Abstract:
A plastic container having a thin outer layer that protects against stress concentration effects arising from either surface irregularities or flexing when dropped and a method of making that container. The outer layer helps avoid bottle failure resulting from cracks that start at such areas of stress. The outer layer may incorporate both the stress inducing irregularities and the properties to minimize possible, resulting catastrophes. The surface irregularities giving birth to stress include edges of in-mold-applied labels, ridges, stippling, and grip bumps. Dropping the container gives rise to flexing which can also induce stress concentrations. The outer layer, to accomplish the protective function, should display lesser brittleness or, in some cases, lesser density than the bulk of the container wall. Making the container involves forming the bulk of the wall with a first, inner, structural plastic. A thin layer displaying lesser brittleness or, in some cases, lesser density is formed as the container&#39;s outer layer.

Description:
BACKGROUND  
         [0001]    Plastic containers have become useful, dependable, and consequently ubiquitous in modem society. Their very success, however, has resulted in demands for their continuing improvement. The impetus underlying the research and developmental efforts derive from three separate concerns. These include the economic, ergonomic, and environmental impacts of and aspirations for these containers.  
           [0002]    The in-mold labeling of containers constituted a major improvement in the manufacture and use of plastic containers. This advance accomplishes two tasks in one processing step with substantial aesthetic and economic savings. The amounts of resins used in containers regularly undergo reductions. This effort, by itself, accomplishes both economic and environmental benefits.  
           [0003]    Plastic containers now also possess features not seen in the early years of their use. Incorporating ridges may provide structural strength and rigidity especially in thin-walled bottles. They can also enhance the appearance and grippability and provide graphic and alphanumeric information. Similarly, placing bumps on the surface may similarly avoid their slipping out of the users&#39; hands. All of these surface treatments, as well as many others, can provide the containers with a pleasing esthetic appearance and improved functionality.  
           [0004]    Not surprisingly, however, plastic bottles are susceptible to outside influences resulting in the development of leaks, cracks, and other undesirable losses of complete integrity. The developments discussed above, by either decreasing the amount of plastic in a container and thus the container&#39;s wall thickness or by incorporating deviations from absolutely smooth walls, have aggravated the likelihood for container failure. While not posing any question as to the superiority and desirability of plastic bottles over other containers, they do pose a challenge to manufacturers to develop even better products for their customers and, of course, the ultimate consumers of the product.  
           [0005]    In particular, Gregory M. Fehn, in his U.S. Pat. No. 5,814,383, issued Sep. 29, 1998, realized that occasional failures of containers, especially those holding oil and stacked several layers upon each other, resulted from cracks that started at incipient creases in the containers&#39; walls. He also discovered that applying a relatively thin layer of a particular type of resin on one or both sides of the container wall may obviate the disastrous cracking. In general, the additional, protective layer should display less stiffness or crease cracking than the bulk of the container wall. If the container&#39;s contents achieve a softening of the container&#39;s interior, then the container only requires a protective layer at the outer surface.  
           [0006]    As with virtually any manufactured product, occasional problems continue to concern the companies producing plastic bottles. One such problem involves very occasional fracture and subsequent leakage through unexpected areas on the bottles&#39; surfaces. Though happening with extremely low frequency, service to the purchasers and to the ultimate consumers strongly suggests the undertaking of efforts to ameliorate the problem.  
         SUMMARY  
         [0007]    The effort to understand, and hopefully reduce, the source of such surface fracturing now suggests that typical, important, and highly desirable features on the bottles&#39; surfaces themselves create stress areas that can lead to leaks. These features may include the outline edges of the plastic that surround labels applied during the bottle&#39;s molding process. Stippling placed for its aesthetic appeal similarly can create surface stresses that carry the seeds of failures. Bumps and ridges employed for grippability may inadvertently produce the same result. Finally, identifying logos, graphics, or alphanumeric information molded into the container may lead to surface stress fracture. Similarly, dropping the container can cause flexing which, in turn, may possibly produce momentary or even long-term stress concentrations with concomitant surface fracturing.  
