How can I tell if a Caster Wheel is made of rubber or polyurethane?

I have a lot of wheels that I want to sell, but do not know if the wheels of rubber or polyurethane. Is there any evidence or touch the main visual difference / between the two materials that will help me determine what are those facts?

Rubber strain crystallizes. One of you put wheels under intense pressure, for example a press, then, if it crystallizes, which means they shatter, then you have rubber. Both materials are polymers and this is the only way I can think of giving the information I have. Hopefully, a expert will come and handle this.

200 x 40mm Rubber Caster Wheel Red Black for Industrial Machines $44.02

Caster 5″. Swivel Plate 3-3/4×4-1/2. Rubber on Poly $6.80

Caster 5″. Swivel Plate 3-1/2×3-1/2.Rubber on Poly $6.80

Caster 3″. Total Lock Swivel Plate. Rubber Wheel $6.80

5″ Black Rubber Wheel Swivel Roller Caster for Trolley $31.41

Caster 4″. Total Lock Swivel Plate. Rubber Wheel $7.94

Caster 5″. Total Lock Swivel Plate. Rubber Wheel $8.47

Replacement 2″ Dia Gray Rubber Wheel Rigid Fixed Industrial Caster $6.47

Caster 6″. Swivel Plate 4×4-1/2. Rubber on Steel Wheel $12.94

Caster 3″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.30

Caster 4″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.56

Caster 5″ .Threaded Stem 3/8-16×1-1/2.TP Rubber $3.85

Caster 3″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.30

Faultless 3″ Gray Rubber Grip Stem Caster with Brake! $4.14

Caster 4″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.56

Caster 5″ .Threaded Stem 1/2-13×1-1/2.TP Rubber $3.85

Caster 3″ .Threaded Stem 1/2-13×1. Rubber on Poly Wheel $5.63

Caster 4″. Threaded Stem 1/2-13×1. Rubber on Poly Wheel $5.92

Caster 5″ .Threaded Stem 1/2-13×1. Rubber on Poly Wheel $6.49

Caster 3″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $5.63

Caster 4″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $5.92

Caster 5″. Threaded Stem 1/2-13×1-1/2. Rubber Wheel $6.49

Caster 3″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $5.92

1 Hard BLack Rubber Colson Replacement Caster 5″ wheel ROLLER CASTER TIRE $8.00

Caster 5″. Threaded Stem 5/8-11×1-3/4. Rubber Wheel $6.49

Caster 3″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $5.92

1 Hard BLack Rubber Colson Replacement Caster 5″wheel ROLLER TIRE CHAIR $8.00

Caster 5″. Threaded Stem 5/8-18×1-3/4. Rubber Wheel $6.49

Caster 3″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $5.63

Caster 4″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $4.91

Caster 5″. Threaded Stem 3/4-10×1-3/4. Rubber Wheel $6.49

Caster 3″. Total Lock Threaded Stem. Rubber Wheel $6.73

Caster 4″. Total Lock Threaded Stem. Rubber Wheel $9.24

Caster 5″. Total Lock Threaded Stem. Rubber Wheel $8.29

Caster 3″. Grip Ring Stem 7/16×1-3/8. Rubber on Poly $3.30

Caster 4″. Grip Ring Stem 7/16×1-3/8. Rubber on Poly $3.56

Caster 5″. Grip Ring Stem 7/16×1-3/8. Rubber on Poly $3.85

Caster 3″. Grip Ring Stem 7/16×1-15/16. Rubber on Poly $3.30

Caster 4″. Grip Ring Stem 7/16×1-15/16. Rubber on Poly $3.56

Caster 5″. Grip Ring Stem 7/16×1-15/16. Rubber on Poly $3.85

Caster 5″. Sheet Pan Rack. Rubber Wheel $4.25

Caster 5″. Metro & Eagle StemCaster. Rubber Wheel $6.93

Handling Truck 5″ Black Rubber Wheel Fixed Industrial Caster $27.58

Caster 3″. Stemless. Rubber on Poly Wheel $4.86

Caster 4″. Stemless. Rubber on Poly Wheel $5.19

Caster 5″. Stemless. Rubber on Poly Wheel $6.05

Caster 3″. Total Lock Stemless. Rubber Wheel $7.30

Caster 4″. Total Lock Stemless. Rubber on Poly Wheel $7.79

Caster 5″. Total Lock Stemless. Rubber on Poly Wheel $8.40

Caster 4″. Expandable Rubber Applicator. Rubber Wheel $4.06

Caster 5″. Expandable Rubber Applicator. Rubber Wheel $4.35

Caster 3″. Expandable Rubber Applicator. Rubber Wheel $5.78

Caster 3″. Expandable Rubber Applicator. Poly Wheel $4.75

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Inter Metro Optional Black Zinc-plated Fork 3 inch Rubber Caster with Brake $13.24 The creative business center that lets you organize the workplace in style….

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Bamboo Kitchen Island Cart with Wood & Black Granite Top $444.56 Expand your kitchen workspace with this natural finish work island featuring an elegant granite top inlay. The cart is complete with chrome finished hardware, towel rack two fixed shelves, one fixed shelf behind doors, two spacious pull out storage drawers, and heavy duty locking rubber casters for easy mobility and safety. The back of the cart is fully finished….

Rubbermaid 5E28 Deluxe Tool Tower Rack with Casters, Holds 40 Tools $48.00 Store and organize over 36 tools for easy access and more floor space with this tool rack from Rubbermaid. The tool tower stores rakes, brooms, snow shovels, and gas-powered weed eaters for easy access and more floor space in the garage or shed. Front clips allow head-down storage of spades and brooms to simplify access and so that the rack doesn’t get top heavy. The clips also enable easy slide-i…

Slipstick CB840 Large Castor Cup Gripper, Caramel $8.46 The patented Large Castor Cup is designed for castor wheels. It contains an O-ring specially formulated to protect floors and stop furniture on wheels from moving….

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