Tackifying rubber compositions.

Of the material characteristics that we encounter every day, tack is the least understood. We can describe the concepts of stiffness, hardness and strength, for example, but very few people can explain tack. Tack is an important property of pressure-sensitive tapes, which we use routinely in our homes and offices. Tapes are used to seal envelopes, boxes, shipping crates, as painting masks and in many similar applications. We buy tapes in many forms, such as the familiar clear plastic tape, masking tape, shipping tape and duct tape. Pressure sensitivity is also important in affixing lightweight decorative items to showcases and walls. In the rubber industry, tack is required to build tires and other rubber articles. In this application, "building tack" is required to adhere adjacent layers together until a more permanent bond is developed, usually by vulcanization.

One of the first commercial applications of tack was formally disclosed about 150 years ago. In U.S. Patent 3,965 issued March 6, 1845, William Shecut and Horace Day described the preparation and use of a medicinal plaster.

The patent describes dissolving natural rubber and pine gum in turpentine and adding the medicinal ingredients. The resulting solution is then coated onto muslin, and dried by evaporation of the solvent. This composition adheres very well to skin, and presumably to other surfaces. In considering Shecut and Day's procedure, it should be noted how little has changed in current practice for manufacturing solvent-based pressure-sensitive adhesives and solvent cements.

The pressure-sensitive tape industry began in the early 1900s, along with the auto industry. An improved method was needed to mask parts of an automobile body for painting, to protect the unpainted surface. Masking tape was developed for this application. The primary requirement was that the tape could be removed from the unpainted surface, and leave a clean, unmarred paint line.

Definition of tack

Laboratory investigators describe tack as the ability of a surface to form an instantaneous bond when brought into contact with another surface for a short time, under a light stress. Pressure-sensitive tack and building tack have different requirements. Pressure-sensitive tack is expected to support only a low load, but for relatively long time periods. Building tack is needed to support a load for only a short time, until a permanent bond is formed between the layers.

Tack is a surface property, not a bulk property of a material. However, bulk properties may change if an additive is used to develop tack. This occurs in pressure-sensitive adhesives. However, a change of bulk properties is not a requirement for good tack. For example, the bulk properties of a compounded rubber layer are relatively unaffected by adding a material to produce a tacky surface. A pressure-sensitive adhesive composition often contains 50% tackifier, while there is usually less than 5% tackifier in a tackified rubber composition. Both systems produce the level of tack required for the intended use.

How do we differentiate between a bond and tack? The term "bond" usually implies structural properties, and is considered to be permanent. Tack is the nonstructural, often temporary, weak bond that is formed by sticky surfaces. Both structural and pressure-sensitive bonds are formed by a bonding process. A physical or chemical change is required to form a structural bond, which takes measurable time. Tack is instantaneous. When a surface bonds to another having the same composition, the maximum strength of the bond is the cohesive strength of the material. This is also the maximum value of tack between layers of the same material.

A bond is formed by increasing the modulus of an adhesive layer, or of the composition itself. This may be the result of a chemical reaction, as in an epoxy cement; polymerization, as in an oxygen-activated, cyano-acrylate adhesive; or evaporation of a carrier solvent, such as water from a polyvinyl acetate (PVA) white glue. A hot-melt adhesive also functions by a change in modulus. A bond is formed when the modulus is reduced by heating. When the adhesive is cooled, the modulus increases, and the adhesive supports stress.

The bond formed by a pressure-sensitive adhesive cannot be explained by the reasoning above. The modulus of a pressure-sensitive adhesive does not change when a bond is formed. There is no reaction, polymerization or loss of solvent to change the modulus of the adhesive surface.

Theory of pressure-sensitive performance

Tack is recognized as a viscoelastic process. A pressure-sensitive adhesive bond forms at a low rate and extent of deformation. The bond is separated at a high rate and high deformation. Properties, which depend upon rate and extent of deformation, originate from viscoelastic characteristics.