           [0008]    Placing a carefully chosen, additional layer of plastic on or near the bottle&#39;s exterior surface portends the reduction of fracturing resulting from surface stress exploitation. In fact, the additional layer, when sitting on the surface, may include the very features giving rise to the stresses and the resulting failures. Although not necessary, incorporating the desirable features into a protective layer permits their continued use with a high reduction of adverse stress and failure.  
           [0009]    In general, a container has a wall defining an interior and an exterior. To incorporate the protective layer, at least a part of the container&#39;s wall should have a plurality of layers. A first layer, generally lying toward, but not necessarily on, the interior has a composition of a thermoplastic polymer. A second layer lies toward or on the exterior of the container wall and occurs at the portion of the part of the wall which has the first layer. To provide its protective function, the second layer should prove less brittle than the first layer or, as an alternate criterion, have a density at least about 0.002 gm./cc. less than that of the first layer. The density difference may amount to 0.005 gm./cc. or more where both layers have a polyethylene composition. Where the less brittle layer lies on the exterior of the part of the container wall, it can include surface irregularities or, as an alternate criterion, areas of stress concentration that give cause for concern.  
           [0010]    Since it performs a protective function, the second layer will constitute less than about one half of the thickness of the part of the wall where it occurs. Alternately, the first or inner layer, since it generally provides the structural strength of a container, will constitute more than about one tenth of thickness of the entire part of the wall where the multiple layers occur.  
           [0011]    The second layer can achieve a similar protective function in those situations where it does not lie on the actual exterior of the container wall. In this situation, the second layer typically will not incorporate areas of stress concentration or surface irregularities. However, it will continue to show either less brittleness or a density at least about 0.002 gm./cc. less than the first layer.  
           [0012]    A method for making a container with the protective layer involves molding a container with the wall defining an interior and exterior. Generally, the molding process includes forming a first thermoplastic layer of at least part of the wall. It also includes forming, at that part of the wall with the first layer, a second thermoplastic layer. The second layer occurs toward the exterior of the container where the first layer exists. The two layers thus formed should display at least one of the sets of characteristics described above for the container. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]    [0013]FIG. 1 gives a cross-sectional view of a container made with two layers of plastic with the outer layer incorporating stress concentrating areas induced by surface irregularities in the form of gripping bumps but also serving to protect the container wall from incipient cracks resulting from such irregularities.  
         [0014]    [0014]FIG. 2 provides an enlarged view of the layers of plastic forming the container of FIG. 1 in an area of surface irregularities.  
         [0015]    [0015]FIG. 3 shows, in a cross-sectional view, a container wall similar to that of FIG. 2 but where in-mold labeling has created irregularities on the container surface.  
         [0016]    [0016]FIG. 4 gives a cross-sectional view of a container wall similar to that of FIG. 3 but including an extra layer of reground plastic.  
         [0017]    [0017]FIG. 5 shows, in cross-sectional view, a plastic container wall which, although not incorporating surface irregularities, sandwiches a protective layer similar to that of the prior figures between two layers of plastic.  
         [0018]    [0018]FIG. 6 provides a cross-sectional view of a portion of a container wall having an outer protective layer, an oxygen barrier layer with adhesive layers, and a layer of reground plastic. 
     
    
     DETAILED DESCRIPTION  
       [0019]    [0019]FIG. 1 shows a container generally at  15  that will alleviate failures caused by stresses originating at surface irregularities. The container  15  includes the bottom  16 , the side  17 , and the neck  18  at the top. As seen in FIGS. 1 and 2, the construction of the container  15  utilizes the thicker interior layer  20  covered by the thinner exterior layer  21 . The gripping bumps  22 , shown in exaggerated size, form part of the exterior layer  21 .  
