Foam Rubber vs. Sponge Rubber: What’s the Difference?

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Doug Sharpe
President of Elasto Proxy

Did you know that there’s a difference between foam rubber and sponge rubber? Sure, the two terms are often used interchangeably. Unless you work in the rubber and plastics industry, the sponge next to your kitchen sink may seem similar enough to the foam mattress pad on your bed. Both substances are soft and squishy, right? Upon closer examination, however, saying that foams are identical to sponges is like saying that doing the dishes is the same as getting a good night’s sleep.

For technical buyers, choosing the right material is a lot more important than finding the right analogy. The memory foam from a mattress might help with after-dinner cleanups, but a polymer kitchen sponge is a better choice. For safety-related applications, selecting the right rubber material may also mean meeting requirements for flame, smoke, and toxicity (FST). In the mass transit industry, for example, some silicone foams meet FST standards but many carbon black foams do not.

What’s so different about these foams, and how does foam rubber compare to sponge rubber anyway? Let’s take a look at how these polymers are made, and consider how raw materials, chemical reactions, and production processes can affect the characteristics of foam and sponge rubber.

How Foam Rubber Is Made

The differences between foam and sponge rubber begin with ingredients and end with molecular structure. Foam rubbers use a blowing agent, typically a gas or a chemical that produces a gas, to create a mass of small bubbles in a liquid mixture. Typically, this mixture contains polyols, polyisocyanates, water, and chemicals or additives such as flame retardants, fillers, and colorants. There are many different types of blowing agents, and foaming is used for both rubber molding and rubber extrusion.

The polyols and polyisocyanates in foam rubber are liquid polymers that, when combined with water, produce a heat-generating or exothermic reaction. By using specific types and combinations of liquid polymers, a material compounder can create flexible or rigid foam rubbers. During polymerization, molecules from the polyols and polyisocyanates crosslink to form three-dimensional structures. The compounder can control foaming by adjusting the amount of water, or by using surfactants.

The importance of blowing agents in the production of foam rubber cannot be overstated. Although you can do the dishes without a sponge and get a good night’s rest without a mattress pad, a compounder cannot create foam rubber without a blowing agent. Typically, flexible foams use the carbon dioxide gas formed by the reaction of water with the polyisocynate. Most rigid foams use hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), gases with higher levels of toxicity and flammability than found in chlorofluorocarbons (CFCs).

How Sponge Rubber Is Made

Sponge rubber may be similar to foam rubber, but the two are not one and the same. For starters, there are two main types of sponge rubber: open-cell and closed-cell. Open-cell sponge rubber contains open, interconnected pockets that permit the passage of air, water, and other chemicals when the material is not compressed. Closed-cell sponge rubber contains balloon-like cells that hold nitrogen gas and thus prevent the passage of these substances at low pressures.

To produce open-cell sponge rubber, sodium bicarbonate is added to other ingredients in a heated mold. As the uncured sponge rises like a cake, the baking soda creates open, interconnected cells. To make closed-cell sponge rubber, a chemical powder that decomposes under heat and pressure is added. The nitrogen gas that’s released helps to give closed-cell sponge rubber its strong compression set and recovery characteristics.

Although nitrogen is a gas, it doesn’t produce a foam like the gaseous blowing agents used with foam rubber. Foaming is specific production process, and foam rubber contains mostly open cells. Although some of the cells in foam rubber are closed, these rubber materials would not pass ASTM tests for water absorption, a standard requirement for closed-cell materials.

Join the Conversation

Do you need to source foam rubber or sponge rubber profiles? How can we help you? For over 20 years, Elasto Proxy has been solving challenges and providing solutions. Join the conversation today. Look for a post with a link to this blog entry on LinkedIn, Facebook, Google+, and Twitter.

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Silicone EPDM Blends: Rubber Compounds for Expansion and Resistance

Doug Sharpe
President of Elasto Proxy


Do people say that you look like your father? How about your mother? Maybe you resemble both of your parents, but in different ways. Human genetics can be complex, but so can polymer chemistry. Like DNA strands, long-chain polymeric molecules consist of base units (monomers) and groups. The double-helix structure of DNA is well-known, but the tangled configurations of polymer molecules are difficult to describe. That’s not the hard part, however. The real challenge is creating a plastic alloy from them.

