Water-based SPMRAs: Improved productivity, quality and environmentally safe.

Mold release agents have always been viewed as a necessary evil; no one wanting to use them, but most polymer processors having to. To the release agent manufacturer, therefore, the goal has always been to provide a benefit to the user such as reducing the cost or inconvenience associated with release agent application. What has often been a side issue, or one even totally overlooked, has been the environmental impact of release agent usage. This article considers release agent choice within the rubber industry and how it can impact part cost, quality and the environment.

Mold release agents

Sacrificial mold release agents (SMRA)

There are three types of mold release agent; each used extensively in the rubber industry. The first type we shall consider is what may be termed a sacrificial mold release agent (SMRA). An SMRA achieves release by its cohesive failure when the molded article is removed, i.e., it acts sacrificially. The very nature of its release means that a significant portion of the release coating transfers to the released surface. Consequently, if an adhesive process is involved, such as rubber to metal in-mold bonding, bond failure will usually result. The most commonly used SMRAs are silicones, specifically nonreactive polydimethylsiloxanes (PDMS) varying in molecular weight from 6,000 to 100,000 (ref. 1). However, PDMSs can cause distinct problems for the rubber molder and should therefore be used with caution. The most common problem with silicones is often termed "knit line" failure. As the rubber is injected (or otherwise molded), some release agent is transported with the rubber as it flows around the mold. When the rubber tries to merge with itself, a thin film of release agent prevents the rubber from adhering to itself. The result is a defective and therefore rejected part. This usually occurs when the quantity of release agent on the mold is greatest, i.e., immediately after the release agent has been applied. As reapplication occurs quite frequently with SMRAs, this mode of failure can be relatively common. Although the surface-active nature of the silicone can be reduced by the introduction of more polar side groups into the polymer chain (such as aminopropyl or long chain alkyl groups), problems can still occur resulting in part failure.

Other SMRAs used in rubber molding include solutions and suspensions based on soaps, mica, talc, polytetrafluoroethylene (PTFE) and even carbon black. These are often "in- house" formulations which provide as much lubrication as they do actual release. Also common are mixtures of these substances with silicones.

Almost all SMRAs are available as either solvent-based or water-based products. As SMRAs have to be applied frequently, typically every cycle, a rubber molder may consume many tons of SMRAs per year. These products are usually simple blends and are generally regarded as commodity items.

Internal mold release agents (IMRA)

Internal release agents are surface-active materials that may be added to the rubber during compounding. These internal release agents migrate to the rubber/mold interface during the rubber molding process where they behave in a similar manner to sacrificial release agents. Unfortunately, they do not only migrate to the rubber/mold interface, but also to other interfaces within the rubber itself where they can cause cohesive failure in the rubber; for example, if the component is stressed.

The most common internal release agent used in the rubber industry is zinc stearate and is present in the majority of rubber compounds. However, its primary function is not as a release agent at all; the zinc ions are necessary to activate the organic accelerators used in the rubber formulation. Stearic acid is used to make the zinc ion available for the vulcanization process through the formation of the zinc stearate; the latter may be present at up to 5% by weight (but typically 1-2%). Internal release agents such as zinc stearate are relatively ineffective, and as a result, rarely affect the component properties. This also means that an external release agent is often required, as well as the internal, with the internal assisting release rather than being solely responsible for it. For example, rubbers containing IMRAs are generally easier to release than ones without internals, but invariably an external is still required.

Semi-permanent mold release agents (SPMRA)

The third category of release agent can be termed a semi-permanent mold release agent. An ideal SPMRA overcomes almost all of the disadvantages associated with sacrificial and internal mold release agents. SPMRAs are designed to enable more than one release to be obtained per release agent application and to provide minimal transfer of the release coating to the polymer. These objectives can be achieved by increasing the cohesive strength of the release agent film. SPMRAs are typically crosslinkable polymers formulated or dissolved in various inert solvents or another carrier. They are generally applied to the mold surface from solution and, by a process of solvent/carrier evaporation and solute cure, form a complete, uniform film over the entire mold surface. Although the production of a crosslinked film tends to slightly increase the force required for release (compared to an SMRA), it also increases the durability of the film and thus enables many releases to be performed from one application. The increased durability also results in a significant decrease in the amount of transfer of the release agent to the polymer and prevents release agent migration. Both properties prevent knit-line failure and make them ideally suited for rubber to metal bonding applications where any migration of release agent will corrupt the rubber/metal bond.

