Paint Technology and Tests Part 9

The supply of mercury being shut off, the operator is now in a position to determine the total sum of both the elasticity and ductility of the paint film, and also the pressure at which the film broke. The breaking pressure of course is read directly upon the pressure column, which is divided into centimeters as has been described above, the piston indicating the maximum height of the mercury column. What may be termed the elasticity of the film can now be calculated. As is perfectly evident, the film in stretching does so by distending from a flat surface to a curved or cup-like surface. If the pressure tube is calibrated in cubic centimeters reckoned from a flat surface where the film was introduced, the stretch of the paint film in distending from a flat surface to a curved surface may be determined. The cubic contents of the pressure tube and side-arm become increased, owing to the cup-like shape the paint film takes on. By subtracting the amount of mercury indicated by the piston in the pressure tube from the amount of mercury delivered from the burette, the amount contained in the distended paint film is obtained, which serves as a measure of elasticity. The temperature is a most important point to consider in running daily tests upon the filmometer. The tests made by the writer were conducted at 70 degrees Fahrenheit throughout.

[Ill.u.s.tration: Gardner-de Horvath film testing apparatus]

=Gardner-de Horvath Filmometer.= Another type of filmometer which gives very concordant results was recently devised by the writer and de Horvath. This apparatus is shown above.

It consists of a three-necked Wolff bottle having provision at one of its necks for exhausting the air from the bottle. The reverse neck is provided with a gauged gla.s.s tube dipping into a porcelain crucible containing mercury, thus acting as a manometer. The middle neck is fitted to accommodate two ground gla.s.s plates. Both these plates are provided with a central orifice one millimeter in diameter. Between the plates is placed a small section of paint film. The plates may be pressed together or clamped together and placed over the middle neck of the bottle, a close contact being made with Canada balsam. As the air is exhausted from the bottle, the mercury in the tube will rise and continue in its ascent until the film, which is exposed to atmospheric pressure, has offered it maximum resistance, which is shown by the breaking point. This point is observed on the manometer and the result expressed in centimeters of mercury.

=Table of Film Testing Results.= By means of the Perry film-testing apparatus, described in the above, interesting results have been obtained, which are embodied in the following table:

COMPARATIVE STRENGTHS OF FILMS AS OBTAINED BY THE BREAKING MACHINE

============================+=========+==========+===========+======== |No. Coats| Pressure | Thickness | Stretch ----------------------------+---------+----------+-----------+-------- 1. Zinc oxide | 3 | 33.2 | 0028 | .30 2. Zinc lead | 3 | 32.7 | 0034 | .35 3. Asbestine | 3 | 28.0 | 0045 | .15 4. Sublimed white lead | 3 | 17.9 | 0024 | .38 5. Barytes | 3 | 13.3 | 0042 | .33 6. Lithopone | 3 | 13.1 | 0024 | .49 7. Whiting | 3 | 13.0 | 0033 | .32 8. Quick process white lead| 3 | 11.3 | 0025 | .38 9. Gypsum | 3 | 10.8 | 0039 | .29 10. China clay | 3 | 10.8 | 0035 | .16 11. Silex | 3 | 9.6 | 0032 | .32 12. Blanc fixe | 3 | 8.5 | 0030 | .28 13. Corroded white lead | 3 | 7.3 | 0020 | .33 14. Barium carbonate | 3 | 7.2 | 0028 | .16 ============================+=========+==========+===========+========

By means of this machine it is possible to obtain very valuable information concerning the effect of age upon a paint as influencing its strength and elasticity. These are two vital qualities in a paint, as it is through its strength that a paint resists abrasion, cracking, peeling, and blistering. That elasticity is a vital qualification of a paint may easily be seen through the checking of oil paintings, which, as Ostwalt has pointed out, is due to the unequal coefficients of expansion between the ground and the paint. This is particularly noticeable in the alligatoring of many enamels which contain large percentages of zinc.

Curves have been prepared having pressure as an abscissa and elasticity as ordinate. These curves show remarkable differences in different pigments. For instance, in the case of white lead, the curve takes a steep upward trend when it apparently reaches a maximum, the curve then flattening out and finally taking another steep upward trend just before breaking. This may be construed as follows: That under low pressures the white lead film is perfectly elastic, when a maximum is obtained, beyond which elasticity does not extend. This point is the maximum point of the upward trend. From here on pressure may be applied without any increase in stretch, this being represented by the flat part of the curve, while the steep upward trend just before breaking shows where the paint begins to tear, finally culminating in breaking. In the case of asbestine, however, the curve is more of a straight line up to the breaking point, which would go to prove that elasticity is proportionate to pressure in the case of this pigment.

