Characteristics, Properties and Processing of EPDM

Characteristics, Properties and processing of EPDM (EPDM)

EPDM is a terpolymer of ethylene, propylene and non-conjugated diolefins, which was commercially produced in 1963. The world consumes millions of tons every year. The most important characteristic of EPDM is its superior ability to resist oxidation, ozone and erosion. Because EPDM belongs to polyolefin family, it has excellent vulcanization property. Of all rubber, EPDM has the lowest proportion. It can absorb a large number of fillers and oil without affecting the characteristics. Therefore, low cost rubber compounds can be made. (note: EPDM Chinese name: EPDM)

Properties and advantages of EPDM

The main chain of EPDM is composed of chemically stable saturated hydrocarbons and contains only unsaturated double bonds in the side chain, so it is basically a kind of saturated rubber. Because there are no polar substituents in the molecular structure and the intermolecular cohesion energy is low, the molecular chain can maintain flexibility in a wide temperature range. The chemical structure of ethylene-propylene rubber makes its vulcanized products have unique properties.

1, low density and high filling: EPDM is a low density rubber with a density of 0.87. In addition, a large amount of oil and fillers can be added to reduce the cost of rubber products, make up for the high price of EPDM rubber, and for high Mooney value EPDM rubber, The reduction of physical and mechanical properties is not significant after high filling.

2, aging resistance: EPDM rubber has excellent weather resistance, ozone resistance, heat resistance, acid and alkali resistance, water vapor resistance, color stability, electrical properties, oil filling and room temperature fluidity. EPDM products can be used for long-term at 120 ℃ and briefly or intermittently at 150 C. C. The application temperature can be raised by adding suitable antioxidant. EPDM crosslinked by peroxide can be used under more severe conditions. Ethylene-propylene-diene rubber (EPDM) can not crack at the ozone concentration of 50 × 10 ~ (-1), stretching 30% and over 150 h.

3, corrosion resistance: because ethylene-propylene rubber lacks polarity and low unsaturation, it has good resistance to various polar chemicals such as alcohol, acid, alkali, oxidant, refrigerant, detergent, animal and vegetable oil, ketone and fat. However, the stability of lipids and aromatic solvents (such as gasoline, benzene, etc.) and mineral oils is poor. Under the action of concentrated acid for a long time, the performance will also decrease. Data on the effects of corrosive gaseous and liquid chemicals on rubber properties were collected in ISO / TR7620. There are more than 80 chemicals of grade 1 action of Liu ethylene-propylene rubber. List one by one.

4, water vapor resistance: ethylene propylene rubber has excellent water vapor resistance and superior to its heat resistance. In the superheated steam at 230 ℃, the appearance did not change after nearly 100 hours. But fluorine rubber, silicone rubber, fluorosilicone rubber, butadiene rubber, nitrile rubber and natural rubber experienced obvious deterioration of appearance under the same conditions.

5, hot water resistance: ethylene propylene diene rubber has better water resistance, but it is closely related to the vulcanization system used. The mechanical properties of ethylene-propylene rubber with dithiodimorphine and TMTD as vulcanization system were slightly changed after soaking in overheated water at 125 ℃ for 15 months, and the volume expansion rate was only 0.3%.

6, electrical properties: EPDM has excellent electrical insulation and corona resistance, and its electrical properties are superior to or close to those of SBR, chlorosulfonated polyethylene, polyethylene and crosslinked polyethylene.

7, elasticity: there are no polar substituents in the molecular structure of EPDM, the intramolecular cohesion energy is low, the molecular chain can maintain flexibility in a wide range, second only to natural rubber and cis-butadiene rubber, and can still be maintained at low temperature.

8, adhesion: EPDM rubber due to the lack of active groups in the molecular structure, low cohesive energy, plus easy to spray rubber, self-adhesive and mutual adhesion is very poor.

Molecular structure and properties

EPDM is a terpolymer of ethylene, propylene and unconjugated diolefins. Diolefin has a special structure and only one of the two bonds can be copolymerized. The unsaturated double bond is mainly used as the cross chain. The other unsaturated does not become the main chain of the polymer, but only the side chain. The main polymer chains of EPDM are completely saturated. This property allows EPDM to resist heat, light, oxygen, and especially ozone. EPDM is essentially non-polar, resistant to polar solutions and chemicals, with low water absorption and good insulating properties. In the process of producing ethylene-propylene, by changing the amount of trimonomer, ethylene and propylene are produced. The alkene ratio, molecular weight and its distribution, as well as the vulcanization method, can be adjusted for their properties.

