- Extrusion recommendations for Genflam OH Sheathing Compounds
- Control your melt zone
- What is a Thermoplastic?
- Thermoplastic Processing - How To Generate Properties
- Crosslinking - Part 1
- Crosslinking - Part 2
- Crosslinking - Part 3
- A Quick Overview of Ethylene Propylene Rubber (EPR)
1. Extrusion recommendations for Genflam OH Sheathing Compounds
The Genflam OH series of products are Thermoplastic Low Smoke Halogen Free insulation and sheathing compounds designed for rugged service in the wire and cable industry. These materials differ from other commercially available thermoplastic materials in that they are designed specifically for an excellent balance of performance properties spanning a wide range of applications. Due to the unique nature of these compounds, the Genflam OH series will process differently than other commercially available thermoplastic materials. The Genflam OH series typically attain maximum output when subjected to relatively low levels of shear in the extruder. The recommended screw design for the Genflam OH series is a 1.25:1 compression ratio, deep flight style screw (typically referred to in the industry as a "rubber screw"). High compression style screws, as typically used for thermoplastic materials, will limit extruder output due to the high shear rates developed in the system. Care should also be taken to ensure the drives on the extruder used are sufficient for the Genflam OH series. These materials tend to be more viscous than the commercially available products, and as such require more torque than competitive materials. Although the higher level of torque may limit ultimate screw speeds, it is often common to exceed output (pounds per hour) with the Genflam OH series due to the low shear/high output design of the screw.
2. Control your melt zone
A typical rubber extruder has three important zones - Feed, Melt, and Metering. The feed zone provides forward pressure, while the melt and metering zones provide a consistent flow of material through the head. If the melt starts to move back to the feed zone, flow to the head will become erratic, and eventually cease. When extruding higher viscosity materials, such as the Gendon OH and XLPO series, the best way to control the melt zone is to use a combination of low to high temperature levels from feed pocket to head, and screw cooling. Of the two, screw cooling is the more important, as this will counteract any heat buildup on the screw due to shear heating as the extruder is in operation. Head pressure is perhaps the best way to monitor the position of the melt zone in the extruder. As the extruder operates and shear heat builds, the melt zone can start to move back towards the feed zone, lowering the back pressure in the extruder. When this occurs, head pressure will start to become erratic, eventually falling to minimum levels as the the flow stops. This can be somewhat counteracted by increasing the temperature delta between the feed and metering section, reducing the temperature of the water on the screw, reducing the screw speed, or a combination of the three.
3. What is a Thermoplastic?
What is a Thermoplastic? May 2014 Thermoplastic materials are characterized by the need to heat the polymer to a temperature above its softening point, form the material to its desired final geometry, then cool to a temperature below the melting point to lock the polymer into the desired geometry. This transformation is characterized as a physical reaction, as there is no chemical transformation of the polymer during the process. Since this is a physical process, thermoplastic materials can often be reground and added to virgin resin, thus reducing the overall cost of manufacture. Thermoplastic materials are unique in that the base polymer used often determines the performance characteristics of the finished part. With thermoplastics, there is typically no chemical modification of the polymer system during processing. The physical properties developed in a thermoplastic material are highly dependent on the polymer morphology – which can be engineered through processing methods. Thermoplastics can be classified as either amorphous or crystalline. Crystalline materials are characterized as polymer chains that are essentially aligned in a similar direction, and neatly packed. Amorphous materials are characterized as polymers chains that are randomly oriented. For a visual, think of spaghetti noodles before and after cooking. Out of the package, the spaghetti is aligned in a linear orientation – similar to a crystalline formation in a polymer chain. After cooking – the spaghetti orientation is random – similar to an amorphous formation in a polymer chain. Crystalline orientation not only provides strength based on the properties of the individual polymer chain, but also through the interaction of the polymer chains with those adjacent to it, as well as chain entanglement in the three dimensional matrix. In these polymers, force applied in the linear direction must be overcome not only by the individual polymer chains, but also the interaction of the chains with each other. To an extent, these intermolecular forces also provide stability when force is applied transverse to the crystalline structure. In an Amorphous orientation, the polymer chains again determine the base properties, but there is much less interaction of these chains with the adjacent chains in the matrix. The net result is the physical properties are more dependent on the polymer itself, with a small positive influence due to polymer chain entanglement added. The overall result is a decrease in physical properties on the finished product. Next month, we will discuss processing methods to encourage crystalline or amorphous orientation, and the overall impact on the finished product.
