OBJECTIVE

The goal of this work was to demonstrate the feasibility of characterizing standard PSA properties with the use of a single probe analyzer test. The use of this instrument is intended to reduce the time required for quality assurance testing and to also serve as a research and development tool for PSA evaluations. A series of adhesives were chosen with the same chemical structure but varying degree of standard adhesive properties to prove that the Probe Material Analyzer can distinguish between these multiple test properties in a single repeatable manner.

MATERIALS

The adhesives evaluated in this work were commercially available solvent based acrylic pressure sensitive formulas selected to cover a range of peel, tack and shear properties. The adhesive dry coating thickness was 1 mil +/- 0.1. The backing used was a 2 mil polyester film. The adhesives were received from the supplier with solids contents of 46 +/- 5 %. The coatings were made on a ChemInstruments LC-100 with a gap of 6 mils. The coatings were then dried at room temperature for 15 min and then placed in a 200°F oven for 6 minutes. The dry coated sample were then laminated to an easy release liner under 20 psi and then allowed to equilibrate for 24 hours at room temperature before testing.

STANDARD TEST METHODS

Caliper

Caliper or thickness was determined by a modified ASTM D 3652 (PSTC-33) method. The modification involved the laminating of a known thickness stable film to the adhesive prior to testing to prevent the adhesive from contacting the foot of the micrometer. Chemsultants is A2LA accredited for this test method. Caliper was measured on a ChemInstruments MI-1000 digital micrometer with 0.0001-inch resolution. The test material was laminated to 2.0-mil polyester film. The thickness was measured with the micrometer. The thickness of the PET film was subtracted from the measurement. Ten replicates of each sample were measured.

Peel Adhesion

Peel adhesion was tested according to a modified ASTM D 3330 (PSTC-101) method. The modification included the dwell time. Chemsultants is A2LA accredited for this method. Peels were performed on a ChemInstruments AR-1000 Adhesion Release Tester in conjunction with the EZ Lab software program. Exactly one (1.0) inch wide samples were applied to a standard stainless steel substrate at a rate of 24 in./min. with a 4 -pound rubber covered roller according to the method. The tape was then peeled from the substrate at a 180° angle with a dwell time of fifteen minutes. The force required for removal was measured, averaged, and the mode of failure noted. Five replicates of each sample were tested.

Loop Tack

Loop tack was tested according to the ASTM D 6195 (PSTC-16) test method on a ChemInstruments LT-1000 Loop Tack Tester. Chemsultants is A2LA accredited for this method.

A 1" X 5" sample was prepared then formed into a loop with the adhesive exposed on the outside. A half-inch strip of masking tape was used to hold the loop together at the top. The loop was placed in the tester and lowered at a rate of 12 in./min. toward the substrate below. The adhesive made a square inch contact patch with the substrate. When the jaw was at the bottom of its travel the direction of the jaw automatically reversed. As the adhesive was pulled from the substrate, the maximum force experienced during its removal was measured and recorded. Five replicates of each sample were tested.

Plasticity

Plasticity measurements were supplied by the manufactures of the adhesives. Plasticity was measured according to ASTM D926-98. Two metal plates at least 10 mm in thickness and 40 mm in diameter were mounted parallel to each other with bottom plate being in a fixed position. A dial indicator was set to zero. An adhesive sample having a volume of 2.00 +/- 0.02 cubic centimeters was placed in between the two parallel plates and a force of 49 N was then applied to the top plate and allowed to dwell for a specific period of time (10 minutes). The distance the top plate travels toward the bottom plate was recorded in hundredths of millimeters. Plasticity was then determined by multiplying the distance the plate traveled by 100. Therefore an adhesive with a high degree of flow will have a lower plasticity number.

PROBE MATERIAL ANALYZER METHOD

The Probe Material Analyzer (PMA-1000) used in this work was manufactured by ChemInstruments. A photograph of the instrument is shown in figure 1. A stainless steel probe with radius of .562" was mounted to a load cell transducer that measured the forces acting on the probe. The probe that was perpendicular to the sample surface was driven by a screw connected to an electric motor. The load on the probe and the displacement were recorded throughout the test. A photograph of the instrument is shown in figure 1.

