Next time you leave wet footprints on a dry floor notice how little of your sole actually contacts the surface. It’s mainly just the heel and the area in front of your toes. In most people those parts are well-padded, but diseases like diabetes reduce that cushioning, and painful ulcers can result. The solution is to increase the load-bearing area under the foot, and that’s a perfect application for PORON Medical® urethane foam.
Flexible urethane foam has many uses, from mattresses and pillows to gaskets in household appliances. Most of these applications don’t place great demands on how the foam performs, but medical applications are different. Whether used for wheelchair cushions, shoe inserts or wrist rests, foam for medical uses must be durable, compression set-resistant, and consistent from batch-to-batch.
Rogers PORON Medical® urethane foams are specially formulated to meet these challenging requirements. For a better understanding of their special properties this blog post addresses:
- How Urethane Foam is Made
- Flexible Foam Fabrication
- Foam Properties
- Medical Applications
- Characteristics of PORON Medical® Foam
How Urethane Foam is Made
When a polyol, (a compound of oxygen and hydrogen atoms) is mixed with isocyanate, (a compound of oxygen, nitrogen and carbon) and water, they react immediately, producing polyurethane and carbon dioxide gas. The process is similar to baking dough into bread, with the gas creating bubbles or pores in the material as it sets. Additives like surfactants and filler materials influence hardness, pore size and distribution, and with very precise control it’s possible to create foam with predictable properties and uniform small pores. In the case of PORON®, those pores are around 100 microns in size.
Making PORON® foam starts with creating a polyurethane ‘froth’ that gets deposited onto a moving web or belt. A gate controls the height of the froth, allowing a thin layer through to heating and then cooling stages before being coiled up on a roll for transport.
Flexible Foam Fabrication
Urethane foams like PORON Medical® foam can be fabricated several different ways. Shapes are cut from thin sheets using steel rule dies, thicker sheets are cut by waterjet cutter. Many different pressure sensitive adhesives can also easily be applied to urethane foams.
Polyurethane foam is usually specified in terms of density. This is referenced in terms of the weight in pounds of a cubic foot. Denser foam supports greater loads and retains its properties better over time. However, density does not necessarily equate to firmness.
Firmness describes how much load it takes to compress a piece of foam material. The actual measurement used is Indentation Force Deflection (IFD). IFD testing, (standardized in ASTM D3574) involves placing a disk on the foam and increasing the load until a target level of compression is achieved.
The most widely quoted measure of firmness is 25% IFD, This is the load required to compress the material to 25% of it’s free or unloaded height. Foam with a low IFD number is easily compressed, so feels softer than one with a higher number.
“Resilience” or surface feel describes the springiness of a foam material. In a foam pillow low resilience is desirable while people usually expect more springiness in their seat cushions. Resilience is determined by a foam’s formulation and is calculated by dropping a ball onto the surface. A material causing a rebound less than 40% of the drop height is considered to have a “dead” feel.
Tensile strength, tear strength and elongation are all important performance measures. Foams that tear easily and are difficult to handle tend not to make durable support products and create considerable waste for their manufacturers. The ASTM D3574 standard provides methods for testing for tensile strength, tear strength and elongation.
Foam is used in many ways in medicine, from medical device packaging to wound control. For PORON® foam the principal application is to provide support. A few of the places you might find PORON Medical® foam are:
- Wheelchair seat cushions, where users not only need comfort over extended periods but also benefit from the increased stability contoured polyurethane foam provides.
- Orthotic inserts, which provide relief from diabetes-induced ulcers through load spreading and impact cushioning. One study found PORON® provided superior dampening and durability to other cushioning materials, especially when bonded to a second material that resists shear forces.
- Running injuries – a study published in the British Journal of Sports Medicine showed that polyurethane running shoe inserts provided relief from chronic running-induced injuries.
- Bed Cushions, which prevent pressure ulcers, commonly called bed sores, by distributing pressure away from bone protrusions.
- Prosthetic devices, where they improve comfort for amputees.
- Slings & ankle supports, also to improve comfort.
- Wrist Rests, to provide support
- Head Restraints, providing cushioning to protect against vibration or impact forces.
Special Characteristics of PORON Medical® Foam
Rogers Corporation developed PORON® as an exceptionally elastic yet also highly predictable material. With an open structure porous structure it has excellent recovery characteristics and low compression set values. It has good resistance to solvents along with low outgassing, which means less odor than other foams. Being flame retardant, it’s also good in situations where fire is a possibility.
Rogers Corporation makes three types of PORON Medical® foam, plus a related product, DermaBak®, a skin care material designed for use in the backings of wound dressings. PORON Medical® is available as:
- PORON Medical® Urethane – Firm
This is an open cell material made in densities from 15 to 20 pounds with 25% IFD in the range 6 – 25 psi. Its formulation makes it suitable for applications where energy absorbency is needed.
- PORON Medical® Urethane – Slow Recovery
Formulated for low resilience, which means a very slow rebound after compression, this is ideal for creating a custom fit after compression. Density is 15 pounds and the material comes in four grades of firmness with 25% IFD numbers ranging from 0.3 – 22 psi.
- PORON® Medical® Urethane – Soft
Available in 15 and 20 pound formulations with 25% IFD ranging from 4 – 14 psi, this versatile material is suitable for most cushioning applications.
Depending on formulation and pore size, most open cell foam provides some degree of cushioning. However, low compression set resistance and poor durability render them inadequate for many medical applications. PORON Medical® foam is specially formulated to have a uniform structure of very small pores along with excellent compression set resistance. That results in predictable, and durable performance, making them the preferred choice when injury or disability necessitates cushioning. Whether the need is for a stable wheelchair seat or a long-lasting, pressure-relieving shoe insert, PORON Medical® foam should be a first choice. Have questions or would you like to schedule a call to discuss more? Please contact us today!