Rubber Adhesive 65 is a solvent-borne polychloroprene contact adhesive supplied as a single-component amber liquid with a nominal non-volatile content of 65% by weight. The model designation 65 refers to the release solids band of 63%–67% and not to a cure speed or service temperature. The formulation combines medium-gel chloroprene rubber, alkyl phenol-formaldehyde tackifying resin, magnesium oxide, zinc oxide, and a hindered phenolic antioxidant in a solvent blend of cyclohexane, methyl ethyl ketone, and ethyl acetate. This solvent balance provides a wet film that remains bondable for 20–45 min at 23 °C and 50% relative humidity under still air. Because the adhesive is a contact type, bond formation depends on solvent evaporation, tackifier coalescence, and chain entanglement under pressure; no moisture cure, heat cure, or second component is required.

Typical substrates include vulcanized EPDM, SBR, neoprene, nitrile rubber, polyurethane foam, leather, canvas, and rigid PVC. Direct bonding of flexible PVC containing more than 20 parts per hundred of dibutyl phthalate or diisononyl phthalate is not recommended because plasticizer migration into the adhesive layer produces tack loss within 14 days. The product is used in roll covering, gasket fabrication, automotive door-seal splicing, footwear sole attachment, and laminate panel assembly where high initial grab and a bond line that follows substrate flexure are required.

The open time of 20–45 min at 23 °C and 50% relative humidity is a manufacturer internal no-tack point determination because ASTM does not define a direct open-time method for contact adhesives. At 125 µm wet film thickness in two crossed coats, Rubber Adhesive 65 typically produces a dry film mass of 120–160 g/m², depending on substrate porosity. Coverage under those conditions is approximately 4.5–6.0 m²/L. Lower coverage below 3.5 m²/L on open-cell polyurethane foam indicates excessive penetration; a preliminary seal coat at 30–40% dilution with the manufacturer-approved solvent is recommended for such substrates.

High-solids polychloroprene chemistry and solvent release control

The elevated solids content compared with lower-solids chloroprene grades reduces total solvent volume per bonded square metre and permits a higher dry film build per coat. The viscosity range of 2,500–4,500 mPa·s at 25 °C is intended for fine atomisation by HVLP and pressure-pot spray equipment with nozzle diameters from 1.8 mm to 2.2 mm. At this viscosity, the adhesive can also be applied by brush, roller, and notched trowel without excessive sag on vertical substrates. The solvent blend evaporates in a defined sequence: methyl ethyl ketone and ethyl acetate flash at ambient temperature, leaving cyclohexane as the tail solvent that keeps the film open and soft during the mating window. Forced drying at 50–60 °C with air velocity of 0.6–1.0 m/s shortens open time to 6–10 min, which is used for roll lamination lines where steady-state tack is more important than long hand-assembly time.

The chloroprene binder is a medium-crystallization grade with gel content controlled to give a balance between solution stability and rapid strength development. The tackifier resin has a softening point of 120–140 °C, and the metal-oxide package functions both as acid scavenger and as mild crosslink promoter during long-term ageing. When the dried film is mated under pressure, chain diffusion across the bond line develops immediate handling strength; ultimate peel strength increases over 7 days as residual solvent leaves the bond line and the chloroprene network undergoes slow strain crystallisation.

The product contains flammable solvents; flash point is -14 °C by ASTM D3278-21. Store in closed containers below 25 °C and away from direct sunlight. Ground and bond all transfer equipment. The product is not suitable for continuous immersion in gasoline, diesel, methylene chloride, or oxygenated fuels; immersion in ASTM reference fuel B at 23 °C for 48 h reduces peel strength by more than 30%.

Rubber Adhesive 65 should not be mixed with amine-based accelerators or ammonia-containing latex. Amines accelerate dehydrochlorination of the chloroprene binder and cause color shift and loss of tack. In co-cured rubber assemblies, sulfur donors and thiuram accelerators at high loadings may migrate into the adhesive layer and cause post-bond softening; published data for this specific interaction is limited.

What production conditions reduce bond strength after open time exceeds 45 minutes?

Open time is not a fixed material constant; it is a function of ambient temperature, relative humidity, air movement, and substrate heat sink. In a production spray booth at 28 °C and 40% relative humidity, the film may become non-tacky within 25 min, whereas at 18 °C and 65% relative humidity the open time can extend beyond 60 min. High relative humidity slows solvent release because water vapour competes at the evaporating surface, but it also condenses on chilled rubber substrates, creating a weak boundary layer if the substrate temperature falls below the dew point. If the dried film has exceeded open time, pressing the parts together without re-application or solvent reactivation produces low peel strength and intermittent bond line failure. Reactivation by heating the adhesive film to 50–60 °C for 5–10 min is usually sufficient for EPDM and SBR, but polyurethane foam should not be heated above 50 °C because cell collapse and dimensional shrinkage occur.