In addition to the viscoelastic requirements, a pressure-sensitive adhesive or a tackified rubber composition must have the polarity required to wet the contacting surface. This provides maximum contact between the layers. The strength of a bond is directly proportional to the contacted surface area. If only 50% of a bonded area is in close contact, the measured strength would be 50% of what it would be if 100% were in contact.

Surface roughness is also important for bonding, but for opposing reasons. It is difficult to develop close contact if surfaces are rough. This reduces bond strength. However, a strong bond can be formed by interlocking the rough surfaces by a gap-filling adhesive layer. In this case, the adhesive displaces trapped air and gases that would interfere with developing maximum bond strength.

There are three steps in developing and recognizing tack. Bonding is the first step. When two surfaces are brought into contact, there must be flow and wetting at the interface, produced by viscous flow of at least one of the contacting materials. For maximum strength, the two surfaces must be in contact on a molecular scale. There should be minimal trapped air and gases. Trapped air or gas reduces the surface area in contact, which reduces bond strength. Flow and wetting during bonding provide the intimate contact, and help displace air and gases from the interface. Bonding occurs at a low rate and extent of deformation. The contacting surfaces do not have to deform significantly to form a bond.

The second step is dwell. This is the time two surfaces are held together to form a bond. Flow and wetting during dwell increase with time, producing a stronger bond. Also, if both contacting surfaces are viscoelastic rubber, the dwell time allows polymer chains to diffuse across the interface, which increases bond strength. Polymer diffusion creates a narrow region between the original surfaces, which differs in composition from both original surfaces.

The third step is debonding, or the resistance to separation. For separation to occur, the material at the interface must deform at a high rate and high deformation. Debonding is carried out when measuring tack. The tack value is a measure of the force required to destroy the bond when in use.

Based on the bonding, dwell and debonding steps, we can identify the criteria required for tack. The processes during bonding and dwell require that at least one of the bonding surfaces has a low modulus. This is a low-rate, low-deformation process, because the bond line is very thin. At these conditions, the viscoelastic material at the bond line will have a low modulus. This allows wetting and spreading to occur. The debonding step requires a high modulus for maximum tack values. Debonding occurs at a high rate and high deformation. The viscoelastic material at the bond line must have a high modulus at these conditions, which gives higher tack.

As discussed earlier, tack requires a viscoelastic response for at least one of the bonding surfaces. We can explain this further by considering the testing time, and the time constant of the material at the bond line. The testing time, [Tau], is the length of time an imposed stress is applied to a viscoelastic composition. The time constant, [Tau], is the time at which the response of a viscoelastic material, to an imposed stress, passes from being predominantly elastic to being predominantly viscous. At [Tau], the response is equally elastic and viscous. The viscous response increases with time. The important consideration is the ratio of the testing time to the time constant, t/[Tau]. The time constant is a property of the viscoelastic material, and is independent of the applied stress. It is a function only of temperature.

If the testing time is very long compared to the time constant of the viscoelastic material (t/[Tau] [much greater than] 1) the response is primarily viscous flow. If this occurs during bonding and dwell, the increased flow will improve bonding. If (t/[Tau] [much greater than] 1) occurs during debonding, we would observe lower modulus and strength. If the testing time is much shorter than the time constant (t/[Tau] [much less than] 1) the material responds elastically. If this occurs during bonding and dwell, flow will be poor, which will produce a weak bond. If (t/[Tau] [much less than] 1) occurs during debonding, we see higher modulus and strength.

The effect of testing time on modulus can be seen when testing viscoelastic materials. For example, examine the plots of natural rubber:aliphatic oil blends containing from 20% to 70% oil (figure 1). The compositions, including the undiluted rubber, were tested in torsion by sinusoidal deformation at frequencies from 0.1 to 100 rad/sec. Frequency is a rate of deformation. It is the reciprocal of testing time, i.e., a low rate is a long testing time. The resistance to deformation (modulus), G', is plotted vs. frequency for each of the samples. Deformation is the same at all testing frequencies. The addition of oil reduced the modulus of the material regardless of the frequency. For a specific ratio of oil to natural rubber, the modulus at 100 rad/sec. is about three to ten times greater than the modulus at 0.1 rad/sec. This test demonstrates the predicted effect of the t/[Tau] ratio.

