The Good, the Bad and the Ugly…Rubber
Quality recycled rubber flooring is manufactured using a series of complex processes and with equipment that is custom made for molding and cutting rubber. Remarkably, millions of pounds of recycled rubber are used to make hundreds of thousands of linear feet of rubber flooring in different material specifications, thicknesses and colors with only a small percentage of flooring that is unusable.
The uniqueness of the manufacturing process for rubber flooring allows the unusable product to be recycled and re-used to make more quality flooring! So, how does recycled rubber flooring become unusable in the first place?
In this article, RFS will look at several examples of different manufacturing conditions that produce undesirable recycled rubber flooring. Let’s look at the good, the bad and the ugly rubber flooring.
Good Skive, Bad Skive
What are skive marks?
Skive marks (also referred to as knife marks) are shallow, visible lines (striations) on the surface of recycled rubber flooring that are created during the cutting (splitting) process.
Specialized equipment is used to cut recycled rubber flooring into different sheet widths and thicknesses. A long steel blade is continuously sharpened while a solid rubber cylinder (log) is turned. The blade cuts into the rubber cylinder according to the programmed thickness and peels the rubber into a long, precise sheet of rubber flooring. The sheet flooring is then rewound back into a cylinder and moved to be cut into more manageable roll lengths. See Rubber Rolls or Rubber Tiles? for additional information.
Skive marks are an inherent characteristic of the manufacturing process and virtually unavoidable. Commercial flooring standards do not classify skive marks as a defect. In addition, skive marks do not affect the floor’s durability or structural integrity and are not grounds for rejecting the product.
Skive marks are most noticeable on solid black rubber and more visible at certain angles to the lighting in the room. Recycled rubber flooring that contains color flecks often break up the skive pattern and may not even be perceptible after installation.
Are all skive marks the same?
Skive marks can vary depending on the sharpness of the cutting blade; the rotation speed of the rubber log as it is being cut; the presence of inclusions in the rubber and environmental factors.
The most common reason for skive marks is the lack of smoothness of the cutting blade. A newly installed blade will have the smoothest edge without any wear. Once the blade starts slicing through solid rubber, it can come into contact with different inclusions in the molded rubber. Objects such as tiny metal fragments (unfiltered tire belting), grit or stones (stuck in tire rubber) and hardened polyurethane (glue) can damage the blade edge. Even though the blade is continuously being sharpened, the blade grinding may not completely smooth the cutting edge as it moves through the rubber. The wear and tear on the cutting blade is continuous as well.
Imagine using a razor blade for shaving with nicks and notches in the blade. Yikes! The grooves in the blade would push into your skin and result in an uneven shave. Think of those nicks and notches in the blade that cuts the rubber. The deeper the flaw in the smoothness of the blade, the larger the skive mark will be on the rubber surface. The skive mark will disappear once the blade is ground down enough to eliminate the notch.
Skive marks that are large and pronounced can affect the aesthetic of your rubber flooring, but would not change the durability or subfloor protection that you can expect for the thickness of the product.
Rough Texture
Recycled rubber flooring already has a unique surface quality with exceptional grip and slip resistance.
Occasionally, however, a roughened surface texture can develop during the cutting process. Similar to skive marks, this rough texture doesn’t affect the performance characteristics of the rubber floor and will naturally wear during use.
A noticeably rough texture on the surface of recycled rubber flooring can be caused by different variables in the manufacturing process. Flooring formulas require different amounts and types of crumb rubber and polyurethane adhesive. How well the formulation mixes combined with other factors such as the blade position during cutting can lead to roughness on the rubber surface. More often than not, this issue is corrected during the cutting process. If the problem persists during cutting, the entire rubber log may be recycled.
Mixed Colors
RFS recycled rubber rolls and rubber tiles are available with different color flecks at certain percentages. The color fleck creates a random pattern over the entire flooring, helps break up seams, and adds color to an otherwise solid black surface.
