MTS Adhesives Project 5 Measurements For Optimizing Adhesives Processing

Task 3d Tack

Report 17 Tack - Part 2

J.A. Bingham SATRA Footwear Technology Centre

This report forms part of the deliverable for Task 3

Reports for Task 3d

Report 14; An instrument for the measurement of “Tack” for the footwear industry

Report 15; Tack - Summary Report

Report 16; Tack - Part 1

Report 17; Tack - Part 2 I

MTS PROJECT 5

MEASUREMENTS FOR OPTIMIZING ADHESIVE PROCESSING: TACK

A PROJECT SUPPORTED BY THE DTI

Report covering the final phase of the work - July 1996

Author: J A Bingham

Contents

1, Introduction

2. Conclusions

3. Establishment of the appropriate spotting pressure range 3.1 Review of pressure measurements 3.2 Measurement of contact time in

spotting 3.3 Shoe factory experience

4. Examination of relationships between bonding variables and tack strength 4.1 The effect of spotting pressure on

tack strength

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2 2

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4 4.2 The effect of activation temperature

on tack strength 4 4.3 The effect of open time on tack

strength 6

5. Comparison of the SATRA/NPL test with SATRA Test MethodAM19 “Spotting tack of adhesive bonds” 6

6. Interpretation of the results 7

7. Study of relative sensitivities of tack and peel strengths to variations in bonding conditions 8 7.1 General 8 7.2 Temperature sensitivity 8 7.3 Open time sensitivity 9 7.4 Pressure sensitivity 9

8. Comments on the performance of the adhesives 10 8.1 Group of adhesives carried forward

from the earlier work 10 8.2 The second group of adhesives 11

1. Introduction

This report covers the work carried out since the completion of the November 1995 ‘Second Interim Report’. This comprised an evaluation of the SATRA/NPL tack tester and its method of use; identification of suitable test conditions for assessing the tack of sole bonding adhesives; an examination of the effect of varying bonding conditions such as activation temperature on tack a study of the relative sensitivities of peel strength and tack to variations in bonding conditions; and an evaluation of four water-borne adhesives in addition to further work on the first group of adhesives described in the November 1995 report.

NB: All the laboratory tests discussed in this report were conducted with the resin rubber soling and standard upper leather described in the November 1995 report.

2. Conclusions

The SATRA/NPL equipment provides a reliable means of quantifying the tack of sole bonding adhesives.

The SATRA/NPL test results correlate well with those given by the SATRA Test Method AM 19, ‘Spotting Tack of Adhesive Bonds’ which had existed in draft form for several years before its publication in 1995. The correlation between shoe factory experience with four of the adhesives and the results of the SATRA/NPL test with a spotting pressure of 2 kg/cm2 also was good.

A number of enhancements to the equipment have been identified that will a) shorten the test time and simplify the procedure; b) reduce the risk of invalid results through slippage of a test piece from its clamp and c) extend the scope of the test to cater for the growing practice of activating both adherends.

A tack classification for sole bonding adhesives has been drawn up. This is intended to be simple for adhesive suppliers to apply and straightforward for adhesive users to understand.

Tack strength is highly dependent upon spotting pressure, open time, and in the case of the polyurethanes, on activation temperature.

In general, tack strength is more vulnerable to deviations from normal conditions than peel strength. I

The level of tack achieved in spotting is not a reliable guide to the likely peel strength of the consolidated bond,

The adhesive referenced polychloroprene A gave the best tack results among the eight adhesives examined in this work and was the least vulnerable to deviations from normal conditions.

The polyurethane adhesive referenced 2-part polyurethane A - a solvent-borne type - gave the worst results overall.

The water-borne polyurethane adhesives generally gave good tack results. The only instances of non-coalescence and low tack values were found with two examples when open times were extended to a week. No evidence was found to indicate that water- borne polyurethanes as a class are inferior to solvent-borne types with respect to tack.

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3. Establishment of the appropriate spotting pressure range

3.1 REVIEW OF PRESSURE MEASUREMENTS

Spotting pressure in shoe factory trials of around 4-6 kg/cm2 and 7.5 kg/cm2, depending upon the measurement technique, had been reported. These contrast with values of 0.7 kg/cm2 and 1,1 kg/cm2 reported for two technicians carrying out the SATRA AM 19 tack test. This discrepancy led to a decision to re-examine the retained foils and films used for these measurements.

