Adhesion test instrument

Method and apparatus for measuring the normal stress required to remove an adhering thermoplastic rod from a substrate. Tensile strength of a bond can be measured reproducibly under controlled conditions. The tip of a thermoplastic rod, machined to a cone, is lowered into contact with a heated substrate and melting of the rod is allowed to proceed to a steady-state condition. After a bond is formed, the force required to break the bond is measured. The method permits rapid simulation of the essential conditions present during bond formation between a mineral filler surface and a polymer matrix.

BACKGROUND OF THE INVENTION 
This invention relates to a method and apparatus for measuring the tensile 
bond strength between one end of a polymer rod and a flat substrate. 
In the search for suitable materials which will be competitive with metals 
in articles of manufacture that utilize molding techniques during one of 
the manufacturing stages, it is desirable to find a material which has a 
high modulus of elasticity, a high degree of toughness, and which is 
suitable for low cost injection molding processes. Two possible candidates 
are epoxy resin composites and thermoplastic or polymer composites. With 
epoxy resin composites, strong adhesion to a substrate is virtually 
assured, but in the area of thermoplastic or polymer composites, weak 
bonds are to be anticipated. Since thermoplastic composites may be 
developed much more cheaply than epoxy resin composites, it is desirable 
to qualitatively and quantitatively measure the pertinent parameters of 
the former. 
Design of reinforced polymer composites requires knowledge of the strength 
of bond between a mineral filler surface and a polymer matrix. In prior 
art methods, this may be somewhat laboriously determined by testing the 
composites in their final use conditions. However, this is a time 
consuming process, unsuitable for screening a host of parameters affecting 
the adhesion process. It is desirable, therefore, to develop a testing 
method that would permit rapid simulation of the essential conditions 
present during bond formation in an actual composite, in a short period of 
time. This would have the advantage of enabling one to measure the tensile 
strength of a bond reproducibly under controlled conditions. Numerous 
areas of interest would benefit from such a method. For example, it may be 
desirable to screen certain surface-chemical agents which are reputed to 
have adhesion promoting properties but for which no adhesion test data is 
available for the system of interest, viz., a silicate mineral in a 
polypropylene matrix. Another application would be testing of microscopic 
polymer/filler dispersions, such as fine vermiculite flakes in a 
polycarbonate matrix. Other areas where methods of this nature would be 
particularly useful are the evaluation of surface preparation techniques 
and optimization of temperature-time aging treatments in polymer-inter 
layer systems. 
SUMMARY OF THE INVENTION 
According to the invention, a method and apparatus is provided for 
measuring the tensile bond strength of a bond formed by adhesion of a test 
member to a substrate. 
The apparatus used to measure the tensile strength of the bond formed by 
the adhesion of the member to a substrate comprises a surface which is 
supported generally below the member, which surface is used for adhering 
the member thereto. Means for gripping the member is provided, as well as 
means for bringing the member and the surface into contact and means for 
separating the member from the surface. This last mentioned means for 
separating the member from the surface may be a part of the means for 
bringing the member and the surface into contact. Means for melting the 
member when it is brought into contact with the surface is provided in 
order to initiate bond formation. Means are provided for measuring the 
force required to break the bond formed between the member and the surface 
after sufficient cooling has occurred to form a bond. It is preferred to 
have the force measuring means connected in an operative manner to the 
means for separating the member from the surface, thereby providing a 
convenient way to measure the force, although the force-measuring means 
can be connected to the surface or its support means if desired. 
A support may be provided for mounting the various components which 
comprise the apparatus of the invention, including the means for gripping 
the member, the means for supplying heat and the surface. 
To provide reproducibility, it is particularly useful to provide means for 
restricting the member to a single degree of linear motion in a direction 
normal to the surface. 
The method of the invention comprises the following steps: The member being 
tested is supported above the surface, and heat which is sufficient to 
melt the member is applied to the surface. To permit proper bond 
formation, the end of the member which is brought into contact with the 
surface should be tapered to a small cross-sectional area. After the 
tapered end is brought into contact with the surface, melting takes place 
as the member and the surface are brought closer together. Melting is 
allowed to progress to a steady state so that the cross-sectional area of 
the melted area, or melt, is less than the cross-sectional area of the 
test member. After a bond is formed by allowing the melt to cool, a force 
is exerted sufficient to break the bond. The magnitude of the force which 
is required to break the bond is a representation of bond strength. In 
bringing the member and the surface into contact, it is desirable for the 
purpose of enhancing the reproducibility of the method to restrict the 
member to motion in a direction perpendicular to the surface.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The principle of the invention will now be described with reference to FIG. 