=Moisture Absorption.= The structure of certain pigments is such that when they are ground in linseed oil and painted out, films are produced which are very water-resistant. This action is possibly due to the filling of the voids in the oil, thus making a compact and water-resistant film. Pigments which are coa.r.s.e and which present an angular crystalline structure, often produce films which contain a relatively large number of voids and are less waterproof. Certain pigments are chemically active and tend to produce, when ground in oil, metallic soaps which act for a time more or less as varnish gums, in keeping out moisture. Later on, however, such films are apt to break down and admit moisture in quant.i.ty. The tests herein described were designed by the author to determine the water-excluding value of a number of typical pigments when ground in linseed oil and painted out into films. Unfortunately, no method has been devised by which films of the same gauge could be prepared. The variations in the thickness of the films used in these experiments, however, are not very great.

[Ill.u.s.tration: Apparatus for Determining Excluding Properties of Paint Films]

A series of small gla.s.s bottles with wide mouths, holding about two ounces, were half filled with concentrated sulphuric acid, and paint films were tightly sealed over the mouths of the bottles with Canada balsam. The bottles were then carefully labeled, numbered, and accurately weighed upon chemical balances. Later they were exposed under a large gla.s.s bell jar containing air saturated with moisture and kept at a constant temperature. The bottles were removed from the receptacle every week and reweighed. The increase in weight, due to the amount of moisture which had penetrated through the films, and absorbed by the sulphuric acid, owing to its hygroscopic nature, was thus determined. In another series of bottles, lumps of calcium chloride were subst.i.tuted for the sulphuric acid. The results obtained from these tests correspond to those of the former tests, and led to the conclusion that the porosity of linseed oil films varied when different pigments were used in the oil.

MOISTURE EXPERIMENTS

Figures Given Express Percentage Gain in Weight, e.g., Water Absorbed

==========================+=========+=========+========= | 7 days | 21 days | 49 days --------------------------+---------+---------+--------- White lead and zinc oxide | 0.043% | 0.115% | 0.266% Zinc lead white | 0.049 | 0.130 | 0.284 Red lead | 0.049 | 0.130 | 0.295 Sublimed white lead | 0.049 | 0.128 | 0.292 Zinc chromate | 0.064 | 0.176 | 0.417 Zinc oxide | 0.065 | 0.172 | 0.391 Barytes | 0.074 | 0.202 | 0.466 Willow charcoal | 0.077 | 0.236 | 0.694 Lithopone | 0.083 | 0.228 | 0.550 Chinese blue | 0.092 | 0.276 | 0.671 Natural graphite | 0.104 | 0.350 | 0.951 Ultramarine | 0.119 | 0.336 | 0.814 ==========================+=========+=========+=========

Another series of tests was started, in which were used films prepared from various oils and varnishes made especially for the test from different gums. The results of this series are very interesting, as they indicate that certain gums are more powerful than others in making oils resistant to moisture. The reader should study with care the data on treated Chinese wood oil, most excellent results having been obtained when it was used in the proper percentage.

