The Ultimate Guide for Polyurethane Elastomers

What is Polyurethane?

 

Polyurethane (PU), whose full name is polyurethane, is a polymer compound. It was produced by Otto Bayer and others in 1937. Polyurethane is divided into two categories: polyester type and polyether type. They can be made into polyurethane plastics (mainly foam plastics), polyurethane fibers (called spandex in China), polyurethane rubber and elastomers. Soft polyurethane is mainly a linear structure with thermoplasticity. It has better stability, chemical resistance, resilience and mechanical properties than PVC foam materials, and has smaller compression deformation. It has good heat insulation, sound insulation, earthquake resistance and anti-toxic properties. Therefore, it is used as packaging, sound insulation and filtration materials.

Hard polyurethane plastic is light, sound insulation, excellent heat insulation, chemical resistance, good electrical properties, easy processing and low water absorption. It is mainly used in construction, automobile, aviation industry, and thermal insulation structural materials.

The performance of polyurethane elastomer is between plastic and rubber, oil resistance, wear resistance, low temperature resistance, aging resistance, high hardness and elasticity. It is mainly used in the shoemaking industry and medical industry. Polyurethane can also be used to make adhesives, coatings, synthetic leather, etc.

 

What is Polyurethane Elastomer

 

Polyurethane elastomer has two soft and hard segments in its structure, so it can be designed through the molecular chain to give the material excellent properties such as high strength, good toughness, wear resistance, and oil resistance. Polyurethane, known as "wear-resistant rubber", has both the high elasticity of rubber and the rigidity of plastic.

There are three main types of raw materials for polyurethane elastomers, namely oligomer polyols, polyisocyanates, and chain extenders. In addition, sometimes in order to increase the reaction rate, improve processing performance, product performance, and reduce costs, it is necessary to add certain compounding agents (catalysts, hydrolysis stabilizers, flame retardants, solvents, mold release agents, antistatic agents, colorants, mildew inhibitors, etc.).

 

Common processing methods and characteristics of polyurethane elastomers

 

PU Elastomer Type	Advantages	Disadvantages
Maximizing Polyurethane Elastomer Characteristics	- Maximizes the characteristic properties of polyurethane elastomers	- High material loss for small products, prone to bubbles
Castable (CPU)	- Simple process	- Difficult to mold tubular and linear products
	- Low cost of processing equipment and molds	- Open mold requires post-processing
	- Can be operated mechanically or manually	- Need to pay attention to raw material storage
Foam Castable (Microcellular PUE)	- Fast reaction, good productivity	- Requires careful maintenance and management of equipment, especially temperature control
	- Suitable for small, medium, and large products	- Need to pay attention to raw material storage
	- Excellent vibration absorption
Thermoplastic (TPU)	- Suitable for the production of small products	- High mold costs
	- Can use plastic processing equipment and techniques to form products	- Poor heat resistance and permanent deformation
	- Can be used in film and leather production	- Difficult to mold large products
Mixed Type (MPU)	- Suitable for medium-scale production	- Hard products perform poorly
	- Can use rubber processing equipment to form products	- Not suitable for large product production
	- Good performance for low-hardness products

 

What is Cast Polyurethane Elastomer (CPU)

 

Among them, cast polyurethane elastomer (CPUE) is referred to as cast polyurethane (CPU) or polyurethane casting glue. "Casting" refers to a chemical processing method in which the material system of the product is liquid before molding, which can be cast, and the product is directly molded by reaction curing, and in principle, the material system does not contain volatile liquids. Because the material system is liquid before molding, and TPU and MPU products are solid before molding, CPU can also be called liquid rubber or liquid elastomer. Compared with TPU and MPU, CPU has a wider range of raw material selection and a wider range of product hardness (Shore 5A to Shore 85D), which is particularly suitable for the production of large and medium-sized products. It makes up for the limitations and shortcomings of TPU and MPU product processes, can maximize the performance advantages of polyurethane elastomers, and broaden the application field of polyurethane elastomers.

(Note: The above is from the second edition of the Handbook of Polyurethane Elastomers, or http://www.cip.com.cn)

 

Classification of CPU Elastomers

 

CPU elastomers are chemical systems that are formed by pouring a liquid reaction mixture into a mold cavity. They are classified differently according to the raw material system and processing technology used.

 

Castable Polyurethane Elastomer (CPU)

 

By Polyol Types

 

1.1. By Polyester Polyol

• • • 1.1.1. Polyadipate
    • 1.1.2. Polycaprolactone
    • 1.1.3. Other Polyester Types

1.2. By Polyether Polyol

1.3. Other Types

 

By Isocyanate Types

2.1. TDI Type

2.2. MDI Type

2.3. Other Isocyanate Types

 

By Chain Extender Types

3.1. Amine Chain Extender

3.2. Glycol Chain Extender

3.3. Amine-Glycol Hybrid

 

By Synthesis Process

4.1. One-step Process

4.2. Two-step Process

• • • 4.2.1. Prepolymer Method
    • 4.2.2. Semi-prepolymer Method

 

By Curing Temperature

5.1. Heat Curing

5.2. Cold Curing (Room Temperature Curing)

 

Main Process Routes of CPU Elastomers

 

The most commonly used process at present is the prepolymer method. The so-called prepolymer method is to synthesize prepolymers from low prepolymer polyols and diisocyanates under certain conditions, and then mix the prepolymers with chain extenders and crosslinkers for casting.