2. The essence of an adhesion test is that a tensile force is applied to a 
bonded configuration and increased until mechanical failure takes place. 
Where this failure takes place, the force is measured and a mode of 
failure identified by observing such things as fracture appearance 
characteristics. It is desirable to simplify mechanical design so that as 
few variables as possible affect interpretation of results. The principle 
of the method of the invention is to form an adhesive bond between the end 
of a thermoplastic rod 21 and a plate 22, over a circular area A.sub.o 
(indicated by arrows 25 of FIG. 2(d)). Thereafter, while maintaining 
alignment of rod 21 perpendicular to plate 22, an axial load P is slowly 
increased in a direction away from plate 22 until normal failure occurs. 
Tensile stress P/A.sub.o provides a measure of bond tensile strength. 
For reasons which have been briefly mentioned and which will now be fully 
explained with reference to FIG. 1, it is necessary for accuracy and 
repeatability of the test method to taper the tip of the test member or 
rod. 
If the member has a generally cylindrical shape, the end may take the form 
of a cone or a frustum of a cone. The end of a member of another shape 
could likewise have a conical shape or a pyramidal or a tetrahedral shape. 
This shaping may be accomplished by any method, for instance by machining 
or molding. It has been found especially useful to utilize a rod whose tip 
has been machined or shaped to a 90.degree. cone. 
If the tip of the rod has a cross section equal to that of the rod itself, 
it is difficult to prevent entrapment of microscopically small air bubbles 
at the bond interface. Tapering of the end improves the bond layer formed 
by preventing the entrapment of these small air bubbles in the bond 
interface. In addition, when a rod of uniform cross section is brought 
into contact with the hot substrate, such as plate 4, heat conduction 
processes tend to create a droplet whose diameter is greater than the 
initial rod diameter. In other words, the melting process geometry is 
difficult to control. A third reason for using a coned tip is that the 
bond interface of plate 4 and the thermoplastic member constitute a 
series-mechanical arrangement. Where a droplet of greater diameter exists 
at the interface of plate 4 and a rod of uniform cross section, the yield 
stress will decrease in a direction away from the plate. The rod would 
tend to fail at a location some distance from the bond interface. 
By using a tapered or coned rod tip, these problems may be eliminated. The 
coned tip tends to melt progressively when lowered into contact with the 
plate, without entrapment of air bubbles. The heat flow is more rapid up 
the axis of a rod with a coned tip compared to one of uniform cross 
section because the mass of a cone-tipped rod above the melt is larger for 
a given contact area than a rod of uniform cross section. When tensile 
load is applied, the average stress is highest where the area is smallest, 
at the bond interface. Thus, the bond interface and the thermoplastic 
material immediately adjacent the interface experience virtually the same 
stress distribution. Failure may occur by de-adhesion or de-cohesion but 
geometry does not favor one over the other since both experience the same 
stress, which is a maximum in the region of interest. 
The surface which is brought into contact with the rod may take any of 
several forms, as long as it is non-meltable at the melting temperature of 
the rod. Although a flat glass plate is referred to in this description, 
any shape will suffice if the surface area brought into contact with the 
coned-tip is flat. The means for melting the rod may be any heat source 
which is capable of supplying the requisite amount of heat, and may be 
located adjacent to the surface. However, it may be equally efficient to 
utilize a plate with an electrically conductive and resistive element 
placed therein such that internal heating of the plate would occur by 
application of electric current to the element. 
Bringing the rod into contact and separating it from the surface may be 
accomplished equally well by moving the member into contact with a 
stationary surface, by moving the surface into contact with the stationary 
member, or by simultaneously moving the member and the surface into 
contact with each other. 
Referring now to FIG. 1, a representation of the test apparatus appears 
wherein a short length of a thermoplastic test member 3 is tightly held in 
grip 2. Grip 2 may be limited to only one degree of linear motion in the 
direction indicated by the double arrow, by providing a sliding fit in a 
cylindrical hole 9 in rigid support frame 1. Grip 2 is operatively 
attached to load cell 8, which is fixed rigidly in place, and support 
frame 1 is attached to cross head 10. Cross head 10 may be raised or 
lowered in a controlled manner to move the surface of glass plate 4 toward 
or away from member 3. Directly below meltable member 3, plate 4 is held 
in place by heater 5, spring 6 and adjustable nut 7. Heater 5 may comprise 
a 50 watt electric cartridge heater inserted in aperture 11 of a heating 
block. 
Cross head 10 and load cell 8 may comprise a commercially available tensile 
testing machine such as one manufactured by Instron Corporation. 
