Flame-retardant polyolefins don't need halogen.

Flame-Retardant Polyolefins Don't Need Halogen 

Efforts to improve the flame and smoke properties of plastics typically consist of heavily loading the base resin with fillers and additives until the desired level of flame retardance is reached. This approach is not without disadvantages: the physical and mechanical properties of the base polymer are affected, and generally not for the better. Furthermore, systems heavily loaded with halogen-containing materials and metal compounds are coming under increased scrutiny in many industries for safety and environmental reasons.

Stricter regulations on plastics' flame and smoke properties, and hesitation in some markets over use of halogens and certain metal compounds, have spawned a search for alternative methods of controlling flame and smoke production. A unique silicone resin offers one such alternative for polyolefins.

Called SFR-100, it was first introduced in 1984 by GE Silicones, Waterford, N.Y. More recently, the growth of interest in halogen-free flame-retardant systems has prompted considerable new research on formulations to take advantage of this non-traditional approach to flame retardance, which is no doubt unfamiliar to many processors. SFR-100 has thus re-emerged as a halogen-free additive for polyolefins that not only improves flame retardancy but, when properly compounded, provides other benefits:

* Use of the resin in conjunction with typical co-additives can offer improved flame properties at lower additive levels In an alumina tri-hydrate system, an 18-20% reduction in ATH can be realized. A UL 94 Standard V-1 rating can be achieved by using a mixture of 42% polyethylene, 42% ATH, 10.5% SFR-100 and 5.5% magnesium stearate. Without the SFR-100 resins, a compound of 40% polyethylene and 60% ATH is required to achieve the same result. Processors have the option of meeting required ratings using less additives and with less sacrifice to base-resin properties, or achieving higher ratings by using more additives.

* Mechanical, physical and processing properties of the base polymer are actually enhanced. SFR-100 resin typically improves the impact resistance of polyolefins, especially at low temperatures. It improves thermal stability and enhances surface gloss. And perhaps most important, it can improve the meltflow properties of the polyolefin composition, even in a highly loaded formulation.

UNIQUE FUNCTIONALITY

While SFR-100 resin provides many of the same properties as silanes (used to improve coupling of fillers with a base polymer) and dimethyl-silicone fluids (used as flow enhancers, mold-release agents and lubricants), it is a very different polymer, both chemically and functionally.

For instance, SFR-100 resin tends to be incorporated into the polyolefin's base-polymer matrix in a partial crosslinking mechanism believed to yield a structure similar to an interpenetrating polymer network (IPN). Hence, the silicone's mobility is greatly restricted, and under normal conditions it does not migrate toward the surface of a part.

And while the presence of SFR-100 resin in the polyolefin matrix improves surface gloss and enhances color, it does not necessarily improve the part's release properties nor does it affect adhesion of dust or other particles to the part.

Finally, this silicone resin has not been shown to affect the regrind stability of a polyolefin, even though the silicone is believed to act at least partially as a crossliner in the presence of the base polymer. In subsequent processing, the silicone polymer behaves like an inert mineral or glass filler. Moreover, because reprocessing at typical polyolefin temperatures does not alter the physical or chemical composition of SFR-100 resin, it cannot be corrected by subjecting the regrind to subsequent passes if poor dispersion is achieved during compounding.

SFR-100 resin is supplied as a one-component, highly viscous, transparent fluid. Compositions with approximately 10% SFR-100 resin seem to yield, the best overall combination of flame, flow and mechanical properties. SFR-100 is generally used in conjunction with one or more co-additives. Typically, these will be a Group II-A organic metal soap such as magnesium steareate, or alumina trihydrate, or a combination of ammonium polyphosphate and pentaerythritol. These co-additives seem to function as synergists with both the base polymer and SFR-100 resin, not only helping compatibilize the diverse materials, but also contributing to char formation and flame retardance.

Depending on the application's requirements and processor's preferences, SFR-100 resin may be compounded with any number of polyolefin systems, yielding different flame and mechanical properties. Three commercially interesting systems and their unique advantages will be discussed here.

WITH MAGNESIUM STEARATE

When properly compounded, use of SFR-100 resin with around 5 phr magnesium stearate (MgSt) in a high- or low-density polyethylene or polypropylene matrix can perform according to the requirements described in UL Standard 94 as V-1.

Magnesium stearate is a metal soap commonly used as a lubricant in polyolefins. Ordinarily, magnesium stearate is not miscible in PE or PP. However, when properly compounded with SFR-100 resin, the C-18 stearate chain seems to compatibilize the magnesium with the silicone. It is believed that the magnesium-silicone portion of the complex is responsible for char formation, helping reduce flame spread during burning. When a minimum of 7.2% talc is added, dripping is reduced sufficiently to lower the UL Standard 94 rating from V-2 to V-1 dripping.

