Abstract:
A method of making a fender protective structure having a layer of resilient plastic/elastomer alloy having a delayed elastic response, comprising providing a mold, placing elastomer spacers in the mold, adding the plastic and elastomer to the mold under heat and pressure, and opening the mold and removing the spacers to create voids the alloy.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to an impact pad for absorbing forces, and more particularly to a method of making an impact pad for absorbing forces having improved energy absorbent materials.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the operation of docking a ship, because of the speed of approach, swells, currents and winds it is imperative to provide a protective docking system to reduce potential damage and impact to the ship, dock or pier. In the case of holding ships, such impact pads are used to protect both the holding ship and the smaller vessel transported therein.  
           [0003]    Examples of fender protective structures for these types of applications can be found in commonly owned U.S. Pat. Nos. 4,923,550, 4,596,734, 4,679,517 all of which are issued to Kramer and are hereby fully incorporated herein by reference. The fender protective structures of these systems are comprised of a very hard outer plastic layer of ultra high molecular weight polyethylene (UHMWPE), a highly flexible, i.e. rubbery, intermediate elastomeric layer and a very hard base layer comprised of plastic. The base layer is necessary for installation purposes because a system with a conventional elastomeric layer bonded to a plastic layer is too flexible to work with, particularly when the elastomeric layer is partially counter bored.  
           [0004]    Elastomer and plastic alloys are known in the marine art for utilization in the journal bearings that support the propeller shafts. More particularly, the alloy is used as stave material in the journal bearing. An example of such alloys is described in commonly owned U.S. Pat. No. 4,735,982 to Orndorff, Jr. Orndorff, Jr. teaches mixing a thermoset rubber compound and a thermoplastic, with the rubber compound having low friction as well as good oil and water resistant properties. Low friction is defined as material which develops hydrodynamic lubrication at normal shaft operating speeds. Use of low friction materials is important in bearing applications because shaft wear must be minimized. For this reason, higher friction rubber compounds are inappropriate in the alloy described in Orndorff, Jr.  
           [0005]    Efforts to improve such fender protective systems have led to continuing developments to improve their versatility, practicality and efficiency.  
         DISCLOSURE OF THE INVENTION  
         [0006]    An object of the present invention is to provide a method of making two layer bonded fender protective structures.  
           [0007]    Another object is to provide a less expensive method to manufacture a bonded fender protective structure.  
           [0008]    A general object is to provide an efficient and effective method of manufacturing a fender protective structure which is relatively hard, has a resilient layer with a delayed elastic response and is able to bulge upon impact for improved energy absorption.  
           [0009]    According to the preferred embodiment of the present invention, a method of making a fender protective structure comprising the steps of providing a mold, disposing cured elastomeric spacers in the mold, pouring an alloy mix of crumb elastomer and plastic powder into the mold over said elastomeric spacers, pouring a plastic layer over the alloy mix, curing both the plastic layer (to make a very hard top layer) and the elastomer and plastic alloy mix (to thereby make a relatively hard elastomer and plastic alloy, the alloy being resilient and having a delayed elastic response) and, removing the elastomeric spacers. Removal of the elastomeric spacers creates voids in the alloy to reduce weight and to allow for expansion of the alloy upon impact. The fender protective structure thus includes an improved energy absorbing elastomer plastic alloy layer.  
           [0010]    The present invention provides a superior method that makes fender protective structures practical, while maintaining optimum resiliency for energy absorption and return and optimum rigidity for ease of installation.  
           [0011]    The present invention further provides a method for making a two layer fender protective structure having a relatively hard, resilient with a delayed elastic response, structure which has room to expand on impact, and which slowly returns to its original shape for receiving subsequent impacts. The structure is instantaneously low in resilience, meaning it has high energy absorbing characteristics, and has a delayed elastic response or resilience, meaning it is able to regain its original dimensions in time delay after impact upon removal of an applied impact force, the delay being between 1 second to over an hour depending on circumstances.  
           [0012]    These and other objects, features and advantages of the present invention will become more apparent in the light of the detailed description of exemplary embodiments thereof, as illustrated by the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a perspective view illustrating the present invention wherein a larger vessel only partly shown, has fenders disposed along the water level under the vessel, with a second vessel being received in the hull of such first vessel.  
