Bonding melt processible rubber.

Bonding melt processible rubber

Many of today's commercial applications require adhering melt processible rubber (MPR) to itself or to other materials, such as metal, textiles, plastics, thermoset rubber, wood or leather. Some industry experts estimate that 80 percent of the rubber goods produced in the United States involve the use of another material, usually for reinforcement. Examples are injection-molded parts with metal or plastic inserts, reinforced hose, footwear, wiring harnesses, pump impellers, and single-ply and reinforced-membranes, such as liners and coated fabrics.

Due to differences in manufacturing processes and adhesion requirements, bonding techniques must be tailored for each substrate and application. Du Pont has programs underway, both in-house and with adhesive suppliers, to develop basic adhesion information for its MPR - Alcryn. In such evaluations, stock-tearing bonds are the measure of successful bonding. Our experience to date indicates that many of the adhesives and bonding techniques that have many of the adhesives and bonding techniques that have been successfully used with rigid and flexible polyvinylchloride (PVC) are also useful with Alcryn.

This article summarizes the bonding information currently available. Because these results are based on laboratory tests, they should only be used as general recommendations. In a production environment, evaluations should be conducted to ensure optimum performance.

Adhesion test methods

Specifications for many parts involving adhesive bonding include both bond strength requirements and bond strength measurement procedures. Where specifications are not applicable, ASTM Standards D429B and D751 are useful for evaluating and comparing adhesive systems. In our evaluations, we used 180 [degrees] C peel adhesion tests with flexible substrates and either 45 [degrees] or 90 [degrees] peel tests with rigid substrates.

Bonding MPR to metal

When adhering to metal, the metal surface should first be prepared by blasting with sand, grit or aluminum oxide, or by treating with zinc phosphate (carbon steel) or persulfate (brass). For most applications, a primer should be applied before the adhesive.

At room temperature Alcryn can be bonded to metals at room temperature with second generation acrylic (SGA) adhesives, using the following procedure:

* Prepare the metal surface. Wiping with solvent may be sufficient.

* Apply primer to Alcryn and dry for 30 seconds; then, apply adhesive to the metal surface.

* Join the surfaces immediately, before the adhesive dries or sets.

* Apply holding pressure for a minimum of 30 seconds, or until the adhesive sets.

* Allow 24 hours for full bond strength to develop.

With solvent adhesives Good bonds have been obtained between MPR and steel, aluminum and brass during insert injection molding using Lord EP 5707 single coat adhesive system and the following procedure:

* Prepare the metal surface by blasting or chemical treatment, as noted above.

* Wipe the prepared surface with solvent.

* Add 10 parts Lord EP 4902-695 curative to 90 parts EP 5707-52A adhesive.

* Apply the mixed adhesive (by brushing or spraying, for example) and allow the surface to dry for at least one hour.

* Place the adhesive-coated insert in the mold. (Preheating the insert in an oven for 30 seconds at 212 [degrees] F to 250 [degrees] F (100 [degrees] C to 121 [degrees] C) may improve bond strength in some applications. This is particularly important if coated parts cannot be bonded the same day that they are coated.

* Inject hot Alcryn (melt temperature 340 [degrees] F to 360 [degrees] F [170 [degrees] C to 180 [degrees] C]) into the mold. The normal molding cycle should be sufficient to achieve satisfactory bond strength. Extended heating times, either in the mold or in a post-cure oven, may improve adhesion.

Excellent bonds with stock-tearing of Alcryn have been achieved by using the Lord EP 5707-52A adhesive system in a profile extrusion of Alcryn over steel. The adhesive was applied on the degreased metal surface and dried online. Then, Alcryn was extruded with a crosshead onto the steel. For best results, the adhesive coating should be uniform over the entire surface that is to be bonded.

Alcryn can be bonded to brass-coated steel wire using the same adhesive system.

With foam tape Foam tape has been used successfully to bond "Alcryn" to bare and painted steel surfaces. Initial work indicates that the MPR should first be coated with a primer or barrier coat to prevent migration of plasticizer to the adhesive interface. Good results have been obtained using Lord's Chemglaze barrier coated with Norton's Tite-R-Bond 2287 adhesion promoter.

Bonding to textiles

With solvent adhesives Excellent bonds have been obtained between Alcryn and various fabrics, such as cotton, polyester and nylon, during extrusion, calendering or molding using solvent-based, urethane adhesives. Upaco 2200 or 2400 works best with cotton, while either Lord's EP 5707-52A or Mobay's Desmocoll 530 is preferred for polyester or nylon. An isocyanate may be added to increase bond strength in some applications. Lord's Tyrite 7610, a moisture-cure system, has also given satisfactory results in limited evaluations.

The samples chosen for our evaluations were reinforcing scrims used to make coated fabrics. Successful bonding has also been achieved to canvas shoe uppers. The following procedure was used in our testing:

* Paint the adhesive (or adhesive/isocyanate combination) onto the textile surface and allow it to dry according to the manufacturer's recommendations. The adhesive may be thinned with solvent to achieve a suitable consistency for application.

* Extrude or calendar Alcryn onto the fabric and pass it through a heated (250 [degrees] F to 300 [degrees] F [121 [degrees] C to 149 [degrees] C]) nip roll to achieve bonding.

Other systems Cotton and polyester have been successfully bonded to Alcryn using a PVC plastisol combined with isocyanate. Good bonds were achieved at typical PVC/plasticizer ratios.

