Melt-processible rubber - an alternative to vulcanized rubber and soft thermoplastic elastomers.

Since its market introduction in the mid-1980s, Alcryn melt-processible rubber (MPR) has been developing a distinctive record of performance-proven applications. Engineers across various industries are improving the quality of their products by cost-effectively converting to the high performance properties and soft feel of MPR. The technology has been so well-received that global MPR usage is growing at a rate of approximately 7% annually.

Alcryn MPR is based on a partially cross-linked halogenated polyolefin alloy compounded with plasticizers and stabilizers (no fillers). The product is a neutral (golden), translucent color and can be pigmented to any desired shade in very bright colors. All grades have unlimited storage stability under normal storage conditions. Alcryn has several registered patents; information is available by calling [phone].

MPR was designed to enable manufacturers to produce rubber pans with high productivity on thermoplastic processing equipment. Years of performance testing against soft thermoplastic elastomers (TPEs) proves conclusively that Alcryn outperforms all soft TPEs in durability, weatherability and environmental resistance, chemical and oil resistance, stress/strain, abrasion resistance, high coefficient of friction, injection-molding cycle times, aging and colorability.

MPR enables manufacturers to cost-effectively produce quality parts with the durable, physical properties and warm feel of rubber, while increasing processing efficiency using standard thermoplastic equipment. Increased consumer demand for durable, high quality products has spurred the strong growth of MPR in industry applications such as:

* Automotive (seals, bumpers, window lace, housings, gaskets, ducts, trims);

* Architectural (weather stripping, window and door seals);

* Consumer (inflatable boats, goggles, sheet goods, athletic equipment, sporting goods);

* Industrial (seals, goggles, couplings, adapters, latches, masks);

* Lawn and garden (irrigation system seals, marine fabrics, lawn mower pans);

* Tools and appliances (soft-touch, over-molded bodies and handles for items such as flashlights, sanders and steam irons);

* Wire and cable (wire insulation, flexible cable, molded connectors).

Performance characteristics and comparative information

Unlike common soft TPEs, which look, feel and wear like plastic, Alcryn offers new options for designing high-quality products with exceptional wear and beauty. MPR outperforms soft TPEs in terms of its durability, tough abrasion resistance, high coefficient of friction and resistance to oil, chemicals, gasoline, ozone, UV light and sunlight. In addition, it colors more brilliantly and with less pigment than most soft TPEs.

MPR outperforms vulcanized rubber in many applications under 120 [degrees] C. Accelerated Weatherometer and outdoor exposure tests of MPR in the abrasive environments of Florida and Arizona predict a long product life without significant deterioration in surface appearance or physical properties.

Table 1 offers a good reference when evaluating and comparing specific properties of TPEs to MPR.

Table 1 - property comparison of MPR to TPE

Property                                       TPE             MPR
Typical specific gravity                   0.9-1.0         1.1-1.3
Hardness, Shore ND                         35A-50D         45A-80A
Abrasion resistance                      Fair-Good       Excellent
Low temperature limit,
 [degrees] C                            -60 to -40      -80 to -40
High temperature limit,
  cont./service, [degrees] C                   135             120
Hydrocarbon oil
  resistance                        Good-Excellent       Excellent
Aqueous fluid resistance                 Excellent       Excellent
Weathering (UV)                     Good-Excellent       Excellent
Ozone                               Good-Excellent       Excellent
Gas permeability                              Fair            Good
Processing cycle time                         Good       Excellent
Recyclable                                     Yes             Yes
Cost $/lb. (T/L)                         1.65-2.50       2.40-3.50

MPR has an impressive roster of successful applications, including a ten-year track record in environmental durability and weathering as perimeter seals for OEM automotive windshields (windshield lace) and backlights. Automakers worldwide use MPR in window lace trim to reduce squeaks and rattles caused by front and rear windows rubbing against the lace. It has been used in this capacity on millions of cars built by virtually every major American and Japanese car manufacturer with no known service failures or need for replacement.

This application has been so successful because MPR's itch-noise performance in windshield lace is two to five times better than any alternative flexible sealing materials. By combining flexibility and energy absorption to reduce noise and vibration, Alcryn MPR sets the industry performance standard when itch noise is an issue.

Other automotive applications include outer-belt strip and reveal moldings, door-latch seals, fuse holders, fuel filler gaskets and seat belt guides, and hinge bumpers. MPR delivers noise reduction, excellent weatherability for exteriors, excellent sealing, low-temperature flexibility, energy absorption capability, resistance to wax and grease, and the ability to be recycled. And, it can be reground and preprocessed as a similar-use product.