         [0020]    The container-wall portion shown FIG. 3 appears similar to that of FIGS. 1 and 2 in that it has the inner layer of plastic  25  with a thinner outer layer of plastic  26 . Rather than the gripping bumps  22 , the container wall portion of FIG. 3 includes the label  27  applied during the actual molding of the container itself. The process of in-mold labeling commences with the placement of an actual label  27  inside the mold and against its wall prior to the insertion of any plastic material. A vacuum applied to small holes in the mold wall serve to hold the label  27  in place during the subsequent bottle-molding process. The result appears very similar to that shown in FIG. 3 in which the exterior surface  28  of the label  27  forms a continuous plane with the exterior surface  29  of the outer plastic layer  26 .  
         [0021]    Naturally, in-mold labeling constitutes a very significant improvement in the process of manufacturing plastic containers. However, careful study and extensive thought now suggest that the process itself may result in stress concentrating areas on the container&#39;s surface that may result in disturbing failures. Fortunately, the frequency with which this has occurred remained sufficiently low that it clearly did not outweigh the benefits of in-mold labeling,  
         [0022]    Returning to FIG. 3, the molten plastic of the outer layer  26 , during the molding of the container, flows around and adjacent to the label  27 . This produces the irregularities  31  and  32  in the outer layer  26  at the outer edges of the label  27 . These irregularities  31  and  32  have the potential of creating very substantial surface stress concentrations. On occasion, these stress concentrations can result in surface failures. In the simple two-layer container walls of FIGS. 2 and 3, the bumps  22  in the former and the irregularities  31  and  32  resulting from the in-mold labeling in the latter can create localized surface stress concentrations that may lead, if untreated, to failures permitting the egress of the container&#39;s contents. However, the selection of the appropriate resins, especially for the outer layer  21  or  26 , can reduce the occurrence of such failures. Thus, the outer layer  21  or  26  may display less brittleness than the inner layers  20  or  25 , respectively. This lower brittleness helps to prevent stresses from developing into cracks and to keep cracks, when created, from spreading and causing failures.  
         [0023]    The protective layers  21  and  26  do not provide the primary structural strength of the container of which they form part. There, they should typically constitute less than about one half of the wall thickness where they occur. More typically, the protective layer will provide even less than half of the wall thickness which may decrease to fourth tenths, one quarter, one tenth, or even less of the wall thickness.  
         [0024]    The inner layers  20  and  25  provide the main structural strength and rigidity of the containers. They should have at least one tenth of the container&#39;s thickness. Normally, the inner or strength layers  20  and  25  will constitute at least about five tenths, six tenths, three quarters or even more of the wall thickness.  
         [0025]    Alternately, with containers made from some thermoplastic polymers such as polyethylene or polypropylene, the relative densities between the protective layers  21  and  26  compared to the other layers may indicate that they will perform the desired functions. The term “polyethylene” refers to ethylene homopolymers and copolymers and multiphasic blends of these with other polymers. A similar definition applies to the word “polypropylene.” 
         [0026]    In referring to containers having layers with differential densities, the structural layers  20  and  25  should constitute at least one tenth of the wall thickness and the protective layers  21  and  26  less than about half of the wall thickness. Where the protective and structural layers each has a composition of a polyethylene or a polypropylene, a density difference of at least 0.002 gm./cc. may, dependent upon the specific compositions of the two layers, suffice for this purpose. The differential of at least 0.002 gm./cc. has particular applicability where the structural and protective layers both contain some form of polypropylene.  
         [0027]    For polyethylene as the plastic in the protective layer, utilizing a resin with a density of at least about 0.005 gm./cc. less than the structural layer will help minimize the development and propagation of cracks resulting from surface irregularities. While this density difference of 0.005 gm./cc. provides some protective function with polyethylene, even greater density differentials may result in greater assurance. Thus, the inner layers  21  and  26  may have a density less than that of the structural layers  20  and  25 , respectively, of 0.010 gm./cc. or even 0.015 gm./cc.  
