Recycling ETPEs.

In the light of increasing environmental awareness there is growing recognition that recyclability will be a major criterion for material selection.

This article will focus on a case study in material recycling (as opposed to energy or chemical recycling) and aims to give some insight into the value of ETPE recyclate. (In this article ETPE refers to engineering thermoplastic elastomer consisting of a dynamically vulcanized elastomer-thermoplastic blend and TPO refers to thermoplastic olefin consisting of a mechanical elastomer-thermoplastic blend.) The study is believed to be the first of its kind to deal with after-service recycling of engineering polymer parts.

Obviously recyclate value is not the only factor providing the incentive to recycle materials. Other considerations outside the scope of this article include: legislation, total environmental cost, disposal costs, suitable source availability, recyclate quality assurance, dismantling collection and reprocessing costs.

In the field of rubber materials ETPEs have recently seen spectacular growth (largely at the expense of thermoset rubbers) thanks to their thermoplastic processing attributes. Compared to thermoset rubbers the recyclability of ETPEs offers potential cost savings through elimination of production and after-service waste and associated disposal costs. Indeed many ETPE users already benefit from recyclability in production where recovery of the full value of the material is common practice.

The case study was initiated to help answer customer queries about whether ETPE could be recycled separately or with TPO. The objectives of the study, therefore, were to determine the value of ETPE recyclate sourced from a severe service application and to investigate the effect of the recyclate of TPO properties.

Taking into account the industry trend towards fewer material sources and the relative magnitudes of ETPE and TPO volumes then the preferred recycling route for ETPE will probably be through blending with TPO.

Summary

The case study found that after-service automotive steering gear bellows could be reprocessed to yield a recyclate with greater than 80% of the mechanical properties of virgin ETPE material. The study also showed that recyclate dry blended with virgin TPO significantly enhanced certain TPO properties - especially compression set and oil resistance. Blend properties were found to be directly proportional to recyclate content.

The case study

Identification of an after-service part source

To study the value of ETPE recyclate several criteria were drawn up in order to select an appropriate raw material source, namely:

* Part service life preferably more than five,years;

* part subject to demanding service environment;

* feasible part dismantling and recovery.

Bearing in mind the demanding service requirement for the study, then less demanding applications could potentially yield better recyclate material characteristics.

All the above criteria were met in the rack and pinion steering gear bellow. This part, a technical and commercial success since 1983, is not only subject to the car underbody environment externally (road dirt, salt, grit etc.) but also internally to a combination of greases necessary for the functional requirements of the gear.

The part is readily available from a steering gear remanufacturing plant where bellows are currently stripped and discarded prior to remanufacturing the gear. The ETPE material used in this application is Santoprene 103-40.

Part reprocessing steps

Having recovered the parts the following process steps were implemented in order to assess the value of the ETPE recyclate: Cleaning, grinding, blending, pre-drying and injection molding.

Identification of a suitable cleaning technique

Once the parts had been gathered, a cleaning technique had to be developed. In addition to considerable contamination, typically 20% of clean bellow weight, the part geometry further amplified the need for an effective cleaning process. Several part cleaning techniques were evaluated as grinding of the raw part was dismissed through fear of contamination and potential damage of standard thermoplastics processing equipment.

The part cleaning techniques screened included:

* Manual cleaning and various aqueous based cleaning agents (to 60 [degrees] C);

* industrial washing machine and cleaning agent (90 [degrees] C);

* ultrasonic cleaning batches and cleaning agent (60 [degrees] C);

* high pressure hot water and steam (110 [degrees] C @ 100 bar).

The high pressure hot water and steam technique was retained for several reasons:

* No need for an aggressive cleaning agent or solvent;

* Minimal effluent generation;

* cleaning effectiveness: assessed visually and confirmed through comparison of physical properties.

Although the above method proved adequate for the purposes of the case study it should be noted that cleaning was the most difficult processing step.

Identification of a suitable grinding technique

In line with standard converter practice for re-use of production scrap and also a recycling industry reference, a method for granulating the cleaned bellows was selected. Several types of grinding machine worked well.

The type used consisted of three rotating blades, running at 100 rpm over two fixed blades. Feed inlet dimensions: 300 x 150 mm. A mesh size of 8 mm was selected to give a particle size compatible with the injection molding machine feeding needs. Dried and wet material were satisfactorily ground at a rate of 2 kg/min.

Blending

A test matrix was developed to evaluate the blends required to meet the case study objectives. For comparison purposes the TPO based blends used Vistaflex 911B as the TPO material having similar hardness and modulus to the virgin ETPE. A dry blending method was selected for reasons of simplicity. The equipment used consisted of a low speed, rotating two bar mixing blade. Mixing time was five minutes.

Pre-drying

In accordance with standard practice all ETPE containing blends were pre-dried for 3 hrs. at 80 [degrees] C in a circulating air oven.

Injection molding

In order to characterize the blends the materials were injection molded. The injection molding machine used was of 175 T clamp force, toggle closing action, 340 g shot capacity fitted with a 25 1/d thermoplastic processing screw.

Melt temperatures were set to standard conditions for virgin materials at 230 [degrees] C for Vistaflex 911 B and 200 [degrees] C for Santoprene 103-40, and blends set at intermediate temperatures. Cycle time was 50 seconds. The mold used was a two cavity 3 mm plaque mold, temperature controlled to 35 [degrees] C.

Material characterization

Plaques were then die cut to the required standard sample sizes for property testing. Property values were tabulated. Overall the results show that compared to virgin ETPE the recyclate property retention is greater than 80%. Compared to virgin TPO the ETPE recyclate is significantly superior in key property areas such as oil resistance and compression set.

The ETPE recyclate/virgin ETPE dry blend results show that some recyclate properties are slightly lower than virgin material and some are virtually unaffected, such as compression set and retention of properties on aging.

The ETPE recyclate/virgin TPO dry blend demonstrates that property level is proportional to recyclate level and that when blended with TPO, ETPE recyclate enhances especially oil resistance and compression set. Consequently ETPE recyclate could be used as a kind of TPO modifier to meet specific application needs.

When considering the relative magnitudes of ETPE recyclate properties versus virgin TPO and virgin ETPE compression set the ETPE recyclate value is very close to that of virgin ETPE and is superior to that of virgin TPO. In fact TPO has a compression set about one third higher than the ETPE materials.

Likewise, weight gain after oil aging is similar for the, recyclate and virgin ETPE materials, whereas the TPO material showed an inferior, almost six fold, increase in weight.

Furthermore, tensile property comparisons including heat and oil aged data show, in absolute terms the ETPE recyclate has about 10 to 20% lower properties than the virgin ETPE material. However, property retention on aging is virtually identical. Compared to virgin TPO, the ETPE recyclate exhibits better aged properties, especially elongation retention after air aging and all oil aged properties.

Further work

The findings of the study have already highlighted several interesting areas for further work:

* development of a cost effective and environmentally acceptable cleaning process to industrial scale;

* evaluation of reprocessing window;

* additional material characterization to meet specific application needs;

* evaluation of ETPE recyclate blended with PP;

* evaluation of recyclate in other thermoplastic processing technologies (e.g. blow molding).

Conclusions

This study shows that ETPE recyclate has excellent value retention even after severe service history. Furthermore as the after service recycling issue grows in importance ETPEs are not only well positioned as fundamentally recyclable thermoplastic materials but they are also compatible with and can contribute to property enhancement of the TPO volume mainstream.