Figure 1

The initial phase of the test occurred when the probe was inserted into the adhesive (see figure 2). Data collection was initiated when the transducer sensed a predetermined load. In this case the starting force was set at 5 grams. The insertion speed was set at 0.02 in/min. The Probe continued to travel though the adhesive until the measured force was equal to 454 grams. Once the maximum load force was seen the probe was retracted. However, the Probe Material Analyzer allows for an optional dwell time. In this work the dwell time was set at 0 seconds.

The second phase of the test occurred when the probe was allowed to retract from the adhesive until no contact remained between the two surfaces (see figure 2). The force required to separate the probe from the adhesive was recorded throughout the retraction phase. The retraction speed of the probe was set at 0.1 in/ min. The retraction curve may consist of one or more peaks, which are described, in the results and discussion part of this paper. The tests were concluded when the adhesive was no longer attached to the probe and the force reached zero.

Figure 2

RESULTS and DISCUSSION

The adhesive properties were first determined using traditional test methods for peel, tack and plasticity as described above. The results of the traditional test methods were then compared with data from the Probe Material Analyzer. An attempt was made to correlate the peel, tack and plasticity results to certain aspects of the probe analyzer curve based on prior work reported by Chuang, et al (1.). This work suggested that relationships exist between traditional PSA test method results and information obtained from the probe analyzer curve. However, the characteristic and useful aspects of the PMA retraction curve should be discussed before examining these relationships.

The following parameters were considered the relevant information obtained from a typical PMA retraction curve in this study.

• Total area under the retraction curve (positive work)
• Area under the first peak (positive work)
• Displacement at total adhesive debonding

The retraction curve consists of an initial elastic region where the adhesive deformation is roughly linear following Hookeís law up to an initial peak. This peak or yield point indicates where the adhesive typically begins to debond locally from the probe surface. Softer or more pliable adhesives are believed to form filaments as the adhesive in the areas of remaining contact begin to orient and tensilize. As the filaments in softer adhesives reach their tensile strength, a second peak may be observed before the material finally debonds from the probe surface. In some softer adhesives the second peak will not be observed because the cross-link density or the level of chain entanglement is insufficient to cause resistance to debonding through orientation of the developed filaments. Contrarily, high modulus or elastic adhesives may not exhibit a second peak as the energy cannot be absorbed in the adhesive mass and is therefore transferred to the adhesive/probe interface causing complete debonding at the initial peak. The displacement of the probe at the point of complete debonding is also related to the level of adhesive deformation during the retraction process.

The relationships previously established between traditional test method results and information from the retraction curve include:

Peel adhesion is related to the positive work or energy absorbed during the retraction phase of the probe analyzer test. This was defined as the total area under the retraction curve. The energy absorbed during the retraction process is related to the adhesive bond to the probe surface and the extent of deformation of the adhesive mass before debonding.

Tack is related to the positive work under the first curve, which indicates the extent of resistance to debonding and adhesive deformation as the probe is retracted. Tack is also related to the total positive work under the curve, which includes any secondary peaks. This is related to the concept that high modulus or elastic adhesives absorb little energy upon debonding which will be evident from small relative area under the initial peak. High modulus adhesives also show little tendency to form filaments upon debonding but rather separate from the probe surface quickly and cleanly. This is evident from the lack of secondary peaks when testing this adhesive type.

Plasticity is related to probe displacement at complete debonding. Prior work has also related shear properties to displacement, however plasticity results proved to be more consistent for the purpose of this study. Higher cross-link density in an adhesive material will manifest itself in high modulus and a limited capacity to deform. This inability to deform and absorb energy prohibits the formation of filaments upon retraction of the probe and results in lower displacement distances when the adhesive completely separates from the probe.

The following data was obtained through traditional test methods and PMA testing: TABLE 1.

PMA Retraction Curves Typical PMA response curves for each of the adhesive types are shown below in figures 3 to 7.

