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Compound Interest

We refine rubber formulas through research and testing.

By RICK HUDSON

As I’m sure you’ve gathered from other articles in this issue, RL Hudson’s Engineering & Research Center is now fully functional. It’s beautiful, but it’s still just a building, right? Not exactly. I think this new facility will revolutionize our products.

A great product is the confluence of two processes: design and material selection. Anyone who has ever listened to me for two minutes knows how proud I am of RL Hudson’s in-house engineering department. The ingenuity of our engineers constantly amazes me. But the best design in the world will still fail if it’s not manufactured using appropriate materials. That’s where our new building – or, more specifically, the equipment it houses – comes in. We have, for quite some time, been testing and verifying material samples submitted by our factories. Now we also have the ability to develop our own compounds from scratch. Let me step you through our compounding capabilities.

MIXING You can’t expect a great cake if you don’t start with a great recipe. The same holds true of rubber parts. You have to mix the right ingredients in the right amounts if you hope to end up with a compound that will function effectively as the basis for an O-ring, shaft seal, hose, or other part. Until recently, our Director of Material Technology, Sam Burgess, could confer with our factories to tweak chemical formulas, but he had no way to directly experiment with those formulations himself. Now he does!

We now have our very own Banbury® mixer, a BR1600 model manufactured by Farrel Corporation. Rubber molders typically use production-size mixers to prepare batches of compound within a factory. Our BR1600 is designed to facilitate research and development in a lab setting, so it’s not as big as a production mixer. It is, however, the finest 1.5-liter (3-pound) mixer on the market, incorporating many advanced features typically found only on larger machines. These features include a programmable logic controller (PLC). The PLC allows operating parameters – such as temperature and speed of the mixer’s internal rotors – to be
closely controlled, stored in memory, and easily retrieved, thus facilitating repeatability and batch-to-batch consistency.

We use the BR1600 to mix pre-weighed rubber ingredients to a specified temperature or time at a set rotor speed. But what do we do with a batch of rubber once it’s mixed?.

MILLING The batch of rubber is conveyed to our new rubber mill, which, like the mixer, was made by Farrel and customized to our specifications by Rubber City Machinery. It is a variable speed, variable friction, cabinet-style lab mill. The mill serves two main functions. First, it gives the batch a chance to cool. Mixing generates a great deal of heat! Second, the mill facilitates sheeting (flattening) of the rubber to a specified thickness. Once this thickness is achieved and the rubber comes off the mill, samples are taken. What are these samples used for? Testing.

RHEOMETRIC TESTS After mixing a batch of rubber, you still have to process it – mold it – before it’s useful as a finished part. But how do you know the length of time, for example, that a given batch needs to spend in the mold to be properly shaped? You must determine its processing characteristics. Our moving die rheometer (MDR) helps us do just that.

Manufactured by TECH/PRO, our MDR is designed in accordance with the American Society for Testing and Materials (ASTM) D 5289 standard, as well as the International Organization for Standardization (ISO) 6502 standard. Our MDR holds a rubber sample firmly between a pair of heated dies (metal plates forming a cavity). As one of the dies rotates across a small arc, the other die gauges the reaction torque generated in the sample. The machine calculates a “cure curve” showing a number of processing characteristics, including optimum cure time for the sample.

VISCOSITY TESTS We can also gauge the viscosity (resistance to flow) of a rubber batch. This is important because a compound’s viscosity determines its ability to fill a mold properly. Different molding methods – compression, transfer, and injection molding are the big three – require different material viscosities in order to work well. If we know a part is going to be injection molded, we can compound a material so that its viscosity facilitates the injection process.

We use a Mooney Viscometer (MV) to gauge viscosity of both raw rubber stock and compounded rubber. Our MV was designed in accordance with the ASTM D 1676 and ISO 289 standards. Our MV can also conduct stress relaxation and pre-vulcanization tests.

PRESS CURING The tests I have outlined thus far are all performed on uncured compound, but there is also much to be learned by testing cured rubber. Curing (also known as vulcanization) is the heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, non-cross-linked materials into strong elastic products. We cure rubber samples in a compression molding hydraulic press with a clamping force of 65 tons. Though our press can mold prototypes, we primarily use it to prepare cured slabs and buttons. These slabs and buttons are used for testing of original physical properties, low and high temperature resistance, compression set resistance, and fluid resistance.

LOW TEMPERATURE TESTS Unless specially formulated, many rubber compounds shrink and harden as a result of extended exposure to low temperatures. We conduct low temperature tests using a Thermotron® environmental test chamber. Our Thermotron also allows us to conduct low-to-high cycling tests from -73° to 177° C (-100° to 350° F) at a rate of 3° C per minute. ASTM D 2137 outlines the measurement of brittleness point, one of the most common low temperature tests.

HIGH TEMPERATURE TESTS We conduct heat aging tests of non-volatiles using five Blue M® horizontal airflow convection ovens. Air is heated to a precisely controlled temperature, then passed over rubber samples inside a specially designed chamber. These tests are important because most of the physical and chemical properties of rubber are impacted when it meets high temperatures, especially for prolonged periods. Studying whether a sample hardens, cracks, or undergoes other changes after being in a heated test environment gives valuable clues as to how that material will perform in high temperature service conditions. Air oven tests are detailed in ASTM D 573. We can also conduct life cycling tests ranging from room temperature to 180° C.

COMPRESSION SET TESTS Cured rubber buttons are used to gauge compression set, which is the result of progressive stress relaxation. We place the buttons between the steel plates of a test fixture, then force the plates together using a bolt-tightening device and steel spacers. The buttons are compressed a particular amount (typically 25% of original thickness) for a specific time (such as 22 hours) at a given temperature (such as 100° C). These time and temperature variables are based on anticipated service conditions. We control the temperature by conducting our compression tests inside the same Blue M ovens used for high temperature tests. Once the compression is released, we measure the button. This measurement reveals to what extent the rubber has “set”; that is, not returned to its original thickness. ASTM D 395 fully describes compression set tests.

FLUID RESISTANCE TESTS We conduct fluid resistance tests using an aluminum heat block made by the Akron Rubber Development Laboratory (ARDL). Our heat block has a digital temperature controller (up to 400° C maximum operating temperature) and holds ten test tubes. We use the block to study elevated temperature aging of dumbbell samples in high flash point or non-volatile fluids. ASTM D 471 details fluid resistance tests.

And I’m sure we will add other test devices in the future. Call us at 1-800-722-6766 if we can develop a new compound for you. We’ll be happy to help!