High-density open-cell
sponge rubbers:
Parts always in shape

Open-cell sponge rubbers are rubbers with low hardness levels of 35 Shore and below, formulated from natural rubber and manufactured using compression presses. Essentially, they are formulated with natural rubber and have low hardness. That’s what sponge rubber is.

Well, from the previous, I forgot to mention that open-cell, I mean open-cell, means that the cellular structure of the rubber cells communicate with each other, which is very important within open-cell.

And then Shore is the hardness that defines sponge rubber materials. There is a scale that ranges from 0 to 100, and as it goes up in grade, it indicates a different hardness; the higher the Shore, the harder the material.

For example, polyurethane foam. If it has a lot of cells, the hardness is very difficult to measure, for people to understand, when it’s very soft, density is talked about more than hardness.

The hardness of a polyurethane foam is usually 5 Shore.

Now, let’s move on to slightly harder materials, for example, a product known as EVA rubber puzzle mats; in that case, we’re talking about a hardness of around 15 Shore. Then, even harder, we have materials with 20 Shore and upwards.

Starting from 40, between 30 and 40, it goes from sponge rubber to compact rubber. It no longer has cells, and the material ranges from 40 to 50 Shore.

For instance, a tire might have a hardness of around 65 Shore, and if you want even harder, rubber can go up to 80 Shore. If you want to go beyond that, starting from 80 to 85, you’re dealing with compact polyurethanes.

Below the Shore scale, there are softer materials, such as semi-closed cell EPDM, which is at the zero-zero end of the scale. And if you want to go higher, there’s a scale for very hard materials that measure over 90, 99, or 100 Shore.

Well, what we manufacture is primarily natural rubber.

What’s the composition? The formulation of natural rubber is quite artisanal, and there are very few people who do it. For example, here in Spain, there are only two manufacturers.

The ingredients they use include oils, calcium carbonate, rubber, and then they add accelerators, antioxidants, in other words, a series of materials that give a specific property to the finished product, which can be utilized.

We basically manufacture using natural rubber, but in some situations, you have to give natural rubber a property that other rubbers have, so we mix them.

For example, if you want it to be more resistant to environmental conditions, the rubber that excels in environmental resistance is EPDM. So, what do we do? We mix natural rubber with EPDM.

For instance, if you want it to be more resistant to low temperatures, we mix natural rubber with neoprene rubber.

If you want it to be resistant to oils, then it’s mixed with nitrile rubber.

Depending on the intended use or where it will be placed, open-cell rubber is mixed with rubbers that provide the property that natural rubber lacks.

Well, the most important properties of natural rubbers include their resilience and memory. For instance, one application of natural rubber, as we understand it, is in the die-cutting used in graphic arts. What happens with the graphic arts die?

It’s a blade and a wooden component that cuts paper. So, to eject the paper, you need an element. What do you use? Natural rubber. What does natural rubber have? It returns to its original state. Its function is to return to the original state after being compressed. It returns to the original state many times, much more in this regard than, for example, EPDM.

EPDM, on the other hand, is the opposite. When you compress EPDM, it stays compressed after two or three repetitions. Natural rubber has elasticity, meaning it returns to its original state again and again. It’s the one that withstands the most.

For abrasion resistance, for example, when there’s something abrasive and it has to endure friction, natural rubber is used.

Yes, in printing, which is a component that presses against a roller, with paper in between, and it prints by applying pressure repeatedly. The design always remains the same; it doesn’t change. In another case, if it were a different material, it would change the design as it would get flattened, and each time, it would print differently.

You’ve already mentioned some, but if you could provide more examples.

These rubbers are very special, as mentioned, there are very few manufacturers, perhaps some Japanese ones. They are very specific and unique.

In terms of usage, they are used in agriculture quite a bit due to their properties, as they endure abrasion well. They are used for making closures on trailers and other such applications. It’s rubber for very basic and traditional uses.

For example, they are also used extensively in large ships for sealing hatches.

They are also used for a line that is from Germany, used for making rollers for food processing machinery. In the graphic arts market. They are used for carrying religious statues during Holy Week processions to prevent damage to the shoulders, for example.

For closures, they were used a lot in the past, but now EPDM works better for this type of use, closed-cell rubber better than natural rubber. So, the usage has remained for what is essential. If no other material has replaced it and where it can’t be replaced, natural rubber is still used.

So, this natural rubber can be considered the pioneer, it’s been around forever, and now new materials are coming out to replace it in some applications, while in others, they haven’t been able to replace it yet, until something new is invented.

Open-cell sponge rubbers, as I mentioned, are crafted in a similar way to baking bread.

We take the flour, oil, salt, sugar, butter, mix it in a cylinder. This is done from a preform, like cutting bread, placing it in the appropriate shape, and then putting it in a machine like an oven that provides heat. It vulcanizes and expands in volume, much like a muffin that rises when you pour a little batter.

We do the same, we create the dough, put it in the mold with much less material than the mold provides, and it expands. And what happens when it expands? It forms bubbles, like a muffin. That’s what open-cell is. The challenge lies in getting the bread dough, so to speak, to form a piece that is square, well-finished, and properly vulcanized, etc.

I think it’s easy to understand with this analogy, made somewhat like a baker. You make the dough and put it in the oven, and in the oven, it bakes, leaving the piece ready. Do you have any more questions?

You’ve mentioned some applications, but whether it’s adhesive, screwed, it depends.

Well, a very important application that is widely used is conveyor belts, which I haven’t mentioned before. Typically, they are adhesive. I mean, you place the belt, and it’s glued. Why is it used so much and how is it applied? For example, for handling food, for fruit picking. In other words, what natural rubber does, as you’ve explained so well, is an important use, preventing damage. For instance, when fruit falls into a container during harvesting, it shouldn’t hit metal but rather a material that protects it from getting damaged.