Figure 3

The PMA curve for adhesive 'A' is shown in figure 3. The degree of cross-linking and therefore the elasticity for this adhesive is very high which is evident from table 1 which shows a low tack value and a high plasticity number. The high degree of elasticity is also seen in the PMA curve in the form of one major peak, the lack of significant secondary peaks (little deformation and absorbed energy), and short displacement at debonding (0.005 in.).

Figure 4

The PMA curve for adhesive 'B' (figure 4.) shows that as the plasticity number drops from approximately 6 to 3.8, a secondary peak is developed. This secondary peak is related to the deformation of the adhesive and development of filaments during debonding as described above. The peel adhesion and tack level of adhesive 'B' has also increased from that of adhesive 'A' which correlates to increased energy absorption in the adhesive upon deformation. This also coincides with a longer displacement at adhesive debonding (0.011 in.).

Figure 5

Adhesive 'C' (figure 5.) also shows a secondary peak similar to adhesive 'B'. However, the secondary peak is more pronounced and the displacement has also increased further. This coincides with an increase in peel and tack. The shape of the curve is similar which is consistent with only a slight decrease in plasticity number.

Figure 6

A more significant decrease in plasticity number and therefore and increase in softness is seen when adhesive 'D' is evaluated with PMA testing. The plasticity number decreased to 1.7 from 3.5 as seen in adhesive 'C' previously. As the softness of the adhesive material increases further, a third peak or shoulder is developed as seen in figure 6. The flexibility of the adhesive allows the filaments to form again (second peak). It is speculated that the lower cross-link density allows the polymer chains to elongate or slide even further resulting in secondary filamentation and selective separation from the probe cause the decline in force to zero. This extended deformation also results in an increase in displacement (up to 0.030 in.).

Figure 7

Adhesive 'E' has a similar plasticity number and also shows a similar PMA profile to adhesive 'D'.

The final debonding process seen in the third peak is extended further in adhesive 'E' which is apparent in the longer displacement. This is related to better surface wetting and resistance to separation from the probe during the final debonding process. It also coincides with higher tack exhibited by adhesive 'E'.

Correlation of Standard and PMA Results

The first significant relationship between standard adhesive testing and probe analysis previously discussed is the correlation between peel adhesion and total work. A comparison was made between peel adhesion results and total work defined as the positive area under the curve in the retraction phase of the test. The results are summarized in figure 8.

Figure 8

A clear relationship was seen between total work under the PMA curve and the peel adhesion measured for each adhesive. The relationship for the adhesive type tested appeared linear and somewhat predictable. Both of these properties are a measure of the total energy required to separate the adhesive from a surface resulting in good correlation of the data.

Figure 9

According to Chuang, et al, tack can be derived from the Probe Material Analyzer method by taking the sum of twice the total positive work and the addition of the positive work under the first peak, which is defined as composite work in this paper. The total work is an indication of how much energy is absorbed in the adhesive throughout the testing process. The area under the fist peak is an indication of the initial debonding strength. It is believe that a weighted composite of these two parameters is related to tack because it accounts for the resistance to initial debonding from the probe surface (first peak) and the total energy required to completely separated the adhesive from the probe (total area under the curve).

Figure 10

The displacement of the probe at the point were the adhesive completely separates is an indication of the adhesive elastic properties. Adhesive that exhibit high flow or deformation properties show higher displacement values than adhesives that are harder or more resistant to flow. This displacement is therefore related to the degree of plasticity of the adhesive. Since low plasticity numbers indicate high flow adhesives the displacement is inversely proportional to the plasticity number of the adhesive. Figure 10 shows a good correlation between probe displacement at debonding and the inverse of plasticity number.

CONCLUSIONS

The results of this study show that good correlations can be established between traditional PSA test methods and the PMA test method. Proportional and fairly predictable relationships were established between peel adhesion, tack and plasticity results and certain aspects of the PMA retraction curve. This allows the consolidation of several tests in to one comprehensive significantly reducing cycle time. The PMA test can distinguish various peel, tack and plasticity levels in various adhesives.

Bibliography

Chuang, et al, PSTC proceedings, 1997, p.39

Handbook of PSA Technology, Satas ed., Van Nostrand Reinhold, 1999

Hand book of Adhesive Technology, Pizzi & Mittal ed., Marcel Dekker, 1994