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This article gives detailed insights into polyurethane rollers. Read further to learn more about:
What is a Polyurethane Roller?
Advantages of Polyurethane Rollers
The Polyurethane Polymer System
Properties of Polyurethane
And much more…
Chapter 1: What is a Polyurethane Roller?
Polyurethane rollers are cylindrical rollers covered by a layer of elastomer material called polyurethane. Depending on the application, the inner roller core is prone to scratches, dents, corrosion, and other types of damage. The layer of polyurethane has intrinistic properties the inner roller core, such as abrasion resistance and impact strength. Polyurethane rollers are used in many manufacturing processes for performing operations such as:
Polyurethane is the most extensively used elastomer material for rollers. It is possible to make a variety of blends from different types and proportions of its compounding ingredients. Through blending, varying mechanical properties are obtained to suit a particular use. The most desirable properties of polyurethane are its toughness, high impingement resistance, shock absorption, and fatigue resistance.
Formulations of polyurethane can produce rollers that are hard and firm or soft and pliable. Polyurethane rollers are processed in hard to soft durometers depending on the requirements of an application. Polyurethane's durability and shock resistance make it one of the most popular types of roller compositions. In addition, polyurethane is an elastomer that can be thermoformed into any possible or conceivable shape.
Chapter 2: Advantages of Polyurethane Rollers
Polyurethane and rubbers, such as nitrile and neoprene, belong to a family of materials called elastomers. Their elastic properties make them an ideal choice as roller-covering material. Polyurethane rollers have several advantages over ordinary rubber rollers. Their impressive mechanical properties make them more viable than other types. The many benefits of polyurethane rollers include:
Wide range of physical properties - As mentioned earlier, with its different types and proportions of polyols, diisocyanates, and curatives, polyurethane can have almost any property to suit a particular use. It is a very versatile material that can be engineered with different mechanical properties.
Better durability - Aside from being versatile, polyurethane’s mechanical properties are also superior to other rubber types. Regardless of its hardness, the toughness and resilience of polyurethane make it a very durable material. While the useful life of rubber rollers is about 1000 hours, polyurethane rollers last four times as long at 4000 hours before they show any signs of wear or aging. Their longevity decreases downtimes and work stoppages for repairs since replacement rollers are seldom needed.
Approved by the Food and Drug Administration (FDA) - For the protection of the public, the FDA has enacted strict standards regarding the equipment and machines used in food processing. Every step of food production is closely examined for contaminants, bacteria, and cleanliness. Due to these rigid requirements, polyurethane rollers are used in place of rubber or plastic rollers since polyurethane rollers do not leave marks or residue that could potentially contaminate food.
Carbon Blacking – Rubber roller linings have carbon black added as a filler and reinforcing material. When rolled against a hard surface with sufficient force, they leave black streaks and marks damaging in applications such as finished goods handling and printing. Polyurethane rollers do not require carbon roller linings as extra support, which avoids the potential of carbon blacking.
Water and oil resistance – Depending on the type of polymer system used, polyurethane can withstand water, oils, and other petroleum-based solvents. Water resistance is required for rollers under wet service or in environments with frequent washdown. Oil resistance is required for handling hydrocarbon-based solvents or chemicals such as inks.
Ease of processing
Processing polyurethane does not require expensive and complicated equipment. A simple blending operation may only require a batch mixer (no heating equipment needed). This is in contrast with other rubber types, which employ equipment for mastication and heating.
Do not mark products and surfaces
Rubber roller linings typically have carbon black added as filler and reinforcing components. When rolled against a surface with sufficient force, they can leave marks that are unpleasant in applications such as finished goods handling and printing. Polyurethane rollers do not have this problem since they do not use carbon black.
Water and oil resistance
Depending on the type of polymer system used, polyurethane can withstand water, oils, and other petroleum-based solvents. Water resistance is a required property for rollers under wet service or in environments with frequent washdown. Oil resistance is required for handling hydrocarbon-based solvents or chemicals such as inks.
Most of the engineering work for polyurethane rollers happens during the production of the elastomer lining. Four components make up the material: the polyol compound, the diisocyanate compound, the chain extender or curatives, and the additives.
