Cast polyurethane in materials handling

With increasing production demands, more competitive markets, labour costs and general safety awareness, the extractive industries have researched material technologies to satisfy these needs. Metals, steels and rubbers were selected due to their abundance and recognised performance properties around wear and reliability. Industrial chemists and engineers are exploring new ways of engineering materials to substitute for improved efficiencies. Polyurethane is one of these substitutes, providing an increased life span compared to steel and rubber. With the use of case studies, this paper explains how polyurethane’s resilient properties – albeit with limitation – allow it to excel in harsh environments. This is particularly evident when it is combined with materials of high abrasion resistance yet brittle properties.

{{image2-a:r-w:200}}The extractive industries have examined different materials and innovations for a variety of reasons. Systems and newly invented materials have been developed and adapted to improve production and safety, decrease labour and breakdowns, and competitively reduce costs and maximise profits.

Metals, steels and rubber have contributed to the foundations of our society and industries today. Co-existing with the development of metals and steels, industrial chemists and engineers in progressive countries have been able to break down and engineer the internal structure of polymers and develop application specific composite materials.

Notable industries that have benefited from these advancements include:

The automotive industry, with its use of polymers and composites replacing steel car bodies. This improves fuel efficiency by reducing the car’s weight, and its safety rating by incorporating a crumple zone.
The aerospace industry, with its use of engineered polymers and composites. These provide freedom in design, improve fuel economy and reduce areas susceptible to corrosion.
The Department of Defence, which has incorporated engineered polymers to improve mobility, ergonomics and the survival of its infantry.

This paper outlines how the utilisation of polyurethane will enhance the bulk materials handling industry, in reference to asset containment, chute lining and conveyor belt cleaning.

Background

Researchers have estimated that the first minerals were mined in the Aswan Period, using stone, bronze and iron. Egyptology researchers have estimated that the Great Pyramids took between 10 and 20 years of hard labour, with an average workforce of 14,657 people (maximum of 40,000) and an estimated lump total of 5.9 million tonnes of limestone and granite moved. Presently, this sort of tonnage would be almost trivial for an average quarry – operating at 1.2 million tonnes per annum – to achieve in only a few years, with a workforce of up to 30 people.

It is believed that hand-driven conveyors were first developed in the late 1700s, designed of leather and wood. One hundred years later came the conveyor belt, made from a combination of rubber, leather and canvas, transporting coal, gravels and other ores. Winn3, Walker4 and the Merriam-Webster Collegiate Encyclopaedia5 recognise different inventors, including Thomas Robins, Henry Ford (founder of Ford Motor Company), Richard Sudcliffe, Henry Bessemer and the Swedish company Sandvik (famous for launching the Bessemer method for industrial scale steel production), for this new technology improving the bulk materials handling industry. Progress utilising steels and alloys has been another key component of the bulk materials handling industry, with improvements from raw materials such as tin and bronze to alloy engineered and cast type materials such as NiHard, Bisalloy and chromium carbide.

Polyurethanes

Following the incorporation of natural and synthetic rubbers, Clemitson and Prisacariu recognise that Dr Otto Bayer and his team developed polyurethanes in 1937 as part of the movement to formulate synthetic materials – a cheaper, more abundant replacement for natural rubber in the Second World War. Rubbers had and have been used in the bulk materials handling industry for many years.

The manufacturing and production processes for polyurethanes vary immensely. Along with the materials’ infancy and introduction to the world, advances in technology have allowed the manufacturing process to be refined and developed. These developments, according to laboratory testing, result in a variety of performance differences between the old and new generations.

Chemical manufacturing company Chemtura Corporation has indicated that polyurethanes have been used in a variety of applications in the bulk materials handling industry. The material is developed by an exothermic reaction between alcohols of varying hydroxyl group molecules, poly-isocyanate and polyether or polyester polyols.

Polyurethanes have been used over time in applications such as screening media, seals, elevator buckets, pulley lagging and conveyor belt cleaners, usually for areas where material abrasion is an issue.

Like rubbers, polyurethanes are manufactured and produced to fit a specific range of durometers, measured to identify the hardness of the material. Regardless of the durometer, polyurethanes are generally divided into two different classes, ether and ester, which assists with positioning them in an optimal environment. Ether polyurethanes handle higher impact, whereas ester polyurethanes operate better under sliding abrasion, with a greater tear strength.

