Understanding abrasion resistance in polymers

Abrasion/wear resistance pertains to a material's capacity to withstand gradual surface volume loss caused by mechanical actions like repetitive rubbing, sliding, or scraping². The two types of abrasion resistance are wet abrasion and dry abrasion. Both these methods are explained below.

 

Dry abrasion

Dry abrasion occurs when a dry, abrasive material rubs against the surface of a polymer (Figure 1). This type of abrasion is common in applications where the polymer is exposed to solid particles, such as sand, dust, or dirt.

 

Figure 1: Schematic Diagram of the ASTM G65 Dry Sand Rubber Wheel Abrasion Test⁷

Wet abrasion

Wet abrasion involves the presence of a liquid, such as water or a lubricant, during the abrasion process. The liquid can act as a carrier for abrasive particles or can soften the polymer surface, making it more susceptible to wear (Figure 2). 

 

Figure 2: Schematic Diagram of the ASTM G105 Wet Sand/Three-body Abrasion Test Setup⁸

Comparing polymeric samples based on abrasion type
 

The abrasion resistance of three different samples was tested using dry sand (G65) and sand slurry (G105) methods. Figure 3 shows the mass loss of each sample by two different methods of testing. The sand slurry method showed more mass loss than the dry sand test method. Therefore, the test method can make a big difference in evaluating the abrasion resistance behavior.

 

Figure 3: Comparison of Dry Sand vs. Sand Slurry Abrasion Test for the Sample⁹

Abrasion resistance is a critical property for many polymer applications, particularly those exposed to mechanical stress and abrasive environments. Polymer additives play a significant role in enhancing this property¹¹. Here are some additives that can improve the abrasion resistance of polymers.

 

Reinforcement additives⁹
 

Fillers and reinforcements provide a physical barrier to abrasive particles, reducing their penetration into the polymer matrix. This helps to prevent wear and tear. Some of the commonly used reinforcements include:

 

• Synthetic fibers: They offer excellent mechanical properties and can significantly enhance abrasion resistance. For example, glass fibers, carbon fibers, and aramid fibers. Chopped glass fibers such as short and randomly oriented glass fibers can improve impact resistance and abrasion resistance.
• Natural fibers: They can be added to polymers to improve abrasion resistance and provide a more sustainable material. For example, cotton, flax, and other natural fibers.
• Glass flakes: They are flat, elongated particles that provide excellent abrasion resistance and dimensional stability. They can reinforce the polymer matrix and reduce surface friction.
• Silica: It is a common filler known for its high surface area and reinforcing properties. It can improve abrasion resistance by providing a physical barrier to abrasive particles and enhancing the polymer's mechanical strength.
• Alumina: It offers excellent hardness and wear resistance, making it a suitable filler for applications requiring high durability.
• Talc: It is a soft, platy mineral that can improve abrasion resistance by reducing friction and enhancing the polymer's surface smoothness
• Mica: It is a layered silicate mineral with excellent mechanical properties and thermal stability. It can improve abrasion resistance and dimensional stability.
• Carbon black: It is a highly pigmented filler that offers both abrasion resistance and UV protection. Its fine particle size and high surface area contribute to its reinforcing properties.

Lubricants⁹

Lubricants decrease the coefficient of friction between the polymer surface and the abrasive particles, minimizing wear and tear.

 

• Waxes: They can reduce friction between the polymer surface and abrasive particles. For example, paraffin wax, beeswax, and other waxes.
• Fatty acids: They can act as lubricants and improve processability. For example, stearic acid, oleic acid, and other fatty acids.
• Silicones: Silicone oils and greases can provide excellent lubrication and release properties.
• Fatty acid amides: Compounds derived from fatty acids can improve the polymer's processability and reduce internal friction.
• Polymeric lubricants: Polymers like polytetrafluoroethylene (PTFE) can provide low friction and improve abrasion resistance.

Crosslinking agents

Crosslinking agents create a more rigid polymer network, making it less susceptible to deformation and abrasion. This enhances the material's resistance to wear.
 

1. Peroxides: Organic compounds that decompose to form free radicals, initiating crosslinking reactions. Common peroxides include benzoyl peroxide and dicumyl peroxide.
2. Epoxies: Reactive compounds that can form crosslinked structures when mixed with appropriate curing agents. Epoxy resins can be used to enhance the abrasion resistance of polymers.