         [0028]    Naturally, these relative densities have their greatest significance where the inner layers  20  and  25  and the outer layers  21  and  26 , respectively, have a similar chemical composition. Stated in other words, although the layers do not exactly have the same composition as indicated by the difference in densities, they may most conveniently derive from resins employing generally the same monomers. In particular, polyethylene used for the two layers but having different densities provides the desired protection against surface stress propagation. The higher densities of the inner layers  20  and  25  gives them their greater structural strength while the lower densities for the outer layers  21  and  26  helps keep the surface stresses from producing failures. High density polyethylene (HDPE) homopolymer in particular provides a structurally strong and rigid inner layer  20  or  25 , while HDPE copolymers may impart good stress properties in the outer layer. Where both layers have a composition of a polyethylene, the protective layer may appear softer than the structural layer.  
         [0029]    As suggested above, the protective and structural layers need not have the same composition nor even derive generally from the same monomer. For example, where the structural layer includes a polypropylene, the protective layer may be a polypropylene or a polyethylene.  
         [0030]    The container  15  of FIG. I employs the dual layer structure  20  and  21  over its entire extent. However, the gripping bumps  22  of FIGS. 1 and 2 or the label  27  of FIG. 3 generally occur in localized areas of the container. As a result, the protective layers  21  and  26  might only occur in those areas of the container possessing the surface stress features. Or, some areas of the container may prove more susceptible to receiving the stresses of a bottle&#39;s falling on the floor; in these instances, only the portions of the container that may fail need enjoy the security of the protective layer. Naturally, if the container does not have the dual-layer structure throughout, then the juncture between the locations with and without the protective layers  21  and  26  themselves should have a conformation that will avoid the development of separate surface stresses. The use of the dual layer structure throughout the container  15  naturally avoids this problem. Furthermore, in the usual manufacturing techniques often employed when making the container, coextruding the layers  20  and  21  of FIGS. 1 and 2 or the layers  25  and  26  of FIG. 3 over the entire container proceeds most facilely.  
         [0031]    The sections of container wall seen in FIGS.  4  to  6  incorporate additional layers for different purposes. Yet, as discussed immediately below, they all contain a layer responding to the requirements given above for avoiding failures resulting from surface stress concentrations. Thus, the container wall in FIG. 4 includes the usual inner, structural layer  37 , and the outer layer  38  which includes the label  39  incorporated during the in-mold labeling process. The surface stresses created by the placement of the label  39  remain in the outer layer  38  in the same fashion as seen in FIG. 3. Between the inner layer  37  and the outer layer  38  sits the middle layer  41  which can utilize reground scrap produced from flash and other material of prior moldings.  
         [0032]    As a specific example, the inner layer  37  may take the form of Phillips 6007 polyethylene homopolymer having a density of 0.963 gm./cc. The outer layer  38  may derive from Paxon 40-003 copolymer material of polyethylene, and providing it with a density of 0.940 gm./cc., allows it to absorb the surface stresses resulting from the inclusion of the label  39  during the molding process. The outer layer  38  may also include the colorant, in three percent, of Breen 940 also having a density of 0.940 gm./cc. To simulate a typical regrind, the middle layer  41  may include both the Phillips  6007  homopolymer of polyethylene used as the inner layer  37  (69 percent), the Paxon 40-003 copolymer (28 percent) with its dosage of colorant Breen 940 (three percent) used for the outer layer  38 . The relative thicknesses of the layers provide the inner layer  37  with approximately 10 percent of the weight of the wall, the outer layer  38  has approximately 20 percent of the weight of the wall, and the simulated regrind layer  41  provides the remaining 70 percent of the wall weight. The use of the low density polymer for the outer layer  38  has the purpose of reducing the in-mold labeling edge-impact failure sensitivity.  
