AQUEOUS EMULSION OF ASPHALT AND EMULSIFIABLE POLYISOCYANATE.
The instant invention provides an aqueous emulsion of asphalt and emulsifiable polyisocyanate.
US-P-4724245 discloses a method for preparing an aqueous emulsion of asphalt, comprising the steps of
- preparing an emulsion (A) by mixing (a) a first asphalt composition comprising asphalt and a polyisocyanate extender and (b) an aqueous phase comprising water and an emulsifier;
- preparing an emulsion (B) of a polyisocyanate in water; and
- mixing emulsions (A) and (B).
Polyisocyanate extenders are low molecular weight polyols, e.g. PEG, PPG, HTPB, etc. Polyisocyanates disclosed encompass all kinds of polyisocyanates, while the only disclosed example is Suprasec 1042, an emulsifiable MDI.
This process thus makes use of a polyol as a chain extender, which will react with the polyisocyanate contained in the second emulsion. This process also requires that two distinct emulsions be first prepared, the first asphalt emulsion and the second polyisocyanate emulsion, before the final asphalt emulsion is applied to the locus of interest. This requires additional work, which is detrimental to the efficiency of the process. Such a process also involves stability problems, since the second polyisocyanate emulsion is not stable. This problem is specifically noted in US-P-4724245, which indicates that the polyisocyanate emulsion is not stable more than 2 hours. This stability problem makes it necessary to prepare the polyisocyanate emulsion in situ, which again requires additional work and equipment. US-P-4724245 proposes to stabilize the polyisocyanate emulsion by reacting it with part of the emulsion (b) above, i.e. with part of the polyol chain extender, finally leading to a prepolymer emulsion.
The instant invention is based on the surprising effect that, in case no polyol is added to the aqueous asphalt, it is possible to add directly emulsifiable polyisocyanate to the asphalt emulsion while still obtaining surprisingly good results.
Thus, the instant invention provides an asphalt emulsion comprising, by weight based on the total weight of the emulsion:
- about 95 to 50%, preferably about 90 to 60%, of asphalt and water, where the asphalt and water are present according to a weight ratio asphalt to water of about 1 :0.3 to 1 :2.0, preferably of about 1 :0.7 to
1 :1.5; and - about 5 to 50%, preferably about 10 to 40%, of an emulsifiable polyisocyanate. The emulsion may be used to provide an aggregate mixture, where said mixture comprises aggregates (coarse and/or fine) and/or filler (e.g. limestone) and the asphalt emulsion in an aggregate/filler:emulsion weight ratio from 3:1 to 20:1, preferably from 5:1 to 15:1.
This aggregate mixture can be either a low compaction energy mixture or a high compaction energy mixture.
Without wishing to be bound by a theory, applicant believes that the emulsifiable polyisocyanate can react with the phenolic, carboxylic, anhydride and pyrrolic groups of the asphalt, which are NCO-reactive groups. What is formed is a covalently linked polymer gel/ network. This allows not using the polyol and allows adding directly the emulsifiable polyisocyanate to the asphalt emulsion.
The asphalt used in the instant invention is any asphalt known and generally covers any bituminous compound. It can be any of the materials referred to as bitumen or asphalt, e.g. distillate, blown, high vacuum and cut-back bitumen, and also e.g. asphalt concrete, cast asphalt, asphalt mastic and natural asphalt. For example, directly distilled asphalt can be used, having, for example, a penetration of 80/100 or 180/220.
The emulsifiable polyisocyanate that is used is any kind of emulsifiable MDI, such as those disclosed in EP-A-0310345 and US-P-3996154, which are incorporated herein by reference. One preferred class of emulsifiable MDI comprises those products obtained by pre-reacting an polyisocyanate, especially polymeric MDI, with a minor proportion of an alkoxypolyalkylene glycol (e.g. one in which one of the glycol -OH groups has been converted to an alkoxy group, for example by reaction with a lower alcohol such as methanol and/or ethanol), such products being mixtures of the polyisocyanate and a minor proportion of a nonionic surfactant formed by reaction between the polyisocyanate and the alkoxypolyalkylene glycol. Preferred emulsifiable polyisocyanates useful in this invention are monomeric, that is they are not prepolymers. By virtue of being pre-reacted with a minor amount of an alkoxypolyalkylene glycol prior to being added to make the
compositions of this invention, they do not require the external addition of emulsifying agents to form an emulsion with water, and hence are self-emulsifiable. The self- emulsifiable polyisocyanate may be based on any organic polyisocyanate, for example low functionality MDI variants such as uretonimine modified MDI, but is preferably based on the mixtures known as polymethylene polyphenyl polyisocyanates or polymeric MDI. Alkoxypolyalkylene glycols which may be reacted with the polyisocyanate to form the self-emulsifiable polyisocyanate include alkoxypolyethylene glycols, especially those having molecular weights in the range 250 to 4000, particularly 600 to 2000. The alkoxy group suitably contains from one to six carbon atoms, the methoxypolyethylene glycols being preferred. The amount of alkoxypolyalkylene glycol reacted with the polyisocyanate should be sufficient to provide an emulsifiable polyisocyanate which, in turn, yields an aqueous emulsion of the desired stability. In general, suitable self-emulsifiable polyisocyanates contain from 1 to 15 parts by weight of polyisocyanate/ alkoxypolyalkylene glycol reaction product for each 100 parts by weight of polyisocyanate. The self-emulsifiable polyisocyanate when obtained may, if desired, be mixed with a further amount of the same or a different polyisocyanate.