In 2012 Charles Pratt of Kinder Australia introduced a new generation polyurethane conveyor skirting to the Australian bulk materials handling industry after hearing of its successes overseas. According to Pratt’s white paper, this was due to its decreased co-efficient of friction and high wear comparison against a standard styrene butadiene rubber (SBR). Lab and practical testing provided results in which the polyurethane exceeded the rubber’s performance by eight to 10 times, depending on the asset material present and its installation environment, with the added benefit of zero minimal conveyor belt damage compared with the previously installed rubber skirting.

With this new discovery of improved wear life, polyurethane was installed in a variety of applications, not only replacing standard rubbers but as a comparison retrofit against abrasion resistant steels and in environments where impact was another source of this issue.

Unlike rigid materials such as steels and ceramics, polyurethane offers a material property described as rebound resilience, according to ASTM International’s D7121-05 (standard test method for rubber property, resilience using Schob type rebound pendulum) – a test commonly conducted on rubbers, designed to measure the ratio of energy released in deformation recovery to the energy that caused the deformation.

Understanding this principle is key in the application exploration of this material, as it functions very differently from other previously recognised wear materials. Unlike steels and ceramics, which convert almost 100 per cent of the impact energy – due to their stiffness properties – into abrasion wear and deflect the product (or crack/shatter in some cases when the impact force is too great), polyurethanes absorb the supplied potential and kinetic energies and momentum of the material, before deflecting it. Under most circumstances this results in less wear.

US-based polyurethane manufacturer Argonics tested and evaluated the rebound resilience over several different polyurethane durometers, and the results are outlined in Table 1.

From this experiment, it was demonstrated that the even (Argonics defined as) durometers have a higher rebound resilience than the odd (Argonics defined as) durometers.

It was suggested that the ether polyurethane handled large impacts, although the larger rock offset this, compromising the material due to its weaker cut and tear resistance. Case studies conducted at ACT and Victorian quarries later confirm and demonstrate this theory, based on combined use of the low and high durometer polyurethanes.

Polyurethane case studies

Case study 1

In 2013 a concrete plant for one of the world’s leading heavy building materials companies – producing aggregates including stone, gravel and sand to premixed concrete used on residential, commercial, civil or industrial projects – trialled polyurethane.

The site used an aggregate weigh hopper constructed of sheet steel, handling aggregate material of 20mm, 14mm, 10mm, 7mm and sand.

Steel was typically used in hopper construction due to its robust properties, and for this application it needed to withstand high volumes of highly abrasive materials.

The position of the hopper at the Brooklyn site meant the aggregate and sand was dropped from several upper levels down into the hopper, from a two to three metre impact height, causing a high level of abrasion to the internal steel surface. As a result, the product was being lost through cracks and holes that were developing. Steel patches had been welded in as a “quick fix”. However, the appearance of abrasion damage was becoming more frequent, resulting in plant shutdowns. This was significantly accumulating labour costs, due to the need for specialised maintenance with poor accessibility and confined space requirements.

Over three years, this application was regularly monitored. The site made several observations during and after the installation that served as additional benefits. It was suggested that the polyurethane was lightweight and easy to work with. Another noticeable observation was that the hopper experienced a significant reduction in noise.

Table 1 shows all the different types of steels/alloys that were used to line and patch the hopper over an unknown period of time, against the polyurethane wear liner, of which photos were taken 18 months after installation.

In March 2016 a site visit confirmed that the hopper holding the liner had since been removed. Increased demand requirements had resulted in the plant receiving a larger, new hopper with improved productivity, and therefore the study was never concluded.

Case study 2

Following success with the polyurethane wear liner, the company in case study 1 elected to trial the material in another application, this time in a wok. The wok-style apparatus is purpose-built to mix the bulk concrete in preparation, ready for transit in mixer trucks.

The wok lining experiences a narrow and concentrated impact area, handling high volumes of highly abrasive aggregate (20mm, 14mm, 10mm, 7mm) and sand particles. Prior to the polyurethane, the site had performed trials in rubber and ceramics. Due to the concentrated high volume impact of the abrasive aggregate, after a few weeks the tiles became dislodged and fell away. An exposed hotspot and buckling ceramics, caused by the curved surface compromising the bond of the ceramic, resulted in the individual ceramic modules falling off. Rubber was installed as an interim wear liner but failed shortly afterwards.