Polytetrafluoroethylene (PTFE)
 

PTFE has the lowest coefficient of friction among all anti-additives. The PTFE molecules that are ground out during the friction process will form a lubricating film on the surface of the part. It has good lubricity and wear resistance under frictional shear. In high-load applications, PTFE is the best wear-resistant additive. These high-load applications include hydraulic piston ring seals and thrust washers. The most appropriate PTFE content is 15% PTFE for non-crystalline and 20% PTFE for crystalline plastic.

 

Polysiloxane

Polysiloxane fluid is a migratory wear-resistant additive. When added to thermoplastics, the additive will slowly migrate to the surface of the part and form a continuous film. Polysiloxane has a wide range of viscosity which is measured in centistokes. The viscosity of the polysiloxane is extremely low, and it migrates to the surface of the part in a fluid state to provide wear resistance. If the viscosity of the polysiloxane is too low, it will be more volatile and will quickly disappear from the part.

 

Molybdenum disulfide
 

Common name for molybdenum disulfide is "Moly". It is a wear-resistant additive mainly used in nylon plastics. Molybdenum disulfide acts as a crystallization agent to increase the crystallinity of nylon. This makes nylon material produce a harder and more abrasive surface. It has a high affinity for metals. Once adsorbed on the metal surface, the molecules of molybdenum disulfide will fill the metal surface with microscopic pores and make it more slippery. This makes molybdenum disulfide an ideal wear-resistant additive for applications where nylon and metal rub against each other.

 

Other commercial additives

Some additives can alter the surface properties of the polymer, making it more resistant to abrasion. Commercially available additives that enhance polymer properties are highlighted below.
 

• Irgasurf® SR 100 B by BASF: Provides excellent scratch and mar resistance. It lubricates the surface to reduce scratch visibility. It is typically used at a level of 1-3%.  
• ScratchShield™ by Ampacet: It can be used in PET packaging, bottles, and preforms. It can help with scratch and abrasion protection and can also provide slip properties. 
• NM-2T by Chengdu Silike Technology: It is a pelletized formulation that can improve the abrasion resistance of EVA or EVA-compatible resin systems. It contains a 50% ultra-high molecular weight (UHMW) siloxane polymer dispersed in EVA resin.
• SILIKE® LYSI-306 by Chengdu Silike Technology: It is a pelletized formulation with 50% UHMW siloxane polymer dispersed in polypropylene (PP). It is widely used as an efficient additive for PP-compatible resin systems to improve the processing properties and surface quality, such as better resin flow ability, mold filling & release, less extruder torque, lower coefficient of friction, and greater mar and abrasion resistance.

• SILMAPROCESS by Silma: These additives improve the abrasion resistance of rigid plastics (PE, PP, PS, HIPS, PA, PET, etc.,) and thermoplastic rubbers (SBS/SEBS, TPV, TPE, copolyesters, EPR/EPDM, EVA, POE, TPU, etc.,). They can also improve other surface properties, such as surface smoothness, scratch resistance, and hydrophobicity. 
   

   

Several factors can influence the abrasion resistance of polymers. These are explained in detail in the below section.

 

Polymer structure

The molecular structure of a polymer significantly impacts its abrasion resistance. Key factors that influence the polymer structure are given below¹.
 

1. Crystallinity

   
   Highly crystalline polymers generally exhibit better abrasion resistance due to their rigid structure and resistance to deformation. Amorphous polymers, on the other hand, are more susceptible to wear.

    

2. Molecular weight

   
   Higher molecular weight polymers often have stronger intermolecular forces, leading to improved abrasion resistance. However, excessive molecular weight can also make the polymer more brittle. This can negatively affect its wear resistance.
    

3. Branching

   
   Branched polymers can have lower abrasion resistance compared to linear polymers. This is because branching can disrupt the polymer's chain structure and reduce its strength.
    

4. Crosslinking

   
   Crosslinked polymers have a more rigid network structure, which can enhance abrasion resistance. Crosslinking can be achieved through chemical reactions or the addition of crosslinking agents.

Additives
 

Additives can significantly influence the abrasion resistance of polymers. Common additives are as follows.

