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 protects the inner roller core using its intrinsic properties such as abrasion resistance and impact strength. Polyurethane rollers are used in many manufacturing processes for performing operations such as:
- Material Conveying
- Grains Milling
Polyurethane is regarded as 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 can produce products on opposite ends of the spectrum. Polyurethane can become hard and tough. These are employed in high-performance parts such as wheels and rollers. It can also be soft and pliable. These are suitable for shock-absorbing applications such as impact-absorbing pads and cushions. There is an array of formulations available on the market.
Chapter 2: Advantages of Polyurethane Rollers
Polyurethane and rubbers such as nitrile and neoprene belong to a family of materials called elastomers. Because of their elastic properties, both of them are used as roller covers. Polyurethane rollers have several advantages over ordinary rubber rollers. Their impressive mechanical properties make them more viable than other types. Below are some of the benefits of using polyurethane rollers.
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 to have different mechanical properties.
Aside from being versatile, polyurethane’s mechanical properties are also superior to that of other rubber types. Regardless of the obtained hardness, its inherent toughness and resilience make it a very durable material. Rollers with polyurethane covers last longer than the typical rubber rollers.
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.
Chapter 3: The Polyurethane Polymer System
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 form 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) to 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 so on. 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 describes how polyurethane is made. The roles of the four different components are also discussed. In this chapter, the focus is on the different chemicals used to make polyurethane and their implications on the final property of the product.
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.
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. Between the two, PTMEG offers superior quality but is more expensive.
Compared to polyether, polyesters have good abrasion resistance, heat aging resistance, oil resistance, solvent resistance, good shock absorption properties, and better tear resistance.
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. Polycaprolactone, on the other hand, 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.
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).
These types are further divided into NDI, TDI, and MDI.
Naphthalenic Diisocyanates (NDI)
NDIs are known to offer superior performance and long service life for dynamic applications. One downside of using 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 by 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 80/20 blend. Producing proportions other than the 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.
CurativeCuratives 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 these properties are attributed to its highly elastic nature. Below are some of the properties of polyurethane significant to roller applications.
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 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 can be classified into two types: sliding and impingement abrasion. Sliding abrasion occurs when a hard material like metal or ceramic slides or rubs into a softer material with or without contaminants between the surfaces. Impingement abrasion, in contrast, happens when particles impact the surface, causing erosion.
Polyurethanes blended to have a low coefficient of friction and high tear strength offer good sliding abrasion resistance. For impingement abrasion, polyurethanes with good resilience are used. Resilient polyurethanes can yield elastically. This allows forces from the impacting particles to be distributed on the surface.
Abrasion resistance is achieved by the right composition of the polyurethane resin. Among the polyol compounds for making polyurethane, polyesters exhibit better tear and abrasion resistance.
This is the ability of the rubber lining to withstand the application of tensile forces that tend to rip the material apart and propagate the tear throughout the body of the material. Tear propagation can vary depending on how the force is applied and on the microscopic structure of the material. Tear strength can sometimes be correlated with abrasion resistance. Polyurethanes are known to have both good tear strength and abrasion resistance.
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 the impact and return to its shape. It does this while dissipating the energy throughout the structure of the roller.
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 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. Special but more expensive formulations can reach 150°C.
Polyurethanes’ coefficient of friction (COF) tends to correlate with its hardness. The two properties have an inverse relationship. COF increases while hardness decreases. Since the hardness of polyurethanes is easily manipulated through blending, the desired COF can be attained as well.
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.
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. It is typically formed through rolling and welding. These processes are typically done in steel mills, which supply steel pipes and tubes as feedstock materials to polyurethane roller manufacturers.
Aside from the outer cylinder, several flanges and a rigid shaft also comprise the roller core. The flanges are used to support and attach the outer cylinder to the shaft. They are typically formed by machining. Proper sizing and cutting are done to ensure proper fitting into the cylinder.
The shaft’s main function is to transfer motion from the driver. It also provides rigidity to the core. It is fabricated by turning a metal stock in a lathe machine, producing a solid, cylindrical core. The flanges are then welded at or near the ends of the cylinder with the shaft.
Bearings are also installed to 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 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 on its surface.
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.
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.
Preparation of 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 and Building
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 or die where the polymer is transferred or poured. Through casting, economical polyurethane casts can be made. The casting process requires less expensive dies than injection molding.>
The less popular option is injection molding. Injection molding uses more expensive equipment and tooling for shaping the rollers.
Curing and Cooling
Curing is the process of creating crosslinks between the chained molecules of elastomer compounds. This makes the rubber more stable, which enables 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.
This process smooths the surface of the cast polyurethane rollers by removing and 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.
- Polyurethane rollers are cylindrical rollers covered by a layer of elastomer material called polyurethane. The layer of polyurethane protects the inner roller core using its intrinsic properties 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.