Examining the growing importance of zinc-free hydraulic fluids

In an increasing number of end-use applications, zinc-free hydraulic fluids are becoming desirable to minimize impact, particularly in sensitive environments.

By Jared Cornett, Product Manager, Industrial Oils, Lubrizol

Hydraulic fluids play an important role in making the modern world work. They are found in countless applications that many take for granted, providing essential power transmission in a wide variety of industrial machinery. Without them, many of today’s industries would grind to a halt.

And that’s why monitoring the ongoing shift in demand for hydraulic fluid formulations is so important. Throughout much of the history of hydraulics, zinc-based additives have been commonly used to infuse fluids with important characteristics that contribute to reliable performance. But in recent years, zinc has come under greater scrutiny. Especially in environmentally sensitive applications — where hydraulic fluid leaks are more likely to escape into a waterway, for instance— zinc-free fluids are becoming more desirable to a range of users.

While zinc-based fluids will not lose majority market share in the industry, shifting demand has become self-evident. According to recent research from Kline + Company, zinc-free hydraulic fluids in North America and Europe are projected to experience a compound annual growth rate (CAGR) of 4.25% from 2024 to 2029, whereas demand for traditional zinc-based fluids is expected to be flat or decline over that same period.

Let’s explore the impacts of these significant shifts in fluid formulation requirements, as well as the necessity of seeking out high-performance alternative additive chemistries that can reliably replace zinc without compromising performance.

Fluid leaks and spills: what’s at stake

First, it’s worth considering the sheer volume of hydraulic fluids found within critical industrial applications across industries:

• A typical mining shovel, responsible for the high-intensity job of moving rocks and earth on mining sites, is serviced with approximately 900 liters of hydraulic fluid per drain interval.
• A backhoe, found in countless industrial and construction sites around the world, is serviced with anywhere from 19 to 75 liters of hydraulic fluid per drain interval depending on the size of the vehicle.
• A common tractor used in farming applications is serviced with around 180 liters of hydraulic fluid per drain interval.
• An excavator is serviced with approximately 180 liters of hydraulic fluid per drain interval.

Across these applications (and others), leak potential is significant. According to a 2016 study¹, it is estimated that approximately 50% of all lubricants sold worldwide end up within the natural environment rather than being properly disposed of or recycled.

This figure is attributable to a number of factors, including incorrect handling, circuit leaks, spillage during service, or other means. But no matter the source, fluids escaping into the environment is of increasing concern in environmentally sensitive applications — and it’s driving the ongoing increase in demand towards reliable alternatives.

Target markets for zinc-free fluids

While increasingly stringent regulatory measures play some role in the drive toward zinc-free fluids, several market-based factors are also influencing this trend.

For example, major OEMs are increasingly recommending zinc-free fluids, including Komatsu, Hitachi, and others. Elsewhere, corporate responsibility and sustainability initiatives are at play, as well as changing market perceptions of zinc-based products — for example, major paper producer Charmin has pledged to source materials from sustainable forests, where zinc-free fluids are recommended for major certifications.

Elsewhere, several specific industries are seeing a more prominent shift to zinc-free fluids, including:

• Mining. In North America, newer U.S. Mine Safety and Health Administration requirements have pushed site managers toward recommending zinc-free fluids.
• Construction. The U.S. Clean Water Act supports the use of zinc-free fluids in North America, heavily influencing construction projects near waterfronts. Meanwhile, sustainability goals and REACH have the most influence in Europe. End users are adopting such fluids accordingly.
• Marine. Here, zinc-free formulations are used around the globe, driven by OEMs in the space in alignment with international and regional standards and marine guidelines.
• Forestry. Sustainable forestry has become more prominent in both Europe and North America. Specifically, zinc-free fluids are recommended in forests certified by the Forest Stewardship Council (FSC) in the United States as well as those certified by the Programme for the Endorsement of Forest Certification (PEFC) in Europe.

The critical role of zinc in hydraulic fluids

To begin understanding challenges associated with formulating high-performance hydraulic fluids, without the zinc additives that fluid manufacturers have relied upon for decades, it’s worth a refresher on the multipronged role that hydraulic oils play in critical applications.