When solvent-based systems are unsuitable, Alcryn can be bonded to polyester fabric with a polyester hot melt adhesive. To retain satisfactory adhesion, the bonded piece must not be exposed to temperatures greater than 158 [degrees] F (70 [degrees] C).

Bonding to plastics

At room temperature Alcryn can be bonded to PVC with commercial PVC pipe cement at room temperature, using the following procedure:

* Wipe the surfaces to be bonded with solvent.

* Apply adhesive.

* Join the surfaces immediately, before the adhesive dries or sets.

* Apply holding pressure for a minimum of 30 seconds, or until the adhesive sets.

At elevated temperatures Good bonds between Alcryn and nylon have been obtained using either Lord's EP 5707-52A or Mobay's Desmocoll 530 adhesive plus curative, using the following procedure:

* Roughen and/or wipe the plastic surface with solvent.

* Apply adhesive and allow it to dry to the touch. The adhesive may be diluted with solvents to achieve desired consistency for application.

* Inject or extrude "molten" Alcryn (melt temperature 340 [degrees] F to 360 [degrees] F [170 [degrees] C to 180 [degrees] C]) onto the plastic surface.

* Apply pressure to ensure adequate contact (this is usually done with a nip roll).

Melt techniques Alcryn will bond to both flexible and rigid PVC during coextrusion. No primer or adhesive is needed.

Bonding MPR to MPR

At room temperature Successful bonding at room temperature has been accomplished with two classes of adhesives - PVC pipe cements and second generation acrylics (ITW B 0550 system), using the following procedure:

* Wipe the surfaces to be bonded with solvent.

* Apply a thin layer of primer to both surfaces and let them dry for approximately 30 seconds. Then, apply adhesive to either surface. (If pipe cement is used, no primer is required.)

* Join the surfaces immediately, before the adhesive dries or sets.

* Apply holding pressure for a minimum of 30 seconds or until the adhesive sets.

* Allow 24 hours for full bond strength to develop. Stock-tearing bonds of Alcryn to Alcryn have been obtained by laminating sheets of Alcryn painted with a 20 percent (by weight) solution of Desmocoll 530 adhesive (in MEK) activated with Mobay's Desmodur R or RF isocyanate (90 parts adhesive/10 parts activator). Excellent bonds have been formed at laminating temperatures of 176 [degrees] F (80 [degrees] C); however, bonds formed at room temperature (73 [degrees] F [23 [degrees] C]) failed in adhesion. The following procedure was used in our testing:

* Wipe the surfaces to be bonded with solvent.

* Apply adhesive by brushing or spraying to clean dry surfaces. The adhesive may be diluted with solvents to achieve desired consistency for application.

* Allow adhesive to become dry to the touch.

* Bond under heat and pressure. The temperature should be at least 176 [degrees] F (80 [degrees] C); the pressure should be at least 100 psi (690 kPa) and be applied for a minimum of 10 minutes.

Melt techniques Alcryn has been successfully butt welded on hot knife equipment similar to that used for welding extruded shapes of flexible and rigid PVC. Optimum welding temperature and pressure must be established for each particular piece of equipment. The recommended welding temperature for Alcryn is generally in the same range as that used for PVC (500 [degrees] F to 600 [degrees] F [260 [degrees] C to 315 [degrees] C]).

Good bonds have also been formed by heating the surface of Alcryn with a hot-air gun. However, this type of bonding is difficult and requires a very precise technique. Ultrasonic welding has been successful in sections up to 40-mils (approximately 1mm) thick. Dielectric (RF) welding, at a frequency of 27.12 MHz, has also been accomplished, with the best bonds obtained when an insulator or buffer was inserted between the MPR and the electrode. To date, spin welding has not been successful in our bonding trials.

Sheets of Alcryn have been successfully seamed by calendering a thin strip of PVC onto the edges of the sheets, then fusing the PVC like a hot-melt adhesive.

Bonding to thermoset rubber

At room temperature Alcryn can be successfully bonded to some thermoset rubbers at room temperature with second generation acrylic (SGA) adhesives, using the following procedure:

* Wipe the surfaces to be bonded with solvent.

* Apply adhesive.

* Join the surfaces immediately, before the adhesive dries or sets.

* Apply holding pressure for a minimum of 30 seconds, or until the adhesive sets.

* Allow 24 hours for full bond strength to develop.

With solvent adhesives In laboratory tests, hot bonding of Alcryn to polychloroprene has been accomplished with the EP 4802-75 adhesive system, using the following procedure:

* Wipe the surfaces to be bonded with solvent.

* Mix six parts Lord's EP 4802-695 curative with 94 parts EP 4802-75.

* Apply adhesive to clean, dry surfaces. The adhesive may be diluted with solvents to achieve desired consistency for application.

* Allow adhesive to become dry to the touch.

* Bond under heat and pressure. The temperature should be at least 176 [degrees] F (80 [degrees] C); the pressure should be at least 100 psi (690 kPa) and be applied for a minimum of 10 minutes. (Limited experience suggests that better bonds can be obtained by extruding Alcryn onto adhesive-coated polychloroprene and pressing the surfaces together in a nip roll.)

Bonding to wood or leather

Alcryn has been successfully bonded to wood and leather during molding or extrusion using heat-reactivated, solvent-based adhesives. We used Lord's EP 4802-56 for wood and Lord's EP 5707-52A for leather.

As with other substrates, the adhesive should be applied to the wood or leather and allowed to dry thoroughly before bonding.

Conclusion

Because bonding is essential to the manufacture of today's rubber goods, the ease with which rubber can be bonded to various substrates is a very important issue.