         [0014]    [0014]FIG. 2 is a side elevational view of the back side of a fender protective structure in accordance with the present invention, taken on line  2 - 2  of FIG. 1.  
         [0015]    [0015]FIG. 3 is a plan view of a fender protective structure taken in cross section on line  3 - 3  of FIG. 2.  
         [0016]    [0016]FIG. 4 a  is a is a graph illustrating the impact force versus deflection force of wood, showing a point of crushing or permanent deformation.  
         [0017]    [0017]FIG. 4 b  is a graph illustrating the impact force versus deflection force of high damping, low resiliency rubber.  
         [0018]    [0018]FIG. 4 c  is a graph illustrating the impact force versus deflection force of low damping, high resiliency rubber.  
         [0019]    [0019]FIG. 4 d  is a graph illustrating the impact force versus deflection force of the elastomer and plastic alloy of the present invention.  
         [0020]    [0020]FIG. 5 is a schematic cross section taken through a prior art fender panel mold.  
         [0021]    [0021]FIG. 6 is a schematic cross section taken through a mold in which the components of a fender panel embodying the concept of the present invention have been assembled prior to processing.  
         [0022]    [0022]FIG. 7 is a graph of an impact test conducted on the alloy of the invention at one impact load.  
         [0023]    [0023]FIG. 8 is a graph of an impact test conducted on the alloy of the invention at a second impact load.  
         [0024]    [0024]FIG. 9 is a graph of an impact test conducted on rubber at one impact load.  
         [0025]    [0025]FIG. 10 is a graph of an impact test conducted on rubber at a second impact load. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Referring now to the drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a portion of a ship or larger vessel  10  whose hull is designated by the numeral  11 . Such vessel  10  is a docking vessel with an open hull capable of lowering a door, not shown, to allow water into the hull to receive smaller vessels  12  such as landing craft, whereby the larger vessels may transport such smaller vessels for ferrying operations close to land.  
         [0027]    Located above and below the water line as depicted in FIG. 1, there are a plurality of impact pads or fender protective structures  15  extending in a horizontal direction and suitably attached to the hull of the larger vessel  10 . Each fender  15  is a composite member which includes a longitudinally extending, inwardly disposed (as viewed in the larger vessel  10 ) first layer of plastic material  16  and a resilient layer  17  with a delayed elastic response attached thereto. The top layer  16  of the present invention may not be necessary in certain applications of the present fender protector of the present invention.  
         [0028]    Referring now to FIG. 2, the resilient, elastic layer  17  has a plurality of rectangular bore holes  30  provided therethrough, for providing space to allow the material of layer  17  to bulge upon impact force being applied thereto. It should be understood that resilient, elastic layer  17  is relatively inflexible; it is not rubbery and is fairly hard. It rapidly bulges upon impact, but slowly returns to its original configuration. Layer  17  also has a plurality of circular mounting bore holes  35  provided therein for the mounting of the pads. Nut and bolt connection assemblies  36  hold the pads in place. It is significant that layer  17  is hard enough to hold assemblies  36  sufficiently so that they can properly hold the fender protective structure in place in the hull. It is to be noted that holes  30 ,  35  may also be any of a number of other shapes not specifically shown herein. Holes  30  are preferably molded into layer  17 . Mounting bore holes  35  are very important for isolating the fasteners (nut and bolt connectors) from impact forces. Bolt stress reduction is well over 75% due to providing the radial bulge area (elastomer compounds are not compressible like gases, at pressures less than 10,000 psi). Impact stresses on fender protective structures or impact pads are well under 5,000 psi. If there is no bulge area, the fender protective structure is virtually incompressible and might as well be made of steel. The radial bulge area allows the fastener bolts or studs to be adequately tightened or prestressed, and prevents damage to the fasteners upon impact to the fender protective structure.  