Primarily, hydraulic fluids are responsible for the smooth and reliable transmission of power throughout the hydraulic circuit that enables a piece of industrial equipment to perform its intended function. For example, it enables hydraulic forklifts to lift heavy weights. However, these fluids maintain critical secondary roles that are no less important, including:

• Effective lubrication and wear protection for metallic parts throughout the hydraulic circuit.
• Heat dissipation to prevent heat-related malfunction or compromised performance.
• Contamination control and filterability to protect the circuit from unwanted particulates.

Zinc additives have long been important to maintain these performance properties. In particular, zinc has delivered reliable wear protection both in hydraulic applications as well as many other industrial fluids, and formulating without a reliable alternative has historically been a challenge.

Indeed, early-generation zinc-free fluids struggled to gain significant market traction. For hydraulic fluid formulators, zinc alternatives were required at cost-prohibitive treat rates in order to maintain adequate wear performance. Meanwhile, many early zinc-free formulations failed to meet stringent OEM specifications (including Bosch-Rexroth, Parker Denison, and Danfoss), ruling out their use in machinery under threat of voided warranties.

Consider the consequences of underperforming fluids. Inadequate wear protection can lead to premature hydraulic pump failure, for example. Such a failure could drag a mining site to a halt, leading to significant lost profits for operators.

Advancements in zinc alternatives

As demand for zinc-free hydraulic fluids continues to grow, the global lubricants industry has been pursuing the development of reliable alternatives that can deliver the performance benefits OEMs and end users have come to expect from traditional hydraulic fluids.

With such a challenge comes opportunity, however. Lubrizol has approached zinc alternatives as a way to deliver overall performance enhancements in zinc-free fluids while enabling flexibility for formulators. These include:

• Formulation flexibility. There is also the matter of efficient manufacturing and production, and optimized additive science can help here as well. Ideally, zinc-free additive solutions should help maximize formulation flexibility, with compatibility and read across a range of Group II base oils. These attributes can help formulators go to market with zinc-free products more quickly and efficiently, contributing to more cost-effective product launches for a market where demand for such products is increasing.
• Antiwear properties. New zinc-free alternatives have been identified that deliver outstanding performance earlier generation fluids were historically unable to achieve. Today’s additives have demonstrated minimal wear in a series of industry-standard and OEM-specific testing protocols, including the Parker Dennison T6H20C pump test, the Eaton 35Vq and 104C pump tests, and most impressively, the Bosch Rexroth pump test. For OEMs and end users, such performance translates to reduced downtime, extended equipment life, and lower overall cost of ownership for critical machinery.
• Oxidative performance and thermal stability. By demonstrating advanced oxidation performance and thermal stability, new zinc-free additive technology has been shown to contribute to extended oil drain intervals. Such performance has been observed in several stringent testing protocols, including the Turbine Oil Oxidation Stability Test (TOST) per the ASTM D943 standard and the Bosch Rexroth A2F10 pump test.
• Cleanliness and filterability. Optimal zinc-free fluids must be formulated to handle water, air, and other contaminants that may exist in dirty application environments. Here, a zinc-free alternative should show optimal air release performance (via ASTM D3427 testing standards), lower water acidity generation (ASTM D2619), excellent wet and dry filterability (ISO 13357), and minimal sludge production (ASTM D4310).
• Smooth operation and control. Zinc-free additives must be able to contribute to precise operation and smooth control of hydraulic equipment. Additives can provide such performance with excellent air release and foam control attributes to enable this kind of performance, allowing OEMs and end users to experience efficient operation and responsive control.

For hydraulic fluid formulators, these attributes cannot be compromised for reliable end-use performance in demanding applications. Zinc-free hydraulic fluids stand to become increasingly important in the coming years as sustainability initiatives and environmental policy evolve. For fluid formulators, it is worth proactive investigation into the advanced chemistries that can help you capitalize the need for such products, contributing to effective and reliable hydraulic performance everywhere.

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