The re-examination confirmed the reported values for the technicians but indicated that the values reported for the shoe factory work were, in fact, peak values associated with small areas located at the extreme front edge and the extreme back edge of each unit. It has now been established that these localised areas of relatively high pressure can be attributed to the fact that the sole units had had to be ‘stretched’ slightly to fit the lasted uppers. The pressure over most of the contact area in the bonding margins of the units, which is a more appropriate indication of spotting pressure, was found to be in the range 1-2 kg/cm2 and thus broadly equated with the spotting pressure in SATRA tack test.

Given the findings of this re-examination of the pressure maps, it was decided not to pursue the provisional plan to include high pressure testing in the programme of work with the SATRA/NPL tack tester but instead to concentrate solely on the more realistic pressure range of 1-2 kg/cm2.

3.2 MEASUREMENT OF CONTACT TIME IN SPOTTING

A trial was carried out in which sole units were spotted onto lasted uppers. The average time to complete spotting was found to be three seconds.

A preliminary study of the SATRA/NPL apparatus in action indicated that contact time - which cannot be preset - is controlled by i) the spotting pressure - when the preset level is reached reversal of the cross-head is triggered - and ii) the skill of the operative in manipulating the manual speed control as the descending adherend approaches its stationary counterpart.

The relationship between the first of these two factors, that is contact time, and spotting pressure was subsequently examined in trials conducted by an experienced user of the equipment. In this work the time between the compressive load first registering and cross-head reversal was measured. This was done at three pressures. The results are given in Table 1.

Table 1. Spotting Pressure v Contact Time

Preset Spotting pressure (kg/cm2) Contact Time(s)

1 4 2 5 4 7

The results in Table 1 clearly show that the contact time at 1 kg/cm2 pressure most closely equates to that found in the work in which the sole units were spotted onto lasted uppers.

This finding led to the adoption of 1 kg/cm2 as the spotting pressure for the work in which the relationship between activation temperature and tack was studied. However, the later findings described in section 3.3 led to a reconsideration of the recommended test pressure.

3.3 SHOE FACTORY EXPERIENCE

Research into the use of water-based adhesives in shoemaking conducted independently of this work by SATRA colleagues, yielded an assessment of the tack properties of four adhesives across a range of shoe styles. This was provided by an experienced shoemaking operative and took the form of a 1 to 4 ranking.

It was found that the ranking order of these adhesives in the SATRA/NPL tack test when the spotting pressure was set to 2 kg/cm2 exactly matched that based on shoe factory experience. It was further found that when the spotting pressure was reduced to 1 kg/cm2, the agreement between test rankings and those based on factory experience was lost.

It was therefore decided that the specified spotting pressure for the test should be 2 kg/cm2.

4. Examination of relationships between bonding variables and tack strength

4.1 THE EFFECT OF SPOTTING PRESSURE ON TACK STRENGTH

An examination was carried out to compare tack values at spotting pressures of 1 kg/cm2 and 2 kg/cm2. Eight adhesives were used in this exercise - the group of three polyurethane and a polychloroprene utilised in the earlier work and the group of four water-borne adhesives for which shoe factory experience was available. By varying open times, a total of twenty pairs of results was generated for each of which the ratios of the tack strength values at the two contact pressures were calculated. An analysis of the paired results is given in Table 2.

Table 2 Ratios of tack strength at higher contact pressure to tack strength at lower contact pressure

Ratio of tack strength values

Range 0.8 to 2.3 Mean 1.55 Median 1.55 Standard Deviation 0.4

With eighteen of the twenty paired results, higher tack values were obtained when the spotting pressure was set to 2 kg/cm2. In only one case was a lower result obtained at the higher spotting pressure. Three quarters of the results fell within one standard deviation each side of the mean and all fell within two standard deviations each side.

In a simpler analysis of the tack test results, the overall mean tack strengths at the two test pressures were calculated. This yielded the values 7.7 N/cm2 at the lower pressure and 11.5 N/cm2 at the higher pressure.

This work indicates that a) the spotting pressure has a significant effect on the level of tack and b) doubling the spotting pressure from 1 kg/cm2 to 2 kg/cm2 typically will increase the tack by around 50% with the adhesives studied in this work.