Environmental considerations

Traditionally, industrial mold release agents (those externally applied) have invariably been solvent-based and as such have always had a deleterious impact on the environment. Prior to 1990, chlorofluorocarbons (CFCs) were used extensively in release agent formulations, either as non-flammable diluents or as aerosol propellants. The Montreal Protocol (1987) set guidelines for the gradual reduction in the use of CFCs. In 1987, the release agent industry was one of the main sources of CFC emissions, releasing as much trichlorotrifluoroethane (Freon 113) into the atmosphere as the entire electronic industry (where it was used as a cleaning solvent). Although the latter was recognized as a major source of CFC emissions, the former (its use in release agents) was often overlooked. Since then, release agent manufacturers have replaced the trichlorotrifluoroethane by simply adding more organic solvents to the formulation. The typical solvents used are petroleum-based aliphatic hydrocarbons, and although not affecting the ozone layer, they do contribute to the greenhouse effect and to general pollution levels.

It is estimated that, on a global basis, the release agent industry is responsible for some 50,000 metric tons of organic solvent being emitted into the atmosphere each year; 95% of this from sacrificial release agents and lubricants.

Over the last decade, the change to water-based release agents has been a slow but steady progression. Many factors influence the ability of specific markets and applications to change. Ultimately it will be legislation that will force such a conversion.

So why doesn't everyone use water-based release agents?

For the release agent manufacturer, the formulation of a water-based release agent is not as straightforward as it may seem. Similarly for the user, the change from a solvent-based product to a water-based one often has its drawbacks. For example, one of the disadvantages with water-based release agents is that ingredients other than the release agent and the carrier have to be used. Typically these are surfactants that are used to emulsify the release agent. Their function is largely to enable the hydrophobic release agent to be suspended in an aqueous phase. However, unless these surfactants are chosen correctly and used in controlled amounts, they reduce the effectiveness of the release agent. This occurs when the surfactant dissolves in the polymer being molded and then aids the wetting of the release agent by the polymer. Consequently, release is either difficult or even impossible; difficult release usually being accompanied by a small deposit of rubber being left on the mold after the part is released. Surfactants also have a relatively low thermal stability (160-190 [degrees] C) and can degrade in many high temperature processes. The decrease in effectiveness of the release agent and the low thermal stability of surfactant are both causes of rubber and release agent build-up on the mold, i.e., mold fouling. The rate at which this occurs is a major consideration in release agent choice as it determines the mold cleaning cycle, i.e., the length of time a press can be operated before the build-up on the mold surface has an unacceptably deleterious effect on part quality.

For room temperature application, water-based release agents have to be able to wet the mold surface. As water has a high surface tension, good wetting of the emulsion on a surface can only be provided by the surfactants. The right surfactant choice is required to decrease the surface tension to less than that of the mold surface. However, the more efficient the surfactant, the more likely it is to reduce release agent performance by dissolving into the rubber (as described above). This again highlights the importance of surfactant choice.

Once a suitable formulation has been developed, the actual production of water-based sacrificial release agents is relatively straightforward. The main difficulty is the emulsification required to force the hydrophobic release agent into a water phase. This process requires the use of some sort of intensive mixer or homogenizer to produce the emulsion. For solvent-based sacrificial manufacture, a simple blending process is invariably all that is required. The higher processing cost of a water-based release agent usually offsets its lower raw material cost and consequently they are generally similar in price to their solvent-based counterparts.

The biggest market for solvent-based release agents is the urethane industry, accounting for approximately 50% of release agent associated solvent emissions (ref. 2). The use of water-based formulations within this industry is prone to problems resulting from the foaming of the urethane and the relatively low (45-65 [degrees] C) process temperatures. Consequently, solvent-based products remain the norm.

The rubber industry, however, due to its high rubber processing temperature, has been able to utilize water-based formulations. Consequently, it is this industry that accounts for 80% (by volume) of all water-based release agents (ref. 3). Similarly, the same high processing temperature also enables the use of waterbased semi-permanent release agents with the high temperature being used to dry the product quickly and to crosslink a heat curing resin. This type of chemistry was first developed in the late 1980s and now accounts for approximately 70% of the technical (non-tire) rubber industry (ref. 3).

Water-based semi-permanent mold release agents

Water-based SPMRAs require some sort of cure mechanism to be incorporated into the emulsified resin. This increases the complexity of the system ten fold. Most solvent-based semi-permanent release agents use a moisture cure resin. Clearly, this is inappropriate for waterbased systems and a heat cure is more suitable. As with water-based sacrificial RA formulation, choice of surfactant(s) is critical. The required heat cure limits the product to processes that use a heated mold, usually rubber manufacturing.

The latest developments in release agent technology have concentrated on water-based semi-permanent release agents, such as the latest products in the Aqualine range from Dexter. Aqualine R150 and R180 are micro-emulsions formed by the high-pressure homogenization of a multifunctional resin with water in the presence of a surfactant blend. Both microemulsions are non-flammable and stable to multiple freeze/thaw cycling. Being micro-emulsions with a particle size of 80-100 nm, neither product requires agitation or other means of resuspending resin particles.