 

 

Characteristics of Polyurethane Elastomers

 

Polyurethane elastomers have outstanding comprehensive performance, which is unmatched by other rubbers and plastics. Moreover, polyurethane elastomers can be processed by almost any conventional process of polymer materials, so they are widely used, and their products are almost in all fields.

Polyurethane elastomers have a wide range of raw materials, formulas and molding methods, and have good comprehensive performance. They can combine many characteristics of rubber, plastics, coatings, adhesives, and fibers.

1. Wide hardness range, from below Shore A0 to Shore D90. Moreover, it still has good rubber elasticity and elongation at break at high hardness, and has good mechanical properties at low hardness.

(Hardness range of polyurethane elastomers)

(Note: The above pictures are from the "Handbook of Polyurethane Elastomers" (Second Edition), or http://www.cip.com.cn)

 

2. Good mechanical properties. At rubber hardness, the tensile strength and tear strength are higher than those of general rubber; at plastic hardness, their impact strength and bending strength are much higher than those of plastic.

3. Wear-resistant. It enjoys the good name of "wear-resistant rubber".

4. Oil-resistant. The oil resistance of polyester polyurethane elastomer is not lower than that of nitrile rubber, and is equivalent to polysulfide rubber.

5. Good bonding performance. It has good bonding with materials such as metal, wood, ceramics, and polar polymers.

6. Excellent radiation resistance and ozone resistance.

7. Polyether polyurethane has good water resistance and mildew resistance.

 

Generally, the shortcomings of polyurethane elastomers are high endogenous heat, poor heat resistance, especially moisture and heat resistance, and high raw material costs.

 

 

Application of Polyurethane Elastomers

 

Polyurethane elastomers have many valuable properties and are widely used. In application research, in addition to correctly selecting the type of rubber according to the use environment, the appropriate hardness should also be selected according to the stress of the product during use.

(Relationship diagram between polyurethane elastomer hardness and application)

(Note: The above pictures are from the Handbook of Polyurethane Elastomers (Second Edition), or http://www.cip.com.cn)

 

Common FA factory automation polyurethane-related parts are as follows:

 

1. Polyurethane shock absorbing material

 

Polyurethane shock-absorbing blocks	Polyurethane positioning supports	Polyurethane foam isolation pads	Polyurethane washers

 

2. Polyurethane pneumatic tube

 

Polyurethane pneumatic tube	Polyurethane pneumatic hose	Polyurethane pneumatic hose

 

3. Polyurethane timing belt

 

Conveyor type polyurethane timing belt	Connecting type polyurethane timing belt	Anti-static polyurethane timing belt

 

4. Pneumatic accessories

 

Internal thread type polyurethane clamp	External thread type polyurethane clamp

 

5. Polyurethane casters

 

Extra Heavy Duty Polyurethane Casters	Heavy Duty Polyurethane Casters	Medium and light load polyurethane casters	Functional casters (shock absorption, anti-static, etc.)

 

6. Polyurethane rollers

 

Polyurethane pulleys for flat belts	Polyurethane coated lithium battery roller	Economical polyurethane roller	Functional polyurethane coated roller (anti-static, wear-resistant, etc.)

 

7. Polyurethane coated bearings

 

Polyurethane coated follower	CPU polyurethane coated bearings	TPU polyurethane coated bearings	Functional polyurethane bearings

 

8. Polyurethane coated fasteners

 

Adjustable stop screw	Cylindrical head set screws	Economical set screws	Conventional polyurethane coated screws

 

Precautions for Polyurethane Elastomers

 

1. Polyurethane products are non-toxic and have almost no harm or pollution to the human body, the atmosphere, water quality and soil; Polyurethane elastomer waste and scraps are non-toxic and harmless to the human body, and can be crushed into particles and pressed into floor paving materials;

2. Regarding the color of polyurethane, since the color of polyurethane is affected by the different chain extenders and cross-linkers used, the products will show different colors. The common natural color of polyurethane is transparent light green (also called green, transparent or natural color), and some products made of cross-linkers are colorless, transparent or milky white, etc.;

Since polyurethane is affected by ultraviolet rays, it will turn yellow after long-term exposure. This is a normal phenomenon and has almost no effect on the performance of the product. Of course, some light stabilizers can be added to extend the time of yellowing;

(Note: The above is from the second edition of the "Handbook of Polyurethane Elastomers", or http://www.cip.com.cn)