US 8,188,171 B2  12  11  aliphatic, cycloaliphatic or arylaliphatic isocyanate mono- mers:  The homocondensation products are the products derived  from the condensation of one of the above-listed isocyanate  monomers with itself. The heterocondensation products are  the products derived from the condensation of two or more of  the above-listed monomers, with one another and/or option› ally with one or more mobile hydrogen compounds, such as  for example an alcohol, a dial and other similar compounds.  The polyisocyanates included in the composition of the  present invention may also be polyisocyanate derivatives  derived from aromatic isocyanates used alone or mixed with  aliphatic compounds.  However, the use of these aromatic derivatives is limited in  terms of amount or even is not preferred as it generally leads  to coatings which may become discoloured, generally turn  yellow, as they age, in particular if the coatings are markedly  exposed to UV radiation, for example solar UV radiation.  The following may be cited as non-limiting examples of  aromatic isocyanates:  Mixtures of these ( cyclo )aliphatic and/or aromatic poly› isocyanates may also be used.  The polyisocyanates used to prepare the (poly)isocyanate  compositions for the coating according to the invention have  an average functionality in isocyanate groups at least equal to  2 and at most equal to 10, preferably greater than 2.5 and at  most equal to 8, advantageously between 2.8 and 6.5.  The viscosity of the non-masked polyisocyanate com› pounds which can be used in the invention is within a very  broad viscosity range owing to the structure of the polyiso› cyanate compounds which may be involved. The viscosity is  generally greater than 10 mPa•s, at 25°  C. with 100% dry  extract, preferably greater than 100 mPa•s, at 25°  C. and with  100%  dry  extract.  Examples include the viscosity of products from Rhodia  such as Tolonatefi HDT-LV2 having a viscosity of approxi› mately 600 mPa•s–150 mPa•s, at 25°  C., or else Tolonatefi  HDT having a viscosity of2,400 mPa•s–400 mPa•s, at 25°  C.,  or else Tolonatefi  HDB having a viscosity of 9,000  mPa•s–2000 mPa•s, at 25°  C., or else Tolonatefi HDT HR  More specifically, in order to optimise the results of the  present invention, said pKa ( or one thereof if several may be  defined) of the group carrying one or more labile hydrogens is  at least equal to 4, advantageously 5, preferably 6 and is at  most equal to 14, advantageously 13, preferably 12 and more  preferably 10. However, one exception is the lactams, the pKa  of which is greater than these values and which constitute  possible (although not preferred) masking agents for the  invention.  The masking agent or mixture of masking agents which  temporarily, or even permanently, protects the isocyanate  groups is/are compounds having at least one group carrying a  labile hydrogen, generally a group carrying a labile hydrogen,  preferably a single group carrying a labile hydrogen, and  which are reactive towards the isocyanate group. There may  be associated with this group carrying a labile hydrogen a pKa  value corresponding either to ionisation of an acid [including  the hydrogen atom of the -ol groups (in the present descrip› tion, the term "-ol(s )" refers to phenols and alcohols)] orto the  associated acid of a (generally nitrogenous) base.  In the present disclosure, the term "masked (poly)isocyan- ate" refers to a (poly)isocyanate for which at least 50%,  preferably 80%, advantageously 90% and more preferably  still all of the isocyanate groups are masked.  The (poly)isocyanates present in the composition accord› ing to the invention may be in masked form, i.e. the isocyanate  groups are not free but rather masked using a masking agent  or a mixture of masking agents as defined hereinafter. Par› ticularly preferred are compositions of masked polyisocyan› ates for the preparation of an one-component-type coating  formulation (formulation lK).  wherein: Mn represents the number-average molecular  weight obtained by gel permeation and  [iNCO] represents the concentration in isocyanate groups in  grammes per 100 grammes.  In the present invention, examples and claims, the average  functionality in isocyanate groups f(iNCO) is defined by the  following formula:  1,6-hexamethylene diisocyanate,  1, 12-dodecane diisocyanate,  cyclobutane-1,3-diisocyanate  cyclohexane-1,3 and/or 1,4-diisocyanate,  l-isocyanato-3,3,5-trimethyl-5-diisocyanatomethyl  cyclo- hexane (isophorone diisocyanate, IPDI),  isocyanatomethyloctylene diisocyanates (TTI), in particular  4-isocyanatomethy 1-1,8-octy lenediisocyanate  2,4 and/or 2,6-hexahydrotoluylene diisocyanate (H$_{6 }$TDI),  hexahydro-1,3 and/or 1,4-phenylene diisocyanate  perhydro-2,4’ and/or 4,4’-diphenylmethane diisocyanate  (H $_{12}$MD I), and in general aromatic amine precursors or the  perhydrogenated carbamates,  bis-isocyanatomethyl cyclohexanes (in particular 1,3 and  1,4) (BIC),  bis-isocyanatomethyl norbornanes (NBDI),  2-methylpentamethylene diisocyanate (MPDI),  tetramethylxylylene diisocyanates (TMXDI), and  lysine diisocyanate and also the esters oflysine diisocyanate  or triisocyanate (LDI or LTI).  Moreover, it has been found that when the average func- tionality in isocyanate groups of the polyisocyanate  increases, the grit resistance and the hardness of the coating  are improved; this phenomenon is particularly marked during  touch-up operations.  In view of their capacity to impart high grit resistance to the  coatings,  homocondensation and/or heterocondensation  products of an, in particular non-cyclic, aliphatic isocyanate  monomer, preferably HDI, are preferred.  Specific polyisocyanate compounds are solid at 100% dry  extract. This is the case, for example, for the isocyanurate  trimer ofIPDI or for the dimer ofIPDI. Examples include the  viscosities of some of these compounds in organic solutions;  Tolonatefi IDT 70 S (isocyanurate trimer ofIPDI) thus has a  viscosity of approximately 1,700mPa•s–600 mPa•s at 25°  C.  in the case of a formulation having 70% dry extract in  Solvessofi 100; Tolonatefi IDT 70 B (isocyanurate trimer of  IPDI) has a viscosity of approximately 600 mPa•s–300 mPa•s  at 25°  C. in the case of a formulation having 70%  dry  extract  inn-butyl acetate.  having a viscosity of approximately 20,000 mPa•s, at 25°  C.  with 100%  dry  extract, or 2,000 mPa•s at 25°  C. at 90% dry  extract inn-butyl acetate.  .  Mnx[iNCO]  f(tNCO)  =  42x100  2,4- and/or 2,6-toluylene diisocyanate,  diphenylmethane-2,4’ and/or 4,4’-diisocyanate (MDI),  1,3- and/or 1,4-phenylene diisocyanate,  triphenylmethane-4,4’,4"-triisocyanate, and  the oligomers of MDI or TDI. 