         [0029]    Referring now to FIG. 3, the first layer or outermost layer  16  as viewed from inside the larger vessel  10  has an outer surface  20  and an inner surface  21 . First layer  16  is a very hard plastic, preferably a thermoplastic, and most preferably ultra high molecular weight polyethylene (UHMWPE) which has a melt flow index less than 0.15 measured in accordance with the test procedure of ASTM 1238-65T modified by an additional 3 kilogram load. UHMWPE is understood to be a polyolefin having an ultra high molecular weight which measured by the solution viscosity method is greater than 2.5 million. Polyolefin is understood to be a polymer or copolymer of one or more mono-olefins having no more than four carbon atoms, as well as mixtures of these polymers and/or copolymers, between themselves. It may also be a thermoplastic-rubber polymer alloy or blend comprised of UHMWPE and a low friction thermoset rubber compound. Such alloys are described in commonly owned U.S. Pat. No. 4,735,982, the disclosure of which is hereby fully incorporated herein by reference. It is to be noted that the alloy described in U.S. Pat. No. 4,735,982 can only be utilized for the top layer of the present invention. Alternatively, layer  16  may be comprised of a compression molded flame retardant high impact strength UHMWPE composition as described in commonly owned U.S. Pat. No. 5,286,576, the disclosure of which is also hereby fully incorporated herein by reference. In the finished molded or densified form, this composition is comprised of 86 volume percent UHMWPE, a minimum of 4.40 volume percent of a flame retardant compound (10 pph based on UHMWPE for ammonium polyphosphate on a weight bases) in a minimum of 6 volume percent of chopped reinforcing fiber (19.0 pph based on UHMWPE for chopped fiberglass on a weight bases). In addition, the bulk volume of the chopped reinforcing fibers used in the above composition has measured by a tapped density test described herein must be at least about 27 volume percent of the finished compression molded volume of flame retardant high impact composition. The maximum volume loadings of the flame retardant compound and chopped reinforcing fibers are only limited by the minimum requirement of 7 ft-lb/inch width of notched izod impact strength, as long as both components are present above the minimum leadings described above.  
         [0030]    The resilient, elastic layer  17  is relatively hard compared to layer  16 , and not highly flexible or rubbery. Layer  17  has an outer surface  22  that is in abutting contact with and bonded to the inner surface  21  of the first layer  16 . To attach the fender structure  15  to the hull  11  of a vessel, the respective first and second layers have a plurality of mounting bores  35 , extending in alignment with a narrower bore  34  extending into the hull  11 . Bore  35  extends to approximately the middle to two thirds the thickness of the resilient, elastic layer, thereby defining a shoulder or bottom  38  to receive the flat base of a washer  39  and the head of a nut  40  secured to the threaded head of a bolt  41  that is suitably fastened to the hull  11 . Such bores  35  are molded into the resilient, elastic layer  17  and the top layer  16  and extend about one half to two thirds of the way through such resilient layer leaving a space  27  that facilitates the attachment of the fender to the bulkhead or hull  11 . Bore  34  is drilled through area  27  of layer  17  for receiving bolt  41 . The open area of the bores of the resilient, elastic layer to the total layer thereof is on the order of 30% to 60%, thus permitting sufficient bulging of the elastic layer so that damage to the connection assemblies  36  is avoided. In addition, the open area of the bores lightens the resilient, elastic layer.  
         [0031]    A preferred thickness for use of the composite laminate structure as a fender protective device, the UHMWPE layer  16  is approximately 3.175 centimeters to 1.27 centimeters thick.  
         [0032]    Although the preferred form of the present invention shows the first layer of UHMWPE as solid, it could be perforated or have bores; however, there is risk that sharp projection on the impacting vessel could catch in the bores and rip the pad if the force exceeded the strength of either the first or second layers, or the bond between.  
         [0033]    Heretofore, it was desirable to include a third layer between the elastomeric layer and the hull in order to provide rigid integrity to the composite laminate fender structure predominantly because prior elastomeric layers were too flexible. The present invention provides an improvement to prior fender structures as will be described in further detail hereinafter. By virtue of the improvement, the elastomeric layer is eliminated and replaced by a relatively hard, resilient layer with a delayed elastic response, and the third layer between the elastomeric layer and the hull is eliminated.  