The mixture of the polyol and the diisocyanate compounds forms the prepolymer resin. They combine to form a simple polymer chain from the reaction of a polyol component (a carbon-chained molecule with alcohol on both ends) with a diisocyanate component (a molecule with isocyanate on both ends). This results in a molecule with reactive alcohol on one end and a reactive isocyanate on the other. The alcohol end further links to another isocyanate end or terminal, while the isocyanate end of the same chain further reacts with chain extender compounds or curatives such as hydroxyl and amines. This process continues, making a long, chained polyurethane molecule.
The mechanical properties depend on the formulation of the prepolymer resin and the curatives. Additives are used to further improve the properties of the polyurethane, such as resin curing time, machinability, color, UV protection, and etc. Careful proportioning of additives relative to the amount of resin in the mixture is required since it can weaken the properties of the product.
Chapter 4: Components of Polyurethanes
The previous chapter briefly described how polyurethane is made and discussed the roles of the four different components. In this chapter, the focus is on the different chemicals used to make polyurethane and their implications on the final property of the product.
The Polyol - A polyol is an organic molecule containing one or more hydroxyl (OH) group(s). Polyols used in urethane casting are either polyether or polyester types.
Polyether: Polyethers are characterized by good resilience, high impact resistance, low heat build-up for dynamic applications, hydrolysis resistance, and good low-temperature performance. Common types of polyether used for polyurethane rollers are PTMEG and PPG. Of the two, PTMEG offers superior quality but is more expensive.
Polyester: Compared to polyether, polyesters have good abrasion resistance, heat aging resistance, oil resistance, solvent resistance, good shock absorption properties, and better tear resistance.
Specialty polyols: The most common are polycarbonate and polycaprolactone polyols. These two polyols are also sometimes classified as polyesters. Polycarbonates are used as engineering materials due to their strength and toughness. On the other hand, polycaprolactone gives the polyurethane good water, oil, solvent, and chlorine resistance.
Like the polyols, diisocyanate compounds form the resin side of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
Aliphatic Diisocyanates - The most popular characteristic of these types is that they are non-yellowing. This makes them suitable for rollers where color stability is required. The most common ADIs are hexamethylene (HDI), hexamethylene (HMDI), and isophorone (IPDI).
Aromatic Diisocyanates - These types are further divided into NDI, TDI, and MDI.
Naphthalenic Diisocyanates (NDI) - NDIs offer superior performance and long service life for dynamic applications. One downside of NDIs is their high melting point, which makes them difficult to process. Moreover, they are highly reactive; this results in lower storage stability. Thus, they are usually manufactured with special equipment at the custom molder.
Toluene Diisocyanate (TDI) - In contrast with MDIs, this type is popularly used for high-hardness applications such as guide rollers. Typical forms of TDIs used on an industrial scale are the 2,4 and 2,6 isomers at an 80/20 blend. Producing proportions other than 80/20 requires an additional process.
Methylene Diphenyl Diisocyanate (MDI) - MDIs are known for imparting high resilience and impact strength to polyurethane casts. That is why MDIs, paired with either polyethers or polyesters, are used in dynamic, high impingement applications such as wheels, grains milling rollers, and the like. The most common isomer used in casting is purified 4,4 isomers.
Curative - Curatives are mixed with the polyol and diisocyanate prepolymer to form a solid or semi-solid elastomer. There are two basic types of curatives: hydroxyls and amines.
These curatives have hydroxyl groups (OH) at the molecule terminals that link prepolymers. The standard hydroxyl curative is 1,4-butanediol (BDO); it is commonly used in MDI prepolymer systems at room temperature.
Aside from hydroxyl groups, amine groups (NH2) can also bond on the terminals of the prepolymer. The widely used amine curative was 4,4-methylene bis (2-chloroaniline) (MOCA) as the base curative for TDI prepolymer systems. However, this type was then identified as a carcinogen by OSHA. Other amine chain extenders are now being used such as 4,4-methylene bis (3-chloro-2,6-diethylaniline) (MCDEA).