         [0033]    The container wall of FIG. 5 also possesses three layers none of which displays any intentionally placed surface irregularities. The inner layer  45  provides the structural strength and rigidity. This may serve to protect against stresses developed if the bottle is dropped from an appreciable height. Alternately the container may include some sort of surface stress concentration located elsewhere in its outer layer  46  but not shown in FIG. 5. In either case, the outer layer  46  may have functions in the container that do not permit the choice of a resin which will absorb surface stress concentrations. In this instance, the inner layer  47  may help absorb the surface stress concentrations in addition to the outer layer  46 . In effect, the middle layer  47  helps insulate the structural layer  45  from cracks or other failures generated by surface stresses in the outer layer  46 . To accomplish this objective, the middle layer  47  would typically display the characteristics of the outer layers  21 ,  26 , and  38  of the prior figures.  
         [0034]    The container wall of FIG. 6 includes the layers of the less complex wall of FIG. 4. Thus, its inner layer  51  incorporates a virgin resin that will contact the bottle&#39;s contents. The outer layer  52  again has some feature that produces surface stress concentrations or may suffer surface stress concentration when dropped. In this case, it also displays the characteristics discussed above for absorbing the stress concentrations and preventing their propagation throughout the container wall to create failures. The thicker layer  53  permits the utilization of reground resin derived from plastic produced in prior moldings and either not actually required for the final container or possibly from containers not meeting specification and then recycled. The three layers  51  to  53  of course appear in the container wall of FIG. 4. However, FIG. 6 also shows the utilization of a barrier layer  54  that will prevent or at least retard the passage of gas.  
         [0035]    In many instances, the oxygen in air may deleteriously affect the contents that the bottle will hold. In this instance, the barrier layer  54  may have a composition of ethylene vinyl alcohol copolymer (“EVOH”) which has found widespread use as a barrier layer in plastic containers. To guard against the delamination of the sundry layers, the container wall of FIG. 6 includes the adhesive layers  55  and  56  which attach the barrier layer  54  to its surrounding layers  52  and  53 , respectively.  
         [0036]    A specific construction for the container wall of FIG. 6 may incorporate an inner layer  51  having a composition of virgin homopolymer polyethylene with a density of 0.960 gm./cc. or even greater. This layer may constitute approximately 20 percent of the overall wall thickness. The regrind layer  53 , providing about 55 percent of the wall thickness, may have a composition of 50 percent reground trim-with the other 50 percent provided by virgin polyethylene of density of 0.955 gm./cc. The outer, or protective, layer  52  may take the form of virgin polyethylene with a density of 0.955 gm./cc. Its lower density compared to the inner layer  51  protects the container from failures caused by surface stress concentrations. It may constitute approximately 20 percent of the wall thickness. The remaining sandwich layer of adhesive  55 , EVOH  54 , and adhesive  56  would constitute the remaining five percent of the wall thickness.  
         [0037]    This structure of FIG. 6 actually permits the use of stronger, higher density polyethylene (HDPE) resins or high stiffness polypropylene resins as its inner layer  51  as well as portions of the reground layer  53  and the outer layer  52  which may include some reground material. This construction permits an overall thinner container wall utilizing less plastic but producing the same strength as prior, thicker container walls. The exterior, protective layer  52  helps prevent the progression of surface stress concentrations into container failures.  
         [0038]    A container wall having a similar structure to that discussed above for FIG. 6 may extend the differences between the various layers. Thus, the inner layer  51  may include an HDPE with the density of at least, and preferably greater than, 0.960 gm,/cc. The outer, protective layer  52  may include the virgin lower density polyethylene having a density of 0.950 gm./cc., 0.940 gm./cc., or lower. This polyethylene may constitute 50 percent of the outer layer  52  with the remainder of that layer derived from reground material The middle layer  53  may have a composition of one half of reground trim while the remaining one half utilizes virgin polyethylene having a higher density of 0.960 gm./cc. The EVOH sandwich of the layers  54  to  56  would remain the same. The higher density of the inner layer  51  and the reground layer  53  provide a container with the same or even higher structural strength while the lower density of the outer layer  52  protects against failures resulting from surface stresses.