         [0034]    The preferred material for the resilient, elastic layer  17  of the present invention consists of an elastomer and plastic blend or alloy, preferably crumb natural rubber held together by a matrix of the thermoplastic UHMWPE. An elastomer is defined as a substance that can be stretched at room temperature to at least twice its original length and, after having been stretched and the stress removed, returns with force to approximately its original length in a short time. (See Glossary of Terms as prepared by ASTM Committee D-11 on Rubber and Rubber-like Materials, published by the American Society of Testing Materials). The elastomeric or rubber material that can be used in constructing the present invention includes any of the well known elastomers, such as natural rubber, SBR rubber, copolymers of butadiene and acrylonitrile, copolymers of butadiene and styrene, copolymers of butadiene and alkyl acrylates, butyl rubber, olefin rubbers such as ethylene-propylene and EPDM rubber, fluorocarbon rubbers, fluorosilicone rubbers, silicone rubber, chlorosulfonated polyethylene, polyacrylates, polybutadiene, polychloroprene and the like. As noted before, however, natural rubber and other elastomers that have high elasticity are most preferred. Such elastomers have a shore A hardness of less than 65 before grinding. Of particular interest in the present invention is crumb rubber obtained by grinding used automotive or truck tires, because they are predominantly natural rubber and ground tire material is inexpensive. Fabric and steel wire particles originally in the tires should be removed.  
         [0035]    The new alloy is hard, preferably having a hardness of 48 shore D, and fasteners can effectively be tightened against it. It is not “rubbery.” As explained earlier, the alloy has a delayed elastic response, and if it is impacted by a large object such as a ship it will absorb the energy imparted by the force and deform; however, it will over an extended period of time (from seconds to over an hour) recover its initial shape and dimensions.  
         [0036]    Referring now to FIG. 4 a , wherein an impact force versus deflection curve for wood is illustrated. It can be seen that force is applied up to a maximum point  60 . When force is relaxed, the material is permanently set and does not recover. This curve illustrates that wood has high damping, low resiliency and very low recovery. The amount of energy absorption of the material is represented by the area  62  under the curve. It can be seen that wood has high energy absorption but is good for only one severe impact, after which it loses its favorable characteristics.  
         [0037]    Referring now to FIG. 4 b , wherein an impact force versus deflection curve for a high damping, low resiliency rubber is illustrated. It can be seen that force is applied up to a maximum point  64 . When force is relaxed, the material is permanently set and does not recover. This curve is similar to the one for wood, meaning that the material has high damping, low resiliency, low recovery, and that it has high energy absorption (area  66 ) but is good for only one severe impact, after which it loses its favorable characteristics.  
         [0038]    Referring now to FIG. 4 c , wherein an impact force versus deflection curve for a low damping, high resiliency rubber is illustrated. It can be seen that force is applied up to a maximum point  64 . When force is relaxed, the material returns reasonably close to its original shape, meaning the material has low permanent set. This material is therefore good for multiple severe impacts. Energy absorption (represented by hysteresis area  70 ) is, however, quite low, meaning that the material has high rebound and produces a slingshot effect, which is highly undesirable.  
         [0039]    Referring now to FIG. 4 d , wherein an impact force versus deflection curve for the elastomer and plastic alloy of the present invention is illustrated. It can be seen that force is applied up to a maximum point  72 . When force is relaxed, the material returns close to its original shape, meaning the material has low permanent set. In addition, energy absorption is high, since hysteresis area  74  is large. It can be seen that a factor contributing to the high energy absorption is that the top curve  76  for the present invention is convex, whereas the top curves  78 ,  80  for the materials for FIGS. 4 b  and  4   c , respectively, are concave. It is to be noted that the hysteresis area  74  is about twice as large as that of FIG. 4 c.    
         [0040]    [0040]FIG. 4 d  therefore illustrates that the elastomer plastic alloy of the present invention provides the favorable characteristics for a fender protective structure without the drawbacks of prior materials. That is, the present elastomer plastic alloy has high damping, high resiliency, low permanent set, high energy absorption and a delayed elastic response. The material is good for multiple severe impacts and does not give a sling shot effect due to the delayed response.  
         [0041]    As mentioned in the Background of the Invention section, commonly owned U.S. Pat. No. 4,735,982 to Orndorff, Jr. teaches mixing a thermoset rubber compound and a thermoplastic for use as staves in journal bearings. Low friction is critical property in the Orndorff, Jr. application. The friction properties of the alloy of the present invention are not necessarily as important as high elasticity, low hardness and high energy absorption. However, highly elastic, soft elastomers with high energy absorption inherently have high friction properties. The low friction elastomers of U.S. Pat. No. 4,735,982 inherently have low elasticity, high hardness and low energy absorption. The teachings of Orndorff, Jr. are therefore not compatible with the requirements of the present invention.  