4. Thermoplastic Processing - How To Generate Properties
June, 2014 Last month, we gave a brief definition of a thermoplastic, and the various morphologies that define the physical, chemical and performance properties of these materials. This month, we will go into a few basic methods on how to develop these morphologies in an extrusion setup. Crystalline: As discussed earlier, a crystalline formation is characterized by a high degree of alignment of the polymers, typically in the direction of extrusion. As a result of this alignment, crystalline morphologies generally give excellent tensile values, as well as resistance to certain fluids. This alignment can be attained in the extrusion using a number of methods – the more prominent outlined below: Tooling: Perhaps the easiest method to assure a crystalline morphology at extrusion is through the proper selection of tooling. Typical extrusion setups for generating maximum crystalline morphology involve a drawdown of the material during extrusion. In a drawdown setup, the material is extruded through tooling that is both larger in overall diameter, and thicker in wall than desired in the finished product. The material is then extruded at a rate that allows the elastomer to be stretched, or Drawn Down, over the core as it is extruded. This stretching, or Drawdown, imparts the needed shear to align the polymer chain in the direction of extrusion – promoting a crystalline morphology. You may encounter the term Draw Down Ratio (DDR) in regards to this type of setup. The DDR is related to the amount of shear added to the system during extrusion. The higher the number, the higher the shear – and as a result – the more crystalline the structure. For Gendon products, we commonly recommend a DDR of approximately 2:1 for the best balance of properties Cooling: After extrusion, the rate of cooling of the sheath also has an impact on morphology. For this, you need to think of a polymer as a very thick fluid. At typical extrusion temperatures, this fluid is above its melting point, and acts very much like a liquid. Above the melting point, the polymer chains in this liquid will attempt to revert to the lowest energy state possible – which is the crystalline morphology. To promote this, extruders typically use a gradient cooling system after extrusion to give the polymer as much time as possible to develop this crystalline structure. The temperatures required for gradient cooling vary with the polymer type used, but for Gendon products, we suggest 120-140 degrees F for the first section, dropping to 80-100 degrees F in the second, and finally 50 degrees F in the third. The lengths of the cooling sections will be determined by the line speed of the process, as well as the resonance time desired to attain the proper orientation. In production, it is common to use a combination of both of these methods to get the best balance of properties. Most extrusion operations will use a Draw Down setup for tooling, coupled with gradient cooling for the finished product. Amorphous: For an amorphous orientation, we are looking at development of precisely the opposite structure as described above. It is not surprising then, that the methods are essentially the opposite as those used to develop a crystalline structure. The benefits of an amorphous structure are typically improved elongation values, and a slightly better resistance to external damage. Tooling: Tooling designed to impart an amorphous structure is designed specifically to eliminate and introduction of shear after extrusion. In this case, it is common to use either a tubing or pressure setup for the product. In a tubing setup, the tip and die sizes are selected to be as close as possible to the finished dimensions desired on the final part. The output of the extruder is matched to the line speed of the core, and the sheath is “Sleeved” onto the core. Alternatively, a pressure setup can be used. In a pressure setup, the tip is selected is the same as the tubing setup, but the die is slightly undersized for the wall thickness. In this setup, the extruder output is set so that the material is extruded at a slightly higher rate – resulting in a higher overall diameter than the die selected. Cooling: As you may have guessed, the cooling requirements to attain an amorphous orientation are also the opposite as what was described for the crystalline structure. To develop an amorphous orientation, it is necessary to cool down the material as quickly as possible to limit the amount of polymer mobility. This is typically done by quenching the material immediately after the extrusion head, and using chilled water in the cooling troughs. As we noted with the crystalline setup, it is also common for both of these methods to be used to develop the desired balance of properties. Next month – we will switch gears so to speak – and introduce the concept of crosslinking to these polymers.
