Coated abrasive article and process for producing the same

A coated abrasive product comprising a backing, a first adhesive layer formed on the backing, a layer of abrasive grains formed on the first adhesive layer, and a second adhesive layer covering the layer of abrasive grains, wherein the second adhesive layer is formed from a composition comprising a water soluble resol phenolic resin having a weight-average molecular weight of from 1,400 to 3,500 and an organic solvent having a boiling point of 110.degree. to 175.degree. C., and a process for producing the same. The present invention remarkably shortens a drying time of the second adhesive used in the production of the coated abrasive product of the present invention and improves productivity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a coated abrasive article and a process 
for producing the same. 
2. Discussion of the Art 
Coated abrasive articles (e.g., abrasive cloth and abrasive paper, etc.) 
are produced by applying a first adhesive on a backing (e.g., paper, 
cloth, etc.), applying abrasive grains to the first adhesive by 
electrostatic coating or gravity coating, curing the first adhesive with 
drying, applying a second adhesive over the first adhesive and abrasive 
grains, and then curing the second adhesive with drying. 
In this case, as the second adhesive, for example, a synthetic resin (e.g., 
phenolic resin, epoxy resin, urea resin, etc.) or glue is used. Among the 
foregoing, a water soluble resol phenolic resin is widely used, because it 
is cheap and is superior in strength and heat resistance. However, the 
curing rate of the resol phenolic resin is low, and it requires long time 
for drying and curing. Further, when the resin is cured at a high 
temperature in order to accelerate drying and curing, boiling of water (as 
a solvent) as well as vapor of condensate formed on curing cause blisters 
(or little foams) in the cured resin layer, which results in a decrease of 
strength. As described above, it is difficult to reduce the drying time. 
A relation between percentage of solids in and viscosity of typical resol 
phenolic resins commercially available in Japan for use as phenolic resins 
for abrasive cloth and paper is shown in FIG. 1. As is apparent from FIG. 
1, resol phenolic resins containing about 65 to 82% solids and having a 
viscosity of about 700 to 10,000 cps are used for abrasive cloth and 
paper. Most popularly, water is used as a solvent. A resin in which 
alcoholic solvent (e.g., methanol, etc.) is partially or exclusively used 
is also commercially available. 
After the second adhesive containing phenolic resin is applied, the second 
adhesive is cured by heating at a temperature of from about 50.degree. to 
110.degree. C. for about 50 minutes to several hours in a festoon oven, 
which can contain hundreds or thousands meters of web. After the web is 
wound, the second adhesive is completely cured by heating to a temperature 
of about 60.degree. to 130.degree. C. for several hours to 20 hours. 
On the other hand, in the case of using a short tunnel oven in place of the 
festoon oven, coating speed and drying speed must be remarkably decreased, 
and, therefore, productivity becomes low. Accordingly, it is difficult to 
reduce curing time with the current formulation and curing step. 
To improve productivity, there is proposed a method for accelerating 
heating and curing by applying some modifications and additives to the 
resol phenolic resin. For example, there is a method comprising the steps 
of adding another resin to the phenolic resin to accelerate gelation, 
winding the coated abrasive product, and then curing the phenolic resin by 
after-cure (post-cure). In Japanese Patent Kokai No. 55-83573, there is 
proposed that a mixture of a water based resol and a condensate product 
which is made from a phenol component including phenol or polyfunctional 
phenol having high reactivity and formaldehyde be used as a binder. 
Further, a method for accelerating curing by acidifying the resol phenolic 
resin with an acid such as p-toluenesulfonic acid is also used. 
However, the modification of resin or the acceleration of curing causes a 
decrease of crosslinking density, which results in deterioration of heat 
resistance. Further, hardness and strength are also decreased. 
It would be desirable to shorten the curing time of the second adhesive and 
to remarkably improve productivity by increasing the molecular weight of a 
water soluble resol phenolic resin formulated in the second adhesive and 
selecting an organic solvent suitable for the resin. 
SUMMARY OF THE INVENTION 
The present invention provides a coated abrasive article comprising a 
backing, a first adhesive layer formed on the backing, a layer of abrasive 
grains formed on the first adhesive layer, and a second adhesive layer 
covering the layer of abrasive grains, wherein the second adhesive layer 
is formed from an adhesive comprising a water soluble resol phenolic resin 
having a weight-average molecular weight of from 1,400 to 3,500 and an 
organic solvent having a boiling point of from 110.degree. to 175.degree. 
C. 