 

1. Reinforcing fillers and fibers

   
   Abrasion resistance can be improved by providing a physical barrier to abrasive particles and reinforcing the polymer matrix. For example, inorganic materials like silica, alumina, and talc. Fibers and particles can enhance abrasion resistance by providing additional strength and rigidity.

    

2. Lubricants

   
   Lubricants reduce friction between the polymer surface and abrasive particles can improve abrasion resistance.

    

3. Antioxidants

   
   Antioxidants can protect the polymer from degradation caused by oxidative processes, which can reduce its abrasion resistance.

    

4. UV-stabilizers

   
   UV stabilizers can protect the polymer from degradation caused by ultraviolet radiation, which can also affect abrasion resistance.

The amount or concentration of additive used affects the degree of improvement in abrasion resistance. Processing conditions such as mixing, compounding, and molding conditions can impact the dispersion and effectiveness of additives.

 

Environmental factors

Some environmental factors can affect the material's performance including:

 

1. Temperature

   
   High temperatures can accelerate the degradation of polymers and reduce their abrasion resistance. Low temperatures may also affect the polymer's properties, such as its flexibility and hardness, which can influence its wear resistance. However, a hard material is not necessarily highly abrasion resistant if it lacks other necessary properties. For example, HDPE is softer than PVC, but HDPE is much better at wear resistance³.
    

2. Humidity

   
   Humidity can affect the polymer's properties, such as its moisture absorption and swelling, which can impact its abrasion resistance.
    

3. Chemicals

   
   Exposure to chemicals can cause degradation of the polymer, leading to a reduction in abrasion resistance. The specific chemicals involved, and their concentration can affect the degree of degradation.

An abrasion-resistant plastic is a material that exhibits high resistance to wear and damage caused by friction and abrasive particles. The specific choice of polymer depends on the type of application. Factors such as the type of abrasion, environment, and desired properties of the material⁹ should also be considered.

 

Polymers with different levels of abrasion resistance
 

Different polymers have varying levels of abrasion resistance¹⁰. Some of them are explained below.
 

1. Nylon or Polyamide (PA) is known for its excellent abrasion resistance and for applications requiring durability. The specific abrasion resistance can vary depending on the type of nylon and the formulation. Nylon is a popular choice for activewear, backpacks, and luggage items.
2. Epoxy has a higher abrasion resistance than other polymers, such as polyurethane.
3. Polysiloxane has a higher abrasion resistance than polyurethane.
4. Polyethylene (PE) has a low coefficient of friction, which allows it to glide over surfaces instead of scouring them. 
5. Polyurethane (PU) has a lower abrasion resistance than epoxy and polysiloxane and may not protect underlying coats. PU with a durometer of 90 Shore A has more abrasion resistance than ultra-high molecular weight polyethylene (UHMWPE).
6. Polyvinylchloride (PVC) can exhibit moderate abrasion resistance, but its performance can vary depending on the specific formulation and additives.
7. Polyether ether ketone (PEEK) is a thermoplastic known for its naturally low-friction and well-rounded, abrasive, and fatigue wear resistance. Its impressive durability and performance in punishing environments make it a choice material for components across a wide variety of industries.
8. Polydicyclopentadiene (PDCPD) is a liquid plastic raw material with excellent abrasion resistance and low friction. This thermoset resin shows elasticity, lightweight, impact- and corrosion resistance.
9. Acetal (POM) shows low friction, high abrasion resistance, strength, and excellent performance in wear applications. High tensile strength, fatigue endurance, and creep resistance make it ideal for high-performance parts.
10. Polyester is a synthetic fiber with high abrasion resistance. It is commonly used in upholstery, workwear, and outdoor gear.

Applications of abrasion-resistant polymers

The optimal abrasion resistance of plastic makes it a good choice for components that undergo frequent wear and friction stresses. Some of the applications of abrasion-resistant plastics are given below¹².

 

Industrial machinery parts

Abrasion-resistant plastics are beneficial in conveyor belts, bearings, rollers, gears, and sprockets. These plastics are useful in smooth surfaces and are resilient to constant grinding motions. The material significantly increases part lifespans in high-impact zones such as industrial gears and abrasion-resistant conveyor rollers.

