How to produce modified bitumen
 Bitumen is a mixture mainly consisting of aliphatic, aromatic and naphthene hydrocarbons, smaller quantities of acids and organic bases and hetero-cyclic compounds containing nitrogen and sulfur. It is a substance with colloidal characteristics, consisting of asphaltene micelles shielded by a stabilizing layer of polar resins dispersed in an oily phase. The chemical nature of the various phases cannot be univocally defined. For hundreds of years, bitumen was used as a binder and waterproofing agent in many applications. Among recent applications, road paving is probably the most important. Bitumen was initially obtained from natural deposits on the earth's surface and applied as such. Nowadays, bitumen and bituminous materials are produced starting from oil, mainly as a heavy residue of the vacuum distillation of crude oil.
 Bitumen is a very appropriate material for the purposes of paving and waterproofing mentioned above, due to its properties and low cost, but according to the climate of the area, the temperature of the road paving can vary from a few tens of degrees centigrade below zero to over 60-70 degrees centigrade and it is often not possible to use bitumen as such as a result of its excessive fragility at low temperatures and poor consistency at high temperatures. In order to overcome these drawbacks and alter its rheological properties, various kinds of materials are therefore added to bitumens, among which polymeric materials and in particular thermoplastic elastomers. In this latter case, so-called polymer modified bitumens are obtained (PMB).
 In order to be used as a modifying agent, a polymer must satisfy a wide number of requisites. Firstly, it must change the rheological properties of the bitumen, reducing its thermal susceptibility and increasing its mechanical and elastic properties. At the same time, however, the polymer must not cause an excessive increase in the viscosity of the bitumen in the molten state, so that, when operating, the conventional laying machines can be used without drawbacks.
 Furthermore, the polymer must be sufficiently compatible with the asphalt so that the PMBs are not subject to phenomena of the phase separation during storage under heat. The polymer-bitumen mixture is unstable to storage under heat if, during a long residence time at high temperatures under static conditions, it tends to form a phase rich in polymer and a phase with a low polymer content, but rich in asphaltenes. Due to the difference in density, the former generally accumulates in the upper part of the storage container whereas the latter in the lower part. This leads to considerable problems relating to processability and lack of homogeneity of the end-product. Finally, the use of polymers must be convenient from an economical point of view, in the sense that the highest initial investment envisaged by their use, must be compensated by the possibility of obtaining better asphalt performances, but also by a reduction in the maintenance and re-asphalting costs, as well as, if possible, by the possibility of recycling the milled product at life-end.
 Many types of polymeric materials have been used for producing PMB, each with its advantages and disadvantages depending on the physico-chemical characteristics. For example, polyolefins, such as polyethylene, polypropylene and their copolymers can be used for increasing the consistency of the bitumen; elastomers such as block styrene copolymers can be used for increasing the elastic recovery and viscosity at high temperatures. The most-widely used polymer at present is the styrene-butadiene-styrene (SBS) block thermoplastic elastomer, generally added in percentages varying from 3 to 8% by weight with respect to the bituminous phase depending on the bituminous base and performances required of the end-product. SIS (styrene-isoprene-styrene) and SB (styrene-butadiene), belonging to the same group of copolymers, are also used quite frequently. These copolymers are often capable of giving PMB stable to storage and convenient from an economical point of view.
 SBS has a biphasic structure consisting of microdomains rich in polystyrene blocks interconnected by means of polybutadiene segments. At ordinary temperatures, the polystyrene domains are under their glass transition temperature (about 80°C), whereas the butadiene dispersing phase is in the rubber state, i.e. flexible and is that which allows the material to be significantly deformed. This leads to a cross-linking structure of the physical type which gives the polymers typical characteristics of elastic materials. When SBS comes in contact with the bitumen, at a high temperature, the latter makes the polymer "swell" and is distributed in its inside. The interactions between bitumen and SBS take place initially, preferably involving the flexible phase of the polymer, as this represents its matrix and also due to its greater free volume. It cannot be excluded, however, that the polystyrene domains also become swollen, mainly as a result of the diffusion in their inside, of aromatic components with a lower molecular weight present in the bitumen. The mixing process must be carried out in such a way as to mainly involve the most flexible, butadiene phase of the polymer, with the styrene domains only partially swollen and therefore still segregated and capable of exerting their function as knots of a three-dimensional lattice. In this way, the polymer, even if added in small quantities, can confer elastomeric properties to the whole system. It is consequently important for the modifications to be effected under non-excessive thermo-mechanical stress conditions so that the system can "memorize" the original structure of the polymer and therefore maintain the highest elasticity and hardness properties. These mixtures, however, are not always sufficiently "closely bound" as to preserve their structure at the storage temperature and in the absence of stirring. In other words, the necessary of forming a partial mixture leads to a thermodynamically unstable (or meta-stable) state which does not correspond to a minimum of energy and which spontaneously tends to evolve towards phase separation. These systems are therefore intrinsically bound to a risk situation relating to stability to storage, which is always the critical aspect for their use.
 As the structure, and therefore the elastomeric properties of the polymer and modified bitumen, depend on a physical rather than chemical cross-linking, this can be destroyed either by the use of solvents, or by heating to temperatures higher than the glass transition temperature of the styrene domains. In both cases, the process is reversible and after evaporation of the solvent or cooling below the Tg, the material recovers its structure and original properties.