         [0042]    Manufacture of the fender protective structure of the present invention is as follows. For the elastomer used in the present alloy, a rubber compound is cured and, (in order to mix the two components), is ground to a suitable size to make a crumb rubber. Conventional grinding methods can be utilized, such as mechanical or cryogenic grinding. The crumb rubber is thoroughly dry mixed with a UHMWPE powder until a generally uniform random dispersion of the rubber is achieved. The particle size of the rubber and UHMWPE preferably pass through a Tyler mesh screen below 35, with the 20 mesh particles being more preferable. Because of the free flowing nature of the UHMWPE, the rubber and UHMWPE powder mix very easily. The preferred ratio of constituents is on the order of 30% UHMWPE powder and 70% crumb rubber by weight. Thus a elastomer plastic mixture  94  is created.  
         [0043]    Referring now to FIG. 5, heretofore molds  84  were provided for curing fender protective structures. Projections  86  were provided integrally therewith for displacing the rubber compound for creating expansion bores in the cured resilient layer. The projections  86  had to be treated with a mold release material and had to have tapered sides  88  in order to facilitate separation of the mold from the material after the curing cycle. The necessary tapers and large number of projections  86  unduly increased tooling costs for producing fender protective structures. Even with the tapered projections and mold release preparation, though, it is still difficult to separate the mold and finished product, resulting in increased manufacturing costs due to time and labor.  
         [0044]    Referring now to FIG. 6, a mold  90 , having the desired shape of the fender panels is provided. In accordance with another aspect of the present invention, a plurality of cured rubber blocks, plugs or projections  92  (having the desired shape of the bores  30  in FIG. 2) are placed in the mold at the desired locations for the bores  30 . Other cured rubber projections are provided for bores  35 . The rubber and UHMWPE mix  94  is then poured into the mold body over the rubber blocks  92  and the blocks for bores  35 . The rubber and UHMWPE mix  94  is then covered with a layer  96  of UHMWPE powder. A mold cover  98  is then secured onto the mold body and sufficient pressure and heat are applied thereto to mold the elastomer and rubber together and melt the polyethylene powder. The heating temperature must be above the glass transition temperature (Tg) of the plastic. Desirable heating temperatures range from about 290° F. to about 360° F. and preferably between 310° and 350° F. After the melted polyethylene powder has: a) coalesced to form a top polyethylene layer  16 ; and b) sufficiently bonded the crumb rubber particles together, the mixture is cooled under pressure (at least 600 psi and desirably more than 1000 psi) to ambient temperature in order to prevent cracking or strain failure of the alloy. The mold is then opened, and the finished composite is removed. This procedure makes for a very even and high quality molded article because the flow distortions problematic of conventional thermoplastic compounds are eliminated.  
         [0045]    It is to be noted that the rubber projections  92  are preferably made of the same elastomeric material as defined above for use in the elastomer plastic alloy, such as a nitrile rubber. The rubber projections  92  are easy to manufacture and inexpansive to make, regardless of the required shape. Rubber projections or spacers  92  shrink more than the rubber plastic alloy during cooling, and therefore are easy to remove. That is the coefficient of thermal expansion of the rubber projections is greater than the very low coefficient of thermal expansion for the rubber plastic alloy and the rubber projections are more elastic than the alloy (after molding). These characteristics facilitate easy removal of the projections from the alloy after the fender protective structure is cooled. To this end, it has been found that the rubber projections “pop” out of the cured alloy when urged slightly. Sometimes the projections  92  simply fall out. It is now apparent that use of the removable rubber projections  92  reduces manufacturing costs of the fender protective structure of the present invention. It should be noted that spacers in prior versions of fender protectors were made from steel, which did not shrink at all during cooling—and whose removal was extremely difficult.  
         [0046]    A way to improve the notoriously poor thermal conductivity (heat transfer) of the cured rubber void forming projections  92  involves increasing the slightly greater coefficient of thermal expansion of the rubber projections  92  so that they will shrink dimensionally even more than the UHMWPE/rubber polymer alloy. This should assure an easy release of the rubber projections  92  at the cooled-down temperature. One could not advantageously add powdered aluminum to the uncured rubber compound, similar to what is often done in tires for high speed (Mach 2) aircraft bombers, because this would decrease the coefficient of thermal expansion, causing the rubber projections  92  to be tightly locked in place instead of easily releasing. The inventor has determined that aluminum metal cylinders can be inserted into circular holes spaced periodically across the plan (top) view of the projections  92  and the projections for bores  35 . Aluminum has roughly 1000 times the thermal conductivity of rubber. Replacing only ten percent of the rubber plan view area with the aluminum cylinders would significantly increase the heat flow through the blocks as shown in the Table discussed below, without noticeably reducing the coefficient of thermal expansion.  