Typical commercially available self-emulsifiable organic polyisocyanates include emulsifiable polymeric MDI sold under various trade names, for example Suprasec 1042 from Imperial Chemical Industries PLC. For example, the self-emulsifiable isocyanate is a reaction product of polymeric
MDI, containing about 50 % of diisocyanates, with a methoxypolyethylene glycol having a molecular weight of approximately 650.
The aqueous emulsions of the invention can also comprise traditional emulsifiers, which can be present in amounts by weight, based on the total weight of the emulsion, of about 0.1 to 10%, preferably about 0.5 to 5%. Such emulsifiers can be any emulsifiers traditionally used in the art, such as bentonite, anionic, cationic and non-ionic surfactants. Specific examples are polyamine hydrochloride, amido-amine hydrochloride and quaternary ammonium salt. These surfactants are disclosed in US-P-4724245, the respective part of the description being incorporated by reference. These further emulsifiers are generally present, when used, in the first emulsion prepared.
The invention also relates to a method for preparing said asphalt emulsion of the invention, comprising the steps of:
- preparing an emulsion (A) by mixing (a) asphalt and (b) water, optionally in the presence of an emulsifier; and
- mixing emulsion (A) and the emulsifiable polyisocyanate.
The first step for preparing the intermediate asphalt emulsion and the final mixing step can be carried out using any suitable apparatus known in the art. The first step is classical and known in the art; the second mixing step can be carried out, e.g., at room temperature.
The aqueous asphalt emulsions of the invention will be used as any classical asphalt emulsions of the prior art. The aqueous asphalt emulsions of the invention will notably be useful for the production of:
- paints and coatings, particularly for waterproofing,
- mastics for filling joints and sealing cracks
- grouts and cold-poured surfaces for surfacing of roads, aerodromes, sports grounds, etc. - cold coatings for surfacing as above
- surface coatings for surfacing as above
The aqueous asphalt emulsions of the invention exhibit improved fatigue properties and higher resistance to permanent deformation, when compared to prior art aqueous asphalt emulsions. The invention also provides a process for preparing the low and high compaction energy mixtures, comprising the step of mixing the aggregate and/or filler with the asphalt emulsion at room temperature, using for example a mechanical shear mixer. The following examples illustrate the invention without limiting it.
Example 1 : Low compaction energy mixtures.
A range of low compaction energy mixtures were prepared using a cationic bitumen emulsion (from Nynas), an emulsifiable isocyanate (Suprasec 1042 from ICI), coarse aggregate (14mm), fine aggregate (0 to 5 mm) and limestone filler (<75 microns). The range of compositions made is shown in table 1 below.
The bitumen emulsion and emulsifiable isocyanate were mixed using a low speed mechanical stirrer at room temperature. This mixture was then mixed with the aggregates using a mechanical shear mixer. The compositions were gently compacted by hand pressure (i.e. tapping). Following compaction the samples were left to cure at room temperature for several weeks.
Stiffness modulus values (MPa) were determined for the compositions identified in table 1. The results obtained are presented in table 2.
Example 2: High compaction energy mixtures.
High compaction energy mixtures were prepared as in example 1. The compositions evaluated are given in table 3 below.
The bitumen emulsion and emulsifiable isocyanate were mixed using a low speed mechanical stirrer at room temperature. This mixture was then mixed with the aggregates using a mechanical shear mixer. The compositions were compacted for 2 minutes using a Howard Frame which combines a limited static load and vibration.. Following compaction the samples were left to cure at room temperature for several weeks. Stiffness modulus values (MPa) were determined for the compositions identified in table 3. The results obtained are presented in table 4.
The fatigue properties (number of cycles to failure) of the compacted mixtures were evaluated by subjecting the samples to dynamic loading using a Nottingham Asphalt Tester. The results obtained are presented in table 5.