When recycled crumb rubber is molded, color granules are added to the black tire rubber mixture and blended with polyurethane adhesive. Once the rubber mixture has been pressed and cured, the rubber log is removed from the mold. The mold is then cleaned and prepared to receive the next rubber mixture.
Different parts of the molding equipment come into contact with the blended rubber mixture. If any of the surfaces are not thoroughly cleaned, residue from the previous molded color can contaminate the next rubber mold. Color flecks that should not be present in a rubber cylinder are usually minimal and can be trimmed from the rubber log after it is peeled into sheeting.
Rubber rolls or tiles that contain different color flecks do not affect the performance of the rubber flooring, just the aesthetic.
Miscut Rubber
Rubber tiles provided by RFS are cut from the same rubber that is molded into large cylinder logs and peeled into sheets. Smaller rubber sheets are then cut into puzzle-piece interlocking shapes using a high-pressure water stream. Occasionally, a rubber tile can be miscut and accidently stacked with hundreds of other tiles.
When sheets of rubber are waterjet cut, they are placed on a large, flat cutting table. An operator programs the machine to cut the tile shape. Multiple waterjet heads pass over the sheets and cut through the rubber with water to create each tile. If the flat sheets are not properly positioned on the cutting table, the water stream could inadvertently cut outside of the sheet. This would result in a miscut tile as shown above.
A miscut tile should be replaced as it would not allow the tile to properly lock to an adjacent tile. Otherwise, the miscut tabs could be cut off completely and used for an outside border tile.
Dry Spots
Dry spots are the Darth Vader of recycled rubber flooring. These evil voids are created inside of the rubber cylinder mold (think Death Star) and can be difficult to eliminate even if the Force is strong. Nothing wrong with a little Star Wars talk to liven up an article about rubber, right?
Dry spots can form inside of a rubber cylinder mold when a gap, or void, is created. This void occurs when crumb rubber becomes clumped together (not loose and flowing) and is added to the mold. The cluster of crumb rubber does not allow for proper mixture with the polyurethane adhesive and results in an area of the rubber cylinder that is dry and uncured. These dry spots can form in different areas of the mold and produce different results. Let’s review some examples:
Exterior Dry Spot
At the beginning stages of molding rubber, millions of rubber granules (crumb) are transported on a conveyor to a large mixing funnel where they are combined with liquid polyurethane adhesive. The slurry of mixed rubber and polyurethane is then added to the cylinder mold. Once the mold is filled, a hydraulic press compacts the rubber in the mold and the urethane begins to cure and harden into a solid rubber cylinder, or log.
When the mold is being filled, some granules that do mix with the polyurethane glue can be pushed to the outside of the mold. Since these granules are “dry” (without glue), they cannot cure properly into a solid mass of rubber. The effect is what you see in the photo above. The smooth area around the “hairy caterpillar” is where the rubber mixture cured against the wall of the mold. The uncured rubber is the location where it separated from the wall.
Since this occurred on the outside of the cylinder, it would be peeled from the rubber log first and then recycled.
Interior Dry Spot
Dry spots are not limited to just the outside of the rubber log. They can also form inside of the molded log. These hidden surprises are only exposed during the cutting stage when the rubber log is peeled to specific thicknesses. This dry spot defect indicates a failure in the bonding process between the crumb rubber and the polyurethane glue. Some causes for this can include:
• Mixing Clumpy Rubber Granules: Rubber granules can become packed during storage and clump together when humidity is high. If the mixing of crumb rubber and glue does not break up the clumping, then the interior granules will stay dry and loose.
• Low Binder Proportion: If an insufficient amount of polyurethane glue is used for the volume of rubber granules in the mold, this would lead to a number of dry spots throughout the molded rubber and the rubber sheeting would simply crumble.
Unfortunately, some manufacturers try to lessen the volume of urethane in their rubber products as a way to cut costs. This results in an inferior product and ultimately poor performance from the flooring.