[Figure 1 ILLUSTRATION OMITTED]

We can now summarize the effect of testing variables on tack, based on viscoelastic characteristics (figure 2). In the left diagram, an increase in bonding pressure, bonding temperature or dwell time during the bonding and dwell steps will increase bond strength. The viscoelastic material at the interface will wet and spread to increase surface contact. During the debonding step, an increase in the testing rate (shorter testing time) will increase bond strength and tack values. If the temperature is increased during the debonding step, strength will decrease. The time constant, [Tau], will be reduced as the debonding temperature is increased. The higher temperature increases flow of the viscoelastic material at the interface, which reduces its resistance to the debonding stress. This reduces bond strength.

[Figure 2 ILLUSTRATION OMITTED]

Function of a tackifier

In both pressure-sensitive adhesives and building tack, a tackifying ingredient is usually added to improve tack. The function of the tackifier is to modify the viscoelastic properties of the composition. For a pressure-sensitive adhesive, the tackifier provides a lower modulus at low rates of deformation (longer time during bonding and dwell), and a higher modulus at high rates of deformation (shorter time during debonding), compared to the untackified composition. In a pressure-sensitive adhesive, this occurs at comparatively high concentrations, approximately equal amounts of tackifier and rubber. The physical properties of the rubber are changed because of the added tackifier.

The tackifying resin for providing building tack, however, is added at a relatively low concentration, less than 5%. The resin is believed to be partially compatible with the rubber composition. A fully compatible resin would completely dissolve in the rubber at the low addition level and would not provide pressure-sensitive properties. A fully incompatible resin would disperse, and also would not provide pressure-sensitive properties. A partially compatible resin provides a dispersed phase at low concentration, which consists of a blend of resin and low-molecular-weight ends of the polymer. This dispersed phase has the viscoelastic properties of a pressure-sensitive adhesive.

Maximum compatibility of the tackifying resin is desirable in a pressure-sensitive adhesive, because this provides the greatest efficiency. However, bulk properties are affected. For building tack, a low level of tackifier is desired, because it has only minimal effects on the bulk of properties of the material.

The function of a tackifier can be seen by examining plots of modulus vs. frequency of simple compositions (figure 1). This graph shows the effect of adding an aliphatic oil to natural rubber. Here, the top line represents undiluted natural rubber. The oil is completely compatible, and it reduces the modulus of the natural rubber over the entire range of test frequencies. These compositions would flow and wet well, but would have low strength.

Figure 3 shows the effect of adding an incompatible resin to natural rubber. Undiluted natural rubber is now the line at the bottom of the graph. There is a minimal effect at low addition levels, because a small amount of the "incompatible" resin is soluble in the polymer. As the concentration of the incompatible resin is increased, there is a large increase in modulus. This occurs because the resin provides reinforcement. These compositions are stronger than unmodified natural rubber, but will not flow and wet during bonding and dwell, so a bond cannot form.

[Figure 3 ILLUSTRATION OMITTED]

Figure 4 shows the effect of adding a compatible, or mostly compatible, resin to natural rubber. In this case, as the resin concentration increases to about 40%, the modulus decreases over the entire frequency range. However, at higher resin concentrations, the slope of the plots increases at higher frequencies. Note that the modulus of the composition containing 70% resin is about 1/10 that of the unmodified natural rubber when measured at low frequencies, but is almost 10 times that of the natural rubber at high frequencies. This large change in modulus with rate is required for pressure-sensitive performance. It also is required for the dispersed phase of a tackified rubber compound.

