Development of VOC compliant flock adhesive for EPDM weatherstrip.

Weatherstrip is an integral part of all automobiles. As the name implies, weatherstrip is a strip of an insulating material which forms a seal around automobile windows (glass run channels), doors, trunks, sunroofs, etc.

In addition to allowing glass to move freely in channels, weatherstrip prevents noise, inclement weather and dirt from entering into the automobile. Until 1960, felts of various kinds were used for this purpose. However, due to inherent deficiencies associated with felts (poor abrasion, environment resistance), these were replaced with flocked weatherstrips.

Until recently, flocked weatherstrips were prepared by electrostatically applying small cut fibers of polyester or polyamide (flock) onto adhesive coated synthetic rubber (styrene butadiene) substrates. Today, in addition to being functional, weatherstrip for window channels should also be durable to UV light and inclement weather conditions. This has caused the automotive manufacturers to mandate high quality components be used in weatherstrip to ensure durable performance throughout the life of the vehicle. Substitution of EPDM elastomer for SBR and natural rubber has solved most of these problems.

EPDM, because of its low surface energy, has proven difficult to bond flock fibers with commonly used adhesives. These demands can be met with solvent-based urethane adhesives. While weatherstrip prepared with these solvent-based adhesives meet and exceed performance specifications of the automotive manufacturers, recent government regulations (1990 Clean Air Act) demand that weatherstrip manufacturers control emissions of organic pollutants to the atmosphere. Hence, the need for low VOC (volatile organic compound) containing adhesives.

While there are several flock adhesives marketed in the field, none of these adhesives have met the requirements of VOC compliance until now. This article describes work done to achieve this objective.

Discussion

The objective was to develop a VOC compliant flock adhesive to help satisfy the demands of the 1990 Clean Air Act, for example, non-chlorinated solvents and a low VOC content, while ensuring the adhesive would process on standard equipment under normal manufacturing conditions without sacrificing the required performance properties. Specifically:

* Ease of application;

* good wetting on warm EPDM in-line and off-line;

* good adhesion/abrasion resistance of flock fibers to EPDM; and

* comparable cure rate under existing in-line and off-line conditions.

To accomplish the objective, we selected a high solids polyurethane approach. Initially, a one component prepolymer technique was investigated. Because of the difficulties in handling and processing high solids prepolymers and/or lack of the desirable performance properties, a two-part high solids polyurethane system was investigated. Our work revealed this approach offered potential.

Preliminary investigation with a two-part polyurethane system on EPDM substrate revealed that even though the adhesive was cross-linked, it showed poor adhesion to EPDM. Improvement in the adhesion of the polyurethane system was noticed when the adhesive was applied to corona treated EPDM. However, the improvement was not enough to meet end use requirements.

Variations in the ratio of polyols to polymeric isocyanate also improved adhesion to EPDM, but not enough to meet end use requirements. None of the investigated adhesion promoters and/or cross-linking agents and/or chlorinated polyolefins resulted in a desirable adhesive for EPDM.

One additive that showed significant improvement in adhesion was a proprietary cross-bridging additive. Addition of this additive improved

adhesion of polyurethane to EPDM. This resulted in a two-part formulation in which part one contained a mixture of polyols, a cross-bridging agent, pigments, plasticizers and surface-active agents. Part two contained diluted polymeric isocyanate.

Having determined the concentration of cross-bridging additive needed for the desired adhesion to EPDM, next, the effects of ratios of diols, triols and polymeric isocyanate needed for desirable adhesion to EPDM were investigated. Once a desirable formulation was established, the effects of processing parameters on adhesion properties were investigated.

Based on our preliminary investigations, the following observations were made:

* The presence of a cross-bridging agent in the formulation is essential for good adhesion to the EPDM substrate.

* Adhesion is further improved if the adhesive is applied on corona treated or abraded EPDM.

* Further improvement in adhesion was noticed when the adhesive was applied to a corona treated, warm EPDM rather than at ambient conditions.

* The ratio of triols to diols in the formulation was important for adhesion.

