Adhesive product for the bonding of steel, particularly in the automotive sheetwork operations

Adhesive product for the bonding of steel, particularly in the automotive sheetwork processes proper to be used on ordinary non-degreased or oiled steel as well as on precoated steels of the electroplated or electrotinned type, even after being regreased, the said adhesive product being polyurethane-based and implementing polyol systems, comprising either one poylol or binary or ternary polyol mixtures or mixtures of more than three polyols modified by reaction with an epoxy resin.

This invention relates to an adhesive product intended for the bonding of 
steel, particularly in the automotive sheetwork operations, such as steel 
cladding, fitting and trimming. 
In the automotive industry, it is known to resort to epoxy adhesives in 
order to bond together metal surfaces which may be coated or soiled with 
grease. 
The bonding or binding strength thus obtained, as determined on standard 
test pieces or specimens in accordance with the Manufacturers' testing 
method Nr. 1108--junction of two sheet metal plates by means of the 
adhesive (glued joint: 25 mm.times.20 mm.times.0.2 mm) and measurement of 
the force required in order to part them by pulling efforts exerted in the 
plane of the sheet metal plates, whereby the mode of rupture, i.e. 
parting, may be "cohesive", "adhesive" or "combined cohesive-adhesive", it 
being understood that "cohesive parting" is soughtafter--is higher than 8 
MPa. 
The drawback encountered with epoxy adhesive bonding lies especially in the 
fact that, on the one hand, in the lapse of time between the application 
of the adhesive and the arrival of the bonded parts in the surface 
treatment area of the assembly line, i.e. for a time duration of about 2 
to 4 hours, the epoxy adhesive does not have any adherence properties, the 
relative shifting or dislocations of parts one relative to the other thus 
remaining possible and, on the other hand, in the surface treatment areas, 
the baths implemented bring about a carryover of the "unbaked" adhesive, 
whereby resulting in contamination and weakening of the glued joint. 
It has been proposed to remedy these drawbacks by providing a resin 
prebaking process, the disadvantage of which is an expensive and difficult 
implementation. 
The object of the present invention is therefore to provide an adhesive 
product of the aforesaid type, which better meets the various requisites 
of the technique than the ones already available. 
It is to the credit of the Applicant to have found at the outcome of 
thorough research that this object could be reached by means of a 
polyurethane-based adhesive substance, either in the form of a 
two-component system, or as a one-component system with blocked hardening 
agent. 
The adhesive product according to this invention for the bonding of steel, 
particularly in the automotive sheetwork processes is polyurethane-based, 
implementing polyol systems, comprising either one polyol or binary or 
ternary polyol mixtures or mixtures of more than three polyols modified by 
reaction with an epoxy resin. 
The adhesive products according to the invention provide a bonding strength 
in the order of 1 to 2 MPa when entering the surface treatment area, 
without any previous baking. 
The epoxy resin may be an epoxy resin available in the market, e.g. the one 
known under the "EPIKOTE 828" trademark and sold by Shell. 
The polyols incorporated in the systems, especially in the binary and 
ternary systems of modified polyols may be polyether polyols or polyester 
polyols or also mixed polyols.