Because of their complex structures, many polymers do not lend themselves to blends. Human history includes a Bronze Age and an Iron Age, but have you read any books that call our current era the Age of Long-Chain Polymer Blends? Through advances in material science, however, mixers are able to create compounds from rubbers such as silicone and EPDM. Blending these long-chain polymers is challenging, but imparting the best characteristics from each is essential.

Let’s look at the advantages – and disadvantages – of silicone and EPDM. Next, we’ll learn how mixers make silicone/EPDM blends, and how creating a blend for an application means getting the recipe for rubber just right.

Silicone Rubber

Silicone resists ozone, sunlight, oxidation, and weathering while providing excellent electrical insulation and flexibility at low temperatures. Although silicone is valued for its low compression set and superior color stability, this synthetic rubber is best known for its usefulness at high or low service temperatures. Silicone is more expensive than EPDM, but silicone seals generally last longer.

Silicone isn’t ideal for all applications, however. Because of its chemical properties, this type of rubber isn’t recommended for use with acids, alkalis, solvents, oil, or gasoline. Silicone lacks strong resistance to abrasion and tearing, too. Although many excellent silicone compounds are available, pure silicone isn’t the right choice for applications that require high tensile strength.

EPDM Rubber

EPDM also has its strengths and weaknesses. A popular polymer, this synthetic rubber is used in belts, hoses, and O-rings for the automotive and mass transit industries. With its excellent resistance to aging, ozone, and weathering, EPDM also provides strong resistance to acids, alkalis, and some solvents. EPDM parts are color stable, remain flexible at low temperatures, and resist both water and steam.

EPDM isn’t perfect however. In fact, this synthetic rubber is no match for mineral oils or aromatic hydrocarbons. EPDM compounds aren’t recommended for applications involving petroleum derivatives then. Though suitable for use as a high voltage insulation material, EPDM can conduct electricity if carbon black is added to improve weathering.

Silicone EPDM Blend

To produce blends with the best properties of silicone and EPDM, compounders add the two rubbers in proportions such as 50:50 or 70:30 to a two-roll mixing mill. Typically, dicumyl peroxide is used as the vulcanizing agent. After the materials are compression-molded into sheets and cured, the mixers test the blends and measure properties such as dielectric strength, tensile strength, and percentage elongation at break.

According to test results published by the IEEE, increasing the proportion of silicone improves a blend’s electrical insulation. By increasing the weight percentage of EPDM, mixers can boost mechanical strength instead. As with other compounds, there are tradeoffs. For example, silicone-modified EPDM blends can withstand higher temperatures than EPDM alone, but provide less temperature resistance than pure silicone. EPDM/silicone blends are tougher than silicone, but not as tough as EPDM alone.

Expansion, Resistance – and Conversation

Do you need to source expansion joint systems for bridges and highways? How about electrically insulating materials for outdoor environments with solid airborne particles?  Let’s talk about how a silicone EPDM blend might meet your application requirements. For over 20 years, Elasto Proxy has been solving challenges and providing solutions. Join the conversation today.

Look for a post with a link to this blog entry on LinkedIn, Facebook, Google+, and Twitter. Elasto Proxy has pages on all of these social media websites, so all that’s missing is you. I hope you’ll subscribe to our free e-newsletters, too. They’re a great source of information delivered right to your email inbox, and provide links to blog entries like this one.

Pucks, Polymers and Olympic Ice Hockey (Part 2)

In Part 1 of this series, we examined how polymers are used in hockey skates and sticks. With the Sochi Winter Olympics underway, let’s take a look at hockey protective equipment – and that hard rubber “biscuit” that glides along the ice.

Elbow Pads and Gloves

Elbow pads are molded guards that protect a player’s elbow while providing forearm protection. They’re made of a hard, impact-resistant plastic and coated in padded fabric.  Some elbow pads use expanded polypropylene (EPP) foam because it’s lightweight, elastic, and regains its shape when deformed. This is the same foam that’s used in car seats to help protect occupants in a collision.

Hockey gloves also contain foam, especially the blocker worn by  the goalie. Built with a rectangular foam board, a goaltender’s blocking glove must fit tightly around the hand without causing discomfort or cramping. Blocker gloves may also have foam inserts that fit between the top of the goalie’s hand and the foam board. These inserts cause the board to be angled properly when the goalie faces a shooter.

Shin Guards and Shoulder Pads

Shin guards are designed to protect the shins, knees, and calves in Olympic ice hockey. Molded and contoured, this type of hockey protective equipment contains several types of polymers. The front of the shin guard is usually made of a hard plastic and lined with foam padding. High-density (HD) foam, a type of polyurethane that consists of open cells packed tightly together, is often used in the knee extension.