Substrate preparation follows solvent wipe and abrasion. On EPDM and SBR, wet-out is improved by wiping with methyl ethyl ketone followed by abrasion with 60–80 grit aluminum oxide. On cured nitrile, methyl ethyl ketone wipe alone may cause surface hardening; use acetone or isopropanol wipe instead. On polyurethane foam, mechanical abrasion is not possible; use a light solvent wipe with ethyl acetate and apply a seal coat. At relative humidity above 60%, pre-drying porous substrates at 50 °C for 10–15 min is required to prevent trapped moisture from displacing adhesive at the substrate interface.

When a three-roll offset gravure coater is used at line speed above 8 m/min

On a production-scale three-roll offset gravure coater with a 180-line screen and 60 µm wet deposition, Rubber Adhesive 65 delivers uniform film thickness at line speeds up to 8 m/min. Above that speed, the adhesive film may not release cleanly from the gravure cells, producing visible ribbing and a reduction in measured T-peel strength. In trials on 80 Shore A EPDM sheet, increasing line speed from 8 m/min to 12 m/min lowered ISO 11339 T-peel from 5.8 N/mm to 4.9 N/mm; published data for this specific configuration is limited, and results will vary with drying tunnel length and solvent balance. The issue is control of solvent tail retention: at higher line speed, insufficient dwell before mating traps cyclohexane in the bond line, causing foaming and creep. Reducing line speed or increasing drying tunnel air temperature from 50 °C to 60 °C restores peel strength.

Edge lifting is the dominant field failure reported in gasket fabrication. It is commonly caused by insufficient pressure during the mating step or by skimming over the adhesive film after its open time. A two-roll pressure nip set at 0.4–0.6 MPa line pressure and rubber roll hardness 70–80 Shore A produces the most consistent contact over continuous sheet length. If only hand pressure is used, immediate handling strength is retained but final T-peel can be 15–20% lower.

Comparing Rubber Adhesive 65 with lower-solids and waterborne contact grades

The primary alternative to Rubber Adhesive 65 is Rubber Adhesive 40, a lower-solids chloroprene grade with non-volatile content 38%–42% and viscosity 300–800 mPa·s. That grade is suitable for porous leather and canvas because it penetrates and wets without excessive dry film build, but it requires three to four coats to achieve the same dry film coverage as two coats of Rubber Adhesive 65. Its open time is shorter, 15–30 min, and its lower tackifier content gives lower initial grab on EPDM. Rubber Adhesive 65 is selected when bond line thickness and high initial tack on low-surface-energy rubber are critical.

Waterborne polychloroprene grades have lower VOC but are not drop-in replacements in high-humidity production. Below 15 °C substrate temperature, waterborne films may not coalesce, while Rubber Adhesive 65 retains spray-applied tack. However, waterborne grades are preferred where SCAQMD Rule 1168 VOC limits apply below 150 g/L for adhesive application. Reactive polyurethane hot melts are used when service temperature above 100 °C is required, but they need 110–130 °C application temperature and moisture-cure time, whereas Rubber Adhesive 65 reaches handling strength immediately after mating and full strength after solvent evaporation.

Spray booth experience indicates viscosity drift is the main source of batch-to-batch variance. A drum opened for 8 h at 25 °C can gain 5–10% viscosity due to loss of low-boiling solvent. Spray equipment should be charged from sealed pressure pots, and unused material should not be returned to the original container. If viscosity exceeds 5,000 mPa·s with a 1.8 mm HVLP nozzle, the spray pattern shows cobwebbing and dry spray. Dilution with the approved solvent blend, not pure acetone, restores atomisation without destabilizing the polychloroprene. Spray application is typically run at pot pressure 0.25–0.40 MPa and atomization air 0.15–0.25 MPa.

RoHS Directive 2011/65/EU does not list adhesives as homogeneous materials; when used in electrical/electronic equipment, the cured adhesive layer can be assessed by IEC 62321-5 for cadmium, IEC 62321-4 for mercury, and IEC 62321-6 for lead. REACH SVHC declaration is available from the manufacturer and lists no substance above 0.1% w/w in the supplied product. No FDA 21 CFR 177.105 food-adhesive status is claimed for the standard grade.

For molded rubber sole attachment, the adhesive is applied to both sole and upper, dried for 30 min at 23 °C, heat-activated at 55 °C, and bonded at 0.5 MPa press pressure for 10 s. On continuous EPDM door-seal splicing, a 25 mm overlap and 0.4 MPa nip pressure provide a splice strength sufficient to survive die-cutting and subsequent handling; full peel strength is reached after 72 h at 23 °C.