongation, %                         680      650
Hardness, A                            65      67

Dibasic lead phosphite

Dibasic lead phosphite is commonly used as a partial or total replacement for the corresponding phthalate where increased light stability or weatherability is needed, or where the combination of acid scavenging and a high temperature antioxidant is called for. Thus, ECO copolymer commonly uses a mixture of dibasic lead phthalate and phosphite, even when black filled. Similarly, water-resistant FKM compounds, although black filled, have made use of dibasic lead phosphite as an acid scavenger, in place of magnesium oxide.

The same additive can also be blended with dibasic lead phosphite, in this case as X-1179. A summary of the available lead stabilizers is shown in table 2.

Table 2

Lead stabilizer          Low extractable blend
Dibasic lead phthalate          X-1129
Dibasic lead phosphite          X-1179
Tribasic lead sulfate           X-1132
Litharge                        X-1141
Red lead                        X-1185

Tribasic lead sulfate

The use of tribasic lead sulfate as a stabilizer is limited to polar polymers. It is the largest volume stabilizer used worldwide in polyvinyl chloride (PVC). X-1132 modified tribasic lead sulfate is suggested for use in elastomeric PVC/NBR cable jackets where low lead extractability from scrap is needed, at levels of 4-6 parts per 100 of PVC in the blend.

Litharge/red lead

The compounds, shown in table 3, were tested for lead extractability via the EPA acetic acid extraction referred to as the Toxic Characteristic Leaching Procedure (TCLP). These compounds also contained an antioxidant and a peroxide crosslinking agent. After vulcanization, compound A yielded TCLP extraction values of 20-30 mg/l lead (ref. 10). Compound B gave extraction values of 2-3 mg/1. This ten fold improvement could enable many carefully formulated CPE or CSM compounds to qualify as non-hazardous waste when scrap disposal or transport is required. A similar gain in decreased extractability is obtained on substitution of X-1185 for pure red lead. The magnitude of the improvement depends on the quantity of lead oxide in the compound. For example, a CSM jacket containing 44 phr litharge exhibited TCLP values around 100 mg/l. Replacement of litharge with X-1141 led to a TCLP value of 8 mg/1. Although this exceeds the 5 mg/l needed for certification as nonhazardous waste, recompounding to lower levels of stabilizer or replacement with X-1129 dibasic lead phthalate could overcome this. In another example, a CSM jacket containing 25 phr red lead gave TCLP values in the range of 10-22 mg/1. Replacement with X-1185 yielded values of 3-4 mg/1.

Table 3

                         A     B

CPE, 36% CI             100   100
Calcined clay           60    60
CaCO3                   40    40
Ester plasticizer       20    20
Litharge                12    -
X-1141 litharge blend   -     12

Effects on recycling

At issue is a "Catch-22" situation: The most desirable means of disposal of compositions containing lead (or other regulated substance) is recycling. After fabrication, and especially after vulcanization, this is best done at specialized sites, engineered to preclude worker exposure or leakage. The added cost of transport of hazardous waste, however, works against this alternative. Formulation so as to qualify as nonhazardous waste, therefore, broadens

disposal choices.

Conclusions

In most rubber compounds, extractability of lead-based additives can be reduced to very low levels through two changes in formulation:

* Replacement of litharge or red lead with lower solubility stabilizers such as dibasic lead phthalate, dibasic lead phosphite or tribasic lead sulfate.

* Use instead of blends of these stabilizers with now available additives that form extremely insoluble reaction products.

References

(1.) For example, The Vanderbilt Rubber Handbook, 13th Ed., 163, 236, R.T. Vanderbilt Co., Inc., Norwalk, CT, 1990.

(2.) DuPont Materials For Wire & Cable, Compound EA-5.

(3.) The Mixing of Rubber, 205, Chapman & Hall, New York, 1997.

(4.) DuPont Bulletin H-00131, 1987.

(5.) Ref 3, p. 134.

(6.) For example, Safety in Handling Lead Compounds, in

Richard Grossman is Technical Director with Halstab, a supplier of lead-based and other heat stabilizers. He began his career in the rubber industry in 1957 with Anaconda Wire. He has also work for Polysar, Synthetic Products, Cooke Color and Polymer Services.3