It's time to get to know N-butyl acrylate copolymers.

Now available in the U.S., these new adhesive resins offer improved thermal stability, toughness, heat-sealing, and optical properties for extrusion coatings and coex packaging films.

Ethylene normal-butyl acrylate copolymers are a new family of olefinic adhesive resins for use as heat-seal layers in multilayer extrusion coatings and blown film. EnBA copolymers have better heat stability than ethylene vinyl acetate (EVA) and better melt strength at high temperatures than ethylene methyl acrylate (EMA) copolymers. These advantages are particularly suited to multilayer applications with high-temperature resins like PET and polycarbonate.

Melt temperatures for extrusion coating with EnBA can be maintained at temperatures as high as 610 F. Processors also find improved drawdown of EnBA, facilitating its use in thin films, coatings and laminations. In specific film applications, EnBA copolymers may have other advantages. For instance, converters have found EnBA to be more ductile than EVA for low-temperature applications. Notched low-temperature brittleness data indicate that TABULAR DATA OMITTED an EnBA with 5% comonomer has about the same low-temperature brittleness as an EVA of 13% comonomer. EnBA resins also displayed good monolayer film impact properties at low temperatures. The EnBA copolymers performed better than EMA (20% MA) and EVA (9% VA) at 32 F and comparable or better at -4 F. As an extrusion coating, EnBA has been tested on various substrates and shown to adhere as well as or better than LDPE, EMA and EVA.

EnBA copolymers have been produced in Europe for over six years and are estimated to total about 15 million lb/yr. Current European producers include BASF in Germany, Neste Chemical Co. in Finland, and Elf Atochem SA in France. EnBA was first made commercially in the U.S. two years ago by Quantum Chemical Co. at its Clinton, Iowa, plant. Quantum currently offers two grades for coextruded film and one for extrusion coating, with an nBA comonomer content of 5%, 19% and 20% by weight (see Table 1). In melt index, the three TABULAR DATA OMITTED commercial EnBAs range from 0.3 to 6.0 g/10 min. Specialty hot-melt adhesive and sealant grades with 28-35% nBA content and 40-110 MI are also available.

Physical properties of EnBA resins are influenced by the ratio of ethylene and nBA comonomers. The nBA comonomer units, which are distributed randomly along the main polyethylene chain, lower the polymer's crystallinity in proportion to the amount of nBA incorporated. The higher the nBA content, the lower the crystallinity. And since crystallinity controls properties like stiffness and impact strength, lower crystallinity translates into higher flexibility and impact strength.

EXTRUSION COATING ADVANTAGES

In extrusion coating, EnBA copolymers have two advantages over EVA and EMA copolymers: higher heat stability and better adhesion. EnBA can be processed at temperatures exceeding 600 F, whereas the maximum extrusion temperature for EVAs is only 450 F. The higher thermal stability of EnBA compared to EVA is an advantage in extrusion coating using materials with higher melt temperatures.

Figure 2 shows the results of a multiple-pass extrusion test comparing the thermal stability of EnBA, EVA, EMA and LDPE. The test monitored the melt-flow rate of each during five passes through an extruder at 340 F melt temperature. The EVA melt index was evaluated at a lower temperature (257 F) so that any melt-flow shift would be due to the extruder heat history and not due to processing temperature. Standard melt-index test conditions (374 F and 4.76 lb) were used for EnBA, EMA and LDPE.

Results showed a slight increase in the MI of EnBA resin over the five extrusions, a lesser increase for the EMA resin, a steady, significant decrease in the MI of the EVA and essentially no change in the MI of LDPE. The increases in MI for EnBA and EMA are due to a degree of polymer chain disentanglement or "melt refining" during the extrusion process, while the sharp decrease in MI for EVA indicates crosslinking of the polymer because of its lower thermal stability than the other resins tested.

In this lab-scale study EnBA and EMA might appear similar in heat stability, but an important difference shows up in commercial trials on an extrusion coating line. EMA's low melt strength at 610 F made it unsuitable for coating. To run the EMA, its melt temperature had to be decreased from 610 F to 560 F, while EnBA performed well even at high temperatures. EnBA's higher processing temperature may be a factor in its improved adhesion, since adhesion generally increases with temperature.

