Application of water reducible adhesives in rubber footwear manufacturing.

In the manufacturing of hand-laid rubber footwear, solvent based rubber adhesives have been used for several years. Although solvent based adhesives have proven to be quite effective, environmental and health hazard concerns have caused many manufacturers today to opt for water, reducible (latex) adhesives in the processes. This article briefly reviews the application of water reducible adhesives as opposed to solvent based adhesives in rubber footwear manufacturing.

Historical overview

Solvent based adhesives have been employed in rubber footwear manufacturing for a long period of time. Solvent based adhesives are solvated rubber compounds with added tackifiers (ref. 1). The adhesive compound is mixed in an internal mixer. To avoid scorching, cure packages are added later on during dissolving of the adhesive batch. The compounded adhesive rubber is then dissolved into a rubber solvent such as toluene (ref. 2). Dissolution of the adhesive rubber is carried out in a large variable speed water-cooled mixer. During dissolution it is important that the temperature of the solution does not go over 150 [degrees] F. This takes approximately six to seven hours. After all the rubber is dissolved into the solvent the solution is allowed to cool down to room temperature. At this point the cure package is added to the cement batch. This mixture is then allowed to mix for one half hour. Now the adhesive is ready to be used or further diluted. Dilution to various viscosities is carried out using rubber solvents. Small candy churns are used to carry out the dilution. The reduction in viscosity depends on the application of the adhesive, e.g. adhesive applied manually is at a much heavier viscosity as opposed to adhesive applied by a roller (ref. 3). Conventional adhesive applicators (ref. 4) include two-roll machines where bottom and/or top rolls are cement rolls. The adhesive is normally fed to the cement roll.

Why are the solvent based adhesives so popular?

One of the many reasons for the overwhelming popularity of solvent based adhesives is that for the most part they do not require drying equipment since the solvents flash off very rapidly at ambient temperatures.

Secondly, apart from being a carrier or the diluent for the adhesive, the solvents also tend to increase the building tack of the uncured rubber sheets. This increased building tack is an advantage in rubber boot building.

The downside of solvent based adhesives comes mainly from environmental, operational and health hazards posed by the carrier solvents such as toluene.

As a result of these considerations, as well as EPA regulations demanding the reduction in volatile organic compounds in air emissions, the need for the use of water reducible adhesives was generated.

The big question was about the feasibility of using water reducible (latex) adhesives in place of solvent based adhesives. Would they work? And if they did, would there be a downside to their use? Needless to say, there was a lot of skepticism and premature predictions of failures.

Although different types of water reducible adhesives were available, we decided to go with latex adhesives for our process. The task was to develop a latex adhesive that would be equivalent to normally used solvent based adhesive in tack, bond strength and cure properties. After months of trials a satisfactory natural latex formulation was developed. Later in the process, experiments were conducted using various viscosities of the same formulation. Different viscosities were chosen to facilitate use of existing equipment. Overall, the outcome of the project was further development of the LG series of natural latex adhesives to replace solvent based adhesives used in our footwear manufacturing process. The feasibility of using latex adhesive in our process was checked by quantifying the adhesive bonding obtained by latex adhesives.

Once the feasibility was established, we went on to evaluate the economic impact of using these adhesives. Initial study and the cost comparison indicated that the use of water reducible (latex) adhesives instead of solvent borne (rubber) adhesives would be cost prohibitive. However, factoring in the cost of disposal and the possible health hazards due to exposure to solvents, we concluded that in the long run water reducible adhesive (latex) is the proper way to go. Last came the implementation of water reducible adhesives in our process. Implementation required modification of existing equipment, purchasing new equipment as well as modification of conventional processing techniques. In addition to equipment and processing techniques that required changing during the implementation phase, the human element came into play. The mindset of the assembly line personnel had to undergo an adjustment also. This proved to be the most difficult part of the transition. Old habits die hard.

Initially, latex adhesives with various viscosities were chosen. These viscosities were selected based on the viscosities of their counterpart solvent based adhesives. A major difference here was that although these latex adhesives were at different viscosities (same as solvent based adhesives used in the process), the percent total solids for each were between 55 to 60. In solvent based adhesives the higher the viscosity, the higher the percent total solids (ref. 4). This is illustrated in figure 1. This particular difference results from the fact that viscosity in latex adhesives is altered using a flow modifier where as in solvent based adhesives the diluent or the solvent is the viscosity modifier. While these latex adhesives at different viscosities seemed to work quite well, their compatibility with the existing equipment raised concerns. Existing equipment allowed the adhesive reservoir to be open to air, which in the case of latex adhesive proved to be detrimental. The adhesive ended up in coagulation lumps and made for messy clean-ups. This problem was overcome by some modifications. Once all the bugs were worked out of the system, we conducted the field trials with footwear manufactured using the latex adhesives. Although in rubber footwear all parts are co-cured to make them integral, there seemed to be doubts about the integrity of the adhesive bonding obtained using water reducible adhesives. Our initial trials indicate that the water reducible (latex) adhesives will perform as well as the conventionally used solvent based adhesives, if not better.

From an environmental standpoint the positive impact of using water reducible adhesives can be seen from tables 1 and 2 and their respective figures 3 and 4. Figure 3 shows that for the year 1994-1995 the theoretical amount of VOCs emitted was 502.5 tons. Over 90% (i.e. 457 tons) of this amount was contributed by solvent based adhesives alone. Figure 4 shows that full implementation of water reducible adhesives in our process results in reduction of VOCs from 502.5 tons/year to 51.65 tons/year (ref. 5). It should be noted that unlike solvent based adhesives the latex adhesives contribute the least in VOCs emission. This is also evident from figure 2.

Table 1 - VOCs distribution in air emission -- solvent based adhesives

Location                     VOCs      % of total
                          tons/year     emission

Internal mixer                7.5          1.49
Boilers                        21          4.18
Presses                        17          3.38
Solvent based adhesives       457         90.95
Location                   VOCs       % of total
                        tons/year       emission

Internal mixer              7.5          14.52
Boilers                      21          40.66
Presses                      17          32.91
Latex adhesives            6.15          11.91

Some of the advantages of the latex adhesives over solvent based adhesives are:

* Improved air-quality in the work place. Comparison of LG series adhesives with solvent based adhesives based on their volatile organic compounds can be seen in figure 2.

* Reduced fire hazard, through elimination of solvent usage in the work place.

* Reduced health hazard for personnel.

* Reduction in classified hazardous waste.

* Easier clean-up: When dry it is peeleable from hands and machinery.

In summary, we can say that from an environmental standpoint, use of water reducible adhesives definitely is a step in the right direction.

References

[1.] A.F. Chiulli, "The Vanderbilt Rubber Handbook," George G. Winspear, ed., R.T Vanderbilt Inc. New York, NY, Ch. 7, p. 535 (1968).

[2.] Jacob Lichman, "Handbook of Adhesives," Irving Skeist ed, Reinhold Publishing Corporation, New York, ch. 35, p. 436 (1964).

[3.] Arthur Hirsh, "Handbook of Adhesives," Irving Skeist ed., Reinhold Publishing Corporation, New York, ch. 46.

[4.] Jacob Lichman, "Handbook of Adhesives," Irving Skeist ed, Reinhold Publishing Corporation, New York, ch. 35, p. 435-440.

[5.] "Title V clean air act permit program," application, Norcross Safety Products L.L.C., (Dec. 1995).