Properties and Testing of Rubber

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Release time:

Oct 26,2021

1. raw rubber performance

Unprocessed raw rubber is commonly known as raw rubber, which is not 100 percent pure,

For example, non-rubber hydrocarbons (about 5%) contained in natural rubber include resin acid protein and other substances. Anti-aging agents and unexhausted synthetic additives, such as molecular weight control agents, terminating agents and dispersing agents, are also added to SR. Generally speaking, however, raw rubber is more representative of the inherent properties of rubber than rubber, including the following:

1. Molecular weight. Refers to the average molecular weight of rubber macromolecules, rubber should be regarded as a system of polymers with different molecular weights, both high molecular weight and some low molecular weight fractions, which is inevitable, so it can only be described by the concept of average molecular weight. According to different test methods, it is divided into viscosity average molecular weight, dispersion average molecular weight and weight average molecular weight. The viscosity average molecular weight is more commonly used, because it is easier to measure, and it is more intuitive to use different viscosities to characterize different molecular weights (the greater the molecular weight, the higher the viscosity). There is a direct and close relationship between molecular weight and the properties of raw rubber. Generally speaking, the greater the molecular weight, the higher the strength of raw rubber, the better the mechanical properties, but with the increase of molecular weight, the fluidity of the processing becomes worse.

2. Molecular weight distribution. Rubber is actually a mixture of molecular chains with different proportions of different sizes and molecular weights. If the different molecular weights are arranged according to the frequency of occurrence, the molecular weight distribution curve can be obtained.

NR molecular weight distribution characteristics:

Medium molecular weight dominates, high molecular weight and low molecular weight each account for a small number, of which the high part is conducive to mechanical properties and the low part is conducive to processing, thus taking into account the performance and processing.

SR molecular weight distribution characteristics:

The molecular weight distribution is very narrow and limited to a small range. Because of the lack of low molecular weight parts, the processability is not as good as NR, but the performance uniformity is good. The reason is that the molecular weight of synthetic rubber is artificially controlled, so the model is simple, and it is difficult to achieve both large, medium and small.

3. Gel content. This generally occurs only in SR. When the polymerization process, due to different structural control, the formation of too much branched structure, the result of this part of the gel, with a solvent can not be dissolved so called gel. Additives are difficult to enter when mixing, affecting performance.

4, side hanging group. Different groups on the rubber monomer give different properties to the rubber. Such as:-COOH (carboxyl): can give good viscosity;-CL: with polarity and electronegativity; Phenyl: bulky can block rays, so it has good ray resistance.

5. Polarity. And the group is closely related, all with nitrile (-CN) hydroxyl (-OH) and carboxyl (-COOH) groups such as rubber has a strong polarity, known as polar rubber. They have good bonding with metals, and rubber, which is close in polarity, is easily blended with each other.

Properties of 2. unvulcanized rubber

Raw rubber is mixed with additives, but unvulcanized rubber is called unvulcanized rubber, also known as rubber compound. Can be understood as semi-finished products, they are closely related to the processing process:

1) Mobility. Related to plasticity, the greater the plasticity, the better the fluidity and the easier it is to eat powder. In the process of calendering and extrusion, it is very smooth. When vulcanized, the mold cavity can be filled quickly during the heating process, while when the fluidity is not good, it is easy to lack glue.

2. Spray frost. In the formula, if an additive is added in excess and exceeds its solubility in rubber, it will gradually spray out the surface of the rubber to form a frost layer, so it is called spray frost. Oil, paraffin, antioxidant, sulfur, accelerator and other excessive addition will appear spray, affect the appearance, and affect the interface.

3, scorched. The phenomenon of vulcanization in advance during processing or parking is called scorch. This phenomenon is easy to occur in the high temperature season, the countermeasure is to add anti-scorch agent, strictly control the processing temperature.

4. Contraction, expansion and deformation. Size shrinkage and expansion occur after the rubber material is exported, which brings inconvenience to molding. Insufficient plasticity or high use ratio of synthetic rubber are the main reasons. Adding softener is beneficial to reduce shrinkage and deformation.

3. vulcanizate performance

If the performance of raw rubber and unvulcanized rubber is mainly for processing and production services, then the performance of vulcanized rubber is mainly for customers and practical applications. The properties of vulcanizates can be summarized into two categories, namely mechanical properties and environmental resistance. The former is a measure of the performance of rubber under stress, mainly including tensile strength, constant elongation strength, elongation at break, tensile permanent deformation (all carried out on a tensile machine), hardness, resilience, compression permanent deformation, tear strength, adhesion and strength, etc. The latter is to measure the performance changes of rubber in the external environment, including thermal aging performance, anti-ozone performance, flame retardant performance, anti-mold performance, etc.

First, the commonly used vulcanizate determination items are briefly described as follows

1. Tensile strength. The rubber test piece is stretched by the pulling machine, and the strength at break is expressed in Mpa, which is the most important and basic item to measure the mechanical properties of rubber. The greater the value, the greater the strength, generally 10 ~ 30Mpa.

2. Fixed elongation strength. The force required per unit area when the specimen is stretched to a certain length. Can reflect the degree of crosslinking of rubber. The higher the value, the tougher the rubber, the unit MPa.

3. Elongation at break. When the sample is broken, the percentage of the elongation part and the original length is used to indicate the limit of the strain capacity of the rubber during elongation, expressed in%.

4. Permanent deformation. The smaller the percentage of the deformed part of the sample that cannot be restored after a certain time of external force, the better the elastic recovery of the rubber.

5. Tear strength. The performance of rubber to resist tearing at a crack, expressed in KN/m as the tearing force per unit length

6. Hardness. The ratio of the load and elastic modulus required to press a steel sphere of a certain diameter into the rubber sample to a certain depth is used to reflect the elastic modulus of the rubber, and the vulcanization state can also be judged.

7. Wear resistance. The commonly used Akron abrasion machine is the most suitable for rolling abrasion parts. The test method is to position the disc-shaped test piece on the abrasion test machine and measure the rubber powder ground in the 1.61km(1 mile) stroke under a certain pressure by relative friction with the grinding wheel at a certain inclination angle (15). The rubber powder is converted into volume according to density, so the value unit is cm3/1.61km, the smaller the value, the better the wear resistance.

8, resilience. Also known as impact elasticity, refers to the degree to which rubber can recover after impact, usually expressed as rebound rate, there are two methods of determination:

⑴ The falling ball method drops a steel ball of a certain quality from a certain height and hits the rebound height on the upper side of the rubber sample.

⑵ The pendulum method uses a steel hammer with a fulcrum to impact the rubber sample, and the rebound amplitude of the side is expressed as a percentage before and after.

9. Aging resistance. Commonly used hot air aging oven method, the rubber sample is placed in an aging oven heated by hot air, and after constant temperature and timing aging (example 70 ℃ × 100h), the performance retention rate is measured to indicate the aging degree, which is called the aging coefficient. Generally, the closer the coefficient is to the 1.0, the better.

10, cold resistance. It is usually expressed in terms of brittleness temperature, and the lower the brittleness temperature, the better the cold resistance of the rubber. The method is as follows: put boiling water into a thermos flask for refrigeration. After reaching the required temperature, use a clamper to vertically feed the sample and keep it for 3 minutes to take it out. Use an impactor to impact the sample within 0.5 seconds. The highest temperature at which cracks or tears occur is the brittle temperature, which indicates the low temperature resistance level of the rubber.

11, flame retardant, insulation, acid and alkali resistance, oil resistance and other tests.