Curing and Crosslinks
In the raw state, rubber has a relatively low strength since it contains weak intermolecular bonds. To make rubber more suitable for industrial uses, the strength of the intermolecular bonds can be increased by crosslinking the polymer molecules.
Crosslinking is when separate polymer chains are bound together by a chemical bond. This process is known as curing or vulcanisation of rubber, and it produces a rubber with high tensile strength, high elasticity, and a non-sensitivity to a wide change of temperature. Figure 1 shows how polymer chains are held together by the formation of crosslinks.
The crosslinked rubber molecules develop a net-like rubber structure that reinforces the material and increases its stiffness by preventing intermolecular movement of the rubber polymer chains. The more cross-links there are the less mobility there will be for the polymer molecules. Elastomers are usually lightly crosslinked. This allows the polymers to stretch whilst still retaining their structure.
Before being vulcanised, the polymer chains in a rubber are held together with weak intermolecular forces. Curing the rubber causes the boiling and melting points to increase as the covalent crosslinks formed require a higher amount of energy to break. Rubbers with longer polymer chains also have higher melting points as there will be more crosslinks between the polymer chains.
The vulcanisation process also increases the elasticity of the rubber. The flexibility of elastomers comes from the ability of the long chains to reconfigure themselves to distribute an applied stress. The covalent cross-links ensure that the elastomer will spring back to its original shape once the stress is removed. The crosslink network formation means that cured rubber becomes insoluble in any solvent.
The process of sulphur vulcanisation was first discovered by Charles Goodyear when he accidently heated natural rubber and sulphur together. At high temperatures the carbon-to-carbon double bonds “open up”. This allows the sulphur molecules to react with the carbon atoms to form covalent polysulphide links between the polymer chains. The chemical formula for the reaction between sulphur and a polymer chain are shown in Figure 2.
The amount of sulphur added determines the extent of the stiffness of the vulcanised rubber. A wide range of physical properties of rubber can be obtained by controlling the amount of sulphur and accelerator concentrations in the process of vulcanisation. Normally about 2 parts by weight of sulphur is used per 100 parts of rubber, this stops the material from becoming too brittle. This method of sulphur curing can only occur when there are double bonds present in the polymer chain. Nowadays, other methods of vulcanisation exist to create crosslinks in different rubbers. To read about this, see our article on Vulcanisation.
Vulcanised rubber will retain its shape and cannot be processed by any means that requires it to flow such as processing in a mixer, moulder, or extruder. This means that vulcanisation must only be performed once the rubber product is in its final geometric form.
If too many crosslinks are formed between the polymers, the material can become over cured. This results in the hardening of the rubber and loss of flexibility as well as a reduction in tensile strength. On the other hand, under cure of a rubber can lead to reduced strength and poorer compression set resistance, which could ultimately reduce the quality of the rubber product and cause it to fail. Compression set resistance is the measurement of the strain retained in a material once the compression force is removed and it indicates how well the rubber can revert to its original shape. It follows that the number of crosslinks must be controlled to ensure that the vulcanised rubber has the desired properties.
The optimum cure of rubber is the degree of vulcanisation or number of crosslinks which leads to the ideal material properties. The optimum cure time is the amount of time which a rubber requires to be vulcanised for to reach this level of cure.
A common way to test the cure characteristics of a rubber compound is by using a Moving Die Rheometer (MDR). The MDR can be used to determine the onset of cure, the cure rate, and the time to optimum cure level. MDR tests are often used to verify that a material meets the desired specifications.
During the test an uncured sample is heated and pressurised inside a sealed test chamber whilst being sheared by the oscillation of the lower die. The heat and pressure cause crosslinks to form between the rubber molecules. As the number of crosslinks increases, the stiffness of the sample will also increase. This increase of stiffness can be calculated by measuring the torque response of the material. The torque response is related to the resistance of the material to being sheared by the oscillating die and is proportional to an increase in low strain modulus of elasticity. The MDR measures the torque of the material using a torque transducer and a graph of torque against time can then be plotted. A typical MDR test graph is shown in Figure 3.
The curve on the MDR graph is referred to as the S’ curve and shows the cure rate of the material. The cure rate of the material will depend on the temperature at which the test is carried out. The graph shows the three phases of the curing process: induction period, curing and over curing.
During the induction period, there are no crosslinks between the rubber polymers. The rubber is very fluid at this point and as the rubber material is heated, its torque response decreases until it reaches a minimum value (ML).
As crosslinks begin to form in the rubber, the torque increases. The curing period of the rubber begins at ts1. This is the time at which the torque increases from the minimum value by 1 unit (M,L + 1), and it gives an indication as to when crosslinks begin to form in the rubber. The time in which rubber can be heated without vulcanizing is known as the scorch time. Scorching is the undesired early vulcanisation of rubber during processing and can lead to a decrease of plasticity and mobility, meaning that the material cannot be easily shaped or moulded. The longer the scorch time, the less likely that scorching will occur during processing.
The optimum cure for the rubber occurs at tc90. This is the cure time at which 90% of the cure has taken place. After tc90 time has elapsed, the material will enter the overcure period. Continuing to heat the rubber past the optimum cure can cause marching or reversion. Marching occurs due to a continuous occurrence of crosslinking reactions and can lead to instability of the final cured material. Conversely, a decrease in torque after the maximum has been reached indicates a reversion of the crosslinking process in the compound. This reversion is caused by thermal aging of the rubber. To read more about this phenomenon, have a look at our article on Ageing of Rubber Materials.
The results from a Moving Die Rheometer test can be useful to indicate the scorch time and the optimum cure time of the rubber at a given temperature and pressure. Other properties of the rubber can be determined from the MDR graph such as the hardness of a cured material which can be estimated from the max value of the cure rate curve.