4.2 THE EFFECT OF ACTIVATION TEMPERATURE ON TACK STRENGTH

The group of four adhesives from the earlier work was used for this study. Tack strengths were measured at five temperatures in the range 30°C to 83°C, the latter of which is the typical level for sole bonding adhesives. The average values for the group at each temperature are given in Table 3.

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Table 3 Activation temperature v Tack strength

Activation temperature Tack strength (°c) (N/cm2)

83 6.8 60 7.1 50 6.7 40 4.6 30 3.2

The values in Table 3 suggest that tack strength with this group of adhesives is insensitive to changes in activation temperature within the range 50°C to 83°C. However, when the results for the polychloroprene are stripped out of the averages, a clear downward trend in tack strength is apparent, as Table 4 shows.

Table 4 Activation Temperature v Tack Strength By Adhesive Type

Tack strength (N/cm2) Activation

temperature Value for the (“C) Average for the three polychloroprene

polyurethane adhesives adhesive

83 6.0 9.1 60 3.6 17.8 50 4.0 14.8 40 1.8 13.0 30 0.8 10.4

Interestingly no signs of non-coalescence were found at any of the activation temperatures with the polychloroprene adhesive whereas non-coalescence was apparent with all three polyurethane at the reduced activation temperatures - and in the case of the 2- part adhesive non-coalescence also occurred at 83°C. These observations, together with tack strength results, suggest that increasing the thermally induced softening of the adhesive film is beneficial up to a point at which the loss in cohesive strength of the adhesive outweighs the increased tackiness of the film.

The tack strength values found in this work indicate that the transition occurs at a lower temperature with the polychloroprene than with the polyurethane. While there is insufficient data to enable the differential to be quantified with confidence, it seems reasonable to speculate that the transition point associated with this polychloroprene is of the order of 20°C lower than that of the l-part polyurethane.

In the case of the 2-part polyurethane, the mechanism discussed above is absent as, even at the highest activation temperature tried, the adhesive films did not coalesce. NB: No attempt was made to study the higher temperature behaviour of this adhesive as the potential value of such work was considered to be insufficient to justify the effort required.

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4.3 THE EFFECT OF OPEN TIME ON TACK STRENGTH

The effect of extending the open time from the standard time - one or two hours as appropriate - to one week was examined in an exercise involving eight adhesives at two spotting pressures to give a total of 16 pairs of results. A more limited study of extending the open time to one day was also carried out; this utilised four adhesives from the earlier work at two spotting pressures. The findings are summarised in Table 5.

Table 5 Open time v tack strength

Average tack strength (N/cm2)

Open time Open time Standard extended extended open time to one week to one day

Tests with eight adhesives 12.2 7.1 Tests with four adhesives 12.9 7.4 9.5

There is good agreement between the two sets of data with respect to the loss of tack strength when the open time is extended to one week - in both cases a loss of about 42-43% is apparent. The more limited work indicates that an extension of the open time to one day leads to a 26% loss in tack strength. Clearly open time is a significant factor in influencing tack levels.

NB: With few exceptions - primarily with the 2-part solvent borne polyurethane adhesive - little or no non-coalescence was apparent in these tests.

Comparison of the SATRA/NPL Test with SATRA Test Method AM 19, ‘Spotting Tack of Adhesive Bonds’

The SATRA tack test was carried out on the same combinations of adherends, adhesives and open times as those used in the spotting pressure comparison work. ,

In seventeen of the twenty tests the performance satisfied the criteria for the tack to be classified as ‘good’; in two tests the tack was classified as “poor’; and in one test the tack was ‘adequate’. The tests that yielded ‘good’ tack could be split into two groups - with twelve of the tests no separation occurred between the adherends; while in the other five tests separations of 2mm were measured.

The average tack strengths, as determined in the SATRA/NPL test, for each of the groupings were calculated. These are given in Table 6. Clearly there is good agreement between the results of the two tests.

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Table 6 SATRA AM1O V SATRA/NPL Tack Test Results

Average tack strength determined in 6ATRA/NPL Test

(N/cm2)

AM 10 Tack Assessment Test at 1 kg/cm2 Test at 2 kg/cm2

Good tack - no separation 8.8 12.3 Good tack -2 mm separation 8.9 15.2 Adequate tack 2.3 3.6 Poor tack 0.8 1.4

Interestingly a higher average tack strength value was obtained in SATRA/NPL tests corresponding to the AM 10 tests where there was a 2mm separation than was obtained in the corresponding tests in which no separation occurred.

