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New flexible thermoplastic alloys achieve high-level synergism.

New Flexible Thermoplastic Alloys Achieve High-Level Synergism

Polymer alloys provide a versatile means of obtaining unique, readily tailored materials. But because of nearly inherent incompatibility between dissimilar polymers, simple blending typically does not lead to the desired combination of properties. A proprietary grafting technology developed by Du Pont has led to a new family of flexible thermoplastics that synergistically combine functionalized engineering thermoplastics, such as polyamides, with functionalized tough, flexible polymers, such as acrylic rubbers.

In general, this technology maintains many of the characteristics of the engineering thermoplastic (which remains the continuous phase), such as strength, the ability to withstand high temperature excursions (over 200 [Degrees] C), and resistance to a wide variety of chemicals (such as automotive fluids, refrigerants, and hot aqueous fluids). The dispersed soft phase provides these alloys with outstanding low temperature toughness and flexibility without plasticizers. The high temperature aging performance of the alloys is superior to that of the base engineering thermoplastic. This unique combination of properties places the alloys among the premium flexible thermoplastics, such as TPUs and flexible nylons.

Developmental materials that have emerged from this technology are named elastomeric thermoplastic polymers (ETP). This article describes the initial candidates for customer testing and products very recently commercialized under the Zytel FN trademark - categorized as flexible, plasticizer-free nylon alloys. Their high temperature aging performance is detailed. Further, these materials can be easily modified to function in a wide variety of thermoplastic fabrication methods; injection molding, extrusion, and blowmolding grades are here characterized.

Morphology

The grafting technology was presented in detail at ANTEC '88 by R. Saltman and B. Varnell. The grafting reaction takes place in a way that ensures that the engineering thermoplastic is the continuous phase despite its being the minority component. (In Fig. 1, a TEM micrograph of a typical alloy, the dark, continuous phase is the polyamide.) Therefore, the engineering plastic dominates the physical characteristics of the material and its processability - the materials are processed under typical nylon operating conditions. The soft acrylic-rubber phase, dispersed in the engineering polymer matrix, provides low temperature toughness and flexibility without the aid of a plasticizer.

High Temperature Aging

A unique characteristic of the ETP grafting technology is the surprising retention of properties (especially toughness and elongation) after high temperature aging. In Fig. 2, a comparison of a representative ETP alloy to a toughened, heat stabilized nylon in retention of elongation (an indication of toughness) after aging at 150 [Degrees] C, the ETP is shown to maintain over 60% of its original elongation after 14 days while the nylon loses nearly all of its elongation after only a few days. At longer times and at various temperatures (Fig. 3), the ETP alloy maintains nearly 100% of its original elongation after 2000 hrs at 125 [Degrees] C, and over 50% after 2000 hrs at 135 [Degrees] C or 1000 hrs at 150 [degrees] C.

Other indications of maintenance of integrity with high temperature exposure are changes in low temperature impact and flexibility. In Fig. 4, the tensile impact and notched Izod impact at -20 [degrees] C for a commercial, toughened nylon are shown to drop over 50% after aging at 135 [degrees] C for 3 days while the values for the ETP are virtually unchanged after 14 days. In Fig. 5, the flexural modulus of the nylon is seen to increase from 274 MPa to over 1370 MPa after 3 days' aging at 135 [Degrees] C - a drastic change. Again, the ETP is virtually unchanged after 14 days at 135 [Degrees] C. This unique and dramatic improvement in high temperature aging performance is characteristic of all ETP grafted alloys, regardless of the type of engineering thermoplastic or soft phase used.

Tailorability for Market

Applications

An important feature of the ETP grafting technology is the ability to tailor the alloys to optimize processability for various thermoplastic fabrication methods without significantly affecting their characteristic physical properties. ETP alloys have found applications in injection molding, extrusion, and blowmolding. In all cases, modifications to the alloy were made to obtain optimal performance. Specific examples follow.

Injection Molding. The initial ETP alloys for customer testing were based on a high melting nylon ([T.sub.m] = 265 [degrees] C) targeted for applications requiring high temperature resistance, low temperature toughness, and engineering thermoplastic performance at a much lower flex modulus. These candidates - three compositions with a flex modulus range of 275 to 450 MPa at 50% RH - were enthusiastically received by customers for several injection molding applications, such as pipe clamps, oil filters, fasteners, and grease seals. In an application for injection molded ski boots, the ETP alloy met all requirements except for surface gloss. Minor alterations in the formula and optimization of molding conditions doubled the 20-degree gloss index from 40 to 80, and ETP ski boots are now ready for commercialization.