The present invention also provides a process for producing a coated 
abrasive article comprising the steps of: 
(a) applying a first adhesive on a backing to form a first adhesive layer; 
(b) applying a layer of abrasive grains on the first adhesive layer; 
(c) curing the first adhesive by heating; 
(d) applying a second adhesive over the first adhesive layer and the layer 
of abrasive grains to form a second adhesive layer; 
(e) curing the second adhesive layer to form a coated abrasive article; 
(f) winding the coated abrasive article; and 
(g) after-curing (post-curing) the article at a temperature of 60.degree. 
to 130.degree. C.; 
wherein the second adhesive contains a water soluble resol phenolic resin 
having a weight-average molecular weight of from 1,400 to 3,500 and an 
organic solvent having a boiling point of from 110.degree.to 175.degree. 
C. Curing step (e) is preferably conducted at a temperature of from 
120.degree. to 160 C. for 1.5 to 3 minutes after the second adhesive is 
applied.

DETAILED DESCRIPTION 
The coated abrasive article of the present invention comprises a backing, a 
first adhesive layer formed on the backing, a layer of abrasive grains 
formed on the first adhesive layer, and a second adhesive layer covering 
the layer of abrasive grains. 
As the backing, for example, paper, cloth, or the like is generally used. 
Film or non-woven fabric, for example, polyethylene terephthalate, may 
also be used. Further, it is important that the backing have sufficient 
strength to stand use in a coated abrasive article, as well as suitable 
flexibility. 
The first adhesive layer formed on the backing comprises a synthetic resin 
(e.g., phenolic resin, epoxy resin, urea resin, etc.) or glue. It is 
preferred that the first adhesive be dried at a temperature of from 
50.degree. to 110.degree. C. for about 30 seconds to several minutes for 
phenolic resin, at a temperature of from 70.degree. to 140.degree. C. for 
about 30 seconds to several minutes for urea resin, and at a temperature 
of from 50.degree. to 110.degree. C. for 30 seconds to several tens of 
minutes for glue. 
A layer of abrasive grains is formed on the first adhesive layer. The layer 
of abrasive grains can be composed of abrasive grains containing aluminum 
oxide, silicon nitride, silicon carbide, alumina-zirconia, alumina 
obtained by sol-gel method as a main component [e.g., "Cubitron" (trade 
name), manufactured by Minnesota Mining and Manufacturing Company, etc.], 
garnet, diamond, CBN (cubic boron nitride), and the like, or combinations 
thereof. 
Normally, the coated abrasive article can be formed by applying a layer of 
the second adhesive on the aforementioned layer of abrasive grains and 
curing the second adhesive by heating. The second adhesive used in the 
present invention contains a water soluble resol phenolic resin having a 
weight-average molecular weight of from 1,400 to 3,500 and an organic 
solvent having a boiling point of 110.degree. to 175.degree. C. The water 
soluble resol phenolic resin may be prepared by mixing phenol with 
formaldehyde to form an aqueous solution and subjecting the solution to 
the condensation reaction by heating in the presence of a basic catalyst 
(e.g., sodium hydroxide, potassium hydroxide, etc.). In this process, 
viscosity and molecular weight of the resin solution can be adjusted by 
controlling the heating temperature and the heating time. The molecular 
weight can be measured by GPC (gel permeation chromatography) and the 
viscosity can be measured by a B type viscometer. A relation between 
viscosity and weight-average molecular weight of the resol phenolic resin 
is shown in FIG. 2. In the present invention, the weight-average molecular 
weight of the resol phenolic resin is from 1,400 to 3,500, that is, it has 
the viscosity of 10,000 to 100,000, preferably 15,000 to 50,000 cps 
(25.degree. C.) at 75% solids. When the weight-average molecular weight 
exceeds 3,500, viscosity becomes high, whereby a large amount of the 
solvent is required for dilution so as to adjust the viscosity within an 
appropriate range, and variability in properties of the product becomes 
large, which is not preferred. On the other hand, when the weight-average 
molecular weight is less than 1,400, the property of the phenolic resin is 
the same as that of a conventional second adhesive as shown in FIG. 1, and 
the drying step is prolonged. 
The second adhesive of the present invention also contains an organic 
solvent having a high boiling point, particularly 110.degree. to 
175.degree. C. Examples of organic solvents suitable for the adhesive 
compositions include cyclohexanol, cyclohexanone, 2-buthoxyethanol, 
2-ethoxyethanol, n-butanol, and the like. The organic solvents that are 
most preferred for the second adhesive of the present invention are 
cyclohexanone (boiling point of 156.degree. C.) and cyclohexanol (boiling 
point of 161.1.degree. C.). When a solvent having an excessively high 
boiling point is used, the solvent is liable to remain in the adhesive 
layer, which results in insufficient drying. When the boiling point of the 
solvent is too low, little foams are liable to arise. The larger the force 
of dissolving the above-mentioned high molecular weight phenolic resin, 
the better. 
