Ever order a steak and find yourself overwhelmed by the choices? How do you want it done? Fries or baked potato? Butter and sour cream with the potato? It goes on and on. Well buying foam gets like that too, and unless you have a background in polymer chemistry some of the questions can be quite baffling.
One of the options foam fabricators throw at buyers is cross-linking. It's actually an important question as cross-linking has quite an effect on the properties of the foam. There are applications that might benefit from it while others won't. And should cross-linking be right for your application you may even be hit with another question: chemically cross-linked or irradiated?
To help you understand the questions, and decide if it's even something to consider, here's a primer on cross-linked polyethylene foam. It explains what the words mean and why you might want your next foam fabrication made from it. Individual sections address:
- Understanding polyethylene
- What is cross-linking?
- Characteristics of XLPE foam
- Benefits and applications
Polyethylene foam is made from polyethylene. The foam comes about because gas is produced during the chemical reaction that makes the polyethylene. Much as yeast produces the CO2 that makes bubbles in your bread (and beer,) the same gas produces the porous, spongy structure we call foam.
Now let's back up a step. The material around the bubbles is polyethylene. Poly is from Greek and means "many," so that tells us this material is composed of lots of ethylene molecules. Ethylene is two atoms of carbon joined to four atoms of hydrogen. Mix them up though and the molecules rearrange themselves into long chains where the carbon atoms all link together and each has two hydrogen atoms attached. This process is called polymerization and the result is polyethylene.
At room temperature polyethylene is solid, but it's soft, almost waxy, because those chains, which could be 5,000 links long, can slide over one another under just a little load. Apply some heat and they become very slippery. For most polyethylene the upper working limit is around 75oC, so it shouldn't even be exposed to boiling water.
It's quite possible to make foam from this polyethylene base, and it will perform adequately in many applications, but it becomes stronger through cross-linking.
What is Cross-linking?
Joining the long chains of polyethylene prevents them sliding over one another. If you picture the chains as lengths of wire, cross-linked polyethylene is like a chain link fence. Pull on one link and it can't move far because the others are all holding it in place.
Most polyethylene is chemically cross-linked. That means the chains were encouraged to join up during or after the polymerization process. Typically a catalyst or additive was used that let some of the carbon atoms link to hydrogen atoms in a different chain. The more cross-links like this that are created, the stronger the end product, which is why some polymer specialists will talk about cross-link density.
An alternative method of creating cross-links is through irradiation. This entails shooting an electron beam into the polyethylene. The energy knocks some of the hydrogen atoms out of their chains, which allows links to form with other chains. Irradiation is a somewhat slow and expensive way of cross-linking polyethylene, but it avoids the use of hazardous chemicals, and results in a longer-lasting material with other desirable characteristics.
Characteristics of XLPE Foam
Polyethylene that's been chemically cross-linked often goes by the acronym XLPE. XLPE foam differs from regular polyethylene foam in several ways.
- Tensile strength is increased.
- Increased temperature range (more strength at elevated temperatures.)
- Harder wearing/more abrasion resistant.
- Reduced flexibility/stiffer.
- Water-resistant (because of the closed-cell nature of the foam.)
- Can't be recycled (cross-linking prevents this.)
Irradiated cross-linked foam has all these characteristics, but in addition, is smoother with a finer cellular structure. It's often sold in thin sheets suitable for die or laser cutting.
Benefits and Applications
As a stiffer, stronger polyethylene able to withstand elevated temperatures, XLPE foam has many uses. It makes an excellent gasket material, thanks to polyethylene's ability to withstand many chemicals and solvents. It's widely used as protective packaging, even for "Class A" surfaces. When given a coating of pressure sensitive adhesive it's easy to mount in place, making it ideal for sound deadening and insulation.
Cushioning is another common XLPE application, thanks to its long-lasting, resilient nature. Wheelchair seats and arms are often XLPE-filled, while sports and leisure equipment is another big market.
When shopping for flexible foam fabrication services there's a lot to consider. Foam comes in many different forms and it's important to understand the pros and cons. Cross-linked polyethylene foam has a lot going for it. It's stronger and harder wearing than regular polyethylene foam and makes an excellent protective packaging material. It could however be more than you need. This primer has explained what's different about XLPE and why you might want to consider it. Have questions? Let us know and we'd be happy to answer them.
Now, how do you want that steak?