Referring to FIG. 2 in combination with FIG. 1, a typical test cycle is 
illustrated. With glass plate 22 installed between frame 1 and heater 5 of 
FIG. 1, and a suitable rod 21 mounted in grip 2 of FIG. 1, frame 1 is 
attached to cross head 10. Grip 2 is then suspended from fixed load cell 
8. Grip 2 is slidably fitted in cylindrical hole 9 of frame 1, and conical 
tip 23 of rod 21 of FIG. 2 is brought almost into contact with plate 22 as 
illustrated in Step (a) of FIG. 2. Glass plate 22 is heated to a suitable 
temperature above the melting point of rod 21 by heater 5. Of course, 
heating of plate 22 may also take place after tip 23 is in contact 
therewith. In Step (b) of FIG. 2, tip 23 is pressed against now-heated 
surface 22, and the thermoplastic meniscus is allowed to form at interface 
24. Cross head 10 is only raised a sufficient amount to provide an ample 
melted area at interface 24 for a proper bond, since it is important to 
maintain the cross-sectional area of the meniscus less than the 
cross-sectional area of the rod. The conical shape of tip 23 permits a 
stable geometric form to be retained by molten thermoplastic in the steady 
state. Cooling then is allowed to occur in Step (c) of FIG. 2 which may be 
facilitated by disabling heater 5, such as by removing the 50 watt 
cartridge heater from aperture 11 of FIG. 1, and inserting into the 
aperture a solid copper rod which has been pre-cooled to some point below 
ambient temperature, for instance, 0.degree. C. Other methods of cooling 
will be equally successful, such as application of a coolant or forced 
air. When the temperature becomes unifrom, cross head 10 is lowered at a 
slow rate, such as 0.002" to 0.005"/minute, and tensile load versus time 
is recorded. After failure has occurred, illustrated by Step (d) of FIG. 
2, plate 22 is removed and the cross-sectional area of area 25 is 
determined. Bond strength may be determined by conventional force 
equations, as previously mentioned. Plate 22 may be examined for 
indications whether the fracture occurred in the plate 22, along interface 
24, or in rod 21. 
With the method and apparatus of the invention, one may study the effects 
of certain chemical pre-treatments on bond strength by utilizing the 
invention. A chemical pre-treatment containing filler particles may be 
thought of as forming one or more thin parallel layers of the filler 
particle on the test surface. These thin layers may range from a molecular 
film to a coating several microns in thickness. This "thin layer concept" 
may be applied in the field of reinforced polymer composites, where it is 
important to determine such things as the cohesive strength within a 
reinforcing filler particle, adhesive strength of the filler particles to 
the polymer matrix and the state of interpenetration of particle 
agglomerates by the polymer during processing. Referring to FIG. 3, a 
dilute suspension of filler particles in a solvent containing a polymer of 
interest is cast on plate 33 to form a thin film 32. The solvent is then 
extracted by vacuum annealing. If the particles are in the form of flakes 
or needles, they will align parallel to plate 33. The rod used in the test 
will be made of the same polymer that has been left on plate 33 after the 
solvent is removed. A bond is formed by lowering the tip of rod 31 into 
contact with heated plate 33 and cooling, as previously described with 
reference to FIG. 2. The molecules of polymer from the plate and the rod 
commingle and the filler particles are trapped in close proximity to the 
surface of plate 33. The strength measured by the method previously 
described is then a measure of strength of the composite of the polymer 
and the filler particles where failure occurs within the composite. As the 
filler-polymer adhesion is increased, failure may occur in the polymer 
rod. This latter point of failure then defines a lower limit to composite 
strength. Of course failure may also occur at the plate surface, or within 
the plate itself, either of which would give, at least, a lower limit to 
composite strength. 
To insure a proper bond at the surface of a glass plate when utilizing the 
invention, it is especially useful to treat the plate to remove all 
contaminants. The glass plate is first rubbed with a wetted metallurgical 
polishing cloth using levigated alumina as a fine abrasive. This removes a 
thin layer of glass, but still retains overall flatness and a scratch-free 
condition. Next, the surface is flushed with a fluid such as air or water 
to dispel any alumina particles still adhering to the surface. As a final 
step, the plate is flushed with absolute ethanol and dried. This process 
provides a glass plate particularly suitable for the testing method and 
apparatus of the invention. 
It will be understood that various modifications of this test method and 
apparatus may occur to those skilled in the art, e.g., the rod may be 
twisted in torsion rather than pulled in tension, and it is intended that 
this invention be limited only by the scope of the appended claims.