If desired any additives such as fillers (e.g., calcium carbonate, silicon 
oxide, talc, etc.), grinding aids (e.g., cryolite, potassium borofluoride, 
etc.), colorants (e.g., pigment, dye, etc.), and the like, may be added to 
the second adhesive in the present invention. The amount of these 
additives is preferably not more than 70% by weight, based on % solids of 
the second adhesive. When the amount of additives exceeds 70% by weight, 
strength of the second adhesive is liable to be decreased. 
After the second adhesive thus obtained is coated, for example, by a roll 
coater, curing is conducted by heating at a temperature of 120.degree. to 
160.degree. C. for a short period of time (1.5 to 3 minutes). The coated 
abrasive article is wound on a jumbo roll according to the same manner as 
that of a conventional method, and then the coated abrasive article is 
subjected to an after-cure (post-cure) treatment to obtain a coated 
abrasive article. 
According to the present invention, the curing step following application 
of the second adhesive can be completed in a remarkably short period of 
time and productivity can be largely improved. Further, a large drying 
device is not required as a production facility and the product can be 
produced by a simple device. The resulting coated abrasive article has the 
same performance as that of a conventional coated abrasive article. 
EXAMPLES 
The following Examples and Comparative Examples further illustrate the 
present invention in detail but are not to be construed to limit the scope 
thereof. All percentages and parts are by weight unless indicated 
otherwise. 
Preparation of High Molecular Weight Phenolic Resin 
A water soluble resol phenolic resin (PR-53074, manufactured by Sumitomo 
Durez K.K.) was reacted by heating to obtain a high molecular weight 
resin. Samples of resin PR-53074 were reacted at 45.degree. C. for 5 days, 
6 days, and 7 days to obtain a "resin 1", "resin 2", "resin 3", 
respectively Properties of resin PR-53074 are shown below: 
Water soluble phenolic resin PR-53074; 
75% solids; viscosity of 720 cps. 
Viscosities of resins 1, 2, and 3 and viscosities of solutions thereof when 
diluted with cyclohexanone (60% solids) are shown in Table 1. 
TABLE 1 
______________________________________ 
Viscosity (Cps) 
Resin 1 Resin 2 Resin 3 
Measuring [reacted at [reacted at 
[reacted at 
temperature 
45.degree. C., 5 
45.degree. C., 
45.degree. C., 7 
(% solids) days] 6 days] days] 
______________________________________ 
45.degree. C. (75 %) 
1,850 3,640 9,540 
(undiluted) 
25.degree. C. (75 %) 
20,000 41,200 123,600 
(undiluted) 
25.degree. C. (60%) 
1,135 1,640 3,630 
(diluted 
with cyclo- 
hexanone) 
______________________________________ 
Examples 1 and 2 and Comparative Examples 1 through 4 Coating Condition of 
the Second Adhesive 
The composition for the second adhesive solution was made by using PR-53074 
and resins 1 to 3. Formulations are shown in Table 2. 
TABLE 2 
______________________________________ 
Formulation of the second adhesive (parts by weight) 
Comp. Comp. Comp. 
Raw Materials Ex. 1 Ex. 2 Ex. 1 
Ex. 2 
Ex. 3 
______________________________________ 
PR-53074 (75% solids) 
64 64 -- -- -- 
Resin 1 (60% solids, 
-- -- 80 -- -- 
diluted with 
cyclohexanone) 
Resin 2 (60% solids, 
-- -- -- 80 -- 
diluted with 
cyclohexanone) 
Resin 3 (60% solids, 
-- -- -- -- 80 
diluted with 
cyclohexanone) 
Water 17.33 -- -- -- -- 
Cyclohexanone -- 17.33 21.85 
10.86 
34.67 
CaCO, (filler) 
52 52 52 52 52 
% solids 75 75 65 70 60 
______________________________________ 
The second adhesive of the articles of Comparative Examples 1 and 2 and 
Examples 1 and 2 was coated on the sample, which had been subjected to the 
first adhesive coating step and abrasive grain coating step. In the first 
adhesive coating step, the first adhesive, an epoxy resin, was applied on 
a paper backing having a weight of 160 g/m.sup.2. Then aluminum oxide 
abrasive grains (SA FEPA #100; single crystal alumina, manufactured by 
Showa Denko K.K.) were deposited over the first adhesive layer. The 
adhesive in the first adhesive layer was cured by heating at a temperature 
of 120.degree. to 150.degree. C. Changes of the state of cure with time 
were observed. The state of cure was evaluated according to the following 
criteria: A, hard; B, tack free; C, slightly tacky; D, tacky. The results 
are shown in Table 3. 