Cross-linked Polymers – an Overview July, 2014 For the last two months, we have been discussing Thermoplastic materials, and how the performance properties can be engineered through a combination of material selection and processing techniques. This month, we will explore the process of crosslinking of the polymers, and how this can open new possibilities for performance enhancement for polymeric compounds. First, a quick review of Thermoplastics: - Performance properties are based on polymer morphology – i.e. crystalline or amorphous orientation of the polymer chain - Materials are processed by heating above the crystalline melting point of the polymer, forming into the desired geometry, then cooled below to the crystalline melting point to retain desired shape - The process is reversible – the material can be reheated above its crystalline melting point, then re-used - Typically, no chemical modification occurs during or after processing to the polymer system The main difference between a Thermoplastic and Cross-linked Polymer is the generation of a network of chemical bonds that effectively tie each individual polymer chain to the adjacent chains in a three dimensional matrix. The generation of these crosslinks serve to convert the polymer system into one very large polymer as opposed to several independent polymer chains we see in the Thermoplastic system. The crosslinking process is a chemical change as opposed to the physical change we discussed earlier in their thermoplastic cousins. Since this is a chemical change, the process is not easily reversed. As a result, polymers that have been cross-linked are generally not re-used. The presence of the cross-linked network provides several advantages when compared to a thermoplastic material: - Elevated temperature performance is greatly improved, as the polymer will not melt when exposed to temperatures above the crystalline melting point - Fluid resistance is greatly improved, especially at elevated temperature - Flame performance is improved, as the polymer network enhances the formation of char - Dynamic performance properties are improved as a result of the exponential increase in molecular weight of the polymer - Polymer cold flow is greatly reduced, especially at elevated temperature and high differential pressures The driving force for all of the performance improvements can be attributed to the presence of the crosslinks in the polymer system. Processing of the cross-linked materials follow the same general guidelines as described in the Thermoplastic discussion, with one major exception – the processing temperature. Several of the crosslinking mechanisms we will discuss below are driven by energy in the form of heat. If the activation energy of the crosslinking agent is exceeded during processing, the material will begin to crosslink prematurely, and result in scrap. This phenomenon is commonly referred to as scorch. As stated, crosslinking in polymers can be attained using a variety of methods. For this month’s discussion, we will concentrate on those methods driven by the addition of heat – commonly referred to as Thermoset Products in the industry. Methods of Crosslinking: Sulfur: The use of sulfur as a vulcanizing agent dates back to Charles Goodyear in the early 1800’s. Sulfur vulcanization continues today, and is the most prevalent method used in Thermoset polymers. As the name implies, this system uses sulfur as the vulcanizing agent, and is commonly coupled with a series of activators and accelerators that serve to lower the activation energy of the reaction, reducing the time needed to complete the crosslinking process. The major drawback to this system however, is that the polymer to be cross-linked must contain an alkene (double bond) functional group in the polymer make-up to allow the crosslinking to occur. The result of the crosslinking reaction is the formation of a Carbon-Sulfur(x)-Carbon bond between the polymer chains forming the three dimensional network. Note in this system that the sulfur portion has a subscript of x, denoting that there can be a number of sulfur atoms contained in the crosslink. We will save this discussion however for a later tip. The main benefits of using sulfur as the crosslinking agent are: - Comparatively lower cost than alternate methods - Multiple methods available for driving crosslinking reaction - Typically lower heats needed to drive reaction to completion - Possible to attain excellent performance properties in dynamic applications Organic Peroxide: Peroxide vulcanization is a somewhat newer method that was driven by the need to vulcanize polymers that did not contain a functional group within the polymer chain. The peroxide vulcanization method is driven by the decomposition of the organic peroxide to form a radical, extraction of a proton from the polymer chain to form a polymeric radical, then reaction of two adjacent radicals to form the crosslink. The main differences between the Sulfur and Peroxide methods are: - The Peroxide used is not incorporated into the crosslink as we noted in the Sulfur system. Peroxides are simply methods to generate the radicals, which in turn generate the crosslinks - Peroxide crosslinks are always Carbon-Carbon bonds Processing of peroxide vulcanized systems follows the same general guidelines as the Sulfur methods. With peroxides however, typical processing temperatures can be slightly higher than the sulfur systems, allowing for more latitude in overall compound design. The main benefits of using Peroxide as the crosslinking agent are: - Much improved resistance to premature crosslink formation (scorch) - Much improved high temperature performance due to Carbon-Carbon crosslinks - Much better compression set - Minimal discoloration on nonblack materials - Typically a much faster cure reaction as compared to Sulfur Other Methods: Several additional methods exist for the crosslinking of Thermoset materials. Additional methods include Metal Oxides, Urethane, and Organic Silanes. The metal oxide method is common in polymers such as Polychloroprene and Chlorosulfonated Polyethylene. We will save this discussion for a future tip. Next month, we will discuss crosslinking methods that do not require the use of heat to drive the reaction.