Selection of the third monomer of EPDM

The monomer of the third olefin type is vulcanized by copolymerization of ethylene and propylene to produce unsaturated in the polymer. The selection of the third monomer must meet the following requirements:

Up to two bonds: one polymerizable, a sulfidable reaction similar to two basic monomer primary bond random polymerization to generate sufficiently uniformly distributed volatility, Effect of suitable curing rate and content of Diolefin on Polymer Properties ENB and DCPD are mainly used in the production of EPDM.

The most widely used in EPDM is ENB, which vulcanizes much faster than DCPD. Under the same polymerization conditions, the nature of the third monomer affects long chain branching, increasing in the following order:

EPM-ENB-Rapid vulcanization, High tensile strength, low permanent deformation

DCPD-Anti-focus, low permanent strain, low cost

As the third monomer of diolefins increases, the following effects will occur: faster vulcanization, lower compression deformation, higher elongation, diversity of accelerator selection, reduced coke resistance and extension, and higher polymer costs.

Ethylene propylene ratio

The ethylene-propylene ratio can be changed during the vulcanization stage, and the commercial ethylene propylene ratio of the ternary ethylene propylene polymer ranges from 80 / 20 to 50 / 50. When the ethylene-propylene ratio changes from 50 / 50 to 80 / 20, the positive effects are: higher billet strength, higher tensile strength, higher crystallization, lower vitreous transformation temperature, and the ability to turn raw material polymers into pellets. and better extrusion properties. The bad effects are bad calender mixing, poor low-temperature properties, and bad compression deformation.

When the ratio of propylene is higher, the advantages are better processing performance, better low temperature characteristic and better compression deformation.

Molecular weight and molecular weight distribution

The molecular weight of ethylene-propylene rubber is usually expressed by Mooney viscosity. In the Mooney viscosity of EPDM, these values are obtained at high temperature, usually at 125 ℃. The main reason for this is to eliminate any effect (crystallization) caused by high ethylene content, thus masking the true molecular weight of the polymer. The Mooney viscosity of EPDM ranges from 20 to 100. There are also higher molecular weight commercial EPDM also produced, but generally filled with oil to facilitate mixing. Molecular weight and distribution in EPDM can be polymerized in the following ways:

The molecular weight distribution of catalyst and co-catalyst temperature modifier, such as hydrogen concentration ternary ethylene-propylene, can be measured by gel permeation chromatography using dichlorobenzene as solvent at high temperature (150 ℃). Molecular weight distribution is usually referred to as the ratio of weight average molecular weight to quantity average molecular weight. Depending on the normal and highly branched structure, this value varies between 2 and 5. Because of the bonding, EPDM containing DCPD has a wider molecular weight distribution. By increasing the molecular weight of EPDM, the positive effects are: higher tensile and tear strength, at high temperatures Higher green strength can absorb more oil and fillers (low cost). With the increase of molecular weight distribution, the positive effects are as follows: increased mixing and milling processability. However, narrow molecular weight distribution can improve vulcanization rate, vulcanization state and injection behavior.

Vulcanization type

EPDM can be vulcanized by organic peroxide or sulfur. However, compared with sulfur vulcanization, the peroxide-crosslinked EPDM has higher temperature resistance, lower compression deformation and improved vulcanization properties in wire and cable industry. The bad thing about peroxide vulcanization is higher costs. As mentioned earlier, the crosslinking rate and curing time of EPDM vary with the type and content of vulcanization. When EPDM is mixed with Ding Ji, natural rubber and styrene-butadiene rubber, the following factors must be taken into account in selecting suitable EPDM products:

When mixing with Ding Ji, due to the relatively low unsaturation of Ding Ji,

In order to adapt to Ding Ji''''s vulcanization rate, it is better to choose relatively low content of DCPD and ENB content of EPDM.

When mixed with natural rubber and styrene-butadiene rubber, it is best to choose EPDM of 8% to 10%ENB content to meet the vulcanization rate.

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