Upon spray application to a clean substrate, the water evaporates to leave a film that further condenses to leave an inert, insoluble, crosslinked coating chemically bonded to the mold surface. As the cure time is only 90 seconds at 180 [degrees] C, production is not disrupted. For the past two years, R150 and R180 have been shown to actually out-perform (in terms of number of releases per application) solvent-based SPMRAs.

Despite the obvious and numerous advantages of this type of technology, there are many factors that influence release agent choice. Detailed below are several situations that illustrate these advantages, each derived from actual trial or customer data.

Case studies

Typical seal/gasket manufacture

Table 1 illustrates the differences observed by a seal/gasket manufacturer when changing from a solvent-based sacrificial to R150, a water-based, semi-permanent mold release agent. The company is working a 24 hour shift, 360 days per year with a cycle time of 20 minutes, i.e., 72 cycles/day. For simplicity, assume one part is molded per cycle, i.e., a one-cavity mold, and there are 10 presses.

Table 1

Release agent         Solvent-based   Water-based
  type                 sacrificial    sacrificial

Cost/litre                $2.5            $2
Solids (%)                 10             10
Typical
  application (ml)         100            100
Application
  frequency            Every cycle    Every cycle
Release agent
  used/day (litres)        72             72
Cost of release
  agent/day               $180           $144
Cost/part                $0.25          $0.20
Parts released/$            4              5
Solvent emissions
  (kg/year)(1)           17,730            0

Release agent          Solvent-based       Aqualine
  type                semi-permanent         R150

Cost/litre                 $15              $12.5
Solids (%)                   1                0.6
Typical
  application (ml)          100               100
Application
  frequency           Every 50 cycles   Every 80 cycles
Release agent
  used/day (litres)        1.44              0.90
Cost of release
  agent/day               $21.60             $11.25
Cost/part                 $0.03             $0.0156
Parts released/$            33                64
Solvent emissions
  (kg/year)(1)             390               3.3

(1) - Assuming specific gravity of solvent to be 0.76 g/l

The economics of the above situation are clear; as is the environmental impact. It is this justification that is responsible for the current widespread use of this type of technology in the "technical" rubber sector.

Tire release

The main users of solvent-based release agents in the rubber industry are tire manufacturers. This results from it being one of the few rubber industries where the release agent is applied to a room temperature surface and therefore rapid drying at room temperature is desired.

Within tire manufacturing, there are many stages that require a release agent or other process aid. The simplified schematic shown in figure 1 illustrates the surfaces that may require a release agent or process aid to assist air bleed and rubber flow.

[ILLUSTRATION OMITTED]

In figure 1, points 2 and 4, the main requirement of the coating is release. In these processes, the ability of the release agent to disperse air (during the molding process) or to assist rubber flow could be considered as secondary. With the tire paints, the release character is less important and it is the ability of the process aid to disperse air that is more critical. In these cases, particulate suspensions of mica, carbon black or even ground, recycled rubber can be found. Many of these products are in-house formulations that are dispensed from a solvent carrier and often contain a few percent of silicone to aid release. These products are applied directly to the green tire before molding, usually in an automated process.

As with all systems, the relative performance of the release agent or process aid is a compromise; the final choice of product being dependant on many factors. This includes (but is not limited to): mold design (air vents and mold complexity); tire size and complexity; rubber formulation; the relative importance of quality (performance and appearance); mold down time (time required for mold cleaning and its frequency); cost; and environmental factors. Clearly, these factors are interpreted differently by different tire manufacturers. Similarly, these manufacturers obtain quality tires by using combinations of release agents and process aids, often differing with each tire design.

The trend in the tire industry is towards a release agent free process where the formulation of the rubber and the design of the mold allow for uninterrupted tire manufacture. Clearly, a process that eliminates the need of release agents or other process aids is desirable; but is there a cost? How efficient is the release in these processes and is there a negative impact on tire quality and mold build-up? Considered below are three examples from the tire industry.

Case 1: Outside tire paint vs. R150

The following illustrates an example where a water-based semi-permanent release agent replaced a solvent-based outside tire paint. Table 2 presents actual figures from a major tire manufacturer. Currently, this manufacturer produces approximately 100 tires/press/24 hours. Unless otherwise stated, data are for one press.