[Figure 4 ILLUSTRATION OMITTED]

Test methods for measuring tack

The strength of a pressure-sensitive bond is defined by the test procedure shown in figure 5, or the use requirements. A change in testing conditions, such as rates and dwell time, can change the ranking of adhesives. In all tests, a bond must be created and destroyed. Tack is measured when the bond is destroyed. Various test procedures have been used in the past. The reliable, but qualitative, finger tack is still useful. Present versions reflect earlier methods.

[Figure 5 ILLUSTRATION OMITTED]

The inverted probe tack test was developed for pressure-sensitive adhesives. The test involves pressing the surface of a flat probe against the surface to be tested, and measuring the force required to separate the probe from the pressure-sensitive surface. In automated equipment, the probe is raised to touch the surface of the adhesive at a predetermined rate, remains in contact with the surface for a specified time and load, and is then pulled away at a specified rate. The probe is about 1/4" in diameter. The surface of the probe can be easily cleaned, and multiple measurements can be made in a few minutes. This test is predominantly for pressure-sensitive adhesives. It is also useful to measure stickiness of rubber compounds to steel.

The rolling ball test involves rolling a stainless steel ball down a ramp onto an adhesive surface. The tack value recorded is the distance the ball rolls before stopping. The shorter the rolling distance, the greater the tack. The predominant use of this test is in the pressure-sensitive adhesive industry, but it also can be found in the tire industry.

The quick-stick test is used on pressure-sensitive tapes. A piece of tape is laid onto a steel plate. The weight of the tape is the applied force. The end of the tape is pulled up, while the plate moves horizontally across the surface of the instrument so that the peel remains at 90 [degrees]. The data are peel strength developed at a low stress for a short contact time.

The loop tack test involves lowering a loop of tape onto a surface of standard dimensions. It remains for a specified time, and the loop ends are pulled away from the surface to measure tack. As in the quick-stick test, the applied stress is the weight of the tape.

The Monsanto Tel-Tak equipment, presented here without the equipment visible, brings two surfaces into contact at a set rate and stress for a set time. The separation force is recorded. This test is designed for measuring building tack.

The T-peel test is not a true tack test, but is a test for peel strength. It is included here because of its use for measuring building tack. In this test, two surfaces are brought together and held at a specified stress for a specified time. The force required to separate the surfaces is measured. The applied stress may be significantly higher, and the debonding rate significantly slower than in the preceding tack tests. The T-peel test is also used to measure bond strength between two surfaces.

In addition to tests for tack, bonding tests are used to evaluate pressure-sensitive adhesives (figure 6). In the peel test, a bond is made by pressing an adhesive surface against a rigid substrate, such as a steel strip. The bond is formed under conditions appropriate for the application. The bond also may be aged. The bond strength is measured by peeling the flexible member of the construction at 180 [degrees].

[Figure 6 ILLUSTRATION OMITTED]

The last test is the shear failure test. A bond is formed between a flexible tacky surface and an inflexible substrate. A specified stress is applied by placing a weight on the unattached end of the flexible member. The time to separate the surfaces is recorded. The bond is 2 [degrees] from vertical to eliminate peeling. This test is used for pressure-sensitive adhesives.

Theory vs. performance

As discussed earlier, the important factor for understanding tack is the ratio of testing time, t, which is the time an imposed stress is applied, to the time constant, [Tau], of the viscoelastic material at the bond line. The isothermal plots of modulus vs. frequency (testing rate) (figures 1 and 2) demonstrated the effect of t/[Tau], when t was varied and [Tau] was constant. Remember that [Tau]is affected by temperature only, and the data were obtained at constant temperature. We will now examine t/[Tau], when t is constant and [Tau] varies. This is done by collecting data over a temperature range at a constant frequency. When the sample is cooled, [Tau]increases and t/[Tau] becomes smaller. This increases the modulus of the sample. Storage modulus (G') and tan [Delta] are plotted vs. temperature. Tan [Delta] is the ratio of energy lost (viscous flow) to energy stored and returned (elastic deformation) during a deformation cycle.