Chapter 5: Properties of Polyurethane
Polyurethane is regarded as an engineering material because of its excellent properties. Most of which are attributed to its highly elastic nature. Below are some of the properties of polyurethane significant to roller applications.
Hardness - Hardness is the relative resistance of a material to localized surface deformation. It is usually determined by measuring the depth of indentation on the material by a standard indenter, ball, or presser foot.
Materials are graded according to their hardness relative to one another. For elastomers, hardness is characterized by the Shore Hardness Number. This is measured by a durometer. There are 12 different Shore Hardness Scales; each scale has its indenter configuration, profile, and force applied. The Shore scales used for polyurethanes are Shore A and D. Shore A scale measures the hardness of soft, semi-rigid polyurethanes, while Shore D measures hard rubbers and rigid polyurethanes. However, keep in mind that high hardness does not correspond with high rigidity or strength.
Abrasion Resistance The two types of abrasion are sliding and impingement; sliding is one surface passing over another, while impingement, or slurry abrasion, includes the impact of particles. Sliding abrasion can be two-way or three-way, with two-way being two surfaces meeting and three-way being two surfaces separated by dry particles. Both types of siding abrasion experience high strain that creates tears in the surface of materials.
With impingement abrasion, the paths of the particles impacting the surface of a material can be perpendicular or at an angle. The attacks of the particles strike specific areas, causing high strain and breaking off small bits of the surface.
Regardless of the type of abrasion, polyurethane is highly capable of resisting both sliding and impingement. Since it has a low friction coefficient and high strength, it is unaffected by sliding abrasion. With impingement abrasion, it absorbs the energy from the particulate attacks, distributes the stress, and immediately recovers without any signs of damage.
Polyurethane is blended to produce a low coefficient of friction, high tear strength, and elasticity, properties that help it withstand abrasion. The composition of its resin produces its exceptional abrasion resistance. Among the polyol compounds for making polyurethane, polyesters have the best tear and abrasion resistance.
Tear Strength - Tear strength is the ability of polyurethane to withstand tensile forces that rip the material apart and tear through its body. The different types of tear tests can vary depending on how the force is applied, the microscopic structure of the material, and is correlated with abrasion resistance. The highest level of tear strength for polyurethanes is found with polyester polyurethanes. The tear strength for polyurethane is as high as 1000 pounds per linear inch using the ASTM D-624, Type C test.
Polyurethane tear strength is measured in pounds per inch of thickness. The testing for tear strength is dependent on the application for which the polyurethane will be used. Tear strength tests have been established by the American Society for Testing Materials (ASTM). There are three ASTM tests used to determine the tear strength of polyurethane, which are:
ASTM D-624, Type C - measures resistance to tearing
ASTM D-1938 – measures resistance to cut or tear growth
ASTM D-470 – measures resistance to cut or tear growth
Impact Strength - In addition to abrasion resistance, polyurethanes possess good impact strength because of their excellent resilience. The polyurethane lining used in rollers can elastically deform to absorb an impact and return to its shape. It does this while dissipating the energy throughout the structure of the roller.
Fatigue Resistance - Polyurethane has high fatigue resistance because of its flexural strength. It can elastically deform under cyclic conditions without failing. This makes it suitable for high-speed applications such as printing and milling. The only problem with polyurethanes used in these conditions is their low-heat dissipation, especially for thicker roller linings. High heat can eventually accelerate creep, which weakens the material.
Thermal Aging Resistance - Thermal aging is the gradual degradation of elastomers under conditions of high temperature and oxygen abundance. It is characterized by a loss of strength and elasticity. This irreversible process dictates the operating temperature limits of the material.
Polyurethane exhibits good thermal aging resistance when formulated with certain compounds such as PPDI and CHDI. Typical polyurethanes have a maximum operating temperature of about 90 to 100°F (32 to 37.7°C). Special but more expensive formulations can reach 302° F (150°C).
Friction - Polyurethanes‘ coefficient of friction (COF) tends to correlate with their hardness. The two properties have an inverse relationship, when COF increases, hardness decreases. Since the hardness of polyurethanes is easily manipulated through blending, the desired COF can also be attained.
