Many variables exist in the rubber compound mixing process that have a direct effect on end compound quality. These variables must be carefully controlled to ensure good processability and quality of the final part.
When it comes to mixing, there are important parameters that must be monitored and controlled during the mixing process to drive consistent compound performance. Failing to do so can have a big impact on processability and product function. The following are four common problems that can arise in a formulation when mixing processes are not properly controlled.
Viscosity is a rubber compound’s resistance to flow. Compounds with very high viscosity can be nearly impossible to process in extrusion, molding or calendaring operations. Conversely, compounds with a very low viscosity may lack proper strength and have issues holding and keeping shape during the curing process.
Compound viscosity is influenced by the formulation itself, as well as the shear rates and duration of the mixing process. At LMI, the incoming quality of the raw materials that make up the formulation is carefully monitored and controlled by the Quality Assurance Department.
Once the raw materials are put into the mixing process, a control system carefully monitors mixer rotor speed, ram pressure, batch temperature and energy input throughout the entire mix cycle. This control helps to ensure that each batch is mixed the same way, with the same heat history and cycle time. Together, this helps to keep the mixed compound’s viscosity in the proper range for which it was designed and consistent batch to batch.
Dispersion can be defined as the degree to which all of the raw materials of a rubber formulation are uniformly mixed into a single homogeneous mixer. Dispersion is impacted by the quality of the raw materials used, age and condition of the mixing equipment, and the various parameters selected for mixing.
Proper dispersion will promote appearance, processability and physical properties of the rubber compound. Conversely, poor dispersion can lead to imperfections in product appearance as well as failure to meet required properties of the end product.
Today, most rubber formulations can be sufficiently mixed in a single mixing cycle. In special cases, optimal dispersion can be best accomplished using a two stage or masterbatch/final mixing procedure. In the masterbatch stage, all raw materials except the curatives are added and dispersed. The masterbatch is returned to the mixer for a second stage, where the curatives are added and dispersed at very controlled conditions.
In single and two stage mixing, it is imperative that the shear rates and temperature inside the mixer are carefully controlled. If local or aggregate zones of the compound get too high in temperature for too long, the conditions can prematurely kick off the cure reaction, introducing the possibility of pre-vulcanized or “scorched” material within the compound. The scorched material will cause processing and performance issues during the curing process. A rubber compound that is completely “scorched” cannot be used and must be scrapped.
Once compounds are mixed with curatives, they will be subjected to a shelf life. The shelf life of a compound is a function of both its cure speed and the storage conditions of the compound. At LMI, mixed compound is stored in a cool and temperature-controlled environment to preserve the integrity of the compound before it is shipped to the customer.
Bloom is a phenomenon that occurs on the surface of a cured rubber part when raw materials in a rubber compound exceed their solubility limits and gradually migrate to the surface over time. Most bloom is generally considered undesirable, affecting both the appearance and performance.
Bloom can appear in many forms, from a light powdery residue to a dark oily bleed on the surface of parts. It can cause a loss of tack and create a mode of failure in either static or dynamic applications. The rate of bloom is highly influenced by temperature.
To help avoid bloom, a rubber chemist will typically select raw materials for a formulation that will retain compatibility with each other for the life of the part. Even still, bloom can also be impacted by the purity of raw materials and dispersion quality.
Systems that Ensure High Quality Rubber Compounds
LMI’s climate-controlled facility, state-of-the-art equipment and mixing control system provide exceptional control of the mixing process from start to finish, from the moment raw materials arrive to the moment product leaves for shipment. This drives the unparalleled consistency needed for the reliable production of high quality technical goods.
In-house compound development and testing ensure quality are built into the design of a new compound as well. LMI’s technical team has over 100 years of combined experience in the rubber industry and provides exceptional technical support to customers. Contact LMI to discuss your rubber compound needs today.