         [0047]    Since the metal cylinders are not adhered to the rubber (inserted after cure), the cylinders will not affect the elastic deflection. Copper cylinders (2,000 times the thermal conductivity of rubber) or heat pipes (over 100,000 times the thermal conductivity of rubber) would work even better, but at a significant increase in cost.  
         [0048]    Accordingly, high thermal conductivity materials  99  are combined with projections  92  to increase their coefficient of thermal or heat conductivity. Materials  99  can be cylindrical aluminum containers as shown in FIG. 6. Materials  99  increase the heat flow to the alloy being cured. Upon cooling, the bore-producing projections shrink more than the plastic/elastomer alloy due to their higher coefficient of thermal expansion, to make the separation of the projections expedient. The following Table and calculations show how to determine the number of plugs required if made from aluminum or copper.  
                                         TABLE I                                       Thermal Conductivity, k           Material   (BTU - ft/Hour - ft 2  - ° F.)                                        Nitrile Rubber   0.098           Aluminum   117           Copper   224           Steel   26           Heat Pipes   100,000+                      
 
         [0049]    Heat conducted through 1 ft 2  of area=k (1 ft 2 )  
         [0050]    A. Nitrile Rubber=0.098 (1)=0.098 BTU-ft/Hour-°F.  
         [0051]    Replacing 10% of area with aluminum plugs:  
         [0052]    Conducted Heat=0.098 (0.9)+117 (0.1)  
         [0053]    =0.088+11.7  
         [0054]    =11.788 BTU-ft/Hour-°F.  
         [0055]    Conducted Heat Ratio=aluminum plugs/nitrile rubber=11.788/0.098=120.3  
         [0056]    B. Replacing 10% of area with copper plugs:  
         [0057]    Conducted Heat=0.098 (0.9)+224 (0.1)  
         [0058]    =0.088+22.4  
         [0059]    =22.488 BTU-ft/hour-°F.  
         [0060]    This is not quite as good as for steel.  
         [0061]    Heat conducted through 1 ft 2  of area Steel=26 (1)=26 BTU-ft/hour-°F.  
         [0062]    C. Replacing 20% of area with copper plugs:  
         [0063]    Conducted Heat=0.098 (0.8)+224 (0.2)  
         [0064]    =0.0784+44.8  
         [0065]    =44.88 BTU-ft/hour-°F.  
         [0066]    Conducted Heat Ratio=copper plugs/steel=44.88/26=1.73 (an improvement)  
         [0067]    D. No. of 1″ diameter copper plugs required for 20% coverage:  
         [0068]    =0.2 (144)/II(1)2/4=0.2 (144) (4)/Π=36.67  
         [0069]    =37 Plugs  
         [0070]    It should also be noted that the present alloy shows no loss of adhesion to the top plastic layer or other plates. No adhesives or other special materials or procedures are needed to cause the top layer to bond to the resilient layer.  
         [0071]    The preferred characteristics of the elastomer/plastic alloy of the present invention are a shore A hardness of 95, an elastic or Young&#39;s modulus in compression of on the order of 5,469 psi for a shape factor=0.381, and a specific gravity of 1.09 (30% UHMWPE). It has been discovered that the present elastomer and plastic alloy as a unique combination of high damping, low resiliency and high recovery not found in conventional materials utilized before.  
         [0072]    Immediate resiliency is defined herein as the percentage of impacting energy that is returned to the impacting object on the immediate rebound (ASTM-D-2632). In the case of a fender protective structure, a low resilience percentage is desired in order to eliminate the possibility of sling shot or whiplash occurring to the occupants of impacting vessels. To this end, the present alloy has similar deflection values and similar resiliency percentages (23-25%) of prior elastomeric compounds used in fender protective structures. An advantage of the present alloy though, because of its delayed elastic response, is that it return within several minutes to very near its original thickness whereas former high damping elastomeric compounds exhibited considerable permanent set, because conventional elastomeric compounds having high damping (low resilience) invariably have high permanent set.  