They are identified by their hydroxyl number. 
The following polyols have been tested: 
polyether polyols having hydroxyl numbers from 34 to 360, especially of 34, 
26, 25 and 360 (all of them being branched triols), 
polyester polyols having hydroxyl numbers from 40 to 215, especially of 
respectively 40 (linear), 56 (linear), 60 (slightly branched), 170 
(half-branched), 160 (branched), 215 (branched), 
mixed polyol having a hydroxyl number from 120 to 200, especially of 
160-170 (branched). 
The shear resistances of the adhesive products embodied by the invention, 
based on the polyurethanes obtained by means of the following three binary 
mixtures of modified polyols are given hereunder by way of example, it 
being understood that the composition of the polyurethanes comprises 
diisocyanate as known in the art. 
The three binary mixtures are as follows: 
(a) 50% of diol having a molecular mass of 1000 and 50% of triol having a 
molecular mass of 450, 
(b) 65% of polyether diol having a molecular mass of 1000 and 35% of 
polyether diol having a molecular mass of 450, 
(c) 66.6% of polyether diol having a molecular mass of 400 and 33.3% of a 
triol having a molecular mass of 400. 
The shear resistances determined by the aforesaid test are as follows: 
on degreased steel, 18 MPa, 8.8 MPa and 20.6 MPa for (a), (b) and (c) 
respectively, the parting being cohesive, 
on oiled steel, 6.9 MPa, 8.3 MPa and 20.4 MPa for (a), (b) and (c) 
respectively, the parting being cohesive for (b), and mixed for (a) and 
(c). 
The values of the shear resistance obtained with the adhesive products 
based on the two ternary mixtures (d) and (e) of the following modified 
polyols are indicated further below: 
(d) 50% of diol having a molecular mass of 1000 
25% of diol having a molecular mass of 400 
25% of triol having a molecular mass of 400 
(e) 50% of diol having a molecular mass of 1000 
15% of diol having a molecular mass of 400 
25% of triol having a molecular mass of 400. 
The shear resistances are as follows: 
on degreased steel, 13.7 MPa for (d) and 17.8 MPa for (e) with cohesive 
parting, 
on oiled steel, 13.3 MPa for (d) and 18 MPa for (e) with mixed parting in 
the first case and cohesive parting in the second one. 
The ageing stability of the adhesive products embodied by this invention 
are in the whole satisfactory. 
With a view to enhancing it still further, there can, according to the 
invention, either be acted on the technique relating to the modification 
of the polyols contained in the constitution of the adhesive products of 
the invention, or there can be added a low amount of suitable additives to 
said adhesive products. 
In the first case, it is a matter of properly selecting the conditions of 
temperature and time of the reaction of modification, whereby it should 
remain understood that, generally, 0.5 to 4 moles of polyol are reacted 
with about 1 mole of epoxy resin and more particularly about 2 moles of 
polyol are reacted with about 1 mole of epoxy resin. 
Good results could be achieved by selecting the temperature in the range of 
130 to 200.degree. C. and the reaction time in the interval of 3 to 24 
hours. 
By following such a procedure, the ageing stability can be improved (as 
measured by an accelerated ageing test) to such a point that the parting 
resistance after several years (e.g. 5 years) is doubled and the parting 
is improved. 
In the second case, to the adhesive product is added an amount generally 
lower than 3% of an additive which can be selected from the silanes, alkyl 
titanates, ketonic resins, acrylic resins and others. 
By way of example, it may be pointed out that: 
among the silanes, have successfully been used those sold under the 
trademark "GLYMO", "MERCAPTO" and "DAMMO" by Dynamit Nobel and those sold 
under the designations "A 187", "A 189", "A 1120" and "A 1100" by Union 
Carbide, the amounts implemented being from 0.1 to 1 % by weight related 
to the mass of adhesive product; 
among the ketonic resins, good results could be achieved with the resin SK 
sold by Huls and the resin sold under the trademark "MACRYNAL" by Hoechst, 
the amount being from 1 to 2%; 
among the acrylic resins, good results could be achieved with the resin 
"DESMOPHEN A" sold by Bayer, used in the amount of 1 to 2%. 
Here again, the parting resistance can be doubled after several years' 
ageing and the nature of the parting is improved. 
Besides the base consisting of polyurethane, the additives in accordance 
with the invention may contain (expressed in weight percent related to the 
mass of adhesive product) 
fillers (10 to 55%) consisting e.g. of CO.sub.3 Ca, kaolin and others, 
plasticizers (2 to 10%) of the phthalate-type, 
catalysts of the tin salt-type, 
solvents (0.1 to 20%) in order to control the viscosity, such as xylene, 
thickening agents (0.1 to 5%) such as silica derivates, 
pigments (0.1 to 15%) such as titania or titanium (IV) oxide and carbon 
blacks. 
The additives according to this invention make it possible to bond not only 
ordinary non-degreased or oiled steel, but also pre-coated steels of the 
electroplated or electrotinned type, even after being regreased.