Figures 1 and 2 show scanning electron micrographs of magnesium stearate with and without SFR-100 resin in LDPE. When the silicone is not present (Fig. 1), dispersion of magnesium stearate in this LDPE matrix is less than ideal as witnessed by the nodules of MgSt that stand apart from the base polymer. However, when SFR-100 resin is added, dispersion in the LDPE improves dramatically. At the same magnification there are no nodules present, and at much higher magnification (Fig. 2), a homogeneous structure is seen. This combination of magnesium stearate and SFR-100 resin also improves the rheology of elastomeric polyolefins.

This particular formulation not only improves the base polymer's flammability without halogens, but enhances processability and impact resistance. Such a compound may be useful for large structural parts such as marine gas tanks, toys, drums, pails and stadium seating.

All commercially available impact modifiers except SFR-100 resin lower the flame retardance of the base polymer. In parts requiring impact as well as flame resistance, a situation results where the addition of more impact modifiers necessitates adding more flame retardants. However, use of SFR-100 resin with a co-additive such as MgSt can improve impact while maintaining or lowering the amount of flame retardant required.

Use of SFR-100 resin and magnesium stearate also offers processing advantages. The compound lets blow molders maintain a high "R-star" value, generally considered indicative of a material with good moldability, while allowing the material to be processed at a lower temperature, viscosity and shear rate. It should also be easier to blow out the material into difficult-to-fill corners when SFR-100 resin is used. This means the final part will have lower stress levels and can be produced at a lower cost since less heat and pressure are required.

The fact that SFR-100 resin improves processability while imparting flame retardance may also prove interesting to manufacturers of PP films and fibers.

WITH COMBINED

SYNERGISTS

Compounding SFR-100 resin in a PP base with ammonium polyphosphate and pentaerythritol (AMP/PTT) mixed in a 26:1 ratio meets the V-0 requirements detailed in UL Standard 94 and provides a good limiting oxygen index value. Mixtures of 17.4% AMP and 5% PTT and 18.4% AMP/4.8% PTT have been tested. Ammonium polyphosphate is a salt commonly used as a solid flame retardant in fire extinguishers and as an additive in intumescent paints. While it has good flame-retardant properties for plastics, its low thermal stability causes it to form bubbles in the melt at polyolefin in the melt at polyolefin processing temperatures. However, when AMP is compounded with SFR-100 resin, its thermal stability is improved by an average of 30-50[degrees]F, allowing processors to take advantage of the salt's flame-retardant properties without sacrificing processability of the base polymer.

Pentaerythritol is a plasticizer commonly used in plastics and paints. In the presence of a flame, it reacts with AMP to produce water and an intumescent char.

Use of this formulation provides non-halogenated flame retardance at lower additive loadings. Fewer additives mean the electrical and mechanical properties of the base resin are not as severely compromised. Formulations with SFR-100 resin process well and can be injection molded or extruded over less costly (and less flame retardant) wire insulation or jacketing material used in such items as smoke detectors and small appliances.

WITH ALUMINA TRIHYDRATE

SFR-100 resin can be compounded with alumina trihydrate (ATH) in a cofunctionalized polyethylene copolymer such as ethylene vinyl acetate (EVA) or other ethylene-copolymer matrices. This formulation provides excellent flame retardance in large-scale-burn and flame-spread tests such as the Steiner Tunnel test (ASTM E-84). However, performance is dependent on the type and grade of ethylene copolymer used; the determining factor being the proportion of vinyl acetate (VA) in the formulation. A copolymer with 12-20% VA yields good results.

Use of SFR-100 resin enables molders to achieve superior flame retardance in a compound with very high filler loading (in excess of 60%) that is still melt processable. Generally, at high ATH loadings, EVA is preferable to straight polyethylene because vinyl acetate, being somewhat sticky and elastomeric, binds better with the filler.

In this compound, use of SFR-100 resin not only increases the flame properties of the copolymer, but enhances processability and allows higher ATH loading. Alternatively, less ATH can be used to achieve required flame retardance while maintaining more of the base polymer's inherent properties. Such a formulation will not be an ideal candidate for parts required to pass the UL 94 flame-spread tests because the EVA component drips when it burns.

An SFR-100/ATH formulation could prove useful to manufacturers of components used in the building and construction industry where flame property performance in large-scale burn tests is more demanding. This compound can either be injection molded or extruded, and it may be useful for wire and cable since it can be extruded over a less costly, less-flame-retardant insulation or jacketing.

PERFORMANCE TEST DATA

Table 1 compares a number of properties for an unfilled PP homopolymer with non-halogenated V-0 and V-1 formulations containing SFR-100 resin and a commercial, halogenated V-0 PP composition.