         [0073]    The present alloy permits a lower cost fender protective structure which because a third layer is unnecessary and the present alloy is less than half the cost of prior elastomeric compounds.  
         [0074]    Energy absorption of the present invention is also maximized because of the greater thickness of the elastomer alloy layer compared with former three layer designs. The result is that the bolts/studs are better isolated from impact forces while reducing the complexity of the system. The bonded two layer design of the present invention also has a much larger circumferential area (for the rubber to bulge upward) between the washer OD and the hole ID then is the case with conventional three layer designs. This helps to better isolate the bolts from impact forces.  
       EXAMPLE  
       [0075]    Tests were conducted to determine the relative energy absorption valves between an EPDM rubber (S-401) developed by The B. F. Goodrich Company for bonded 3-layer impact pads (or batterboard) use with the 2-layer batterboard incorporating crumb rubber (such as made from ground up truck tire rubber). The S-401 rubber cannot be bolted against a rigid support because it is a relatively soft elastomer, and could not be used in the bonded 2-layer design.  
         [0076]    In the graphs of test plats shown in FIGS.  7 - 10 , the area shown between the load and unload (or rebound) curves shows the absorbed energy. The experiments were conducted to show and compare the absorbed energy.  
         [0077]    It has been determined that high speed drop test impacts on batterboard could be simulated with equivalent damage by subjecting the test samples to very high compressive loads at much slower speeds using a large, conventional physical property test machine. 50,000 pounds has been found to be a high load, and 100,000 pounds has been found to be an extremely high load. (The impact energy is the same on both the drop test and the compressive test.)  
                                                                       TABLE II                                       UHMWPE/CRUMB   EPDM RUBBER           RUBBER ALLOY   (S-401)                50,000   100,000   50,000   100,000           LBS   LBS.   LBS   LBS                        Overall Dimensions   4 × 4 ×   4 × 4 ×   4 × 4 ×   4 × 4 ×       (Plan View and   3.3125 in.    3.3125 in.    3 563 in.   3.563 in       Thickness)       Elastomer Thickness   2.625 in.   2.625 in.   2.325 in   2.325 in.       Elastomer Hardness   75   75   65   65       Overall Dimensions   4.625 in. ×   5.25 in. ×   4.125 ×   4.313 ×       (Plan View)   4.625 in.    5.25 in.   4.125 in.    4 5 in.       (After Test)       Overall Thickness   2.875 in.    2.5 in.   3.313 in.   3.063 in.       Minutes After       Testing       Recovery After   85.3%   75.5%   91.0%   86.0%       5 Minutes       Recovery After       (1.5 Hours)       (2 Hours)       1.5 Hours and       85.3%       91.0%       2.0 Hours       Comments   No cracks or   Started           splits   cracking at               70,000 lbs.       Amount of   25,769   47,692   13,461   30,769       Energy Absorbed   in. - lbs.   in. - lbs.   in. - lbs.   in. - lbs.                  
 
         [0078]    (The same sample in each case was consecutively tested at 50,000 and 100,000 lbs.)  
         [0079]    The results of the tests are significant. The bonded 2-layer with the alloy absorbed 91.4% more energy than the S-401 rubber (25,769 vs. 13,461 in.-lbs.) when exposed to the 50,000 lb. force, and absorbed 55.5% more energy than the S-401 rubber (47,692 vs. 30,769 in.-lbs.) when exposed to the 100,000 lb. force. The S-401 rubber compressed more and returned much more energy on the rebound (sling-shot effect). The destructive reaction pressure on the back-up metal supporting structure would also be significantly reduced with the bonded 2-layer because of its greater energy absorption.  
         [0080]    The results of the tests are shown for multiple or repeated impact capability. The test shown in FIG. 4C shows immediate elastic recovery for the S-401 rubber, whereas the new alloy&#39;s tests shown in FIG. 4D shows a delayed elastic response or resiliency.  
         [0081]    The test results shown on the foregoing Table and FIGS.  7 - 10  indicate that the new alloy was recovering its thickness at a faster rate than the S-401 rubber (see both “Recovery” rows in Table II).  
         [0082]    Although the invention has been shown and described with exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto without departing from the spirit and scope of the invention.