An unusual match: nylon & PVC can go together.

With the right compatibilizer and right processing conditions, nylon can be alloyed with less thermally stable PVC to yield novel engineering alloys.

Technology for producing alloys of nylon and PVC compatibilized with proprietary ethylene terpolymers was recently developed by DuPont Packaging & Industrial Polymers in Wilmington, Del. Such alloys display an attractive combination of properties that include the flame resistance and toughness of lightly plasticized PVC together with the chemical resistance of nylon. These blends also display outstanding abrasion resistance, strength, and processability.

Formulations can vary from soft and flexible elastomers to semirigid grades. Potential applications for this technology, for which licensing is now available, include wire and cable insulation and jacketing, chemical-resistant liners, membranes, and containers.

Since PVC is prone to decompose if exposed to temperatures much above 392 F, suitable nylons for alloying are those that melt at temperatures no higher than 419 F. That category excludes nylon 66, which melts at about 500 F. Of the nylons investigated for PVC alloys, the highest-melting was nylon 6 and the lowest-melting was amorphous nylon. In between are other commercial nylons such as 12, 612, and 1212. This alloying technology has also been demonstrated with two proprietary nylons, which are similar to nylon 66 but with lower melting temperatures. One, designated PPA-160, melts at 320 F, and PPA-180 melts at 356 F.

The compatibilizing agents that are critical to this technology are DuPont's Elvaloy terpolymers of ethylene, carbon monoxide, and acrylate monomers. These products have been offered commercially as polymeric plasticizers for PVC and are designed to be fully miscible with PVC. For use in alloying PVC with nylon, these terpolymers have been functionalized [TABULAR DATA FOR TABLE 1 OMITTED] further by grafting anhydride groups onto the copolymer backbone. The resultant acid-functionalized polymer has the capability to react with base-functional polymers such as nylon. These further modified terpolymers are part of DuPont's Fusabond family of modifiers.

The compounding step

Experiments with a Haake laboratory batch mixer revealed that the best procedure to minimize PVC degradation was to first mix the ethylene-terpolymer modifier with the nylon at a temperature about 50 F above the melting point of the nylon.

This facilitated the reaction between the acid-functional terpolymer and the amine end groups of the nylon. The presence of the low-melting terpolymer also reduced the viscosity of the blend so that the temperature could then be lowered without encountering excessive torque. After the temperature dropped to or below 392 F, the stabilized PVC powder was added and the blend mixed until homogeneous.

Scale-up to a Buss Kneader continuous mixer was straightforward. The nylon and terpolymer were fed into the barrel throat and reacted and homogenized at high temperature. The melt temperature was decreased before the next feed port, which was about halfway down the barrel. The PVC powder was added at this second feed port, and the final blending was completed without letting the melt temperature exceed 392 F for any significant period of time.

TPE alloys with nylon 6

In general, alloys of nylon 6 require a relatively high level of ethylene-terpolymer modifier to avoid decomposition of the PVC. Nylon 6's melting point of 419 F requires compounding temperatures in the region where PVC is unstable.

TABLE 2 - CHEMICAL-RESISTANT PVC/NYLON 12 FORMULATION

                                                   % Retention
Chemical                     Days @ 73 F    Tensile Str.     Elong.

ASTM Fuel C                       7             60             85
Fuel C/Methanol (70/30)           7             65            127
JP-4 Jet Fuel                     7            108            112
Methanol                          7             80            130
20% Sodium Hydroxide              7            101            110
10% Sulfuric Acid                 7             97             99
Acetone/Water (10/90)            30             87            127
Ethylene Chloride/
Water (10/90)                    30            100            104
Chloroform/Water (5/95)          30             71             88
Phenol/Water (10/90)             30             53             97
Cresol/Water (10/90)             30             52             88

Days @ 140 F

10% Hydrochloric Acid            30             82             82
10% Acetic Acid                  30             95            110
10% Sulfuric Acid                30             55             77
Nickel Chloride/
Water (10/90)                    30             93             88

Days @ 212 F

Glycol/Water (50/50)             30             89             95
Water                            30             77             54

Days @ 257 F

ASTM #1 Oil                       7             80             34
ASTM #2 Oil                       7             87             63
ASTM #3 Oil                        7             69             26

After completion of the nylon-modifier reaction, the reduction in melt temperature (necessary to avoid PVC degradation) can result in extremely high melt viscosity. In order to reduce viscosity and facilitate incorporation of the PVC, the ethylene terpolymer is added at a level of about one-half of the final polymer composition. A typical blend (see Table 1, Formula A) has thermoplastic elastomer (TPE) properties and, with the addition of antimony oxide, shows a UL 94V-0 flammability rating.

Tests of chemical and solvent resistance (Table 2) show good to excellent retention ([greater than]50%) of tensile strength in most environments except Fuel C. The latter contains 50% toluene, which is an excellent solvent for PVC. The alloy is attacked by Fuel C because the PVC is plasticized by it. The PVC together with ethylene terpolymer forms the continuous phase and the nylon is the discontinuous phase in these particular blends.

Chemical-resistant grades with nylon 12

Because nylon 12 melts about 95 [degrees] F lower than nylon 6, the mixing temperature can be greatly reduced. Consequently, less ethylene-terpolymer modifier is needed to avoid PVC degradation. This allows incorporation of nylon 12 at levels approaching 50%.

The morphology of the blend, as confirmed by transmission electron microscopy, shows that the nylon and PVC phases are co-continuous. This results in substantial improvements in physical properties (Table 1, Formula B). This blend also displays outstanding resistance to a wide variety of chemicals (Table 2), including simulated aqueous waste streams containing high levels of chemical contaminants. As expected, Fuel C resistance is excellent as a result of increased nylon content in the blend.

A third composition (Table 1, Formula C) was formulated as a typical electrical-jacketing grade displaying toughness, flexibility, and excellent aging resistance.

All of these blends exhibit excellent extrusion and molding processability. In addition, blend B was extrusion blow molded into cylindrical one-liter bottles (see photo on facing page). Production of these bottles was made easy by the composition's exceptional melt strength at 374 F.