A close study of SATRA tack tests being carried out revealed that operator spotting technique was highly significant in influencing the incidence of minor separations. It was apparent that in many cases lifting of the test piece ends could be eliminated by ensuring that they were pushed down firmly by sliding the fingers along the test piece as the soling was spotted onto the upper.

It can therefore be concluded with reasonable confidence that the presence of minor separations, viz 2mm, is not significant in terms of tack strength and that the values obtained for the two groups of ‘good results in the higher spotting pressure tests belong to the same population.

6. Interpretation of the results

It should be noted that tack strength is not the sole determinate of whether spotted components hold together. Factors such as the degree of deformation introduced in bringing the components together and their tensile and compression properties will influence the risk of the two surfaces springing apart. For this reason, the development of a universal adhesive tack specification for footwear is considered to be unrealistic. A more realistic approach is to develop a tack classification system that will enable shoemakers to make an informed choice when selecting an adhesive for a particular application. The SATRA/NPL test provides a good basis for such a scheme. A possible format is given below for illustrative purposes.

Tack strength under standard conditions

(N/cm2) Tack classification

Less than 5 Low tack 5 to 15 Medium tack Greater than 15 High tack

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For maximum benefit the basic classification could be could be supplemented by the addition of an indication of the adhesive’s sensitivity to deviations from ‘ideal’ bonding conditions. However, almost certainly it would be difficult to construct a simple system that would satisfy all interested parties. In the absence of such a system, the use of brief advisory notes, for example Tack adversely affected by extended open times’ would be commendable.

7. Study of relative sensitivities of tack and peel strengths to variations in bonding conditions

7.1 GENERAL

SATRA Test Method AM 1:1992 was used to determine the peel strengths in these studies.

7.2 TEMPERATURE SENSITIVITY

A set of bonds was made up to correspond to a range of the combinations examined in the study of the relationship between tack strength and activation temperature. The results are presented in summary form in Table 7 together with equivalent SATRA/NPL tack test values.

Table 7 Summary of peel test and SATRA/NPL tack test results at a range of activation temperatures

Average for the Value for the three polyurethane polychloroprene

adhesives adhesive Activation

temperature Peel strength Tack strength Peel strength Tack strength (°C) (N/mm) (N/cm2) (N/mm) N/cm2)

83 3.8 6.0 3.2 9.1 60 3.5 3.6 3.5 17.8 50 3.7 4.0 , 2.8 14.8

It is apparent from Table 7 that on average, the polyurethane retained their peel strength across the test temperature range, whereas tack strength fell away as the temperature was lowered, thus indicating that peel strengths are less sensitive to temperature variation than tack levels.

None, or at worst little, non-coalescence was found with the l-part polyurethane in any of the fully consolidated bonds at any of the activation temperatures. In contrast, high levels of non-coalescence were found with these adhesives in the SATRA/NPL tack tests at all temperatures below 83°C. This demonstrates that, even though the adhesive films had not been softened sufficiently at these temperatures to coalesce under spotting pressure, they had become soft enough to coalesce when subjected to the higher pressure and longer dwell time of bond consolidation.

The bond achieved with the 2-part polyurethane when the adhesive was heated to was 83°C was free from non-coalescence. However, at lower temperatures this type of separation was present, which further emphasising the difficulty of achieving adequate softening of this particular adhesive.

The values in Table 7 for the polychloroprene suggest that the apparent advantage in tack strength obtainable by using an activation temperature of 60°C rather than 83°C is reflected in the corresponding peel strength performances of the consolidated bonds. However, the difference in peel strengths is well within the normal levels of variability found in bond tests and almost certainly this apparent difference is illusory and, thus, there is no real benefit in terms of bond strength in reducing the activation temperature with this adhesive.

7.3 OPEN TIME SENSITIVITY

This comparison was limited to the group of four adhesives carried forward from the earlier work. The results are given in Table 8.

Table 8 Summary of peel test and SATRA/NPL tack test results at a range of open times

Peel strength Tack strength Open Time (N/mm) (N/cm2)

Normal 3.3 11.6 1 Week 2.8 6.7

On average the peel strength fell by only 15% when the open time was extended to one week. The corresponding fall in tack strength was 42% indicating that tack levels are significantly more sensitive than peel strength values to open time extension.