EXCLUDING TESTS ON OIL VEHICLES AND VARNISHES SHOWING PERCENTAGE OF MOISTURE ABSORBED AT VARIOUS PERIODS

===================================+=========+=========+========= | 6 days | 18 days | 24 days -----------------------------------+---------+---------+--------- Linseed oil, 100% | .233 | .686 | .895 Soya bean oil, 100% | .340 | 1.06 | 1.39 Linseed oil, 80% } | .250 | .755 | .987 Soya bean oil, 20%} | | | Linseed oil, 60% } | .289 | .857 | 1.125 Soya bean oil, 40% } | | | Linseed oil, 40% } | .355 | 1.05 | 1.39 Soya bean oil, 60%} | | | Linseed oil, 20% } | .260 | .789 | 1.03 Soya bean oil, 80% } | | | China wood oil treated, 100% | .130 | .297 | .375 Linseed oil, 80% } | .182 | .559 | .728 China wood oil treated, 20%} | | | Linseed oil, 60% } | .173 | .540 | .708 China wood oil treated, 40% } | | | Linseed oil, 40% } | .119 | .348 | .450 China wood oil treated, 60%} | | | Linseed oil, 20% } | .127 | .375 | .494 China wood oil treated, 80% } | | | Kauri gum, 33% } | | | Linseed oil, 33%} | .061 | .191 | .302 Turpentine, 33% } | | | Kauri gum, 25% } | | | Linseed oil, 50% } | .096 | .266 | .346 Turpentine, 25% } | | | Kauri gum, 20% } | | | Linseed oil, 60%} | .122 | .367 | .449 Turpentine, 20% } | | | Kauri gum, 15% } | | | Linseed oil, 70% } | .187 | .421 | .601 Turpentine, 15% } | | | Congo copal gum, 20% } | | | Linseed oil, 50% } | .228 | -- | -- Turpentine, 30% } | | | Sierra Leone copal, 20% } | | | Linseed oil, 50% } | .099 | -- | -- Turpentine, 30% } | | | Zanzibar gum, 20% } | | | Linseed oil, 50% } | .082 | -- | -- Turpentine, 30% } | | | Amimi gum, 20% } | | | Linseed oil, 50% } | .080 | -- | -- Turpentine, 30% } | | | Boiled linseed oil (linoleate type)| .210 | -- | -- Collodion solution (6 oz.), 80% } | .201 | -- | -- Boiled linseed oil, 20% } | | | ===================================+=========+=========+=========

[Ill.u.s.tration: Microscopic view of section of cedar]

[Ill.u.s.tration: Microscopic view of section of maple]

[Ill.u.s.tration: Microscopic view of section of white pine]

[Ill.u.s.tration: Gardner photomicroscope in position against painted surface]

[Ill.u.s.tration: Inside White on White Pine]

=Use of the Microscope.= 4. The microscope is a necessary adjunct of every well-ordered paint laboratory, as has been recognized throughout the whole paint industry. The writer has attempted to collect certain data which may materially a.s.sist those manufacturers who employ this instrument to judge of the quality of their raw and finished products.

The fineness of grinding considerably affects the quality of the paint, and this can be easily controlled through the intelligent use of the microscope. This instrument may also be used to detect certain adulterations. Appended is a table giving the fineness of grinding of the various pigments, together with their characteristics under the microscope. In this table measurements are given both in millimeters and in inches, in order to accommodate itself to the use of those chemists employing a millimeter stage micrometer, or those employing the English or inch system. Although it is not yet certain that any and all combinations of pigments may be detected under the microscope the writer is working toward a method which will allow a manipulator to judge of the composition of the paint under observation.

In order to properly prepare a paint for microscopic examination, the following method is recommended: A microscopic turn-table is a convenient accessory of the microscope, and its use is to be recommended. A gla.s.s slide being placed in position upon the turn-table, a very small amount of either the pigment rubbed up in oil, or the paint, is applied to the slide; a small drop of Canada balsam is then applied by means of a gla.s.s rod dipped in a solution of balsam in xylol, and dropped upon the slide. The rod is then used to thoroughly incorporate the pigment with the balsam, and a cleaned cover gla.s.s is dropped over the whole and pressed down tightly, so that a small amount of balsam will exude from under the edges and thus firmly seal the gla.s.s. In order to make permanent slides it has been found advisable to rim the cover gla.s.s with balsam and even follow this up with some suitable black varnish, the slide being then carefully labeled and placed in the collection. Following is a table of the characteristics of the fourteen chief pigments:

TABLE OF THE SIZE OF PARTICLES OF THE CHIEF PIGMENTS WITH THEIR CHARACTERISTICS UNDER THE MICROSCOPE

===+===================+======================+======================= | | Diameter in | Diameter in | | Millimeters | Inches | +-------+-------+------+-------+------+-------- No.|Name | Small | Aver. |Large | Small | Aver.| Large ---+-------------------+-------+-------+------+-------+------+-------- 1|Asbestine |.002 | -- |.12 |.00015 | -- |.049 2|China clay |.003 | -- |.065 |.00009 | -- |.025 3|Barium carbonate |.00076 |.0055 |.0172 |.00003 |.00024|.0011 4|Blanc fixe |.00073 |.0037 |.0073 |.00003 |.00014|.0003 5|Silex |.0037 |.0092 |.03 |.00014 |.00036|.0012 6|Gypsum |.0037 |.011 |.05 |.00014 |.00044|.0022 7|Amer.-Paris white |.0015 |.0050 |.04 |.00006 |.00022|.0018 8|Barytes |.0015 |.0092 |.05 |.00006 |.00036|.0021 9|Zinc lead |.00037 |.0018 |.0037 |.000014|.00007|.00014 10|Sublimed white lead|.00037 |.0018 |.0037 |.000014|.00007|.00014 11|Lithopone |.00076 |.0018 | -- |.00003 |.00007| -- 12|Zinc oxide |.00046 |.0018 |.00037|.00002 |.00007|.00014 13|Quick Pro. lead |.00061 |.0030 |.0048 |.00002 |.00012|.00018 14|Dutch Pro. lead |.00061 |.0018 |.0066 |.00002 |.00007|.00026 ===+===================+=======+=======+======+=======+======+========

=Film Sectioning and Deductions to be Drawn Therefrom.= 5.

Investigations were undertaken into the innermost structure of paint films as revealed under the microscope. Up to the present time, work has been done upon barytes, asbestine, blanc fixe, and white lead, painted upon wood, and a combination paint upon wood. The films, the preparation of which has been described in the foregoing, were sectioned and prepared for microscopic examination in the following manner:

A solidifying dish was partly filled with low melting-point paraffin which was allowed to harden, when a small piece of paint was thrown upon it and then more paraffin poured over it. After hardening, sections were obtained of the paint film by means of a microtome.

[Ill.u.s.tration: Section Barytes Film]

A view of these sections of paint films under the microscope gave to the operator a better idea of the structure of a paint than had ever been afforded heretofore. It was easy to perceive the relative position of the pigment particles and the three coats. The penetration of one coat into another was easily discernible, and measurements were made upon the sections in order to determine the average thickness of coat and its general appearance.

Sections were also made of Inside and Outside White upon wood. These sections revealed under the microscope the thickness of the coats and also the penetration of the priming coat into the wood. Appended is a table giving microscopic measurements.

PAINT SECTION MEASUREMENTS UNDER MICROSCOPE

======================+=============+===========+====== | |Millimeters|Inches ----------------------+-------------+-----------+------ Barytes |3 coats (sum)| .1068 |.00421 |Single coat | .0356 |.00140 | | | Inside. White on wood |3 coats (sum)| .1624 |.00639 |Outside coat | .0230 |.00091 |Next coat | .0443 |.00175 Field in photographs |Next coat | .0620 |.00245 |Penetration | .0294 |.00116 White lead |Inside | .0215 |.00085 |Middle | .0405 |.00159 |Outside | .0184 |.00073 |3 coats (sum)| .0811 |.00319 Asbestine |3 coats (sum)| .1840 |.00725 | | | Blanc fixe |3 coats (sum)| .1068 |.0042 |Single coat | .0356 |.00014 | | | Outside. White on wood|Outside coat | .1329 |.00523 |Inside | .1845 |.00727 |Penetration | .0737 |.00290 ======================+=============+===========+======

=Polar Micro-Examinations and Photomicrographs.= By Polar Micro-Examination is meant the examination of pigments under polarized light. A polarizing apparatus, though not an essential in the hands of the paint chemist, is nevertheless much to be desired, for by its help deductions may be drawn as to the contents of a paint, which by other means might not be possible. The polarizing apparatus as marketed by most manufacturers of the microscope is attached in the following manner:

The diaphragm immediately under the sub-stage container is swung out and opened to its widest limit, allowing the insertion of the polarizer.

This polarizer carries one of the pair of Nicols prisms and is countersunk to allow of the introduction of gypsum or selenite plates.

The a.n.a.lyzer fits over the eyepiece on the tube.

The use of polarized light upon paint is valuable on account of its action upon crystalline substances. The re-enforcing pigments, such as Asbestine, China Clay, Gypsum, Silex, Barytes, etc., are crystalline and consequently act upon the polarized light. In most cases these pigments are used in ready-mixed paints in small amounts, varying between 5 and 25%. When a slide containing a small amount--for example, less than 3%--of these crystalline pigments is examined under the microscope by ordinary transmitted light, they will often escape observation, owing to the small amount in which they are present. However, in the case of polarized light, this could hardly happen.

