Tailored flexible nylon alloys.

Tailored flexible nylon alloys

Increasing performance requirements are being placed on thermoplastic elastomers today as they find opportunities in a broader range of end use applications. Specifiers looking to take advantage of the thermoplastic processing nature of these flexible, rubbery materials are unable, however, to sacrifice previous product capability and are in many cases looking to move to the next performance plateau. Extended use temperature windows, longer operating lives and harsher environments are today all too often the reality the TPE supplier faces.

In response to these sometimes divergent demands, resin producers have turned to techniques of combining different polymers in order to utilize the strengths of the individual components. As a result compounds, blends and alloys are increasingly penetrating the mainstream of candidates being considered for these challenging applications.

Du Pont has developed a proprietary grafting technology (referred to as ETP) that enables us to combine a range of thermoplastic hard segments with rubbery soft segments to produce a family of resins that directly responds to these needs. The initial grades have a polyamide backbone, plasticizer free flexibility, and exhibit the strength and chemical resistance of the nylon hard phase. Excellent low temperature toughness is provided by the dispersed soft phase. A key performance attribute of these alloys is their enhanced retention of physical properties after high temperature aging.

ETP grafting technology

ETP (elastomeric thermoplastic polymers) is a proprietary Du Pont grafting technology (a chemical reaction occurs during compounding) for combining engineering thermoplastics with an acryling rubber soft phase using a polymeric grafting agent. There are, however, three specific conditions which must be met in order for this combination of materials to yield ETP product characteristics. First, the engineering thermoplastic is the minority component (that is, it is less than 50% of the alloy); second, a sufficient level of grafting and mixing must occur in order to ensure that the engineering thermoplastic is the continuous phase; and finally, the grafting must be controlled in order to retain thermoplastic processability.

Operating within these three criteria provides a range of acceptable limits allowing tailorability of physical and rheological properties depending on the needs of the specific application.

Figure 1 shows the effect of the proper level of grafting on the morphology of the alloy. All three materials shown have the same relative amount of nylon and acrylic rubber.

On the left you see a blend without any grafting and the rubber (light color) is the continuous phase with dispersed nylon droplets. The center photo shows a low level of grafting with elongated nylon droplets, but the nylon is still discontinuous. This shows an insufficient level of grafting for the desired morphology. The picture on the right shows the result of a sufficient level of grafting to make the nylon the continuous phase. Since the nylon is the continuous phase, it now dominates the characteristics of the resulting polymer (i.e. melt point, processing conditions, chemical resistance) yet with the flexibility and toughness imparted by the dispersed rubber phase.

A way to illustrate the significance of the ETP morphology is shown in the torsion modulus versus temperature curve shown in figure 2. As the alloy obtains the proper level of grafting it begins to approach the profile of the nylon hard phase. When inadequate grafting is present, the blend parallels the soft phase performance and melts at a much lower temperature. The desired alloy parallels the hard phase but at a much lower modulus range and is therefore a much more flexible version, without the aid of plasticizers.

Thus, we find that an ETP graft is a route to synergistically combine a range of hard and soft phase materials. By altering the components and chemistry of the resulting alloy we are able to tailor products for specific product and process requirements. A key attribute of these plasticizer free flexible plastics is their retention of properties after extended heat aging. Coupling this with their low temperature impact resistance and good environmental exposure capabilities, we have more than just a toughened nylon, we have a high performance TPE.

Since the initial focus of this work was targeted at ever increasing upper temperature requirements, polyamides were selected as the initial hard phase candidate. The first four nylon based grades have been commercialized as Zytel FN flexible nylon alloys, and the remainder of this article will describe some of their characteristics and targeted applications.

Typical FN alloy properties

As shown in table 1, FN grades have hardness from 55-65 Shore D with 50% RH flexural modulus in the 290-450 MPa range. All four grades exhibit good tensile and elongations, have near 1.0 specific gravity, and "no break" notched izods at -29 [degrees] C. The melting points reflect the type of nylon in the backbone, with the lower melting 726 targeted for extrusion and blow molding, while the 714, 716 and 718 are primarily for injection molding applications.