* The type of polymeric isocyanate in the formulation has a contributory effect.

* The ratio of NCO/OH was critical for adhesion.

* Cure conditions and other processing variables have a significant effect on the overall properties of the flocked materials.

By applying the adhesive via a lab extrusion line (a scaled down version of a commercial unit), we were able to determine not only the effects of various variables, but also an applicability of the developed adhesive under dynamic application conditions. A factorial designed experiment was used to assist in this study (table 1).

Table 1 - factorial designed experiment

Variables                    Description        Low
                                                value

Surface treatment            Corona discharge   None
EPDM extrusion temp.         Degree             43 [degrees] C
Wet film thickness           Mills              3
Adhesive open time           Seconds            10
Cure time @ 232 [degrees]C   Minutes            2.5

Variables                    High
                             value

Surface treatment            Yes
EPDM extrusion temp.         54 [degrees] C
Wet film thickness           5
Adhesive open time           30
Cure time @ 232 [degrees]C   3.0

The effects of the listed variables were measured and data were generated for the following:

* Flock density;

* primary adhesion (peel strength);

* adhesion after exposure for eight hours at 125 [degrees] C (257 [degrees] F);

* adhesion after exposure for seven days at 80 [degrees] C (176 [degrees] F);

* adhesion after exposure for 24 hours in D.I. water.

Based on the generated data, the following conclusions were drawn:

* Surface treatment has a significant effect on flock density, adhesion and environmental resistance. Improved results were obtained with the corona treated surface than with the untreated surface.

* Extrusion temperature also had a contributory effect, but the effect was not pronounced. Higher extrusion temperatures improved flock density and adhesion (peel values).

* Wet film thickness showed little or no effect on the response variables.

* Adhesive open time showed a significant negative effect. Less open time resulted in improved adhesion and flock density.

* Cure time showed a contributory effect. Generally, the longer the cure time, the greater the peel value.

Using the above parameters, we have developed an environmentally compliant flock adhesive (VOC content of 96 kilogram/[meter.sup.3] = 0.8 lb./gal.) that is processable on standard equipment under normal manufacturing environments.

Experimental

A commercially used EPDM stock was continuously extruded through a 1 Qr "wide by Qi "thick die. The EPDM was extruded using processing conditions typical for that type of elastomer. To duplicate the trend of the industry, the extruded EPDM was vulcanized to approximately 90% of full cure by the use of a hot air oven. Next, the vulcanized extrusion was exposed to a "corona discharge" (Tantec EST surface treatment unit, operating at ~100 watts). The adhesive was applied to the extrusion using a peristaltic pump and a common industry technique called drip/brush (a.k.a. drip/drag). With a controlled pump rate (i.e., ml/min.) a continuous wet film was applied to the appropriate thickness. The adhesive application equipment was physically set-up to allow for a 10 second or 30 second adhesive flash prior to entering the flock unit. As the adhesive coated extrusion traveled through the flock unit, polyester fibers (flock) were electrostatically charged using 40 kV and embedded into the wet adhesive film. Excess flock fibers were removed by an air blow-off unit. In a curing oven set at 232 [degrees] C (450 [degrees] F), the flock adhesive is cured and the elastomer is completely vulcanized. The cured, hot [~154 [degrees] C (~310 [degrees] F)], flocked EPDM extrusion was cooled by passing through a cold water quench tank. The wet flocked extrusion was dried using a compressed air blow-off and cut into strips of predetermined size.

The strips were checked for flock density and tested for adhesion, abrasion and environmental resistant properties by the procedures outlined in the following test specifications:

* GM6275M (General Motors);

* WSB-M8H13-A (Ford);

* MS-AK87 (Chrysler).

[Figures 1-8 ILLUSTRATION OMITTED]

Dru W. Alwani has worked in polymer synthesis, formulations of coatings and adhesives for 35 years. A holder of 13 patents, he is currently a staff scientist for Lord. George M. Klapsinos has worked on the development of chemical products for automotive weatherstripping since joining Lord nine years ago. He currently supervises the operation of Lord's experimental extrusion line.3