Cross-linked Polymers – an Overview September, 2014 Last month, we started a discussion of the various methods to crosslink a polymer system to enhance the performance properties of the finished part. As a review, the crosslinking mechanism consists of a suitable agent which, when exposed to sufficient energy, will react with the polymer to form a radical, which then combines with an adjacent radical to form a chemical bond across the two polymer chains. This chemical bond is typically referred to a crosslink. The presence of the crosslink enhances the polymer through improvements in: - Elevated temperature performance, as the polymer will not melt when exposed to temperatures above the crystalline melting point - Fluid resistance, especially at elevated temperature - Flame performance, as the polymer network enhances the formation of char - Dynamic performance properties as a result of the exponential increase in molecular weight of the polymer - Greatly reduced Polymer cold flow, especially at elevated temperature and high differential pressures We discussed two main methods for achieving this crosslinking reaction – using both sulfur and organic peroxide systems. Both of these systems are commonly used in the wire and cable industry, but have one potential drawback – both require the cable to be exposed to heat at a temperature above the activation energy of the cure system used. The application of heat to drive the reaction can have detrimental effects on some cable designs – especially those utilizing thermoplastic components in the core that are prone to flow when exposed to these heat levels. There are two main methods of crosslinking polymers in the cable industry which do not require the use of high heat levels to drive the reaction. A brief overview of the radiation crosslink system is discussed below: Radiation Crosslinking (E-Beam) Radiation crosslinking is the practice of exposing the polymer to a high energy electrons generated and accelerated to near the speed of light in the electron beam – which then bombards the polymer, removing a hydrogen atom from the polymer backbone to form a polymer radical. Once the radical is formed, two reactions typically occur: - Two adjacent radicals react - Crosslinking - The radical formed initiates chain cleavage – Chain Scission In the E Beam process, both reactions are occurring – it is the ultimate design of the polymer to be cross linked and the processing characteristics of the beaming process that determine which reaction will dominate. As a general rule, olefin based polymer systems will crosslink, whereas polymer systems utilizing more polar components on the backbone will result in chain scission. For our discussion – we will concentrate on crosslinking as the desired outcome. The mechanism for E-Beam crosslinking is very similar to that of the peroxide systems discussed last month. In the E-Beam method, the high energy electrons generated in the beam are used to remove a hydrogen atom from the polymer backbone, forming a polymer radical. When two of these radicals are generated in close proximity, the radicals will react with each other, forming the Carbon-Carbon bond we call a crosslink. Since the general method of crosslinking follows the same general chemistry as that of the peroxide system – why use radiation based systems? A couple of benefits outlined below should answer that question: - The radiation crosslinking process does not need elevated temperatures to complete. Because of this, the finished cable can be designed to utilize lower cost thermoplastic materials in the core without heat deformation issues seen with the peroxide systems - Radiation crosslinking is much more energy efficient than the thermoset alternatives (sulfur or peroxide). Exposure time of the part to the radiation beam is measured in seconds as opposed to minutes/hours with other thermoset methods - Output of the radiation crosslinking systems are much higher than the thermoset systems – line speeds in the hundreds of feet per minute are possible - In radiation crosslinking – there exists a high correlation between dosage level and crosslink density. Balancing the beam dosage and energy can be used to tailor the level of crosslinks desired Remember though, for every benefit there is a corresponding drawback. A few of the major drawbacks to radiation crosslinking are outlined below: - Not all polymers can be cross linked using radiation methods. Aromatic based polymers are a good example - Some polymers, specifically highly halogenated materials, will favor chain scission as the dominate reaction when exposed to radiation. For this reason, radiation is generally used to decompose these polymers for further processing - Depth of penetration of the electron beam driving the crosslinking reaction is limited – making radiation crosslinking of thick cross sections difficult if not impossible. Variable cross sections are also an issue with radiation crosslinking – as the higher energy needed for the thick sections will drive a higher cross link density in the thinner section - Although there is a drastic improvement in output for radiation crosslinking – the initial capital cost is relatively high - Care must be taken to protect personnel from exposure to the radiation Next month we will delve into the world of moisture cure – stay tuned!