• High Temps/Low Viscosity: Ambient heat combined with equipment-generated heat can make the polyurethane binder thin and more fluid. The low viscosity of the binder can filter more easily to the bottom of the mold, leaving the top layers without sufficient glue coverage.
• Environmental Dust: Processing rubber creates an environment filled with dust and particulates. This can infiltrate crumb rubber stock or coat raw material waiting to be re-ground. When mixed with granules, the binder can be absorbed by the dust rather than bond to the granules and prevent proper adhesion.
Large dry spots concentrated on the interior of a rubber log will show on each layer of rubber that is peeled. In some cases, the dry spots will create holes in the rubber sheeting. Depending on the depth of the dry spot, this defect can show on thicker commercial flooring or thin underlayment.
Dry spot defects are not repairable or usable and sections of rubber flooring with dry spots are cut from the log and recycled.
Hardened Urethane
Rubber Void
A rubber void is nearly the opposite of a dry spot. When the polyurethane glue is mixed with the crumb rubber, a hollow void can form from rubber clumped together. The glue can filter into this void and create a hardened mass of urethane with little crumb rubber in it. This heavy concentration of glue creates a noticeable defect when the rubber is cut.
A rubber void defect is not desirable and would be removed from the sheet and discarded.
Bubble Spots
Bubble spots in cured recycled rubber are not a common defect but can produce unwanted results. They form near the wall of the cylinder mold when clumps of rubber granules create pockets. The pockets become filled with polyurethane glue and then squeeze out when the press pushes down on the rubber mixture inside of the mold. The urethane creates rounded globules that harden on the surface of the rubber.
Bubble spots are more likely to form when molding less dense rubber flooring products and where reground granules are used in larger quantities, such as underlayment.
Foreign Objects
When crumb rubber and EPDM color granules mix together with polyurethane adhesive, the bond that is formed under high pressure creates a solid mass of rubber. This dense mass relies on the similarity and size of the granulate used in the rubber mixture to cure together. When a foreign object accidently mixes with the granules and urethane, a separation of the bond forms inside of the molded rubber.
What type of foreign objects can get inside of a rubber mold?
Foreign objects that make their way into the final molded rubber are uncommon. The molding process is measured, repetitive and automated; however, crumb rubber contaminants, poor mold preparation and/or human error can happen. Here are a few different foreign object examples:
• Crumb Rubber Contaminants: Crumb rubber stock is filtered from oversize granulate, metal pieces and other objects (like stones). The vast majority of millions of pounds of crumb rubber is clean, but processing equipment may not always remove foreign objects before it gets packaged. If the contaminant is not spotted before entering the mold line, it will become part of the molded rubber. In the image above, a large piece of an EPDM rubber seal did not get shredded into smaller granules and passed through the filtering process. The piece was revealed after the log was cut into sheets.
• Poor Mold Preparation: All large cylinder molds are cleaned after each rubber log is removed. Cleaning can involve scraping, washing and rinsing the interior of the mold. In addition, a liquid wax is applied to prep the center core of the mold. The wax is used as a release agent and helps to remove the solid rubber log. Excess wax can separate from the mold and combine with the rubber/urethane mixture. The wax will prevent proper bonding and can create voids in the molded rubber.
• Human Error: Even though the molding equipment is automated, human error can enter the molding process. Decisions made in the moment can determine if quality standards are met or lead to problems with the molded rubber. In one such example, a large, woven poly holding bag was sliced open to unload the crumb rubber into the molding line and a small piece of the bag was buried in the granules. The bag material was not noticed and became part of the molded rubber. Only after the log was split into sheets, was the poly bag piece found. It resulted in a large hole in the center of the rubber sheet.
For a seemingly simple product like recycled rubber flooring, the challenges of manufacturing quality flooring is very real. The technical know-how and the ability to adjust to continuously changing conditions are a necessity when the results are good, bad or ugly.
RFS is committed to supplying the highest-quality, American-made rubber flooring at affordable prices.
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