Table 2

Release agent type          Conventional outside    Aqualine R150
                            tire paint (solvent/     (water-based
                               carbon black)       semi-permanent)

Cost/litre                        $0.50                $12.50
Solids (%)                          10                   0.6
Typical application (ml)           125                   250
Application frequency           Every tire         Sprayed onto mold
                                                    every 100 cycles
Release agent used/
  day/press (litres)               12.5                  0.25
Cost of release agent/
  day                             $6.25                 $3.12
Cost/tire                         $0.063                $0.031
Tires released/$                    16                    32
Cycles between
  cleaning(1)                     2,000                 3,500
Solvent emmissions
  (kg/year) (2)                  300,000                  50
Appearance of tire          Moderate, uneven,         Excellent
                              matte finish

(1) Actual data;

(2) Total from 100 presses; solvent having
specific gravity of 0.76 g/ml

The quality (appearance) of the type is of major importance. Figure 2 clearly illustrates the difference between the two types of system.

[ILLUSTRATION OMITTED]

Case 2: No release agent/tire paint vs. R150

Table 3 illustrates the differences between using no release agent (or outside tire paint) and a water-based semi-permanent release agent. Again, assume a production of 100 tires/press/24 hours.

Table 3

Release agent type        No outside tire paint/     Aqualine R150
                          no tread release agent   (water-based SPMRA)

Cost/litre                          $0                   $12.5
Application frequency         Not applicable       250 ml sprayed onto
                                                     mold every 100
                                                        cycles
Application time                    0                    2 min.
Release agent cost/tire             $0                   $0.031
Ease of demolding            Depends on mold           Always good
Additional investments       Modification of         Spray equipment
                            molds/mold design
Rejects                    Some, especially on        Close to zero
                          larger molds or with
                          deeper, more intricate
                                 treads
Cycles between                2,000 (3 weeks)        3,500 (5 weeks)
  cleaning(1)
Solvent emissions                   0                      50
  (kg/year)(2)
Appearance of tire                 Fair                 Excellent

(1) Actual data;

(2) Total from 100 presses; solvent having specific
gravity of 0.76 g/ml

For the above comparison, it is the quality of the tire and the decrease in the frequency of mold cleaning that are the main advantages in using the SPMRA. The difference in appearance in the tires is similar to figure 2. Against the use of the SPMRA is the associated direct cost and application time.

Case 3: Other water-based SPMRAs vs. R150

Table 4 illustrates the differences between a different waterbased SPMRA and R150. Again the data are from an actual situation and, for the sake of comparison, production is 100 tires/press/24 hours. Unless otherwise stated, data are for one press.

Table 4

Release agent type     Semi-permanent water-based      Aqualine R150
                         mold release agent (two       (water-based
                      products used in conjunction)   semi-permanent)

Application           Sprayed onto   Brush applied    Sprayed onto
frequency              mold every    to sidewalls      mold every
                         4 hrs.         of every       200 cycles
                       (16 cycles)     green tire
Cost/litre                $13             $12            $12.5
Solids (%)                 3               5               0.6
Typical application       200             10               200
  (ml)
Release agent
  used/day/press          1.20           1.00             0.20
  (litres)
Cost of release
  agent/day              $15.6            $12             $2.5
Total cost/part                                          $0.025
Parts released/$                                           40
Cycles between
  cleaning(1)                                             3,500
Solvent emissions
  (kg/year)(2)                                             50
Appearance of tire                                      Excellent

(1) Actual data;

(2) Total from 100 presses; solvent having specific
gravity of 0.76 g/ml

Again, the economic advantages are clear. The tire manufacturer claimed that the tire paint was necessary to assist rubber flow over the dry solvent-based SPMRA. The blemishes on the molded tire are the result of brush applying the water-based emulsion to the green tire. This illustrates one of the problems with using some types of water-based product at room temperature. It also illustrates the supremacy of spray application in terms of finish.

Conclusions

Presented in the above tables are some, but certainly not all, of the factors that influence release agent choice. Clearly, the relative importance of each factor will differ from manufacturer to manufacturer, and even between different production lines. It is only when all aspects of release agent influence are considered that the right decision can be made.

The understanding that "no release agent must be best" is not always valid. If the rubber formulation and mold design are such that release can be obtained without any assistance, other factors, such as mold fouling and appearance (customer perceived quality) must be evaluated. Within the tire industry it is likely that some sort of release agent will always be necessary for the harder to release tires, such as truck, winter or high-performance tires. In this case, it is clear that waterbased semi-permanent mold release agents can provide superior cosmetics, lower cost and higher productivity than the alternatives. An increasingly important benefit is that these release agents are also the most environmentally responsible.

With the many alternatives now available to the rubber molder, there is little excuse for using solvent-based release agents or other process aids. The change to water-based semi-permanent products can be made simply, with no additional investment; the possible exception being the time required for the initial mold cleaning. As well as the environmental advantages, lower part cost, improved quality and greater productivity can also be realized.

References

(1.) Noll, W., "Chemistry and technology of silicones," Academic Press, 1968.

(2.) Chemtel, "Mold release agents for urethanes," market study for Dexter, 1997.

(3.) Chemtel, "U.S. and European mold release agents for rubber," market study for Dexter 1998.