7.4 PRESSURE SENSITIVITY

The approach taken in examining the relative pressure sensitivity of tack and bond strengths was similar to that taken in assessing the relative temperature and open time sensitivities except that, for obvious reasons, the absolute pressure levels used for the two operations were markedly different. A direct comparison has been achieved by expressing the pressure levels in terms of percentages of the normal for the operation - and for this exercise 2 kg/cm2 has been assumed to be normal for spotting, The data is given in Table 9.

Table 9 Summary of peel test and SATRA/NPL tack test results at two pressure levels

Applied Peel strength Tack strength pressure (N/mm) (N/cm2)

Normal 3.6 10.8 50% of Normal 2.8 7.4 25% of Normal 2.3

Halving of the spotting pressure lead to a loss of almost a third of the tack strength but only a loss of just over a fifth of the peel strength when the bonding pressure was halved. The bonding pressure has to be reduced by three-quarters before the strength loss approximates to that found on halving the spotting pressure. Thus tack strength appears to be significantly more sensitive than peel strength to pressure variation.

These observations and those relating to temperature and open time sensitivities highlight the danger of attempting to apply the findings of tack tests to predict ultimate bond strengths.

8. Comments on the performances of the adhesives

8.1 GROUP OF ADHESIVE8 CARRIED FORWARD FROM THE EARLIER WORK

8.1.1 One-part solvent-borne polyurethane A

This gave at least reasonably good tack results under most conditions - it only gave low values when subject to extreme deviations from normal conditions, for example very low activation temperature and long open time when the spotting pressure was low,

In general the peel strength results were also reasonably satisfactory. The adhesive was found to be tolerant of extensions of open times up to one week, provided that normal bonding pressure was applied. It was also found to be insensitive to variation in temperature in the range examined.

8.1.2 Two-part solvent-borne polyurethane A

This adhesive gave generally poor results in the tack tests - low tack values were obtained under all conditions and in all cases non-coalescence was apparent.

Reasonably satisfactory sole bonds were obtained under normal bonding conditions. However, the adhesive was found to be very sensitive to reductions in bonding pressure - lowering the pressure by 25% resulted in the peel strength falling by over half and non-coalescence becoming the dominant separation mode. Extending the open time of the adhesive film on the soling also led to the development of non- coalescence and falling peel strengths despite the fact that the adhesive on the leather was ‘fresh' ie an normal open time had been used, Sensitivity to reduction in activation temperature was also apparent. This was manifested in both the development of non-coalescence and a reduction in peel strength.

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8.1.3 Water-borne Polyurethane A

This adhesive gave at least reasonably good tack results under all conditions of test except at the lowest activation temperatures. As such, it performed better than either variant of the solvent-borne adhesive A.

The adhesive gave a reasonably satisfactory bond when normal bonding conditions were used and was found to be tolerant of reductions in activation temperature and moderate reductions in bonding pressure. However, extending the open time led to a reduction in bond strengths but not to major levels of non-coalescence at times up to one week.

8.1.4 Polychloroprene A

Good tack was obtained under all conditions. The overall performance of this adhesive in the tack tests was not matched by any other the others examined.

The adhesive was found to be tolerant of reductions in bonding pressure and in activation temperature. Reasonably satisfactory peel loads were achieved at all open times when the normal activation temperature and bonding pressure were used - though a degree of non-coalescence was apparent in all the extended open time bonds.

8.2 THE SECOND GROUP OF ADHESIVES

These have not been studied as intensively as the first group of adhesives, for example the effect on tack of varying activation temperature has not been examined, and peel strength tests have been restricted to normal bonding pressures and open times.

Good tack results were obtained with all the adhesives at both spotting pressures and at normal open times. With two of the adhesives - both 1-part - good results were obtained when the open time was extended up to one week thus matching the performance of the water-borne polyurethane in the first group of adhesives. With the other two adhesives - one a 2-part and the other a 1-part polyurethane - good tack was apparent when a one-day open time was used, but the adhesive films failed to coalesce when the open time was increased to one week and low tack strengths resulted.

All four adhesives gave reasonably satisfactory levels of peel strength with no non- coalescence apparent when tested under normal conditions.

Ref. SATRA/DTI Reports (4)

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