Machinability - Machinability is a property observed in hard polyurethanes. This property allows polyurethanes to be shaped into perfect geometries. This is particularly useful for polyurethane rollers since they must undergo machining to create the desired profile, especially for crowned rollers.
Chemical Resistance - The chemical resistance of polyurethanes depends on the type of polyol used in their polymer system. Ether-based systems are more resistant to water, making them suitable for wet applications. Ester-based, on the other hand, are best against oils, solvents, and most petroleum compounds.
Chapter 6: Manufacturing Process
The manufacture of a polyurethane roller is a straightforward process that involves the fabrication of the roller core, balancing, polyurethane blending, bonding, building, curing, machining, and quality testing. The fabrication process is similar to that of other rubber roller types. The main difference is cover building since the polyurethane resin is in liquid form.
Roller Core Fabrication and Preparation - Steel is the most common type of material for making roller cores. Steel roller cores are formed through a series of sheet rolling, milling, cutting, and welding processes. The main part is the outer cylinder, which holds the polyurethane. It is typically formed through rolling and welding processes completed in steel mills, which supply steel pipes and tubes as feedstock materials to polyurethane roller manufacturers.
Polyurethane rollers have a solid core or a keyway and space for bearings, which are designed for one or both ends of the roller. Bearings reduce friction against the static and rotating parts. The configuration, mounting, and type of bearing can vary depending on the design of the roller.
All dimensions of a polyurethane roller must be accurate to attain the required diameter, roundness, and balance of the roller. After fabrication, the roller core is subjected to secondary processes such as blasting, lapping, and cleaning to remove any traces of corrosion and contaminants.
Roller Balancing - A roller core can become imbalanced in two ways: static and dynamic imbalance. Static imbalance is described as the roller rolling to its heavy side when made to rotate freely. Dynamic imbalance is the generation of a rocking motion or vibration when the roller is rotated to its operating speed. Polyurethane rollers are usually inspected and corrected for dynamic imbalance. Dynamic balancing is done by testing the roller with a computer-controlled dynamic balancing system. It determines the location and amount of counterweight needed to ensure proper balancing.
Polyurethane Preparation - The components of the polyurethane polymer system were tackled in chapters two and three. As previously mentioned, polyurethane is a combination of chemicals, namely polyols, diisocyanates, curatives, and additives. Specific formulations are used to create a product with the desired mechanical and chemical properties.
The formulation can be done through different processes. These are known as the single shot, prepolymer, and quasi-prepolymer processes.
The single-shot process involves having all components in separate chambers. These will then be blended by a mixing head and poured or injected into the mold.
The second option is the prepolymer process. This process is carried out by mixing the polyols and diisocyanates before pouring them into the mold. This process helps dissipate the heat produced from the exothermic reaction of the compounds.
Last is the quasi-prepolymer process. Quasi-prepolymers consist of polyols partially reacted with the diisocyanate compounds. This simplifies the formulation process since the quasi-prepolymers are less viscous and require low processing temperature.
Bonding - Bonding is the process that involves adhering the rubber cover to the surface of the roller core. It is done by using a chemical bonding agent that strongly adheres to the outer surface of the roller core. Once the bonding components are applied, the polyurethane building process can begin. The building is the process of covering or lining the rigid roller core with a rubber compound.
Other rubber compounds used for roller linings are in the form of calendered sheets and strips. They are joined to the roller core by plying and extrusion. Polyurethane formulations are available as liquid mixtures. Thus, the processes of building polyurethane are casting and injection molding. Both of these methods use liquid resins.
Casting involves placing the roller core into a mold where the polyurethane polymer is poured. The process of casting is an economical method for manufacturing polyurethane rollers and is much less expensive than injection molding.
Injection molding requires expensive dies that must be machined and configured to the shape of the desired polyurethane roller. It involves a great deal of time and the use of expensive machinery and equipment.
Curing and Cooling - Curing is the process of creating cross-links between the chained molecules of elastomer compounds. This makes the rubber more stable, enabling it to resist the effects of heat, cold, and solvents. Curing is done by applying heat to the system, which initiates the bonding of the curative agents. In some polyurethane polymer systems, curing can also be done at room temperature. After heating, the polyurethane is allowed to cure for several minutes or hours. Finally, the polyurethane roller is cooled and released from the mold.