A key performance characteristic of ETP grafting and the FN product line is the unexpected retention of properties (especially toughness and elongation) after high temperature aging. Table 2 compares a typical FN alloy to toughened heat stabilized nylons after aging at either 135 or 150 [degrees] C. Note that FN retains over 60% of its original elongation after 14 days at 150 [degrees] C, while the heat stabilized nylons lose nearly all of their elongation after only a few days. Table 2 also shows that after a commercial toughened nylon is aged at 135 [degrees] C, the flex modulus has increased almost six fold within three days and the tensile impact is 1/3 the original value. In comparison the FN alloy shows virtually no change in flexibility and impact after 14 days. Figure 3 shows the retention of elongation of FN at longer times at various temperatures. FN alloys maintain nearly 100% of their original elongation after 2,000 hours at 125 [degrees] C and over 60% of its original elongation after 2,000 hours at 135 [degrees] C. This unique and dramatic improvement in high temperature aging performance is characteristic of all ETP grafting alloys, regardless of the type of engineering thermoplastic or soft phase used.

Typical FN applications

The combination of a plasticizer free, flexible TPE with good high and low temperature capability, environmental resistance, and good processability on typical thermoplastic equipment is desirable in a rage of commercial applications and has resulted in a variety of evaluations and adoptions. The initial nylon based grades are finding interest in diverse applications such as ski boots, where the gloss and colorability associated with nylon is coupled with the desired balance of stiffness and flexibility provided by this alloy at room and at sub-freezing temperatures.

Injection molded oil and grease seals used in a variety of aggressive lubricants where temperatures exceed 135 [degrees] C is another area of increasing activity. The necessary flexibility to allow adequate sealing along with the ease of fabrication complement the heat resistance performance to make FN a strong candidate.

In extrusion applications, automotive hose and tube and cable jacketing are being exposed to increasingly higher temperatures in under-the-hood applications. In addition, changing chemical exposures require these materials to perform in environments that did not exist a few years ago. A good example of this is in air conditioning hose where the needs for improved flexibility, and retention of elastomeric characteristics at high temperatures is compounded by the changes to more environmentally safe refrigerant types. FN 726 has excellent permeation resistance to the refrigerants used today in auto air conditioners as well as the projected alternatives that will be phased in during the latter part of this decade.

ETP product tailorability

An inherent attribute of ETP grafting technology is the ease WITH which it permits product tailoring to enable performance and processing fine tuning to respond to specific application requirements. Table 3 highlights modifications currently being pursued in response to market feedback obtained during the initial stages of the ETP development.

A key determinant in ETP product performance is the selection of the thermoplastic hard segment which through the grafting process becomes the continuous phase of the end product. Typical of that process is the selection of 1212 nylon as a replacement for ether 6,6 or 6 nylon found in the initial offering. Enhanced resistance to fuels and other chemicals along with significantly reduced moisture sensitivity are features of the nylon 1212 based alloy candidate with an even lower flexural modulus (490 MPa DAM vs. 540 for FN 726). Extruded hose and tube applications are potential targets for this enhancement.

Another option is replacement of the polyamide with a polyester to provide improved flex durability for applications such as shoe soles. The resultant lower density and toughness may offer this performance oriented market a cost effective route to improved durability and longer life.

A growing opportunity in the automotive market is for thermoplastic blow molded parts for use in air handling systems, fluid reservoirs and heat deflectors. These parts provide the auto maker with ease of assembly and one part construction incorporating both flexible and semirigid sections. Adjusting the composition and chemistry of an ETP candidate has enabled us to significantly extend large parison hang time (3-5x) with an associated reduction in melt flow and increase in melt strength for proper blow molding rheology. Parts from two liter to automotive gas tank size are now readily achievable with low parison sag.

The use of high filler loading to enhance selected properties such as flame resistance without a resultant loss in key physical properties is under evaluation. Ability to obtain a UL-94 V-O rating at 1/32" while retaining "super tough" impact performance at -20 [degrees] C, a 200% elongation at room temperature and the ETP type heat aging characteristics are indicative of the capability of a next generation development grade.

Summary

FN alloys are flexible, plasticizer free nylon alloys with good high-temperature aging, low temperature toughness and environmental resistance. They are the first commercial examples of a new generation of flexible plastics resulting from a new grafting technology that allows a wide range of tailorability to optimize product and process performance. [Tabular Data 2 and 3 Omitted]

Table 1 - FN product line

                                  714    716    718    726
Flex. mod. (MPa, 50% RH)          300    365    450    270
Hardness (Shore D)                 55     59     64     58
Tensile strength (MPa)           27.2   29.9   37.9   31.7
Elongation at break               260    250    260    300
Notched izod (-29 [degrees] C)     NB     NB     NB     NB
Density (g/[cm.sup.3])           1.02   1.03   1.04   1.01
Melting point (C)                 262    262    262    225

PHOTO : Figure 1 - transition from blend to ETP graft morphology

PHOTO : Figure 2 - advantage of graft vs. conventional blend technology

PHOTO : Figure 3 - FN property retention after thermal aging