Testing and quality control of foam rubber

Foam rubber is a consumer product. Therefore various national standards organizations have taken steps to ensure the production and marketing of good quality foam rubber . Important properties tested for quality control and their specification limits are given in that follow.

• Indentation hardness index

• It is the load in kilograms required to give an indentation  in the sample equivalent to 40% of the original thickness of the sample under specified condition. It is a measure of the load bearing capacity of the foam rubber. Foam rubber product are graded by indentation hardness such as Code A has 7-14 indentation hardness.

• Compression set

• It is a measure of state of cure of the foam rubber . The test consists of compressong the test piece under specified condition of time, temperature and constant deflation and noting the effect on the height of the released test sample.The compression set of the sample when determined by the first method shall not exceed 15% and by the second method shall noe exceed 5%

• The color of latex foam rubber shall be as agreed between the purchaser and supplier.
• Flexing test that involves submitting the whole sample to a continuous flexing with an indenter for 2,500,000 cycles at 4 Hz and measuring the loss in hardness and thickness.
• Aging test 

• The test sample of aging test is kept in an oven at 70 +/- 1 *C for 168 hr. Upon removal from the oven after 10 hr, the hardness is measured. The hardness of the test sample after aging shall not vary by more than +/- 20% of the value obthained with the unaged test sample.

• Metallic Impurities

• The copper and manganese content in the test sample shall not be more than 0.001 and 0.005% respectively.

Rubber mounts

Rubber mounts or anti vibration mounts can reduce the intensity , or more precisely the amplitude, of an externally imposed oscillation or vibration in two way

1. If the frequency of the oscillation is very different from the natural frequency of the system comprising the supported mass plus the rubber mounts support, the rubber material will not deform freely at this impose frequency ; we might indeed say that it is unwilling to vibration at an “unnatural” frequency
2. When the imposed frequency is very high , say 1000 Hz or more, the imposed oscillations are more or less damped out by the viscous element in the deformation of rubber material ; this effect is call “attenuation” and is particularly inportant with sound vibrations.

Pic rubber mounts sample

Relate Topic

The flat rubber mounts

Why is the many application of natural rubber ?

Most natural rubber is used in tires (about 70% of the total production is natural rubber) because natural rubber has outstanding properties in terms of flexibility high mechanical properties and low heat buiding up during use. In addition to the properties of natural rubber and sticky stick (tack properties) makes a good tire assembly process to be easy. Therefore, natural rubber remains a key option in the production of tires. Especially truck tires, tire plane and tire forklifts etc.

In addition to tires . Many natural rubber to be used in the manufacture of rubber cushioning port (docking fender) and rubber cosapoan (bridge bearing) . Also has high mechanical properties and low creep under the force as well. In the same way natural rubber has been used in the manufacture of rubber to reduce vibration. (anti-vibration dampers) are due to these products to good mechanical properties and low heat buiding up accumulated during use.

Silicone Rubber

Which have carbon-carbon backbones, silicone rubber contain very flexible siloxane backbones,and have very low glass transition temperature. The most common silicone rubber is polydimethyl siloxane with a Tg of –127 *C . Silicone rubbers have both excellent hight temperature resistance and low temperature flexibility. In addition, they process good biocompatibility and thus are use in implants and prosthese. Other uses include gasket , seal O-ring , Ice Tyre.

Rubber Tire : Basic function of a tire

1. Provide Load carrying capacity
2. Provide cushioning and damping
3. Transmit driving and breaking torque
4. Provide cornerring force
5. Provide dimensional stability
6. Resist abrasion
7. Generate steering response
8. Have low rolling resistance
9. Provide minimum noise and minimum vibration
10. Be durable throughout the expected life span

Prinary Purpose of the Tire

The primary purpose of the tire is to transfer the driver’s action such as accelerating,steering and breaking to the road surface.

The part of the tire that rolls on the road is know as the contact patch

The friction between the road surface and the contact patch is all that ensures the vehicle follows the driver’s commands.

Picture 1 Contact Patch

Three major performances are demanded

1. Safety
2. Economy
3. Comfort

Rubber Properties: Thermal Expansivity , Pressure

The coefficient of thermal expansion for elastomer is approximately 4.8 x 10E-04 /K similar to a hydrocarbon liquid. Addition of fillers reduces the value slightly. Comparing this expansivity to that of steel (3.5 x E-05 /K) ,a tenfold difference, one begins to understand the built-in interfacial strain in a bonded rubber-metal or composite structure.

The high coefficient of expansion couple with the high bulk modulus mean that a potential exists for subtantial thermal pressure .If the elastomer is confined and subsequently heated. This describes the mouldind enveronment, with thermal pressure a neccessity for replicating mould contours and surface finish. The moulding procedure of “bumping” vent excess rubber create by this thermal expansion. If not relieved, the thermal pressure could exceed the press clamping force. This can make the mould open at the parting line, creating a condition referred to as “Backrinding”.

Beerbower has calculate these pressure using the ratio of thermal expansion at constant pressure to compressibility at constant temperature .Using Beerbower's Equation. It is possible to predict the pressure developed .

= 1.13 for saturate backbone hydrocarbon that identify according to ASTM D 1418 designation "M" ; e.g., EPDM and 1.22 for unsaturated elastomer ASTM D1418 designation " R" ; e.g., NBR , SBR , NR

T is temperature (K)

Natural Rubber

Of the range of elastomers available to technologists natural rubber (NR) is among the most important , because it is the building block of most rubber compound used in product today. It is a material that is capable of rapid deformation and recovery , and it is insoluble in a range of solvents, though it will swell when immersed in organics solvents at elevated temperature. Its many attributes include abrasion resistance , good hysteretic properties, high tear strength, high tensile strength and high green strength. However , it may also display poor fatigue resistance. It is difficult to process in factories, and it can show poor tire performance in areas such as traction or wet skid compared to selected synthetic elastomer.