Imbedded polyurethane

With the evolution of polyurethane in the industry, understanding of the rebound resilience and experience of the capabilities of ceramics in the bulk materials handling industry, combinations of the materials have been utilised to further prolong wear liner life.

Similar to the rubber-backed ceramic tiles developed earlier, a ceramic imbedded polyurethane was designed to handle impact. Ceramics are suggested to have wear resistant features such as extreme hardness, and have been recorded to excel in applications involving punishing abrasion. With the flexibility of polyurethane (used to absorb and disperse the assets’ energy), combined with the hardness of the ceramics, several case studies proved a successful result.

Case study 1

An initial trial was conducted in a smaller Tasmanian quarry producing more than 700,000 tonnes a year of hard rock dolerite products. In addition to its fixed plants, the quarry operates a modern fleet of track-mounted mobile crushing and screening plants, comprising primary, secondary and tertiary units, screens and pugmills. The plant can be configured to suit many applications, with crushing and screening units having a production capacity of up to 250 tonnes per hour. A consequence of the crushing operations’ volume was the high frequency replacement of the steel Hardox wear liner, having a wear life on average of five months.

Continuous site visits and interviews were conducted over the three-year failure mode study. A replacement was installed shortly after the conclusion of the investigation. According to results and observations:

At 15 months post-installation, a business partner reported that the tile showed wear in some positions, but not in others.
At 24 months, during a feedback interview, the tile was rotated to a position where no visible wear was present.
At 36 months, a replacement tile was installed.

Case study 2

After the success of the Tasmanian quarry, other opportunities appeared to trial polyurethane, including at a busy metropolitan quarry for an ACT-based building and construction materials supplier. In this case, the maintenance supervisor was receiving regular reports of failed worn liners and consequently was looking for a way to improve the efficiencies of the transfer chute. It was observed that 20mm Hardox steel lining had previously been installed, but was not coping with the large bulk volume being conveyed at this site. The severe two-metre drop height of the chute led to the alloy lasting just 10 to 14 days before a replacement liner was required.

Following initial installation of the plain polyurethane tiles, the maintenance supervisor advised that environmental conditions had proved too severe for the 83 durometer material, further supporting the recommendations made in Table 2.

It was also observed (Figure 4) that the path of the material had been impacting the tile’s edge rather than the front. This in turn contributed to compromising the liner panel and thus increased the wear rate of the tile.

Case study 3

In 2013 trials at a granite quarry in south-east Victoria recorded an estimated production of 650,000 tonnes per annum.

It was advised by the quarry manager that the fixed plant continued to experience high amounts of wear, complicated further by a poorly designed chute system. It was advised that the site had been using 16mm Bisalloy plates that, suffering extreme abrasion damage, had been offering a wear life of five days before requiring replacement or patching. The quarry manager believed this to be a result of the lump size (400mm minus), combined with the witnessed impact height of 2.5m.

{{image21-a:r-w:200}}Initially, plain 83A durometer polyurethane tiles were installed in this application, however, they were removed shortly afterwards due to the extreme abrasive properties of the granite, further supporting the data listed in Table 2.

With the two key elements of impact and abrasion present in this environment, a dual durometer ceramic imbedded polyurethane panel was installed. A 63 durometer polyurethane was trialled as a backing to absorb energy provided by the constant granite flow, due to its high rebound resilience, combined with a 93A ceramic imbedded polyurethane (which handles maximum lump size). Due to these environment parameters, this liner was developed application specific.

Several site visits and interviews were conducted over a period of 24 months until the fixed plant was upgraded. The quarry manager recorded that he had replaced the panels once and considered this installation to be a significant success.

Conclusion

Due to its rebound resilience and ability to absorb energy when installed in applications involving material impact, polyurethane in the extractive industries has proven to offer a variety of different solutions.

Like any material, it’s important to understand its optimised environment capabilities and limitations before recommending or installing. When exposed to applications involving impact and abrasive environments with smaller lumped material, it was concluded that polyurethanes provided higher abrasion and impact resistance than rubbers and ceramics. However, when larger lumped materials were present, liners containing polyurethane could be used, but a combination of polyurethane (to absorb impact) and ceramic (to handle the material abrasion) functioned even better, with exceptional results.

Sean E Kinder is an IQA member and a sales engineer for Kinder Australia.