Figure 7 is the temperature sweep of natural rubber, and a 1:1 blend of natural rubber and an incompatible resin. The solid lines are the data for natural rubber; the broken lines represent the blend.

[Figure 7 ILLUSTRATION OMITTED]

The modulus plots, G', are the curves that drop as the temperature is increased, although at different rates depending on the temperature. At ambient temperature, about 20-25 [degrees] C, the modulus of the sample containing resin is about 10 times that of the natural rubber. This material would be too stiff to flow at ambient temperatures, which would prevent formation of a pressure-sensitive bond.

The tan [Delta] curves are those that have a peak or peaks. The natural rubber:resin blend has two peaks, indicating that two phases are present. The continuous phase, which has the tan [Delta] peak temperature at about -50 [degrees] C, consists primarily of natural rubber and a low concentration of low-molecular-weight ends of the resin. The dispersed phase, which has the tan [Delta] temperature peak at about 4 60 [degrees] C consists primarily of resin and a low concentration of low-molecular-weight ends of the rubber.

Figure 8 is a temperature sweep of a 1:1 blend of natural rubber and a compatible resin (broken lines), and the undiluted natural rubber (solid lines). In this case, the modulus at ambient temperature is substantially reduced below that of the undiluted natural rubber. The lower modulus increases the flow of this sample, which makes it easier to form a pressure-sensitive bond. The temperature of the major tan [Delta] peak has increased from about -60 [degrees] C to 0 [degrees] C because of the resin addition. This composition is a good pressure-sensitive adhesive.

[Figure 8 ILLUSTRATION OMITTED]

These plots of G' and tan [Delta] vs. temperature have rheological features, which makes it possible to construct a "window" for pressure-sensitive adhesive performance. Resins that are effective tackifiers have specific concentration ranges for optimum performance. Performance is poor both above and below this range. Rheological and pressure-sensitive test data have been collected for effective tackifiers over the entire concentration range. These data can be plotted to construct a rheological window for pressure-sensitive adhesive performance.

Figure 9 is an example of one approach to a rheological window for pressure-sensitive adhesive performance. The points represent different compositions, which follow a line on the plot. The window contains those compositions that are effective pressure-sensitive adhesives. Further review of this study is beyond the scope of this discussion.

[Figure 9 ILLUSTRATION OMITTED]

One of the series of natural rubber:resin compositions, covering a range of concentrations, was examined further. Pressure sensitive adhesive performance data from the inverted probe tack test, 180 [degrees] peel test and the shear failure time are plotted on figure 10. We see that tack rises as resin concentration is increased to about 50%. At higher resin levels, the tack drops to zero. The drop in tack is caused by a phase inversion, where the continuous phase is predominantly resin, and rubber is the dispersed phase.

[Figure 10 ILLUSTRATION OMITTED]

Typical photomicrographs show the changes in the composition. As the resin concentration is increased from 25% to 50%, to 60% and to 80%, the tack increases to a maximum of 1,250 g, and then drops to zero. We see that the resin is relatively compatible with the rubber at low concentrations. As the resin concentration increases, a dispersed phase appears, which increases in volume as the resin concentration is increased. At 80% resin, we see a continuous phase, which is predominantly resin. Although the tack level and resin concentration are different, the separation of a dispersed tacky phase is necessary for building tack.

Resin selections

Rosin derivatives, polyterpenes and petroleum resins can be used for pressure-sensitive adhesives. Coal tar resins and phenolic resins are not recommended. For building tack, where partial compatibility is required, resins of all types, except polyterpenes, have been recommended.

Conclusions

After reviewing pressure sensitive adhesives and building tack, we can agree on the following:

* The measurement of tack requires selecting the proper test, and testing at the appropriate conditions for bonding, dwell and debonding. Ranking of tackifiers will depend upon test conditions.

* The role of the tackifier is to adjust the viscoelastic response of a rubber compound when it is subjected to a deforming stress. In building tack, the viscoelastic response occurs in the dispersed phase.

* The effects of compositional variables on performance can be explained by molecular considerations.