Since introduced, MPR has been improved to withstand rigorous and demanding applications. Flame-retarding additives have been designed for blending prior to processing to achieve any level of flame-retardance required. Pellet forms can be tumble blended with MPR pellets and incorporated during normal processing. The additives can be used with both extrusion grades and injection molding grades. Also, Alcryn has Yellow Card listings for various Alcryn grades to provide superior electrical properties and is UL-approved (UL94) for applications in components and parts of end-product devices such as cords, appliances, antennas, etc.

Comparison test results

When a product application requires the look, feel and properties of rubber, manufacturers consider softness and flexibility (low stiffness), and determine if the material shows elastic recovery from deformation. Stress-strain comparison tests show that soft TPEs fail to meet the performance properties of rubber. In tests of elasticity, mechanical resiliency, wear and frictional properties, only MPR exhibits the linear stress-strain performance characteristics that provide the outstanding flexibility and the true soft feel and appearance of a quality rubber. Alcryn recovers elastically from 100% strain and demonstrates resistance to creep in tension equal to that of cured rubber.

Stress-strain properties are useful indices of flexibility and elasticity when comparing MPR, TPEs and conventional thermoset rubber (TSE). The slope of the curve is a measure of flexibility or stiffness of a material. This curve (figure 1) is for a very flexible (low stiffness) material - the tensile stress at 100% strain is around 800 PSI. Note that the graph is a straight line, which indicates the material will elastically recover from elongations as high as 100%. This curve represents the stress-strain for a 70A-hardness vulcanized nitrile rubber compound.

[Figure 1 ILLUSTRATION OMITTED]

Figure 2 compares the stress-strain curves of a 70A-hardness nitrile rubber compound and a 70A-hardness thermoplastic vulcanizate (TPV). Keeping in mind that MPR maintains the performance properties of nitrile rubber, note that the TPV has a different stress-strain curve than rubber. The slope is much higher. At 25% strain, the TPV has a tensile stress about three times as high as rubber. Hence, at equal hardness, we should expect TPVs to feel much stiffer than rubber and MPR.

[Figure 2 ILLUSTRATION OMITTED]

In addition to being stiffer, TPVs are not as elastic as nitrile rubber or MPR. Typical TPVs display a "plastic-like" yield at around 35% strain. From these data, one can predict that switching a 70A rubber part to a 70A TPV would cause many users to ask for something softer and more elastic.

Figure 3 compares stress-strain data for 70A hardness MPR with 70A nitrile and polychloroprene vulcanized rubber compounds.

[Figure 3 ILLUSTRATION OMITTED]

Note that the slope of all three curves is very similar - MPR has the same flexibility (stiffness) as nitrile. The stress-strain behavior of MPR is linear with no evidence of a "plastic yield" point like TPEs, and MPR recovers elastically from 100% strain.

When tested against soft TPEs and vulcanized rubber, MPR demonstrates faster injection-molding cycle times, superior dimensional stability and chemical resistance, and long-term weatherability. MPR recovers elastically from 100% strain and demonstrates resistance to creep in tension equal to that of cured rubber. Alcryn has also proved that it colors more brilliantly using less pigment than most soft TPEs.

Summary

Because MPR combines the dynamic properties of rubber with the clean, easy processing associated with plastics, product manufacturers and part fabricators gain a marketing advantage by improving the customer's product quality and simplifying the manufacturing process with MPR technology. Alcryn can be extruded, injection-molded, blow-molded, calendered, vacuum-formed, over-molded on rigid substrates, and co-extruded with PVC. It also bonds well with TPUs, copolyesters and PC/ABS alloys without adhesives. MPR does not require vulcanization, pre-drying or any compounding prior to processing, and scrap can be recycled over and over with no detrimental effect on performance or appearance.

Manufacturers have identified Alcryn MPR as a unique, cost-effective alternative to vulcanized rubber and soft TPEs. With MPR technology, manufacturers can achieve new quality characteristics by adding the soft feel and high-performance properties of rubber while controlling manufacturing costs.

Dave Santoleri is vice president of Advance Polymer Alloys responsible for global marketing and sales. Prior to the founding of APA he worked for Du Pont. Jeff Armour is technical manager of APA where he is involved with application development.