Cross-linked Polymers – an Overview October 2014 For the past two months, we have been looking at methods used to crosslink polymers in the wire and cable industry. The first installment covered chemical crosslinking utilizing sulfur and peroxide crosslinking agents, accelerated by heat. Last month, we discussed Radiation Crosslinking (E-Beam), where high energy electrons are generated and bombard the polymer at near the speed of light to generate the radicals, which in turn react with adjacent radicals to form the crosslinks. The benefits of both systems have been discussed – as well as the drawbacks. From a business standpoint, the main drawback of the methods above is the capital requirement needed to drive the reactions. The capital outlay for these systems can exceed several million dollars, and present a significant barrier to entry for producers wishing to add the crosslinked products to their current thermoplastic line of products There is another way however Silane Crosslinking Silane crosslinking – or known more commonly as moisture cure – is a method by which the crosslinks are formed by reaction of a silane group in the backbone of the polymer with moisture to form the crosslink. There are two distinct methods for moisture cure – each summarized below: Siloplas Method: The Siloplas method was developed in the 1960’s, and is characterized by a two step process to generate the crosslinks. The first step involves modification of the polymer backbone by incorporating a silicone based organic compound into the backbone. This silicone based material contains both a functional organic section, as well as a hydrolysable component on the molecule. The component is added to the molten resin during processing, along with a small amount of peroxide. The peroxide decomposes to generate a radical on the backbone of the polymer – which then reacts with the organic functional group on the silicone based additive chemically incorporating this into the backbone. If this sounds familiar, it should – so far, we have simply duplicated the reaction for a typical peroxide crosslink. The big difference is in this case, we have not formed a crosslink – we have simply added a functional group to the backbone of the polymer which will be used later. Once the grafting has been completed, the material is generally pelletized and stored in an airtight container. The second step to add the catalyst to drive the crosslinking reaction. This step is typically done in an extruder as the final product is produced. The catalyst is typically added at a level of approximately 5 percent, and serves to accelerate the reaction for crosslinking. Once produced, the final product is typically exposed to moisture using either water baths or low pressure steam. The incorporation of moisture into the system causes a dehydration reaction with the hydrolysable portion of the incorporated graft, forming a radical that then crosslinks with an adjacent radical to form a stable siloxane crosslink. Monosil Method: The Monosil method was developed in the early 1970’s as an alternative to the Siloplas method described above. The main benefit of this method is that this process incorporated both the grafting and catalyst addition in one step, thus eliminating the second process step to form the crosslink. The chemistry for grafting and crosslinking is the same for both methods. Why use Moisture Cure: The Siloplas method is typically used in wire and cable manufacturing – so we will limit our discussion to this specific process. The obvious benefit of moisture cure is in cost. Since this reaction can take place at room temperature, the capital required to set up a processing line is minimized. Typical extruders used in processing of thermoplastic products can be utilized to run moisture cure products with little or no modification. Another benefit is in the flexibility of the process. Since the grafting step can be performed with a variety of polymer bases, the processer has several options for base polymer available to select the proper base material for performance of the final product. The process is also very easy to run on standard equipment – and lends itself well to those who are well versed in thermoplastic processing. So why does everyone not use this? As with all benefits, there are also drawbacks. Firstly, the cost of the raw materials are significantly higher than the standard thermoplastic counterparts, as well as materials designed for either peroxide or E-Beam crosslinking. Another big drawback is in the reaction time needed for the crosslinking reaction. Although this can be accelerated by exposure to low pressure steam – reaction times for crosslinking can take days to complete. Another drawback is in the sensitivity to variability in processing. As stated above, this method involves the incorporation of two materials added to the extruder during processing. If the ratios of these materials vary during processing, or these materials are not evenly distributed in the mixer – there can be wide variation in performance in the final product. Processing also presents some challenges not normally seen with typical thermoplastic or peroxide/E-Beam products. The first important point is that the base resin contains a graft that will react with moisture at room temperature. This requires that the material be stored prior to processing in moisture proof bags, then resealed after use. It is imperative that the base resin be kept dry to prevent crosslinking. The processer must also remember that immediately after incorporation of catalyst in the extruder, the reaction has essentially started. It is typically not recommended to shut down the extruder during the run without a complete purge of material to prevent crosslink formation in the barrel. Moisture cure offers the cable producers another option to get the advantage of a polymer crosslinked network for improved performance, without the need for intensive capital investment for downstream equipment. Although this is not a complete substitution for either peroxide or E-Beam systems, it does provide an alternate that is commonly used in the wire and cable field.