 If, on the one hand, these polymers greatly improve the performances and duration of the bitumen, on the other, they unfortunately have the disadvantage of being sensitive to ultraviolet radiations and ozone, and can therefore reduce the resistance to aging of the bituminous conglomerate. In the case of SBS, but also SIS and SB, this problem is mainly due to the unsaturation present in the olefinic chains. This leads to the necessity of availing of a material which preserves the physical properties of these polymers, but which is less sensitive to UV radiations and ozone. The natural candidate for these requisites is SEBS (styrene-ethylene-butene-styrene) which is produced by the catalytic hydrogenation of C=C double bonds of SBS. With respect to SBS and analogous unsaturated rubbers, SEBS does in fact have a better resistance to UV and ozone, as well as to thermal cycles, and also has a lower permeability to humidity, fats and oils. The saturated nature of the aliphatic chain, however, creates a great limit to the compatibility of SEBS with bitumen and, other parameters such as molecular weight of the polymer and mixing conditions, being equal, SEBS is much less compatible with bitumen than SBS. The main problem in the use of a copolymer of the SEBS type for the modification of bitumens therefore consists in its poor miscibility with the latter.
 Many attempts have been made to try and increase the compatibility of bitumen and modifying polymers, such as the addition of oils or low molecular weight polymers, or an attempt to promote the formation of a chemical bond between polymer and some of the bitumen molecules or cross-linking the polymer after mixing it with bitumen. The case of SEBS however seems to be particularly complex as, in spite of the great interest in this polymer on the part of the BMP field, both scientific and patent literature cite an extremely limited number of case in which bitumen is modified with SEBS to give a mixture which is sufficiently stable as to satisfy the specifications relating to storage. The fact that the market of bitumens modified with SEBS is still being developed, represents a further confirmation of this difficulty.
 With respect to patent literature, there are some exceptions relating to patents filed by John Field in which bitumen is modified by the simple addition of SEBS copolymer. For example,US-A-5,973,037discloses how to modify both oxidized and non-oxidized bitumens by the addition of SEBS in the form of pellets or powder by means of a simple mixing process with a high shear stress.US-A-5,929,144, filed by the same author, describes a completely analogous process, in which the modifying agent is a SEBS plasticized with 40% by weight of naphthene oil and the mixing can be effected with both a high and low shear stress. In any case, it should be pointed out that the success or failure of the mixing depends on a wide number of variables which can be basically summarized in three points: the characteristics of the polymer (composition, molecular weight, etc.), the mixing conditions (temperature, duration, equipment and procedures used) and finally, which is probably the most important, the characteristics of the bitumen (composition, origin, etc.) In this context, it should also be pointed out that it is almost never possible to establish a priori if a particular bitumen-polymer pair is compatible, specifically as a result of the variability of the composition of the bitumen. Consequently, compatibilizing techniques which have proved to be valid for a certain bitumen, may not necessarily be so for different bitumens.
 With reference to the solubility parameters of the polymers and bitumens, however, it can be asserted that for most of the latter, compatibility with respect to SBS is higher than that with respect to SEBS.
 The necessity therefore remains of finding as general as possible a method of use for improving the compatibility between bitumen and block copolymers of the SEBS type. Considering that mixing conditions and bitumen must obviously resort to machines and materials normally available in modification plants, the best method seems to be that of intervening on the polymer. As far as SEBS is concerned, the ratio between the styrene part and the butadiene part cannot be varied much in order to maintain the elastomeric properties based on the correct balance between these two components. Furthermore, the molecular weight cannot be reduced too much (it is a general rule that with an increase in the molecular weight, the compatibility between bitumen and polymer decreases), as the good physical characteristics of the final modified product could no longer be guaranteed. The most promising method therefore appears to be that of the chemical modification of the polymer, suitable for introducing functional groups into the chain, which are capable of increasing the polarity of the macromolecules and possibly also capable of reacting with other functional groups belonging to bitumen molecules (so as to establish chemical bonds with the latter) and/or belonging to the polymer itself (in order to produce a structure in which, in addition to physical cross-linking, there is also a chemical cross-linking). Copolymers of the SEBS type modified by means of the grafting of maleic anhydride, or the introduction of phthalic anhydride inside the macromolecule, are currently available on the market. These copolymers are also produced and commercialized for the modification of bitumens but, in certain cases, they can prove to be "too" compatible, which causes their dissolution in the bitumen itself, with a loss of the three-dimensional structure responsible for the elastic properties and a consequent reduction in the performances of the PMB obtained.US-A-5,246,987describes the use of a block copolymer in which one of the blocks consists of halogenated vinylaromatic compounds and the other of olefinic chains, the latter preferably being halogenated. A similar philosophy is proposed inUS-A-6,211,292which suggests the use of a hydrogenated block elastomer, modified by means of the grafting of an alcohol, an acid or an amine subsequently reacted with an isocyanate. In both cases, the functionalization of the polymer requires the use of compounds which are difficult to handle and extremely complex and costly operations with respect to the plant.
 It is therefore necessary to find a simple and economical functionalization method of SEBS, in order to increase its compatibility with bitumen to enable the production of modified bitumens suitable for road paving which are resistant to ozone, UV rays, thermal cycles and which also have a high elasticity, and high softening temperature.
- (a) bitumen;
- (b) block copolymers having the general formula (A-B)n-A or (A-B)n, A being a vinyl aromatic polymer block and B being a polymer block obtained by polymerization and subsequent hydrogenation of a conjugated diene, the above block copolymer being functionalized with one or more acrylates having general formula (I)