BETTER ADHESION

EnBA also outperforms EVA, EMA and LDPE in adhesion. EnBAs adhere better to many substrates than EVAs or EMAs, especially to nylon, PET, chemically primed PET and PVdC-coated PET (see Table 2). EnBA has a much stronger bond to nylon, even without corona treating, than EVAs and EMAs, which adhere poorly to nylon. With the necessary corona treating, EnBA also adheres better to PET, chemically primed PET and metallized BOPP than do EMA and EVA. However, the most dramatic advantages are found with corona-treated PET. And adhesion of all three copolymers (EnBA, EVA and EMA) to PVdC-coated PET film is strong enough to delaminate the PVdC coating from the PET substrate.

EnBA's adhesion to BOPP is also comparable to the best adhesion levels demonstrated by EVA and EMA copolymers. Like EVA and EMA, EnBA isn't a competitive material for bonding to foils.

Generally, adhesive strength increases with increased melt temperature because higher temperature creates more oxidation in the air gap or vertical distance between the die and chill roll. So the ability to extrusion coat with an EnBA copolymer at a final melt temperature of 610 F helps achieve higher adhesion. Both EVA and EMA, on the other hand, require lower melt temperatures (445 F for EVA, 565 F for EMA) in order to maintain adequate melt stability, so less oxidation occurs in the air gap.

When running at a constant line speed and coating thickness, other EnBA processing conditions are only slightly different from those for EVA and EMA copolymers. In general, melt pressure will be lower and screw speed will be higher with EnBA. Also, the neck-in for EnBA will be slightly less than for EVA at typical extrusion conditions. Because extrusion coating with EnBA is possible at much higher temperatures than with EVA, greater drawdown and thus a thinner coating is possible. EnBA also requires no modification of standard extrusion coating equipment. Matte chill rolls are recommended if the EnBA is to be against the chill roll.

BLOWN FILM COEXTRUSION APPLICATIONS

Most EnBA film applications at present are in coextrusion. Monolayer EnBA film has been made in a lab for purposes of testing inherent EnBA resin properties. Compared with EVA resins of the same co-monomer percentages (5-20%), EnBA copolymers have lower Shore A hardness, better notched low-temperature brittleness, comparable Vicat softening point, and similar heat-seal strength.

Comparing properties of film samples blown from EnBA, EVA and EMA copolymer films, all with 20% comonomer content, several key physical properties of EnBA film are better. Notably, EnBA film has superior optical properties--haze and gloss--relative to the corresponding EMA film (Table 3). The film samples were blown on an 8-in. die with a 24:1 L/D extruder and 2:1 blow-up ratio. All three resins contained the same antiblock additive.

Operating conditions for EnBA, EVA and EMA in blown film are similar. Target melt temperature is slightly higher for the EnBAs, but the difference isn't as pronounced as in extrusion coating. Target melt temperature for EnBAs varies with melt index and comonomer content--e.g., a 3.0 MI resin with 5% nBA has a target melt temperature of 360 F, while a 0.3 MI resin with 19% nBA extrudes at 390 F.

Excellent adhesion is possible between EnBA and PET, PP or other polyolefins in coextruded film layers. However, in some cases EnBA doesn't have as great an adhesion advantage in coex blown film as it does in extrusion coating on cast film. For instance, EnBA adhesion to nylon is only marginally better than EVA in blown films.

Two factors contribute to lower levels of EnBA adhesion in co-extruded blown film than in extrusion coating. First, a lower melt TABULAR DATA OMITTED temperature is typically used in blown film than is used in extrusion coating. Second, in coextrusion of film there is generally no oxygen at the adhesive polymer interface, so little oxidation occurs to promote adhesion.

SEEKING FDA COMPLIANCE

At present, Quantum Chemical's EnBA copolymers are approved (under FDA Regulation 21 CFR Section 175.105) as adhesives in food packaging where a functional barrier exists between the EnBA and the food. Quantum is actively pursuing FDA approval for direct food contact. The timing of further approval depends on FDA priorities and any additional data that may be needed.