1. Field of the Invention
This invention relates to improved adhesives containing an ethylene copolymer resin having a network structure, a modified polyolefin and, optionally, an elastomeric homopolymer or copolymer. More particularly, the invention relates to extrudable adhesive compositions having acid or acid derivative functionality useful as tie-layer resins for multi-layer constructions, such as films, tubes, blow molded articles and sheets and especially for polyolefin/metal composite pressure pipe.
2. Description of the Prior Art
The use of tie-layer adhesives to improve adhesion between polyolefins, such as polyethylene (PE) and polypropylene (PP), and dissimilar substrates, such as polyamides, ethylene-vinyl alcohol (EVOH) copolymers and metals, and multi-layer constructions of these materials is well known. While numerous acid and acid derivative tie-layer resins are known, new adhesive compositions are always in demand to meet the requirements of new polyolefin laminate applications.
Tie-layer adhesives useful for high-stress/high-demand applications are particularly desirable. Such high-stress/high-demand conditions may occur as a result of the processing or manufacturing operation or the environment to which the finished article will be exposed.
In thermoforming, blow molding and film blowing operations, the adhesive may be exposed to high temperature, high shear and other detrimental process conditions. For example, when blow molding large articles such as multi-layer thermoplastic gas tanks, the parisons may be suspended at elevated temperatures for extended periods before mold closure and blowing thus creating significant stresses and placing extraordinary demands on the adhesives being used. Similarly, in various post-processing procedures the adhesive may be exposed to irradiation or other stresses, such as the stresses encountered during orientation using double-bubble or tenter frame processes. Any deterioration of the adhesive during processing or post-processing results in a corresponding reduction in adhesive strength in the finished product which can reduce service life and result in premature failure.
As indicated above, in numerous end-use applications the finished articles containing tie-layer adhesives are subjected to high-stress/high-demand conditions. One such application is multi-layer tubing used for under-the-hood automotive applications, such as for fuel lines. These lines can be exposed to high temperatures for extended periods and an adhesive composition resistant to thermal degradation is therefore essential. Other applications where heat-resistant tie-layer adhesives are beneficial are for heat shrinkable multi-layer films and films used for cook-in applications.
Another such end-use application where composite structures utilizing tie-layers must withstand the repeated stresses of temperature and pressure is for composite polyolefin/metal pipe and tubing for plumbing applications and heating systems. Crosslinked PE (PEX) and aluminum are most commonly used for these applications, but copper and conventional PE are also utilized by some manufacturers. These constructions would have the structure PEX/tie-layer/metal/tie-layer/PEX or PE/tie-layer/metal/tie-layer/PE. Even though the metal provides most of the mechanical strength in these constructions, mechanical integrity and pressure ratings are dependent on the quality of the bond between the metal and PE.
Failure of the PE or PEX to metal bond results in delamination, which usually takes the form of blister formation at the inner metal/tie-layer interface and severely detracts from the service life of the pipe. While various theories have been advanced as to how the blisters are formed, most agree that the thermal shocks associated with repeated heating and cooling play a major role. In fact, thermal cycle tests such as those defined in ASTM F-1281-97 and F-1282-97, DVGW W542 (Deutscher Verein das Gasund Wasserfaches) and KIWA BRL-K536/03 G/98 (Keurings Instituut voor Waterleidung Artikelen) are used to evaluate coextruded PE or PEX composite pressure pipe having a welded aluminum tube reinforcement between the inner and outer layers.