Because it had been previously discovered that adding MgSt to SFR-100 resin often affected a polyolefin composition's rheology, a number of additional tests were performed comparing SFR-100 systems with and without the organic metal soap. In highly filled ATH systems, adding MgSt with the silicone was helpful since the soap acted as a flow enhancer while maintaining other properties. SFR-100 resin by itself actually reduced flow in the elastomeric EEA/ATH systems. But when MgSt was added, the effect was reversed.

A number of general trends were observed in the melt properties of polyolefin compounds. The rheology of both unfilled and filled PP shows a lowering of viscosity with the addition of SFR-100 resin with or without MgSt. In ATH-filled LDPE, viscosity increases as the amount of SFR-100 resin increases. This trend is reversed if MgSt is also added because the soap functions as a plasticizer.

Figure 3 plots the rheological behavior of unfilled LDPE against compositions containing 60% ATH and increasing amounts of both SFR-100 resin and MgSt. Flow benefits seem to maximize around 10 pph SFR-100 resin/5 pph MgSt, allowing about the same viscosity to be achieved with a highly filled composition as with a neat polyolefin.

Melt index and melt flow (MI/MF) properties increase in a linear fashion with the addition of SFR-100 resin in unfilled polyolefin systems. Yet, in ATH-filled LDPE, SFR-100 resin alone does not increase the MI/MF properties. However, in both filled and neat polyolefins, adding MgSt with the SFR-100 resin increases MI/MF properties (Table 2).

In general, both the limiting oxygen index and UL 94 performance of filled and unfilled polyolefins seems to improve with SFR-100 resin due to the silicone's char-formation characteristics. Again, the specific level of flame retardance achieved will be dependent on the particular combination of polyolefin blends and additives incorporated into the composition.

SFR-100 resin improves the impact strength of both filled and unfilled polyolefins at room temperature and, more important, at low temperatures. This can be attributed to the low glass-transition temperature ([T.sub.g]) characteristic of silicone polymers (-85 to -193 F).

Because of their low [T.sub.g], silicones offer significantly lower temperature capabilities than organic polymers. In addition, the physical and mechanical property profile of a silicone shows little change until the temperature is in close proximity to its [T.sub.g]. Therefore, when a silicone polymer is incorporated into the matrix of an organic polymer as either a copolymer or an additive, it helps improve that composition's low-temperature impact performance. Table 3 compares mechanical properties for highly filled compositions vs. neat LDPE.

Gardner and notched Izod impact properties follow similar trends in polyolefin compositions containing SFR-100 resin. As the level of the silicone increases in rigid PP, Gardner and Izod values also increase because the SFR-100 resin is functioning as an impact modifier.

The level of impact properties increases as the content of the silicone increases, up to approximately 15%. Thereafter, addition of the silicone either has no greater effect or property benefits tend to be reversed, and the silicone has a negative rather than positive effect on specific mechanical and physical properties in the polyolefin composition.

Dynamic mechanical analysis (DMA) data demonstrate an increase in low-temperature mechanical performance. Depending on the formulation tested, either a new [T.sub.g] in the silicone region of the plot appeared; or a single [T.sub.g] lower and broader than that normally associated with the particular polyolefin was observed. In fact, in such cases, SFR-100 resin seemed to be compatibilizing itself within the polyolefin matrix, accounting for the single, lower and broader [T.sub.g] (Fig. 4), and the further improvement in low-temperature impact performance (Fig. 5).

As would be expected, the mechanical properties of elastomeric (EEA-type) polyolefins are affected to a greater extent than those of more rigid polyolefins. But even at high filler loadings, the elastomer tends to maintain more of its flexibility than rigid blends with lower filler levels.

IMPLICATIONS FOR PROCESSORS

Difficult-to-handle formulations can be more readily compounded and molded with the addition of SFR-100 resin. The recommended compounding process is via corotating twin-screw extrusion with specific screw design and geometry. Downstream incorporation of SFR-100 resin (metered by a gear pump) into the polyolefin melt is desirable in all cases. (In batch compounding, SFR-100 should be added at a later stage to the base-resin melt.) ATH should be incorporated at the extruder's vacuum vent.

If SFR-100 resin is added too early in the compounding process it will coat the still-solid resin and additives, causing the mass to become slippery, making it difficult to move the material since the screw has trouble grasping the slippery resin. Consequently, materials will not be properly melted and dispersed, and the mass will begin to roll in the throat of the machine until it eventually builds, bridges and blocks the throat. Therefore, it is essential to melt and mix all solid materials prior to incorporating SFR-100 resin at a later stage.

The presence of the silicone polymer will improve processability by about 10-20%, depending on the formulation and the process. In extrusion and injection molding, the addition of SFR-100 resin allows the use of lower machine torque, melt pressure, injection pressure, and cycle time.

As experience with the materials grows, this silicone resin will likely be used to enhance properties in an even broader family of organic compounds.