[Ill.u.s.tration: Microscopic View of Barytes under Polarized Light]

A slide of paint containing these re-enforcing pigments is prepared in the usual manner. On examining this under the microscope and using the polarizing apparatus, the crystalline pigments are at once detected by revolving the a.n.a.lyzer. At one position of the a.n.a.lyzer, one sees an ordinary field, as with transmitted light, but if one revolves the a.n.a.lyzer, the field gradually becomes darker until total darkness is obtained throughout, except in such places where crystalline substances are present, when the crystal is shown up with beautiful distinctness.

Photomicrographs of various single pigments and pigment combinations are shown under Chapter III.

=Effect of Pigments on Oil.= Certain pigments have the property of acting upon the linseed oil in which they are ground, forming metallic linoleates which accelerate the drying of oil. This is especially true of lead and zinc pigments. The inert crystalline pigments, when ground in linseed oil and painted out, distribute the oil so as to allow a great surface to be exposed to the air. Thus by physical action, and possibly catalytic or contact action, these inert pigments stimulate the drying of oil paints in which they are ground. Lead and zinc paints, of course, have the greatest drying values on account of the added effect of the linoleates formed, as outlined above. The writer has made a series of tests in which the action of various pigments upon linseed oil is shown. The tests were made in the following manner:

Five grams of each of a series of commonly used paint pigments, including those of inert crystalline nature as well as the more valuable amorphous pigments which are considered more or less chemically active, were ground separately in an agate mortar, with 5 grams of raw linseed oil. The ground paste in each case was placed in a marked gla.s.s beaker, and allowed to stand in a dustless section of the laboratory for one month. The oil-pigment paste from each beaker was then separately extracted with benzine to remove the linseed oil from the pigment. The benzine solutions of oil were then heated to remove the benzine and the residue of oil burned to ash in crucibles. The ash from each test was weighed, and if it ran above the percentage of ash determined on a blank sample of linseed oil (namely, .003%), the ash was a.n.a.lyzed qualitatively for metallic const.i.tuents. The following table of results shows the percentage increase in ash, as well as the const.i.tuents of ash on the various samples tested:

TABLE OF RESULTS

===============================+==============+======================== | Per cent. of | | Ash in Oil | Pigment in Oil |Extracted from|a.n.a.lysis of Ash | Oil-Pigment | | Paste | -------------------------------+--------------+------------------------ Raw linseed oil without pigment| 0.003 | -- Barytes | 0.003 | -- Blanc fixe | 0.003 | -- Silica | 0.003 | -- Asbestine | 0.005 | -- China clay | 0.007 | -- Whiting | 0.008 | -- Chrome yellow | 0.025 |Lead oxide (PbO) Lithopone | 0.031 |Zinc oxide (ZnO) Prussian blue | 0.032 |Iron oxide (Fe_{2}O_{3}) Sublimed white lead | 0.033 |Lead oxide (PbO) Zinc oxide | 0.105 |Zinc oxide (ZnO) Corroded white lead | 0.116 |Lead oxide (PbO) Red lead | 0.2112 |Lead oxide (PbO) ===============================+==============+========================

Observation of these results shows that pigments such as Barytes, Blanc Fixe, and Silica have no chemical action on the linseed oil. The results on Asbestine and China Clay also are negative, the extremely slight increase in amount of ash from these samples probably being due to traces carried over mechanically into the oil mixture; the last named pigments being more fluffy and difficult to separate from oil. Slight action seemed to be apparent in the case of whiting, a pigment somewhat alkaline in nature. A longer test might have shown this pigment to have possessed still greater action. Corroded white lead showed considerable action, resulting in the formation of lead linoleate or some other organic compound. Zinc oxide and lithopone, the latter pigment containing 30% of zinc sulphide, both indicated action on the oil.

Chrome yellow (chromate of lead) showed some action, as did also Prussian blue, the ash from the last named pigment showing a heavy percentage of iron oxide.

Red Lead showed a most astounding gain in these tests, chemical action of the pigment on the oil being apparent soon after the tests were started, as shown by the formation of a hard cake with the linseed oil.

The Raw Linseed Oil which was used in these tests had an acid value of 1.84%, which is very low. The neutralization of this free fatty acid by some of the alkaline pigments used, may account for part of the increased percentage of ash, but in most cases the pigments, and more especially the basic pigments, had a direct saponifying action upon the glycerides of the oil.