 

Food processing equipment

The non-corrosive, food-safe (FDA-approved) materials like nylon are used in several food processing equipment. For example, agitator tanks (mixing paddles) in pharmaceutical plants and cutting blades for food processing. It holds up to caustic cleaners and sanitization chemicals.

 

Electronic device housings

Every day drops and scratches degrade plastic coverings for phones (phone cases), laptops, and other electronics. Abrasive-resistant plastics provide exterior protection from surface scuffing.


 

Automotive components

Gears, bearings, and clutch parts in transmissions and engines withstand tremendous friction, heat, and pressure over time. Nylon's hardness and thermal resistance keep these components functioning.


 

Protective work gear

Goggles, helmets, and gloves used in industrial workplace settings are subject to regular wear and impact. Abrasion-resistant plastics can offer cut, chemical, and abrasion protection.


 

Consumer goods

Knife handles, plastic pails, and floor mats experience surface scratching during daily use. Parts molded from abrasion-resistant plastics have superior longevity.


 

Abrasion resistance is typically measured in terms of mass loss, volume loss, or depth of wear. The units used depend on the specific test method. However, the commonly used units include⁶:
 

• Mass loss: grams/1000 cycles
• Volume loss: cubic centimeters/1000 cycles
• Depth of wear: millimeters/1000 cycles

The formula for abrasion resistance can vary depending on the specific test method and the desired units of measurement. However, a common approach involves calculating the rate of mass loss or volume loss per unit of abrasion.

Many test methods are employed to evaluate the abrasion resistance of plastics. These tests measure the polymer's ability to resist wear caused by friction and abrasion. Some of these are explained below.

 

Taber abrasion test

The Taber rotary platform abraser is a long established industry standard tester⁵. Taber abrasion is a common method that uses a rotating wheel with abrasive paper to simulate wear. This method is used to determine the resistance of different polymers by the action of abrasive wheels and weights. It can be modified to meet a variety of scenarios by choosing the appropriate abrasive wheel and weights for testing. The standard methods for conducting Taber abrasion test are: 
 

1. ASTM D1044 - Standard Test Method for Resistance of Transparent Plastics to Abrasion
2. ISO 9352 - Plastics - Determination of Abrasion Resistance

Sand/rubber wheel abrasion test

The Sand Rubber Wheel Abrasion Test is a standardized method to evaluate the abrasion resistance of materials. The sample is pressed against a rotating rubber wheel while sand flows between them. It measures volume loss over time. This method uses relevant standards such as:
 

1. ASTM G65 - Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus
2. ISO 28065 - Rubber-covered abrasive wheels - Determination of abrasion resistance

Scratch test
 

Scratch testing for abrasion testing evaluates a material's resistance to wear and tear caused by scratching or rubbing against another surface.
 

1. ASTM D7027-13 - Standard Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Paints Using a Linear Scratch Tester". Measures scratch resistance using a linear scratch tester with a conical or spherical indenter.
2. ISO 20502 - Plastics - Scratch resistance - Part 1: General requirements and test conditions. Provides general requirements and test conditions for evaluating scratch resistance.
3. ISO 19252 - Plastics - Determination of scratch properties

Martindale abrasion test¹⁴

It refers to the testing of textile products according to Martindale's standard system and tests the abrasion resistance of the fabric through the test. Abrasion resistance refers to the resistance of a fabric to other materials in the process of repeated friction with other materials. 

Pilling resistance is an important quality index of textile products, which directly affects the durability and application effect of the product. Martindale abrasion tester is used to test the abrasion and pilling resistance of fabric. Some test methods include:
 

1. ISO12947.2 - The abrasion and pilling resistance testing of fabrics by the Martindale method-part 2: measurement of specimen breakage.
2. ISO12947.3 - The abrasion and pilling resistance testing of fabrics by the Martindale method-part 3: measurement of mass loss.
3. ISO12947.4 - The abrasion and pilling resistance testing of fabrics by the Martindale method-part 4: measurement of appearance change.

Abrasion resistance is crucial for polymers in demanding environments. It can be enhanced through careful selection of polymer types, additives, and processing conditions. Factors such as polymer structure and environmental conditions also influence abrasion resistance. By understanding these factors and utilizing appropriate testing methods, you can develop durable polymer materials tailored to withstand wear and friction in various industrial and consumer applications.