How is it installed? Typically, it’s done with adhesive. We can shape it, cut it to the customer’s specifications, apply adhesive, create the necessary holes. For example, we’ve recently been asked for rubber cut to fit certain metal structures, so it’s put through a water jet cutting machine to produce a piece designed to the customer’s specifications.

From what you’ve told us, these open-cell sponge rubbers are used in various sectors, such as agriculture, as you mentioned, and also in graphic arts, as you’ve explained. But this new information you’re providing about fruit harvesting…

Our customers are large industrial suppliers. In other words, our products are sold in small quantities, and a significant portion goes to very large industrial suppliers. Another smaller portion goes directly to the customer, such as a bodybuilder who manufactures tractor trailers and directly requests the appropriate profile from us.

The conveyor belt is also widely used, as I mentioned earlier. But primarily, our customers are large industrial suppliers. Lately, we’ve been introducing this product to companies that manufacture metal structures for transportation because, recently, to prevent friction of what they transport, this type of material is used due to its good resistance, meaning it recovers well, doesn’t degrade, and endures well.

Yes, you might serve an individual customer who needs to assemble a particular structure or a shipping company that wants to install rubber or something else on their transport ship because they carry a specific cargo and want to install a rubber or similar material. You may receive inquiries from both companies and individuals.

Well, rubber, I mean, the rubber products we make, especially natural rubber, are primarily affected by exposure to the elements. In other words, if you want the rubber to last longer, it should be kept as dry as possible, away from direct sunlight, and shielded from external agents.

It’s basic. Above all, one thing that degrades them significantly is temperature. So, if you subject these natural rubber open-cell rubbers to high temperatures, they’re no longer effective. You have to use silicone instead. Of course, there are different formulations for each type.

Temperature, in particular, forces us to switch to different types of rubber: chloroprene, natural rubbers, and silicone, which I didn’t mention earlier, but there is also open-cell silicone rubber that withstands temperature very well.

Yes, as for what you mentioned, I have a spatula in my kitchen that’s probably made of silicone. Those that you can leave on top of a pan and it can withstand the heat. Is that an open-cell sponge rubber?

No, those are compact rubbers because they don’t have cells. I mean, if you were to cut the spatula, you wouldn’t see any cells, there’s no air space in between. It’s a compact material. What we manufacture are materials where, if you were to cut them, you would see the cells perfectly. They have air cavities of 1, 2, 3, 4 millimeters. It depends on the type of formulation we’re dealing with, the internal structure.

As for your example, it’s a compact silicone rubber, something we also produce, but that’s a slightly different category. We make compact rubber, but perhaps we represent around 3% of the global market here in Spain. We make it because any rubber manufacturer knows how to produce compact rubber. In reality, compact rubber is something that 99% of manufacturers produce because, in the end, it’s a formulation, you apply heat, and the product is ready.

However, with open-cell rubber, the challenging part is to take it, vulcanize it, and have the finished piece come out well. In this type of rubber, we represent about 70% to 80% of the national production.

Yes, you’re telling me that you’ve mentioned this before, that it’s a somewhat artisanal product, which is why you likened it to a baker, saying it has a similar process. Is that what sets you apart? Is that your competitive advantage, the difference you have in the market? Because there are few people doing it the way you do.

The market for open-cell natural rubber is very small. There’s a product called ‘Por Alemán,’ which is also natural rubber. To my knowledge, there’s an Italian company around here that produces it, an Austrian company, and us. So, there are only three, and we’re not that many in Europe. These products, something important to note, are only used in developed countries. In Africa and third-world countries, they are hardly used, which also limits the market.

I can’t say for certain why, but it’s true that the customers who consume these products are in Western countries, so to speak.

As for natural rubber other than ‘Por Alemán,’ there’s a factory in Denmark, the Japanese company I mentioned earlier, and I’m not aware of other companies that produce it. I have no idea, but it’s true that, for example, the Japanese company that showed us what they do didn’t go beyond 20 millimeters. We, it’s not that we’ve achieved a milestone because it’s something we do routinely. If we make 30 millimeters thick, 40, 50…

I mean, it’s not difficult for us to make it thicker, so maybe there are two or three companies in the world making 20mm, but when it comes to making 40mm, maybe there are only two. We don’t know for sure, but very few people do it, and the thicker it gets, the fewer there are.

If someone listening suddenly sees an opportunity there, and says, ‘Well, there’s a market for that,’ to you, of course, it’s artisanal, and you make it to order, to the specifications they request, as you’ve mentioned regarding thickness, sizes, and so on. In this case, it won’t necessarily be engineers or architects; I don’t know who would request it, and you would custom manufacture it for them.

For this product, there are rectangular or square measurements, typically profiles or rolls. It’s not like supports; those have standard measurements, rectangular, rolls, or sheets of two by one. So, there isn’t as much variety in measurements; there are manufacturing standards, and we stick to them.

Because, for example, if a piece needs to be a specific shape, there are other materials that fill the mold gap much better and are much cheaper than natural rubber. In other words, natural rubber is used for specific purposes, but there are always defined shapes. Each customer doesn’t get a custom design; instead, customers adapt to the available sizes.

Regarding what you mentioned, indeed, heat can cause deterioration. What problems can arise if open-cell sponge rubbers are not used or maintained correctly in an application?

Improperly applied sponge rubber deteriorates, degrades, and turns into dust. It loses elasticity and all its functionality, becoming unusable.