Other polyurethane systems employ an additional curing process known as post-curing. Post-curing further improves the mechanical properties of the cast, as well as its temperature aging resistance.
Machining - This process smooths the surface of the cast polyurethane rollers by removing protruding areas and flashings. Grinding is the typical process; it is done by rolling the polyurethane roller against an abrasive wheel. Other machining processes can be involved, such as cutting and laser engraving to produce surfaces with customized profiles.
Polyurethane Compound Quality Testing - Most large-scale polyurethane roller manufacturers have in-house testing capabilities to monitor the quality of cast polyurethanes on their roller products. Polyurethanes are tested to evaluate their basic properties, such as hardness, abrasion resistance, and tear strength. Other test methods are utilized for more specific applications, such as accelerated aging and heat resistance tests for high-temperature applications.
Chapter 7: Uses for Polyurethane Rollers
The choice of polyurethane rollers for an industrial application is mainly based on their durability and wide range of durometers. While tooling for manufacturing rubber rollers is expensive and time-consuming, polyurethane casting uses aluminum molds that are easily formed and shaped to the proper dimensions of the designed roller.
Although polyurethane rollers are easily produced, they have exceptional durability, are resistant to abrasion damage, and come in sizes to fit all types of roller applications.
The choice of polyurethane rollers as conveyor rollers is due to their noise abatement and abrasion and solvent resistance. In packaging and shipping, conveying systems are the foundation of efficient and smooth operations. Materials have to move quickly without being damaged or mishandled. Polyurethane rollers are an important part of the effortless conveyance of parts, tools, customer orders, and equipment. Their noise suppression properties assist in keeping the work environment stress free for workers.
An essential part of a conveyor roller system is idler rollers, which perform different functions to assist in the movement of materials along a conveying belt. The two types of idler rollers are carrying and return, with carrying rollers moving materials while return rollers support the conveying belt during its return.
Trough - A common type of carrying roller is a set of trough idler rollers in the shape of a trough, with a trough frame on the load side of the conveying system. They have a central idler and two wing idlers, with central idlers coming in several widths. The wing idlers of a trough idler come at 20°, 35°, and 45° angles.
Impact Idlers - Impact idlers are used for conveyor belts that have loads dropped on them. Polyurethane rollers designed for impact have a ring shape to buffer and absorb the weight of heavy loads and reduce the amount of damage to the conveyor belt. The center roller for an impact idler is longer, while the wing idlers are shorter at 20° angles to handle the dispersal of the load material and make materials available for inspection.
Flat Idlers - Flat idlers are the most common form of idlers and are found supporting nearly every type of conveyor belt. They are designed for high-speed conveying processes and come in different lengths to match the width of the conveyor. Flat idlers are designed to withstand the weight and impact associated with production and shipping operations.
On the return side, flat return idlers are used to support the return of the conveyor belt. They have a steel support rod and lifting brackets to keep the belt from stretching, deforming, and slacking.
Industrial polyurethane coated casters are a rugged, durable, tough, and strong type of caster capable of carrying up to five tons. They have a higher capacity than rubber wheels and are made of high density thick polyurethane. These polyurethane casters are chosen because they can carry exceptionally heavy loads without damaging floors or other surfaces.
As with polyurethane rollers used on conveying systems, industrial polyurethane casters operate noiselessly and have a larger footprint, which reduces stress on the floor. The longevity of polyurethane industrial casters is due to their elasticity, which provides ergonomic benefits.
Polyurethane rollers are cylindrical rollers covered by a layer of elastomer material called polyurethane. The layer of polyurethane has intrinsic properties that protect the inner roller core, such as abrasion resistance and impact strength.
Polyurethane rollers have several advantages over ordinary rubber rollers. Some of these advantages are versatility, durability, simpler processing, and water and oil resistance.
There are four components that make up polyurethane. These are polyols, diisocyanates, chain extenders or curatives, and additives.
Polyurethane is regarded as an engineering material because of its excellent properties. Most of these properties are attributed to its highly elastic nature.
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