In a conventional pneumatic tire of the “tubeless” type (that is to say of the type without an inner tube), the radially internal face comprises an airtight layer (or more generally a layer that is impermeable to any inflation gas) which enables the pneumatic tire to be inflated and kept under pressure. Its airtightness properties enable it to guarantee a relatively low rate of pressure loss, making it possible to keep the tire inflated, in the normal operating state, for a sufficient time, normally several weeks or several months. It also has the role of protecting the carcass reinforcement from diffusion of air coming from the internal space of the tire.
This role of airtight inner layer or “inner liner” is today essentially fulfilled by compositions based on an elastomer or butyl rubber, long renowned for their excellent airtightness properties.
Moreover, in recent years, pneumatic tire manufacturers have made particularly strenuous efforts to develop novel ways of solving a problem dating back from the very start of the use of wheels fitted with inflated pneumatic tires, namely how to allow a vehicle to continue to travel despite a substantial or total loss of pressure of one or more pneumatic tires. For decades, the spare wheel was considered to be the sole and universal solution. Then, more recently, the considerable advantages of possibly dispensing with the spare wheel have appeared. The concept of “extended mobility” was developed. The associated techniques allow the vehicle to run with the same pneumatic tire, dependent on certain limitations to be respected, after a puncture or a pressure drop. This makes it possible for example to get to a point of repair without having to stop, in often hazardous circumstances, to fit the spare wheel.
Self-sealing compositions that can achieve such an objective, which by definition are capable automatically, i.e. without any external intervention, of rendering a pneumatic tire airtight in the event of it being punctured by a foreign body, such as a nail, are particularly difficult to develop.
In order to be able to be used, a self-sealing layer must satisfy many conditions of a physical and chemical nature. It must in particular be effective over a very wide range of operating temperatures and over the entire lifetime of the pneumatic tires. It must be capable of sealing the hole when the puncturing body remains in place; following expulsion of the latter, it must be able to fill in the hole and render the pneumatic tire airtight.
Many solutions have been envisioned but have not been able to be developed in pneumatic tires for vehicles, in particular due to the lack of stability over time or lack of effectiveness under extreme operating temperature conditions.
To help to maintain good effectiveness at high temperature, document U.S. Pat. No. 4,113,799 (or FR-A-2 318 042) has proposed, as self-sealing layer, a composition comprising a combination of partially crosslinked butyl rubbers of high and low molecular weights, optionally in the presence of a small portion of a thermoplastic styrene elastomer. For good sealing effectiveness, said composition comprises from 55 to 70% by weight of a tackifying agent.
Document U.S. Pat. No. 4,228,839 has proposed, as self-sealing layer for a tire, a rubber compound containing a first polymer material that degrades when irradiated, such as polyisobutylene, and a second polymer material that crosslinks when irradiated, preferably a butyl rubber.
Document U.S. Pat. No. 4,426,468 has itself also proposed a self-sealing composition for a tire based on crosslinked butyl rubber of very high molecular weight.
A known drawback of butyl rubbers is that they suffer large hysteresis losses, furthermore over a wide temperature range, which drawback has repercussions on the layers or compositions themselves, whether they are of gastight type or of the self-sealing type, giving them a large increase in hysteresis and considerably degrading the rolling resistance of pneumatic tires using such compositions.
Reducing the hysteresis of gastight and self-sealing multilayer laminates, and therefore in fine the fuel consumption of motor vehicles, is a general objective that the current technology comes up against.
Moreover, the Applicants have observed that the self-sealing compositions based on butyl rubber may be insufficiently effective after the delayed expulsion or removal of a puncturing article left in place for a long period in the structure of the pneumatic tire.
Document EP-B 1-1 090 069 has indeed itself proposed self-sealing compositions that are free of butyl rubber, the specific formulation of which comprises, per 100 parts by weight of a styrene-based thermoplastic elastomer, 80 to 140 parts of a liquid plasticizer, 110 to 190 parts of a tackifying resin and from 2 to 20 parts of an additive.
A large amount of tackifying resin, besides the higher industrial cost that it induces for the tires, may itself also degrade the rolling resistance of the tires due to a risk of excessive stiffening of the self-sealing composition.