Hockey shin guards may also contain U-Foam, a rigid two-component urethane foam system. Thigh guards typically contain molded, removable U-Foam. The shin guard’s calf wrap section may contain molded, segmented urethane foam. Other polymeric parts for shin guards include a neoprene lock zone in the knee bed.

To protect the upper body, hockey players wear padding on critical points of the shoulder, biceps, sternum, shoulder blade, and spine. Known simply as shoulder pads, this type of hockey protective equipment is usually made of a hard, impact-resistant plastic and covered in a padded fabric. Worn under the jersey, they’re bulky but durable.

The Pucks Stops Here

Hockey pucks aren’t part of a player’s equipment, but they’re an indispensable part of the game. According to the Olympic organization, these durable disks must be made of vulcanized rubber that’s approved by the International Hockey Federation (IIHG). Predominantly black in color, Olympic hockey pucks are 2.54-cm thick, 7.62-cm in diameter, and must weigh between 156 and 179 g.

On average, as many as 80 rubber pucks are needed for an Olympic event. Moreover, before each hockey game, the pucks must be frozen in order to reduce friction and limit rebounds off the ice’s surface. In North America, the National Hockey League (NHL) follows this same rule – and even specifies that NHL pucks must be kept in a cooler at the penalty bench.

As with sticks, skates, and protective equipment, material science helps us to understand aspects of Olympic hockey that even some diehard fans don’t know. The reason that frozen hockey pucks bounce less is that rubber’s physical properties change with temperature. When a rubber puck is exposed to low temperatures, it becomes harder and slides better.

If you’ve ever been hit by a hockey puck, you probably remember how hard a “biscuit” can be. With a hardness of approximately 90 durometer (duro), pucks can move at speeds of more than 150 km/h. So when you watch the world’s greatest hockey players in the Winter Olympics next month, follow that fast-moving puck – and remember it’s not the only polymer in the game.


Doing Business in Brazil: Québec Companies and the Rubber and Plastics Industry


Doug Sharpe
President of Elasto Proxy

Did you know that Québec is home to over 700 companies in the plastics industry? There are also plenty of businesses in the rubber industry, including toolmakers, mixers, suppliers, and distributors. Canada’s National Research Council (NRC) Industrial Material Institute is here, along with leading engineering schools such as Polytechnique Montréal. For polymers, Québec is a world-class center of innovation.

As the co-founder and co-owner of a Boisbriand company with offices in Ontario, the United States, and China, I’m proud to be part of such a thriving economic sector.  Since our founding in 1989, Elasto Proxy has listened to its partners, analyzing all of their needs before recommending sealing solutions. Today, we provide a full range of custom-fabricated rubber and plastic products to numerous industries.

Growing Globally in Québec

To grow globally, Elasto Proxy has participated in the SME Passport program, attended tradeshows in Europe, and participated in trade missions to Brazil. Just as we enjoy sharing our application knowledge and technical expertise with customers, we’re eager to explain what we’ve learned about global markets. That’s why next month, I’ll be part of a panel of rubber and plastics industry experts who will answer questions from Québec companies about doing business abroad.

Sponsored by Elastomer Valley, CSMO, Canada Economic Development (CED), Export Québec, and the government of Québec, this World Alliance event is scheduled for February 11 and 12. The panel I’ll be part of will meet on Day 1 just after lunch, from 1:00 to 2:30 PM, and focus on doing business in Brazil. With 35 speakers in all, the entire two-day event will also include innovations in the plastic and rubber industry, sales trends, developing markets, and human resources.  Event participants can network with peers and uncover new business opportunities, too.

Join the Conversation

Does your company want to increase exports or reach global markets for the first time? What would you like to know about business practices in places like Brazil? Elasto Proxy’s President of International Sales, Clyde Sharpe, visited South America’s largest nation earlier this month, and has some new insights I’ll share in February.

Do you have questions about doing business in Brazil that can’t wait until then? Then join the conversation today. Look for a post with a link to this blog entry on LinkedIn, Facebook, Google+, and Twitter. Elasto Proxy has pages on all of these social media websites, so all that’s missing is you.
I hope you’ll subscribe to our free e-newsletters, too. They’re a great source of information delivered right to your email inbox, and provide links to blog entries like this one.