TABLE 3 
______________________________________ 
Relation between heating condition and 
state of cure of the first adhesive 
______________________________________ 
Comparative Example 1 
Time Temperature of Cure 
(min.) 120.degree. C. 
130.degree. C. 
140.degree. C. 
______________________________________ 
1 D D D 
2 D D -- 
3 D A,D A,D* 
5 A A (NG) A (NG) 
10 A A (NG A (NG) 
Comparative Example 2 
Time Temperature of Cure 
(min.) 120.degree. C. 
130.degree. C. 
140.degree. C. 
______________________________________ 
1 D D D 
2 D D D 
3 D D A-C (NG) 
5 C C A (NG) 
10 A A (NG) A (NG) 
Example 1 
Time Temperature of Cure 
(min.) 120.degree. C. 
130.degree. C. 
140.degree. C. 
150.degree. C. 
______________________________________ 
1 D D D D 
C (1.5 min.) 
2 D D C B 
3 B C B A (NG) 
5 B B A A (NG) 
10 B A A (NG) -- 
Example 2 
Time* Temperature of Cure 
(min.) 120.degree. C. 
130.degree. C. 
140.degree. C. 
150.degree. C. 
______________________________________ 
1 D D D D 
D (1.5 min.) 
2 C B B B 
3 C B A A 
5 B B A A 
10 A A A (NG) -- 
______________________________________ 
*: hard but tacky 
NG: no good because of blister 
In the case of using resin PR-53074 without cure or dilution, requisite 
drying time is 5 minutes or more at a temperature of less than 130.degree. 
C. although drying time varies depending on the kind of a solvent. On the 
other hand, in Examples 1 and 2, the cured state (tack free) suitable for 
winding can be obtained by drying at 150.degree. C. for 2 minutes. In 
Comparative Example 3, the adhesive had high viscosity even though % 
solids was decreased to 60%, and, therefore, coating was not conducted. 
Sanding Test 
A sample of the article of Example 1 and a sample of the article of 
Comparative Example 4 (formulation of which is shown in Table 4) were 
prepared by applying the second adhesive on a sample coated with the same 
first adhesive as that of Example 1 and curing the second adhesive at a 
temperature of 80.degree. C. for 2 hours, then at a temperature of 
90.degree. C. for 2 hours, and then at a temperature of 100.degree. C. for 
15 hours. Then, a loop material was laminated on the back surface of the 
backing, which was punched to form a disc having a diameter of 5 inches. 
Then, a sanding test was conducted. The results of the test are shown in 
Table 5. 
TABLE 4 
______________________________________ 
Formulation of Comparative Example 4 
Raw Materials Amount (part by weight) 
______________________________________ 
Phenolic resin (78% solids, 
61.54 
600 cps) 
Water 13.86 
2-Ethoxyethanol 5.93 
CaCO, (filler) 52 
% Solids 75 
______________________________________ 
TABLE 5 
______________________________________ 
Results of sanding test 
Stock removal Example 1 Comp. Example 4 
______________________________________ 
(1st) 13.9 14.0 
(2nd) 14.5 14.8 
Average 14.2 14.4 
______________________________________ 
Work piece: steel 
Sander: dual action air sander 
Load: sander weight + 2 kg 
Air pressure: 5 kg/cm.sup.2 
Sanding time: 5 min. .times. 6 times (total 30 min.) 
As is apparent from Table 5, stock removal by a sample of the article of 
Example 1 is almost the same as that of a sample of the article of 
Comparative Example 4 wherein the current second adhesive is used. 
Further, physical properties of cured resins were compared. PR-53074 resin 
and resin 1 were cured at 105.degree. C. for 16 hours by heating and then 
subjected to a bending test. The bending strength and Young's modulus are 
shown in Table 6. 
TABLE 6 
______________________________________ 
Results of Bending Test 
PR-53074 
Resin 1 
______________________________________ 
Bending Strength 1259 1331 
(kg/cm.sup.2) 
Young's Modulus (kg/cm.sup.2) 
534 516 
______________________________________ 
As is apparent from Table 6, physical property of resin 1 is almost the 
same as that of original PR-53074 and it has high strength.