Extrusion. Early evaluation of ETP alloys has identified excellent resistance to Freon (Du Pont) refrigerant permeation, making ETP a strong candidate for automotive air conditioning hose. The alloys based on high melting nylon have limited applicability in extrusion because of a narrow processing window. Replacement of the high melting nylon with a lower melting nylon ([T.sub.m] = 225 [Degrees] C) significantly broadened the processing window without altering the permeation performance. The lower melting alloy, ETP 65, maintains the high temperature aging and low temperature toughness characteristics of the initial ETP alloys, thus demonstrating the forgiving nature of ETP technology. The permeation data, given in Table 1, show a significant improvement in permeation loss for ETP 65 compared with two commercially used hose materials - thermoset rubber and plasticized nylon - for both the current automotive refrigerant, Freon 12, and an environmentally safe replacement candidate, Freon 134a. [Tabular Data Omitted]

Blowmolding. A study has revealed significant opportunities for ETP alloys in blowmolding applications for automotive markets, especially for large under-the-hood parts. While ETP 65 is acceptable for making small parts ([is less than] 2 liters), it has inadequate melt strength for processing large parts ([is greater than or equal to] 1-meter parisons). The ETP grafting technology again proved very versatile in obtaining a material with acceptable melt strength for blowmolding while maintaining all characteristic physical properties. The only important difference between ETP 65 (see Table 3 for its properties) and ETP Blow Molding Resin (BMR) is that the melt flow has been reduced from 6.5 g/ 10 min (240 [Degrees] C, 2.16 kg) for ETP 65 to 0.4 g/10 min for ETP BMR - indicative of a significant increase in the low-shear viscosity required for blowmolding. ETP BMR is being evaluated for air ducts, heat shields, and fuel tanks in automotive market applications, and also for applications in other markets such as packaging. [Tabular Data Omitted]

Commercial Products

Four polyamide-based compositions have been released under the trade name Zytel FN, flexible nylon alloy. Their typical physical properties are show in Table 2. The higher melting grades are predominantly utilized for injection molding applications. The lower melting Zytel FN 726 is the same as ETP 65 and is utilized in extrusion and blowmolding applications.

Table : TABLE 2. Properties of Commercial Nylon-Based Alloys.
 Zytel FN
Property 714 716 718 726
Melting point, [Degree] C 262 262 262 225
Density, g/cc 1.02 1.03 1.04 1.01
Flex modulus, MPa, 50% RH 290 365 448 269
Tensile strength, MPa 28 30 38 32
Elongation at break, % 260 250 260 300
Hardness, Shore D 55 59 64 58
Notched Izod, -29 [Degrees] C NB NB NB NB


Nylon 1212-Based Alloys

While the ETP technology is relatively generic and can be used with a vast array of polymer combinations, compositions based on Du Pont Canada's nylon 1212 are particularly intriguing because nylon 1212 offers better dimensional stability (very low moisture uptake) and lower flex modulus (about 1/2, DAM) than nylon 6/6 or nylon 6. The properties of pure nylon 1212, two candidate nylon 1212-based ETP alloys (ETP-1 and ETP-2), and ETP 65 are compared in Table 3. Note the dramatic improvement in retention of properties after aging at 150 [Degrees] C and 165 [Degrees] C of all ETP alloys compared with nylon 1212 alone, again demonstrating this fundamental characteristic of the ETP grafting technology.

Comparison of the two nylon 1212 ETP alloys with ETP 65 has led to some important observations. Nylon 1212 ETP-1 has the necessary improvement in Fuel C/Methanol swell (16.7% swell) to qualify for applications with gasoline exposure. However, use of a different soft segment to achieve this level of chemical resistance has resulted in a reduction in low temperature toughness, as indicated by the change in notched Izod. If no-break notched Izod at -40 [Degrees] C is required, nylon 1212 ETP-2 has the appropriate toughness, but resistance to Fuel C/Methanol is sacrificed.

Both nylon 1212 ETP alloys have excellent moisture resistance (low water swell) and better heat aging than ETP 65, as indicated by retention of properties at 165 [Degrees] C. These alloys maintain the tensile strength and flexibility without plasticizer that is characteristic of ETP alloys. Also, the same modifications as those discussed previously can be made with nylon 1212 EPT alloys to optimize for processability over the range of thermoplastic fabrication methods.

PHOTO : FIGURE 1. TEM micrograph (4640X) of nylon/acrylic rubber alloy shows the dark nylon as the continuous phase although it is the minor component.

PHOTO : FIGURE 2. The ETP retains 60% of its elongation after 14 days aging at 150 [Degrees] C while the nylon loses 80% after only 3 days.

PHOTO : FIGURE 3. Property retention after long-term thermal aging is a fundamental characteristic of ETP alloys.

PHOTO : FIGURE 4. The nylon impact strength drops 50% after 3 days' aging at 135 [Degrees] C. The ETP is virtually unaffected.

PHOTO : FIGURE 5. The ETP maintains its flexibility on aging at 135 [Degrees] C while that of the nylon is drastically reduced.
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Author:Katsaros, J.D.; Grimes, D.G.
Publication:Plastics Engineering
Date:Aug 1, 1990
Words:1644
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