What Causes Hot Tyre Staining (Paw Printing)?
Tyres are made of a mixture of synthetic and natural rubbers, fillers and additives. The major fillers are carbon black and silica. Carbon black not only gives the tyre its characteristic black colour but also improves the strength of the tyre. If there is friction between the tyre and the floor surface, a black mark sometimes appears. This is not the type of tyre marking that this Technical Note refers to, however. This note refers to the yellow to brown stain that occurs after certain types of the tyre has been in direct contact with a floor covering, such as vinyl sheeting or vinyl tiles, or floor coatings over some time.
In the manufacture of all tyre rubber, an ingredient known as and antioxidant-antiozonant is used. This chemical functions as a preservative that allows the tyre to remain flexible, withstand the elements of the ozone and UV degradation and enjoy a long tread life. Chemicals such as diaryl-p-phenylenediamines and N-(1,3-dimethylbutyl)-N-'-phenyl-p-phenylenediamine are commonly used. However, depending on the manufacturer, many derivatives of this type of chemical can also be used. The use of these chemicals is so common that the chemicals are referred to by codes such as 6PPD or IPPD, depending on the manufacturer. Major tyre manufacturers are fully aware of the problem of paw printing in the coating industry. Accordingly, many tyre manufacturers have selected the least detrimental (to flooring) chemical in the manufacture of tyres. However, many manufacturers are still using products such as the IPPD which is far more prone to discolour a coating than the 6PPD. Many chemical reasons exist that account for the paw printing, however, in layman terms, here is what information is known.
Products such as the 6PPD are colourless when introduced into the tyre formulation and do not by itself cause the paw printing to occur. However, the moment that the 6PPD becomes oxidized, it only takes a few parts per million of the 6PPD to create the typical paw print on a coating. To further compound this problem, a high aromatic greenish-brown oil commonly used in the manufacture of tyres continually leaches from the tyre and can contribute to the exudation of the 6PPD from the rubber. Unfortunately, the 6PPD can be easily oxidized. Once the material is oxidized, it turns to a purple/grey/black colour depending on the severity of the oxidation. This colour variance in conjunction with the actual concentration of the 6PPD accounts for the differing types of paw printing from slight deglossing to actual colour changes. Also, atmospheric contaminants can cause sufficient oxidation to occur, resulting in a paw print. Surface contact is enough concentration results in surface paw printing. Subsequently, the longer the tyre remains in contact with the area, the greater the chance that enough 6PPD will leach out of the tyre and into the coating itself. Given enough time in one location, not only will enough 6PPD migrate out of the tyre, but the constant exposure may result in the actual migration of the chemical into the coating creating a more pronounced and deeper stain.
Many other parameters play an important role in this phenomenon. These are mostly too technical to go into. Suffice to say that the problem is worse in synthetic rubber tyres.
In the past, the products such as 6PPD or IPPD added to the cost of tyre production and hence was limited to more high-performance tyres. This is no longer the case with many smaller economical cars coming out with low profile high-performance tyres.
Are water-based or solvent-based coatings better at hot tyre marking resistance?
Coatings that contain solvent or water simply to allow the product to be worked and applied without special equipment or skills. During the curing process, 100% of the water or solvent leaves the film. The main cause of this issue is the chemistry of the tyres, both water-based and solvent-based products are susceptible to tyre staining.
Does the degree of cure of the coating play a role?
Most coatings are only 95% cured after two days. Full cure typically is after 7 -14 days. The window of penetration is at its greatest when the coating is new and not fully cross-linked. Add in all the other variances such as the age of the tyre, manufacture of the tyre, the temperature of the floor, atmospheric contaminants, colour of the coating, length of cure, migration rate for each type of tyre, specific chemicals used for each tyre, weight of the vehicle, UV exposure, relative humidity, temperature of the tyre and so forth and so forth for endless possibilities. It is always recommended no parking of tyres on a new coating within the first 7 days.
What can be done to Prevent Tyre Staining?
Unfortunately, this is a problem that coating manufacturers can not presently cure any easier than the tyre industry can make tyres without the use of these chemicals. Firstly, any customer with the potential for this problem should seek specialist advice on product range, options, application requirements as well as be educated about the potential cause of this problem.
As part of this process, both customers and/or applicators should review “high risk” areas. Follow all preparation and application instructions per Product Data Sheets and allow all coatings to fully cure before being exposed to tyres and the chemicals that can potentially migrate.
Finally, because we know the problem is random and we are unable to fully prevent these occasions, it would be advisable in high or at-risk areas to park vehicles on mats, branded carpets or any suitable material that prevents at least long-term contact to the coating.
Various chemistries are known to perform better as we recommend you talk to our team.
Given the wide variation in tyre composition and age, Fortis Adhesives & Coatings makes no warranties as to the performance of the coatings and potential tyre staining and will not be held liable for any claims made where tyre staining occurs.
- How a tyre is made
- Chemical name is N- (1,3-dimethylbutyl) - N'-phenyl-p-phenylenediamine.
- An antioxidant is a chemical additive that inhibits oxidation of the base material (in this case, rubber), by oxidizing itself sacrificially.
- An antiozonant is a chemical additive that prevents the degradation of the base material (in this case, rubber) with ground-level ozone