8. A Quick Overview of Ethylene Propylene Rubber (EPR)
February, 2015 As we begin a new year, we will start for focus on some of the polymer types that are typically used in the wire and cable marketplace. For primary insulation uses, where the performance requirements require a good balance of Physical properties, Heat Resistance, Abrasion Resistance, Fluid Resistance, and Environmental performance – EPR’s tend to be the workhorse. Ethylene Propylene Rubber exists in two basic forms. The Ethylene Propylene version (EP) is a copolymer of ethylene and propylene monomers. The polymerization process utilizes the point of unsaturation on each of the monomers as a reactive point to drive the polymerization reaction – yielding a completely saturated material as the end product. The absence of any unsaturation in the polymer is the main driver for the excellent heat resistance properties of this material. As an insulation – Underwriters Laboratories recognizes EP based compounds at temperatures up to 125 degrees C in continuous service. With proper compound design, these polymers have been approved for up to 150 degrees C in service. The absence of unsaturation in the polymer backbone also gives this polymer excellent resistance to attack from oxidative materials in the environment such as ozone and ultraviolet light. EP based compounds can easily pass the requirements for Sunlight Resistance as defined in many of the standards commonly used in the wire and cable industry. With the completely saturated molecular architecture, the majority of crosslinking is done through either organic peroxides or radiation. Although other methods are available, these are the two most commonly used in wire and cable. The second form of EPR polymer available adds a third monomer to the polymerization process. This third monomer is a diene – which adds the necessary reaction point for inclusion in the backbone during polymerization – and a second point of unsaturation that remains available pendant to the backbone of the polymer. This polymer is typically referred to as EPDM (Ethylene Propylene Diene Monomer) in the literature. Of all the EPR based materials used in wire and cable, the majority are actually classified as an EPDM. The diene content for EPDM based materials typically ranges from 2 to 10 percent of the polymer – with the higher levels designed for faster crosslinking in production. The inclusion of the diene in the backbone of the polymer allows more flexibility in the crosslinking methods that can be used with the polymer. With the available double bond, it now becomes possible to utilize a sulfur based system for crosslinking – which has the benefit of reducing overall costs for production of the finished part. Since the polymer backbone remains fully saturated, the performance when exposed to free radicals such as ozone and ultraviolet light remain unchanged as compared to the EP based compounds. Heat age properties can be impacted due to the presence of sulfur in the crosslinked system, although with proper design, the change in high temperature performance is minimal. The mechanism for the slight reduction in high temperature performance is due to the bond energies of the C-C vs the C-S-C bonds resulting from the crosslink. The bond energy for the C–S-C bond averages 65 kcal/mole as opposed to a C–C bond at 83 kcal/mole. Through proper compound design however, it is possible to attain close to the same heat performance with a sulfur based system as with either a peroxide or radiation cross-linked material. So, other than the inclusion of a diene – how do EPR materials differ from one manufacturer to another? The answer is greatly! As stated, both the copolymer and terpolymer are based on polymerization of ethylene and propylene to form the backbone of the polymer. The ratio of the ethylene component to the propylene component (typically referred to as the E/P ratio) – has a direct bearing on the performance characteristics of the finished material. The E/P ratio for standard EPR polymers range from 45 to 75 percent ethylene to propylene. For the final compound, the higher the ethylene content, the more crystalline the polymer. If you recall the discussion on polymer morphology (What is a Thermoplastic, May 2014) – the presence of this crystalline structure allows for much higher polymer chain interaction – which serves to improve such properties as abrasion and fluid resistance. This polymer chain interaction also serves to improve the physical property capabilities of the polymer – allowing the compounder to increase the filler level without sacrificing performance characteristics of the final compound. On the other hand, a lower ethylene / propylene ratio ends to produce a material that is somewhat more flexible in service - a trait that is highly valued in wire and cable. Next month we will take a look at the performance characteristics of EPR based materials, and how these can be engineered to meet specific attributes needed in the field.